Photoluminescence-detected magnetic-resonance study of fullerene-doped π-conjugated polymers

NASA Astrophysics Data System (ADS)

Lane, P. A.; Shinar, J.; Yoshino, K.

1996-10-01

X-band photoluminescence (PL)-detected magnetic resonance (PLDMR) spectra of C60- and C70-doped 2,5-dihexoxy poly(p-phenylenevinylene) (DHO-PPV), 2,5-dibutoxy poly(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 11Bu singlet excitons to polaron pairs and intersystem crossing to 13Bu triplet excitons. The triplet resonance in all polymers is quenched at relatively low doping levels of C60 and C70, which is attributed to quenching of triplets by positive polarons injected onto the polymer chain. Increased doping by C60, but not C70, quenches the polaron resonance, also due to photoinduced charge transfer.

Relaxation of photoexcitations in polaron-induced magnetic microstructures

NASA Astrophysics Data System (ADS)

Köhler, Thomas; Rajpurohit, Sangeeta; Schumann, Ole; Paeckel, Sebastian; Biebl, Fabian R. A.; Sotoudeh, Mohsen; Kramer, Stephan C.; Blöchl, Peter E.; Kehrein, Stefan; Manmana, Salvatore R.

2018-06-01

We investigate the evolution of a photoexcitation in correlated materials over a wide range of time scales. The system studied is a one-dimensional model of a manganite with correlated electron, spin, orbital, and lattice degrees of freedom, which we relate to the three-dimensional material Pr1 -xCaxMnO3 . The ground-state phases for the entire composition range are determined and rationalized by a coarse-grained polaron model. At half doping a pattern of antiferromagnetically coupled Zener polarons is realized. Using time-dependent density-matrix renormalization group (tDMRG), we treat the electronic quantum dynamics following the excitation. The emergence of quasiparticles is addressed, and the relaxation of the nonequilibrium quasiparticle distribution is investigated via a linearized quantum-Boltzmann equation. Our approach shows that the magnetic microstructure caused by the Zener polarons leads to an increase of the relaxation times of the excitation.

Polaron-like vortices, dissociation transition, and self-induced pinning in magnetic superconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bulaevskii, L. N., E-mail: lnb@lanl.gov; 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 inmore » 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.« less

Piezo electric polaron and polaron pinning in n-CdS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nagasaka, K.

1976-05-01

The cyclotron resonance of the piezoelectric polaron in n-CdS has been investigated using far infrared spectroscopy at magnetic fields to 90 kOe. Both lamellar grating and Michelson Fourier transform spectrometers were used with a 0.3/sup 0/K Ge bolometer to study the photon energy region from 10 cm/sup -1/ to 60 cm/sup -1/. The theory of Miyake predicts that the frequency of the polaron's cyclotron resonance is shifted from the bare hand electron resonance frequency according to the expression: ..delta omega../sup p//sub c// ..cap omega../sub c/ varies as H/sup -1/ T/sup /sup 2///sup 3//. The magnetic field dependence of the presentmore » cyclotron resonance confirms this expression; the cyclotron mass isiezoelectric polaron effects. The bare band mass in n-CdS has also been determined by taking into account the Froehlich polaron interaction in addition to the piezoelectric polaron effects. For H parallel to the c-axis this cyclotron mass is 0.155 +- 0.005 m. The polaron pinning due to the 43 cm/sup -1/ optically inactive phonon has been observed.« less

Transport of Indirect Excitons in High Magnetic Fields

NASA Astrophysics Data System (ADS)

Dorow, C. J.; Kuznetsova, Y. Y.; Calman, E. V.; Butov, L. V.; Wilkes, J.; Campman, K. L.; Gossard, A. C.

Spatially- and spectrally-resolved photoluminescence measurements of indirect excitons in high magnetic fields are presented. The high magnetic field regime for excitons is realized when the cyclotron splitting compares to the exciton binding energy. Due to small mass and binding energy, the high magnetic field regime for excitons is achievable in lab, requiring a few Tesla. Long indirect exciton lifetimes allow large exciton transport distances before recombination, giving an opportunity to study transport and relaxation kinetics of indirect magnetoexcitons via optical imaging. Indirect excitons in several Landau level states are realized. 0e -0h indirect magnetoexcitons (formed from electrons and holes at zeroth Landau levels) travel over large distances and form an emission ring around the excitation spot. In contrast, the 1e -1h and 2e -2h states do not exhibit long transport distances, and the spatial profiles of the emission closely follow the laser excitation. The 0e -0h indirect magnetoexciton transport distance reduces with increasing magnetic field. Accompanying theoretical work explains these effects in terms of magnetoexciton energy relaxation and effective mass enhancement. Supported by NSF Grant No. 1407277. J.W. was supported by the EPSRC (Grant EP/L022990/1). C.J.D. was supported by the NSF Graduate Research Fellowship Program under Grant No. DGE-1144086.

Effect of Dangling Bond Spins on the Dark Exciton Recombination and Spin Polarization in CdSe Colloidal Nanostructures

NASA Astrophysics Data System (ADS)

Rodina, A. V.; Golovatenko, A. A.; Shornikova, E. V.; Yakovlev, D. R.; Efros, Al. L.

2018-04-01

We present theoretical aspects of the exchange interaction between the ground optically-forbidden "dark" exciton state and surface dangling bonds in colloidal CdSe spherical nanocrystals and nanoplatelets. The influence of the dangling bond spins on the radiative recombination and spin splitting of the dark exciton is shown. Processes of optically-driven and external magnetic field-driven formation of the dangling bond magnetic polaron (DBMP) are considered. Thermodynamic and dynamic polarization mechanisms of the DBMP formation within these two processes and corresponding critical temperatures are compared. Experimental manifestations of the DBMP formation in CdSe nanocrystals and nanoplatelets are discussed.

The effect of interface hopping on inelastic scattering of oppositely charged polarons in polymers

NASA Astrophysics Data System (ADS)

Di, Bing; Wang, Ya-Dong; Zhang, Ya-Lin; An, Zhong

2013-06-01

The inelastic scattering of oppositely charge polarons in polymer heterojunctions is believed to be of fundamental importance for the light-emitting and transport properties of conjugated polymers. Based on the tight-binding SSH model, and by using a nonadiabatic molecular dynamic method, we investigate the effects of interface hopping on inelastic scattering of oppositely charged polarons in a polymer heterojunction. It is found that the scattering processes of the charge and lattice defect depend sensitively on the hopping integrals at the polymer/polymer interface when the interface potential barrier and applied electric field strength are constant. In particular, at an intermediate electric field, when the interface hopping integral of the polymer/polymer heterojunction material is increased beyond a critical value, two polarons can combine to become a lattice deformation in one of the two polymer chains, with the electron and the hole bound together, i.e., a self-trapped polaron—exciton. The yield of excitons then increases to a peak value. These results show that interface hopping is of fundamental importance and facilitates the formation of polaron—excitons.

Quantitative measurements of magnetic polaron binding on acceptors in CdMnTe alloys

NASA Astrophysics Data System (ADS)

Nhung, Tran Hong; Planel, R.

1983-03-01

The acceptor binding energy is measured as a function of Temperature and composition in Cd1-x Mnx Te alloys, by time resolved spectroscopy. The Bound magnetic polaron effect is measured and compared with a theory accouting for magnetic saturation and fluctuations.

Crossover from Polaronic to Magnetically Phase-Separated Behavior in La1-xSrxCoO3

NASA Astrophysics Data System (ADS)

Phelan, D.; El Khatib, S.; Wang, S.; Barker, J.; Zhao, J.; Zheng, H.; Mitchell, J. F.; Leighton, C.

2013-03-01

Dilute hole-doping in La1-xSrxCoO3 leads to the formation of ``spin-state polarons'' where a non-zero spin-state is stabilized on the nearest Co3+ ions surrounding a hole. Here, we discuss the development of electronic/magnetic properties of this system from non-magnetic x=0, through the regime of spin-state polarons, and into the region where longer-range spin correlations and phase separation develop. We present magnetometry, transport, heat capacity, and small-angle neutron scattering (SANS) on single crystals. Magnetometry indicates a crossover with x from Langevin-like behavior (polaronic) to a state with a freezing temperature and finite coercivity. Fascinating correlations with this behavior are seen in transport measurements, the evolution from polaronic to clustered states being accompanied by a crossover from Mott variable range hopping to intercluster hopping. SANS data shows Lorentzian scattering from short-range ferromagnetic clusters first emerging around x = 0.03 with correlation lengths of order two unit cells. We argue that this system provides a unique opportunity to understand in detail the crossover from polaronic to truly phase-separated states.

Singlet and triplet excitons and charge polarons in cycloparaphenylenes. A density functional theory study

DOE PAGES

Liu, Jin; Adamska, Lyudmyla; Doorn, Stephen K.; ...

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

Exciton diamagnetic shift and optical properties in CdSe nanocrystal quantum dots in magnetic fields

NASA Astrophysics Data System (ADS)

Wu, Shudong; Cheng, Liwen

2018-04-01

The magnetic field dependence of the optical properties of CdSe nanocrystal quantum dots (NQDs) is investigated theoretically using a perturbation method within the effective-mass approximation. The results show that the magnetic field lifts the degeneracy of the electron (hole) states. A blue-shift in the absorption spectra of m ≥ 0 exciton states is observed while the absorption peak of m < 0 exciton states is first red-shifted and then blue-shifted with increasing the magnetic field strength B. This is attributed to the interplay of the orbital Zeeman effect and the additive confinement induced by the magnetic field. The excitonic absorption coefficient is almost independent of B in the strong confinement regime. The applied magnetic field causes the splitting of degenerated exciton states, resulting in the new absorption peaks. Based on the first-order perturbation theory, we propose the analytical expressions for the exciton binding energy, exciton transition energy and exciton diamagnetic shift of 1s, 1p-1, 1p0, 1p1, 1d-2, 1d-1, 1d0, 1d1, 1d2 and 2s exciton states on the applied magnetic field in the strong confinement regime.

Chain Length Dependence of Energies of Electron and Triplet Polarons in Oligofluorenes

DOE PAGES

Chen, Hung Cheng; Sreearunothai, Paiboon; Cook, Andrew R.; ...

2017-03-01

Bimolecular equilibria measured the one-electron reduction potentials and triplet free energies (ΔG° T) of oligo(9,9-dihexyl)fluorenes and a polymer with lengths of n = 1–10 and 57 repeat units. We can accurately measure one-electron potentials electrochemically only for the shorter oligomers. Starting at n = 1 the free energies change rapidly with increasing length and become constant for lengths longer than the delocalization length. Both the reduction potentials and triplet energies can be understood as the sum of a free energy for a fixed polaron and a positional entropy. Furthermore, the positional entropy increases gradually with length beyond the delocalization lengthmore » due to the possible occupation sites of the charge or the triplet exciton. Our results reinforce the view that charges and triplet excitons in conjugated chains exist as polarons and find that positional entropy can replace a popular empirical model of the energetics.« less

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.

On the condensation of exciton polaritons in microcavities induced by a magnetic field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kochereshko, V. P., E-mail: Vladimir.Kochereshko@mail.ioffe.ru; Avdoshina, D. V.; Savvidis, P.

2016-11-15

The photoluminescence spectra of exciton polaritons in microcavities under conditions of three-dimensional quantization are studied as a factor of the density of the optical excitation and magnetic field. The behavior of the degree of circular polarization of the exciton luminescence in a magnetic field shows that, when the concentration of excitons increases, they condense at the lowest Zeeman sublevel.

Excitons, trions, and biexcitons in transition-metal dichalcogenides: Magnetic-field dependence

NASA Astrophysics Data System (ADS)

Van der Donck, M.; Zarenia, M.; Peeters, F. M.

2018-05-01

The influence of a perpendicular magnetic field on the binding energy and structural properties of excitons, trions, and biexcitons in monolayers of semiconducting transition metal dichalcogenides (TMDs) is investigated. The stochastic variational method (SVM) with a correlated Gaussian basis is used to calculate the different properties of these few-particle systems. In addition, we present a simplified variational approach which supports the SVM results for excitons as a function of magnetic field. The exciton diamagnetic shift is compared with recent experimental results, and we extend this concept to trions and biexcitons. The effect of a local potential fluctuation, which we model by a circular potential well, on the binding energy of trions and biexcitons is investigated and found to significantly increase the binding of those excitonic complexes.

Spatially resolved and time-resolved imaging of transport of indirect excitons in high magnetic fields

NASA Astrophysics Data System (ADS)

Dorow, C. J.; Hasling, M. W.; Calman, E. V.; Butov, L. V.; Wilkes, J.; Campman, K. L.; Gossard, A. C.

2017-06-01

We present the direct measurements of magnetoexciton transport. Excitons give the opportunity to realize the high magnetic-field regime for composite bosons with magnetic fields of a few tesla. Long lifetimes of indirect excitons allow the study of kinetics of magnetoexciton transport with time-resolved optical imaging of exciton photoluminescence. We performed spatially, spectrally, and time-resolved optical imaging of transport of indirect excitons in high magnetic fields. We observed that an increasing magnetic field slows down magnetoexciton transport. The time-resolved measurements of the magnetoexciton transport distance allowed for an experimental estimation of the magnetoexciton diffusion coefficient. An enhancement of the exciton photoluminescence energy at the laser excitation spot was found to anticorrelate with the exciton transport distance. A theoretical model of indirect magnetoexciton transport is presented and is in agreement with the experimental data.

Parton Theory of Magnetic Polarons: Mesonic Resonances and Signatures in Dynamics

NASA Astrophysics Data System (ADS)

Grusdt, F.; Kánasz-Nagy, M.; Bohrdt, A.; Chiu, C. S.; Ji, G.; Greiner, M.; Greif, D.; Demler, E.

2018-01-01

When a mobile hole is moving in an antiferromagnet it distorts the surrounding Néel order and forms a magnetic polaron. Such interplay between hole motion and antiferromagnetism is believed to be at the heart of high-temperature superconductivity in cuprates. In this article, we study a single hole described by the t -Jz model with Ising interactions between the spins in two dimensions. This situation can be experimentally realized in quantum gas microscopes with Mott insulators of Rydberg-dressed bosons or fermions, or using polar molecules. We work at strong couplings, where hole hopping is much larger than couplings between the spins. In this regime we find strong theoretical evidence that magnetic polarons can be understood as bound states of two partons, a spinon and a holon carrying spin and charge quantum numbers, respectively. Starting from first principles, we introduce a microscopic parton description which is benchmarked by comparison with results from advanced numerical simulations. Using this parton theory, we predict a series of excited states that are invisible in the spectral function and correspond to rotational excitations of the spinon-holon pair. This is reminiscent of mesonic resonances observed in high-energy physics, which can be understood as rotating quark-antiquark pairs carrying orbital angular momentum. Moreover, we apply the strong-coupling parton theory to study far-from-equilibrium dynamics of magnetic polarons observable in current experiments with ultracold atoms. Our work supports earlier ideas that partons in a confining phase of matter represent a useful paradigm in condensed-matter physics and in the context of high-temperature superconductivity in particular. While direct observations of spinons and holons in real space are impossible in traditional solid-state experiments, quantum gas microscopes provide a new experimental toolbox. We show that, using this platform, direct observations of partons in and out of equilibrium are

Observation of magnetic excitons in LaCoO3

NASA Astrophysics Data System (ADS)

Giblin, S. R.; Terry, I.; Clark, S. J.; Prokscha, T.; Prabhakaran, D.; Boothroyd, A. T.; Wu, J.; Leighton, C.

2005-06-01

An impurity-driven magnetic phase transition has been investigated in LaCoO3 at temperatures below that of the thermally induced spin state transition of the Co3+ ion. We have discovered a saturating component of the magnetisation, which we attribute to previously unobserved interactions between magnetic excitons. These conclusions are confirmed by muon spin spectroscopy which indicates an ordering temperature of 50 K in both the transverse and zero-field configurations. Low-energy muon measurements demonstrate that the magnetic behaviour is independent of implantation energy and hence a property of the bulk of the material. The magnetic exciton formation is attributed to the interaction between electrons bound at oxygen vacancies and neighbouring cobalt ions, and is proposed as the precursor to the magneto-electronic phase separation recently observed in doped lanthanum cobaltite.

Magnetic field effect on the energy levels of an exciton in a GaAs quantum dot: Application for excitonic lasers.

PubMed

Jahan, K Luhluh; Boda, A; Shankar, I V; Raju, Ch Narasimha; Chatterjee, Ashok

2018-03-22

The problem of an exciton trapped in a Gaussian quantum dot (QD) of GaAs is studied in both two and three dimensions in the presence of an external magnetic field using the Ritz variational method, the 1/N expansion method and the shifted 1/N expansion method. The ground state energy and the binding energy of the exciton are obtained as a function of the quantum dot size, confinement strength and the magnetic field and compared with those available in the literature. While the variational method gives the upper bound to the ground state energy, the 1/N expansion method gives the lower bound. The results obtained from the shifted 1/N expansion method are shown to match very well with those obtained from the exact diagonalization technique. The variation of the exciton size and the oscillator strength of the exciton are also studied as a function of the size of the quantum dot. The excited states of the exciton are computed using the shifted 1/N expansion method and it is suggested that a given number of stable excitonic bound states can be realized in a quantum dot by tuning the quantum dot parameters. This can open up the possibility of having quantum dot lasers using excitonic states.

Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants

NASA Astrophysics Data System (ADS)

Pinchetti, Valerio; Di, Qiumei; Lorenzon, Monica; Camellini, Andrea; Fasoli, Mauro; Zavelani-Rossi, Margherita; Meinardi, Francesco; Zhang, Jiatao; Crooker, Scott A.; Brovelli, Sergio

2018-02-01

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag+ is an emerging electronic dopant in iii-v and ii-vi nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag+ is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag+. This optical activation process and the associated modification of the electronic configuration of Ag+ remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag+ to paramagnetic Ag2+. The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

Reduced Charge Transfer Exciton Recombination in Organic Semiconductor Heterojunctions by Molecular Doping

NASA Astrophysics Data System (ADS)

Deschler, Felix; da Como, Enrico; Limmer, Thomas; Tautz, Raphael; Godde, Tillmann; Bayer, Manfred; von Hauff, Elizabeth; Yilmaz, Seyfullah; Allard, Sybille; Scherf, Ullrich; Feldmann, Jochen

2011-09-01

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ˜70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.

Non-canonical distribution and non-equilibrium transport beyond weak system-bath coupling regime: A polaron transformation approach

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.

Overflow of a dipolar exciton trap at high magnetic fields

NASA Astrophysics Data System (ADS)

Dietl, Sebastian; Kowalik-Seidl, Katarzyna; Hammer, Lukas; Schuh, Dieter; Wegscheider, Werner; Holleitner, Alexander; Wurstbauer, Ursula

We study the photoluminescence of trapped dipolar excitons (IX) in coupled double GaAs quantum wells at low temperatures and high magnetic fields. A voltage-tunable electrode geometry controls the strength of the quantum confined Stark effect and defines the lateral trapping potential. Furthermore, it enhances the IX lifetime, enabling them to cool down to lattice temperature. We show that a magnetic field in Faraday configuration effectively prevents the escape of unbound photogenerated charge carriers from the trap area, thus increasing the density of dipolar excitons. For large magnetic fields, we observe an overflow of the IX trap and an effectively suppressed quantum confined Stark effect. We acknowledge financial support by the German Excellence Initiative via the Nanosystems Initiative Munich (NIM).

Photophysics of the geminate polaron-pair state in copper phthalocyanine organic photovoltaic blends: evidence for enhanced intersystem crossing.

PubMed

Snedden, Edward W; Monkman, Andrew P; Dias, Fernando B

2013-04-04

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. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Absolute instability of polaron mode in semiconductor magnetoplasma

NASA Astrophysics Data System (ADS)

Paliwal, Ayushi; Dubey, Swati; Ghosh, S.

2018-01-01

Using coupled mode theory under hydrodynamic regime, a compact dispersion relation is derived for polaron mode in semiconductor magnetoplasma. The propagation and amplification characteristics of the wave are explored in detail. The analysis deals with the behaviour of anomalous threshold and amplification derived from dispersion relation, as function of external parameters like doping concentration and applied magnetic field. The results of this investigation are hoped to be useful in understanding electron-longitudinal optical phonon interplay in polar n-type semiconductor plasmas under the influence of coupled collective cyclotron excitations. The best results in terms of smaller threshold and higher gain of polaron mode could be achieved by choosing moderate doping concentration in the medium at higher magnetic field. For numerical appreciation of the results, relevant data of III-V n-GaAs compound semiconductor at 77 K is used. Present study provides a qualitative picture of polaron mode in magnetized n-type polar semiconductor medium duly shined by a CO2 laser.

Theoretical investigation of excitonic magnetism in LaSrCoO4

NASA Astrophysics Data System (ADS)

Fernández Afonso, J.; Sotnikov, A.; Kuneš, J.

2018-04-01

We use the LDA+U approach to search for possible ordered ground states of LaSrCoO4. We find a staggered arrangement of magnetic multipoles to be stable over a broad range of Co 3d interaction parameters. This ordered state can be described as a spin-density-wave-type condensate of dxy \\otimes dx^2-y^2 excitons carrying spin S  =  1. Further, we construct an effective strong-coupling model, calculate the exciton dispersion and investigate closing of the exciton gap, which marks the exciton condensation instability. Comparing the layered LaSrCoO4 with its pseudo cubic analog LaCoO3, we find that for the same interaction parameters the excitonic gap is smaller (possibly vanishing) in the layered cobaltite.

Exciton-phonon system on a star graph: A perturbative approach.

PubMed

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.

Magnetic brightening and control of dark excitons in monolayer WSe2.

PubMed

Zhang, Xiao-Xiao; Cao, Ting; Lu, Zhengguang; Lin, Yu-Chuan; Zhang, Fan; Wang, Ying; Li, Zhiqiang; Hone, James C; Robinson, Joshua A; Smirnov, Dmitry; Louie, Steven G; Heinz, Tony F

2017-09-01

Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX 2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe 2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

Acceptor and Excitation Density Dependence of the Ultrafast Polaron Absorption Signal in Donor-Acceptor Organic Solar Cell Blends.

PubMed

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.

Strong valley Zeeman effect of dark excitons in monolayer transition metal dichalcogenides in a tilted magnetic field

NASA Astrophysics Data System (ADS)

Van der Donck, M.; Zarenia, M.; Peeters, F. M.

2018-02-01

The dependence of the excitonic photoluminescence (PL) spectrum of monolayer transition metal dichalcogenides (TMDs) on the tilt angle of an applied magnetic field is studied. Starting from a four-band Hamiltonian we construct a theory which quantitatively reproduces the available experimental PL spectra for perpendicular and in-plane magnetic fields. In the presence of a tilted magnetic field, we demonstrate that the dark exciton PL peaks brighten due to the in-plane component of the magnetic field and split for light with different circular polarizations as a consequence of the perpendicular component of the magnetic field. This splitting is more than twice as large as the splitting of the bright exciton peaks in tungsten-based TMDs. We propose an experimental setup that will allow for accessing the predicted splitting of the dark exciton peaks in the PL spectrum.

Magnetic brightening and control of dark excitons in monolayer WSe 2

DOE PAGES

Zhang, Xiao -Xiao; Cao, Ting; Lu, Zhengguang; ...

2017-06-26

Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light–matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX 2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitonsmore » are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. Furthermore, these studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.« less

Spin-exciton interaction and related micro-photoluminescence spectra of ZnSe:Mn DMS nanoribbon

NASA Astrophysics Data System (ADS)

Hou, Lipeng; Zhou, Weichang; Zou, Bingsuo; Zhang, Yu; Han, Junbo; Yang, Xinxin; Gong, Zhihong; Li, Jingbo; Xie, Sishen; Shi, Li-Jie

2017-03-01

For their spintronic applications the magnetic and optical properties of diluted magnetic semiconductors (DMS) have been studied widely. However, the exact relationships between the magnetic interactions and optical emission behaviors in DMS are not well understood yet due to their complicated microstructural and compositional characters from different growth and preparation techniques. Manganese (Mn) doped ZnSe nanoribbons with high quality were obtained by using the chemical vapor deposition (CVD) method. Successful Mn ion doping in a single ZnSe nanoribbon was identified by elemental energy-dispersive x-ray spectroscopy mapping and micro-photoluminescence (PL) mapping of intrinsic d-d optical transition at 580 nm, i.e. the transition of 4 T 1(4 G) → 6 A 1(6 s),. Besides the d-d transition PL peak at 580 nm, two other PL peaks related to Mn ion aggregates in the ZnSe lattice were detected at 664 nm and 530 nm, which were assigned to the d-d transitions from the Mn2+-Mn2+ pairs with ferromagnetic (FM) coupling and antiferromagnetic (AFM) coupling, respectively. Moreover, AFM pair formation goes along with strong coupling with acoustic phonon or structural defects. These arguments were supported by temperature-dependent PL spectra, power-dependent PL lifetimes, and first-principle calculations. Due to the ferromagnetic pair existence, an exciton magnetic polaron (EMP) is formed and emits at 460 nm. Defect existence favors the AFM pair, which also can account for its giant enhancement of spin-orbital coupling and the spin Hall effect observed in PRL 97, 126603(2006) and PRL 96, 196404(2006). These emission results of DMS reflect their relation to local sp-d hybridization, spin-spin magnetic coupling, exciton-spin or phonon interactions covering structural relaxations. This kind of material can be used to study the exciton-spin interaction and may find applications in spin-related photonic devices besides spintronics.

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.

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.

Finite temperature dynamics of a Holstein polaron: The thermo-field dynamics approach

NASA Astrophysics Data System (ADS)

Chen, Lipeng; Zhao, Yang

2017-12-01

Combining the multiple Davydov D2 Ansatz with the method of thermo-field dynamics, we study finite temperature dynamics of a Holstein polaron on a lattice. It has been demonstrated, using the hierarchy equations of motion method as a benchmark, that our approach provides an efficient, robust description of finite temperature dynamics of the Holstein polaron in the simultaneous presence of diagonal and off-diagonal exciton-phonon coupling. The method of thermo-field dynamics handles temperature effects in the Hilbert space with key numerical advantages over other treatments of finite-temperature dynamics based on quantum master equations in the Liouville space or wave function propagation with Monte Carlo importance sampling. While for weak to moderate diagonal coupling temperature increases inhibit polaron mobility, it is found that off-diagonal coupling induces phonon-assisted transport that dominates at high temperatures. Results on the mean square displacements show that band-like transport features dominate the diagonal coupling cases, and there exists a crossover from band-like to hopping transport with increasing temperature when including off-diagonal coupling. As a proof of concept, our theory provides a unified treatment of coherent and incoherent transport in molecular crystals and is applicable to any temperature.

Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: First-Principles Quantum Dynamical Study

NASA Astrophysics Data System (ADS)

Binder, Robert; Lauvergnat, David; Burghardt, Irene

2018-06-01

We report on high-dimensional quantum dynamical simulations of photoinduced exciton migration in a single-chain oligothiophene segment, in view of elucidating the controversial nature of the elementary exciton transport steps in semiconducting polymers. A novel first-principles parametrized Frenkel J aggregate Hamiltonian is employed that goes significantly beyond the standard Frenkel-Holstein Hamiltonian. Departing from a nonequilibrium state created by photoexcitation, these simulations provide evidence of an ultrafast two-timescale process at low temperatures, involving exciton-polaron formation within tens of femtoseconds (fs), followed by torsional relaxation on an ˜400 fs timescale. The second step is the driving force for exciton migration, as initial conjugation breaks are removed by dynamical planarization. The quantum coherent nature of the elementary exciton migration step is consistent with experimental observations highlighting the correlated and vibrationally coherent nature of the dynamics on ultrafast timescales.

Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: First-Principles Quantum Dynamical Study.

PubMed

Binder, Robert; Lauvergnat, David; Burghardt, Irene

2018-06-01

We report on high-dimensional quantum dynamical simulations of photoinduced exciton migration in a single-chain oligothiophene segment, in view of elucidating the controversial nature of the elementary exciton transport steps in semiconducting polymers. A novel first-principles parametrized Frenkel J aggregate Hamiltonian is employed that goes significantly beyond the standard Frenkel-Holstein Hamiltonian. Departing from a nonequilibrium state created by photoexcitation, these simulations provide evidence of an ultrafast two-timescale process at low temperatures, involving exciton-polaron formation within tens of femtoseconds (fs), followed by torsional relaxation on an ∼400  fs timescale. The second step is the driving force for exciton migration, as initial conjugation breaks are removed by dynamical planarization. The quantum coherent nature of the elementary exciton migration step is consistent with experimental observations highlighting the correlated and vibrationally coherent nature of the dynamics on ultrafast timescales.

Magnetic order and polaron formation in hole-doped LaMnO_3

NASA Astrophysics Data System (ADS)

Terashita, Hirotoshi; Neumeier, John J.; Mitchell, J. F.

2003-03-01

We report the magnetic properties of hole-doped La_1-xCa_xMnO3 (0 <= x <= 0.14). A ferromagnetic saturation moment M_sat develops linearly with Mn^4+ concentration. The slope of M_sat versus Mn^4+ concentration is 27 μ_B/(Mn-ion) per substututed Mn^4+, which is about 3 times larger in magnitude than that of electron-doped CaMnO3 [1]. This result suggests differences in the formation of magnetic polarons of the A-type antiferromagnet LaMnO3 versus that of the G-type antiferromagnet CaMnO_3. Supported by NSF Grant DMR9982834 and the USDOE under contract W-31-109-ENG-38. [1] J. J. Neumeier and J. L. Cohn, Phys. Rev. B 61, 14319 (2000).

Diamagnetic excitons and exciton magnetopolaritons in semiconductors

NASA Astrophysics Data System (ADS)

Seisyan, R. P.

2012-05-01

Interband magneto-absorption in semiconductors is reviewed in the light of the diamagnetic exciton (DE) concept. Beginning with a proof of the exciton nature of oscillating-magnetoabsorption (the DE discovery), development of the DE concept is discussed, including definition of observation conditions, quasi-cubic approximation for hexagonal crystals, quantum-well effects in artificial structures, and comprehension of an important role of the DE polariton. The successful use of the concept application to a broad range of substances is reviewed, namely quasi-Landau magnetic spectroscopy of the ‘Rydberg’ exciton states in cubic semiconductors such as InP and GaAs and in hexagonal ones such as CdSe, the proof of exciton participation in the formation of optical spectra in narrow-gap semiconductors such as InSb, InAs, and, especially, PbTe, observation of DE spectra in semiconductor solid solutions like InGaAs. The most fundamental findings of the DE spectroscopy for various quantum systems are brought together, including the ‘Coulomb-well’ effect, fine structure of discrete oscillatory states in the InGaAs/GaAs multiple quantum wells, the magneto-optical observation of above-barrier exciton. Prospects of the DE physics in ultrahigh magnetic field are discussed, including technological creation of controllable low-dimensional objects with extreme oscillator strengths, formation of magneto-quantum exciton polymer, and even modelling of the hydrogen behaviour in the atmosphere of a neutron star.

Neutron-scattering-based evidence for interacting magnetic excitons in LaCo O 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

El-Khatib, S.; Phelan, D.; Barker, J. G.

Recent progress with the thermally-driven spin-state crossover in LaCoO3 has made it increasingly apparent that the nominally non-magnetic low spin ground state of this material actually hosts novel defect-'based magnetism. This is investigated here via a small-angle neutron scattering (SANS) study of LaCoO3-s crystals. The results provide: (i) the surprising finding that the spin-state crossover is clearly reflected in SANS via quasielastic/inelastic scattering from paramagnetic spin fluctuations/excitations, and (ii) evidence for the formation, likely around oxygen defects, of local entities known as magnetic excitons. The latter generate distinct magnetic scattering below 60 K, providing valuable quantitative information on exciton densitiesmore » and interactions. Potential relevance to the unexpected ferromagnetism recently discovered in epitaxial LaCoO3 films is discussed.« less

Polaron melting and ordering as key mechanisms for colossal resistance effects in manganites

PubMed Central

Jooss, Ch.; Wu, L.; Beetz, T.; Klie, R. F.; Beleggia, M.; Schofield, M. A.; Schramm, S.; Hoffmann, J.; Zhu, Y.

2007-01-01

Polarons, the combined motion of electrons in a cloth of their lattice distortions, are a key transport feature in doped manganites. To develop a profound understanding of the colossal resistance effects induced by external fields, the study of polaron correlations and the resulting collective polaron behavior, i.e., polaron ordering and transition from polaronic transport to metallic transport is essential. We show that static long-range ordering of Jahn–Teller polarons forms a polaron solid which represents a new type of charge and orbital ordered state. The related noncentrosymmetric lattice distortions establish a connection between colossal resistance effects and multiferroic properties, i.e., the coexistence of ferroelectric and antiferromagnetic ordering. Colossal resistance effects due to an electrically induced polaron solid–liquid transition are directly observed in a transmission electron microscope with local electric stimulus applied in situ using a piezo-controlled tip. Our results shed light onto the colossal resistance effects in magnetic field and have a strong impact on the development of correlated electron-device applications such as resistive random access memory (RRAM). PMID:17699633

Magnon Polarons in the Spin Seebeck Effect.

PubMed

Kikkawa, Takashi; Shen, Ka; Flebus, Benedetta; Duine, Rembert A; Uchida, Ken-Ichi; Qiu, Zhiyong; Bauer, Gerrit E W; Saitoh, Eiji

2016-11-11

Sharp structures in the magnetic field-dependent spin Seebeck effect (SSE) voltages of Pt/Y_{3}Fe_{5}O_{12} at low temperatures are attributed to the magnon-phonon interaction. Experimental results are well reproduced by a Boltzmann theory that includes magnetoelastic coupling. The SSE anomalies coincide with magnetic fields tuned to the threshold of magnon-polaron formation. The effect gives insight into the relative quality of the lattice and magnetization dynamics.

Even exciton series in Cu2O

NASA Astrophysics Data System (ADS)

Schweiner, Frank; Main, Jörg; Wunner, Günter; Uihlein, Christoph

2017-05-01

Recent investigations of excitonic absorption spectra in cuprous oxide (Cu2O ) have shown that it is indispensable to account for the complex valence-band structure in the theory of excitons. In Cu2O , parity is a good quantum number and thus the exciton spectrum falls into two parts: the dipole-active exciton states of negative parity and odd angular momentum, which can be observed in one-photon absorption (Γ4- symmetry), and the exciton states of positive parity and even angular momentum, which can be observed in two-photon absorption (Γ5+ symmetry). The unexpected observation of D excitons in two-photon absorption has given first evidence that the dispersion properties of the Γ5+ orbital valence band are giving rise to a coupling of the yellow and green exciton series. However, a first theoretical treatment by Uihlein et al. [Phys. Rev. B 23, 2731 (1981), 10.1103/PhysRevB.23.2731] was based on a simplified spherical model. The observation of F excitons in one-photon absorption is a further proof of a coupling between yellow and green exciton states. Detailed investigations on the fine structure splitting of the F exciton by F. Schweiner et al. [Phys. Rev. B 93, 195203 (2016), 10.1103/PhysRevB.93.195203] have proved the importance of a more realistic theoretical treatment including terms with cubic symmetry. In this paper we show that the even and odd parity exciton system can be consistently described within the same theoretical approach. However, the Hamiltonian of the even parity system needs, in comparison to the odd exciton case, modifications to account for the very small radius of the yellow and green 1 S exciton. In the presented treatment, we take special care of the central-cell corrections, which comprise a reduced screening of the Coulomb potential at distances comparable to the polaron radius, the exchange interaction being responsible for the exciton splitting into ortho and para states, and the inclusion of terms in the fourth power of p in the

Multi-excitonic (N=1,2 and 3) quantum dots in magnetic field: Analytical mapping of correlations (exchange) by multipole expansion

NASA Astrophysics Data System (ADS)

Singh, Sunny; Kaur, Harsimran; Sharma, Shivalika; Aggarwal, Priyanka; Hazra, Ram Kuntal

2017-04-01

The understanding of the physics of exciton, bi-exciton, tri-exciton and the subsequent insight into controlling the properties of mesoscopic systems holds the key to various exotic optical, electrical and magnetic phenomena like superconductivity, Mott insulation, Quantum Hall effect etc. Many of exciton properties are similar to atomic hydrogen that attracts researchers to explore electronic structure of exciton in quantum dots, but nontriviality arises due to coulombic interactions among electrons and holes. We propose an exact integral of coulomb (exchange) correlation in terms of finitely summed Lauricella functions to examine 3-D exciton of harmonic dots confined in zero and non-zero arbitrary magnetic field. The highlight of our work is the use of exact variational solution for coloumbic interaction between the hole and the electron and evaluation of the cross terms arising out of the coupling among centre-of-mass and relative coordinates. We also have extended the size of the system to generalized N-body problem with N=3,4 for tri-exciton (e-e-h/e-h-h)

Excitonic magnet in external field: Complex order parameter and spin currents

NASA Astrophysics Data System (ADS)

Geffroy, D.; Hariki, A.; Kuneš, J.

2018-04-01

We investigate spin-triplet exciton condensation in the two-orbital Hubbard model close to half-filling by means of dynamical mean-field theory. Employing an impurity solver that handles complex off-diagonal hybridization functions, we study the behavior of excitonic condensate in stoichiometric and doped systems subject to external magnetic field. We find a general tendency of the triplet order parameter to lie perpendicular with the applied field and identify exceptions from this rule. For solutions exhibiting k -odd spin textures, we discuss the Bloch theorem, which, in the absence of spin-orbit coupling, forbids the appearance of spontaneous net spin current. We demonstrate that the Bloch theorem is not obeyed by the dynamical mean-field theory.

Mobility balance in the light-emitting layer governs the polaron accumulation and operational stability of organic light-emitting diodes

NASA Astrophysics Data System (ADS)

Kim, Jae-Min; Lee, Chang-Heon; Kim, Jang-Joo

2017-11-01

Organic light-emitting diode (OLED) displays are lighter and more flexible, have a wider color gamut, and consume less power than conventional displays. Stable materials and the structural design of the device are important for OLED longevity. Control of charge transport and accumulation in the device is particularly important because the interaction of excitons and polarons results in material degradation. This research investigated the charge dynamics of OLEDs experimentally and by drift-diffusion modeling. Parallel capacitance-voltage measurements of devices provided knowledge of charge behavior at different driving voltages. A comparison of exciplex-forming co-host and single host structures established that the mobility balance in the emitting layers determined the amount of accumulated polarons in those layers. Consequently, an exciplex-forming co-host provides a superior structure in terms of device lifetime and efficiency because of its well-balanced mobility. Minimizing polaron accumulation is key to achieving long OLED device lifetimes. This is a crucial aspect of device physics that must be considered in the device design structure.

Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

DOE PAGES

Thomson, Stuart A. J.; Niklas, Jens; Mardis, Kristy L.; ...

2017-09-13

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh 2) 2, DTS(F2BTTh 2) 2, DTS(PTTh 2) 2, DTG(FBTTh 2) 2 and DTG(F2BTTh 2) 2) with the fullerene derivative PCmore » 61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh 2) 2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh 2) 2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. As a result, the higher BET triplet exciton population in the DTS(PTTh 2) 2 blend is in accordance with the slower charge separation dynamics observed in this blend.« less

Charge Separation and Triplet Exciton Formation Pathways in Small Molecule Solar Cells as Studied by Time-resolved EPR Spectroscopy.

PubMed

Thomson, Stuart A J; Niklas, Jens; Mardis, Kristy L; Mallares, Christopher; Samuel, Ifor D W; Poluektov, Oleg G

2017-10-19

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh 2 ) 2 , DTS(F 2 BTTh 2 ) 2 , DTS(PTTh 2 ) 2 , DTG(FBTTh 2 ) 2 and DTG(F 2 BTTh 2 ) 2 ) with the fullerene derivative PC 61 BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh 2 ) 2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh 2 ) 2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh 2 ) 2 blend is in accordance with the slower charge separation dynamics observed in this blend.

Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thomson, Stuart A. J.; Niklas, Jens; Mardis, Kristy L.

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh 2) 2, DTS(F2BTTh 2) 2, DTS(PTTh 2) 2, DTG(FBTTh 2) 2 and DTG(F2BTTh 2) 2) with the fullerene derivative PCmore » 61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh 2) 2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh 2) 2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. As a result, the higher BET triplet exciton population in the DTS(PTTh 2) 2 blend is in accordance with the slower charge separation dynamics observed in this blend.« less

Optically detected magnetic resonance studies on pi-conjugated polymers and novel carbon allotropes

NASA Astrophysics Data System (ADS)

Partee, Jonathan Farel

1997-12-01

The photophysics of poly(p-phenylene)-type ladder polymers (m-LPPP) and 2,5-dibutoxy poly(p-phenylene ethynylene) (DBO-PPE) films and solutions were studied by X-band photoluminescence detected magnetic resonance (PLDMR). Frequency resolved PLDMR measurements on LPPP, DBO-PPE, poly(3-hexylthiophene) (P3HT), poly(p-phenylene-vinylene) (PPV), and Csb{70} are also reported and discussed. All the polymer samples exhibit three distinct features when excited at wavelengths lambda≥ 458nm: (i) A narrow PL-enhancing spin-1/2 polaron resonance, (ii) broad full- and (iii) half-field spin-1 triplet exciton powder patterns due to the Deltamsbs = 1 and Deltamsbs = 2 transitions among the triplet sublevels, respectively. The full-width at half maximum (FWHM) of the spin-1/2 resonance in LPPP decreased from film to solution. However, the FWHM of that resonance in PPE was identical at all concentrations. This spin-1/2 resonance is assigned to the magnetic resonance enhancement of the recombination of both interchain and intrachain-intersegment polaron pairs which quench singlet exciton recombination. In solid m-LPPP samples, the aggregate PL gives rise to a proportionally higher magnetic resonance effect than other parts of the PL spectrum. In DBO-PPE and m-LPPP solutions, the triplet resonance decreased with decreasing concentration. This suggests that the triplet state is an intrinsic long-lived (˜30mus) trapped state localized on a phenylene ring and stabilized by coupling to a unit of an adjacent chain. Frequency resolved measurements of the lifetime of the species affected by the resonance conditions for all the polymers are described and discussed. The lifetimes appeared to include: (i) fast (9mus ≤ tausb1 ≤ 40mus) and (ii) slow (575mus\\ ≤ tausb2≤ 1868mus) components. The lifetimes increased with increasing concentration of the polymers in toluene solutions. These results can be interpreted to provide support for the interchain/intersegment polaron model or a

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

Measuring the Influence of Dielectric Environment on 2D Excitons in Monolayer Semiconductors: Insight from High Magnetic Fields1

NASA Astrophysics Data System (ADS)

Stier, Andreas

The relatively heavy electrons and holes in monolayer semiconductors such as MoS2 form tightly-bound excitons with large binding energies, thus motivating magneto-optical studies in high magnetic fields. Because 2D excitons in these materials necessarily lie close to a surface, their properties are expected to be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role are challenging, in part because the most readily accessible exciton parameter - the exciton's optical transition energy - is largely unaffected by the surrounding medium. Here we show that the role of the dielectric environment can be revealed through its systematic influence on the size of the exciton, which can be directly measured via the diamagnetic shift of the exciton transition in high magnetic fields. Using exfoliated WSe2 monolayers affixed to single-mode optical fibers, we tune the surrounding dielectric environment by encapsulating the monolayers with different materials, and perform polarization resolved low-temperature magneto-absorption studies to 65 tesla. The systematic increase of the exciton's size with dielectric screening, and concurrent two-fold reduction in binding energy (also inferred from these measurements), is quantitatively compared with leading theoretical models based on the Keldysh potential. These results demonstrate how exciton properties can be tuned in future 2D devices and van der Waals heterostructures. 1In collaboration with S.A. Crooker (NHMFL); J. Kono (Rice University); K.M. McCreary, B.T. Jonker (Naval Research Lab); N.P. Wilson, G. Clark, X. Xu (University of Washington).

Magnetic excitation and local magnetic susceptibility of the excitonic insulator Ta2NiSe5 investigated by 77Se NMR

NASA Astrophysics Data System (ADS)

Li, Shang; Kawai, Shunsuke; Kobayashi, Yoshiaki; Itoh, Masayuki

2018-04-01

77Se NMR measurements were made on polycrystalline and single-crystalline samples to elucidate local magnetic susceptibility and magnetic excitation of Ta2NiSe5 , which is proposed to undergo an exciton condensation accompanied by a structural transition at Tc=328 K . We determine the 77Se Knight shift tensors for the three Se sites and analyze their anisotropy based on the site symmetry. The temperature dependence of the Knight shift is discussed on the basis of spin and orbital susceptibilities calculated for two-chain and two-dimensional three-band models. The large fraction of the Se 4 p orbital polarization due to the mixing between Ni 3 d and Se 4 p orbitals is estimated from the analysis of the transferred hyperfine coupling constant. Also the nuclear spin-lattice relaxation rate 1 /T1 is found not to show a coherent peak just below Tc and to obey the thermally activated temperature dependence with a spin gap energy of 1770 ±40 K . This behavior of 1 /T1 monitors the exciton condensation as proposed by the theoretical study of 1 /T1 based on the three-chain Hubbard model for the excitonic insulator.

Excitons in coupled type-II double quantum wells under electric and magnetic fields: InAs/AlSb/GaSb

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lyo, S. K., E-mail: sklyo@uci.edu; 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 themore » electric and magnetic fields.« less

Quantum transition and decoherence of levitating polaron on helium film thickness under an electromagnetic field

NASA Astrophysics Data System (ADS)

Kenfack, S. C.; Fotue, A. J.; Fobasso, M. F. C.; Djomou, J.-R. D.; Tiotsop, M.; Ngouana, K. S. L.; Fai, L. C.

2017-12-01

We have studied the transition probability and decoherence time of levitating polaron in helium film thickness. By using a variational method of Pekar type, the ground and the first excited states of polaron are calculated above the liquid-helium film placed on the polar substrate. It is shown that the polaron transits from the ground to the excited state in the presence of an external electromagnetic field in the plane. We have seen that, in the helium film, the effects of the magnetic and electric fields on the polaron are opposite. It is also shown that the energy, transition probability and decoherence time of the polaron depend sensitively on the helium film thickness. We found that decoherence time decreases as a function of increasing electron-phonon coupling strength and the helium film thickness. It is seen that the film thickness can be considered as a new confinement in our system and can be adjusted in order to reduce decoherence.

Magnetic polarons in antiferromagnetic CaMnO3-x (x<0.01) probed by O17 NMR

NASA Astrophysics Data System (ADS)

Trokiner, A.; Verkhovskii, S.; Yakubovskii, A.; Gerashenko, A.; Monod, P.; Kumagai, K.; Mikhalev, K.; Buzlukov, A.; Litvinova, Z.; Gorbenko, O.; Kaul, A.; Kartavtzeva, M.

2009-06-01

We study with O17 NMR and bulk magnetization a lightly electron doped CaMnO3-x (x<0.01) polycrystalline sample in the G -type antiferromagnetic state. The O17 NMR spectra show two lines with very different intensities corresponding to oxygen sites with very different local magnetic environments. The more intense unshifted line is due to the antiferromagnetic (AF) matrix. The thermal dependence of the magnetic moment of the AF sublattice deduced from the O17 linewidth is typical of insulating three-dimensional Heisenberg antiferromagnets. The less intense, strongly shifted line directly evidences the existence of ferromagnetic (FM) domains embedded in the AF spin lattice. The extremely narrow line in zero magnetic field indicates a nearly perfect alignment of the manganese spins in the FM domains which also display an unusually weak temperature dependence of their magnetic moment. We show that these FM entities start to move above 40 K in a slow-diffusion regime. These static and dynamic properties bear a strong similarity with those of a small size self-trapped magnetic polaron.

Theory of exciton transfer and diffusion in conjugated polymers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barford, William, E-mail: william.barford@chem.ox.ac.uk; Tozer, Oliver Robert; University College, University of Oxford, Oxford OX1 4BH

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 themore » 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

Theory of exciton transfer and diffusion in conjugated polymers.

PubMed

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

Spectroscopic study of excitations in pi-conjugated polymers

NASA Astrophysics Data System (ADS)

Yang, Cungeng

This dissertation deals with spin-physics of photo excitations in pi-conjugated polymers. Optical and magneto-optical spectroscopies, including continuous wave and time-resolved photo-induced absorption, photoluminescence, electroluminescence, and their optically detected magnetic resonance, were used to study steady state and transient photogeneration, energy transfer, spin relaxation, and spin dependent recombination process in the time domain from tens of nanoseconds to tens of milliseconds in polymer materials including regio-random poly (3-hexyl-thiophene-2,5-diyl), regio-regular poly (3-hexyl-thiophene-2,5-diyl), poly (9,9-dioctyl-fluorenyl-2,7-diyl), poly (poly (2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) of various morphologies, and transition metal complex poly (Pt-quinoxene). Our studies provided the tools to clarify the physical pictures regarding two types of long-lived photoexcitations, namely polarons (both germinate polaron-pairs, and unpaired polarons) and triplet excitons, which are the major excitations in these exotic semiconductors in electrical and optical related applications. From measurements of transient fluorescence and transient fluorescence detected magnetic resonance we show that photogenerated geminate polaron pairs live up to hundreds of microseconds following laser pulsed excitation. This conclusion is in agreement with the delayed formation of triplet excitons that we measured by transient photoinduced absorption. It also agrees with the weak spin-lattice relaxation rate in polymers that we measured using the optically detected magnetic resonance dynamic in thin films and organic light emitting devices. Randomly captured nongeminate polaron pairs were shown to be the major source of optically detected magnetic resonance signal at steady, state. We found that the dynamics and magnitude of the signal depend on the spin-relaxation rate, generation rate and decay rate of the geminate pairs and nongeminate pairs. Importantly we

Probing the influence of dielectric environment on excitons in monolayer WSe 2: Insight from high magnetic fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stier, Andreas V.; Wilson, Nathan P.; Clark, Genevieve

Excitons in atomically thin semiconductors necessarily lie close to a surface, and therefore their properties are expected to be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role are challenging, in part because the most readily accessible exciton parameter—the exciton’s optical transition energy—is largely unaffected by the surrounding medium. Here we show that the role of the dielectric environment is revealed through its systematic influence on the size of the exciton, which can be directly measured via the diamagnetic shift of the exciton transition in high magnetic fields. Using exfoliated WSe 2 monolayers affixed to single-modemore » optical fibers, we tune the surrounding dielectric environment by encapsulating the flakes with different materials and perform polarized low-temperature magneto-absorption studies to 65 T. The systematic increase of the exciton’s size with dielectric screening, and concurrent reduction in binding energy (also inferred from these measurements), is quantitatively compared with leading theoretical models. Furthermore, these results demonstrate how exciton properties can be tuned in future 2D optoelectronic devices.« less

Probing the influence of dielectric environment on excitons in monolayer WSe 2: Insight from high magnetic fields

DOE PAGES

Stier, Andreas V.; Wilson, Nathan P.; Clark, Genevieve; ...

2016-11-09

Excitons in atomically thin semiconductors necessarily lie close to a surface, and therefore their properties are expected to be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role are challenging, in part because the most readily accessible exciton parameter—the exciton’s optical transition energy—is largely unaffected by the surrounding medium. Here we show that the role of the dielectric environment is revealed through its systematic influence on the size of the exciton, which can be directly measured via the diamagnetic shift of the exciton transition in high magnetic fields. Using exfoliated WSe 2 monolayers affixed to single-modemore » optical fibers, we tune the surrounding dielectric environment by encapsulating the flakes with different materials and perform polarized low-temperature magneto-absorption studies to 65 T. The systematic increase of the exciton’s size with dielectric screening, and concurrent reduction in binding energy (also inferred from these measurements), is quantitatively compared with leading theoretical models. Furthermore, these results demonstrate how exciton properties can be tuned in future 2D optoelectronic devices.« less

Ground-state energy of an exciton-(LO) phonon system in a parabolic quantum well

NASA Astrophysics Data System (ADS)

Gerlach, B.; Wüsthoff, J.; Smondyrev, M. A.

1999-12-01

This paper presents a variational study of the ground-state energy of an exciton-(LO) phonon system, which is spatially confined to a quantum well. The exciton-phonon interaction is of Fröhlich type, the confinement potentials are assumed to be parabolic functions of the coordinates. Making use of functional integral techniques, the phonon part of the problem can be eliminated exactly, leading us to an effective two-particle system, which has the same spectral properties as the original one. Subsequently, Jensen's inequality is applied to obtain an upper bound on the ground-state energy. The main intention of this paper is to analyze the influence of the quantum-well-induced localization of the exciton on its ground-state energy (or its binding energy, respectively). To do so, we neglect any mismatch of the masses or the dielectric constants, but admit an arbitrary strength of the confinement potentials. Our approach allows for a smooth interpolation of the ultimate limits of vanishing and infinite confinement, corresponding to the cases of a free three-dimensional and a free two-dimensional exciton-phonon system. The interpolation formula for the ground-state energy bound corresponds to similar formulas for the free polaron or the free exciton-phonon system. These bounds in turn are known to compare favorably with all previous ones, which we are aware of.

Scaling laws of Rydberg excitons

NASA Astrophysics Data System (ADS)

Heckötter, J.; Freitag, M.; Fröhlich, D.; Aßmann, M.; Bayer, M.; Semina, M. A.; Glazov, M. M.

2017-09-01

Rydberg atoms have attracted considerable interest due to their huge interaction among each other and with external fields. They demonstrate characteristic scaling laws in dependence on the principal quantum number n for features such as the magnetic field for level crossing or the electric field of dissociation. Recently, the observation of excitons in highly excited states has allowed studying Rydberg physics in cuprous oxide crystals. Fundamentally different insights may be expected for Rydberg excitons, as the crystal environment and associated symmetry reduction compared to vacuum give not only optical access to many more states within an exciton multiplet but also extend the Hamiltonian for describing the exciton beyond the hydrogen model. Here we study experimentally and theoretically the scaling of several parameters of Rydberg excitons with n , for some of which we indeed find laws different from those of atoms. For others we find identical scaling laws with n , even though their origin may be distinctly different from the atomic case. At zero field the energy splitting of a particular multiplet n scales as n-3 due to crystal-specific terms in the Hamiltonian, e.g., from the valence band structure. From absorption spectra in magnetic field we find for the first crossing of levels with adjacent principal quantum numbers a Br∝n-4 dependence of the resonance field strength, Br, due to the dominant paramagnetic term unlike for atoms for which the diamagnetic contribution is decisive, resulting in a Br∝n-6 dependence. By contrast, the resonance electric field strength shows a scaling as Er∝n-5 as for Rydberg atoms. Also similar to atoms with the exception of hydrogen we observe anticrossings between states belonging to multiplets with different principal quantum numbers at these resonances. The energy splittings at the avoided crossings scale roughly as n-4, again due to crystal specific features in the exciton Hamiltonian. The data also allow us to

Spin polarized semimagnetic exciton-polariton condensate in magnetic field.

PubMed

Król, Mateusz; Mirek, Rafał; Lekenta, Katarzyna; Rousset, Jean-Guy; Stephan, Daniel; Nawrocki, Michał; Matuszewski, Michał; Szczytko, Jacek; Pacuski, Wojciech; Piętka, Barbara

2018-04-27

Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, which precludes the observation of condensates with a well defined spin projection along the axis of the system. The enhancement of the Zeeman splitting can be achieved by introducing magnetic ions to the quantum wells, and consequently forming semimagnetic polaritons. In this system, increasing magnetic field can induce polariton condensation at constant excitation power. Here we evidence the spin polarization of a semimagnetic polaritons condensate exhibiting a circularly polarized emission over 95% even in a moderate magnetic field of about 3 T. Furthermore, we show that unlike nonmagnetic polaritons, an increase on excitation power results in an increase of the semimagnetic polaritons condensate spin polarization. These properties open new possibilities for testing theoretically predicted phenomena of spin polarized condensate.

Hole polaron-polaron interaction in transition metal oxides and its limit to p-type doping

NASA Astrophysics Data System (ADS)

Chen, Shiyou; Wang, Lin-Wang

2014-03-01

Traditionally the origin of the poor p-type conductivity in some transition metal oxides (TMOs) was attributed to the limited hole concentration: the charge-compensating donor defects, such as oxygen vacancies and cation interstitials, can form spontaneously as the Fermi energy shifts down to near the valence band maximum. Besides the thermodynamic limit to the hole concentration, the limit to the hole mobility can be another possible reason, e.g., the hole carrier can form self-trapped polarons with very low carrier mobility. Although isolated hole polarons had been found in some TMOs, the polaron-polaron interaction is not well-studied. Here we show that in TMOs such as TiO2 and V2O5, the hole polarons prefer to bind with each other to form bipolarons, which are more stable than free hole carriers or separated polarons. This pushes the hole states upward into the conduction band and traps the holes. The rise of the Fermi energy suppresses the spontaneous formation of the charge-compensating donor defects, so the conventional mechanism becomes ineffective. Since it can happen in the impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs. This material is based upon work performed by the JCAP, a US DOE Energy Innovation Hub, the NSFC (No. 61106087 and 91233121) and special funds for major state basic research (No. 2012CB921401).

Exciton Absorption Spectra by Linear Response Methods:Application to Conjugated Polymers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mosquera, Martin A.; Jackson, Nicholas E.; Fauvell, Thomas J.

The theoretical description of the timeevolution of excitons requires, as an initial step, the calculation of their spectra, which has been inaccessible to most users due to the high computational scaling of conventional algorithms and accuracy issues caused by common density functionals. Previously (J. Chem. Phys. 2016, 144, 204105), we developed a simple method that resolves these issues. Our scheme is based on a two-step calculation in which a linear-response TDDFT calculation is used to generate orbitals perturbed by the excitonic state, and then a second linear-response TDDFT calculation is used to determine the spectrum of excitations relative to themore » excitonic state. Herein, we apply this theory to study near-infrared absorption spectra of excitons in oligomers of the ubiquitous conjugated polymers poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), and poly(benzodithiophene-thieno[3,4-b]thiophene) (PTB7). For P3HT and MEH-PPV oligomers, the calculated intense absorption bands converge at the longest wavelengths for 10 monomer units, and show strong consistency with experimental measurements. The calculations confirm that the exciton spectral features in MEH-PPV overlap with those of the bipolaron formation. In addition, our calculations identify the exciton absorption bands in transient absorption spectra measured by our group for oligomers (1, 2, and 3 units) of PTB7. For all of the cases studied, we report the dominant orbital excitations contributing to the optically active excited state-excited state transitions, and suggest a simple rule to identify absorption peaks at the longest wavelengths. We suggest our methodology could be considered for further evelopments in theoretical transient spectroscopy to include nonadiabatic effects, coherences, and to describe the formation of species such as charge-transfer states and polaron pairs.« less

Superconducting Polarons and Bipolarons

NASA Astrophysics Data System (ADS)

Alexandrov, A. S.

The seminal work by Bardeen, Cooper and Schrieffer (BCS) extended further by Eliashberg to the intermediate coupling regime solved one of the major scientific problems of Condensed Matter Physics in the last century. The BCS theory provides qualitative and in many cases quantitative descriptions of low-temperature superconducting metals and their alloys, and some novel high-temperature superconductors like magnesium diboride. The theory has been extended by us to the strong-coupling regime where carriers are small lattice polarons and bipolarons. Here I review the multi-polaron strong-coupling theory of superconductivity. Attractive electron correlations, prerequisite to any superconductivity, are caused by an almost unretarded electron-phonon (e-ph) interaction sufficient to overcome the direct Coulomb repulsion in this regime. Low energy physics is that of small polarons and bipolarons, which are real-space electron (hole) pairs dressed by phonons. They are itinerant quasiparticles existing in the Bloch states attemperatures below the characteristic phonon frequency. Since there is almost no retardation (i.e. no Tolmachev-Morel-Anderson logarithm) reducing the Coulomb repulsion, e-ph interactions should be relatively strong to overcome the direct Coulomb repulsion, so carriers mustbe polaronic to form pairs in novel superconductors. I identify the long-range Fröhlich electron-phonon interaction as the most essential for pairing in superconducting cuprates. A number of key observations have been predicted or explained with polarons and bipolarons including unusual isotope effects and upper critical fields, normal state (pseudo)gaps and kinetic properties, normal state diamagnetism, and giant proximity effects. These and many other observations provide strong evidence for a novel state of electronic matter in layered cuprates, which is a charged Bose-liquid of small mobile bipolarons.

Strong influence of polaron-polaron interaction on the magnetoresistance effect in La0.7A0.3MnO3 thin films

NASA Astrophysics Data System (ADS)

Zhang, Haijuan; Wang, Kuidong; Zhang, Yuanyuan; Dong, Wenxia; Chen, Long; Tang, Xiaodong; Chen, Jie

2017-11-01

The colossal magnetoresistance effect endows La0.7A0.3MnO3 manganites distinctive fascination. Both theoretical and experimental studies demonstrated that the interplay among polarons could significantly influence magnetoresistance. However, the underlying microscopic mechanism of the influence remains elusive due to the lack of experimental evidences. Utilizing ultrafast optical spectroscopy to track the polaron dynamics around Curie temperatures, we observed a diverse two-step recovery process in three sibling manganite thin films with various magnetoresistance effects and Curie temperatures, while the slow step was proposed to be the formation evolution of correlated polarons through the polaron-polaron interaction. Polarons in La0.7Ca0.3MnO3 equilibrate much faster than those in La0.7(Ca0.58Sr0.42)0.3MnO3 and La0.7Sr0.3MnO3, indicating a comparatively tighter interaction between polarons and subsequently a stronger magnetoresistance effect.

Magneto-optical quantum interferences in a system of spinor excitons

NASA Astrophysics Data System (ADS)

Kuan, Wen-Hsuan; Gudmundsson, Vidar

2018-04-01

In this work we investigate magneto-optical properties of two-dimensional semiconductor quantum-ring excitons with Rashba and Dresselhaus spin-orbit interactions threaded by a magnetic flux perpendicular to the plane of the ring. By calculating the excitonic Aharonov-Bohm spectrum, we study the Coulomb and spin-orbit effects on the Aharonov-Bohm features. From the light-matter interactions of the excitons, we find that for scalar excitons, there are open channels for spontaneous recombination resulting in a bright photoluminescence spectrum, whereas the forbidden recombination of dipolar excitons results in a dark photoluminescence spectrum. We investigate the generation of persistent charge and spin currents. The exploration of spin orientations manifests that by adjusting the strength of the spin-orbit interactions, the exciton can be constructed as a squeezed complex with specific spin polarization. Moreover, a coherently moving dipolar exciton acquires a nontrivial dual Aharonov-Casher phase, creating the possibility to generate persistent dipole currents and spin dipole currents. Our study reveals that in the presence of certain spin-orbit generated fields, the manipulation of the magnetic field provides a potential application for quantum-ring spinor excitons to be utilized in nano-scaled magneto-optical switches.

Spectrally resolved hyperfine interactions between polaron and nuclear spins in organic light emitting diodes: Magneto-electroluminescence studies

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.

Polaron spin echo envelope modulations in an organic semiconducting polymer

DOE PAGES

Mkhitaryan, V. V.; Dobrovitski, V. V.

2017-06-01

Here, we present a theoretical analysis of the electron spin echo envelope modulation (ESEEM) spectra of polarons in semiconducting π -conjugated polymers. We show that the contact hyperfine coupling and the dipolar interaction between the polaron and the proton spins give rise to different features in the ESEEM spectra. Our theory enables direct selective probe of different groups of nuclear spins, which affect the polaron spin dynamics. Namely, we demonstrate how the signal from the distant protons (coupled to the polaron spin via dipolar interactions) can be distinguished from the signal coming from the protons residing on the polaron sitemore » (coupled to the polaron spin via contact hyperfine interaction). We propose a method for directly probing the contact hyperfine interaction, that would enable detailed study of the polaron orbital state and its immediate environment. Lastly, we also analyze the decay of the spin echo modulation, and its connection to the polaron transport.« less

Magneto-exciton transitions in laterally coupled quantum dots

NASA Astrophysics Data System (ADS)

Barticevic, Zdenka; Pacheco, Monica; Duque, Carlos A.; Oliveira, Luiz E.

2008-03-01

We present a study of the electronic and optical properties of laterally coupled quantum dots. The excitonic spectra of this system under the effects of an external magnetic field applied perpendicular to the plane of the dots is obtained, with the potential of every individual dot taken as the superposition of a quantum well potential along the axial direction with a lateral parabolic confinement potential, and the coupled two- dot system then modeled by a superposition of the potentials of each dot, with their minima at different positions and truncated at the intersection plane. The wave functions and eigenvalues are obtained in the effective-mass approximation by using an extended variational approach in which the magneto- exciton states are simultaneously obtained [1]. The allowed magneto-exciton transitions are investigated by using circularly polarized radiation in the plane perpendicular to the magnetic field. We present results on the excitonic absorption coefficient as a function of the photon energy for different geometric quantum-dot confinement and magnetic-field values. Reference: [1] Z. Barticevic, M. Pacheco, C. A. Duque and L. E. Oliveira, Phys. Rev. B 68, 073312 (2003).

Hyperfine spin interactions between polarons and nuclei in organic light emitting diodes: Magneto-EL measurements

NASA Astrophysics Data System (ADS)

Crooker, S. A.; Kelley, M. R.; Martinez, N.; Nie, W.; Mohite, A. D.; Smith, D. L.; Tretiak, S.; Ruden, P. P.

2014-03-01

Considerable attention in recent years has focused on the effects of applied magnetic fields on the conductance, photocurrent, electroluminescence (EL), and photoluminescence of nominally nonmagnetic organic semiconductor materials and devices. These magnetic field effects have proven useful in revealing the underlying physical mechanisms and relevant spin interactions that influence the electrical and optical properties in these organic systems (e.g., hyperfine coupling, exchange interactions, and spin-orbit coupling). Here we study the field-dependent properties of organic light-emitting diode (OLEDs) based on MTDATA/LiF/Bphen layered structures, in which exciplex recombination at the interface dominates the EL spectra. Small applied magnetic fields (~10 mT) are found to boost the net EL yield by up to 10%, due to a suppression of the mixing between singlet and triplet polaron pairs which, in turn, arises from hyperfine spin coupling of the polarons to the underlying nuclei of the host molecules. We discuss the dependence of these field-induced effects on the LiF barrier thickness, device bias, and on the orientation of the applied magnetic field, as well as the mechanisms responsible.

Dynamics of Polarons in Organic Conjugated Polymers with Side Radicals.

PubMed

Liu, J J; Wei, Z J; Zhang, Y L; Meng, Y; Di, B

2017-03-16

Based on the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model, and using the molecular dynamics method, we discuss the dynamics of electron and hole polarons propagating along a polymer chain, as a function of the distance between side radicals and the magnitude of the transfer integrals between the main chain and the side radicals. We first discuss the average velocities of electron and hole polarons as a function of the distance between side radicals. It is found that the average velocities of the electron polarons remain almost unchanged, while the average velocities of hole polarons decrease significantly when the radical distance is comparable to the polaron width. Second, we have found that the average velocities of electron polarons decrease with increasing transfer integral, but the average velocities of hole polarons increase. These results may provide a theoretical basis for understanding carriers transport properties in polymers chain with side radicals.

Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry

PubMed Central

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.

PubMed

Chestnov, Igor Yu; Demirchyan, Sevak S; Alodjants, Alexander P; Rubo, Yuri G; Kavokin, Alexey V

2016-01-21

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.

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.

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

Creation of Rydberg Polarons in a Bose Gas

NASA Astrophysics Data System (ADS)

Schmidt, Richard

2017-04-01

In this talk we review the theory of various types of Bose polarons that can be realized in ultracold atomic systems. We then report the spectroscopic observation of Rydberg polarons in a Bose gas which is in excellent agreement with theoretical predictions. This novel type of polaron is created by excitation of Rydberg atoms in a strontium Bose-Einstein condensate and it is distinguished by the occupation of a large number bound molecular states. The cross-over from few-body bound molecular oligomers to many-body polaron features is described with a functional determinant theory that solves an extended Froehlich Hamiltonian for an impurity in a Bose gas. The detailed analysis of the red-detuned tail of the excitation spectrum describes the contribution from the region of highest density in the condensate and provides a clear signature of Rydberg polarons. This work has been performed in collaboration with groups at Rice University, Harvard University, and the TU Vienna.

Polarized excitons and optical activity in single-wall carbon nanotubes

NASA Astrophysics Data System (ADS)

Chang, Yao-Wen; Jin, Bih-Yaw

2018-05-01

The polarized excitons and optical activity of single-wall carbon nanotubes (SWNTs) are studied theoretically by π -electron Hamiltonian and helical-rotational symmetry. By taking advantage of the symmetrization, the single-particle energy and properties of a SWNT are characterized with the corresponding helical band structure. The dipole-moment matrix elements, magnetic-moment matrix elements, and the selection rules can also be derived. Based on different selection rules, the optical transitions can be assigned as the parallel-polarized, left-handed circularly-polarized, and right-handed circularly-polarized transitions, where the combination of the last two gives the cross-polarized transition. The absorption and circular dichroism (CD) spectra are simulated by exciton calculation. The calculated results are well comparable with the reported measurements. Built on the foundation, magnetic-field effects on the polarized excitons and optical activity of SWNTs are studied. Dark-bright exciton splitting and interband Faraday effect in the CD spectrum of SWNTs under an axial magnetic field are predicted. The Faraday rotation dispersion can be analyzed according to the selection rules of circular polarizations and the helical band structure.

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.

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.

Quantum Hall signatures of dipolar Mahan excitons

NASA Astrophysics Data System (ADS)

Schinner, G. J.; Repp, J.; Kowalik-Seidl, K.; Schubert, E.; Stallhofer, M. P.; Rai, A. K.; Reuter, D.; Wieck, A. D.; Govorov, A. O.; Holleitner, A. W.; Kotthaus, J. P.

2013-01-01

We explore the photoluminescence of spatially indirect, dipolar Mahan excitons in a gated double quantum well diode containing a mesoscopic electrostatic trap for neutral dipolar excitons at low temperatures down to 250 mK and in quantizing magnetic fields. Mahan excitons in the surrounding of the trap, consisting of individual holes interacting with a degenerate two-dimensional electron system confined in one of the quantum wells, exhibit strong quantum Hall signatures at integer filling factors and related anomalies around filling factor ν=(2)/(3),(3)/(5), and (1)/(2), reflecting the formation of composite fermions. Interactions across the trap perimeter are found to influence the energy of the confined neutral dipolar excitons by the presence of the quantum Hall effects in the two-dimensional electron system surrounding the trap.

Topologically protected excitons in porphyrin thin films.

PubMed

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.

Magnetooptics of Exciton Rydberg States in a Monolayer Semiconductor

NASA Astrophysics Data System (ADS)

Stier, A. V.; Wilson, N. P.; Velizhanin, K. A.; Kono, J.; Xu, X.; Crooker, S. A.

2018-02-01

We report 65 T magnetoabsorption spectroscopy of exciton Rydberg states in the archetypal monolayer semiconductor WSe2 . The strongly field-dependent and distinct energy shifts of the 2 s , 3 s , and 4 s excited neutral excitons permits their unambiguous identification and allows for quantitative comparison with leading theoretical models. Both the sizes (via low-field diamagnetic shifts) and the energies of the n s exciton states agree remarkably well with detailed numerical simulations using the nonhydrogenic screened Keldysh potential for 2D semiconductors. Moreover, at the highest magnetic fields, the nearly linear diamagnetic shifts of the weakly bound 3 s and 4 s excitons provide a direct experimental measure of the exciton's reduced mass mr=0.20 ±0.01 m0.

Optical, electrical, and magnetic field studies of organic materials for light emitting diodes and photovoltaic applications

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

Excitonic energy transfer in light-harvesting complexes in purple bacteria

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ye Jun; Sun Kewei; Zhao Yang

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 beenmore » 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.« less

Polarons in acetanilide

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.

Polarons in acetanilide

DOE Office of Scientific and Technical Information (OSTI.GOV)

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.

Optical studies of native defects in π-conjugated donor-acceptor copolymers

NASA Astrophysics Data System (ADS)

Baniya, Sangita; Khanal, Dipak; Lafalce, Evan; You, Wei; Valy Vardeny, Z.

2018-04-01

We used multiple spectroscopies such as photoinduced absorption (PIA), magneto photoinduced absorption, and doping induced absorption for studying native defects in π-conjugated donor-acceptor copolymer chains of benzodithio-phene fluorinated benzotriazole. The PIA spectrum contains characteristic photoinduced absorption bands that are due to polarons and triplet exciton species, whose strengths have different dependencies on the modulation frequency, temperature, and laser excitation, as well as magnetic field response. We found that the native defects in the copolymer chains serve as efficient traps that ionize the photoexcited excitons, thereby generating charge carriers whose characteristic optical properties are similar, but not equal to those of intrachain polarons formed by doping. The native defects density is of the order of 1017 cm-3 indicating that most of the copolymer chains contain native defects upon synthesis; however, this does not preclude their used-for photovoltaic applications.

Screening effect on the polaron by surface plasmons

NASA Astrophysics Data System (ADS)

Xu, Xiaoying; Xu, Xiaoshan; Seal, Katyayani; Guo, Hangwen; Shen, Jian; Low Dimensional Materials Physics, Oak Ridge National Lab Team; University of Tennessee Team; Physics Department, Fudan University Team

2011-03-01

Surface plasmons occur when the conduction electrons at a metal/dielectric interface resonantly interact with external electromagnetic fields. While surface plasmons in vicinity of a polaron in the dielectric material, a strong screening effect on polaron characteristics is introduced. In this work, we observed the reduction of polarons in multiferroic LuFe2O4, which is mainly contributed by surface plasmons. Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy.

Observation of Spin-Polarons in a strongly interacting Fermi liquid

NASA Astrophysics Data System (ADS)

Zwierlein, Martin

2009-03-01

We have observed spin-polarons in a highly imbalanced mixture of fermionic atoms using tomographic RF spectroscopy. Feshbach resonances allow to freely tune the interactions between the two spin states involved. A single spin down atom immersed in a Fermi sea of spin up atoms can do one of two things: For strong attraction, it can form a molecule with exactly one spin up partner, but for weaker interaction it will spread its attraction and surround itself with a collection of majority atoms. This spin down atom ``dressed'' with a spin up cloud constitutes the spin-polaron. We have observed a striking spectroscopic signature of this quasi-particle for various interaction strengths, a narrow peak in the spin down spectrum that emerges above a broad background. The narrow width signals a long lifetime of the spin-polaron, much longer than the collision rate with spin up atoms, as it must be for a proper quasi-particle. The peak position allows to directly measure the polaron energy. The broad pedestal at high energies reveals physics at short distances and is thus ``molecule-like'': It is exactly matched by the spin up spectra. The comparison with the area under the polaron peak allows to directly obtain the quasi-particle weight Z. We observe a smooth transition from polarons to molecules. At a critical interaction strength of 1/kFa = 0.7, the polaron peak vanishes and spin up and spin down spectra exactly match, signalling the formation of molecules. This is the same critical interaction strength found earlier to separate a normal Fermi mixture from a superfluid molecular Bose-Einstein condensate. The spin-polarons determine the low-temperature phase diagram of imbalanced Fermi mixtures. In principle, polarons can interact with each other and should, at low enough temperatures, form a superfluid of p-wave pairs. We will present a first indication for interactions between polarons.

From Mahan excitons to Landau levels at high magnetic fields: 2DFT spectroscopy reveals hidden quantum correlations (Conference Presentation)

NASA Astrophysics Data System (ADS)

Karaiskaj, Denis

2017-02-01

Two-dimensional electron gases have been the subject of research for decades. Modulation doped GaAs quantum wells in the absence of magnetic fields exhibit interesting many-body physics such as the Fermi edge singularity or Mahan exciton and can be regarded as a collective excitation of the system. Under high magnetic fields Landau levels form which have been studied using transport and optical measurements. Nonlinear coherent two-dimensional Fourier transform (2DFT) spectroscopy however provides new insights into these systems. We present the 2DFT spectra of Mahan Excitons associated with the heavy-hole and light-hole resonances observed in a modulation doped GaAs/AlGaAs single quantum well [1]. These resonances are observed to be strongly coupled through many-body interactions. The 2DFT spectra were measured using co-linear, cross-linear, and co-circular polarizations and reveal striking differences. Furthermore, 2DFT spectra at high magnetic fields performed at the National High Magnetic Field Lab (NHMFL) in Tallahassee, Florida will be discussed. The spectra exhibit new features and peculiar line shapes suggesting interesting underlying physics. [1] J. Paul, C. E. Stevens, C. Liu, P. Dey, C. McIntyre, V. Turkowski, J. L. Reno, D. J. Hilton, and D. Karaiskaj, Phys. Rev. Lett.116, 157401 (2016).

Excitonic giant-dipole potentials in cuprous oxide

NASA Astrophysics Data System (ADS)

Kurz, Markus; Grünwald, Peter; Scheel, Stefan

2017-06-01

In this paper we predict the existence of a novel species of Wannier excitons when exposed to crossed electric and magnetic fields. In particular, we present a theory of giant-dipole excitons in Cu2O in crossed fields. Within our theoretical approach we perform a pseudoseparation of the center-of-mass motion for the field-dressed excitonic species, thereby obtaining an effective single-particle Hamiltonian for the relative motion. For arbitrary gauge fields we exactly separate the gauge-dependent kinetic-energy terms from the effective single-particle interaction potential. Depending on the applied field strengths and the specific field orientation, the potential for the relative motion of electron and hole exhibits an outer well at spatial separations up to several micrometers and depths up to 380 μ eV , leading to possible permanent excitonic electric dipole moments of around 3 ×106 D.

Impact of charge-transfer excitons in regioregular polythiophene on the charge separation at polythiophene-fullerene heterojunctions

NASA Astrophysics Data System (ADS)

Polkehn, M.; Tamura, H.; Burghardt, I.

2018-01-01

This study addresses the mechanism of ultrafast charge separation in regioregular oligothiophene-fullerene assemblies representative of poly-3-hexylthiophene (P3HT)-[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) heterojunctions, with special emphasis on the inclusion of charge transfer excitons in the oligothiophene phase. The formation of polaronic inter-chain charge separated species in highly ordered oligothiophene has been demonstrated in recent experiments and could have a significant impact on the net charge transfer to the fullerene acceptor. The present approach combines a first-principles parametrized multi-site Hamiltonian, based on time-dependent density functional theory calculations, with accurate quantum dynamics simulations using the multi-layer multi-configuration time-dependent Hartree method. Quantum dynamical studies are carried out for up to 182 electronic states and 112 phonon modes. The present analysis follows up on our previous study of (Huix-Rotllant et al 2015 J. Phys. Chem. Lett. 6 1702) and significantly expands the scope of this analysis by including the dynamical role of charge transfer excitons. Our investigation highlights the pronounced mixing of photogenerated Frenkel excitons with charge transfer excitons in the oligothiophene domain, and the opening of new transfer channels due the creation of such charge-separated species. As a result, it turns out that the interfacial donor/acceptor charge transfer state can be largely circumvented due to the presence of charge transfer excitons. However, the latter states in turn act as a trap, such that the free carrier yield observed on ultrafast time scales is tangibly reduced. The present analysis underscores the complexity of the transfer pathways at P3HT-PCBM type junctions.

Cotunneling and polaronic effect in granular systems

NASA Astrophysics Data System (ADS)

Ioselevich, A. S.; Sivak, V. V.

2017-06-01

We theoretically study the conductivity in arrays of metallic grains due to the variable-range multiple cotunneling of electrons with short-range (screened) Coulomb interaction. The system is supposed to be coupled to random stray charges in the dielectric matrix that are only loosely bounded to their spatial positions by elastic forces. The flexibility of the stray charges gives rise to a polaronic effect, which leads to the onset of Arrhenius-type conductivity behavior at low temperatures, replacing conventional Mott variable-range hopping. The effective activation energy logarithmically depends on temperature due to fluctuations of the polaron barrier heights. We present the unified theory that covers both weak and strong polaron effect regimes of hopping in granular metals and describes the crossover from elastic to inelastic cotunneling.

Observation of rapid exciton-exciton annihilation in monolayer molybdenum disulfide.

PubMed

Sun, Dezheng; Rao, Yi; Reider, Georg A; Chen, Gugang; You, Yumeng; Brézin, Louis; Harutyunyan, Avetik R; Heinz, Tony F

2014-10-08

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature.

Exciton-photon correlations in bosonic condensates of exciton-polaritons.

PubMed

Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G

2015-07-08

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.

Exciton-photon correlations in bosonic condensates of exciton-polaritons

PubMed Central

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

Magnetic field tuning of an excitonic insulator between the weak and strong coupling regimes in quantum limit graphite [Tunable excitonic insulator in quantum limit graphite

DOE PAGES

Zhu, Zengwei; McDonald, R. D.; Shekhter, A.; ...

2017-05-04

Here, the excitonic insulator phase has long been predicted to form in proximity to a band gap opening in the underlying band structure. The character of the pairing is conjectured to crossover from weak (BCS-like) to strong coupling (BEC-like) as the underlying band structure is tuned from the metallic to the insulating side of the gap opening. Here we report the high-magnetic field phase diagram of graphite to exhibit just such a crossover. By way of comprehensive angle-resolved magnetoresistance measurements, we demonstrate that the underlying band gap opening occurs inside the magnetic field-induced phase, paving the way for a systematicmore » study of the BCS-BEC-like crossover by means of conventional condensed matter probes.« less

Magnetic field tuning of an excitonic insulator between the weak and strong coupling regimes in quantum limit graphite [Tunable excitonic insulator in quantum limit graphite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Zengwei; McDonald, R. D.; Shekhter, A.

Here, the excitonic insulator phase has long been predicted to form in proximity to a band gap opening in the underlying band structure. The character of the pairing is conjectured to crossover from weak (BCS-like) to strong coupling (BEC-like) as the underlying band structure is tuned from the metallic to the insulating side of the gap opening. Here we report the high-magnetic field phase diagram of graphite to exhibit just such a crossover. By way of comprehensive angle-resolved magnetoresistance measurements, we demonstrate that the underlying band gap opening occurs inside the magnetic field-induced phase, paving the way for a systematicmore » study of the BCS-BEC-like crossover by means of conventional condensed matter probes.« less

Spin-polarized exciton quantum beating in hybrid organic-inorganic perovskites

NASA Astrophysics Data System (ADS)

Odenthal, Patrick; Talmadge, William; Gundlach, Nathan; Wang, Ruizhi; Zhang, Chuang; Sun, Dali; Yu, Zhi-Gang; Valy Vardeny, Z.; Li, Yan S.

2017-09-01

Hybrid organic-inorganic perovskites have emerged as a new class of semiconductors that exhibit excellent performance as active layers in photovoltaic solar cells. These compounds are also highly promising materials for the field of spintronics due to their large and tunable spin-orbit coupling, spin-dependent optical selection rules, and their predicted electrically tunable Rashba spin splitting. Here we demonstrate the optical orientation of excitons and optical detection of spin-polarized exciton quantum beating in polycrystalline films of the hybrid perovskite CH3NH3PbClxI3-x. Time-resolved Faraday rotation measurement in zero magnetic field reveals unexpectedly long spin lifetimes exceeding 1 ns at 4 K, despite the large spin-orbit couplings of the heavy lead and iodine atoms. The quantum beating of exciton states in transverse magnetic fields shows two distinct frequencies, corresponding to two g-factors of 2.63 and -0.33, which we assign to electrons and holes, respectively. These results provide a basic picture of the exciton states in hybrid perovskites, and suggest they hold potential for spintronic applications.

Polaron theory of high- Tc superconductors

NASA Astrophysics Data System (ADS)

Alexandrov, A. S.

1989-05-01

It is shown that the ordinary electron-phonon interaction can produce a high Tc as a result of the polaron narrowing of the band, which is not considered by the traditional theory of strong-coupling superconductors based on Migdal-Eliashberg equations, which are violated even in the range of moderate values λ ⪖1. Numerous experimental data are discussed which seem to favour a phonon-mediated attraction, polaron mass enhancement, narrow band and nonadiabatic motion of carriers and charged Bose-like excitations in high Tc metallic oxides.

Extreme electron polaron spatial delocalization in π-conjugated materials

DOE PAGES

Rawson, Jeff; Angiolillo, Paul J.; Therien, Michael J.

2015-10-28

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. In this paper, 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. Finally, because the electron polaron dimension can be linked tomore » 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.« less

Thermodynamics of Polaronic States in Artificial Spin Ice

NASA Astrophysics Data System (ADS)

Farhan, Alan

Artificial spin ices represent a class of systems consisting of lithographically patterned nanomagnets arranged in two-dimensional geometries. They were initially introduced as a two-dimensional analogue to geometrically frustrated pyrochlore spin ice, and the most recent introduction of artificial spin ice systems with thermally activated moment fluctuations not only delivered the possibility to directly investigate geometrical frustration and emergent phenomena with real space imaging, but also paved the way to design and investigate new two-dimensional magnetic metamaterials, where material properties can be directly manipulated giving rise to properties that do not exist in nature. Here, taking advantage of cryogenic photoemission electron microscopy, and using the concept of emergent magnetic charges, we are able to directly visualize the creation and annihilation of screened emergent magnetic monopole defects in artificial spin ice. We observe that these polaronic states arise as intermediate states, separating an energetically excited out-of-equilibrium state and low-energy equilibrium configurations. They appear as a result of a local screening effect between emergent magnetic charge defects and their neighboring magnetic charges, thus forming a transient minimum, before the system approaches a global minimum with the least amount of emergent magnetic charge defects. This project is funded by the Swiss National Science Foundation.

TOPICAL REVIEW: Electron small polarons and bipolarons in LiNbO3

NASA Astrophysics Data System (ADS)

Schirmer, O. F.; Imlau, M.; Merschjann, C.; Schoke, B.

2009-03-01

An overview of the properties of electron small polarons and bipolarons is given, which can occur in the congruently melting composition of LiNbO3 (LN). Such polarons influence the performance of this important optical material decisively. Since coupling to the lattice strongly quenches the tunnelling of free small polarons in general, they are easily localized at one site even by weak irregularities of a crystal. The mechanism of their optical absorptions is thus shared with those of small polarons localized by binding to selected defects. It is shown that the optical properties of free electrons in LN as well as those bound to NbLi antisite defects can be attributed consistently to small polarons. This is extended to electron pairs forming bipolarons bound to NbLi-NbNb nearest neighbours in the LN ground state. On the basis of an elementary phenomenological approach, relying on familiar concepts of defect physics, the peak energies, lineshapes, widths of the related optical absorption bands as well as the defect binding energies induced by lattice distortion are analysed. A criterion universally identifying small polaron absorption bands in oxide materials is pointed out. For the bipolarons, the dissociation energy, 0.27 eV, derived from a corresponding study of the mass action behaviour, is shown to be consistent with the data on isolated polarons. Based on experience with simple O- hole small polaron systems, a mechanism is proposed which explains why the observed small polaron optical absorptions are higher above the peak energies of the bands than those predicted by the conventional theory. The parameters characterizing the optical absorptions are seen to be fully consistent with those determining the electrical conductivity, i.e. the bipolaron dissociation energy and the positions of the defect levels as well as the activation energy of mobility. A reinterpretation of previous thermopower data of reduced LN on the basis of the bipolaron model confirms that

Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes.

PubMed

Ma, Xuedan; Roslyak, Oleskiy; Duque, Juan G; Pang, Xiaoying; Doorn, Stephen K; Piryatinski, Andrei; Dunlap, David H; Htoon, Han

2015-07-03

Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization.

Low-Energy Excitation Spectra in the Excitonic Phase of Cobalt Oxides

NASA Astrophysics Data System (ADS)

Yamaguchi, Tomoki; Sugimoto, Koudai; Ohta, Yukinori

2017-04-01

We study the excitonic phase and low-energy excitation spectra of perovskite cobalt oxides. Constructing the five-orbital Hubbard model defined on the three-dimensional cubic lattice for the 3d bands of Pr0.5Ca0.5CoO3, we calculate the excitonic susceptibility in the normal state in the random-phase approximation (RPA) to show the presence of the instability toward excitonic condensation. On the basis of the excitonic ground state with a magnetic multipole obtained in the mean-field approximation, we calculate the dynamical susceptibility of the excitonic phase in the RPA and find that there appear a gapless collective excitation in the spin-transverse mode (Goldstone mode) and a gapful collective excitation in the spin-longitudinal mode (Higgs mode). The experimental relevance of our results is discussed.

Analytical solutions of the Schroedinger equation for a two-dimensional exciton in magnetic field of arbitrary strength

DOE Office of Scientific and Technical Information (OSTI.GOV)

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 variousmore » physical analyses and the method used here could also be applied to other atomic systems.« less

Spin-state transitions and magnetic polaron in lightly doped La1-xSrxCoO3.

NASA Astrophysics Data System (ADS)

Podlesnyak, A.; Haverkort, M. W.; Conder, K.; Pomyakushina, E.; Khomskii, Daniel

2007-03-01

Using the inelastic neutron scattering (INS) technique, we identified the energy levels of the thermally excited states of Co^3+ ions in both LaCoO3 and La0.998Sr0.002CoO3. In LaCoO3 an excitation at ˜0.6 meV appears at T>30K, whose intensity follows the bulk magnetization. Within a model including crystal field interaction and spin-orbit coupling we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. Since the g-factor obtained from the field dependence of the INS is g˜ 3, we interpret this state as a high-spin state of Co^3+ . The lightly doped material x˜0:002 exhibits paramagnetic properties at low temperatures. An INS peak at energy transfer ˜0.75 meV was observed in it already at T = 1:5 K. We propose that the holes introduced in the LS state of LaCoO3 by doping are extended over the neighboring Co sites, forming thus magnetic polaron and transforming all the involved Co ions (e.g. 6 of them) to the high-spin state. Similarly to LaCoO3, we interpret the INS transition at 0.75 meV as that on these high-spin Co^3+ ions.

Influence of lithium vacancies on the polaronic transport in olivine phosphate structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Murugavel, Sevi, E-mail: murug@physics.du.ac.in; Sharma, Monika; Shahid, Raza

2016-01-28

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 betweenmore » 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 LiFePO{sub 4} (LFP) and delithiated heterosite FePO{sub 4} (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.« less

Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

NASA Astrophysics Data System (ADS)

Robert, C.; Amand, T.; Cadiz, F.; Lagarde, D.; Courtade, E.; Manca, M.; Taniguchi, T.; Watanabe, K.; Urbaszek, B.; Marie, X.

2017-10-01

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting δ =0.6 ±0.1 meV between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at Bz=0 , while the upper state is partially coupled to light with z polarization ("gray" exciton). The first determination of the dark neutral exciton lifetime τD in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τD˜110 ±10 ps for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T =12 K .

Singlet exciton fission photovoltaics.

PubMed

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

Polaron-mediated surface reconstruction in the reduced Rutile TiO2 (110) surface

NASA Astrophysics Data System (ADS)

Reticcioli, Michele; Setvin, Martin; Hao, Xianfeng; Diebold, Ulrike; Franchini, Cesare

The role of polarons is of key importance for the understanding of the fundamental properties and functionalities of TiO2. We use density functional theory with an on-site Coulomb interaction and molecular dynamics to study the formation and dynamics of small polarons in the reduced rutile (110) surface. We show that excess electrons donated by oxygen-vacancies (VO) form mobile small polarons that hop easily in subsurface and surface Ti-sites. The polaron formation becomes more favorable by increasing the VO concentration level (up to 20%) due to the progressively lower energy cost needed to distort the lattice. However, at higher VO concentration the shortening of the averaged polaron-polaron distance leads to an increased Coulomb repulsion among the trapped charges at the Ti-sites, which weakens this trend. This instability is overtaken by means of a structural 1 × 2 surface reconstruction, characterized by a distinctively more favorable polaron distribution. The calculations are validated by a direct comparison with experimental AFM and STM data. Our study identifies a fundamentally novel mechanism to drive surface reconstructions and resolves a long standing issue on the origin of the reconstruction in rutile (110) surface.

Creation of Rydberg Polarons in a Bose Gas

NASA Astrophysics Data System (ADS)

Camargo, F.; Schmidt, R.; Whalen, J. D.; Ding, R.; Woehl, G.; Yoshida, S.; Burgdörfer, J.; Dunning, F. B.; Sadeghpour, H. R.; Demler, E.; Killian, T. C.

2018-02-01

We report spectroscopic observation of Rydberg polarons in an atomic Bose gas. Polarons are created by excitation of Rydberg atoms as impurities in a strontium Bose-Einstein condensate. They are distinguished from previously studied polarons by macroscopic occupation of bound molecular states that arise from scattering of the weakly bound Rydberg electron from ground-state atoms. The absence of a p -wave resonance in the low-energy electron-atom scattering in Sr introduces a universal behavior in the Rydberg spectral line shape and in scaling of the spectral width (narrowing) with the Rydberg principal quantum number, n . Spectral features are described with a functional determinant approach (FDA) that solves an extended Fröhlich Hamiltonian for a mobile impurity in a Bose gas. Excited states of polyatomic Rydberg molecules (trimers, tetrameters, and pentamers) are experimentally resolved and accurately reproduced with a FDA.

Polaron in the dilute critical Bose condensate

NASA Astrophysics Data System (ADS)

Pastukhov, Volodymyr

2018-05-01

The properties of an impurity immersed in a dilute D-dimensional Bose gas at temperatures close to its second-order phase transition point are considered. Particularly by means of the 1/N-expansion, we calculate the leading-order polaron energy and the damping rate in the limit of vanishing boson–boson interaction. It is shown that the perturbative effective mass and the quasiparticle residue diverge logarithmically in the long-length limit, signalling the non-analytic behavior of the impurity spectrum and pole-free structure of the polaron Green’s function in the infrared region, respectively.

Identification of effective exciton-exciton annihilation in squaraine-squaraine copolymers.

PubMed

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.

Phase-breaking effect on polaron transport in organic conjugated polymers

DOE PAGES

Meng, Ruixuan; Yin, Sun; Zheng, Yujun; ...

2017-06-15

Despite intense investigations and many accepted viewpoints on theory and experiment, the coherent and incoherent carrier transport in organic semiconductors remains an unsettled topic due to the strong electron-phonon coupling. Based on the tight-binding Su-Schrieffer-Heeger (SSH) model combined with a non-adiabatic dynamics method, we study the effect of phase-breaking on polaron transport by introducing a group of phase-breaking factors into π-electron wave-functions in organic conjugated polymers. Two approaches are applied: the modification of the transfer integral and the phase-breaking addition to the wave-function. Within the former, it is found that a single site phase-breaking can trap a polaron. However, withmore » a larger regular phase-breaking a polaron becomes more delocalized and lighter. Additionally, a group of disordered phase-breaking factors can make the polaron disperse in transport process. Within the latter approach, we show that the phase-breaking can render the delocalized state in valence band discrete and the state in the gap more localized. Consequently, the phase-breaking frequency and intensity can reduce the stability of a polaron. Furthermore, the phase-breaking in organic systems is the main factor that degrades the coherent transport and destroys the carrier stability.« less

Phase-breaking effect on polaron transport in organic conjugated polymers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meng, Ruixuan; Yin, Sun; Zheng, Yujun

Despite intense investigations and many accepted viewpoints on theory and experiment, the coherent and incoherent carrier transport in organic semiconductors remains an unsettled topic due to the strong electron-phonon coupling. Based on the tight-binding Su-Schrieffer-Heeger (SSH) model combined with a non-adiabatic dynamics method, we study the effect of phase-breaking on polaron transport by introducing a group of phase-breaking factors into π-electron wave-functions in organic conjugated polymers. Two approaches are applied: the modification of the transfer integral and the phase-breaking addition to the wave-function. Within the former, it is found that a single site phase-breaking can trap a polaron. However, withmore » a larger regular phase-breaking a polaron becomes more delocalized and lighter. Additionally, a group of disordered phase-breaking factors can make the polaron disperse in transport process. Within the latter approach, we show that the phase-breaking can render the delocalized state in valence band discrete and the state in the gap more localized. Consequently, the phase-breaking frequency and intensity can reduce the stability of a polaron. Furthermore, the phase-breaking in organic systems is the main factor that degrades the coherent transport and destroys the carrier stability.« less

Self-induced pinning of vortices in the presence of ac driving force in magnetic superconductors

NASA Astrophysics Data System (ADS)

Bulaevskii, Lev N.; Lin, Shi-Zeng

2012-12-01

We derive the response of the magnetic superconductors in the vortex state to the ac Lorentz force, FL(t)=Facsin(ωt), taking into account the interaction of vortices with the magnetic moments described by the relaxation dynamics (polaronic effect). At low amplitudes of the driving force Fac the dissipation in the system is suppressed due to the enhancement of the effective viscosity at low frequencies and due to formation of the magnetic pinning at high frequencies ω. In the adiabatic limit with low frequencies ω and high amplitude of the driving force Fac, the vortex and magnetic polarization form a vortex polaron when FL(t) is small. When FL increases, the vortex polaron accelerates and at a threshold driving force, the vortex polaron dissociates and the motion of vortex and the relaxation of magnetization are decoupled. When FL decreases, the vortex is retrapped by the background of remnant magnetization and they again form vortex polaron. This process repeats when FL(t) increases in the opposite direction. Remarkably, after dissociation, decoupled vortices move in the periodic potential induced by magnetization which remains for some periods of time due to retardation after the decoupling. At this stage vortices oscillate with high frequencies determined by the Lorentz force at the moment of dissociation. We derive also the creep rate of vortices and show that magnetic moments suppress creep rate.

Modulation of manganite nano-film properties mediated by strong influence of strontium titanate excitons.

PubMed

Yin, Xinmao; Tang, Chi Sin; Majidi, Muhammad Aziz; Ren, Peng; Wang, Le; Yang, Ping; Diao, Caozheng; Yu, Xiaojiang; Breese, Mark B H; Wee, Andrew Thye Shen; Wang, Junling; Rusydi, Andrivo

2017-12-06

Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic and magnetic properties induced by the interplay between spin, charge, orbital and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic and magnetic properties of La0.7Sr0.3MnO3 thin-films on SrTiO3 (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin-film nanostructure LSMO/STO at ~4eV, which could be attributed to the formation of anomalous charged excitonic complexes. Based on spectral-weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic-correlation in LSMO films. This results in the occurrence of optical spectral changes related to the intrinsic Mott-Hubbard properties in manganites. We find that while lattice strain from the substrate influences the optical properties of the LSMO thin-films, the coexistence of strong electron-electron (e-e) and electron-hole (e-h) interactions which leads to the resonant excitonic effects from the substrate play a much more significant role. Our result shows that the onset of anomalous excitonic dynamics in manganite oxides may potentially generate new approaches in manipulating exciton-based optoelectronic applications.

Valley-Selective Exciton Bistability in a Suspended Monolayer Semiconductor.

PubMed

Xie, Hongchao; Jiang, Shengwei; Shan, Jie; Mak, Kin Fai

2018-05-09

We demonstrate robust optical bistability, the phenomenon of two well-discriminated stable states depending upon the history of the optical input, in fully suspended monolayers of WSe 2 at low temperatures near the exciton resonance. Optical bistability has been achieved under continuous-wave optical excitation that is red-detuned from the exciton resonance at an intensity level of 10 3 W/cm 2 . The observed bistability is originated from a photothermal mechanism, which provides both optical nonlinearity and passive feedback, two essential elements for optical bistability. The low thermal conductance of suspended samples is primarily responsible for the low excitation intensities required for optical bistability. Under a finite out-of-plane magnetic field, the exciton bistability becomes helicity dependent due to the exciton valley Zeeman effect, which enables repeatable switching of the sample reflectance by light polarization. Our study has opened up exciting opportunities in controlling light with light, including its wavelength, power, and polarization, using monolayer semiconductors.

Polaronic transport in Ag-based quaternary chalcogenides

NASA Astrophysics Data System (ADS)

Wei, Kaya; Khabibullin, Artem R.; Stedman, Troy; Woods, Lilia M.; Nolas, George S.

2017-09-01

Low temperature resistivity measurements on dense polycrystalline quaternary chalcogenides Ag2+xZn1-xSnSe4, with x = 0, 0.1, and 0.3, indicate polaronic type transport which we analyze employing a two-component Holstein model based on itinerant and localized polaron contributions. Electronic structure property calculations via density functional theory simulations on Ag2ZnSnSe4 for both energetically similar kesterite and stannite structure types were also performed in order to compare our results to those of the compositionally similar but well known Cu2ZnSnSe4. This theoretical comparison is crucial in understanding the bonding that results in polaronic type transport for Ag2ZnSnSe4, as well as the structural and electronic properties of both crystal structure types. In addition to possessing this unique electronic transport, the thermal conductivity of Ag2ZnSnSe4 is low and decreases with increasing silver content. This work reveals unique structure-property relationships in materials that continue to be of interest for thermoelectric and photovoltaic applications.

Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI 2: a key to high-performance scintillation [First-principles study of hole polaron formation and migration in strontium iodide

DOE PAGES

Zhou, Fei; Sadigh, Babak; Aberg, Daniel; ...

2016-08-12

The excellent light yield proportionality of europium-doped strontium iodide (SrI 2:Eu) has resulted in state-of-the-art γ-ray detectors with remarkably high-energy resolution, far exceeding that of most halide compounds. In this class of materials, the formation of self-trapped hole polarons is very common. However, polaron formation is usually expected to limit carrier mobilities and has been associated with poor scintillator light-yield proportionality and resolution. Here using a recently developed first-principles method, we perform an unprecedented study of polaron transport in SrI 2, both for equilibrium polarons, as well as nascent polarons immediately following a self-trapping event. We propose a rationale formore » the unexpected high-energy resolution of SrI 2. We identify nine stable hole polaron configurations, which consist of dimerised iodine pairs with polaron-binding energies of up to 0.5 eV. They are connected by a complex potential energy landscape that comprises 66 unique nearest-neighbour migration paths. Ab initio molecular dynamics simulations reveal that a large fraction of polarons is born into configurations that migrate practically barrier free at room temperature. Consequently, carriers created during γ-irradiation can quickly diffuse away reducing the chance for nonlinear recombination, the primary culprit for non-proportionality and resolution reduction. We conclude that the flat, albeit complex, landscape for polaron migration in SrI 2 is a key for understanding its outstanding performance. This insight provides important guidance not only for the future development of high-performance scintillators but also of other materials, for which large polaron mobilities are crucial such as batteries and solid-state ionic conductors.« less

Bound exciton and free exciton states in GaSe thin slab.

PubMed

Wei, Chengrong; Chen, Xi; Li, Dian; Su, Huimin; He, Hongtao; Dai, Jun-Feng

2016-09-22

The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extracted as 0.5 meV, dominates PL emission.

Probing excitons in transition metal dichalcogenides by Drude-like exciton intraband absorption.

PubMed

Zhao, Siqi; He, Dawei; He, Jiaqi; Zhang, Xinwu; Yi, Lixin; Wang, Yongsheng; Zhao, Hui

2018-05-24

Understanding excitonic dynamics in two-dimensional semiconducting transition metal dichalcogenides is important for developing their optoelectronic applications. Recently, transient absorption techniques based on resonant excitonic absorption have been used to study various aspects of excitonic dynamics in these materials. The transient absorption in such measurements originates from phase-space state filling, bandgap renormalization, or screening effects. Here we report a new method to probe excitonic dynamics based on exciton intraband absorption. In this Drude-like process, probe photons are absorbed by excitons in their intraband excitation to higher energy states, causing a transient absorption signal. Although the magnitude of the transient absorption is lower than that of the resonant techniques, the new method is less restrictive on the selection of probe wavelength, has a larger linear range, and can provide complementary information on photocarrier dynamics. Using the WS2 monolayer and bulk samples as examples, we show that the new method can probe exciton-exciton annihilation at high densities and reveal exciton formation processes. We also found that the exciton intraband absorption cross section of the WS2 monolayer is on the order of 10-18 cm2.

First-principles supercell calculations of small polarons with proper account for long-range polarization effects

NASA Astrophysics Data System (ADS)

Kokott, Sebastian; Levchenko, Sergey V.; Rinke, Patrick; Scheffler, Matthias

2018-03-01

We present a density functional theory (DFT) based supercell approach for modeling small polarons with proper account for the long-range elastic response of the material. Our analysis of the supercell dependence of the polaron properties (e.g., atomic structure, binding energy, and the polaron level) reveals long-range electrostatic effects and the electron–phonon (el–ph) interaction as the two main contributors. We develop a correction scheme for DFT polaron calculations that significantly reduces the dependence of polaron properties on the DFT exchange-correlation functional and the size of the supercell in the limit of strong el–ph coupling. Using our correction approach, we present accurate all-electron full-potential DFT results for small polarons in rocksalt MgO and rutile TiO2.

Strong-coupling Bose polarons out of equilibrium: Dynamical renormalization-group approach

NASA Astrophysics Data System (ADS)

Grusdt, Fabian; Seetharam, Kushal; Shchadilova, Yulia; Demler, Eugene

2018-03-01

When a mobile impurity interacts with a surrounding bath of bosons, it forms a polaron. Numerous methods have been developed to calculate how the energy and the effective mass of the polaron are renormalized by the medium for equilibrium situations. Here, we address the much less studied nonequilibrium regime and investigate how polarons form dynamically in time. To this end, we develop a time-dependent renormalization-group approach which allows calculations of all dynamical properties of the system and takes into account the effects of quantum fluctuations in the polaron cloud. We apply this method to calculate trajectories of polarons following a sudden quench of the impurity-boson interaction strength, revealing how the polaronic cloud around the impurity forms in time. Such trajectories provide additional information about the polaron's properties which are challenging to extract directly from the spectral function measured experimentally using ultracold atoms. At strong couplings, our calculations predict the appearance of trajectories where the impurity wavers back at intermediate times as a result of quantum fluctuations. Our method is applicable to a broader class of nonequilibrium problems. As a check, we also apply it to calculate the spectral function and find good agreement with experimental results. At very strong couplings, we predict that quantum fluctuations lead to the appearance of a dark continuum with strongly suppressed spectral weight at low energies. While our calculations start from an effective Fröhlich Hamiltonian describing impurities in a three-dimensional Bose-Einstein condensate, we also calculate the effects of additional terms in the Hamiltonian beyond the Fröhlich paradigm. We demonstrate that the main effect of these additional terms on the attractive side of a Feshbach resonance is to renormalize the coupling strength of the effective Fröhlich model.

Valley-Selective Exciton Bistability in a Suspended Monolayer Semiconductor

NASA Astrophysics Data System (ADS)

Xie, Hongchao; Jiang, Shengwei; Shan, Jie; Mak, Kin Fai

2018-05-01

We demonstrate robust power- and wavelength-dependent optical bistability in fully suspended monolayers of WSe2 near the exciton resonance. Bistability has been achieved under continuous-wave optical excitation at an intensity level of 10^3 W/cm^2. The observed bistability is originated from a photo-thermal mechanism, which provides both optical nonlinearity and passive feedback, two essential elements for optical bistability. Under a finite magnetic field, the exciton bistability becomes helicity dependent, which enables repeatable switching of light purely by its polarization.

Detailed relationship between local structure, polarons, and magnetizationfor La1-xCaxMnO3 (0.21≤x≤0.45)

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 (0.21magnetic field. The local distortions of the Mn-O bonds are parameterized using σ , the width of the Mn-O pair-distribution function (PDF). After subtracting thermal phonon contributions, we show that the contributions to σ2 from polaron and Jahn-Teller (JT) distortions, σJT/polaron2 , are a universal function of the magnetization, independent of how the magnetization is achieved via changes in temperature or magnetic field. However this universal behavior is only observed for Bfields≥2T , likely as a result of domain canting in low B fields. The resulting curve is well described by two straight lines with significantly different slopes. These regimes represent two distinctly differ distortions of the oxygen octahedra about the Mn. For low magnetizations up to ˜65% of the theoretical maximum magnetization, MT , the slope is low and the distortion removed as the sample becomes magnetized is small—we argue this arises from polarons which have a low distortion around two (or possibly three) Mn sites. At high magnetizations large distortions per Mn site are removed as these sites become magnetized. The data are also analyzed in terms of a two Mn-O peak distribution using experimental standards for Mn-O. The results agree well with recent neutron PDF results but not with some earlier results. We discuss the limitations of assuming a two peak distribution in view of the two distortions needed to describe the Mn-O distortions as a function of T and B for B≥2T . It is likely that there is a distribution of longer bonds. Finally we show that with increasing B field, the Mn-Mn peak also has a small B -field-induced change—a measure at the unit cell level of magnetostriction but find that there is no observable B -field-induced change in the Mn

Magnetic field and dielectric environment effects on an exciton trapped by an ionized donor in a spherical quantum dot

NASA Astrophysics Data System (ADS)

Aghoutane, N.; Feddi, E.; El-Yadri, M.; Bosch Bailach, J.; Dujardin, F.; Duque, C. A.

2017-11-01

Magnetic field and host dielectric environment effects on the binding energy of an exciton trapped by an ionized donor in spherical quantum dot are investigated. In the framework of the effective mass approximation and by using a variational method, the calculations have been performed by developing a robust ten-terms wave function taking into account the different inter-particles correlations and the distortion of symmetry induced by the orientation of the applied magnetic field. The binding and the localization energies are determined as functions of dot size and magnetic field strength. It appears that the variation of magnetic shift obeys a quadratic law for low magnetic fields regime while, for strong magnetic fields, this shift tends to be linear versus the magnetic field strength. The stability of this complex subjected to a magnetic field is also discussed according to the electron-hole ratio and the dielectric constant of the surrounding medium. A last point to highlight is that the Haynes' rule remains valid even in the presence of an applied magnetic field.

Relative ordering between bright and dark excitons in single-walled carbon nanotubes.

PubMed

Zhou, Weihang; Nakamura, Daisuke; Liu, Huaping; Kataura, Hiromichi; Takeyama, Shojiro

2014-11-11

The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1(st) sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2(nd) sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

Tailoring Quantum Dot Assemblies to Extend Exciton Coherence Times and Improve Exciton Transport

NASA Astrophysics Data System (ADS)

Seward, Kenton; Lin, Zhibin; Lusk, Mark

2012-02-01

The motion of excitons through nanostructured assemblies plays a central role in a wide range of physical phenomena including quantum computing, molecular electronics, photosynthetic processes, excitonic transistors and light emitting diodes. All of these technologies are severely handicapped, though, by quasi-particle lifetimes on the order of a nanosecond. The movement of excitons must therefore be as efficient as possible in order to move excitons meaningful distances. This is problematic for assemblies of small Si quantum dots (QDs), where excitons quickly localize and entangle with dot phonon modes. Ensuing exciton transport is then characterized by a classical random walk reduced to very short distances because of efficient recombination. We use a combination of master equation (Haken-Strobl) formalism and density functional theory to estimate the rate of decoherence in Si QD assemblies and its impact on exciton mobility. Exciton-phonon coupling and Coulomb interactions are calculated as a function of dot size, spacing and termination to minimize the rate of intra-dot phonon entanglement. This extends the time over which more efficient exciton transport, characterized by partial coherence, can be maintained.

Spin-state polarons as a precursor to ferromagnetism and metallicity in hole-doped LaCoO3

NASA Astrophysics Data System (ADS)

Podlesnyak, A.; Russina, M.; Pomjakushina, E.; Conder, K.; Khomskii, D.

2008-03-01

Lightly doped cobaltites La1-xSrxCoO3 exhibit magnetic properties at low temperatures, in strong contrast to the diamagnetic LaCoO3. We undertook an inelastic neutron scattering study with the goal to identify the energy spectrum and magnetic state of cobalt ions in the doped system with x=0.002. In distinguish to the parent compound, where no excitations have been found for T<30 K, an inelastic peak at δE ˜0.75 meV was observed in La0.998Sr0.002CoO3 at T=1.5 K. The intensity of this excitation is much higher than what is expected from an estimated concentration of doped holes. Furthermore, strong Zeeman splitting of the inelastic peak corresponds to an unusually high effective magnetic moment ˜15 μB. Neighboring low-spin (LS) Co^4+ and intermediate-spin Co^3+ ions can share an eg electron by swapping configuration. The t2g electrons, in their turn, couple ferromagnetically. Therefore, we propose that the holes introduced in the LS state of LaCoO3 are extended over the neighboring Co sites forming spin-state polarons and transforming the involved Co^3+ ions to the higher spin state. Grows of spin-state polarons with hole doping finally results in a metallic ferromagnetic state for x > 0.3.

Atomistic model for excitons: Capturing Strongly Bound Excitons in Monolayer Transition-Metal Dichalcogenides

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.

Exciton multiplication from first principles.

PubMed

Jaeger, Heather M; Hyeon-Deuk, Kim; Prezhdo, Oleg V

2013-06-18

Third-generation photovolatics require demanding cost and power conversion efficiency standards, which may be achieved through efficient exciton multiplication. Therefore, generating more than one electron-hole pair from the absorption of a single photon has vast ramifications on solar power conversion technology. Unlike their bulk counterparts, irradiated semiconductor quantum dots exhibit efficient exciton multiplication, due to confinement-enhanced Coulomb interactions and slower nonradiative losses. The exact characterization of the complicated photoexcited processes within quantum-dot photovoltaics is a work in progress. In this Account, we focus on the photophysics of nanocrystals and investigate three constituent processes of exciton multiplication, including photoexcitation, phonon-induced dephasing, and impact ionization. We quantify the role of each process in exciton multiplication through ab initio computation and analysis of many-electron wave functions. The probability of observing a multiple exciton in a photoexcited state is proportional to the magnitude of electron correlation, where correlated electrons can be simultaneously promoted across the band gap. Energies of multiple excitons are determined directly from the excited state wave functions, defining the threshold for multiple exciton generation. This threshold is strongly perturbed in the presence of surface defects, dopants, and ionization. Within a few femtoseconds following photoexcitation, the quantum state loses coherence through interactions with the vibrating atomic lattice. The phase relationship between single excitons and multiple excitons dissipates first, followed by multiple exciton fission. Single excitons are coupled to multiple excitons through Coulomb and electron-phonon interactions, and as a consequence, single excitons convert to multiple excitons and vice versa. Here, exciton multiplication depends on the initial energy and coupling magnitude and competes with electron

Subnanosecond control of excitons in coupled quantum well nanostructures: Photonic storage and Exciton Conveyer devices

NASA Astrophysics Data System (ADS)

Winbow, Alexander Graham

Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons --- a bound electron--hole pair --- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated: Photon storage --- an optical memory --- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization--delocalization transitions for the excitons in the conveyer: In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton

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).

Excitonic processes at organic heterojunctions

NASA Astrophysics Data System (ADS)

He, ShouJie; Lu, ZhengHong

2018-02-01

Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.

Tracking polaron generation in electrochemically doped polyaniline thin films

NASA Astrophysics Data System (ADS)

Kalagi, S. S.; Patil, P. S.

2018-04-01

Electrochemically deposited polyaniline films on ITO substrates have been studied for their optical properties. π-π*transitions inducing the formation of polarons and bipolarons have been studied from the optical spectra. The generation of these quasiparticles and the corresponding quantum of energy stored has been analysed and calculated from the experimental data. The evolution of polaron with increased levels of protonation has been identified and the necessary energy required for the transitions have been explained with the help of band structure diagram.

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.

Percolation Magnetism in Ferroelectric Nanoparticles

NASA Astrophysics Data System (ADS)

Golovina, Iryna S.; Lemishko, Serhii V.; Morozovska, Anna N.

2017-06-01

Nanoparticles of potassium tantalate (KTaO3) and potassium niobate (KNbO3) were synthesized by oxidation of metallic tantalum in molten potassium nitrate with the addition of potassium hydroxide. Magnetization curves obtained on these ferroelectric nanoparticles exhibit a weak ferromagnetism, while these compounds are nonmagnetic in a bulk. The experimental data are used as a start point for theoretical calculations. We consider a microscopic mechanism that leads to the emerging of a ferromagnetic ordering in ferroelectric nanoparticles. Our approach is based on the percolation of magnetic polarons assuming the dominant role of the oxygen vacancies. It describes the formation of surface magnetic polarons, in which an exchange interaction between electrons trapped in oxygen vacancies is mediated by magnetic impurity Fe3+ ions. The dependences of percolation radius on concentration of the oxygen vacancies and magnetic defects are determined in the framework of percolation theory.

Observation of Spin Polarons in a Tunable Fermi Liquid of Ultracold Atoms

NASA Astrophysics Data System (ADS)

Zwierlein, Martin

2009-05-01

We have observed spin polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms via tomographic RF spectroscopy. Feshbach resonances allow to freely tune the interactions between the two spin states involved. A single spin down atom immersed in a Fermi sea of spin up atoms can do one of two things: For strong attraction, it can form a molecule with exactly one spin up partner, but for weaker interaction it will spread its attraction and surround itself with a collection of majority atoms. This spin down atom dressed with a spin up cloud constitutes the spin- or Fermi polaron. We have observed a striking spectroscopic signature of this quasi-particle for various interaction strengths, a narrow peak in the spin down spectrum that emerges above a broad background. The spectra allow us to directly measure the polaron energy and the quasi-particle residue Z. The polarons are found to be only weakly interacting with each other, and can thus be identified with the quasi-particles of Landau's Fermi liquid theory. At a critical interaction strength, we observe a transition from spin one-half polarons to spin zero molecules. At this point the Fermi liquid undergoes a phase transition into a superfluid Bose liquid.

Colloquium: Excitons in atomically thin transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.; Heinz, Tony F.; Marie, Xavier; Amand, Thierry; Urbaszek, Bernhard

2018-04-01

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Exciton characteristics in graphene epoxide.

PubMed

Zhu, Xi; Su, Haibin

2014-02-25

Exciton characteristics in graphene epoxide (GE) are investigated by density functional theory with quasi-particle corrections and many-body interactions. The nature of the exciton is influenced by epoxide content and detailed geometric configurations. Two kinds of excitons are identified in GE: Frenkel-like exciton originated from the sp(2) carbon cluster and charge-transfer exciton formed by localized states involving both oxygen and carbon atoms. The unusual blue shift associated with the Frenkel-like exciton leaking is highlighted. One scaling relationship is proposed to address the power-law dependence of Frenkel-like exciton binding strength on its size. The charge-transfer exciton appears in GE samples with the high oxygen coverage. Particularly, the exciton in GE structures exhibits long lifetime by analyzing both radiative and nonradiative decay processes. This study sheds light on the potential applications of GE-based structures with attractive high quantum yield in light emission and optoelectronic technology.

Mahan excitons in Weyl semimetals

NASA Astrophysics Data System (ADS)

Garate, Ion; Bertrand, Simon; Côté, René

We report on a theoretical study of excitons in weakly doped Weyl semimetals. Solving a two-body Coulomb problem in the presence of a monopole Berry vector potential, we obtain the binding energies of electron-hole pairs and establish their dependence on the monopole charge and on the sign of the magnetic quantum number. We discuss the implications of our results for optical absorption experiments. This research has been supported by Canada's NSERC and Québec's RQMP.

Frenkel-Charge-Transfer exciton intermixing theory for molecular crystals with two isolated Frenkel exciton states.

NASA Astrophysics Data System (ADS)

Bondarev, Igor; Popescu, Adrian

We develop an analytical theory for the intra-intermolecular exciton intermixing in periodic 1D chains of planar organic molecules with two isolated low-lying Frenkel exciton states, typical of copper phthalocyanine (CuPc) and other transition metal phthalocyanine molecules. We formulate the Hamiltonian and use the exact Bogoliubov diagonalization procedure to derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer (CT) exciton state. By comparing our theoretical spectrum with available experimental CuPc absorption data, we obtain the parameters of the Frenkel-CT exciton intermixing in CuPc thin films. The two Frenkel exciton states here are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the CT exciton, showing the coupling constant 0.17 eV in agreement with earlier electron transport experiments. Our results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines. DOE-DE-SC0007117 (I.B.), UNC-GA ROI Grant (A.P.).

Displacement of polarons by vibrational modes in doped conjugated polymers

NASA Astrophysics Data System (ADS)

Anderson, M.; Ramanan, C.; Fontanesi, C.; Frick, A.; Surana, S.; Cheyns, D.; Furno, M.; Keller, T.; Allard, S.; Scherf, U.; Beljonne, D.; D'Avino, G.; von Hauff, E.; Da Como, E.

2017-10-01

Organic pi-conjugated polymers are deemed to be soft materials with strong electron-phonon coupling, which results in the formation of polarons, i.e., charge carriers dressed by self-localized distortion of the nuclei. Universal signatures for polarons are optical resonances below the band gap and intense vibrational modes (IVMs), both found in the infrared (IR) spectral region. Here, we study p -doped conjugated homo- and copolymers by combining first-principles modelling and optical spectroscopy from the far-IR to the visible. Polaronic IVMs are found to feature absorption intensities comparable to purely electronic transitions and, most remarkably, show only loose resemblance to the Raman or IR-active modes of the neutral polymer. The IVM frequency is dramatically scaled down (up to 50%) compared to the backbone carbon-stretching modes in the pristine polymers. The very large intensity of IVMs is associated with displacement of the excess positive charge along the backbone driven by specific vibrational modes. We propose a quantitative picture for the identification of these polaron shifting modes that solely based on structural information, directly correlates with their IR intensity. This finding finally discloses the elusive microscopic mechanism behind the huge IR intensity of IVMs in doped polymeric semiconductors.

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.

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.

Polaronic and dressed molecular states in orbital Feshbach resonances

NASA Astrophysics Data System (ADS)

Xu, Junjun; Qi, Ran

2018-04-01

We consider the impurity problem in an orbital Feshbach resonance (OFR), with a single excited clock state | e ↑⟩ atom immersed in a Fermi sea of electronic ground state | g ↓⟩. We calculate the polaron effective mass and quasi-particle residue, as well as the polaron to molecule transition. By including one particle-hole excitation in the molecular state, we find significant correction to the transition point. This transition point moves toward the BCS side for increasing particle densities, which suggests that the corresponding many-body physics is similar to a narrow resonance.

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

Bipolaronic charge density waves, polaronic spin density waves and high Tc superconductivity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aubry, S.

1992-01-01

At large enough electron phonon coupling, the existence of bipolaronic, polaronic and mixed states is rigorously proven for the adiabatic Holstein model at any dimension and any band filling. The ground-state is one of them which then prove the existence of insulating Bipolaronic Charge Density Waves. The role of the quantum lattice fluctuations is analysed and found to be neglegible in that regime but to become essential in case of phonon softening then favoring the occurence of superconductivity. When a strong Hubbard term is also present, the bipolarons break into polorons and the ground state is expected to be amore » polaronic spin density wave. If the repulsive Hubbard term is comparable to the electron-phonon coupling, the energy for breaking a bipoloron into two polarons can become small and we get instead of these two degenerate structures, a pait of polarons bounded by a spin resonance which we call spin resonant bipolaron''. This resonant bipolaron is still strongly bound, but the role of the quantum lattice fluctuations becomes now very important and yields a sharp broadening of the bandwidth of this resonant bipolarona. Thus, the strong quantum character of these resonant bipolarons could prevent their localization into real space structures which could be insulating bipolaronic CDWs or polaronic SDWS, then favoring the formation of a superconducting coherent state with a possible high {Tc}.« less

Bipolaronic charge density waves, polaronic spin density waves and high {Tc} superconductivity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aubry, S.

1992-09-01

At large enough electron phonon coupling, the existence of bipolaronic, polaronic and mixed states is rigorously proven for the adiabatic Holstein model at any dimension and any band filling. The ground-state is one of them which then prove the existence of insulating Bipolaronic Charge Density Waves. The role of the quantum lattice fluctuations is analysed and found to be neglegible in that regime but to become essential in case of phonon softening then favoring the occurence of superconductivity. When a strong Hubbard term is also present, the bipolarons break into polorons and the ground state is expected to be amore » polaronic spin density wave. If the repulsive Hubbard term is comparable to the electron-phonon coupling, the energy for breaking a bipoloron into two polarons can become small and we get instead of these two degenerate structures, a pait of polarons bounded by a spin resonance which we call ``spin resonant bipolaron``. This resonant bipolaron is still strongly bound, but the role of the quantum lattice fluctuations becomes now very important and yields a sharp broadening of the bandwidth of this resonant bipolarona. Thus, the strong quantum character of these resonant bipolarons could prevent their localization into real space structures which could be insulating bipolaronic CDWs or polaronic SDWS, then favoring the formation of a superconducting coherent state with a possible high {Tc}.« less

Excitons at the center of the Brillouin zone in CuB{sub 2}O{sub 4} magnetoelectric

DOE Office of Scientific and Technical Information (OSTI.GOV)

Menshenin, V. V., E-mail: menshenin@imp.uran.ru

2017-02-15

The possibility of exciton excitation in CuB{sub 2}O{sub 4} magnetoelectric at point Γ(0, 0, 0) of the Brillouin zone has been analyzed using group theory. All possible orientations of the electric and magnetic fields that permit the excitation of these excitons have been determined.

Optical, Electrical and Magnetic Studies of Pi-Conjugated Organic Semiconductor Systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vardeny, Zeev Valentine

Over the duration of this grant our group has studied the transient and cw optical response of various π-conjugated polymers, oligomers, single crystals, fullerene molecules and blends of organic donor-acceptor molecules. We have been also involved in complementary experiments such as magneto-optical studies and spin-physics. We have advanced the field of photophysics of these materials by providing information on their excited state energies and primodal and long-lived photoexcitations such as singlet excitons, triplet excitons, polaron-pairs, excimers and exciplexes. We also fabricated various organic optoelectronic devices such as organic light emitting diodes (OLED), electrochemical cells, organic diodes, organic spin-valves (OSV), andmore » organic photovoltaic (OPV) solar cells. These devices benefited the society in terms of cheap and energy saving illumination, as well as harnessing the solar energy.« less

Magnetic field induced mixing of light hole excitonic states in (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires

NASA Astrophysics Data System (ADS)

Płachta, Jakub; Grodzicka, Emma; Kaleta, Anna; Kret, Sławomir; Baczewski, Lech T.; Pietruczik, Aleksiej; Wiater, Maciej; Goryca, Mateusz; Kazimierczuk, Tomasz; Kossacki, Piotr; Karczewski, Grzegorz; Wojtowicz, Tomasz; Wojnar, Piotr

2018-05-01

A detailed magneto-photoluminescence study of individual (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires grown by molecular beam epitaxy is performed. First of all, an enhancement of the Zeeman splitting due to sp-d exchange interaction between band carriers and Mn-spins is evidenced in these nanostructures. Then, it is found that the value of this splitting depends strongly on the magnetic field direction with respect to the nanowire axis. The largest splitting is observed when the magnetic field is applied perpendicular and the smallest when it is applied parallel to the nanowire axis. This effect is explained in terms of magnetic field induced valence band mixing and evidences the light hole character of the excitonic emission. The values of the light and heavy hole splitting are determined for several individual nanowires based on the comparison of experimental results to theoretical calculations.

Magnetic field induced mixing of light hole excitonic states in (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires.

PubMed

Płachta, Jakub; Grodzicka, Emma; Kaleta, Anna; Kret, Sławomir; Baczewski, Lech T; Pietruczik, Aleksiej; Wiater, Maciej; Goryca, Mateusz; Kazimierczuk, Tomasz; Kossacki, Piotr; Karczewski, Grzegorz; Wojtowicz, Tomasz; Wojnar, Piotr

2018-05-18

A detailed magneto-photoluminescence study of individual (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires grown by molecular beam epitaxy is performed. First of all, an enhancement of the Zeeman splitting due to sp-d exchange interaction between band carriers and Mn-spins is evidenced in these nanostructures. Then, it is found that the value of this splitting depends strongly on the magnetic field direction with respect to the nanowire axis. The largest splitting is observed when the magnetic field is applied perpendicular and the smallest when it is applied parallel to the nanowire axis. This effect is explained in terms of magnetic field induced valence band mixing and evidences the light hole character of the excitonic emission. The values of the light and heavy hole splitting are determined for several individual nanowires based on the comparison of experimental results to theoretical calculations.

Tight-Binding Study of Polarons in Two-Dimensional Systems: Implications for Organic Field-Effect Transistor Materials

NASA Astrophysics Data System (ADS)

Lei, Jie

2011-03-01

In order to understand the electronic and transport properties of organic field-effect transistor (FET) materials, we theoretically studied the polarons in two-dimensional systems using a tight-binding model with the Holstein type and Su--Schrieffer--Heeger type electron--lattice couplings. By numerical calculations, it was found that a carrier accepts four kinds of localization, which are named the point polaron, two-dimensional polaron, one-dimensional polaron, and the extended state. The degree of localization is sensitive to the following parameters in the model: the strength and type of electron--lattice couplings, and the signs and relative magnitudes of transfer integrals. When a parameter set for a single-crystal phase of pentacene is applied within the Holstein model, a considerably delocalized hole polaron is found, consistent with the bandlike transport mechanism.

Colloquium: Excitons in atomically thin transition metal dichalcogenides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weakmore » dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. In this article, recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.« less

Colloquium: Excitons in atomically thin transition metal dichalcogenides

DOE PAGES

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.; ...

2018-04-04

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weakmore » dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. In this article, recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.« less

A new DFT approach to model small polarons in oxides with proper account for long-range polarization

NASA Astrophysics Data System (ADS)

Kokott, Sebastian; Levchenko, Sergey V.; Scheffler, Matthias; Theory Department Team

In this work, we address two important challenges in the DFT description of small polarons (excess charges localized within one unit cell): sensitivity to the errors in exchange-correlation (XC) treatment and finite-size effects in supercell calculations. The polaron properties are obtained using a modified neutral potential-energy surface (PES). Using the hybrid HSE functional and considering the whole range 0 <= α <= 1 , we show that the modified PES model significantly reduces the dependence of the polaron level and binding energy in MgO and TiO2 on the XC functional. It does not eliminate the dependence on supercell size. Based on Pekar's model, we derive the proper long-range behavior of the polaron and a finite-size correction that allows to obtain the polaron properties in the dilute limit (tested for supercells containing up to 1,000 atoms). The developed approach reduces drastically the computational time for exploring the polaron PES, and gives a consistent description of polarons for the whole range of α. It allowed us to find a self-trapped hole in MgO that is noticeably more stable than reported previously. partially supported by UniCat (Deutsche Forschungsgemeinschaft).

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.

Small polaron hopping conduction mechanism in LiFePO4 glass and crystal

NASA Astrophysics Data System (ADS)

Banday, Azeem; Murugavel, Sevi

2017-01-01

The optimization of a cathode material is the most important criterion of lithium ion battery technology, which decides the power density. In order to improve the rate capability, a cathode material must possess high electronic and ionic conductivities. Therefore, it is important to understand the charge transport mechanism in such an advanced cathode material in its intrinsic state before modifying it by various means. In this work, we report the thermal, structural, and electrical conductivity studies on lithium iron phosphate, LiFePO4, both in its polycrystalline (LFPC) and glassy (LFPG) counterpart states. The vibrational spectroscopic measurements reveal the characteristic vibrational modes, which are the intrinsic part of LFPC, whereas in LFPG, the phonon modes become broader and overlap with each other due to the lattice disorder. The electrical conductivity measurements reveal that LFPG exhibits a higher polaronic conductivity of 1.6 orders than the LFPC sample. The temperature dependent dc conductivity has been analyzed with the Mott model of polarons and reveals the origin of enhanced polaronic conductivity in LFPG. Based on the analysis, the enhanced polaronic conductivity in LFPG has been attributed to the combined effect of reduced hopping length, decreased activation energy, and enhanced polaron concentration.

Single photon generation through exciton-exciton annihilation in air-suspended carbon nanotubes

NASA Astrophysics Data System (ADS)

Ishii, Akihiro; Uda, Takushi; Kato, Yuichiro K.

Carbon nanotubes have great potential for single photon sources as they have stable exciton states even at room temperature and their emission wavelengths cover the telecommunication bands. In recent years, single photon emission from carbon nanotubes has been achieved by creating localized states of excitons. In contrast to such an approach, here we utilize mobile excitons and show that single photons can be generated in air-suspended carbon nanotubes, where exciton diffusion length is as long as several hundred nanometers and exciton-exciton annihilation is efficient. We perform photoluminescence microscopy on as-grown air-suspended carbon nanotubes in order to determine their chirality and suspended length. Photon correlation measurements are performed on nanotube emission at room temperature using a Hanbury-Brown-Twiss setup with InGaAs/InP single photon detectors. We observe antibunching with a clear excitation power dependence, where we obtain g (2) (0) value less than 0.5 at low excitation powers, indicating single photon generation. We show such g (2) (0) data with different chiralities and suspended lengths, and the effects of exciton diffusion on single photon generation processes are discussed. Work supported by KAKENHI (26610080, 16H05962), The Canon Foundation, and MEXT (Photon Frontier Network Program, Nanotechnology Platform). A.I. is supported by MERIT and JSPS Research Fellowship, and T.U. is supported by ALPS.

Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS 2 and MoS 2 to 65 Tesla

DOE PAGES

Stier, Andreas V.; McCreary, Kathleen M.; Jonker, Berend T.; ...

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 WS 2 and MoS 2 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 WS 2,more » 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). Lastly, these results highlight the utility of high magnetic fields for understanding new two-dimensional materials.« less

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.

Direct Evidence of Exciton-Exciton Annihilation in Single-Crystalline Organic Metal Halide Nanotube Assemblies.

PubMed

Ma, Ying-Zhong; Lin, Haoran; Du, Mao-Hua; Doughty, Benjamin; Ma, Biwu

2018-05-03

Excitons in low-dimensional organic-inorganic metal halide hybrid structures are commonly thought to undergo rapid self-trapping following creation due to strong quantum confinement and exciton-phonon interaction. Here we report an experimental study probing the dynamics of these self-trapped excitons in the single-crystalline bulk assemblies of 1D organic metal halide nanotubes, (C 6 H 13 N 4 ) 3 Pb 2 Br 7 . Through time-resolved photoluminescence (PL) measurements at different excitation intensities, we observed a marked variation in the PL decay behavior that is manifested by an accelerated decay rate with increasing excitation fluence. Our results offer direct evidence of the occurrence of an exciton-exciton annihilation process, a nonlinear relaxation phenomenon that takes place only when some of the self-trapped excitons become mobile and can approach either each other or those trapped excitons. We further identify a fast and dominant PL decay component with a lifetime of ∼2 ns with a nearly invariant relative area for all acquired PL kinetics, suggesting that this rapid relaxation process is intrinsic.

Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates.

PubMed

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.

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.

Exciton-exciton annihilation in a disordered molecular system by direct and multistep Förster transfer

NASA Astrophysics Data System (ADS)

Fennel, Franziska; Lochbrunner, Stefan

2015-10-01

Exciton annihilation dynamics in a disordered organic model system is investigated by ultrafast absorption spectroscopy. We show that the temporal evolution of the exciton density can be quantitatively understood by applying Förster energy transfer theory to describe the diffusion of the excitons as well as the annihilation step itself. To this end, previous formulations of Förster theory are extended to account for the inhomogeneous distribution of the S0-S1 transition energies resulting in an effective exciton diffusion constant. Two annihilation pathways are considered, the direct transfer of an exciton between two excited molecules and diffusive motion by multiple transfer steps towards a second exciton preceding the annihilation event. One pathway can be emphasized with respect to the other by tuning the exciton diffusion constant via the chromophore concentration. The investigated system allows one to extract all relevant parameters for the description and provides in this way a proof that the annihilation dynamics can be entirely understood and modeled by Förster energy transfer.

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.

Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla

PubMed Central

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.

PubMed

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 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.

Hole polarons and p -type doping in boron nitride polymorphs

NASA Astrophysics Data System (ADS)

Weston, L.; Wickramaratne, D.; Van de Walle, C. G.

2017-09-01

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for p -type doping of hexagonal (h -BN ), wurtzite (w z -BN ), and cubic (c -BN ) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in h -BN compared to w z -BN or c -BN . Despite forming small hole polarons, Be acceptors enable p -type doping, with ionization energies of 0.31 eV for w z -BN and 0.24 eV for c -BN ; these values are comparable to the Mg ionization energy in GaN.

Excitons emissions and Raman scattering of ZnO nanoparticles embedded in BaF2 matrices by reactive magnetron sputtering.

PubMed

Zang, C H; Su, J F; Liu, Y C; Tang, C J; Fang, S J; Zhang, D M; Zhang, Y S

2011-11-01

ZnO nanoparticles embedded in BaF2 matrix were fabricated by rf magnetic sputtering technology. The optical properties of high quality ZnO nanoparticles, thermally post treated in a N2 atmosphere, were investigated by temperature-dependence photoluminescence measurement. Free exciton and localized exciton were observed at the low temperature. Free exciton peak was at 3.374 eV and localized exciton peak was at 3.420 eV, dominating the PL spectrum at 77 K. Free exciton transition was observed at 3.310 eV at room temperature, whereas the localized exciton transition was at 3.378 eV. The multiple-phonon Raman scattering spectrum showed that ZnO nanoparticles embedded in BaF2 matrix had a large deformation energy originated from lattice mismatch between ZnO and BaF2 matrix. Analysis of the fitting results from the temperature dependence of FWHM of ZnO exciton illustrated that the large value of gamma(ph) was good qualitative agreement with the large deformation potential.

Effects of system-bath coupling on a photosynthetic heat engine: A polaron master-equation approach

NASA Astrophysics Data System (ADS)

Qin, M.; Shen, H. Z.; Zhao, X. L.; Yi, X. X.

2017-07-01

Stimulated by suggestions of quantum effects in energy transport in photosynthesis, the fundamental principles responsible for the near-unit efficiency of the conversion of solar to chemical energy became active again in recent years. Under natural conditions, the formation of stable charge-separation states in bacteria and plant reaction centers is strongly affected by the coupling of electronic degrees of freedom to a wide range of vibrational motions. These inspire and motivate us to explore the effects of the environment on the operation of such complexes. In this paper, we apply the polaron master equation, which offers the possibilities to interpolate between weak and strong system-bath coupling, to study how system-bath couplings affect the exciton-transfer processes in the Photosystem II reaction center described by a quantum heat engine (QHE) model over a wide parameter range. The effects of bath correlation and temperature, together with the combined effects of these factors are also discussed in detail. We interpret these results in terms of noise-assisted transport effect and dynamical localization, which correspond to two mechanisms underpinning the transfer process in photosynthetic complexes: One is resonance energy transfer and the other is the dynamical localization effect captured by the polaron master equation. The effects of system-bath coupling and bath correlation are incorporated in the effective system-bath coupling strength determining whether noise-assisted transport effect or dynamical localization dominates the dynamics and temperature modulates the balance of the two mechanisms. Furthermore, these two mechanisms can be attributed to one physical origin: bath-induced fluctuations. The two mechanisms are manifestations of the dual role played by bath-induced fluctuations depending on the range of parameters. The origin and role of coherence are also discussed. It is the constructive interplay between noise and coherent dynamics, rather

Exciton-exciton scattering: Composite boson versus elementary boson

NASA Astrophysics Data System (ADS)

Combescot, M.; Betbeder-Matibet, O.; Combescot, R.

2007-05-01

This paper shows the necessity of introducing a quantum object, the “coboson,” to properly describe, through a fermion scheme, any composite particle, such as the exciton, which is made of two fermions. Although commonly dealt with as elementary bosons, these composite bosons—cobosons in short—differ from them due to their composite nature which makes the handling of their many-body effects quite different from the existing treatments valid for elementary bosons. As a direct consequence of this composite nature, there is no correct way to describe the interaction between cobosons as a potential V . This is rather dramatic because, with the Hamiltonian not written as H=H0+V , all the usual approaches to many-body effects fail. In particular, the standard form of the Fermi golden rule, written in terms of V , cannot be used to obtain the transition rates of two cobosons. To get them, we have had to construct an unconventional expression for this Fermi golden rule in which H only appears. Making use of this expression, we give here a detailed calculation of the time evolution of two excitons. We compare the results of this exact approach with the ones obtained by using an effective bosonic Hamiltonian in which the excitons are considered as elementary bosons with effective scatterings between them, these scatterings resulting from an elaborate mapping between the two-fermion space and the ideal boson space. We show that the relation between the inverse lifetime and the sum of the transition rates for elementary bosons differs from the one of the composite bosons by a factor of 1/2 , so that it is impossible to find effective scatterings between bosonic excitons giving these two physical quantities correctly, whatever the mapping from composite bosons to elementary bosons is. The present paper thus constitutes a strong mathematical proof that, in spite of a widely spread belief, we cannot forget the composite nature of these cobosons, even in the extremely low

Field-induced exciton condensation in LaCoO3

PubMed Central

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

Control of exciton spin statistics through spin polarization in organic optoelectronic devices

PubMed Central

Wang, Jianpu; Chepelianskii, Alexei; Gao, Feng; Greenham, Neil C.

2012-01-01

Spintronics based on organic semiconductor materials is attractive because of its rich fundamental physics and potential for device applications. Manipulating spins is obviously important for spintronics, and is usually achieved by using magnetic electrodes. Here we show a new approach where spin populations can be controlled primarily by energetics rather than kinetics. We find that exciton spin statistics can be substantially controlled by spin-polarizing carriers after injection using high magnetic fields and low temperatures, where the Zeeman energy is comparable with the thermal energy. By using this method, we demonstrate that singlet exciton formation can be suppressed by up to 53% in organic light-emitting diodes, and the dark conductance of organic photovoltaic devices can be increased by up to 45% due to enhanced formation of triplet charge-transfer states, leading to less recombination to the ground state. PMID:23149736

Fractal dimension study of polaron effects in cylindrical GaAs/Al x Ga1- x As core-shell nanowires

NASA Astrophysics Data System (ADS)

Sun, Hui; Li, Hua; Tian, Qiang

2018-04-01

Polaron effects in cylindrical GaAs/Al x Ga1- x As core-shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/Al x Ga1- x As core-shell nanowires with different core radii and aluminum concentrations are discussed. The polaron binding energy, polaron mass shift, and fractal dimension parameter are numerically determined as functions of shell width. The calculation results reveal that the binding energy and mass shift of the polaron first increase and then decrease as the shell width increases. A maximum value appears at a certain shell width for different aluminum concentrations and a given core radius. By using the fractal dimension method, polaron problems in cylindrical GaAs/Al x Ga1- x As core-shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.

Holstein polaron in a valley-degenerate two-dimensional semiconductor.

PubMed

Kang, Mingu; Jung, Sung Won; Shin, Woo Jong; Sohn, Yeongsup; Ryu, Sae Hee; Kim, Timur K; Hoesch, Moritz; Kim, Keun Su

2018-05-28

Two-dimensional (2D) crystals have emerged as a class of materials with tunable carrier density 1 . Carrier doping to 2D semiconductors can be used to modulate many-body interactions 2 and to explore novel composite particles. The Holstein polaron is a small composite particle of an electron that carries a cloud of self-induced lattice deformation (or phonons) 3-5 , which has been proposed to play a key role in high-temperature superconductivity 6 and carrier mobility in devices 7 . Here we report the discovery of Holstein polarons in a surface-doped layered semiconductor, MoS 2 , in which a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently 8-11 . Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons 12-18 . The short-range nature of electron-phonon (e-ph) coupling in MoS 2 can be explained by its valley degeneracy, which enables strong intervalley coupling mediated by acoustic phonons. The coupling strength is found to increase gradually along the superconducting dome up to the intermediate regime, which suggests a bipolaronic pairing in the 2D superconductivity.

Chromophore-Dependent Intramolecular Exciton-Vibrational Coupling in the FMO Complex: Quantification and Importance for Exciton Dynamics.

PubMed

Padula, Daniele; Lee, Myeong H; Claridge, Kirsten; Troisi, Alessandro

2017-11-02

In this paper, we adopt an approach suitable for monitoring the time evolution of the intramolecular contribution to the spectral density of a set of identical chromophores embedded in their respective environments. We apply the proposed method to the Fenna-Matthews-Olson (FMO) complex, with the objective to quantify the differences among site-dependent spectral densities and the impact of such differences on the exciton dynamics of the system. Our approach takes advantage of the vertical gradient approximation to reduce the computational demands of the normal modes analysis. We show that the region of the spectral density that is believed to strongly influence the exciton dynamics changes significantly in the timescale of tens of nanoseconds. We then studied the impact of the intramolecular vibrations on the exciton dynamics by considering a model of FMO in a vibronic basis and neglecting the interaction with the environment to isolate the role of the intramolecular exciton-vibration coupling. In agreement with the assumptions in the literature, we demonstrate that high frequency modes at energy much larger than the excitonic energy splitting have negligible influence on exciton dynamics despite the large exciton-vibration coupling. We also find that the impact of including the site-dependent spectral densities on exciton dynamics is not very significant, indicating that it may be acceptable to apply the same spectral density on all sites. However, care needs to be taken for the description of the exciton-vibrational coupling in the low frequency part of intramolecular modes because exciton dynamics is more susceptible to low frequency modes despite their small Huang-Rhys factors.

Highly Sensitive Determination of the Polaron-Induced Optical Absorption of Organic Charge-Transport Materials

NASA Astrophysics Data System (ADS)

Rabe, T.; Görrn, P.; Lehnhardt, M.; Tilgner, M.; Riedl, T.; Kowalsky, W.

2009-04-01

We examine polaron-induced absorption in organic transport materials using a highly sensitive measurement technique. A hole only device is embedded into a low-loss TE2 waveguide structure, and the current induced change of the waveguide absorption is measured. The exemplary study of 2,2',7,7'-tetrakis(N,N-diphenylamine)-9,9'-spiro-bifluorene (S-TAD) reveals a very low polaron absorption cross section of σp≤2.6×10-18cm2 for 560 nm ≤λ≤660nm. The accuracy of this data is unsurpassed by other techniques used for the unambiguous study of polaronic species in organic thin films.

Existence of bound states of a polaron with a breather in soft potentials

NASA Astrophysics Data System (ADS)

Cuevas, J.; Kevrekidis, P. G.; Frantzeskakis, D. J.; Bishop, A. R.

2006-08-01

We consider polarons in models of coupled electronic and vibrational degrees of freedom, in the presence of a soft nonlinear substrate potential (Morse potential). In particular, we focus on a bound state of a polaron with a breather, a so-called “polarobreather.” We analyze the existence of these states based on frequency resonance conditions and illustrate their stability using Floquet spectrum techniques. Multisite solutions of this type are also obtained both in the stationary case (bond-centered and twisted polarons) and in the breathing case (bond-centered and twisted polarobreathers). For all the branches examined, the dynamical evolution of instabilities pertinent to the corresponding solutions are also briefly discussed. Finally, a different branch of so-called phantom polarobreathers is also demonstrated.

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.

Spatially indirect excitons in coupled quantum wells

DOE Office of Scientific and Technical Information (OSTI.GOV)

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 opportunitiesmore » 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) 2 were

Exciton Seebeck effect in molecular systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Yun-An, E-mail: yunan@nano.gznc.edu.cn; 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. Thismore » phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.« less

Exciton size and quantum transport in nanoplatelets.

PubMed

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.

Size dependent polaronic conduction in hematite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sharma, Monika; Banday, Azeem; Murugavel, Sevi

2016-05-23

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-Fe{sub 2}O{sub 3} has been considered as a potential anode material due to high specific capacity, low cost, high abundance and environmental benignity. We have synthesized α-Fe{sub 2}O{sub 3} 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 50 nm. It has been observedmore » 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-Fe{sub 2}O{sub 3} may be a better alternative anode material for lithium ion batteries than earlier reported systems.« less

Exciton interference revealed by energy dependent exciton transfer rate for ring-structured molecular systems.

PubMed

Yan, Yun-An

2016-01-14

The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a new short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today's nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored.

DNA-mediated excitonic upconversion FRET switching

DOE PAGES

Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; ...

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

First-Principles Modeling of Polaron Formation in TiO2 Polymorphs.

PubMed

Elmaslmane, A R; Watkins, M B; McKenna, K P

2018-06-21

We present a computationally efficient and predictive methodology for modeling the formation and properties of electron and hole polarons in solids. Through a nonempirical and self-consistent optimization of the fraction of Hartree-Fock exchange (α) in a hybrid functional, we ensure the generalized Koopmans' condition is satisfied and self-interaction error is minimized. The approach is applied to model polaron formation in known stable and metastable phases of TiO 2 including anatase, rutile, brookite, TiO 2 (H), TiO 2 (R), and TiO 2 (B). Electron polarons are predicted to form in rutile, TiO 2 (H), and TiO 2 (R) (with trapping energies ranging from -0.02 eV to -0.35 eV). In rutile the electron localizes on a single Ti ion, whereas in TiO 2 (H) and TiO 2 (R) the electron is distributed across two neighboring Ti sites. Hole polarons are predicted to form in anatase, brookite, TiO 2 (H), TiO 2 (R), and TiO 2 (B) (with trapping energies ranging from -0.16 eV to -0.52 eV). In anatase, brookite, and TiO 2 (B) holes localize on a single O ion, whereas in TiO 2 (H) and TiO 2 (R) holes can also be distributed across two O sites. We find that the optimized α has a degree of transferability across the phases, with α = 0.115 describing all phases well. We also note the approach yields accurate band gaps, with anatase, rutile, and brookite within six percent of experimental values. We conclude our study with a comparison of the alignment of polaron charge transition levels across the different phases. Since the approach we describe is only two to three times more expensive than a standard density functional theory calculation, it is ideally suited to model charge trapping at complex defects (such as surfaces and interfaces) in a range of materials relevant for technological applications but previously inaccessible to predictive modeling.

Quantum chaos and breaking of all anti-unitary symmetries in Rydberg excitons.

PubMed

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.

The elusive role of Nb_Li bound polaron energy in hopping charge transport in Fe : LiNbO₃.

PubMed

Guilbert, Laurent; Vittadello, Laura; Bazzan, Marco; Mhaouech, Imed; Messerschmidt, Simon; Imlau, M

2018-02-06

Charge transport due to small polarons hopping among defective (bound polarons) and regular (free polarons) sites is shown to depend in a non-trivial way from the value of the stabilization energy provided by the lattice distortion surrounding the charge carriers. This energy, normally not directly accessible for bound polarons by spectroscopic techniques, is here determined by a combination of experimental and numerical methods for the important case of small electron polarons bound to \\mathrm{Nb}_{\\mathrm{Li}} defects in the prototype ferroelectric oxide lithium niobate. Our findings provide an estimation of the \\mathrm{Nb}_{\\mathrm{Li}} polaron stabilization energy E_{GP}=\\unit[(0.75\\pm0.05)]{eV} and point out that in lithium niobate both free and bound polarons contributes to charge transport already at room temperature, explaining the fast decays of the light-induced bound polaron population observed by transient absorption spectroscopy. © 2018 IOP Publishing Ltd.

Exciton size and quantum transport in nanoplatelets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pelzer, Kenley M., E-mail: kpelzer@anl.gov; Gray, Stephen K.; Darling, Seth B.

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 exploremore » 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.« less

Combined ab initio and density functional study on polaron to bipolaron transitions in oligophenyls and oligothiophenes

NASA Astrophysics Data System (ADS)

Irle, Stephan; Lischka, Hans

1997-08-01

Ab initio self-consistent-field (SCF), two-configuration SCF (TCSCF), and density functional theory (DFT) calculations on the charge-transfer complexes of doubly Li/Cl-doped oligothiophenes and oligo(p-phenyls) and on respective charged systems without counterions have been carried out in order to study polaron to bipolaron transitions. Oligomer chains up to octamers and the ring structures cyclo-dodecathiophene and cyclo-dodeca(p-phenyl) have been investigated. Special attention is paid to the open-shell biradical character of two isolated polaronic defects. It is found that the TCSCF and the spin-unrestricted DFT methods can be successfully applied. A bipolaron structure is obtained when the doping atoms are located on neighboring rings and when there is one undoped ring separating the two doped ones. If there are two or more undoped rings in between a two-polaron configuration (biradical) is found. The bipolaron system is calculated to be more stable than the two-polaron case when counterions are taken into account. The stabilities are reversed if the bare, doubly-charged systems are considered. A theoretical estimate for the barrier height of the polaron to bipolaron transition is given using model reaction coordinates.

Exciton interference revealed by energy dependent exciton transfer rate for ring-structured molecular systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Yun-An, E-mail: yunan@gznc.edu.cn

2016-01-14

The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a newmore » short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today’s nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored.« less

Polaron-to-Polaron Transitions in the Radio-Frequency Spectrum of a Quasi-Two-Dimensional Fermi Gas

NASA Astrophysics Data System (ADS)

Zhang, Y.; Ong, W.; Arakelyan, I.; Thomas, J. E.

2012-06-01

We measure radio-frequency spectra for a two-component mixture of a Li6 atomic Fermi gas in a quasi-two-dimensional regime with the Fermi energy comparable to the energy level spacing in the tightly confining potential. Near the Feshbach resonance, we find that the observed resonances do not correspond to transitions between confinement-induced dimers. The spectral shifts can be fit by assuming transitions between noninteracting polaron states in two dimensions.

Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI2: a key to high-performance scintillation

NASA Astrophysics Data System (ADS)

Zhou, Fei; Sadigh, Babak; Erhart, Paul; Åberg, Daniel

2016-08-01

The excellent light yield proportionality of europium-doped strontium iodide (SrI2:Eu) has resulted in state-of-the-art γ-ray detectors with remarkably high-energy resolution, far exceeding that of most halide compounds. In this class of materials, the formation of self-trapped hole polarons is very common. However, polaron formation is usually expected to limit carrier mobilities and has been associated with poor scintillator light-yield proportionality and resolution. Here using a recently developed first-principles method, we perform an unprecedented study of polaron transport in SrI2, both for equilibrium polarons, as well as nascent polarons immediately following a self-trapping event. We propose a rationale for the unexpected high-energy resolution of SrI2. We identify nine stable hole polaron configurations, which consist of dimerised iodine pairs with polaron-binding energies of up to 0.5 eV. They are connected by a complex potential energy landscape that comprises 66 unique nearest-neighbour migration paths. Ab initio molecular dynamics simulations reveal that a large fraction of polarons is born into configurations that migrate practically barrier free at room temperature. Consequently, carriers created during γ-irradiation can quickly diffuse away reducing the chance for non-linear recombination, the primary culprit for non-proportionality and resolution reduction. We conclude that the flat, albeit complex, landscape for polaron migration in SrI2 is a key for understanding its outstanding performance. This insight provides important guidance not only for the future development of high-performance scintillators but also of other materials, for which large polaron mobilities are crucial such as batteries and solid-state ionic conductors.

Dynamics of exciton transfer in coupled polymer chains.

PubMed

Zhang, Y L; Liu, X J; Sun, Z; An, Z

2013-05-07

The dynamics of singlet and triplet exciton transfer in coupled polymer chains are investigated within the Su-Schrieffer-Heeger+Pariser-Parr-Pople model including both electron-phonon (e-p) coupling and electron-electron (e-e) interactions, using a multi-configurational time-dependent Hartree-Fock dynamic method. In order to explain the processes involved, the effects of on-site and long-range e-e interactions on the locality of the singlet and triplet excitons are first investigated on an isolated chain. It is found that the locality of the singlet exciton decreases, while the locality of the triplet exciton increases with an increase in the on-site e-e interactions. On the other hand, an increase in the long-range e-e interaction results in a more localized singlet exciton and triplet exciton. In coupled polymer chains, we then quantitatively show the yields of singlet and triplet exciton transfer products under the same interchain coupling. It is found that the yield of singlet interchain excitons is much higher than that of triplet interchain excitons, that is to say, singlet exciton transfer is significantly easier than that for triplet excitons. This results from the fact that the singlet exciton is more delocalized than the triplet exciton. In addition, hopping of electrons with opposite spins between the coupled chains can facilitate the transfer of singlet excitons. The results are of great significance for understanding the photoelectric conversion process and developing high-power organic optoelectronic applications.

Bose-Einstein condensation and indirect excitons: a review.

PubMed

Combescot, Monique; Combescot, Roland; Dubin, François

2017-06-01

We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is 'gray'. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath

Exciton dispersion in molecular solids

NASA Astrophysics Data System (ADS)

Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo

2015-03-01

The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.

The Zeeman splitting of bulk 2H-MoTe2 single crystal in high magnetic field

NASA Astrophysics Data System (ADS)

Sun, Yan; Zhang, Junpei; Ma, Zongwei; Chen, Cheng; Han, Junbo; Chen, Fangchu; Luo, Xuan; Sun, Yuping; Sheng, Zhigao

2017-03-01

A high magnetic field magneto-optical spectrum is utilized to study the A exciton of bulk 2H-MoTe2 single crystal. A clear Zeeman splitting of the A exciton is observed under high magnetic fields up to 41.68 T, and the g-factor (-2.09 ± 0.08) is deduced. Moreover, a high magnetic field enables us to obtain the quadratic diamagnetic shifts of the A exciton (0.486 μeV T-2). Accordingly, the binding energy, reduced mass, and radius of the A exciton were obtained by using both two and three dimensional models. Compared with other transition metal dichalcogenides (TMDs), the A exciton of bulk 2H-MoTe2 has a relatively small binding energy and larger exciton radius, which provide fundamental parameters for comprehensive understanding of excitons in TMDs as well as their future applications.

Can Disorder Enhance Incoherent Exciton Diffusion?

PubMed

Lee, Elizabeth M Y; Tisdale, William A; Willard, Adam P

2015-07-30

Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we introduce a general model, based upon Förster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates, which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific hopping rates is broadened in a manner that results in a decrease in average exciton diffusivity relative to that in a perfectly ordered film. Second, since excitons prefer to make transitions that are downhill in energy, the steady state distribution of exciton energies is biased toward low-energy molecular subunits, those that exhibit reduced diffusivity relative to a perfectly ordered film. These effects combine to reduce the net diffusivity in a manner that is time dependent and grows more pronounced as disorder is increased. Notably, however, we demonstrate that the presence of energetic disorder can give rise to a population of molecular subunits with exciton transfer rates exceeding those of subunits in an energetically uniform material. Such enhancements may play an important role in processes that are sensitive to molecular-scale fluctuations in exciton density field.

X-ray Characterization of Oxide-based Magnetic Semiconductors

NASA Astrophysics Data System (ADS)

Idzerda, Yves

2008-05-01

Although the evidence for magnetic semiconductors (not simply semiconductors which are ferromagnetic) is compelling, there is much uncertainty in the mechanism for the polarization of the carriers, suggesting that it must be quite novel. Recent experimental evidence suggests that this mechanism is similar to the polaron percolation theory proposed by Kaminski and Das Sarma,ootnotetextKaminski and S. Das Sarma, Physical Review Letters 88, 247202 (2002). which was recently applied specifically to doped oxides by Coey et al.ootnotetextJ. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald, Nature Materials 4, 173 (2005). where the ferromagnetism is driven by the percolation of polarons generated by defects or dopants. We have used X-ray absorption spectroscopy at the L-edges and K-edges for low concentrations transition metal (TM) doped magnetic oxides (including TiO2, La1-xSrxO3, HfO2, and In2O3). We have found that in most cases, the transition metal assumes a valence consistent with being at a substitutional, and not interstitial site. We have also measured the X-ray Magnetic Circular Dichroism spectra. Although these materials show strong bulk magnetization, we are unable to detect a robust dichroism feature associated with magnetic elements in the host semiconductor. In the cases where a dichroism signal was observed, it was very weak and could be ascribed to a distinct ferromagnetic phase (TM metal cluster, TM oxide particulate, etc.) separate from the host material. This fascinating absence of a dichroic signal and its significant substantiation of important features of the polaron percolation model may help to finally resolve the issue of ferromagnetism in magnetically doped oxides.

Charged excitons in a dilute two-dimensional electron gas in a high magnetic field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wojs, Arkadiusz; Institute of Physics, Wroclaw University of Technology, Wroclaw 50-370,; Quinn, John J.

2000-08-15

A theory of charged excitons X{sup -} in a dilute two-dimensional (2D) electron gas in a high-magnetic field is presented. In contrast to previous calculations, three bound X{sup -} states (one singlet and two triplets) are found in a narrow and symmetric GaAs quantum well. The singlet and a ''bright'' triplet are the two optically active states observed in experiments. The bright triplet has the binding energy of about 1 meV, smaller than the singlet and a ''dark'' triplet. The interaction of bound X{sup -}'s with a dilute 2D electron gas is investigated using exact diagonalization techniques. It is foundmore » that the short-range character of the e-X{sup -} interactions effectively isolates bound X{sup -} states from a dilute e-h plasma. This results in the insensitivity of the photoluminescence spectrum to the filling factor {nu}, and a rapid decrease of the oscillator strength of the dark triplet X{sup -} as a function of {nu}{sup -1}. (c) 2000 The American Physical Society.« less

Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in a same device

DOE PAGES

Hu, Miao; Bi, Cheng; Yuan, Yongbo; ...

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.

Excitonic Effects in Methylammonium Lead Halide Perovskites.

PubMed

Chen, Xihan; Lu, Haipeng; Yang, Ye; Beard, Matthew C

2018-05-17

The exciton binding energy in methylammonium lead iodide (MAPbI 3 ) is about 10 meV, around 1/3 of the available thermal energy ( k B T ∼ 26 meV) at room temperature. Thus, exciton populations are not stable at room temperature at moderate photoexcited carrier densities. However, excitonic resonances dominate the absorption onset. Furthermore, these resonances determine the transient absorbance and transient reflectance spectra. The exciton binding energy is a reflection of the Coulomb interaction energy between photoexcited electrons and holes. As such, it serves as a marker for the strength of electron/hole interactions and impacts a variety of phenomena, such as, absorption, radiative recombination, and Auger recombination. In this Perspective, we discuss the role of excitons and excitonic resonances in the optical properties of lead-halide perovskite semiconductors. Finally, we discuss how the strong light-matter interactions induce an optical stark effect splitting the doubly spin degenerate ground exciton states and are easily observed at room temperature.

Subdiffusive exciton transport in quantum dot solids.

PubMed

Akselrod, Gleb M; Prins, Ferry; Poulikakos, Lisa V; Lee, Elizabeth M Y; Weidman, Mark C; Mork, A Jolene; Willard, Adam P; Bulović, Vladimir; Tisdale, William A

2014-06-11

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

Polaronic effects due to quasi-confined optical phonons in wurtzite nitride nanowire in the presence of an electric field

NASA Astrophysics Data System (ADS)

Vardanyan, Karen A.; Asatryan, Anna L.; Vartanian, Arshak L.

2015-07-01

Considering the effect of an external electric field in wurtzite nitride cylindrical nanowire (NW), the polaron self-energy and effective mass due to the electron interaction with the quasi-confined optical phonons are studied theoretically by means of Lee-Low-Pines variational approach. The analytical expressions for the quasi-one-dimensional Fröhlich polaron self-energy and effective mass are obtained as functions of the wire radius and the strength of the electric field applied perpendicular to the wire axis. It is found that the main contribution to polaron basic parameters is from higher frequency optical phonon modes. The numerical results on the GaN material show that the polaron self-energy increases with the increase of the electric field and is more sensitive to the field when the wire radius is larger. It is also found that the polaron self-energy in GaN NWs is higher than that in zinc-blende GaAs-based cylindrical NWs.

Observation of a two-dimensional liquid of Fröhlich polarons at the bare SrTiO3 surface

PubMed Central

Chen, Chaoyu; Avila, José; Frantzeskakis, Emmanouil; Levy, Anna; Asensio, Maria C.

2015-01-01

The polaron is a quasi-particle formed by a conduction electron (or hole) together with its self-induced polarization in a polar semiconductor or an ionic crystal. Among various polarizable examples of complex oxides, strontium titanate (SrTiO3) is one of the most studied. Here we examine the carrier type and the interplay of inner degrees of freedom (for example, charge, lattice, orbital) in SrTiO3. We report the experimental observation of Fröhlich polarons, or large polarons, at the bare SrTiO3 surface prepared by vacuum annealing. Systematic analyses of angle-resolved photoemission spectroscopy and X-ray absorption spectra show that these Fröhlich polarons are two-dimensional and only exist with inversion symmetry breaking by two-dimensional oxygen vacancies. Our discovery provides a rare solvable field theoretical model, and suggests the relevance of large (bi)polarons for superconductivity in perovskite oxides, as well as in high-temperature superconductors. PMID:26489376

Low temperature magnetic characterization of EuO1-x

NASA Astrophysics Data System (ADS)

Rimal, Gaurab; Tang, Jinke

EuO is a widely studied magnetic semiconductor. It is an ideal case of a Heisenberg ferromagnet as well as a model magnetic polaron system. The interesting aspect of this material is the existance of magnetic polarons in the low temperature region. We study the properties of oxygen deficient EuO prepared by pulsed laser deposition. Besides normal ferromagnetic transitions near 70K and 140K, we observe a different transition at 16K. We also observe a shift in the coercivity for field cooling versus zero field cooling. Possible mechanisms driving these behaviors will be discussed. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (DEFG02-10ER46728) and by the School of Energy Resources of the University of Wyoming.

Effective Mass Theory of 2D Excitons Revisited

NASA Astrophysics Data System (ADS)

Gonzalez, Joseph; Oleynik, Ivan

Two-dimensional (2D) semiconducting materials possess an exceptionally unique set of electronic and excitonic properties due to the combined effects of quantum and dielectric confinement. Reliable determination of exciton binding energies from both first-principles many-body perturbation theory (GW/BSE) and experiment is very challenging due to the enormous computational expense as well as the tremendous technical difficulties in experiment.. Very recently, effective mass theories of 2D excitons have been developed as an attractive alternative for inexpensive and accurate evaluation of the exciton binding energies. In this presentation, we evaluate two effective mass theory approaches by Velizhanin et al and Olsen et al in predicting exciton binding energies across a wide range of 2D materials. We specifically analyze the trends related to the varying screening lengths and exciton effective masses. We also extended the effective mass theory of 2D excitons to include effects of electron and hole mass anisotropies (mx ≠ my) , the latter showing a substantial influence on exciton binding energies. The recent predictions of exciton binding energies being independent of the exciton effective mass and a linear correlation with the band gap of a specific material are also critically reexamined.

Optically dark excitonic states mediated exciton and biexciton valley dynamics in monolayer WSe2

NASA Astrophysics Data System (ADS)

Zhang, Minghua; Fu, Jiyong; Dias, A. C.; Qu, Fanyao

2018-07-01

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe2, under the impact of excitonic dark states of both excitons and biexcitons. We find that the PL intensity of all excitonic channels including intravalley exciton (Xb), intravalley biexciton (XXk,k) and intervalley biexciton (XX) in particular for the XXk,k PL is enhanced by laser excitation fluence. In addition, our results indicate the anomalous temperature dependence of PL, i.e. increasing with temperature, as a result of favored phonon assisted dark-to-bright scatterings at high temperatures. Moreover, we observe that the PL is almost immune to intervalley scatterings, which trigger the exchange of excitonic states between the two valleys. As far as the valley polarization is concerned, we find that the VP of Xb shrinks as temperature increases, exhibiting opposite temperature response to PL, while the intravalley XXk,k VP is found almost independent of temperature. In contrast to both Xb and XXk,k, the intervalley XX VP identically vanishes, because of equal populations of excitons in the K and valleys bounded to form intervalley biexcitons. Notably, it is found that the Xb VP much more strongly depends on bright–dark scattering than that of XXk,k, making dark state act as a robust reservoir for valley polarization against intervalley scatterings for Xb at strong bright–dark scatterings, but not for XXk,k. Dark excitonic states enabled enhancement of VP benefits quantum technology for information processing based on the valley degree of freedom in valleytronic devices. Furthermore, the VP has strong dependence on intervalley scattering but maintains essentially constant with excitation fluence. Finally, the dependence of time evolution of PL and VP on temperature and excitation fluence is discussed.

Polaronic Nonmetal-Correlated Metal Crossover System β'-CuxV2O5 with Anharmonic Copper Oscillation and Thermoelectric Conversion Performance

NASA Astrophysics Data System (ADS)

Onoda, Masashige; Sato, Takuma

2017-12-01

The crystal structures and electronic properties of β'CuxV2O5 are explored through measurements of X-ray four-circle diffraction, electrical resistivity, thermoelectric power, thermal conductivity, magnetization, and electron paramagnetic resonance. For various compositions with 0.243 ≤ x ≤ 0.587, the crystal structures are redetermined through the anharmonic approach of the copper displacement factors, where the anharmonicity is reduced with increasing Cu concentration. The electron transport for x ≤ 0.45 is nonmetallic due to polaron hopping and the random potential of Cu ions, while for x = 0.60, a correlated Fermi-liquid state appears with a Wilson ratio of 1.3 and a Kadowaki-Woods ratio close to the universal value for heavy-fermion systems. At around x = 0.50, the polaronic bandwidth may broaden so that the Hubbard subbands caused by the electron correlation will overlap. The nonmetallic composition in the proximity of the nonmetal-metal crossover shows a dimensionless thermoelectric power factor of 10-2 at 300 K, partly due to the anharmonic copper oscillation.

Exciton dynamics and annihilation in WS2 2D semiconductors.

PubMed

Yuan, Long; Huang, Libai

2015-04-28

We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10(-3) and (1.88 ± 0.47) × 10(-3) cm(2) s(-1), respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons.

Probing exciton density of states through phonon-assisted emission in GaN epilayers: A and B exciton contributions

NASA Astrophysics Data System (ADS)

Cavigli, Lucia; Gabrieli, Riccardo; Gurioli, Massimo; Bogani, Franco; Feltin, Eric; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas; Vinattieri, Anna

2010-09-01

A detailed experimental investigation of the phonon-assisted emission in a high-quality c -plane GaN epilayer is presented up to 200 K. By performing photoluminescence and reflectivity measurements, we find important etaloning effects in the phonon-replica spectra, which have to be corrected before addressing the lineshape analysis. Direct experimental evidence for free exciton thermalization is found for the whole temperature range investigated. A close comparison with existing models for phonon replicas originating from a thermalized free exciton distribution shows that the simplified and commonly adopted description of the exciton-phonon interaction with a single excitonic band leads to a large discrepancy with experimental data. Only the consideration of the complex nature of the excitonic band in GaN, including A and B exciton contributions, allows accounting for the temperature dependence of the peak energy, intensity, and lineshape of the phonon replicas.

Diagrammatic Monte Carlo study of Fröhlich polaron dispersion in two and three dimensions

NASA Astrophysics Data System (ADS)

Hahn, Thomas; Klimin, Sergei; Tempere, Jacques; Devreese, Jozef T.; Franchini, Cesare

2018-04-01

We present results for the solution of the large polaron Fröhlich Hamiltonian in 3 dimensions (3D) and 2 dimensions (2D) obtained via the diagrammatic Monte Carlo (DMC) method. Our implementation is based on the approach by Mishchenko [A. S. Mishchenko et al., Phys. Rev. B 62, 6317 (2000), 10.1103/PhysRevB.62.6317]. Polaron ground state energies and effective polaron masses are successfully benchmarked with data obtained using Feynman's path integral formalism. By comparing 3D and 2D data, we verify the analytically exact scaling relations for energies and effective masses from 3 D →2 D , which provides a stringent test for the quality of DMC predictions. The accuracy of our results is further proven by providing values for the exactly known coefficients in weak- and strong-coupling expansions. Moreover, we compute polaron dispersion curves which are validated with analytically known lower and upper limits in the small-coupling regime and verify the first-order expansion results for larger couplings, thus disproving previous critiques on the apparent incompatibility of DMC with analytical results and furnishing useful reference for a wide range of coupling strengths.

Polaron Thermodynamics of Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases

NASA Astrophysics Data System (ADS)

Ong, Willie; Cheng, Chingyun; Arakelyan, Ilya; Thomas, John

2015-05-01

We present the first spatial profile measurements for spin-imbalanced mixtures of atomic 6Li fermions in a quasi-2D geometry with tunable strong interactions. The observed minority and majority profiles are not correctly predicted by BCS theory for a true 2D system, but are reasonably well fit by a 2D-polaron model of the free energy. Density difference profiles reveal a flat center with two peaks at the edges, consistent with a fully paired core of the corresponding 2D density profiles. These features are more prominent for higher interaction strengths. Not predicted by the polaron model is an observed transition from a spin-imbalanced normal fluid phase to a spin-balanced central core above a critical imbalance. Supported by ARO, DOE, AFOSR, NSF.

Excitons in Single-Walled Carbon Nanotubes and Their Dynamics

NASA Astrophysics Data System (ADS)

Amori, Amanda R.; Hou, Zhentao; Krauss, Todd D.

2018-04-01

Understanding exciton dynamics in single-walled carbon nanotubes (SWCNTs) is essential to unlocking the many potential applications of these materials. This review summarizes recent progress in understanding exciton photophysics and, in particular, exciton dynamics in SWCNTs. We outline the basic physical and electronic properties of SWCNTs, as well as bright and dark transitions within the framework of a strongly bound one-dimensional excitonic model. We discuss the many facets of ultrafast carrier dynamics in SWCNTs, including both single-exciton states (bright and dark) and multiple-exciton states. Photophysical properties that directly relate to excitons and their dynamics, including exciton diffusion lengths, chemical and structural defects, environmental effects, and photoluminescence photon statistics as observed through photon antibunching measurements, are also discussed. Finally, we identify a few key areas for advancing further research in the field of SWCNT excitons and photonics.

Direct observation of anisotropic small-hole polarons in an orthorhombic structure of BiV O4 films

NASA Astrophysics Data System (ADS)

Chaudhuri, A.; Mandal, L.; Chi, X.; Yang, M.; Scott, M. C.; Motapothula, M.; Yu, X. J.; Yang, P.; Shao-Horn, Y.; Venkatesan, T.; Wee, A. T. S.; Rusydi, A.

2018-05-01

Here, we report an anisotropic small-hole polaron in an orthorhombic structure of BiV O4 films grown by pulsed-laser deposition on yttrium-doped zirconium oxide substrate. The polaronic state and electronic structure of BiV O4 films are revealed using a combination of polarization-dependent x-ray absorption spectroscopy at V L3 ,2 edges, spectroscopic ellipsometry, x-ray photoemission spectroscopies, and high-resolution x-ray diffraction with the support of first-principles calculations. We find that in the orthorhombic phase, which is slightly different from the conventional pucherite structure, the unoccupied V 3d orbitals and charge inhomogeneities lead to an anisotropic small-hole polaron state. Our result shows the importance of the interplay of charge and lattice for the formation of a hole polaronic state, which has a significant impact in the electrical conductivity of BiV O4 , hence its potential use as a photoanode for water splitting.

Exciton-Exciton Annihilation as a Probe of Interchain Interactions in PPV-Oligomer Aggregates.

PubMed

Peteanu, Linda A; Chowdhury, Sanchari; Wildeman, Jurjen; Sfeir, Matthew Y

2017-02-23

One measure of exciton mobility in an aggregate is the efficiency of exciton-exciton annihilation (EEA). Both exciton mobilities and EEA are enhanced for aggregate morphologies in which the distances between chromophores and their relative orientations are favorable for Förster energy transfer. Here this principle is applied to gauge the strength of interchain interactions in aggregates of two substituted PPV oligomers of 7 (OPPV7) and 13 (OPPV13) phenylene rings. These are models of the semiconducting conjugated polymer MEH-PPV. The aggregates were formed by adding a poor solvent (methanol or water) to the oligomers dissolved in a good solvent. Aggregates formed from the longer-chain oligomer and/or by addition of the more polar solvent showed the largest contribution of EEA in their emission decay dynamics. This was found to correlate with the degree to which the steady-state emission spectrum of the monomer is altered by aggregation. The wavelength dependence of the EEA signal was also shown to be useful in differentiating emission features due to monomeric and aggregated chains when their spectra overlap significantly.

Possibility of observation of polaron normal modes at the far-infrared spectrum of acetanilide and related organics

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.

Exciton exciton annihilation dynamics in chromophore complexes. II. Intensity dependent transient absorption of the LH2 antenna system.

PubMed

Bruggemann, B; May, V

2004-02-01

Using the multiexciton density matrix theory of excitation energy transfer in chromophore complexes developed in a foregoing paper [J. Chem. Phys. 118, 746 (2003)], the computation of ultrafast transient absorption spectra is presented. Beside static disorder and standard mechanisms of excitation energy dissipation the theory incorporates exciton exciton annihilation (EEA) processes. To elucidate signatures of EEA in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. As main indications for two-exciton population and resulting EEA we found (i) a weakening of the dominant single-exciton bleaching structure in the transient absorption, and (ii) an intermediate suppression of long-wavelength and short-wavelength shoulders around the bleaching structure. The suppression is caused by stimulated emission from the two-exciton to the one-exciton state and the return of the shoulders follows from a depletion of two-exciton population according to EEA. The EEA-signature survives as a short-wavelength shoulder in the transient absorption if orientational and energetic disorder are taken into account. Therefore, the observation of the EEA-signatures should be possible when doing frequency resolved transient absorption experiments with a sufficiently strongly varying pump-pulse intensity. Copyright 2004 American Institute of Physics

Exciton Dynamics in Monolayer Transition Metal Dichalcogenides.

PubMed

Moody, Galan; Schaibley, John; Xu, Xiaodong

2016-07-01

Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population recombination and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond. At present, however, a consensus on the microscopic mechanisms governing exciton radiative and non-radiative recombination is lacking. The strong exciton oscillator strength resulting in up to ~ 20% absorption for a single monolayer points to ultrafast radiative recombination. However, the low quantum yield and large variance in the reported lifetimes suggest that non-radiative Auger-type processes obscure the intrinsic exciton radiative lifetime. In either case, the electron-hole exchange interaction plays an important role in the exciton spin and valley dynamics. In this article, we review the experiments and theory that have led to these conclusions and comment on future experiments that could complement our current understanding.

Exciton Dynamics in Monolayer Transition Metal Dichalcogenides

PubMed Central

Moody, Galan; Schaibley, John; Xu, Xiaodong

2017-01-01

Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population recombination and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond. At present, however, a consensus on the microscopic mechanisms governing exciton radiative and non-radiative recombination is lacking. The strong exciton oscillator strength resulting in up to ~ 20% absorption for a single monolayer points to ultrafast radiative recombination. However, the low quantum yield and large variance in the reported lifetimes suggest that non-radiative Auger-type processes obscure the intrinsic exciton radiative lifetime. In either case, the electron-hole exchange interaction plays an important role in the exciton spin and valley dynamics. In this article, we review the experiments and theory that have led to these conclusions and comment on future experiments that could complement our current understanding. PMID:28890600

Ultra-fast relaxation, decoherence, and localization of photoexcited states in π-conjugated polymers

NASA Astrophysics Data System (ADS)

Mannouch, Jonathan R.; Barford, William; Al-Assam, Sarah

2018-01-01

The exciton relaxation dynamics of photoexcited electronic states in poly(p-phenylenevinylene) are theoretically investigated within a coarse-grained model, in which both the exciton and nuclear degrees of freedom are treated quantum mechanically. The Frenkel-Holstein Hamiltonian is used to describe the strong exciton-phonon coupling present in the system, while external damping of the internal nuclear degrees of freedom is accounted for by a Lindblad master equation. Numerically, the dynamics are computed using the time evolving block decimation and quantum jump trajectory techniques. The values of the model parameters physically relevant to polymer systems naturally lead to a separation of time scales, with the ultra-fast dynamics corresponding to energy transfer from the exciton to the internal phonon modes (i.e., the C-C bond oscillations), while the longer time dynamics correspond to damping of these phonon modes by the external dissipation. Associated with these time scales, we investigate the following processes that are indicative of the system relaxing onto the emissive chromophores of the polymer: (1) Exciton-polaron formation occurs on an ultra-fast time scale, with the associated exciton-phonon correlations present within half a vibrational time period of the C-C bond oscillations. (2) Exciton decoherence is driven by the decay in the vibrational overlaps associated with exciton-polaron formation, occurring on the same time scale. (3) Exciton density localization is driven by the external dissipation, arising from "wavefunction collapse" occurring as a result of the system-environment interactions. Finally, we show how fluorescence anisotropy measurements can be used to investigate the exciton decoherence process during the relaxation dynamics.

Inapplicability of small-polaron model for the explanation of infrared absorption spectrum in acetanilide.

PubMed

Zeković, Slobodan; Ivić, Zoran

2009-01-01

The applicability of small-polaron model for the interpretation of infrared absorption spectrum in acetanilide has been critically reexamined. It is shown that the energy difference between the normal and anomalous peak, calculated by means of small-polaron theory, displays pronounced temperature dependence which is in drastic contradiction with experiment. It is demonstrated that self-trapped states, which are recently suggested to explain theoretically the experimental absorption spectrum in protein, cannot cause the appearance of the peaks in absorption spectrum for acetanilide.

High-density Two-Dimensional Small Polaron Gas in a Delta-Doped Mott Insulator

PubMed Central

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

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

PubMed Central

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

Mapping polaronic states and lithiation gradients in individual V 2O 5 nanowires

DOE PAGES

De Jesus, Luis R.; Horrocks, Gregory A.; Liang, Yufeng; ...

2016-06-28

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 LiCoO 2 , lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO 4 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 V 2 O 5 , lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V 2 O 5 framework couple to localmore » 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.« less

Exciton Level Structure and Dynamics in Tubular Porphyrin Aggregates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wan, Yan; Stradomska, Anna; Fong, Sarah

2014-10-30

We present an account of the optical properties of the Frenkel excitons in self-assembled porphyrin tubular aggregates that represent an analog to natural photosynthetic antennae. Using a combination of ultrafast optical spectroscopy and stochastic exciton modeling, we address both linear and nonlinear exciton absorption, relaxation pathways, and the role of disorder. The static disorder-dominated absorption and fluorescence line widths show little temperature dependence for the lowest excitons (Q band), which we successfully simulate using a model of exciton scattering on acoustic phonons in the host matrix. Temperature-dependent transient absorption of and fluorescence from the excitons in the tubular aggregates aremore » marked by nonexponential decays with time scales ranging from a few picoseconds to a few nanoseconds, reflecting complex relaxation mechanisms. Combined experimental and theoretical investigations indicate that nonradiative pathways induced by traps and defects dominate the relaxation of excitons in the tubular aggregates. We model the pumpprobe spectra and ascribe the excited-state absorption to transitions from one-exciton states to a manifold of mixed one- and two-exciton states. Our results demonstrate that while the delocalized Frenkel excitons (over 208 (1036) molecules for the optically dominant excitons in the Q (B) band) resulting from strong intermolecular coupling in these aggregates could potentially facilitate efficient energy transfer, fast relaxation due to defects and disorder probably present a major limitation for exciton transport over large distances.« less

Influence of molecular designs on polaronic and vibrational transitions in a conjugated push-pull copolymer

NASA Astrophysics Data System (ADS)

Cobet, Christoph; Gasiorowski, Jacek; Menon, Reghu; Hingerl, Kurt; Schlager, Stefanie; White, Matthew S.; Neugebauer, Helmut; Sariciftci, N. Serdar; Stadler, Philipp

2016-10-01

Electron-phonon interactions of free charge-carriers in doped pi-conjugated polymers are conceptually described by 1-dimensional (1D) delocalization. Thereby, polaronic transitions fit the 1D-Froehlich model in quasi-confined chains. However, recent developments in conjugated polymers have diversified the backbones to become elaborate heterocylcic macromolecules. Their complexity makes it difficult to investigate the electron-phonon coupling. In this work we resolve the electron-phonon interactions in the ground and doped state in a complex push-pull polymer. We focus on the polaronic transitions using in-situ spectroscopy to work out the differences between single-unit and push-pull systems to obtain the desired structural- electronic correlations in the doped state. We apply the classic 1D-Froehlich model to generate optical model fits. Interestingly, we find the 1D-approach in push-pull polarons in agreement to the model, pointing at the strong 1D-character and plain electronic structure of the push-pull structure. In contrast, polarons in the single-unit polymer emerge to a multi- dimensional problem difficult to resolve due to their anisotropy. Thus, we report an enhancement of the 1D-character by the push-pull concept in the doped state - an important view in light of the main purpose of push-pull polymers for photovoltaic devices.

Ultrafast dynamics of exciton fission in polycrystalline pentacene.

PubMed

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. © 2011 American Chemical Society

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

NASA Astrophysics Data System (ADS)

Tautz, Raphael; da Como, Enrico; Limmer, Thomas; Feldmann, Jochen; Egelhaaf, Hans-Joachim; von Hauff, Elizabeth; Lemaur, Vincent; Beljonne, David; Yilmaz, Seyfullah; Dumsch, Ines; Allard, Sybille; Scherf, Ullrich

2012-07-01

Polymeric semiconductors are materials where unique optical and electronic properties often originate from a tailored chemical structure. This allows for synthesizing conjugated macromolecules with ad hoc functionalities for organic electronics. In photovoltaics, donor-acceptor co-polymers, with moieties of different electron affinity alternating on the chain, have attracted considerable interest. The low bandgap offers optimal light-harvesting characteristics and has inspired work towards record power conversion efficiencies. Here we show for the first time how the chemical structure of donor and acceptor moieties controls the photogeneration of polaron pairs. We show that co-polymers with strong acceptors show large yields of polaron pair formation up to 24% of the initial photoexcitations as compared with a homopolymer (η=8%). π-conjugated spacers, separating the donor and acceptor centre of masses, have the beneficial role of increasing the recombination time. The results provide useful input into the understanding of polaron pair photogeneration in low-bandgap co-polymers for photovoltaics.

Ultrafast exciton relaxation in monolayer transition metal dichalcogenides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thilagam, A., E-mail: thilaphys@gmail.com

2016-04-28

We examine a mechanism by which excitons undergo ultrafast relaxation in common monolayer transition metal dichalcogenides. It is shown that at densities ≈1 × 10{sup 11 }cm{sup −2} and temperatures ≤60 K, excitons in well known monolayers (MoS{sub 2}, MoSe{sub 2}, WS{sub 2}, and WSe{sub 2}) 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 increasemore » 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.« less

Exciton management in organic photovoltaic multidonor energy cascades.

PubMed

Griffith, Olga L; Forrest, Stephen R

2014-05-14

Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.

Who Named the -On's?

ERIC Educational Resources Information Center

Walker, Charles T.; Slack, Glen A.

1970-01-01

Originators of the concept coiners of the name are discussed for the following particles": boson, electron, exciton, fermion, magnon, neutron, phonon, photon, plasmon, polariton, polaron, proton, and roton. (Author/DS)

Magnetic control of dipolaritons in quantum dots.

PubMed

Rojas-Arias, J S; Rodríguez, B A; Vinck-Posada, H

2016-12-21

Dipolaritons are quasiparticles that arise in coupled quantum wells embedded in a microcavity, they are a superposition of a photon, a direct exciton and an indirect exciton. We propose the existence of dipolaritons in a system of two coupled quantum dots inside a microcavity in direct analogy with the quantum well case and find that, despite some similarities, dipolaritons in quantum dots have different properties and can lead to true dark polariton states. We use a finite system theory to study the effects of the magnetic field on the system, including the emission, and find that it can be used as a control parameter of the properties of excitons and dipolaritons, and the overall magnetic behaviour of the structure.

Multiphonon contribution to the polaron formation in cuprates with strong electron correlations and strong electron-phonon interaction

NASA Astrophysics Data System (ADS)

Ovchinnikov, Sergey G.; Makarov, Ilya A.; Kozlov, Peter A.

2017-03-01

In this work dependences of the electron band structure and spectral function in the HTSC cuprates on magnitude of electron-phonon interaction (EPI) and temperature are investigated. We use three-band p-d model with diagonal and offdiagonal EPI with breathing and buckling phonon mode in the frameworks of polaronic version of the generalized tight binding (GTB) method. The polaronic quasiparticle excitation in the system with EPI within this approach is formed by a hybridization of the local multiphonon Franck-Condon excitations with lower and upper Hubbard bands. Increasing EPI leads to transfer of spectral weight to high-energy multiphonon excitations and broadening of the spectral function. Temperature effects are taken into account by occupation numbers of local excited polaronic states and variations in the magnitude of spin-spin correlation functions. Increasing the temperature results in band structure reconstruction, spectral weight redistribution, broadening of the spectral function peak at the top of the valence band and the decreasing of the peak intensity. The effect of EPI with two phonon modes on the polaron spectral function is discussed.

Exciton coupling between enones: Quassinoids revisited.

PubMed

Pescitelli, Gennaro; Di Bari, Lorenzo

2017-09-01

The electronic circular dichroism (ECD) spectra of two previously reported quassinoids containing a pair of enone chromophores are revisited to gain insight into the consistency and applicability of the exciton chirality method. Our study is based on time-dependent Density Functional Theory calculations, transition and orbital analysis, and numerical exciton coupling calculations. In quassin (1) the enone/enone exciton coupling is quasi-degenerate, yielding strong rotational strengths that account for the observed ECD spectrum in the enone π-π* region. In perforalactone C (2) the nondegenerate coupling produces weak rotational strengths, and the ECD spectrum is dominated by other mechanisms of optical activity. We remark the necessity of a careful application of the nondegenerate exciton coupling method in similar cases. © 2017 Wiley Periodicals, Inc.

Direct Imaging of Frenkel Exciton Transport by Ultrafast Microscopy.

PubMed

Zhu, Tong; Wan, Yan; Huang, Libai

2017-07-18

Long-range transport of Frenkel excitons is crucial for achieving efficient molecular-based solar energy harvesting. Understanding of exciton transport mechanisms is important for designing materials for solar energy applications. One major bottleneck in unraveling of exciton transport mechanisms is the lack of direct measurements to provide information in both spatial and temporal domains, imposed by the combination of fast energy transfer (typically ≤1 ps) and short exciton diffusion lengths (typically ≤100 nm). This challenge requires developing experimental tools to directly characterize excitation energy transport, and thus facilitate the elucidation of mechanisms. To address this challenge, we have employed ultrafast transient absorption microscopy (TAM) as a means to directly image exciton transport with ∼200 fs time resolution and ∼50 nm spatial precision. By mapping population in spatial and temporal domains, such approach has unraveled otherwise obscured information and provided important parameters for testing exciton transport models. In this Account, we discuss the recent progress in imaging Frenkel exciton migration in molecular crystals and aggregates by ultrafast microscopy. First, we establish the validity of the TAM methods by imaging singlet and triplet exciton transport in a series of polyacene single crystals that undergo singlet fission. A new singlet-mediated triplet transport pathway has been revealed by TAM, resulting from the equilibrium between triplet and singlet exciton populations. Such enhancement of triplet exciton transport enables triplet excitons to migrate as singlet excitons and leads to orders of magnitude faster apparent triplet exciton diffusion rate in the picosecond and nanosecond time scales, favorable for solar cell applications. Next we discuss how information obtained by ultrafast microscopy can evaluate coherent effects in exciton transport. We use tubular molecular aggregates that could support large exciton

Spontaneous and superfluid chiral edge states in exciton-polariton condensates

NASA Astrophysics Data System (ADS)

Sigurdsson, H.; Li, G.; Liew, T. C. H.

2017-09-01

We present a scheme of interaction-induced topological band structures based on the spin anisotropy of exciton-polaritons in semiconductor microcavities. We predict theoretically that this scheme allows the engineering of topological gaps, without requiring a magnetic field or strong spin-orbit interaction (transverse electric-transverse magnetic splitting). Under nonresonant pumping we find that an initially topologically trivial system undergoes a topological transition upon the spontaneous breaking of phase symmetry associated with polariton condensation. Under either nonresonant or resonant coherent pumping we find that it is also possible to engineer a topological dispersion that is linear in wave vector—a property associated with polariton superfluidity.

Exciton-vibrational coupling in the dynamics and spectroscopy of Frenkel excitons in molecular aggregates

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

Unexpectedly Fast Phonon-Assisted Exciton Hopping between Carbon Nanotubes

DOE PAGES

Davoody, A. H.; Karimi, F.; Arnold, M. S.; ...

2017-06-05

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. We theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (~1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons withmore » large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. Furthermore, as dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT composites.« less

Unexpectedly Fast Phonon-Assisted Exciton Hopping between Carbon Nanotubes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davoody, A. H.; Karimi, F.; Arnold, M. S.

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. We theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (~1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons withmore » large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. Furthermore, as dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT composites.« less

Excitons in Cuprous Oxide: Photoionization and Other Multiphoton Processes

NASA Astrophysics Data System (ADS)

Frazer, Nicholas Laszlo

In cuprous oxide (Cu2O), momentum from the absorption of two infrared photons to make an orthoexciton is conserved and detected through the photon component of a resulting mixed exciton/photon (quadrupole exciton polariton) state. I demonstrated that this process, which actually makes the photon momentum more precisely defined, is disrupted by photoionization of excitons. Some processes are known to affect exciton propagation in both the pump and exciton stages, such as phonon emission, exciton-exciton (Auger) scattering, and third harmonic generation. These processes alone were not able to explain all observed losses of excitons or all detected scattering products, which lead me to design an optical pump-probe experiment to measure the exciton photoionization cross section, which is (3.9+/-0.2) x 10-22 m2. This dissertation describes the synthesis of cuprous oxide crystals using oxidation of copper, crystallization from melt with the optical floating zone method, and annealing. The cuprous oxide crystals were characterized using time and space resolved luminescence, leading to the discovery of new defect properties. Selection rules and overall efficiency of third harmonic generation in these crystals were characterized. Exciton photoionization was demonstrated through the depletion of polariton luminescence by an optical probe, the production of phonon linked luminescence as a scattering product, temporal delay of the probe, and time resolved luminescence. The results are integrated with the traditional dynamical model of exciton densities. An additional investigation of copper/cuprous oxide/gold photovoltaic devices is appended.

Transition of a small-bipolaron gas to a Fröhlich polaron in a deformable lattice

NASA Astrophysics Data System (ADS)

Hettiarachchi, Gayan Prasad; Muhid, Mohd Nazlan Mohd; Hamdan, Halimaton

2018-04-01

The electronic properties of guest Cs atoms in a deformable lattice are investigated at various densities n . Low values of n show optical absorptions of small bipolarons. At intermediate n values, new bands appear in the midinfrared (MIR) and high-frequency regions, which coexist with the small bipolaron bands. With a further increase in n , the small bipolaron bands become less discernible and subsequently disappear, resulting in the appearance of a Drude component superimposed on a MIR sideband suggesting a phase transition to a polaronic metal. In this itinerant phase, an approximately twofold mass enhancement is observed. This continuous transition of a gas of small bipolarons to a polaronic metal characterized by a Fröhlich polaron reveals an important part of the complex phase diagram of the metal-insulator transition in a deformable lattice.

Excitonic luminescence upconversion in a two-dimensional semiconductor

DOE PAGES

Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; ...

2015-12-21

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. Furthermore, this phenomenon lies at the heart of optical refrigeration in solids(1), where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities(2) or exciton-phonon coupling(3). 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(4). Since the charged exciton binding energy(5) closelymore » matches the 31 meV A(1)' optical phonon(6-9), 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. Additionally, 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.« less

Mapping the exciton diffusion in semiconductor nanocrystal solids.

PubMed

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.

Exciton fission in monolayer transition metal dichalcogenide semiconductors.

PubMed

Steinhoff, A; Florian, M; Rösner, M; Schönhoff, G; Wehling, T O; Jahnke, F

2017-10-27

When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a plasma of unbound fermionic particles or a gas of composite bosons called excitons. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission-fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which present direct solid-state counterparts of high-energy collider experiments on the induced fission of composite particles.

Polaronic Effect on Electrical Conductivity and Thermoelectric Power in Ga(Cu)V4S8

NASA Astrophysics Data System (ADS)

Naik, I.

2018-01-01

Polycrystalline V4-cluster compounds of GaV4S8 and its derivatives Ga0.90 Cu0.10V4S8 and Ga0.90Cu0.20V4S8 have been prepared at 800°C by solid-state reaction method. Although the cubic-rhombohedral phase transformation at 45 K was found to be absent in the derivatives of GaV4S8, low-temperature hopping conduction occurred in all the materials. In the present context, we explain the conduction mechanism for all the materials using polaron theory. The polaron size was found to be large above 260 K but small below 260 K in GaV4S8, as confirmed by the Seebeck coefficient. From the activation energies and polaron size, the anomaly at 260 K is interpreted as associated with crossover from thermally activated to nearest-neighbor hopping upon cooling.

Nonadiabatic small-polaron hopping electron transport in diphenoquinone-doped polycarbonate

NASA Astrophysics Data System (ADS)

Yamaguchi, Yasuhiro; Yokoyama, Masaaki

1991-10-01

The dependences of electron mobility on the electric field F, temperature T, and hopping site distance R have been characterized in 3,5-dimethyl-3',5'-di-tert-butyl-4,4'-diphenoquinone dispersed molecularly in a polycarbonate according to Schein's analytical technique. The electron mobility can be described in the form a0R2 exp(-2R/R0) exp(-E0/kT) × exp[β(1/kT-1/kT0)F1/2], where a0, R0, β, and T0 are constants. Moreover, it is found that the zero-field activation energy E0 is independent of R. The invariable E0 and the exponential dependence of the Arrhenius prefactor on R strongly suggest that the electron transport therein is due to nonadiabatic small-polaron hopping. Based on the small-polaron theory, the transport properties are qualitatively discussed in terms of molecular properties.

Hartree-Fock treatment of Fermi polarons using the Lee-Low-Pine transformation

NASA Astrophysics Data System (ADS)

Kain, Ben; Ling, Hong Y.

2017-09-01

We consider the Fermi polaron problem at zero temperature, where a single impurity interacts with noninteracting host fermions. We approach the problem starting with a Fröhlich-like Hamiltonian where the impurity is described with canonical position and momentum operators. We apply the Lee-Low-Pine (LLP) transformation to change the fermionic Fröhlich Hamiltonian into the fermionic LLP Hamiltonian, which describes a many-body system containing host fermions only. We adapt the self-consistent Hartree-Fock (HF) approach, first proposed by Edwards, to the fermionic LLP Hamiltonian in which a pair of host fermions with momenta k and k' interact with a potential proportional to k .k' . We apply the HF theory, which has the advantage of not restricting the number of particle-hole pairs, to repulsive Fermi polarons in one dimension. When the impurity and host fermion masses are equal our variational ansatz, where HF orbitals are expanded in terms of free-particle states, produces results in excellent agreement with McGuire's exact analytical results based on the Bethe ansatz. This work raises the prospect of using the HF ansatz and its time-dependent generalization as building blocks for developing all-coupling theories for both equilibrium and nonequilibrium Fermi polarons in higher dimensions.

Influence of quasi-particle density over polaron mobility in armchair graphene nanoribbons.

PubMed

Silva, Gesiel Gomes; da Cunha, Wiliam Ferreira; de Sousa Junior, Rafael Timóteo; Almeida Fonseca, Antonio Luciano; Ribeiro Júnior, Luiz Antônio; E Silva, Geraldo Magela

2018-06-20

An important aspect concerning the performance of armchair graphene nanoribbons (AGNRs) as materials for conceiving electronic devices is related to the mobility of charge carriers in these systems. When several polarons are considered in the system, a quasi-particle wave function can be affected by that of its neighbor provided the two are close enough. As the overlap may affect the transport of the carrier, the question concerning how the density of polarons affect its mobility arises. In this work, we investigate such dependence for semiconducting AGNRs in the scope of nonadiabatic molecular dynamics. Our results unambiguously show an impact of the density on both the stability and average velocity of the quasi-particles. We have found a phase transition between regimes where increasing density stops inhibiting and starts promoting mobility; densities higher than 7 polarons per 45 Å present increasing mean velocity with increasing density. We have also established three different regions relating electric field and average velocity. For the lowest electric field regime, surpassing the aforementioned threshold results in overcoming the 0.3 Å fs-1 limit, thus representing a transition between subsonic and supersonic regimes. For the highest of the electric fields, density effects alone are responsible for a stunning difference of 1.5 Å fs-1 in the mean carrier velocity.

Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kovalev, V. M.; Chaplik, A. V., E-mail: chaplik@isp.nsc.ru

2016-03-15

A theory for the linear and quadratic responses of a 2D gas of indirect dipolar excitons to an external surface acoustic wave perturbation in the presence of a static random potential is considered. The theory is constructed both for high temperatures, definitely greater than the exciton gas condensation temperature, and at zero temperature by taking into account the Bose–Einstein condensation effects. The particle Green functions, the density–density correlation function, and the quadratic response function are calculated by the “cross” diagram technique. The results obtained are used to calculate the absorption of Rayleigh surface waves and the acoustic exciton gas dragmore » by a Rayleigh wave. The damping of Bogoliubov excitations in an exciton condensate due to theirs scattering by a random potential has also been determined.« less

Ultrafast exciton fine structure relaxation dynamics in lead chalcogenide nanocrystals.

PubMed

Johnson, Justin C; Gerth, Kathrine A; Song, Qing; Murphy, James E; Nozik, Arthur J; Scholes, Gregory D

2008-05-01

The rates of fine structure relaxation in PbS, PbSe, and PbTe nanocrystals were measured on a femtosecond time scale as a function of temperature with no applied magnetic field by cross-polarized transient grating spectroscopy (CPTG) and circularly polarized pump-probe spectroscopy. The relaxation rates among exciton fine structure states follow trends with nanocrystal composition and size that are consistent with the expected influence of material dependent spin-orbit coupling, confinement enhanced electron-hole exchange interaction, and splitting between L valleys that are degenerate in the bulk. The size dependence of the fine structure relaxation rate is considerably different from what is observed for small CdSe nanocrystals, which appears to result from the unique material properties of the highly confined lead chalcogenide quantum dots. Modeling and qualitative considerations lead to conclusions about the fine structure of the lowest exciton absorption band, which has a potentially significant bearing on photophysical processes that make these materials attractive for practical purposes.

Observation of Exciton-Exciton Interaction Mediated Valley Depolarization in Monolayer MoSe2.

PubMed

Mahmood, Fahad; Alpichshev, Zhanybek; Lee, Yi-Hsien; Kong, Jing; Gedik, Nuh

2018-01-10

The valley pseudospin in monolayer transition metal dichalcogenides (TMDs) has been proposed as a new way to manipulate information in various optoelectronic devices. This relies on a large valley polarization that remains stable over long time scales (hundreds of nanoseconds). However, time-resolved measurements report valley lifetimes of only a few picoseconds. This has been attributed to mechanisms such as phonon-mediated intervalley scattering and a precession of the valley pseudospin through electron-hole exchange. Here we use transient spin grating to directly measure the valley depolarization lifetime in monolayer MoSe 2 . We find a fast valley decay rate that scales linearly with the excitation density at different temperatures. This establishes the presence of strong exciton-exciton Coulomb exchange interactions enhancing the valley depolarization. Our work highlights the microscopic processes inhibiting the efficient use of the exciton valley pseudospin in monolayer TMDs.

Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials

DOE PAGES

Neukirch, Amanda J.; Nie, Wanyi; Blancon, Jean-Christophe; ...

2016-05-25

Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. In spite of 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. Wemore » 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 CsPbI 3 cluster creates a polaron with binding energy of around 300 and 900 meV for holes and electrons, respectively. In the MAPbI 3 (MA = CH 3NH 3) 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. Furthermore, 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(NH 2) 2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications.« less

Charge and energy transports via poly-phenylacetylene based dendrimers

NASA Astrophysics Data System (ADS)

Shin, Yongwoo; Li, Minghai; Lin, Xi

2010-03-01

Poly-Phenylacetylene (PPA) is widely used in photoconductivity, photoluminescence, and light harvesting applications. In this work, we investigate the charge and exciton transport energetics and mechanisms in the PPA-based dendrimers using our recently developed adapted Su-Schrieffer-Heeger (SSH) model Hamiltonians and ab initio Hartree-Fock (HF) calculations. We found both doping and photo-excitation lead to the formation of optical phonon dressed pi electron states, namely the self-localized polarons, in the energy gap. Independent from their origins, these polarons can be self-trapped at multiple lattice locations along the PPA chain, and migrate from one to the next with an activation barrier of ˜0.006 eV, slightly higher than the corresponding barrier found in trans-polyacetylene. The PPA-based dendrimers can be constructed via the meta-positions of phenyl rings. In this case, we found the dendrimer junctions form attractive potential wells for both polarons and excitons, and the width and height of these junction potential wells can be controlled by the geometry of the dendrimers.

Exciton transport in π-conjugated polymers with conjugation defects.

PubMed

Meng, Ruixuan; Li, Yuan; Li, Chong; Gao, Kun; Yin, Sun; Wang, Luxia

2017-09-20

In π-conjugated polymers for photovoltaic applications, intrinsic conjugation defects are known to play crucial roles in impacting exciton transport after photoexcitation. However, the understanding of the associated microscopic processes still remains limited. Here, we present a theoretical investigation of the effects of different conjugation defects on the dynamics of exciton transport in two linearly coupled poly(p-phenylene vinylene) (PPV) molecules. The model system is constructed by employing an extended version of the Su-Schrieffer-Heeger model and the exciton behaviors are simulated by means of a quantum nonadiabatic dynamics. We identify two types of conjugation defects, i.e., weakening conjugation and strengthening conjugation, which are demonstrated to play different roles in impacting the dynamics of exciton transport in the system. The weakening conjugation acts as an energy well inclined to trap a moving exciton, while the strengthening conjugation acts as an energy barrier inclined to block the exciton. We also systematically simulate both intrachain and interchain dynamics of exciton transport, and find that an exciton could experience a "short-time delaying", "trapping", "blocking", or "hopping" process, which is determined by the defect type, strength, and position. These findings provide a microscopic understanding of how the exciton transport dynamics can be impacted by conjugation defects in an actual polymer system.

Directing energy transport in organic photovoltaic cells using interfacial exciton gates.

PubMed

Menke, S Matthew; Mullenbach, Tyler K; Holmes, Russell J

2015-04-28

Exciton transport in organic semiconductors is a critical, mediating process in many optoelectronic devices. Often, the diffusive and subdiffusive nature of excitons in these systems can limit device performance, motivating the development of strategies to direct exciton transport. In this work, directed exciton transport is achieved with the incorporation of exciton permeable interfaces. These interfaces introduce a symmetry-breaking imbalance in exciton energy transfer, leading to directed motion. Despite their obvious utility for enhanced exciton harvesting in organic photovoltaic cells (OPVs), the emergent properties of these interfaces are as yet uncharacterized. Here, directed exciton transport is conclusively demonstrated in both dilute donor and energy-cascade OPVs where judicious optimization of the interface allows exciton transport to the donor-acceptor heterojunction to occur considerably faster than when relying on simple diffusion. Generalized systems incorporating multiple exciton permeable interfaces are also explored, demonstrating the ability to further harness this phenomenon and expeditiously direct exciton motion, overcoming the diffusive limit.

Topological Exciton Bands in Moire Heterojunctions.

DOE PAGES

Wu, Fengcheng; Lovorn, Timothy; MacDonald, A. H.

2017-04-05

Moire patterns are common in Van der Waals heterostructures and can be used to apply periodic potentials to elementary excitations. Here, we show that the optical absorption spectrum of transition metal dichalcogenide bilayers is profoundly altered by long period moire patterns that introduce twist-angle dependent satellite excitonic peaks. Topological exciton bands with non-zero Chern numbers that support chiral excitonic edge states can be engineered by combining three ingredients: i) the valley Berry phase induced by electron-hole exchange interactions, ii) the moire potential, and iii) the valley Zeeman field.

Exciton Dynamics, Transport, and Annihilation in Atomically Thin Two-Dimensional Semiconductors.

PubMed

Yuan, Long; Wang, Ti; Zhu, Tong; Zhou, Mingwei; Huang, Libai

2017-07-20

Large binding energy and unique exciton fine structure make the transition metal dichalcogenides (TMDCs) an ideal platform to study exciton behaviors in two-dimensional (2D) systems. While excitons in these systems have been extensively researched, there currently lacks a consensus on mechanisms that control dynamics. In this Perspective, we discuss extrinsic and intrinsic factors in exciton dynamics, transport, and annihilation in 2D TMDCs. Intrinsically, dark and bright exciton energy splitting is likely to play a key role in modulating the dynamics. Extrinsically, defect scattering is prevalent in single-layer TMDCs, which leads to rapid picosecond decay and limits exciton transport. The exciton-exciton annihilation process in single-layer TMDCs is highly efficient, playing an important role in the nonradiative recombination rate in the high exciton density regime. Future challenges and opportunities to control exciton dynamics are discussed.

Anisotropic polaron localization and spontaneous symmetry breaking: Comparison of cation-site acceptors in GaN and ZnO

NASA Astrophysics Data System (ADS)

Sun, Y. Y.; Abtew, Tesfaye A.; Zhang, Peihong; Zhang, S. B.

2014-10-01

The behavior of cation substitutional hole doping in GaN and ZnO is investigated using hybrid density functional calculations. Our results reveal that Mg substitution for Ga (MgGa) in GaN can assume three different configurations. Two of the configurations are characterized by the formation of defect-bound small polaron (i.e., a large structural distortion accompanied by hole localization on one of the neighboring N atoms). The third one has a relatively small but significant distortion that is characterized by highly anisotropic polaron localization. In this third configuration, MgGa exhibits both effective-mass-like and noneffective-mass-like characters. In contrast, a similar defect in ZnO, LiZn, cannot sustain the anisotropic polaron in the hybrid functional calculation, but undergoes spontaneous breaking of a mirror symmetry through a mechanism driven by the hole localization. Finally, using NaZn in ZnO as an example, we show that the deep acceptor levels of the small-polaron defects could be made shallower by applying compressive strain to the material.

Frenkel versus charge-transfer exciton dispersion in molecular crystals

NASA Astrophysics Data System (ADS)

Cudazzo, Pierluigi; Gatti, Matteo; Rubio, Angel; Sottile, Francesco

2013-11-01

By solving the many-body Bethe-Salpeter equation at finite momentum transfer, we characterize the exciton dispersion in two prototypical molecular crystals, picene and pentacene, in which localized Frenkel excitons compete with delocalized charge-transfer excitons. We explain the exciton dispersion on the basis of the interplay between electron and hole hopping and electron-hole exchange interaction, unraveling a simple microscopic description to distinguish Frenkel and charge-transfer excitons. This analysis is general and can be applied to other systems in which the electron wave functions are strongly localized, as in strongly correlated insulators.

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.

Multipole ordering and collective excitations in the excitonic phase of Pr0.5Ca0.5CoO3

NASA Astrophysics Data System (ADS)

Yamaguchi, Tomoki; Sugimoto, Koudai; Ohta, Yukinori

2018-05-01

As an extension of our previous paper (Yamaguchi et al., 2017) [24], we study the carrier doping dependence of the excitonic condensation in Pr0.5Ca0.5CoO3 using the random-phase and mean-field approximations for the realistic five-orbital Hubbard model. We show that the spin-triplet excitonic phase with a magnetic multipole ordering is stable against the doping of carriers in a considerable range around Co3+ (or 3d6). We discuss experimental relevance of our results.

Optically dark excitonic states mediated exciton and biexciton valley dynamics in monolayer WSe₂.

PubMed

Zhang, Minghua; Fu, Jiyong; Dias, A C; Qu, Fanyao

2018-05-18

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe$_2$, under the impact of excitonic dark states of both excitons and biexcitons. We find that the PL intensity of all excitonic channels including intravalley exciton (X$_{\\rm b}$), intravalley biexciton (XX$_{\\rm k,k}$) and intervalley biexciton (XX$_{\\rm k,k^\\prime}$) in particular for the {XX$_{\\rm k,k}$} PL is enhanced by laser excitation fluence. In addition, our results indicate the anomalous temperature dependence of PL, i.e., increasing with temperature, as a result of favored phonon assisted dark-to-bright scatterings at high temperatures. Moreover, we observe that the PL is almost immune to intervalley scatterings, which trigger the exchange of excitonic states between the two valleys. As far as the valley polarization is concerned, we find that the VP of X$_{\\rm b}$ shrinks as temperature increases, exhibiting opposite temperature response to PL, while the intravalley XX$_{\\rm k,k}$ VP is found almost independent of temperature. In contrast to both X$_{\\rm b}$ and XX$_{\\rm k,k}$, the intervalley XX$_{\\rm k,k^\\prime}$ VP identically vanishes, because of equal populations of excitons in the $K$ and $K^\\prime$ valleys bounded to form intervalley biexcitons. Notably, it is found that the X$_{\\rm b}$ VP much more strongly depends on bright-dark scattering than that of {XX$_{\\rm k,k}$}, making dark state act as a robust reservoir for valley polarization against intervalley scatterings for X$_{\\rm b}$ at strong bright-dark scatterings, but not for XX$_{\\rm k,k}$. Dark excitonic states enabled enhancement of VP benefits quantum technology for information processing based on the valley degree of freedom in valleytronic devices. Furthermore, the VP has strong dependence on intervalley scattering but maintains essentially constant with excitation fluence. Finally, the time evolution of PL and VP, depending on temperature and

Exciton Binding Energy of Monolayer WS2

PubMed Central

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

Bright triplet excitons in caesium lead halide perovskites

NASA Astrophysics Data System (ADS)

Becker, Michael A.; Vaxenburg, Roman; Nedelcu, Georgian; Sercel, Peter C.; Shabaev, Andrew; Mehl, Michael J.; Michopoulos, John G.; Lambrakos, Samuel G.; Bernstein, Noam; Lyons, John L.; Stöferle, Thilo; Mahrt, Rainer F.; Kovalenko, Maksym V.; Norris, David J.; Rainò, Gabriele; Efros, Alexander L.

2018-01-01

Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund’s rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the ‘dark exciton’. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Multiple exciton generation and recombination in carbon nanotubes and nanocrystals.

PubMed

Kanemitsu, Yoshihiko

2013-06-18

Semiconducting nanomaterials such as single-walled carbon nanotubes (SWCNTs) and nanocrystals (NCs) exhibit unique size-dependent quantum properties. They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications. SWCNTs and NCs also provide an excellent new stage for experimental studies of many-body effects of electrons and excitons on optical processes in nanomaterials. In this Account, we discuss multiple exciton generation and recombination in SWCNTs and NCs for next-generation photovoltaics. Strongly correlated ensembles of conduction-band electrons and valence-band holes in semiconductors are complex quantum systems that exhibit unique optical phenomena. In bulk crystals, the carrier recombination dynamics can be described by a simple model, which includes the nonradiative single-carrier trapping rate, the radiative two-carrier recombination rate, and the nonradiative three-carrier Auger recombination rate. The nonradiative Auger recombination rate determines the carrier recombination dynamics at high carrier density and depends on the spatial localization of carriers in two-dimensional quantum wells. The Auger recombination and multiple exciton generation rates can be advantageously manipulated by nanomaterials with designated energy structures. In addition, SWCNTs and NCs show quantized recombination dynamics of multiple excitons and carriers. In one-dimensional SWCNTs, excitons have large binding energies and are very stable at room temperature. The extremely rapid Auger recombination between excitons determines the photoluminescence (PL) intensity, the PL linewidth, and the PL lifetime. SWCNTs can undergo multiple exciton generation, while strong exciton-exciton interactions and complicated exciton structures affect the quantized Auger rate and the multiple exciton generation efficiency. Interestingly, in zero-dimensional NC quantum dots, quantized Auger recombination causes unique

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.

Pentacene Excitons in Strong Electric Fields.

PubMed

Kuhnke, Klaus; Turkowski, Volodymyr; Kabakchiev, Alexander; Lutz, Theresa; Rahman, Talat S; Kern, Klaus

2018-02-05

Electroluminescence spectroscopy of organic semiconductors in the junction of a scanning tunneling microscope (STM) provides access to the polarizability of neutral excited states in a well-characterized molecular geometry. We study the Stark shift of the self-trapped lowest singlet exciton at 1.6 eV in a pentacene nanocrystal. Combination of density functional theory (DFT) and time-dependent DFT (TDDFT) with experiment allows for assignment of the observation to a charge-transfer (CT) exciton. Its charge separation is perpendicular to the applied field, as the measured polarizability is moderate and the electric field in the STM junction is strong enough to dissociate a CT exciton polarized parallel to the applied field. The calculated electric-field-induced anisotropy of the exciton potential energy surface will also be of relevance to photovoltaic applications. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

Analytic derivative couplings and first-principles exciton/phonon coupling constants for an ab initio Frenkel-Davydov exciton model: Theory, implementation, and application to compute triplet exciton mobility parameters for crystalline tetracene.

PubMed

Morrison, Adrian F; Herbert, John M

2017-06-14

Recently, we introduced an ab initio version of the Frenkel-Davydov exciton model for computing excited-state properties of molecular crystals and aggregates. Within this model, supersystem excited states are approximated as linear combinations of excitations localized on molecular sites, and the electronic Hamiltonian is constructed and diagonalized in a direct-product basis of non-orthogonal configuration state functions computed for isolated fragments. Here, we derive and implement analytic derivative couplings for this model, including nuclear derivatives of the natural transition orbital and symmetric orthogonalization transformations that are part of the approximation. Nuclear derivatives of the exciton Hamiltonian's matrix elements, required in order to compute the nonadiabatic couplings, are equivalent to the "Holstein" and "Peierls" exciton/phonon couplings that are widely discussed in the context of model Hamiltonians for energy and charge transport in organic photovoltaics. As an example, we compute the couplings that modulate triplet exciton transport in crystalline tetracene, which is relevant in the context of carrier diffusion following singlet exciton fission.

Charge-transfer excitons at organic semiconductor surfaces and interfaces.

PubMed

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

InAs Band-Edge Exciton Fine Structure

DTIC Science & Technology

2015-07-29

Chapter 1 InAs Band-Edge Exciton Fine Structure 1.1 Contributions This work was carried out in collaboration with Oscar Sandoval, a summer student at...diffusion,1,2 charg- ing,2,3 and excitonic fine structure.1,3–9 While spectral diffusion and charging are most likely photoinduced effects and thus can be...unavoidable. A complete understanding of the excitonic 1 Distribution A: Public Release energy landscape enables us to determine dephasing rates

Exciton binding energy in a pyramidal quantum dot

NASA Astrophysics Data System (ADS)

Anitha, A.; Arulmozhi, M.

2018-05-01

The effects of spatially dependent effective mass, non-parabolicity of the conduction band and dielectric screening function on exciton binding energy in a pyramid-shaped quantum dot of GaAs have been investigated by variational method as a function of base width of the pyramid. We have assumed that the pyramid has a square base with area a× a and height of the pyramid H=a/2. The trial wave function of the exciton has been chosen according to the even mirror boundary condition, i.e. the wave function of the exciton at the boundary could be non-zero. The results show that (i) the non-parabolicity of the conduction band affects the light hole (lh) and heavy hole (hh) excitons to be more bound than that with parabolicity of the conduction band, (ii) the dielectric screening function (DSF) affects the lh and hh excitons to be more bound than that without the DSF and (iii) the spatially dependent effective mass (SDEM) affects the lh and hh excitons to be less bound than that without the SDEM. The combined effects of DSF and SDEM on exciton binding energy have also been calculated. The results are compared with those available in the literature.

Visualization of exciton transport in ordered and disordered molecular solids.

PubMed

Akselrod, Gleb M; Deotare, Parag B; Thompson, Nicholas J; Lee, Jiye; Tisdale, William A; Baldo, Marc A; Menon, Vinod M; Bulović, Vladimir

2014-04-16

Transport of nanoscale energy in the form of excitons is at the core of photosynthesis and the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal and spectral visualization of exciton transport in molecular crystals and disordered thin films. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of the thin film, we show that this transition to subdiffusive transport occurs at earlier times as disorder is increased. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology, which has wide implications for the design of excitonic materials and devices.

Multi-exciton emission from solitary dopant states of carbon nanotubes.

PubMed

Ma, Xuedan; Hartmann, Nicolai F; Velizhanin, Kirill A; Baldwin, Jon K S; Adamska, Lyudmyla; Tretiak, Sergei; Doorn, Stephen K; Htoon, Han

2017-11-02

By separating the photons from slow and fast decays of single and multi-exciton states in a time gated 2 nd order photon correlation experiment, we show that solitary oxygen dopant states of single-walled carbon nanotubes (SWCNTs) allow emission of photon pairs with efficiencies as high as 44% of single exciton emission. Our pump dependent time resolved photoluminescence (PL) studies further reveal diffusion-limited exciton-exciton annihilation as the key process that limits the emission of multi-excitons at high pump fluences. We further postulate that creation of additional permanent exciton quenching sites occurring under intense laser irradiation leads to permanent PL quenching. With this work, we bring out multi-excitonic processes of solitary dopant states as a new area to be explored for potential applications in lasing and entangled photon generation.

Excitons in boron nitride single layer

NASA Astrophysics Data System (ADS)

Galvani, Thomas; Paleari, Fulvio; Miranda, Henrique P. C.; Molina-Sánchez, Alejandro; Wirtz, Ludger; Latil, Sylvain; Amara, Hakim; Ducastelle, François

2016-09-01

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π ) and conduction (π*) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.

Magnetic, electronic, dielectric and optical properties of Pr(Ca:Sr)MnO 3

NASA Astrophysics Data System (ADS)

Sichelschmidt, J.; Paraskevopoulos, M.; Brando, M.; Wehn, R.; Ivannikov, D.; Mayr, F.; Pucher, K.; Hemberger, J.; Pimenov, A.; Krug von Nidda, H.-A.; Lunkenheimer, P.; Ivanov, V. Yu.; Mukhin, A. A.; Balbashov, A. M.; Loidl, A.

2001-03-01

The charge-ordered perovskite Pr0.65Ca0.28Sr0.07MnO3 was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping.

Excitonic instability in optically pumped three-dimensional Dirac materials

NASA Astrophysics Data System (ADS)

Pertsova, Anna; Balatsky, Alexander V.

2018-02-01

Recently it was suggested that transient excitonic instability can be realized in optically pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by nonequilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a rich phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission, and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.

Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers.

PubMed

Andernach, Rolf; Utzat, Hendrik; Dimitrov, Stoichko D; McCulloch, Iain; Heeney, Martin; Durrant, James R; Bronstein, Hugo

2015-08-19

We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple time scales and investigated the mechanism of triplet exciton formation. During sensitization, singlet exciton diffusion is followed by exciton transfer from the polymer backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the triplet exciton back transfer from the Pt-porphyrin to the polymer and found that 60% of the complex triplet excitons were transferred with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased triplet-triplet annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and upconversion layers.

Water-induced polaron formation at the pentacene surface: Quantum mechanical molecular mechanics simulations

NASA Astrophysics Data System (ADS)

Cramer, Tobias; Steinbrecher, Thomas; Koslowski, Thorsten; Case, David A.; Biscarini, Fabio; Zerbetto, Francesco

2009-04-01

Water is an omnipresent polar impurity that is expected to be the origin of many electric degradation phenomena observed in organic semiconductors. Here, we describe a microscopic model for polaron formation in the outermost layer of a pentacene crystal due to the polarization of a nearby water layer. The efficient coupling of a classical force field that describes the liquid with a tight-binding model that represents the π system of the organic layer permits the calculation of nanosecond length trajectories. The model predicts that the reorientation of water dipoles stabilizes positive charge carriers on average by 0.6 eV and thus leads to a polaron trap state at the liquid interface. Thermal fluctuations of the water molecules provoke two-dimensional diffusive hopping of the charge carrier parallel to the interface with mobilities of up to 0.6cm2s-1V-1 and lead to an amorphous broadening of the valence-band tail. As a consequence, water-filled nanocavities act as trapping sites in pentacene transistors. Instead, a complete wetting of the organic film is expected to result in fast thermally activated hopping transport. Polaron trapping is thus not expected to be a limiting factor for transistor-based sensors that operate under water.

Polarization-dependent exciton dynamics in tetracene single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Bo; Zhang, Chunfeng, E-mail: cfzhang@nju.edu.cn; Xu, Yanqing

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.

Plasmon-Exciton Coupling Interaction for Surface Catalytic Reactions.

PubMed

Wang, Jingang; Lin, Weihua; Xu, Xuefeng; Ma, Fengcai; Sun, Mengtao

2018-05-01

In this review, we firstly reveal the physical principle of plasmon-exciton coupling interaction with steady absorption spectroscopy, and ultrafast transition absorption spectroscopy, based on the pump-prop technology. Secondly, we introduce the fabrication of electro-optical device of two-dimensional semiconductor-nanostructure noble metals hybrid, based on the plasmon-exciton coupling interactions. Thirdly, we introduce the applications of plasmon-exciton coupling interaction in the field of surface catalytic reactions. Lastly, the perspective of plasmon-exciton coupling interaction and applications closed this review. © 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of defects on the small polaron formation and transport properties of hematite from first-principles calculations

NASA Astrophysics Data System (ADS)

Smart, Tyler J.; Ping, Yuan

2017-10-01

Hematite (α-Fe2O3) is a promising candidate as a photoanode material for solar-to-fuel conversion due to its favorable band gap for visible light absorption, its stability in an aqueous environment and its relatively low cost in comparison to other prospective materials. However, the small polaron transport nature in α-Fe2O3 results in low carrier mobility and conductivity, significantly lowering its efficiency from the theoretical limit. Experimentally, it has been found that the incorporation of oxygen vacancies and other dopants, such as Sn, into the material appreciably enhances its photo-to-current efficiency. Yet no quantitative explanation has been provided to understand the role of oxygen vacancy or Sn-doping in hematite. We employed density functional theory to probe the small polaron formation in oxygen deficient hematite, N-doped as well as Sn-doped hematite. We computed the charged defect formation energies, the small polaron formation energy and hopping activation energies to understand the effect of defects on carrier concentration and mobility. This work provides us with a fundamental understanding regarding the role of defects on small polaron formation and transport properties in hematite, offering key insights into the design of new dopants to further improve the efficiency of transition metal oxides for solar-to-fuel conversion.

Enabling valley selective exciton scattering in monolayer WSe2 through upconversion

PubMed Central

Manca, M.; Glazov, M. M.; Robert, C.; Cadiz, F.; Taniguchi, T.; Watanabe, K.; Courtade, E.; Amand, T.; Renucci, P.; Marie, X.; Wang, G.; Urbaszek, B.

2017-01-01

Excitons, Coulomb bound electron–hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb interaction in transition metal dichalcogenides such as WSe2 monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy; we also observe the excited A-excitons state 2s. Detuning of the continuous wave, low-power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal. PMID:28367962

The excited spin-triplet state of a charged exciton in quantum dots.

PubMed

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.

Theoretical and computational studies of excitons in conjugated polymers

NASA Astrophysics Data System (ADS)

Barford, William; Bursill, Robert J.; Smith, Richard W.

2002-09-01

We present a theoretical and computational analysis of excitons in conjugated polymers. We use a tight-binding model of π-conjugated electrons, with 1/r interactions for large r. In both the weak-coupling limit (defined by W>>U) and the strong-coupling limit (defined by W<excitons, i.e., conduction-band electrons bound to valence-band holes. Singlet and triplet excitons whose relative wave functions are odd under a reflection of the relative coordinate are degenerate. Thus, the 2 1A+g and 1 3A-g states are degenerate in this limit. (2) In the strong-coupling limit the bound states are Mott-Hubbard excitons, i.e., particles in the upper Hubbard band bound to holes in the lower Hubbard band. These bound states occur in doublets of even and odd parity excitons. Triplet excitons are magnons bound to the singlet excitons, and hence are degenerate with their singlet counterparts. (3) In the intermediate-coupling regime Mott-Wannier excitons are the more appropriate description for large dimerization, while for the undimerized chain Mott-Hubbard excitons are the correct description. For dimerizations relevant to polyacetylene and polydiacetylene both Mott-Hubbard and Mott-Wannier excitons are present. (4) For all coupling strengths an infinite number of bound states exist for 1/r interactions for an infinite polymer. As a result of the discreteness of the lattice and the restrictions on the exciton wave functions in one dimension, the progression of states does not follow

Magnetic and electronic properties of La3 MO7 and possible polaron formation in hole-doped La3 MO7 (M  =  Ru and Os)

NASA Astrophysics Data System (ADS)

Gao, Bin; Weng, Yakui; Zhang, Jun-Jie; Zhang, Huimin; Zhang, Yang; Dong, Shuai

2017-03-01

Oxides with 4d/5d transition metal ions are physically interesting for their particular crystalline structures as well as the spin-orbit coupled electronic structures. Recent experiments revealed a series of 4d/5d transition metal oxides R 3 MO7 (R: rare earth; M: 4d/5d transition metal) with unique quasi-one-dimensional M chains. Here first-principles calculations have been performed to study the electronic structures of La3OsO7 and La3RuO7. Our study confirm both of them to be Mott insulating antiferromagnets with identical magnetic order. The reduced magnetic moments, which are much smaller than the expected value for ideal high-spin state (3 t 2g orbitals occupied), are attributed to the strong p  -  d hybridization with oxygen ions, instead of the spin-orbit coupling. The Ca-doping to La3OsO7 and La3RuO7 can not only modulate the nominal carrier density but also affect the orbital order as well as the local distortions. The Coulombic attraction and particular orbital order would prefer to form polarons, which might explain the puzzling insulating behavior of doped 5d transition metal oxides. In addition, our calculations predict that the Ca-doping can trigger ferromagnetism in La3RuO7 but not in La3OsO7.

Observation of interlayer excitons in MoSe2 single crystals

NASA Astrophysics Data System (ADS)

Horng, Jason; Stroucken, Tineke; Zhang, Long; Paik, Eunice Y.; Deng, Hui; Koch, Stephan W.

2018-06-01

Interlayer excitons with direct optical transitions are observed coexisting with intralayer excitons in the same K valleys in bilayer, few-layer, and bulk MoSe2 single crystals by confocal reflection contrast spectroscopy. Quantitative analysis using the Dirac-Bloch equations provides unambiguous state assignment of all the measured resonances. The interlayer excitons in bilayer MoSe2 have a large binding energy of 153 meV and a narrow linewidth of 20 meV. Their spectral weight is comparable to the commonly studied higher-order intralayer excitons. At the same time, the interlayer excitons are characterized by distinct transition energies and permanent dipole moments, providing a promising high temperature and optically accessible platform for dipolar exciton physics.

Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

DOE PAGES

Selig, Malte; Berghäuser, Gunnar; Raja, Archana; ...

2016-11-07

Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. We investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS 2 and MoSe 2) through a study combining microscopic theory with spectroscopic measurements. We also show that the excitonicmore » coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. Particularly, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures, in WS 2.« less

Cross-circularly polarized two-exciton states in one to three dimensions.

PubMed

Ajiki, Hiroshi

2015-03-14

Biexciton and two-exciton dissociated states of Frenkel-type excitons are studied theoretically using an exciton tight-binding (TB) model including a polarization degree of freedom. Because the biexciton consists of two cross-circularly polarized excitons, an on-site interaction (V) between the two excitons should be considered in addition to a nearest-neighbor two-exciton attractive interaction (δ). Although there are an infinitely large number of combinations of V and δ providing the observed binding energy of a biexciton, the wave function of the biexciton and two-exciton dissociated states is nearly independent of these parameter sets. This means that all the two-exciton states are uniquely determined from the exciton TB model. There are a spatially symmetric and an antisymmetric biexciton state for a one-dimensional (1D) lattice and two symmetric and one antisymmetric biexciton states at most for two- (2D) and three-dimensional (3D) lattices. In contrast, when the polarization degree of freedom is ignored, there is one biexciton state for 1D, 2D, and 3D lattices. For this study, a rapid and memory-saving calculation method for two-exciton states is extended to include the polarization degree of freedom.

Cross-circularly polarized two-exciton states in one to three dimensions

NASA Astrophysics Data System (ADS)

Ajiki, Hiroshi

2015-03-01

Biexciton and two-exciton dissociated states of Frenkel-type excitons are studied theoretically using an exciton tight-binding (TB) model including a polarization degree of freedom. Because the biexciton consists of two cross-circularly polarized excitons, an on-site interaction (V) between the two excitons should be considered in addition to a nearest-neighbor two-exciton attractive interaction (δ). Although there are an infinitely large number of combinations of V and δ providing the observed binding energy of a biexciton, the wave function of the biexciton and two-exciton dissociated states is nearly independent of these parameter sets. This means that all the two-exciton states are uniquely determined from the exciton TB model. There are a spatially symmetric and an antisymmetric biexciton state for a one-dimensional (1D) lattice and two symmetric and one antisymmetric biexciton states at most for two- (2D) and three-dimensional (3D) lattices. In contrast, when the polarization degree of freedom is ignored, there is one biexciton state for 1D, 2D, and 3D lattices. For this study, a rapid and memory-saving calculation method for two-exciton states is extended to include the polarization degree of freedom.

Energy and Information Transfer Via Coherent Exciton Wave Packets

NASA Astrophysics Data System (ADS)

Zang, Xiaoning

Electronic excitons are bound electron-hole states that are generated when light interacts with matter. Such excitations typically entangle with phonons and rapidly decohere; the resulting electronic state dynamics become diffusive as a result. However, if the exciton-phonon coupling can be reduced, it may be possible to construct excitonic wave packets that offer a means of efficiently transmitting information and energy. This thesis is a combined theory/computation investigation to design condensed matter systems which support the requisite coherent transport. Under the idealizing assumption that exciton-phonon entanglement could be completely suppressed, the majority of this thesis focuses on the creation and manipulation of exciton wave packets in quasi-one-dimensional systems. While each site could be a silicon quantum dot, the actual implementation focused on organic molecular assemblies for the sake of computational simplicity, ease of experimental implementation, potential for coherent transport, and promise because of reduced structural uncertainty. A laser design was derived to create exciton wave packets with tunable shape and speed. Quantum interference was then exploited to manipulate these packets to block, pass, and even dissociate excitons based on their energies. These developments allow exciton packets to be considered within the arena of quantum information science. The concept of controllable excitonic wave packets was subsequently extended to consider molecular designs that allow photons with orbital angular momentum to be absorbed to create excitons with a quasi-angular momentum of their own. It was shown that a well-defined measure of topological charge is conserved in such light-matter interactions. Significantly, it was also discovered that such molecules allow photon angular momenta to be combined and later emitted. This amounts to a new way of up/down converting photonic angular momentum without relying on nonlinear optical materials. The

Interlayer excitons in a bulk van der Waals semiconductor.

PubMed

Arora, Ashish; Drüppel, Matthias; Schmidt, Robert; Deilmann, Thorsten; Schneider, Robert; Molas, Maciej R; Marauhn, Philipp; Michaelis de Vasconcellos, Steffen; Potemski, Marek; Rohlfing, Michael; Bratschitsch, Rudolf

2017-09-21

Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments and ab initio calculations for 2H-MoTe 2 , we explain their salient features: the positive sign of the g-factor and the large diamagnetic shift. Our investigations solve the long-standing puzzle of positive g-factors in transition metal dichalcogenides, and pave the way for studying collective phenomena in these materials at elevated temperatures.Excitons, quasi-particles of bound electron-hole pairs, are at the core of the optoelectronic properties of layered transition metal dichalcogenides. Here, the authors unveil the presence of interlayer excitons in bulk van der Waals semiconductors, arising from strong localization and spin-valley coupling of charge carriers.

Formation and decay of charge carriers in aggregate nanofibers consisting of poly(3-hexylthiophene)-coated gold nanoparticles.

PubMed

Lee, Dongki; Lee, Jaewon; Song, Ki-Hee; Rhee, Hanju; Jang, Du-Jeon

2016-01-21

Thin nanofibers (NFs) of J-dominant aggregates with a thickness of 15 nm and thick NFs of H-dominant aggregates with a thickness of 25 nm were fabricated by the self-assembly of poly(3-hexylthiophene)-coated gold nanoparticles. The formation and decay dynamics of the charge carriers, which are dependent on the aggregate types of NFs, was investigated by time-resolved emission and transient-absorption spectroscopy. With increasing excitation energy, the fraction of the fast emission decay component decreased, suggesting that the fast formation of polaron pairs (PP), localized (LP), and delocalized polarons (DP) results from higher singlet exciton states produced by the singlet fusion. The faster decay dynamics of DP and LP in the thick NFs than in thin NFs is due to the increased delocalization of DP and LP. As the interchain aggregation is weaker than intrachain aggregation, PP decays faster in thin NFs than in thick NFs. In both thin and thick NFs, although triplet (T1) excitons were barely observed with excitation at 532 nm on a nanosecond time scale, they were observed with excitation at 355 nm, showing that T1 excitons within NFs are generated mainly through the singlet fission from a higher singlet exciton state rather than through intersystem crossing.

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.

Effects of surface and interface traps on exciton and multi-exciton dynamics in core/shell quantum dots

NASA Astrophysics Data System (ADS)

Bozio, Renato; Righetto, Marcello; Minotto, Alessandro

2017-08-01

Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption, fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS, CdSe/Cd0.5Zn0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination processes.

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.

Reconfigurable exciton-plasmon interconversion for nanophotonic circuits

PubMed Central

Lee, Hyun Seok; Luong, Dinh Hoa; Kim, Min Su; Jin, Youngjo; Kim, Hyun; Yun, Seokjoon; Lee, Young Hee

2016-01-01

The recent challenges for improving the operation speed of nanoelectronics have motivated research on manipulating light in on-chip integrated circuits. Hybrid plasmonic waveguides with low-dimensional semiconductors, including quantum dots and quantum wells, are a promising platform for realizing sub-diffraction limited optical components. Meanwhile, two-dimensional transition metal dichalcogenides (TMDs) have received broad interest in optoelectronics owing to tightly bound excitons at room temperature, strong light-matter and exciton-plasmon interactions, available top-down wafer-scale integration, and band-gap tunability. Here, we demonstrate principal functionalities for on-chip optical communications via reconfigurable exciton-plasmon interconversions in ∼200-nm-diameter Ag-nanowires overlapping onto TMD transistors. By varying device configurations for each operation purpose, three active components for optical communications are realized: field-effect exciton transistors with a channel length of ∼32 μm, field-effect exciton multiplexers transmitting multiple signals through a single NW and electrical detectors of propagating plasmons with a high On/Off ratio of∼190. Our results illustrate the unique merits of two-dimensional semiconductors for constructing reconfigurable device architectures in integrated nanophotonic circuits. PMID:27892463

Magnetic control of valley pseudospin in monolayer WSe 2

DOE PAGES

Aivazian, G.; Gong, Zhirui; Jones, Aaron M.; ...

2015-01-26

Local energy extrema of the bands in momentum space, or valleys, can endow electrons in solids with pseudo-spin in addition to real spin 1-5. In transition metal dichalcogenides this valley pseudo-spin, like real spin, is associated with a magnetic moment1,6 which underlies the valley-dependent circular dichroism 6 that allows optical generation of valley polarization 7-9, intervalley quantum coherence 10, and the valley Hall effect 11. However, magnetic manipulation of valley pseudospin via this magnetic moment 12-13, analogous to what is possible with real spin, has not been shown before. Here we report observation of the valley Zeeman splitting and magneticmore » tuning of polarization and coherence of the excitonic valley pseudospin, by performing polarization-resolved magneto-photoluminescence on monolayer WSe 2. Our measurements reveal both the atomic orbital and lattice contributions to the valley orbital magnetic moment; demonstrate the deviation of the band edges in the valleys from an exact massive Dirac fermion model; and reveal a striking difference between the magnetic responses of neutral and charged valley excitons which is explained by renormalization of the excitonic spectrum due to strong exchange interactions.« less

Phonon-Driven Oscillatory Plasmonic Excitonic Nanomaterials

DOE PAGES

Kirschner, Matthew S.; Ding, Wendu; Li, Yuxiu; ...

2017-12-01

In this study, we demonstrate that coherent acoustic phonons derived from plasmonic nanoparticles can modulate electronic interactions with proximal excitonic molecular species. A series of gold bipyramids with systematically varied aspect ratios and corresponding localized surface plasmon resonance energies, functionalized with a J-aggregated thiacarbocyanine dye molecule, produce two hybridized states that exhibit clear anti-crossing behavior with a Rabi splitting energy of 120 meV. In metal nanoparticles, photoexcitation generates coherent acoustic phonons that cause oscillations in the plasmon resonance energy. In the coupled system, these photo-generated oscillations alter the metal nanoparticle’s energetic contribution to the hybridized system and, as a result,more » change the coupling between the plasmon and exciton. We demonstrate that such modulations in the hybridization is consistent across a wide range of bipyramid ensembles. We also use Finite-Difference Time Domain calculations to develop a simple model describing this behavior. Lastly, such oscillatory plasmonic-excitonic nanomaterials (OPENs) offer a route to manipulate and dynamically-tune the interactions of plasmonic/excitonic systems and unlock a range of potential applications.« less

Polaron formation in normal state optical conductivity of iron-based superconductor

NASA Astrophysics Data System (ADS)

Choudhary, K. K.; Lodhi, Pavitra Devi; Kaurav, Netram

2018-05-01

Normal state Optical conductivity σ(ω) of Iron-Based superconductor LaFeAsO have been investigated using polaron formation mechanism. The coherent Drude free carrier excitations as well as the incoherent motion of carriers leading to a polaron formation, originated from inter and intra layer transitions of charge carriers are incorporated in the present model. Coherent motion of Drude carriers obtained from an effective interaction potential leads to a peak at zero frequency regime which is an indication of metallic conduction in superconducting materials and also produces a long tail at higher frequencies infrared region. Whereas, the incoherent motion i.e. hopping of carriers from Fe to Fe in the FeAs layer and from FeAs layer to LaO layer produces two different peaks at around 100 cm-1 and 430 cm-1 respectively. Two contributions, Drude and hopping carriers successfully explain the anomalies observed in the optical conductivity of metallic state of the iron-based superconductors.

Strain Control of Exciton-Phonon Coupling in Atomically Thin Semiconductors.

PubMed

Niehues, Iris; Schmidt, Robert; Drüppel, Matthias; Marauhn, Philipp; Christiansen, Dominik; Selig, Malte; Berghäuser, Gunnar; Wigger, Daniel; Schneider, Robert; Braasch, Lisa; Koch, Rouven; Castellanos-Gomez, Andres; Kuhn, Tilmann; Knorr, Andreas; Malic, Ermin; Rohlfing, Michael; Michaelis de Vasconcellos, Steffen; Bratschitsch, Rudolf

2018-03-14

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton-phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe 2 , WSe 2 , WS 2 , and MoS 2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS 2 . For MoS 2 monolayers, the line width increases. These effects are due to a modified exciton-phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton-phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.

Exciton-plasmon coupling interactions: from principle to applications

NASA Astrophysics Data System (ADS)

Cao, En; Lin, Weihua; Sun, Mengtao; Liang, Wenjie; Song, Yuzhi

2018-01-01

The interaction of exciton-plasmon coupling and the conversion of exciton-plasmon-photon have been widely investigated experimentally and theoretically. In this review, we introduce the exciton-plasmon interaction from basic principle to applications. There are two kinds of exciton-plasmon coupling, which demonstrate different optical properties. The strong exciton-plasmon coupling results in two new mixed states of light and matter separated energetically by a Rabi splitting that exhibits a characteristic anticrossing behavior of the exciton-LSP energy tuning. Compared to strong coupling, such as surface-enhanced Raman scattering, surface plasmon (SP)-enhanced absorption, enhanced fluorescence, or fluorescence quenching, there is no perturbation between wave functions; the interaction here is called the weak coupling. SP resonance (SPR) arises from the collective oscillation induced by the electromagnetic field of light and can be used for investigating the interaction between light and matter beyond the diffraction limit. The study on the interaction between SPR and exaction has drawn wide attention since its discovery not only due to its contribution in deepening and broadening the understanding of SPR but also its contribution to its application in light-emitting diodes, solar cells, low threshold laser, biomedical detection, quantum information processing, and so on.

Electronic and vibrational exciton coupling in oxidized trianglimines.

PubMed

Szymkowiak, Joanna; Kwit, Marcin

2018-02-01

Readily available chiral trianglimine and their (poly)oxygenated congeners represent a unique class of macrocyclic rigid compounds optimal for testing electronic and vibrational circular dichroism exciton chirality methods. Electronic and vibrational circular dichroism spectra of such trianglimines are strongly affected by polar substituents in macrocycle skeletons. Double substitution by OH groups in each aromatic fragment of the macrocycle causes sign reversal of the exciton couplet in the region of the strongest UV absorption. On the other hand, electronic circular dichroism spectrum of the macrocycle having 2 methoxy groups shows 2 exciton couplets-the long-wavelength positive and the second of the negative sign, observed at the shorter wavelengths. VCD spectra of macrocyclic imines show vibrational exciton couplets in the region of strong C=N stretches. The signs of these couplets are positive and the opposite of the diamine chirality. For trianglimine macrocycles the interpretation of VCD spectra in terms of excitons is much more convincing than for electronic circular dichroism spectra. By contrast, trans-1,2-diaminocyclohexane-based vicinal diimines, being a one-third of the respective macrocycle, do not exhibit any vibrational exciton effect. Experimental data were confronted with DFT calculations. We observed good-to-excellent agreement between experimental and computed data. © 2017 Wiley Periodicals, Inc.

Shallow trapping vs. deep polarons in a hybrid lead halide perovskite, CH3NH3PbI3.

PubMed

Kang, Byungkyun; Biswas, Koushik

2017-10-18

There has been considerable speculation over the nature of charge carriers in organic-inorganic hybrid perovskites, i.e., whether they are free and band-like, or they are prone to self-trapping via short range deformation potentials. Unusually long minority-carrier diffusion lengths and moderate-to-low mobilities, together with relatively few deep defects add to their intrigue. Here we implement density functional methods to investigate the room-temperature, tetragonal phase of CH 3 NH 3 PbI 3 . We compare charge localization behavior at shallow levels and associated lattice relaxation versus those at deep polaronic states. The shallow level originates from screened Coulomb interaction between the perturbed host and an excited electron or hole. The host lattice has a tendency towards forming these shallow traps where the electron or hole is localized not too far from the band edge. In contrast, there is a considerable potential barrier that must be overcome in order to initiate polaronic hole trapping. The formation of a hole polaron (I 2 - center) involves strong lattice relaxation, including large off-center displacement of the organic cation, CH 3 NH 3 + . This type of deep polaron is energetically unfavorable, and active shallow traps are expected to shape the carrier dynamics in this material.

Excitonic effects and related properties in semiconductor nanostructures: roles of size and dimensionality

NASA Astrophysics Data System (ADS)

Wu, Shudong; Cheng, Liwen; Wang, Qiang

2017-08-01

The size- and dimensionality-dependence of excitonic effects and related properties in semiconductor nanostructures are theoretically studied in detail within the effective-mass approximation. When nanostructure sizes become smaller than the bulk exciton Bohr radius, excitonic effects are significantly enhanced with reducing size or dimensionality. This is as a result of quantum confinement in more directions leading to larger exciton binding energies and normalized exciton oscillator strengths. These excitonic effects originate from electron-hole Coulombic interactions, which strongly enhance the oscillator strength between the electron and hole. It is also established that the universal scaling of exciton binding energy versus the inverse of the exciton Bohr radius follows a linear scaling law. Herein, we propose a stretched exponential law for the size scaling of optical gap, which is in good agreement with the calculated data. Due to differences in the confinement dimensionality, the radiative lifetime of low-dimensional excitons becomes shorter than that of bulk excitons. The size dependence of the exciton radiative lifetimes is in good agreement with available experimental data. This strongly enhanced electron-hole exchange interaction is expected in low-dimensional structures due to enriched excitonic effects. The main difference in nanostructures compared to the bulk can be interpreted in terms of the enhanced excitonic effects induced by exciton localization. The enhanced excitonic effects are expected to be of importance in developing stable and high-efficiency nanoscale excitonic optoelectronic devices.

Exciton-dominant electroluminescence from a diode of monolayer MoS{sub 2}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ye, Yu; Ye, Ziliang; Gharghi, Majid

2014-05-12

In two-dimensional monolayer MoS{sub 2}, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS{sub 2} fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of amore » higher energy exciton peak of 2.255 eV in the monolayer MoS{sub 2} diode, attributed to the excited exciton state of a direct-exciton transition.« less

Polaronic and ionic conduction in NaMnO2: influence of native point defects

NASA Astrophysics Data System (ADS)

Zhu, Zhen; Peelaers, Hartwin; van de Walle, Chris G.

Layered NaMnO2 has promising applications as a cathode material for sodium ion batteries. We will discuss strategies to improve the electrical performance of NaMnO2, including how to optimize the conditions of synthesis and how impurity doping affects the performance. Using hybrid density functional theory, we explored the structural, electronic, and defect properties of bulk NaMnO2. It is antiferromagnetic in the ground state with a band gap of 3.75 eV. Small hole and electron polarons can form in the bulk either through self-trapping or adjacent to point defects. We find that both Na and Mn vacancies are shallow acceptors with the induced holes trapped as small polarons, while O vacancies are deep defect centers. Cation antisites, especially MnNa, are found to have low formation energies. As a result, we expect that MnNa exists in as-grown NaMnO2 in moderate concentrations, rather than forming only at a later stage of the charging process, at which point it causes undesirable structural phase transitions. Both electronic conduction, via polaron hopping, and ionic conduction, through VNa migration, are significantly affected by the presence of point defects. This work was supported by DOE.

Exciton intrachain transport induced by interchain packing configurations in conjugated polymers.

PubMed

Meng, Ruixuan; Gao, Kun; Zhang, Gaiyan; Han, Shixuan; Yang, Fujiang; Li, Yuan; Xie, Shijie

2015-07-28

Based on a tight binding model combined with a nonadiabatic dynamics approach, we theoretically investigate the exciton intrachain transport in conjugated polymers with different interchain packing configurations. We construct two different interchain packing configurations, i.e. linear and exponential forms, and simulate the dynamical processes of the exciton transport in these systems. We find that, in both cases, there exists a distribution of driving force for exciton transport, which stems from the gradient of the exciton creation energy along the chains. This finding enriches the picture of exciton transport in polymers and provides a new idea to improve the exciton transport length in polymeric photovoltaic devices.

Optically Induced Nuclear Spin Polarization in the Quantum Hall Regime: The Effect of Electron Spin Polarization through Exciton and Trion Excitations.

PubMed

Akiba, K; Kanasugi, S; Yuge, T; Nagase, K; Hirayama, Y

2015-07-10

We study nuclear spin polarization in the quantum Hall regime through the optically pumped electron spin polarization in the lowest Landau level. The nuclear spin polarization is measured as a nuclear magnetic field B(N) by means of the sensitive resistive detection. We find the dependence of B(N) on the filling factor nonmonotonic. The comprehensive measurements of B(N) with the help of the circularly polarized photoluminescence measurements indicate the participation of the photoexcited complexes, i.e., the exciton and trion (charged exciton), in nuclear spin polarization. On the basis of a novel estimation method of the equilibrium electron spin polarization, we analyze the experimental data and conclude that the filling factor dependence of B(N) is understood by the effect of electron spin polarization through excitons and trions.

Magneto-optical spectrum and the effective excitonic Zeeman splitting energies of Mn and Co-doped CdSe nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiong, Wen, E-mail: wenxiong@cqu.edu.cn; Chen, Wensuo

2013-12-21

The electronic structure of Mn and Co-doped CdSe nanowires are calculated based on the six-band k·p effective-mass theory. Through the calculation, it is found that the splitting energies of the degenerate hole states in Mn-doped CdSe nanowires are larger than that in Co-doped CdSe nanowires when the concentration of these two kinds of magnetic ions is the same. In order to analysis the magneto-optical spectrum of Mn and Co-doped CdSe nanowires, the four lowest electron states and the four highest hole states are sorted when the magnetic field is applied, and the 10 lowest optical transitions between the conduction subbandsmore » and the valence subbands at the Γ point in Mn and Co-doped CdSe nanowires are shown in the paper, it is found that the order of the optical transitions at the Γ point almost do not change although two different kinds of magnetic ions are doped in CdSe nanowires. Finally, the effective excitonic Zeeman splitting energies at the Γ point are found to increase almost linearly with the increase of the concentration of the magnetic ions and the magnetic field; meanwhile, the giant positive effective excitonic g factors in Mn and Co-doped CdSe nanowires are predicted based on our theoretical calculation.« less

Interlayer exciton optoelectronics in a 2D heterostructure p–n junction

DOE PAGES

Ross, Jason S.; Rivera, Pasqual; Schaibley, John; ...

2016-12-22

Semiconductor heterostructures are backbones for solid-state-based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures have enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and a long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p–n junctions in a MoSe 2–WSe 2 heterobilayer. Applying a forward bias enablesmore » the first observation of electroluminescence from interlayer excitons. At zero bias, the p–n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller than the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is 2 orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to the opposite layers. Lastly, these results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.« less

Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n Junction.

PubMed

Ross, Jason S; Rivera, Pasqual; Schaibley, John; Lee-Wong, Eric; Yu, Hongyi; Taniguchi, Takashi; Watanabe, Kenji; Yan, Jiaqiang; Mandrus, David; Cobden, David; Yao, Wang; Xu, Xiaodong

2017-02-08

Semiconductor heterostructures are backbones for solid-state-based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures have enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and a long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p-n junctions in a MoSe 2 -WSe 2 heterobilayer. Applying a forward bias enables the first observation of electroluminescence from interlayer excitons. At zero bias, the p-n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller than the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is 2 orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to the opposite layers. These results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.

Excitons in atomically thin 2D semiconductors and their applications

DOE PAGES

Xiao, Jun; Zhao, Mervin; Wang, Yuan; ...

2017-01-01

The research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS 2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. Here in this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical meansmore » is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.« less

Excitons in atomically thin 2D semiconductors and their applications

NASA Astrophysics Data System (ADS)

Xiao, Jun; Zhao, Mervin; Wang, Yuan; Zhang, Xiang

2017-06-01

The research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. In this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical means is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.

Crossovers from excitons to plasmons in narrow-gap carbon nanotubes

NASA Astrophysics Data System (ADS)

Uryu, Seiji

2018-06-01

Plasmons and excitons, bound states of electrons and holes, are collective charge excitations in solids. In this study, we numerically show that in most metallic carbon nanotubes, which are called narrow-gap carbon nanotubes, excitons cross over to plasmons as the wave vector increases. This indicates that resonance with the excitons changes to that with the plasmons by changing the nanotube length, which can explain the origin of observed peaks in the terahertz or far-infrared region in the optical absorption spectra of metallic carbon nanotubes. In the crossovers from excitons to plasmons, a depolarization effect on the many-body wave functions of the plasmons and excitons is clarified.

Implications of the formation of small polarons in Li2O2 for Li-air batteries

NASA Astrophysics Data System (ADS)

Kang, Joongoo; Jung, Yoon Seok; Wei, Su-Huai; Dillon, Anne C.

2012-01-01

Lithium-air batteries (LABs) are an intriguing next-generation technology due to their high theoretical energy density of ˜11 kWh/kg. However, LABs are hindered by both poor rate capability and significant polarization in cell voltage, primarily due to the formation of Li2O2 in the air cathode. Here, by employing hybrid density functional theory, we show that the formation of small polarons in Li2O2 limits electron transport. Consequently, the low electron mobility μ = 10-10-10-9 cm2/V s contributes to both the poor rate capability and the polarization that limit the LAB power and energy densities. The self-trapping of electrons in the small polarons arises from the molecular nature of the conduction band states of Li2O2 and the strong spin polarization of the O 2p state. Our understanding of the polaronic electron transport in Li2O2 suggests that designing alternative carrier conduction paths for the cathode reaction could significantly improve the performance of LABs at high current densities.

Dynamics of charge-transfer excitons in type-II semiconductor heterostructures

NASA Astrophysics Data System (ADS)

Stein, M.; Lammers, C.; Richter, P.-H.; Fuchs, C.; Stolz, W.; Koch, M.; Vänskä, O.; Weseloh, M. J.; Kira, M.; Koch, S. W.

2018-03-01

The formation, decay, and coherence properties of charge-transfer excitons in semiconductor heterostructures are investigated by applying four-wave-mixing and terahertz spectroscopy in combination with a predictive microscopic theory. A charge-transfer process is identified where the optically induced coherences decay directly into a charge-transfer electron-hole plasma and exciton states. It is shown that charge-transfer excitons are more sensitive to the fermionic electron-hole substructure than regular excitons.

Fractional Solitons in Excitonic Josephson Junctions

NASA Astrophysics Data System (ADS)

Su, Jung-Jung; Hsu, Ya-Fen

The Josephson effect is especially appealing because it reveals macroscopically the quantum order and phase. Here we study this effect in an excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. Such a junction is proposed to take place in the quantum Hall bilayer (QHB) that makes it subtler than in superconductor because of the counterflow of excitonic supercurrent and the interlayer tunneling in QHB. We treat the system theoretically by first mapping it into a pseudospin ferromagnet then describing it by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, the excitonic Josephson junction can possess a family of fractional sine-Gordon solitons that resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Interestingly, each fractional soliton carries a topological charge Q which is not necessarily a half/full integer but can vary continuously. The resultant current-phase relation (CPR) shows that solitons with Q =ϕ0 / 2 π are the lowest energy states for small ϕ0. When ϕ0 > π , solitons with Q =ϕ0 / 2 π - 1 take place - the polarity of CPR is then switched.

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.

Spectral properties of excitons in the bilayer graphene

NASA Astrophysics Data System (ADS)

Apinyan, V.; Kopeć, T. K.

2018-01-01

In this paper, we consider the spectral properties of the bilayer graphene with the local excitonic pairing interaction between the electrons and holes. We consider the generalized Hubbard model, which includes both intralayer and interlayer Coulomb interaction parameters. The solution of the excitonic gap parameter is used to calculate the electronic band structure, single-particle spectral functions, the hybridization gap, and the excitonic coherence length in the bilayer graphene. We show that the local interlayer Coulomb interaction is responsible for the semimetal-semiconductor transition in the double layer system, and we calculate the hybridization gap in the band structure above the critical interaction value. The formation of the excitonic band gap is reported as the threshold process and the momentum distribution functions have been calculated numerically. We show that in the weak coupling limit the system is governed by the Bardeen-Cooper-Schrieffer (BCS)-like pairing state. Contrary, in the strong coupling limit the excitonic condensate states appear in the semiconducting phase, by forming the Dirac's pockets in the reciprocal space.

Exciton Transport Simulations in Phenyl Cored Thiophene Dendrimers

NASA Astrophysics Data System (ADS)

Kim, Kwiseon; Erkan Kose, Muhammet; Graf, Peter; Kopidakis, Nikos; Rumbles, Garry; Shaheen, Sean E.

2009-03-01

Phenyl cored 3-arm and 4-arm thiophene dendrimers are promising materials for use in photovoltaic devices. It is important to understand the energy transfer mechanisms in these molecules to guide the synthesis of novel dendrimers with improved efficiency. A method is developed to estimate the exciton diffusion lengths for the dendrimers and similar chromophores in amorphous films. The approach exploits Fermi's Golden Rule to estimate the energy transfer rates for an ensemble of bimolecular complexes in random orientations. Using Poisson's equation to evaluate Coulomb integrals led to efficient calculation of excitonic couplings between the transition densities. Monte-Carlo simulations revealed the dynamics of energy transport in the dendrimers. Experimental exciton diffusion lengths of the dendrimers range 10 ˜ 20 nm, increasing with the size of the dendrimer. Simulated diffusion lengths correlate well with experiments. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors that determine the exciton diffusion length in amorphous films.

Angular momentum transport with twisted exciton wave packets

NASA Astrophysics Data System (ADS)

Zang, Xiaoning; Lusk, Mark T.

2017-10-01

A chain of cofacial molecules with CN or CN h symmetry supports excitonic states with a screwlike structure. These can be quantified with the combination of an axial wave number and an azimuthal winding number. Combinations of these states can be used to construct excitonic wave packets that spiral down the chain with well-determined linear and angular momenta. These twisted exciton wave packets can be created and annihilated using laser pulses, and their angular momentum can be optically modified during transit. This allows for the creation of optoexcitonic circuits in which information, encoded in the angular momentum of light, is converted into excitonic wave packets that can be manipulated, transported, and then reemitted. A tight-binding paradigm is used to demonstrate the key ideas. The approach is then extended to quantify the evolution of twisted exciton wave packets in a many-body, multilevel time-domain density functional theory setting. In both settings, numerical methods are developed that allow the site-to-site transfer of angular momentum to be quantified.

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires.

PubMed

Yan, Jie-Yun

2018-06-13

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires is studied. Based on the excitonic theory, the numerical method to calculate the photoconductivity spectrum in the nanowires is developed, which can simulate optical pump terahertz-probe spectroscopy measurements on real nanowires and thereby calculate the typical photoconductivity spectrum. With the help of the energetic structure deduced from the calculated linear absorption spectrum, the numerically observed shift of the resonant peak in the photoconductivity spectrum is found to result from the dominant exciton transition between excited or continuum states to the ground state, and the quantitative analysis is in good agreement with the quantum plasmon model. Besides, the dependence of the photoconductivity on the polarization of the terahertz field is also discussed. The numerical method and supporting theoretical analysis provide a new tool for experimentalists to understand the terahertz photoconductivity in intrinsic semiconductor nanowires at low temperatures or for nanowires subjected to below bandgap photoexcitation, where excitonic effects dominate.

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires

NASA Astrophysics Data System (ADS)

Yan, Jie-Yun

2018-06-01

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires is studied. Based on the excitonic theory, the numerical method to calculate the photoconductivity spectrum in the nanowires is developed, which can simulate optical pump terahertz-probe spectroscopy measurements on real nanowires and thereby calculate the typical photoconductivity spectrum. With the help of the energetic structure deduced from the calculated linear absorption spectrum, the numerically observed shift of the resonant peak in the photoconductivity spectrum is found to result from the dominant exciton transition between excited or continuum states to the ground state, and the quantitative analysis is in good agreement with the quantum plasmon model. Besides, the dependence of the photoconductivity on the polarization of the terahertz field is also discussed. The numerical method and supporting theoretical analysis provide a new tool for experimentalists to understand the terahertz photoconductivity in intrinsic semiconductor nanowires at low temperatures or for nanowires subjected to below bandgap photoexcitation, where excitonic effects dominate.

Theory for electric dipole superconductivity with an application for bilayer excitons.

PubMed

Jiang, Qing-Dong; Bao, Zhi-qiang; Sun, Qing-Feng; Xie, X C

2015-07-08

Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we derive the London-type and Ginzburg-Landau-type equations for the electric dipole superconductors. By using these equations, we discover the Meissner-type effect and the electric dipole current Josephson effect. These effects can provide direct evidence for the formation of the exciton superfluid state in bilayer systems and pave new ways to drive an electric dipole current.

Theory for electric dipole superconductivity with an application for bilayer excitons

PubMed Central

Jiang, Qing-Dong; Bao, Zhi-qiang; Sun, Qing-Feng; Xie, X. C.

2015-01-01

Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we derive the London-type and Ginzburg-Landau-type equations for the electric dipole superconductors. By using these equations, we discover the Meissner-type effect and the electric dipole current Josephson effect. These effects can provide direct evidence for the formation of the exciton superfluid state in bilayer systems and pave new ways to drive an electric dipole current. PMID:26154838

Magnetic field induced enhancement of resistance in polycrystalline ZrTe5

NASA Astrophysics Data System (ADS)

Behera, Prakash; Bera, Sumit; Patidar, Manju Mishra; Singh, Durgesh; Mishra, A. K.; Krishnan, M.; Gangrade, M.; Deshpande, U. P.; Venkatesh, R.; Ganesan, V.

2018-04-01

Transport properties of the polycrystalline ZrTe5 showing a considerable positive Magneto-Resistance (MR) in the intermediate temperatures has been reported. Substantial shift of peak temperature by approximately 65 K with an applied magnetic field of 13.5 Tesla has been observed. Magneto resistance of this polycrystalline sample (˜100%) is comparable with its single crystalline counterpart reported in literature. The peak intensity scales with peak temperature and obeys reasonably the Dionne relationship that is a clear indication of polaron mediated conduction in this system. Magneto Resistance (MR) in this system is attributed to the two carrier polaronic conduction model similar to the Holstein's approach. The results are further complemented with the Peak shift in magnetic field expected for a system having a fraction of localized carrier density. This observation places this famous thermoelectric material that displays a topological Dirac to Weyl transition in magnetic field in to the family of materials that have potential technological applications in the liquid nitrogen temperature range viz. 85-150 K.

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.

Discernment of Possible Organic Magnetic Field Effect Mechanisms Using Polymer Light-Emitting Electrochemical Cells

NASA Astrophysics Data System (ADS)

Geng, R.; Subedi, R. C.; Liang, S.; Nguyen, T. D.

2014-07-01

We report studies of magnetic field effect (MFE) in polymer light-emitting electrochemical cells (PLEC) using the "super-yellow" poly-(phenylene vynilene) (SY-PPV) polymer in vertical and planar device configurations. The purpose is to discern the existing MFE mechanisms in organic light emitting diodes (OLEDs) where the current and electroluminescence are strongly modulated by a small applied magnetic field. In particular, we investigate the mutual relationship between magneto-conductance (MC) and magneto-electroluminescence (MEL) by studying the role of polaron density dissociated from polaron pairs (PP) on these magnetic responses. In general, the dissociated polaron density is determined by the PP dissociation rate and the PP density. For the planar PLEC, which possesses a small dissociation rate, we observe small and negative MC at all applied voltages regardless of the emission intensity, while MEL becomes positive when electroluminescence quantum efficiency increases. The MC has a much narrower width than the MEL, indicating that the MC and MEL do not share a common origin. However, MC reverses and has the same width as MEL when the device is exposed to a threshold laser power. For the vertical PLEC, characterized by a large dissociation rate, MC and MEL are positive and have the same width. We discuss the results using the existing MFE mechanism in OLEDs. We show that the PP model can explain the positive MEL and MC, while the negative MC can be explained by the bipolaron model. Finally, we present a possibility to complete an all-organic PLEC magnetic sensor by using an inkjet printer.

Lead halide perovskites: Crystal-liquid duality, phonon glass electron crystals, and large polaron formation

PubMed Central

Miyata, Kiyoshi; Atallah, Timothy L.; Zhu, X.-Y.

2017-01-01

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. This “crystal-liquid” duality implies that lead halide perovskites belong to phonon glass electron crystals, a class of materials believed to make the most efficient thermoelectrics. We show that the crystal-liquid duality and the resulting dielectric response are responsible for large polaron formation and screening of charge carriers, leading to defect tolerance, moderate charge carrier mobility, and radiative recombination properties. Large polaron formation, along with the phonon glass character, may also explain the marked reduction in hot carrier cooling rates in these materials. PMID:29043296

An Ab Initio Exciton Model Including Charge-Transfer Excited States

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Xin; Parrish, Robert M.; Liu, Fang

Here, the Frenkel exciton model is a useful tool for theoretical studies of multichromophore systems. We recently showed that the exciton model could be used to coarse-grain electronic structure in multichromophoric systems, focusing on singly excited exciton states. However, our previous implementation excluded charge-transfer excited states, which can play an important role in light-harvesting systems and near-infrared optoelectronic materials. Recent studies have also emphasized the significance of charge-transfer in singlet fission, which mediates the coupling between the locally excited states and the multiexcitonic states. In this work, we report on an ab initio exciton model that incorporates charge-transfer excited statesmore » and demonstrate that the model provides correct charge-transfer excitation energies and asymptotic behavior. Comparison with TDDFT and EOM-CC2 calculations shows that our exciton model is robust with respect to system size, screening parameter, and different density functionals. Inclusion of charge-transfer excited states makes the exciton model more useful for studies of singly excited states and provides a starting point for future construction of a model that also includes double-exciton states.« less

An Ab Initio Exciton Model Including Charge-Transfer Excited States

DOE PAGES

Li, Xin; Parrish, Robert M.; Liu, Fang; ...

2017-06-15

Here, the Frenkel exciton model is a useful tool for theoretical studies of multichromophore systems. We recently showed that the exciton model could be used to coarse-grain electronic structure in multichromophoric systems, focusing on singly excited exciton states. However, our previous implementation excluded charge-transfer excited states, which can play an important role in light-harvesting systems and near-infrared optoelectronic materials. Recent studies have also emphasized the significance of charge-transfer in singlet fission, which mediates the coupling between the locally excited states and the multiexcitonic states. In this work, we report on an ab initio exciton model that incorporates charge-transfer excited statesmore » and demonstrate that the model provides correct charge-transfer excitation energies and asymptotic behavior. Comparison with TDDFT and EOM-CC2 calculations shows that our exciton model is robust with respect to system size, screening parameter, and different density functionals. Inclusion of charge-transfer excited states makes the exciton model more useful for studies of singly excited states and provides a starting point for future construction of a model that also includes double-exciton states.« less

An Ab Initio Exciton Model Including Charge-Transfer Excited States.

PubMed

Li, Xin; Parrish, Robert M; Liu, Fang; Kokkila Schumacher, Sara I L; Martínez, Todd J

2017-08-08

The Frenkel exciton model is a useful tool for theoretical studies of multichromophore systems. We recently showed that the exciton model could be used to coarse-grain electronic structure in multichromophoric systems, focusing on singly excited exciton states [ Acc. Chem. Res. 2014 , 47 , 2857 - 2866 ]. However, our previous implementation excluded charge-transfer excited states, which can play an important role in light-harvesting systems and near-infrared optoelectronic materials. Recent studies have also emphasized the significance of charge-transfer in singlet fission, which mediates the coupling between the locally excited states and the multiexcitonic states. In this work, we report on an ab initio exciton model that incorporates charge-transfer excited states and demonstrate that the model provides correct charge-transfer excitation energies and asymptotic behavior. Comparison with TDDFT and EOM-CC2 calculations shows that our exciton model is robust with respect to system size, screening parameter, and different density functionals. Inclusion of charge-transfer excited states makes the exciton model more useful for studies of singly excited states and provides a starting point for future construction of a model that also includes double-exciton states.

Electrical Tuning of Interlayer Exciton Gases in WSe2 Bilayers.

PubMed

Wang, Zefang; Chiu, Yi-Hsin; Honz, Kevin; Mak, Kin Fai; Shan, Jie

2018-01-10

van der Waals heterostructures formed by stacking two-dimensional atomic crystals are a unique platform for exploring new phenomena and functionalities. Interlayer excitons, bound states of spatially separated electron-hole pairs in van der Waals heterostructures, have demonstrated potential for rich valley physics and optoelectronics applications and been proposed to facilitate high-temperature superfluidity. Here, we demonstrate highly tunable interlayer excitons by an out-of-plane electric field in homobilayers of transition metal dichalcogenides. Continuous tuning of the exciton dipole from negative to positive orientation has been achieved, which is not possible in heterobilayers due to the presence of large built-in interfacial electric fields. A large linear field-induced redshift up to ∼100 meV has been observed in the exciton resonance energy. The Stark effect is accompanied by an enhancement of the exciton recombination lifetime by more than two orders of magnitude to >20 ns. The long recombination lifetime has allowed the creation of an interlayer exciton gas with density as large as 1.2 × 10 11 cm -2 by moderate continuous-wave optical pumping. Our results have paved the way for the realization of degenerate exciton gases in atomically thin semiconductors.

Is there any Exciton (bottleneck) in an Excitonic Solar Cell: Revisiting the Prospects of Single-Semiconductor OPV

NASA Astrophysics Data System (ADS)

Alam, Muhammad

2014-03-01

The discovery dye sensitized and bulk heterojunction (BHJ) solar cells in early 1990s introduced a new class of PV technology that rely on (i) distributed photogeneration of excitons, (ii) dissociation of excitons into free carriers by the heterojunction between two organic semiconductors (OSC), and (iii) collection of free carriers through electron and hole transport layers. The success of the approach is undisputed: the highest efficiency OPV cells have all relied on variants of BHJ approach. Yet, three concerns related to the use of a pair of OSCs, namely, low Voc, process sensitivity, and reliability, suggest that the technology may never achieve efficiency-variability-reliability metrics comparable to inorganic solar cells. This encourages a reconsideration of the prospects of Single semiconductor OPV (SS-OPV), a system presumably doomed by the exciton bottleneck. In this talk, we use an inverted SS-OPV to demonstrate how the historical SS-OPV experiments may have been misinterpreted. No one disputes the signature of excitons in polymer under narrowband excitation, but our experiments show that exciton dissociation need not be a bottleneck for OPV under broadband solar illumination. We demonstrate that an alternate collection-limited theory consistently interprets the classical and new experiments, resolves puzzles such as efficiency loss with increasing light intensity, and voltage-dependent reverse photo-current, etc. The theory and experiments suggest a new ``perovskite-like'' strategy to efficiency-variability-reliability of organic solar cells. The work was supported by the Columbia DOE-EFRC (DE-SC0001085) and NSF-NCN (EEC-0228390).

Organic-Inorganic Composites of Semiconductor Nanocrystals for Efficient Excitonics.

PubMed

Guzelturk, Burak; Demir, Hilmi Volkan

2015-06-18

Nanocomposites of colloidal semiconductor nanocrystals integrated into conjugated polymers are the key to soft-material hybrid optoelectronics, combining advantages of both plastics and particles. Synergic combination of the favorable properties in the hybrids of colloidal nanocrystals and conjugated polymers offers enhanced performance and new functionalities in light-generation and light-harvesting applications, where controlling and mastering the excitonic interactions at the nanoscale are essential. In this Perspective, we highlight and critically consider the excitonic interactions in the organic-inorganic nanocomposites to achieve highly efficient exciton transfer through rational design of the nanocomposites. The use of strong excitonic interactions in optoelectronic devices can trigger efficiency breakthroughs in hybrid optoelectronics.

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 diffusion in disordered small molecules for organic photovoltaics: insights from first-principles simulations.

PubMed

Li, Z; Zhang, X; Lu, G

2014-05-07

Exciton diffusion in small molecules 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione [DPP(TBFu)2] is studied using first-principles simulations. We have examined dependence of exciton diffusion on structure disorder, temperature and exciton energy. We find that exciton diffusion length and diffusivity increase with structural order, temperature and the initial exciton energy. Compared to conjugated polymer poly(3-hexylthiophene) (P3HT), DPP(TBFu)2 small molecules exhibit a much higher exciton diffusivity, but a shorter lifetime. The exciton diffusion length in DPP(TBFu)2 is 50% longer than that in P3HT, yielding a higher exciton harvesting efficiency; the physical origin behind these differences is discussed. The time evolutions of exciton energy, electron-hole distance, and exciton localization are explored, and the widely speculated exciton diffusion mechanism is confirmed theoretically. The connection between exciton diffusion and carrier mobilities is also studied. Finally we point out the possibility to estimate exciton diffusivity by measuring carrier mobilities under AC electric fields.

Resolving ultrafast exciton migration in organic solids at the nanoscale

NASA Astrophysics Data System (ADS)

Penwell, Samuel B.; Ginsberg, Lucas D. S.; Noriega, Rodrigo; Ginsberg, Naomi S.

2017-11-01

Effectiveness of molecular-based light harvesting relies on transport of excitons to charge-transfer sites. Measuring exciton migration, however, has been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. Instead of using bulk substrate quenching methods, here we define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native nanometre and picosecond scales. By transforming stimulated emission depletion microscopy into a time-resolved ultrafast approach, we measure a 16-nm migration length in poly(2,5-di(hexyloxy)cyanoterephthalylidene) conjugated polymer films. Combined with Monte Carlo exciton hopping simulations, we show that migration in these films is essentially diffusive because intrinsic chromophore energetic disorder is comparable to chromophore inhomogeneous broadening. Our approach will enable previously unattainable correlation of local material structure to exciton migration character, applicable not only to photovoltaic or display-destined organic semiconductors but also to explaining the quintessential exciton migration exhibited in photosynthesis.

Resolving ultrafast exciton migration in organic solids at the nanoscale.

PubMed

Penwell, Samuel B; Ginsberg, Lucas D S; Noriega, Rodrigo; Ginsberg, Naomi S

2017-11-01

Effectiveness of molecular-based light harvesting relies on transport of excitons to charge-transfer sites. Measuring exciton migration, however, has been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. Instead of using bulk substrate quenching methods, here we define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native nanometre and picosecond scales. By transforming stimulated emission depletion microscopy into a time-resolved ultrafast approach, we measure a 16-nm migration length in poly(2,5-di(hexyloxy)cyanoterephthalylidene) conjugated polymer films. Combined with Monte Carlo exciton hopping simulations, we show that migration in these films is essentially diffusive because intrinsic chromophore energetic disorder is comparable to chromophore inhomogeneous broadening. Our approach will enable previously unattainable correlation of local material structure to exciton migration character, applicable not only to photovoltaic or display-destined organic semiconductors but also to explaining the quintessential exciton migration exhibited in photosynthesis.

One-Dimensional Singlet Exciton Diffusion in Poly(3-hexylthiophene) Crystalline Domains.

PubMed

Tamai, Yasunari; Matsuura, Yuu; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo

2014-01-16

Singlet exciton dynamics in crystalline domains of regioregular poly(3-hexylthiophene) (P3HT) films was studied by transient absorption spectroscopy. Upon the selective excitation of crystalline P3HT at the absorption edge, no red shift of the singlet exciton band was observed with an elapse of time, suggesting singlet exciton dynamics in relatively homogeneous P3HT crystalline domains without downhill relaxation in the energetic disorder. Even under such selective excitation conditions, the annihilation rate coefficient γ(t) was still dependent on time, γ(t) ∝ t(-1/2), which is attributed to anisotropic exciton diffusion in P3HT crystalline domains. From the annihilation rate coefficient, the singlet exciton diffusion coefficient D and exciton diffusion length LD in the crystalline domains were evaluated to be 7.9 × 10(-3) cm(2) s(-1) and 20 nm, respectively. The origin of the time-dependent exciton dynamics is discussed in terms of dimensionality.

Single-exciton optical gain in semiconductor nanocrystals.

PubMed

Klimov, Victor I; Ivanov, Sergei A; Nanda, Jagjit; Achermann, Marc; Bezel, Ilya; McGuire, John A; Piryatinski, Andrei

2007-05-24

Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size--owing to the quantum-confinement effect--and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant ( approximately 100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.

Exciton size and binding energy limitations in one-dimensional organic materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kraner, S., E-mail: stefan.kraner@iapp.de; Koerner, C.; Leo, K.

2015-12-28

In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent densitymore » functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.« less

Exciton size and binding energy limitations in one-dimensional organic materials.

PubMed

Kraner, S; Scholz, R; Plasser, F; Koerner, C; Leo, K

2015-12-28

In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.

Two Impurities in a Bose-Einstein Condensate: From Yukawa to Efimov Attracted Polarons

NASA Astrophysics Data System (ADS)

Naidon, Pascal

2018-04-01

The well-known Yukawa and Efimov potentials are two different mediated interaction potentials. The first one arises in quantum field theory from the exchange of virtual particles. The second one is mediated by a real particle resonantly interacting with two other particles. This Letter shows how two impurities immersed in a Bose-Einstein condensate can exhibit both phenomena. For a weak attraction with the condensate, the two impurities form two polarons that interact through a weak Yukawa attraction mediated by virtual excitations. For a resonant attraction with the condensate, the exchanged excitation becomes a real boson and the mediated interaction changes to a strong Efimov attraction that can bind the two polarons. The resulting bipolarons turn into in-medium Efimov trimers made of the two impurities and one boson. Evidence of this physics could be seen in ultracold mixtures of atoms.

Optical phonon modes and polaron related parameters in GaxIn1-xP

NASA Astrophysics Data System (ADS)

Bouarissa, N.; Algarni, H.; Al-Hagan, O. A.; Khan, M. A.; Alhuwaymel, T. F.

2018-02-01

Based on a pseudopotential approach under the virtual crystal approximation that includes the effect of compositional disorder, the optical lattice vibration frequencies and polaron related parameters in zinc-blende GaxIn1-xP have been studied. Our findings showed generally reasonably good accord with data in the literature. Other case, our results are predictions. The composition dependence of longitudinal optical (LO) and transverse optical (TO) phonon modes, LO-TO splittings, Frӧhlich coupling parameter, Debye temperature of LO phonon frequency, and polaron effective mass has been analyzed and discussed. While a non-monotonic behavior has been noticed for the LO and TO phonon frequencies versus Ga concentration x, a monotonic behavior has been observed for the rest of the features of interest. The information derived from this investigation may be useful for optoelectronic technological applications.

Hole-exciton interaction induced high field decay of magneto-electroluminescence in Alq3-based organic light-emitting diodes at room temperature

NASA Astrophysics Data System (ADS)

Zhang, Tingting; Holford, D. F.; Gu, Hang; Kreouzis, T.; Zhang, Sijie; Gillin, W. P.

2016-01-01

The magnetic field effects on the electroluminescence of aluminium tris-(8-hydroxyqinoline) (Alq3) based organic light emitting diodes have been investigated by varying the electron/hole ratio in the emissive layer. Experimental results reveal that a negative high field effect in the magneto-electroluminescence (MEL) can be found in devices with very low triplet exciton concentration at room temperature. This suggests triplet-triplet annihilation cannot be used to explain the negative high field MEL in the Alq3 system. Our results suggest that hole-exciton interaction may be the origin of the negative high field MEL and also, in parallel with this interaction, there is also the more common positive high field process occurring which has been tentatively attributed to electron-exciton interactions. The competition between these different processes decides the final shape of the MEL at high fields.

Decay channels of Al L sub 2,3 excitons and the absence of O K excitons in. alpha. -Al sub 2 O sub 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

O'Brien, W.L.; Jia, J.; Dong, Q.

1991-12-15

The Al {ital L}{sub 2,3} and O {ital K} thresholds for single-crystal {alpha}-Al{sub 2}O{sub 3} have been studied by photoemission. Energy-distribution curves, constant-initial-state (CIS), and constant-final-state (CFS) spectra are reported and compared to the absorption spectrum reported previously. An exciton appears as a doublet at threshold in the Al {ital L}{sub 2,3} CFS, CIS, and absorption spectra. The details of the Al {ital L}{sub 2,3} CFS spectrum and absorption spectrum are similar, while the exciton is the only feature present in the CIS spectrum. Comparisons of the various Al {ital L}{sub 2,3} spectra allow the probabilities of different exciton decaymore » channels to be determined. The probability for nonradiative direct recombination of the exciton is found to be (8{plus minus}1)% and the probability for Auger decay of the exciton is found to be (72{plus minus}20)%. Comparisons of the O {ital K} CIS and CFS spectra suggest that no O {ital K} exciton is formed.« less

Madelung and Hubbard interactions in polaron band model of doped organic semiconductors

PubMed Central

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

Density of states and extent of wave function: two crucial factors for small polaron hopping conductivity in 1D

NASA Astrophysics Data System (ADS)

Dimakogianni, M.; Simserides, C.; Triberis, G. P.

2013-07-01

We introduce a theoretical model to scrutinize the conductivity of small polarons in 1D disordered systems, focusing on two crucial - as will be demonstrated - factors: the density of states and the spatial extent of the electronic wave function. The investigation is performed for any temperature up to 300 K and under electric field of arbitrary strength up to the polaron dissociation limit. To accomplish this task, we combine analytical work with numerical calculations.

Multidimensional Coherent Spectroscopy of GaAs Excitons and Quantum Microcavity Polaritons

NASA Astrophysics Data System (ADS)

Wilmer, Brian L.

Light-matter interactions associated with excitons and exciton related complexes are explored in bulk GaAs and semiconductor microcavities using multidimensional coherent spectroscopy (MDCS). This approach provides rich spectra determining quantum excitation pathways, structural influences on the excitons, and coherence times. Polarization, excitation density, and temperature-dependent MDCS is performed on excitons in strained bulk GaAs layers, probing the coherent response for differing amounts of strain. Biaxial tensile strain lifts the degeneracy of heavy-hole and light-hole valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the heavy-/light- hole exciton peak splitting, induces an asymmetry in the off-diagonal interaction coherences, increases the difference in the heavy- and light- hole exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound light-, heavy- and mixed- hole biexcitons. 2DCS maps the anticrossing associated with normal mode splitting in a semiconductor microcavity. For a detuning range near zero, it is observed that there are two diagonal features related to the intra-action of exciton-polariton branches and two off-diagonal features related to coherent interaction between the polaritons. At negative detuning, the line shape properties of the diagonal intra-action features are distinguishable and can be associated with cavity-like and exciton-like modes. A biexcitonic companion feature is observed, shifted from the exciton feature by the biexciton binding energy. Closer to zero detuning, all features are enhanced and

Low temperature exciton dynamics and structural changes in perylene bisimide aggregates

NASA Astrophysics Data System (ADS)

Wolter, Steffen; Magnus Westphal, Karl; Hempel, Magdalena; Würthner, Frank; Kühn, Oliver; Lochbrunner, Stefan

2017-09-01

The temperature dependent exciton dynamics of J-aggregates formed by a perylene bisimide dye is investigated down to liquid nitrogen temperature (77 K) by femtosecond pump-probe spectroscopy. The analysis of the transient absorption data using a diffusion model for the excitons does not only reveal an overall decrease of the exciton mobility, but also a change in the dimensionality of the exciton transport at low temperatures. This change in dimensionality is further investigated by kinetic Monte Carlo simulations, identifying weakly interlinked one-dimensional aggregate chains as the most likely structure at low temperatures. This causes the exciton transport to be highly anisotropic.

Tailorable Exciton Transport in Doped Peptide–Amphiphile Assemblies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Solomon, Lee A.; Sykes, Matthew E.; Wu, Yimin A.

Light-harvesting biomaterials are an attractive target in photovoltaics, photocatalysis, and artificial photosynthesis. Through peptide self-assembly, complex nanostructures can be engineered to study the role of chromophore organization during light absorption and energy transport. To this end, we demonstrate the one-dimensional transport of excitons along naturally occurring, light-harvesting, Zn-protoporphyrin IX chromophores within self-assembled peptide-amphiphile nanofibers. The internal structure of the nanofibers induces packing of the porphyrins into linear chains. We find that this peptide assembly can enable long-range exciton diffusion, yet it also induces the formation of excimers between adjacent molecules, which serve as exciton traps. Electronic coupling between neighboring porphyrinmore » molecules is confirmed by various spectroscopic methods. The exciton diffusion process is then probed through transient photoluminescence and absorption measurements and fit to a model for one-dimensional hopping. Because excimer formation impedes exciton hopping, increasing the interchromophore spacing allows for improved diffusivity, which we control through porphyrin doping levels. We show that diffusion lengths of over 60 nm are possible at low porphyrin doping, representing an order of magnitude improvement over the highest doping fractions.« less

Tailorable Exciton Transport in Doped Peptide-Amphiphile Assemblies.

PubMed

Solomon, Lee A; Sykes, Matthew E; Wu, Yimin A; Schaller, Richard D; Wiederrecht, Gary P; Fry, H Christopher

2017-09-26

Light-harvesting biomaterials are an attractive target in photovoltaics, photocatalysis, and artificial photosynthesis. Through peptide self-assembly, complex nanostructures can be engineered to study the role of chromophore organization during light absorption and energy transport. To this end, we demonstrate the one-dimensional transport of excitons along naturally occurring, light-harvesting, Zn-protoporphyrin IX chromophores within self-assembled peptide-amphiphile nanofibers. The internal structure of the nanofibers induces packing of the porphyrins into linear chains. We find that this peptide assembly can enable long-range exciton diffusion, yet it also induces the formation of excimers between adjacent molecules, which serve as exciton traps. Electronic coupling between neighboring porphyrin molecules is confirmed by various spectroscopic methods. The exciton diffusion process is then probed through transient photoluminescence and absorption measurements and fit to a model for one-dimensional hopping. Because excimer formation impedes exciton hopping, increasing the interchromophore spacing allows for improved diffusivity, which we control through porphyrin doping levels. We show that diffusion lengths of over 60 nm are possible at low porphyrin doping, representing an order of magnitude improvement over the highest doping fractions.

Stability of the two-dimensional Fermi polaron

NASA Astrophysics Data System (ADS)

Griesemer, Marcel; Linden, Ulrich

2018-02-01

A system composed of an ideal gas of N fermions interacting with an impurity particle in two space dimensions is considered. The interaction between impurity and fermions is given in terms of two-body point interactions whose strength is determined by the two-body binding energy, which is a free parameter of the model. If the mass of the impurity is 1.225 times larger than the mass of a fermion, it is shown that the energy is bounded below uniformly in the number N of fermions. This result improves previous, N-dependent lower bounds, and it complements a recent, similar bound for the Fermi polaron in three space dimensions.

Excitonic AND Logic Gates on DNA Brick Nanobreadboards.

PubMed

Cannon, Brittany L; Kellis, Donald L; Davis, Paul H; Lee, Jeunghoon; Kuang, Wan; Hughes, William L; Graugnard, Elton; Yurke, Bernard; Knowlton, William B

2015-03-18

A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems.

Excitonic AND Logic Gates on DNA Brick Nanobreadboards

PubMed Central

2015-01-01

A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems. PMID:25839049

Reducing exciton binding energy by increasing thin film permittivity: an effective approach to enhance exciton separation efficiency in organic solar cells.

PubMed

Leblebici, Sibel Y; Chen, Teresa L; Olalde-Velasco, Paul; Yang, Wanli; Ma, Biwu

2013-10-23

Photocurrent generation in organic solar cells requires that excitons, which are formed upon light absorption, dissociate into free carriers at the interface of electron acceptor and donor materials. The high exciton binding energy, arising from the low permittivity of organic semiconductor films, generally causes low exciton separation efficiency and subsequently low power conversion efficiency. We demonstrate here, for the first time, that the exciton binding energy in B,O-chelated azadipyrromethene (BO-ADPM) donor films is reduced by increasing the film permittivity by blending the BO-ADPM donor with a high dielectric constant small molecule, camphoric anhydride (CA). Various spectroscopic techniques, including impedance spectroscopy, photon absorption and emission spectroscopies, as well as X-ray spectroscopies, are applied to characterize the thin film electronic and photophysical properties. Planar heterojunction solar cells are fabricated with a BO-ADPM:CA film as the electron donor and C60 as the acceptor. With an increase in the dielectric constant of the donor film from ∼4.5 to ∼11, the exciton binding energy is reduced and the internal quantum efficiency of the photovoltaic cells improves across the entire spectrum, with an ∼30% improvement in the BO-ADPM photoactive region.

Influence of Energetic Disorder on Exciton Lifetime and Photoluminescence Efficiency in Conjugated Polymers.

PubMed

Rörich, Irina; Mikhnenko, Oleksandr V; Gehrig, Dominik; Blom, Paul W M; Crăciun, N Irina

2017-02-16

Using time-resolved photoluminescence (TRPL) spectroscopy the exciton lifetime in a range of conjugated polymers is investigated. For poly(p-phenylenevinylene) (PPV)-based derivatives and a polyspirobifluorene copolymer (PSBF) we find that the exciton lifetime is correlated with the energetic disorder. Better ordered polymers exhibit a single exponential PL decay with exciton lifetimes of a few hundred picoseconds, whereas polymers with a larger degree of disorder show multiexponential PL decays with exciton lifetimes in the nanosecond regime. These observations are consistent with diffusion-limited exciton quenching at nonradiative recombination centers. The measured PL decay time reflects the time that excitons need to diffuse toward these quenching sites. Conjugated polymers with large energetic disorder and thus longer exciton lifetime also exhibit a higher photoluminescence quantum yield due to the slower exciton diffusion toward nonradiative quenching sites.

Cooperative Singlet and Triplet Exciton Transport in Tetracene Crystals Visualized by Ultrafast Microscopys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wan, Yan; Guo, Zhi; Zhu, Tong

2015-09-14

Singlet fission presents an attractive solution to overcome the Shockley–Queisser limit by generating two triplet excitons from one singlet exciton. Although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. We report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. Moreover, these measurements revealmore » a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.« less

Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy

NASA Astrophysics Data System (ADS)

Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

2015-10-01

Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

Superfluidity of dipolar excitons in a transition metal dichalcogenide double layer

NASA Astrophysics Data System (ADS)

Berman, Oleg L.; Kezerashvili, Roman Ya.

2017-09-01

We study formation and superfluidity of dipolar excitons in double layer heterostructures formed by two transition metal dichalcogenide (TMDC) atomically thin layers. Considering screening effects for an electron-hole interaction via the harmonic oscillator approximation for the Keldysh potential, the analytical expressions for the exciton energy spectrum and the mean field critical temperature Tc for the superfluidity are obtained. It is shown that binding energies of A excitons are larger than for B excitons. The mean field critical temperature for a two-component dilute exciton system in a TMDC double layer is analyzed and shown that the latter is an increasing function of the factor Q , determined by the effective masses of A and B excitons and their reduced mass. Comparison of the calculations for Tc performed by employing the Coulomb and Keldysh interactions demonstrates the importance of screening effects in TMDC.

Correction of the exciton Bohr radius in monolayer transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Li, Run-Ze; Dong, Xi-Ying; Li, Zhi-Qing; Wang, Zi-Wu

2018-07-01

We theoretically investigate the correction of exciton Bohr radius in monolayer transition metal dichalcogenides (TMDCs) on different polar substrates arising from the exciton-optical phonon coupling, in which both the intrinsic longitudinal optical phonon and surface optical phonon modes couple with the exciton are taken into account. We find that the exciton Bohr radius is enlarged markedly due to these coupling. Moreover, it can be changed on a large scale by modulating the polarizability of polar substrate and the internal distance between the monolayer TMDCs and polar substrate. Theoretical result provides a potential explanation for the variation of the exciton Bohr radius in experimental measurement.

Control of Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers

NASA Astrophysics Data System (ADS)

Wang, Gang

Current research on Transition Metal Dichalcogenide (TMD) Monolayers is stimulated by their strong light-matter interaction and the possibility to use the valley index in addition to spin as an information carrier. The direct gap interband transitions in TMD monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the K+ or K- valley in momentum space are induced. Very recently the optical generation of valley polarization and valley coherence (coherent superposition of valley states) have been reported. In this work we go a step further by discussing the coherent manipulation of valley states. Linearly polarized laser excitation prepares a coherent superposition of valley states. We demonstrate the control of the exciton valley coherence in monolayer WSe2 by tuning the applied magnetic field perpendicular to the monolayer plane. The induced valley Zeeman splitting between K+ and K- results in a change of the oscillation frequency of the superposition of the valley states, which corresponds to a rotation of the exciton valley pseudo-spin. We show rotation of this coherent superposition of valley states by angles as large as 30 degrees in applied fields up to 9T and discuss valley coherence in other TMD monolayer materials. This exciton valley coherence control on ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom. In collaboration with X. Marie, T. Amand, C. Robert, F. Cadiz, P. Renucci, B. Urbaszek (Université de Toulouse, INSA-CNRS-UPS, LPCNO, France), B. L. Liu (Institute of Physics, Chinese Academy of Sciences, China) and we acknowledge ERC Grant No. 306719.

Multistability of cavity exciton polaritons affected by the thermally generated exciton reservoir

NASA Astrophysics Data System (ADS)

Vishnevsky, D. V.; Solnyshkov, D. D.; Gippius, N. A.; Malpuech, G.

2012-04-01

Recently the buildup of an excitonic reservoir in a GaAs cavity polariton system under quasiresonant pumping has been demonstrated experimentally [S. S. Gavrilov , JETP Lett.JTPLA20021-364010.1134/S0021364010150105 92, 171 (2010)]. We show that in microcavities having a small Rabi splitting (typically GaAs cavities with a single quantum well), this reservoir can be efficiently populated by polariton-phonon scattering. We consider the influence of the exciton reservoir on the energy shifts of the resonantly pumped polariton modes. We show that the presence of this reservoir effectively reduces the spin anisotropy of the polariton-polariton interaction, in agreement with recent experimental measurements, where the multistability of cavity polaritons has been analyzed [T. K. Paraiso , Nat. Mater.1476-112210.1038/nmat2787 9, 655 (2010)].

Identification of a triplet pair intermediate in singlet exciton fission in solution

PubMed Central

Stern, Hannah L.; Musser, Andrew J.; Gelinas, Simon; Parkinson, Patrick; Herz, Laura M.; Bruzek, Matthew J.; Anthony, John; Friend, Richard H.; Walker, Brian J.

2015-01-01

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley–Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]—tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process. PMID:26060309

Electronic Two-Transition-Induced Enhancement of Emission Efficiency in Polymer Light-Emitting Diodes

PubMed Central

Chen, Ren-Ai; Wang, Cong; Li, Sheng; George, Thomas F.

2013-01-01

With the development of experimental techniques, effective injection and transportation of electrons is proven as a way to obtain polymer light-emitting diodes (PLEDs) with high quantum efficiency. This paper reveals a valid mechanism for the enhancement of quantum efficiency in PLEDs. When an external electric field is applied, the interaction between a negative polaron and triplet exciton leads to an electronic two-transition process, which induces the exciton to emit light and thus improve the emission efficiency of PLEDs. PMID:28809346

Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure

NASA Astrophysics Data System (ADS)

Cadiz, F.; Robert, C.; Courtade, E.; Manca, M.; Martinelli, L.; Taniguchi, T.; Watanabe, K.; Amand, T.; Rowe, A. C. H.; Paget, D.; Urbaszek, B.; Marie, X.

2018-04-01

We have combined spatially resolved steady-state micro-photoluminescence with time-resolved photoluminescence to investigate the exciton diffusion in a WSe2 monolayer encapsulated with hexagonal boron nitride. At 300 K, we extract an exciton diffusion length of LX = 0.36 ± 0.02 μm and an exciton diffusion coefficient of DX = 14.5 ± 2 cm2/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species: bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of LXD=1.5 ±0.02 μ m.

Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

NASA Astrophysics Data System (ADS)

Avella, Adolfo; Oleś, Andrzej M.; Horsch, Peter

2018-04-01

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the R1 -xCaxVO3 perovskites, where R = La, ⋯, Lu. A fundamental characteristic of these vanadium d2 compounds with partly filled t2 g valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-Tc cuprates at low defect concentration. We study the disordered electronic structure of such doped Mott-Hubbard insulators within the unrestricted Hartree-Fock approximation and, as a result, manage to explain the spectral features that occur in photoemission and inverse photoemission. In particular, (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are qualitatively well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect-states gap, which separates the highest occupied from the lowest unoccupied states, can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. These parameters provide a criterion and a comprehensive schematization for the insulator-metal transition in disordered systems. Our results provide clear indications that doped holes are bound to charged defects and form small spin-orbital polarons whose internal kinetic energy is responsible for the opening of the soft defect-states gap. We show that this kinetic gap survives disorder fluctuations of defects and is amplified by the long-range electron-electron interactions, whereas we observe a Coulomb singularity in the atomic limit. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio and by

Anisotropic Exciton Rabi Oscillation in Single Telecommunication-Band Quantum Dot

NASA Astrophysics Data System (ADS)

Miyazawa, Toshiyuki; Nakaoka, Toshihiro; Watanabe, Katsuyuki; Kumagai, Naoto; Yokoyama, Naoki; Arakawa, Yasuhiko

2010-06-01

Anisotropic Rabi oscillation in the exciton state in a single InAs/GaAs quantum dot (QD) was demonstrated in the telecommunication-band by selecting two orthogonal polarization angles of the excitation laser. Our InAs QDs were embedded in an intrinsic layer of an n-i-Schottky diode, which provides an electric field to extract photoexcited carriers from QDs. Owing to the potential anisotropy of QDs, the fine structure splitting (FSS) energy in the exciton state in single InAs QDs was ˜110 µeV, measured by polarization-resolved photocurrent spectroscopy. The ratio between two different Rabi frequencies, which reflect anisotropic dipole moments of two orthogonal exciton states, was estimated to be ˜1.2. This demonstrates that the selective control of two orthogonal polarized exciton states is a promising technique for exciton-based-quantum information devices compatible with fiber optics.

Understanding molecular structure dependence of exciton diffusion in conjugated small molecules

NASA Astrophysics Data System (ADS)

Li, Zi; Zhang, Xu; Woellner, Cristiano F.; Lu, Gang

2014-04-01

First-principles simulations are carried out to understand molecular structure dependence of exciton diffusion in a series of small conjugated molecules arranged in a disordered, crystalline, and blend structure. Exciton diffusion length (LD), lifetime, and diffusivity in four diketopyrrolopyrrole derivatives are calculated and the results compare very well with experimental values. The correlation between exciton diffusion and molecular structure is examined in detail. In the disordered molecule structure, a longer backbone length leads to a shorter exciton lifetime and a higher exciton diffusivity, but it does not change LD substantially. Removal of the end alkyl chains or the extra branch on the side alkyl chains reduces LD. In the crystalline structure, exciton diffusion exhibits a strong anisotropy whose origin can be elucidated from the intermolecular transition density interaction point of view. In the blend structure, LD increases with the crystalline ratios, which are estimated and consistent with the experimental results.

Many-body effects and excitonic features in 2D biphenylene carbon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lüder, Johann, E-mail: johann.luder@physics.uu.se; Puglia, Carla; Eriksson, Olle

2016-01-14

The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV asmore » well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon’s excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.« less

How Markovian is exciton dynamics in purple bacteria?

NASA Astrophysics Data System (ADS)

Vaughan, Felix; Linden, Noah; Manby, Frederick R.

2017-03-01

We investigate the extent to which the dynamics of excitons in the light-harvesting complex LH2 of purple bacteria can be described using a Markovian approximation. To analyse the degree of non-Markovianity in these systems, we introduce a measure based on fitting Lindblad dynamics, as well as employing a recently introduced trace-distance measure. We apply these measures to a chromophore-dimer model of exciton dynamics and use the hierarchical equation-of-motion method to take into account the broad, low-frequency phonon bath. With a smooth phonon bath, small amounts of non-Markovianity are present according to the trace-distance measure, but the dynamics is poorly described by a Lindblad master equation unless the excitonic dimer coupling strength is modified. Inclusion of underdamped, high-frequency modes leads to significant deviations from Markovian evolution in both measures. In particular, we find that modes that are nearly resonant with gaps in the excitonic spectrum produce dynamics that deviate most strongly from the Lindblad approximation, despite the trace distance measuring larger amounts of non-Markovianity for higher frequency modes. Overall we find that the detailed structure in the high-frequency region of the spectral density has a significant impact on the nature of the dynamics of excitons.

Interface exciton at lateral heterojunction of monolayer semiconductors

NASA Astrophysics Data System (ADS)

Lau, Ka Wai; Gong, Zhirui; Yu, Hongyi; Yao, Wang

Heterostructures based on 2D transition metal dichalcogenides (TMDs) have attracted extensive research interest recently due to the appealing physical properties of TMDs and new geometries for forming heterostructures. One such heterostructure is the lateral heterojunctions seamlessly formed in a monolayer crystal between two different types of TMDs, e.g. WSe2 and MoSe2. Such heterojunction exhibits a type II band alignment, with electrons (holes) having lower energy on the MoSe2 (WSe2) region. Here we present the study of an interface exciton at the 1D lateral junction of monolayer TMDs. With the distance dependent screening, we find that the interface exciton can have strong binding even though the electron-hole separation is much larger compare to the 2D excitons in TMDs. Neutral excitons are studied using two different approaches: the solution based on a real-space tight binding model, and the perturbation expansion in a hydrogen-like basis in an effective mass model. We have also used the latter method to study charged excitons at a MoSe2-WSe2-MoSe2 nanoscale junction. The work is supported by the Research Grant Council of Hong Kong (HKU705513P, HKU9/CRF/13G), the Croucher Foundation, and the HKU OYRA.

Theory of Exciton Energy Transfer in Carbon Nanotube Composites

DOE PAGES

Davoody, A. H.; Karimi, F.; Arnold, M. S.; ...

2016-06-24

Here, we compute the exciton transfer (ET) rate between semiconducting single-wall carbon nanotubes (SWNTs). We show that the main reasons for the wide range of measured ET rates reported in the literature are (1) exciton confinement in local quantum wells stemming from disorder in the environment and (2) exciton thermalization between dark and bright states due to intratube scattering. The SWNT excitonic states are calculated by solving the Bethe–Salpeter equation using tight-binding basis functions. The ET rates due to intertube Coulomb interaction are computed via Fermi’s golden rule. In pristine samples, the ET rate between parallel (bundled) SWNTs of similarmore » chirality is very high (~10 14 s –1), while the ET rate for dissimilar or nonparallel tubes is considerably lower (~10 12 s –1). Exciton confinement reduces the ET rate between same-chirality parallel SWNTs by 2 orders of magnitude but has little effect otherwise. Consequently, the ET rate in most measurements will be on the order of 10 12 s –1, regardless of the tube relative orientation or chirality. Exciton thermalization between bright and dark states further reduces the ET rate to ~10 11 s –1. The ET rate also increases with increasing temperature and decreases with increasing dielectric constant of the surrounding medium.« less

Theory of Exciton Energy Transfer in Carbon Nanotube Composites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davoody, A. H.; Karimi, F.; Arnold, M. S.

Here, we compute the exciton transfer (ET) rate between semiconducting single-wall carbon nanotubes (SWNTs). We show that the main reasons for the wide range of measured ET rates reported in the literature are (1) exciton confinement in local quantum wells stemming from disorder in the environment and (2) exciton thermalization between dark and bright states due to intratube scattering. The SWNT excitonic states are calculated by solving the Bethe–Salpeter equation using tight-binding basis functions. The ET rates due to intertube Coulomb interaction are computed via Fermi’s golden rule. In pristine samples, the ET rate between parallel (bundled) SWNTs of similarmore » chirality is very high (~10 14 s –1), while the ET rate for dissimilar or nonparallel tubes is considerably lower (~10 12 s –1). Exciton confinement reduces the ET rate between same-chirality parallel SWNTs by 2 orders of magnitude but has little effect otherwise. Consequently, the ET rate in most measurements will be on the order of 10 12 s –1, regardless of the tube relative orientation or chirality. Exciton thermalization between bright and dark states further reduces the ET rate to ~10 11 s –1. The ET rate also increases with increasing temperature and decreases with increasing dielectric constant of the surrounding medium.« less

Localized diabatization applied to excitons in molecular crystals

NASA Astrophysics Data System (ADS)

Jin, Zuxin; Subotnik, Joseph E.

2017-06-01

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localized diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. These methods should be very useful for studying energy transfer through solids with ab initio calculations.

Strong quantum coherence between Fermi liquid Mahan excitons

DOE PAGES

Paul, J.; Stevens, C. E.; Liu, C.; ...

2016-04-14

In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called “Mahan excitons.” The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the opticalmore » Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Furthermore, time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.« less

Localized diabatization applied to excitons in molecular crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jin, Zuxin; Subotnik, Joseph E.

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localizedmore » diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. Lastly, these methods should be very useful for studying energy transfer through solids with ab initio calculations.« less

Strong Quantum Coherence between Fermi Liquid Mahan Excitons

NASA Astrophysics Data System (ADS)

Paul, J.; Stevens, C. E.; Liu, C.; Dey, P.; McIntyre, C.; Turkowski, V.; Reno, J. L.; Hilton, D. J.; Karaiskaj, D.

2016-04-01

In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

Strong Quantum Coherence between Fermi Liquid Mahan Excitons.

PubMed

Paul, J; Stevens, C E; Liu, C; Dey, P; McIntyre, C; Turkowski, V; Reno, J L; Hilton, D J; Karaiskaj, D

2016-04-15

In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

Localized diabatization applied to excitons in molecular crystals

DOE PAGES

Jin, Zuxin; Subotnik, Joseph E.

2017-06-28

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localizedmore » diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. Lastly, these methods should be very useful for studying energy transfer through solids with ab initio calculations.« less

Large-k exciton dynamics in GaN epilayers: Nonthermal and thermal regimes

NASA Astrophysics Data System (ADS)

Vinattieri, Anna; Bogani, Franco; Cavigli, Lucia; Manzi, Donatella; Gurioli, Massimo; Feltin, Eric; Carlin, Jean-François; Martin, Denis; Butté, Raphaël; Grandjean, Nicolas

2013-02-01

We present a detailed investigation performed at low temperature (T<50 K) concerning the exciton dynamics in GaN epilayers grown on c-plane sapphire substrates, focusing on the exciton formation and the transition from the nonthermal to the thermal regime. The time-resolved kinetics of longitudinal-optical-phonon replicas is used to address the energy relaxation in the excitonic band. From picosecond time-resolved spectra, we bring evidence for a long lasting nonthermal excitonic distribution, which accounts for the first 50 ps. Such a behavior is confirmed in different experimental conditions when both nonresonant and resonant excitations are used. At low excitation power density, the exciton formation and their subsequent thermalization are dominated by impurity scattering rather than by acoustic phonon scattering. The estimate of the average energy of the excitons as a function of delay after the excitation pulse provides information on the relaxation time, which describes the evolution of the exciton population to the thermal regime.

Organic photovoltaics: elucidating the ultra-fast exciton dissociation mechanism in disordered materials.

PubMed

Heitzer, Henry M; Savoie, Brett M; Marks, Tobin J; Ratner, Mark A

2014-07-14

Organic photovoltaics (OPVs) offer the opportunity for cheap, lightweight and mass-producible devices. However, an incomplete understanding of the charge generation process, in particular the timescale of dynamics and role of exciton diffusion, has slowed further progress in the field. We report a new Kinetic Monte Carlo model for the exciton dissociation mechanism in OPVs that addresses the origin of ultra-fast (<1 ps) dissociation by incorporating exciton delocalization. The model reproduces experimental results, such as the diminished rapid dissociation with increasing domain size, and also lends insight into the interplay between mixed domains, domain geometry, and exciton delocalization. Additionally, the model addresses the recent dispute on the origin of ultra-fast exciton dissociation by comparing the effects of exciton delocalization and impure domains on the photo-dynamics.This model provides insight into exciton dynamics that can advance our understanding of OPV structure-function relationships. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Anisotropic Exciton Rabi Oscillation in Single Telecommunication-Band Quantum Dot

NASA Astrophysics Data System (ADS)

Toshiyuki Miyazawa,; Toshihiro Nakaoka,; Katsuyuki Watanabe,; Naoto Kumagai,; Naoki Yokoyama,; Yasuhiko Arakawa,

2010-06-01

Anisotropic Rabi oscillation in the exciton state in a single InAs/GaAs quantum dot (QD) was demonstrated in the telecommunication-band by selecting two orthogonal polarization angles of the excitation laser. Our InAs QDs were embedded in an intrinsic layer of an n-i-Schottky diode, which provides an electric field to extract photoexcited carriers from QDs. Owing to the potential anisotropy of QDs, the fine structure splitting (FSS) energy in the exciton state in single InAs QDs was ˜110 μeV, measured by polarization-resolved photocurrent spectroscopy. The ratio between two different Rabi frequencies, which reflect anisotropic dipole moments of two orthogonal exciton states, was estimated to be ˜1.2. This demonstrates that the selective control of two orthogonal polarized exciton states is a promising technique for exciton-based-quantum information devices compatible with fiber optics.

Optical nonlinearities of excitons in monolayer MoS2

NASA Astrophysics Data System (ADS)

Soh, Daniel B. S.; Rogers, Christopher; Gray, Dodd J.; Chatterjee, Eric; Mabuchi, Hideo

2018-04-01

We calculate linear and nonlinear optical susceptibilities arising from the excitonic states of monolayer MoS2 for in-plane light polarizations, using second-quantized bound and unbound exciton operators. Optical selection rules are critical for obtaining the susceptibilities. We derive the valley-chirality rule for the second-order harmonic generation in monolayer MoS2 and find that the third-order harmonic process is efficient only for linearly polarized input light while the third-order two-photon process (optical Kerr effect) is efficient for circularly polarized light using a higher order exciton state. The absence of linear absorption due to the band gap and the unusually strong two-photon third-order nonlinearity make the monolayer MoS2 excitonic structure a promising resource for coherent nonlinear photonics.

Simulations of singlet exciton diffusion in organic semiconductors: a review

DOE PAGES

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 simulationmore » results with those of experiment.« less

Spin-polaron nature of fermion quasiparticles and their d-wave pairing in cuprate superconductors

NASA Astrophysics Data System (ADS)

Val'kov, V. V.; Dzebisashvili, D. M.; Barabanov, A. F.

2016-11-01

In the framework of the spin-fermion model, to which the Emery model is reduced in the limit of strong electron correlations, it is shown that the fermion quasiparticles in cuprate high- T c superconductors (HTSCs) arise under a strong effect of exchange coupling between oxygen holes and spins of copper ions. This underlies the spin-polaron nature of fermion quasiparticles in cuprate HTSCs. The Cooper instability with respect to the d-wave symmetry of the order parameter is revealed for an ensemble of such quasiparticles. For the normal phase, the spin-polaron concept allows us to reproduce the fine details in the evolution of the Fermi surface with the changes in the doping level x observed in experiment for La2-xSrxCuO4. The calculated T-x phase diagram correlates well with the available experimental data for cuprate HTSCs.

Engineering and manipulating exciton wave packets

NASA Astrophysics Data System (ADS)

Zang, Xiaoning; Montangero, Simone; Carr, Lincoln D.; Lusk, Mark T.

2017-05-01

When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction, and spectral make-up that allows them to be selectively passed, rejected, or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage, and remove structured excitons comprises the foundation for optoexcitonic circuits with application to a wide range of quantum information, energy, and light-flow technologies. The paradigm is demonstrated using both tight-binding and time-domain density functional theory simulations.

Radiative energy transfer from MoS2 excitons to surface plasmons

NASA Astrophysics Data System (ADS)

Kang, Yimin; Li, Bowen; Fang, Zheyu

2017-12-01

In this work, we demonstrated the energy transfer process from few-layer MoS2 to gold dimer arrays via ultrafast pump-probe spectroscopy. With the overlap between the MoS2 exciton and the designed plasmon dipolar modes in the frequency domain, the exciton energy can be radiatively transferred to plasmonic structures, excited the localized surface plasmon resonance, and then enhanced the oscillation of coherent acoustic phonons. Power-dependent differential reflection signals and an analytical model based on the rate equation of exciton density were carried out to quantitatively study the energy transfer process. Our finding explores the energy flow between MoS2 excitons and surface plasmons, and can be contributed to the design of exciton-plasmon structures utilizing ultrathin materials.

Exciton-polaritons in cuprous oxide: Theory and comparison with experiment

NASA Astrophysics Data System (ADS)

Schweiner, Frank; Ertl, Jan; Main, Jörg; Wunner, Günter; Uihlein, Christoph

2017-12-01

The observation of giant Rydberg excitons in cuprous oxide (Cu2O ) up to a principal quantum number of n =25 by T. Kazimierczuk et al. [Nature (London) 514, 343 (2014), 10.1038/nature13832] inevitably raises the question whether these quasiparticles must be described within a multipolariton framework since excitons and photons are always coupled in the solid. In this paper we present the theory of exciton-polaritons in Cu2O . To this end we extend the Hamiltonian which includes the complete valence-band structure, the exchange interaction, and the central-cell corrections effects, and which has been recently deduced by F. Schweiner et al. [Phys. Rev. B 95, 195201 (2017), 10.1103/PhysRevB.95.195201], for finite values of the exciton momentum ℏ K . We derive formulas to calculate not only dipole but also quadrupole oscillator strengths when using the complete basis of F. Schweiner et al., which has recently been proven as a powerful tool to calculate exciton spectra. Very complex polariton spectra for the three orientations of K along the axes [001 ] , [110 ] , and [111 ] of high symmetry are obtained and a strong mixing of exciton states is reported. The main focus is on the 1 S ortho-exciton-polariton, for which pronounced polariton effects have been measured in experiments. We set up a 5 ×5 matrix model, which accounts for both the polariton effect and the K -dependent splitting, and which allows treating the anisotropic polariton dispersion for any direction of K . We especially discuss the dispersions for K being oriented in the planes perpendicular to [1 1 ¯0 ] and [111 ] , for which experimental transmission spectra have been measured. Furthermore, we compare our results with experimental values of the K -dependent splitting, the group velocity, and the oscillator strengths of this exciton-polariton. The results are in good agreement. This proves the validity of the 5 ×5 matrix model as a useful theoretical model for further investigations on the 1 S

Exciton-Delocalizing Ligands Can Speed Up Energy Migration in Nanocrystal Solids.

PubMed

Azzaro, Michael S; Dodin, Amro; Zhang, Diana Y; Willard, Adam P; Roberts, Sean T

2018-05-09

Researchers have long sought to use surface ligands to enhance energy migration in nanocrystal solids by decreasing the physical separation between nanocrystals and strengthening their electronic coupling. Exciton-delocalizing ligands, which possess frontier molecular orbitals that strongly mix with nanocrystal band-edge states, are well-suited for this role because they can facilitate carrier-wave function extension beyond the nanocrystal core, reducing barriers for energy transfer. This report details the use of the exciton-delocalizing ligand phenyldithiocarbamate (PDTC) to tune the transport rate and diffusion length of excitons in CdSe nanocrystal solids. A film composed of oleate-terminated CdSe nanocrystals is subjected to a solid-state ligand exchange to replace oleate with PDTC. Exciton migration in the films is subsequently investigated by femtosecond transient absorption. Our experiments indicate that the treatment of nanocrystal films with PDTC leads to rapid (∼400 fs) downhill energy migration (∼80 meV), while no such migration occurs in oleate-capped films. Kinetic Monte Carlo simulations allow us to extract both rates and length scales for exciton diffusion in PDTC-treated films. These simulations reproduce dynamics observed in transient absorption measurements over a range of temperatures and confirm excitons hop via a Miller-Abrahams mechanism. Importantly, our experiments and simulations show PDTC treatment increases the exciton hopping rate to 200 fs, an improvement of 5 orders of magnitude relative to oleate-capped films. This exciton hopping rate stands as one of the fastest determined for CdSe solids. The facile, room-temperature processing and improved transport properties offered by the solid-state exchange of exciton-delocalizing ligands show they offer promise for the construction of strongly coupled nanocrystal arrays.

Hole Polaron Diffusion in the Final Discharge Product of Lithium–Sulfur Batteries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Zhixiao; Balbuena, Perla B.; Mukherjee, Partha P.

Poor electronic conductivity of bulk lithium sulfide (Li 2S) is a critical challenge for the debilitating performance of the lithium–sulfur battery. In this study we focus on investigating the thermodynamic and kinetic properties of native defects in Li 2S based on a first-principles approach. It is found that the hole polaron p + can form in Li 2S by removing a 3p electron from an S 2– anion. The p + diffusion barrier is only 90 meV, which is much lower than the Li vacancy (V Li –) diffusion barrier. Hence p + has the potential to serve as amore » charge carrier in the discharge product. Once the vacancy–polaron complex (V Li -––2p +) forms, the charge transport will be hindered due to the relatively higher diffusion barrier of the complex. Heteroatom dopants, which can decrease the p + formation energy and increase V Li – formation energy, are expected to be introduced to the discharge product to improve the electronic conductivity.« less

Hole Polaron Diffusion in the Final Discharge Product of Lithium–Sulfur Batteries

DOE PAGES

Liu, Zhixiao; Balbuena, Perla B.; Mukherjee, Partha P.

2017-07-24

Poor electronic conductivity of bulk lithium sulfide (Li 2S) is a critical challenge for the debilitating performance of the lithium–sulfur battery. In this study we focus on investigating the thermodynamic and kinetic properties of native defects in Li 2S based on a first-principles approach. It is found that the hole polaron p + can form in Li 2S by removing a 3p electron from an S 2– anion. The p + diffusion barrier is only 90 meV, which is much lower than the Li vacancy (V Li –) diffusion barrier. Hence p + has the potential to serve as amore » charge carrier in the discharge product. Once the vacancy–polaron complex (V Li -––2p +) forms, the charge transport will be hindered due to the relatively higher diffusion barrier of the complex. Heteroatom dopants, which can decrease the p + formation energy and increase V Li – formation energy, are expected to be introduced to the discharge product to improve the electronic conductivity.« less

Frenkel-exciton decomposition analysis of circular dichroism and circularly polarized luminescence for multichromophoric systems.

PubMed

Shiraogawa, Takafumi; Ehara, Masahiro; Jurinovich, Sandro; Cupellini, Lorenzo; Mennucci, Benedetta

2018-06-15

Recently, a method to calculate the absorption and circular dichroism (CD) spectra based on the exciton coupling has been developed. In this work, the method was utilized for the decomposition of the CD and circularly polarized luminescence (CPL) spectra of a multichromophoric system into chromophore contributions for recently developed through-space conjugated oligomers. The method which has been implemented using rotatory strength in the velocity form and therefore it is gauge-invariant, enables us to evaluate the contribution from each chromophoric unit and locally excited state to the CD and CPL spectra of the total system. The excitonic calculations suitably reproduce the full calculations of the system, as well as the experimental results. We demonstrate that the interactions between electric transition dipole moments of adjacent chromophoric units are crucial in the CD and CPL spectra of the multichromophoric systems, while the interactions between electric and magnetic transition dipole moments are not negligible. © 2018 Wiley Periodicals, Inc. © 2018 Wiley Periodicals, Inc.

Many-body exciton states in self-assembled quantum dots coupled to a Fermi sea

NASA Astrophysics Data System (ADS)

Kleemans, N. A. J. M.; van Bree, J.; Govorov, A. O.; Keizer, J. G.; Hamhuis, G. J.; Nötzel, R.; Silov, A. Yu.; Koenraad, P. M.

2010-07-01

Many-body interactions give rise to fascinating physics such as the X-ray Fermi-edge singularity in metals, the Kondo effect in the resistance of metals with magnetic impurities and the fractional quantum Hall effect. Here we report the observation of striking many-body effects in the optical spectra of a semiconductor quantum dot interacting with a degenerate electron gas. A semiconductor quantum dot is an artificial atom, the properties of which can be controlled by means of a tunnel coupling between a metallic contact and the quantum dot. Previous studies concern mostly the regime of weak tunnel coupling, whereas here we investigate the regime of strong coupling, which markedly modifies the optical spectra. In particular we observe two many-body exciton states: Mahan and hybrid excitons. These experimental results open the route towards the observation of a tunable Kondo effect in excited states of semiconductors and are of importance for the technological implementation of quantum dots in devices for quantum information processing.

Optical orientation of the homogeneous nonequilibrium Bose-Einstein condensate of exciton polaritons

NASA Astrophysics Data System (ADS)

Korenev, V. L.

2012-07-01

A simple model, describing the steady state of the nonequilibrium polarization of a homogeneous Bose-Einstein condensate of exciton polaritons, is considered. It explains the suppression of spin splitting of a nonequilibrium polariton condensate in an external magnetic field, the linear polarization, the linear-to-circular polarization conversion, and the unexpected sign of the circular polarization of the condensate all on equal footing. It is shown that inverse effects are possible, to wit, spontaneous circular polarization and the enhancement of spin splitting of a nonequilibrium condensate of polaritons.

Photoinduced Bandgap Renormalization and Exciton Binding Energy Reduction in WS2.

PubMed

Cunningham, Paul D; Hanbicki, Aubrey T; McCreary, Kathleen M; Jonker, Berend T

2017-12-26

Strong Coulomb attraction in monolayer transition metal dichalcogenides gives rise to tightly bound excitons and many-body interactions that dominate their optoelectronic properties. However, this Coulomb interaction can be screened through control of the surrounding dielectric environment as well as through applied voltage, which provides a potential means of tuning the bandgap, exciton binding energy, and emission wavelength. Here, we directly show that the bandgap and exciton binding energy can be optically tuned by means of the intensity of the incident light. Using transient absorption spectroscopy, we identify a sub-picosecond decay component in the excited-state dynamics of WS 2 that emerges for incident photon energies above the A-exciton resonance, which originates from a nonequilibrium population of charge carriers that form excitons as they cool. The generation of this charge-carrier population exhibits two distinct energy thresholds. The higher threshold is coincident with the onset of continuum states and therefore provides a direct optical means of determining both the bandgap and exciton binding energy. Using this technique, we observe a reduction in the exciton binding energy from 310 ± 30 to 220 ± 20 meV as the excitation density is increased from 3 × 10 11 to 1.2 × 10 12 photons/cm 2 . This reduction is due to dynamic dipolar screening of Coulomb interactions by excitons, which is the underlying physical process that initiates bandgap renormalization and leads to the insulator-metal transition in monolayer transition metal dichalcogenides.

Generalized formula for electron emission taking account of the polaron effect

NASA Astrophysics Data System (ADS)

Barengolts, Yu A.; Beril, S. I.; Barengolts, S. A.

2018-01-01

A generalized formula is derived for the electron emission current as a function of temperature, field, and electron work function in a metal-dielectric system that takes account of the quantum nature of the image forces. In deriving the formula, the Fermi-Dirac distribution for electrons in a metal and the quantum potential of the image obtained in the context of electron polaron theory are used.

Resolving ultrafast exciton migration in organic solids at the nanoscale

NASA Astrophysics Data System (ADS)

Ginsberg, Naomi

The migration of Frenkel excitons, tightly-bound electron-hole pairs, in photosynthesis and in organic semiconducting films is critical to the efficiency of natural and artificial light harvesting. While these materials exhibit a high degree of structural heterogeneity on the nanoscale, traditional measurements of exciton migration lengths are performed on bulk samples. Since both the characteristic length scales of structural heterogeneity and the reported bulk diffusion lengths are smaller than the optical diffraction limit, we adapt far-field super-resolution fluorescence imaging to uncover the correlations between the structural and energetic landscapes that the excitons explore. By combining the ultrafast super-resolved measurements with exciton hopping simulations we furthermore specify the nature (in addition to the extent) of exciton migration as a function of the intrinsic and ensemble chromophore energy scales that determine a spatio-energetic landscape for migration. In collaboration with: Samuel Penwell, Lucas Ginsberg, University of California, Berkeley and Rodrigo Noriega University of Utah.

Exciton center-of-mass localization and dielectric environment effect in monolayer WS2

NASA Astrophysics Data System (ADS)

Hichri, Aïda; Ben Amara, Imen; Ayari, Sabrine; Jaziri, Sihem

2017-06-01

The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. In this paper, we study the strong localization of exciton center-of-mass motion within random potential fluctuations caused by the monolayer defects. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping, and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.

Exciton confinement in strain-engineered metamorphic InAs/I nxG a1 -xAs quantum dots

NASA Astrophysics Data System (ADS)

Khattak, S. A.; Hayne, M.; Huang, J.; Vanacken, J.; Moshchalkov, V. V.; Seravalli, L.; Trevisi, G.; Frigeri, P.

2017-11-01

We report a comprehensive study of exciton confinement in self-assembled InAs quantum dots (QDs) in strain-engineered metamorphic I nxG a1 -xAs confining layers on GaAs using low-temperature magnetophotoluminescence. As the lattice mismatch (strain) between QDs and confining layers (CLs) increases from 4.8% to 5.7% the reduced mass of the exciton increases, but saturates at higher mismatches. At low QD-CL mismatch there is clear evidence of spillover of the exciton wave function due to small localization energies. This is suppressed as the In content x in the CLs decreases (mismatch and localization energy increasing). The combined effects of low effective mass and wave-function spillover at high x result in a diamagnetic shift coefficient that is an order of magnitude larger than for samples where In content in the barrier is low (mismatch is high and localization energy is large). Finally, an anomalously small measured Bohr radius in samples with the highest x is attributed to a combination of thermalization due to low localization energy, and its enhancement with magnetic field, a mechanism which results in small dots in the ensemble dominating the measured Bohr radius.

Electrically Controlled Coherent Excitonic Steady States in Semiconductor Bilayers

NASA Astrophysics Data System (ADS)

Xie, Ming; MacDonald, Allan

Spatially indirect excitons are long lived bosonic quasiparticles that can form quasi-equilibrium condensed states. Optical access to these excitons has been limited by their small optical matrix elements. Here we propose a promising electrical process that can be used both to populate and to probe fluids of indirect excitons, and is analogous to the crossed Andreev reflection (CAR) process of Cooper pairs in superconductors. We consider vertically stacked multilayer heterostructures containing two transition metal dichalcogenide (TMD) layers that host the indirect excitons, graphene layers on the top and the bottom of the heterostructure, and hBN tunnel barrier layers of variable thickness. When the bias voltage between the graphene leads is smaller than the indirect gap, tunneling between the graphene leads and the TMD hetero-bilayer is possible only through the CAR process. Both DC and low frequency AC bias cases are explored and establish that electrical measurements can be used to determine crucial properties such as the condensate density, interaction strength and CAR tunneling amplitudes. We have also proposed a way to electrically manipulate another type of bosonic quasiparticles, cavity exciton-polaritons, in a laterally contacted structure.

Tunable optical and excitonic properties of phosphorene via oxidation

NASA Astrophysics Data System (ADS)

Sadki, S.; Drissi, L. B.

2018-06-01

The optical properties and excitonic wave function of phosphorene oxides (PO) are studied using the first principle many-body Green function and the Bethe–Salpeter equation formalism. In this work, the optical properties are determined using ab initio calculations of the dielectric function. At the long wavelength limit q of EM wave (i.e. ), the dielectric function, the absorption spectrum, the lectivity, the electron energy loss spectra (EELS) and the wave function are calculated. The results show an excitonic binding energy of 818 meV with a bright exciton located in the armchair direction in pristine phosphorene. For PO, the arrangement of the oxygen atoms significantly influences the optical properties. In particular, the absorption spectrum is extended along the solar spectrum, with a high absorption coefficient observed in the dangling structures. The maximum lectivity values are observed for the high energies of the light spectrum. Moreover, the first EELS peak is located in the visible region in all the structures except for one configuration that exhibits the same behavior as pure phosphorene. Finally, the exciton effect reveals that all PO conformers have a dark exciton state, which is suitable for long-lived applications.

Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers

DOE PAGES

Wu, Fengcheng; Lovorn, Timothy; MacDonald, A. H.

2018-01-22

In this paper, we present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moire pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moire Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moire potential energy restores circular opticalmore » selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. Finally, we discuss the possibility of using the moire pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.« less

Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers

NASA Astrophysics Data System (ADS)

Wu, Fengcheng; Lovorn, Timothy; MacDonald, A. H.

2018-01-01

We present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moiré pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moiré Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moiré potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. We discuss the possibility of using the moiré pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.

Direct Imaging of Exciton Transport in Tubular Porphyrin Aggregates by Ultrafast Microscopy.

PubMed

Wan, Yan; Stradomska, Anna; Knoester, Jasper; Huang, Libai

2017-05-31

Long-range exciton transport is a key challenge in achieving efficient solar energy harvesting in both organic solar cells and photosynthetic systems. Self-assembled molecular aggregates provide the potential for attaining long-range exciton transport through strong intermolecular coupling. However, there currently lacks an experimental tool to directly characterize exciton transport in space and in time to elucidate mechanisms. Here we report a direct visualization of exciton diffusion in tubular molecular aggregates by transient absorption microscopy with ∼200 fs time resolution and ∼50 nm spatial precision. These direct measurements provide exciton diffusion constants of 3-6 cm 2 s -1 for the tubular molecular aggregates, which are 3-5 times higher than a theoretical lower bound obtained by assuming incoherent hopping. These results suggest that coherent effects play a role, despite the fact that exciton states near the band bottom crucial for transport are only weakly delocalized (over <10 molecules). The methods presented here establish a direct approach for unraveling the mechanisms and main parameters underlying exciton transport in large molecular assemblies.

Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Fengcheng; Lovorn, Timothy; MacDonald, A. H.

In this paper, we present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moire pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moire Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moire potential energy restores circular opticalmore » selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. Finally, we discuss the possibility of using the moire pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.« less

Orientation-Dependent Exciton-Plasmon Coupling in Embedded Organic/Metal Nanowire Heterostructures.

PubMed

Li, Yong Jun; Hong, Yan; Peng, Qian; Yao, Jiannian; Zhao, Yong Sheng

2017-10-24

The excitation of surface plasmons by optical emitters based on exciton-plasmon coupling is important for plasmonic devices with active optical properties. It has been theoretically demonstrated that the orientation of exciton dipole can significantly influence the coupling strength, yet systematic study of the coupling process in nanostructures is still hindered by the lack of proper material systems. In this work, we have experimentally investigated the orientation-dependent exciton-plasmon coupling in a rationally designed organic/metal nanowire heterostructure system. The heterostructures were prepared by inserting silver nanowires into crystalline organic waveguides during the self-assembly of dye molecules. Structures with different exciton orientations exhibited varying coupling efficiencies. The near-field exciton-plasmon coupling facilitates the design of nanophotonic devices based on the directional surface plasmon polariton propagations.

Selective Amplification of the Primary Exciton in a MoS_{2} Monolayer.

PubMed

Lee, Hyun Seok; Kim, Min Su; Jin, Youngjo; Han, Gang Hee; Lee, Young Hee; Kim, Jeongyong

2015-11-27

Optoelectronics applications for transition-metal dichalcogenides are still limited by weak light absorption and their complex exciton modes are easily perturbed by varying excitation conditions because they are inherent in atomically thin layers. Here, we propose a method of selectively amplifying the primary exciton (A^{0}) among the exciton complexes in monolayer MoS_{2} via cyclic reexcitation of cavity-free exciton-coupled plasmon propagation. This was implemented by partially overlapping a Ag nanowire on a MoS_{2} monolayer separated by a thin SiO_{2} spacer. Exciton-coupled plasmons in the nanowire enhance the A^{0} radiation in MoS_{2}. The cumulative amplification of emission enhancement by cyclic plasmon traveling reaches approximately twentyfold selectively for the A^{0}, while excluding other B exciton and multiexciton by significantly reduced band filling, without oscillatory spectra implying plasmonic cavity effects.

Room-Temperature Micron-Scale Exciton Migration in a Stabilized Emissive Molecular Aggregate.

PubMed

Caram, Justin R; Doria, Sandra; Eisele, Dörthe M; Freyria, Francesca S; Sinclair, Timothy S; Rebentrost, Patrick; Lloyd, Seth; Bawendi, Moungi G

2016-11-09

We report 1.6 ± 1 μm exciton transport in self-assembled supramolecular light-harvesting nanotubes (LHNs) assembled from amphiphillic cyanine dyes. We stabilize LHNs in a sucrose glass matrix, greatly reducing light and oxidative damage and allowing the observation of exciton-exciton annihilation signatures under weak excitation flux. Fitting to a one-dimensional diffusion model, we find an average exciton diffusion constant of 55 ± 20 cm 2 /s, among the highest measured for an organic system. We develop a simple model that uses cryogenic measurements of static and dynamic energetic disorder to estimate a diffusion constant of 32 cm 2 /s, in agreement with experiment. We ascribe large exciton diffusion lengths to low static and dynamic energetic disorder in LHNs. We argue that matrix-stabilized LHNS represent an excellent model system to study coherent excitonic transport.

Magnetic field induced polarization enhancement in monolayers of tungsten dichalcogenides: effects of temperature

NASA Astrophysics Data System (ADS)

Smoleński, T.; Kazimierczuk, T.; Goryca, M.; Molas, M. R.; Nogajewski, K.; Faugeras, C.; Potemski, M.; Kossacki, P.

2018-01-01

Optical orientation of localized/bound excitons is shown to be effectively enhanced by the application of magnetic fields as low as 20 mT in monolayer WS2. At low temperatures, the evolution of the polarization degree of different emission lines of monolayer WS2 with increasing magnetic fields is analyzed and compared to similar results obtained on a WSe2 monolayer. We study the temperature dependence of this effect up to T=60 K for both materials, focusing on the dynamics of the valley pseudospin relaxation. A rate equation model is used to analyze our data and from the analysis of the width of the polarization dip in magnetic field we conclude that the competition between the dark exciton pseudospin relaxation and the decay of the dark exciton population into the localized states are rather different in these two materials which are representative of the two extreme cases for the ratio of relaxation rate and depolarization rate.