Dynamics of bright and dark localized excitonic magnetic polarons in CdMnTe spin glass compound
NASA Astrophysics Data System (ADS)
Gnatenko, Yuriy; Bukivskij, Petro; Piryatinski, Yuriy
2015-03-01
The measurements of the magnetic properties of spin glass (SG) system indicate that the magnetic relaxation is characterized by a broad range of times below Tf. Here, for the first time, we have investigated time-resolved photoluminescence spectra of Cd0.70Mn0.30Te SG compound at the temperature below the freezing temperature Tf. This enables us to study the dynamics of different localized excitonic magnetic polarons (LEMPs) at T = 0.7Tfexcited in the crystal regions where various microscopic magnetic spin states (MMSSs) are formed. It was found that there is a broad distribution of the lifetimes of the LEMPs which have different lifetimes but same energies. It was shown that the presence of the long-lived LEMPs is caused by the admixture of the optically active bright exciton states to the dark exciton states as a result of the local magnetic fields formation. The lifetimes of these dark LEMPs correspond to hundreds of nanoseconds. It was also found that the decay process of the PL exciton band intensity is described by the Kohlrausch-Williams-Watts stretched exponential function which describes the recombination processes which correspond to the emission of the LEMPs formed in the crystal region of the finite clusters as well as the infinite cluster. These complex dynamical phenomena, observed for Cd1-XMnXTe at low temperatures, reflect the spatially heterogeneous dynamics in the SG system which is due to the presence of different MMSSs below Tf.
Nelson, Heidi D; Bradshaw, Liam R; Barrows, Charles J; Vlaskin, Vladimir A; Gamelin, Daniel R
2015-11-24
Spontaneous magnetization is observed at zero magnetic field in photoexcited colloidal Cd(1-x)Mn(x)Se (x = 0.13) quantum dots (QDs) prepared by diffusion doping, reflecting strong Mn(2+)-exciton exchange coupling. The picosecond dynamics of this phenomenon, known as an excitonic magnetic polaron (EMP), are examined using a combination of time-resolved photoluminescence, magneto-photoluminescence, and Faraday rotation (TRFR) spectroscopies, in conjunction with continuous-wave absorption, magnetic circular dichroism (MCD), and magnetic circularly polarized photoluminescence (MCPL) spectroscopies. The data indicate that EMPs form with random magnetization orientations at zero external field, but their formation can be directed by an external magnetic field. After formation, however, external magnetic fields are unable to reorient the EMPs within the luminescence lifetime, implicating anisotropy in the EMP potential-energy surfaces. TRFR measurements in a transverse magnetic field reveal rapid (<5 ps) spin transfer from excitons to Mn(2+) followed by coherent EMP precession at the Mn(2+) Larmor frequency for over a nanosecond. A dynamical TRFR phase inversion is observed during EMP formation attributed to the large shifts in excitonic absorption energies during spontaneous magnetization. Partial optical orientation of the EMPs by resonant circularly polarized photoexcitation is also demonstrated. Collectively, these results highlight the extraordinary physical properties of colloidal diffusion-doped Cd(1-x)Mn(x)Se QDs that result from their unique combination of strong quantum confinement, large Mn(2+) concentrations, and relatively narrow size distributions. The insights gained from these measurements advance our understanding of spin dynamics and magnetic exchange in colloidal doped semiconductor nanostructures, with potential ramifications for future spin-based information technologies. PMID:26417918
Impurity effects on polaron-exciton formation in conjugated polymers
Ribeiro, Luiz Antonio Ferreira da Cunha, Wiliam; Neto de Oliveira, Pedro Henrique; Gargano, Ricardo; Magela e Silva, Geraldo
2013-11-07
Combining the one-dimensional tight-binding Su-Schrieffer-Heeger model and the extended Hubbard model, the collision of two oppositely charged polarons is investigated under the influence of impurity effects using a non-adiabatic evolution method. Results show that electron-electron interactions have direct influence on the charge distribution coupled to the polaron-exciton lattice defect. Additionally, the presence of an impurity in the collisional process reduces the critical electric field for the polaron-exciton formation. In the small electric field regime, the impurity effects open three channels and are of fundamental importance to favor the polaron-exciton creation. The results indicate that the scattering between polarons in the presence of impurities can throw a new light on the description of electroluminescence in conjugated polymer systems.
Exciton and polaron interactions in self-assembled conjugated polymer aggregates
NASA Astrophysics Data System (ADS)
Grey, John K.; Thomas, Alan K.; Gao, Jian
2015-08-01
We study exciton coupling and interconversion between neutral and charged states of different spin in pi-stacked conjugated polymer aggregates. Rigorous self-assembly approaches are used to prepare aggregate nanofibers that permit reliable control of polymer chain conformational and packing (intra- and interchain) order within these structures. Exciton coupling can be tuned between the H- and J-aggregate limits, which has important implications for determining the fates of excitons and polarons. Single molecule intensity modulation spectroscopy was performed on individual nanofibers and large quenching depths of emissive singlet excitons by triplets are found in J-aggregate type structures. We propose that high intrachain order leads to exciton delocalization that effectively lowers singlet-triplet energy splittings thus increasing triplet yields. Exciton-polaron and polaron-polaron interactions are next investigated in both H- and J-type nanofibers where polarons are injected by charge transfer doping. We find that the enhanced intrachain order of J-aggregates enables efficient intrachain polaron transport and leads to significantly larger doping efficiencies than less ordered H-aggregates. As polaron densities increase, signatures of spin-spin interactions between polarons on adjacent chains become appreciable leading to the formation of a spinless bipolaron. Overall, these studies demonstrate the potential for controlling and directing exciton and polaron interactions via tuning of subtle intra- and interchain ordering characteristics of aggregates, which could benefit various polymeric optoelectronic applications.
Dangling Bond Magnetic Polaron in CdSe nanocrystals
NASA Astrophysics Data System (ADS)
Efros, Alexander; Rodina, Anna
In this work we study theoretically the effect of the spins of the surface dangling bonds on the PL of CdSe nanocrystals (NCs). We show that spins of dangling bonds open new recombination channels for the dark exciton recombination which is connected with flip-flip and flip-flop spin-assisted recombination of the dark exciton. Calculations show that at low temperatures the interaction between dangling bonds and NC excitons leads to the dynamical polarization of the dangling bond spins along the anisotropic axis following by the formation of a dangling bond magnetic polaron. An increase of the temperature, or of the external magnetic field perpendicular to the anisotropic axis, destroys the polaron state. This results in a shift of the transition energy and an increase of its recombination rate. Thus thermal depolarization of the polaron state may explain the small activation energies observed in the temperature dependences of the exciton lifetimes in CdSe NCs. The exchange interaction of the electron spin with spins of the surface dangling bonds explains also radiative recombination of the dark excitons in nanowires, nanorods and nanoplatelets.
Polaronic trapping in magnetic semiconductors
NASA Astrophysics Data System (ADS)
Raebiger, Hannes
2012-02-01
GaN doped with iron is an interesting candidate material for magnetic semiconductors, as p-d coupling between the localized Fe-d and extended N-p hole states is expected to facilitate long-range ferromagnetic alignment of the Fe spins [1]. This picture of extended states in GaN:Fe, however, falls apart due to a polaronic localization of the hole carriers nearby the Fe impurities. To elucidate the carrier localization in GaN:Fe and related iron doped III-V semiconductors, I present a systematic study using self-interaction corrected density-functional calculations [2]. These calculations predict three distinct scenarios. (i) Some systems do sustain extended host-like hole states, (ii) some exhibit polaronic trapping, (iii) and some exhibit carrier trapping at Fe-d orbitals. These behaviors are described in detail to give an insight as to how to distinguish them experimentally. I thank T. Fujita, C. Echeverria-Arrondo, and A. Ayuela for their collaboration.[4pt] [1] T. Dietl et al, Science, 287, 1019 (2000).[0pt] [2] S. Lany and A. Zunger, Phys. Rev. B, 80, 085202 (2009).
Ultralong-Range Polaron-Induced Quenching of Excitons in Isolated Conjugated Polymers
NASA Astrophysics Data System (ADS)
Bolinger, Joshua C.; Traub, Matthew C.; Adachi, Takuji; Barbara, Paul F.
2011-02-01
In conjugated polymers, radiative recombination of excitons (electron-hole pairs) competes with nonradiative thermal relaxation pathways. We visualized exciton quenching induced by hole polarons in single-polymer chains in a device geometry. The distance-scale for quenching was measured by means of a new subdiffraction, single-molecule technique—bias-modulated intensity centroid spectroscopy—which allowed the extraction of a mean centroid shift of 14 nanometers for highly ordered, single-polymer nanodomains. This shift requires energy transfer over distances an order of magnitude greater than previously reported for bulk conjugated polymers and far greater than predicted by the standard mechanism for exciton quenching, the unbiased diffusion of free excitons to quenching sites. Instead, multistep “energy funneling” to trapped, localized polarons is the probable mechanism for polaron-induced exciton quenching.
Reducing exciton-polaron annihilation in organic planar heterojunction solar cells
NASA Astrophysics Data System (ADS)
Verreet, Bregt; Bhoolokam, Ajay; Brigeman, Alyssa; Dhanker, Rijul; Cheyns, David; Heremans, Paul; Stesmans, Andre; Giebink, Noel C.; Rand, Barry P.
2014-09-01
We investigate the relationship between charge concentration, exciton concentration, and photocurrent generation in fullerene-containing heterojunction diodes. Impedance measurements on C60 diodes reveal a charge buildup at the C60/bathocuproine (BCP) interface that can be swept out under reverse bias. In solar cell structures, a similar charge buildup is observed in dark conditions, and increases as a function of incident light intensity. Photoluminescence measurements reveal that the C60 exciton concentration is voltage dependent, explained via the process of exciton-polaron annihilation. This process has a negative impact on the generated photocurrent of the solar cells and thereby decreases the fill factor. A combination of electroabsorption, photoluminescence, and impedance measurements reveal a decrease in charge buildup and the associated exciton-polaron annihilation through the use of a BCP/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag cathode.
Magnetic polarons in MnTe layers
NASA Astrophysics Data System (ADS)
Schinagl, F.; Bonanni, A.; Holl, S.; Prechtl, G.; Krenn, H.
1999-06-01
We investigate the crossover from antiferromagnetic to ferromagnetic behaviour in semimagnetic semiconductor structures. Systems with two dimensional magnetic layers of (1 0 0)-MnTe equispatially inserted four times in a CdTe quantum well, embedded between Cd 0.75Mg 0.25Te barriers are studied in the ideal paramagnetic limit. The interaction between localized magnetic spins and quantum confined electronic states is shown to give rise to the formation of free magnetic polarons.
Negative Polaron and Triplet Exciton Diffusion inOrganometallic “Molecular Wires”
Schanze, K.S.; Miller, J.; Keller, J.M.; Sean McIlroy, S.; Sreearuothai, P.; Danilov, E.O.; Jiang, H.; Glusac, K.D.; Miller, J.R.
2011-07-27
The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL{sub 2}-C {triple_bond} C-Ph-C {triple_bond} C-]{sub n} (where L = PBu{sub 3} and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of 3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of 27 ps for triplets and <10 ps for electrons.
Conventional versus unconventional magnetic polarons: ZnMnTe/ZnSe and ZnTe/ZnMnSe quantum dots
NASA Astrophysics Data System (ADS)
Barman, B.; Tsai, Y.; Scrace, T.; Murphy, J. R.; Cartwright, A. N.; Pientka, J. M.; Zutic, I.; McCombe, B. D.; Petrou, A.; Sellers, I. R.; Oszwaldowski, R.; Petukhov, A.; Fan, W. C.; Chou, W. C.; Yang, C. S.
2014-08-01
We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, EMP , in these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with EMP=35 meV. Furthermore, EMP has unconventionally weak dependence of on both temperature T and magnetic field Bappl . In contrast, magnetic polarons in sample B show conventional characteristics with EMP decreasing with increasing temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between the two types of QDs to the difference in the location of the Mn ions in the respective structures.
Optical studies of magnons, excitons and polarons in CuO{sub 2}-layer compounds
Kastner, M.A.; Birgeneau, R.J.
1996-12-31
The optical properties of undoped and lightly doped lamellar copper oxides are reviewed. In the undoped materials the absorption below the charge-transfer gap is dominated by magnetic and crystal field excitations of the CuO{sub 2} layers. The temperature dependence of the charge-transfer absorption provides evidence that free charged excitations form large polarons. However, the optical ionization energy of holes bound to acceptors is much larger than the thermal ionization energy, indicating that the bound polarons are small. The parameters extracted from optical measurements predict the Hall mobility of holes in lightly doped La{sub 2}CuO{sub 4}, with no adjustable parameters, confirming that the carriers are polarons.
NASA Astrophysics Data System (ADS)
Aziz, Hany; Wang, Qi
2015-10-01
The limited performance stability and gradual loss in the electroluminescence efficiency of OLEDs utilizing wide band-gap materials, such as blue-emitting phosphorescent and fluorescent devices, continues to be a challenge for wider technology adoption. We recently found that interactions between excitons and polarons play an important role in the aging behavior of electroluminescent materials, and that a correlation exists between the susceptibility of these materials to this aging mode and their band-gap. This degradation mode is also found to be often associated with the emergence of new bands - at longer wavelength - in the electroluminescence spectra of the materials, that can often be detected after prolonged electrical driving. Such bands contribute to the increased spectral broadening and color purity loss often observed in these devices over time. Exciton-polaron interactions, and the associated degradation, are also found to occur most significantly in the vicinity of device inter-layer interfaces such as at the interface between the emitter layer and the electron or hole transport layers. New results obtained from investigations of these phenomena in a wide range of commonly used host and guest OLED materials will be presented.
NASA Astrophysics Data System (ADS)
Damjanović, Ana; Kosztin, Ioan; Kleinekathöfer, Ulrich; Schulten, Klaus
2002-03-01
The dynamics of pigment-pigment and pigment-protein interactions in light-harvesting complexes is studied with an approach that combines molecular dynamics simulations with quantum chemistry calculations and a polaron model analysis. The molecular dynamics simulation of light-harvesting (LH) complexes was performed on an 87 055 atom system comprised of a LH-II complex of Rhodospirillum molischianum embedded in a lipid bilayer and surrounded with appropriate water layers. For each of the 16 B850 bacteriochlorophylls (BChls), we performed 400 ab initio quantum chemistry calculations on geometries that emerged from the molecular dynamical simulations, determining the fluctuations of pigment excitation energies as a function of time. From the results of these calculations we construct a time-dependent Hamiltonian of the B850 exciton system from which we determine within linear response theory the absorption spectrum. Finally, a polaron model is introduced to describe both the excitonic and coupled phonon degrees of freedom by quantum mechanics. The exciton-phonon coupling that enters into the polaron model, and the corresponding phonon spectral function, are derived from the molecular dynamics and quantum chemistry simulations. The model predicts that excitons in the B850 BChl ring are delocalized over five pigments at room temperature. Also, the polaron model permits the calculation of the absorption and circular dichroism spectra of the B850 excitons from the sole knowledge of the autocorrelation function of the excitation energies of individual BChls, which is readily available from the combined molecular dynamics and quantum chemistry simulations. The obtained results are found to be in good agreement with the experimentally measured absorption and circular dichroism spectra.
Magnetization steps and bound magnetic polarons in diluted magnetic semiconductors
NASA Astrophysics Data System (ADS)
McCabe, Gao Hua
1997-09-01
Magnetization measurements and computer simulations were used to address several current problems in the area of the Diluted Magnetic Semiconductors (DMS). The method of Magnetization Steps (MSTs) was used to study Jahn-Teller Distortions in Zn1-xCrxTe, and exchange constants in Pb1-xEuxSe and Pb1- xEuxTe. Bound magnetic polarons in Cu2MnxZn1-xSnS4 were studied by conventional magnetometry. Jahn-Teller Distortions in cubic Zn1- xCrxTe were studied using MSTs. Possible Jahn- Teller Distortions in this material are along the three equivalent /langle 100/rangle axes. Energy states for the three distortion directions are equivalent at H = 0, but are different for finite H. The main issue is whether the populations of three possible distortions will vary with field, or remain frozen at their values in zero- field. The data showed that the populations of the distortions changed significantly. The dominant antiferomagnetic exchange constants J between Eu2+ ions in IV-VI Pb1- xEuxSe and Pb1-xEuxTe were determined using the MST method. Measurements were made at 0.6 K in fields up to 150 kOe. Supplementary data by our collaborators were taken at much lower temperatures. Simulations of various MSTs (from isolated Eu2+ ions, pairs of ions, and triplets) were performed to fit the experimental data. Because J is determined from the MSTs for pairs, the effects of other anisotropies and exchange interactions on these MSTs were considered. They were found to bring little change to the values of J. The exchange constants were J/kB = -0/24 ± 0.03 K for Ph 1-xEu xSe amd os J/KB = -0.264 ± 0.018 K for Ph1-xEuxTe. The dominant AF exchange constants were identified as J1, between nearest-neighbors, by comparing the experimental magnetization curves to the theoretical simulations using the single J model. The dominant antiferromagnetic exchange constants J between Eu2+ ions in IV-VI Pb1- xEuxSe and Pb1-xEuxTe were determined using the MST method. Measurements were made at 0.6 K in
Optically induced magnetic polarons in EuTe
Henriques, A. B.; Galgano, G. D.; Abramof, E.; Rappl, P. H. O.
2013-12-04
Direct measurements of the photoinduced magnetization in EuTe, using a two color pump-and-probe technique, are presented. The photoinduced effect was pumped using photons of above-the-bandgap energy, and detected by the Faraday rotation of a probe beam of energy below-the-bandgap. The photoinduced Faraday rotation changes sign, as expected from our model for the optically induced magnetic polaron. The EuTe spin-flop transition at low fields is also detected as a sharp step in the photoinduced Faraday rotation, and its observation provides additional supports for the photoinduced polaron model.
Magnetic frustration in itinerant systems: the Kondo polaron problem
NASA Astrophysics Data System (ADS)
Isaev, Leonid; Batista, Cristian; Vekhter, Ilya
2013-03-01
We study the interplay between magnetic frustration and Kondo screening in Kondo lattices by analyzing the J1-J2 antiferromagnetic chain coupled to a conduction band. The system is tuned to the Majumdar-Ghosh point J2 =J1 / 2 which stabilizes a dimer valence-bond solid at weak Kondo coupling JK. We use an effective low-energy theory to demonstrate that sufficiently large JK results in a proliferation of ``Kondo polarons'', i.e. Kondo-screened domain-wall excitations of the dimer state, and collapse of the dimer order via a 2nd order quantum phase transition. At the quantum critical point, JK =JKc , these polarons become gapless, and we argue that the transition itself belongs to a 2D Ising universality class. For JK >JKc increasing concentration of the polarons leads to a continuous growth of the electron Fermi momentum until all spins are absorbed by the Fermi sea.
Liu, Jin; Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei
2015-05-14
Conformational structure and the electronic properties of various electronic excitations in cycloparaphenylenes (CPPs) are calculated using hybrid Density Functional Theory (DFT). The results demonstrate that wavefunctions of singlet and triplet excitons as well as the positive and negative polarons remain fully delocalized in CPPs. In contrast, these excitations in larger CPP molecules become localized on several phenyl rings, which are locally planarized, while the undeformed ground state geometry is preserved on the rest of the hoop. As evidenced by the measurements of bond-length alternation and dihedral angles, localized regions show stronger hybridization between neighboring bonds and thus enhanced electronic communication. This effect is even more significant in the smaller hoops, where phenyl rings have strong quinoid character in the ground state. Thus, upon excitation, electron–phonon coupling leads to the self-trapping of the electronic wavefunction and release of energy from fractions of an eV up to two eVs, depending on the type of excitation and the size of the hoop. The impact of such localization on electronic and optical properties of CPPs is systematically investigated and compared with the available experimental measurements.
Liu, Jin; Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei
2015-05-14
Conformational structure and the electronic properties of various electronic excitations in cycloparaphenylenes (CPPs) are calculated using hybrid Density Functional Theory (DFT). The results demonstrate that wavefunctions of singlet and triplet excitons as well as the positive and negative polarons remain fully delocalized in CPPs. In contrast, these excitations in larger CPP molecules become localized on several phenyl rings, which are locally planarized, while the undeformed ground state geometry is preserved on the rest of the hoop. As evidenced by the measurements of bond-length alternation and dihedral angles, localized regions show stronger hybridization between neighboring bonds and thus enhanced electronic communication.more » This effect is even more significant in the smaller hoops, where phenyl rings have strong quinoid character in the ground state. Thus, upon excitation, electron–phonon coupling leads to the self-trapping of the electronic wavefunction and release of energy from fractions of an eV up to two eVs, depending on the type of excitation and the size of the hoop. The impact of such localization on electronic and optical properties of CPPs is systematically investigated and compared with the available experimental measurements.« less
Thermoelectric power of small polarons in magnetic semiconductors
Liu, N.H.; Emin, D.
1984-09-15
The thermoelectric power (Seebeck coefficient) ..cap alpha.. of a small polaron in both ferromagnetic and antiferromagnetic semiconductors and insulators is calculated for the first time. In particular, we obtain the contribution to the Seebeck coefficient arising from exchange interactions between the severely localized carrier (i.e., small polaron) of charge q and the spins of the host lattice. In essence, we study the heat transported along with a carrier. This heat, the Peltier heat, Pi, is related to the Seebeck coefficient by the Kelvin relation: Pi = qT..cap alpha.., where T is the temperature. The heat per carrier is simply the product of the temperature and the change of the entropy of the system when a small polaron is added to it. The magnetic contribution to the Seebeck coefficient is therefore directly related to the change of the magnetic entropy of the system upon introduction of a charge carrier. We explicitly treat the intrasite and intersite exchange interactions between a small polaron and the spins of a spin-1/2 system. These magnetic interactions produce two competing contributions to the Seebeck coefficient. First, adding the carrier tends to provide extra spin freedom (e.g., spin up or spin down of the carrier). This effect augments the entropy of the system, thereby producing a positive contribution to the Peltier heat. Second, however, the additional exchange between the carrier and the sites about it enhances the exchange binding among these sites. This generally reduces the energetically allowable spin configurations. The concomitant reduction of the system's entropy provides a negative contribution to the Peltier heat. At the highest of temperatures, when kT exceeds the intrasite exchange energy, the first effect dominates. Then, the Peltier heat is simply augmented by kT ln2.
Orbital effects of strong magnetic field on a two-dimensional Holstein polaron
NASA Astrophysics Data System (ADS)
Pradhan, Subhasree; Chakraborty, Monodeep; Taraphder, A.
2016-03-01
We investigate the orbital effects of a strong external magnetic field on the ground-state properties of a two-dimensional (2D) Holstein polaron, employing variational approaches based on exact diagonalization. From the ground-state energy and the wave function, we calculate the electron-phonon correlation function, the average phonon number, and the Drude weight and investigate the evolution of a 2D Holstein polaron as a function of the magnetic flux. Although the external magnetic field affects the polaron throughout the parameter regime, we show that the magnetic field has a stronger effect on a loosely bound (spatially extended) polaron. We also find that the magnetic field can be used as a tuning parameter, particularly for a weakly coupled polaron, to reduce the spatial extent of a large polaron.
NASA Astrophysics Data System (ADS)
Bednarski, Henryk
2014-01-01
We present a detailed analysis of the role of various interaction mechanisms contributing to the bound magnetic polaron (BMP) molecule Hamiltonian with the purpose of gaining an insight into the origin of the ferromagnetic interaction between BMPs. Explicitly, it appears that the BMP molecule Hamiltonian without interatomic direct exchange interaction does not lead to appearance of the parallel alignment of polaronic clouds in the lowest energy state even for large magnitudes of the polaronic exchange fields (up to ~65 meV in Cd0.95Mn0.05Se). Also, it appears that for the range of moderate values of polaronic exchange fields (16-25 meV in Cd0.95Mn0.05Se), the regular Hubbard BMP pair Hamiltonian must be supplemented with the direct interatomic (Heisenberg) exchange interaction to form a minimal model Hamiltonian of BMP molecule with the parallel alignment of the polaronic clouds in the lowest energy state.
Magnetic monopole polarons in artificial spin ices
NASA Astrophysics Data System (ADS)
Chern, Gia-Wei; Mellado, Paula
2016-05-01
Emergent quasiparticles that arise from the fractionalization of the microscopic degrees of freedom have been one of the central themes in modern condensed-matter physics. The notion of magnetic monopoles, freely moving quasiparticles fragmented from local dipole excitations, has enjoyed much success in understanding the thermodynamic, static, and transport properties of the so-called spin-ice materials. The artificial version of spin ice, where a lattice of nanoscale magnetic dipoles is sculpted out of a ferromagnetic film, provides a unique opportunity to study these unusual quasiparticles in a material-by-design approach. Here we show that the elementary excitations in the ice phase of a nano-magnetic array arranged in the pentagonal lattice are composite objects comprised of the emergent monopole and a surrounding cloud of opposite uncompensated magnetic charges.
Polaronic pinning of vortex in magnetic superconductors and magnetic-superconducting multilayers
NASA Astrophysics Data System (ADS)
Lin, Shi-Zeng; Bulaevskii, Lev
2013-03-01
We present a new type of vortex pinning by enhancing the viscosity of vortex in magnetic superconductors with long relaxation time of magnetization and large magnetic susceptibility. In the absence of current, vortices are dressed by nonuniform magnetic polarization and form vortex-polarons. Under a small current and consequently low Lorentz force, the magnetic polarization follows the vortex motion. However, at long magnetic relaxation time of magnetization, there is additional dragging force by the magnetization besides the Bardeen-Stephen one, thus the effective viscosity of vortex is significantly enhanced resulting in suppression of dissipation. For a large current, the magnetic polarization cannot follow the vortex motion and the vortex-polaron dissociates, i.e. the magnetization and vortex become decoupled. In the IV characteristic, the decoupling transition shows as a voltage jump and can be identified as a depinning transition. The polaronic pinning mechanism successfully explains the observed enhancement of critical current in the ErNiBC superconductor at low temperatures. The polaronic pinning can be optimized in magnetic-superconducting multilayers. We show also that vortex-polaron creep is suppressed at low temperatures. This publication was made possible by funding from the Los Alamos Laboratory Directed Research and Development Program, project number 20110138ER.
Peltier heat of a small polaron in a magnetic semiconductor
Liu, N.H.; Emin, D.
1985-04-15
For the first time the heat transported with a small polaron in both antiferromagnetic and ferromagnetic semiconductors is calculated. This heat, the Peltier heat, ..pi.., is obtained from the change of the entropy of the total system upon introduction of a charge carrier. We explicitly consider both the intrasite and intersite exchange interactions between a small polaron and the interacting spins of a spin-1/2 magnet. There are two competing magnetic contributions to the Peltier heat. First, adding the carrier increases the spin entropy of the system. This provides a positive contribution to ..pi... Second, the exchange between the carrier and the sites about it enhances the exchange binding between these sites. This reduces the energetically allowable spin configurations and provides a negative contribution to ..pi... At extremely high temperatures when kT exceeds the intrasite exchange energy, the first effect dominates. Then ..pi.. is simply augmented by kT ln 2. However, well below the magnetic transition temperature the second effect dominates. In the experimentally accessible range between these limits both effects are comparable and sizable. The net magnetic contribution to the Peltier heat rises with temperature. Thus, a carrier's interactions with its magnetic environment produces a significant and distinctive contribution to its Peltier heat.
Peltier heat of a small polaron in a magnetic semiconductor
Liu, N.L.H.; Emin, D.
1984-01-01
The heat transported with a small polaron in both antiferromagnetic and ferromagnetic semiconductors is calculated. This heat, the Peltier heat, ..pi.., is obtained from the change of the entropy of the total system upon introduction of a charge carrier. We explicitly consider both the intrasite and intersite exchange interactions between a small polaron and the interacting spins of a spin-1/2 magnet. There are two competing magnetic contributions to the Peltier heat. First, adding the carrier increases the spin entropy of the system. This provides a positive contribution to ..pi... Second, the exchange between the carrier and the sites about it enhances the exchange binding between these sites. This reduces the energetically allowable spin configurations and provides a negative contribution to ..pi... At extremely high temperature when kT exceeds the intrasite exchange energy, the first effect dominates. Then ..pi.. is simply augmented by kTln2. However, well below the magnetic transition temperature the second effect dominates. In the experimentally accessible range between these limits both effects are comparable and sizable. The net magnetic contribution to the Peltier heat rises with temperature. Thus, a carrier's interactions with its magnetic environment produces a significant and distinctive contribution to its Peltier heat.
Magnetic-Polaron-Induced Enhancement of Surface Raman Scattering.
Shao, Qi; Liao, Fan; Ruotolo, Antonio
2016-01-01
The studies of the effects of magnetic field on surface enhanced Raman scattering (SERS) have been so far limited to the case of ferromagnetic/noble-metal, core/shell nano-particles, where the influence was always found to be negative. In this work, we investigate the influence of magnetic field on a diluted magnetic semiconductor/metal SERS system. Guided by three dimensional finite-difference time-domain simulations, a high efficient SERS substrate was obtained by diluting Mn into Au-capped ZnO, which results in an increase of the dielectric constant and, therefore, an enhancement of Raman signals. More remarkably, an increase of intensities as well as a reduction of the relative standard deviation (RSD) of Raman signals have been observed as a function of the external magnetic strength. We ascribe these positive influences to magnetic-field induced nucleation of bound magnetic polarons in the Mn doped ZnO. The combination of diluted magnetic semiconductors and SERS may open a new avenue for future magneto-optical applications. PMID:26754049
Magnetic-Polaron-Induced Enhancement of Surface Raman Scattering
Shao, Qi; Liao, Fan; Ruotolo, Antonio
2016-01-01
The studies of the effects of magnetic field on surface enhanced Raman scattering (SERS) have been so far limited to the case of ferromagnetic/noble-metal, core/shell nano-particles, where the influence was always found to be negative. In this work, we investigate the influence of magnetic field on a diluted magnetic semiconductor/metal SERS system. Guided by three dimensional finite-difference time-domain simulations, a high efficient SERS substrate was obtained by diluting Mn into Au-capped ZnO, which results in an increase of the dielectric constant and, therefore, an enhancement of Raman signals. More remarkably, an increase of intensities as well as a reduction of the relative standard deviation (RSD) of Raman signals have been observed as a function of the external magnetic strength. We ascribe these positive influences to magnetic-field induced nucleation of bound magnetic polarons in the Mn doped ZnO. The combination of diluted magnetic semiconductors and SERS may open a new avenue for future magneto-optical applications. PMID:26754049
Correlation between exciton decay time and Stokes shift in digital magnetic heterostructures
NASA Astrophysics Data System (ADS)
Heiss, W.; Prechtl, G.; Mackowski, S.; Janik, E.
2002-03-01
The exciton recombination time and the exciton photoluminescence Stokes shift are systematically investigated in a set of CdTe/Cd1-xMgxTe quantum-well samples containing MnTe layers with the thickness below or equal to 1 ML. Similar to diluted magnetic semiconductors, these digital magnetic heterostructures exhibit strongly magnetic-field-dependent photoluminescence lines which are inhomogeneously broadened. Both quantities under investigation depend on the Mn content, on the details of the Mn distribution within the quantum wells, and on the energy of detection. Three sets of samples are investigated: digital magnetic heterostructures containing four MnTe layers with equidistant spacing and various thickness of the MnTe layers, quantum wells containing 1/4-ML MnTe at various positions in the quantum well, and quantum wells containing in total 1-ML MnTe but distributed in the quantum well in different ways. For all samples, the exciton recombination time is strongly correlated to the Stokes shift. This correlation is caused by exciton localization, leading to a mixing of radiatively decaying states with small momentum with nonradiative states with large momentum. In our experiment, however, a linear dependence between the exciton decay time and the Stokes shift is observed, which is not yet explained by theory, to our knowledge. Although, the luminescence properties in semimagnetic semiconductors are affected by exchange interactions leading to the formation of magnetic polarons, this process can be ruled out as the origin of the observed correlation.
Light-induced polaron magnetization in EuTe at temperatures reaching 150 K
NASA Astrophysics Data System (ADS)
Henriques, A. B.; Galgano, G. D.; Rappl, P. H. O.; Abramof, E.
2016-05-01
We demonstrate that light creates a highly magnetized region in a magnetic semiconductor far above its critical temperature. A near-gap photon generates a quasiparticle of nonzero magnetic moment, named magnetic polaron, which is constituted by the photoexcited electron and about 1000 spin-polarized lattice atoms surrounding the photoexcited electron. The photoinduced magnetization follows a Langevin function, whose shape uniquely determines the magnetic moment of an individual polaron. In EuTe at 5 K the magnetic moment reaches a giant value of over 500 Bohr magnetons, thus the photoinduced magnetization saturates with a magnetic field of only 50 mT, which characterizes the magnetic polaron system as superparamagnetic. The polaron has an average lifetime of 15 μ s . When temperature is increased its magnetic moment decreases, but at 150 K it still has a large value of about 80 Bohr magnetons. The paramagnet of polarons is fully controlled by light. Because the magnetic polaron affects only spin orientation, but not the charge distribution, in the superparamagnetic state the ideal optical quality of the host semiconductor is preserved.
Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots
NASA Astrophysics Data System (ADS)
Barman, B.; Oszwałdowski, R.; Schweidenback, L.; Russ, A. H.; Pientka, J. M.; Tsai, Y.; Chou, W.-C.; Fan, W. C.; Murphy, J. R.; Cartwright, A. N.; Sellers, I. R.; Petukhov, A. G.; Žutić, I.; McCombe, B. D.; Petrou, A.
2015-07-01
We used continuous wave photoluminescence (cw-PL) and time-resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe /(Zn ,Mn )Se system the holes are confined in the nonmagnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn ,Mn )Te /ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding nonmagnetic ZnSe matrix. The magnetic polaron formation energies EMP in both systems were measured from the temporal redshift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe /(Zn ,Mn )Se system the magnetic polaron shows conventional behavior with EMP decreasing with increasing temperature T and increasing magnetic field B . In contrast, EMP in the (Zn ,Mn )Te /ZnSe system has unconventional dependence on temperature T and magnetic field B ; EMP is weakly dependent on T as well as on B . We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.
Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells
NASA Astrophysics Data System (ADS)
Zhukov, E. A.; Kusrayev, Yu. G.; Kavokin, K. V.; Yakovlev, D. R.; Debus, J.; Schwan, A.; Akimov, I. A.; Karczewski, G.; Wojtowicz, T.; Kossut, J.; Bayer, M.
2016-06-01
The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field.
NASA Astrophysics Data System (ADS)
Xu, Kai; Ma, Dongge
2014-08-01
The magnetic field effects of electroluminescence (MEL) in 4-[dicyanomethylene]-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB) doped tri-(8-hydroxyquinoline)-aluminum (Alq3) based organic light-emitting diodes were investigated. A linear decrease in MEL with the increase of magnetic field was observed at high magnetic field (>20 mT) in this doping devices, which has been attributed to the singlet-polaron quenching effect. It was found that the singlet-polaron quenching is magnetic field dependent. Our results showed that singlet-polaron quenching commonly exists in fluorescence OLEDs and induces the linear decrease in MEL.
Polaron-like vortices, dissociation transition, and self-induced pinning in magnetic superconductors
Bulaevskii, L. N. Lin, S.-Z.
2013-09-15
Vortices in magnetic superconductors polarize spins nonuniformly and repolarize them when moving. At a low spin relaxation rate and at low bias currents, vortices carrying magnetic polarization clouds become polaron-like and their velocities are determined by the effective drag coefficient that is significantly bigger than the Bardeen-Stephen (BS) one. As the current increases, vortices release polarization clouds and the velocity as well as the voltage in the I-V characteristics jump to values corresponding to the BS drag coefficient at a critical current J{sub c}. The nonuniform components of the magnetic field and magnetization drop as the velocity increases, resulting in weaker polarization and a discontinuous dynamic dissociation depinning transition. Experimentally, the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. As the current decreases, on the way back, vortices are retrapped by polarization clouds at the current J{sub r} < J{sub c}. As a result, the polaronic effect suppresses dissipation and enhances the critical current. Borocarbides (RE)Ni{sub 2}B{sub 2}C with a short penetration length and highly polarizable rare earth spins seem to be optimal systems for a detailed study of vortex polaron formation by measuring I-V characteristics. We also propose to use a superconductor-magnet multilayer structure to study polaronic mechanism of pinning with the goal to achieve high critical currents. The magnetic layers should have large magnetic susceptibility to enhance the coupling between vortices and magnetization in magnetic layers while the relaxation of the magnetization should be slow. For Nb and a proper magnet multilayer structure, we estimate the critical current density J{sub c} {approx} 10{sup 9} A/m{sup 2} at the magnetic field B Almost-Equal-To 1 T.
Modeling of magnetic polaron properties in (Zn,Mn)Te quantum dots
NASA Astrophysics Data System (ADS)
Pientka, James; Barman, B.; Schweidenback, L.; Russ, A. H.; Tsai, Y.; Murphy, J. R.; Cartwright, A. N.; Zutic, I.; McCombe, B. D.; Petrou, A.; Chou, W.-C.; Fan, W. C.; Sellers, I. R.; Petukhov, A. G.; Oszwaldowski, R.
Magnetic polarons in (Zn,Mn)Te quantum dots (QD) show unconventional behavior. These structures exhibit a small red shift of the photoluminescence peak energy in the presence of a magnetic field B and they also have a weak dependence of the polaron energy EMP on temperature T and B. We attribute these properties to a large molecular field Bm that is proportional to the heavy holes spin density. We have calculated Bm using the QD diameter and height as adjustable parameters. Assuming hole localization, this calculation yields values of Bm >20 T. The assumption that the hole localization diameter can be smaller than the QD diameter is justified due to alloy and spin disorder scattering. Using the magnetic polaron free energy, we calculate EMP as function of T and B for a variety of Bm values. To get a weak dependence of EMP on T and Bwe must assume that the polaron temperature is higher than T. This work was supported by U.S. DOE BES, Award DE-SC0004890, NSF DMR-1305770 and U.S. ONR N000141310754.
Anomalous magnetization of a carbon nanotube as an excitonic insulator
NASA Astrophysics Data System (ADS)
Rontani, Massimo
2014-11-01
We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.
NASA Astrophysics Data System (ADS)
Manson, Ross; Roy-Choudhury, Kaushik; Hughes, Stephen
2016-04-01
We introduce an intuitive and semianalytical polaron master equation approach to model pulse-driven population inversion and emitted single photons from a quantum dot exciton. The master equation theory allows one to identify important phonon-induced scattering rates analytically and fully includes the role of the time-dependent pump field. As an application of the theory, we first study a quantum dot driven by a time-varying laser pulse on and off resonance, showing the population inversion caused by acoustic phonon emission in direct agreement with recent experiments of Quilter et al. [Phys. Rev. Lett. 114, 137401 (2015), 10.1103/PhysRevLett.114.137401]. We then model quantum dots in weakly coupled cavities and show the difference in population response between exciton-driven and cavity-driven systems. Finally, we assess the nonresonant phonon-assisted loading scheme with a quantum dot resonantly coupled to a cavity as a deterministic single-photon source. We also compare and contrast the important single photon figures of merit with direct Rabi oscillation of the population using a resonant π pulse, and show that the resonant scheme is much more efficient.
Controlling the dark exciton spin eigenstates by external magnetic field
NASA Astrophysics Data System (ADS)
Gantz, L.; Schmidgall, E. R.; Schwartz, I.; Don, Y.; Waks, E.; Bahir, G.; Gershoni, D.
2016-07-01
We study the dark exciton's behavior as a coherent physical two-level spin system (qubit) using an external magnetic field in the Faraday configuration. Our studies are based on polarization-sensitive intensity autocorrelation measurements of the optical transition resulting from the recombination of a spin-blockaded biexciton state, which heralds the dark exciton and its spin state. We demonstrate control over the dark exciton eigenstates without degrading its decoherence time. Our observations agree well with computational predictions based on a master equation model.
Magnetic brightening of dark excitons in transitional metal dichalcogenides
NASA Astrophysics Data System (ADS)
Zhang, Xiao-Xiao; Lu, Zhengguang; Cao, Ting; Zhang, Fan; Hone, James; Louie, Steven G.; Li, Zhiqiang; Smirnov, Dmitry; Heinz, Tony
Transitional metal dichalcogenides (TMDC) in the MX2 (M = Mo, W, X = S, Se) family represent an excellent platform to study of excitonic effects. At monolayer thickness, these materials exhibit both direct band-gap character and enhanced excitonic interactions. Theoretical studies suggest that both the valence and conduction bands are split and exhibit spin polarized character at the K/K' valleys. The lowest energy band-edge excitons are predicted to have different spin configurations for different materials in this family. When the lowest lying exciton has parallel electron and hole spin, radiative decay is forbidden and the state is dark. Here we demonstrate that by applying an in-plane magnetic field we can perturb the exciton spin configuration and brighten this state, allowing it to undergo radiative decay. We identify such a brightened dark state by the emergence of a new emission peak lying below the absorption peak, with a strength growing with applied in-plane magnetic field. On the other hand, for monolayer MoSe2, where no low-lying dark state is expected, we do not see the growth of a new emission feature under application of an in-plane magnetic field. Our experimental findings are in agreement with the calculated properties of dark excitons based on GW plus Bethe-Salpeter equation approach
NASA Astrophysics Data System (ADS)
Liu, Jia; Xiao, Jing-Ling
2006-10-01
We study theoretically the ground state energy of a polaron near the interface of a polar-polar semiconductor by considering the Rashba spin-orbit (SO) coupling with the Lee-Low-Pines intermediate coupling method. Our numerical results show that the Rashba SO interaction originating from the inversion asymmetry in the heterostructure splits the ground state energy of the polaron. The electron areal density and vector dependence of the ratio of the SO interaction to the total ground state energy or other energy composition are obvious. One can see that even without any external magnetic field, the ground state energy can be split by the Rashba SO interaction, and this split is not a single but a complex one. Since the presents of the phonons, whose energy gives negative contribution to the polaron's, the spin-splitting states of the polaron are more stable than electron's.
Li, Haifeng; Xiao, Yinguo; Schmitz, Berthold; Persson, Jörg; Schmidt, Wolfgang; Meuffels, Paul; Roth, Georg; Brückel, Thomas
2012-01-01
Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around TN the PMRV translates to negative, down to ~−10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices. PMID:23087815
Li, Haifeng; Xiao, Yinguo; Schmitz, Berthold; Persson, Jörg; Schmidt, Wolfgang; Meuffels, Paul; Roth, Georg; Brückel, Thomas
2012-01-01
Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around T(N) the PMRV translates to negative, down to ~-10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices. PMID:23087815
Magnetic Brightening of Dark Excitons in Carbon Nanotubes
NASA Astrophysics Data System (ADS)
Kono, Junichiro
2007-03-01
To gain insight into the internal energy structure and radiative properties of excitons in single-walled carbon nanotubes (SWNTs), we have studied photoluminescence (PL) from individualized HiPco and CoMoCAT samples as a function of magnetic field (B) and temperature (T). The PL intensity increased, or ``brightened,'' with B applied along the tube axis and the amount of brightening increased with decreasing T. These results are consistent with the existence of a dark state below the first bright state [1]. In the presence of time reversal symmetry, exchange-interaction-induced mixing between excitons in two equivalent valleys (the K and K' valleys) is expected to result in a set of exciton states, only one of which is optically active. This predicted bright state, however, is not the lowest in energy. Excitons would be trapped in the dark, lowest-energy state without a radiative recombination path. When a tube-threading B is applied, addition of an Aharonov-Bohm phase modifies the circumferential boundary conditions on the wave functions and lifts time reversal symmetry [2,3]. This symmetry breaking splits the K and K' valley transitions, lessening the intervalley mixing and causing the recovery of the unmixed direct K and K' excitons, which are both optically active. We have calculated PL spectra through B-dependent effective masses, populations of finite-k states, and acoustic phonon scattering, which quantitatively agree with the observations. These results demonstrate the existence of dark excitons, their influence on the PL quantum yield, and their elimination through symmetry manipulation by a B. This work was performed in collaboration with J. Shaver, S. Zaric, O. Portugall, V. Krstic, G. L. J. A. Rikken, X. Wei, S. A. Crooker, Y. Miyauchi, S. Maruyama, and V. Perebeinos and supported by the Robert A. Welch Foundation, the NSF, and EuroMagNET. [1] V. Perebeinos et al., Phys. Rev. Lett. 92, 257402 (2004); H. Zhao and S. Mazumdar, Phys. Rev. Lett. 93, 157402
Ding, Baofu Alameh, Kamal
2014-07-07
The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.
Quantum vibrational polarons: Crystalline acetanilide revisited
NASA Astrophysics Data System (ADS)
Hamm, Peter; Edler, Julian
2006-03-01
We discuss a refined theoretical description of the peculiar spectroscopy of crystalline acetanilide (ACN). Acetanilide is a molecular crystal with quasi-one-dimensional chains of hydrogen-bonded units, which is often regarded as a model system for the vibrational spectroscopy of proteins. In linear spectroscopy, the CO stretching (amide I) band of ACN features a double-peak structure, the lower of which shows a pronounced temperature dependence which has been discussed in the context of polaron theory. In nonlinear spectroscopy, both of these peaks respond distinctly differently. The lower-frequency band exhibits the anharmonicity expected from polaron theory, while the higher-frequency band responds as if it were quasiharmonic. We have recently related the response of the higher-frequency band to that of a free exciton [J. Edler and P. Hamm, J. Chem. Phys. 117, 2415 (2002)]. However, as discussed in the present paper, the free exciton is not an eigenstate of the full quantum version of the Holstein polaron Hamiltonian, which is commonly used to describe these phenomena. In order to resolve this issue, we present a numerically exact solution of the Holstein polaron Hamiltonian in one dimension (1D) and 3D. In 1D, we find that the commonly used displaced oscillator picture remains qualitatively correct, even for relatively large exciton coupling. However, the result is not in agreement with the experiment, as it fails to explain the free-exciton band. In contrast, when taking into account the 3D nature of crystalline acetanilide, certain parameter regimes exist where the displaced oscillator picture breaks down and states appear in the spectrum that indeed exhibit the characteristics of a free exciton. The appearance of these states is a speciality of vibrational polarons, whose source of exciton coupling is transition dipole coupling which is expected to have opposite signs of interchain and intrachain coupling.
Finite-momentum condensate of magnetic excitons in a bilayer quantum Hall system
NASA Astrophysics Data System (ADS)
Doretto, R. L.; Morais Smith, C.; Caldeira, A. O.
2012-07-01
We study the bilayer quantum Hall system at total filling factor νT=1 within a bosonization formalism which allows us to approximately treat the magnetic exciton as a boson. We show that in the region where the distance between the two layers is comparable to the magnetic length, the ground state of the system can be seen as a finite-momentum condensate of magnetic excitons provided that the excitation spectrum is gapped. We analyze the stability of such a phase within the Bogoliubov approximation first assuming that only one momentum Q is macroscopically occupied and later we consider the same situation for two modes ±Q. We find strong evidences that a first-order quantum phase transition at small interlayer separation takes place from a zero-momentum condensate phase, which corresponds to Halperin 111 state, to a finite-momentum condensate of magnetic excitons.
Jahan K, Luhluh Boda, Aalu; Chatterjee, Ashok
2015-05-15
The problem of an exciton trapped in a three dimensional Gaussian quantum dot is studied in the presence of an external magnetic field. A variational method is employed to obtain the ground state energy of the exciton as a function of the quantum dot size, the confinement strength and the magnetic field. It is also shown that the variation of the size of the exciton with the radius of the quantum dot.
A Comparison Between Magnetic Field Effects in Excitonic and Exciplex Organic Light-Emitting Diodes
NASA Astrophysics Data System (ADS)
Sahin Tiras, Kevser; Wang, Yifei; Harmon, Nicholas J.; Wohlgenannt, Markus; Flatte, Michael E.
In flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25% because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.
Effective Landé factor in a GaMnAs quantum dot; with the effects of sp-d exchange on a bound polaron
NASA Astrophysics Data System (ADS)
Lalitha, D.; Peter, A. John
2014-04-01
The effective g-factor of conduction (valence) band electron (hole) is obtained in the GaMnAs quantum dot. Magneto bound polaron in a GaMnAs/Ga0.6Al0.4As quantum dot is investigated with the inclusion of exchange interaction effects due to Mn alloy content and the geometrical confinement. The spin polaronic energy of the heavy hole exciton is studied with the spatial confinement using a mean field theory in the presence of magnetic field strength.
Effective Landé factor in a GaMnAs quantum dot; with the effects of sp-d exchange on a bound polaron
Lalitha, D. Peter, A. John
2014-04-24
The effective g-factor of conduction (valence) band electron (hole) is obtained in the GaMnAs quantum dot. Magneto bound polaron in a GaMnAs/Ga{sub 0.6}Al{sub 0.4}As quantum dot is investigated with the inclusion of exchange interaction effects due to Mn alloy content and the geometrical confinement. The spin polaronic energy of the heavy hole exciton is studied with the spatial confinement using a mean field theory in the presence of magnetic field strength.
Vector polarons in a degenerate electron system
NASA Astrophysics Data System (ADS)
Clougherty, Dennis P.; Foell, Charles A.
2004-08-01
We consider a one-dimensional model of an electron in a doubly (or nearly) degenerate band that interacts with elastic distortions. We show that the electron equations of motion reduce to a set of coupled nonlinear Schrödinger equations. For the case of interband electron-phonon coupling stemming from local Jahn-Teller interactions, multicomponent self-localized polaron solutions-vector polarons- are described and classified. The phase diagram for the different types of vector polarons in this model is presented. By interpreting the components of the orbital doublet as those of spin- (1)/(2) , our results can also be used to describe bound magnetic polarons.
Podlesnyak, Andrey A; Ehlers, Georg; Frontzek, Matthias D; Sefat, A. S.; Furrer, Albert; Straessle, Thierry; Pomjakushina, Ekaterina; Conder, Kazimierz; Demmel, F.; Khomskii, D. I.
2011-01-01
We investigate the doping dependence of the nanoscale electronic and magnetic inhomogeneities in the hole-doping range 0.002 < x < 0.1 of cobalt based perovskites, La{sub 1-x}Sr{sub x}CoO{sub 3}. Using single-crystal inelastic neutron scattering and magnetization measurements we show that the lightly doped system exhibits magnetoelectronic phase separation in the form of spin-state polarons. Higher hole doping leads to a decay of spin-state polarons in favor of larger scale magnetic clusters, due to competing ferromagnetic correlations of Co{sup 3+} ions which are formed by neighboring polarons. The present data give evidence for two regimes of magnetoelectronic phase separation in this system: (i) x {approx}< 0.05, dominated by ferromagnetic intrapolaron interactions, and (ii) x {approx}> 0.05, dominated by Co{sup 3+}-Co{sup 3+} intracluster interactions. Our conclusions are in good agreement with a recently proposed model of the phase separation in cobalt perovskites.
NASA Astrophysics Data System (ADS)
Sancho, S.; Chaouache, M.; Maaref, M. A.; Bernardot, F.; Eble, B.; Lemaître, A.; Testelin, C.
2011-10-01
We study the injection of polarized bright and dark excitons in quantum dots, under nonresonant or resonant excitation, by polarization-resolved photoluminescence experiments on an ensemble of self-assembled InAs/GaAs quantum dots. The importance of the polarized dark exciton creation on the optical emission under magnetic field is discussed. Under circular excitation, we observe the expected increase and saturation of the polarization rate with a magnetic field applied in Faraday geometry. Strikingly, the polarization rate slightly decreases for magnetic fields greater than ˜1.5 T; the feature is more pronounced for higher interband energies and is attributed to a more efficient initial polarization of the dark exciton states. This interpretation is confirmed by the lack of decrease of the polarization rate for quantum dots excited at exact resonance through a 1LO-phonon-assisted transition. Finally, we measure the bright exciton exchange energy as a function of interband emission energy, we measure a decrease from 65 to 30 μeV in the range 1.28-1.35 eV, and we obtain an estimate of the dark exciton splitting.
NASA Astrophysics Data System (ADS)
Yannopapas, V.; Vitanov, N. V.
2006-11-01
A type of magnetic metamaterial which exhibits strong magnetic activity within and below the optical region is presented. The metamaterial consists of semiconductor nanoparticles such as CuCl or Cu2O particles. The magnetic activity is attributed to the strong oscillator strength stemming from the exciton absorption line. The magnetic permeability of the proposed metamaterial is calculated from the extended Maxwell-Garnett theory, and its validity is compared against ab initio layer multiple-scattering calculations. The proposed structure is a low-loss, subwavelength, isotropic magnetic metamaterial, and its response is robust against stacking and point disorder.
Excitonic spin-splitting in quantum wells with a tilted magnetic field
NASA Astrophysics Data System (ADS)
Fernandes dos Santos, L.; Castelano, L. K.; Padilha, J. X.; Pusep, Y.; Marques, G. E.; Smirnov, D.; Bakarov, A. K.; Toropov, A. I.; Lopez-Richard, V.
2016-02-01
This work aims to investigate the effects of magnetic field strength and direction on the electronic properties and optical response of GaAs/AlGaAs-based heterostructures. An investigation of the excitonic spin-splitting of a disordered multiple quantum well embedded in a wide parabolic quantum well is presented. The results for polarization-resolved photoluminescence show that the magnetic field dependencies of the excitonic spin-splitting and photoluminescence linewidth are crucially sensitive to magnetic field orientation. Our experimental results are in good agreement with the calculated Zeeman splitting obtained by the Luttinger model, which predicts a hybridization of the spin character of states in the valence band under tilted magnetic fields.
Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates.
Ohadi, H; Del Valle-Inclan Redondo, Y; Dreismann, A; Rubo, Y G; Pinsker, F; Tsintzos, S I; Hatzopoulos, Z; Savvidis, P G; Baumberg, J J
2016-03-11
Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders. PMID:27015497
Excitonic diamagnetic shifts and linewidths in Al_xGa_1-xAs alloys in high magnetic fields
NASA Astrophysics Data System (ADS)
Senger, R. T.; Coli, G.; Bajaj, K. K.; Reno, J. L.; Jones, E. D.; Crooker, Scott
2002-03-01
We have measured both the diamagnetic shifts and the linewidths of excitonic transitions in Alx Ga_1-xAs alloys as a function of aluminum concentration and magnetic field at 1.4 K using photoluminescence spectroscopy. The aluminum composition in our samples ranged from 0 to 300 to 50 tesla. The samples were grown on GaAs substrates oriented along [001] direction using molecular beam epitaxy at 590ºC. We find that for a given value of alloy composition, both the diamagnetic shift and excitonic linewidth increase as a function of magnetic field. The observed variations of the diamagnetic shifts and excitonic linewidths with magnetic field are considerably smaller than those calculated by Lee and Bajaj [J. Appl. Phys. 73, 1788 (1993)] using a free exciton model. To explain our experimental data we propose that the excitons are localized in a very specific manner. To simulate exciton localization, we assume that the exciton reduced mass is increased and is obtained by using the alloy dependent heavy-hole mass along [001] direction treated isotropically. The calculated values of the variations of the diamagnetic shift and excitonic linewidth as a function of magnetic field obtained using this model agree very well with those reported here. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.
NASA Astrophysics Data System (ADS)
Zhang, Xi-Cheng
1986-12-01
field accelerates the localized exciton recombination processes. The dynamics of the excitons in CdMnTe MQW samples suggests that quasi-2D bound magnetic polarons (BMP) exist. A discussion of this quasi-2D BMP and the influence of an external magnetic field on exciton lifetime and exciton dynamics in Cd(,1 -x)Mn(,x)Te MQW is included.
Schaller, Richard D.; Crooker, Scott A.; Bussian, David A.; Pietryga, Jeffrey M.; Joo, Jin; Klimov, Victor I.
2010-08-04
We measure the photoluminescence lifetime τ of excitons in colloidal PbSe nanocrystals (NCs) at low temperatures to 270 mK and in high magnetic fields to 15 T. For all NCs, τ increases sharply below 10 K but saturates by 500 mK. In contrast to the usual picture of well-separated “bright” and “dark” exciton states (found, e.g., in CdSe NCs), these dynamics fit remarkably well to a system having two exciton states with comparable—but small—oscillator strengths that are separated by only 300–900 μeV depending on NC size. Importantly, magnetic fields reduce τ below 10 K, consistent with field-induced mixing between the two states. Magnetic-circular dichroism studies reveal exciton g factors from 2–5, and magnetophotoluminescence shows >10% circularly polarized emission.
Alexander-Webber, Jack A; Faugeras, Clement; Kossacki, Piotr; Potemski, Marek; Wang, Xu; Kim, Hee Dae; Stranks, Samuel D; Taylor, Robert A; Nicholas, Robin J
2014-09-10
Semiconducting carbon nanotubes (CNTs) provide an exceptional platform for studying one-dimensional excitons (bound electron-hole pairs), but the role of defects and quenching centers in controlling emission remains controversial. Here we show that, by wrapping the CNT in a polymer sheath and cooling to 4.2 K, ultranarrow photoluminescence (PL) emission line widths below 80 μeV can be seen from individual solution processed CNTs. Hyperspectral imaging of the tubes identifies local emission sites and shows that some previously dark quenching segments can be brightened by the application of high magnetic fields, and their effect on exciton transport and dynamics can be studied. Using focused high intensity laser irradiation, we introduce a single defect into an individual nanotube which reduces its quantum efficiency by the creation of a shallow bound exciton state with enhanced electron-hole exchange interaction. The emission intensity of the nanotube is then reactivated by the application of the high magnetic field. PMID:25158099
Highly accurate analytical energy of a two-dimensional exciton in a constant magnetic field
NASA Astrophysics Data System (ADS)
Hoang, Ngoc-Tram D.; Nguyen, Duy-Anh P.; Hoang, Van-Hung; Le, Van-Hoang
2016-08-01
Explicit expressions are given for analytically describing the dependence of the energy of a two-dimensional exciton on magnetic field intensity. These expressions are highly accurate with the precision of up to three decimal places for the whole range of the magnetic field intensity. The results are shown for the ground state and some excited states; moreover, we have all formulae to obtain similar expressions of any excited state. Analysis of numerical results shows that the precision of three decimal places is maintained for the excited states with the principal quantum number of up to n=100.
NASA Astrophysics Data System (ADS)
Meier, T.; Thomas, P.; Koch, S. W.
2001-07-01
Linear and nonlinear optical absorption spectra are studied theoretically for semiconductor nanorings penetrated by a magnetic field. Due to the Aharanov-Bohm effect the spectral position as well as the oscillator strength of the exciton change periodically as function of the magnetic flux enclosed by the ring. In the nonlinear differential absorption spectra it is found that the magnetic field strongly modifies Coulomb many-body correlations. In particular, the magnetic-field-induced increase of the exciton binding energy is accompanied by a decrease of the biexciton binding energy. The persistence of these effects in the presence of energetic disorder is analyzed.
Riva, C.; Peeters, F. M.; Varga, K.
2001-03-15
We present a variational calculation of the spin-singlet and spin-triplet states of a negatively charged exciton (trion) confined to a single quantum well in the presence of a perpendicular magnetic field. We calculated the probability density and the pair correlation function of the singlet and triplet trion states. The dependence of the energy levels and of the binding energy on the well width and on the magnetic field strength was investigated. We compared our results with the available experimental data on GaAs/AlGaAs quantum wells and find that in the low-magnetic-field region (B<18 T) the observed transitions are those of the singlet and the dark triplet trion (with angular momentum L{sub z}=-1), while for high magnetic fields (B>25 T) the dark trion becomes optically inactive and possibly a transition to a bright triplet trion (angular momentum L{sub z}=0) state is observed.
Evidence for photogenerated intermediate hole polarons in ZnO.
Sezen, Hikmet; Shang, Honghui; Bebensee, Fabian; Yang, Chengwu; Buchholz, Maria; Nefedov, Alexei; Heissler, Stefan; Carbogno, Christian; Scheffler, Matthias; Rinke, Patrick; Wöll, Christof
2015-01-01
Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. Polarons represent a crucial intermediate step populated immediately after dissociation of the excitons formed in the primary photoabsorption process. Here we present a novel approach to studying photoexcited polarons in an important photoactive oxide, ZnO, using infrared (IR) reflection-absorption spectroscopy (IRRAS) with a time resolution of 100 ms. For well-defined (10-10) oriented ZnO single-crystal substrates, we observe intense IR absorption bands at around 200 meV exhibiting a pronounced temperature dependence. On the basis of first-principles-based electronic structure calculations, we assign these features to hole polarons of intermediate coupling strength. PMID:25902307
NASA Astrophysics Data System (ADS)
Qu, Fanyao; Moura, Fábio Vieira; Alves, Fabrizio M.; Gargano, Ricardo
2013-03-01
Optical control of magnetic property of a magnetic polaron (MP) in Mn-doped bulk GaAs and GaAs/AlGaAs quantum dots (QDs) have been studied. We have developed basis optimization technique for the method of linear combination of atomic orbitals (LCAOs), which significantly improve the accuracy of the conventional LCAO calculation. We have demonstrated that a monochromatic, linearly polarized, intense pulsed laser field induces a collapse of the MP and an ionization of Mn-acceptor in Mn-doped GaAs materials due to a dichotomy of hole wave function. We find this optical tunability of MP stability can be adjusted by confinement introduced in GaAs QDs.
NASA Astrophysics Data System (ADS)
Grochol, M.; Grosse, F.; Zimmermann, R.
2006-09-01
The optical exciton Aharonov-Bohm effect—i.e., an oscillatory component in the energy of optically active (bright) states—is investigated in nanorings. It is shown that a small effective electron mass, strong confinement of the electron, and high barrier for the hole, achieved, e.g., by an InAs nanoring embedded in an AlGaSb quantum well, are favorable for observing the optical exciton Aharonov-Bohm effect. The second derivative of the exciton energy with respect to the magnetic field is utilized to extract Aharonov-Bohm oscillations even for the lowest bright state unambiguously. A connection between the theories for infinitesimal narrow and finite width rings is established. Furthermore, the magnetization is compared to the persistent current, which oscillates periodically with the magnetic field and confirms thus the nontrivial (connected) topology of the wave function in the nanoring.
A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects.
Lee, Chee Kong; Shi, Liang; Willard, Adam P
2016-06-16
In this Letter, we explore how the microscopic dynamics of charge-transfer (CT) excitons are influenced by the presence of an external magnetic field in disordered molecular semiconductors. This influence is driven by the dynamic interplay between the spin and spatial degrees of freedom of the electron-hole pair. To account for this interplay, we have developed a numerical framework that combines a traditional model of quantum spin dynamics with a stochastic coarse-grained model of charge transport. This combination provides a general and efficient methodology for simulating the effects of magnetic field on CT state dynamics, therefore providing a basis for revealing the microscopic origin of experimentally observed magnetic field effects. We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials. PMID:27237448
NASA Astrophysics Data System (ADS)
Patterson, C. H.
2008-03-01
We report hybrid density functional theory calculations on hole doped Ca2-xNaxCuO2Cl2 performed in 4×4 , 42×42 , and 8×2 supercells with hole concentrations x=0.0625 and x=0.125 . Holes at the lower concentration form small polarons, in which the hole is mainly localized on four oxygen ions surrounding one copper ion. The polaron is a spin one-half ferromagnetic polaron (Cu5O4) , in which the moment on the central copper ion is parallel to those on the four neighboring copper ions and the moment on the oxygen ions is opposed to that on the copper ions. This is therefore an Emery-Reiter spin polaron rather than a Zhang-Rice singlet. At the higher hole concentration (x=0.125) , many cuprates form stripes. Hybrid density functional theory calculations on linear chains of spin polarons separated by 4a0 show a group of bands localized mainly on the stripe. Spins on neighboring copper ions in the stripe are parallel and so the stripe forms a magnetic antiphase boundary between antiferromagnetically ordered blocks of copper spins. Stripes of this kind, which run in one direction only, may explain recent scanning tunneling microscopy data from Ca2-xNaxCuO2Cl2 by Kohsaka [Science 315, 1380 (2007)]. We also consider an ordered spin polaron phase where magnetic antiphase boundaries intersect at right angles. In this case, sets of four copper ions in squares at stripe intersections have parallel spins. This phase may be the 4×4 checkerboard pattern reported by Hanaguri [Nature (London) 430, 1001 (2004)].
An exciton on coupled concentric double nanorings in a magnetic field
NASA Astrophysics Data System (ADS)
Song, Weiwei; Jin, Guojun; Ma, Yu-qiang
2008-09-01
We study the influence of coupling between two concentric nanorings, embedded in two different layers separated by an insulating barrier, on the exciton's energy levels by the attractive Fermionic Hubbard model. The hopping coefficients between different rings are derived and the carriers' cyclotron movement caused by the magnetic field are taken into account. We find the amplitude of the Aharonov-Bohm oscillation is suppressed by the emergence of the Coulomb interaction. The coupling leads to a decrease of the energy of the excited states and this decrease varies significantly for different types of configuration for the electron and the hole. The study of the energy dependence on the radii of the two rings shows that the lowest energy level is determined by the small ring and the deviations of the exciton's energy from that of the single ring decreases with increasing magnetic field. The results also show that the energy of the AB oscillation can be modulated by changing the thickness of the spacer.
Charge-transfer excitons in DNA.
Conwell, E M; McLaughlin, P M; Bloch, S M
2008-02-21
There have been a number of theoretical treatments of excitons in DNA, most neglecting both the intrachain and interchain wavefunction overlaps of the electron and hole, treating them as Frenkel excitons. Recently, the importance of the intrachain and interchain coupling has been highlighted. Experiments have shown that in (dA)n oligomers and in duplex (dA)n.(dT)n, to be abbreviated (A/T), where A is adenine and T is thymine, the exciton wavefunction is delocalized over several bases. In duplexes it is possible to have charge-transfer (CT) excitons. Theoretical calculations have suggested that CT excitons in DNA may have lower energy than single chain excitons. In all the calculations of excitons in DNA, the polarization of the surrounding water has been neglected. Calculations have shown, however, that polarization of the water by an excess electron or a hole in DNA lowers its energy by approximately 1/2 eV, causing it to become a polaron. It is therefore to be expected that polarization charge induced in the surrounding water has a significant effect on the properties of the exciton. In what follows, we present calculations of some properties CT excitons would have in an A/T duplex taking into account the wavefunction overlaps, the effect of the surrounding water, which results in the electron and hole becoming polarons, and the ions in the water. As expected, the CT exciton has lowest energy when the electron and hole polarons are directly opposite each other. By appropriate choice of the dielectric constant, we can obtain a CT exciton delocalized over the number of sites found in photoinduced absorption experiments. The absorption threshold that we then calculate for CT exciton creation in A/T is in reasonable agreement with the lowest singlet absorption deduced from available data. PMID:18232682
NASA Astrophysics Data System (ADS)
Scott, Alwyn C.; Bigio, Irving J.; Johnston, Clifford T.
1989-06-01
The best available data are presented of the integrated intensity of the 1650-cm-1 band in crystalline acetanilide as a function of temperature. A concise theory of polaron states is presented and used to interpret the data.
Scott, A. C.; Bigio, I. J.; Johnston, C. T.
1989-06-15
The best available data are presented of the integrated intensity of the1650-cm/sup /minus/1/ band in crystalline acetanilide as a function oftemperature. A concise theory of polaron states is presented and used tointerpret the data.
Semiclassical and quantum polarons in crystalline acetanilide
NASA Astrophysics Data System (ADS)
Hamm, P.; Tsironis, G. P.
2007-08-01
Crystalline acetanilide is a an organic solid with peptide bond structure similar to that of proteins. Two states appear in the amide I spectral region having drastically different properties: one is strongly temperature dependent and disappears at high temperatures while the other is stable at all temperatures. Experimental and theoretical work over the past twenty five years has assigned the former to a selftrapped state while the latter to an extended free exciton state. In this article we review the experimental and theoretical developments on acetanilide paying particular attention to issues that are still pending. Although the interpretation of the states is experimentally sound, we find that specific theoretical comprehension is still lacking. Among the issues that that appear not well understood is the effective dimensionality of the selftrapped polaron and free exciton states.
NASA Astrophysics Data System (ADS)
Fenniche, H.; Jaziri, S.; Bennaceur, R.
1998-12-01
We study theoretically a particular type of semiconductor microcavity formed by a quantum well embedded inside it and the distributed Bragg reflectors presenting a gradual structure. We apply to this structure a static magnetic field along the growth direction. In the strong coupling regime between the confined exciton and cavity modes, we evaluate the polariton Rabi splitting corresponding to the two lowest lying exciton states: HH1-CB1 and HH2-CB2 as a function of the applied magnetic field. In high magnetic field and for distinct reflectivities, we find that the Rabi splitting magnitude of the HH2-CB2 exciton is close to the fundamental one (HH1-CB1). In the presence of the magnetic field, the polariton Rabi splitting can be obtained even in low reflectivity. The dispersion polariton radiative decay rates related to the two lowest lying exciton states: HH1-CB1 and HH2-CB2 are calculated for different magnetic field values. At k //=0 and in the weak coupling regime, the polariton radiative decay rates are evaluated for both the HH1-CB1 and HH2-CB2 excitons. We show that for the fundamental excitonic state, the magnetic field value which determines the transition from the weak to the strong coupling regime is different from the HH2-CB2 exciton state.
NASA Astrophysics Data System (ADS)
Lany, Stephan
2010-03-01
The formation of a small polaron, i.e. of a localized (electron or hole) quasi-particle state that is stabilized by a lattice distortion, is a problem in solid state physics that has eluded a quantitative description by first principles Hamiltonians for a long time. Specifically, conventional density functional theory calculations typically predict a much too delocalized state and usually fail to correctly predict the lattice distortions of localized hole-states in semiconductors and insulators. While this problem has been studied in detail for some prototypical cases like the Al impurity in SiO2, it has at the same time precluded an extensive theoretical literature on the phenomenology of systems with localized hole states, despite the potentially dramatic effect of hole localization on such timely research topics as p-type doping of oxides or that of diluted magnetic semiconductors. Indeed, many predictions for hole-introducing defects and impurities that were based on local density approximations have led to a qualitatively wrong physical picture about the lattice distortion, the energies of the hole-bearing acceptor levels in the gap, and about ferro-magnetic interactions between defects. In order to stabilize the polaronic localized states in the gap, we define a parameterized hole- (or electron-) state potential which increases the energy splitting between occupied and unoccupied orbitals, where we further require that a fundamental physical condition is satisfied, i.e., the piecewise linearity of the energy as a function of the occupation number. This requirement takes the form of a generalized Koopmans conditions, which uniquely determines the one free parameter of the hole- (electron-) state potential. Applying this method to the anion-p orbitals within the II-VI series of ZnO, ZnS, ZnSe, and ZnTe, we demonstrate electronic correlation effects remove the partial band occupation and the metallic band-structure character that is predicted by local density
Robust ground state and artificial gauge in DQW exciton condensates under weak magnetic field
NASA Astrophysics Data System (ADS)
Hakioğlu, T.; Özgün, Ege; Günay, Mehmet
2014-08-01
An exciton condensate is a vast playground in studying a number of symmetries that are of high interest in the recent developments in topological condensed matter physics. In double quantum wells (DQWs) they pose highly nonconventional properties due to the pairing of non-identical fermions with a spin dependent order parameter. Here, we demonstrate a new feature in these systems: the robustness of the ground state to weak external magnetic field and the appearance of the artificial spinor gauge fields beyond a critical field strength where negative energy pair-breaking quasi particle excitations, i.e. de-excitation pockets (DX-pockets), are created in certain k regions. The DX-pockets are the Kramers symmetry broken analogs of the negative energy pockets examined in the 1960s by Sarma. They respect a disk or a shell-topology in k-space or a mixture between them depending on the magnetic field strength and the electron-hole density mismatch. The Berry connection between the artificial spinor gauge field and the TKNN number is made. This field describes a collection of pure spin vortices in real space when the magnetic field has only inplane components.
Localized and bound excitons in type-II ZnMnSe/ZnSSe quantum wells.
Chernenko, A V; Brichkin, A S
2014-10-22
Photoluminescence of ZnMnSe/ZnSSe multiple quantum wells under a bandgap continuous wave and fs-pulsed excitations is measured in magnetic fields up to 10 T in Faraday geometry at temperatures within the range of 1.6-20 K. The measurements reveal two dominant lines in the spectra and LO-phonon replicas of the lower-energy line. The photoluminescence and time-resolved studies show dramatically different behaviour of the lines. Analysis of their properties reveals that they correspond to recombination of indirect localized excitons and indirect acceptor-bound excitons (A0X). Crossing of exciton and A0X lines because of the difference in magnitudes of their Zeeman shifts is observed. Analysis of LO-phonon replicas of photoluminescence lines provides additional evidence for strong carrier localization bound to A0X. A model of phonon-assisted recombination of indirect acceptor-bound excitons is proposed. The fitting of photoluminescence lines with this model gives the Huang-Rhys factor S≃0.25 for A0X and the hole localization size ah≃30 Å. Contrary to expectations the exciton magnetic polaron effect is hardly observed in these structures. PMID:25273841
Verma, Kuldeep Chand; Kotnala, R K
2016-02-21
Zn0.94TM0.03Ce0.03O [Zn0.94Fe0.03Ce0.03O (ZFCeO) and Zn0.94Co0.03Ce0.03O (ZCCeO)] nanoparticles were synthesized by a sol-gel process. Elemental analysis of these nanoparticles detects the weight percentage of Zn, Co, Fe, Ce and O in each sample. The Rietveld refinement of the X-ray diffraction pattern obtains the occupancy of dopant atoms, Wurtzite ZnO structure, crystallinity and lattice deformation with doping. The Ce doping into ZFO and ZCO form nanoparticles than nanorods was observed in pure ZnO, ZFO and ZCO samples that described due to chemical and ionic behavior of Ce, Fe, Co and Zn ions. The Raman active modes have peak broadening, intensity changes and peak shifts with metal doping that induces lattice defects. Photoluminescence spectra show blue-shifts at near-band edges and defects that influence broad visible emission with Ce doping. An enhancement in ferromagnetism in the magnetic hysteresis at 5 K is measured. The zero-field cooling and field cooling at H = 500 Oe and T = 300-5 K could confirm antiferromagnetic interactions mediated by defect carriers. The bound magnetic polaron at defect sites is responsible for the observed ferromagnetism. The ac magnetic susceptibility measurements determine the antiferromagnetic to ferromagnetic transition with some magnetic clustered growth in the samples and reveal a frequency independent peak that shows the Neel temperature. Weak room temperature ferromagnetism and optical quenching in ZFCeO are described by valance states of Fe and Ce ions, respectively. Using first-principle calculations, we studied the occupancy of Ce (replacing Zn atoms) in the Wurtzite structure. PMID:26831598
Solnyshkov, D. D.; Malpuech, G.; Shelykh, I. A.
2009-10-15
We establish a phase diagram of a spinor exciton-polariton condensate in a disordered microcavity in the presence of an external magnetic field. We find that the combination of the full paramagnetic screening and Anderson localization leads to the formation of a condensed phase having both localized and superfluid components. This is reflected by different dispersions of elementary excitations for the two polarization components.
NASA Astrophysics Data System (ADS)
Shamirzaev, T. S.; Debus, J.; Yakovlev, D. R.; Glazov, M. M.; Ivchenko, E. L.; Bayer, M.
2016-07-01
The exciton recombination dynamics is studied experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells characterized by an indirect band gap and a type-II band alignment. At cryogenic temperatures, the lifetimes of the excitons that are indirect both in real and k space are in the millisecond range. The exciton recombination time and the photoluminescence (PL) intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to the very weak influence of an in-plane field, at 2 K temperature a field applied parallel to the growth axis drastically slows down the recombination and reduces the PL intensity. With increasing temperature the magnetic field effects on PL intensity and decay time are vanishing. The experimental data are well described by a model for the exciton dynamics that takes into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states. It allows us to evaluate the lower bound of the heavy-hole longitudinal g factor of 2.5, the radiative recombination time for the bright excitons of 0.34 ms, and the nonradiative recombination time of the bright and dark excitons of 8.5 ms.
NASA Astrophysics Data System (ADS)
Siebers, B.; Biadala, L.; Yakovlev, D. R.; Rodina, A. V.; Aubert, T.; Hens, Z.; Bayer, M.
2015-04-01
The exciton spin dynamics and polarization properties of the related emission are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL) spectroscopy in magnetic fields up to 15 T. It is shown that the degree of circular polarization (DCP) of the exciton emission induced by the magnetic field is affected by the NC geometry as well as the exciton fine structure and can provide information on nanorod orientation. A theory to describe the circular and linear polarization properties of the NC emission in a magnetic field is developed. It takes into account phonon mediated coupling between the exciton fine structure states as well as the dielectric enhancement effect resulting from the anisotropic shell of DiR NCs. This theoretical approach is used to model the experimental results and allows us to explain most of the measured features. The spin dynamics of the dark excitons is investigated in magnetic fields by time-resolved photoluminescence. The results highlight the importance of confined acoustic phonons in the spin relaxation of dark excitons. The bare core surface as well as the core/shell interface give rise to an efficient spin-relaxation channel, while the surface of core/shell NCs seems to play only a minor role.
Dynamics of exciton dissociation in donor-acceptor polymer heterojunctions.
Sun, Zhen; Stafström, Sven
2013-04-28
Exciton dissociation in a donor-accepter polymer heterojunction has been simulated using a nonadiabatic molecular dynamics approach, which allows for the coupled evolution of the nuclear degrees of freedom and the electronic degrees of freedom described by multiconfigurational electronic wavefunctions. The simulations reveal important details of the charge separation process: the exciton in the donor polymer first dissociates into a "hot" charge transfer state, which is best described as a polaron pair. The polaron pair can be separated into free polaron charge carriers if a sufficiently strong external electric field is applied. We have also studied the effects of inter-chain interaction, temperature, and the external electric field strength. Increasing inter-chain interactions makes it easier for the exciton to dissociate into a polaron pair state, but more difficult for the polaron pair to dissociate into free charge carriers. Higher temperature and higher electric field strength both favor exciton dissociation as well as the formation of free charge carriers. PMID:23635169
High magnetic field studies of charged exciton localization in GaAs/AlxGa1-xAs quantum wells
NASA Astrophysics Data System (ADS)
Jadczak, J.; Bryja, L.; Ryczko, K.; Kubisa, M.; Wójs, A.; Potemski, M.; Liu, F.; Yakovlev, D. R.; Bayer, M.; Nicoll, C. A.; Farrer, I.; Ritchie, D. A.
2014-09-01
We report on low temperature, polarization resolved, high magnetic field (up to 23 T) photoluminescence experiments on high mobility asymmetric GaAs quantum wells. At high magnetic fields, we detect two strong emission lines of the neutral and positively charged excitons (X and X+) and a series of weaker lines of the excitons bound to ionized acceptors (AX-). From polarization energy splittings of these lines, we determine the hole Landé factors (gh) of different complexes. For X and X+, gh initially grows with magnetic field and then saturates at gh = 0.88 and 1.55, respectively; for AX-'s, gh begins from a high value (from 6 to 11 at zero field) and decreases with the field growth. This contrasting behavior is traced to the structure of valence band Landau levels, calculated numerically in the Luttinger model, beyond axial approximation. This points to the coexistence (in the same well) of mobile X and X+ with localized and interface-pressed AX- states.
Hoang-Do, Ngoc-Tram; Hoang, Van-Hung; Le, Van-Hoang
2013-05-15
The Feranchuk-Komarov operator method is developed by combining with the Levi-Civita transformation in order to construct analytical solutions of the Schroedinger equation for a two-dimensional exciton in a uniform magnetic field of arbitrary strength. As a result, analytical expressions for the energy of the ground and excited states are obtained with a very high precision of up to four decimal places. Especially, the precision is uniformly stable for the whole range of the magnetic field. This advantage appears due to the consideration of the asymptotic behaviour of the wave-functions in strong magnetic field. The results could be used for various physical analyses and the method used here could also be applied to other atomic systems.
Spontaneous polaron transport in biopolymers
NASA Astrophysics Data System (ADS)
Chakrabarti, B.; Piette, B. M. A. G.; Zakrzewski, W. J.
2012-02-01
Polarons, introduced by Davydov to explain energy transport in α-helices, correspond to electrons localised on a few lattice sites because of their interaction with phonons. While the static polaron field configurations have been extensively studied, their displacement is more difficult to explain. In this paper we show that, when the next-to-nearest-neighbour interactions are included, for physical values of the parameters, polarons can spontaneously move, at T=0, on bent chains that exhibit a positive gradient in their curvature. At room temperature polarons perform a random walk but a curvature gradient can induce a non-zero average speed similar to the one observed at zero temperature. We also show that, at zero temperature, a polaron bounces on sharply kinked junctions. We interpret these results in the light of the energy transport by transmembrane proteins.
Photoluminescence-detected magnetic-resonance study of fullerene-doped {pi}-conjugated polymers
Lane, P.A.; Shinar, J.; Yoshino, K.
1996-10-01
{ital X}-band photoluminescence (PL)-detected magnetic resonance (PLDMR) spectra of C{sub 60}- and C{sub 70}-doped 2,5-dihexoxy poly({ital p}-phenylenevinylene) (DHO-PPV), 2,5-dibutoxy poly({ital p}-phenylene ethynylene) (DBO-PPE), and poly(3-dodecylthiophene) (P3DT) are described and discussed. While light doping of DHO-PPV by both fullerenes quenches the PL, both the polaron and triplet exciton resonances are dramatically enhanced. This is attributed to the creation of conformational defects which enhance the fission of 1{sup 1}{ital B}{sub {ital u}} singlet excitons to polaron pairs and intersystem crossing to 1{sup 3}{ital B}{sub {ital u}} triplet excitons. The triplet resonance in all polymers is quenched at relatively low doping levels of C{sub 60} and C{sub 70}, which is attributed to quenching of triplets by positive polarons injected onto the polymer chain. Increased doping by C{sub 60}, but not C{sub 70}, quenches the polaron resonance, also due to photoinduced charge transfer.
Excitons in coupled type-II double quantum wells under electric and magnetic fields: InAs/AlSb/GaSb
Lyo, S. K.; Pan, W.
2015-11-21
We calculate the wave functions and the energy levels of an exciton in double quantum wells under electric (F) and magnetic (B) fields along the growth axis. The result is employed to study the energy levels, the binding energy, and the boundary on the F–B plane of the phase between the indirect exciton ground state and the semiconductor ground state for several typical structures of the type-II quasi-two-dimensional quantum wells such as InAs/AlSb/GaSb. The inter-well inter-band radiative transition rates are calculated for exciton creation and recombination. We find that the rates are modulated over several orders of magnitude by the electric and magnetic fields.
Mrowiński, P.; Musiał, A.; Maryński, A.; Syperek, M.; Misiewicz, J.; Sęk, G.; Somers, A.; Reithmaier, J. P.; Höfling, S.
2015-02-02
We investigated the neutral and charged exciton fine structure in single InAs/InGaAlAs/InP quantum dashes emitting at 1.55 μm using polarization-resolved microphotoluminescence in a magnetic field. Inverted spin configuration of horizontally [1–10] and vertically [110] polarized transitions has been observed. An in-plane magnetic field of up to 5 Tesla has been applied to tailor the fine structure, and eventually to reduce the splitting of the bright exciton states down to zero. This inverted structure has been observed for all the investigated excitons, making it a characteristic feature for this class of nanostructures with the largest splitting reduction of 170 μeV.
NASA Astrophysics Data System (ADS)
Liu, Feng; Rodina, A. V.; Yakovlev, D. R.; Golovatenko, A. A.; Greilich, A.; Vakhtin, E. D.; Susha, A.; Rogach, A. L.; Kusrayev, Yu. G.; Bayer, M.
2015-09-01
We present a systematic experimental study along with theoretical modeling of the energy transfer in an ensemble of closely packed CdTe colloidal nanocrystals identified as the Förster resonant energy transfer (FRET). We prove that at low temperature of 4.2 K, mainly the ground dark exciton states in the initially excited small-size (donor) nanocrystals participate in the dipole-dipole FRET leading to additional excitation of the large-size (acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of the bright exciton states to the dark states. The admixture takes place even in zero magnetic field and allows the radiative recombination of the dark excitons. An external magnetic field considerably enhances this admixture, thus increasing the energy transfer rate by a factor of 2-3 in a field of 15 T, as well as the radiative rates of the dark excitons in the donor and acceptor nanocrystals. The theoretical modeling allows us to determine the spectral dependence of the probability for the NC to serve as a donor for larger nanocrystals, to evaluate the energy transfer rates as well as to predict their dependencies on the magnetic field, to describe the spectral shift of the photoluminescence maximum due to the energy transfer, and to reproduce the experimentally observed spectral dependencies of the photoluminescence recombination dynamics in the magnetic field.
Effective Masses of Vector Polarons
NASA Astrophysics Data System (ADS)
Foell, Charles; Clougherty, Dennis
2006-03-01
We consider the vector polarons of a one-dimensional model of an electron in a doubly (or nearly) degenerate band that couples to two elastic distortions, as described previously by Clougherty and Foell [1]. A variational approach is used to analytically and numerically calculate effective masses of the three types of vector polarons. [1] D. P. Clougherty and C. A. Foell, Phys. Rev. B 70, 052301 (2004).
Monotonicity of the Polaron Energy
NASA Astrophysics Data System (ADS)
Miyao, Tadahiro
2014-12-01
In condensed matter physics, the polaron is described by the Hamiltonian of H. Fröhlich. In this paper, the Fröhlich Hamiltonian is investigated from a viewpoint of operator inequalities proposed in [36]. This point of view clarifies the monotonicity of polaron energy, i.e. denoting the lowest energy of the Fröhlich Hamiltonian with the ultraviolet cutoff Λ by EΛ, we prove that EΛ, >EΛ‧ for Λ < Λ‧.
Ultrafast generation of pseudo-magnetic field for valley excitons in WSe2 monolayers
NASA Astrophysics Data System (ADS)
Kim, Jonghwan; Hong, Xiaoping; Jin, Chenhao; Shi, Su-Fei; Chang, Chih-Yuan S.; Chiu, Ming-Hui; Li, Lain-Jong; Wang, Feng
2015-03-01
The valley pseudospin emerges as a new degree of freedom in atomically thin two-dimensional transition metal dichalcogenides (MX2). In analogy to the control of spin in spintronics, the capability to manipulate the valley pseudospin can provide exciting opportunities in valleytronics. Here we present that femtosecond pulses with circular polarization can generate ultrafast and ultrahigh valley pseudomagnetic field in a monolayer MX2. Our polarization-resolved transient absorption measurement shows that the degeneracy of valley exciton transitions at K and K' valley in WSe2 monolayers can be lifted by optical Stark effect from the non-resonant pump. Energy splitting due to the optical Stark effect is linear with both the pump intensity and the inverse of pump detuning. We observe that valley-selective optical Stark effect can create an energy splitting more than 10 meV which corresponds to a pseudomagnetic field over 60 Tesla. Our study demonstrates efficient and ultrafast control of the valley excitons with optical light which can open up the possibility of coherent manipulation of the valley polarization in MX2.
TOPICAL REVIEW: O- bound small polarons in oxide materials
NASA Astrophysics Data System (ADS)
Schirmer, O. F.
2006-11-01
Holes bound to acceptor defects in oxide crystals are often localized by lattice distortion at just one of the equivalent oxygen ligands of the defect. Such holes thus form small polarons in symmetric clusters of a few oxygen ions. An overview on mainly the optical manifestations of those clusters is given. The article is essentially divided into two parts: the first one covers the basic features of the phenomena and their explanations, exemplified by several paradigmatic defects; in the second part numerous oxide materials are presented which exhibit bound small polaron optical properties. The first part starts with summaries on the production of bound hole polarons and the identification of their structure. It is demonstrated why they show strong, wide absorption bands, usually visible, based on polaron stabilization energies of typically 1 eV. The basic absorption process is detailed with a fictitious two-well system. Clusters with four, six and twelve equivalent ions are realized in various oxide compounds. In these cases several degenerate optically excited polaron states occur, leading to characteristic final state resonance splittings. The peak energies of the absorption bands as well as the sign of the transfer energy depend on the topology of the clusters. A special section is devoted to the distinction between interpolaron and intrapolaron optical transitions. The latter are usually comparatively weak. The oxide compounds exhibiting bound hole small polaron absorptions include the alkaline earth oxides (e.g. MgO), BeO and ZnO, the perovskites BaTiO3 and KTaO3, quartz, the sillenites (e.g. Bi12TiO20), Al2O3, LiNbO3, topaz and various other materials. There are indications that the magnetic crystals NiO, doped with Li, and LaMnO3, doped with Sr, also show optical features caused by bound hole polarons. Beyond being elementary paradigms for the properties of small polarons in general, the defect species treated can be used to explain radiation and light
NASA Astrophysics Data System (ADS)
Xu, Dazhi; Cao, Jianshu
2016-08-01
The concept of polaron, emerged from condense matter physics, describes the dynamical interaction of moving particle with its surrounding bosonic modes. This concept has been developed into a useful method to treat open quantum systems with a complete range of system-bath coupling strength. Especially, the polaron transformation approach shows its validity in the intermediate coupling regime, in which the Redfield equation or Fermi's golden rule will fail. In the polaron frame, the equilibrium distribution carried out by perturbative expansion presents a deviation from the canonical distribution, which is beyond the usual weak coupling assumption in thermodynamics. A polaron transformed Redfield equation (PTRE) not only reproduces the dissipative quantum dynamics but also provides an accurate and efficient way to calculate the non-equilibrium steady states. Applications of the PTRE approach to problems such as exciton diffusion, heat transport and light-harvesting energy transfer are presented.
Chang, Hung-Tzu; Cheng, Yuan-Chung; Zhang, Pan-Pan
2013-12-14
The small polaron quantum master equation (SPQME) proposed by Jang et al. [J. Chem. Phys. 129, 101104 (2008)] is a promising approach to describe coherent excitation energy transfer dynamics in complex molecular systems. To determine the applicable regime of the SPQME approach, we perform a comprehensive investigation of its accuracy by comparing its simulated population dynamics with numerically exact quasi-adiabatic path integral calculations. We demonstrate that the SPQME method yields accurate dynamics in a wide parameter range. Furthermore, our results show that the accuracy of polaron theory depends strongly upon the degree of exciton delocalization and timescale of polaron formation. Finally, we propose a simple criterion to assess the applicability of the SPQME theory that ensures the reliability of practical simulations of energy transfer dynamics with SPQME in light-harvesting systems.
Bound polarons in semiconductor nanostructures
NASA Astrophysics Data System (ADS)
Woggon, U.; Miller, D.; Kalina, F.; Gerlach, B.; Kayser, D.; Leonardi, K.; Hommel, D.
2003-01-01
Bound polarons are discrete, confined electronic states, spatially localized due to a local potential V(r) but sharing a common phonon state of the surrounding crystal. We study the energy states of polarons bound in a potential and determine the local optical absorption spectrum up to first-order time-dependent perturbation theory with respect to the electron-photon interaction. The model is applied to describe the optical properties of submonolayer CdSe insertions epitaxially grown between ZnSe layers. As a typical signature of bound polarons we found excited-state energies equidistantly separated by the LO phonon energy and with optical transition probabilities determined by the anisotropies in V(r).
Some approaches to polaron theory
NASA Astrophysics Data System (ADS)
Bogolubov, N. N.; Bogolubov, N. N.
1985-11-01
Here, in our approximation of polaron theory, we examine the importance of introducing the T product, which turn out to be a very convenient theoretical approach for the calculation of thermodynamical averages. We focus attention on the investigation of the so-called linear polaron Hamiltonian and present in detail the calculation of the correlation function, spectral function, and Green function for such a linear system. It is shown that the linear polaron Hamiltonian provides an exactly solvable model of our system, and the result obtained with this approach holds true for an arbitrary coupling constant which describes the strength of interaction between the electron and the lattice vibrations. Then, with the help of a variational technique, we show the possibility of reducing the real polaron Hamiltonian to a socalled trial or approximate linear model Hamiltonian. We also consider the exact calculation of free energy with a special technique that reduces calculations with the help of the T product, which, in our opinion, works much better and is easier than other analogous considerations, for example, the path-integral or Feynman-integral method.(1,2) Here we furthermore recall our own work,(4) where it was shown that the results of Refs. 7 and 8 concerning the impedance calculation in the polaron model may be obtained directly without the use of the path-integral method. The study of the polaron system's thermodynamics is carried out by us in the framework of the functional method. A calculation of the free energy and the momentum distribution function is proposed. Note also that the polaron systems with strong coupling(9) proved to be useful in different quantum field models in connection with the construction of dynamical models of composite particles. A rigorous solution of the special strong-coupling polaron problem, describing the interaction of a nonrelativistic particle with a quantum field, was given by Bogolubov.(3) The works of Tavkhelidze, Fedyanin
NASA Astrophysics Data System (ADS)
Li, Guangqi; Movaghar, Bijan
2015-06-01
We describe electron transfer and localization in a finite two-dimensional transporting layer (15 × 15) using a tight binding Hamiltonian where each site is coupled to phonons. For a narrow electronic band, a polaron is formed with a population that peaks in the middle of the layer and exhibits a concomitant energy lowering. A "local defect" can be simulated by lowering or raising the corresponding site energy. As an example, if we put the defect in one corner, the consequence is that the electron population builds up a polaron which is repelled from this region. The model has been applied to describe the experimentally observed real time polaron formation process in organic layers and in particular in the surface bands of ice-covered metal. We simulate the polaron formation, population distribution and energy relaxation in time. We also investigate the effect of local fluctuations on polaron formation. The formalism can be generalized to excitonic trapping, and has many potential applications. Supplementary material in the form of one pdf file available from the Journal web page at: http://dx.doi.org/10.1140/epjb/e2015-60258-3.
Spin polarization of polaron in quasi-one dimensional organic system
NASA Astrophysics Data System (ADS)
Jiang, Hong; Zhang, Chao; Hu, Xuening; Hu, Guichao; Xie, Shijie
2015-01-01
The spin polarization of polarons in quasi-1D organic materials has been investigated by using the extended Su-Schrieffer-Heeger (SSH) model with spin-orbit coupling. Results show that the polaron is partly spin polarized, and that the electron-electron interaction and spin-orbit coupling compete with each other during the formation of spin polarization. The dependence of spin polarization on electron-phonon coupling is also revealed. Our results demonstrate that spin polarization is well correlated with polaron localization, thus providing useful guidance for exploring magnetic effects in organic materials.
NASA Astrophysics Data System (ADS)
Inpasalini, M. S.; Choubey, Ravi Kant; Mukherjee, Samrat
2016-04-01
Phase-pure rutile SnO2 nanoparticles doped with various pairs of transition metals such as Fe-Ni, Fe-Mn, Ni-Mn, and Co-Mn have been synthesized using a sol-gel method. Average crystallite size of 22 nm to 29 nm with no impurity secondary phase was observed by x-ray diffraction (XRD) analysis. The morphologies of the particles were studied and their sizes calculated by field-emission scanning electron microscopy (FESEM). Emission at 338 nm in photoluminescence (PL) spectroscopy was observed and attributed to band-edge emission, while the peak at 470 nm was ascribed to doubly ionized oxygen vacancies. Raman spectroscopy confirmed the phase purity of the sample. The effects of oxygen vacancies broaden the E g mode of the Raman spectrum. In 119Sn Mössbauer spectra, broadened line width was observed and the singlet was attributed to Sn4+ valence state. The interaction between singly ionized oxygen vacancies and 3d dopant ion is the reason behind the ferromagnetic ordering observed in magnetic studies. Spins pinned at the surface orient randomly, which decreases the total moment.
NASA Astrophysics Data System (ADS)
Inpasalini, M. S.; Choubey, Ravi Kant; Mukherjee, Samrat
2016-07-01
Phase-pure rutile SnO2 nanoparticles doped with various pairs of transition metals such as Fe-Ni, Fe-Mn, Ni-Mn, and Co-Mn have been synthesized using a sol-gel method. Average crystallite size of 22 nm to 29 nm with no impurity secondary phase was observed by x-ray diffraction (XRD) analysis. The morphologies of the particles were studied and their sizes calculated by field-emission scanning electron microscopy (FESEM). Emission at 338 nm in photoluminescence (PL) spectroscopy was observed and attributed to band-edge emission, while the peak at 470 nm was ascribed to doubly ionized oxygen vacancies. Raman spectroscopy confirmed the phase purity of the sample. The effects of oxygen vacancies broaden the E g mode of the Raman spectrum. In 119Sn Mössbauer spectra, broadened line width was observed and the singlet was attributed to Sn4+ valence state. The interaction between singly ionized oxygen vacancies and 3 d dopant ion is the reason behind the ferromagnetic ordering observed in magnetic studies. Spins pinned at the surface orient randomly, which decreases the total moment.
NASA Astrophysics Data System (ADS)
Zhai, Yaxin; Olejnik, Ella; Vardeny, Z. Valy
2015-09-01
We used steady state and picosecond transient photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot-excitons into two separated triplet excitons in luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of poly(dioctyloxy)-phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≍2ET (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The picosecond transient PA with 3.1 eV pump excitation shows that in DOO-PPV film a triplet exciton is generated at time, t<500 ps. However the ultrafast triplet generation is missing in DOO-PPV solution, indicating that the HE-SF is predominantly interchain in nature. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2ET, which shows a typical SF response. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously.
Investigating the magnetic field effect on electron-hole pair in organic semiconductor devices
NASA Astrophysics Data System (ADS)
Qin, W.; Gao, K.; Yin, S.; Xie, S. J.
2013-05-01
By constructing dynamic equations including electrons, holes and their pair densities, we calculate the magnetoconductance (MC) and the magnetoelectroluminescence (MEL) separately. It is indicated that MC and MEL may result from different response on the applied magnetic field. MC is from the scattering of polarons by magnetic field related triplet excitons, while MEL is mainly from magnetic field related conversion between singlet and triplet electron-hole pairs. Furthermore, we discuss the relation between MC and MEL. The theoretical calculations are well consistent with the experimental results.
Snedden, Edward W; Monkman, Andrew P; Dias, Fernando B
2013-04-01
Geminate polaron-pair recombination directly to the triplet state of the small dye molecule copper(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H- phthalocyanine (CuPC) and exciton trapping in CuPC domains, combine to reduce the internal quantum efficiency of free polaron formation in the bulk-heterojunction blends of CuPC doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor. PMID:22933249
Magnetic Field Induced Charged Exciton Studies in a GaAs/Al(0.3)Ga(0.7)As Single Heterojunction
Kim, Yongmin; Munteanu, F.M.; Perry, C.H.; Reno, J.L.; Rickel, D.G.; Simmons, J.A.
1999-05-25
The magnetophotoluminescence (MPL) behavior of a GaAs/Al_{0.3}Ga_{0.7}As single heterojunction has been investigated to 60T. We observed negatively charged singlet and triplet exciton states that are formed at high magnetic fields beyond the {nu}=l quantum Hall state. The variation of the charged exciton binding energies are in good agreement with theoretical predictions. The MPL transition intensities for these states showed intensity variations (maxima and minima) at the {nu}=l/3 and 1/5 fractional quantum Hall (FQH) state as a consequence of a large reduction of electron-hole screening at these filling factors.
Jadczak, J.; Bryja, L. Ryczko, K.; Kubisa, M.; Wójs, A.; Potemski, M.; Liu, F.; Yakovlev, D. R.; Bayer, M.; Nicoll, C. A.; Farrer, I.; Ritchie, D. A.
2014-09-15
We report on low temperature, polarization resolved, high magnetic field (up to 23 T) photoluminescence experiments on high mobility asymmetric GaAs quantum wells. At high magnetic fields, we detect two strong emission lines of the neutral and positively charged excitons (X and X{sup +}) and a series of weaker lines of the excitons bound to ionized acceptors (AX{sup −}). From polarization energy splittings of these lines, we determine the hole Landé factors (g{sub h}) of different complexes. For X and X{sup +}, g{sub h} initially grows with magnetic field and then saturates at g{sub h} = 0.88 and 1.55, respectively; for AX{sup −}'s, g{sub h} begins from a high value (from 6 to 11 at zero field) and decreases with the field growth. This contrasting behavior is traced to the structure of valence band Landau levels, calculated numerically in the Luttinger model, beyond axial approximation. This points to the coexistence (in the same well) of mobile X and X{sup +} with localized and interface-pressed AX{sup −} states.
NASA Astrophysics Data System (ADS)
Martucci, Mary B.
The field of spintronics, the development of spin-based devices that utilize the spin degree of freedom to increase memory capacity, has emerged as a solution to faster more efficient memory storage for electronic devices. One class of materials that has been extensively studied is the half-metallic ferromagnets, compounds that are 100% spin-polarized at the Fermi level. One material in this group that has been investigated is chromium telluride (Cr 1-xTe), whose family of compounds is known to exhibit a wide range of interesting magnetic and electronic properties. We have developed a hot injection solution synthesis of Cr5Te8 nanoplatlets which show similar magnetic behavior to the bulk material. It has also been shown that selenium and sulfur analogues can be obtained without changing the reaction conditions, making progress toward a better understanding of the reaction as well as an interesting family of compounds. Using real-space simulations, the effect of polarons in the high-Tc superconducting cuprates has been studied. The simulations demonstrate energetically favorable sites for the defects and show evidence of longer-range pairing interactions. Variations of the stripe show similar energetic results. X-ray absorption fine structure spectroscopy and neutron scattering have been utilized to examine the local structure of Ni-doped Mg nanoparticles, a hydrogen storage material as well as Cu2ZnSnS4 (CZTS) nanoparticles, a photovoltaic material. The Mg-Ni material shows much local disorder upon hydrogen cycling. The CZTS data demonstrate a loss of sulfur from around the copper sites upon annealing, helping to explain the changes observed in the optical absorption properties resulting from the annealing process.
NASA Astrophysics Data System (ADS)
Tatsuno, Taro; Mizoguchi, Eriko; Nasu, Joji; Naka, Makoto; Ishihara, Sumio
2016-08-01
Magnetic field (H) effects on a correlated electron system with a spin-state degree of freedom are examined. The effective Hamiltonian derived from the two-orbital Hubbard model is analyzed by the mean-field approximation. Applying a magnetic field to the low-spin (LS) phase induces an excitonic insulating phase as well as a spin-state ordered phase where the LS and high-spin (HS) states are ordered alternately. When H is applied to the HS phase, a reentrant transition for the HS phase appears. A rich variety of the phase diagrams is attributed to the spin-state degree of freedom and their combinations in the wave function as well as in the real-space configuration. The present results provide a possible interpretation for the recent experimental observation of LaCoO3 under a strong magnetic field.
The properties of strong couple bound polaron in monolayer graphene
NASA Astrophysics Data System (ADS)
Ding, Zhao-Hua; Zhao, Ying; Xiao, Jing-Lin
2016-09-01
Based on the Hamiltonian of the interaction energy between electron on the surface of the graphene and longitudinal acoustic phonon on the surface of the substrate, the paper studies the properties of strong couple polaron in monolayer graphene considering the coulomb doping problem. The conventional Lee-Low-Pine unitary transformation method and linear combination operator method are used to calculate the ground state energy of the polaron. The results show that the ground state energy of the system has a linear relationship with the magnetic field strength, the cut-off wave number, the coulomb bound parameter, the distance between the graphene and the substrates, meanwhile, the ground state energy will split into two branches near the Dirac point.
Microscopic theories of excitons and their dynamics
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.
those that evolve "fast" and "slow," as compared to the internal system dynamics. This method is tested and applied to the spin-boson model, a two-site Frenkel exciton model, and the seven-site Fenna-Matthews-Olson complex. I conclude with a collaborative analysis of a recently developed polaron-transformed quantum master equation, which is shown to accurately interpolate between the well-known Redfield and Forster theories, even in challenging donor-bridge-acceptor arrangements.
Giant Optical Polarization Rotation Induced by Spin-Orbit Coupling in Polarons
NASA Astrophysics Data System (ADS)
Casals, Blai; Cichelero, Rafael; García Fernández, Pablo; Junquera, Javier; Pesquera, David; Campoy-Quiles, Mariano; Infante, Ingrid C.; Sánchez, Florencio; Fontcuberta, Josep; Herranz, Gervasi
2016-07-01
We have uncovered a giant gyrotropic magneto-optical response for doped ferromagnetic manganite La2 /3Ca1 /3MnO3 around the near room-temperature paramagnetic-to-ferromagnetic transition. At odds with current wisdom, where this response is usually assumed to be fundamentally fixed by the electronic band structure, we point to the presence of small polarons as the driving force for this unexpected phenomenon. We explain the observed properties by the intricate interplay of mobility, Jahn-Teller effect, and spin-orbit coupling of small polarons. As magnetic polarons are ubiquitously inherent to many strongly correlated systems, our results provide an original, general pathway towards the generation of magnetic-responsive gigantic gyrotropic responses that may open novel avenues for magnetoelectric coupling beyond the conventional modulation of magnetization.
Singlet exciton fission photovoltaics.
Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A
2013-06-18
Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses
NASA Astrophysics Data System (ADS)
Čápek, V.
1993-11-01
Starting from the Mori formalism, memory functions of excitons coupled to harmonic phonons in periodic crystals with linear exciton-phonon coupling are calculated in two limiting cases: that of the naxrow unrenormalized exciton band with only site local coupling and that of the weak exciton-phonon coupling. Mutual correspondence as well as that with results of analogous works is discussed. By a mathematical trick, perturbational series for memory kernel of initially unrela-xed excitons interacting locally with harmonic phonons is then reorganized. Using that, full agreement is achieved with previous results obtained by Generalized Master Equations for initially relaxed excitons in case of narrow exciton band width. For the weak exciton-phonon coupling case, appreciable reduction of damping of the quasicoherent channel may be achieved on account of polaron effects. Crucial role of the exciton bandwidth beyond the lowest order is found in both cases for the low-temperature exciton diffusivity. Some predictions on the temperature dependence of the phonon-scattering limited exciton diffusion constant are given.
NASA Astrophysics Data System (ADS)
Sun, Yong; Ding, Zhao-Hua; Xiao, Jing-Lin
2016-07-01
On the condition of strong electron-LO phonon coupling in a RbCl quantum pseudodot (QPD), the ground state energy and the mean number of phonons are calculated by using the Pekar variational method and quantum statistical theory. The variations of the ground state energy and the mean number with respect to the temperature and the cyclotron frequency of the magnetic field are studied in detail. We find that the absolute value of the ground state energy increases (decreases) with increasing temperature when the temperature is in the lower (higher) temperature region, and that the mean number increases with increasing temperature. The absolute value of the ground state energy is a decreasing function of the cyclotron frequency of the magnetic field whereas the mean number is an increasing function of it. We find two ways to tune the ground state energy and the mean number: controlling the temperature and controlling the cyclotron frequency of the magnetic field.
Zarrabi, Nasim; Burn, Paul L; Meredith, Paul; Shaw, Paul E
2016-07-21
Transient absorption spectroscopy on organic semiconductor blends for solar cells typically shows efficient charge generation within ∼100 fs, accounting for the majority of the charge carriers. In this Letter, we show using transient absorption spectroscopy on blends containing a broad range of acceptor content (0.01-50% by weight) that the rise of the polaron signal is dependent on the acceptor concentration. For low acceptor content (<10% by weight), the polaron signal rises gradually over ∼1 ps with most polarons generated after 200 fs, while for higher acceptor concentrations (>10%) most polarons are generated within 200 fs. The rise time in blends with low acceptor content was also found to be sensitive to the pump fluence, decreasing with increasing excitation density. These results indicate that the sub-100 fs rise of the polaron signal is a natural consequence of both the high acceptor concentrations in many donor-acceptor blends and the high excitation densities needed for transient absorption spectroscopy, which results in a short average distance between the exciton and the donor-acceptor interface. PMID:27355877
Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.
1980-01-01
The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)
Bose Polarons in the Strongly Interacting Regime.
Hu, Ming-Guang; Van de Graaff, Michael J; Kedar, Dhruv; Corson, John P; Cornell, Eric A; Jin, Deborah S
2016-07-29
When an impurity is immersed in a Bose-Einstein condensate, impurity-boson interactions are expected to dress the impurity into a quasiparticle, the Bose polaron. We superimpose an ultracold atomic gas of ^{87}Rb with a much lower density gas of fermionic ^{40}K impurities. Through the use of a Feshbach resonance and radio-frequency spectroscopy, we characterize the energy, spectral width, and lifetime of the resultant polaron on both the attractive and the repulsive branches in the strongly interacting regime. The width of the polaron in the attractive branch is narrow compared to its binding energy, even as the two-body scattering length diverges. PMID:27517776
Bose Polarons in the Strongly Interacting Regime
NASA Astrophysics Data System (ADS)
Hu, Ming-Guang; Van de Graaff, Michael J.; Kedar, Dhruv; Corson, John P.; Cornell, Eric A.; Jin, Deborah S.
2016-07-01
When an impurity is immersed in a Bose-Einstein condensate, impurity-boson interactions are expected to dress the impurity into a quasiparticle, the Bose polaron. We superimpose an ultracold atomic gas of 87Rb with a much lower density gas of fermionic 40 impurities. Through the use of a Feshbach resonance and radio-frequency spectroscopy, we characterize the energy, spectral width, and lifetime of the resultant polaron on both the attractive and the repulsive branches in the strongly interacting regime. The width of the polaron in the attractive branch is narrow compared to its binding energy, even as the two-body scattering length diverges.
Polarons and Mobile Impurities Near a Quantum Phase Transition
NASA Astrophysics Data System (ADS)
Shadkhoo, Shahriar
aforementioned polaronic and solitonic states. We eventually generalize the polaron formalism to the case of impurities that couple quadratically to a nearly-critical field; hence called the ''quadratic polaron''. The Hertz-Millis field theory and its generalization to the case of magnetic transition in helimagnets, is taken as a toy model. The phase diagram of the bare model contains both second-order and fluctuation-induced first-order quantum phase transitions. We propose a semi-classical scenario in which the impurity and the field couple quadratically. The polaron properties in the vicinity of these transitions are calculated in different dimensions. We observe that the quadratic coupling in three dimensions, even in the absence of the critical modes with finite wavelength, leads to a jump-like localization of the polaron. In lower dimensions, the transition behavior remains qualitatively similar to those in the case of linear coupling, namely the critical modes must have a finite wavelength to localize the particle.
Bose polarons in the strongly interacting regime
NASA Astrophysics Data System (ADS)
Kedar, Dhruv; Hu, Ming-Guang; van de Graaff, Michael; Corson, John; Cornell, Eric; Jin, Deborah
2016-05-01
Impurities immersed in and interacting with a Bose-Einstein condensate (BEC) are predicted to form quasiparticle excitations called Bose polarons. I will present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic K-40 impurities interacting with a BEC of Rb-87 atoms. We use an interspecies Feshbach resonance to tune the interactions between the impurities and the bosons, and we take data in the strongly interacting regime.
The Structure and Dynamics of Molecular Excitons
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-04-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet -- triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion.
The Holstein polaron problem revisited
NASA Astrophysics Data System (ADS)
Tayebi, Amin; Zelevinsky, Vladimir
2016-06-01
The Holstein Hamiltonian was proposed half a century ago; since then, decades of research have come up empty handed in the pursuit of a closed-form solution. An exact solution to the two-site Holstein model is presented in this paper. The obtained results provide a clear image of the Hamiltonian structure and allow for the investigation of the symmetry, energy level crossings and polaronic characteristics of the system. The main mathematical tool is a three-term recurrence relation between the wave function amplitudes, which was obtained using the properties of a family of orthogonal functions, namely the Poisson–Charlier polynomials. It is shown that, with the appropriate choice of basis, the eigenfunctions of the problem naturally fall into two families (parities) associated with the discrete {{{Z}}}2 symmetry of the Hamiltonian. The asymptotic solution to the recurrence relation is found by using the Birkhoff expansion. The asymptotic sets the truncation criterion for the wave function, which ensures the accurate calculation of the energy levels for any strength of electron–phonon interaction. The level crossing of states with different parities is discussed and the exact points of broken symmetry are found analytically. The results are used as the building blocks for studying a four-site system. The inherited symmetries lead to the formation of a sparse matrix that is convenient for numerical calculations.
Theory of exciton transfer and diffusion in conjugated polymers
Barford, William; Tozer, Oliver Robert
2014-10-28
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ∼10 nm are in good agreement with experiment. The spectral
Orbital diamagnetic susceptibility in excitonic condensation phase
NASA Astrophysics Data System (ADS)
Sugimoto, Koudai; Ohta, Yukinori
2016-08-01
We study the orbital diamagnetic susceptibility in excitonic condensation phase using the mean-field approximation for a two-band model defined on a square lattice. We find that, in semiconductors, the excitonic condensation acquires a finite diamagnetic susceptibility due to spontaneous hybridization between the valence and the conduction bands, whereas in semimetals, the diamagnetic susceptibility in the normal phase is suppressed by the excitonic condensation. We also study the orbital diamagnetic and Pauli paramagnetic susceptibilities of Ta2NiSe5 using a two-dimensional three-band model and find that the calculated temperature dependence of the magnetic susceptibility is in qualitative agreement with experiment.
Excitons in the Fractional Quantum Hall Effect
DOE R&D Accomplishments Database
Laughlin, R. B.
1984-09-01
Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.
Exciton-phonon system on a star graph: A perturbative approach
NASA Astrophysics Data System (ADS)
Yalouz, Saad; Pouthier, Vincent
2016-05-01
Based on the operatorial formulation of the perturbation theory, the properties of an exciton coupled with optical phonons on a star graph are investigated. Within this method, the dynamics is governed by an effective Hamiltonian, which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. In spite of the coupling with the phonons, it is shown that the energy spectrum of the dressed exciton resembles that of a bare exciton. The only differences originate in a polaronic mechanism that favors an energy shift and a decay of the exciton hopping constant. By contrast, the motion of the exciton allows the phonons to propagate over the graph so that the dressed normal modes drastically differ from the localized modes associated to bare phonons. They define extended vibrations whose properties depend on the state occupied by the exciton that accompanies the phonons. It is shown that the phonon frequencies, either red shifted or blue shifted, are very sensitive to the model parameter in general, and to the size of the graph in particular.
Polaron Properties in Armchair Graphene Nanoribbons.
da Cunha, Wiliam F; Acioli, Paulo H; de Oliveira Neto, Pedro H; Gargano, Ricardo; E Silva, Geraldo M
2016-07-14
By means of a 2-D tight-binding model with lattice relaxation in a first-order expansion, we report different polaron properties depending on the armchair graphene nanoribbons width family as well as on its size. We find that representatives of the 3p+2 family do not present a polaronic-mediated charge transport. As for 3p and 3p+1 families, the polaron behavior was completely dependent on the system's width. In particular, we observed a greater degree of delocalization for broader nanoribbons; narrower nanoribbons of both families, on the contrary, typically presented a more localized polaronic-type transport. Energy levels and occupation numbers analysis are performed to rigorously assess the nature of the charge carrier. Time evolution in the scope of the Ehrenfest molecular dynamics was also carried out to confirm the collective behavior and stability of the carrier as a function of time. We were able to determine that polarons in nanoribbons of 3p family present higher mobility than those in 3p+1 nanoribbons. These results identify the transport process that takes place for each system, and they allow the prediction of the mobility of the charge carriers as a function of the structural properties of the system, thus providing guidance on how to improve the efficiency of graphene nanoribbon-based devices. PMID:26918483
Localization length scales of triplet excitons in singlet fission materials
NASA Astrophysics Data System (ADS)
Bayliss, Sam L.; Thorley, Karl J.; Anthony, John E.; Bouchiat, Hélène; Greenham, Neil C.; Chepelianskii, Alexei D.
2015-09-01
We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.
NASA Astrophysics Data System (ADS)
Oyama, Shiho; Sakai, Heisuke; Murata, Hideyuki
2016-03-01
We observed the quenching of tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3] excitons by polarons (holes or electrons) by time-resolved photoluminescence (PL) spectroscopy to clarify the dynamics of the triplet-polaron quenching of excitons. We employed a hole-only device (HOD) and an electron-only device (EOD), where the emitting layer consists of Ir(ppy)3 doped in 4,4‧-bis(carbazol-9-yl)biphenyl. Time-resolved PL spectroscopy of the EOD and HOD were measured under a constant current density. The results showed that the excitons of Ir(ppy)3 were significantly quenched only by holes. The PL decay curves of HOD were well fitted by the biexponential function, where lifetimes (τ1 and τ2) remain unchanged but the coefficient of each exponential term depends on hole current density. From the results, we proposed a model of exciton quenching where the exciton-hole quenching area expands with increasing hole current density. On the basis of the model, the triplet-polaron quenching rate constant Kq was determined.
Polarons and solitons in Jahn Teller systems
NASA Astrophysics Data System (ADS)
Clougherty, Dennis P.
2007-07-01
Using a semiclassical continuum model of an electron in a deformable molecular crystal, some properties of multicomponent generalizations of the polaron - "vector polarons" - are elucidated. Analytical solutions for the case of two electronic bands coupled to two vibrational modes are given in detail. Within the model considered, the vector polaron can be classified by its wavefunction into several types and can have features that include: (1) a spatial variation in the electronic and vibrational character, and (2) low-energy internal degrees of freedom. For the case of electronic and vibrational degeneracy, local Jahn-Teller interactions can also lead to a novel spatiotemporal soliton, a long-lived excited state of the many-electron system stabilized by the conservation law resulting from degeneracy.
Small polaron hopping transport along DNA molecules
NASA Astrophysics Data System (ADS)
Triberis, G. P.; Simserides, C.; Karavolas, V. C.
2005-05-01
We present a small polaron hopping model for interpreting the strong temperature (T) dependence of the electrical conductivity, σ, observed at high (h) temperatures along DNA molecules. The model takes into account the one-dimensional character of the system and the presence of disorder in the DNA double helix. Percolation-theoretical considerations lead to analytical expressions for the high temperature multiphonon-assisted small polaron hopping conductivity, the hopping distance and their temperature dependence. The experimental data for lambda phage DNA (λ-DNA) and poly(dA)-poly(dT) DNA follow nicely the theoretically predicted behaviour (lnσh~T-2/3). Moreover, our model leads to realistic values of the maximum hopping distances, supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbours of the DNA molecular 'wire'. The low temperature case is also investigated.
NASA Astrophysics Data System (ADS)
Perebeinos, Vasili
2001-12-01
Self-trapped states occur in many insulating solids but are not especially well-understood. There is a need for better theoretical models and better experimental tools for exploring these states. This thesis provides models for two kinds of materials LaMnO3 and NaCl, and predicts experimental effects which can be used to characterize such states. LaMnO3 is an insulating antiferromagnet which can be doped with holes over a wide concentration range, as in La1- xCaxMnO3. Here I study the regime x << 1 where particularly interesting and simple behavior is predicted. The model has electronic and lattice-vibrational degrees of freedom chosen to represent the Mn ion outer electronic states and their interaction with oxygen motions in the three dimensional perovskite crystal structure. Four independent types of data are available to choose three adjusted parameters. Using electronic structure calculations, optical conductivity and Raman spectra for this choice the predicted magnitude of the static Jahn-Teller distortion agrees within 10-15% with neutron diffraction data. I use the model to analyze and predict the self-localized states which form under optical excitation and under hole doping. In particular five types of behavior are analyzed: (1)the insulating nature of lightly doped LaMnO3 due to the anti-Jahn-Teller polaron formation; (2)phonon broadening due to the exciton formation; (3)polaronic angle-resolved- photoemission-spectra (ARPES); (4)Raman spectra due to the Franck-Condon mechanism; (5)the self-trapped exciton in NaCl and its optical properties including the Franck-Condon effect using the first-principles Density Functional Theory (DFT) calculations. Experimental confirmation of the predicted behavior for LaMnO3 will differentiate the Jahn-Teller model studied here from competing versions. The results given here are novel in five ways. (1)Essentially exact analytical polaronic spectra of the two-orbital model Hamiltonian have been obtained. (2)Self
Polaronic discontinuities induced by off-diagonal coupling.
Zhang, Yuyu; Duan, Liwei; Chen, Qinghu; Zhao, Yang
2012-07-21
In this paper, we study a form of the Holstein molecular crystal model in which the influence of lattice vibrations on the transfers of electronic excitations between neighboring sites (off-diagonal coupling) is taken into account. Using the Toyozawa Ansatz and the Lanczos algorithm, the Holstein Hamiltonian with two types of off-diagonal coupling is studied focusing on a number of analyticity issues in the ground state. For finite-sized lattices and antisymmetric coupling, a sequence of discontinuities are found in the polaron energy dispersion, the size of the ground-state phonon cloud, and the linearized von Neumann entropy used to quantify the quantum entanglement between the exciton and the phonons in the ground state. Such behavior is accompanied by a shift of the ground-state crystal momentum from zero to nonzero values as the coupling strength is increased. In the thermodynamic limit, all discontinuities associated with antisymmetric coupling vanish except the one corresponding to the initial departure of the ground-state wavevector from the Brillouin zone center. For the case of symmetric off-diagonal coupling, a smooth crossover is found to exist in all parameters regimes. PMID:22830684
Polaronic discontinuities induced by off-diagonal coupling
NASA Astrophysics Data System (ADS)
Zhang, Yuyu; Duan, Liwei; Chen, Qinghu; Zhao, Yang
2012-07-01
In this paper, we study a form of the Holstein molecular crystal model in which the influence of lattice vibrations on the transfers of electronic excitations between neighboring sites (off-diagonal coupling) is taken into account. Using the Toyozawa Ansatz and the Lanczos algorithm, the Holstein Hamiltonian with two types of off-diagonal coupling is studied focusing on a number of analyticity issues in the ground state. For finite-sized lattices and antisymmetric coupling, a sequence of discontinuities are found in the polaron energy dispersion, the size of the ground-state phonon cloud, and the linearized von Neumann entropy used to quantify the quantum entanglement between the exciton and the phonons in the ground state. Such behavior is accompanied by a shift of the ground-state crystal momentum from zero to nonzero values as the coupling strength is increased. In the thermodynamic limit, all discontinuities associated with antisymmetric coupling vanish except the one corresponding to the initial departure of the ground-state wavevector from the Brillouin zone center. For the case of symmetric off-diagonal coupling, a smooth crossover is found to exist in all parameters regimes.
Bose polarons in the strongly interacting regime
NASA Astrophysics Data System (ADS)
Hu, Ming-Guang; van de Graaff, Michael; Kedar, Dhruv; Cornell, Eric; Jin, Deborah
Impurities immersed in and interacting with a Bose-Einstein condensate (BEC) are predicted to form quasiparticle excitations called Bose polarons. I will present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic 40K impurities interacting with a BEC of 87Rb atoms. We use an interspecies Feshbach resonance to tune the interactions between the impurities and the bosons, and we take data in the strongly interacting regime. This work is supported by NSF, NASA and NIST.
Chiral plaquette polaron theory of cuprate superconductivity
NASA Astrophysics Data System (ADS)
Tahir-Kheli, Jamil; Goddard, William A., III
2007-07-01
Ab initio density functional calculations on explicitly doped La2-xSrxCuO4 find that doping creates localized holes in out-of-plane orbitals. A model for cuprate superconductivity is developed based on the assumption that doping leads to the formation of holes on a four-site Cu plaquette composed of the out-of-plane A1 orbitals apical Opz , planar Cud3z2-r2 , and planar Opσ . This is in contrast to the assumption of hole doping into planar Cudx2-y2 and Opσ orbitals as in the t-J model. Allowing these holes to interact with the d9 spin background leads to chiral polarons with either a clockwise or anticlockwise charge current. When the polaron plaquettes percolate through the crystal at x≈0.05 for La2-xSrxCuO4 , a Cudx2-y2 and planar Opσ band is formed. The computed percolation doping of x≈0.05 equals the observed transition to the “metallic” and superconducting phase for La2-xSrxCuO4 . Spin exchange Coulomb repulsion with chiral polarons leads to d -wave superconducting pairing. The equivalent of the Debye energy in phonon superconductivity is the maximum energy separation between a chiral polaron and its time-reversed partner. This energy separation is on the order of the antiferromagnetic spin coupling energy, Jdd˜0.1eV , suggesting a higher critical temperature. An additive skew-scattering contribution to the Hall effect is induced by chiral polarons and leads to a temperature dependent Hall effect that fits the measured values for La2-xSrxCuO4 . The integrated imaginary susceptibility, observed by neutron spin scattering, satisfies ω/T scaling due to chirality and spin-flip scattering of polarons along with a uniform distribution of polaron energy splittings. The derived functional form is compatible with experiments. The static spin structure factor for chiral spin coupling of the polarons to the undoped antiferromagnetic Cud9 spins is computed for classical spins on large two-dimensional lattices and is found to be incommensurate with a
Topologically protected excitons in porphyrin thin films
NASA Astrophysics Data System (ADS)
Yuen-Zhou, Joel; Saikin, Semion K.; Yao, Norman Y.; Aspuru-Guzik, Alán
2014-11-01
The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.
Topologically protected excitons in porphyrin thin films.
Yuen-Zhou, Joel; Saikin, Semion K; Yao, Norman Y; Aspuru-Guzik, Alán
2014-11-01
The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials. PMID:25242533
Real Space Imaging of Spin Polarons in Zn-Doped SrCu2(BO3)2
NASA Astrophysics Data System (ADS)
Yoshida, M.; Kobayashi, H.; Yamauchi, I.; Takigawa, M.; Capponi, S.; Poilblanc, D.; Mila, F.; Kudo, K.; Koike, Y.; Kobayashi, N.
2015-02-01
We report on the real space profile of spin polarons in the quasi-two-dimensional frustrated dimer spin system SrCu2(BO3)2 doped with 0.16% of Zn. The 11B nuclear magnetic resonance spectrum exhibits 15 additional boron sites near nonmagnetic Zn impurities. With the help of exact diagonalizations of finite clusters, we have deduced from the boron spectrum, the distribution of local magnetizations at the Cu sites with fine spatial resolution, providing direct evidence for an extended spin polaron. The results are confronted with those of other experiments performed on doped and undoped samples of SrCu2(BO3)2.
Spin-polaron theory of high-{Tc} superconductivity: I, spin polarons and high-{Tc} pairing
Wood, R.F.
1993-06-01
The concept of a spin polaron is introduced and contrasted with the more familiar ionic polaron picture. A brief review of aspects of ionic bipolaronic superconductivity is given with particular emphasis on the real-space pairing and true Bose condensation characteristics. The formation energy of spin polarons is then calculated in analogy with ionic polarons. The spin-flip energy of a Cu spin in an antiferromagnetically aligned CuO{sub 2} plane is discussed. It is shown that the introduction of holes into the CuO{sub 2} planes will always lead to the destruction of long-range AF ordering due to the formation of spin polarons. The pairing of two spin polarons can be expected because of the reestablishment of local (short-range) AF ordering; the magnitude of the pairing energy is estimated using a simplified model. The paper closes with a brief discussion of the formal theory of spin polarons.
Spin Relaxation of an Impurity Polaron in a Parabolic Quantum Dot
NASA Astrophysics Data System (ADS)
Li, Zhi-Xin
2015-01-01
We have studied theoretically the spin relaxation of an impurity polaron, which arises from the electron interactions with the longitudinal optical phonon between the sublevel Zeeman splitting of the ground-state, by employing a variational method for a parabolic quantum dot (QD). In fact, this process occurs by the absorption of a deformation potential acoustic phonon. With Rashba spin-orbit coupling, the expression of the spin relaxation rate of an impurity polaron as functions of the radius of QD, the Lande factor parameter, the magnetic field adjusting length has been derived. Results of the numerical calculation show that the spin relaxation rate decreases with increasing the radius of QD and enlarges with increasing the magnetic field adjusting length when the magnetic field adjusting length . In addition, we find that the spin relaxation rate is an increasing function of the Lande factor parameter.
Study of spin-polaron formation in 1D systems
Arredondo, Y.; Navarro, O.; Vallejo, E.
2014-05-15
We study numerically the formation of spin-polarons in low-dimensional systems. We consider a ferromagnetic Kondo lattice model with Hund coupling J{sub H} and localized spins interacting antiferromagnetically with coupling constant J. We investigate the ground state phase diagram as a function of the exchange couplings J{sub H} and J and as a function of the band filling, since it has been observed that doping either on the ferromagnetic or antiferromagnetic regime lead to formation of magnetic domains [1]. We explore the quasi-particle formation and phase separation using the density-matrix renormalization group method, which is a highly efficient method to investigate quasi-one-dimensional strongly correlated systems.
Bose polarons: Dynamical decay and RF signatures
NASA Astrophysics Data System (ADS)
Corson, John; Bohn, John
2016-05-01
Interactions of a single impurity with a quantum many-body environment are known to alter the character of the impurity, thereby forming a ``quasiparticle''. The condensed matter tradition often identifies quasiparticles as poles of a Green function in the complex plane, a notion whose sophistication sometimes obscures the underlying physics. The problem of a single quantum impurity in a Bose condensate, or Bose polaron, is an illustrative example where the meaning of the impurity Green function, and hence the quasiparticle itself, becomes especially transparent. Using direct diagonalization in a truncated Hilbert space, we examine the dynamical evolution and quasiparticle decay of the repulsive Bose polaron. This approach also allows us to simulate RF spectroscopy across a Feshbach resonance and outside the linear regime, as well as account for motional and thermal effects in a harmonic trap.
Optically programmable excitonic traps
Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François
2013-01-01
With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits. PMID:23546532
NASA Astrophysics Data System (ADS)
Moskalenko, S. A.; Tiginyanu, I. M.
2016-05-01
We present a review of the investigations realized in the last decades of the phenomenon of the Bose-Einstein condensation (BEC) in the system of two-dimensional cavity polaritons in semiconductor nanostructures. The conditions at which the excitons interacting with cavity photons form new type of quasiparticles named as polaritons are described. Since polaritons can form in a microcavity a weakly interacting Bose gas, similarly to the exciton gas in semiconductors, the microcavity exciton-polariton BEC emerged in the last decades as a new direction of the exciton BEC in solids, promising for practical applications. The high interest in BEC of exciton-polaritons in semiconductor microcavities is related to the ultra-low threshold lasing which has been demonstrated, in particular, for an electrically injected polariton laser based on bulk GaN microcavity diode working at room temperature.
Ground state energy of large polaron systems
Benguria, Rafael D.; Frank, Rupert L.; Lieb, Elliott H.
2015-02-15
The last unsolved problem about the many-polaron system, in the Pekar–Tomasevich approximation, is the case of bosons with the electron-electron Coulomb repulsion of strength exactly 1 (the “neutral case”). We prove that the ground state energy, for large N, goes exactly as −N{sup 7/5}, and we give upper and lower bounds on the asymptotic coefficient that agree to within a factor of 2{sup 2/5}.
Biopolymer hairpin loops sustained by polarons
NASA Astrophysics Data System (ADS)
Chakrabarti, B.; Piette, B. M. A. G.; Zakrzewski, W. J.
2012-08-01
We show that polarons can sustain looplike configurations in flexible biopolymers and that the size of the loops depend on both the flexural rigidity of the polymer and the electron-phonon coupling constant. In particular we show that for single stranded DNA (ssDNA) and polyacetylene such loops can have as few as seven monomers. We also show that these configurations are very stable under thermal fluctuations and so could facilitate the formation of hairpin loops of ssDNA.
Exciton dynamics in organic molecular crystals and nanostructures
NASA Astrophysics Data System (ADS)
Bardeen, Chris
2014-03-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet <-- --> triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations.
NASA Astrophysics Data System (ADS)
Crooker, S. A.; Liu, F.; Kelley, M. R.; Martinez, N. J. D.; Nie, W.; Mohite, A.; Nayyar, I. H.; Tretiak, S.; Smith, D. L.; Ruden, P. P.
2014-10-01
We use spectrally resolved magneto-electroluminescence (EL) measurements to study the energy dependence of hyperfine interactions between polaron and nuclear spins in organic light-emitting diodes. Using layered devices that generate bright exciplex emission, we show that the increase in EL emission intensity I due to small applied magnetic fields of order 100 mT is markedly larger at the high-energy blue end of the EL spectrum (ΔI/I ˜ 11%) than at the low-energy red end (˜4%). Concurrently, the widths of the magneto-EL curves increase monotonically from blue to red, revealing an increasing hyperfine coupling between polarons and nuclei and directly providing insight into the energy-dependent spatial extent and localization of polarons.
Crooker, S. A.; Kelley, M. R.; Martinez, N. J. D.; Nie, W.; Mohite, A.; Nayyar, I. H.; Tretiak, S.; Smith, D. L.; Liu, F.; Ruden, P. P.
2014-10-13
We use spectrally resolved magneto-electroluminescence (EL) measurements to study the energy dependence of hyperfine interactions between polaron and nuclear spins in organic light-emitting diodes. Using layered devices that generate bright exciplex emission, we show that the increase in EL emission intensity I due to small applied magnetic fields of order 100 mT is markedly larger at the high-energy blue end of the EL spectrum (ΔI/I ∼ 11%) than at the low-energy red end (∼4%). Concurrently, the widths of the magneto-EL curves increase monotonically from blue to red, revealing an increasing hyperfine coupling between polarons and nuclei and directly providing insight into the energy-dependent spatial extent and localization of polarons.
Energy Gap Dependence on Mn Content in a Diluted Magnetic Quantum Dot
NASA Astrophysics Data System (ADS)
Nalini, P.; John Peter, A.
2011-04-01
Positively charged donor exciton binding energy is computed as a function of quantum-dot size within the single band effective mass approximation for different Mn contents in Cd1-xinMnxin Te/Cd1-xoutMnxout Te. The exciton bound polaron is computed for 0 <= x <= 0.08, on the Mn mole fraction. We determine the energy gap using the mean field approximation and incorporate the exchange interaction between the carrier and the magnetic impurity. The interband emission energy is studied with the height and radius of the cylindrical quantum dot. Valence-band anisotropy is included in our theoretical model using different hole masses in different spatial directions. Spin polaronic shifts as functions of quantum-dot radius and Mn concentration are estimated using the mean field theory. It is found that (i) the energy gap depends on the Mn mole fraction, (ii) it increases linearly with an increase in Mn content, and (iii) the effect is more pronounced for a narrow dot, showing the quantum size effects. Our results are in good agreement with other recently published reports.
Large magnetic field effects in electrochemically doped organic light-emitting diodes
NASA Astrophysics Data System (ADS)
van Reenen, S.; Kersten, S. P.; Wouters, S. H. W.; Cox, M.; Janssen, P.; Koopmans, B.; Bobbert, P. A.; Kemerink, M.
2013-09-01
Large negative magnetoconductance (MC) of ˜12% is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (Vbias). Simultaneously, a positive magnetoefficiency (Mη) of 9% is observed at Vbias = 2 V. At higher bias voltages, both the MC and Mη diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive Mη effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.
High efficiency organic multilayer photodetectors based on singlet exciton fission
NASA Astrophysics Data System (ADS)
Lee, J.; Jadhav, P.; Baldo, M. A.
2009-07-01
We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.
Geminate and Nongeminate Recombination of Triplet Excitons Formed by Singlet Fission
NASA Astrophysics Data System (ADS)
Bayliss, Sam L.; Chepelianskii, Alexei D.; Sepe, Alessandro; Walker, Brian J.; Ehrler, Bruno; Bruzek, Matthew J.; Anthony, John E.; Greenham, Neil C.
2014-06-01
We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV.
Size dependent polaronic conduction in hematite
NASA Astrophysics Data System (ADS)
Sharma, Monika; Banday, Azeem; Murugavel, Sevi
2016-05-01
Lithium Ion Batteries have been attracted as the major renewable energy source for all portable electronic devices because of its advantages like superior energy density, high theoretical capacity, high specific energy, stable cycling and less memory effects. Recently, α-Fe2O3 has been considered as a potential anode material due to high specific capacity, low cost, high abundance and environmental benignity. We have synthesized α-Fe2O3 with various sizes by using the ball milling and sol-gel procedure. Here, we report the dc conductivity measurement for the crystallite size ranging from 15 nm to 50nm. It has been observed that the enhancement in the polaronic conductivity nearly two orders in magnitude while reducing the crystallite size from bulk into nano scale level. The enhancement in the conductivity is due to the augmented to compressive strain developed in the material which leads to pronounced decrease in the hopping length of polarons. Thus, nanocrystaline α-Fe2O3 may be a better alternative anode material for lithium ion batteries than earlier reported systems.
Bloch oscillations of bosonic lattice polarons
NASA Astrophysics Data System (ADS)
Grusdt, F.; Shashi, A.; Abanin, D.; Demler, E.
2014-12-01
We consider a single-impurity atom confined to an optical lattice and immersed in a homogeneous Bose-Einstein condensate (BEC). Interaction of the impurity with the phonon modes of the BEC leads to the formation of a stable quasiparticle, the polaron. We use a variational mean-field approach to study dispersion renormalization and derive equations describing nonequilibrium dynamics of polarons by projecting equations of motion into mean-field-type wave functions. As a concrete example, we apply our method to study dynamics of impurity atoms in response to a suddenly applied force and explore the interplay of coherent Bloch oscillations and incoherent drift. We obtain a nonlinear dependence of the drift velocity on the applied force, including a sub-Ohmic dependence for small forces for dimensionality d >1 of the BEC. For the case of heavy impurity atoms, we derive a closed analytical expression for the drift velocity. Our results show considerable differences with the commonly used phenomenological Esaki-Tsu model.
Polaronic effects in one- and two-band quantum systems
NASA Astrophysics Data System (ADS)
Yin, Tao; Cocks, Daniel; Hofstetter, Walter
2015-12-01
In this work, we study the formation and dynamics of polarons in a system with a few impurities in a lattice immersed in a Bose-Einstein condensate (BEC). This system has been experimentally realized using ultracold atoms and optical lattices. Here, we consider a two-band model for the impurity atoms, along with a Bogoliubov approximation for the BEC, with phonons coupled to impurities via both intraband and interband transitions. We decouple this Fröhlich-type term by an extended two-band Lang-Firsov polaron transformation using a variational method. The new effective Hamiltonian with two (polaron) bands differs from the original Hamiltonian by modified coherent transport, polaron energy shifts, and induced long-range interaction. A Lindblad master-equation approach is used to take into account residual incoherent coupling between polaron and bath. This polaronic treatment yields a renormalized interband relaxation rate compared to Fermi's golden rule. For a strongly coupled two-band Fröhlich Hamiltonian, the polaron is tightly dressed in each band and can not tunnel between them, leading to an interband self-trapping effect.
Topological polaritons and excitons in garden-variety systems
NASA Astrophysics Data System (ADS)
Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Timothy C. H.
2015-04-01
We present a practical scheme for creating topological polaritons in garden-variety systems based, for example, on zinc-blende semiconductor quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as an appealing alternative for topological states in exciton-based systems. Topological polaritons and indirect excitons open a new frontier for topological states in solid-state systems, which can be directly probed and manipulated while offering a system with nonlinear interactions.
Neutral and positively charged excitons in narrow quantum ring
Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.
2014-05-15
We study theoretically quantized states of a neutral and a positively charged exciton (trion X{sup +}) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.
Neutral and positively charged excitons in narrow quantum ring
NASA Astrophysics Data System (ADS)
Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.
2014-05-01
We study theoretically quantized states of a neutral and a positively charged exciton (trion X+) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.
Excitonic surface lattice resonances
NASA Astrophysics Data System (ADS)
Humphrey, A. D.; Gentile, M. J.; Barnes, W. L.
2016-08-01
Electromagnetic resonances are important in controlling light at the nanoscale. The most studied such resonance is the surface plasmon resonance that is associated with metallic nanostructures. Here we explore an alternative resonance, the surface exciton-polariton resonance, one based on excitonic molecular materials. Our study is based on analytical and numerical modelling. We show that periodic arrays of suitable molecular nanoparticles may support surface lattice resonances that arise as a result of coherent interactions between the particles. Our results demonstrate that excitonic molecular materials are an interesting alternative to metals for nanophotonics; they offer the prospect of both fabrication based on supramolecular chemistry and optical functionality arising from the way the properties of such materials may be controlled with light.
Small hole polarons in rare-earth titanates
Bjaalie, L.; Moetakef, P.; Cain, T. A.; Janotti, A.; Himmetoglu, B.; Stemmer, S.; Van de Walle, C. G.; Ouellette, D. G.; Allen, S. J.
2015-06-08
We investigate the behavior of hole polarons in rare-earth titanates by combining optical conductivity measurements with first-principles hybrid density functional calculations. Sr-doped GdTiO{sub 3} (Gd{sub 1−x}Sr{sub x}TiO{sub 3}) was grown by molecular beam epitaxy. We show that a feature in the optical conductivity that was previously identified with the Mott-Hubbard gap is actually associated with the excitation of a small polaron. The assignment is based on an excellent match between the experimental spectra and first-principles calculations for polaron excitation mechanisms.
Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry
Chestnov, Igor Yu.; Demirchyan, Sevak S.; Alodjants, Alexander P.; Rubo, Yuri G.; Kavokin, Alexey V.
2016-01-01
We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators. PMID:26790534
Permanent Rabi oscillations in coupled exciton-photon systems with PT-symmetry.
Chestnov, Igor Yu; Demirchyan, Sevak S; Alodjants, Alexander P; Rubo, Yuri G; Kavokin, Alexey V
2016-01-01
We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators. PMID:26790534
Extreme electron polaron spatial delocalization in π-conjugated materials
Rawson, Jeff; Angiolillo, Paul J.; Therien, Michael J.
2015-01-01
The electron polaron, a spin-1/2 excitation, is the fundamental negative charge carrier in π-conjugated organic materials. Large polaron spatial dimensions result from weak electron-lattice coupling and thus identify materials with unusually low barriers for the charge transfer reactions that are central to electronic device applications. Here we demonstrate electron polarons in π-conjugated multiporphyrin arrays that feature vast areal delocalization. This finding is evidenced by concurrent optical and electron spin resonance measurements, coupled with electronic structure calculations that suggest atypically small reorganization energies for one-electron reduction of these materials. Because the electron polaron dimension can be linked to key performance metrics in organic photovoltaics, light-emitting diodes, and a host of other devices, these findings identify conjugated materials with exceptional optical, electronic, and spintronic properties. PMID:26512097
Low hole polaron migration barrier in lithium peroxide
NASA Astrophysics Data System (ADS)
Ong, Shyue Ping; Mo, Yifei; Ceder, Gerbrand
2012-02-01
We present computational evidence of polaronic hole trapping and migration in lithium peroxide (Li2O2), a material of interest in lithium-air batteries. We find that the hole forms in the π* antibonding molecular orbitals of the peroxide (O22-) anion, and that this trapped hole induces significant local lattice distortion, forming a polaron. Our study finds migration barriers for the free polaron to be between 68 and 152 meV, depending on the hopping direction. This low barrier suggests that this material might not be as insulating as previously assumed, provided that the formation of carriers can be achieved. One transport limitation may arise from lithium vacancies, which we find to strongly bind to the polaron. This result, in combination with previous experimental results, suggests that electronic conductivity in this material is likely to be determined by vacancy diffusion.
electric dipole superconductor in bilayer exciton system
NASA Astrophysics Data System (ADS)
Sun, Qing-Feng; Jiang, Qing-Dong; Bao, Zhi-Qiang; Xie, X. C.
Recently, it was reported that the bilayer exciton systems could exhibit many new phenomena, including the large bilayer counterflow conductivity, the Coulomb drag, etc. These phenomena imply the formation of exciton condensate superfluid state. On the other hand, it is now well known that the superconductor is the condensate superfluid state of the Cooper pairs, which can be viewed as electric monopoles. In other words, the superconductor state is the electric monopole condensate superfluid state. Thus, one may wonder whether there exists electric dipole superfluid state. In this talk, we point out that the exciton in a bilayer system can be considered as a charge neutral electric dipole. And we derive the London-type and Ginzburg-Landau-type equations of electric dipole superconductivity. From these equations, we discover the Meissner-type effect (against spatial variation of magnetic fields), and the dipole current Josephson effect. The frequency in the AC Josephson effect of the dipole current is equal to that in the normal (monopole) superconductor. These results can provide direct evidence for the formation of exciton superfluid state in the bilayer systems and pave new ways to obtain the electric dipole current. We gratefully acknowledge the financial support by NBRP of China (2012CB921303 and 2015CB921102) and NSF-China under Grants Nos. 11274364 and 11574007.
Real space imaging of spin polarons in Zn-doped SrCu(2)(BO(3))(2).
Yoshida, M; Kobayashi, H; Yamauchi, I; Takigawa, M; Capponi, S; Poilblanc, D; Mila, F; Kudo, K; Koike, Y; Kobayashi, N
2015-02-01
We report on the real space profile of spin polarons in the quasi-two-dimensional frustrated dimer spin system SrCu(2)(BO(3))(2) doped with 0.16% of Zn. The (11)B nuclear magnetic resonance spectrum exhibits 15 additional boron sites near nonmagnetic Zn impurities. With the help of exact diagonalizations of finite clusters, we have deduced from the boron spectrum, the distribution of local magnetizations at the Cu sites with fine spatial resolution, providing direct evidence for an extended spin polaron. The results are confronted with those of other experiments performed on doped and undoped samples of SrCu(2)(BO(3))(2). PMID:25699459
Excitonic condensation in spatially separated one-dimensional systems
Abergel, D. S. L.
2015-05-25
We show theoretically that excitons can form from spatially separated one-dimensional ground state populations of electrons and holes, and that the resulting excitons can form a quasicondensate. We describe a mean-field Bardeen-Cooper-Schrieffer theory in the low carrier density regime and then focus on the core-shell nanowire giving estimates of the size of the excitonic gap for InAs/GaSb wires and as a function of all the experimentally relevant parameters. We find that optimal conditions for pairing include small overlap of the electron and hole bands, large effective mass of the carriers, and low dielectric constant of the surrounding media. Therefore, one-dimensional systems provide an attractive platform for the experimental detection of excitonic quasicondensation in zero magnetic field.
Topological Polaritons and Excitons in Garden Variety Systems
NASA Astrophysics Data System (ADS)
Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Tim
2015-03-01
Topological polaritons (aka topolaritons) present a new frontier for topological behavior in solid-state systems. They combine light and matter, which allows to probe and manipulate them in a variety of ways. They can also be made strongly interacting, due to their excitonic component. Here we present a scheme which allows to realize topolaritons in garden variety zinc-blende quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as a particularly appealing alternative for topological states in exciton-based systems. Indirect excitons are robust and long lived (with lifetimes up to milliseconds), and, therefore, provide a flexible platform for the realization, probing, and utilization of topological coupled light-matter states. Funded by: Institute for Quantum Information and Matter, Swiss National Science Foundation, Packard Foundation, NSF.
Spin transport dynamics of excitons in CdTe/Cd{sub 1{minus}x}Mn{sub x}Te quantum wells
Kayanuma, Kentaro; Shirado, Eiji; Debnath, Mukul C.; Souma, Izuru; Chen, Zhanghai; Oka, Yasuo
2001-06-01
Transport properties of spin-polarized excitons were studied in the double quantum well system composed of Cd{sub 0.95}Mn{sub 0.05}Te and CdTe wells. Circular polarization degrees of the time resolved exciton photoluminescence in magnetic field showed that the spin-polarized excitons diffused from the magnetic quantum well and injected to the non-magnetic quantum well by conserving their spins. The spin-polarized excitons injected into the nonmagnetic well reaches 18% of the nonmagnetic well excitons. From the circular polarization degree and the lifetime of the magnetic quantum well excitons, the spin relaxation time of the excitons in the Cd{sub 0.95}Mn{sub 0.05}Te well was determined as 275{endash}10 ps depending on the magnetic field strength. {copyright} 2001 American Institute of Physics.
Attractive and repulsive Fermi polarons in two dimensions.
Koschorreck, Marco; Pertot, Daniel; Vogt, Enrico; Fröhlich, Bernd; Feld, Michael; Köhl, Michael
2012-05-31
The dynamics of a single impurity in an environment is a fundamental problem in many-body physics. In the solid state, a well known case is an impurity coupled to a bosonic bath (such as lattice vibrations); the impurity and its accompanying lattice distortion form a new entity, a polaron. This quasiparticle plays an important role in the spectral function of high-transition-temperature superconductors, as well as in colossal magnetoresistance in manganites. For impurities in a fermionic bath, studies have considered heavy or immobile impurities which exhibit Anderson's orthogonality catastrophe and the Kondo effect. More recently, mobile impurities have moved into the focus of research, and they have been found to form new quasiparticles known as Fermi polarons. The Fermi polaron problem constitutes the extreme, but conceptually simple, limit of two important quantum many-body problems: the crossover between a molecular Bose-Einstein condensate and a superfluid with BCS (Bardeen-Cooper-Schrieffer) pairing with spin-imbalance for attractive interactions, and Stoner's itinerant ferromagnetism for repulsive interactions. It has been proposed that such quantum phases (and other elusive exotic states) might become realizable in Fermi gases confined to two dimensions. Their stability and observability are intimately related to the theoretically debated properties of the Fermi polaron in a two-dimensional Fermi gas. Here we create and investigate Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas, measuring their spectral function using momentum-resolved photoemission spectroscopy. For attractive interactions, we find evidence for a disputed pairing transition between polarons and tightly bound dimers, which provides insight into the elementary pairing mechanism of imbalanced, strongly coupled two-dimensional Fermi gases. Additionally, for repulsive interactions, we study novel quasiparticles--repulsive polarons--the lifetime of which determines the
NASA Astrophysics Data System (ADS)
Ye, Yu; Wong, Zi Jing; Lu, Xiufang; Ni, Xingjie; Zhu, Hanyu; Chen, Xianhui; Wang, Yuan; Zhang, Xiang
2015-11-01
Two-dimensional van der Waals materials have opened a new paradigm for fundamental physics exploration and device applications because of their emerging physical properties. Unlike gapless graphene, monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional semiconductors that undergo an indirect-to-direct bandgap transition, creating new optical functionalities for next-generation ultra-compact photonics and optoelectronics. Although the enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystals and distributed Bragg reflector microcavities, coherent light emission from a TMDC monolayer has not been demonstrated. Here, we report the realization of a two-dimensional excitonic laser by embedding monolayer WS2 in a microdisk resonator. Using a whispering gallery mode with a high quality factor and optical confinement, we observe bright excitonic lasing at visible wavelengths. This demonstration of a two-dimensional excitonic laser marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications.
Excitons in asymmetric quantum wells
NASA Astrophysics Data System (ADS)
Grigoryev, P. S.; Kurdyubov, A. S.; Kuznetsova, M. S.; Ignatiev, I. V.; Efimov, Yu. P.; Eliseev, S. A.; Petrov, V. V.; Lovtcius, V. A.; Shapochkin, P. Yu.
2016-09-01
Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.
The influence of external electromagnetic field on an exciton spin current
NASA Astrophysics Data System (ADS)
Zhang, L.; Liu, F. Q.; Liu, C.
2007-08-01
A pure spin current of a triplet exciton has been proposed for a three-dimensional system based on Dirac relativistic quantum mechanics, considering spin to be an intrinsic relativistic quantum variable. The transport dissipations of the triplet exciton caused by the magnetic excitation and the Coulomb interaction (such as the bremsstrahlung effect) can be minimized, making it possible to produce a pure exciton spin current with minimized charge-induced dissipation for the transport of information in spin-based devices. The exciton spin current can be expressed as a sum of an electron spin current and a hole spin current, modulated by an envelope function. It flows along the tangential direction of the isodense of the exciton. Both external electric and magnetic fields influence the exciton spin current and can be used to separate the triplet exciton and the singlet exciton. A simplification of our model, i.e., considering only an external electric field, yields the same result reported by Shen (2005 Phys. Rev. Lett. 95 187203), in which the spin current was phenomenologically derived by the expectation value of the product of spin and velocity observables. Incorporation and possible application of the exciton spin current in several systems are discussed.
Exciton Transport in Organic Semiconductors
NASA Astrophysics Data System (ADS)
Menke, Stephen Matthew
Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (L D) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on L D are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors.
Numerical simulation of photoexcited polaron states in water
NASA Astrophysics Data System (ADS)
Zemlyanaya, E. V.; Volokhova, A. V.; Lakhno, V. D.; Amirkhanov, I. V.; Puzynin, I. V.; Puzynina, T. P.; Rikhvitskiy, V. S.; Atanasova, P. Kh.
2015-10-01
We consider the dynamic polaron model of the hydrated electron state on the basis of a system of three nonlinear partial differential equations with appropriate initial and boundary conditions. A parallel numerical algorithm for the numerical solution of this system has been developed. Its effectiveness has been tested on a few multi-processor systems. A numerical simulation of the polaron states formation in water under the action of the ultraviolet range laser irradiation has been performed. The numerical results are shown to be in a reasonable agreement with experimental data and theoretical predictions.
Numerical simulation of photoexcited polaron states in water
Zemlyanaya, E. V. Volokhova, A. V.; Amirkhanov, I. V.; Puzynin, I. V.; Puzynina, T. P.; Rikhvitskiy, V. S.; Lakhno, V. D.; Atanasova, P. Kh.
2015-10-28
We consider the dynamic polaron model of the hydrated electron state on the basis of a system of three nonlinear partial differential equations with appropriate initial and boundary conditions. A parallel numerical algorithm for the numerical solution of this system has been developed. Its effectiveness has been tested on a few multi-processor systems. A numerical simulation of the polaron states formation in water under the action of the ultraviolet range laser irradiation has been performed. The numerical results are shown to be in a reasonable agreement with experimental data and theoretical predictions.
Excitonic condensation in bilayer systems
NASA Astrophysics Data System (ADS)
Su, Jung-Jung
Among the many examples of Bose condensation considered in physics, electron-hole-pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because of controversy about condensate properties. In this thesis, we studied the various aspects of spontaneous symmetry broken state of exciton in bilayer using mean field theory. We calculated the photoluminescence of excitonic condensation created by laser. We developed a one-dimensional toy model of excitonic supercurrent using mean field theory plus non-equilibrium Green's function (NEGF) which give qualitatively consistent results with experiments. We proposed graphene bilayer as a novel system for excitonic condensation to occur and estimate it to exist even at temperature as high as room temperature.
NASA Astrophysics Data System (ADS)
Zhang, Qi; Gao, Weilu; Watson, John; Manfra, Michael; Kono, Junichiro
2015-03-01
Density-dependent Coulomb interactions can drive electron-hole (e - h) pairs in semiconductors through an excitonic Mott transition from an excitonic gas into an e - h plasma. Theoretical studies suggest that these interactions can be strongly modified by an external magnetic field, including the absence of inter-exciton interactions in the high magnetic field limit in two dimensions, due to an e - h charge symmetry, which results in ultrastable magneto-excitons. Here, we present a systematic experimental study of e - h pairs in photo-excited undoped GaAs quantum wells in magnetic fields with ultrafast terahertz spectroscopy. We simultaneously monitored the dynamics of the intraexcitonic 1 s-2 p transition (which splits into 1 s-2p+ and 1 s-2p- transitions in a magnetic field) and the cyclotron resonance of unbound electrons and holes up to 10 Tesla. We found that the 1 s-2p- absorption feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. We will discuss the Mott physics of magneto-excitons as a function of temperature, e - h pair density, optical pump delay time, as well as magnetic field, and also compare two-dimensional excitons in GaAs quantum wells with three-dimensional excitons in bulk GaAs.
Transient electrically detected magnetic resonance spectroscopy applied to organic solar cells
Kraffert, Felix; Steyrleuthner, Robert; Meier, Christoph; Bittl, Robert; Behrends, Jan
2015-07-27
The influence of light-induced paramagnetic states on the photocurrent generated by polymer:fullerene solar cells is studied using spin-sensitive techniques in combination with laser-flash excitation. For this purpose, we developed a setup that allows for simultaneous detection of transient electron paramagnetic resonance as well as transient electrically detected magnetic resonance (trEDMR) signals from fully processed and encapsulated solar cells. Combining both techniques provides a direct link between photoinduced triplet excitons, charge transfer states, and free charge carriers as well as their influence on the photocurrent generated by organic photovoltaic devices. Our results obtained from solar cells based on poly(3-hexylthiophene) as electron donor and a fullerene-based electron acceptor show that the resonant signals observed in low-temperature (T = 80 K) trEDMR spectra can be attributed to positive polarons in the polymer as well as negative polarons in the fullerene phase, indicating that both centers are involved in spin-dependent processes that directly influence the photocurrent.
Natori, Kenji
2015-02-07
The concentration of excitons generated in a high-quality diamond p-i-n junction is investigated considering the forward current characteristics of the junction. As the forward current in the junction increases, the exciton concentration increases superlinearly, contrary to the linear increases of the electron and hole concentration. This tendency suggests a superlinear increase in emission intensity due to exciton recombination. The increase rate is more radical than quadratic, in accordance with the observed increase of the integrated intensity of free exciton emission. To estimate the concentration of triplet excitons generated in the p-i-n junction, observation of the paramagnetism due to the exciton spin moment is proposed. The magnetic susceptibility superlinearly increases with the increase in the forward current, unlike any other magnetic property of the device.
A Nonempirical Comparison of the Polaron and Mowat Sensor.
ERIC Educational Resources Information Center
Moore, Karyl A.
1995-01-01
This article compares two electronic aids that send out an elliptical cone of ultrasonic sound that bounces back as a usable information signal for individuals with blindness. The Polaron is better for people who are predominantly route travelers or with limited hand use. The Mowat Sensor is better for travelers in a variety of environments. (JDD)
Deep electron and hole polarons and bipolarons in amorphous oxide
NASA Astrophysics Data System (ADS)
Kaviani, Moloud; Strand, Jack; Afanas'ev, Valery V.; Shluger, Alexander L.
2016-07-01
Amorphous (a)-HfO2 is a prototype high dielectric constant insulator with wide technological applications. Using ab initio calculations we show that excess electrons and holes can trap in a-HfO2 in energetically much deeper polaron states than in the crystalline monoclinic phase. The electrons and holes localize at precursor sites, such as elongated Hf-O bonds or undercoordinated Hf and O atoms, and the polaronic relaxation is amplified by the local disorder of amorphous network. Single electron polarons produce states in the gap at ˜2 eV below the bottom of the conduction band with average trapping energies of 1.0 eV. Two electrons can form even deeper bipolaron states on the same site. Holes are typically localized on undercoordinated O ions with average trapping energies of 1.4 eV. These results advance our general understanding of charge trapping in amorphous oxides by demonstrating that deep polaron states are inherent and do not require any bond rupture to form precursor sites.
Thermal enhancement and stochastic resonance of polaron ratchets.
Brizhik, L S; Eremko, A A; Piette, B M A G; Zakrzewski, W J
2014-06-01
We study the ratchet drift of large polarons (solitons) in molecular diatomic chains induced by unbiased time periodic electric fields at nonzero temperature below its critical value. We show that, at a nonzero temperature, the critical value of the intensity of the electric field above which the ratchet phenomenon takes place is lower than at zero temperature for the same frequency of the field. We show that there is a range of temperatures for which the polaron drift is larger than that at zero temperature. We also show that temperature decreases the value of the lowest critical period of the field. And, finally, we demonstrate that there is a stochastic resonance in a polaron ratchet, namely that there is an optimal temperature at which the polaron drift is a maximum. The values of the stochastic resonance temperature, the lowest critical values of the field intensity, and its period depend on various parameters of the system and, in particular, on the anisotropy of the chain parameters. This temperature induced decrease of the critical value of the field intensity and its period, as well as the stochastic resonance itself, may be important for practical applications of the ratchet phenomenon in systems involving conducting polymers and other low-dimensional materials. They may also be important in some biological macromolecules where the ratchet phenomenon could take place in biomotors and energy and/or charge transport. PMID:25019849
Thermal enhancement and stochastic resonance of polaron ratchets
NASA Astrophysics Data System (ADS)
Brizhik, L. S.; Eremko, A. A.; Piette, B. M. A. G.; Zakrzewski, W. J.
2014-06-01
We study the ratchet drift of large polarons (solitons) in molecular diatomic chains induced by unbiased time periodic electric fields at nonzero temperature below its critical value. We show that, at a nonzero temperature, the critical value of the intensity of the electric field above which the ratchet phenomenon takes place is lower than at zero temperature for the same frequency of the field. We show that there is a range of temperatures for which the polaron drift is larger than that at zero temperature. We also show that temperature decreases the value of the lowest critical period of the field. And, finally, we demonstrate that there is a stochastic resonance in a polaron ratchet, namely that there is an optimal temperature at which the polaron drift is a maximum. The values of the stochastic resonance temperature, the lowest critical values of the field intensity, and its period depend on various parameters of the system and, in particular, on the anisotropy of the chain parameters. This temperature induced decrease of the critical value of the field intensity and its period, as well as the stochastic resonance itself, may be important for practical applications of the ratchet phenomenon in systems involving conducting polymers and other low-dimensional materials. They may also be important in some biological macromolecules where the ratchet phenomenon could take place in biomotors and energy and/or charge transport.
Polaron mass of charge carriers in semiconductor quantum wells
Maslov, A. Yu. Proshina, O. V.
2015-10-15
A theory of the interaction of charge carriers with optical phonons in a quantum well is developed with consideration for interface optical phonons. The dependence of the polaron effective mass on the quantum-well dimensions and dielectric characteristics of barriers is analyzed in detail. It is shown that, in narrow quantum wells, a quasi-two-dimensional polaron can be formed. In this case, however, the interaction parameters are defined by the charge-carrier effective mass in the quantum well and by the frequencies of interface optical phonons. If barriers are made of a nonpolar material, the polaron effective mass depends on the quantum-well width. As the quantum-well width is increased, a new mechanism of enhancement of the electron–phonon interaction develops. The mechanism is implemented, if the optical phonon energy is equal to the energy of one of the electronic transitions. This condition yields an unsteady dependence of the polaron effective mass on the quantum-well width.
Geminate and nongeminate recombination of triplet excitons formed by singlet fission.
Bayliss, Sam L; Chepelianskii, Alexei D; Sepe, Alessandro; Walker, Brian J; Ehrler, Bruno; Bruzek, Matthew J; Anthony, John E; Greenham, Neil C
2014-06-13
We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV. PMID:24972236
Polarons and Mobile Impurities Near a Quantum Phase Transition
NASA Astrophysics Data System (ADS)
Shadkhoo, Shahriar
This dissertation aims at improving the current understanding of the physics of mobile impurities in highly correlated liquid-like phases of matter. Impurity problems pose challenging and intricate questions in different realms of many-body physics. For instance, the problem of ''solvation'' of charged solutes in polar solvents, has been the subject of longstanding debates among chemical physicists. The significant role of quantum fluctuations of the solvent, as well as the break down of linear response theory, render the ordinary treatments intractable. Inspired by this complicated problem, we first attempt to understand the role of non-specific quantum fluctuations in the solvation process. To this end, we calculate the dynamic structure factor of a model polar liquid, using the classical Molecular Dynamics (MD) simulations. We verify the failure of linear response approximation in the vicinity of a hydrated electron, by comparing the outcomes of MD simulations with the predictions of linear response theory. This nonlinear behavior is associated with the pronounced peaks of the structure factor, which reflect the strong fluctuations of the local modes. A cavity picture is constructed based on heuristic arguments, which suggests that the electron, along with the surrounding polarization cloud, behave like a frozen sphere, for which the linear response theory is broken inside and valid outside. The inverse radius of the spherical region serves as a UV momentum cutoff for the linear response approximation to be applicable. The problem of mobile impurities in polar liquids can be also addressed in the framework of the ''polaron'' problem. Polaron is a quasiparticle that typically acquires an extended state at weak couplings, and crossovers to a self-trapped state at strong couplings. Using the analytical fits to the numerically obtained charge-charge structure factor, a phenomenological approach is proposed within the Leggett's influence functional formalism, which
Momentum dependence of the excitons in pentacene
Roth, Friedrich; Schuster, Roman; Koenig, Andreas; Knupfer, Martin; Berger, Helmuth
2012-05-28
We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.
The structure of nanoscale polaron correlations in the layered manganites
NASA Astrophysics Data System (ADS)
Campbell, Branton
2002-03-01
Recent x-ray and neutron scattering experiments have uncovered nanoscale polaron correlations that play an essential role in the colossal magnetoresistive (CMR) behavior of the perovskite manganites. Short-range polaronic order decreases the charge-carrier mobility of the high-temperature paramagnetic state, and subsequently becomes unstable at the ferromagnetic transition, contributing to a pronounced resistivity decrease at T_C. In the bilayered perovskite system La_2-2xSr_1+2xMn_2O7 (0.3 < x < 0.5), weak x-ray diffuse scattering maxima reveal a one-dimensional incommensurate structural modulation with wavevector q = (0.3, 0, ± 1) and a correlation length of 10 to 30 Angstroms. A crystallographic analysis of the diffuse satellite intensities yields a longitudinal Jahn-Teller stretch mode suggestive of charge-density-wave fluctuations. Within the correlated regions, polaronic eg electrons form a striped pattern of occupied d(3x^2-r^2) orbitals. Dynamic polaron correlations of the zig-zag orbital type are also observed above TC and exhibit distinctly glassy behavior. These structures provide unique insights into the nature of strongly correlated polaronic systems. Collaborators: R. Osborn, D.N. Argyriou, S. Rosenkranz, L. Vasiliu-Doloc, J.F. Mitchell, S.K. Sinha, J.W. Lynn, C.D. Ling, Z. Islam, U. Ruett, and A. Berger. This work was supported by the U.S. DOE Office of Science contract No. W-31-109-ENG-38.
Lous, E J; Hoff, A J
1987-09-01
Linear dichroic triplet-minus-singlet [LD-(T - S)] spectra of isolated reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis have been measured at 1.2 K with the linear dichroic absorbance-detected magnetic resonance (LD-ADMR) technique for two mutually perpendicular directions of the preferred axis. The LD-(T - S) spectra have been calibrated with respect to the corresponding (T - S) spectra as a function of applied microwave power and quantitatively interpreted using the formalism of photoselection. The transition moment of the optical transition at 1007 nm makes angles of 72 degrees +/- 5 degrees and 15 degrees +/- 5 degrees with the triplet x and y spin axes, respectively. The experimental spectra have been simulated employing exciton theory and using the atomic coordinates of the resolved crystal structure of the reaction center. The spectral interpretation yields the angles between the transition moments of the various absorption bands of the (T - S) spectra and the triplet axes, and between the moments themselves, with the triplet state of the primary donor (3)P localized on the P-bacteriochlorophyll b in the "active" (L) chain. PMID:16578814
Vinolin, Ada; Peter, A. John
2015-06-24
Magneto-LO-polaron in a cylindrical GaAs{sub 0.9} P{sub 0.1} / GaAs{sub 0.6} P{sub 0.4} quantum dot is investigated taking into consideration of geometrical confinement effect. The effects of phonon on the exciton binding energy and the interband emission energy as a function of dot radius are found. The calculations are performed within the single band effective mass approximation using the variational method based on the Lee-Low-Pine LLP transformation.
Aharonov-Bohm effect of excitons in nanorings
NASA Astrophysics Data System (ADS)
Hu, Hui; Zhu, Jia-Lin; Li, Dai-Jun; Xiong, Jia-Jiong
2001-05-01
The magnetic field effects on excitons in an InAs nanoring are studied theoretically. By numerically diagonalizing the effective-mass Hamiltonian of the problem that can be separated into terms in center-of-mass and relative coordinates, we calculate the low-lying excitonic energy levels and oscillator strengths as a function of the ring width and the strength of an external magnetic field. It is shown that in the presence of Coulomb correlation, the so-called Aharonov-Bohm effect of excitons exists in a finite (but small) width nanoring. However, when the ring width becomes large, the non-simply-connected geometry of nanorings is destroyed, causing the suppression of the Aharonov-Bohm effect. The analytical results are obtained for a narrow-width nanoring in which the radial motion is the fastest one and adiabatically decoupled from the azimuthal motions. The conditional probability distribution calculated for the low-lying excitonic states allows identification of the presence of the Aharonov-Bohm effect. The linear optical susceptibility is also calculated as a function of the magnetic field, to be compared with the future measurements of optical emission experiments on InAs nanorings.
Giant permanent dipole moment of two-dimensional excitons bound to a single stacking fault
NASA Astrophysics Data System (ADS)
Karin, Todd; Linpeng, Xiayu; Glazov, M. M.; Durnev, M. V.; Ivchenko, E. L.; Harvey, Sarah; Rai, Ashish K.; Ludwig, Arne; Wieck, Andreas D.; Fu, Kai-Mei C.
2016-07-01
We investigate the magneto-optical properties of excitons bound to single stacking faults in high-purity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy hole, and we confirm a vanishing in-plane hole g -factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultranarrow photoluminescence emission lines (with a full width at half-maximum ≲80 μ eV ) and reveals a large magnetic nonreciprocity effect that originates from the magneto-Stark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment (≳e ×10 nm ) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases.
Polaron hopping in olivine phosphates studied by nuclear resonant scattering
NASA Astrophysics Data System (ADS)
Tracy, Sally June
Valence fluctuations of Fe2+ and Fe3+ were studied in a solid solution of LixFePO4 by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamiltonian. These frequencies were analyzed to obtain activation energies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an anomalously large activation volume. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons, and Li+ ions that alter the dynamics of both. In a parallel study of NaxFePO4, the interplay between sodium ordering and electron mobility was investigated using a combination of synchrotron x-ray diffraction and nuclear resonant scattering. Conventional Mossbauer spectra were collected while the sample was heated in a resistive furnace. An analysis of the temperature evolution of the spectral shapes was used to identify the onset of fast electron hopping and determine the polaron hopping rate. Synchrotron x-ray diffraction measurements were carried out in the same temperature range. Reitveld analysis of the diffraction patterns was used to determine the temperature of sodium redistribution on the lattice. The diffraction analysis also provides new information about the phase stability of the system. The temperature evolution of the iron site occupancies from the Mossbauer measurements, combined with the synchrotron diffraction results give strong evidence for a relationship between the onset of fast electron dynamics and the redistribution of sodium in the lattice. Measurements of activation barriers for polaron hopping gave fundamental insights about the correlation between electronic carriers and mobile ions. This work established that polaron-ion interactions
Turnover of Exciton Spin States in CdTe/Cd0.88Mn0.12Te Quantum Wells
NASA Astrophysics Data System (ADS)
Kamimura, Shota; Date, Akira; Nakajima, Makoto; Karczewski, Grzegorz; Wojtowicz, Tomasz; Kossut, Jacek; Tsuchiya, Takuma; Mino, Hirofumi
2015-10-01
Photoinduced exciton spin states in diluted magnetic semiconductor CdTe/Cd0.88Mn0.12Te quantum wells (QWs) were investigated by the time-resolved Kerr rotation (TRKR) technique at various temperatures and applied magnetic fields. Two oscillation components were observed in the TRKR signals. One component is ascribed to the spin precession of a localized electron, which was not affected by the Mn ions in the CdMnTe barrier layers. Its frequency increased linearly with the applied magnetic field. The other component was strongly dependent on the temperature, and its frequency showed a minimum at around 20 K in the 8.4 nm QW. This result was well explained by the turnover of exciton spin states in the CdTe QW, which is determined by the competition between the exciton Zeeman effect and an exciton-Mn exchange interaction. The temperature dependence of the exciton spin relaxation is also discussed.
Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites
NASA Astrophysics Data System (ADS)
Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.
2016-03-01
UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.
Tunable spin-polaron state in a singly clamped semiconducting carbon nanotube
NASA Astrophysics Data System (ADS)
Pistolesi, F.; Shekhter, R.
2015-07-01
We consider a semiconducting carbon nanotube (CNT) lying on a ferromagnetic insulating substrate with one end passing the substrate and suspended over a metallic gate. We assume that the polarized substrate induces an exchange interaction acting as a local magnetic field for the electrons in the nonsuspended CNT side. Generalizing the approach of I. Snyman and Yu.V. Nazarov [Phys. Rev. Lett. 108, 076805 (2012), 10.1103/PhysRevLett.108.076805], we show that one can generate electrostatically a tunable spin-polarized polaronic state localized at the bending end of the CNT. We argue that at low temperatures manipulation and detection of the localized quantum spin state are possible.
Polaron luminescence in iron-doped lithium niobate
NASA Astrophysics Data System (ADS)
Harhira, A.; Guilbert, L.; Bourson, P.; Rinnert, H.
2008-09-01
Photoluminescence related to the bound polaron Nb{Li/4+} is investigated as a function of temperature and incident light intensity in iron-doped lithium niobate crystals with various iron concentrations. Experiments are done under constant-wave (CW) and pulsed illumination. Its found that the decay time is always monoexponential. The radiative lifetime, the activation energy of the nonradiative lifetime and the quenching temperature are only weakly sensitive to iron concentration. On the other hand, the magnitude of the photoluminescence signal seems strongly correlated to the Fe2+ concentration, and the superlinear regime evidenced at low CW illumination definitely confirms that polaron excitation in lithium niobate is a two-step process.
Polaron and bipolaron of uniaxially strained one dimensional zigzag ladder
NASA Astrophysics Data System (ADS)
Yavidov, B. Ya.
2016-09-01
An influence of the uniaxial strains in one dimensional zigzag ladder (1DZL) on the properties of polarons and bipolarons is considered. It is shown that strain changes all the parameters of the system, in particular, spectrum, existing bands and the masses of charge carriers. Numerical results obtained by taking into an account the Poisson effect clearly indicate that the properties of the (bi)polaronic system can be tuned via strain. Mass of bipolaron can be manipulated by the strain too which in turn leads to the way of tuning Bose-Einstein condensation temperature TBEC of bipolarons. It is shown that TBEC of bipolarons in strained 1DZL reasonably correlates with the values of critical temperature of superconductivity of certain perovskites.
Vacancies and small polarons in SrTiO3
NASA Astrophysics Data System (ADS)
Janotti, Anderson; Varley, Joel B.; Choi, Minseok; Van de Walle, Chris G.
2014-08-01
Using first-principles calculations we investigate the impact of intrinsic defects and small polarons on the electrical and optical properties of SrTiO3. We pay special attention to the seemingly contradictory role of oxygen vacancies as shallow donor and source of deep-level luminescence, as reported in the literature. We find that oxygen vacancies are double donors, and that one electron is easily ionized, explaining the shallow donor behavior. The second electron is trapped in the form of a small polaron, and this additional binding energy explains the behavior as a deep center that gives rise to blue luminescence. At low temperatures, holes become self-trapped, and recombination of free electrons with self-trapped holes gives rise to green luminescence. These results explain the intricate interplay between the observed green and blue luminescence in SrTiO3, and form a framework for interpreting similar phenomena in other complex oxides.
Small polarons and point defects in barium cerate
NASA Astrophysics Data System (ADS)
Swift, Michael; Janotti, Anderson; Van de Walle, Chris G.
2015-12-01
Barium cerate (BaCeO3) is a well-known ionic conductor of both hydrogen and oxygen. In applications, it is frequently doped (for instance with Y) to increase stability and promote diffusion. However, the effects of doping and native defects are not fully understood. Computational studies have been stymied by the nature of the conduction band, which is made up of cerium 4 f states. These states present a challenge to ab initio techniques based on density functional theory within the standard approximations for exchange and correlation. Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical and optical properties of BaCeO3. We discuss the tendency of excess electrons or holes to localize in the form of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies, and their impact on luminescence properties.
Solution of the Fröhlich polaron problem at intermediate couplings
NASA Astrophysics Data System (ADS)
Grusdt, Fabian; Shchadilova, Yulia E.; Rubtsov, Alexey N.; Demler, Eugene
2015-05-01
We develop a renormalization group approach for analyzing Fröhlich polarons and apply it to a problem of impurity atoms immersed in a Bose-Einstein condensate (BEC) of ultra cold atoms. Polaron energies obtained by our method are in excellent agreement with recent diagrammatic Monte Carlo calculations for a wide range of interaction strengths. We show analytically that the energy of the Fröhlich polaron in a BEC is logarithmically UV divergent, and present a regularization scheme. This allows us to make predictions for the polaron energy, which can be tested in future experiments. Furthermore we calculate the effective mass of polarons and find a smooth crossover from weak to strong coupling regimes. Our method can be generalized to non-equilibrium polaron problems.
Radio-frequency spectroscopy of polarons in ultracold Bose gases
NASA Astrophysics Data System (ADS)
Shashi, Aditya; Grusdt, Fabian; Abanin, Dmitry A.; Demler, Eugene
2014-05-01
Recent experimental advances enabled the realization of mobile impurities immersed in a Bose-Einstein condensate (BEC) of ultracold atoms. Here, we consider impurities with two or more internal hyperfine states, and study their radio-frequency (rf) absorption spectra, which correspond to transitions between two different hyperfine states. We calculate rf spectra for the case when one of the hyperfine states involved interacts with the BEC, while the other state is noninteracting, by performing a nonperturbative resummation of the probabilities of exciting different numbers of phonon modes. In the presence of interactions, the impurity gets dressed by Bogoliubov excitations of the BEC, and forms a polaron. The rf signal contains a δ-function peak centered at the energy of the polaron measured relative to the bare impurity transition frequency with a weight equal to the amount of bare impurity character in the polaron state. The rf spectrum also has a broad incoherent part arising from the background excitations of the BEC, with a characteristic power-law tail that appears as a consequence of the universal physics of contact interactions. We discuss both the direct rf measurement, in which the impurity is initially in an interacting state, and the inverse rf measurement, in which the impurity is initially in a noninteracting state. In the latter case, in order to calculate the rf spectrum, we solve the problem of polaron formation: a mobile impurity is suddenly introduced in a BEC, and dynamically gets dressed by Bogoliubov phonons. Our solution is based on a time-dependent variational ansatz of coherent states of Bogoliubov phonons, which becomes exact when the impurity is localized. Moreover, we show that such an ansatz compares well with a semiclassical estimate of the propagation amplitude of a mobile impurity in the BEC. Our technique can be extended to cases when both initial and final impurity states are interacting with the BEC.
Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics
NASA Astrophysics Data System (ADS)
Chasteen, Stephanie V.; Carter, Sue A.; Rumbles, Garry
2005-10-01
We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO2 solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO2 nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO2 solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO2.
Novel, discontinuous polaron transition in a two-band model
NASA Astrophysics Data System (ADS)
Moeller, Mirko M.; Sawatzky, George A.; Berciu, Mona
The coupling of charge carriers (electrons or holes) to phonons leads to the formation of a polaron, a coherent quasi-particle consisting of the charge carrier and the cloud of phonons surrounding it and moving coherently with it. Here we present exact diagonalization and momentum average approximation results for the single polaron properties of a two-band model with phonon modulated hopping, inspired by the perovskite BaBiO3. For large coupling we find that the ground state momentum changes discontinuously from k = π to k = 0 . Such sharp transitions of the polaron's ground state properties cannot occur in the well-studied models of the Holstein or Fröhlich type in which the carrier-phonon coupling modulates the on-site energies. However, they can occur in models where the carrier-phonon coupling modulates the hopping integrals such as the SSH model for which a similar yet smooth transition of the ground state momentum was recently shown to exist. We compare our findings to the SSH model and point out qualitative differences which we believe to be due to the two band nature of our model versus the single band SSH model. This work was supported by NSERC, QMI and the UBC 4YF.
Topological phases and polaron physics in ultracold quantum gases
NASA Astrophysics Data System (ADS)
Grusdt, Fabian
2016-05-01
The description of quantum many-body systems poses a formidable theoretical challenge. A seemingly simple problem is the coupling of a single impurity atom to non-interacting Bogoliubov phonons in a surrounding Bose-Einstein condensate. The system can be described by a polaron model at intermediate couplings - an 80 year problem. The situation has been realized experimentally, but when the impurity mass is small compared to the Boson mass, neither mean-field nor strong-coupling expansions are valid anymore. Now the impurity acts as an exchange particle, mediating phonon-phonon interactions. In this talk I present a semi-analytical solution to the polaron problem. I will show that the approach can be generalized to solve far-from equilibrium polaron problems, too, and elaborate on connections with recent experiments involving ultracold atoms and photons. A completely different class of many-body problems are systems with topological order. In recent years we have seen an uprise of cold-atomic or photonic implementations of artificial gauge fields, providing a corner stone for the realization of topological phases of matter. In the second part of my talk, I will address the challenging problem how non-local topological orders can be detected. It will be demonstrated that many-body topological invariants can be measured, making use of mobile impurities as coherent probes of the highly entangled groundstates. I will discuss Laughlin states and comment on possible realizations using ultracold atoms.
Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes.
Rettie, Alexander J E; Chemelewski, William D; Emin, David; Mullins, C Buddie
2016-02-01
Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed. PMID:26758715
Excitons and charged excitons in semiconductor quantum wells
Riva, C.; Peeters, F. M.; Varga, K.
2000-05-15
A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width. (c) 2000 The American Physical Society.
Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms
Schirotzek, Andre; Wu, C.-H.; Sommer, Ariel; Zwierlein, Martin W.
2009-06-12
We have observed Fermi polarons, dressed spin-down impurities in a spin-up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid, coexisting with a Fermi sea.
Gate controlled Aharonov-Bohm-type oscillations from single neutral excitons in quantum rings
NASA Astrophysics Data System (ADS)
Ding, F.; Akopian, N.; Li, B.; Perinetti, U.; Govorov, A.; Peeters, F. M.; Bof Bufon, C. C.; Deneke, C.; Chen, Y. H.; Rastelli, A.; Schmidt, O. G.; Zwiller, V.
2010-08-01
We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr3 in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.
Lloyd-Hughes, J. Failla, M.; Ye, J.; Jones, S. P. P.; Teo, K. L.; Jagadish, C.
2015-05-18
The cyclotron resonance of polarons in Zn{sub 1−x}Mg{sub x}O/ZnO heterostructures (with 0.15
Polarized excitons in nanorings and the optical Aharonov-Bohm effect
NASA Astrophysics Data System (ADS)
Govorov, A. O.; Ulloa, S. E.; Karrai, K.; Warburton, R. J.
2002-08-01
The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between ``bright'' exciton ground states and novel ``dark'' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.
Talbot Effect for Exciton Polaritons.
Gao, T; Estrecho, E; Li, G; Egorov, O A; Ma, X; Winkler, K; Kamp, M; Schneider, C; Höfling, S; Truscott, A G; Ostrovskaya, E A
2016-08-26
We demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter waves-an exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic "Talbot carpet" is produced by loading the exciton-polariton condensate into a microstructured one-dimensional periodic array of mesa traps, which creates an array of phase-locked sources for coherent polariton flow in the plane of the quantum wells. The spatial distribution of the Talbot fringes outside the mesas mimics the near-field diffraction of a monochromatic wave on a periodic amplitude and phase grating with the grating period comparable to the wavelength. Despite the lossy nature of the polariton system, the Talbot pattern persists for distances exceeding the size of the mesas by an order of magnitude. Thus, our experiment demonstrates efficient shaping of the two-dimensional flow of coherent exciton polaritons by a one-dimensional "flat lens." PMID:27610883
Exciton coupling in molecular crystals
NASA Technical Reports Server (NTRS)
Ake, R. L.
1976-01-01
The implications of perfect exciton coupling and molecular vibrations were investigated, as well as the effect they have on the lifetime of singlet and triplet excitons coupled in a limiting geometry. Crystalline bibenzyl, Cl4Hl4, provided a situation in which these mechanisms involving exciton coupling can be studied in the limit of perfect coupling between units due to the crystal's geometry. This geometry leads to a coupling between the two halves of the molecule resulting in a splitting of the molecular excited states. The study reported involves an experimental spectroscopic approach and begins with the purification of the bibenzyl. The principal experimental apparatus was an emission spectrometer. A closed cycle cryogenic system was used to vary the temperature of the sample between 20 K and 300 K. The desired results are the temperature-dependent emission spectra of the bibenzyl; in addition, the lifetimes and quantum yields measured at each temperature reveal the effect of competing radiationless processes.
Exciton-photon correlations in bosonic condensates of exciton-polaritons
Kavokin, Alexey V.; Sheremet, Alexandra S.; Shelykh, Ivan A.; Lagoudakis, Pavlos G.; Rubo, Yuri G.
2015-01-01
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979
Exciton-photon correlations in bosonic condensates of exciton-polaritons.
Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G
2015-01-01
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979
NASA Astrophysics Data System (ADS)
Matsunaga, Ryusuke; Matsuda, Kazunari; Kanemitsu, Yoshihiko
2009-03-01
We have performed micro-photoluminescence (PL) spectroscopy for single carbon nanotubes under magnetic fields at various temperatures. Sharp PL spectra of single carbon nanotubes allow us to directly observe the dark exciton PL peak a few meV below the bright exciton PL peak due to the Aharonov-Bohm effect [1]. From the PL intensity ratio of the dark to the bright excitons under magnetic fields, we found that the non-equilibrium (non-Boltzmann) distribution occurs between the bright and dark states, because phonons cannot scatter excitons between the two states with different parities [2]. Furthermore, we discuss the diameter dependence of the exciton population of the bright and dark states in single carbon nanotubes. [1] R. Matsunaga, K. Matsuda, and Y. Kanemitsu, Phys. Rev. Lett. 101, 147404 (2008). [2] V. Perebeinos, J. Tersoff, and Ph. Avouris, Nano Lett. 5, 2495 (2005).
NASA Astrophysics Data System (ADS)
Basel, Tek Prasad
We studied optical, electrical, and magnetic field responses of films and devices based on organic semiconductors that are used for organic light emitting diodes (OLEDs) and photovoltaic (OPV) solar cell applications. Our studies show that the hyperfine interaction (HFI)-mediated spin mixing is the key process underlying various magnetic field effects (MFE) and spin transport in aluminum tris(8-hydroxyquinoline)[Alq3]-based OLEDs and organic spin-valve (OSV). Conductivity-detected magnetic resonance in OLEDs and magneto-resistance (MR) in OSVs show substantial isotope dependence. In contrast, isotope-insensitive behavior in the magneto-conductance (MC) of same devices is explained by the collision of spin ½ carriers with triplet polaron pairs. We used steady state optical spectroscopy for studying the energy transfer dynamics in films and OLEDs based on host-guest blends of the fluorescent polymer and phosphorescent molecule. We have also studied the magnetic-field controlled color manipulation in these devices, which provide a strong proof for the `polaron-pair' mechanism underlying the MFE in organic devices. The critical issue that hampers organic spintronics device applications is significant magneto-electroluminescence (MEL) at room temperature (RT). Whereas inorganic spin valves (ISVs) show RT magneto-resistance, MR>80%, however, the devices do not exhibit electroluminescence (EL). In contrast, OLEDs show substantive EL emission, and are particularly attractive because of their flexibility, low cost, and potential for multicolor display. We report a conceptual novel hybrid organic/inorganic spintronics device (h-OLED), where we employ both ISV with large MR at RT, and OLED that has efficient EL emission. We investigated the charge transfer process in an OPV solar cell through optical, electrical, and magnetic field measurements of thin films and devices based on a low bandgap polymer, PTB7 (fluorinated poly-thienothiophene-benzodithiophene). We found that
Exciton size and quantum transport in nanoplatelets
Pelzer, Kenley M. Gray, Stephen K.; Darling, Seth B.; Schaller, Richard D.
2015-12-14
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.
Exciton size and quantum transport in nanoplatelets.
Pelzer, Kenley M; Darling, Seth B; Gray, Stephen K; Schaller, Richard D
2015-12-14
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport. PMID:26671357
Exciton size and quantum transport in nanoplatelets
NASA Astrophysics Data System (ADS)
Pelzer, Kenley M.; Darling, Seth B.; Gray, Stephen K.; Schaller, Richard D.
2015-12-01
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.
Fine structure of a resonantly excited p -shell exciton in a CdTe quantum dot
NASA Astrophysics Data System (ADS)
Smoleński, T.; Kazimierczuk, T.; Goryca, M.; Wojnar, P.; Kossacki, P.
2016-05-01
We present a polarization-resolved photoluminescence excitation study of the absorption spectrum of a p -shell neutral exciton in a single CdTe/ZnTe quantum dot. We find that the fine structure of the p -shell exciton is completely analogous to the fine structure of the s -shell exciton, including the selection rules and the effects of a magnetic field applied in Faraday and Voigt configurations. The energy spectrum of the p -shell exciton is found to be well described by introducing respective isotropic and anisotropic constants of the exchange interaction between a p -shell electron and a p -shell hole. The typical values of these exchange constants averaged over several randomly selected quantum dots yield δ0p p=(0.92 ±0.16 ) meV and δ1p p=(0.58 ±0.25 ) meV. Additionally, we demonstrate that the nonresonant relaxation of the p -shell exciton conserves the exciton spin to a very high degree for both bright and dark exciton configurations.
NASA Astrophysics Data System (ADS)
Grochol, Michal; Zimmermann, Roland
2007-11-01
Transition energies and oscillator strengths of excitons in dependence on magnetic field are investigated in types I and II semiconductor nanorings. A slight deviation from circular (concentric) shape of the type II nanoring gives a better observability of the Aharonov-Bohm oscillations since the ground state is always optically active. Kinetic equations for the exciton occupation are solved with acoustic phonon scattering as the major relaxation process, and absorption and luminescence spectra are calculated, showing deviations from equilibrium. The presence of a nonradiative exciton decay leads to a quenching of the integrated photoluminescence with magnetic field.
On uniqueness and non-degeneracy of anisotropic polarons
NASA Astrophysics Data System (ADS)
Ricaud, Julien
2016-05-01
We study the anisotropic Choquard-Pekar equation which describes a polaron in an anisotropic medium. We prove the uniqueness and non-degeneracy of minimizers in a weakly anisotropic medium. In addition, for a wide range of anisotropic media, we derive the symmetry properties of minimizers and prove that the kernel of the associated linearized operator is reduced, apart from three functions coming from the translation invariance, to the kernel on the subspace of functions that are even in each of the three principal directions of the medium.
On the LO-polaron dispersion in D dimensions
NASA Astrophysics Data System (ADS)
Gerlach, B.; Kalina, F.; Smondyrev, M.
2003-05-01
We discuss the (LO)polaron dispersion for arbitrary spatial dimension D. Firstly, we review the existing literature; recent numerical work is critically analyzed. Secondly, we derive novel upper bounds for the dispersion, which incorporate the correct behaviour of the dispersion up to third order of the coupling constant . A totally analytical evaluation is performed in the case D = 1. We compare the upper bounds with previously published lower bounds. Apart from a surrounding of zero dispersion, the relative deviation is on a few-percent scale.
The spin-polaron theory of high-Tc superconductivity
NASA Astrophysics Data System (ADS)
Mott, N. F.
1990-01-01
An outline is given of the model for some high-temperature superconductors which assumes that the carriers are holes in the (hybridized) oxygen 2p band and form ‘spin polarons’ with the moments on the copper atoms. A comparison is made with observations of spin polarons in Gd3-xvxS4 and with the properties of La1-xSrxVO3 in relation to those of La2-xSrxCuO4. It is assumed, following several authors, that in the superconductors the polarons form bipolarons, which are bosons, and a comparison is made with some other treatments of this hypothesis. It is proposed that in many such superconductors the boson, essentially a pair of these holes, moves in an impurity band, and that normally all the polarons (fermions) form bipolarons; the fermions repel each other on the same site (positive Hubbard U) but attract when on adjacent sites; the critical temperature Tc is then that at which the Bose gas becomes non-degenerate. In such materials a non-degenerate gas of bosons would carry the current above Tc as first suggested by Alexandrov et al. (1986). The linear increase in the resistivity above Tc is explained on this hypothesis. The effective mass of the bipolaron is, we believe, large (˜20 30me). The copper 3d9 moments in the superconducting range resonate between their two orientations as a consequence of the motion of the carriers, as they do in the description by Brinkman and Rice (1970) of highly correlated metals. Spin polarons, we believe, form only when this is so, but not in the antiferromagnetic range of x. A discussion is given of the resistivity above Tc, thermopower above Tc, and of the nature of the superconducting gap as shown by tunnelling. We confine our discussion to the materials containing copper, excluding for instance cubic Ba1-xKxBiO3, and possibly any superconductor containing bismuth, where the bosons may be Bi3+.
Mobility of Holstein Polaron at Finite Temperature: An Unbiased Approach
NASA Astrophysics Data System (ADS)
Mishchenko, A. S.; Nagaosa, N.; De Filippis, G.; de Candia, A.; Cataudella, V.
2015-04-01
We present the first unbiased results for the mobility μ of a one-dimensional Holstein polaron obtained by numerical analytic continuation combined with diagrammatic and worldline Monte Carlo methods in the thermodynamic limit. We have identified for the first time several distinct regimes in the λ -T plane including a band conduction region, incoherent metallic region, an activated hopping region, and a high-temperature saturation region. We observe that although mobilities and mean free paths at different values of λ differ by many orders of magnitude at small temperatures, their values at T larger than the bandwidth become very close to each other.
Impurities in Bose-Einstein Condensates: From Polaron to Soliton.
Shadkhoo, Shahriar; Bruinsma, Robijn
2015-09-25
We propose that impurities in a Bose-Einstein condensate which is coupled to a transversely laser-pumped multimode cavity form an experimentally accessible and analytically tractable model system for the study of impurities solvated in correlated liquids and the breakdown of linear-response theory [corrected]. As the strength of the coupling constant between the impurity and the Bose-Einstein condensate is increased, which is possible through Feshbach resonance methods, the impurity passes from a large to a small polaron state, and then to an impurity-soliton state. This last transition marks the breakdown of linear-response theory. PMID:26451565
Spatially indirect excitons in coupled quantum wells
Lai, Chih-Wei Eddy
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were
External quantum efficiency exceeding 100% in a singlet-exciton-fission-based solar cell
NASA Astrophysics Data System (ADS)
Baldo, Marc
2013-03-01
Singlet exciton fission can be used to split a molecular excited state in two. In solar cells, it promises to double the photocurrent from high energy photons, thereby breaking the single junction efficiency limit. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the peak external quantum efficiency is (109 +/-1)% at λ = 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160 +/-10)%. Independent confirmation of the high internal efficiency is obtained by analysis of the magnetic field effect on photocurrent, which determines that the triplet yield approaches 200% for pentacene films thicker than 5 nm. To our knowledge, this is the first solar cell to generate quantum efficiencies above 100% in the visible spectrum. Alternative multiple exciton generation approaches have been demonstrated previously in the ultraviolet, where there is relatively little sunlight. Singlet exciton fission differs from these other mechanisms because spin conservation disallows the usual dominant loss process: a thermal relaxation of the high-energy exciton into a single low-energy exciton. Consequently, pentacene is efficient in the visible spectrum at λ = 670 nm because only the collapse of the singlet exciton into twotriplets is spin-allowed. Supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088.
Coulomb-interaction effects on the electronic structure of radially polarized excitons in nanorings
NASA Astrophysics Data System (ADS)
Barticevic, Z.; Pacheco, M.; Simonin, J.; Proetto, C. R.
2006-04-01
The electronic structure of radially polarized excitons in structured nanorings is analyzed, with emphasis in the ground-state properties and their dependence under applied magnetic fields perpendicular to the ring plane. The electron-hole Coulomb attraction has been treated rigorously, through numerical diagonalization of the full exciton Hamiltonian in the noninteracting electron-hole pairs basis. Depending on the relative weight of the kinetic energy and Coulomb contributions, the ground-state of polarized excitons has “extended” or “localized” features. In the first case, corresponding to small rings dominated by the kinetic energy, the ground-state shows Aharonov-Bohm (AB) oscillations due to the individual orbits of the building particles of the exciton. In the localized regime, corresponding to large rings dominated by the Coulomb interaction, the only remaining AB oscillations are due to the magnetic flux trapped between the electron and hole orbits. This dependence of the exciton, a neutral excitation, on the flux difference confirms this feature as a signature of Coulomb dominated polarized excitons. Analytical approximations are provided in both regimes, which accurately reproduce the numerical results.
NASA Astrophysics Data System (ADS)
Yu, Ting
2015-03-01
Two-dimensional (2D) semiconductors, such as transitional-metal-dichalcogenide monolayers (TMD 1Ls), have aroused great interest because of the underlying fundamental physics (e.g. many body effects and wealth excitonic states) and the promising optoelectronic applications such as light-emitting diodes and solar cells. Here, we report excitonic emission and valley splitting of monolayer WS2 and MoS2 under electrical, optical and magnetic manipulation. Through electrical and optical injection of charge carriers, tunable excitonic emission has been realized due to interplay of various excitonic states, and basic binding energies of trions have been extracted. At low temperature, the Zeeman shifts of excitons and trions have been determined by polarization-dependent photoluminescence measurements under perpendicular magnetic fields, which reveal the breaking of valley degeneracy. Our studies provide the fundamental understanding on large excitonic and unique valleytronic effects in TMD 1Ls. Moreover, we also develop multiple strategies for managing the light emission, which opens up many possibilities for improving the performance and creating the multifunction of 2D TMD-based light emitting applications. Also at Department of Physics, Faculty of Science, National University of Singapore 117542, Singapore; Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore 117546, Singapore.
NASA Astrophysics Data System (ADS)
Rodina, Anna V.; Efros, Alexander L.
2016-04-01
We analyze theoretically physical mechanisms responsible for the radiative recombination of the ground optically passive ("dark") exciton (DE), which dominates in photoluminescence (PL) of colloidal nanocrystals (NCs) at low temperatures. The DE becomes optically active due to its mixing with the bright excitons caused by an external magnetic field, dangling-bond spins or by acoustic and optical phonons. These activation mechanisms mix the DE with different bright excitons and, consequently, lead to different PL polarization properties, because they are determined by dipole orientations of the bright excitons, which the DE is coupled with. We show that the PL polarization properties of prolate and oblate shape NCs are different due to different activation mechanisms responsible for the DE recombination.
NASA Astrophysics Data System (ADS)
Meier, Christoph; Teutloff, Christian; Behrends, Jan; Bittl, Robert; Astakhov, Oleksandr; Lips, Klaus
2016-07-01
Electrically detected magnetic resonance (EDMR) spectroscopy is employed to study the influence of triplet excitons on the photocurrent in state-of-the-art microcrystalline silicon thin-film solar cells. These triplet excitons are used as sensitive spin probes for the investigation of their electronic and nuclear environment in this mixed-phase material. According to low-temperature EDMR results obtained from solar cells with different
Exciton formation in monolayer transition metal dichalcogenides.
Ceballos, Frank; Cui, Qiannan; Bellus, Matthew Z; Zhao, Hui
2016-06-01
Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps. PMID:27219022
Contrastive analysis of multiple exciton generation theories
NASA Astrophysics Data System (ADS)
Tan, Hengyu; Chang, Qing
2015-10-01
Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.
Coherent Exciton Dynamics in Atomically Thin Semiconductors
NASA Astrophysics Data System (ADS)
Li, Xiaoqin (Elaine)
The near band-edge optical response of an emerging class of semiconductors, known as the transitional metal dichalcogenides (TMDs), is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is quantum decoherence time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Dephasing time is also equivalent to the intrinsic homogeneous linewidth of exciton resonance. In addition, excitons in TMDs are localized at the corners of the Brillouin zone and provide a convenient way to optical manipulate the valley degree of freedom, which may act as a useful information carrier analogous to electronic charge or spin. Direct measurement of valley coherence time is challenging because it corresponds to a non-radiative coherence between two degenerate states. Using ultrafast multi-dimensional optical spectroscopy, we investigate the intrinsic homogeneous linewidth of excitons, exciton valley coherence as well as coupling between excitons and trions. Our studies reveal coherent electronic dynamics on the order of ~100 fs in monolayer TMDs. We gratefully acknowledge financial support from NSF, AFOSR, and the Welch Foundation.
Exciton formation in monolayer transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Ceballos, Frank; Cui, Qiannan; Bellus, Matthew Z.; Zhao, Hui
2016-06-01
Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps.
Two-exciton excited states of J-aggregates in the presence of exciton-exciton annihilation
NASA Astrophysics Data System (ADS)
Levinsky, B.; Fainberg, B. D.; Nesterov, L. A.; Rosanov, N. N.
2016-07-01
We study decay of two-exciton states of a J-aggregate that is collective in nature. We use mathematical formalism based on effective non-Hermitian Hamiltonian suggested in nuclear theory. We show that decay of two-exciton states is strongly affected by the interference processes in the exciton-exciton annihilation. Our evaluations of the imaginary part of the effective Hamiltonian show that it exceeds the spacing between real energies of the two-exciton states that gives rise to the transition to the regime of overlapping resonances supplying the system by the new collectivity - the possibility of coherent decay in the annihilation channel. The decay of two-exciton states varies from twice bimolecular decay rate to the much smaller values that is associated with population trapping. We have also considered the corresponding experiment in the framework of our approach, the picture of which appears to be more complex and richer than it was reasoned before.
Spectroscopic Evidence of Formation of Small Polarons in Doped Manganites
NASA Astrophysics Data System (ADS)
Moritomo, Yutaka; Machida, Akihiko; Nakamura, Arao
1998-03-01
Temperature dependence of absorption spectra for thin films of doped manganites R_0.6Sr_0.4MnO_3, where R is rare-earth atom, has been investigated systematically changing averaged ionic radius < rA > of perovskite A-site. We have observed a specific absorption band at ~1.5eV due to optical excitations from small polarons (SP)(Machida et al.), submitted.. Spectral weight of the SP band increases with decreasing temperature and eventually disappears at the insulator-metal (IM) transition, indicating that SP in the paramagnetic state (T >= T_C) changes into bare electrons (or large polarons) in the ferromagnetic state due to the enhanced one-electron bandwidth W. We further derived important physical quantities, i.e., W, on-site exchange interaction J and binding energy Ep of SP, and discuss material dependence of stability of SP. This work was supported by a Grant-In-Aid for Scientific Research from the Ministry of Education, Science, Sport and Culture and from PRESTO, Japan Scienece and Technology Corporation (JST), Japan.
Edwards polaron formation : From one to three dimensions
NASA Astrophysics Data System (ADS)
Chakraborty, M.; Mohanta, N.; Taraphder, A.; Min, B. I.; Fehske, H.
2016-04-01
Employing a self-consistent (optimized) variational diagonalization scheme, we investigate the formation of polaronic quasiparticles in a spinless fermion-boson transport model that couples the movement of charge carriers to fluctuations and correlations of a background medium. The background is parameterized by bosonic degrees of freedom. The variational fermion-boson Hilbert space is constructed to achieve high accuracy in one to three spatial dimensions with modest computational requirements. To characterize the ground-state properties of the Edwards model in the single-particle sector, we present exact numerical results for the polaron band dispersion, quasiparticle weight, Drude weight, mass enhancement, and the particle-boson correlations in a wide parameter regime. In the Edwards model, transport will be quasifree, diffusive or boson-assisted in the weakly fermion-boson coupled, fluctuation-dominated or strongly correlated regimes, respectively. Thereby correlated transport is not only assisted but also limited by the bosonic excitations. As a result, the Drude weight remains finite even in the limit of very small boson frequencies. For a strongly correlated background, closed loops are important, in any dimension, to generate a finite effective particle mass even when the free fermion has an infinite mass.
Polaronic effects in monolayer black phosphorus on polar substrates
NASA Astrophysics Data System (ADS)
Mogulkoc, A.; Mogulkoc, Y.; Rudenko, A. N.; Katsnelson, M. I.
2016-02-01
We investigate the effect of charge carrier interaction with surface optical phonons on the band properties of monolayer black phosphorus induced by polar substrates. We develop an analytical method based on the Lee-Low-Pines theory to calculate the spectrum of Fröhlich type continuum Hamiltonian in the long-wavelength limit. We examine the modification of a band gap and renormalization of effective masses due to the substrate-related polaronic effect. Our results show that an energy gap in supported monolayer black phosphorus is enlarged depending on a particular substrate and the interlayer distance z . Among the substrate considered, the largest gap broadening at z =2.5 Å is observed for the Al2O3 substrate, which is found to be ˜50 meV. Carrier-phonon coupling also renormalizes the effective masses which is more pronounced along the zigzag direction. Anisotropy of the effective masses becomes stronger by the influence of the polaronic effect corresponding to direction-dependent carrier-phonon coupling. We conclude that substrate phonons have a non-negligible effect on the static band properties of monolayer black phosphorus, which may be further exploited in its experimental and theoretical studies.
Two-Dimensional Polaronic Behavior in the Binary Oxides m-HfO2 and m-ZrO2
Mckenna, Keith P.; Wolf, Matthew J.; Shluger, Alexander L.; Lany, Stephan; Zunger, Alex
2012-03-14
We demonstrate that the three-dimensional (3D) binary monoclinic oxides HfO2 and ZrO2 exhibit quasi-2D polaron localization and conductivity, which results from a small difference in the coordination of two oxygen sublattices in these materials. The transition between a 2D large polaron into a zerodimensional small polaron state requires overcoming a small energetic barrier. These results demonstrate how a small asymmetry in the lattice structure can determine the qualitative character of polaron localization and significantly broaden the realm of quasi-2D polaron systems.
Two-Dimensional Polaronic Behavior in the Binary Oxides m-HfO2 and m-ZrO2
McKenna, K. P.; Wolf, M. J.; Shluger, A. L.; Lany, S.; Zunger, A.
2012-03-16
We demonstrate that the three-dimensional (3D) binary monoclinic oxides HfO{sub 2} and ZrO{sub 2} exhibit quasi-2D polaron localization and conductivity, which results from a small difference in the coordination of two oxygen sublattices in these materials. The transition between a 2D large polaron into a zero-dimensional small polaron state requires overcoming a small energetic barrier. These results demonstrate how a small asymmetry in the lattice structure can determine the qualitative character of polaron localization and significantly broaden the realm of quasi-2D polaron systems.
NASA Astrophysics Data System (ADS)
Noltemeyer, Martin; Bertram, Frank; Hempel, Thomas; Bastek, Barbara; Christen, Juergen; Brandt, Matthias; Lorenz, Michael; Grundmann, Marius
2012-02-01
The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a combination of cw- and pulsed excitation in two independent measurements are needed.
Exciton Seebeck effect in molecular systems
Yan, Yun-An; Cai, Shaohong
2014-08-07
We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.
Undoped GaAs bilayers for exciton condensation experiments
NASA Astrophysics Data System (ADS)
Lilly, M. P.
2005-03-01
Experimental progress in transport studies of exciton condensation of in electron and hole bilayers at high magnetic fields [1,2] has shown this novel physics can be observed. Fabrication of the bipolar electron-hole bilayers for zero field studies of exciton condensation still remains elusive. We describe a series of experiments on undoped GaAs/AlGaAs heterostructures with the motivation of making electron-hole bilayers. In these undoped devices, external electric fields induce carriers rather than the traditional doping techniques. Single layer electron (or hole) devices demonstrate a high mobility over a wide range of density. More recently, fully undoped bilayers have been made where the density in each layer is independently controlled with gates on the top and bottom of the bilayer. In this talk we present high field transport of undoped electron-electron bilayers, and describe recent progress towards extending the fabrication techniques to creating electron-hole bilayers for exciton condensation studies at zero magnetic field. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. 1. M. Kellogg, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 93 036801 (2004). 2. E. Tutoc, M. Shayegan, and D. A. Huse, Phys. Rev. Lett. 93, 036802 (2004).
Localized Excitons in Carbon Nanotubes.
NASA Astrophysics Data System (ADS)
Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei
2015-03-01
It has been historically known that unintentional defects in carbon nanotubes (CNTs) may fully quench the fluorescence. However, some dopants may enhance the fluorescence by one order of magnitude thus turning the CNTs, which are excellent light absorbers, in good emitters. We have correlated the experimentally observed photoluminescence spectra to the electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the lowest bright exciton peak. Our simulations suggest an association of these peaks with deep trap states tied to different specific chemical adducts. While the wave functions of excitons in undoped CNTs are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with the experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from scattering of acoustic phonons on localized excitons. Our work lays the foundation to utilize doping as a generalized route for wave function engineering and direct control of carrier dynamics in SWCNTs toward enhanced light emission properties for photonic applications.
Exciton Binding Energy of Monolayer WS2
Zhu, Bairen; Chen, Xi; Cui, Xiaodong
2015-01-01
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01 eV around K valley in the Brillouin zone. PMID:25783023
Influence of lithium vacancies on the polaronic transport in olivine phosphate structure
NASA Astrophysics Data System (ADS)
Murugavel, Sevi; Sharma, Monika; Shahid, Raza
2016-01-01
Intercalation and deintercalation of lithium ions in cathode materials are of principal to the operation of current rechargeable lithium ion batteries. The performance of lithium ion batteries highly relies on the active cathode material which includes cell potential, power/energy density, capacity, etc. An important issue in this class of material is to resolve the factors governing the electron and ion transport in olivine phosphate structure. In this class of material, there is still an open debate on the mechanism of charge transport including both polarons and lithium ions. On the one hand, this is due to the large disparity between the experimental results and the theoretical model predictions. On the other hand, this is also due to the lack of precise experimental measurement without any parasitic phases in a given cathode material. Here, we present the polaronic conduction in lithiated triphylite LiFePO4 (LFP) and delithiated heterosite FePO4 (FP) by means of broadband ac impedance spectroscopy over wide range temperatures and frequency. It is found that the LFP phase possess two orders of higher polaronic conductivity than FP phase despite having similar mobility of polarons in both phases. We show that the differences in the polaronic conductivity of two phases are due to the significant differences in concentration of polarons. It is found that the formation energy of polarons in individual phases is mainly determined by the corresponding defect state associated with it. The temperature dependent dc conductivity has been analyzed within the framework of Mott model of polaronic conduction and explored the origin of polaronic conduction mechanism in this class of material.
Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong
2015-01-15
The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. Moreover, it is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.
Cross-polarized excitons in carbon nanotubes.
Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R
2008-05-13
Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E(22) van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active "bright" excitonic band polarized parallel to the tube axis and several optically "weak" cross-polarized excitons. The rest are optically (near) forbidden "dark" transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E(11) transition are more localized along the circumference of a tube, compared with others related to the E(12) and E(21) cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293
Tang, Yanhao; Xie, Wei; McGuire, John A. Lai, Chih Wei; Mandal, Krishna C.
2015-09-21
We analyze exciton spin dynamics in GaSe under nonresonant circularly polarized optical pumping with an exciton spin-flip rate-equation model. The model reproduces polarized time-dependent photoluminescence measurements in which the initial circular polarization approaches unity even when pumping with 0.15 eV excess energy. At T = 10 K, the exciton spin relaxation exhibits a biexponential decay with sub-20 ps and >500 ps time constants, which are also reproduced by the rate-equation model assuming distinct spin-relaxation rates for hot (nonequilibrium) and cold band-edge excitons.
Plasmon and Exciton Coupling and Purcell Enhancement
NASA Astrophysics Data System (ADS)
Rice, Quinton; Rigo, Maria Veronica; Fudala, Rafal; Cho, Hyoyeong; Kim, Wan-Joong; Rich, Ryan; Tabibi, Bagher; Gryczynski, Zygmunt; Gryczynski, Ignacy; Yu, William; Seo, Jaetae
2014-05-01
The photoluminescence from plasmon-coupled exciton is of great interest for optoelectronic applications, because of the large quantum yield with localized field enhancement and reduced nonradiative transition. The Coulomb interaction through plasmon-exciton coupling results in the Purcell enhancement of quantum dots (QDs) in the vicinity of metal nanoparticles (MNPs). With plasmon-exciton coupling, the radiative and non-radiative decay rates and the coupling rates compete with each other. The coupling rate is closely related to the coupling distance between QDs and MNPs. The optimized coupling distance scales the re-excitation density of localized fields and the plasmon-exciton coupling rates. If the plasmon-exciton coupling rate is much faster than the radiative and non-radiative transitions of excitons, the re-excitations of excitons by the localized plasmonic field and the reduction of non-radiative transitions may occur. This presentation includes plasmon-exciton coupling dynamics, large enhancement and temporal properties of PL, and dipole-PL polarization fidelity of hybrid optical materials of plasmonic nanometals and excitonic semiconductor QDs. The work at Hampton University was supported by the National Science Foundation (NSF HRD-1137747), and Army Research Office (ARO W911NF-11-1-0177). The work at University of North Texas was supported by National Institutes of Health (NIH R01EB12003, and 5R21CA14897 (Z.G.)).
Cross-polarized excitons in carbon nanotubes
Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K.; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R.
2008-01-01
Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293
Ground state energy of N Frenkel excitons
NASA Astrophysics Data System (ADS)
Pogosov, W.; Combescot, M.
2009-03-01
By using the composite many-body theory for Frenkel excitons we have recently developed, we here derive the ground state energy of N Frenkel excitons in the Born approximation through the Hamiltonian mean value in a state made of N identical Q = 0 excitons. While this quantity reads as a density expansion in the case of Wannier excitons, due to many-body effects induced by fermion exchanges between N composite particles, we show that the Hamiltonian mean value for N Frenkel excitons only contains a first order term in density, just as for elementary bosons. Such a simple result comes from a subtle balance, difficult to guess a priori, between fermion exchanges for two or more Frenkel excitons appearing in Coulomb term and the ones appearing in the N exciton normalization factor - the cancellation being exact within terms in 1/Ns where Ns is the number of atomic sites in the sample. This result could make us naively believe that, due to the tight binding approximation on which Frenkel excitons are based, these excitons are just bare elementary bosons while their composite nature definitely appears at various stages in the precise calculation of the Hamiltonian mean value.
BLF-SSH polarons coupled to acoustic phonons in the adiabatic limit
NASA Astrophysics Data System (ADS)
Chandler, Carl J.; Marsiglio, F.
2014-12-01
We survey polaron formation in the Barisić-Labbé-Friedel and Su-Schrieffer-Heeger (BLF-SSH) model using acoustic phonons in the adiabatic limit. Multiple different numerical optimization routines and strong-coupling analytical calculations are used to find a robust ground-state energy for a wide range of coupling strengths. The electronic configuration and accompanying ionic distortions of the polaron were determined, as well as a nonzero critical coupling strength for polaron formation in two and three dimensions.
NASA Astrophysics Data System (ADS)
Shchadilova, Yulia E.; Grusdt, Fabian; Rubtsov, Alexey N.; Demler, Eugene
2016-04-01
We propose a class of variational Gaussian wave functions to describe Fröhlich polarons at finite momenta. Our wave functions give polaron energies that are in excellent agreement with the existing Monte Carlo results for a broad range of interactions. We calculate the effective mass of polarons and find smooth crossover between weak- and intermediate-coupling strength. Effective masses that we obtain are considerably larger than those predicted by the mean-field method. A prediction based on our variational wave functions is a special pattern of correlations between host atoms that can be measured in time-of-flight experiments.
Controlling the polarization eigenstate of a quantum dot exciton with light.
Belhadj, Thomas; Simon, Claire-Marie; Amand, Thierry; Renucci, Pierre; Chatel, Beatrice; Krebs, Olivier; Lemaître, Aristide; Voisin, Paul; Marie, Xavier; Urbaszek, Bernhard
2009-08-21
We demonstrate optical control of the polarization eigenstates of a neutral quantum dot exciton without any external fields. By varying the excitation power of a circularly polarized laser in microphotoluminescence experiments on individual InGaAs quantum dots we control the magnitude and direction of an effective internal magnetic field created via optical pumping of nuclear spins. The adjustable nuclear magnetic field allows us to tune the linear and circular polarization degree of the neutral exciton emission. The quantum dot can thus act as a tunable light polarization converter. PMID:19792745
NASA Astrophysics Data System (ADS)
Malyukin, Yu. V.; Sorokin, A. V.; Semynozhenko, V. P.
2016-06-01
We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ɛLR/2β, wherein ɛLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g < 1) and a strong (g > 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.
Josephson effects in condensates of excitons and exciton polaritons
Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.
2008-07-15
We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.
Polaron transport in TiO{sub 2} thin films
Yildiz, Abdullah; Iacomi, Felicia; Mardare, Diana
2010-10-15
Undoped and Fe-doped TiO{sub 2} thin films were obtained by rf-sputtering technique onto heated glass substrates (250 deg. C) covered with indium tin oxide. The temperature dependence of the electrical conductivity was investigated in the temperature range 13-320 K, and it shows that the conduction mechanism in the studied samples is described by small-polaron hopping (SPH) at temperatures higher than half of the Debye temperature ({theta}{sub D}). It was found that the magnitude of the SPH coupling increases by Fe doping in TiO{sub 2} thin films. With decreasing temperature, the conduction behavior transited from SPH conduction to variable-range hopping (VRH) conduction. In the intermediate temperature domain (200 K
Polaron formation in the vicinity of a narrow Feshbach resonance
NASA Astrophysics Data System (ADS)
Casteels, W.; Wouters, M.
2014-10-01
The polaronic system consisting of an impurity in a dilute Bose-Einstein condensate is considered in the presence of a narrow Feshbach resonance. For this purpose a coupled-channel model is used, which at the mean-field level predicts the formation of quasiparticles that are a superposition of the impurity and the molecular states. The impurity-boson interactions and the coupling between the open and closed channels are then considered weak and a perturbative calculation of the corrections to the mean-field results is presented. This allows us to examine the properties of the quasiparticles, such as the lifetime and the effective mass. The relation between the model parameters and the experimental parameters is determined by identifying the low-energy T matrix and applying a proper renormalization scheme.
Radio frequency spectroscopy of polarons in ultracold Bose gases
NASA Astrophysics Data System (ADS)
Shashi, Aditya; Grusdt, Fabian; Abanin, Dmitry; Demler, Eugene
2014-05-01
Recent experimental advances enabled the realization of mobile impurities immersed in a Bose-Einstein condensate (BEC) of ultracold atoms. We consider impurities with two or more internal hyperfine states, and study their radio-frequency (RF) absorption spectra, which correspond to transitions between two different hyperfine states. We calculate RF spectra for the case when one of the hyperfine states involved interacts with the BEC, while the other state is non-interacting, by performing a non-perturbative resummation of the probabilities of exciting different numbers of phonon modes. We discuss both the direct RF measurement, in which the impurity is initially in an interacting state, and the inverse RF measurement, in which the impurity is initially in a non-interacting state. In the latter case, in order to calculate the RF spectrum, we solve the problem of polaron formation: a mobile impurity dynamically gets dressed by Bogoliubov phonons, using a time-dependent variational ansatz of coherent states.
Phase diagram and collective excitations in an excitonic insulator from an orbital physics viewpoint
NASA Astrophysics Data System (ADS)
Nasu, Joji; Watanabe, Tsutomu; Naka, Makoto; Ishihara, Sumio
2016-05-01
An excitonic-insulating system is studied from a viewpoint of the orbital physics in strongly correlated electron systems. An effective model Hamiltonian for low-energy electronic states is derived from the two-orbital Hubbard model with a finite-energy difference corresponding to the crystalline-field splitting. The effective model is represented by the spin operators and the pseudospin operators for the spin-state degrees of freedom. The ground-state phase diagram is analyzed by the mean-field approximation. In addition to the low-spin state and high-spin state phases, two kinds of the excitonic-insulating phases emerge as a consequence of the competition between the crystalline-field effect and the Hund coupling. Transitions to the excitonic phases are classified to an Ising-type transition resulted from a spontaneous breaking of the Z2 symmetry. Magnetic structures in the two excitonic-insulating phases are different from each other: an antiferromagnetic order and a spin nematic order. Collective excitations in each phase are examined using the generalized spin-wave approximation. Characteristics in the Goldstone modes in the excitonic-insulating phases are studied through the calculations of the dynamical correlation functions for the spins and pseudospin operators. Both the transverse and longitudinal spin excitation modes are active in the two excitonic-insulating phases in contrast to the low-spin state and high-spin state phases. Relationships of the present results to the perovskite cobalt oxides are discussed.
Optical spectroscopy of free excitons in a CuInS{sub 2} chalcopyrite semiconductor compound
Mudryi, A. V. Ivanyukovich, A. V.; Yakushev, M. V.; Martin, R.; Saad, A.
2008-01-15
The spectra of reflectance and luminescence of high-quality CuInS{sub 2} single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy {approx}1.5356 eV and doubly degenerate peak BC at the energy {approx}1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A{sub UPB} at {approx}1.5361 eV and A{sub LPB} at {approx}1.5347 eV, with a half-width {approx}1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be {approx}18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.
Hybrid functional studies of defects and hole polarons in oxides
NASA Astrophysics Data System (ADS)
Varley, Joel
Transparent conducting oxides (TCOs) are ubiquitous, appearing in windows, flat-panel displays, solar cells, solid-state lighting, and transistors that all exploit TCOs' combination of high electrical conductivity and optical transparency. Thanks to this large and growing list of applications, there has been a surge of interest in the science of these materials, focusing on the fundamental properties and doping opportunities in traditional TCOs as well as the exploration of promising new candidate materials. Hybrid density functional theory has proven instrumental in elucidating the physics of TCOs. One example is the study of dopants and defects that determine the conductivity. Accurate formation energies and charge-state transition levels can now be obtained thanks to the accurate electronic structure provided by a hybrid functional. This allows us to address the origins of unintentional conductivity: for SnO2, In2O3, and Ga2O3, we demonstrate that this is not due to native defects such as oxygen vacancies, but must be attributed to unintentional incorporation of impurities. We can also provide guidelines for achieving higher doping levels, suggesting several impurities as candidate donors with high solubility. Limitations on doping due to the formation or incorporation of compensating centers are addressed as well. Hybrid functional calculations also overcome the shortcomings associated with traditional local or semi-local functionals, which do not properly describe charge localization. Hybrid functionals accurately describe polaron formation, i.e., the self-trapping of holes when p - type doping of the oxide materials is attempted. Consequences of polaron formation for optical characterization of the material will be discussed. This work was performed in collaboration with Anderson Janotti and Chris G. Van de Walle, and was in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Magneto-Excitons in (411)A and (100)-Oriented GaAs/AlGaAs Multiple Quantum Well Structures
Bajaj, K.K.; Hiyamizu, S.; Jones, E.D.; Krivorotov, I.; Shimomura, S.; Shinohara, K.
1999-01-20
We report magneto-exciton spectroscopy studies of (411)A and (100)-oriented GaAs/Al{sub 0.3}Ga{sub 0.7}As multiquantum well structures. The samples consisted of seven GaAs quantum wells with widths varying between 0.6 and 12nm, were grown on (411)A and (100)-oriented GaAs substrates. The exciton diamagnetic energy shifts and linewidths were measured between 0 and 14T at 1.4K The dependence of the exciton diamagnetic shifts with magnetic field were calculated using a variational approach and good agreement with experiment for both substrate orientations was found.
Landau levels of the C-exciton in CuInSe{sub 2} studied by magneto-transmission
Yakushev, M. V.; Rodina, A. V.; Shuchalin, G. M.; Seisian, R. P.; Abdullaev, M. A.; Rockett, A.; Zhivulko, V. D.; Mudryi, A. V.; Faugeras, C.; Martin, R. W.
2014-10-06
The electronic structure of the solar cell absorber CuInSe{sub 2} is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29 T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7 T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0 T (8.5 meV), band gap (1.2828 eV), and hole effective mass m{sub so} = (0.31 ± 0.12)m{sub 0} for the C valence sub-band.
Excitonic energy transfer in light-harvesting complexes in purple bacteria
NASA Astrophysics Data System (ADS)
Ye, Jun; Sun, Kewei; Zhao, Yang; Yu, Yunjin; Kong Lee, Chee; Cao, Jianshu
2012-06-01
Two distinct approaches, the Frenkel-Dirac time-dependent variation and the Haken-Strobl model, are adopted to study energy transfer dynamics in single-ring and double-ring light-harvesting (LH) systems in purple bacteria. It is found that the inclusion of long-range dipolar interactions in the two methods results in significant increase in intra- or inter-ring exciton transfer efficiency. The dependence of exciton transfer efficiency on trapping positions on single rings of LH2 (B850) and LH1 is similar to that in toy models with nearest-neighbor coupling only. However, owing to the symmetry breaking caused by the dimerization of BChls and dipolar couplings, such dependence has been largely suppressed. In the studies of coupled-ring systems, both methods reveal an interesting role of dipolar interactions in increasing energy transfer efficiency by introducing multiple intra/inter-ring transfer paths. Importantly, the time scale (4 ps) of inter-ring exciton transfer obtained from polaron dynamics is in good agreement with previous studies. In a double-ring LH2 system, non-nearest neighbor interactions can induce symmetry breaking, which leads to global and local minima of the average trapping time in the presence of a non-zero dephasing rate, suggesting that environment dephasing helps preserve quantum coherent energy transfer when the perfect circular symmetry in the hypothetic system is broken. This study reveals that dipolar coupling between chromophores may play an important role in the high energy transfer efficiency in the LH systems of purple bacteria and many other natural photosynthetic systems.
DNA-mediated excitonic upconversion FRET switching
NASA Astrophysics Data System (ADS)
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-01
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.
DNA-mediated excitonic upconversion FRET switching
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-17
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.
DNA-mediated excitonic upconversion FRET switching
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-17
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffersmore » from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.« less
Excitonic superconductivity in copper oxides
Tesanovic, Z.; Bishop, A.R.; Martin, R.L.; Harris, C.
1988-01-01
We discuss the possibility of excitonic superconductivity in high T/sub c/ copper oxides. The Hamiltonians describing CuO/sub 2/ planes supports both antiferromagnetism and low-lying Cu /longleftrightarrow/ O intra- and interband charge fluctuations. One crosses from one regime to another as the number of holes per unit cell increases. The high T/sub c/ superconductivity takes place at hole concentrations most favorable for intraband charge transfer excitations. The dynamic polarizability of the environment surrounding CuO/sub 2/ planes plays an important role in enhancing T/sub c/. 15 refs., 4 figs.
Infrared absorption spectra of molecular crystals: Possible evidence for small-polaron formation?
NASA Astrophysics Data System (ADS)
Pržulj, Željko; Čevizović, Dalibor; Zeković, Slobodan; Ivić, Zoran
2008-09-01
The temperature dependence of the position of the so-called anomalous band peaked at 1650cm in the IR-absorption spectrum of crystalline acetanilide (ACN) is theoretically investigated within the small-polaron theory. Its pronounced shift towards the position of the normal band is predicted with the rise of temperature. Interpretation of the IR-absorption spectra in terms of small-polaron model has been critically assessed on the basis of these results.
Thermodynamic efficiency limit of excitonic solar cells
Giebink, Noel C.; Wiederrecht, Gary P.; Wasielewski, Michael R.; Forrest, Stephen R.
2011-01-01
Excitonic solar cells, comprised of materials such as organic semiconductors, inorganic colloidal quantum dots, and carbon nanotubes, are fundamentally different than crystalline, inorganic solar cells in that photogeneration of free charge occurs through intermediate, bound exciton states. Here, we show that the Second Law of Thermodynamics limits the maximum efficiency of excitonic solar cells below the maximum of 31% established by Shockley and Queisser [J. Appl. Phys. 32, 510 (1961)] for inorganic solar cells (whose exciton-binding energy is small). In the case of ideal heterojunction excitonic cells, the free energy for charge transfer at the interface, ΔG, places an additional constraint on the limiting efficiency due to a fundamental increase in the recombination rate, with typical -ΔG in the range 0.3 to 0.5 eV decreasing the maximum efficiency to 27% and 22%, respectively.
Excitons and optical spectra of phosphorene nanoribbons
NASA Astrophysics Data System (ADS)
Nourbakhsh, Zahra; Asgari, Reza
2016-07-01
On the basis of many-body ab initio calculations, using the single-shot G0W0 method and Bethe-Salpeter equation, we study phosphorene nanoribbons (PNRs) in the two typical zigzag and armchair directions. The electronic structure, optical absorption, electron-hole (exciton) binding energy, exciton exchange splitting, and exciton wave functions are calculated for different sizes of PNRs. The typically strong splitting between singlet and triplet excitonic states make PNRs favorable systems for optoelectronic applications. Quantum confinement occurs in both kinds of PNRs, and it is stronger in the zPNRs, which behave like quasi-zero-dimensional systems. Scaling laws are investigated for the size-dependent behaviors of PNRs. The first bright excitonic state in PNRs is explored in detail.
Excitonic Aharonov-Bohm effect: Unstrained versus strained type-I semiconductor nanorings
NASA Astrophysics Data System (ADS)
Tadić, M.; Čukarić, N.; Arsoski, V.; Peeters, F. M.
2011-09-01
We study how mechanical strain affects the magnetic field dependence of the exciton states in type-I semiconductor nanorings. Strain spatially separates the electron and hole in (In,Ga)As/GaAs nanorings which is beneficial for the occurrence of the excitonic Aharonov-Bohm (AB) effect. In narrow strained (In,Ga)As/GaAs nanorings the AB oscillations in the exciton ground-state energy are due to anticrossings with the first excited state. No such AB oscillations are found in unstrained GaAs/(Al,Ga)As nanorings irrespective of the ring width. Our results are obtained within an exact numerical diagonalization scheme and are shown to be accurately described by a two-level model with off-diagonal coupling t. The later transfer integral expresses the Coulomb coupling between states of electron-hole pairs. We also found that the oscillator strength for exciton recombination in (In,Ga)As/GaAs nanorings exhibits AB oscillations, which are superimposed on a linear increase with magnetic field. Our results agree qualitatively with recent experiments on the excitonic Aharonov-Bohm effect in type-I (In,Ga)As/GaAs nanorings.
NASA Astrophysics Data System (ADS)
Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel
2015-03-01
Monolayer transition-metal dichalcogenides form a direct bandgap predicted in the visible regime making them attractive host materials for various electronic and optoelectronic applications. Due to a weak dielectric screening in these materials, strongly bound electron-hole pairs or excitons have binding energies up to at least several hundred meV's. While the conventional wisdom is to think of excitons as hydrogen-like quasi-particles, we show that the hydrogen model breaks down for these experimentally observed strongly bound, room-temperature excitons. To capture these non-hydrogen-like photo-excitations, we introduce an atomistic model for excitons that predicts both bright excitons and dark excitons, and their broken degeneracy in these two-dimensional materials. For strongly bound exciton states, the lattice potential significantly distorts the envelope wave functions, which affects predicted exciton peak energies. The combination of large binding energies and non-degeneracy of exciton states in monolayer transition metal dichalogendies may furthermore be exploited in room temperature applications where prolonged exciton lifetimes are necessary. This work has been funded by the Office of Naval Research (ONR), directly and through the Naval Research Laboratory (NRL). F.T and E.S acknowledge support from NRL through the NRC Research Associateship Program and ONR Summer Faculty Program, respectively.
NASA Astrophysics Data System (ADS)
Bridges, F.; Downward, L.; Neumeier, J. J.; Tyson, T. A.
2010-05-01
We present detailed local structure measurements (using the extended x-ray absorption fine structure technique) for the colossal magnetoresistive material La1-xCaxMnO3 (0.21
Exciton effects in strained armchair graphene nanoribbons
NASA Astrophysics Data System (ADS)
Jia, Yonglei; Liu, Junlin
2016-01-01
The exciton effects in 1-nm-wide armchair graphene nanoribbons (AGNRs) under the uniaxial strain were studied within the nonorthogonal tight-binding (TB) model, supplemented by the long-range Coulomb interactions. The obtained results show that both the excitation energy and exciton binding energy are modulated by the uniaxial strain. The variation of these energies depends on the ribbon family. In addition, the results show that the variation of the exciton binding energy is much weaker than the variation of excitation energy. Our results provide new guidance for the design of optomechanical systems based on graphene nanoribbons.
Exciton Regeneration at Polymeric Semiconductor Heterojunctions
NASA Astrophysics Data System (ADS)
Morteani, Arne C.; Sreearunothai, Paiboon; Herz, Laura M.; Friend, Richard H.; Silva, Carlos
2004-06-01
Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2 3.1nm) which may collapse into an exciplex and then endothermically (EA=100 200 meV) back transfer towards the exciton.
NASA Astrophysics Data System (ADS)
Sadigh, Babak; Erhart, Paul; Ã berg, Daniel
2015-08-01
We conduct a detailed investigation of the polaron self-interaction (pSI) error in standard approximations to the exchange-correlation (XC) functional within density-functional theory (DFT). The pSI leads to delocalization error in the polaron wave function and energy, as calculated from the Kohn-Sham (KS) potential in the native charge state of the polaron. This constitutes the origin of the systematic failure of DFT to describe the polaron formation in band insulators. It is shown that the delocalization error in these systems is, however, largely absent in the KS potential of the closed-shell neutral charge state. This leads to a modification of the DFT total-energy functional that corrects the pSI in the XC functional. The resulting pSIC-DFT method constitutes an accurate parameter-free ab initio methodology for calculating polaron properties in insulators at a computational cost that is orders of magnitude smaller than hybrid XC functionals. Unlike approaches that rely on parametrized localized potentials such as DFT+U , the pSIC-DFT method properly captures both site and bond-centered polaron configurations. This is demonstrated by studying formation and migration of self-trapped holes in alkali halides (bond-centered) as well as self-trapped electrons in an elpasolite compound (site-centered). The pSIC-DFT approach consistently reproduces the results obtained by hybrid XC functionals parametrized by DFT+G0W0 calculations. Finally, we generalize the pSIC approach to hybrid functionals, and show that in stark contrast to conventional hybrid calculations of polaron energies, the pSIC-hybrid method is insensitive to the parametrization of the hybrid XC functional. On this basis, we further rationalize the success of the pSIC-DFT approach.
Exciton storage in type-II quantum dots using the optical Aharonov-Bohm effect
Climente, Juan I.; Planelles, Josep
2014-05-12
We investigate the bright-to-dark exciton conversion efficiency in type-II quantum dots subject to a perpendicular magnetic field. To this end, we take the exciton storage protocol recently proposed by Simonin and co-workers [Phys. Rev. B 89, 075304 (2014)] and simulate its coherent dynamics. We confirm the storage is efficient in perfectly circular structures subject to weak external electric fields, where adiabatic evolution is dominant. In practice, however, the efficiency rapidly degrades with symmetry lowering. Besides, the use of excited states is likely unfeasible owing to the fast decay rates. We then propose an adaptation of the protocol which does not suffer from these limitations.
Direct optical state preparation of the dark exciton in a quantum dot
NASA Astrophysics Data System (ADS)
Lüker, S.; Kuhn, T.; Reiter, D. E.
2015-11-01
Because of their weak coupling to the electromagnetic field, dark excitons in semiconductor quantum dots possess extremely long lifetimes, which makes them attractive candidates for quantum information processing. On the other hand, the preparation and manipulation of dark states is challenging, because commonly used optical excitation mechanisms are not applicable. We propose an efficient mechanism for the deterministic preparation of the dark exciton exploiting the application of a tilted magnetic field and the optical excitation with a chirped, i.e., frequency modulated, laser pulse.
Multiple Exciton Generation Solar Cells
Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.
2012-01-01
Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.
Singlet exciton fission in solution
NASA Astrophysics Data System (ADS)
Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
Singlet exciton fission in solution.
Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. PMID:24256865
Liquid-state polaron theory of the hydrated electron revisited
NASA Astrophysics Data System (ADS)
Donley, James P.; Heine, David R.; Tormey, Caleb A.; Wu, David T.
2014-07-01
The quantum path integral/classical liquid-state theory of Chandler and co-workers, created to describe an excess electron in solvent, is re-examined for the hydrated electron. The portion that models electron-water density correlations is replaced by two equations: the range optimized random phase approximation (RO-RPA), and the Donley, Rajasekaran, and Liu (DRL) approximation to the "two-chain" equation, both shown previously to describe accurately the static structure and thermodynamics of strongly charged polyelectrolyte solutions. The static equilibrium properties of the hydrated electron are analyzed using five different electron-water pseudopotentials. The theory is then compared with data from mixed quantum/classical Monte Carlo and molecular dynamics simulations using these same pseudopotentials. It is found that the predictions of the RO-RPA and DRL-based polaron theories are similar and improve upon previous theory, with values for almost all properties analyzed in reasonable quantitative agreement with the available simulation data. Also, it is found using the Larsen, Glover, and Schwartz pseudopotential that the theories give values for the solvation free energy that are at least three times larger than that from experiment.
Liquid-state polaron theory of the hydrated electron revisited
Donley, James P.; Heine, David R.; Tormey, Caleb A.; Wu, David T.
2014-07-14
The quantum path integral/classical liquid-state theory of Chandler and co-workers, created to describe an excess electron in solvent, is re-examined for the hydrated electron. The portion that models electron-water density correlations is replaced by two equations: the range optimized random phase approximation (RO-RPA), and the Donley, Rajasekaran, and Liu (DRL) approximation to the “two-chain” equation, both shown previously to describe accurately the static structure and thermodynamics of strongly charged polyelectrolyte solutions. The static equilibrium properties of the hydrated electron are analyzed using five different electron-water pseudopotentials. The theory is then compared with data from mixed quantum/classical Monte Carlo and molecular dynamics simulations using these same pseudopotentials. It is found that the predictions of the RO-RPA and DRL-based polaron theories are similar and improve upon previous theory, with values for almost all properties analyzed in reasonable quantitative agreement with the available simulation data. Also, it is found using the Larsen, Glover, and Schwartz pseudopotential that the theories give values for the solvation free energy that are at least three times larger than that from experiment.
Thermodynamics of the polaron master equation at finite bias
Krause, Thilo Brandes, Tobias; Schaller, Gernot; Esposito, Massimiliano
2015-04-07
We study coherent transport through a double quantum dot. Its two electronic leads induce electronic matter and energy transport and a phonon reservoir contributes further energy exchanges. By treating the system-lead couplings perturbatively, whereas the coupling to vibrations is treated non-perturbatively in a polaron-transformed frame, we derive a thermodynamic consistent low-dimensional master equation. When the number of phonon modes is finite, a Markovian description is only possible when these couple symmetrically to both quantum dots. For a continuum of phonon modes however, also asymmetric couplings can be described with a Markovian master equation. We compute the electronic current and dephasing rate. The electronic current enables transport spectroscopy of the phonon frequency and displays signatures of Franck-Condon blockade. For infinite external bias but finite tunneling bandwidths, we find oscillations in the current as a function of the internal bias due to the electron-phonon coupling. Furthermore, we derive the full fluctuation theorem and show its identity to the entropy production in the system.
Coupled polaronic and ion transport in nanocrystalline metal oxide electrodes
NASA Astrophysics Data System (ADS)
Rosso, Kevin
2012-02-01
We report new computational methods and fundamental understanding in the dynamics of coupled charge and ion transport in nanoscale metal oxides. The methods attack the multi-scale problem of simulating the collective diffusivities of ions and charge compensating e-/h+ carriers in single crystal particles, across particle-particle grain boundaries, and through networks of grains for select systems. Methods include embedded quantum mechanical clusters at the DFT and MP2 levels of theory for atomic-scale polaronic and ion transport kinetics, classical DFT-based free energy calculations for grain-scale conductivity in the framework of the Poisson-Nernst-Planck formalism, and phase field simulation of charged particle diffusivity for conductivity at the grain network scale. This combination of approaches is one of a kind in terms of its multi-scale range, scaling, and computational efficiency. We are presently focused on coupled electron and Li+ ion transport in polymorphs of TiO2, and also in mixed valence spinel oxides, for electrode conductivity optimization and improving energy storage materials performance for Li+ batteries.
Energy spectrum of the optical polaron at finite total momentum
NASA Astrophysics Data System (ADS)
Gerlach, B.; Kalina, F.
1999-10-01
In the following discussion we are concerned with the standard Fröhlich model for an optical polaron. We clarify the qualitative properties of the energy spectrum for arbitrary total momentum Q. Concerning the ground-state energy, we establish an effective lower bound. Until now, we have to assume that the electron-phonon coupling parameter α does not exceed a specified positive value. Using this bound, we demonstrate that the ground-state energy coincides with the continuum edge for \\|Q\\|>=\\|QC\\|, QC being finite. Consequently, it is only for \\|Q\\|<\\|QC\\| that an isolated ground state exists at all. This behavior is strikingly different from that of the corresponding system in lower dimensions, which has been analyzed previously by other authors, the discussion of the three-dimensional case remaining incomplete. Concerning the overall behavior of the ground-state energy as a function of Q and α, we find an increase (strict decrease) with increasing \\|Q\\|(α). In addition, we present an approach to the excited states. Interestingly enough, this can be based entirely on the knowledge of the ground-state energy and ground-state wave function.
Polaronic absorption in Sr2IrO4
NASA Astrophysics Data System (ADS)
Sohn, Chang Hee; Qi, Tong-Fei; Noh, Kyung Joo; Park, Hyun-Ju; Yoo, Hyang Keun; Cao, Gang; Kim, Kyung Wan; Cho, Deok-Yong; Moon, Soon Jae; Noh, Tae Won
2014-03-01
Sr2IrO4 has received much attention as a novel Jeff = 1/2 Mott insulator. Many theorists have supposed that exotic novel ground state such as superconductivity, topological insulator, and quantum spin liquid could emerge in Jeff = 1/2 state. However, despite of great interests on Sr2IrO4, the ground state of this material is elusive up to now. Unlike previous Mott scenario, recent reports support that Sr2IrO4 can be described as Slater insulator rather than Mott insulator. The origin of temperature evolutions of electronic structure shown in many experiments also remains vague until now. Here, we investigated the detail temperature evolution of electronic structure of Sr2IrO4 using infrared spectroscopy. We couldn't observe any anomaly in optical conductivity near the TN, which is not consistent with recent reports. Instead, we observed the continuous changes in our optical data which can be explained in terms of polaronic behavior, closely related to La2CuO4.
Light induced polaron formation in perovskite solar cell devices
NASA Astrophysics Data System (ADS)
Neukirch, Amanda; Nie, Wanyi; Blancon, Jean-Christophe; Appavoo, Kannatassen; Tsai, Hsinhan; Chhowalla, Manish; Alam, Muhammad; Sfeir, Matthew; Katan, Claudine; Even, Jacky; Crochet, Jared; Gupta, Gautum; Mohite, Aditya; Tretiak, Sergei
The need for a low-cost, clean, and abundant source of energy has generated large amounts of research in solution processed solar cell materials. The lead halide perovskite has rapidly developed as a serious candidate for the active layer of photovoltaic devices. The efficiencies of devices made with this material have increased from 3.5% to over 20% in around 5 years. Despite the remarkable progress associated with perovskite materials, there are still fundamental questions regarding their lack of photo-stability over prolonged solar irradiation that need to be addressed. Recent experiments on photo-degradation under constant illumination have found fast self-healing by resting the device in the dark for less than 1 minute. Density functional theory and symmetry analysis show that localized charge states couple to local structural lattice distortions and methyl ammonium quasistatic configurations. Once translational symmetry is lost, additional bonding configurations become symmetry allowed, triggering localized charges in the vicinity over time under constant illumination, thus seeding the formation of macroscopic charged domains and preventing efficient charge extraction. Here we present an in-depth study of polaron formation and binding energy at the atomistic level.
Interferometric measurement of many-body topological invariants using polarons
NASA Astrophysics Data System (ADS)
Grusdt, Fabian; Yao, Norman; Abanin, Dmitry; Demler, Eugene
2014-05-01
We present a scheme for the direct detection of many-body topological invariants in ultra cold quantum gases in optical lattices. We generalize single-particle interferometric schemes developed for the detection of topologically non-trivial band structures [Atala et al., Nature Physics 9, 795 (2013)] by coupling a spin-1/2 impurity to a (topological) excitation of an interacting many-body system. Performing Ramsey interferometry in combination with Bloch oscillations of the resulting polaronic particle allows to directly detect the many body-topological invariant. In particular we consider adiabatic Thouless pumps in the super-lattice Bose-Hubbard model, which transport a quantized amount of particles across a one-dimensional lattice. In the presence of inter-atomic interactions this quantized current is given by a many-body Chern number, which can be measured using our protocol. These systems also support symmetry-protected topological phases, the invariants of which can be obtained from our protocol as well.
Polaronic Superlattice Formed on Oxidised Magnetite (111) Surface
NASA Astrophysics Data System (ADS)
Berdunov, N.; Mariotto, G.; Murphy, S.; Ceballos, S. F.; Jordan, K.; Shvets, I. V.
2003-12-01
We present direct experimental evidence of the formation of a superstructure on the (111) surface of a magnetite, Fe3O4 single crystal. The superstructure, which has a periodicity of 42 A and three-fold symmetry has been observed by means of STM and LEED. Under the correct conditions of oxygen pressure and sample anneal temperature the superstructure is reproducibly formed throughout most of the sample surface. The characteristics of the superstructure, including its dependency on the tunnel bias voltage and its atomic scale periodicity, suggest that it is an electronic effect rather than a mosaic of several iron oxide phases. We explain the results in terms of the formation of giant static polarons, although we notice that other types of electron-lattice instabilities such as charge density wave may offer possible explanations. We suggest two possible scenarios of instability linking the electron band structure and lattice distortions in magnetite: either resulting from reallocation of Fe2+ and Fe3+ valence states between octahedral sites or alternatively from reallocation between octahedral and tetrahedral sites.
Hybrid interlayer excitons with tunable dispersion relation
NASA Astrophysics Data System (ADS)
Skinner, Brian
When two semiconducting materials are layered on top of each other, interlayer excitons can be formed by the Coulomb attraction of an electron in one layer to a hole in the opposite layer. The resulting exciton is a composite boson with a dispersion relation that is a hybrid between the dispersion relations of the electron and the hole separately. In this talk I show how such hybridization is particularly interesting when one layer has a ``Mexican hat''-shaped dispersion relation and the other has a conventional parabolic dispersion. In this case the interlayer exciton can have a range of qualitatively different dispersion relations, which can be continuously altered by an external field. This tunability in principle allows one to continuously tune a collection of interlayer excitons between different quantum many-body phases, including Bose-Einstein condensate, Wigner crystal, and fermion-like ``moat band'' phases.
Triplet excitons: Bringing dark states to light
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-11-01
Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.
Geminate and non-geminate recombination of triplet excitons formed by singlet fission
NASA Astrophysics Data System (ADS)
Bayliss, Sam L.; Chepelianskii, Alexei; Sepe, Alessandro; Ehrler, Bruno; Walker, Brian J.; Bruzek, Matt J.; Anthony, John E.; Greenham, Neil C.
2014-03-01
Singlet fission is a promising route to enhance solar cells by harvesting two electron-hole pairs from high-energy photons. Through singlet fission, an optically generated singlet exciton is transformed into two spin-correlated triplet excitons, which serve as a unique signature of the process. We use optically detected magnetic resonance to identify and study triplet excitons created through singlet fission in the solution-processable small molecule TIPS-tetracene (bis(triisopropylsilylethynyl)tetracene). Through changes in photoluminescence under spin resonance, we identify geminate recombination of triplet pairs directly following singlet fission, as well as recombination from bimolecular triplet-triplet annihilation. We show that both processes can be present in spin-coated films, and correlate the two distinct annihilation pathways to film morphology.
External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell.
Congreve, Daniel N; Lee, Jiye; Thompson, Nicholas J; Hontz, Eric; Yost, Shane R; Reusswig, Philip D; Bahlke, Matthias E; Reineke, Sebastian; Van Voorhis, Troy; Baldo, Marc A
2013-04-19
Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers. PMID:23599489
Srivastava, Ajit; Htoon, Han; Klimov, Victor I; Kono, Junichiro
2008-08-22
We report the direct observation of spin-singlet dark excitons in individual single-walled carbon nanotubes through low-temperature micro-magneto-photoluminescence spectroscopy. A magnetic field (B) applied along the tube axis brightened the dark state, leading to the emergence of a new emission peak. The peak rapidly grew in intensity with increasing B at the expense of the originally dominated bright exciton peak and became dominant at B>3 T. This behavior, universally observed for more than 50 tubes of different chiralities, can be quantitatively modeled by incorporating the Aharonov-Bohm effect and intervalley Coulomb mixing. The directly measured dark-bright splitting values were 1-4 meV for tube diameters 1.0-1.3 nm. Scatter in the splitting value emphasizes the role of the local environment surrounding a nanotube in determining its excitonic fine structure. PMID:18764659
Ultrafast exciton dynamics at molecular surfaces
NASA Astrophysics Data System (ADS)
Monahan, Nicholas R.
Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge
Theory of Orbital Susceptibility on Excitonic Insulator
NASA Astrophysics Data System (ADS)
Matsuura, Hiroyasu; Ogata, Masao
2016-09-01
We study the temperature dependence of the orbital susceptibility of an excitonic insulator on the basis of a two-band model. It is shown that a drastic change (an anomalous enhancement) in susceptibility as a function of temperature occurs owing to the occurrence of additional orbital susceptibility due to the excitonic gap. We calculate explicitly the temperature dependence of orbital susceptibility for a model of Ta2NiSe5, and show that the result is consistent with experimental results.
Exciton properties of selected aromatic hydrocarbon systems
NASA Astrophysics Data System (ADS)
Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin
2013-02-01
We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.
Exciton transport by surface acoustic waves
NASA Astrophysics Data System (ADS)
Rudolph, J.; Hey, R.; Santos, P. V.
2007-05-01
Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.
Tunable Polarons of Slow-Light Polaritons in a Two-Dimensional Bose-Einstein Condensate.
Grusdt, Fabian; Fleischhauer, Michael
2016-02-01
When an impurity interacts with a bath of phonons it forms a polaron. For increasing interaction strengths the mass of the polaron increases and it can become self-trapped. For impurity atoms inside an atomic Bose-Einstein condensate (BEC) the nature of this transition is not understood. While Feynman's variational approach to the Fröhlich model predicts a sharp transition for light impurities, renormalization group studies always predict an extended intermediate-coupling region characterized by large phonon correlations. To investigate this intricate regime and to test polaron physics beyond the validity of the Fröhlich model we suggest a versatile experimental setup that allows us to tune both the mass of the impurity and its interactions with the BEC. The impurity is realized as a dark-state polariton (DSP) inside a quasi-two-dimensional BEC. We show that its interactions with the Bogoliubov phonons lead to photonic polarons, described by the Bogoliubov-Fröhlich Hamiltonian, and make theoretical predictions using an extension of a recently introduced renormalization group approach to Fröhlich polarons. PMID:26894712
Tunable Polarons of Slow-Light Polaritons in a Two-Dimensional Bose-Einstein Condensate
NASA Astrophysics Data System (ADS)
Grusdt, Fabian; Fleischhauer, Michael
2016-02-01
When an impurity interacts with a bath of phonons it forms a polaron. For increasing interaction strengths the mass of the polaron increases and it can become self-trapped. For impurity atoms inside an atomic Bose-Einstein condensate (BEC) the nature of this transition is not understood. While Feynman's variational approach to the Fröhlich model predicts a sharp transition for light impurities, renormalization group studies always predict an extended intermediate-coupling region characterized by large phonon correlations. To investigate this intricate regime and to test polaron physics beyond the validity of the Fröhlich model we suggest a versatile experimental setup that allows us to tune both the mass of the impurity and its interactions with the BEC. The impurity is realized as a dark-state polariton (DSP) inside a quasi-two-dimensional BEC. We show that its interactions with the Bogoliubov phonons lead to photonic polarons, described by the Bogoliubov-Fröhlich Hamiltonian, and make theoretical predictions using an extension of a recently introduced renormalization group approach to Fröhlich polarons.
First-principles study of hole polaron formation and migration in SrI2
NASA Astrophysics Data System (ADS)
Zhou, Fei; Sadigh, Babak; Aberg, Daniel
2015-03-01
We investigate the formation of self-trapped holes (STH) in the high performance scintillator material SrI2 using a recently developed first principles method, polaron self-interaction correction (pSIC). pSIC removes the significant spurious self-interaction of localized polaron states. It is capable of accurately reproduce the configurational energy landscape of polaronic states from optimized hybrid functionals at the computational cost of the local density approximation. We searched for and identified all symmetrically distinct STH states localized on neighboring I-I dimers, i.e. Vk centers, and found non-trivial relation between the STH formation energies and dimer separation. All possible polaron hopping paths of the type IAIB -->IBIC are investigated systematically with pSIC and the elastic band method, and paths with low migration barrier energy of about 0.2 eV were identified, suggesting high mobility in SrI2. We expect that the present approach can be applied to study polaron formation and migration in other materials. Support from the National Nuclear Security Administration Office of Nonproliferation Research and Development (NA-22) is acknowledged. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore N We acknowledge funding from the NA-22 agency.
Orientation of luminescent excitons in layered nanomaterials
NASA Astrophysics Data System (ADS)
Schuller, Jon A.; Karaveli, Sinan; Schiros, Theanne; He, Keliang; Yang, Shyuan; Kymissis, Ioannis; Shan, Jie; Zia, Rashid
2013-04-01
In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling--MoS2 and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS2 mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.
Orientation of luminescent excitons in layered nanomaterials.
Schuller, Jon A; Karaveli, Sinan; Schiros, Theanne; He, Keliang; Yang, Shyuan; Kymissis, Ioannis; Shan, Jie; Zia, Rashid
2013-04-01
In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling-MoS₂ and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS₂ mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations. PMID:23455984
Exciton Resonances in Novel Silicon Carbide Polymers
NASA Astrophysics Data System (ADS)
Burggraf, Larry; Duan, Xiaofeng
2015-05-01
A revolutionary technology transformation from electronics to excitionics for faster signal processing and computing will be advantaged by coherent exciton transfer at room temperature. The key feature required of exciton components for this technology is efficient and coherent transfer of long-lived excitons. We report theoretical investigations of optical properties of SiC materials having potential for high-temperature excitonics. Using Car-Parinello simulated annealing and DFT we identified low-energy SiC molecular structures. The closo-Si12C12 isomer, the most stable 12-12 isomer below 1100 C, has potential to make self-assembled chains and 2-D nanostructures to construct exciton components. Using TDDFT, we calculated the optical properties of the isomer as well as oligomers and 2-D crystal formed from the isomer as the monomer unit. This molecule has large optical oscillator strength in the visible. Its high-energy and low-energy transitions (1.15 eV and 2.56 eV) are nearly pure one-electron silicon-to-carbon transitions, while an intermediate energy transition (1.28 eV) is a nearly pure carbon-to-silicon one-electron charge transfer. These results are useful to describe resonant, coherent transfer of dark excitons in the nanostructures. Research supported by the Air Force Office of Scientific Research.
NASA Astrophysics Data System (ADS)
Reynolds, D. C.; Jogai, B.; Collins, T. C.
2002-05-01
Many of the previous investigations of longitudinal excitons have involved reflection and absorption measurements rather than emission. In these measurements it is more difficult to resolve the longitudinal exciton from the Γ5 and Γ6 free excitons in wurtzite material. The longitudinal excitons have energies and oscillator strengths that depend on the direction of propagation and they are not observable along the principal axis of the crystal. In the wurtzite structure, such as GaN, the Γ5 exciton is the pure transverse mode, whereas the longitudinal is a mixed mode going from pure longitudinal, for the propogation direction K perpendicular to C, to pure transverse for K parallel to C. If more than one orientation is present in the sample, it is clear that more than one longitudinal exciton may be seen since it is a mixed mode. In the current experiment we observe more than one mode, which we associate with more than one crystal orientation. This may result from the columnar growth often observed in GaN.
Exciton formation and diffusion in OLEDs (Presentation Recording)
NASA Astrophysics Data System (ADS)
Ingram, Grayson L.; Lu, Zheng-Hong
2015-10-01
This talk will discuss recent experiments designed to study the formation of excitons and their subsequent diffusions in OLEDs. These experimental results suggest that contrary to conventional wisdom, host singlet exciton diffusion can occur over long distances, while host triplet excitons are confined close to the exciton formation region for the archetype host and hole transport layer CBP. The exciton formation mechanism is studied and we show that the ratio of excitons formed on the host to excitons formed on the dopant varies strongly with the applied voltage. Refinements to models of efficiency roll off are discussed in light of the improved understanding of exciton formation and we suggest design guidelines to improve efficiency by engineering exciton formation.
NASA Astrophysics Data System (ADS)
Kalosakas, G.; Aubry, S.; Tsironis, G. P.
1998-10-01
We use a stationary and normal mode analysis of the semiclassical Holstein model in order to connect the low-frequency linear polaron modes to low-lying far-infrared lines of the acetanilide spectrum and through parameter fitting we comment on the validity of the polaron results in this system.
Interaction of Dirac Fermion excitons and biexciton-exciton cascade in graphene quantum dots
NASA Astrophysics Data System (ADS)
Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel
2015-03-01
We present a microscopic theory of interacting Dirac quasi-electrons and quasi-holes confined in graphene quantum dots. The single particle states of quantum dots are described using a tight binding model and screened direct, exchange, and scattering Coulomb matrix elements are computed using Slater pz orbitals. The many-body ground and excited states are expanded in a finite number of electron-hole pair excitations from the Hartree-Fock ground state and computed using exact diagonalization techniques. The resulting exciton and bi-exciton spectrum reflects the degeneracy of the top of the valence and bottom of the conduction band characteristic of graphene quantum dots with C3 symmetry. We study the interaction of multi-electron and hole complexes as a function of quantum dot size, shape and strength of Coulomb interactions. We identify two degenerate bright exciton (X) states and a corresponding biexciton (XX) state as XX-X cascade candidates, a source of entangled photon pairs. We next calculate the exciton to bi-exciton transitions detected in transient absorption experiments to extract the strength of exciton-exciton interactions and biexciton binding energies. We further explore the possibility of excitonic instability.
Circuit-QED-based superconducting quantum simulator for the Holstein-polaron model
NASA Astrophysics Data System (ADS)
Mei, Feng; Stojanović, Vladimir; Siddiqi, Irfan; Tian, Lin
2014-03-01
We propose an analog quantum simulator for the Holstein molecular-crystal model based on a superconducting circuit-QED system in the dispersive regime. The many-body Hamiltonian of this model includes both bosonic and fermionic degrees of freedom. By varying the driving field on the superconducting resonators, one can readily access both the adiabatic and anti-adiabatic regimes of this model, and reach the strong e-ph coupling limit required for small-polaron formation. We show that small-polaron state of arbitrary quasimomentum can be generated by applying a microwave pulse to the resonators. We also show that significant squeezing in the resonator modes can be achieved in the polaron-crossover regime through a measurement-based scheme. The project was supported by NSF-0956064, NSF-0916303, SNSF, NCCR QSIT, and NSF-0939514.
Importance of Polaronic Effects for Charge Transport in CdSe Quantum Dot Solids.
Prodanović, Nikola; Vukmirović, Nenad; Ikonić, Zoran; Harrison, Paul; Indjin, Dragan
2014-04-17
We developed an accurate model accounting for electron-phonon interaction in colloidal quantum dot supercrystals that allowed us to identify the nature of charge carriers and the electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids, the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping. PMID:26269977
Bloch oscillations as generators of polarons in a 1D crystal
NASA Astrophysics Data System (ADS)
Nazareno, H. N.; Brito, P. E. de
2016-08-01
The main purpose of this work is to characterize the kind of propagation/localization of carriers in a one-dimensional crystalline structure along the tight-binding model while the electron-phonon interaction is taken into account through a deformation potential and the system is under the action of a dc electric field. The lattice was treated in the classical formalism of harmonic vibrations. A remarkable effect is obtained due to the presence of the electric field. On one side the particle performs Bloch oscillations and at the same time it interacts with the lattice and as a result at each turning point of its trajectory phonons are generated that carry with them a fraction of the electronic wave packet, it is the polaron formation. This way the Bloch oscillations pump polarons into the system. We explain why the polaron is formed at returning points of the oscillations.
Eersel, H. van Coehoorn, R.; Bobbert, P. A.; Janssen, R. A. J.
2014-10-06
We present an advanced molecular-scale organic light-emitting diode (OLED) model, integrating both electronic and excitonic processes. Using this model, we can reproduce the measured efficiency roll-off for prototypical phosphorescent OLED stacks based on the green dye tris[2-phenylpyridine]iridium (Ir(ppy){sub 3}) and the red dye octaethylporphine platinum (PtOEP) and study the cause of the roll-off as function of the current density. Both the voltage versus current density characteristics and roll-off agree well with experimental data. Surprisingly, the results of the simulations lead us to conclude that, contrary to what is often assumed, not triplet-triplet annihilation but triplet-polaron quenching is the dominant mechanism causing the roll-off under realistic operating conditions. Simulations for devices with an optimized recombination profile, achieved by carefully tuning the dye trap depth, show that it will be possible to fabricate OLEDs with a drastically reduced roll-off. It is envisaged that J{sub 90}, the current density at which the efficiency is reduced to 90%, can be increased by almost one order of magnitude as compared to the experimental state-of-the-art.
Electronic structure and polaronic excitation in FeVO{sub 4}
Dixit, A.; Lawes, G.; Chen, P.; Musfeldt, J. L.
2011-10-03
We investigated the electronic properties of FeVO{sub 4} films using optical, valence band x-ray photoelectron, and infrared spectroscopies. These studies show that FeVO{sub 4} is a direct bandgap system with a 2.7 eV gap with the Fermi level in the middle of the valence band maximum and conduction band minimum. A polaronic excitation is also observed in the middle infrared, indicating the importance of charge-lattice coupling in this multiferroic material. Fits to a model for the optical response of large polarons yield a binding energy of approximately 130 meV.
High temperature electrical conductivity due to small polaron hopping motion in DNA molecules
NASA Astrophysics Data System (ADS)
Triberis, G. P.; Karavolas, V. C.; Simserides, C. D.
2005-01-01
We present a small polaron hopping model to interpret the high-temperature electrical conductivity measured along the DNA molecules. The model takes into account the one-dimensional character of the system and the presence of disorder in the DNA double helix. The experimental data for the lambda phage DNA (λ-DNA) and the poly(dA)-poly(dT) DNA follow nicely the theoretically predicted behavior leading to realistic values of the maximum hopping distances supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbors of the DNA molecular "wire".
Effects of Shannon entropy and electric field on polaron in RbCl triangular quantum dot
NASA Astrophysics Data System (ADS)
M, Tiotsop; A, J. Fotue; S, C. Kenfack; N, Issofa; H, Fotsin; L, C. Fai
2016-04-01
In this paper, the time evolution of the quantum mechanical state of a polaron is examined using the Pekar type variational method on the condition of the electric-LO-phonon strong-coupling and polar angle in RbCl triangular quantum dot. We obtain the eigenenergies, and the eigenfunctions of the ground state, and the first excited state respectively. This system in a quantum dot can be treated as a two-level quantum system qubit and the numerical calculations are performed. The effects of Shannon entropy and electric field on the polaron in the RbCl triangular quantum dot are also studied.
Improving Li2O2 conductivity via polaron preemption: An ab initio study of Si doping
NASA Astrophysics Data System (ADS)
Timoshevskii, Vladimir; Feng, Zimin; Bevan, Kirk H.; Goodenough, John; Zaghib, Karim
2013-08-01
We report on ab initio electronic structure simulations of Li2O2, where 1.6% of lithium atoms are substituted by silicon. It is demonstrated that this leads to the formation of conducting impurity states in the band gap of Li2O2. We show that these states originate from the antibonding orbitals of the oxygen pairs and are remarkably stable against possible polaron formation (upon electron injection). Through this polaron preemption mechanism, the proposed compound is expected to show significantly higher electronic mobility than stoichiometric Li2O2, which could have significant applications in lithium-air batteries.
Exciton diamagnetic shifts and valley Zeeman effect in monolayer WS2 and MoS2 to 65Tesla
NASA Astrophysics Data System (ADS)
Stier, A. V.; McCreary, K. A.; Jonker, B. T.; Kono, J.; Crooker, S. A.
We report circularly-polarized optical reflection spectroscopy of monolayer WS2 and MoS2 at low temperatures (4 K) and in high magnetic fields to 65 T. Both the A and the B exciton transitions exhibit a clear and very similar Zeeman splitting of approximately -230 μeV/T (g ~= - 4), providing the first measurements of the valley Zeeman effect and associated g-factors in monolayer transition-metal disulphides. These results complement and are compared with recent low-field photoluminescence measurements of valley degeneracy breaking in the monolayer diselenides MoSe2 and WSe2. Further, the very large magnetic fields used in our studies allows us to observe the small quadratic diamagnetic shifts of the A and B excitons in monolayer WS2 (0.32 and 0.11 μeV/T2, respectively), from which we calculate exciton radii of 1.53 nm and 1.16 nm. When analyzed within a model of non-local dielectric screening in monolayer semiconductors, these diamagnetic shifts also constrain and provide estimates of the exciton binding energies (410 meV and 470 meV for the A and B excitons, respectively), further highlighting the utility of high magnetic fields for understanding new 2D materials.
Somoza Márquez, Alejandro; Chen, Lipeng; Sun, Kewei; Zhao, Yang
2016-07-27
The chlorosome antenna complex is a fascinating structure which due to its immense scale, accurate simulation of excitation energy transfer (EET) dynamics supposes a genuine computational challenge. Resonant vibronic modes have been recently identified in 2D spectra of the chlorosome which motivates our present endeavour of modelling electronic and vibrational degrees of freedom on an equal footing. Following the Dirac-Frenkel time-dependent variational principle, we exploit a general theory of polaron dynamics in two-dimensional lattices based on the Holstein molecular crystal model and investigate a single rod model of pigment aggregates. Unlike reduced formalisms, explicit integration of the degrees of freedom of both the system and the bath requires extensive computational resources. We exploit the architecture of graphic processor units (GPUs) by implementing our simulations on this platform. The simulation of dynamic properties of hundreds or even thousands of pigments is thus achievable in just a few hours. The potential investigation and design of natural or engineered two-dimensional pigment networks can thus be accommodated. Due to the lack of consensus regarding the precise arrangement of chromophores in the chlorosome, helicity and dimerization are investigated independently, extracting their contributions to both optical and EET properties. The presence of dimerization is found to slow down the delocalization process. Exciton delocalization is completed in 100 fs in a single rod aggregate whose dimensions (20 nm) fairly exceed the estimated extent of a coherent domain. Ultrafast energy relaxation in the exciton manifold occurs in 50 fs and the duration of super-diffusive transport is found to last for about 80 fs. PMID:26792106
Aharonov-Bohm Beats in Excitonic Luminescence from Quantum Rings and Type-II Quantum Dots
NASA Astrophysics Data System (ADS)
Dias da Silva, Luis; Shahbazyan, Tigran
2005-03-01
We study the absorption spectrum of neutral magnetoexcitons confined in ring-like structures. Despite their neutral character, excitons exhibit strong modulation effects on the energy and oscillator strength in the presence of magnetic fields [1] that have been recently observed [2]. We calculate the absorption coefficient α for neutral excitons confined in circular ring geometries with radii Re for electrons and Rh for holes. A particularly interesting situation comes about when Re!=Rh and a net radial charge polarization arises. In this case, we consider an attractive Coulomb interaction proportional to (Re- Rh)-1 and the excitonic absorption peak shows oscillatory behavior as function of the applied magnetic field both in position and amplitude. Such oscillations strongly depend on the dipole moment P=e(Rh-Re) of the exciton and on the dielectric constant of the system. Such intensity changes could in principle be experimentally observed with single dot spectroscopy in quantum rings [3]. Supported by the NSF-IMC and NSF-RUI [1] A.O. Govorov et al. Phys. Rev. B 66 081309 (2002); A.O. Govorov et al. Physica E 13, 297 (2002). [2] E. Ribeiro et al. Phys Rev. Lett. 92 126402 (2004). [3] R.J. Warburton et al. Nature 405 (6789) 926 (2000).
Excitons in nanorings. The signature of quantum phases in optical emission
NASA Astrophysics Data System (ADS)
Govorov, Alexander; Ulloa, Sergio
2002-03-01
When a quantum charged particle moves along a closed trajectory in an external magnetic field, the Aharonov-Bohm (AB) effect can occur, caused by quantum interference between paths with different phases. However, excitons in solids are neutral and hence a cursory analysis would not predict the AB effect. Nevertheless, we demonstrate here that a strong magnetic interference effect for neutral particles can exist, such as the polarizable excitons in quantum-ring structures. This phenomenon is akin to the Aharonov-Anandan quantum phases acquired under a cyclic evolution of the Hamiltonian. With increasing normal magnetic field, the ground state of the exciton acquires a nonzero angular momentum due to a net microscopic phase in the electron-hole pair. Under these conditions, the photoluminescence becomes suppressed in well-defined intervals of magnetic field, which depend on the degree of correlation of the two-particle motion. We demonstrate theoretically this novel magnetic interference effect using models of type-II quantum dots and InAs self-assembled quantum rings [1]. 1. A. Lorke et al., Phys. Rev. Lett. 84, 2223 (2000).
Optically Detected Magnetic Resonance Studies on π-conjugated semiconductor systems
Chen, Ying
2011-01-01
Optically Detected Magnetic Resonance (ODMR) techniques were used to investigate the dynamics of excitons and charge carriers in π-conjugated organic semiconductors. Degradation behavior of the negative spin-1/2 electroluminescence-detected magnetic resonance (ELDMR) was observed in Alq3 devices. The increase in the resonance amplitude implies an increasing bipolaron formation during degradation, which might be the result of growth of charge traps in the device. The same behavior of the negative spin-1/2 ELDMR was observed in 2wt% Rubrene doped Tris(8-hydroxyquinolinato)aluminium (Alq3) devices. However, with increasing injection current, a positive spin-1/2 ELDMR, together with positive spin 1 triplet powder patterns at Δm_{S}=±1 and Δm_{S}=±2, emerges. Due to the similarities in the frequency dependences of single and double modulated ELDMR and the photoluminescence-detected magnetic resonance (PLDMR) results in poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenyl ene vinylene] (MEH-PPV) films, the mechanism for this positive spin-1/2 ELDMR was assigned to enhanced triplet-polaron quenching under resonance conditions. The ELDMR in rubrene doped Alq3 devices provides a path to investigate charge distribution in the device under operational conditions. Combining the results of several devices with different carrier blocking properties and the results from transient EL, it was concluded trions not only exist near buffer layer but also exist in the electron transport layer. This TPQ model can also be used to explain the positive spin-1/2 PLDMR in poly(3-hexylthiophene) (P3HT) films at low temperature and in MEH-PPV films at various temperatures up to room temperature. Through quantitative analysis, TE-polaron quenching (TPQ) model is shown having the ability to explain most behaviors of the positive spin-1/2 resonance. Photocurrent detected magnetic resonance (PCDMR) studies on MEH-PPV devices revealed a novel transient resonance signal. The signal
Photoinduced gap closure in an excitonic insulator
NASA Astrophysics Data System (ADS)
Golež, Denis; Werner, Philipp; Eckstein, Martin
2016-07-01
We study the dynamical phase transition out of an excitonic insulator phase after photoexcitation using a time-dependent extension of the self-consistent GW method. We connect the evolution of the photoemission spectra to the dynamics of the excitonic order parameter and identify two dynamical phase transition points marked by a slowdown in the relaxation: one critical point is connected with the trapping in a nonthermal state with reduced exciton density and the second corresponds to the thermal phase transition. The transfer of kinetic energy from the photoexcited carriers to the exciton condensate is shown to be the main mechanism for the gap melting. We analyze the low energy dynamics of screening, which strongly depends on the presence of the excitonic gap, and argue that it is difficult to interpret the static component of the screened interaction as the effective interaction of some low energy model. Instead we propose a phenomenological measure for the effective interaction which indicates that screening has minor effects on the low energy dynamics.
Plasmon-Exciton Coupling Using DNA Templates.
Roller, Eva-Maria; Argyropoulos, Christos; Högele, Alexander; Liedl, Tim; Pilo-Pais, Mauricio
2016-09-14
Coherent energy exchange between plasmons and excitons is a phenomenon that arises in the strong coupling regime resulting in distinct hybrid states. The DNA-origami technique provides an ideal framework to custom-tune plasmon-exciton nanostructures. By employing this well controlled self-assembly process, we realized hybrid states by precisely positioning metallic nanoparticles in a defined spatial arrangement with fixed nanometer-sized interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60 nm while keeping their separation distance constant allowed us to precisely adjust the plasmon resonance of the structure to accurately match the energy frequency of a J-aggregate exciton. With this system we obtained strong plasmon-exciton coupling and studied far-field scattering at the single-structure level. The individual structures displayed normal mode splitting up to 170 meV. The plasmon tunability and the strong field confinement attained with nanodimers on DNA-origami renders an ideal tool to bottom-up assembly plasmon-exciton systems operating at room temperature. PMID:27531635
Pairing interaction effects in exciton level densities
Fu, C.Y.
1989-01-01
Recent progress in pairing corrections for exciton state-density formulas used in pre-compound nuclear reaction theories is reviewed. These correction factors are, strictly speaking, dependent on the nuclear excitation energy U and the exciton number n. A simple formula for (U,n)-dependent pairing corrections has been derived, based on the BCS pairing equations for constant single-particle spacing, for the exciton state-density formula for one kind of Fermion. It has been shown that the constant-pairing-energy correction used in standard state-density formulas, such U{sub 0} in Gilbert and Cameron, is a limiting case of the general (U,n)-dependent results. Spin cutoff factors with pairing effects were also obtained using the same theory and parameterized into an explicit (U,n)-dependent function, thereby defining a simple exciton level-density formula for applications in quantum mechanical precompound theories. Preliminary results from extending such simple pairing-interaction representations to level-density formulas for two kinds of Fermions are summarized. The results show that the ratios in the exciton level densities in the one-Fermion and two-Fermion approaches vary with both U and n, thus likely leading to differences in calculated compound to precompound ratios. However, the differences in the spin cutoff factors in the two cases are found to be rather small. 12 refs., 3 figs.
Attractive Coulomb interaction of two-dimensional Rydberg excitons
NASA Astrophysics Data System (ADS)
Shahnazaryan, V.; Shelykh, I. A.; Kyriienko, O.
2016-06-01
We analyze theoretically the Coulomb scattering processes of highly excited excitons in the direct-band-gap semiconductor quantum wells. We find that contrary to the interaction of ground-state excitons, the electron and hole exchange interaction between excited excitons has an attractive character both for s - and p -type two-dimensional (2D) excitons. Moreover, we show that similar to the three-dimensional highly excited excitons, the direct interaction of 2D Rydberg excitons exhibits van der Waals-type long-range interaction. The results predict the linear growth of the absolute value of exchange interaction strength with an exciton principal quantum number and point the way towards enhancement of optical nonlinearity in 2D excitonic systems.
NASA Astrophysics Data System (ADS)
Balanta, M. A. G.; Kopaczek, J.; Orsi Gordo, V.; Santos, B. H. B.; Rodrigues, A. D.; Galeti, H. V. A.; Richards, R. D.; Bastiman, F.; David, J. P. R.; Kudrawiec, R.; Galvão Gobato, Y.
2016-09-01
Raman spectroscopy and magneto-photoluminescence measurements under high magnetic fields were used to investigate the optical and spin properties of GaBiAs/GaAs multiple quantum wells (MQWs). An anomalous negative diamagnetic energy shift was observed at higher temperatures and higher laser intensities, which was associated to a sign inversion of hole effective mass in these structures. In addition, an enhancement of the polarization degree with decreasing of laser intensity was observed (experimental condition where the emission is dominated by localized excitons). This effect was explained by changes of spin relaxation and exciton recombination times due to exciton localization by disorder.
Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires
Ma, Yingzhong; Xiao, Kai; Shaw, Robert W
2012-01-01
Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.
Identification of effective exciton-exciton annihilation in squaraine-squaraine copolymers.
Hader, Kilian; May, Volkhard; Lambert, Christoph; Engel, Volker
2016-05-11
Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton-exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine-squaraine copolymers. Transient absorption time traces measured at different laser powers exhibit an unusual time-dependence. The analysis points towards dynamics taking place on three time-scales. Immediately after laser-excitation a localization of excitons takes place within the femtosecond time-regime. This is followed by exciton-exciton annihilation which is responsible for a fast decay of the exciton population. At later times, excitations being localized on units which are not directly connected remain so that diffusion dominates the dynamics and leads to a slower decay. We thus provide evidence for EEA tracked by time-resolved spectroscopy which has not been reported that clearly before. PMID:27120976
NASA Astrophysics Data System (ADS)
Xue, Fei; Wu, Feng Cheng; MacDonald, Allan
BEC of excitons and polaritons have drawn attention in recent years because of the demonstration of their ability to host macroscopic quantum phenomena and because of their promise for applications. We study the case of a system containing two TMD monolayers that are separated and surrounded by h-BN. Under appropriate conditions this system is expected to support a spatially indirect thermal equilibrium exciton condensate. We combine a microscopic mean-field calculation and a weakly interacting boson model to explore the bilayer exciton condensates phase diagram. By varying the layer separation and exciton density, we find a phase transition occurs between states containing one and two condensate flavors. We also use a microscopic time-dependent mean-field theory to address condensate collective mode spectra and quantum fluctuations. Next we study the case of exciton-polariton formed by strong coupling between quantum well excitons and confined photon modes when the system is placed in a vertical microcavity. We build a microscopic mean-field theory starting from electrons and holes, and account for their coupling to coherent light field. We compare our model with the normal weakly interacting boson model that starts from weakly interacting excitons that are coupled to photons. This work was supported by the SRC and NIST under the Nanoelectronic Research Initiative (NRI) and SWAN, by the Welch Foundation under Grant No. F1473, and by the ARO Grant No. 26-3508-81.
Polaron pair mediated triplet generation in polymer/fullerene blends
Dimitrov, Stoichko D.; Wheeler, Scot; Niedzialek, Dorota; Schroeder, Bob C.; Utzat, Hendrik; Frost, Jarvist M.; Yao, Jizhong; Gillett, Alexander; Tuladhar, Pabitra S.; McCulloch, Iain; Nelson, Jenny; Durrant, James R.
2015-01-01
Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields. PMID:25735188
PREFACE: International Conference on Optics of Excitons in Confined Systems
NASA Astrophysics Data System (ADS)
Viña, Luis; Tejedor, Carlos; Calleja, José M.
2010-01-01
The OECS11 (International Conference on Optics of Excitons in Confined Systems) was the eleventh of a very successful series of conferences that started in 1987 in Rome (Italy). Afterwards the conference was held at Naxos (Sicily, Italy, 1991), Montpellier (France, 1993), Cortona (Italy, 1995), Göttingen (Germany, 1997), Ascona (Switzerland, 1999), Montpellier (France, 2001), Lecce (Italy, 2003), Southampton (UK, 2005) and Patti (Sicily, Italy, 2007). It is addressed to scientists who lead fundamental and applied research on the optical properties of excitons in novel condensed-matter nanostructures. The 2009 meeting (7-11 September 2009) has brought together a large representation of the world leading actors in this domain, with the aim of stimulating the exchange of ideas, promoting international collaborations, and coordinating research on the newest exciton-related issues such as quantum information science and exciton quantum-collective phenomena. The meeting has included invited lectures, contributed oral presentations and posters, covering the following general topics: low-dimensional heterostructures: quantum wells, quantum wires and quantum dots polaritons quantum optics with excitons and polaritons many-body effects under coherent and incoherent excitation coherent optical spectroscopy quantum coherence and quantum-phase manipulation Bose-Einstein condensation and other collective phenomena excitons in novel materials The OECS 11 was held at the campus of the Universidad Autónoma de Madrid in Cantoblanco. The scientific program was composed of more than 200 contributions divided into 16 invited talks, 44 oral contributions and 3 poster sessions with a total of 150 presentations. The scientific level of the presentations was guaranteed by a selection process where each contribution was rated by three members of the Program Committee. The Conference has gathered 238 participants from 21 different countries, with the following distribution: Germany (43
Theory of exciton linewidth in II VI semiconductor mixed crystals
NASA Astrophysics Data System (ADS)
Zimmermann, R.
1990-04-01
The disorder-induced broadening of excitons in mixed crystals is discussed, using a novel expression for the relevant exciton volume. Earlier experimental data on CdS 1-xSe x are successfully explained. The exciton broadening in quantum wells due to well-width fluctuations is obtained along similar lines.
Competition between polaron pair formation and singlet fission observed in amorphous rubrene films
NASA Astrophysics Data System (ADS)
Jankus, Vygintas; Snedden, Edward W.; Bright, Daniel W.; Arac, Erhan; Dai, DeChang; Monkman, Andrew P.
2013-06-01
In this paper, we investigate excited state dynamics in amorphous rubrene vacuum sublimed films. We report the direct observation of singlet fission in amorphous rubrene films. We have determined the fission rate to be >2.5×1012 s-1. Simultaneously, we observe strong polaron pair absorption and propose that polaron pair formation could be competing with singlet fission. Another possible conclusion from our experiments could be that two triplets from singlet fission might arise via polaron pairs. In either case, polaron pairs play an important role in singlet fission in an amorphous rubrene film. We also observe that triplets created by singlet fission fuse to regenerate a singlet, giving delayed fluorescence (DF) scaling linearly with initial laser energy (i.e., one singlet gives two triplets and two triplets give back one singlet). This is a strong evidence of S1n→2T1. We did not observe substantial temperature dependence of DF decay curve shape, indicating that triplet migration in amorphous rubrene films is not hopping limited and that triplets undergo fusion before their migration.
Small polarons and point defects in LaFeO3
NASA Astrophysics Data System (ADS)
Zhu, Zhen; Peelaers, Hartwin; van de Walle, Chris G.
The proton-conductive perovskite-type LaFeO3 is a promising negative-electrode material for Ni/metal-hydride (Ni-MH) batteries. It has a discharge capacity up to 530 mAhg-1 at 333 K, which is significantly higher than commercialized AB5-type alloys. To elucidate the underlying mechanism of this performance, we have investigated the structural and electronic properties of bulk LaFeO3, as well as the effect of point defects, using hybrid density functional methods. LaFeO3 is antiferromagnetic in the ground state with a band gap of 3.54 eV. Small hole and electron polarons can form through self- or point-defect-assisted trapping. We find that La vacancies and Sr substitutional on La sites are shallow acceptors with the induced holes trapped as small polarons, while O and Fe vacancies are deep defect centers. Hydrogen interstitials behave like shallow donors, with the donor electrons localized on nearby iron sites as electron polarons. With a large trapping energy, these polarons can act as electron or hole traps and affect the electrical performance of LaFeO3 as the negative electrode for Ni-MH batteries. We acknowledge DOE for financial support.
Generalized Hartree-Fock-Bogoliubov description of the Fröhlich polaron
NASA Astrophysics Data System (ADS)
Kain, Ben; Ling, Hong Y.
2016-07-01
We adapt the generalized Hartree-Fock-Bogoliubov (HFB) method to an interacting many-phonon system free of impurities. The many-phonon system is obtained from applying the Lee-Low-Pine (LLP) transformation to the Fröhlich model which describes a mobile impurity coupled to noninteracting phonons. We specialize our general HFB description of the Fröhlich polaron to Bose polarons in quasi-one-dimensional cold-atom mixtures. The LLP-transformed many-phonon system distinguishes itself with an artificial phonon-phonon interaction which is very different from the usual two-body interaction. We use the quasi-one-dimensional model, which is free of an ultraviolet divergence that exists in higher dimensions, to better understand how this unique interaction affects polaron states and how the density and pair correlations inherent to the HFB method conspire to create a polaron ground state with an energy in good agreement with and far closer to the prediction from Feynman's variational path integral approach than mean-field theory where HFB correlations are absent.
Exciton dynamics in perturbed vibronic molecular aggregates
Brüning, C.; Wehner, J.; Hausner, J.; Wenzel, M.; Engel, V.
2015-01-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Control of exciton transport using quantum interference
NASA Astrophysics Data System (ADS)
Lusk, Mark T.; Stafford, Charles A.; Zimmerman, Jeramy D.; Carr, Lincoln D.
2015-12-01
It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasiparticle motion and also discriminates between quasiparticles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano antiresonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.
Multiple Exciton Generation in Silicon QD arrays
NASA Astrophysics Data System (ADS)
Kryjevski, Andrei; Kilin, Dmitri
2014-03-01
We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate multiple exciton generation (MEG) in several semiconductor nanosystems. Hydrogen-passivated Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Quantum efficiency, the average number of excitons created by a single photon, has been calculated in these nanoparticles to the leading order in the screened Coulomb interaction. Amorphous nanostructures are predicted to have more effective carrier multiplication.
Exciton dynamics in perturbed vibronic molecular aggregates.
Brüning, C; Wehner, J; Hausner, J; Wenzel, M; Engel, V
2016-07-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla
Stier, Andreas V.; McCreary, Kathleen M.; Jonker, Berend T.; Kono, Junichiro; Crooker, Scott A.
2016-02-09
In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately –230 μeV T–1 (g-factor ≃–4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radiimore » of ~1.53 and ~1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). Lastly, these results highlight the utility of high magnetic fields for understanding new two-dimensional materials.« less
Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla
NASA Astrophysics Data System (ADS)
Stier, Andreas V.; McCreary, Kathleen M.; Jonker, Berend T.; Kono, Junichiro; Crooker, Scott A.
2016-02-01
In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately -230 μeV T-1 (g-factor ~=-4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radii of ~1.53 and ~1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). These results highlight the utility of high magnetic fields for understanding new two-dimensional materials.
NASA Astrophysics Data System (ADS)
Inche Ibrahim, M. L.
2016-04-01
The dissociation of polaron pairs into free charge carriers in organic bulk heterojunction solar cells is a fundamental step in generating photocurrent and is still in debate. In this study, we propose two simple criteria that can be used to test the validity of any polaron pair dissociation model for polymer-fullerene bulk heterojunction solar cells. The first criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density should increase as a function of applied voltage. The second criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density at short circuit should not be larger than 1. We apply these criteria to test the validity of the widely used Onsager-Braun model by using the experimental current-voltage data of poly[2-methoxy-5-(3'-7'-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells. We find that our numerical analysis is not suitable to employ these criteria. Our analytical analysis, on the other hand, clearly demonstrates that the Onsager-Braun model simply cannot fulfill the first criteria. The reason is because the polaron pair dissociation given by the Onsager-Braun model is too strongly influenced by the electric field (i.e., decreases too rapidly as the electric field decreases). The analysis provides a further evidence against the widely used Onsager-Braun model. The proposed criteria can help us to determine the correct model for polaron pair dissociation by serving as a guideline on how strongly the electric field is allowed to influence the polaron pair dissociation.
NASA Astrophysics Data System (ADS)
Bourson, P.; Aillerie, M.; Cochez, M.; Ferriol, M.; Zhang, Y.; Guilbert, L.
2003-10-01
The growth of iron-doped single-crystal fibers of lithium niobate was performed by the Laser Heated Pedestal Growth Technique for different Fe 2O 3 contents in the feed rods. We used the polaron luminescence to explain the processes of iron substitution in iron-doped single-crystal fibers of lithium niobate. The interpretation of the polaron behavior as a function of iron concentration confirms several predicted effects as the decrease of the global amount of vacancies and the predominant role of the niobium in its polaronic or bipolaronic forms in the LN lattice.
Bright and dark excitons in semiconductor carbon nanotubes
Tretiak, Sergei
2008-01-01
We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low
Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.
Rao, Akshay; Wilson, Mark W B; Hodgkiss, Justin M; Albert-Seifried, Sebastian; Bässler, Heinz; Friend, Richard H
2010-09-15
Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs. PMID:20735067
Exciton-exciton annihilation and relaxation pathways in semiconducting carbon nanotubes
NASA Astrophysics Data System (ADS)
Chmeliov, Jevgenij; Narkeliunas, Jonas; Graham, Matt W.; Fleming, Graham R.; Valkunas, Leonas
2016-01-01
We present a thorough analysis of one- and two-color transient absorption measurements performed on single- and double-walled semiconducting carbon nanotubes. By combining the currently existing models describing exciton-exciton annihilation--the coherent and the diffusion-limited ones--we are able to simultaneously reproduce excitation kinetics following both E11 and E22 pump conditions. Our simulations revealed the fundamental photophysical behavior of one-dimensional coherent excitons and non-trivial excitation relaxation pathways. In particular, we found that after non-linear annihilation a doubly-excited exciton relaxes directly to its E11 state bypassing the intermediate E22 manifold, so that after excitation resonant with the E11 transition, the E22 state remains unpopulated. A quantitative explanation for the observed much faster excitation kinetics probed at E22 manifold, comparing to those probed at the E11 band, is also provided.
Nanosecond spin coherence of excitons bound to acceptors in a CdTe quantum well
NASA Astrophysics Data System (ADS)
Grinberg, P.; Bernardot, F.; Eble, B.; Karczewski, G.; Testelin, C.; Chamarro, M.
2016-03-01
We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times.
Quantum chaos and breaking of all anti-unitary symmetries in Rydberg excitons.
Aßmann, Marc; Thewes, Johannes; Fröhlich, Dietmar; Bayer, Manfred
2016-07-01
Symmetries are the underlying principles of fundamental interactions in nature. Chaos in a quantum system may emerge from breaking these symmetries. Compared to vacuum, crystals are attractive for studying quantum chaos, as they not only break spatial isotropy, but also lead to novel quasiparticles with modified interactions. Here we study yellow Rydberg excitons in cuprous oxide which couple strongly to the vacuum light field and interact significantly with crystal phonons, leading to inversion symmetry breaking. In a magnetic field, time-reversal symmetry is also broken and the exciton states show a complex splitting pattern, resulting in quadratic level repulsion for small splittings. In contrast to atomic chaotic systems in a magnetic field, which show only a linear level repulsion, this is a signature of a system where all anti-unitary symmetries are broken simultaneously. This behaviour can otherwise be found only for the electro-weak interaction or engineered billiards. PMID:27064527
Polaronic Transport in Phosphate Glasses Containing Transition Metal Ions
NASA Astrophysics Data System (ADS)
Henderson, Mark
The goal of this dissertation is to characterize the basic transport properties of phosphate glasses containing various amounts of TIs and to identify and explain any electronic phase transitions which may occur. The P2 O5-V2O5-WO3 (PVW) glass system will be analyzed to find the effect of TI concentration on conduction. In addition, the effect of the relative concentrations of network forming ions (SiO2 and P2O5) on transport will be studied in the P2O5-SiO2-Fe2O 3 (PSF) system. Also presented is a numerical study on a tight-binding model adapted for the purposes of modelling Gaussian traps, mimicking TI's, which are arranged in an extended network. The results of this project will contribute to the development of fundamental theories on the electronic transport in glasses containing mixtures of transition oxides as well as those containing multiple network formers without discernible phase separation. The present study on the PVW follows up on previous investigation into the effect on mixed transition ions in oxide glasses. Past research has focused on glasses containing transition metal ions from the 3d row. The inclusion of tungsten, a 5d transition metal, adds a layer of complexity through the mismatch of the energies of the orbitals contributing to localized states. The data have indicated that a transition reminiscent of a metal-insulator transition (MIT) occurs in this system as the concentration of tungsten increases. As opposed to some other MIT-like transitions found in phosphate glass systems, there seems to be no polaron to bipolaron conversion. Instead, the individual localization parameter for tungsten noticeably decreases dramatically at the transition point as well as the adiabaticity. Another distinctive feature of this project is the study of the PSF system, which contains two true network formers, phosphorous pentoxide (P2O 5) and silicon dioxide (SiO2). It is not usually possible to do a reliable investigation of the conduction properties of
NASA Astrophysics Data System (ADS)
Panda, J.; Sasmal, I.; Nath, T. K.
2016-03-01
In this paper we have reported the synthesis of high quality vertically aligned undoped and Mn-doped ZnO single crystalline nanorods arrays on Si (100) substrates using two steps process, namely, initial slow seed layer formation followed by solution growth employing wet chemical hydrothermal method. The shapes of the as grown single crystalline nanorods are hexagonal. The diameter and length of the as grown undoped ZnO nanorods varies in the range of 80-150 nm and 1.0 - 1.4 μm, respectively. Along with the lattice parameters of the hexagonal crystal structure, the diameter and length of Mn doped ZnO nanorods are found to increase slightly as compared to the undoped ZnO nanorods. The X-ray photoelectron spectroscopy confirms the presence of Mn atoms in Mn2+ state in the single crystalline ZnO nanorods. The recorded photoluminescence spectrum contains two emissions peaks having UV exciton emissions along with a green-yellow emission. The green-yellow emissions provide the evidence of singly ionized oxygen vacancies. The magnetic field dependent magnetization measurements [M (H)] and zero field cooled (ZFC) and field cooled (FC) magnetization [M(T)] measurements have been carried out at different isothermal conditions in the temperature range of 5-300 K. The Mn doped ZnO nanorods clearly show room temperature ferromagnetic ordering near room temperature down to 5 K. The observed magnetization may be attributed to the long range ferromagnetic interaction between bound magnetic polarons led by singly charged oxygen vacancies.
Topological aspects of nonlinear excitonic processes in noncentrosymmetric crystals
NASA Astrophysics Data System (ADS)
Morimoto, Takahiro; Nagaosa, Naoto
2016-07-01
We study excitonic processes second order in the electric fields in noncentrosymmetric crystals. We derive formulas for shift current and second harmonic generation produced by exciton creation, by using the Floquet formalism combined with the Keldysh Green's function method. It is shown that (i) the steady dc shift current flows by exciton creation without dissociation into free carriers and (ii) second harmonic generation is enhanced at the exciton resonance. The obtained formulas clarify topological aspects of these second order excitonic processes which are described by Berry connections of the relevant valence and conduction bands.
Photoluminescence properties and exciton dynamics in monolayer WSe2
NASA Astrophysics Data System (ADS)
Yan, Tengfei; Qiao, Xiaofen; Liu, Xiaona; Tan, Pingheng; Zhang, Xinhui
2014-09-01
In this work, comprehensive temperature and excitation power dependent photoluminescence and time-resolved photoluminescence studies are carried out on monolayer WSe2 to reveal its properties of exciton emissions and related excitonic dynamics. Competitions between the localized and delocalized exciton emissions, as well as the exciton and trion emissions are observed, respectively. These competitions are suggested to be responsible for the abnormal temperature and excitation intensity dependent photoluminescence properties. The radiative lifetimes of both excitons and trions exhibit linear dependence on temperature within the temperature regime below 260 K, providing further evidence for two-dimensional nature of monolayer material.
Exciton localization and drift in tailored-potential quantum nanowires
Szeszko, J. Rudra, A.; Kapon, E.; Belykh, V. V.; Sibeldin, N. N.
2014-06-30
Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm{sup 2}/(eVs).
Multiple Exciton Generation in Colloidal Silicon Nanocrystals
Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. M.
2007-01-01
Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap {identical_to} Eg = 1.20 eV) to be 2.4 {+-} 0.1E{sub g} and find an exciton-production quantum yield of 2.6 {+-} 0.2 excitons per absorbed photon at 3.4E{sub g}. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.
NASA Astrophysics Data System (ADS)
Bhunia, Amit; Bansal, Kanika; Datta, Shouvik; Alshammari, Marzook S.; Henini, Mohamed
In contrast to the widely reported optical techniques, there are hardly any investigations on corresponding electrical signatures of condensed matter physics of excitonic phenomena. We studied small signal steady state capacitance response in III-V materials based multi quantum well (AlGaInP) and MBE grown quantum dot (InGaAs) laser diodes to identify signatures of excitonic presence. Conductance activation by forward bias was probed using frequency dependent differential capacitance response (fdC/df), which changes characteristically with the onset of light emission indicating the occurrence of negative activation energy. Our analysis shows that it is connected with a steady state population of exciton like bound states. Calculated average energy of this bound state matches well with the binding energy of weakly confined excitons in this type of structures. Further increase in charge injection decreases the differential capacitive response in AlGaInP based diodes, indicating a gradual Mott transition of excitonic states into electron hole plasma. This electrical description of excitonic Mott transition is fully supplemented by standard optical spectroscopic signatures of band gap renormalization and phase space filling effects.
Indirect excitons in a potential energy landscape created by a perforated electrode
NASA Astrophysics Data System (ADS)
Dorow, C. J.; Kuznetsova, Y. Y.; Leonard, J. R.; Chu, M. K.; Butov, L. V.; Wilkes, J.; Hanson, M.; Gossard, A. C.
2016-02-01
We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at a constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions.
Warping and interactions of vortices in exciton-polariton condensates
NASA Astrophysics Data System (ADS)
Toledo-Solano, M.; Mora-Ramos, M. E.; Figueroa, A.; Rubo, Y. G.
2014-01-01
We investigate the properties of the vortex singularities in two-component exciton-polariton condensates in semiconductor microcavities in the presence of transverse-electric-transverse-magnetic (TE-TM) splitting of the lower polariton branch. This splitting does not change qualitatively the basic (lemon and star) geometry of half-quantum vortices (HQVs), but results in warping of both the polarization field and the supercurrent streamlines around these entities. The TE-TM splitting has a pronounced effect on the HQV energies and interactions, as well as on the properties of integer vortices, especially on the energy of the hedgehog polarization vortex. The energy of this vortex can become smaller than the energies of HQVs. This leads to modification of the Berezinskii-Kosterlitz-Thouless transition from the proliferation of half-vortices to the proliferation of hedgehog-based vortex molecules.
Dynamics of the excitonic coupling in organic crystals.
Aragó, Juan; Troisi, Alessandro
2015-01-16
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene. PMID:25635554
Dynamics of the Excitonic Coupling in Organic Crystals
NASA Astrophysics Data System (ADS)
Aragó, Juan; Troisi, Alessandro
2015-01-01
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene.
Ultrafast dynamics of exciton fission in polycrystalline pentacene.
Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H
2011-08-10
We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. PMID:21755937
Temperature dependence of free excitons in GaN
NASA Astrophysics Data System (ADS)
Reynolds, D. C.; Hoelscher, J.; Litton, C. W.; Collins, T. C.
2002-11-01
The excitons involved in this study are the longitudinal and the Gamma5 and Gamma6 free excitons, as well as the donor bound exciton (D0),X. The temperature dependence of the energy positions of the Gamma5, Gamma6, and (D0),X excitons are well accounted for by the Varshni equation (Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)). In the same temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation. The separation between the longitudinal- and transverse-mode free excitons has been previously reported. One component of this separation is the polarizability, which has a temperature dependence. The longitudinal exciton therefore has a band-gap temperature dependence, predicted by the Varshni equation, as well as an additional dependence due to polarizability. This temperature dependence has been accounted for by the Varshni equation, plus an additional linear and a quadratic temperature dependent term.
Many-polaron description of impurities in a Bose-Einstein condensate in the weak-coupling regime
Casteels, W.; Devreese, J. T.; Tempere, J.
2011-12-15
The weak-coupling many-polaron formalism is applied to the case of the polaronic system consisting of impurities in a Bose-Einstein condensate. This allows investigating the ground-state properties and the response of the system to Bragg spectroscopy. Then, this theory is applied to the system of spin-polarized fermionic lithium-6 impurities in a sodium condensate. The Bragg spectrum reveals a peak that corresponds to the emission of Bogoliubov excitations. Both the ground-state properties and the response spectrum show that the polaronic effect vanishes at high densities. We also look at two possibilities to define the polaronic effective mass and observe that this results in a different quantitative behavior if multiple impurities are involved.
Spin-polaron theory of high-{Tc} superconductivity: 2, electronic structure of the CuO{sub 2} planes
Wood, R.F.
1993-06-01
After an introductory discussion of electronic structure calculations for the CuO{sub 2} planes in the copper-oxide based high-{Tc} superconductors, the method suggested by Slater for studying antiferromagnetic (AF) metals is described. In this method, as applied here, the chemical unit cell is doubled to form a magnetic unit cell which contains one Cu ion with predominantly up spin and one with predominantly down spin. Down spins are kept off up-spin sites, and conversely, by the introduction of a Hubbard U term. As a result, the band structure obtained is typical of that for a Mott-Hubbard (M-H) or, more generally, a charge transfer insulator. Conductivity in the a-b plane results when holes are introduced into the M-H valence band. The band structure as a function of the parameters in Koster-Slater type calculations is discussed and the Fermi surface is described. A calculation of the delocalization energy for spin-polaron formation is carried out within the context of the band calculations.
Renormalization group approach to the Fröhlich polaron model: application to impurity-BEC problem
Grusdt, F.; Shchadilova, Y. E.; Rubtsov, A. N.; Demler, E.
2015-01-01
When a mobile impurity interacts with a many-body system, such as a phonon bath, a polaron is formed. Despite the importance of the polaron problem for a wide range of physical systems, a unified theoretical description valid for arbitrary coupling strengths is still lacking. Here we develop a renormalization group approach for analyzing a paradigmatic model of polarons, the so-called Fröhlich model, and apply it to a problem of impurity atoms immersed in a Bose-Einstein condensate of ultra cold atoms. Polaron energies obtained by our method are in excellent agreement with recent diagrammatic Monte Carlo calculations for a wide range of interaction strengths. They are found to be logarithmically divergent with the ultra-violet cut-off, but physically meaningful regularized polaron energies are also presented. Moreover, we calculate the effective mass of polarons and find a smooth crossover from weak to strong coupling regimes. Possible experimental tests of our results in current experiments with ultra cold atoms are discussed. PMID:26183614
Renormalization group approach to the Fröhlich polaron model: application to impurity-BEC problem.
Grusdt, F; Shchadilova, Y E; Rubtsov, A N; Demler, E
2015-01-01
When a mobile impurity interacts with a many-body system, such as a phonon bath, a polaron is formed. Despite the importance of the polaron problem for a wide range of physical systems, a unified theoretical description valid for arbitrary coupling strengths is still lacking. Here we develop a renormalization group approach for analyzing a paradigmatic model of polarons, the so-called Fröhlich model, and apply it to a problem of impurity atoms immersed in a Bose-Einstein condensate of ultra cold atoms. Polaron energies obtained by our method are in excellent agreement with recent diagrammatic Monte Carlo calculations for a wide range of interaction strengths. They are found to be logarithmically divergent with the ultra-violet cut-off, but physically meaningful regularized polaron energies are also presented. Moreover, we calculate the effective mass of polarons and find a smooth crossover from weak to strong coupling regimes. Possible experimental tests of our results in current experiments with ultra cold atoms are discussed. PMID:26183614
Renormalization group approach to the Fröhlich polaron model: application to impurity-BEC problem
NASA Astrophysics Data System (ADS)
Grusdt, F.; Shchadilova, Y. E.; Rubtsov, A. N.; Demler, E.
2015-07-01
When a mobile impurity interacts with a many-body system, such as a phonon bath, a polaron is formed. Despite the importance of the polaron problem for a wide range of physical systems, a unified theoretical description valid for arbitrary coupling strengths is still lacking. Here we develop a renormalization group approach for analyzing a paradigmatic model of polarons, the so-called Fröhlich model, and apply it to a problem of impurity atoms immersed in a Bose-Einstein condensate of ultra cold atoms. Polaron energies obtained by our method are in excellent agreement with recent diagrammatic Monte Carlo calculations for a wide range of interaction strengths. They are found to be logarithmically divergent with the ultra-violet cut-off, but physically meaningful regularized polaron energies are also presented. Moreover, we calculate the effective mass of polarons and find a smooth crossover from weak to strong coupling regimes. Possible experimental tests of our results in current experiments with ultra cold atoms are discussed.
Exciton radiative lifetime in transition metal dichalcogenide monolayers
NASA Astrophysics Data System (ADS)
Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B.; Marie, X.
2016-05-01
We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measure τrad0=1.8 ±0.2 ps at T =7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WS e2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T ≲50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.
The Orientation of Luminescent Excitons in Layered Organic Nanomaterials
NASA Astrophysics Data System (ADS)
Schuller, Jon; Karavelli, Sinan; He, Keliang; Yang, Shyuan; Shan, Jie; Kymissis, John; Zia, Rashid
2012-02-01
A fundamental understanding of optoelectronics in organic semiconductors is complicated by the diversity of excitons which can exist within a single material system. Measurements that distinguish between different exciton types are crucial for a complete understanding of organic materials. By fitting experimental curves of angle-, polarization-, and energy-dependent PL to analytical Purcell calculations we quantify the relative dipole moments for in-plane and out-of-plane oriented excitons in organic and inorganic layered nanomaterials. In mono- and bi-layers of Molybdenum Disulfide (MoS2) and Graphene Oxide the luminescence arises only from in-plane oriented excitons. In the perylene derivative PTCDA, however, we show that PL arises from both in-plane and out-of-plane excitons. We observe a difference in emission frequency between the dipole orientations which indicates the existence of two distinct exciton species: an in-plane oriented Frenkel exciton and an out-of-plane oriented Charge Transfer exciton. Based on these results we devise and implement a method for isolating luminescence from either exciton species. We observe different temporal dynamics for the two distinct excitons, highlighting the power of this technique for fundamental studies of organic materials.
Exciton Kinetics in Strained II-Vi Semiconductor Multiple Quantum Wells.
NASA Astrophysics Data System (ADS)
Hefetz, Yaron
1987-09-01
Two groups of wide gap II-VI semiconductor superlattices based on ZnSe/Zn(,1-x)Mn(,x)Se and CdTe/ZnTe were investigated using CW and time-resolved photoluminescence, excitation, reflectance, and photomodulated reflectance spectroscopy at various temperatures and under an external magnetic field. All these lattice mismatch strained layer structures were grown by MBE technique and exhibit strong excitonic photoluminescence at low temperatures. By studying the dynamics of the exciton recombination processes, the role of strain, quantum confinement and localization effects were revealed. In the CdTe/ZnTc system where the lattice mismatch is (DELTA)a/a (TURNEQ) 6% the inhomogeneously broadened ((TURN)40 mev) luminescence line is governed by excitonic localization in well width fluctuations. Exchange interactions of the carriers with the Mn('++) ions in the dilute magnetic semiconductor Zn(,1-x)Mn(,x)Se in thin film and the barrier of the MQW structures influence their optical behavior in an exernal magnetic field. "Giant" Zeeman splittings of up to (TURN)10 mev/Tesla were measured in samples with moderate Mn concentration (x = .23). Antiferromagnetic interaction reduces these splittings in samples with higher Mn concentrations. In observing the time evolution of the carrier in Zn(,1-x)Mn(,x)Se MQW we found that the capture time of these carriers into the well is on the order of 1 psec but the last stages of thermalization, exciton formations and localization is (TURN)70 ps. The fast capture of electrons and holes into the quantum wells bypass the energy transfer into the Mn internal transition that is responsible to the efficient "yellow" luminescence in ZnMnSe mixed crystals.
Final Report, DOE grant DE-FG02-99ER45780, "Indirect Excitons in Coupled Quantum Wells"
Snoke, david W.
2014-07-21
The is the final technical report for this project, which was funded by the DOE from 1999 to 2012. The project focused on experimental studies of spatially indirect excitons in coupled quantum wells, with the aim of understanding the quantum physics of these particles, including such effects as pattern formation due to electron-hole charge separation, the Mott plasma-insulator transition, luminescence up-conversion through field-assisted tunneling, luminescence line shifts due to many-body renormalization and magnetic field effects on tunneling, and proposed effects such as Bose-Einstein condensation of indirect excitons and phase separation of bright and dark indirect excitons. Significant results are summarized here and the relation to other work is discussed.
All-coupling theory for the Fröhlich polaron
NASA Astrophysics Data System (ADS)
Grusdt, F.
2016-04-01
The Fröhlich model describes the interaction of a mobile impurity with a surrounding bath of phonons which leads to the formation of a quasiparticle, the polaron. In this paper an efficient renormalization group approach is presented which provides a description of Fröhlich polarons in all regimes ranging from weak to strong coupling. The extended renormalization group approach introduced here is capable of predicting ground state properties for arbitrarily small impurity masses. This allows us to obtain the full phase diagram of the Fröhlich Hamiltonian, which we present concretely for the Bogoliubov-Fröhlich model originally introduced to describe ultracold impurities in a Bose-Einstein condensate. For this model, whose regime of validity in ultracold quantum gases is the subject of debate, we benchmark our method by comparison of the ground state energy to recent diagrammatic quantum Monte Carlo calculations.
Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film
NASA Astrophysics Data System (ADS)
Klimin, Serghei N.; Tempere, Jacques; Misko, Vyacheslav R.; Wouters, Michiel
2016-07-01
Electrons on liquid helium can form different phases depending on density, and temperature. Also the electron-ripplon coupling strength influences the phase diagram, through the formation of so-called "ripplonic polarons", that change how electrons are localized, and that shifts the transition between the Wigner solid and the liquid phase. We use an all-coupling, finite-temperature variational method to study the formation of a ripplopolaron Wigner solid on a liquid helium film for different regimes of the electron-ripplon coupling strength. In addition to the three known phases of the ripplopolaron system (electron Wigner solid, polaron Wigner solid, and electron fluid), we define and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the transitions between these four phases and calculate the corresponding phase diagrams. This reveals a reentrant melting of the electron solid as a function of temperature. The calculated regions of existence of the Wigner solid are in agreement with recent experimental data.
Point defects, impurities, and small hole polarons in GdTiO3
NASA Astrophysics Data System (ADS)
Bjaalie, L.; Janotti, A.; Krishnaswamy, K.; Van de Walle, C. G.
2016-03-01
The electronic structure of native defects and impurities in GdTiO3, a rare-earth titanate Mott insulator, is studied using density functional theory with a hybrid functional. Among native defects, the cation vacancies have the lowest formation energies in oxygen-rich conditions and oxygen vacancies have the lowest formation energy in oxygen-poor conditions. Among the impurities, SrGd,Hi, and CO have low formation energies. A common feature of the native defects and impurities is that they lead to the formation of small hole polarons, which explains the frequent observation of p -type hopping conductivity in the rare-earth titanates. These small hole polarons also lead to optical absorption and act as electron traps in devices.
Polaron Coherence Condensation As the Mechanism for Colossal Magnetoresistance in Layered Manganites
Mannella, N.; Yang, W.L.; Tanaka, K.; Zhou, X.J.; Zheng, H.; Mitchell, J.F.; Zaanen, J.; Devereaux, T.P.; Nagaosa, N.; Hussain, Z.; Shen, Z.-X.; /SLAC
2007-11-20
Angle-resolved photoemission spectroscopy data for the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.
Polaronic contributions to oxidation and hole conductivity in acceptor-doped BaZrO3
NASA Astrophysics Data System (ADS)
Lindman, Anders; Erhart, Paul; Wahnström, Göran
2016-08-01
Acceptor-doped perovskite oxides like BaZrO3 are showing great potential as materials for renewable energy technologies where hydrogen acts an energy carrier, such as solid oxide fuel cells and hydrogen separation membranes. While ionic transport in these materials has been investigated intensively, the electronic counterpart has received much less attention and further exploration in this field is required. Here, we use density functional theory (DFT) to study hole polarons and their impact on hole conductivity in Y-doped BaZrO3. Three different approaches have been used to remedy the self-interaction error of local and semilocal exchange-correlation functionals: DFT +U , pSIC-DFT, and hybrid functionals. Self-trapped holes are found to be energetically favorable by about -0.1 eV and the presence of yttrium results in further stabilization. Polaron migration is predicted to occur through intraoctahedral transfer and polaron rotational processes, which are associated with adiabatic barriers of about 0.1 eV. However, the rather small energies associated with polaron formation and migration suggest that the hole becomes delocalized and bandlike at elevated temperatures. These results together with an endothermic oxidation reaction [A. Lindman, P. Erhart, and G. Wahnström, Phys. Rev. B 91, 245114 (2015), 10.1103/PhysRevB.91.245114] yield a picture that is consistent with experimental data for the hole conductivity. The results we present here provide new insight into hole transport in acceptor-doped BaZrO3 and similar materials, which will be of value in the future development of sustainable technologies.
Neukirch, Amanda J; Nie, Wanyi; Blancon, Jean-Christophe; Appavoo, Kannatassen; Tsai, Hsinhan; Sfeir, Matthew Y; Katan, Claudine; Pedesseau, Laurent; Even, Jacky; Crochet, Jared J; Gupta, Gautam; Mohite, Aditya D; Tretiak, Sergei
2016-06-01
Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. Despite the remarkable progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. High on the list of unexplained phenomena are very modest mobilities despite low charge carrier effective masses. Moreover, experiments elucidate unique degradation of photocurrent affecting stable operation of perovskite solar cells. These puzzles suggest that, while ionic hybrid perovskite devices may have efficiencies on par with conventional Si and GaAs devices, they exhibit more complicated charge transport phenomena. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using both periodic boundary conditions and isolated structures. Using the hybrid density functional theory, we found that volumetric strain in a CsPbI3 cluster creates a polaron with binding energy of around 300 and 900 meV for holes and electrons, respectively. In the MAPbI3 (MA = CH3NH3) cluster, both volumetric strain and MA reorientation effects lead to larger binding energies at around 600 and 1300 meV for holes and electrons, respectively. Such large reorganization energies suggest appearance of small polarons in organometallic perovskite materials. The fact that both volumetric lattice strain and MA molecular rotational degrees of freedom can cooperate to create and stabilize polarons indicates that in order to mitigate this problem, formamidinium (FA = HC(NH2)2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications. PMID:27224519
Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes
Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.
2011-07-01
The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.
High field magneto-spectroscopy of excitons in monolayer WSe2
NASA Astrophysics Data System (ADS)
Lu, Zhengguang; Zhang, Xiaoxiao; Ludwig, Jonathan; Zhang, Fan; Thirunavukkuarasu, Komalavalli; Moon, Seongphill; Hone, James; Heinz, Tony; Smirnov, Dmitry
We have performed circularly polarized photoluminescence (PL) experiments on monolayer WSe2 in magnetic fields up to +/-31T and at temperatures between 2K and 45K, focusing on the emission from the neutral (X0) and negatively charged (X-) excitons. A parallel magnetic field does not affect the exciton energy. At 45K, a perpendicular magnetic field (Faraday geometry) induces linear shift of about 0.12 meV/T ~2 μB for both X0 and X- peaks indicating lifting of the valley degeneracy. The magnitude of this valley Zeeman shift agrees with the valence band edge lifting due to atomic orbital contribution. The change of the X- PL intensity with the magnetic field suggests that the intravalley configuration is the lower energy state of the trion in WSe2. At lower temperatures, the X0 exhibits the same shift with the magnetic field as at 45K, while the X- shows a more pronounced and non-linear shift with respect to magnetic field.
Interlayer excitons with tunable dispersion relation
NASA Astrophysics Data System (ADS)
Skinner, Brian
2016-06-01
Interlayer excitons, comprising an electron in one material bound by Coulomb attraction to a hole in an adjacent material, are composite bosons that can assume a variety of many-body phases. The phase diagram of the bosonic system is largely determined by the dispersion relation of the bosons, which itself arises as a combination of the dispersion relations of the electron and hole separately. Here I show that in situations where either the electron or the hole has a nonmonotonic, "Mexican hat-shaped" dispersion relation, the exciton dispersion relation can have a range of qualitatively different forms, each corresponding to a different many-body phase at low temperature. This diversity suggests a platform for continuously tuning between different quantum phases using an external field.
Resistive transition, polaron dynamics and scaling behavior in Fe doped SrTiO3
NASA Astrophysics Data System (ADS)
Ghosh, Arijit; Masud, Md. G.; Sannigrahi, Jhuma; Chaudhuri, B. K.
2013-04-01
Temperature dependent electrical transport (ac and dc) and dielectric measurements have been performed with SrFexTi(1-x)O3 (0≤x≤1.0) samples prepared by ceramic route. Metal-insulator like transition observed depending on doping concentration. High dc resistivity of the present system is due to localization of polaronic charge carriers and the dc resistivity data, above respective TMI, fit well with Mott's variable range hopping, Arrhenius conduction mechanism, while a power law nature is followed below TMI. The charge transport mechanism is also revealed from the scaled ac conductivity and dielectric modulus spectrum. The MI transition is considered to be associated with polaron dissociation occurring little below polaron ordering (PO) temperature, TPO. The samples also indicate a cross-over from universal dielectric response (UDR) (f<105 Hz) to second universality (SU) (f>105 Hz). This cross over as well as MI like transition temperature shift towards the lower temperature regime with increasing x.
NASA Astrophysics Data System (ADS)
Khalil, M. M. I.
2007-03-01
The effect of electric field strength on conduction in lithium borate glasses doped with CuO with different concentration was studied and the value of the jump distance of charge carrier was calculated. The conductivity measurements indicate that the conduction is due to non-adiabatic hopping of polarons and the activation energies are found to be temperature and concentration dependent. Lithium borate glasses are subjected to carefully-programmed thermal treatments which cause the nucleation and growth of crystalline phases. X-ray diffraction analysis confirmed the amorphous nature for the investigated glass sample and the formation of crystalline phase for annealed samples at 650 °C. The main separated crystalline phase is Li2B8O13. The scanning electron micrographs of some selected glasses showed a significant change in the morphology of the films investigated due to heat treatment of the glass samples. It was found that the dc-conductivity decreases with an increase of the HT temperature. The decrease of dc conductivity, with an increase of the HT temperature, can be related to the decrease in the number of free ions in the glass matrix. There is deviation from linearity at high temperature regions in the logσ-1/T plots for all investigated doped samples at a certain temperature at which the transition from polaronic to ionic conduction occurs. The hopping of small polarons is dominant at low temperatures, whereas the hopping of Li+ ions dominates at high temperatures.
Vast Hole- and Electron-Polaron Spatial Extent in Oligomeric π-Conjugated Porphyrin Arrays
NASA Astrophysics Data System (ADS)
Angiolillo, Paul; Rawson, Jeff; Therien, Michael
meso-Ethyne bridged π-conjugated zinc porphyrin oligomers (PZnn compounds) have been demonstrated to evince lowest excited singlet states that are globally delocalized. It has also previously been shown that hole-polaron states of these oligomers exhibit delocalization lengths that mirror the molecular spatial dimension, 7.5 nm in the case of the heptamer. Here we demonstrate that the electron-polaron states in PZnn compounds also feature vast areal delocalization. This finding is evidenced by concurrent optical and electron spin resonance measurements, coupled with electronic structure calculations that suggest atypically small reorganization energies for one-electron reduction of these materials. These results are buttressed by electron spin relaxation measurements of PZnn electron polarons that show that both T1 and T2 relaxation times are unusually large, on the order of 103 ns and 102 ns, respectively. Since rapid charge delocalization defines an important mechanism that mitigates Coulombic stabilization of photogenerated electron-hole pairs to create separated free charge carriers, and spin polarization lifetimes feature prominently in spin currents, these findings identify conjugated materials with exceptional optical, electronic, and spintronic properties.
Mapping polarons in polymer FETs by charge modulation microscopy in the mid-infrared
NASA Astrophysics Data System (ADS)
Chin, Xin Yu; Yin, Jun; Wang, Zilong; Caironi, Mario; Soci, Cesare
2014-01-01
We implemented spatial mapping of charge carrier density in the channel of a conventional polymer Field-Effect Transistor (FET) by mid-infrared Charge Modulation Spectroscopy (CMS). CMS spectra are recorded with a high sensitivity confocal Fourier Transform Infra-Red (FTIR) microscope by probing electroinduced Infra-Red Active Vibrational (IRAV) modes and low-energy polaron bands in the spectral region 680-4000 cm-1. Thanks to the high specificity and strong oscillator strength of these modes, charge-induced reflectance measurements allow quantitative estimation of charge carrier densities within the FET channel, without the need for amplitude or phase modulation. This is illustrated by identifying the contribution of intrinsic and electrostatically induced polarons to conduction, and by mapping the polaron spatial distribution in a P3HT (Poly(3-hexylthiophene-2,5-diyl)) FET channel under different drain-source bias conditions. This work demonstrates the potential of mid-infrared charge modulation microscopy to characterize carrier injection and transport in semiconducting polymer materials.
Mapping polarons in polymer FETs by charge modulation microscopy in the mid-infrared
Chin, Xin Yu; Yin, Jun; Wang, Zilong; Caironi, Mario; Soci, Cesare
2014-01-01
We implemented spatial mapping of charge carrier density in the channel of a conventional polymer Field-Effect Transistor (FET) by mid-infrared Charge Modulation Spectroscopy (CMS). CMS spectra are recorded with a high sensitivity confocal Fourier Transform Infra-Red (FTIR) microscope by probing electroinduced Infra-Red Active Vibrational (IRAV) modes and low-energy polaron bands in the spectral region 680–4000 cm−1. Thanks to the high specificity and strong oscillator strength of these modes, charge-induced reflectance measurements allow quantitative estimation of charge carrier densities within the FET channel, without the need for amplitude or phase modulation. This is illustrated by identifying the contribution of intrinsic and electrostatically induced polarons to conduction, and by mapping the polaron spatial distribution in a P3HT (Poly(3-hexylthiophene-2,5-diyl)) FET channel under different drain-source bias conditions. This work demonstrates the potential of mid-infrared charge modulation microscopy to characterize carrier injection and transport in semiconducting polymer materials. PMID:24406635
Mapping polaronic states and lithiation gradients in individual V2O5 nanowires
NASA Astrophysics Data System (ADS)
de Jesus, Luis R.; Horrocks, Gregory A.; Liang, Yufeng; Parija, Abhishek; Jaye, Cherno; Wangoh, Linda; Wang, Jian; Fischer, Daniel A.; Piper, Louis F. J.; Prendergast, David; Banerjee, Sarbajit
2016-06-01
The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.
DFT +U Modeling of Hole Polarons in Organic Lead Halide Perovskites
NASA Astrophysics Data System (ADS)
Welch, Eric; Erhart, Paul; Scolfaro, Luisa; Zakhidov, Alex
Due to the ever present drive towards improved efficiencies in solar cell technology, new and improved materials are emerging rapidly. Organic halide perovskites are a promising prospect, yet a fundamental understanding of the organic perovskite structure and electronic properties is missing. Particularly, explanations of certain physical phenomena, specifically a low recombination rate and high mobility of charge carriers still remain controversial. We theoretically investigate possible formation of hole polarons adopting methodology used for oxide perovskites. The perovskite studied here is the ABX3structure, with A being an organic cation, B lead and C a halogen; the combinations studied allow for A1,xA2 , 1 - xBX1,xX2 , 3 - xwhere the alloy convention is used to show mixtures of the organic cations and/or the halogens. Two organic cations, methylammonium and formamidinium, and three halogens, iodine, chlorine and bromine are studied. Electronic structures and polaron behavior is studied through first principle density functional theory (DFT) calculations using the Vienna Ab Initio Simulation Package (VASP). Local density approximation (LDA) pseudopotentials are used and a +U Hubbard correction of 8 eV is added; this method was shown to work with oxide perovskites. It is shown that a localized state is realized with the Hubbard correction in systems with an electron removed, residing in the band gap of each different structure. Thus, hole polarons are expected to be seen in these perovskites.
Mapping polaronic states and lithiation gradients in individual V2O5 nanowires.
De Jesus, Luis R; Horrocks, Gregory A; Liang, Yufeng; Parija, Abhishek; Jaye, Cherno; Wangoh, Linda; Wang, Jian; Fischer, Daniel A; Piper, Louis F J; Prendergast, David; Banerjee, Sarbajit
2016-01-01
The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation. PMID:27349567
Rawson, Jeff; Angiolillo, Paul J; Frail, Paul R; Goodenough, Isabella; Therien, Michael J
2015-06-18
Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZnn compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the PZnn hole polaron ([PZnn](+•)) and electron polaron ([PZnn](-•)) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase [PZnn](+•) and [PZnn](-•) spin-spin (T2) and spin-lattice (T1) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin-spin and spin-lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal-dithiol-PZnn-metal junctions, present meso-to-meso ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications. PMID:25697578
High-density Two-Dimensional Small Polaron Gas in a Delta-Doped Mott Insulator
Ouellette, Daniel G.; Moetakef, Pouya; Cain, Tyler A.; Zhang, Jack Y.; Stemmer, Susanne; Emin, David; Allen, S. James
2013-01-01
Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO3 are delta-doped by substituting a single (GdO)+1 plane with a monolayer of charge neutral SrO to produce a two-dimensional system with high planar doping density. Unlike metallic SrTiO3 quantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally activated DC and optical conductivity that agree in a quantitative manner with predictions of small polaron transport but with an extremely high two-dimensional density of polarons, ~7 × 1014 cm−2. PMID:24257578
Do Hydrogen Bonds Influence Excitonic Splittings?
Balmer, Franziska A; Ottiger, Philipp; Leutwyler, Samuel
2016-01-01
The excitonic splitting and vibronic quenching of the inversion-symmetric homodimers of benzonitrile, (BN)2, and meta-cyanophenol, (mCP)2, are investigated by two-color resonant two-photon ionization spectroscopy. These systems have very different hydrogen bond strengths: the OH···N≡C bonds in (mCP)2 are ∼10 times stronger than the CH···N≡C hydrogen bonds in (BN)2. In (BN)2 the S0((1)Ag) → S1((1)Ag) transition is electric-dipole forbidden, while the S0((1)Ag) → S2((1)Bu) transition is allowed. The opposite holds for (mCP)2 due to the different transition dipole moment vector alignment. The S0 → S1S2 spectra of the dimers are compared and their excitonic splittings and vibronic quenchings are investigated by measuring the (13)C-substituted heterodimer isotopomers, for which the centrosymmetry is broken and both transitions are allowed. The excitonic splittings are determined as Δexc = 2.1 cm(-1) for (BN)2 and Δexc = 7.3 cm(-1) for (mCP)2. The latter exhibits a much stronger vibronic quenching, as the purely electronic splitting resulting from ab initio calculations is determined to be Δcalc = 179 cm-1, while in (BN)2 the calculated splitting is Δcalc = 10 cm(-1). The monomer site-shifts upon dimerization and comparing certain vibrations that deform the hydrogen bonds confirm that the OH···N≡C hydrogen bond is much stronger than the CH···N≡C bond. We show that the H-bonds have large effects on the spectral shifts, but little or no influence on the excitonic splitting. PMID:27131115
Nanotransformation and current fluctuations in exciton condensate junctions.
Soller, H; Dolcini, F; Komnik, A
2012-04-13
We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale. PMID:22587267
Ultrafast dynamics of excitons in tetracene single crystals
NASA Astrophysics Data System (ADS)
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-01
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states Sn on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Ultrafast dynamics of excitons in tetracene single crystals.
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale. PMID:24655187
Exciton states and optical properties of carbon nanotubes.
Ajiki, Hiroshi
2012-12-01
Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k·p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis. PMID:23139202
Excitonic gap formation and condensation in the bilayer graphene structure
NASA Astrophysics Data System (ADS)
Apinyan, V.; Kopeć, T. K.
2016-09-01
We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.
Singlet Exciton Fission in Nanostructured Organic Solar Cells
Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc
2011-04-13
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
Wannier-Mott excitons in semiconductors with a superlattice
Suris, R. A.
2015-06-15
The effect of the motion of a Wannier-Mott exciton in semiconductors with a superlattice formed by heterojunctions on the exciton binding energy and wave function is analyzed. This effect arises as a result of the fact that the dispersion laws of the electron and hole that form an exciton in a superlattice differ from the quadratic law. The investigated one-dimensional superlattice consists of alternating semiconductor layers with different energy positions of the conduction and valence bands, i.e., with one-dimensional wells and barriers. The exciton state in a superlattice consisting of quantum dots is analyzed. It is demonstrated that the closer the electron and hole effective masses, the greater the dependence of the binding energy on the exciton quasi-momentum. The possibility of replacing the tunneling excitation transfer between superlattice cells with the dipole-dipole one at certain exciton quasi-wave vector values is investigated.
Fluorescence spectroscopy, exciton dynamics, and photochemistry of single allophycocyanin trimers
Ying, L.; Sie, X.S.
1998-12-10
The authors report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasi-independent pairs of strong interacting {alpha}84 and {beta}84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.
Exciton mapping at subwavelength scales in two-dimensional materials.
Tizei, Luiz H G; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu
2015-03-13
Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius. PMID:25815966
Ultrafast dynamics of excitons in tetracene single crystals
Birech, Zephania; Schwoerer, Heinrich; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Size-dependent decoherence of excitonic states in semiconductor microcrystallites
Liu Yuxi; Miranowicz, Adam; Oezdemir, Sahin K.; Koashi, Masato; Imoto, Nobuyuki
2003-03-01
The size-dependent decoherence of the exciton states, resulting from the spontaneous emission, is investigated in a semiconductor spherical microcrystallite under the condition a{sub B}<
Spin-charge order and excitonic effects in sawtooth-like graphene nanoribbons
NASA Astrophysics Data System (ADS)
Wu, Sha; Lu, Wengang; Qi, Jingshan
2016-09-01
In this paper we systematically study electronic structures and excitonic effects in one type of the sawtooth-like graphene nanoribbons. A main feature is that the local magnetism is developed for the certain width and changes with the increase of width. A variety of magnetic orders root in the competition of the short range interaction between the same spin-electrons and long range exchange interaction between opposite spin-electrons. For excitonic effects, the binding energy of degenerate spin-triplets T1 and T-1 is higher than that of T0 for all studied nanoribbons and is size dependent. We reveal the underlying physical mechanism from the charge distributions of excitons and its correlation with the spin-resolved charge density distributions in the ground state. We find that the electrons and holes in degenerate spin-triplets T1 and T-1 are closer together and thus the interaction between them is more strong, while the distribution of electrons and holes is relatively more disperse for T0, indicating the weaker interaction. We hope that these interesting results are able to be detected in the experiment and these multi-performance samples are better utilized in future device applications.
Exciton Binding energies and effective masses in Organo-lead Tri-Halide Perovskites
NASA Astrophysics Data System (ADS)
Portugall, Oliver; Miyata, Atsuhiko; Mitioglu, Anatol; Plochocka, Paulina; Wang, Jacob Tse-Wei; Stranks, Samuel; Snaith, Henry; Nicholas, Robin; Lncmi Toulouse Team; Oxford University Team
2015-03-01
Solid-state perovskite-based solar cells have made a dramatic impact on emerging PV research with efficiencies of over 17% already achieved. However, to date the basic electronic properties of the perovskites such as the electron and hole effective masses and the exciton binding energy are not well known. We have measured both for methyl ammonium lead tri-iodide using magneto absorption in very high magnetic fields up to 150T showing that the exciton binding energy at low temperatures is only 16 meV, a value three times smaller than previously thought and sufficiently small to completely transform the way in which the devices must operate. Landau level spectroscopy shows that the reduced effective mass of 0.104 me is also smaller than previously thought. In addition by using a fast pulse 150T magnet we measure the band structure change due to the structural phase transition that occurs in this system at around 160K. We also observe Landau levels in the high temperature phase as used for device production, which has a very similar effective mass and the analysis suggests an exciton binding energy which is even smaller than in the low temperature phase.
Fractional Solitons in Excitonic Josephson Junctions
Hsu, Ya-Fen; Su, Jung-Jung
2015-01-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 – until ϕ0 > π – then the alternative group of solitons with Q = ϕ0/2π − 1 takes place and switches the polarity of CPR. PMID:26511770
Fractional Solitons in Excitonic Josephson Junctions.
Hsu, Ya-Fen; Su, Jung-Jung
2015-01-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR. PMID:26511770
Fractional Solitons in Excitonic Josephson Junctions
NASA Astrophysics Data System (ADS)
Hsu, Ya-Fen; Su, Jung-Jung
2015-10-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR.
Valley excitons in two-dimensional semiconductors
Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang
2014-12-30
Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibitmore » remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.« less
Valley excitons in two-dimensional semiconductors
Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang
2014-12-30
Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibit remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.
Excitonic effects in oxyhalide scintillating host compounds
Shwetha, G.; Kanchana, V.; Valsakumar, M. C.
2014-10-07
Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.
Excitons in semiconducting superlattices, quantum wells, and ternary alloys
Sturge, M.D. . Dept. of Physics); Nahory, R.E.; Tamargo, M.C. )
1992-06-01
Semiconducting layered structures can now be fabricated with precisely defined layer thicknesses down to one monolayer. An example is the superlattice'' (SL) structure, in which to semiconductors with different band gaps are interleaved. The electronic and optical properties of the SL are quite different from those of the constitutents and offer interesting new possibilities both in device design and in basic physics. This proposal aims to improve our understanding of optically excited states in SL's, particularly in the so-called Type 2 indirect'' SL's in which in electron and hole created by optical excitation are separated both in real and in momentum space. We study these structures by time-resolved tunable laser spectroscopy, with and without external perturbations such as magnetic field, electric field, and uniaxial stress. In SLs with only a few atomic layers per period the familiar effective mass model'' of semiconductor states breaks down. We have made precise optical experiments on well-characterized material to test current first principles'' calculations of the band structure. Our work under this grant has shown that the material we are using is of sufficiently high quality to test the theoretical predictions. Comparison of theory and experiment provides a new and sensitive probe of the interface quality on a fine scale. Statistical analysis of the temperature dependence of the exciton decay dynamics provides complementary information. From a careful study of the exciton spectra of the recently discovered mixed type 1- type 2 CdTe/CdZnTe SLs we have obtained the band offset at the CdTe/CdZnTe interface to unprecedented accuracy.
Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.
Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio
2016-01-01
Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈ 250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics. PMID:26984281
Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons
NASA Astrophysics Data System (ADS)
Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio
2016-03-01
Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ~250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.
Field-induced exciton condensation in LaCoO3.
Sotnikov, A; Kuneš, J
2016-01-01
Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton condensation. We show that, while in both cases the transition can be induced by an external field, temperature dependencies of the critical field have opposite slopes. Based on this result we argue that the experimental observations select the exciton condensation scenario. We show that such condensation is possible due to high mobility of the intermediate spin excitations. The estimated width of the corresponding dispersion is large enough to overrule the order of atomic multiplets and to make the intermediate spin excitation propagating with a specific wave vector the lowest excitation of the system. PMID:27461512
Field-induced exciton condensation in LaCoO3
Sotnikov, A.; Kuneš, J.
2016-01-01
Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton condensation. We show that, while in both cases the transition can be induced by an external field, temperature dependencies of the critical field have opposite slopes. Based on this result we argue that the experimental observations select the exciton condensation scenario. We show that such condensation is possible due to high mobility of the intermediate spin excitations. The estimated width of the corresponding dispersion is large enough to overrule the order of atomic multiplets and to make the intermediate spin excitation propagating with a specific wave vector the lowest excitation of the system. PMID:27461512
Field-induced exciton condensation in LaCoO3
NASA Astrophysics Data System (ADS)
Sotnikov, A.; Kuneš, J.
2016-07-01
Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton condensation. We show that, while in both cases the transition can be induced by an external field, temperature dependencies of the critical field have opposite slopes. Based on this result we argue that the experimental observations select the exciton condensation scenario. We show that such condensation is possible due to high mobility of the intermediate spin excitations. The estimated width of the corresponding dispersion is large enough to overrule the order of atomic multiplets and to make the intermediate spin excitation propagating with a specific wave vector the lowest excitation of the system.
Upconverted photoluminescence induced by radiative coupling between excitons
NASA Astrophysics Data System (ADS)
Matsuda, Takuya; Yokoshi, Nobuhiko; Ishihara, Hajime
2016-04-01
We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and light wave prominently enhances the interaction between different excitonic states via radiation beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the upconverted photoluminescence based on the coherent quantum superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas.
Superradiance of High Density Frenkel Excitons at Room Temperature
NASA Astrophysics Data System (ADS)
Wang, H. Z.; Zheng, X. G.; Zhao, F. L.; Gao, Z. L.; Yu, Z. X.
1995-05-01
Superradiance of high density Frenkel excitons in an R-phycoerythrin single crystal is observed at room temperature for the first time. No fluorescence is observed except the emission at the sharp exciton band when the superradiance of excitons occurs, and the higher the pump density, the sharper the emission bandwidth. A redshift and a blueshift are observed at the rise time and the fall time of the emission pulse, respectively. The experimental results also imply deformed-boson properties of high density Frenkel excitons.
Polarization-dependent exciton dynamics in tetracene single crystals.
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. PMID:25554147
Triplet diffusion in singlet exciton fission sensitized pentacene solar cells
NASA Astrophysics Data System (ADS)
Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.
2013-10-01
Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.
Polarization-dependent exciton dynamics in tetracene single crystals
NASA Astrophysics Data System (ADS)
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-01
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Revealing and Characterizing Dark Excitons through Coherent Multidimensional Spectroscopy.
Tollerud, Jonathan O; Cundiff, Steven T; Davis, Jeffrey A
2016-08-26
Dark excitons are of fundamental importance in a broad range of contexts but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify parity-forbidden and spatially indirect excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms, and coupling strengths. The observations of coherent coupling between these states and bright excitons hint at a role for a multistep process by which excitons in the barrier can relax into the quantum wells. PMID:27610881
Valley-Polarized Interlayer Excitons in 2D Semiconductor Heterostructures
NASA Astrophysics Data System (ADS)
Rivera, Pasqual; Seyler, Kyle; Yu, Hongyi; Schaibley, John; Yan, Jiaqiang; Mandrus, David; Xu, Xiaodong
Vertically stacked monolayers of MoSe2 and WSe2 feature a type-II band alignment causing the formation of interlayer excitons, where the Coulomb bound hole and electron reside in different layers. This species of exciton has lifetime many orders of magnitude longer than intralayer valley excitons, providing a unique and advantageous system for investigating valley exciton physics. Here, we optically pump the MoSe2-WSe2 heterostructure with circularly polarized light, creating interlayer valley excitons with gate-tunable spin-valley polarization lifetime up to 40 ns. This long valley lifetime enables the diffusion of the interlayer valley exciton gas to be visualized. Under increasing excitation power we observe the formation of a ring in the spatial distribution of the valley polarization, a manifestation of significant valley-selective exchange interactions at high exciton densities. The combination of long valley polarization and spatial diffusion makes the interlayer exciton in semiconductor heterostructures an exciting platform for studies of valley exciton physics.
Disorder-enhanced exciton delocalization in an extended dendrimer.
Pouthier, Vincent
2014-08-01
The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory. PMID:25215792
Binding energies of indirect excitons in double quantum well systems
NASA Astrophysics Data System (ADS)
Rossokhaty, Alex; Schmult, Stefan; Dietsche, Werner; von Klitzing, Klaus; Kukushkin, Igor
2011-03-01
A prerequisite towards Bose-Einstein condensation is a cold and dense system of bosons. Indirect excitons in double GaAs/AlGaAs quantum wells (DQWs) are believed to be suitable candidates. Indirect excitons are formed in asymmetric DQW structures by mass filtering, a method which does not require external electric fields. The exciton density and the electron-hole balance can be tuned optically. Binding energies are measured by a resonant microwave absorption technique. Our results show that screening of the indirect excitons becomes already relevant at densities as low as ~ 5 × 109 cm-2 and results in their destruction.
Simulations of singlet exciton diffusion in organic semiconductors: a review
Bjorgaard, Josiah A.; Kose, Muhammet Erkan
2014-12-22
Our review describes the various aspects of simulation strategies for exciton diffusion in condensed phase thin films of organic semiconductors. Several methods for calculating energy transfer rate constants are discussed along with procedures for how to account for energetic disorder. Exciton diffusion can be modelled by using kinetic Monte-Carlo methods or master equations. Recent literature on simulation efforts for estimating exciton diffusion lengths of various conjugated polymers and small molecules are introduced. Moreover, these studies are discussed in the context of the effects of morphology on exciton diffusion and the necessity of accurate treatment of disorder for comparison of simulation results with those of experiment.
Surface photovoltage in exciton absorption range in CdS
NASA Technical Reports Server (NTRS)
Morawski, A.; Banisch, R.; Lagowski, J.
1977-01-01
The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120-160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.
Exciton annihilation in dye-sensitized nanocrystalline semiconductor films
NASA Astrophysics Data System (ADS)
Namekawa, Akihiro; Katoh, Ryuzi
2016-08-01
Exciton annihilation in dye-sensitized nanocrystalline semiconductor (Al2O3) films has been studied through laser-induced fluorescence spectroscopy. The relative quantum yield of the fluorescence decreases with increasing excitation light intensity, the indication being that exciton annihilation occurred. The rate constants of the annihilation were estimated for three dyes, N719, D149, and MK2, that are known to be sensitizing dyes for efficient dye-sensitized solar cells. The hopping time between dye molecules and the diffusion length of excitons within their lifetime were also estimated to facilitate discussion of the relevance of exciton annihilation to primary processes in dye-sensitized solar cells.
Measurement of Exciton Binding Energy of Monolayer WS2
NASA Astrophysics Data System (ADS)
Chen, Xi; Zhu, Bairen; Cui, Xiaodong
Excitonic effects are prominent in monolayer crystal of transition metal dichalcogenides (TMDCs) because of spatial confinement and reduced Coulomb screening. Here we use linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE) to measure the exciton binding energy of monolayer WS2. Peaks for excitonic absorptions of the direct gap located at K valley of the Brillouin zone and transitions from multiple points near Γ point of the Brillouin zone, as well as trion side band are shown in the linear absorption spectra of WS2. But there is no gap between distinct excitons and the continuum of the interband transitions. Strong electron-phonon scattering, overlap of excitons around Γ point and the transfer of the oscillator strength from interband continuum to exciton states make it difficult to resolve the electronic interband transition edge even down to 10K. The gap between excited states of the band-edge exciton and the single-particle band is probed by TP-PLE measurements. And the energy difference between 1s exciton and the single-particle gap gives the exciton binding energy of monolayer WS2 to be about 0.71eV. The work is supported by Area of excellency (AoE/P-04/08), CRF of Hong Kong Research Grant Council (HKU9/CRF/13G) and SRT on New Materials of The University of Hong Kong.
Polarization-dependent exciton dynamics in tetracene single crystals
Zhang, Bo; Zhang, Chunfeng Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Granados Del Águila, Andrés; Groeneveld, Esther; Maan, Jan C; de Mello Donegá, Celso; Christianen, Peter C M
2016-04-26
Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core-shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e-h pairs (excitons) that are responsible for the light emission, but also modifies the e-h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e-h exchange interaction, independently of the e-h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron-hole separation is systematically varied. We are able to unravel the separate effects of e-h overlap and e-h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields. PMID:26982795
NASA Astrophysics Data System (ADS)
Kramar, V. M.; Pugantseva, O. V.
2014-08-01
In the approximation of effective masses for electronic and phononic - dielectric continuum - systems, the influence of spatial bounding, self-polarization, and exciton-phonon interactions on the exciton state in a flat double nanoheterostructure (a nanofilm) - lead iodide in a polymer matrix -is theoretically investigated for the model of a single infinitely deep quantum well. It is demonstrated that the dominating factor determining the energy of the bottom of the ground exciton band and its binding energy is spatial bounding. The relationship between two other effects depends on the nanofilm thickness, namely, the influence of the self-polarization effect in ultrathin films significantly exceeds that of exciton-phonon interaction.
NASA Astrophysics Data System (ADS)
Udal'tsov, Alexander V.
2015-11-01
Polaron theory is often used for the study of electrons and holes mobility in semiconductors when longitudinal optical (LO) phonons are generated upon the charge carriers moving. The polaron theory was applied to explain long-wavelength absorptions observed nearby Soret band in the electronic spectra of assemblies of mono-protonated meso-tetraphenylporphine dimer (TPP2H+) that are interpreted as LO-phonons originated due to proton movement. The energy of hole polaron is found to be 1.50 eV at 77 K. Energy of Franck-Condon transitions of LO-phonons generated by hole polaron moving through water confined in the assemblies with distortions of O-H bonds is 0.2653 eV (2138 cm-1). A broad band around 2127 cm-1 corresponding the same energy of O-H bonds vibrations is observed in IR spectra of the assemblies consisting of water and mainly of TPP2H+ species in the solid state indicating the presence of similar distortions of the hydrogen bonds. The radius of protonic sphere of 0.202 Å, which was estimated as a polaron quasiparticle moving through the confined water at 77 K, is found in agreement with earlier evaluated one of 0.265 Å that was obtained for proton diffusion at 298 K in similar assemblies.
NASA Astrophysics Data System (ADS)
Casteels, W.; Tempere, J.; Devreese, J. T.
2011-03-01
We expand the existing polaron response theory, expressed within the Mori-Zwanzig projection operator formalism applicable for the transfer of arbitrary energy and zero momentum, for the case of finite momentum exchange. A general formula is then derived that can be used to calculate the response of a system to a probe that transfers both momentum and energy to the system. The main extension of the existing polaron response theory is the finite momentum exchange, which was not needed until now, since it is negligible for optical absorption. However, this formalism is needed to calculate the response of the polaronic system consisting of an impurity in a Bose-Einstein condensate (BEC) to Bragg spectroscopy. We show that the well-known features that appear in the optical absorption of the solid-state Fröhlich polaron are also present in the Bragg response of the BEC-impurity polaron. The f-sum rule is written in a form suitable to provide an independent consistency test for our results. The effect of lifetime broadening on the BEC-impurity spectrum is examined. The results derived here are discussed in the framework of an experimental realization consisting of a lithium impurity in a sodium condensate.
Band or Polaron: The Hole Conduction Mechanism in the p-Type Spinel Rh 2ZnO4
Nagaraja, A. R.; Perry, N. H.; Mason, T. O.; Tang, Y.; Grayson, M.; Paudel, T. R.; Lany, S.; Zunger, A.
2012-01-01
Given the emerging role of oxide spinels as hole conductors, we discuss in this article the traditional vs. new methodologies of determining the type of conduction mechanism at play - localized polaronic vs. band-like transport. Applying (i) traditional small polaron analysis to our in-situ high temperature four-point conductivity and thermopower measurements, we previously found an activated mobility, which is indicative of the small polaron mechanism. However, (ii) employing the recent developments in correcting density functional methodologies for hole localization, we predict that the self-trapped hole is unstable and that Rh{sub 2}ZnO{sub 4} is instead a band conductor with a large effective mass. The hole mobility measured by high-field room temperature Hall effect also suggests band rather than polaron conduction. The apparent contradiction between the conclusion of the traditional procedure (i) and first-principles theory (ii) is resolved by taking into account in the previous transport analysis the temperature dependence of the effective density of states, which leads to the result that the mobility is actually temperature-independent in Rh{sub 2}ZnO{sub 4}. Our case study on Rh{sub 2}ZnO{sub 4} illustrates the range of experimental and theoretical approaches at hand to determine whether the transport mechanism of a semiconductor is band or small polaron conduction.
Band or polaron: The hole conduction mechanism in the p-type spinel Rh2ZnO4
Nagaraja, Arpun R.; Perry, Nicola H.; Mason, Thomas O.; Tang, Yang; Grayson, Matthew; Paudel, Tula; Lany, Stephan; Zunger, Alex
2011-08-05
Given the emerging role of oxide spinels as hole conductors, we discuss in this article the traditional vs. new methodologies of determining the type of conduction mechanism at play––localized polaronic vs. band-like transport. Applying (i) traditional small polaron analysis to our in-situ high temperature four-point conductivity and thermopower measurements, we previously found an activated mobility, which is indicative of the small polaron mechanism. However, (ii) employing the recent developments in correcting density functional methodologies for hole localization, we predict that the self-trapped hole is unstable and that Rh₂ZnO₄ is instead a band conductor with a large effective mass. The hole mobility measured by high-field room temperature Hall effect also suggests band rather than polaron conduction. The apparent contradiction between the conclusion of the traditional procedure (i) and first-principles theory (ii) is resolved by taking into account in the previous transport analysis the temperature dependence of the effective density of states, which leads to the result that the mobility is actually temperature-independent in Rh₂ZnO₄. Our case study on Rh₂ZnO₄ illustrates the range of experimental and theoretical approaches at hand to determine whether the transport mechanism of a semiconductor is band or small polaron conduction.
NASA Astrophysics Data System (ADS)
Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.
2015-03-01
The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be
Excitonic luminescence upconversion in a two-dimensional semiconductor
NASA Astrophysics Data System (ADS)
Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; Yan, Jiaqiang; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Dery, Hanan; Yao, Wang; Xu, Xiaodong
2016-04-01
Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. This phenomenon lies at the heart of optical refrigeration in solids, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities or exciton-phonon coupling. Here, we demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors. Since the charged exciton binding energy closely matches the 31 meV A1' optical phonon, we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. In addition, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.
Ultrafast exciton relaxation in monolayer transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Thilagam, A.
2016-04-01
We examine a mechanism by which excitons undergo ultrafast relaxation in common monolayer transition metal dichalcogenides. It is shown that at densities ≈1 × 1011 cm-2 and temperatures ≤60 K, excitons in well known monolayers (MoS2, MoSe2, WS2, and WSe2) exist as point-like structureless electron-hole quasi-particles. We evaluate the average rate of exciton energy relaxation due to acoustic phonons via the deformation potential and the piezoelectric coupling mechanisms and examine the effect of spreading of the excitonic wavefunction into the region perpendicular to the monolayer plane. Our results show that the exciton relaxation rate is enhanced with increase in the exciton temperature, while it is decreased with increase in the lattice temperature. Good agreements with available experimental data are obtained when the calculations are extrapolated to room temperatures. A unified approach taking into account the deformation potential and piezoelectric coupling mechanisms shows that exciton relaxation induced by phonons is as significant as defect assisted scattering and trapping of excitons by surface states in monolayer transition metal dichalcogenides.
Measuring and Modeling Exciton Dynamics in Multichromophore Macromolecules
NASA Astrophysics Data System (ADS)
Weingarten, Daniel; Hu, Nan; Lacount, Michael; Ferguson, Andrew; Dessau, Daniel; Walba, David; Vandelagemaat, Jao; Lusk, Mark; Rumbles, Garry; Shaheen, Sean
2014-03-01
Attaining specific control over the dynamics of exciton movement in organic photovoltaics (OPV) has, thus far, been a largely unachieved goal of OPV design. Such an understanding of exciton transfer dynamics would allow for the design of macromolecules whose energetics, bandgaps, and conformational properties allow for control of exciton flow toward specific reaction site chromophores, potentially enabling non-linear improvements in energy harvesting. To better understand exciton movement we synthesized and characterized a multi-chromophoric macromolecule and measured the dynamics of exciton transfer across coupled chromophores. Our model system is a hexabenzocoronene molecule attached to six oligothiophene. We developed a kinetic model and by fitting it to the decay rates of excited states measured via time-correlated single photon counting, we were able to extract rates for exciton transfer between chromophores. Since this macromolecule exhibits liquid crystalline aggregation behavior, observing the dependence of exciton transfer rates on solution concentration yields an improved understanding of exciton movement within a single molecule as well as the dependence of that transfer process on local material structure.
Spin injection effects on exciton distributions in conjugated organic semiconductors
NASA Astrophysics Data System (ADS)
Yunus, Mohammad; Ruden, P. Paul; Smith, Darryl
2008-03-01
Conjugated organic semiconductors are under rapid development as the active material in organic light emitting diodes (OLEDs). Electrons and holes injected into the organic semiconductor form bound singlet or triplet excitons. Singlet excitons may recombine radiatively giving rise to light emission whereas triplet excitons do not recombine radiatively. Thus the quantum efficiency of OLEDs is limited by the fraction of singlet excitons, χS. In this work, we explore theoretically an approach to control χS through spin-polarized electron and hole injection from ferromagnetic contacts. Conventional ferromagnetic transition metals and half-metallic materials, such as LSMO, are considered as candidate electrode materials. Electron and hole transport in the organic semiconductor is treated through the conventional device equations with the formation of excitons described by a Langevin process. Once formed, the excitons may recombine or diffuse. Triplet excitons have a lower recombination probability and hence a longer diffusion length. The model calculations yield steady state spatial profiles for singlet and triplet exciton densities in the organic semiconductor.
Storing excitons in transition-metal dichalcogenides using dark states
NASA Astrophysics Data System (ADS)
Gunlycke, Daniel; Tseng, Frank; Simsek, Ergun
Monolayer transition-metal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electron-hole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electron-hole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.
Mapping the exciton diffusion in semiconductor nanocrystal solids.
Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail
2015-03-24
Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling. PMID:25682881
Temperature Dependence of the Free Excitons in GaN
NASA Astrophysics Data System (ADS)
Reynolds, D. C.; Collins, T. C.
2002-03-01
We are presenting the temperature dependence of the photoluminescence (PL) of the free excitons and donor bound exciton ( D^circ ,X) in GaN. The excitons involved are the longitudinal, the Γ5 and the Γ6 excitons as well as D^circ ,X. The temperature dependence of the energy positions of Γ_5, Γ_6, and D^circ ,X are well mapped out using the Varshni equation in a temperature range of 0 -- 60 K. In this temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation used for the other excitons. If more than one columnar structural direction is present, then in PL, more than one longitudinal mode might be observed. The sample being investigated shows two longitudinal modes. The energy separation between the longitudinal and transverse modes has been reported by Hopfield and Thomas. One component in the separation is the polarizability, which has some temperature dependence. The longitudinal exciton then has a band gap temperature dependence, which is predicted by the Varshni equation and an additional temperature dependence due to the polarizability. We have used the Varshni equation plus a linear and quadratic temperature dependence term to map the energy positions of the longitudinal excitons.
Excitons in atomically thin black phosphorus
NASA Astrophysics Data System (ADS)
Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Tabis, W.; Maude, D. K.; Plochocka, P.
2016-03-01
Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in-plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blueshift of the emission with increasing temperature is well described using a two-oscillator model for the temperature dependence of the band gap.
Electro-optical properties of Rydberg excitons
NASA Astrophysics Data System (ADS)
Zielińska-Raczyńska, Sylwia; Ziemkiewicz, David; Czajkowski, Gerard
2016-07-01
We show how to compute the electro-optical functions (absorption, reflection, and transmission) when Rydberg exciton-polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. With the use of the real density matrix approach, numerical calculations applied for the Cu2O crystal are performed. We also examine in detail and explain the dependence of the resonance displacement on the state number and applied electric field strength. We report a fairly good agreement with recently published experimental data.
Feasibility study of a nuclear exciton laser
NASA Astrophysics Data System (ADS)
ten Brinke, Nicolai; Schützhold, Ralf; Habs, Dietrich
2013-05-01
Nuclear excitons known from Mössbauer spectroscopy describe coherent excitations of a large number of nuclei—analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free-electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology; but our design should be feasible in the UV regime, for example.
Exciton Transport and Perfect Coulomb Drag
NASA Astrophysics Data System (ADS)
Nandi, Debaleena
2013-03-01
Exciton condensation is realized in closely-spaced bilayer quantum Hall systems at νT = 1 when the total density in the two 2D electron layers matches the Landau level degeneracy. In this state, electrons in one layer become tightly bound to holes in the other layer, forming a condensate similar to the Cooper pairs in a superconductor. Being charge neutral, these excitons ought to be free to move throughout the bulk of the quantum Hall fluid. One therefore expects that electron current driven in one layer would spontaneously generate a ``hole'' current in the other layer, even in the otherwise insulating bulk of the 2D system. We demonstrate precisely this effect, using a Corbino geometry to defeat edge state transport. Our sample contains two essentially identical two-dimensional electron systems (2DES) in GaAs quantum wells separated by a thin AlGaAs barrier. It is patterned into an annulus with arms protruding from each rim that provide contact to each 2DES separately. A current drag geometry is realized by applying a drive voltage between the outer and inner rim on one 2DES layer while the two rims on the opposite layer are connected together in a closed loop. There is no direct electrical connection between the two layers. At νT = 1 the bulk of the Corbino annulus becomes insulating owing to the quantum Hall gap and net charge transport across the bulk is suppressed. Nevertheless, we find that in the drag geometry appreciable currents do flow in each layer. These currents are almost exactly equal magnitude but, crucially, flow in opposite directions. This phenomenon reflects exciton transport within the νT = 1 condensate, rather than its quasiparticle excitations. We find that quasiparticle transport competes with exciton transport at elevated temperatures, drive levels, and layer separations. This work represents a collaboration with A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer and K.W. West. This work is supported by the NSF under grant DMR-1003080.
Entangled exciton states in quantum dot molecules
NASA Astrophysics Data System (ADS)
Bayer, Manfred
2002-03-01
Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantum dots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantum dot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For
Charge-transfer excitons at organic semiconductor surfaces and interfaces.
Zhu, X-Y; Yang, Q; Muntwiler, M
2009-11-17
When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D
Hybrid em wave - polar semiconductor interaction: A polaronic study
Paliwal, Ayushi Dubey, Swati; Ghosh, S.
2015-07-31
Present paper considers incidence of a most realistic hybrid pump wave on a weakly polar semiconductor having a very small coupling constant. Possibility of optical parametric interaction has been explored in the presence of an external transverse magnetic field. The effect of doping concentrations and transverse magnetostatic field on threshold characteristics of optical parametric interaction in polar semiconductor plasma has been studied, using hydrodynamic model of semiconductors, in the far infrared regime. Numerical estimations have been carried out by using data of weakly polar III-V GaAs semiconductor and influence of control parameters on electron-LO phonon interaction has been analyzed. A particular range of physical parameters is found to be suitable for minimum threshold. The choice of nonlinear medium and favorable range of operating parameters are crucial aspects in design and fabrication of parametric amplifiers and oscillators. The hybrid mode of the pump is found to be favorable for the onset of the said process and realization of a low cost amplifier.
Exciton complexes in low dimensional transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Thilagam, A.
2014-08-01
We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter α, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of α is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.
Exciton complexes in low dimensional transition metal dichalcogenides
Thilagam, A.
2014-08-07
We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter α, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of α is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.
Temperature effects in excitonic condensation driven by the lattice distortion
NASA Astrophysics Data System (ADS)
Do, Thi-Hong-Hai; Nguyen, Huu-Nha; Nguyen, Thi-Giang; Phan, Van-Nham
2016-06-01
The stability of the excitonic condensation at low temperature driven by a coupling of electrons to vibrational degrees of freedom in semimetal two-dimensional electronic system is discussed. In the framework of the unrestricted Hartree-Fock approximation, we derive a set of equations to determine both the excitonic condensate order parameter and lattice displacement self-consistently. By lowering temperature we find out a semimetal-insulator transition in the system if the coupling is large enough. The insulating state typifies an excitonic condensation accompanied by a finite lattice distortion. Increasing temperature, both excitonic condensate order parameter and the lattice distortion decrease and then disappear in the same manner. Microscopic analysis in momentum space strongly specifies that the excitonic condensate driven by the lattice distortion favours the BCS type.
The excited spin-triplet state of a charged exciton in quantum dots.
Molas, M R; Nicolet, A A L; Piętka, B; Babiński, A; Potemski, M
2016-09-14
We report on spectroscopic studies of resonances related to ladder of states of a charged exciton in single GaAlAs/AlAs quantum dot structures. Polarization-resolved photoluminescence, photoluminescence excitation and photon-correlation measurements were performed at low (T = 4.2 K) temperature also in magnetic field applied in Faraday configuration. The investigated resonances are assigned to three different configurations of a positively charged exciton. Together with a singlet ground state and a conventional triplet state (involving an electron from the ground state electronic s-shell), an excited triplet state, which involved an electron from the excited electronic p-shell was identified in single dots. The appearance of an emission line related to the latter complex is due to a partially suppressed electron relaxation in the investigated dots. An analysis of this emission line allows us to scrupulously determine properties of the excited triplet state and compare them with those of the conventional triplet state. Both triplets exhibit similar patterns of anisotropic fine structure and Zeeman splitting, however their amplitudes significantly differ for those two states. Presented results emphasize the role of the symmetry of the electronic state on the properties of the triplet states of two holes + electron excitonic complex. PMID:27391126
NASA Astrophysics Data System (ADS)
Slobodeniuk, A. O.; Basko, D. M.
2016-09-01
We perform a theoretical study of radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers. This decay necessarily involves an electronic spin flip. The intrinsic decay mechanism due to interband spin–flip dipole moment perpendicular to the monolayer plane, gives a rate about 100–1000 times smaller than that of bright excitons. However, we find that this mechanism also introduces an energy splitting due to a local field effect, and the whole oscillator strength is contained in the higher-energy component, while the lowest-energy state remains dark and needs an extrinsic spin–flip mechanism for the decay. Rashba effect due to a perpendicular electric field or a dielectric substrate, gives a negligible radiative decay rate (about 107 times slower than that of bright excitons). Spin flip due to Zeeman effect in a sufficiently strong in-plane magnetic field can give a decay rate comparable to that due to the intrinsic interband spin–flip dipole.
The excited spin-triplet state of a charged exciton in quantum dots
NASA Astrophysics Data System (ADS)
Molas, M. R.; Nicolet, A. A. L.; Piętka, B.; Babiński, A.; Potemski, M.
2016-09-01
We report on spectroscopic studies of resonances related to ladder of states of a charged exciton in single GaAlAs/AlAs quantum dot structures. Polarization-resolved photoluminescence, photoluminescence excitation and photon-correlation measurements were performed at low (T = 4.2 K) temperature also in magnetic field applied in Faraday configuration. The investigated resonances are assigned to three different configurations of a positively charged exciton. Together with a singlet ground state and a conventional triplet state (involving an electron from the ground state electronic s-shell), an excited triplet state, which involved an electron from the excited electronic p-shell was identified in single dots. The appearance of an emission line related to the latter complex is due to a partially suppressed electron relaxation in the investigated dots. An analysis of this emission line allows us to scrupulously determine properties of the excited triplet state and compare them with those of the conventional triplet state. Both triplets exhibit similar patterns of anisotropic fine structure and Zeeman splitting, however their amplitudes significantly differ for those two states. Presented results emphasize the role of the symmetry of the electronic state on the properties of the triplet states of two holes + electron excitonic complex.
Probing excitonic dark states in single-layer tungsten disulphide
NASA Astrophysics Data System (ADS)
Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang
2014-09-01
Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many
Analog of small Holstein polaron in hydrogen-bonded amide systems
NASA Astrophysics Data System (ADS)
Alexander, D. M.
1985-01-01
A class of amide-I (C = O stretch) related excitations and their contribution to the spectral function for infrared absorption is determined by use of the Davydov Hamiltonian. The treatment is a fully quantum, finite-temperature one. A consistent picture and a quantitative fit to the absorption data for crystalline acetanilide confirms that the model adequately explains the anomalous behavior cited by Careri et al. The localized excitation responsible for this behavior is the vibronic analog of the small Holstein polaron. The possible extension to other modes and biological relevance is examined.
Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla.
Stier, Andreas V; McCreary, Kathleen M; Jonker, Berend T; Kono, Junichiro; Crooker, Scott A
2016-01-01
In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately -230 μeV T(-1) (g-factor ≃-4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radii of ∼1.53 and ∼1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). These results highlight the utility of high magnetic fields for understanding new two-dimensional materials. PMID:26856412
Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla
Stier, Andreas V.; McCreary, Kathleen M.; Jonker, Berend T.; Kono, Junichiro; Crooker, Scott A.
2016-01-01
In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately −230 μeV T−1 (g-factor ≃−4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radii of ∼1.53 and ∼1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). These results highlight the utility of high magnetic fields for understanding new two-dimensional materials. PMID:26856412
Exciton Correlations in Intramolecular Singlet Fission.
Sanders, Samuel N; Kumarasamy, Elango; Pun, Andrew B; Appavoo, Kannatassen; Steigerwald, Michael L; Campos, Luis M; Sfeir, Matthew Y
2016-06-15
We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases. PMID:27183040
Excitons and charges at organic semiconductor heterojunctions.
Friend, Richard H; Phillips, Matthew; Rao, Akshay; Wilson, Mark W B; Li, Zhe; McNeill, Christopher R
2012-01-01
All-organic heterojunction solar cells now provide very high quantum efficiencies for charge generation and rapidly-improving power conversion efficiencies. Charge generation and separation however, must overcome the strong Coulomb interactions between electrons and holes in these materials that is manifest also through the large exchange energies usually observed. We show for a polymer-polymer system with low charge generation efficiency that this arises through intersystem crossing from the photogenerated charge-transfer state to a lower lying triplet state, mediated by the proton hyperfine interaction, and that the activation barrier for full separation of electrons and holes is of the order of 250 meV. We observe, using transient optical spectroscopy, the processes of charge separation, recombination and sweep-out in efficient polymer-fullerene devices. We report also on the process of singlet exciton fission to form a pair of triplet excitons in pentacene that can later be dissociated against a heterojunction formed with C60. PMID:22470984
Robust excitons inhabit soft supramolecular nanotubes
Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.
2014-01-01
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336
Exciton Dynamics in Semiconducting Carbon Nanotubes
Graham, Matt; Chmeliov, Javgenij; Ma, Yingzhong; Shinohara, Nori; Green, Alexander A.; Hersam, Mark C.; Valkunas, Leonas; Fleming, Graham
2010-01-01
We report femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E11) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E22) is excited than that measured by exciting its first transition-allowed state (E11). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E22 excitation, whereas the corresponding amplitude scales linearly with the square root of the E11 excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E22 and E11 excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model.
Robust excitons inhabit soft supramolecular nanotubes.
Eisele, Dörthe M; Arias, Dylan H; Fu, Xiaofeng; Bloemsma, Erik A; Steiner, Colby P; Jensen, Russell A; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G
2014-08-19
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions--prerequisites for efficient energy transport--are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336
Robust excitons inhabit soft supramolecular nanotubes
NASA Astrophysics Data System (ADS)
Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.
2014-08-01
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions-prerequisites for efficient energy transport-are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials.
Interaction between excitons and 2DEG Landau levels in modulation doped GaAs/AlGaAs heterojunctions
NASA Astrophysics Data System (ADS)
Preezant, Yulia; Gabbay, A.; Eitan, A. A.; Ashkinadze, B. M.; Cohen, E.; Pfeiffer, L. N.
2007-04-01
The reflection and photoluminescence spectra of n-type, modulation-doped GaAs/AlxGa1-xAs wide quantum wells (QW) and heterojunctions (HJ) were studied at T = 2K and under a perpendicularly applied magnetic field. The spectra show two groups of very sharp lines that originate in two types of excitations: excitons, whose center of mass motion is quantized, and interband Landau transitions of the 2DEG, that is confined to the QW edges. Abrupt energy and intensity variations of both types of lines are observed at filling factors ν = 1,2 of the 2DEG. These variations are interpreted in terms of an interaction between excitations that are spatially confined in separate parts of the wide QW (or HJ). It leads to energy level splittings and increased exciton dissociation by the magnetized 2DEG layer.
Theory of high-field combined exciton-cyclotron resonance
NASA Astrophysics Data System (ADS)
Dzyubenko, Alexander
2003-03-01
Optical manifestations of many-body effects in low-dimensional electron and electron-hole (e-h) systems in magnetic fields have been the focus of many experimental and theoretical studies during the past decade. An interesting manifestation of many-body effects are shake-up processes (Finkelstein et al. 1996) in the photoluminescence of a two-dimensional electron gas (2DEG): After the recombination of the e-h pair, one electron is excited to one of the higher Landau levels. A closely related phenomenon, combined exciton-cyclotron resonance (ExCR), has also been identified in low-density 2DEG systems: Here, an incident photon creates an exciton and simultaneously excites one electron to higher Landau levels (Yakovlev et al. 1997). These phenomena and the relation between them remain only partially understood. In this work, I develop a theory of ExCR in a low-density strictly-2D electron gas in high magnetic fields. Electrons are assumed to be spin-polarized and occupy zero Landau level. In the low-density limit, ExCR can be considered to be a three-particle resonance involving a charged system of two electrons and one hole, 2e-h, in the final state. Importantly, there is a coupling of the center-of-mass and internal motions for charged e-h complexes in magnetic fields. In order to describe the high-field ExCR, I obtain the complete spectra of the 2e-h eigenstates in higher Landau levels with a consistent treatment of the Coulomb correlations. I derive exact ExCR selection rules that follow from the existing dynamical symmetries, magnetic translations and rotations about the magnetic field axis. This allows one to establish the characteristic features of the high-field ExCR; in particular, the double-peak structure of the transitions to the first electron Landau level is predicted. I also consider combined hole-ExCR in a low-density 2DEG, a resonance in which the hole is excited to higher hole Lnadau levels. It is shown that the high-field hole-ExCR has different
Valley depolarization dynamics and valley Hall effect of excitons in monolayer and bilayer MoS2
NASA Astrophysics Data System (ADS)
Yu, T.; Wu, M. W.
2016-01-01
We investigate the valley depolarization dynamics and valley Hall effect of exciton due to the electron-hole exchange interaction in mono- and bilayer MoS2 by solving the kinetic spin Bloch equations. The effect of the exciton energy spectra by the electron-hole exchange interaction is explicitly considered. For the valley depolarization dynamics, in the monolayer MoS2, it is found that in the strong scattering regime, the conventional motional narrowing picture in the conventional strong scattering regime is no longer valid, and a novel valley depolarization channel is opened. For the valley Hall effect of exciton, in both the mono- and bilayer MoS2, with the exciton equally pumped in the K and K' valleys, the system can evolve into the equilibrium state where the valley polarization is parallel to the effective magnetic field due to the exchange interaction. With the drift of this equilibrium state by applied uniaxial strain, the exchange interaction can induce the momentum-dependent valley/photoluminesence polarization, which leads to the valley/photoluminesence Hall current. Specifically, the disorder strength dependence of the valley Hall conductivity is revealed. In the strong scattering regime, the valley Hall conductivity decreases with the increase of the disorder strength; whereas in the weak scattering regime, it saturates to a constant, which can be much larger than the one in Fermi system due to the absence of the Pauli blocking.
Exciton spin decay modified by strong electron-hole exchange interaction.
Astakhov, G V; Koudinov, A V; Kavokin, K V; Gagis, I S; Kusrayev, Yu G; Ossau, W; Molenkamp, L W
2007-07-01
We study exciton spin decay in the regime of strong electron-hole exchange interaction, which occurs in a wide variety of semiconductor nanostructures. In this regime the electron spin precession is restricted within a sector formed by the external magnetic field and the effective exchange fields triggered by random spin flips of the hole. Using Hanle effect measurements, we demonstrate that this mechanism dominates our experiments in CdTe/(Cd,Mg)Te quantum wells. We present calculations that provide a consistent description of the experimental results, which is supported by independent measurements of the parameters entering the model. PMID:17678176
Coherent coupling of magneto-excitons probed by two-dimensional Fourier transform spectroscopy
NASA Astrophysics Data System (ADS)
Paul, Jagannath; Liu, Cunming; McGill, Stephen; Hilton, David; Karaiskaj, Denis
We present the coherent two dimensional Fourier Transform (2DFT) spectra of magneto-excitons in undoped GaAs quantum wells at high magnetic field up to 10 Tesla. The 2DFT data reveal strong coherent coupling between resonances and line shapes which are strikingly different from the zero field spectra. 2DFT spectra measured using co-linear and co-circular polarizations at low temperatures will be discussed. The work at USF and UAB was supported by the National Science Foundation under Grant Number DMR-1409473. The work at NHMFL, Florida State University was supported by the National Science Foundation under Grant Numbers DMR-1157490 and DMR-1229217.
Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier
LYO,SUNGKWUN K.
1999-12-06
We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.
Exciton Transfer in Carbon Nanotube Aggregates for Energy Harvesting Applications
NASA Astrophysics Data System (ADS)
Davoody, Amirhossein; Karimi, Farhad; Knezevic, Irena
Carbon nanotubes (CNTs) are promising building blocks for organic photovoltaic devices, owing to their tunable band gap, mechanical and chemical stability. We study intertube excitonic energy transfer between pairs of CNTs with different orientations and band gaps. The optically bright and dark excitonic states in CNTs are calculated by solving the Bethe-Salpeter equation. We calculate the exciton transfer rates due to the direct and exchange Coulomb interactions, as well as the second-order phonon-assisted processes. We show the importance of phonons in calculating the transfer rates that match the measurements. In addition, we discuss the contribution of optically inactive excited states in the exciton transfer process, which is difficult to determine experimentally. Furthermore, we study the effects of sample inhomogeneity, impurities, and temperature on the exciton transfer rate. The inhomogeneity in the CNT sample dielectric function can increase the transfer rate by about a factor of two. We show that the exciton confinement by impurities has a detrimental effect on the transfer rate between pairs of similar CNTs. The exciton transfer rate increases monotonically with increasing temperature. Support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0008712.
Dynamical fermion mass generation and exciton spectra in graphene
Zhang Chunxu; Liu Guozhu; Huang Mingqiu
2011-03-15
The Coulomb interaction between massless Dirac fermions may induce dynamical chiral symmetry breaking by forming excitonic pairs in clean graphene, leading to semimetal-insulator transition. If the Dirac fermions have zero bare mass, an exact continuous chiral symmetry is dynamically broken and thus there are massless Goldstone excitons. If the Dirac fermions have a small bare mass, an approximate continuous chiral symmetry is dynamically broken and the resultant Goldstone-type excitons become massive, which is analogous to what happens in QCD. In this paper, after solving the Dyson-Schwinger gap equation in the presence of a small bare fermion mass, we found a remarkable reduction of the critical Coulomb interaction strength for excitonic pair formation and a strong enhancement of dynamical fermion mass. We then calculate the masses of Goldstone-type excitons using the Shifman-Vainshtein-Zakharov sum-rule method and operator product expansion technique developed in QCD and find that the exciton masses are much larger than bare fermion mass but smaller than the width of dynamical fermion mass gap. We also study the spin susceptibilities and estimate the masses of non-Goldstone-type excitons using the same tools.
NASA Astrophysics Data System (ADS)
Triana, C. A.; Granqvist, C. G.; Niklasson, G. A.
2015-07-01
Thin films of Li x WO 3 - z with 0 ≤ x ≤ 0.27 and 0 ≤ z ≤ 0.27 were prepared by sputter deposition followed by electrochemical lithiation. Kramers-Kronig-consistent complex dielectric functions were obtained for these films by numerical inversion of experimental spectra of optical transmittance and reflectance by using a superposition of Tauc-Lorentz and Lorentz oscillator models. Low-energy optical absorption bands were induced by oxygen vacancies and/or by electrochemical intercalation of Li+ species together with charge compensating electrons. The experimental optical conductivity was fitted to a small-polaron model for disordered systems with strong electron-phonon interaction, taking into account transitions near the Fermi level. The optical absorption is due to small-polaron hopping and associated with the formation of W5+ states due to transfer of electrons from oxygen vacancies and/or insertion of Li+ species. The results also show increases in the Fermi level, caused by oxygen deficiency or Li+ insertion, which occur along with a band gap shift towards higher energies for the Li+ intercalated films.
Polaron effects on the dc- and ac-tunneling characteristics of molecular Josephson junctions
NASA Astrophysics Data System (ADS)
Wu, B. H.; Cao, J. C.; Timm, C.
2012-07-01
We study the interplay of polaronic effect and superconductivity in transport through molecular Josephson junctions. The tunneling rates of electrons are dominated by vibronic replicas of the superconducting gap, which show up as prominent features in the differential conductance for the dc and ac current. For relatively large molecule-lead coupling, a features that appears when the Josephson frequency matches the vibron frequency can be identified with an over-the-gap structure observed by Marchenkov [Nat. Nanotech. 1748-338710.1038/nnano.2007.2182, 481 (2007)]. However, we are more concerned with the weak-coupling limit, where resonant tunneling through the molecular level dominates. We find that certain features involving both Andreev reflection and vibron emission show an unusual shift of the bias voltage V at their maximum with the gate voltage Vg as V˜(2/3)Vg. Moreover, due to the polaronic effect, the ac Josephson current shows a phase shift of π when the bias eV is increased by one vibronic energy quantum ℏωv. This distinctive even-odd effect is explained in terms of the different sign of the coupling to vibrons of electrons and of Andreev-reflected holes.
Photochromism and polaronic photocharge localization in diluted KTa1-xNbxO3
NASA Astrophysics Data System (ADS)
Gubaev, A. I.; Kapphan, S. E.; Jastrabik, L.; Trepakov, V. A.; Syrnikov, P. P.
2006-07-01
Ultraviolet (UV)-light-induced optical absorption in the near infrared (NIR) region was observed in diluted KTa1-xNbxO3 single crystals (x =0,0.004,0.007,0.012,0.07) at low temperatures. Illumination by wideband light (3.10-4.13eV, 300-400nm) is accompanied by the appearance of a broad NIR absorption band with the position of the maxima varying in the 0.69-0.8eV (1.54-1.79μ, T =1.3K) region for different Nb concentrations. This UV-light-induced absorption is absent in nominally pure KTaO3, as well as in all Nb diluted specimens at elevated temperatures. The centers responsible for the photochromic NIR absorption bands are tied to interband optical transitions of pair Nb4+ electronic polarons. The photochromic experimental data, supplemented by luminescence studies in the visible range, evidence the strong localization of the photocharge carriers by pair Nb4+ polarons at low temperatures. It is suggested that namely the strong localization of the photocarriers plays a crucial role in photoinduced gigantic dielectric effects and possible phase transitions, which have been recognized recently in incipient ferroelectrics at low temperatures.
Ideal diode equation for organic heterojunctions. II. The role of polaron pair recombination
Giebink, Noel C; Lassiter, Brian E; Wiederrecht, Gary P; Wasielewski, Michael R; Forrest, Stephen R.
2010-10-04
In paper I [N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010)], we proposed that current transport in a donor-acceptor heterojunction (HJ) depends on the balance of polaron pair (PP) dissociation and recombination. Here, we directly investigate these processes in archetype planar copper phthalocyanine (CuPc)/C{sub 60} and boron subpthalocyanine chloride (SubPc)/C{sub 60} HJs. Using intensity-modulated photocurrent spectroscopy (IMPS) along with emission from interfacial Pc/C{sub 60} exciplex states, we monitor the geminate PP density at the HJ as a function of bias and illumination intensity. We find that the SubPc/C{sub 60} PP density is limited by the dynamics of dissociation, where it increases from short circuit, and peaks at open circuit. In contrast, that of CuPc/C{sub 60} is dominated by faster recombination kinetics and declines monotonically over the same voltage domain. We conclude that the PP recombination rate depends on electric field, and propose a simple expression that qualitatively explains the observed exciplex luminescence and IMPS behavior for these HJs. Our results provide insight into polaron pair recombination, which governs the current-voltage characteristics of organic heterojunctions in the dark and under illumination.
Ideal Diode Equation for Organic Heterojunctions. II. The Role of Polaron Pair Recombination
Giebink, Noel C; Wiederrecht, Gary P; Wasielewski, Michael R; Forrest, Stephen R.
2010-10-04
In paper I [N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010)], we proposed that current transport in a donor-acceptor heterojunction (HJ) depends on the balance of polaron pair (PP) dissociation and recombination. Here, we directly investigate these processes in archetype planar copper phthalocyanine (CuPc)/C{sub 60} and boron subpthalocyanine chloride (SubPc)/C{sub 60} HJs. Using intensity-modulated photocurrent spectroscopy (IMPS) along with emission from interfacial Pc/C{sub 60} exciplex states, we monitor the geminate PP density at the HJ as a function of bias and illumination intensity. We find that the SubPc/C{sub 60} PP density is limited by the dynamics of dissociation, where it increases from short circuit, and peaks at open circuit. In contrast, that of CuPc/C{sub 60} is dominated by faster recombination kinetics and declines monotonically over the same voltage domain. We conclude that the PP recombination rate depends on electric field, and propose a simple expression that qualitatively explains the observed exciplex luminescence and IMPS behavior for these HJs. Our results provide insight into polaron pair recombination, which governs the current-voltage characteristics of organic heterojunctions in the dark and under illumination.
Madelung and Hubbard interactions in polaron band model of doped organic semiconductors.
Png, Rui-Qi; Ang, Mervin C Y; Teo, Meng-How; Choo, Kim-Kian; Tang, Cindy Guanyu; Belaineh, Dagmawi; Chua, Lay-Lay; Ho, Peter K H
2016-01-01
The standard polaron band model of doped organic semiconductors predicts that density-of-states shift into the π-π* gap to give a partially filled polaron band that pins the Fermi level. This picture neglects both Madelung and Hubbard interactions. Here we show using ultrahigh workfunction hole-doped model triarylamine-fluorene copolymers that Hubbard interaction strongly splits the singly-occupied molecular orbital from its empty counterpart, while Madelung (Coulomb) interactions with counter-anions and other carriers markedly shift energies of the frontier orbitals. These interactions lower the singly-occupied molecular orbital band below the valence band edge and give rise to an empty low-lying counterpart band. The Fermi level, and hence workfunction, is determined by conjunction of the bottom edge of this empty band and the top edge of the valence band. Calculations are consistent with the observed Fermi-level downshift with counter-anion size and the observed dependence of workfunction on doping level in the strongly doped regime. PMID:27582355
Surface of active polarons: A semiexplicit solvation method for biomolecular dynamics
NASA Astrophysics Data System (ADS)
Kimura, S. Roy; Brower, Richard C.; Zhang, Chao; Sugimori, Masamichi
2000-05-01
We present a strategy for solvating biomolecules in molecular dynamics or Monte Carlo simulations. The method employs a thin layer (often monomolecular) of explicit water with additional external forces representing the electrostatics, pressure, fluctuations, and dissipations caused by the neglected bulk. Long-range electrostatic corrections are supplied through a set of variable surface charges (polarons) that recreates the mean reaction field (or dielectric properties) of an infinite solvent. We refer to this "fictitious" boundary layer as a "surface of active polarons" (or SOAP). Test simulations of the solvation free energies of 15 amino acid analogs and nine ions are in good agreement with experiment (correlation coefficients: 0.995 and 1.000, respectively) despite the use of unaltered published force-fields with only one adjustable parameter. Dynamical capabilities of SOAP are illustrated by application to a six residue peptide with a stable conformation (SYPFDV), as well as a flexible nine residue HIV-1 gp120 peptide (TLTSCNTSV from PDB 1hhg). Future extensions, calibrations, and applications are discussed briefly.
NASA Astrophysics Data System (ADS)
Dahiya, M. S.; Khasa, S.; Yadav, Arti; Agarwal, A.
2016-05-01
Lithium bismuth borate glasses containing different amounts of cobalt and iron oxides having chemical composition xFe2O3•(20-x)CoO•30Li2O•10Bi2O3•40B2O3 (x = 0, 5, 10, 15 and 20 mol% abbreviated as CFLBB1-5 respectively) prepared via melt quench technique have been investigated for their dc electrical conductivity. The amorphous nature of prepared glasses has been confirmed through X-ray diffraction measurements. The dc electrical conductivity has been analyzed by applying Mott's small polaron hopping model. Activation energies corresponding to lower and higher temperature region have been evaluated. The iron ion concentration (N), mean spacing between iron ions (R) and polaron radius (Rp) has been evaluated using the values of phonon radius (Rph) and Debye temperature (θD). The glass sample without iron (CFLBB1) shows ionic conductivity but the incorporation of iron in the glass matrix results in the appearance of electronic conductivity.
Anisotropic small-polaron hopping in W:BiVO{sub 4} single crystals
Rettie, Alexander J. E.; Chemelewski, William D.; Zhou, Jianshi; Lindemuth, Jeffrey; McCloy, John S.; Marshall, Luke G.; Emin, David; Mullins, C. Buddie
2015-01-12
DC electrical conductivity, Seebeck and Hall coefficients are measured between 300 and 450 K on single crystals of monoclinic bismuth vanadate that are doped n-type with 0.3% tungsten donors (W:BiVO{sub 4}). Strongly activated small-polaron hopping is implied by the activation energies of the Arrhenius conductivities (about 300 meV) greatly exceeding the energies characterizing the falls of the Seebeck coefficients' magnitudes with increasing temperature (about 50 meV). Small-polaron hopping is further evidenced by the measured Hall mobility in the ab-plane (10{sup −1 }cm{sup 2 }V{sup −1 }s{sup −1} at 300 K) being larger and much less strongly activated than the deduced drift mobility (about 5 × 10{sup −5 }cm{sup 2 }V{sup −1 }s{sup −1} at 300 K). The conductivity and n-type Seebeck coefficient is found to be anisotropic with the conductivity larger and the Seebeck coefficient's magnitude smaller and less temperature dependent for motion within the ab-plane than that in the c-direction. These anisotropies are addressed by considering highly anisotropic next-nearest-neighbor (≈5 Å) transfers in addition to the somewhat shorter (≈4 Å), nearly isotropic nearest-neighbor transfers.
Anisotropic small-polaron hopping in W:BiVO4 single crystals
NASA Astrophysics Data System (ADS)
Rettie, Alexander J. E.; Chemelewski, William D.; Lindemuth, Jeffrey; McCloy, John S.; Marshall, Luke G.; Zhou, Jianshi; Emin, David; Mullins, C. Buddie
2015-01-01
DC electrical conductivity, Seebeck and Hall coefficients are measured between 300 and 450 K on single crystals of monoclinic bismuth vanadate that are doped n-type with 0.3% tungsten donors (W:BiVO4). Strongly activated small-polaron hopping is implied by the activation energies of the Arrhenius conductivities (about 300 meV) greatly exceeding the energies characterizing the falls of the Seebeck coefficients' magnitudes with increasing temperature (about 50 meV). Small-polaron hopping is further evidenced by the measured Hall mobility in the ab-plane (10-1 cm2 V-1 s-1 at 300 K) being larger and much less strongly activated than the deduced drift mobility (about 5 × 10-5 cm2 V-1 s-1 at 300 K). The conductivity and n-type Seebeck coefficient is found to be anisotropic with the conductivity larger and the Seebeck coefficient's magnitude smaller and less temperature dependent for motion within the ab-plane than that in the c-direction. These anisotropies are addressed by considering highly anisotropic next-nearest-neighbor (≈5 Å) transfers in addition to the somewhat shorter (≈4 Å), nearly isotropic nearest-neighbor transfers.
Point defects, impurities, and small hole polarons in GdTiO3
NASA Astrophysics Data System (ADS)
Bjaalie, Lars; Janotti, Anderson; Krishnaswamy, Karthik; van de Walle, Chris G.
GdTiO3(GTO) has become the focus of great interest because of its use in complex-oxide heterostructures that display two-dimensional electron gases with unprecedented high densities. GTO is a Mott insulator, with a band gap arising within the partially filled Ti 3 d band due to strong electron-electron interactions. GTO often displays hole conductivity, likely attributed to defects or impurities, yet the cause of this unintentional conductivity has not yet been explored. We therefore used density functional theory with a hybrid functional to study their electronic structure. Among native defects, the cation vacancies have the lowest formation energies in oxygen-rich conditions, and oxygen vacancies have the lowest formation energy in oxygen-poor conditions. Among the impurities, rGd, Hi and CO have the lowest formation energies. The defects and impurities are intrinsically stable only in a single ``natural'' charge state, to which various numbers of hole polarons can be bound, which explains the frequent observation of p-type hopping conductivity in the rare-earth titanates. These small hole polarons also lead to optical absorption and act as electron traps in devices. Work supported by NSF and by the LEAST Center.
Coherent transport and manipulation of spins in indirect-exciton nanostructures
NASA Astrophysics Data System (ADS)
Violante, A.; Hey, R.; Santos, P. V.
2015-03-01
We report on the coherent control and transport of indirect-exciton (IX) spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was investigated by optically generating spins using a focused, circularly polarized light spot and by probing their spatial distribution using spatially and polarization resolved photoluminescence spectroscopy. Optically injected IX spins precess while moving over distances exceeding 20 μ m from the excitation spot with a precession length that depends on the spin transport direction as well as on the bias applied across the DQW structure. This behavior is attributed to the spin precession in the effective magnetic field induced by the spin-orbit interaction. From the dependence of the spin dynamics on the transport direction, bias, and external magnetic fields we directly determined the Dresselhaus and Rashba electron spin splitting coefficients for the DQW structure. The precession dynamics is essentially independent of the IX density, thus indicating that the long spin lifetimes are not associated with IX collective effects. The long IX lifetimes, together with the negligible contribution of holes to the spin dynamics, are rather attributed to spatial separation of the electron and hole wave functions by the electric field, which reduces the electron-hole exchange interaction. If extended to the single-exciton regime, the present results on coherent spin precession over long transport distances as well as the control of the spin vector using electric and magnetic fields open the way for the application of IX spins in quantum information processing.
Novel exciton systems in 2D TMD monolayers and heterobilayers
NASA Astrophysics Data System (ADS)
Yu, Hongyi
In this talk, two exciton systems in transition metal dichalcogenides (TMDs) monolayer and heterobilayer will be discussed. In TMD monolayers, the strong e-h Coulomb exchange interaction splits the exciton and trion dispersions into two branches with zero and finite gap, respectively. Each branch is a center-of-mass wave vector dependent coherent superposition of the two valleys, which leads to a valley-orbit coupling and possibly a trion valley Hall effect. The exchange interaction also eliminates the linear polarization of the negative trion PL emission. In TMD heterobilayers with a type-II band alignment, the low energy exciton has an interlayer configuration with the e and h localized in opposite layers. Because of the inevitable twist or/and lattice mismatch between the two layers, the bright interlayer excitons are located at finite center-of-mass velocities with a six-fold degeneracy. The corresponding photon emission is elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity determined by the valley indices of the e and h. Some experimental results on the interlayer excitons in the WSe2-MoSe2 heterobilayers will also be presented. The interlayer exciton exhibits a long lifetime as well as a long depolarization time, which facilitate the observation of a PL polarization ring pattern due to the valley dependent exciton-exciton interaction induced expansion. The works were supported by the Research Grant Council of Hong Kong (HKU17305914P, HKU705513P), the Croucher Foundation, and the HKU OYRA and ROP.
NASA Astrophysics Data System (ADS)
Abolfath, Ramin M.; Trojnar, Anna; Roostaei, Bahman; Brabec, Thomas; Hawrylak, Pawel
2013-06-01
Dynamical magnetic and nuclear polarization in complex spin systems is discussed on the example of transfer of spin from exciton to the central spin of magnetic impurity in a quantum dot in the presence of a finite number of nuclear spins. The exciton is described in terms of electron and heavy-hole spins interacting via exchange interaction with magnetic impurity, via hyperfine interaction with a finite number of nuclear spins and via dipole interaction with photons. The time evolution of the exciton, magnetic impurity and nuclear spins is calculated exactly between quantum jumps corresponding to exciton radiative recombination. The collapse of the wavefunction and the refilling of the quantum dot with a new spin-polarized exciton is shown to lead to the build up of magnetization of the magnetic impurity as well as nuclear spin polarization. The competition between electron spin transfer to magnetic impurity and to nuclear spins simultaneous with the creation of dark excitons is elucidated. The technique presented here opens up the possibility of studying optically induced dynamical magnetic and nuclear polarization in complex spin systems.
Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes.
Hofmann, Matthias S; Noé, Jonathan; Kneer, Alexander; Crochet, Jared J; Högele, Alexander
2016-05-11
We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes. PMID:27105355
Josephson oscillations between exciton condensates in electrostatic traps
Rontani, Massimo
2009-08-15
Technological advances allow for tunable lateral confinement of cold dipolar excitons in coupled quantum wells. We consider theoretically the Josephson effect between exciton condensates in two traps separated by a weak link. The flow of the exciton supercurrent is driven by the dipole-energy difference between the traps. The Josephson oscillations may be observed after ensemble average of the time correlation of photons separately emitted from the two traps. The fringe visibility is controlled by the trap coupling and is robust against quantum and thermal fluctuations.