Placing molecules with Bohr radius resolution using DNA origami
NASA Astrophysics Data System (ADS)
Funke, Jonas J.; Dietz, Hendrik
2016-01-01
Molecular self-assembly with nucleic acids can be used to fabricate discrete objects with defined sizes and arbitrary shapes. It relies on building blocks that are commensurate to those of biological macromolecular machines and should therefore be capable of delivering the atomic-scale placement accuracy known today only from natural and designed proteins. However, research in the field has predominantly focused on producing increasingly large and complex, but more coarsely defined, objects and placing them in an orderly manner on solid substrates. So far, few objects afford a design accuracy better than 5 nm, and the subnanometre scale has been reached only within the unit cells of designed DNA crystals. Here, we report a molecular positioning device made from a hinged DNA origami object in which the angle between the two structural units can be controlled with adjuster helices. To test the positioning capabilities of the device, we used photophysical and crosslinking assays that report the coordinate of interest directly with atomic resolution. Using this combination of placement and analysis, we rationally adjusted the average distance between fluorescent molecules and reactive groups from 1.5 to 9 nm in 123 discrete displacement steps. The smallest displacement step possible was 0.04 nm, which is slightly less than the Bohr radius. The fluctuation amplitudes in the distance coordinate were also small (±0.5 nm), and within a factor of two to three of the amplitudes found in protein structures.
NASA Astrophysics Data System (ADS)
Heyrovska, R.; Narayan, S.
2005-10-01
Recently, the ground state Bohr radius (aB) of hydrogen was shown to be divided into two Golden sections, aB,p = aB/ø2 and aB,e = aB/ø at the point of electrical neutrality, where ø = 1.618 is the Golden ratio (R. Heyrovska, Molecular Physics 103: 877-882, and the literature cited therein). The origin of the difference of two energy terms in the Rydberg equation was thus shown to be in the ground state energy itself, as shown below: EH = (1/2)e2/(κaB) = (1/2)(e2/κ) [(1/aB,p - (1/aB,e)] (1). This work brings some new results that 1) a unit charge in vacuum has a magnetic moment, 2) (e2/2κ) in eq. (1) is an electromagnetic condenser constant, 3) the de Broglie wavelengths of the proton and electron correspond to the Golden arcs of a circle with the Bohr radius, 4) the fine structure constant (α) is the ratio of the Planck's constants without and with the interaction of light with matter, 5) the g-factors of the electron and proton, ge/2 and gp/2 divide the Bohr radius at the magnetic center and 6) the ``mysterious'' value (137.036) of α -1 = (360/ø2) - (2/ø3), where (2/ø3) arises from the difference, (gp - ge).
Exciton binding energy in GaAs V-shaped quantum wires
NASA Astrophysics Data System (ADS)
Rinaldi, R.; Cingolani, R.; Lepore, M.; Ferrara, M.; Catalano, I. M.; Rossi, F.; Rota, L.; Molinari, E.; Lugli, P.; Marti, U.; Martin, D.; Morier-Gemoud, F.; Ruterana, P.; Reinhart, F. K.
1994-11-01
We have determined the main parameters of the quasi-one-dimensional excitons confined in GaAs V-shaped quantum wires, namely exciton Bohr radius and binding energy, by two-photon absorption and magnetoluminescence experiments. The experimental results are in excellent agreement with our calculations based on realistic wave functions for the actual wire geometry.
NASA Astrophysics Data System (ADS)
Narayan, Monishka Rita; Singh, Jai
2013-10-01
Primary steps of exciton formation in organic solar cells are presented here. The rates of absorption of photons to excite singlet and triplet excitons are derived using exciton-photon and exciton-spin-orbit-photon-interaction, respectively, as perturbation operators. In both singlet and triplet absorptions, the rates are found to depend on the absorption energy, excitonic Bohr radius, and the dielectric constant of the donor organic material. Incorporation of heavy metal atoms enhances the exciton-spin-orbit-photon interaction and hence the rate of excitation of triplet excitons because it depends on the square of the heaviest atomic number. The new exciton-spin-orbit-photon interaction operator flips the spin to a singlet form leading to faster dissociation into charge carriers and resulting in higher photon to electron-hole pair conversion efficiency in organic solar cells.
Exciton effective mass enhancement in coupled quantum wells in electric and magnetic fields
NASA Astrophysics Data System (ADS)
Wilkes, J.; Muljarov, E. A.
2016-02-01
We present a calculation of exciton states in semiconductor coupled quantum wells in the presence of electric and magnetic fields applied perpendicular to the QW plane. The exciton Schrödinger equation is solved in real space in three-dimensions to obtain the Landau levels of both direct and indirect excitons. Calculation of the exciton energy levels and oscillator strengths enables mapping of the electric and magnetic field dependence of the exciton absorption spectrum. For the ground state of the system, we evaluate the Bohr radius, optical lifetime, binding energy and dipole moment. The exciton mass renormalization due to the magnetic field is calculated using a perturbative approach. We predict a non-monotonous dependence of the exciton ground state effective mass on magnetic field. Such a trend is explained in a classical picture, in terms of the ground state tending from an indirect to a direct exciton with increasing magnetic field.
Exciton mapping at subwavelength scales in two-dimensional materials.
Tizei, Luiz H G; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu
2015-03-13
Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius. PMID:25815966
Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials
NASA Astrophysics Data System (ADS)
Tizei, Luiz H. G.; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu
2015-03-01
Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.
Two-dimensional excitons in three-dimensional hexagonal boron nitride
Cao, X. K.; Lin, J. Y. Jiang, H. X.; Clubine, B.; Edgar, J. H.
2013-11-04
The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 Å and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54m{sub o}, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.
Scaling of exciton binding energy and virial theorem in semiconductor quantum wells and wires
NASA Astrophysics Data System (ADS)
Zhang, Yong; Mascarenhas, A.
1999-01-01
Recent numerical calculations [F. Rossi, G. Goldoni, and E. Molinari, Phys. Rev. Lett. 78, 3527 (1997)] have revealed a shape-independent hyperbolic scaling rule for the exciton binding energy versus the exciton Bohr radius in semiconductor quantum wires, and an enhancement in the exciton binding energy in a quantum wire with respect to a quantum well for a given exciton Bohr radius. These findings were attributed to the existence of a constant (shape- and/or size-independent) virial theorem value (potential- to kinetic-energy ratio), respectively, for the wires and wells, and its value was found to be larger (=4) for wires than (=2) for wells. In order to elucidate the physics underlying the above results, we reexamine this subject by calculating the exciton binding energy and the corresponding virial theorem value in quantum wells and wires with infinite confinement barriers. We find the following. (i) The virial theorem value is nonconstant but approaches 2 from above when reducing the finite extension of the electron and hole wave functions in the confined directions. This is because the origin of the virial theorem value of 2 lies in the inverse square Coulomb force being the only interaction seen by the electron and hole. (ii) The scaling rule is nonhyperbolic, because the virial theorem value is not a constant. (iii) The virial theorem value and the exciton binding energy are larger in a wire than in a well for a given exciton Bohr radius, because the wire exciton has a smaller kinetic energy in the nonconfined direction. (iv) The origin of the shape-independent scaling rule for wires lies in the close similarity of the effective Coulomb potentials for wires with different shapes and widths. The virial theorem value being or not being a constant is irrelevant to the scaling rule. (v) There exists a more fundamental and practically more useful shape-independent scaling rule.
NASA Astrophysics Data System (ADS)
Crease, Robert P.
2008-05-01
In his book Niels Bohr's Times, the physicist Abraham Pais captures a paradox in his subject's legacy by quoting three conflicting assessments. Pais cites Max Born, of the first generation of quantum physics, and Werner Heisenberg, of the second, as saying that Bohr had a greater influence on physics and physicists than any other scientist. Yet Pais also reports a distinguished younger colleague asking with puzzlement and scepticism "What did Bohr really do?".
ERIC Educational Resources Information Center
Willden, Jeff
2001-01-01
"Bohr's Atomic Model" is a small interactive multimedia program that introduces the viewer to a simplified model of the atom. This interactive simulation lets students build an atom using an atomic construction set. The underlying design methodology for "Bohr's Atomic Model" is model-centered instruction, which means the central model of the…
NASA Astrophysics Data System (ADS)
Litvinenko, K. L.; Li, Juerong; Stavrias, N.; Meaney, A. J.; Christianen, P. C. M.; Engelkamp, H.; Homewood, K. P.; Pidgeon, C. R.; Murdin, B. N.
2016-04-01
We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si:P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the m J = ±3/2 heavy hole is found to be larger than that of the m J = ±1/2 light hole.
Nondispersing Bohr Wave Packets
Maeda, H.; Gurian, J. H.; Gallagher, T. F.
2009-03-13
Long-lived, nondispersing circular, or Bohr, wave packets are produced starting from Li Rydberg atoms by exposing them first to a linearly polarized microwave field at the orbital frequency, 17.6 GHz at principal quantum number n=72, which locks the electron's motion into an approximately linear orbit in which the electron oscillates in phase with the microwave field. The microwave polarization is changed to circular polarization slowly compared to the orbital frequency, and the electron's motion follows, resulting in a nondispersing Bohr wave packet.
Nondispersing Bohr Wave Packets
NASA Astrophysics Data System (ADS)
Maeda, H.; Gurian, J. H.; Gallagher, T. F.
2009-03-01
Long-lived, nondispersing circular, or Bohr, wave packets are produced starting from Li Rydberg atoms by exposing them first to a linearly polarized microwave field at the orbital frequency, 17.6 GHz at principal quantum number n=72, which locks the electron’s motion into an approximately linear orbit in which the electron oscillates in phase with the microwave field. The microwave polarization is changed to circular polarization slowly compared to the orbital frequency, and the electron’s motion follows, resulting in a nondispersing Bohr wave packet.
Multiple Exciton Generation in Colloidal Silicon Nanocrystals
Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. M.
2007-01-01
Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap {identical_to} Eg = 1.20 eV) to be 2.4 {+-} 0.1E{sub g} and find an exciton-production quantum yield of 2.6 {+-} 0.2 excitons per absorbed photon at 3.4E{sub g}. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.
Takagi, Hidetsugu; Kunugita, Hideyuki; Ema, Kazuhiro; Sato, Mikio; Takeoka, Yuko
2013-12-04
We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C{sub 4}H{sub 9}NH{sub 3}){sub 2}PbBr{sub 4} and (C{sub 6}H{sub 5}−C{sub 2}H{sub 4}NH{sub 3}){sub 2}PbBr{sub 4}, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.
NASA Astrophysics Data System (ADS)
Takagi, Hidetsugu; Sato, Mikio; Takeoka, Yuko; Kunugita, Hideyuki; Ema, Kazuhiro
2013-12-01
We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C4H9NH3)2PbBr4 and (C6H5-C2H4NH3)2PbBr4, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.
NASA Astrophysics Data System (ADS)
Bellac, Michel Le
2014-11-01
The final form of quantum physics, in the particular case of wave mechanics, was established in the years 1925-1927 by Heisenberg, Schrödinger, Born and others, but the synthesis was the work of Bohr who gave an epistemological interpretation of all the technicalities built up over those years; this interpretation will be examined briefly in Chapter 10. Although Einstein acknowledged the success of quantum mechanics in atomic, molecular and solid state physics, he disagreed deeply with Bohr's interpretation. For many years, he tried to find flaws in the formulation of quantum theory as it had been more or less accepted by a large majority of physicists, but his objections were brushed away by Bohr. However, in an article published in 1935 with Podolsky and Rosen, universally known under the acronym EPR, Einstein thought he had identified a difficulty in the by then standard interpretation. Bohr's obscure, and in part beyond the point, answer showed that Einstein had hit a sensitive target. Nevertheless, until 1964, the so-called Bohr-Einstein debate stayed uniquely on a philosophical level, and it was actually forgotten by most physicists, as the few of them aware of it thought it had no practical implication. In 1964, the Northern Irish physicist John Bell realized that the assumptions contained in the EPR article could be tested experimentally. These assumptions led to inequalities, the Bell inequalities, which were in contradiction with quantum mechanical predictions: as we shall see later on, it is extremely likely that the assumptions of the EPR article are not consistent with experiment, which, on the contrary, vindicates the predictions of quantum physics. In Section 3.2, the origin of Bell's inequalities will be explained with an intuitive example, then they will be compared with the predictions of quantum theory in Section 3.3, and finally their experimental status will be reviewed in Section 3.4. The debate between Bohr and Einstein goes much beyond a simple controversy, which is after all almost eighty years old and has been settled today. In fact, the concept introduced in this debate, that of entanglement, lies at the heart of many very important developments of modern quantum physics, in particular all those linked to quantum information (Chapter 8). Moreover, we shall see that the phenomenon of non-local correlations compels us to revise in depth our space-time representation of quantum processes. These are the two reasons why a whole chapter is devoted to this debate.
NASA Astrophysics Data System (ADS)
Heilbron, J. L.
1981-03-01
Bohr used to introduce his attempts to explain clearly the principles of the quantum theory of the atom with an historical sketch, beginning invariably with the nuclear model proposed by Rutherford. That was sound pedagogy but bad history. The Rutherford-Bohr atom stands in the middle of a line of work initiated by J.J. Thomson and concluded by the invention of quantum mechanics. Thompson's program derived its inspiration from the peculiar emphasis on models characteristic of British physics of the 19th century. Rutherford's atom was a late product of the goals and conceptions of Victorian science. Bohr's modifications, although ultimately fatal to Thomson's program, initially gave further impetus to it. In the early 1920s the most promising approach to an adequate theory of the atom appeared to be the literal and detailed elaboration of the classical mechanics of multiply periodic orbits. The approach succeeded, demonstrating in an unexpected way the force of an argument often advanced by Thomson: because a mechanical model is richer in implications than the considerations for which it was advanced, it can suggest new directions of research that may lead to important discoveries.
The Aharonov-Bohm effect for an exciton
NASA Astrophysics Data System (ADS)
Römer, R. A.; Raikh, M. E.
2000-03-01
We study theoretically the exciton absorption (luminescence) of a ring-like quantum dot shreded by a magnetic flux. We consider the limit when the width of the ring is smaller than the excitonic Bohr radius a_B. We demonstrate that, despite the electrical neutrality of the exciton, both the spectral position of the exciton peak in the absorption (luminescence), and the corresponding oscillator strength oscillate with magnetic flux with a period Φ0 --- the universal flux quantum. Assuming that the attraction between electron and hole is short-ranged we find analytically the functional form of these oscillations for both quantities.^1 This enables us to trace the magnitude of the effect with changing the ratio 2 π R/aB where R is the radius of the ring. Physically, the origin of the oscillations is the finite probability for electron and hole, created by a photon at the same point, to tunnel in the opposite directions and meet each other on the opposite side of the ring. Possible candidates for the experimental observation of the effect are recently discovered self-assembled quantum ring-like structures of InAs embedded in GaAs.^2,3 ^1R.A. Römer and M.E. Raikh, preprint cond-mat/9906314. ^2A. Lorke et al., Microelectronic Engeneering 47, 95 (1999). ^3H. Petterson et al., Proceedings of EP2DS-13, to be published in Physica E, (1999).
NASA Astrophysics Data System (ADS)
Monozon, B. S.; Schmelcher, P.
2009-04-01
We present an analytical investigation of quasi-one-dimensional excitons in thin uniform (single) and double nanoscaled cylindrical quantum wires (UQWR and DQWR) surrounded by a barrier of infinite height and exposed to external electric and strong magnetic fields. The DQWR is formed by inserting an impenetrable longitudinal barrier in a single-quantum wire. Both external fields are directed parallel to the quantum wire (QWR) axis. The radius of the QWRs and the magnetic length are taken to be much less than the exciton Bohr radius. For the dependencies of the positions and widths of the complex quasidiscrete energy levels of the indirect exciton in the DQWR, in which the carriers are separated by the insertion on the confinement, electric field strength and width of the interwire barrier are derived. The confinement (insertion) leads to an increase (decrease) of the exciton binding energy. The impact of the electric field ionization of the exciton is less pronounced for strongly confined and weakly separated carriers. The coefficient of the exciton absorption in the UQWR as a function of the confinement and electric field is calculated in an explicit form. The effect of the confinement and electric field on the exciton peak closely resembles that on the quasidiscrete level of the indirect exciton in the DQWR. Electron-hole attraction increases remarkably the optical Franz-Keldysh electroabsorption in the frequency region below the edge and distant from the exciton peaks. The coefficient of absorption reflecting the electric field ionization and autoionization caused by the coupling between the discrete and continuous exciton states adjacent to the different size-quantized or Landau levels is obtained analytically. A comparison of our analytical results with numerical data is performed. Estimates of the expected experimental values for the parameters of GaAs/GaAlAs QWR show that the autoionized exciton magnetostates in thin biased QWRs are sufficiently stable to be observed.
NASA Astrophysics Data System (ADS)
Dotson, Allen
2013-07-01
Jon Cartwright's interesting and informative article on quantum philosophy ("The life of psi", May pp26-31) mischaracterizes Niels Bohr's stance as anti-realist by suggesting (in the illustration on p29) that Bohr believed that "quantum theory [does not] describe an objective reality, independent of the observer".
Einstein, Bohr and the Quantum Dilemma
NASA Astrophysics Data System (ADS)
Whitaker, Andrew
2006-06-01
1. Bohr and Einstein: Einstein and Bohr; 2. The peace before the quantum; 3. A glance at relativity; 4. The slow rise of the quantum; 5. Bohr: what does it all mean?; 6. Einstein's negative views; 7. Bohm, Bell and experimental philosophy; 8. A round-up of recent developments; 9. Quantum information theory - an introduction; 10. Bohr or Einstein?; References; Bibliography.
Revisiting Bohr's semiclassical quantum theory.
Ben-Amotz, Dor
2006-10-12
Bohr's atomic theory is widely viewed as remarkable, both for its accuracy in predicting the observed optical transitions of one-electron atoms and for its failure to fully correspond with current electronic structure theory. What is not generally appreciated is that Bohr's original semiclassical conception differed significantly from the Bohr-Sommerfeld theory and offers an alternative semiclassical approximation scheme with remarkable attributes. More specifically, Bohr's original method did not impose action quantization constraints but rather obtained these as predictions by simply matching photon and classical orbital frequencies. In other words, the hydrogen atom was treated entirely classically and orbital quantized emerged directly from the Planck-Einstein photon quantization condition, E = h nu. Here, we revisit this early history of quantum theory and demonstrate the application of Bohr's original strategy to the three quintessential quantum systems: an electron in a box, an electron in a ring, and a dipolar harmonic oscillator. The usual energy-level spectra, and optical selection rules, emerge by solving an algebraic (quadratic) equation, rather than a Bohr-Sommerfeld integral (or Schroedinger) equation. However, the new predictions include a frozen (zero-kinetic-energy) state which in some (but not all) cases lies below the usual zero-point energy. In addition to raising provocative questions concerning the origin of quantum-chemical phenomena, the results may prove to be of pedagogical value in introducing students to quantum mechanics. PMID:17020371
Exciton-exciton annihilation in organic polariton microcavities
Akselrod, G. M.; Tischler, Jonathan R.; Young, E. R.; Nocera, D.G.; Bulovic, Vladimir
2010-09-27
We investigate the incoherent diffusion of excitons in thin films (5.1±0.1 nm thick) of a highly absorbing J-aggregated cyanine dye material (10^{6} cm^{-1} absorption constant) as the excitonic component of a polariton microcavity. Under high-intensity pulsed laser excitation, the J-aggregated molecular films exhibit significant exciton-exciton annihilation, indicating a large exciton diffusion radius of more than 100 nm. When the material is strongly coupled to a cavity, the polaritonic structure also shows exciton-exciton annihilation, which is a competing process against the establishment of a threshold population of polaritons needed for polariton lasing. This study suggests that exciton-exciton annihilation is a loss process which can significantly increase the lasing threshold in polariton microcavities.
A Simple Relativistic Bohr Atom
ERIC Educational Resources Information Center
Terzis, Andreas F.
2008-01-01
A simple concise relativistic modification of the standard Bohr model for hydrogen-like atoms with circular orbits is presented. As the derivation requires basic knowledge of classical and relativistic mechanics, it can be taught in standard courses in modern physics and introductory quantum mechanics. In addition, it can be shown in a class that…
A Simple Relativistic Bohr Atom
ERIC Educational Resources Information Center
Terzis, Andreas F.
2008-01-01
A simple concise relativistic modification of the standard Bohr model for hydrogen-like atoms with circular orbits is presented. As the derivation requires basic knowledge of classical and relativistic mechanics, it can be taught in standard courses in modern physics and introductory quantum mechanics. In addition, it can be shown in a class that
ERIC Educational Resources Information Center
Brunori, Maurizio
2012-01-01
Before the outbreak of World War II, Jeffries Wyman postulated that the "Bohr effect" in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on the…
Exciton-Phonon Interaction Effects in II-Vi Compound Semiconductor Quantum Wells
NASA Astrophysics Data System (ADS)
Pelekanos, Nikolaos Themelis
1992-01-01
In this thesis, we report on two specific examples of exciton-LO phonon Frohlich interaction effects, namely, hot carrier relaxation and temperature dependent exciton linewidth broadening. These phenomena are considered in the context of quasi-two dimensional excitons in strongly polar II-VI semiconductor quantum wells. Hot-exciton luminescence phenomena are investigated in a single quantum well of ZnTe/MnTe where tunneling through thin MnTe barrier layers suppresses the formation of thermalized luminescence. For near resonant photoexcitation, the secondary emission spectrum is modulated by distinct LO-phonon peaks, which, for sufficiently high order of scattering ( >=4), behave like hot luminescence (HPL) as opposed to resonant Raman scattering. This is confirmed by time-resolved spectroscopy as well as by steady-state characteristics such as linewidth broadening and lack of polarization memory. Several novel observations are made: (1) The LO-phonon intermediated energy relaxation involves Coulomb-correlated pairs, i.e. hot excitons, as opposed to independently-relaxing free electrons and holes. (2) The additional weak disorder originating from QW thickness fluctuations plays a major role in the details of the HPL spectra. The major contribution to the ground state exciton linewidth at room temperature originates from LO phonon -intermediated exciton scattering to higher exciton states. A measure of the effect is given by the parameter Gamma_{LO} which increases with the polarity of the material and is independent of dimensionality provided that the LO phonon energy is greater than the exciton binding energy. Measurements of Gamma_{LO} are performed in two quantum well systems: CdTe/MnTe and (Zn,Cd)Se/ZnSe. In the latter system, a strong reduction of Gamma _{LO} is observed as the quantum well width becomes comparable to the three-dimensional exciton Bohr radius. This is explained in terms of a model where quasi-2D confinement effects increase the exciton binding energy to a value greater than the LO phonon energy and hence reduce the available phase space for the exciton -LO phonon scattering process. Direct confirmation of our interpretation is found in magneto-transmission experiments.
Bohr's 1913 molecular model revisited
Svidzinsky, Anatoly A.; Scully, Marlan O.; Herschbach, Dudley R.
2005-01-01
It is generally believed that the old quantum theory, as presented by Niels Bohr in 1913, fails when applied to few electron systems, such as the H2 molecule. Here, we find previously undescribed solutions within the Bohr theory that describe the potential energy curve for the lowest singlet and triplet states of H2 about as well as the early wave mechanical treatment of Heitler and London. We also develop an interpolation scheme that substantially improves the agreement with the exact ground-state potential curve of H2 and provides a good description of more complicated molecules such as LiH, Li2, BeH, and He2. PMID:16103360
Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films.
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E; Van de Walle, Chris G; Nguyen, Nhan V; Zhang, Yong-Wei; Cao, Linyou
2015-01-01
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters. PMID:26598075
Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films
NASA Astrophysics Data System (ADS)
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong-Wei; Cao, Linyou
2015-11-01
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.
Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong-Wei; Cao, Linyou
2015-01-01
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters. PMID:26598075
Multiphoton exciton absorption in a semiconductor superlattice in a dc electric field
NASA Astrophysics Data System (ADS)
Monozon, B. S.; Schmelcher, P.
2007-06-01
An analytical approach to the problem of the multiphoton exciton absorption in biased narrow-well superlattices (SLs) induced by the optical transitions to the localized resonant exciton states is developed. Both the ac electric field of the intense optical wave and the dc electric field are directed parallel to the SL axis. The SL is formed by a periodic sequence of quantum wells (QWs) whose widths are taken to be much less than the exciton Bohr radius. The model of the SL potential employs a limiting form of the Kronig-Penney potential, i.e., a periodic chain of QWs separated by δ -function-type barriers. A sufficiently strong dc electric field provides the localization of the carriers within one period of the SL. Analytical dependencies of the coefficient of the multiphoton exciton absorption on the characteristics of the dc and ac electric fields and on the parameters of the SL in the approximation of both isolated and interacting Wannier-Stark levels are obtained in the nearest-neighbor tight-binding approximations. Our analytical results correlate well with those obtained in numerical investigations. Estimates of the expected experimental values are performed for the parameters of a GaAs/AlGaAs SL.
Exciton-dominated dielectric function of atomically thin MoS2 films
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong -Wei; et al
2015-11-24
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function maymore » dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. Lastly, the knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.« less
Exciton-dominated dielectric function of atomically thin MoS_{2} films
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong -Wei; Cao, Linyou
2015-11-24
We systematically measure the dielectric function of atomically thin MoS_{2} films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS_{2} films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. Lastly, the knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS_{2} films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.
Corda, Christian
2015-03-10
The idea that black holes (BHs) result in highly excited states representing both the “hydrogen atom” and the “quasi-thermal emission” in quantum gravity is today an intuitive but general conviction. In this paper it will be shown that such an intuitive picture is more than a picture. In fact, we will discuss a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. The model is completely consistent with existing results in the literature, starting from the celebrated result of Bekenstein on the area quantization.
Timing and Impact of Bohr's Trilogy
NASA Astrophysics Data System (ADS)
Jeong, Yeuncheol; Wang, Lei; Yin, Ming; Datta, Timir
2014-03-01
In their article- Genesis of the Bohr Atom Heilbron and Kuhn asked - what suddenly turned his [Bohr's] attention, to atom models during June 1912- they were absolutely right; during the short period in question Bohr had made an unexpected change in his research activity, he has found a new interest ``atom'' and would soon produce a spectacularly successful theory about it in his now famous trilogy papers in the Phil Mag (1913). We researched the trilogy papers, Bohr`s memorandum, his own correspondence from that time in question and activities by Moseley (Manchester), Henry and Lawrence Bragg. Our work suggests that Bohr, also at Manchester that summer, was likely to have been inspired by Laue's sensational discovery in April 1912, of X-ray interference from atoms in crystals. The three trilogy papers include sixty five distinct (numbered) references from thirty one authors. The publication dates of the cited works range from 1896 to 1913. Bohr showed an extraordinary skill in navigating thru the most important and up-to date works. Eleven of the cited authors (Bohr included, but not John Nicholson) were recognized by ten Noble prizes, six in physics and four in chemistry.
What classicality? Decoherence and Bohr's classical concepts
NASA Astrophysics Data System (ADS)
Schlosshauer, Maximilian; Camilleri, Kristian
2011-03-01
Niels Bohr famously insisted on the indispensability of what he termed "classical concepts." In the context of the decoherence program, on the other hand, it has become fashionable to talk about the "dynamical emergence of classicality" from the quantum formalism alone. Does this mean that decoherence challenges Bohr's dictum—for example, that classical concepts do not need to be assumed but can be derived? In this paper we'll try to shed some light down the murky waters where formalism and philosophy cohabitate. To begin, we'll clarify the notion of classicality in the decoherence description. We'll then discuss Bohr's and Heisenberg's take on the quantum—classical problem and reflect on different meanings of the terms "classicality" and "classical concepts" in the writings of Bohr and his followers. This analysis will allow us to put forward some tentative suggestions for how we may better understand the relation between decoherence-induced classicality and Bohr's classical concepts.
Excitons in organic multiple quantum well structures
NASA Astrophysics Data System (ADS)
Agranovich, Vladimir M.; Atanasov, Rosen D.; Bassani, Giuseppe F.
1992-11-01
Multiple quantum well structures consisting of alternating layers of two crystalline organic semiconductors PTCDA and NTCDA are considered. It is shown that the blue-shift of the lowest energy PTCDA singlet exciton line with decreasing layer thickness is described quantitatively within the model of "small radius" excitons.
Excitonic effects in GeC hybrid: Many-body Green's function calculations
NASA Astrophysics Data System (ADS)
Drissi, L. B.; Ramadan, F. Z.
2015-11-01
Many-body effects on the electronic and optical absorption properties of a GeC sheet are studied by means of first principle many-body Green's function and Bethe-Salpeter equation formalism. The absence of soft modes in the phonon-spectrum indicates the stability of the system. The inclusion of quasiparticle corrections increases significantly the band gap. The local field effects induce significant change in the absorption spectra for the out-plane polarization rendering the GeC monolayer transparent below 7 eV. The excitonic effects are significant on the optical absorption properties. A detailed analysis of the spectrum shows a strong binding energy of 1.82 eV assigned to the lowest-energy bound excitons that is characterized by an effective mass of 1.68 m0 and a Bohr radius of 2 Å. The results of this study hold the promise for potential applications of the GeC hybrid in optoelectronics.
Niels Bohr and the Third Quantum Revolution
NASA Astrophysics Data System (ADS)
Goldhaber, Alfred
2013-04-01
In the history of science few developments can rival the discovery of quantum mechanics, with its series of abrupt leaps in unexpected directions stretching over a quarter century. The result was a new world, even more strange than any previously imagined subterranean (or in this case submicroscopic) kingdom. Niels Bohr made the third of these leaps (following Planck and Einstein) when he realized that still-new quantum ideas were essential to account for atomic structure: Rutherford had deduced, using entirely classical-physics principles, that the positive charge in an atom is contained in a very small kernel or nucleus. This made the atom an analogue to the solar system. Classical physics implied that negatively charged electrons losing energy to electromagnetic radiation would ``dive in'' to the nucleus in a very short time. The chemistry of such tiny atoms would be trivial, and the sizes of solids made from these atoms would be much too small. Bohr initially got out of this dilemma by postulating that the angular momentum of an electron orbiting about the nucleus is quantized in integer multiples of the reduced quantum constant ℏ = h/2 π. Solving for the energy of such an orbit in equilibrium immediately produces the famous Balmer formula for the frequencies of visible light radiated from hydrogen as an electron jumps from any particular orbit to another of lower energy. There remained mysteries requiring explanation or at least exploration, including two to be discussed here: 1. Rutherford used classical mechanics to compute the trajectory and hence the scattering angle of an α particle impinging on a small positively charged target. How could this be consistent with Bohr's quantization of particle orbits about the nucleus? 2. Bohr excluded for his integer multiples of ℏ the value 0. How can one justify this exclusion, necessary to bar tiny atoms of the type mentioned earlier?
Niels Bohr and the Third Quantum Revolution
NASA Astrophysics Data System (ADS)
Scharff Goldhaber, Alfred
2013-04-01
In the history of science few developments can rival the discovery of quantum mechanics, with its series of abrupt leaps in unexpected directions stretching over a quarter century. The result was a new world, even more strange than any previously imagined subterranean (or in this case submicroscopic) kingdom. Niels Bohr made the third of these leaps (following Planck and Einstein) when he realized that still-new quantum ideas were essential to account for atomic structure: Rutherford had deduced, using entirely classical-physics principles, that the positive charge in an atom is contained in a very small kernel or nucleus. This made the atom an analogue to the solar system. Classical physics implied that negatively charged electrons losing energy to electromagnetic radiation would ``dive in'' to the nucleus in a very short time. The chemistry of such tiny atoms would be trivial, and the sizes of solids made from these atoms would be much too small. Bohr initially got out of this dilemma by postulating that the angular momentum of an electron orbiting about the nucleus is quantized in integer multiples of the reduced quantum constant = h/2π. Solving for the energy of such an orbit in equilibrium immediately produces the famous Balmer formula for the frequencies of visible light radiated from hydrogen as an electron jumps from any particular orbit to another of lower energy. There remained mysteries requiring explanation or at least exploration, including two to be discussed here: 1. Rutherford used classical mechanics to compute the trajectory and hence the scattering angle of an α particle impinging on a small positively charged target. How could this be consistent with Bohr's quantization of particle orbits about the nucleus? 2. Bohr excluded for his integer multiples of the value 0. How can one justify this exclusion, necessary to bar tiny atoms of the type mentioned earlier?
Bohr-Sommerfeld quantization of pseudospin Hamiltonians
Shankar, R.
1980-09-29
It is shown here how to map the problem with pseudospin J into an equivalent one in which 1/J plays the role of h and canonical variables exist at the classical level. Bohr-Sommerfeld quantization of the equivalent theory is found to produce a spectrum in very good agreement with the exact results for the Lipkin-Meshkov-Glick model at J=15 and 25. The method readily extends to the SU(n) case.
Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier
LYO,SUNGKWUN K.
1999-12-06
We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.
Realization of Localized Bohr-like Wavepackets
Mestayer, J. J.; Wyker, B.; Lancaster, J. C.; Dunning, F. B.; Reinhold, Carlos O; Yoshida, S.; Burgdorfer, J.
2008-01-01
We demonstrate a protocol to create localized wavepackets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wavepackets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wavepackets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in circular classical orbit around the nucleus. Possible extension of the approach to create so far elusive "planetary atoms" in highly correlated stable multiply-excited states is discussed.
The operation : The interview of Niels Bohr
NASA Astrophysics Data System (ADS)
Terletskij, Yakov P.
The article represents a stenogramme of the recollections of professor Yakov Petrovich Terletskij (1912, St. Petersburg-1993, Moscow) concerning his meeting with professor Niels Bohr in Kopenhagen from 24 september 1945 till 22 november 1945 under the task of Soviet KGB and personally Lavrentii Berija. Some additional informations concerning previous preparations of the trip, the final stage of the mission, as well as general considerations about the development of Physics, persons of physicists and nuclear technologies has been given by the author of recollections throughout the text
Bohr's Creation of his Quantum Atom
NASA Astrophysics Data System (ADS)
Heilbron, John
2013-04-01
Fresh letters throw new light on the content and state of Bohr's mind before and during his creation of the quantum atom. His mental furniture then included the atomic models of the English school, the quantum puzzles of Continental theorists, and the results of his own studies of the electron theory of metals. It also included the poetry of Goethe, plays of Ibsen and Shakespeare, novels of Dickens, and rhapsodies of Kierkegaard and Carlyle. The mind that held these diverse ingredients together oscillated between enthusiasm and dejection during the year in which Bohr took up the problem of atomic structure. He spent most of that year in England, which separated him for extended periods from his close-knit family and friends. Correspondence with his fianc'ee, Margrethe Nørlund, soon to be published, reports his ups and downs as he adjusted to J.J. Thomson, Ernest Rutherford, the English language, and the uneven course of his work. In helping to smooth out his moods, Margrethe played an important and perhaps an enabling role in his creative process.
The Bohr-Heisenberg Correspondence Principle Viewed from Phase Space
NASA Astrophysics Data System (ADS)
Dahl, Jens P.
2003-09-01
Phase-space representations play an increasingly important role in several branches of physics. Here, we review the author's studies of the Bohr-Heisenberg correspondence principle within the Weyl-Wigner phase-space representation. The analysis leads to refined correspondence rules that can be successfully used far away from the classical limit considered by Bohr and Heisenberg.
Davidson potential and SUSYQM in the Bohr Hamiltonian
Georgoudis, P. E.
2013-06-10
The Bohr Hamiltonian is modified through the Shape Invariance principle of SUper-SYmmetric Quantum Mechanics for the Davidson potential. The modification is equivalent to a conformal transformation of Bohr's metric, generating a different {beta}-dependence of the moments of inertia.
100th anniversary of Bohr's model of the atom.
Schwarz, W H Eugen
2013-11-18
In the fall of 1913 Niels Bohr formulated his atomic models at the age of 27. This Essay traces Bohr's fundamental reasoning regarding atomic structure and spectra, the periodic table of the elements, and chemical bonding. His enduring insights and superseded suppositions are also discussed. PMID:24123759
Resisting the Bohr Atom: The Early British Opposition
NASA Astrophysics Data System (ADS)
Kragh, Helge
2011-03-01
When Niels Bohr's theory of atomic structure appeared in the summer and fall of 1913, it quickly attracted attention among British physicists. While some of the attention was supportive, others was critical. I consider the opposition to Bohr's theory from 1913 to about 1915, including attempts to construct atomic theories on a classical basis as alternatives to Bohr's. I give particular attention to the astrophysicist John W. Nicholson, who was Bohr's most formidable and persistent opponent in the early years. Although in the long run Nicholson's objections were inconsequential, for a short period of time his atomic theory was considered to be a serious rival to Bohr's. Moreover, Nicholson's theory is of interest in its own right.
Bohr effect of hemoglobins: Accounting for differences in magnitude.
Okonjo, Kehinde O
2015-09-01
The basis of the difference in the Bohr effect of various hemoglobins has remained enigmatic for decades. Fourteen amino acid residues, identical in pairs and located at specific 'Bohr group positions' in human hemoglobin, are implicated in the Bohr effect. All 14 are present in mouse, 11 in dog, eight in pigeon and 13 in guinea pig hemoglobin. The Bohr data for human and mouse hemoglobin are identical: the 14 Bohr groups appear at identical positions in both molecules. The dog data are different from the human because three Bohr group positions are occupied by non-ionizable groups in dog hemoglobin; the pigeon data are vastly different from the human because six Bohr group positions are occupied by non-ionizable groups in pigeon hemoglobin. The guinea pig data are quite complex. Quantitative analyses showed that only the pigeon data could be fitted with the Wyman equation for the Bohr effect. We demonstrate that, apart from guinea pig hemoglobin, the difference between the Bohr effect of each of the other hemoglobins and of pigeon hemoglobin can be accounted for quantitatively on the basis of the occupation of some of their Bohr group positions by non-ionizable groups in pigeon hemoglobin. We attribute the anomalous guinea pig result to a new salt-bridge formed in its R2 quaternary structure between the terminal NH3(+) group of one β-chain and the COO(-) terminal group of the partner β-chain in the same molecule. The pKas of this NH3(+) group are 6.33 in the R2 and 4.59 in the T state. PMID:26102019
Realization of Bohr-like circular wavepackets
NASA Astrophysics Data System (ADS)
Mestayer, Jeff; Wyker, B.; Dunning, F. B.; Reinhold, C.; Yoshida, S.; Burgdörfer, J.
2008-05-01
We demonstrate a protocol to create localized wavepackets in very-high n Rydberg states that travel in near-circular orbits around the nucleus. Although these wavepackets slowly dephase and eventually lose their localization, their motion can be followed for several orbital periods. These wavepackets represent the closest analog yet achieved to the original Bohr model of the atom, i.e., an electron in circular classical orbit around the nucleus. The time evolution of the momentum and position of the wavepackets is monitored using, respectively, short half-cycle pulses and sudden field steps, which are applied after a variable time delay and along different directions. The present technique is explained with the aid of CTMC simulations, and possible extension to creation of elusive ``planetary atoms'' in highly correlated stable multiply-excited states is discussed. Research supported by the NSF, the Robert A. Welch Foundation, the OBES, U.S. DoE to ORNL, and by the FWF (Austria).
Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.
1980-01-01
The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)
Electron-hole interaction in the presence of excitons
NASA Astrophysics Data System (ADS)
Collet, J. H.; Amand, T.
1984-10-01
We calculate the effective electron-hole interaction V re in the presence of an exciton gas, which reads in real space: V re(r)= {-e 2}/{r}{1+ limit∑i=14(-1) iC iexp (-Z i{r}/{a}} The parameters Ci and Zi are given explicitly for GaAs. For this material, we show the binding energy of the exciton is weakly modified so long as {8πR 0ϱ exa 03}/{kT⩽1}. ( R0, exciton Rydberg, a0 exciyon radius, ϱ ex exciton density, T temperature).
Paul Ehrenfest, Niels Bohr, and Albert Einstein: Colleagues and Friends
NASA Astrophysics Data System (ADS)
Klein, Martin J.
2010-09-01
In May 1918 Paul Ehrenfest received a monograph from Niels Bohr in which Bohr had used Ehrenfest's adiabatic principle as an essential assumption for understanding atomic structure. Ehrenfest responded by inviting Bohr, whom he had never met, to give a talk at a meeting in Leiden in late April 1919, which Bohr accepted; he lived with Ehrenfest, his mathematician wife Tatyana, and their young family for two weeks. Albert Einstein was unable to attend this meeting, but in October 1919 he visited his old friend Ehrenfest and his family in Leiden, where Ehrenfest told him how much he had enjoyed and profited from Bohr's visit. Einstein first met Bohr when Bohr gave a lecture in Berlin at the end of April 1920, and the two immediately proclaimed unbounded admiration for each other as physicists and as human beings. Ehrenfest hoped that he and they would meet at the Third Solvay Conference in Brussels in early April 1921, but his hope was unfulfilled. Einstein, the only physicist from Germany who was invited to it in this bitter postwar atmosphere, decided instead to accompany Chaim Weizmann on a trip to the United States to help raise money for the new Hebrew University in Jerusalem. Bohr became so overworked with the planning and construction of his new Institute for Theoretical Physics in Copenhagen that he could only draft the first part of his Solvay report and ask Ehrenfest to present it, which Ehrenfest agreed to do following the presentation of his own report. After recovering his strength, Bohr invited Ehrenfest to give a lecture in Copenhagen that fall, and Ehrenfest, battling his deep-seated self-doubts, spent three weeks in Copenhagen in December 1921 accompanied by his daughter Tanya and her future husband, the two Ehrenfests staying with the Bohrs in their apartment in Bohr's new Institute for Theoretical Physics. Immediately after leaving Copenhagen, Ehrenfest wrote to Einstein, telling him once again that Bohr was a prodigious physicist, and again expressing the hope that he soon would see both of them in Leiden.
Solutions of the Bohr Hamiltonian, a compendium
NASA Astrophysics Data System (ADS)
Fortunato, L.
2005-10-01
The Bohr Hamiltonian, also called collective Hamiltonian, is one of the cornerstones of nuclear physics and a wealth of solutions (analytic or approximated) of the associated eigenvalue equation have been proposed over more than half a century (confining ourselves to the quadrupole degree of freedom). Each particular solution is associated with a peculiar form for the V(β,γ) potential. The large number and the different details of the mathematical derivation of these solutions, as well as their increased and renewed importance for nuclear structure and spectroscopy, demand a thorough discussion. It is the aim of the present monograph to present in detail all the known solutions in γ-unstable and γ-stable cases, in a taxonomic and didactical way. In pursuing this task we especially stressed the mathematical side leaving the discussion of the physics to already published comprehensive material. The paper contains also a new approximate solution for the linear potential, and a new solution for prolate and oblate soft axial rotors, as well as some new formulae and comments. The quasi-dynamical SO(2) symmetry is proposed in connection with the labeling of bands in triaxial nuclei.
Excitons in nanoscale systems.
Scholes, Gregory D; Rumbles, Garry
2006-09-01
Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes. PMID:16946728
NASA Astrophysics Data System (ADS)
Scholes, Gregory D.; Rumbles, Garry
2006-09-01
Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.
Triangular lattice exciton model.
Gunlycke, Daniel; Tseng, Frank
2016-03-16
We present a minimalistic equilateral triangular lattice model showing explicitly that the two-dimensional hydrogen model for excitons breaks down for excitons in semiconducting monolayer transition-metal dichalcogenides due to lattice effects and that these excitons are neither Wannier nor Frenkel excitons but rather span an intermediate regime. The model is formulated on sparse form in direct space, allowing it to be solved with great computational efficiency. PMID:26947357
Hemoglobin Bohr effects: atomic origin of the histidine residue contributions.
Zheng, Guishan; Schaefer, Michael; Karplus, Martin
2013-11-26
The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resolution structures, the residue pKa values corresponding to the two structures are calculated. The method is based on determining the electrostatic interactions between residues in the protein, relative to those of the residue in solution, by use of the linearized finite difference Poisson-Boltzmann equation and Monte Carlo sampling of protonation states. Given that good agreement is obtained with the available experimental values for the contribution of His residues in HbA to the Bohr effect, the calculated results are used to determine the atomic origin of the pKa shift between deoxy and carbonmonoxy HbA. The contributions to the pKa shift calculated by means of the linear response approximation show that the salt bridge involving His146 plays an important role in the alkaline Bohr effect, as suggested by Perutz but that other interactions are significant as well. A corresponding analysis is made for the contribution of His143 to the acid Bohr effect for which there is no proposed explanation. The method used is summarized and the program by which it is implemented is described in the Appendix . PMID:24224786
Gravitational Bohr's model with Newton's and Weber's potentials
NASA Astrophysics Data System (ADS)
Tiandho, Yuant; Triyanta
2015-09-01
Similarity between Newtonian gravitational force and Coulomb's force leads one to define gravitational electric and magnetic fields or gravito-electromagnetic fields. Also, one may define a gravitational Bohr's theory in analogy with the electromagnetic Bohr's theory for which the latter is quite successful in explaining some properties of hydrogen atom. Unlike to the Coulomb's force, the Newton`s force is believed to be an approximated theory of gravity as it is unable to explain the Mercury's orbit. This paper discusses the gravitational Bohr's theory by taking into account the Newton's potential and Weber's potential; the later is a modified Newton's gravitational potential that conforms, to some degrees, to the anomaly of the Mercury's orbit.
Jocular Physics: A Tribute to Bohr in Humor
NASA Astrophysics Data System (ADS)
Halpern, Paul
2011-04-01
Copenhagen, starting in the 1920s and 1930s and continuing after the Second World War, was not just a center for extraordinary developments in quantum and nuclear physics; it also provided a perfect stage for physicists' abundant humor. We will examine the Journal of Jocular Physics, a humorou tribute to Bohr published on the occasions of his 50th , 60th and 70th birthdays. We will discuss how the articles in the journal reflected attempts by the contributors, such as Léon Rosenfeld, Victor Weisskopf and others, to interpret and explain aspects of Bohr's philosophy, such as complementarity and the abandonment of pure Laplacian determinism.
Steering quantum states toward classical Bohr-like orbits
NASA Astrophysics Data System (ADS)
Dunning, F. B.; Reinhold, C. O.; Yoshida, S.; Burgdörfer, J.
2010-08-01
We describe how, almost 100 years after the introduction of the Bohr model of the atom, it is now possible using pulsed electric fields to create localized wavepackets in high n (n ≈300) Rydberg atoms that travel in near-circular Bohr-like orbits mimicking the behavior of a classical electron. The protocols employed are explained with the aid of quantum and classical dynamics. Although many aspects of the underlying behavior can be described using classical arguments, purely quantum effects such as revivals can be seen even for n ≈300.
Radiative lifetimes of excitons and trions in monolayers of the metal dichalcogenide MoS2
NASA Astrophysics Data System (ADS)
Wang, Haining; Zhang, Changjian; Chan, Weimin; Manolatou, Christina; Tiwari, Sandip; Rana, Farhan
2016-01-01
We present results on the radiative lifetimes of excitons and trions in a monolayer of metal dichalcogenide MoS2. The small exciton radius and the large exciton optical oscillator strength result in radiative lifetimes in the 0.18-0.30 ps range for excitons that have small in-plane momenta and couple to radiation. Average lifetimes of thermally distributed excitons depend linearly on the exciton temperature and can be in the few picoseconds range at small temperatures and more than a nanosecond near room temperature. Localized excitons exhibit lifetimes in the same range and the lifetime increases as the localization length decreases. The radiative lifetimes of trions are in the hundreds of picosecond range and increase with the increase in the trion momentum. Average lifetimes of thermally distributed trions increase with the trion temperature as the trions acquire thermal energy and larger momenta. We expect our theoretical results to be applicable to most other 2D transition metal dichalcogenides.
NASA Technical Reports Server (NTRS)
Harris, Michael R.
1987-01-01
Resonator configurations permitting operation with large mode radius while maintaining good transverse mode discrimination are considered. Stable resonators incorporating an intracavity telescope and unstable resonator geometries utilizing an output coupler with a Gaussian reflectivity profile are shown to enable large radius single mode laser operation. Results of heterodyne studies of pulsed CO2 lasers with large (11mm e sup-2 radius) fundamental mode sizes are presented demonstrating minimal frequency sweeping in accordance with the theory of laser-induced medium perturbations.
Optically programmable excitonic traps
Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François
2013-01-01
With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits. PMID:23546532
Bohr and Ehrenfest: transformations and correspondences in the early 1920s
NASA Astrophysics Data System (ADS)
Pérez, Enric; Pié i Valls, Blai
2016-04-01
We analyze the collaboration between Bohr and Ehrenfest on the quantum theory in the early 1920s (1920-1923). We focus on their reflections and developments around the adiabatic principle and the correspondence principle, the two pillars of Bohr's quantum theory of 1922-23. We argue that the evolution of Bohr's ideas after 1918 brought the two principles closer, subordinating the former to the latter. The examination of the weight Bohr attributed to each principle along the years illustrates very clearly the vicissitudes of Bohr's theory before the emergence of quantum mechanics, especially with regards to its rejection/inclusion of mechanics.
Steering Quantum States Towards Classical Bohr-Like Orbits
Dunning, F. B.; Reinhold, Carlos O; Yoshida, S.; Burgdorfer, J.
2010-01-01
This article furnishes an introduction to the properties of time-dependent electronic wavefunctions in atoms and to physics at the interface between the quantum and classical worlds. We describe how, almost 100 years after the introduction of the Bohr model of the atom, it is now possible using pulsed electric fields to create in the laboratory localized wavepackets in high-n (n ~ 300) Rydberg atoms that travel in near-circular Bohr-like orbits mimicking the behavior of a classical electron. The control protocols employed are explained with the aid of quantum and classical dynamics. Remarkably, while many aspects of the underlying behavior can be described using classical arguments, even at n ~ 300 purely quantum effects such as revivals can be seen.
Analytical solutions of the Bohr Hamiltonian with the Morse potential
Boztosun, I.; Inci, I.; Bonatsos, D.
2008-04-15
Analytical solutions of the Bohr Hamiltonian are obtained in the {gamma}-unstable case, as well as in an exactly separable rotational case with {gamma}{approx_equal}0, called the exactly separable Morse (ES-M) solution. Closed expressions for the energy eigenvalues are obtained through the asymptotic iteration method (AIM), the effectiveness of which is demonstrated by solving the relevant Bohr equations for the Davidson and Kratzer potentials. All medium mass and heavy nuclei with known {beta}{sub 1} and {gamma}{sub 1} bandheads have been fitted by using the two-parameter {gamma}-unstable solution for transitional nuclei and the three-parameter ES-M for rotational ones. It is shown that bandheads and energy spacings within the bands are well reproduced for more than 50 nuclei in each case.
Adiabatic Bohr-Sommerfeld calculations for the hydrogenic Stark effect
Byrd, David; Mitchell, David
2004-12-01
A general prescription is presented for the semiclassical calculation of quantized energy levels of a class of parametric quantum systems. Starting with the Bohr-Sommerfeld quantization conditions of an exactly solvable system in parabolic coordinates, the calculation evolves the classical equations of motion while adiabatically introducing a nonzero parameter. We present accurate results for the quadratic Stark effect in addition to the linear Stark effect, an improvement over previous semiclassical studies.
Creation of nondispersive Bohr-like wave packets
NASA Astrophysics Data System (ADS)
Mestayer, J. J.; Wyker, B.; Dunning, F. B.; Yoshida, S.; Reinhold, C. O.; Burgdörfer, J.
2009-03-01
We demonstrate the use of a periodic train of half-cycle pulses to maintain strongly-localized wave packets in very-high- n (ñ300) Rydberg atoms that travel in near-circular orbits about the nucleus. This motion can be followed for hundreds of orbital periods and mimics the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about the nucleus.
Bohr-Sommerfeld Lagrangians of moduli spaces of Higgs bundles
NASA Astrophysics Data System (ADS)
Biswas, Indranil; Gammelgaard, Niels Leth; Logares, Marina
2015-08-01
Let X be a compact connected Riemann surface of genus at least two. Let MH(r, d) denote the moduli space of semistable Higgs bundles on X of rank r and degree d. We prove that the compact complex Bohr-Sommerfeld Lagrangians of MH(r, d) are precisely the irreducible components of the nilpotent cone in MH(r, d) . This generalizes to Higgs G-bundles and also to the parabolic Higgs bundles.
Neutral and positively charged excitons in narrow quantum ring
Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.
2014-05-15
We study theoretically quantized states of a neutral and a positively charged exciton (trion X{sup +}) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.
Electronic properties of exciton and biexciton in a CdTe/ZnTe nano-heterostructure
NASA Astrophysics Data System (ADS)
Sujanah, P.; Peter, A. John
2015-06-01
Energy eigen values and the binding energies of exciton and biexciton in a CdTe/ZnTe quantum dot are studied with the geometrical confinement effect. The single exciton binding energy and the biexciton binding energy are calculated as a function of dot radius. Overlap integral of the exciton and the biexciton is found in the CdTe/ZnTe quantum dot. The electron and hole potentials are calculated from the Poisson equations. Self-consistent method is to compute the energy eigenvalues of the exciton and the biexciton. The Hartree potential is employed to obtain the Coulomb interaction energy.
NASA Astrophysics Data System (ADS)
Downie, E. J.
2016-03-01
The proton radius puzzle is the difference between the proton radius as measured with electron scattering and in the excitation spectrum of atomic hydrogen, and that measured with muonic hydrogen spectroscopy. Since the inception of the proton radius puzzle in 2010 by the measurement of Pohl et al.[1], many possible resolutions to the puzzle have been postulated, but, to date, none has been generally accepted. New data are therefore necessary to resolve the issue. We briefly review the puzzle, the proposed solutions, and the new electron scattering and spectroscopy experiments planned and underway. We then introduce the MUSE experiment, which seeks to resolve the puzzle by simultaneously measuring elastic electron and muon scattering on the proton, in both charge states, thereby providing new information to the puzzle. MUSE addresses issues of two-photon effects, lepton universality and, possibly, new physics, while providing simultaneous form factor, and therefore radius, measurements with both muons and electrons.
Jahan K, Luhluh Boda, Aalu; Chatterjee, Ashok
2015-05-15
The problem of an exciton trapped in a three dimensional Gaussian quantum dot is studied in the presence of an external magnetic field. A variational method is employed to obtain the ground state energy of the exciton as a function of the quantum dot size, the confinement strength and the magnetic field. It is also shown that the variation of the size of the exciton with the radius of the quantum dot.
Realization of localized Bohr-like wave packets.
Mestayer, J J; Wyker, B; Lancaster, J C; Dunning, F B; Reinhold, C O; Yoshida, S; Burgdörfer, J
2008-06-20
We demonstrate a protocol to create localized wave packets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wave packets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wave packets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in a circular classical orbit around the nucleus. The possible extension of the approach to create "planetary atoms" in highly correlated stable multiply excited states is discussed. PMID:18643582
Realization of Localized Bohr-Like Wave Packets
NASA Astrophysics Data System (ADS)
Mestayer, J. J.; Wyker, B.; Lancaster, J. C.; Dunning, F. B.; Reinhold, C. O.; Yoshida, S.; Burgdörfer, J.
2008-06-01
We demonstrate a protocol to create localized wave packets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wave packets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wave packets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in a circular classical orbit around the nucleus. The possible extension of the approach to create “planetary atoms” in highly correlated stable multiply excited states is discussed.
Bohr Hamiltonian with Eckart potential for triaxial nuclei
NASA Astrophysics Data System (ADS)
Naderi, L.; Hassanabadi, H.
2016-05-01
In this paper, the Bohr Hamiltonian has been solved using the Eckart potential for the β-part and a harmonic oscillator for the γ-part of the Hamiltonian. The approximate separation of the variables has been possible by choosing the convenient form for the potential V(β,γ). Using the Nikiforov-Uvarov method the eigenvalues and eigenfunctions of the eigenequation for the β-part have been derived. An expression for the total energy of the levels has been represented.
Challenges to Bohr's Wave-Particle Complementarity Principle
NASA Astrophysics Data System (ADS)
Rabinowitz, Mario
2013-02-01
Contrary to Bohr's complementarity principle, in 1995 Rabinowitz proposed that by using entangled particles from the source it would be possible to determine which slit a particle goes through while still preserving the interference pattern in the Young's two slit experiment. In 2000, Kim et al. used spontaneous parametric down conversion to prepare entangled photons as their source, and almost achieved this. In 2012, Menzel et al. experimentally succeeded in doing this. When the source emits entangled particle pairs, the traversed slit is inferred from measurement of the entangled particle's location by using triangulation. The violation of complementarity breaches the prevailing probabilistic interpretation of quantum mechanics, and benefits Bohm's pilot-wave theory.
Triplet exciton dissociation in singlet exciton fission photovoltaics.
Jadhav, Priya J; Brown, Patrick R; Thompson, Nicholas; Wunsch, Benjamin; Mohanty, Aseema; Yost, Shane R; Hontz, Eric; Van Voorhis, Troy; Bawendi, Moungi G; Bulović, Vladimir; Baldo, Marc A
2012-12-01
Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption. PMID:22968762
Distal radius triplane fracture
Marya, S; Auplish, S
2014-01-01
A triplane fracture is so named because of the three planes traversed by the fracture line. These are physeal fractures that result from injury during the final phase of maturation and cessation of growth. This fracture pattern typically involves the distal tibia. We present a rare case of a triplane fracture involving the distal radius. PMID:25350166
Molecular Basis of the Bohr Effect in Arthropod Hemocyanin
Hirota, S.; Kawahara, T; Beltramini, M; Di Muro, P; Magliozzo, R; Peisach, J; Powers, L; Tanaka, N; Nagao, S; Bubacco, L
2008-01-01
Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, kon, rather than changes in koff. In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s ? 4pz transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs.
Experimental Observation of Bohr's Nonlinear Fluidic Surface Oscillation.
Moon, Songky; Shin, Younghoon; Kwak, Hojeong; Yang, Juhee; Lee, Sang-Bum; Kim, Soyun; An, Kyungwon
2016-01-01
Niels Bohr in the early stage of his career developed a nonlinear theory of fluidic surface oscillation in order to study surface tension of liquids. His theory includes the nonlinear interaction between multipolar surface oscillation modes, surpassing the linear theory of Rayleigh and Lamb. It predicts a specific normalized magnitude of 0.416η(2) for an octapolar component, nonlinearly induced by a quadrupolar one with a magnitude of η much less than unity. No experimental confirmation on this prediction has been reported. Nonetheless, accurate determination of multipolar components is important as in optical fiber spinning, film blowing and recently in optofluidic microcavities for ray and wave chaos studies and photonics applications. Here, we report experimental verification of his theory. By using optical forward diffraction, we measured the cross-sectional boundary profiles at extreme positions of a surface-oscillating liquid column ejected from a deformed microscopic orifice. We obtained a coefficient of 0.42 ± 0.08 consistently under various experimental conditions. We also measured the resonance mode spectrum of a two-dimensional cavity formed by the cross-sectional segment of the liquid jet. The observed spectra agree well with wave calculations assuming a coefficient of 0.414 ± 0.011. Our measurements establish the first experimental observation of Bohr's hydrodynamic theory. PMID:26803911
Engineering directed excitonic energy transfer
Perdomo, Alejandro; Vogt, Leslie; Najmaie, Ali; Aspuru-Guzik, Alan
2010-01-01
We provide an intuitive platform for engineering exciton transfer dynamics. We show that careful consideration of the spectral density, which describes the system-bath interaction, leads to opportunities to engineer exciton transfer. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dynamics. We carefully describe the validity of the model and use experimentally relevant material parameters to show counter-intuitive examples of directed exciton transfer in a linear chain of quantum dots.
Exciton multiplication from first principles.
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-phonon energy relaxation. Multiple excitons are generated through impact ionization within picoseconds. The basis of exciton multiplication in quantum dots is the collective result of photoexcitation, dephasing, and nonadiabatic evolution. Each process is characterized by a distinct time-scale, and the overall multiple exciton generation dynamics is complete by about 10 ps. Without relying on semiempirical parameters, we computed quantum mechanical probabilities of multiple excitons for small model systems. Because exciton correlations and coherences are microscopic, quantum properties, results for small model systems can be extrapolated to larger, realistic quantum dots. PMID:23459543
A comparative study of the kinetics of the Bohr effect in vertebrates.
Maren, T H; Swenson, E R
1980-01-01
1. The kinetics of the Bohr effect and the role of carbonic anhydrase were studied in a series of representative vertebrates using a continuous flow rapid reaction apparatus. 2. The rates of the Bohr effect in vertebrates are very similar, and differences among classes are manifestations of the ambient temperature. 3. Complete carbonic anhydrase inhibition causes a fifteen to fortyfold reduction in the rate of the Bohr effect, sufficient to abolish its occurrence within capillary transit. 4. There is a twenty-three-fold (duck) to 360-fold (man) excess of carbonic anhydrase activity in vertebrate red cells for the normal generation of the Bohr effect. 5. When carbonic anhydrase is inhibited and CO2 hydration becomes rate limiting, the stoichiometry of the Bohr effect (delta log pO2/delta pH) is revealed in the ratio of the rates of proton formation in red cells to O2 release from haemoglobin. PMID:6776264
Singlet exciton fission photovoltaics.
Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A
2013-06-18
Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption. PMID:23611026
Exciton Fine Structure in Single CdSe Nanorods
NASA Astrophysics Data System (ADS)
Le Thomas, N.; Herz, E.; Schöps, O.; Woggon, U.; Artemyev, M. V.
2005-01-01
We study the optical properties of excitons in one-dimensional (1D) nanostructures at low temperatures. In single CdSe/ZnS core-shell nanorods we observe a fine structure splitting and explain it by exchange interaction. Two peaks are observed with different degrees of linear polarization of DLP<0.85 and DLP>0.95. For small nanorod radii R≤aB/2, an increase in the photoluminescence decay time is found when the temperature increases from 10 to 80K. The observations are explained by a radius-dependent change in the symmetry of the 1D-exciton ground state which transforms from a dark state into bright states below a critical radius of Rcrit≈3.7 nm.
NASA Astrophysics Data System (ADS)
Ye, Yu; Wong, Zi Jing; Lu, Xiufang; Ni, Xingjie; Zhu, Hanyu; Chen, Xianhui; Wang, Yuan; Zhang, Xiang
2015-11-01
Two-dimensional van der Waals materials have opened a new paradigm for fundamental physics exploration and device applications because of their emerging physical properties. Unlike gapless graphene, monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional semiconductors that undergo an indirect-to-direct bandgap transition, creating new optical functionalities for next-generation ultra-compact photonics and optoelectronics. Although the enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystals and distributed Bragg reflector microcavities, coherent light emission from a TMDC monolayer has not been demonstrated. Here, we report the realization of a two-dimensional excitonic laser by embedding monolayer WS2 in a microdisk resonator. Using a whispering gallery mode with a high quality factor and optical confinement, we observe bright excitonic lasing at visible wavelengths. This demonstration of a two-dimensional excitonic laser marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications.
Ma, Xuedan; Roslyak, Oleskiy; Duque, Juan G; Pang, Xiaoying; Doorn, Stephen K; Piryatinski, Andrei; Dunlap, David H; Htoon, Han
2015-07-01
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. PMID:26182119
Exciton Transport in Organic Semiconductors
NASA Astrophysics Data System (ADS)
Menke, Stephen Matthew
Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (L D) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on L D are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors.
Exciton Energy Transfer between Asymmetric Quantum Wires
NASA Astrophysics Data System (ADS)
Lyo, S. K.; Karlsson, K. F.; Weman, H.; Leifer, K.; Rudra, A.; Kapon, E.
2006-03-01
We present theoretical result and data for the Stokes exciton transfer rate from a narrow quantum wire (n-QWR) to a parallel wide QWR (w-QWR) separated by a wide barrier and also to an array of parallel w-QWRs. The transfer rate is calculated as a function of the distance d between n-QWR and w-QWR and also the array. The dependence of the rate on the temperature and the localization radius is studied for free and localized excitons, respectively. Both the resonant and non-resonant rates are considered. We find that, for energy transfer between two QWRs, the Förster dipole-dipole transfer dominates the transfer rate at short and intermediate distances. The photon-exchange transfer prevails only at an extremely long distance where the rate is negligibly small. This behavior is in contrast with the two-dimensional quantum wells, where the photon-exchange mechanism is dominant except at a very short distance. However, the photon-exchange transfer rate continues to increase as the array size grows to a macroscopic scale due to its slow range dependence while the dipolar rate saturates quickly with the array size. The prediction of the theory is consistent with the data from V-groove GaAs/AlxGa1-xAs double QWRs. Supported by the US DOE (SKL), Swedish Foundation for Strategic Research, Swedish Research Council, and Ericsson's Research Foundation.
Turnaround radius in modified gravity
NASA Astrophysics Data System (ADS)
Faraoni, Valerio
2016-03-01
In an accelerating universe in General Relativity there is a maximum radius above which a shell of test particles cannot collapse, but is dispersed by the cosmic expansion. This radius could be used in conjunction with observations of large structures to constrain the equation of state of the universe. We extend the concept of turnaround radius to modified theories of gravity for which the gravitational slip is non-vanishing.
Van der Waals Interactions and Exciton Condensation
Handel, P. H.; Kittel, C.
1971-01-01
It is shown that the van der Waals interaction can lead at low temperatures to a condensed state of excitons with properties in qualitative agreement with the observations of exciton droplets. Our calculation gives a binding energy of the correct sign and magnitude for the exciton condensate. In a diclectric medium, the strong enhancement of the exciton polarizability leads to a giant van der Waals interaction, and this interaction appears to make possible a condensed exciton phase. PMID:16591958
Momentum dependence of the excitons in pentacene
Roth, Friedrich; Schuster, Roman; Koenig, Andreas; Knupfer, Martin; Berger, Helmuth
2012-05-28
We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.
Bohr's Electron was Problematic for Einstein: String Theory Solved the Problem
NASA Astrophysics Data System (ADS)
Webb, William
2013-04-01
Neils Bohr's 1913 model of the hydrogen electron was problematic for Albert Einstein. Bohr's electron rotates with positive kinetic energies +K but has addition negative potential energies - 2K. The total net energy is thus always negative with value - K. Einstein's special relativity requires energies to be positive. There's a Bohr negative energy conflict with Einstein's positive energy requirement. The two men debated the problem. Both would have preferred a different electron model having only positive energies. Bohr and Einstein couldn't find such a model. But Murray Gell-Mann did! In the 1960's, Gell-Mann introduced his loop-shaped string-like electron. Now, analysis with string theory shows that the hydrogen electron is a loop of string-like material with a length equal to the circumference of the circular orbit it occupies. It rotates like a lariat around its centered proton. This loop-shape has no negative potential energies: only positive +K relativistic kinetic energies. Waves induced on loop-shaped electrons propagate their energy at a speed matching the tangential speed of rotation. With matching wave speed and only positive kinetic energies, this loop-shaped electron model is uniquely suited to be governed by the Einstein relativistic equation for total mass-energy. Its calculated photon emissions are all in excellent agreement with experimental data and, of course, in agreement with those -K calculations by Neils Bohr 100 years ago. Problem solved!
Exactly separable Bohr Hamiltonian with the Killingbeck potential for triaxial nuclei
NASA Astrophysics Data System (ADS)
Neyazi, H.; Rajabi, A. A.; Hassanabadi, H.
2016-01-01
After pioneering work by Bohr, Mottelson and their numerous colleagues, the essential framework for understanding collective model is introduced. One of the applications of this framework is the study of shape phase transition, vibrational and rotational energy spectrum of nuclei. We consider the Bohr Hamiltonian and solve the beta and gamma part equation of it, by considering that reduced potential and wave function are exactly separable. In the beta part equation we consider the Killingbeck potential and derive the wave function and energy spectrum of it.
Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites
NASA Astrophysics Data System (ADS)
Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.
2016-03-01
UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.
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.
Exciton dispersion in molecular solids.
Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo
2015-03-25
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. PMID:25738755
Revised Fine Splitting of Excitons in Diamond
NASA Astrophysics Data System (ADS)
Sauer, R.; Sternschulte, H.; Wahl, S.; Thonke, K.; Anthony, T. R.
2000-05-01
We study low-strain synthetic high pressure, high temperature diamonds by cathodoluminescence and observe novel fine structure in the free exciton and the boron-bound exciton emission. The basic spectral structure is a doublet with ΔE~11 meV common to both exciton spectra. This resolves the previously found inequivalence of free exciton ( ~7 meV) and bound exciton ( ~12 meV) fine splitting. It is argued that for a spin-orbit interaction Δ0 much smaller than the excitonic binding ( EX~80 meV) and the excitonic localization ( Eloc~51 meV) at the boron acceptor, the orbital momentum and the spin of the particles constituting the electron-hole pair are recoupled to form spin singlet and triplet exciton states as the elementary excitations.
The Pb radius experiment (PREX)
NASA Astrophysics Data System (ADS)
Mammei, Juliette M.
2013-10-01
We report the first measurement of the parity-violating asymmetry APV in the elastic scattering of polarized electrons from 208Pb from the Lead Radius Experiment PREX which ran in Hall A at the Thomas Jefferson National Accelerator Facility (JLab). APV is sensitive to the radius of the neutron distribution Rn. The Z boson that mediates the weak neutral interaction couples mainly to neutrons and provides a clean, model-independent measurement of the RMS radius Rn of the neutron distribution in the nucleus and is a fundamental test of nuclear structure theory. The result, APV = 0.656±0.060(stat)±0.014(syst) ppm, corresponds to a difference between the radii of the neutron and proton distributions Rn-Rp = 0.33-0.18+0.16 fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.
Exciton dynamicsstudied via internal THz transitions
Kaindl, R.A.; Hagele, D.; Carnahan, M.A.; Lovenich, R.; Chemla,D.S.
2003-02-26
We employ a novel, ultrafast terahertz probe to investigatethe dynamical interplay of optically-induced excitons and unboundelectron-hole pairs in GaAs quantum wells. Resonant creation ofheavy-hole excitons induces a new low-energy oscillator linked totransitions between the internal exciton degrees of freedom. The timeresolved terahertz optical conductivity is found to be a probe wellsuited for studies of fundamental processes such as formation, relaxationand ionization of excitons.
Caprio, M. A.
2011-06-15
Detailed quantitative predictions are obtained for phonon and multiphonon excitations in well-deformed rotor nuclei within the geometric framework, by exact numerical diagonalization of the Bohr Hamiltonian in an SO(5) basis. Dynamical {gamma} deformation is found to significantly influence the predictions through its coupling to the rotational motion. Basic signatures for the onset of rigid triaxial deformation are also obtained.
Why We Should Teach the Bohr Model and How to Teach it Effectively
ERIC Educational Resources Information Center
McKagan, S. B.; Perkins, K. K.; Wieman, C. E.
2008-01-01
Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits students' ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary school…
What Can the Bohr-Sommerfeld Model Show Students of Chemistry in the 21st Century?
ERIC Educational Resources Information Center
Niaz, Mansoor; Cardellini, Liberato
2011-01-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. To increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study aims to elaborate a framework…
Why We Should Teach the Bohr Model and How to Teach it Effectively
ERIC Educational Resources Information Center
McKagan, S. B.; Perkins, K. K.; Wieman, C. E.
2008-01-01
Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits students' ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary school
Quantum Explorers: Bohr, Jordan, and Delbrück Venturing into Biology
NASA Astrophysics Data System (ADS)
Joaquim, Leyla; Freire, Olival; El-Hani, Charbel N.
2015-09-01
This paper disentangles selected intertwined aspects of two great scientific developments: quantum mechanics and molecular biology. We look at the contributions of three physicists who in the 1930s were protagonists of the quantum revolution and explorers of the field of biology: Niels Bohr, Pascual Jordan, and Max Delbrück. Their common platform was the defense of the Copenhagen interpretation in physics and the adoption of the principle of complementarity as a way of looking at biology. Bohr addressed the problem of how far the results reached in physics might influence our views about life. Jordan and Delbrück were followers of Bohr's ideas in the context of quantum mechanics and also of his tendency to expand the implications of the Copenhagen interpretation to biology. We propose that Bohr's perspective on biology was related to his epistemological views, as Jordan's was to his political positions. Delbrück's propensity to migrate was related to his transformation into a key figure in the history of twentieth-century molecular biology.
EPR before EPR: A 1930 Einstein-Bohr thought Experiment Revisited
ERIC Educational Resources Information Center
Nikolic, Hrvoje
2012-01-01
In 1930, Einstein argued against the consistency of the time-energy uncertainty relation by discussing a thought experiment involving a measurement of the mass of the box which emitted a photon. Bohr seemingly prevailed over Einstein by arguing that Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisit
EPR before EPR: A 1930 Einstein-Bohr thought Experiment Revisited
ERIC Educational Resources Information Center
Nikolic, Hrvoje
2012-01-01
In 1930, Einstein argued against the consistency of the time-energy uncertainty relation by discussing a thought experiment involving a measurement of the mass of the box which emitted a photon. Bohr seemingly prevailed over Einstein by arguing that Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisit…
What Can the Bohr-Sommerfeld Model Show Students of Chemistry in the 21st Century?
ERIC Educational Resources Information Center
Niaz, Mansoor; Cardellini, Liberato
2011-01-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. To increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study aims to elaborate a framework
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.
Excitonic polaritons in Fibonacci quasicrystals.
Hendrickson, J; Richards, B C; Sweet, J; Khitrova, G; Poddubny, A N; Ivchenko, E L; Wegener, M; Gibbs, H M
2008-09-29
The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission. PMID:18825174
Exciton-photon correlations in bosonic condensates of exciton-polaritons.
Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G
2015-01-01
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979
Exciton-photon correlations in bosonic condensates of exciton-polaritons
Kavokin, Alexey V.; Sheremet, Alexandra S.; Shelykh, Ivan A.; Lagoudakis, Pavlos G.; Rubo, Yuri G.
2015-01-01
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979
NASA Astrophysics Data System (ADS)
Iizuka, Hideyuki; Nakayama, Takashi
2016-02-01
Quantum processes of bound exciton dissociation at organic donor/acceptor interfaces were studied by the time-evolution simulation of a wave packet, focusing on the effects of interface roughness and hot excitons. It is shown that the dissociation is remarkably enhanced by the interface roughness that is comparable to the exciton diameter. Moreover, the dissociation probability increases for the hot excitons compared with the ground-state exciton owing to their small binding energies and large diameters.
Wannier-Mott excitons in isotope-disordered crystals
NASA Astrophysics Data System (ADS)
Plekhanov, V. G.
1998-08-01
Most of the physical properties of a solid depend to a greater or lesser degree on its isotopic composition. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor and insulator crystals. A systematic analysis is for the first time presented of isotopic and disorder effects observed in crystals of various isotopic composition via low-temperature large-radius exciton spectroscopy. Substituting a light isotope with a heavy one increases the interband transition energy and the binding energy of the Wannier-Mott exciton as well as the magnitude of the longitudinal-transverse splitting. The nonlinear variation of these quantities with the isotope concentration is due to the isotopic disordering of the crystal lattice and is consistent with the concentration dependence of line half-widths in exciton reflection and luminescence spectra. The common nature of the isotopic and disorder effects in the crystals of C, LiH, ZnO, ZnSe, CuCl, CdS, 0034-4885/61/8/003/img2, GaAs, Si and Ge is emphasized. The review closes with an outlook on the exciting future possibilities offered through isotope control of a wide range of semiconductor and insulator crystals.
Exciton size and quantum transport in nanoplatelets.
Pelzer, Kenley M; Darling, Seth B; Gray, Stephen K; Schaller, Richard D
2015-12-14
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport. PMID:26671357
Exciton size and quantum transport in nanoplatelets
NASA Astrophysics Data System (ADS)
Pelzer, Kenley M.; Darling, Seth B.; Gray, Stephen K.; Schaller, Richard D.
2015-12-01
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.
Measurement of solar radius changes
NASA Technical Reports Server (NTRS)
Labonte, B. J.; Howard, R.
1981-01-01
Results of daily photometric measurements of the solar radius from Mt. Wilson over the past seven years are reported. Reduction of the full disk magnetograms yields a formal error of 0.1 arcsec in the boustrophedonic scans in the 5250.2 A FeI line. 150 scan lines comprise each observation; 1,412 observations were made from 1974-1981. Measurement procedures, determination of the scattered light of the optics and the atmosphere, and error calculations are described, noting that days of poor atmospheric visibility are omitted from the data. The horizontal diameter of the sun remains visually fixed while the vertical component changes due to atmospheric diffraction; error accounting for thermal effects, telescope aberrations, and instrument calibration are discussed, and results, within instrument accuracy, indicate no change in the solar radius over the last seven years.
Electric arc radius and characteristics
Fang, T.M.
1980-09-30
The heat transfer equation of an arc discharge has been solved. The arc is assumed to be a cylinder with negligible axial variation and the dominant heat transfer process is conduction radially inside the column and radiation/convection at the outside edge. The symmetric consideration allows a simple one-dimensional formulation. By taking into account proper variation of the electrical conductivity as function of temperature, the heat balance equation has been solved analytically. The radius of the arc and its current-field characteristics have also been obtained. The conventional results that E approx. I/sup 0/ /sup 5385/ and R approx. I/sup 0/ /sup 7693/ with E being the applied field, I the current, and R the radius of the cylindrical arc, have been proved to be simply limiting cases of our more general characteristics. The results can be applied quite widely including, among others, the neutral beam injection project in nuclear fusion and MHD energy conversion.
Rate limiting processes in the bohr shift in human red cells
Forster, R. E.; Steen, J. B.
1968-01-01
1. The rates of the Bohr shift of human red cells and some of its constituent reactions have been studied with a modified Hartridge—Roughton rapid reaction apparatus using an oxygen electrode to measure the progress of the reaction. 2. The rate of the Bohr shift was compatible with the hypothesis that the transfer of H+ across the membrane by means of CO2 exchange and reaction with buffers is generally the rate-limiting step. (a) When the Bohr off-reaction was produced by a marked increase in PCO2 around the cells, the half-time at 37° C was 0·12 sec. In this case CO2 was available initially to diffuse into the cells, the process being predominantly limited by the rate of intracellular CO2 hydration. (b) When the Bohr off-shift was produced by an increase of [H+] outside the cell, PCO2 being low and equal within and outside the cells, the half time became 0·31 sec. In this case, even at the start, the H2CO3 formed by the almost instantaneous neutralization reaction of H+ and HCO3- had to dehydrate to form CO2 and this in turn had to diffuse into and react within the red cell before the [HbO2] could change. When a carbonic anhydrase inhibitor was added to slow the CO2 reaction inside the cell, the half-time rose to 10 sec. (c) The Bohr off-shift in a haemolysed cell suspension produced by an increase in PCO2 appeared to be limited by the rate at which the CO2 could hydrate to form H+. 3. The Bohr off-shift has an average Q10 of 2·5 between 42·5 and 28° C with an activation energy of 8000 cal. 4. The pronounced importance of the CO2-bicarbonate system for rapid intracellular pH changes is discussed in connexion with some physiological situations. PMID:5664232
Spatially indirect excitons in coupled quantum wells
Lai, Chih-Wei Eddy
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.
Coherent Exciton Dynamics in Atomically Thin Semiconductors
NASA Astrophysics Data System (ADS)
Li, Xiaoqin (Elaine)
The near band-edge optical response of an emerging class of semiconductors, known as the transitional metal dichalcogenides (TMDs), is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is quantum decoherence time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Dephasing time is also equivalent to the intrinsic homogeneous linewidth of exciton resonance. In addition, excitons in TMDs are localized at the corners of the Brillouin zone and provide a convenient way to optical manipulate the valley degree of freedom, which may act as a useful information carrier analogous to electronic charge or spin. Direct measurement of valley coherence time is challenging because it corresponds to a non-radiative coherence between two degenerate states. Using ultrafast multi-dimensional optical spectroscopy, we investigate the intrinsic homogeneous linewidth of excitons, exciton valley coherence as well as coupling between excitons and trions. Our studies reveal coherent electronic dynamics on the order of ~100 fs in monolayer TMDs. We gratefully acknowledge financial support from NSF, AFOSR, and the Welch Foundation.
Contrastive analysis of multiple exciton generation theories
NASA Astrophysics Data System (ADS)
Tan, Hengyu; Chang, Qing
2015-10-01
Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.
Darwinism in disguise? A comparison between Bohr's view on quantum mechanics and QBism.
Faye, Jan
2016-05-28
The Copenhagen interpretation is first and foremost associated with Niels Bohr's philosophy of quantum mechanics. In this paper, I attempt to lay out what I see as Bohr's pragmatic approach to science in general and to quantum physics in particular. A part of this approach is his claim that the classical concepts are indispensable for our understanding of all physical phenomena, and it seems as if the claim is grounded in his reflection upon how the evolution of language is adapted to experience. Another, recent interpretation, QBism, has also found support in Darwin's theory. It may therefore not be surprising that sometimes QBism is said to be of the same breed as the Copenhagen interpretation. By comparing the two interpretations, I conclude, nevertheless, that there are important differences. PMID:27091172
Quantum Humor: The Playful Side of Physics at Bohr's Institute for Theoretical Physics
NASA Astrophysics Data System (ADS)
Halpern, Paul
2012-09-01
From the 1930s to the 1950s, a period of pivotal developments in quantum, nuclear, and particle physics, physicists at Niels Bohr's Institute for Theoretical Physics in Copenhagen took time off from their research to write humorous articles, letters, and other works. Best known is the Blegdamsvej Faust, performed in April 1932 at the close of one of the Institute's annual conferences. I also focus on the Journal of Jocular Physics, a humorous tribute to Bohr published on the occasions of his 50th, 60th, and 70th birthdays in 1935, 1945, and 1955. Contributors included Léon Rosenfeld, Victor Weisskopf, George Gamow, Oskar Klein, and Hendrik Casimir. I examine their contributions along with letters and other writings to show that they offer a window into some issues in physics at the time, such as the interpretation of complementarity and the nature of the neutrino, as well as the politics of the period.
Conceptual objections to the Bohr atomic theory — do electrons have a "free will" ?
NASA Astrophysics Data System (ADS)
Kragh, Helge
2011-11-01
The atomic model introduced by Bohr in 1913 dominated the development of the old quantum theory. Its main features, such as the radiationless stationary states and the discontinuous quantum jumps between the states, were hard to swallow for contemporary physicists. While acknowledging the empirical power of the theory, many scientists criticized its foundation or looked for ways to reconcile it with classical physics. Among the chief critics were A. Crehore, J.J. Thomson, E. Gehrcke and J. Stark. This paper examines from a historical perspective the conceptual objections to Bohr's atom, in particular the stationary states (where electrodynamics was annulled by fiat) and the mysterious, apparently teleological quantum jumps. Although few of the critics played a constructive role in the development of the old quantum theory, a history neglecting their presence would be incomplete and distorted.
The cognitive nexus between Bohr's analogy for the atom and Pauli's exclusion schema.
Ulazia, Alain
2016-03-01
The correspondence principle is the primary tool Bohr used to guide his contributions to quantum theory. By examining the cognitive features of the correspondence principle and comparing it with those of Pauli's exclusion principle, I will show that it did more than simply 'save the phenomena'. The correspondence principle in fact rested on powerful analogies and mental schemas. Pauli's rejection of model-based methods in favor of a phenomenological, rule-based approach was therefore not as disruptive as some historians have indicated. Even at a stage that seems purely phenomenological, historical studies of theoretical development should take into account non-formal, model-based approaches in the form of mental schemas, analogies and images. In fact, Bohr's images and analogies had non-classical components which were able to evoke the idea of exclusion as a prohibition law and as a preliminary mental schema. PMID:26803549
Verification of Bohr's frequency condition and Moseley's law: An undergraduate laboratory experiment
NASA Astrophysics Data System (ADS)
Gudennavar, S. B.; Badiger, N. M.; Thontadarya, S. R.; Hanumaiah, B.
2003-08-01
We describe an undergraduate laboratory experiment to verify Bohr's frequency condition and Moseley's law using a thin NaI(Tl) detector spectrometer and a weak 57Co source. The slope of the plot of Kα x-ray energy versus (Z-1)2 yields a value for the Rydberg constant, R=(1.19±0.01)×107 m-1, which is in fair agreement with the best literature value, R=10 973 731.534(13) m-1.
How Sommerfeld extended Bohr's model of the atom (1913-1916)
NASA Astrophysics Data System (ADS)
Eckert, Michael
2014-04-01
Sommerfeld's extension of Bohr's atomic model was motivated by the quest for a theory of the Zeeman and Stark effects. The crucial idea was that a spectral line is made up of coinciding frequencies which are decomposed in an applied field. In October 1914 Johannes Stark had published the results of his experimental investigation on the splitting of spectral lines in hydrogen (Balmer lines) in electric fields, which showed that the frequency of each Balmer line becomes decomposed into a multiplet of frequencies. The number of lines in such a decomposition grows with the index of the line in the Balmer series. Sommerfeld concluded from this observation that the quantization in Bohr's model had to be altered in order to allow for such decompositions. He outlined this idea in a lecture in winter 1914/15, but did not publish it. The First World War further delayed its elaboration. When Bohr published new results in autumn 1915, Sommerfeld finally developed his theory in a provisional form in two memoirs which he presented in December 1915 and January 1916 to the Bavarian Academy of Science. In July 1916 he published the refined version in the Annalen der Physik. The focus here is on the preliminary Academy memoirs whose rudimentary form is better suited for a historical approach to Sommerfeld's atomic theory than the finished Annalen-paper. This introductory essay reconstructs the historical context (mainly based on Sommerfeld's correspondence). It will become clear that the extension of Bohr's model did not emerge in a singular stroke of genius but resulted from an evolving process.
Bohr-Sommerfeld quantization condition for Dirac states derived from an Ermakov-type invariant
Thylwe, Karl-Erik; McCabe, Patrick
2013-05-15
It is shown that solutions of the second-order decoupled radial Dirac equations satisfy Ermakov-type invariants. These invariants lead to amplitude-phase-type representations of the radial spinor solutions, with exact relations between their amplitudes and phases. Implications leading to a Bohr-Sommerfeld quantization condition for bound states, and a few particular atomic/ionic and nuclear/hadronic bound-state situations are discussed.
Electric quadrupole transitions of the Bohr Hamiltonian with the Morse potential
Inci, I.; Bonatsos, D.; Boztosun, I.
2011-08-15
Eigenfunctions of the collective Bohr Hamiltonian with the Morse potential have been obtained by using the asymptotic iteration method (AIM) for both {gamma}-unstable and rotational structures. B(E2) transition rates have been calculated and compared to experimental data. Overall good agreement is obtained for transitions within the ground-state band, while some interband transitions appear to be systematically underpredicted in {gamma}-unstable nuclei and overpredicted in rotational nuclei.
Discreteness of the volume of space from Bohr-Sommerfeld quantization.
Bianchi, Eugenio; Haggard, Hal M
2011-07-01
A major challenge for any theory of quantum gravity is to quantize general relativity while retaining some part of its geometrical character. We present new evidence for the idea that this can be achieved by directly quantizing space itself. We compute the Bohr-Sommerfeld volume spectrum of a tetrahedron and show that it reproduces the quantization of a grain of space found in loop gravity. PMID:21797533
Closed analytical solutions of Bohr Hamiltonian with Manning-Rosen potential model
NASA Astrophysics Data System (ADS)
Chabab, M.; Lahbas, A.; Oulne, M.
2015-11-01
In the present paper, we have obtained closed analytical expressions for eigenvalues and eigenfunctions of the Bohr Hamiltonian with the Manning-Rosen potential for γ-unstable nuclei as well as exactly separable rotational ones with γ ≈ 0. Some heavy nuclei with known β and γ bandheads have been fitted by using two parameters in the γ-unstable case and three parameters in the axially symmetric prolate deformed one. A good agreement with experimental data has been achieved.
The theory of the Bohr-Weisskopf effect in the hyperfine structure
NASA Astrophysics Data System (ADS)
Karpeshin, F. F.; Trzhaskovskaya, M. B.
2015-09-01
Description of the Bohr-Wesskopf effect in the hyperfine structure of few-electron heavy ions is a challenging problem, which can be used as a test of both QED and atomic calculations. However, for twenty years the research has actually been going in a wrong direction, aimed at fighting the Bohr-Weisskopf effect through its cancellation in specific differences. Alternatively, we propose the constructive model-independent way, which enables the nuclear radii and their momenta to be retrieved from the hyper-fine splitting (HFS). The way is based on analogy of HFS to internal conversion coefficients, and the Bohr-Weisskopf effect - to the anomalies in the internal conversion coefficients. It is shown that the parameters which can be extracted from the data are the even nuclear momenta of the magnetization distribution. The radii R2 and - for the first time - R4 are obtained in this way by analysis of the experimental HFS values for the H- and Li-like ions of 209Bi. The critical prediction concerning the HFS for the 2p1/2 state is made. The present analysis shows high sensitivity of the method to the QED effects, which offers a way of precision test of QED. Experimental recommendations are given, which are aimed at retrieving data on the HFS values for a set of a few-electron configurations of each atom.
Bohr Hamiltonian with a deformation-dependent mass term: physical meaning of the free parameter
NASA Astrophysics Data System (ADS)
Bonatsos, Dennis; Minkov, N.; Petrellis, D.
2015-09-01
Embedding the five-dimensional (5D) space of the Bohr Hamiltonian with a deformation-dependent mass (DDM) into a six-dimensional (6D) space shows that the free parameter in the dependence of the mass on the deformation is connected to the curvature of the 5D space, with the special case of constant mass corresponding to a flat 5D space. Comparison of the DDM Bohr Hamiltonian to the 5D classical limit of Hamiltonians of the 6D interacting boson model (IBM), shows that the DDM parameter is proportional to the strength of the pairing interaction in the U(5) (vibrational) symmetry limit, while it is proportional to the quadrupole-quadrupole interaction in the SU(3) (rotational) symmetry limit, and to the difference of the pairing interactions among s, d bosons and d bosons alone in the O(6) (γ-soft) limit. The presence of these interactions leads to a curved 5D space in the classical limit of IBM, in contrast to the flat 5D space of the original Bohr Hamiltonian, which is made curved by the introduction of the DDM.
Quantum size effects on exciton states in indirect-gap quantum dots
NASA Astrophysics Data System (ADS)
Feng, D. H.; Xu, Z. Z.; Jia, T. Q.; Li, X. X.; Gong, S. Q.
2003-07-01
We investigate exciton ground states in Si and 3C-SiC quantum dots by using the effective mass theory, taking account of the conduction- and valence-band mass anisotropy as well as the small spin-orbit splitting energy. The degenerate hole and exciton states are partly split by the mass anisotropy. The anisotropy splitting energies in quantum dots are different dramatically from their bulk value due to quantum size effects. The assumed changeable spin-orbit splitting energy may change the ordering of the anisotropy-split energy levels. Taking account of the exchange interaction, the degeneracy of the exciton states is further lifted. Due to the anisotropy and exchange splitting, the 48-fold exciton ground state will be split into two 18-fold triplets and two 6-fold singlets. The lowest three states are optically forbidden for Si quantum dots, which leads to a Stokes shift of luminescence. The theroretical shift agrees well with the experimental data. Furthermore, the exciton band gap and binding energy as a function of dot radius are presented both for Si and for 3C-SiC quantum dots. The band gap of Si quantum dots agrees well with the recent photoluminescence results of size-separated quantum dots by Ledoux et al. and absorption data of Furukawa et al.
Exciton Seebeck effect in molecular systems
Yan, Yun-An; Cai, Shaohong
2014-08-07
We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.
NASA Astrophysics Data System (ADS)
Tanona, Scott Daniel
I develop a new analysis of Niels Bohr's Copenhagen interpretation of quantum mechanics by examining the development of his views from his earlier use of the correspondence principle in the so-called 'old quantum theory' to his articulation of the idea of complementarity in the context of the novel mathematical formalism of quantum mechanics. I argue that Bohr was motivated not by controversial and perhaps dispensable epistemological ideas---positivism or neo-Kantianism, for example---but by his own unique perspective on the difficulties of creating a new working physics of the internal structure of the atom. Bohr's use of the correspondence principle in the old quantum theory was associated with an empirical methodology that used this principle as an epistemological bridge to connect empirical phenomena with quantum models. The application of the correspondence principle required that one determine the validity of the idealizations and approximations necessary for the judicious use of classical physics within quantum theory. Bohr's interpretation of the new quantum mechanics then focused on the largely unexamined ways in which the developing abstract mathematical formalism is given empirical content by precisely this process of approximation. Significant consistency between his later interpretive framework and his forms of argument with the correspondence principle indicate that complementarity is best understood as a relationship among the various approximations and idealizations that must be made when one connects otherwise meaningless quantum mechanical symbols to empirical situations or 'experimental arrangements' described using concepts from classical physics. We discover that this relationship is unavoidable not through any sort of a priori analysis of the priority of classical concepts, but because quantum mechanics incorporates the correspondence approach in the way in which it represents quantum properties with matrices of transition probabilities, the empirical meaning of which depend on the situation but in general are tied to the correspondence connection to the spectra. For Bohr, it is then the commutation relations, which arise from the formalism, which inform us of the complementary nature of this approximate representation of quantum properties via the classical equations through which we connect them to experiments.
Exciton Binding Energy of Monolayer WS2
Zhu, Bairen; Chen, Xi; Cui, Xiaodong
2015-01-01
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 0.01?eV around K valley in the Brillouin zone. PMID:25783023
Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong
2015-01-15
The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. Moreover, it is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.
Plasmon and Exciton Coupling and Purcell Enhancement
NASA Astrophysics Data System (ADS)
Rice, Quinton; Rigo, Maria Veronica; Fudala, Rafal; Cho, Hyoyeong; Kim, Wan-Joong; Rich, Ryan; Tabibi, Bagher; Gryczynski, Zygmunt; Gryczynski, Ignacy; Yu, William; Seo, Jaetae
2014-05-01
The photoluminescence from plasmon-coupled exciton is of great interest for optoelectronic applications, because of the large quantum yield with localized field enhancement and reduced nonradiative transition. The Coulomb interaction through plasmon-exciton coupling results in the Purcell enhancement of quantum dots (QDs) in the vicinity of metal nanoparticles (MNPs). With plasmon-exciton coupling, the radiative and non-radiative decay rates and the coupling rates compete with each other. The coupling rate is closely related to the coupling distance between QDs and MNPs. The optimized coupling distance scales the re-excitation density of localized fields and the plasmon-exciton coupling rates. If the plasmon-exciton coupling rate is much faster than the radiative and non-radiative transitions of excitons, the re-excitations of excitons by the localized plasmonic field and the reduction of non-radiative transitions may occur. This presentation includes plasmon-exciton coupling dynamics, large enhancement and temporal properties of PL, and dipole-PL polarization fidelity of hybrid optical materials of plasmonic nanometals and excitonic semiconductor QDs. The work at Hampton University was supported by the National Science Foundation (NSF HRD-1137747), and Army Research Office (ARO W911NF-11-1-0177). The work at University of North Texas was supported by National Institutes of Health (NIH R01EB12003, and 5R21CA14897 (Z.G.)).
Exciton circular dichroism in channelrhodopsin.
Pescitelli, Gennaro; Kato, Hideaki E; Oishi, Satomi; Ito, Jumpei; Maturana, Andrés Daniel; Nureki, Osamu; Woody, Robert W
2014-10-16
Channelrhodopsins (ChRs) are of great interest currently because of their important applications in optogenetics, the photostimulation of neurons. The absorption and circular dichroism (CD) spectra of C1C2, a chimera of ChR1 and ChR2 of Chlamydomonas reinhardtii, have been studied experimentally and theoretically. The visible absorption spectrum of C1C2 shows vibronic fine structure in the 470 nm band, consistent with the relatively nonpolar binding site. The CD spectrum has a negative band at 492 nm (Δε(max) = -6.17 M(-1) cm(-1)) and a positive band at 434 nm (Δε(max) = +6.65 M(-1) cm(-1)), indicating exciton coupling within the C1C2 dimer. Time-dependent density functional theory (TDDFT) calculations are reported for three models of the C1C2 chromophore: (1) the isolated protonated retinal Schiff base (retPSB); (2) an ion pair, including the retPSB chromophore, two carboxylate side chains (Asp 292, Glu 162), modeled by acetate, and a water molecule; and (3) a hybrid quantum mechanical/molecular mechanical (QM/MM) model depicting the binding pocket, in which the QM part consists of the same ion pair as that in (2) and the MM part consists of the protein residues surrounding the ion pair within 10 Å. For each of these models, the CD of both the monomer and the dimer was calculated with TDDFT. For the dimer, DeVoe polarizability theory and exciton calculations were also performed. The exciton calculations were supplemented by calculations of the coupling of the retinal transition with aromatic and peptide group transitions. For the dimer, all three methods and three models give a long-wavelength C2-axis-polarized band, negative in CD, and a short-wavelength band polarized perpendicular to the C2 axis with positive CD, differing in wavelength by 1-5 nm. Only the retPSB model gives an exciton couplet that agrees qualitatively with experiment. The other two models give a predominantly or solely positive band. We further analyze an N-terminal truncated mutant because it was assumed that the N-terminal domain has a crucial role in the dimerization of ChRs. However, the CD spectrum of this mutant has an exciton couplet comparable to that of the wild-type, demonstrating that it is dimeric. Patch-clamp experiments suggest that the N-terminal domain is involved in protein stabilization and channel kinetics rather than dimerization or channel activity. PMID:25247388
Exciton Brightening in Monolayer Phosphorene via Dimensionality Modification.
Xu, Renjing; Yang, Jiong; Myint, Ye Win; Pei, Jiajie; Yan, Han; Wang, Fan; Lu, Yuerui
2016-05-01
Exciton brightening in monolayer phosphorene is achieved via the dimensionality modification of excitons from quasi-1D to 0D. The luminescence quantum yield of 0D-like excitons is >33.6 times larger than that of quasi-1D free excitons. 2D phosphorene with quasi-1D free excitons and 0D-like excitons provides a unique platform to investigate the fundamental phenomena in the ideal 2D-1D-0D hybrid system. PMID:26990082
Radius of curvature controlled mirror
Neil, George R.; Rathke, John Wickham; Schultheiss, Thomas John; Shinn, Michelle D.; Dillon-Townes, Lawrence A.
2006-01-17
A controlled radius of curvature mirror assembly comprising: a distortable mirror having a reflective surface and a rear surface; and in descending order from the rear surface; a counter-distortion plate; a flow diverter having a flow diverter aperture at the center thereof; a flow return plate having a flow return aperture at the center thereof; a thermal isolation plate having a thermal isolation plate aperture at the center thereof and a flexible heater having a rear surface and a flexible heater aperture at the center thereof; a double walled tube defining a coolant feed chamber and a coolant return chamber; said coolant feed chamber extending to and through the flow diverter aperture and terminating at the counter-distortion plate and the coolant return chamber extending to and through the thermal isolation backplate and terminating at the flow diverter; and a coolant feed and a coolant return exit at the rear of said flexible heater.
Josephson effects in condensates of excitons and exciton polaritons
Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.
2008-07-15
We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.
Baranov, P. G.; Poluektov, O. G.; Schmidt, J.
2010-01-01
Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4− complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra. PMID:21151483
DNA-mediated excitonic upconversion FRET switching
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-17
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffersmore » from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.« less
DNA-mediated excitonic upconversion FRET switching
NASA Astrophysics Data System (ADS)
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-01
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.
DNA-mediated excitonic upconversion FRET switching
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-17
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.
Bohr effect and temperature sensitivity of hemoglobins from highland and lowland deer mice.
Jensen, Birgitte; Storz, Jay F; Fago, Angela
2016-05-01
An important means of physiological adaptation to environmental hypoxia is an increased oxygen (O2) affinity of the hemoglobin (Hb) that can help secure high O2 saturation of arterial blood. However, the trade-off associated with a high Hb-O2 affinity is that it can compromise O2 unloading in the systemic capillaries. High-altitude deer mice (Peromyscus maniculatus) have evolved an increased Hb-O2 affinity relative to lowland conspecifics, but it is not known whether they have also evolved compensatory mechanisms to facilitate O2 unloading to respiring tissues. Here we investigate the effects of pH (Bohr effect) and temperature on the O2-affinity of high- and low-altitude deer mouse Hb variants, as these properties can potentially facilitate O2 unloading to metabolizing tissues. Our experiments revealed that Bohr factors for the high- and low-altitude Hb variants are very similar in spite of the differences in O2-affinity. The Bohr factors of deer mouse Hbs are also comparable to those of other mammalian Hbs. In contrast, the high- and low-altitude variants of deer mouse Hb exhibited similarly low temperature sensitivities that were independent of red blood cell anionic cofactors, suggesting an appreciable endothermic allosteric transition upon oxygenation. In conclusion, high-altitude deer mice have evolved an adaptive increase in Hb-O2 affinity, but this is not associated with compensatory changes in sensitivity to changes in pH or temperature. Instead, it appears that the elevated Hb-O2 affinity in high-altitude deer mice is compensated by an associated increase in the tissue diffusion capacity of O2 (via increased muscle capillarization), which promotes O2 unloading. PMID:26808972
The Structure and Dynamics of Molecular Excitons
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-04-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet -- triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion.
The structure and dynamics of molecular excitons.
Bardeen, Christopher J
2014-01-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet ↔ triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion. PMID:24313684
Intrinsic frame inverse mass tensor as a function of {beta} and {gamma} in the Bohr Hamiltonian
Jolos, R. V.; Brentano, P. von
2012-04-15
Analytical expressions are derived for the components of the intrinsic frame inverse mass tensor of the Bohr Hamiltonian. These expressions contain parameters which are determined by the experimental data on the B(E2)'s and the excitation energies of the low-lying collective states. It is shown that the nondiagonal component of the intrinsic frame mass tensor has a small effect on the collective motion. It is shown also that the values of the B{sub {beta}{beta}}, B{sub {gamma}{gamma}} and the rotational mass coefficientB{sub 1} differ in the well-deformed nuclei by factor 3 or more.
AGU's historical records move to the Niels Bohr Library and Archives
NASA Astrophysics Data System (ADS)
Harper, Kristine C.
2012-11-01
As scientists, AGU members understand the important role data play in finding the answers to their research questions: no data—no answers. The same holds true for the historians posing research questions concerning the development of the geophysical sciences, but their data are found in archival collections comprising the personal papers of geophysicists and scientific organizations. Now historians of geophysics—due to the efforts of the AGU History of Geophysics Committee, the American Institute of Physics (AIP), and the archivists of the Niels Bohr Library and Archives at AIP—have an extensive new data source: the AGU manuscript collection.
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.
Einstein-Bohr recoiling double-slit gedanken experiment performed at the molecular level
NASA Astrophysics Data System (ADS)
Liu, Xiao-Jing; Miao, Quan; Gel'Mukhanov, Faris; Patanen, Minna; Travnikova, Oksana; Nicolas, Christophe; Ågren, Hans; Ueda, Kiyoshi; Miron, Catalin
2015-02-01
Double-slit experiments illustrate the quintessential proof for wave-particle complementarity. If information is missing about which slit the particle has traversed, the particle, behaving as a wave, passes simultaneously through both slits. This wave-like behaviour and corresponding interference is absent if ‘which-slit’ information exists. The essence of Einstein-Bohr's debate about wave-particle duality was whether the momentum transfer between a particle and a recoiling slit could mark the path, thus destroying the interference. To measure the recoil of a slit, the slits should move independently. We showcase a materialization of this recoiling double-slit gedanken experiment by resonant X-ray photoemission from molecular oxygen for geometries near equilibrium (coupled slits) and in a dissociative state far away from equilibrium (decoupled slits). Interference is observed in the former case, while the electron momentum transfer quenches the interference in the latter case owing to Doppler labelling of the counter-propagating atomic slits, in full agreement with Bohr's complementarity.
Russell, Bianca; Johnston, Jennifer J; Biesecker, Leslie G; Kramer, Nancy; Pickart, Angela; Rhead, William; Tan, Wen-Hann; Brownstein, Catherine A; Kate Clarkson, L; Dobson, Amy; Rosenberg, Avi Z; Vergano, Samantha A Schrier; Helm, Benjamin M; Harrison, Rachel E; Graham, John M
2015-09-01
Bohring-Opitz syndrome is a rare genetic condition characterized by distinctive facial features, variable microcephaly, hypertrichosis, nevus flammeus, severe myopia, unusual posture (flexion at the elbows with ulnar deviation, and flexion of the wrists and metacarpophalangeal joints), severe intellectual disability, and feeding issues. Nine patients with Bohring-Opitz syndrome have been identified as having a mutation in ASXL1. We report on eight previously unpublished patients with Bohring-Opitz syndrome caused by an apparent or confirmed de novo mutation in ASXL1. Of note, two patients developed bilateral Wilms tumors. Somatic mutations in ASXL1 are associated with myeloid malignancies, and these reports emphasize the need for Wilms tumor screening in patients with ASXL1 mutations. We discuss clinical management with a focus on their feeding issues, cyclic vomiting, respiratory infections, insomnia, and tumor predisposition. Many patients are noted to have distinctive personalities (interactive, happy, and curious) and rapid hair growth; features not previously reported. PMID:25921057
Mirror with thermally controlled radius of curvature
Neil, George R.; Shinn, Michelle D.
2010-06-22
A radius of curvature controlled mirror for controlling precisely the focal point of a laser beam or other light beam. The radius of curvature controlled mirror provides nearly spherical distortion of the mirror in response to differential expansion between the front and rear surfaces of the mirror. The radius of curvature controlled mirror compensates for changes in other optical components due to heating or other physical changes. The radius of curvature controlled mirror includes an arrangement for adjusting the temperature of the front surface and separately adjusting the temperature of the rear surface to control the radius of curvature. The temperature adjustment arrangements can include cooling channels within the mirror body or convection of a gas upon the surface of the mirror. A control system controls the differential expansion between the front and rear surfaces to achieve the desired radius of curvature.
Can Disorder Enhance Incoherent Exciton Diffusion?
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. PMID:26106811
Superfluorescence spectra of excitons in quantum wells
NASA Astrophysics Data System (ADS)
Grünwald, P.; Burau, G. K. G.; Stolz, H.; Vogel, W.
2013-11-01
We study the fluorescence light emitted from GaAs excitons in semiconductor quantum wells. The excitons are modeled as interacting bosons. By combining quantum optical methods for the excitonic emission spectrum with many particle descriptions of the transmission through the medium, we can evaluate the spectra outside the well. Comparing with experimental spectra, we get a very good agreement. The method helps to explain the main features of the observed spectra. It is demonstrated that the observed spectra show clear evidence of superfluorescent emission.
Ultrahigh exciton diffusion in intrinsic diamond
NASA Astrophysics Data System (ADS)
Morimoto, Hikaru; Hazama, Yuji; Tanaka, Koichiro; Naka, Nobuko
2015-11-01
We observe exceptionally high diffusion of excitons in diamond using time-resolved photoluminescence imaging. The diffusivity is found to increase with decreasing temperature more rapidly than well-established predictions. The highest diffusion constant, 9.2 ×103cm2/s , measured for thermalized excitons is 150 times that recorded previously in diamond. We elucidate the momentum relaxation mechanisms determining transport: intraband and interband scattering by acoustic phonons in the exciton fine-structure levels. The enhanced diffusivity above 100 K is explained by a free-carrier contribution.
Excitons in the Fractional Quantum Hall Effect
DOE R&D Accomplishments Database
Laughlin, R. B.
1984-09-01
Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.
Exciton Regeneration at Polymeric Semiconductor Heterojunctions
NASA Astrophysics Data System (ADS)
Morteani, Arne C.; Sreearunothai, Paiboon; Herz, Laura M.; Friend, Richard H.; Silva, Carlos
2004-06-01
Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2 3.1nm) which may collapse into an exciplex and then endothermically (EA=100 200 meV) back transfer towards the exciton.
Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong
2015-05-13
The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. 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. PMID:25641931
Singlet exciton fission in solution.
Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. PMID:24256865
Singlet exciton fission in solution
NASA Astrophysics Data System (ADS)
Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
Multiple Exciton Generation Solar Cells
Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.
2012-01-01
Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.
Optical nutation in the exciton range of spectrum
Khadzhi, P. I.; Vasiliev, V. V.
2013-08-15
Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.
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.
NASA Astrophysics Data System (ADS)
Mouri, Shinichiro; Miyauchi, Yuhei; Toh, Minglin; Zhao, Weijie; Eda, Goki; Matsuda, Kazunari
2014-10-01
We studied multiexciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.
Cavity-Enhanced Transport of Excitons
NASA Astrophysics Data System (ADS)
Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido
2015-05-01
We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions.
Triplet excitons: Bringing dark states to light
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-11-01
Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.
Hybrid interlayer excitons with tunable dispersion relation
NASA Astrophysics Data System (ADS)
Skinner, Brian
When two semiconducting materials are layered on top of each other, interlayer excitons can be formed by the Coulomb attraction of an electron in one layer to a hole in the opposite layer. The resulting exciton is a composite boson with a dispersion relation that is a hybrid between the dispersion relations of the electron and the hole separately. In this talk I show how such hybridization is particularly interesting when one layer has a ``Mexican hat''-shaped dispersion relation and the other has a conventional parabolic dispersion. In this case the interlayer exciton can have a range of qualitatively different dispersion relations, which can be continuously altered by an external field. This tunability in principle allows one to continuously tune a collection of interlayer excitons between different quantum many-body phases, including Bose-Einstein condensate, Wigner crystal, and fermion-like ``moat band'' phases.
Ultrafast exciton dynamics at molecular surfaces
NASA Astrophysics Data System (ADS)
Monahan, Nicholas R.
Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge separation mechanism should occur at all donor-acceptor interfaces. My results show that entropy facilitates charge separation and indicate that the density of acceptor states should be a design consideration when constructing organic solar cells.
Exciton-polariton wakefields in semiconductor microcavities
NASA Astrophysics Data System (ADS)
Terças, H.; Mendonça, J. T.
2016-02-01
We consider the excitation of polariton wakefields due to a propagating light pulse in a semiconductor microcavity. We show that two kinds of wakes are possible, depending on the constituents fraction (either exciton or photon) of the polariton wavefunction. The nature of the wakefields (pure excitonic or polaritonic) can be controlled by changing the speed of propagation of the external pump. This process could be used as a diagnostic for the internal parameters of the microcavity.
Excitonic molecules in type-II superlattices
NASA Astrophysics Data System (ADS)
Tsuchiya, T.; Katayama, S.; Ando, T.
1998-01-01
Excitonic molecules in GaAs/AlAs type-II superlattices are numerically investigated. In spite of large difference of electronic structures between type-II and type-I superlattices, variational calculations show that the configuration of particles is similar to that in type-I superlattices. This is because the layer width is smaller than the extent of excitonic wavefunctions in the direction parallel to the layers in the present superlattices.
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.
Exciton quasicondensation in one-dimensional systems
NASA Astrophysics Data System (ADS)
Werman, Yochai; Berg, Erez
2015-06-01
Two Luttinger liquids, with an equal density and opposite sign of charge carriers, may exhibit enhanced excitonic correlations. We term such a system an exciton quasicondensate, with a possible realization being two parallel oppositely doped quantum wires, coupled by repulsive Coulomb interactions. We show that this quasiexciton condensate can be stabilized in an extended range of parameters, in both spinless and spinful systems. We calculate the interwire tunneling current-voltage characteristic, and find that a negative differential conductance is a signature of excitonic correlations. For spinful electrons, the excitonic regime is shown to be distinct from the usual quasi-long-range ordered Wigner crystal phase characterized by power-law density wave correlations. The two phases can be clearly distinguished through their interwire tunneling current-voltage characteristics. In the quasiexciton condensate regime the tunneling conductivity diverges at low temperatures and voltages, whereas in the Wigner crystal it is strongly suppressed. Both the Wigner crystal and the excitonic regime are characterized by a divergent Coulomb drag at low temperature. Finally, metallic carbon nanotubes are considered as a special case of such a one-dimensional setup, and it is shown that exciton condensation is favorable due to the additional valley degree of freedom.
Exciton formation and diffusion in OLEDs (Presentation Recording)
NASA Astrophysics Data System (ADS)
Ingram, Grayson L.; Lu, Zheng-Hong
2015-10-01
This talk will discuss recent experiments designed to study the formation of excitons and their subsequent diffusions in OLEDs. These experimental results suggest that contrary to conventional wisdom, host singlet exciton diffusion can occur over long distances, while host triplet excitons are confined close to the exciton formation region for the archetype host and hole transport layer CBP. The exciton formation mechanism is studied and we show that the ratio of excitons formed on the host to excitons formed on the dopant varies strongly with the applied voltage. Refinements to models of efficiency roll off are discussed in light of the improved understanding of exciton formation and we suggest design guidelines to improve efficiency by engineering exciton formation.
Mass tensor in the Bohr Hamiltonian from the nondiagonal energy weighted sum rules
Jolos, R. V.; Brentano, P. von
2009-04-15
Relations are derived in the framework of the Bohr Hamiltonian that express the matrix elements of the deformation-dependent components of the mass tensor through the experimental data on the energies and the E2 transitions relating the low-lying collective states. These relations extend the previously obtained results for the intrinsic mass coefficients of the well-deformed axially symmetric nuclei on nuclei of arbitrary shape. The expression for the mass tensor is suggested, which is sufficient to satisfy the existing experimental data on the energy weighted sum rules for the E2 transitions for the low-lying collective quadrupole excitations. The mass tensor is determined for {sup 106,108}Pd, {sup 108-112}Cd, {sup 134}Ba, {sup 150}Nd, {sup 150-154}Sm, {sup 154-160}Gd, {sup 164}Dy, {sup 172}Yb, {sup 178}Hf, {sup 188-192}Os, and {sup 194-196}Pt.
NASA Astrophysics Data System (ADS)
Chabab, M.; Lahbas, A.; Oulne, M.
2015-06-01
In a recent work [Phys. Rev. C 84, 044321 (2011), 10.1103/PhysRevC.84.044321] M. J. Ermamatov and P. R. Fraser have studied rotational and vibrational excited states of axially symmetric nuclei within the Bohr Hamiltonian with different mass parameters. However, the energy formula that the authors have used contains some inaccuracies. So the numerical results they obtained seem to be controversial. In this paper, we revisit all calculations related to this problem and determine the appropriate formula for the energy spectrum. Moreover, in order to improve such calculations, we reconsider this problem within the framework of the deformation-dependent mass formalism. Also, unlike the work of Bonatsos et al. [Phys. Rev. C 83, 044321 (2011), 10.1103/PhysRevC.83.044321], in which the mass parameter has not been considered, we will show the importance of this parameter and its effect on numerical predictions.
Inspirations from the theories of Bohr and Mottelson: a Canadian perspective
NASA Astrophysics Data System (ADS)
Ward, D.; Waddington, J. C.; Svensson, C. E.
2016-03-01
The theories developed by Bohr and Mottelson have inspired much of the world-wide experimental investigation into the structure of the atomic nucleus. On the occasion of the 40th anniversary of the awarding of their Nobel prize, we reflect on some of the experimental developments made in understanding the structure of nuclei. We have chosen to focus on experiments performed in Canada, or having strong ties to Canada, and the work included here spans virtually the whole of the second half of the 20th century. The 8π Spectrometer, which figures prominently in this story, was a novel departure for funding science in Canada that involved an intimate collaboration between a Crown Corporation (Atomic Energy of Canada Ltd) and University research, and enabled many of the insights discussed here.
Wo die Quantentheorie Wurzeln schlug: Die Schulen von Sommerfeld, Bohr und Born
NASA Astrophysics Data System (ADS)
Eckert, Michael
Die Geburtsstunde der Quantentheorie liegt hundert Jahre zurück, aber es dauerte ein Vierteljahrhundert, bis die neue Physik Wurzeln schlug und daraus die Quantenmechanik hervorging. Auf dem Weg dahin spielten die frühen Schulen der theoretischen Physik eine wichtige Rolle. Das erste dieser Zentren war die von Arnold Sommerfeld an der Münchner Universität gegründete Pflanzstätte für theoretische Physik. In Niels Bohrs 1921 eröffneten Institut für theoretische Physik gaben sich die hoffnungsvollen Nachwuchswissenschaftler die Klinke in die Hand, und in Deutschland war nach dem Ersten Weltkrieg die Universität Göttingen eine der ersten Adressen für moderne Physik. Der dortige Mentor war Max Born.
ERIC Educational Resources Information Center
Gjedde, Albert
2010-01-01
The year 2010 is the centennial of the publication of the "Seven Little Devils" in the predecessor of "Acta Physiologica". In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity…
Inci, I.; Boztosun, I.; Bonatsos, D.
2008-11-11
Analytical solutions of the collective Bohr Hamiltonian with the Morse potential have been obtained for the U(5)-O(6) and U(5)-SU(3) transition regions through the Asymptotic Iteration Method (AIM). The obtained energy eigenvalue equations have been used to get the experimental excitation energy spectrum of Xe and Yb isotopes. The results are in good agreement with experimental data.
ERIC Educational Resources Information Center
Gjedde, Albert
2010-01-01
The year 2010 is the centennial of the publication of the "Seven Little Devils" in the predecessor of "Acta Physiologica". In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity
Microscopic theories of excitons and their dynamics
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.
This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment. In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series. In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into those that evolve "fast" and "slow," as compared to the internal system dynamics. This method is tested and applied to the spin-boson model, a two-site Frenkel exciton model, and the seven-site Fenna-Matthews-Olson complex. I conclude with a collaborative analysis of a recently developed polaron-transformed quantum master equation, which is shown to accurately interpolate between the well-known Redfield and Forster theories, even in challenging donor-bridge-acceptor arrangements.
Management of Complications of Distal Radius Fractures
Chung, Kevin C.; Mathews, Alexandra L.
2015-01-01
Synopsis Treating a fracture of the distal radius may require the surgeon to make a difficult decision between surgical treatment and nonsurgical management. The use of surgical fixation has recently increased owing to complications associated with conservative treatment. However, conservative action may be necessary depending on certain patient factors. The treating surgeon must be aware of the possible complications associated with distal radius fracture treatments to prevent their occurrence. Prevention can be achieved with a proper understanding of the mechanism of these complications. This article discusses the most recent evidence on how to manage and prevent complications following a fracture of the distal radius. PMID:25934197
NASA Astrophysics Data System (ADS)
Yan, Yun-An
2016-01-01
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.
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. PMID:26772569
Exciton transport and dissociation at organic interfaces
NASA Astrophysics Data System (ADS)
Beljonne, David
2011-03-01
This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length LD in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger LD values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules (> 50nm) to values actually measured in CPs (<10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how these interactions in turn affect the energy landscape around the interface and therefore also the energy barrier for exciton dissociation into free carriers.
Large-Larmor-radius interchange instability
Ripin, B.H.; McLean, E.A.; Manka, C.K.; Pawley, C.; Stamper, J.A.; Peyser, T.A.; Mostovych, A.N.; Grun, J.; Hassam, A.B.; Huba, J.
1987-11-16
We observe linear and nonlinear features of a strong plasma/magnetic field interchange Rayleigh-Taylor instability in the limit of large ion Larmor radius. The instability undergoes rapid linear growth culminating in free-streaming flute tips.
Laser differential confocal paraboloidal vertex radius measurement.
Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Shen, Yang; Jiang, Hongwei
2014-02-15
This Letter proposes a laser differential confocal paraboloidal vertex radius measurement (DCPRM) method for the high-accuracy measurement of the paraboloidal vertex radius of curvature. DCPRM constructs an autocollimation vertex radius measurement light path for the paraboloid by placing a reflector in the incidence light path. This technique is based on the principle that a paraboloid can aim a parallel beam at its focus without aberration and uses differential confocal positioning technology to identify the paraboloid focus and vertex accurately. Measurement of the precise distance between these two positions is achieved to determine the paraboloid vertex radius. Preliminary experimental results indicate that DCPRM has a relative expanded uncertainty of less than 0.001%. PMID:24562218
Photoexcitation and Exciton Transport in Molecular Crystals
NASA Astrophysics Data System (ADS)
Irkhin, Pavel
This work is dedicated to the investigation of exciton transport processes in organic molecular crystals, and to the optical characterization of an important organic semiconductor, rubrene single crystal. Rubrene is a high quality molecular crystal that has been found to have exceptional functionalities in electronic applications such as field effect transistors. I present the intrinsic absorption and photoluminescence spectra of rubrene single crystals, deriving them from a series of experiments performed in different experimental geometries. I describe the absorption spectra for all three principal light polarizations in the crystal, and discuss how the strongly anisotropic absorption and emission properties affect the spectral characteristics of observed photoluminescence spectra. I identify vibronic progressions both in absorption and emission and discuss their parameters and the main vibrational modes that are responsible for them. Through careful analysis of the data, I arrive at a conclusion that absorption and emission of rubrene that is not polarized along a specific crystallographic and molecular direction (c-axis of the crystal, corresponding to the M-axis of the molecule) is not due to an electronic dipole matrix element that has components in that direction. Instead, it is caused by vibronically-induced depolarization of the electronic HOMO-LUMO transition that is described by a dipole matrix element that has components along the c-axis. Further, this work developed and demonstrated a direct imaging technique that allowed to directly observe the diffusion of excitons in rubrene single crystals. This simple and intuitive technique uses localized photoexcitation and spatially resolved detection of the excitonic luminescence to visualize the spatial distribution of excitons. The technique was then used to directly measure the diffusion length of triplet excitons in rubrene. To do this, I exploited the fact that in rubrene photoexcited singlet excitons undergo efficient fission into triplet excitons, and the latter can interact with each other to create photoluminescing singlet excitons again. I show that the exciton mobility in rubrene is strongly anisotropic, with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, the same direction for which a large charge carrier mobility has been observed in experiments with field effect transistors.
Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires
Ma, Yingzhong; Xiao, Kai; Shaw, Robert W
2012-01-01
Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.
Spin-dependent coherent transport of two-dimensional excitons
NASA Astrophysics Data System (ADS)
Durnev, M. V.; Glazov, M. M.
2016-04-01
We propose a theory of interference contributions to the two-dimensional exciton diffusion coefficient. The theory takes into account four spin states of the heavy-hole exciton. The interplay of the single-particle (electron and hole) spin splittings with the electron-hole exchange interaction gives rise to either localization or antilocalization behavior of excitons, depending on the system parameters. Possible experimental manifestations of exciton interference are discussed.
Image excitons and plasmon-exciton strong coupling in two-dimensional perovskite semiconductors
NASA Astrophysics Data System (ADS)
Niu, Wendy; Ibbotson, Lindsey A.; Leipold, David; Runge, Erich; Prakash, G. Vijaya; Baumberg, Jeremy J.
2015-04-01
We present evidence for "image biexcitons" within organic-inorganic perovskite-coated silver gratings. These composite quasiparticles are formed by the interaction between an exciton and its image in the metal mirror below, with binding energy 100 meV at room temperature. By changing the polar and azimuthal angles of the incident light, we observe strong coupling between excitons and surface plasmon polaritons on the grating, with Rabi splittings of 150 and 125 meV for the exciton and biexciton, respectively. A detailed analysis of the field polarizations and dipole orientations shows how these Rabi couplings arise from the strongly compressed field volume.
Competition of Excitonic Transitions Under Hydrostatic Pressure
NASA Astrophysics Data System (ADS)
Weinstein, B. A.; Tischler, J. G.; McCombe, B. D.
2000-03-01
Photoluminescence studies of neutral, bound and charged excitons in modulation doped GaAs/AlGaAs quantum wells were performed as functions of applied pressure, temperature, excitation power and photon energy. Varying both pressure and incident power allows sensitive selection of the different exciton transitions. The Γ-X crossover within the barriers at 7-9kbar accelerates photo-pumping of electrons to the barriers. A semi-empirical kinetic model is able to account for the power, temperature and pressure dependencies of this process. Supported by NSF grant 942409.
Exciton dynamics in perturbed vibronic molecular aggregates
Brüning, C.; Wehner, J.; Hausner, J.; Wenzel, M.; Engel, V.
2015-01-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Exciton dynamics in perturbed vibronic molecular aggregates.
Brüning, C; Wehner, J; Hausner, J; Wenzel, M; Engel, V
2016-07-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Control of exciton transport using quantum interference
NASA Astrophysics Data System (ADS)
Lusk, Mark T.; Stafford, Charles A.; Zimmerman, Jeramy D.; Carr, Lincoln D.
2015-12-01
It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasiparticle motion and also discriminates between quasiparticles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano antiresonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.
Device design for optimal exciton harvesting
NASA Astrophysics Data System (ADS)
Ingram, G. L.; Lu, Z. H.
2014-10-01
Organic light emitting diodes (OLEDs) show potential as the next generation solid state lighting technology. A major barrier to widespread adoption at this point is the efficiency droop that occurs for OLEDs at practical brightness (~ 5000 cd/m2) levels necessary for general lighting. We highlight recent progress in highly efficient OLEDs at high brightness, where improvements are made by managing excitons in these devices through rational device design. General design principles for monochrome OLEDs are discussed based on recent device architectures that have been successfully implemented. We expect that an improved understanding of exciton dynamics in OLEDs in combination with innovative device design will drive future development.
Triplet exciton dynamics in rubrene single crystals
NASA Astrophysics Data System (ADS)
Ryasnyanskiy, Aleksandr; Biaggio, Ivan
2011-11-01
The decay of the photoluminescence excited in rubrene single crystals by picosecond pulses is measured over 7 orders of magnitude and more than 4 time decades. We identify the typical decay dynamics due to triplet-triplet interaction. We show that singlet exciton fission and triplet fusion quantum yields in rubrene are both very large, and we directly determine a triplet exciton lifetime of 100±20 μs, which explains the delayed buildup of a large photocurrent that has been reported earlier for low excitation densities.
Multiple Exciton Generation in Silicon QD arrays
NASA Astrophysics Data System (ADS)
Kryjevski, Andrei; Kilin, Dmitri
2014-03-01
We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate multiple exciton generation (MEG) in several semiconductor nanosystems. Hydrogen-passivated Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Quantum efficiency, the average number of excitons created by a single photon, has been calculated in these nanoparticles to the leading order in the screened Coulomb interaction. Amorphous nanostructures are predicted to have more effective carrier multiplication.
Excess equimolar radius of liquid drops.
Horsch, Martin; Hasse, Hans; Shchekin, Alexander K; Agarwal, Animesh; Eckelsbach, Stefan; Vrabec, Jadran; Müller, Erich A; Jackson, George
2012-03-01
The curvature dependence of the surface tension is related to the excess equimolar radius of liquid drops, i.e., the deviation of the equimolar radius from the radius defined by the macroscopic capillarity approximation. Based on the Tolman [J. Chem. Phys. 17, 333 (1949)] approach and its interpretation by Nijmeijer et al. [J. Chem. Phys. 96, 565 (1991)], the surface tension of spherical interfaces is analyzed in terms of the pressure difference due to curvature. In the present study, the excess equimolar radius, which can be obtained directly from the density profile, is used instead of the Tolman length. Liquid drops of the truncated and shifted Lennard-Jones fluid are investigated by molecular dynamics simulation in the canonical ensemble, with equimolar radii ranging from 4 to 33 times the Lennard-Jones size parameter σ. In these simulations, the magnitude of the excess equimolar radius is shown to be smaller than σ/2. This suggests that the surface tension of liquid drops at the nanometer length scale is much closer to that of the planar vapor-liquid interface than reported in studies based on the mechanical route. PMID:22587106
Baym, Gordon; Ozawa, Tomoki
2009-01-01
We analyze Niels Bohr's proposed two-slit interference experiment with highly charged particles which argues that the consistency of elementary quantum mechanics requires that the electromagnetic field must be quantized. In the experiment a particle's path through the slits is determined by measuring the Coulomb field that it produces at large distances; under these conditions the interference pattern must be suppressed. The key is that, as the particle's trajectory is bent in diffraction by the slits, it must radiate and the radiation must carry away phase information. Thus, the radiation field must be a quantized dynamical degree of freedom. However, if one similarly tries to determine the path of a massive particle through an inferometer by measuring the Newtonian gravitational potential the particle produces, the interference pattern would have to be finer than the Planck length and thus indiscernible. Unlike for the electromagnetic field, Bohr's argument does not imply that the gravitational field must be quantized. PMID:19218440
A Maximum Radius for Habitable Planets
NASA Astrophysics Data System (ADS)
Alibert, Yann
2015-09-01
We compute the maximum radius a planet can have in order to fulfill two constraints that are likely necessary conditions for habitability: 1- surface temperature and pressure compatible with the existence of liquid water, and 2- no ice layer at the bottom of a putative global ocean, that would prevent the operation of the geologic carbon cycle to operate. We demonstrate that, above a given radius, these two constraints cannot be met: in the Super-Earth mass range (1-12 Mearth), the overall maximum that a planet can have varies between 1.8 and 2.3 Rearth. This radius is reduced when considering planets with higher Fe/Si ratios, and taking into account irradiation effects on the structure of the gas envelope.
Mass and radius of cosmic balloons
NASA Technical Reports Server (NTRS)
Wang, Yun
1994-01-01
Cosmic balloons are spherical domain walls with relativistic particles trapped inside. We derive the exact mass and radius relations for a static cosmic balloon using Gauss-Codazzi equations. The cosmic balloon mass as a function of its radius, M(R), is found to have a functional form similar to that of fermion soliton stars, with a fixed point at 2GM(R)/R approximately or equal to 0.486 which corresponds to the limit of infinite central density. We derive a simple analytical approximation for the mass density of a spherically symmetric relativistic gas star. When applied to the computation of the mass and radius of a cosmic balloon, the analytical approximation yields fairly good agreement with the exact numerical solutions.
A Maximum Radius for Habitable Planets.
Alibert, Yann
2015-09-01
We compute the maximum radius a planet can have in order to fulfill two constraints that are likely necessary conditions for habitability: 1- surface temperature and pressure compatible with the existence of liquid water, and 2- no ice layer at the bottom of a putative global ocean, that would prevent the operation of the geologic carbon cycle to operate. We demonstrate that, above a given radius, these two constraints cannot be met: in the Super-Earth mass range (1-12 Mearth), the overall maximum that a planet can have varies between 1.8 and 2.3 Rearth. This radius is reduced when considering planets with higher Fe/Si ratios, and taking into account irradiation effects on the structure of the gas envelope. PMID:26159097
Transporting near-circular Bohr-like wave packets using chirped pulse trains
NASA Astrophysics Data System (ADS)
Yoshida, S.; Burgdörfer, J.; Reinhold, C. O.; Wyker, B.; Ye, S.; Dunning, F. B.
2011-05-01
Protocols for driving localized high- n (n 300) wave packets in near-circular Bohr-like orbits to higher n states using chirped sine waves are described. While Rydberg wave packets involving several n levels are known to be dispersive, circular wave packets can be stabilized by the application of a circularly- or linearly-polarized resonant sine wave. A similar stabilization mechanism is known for the formation of the Trojan asteroids in celestial mechanics. We demonstrate that by slowly chirping the drive frequency parent high- n wave packets can be transported to a narrow distribution of much higher n states because the motion of the wave packet remains locked to the sine wave during the chirping. Use of a chirped HCP train instead of a sine wave allows similar transport but also provides some control of the orbit eccentricity. Research supported by the NSF, the Robert A. Welch Foundation, the OBES US DoE to ORNL, and by the FWF (Austria)
Generation of localized ``Bohr-like'' wavepackets in near-circular orbit about the nucleus
NASA Astrophysics Data System (ADS)
Dunning, F. Barry
2009-05-01
Atoms in high-lying (n ˜ 300) Rydberg states provide a valuable laboratory in which to explore the engineering of electronic wavefunctions using carefully-tailored sequences of short electric field pulses whose characteristic times (duration and/or rise/fall times) are less than the classical electron orbital period. The level of control that can be exercised is illustrated with reference to the generation of localized wavepackets in ``Bohr-like'' near circular orbits. While such wavepackets slowly dephase and undergo dispersion, their localization can be maintained for extended periods (many hundreds of orbits) through external driving using a periodic train of pulses. The wavepackets can be further manipulated by slowly varying, or ``chirping,'' the pulse repetition frequency. The physics underlying these control protocols is explained using classical trajectory Monte Carlo simulations. Even in the absence of external driving, however, wavepacket relocalization is expected at late times due to quantum revivals. The observation of such relocalization is described and demonstrates that quantum phenomena can be seen even in mesoscopic very-high-n atoms. Research undertaken in collaboration with J. J. Mestayer, B. Wyker, C. O. Reinhold, S. Yoshida and J. Burgdörfer.
A rigorous proof of the Bohr-van Leeuwen theorem in the semiclassical limit
NASA Astrophysics Data System (ADS)
Savoie, Baptiste
2015-10-01
The original formulation of the Bohr-van Leeuwen (BvL) theorem states that, in a uniform magnetic field and in thermal equilibrium, the magnetization of an electron gas in the classical Drude-Lorentz model vanishes identically. This stems from classical statistics which assign the canonical momenta all values ranging from -∞ to ∞ that makes the free energy density magnetic-field-independent. When considering a classical (Maxwell-Boltzmann) interacting electron gas, it is usually admitted that the BvL theorem holds upon condition that the potentials modeling the interactions are particle-velocities-independent and do not cause the system to rotate after turning on the magnetic field. From a rigorous viewpoint, when treating large macroscopic systems, one expects the BvL theorem to hold provided the thermodynamic limit of the free energy density exists (and the equivalence of ensemble holds). This requires suitable assumptions on the many-body interactions potential and on the possible external potentials to prevent the system from collapsing or flying apart. Starting from quantum statistical mechanics, the purpose of this paper is to give, within the linear-response theory, a proof of the BvL theorem in the semiclassical limit when considering a dilute electron gas in the canonical conditions subjected to a class of translational invariant external potentials.
An investigation of the nature of Bohr, Root, and Haldane effects in Octopus dofleini hemocyanin.
Miller, K I; Mangum, C P
1988-01-01
1. The pH dependence of Octopus dofleini hemocyanin oxygenation is so great that below pH 7.0 the molecule does not become fully oxygenated, even in pure O2 at 1 atm pressure. However, the curves describing percent oxygenation as a function of PO2 appear to be gradually increasing in oxygen saturation, rather than leveling out at less than full saturation. Hill plots indicate that at pH 6.6 and below the molecule is stabilized in its low affinity conformation. Thus, the low saturation of this hemocyanin in air is due to the very large Bohr shift, and not to the disabling of one or more functionally distinct O2 binding sites on the native molecule. 2. Experiments in which pH was monitored continuously while oxygenation was manipulated in the presence of CO2 provide no evidence of O2 linked binding of CO2. While CO2 does influence O2 affinity independently of pH, its effect may be due to high levels of HCO3- and CO3-, rather than molecular CO2, and it may entail a lowering of the activities of the allosteric effectors Mg2+ and Ca2+. PMID:3150406
What is complementarity?: Niels Bohr and the architecture of quantum theory
NASA Astrophysics Data System (ADS)
Plotnitsky, Arkady
2014-12-01
This article explores Bohr’s argument, advanced under the heading of ‘complementarity,’ concerning quantum phenomena and quantum mechanics, and its physical and philosophical implications. In Bohr, the term complementarity designates both a particular concept and an overall interpretation of quantum phenomena and quantum mechanics, in part grounded in this concept. While the argument of this article is primarily philosophical, it will also address, historically, the development and transformations of Bohr’s thinking, under the impact of the development of quantum theory and Bohr’s confrontation with Einstein, especially their exchange concerning the EPR experiment, proposed by Einstein, Podolsky and Rosen in 1935. Bohr’s interpretation was progressively characterized by a more radical epistemology, in its ultimate form, which was developed in the 1930s and with which I shall be especially concerned here, defined by his new concepts of phenomenon and atomicity. According to this epistemology, quantum objects are seen as indescribable and possibly even as inconceivable, and as manifesting their existence only in the effects of their interactions with measuring instruments upon those instruments, effects that define phenomena in Bohr’s sense. The absence of causality is an automatic consequence of this epistemology. I shall also consider how probability and statistics work under these epistemological conditions.
Bright and dark excitons in semiconductor carbon nanotubes
Tretiak, Sergei
2008-01-01
We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.
Multilevel fitting of {sup 235}U resonance data sensitive to Bohr-and Brosa-fission channels
Moore, M.S.
1995-05-01
The recent determination of the K, J dependence of the neutron induced fission cross section of {sup 235}U by the Dubna group has led to a renewed interest in the mechanism of fission from saddle to scission. The K quantum numbers designate the so-called Bohr fission channels, which describe the fission properties at the saddle point. Certain other fission properties, e.g., the fragment mass and kinetic-energy distribution, are related to the properties of the scission point. The neutron energy dependence of the fragment kinetic energies has been measured by Hambsch et al., who analyzed their data according to a channel description of Brosa et al. How these two channel descriptions, the saddle-point Bohr channels and the scission-point Brosa channels, relate to one another is an open question, and is the subject matter of the present paper. We use the correlation coefficient between various data sets, in which variations are reported from resonance to resonance, as a measure of both-the statistical reliability of the data and of the degree to which different scission variables relate to different Bohr channels. We have carried out an adjustment of the ENDF/B-VI multilevel evaluation of the fission cross section of {sup 235}U, one that provides a reasonably good fit to the energy dependence of the fission, capture, and total cross sections below 100 eV, and to the Bohr-channel structure deduced from an earlier measurement by Pattenden and Postma. We have also further explored the possibility of describing the data of Hambsch et al. in the Brosa-channel framework with the same set of fission-width vectors, only in a different reference system. While this approach shows promise, it is clear that better data are also needed for the neutron energy variation of the scission-point variables.
Exciton-exciton annihilation and relaxation pathways in semiconducting carbon nanotubes
NASA Astrophysics Data System (ADS)
Chmeliov, Jevgenij; Narkeliunas, Jonas; Graham, Matt W.; Fleming, Graham R.; Valkunas, Leonas
2016-01-01
We present a thorough analysis of one- and two-color transient absorption measurements performed on single- and double-walled semiconducting carbon nanotubes. By combining the currently existing models describing exciton-exciton annihilation--the coherent and the diffusion-limited ones--we are able to simultaneously reproduce excitation kinetics following both E11 and E22 pump conditions. Our simulations revealed the fundamental photophysical behavior of one-dimensional coherent excitons and non-trivial excitation relaxation pathways. In particular, we found that after non-linear annihilation a doubly-excited exciton relaxes directly to its E11 state bypassing the intermediate E22 manifold, so that after excitation resonant with the E11 transition, the E22 state remains unpopulated. A quantitative explanation for the observed much faster excitation kinetics probed at E22 manifold, comparing to those probed at the E11 band, is also provided.
Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.
Rao, Akshay; Wilson, Mark W B; Hodgkiss, Justin M; Albert-Seifried, Sebastian; Bässler, Heinz; Friend, Richard H
2010-09-15
Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs. PMID:20735067
Inside the Bondi radius of M87
NASA Astrophysics Data System (ADS)
Russell, H. R.; Fabian, A. C.; McNamara, B. R.; Broderick, A. E.
2015-07-01
Chandra X-ray observations of the nearby brightest cluster galaxy M87 resolve the hot gas structure across the Bondi accretion radius of the central supermassive black hole (SMBH), a measurement possible in only a handful of systems but complicated by the bright nucleus and jet emission. By stacking only short frame-time observations to limit pileup, and after subtracting the nuclear point spread function, we analysed the X-ray gas properties within the Bondi radius at 0.12-0.22 kpc (1.5-2.8 arcsec), depending on the black hole mass. Within 2 kpc radius, we detect two significant temperature components, which are consistent with constant values of 2 and 0.9 keV down to 0.15 kpc radius. No evidence was found for the expected temperature increase within ˜ 0.25 kpc due to the influence of the SMBH. Within the Bondi radius, the density profile is consistent with ρ ∝ r-1. The lack of a temperature increase inside the Bondi radius suggests that the hot gas structure is not dictated by the SMBH's potential and, together with the shallow density profile, shows that the classical Bondi rate may not reflect the accretion rate on to the SMBH. If this density profile extends in towards the SMBH, the mass accretion rate on to the SMBH could be at least two orders of magnitude less than the Bondi rate, which agrees with Faraday rotation measurements for M87. We discuss the evidence for outflow from the hot gas and the cold gas disc and for cold feedback, where gas cooling rapidly from the hot atmosphere could feed the cirumnuclear disc and fuel the SMBH. At 0.2 kpc radius, the cooler X-ray temperature component represents ˜20 per cent of the total X-ray gas mass and, by losing angular momentum to the hot gas component, could provide a fuel source of cold clouds within the Bondi radius.
Okonjo, Kehinde O; Olatunde, Abimbola M; Fodeke, Adedayo A; Babalola, J Oyebamiji
2014-06-01
We have measured the affinity of the CysF9[93]β sulfhydryl group of human deoxyhemoglobin and oxyhemoglobin for 5,5'-dithiobis(2-nitrobenzoate), DTNB, between pH ≈5.6 and 9 in order to understand the basis of the reported reduction of the Bohr effect induced by chemical modification of the sulfhydryl. We analyzed the results quantitatively on the basis of published data indicating that the sulfhydryl exists in two conformations that are coupled to the transition between two tertiary structures of hemoglobin in dynamic equilibrium. Our analyses show that the ionizable groups linked to the DTNB reaction have lower pKas of ionization in deoxyhemoglobin compared to oxyhemoglobin. So these ionizable groups should make negative contributions to the Bohr effect. We identify these groups as HisNA2[2]β, HisEF1[77]β and HisH21[143]β. We provide explanations for the finding that hemoglobin, chemically modified at CysF9[93]β, has a lower Bohr effect and a higher oxygen affinity than unmodified hemoglobin. PMID:24824171
Singlet exciton fission in a hexacene derivative.
Lee, Jiye; Bruzek, Matthew J; Thompson, Nicholas J; Sfeir, Matthew Y; Anthony, John E; Baldo, Marc A
2013-03-13
Hexacene, an acene with six benzene rings, is notable for its exceptionally small triplet energy, around one third of the singlet energy. Herein, singlet fission, i.e., conversion of a singlet exciton into two triplets, is demonstrated in a thin film of hexacene derivative, employing both transient absorption spectroscopy and magnetic field effects on photocurrent. PMID:23293054
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.
NASA Astrophysics Data System (ADS)
Bhunia, Amit; Bansal, Kanika; Datta, Shouvik; Alshammari, Marzook S.; Henini, Mohamed
In contrast to the widely reported optical techniques, there are hardly any investigations on corresponding electrical signatures of condensed matter physics of excitonic phenomena. We studied small signal steady state capacitance response in III-V materials based multi quantum well (AlGaInP) and MBE grown quantum dot (InGaAs) laser diodes to identify signatures of excitonic presence. Conductance activation by forward bias was probed using frequency dependent differential capacitance response (fdC/df), which changes characteristically with the onset of light emission indicating the occurrence of negative activation energy. Our analysis shows that it is connected with a steady state population of exciton like bound states. Calculated average energy of this bound state matches well with the binding energy of weakly confined excitons in this type of structures. Further increase in charge injection decreases the differential capacitive response in AlGaInP based diodes, indicating a gradual Mott transition of excitonic states into electron hole plasma. This electrical description of excitonic Mott transition is fully supplemented by standard optical spectroscopic signatures of band gap renormalization and phase space filling effects.
Indirect excitons in a potential energy landscape created by a perforated electrode
NASA Astrophysics Data System (ADS)
Dorow, C. J.; Kuznetsova, Y. Y.; Leonard, J. R.; Chu, M. K.; Butov, L. V.; Wilkes, J.; Hanson, M.; Gossard, A. C.
2016-02-01
We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at a constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions.
Dynamics of the excitonic coupling in organic crystals.
Aragó, Juan; Troisi, Alessandro
2015-01-16
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene. PMID:25635554
Dynamics of the Excitonic Coupling in Organic Crystals
NASA Astrophysics Data System (ADS)
Aragó, Juan; Troisi, Alessandro
2015-01-01
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene.
Coupled exciton-photon Bose condensate in path integral formalism
NASA Astrophysics Data System (ADS)
Elistratov, A. A.; Lozovik, Yu. E.
2016-03-01
We study the behavior of exciton polaritons in an optical microcavity with an embedded semiconductor quantum well. We use a two-component exciton-photon approach formulated in terms of path integral formalism. In order to describe spatial distributions of the exciton and photon condensate densities, the two coupled equations of the Gross-Pitaevskii type are derived. For a homogeneous system, we find the noncondensate photon and exciton spectra, calculate the coefficients of transformation from the exciton-photon basis to the lower-upper polariton basis, and obtain the exciton and photon occupation numbers of the lower and upper polariton branches for nonzero temperatures. For an inhomogeneous system, the set of coupled equations of the Bogoliubov-de Gennes type is derived. The equations govern the spectra and spatial distributions of noncondensate photons and excitons.
Ultrafast dynamics of exciton fission in polycrystalline pentacene.
Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H
2011-08-10
We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. PMID:21755937
Intracavity optical pumping of J-aggregate microcavity exciton polaritons
Bradley, M. Scott; Bulovic, Vladimir
2010-07-29
We demonstrate intracavity optical pumping of J-aggregate microcavity exciton polaritons. The use of ultrathin layer-by-layer J-aggregate thin films as the strongly coupled exciton medium allows for inclusion of a thermally evaporated luminescent cavity spacer layer, through which the lower-branch exciton-polariton states are resonantly pumped. We present a measurement of the lower-branch exciton-polariton occupation in room-temperature microcavity devices containing J-aggregated molecular thin films under low-density steady-state excitation. The observed exciton-polariton occupation shows a Maxwell-Boltzmann distribution at T=300 K , indicating thermalization of exciton polaritons in the lower energy branch. This device design enables us to propose a new type of polariton laser architecture for microcavity exciton polaritons.
The Tidal Radius of the Arches Cluster
NASA Astrophysics Data System (ADS)
Hosek, Matthew; Lu, Jessica R.; Anderson, Jay; Ghez, Andrea; Morris, Mark; Clarkson, William
2015-08-01
At a projected distance of just ˜26 pc from the center of the Milky Way, the Arches cluster allows us to examine the structure of a young massive cluster in the strong tidal environment of the Galactic center (GC). We use the HST WFC3IR camera to conduct an astrometric and photometric study of the outer region of the Arches cluster (R > 6.25”) in order to measure its radial profile. Using proper motions we separate cluster members from field stars down to F153M = 20 mag (˜2.5 M_sun) over a 120” x 120” field of view, covering an area 144 times larger than previous proper motion studies. This is a significant improvement over photometrically-determined cluster membership, which is complicated by the high degree of differential reddening across the field. Using cluster membership probabilities, a derived extinction map, and extensive completeness simulations, we construct the radial profile of the Arches cluster to a radius of ˜80” (˜3.1 pc assuming a distance of 8 kpc). Evidence of mass segregation out to this radius is observed, and no significant tidal tail structure is apparent. We find that the projected radial extent of the Arches cluster is significantly larger than its expected tidal radius. This result suggests either that the cluster is not as close to the GC as previously thought or that it is inflated beyond its nominal tidal radius.
NASA Astrophysics Data System (ADS)
Kowalski, Piotr; Machnikowski, Paweł
2015-12-01
We study the coherent mixing between two-particle (single exciton) and four-particle (biexciton) states of a semiconductor nanocrystal resulting from the Coulomb coupling between states with different numbers of electron-hole pairs. Using a simple model of the nanocrystal wave functions and an envelope function approach, we estimate the efficiency of the multiple exciton generation (MEG) process resulting from such coherent admixture mechanism, including all the relevant states in a very broad energy interval. We show that in a typical ensemble of nanocrystals with an average radius of 3nm, the onset of the MEG process appears about 1 eV above the lower edge of the biexciton density of states. This is due to the angular momentum conservation that imposes selection rules and limits the available MEG pathways, thus taking over the role of momentum conservation that hinders this process in bulk. The efficiency of the MEG process reaches 50% for photon energies around 5 eV. The MEG onset shifts to lower energies and therefore the efficiency increases in a certain energy range as the radius grows. The energy dependence of the MEG efficiency differs considerably between ensembles with small and large inhomogeneity of nanocrystal sizes.
MASS-RADIUS RELATIONSHIPS FOR EXOPLANETS
Swift, D. C.; Eggert, J. H.; Hicks, D. G.; Hamel, S.; Caspersen, K.; Schwegler, E.; Collins, G. W.; Nettelmann, N.; Ackland, G. J.
2012-01-01
For planets other than Earth, particularly exoplanets, interpretation of the composition and structure depends largely on comparing the mass and radius with the composition expected given their distance from the parent star. The composition implies a mass-radius relation which relies heavily on equations of state calculated from electronic structure theory and measured experimentally on Earth. We lay out a method for deriving and testing equations of state, and deduce mass-radius and mass-pressure relations for key, relevant materials whose equation of state (EOS) is reasonably well established, and for differentiated Fe/rock. We find that variations in the EOS, such as may arise when extrapolating from low-pressure data, can have significant effects on predicted mass-radius relations and on planetary pressure profiles. The relations are compared with the observed masses and radii of planets and exoplanets, broadly supporting recent inferences about exoplanet structures. Kepler-10b is apparently 'Earth-like', likely with a proportionately larger core than Earth's, nominally 2/3 of the mass of the planet. CoRoT-7b is consistent with a rocky mantle over an Fe-based core which is likely to be proportionately smaller than Earth's. GJ 1214b lies between the mass-radius curves for H{sub 2}O and CH{sub 4}, suggesting an 'icy' composition with a relatively large core or a relatively large proportion of H{sub 2}O. CoRoT-2b is less dense than the hydrogen relation, which could be explained by an anomalously high degree of heating or by higher than assumed atmospheric opacity. HAT-P-2b is slightly denser than the mass-radius relation for hydrogen, suggesting the presence of a significant amount of matter of higher atomic number. CoRoT-3b lies close to the hydrogen relation. The pressure at the center of Kepler-10b is 1.5{sup +1.2}{sub -1.0} TPa. The central pressure in CoRoT-7b is probably close to 0.8 TPa, though may be up to 2 TPa. These pressures are accessible by planar shock and ramp-loading experiments at large laser facilities. The center of HAT-P-2b is probably around 210 TPa, in the range of planned National Ignition Facility experiments, and that of CoRoT-3b around 1900 TPa.
Exciton radiative lifetime in transition metal dichalcogenide monolayers
NASA Astrophysics Data System (ADS)
Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B.; Marie, X.
2016-05-01
We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measure τrad0=1.8 ±0.2 ps at T =7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WS e2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T ≲50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.
Taming excitons in II-VI semiconductor nanowires and nanobelts
NASA Astrophysics Data System (ADS)
Xu, Xinlong; Zhang, Qing; Zhang, Jun; Zhou, Yixuan; Xiong, Qihua
2014-10-01
Excitons are one of the most important fundamental quasi-particles, and are involved in a variety of processes forming the basis of a wide range of opto-electronic and photonic devices based on II-VI semiconductor nanowires and nanobelts, such as light-emitting diodes, photovoltaic cells, photodetectors and nanolasers. A clear understanding of their properties and unveiling the potential engineering for excitons is of particular importance for the design and optimization of nanoscale opto-electronic and photonic devices. Herein, we present a comprehensive review on discussing the fundamental behaviours of the excitons in one-dimensional (1D) II-VI semiconductor nanomaterials (nanowires and nanobelts). We will start with a focus on the unique properties (origin, generation, etc) and dynamics of excitons and exciton complexes in the II-VI semiconductor nanowires and nanobelts. Then we move to the recent progress on the excitonic response in 1D nanomaterials and focus on the tailoring and engineering of excitonic properties through rational controlling of the physical parameters and conditions, intrinsically and extrinsically. These include (1) exciton-exciton interaction, which is important for 1D nanomaterial nanolasing; (2) exciton-phonon interaction, which has interesting applications for laser cooling; and (3) exciton-plasmon interaction, which is the cornerstone towards the realization of plasmonic lasers. The potential of electric field, morphology and size control for excitonic properties is also discussed. Unveiling and controlling excitonic properties in II-VI semiconductor nanowires and nanobelts would promote the development of 1D nanoscience and nanotechnology.
Surgical exposures of the radius and ulna.
Catalano, Louis W; Zlotolow, Dan A; Hitchcock, Phillip B; Shah, Suparna N; Barron, O Alton
2011-07-01
The forearm contains many muscles, nerves, and vascular structures that change position on forearm rotation. Exposure of the radial shaft is best achieved with the Henry (volar) or Thompson (dorsal) approach. The volar flexor carpi radialis approaches are used increasingly for exposure of the distal radius. Although the dorsal approach is a safe utilitarian option with many applications, its use for managing fracture of the distal radius has waned. Potential complications associated with radial exposure include injury to the superficial branch of the radial nerve, the lateral antebrachial cutaneous nerve, and the cephalic vein. Dorsal and ulnar proximal radial exposures are associated with increased risk of injury to the posterior interosseous nerve. With surgical exposure of the ulna, care is required to avoid injuring the dorsal cutaneous branch of the ulnar nerve. PMID:21724922
Radius measurement by laser confocal technology.
Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Zhang, Xin; Wang, Xu
2014-05-01
A laser confocal radius measurement (LCRM) method is proposed for high-accuracy measurement of the radius of curvature (ROC). The LCRM uses the peak points of confocal response curves to identify the cat eye and confocal positions precisely. It then accurately measures the distance between these two positions to determine the ROC. The LCRM also uses conic fitting, which significantly enhances measurement accuracy by restraining the influences of environmental disturbance and system noise on the measurement results. The experimental results indicate that LCRM has a relative expanded uncertainty of less than 10 ppm for both convex and concave spheres. Thus, LCRM is a feasible method for ROC measurements with high accuracy and concise structures. PMID:24921872
The Epidemiology of Distal Radius Fractures
Nellans, Kate W.; Kowalski, Evan; Chung, Kevin C.
2012-01-01
Distal radius fractures are one of the most common types of fractures, accounting for around 25% of fractures in the pediatric population and up to 18% of all fractures in the elderly age group. Although the pediatric and elderly populations are at the greatest risk for this injury, distal radius fractures still have a significant impact on the health and well-being of young adults. Data from the past 40 years has documented a trend towards an overall increase in the prevalence of this injury. For the pediatric population, this increase can likely be attributed to a surge in sports related activities. The growth of the elderly population and a rise in the number of active elderly are directly responsible for the increase seen in this age group. Understanding the epidemiology of this fracture is an important step towards the improvement of the treatment strategies and preventative measures which target this debilitating injury. PMID:22554654
Ulnar Shortening Osteotomy for Distal Radius Malunion
Kamal, Robin N.; Leversedge, Fraser J.
2014-01-01
Background Malunion is a common complication of distal radius fractures. Ulnar shortening osteotomy (USO) may be an effective treatment for distal radius malunion when appropriate indications are observed. Methods The use of USO for treatment of distal radius fracture malunion is described for older patients (typically patients >50 years) with dorsal or volar tilt less than 20 degrees and no carpal malalignment or intercarpal or distal radioulnar joint (DRUJ) arthritis. Description of Technique Preoperative radiographs are examined to ensure there are no contraindications to ulnar shortening osteotomy. The neutral posteroanterior (PA) radiograph is used to measure ulnar variance and to estimate the amount of ulnar shortening required. An ulnar, mid-sagittal incision is used and the dorsal sensory branch of the ulnar nerve is preserved. An USO-specific plating system with cutting jig is used to create parallel oblique osteotomies to facilitate shortening. Intraoperative fluoroscopy and clinical range of motion are checked to ensure adequate shortening and congruous reduction of the ulnar head within the sigmoid notch. Results Previous outcomes evaluation of USO has demonstrated improvement in functional activities, including average flexion-extension and pronosupination motions, and patient reported outcomes. Conclusion The concept and technique of USO are reviewed for the treatment of distal radius malunion when specific indications are observed. Careful attention to detail related to surgical indications and to surgical technique typically will improve range of motion, pain scores, and patient-reported outcomes and will reduce the inherent risks of the procedure, such as ulnar nonunion or the symptoms related to unrecognized joint arthritis. Level of Evidence: Level IV PMID:25097811
Fractures of Distal Radius: An Overview
Meena, Sanjay; Sharma, Pankaj; Sambharia, Abhishek Kumar; Dawar, Ashok
2014-01-01
Fractures of distal radius account for up to 20% of all fractures treated in emergency department. Initial assessment includes a history of mechanism of injury, associated injury and appropriate radiological evaluation. Treatment options include conservative management, internal fixation with pins, bridging and non-bridging external fixation, dorsal or volar plating with/without arthroscopy assistance. However, many questions regarding these fractures remain unanswered and good prospective randomized trials are needed. PMID:25657938
Solar Radius Variations: An Inquisitive Wavelength Dependence
NASA Astrophysics Data System (ADS)
Rozelot, Jean Pierre; Kosovichev, Alexander; Kilcik, Ali
2015-10-01
Recent solar radius determinations from space observations of Mercury and Venus transits have been made by different teams in 2003, 2006, 2012, and 2014. Seemingly the results are not consistent: the authors interpreted the discrepancies as caused by the different methods of analysis. However, looking at the wavelength dependence and adding other available observations from X-EUV up to radio, a typical wavelength dependence can be found, reflecting the different heights at which the lines are formed. Measurements obtained during different periods of time would, in principle, allow us to detect a signature of radius temporal dependence. However, the available data are not sufficiently numerous to detect a significant dependence, at least at the level of the uncertainty at which the observations were made. Lastly, no unique theoretical model is available today to reproduce the strong wavelength dependence of the solar radius, which shows an unexpected minimum at around (6.6 ± 1.9) μm, after a parabolic fit.
Holographic radius test plates for spherical surfaces with large radius of curvature.
Wang, Quandou; Griesmann, Ulf; Soons, Johannes A
2014-07-10
We describe a novel interferometric method, based on nested Fresnel zone lenses or photon sieves, for testing and measuring the radius of curvature of precision spherical surfaces that have radii in a range between several meters and a few hundred meters. We illustrate the measurement concept with radius measurements of a spherical mirror with a radius of about 10 m. The measured radius is 9877 mm±10 mm for a coverage factor k=2. Our measurements also demonstrate, for the first time to the best of our knowledge, the utility of photon sieves for precision surface metrology because they diffuse higher diffraction orders of computer generated holograms, which reduces coherent noise. PMID:25090075
Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes
Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.
2011-07-01
The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.
Intrachain exciton dynamics in conjugated polymer chains in solution
NASA Astrophysics Data System (ADS)
Tozer, Oliver Robert; Barford, William
2015-08-01
We investigate exciton dynamics on a polymer chain in solution induced by the Brownian rotational motion of the monomers. Poly(para-phenylene) is chosen as the model system and excitons are modeled via the Frenkel exciton Hamiltonian. The Brownian fluctuations of the torsional modes were modeled via the Langevin equation. The rotation of monomers in polymer chains in solution has a number of important consequences for the excited state properties. First, the dihedral angles assume a thermal equilibrium which causes off-diagonal disorder in the Frenkel Hamiltonian. This disorder Anderson localizes the Frenkel exciton center-of-mass wavefunctions into super-localized local exciton ground states (LEGSs) and higher-energy more delocalized quasi-extended exciton states (QEESs). LEGSs correspond to chromophores on polymer chains. The second consequence of rotations—that are low-frequency—is that their coupling to the exciton wavefunction causes local planarization and the formation of an exciton-polaron. This torsional relaxation causes additional self-localization. Finally, and crucially, the torsional dynamics cause the Frenkel Hamiltonian to be time-dependent, leading to exciton dynamics. We identify two distinct types of dynamics. At low temperatures, the torsional fluctuations act as a perturbation on the polaronic nature of the exciton state. Thus, the exciton dynamics at low temperatures is a small-displacement diffusive adiabatic motion of the exciton-polaron as a whole. The temperature dependence of the diffusion constant has a linear dependence, indicating an activationless process. As the temperature increases, however, the diffusion constant increases at a faster than linear rate, indicating a second non-adiabatic dynamics mechanism begins to dominate. Excitons are thermally activated into higher energy more delocalized exciton states (i.e., LEGSs and QEESs). These states are not self-localized by local torsional planarization. During the exciton's temporary occupation of a LEGS—and particularly a quasi-band QEES—its motion is semi-ballistic with a large group velocity. After a short period of rapid transport, the exciton wavefunction collapses again into an exciton-polaron state. We present a simple model for the activated dynamics which is in agreement with the data.
Excitonic gap formation in neutral bilayer structures
NASA Astrophysics Data System (ADS)
Apinyan, V.; Kopeć, T. K.
2015-08-01
We consider the pairing between conduction band electrons, and the valence band holes in the neutral bilayer-type structures. By employing the bilayer Hubbard model, we show the possibility of the inter-plane exciton formation in the system without applied external field. The in-plane and inter-plane Coulomb interaction effects on the pairing mechanism are considered, and the role of the in-plane particle hopping asymmetry on the gap behavior is analyzed in the paper. We show that both Frenkel-type pairing channel and Wannier-Mott-type excitonic pairings are present in the considered system. We analyze also the structure of the chemical potential in the bilayer system. The temperature effects, and the tunable inter-plane electron hopping effects are discussed. For the Frenkel channel, we have shown a particular behavior of the chemical potential at very low temperatures, which is related to the degenerated Frenkel-gap.
Exciton Spectra of the Nanostructured Zinc Oxide
NASA Astrophysics Data System (ADS)
Kapustianyk, V.; Panasiuk, M.; Lubochkova, G.; Turko, B.; Rudyk, V.; Partyka, M.; Serkiz, R.; Voznyuk, D.
A simple two-step approach to produce ZnO nanorods and nanotubes by electron sputtering of the metallic zinc in vacuum and then oxidizing it to ZnO in air is reported. It has been found that the samples synthesised by this method exhibited much more intense exciton photoluminescence around ˜370nm in comparison with the usual flat zinc oxide thin films. The clearly distinguished bands corresponding to localised exciton states and phonon replicas testify to quite high crystallinity of the obtained nanorods (nanotubes). The slight but noticeable blueshift (4meV) in the near band edge luminescence for the sample with nanotubes in comparison with the samples with nanorods and flat films could reflect the quantum confinement effect.
Haines, D.E.; Watson, D.D.; Verow, A.F. )
1990-07-01
Myocardial heating by transcatheter delivery of radiofrequency (RF) energy has been proposed as an effective means of arrhythmia ablation. A thermodynamic model describing the radial temperature gradient at steady state during RF-induced heating is proposed. If one assumes that RF power output is adjusted to maintain a constant electrode-tissue interface temperature at all times, then this thermodynamic model predicts that the radius of the RF-induced lesion will be directly proportional to the electrode radius. A total of 76 RF-induced lesions were created in a model of isolated canine right ventricular free wall perfused and superfused with oxygenated Krebs-Henseleit buffer. Electrode radius was varied between 0.75 and 2.25 mm. RF energy (500 kHz) was delivered for 90 seconds, and the power output was adjusted to maintain a constant electrode-tissue interface temperature of 60 degrees C. A strong linear correlation was observed between electrode radius and lesion radius in two dimensions: transverse (p = 0.0001, r = 0.85) and transmural (p = 0.0001, r = 0.89). With these data, the temperature correlation with irreversible myocardial injury in this model was calculated at 46.6-48.8 degrees C. Therefore, the proposed thermodynamic model closely predicts the observed relation between electrode radius and lesion size during RF myocardial heating.
Size-dependent decoherence of excitonic states in semiconductor microcrystallites
Liu Yuxi; Miranowicz, Adam; Oezdemir, Sahin K.; Koashi, Masato; Imoto, Nobuyuki
2003-03-01
The size-dependent decoherence of the exciton states, resulting from the spontaneous emission, is investigated in a semiconductor spherical microcrystallite under the condition a{sub B}<
Ultrafast dynamics of excitons in tetracene single crystals
Birech, Zephania; Schwoerer, Heinrich; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Excitonic properties of ZnSe/ZnSeS superlattices
NASA Astrophysics Data System (ADS)
Cingolani, R.; Lomascolo, M.; Lovergine, N.; Dabbicco, M.; Ferrara, M.; Suemune, I.
1994-05-01
We report an optical investigation of the excitons in ZnSe/ZnSeS superlattices of well widths ranging between 2 and 15 nm. An almost constant exciton binding energy is found. The exciton confinement is found to be dominated by the hole quantization, consistent with the expectation of negligible conduction-band discontinuity in these heterostructures. The effect of strain has been included to properly reproduce the well width dependence of the light-hole energies. Finally strong evidence of hot exciton photogeneration is obtained from the oscillatory behavior of the photoluminescence excitation spectra. This is consistent with the strong exciton-phonon coupling deduced from the temperature dependence of the excitonic linewidth measured by transmission experiments.
Exciton states and optical properties of carbon nanotubes.
Ajiki, Hiroshi
2012-12-01
Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k·p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis. PMID:23139202
Exciton states and optical properties of carbon nanotubes
NASA Astrophysics Data System (ADS)
Ajiki, Hiroshi
2012-12-01
Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k ṡp approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis.
Singlet Exciton Fission in Nanostructured Organic Solar Cells
Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc
2011-04-13
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
Ultrafast dynamics of excitons in tetracene single crystals.
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale. PMID:24655187
Ultrafast dynamics of excitons in tetracene single crystals
NASA Astrophysics Data System (ADS)
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-01
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states Sn on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Observation of High Angular Momentum Excitons in Cuprous Oxide
NASA Astrophysics Data System (ADS)
Thewes, J.; Hecktter, J.; Kazimierczuk, T.; Amann, M.; Frhlich, D.; Bayer, M.; Semina, M. A.; Glazov, M. M.
2015-07-01
The recent observation of dipole-allowed P excitons up to principal quantum numbers of n =25 in cuprous oxide has given insight into exciton states with unprecedented spectral resolution. While so far the exciton description as a hydrogenlike complex has been fully adequate for cubic crystals, we demonstrate here distinct deviations: The breaking of rotational symmetry leads to mixing of high angular momentum F and H excitons with the P excitons so that they can be observed in absorption. The F excitons show a threefold splitting that depends systematically on n , in agreement with theoretical considerations. From detailed comparison of experiment and theory we determine the cubic anisotropy parameter of the Cu2O valence band.
Singlet exciton fission in nanostructured organic solar cells.
Jadhav, Priya J; Mohanty, Aseema; Sussman, Jason; Lee, Jiye; Baldo, Marc A
2011-04-13
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale. PMID:21355536
Picosecond dynamics of free and bound excitons in doped diamond
NASA Astrophysics Data System (ADS)
Barjon, J.; Valvin, P.; Brimont, C.; Lefebvre, P.; Brinza, O.; Tallaire, A.; Achard, J.; Jomard, F.; Pinault-Thaury, M. A.
2016-03-01
The dynamics of the free-exciton capture by boron acceptors and phosphorus donors in diamond is observed in the picosecond range by time-resolved photoluminescence experiments at low temperature. The formation of boron-bound excitons is observed with a delay of 410 ps after the formation of free excitons. For phosphorus, this delay is 120 ps. This is the result of the free-exciton capture by B0 and P0 impurities. The lifetimes of boron- and phosphorus-bound excitons are measured and found to be equal to 270 and 70 ps, respectively. These values are about four orders of magnitude shorter than for the same impurities in silicon. Ei being the ionization energy of dopants, these results scale well with the Ei4 dependence of the Auger recombination rate expected for bound excitons in indirect band-gap semiconductors.
Excitonic ring formation in ultrapure bulk GaAs
NASA Astrophysics Data System (ADS)
Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.
2014-11-01
We report on spatially resolved low-temperature photoluminescence (PL) measurements of excitons in ultrapure bulk GaAs. At moderate excitation densities we observe butterfly-shaped luminescence images in the wavelength-radial distance plane with a pronounced quench of the exciton PL intensity at the excitation center. The shapes of the PL images show a delicate dependence on excitation wavelength and pump power. We present a model that quantitatively explains the PL intensity quench by a localized overheating of the exciton ensemble due to nonresonant optical excitation. Our model allows us to extract absolute exciton temperatures and to trace the influence of excitation excess energy on the spatial dependence of the exciton energy relaxation. We observe temperature gradients in the exciton system which persist over distances ≥10 μ m away from the excitation spot.
Fractional Solitons in Excitonic Josephson Junctions.
Hsu, Ya-Fen; Su, Jung-Jung
2015-01-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR. PMID:26511770
Fractional Solitons in Excitonic Josephson Junctions
Hsu, Ya-Fen; Su, Jung-Jung
2015-01-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 – until ϕ0 > π – then the alternative group of solitons with Q = ϕ0/2π − 1 takes place and switches the polarity of CPR. PMID:26511770
Microcavity controlled coupling of excitonic qubits.
Albert, F; Sivalertporn, K; Kasprzak, J; Strauß, M; Schneider, C; Höfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W
2013-01-01
Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits--like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots--is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288
Fractional Solitons in Excitonic Josephson Junctions
NASA Astrophysics Data System (ADS)
Hsu, Ya-Fen; Su, Jung-Jung
2015-10-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR.
Excitonic effects in oxyhalide scintillating host compounds
Shwetha, G.; Kanchana, V.; Valsakumar, M. C.
2014-10-07
Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.
Sakai, Yoshitada; Miwa, Masahiko; Oe, Keisuke; Ueha, Takeshi; Koh, Akihiro; Niikura, Takahiro; Iwakura, Takashi; Lee, Sang Yang; Tanaka, Masaya; Kurosaka, Masahiro
2011-01-01
Background Carbon dioxide (CO2) therapy refers to the transcutaneous administration of CO2 for therapeutic purposes. This effect has been explained by an increase in the pressure of O2 in tissues known as the Bohr effect. However, there have been no reports investigating the oxygen dissociation of haemoglobin (Hb) during transcutaneous application of CO2 in vivo. In this study, we investigate whether the Bohr effect is caused by transcutaneous application of CO2 in human living body. Methods We used a novel system for transcutaneous application of CO2 using pure CO2 gas, hydrogel, and a plastic adaptor. The validity of the CO2 hydrogel was confirmed in vitro using a measuring device for transcutaneous CO2 absorption using rat skin. Next, we measured the pH change in the human triceps surae muscle during transcutaneous application of CO2 using phosphorus-31 magnetic resonance spectroscopy (31P-MRS) in vivo. In addition, oxy- and deoxy-Hb concentrations were measured with near-infrared spectroscopy in the human arm with occulted blood flow to investigate O2 dissociation from Hb caused by transcutaneous application of CO2. Results The rat skin experiment showed that CO2 hydrogel enhanced CO2 gas permeation through the rat skin. The intracellular pH of the triceps surae muscle decreased significantly 10 min. after transcutaneous application of CO2. The NIRS data show the oxy-Hb concentration decreased significantly 4 min. after CO2 application, and deoxy-Hb concentration increased significantly 2 min. after CO2 application in the CO2-applied group compared to the control group. Oxy-Hb concentration significantly decreased while deoxy-Hb concentration significantly increased after transcutaneous CO2 application. Conclusions Our novel transcutaneous CO2 application facilitated an O2 dissociation from Hb in the human body, thus providing evidence of the Bohr effect in vivo. PMID:21931656
Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.
Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio
2016-01-01
Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics. PMID:26984281
Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons
NASA Astrophysics Data System (ADS)
Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio
2016-03-01
Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ~250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.
One-dimensional excitons in V-shaped quantum wires
NASA Astrophysics Data System (ADS)
Rinaldi, R.; Cingolani, R.; Lepore, M.; Ferrara, M.; Catalano, I. M.; Marti, U.; Martin, D.; Morier-Gemoud, U.; Ruterana, P.; Reinhart, F. K.
We report a detailed study of one-dimensional excitons in a planar array of single V-shaped GaAs quantum wires. Two-photon absorption, magnetoluminescence and linear photoluminescence spectroscopy have been used to measure the exciton binding energy, the excited 2 p states of the excitons, the higher index transitions and the extension of the confined wavefunctions in the V-shaped region of the quantum wires.
NASA Astrophysics Data System (ADS)
Andreev, A. V.; Kozhevnikov, A. B.; Yavelov, Boris E.
The authors describes the Soveit KGB operation of interviewing Niels Bohr by soviet scientist Yakov. P. Terletskii(1912-1993) and KGB kolonel Lev Petrovich Vasilevskii (b. 1903) on 24 september 1945-20 november 1945 concerning the American Nuclear weapons (Manhattan project)undertaken under the project of the Soviet KGB Lieder Lavrentij P. Berija and supervised by Soviet KGB generals Pavel A. Sudoplatov (b. 1907) and Nikolay S. Sazykin (1910-1985) after the detailed magnetophone interview of Professor Ya. P. Terletskij before his die in Moscow.
Schmidt, L Ph H; Lower, J; Jahnke, T; Schößler, S; Schöffler, M S; Menssen, A; Lévêque, C; Sisourat, N; Taïeb, R; Schmidt-Böcking, H; Dörner, R
2013-09-01
We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both. PMID:25166663
Aruda, Kenneth O; Amin, Victor A; Thompson, Christopher M; Lau, Bryan; Nepomnyashchii, Alexander B; Weiss, Emily A
2016-04-12
This work describes the quantitative characterization of the interfacial chemical and electronic structure of CdSe quantum dots (QDs) coated in one of five p-substituted thiophenolates (X-TP, X = NH2, CH3O, CH3, Cl, or NO2), and the dependence of this structure on the p-substituent X. (1)H NMR spectra of mixtures of CdSe QDs and X-TPs yield the number of X-TPs bound to the surface of each QD. The binding data, in combination with the shift in the energy of the first excitonic peak of the QDs as a function of the surface coverage of X-TP and Raman and NMR analysis of the mixtures, indicate that X-TP binds to CdSe QDs in at least three modes, two modes that are responsible for exciton delocalization and a third mode that does not affect the excitonic energy. The first two modes involve displacement of OPA from the QD core, whereas the third mode forms cadmium-thiophenolate complexes that are not electronically coupled to the QD core. Fits to the data using the dual-mode binding model also yield the values of Δr1, the average radius of exciton delocalization due to binding of the X-TP in modes 1 and 2. A 3D parametrized particle-in-a-sphere model enables the conversion of the measured value of Δr1 for each X-TP to the height of the potential barrier that the ligand presents for tunneling of excitonic hole into the interfacial region. The height of this barrier increases from 0.3 to 0.9 eV as the substituent, X, becomes more electron-withdrawing. PMID:27002248
Sugimoto, Toshikazu; Habuchi, Satoshi; Ogino, Kenji; Vacha, Martin
2009-09-10
We study conformation-dependent photophysical properties of polythiophene (PT) by molecular dynamics simulations and by ensemble and single-molecule optical experiments. We use a graft copolymer consisting of a polythiophene backbone and long polystyrene branches and compare its properties with those obtained on the same polythiophene derivative without the side chains. Coarse-grain molecular dynamics simulations show that in a poor solvent, the PT without the side chains (PT-R) forms a globulelike conformation in which distances between any two conjugated segments on the chain are within the Forster radius for efficient energy transfer. In the PT with the polystyrene branches (PT-PS), the polymer main PT chain retains an extended coillike conformation, even in a poor solvent, and the calculated distances between conjugated segments favor energy transfer only between a few neighboring chromophores. The theoretical predictions are confirmed by measurements of fluorescence anisotropy and fluorescence blinking of the polymers' single chains. High anisotropy ratios and two-state blinking in PT-R are due to localization of the exciton on a single conjugated segment. These signatures of exciton localization are absent in single chains of PT-PS. Electric-field-induced quenching measured as a function of concentration of PT dispersed in an inert matrix showed that in well-isolated chains of PT-PS, the exciton dissociation is an intrachain process and that aggregation of the PT-R chains causes an increase in quenching due to the onset of interchain interactions. Measurements of the field-induced quenching on single chains indicate that in PT-R, the exciton dissociation is a slower process that takes place only after the exciton is localized on one conjugated segment. PMID:19691332
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.
Upconverted photoluminescence induced by radiative coupling between excitons
NASA Astrophysics Data System (ADS)
Matsuda, Takuya; Yokoshi, Nobuhiko; Ishihara, Hajime
2016-04-01
We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and light wave prominently enhances the interaction between different excitonic states via radiation beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the upconverted photoluminescence based on the coherent quantum superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas.
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.
Polarization-dependent exciton dynamics in tetracene single crystals
NASA Astrophysics Data System (ADS)
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-01
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Polarization-dependent exciton dynamics in tetracene single crystals
Zhang, Bo; Zhang, Chunfeng Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Localization length scales of triplet excitons in singlet fission materials
NASA Astrophysics Data System (ADS)
Bayliss, Sam L.; Thorley, Karl J.; Anthony, John E.; Bouchiat, Hélène; Greenham, Neil C.; Chepelianskii, Alexei D.
2015-09-01
We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.
Polarization-dependent exciton dynamics in tetracene single crystals.
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. PMID:25554147
Exciton dynamics in organic molecular crystals and nanostructures
NASA Astrophysics Data System (ADS)
Bardeen, Chris
2014-03-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet <-- --> triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations.
Triplet diffusion in singlet exciton fission sensitized pentacene solar cells
NASA Astrophysics Data System (ADS)
Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.
2013-10-01
Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.
Superradiance of High Density Frenkel Excitons at Room Temperature
NASA Astrophysics Data System (ADS)
Wang, H. Z.; Zheng, X. G.; Zhao, F. L.; Gao, Z. L.; Yu, Z. X.
1995-05-01
Superradiance of high density Frenkel excitons in an R-phycoerythrin single crystal is observed at room temperature for the first time. No fluorescence is observed except the emission at the sharp exciton band when the superradiance of excitons occurs, and the higher the pump density, the sharper the emission bandwidth. A redshift and a blueshift are observed at the rise time and the fall time of the emission pulse, respectively. The experimental results also imply deformed-boson properties of high density Frenkel excitons.
Minimal model for charge transfer excitons at the dielectric interface
NASA Astrophysics Data System (ADS)
Ono, Shota; Ohno, Kaoru
2016-03-01
A theoretical description of the charge transfer (CT) exciton across the donor-acceptor interface without the use of a completely localized hole (or electron) is a challenge in the field of organic solar cells. We calculate the total wave function of the CT exciton by solving an effective two-particle Schrödinger equation for the inhomogeneous dielectric interface. We formulate the magnitude of the CT and construct a minimal model of the CT exciton under the breakdown of inversion symmetry. We demonstrate that both a light hole mass and a hole localization along the normal to the dielectric interface are crucial to yield the CT exciton.
[Vascularized iliac crest and distal radius reconstruction].
Pic Gomis, L; Gomis, R
2010-12-01
The authors relate their experience concerning the vascularized iliac crest flap. In the first chapter, they detail the anatomic vascularized osteocutaneous iliac crest. Blood supply arises from the deep and superficial circonflexe iliac artery. Many anastomoses connect the two systems. In the second chapter, they detail the operative technique of free and pedicule hone iliac crest flap. Composite cutaneous bone flaps are also detailed. In the third chapter, they detail informations about treatment of distal radius bone defects with associated skeen flap if necessary. PMID:21084210
Abramavicius, Darius; Nemeth, Alexandra; Milota, Franz; Sperling, Jaroslaw; Mukamel, Shaul; Kauffmann, Harald F.
2013-01-01
The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry. PMID:22401120
NASA Astrophysics Data System (ADS)
Kramar, V. M.; Pugantseva, O. V.
2014-08-01
In the approximation of effective masses for electronic and phononic - dielectric continuum - systems, the influence of spatial bounding, self-polarization, and exciton-phonon interactions on the exciton state in a flat double nanoheterostructure (a nanofilm) - lead iodide in a polymer matrix -is theoretically investigated for the model of a single infinitely deep quantum well. It is demonstrated that the dominating factor determining the energy of the bottom of the ground exciton band and its binding energy is spatial bounding. The relationship between two other effects depends on the nanofilm thickness, namely, the influence of the self-polarization effect in ultrathin films significantly exceeds that of exciton-phonon interaction.
An Exoplanet Radius and Transit Timing Survey
NASA Astrophysics Data System (ADS)
Deming, Drake; Jennings, Jonald; Sada, Pedro
2010-02-01
Many exoplanet systems contain Jupiter-mass planets on close-in orbits. Theories of planetary system formation account for these hot Jupiters as being end states of inward migration. Variants of those theories also predict terrestrial planets to be captured in mean motion resonance with the hot Jupiters. A continuing explosion of discoveries by transit surveys have given us a sample of 45 hot Jupiters transiting planets brighter than V=13. A transit timing survey of these systems could detect hot Earths in resonance, via the large (~ 180 second) perturbations they induce on the giant planet transits. Moreover, the discovery photometry for these systems usually provides only relatively coarse photometric precision, but larger-aperture follow-up can determine the giant planet radius to a precision limited only by knowledge of the stellar mass, and thereby reveal the diversity of giant exoplanet structure, such as the presence of heavy element cores. The relatively large sample now available means that a radius- and transit timing-survey is well matched to classical observing and telescope scheduling. We propose continued observations to perform transit photometry using FLAMINGOS on the 2.1-meter in the J-band, where stellar limb darkening is minimal and transit photometry has excellent sensitivity to planetary radii and shifts in transit time.
An Exoplanet Radius and Transit Timing Survey
NASA Astrophysics Data System (ADS)
Deming, Drake; Jennings, Jonald; Sada, Pedro
2009-08-01
Many exoplanet systems contain Jupiter-mass planets on close-in orbits. Theories of planetary system formation account for these hot Jupiters as being end states of inward migration. Variants of those theories also predict terrestrial planets to be captured in mean motion resonance with the hot Jupiters. A recent explosion of discoveries by transit surveys have given us a sample of 37 hot Jupiters transiting planets brighter than V=13. A transit timing survey of these systems could detect hot Earths in resonance, via the large (~ 180 second) perturbations they induce on the giant planet transits. Moreover, the discovery photometry for these systems usually provides only relatively coarse photometric precision, but larger-aperture follow-up can determine the giant planet radius to a precision limited only by knowledge of the stellar mass, and thereby reveal the diversity of giant exoplanet structure, such as the presence of heavy element cores. The relatively large sample now available means that a radius- and transit timing-survey is well matched to classical observing and telescope scheduling. We propose continued observations to perform transit photometry using FLAMINGOS on the 2.1-meter in the J-band, where stellar limb darkening is minimal and transit photometry has excellent sensitivity to planetary radii and shifts in transit time.
The role of excitons and interfaces in molecular organic devices
NASA Astrophysics Data System (ADS)
Bulovic, Vladimir
Operating characteristics of molecular organic devices are largely governed by the formation of excitons, and exciton interactions with interfaces between dissimilar materials. The study of these excitonic processes and their implementation in practical optoelectronic applications is the focus of this work. We demonstrate a number of novel molecular organic devices by utilizing unique optical and electronic properties of this class of solids. Excitons dominate the fundamental processes in organic materials determining their absorption, photoconduction, luminescence, and lasing characteristics. The spatial extent of the exciton governs its dynamics and depends on the strength of intermolecular interaction. Different types of excitons are illustrated in our spectroscopic study of the archetype molecular compound 3,4,9,10-perylineteracarboxylic dianhydride. Through our discussion of exciton self-trapping, line broadening, diffusion, and inter-level transitions in this archetype molecular organic crystal, we access a rich array of excitonic processes. The exciton-interface interaction influences the luminescence and photogeneration efficiency, energy quenching, exciton quantum confinement, and exciton lifetime. After examining these phenomena, we tailor our molecular organic structures to demonstrate photovoltaic cells and efficient organic light emitting devices (OLEDs). We demonstrate transparent OLEDs that can be used in lightweight, conformable, head-up displays, and inverted OLEDs that can be integrated with conventional electronics. We also demonstrate a stacked OLED that integrates three transparent OLEDs to generate a color-tunable, true color device. Bright and efficient electroluminescence (EL) is a general property of many organic thin films. It is generated by radiative recombination of an exciton formed by electrically injected carriers. We investigate the formation of excitons in the EL process by analyzing our measurements in terms of trap-limited conduction in amorphous materials. We infer that the traps are due to molecular polarons, which also determine the energy distribution of excitons, and hence the EL emission spectrum. We also show that spectral emission can be modified when the luminescent center is in the vicinity of a strong electric dipole, where by adjusting the strength of the dipole field the EL spectrum can be altered. The radiative recombination of Frenkel excitons in luminescent devices is also influenced by the presence of multilayer structures which introduce microcavity effects. We develop a comprehensive theoretical description of microcavity effects in OLEDs which accounts for the spectral shape and intensity as a function of the emission angle, treating both radiative and waveguided modes. We finally show that optically excited organic material in a microcavity can undergo population inversion and lase.
Exciton dissociation at phthalocyanine-C60 interfaces
NASA Astrophysics Data System (ADS)
Robey, S. W.; Dutton, G. J.
2014-03-01
Exciton dissociation and charge transfer processes occurring within 10's of nanometers of donor-acceptor interfaces are critical for the performance of organic photovoltaic (OPV) structures. We investigated fundamental issues of exciton dissociation near prototypical donor-acceptor interface using time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine (Pc)-C60 interfaces with known structures were formed using organic molecular beam epitaxy. Pc π --> π * (Q-band) transitions were created by a sub-picosecond pump pulse, producing a population of singlet (S1) Pc excitons. The dynamics of this population were then probed via photoemission by a time-delayed UV pulse. For CuPc ∖C60 interfaces, the dynamics for excitons created far from the interface were modeled with a combination of vibrational or intraband relaxation plus intersystem crossing (ISC) to triplet levels. Relaxation leads predominantly to triplet (T1) exciton levels on timescales of ~ 1-2 ps. The decay dynamics of S1 excitons excited in the CuPc layer adjacent to C60 were increased due to the additional channel leading to exciton dissociation, occurring with a rate of ~ 7 x 10 12 sec-1. However, excitons that relax to T1 levels at the interface dissociate with a rate ~ 500 to 1000 times slower, providing a picture of the energy dependence of exciton dissociation at this interface. The dependence of exciton dissociation versus Pc thickness at analogous H2Pc ∖C60 interfaces will also be presented. The results indicate that, for this interface, exciton dissociation is much faster for the interfacial layer with dissociation from the 2nd, and subsequent layers of H2Pc, reduced by at least a factor of 10.
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 given in terms of a stochastic decoupling ansatz. This method has become the standard in exciton-vibrational theory and illustrative examples will be presented as well as a comparison with ML-MCTDH. Applications will be shown for generic model systems as well as for small aggregates mimicking those formed by perylene bisimide dyes. Further, photosynthetic antenna complexes will be discussed, including spectral densities and the role of exciton-vibrational coupling in two-dimensional electronic spectroscopy.
Effect of exciton dragging by a surface acoustic wave
NASA Astrophysics Data System (ADS)
Kovalev, V. M.; Chaplik, A. V.
2015-02-01
We present the theory of the effect of exciton dragging by a Rayleigh surface acoustic wave at temperatures above the condensation temperature of the exciton gas and at zero temperature, where the effects of the Bose-Einstein condensation of the exciton gas are most pronounced. The magnitude of the acoustic drag flux in the exciton gas at high temperatures has been calculated taking into account the exciton-exciton interaction. It has been shown that the drag flux at typical experimental parameters (at a given intensity of the surface acoustic wave (SAW)) is independent of the frequency of the acoustic wave, whereas the interaction between excitons leads to screening of the SAW-induced perturbation, which results in an exponentially fast decrease in the drag flux with an increase in the exciton density. At low temperatures, in the presence of a condensate, the drag flux of condensate particles exhibits a resonance character when the velocity of Bogoliubov excitations approaches the velocity of the acoustic wave and the magnitude of the flux is linear in the SAW frequency. The drag flux of the above-condensate particles has a threshold character: the above-condensate particles are dragged by the wave at a velocity of the acoustic wave higher than the bogolon velocity. The magnitude of the above-condensate flux is inversely proportional to the SAW frequency.
Excitonic luminescence upconversion in a two-dimensional semiconductor
NASA Astrophysics Data System (ADS)
Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; Yan, Jiaqiang; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Dery, Hanan; Yao, Wang; Xu, Xiaodong
2016-04-01
Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. This phenomenon lies at the heart of optical refrigeration in solids, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities or exciton-phonon coupling. Here, we demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors. Since the charged exciton binding energy closely matches the 31 meV A1' optical phonon, we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. In addition, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.
Architectures for enhanced exciton collection in organic photovoltaic cells
NASA Astrophysics Data System (ADS)
Holmes, Russell
2010-03-01
Organic semiconductors have received considerable attention for application in a variety of optoelectronic systems including light-emitting devices, lasers, and photovoltaic cells. Due to their compatibility with lightweight flexible substrates and high throughput processing techniques, organic photovoltaic cells (OPVs) represent an intriguing renewable energy option. In these materials, photogenerated excitons must be dissociated in order to generate a photocurrent. Exciton dissociation is typically realized using a donor-acceptor (D-A) heterojunction, where the energy level offset exceeds the exciton binding energy. Mobile excitons diffuse to the D-A heterojunction and are dissociated into their component charge carriers. In most organic materials, the exciton diffusion length is much shorter than the optical absorption length. Consequently, not all of the photogenerated excitons reach the D-A interface, limiting cell efficiency. For small molecule active materials, routes around this bottleneck have centered on the use of mixed D-A film morphologies to increase the area of the dissociating interface. This work instead focuses on the use of OPVs with continuously graded film composition and morphology as a means to simultaneously optimize the exciton diffusion and charge collection efficiencies. In these graded heterojunction OPVs, the power conversion efficiency is noted to exceed that of comparable devices containing a planar or mixed heterojunction. Overall, this approach provides the ability to tune the exciton diffusion and charge collection efficiencies based on the composition profile, permitting greater control over device performance.
Ultrafast exciton relaxation in monolayer transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Thilagam, A.
2016-04-01
We examine a mechanism by which excitons undergo ultrafast relaxation in common monolayer transition metal dichalcogenides. It is shown that at densities ≈1 × 1011 cm-2 and temperatures ≤60 K, excitons in well known monolayers (MoS2, MoSe2, WS2, and WSe2) exist as point-like structureless electron-hole quasi-particles. We evaluate the average rate of exciton energy relaxation due to acoustic phonons via the deformation potential and the piezoelectric coupling mechanisms and examine the effect of spreading of the excitonic wavefunction into the region perpendicular to the monolayer plane. Our results show that the exciton relaxation rate is enhanced with increase in the exciton temperature, while it is decreased with increase in the lattice temperature. Good agreements with available experimental data are obtained when the calculations are extrapolated to room temperatures. A unified approach taking into account the deformation potential and piezoelectric coupling mechanisms shows that exciton relaxation induced by phonons is as significant as defect assisted scattering and trapping of excitons by surface states in monolayer transition metal dichalcogenides.
Mapping the exciton diffusion in semiconductor nanocrystal solids.
Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail
2015-03-24
Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling. PMID:25682881
Storing excitons in transition-metal dichalcogenides using dark states
NASA Astrophysics Data System (ADS)
Gunlycke, Daniel; Tseng, Frank; Simsek, Ergun
Monolayer transition-metal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electron-hole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electron-hole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.
Directing energy transport in organic photovoltaic cells using interfacial exciton gates.
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. PMID:25798712
Excitons in atomically thin black phosphorus
NASA Astrophysics Data System (ADS)
Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Tabis, W.; Maude, D. K.; Plochocka, P.
2016-03-01
Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in-plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blueshift of the emission with increasing temperature is well described using a two-oscillator model for the temperature dependence of the band gap.
Optical diode based on exciton-polaritons
NASA Astrophysics Data System (ADS)
Espinosa-Ortega, T.; Liew, T. C. H.; Shelykh, I. A.
2013-11-01
We propose theoretically an optical diode based on exciton-polaritons in semiconductor microcavities. A flow of polaritons in the bistable regime is used to send signals through an asymmetric fixed potential that favours the bridging of particles in one direction. Through dynamic modelling of the coherent polariton field, we demonstrate the characteristics of an ideal diode, namely, that the forward signal is fully transmitted while the transmission in the reverse direction tends to zero, without any additional external control. Moreover, the system proves to be robust to the presence of disorder, intrinsic to microcavities, and can function at gigahertz repetition rates.
Charge-transfer excitons at organic semiconductor surfaces and interfaces.
Zhu, X-Y; Yang, Q; Muntwiler, M
2009-11-17
When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT(1s) as expected from energy resonance arguments. We suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface. PMID:19378979
Understanding the mass-radius relation for sub-Neptunes: radius as a proxy for composition
Lopez, Eric D.; Fortney, Jonathan J.
2014-09-01
Transiting planet surveys like Kepler have provided a wealth of information on the distribution of planetary radii, particularly for the new populations of super-Earth- and sub-Neptune-sized planets. In order to aid in the physical interpretation of these radii, we compute model radii for low-mass rocky planets with hydrogen-helium envelopes. We provide model radii for planets 1-20 M {sub ⊕}, with envelope fractions 0.01%-20%, levels of irradiation 0.1-1000 times Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide simple analytic fits that summarize how radius depends on each of these parameters. Most importantly, we show that at fixed H/He envelope fraction, radii show little dependence on mass for planets with more than ∼1% of their mass in their envelope. Consequently, planetary radius is to a first order a proxy for planetary composition, i.e., H/He envelope fraction, for Neptune- and sub-Neptune-sized planets. We recast the observed mass-radius relationship as a mass-composition relationship and discuss it in light of traditional core accretion theory. We discuss the transition from rocky super-Earths to sub-Neptune planets with large volatile envelopes. We suggest ∼1.75 R {sub ⊕} as a physically motivated dividing line between these two populations of planets. Finally, we discuss these results in light of the observed radius occurrence distribution found by Kepler.
Excitonic correlation in the Mott crossover regime in Ge
NASA Astrophysics Data System (ADS)
Sekiguchi, Fumiya; Shimano, Ryo
2015-04-01
Exciton Mott transition (EMT) in Ge was investigated by using optical-pump and terahertz-probe spectroscopy. From the quantitative analysis of optical conductivity and dielectric function, we evaluated the densities of unbound electron-hole pairs and excitons after the photoexcitation, from which we determined the ionization ratio of excitons α. The Mott crossover density region in Ge was elucidated from the density dependence of α in the temperature range above the critical temperature of electron-hole droplets. The 1 s -2 p excitonic transition energy hardly shifted with increasing density toward the EMT. Combined with the similar results recently observed in bulk Si, we suggest that the robustness of excitonic correlation against the Coulomb screening is a universal feature in bulk semiconductors in the Mott crossover regime.
Multiphonon excitonic absorption in semiconductors and quantum wells
Bardyszewski, Witold; Prywata, Miroslaw; Yevick, David
2001-07-01
In this article, we present a straightforward procedure based on the cumulant expansion for computing the contribution of exciton{endash}phonon coupling to the exciton line-shape function and thus absorption in bulk and quantum well semiconductors. Using the functional integral technique we derive the effective equation of motion for the two-particle Green{close_quote}s function in the presence of interaction with phonons. We show that our many body formulation in the limit of linear coupling of carriers with polar longitudinal optical phonons in bulk semiconductors is equivalent to the model exciton{endash}phonon Hamiltonian approach. Our calculations for the hydrogenic model of excitons in three- and two-dimensional systems reproduce not only the correct polaron induced shift (renormalization) of the exciton energy but also the observed asymmetric spectral line shape and exponential Urbach absorption tail below the fundamental absorption edge. {copyright} 2001 American Institute of Physics.
Calculating exciton downconversion rates in Coulombically coupled chromophores
NASA Astrophysics Data System (ADS)
Chapman, Craig; Schatz, George
2015-03-01
Exciton downconversion is a second order energy transfer process that splits a high energy exciton in a donor chromophore into multiple lower energy excitons in acceptor chromophores. Downconversion has been seen in a variety of materials including rare-earth doped glassy matrices, organic crystals, and semiconductor nanocrystals, and has the potential to efficiently convert a single high energy photon into a broad distribution of lower energy excitons. A comprehensive mechanistic understanding of the energy conversion process will allow for the rational engineering of materials that can control the flow of energy in a guided fashion. To this end we formulate and implement a method for calculating multi-chromophore Förster-like exciton transfer rates using transition charges obtained from time-dependent density functional theory.
High efficiency organic multilayer photodetectors based on singlet exciton fission
NASA Astrophysics Data System (ADS)
Lee, J.; Jadhav, P.; Baldo, M. A.
2009-07-01
We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.
Excitonic states of an impurity in a Fermi gas
NASA Astrophysics Data System (ADS)
Lan, Zhihao; Lobo, Carlos
2015-11-01
We study excitonic states of an atomic impurity in a Fermi gas, i.e., bound states consisting of the impurity and a hole. Previous studies considered bound states of the impurity with particles from the Fermi sea where the holes formed only part of the particle-hole dressing. Within a two-channel model, we find that, for a wide range of parameters, excitonic states are not ground but metastable states. We further calculate the decay rates of the excitonic states to polaronic and dimeronic states and find they are long-lived, scaling as ΓPolExc∝(Δω ) 5.5 and ΓDimExc∝(Δω ) 4 . We also find that a new continuum of exciton-particle states should be considered alongside the previously known dimeron-hole continuum in spectroscopic measurements. Excitons must therefore be considered a new ingredient in the study of metastable physics currently being explored experimentally.
Bose-Einstein Condensation of Excitons and Biexcitons
NASA Astrophysics Data System (ADS)
Moskalenko, S. A.; Snoke, D. W.
2000-02-01
Bose-Einstein condensation of excitons is a unique effect in which the electronic states of a solid can self-organize to acquire quantum phase coherence. The phenomenon is closely linked to Bose-Einstein condensation in other systems such as liquid helium and laser-cooled atomic gases. Covering theoretical aspects as well as recent experimental work, the book provides a comprehensive survey of the field. After introducing the relevant basic physics of excitons, the authors discuss exciton-phonon interactions as well as the behavior of biexcitons. They also cover exciton phase-transitions and give particular attention to nonlinear optical effects including the optical Stark effect and chaos in excitonic systems. The thermodynamics of equilibrium, quasiequilibrium, and nonequilibrium systems are examined in detail. Throughout, the authors interweave theoretical and experimental results. The book will be of great interest to graduate students and researchers in semiconductor and superconductor physics, quantum optics, and atomic physics.
21 CFR 886.1450 - Corneal radius measuring device.
Code of Federal Regulations, 2014 CFR
2014-04-01
... 21 Food and Drugs 8 2014-04-01 2014-04-01 false Corneal radius measuring device. 886.1450 Section 886.1450 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES OPHTHALMIC DEVICES Diagnostic Devices § 886.1450 Corneal radius measuring device. (a) Identification. A corneal radius...
NASA Astrophysics Data System (ADS)
Szczytko, J.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M. T.; Deveaud, B.
2005-05-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum well: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We demonstrate, by comparing the temperature dependence of 1s and 2s excitonic transitions, that excitons provide the dominant contribution to the luminescence signal. Furthermore, our analysis gives evidence that the Coulomb correlated plasma contribution to the luminescence signal might be neglected for densities, temperatures, and time scales actually used in time-resolved experiments.
Trofymchuk, Kateryna; Prodi, Luca; Reisch, Andreas; Mély, Yves; Altenhöner, Kai; Mattay, Jochen; Klymchenko, Andrey S
2015-06-18
Photoswitching of bright fluorescent nanoparticles opens new possibilities for bioimaging with superior temporal and spatial resolution. However, efficient photoswitching of nanoparticles is hard to achieve using Förster resonance energy transfer (FRET) to a photochromic dye, because the particle size is usually larger than the Förster radius. Here, we propose to exploit the exciton diffusion within the FRET donor dyes to boost photoswitching efficiency in dye-doped polymer nanoparticles. To this end, we utilized bulky hydrophobic counterions that prevent self-quenching and favor communication of octadecyl rhodamine B dyes inside a polymer matrix of poly(D,L-lactide-co-glycolide). Among tested counterions, only perfluorinated tetraphenylborate that favors the exciton diffusion enables high photoswitching efficiency (on/off ratio ∼20). The switching improves with donor dye loading and requires only 0.1-0.3 wt % of a diphenylethene photochromic dye. Our nanoparticles were validated both in solution and at the single-particle level. The proposed concept paves the way to new efficient photoswitchable nanomaterials. PMID:26266601
Exciton energy transfer between asymmetric quantum wires: Effect of transfer to an array of wires
NASA Astrophysics Data System (ADS)
Lyo, S. K.
2006-05-01
We study the Stokes exciton transfer rate from a narrow quantum wire (QWR) to a parallel wide QWR separated by a wide barrier and also to a planar array of parallel wide QWRs. The transfer rate is calculated as a function of the distance d between the narrow QWR and the wide QWR and also the array. The dependence of the rate on the temperature and the localization radius is studied for free and localized excitons, respectively. Both the resonant and nonresonant rates are considered. We find that, for energy transfer between two QWRs, the Förster dipole-dipole transfer dominates the transfer rate at short and intermediate distances. The photon-exchange transfer prevails only at an extremely long distance where the rate is negligibly small. This behavior is in contrast with the two-dimensional quantum wells, where the photon-exchange mechanism is dominant except at a very short distance. However, for the energy transfer to an array of QWRs, the photon-exchange transfer rate continues to increase as the array size grows to a macroscopic scale due to its slow range dependence while the dipolar rate saturates quickly with the array size. As a result, the photon-exchange transfer can become dominant in a system consisting of stacks of QWRs or arrays distributed over a wide range. The prediction of the theory is consistent with recent data from V-groove GaAs/AlxGa1-xAs double quantum wires.
Lebedev, Vladimir S; Medvedev, A S
2012-08-31
This paper examines plasmon - exciton coupling effects in light absorption and scattering by hybrid nanoparticles consisting of a metallic core and organic dye J-aggregate shell. The spectroscopic characteristics of such particles are calculated using generalised Mie theory for two concentric spheres in a wide spectral range for various geometric parameters of the system, core materials (Ag, Au, Cu and Al) and cyanine dyes (TC, OC and PIC). We determine the eigenfrequencies of hybrid modes in the system and photoabsorption peak heights as functions of the oscillator strength of the transition in the J-band of the dye, core radius and shell thickness, and demonstrate that the interactions of a Frenkel exciton with dipole and multipole plasmons have radically different effects on the optical properties of the composite nanoparticles. Varying the particle size and the optical constants of the core and shell materials influences the number of peaks in the spectra of the particles and leads to a significant redistribution of peak heights. We identify regions where the extinction spectrum of the particles is dominated by light absorption or scattering processes. (nanophotonics)
Microcavity controlled coupling of excitonic qubits
Albert, F.; Sivalertporn, K.; Kasprzak, J.; Strauß, M.; Schneider, C.; Höfling, S.; Kamp, M.; Forchel, A.; Reitzenstein, S.; Muljarov, E.A.; Langbein, W.
2013-01-01
Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288
Excitons and charges at organic semiconductor heterojunctions.
Friend, Richard H; Phillips, Matthew; Rao, Akshay; Wilson, Mark W B; Li, Zhe; McNeill, Christopher R
2012-01-01
All-organic heterojunction solar cells now provide very high quantum efficiencies for charge generation and rapidly-improving power conversion efficiencies. Charge generation and separation however, must overcome the strong Coulomb interactions between electrons and holes in these materials that is manifest also through the large exchange energies usually observed. We show for a polymer-polymer system with low charge generation efficiency that this arises through intersystem crossing from the photogenerated charge-transfer state to a lower lying triplet state, mediated by the proton hyperfine interaction, and that the activation barrier for full separation of electrons and holes is of the order of 250 meV. We observe, using transient optical spectroscopy, the processes of charge separation, recombination and sweep-out in efficient polymer-fullerene devices. We report also on the process of singlet exciton fission to form a pair of triplet excitons in pentacene that can later be dissociated against a heterojunction formed with C60. PMID:22470984
Robust excitons inhabit soft supramolecular nanotubes
Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.
2014-01-01
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336
Theory of exciton transfer and diffusion in conjugated polymers
Barford, William; Tozer, Oliver Robert
2014-10-28
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ∼10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.
PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS
Watts, Anna L.; Van der Klis, Michiel; Wijers, Ralph A. M. J.; Kouveliotou, Chryssa; Van der Horst, Alexander J.; Goegues, Ersin; Kaneko, Yuki; Harding, Alice K.; Baring, Matthew G.
2010-08-10
On 2008 August 24 the new magnetar SGR 0501+4516 (discovered by Swift) emitted a bright burst with a pronounced double-peaked structure in hard X-rays, reminiscent of the double-peaked temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However, for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24 burst.
Experimental study of finite Larmor radius effects
Struve, K.W.
1980-08-01
Linear Z-pinches in Ar, Kr, Xe, N/sub 2/, and He are experimentally studied in regimes where strong finite Larmor radius effects could provide a significant stabilizing effect. Scaling arguments show that for deuterium such a pinch has an electron line density of order 2 x 10/sup 15//cm. For higher Z plasmas a higher line density is allowed, the exact value of which depends on the average ion charge. The pinch is formed by puffing gas axially through the cathode towards the anode of an evacuated pinch chamber. When the gas reaches the anode, the pinch bank is fired. The pinch current rises in 2 to 3 ..mu..sec to a maximum of 100 to 200 kA. The pinch bank capacitance is 900 ..mu..F, and the external inductance is 100 nH. Additionally, the bank is fused to increase dI/dt. The primary diagnostics are a framing camera, a spatially resolved Mach-Zehnder interferometer, and X-ray absorption.
Broadband THz response of high-density excitons
NASA Astrophysics Data System (ADS)
Huber, R.
2005-03-01
Many-body interactions in electron-hole (e-h) gases determine their nature as a conductive unbound e-h plasma or insulating exciton gas. THz spectroscopy, unlike bandgap luminescence or absorption, probes transitions between internal exciton states. In this way, exciton formation in a dilute, optically-generated e-h gas was recently investigated in GaAs quantum wells (R. Kaindl, et al. Nature 423, 734, 2003). Here, we report THz studies of a high-density exciton gas. For dilute, insulating excitons, the THz conductivity peaks around 7 meV due to 1s-2p transitions. With increasing density, the peak shows a striking red-shift and broadening, and finally evolves into a Drude shape. Quantitative analysis reveals a broadening that gradually exceeds the level spacing and is larger than expected for 1s excitons. This agrees with enhanced scattering from p-like states. In contrast to optical studies, which are limited due to counteracting bandgap renormalization and reduced e-h attraction, THz radiation provides a direct way to measure the impact of phase-space filling and screening on exciton levels at high densities.
Dynamical fermion mass generation and exciton spectra in graphene
Zhang Chunxu; Liu Guozhu; Huang Mingqiu
2011-03-15
The Coulomb interaction between massless Dirac fermions may induce dynamical chiral symmetry breaking by forming excitonic pairs in clean graphene, leading to semimetal-insulator transition. If the Dirac fermions have zero bare mass, an exact continuous chiral symmetry is dynamically broken and thus there are massless Goldstone excitons. If the Dirac fermions have a small bare mass, an approximate continuous chiral symmetry is dynamically broken and the resultant Goldstone-type excitons become massive, which is analogous to what happens in QCD. In this paper, after solving the Dyson-Schwinger gap equation in the presence of a small bare fermion mass, we found a remarkable reduction of the critical Coulomb interaction strength for excitonic pair formation and a strong enhancement of dynamical fermion mass. We then calculate the masses of Goldstone-type excitons using the Shifman-Vainshtein-Zakharov sum-rule method and operator product expansion technique developed in QCD and find that the exciton masses are much larger than bare fermion mass but smaller than the width of dynamical fermion mass gap. We also study the spin susceptibilities and estimate the masses of non-Goldstone-type excitons using the same tools.
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.
Excitonic AND Logic Gates on DNA Brick Nanobreadboards
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
Topological polaritons and excitons in garden-variety systems
NASA Astrophysics Data System (ADS)
Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Timothy C. H.
2015-04-01
We present a practical scheme for creating topological polaritons in garden-variety systems based, for example, on zinc-blende semiconductor quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as an appealing alternative for topological states in exciton-based systems. Topological polaritons and indirect excitons open a new frontier for topological states in solid-state systems, which can be directly probed and manipulated while offering a system with nonlinear interactions.
Compensation of dipolar-exciton spin splitting in magnetic field
NASA Astrophysics Data System (ADS)
Gorbunov, A. V.; Timofeev, V. B.
2013-03-01
Magnetoluminescence of spatially indirect dipolar excitons in 25 nm GaAs/AlGaAs single quantum well collected within a lateral potential trap has been studied in Faraday geometry. The paramagnetic spin splitting of the luminescence line of the heavy-hole excitons in the trap centre is completely compensated at magnetic field below critical value ≈2 Т. The effect of spin splitting compensation is caused by the exchange interaction in dense exciton Bose gas which is in qualitative agreement with the existing theoretical concepts.
Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals
NASA Astrophysics Data System (ADS)
Lyu, ByungGook
This work discusses the photocurrent and photoluminescence that can be induced by short-pulse illumination in rubrene single crystals. The pulsed illumination excites a rubrene molecule from the ground state to its first optically accessible excited state, resulting in a singlet exciton state. In rubrene, a singlet exciton can transform into two triplet excitons - which together have a spin of zero - by an efficient spin-conserved fission process. On the other hand, two triplet excitons can interact to again form a singlet exciton by a fusion process. Quantitative modeling of the transformation of singlet excitons into triplet excitons and vice-versa shows that both photoconductivity dynamics and photocurrent dynamics after pulsed excitations can be understood within the same framework. The photoluminescence observed after pulsed excitation is only emitted upon radiative recombination of singlet excitons. A simple model of fission and fusion based on rate equations leads to a qualitatively different photoluminescence dynamics depending on the time scale. In particular, it predicts a fast exponential decay corresponding to the initial fission process, later a power-law (quadratic) decay corresponding to a regime when triplet-triplet interaction is dominant, and a final exponential decay with a time-constant which is half the triplet exciton lifetime. This last exponential decay corresponds to the case when only a lower density of triplet excitons is left. The same model can be used to predict the photocurrent dynamics after pulsed excitation. Experimental observations after pulsed illumination show that, for low excitation pulse energies, a large photocurrent grows exponentially with a time constant of the order of 100 microseconds. This photocurrent build-up time then becomes shorter at higher excitation energies, with the peak photocurrent also saturating. One finds that the observed photocurrent dynamics can be reproduced with the same model based on exciton fission and fusion that successfully explained photoluminescence dynamics. The only additional assumption that is required to do so is that triplet excitons be able dissociate and release free holes by direct interaction with a defect state. The 100 microsecond build-up time of the impulsively induced photocurrent then corresponds to the triplet lifetime.
Spin-excitons in heavy-fermion semimetals
NASA Astrophysics Data System (ADS)
Riseborough, Peter S.; Magalhaes, S. G.
2016-02-01
Spin-excitons are sharp and dispersive magnetic fluctuations in paramagnetic semiconductors where the dispersion relation lies within the semiconducting gap. Spin-excitons are found in the vicinity of magnetic quantum critical points in semiconductors, much the same as antiparamagnons are precursor fluctuations for quantum critical points in metals. Here we show that this concept of spin-exciton excitations can be extended to heavy-fermion semimetals and provides a natural explanation of the magnetic modes found by inelastic neutron scattering experiments on paramagnetic CeFe2Al10.
Electron-exciton complex formation in organic solids
NASA Astrophysics Data System (ADS)
Bader, G.; Caron, L.; Sanche, L.
1981-06-01
Sharp structure in the doubly differentiated electron transmission spectra of thin (≅ 150 Å) benzene films is shown to result from electron-exciton complex formation. The large energy shift between the electronic energy levels of the molecule and the transmission maxima is related to the electron binding energy. This is due to the polarizability difference between the Frenkel exciton and the organic molecule. From a simple Wigner-Seitz model, we show that the captured electron is localized between the exciton and the sorrounding molecules.
Confocal shift interferometry of coherent emission from trapped dipolar excitons
Repp, J.; Schinner, G. J.; Schubert, E.; Rai, A. K.; Wieck, A. D.; Reuter, D.; Wurstbauer, U.; Holleitner, A. W.; and others
2014-12-15
We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.
Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes.
Hofmann, Matthias S; Noé, Jonathan; Kneer, Alexander; Crochet, Jared J; Högele, Alexander
2016-05-11
We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes. PMID:27105355
Role of phonons in Josephson oscillations of excitonic and polaritonic condensates
Magnusson, E. B.; Flayac, H.; Malpuech, G.; Shelykh, I. A.
2010-11-15
We analyze theoretically the role of the exciton-phonon interactions in phenomena related to the Josephson effect between two spatially separated exciton and exciton-polariton condensates. We consider the role of the dephasing introduced by phonons in such phenomena as Josephson tunneling, self-trapping and spontaneous polarization separation. In the regime of cw pumping we find a remarkable bistability effect arising from exciton-exciton interactions as well as regimes of self-sustained regular and chaotic oscillations.
Tidal radius estimates for three open clusters
NASA Astrophysics Data System (ADS)
Danilov, V. M.; Loktin, A. V.
2015-10-01
A new method is developed for estimating tidal radii and masses of open star clusters (OCL) based on the sky-plane coordinates and proper motions and/or radial velocities of cluster member stars. To this end, we perform the correlation and spectral analysis of oscillations of absolute values of stellar velocity components relative to the cluster mass center along three coordinate planes and along each coordinate axis in five OCL models. Mutual correlation functions for fluctuations of absolute values of velocity field components are computed. The spatial Fourier transform of the mutual correlation functions in the case of zero time offset is used to compute wavenumber spectra of oscillations of absolute values of stellar velocity components. The oscillation spectra of these quantities contain series of local maxima at equidistant wavenumber k values. The ratio of the tidal radius of the cluster to the wavenumber difference Δ k of adjacent local maxima in the oscillation spectra of absolute values of velocity field components is found to be the same for all five OCL models. This ratio is used to estimate the tidal radii and masses of the Pleiades, Praesepe, and M67 based on the proper motions and sky-plane coordinates of the member stars of these clusters. The radial dependences of the absolute values of the tangential and radial projections of cluster star velocities computed using the proper motions relative to the cluster center are determined, along with the corresponding autocorrelation functions and wavenumber spectra of oscillations of absolute values of velocity field components. The Pleiades virial mass is estimated assuming that the cluster is either isolated or non-isolated. Also derived are the estimates of the Pleiades dynamical mass assuming that it is non-stationary and non-isolated. The inferred Pleiades tidal radii corresponding to these masses are reported.
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. PMID:15268371
Control of Exciton Photon Coupling in Nano-structures
NASA Astrophysics Data System (ADS)
Liu, Xiaoze
In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications.
Exciton dynamics in emergent Rydberg lattices
NASA Astrophysics Data System (ADS)
Bettelli, S.; Maxwell, D.; Fernholz, T.; Adams, C. S.; Lesanovsky, I.; Ates, C.
2013-10-01
The dynamics of excitons in a one-dimensional ensemble with partial spatial order are studied. During optical excitation, cold Rydberg atoms spontaneously organize into regular spatial arrangements due to their mutual interactions. This emergent lattice is used as the starting point to study resonant energy transfer triggered by driving a nS to n'P transition using a microwave field. The dynamics are probed by detecting the survival probability of atoms in the nS Rydberg state. Experimental data qualitatively agree with our theoretical predictions including the mapping onto the XXZ spin model in the strong-driving limit. Our results suggest that emergent Rydberg lattices provide an ideal platform to study coherent energy transfer in structured media without the need for externally imposed potentials.
Hot Exciton Dissociation in a Conjugated Polymer
NASA Astrophysics Data System (ADS)
Arkhipov, V. I.; Emelianova, E. V.; Bässler, H.
1999-02-01
A model has been developed to treat field-assisted ultrafast on-chain dissociation of optical excitations in a conjugated polymer. Conversion of the excess photon energy above the singlet exciton into the vibrational heat bath of a polymer segment is considered as a main source of the energy required for charges to escape from a potential well formed by a superposition of the Coulomb and external electric fields. The calculated yield of carrier photogeneration increases strongly with increasing field but is independent of the temperature in good quantitative agreement with data obtained by Barth et al. [Chem. Phys. Lett. 288, 147 (1998)] on weakly disordered conjugated ladder-type poly-phenylene.
Decoherence in weakly coupled excitonic complexes.
Mančal, Tomáš; Balevičius, Vytautas; Valkunas, Leonas
2011-04-28
Equations of motion for weakly coupled excitonic complexes are derived. The description allows one to treat the system in the basis of electronic states localized on individual chromophores while at the same time accounting for experimentally observable delocalization effects in optical spectra. The equations are shown to be related to the well-known Förster type energy-transfer rate equations, but unlike Förster equations, they provide a description of the decoherence processes leading to suppression of the resonance coupling by bath fluctuations. Linear absorption and two-dimensional photon echo correlation spectra are calculated for simple model systems in the homogeneous limit, demonstrating a distinct delocalization effect and reduction of the resonance coupling due to interaction with the bath. PMID:21338152
Excitons in a surface quantum well
NASA Astrophysics Data System (ADS)
Arulmozhi, M.; Anitha, A.
2014-11-01
Binding energies of excitons in a Surface Quantum Well (SQW) composed of vacuum/GaAs/AlxGa1-xAs as a function of wellwidth are calculated. The effect of non-parabolicity is considered by using an energy dependent effective mass. The effect of mass anisotropy and the effect of image charges which arise due to the large dielectric discontinuity at the vacuum/GaAs interface are also considered. The average distances of the electron
Optical properties of Rydberg excitons and polaritons
NASA Astrophysics Data System (ADS)
Zielińska-Raczyńska, Sylwia; Czajkowski, Gerard; Ziemkiewicz, David
2016-02-01
We show how to compute the optical functions when Rydberg excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the real density matrix approach (RDMA), which, combined with the Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a Cu20 crystal, being a semiconductor with an indirect gap. The effect of the coherence is displayed in the line shape. We also examine in detail and explain the dependence of the oscillator strength and the resonance placement on the state number. We report good agreement with recently published experimental data. We also show that the presented method can be applied to semiconductors with a direct gap.
Synthesis and Characterization of Quantum Dots: A Case Study Using PbS
ERIC Educational Resources Information Center
Pan, Yi; Li, Yue Ru; Zhao, Yu; Akins, Daniel L.
2015-01-01
A research project for senior undergraduates of chemistry has been developed to introduce syntheses of a series of monodispersed semiconductor PbS quantum dots (QDs) and their characterization methodologies. In this paper, we report the preparation of monodispersed semiconductor PbS QDs with sizes smaller than the exciton Bohr radius using a…
Synthesis and Characterization of Quantum Dots: A Case Study Using PbS
ERIC Educational Resources Information Center
Pan, Yi; Li, Yue Ru; Zhao, Yu; Akins, Daniel L.
2015-01-01
A research project for senior undergraduates of chemistry has been developed to introduce syntheses of a series of monodispersed semiconductor PbS quantum dots (QDs) and their characterization methodologies. In this paper, we report the preparation of monodispersed semiconductor PbS QDs with sizes smaller than the exciton Bohr radius using a
Min, Kyung-Gu; Jho, Young-Dahl; Yee, Ki-Ju; Stanton, C. J.; Song, Jin-Dong
2013-12-04
We have studied the characteristics of longitudinal-optical-phonon--plasmon coupled (LOPC) mode as a function of thickness in InAs epilayers, ranging from 10 to 900 nm. The absence of LOPC modes in a scale less than exciton Bohr radius manifests the role of electron diffusion rather than the carrier screening via drift motion in surface depletion region.
An iterative algorithm for calculating stylus radius unambiguously
NASA Astrophysics Data System (ADS)
Vorburger, T. V.; Zheng, A.; Renegar, T. B.; Song, J.-F.; Ma, L.
2011-08-01
The stylus radius is an important specification for stylus instruments and is commonly provided by instrument manufacturers. However, it is difficult to measure the stylus radius unambiguously. Accurate profiles of the stylus tip may be obtained by profiling over an object sharper than itself, such as a razor blade. However, the stylus profile thus obtained is a partial arc, and unless the shape of the stylus tip is a perfect sphere or circle, the effective value of the radius depends on the length of the tip profile over which the radius is determined. We have developed an iterative, least squares algorithm aimed to determine the effective least squares stylus radius unambiguously. So far, the algorithm converges to reasonable results for the least squares stylus radius. We suggest that the algorithm be considered for adoption in documentary standards describing the properties of stylus instruments.
Mechanism for radiative recombination in ZnCdO alloys
Buyanova, I. A.; Bergman, J. P.; Pozina, G.; Chen, W. M.; Rawal, S.; Norton, D. P.; Pearton, S. J.; Osinsky, A.; Dong, J. W.
2007-06-25
Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content.
Electrical Activation of Dark Excitonic States in Carbon Nanotubes
NASA Astrophysics Data System (ADS)
Uda, Takushi; Yoshida, Masahiro; Ishii, Akihiro; Kato, Yuichiro K.
Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric field effects on various excitonic states by simultaneously measuring both photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of E11 excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. A simple field-induced exciton dissociation model is introduced to explain the photocurrent threshold fields, and the edge of the E11 continuum states have been identified using this model. Work supported by JSPS (KAKENHI 24340066, 26610080), MEXT (Photon Frontier Network Program, Nanotechnology Platform), Canon Foundation, and Asahi Glass Foundation.
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.
Topological Polaritons and Excitons in Garden Variety Systems
NASA Astrophysics Data System (ADS)
Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Tim
2015-03-01
Topological polaritons (aka topolaritons) present a new frontier for topological behavior in solid-state systems. They combine light and matter, which allows to probe and manipulate them in a variety of ways. They can also be made strongly interacting, due to their excitonic component. Here we present a scheme which allows to realize topolaritons in garden variety zinc-blende quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as a particularly appealing alternative for topological states in exciton-based systems. Indirect excitons are robust and long lived (with lifetimes up to milliseconds), and, therefore, provide a flexible platform for the realization, probing, and utilization of topological coupled light-matter states. Funded by: Institute for Quantum Information and Matter, Swiss National Science Foundation, Packard Foundation, NSF.
Effect of correlation of local fluctuations on exciton coherence
NASA Astrophysics Data System (ADS)
Chen, Xin; Silbey, Robert J.
2010-05-01
Recent experimental studies have shown both oscillations of exciton populations and long lasting coherence in multichromophoric systems such as photosynthetic light harvesting systems and conjugated polymers. It has been suggested that this quantum effect is due to correlations of the fluctuations of site energies among the closely packed chromophores in the protein environment. In addition to these, there is the strong possibility of correlations between site energies and transfer matrix elements. In order to understand the role of such correlations we generalize the Haken-Strobl-Reineker (HSR) model to include the energetic correlations and the site diagonal-off-diagonal correlations in a systematic way. The extended HSR model in the exciton basis is also constructed and allows us to study the dynamics of the exciton populations and coherences. With the extended model, we can provide insight into how these correlations affect the evolution of the populations and coherences of excitons by comparing to the original HSR model with uncorrelated fluctuating environments.
Effect of correlation of local fluctuations on exciton coherence
Chen, Xin; Silbey, Robert J.
2010-01-01
Recent experimental studies have shown both oscillations of exciton populations and long lasting coherence in multichromophoric systems such as photosynthetic light harvesting systems and conjugated polymers. It has been suggested that this quantum effect is due to correlations of the fluctuations of site energies among the closely packed chromophores in the protein environment. In addition to these, there is the strong possibility of correlations between site energies and transfer matrix elements. In order to understand the role of such correlations we generalize the Haken–Strobl–Reineker (HSR) model to include the energetic correlations and the site diagonal-off-diagonal correlations in a systematic way. The extended HSR model in the exciton basis is also constructed and allows us to study the dynamics of the exciton populations and coherences. With the extended model, we can provide insight into how these correlations affect the evolution of the populations and coherences of excitons by comparing to the original HSR model with uncorrelated fluctuating environments.
Excitons in ultrathin organic-inorganic perovskite crystals
NASA Astrophysics Data System (ADS)
Yaffe, Omer; Chernikov, Alexey; Norman, Zachariah M.; Zhong, Yu; Velauthapillai, Ajanthkrishna; van der Zande, Arend; Owen, Jonathan S.; Heinz, Tony F.
2015-07-01
We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.
Excitonic condensation in systems of strongly correlated electrons
NASA Astrophysics Data System (ADS)
Kuneš, Jan
2015-08-01
The idea of exciton condensation in solids was introduced in the 1960s with the analogy of superconductivity in mind. While exciton supercurrents have been realised only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g. cold atoms physics. One of our goals is to provide a ‘dictionary’ that would allow the reader to efficiently combine results obtained in these different fields.
Singlet exciton fission-sensitized infrared quantum dot solar cells.
Ehrler, Bruno; Wilson, Mark W B; Rao, Akshay; Friend, Richard H; Greenham, Neil C
2012-02-01
We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells. PMID:22257168
Exciton separation at the interface between pentacene and dipolar chromophore
NASA Astrophysics Data System (ADS)
Park, Byoungnam; Park, Jonghoo
2013-11-01
We demonstrate photo-induced charge transfer between pentacene and dipolar chromophore, disperse red 19 (DR19), which can be applied in fabricating optoelectronic devices with abundant flexibility due to the functionality of the dipolar chromophore at the interface. Photo-induced charge transfer phenomenon between pentacene and DR19 is explored through pentacene thickness-dependent threshold voltage measurements using pentacene/DR19 bilayer field effect transistors under illumination. Threshold voltage increases as pentacene thickness increases up to a certain thickness followed by a decrease, resulting in a peak threshold voltage. The presence of the peak explains competition between optical absorption in the pentacene layer and exciton diffusion followed by exciton separation at the pentacene/DR19 interface. The exciton diffusion length in pentacene is estimated using an exciton diffusion-dissociation model.
Large excitonic effects in group-IV sulfide monolayers
NASA Astrophysics Data System (ADS)
Tuttle, Blair R.; Alhassan, Saeed M.; Pantelides, Sokrates T.
2015-12-01
Large exciton binding energies are a distinguishing feature of two-dimensional semiconductors because of reduced screening, potentially leading to unique optoelectronic applications. Here we use electronic structure methods to calculate the properties of a two-dimensional material class: group-IV monosulfides including SiS, GeS, and SnS. Bulk SiS is predicted to be a metastable layered material. Quasiparticle excitations are calculated with the G0W0 method and the Bethe-Salpeter equation is are used to include electron-hole interactions. For monolayers, strongly bound excitons are found below the quasiparticle absorption edge. The predicted excitonic binding energies are as high as 0.7 eV. Due to large excitonic effects, these group-IV sulfide monolayers have great potential for nanoscale optoelectronic applications.
Excitonic condensation in spatially separated one-dimensional systems
Abergel, D. S. L.
2015-05-25
We show theoretically that excitons can form from spatially separated one-dimensional ground state populations of electrons and holes, and that the resulting excitons can form a quasicondensate. We describe a mean-field Bardeen-Cooper-Schrieffer theory in the low carrier density regime and then focus on the core-shell nanowire giving estimates of the size of the excitonic gap for InAs/GaSb wires and as a function of all the experimentally relevant parameters. We find that optimal conditions for pairing include small overlap of the electron and hole bands, large effective mass of the carriers, and low dielectric constant of the surrounding media. Therefore, one-dimensional systems provide an attractive platform for the experimental detection of excitonic quasicondensation in zero magnetic field.
Excitons in organic molecular nanostructures: Physics and applications
NASA Astrophysics Data System (ADS)
Shen, Zilan
Excitonic processes dominate the electric and optical properties of organic materials. From delocalized charge-transfer (CT) excitons in very closely packed organic molecular crystals (OMCs) to localized Frenkel excitons in loosely packed amorphous organic solids, they determine material characteristics such as absorption, photo-conduction and luminescence. Recently, organic light emitting displays (OLEDs) and organic thin film transistors whose functionality partially depends on fundamental excitations have attracted substantial interest due to their unique properties unattainable with conventional semiconductors. To optimize the device performance such as shifting absorption peak wavelength, enhancing current injection, controlling channel conduction, maximizing electroluminescence (EL) efficiency, and obtaining saturated red, green and blue emission colors, it is essential to understand how excitonic processes is modified in organic nanostructures. In this work, we examine CT and Frenkel excitons in OMCs and amorphous organic materials. A quantum mechanical model is developed to study electrooptical properties of delocalized CT excitons in closely packed OMC nanostructures. Based on this model, we analyze the electroabsorption (EA) spectrum in bulk PTCDA and the absorption spectral shifts in PTCDA/NTCDA multilayers, and obtain consistent values of effective masses and exciton radii along difference crystalline axes. The same treatment is extended to fit the GaAs EA spectrum, suggesting a common physical origin for both CT and Wannier excitons. We also examine the Frenkel excitons in more decoupled amorphous organic solids used for OLEDs. Assuming an exponentially distributed trap states in the lowest unoccupied molecular orbital and the highest occupied molecular orbital gap, we are able to explain current-voltage characteristics and EL efficiencies observed under various temperature and charge injection conditions. We infer that the traps are due to molecular polarons, which also determine the energy distribution of excitons, and hence the EL emission spectrum. To utilize some unique properties of OMCs, we demonstrate a vertically stacked, three color OLED which allows for independent tuning of color, gray scale and intensity. The radiative recombination of Frenkel excitons is influenced by the heterogeneous multilayer structure via microcavity interference. Controlling the recombination environment by varying the layer thickness, and emissive layer positions, color saturation and EL efficiency can be optimized.
Improving optical bench radius measurements using stage error motion data
Schmitz, Tony L.; Gardner, Neil; Vaughn, Matthew; Medicus, Kate; Davies, Angela
2008-12-20
We describe the application of a vector-based radius approach to optical bench radius measurements in the presence of imperfect stage motions. In this approach, the radius is defined using a vector equation and homogeneous transformation matrix formulism. This is in contrast to the typical technique, where the displacement between the confocal and cat's eye null positions alone is used to determine the test optic radius. An important aspect of the vector-based radius definition is the intrinsic correction for measurement biases, such as straightness errors in the stage motion and cosine misalignment between the stage and displacement gauge axis, which lead to an artificially small radius value if the traditional approach is employed. Measurement techniques and results are provided for the stage error motions, which are then combined with the setup geometry through the analysis to determine the radius of curvature for a spherical artifact. Comparisons are shown between the new vector-based radius calculation, traditional radius computation, and a low uncertainty mechanical measurement. Additionally, the measurement uncertainty for the vector-based approach is determined using Monte Carlo simulation and compared to experimental results.
Quantum confinement of excitons in wurtzite InP nanowires
Pemasiri, K.; Jackson, H. E.; Smith, L. M.; Wong, B. M.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.
2015-05-21
Exciton resonances are observed in photocurrent spectra of 80 nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30 nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.
Excitonic coupling in van der waals complexes: The anisole dimers
NASA Astrophysics Data System (ADS)
Pasquini, Massimiliano; Pietraperzia, Giangaetano; Piani, Giovanni; Becucci, Maurizio
2011-05-01
We report on the determination of the excitonic coupling occurring between the two anisole molecular units bound to form the anisole dimer in the gas phase. The experimental data are summarized and then analyzed in terms of time-independent perturbation theory. The effective perturbation term for the molecular hamiltonian giving rise to the excitonic coupling is determined. The relative relevance of different coupling terms in the hamiltonian of the system is discussed as well.
Kekule-distortion-induced exciton instability in graphene
Dillenschneider, Raoul
2008-09-15
Effects of a Kekule distortion on exciton instability in single-layer graphene are discussed. In the framework of quantum electrodynamics the mass of the electron generated dynamically is worked out using a Schwinger-Dyson equation. For homogeneous lattice distortion it is shown that the generated mass is independent of the amplitude of the lattice distortion at the one-loop approximation. Formation of excitons induced by the homogeneous Kekule distortion could appear only through direct dependence of the lattice distortion.
Femtosecond THz Studies of Intra-Excitonic Transitions
Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.
2007-10-02
Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.
Exciton Band Structure in Two-Dimensional Materials.
Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo
2016-02-12
Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene. PMID:26919006
Monte Carlo Simulation of Exciton Dynamics in Supramolecular Semiconductor Architectures
NASA Astrophysics Data System (ADS)
Silva, Carlos; Beljonne, David; Herz, Laura; Hoeben, Freek
2005-03-01
Supramolecular chemistry is useful to construct molecular architectures with functional semiconductor properties. To explore the consequences of this approach in molecular electronics, we have carried out ultrafast measurements of exciton dynamics in supramolecular assemblies of an oligo-p-phenyl-ene-vinyl-ene derivative functionalized to form chiral stacks in dodecane solution in a thermotropically reversible manner. We apply a model of incoherent exciton hopping within a Monte Carlo scheme to extract microscopic physical quantities. The simulation first builds the chiral stacks with a Gaussian disorder of site energies and then simulates exciton hopping on the structure and exciton-exciton annihilation to reproduce ensemble-averaged experimental data. The exciton transfer rates are calculated beyond the point-dipole approximation using the so-called line-dipole approach in combination with the Förster expression. The model of incoherent hopping successfully reproduces the data and we extract a high diffusion coefficient illustrating the polymeric properties of such supramolecular assemblies. The scope and limitations of the line-dipole approximation as well as the resonance energy transfer concept in this system are discussed.
Molecular packing determines singlet exciton fission in organic semiconductors.
Kolata, Kolja; Breuer, Tobias; Witte, Gregor; Chatterjee, Sangam
2014-07-22
Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission. PMID:24957197
Exciton Band Structure in Two-Dimensional Materials
NASA Astrophysics Data System (ADS)
Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo
2016-02-01
Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.
Singlet fission in pentacene through multi-exciton quantum states.
Zimmerman, Paul M; Zhang, Zhiyong; Musgrave, Charles B
2010-08-01
Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here. PMID:20651727
Exciton energy-momentum map of hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Fugallo, Giorgia; Aramini, Matteo; Koskelo, Jaakko; Watanabe, Kenji; Taniguchi, Takashi; Hakala, Mikko; Huotari, Simo; Gatti, Matteo; Sottile, Francesco
2015-10-01
Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe-Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum q , as previously reported, but also at large q . We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials.
Exciton Dynamics in Hexagonal InP Nanowires
NASA Astrophysics Data System (ADS)
Kaveh-Baghbadorani, Masoud; Langbein, Wolfgang; Gao, Qiang; Jagadish, Chennupati; Wagner, Hans-Peter
2013-03-01
We study the exciton dynamics in InP nanowire ensembles by intensity- and temperature-dependent photoluminescence (PL) measurements, time-correlated-single-photon-counting (TCSPC) and heterodyne detected four-wave-mixing experiments (HFWM). The InP nanowires were grown on fused silica substrate by 50 nm gold catalyst metal-organic-vapor-phase-epitaxy at a temperature of 450 C resulting in nearly wurtzite type nanowires. The PL measurements at 15 K show a strong emission band at 837 nm and two weak side bands at nearly 820 and 860 nm. The bands are tentatively attributed to trapped, free and zinc-blende related exciton transitions, respectively. With increasing temperature the free-exciton band gains importance relative to the dominating trapped exciton band while the low energy band vanishes. TCSPC measurements show an increasing PL decay rate of all emission bands with increasing temperature most pronounced for the low energy band. The result agrees with the exciton population dynamics obtained from three-beam HFWM measurements. Photon echo experiments at 80 K reveal an ultrafast exciton dephasing time of less than 100 fs which is attributed to scattering with a high carrier background in these nanowires. The support of the Australian Research Council is kindly acknowledged.
Excitonic effects in the luminescence of quantum wells
NASA Astrophysics Data System (ADS)
Deveaud, B.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M. T.; Szczytko, J.; Piermarocchi, C.
2005-11-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high-quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum wells: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We are also able to give the boundaries of the Mott transition in our system, and to show the absence of observable renormalization of the gap below the onset of this transition. We detail the characteristics of the trion formation and evidence the possible formation of both positive and negative trions in the absence of any resident free carrier populations.
Probing the origin of excitonic states in monolayer WSe2
Huang, Jiani; Hoang, Thang B.; Mikkelsen, Maiken H.
2016-01-01
Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications. PMID:26940069
Schweitzer-Stenner, Reinhard; Bosenbeck, Michael; Dreybrodt, Wolfgang
1993-01-01
The depolarization ratios of heme protein Raman lines arising from vibrations of the heme group exhibit significant dependence on the excitation wavelength. From the analysis of this depolarization ratio dispersion, one obtains information about symmetry-lowering distortions ?Q? of the heme group that can be classified in terms of the symmetry races ? = A1g, B1g, B2g, and A2g in D4h symmetry. The heme-protein interaction can be changed by the protonation of distinct amino acid side chains (i.e., for instance the Bohr groups in hemoglobin derivates), which gives rise to specific static heme distortions for each protonation state. From the Raman dispersion data, it is possible to obtain parameters by fitting to a theoretical expression of the Raman tensor, which provide information on these static distortions and also about the pK values of the involved titrable side chains. We have applied this method to the ?4 (1,355 cm-1) and ?10 (1,620 cm-1) lines of deoxygenated hemoglobin of the fourth component of trout and have measured their depolarization ratio dispersion as a function of pH between 6 and 9. From the pH dependence of the thus derived parameters, we obtain pK values identical to those of the Bohr groups, which were earlier derived from the corresponding O2-binding isotherms. These are pK?1 = pK?2 = 8.5 for the ? and pK?1 = 7.5, pK?2 = 7.4 for the ? chains. We also obtain the specific distortion parameters for each protonation state. As shown in earlier studies, the ?4 mode mainly probes distortions from interactions between the proximal histidine and atoms of the heme core (i.e., the nitrogens and the C? atoms of the pyrroles). Group theoretical argumentation allows us to relate specific changes of the imidazole geometry as determined by its tilt and azimuthal angle and the iron-out-of-plane displacement to distinct variations of the normal distortions ?Q? derived from the Raman dispersion data. Thus, we found that the pH dependence of the heme distortions ?QA1g (totally symmetric) and ?QB1g (asymmetric) is caused by variations of the azimuthal rather than the tilt angle of the Fe-His (F8) bond. In contrast to this, the ?10 line mainly monitors changes resulting from the interaction between peripheral substituents of the porphyrin macrocycle (vinyl). From the pH dependence of the parameters, it is possible to separately identify distortions ?Q? affecting the hemes in the ? and ? chains, respectively. From this, we find that in the ? subunit structural changes induced on protonation of the corresponding Bohr groups are mainly transferred via the FeN? bond and give rise to changes in the azimuthal angle. In the ? subunit, however, in addition, structural changes of the heme pocket arise, which most probably result from protonation of the imidazole of the COOH-terminal His (HC3 ?). This rearranges the net of H bonds between His HC3 ?, Ser (F9 ?), and Glu (F7 ?). PMID:19431886
Exciton-dominant electroluminescence from a diode of monolayer MoS{sub 2}
Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Wang, Yuan; Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang
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 a 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.
Exciton dynamics in atomically thin MoS2: Interexcitonic interaction and broadening kinetics
NASA Astrophysics Data System (ADS)
Sim, Sangwan; Park, Jusang; Song, Jeong-Gyu; In, Chihun; Lee, Yun-Shik; Kim, Hyungjun; Choi, Hyunyong
2013-08-01
We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS2. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons (A and B) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the B-exciton linewidth in turn lowers the peak spectral amplitude of the A exciton. Resonant excitation at the B-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.
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. PMID:26391077
Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai
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 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.
Probing excitonic dark states in single-layer tungsten disulphide.
Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang
2014-09-11
Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (∼0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing. PMID:25162523
Determination of Radius of Curvature for Teeth With Cycloid Profile
NASA Astrophysics Data System (ADS)
Shatalov, E. V.; Efremenkov, E. A.; Shibinskiy, K. G.
2016-04-01
In the article the geometric determination of curvature radius is considered for teeth with cycloid profile. The equations are obtained for the determination of a radius of curvature with point coordinates of a cycloid profile. The conditions of convexo-concavity of a teeth profile are defined for transmission with intermediate rollers.
Precise root-mean-square radius of {sup 4}He
Sick, Ingo
2008-04-15
We study the world data on elastic electron-helium scattering to determine the {sup 4}He charge root-mean-square radius. A precise value for this radius is needed as a reference for a number of ongoing studies in nuclear and atomic physics.
The Origin of the Ionic-Radius Ratio Rules
ERIC Educational Resources Information Center
Jensen, William B.
2010-01-01
In response to a reader query, this article traces the origins of the ionic-radius ratio rules and their incorrect attribution to Linus Pauling in the chemical literature and to Victor Goldschmidt in the geochemical literature. In actual fact, the ionic-radius ratio rules were first proposed within the context of the coordination chemistry…
The Origin of the Ionic-Radius Ratio Rules
ERIC Educational Resources Information Center
Jensen, William B.
2010-01-01
In response to a reader query, this article traces the origins of the ionic-radius ratio rules and their incorrect attribution to Linus Pauling in the chemical literature and to Victor Goldschmidt in the geochemical literature. In actual fact, the ionic-radius ratio rules were first proposed within the context of the coordination chemistry
21 CFR 886.1450 - Corneal radius measuring device.
Code of Federal Regulations, 2012 CFR
2012-04-01
... 21 Food and Drugs 8 2012-04-01 2012-04-01 false Corneal radius measuring device. 886.1450 Section 886.1450 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES OPHTHALMIC DEVICES Diagnostic Devices § 886.1450 Corneal radius measuring...
Fragment-Specific Fixation for Fractures of the Distal Radius.
Geissler, William B; Clark, Sonja M
2016-03-01
This article summarizes the management of distal fractures utilizing Acumed fragment-specific family of plates. No single plate option can address every fracture pattern of the distal radius. These fragment-specific plates are usually adjuncts to allow the surgeon to expand the armamentarium in the management of complex volar and dorsal comminuted distal radius fracture patterns. PMID:26855832
Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy
Huber, Rupert; Kaindl, Robert A.; Schmid, Ben A.; Chemla, Daniel S.
2005-06-25
We report the density-dependent crossover of a resonantly photoexcited exciton gas from insulating to conducting phases. Broadband terahertz spectroscopy gives direct access to the exciton binding energy via intra-excitonic 1s-2p transitions. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with decreasing inter-particle distance. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite free-carrier and exciton terahertz conductivity. Comparison with near-infrared absorption changes illustrates a significantly enhanced energy shift and broadening of the intra-excitonic resonance.
Decreasing the spectral radius of a graph by link removals.
Van Mieghem, Piet; Stevanović, Dragan; Kuipers, Fernando; Li, Cong; van de Bovenkamp, Ruud; Liu, Daijie; Wang, Huijuan
2011-07-01
The decrease of the spectral radius, an important characterizer of network dynamics, by removing links is investigated. The minimization of the spectral radius by removing m links is shown to be an NP-complete problem, which suggests considering heuristic strategies. Several greedy strategies are compared, and several bounds on the decrease of the spectral radius are derived. The strategy that removes that link l=i~j with largest product (x(1))(i)(x(1))(j) of the components of the eigenvector x(1) belonging to the largest adjacency eigenvalue is shown to be superior to other strategies in most cases. Furthermore, a scaling law where the decrease in spectral radius is inversely proportional to the number of nodes N in the graph is deduced. Another sublinear scaling law of the decrease in spectral radius versus the number m of removed links is conjectured. PMID:21867251
Plasmon transmission through excitonic subwavelength gaps.
Sukharev, Maxim; Nitzan, Abraham
2016-04-14
We study the transfer of electromagnetic energy across a subwavelength gap separating two co-axial metal nanorods. In the absence of spacer in the gap separating the rods, the system exhibits strong coupling behavior between longitudinal plasmons in the two rods. The nature and magnitude of this coupling are studied by varying various geometrical parameters. As a function of frequency, the transmission is dominated by a split longitudinal plasmon peak. The two hybrid modes are the dipole-like "bonding" mode characterized by a peak intensity in the gap and a quadrupole-like "antibonding" mode whose amplitude vanishes at the gap center. When the length of one rod is varied, this mode spectrum exhibits the familiar anti-crossing behavior that depends on the coupling strength determined by the gap width. When off-resonant 2-level emitters are placed in the gap, almost no effect on the frequency dependent transmission is observed. In contrast, when the molecular system is resonant with the plasmonic line shape, the transmission is strongly modified, showing characteristics of strong exciton-plasmon coupling. Most strongly modified is the transmission near the lower frequency "bonding" plasmon mode. The presence of resonant molecules in the gap affects not only the molecule-field interaction but also the spatial distribution of the field intensity and the electromagnetic energy flux across the junction. PMID:27083741
Plasmon transmission through excitonic subwavelength gaps
NASA Astrophysics Data System (ADS)
Sukharev, Maxim; Nitzan, Abraham
2016-04-01
We study the transfer of electromagnetic energy across a subwavelength gap separating two co-axial metal nanorods. In the absence of spacer in the gap separating the rods, the system exhibits strong coupling behavior between longitudinal plasmons in the two rods. The nature and magnitude of this coupling are studied by varying various geometrical parameters. As a function of frequency, the transmission is dominated by a split longitudinal plasmon peak. The two hybrid modes are the dipole-like "bonding" mode characterized by a peak intensity in the gap and a quadrupole-like "antibonding" mode whose amplitude vanishes at the gap center. When the length of one rod is varied, this mode spectrum exhibits the familiar anti-crossing behavior that depends on the coupling strength determined by the gap width. When off-resonant 2-level emitters are placed in the gap, almost no effect on the frequency dependent transmission is observed. In contrast, when the molecular system is resonant with the plasmonic line shape, the transmission is strongly modified, showing characteristics of strong exciton-plasmon coupling. Most strongly modified is the transmission near the lower frequency "bonding" plasmon mode. The presence of resonant molecules in the gap affects not only the molecule-field interaction but also the spatial distribution of the field intensity and the electromagnetic energy flux across the junction.
Benchmarking Calculations of Excitonic Couplings between Bacteriochlorophylls.
Kenny, Elise P; Kassal, Ivan
2016-01-14
Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. Understanding these uncertainties can guard against striving for unrealistic precision; at the same time, detailed benchmarks can allow important qualitative questions-which do not depend on the precise values of the simulation parameters-to be addressed with greater confidence about the conclusions. PMID:26651217
Photosynthetic light harvesting: excitons and coherence.
Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C; Scholes, Gregory D
2014-03-01
Photosynthesis begins with light harvesting, where specialized pigment-protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques. PMID:24352671
Exciton condensation and perfect Coulomb drag.
Nandi, D; Finck, A D K; Eisenstein, J P; Pfeiffer, L N; West, K W
2012-08-23
Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, double quantum well systems can support exciton condensates, which consist of electrons in one well tightly bound to holes in the other. 'Perfect' drag is therefore expected; a steady transport current of electrons driven through one quantum well should be accompanied by an equal current of holes in the other. Here we demonstrate this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunnelling of charge between the quantum wells, which can readily compromise drag measurements, is negligible. We note that, from an electrical engineering perspective, perfect Coulomb drag is analogous to an electrical transformer that functions at zero frequency. PMID:22914164
Photosynthetic light harvesting: excitons and coherence
Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C.; Scholes, Gregory D.
2014-01-01
Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques. PMID:24352671
Singlet fission of hot excitons in π-conjugated polymers.
Zhai, Yaxin; Sheng, Chuanxiang; Vardeny, Z Valy
2015-06-28
We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2ET (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2ET, which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=Eg (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2Ag exciton, dominates the triplet photogeneration in PDA polymer as Eg>2ET. The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously. PMID:25987576
Multiple exciton generation and ultrafast exciton dynamics in HgTe colloidal quantum dots.
Al-Otaify, Ali; Kershaw, Stephen V; Gupta, Shuchi; Rogach, Andrey L; Allan, Guy; Delerue, Christophe; Binks, David J
2013-10-21
The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model. PMID:23999734
Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation.
Scholes, Gregory D
2015-12-24
Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet-triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes-the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin |X1⟩ ≈ (1)[TT] and quintet spin |Q⟩ ≈ (5)[TT]. It is shown that triplet pair states can be separated by a triplet-triplet energy-transfer mechanism to give a separated, yet entangled triplet pair (1)[T···T]. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton-exciton annihilation to form the correlated triplet pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the separated correlated triplet pair (geminate recombination). PMID:26595530
Intermolecular exciton-exciton annihilation in phospholipid vesicles doped with [Ru(bpy)2dppz]2+
NASA Astrophysics Data System (ADS)
De la Cadena, Alejandro; Pascher, Torbjörn; Davydova, Dar'ya; Akimov, Denis; Herrmann, Felix; Presselt, Martin; Wächtler, Maria; Dietzek, Benjamin
2016-01-01
The ultrafast photophysics of [Ru(bpy)2dppz]2+ (dppz = dipyrido[3,2-a:2‧,3‧-c]-phenazine) embedded into the walls of phospholipid vesicles has been studied by femtosecond time-resolved pump-probe spectroscopy. While [Ru(bpy)2dppz]2+ has been studied intensively with respect to its intramolecular charge transfer processes, which are associated with the well known light-switch effect, this study focuses on intermolecular energy transfer processes taking place upon dense packing of the complexes into a phospholipid membrane composed of dipalmitoyl-L-α-phosphatidylglycerol, which can be thought of as a simplistic model of a cellular membrane. The data indicate additional quenching of excited [Ru(bpy)2dppz]2+ upon increasing the pump-pulse intensity. Hence, the observed photophysics, which is assigned to the presence of intermolecular exciton-exciton annihilation at high pump-intensities, might be related to the ultrafast photophysics of [Ru(bpy)2dppz]2+ when used as a chromophore to stain cells, an effect that may be taken into account during the employment of novel cellular markers based on Ru polypyridine complexes.
ON THE VARIATION OF SOLAR RADIUS IN ROTATION CYCLES
Qu, Z. N.; Kong, D. F.; Xiang, N. B.; Feng, W.
2015-01-10
The Date Compensated Discrete Fourier Transform and CLEANest algorithm are used to study the temporal variations of the solar radius observed at Rio de Janeiro Observatory from 1998 March 2 to 2009 November 6. The CLEANest spectra show several significant periodicities around 400, 312, 93.5, 86.2, 79.4, 70.9, 53.2, and 26.3 days. Then, combining the data on the daily solar radius measured at Calern Observatory and Rio de Janeiro Observatory and the corresponding daily sunspot areas, we study the short-term periodicity of the solar radius and the role of magnetic field in the variation of the solar radius. The rotation period of the daily solar radius is determined to be statistically significant. Moreover, its temporal evolution is anti-phase with that of sunspot activity, and it is found anti-phase with solar activity. Generally, the stronger solar activity is, the more obvious is the anti-phase relation of radius with solar activity. This indicates that strong magnetic fields have a greater inhibitive effect than weak magnetic fields on the variation of the radius.
Thermoconvective vortices in a cylindrical annulus with varying inner radius
NASA Astrophysics Data System (ADS)
Castaño, D.; Navarro, M. C.; Herrero, H.
2014-12-01
This paper shows the influence of the inner radius on the stability and intensity of vertical vortices, qualitatively similar to dust devils and cyclones, generated in a cylindrical annulus non-homogeneously heated from below. Little relation is found between the intensity of the vortex and the magnitude of the inner radius. Strong stable vortices can be found for both small and large values of the inner radius. The Rankine combined vortex structure, that characterizes the tangential velocity in dust devils, is clearly observed when small values of the inner radius and large values of the ratio between the horizontal and vertical temperature differences are considered. A contraction on the radius of maximum azimuthal velocity is observed when the vortex is intensified by thermal mechanisms. This radius becomes then nearly stationary when frictional force balances the radial inflow generated by the pressure drop in the center, despite the vortex keeps intensifying. These results connect with the behavior of the radius of the maximum tangential wind associated with a hurricane.
Thermoconvective vortices in a cylindrical annulus with varying inner radius.
Castaño, D; Navarro, M C; Herrero, H
2014-12-01
This paper shows the influence of the inner radius on the stability and intensity of vertical vortices, qualitatively similar to dust devils and cyclones, generated in a cylindrical annulus non-homogeneously heated from below. Little relation is found between the intensity of the vortex and the magnitude of the inner radius. Strong stable vortices can be found for both small and large values of the inner radius. The Rankine combined vortex structure, that characterizes the tangential velocity in dust devils, is clearly observed when small values of the inner radius and large values of the ratio between the horizontal and vertical temperature differences are considered. A contraction on the radius of maximum azimuthal velocity is observed when the vortex is intensified by thermal mechanisms. This radius becomes then nearly stationary when frictional force balances the radial inflow generated by the pressure drop in the center, despite the vortex keeps intensifying. These results connect with the behavior of the radius of the maximum tangential wind associated with a hurricane. PMID:25554036
New radius of curvature formulae and their applications
NASA Astrophysics Data System (ADS)
Friedman, M. H.
1981-09-01
The shape of a surface influences reflectance properties; therefore, shape is one factor among many which determines how difficult it is to 'see' a surface-laid mine from the air. The shape of the surface can be locally characterized using the radius of curvature concept. A survey of the literature showed no systematic radius of curvature treatment which displayed explicit formulae for calculating radius of curvature in three dimensions and which emphasized applications of this concept. In this report a new and physically motivated definition for radius of curvature is given and from this definition the well-known and some hitherto unknown radius of curvature formulae are derived. The problem of computing radius of curvature in three dimensions for curves known at discrete points is solved and applied to the problem of measuring one parameter characterizing enemy aircraft maneuverability from radar data. Other applications include: the design of a device for measuring road or railroad track radius of curvature, the design of a device for checking highspeed roads for proper banking and a technique for the use of curvature concepts in the design of a land navigational system.
Exciton-plasmon interactions and energy transfer in nanoparticles
NASA Astrophysics Data System (ADS)
Govorov, Alexander
2009-03-01
Energy transfer between optically-excited nanocrystals coupled by the Coulomb interaction can be very efficient. The interaction of excitons and plasmons in nanocrystals leads to several effects: energy transfer between nanoparticles (NPs), electromagnetic enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, and interference and non-linear phenomena [1-3]. Using kinetic equations for excitons, we model exciton transport in a NW and explain the origin of the blue shift of exciton emission observed in the recent experiments on hybrid NW-NP assemblies [2]. We also model artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. Using superior optical properties of metal and semiconductor NPs, one can strongly enhance the efficiency of light harvesting [3]. An interaction between a discrete state of exciton and a continuum of plasmonic states can give rise to interference effects (Fano-like asymmetric resonances). These interference effects greatly enhance visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticle and molecules. In the nonlinear regime, the Fano effect becomes strongly amplified [4]. In conclusion, our theory explains present experimental results and also provides motivation for future experiments and applications. Potential applications of dynamical exciton-plasmon systems include sensors and light-harvesting. The above theoretical studies were performed in collaboration with several groups [1-4]. [4pt] [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters 6, 984 (2006).[0pt] [2] J. Lee, P. Hernandez, J. Lee, A. Govorov, and N. Kotov, Nature Materials 6, 291 (2007).[0pt] [3] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007); S. Mackowski, S. W"ormke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Br"auchle, Nano Lett. 8, 558 (2008). [0pt] [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R.Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature 451, 311 (2008).
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.
Dynamics of exciton formation and relaxation in photoexcited semiconductors
NASA Astrophysics Data System (ADS)
Janković, Veljko; Vukmirović, Nenad
2015-12-01
We investigate the dynamics of the exciton formation and relaxation on a picosecond time scale following a pulsed photoexcitation of a semiconductor. The study is conducted in the framework of the density matrix theory complemented with the dynamics controlled truncation scheme. We truncate the phonon branch of the resulting hierarchy of equations and propose the form of coupling among single-phonon-assisted and higher-order phonon-assisted density matrices so as to ensure the energy and particle-number conservation in a closed system. Time scales relevant for the exciton formation and relaxation processes are determined from numerical investigations performed on a one-dimensional model for the values of model parameters representative of a typical organic and inorganic semiconductor. The exciton dynamics is examined for different values of central frequency of the exciting field, temperature, and microscopic model parameters, such as the strengths of carrier-carrier and carrier-phonon couplings. We find that for typical organic semiconductor parameters, formation of bound excitons occurs on a several-hundred-femtosecond time scale, while their subsequent relaxation and equilibration take at least several picoseconds. These time scales are consistent with recent experimental studies of the exciton formation and relaxation in conjugated polymer-based materials.
Brightening of excitons in carbon nanotubes on dimensionality modification
NASA Astrophysics Data System (ADS)
Miyauchi, Yuhei; Iwamura, Munechiyo; Mouri, Shinichiro; Kawazoe, Tadashi; Ohtsu, Motoichi; Matsuda, Kazunari
2013-09-01
Despite the attractive one-dimensional characteristics of carbon nanotubes, their typically low luminescence quantum yield, restricted because of their one-dimensional nature, has limited the performance of nanotube-based light-emitting devices. Here, we report the striking brightening of excitons (bound electron-hole pairs) in carbon nanotubes through an artificial modification of their effective dimensionality from one dimension to zero dimensions. Exciton dynamics in carbon nanotubes with luminescent, local zero-dimension-like states generated by oxygen doping were studied as model systems. We found that the luminescence quantum yield of the excitons confined in the zero-dimension-like states can be more than at least one order larger (~18%) than that of the intrinsic one-dimensional excitons (typically ~1%), not only because of the reduced non-radiative decay pathways but also due to an enhanced radiative recombination probability beyond that of intrinsic one-dimensional excitons. Our findings are extendable to the realization of future nanoscale photonic devices including a near-infrared single-photon emitter operable at room temperature.
Multiple Exciton Generation in Semiconductor Nanostructures: DFT-based Computation
NASA Astrophysics Data System (ADS)
Mihaylov, Deyan; Kryjevski, Andrei; Kilin, Dmitri; Kilina, Svetlana; Vogel, Dayton
Multiple exciton generation (MEG) in nm-sized H-passivated Si nanowires (NWs), and quasi 2D nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbation theory calculations based on the DFT simulations. In perturbation theory, we work to the 2nd order in the electron-photon coupling and in the (approximate) RPA-screened Coulomb interaction. We also include the effect of excitons for which we solve Bethe-Salpeter Equation. To describe MEG we calculate exciton-to-biexciton as well as biexciton-to-exciton rates and quantum efficiency (QE). We consider 3D arrays of Si29H36 quantum dots, NWs, and quasi 2D silicon nanofilms, all with both crystalline and amorphous core structures. Efficient MEG with QE of 1.3 up to 1.8 at the photon energy of about 3Egap is predicted in these nanoparticles except for the crystalline NW and film where QE ~=1. MEG in the amorphous nanoparticles is enhanced by the electron localization due to structural disorder. The exciton effects significantly red-shift QE vs. photon energy curves. Nm-sized a-Si NWs and films are predicted to have effective MEG within the solar spectrum range. Also, we find efficient MEG in the chiral single-wall Carbon nanotubes and in a perovskite nanostructure.
How to Draw Energy Level Diagrams in Excitonic Solar Cells.
Zhu, X-Y
2014-07-01
Emerging photovoltaic devices based on molecular and nanomaterials are mostly excitonic in nature. The initial absorption of a photon in these materials creates an exciton that can subsequently dissociate in each material or at their interfaces to give charge carriers. Any attempt at mechanistic understanding of excitonic solar cells must start with drawing energy level diagrams. This seemingly elementary exercise, which is described in textbooks for inorganic solar cells, has turned out to be a difficult subject in the literature. The problem stems from conceptual confusion of single-particle energy with quasi-particle energy and the misleading practice of mixing the two on the same energy level diagram. Here, I discuss how to draw physically accurate energy diagrams in excitonic solar cells using only single-particle energies (ionization potentials and electron affinities) of both ground and optically excited states. I will briefly discuss current understanding on the electronic energy landscape responsible for efficient charge separation in excitonic solar cells. PMID:26279547
Single and multi-exciton dynamics in aqueous protochlorophyllide aggregates.
Sytina, Olga A; van Stokkum, Ivo H M; van Grondelle, Rienk; Groot, Marie Louise
2011-04-28
In plants, the oxidoreductase enzyme POR reduces protochlorophyllide (Pchlide) into chlorophyllide (Chlide), using NADPH as a cofactor. The reduction involves the transfer of two electrons and two protons to the C17═C18 double bond of Pchlide, and the reaction is initiated by the absorption of light by Pchlide itself. In this work we have studied the excited state dynamics of Pchlide dissolved in water, where it forms excitonically coupled aggregates, by ultrafast time-resolved transient absorption and fluorescence experiments performed in the 480-720 nm visible region and in the 1780-1590 cm(-1) mid-IR region. The ground state visible absorption spectrum of aqueous Pchlide red shifts and broadens in comparison to the spectrum of monomeric Pchlide in organic solvents. The population of the one-exciton state occurs at low excitation densities, of <1 photon per aggregate. We characterized the multiexciton manifolds spectra by measuring the absorption difference spectra at increasingly higher photon densities. The multiexciton states are characterized by blue-shifted stimulated emission and red-shifted excited state absorption in comparison to those of the one-exciton manifold. The relaxation dynamics of the multiexciton manifolds into the one-exciton manifold is found to occur in ∼10 ps. This surprisingly slow rate we suggest is due to the intrinsic charge transfer character of the PChlide excited state that leads to solvation, stabilizing the CT state, and subsequent charge recombination, which limits the exciton relaxation. PMID:21171640
New Precision Measurement for Proton Zemach Radius with Laser Spectroscopy
NASA Astrophysics Data System (ADS)
Ma, Y.; Ishida, K.; Iwasaki, M.; Matsuzaki, Y.; Oishi, Y.; Okada, S.; Sato, M.; Midorikawa, K.; Saito, N.; Wada, S.; Aikawa, S.; Kanda, S.; Matsuda, Y.; Tanaka, K.; Takamine, A.
2016-02-01
In this proceeding, a new proposal aiming to improve the precision of the proton Zemach radius will be presented. A circularly polarized laser will be shed on a sample of muonic hydrogen in its ground state. By observing the maximum muon decay asymmetry during scanning laser wave length, the ground-state hyperfine splitting energy can be identified, which is directly related to Zemach radius.citedupays The precision of Zemach radius by this measurement is estimated to be three times better compared to PSI experiment. This result will contribute to the solution of proton size puzzle.
Primary nonunion of the distal radius fractures in healthy children.
Song, Kwang Soon; Lee, Si Wook; Bae, Ki Cheor; Yeon, Chang Jin; Naik, Premal
2016-03-01
There are no published case series of nonunion of distal radius fractures in healthy children because of the rarity of its occurrence. We searched for all reported cases of this condition in Pubmed, Google scholar, and SCOPUS. We found three series, which included one previously reported by our group. The aim of the present study was to define the predisposing factors leading to nonunion after treatment of distal radius fractures in healthy children. We also aimed to emphasize that nonunion should be included in the list of complications of distal radius fractures in children and be mentioned in the textbook of pediatric trauma. PMID:26583931
Homogeneous linewidth broadening and exciton dephasing mechanism in MoT e2
NASA Astrophysics Data System (ADS)
Koirala, Sandhaya; Mouri, Shinichiro; Miyauchi, Yuhei; Matsuda, Kazunari
2016-02-01
Spectroscopic studies of mechanically exfoliated monolayer MoT e2 were performed over a wide temperature range from 4.2 to 300 K. At a low temperature, the photoluminescence spectra for monolayer MoT e2 showed two sharp peaks for excitons and charged excitons (trions). The homogeneous linewidth of the exciton peak broadened linearly as the temperature increased. This linear linewidth broadening was caused by acoustic-phonon scattering of the exciton, i.e., shortening of exciton dephasing. The broadening factor due to exciton-acoustic-phonon interactions was found to be 0.11 meV K-1 . This small value for the exciton-phonon coupling coefficient and the lack of a Stokes shift suggest that exciton-phonon interactions in monolayer MoT e2 are in the weak coupling regime.
Body segment mass, radius and radius of gyration proportions of children.
Jensen, R K
1986-01-01
The segment inertial parameters of children are fundamental to the analysis and simulation of their movements. Generally it has been recognized that adult parameters cannot be extrapolated and most of the anthropometric data on children are of little or no use for determining inertias. Consequently, there have been few studies of children's kinetics. In response to this problem a longitudinal investigation, the Laurentian Study of Biomechanical Development, was launched and in this paper the effects of growth on selected segmental size and inertial parameters are reported for boys between the ages of 4 and 15 yr. The twelve subjects, representing heterogeneous body types were followed over 3 yr for a total of 36 observations. Elliptical zones 2 cm wide were used to model the body and segment inertias calculated using segment densities from the literature. These inertias were the mass, moment of inertia and mass centroid location for a fourteen segment planar representation of the body. The general accuracy mean error based on body mass was 0.203% which is consistent with reports from similar studies and techniques. Plots of segment mass proportions with respect to age showed a decrease in the head proportion balanced by increases in the thigh, shank, foot and upper arm proportions in particular. The trends for each segment were consistent with the trends for linear measures reported in the anthropometry literature. Radius proportions to the mass centroid and radius of gyration proportions were also plotted and showed smaller but consistent changes with respect to age. Linear regressions were then fitted to the distributions and standard errors calculated. The magnitude and slope of the regressions were for the most part consistent with a reported cross-sectional study of Japanese children. Where data were available, predicted parameters were compared with the reported parameters for a 12 yr old analyzed using a different mathematical model. Comparisons were also made between the predicted parameters at 15 yr and the reported parameters for healthy young adults who had been scanned using a gamma-radiation technique. For most parameters there was either good agreement or differences could be explained logically. The traditionally used parameters from older cadavers were quite inconsistent with the above. The variances of the 36 observations about the regression lines as indicated by the standard errors were small. As an illustration of the effect of these variances, the trunk parameters for a 10 yr old performing a standing jump for distance were decreased by 1 S.E. and this matched by increases for the thigh, shank and head.(ABSTRACT TRUNCATED AT 400 WORDS) PMID:3733761
Novel Quantum Condensates in Excitonic Matter
Littlewood, P. B.; Keeling, J. M. J.; Simons, B. D.; Eastham, P. R.; Marchetti, F. M.; Szymanska, M. H.
2009-08-20
These lectures interleave discussion of a novel physical problem of a new kind of condensate with teaching of the fundamental theoretical tools of quantum condensed matter field theory. Polaritons and excitons are light mass composite bosons that can be made inside solids in a number of different ways. As bosonic particles, they are liable to make a phase coherent ground state - generically called a Bose-Einstein condensate (BEC) - and these lectures present some models to describe that problem, as well as general approaches to the theory. The focus is very much to explain how mean-field-like approximations that are often presented heuristically can be derived in a systematic fashion by path integral methods. Going beyond the mean field theory then produces a systematic approach to calculation of the excitation energies, and the derivation of effective low energy theories that can be generalised to more complex dynamical and spatial situations than is practicable for the full theory, as well as to study statistical properties beyond the semi-classical regime. in particular, for the polariton problem, it allows one to connect the regimes of equilibrium BEC and non-equilibrium laser. The lectures are self-sufficient, but not highly detailed. The methodological aspects are covered in standard quantum field theory texts and the presentation here is deliberately cursory: the approach will be closest to the book of Altland and Simons. Since these lectures concern a particular type of condensate, reference should also be made to texts on BEC, for example by Pitaevskii and Stringari. A recent theoretically focussed review of polariton systems covers many of the technical issues associated with the polariton problem in greater depth and provides many further references.
Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon.
Witzel, Wayne M; Shabaev, Andrew; Hellberg, C Stephen; Jacobs, Verne L; Efros, Alexander L
2010-09-24
We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time. PMID:21230809
Quantum Simulation of Multiple-Exciton Generation in a Nanocrystal by a Single Photon
Witzel, Wayne M.; Shabaev, Andrew; Hellberg, C. Stephen; Jacobs, Verne L.; Efros, Alexander L.
2010-09-22
We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time.
Long-range exciton dissociation in organic solar cells.
Caruso, Domenico; Troisi, Alessandro
2012-08-21
It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range exciton dissociation is likely (using a kinetic model and a microscopic model for the calculation of the long-range electron transfer rate). Then, defining a rather general model Hamiltonian for the donor material, we show that the phenomenon is extremely common in the majority of polymer:fullerene bulk heterojunction solar cells. PMID:22869702
Excitonic condensation of massless fermions in graphene bilayers
Zhang, C.-H.; Joglekar, Yogesh N.
2008-06-15
Graphene, a single sheet of graphite with honeycomb lattice structure, has massless carriers with tunable density and polarity. We investigate the ground-state phase diagram of two graphene sheets (embedded in a dielectric), separated by distance d where the top layer has electrons and the bottom layer has holes, using mean-field theory. We find that a uniform excitonic condensate occurs over a large range of carrier densities and is weakly dependent on the relative orientation of the two sheets. We obtain the excitonic gap, quasiparticle energy, and the density of states. We show that both the condensate phase stiffness and the mass of the excitons with massless particles as constituents, vary as the square root of the carrier density, and predict that the condensate will not undergo Wigner crystallization.
Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics
NASA Astrophysics Data System (ADS)
Scully, Shawn Ryan
Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures. In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material. After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures. After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of the primary losses that puts stringent requirements on the charge carrier mobilities in these cells is the recombination losses due to space charge build up at the heterojunction. Because electrons are confined to the acceptor and holes to the donor, net charge density always exists even when mobilities are matched, in contrast to bulk heterojunctions wherein matched mobilities lead to zero net charge. This net charge creates an electric field which opposes the built-in field and limits the current that can be carried away from this heterojunction. Using simulations we show that for relevant current densities charge carrier mobilities must be higher than 10-4 cm2/V.s to avoid significant losses due to space charge formation. In the last part of this work, we will focus on the second class of architectures in which exciton harvesting is efficient. We will present a systematic analysis of one of the leading polymer:fullerene bulk heterojunction cells to show that losses in this architecture are due to charge recombination. Using optical measurements and simulations, exciton harvesting measurements, and device characteristics we will show that the dominant loss is likely due to field-dependent geminate recombination of the electron and hole pair created immediately following exciton dissociation. No losses in this system are seen due to bimolecular recombination or space charge which provides information on charge-carrier mobility targets necessary for the future design of high efficiency organic photovoltaics.
Excitonic positronium emission from n-Si(111)
NASA Astrophysics Data System (ADS)
Cassidy, D. B.; Hisakado, T. H.; Tom, H. W. K.; Mills, A. P., Jr.
2012-10-01
Recently it was found that energetic positronium (Ps) emission from both n- and p-type Si(100) occurs primarily via an exciton-like surface state PsX [D. B. Cassidy, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr., Phys. Rev. B1098-012110.1103/PhysRevB.84.195312 84, 195312 (2011)]. The underlying Ps production mechanism was deduced from the observed Ps emission yield and kinetic energy, using laser and thermally generated electrons to populate the PsX state, but not all of the observed effects have been explained. In this brief report we show that excitonic Ps emission also occurs from n-Si(111), but with characteristics that more closely resemble p-Si(100) than n-Si(100). Considering both (100) and (111) crystal orientations may help in the formulation of a more complete theoretical description of the excitonic Ps production mechanism.
Microscopic theory of singlet exciton fission. I. General formulation.
Berkelbach, Timothy C; Hybertsen, Mark S; Reichman, David R
2013-03-21
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach. PMID:23534622
Impurity effects on polaron-exciton formation in conjugated polymers
Ribeiro, Luiz Antonio Ferreira da Cunha, Wiliam; Neto de Oliveira, Pedro Henrique; Gargano, Ricardo; Magela e Silva, Geraldo
2013-11-07
Combining the one-dimensional tight-binding Su-Schrieffer-Heeger model and the extended Hubbard model, the collision of two oppositely charged polarons is investigated under the influence of impurity effects using a non-adiabatic evolution method. Results show that electron-electron interactions have direct influence on the charge distribution coupled to the polaron-exciton lattice defect. Additionally, the presence of an impurity in the collisional process reduces the critical electric field for the polaron-exciton formation. In the small electric field regime, the impurity effects open three channels and are of fundamental importance to favor the polaron-exciton creation. The results indicate that the scattering between polarons in the presence of impurities can throw a new light on the description of electroluminescence in conjugated polymer systems.
Surface plasmon mediated energy transfer of electrically-pumped excitons.
An, Kwang Hyup; Shtein, Max; Pipe, Kevin P
2010-03-01
We report strong surface plasmon polariton mediated transfer of energy between molecular excitons across the metallic cathode of an electrically-pumped organic heterostructure. The donor molecular excitons at the organic heterojunction resonantly excite surface plasmon modes on both sides of the optically thick metal electrode, which evanescently couple to dye molecules near the electrode's exterior surface. Dye fluorescence in the capping layer on the exterior of the device shows a 6.5-fold increase in intensity due to this effect, far exceeding any enhancement attributable to Purcell or optical microcavity effects. Demonstration of this energy transfer mechanism for electrically-pumped excitons suggests new sensing and imaging applications with high signal to noise ratio and new routes for performance improvement in energy harvesting devices, plasmonic devices, and organic LEDs (including white light emission). PMID:20389418
Microscopic theory of singlet exciton fission. I. General formulation
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.; Hybertsen, Mark S.; Reichman, David R.
2013-03-01
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach.
Optical control of charged exciton states in tungsten disulfide
Currie, M.; Hanbicki, A. T.; Jonker, B. T.; Kioseoglou, G.
2015-05-18
A method is presented for optically preparing WS{sub 2} monolayers to luminescence from only the charged exciton (trion) state–completely suppressing the neutral exciton. When isolating the trion state, we observed changes in the Raman A{sub 1g} intensity and an enhanced feature on the low energy side of the E{sup 1}{sub 2g} peak. Photoluminescence and optical reflectivity measurements confirm the existence of the prepared trion state. This technique also prepares intermediate regimes with controlled luminescence amplitudes of the neutral and charged exciton. This effect is reversible by exposing the sample to air, indicating the change is mitigated by surface interactions with the ambient environment. This method provides a tool to modify optical emission energy and to isolate physical processes in this and other two-dimensional materials.
Binding energies of excitons in ionic quantum well structures
NASA Astrophysics Data System (ADS)
Antonelli, A.; Cen, J.; Bajaj, K. K.
1996-01-01
We have calculated the binding energies of excitons in quantum well structures based on ionic semiconductors by including the electron - hole interactions with the longitudinal optical phonon field. We have taken into account these interactions by using different effective interaction potentials between the electron and the hole as derived by Haken, by Aldrich and Bajaj, and by Pollman and Büttner. We have calculated the binding energies of excitons in several ionic quantum well structures as functions of well width using these effective potentials by following a variational approach. We find that the values of the exciton binding energies calculated using these potentials are always larger than those obtained using a Coulomb potential screened by a static dielectric constant. We compare our results with those of some recent calculations.
Quantum confinement-induced tunable exciton states in graphene oxide
Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin
2013-01-01
Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology. PMID:23872608
Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots.
Ardelt, P-L; Gawarecki, K; Müller, K; Waeber, A M; Bechtold, A; Oberhofer, K; Daniels, J M; Klotz, F; Bichler, M; Kuhn, T; Krenner, H J; Machnikowski, P; Finley, J J
2016-02-19
We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k·p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule. PMID:26943557
Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots
NASA Astrophysics Data System (ADS)
Ardelt, P.-L.; Gawarecki, K.; Müller, K.; Waeber, A. M.; Bechtold, A.; Oberhofer, K.; Daniels, J. M.; Klotz, F.; Bichler, M.; Kuhn, T.; Krenner, H. J.; Machnikowski, P.; Finley, J. J.
2016-02-01
We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k .p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule.
Charged two-exciton emission from a single semiconductor nanocrystal
Hu, Fengrui; Zhang, Qiang; Zhang, Chunfeng; Wang, Xiaoyong; Xiao, Min
2015-03-30
Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the “on” and “off” levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate “grey” level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.
Surface plasmon mediated energy transfer of electrically-pumped excitons
An, Kwang Hyup; Shtein, Max; Pipe, Kevin P.
2010-02-16
We report strong surface plasmon polariton mediated transfer of energy between molecular excitons across the metallic cathode of an electrically-pumped organic heterostructure. The donor molecular excitons at the organic heterojunction resonantly excite surface plasmon modes on both sides of the optically thick metal electrode, which evanescently couple to dye molecules near the electrode’s exterior surface. Dye fluorescence in the capping layer on the exterior of the device shows a 6.5-fold increase in intensity due to this effect, far exceeding any enhancement attributable to Purcell or optical microcavity effects. Demonstration of this energy transfer mechanism for electrically-pumped excitons suggests new sensing and imaging applications with high signal to noise ratio and new routes for performance improvement in energy harvesting devices, plasmonic devices, and organic LEDs (including white light emission).
Effects of fermion exchange on the polarization of exciton condensates.
Combescot, Monique; Combescot, Roland; Alloing, Mathieu; Dubin, Franois
2015-03-01
Exchange interaction is responsible for the stability of elementary boson condensates with respect to momentum fragmentation. This remains true for composite bosons when single fermion exchanges are included but spin degrees of freedom are ignored. Here, we show that their inclusion can produce a spin fragmentation of the dark exciton condensate, i.e., an unpolarized condensate with an equal amount of spin (+2) and (-2) excitons not coupled to light. The composite boson many-body formalism allows us to predict that, for spatially indirect excitons, the condensate polarization switches from unpolarized to fully polarized when the distance between the layers confining electrons and holes increases. Importantly, the threshold distance for this switch lies in a regime fully accessible to experiments. PMID:25793784
Exciton absorption of entangled photons in semiconductor quantum wells
NASA Astrophysics Data System (ADS)
Rodriguez, Ferney; Guzman, David; Salazar, Luis; Quiroga, Luis; Condensed Matter Physics Group Team
2013-03-01
The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers. Research funds from Facultad de Ciencias, Universidad de los Andes
Vacuum Field Ellipticity Dependence on Radius in Quadrupolar Mirror Machines
NASA Astrophysics Data System (ADS)
Hagnestål, A.; Ågren, O.
2012-10-01
The vacuum field flux tube ellipticity dependence on radius for quadrupolar mirror machines has been investigated. A third order expression in the paraxial approximation has been derived for the vacuum field ellipticity. The dependence of ellipticity on midplane radius has been examined in the SFLM Hybrid and the outermost plasma flux tube is 3.5 cm wider than predicted by the first order paraxial approximation, which is within boundaries set by the first wall. The third order approximation has a high accuracy for the ellipticity for long-thin mirrors such as the SFLM Hybrid, and even the first order approximation that is independent of radius is sufficient in many applications. The ellipticity dependence on midplane radius for mirrors with more strongly localized quadrupolar fields than the SFLM Hybrid is also shown to be minor.
Experiments towards resolving the proton charge radius puzzle
NASA Astrophysics Data System (ADS)
Antognini, A.; Schuhmann, K.; Amaro, F. D.; Amaro, P.; Abdou-Ahmed, M.; Biraben, F.; Chen, T.-L.; Covita, D. S.; Dax, A. J.; Diepold, M.; Fernandes, L. M. P.; Franke, B.; Galtier, S.; Gouvea, A. L.; Götzfried, J.; Graf, T.; Hänsch, T. W.; Hildebrandt, M.; Indelicato, P.; Julien, L.; Kirch, K.; Knecht, A.; Kottmann, F.; Krauth, J. J.; Liu, Y.-W.; Machado, J.; Monteiro, C. M. B.; Mulhauser, F.; Nez, F.; Santos, J. P.; dos Santos, J. M. F.; Szabo, C. I.; Taqqu, D.; Veloso, J. F. C. A.; Voss, A.; Weichelt, B.; Pohl, R.
2016-03-01
We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy.
Radius of Curvature of Off-Axis Paraboloids
NASA Technical Reports Server (NTRS)
Robinson, Brian; Reardon, Patrick; Hadaway, James; Geary, Joseph; Russell, Kevin (Technical Monitor)
2002-01-01
We present several methods for measuring the vertex radius of curvature of off-axis paraboloidal mirrors. One is based on least-squares fitting of interferometer output, one on comparison of sagittal and tangential radii of curvature, and another on measurement of displacement of the nulled test article from the ideal reference wave. Each method defines radius of curvature differently and, as a consequence, produces its own sort of errors.
How Do f-Mode Frequencies Change with Solar Radius?
NASA Astrophysics Data System (ADS)
Chatterjee, Piyali; Antia, H. M.
2008-12-01
We test the relation between relative f-mode frequency variation (δ ν/ν) and Lagrangian perturbation in the solar radius (δ r/r) obtained by Dziembowski and Goode using several pairs of solar models and show that it does not hold true for any of the model pairs we have used. We attempt to derive a better approximation for the kernel linking the relative frequency changes and the solar radius variation in the subsurface layers.
Palmar cortical angle of the distal radius: a radiological study.
Bassi, R S; Krishnan, K M; Dhillon, S S; Deshmukh, S C
2003-04-01
Fifty lateral radiographs of normal wrists were viewed to determine the palmar cortical angle of the distal radius. The palmar cortical angle is different to its previously described palmar tilt or angulation. The mean value for the palmar cortical angle was 37 degrees (range, 26-50 degrees). This may be clinically important in the design of palmar plates for the distal radius. PMID:12631490
NASA Astrophysics Data System (ADS)
Bamba, Motoaki; Wakaiki, Shuji; Ichida, Hideki; Mizoguchi, Kohji; Kim, DaeGwi; Nakayama, Masaaki; Kanematsu, Yasuo
2015-06-01
When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process because there is no exciton level at that energy. However, according to the recent time-resolved measurements of P emission originating from inelastic exciton-exciton scattering, it looks rather natural to consider that the exciton-polariton is created in a finite time scale which is restricted by a coherence volume of the exciton after the scattering. In this interpretation, the exciton remains in this time scale far below its resonance energy as a transient state in a series of processes. We propose an expression of the P-emission lifetime depending on the coherence volume of the scattered excitons through the conversion process from them to the polaritons. The coherence volume of the scattered excitons appears in the calculation of the inelastic scattering process on the assumption of a finite coherence volume of the bottleneck excitons. Time-resolved optical-gain measurements could be a way for investigating the validity of our interpretation.
Excitonic splittings in jet-cooled molecular dimers.
Ottiger, Philipp; Leutwyler, Samuel
2011-01-01
In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced. PMID:21678767
Coulomb screening and exciton binding energies in conjugated polymers
NASA Astrophysics Data System (ADS)
Moore, Eric; Gherman, Benjamin; Yaron, David
1997-03-01
Hartree-Fock solutions of the Pariser-Parr-Pople and MNDO Hamiltonians are shown to give reasonable predictions for the ionization potentials and electron affinities of gas-phase polyenes. However, the energy predicted for formation of a free electron-hole pair on an isolated chain of polyacetylene is much larger than that seen in the solid state. The prediction is 6.2 eV if soliton formation is ignored and about 4.7 eV if soliton formation is included. The effects of interchain interactions on the exciton binding energy are then explored using a model system consisting of one solute and one solvent polyene, that are coplanar and separated by 4 Å. The lowering of the exciton binding energy is calculated by comparing the solvation energy of the exciton state to that of a single hole (a cationic solute polyene) and a single electron (an anionic solute polyene). It is argued that when the relative timescales of charge fluctuations on the solute and solvent chains are taken into account, it is difficult to rationalize the electron-electron screening implicit in the parametrization of a single-chain Hamiltonian to solid-state data. Instead, an electron-hole screening model is developed that includes the time scales of both the electron-hole motion and the solvent polarization. The predicted solvation energies, which are saturated with respect to solute and solvent chain length, are 0.07 eV for the exciton and 0.50 eV for a well separated electron-hole pair. Given this large, 0.43 eV reduction in the exciton binding energy due to interaction with a single chain, it seems likely that interchain interactions play a central role in establishing the solid-state exciton binding energy.
Isolated Diaphyseal Fractures of the Radius in Skeletally Immature Patients
Guitton, Thierry G.; Van Dijk, Niek C.; Raaymakers, Ernst L.
2009-01-01
Diaphyseal radius fractures without associated ulna fracture or radioulnar dislocation (isolated fracture of the radius) are recognized in adults but are rarely described in skeletally immature patients. A search of our database (1974–2002) identified 17 pediatric patients that had an isolated fracture of the radius. Among the 13 patients with at least 1 year follow-up, ten were treated with manipulative reduction and immobilization in an above elbow cast and three had initial operative treatment with plate and screw fixation. These 13 patients were evaluated for an average of 18 months (range, 12 to 45 months) after injury using the system of Price and colleagues. The incidence of isolated diaphyseal radius fractures in skeletally immature patients was 0.56 per year in our database and represented 27% of the 63 patients with a diaphyseal forearm fracture. All 13 patients, with at least 1 year follow-up, regained full elbow flexion and extension and full forearm rotation. According to the classification system of Price, all 13 patients (100%) had an excellent result. As in adults, isolated radius fractures seem to occur in children more frequently than previously appreciated. Treatment of isolated radius fractures in skeletally immature patients has a low complication rate, and excellent functional outcomes are the rule. PMID:19859772
Exciton localization-delocalization transition in an extended dendrimer
NASA Astrophysics Data System (ADS)
Pouthier, Vincent
2013-12-01
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number Gc ? 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.
Resolving multi-exciton generation by attosecond spectroscopy.
Neukirch, A J; Neumark, D M; Kling, M F; Prezhdo, O V
2014-10-20
We propose an experimentally viable attosecond transient absorption spectroscopy scheme to resolve controversies regarding multiexciton (ME) generation in nanoscale systems. Absence of oscillations indicates that light excites single excitons, and MEs are created by incoherent impact ionization. An oscillation indicates the coherent mechanism, involving excitation of superpositions of single and MEs. The oscillation decay, ranging from 5 fs at ambient temperature to 20 fs at 100 K, gives the elastic exciton-phonon scattering time. The signal is best observed with multiple-cycle pump pulses. PMID:25401661
Crossed excitons in a semiconductor nanostructure of mixed dimensionality
Owschimikow, Nina Kolarczik, Mirco; Kaptan, Yücel I.; Grosse, Nicolai B.; Woggon, Ulrike
2014-09-08
Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of “crossed” excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.
Enhanced energy transport in genetically engineered excitonic networks
NASA Astrophysics Data System (ADS)
Park, Heechul; Heldman, Nimrod; Rebentrost, Patrick; Abbondanza, Luigi; Iagatti, Alessandro; Alessi, Andrea; Patrizi, Barbara; Salvalaggio, Mario; Bussotti, Laura; Mohseni, Masoud; Caruso, Filippo; Johnsen, Hannah C.; Fusco, Roberto; Foggi, Paolo; Scudo, Petra F.; Lloyd, Seth; Belcher, Angela M.
2016-02-01
One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.
Exciton localization-delocalization transition in an extended dendrimer
Pouthier, Vincent
2013-12-21
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G{sub c} ≈ 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.
Frequency combs with weakly lasing exciton-polariton condensates.
Rayanov, K; Altshuler, B L; Rubo, Y G; Flach, S
2015-05-15
We predict the spontaneous modulated emission from a pair of exciton-polariton condensates due to coherent (Josephson) and dissipative coupling. We show that strong polariton-polariton interaction generates complex dynamics in the weak-lasing domain way beyond Hopf bifurcations. As a result, the exciton-polariton condensates exhibit self-induced oscillations and emit an equidistant frequency comb light spectrum. A plethora of possible emission spectra with asymmetric peak distributions appears due to spontaneously broken time-reversal symmetry. The lasing dynamics is affected by the shot noise arising from the influx of polaritons. That results in a complex inhomogeneous line broadening. PMID:26024173
Plasmon and exciton superconductivity mechanisms in layered structures
NASA Technical Reports Server (NTRS)
Gabovich, A. M.; Pashitskiy, E. A.; Uvarova, S. K.
1977-01-01
Plasmon and exciton superconductivity mechanisms are discussed. Superconductivity in a three layer metal semiconductor metal and insulator semimetal insulator sandwich structure was described in terms of the temperature dependent Green function of the longitudinal (Coulomb) field. The dependences of the superconducting transition temperature on structure parameters were obtained. In a semiconducting film, as a result of interactions of degenerate free carriers with excitons, superconductivity exists only in a certain range of parameter values, and the corresponding critical temperature is much lower than in the plasmon mechanism of superconductivity.
Coherent exciton delocalization in strongly coupled quantum dot arrays.
Crisp, Ryan W; Schrauben, Joel N; Beard, Matthew C; Luther, Joseph M; Johnson, Justin C
2013-10-01
Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical transitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron-hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter. PMID:24041088
Enhanced energy transport in genetically engineered excitonic networks.
Park, Heechul; Heldman, Nimrod; Rebentrost, Patrick; Abbondanza, Luigi; Iagatti, Alessandro; Alessi, Andrea; Patrizi, Barbara; Salvalaggio, Mario; Bussotti, Laura; Mohseni, Masoud; Caruso, Filippo; Johnsen, Hannah C; Fusco, Roberto; Foggi, Paolo; Scudo, Petra F; Lloyd, Seth; Belcher, Angela M
2016-02-01
One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime. PMID:26461447
Realization of an all optical exciton-polariton router
NASA Astrophysics Data System (ADS)
Marsault, Félix; Nguyen, Hai Son; Tanese, Dimitrii; Lemaître, Aristide; Galopin, Elisabeth; Sagnes, Isabelle; Amo, Alberto; Bloch, Jacqueline
2015-11-01
We report on the experimental realization of an all optical router for exciton-polaritons. This device is based on the design proposed by Flayac and Savenko [Appl. Phys. Lett. 103, 201105 (2013)], in which a zero-dimensional island is connected through tunnel barriers to two periodically modulated wires of different periods. Selective transmission of polaritons injected in the island, into either of the two wires, is achieved by tuning the energy of the island state across the band structure of the modulated wires. We demonstrate routing of ps polariton pulses using an optical control beam which controls the energy of the island quantum states, thanks to polariton-exciton interactions.
Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.
Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H
2013-06-18
Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit. PMID:23656886
NASA Astrophysics Data System (ADS)
Chen, R.; Bajaj, K. K.
1997-02-01
We have derived an expression for the effective interaction potential between the constituents of an exciton, namely an electron and a hole in an ionic quantum well structure by taking into account the electron-hole interactions with the confined optical, interface and half-space longitudinal optical phonon modes using a variational method. We follow an approach similar to that used by Haken to derive an effective potential between an electron and a hole in bulk ionic crystals. Using our interaction potential we have calculated the binding energies of excitons in several ionic quantum well structures. We find that the values of the exciton binding energies calculated using this effective interaction potential are always larger than those obtained using a Coulomb potential screened by the static dielectric constant. We compare our results with those of some recent calculations.
Truncation Errors and Proton Radius Extractions from Form Factor Data
NASA Astrophysics Data System (ADS)
Mesick, Katherine; MUSE Collaboration
2014-09-01
The MUon Scattering Experiment (MUSE) Collaboration plans to extract the proton charge radius from new O (0 . 1 %) measurements of the μ+/- p and e+/- p electromagnetic form factors in the region of Q2 = 0 . 002 - 0 . 07 GeV2. The aim is to determine if there is a fundamental difference between e and μ, adding new insight into the proton radius puzzle - the ~7 σ discrepancy between the charge radius as determined from muonic hydrogen and that from atomic hydrogen spectroscopy and ep elastic scattering form factor data. One important consideration in extracting the radius from form factor data is errors in the fitting procedure, one of which is a so-called truncation offset, which results from the truncation of a series expansion to fit a finite range of Q2 data. The truncation offset resulting from a Taylor series expansion and from an inverse polynomial series will be compared and discussed. An example of how the MUSE experiment plans to extract the proton radius and the anticipated significance will also be presented. The MUon Scattering Experiment (MUSE) Collaboration plans to extract the proton charge radius from new O (0 . 1 %) measurements of the μ+/- p and e+/- p electromagnetic form factors in the region of Q2 = 0 . 002 - 0 . 07 GeV2. The aim is to determine if there is a fundamental difference between e and μ, adding new insight into the proton radius puzzle - the ~7 σ discrepancy between the charge radius as determined from muonic hydrogen and that from atomic hydrogen spectroscopy and ep elastic scattering form factor data. One important consideration in extracting the radius from form factor data is errors in the fitting procedure, one of which is a so-called truncation offset, which results from the truncation of a series expansion to fit a finite range of Q2 data. The truncation offset resulting from a Taylor series expansion and from an inverse polynomial series will be compared and discussed. An example of how the MUSE experiment plans to extract the proton radius and the anticipated significance will also be presented. This work was supported in part by the US National Science Foundation Grant PHY 1306126.
Nogues, Gilles Den Hertog, Martien; Auzelle, Thomas; Gayral, Bruno; Daudin, Bruno
2014-03-10
We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42 eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location.
All-optical depletion of dark excitons from a semiconductor quantum dot
Schmidgall, E. R.; Schwartz, I.; Cogan, D.; Gershoni, D.; Gantz, L.; Heindel, T.; Reitzenstein, S.
2015-05-11
Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand sources of single photons. However, the generation of long-lived dark excitons imposes significant limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark exciton population and converts it to a bright exciton population, using intermediate excited biexciton states. We show experimentally that our method considerably reduces the dark exciton population while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of quantum dot depletion.
Nanosecond exciton recombination dynamics in colloidal CdSe quantum dots under ambient conditions
NASA Astrophysics Data System (ADS)
Javier, Artjay; Magana, Donny; Jennings, Travis; Strouse, Geoffrey F.
2003-08-01
In solution, CdSe quantum dot exciton recombination is composed of an intrinsic band edge exciton decay and a photoinduced charged exciton, which produces the observation of biexponential decay dynamics. The nearly identical radiative lifetimes of both intrinsic and extrinsic decays scale with the cube of the size, and the nonradiative rate for the intrinsic decay follows the energy gap law, while the charged exciton exhibits strong lattice perturbations arising from Frölich coupling to optical phonons. The charged exciton, which arises from recombination in a particle that has a photo-induced trapped carrier has a temporally resolved Stark shift of ˜20 meV.
Yamada, Yasuhiro; Yamaji, Youhei; Imada, Masatoshi
2015-11-01
Energy dissipation and decoherence are at first glance harmful to acquiring the long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental, as suggested in photosynthesis. By simulating the quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of a carbon nanotube, the exciton lifetime increases by nearly 2 orders of magnitude when the crossover triggers a stable high population in the dark excitons. PMID:26588415
Plasmon-exciton coupling in neighboring metal nanoparticles and a semiconductor quantum well: Theory
NASA Astrophysics Data System (ADS)
Kosobukin, V. A.
2016-02-01
Spectroscopic effects of plasmon-exciton coupling are studied theoretically for metal nanoparticles and a semiconductor quantum-well located nearby. Resonant polarizability is self-consistently calculated for dipole plasmons supported by nanoparticles and influenced by image dipoles due to exciton polarization of the quantum well. Spectra of elastic scattering and reflection of light are analyzed in conformity with high-quality plasmons of silver nanoparticles and quasi-two-dimensional excitons of AlGaAs/GaAs quantum well. Essential two-peak resonant structure associated with plasmon-exciton Rabi splitting is found. The related spectral dip is interpreted in terms of excitonic response enhancement by plasmons of nanoparticles.
NASA Astrophysics Data System (ADS)
Yamada, Yasuhiro; Yamaji, Youhei; Imada, Masatoshi
2015-11-01
Energy dissipation and decoherence are at first glance harmful to acquiring the long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental, as suggested in photosynthesis. By simulating the quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of a carbon nanotube, the exciton lifetime increases by nearly 2 orders of magnitude when the crossover triggers a stable high population in the dark excitons.
NASA Astrophysics Data System (ADS)
Kramar, V. M.; Pugantseva, O. V.; Derevyanchuk, A. V.
2014-08-01
Theoretical investigation of the spatial confinement, self-polarization and exciton-phonon interaction influence on the exciton state in plane double nanoheterostructure (nanofilm)-lead iodide in polymeric matrix is performed within the effective mass approximation for the electron and dielectric continuum for the phonons in the framework of infinitely deep single quantum well. It is shown that spatial confinement is the dominating feature determining the energy of the bottom of exciton ground band and its binding energy. The relationship of two others depends on nanofilm thickness: in ultrathin films the influence of self-polarization effect is essentially bigger than the role of exciton-phonon interaction.
Magnetic control of Coulomb scattering and terahertz transitions among excitons
NASA Astrophysics Data System (ADS)
Bhattacharyya, J.; Zybell, S.; Eer, F.; Helm, M.; Schneider, H.; Schneebeli, L.; Bttge, C. N.; Breddermann, B.; Kira, M.; Koch, S. W.; Andrews, A. M.; Strasser, G.
2014-03-01
Time-resolved terahertz quenching studies of the magnetoexcitonic photoluminescence from GaAs/AlGaAs quantum wells are performed. A microscopic theory is developed to analyze the experiments. Detailed experiment-theory comparisons reveal a remarkable magnetic-field controllability of the Coulomb and terahertz interactions in the excitonic system.
Strong Quantum Coherence between Fermi Liquid Mahan Excitons.
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. PMID:27127985
Excitons in the LH3 complexes from purple bacteria.
Chmeliov, Jevgenij; Songaila, Egidijus; Rancova, Olga; Gall, Andrew; Robert, Bruno; Abramavicius, Darius; Valkunas, Leonas
2013-09-26
The noncovalently bound and structurally identical bacteriochlorophyll a chromophores in the peripheral light-harvesting complexes LH2 (B800-850) and LH3 (B800-820) from photosynthetic purple bacteria ensure the variability of the exciton spectra in the near-infrared (820-850 nm) wavelength region. As a result, the spectroscopic properties of the antenna complexes, such as positions of the maxima in the exciton absorption spectra, give rise to very efficient excitation transfer toward the reaction center. In this work, we investigated the possible molecular origin of the excitonically coupled B820 bacteriochlorophylls in LH3 using femtosecond transient absorption spectroscopy, deconvolution of steady-state absorption spectra, and modeling of the electrostatic intermolecular interactions using a charge density coupling approach. Compared to LH2, the upper excitonic level is red-shifted from 755 to 790 nm and is associated with an approximate 2-fold decrease of B820 intrapigment coupling. The absorption properties of LH3 cannot be reproduced by only changing the B850 site energy but also require a different scaling factor to be used to calculate interpigment couplings and a change of histidine protonation state. Several protonation patterns for distinct amino acid groups are presented, giving values of 162-173 cm(-1) at 100 K for the intradimer resonance interaction in the B820 ring. PMID:23570515
Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers
NASA Astrophysics Data System (ADS)
Daniel, Clément; Herz, Laura M.; Silva, Carlos; Hoeben, Freek J.; Jonkheijm, Pascal; Schenning, Albertus P.; Meijer, E. W.
2003-12-01
We present femtosecond transient absorption measurements on π-conjugated supramolecular assemblies in a high-pump-fluence regime. Oligo(p-phenylenevinylene) monofunctionalized with ureido-s-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75 °C at a concentration of 4×10-4 M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of 11Bu-exciton spectral signatures and by the sublinear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors.
Exploration of exciton delocalization in organic crystalline thin films
NASA Astrophysics Data System (ADS)
Hua, Kim; Manning, Lane; Rawat, Naveen; Ainsworth, Victoria; Furis, Madalina
The electronic properties of organic semiconductors play a crucial role in designing new materials for specific applications. Our group recently found evidence for a rotation of molecular planes in phthalocyanines that is responsible for the disappearance of a delocalized exciton in these systems for T >150K.................()().......1 In this study, we attempt to tune the exciton delocalization of small organic molecules using strain effects and alloying different molecules in the same family. The exciton behavior is monitored using time- and polarization resolved photolumniscence (PL) spectroscopy as a function of temperature. Specifically, organic crystalline thin films of octabutoxy phthalocyanine (H2OBPc), octyloxy phthalocyanines and H-bonded semiconductors such as the quinacridone and indigo derivatives are deposited on flexible substrates (i.e. Kapton and PEN) using an in-house developed pen-writing method.........2 that results in crystalline films with macroscopic long range order. The room temperature PL studies show redshift and changes in polarization upon bending of the film. Crystalline thin films of alloyed H2OBPc and octabutoxy naphthalocyanine with ratios ranging from 1:1 to 100:1 fabricated on both sapphire and flexible substrates are also explored using the same PL spectroscopy to elucidate the behaviors of delocalized excitons. .1N. Rawat, et al., J Phys Chem Lett 6, 1834 (2015). 2R. L. Headrick, et al., Applied Physics Letters 92, 063302 (2008). NSF DMR-1056589, NSF DMR-1062966.
Effect of correlation of local fluctuations on exciton coherence.
Chen, Xin; Silbey, Robert J
2010-05-28
Recent experimental studies have shown both oscillations of exciton populations and long lasting coherence in multichromophoric systems such as photosynthetic light harvesting systems and conjugated polymers. It has been suggested that this quantum effect is due to correlations of the fluctuations of site energies among the closely packed chromophores in the protein environment. In addition to these, there is the strong possibility of correlations between site energies and transfer matrix elements. In order to understand the role of such correlations we generalize the Haken-Strobl-Reineker (HSR) model to include the energetic correlations and the site diagonal-off-diagonal correlations in a systematic way. The extended HSR model in the exciton basis is also constructed and allows us to study the dynamics of the exciton populations and coherences. With the extended model, we can provide insight into how these correlations affect the evolution of the populations and coherences of excitons by comparing to the original HSR model with uncorrelated fluctuating environments. PMID:20515096
Heavy ion collisions and the pre-equilibrium exciton model
Betak, E.
2012-10-20
We present a feasible way to apply the pre-equilibrium exciton model in its masterequation formulation to heavy-ion induced reactions including spin variables. Emission of nucleons, {gamma}'s and also light clusters is included in our model.
Ultrafast exciton dissociation at donor/acceptor interfaces
NASA Astrophysics Data System (ADS)
Grancini, G.; Fazzi, D.; Binda, M.; Maiuri, M.; Petrozza, A.; Criante, L.; Perissinotto, S.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.
2013-09-01
Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV) community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very intriguing their full understanding. In particular charge generation is believed to occur in < 200 fs, but no clear picture emerged so far. In this work we reveal for the first time the actual charge generation mechanism following in real time the exciton dissociation mechanism by means of sub-22 fs pump-probe spectroscopy. We study a low-band-gap polymer: fullerene interface as an ideal system for OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50 fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher fraction of free charges.
Modeling ultrafast exciton deactivation in oligothiophenes via nonadiabatic dynamics.
Fazzi, Daniele; Barbatti, Mario; Thiel, Walter
2015-03-28
Ultrafast excited-state processes play a key role in organic electronics and photovoltaics, governing the way of how excitons can relax and separate. Through the use of nonadiabatic excited-state dynamics, relaxation processes were investigated at the sub-picosecond timescale in thiophene and oligothiophenes (nT, n = 2, 3, 4), prototype oligomers for efficient π-electron conjugated polymers adopted in photovoltaics. For thiophene, TDDFT and TDA nonadiabatic excited-state dynamics revealed ultrafast nonradiative relaxation processes through ring opening and ring puckering, bringing the system to an S1/S0 conical intersection seam. The computed relaxation time is 110 fs, matching well the experimental one (∼105 fs). In oligothiophenes (n = 2-4), high-energy (hot) excitations were considered. Exciton relaxation through the manifold of excited states to the lowest excited state is predicted to occur within ∼150-200 fs, involving bond stretching, ring puckering, and torsional oscillations. For the longer oligomer (4T), the ultrafast relaxation process leads to exciton localization over three thiophene rings in 150 fs. These data agree with the self-localization mechanism (∼100-200 fs) observed for poly(3-hexylthiophene) (P3HT) and shed light on the complex exciton relaxation dynamics occurring in π-conjugated oligomers of potential interest for optoelectronic applications. PMID:25714233
Using dispersive medium to control excitons in 2D materials
NASA Astrophysics Data System (ADS)
Klots, Andrey; Bolotin, Kirill I.
Excitons in 2D materials (2DMs) are known to be sensitive to the surrounding environment. This makes it possible to modify 2D excitons by depositing materials with controlled dielectric constant on top of 2DMs. This possibility becomes especially interesting if we consider materials with dielectric permittivity ɛ that depends both on wavevector k (this happens if the medium is spatially non-uniform) and frequency ω. Here, we develop platforms to control ɛ (k , ω) and explore resulting changes in light-matter interactions of 2DMs. To examine the effect of wavevector-dependent permittivity of the medium, we study absorption/photoluminescence of graphene and MoS2 in the vicinity of highly non-uniform medium - an array of metal nanoparticles, 3-5 nm in diameter. In this case absorption of light can lead to creation of excitons with non-zero momentum. These dark states are not accessible via regular absorption spectroscopy. We study the case of frequency-dependent permittivity by surrounding MoS2 by a highly-dispersive media (e.g. dielectric liquids, graphene and VO2) . We demonstrate non-trivial frequency-dependent renormalization of the quasiparticle bandgap and exciton binding energies.
Singlet fission in pentacene through multiple exciton quantum states
NASA Astrophysics Data System (ADS)
Zhang, Zhiyong; Zimmerman, Paul; Musgrave, Charles
2010-03-01
Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states have single-exciton character, singlet fission to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple electron-hole pairs. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through a dark state (D) of ME character that lies just below S1, satisfies the fission energy requirement (ED>2ET0), and splits into two triplets (2xT0). In tetracene, D lies just above S1, consistent with the observation that singlet fission is thermally activated in tetracene. Rational design of photovoltaic systems that exploit singlet fission will require ab initio analysis of ME states such as D.
Singlet Fission and Multi-Exciton Generation in Organic Systems
NASA Astrophysics Data System (ADS)
Musgrave, Charles
2012-02-01
Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2xT0). D satisfies the energy requirement for SF (ED>2ET0) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into triplets proceeds through the excimer. The predicted energetics, wavefunctions and excimer interaction support the proposed mechanism, which accounts for the observed rapid, unactivated SF in pentacene. Results for SF in polyacenes, grapheme nanoribbons, rubrene and carbon nanotubes will be presented.
Vibrational exciton mediated quantum state transfer: Simple model
NASA Astrophysics Data System (ADS)
Pouthier, Vincent
2012-06-01
A communication protocol is proposed in which quantum state transfer is mediated by a vibrational exciton. We consider two distant molecular groups grafted on the sides of a one-dimensional lattice. These groups behave as two quantum computers where the information in encoded and received. The lattice plays the role of a communication channel along which the exciton propagates and interacts with a phonon bath. Special attention is paid to describing the system involving an exciton dressed by a single phonon mode. The Hamiltonian is thus solved exactly so that the relevance of the perturbation theory is checked. Within the nonadiabatic weak-coupling limit, it is shown that the system supports three quasidegenerate states that define the relevant paths followed by the exciton to tunnel between the computers. When the model parameters are judiciously chosen, constructive interferences take place between these paths. Phonon-induced decoherence is minimized and a high-fidelity quantum state transfer occurs over a broad temperature range.
Order-disorder oscillations in exciton-polariton superfluids.
Saito, Hiroki; Aioi, Tomohiko; Kadokura, Tsuyoshi
2013-01-11
The dynamics of an exciton-polariton superfluid resonantly pumped in a semiconductor microcavity are investigated by mean-field theory. Modulational instability develops into crystalline order, and then, ordered and disordered states alternately form. It is found that the crystalline order at rest can coexist with superflow. PMID:23383923
National solar technology roadmap: Multiple-exciton-generation PV
Ellingson, Randy
2007-06-01
This roadmap addresses the development of solar cells based on inorganic semiconductor nanocrystals (NCs)—such as spherical quantum dots (QDs), quantum rods (QRs), or quantum wires (QWs)—focusing on their potential to improve upon bulk semiconductor cell efficiencies by efficient multiple-exciton generation (MEG
Enhanced Multiple Exciton Generation in Amorphous Silicon Nanoparticles
NASA Astrophysics Data System (ADS)
Kryjevski, Andrei; Mihaylov, Deyan; Kilin, Dmitri
2015-03-01
Multiple exciton generation (MEG) in nm-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the many-body perturbation theory (MBPT) calculations based on the DFT simulations. Here, we use the HSE exchange correlation functional. In MBPT, we work to the 2nd order in the electron-photon coupling and in the approximate screened Coulomb interaction. We also include the effect of excitons for which we solve Bethe-Salpeter Equation. We calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in 3D arrays of Si29H36 quantum dots, NWs, and quasi 2D silicon nanofilms, all with both crystalline and amorphous core structures. Efficient MEG with QE of 1.3 up to 1.8 at the photon energy of about 3Eg , where Eg is the gap, is predicted in these nanoparticles except for the crystalline NW and film where QE ~= 1 . MEG in the amorphous nanoparticles is enhanced by the electron localization due to structural disorder. The exciton effects significantly red-shift QE (Ephoton) curves. Nanometer-sized amorphous silicon NWs and films are predicted to have effective MEG within the solar spectrum range. We acknowledge NSF support (CHE-1413614) for method development.
Organic photosensitive optoelectronic device having a phenanthroline exciton blocking layer
Thompson, Mark E.; Li, Jian; Forrest, Stephen; Rand, Barry
2011-02-22
An organic photosensitive optoelectronic device, having an anode, a cathode, and an organic blocking layer between the anode and the cathode is described, wherein the blocking layer comprises a phenanthroline derivative, and at least partially blocks at least one of excitons, electrons, and holes.
Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides.
Palummo, Maurizia; Bernardi, Marco; Grossman, Jeffrey C
2015-05-13
Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS2 and in two MX2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 heterobilayers exhibit very long-lived (∼20-30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion. PMID:25798735
The radius distribution of planets around cool stars
Morton, Timothy D.; Swift, Jonathan
2014-08-10
We calculate an empirical, non-parametric estimate of the shape of the period-marginalized radius distribution of planets with periods less than 150 days using the small yet well-characterized sample of cool (T{sub eff} < 4000 K) dwarf stars in the Kepler catalog. In particular, we present and validate a new procedure, based on weighted kernel density estimation, to reconstruct the shape of the planet radius function down to radii smaller than the completeness limit of the survey at the longest periods. Under the assumption that the period distribution of planets does not change dramatically with planet radius, we show that the occurrence of planets around these stars continues to increase to below 1 R{sub ⊕}, and that there is no strong evidence for a turnover in the planet radius function. In fact, we demonstrate using many iterations of simulated data that a spurious turnover may be inferred from data even when the true distribution continues to rise toward smaller radii. Finally, the sharp rise in the radius distribution below ∼3 R{sub ⊕} implies that a large number of planets await discovery around cool dwarfs as the sensitivities of ground-based transit surveys increase.
Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE
NASA Astrophysics Data System (ADS)
Bouzidi, M.; Soltani, S.; Halidou, I.; Chine, Z.; El Jani, B.
2016-04-01
In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.
Using dark states for exciton storage in transition-metal dichalcogenides.
Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel
2016-01-27
We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. PMID:26704568
Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry
NASA Astrophysics Data System (ADS)
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.
Excitons in a hexagonal graphene-like donor-acceptor lattice
NASA Astrophysics Data System (ADS)
Lalov, I. J.; Zhelyazkov, I.
2016-03-01
Spectra of excitonic excitations in a high-symmetry 2D molecular lattice with two different entities—donors and acceptors have been studied. We use the geometry of two hexagonal sub-lattices in graphene created by donors and acceptors, respectively, and investigate both Frenkel excitons (FEs) and charge-transfer excitons (CTEs) along with their coupling and manifestation in the linear absorption spectra. Vibronic spectra that consist of excitons and one-mode intramolecular vibrations of the donor or acceptor molecules are explored both analytically and numerically. Our calculations model the excitonic density of state and the linear absorption spectra in excitonic and vibronic regimes for two polarization of light, notably in the lattice plane and perpendicular to it. Lineshapes of the absorption depend upon the FE-CTEs-phonon coupling and exhibit bound and unbound exciton-phonon states in one-phonon and two-phonon vibronic spectra.
Using dark states for exciton storage in transition-metal dichalcogenides
NASA Astrophysics Data System (ADS)
Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel
2016-01-01
We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing.
Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots
NASA Astrophysics Data System (ADS)
Damtie, Fikeraddis A.; Wacker, Andreas
2016-03-01
We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton ESX in resonance to a double exciton EDX. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a 4 nm PbS quantum dot.
Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry.
Chestnov, Igor Yu; Demirchyan, Sevak S; Alodjants, Alexander P; Rubo, Yuri G; Kavokin, Alexey V
2016-01-01
We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators. PMID:26790534
Physical model of the vapor-liquid (insulator-metal) transition in an exciton gas
NASA Astrophysics Data System (ADS)
Khomkin, A. L.; Shumikhin, A. S.
2015-04-01
We propose a simple physical model describing the transition of an exciton gas to a conducting exciton liquid. The transition occurs due to cohesive coupling of excitons in the vicinity of the critical point, which is associated with transformation of the exciton ground state to the conduction band and the emergence of conduction electrons. We calculate the cohesion binding energy for the exciton gas and, using it, derive the equations of state, critical parameters, and binodal. The computational method is analogous to that used by us earlier [5] for predicting the vapor-liquid (insulator-metal) phase transition in atomic (hypothetical, free of molecules) hydrogen and alkali metal vapors. The similarity of the methods used for hydrogen and excitons makes it possible to clarify the physical nature of the transition in the exciton gas and to predict more confidently the existence of a new phase transition in atomic hydrogen.
Photophysics of pentacene thin films: The role of exciton fission and heating effects
NASA Astrophysics Data System (ADS)
Rao, Akshay; Wilson, Mark W. B.; Albert-Seifried, Sebastian; di Pietro, Riccardo; Friend, Richard H.
2011-11-01
There is evidence that the photo-generated singlet exciton in polycrystalline pentacene films undergoes rapid and efficient fission to form two triplet excitons. However, the role of exciton fission in pentacene has been controversial, with previous studies putting forward alternate relaxation pathways for the singlet exciton, such as excimer or charge formation, or internal conversion to a doubly excited exciton. We report temperature- and angular-dependent ultrafast transient optical absorption measurements in a wide spectral and temporal window. Angular-dependent transient spectra identify a common origin to photo-induced absorptions at 530 and 860 nm, which we associate with triplet excitons. These constitute the dominant relaxation channel for singlet excitons. Other features, particularly near 620 nm, previously assigned to excimers or electronic charges, are shown to be caused by thermal modulation from the optical pump.
Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry
Chestnov, Igor Yu.; Demirchyan, Sevak S.; Alodjants, Alexander P.; Rubo, Yuri G.; Kavokin, Alexey V.
2016-01-01
We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators. PMID:26790534
Physical model of the vapor-liquid (insulator-metal) transition in an exciton gas
Khomkin, A. L. Shumikhin, A. S.
2015-04-15
We propose a simple physical model describing the transition of an exciton gas to a conducting exciton liquid. The transition occurs due to cohesive coupling of excitons in the vicinity of the critical point, which is associated with transformation of the exciton ground state to the conduction band and the emergence of conduction electrons. We calculate the cohesion binding energy for the exciton gas and, using it, derive the equations of state, critical parameters, and binodal. The computational method is analogous to that used by us earlier [5] for predicting the vapor-liquid (insulator-metal) phase transition in atomic (hypothetical, free of molecules) hydrogen and alkali metal vapors. The similarity of the methods used for hydrogen and excitons makes it possible to clarify the physical nature of the transition in the exciton gas and to predict more confidently the existence of a new phase transition in atomic hydrogen.
Transient Optical Studies of Exciton Dynamics in Organic Solar Cells
NASA Astrophysics Data System (ADS)
Reynolds, Luke X.
There is increasing evidence that the initially generated excited state species in bulk heterojunction solar cell photoactive layers are critical to device performance. At present however, an understanding of the nature and dynamics of such excited states still remains limited. This thesis presents a study of the ultrafast exciton dynamics in bulk heterojunction organic and hybrid organic-inorganic solar cells. Fluorescence upconversion is used to elucidate the dynamics of such transient species allowing internal properties of the blend systems to be probed including changes in film morphology and ultrafast energy loss mechanisms. An understanding of such processes is an important step forward in the evolution of molecular semiconductor based solar cells. The first chapter focuses on the main experimental technique, fluorescence upconversion, and how this can be employed to study excited states. In particular, this section addresses one of the main unanswered questions in the field and attempts to correlate the exciton dynamics with the structure of the common photoactive polymer poly(3-hexylthiophene) (P3HT). Three structural variations of P3HT are studied and their exciton dynamics associated with differing internal processes occurring within the polymers. These include self localisation, and different types of long-range energy transfer mechanisms. The following two chapters build upon the knowledge of exciton dynamics obtained from the first chapter. First, a study is made of amorphous polymers with different acceptors, all based on phenyl-C61-butyric acid methyl ester (PCBM). The distinct interactions of the PCBM-type molecules with the polymer results in different electron transfer dynamics, from which the exciton diffusion length of the polymer in real bulk heterojunction blends is extracted using a simple model. Second, the ultrafast excited state dynamics of a crystalline polymer with the same PCBM-type acceptors is studied. Correlation of these dynamics with thermal analysis of the blend films allows the morphology of the films to be extracted and allows two different mechanisms of microstructure development to be identified. In the final chapter, the effect of acceptor aggregation on exciton dynamics and charge generation yields in hybrid organic-inorganic blend films has been studied. Such aggregation has been shown to be essential for efficient charge generation in all-organic solar cells but has often been assumed to be less important in such inorganic hybrids. More aggregated acceptor nanoparticles are shown to not only result in greater than expected exciton quenching but are also shown to result in a greater yield of long-lived charges. This study is extended to show that in-situ grown inorganic nanoparticles exhibit superior performance to standard pre-synthesised inorganics.
Zimmer, Mindy M.; Naes, Benjamin E.; Willingham, David G.; Cloutier, Janet M.
2015-09-15
PNNL has been procured a Cameca 1280 Large Radius Secondary Ions Mass Spectrometer (LRSIMS) from the Amtek corporation out of France. This state-of-the-art instrument is aligning PNNL to deliver to NNSA the ability to address issues from proliferation detection to nuclear archeology of reactor operation and cascade enrichment history verification pushing beyond the limits of currently available methods and instrumentation at PNNL.
Exciton Relaxation and Electron Transfer Dynamics of Semiconductor Quantum Dots
NASA Astrophysics Data System (ADS)
Liu, Cunming
Quantum dots (QDs), also referred to as colloidal semiconductor nanocrystals, exhibit unique electronic and optical properties arising from their three-dimensional confinement and strongly enhanced coulomb interactions. Developing a detailed understanding of the exciton relaxation dynamics within QDs is important not only for sake of exploring the fundamental physics of quantum confinement processes, but also for their applications. Ultrafast transient absorption (TA) spectroscopy, as a powerful tool to explore the relaxation dynamics of excitons, was employed to characterize the hot single/multiexciton relaxation dynamics at the first four exciton states of CdSe/CdZnS QDs. We observed for the first time that the hot hole can relax through two possible pathways: Intraband multiple phonon coupling and intrinsic defect trapping, with a lifetime of ˜7 ps. Additionally, an ultra-short component of ˜ 8 ps, directly associated with the Auger recombination of highly energetic exciton states, was discovered. After exploring the exciton relaxation inside QDs, ultrafast TA spectroscopy was further applied to study the electron transferring outside from QDs. By using a brand-new photocatalytic system consisting of CdSe QDs and Ni-dihydrolipoic acid (Ni-DHLA) catalyst, which has represented a robust photocatalysis of H2 from water, the photoinduced electron transfer (ET) dynamics between QD and the catalyst, one of most important steps during H2 generation, was studied. We found smaller bare CdSe QDs exhibit a better ET performance and CdS shelling on the bare QDs leads to worsen the ET. The calculations of effective mass approximation (EMA) and Marcus theory show the ET process is mainly dominated by driving force, electronic coupling strength and reorganization energy between QD and the catalyst.
Seven Excitons per Single Photon Using Semiconductor Nanocrystals
NASA Astrophysics Data System (ADS)
Klimov, Victor
2007-03-01
The efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology. A usual assumption is that absorption of a single photon by a material produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat. In 2004, we reported for the first time that nanocrystals (NCs) of PbSe could respond to absorption of a single photon by producing two or more excitons with the unity probability (Phys. Rev. Lett. 92, 186601, 2004). Our more recent findings indicate that this carrier multiplication process can generate multiple charges with quantum efficiencies that correspond to the ultimate limit dictated by energy conservation. For example, for photon energy of 7.8 energy gaps, a maximal possible number of photogenerated excitons based on energy considerations is 7, which is exactly the number measured in our experiments (Nano Lett. 6, 424, 2006). Another unexpected feature of carrier multiplication is that it results in unusual distributions of carrier populations that cannot be described by Poisson statistics. Specifically, by selecting certain photon energies, we obtain photoexcited NC ensembles with nearly pure single multiplicities (i.e., all excited NCs contain the same number of excitons) that can be tuned in the controlled way from 1 to 7 (Phys. Rev. Lett. 96, 097402, 2006). While the exact mechanism for carrier multiplication in NCs is still under debate, one factor, which likely contributes to high efficiencies of this process, is a unique property of the NCs to produce significant carrier-carrier interactions as indicated, e.g., by our previous Auger recombination studies (Science 287, 1011, 2000). This confinement-enhanced Coulomb coupling can lead to the unusual mechanism for direct photogeneration of multiexcitons via virtual single-exciton states, which can explain our observations of very short, sub-200 femtosecond buildup times of multiexciton populations in the regime of carrier multiplication (Nature Phys. 1, 189, 2005).
Closed reduction of radius refracture: A case report
Shahid, Mohammad; Yeo, Mildrid; Smibert, John Graham
2011-01-01
INTRODUCTION Refractures of the radius and ulna in the paediatric patient with flexible intramedullary nails in situ are known to occur. There are no formal guidelines currently in the literature to guide the management of such fractures. PRESENTATION OF CASE A 10-year-old Caucasian girl, sustained a closed refracture of the radius at the same level, with the flexible intramedullary nails in situ to treat her recent ulna and radius fractures. DISCUSSION We proposed a new non-invasive way of reducing and maintaining such fractures without removal of the bent nail completely. CONCLUSION This method does not significantly reduce the mechanical strength of the nail, as we do not advocate applying an external lateral force, which would compromise nail mechanical strength and eventually lead to breakage of the nail in situ. PMID:22096752
Attractor radius, a new determination criterion of predictability limit
NASA Astrophysics Data System (ADS)
Liu, Deqiang; Ding, Ruiqiang; Li, Jianping; Feng, Jie
2014-05-01
Firstly, the definition of the attractor radius was given and then the property of that the attractor radius (AR) in a given n-dimensional attractor A is a constant was proved in theory. Secondly, the SV of the square of the RMS difference was separated into two components - the systematic error and the attractor radius, and it was proved that the observed global climatological RMS (OCR) difference is not equal to 71% of the SV of the RMS difference when the systematic error is existed, however, it is always equal to 71% of the AR. Then the physical understanding of the AR and also the predictability limit determinated by it were discussed. Finally, the spatial distributions of the predictability limit calculated from CFSv2 data by different criterions were compared.
Solar Radius Variations: a New Look on the Wavelength Dependence
NASA Astrophysics Data System (ADS)
Rozelot, Jean-Pierre; Kosovichev, Alexander; Kilcik, Ali
2015-08-01
The possibility that the the Sun's radius is changing, even at a faint level, has been talk over a long time. As solar radius is certainly one of the most important basic pieces of astrophysical information, it is crucial to determine the physical mechanisms that may cause shrinking or expansion of the solar envelope. The wavelength dependence has been poorly examine up to now. In this talk, we examine recent solar radius determinations from space observation of Mercury and Venus transits that have been made by different teams, in 2006, 2012 and 2014. Seemingly results are not consistent: authors interpreted the discrepancies by the different methods of analysis used. However, looking at the wavelength dependence, adding other available observations, from X-EUV up to radio, a typical wavelength dependence can be found, reflecting the different heights at which the lines are formed. A possible explanation is proposed. Such results can be interesting for solar-stellar connections.
Hominid radius from the middle Pliocene of Lake Turkana, Kenya.
Heinrich, R E; Rose, M D; Leakey, R E; Walker, A C
1993-10-01
A nearly complete left radius, KNM-ER 20419, was recovered from middle Pliocene sediments east of Lake Turkana, Kenya in 1988. Ape-like characteristics of the fossil include an eccentrically positioned articular fovea, relatively long radial neck, wide distal metaphysis, and large brachioradialis crest. The robustness of the radial neck in proportion to the radial head, and the semilunar shape of the distal diaphysis, however, clearly distinguish KNM-ER 20419 as hominid. The distal articular surface possesses a larger area for radius-lunate articulation than for radius and scaphoid, a radiocarpal arrangement that is associated with increased wrist adduction among quadrumanous climbers. Since this morphology is also found in hylobatids, Pongo, and other early australopithecines, it is argued to be plesiomorphic for hominoids. This further supports the argument that vertical climbing was an important locomotor behavior among both early hominoids and our more immediate prebipedal ancestors. PMID:8273826