Sample records for bohr exciton radius

  1. On the Bohr radius relationship to spin-orbit interaction, spin magnitude, and Thomas precession

    E-print Network

    David C. Lush

    2009-03-24

    The dynamics of the spin-orbit interaction in atomic hydrogen are studied in a classical electrodynamics-like setting. A Rutherfordian atomic model is used assuming a circular electron orbit, without the quantum principle as imposed arbitrarily in the Bohr model, but with an ad hoc incorporation in the electron of intrinsic spin and associated magnetic dipole moment. Analyzing the motions of the electron spin and orbital angular momenta, it is found that in the presence of Thomas precession, the total angular momentum averaged over the orbit is not generally a constant of the motion. It is noted this differs from the finding of Thomas in a similar assessment of 1927, and the reason for this difference is provided. It is found that although the total orbit-averaged angular momentum is not a constant of the motion, it precesses around a fixed axis similarly to the precession of the total angular momentum vector seen in spin-orbit coupling in quantum theory. The magnitude of the angular velocity of the total orbit-averaged angular momentum is seen to vanish only when the spin and orbital angular momenta are antiparallel and their mutual precession frequencies equate. It is then found, there is a unique radius where the mutual precession frequencies equate. Assuming the electron magnetic moment is the Bohr magneton, and an electron g-factor of two, this radius corresponds to where the orbital angular momentum is the reduced Planck's constant. The orbit radius for stationary total angular momentum for the circular orbit model with nonzero orbital angular momentum is thus the ground-state radius of the Bohr model.

  2. The Aharonov-Bohm effect for an exciton

    Microsoft Academic Search

    R. A. Römer; M. E. Raikh

    2000-01-01

    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

  3. Radiative lifetime of excitons in ZnO nanocrystals: The dead-layer effect Vladimir A. Fonoberov* and Alexander A. Balandin

    E-print Network

    Fonoberov, Vladimir

    Radiative lifetime of excitons in ZnO nanocrystals: The dead-layer effect Vladimir A. Fonoberov investigate exciton states of colloidal nearly spherical ZnO nanocrystals with diameters from 2 nm to 6 nm. The sizes of considered ZnO nanocrystals are chosen to be slightly larger than the exciton Bohr radius

  4. Wave functions of hot excitons in semiconductors with degenerate bands

    SciTech Connect

    Efanov, A. V. [Russian Academy of Sciences, Institute of Semiconductor Physics, Siberian Branch (Russian Federation)], E-mail: efanov@isp.nsc.ru

    2008-06-15

    Hot excitons in direct-gap cubic semiconductors with a degenerate valence band are considered. Corrections to the model of independent excitonic branches in terms of the small parameter {Dirac_h}/Ka{sub B}, where K is the exciton momentum and a{sub B} is the Bohr radius, are determined. The corrections take into account the internal motion of particles in the exciton. It is shown that the internal motion mixes the states of light and heavy holes in the exciton wave function. As a result, the processes of forward scattering of excitons with transitions between different excitonic branches become allowed. The consideration is concerned with the region of kinetic energies no higher than the spin-orbit splitting in the valence band. The dispersion relation for holes is described in the spherical Kohn-Luttinger model (the 4 x 4 matrix)

  5. Excitons in monolayer transition metal dichalcogenides.

    PubMed

    Li, J; Zhong, Y L; Zhang, Dong

    2015-08-12

    We theoretically investigate the exciton formed with two massive Dirac particles in monolayer [Formula: see text] and other transition metal dichalcogenides as well as two layers separated by a dielectric layer. In the low-energy limit, the separation of the center-of-mass and relative motions is obtained. Analytical solutions for the exciton wave function and energy dispersion are obtained including the Coulomb interaction between electron and hole, the exciton Bohr radius, binding energy and its effective mass are obtained in monolayer transition metal dichalcogenides. In the case of two monolayers separated by a dielectric layer, we find that the exciton effective mass can be continuously tuned by the interlayer separation. PMID:26190703

  6. Two-dimensional excitons in three-dimensional hexagonal boron nitride

    SciTech Connect

    Cao, X. K.; Lin, J. Y., E-mail: hx.jiang@ttu.edu; Jiang, H. X., E-mail: jingyu.lin@ttu.edu [Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas 79409 (United States); Clubine, B.; Edgar, J. H. [Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506 (United States)] [Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506 (United States)

    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.

  7. ZnSe\\/ZnTe(shell\\/shell) radial quantum-wire heterostructures: the excitonic properties

    Microsoft Academic Search

    Mehran Bagheri

    2010-01-01

    The ground-state characteristics of spatially indirect excitons trapped in radially heteronanostructured type-II band alignment ZnSe\\/ZnTe nanotubes as functions of the magnetic field for nanotubes with a radial size both smaller and larger than the effective Bohr radius are theoretically investigated. In the former case, dominated by the net kinetic energy of the electron and hole, the magnetic field modifies the

  8. Separating homogeneous and inhomogeneous line widths of heavy- and light-hole excitons in weakly disordered semiconductor quantum wells.

    PubMed

    Bristow, Alan D; Zhang, Tianhao; Siemens, Mark E; Cundiff, Steven T; Mirin, R P

    2011-05-12

    Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in weakly disordered GaAs quantum wells. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the reverse. This situation occurs because of the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged. PMID:21384940

  9. Bohr's power series theorem in several variables

    Microsoft Academic Search

    Harold P. Boas; Dmitry Khavinson

    1996-01-01

    Generalizing a classical one-variable theorem of Harald Bohr, we show that if an n-variable power series has modulus less than 1 in the unit polydisc, then the sum of the moduli of the terms is less than 1 in the polydisc of radius 1\\/(3*n^{1\\/2}).

  10. Multiple Exciton Generation in Colloidal Silicon Nanocrystals

    SciTech Connect

    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.

  11. Multiple exciton generation in colloidal silicon nanocrystals.

    PubMed

    Beard, Matthew C; Knutsen, Kelly P; Yu, Pingrong; Luther, Joseph M; Song, Qing; Metzger, Wyatt K; Ellingson, Randy J; Nozik, Arthur J

    2007-08-01

    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 with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. 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. PMID:17645368

  12. Influence of image charge effect on exciton fine structure in an organic-inorganic quantum well material

    SciTech Connect

    Takagi, Hidetsugu; Kunugita, Hideyuki; Ema, Kazuhiro [Department of Physics, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 (Japan); Sato, Mikio; Takeoka, Yuko [Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 (Japan)

    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.

  13. 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 ?0 --- 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 ? 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).

  14. Exciton-exciton annihilation in organic polariton microcavities

    SciTech Connect

    Akselrod, G. M.; Tischler, Jonathan R.; Young, E. R.; Nocera, D. G.; Bulovic, Vladimir

    2010-01-01

    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{sup 6} ?cm{sup ?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.

  15. FAST TRACK COMMUNICATION: ZnSe/ZnTe(shell/shell) radial quantum-wire heterostructures: the excitonic properties

    NASA Astrophysics Data System (ADS)

    Bagheri, Mehran

    2010-07-01

    The ground-state characteristics of spatially indirect excitons trapped in radially heteronanostructured type-II band alignment ZnSe/ZnTe nanotubes as functions of the magnetic field for nanotubes with a radial size both smaller and larger than the effective Bohr radius are theoretically investigated. In the former case, dominated by the net kinetic energy of the electron and hole, the magnetic field modifies the exciton spectrum through the well-known Zeeman splitting, intra-orbital-state Aharonov-Bohm oscillations and inter-orbital-state crossovers occurring in very strong magnetic field strengths. However, in the latter case, dominated by the electron-hole Coulomb attraction, the magnetic field adjusts the exciton lines only by means of the Zeeman splitting and inter-orbital-state transitions happening in typical magnetic fields. As a result, the angular momentum transitions occurr at lower magnetic fields when the radial size of the nanotube is increased. Most importantly, another consequence is the substantially unusual exciton oscillator strength in such heteronanostructures. It is shown that when the exciton is optically active, due to the full cylindrical symmetry of the problem, the exciton oscillator strength shows undamped oscillations. This effect is associated with the periodic redistribution of the exciton density as the magnetic field is varied. Also, the magnitude of the magnetically induced excitonic persistent current is decreased with increasing radial size of the nanotube. This study may provide a platform to investigate new photonic quantum interference as well as polarization-sensitive photodetector and photovoltaic devices based on the Aharonov-Bohm effect.

  16. Photonic Crystal Architecture for Room-Temperature Equilibrium Bose-Einstein Condensation of Exciton Polaritons

    NASA Astrophysics Data System (ADS)

    Jiang, Jian-Hua; John, Sajeev

    2014-07-01

    We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This goal is achieved through a careful balance between strong light trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with a moderate dielectric constant. This approach enables the formation of a long-lived, dense 10-?m-1-cm- scale cloud of exciton polaritons with vacuum Rabi splitting that is roughly 7% of the bare exciton-recombination energy. We introduce a woodpile photonic crystal made of Cd0.6 Mg0.4Te with a 3D PBG of 9.2% (gap-to-central-frequency ratio) that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3-nm QWs with 5-nm barrier width, the exciton-photon coupling can be as large as ??=55 meV (i.e., a vacuum Rabi splitting of 2??=110 meV). The exciton-recombination energy of 1.65 eV corresponds to an optical wavelength of 750 nm. For N =106 QWs embedded in the cavity, the collective exciton-photon coupling per QW (??/?N =5.4 meV) is much larger than the state-of-the-art value of 3.3 meV, for the CdTe Fabry-Pérot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass of approximately 10-5m0, where m0 is the electron mass in vacuum. By detuning the bare exciton-recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC. The BEC transition temperature ranges as high as 500 K when the polariton density per QW is increased to (11aB)-2, where aB?3.5 nm is the exciton Bohr radius and the exciton-cavity detuning is increased to 30 meV. A high-quality PBG can suppress exciton radiative decay and enhance the polariton lifetime to beyond 150 ps at room temperature, sufficient for thermal equilibrium BEC.

  17. Applications of Bohr's correspondence principle

    Microsoft Academic Search

    Frank S. Crawford

    1989-01-01

    The Bohr correspondence-principle (cp) formula \\/ital dE\\/\\/\\/ital dn\\/=\\/h bar\\/..omega.. is presented (..omega.. is the classical angular frequency) and its predicted energy levels \\/ital E\\/\\/sub \\/ital n\\/\\/ are compared to those given by the stationary state solutions of the Schr\\/umlt o\\/dinger equation, first for several examples in one dimension (1D), including the ''quantum bouncer,'' and then for several examples in three

  18. Bohr-like black holes

    E-print Network

    Christian Corda

    2015-03-11

    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.

  19. The Bohr effect before Perutz.

    PubMed

    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 chemistry and structure of the protein was essentially nil. The magnetochemical properties of hemoglobin led Linus Pauling to hypothesize that the (so called) Bohr histidines were coordinated to the heme iron in the fifth and sixth positions; and Wyman shared this opinion. However, this structural hypothesis was abandoned in 1951 when J. Wyman and D. W. Allen proposed the pK shift of the oxygen linked histidines to be the result of "...a change of configuration of the hemoglobin molecule as a whole accompanying oxygenation." This shift in paradigm, that was published well before the 3D structure of hemoglobin was solved by M.F. Perutz, paved the way to the concept of allostery. After 1960 the availability of the crystallographic structure opened new horizons to the interpretation of the allosteric properties of hemoglobin. PMID:22987550

  20. Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier

    SciTech Connect

    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.

  1. Applications of Bohr's correspondence principle

    SciTech Connect

    Crawford, F. S.

    1989-07-01

    The Bohr correspondence-principle (cp) formula /ital dE///ital dn/=/h bar/..omega.. is presented (..omega.. is the classical angular frequency) and its predicted energy levels /ital E//sub /ital n// are compared to those given by the stationary state solutions of the Schr/umlt o/dinger equation, first for several examples in one dimension (1D), including the ''quantum bouncer,'' and then for several examples in three dimensions (3D), including the hydrogen atom and the isotropic harmonic oscillator. For the 3-D cases, the cp predictions based on classical circular orbits are compared with the ''circlelike'' Schr/umlt o/dinger solutions (those with the lowest energy eigenvalue for a given /ital l/) and the cp predictions based on classical ''needle'' orbits (having zero angular momentum) with the Schr/umlt o/dinger /ital l/=0 solutions. For the H atom and the isotropic oscillator, the cp prediction does not depend on the classical orbit chosen because of a ''degeneracy'': the fact that for these systems ..omega.. is independent of the orbit. As a more stringent test of the cp, analogous nondegenerate systems /ital V/=/minus//ital k///ital r//sup 3/2/ in place of the H-atom potential /ital V/=/minus//ital e//sup 2///ital r/ and /ital V/=/ital kr//sup 4/ in place of the oscillator potential /ital V/=(1/2)/ital m/..omega../sup 2//ital r2/ are therefore considered. Aninteresting anomaly that occurs for the harmonic oscillator and itsnondegenerate analog /ital V/=/ital kr//sup 4/ is encountered (but not for the H atomnor its nondegenerate analog /ital V/=/minus//ital k///ital r//sup 3/2/), wherein half of thestates predicted by application of the cp to the needle orbits are ''spurious''in that there are no corresponding Schr/umlt o/dinger /ital l/=0 states.

  2. 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/2?. 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 ? 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?

  3. 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 ? = h/2 ?. 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 ? 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?

  4. Frenkel-like Wannier-Mott excitons in few-layer Pb I2

    NASA Astrophysics Data System (ADS)

    Toulouse, Alexis S.; Isaacoff, Benjamin P.; Shi, Guangsha; Matuchová, Marie; Kioupakis, Emmanouil; Merlin, Roberto

    2015-04-01

    Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer Pb I2 indicate that the n =1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.

  5. The Bohr Model of the Earth-Sun System* Data for the earth-sun system assuming a circular earth orbit

    E-print Network

    Rioux, Frank

    energy expression below. The potential energy of the earth-sun interaction is wellThe Bohr Model of the Earth-Sun System* Data for the earth-sun system assuming a circular earth orbit: Mass of the earth: Me 5.974 10 24 kg Mass of the sun: Ms 1.989 10 30 kg Earth orbit radius: r 1

  6. 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.

  7. Bohr model and dimensional scaling analysis of atoms and molecules 

    E-print Network

    Urtekin, Kerim

    2007-04-25

    and applications the implementation of a simple and successful extension of Bohr�s planetary model of the hydrogenic atom, which has recently been developed by an atomic and molecular theory group from Texas A&M University. This �extended� Bohr model...

  8. Davidson potential and SUSYQM in the Bohr Hamiltonian

    SciTech Connect

    Georgoudis, P. E. [Institute of Nuclear and Particle Physics, National Center for Scientific Research Demokritos, GR-15310 Athens (Greece)

    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.

  9. Bohr's correspondence principle for atomic transport calculations

    E-print Network

    Viviana P. Ramunni; Alejandro M. F. Rivas

    2015-03-26

    In this work we perform a comparison between Classical Molecular Static (CMS) and quantum Density Functional Theory (DFT) calculations in order to obtain the diffusion coefficients for diluted \\emph{Fe-Cr} alloys. We show that, in accordance with Bohr's correspondence principle, as the size of the atomic cell (total number of atoms) is increased, quantum results with DFT approach to the classical ones obtained with CMS. Quantum coherence effects play a crucial role in the difference arising between CMS and DFT calculations. Also, thermal contact with the environment destroys quantum coherent effects making the classical behavior to emerge. Indeed, CMS calculations are in good agreement with available experimental data. We claim that, the atomic diffusion process in metals is a classical phenomena. Then, if reliable semi empirical potentials are available, a classical treatment of the atomic transport in metals is much convenient than DFT.

  10. Enhanced free exciton and direct band-edge emissions at room temperature in ultrathin ZnO films grown on Si nanopillars by atomic layer deposition.

    PubMed

    Chang, Yuan-Ming; Shieh, Jiann; Chu, Pei-Yuan; Lee, Hsin-Yi; Lin, Chih-Ming; Juang, Jenh-Yih

    2011-11-01

    Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions. PMID:21967063

  11. Quantum discord is Bohr’s notion of non-mechanical disturbance introduced to counter the Einstein–Podolsky–Rosen argument

    SciTech Connect

    Wiseman, Howard M., E-mail: H.Wiseman@Griffith.edu.au

    2013-11-15

    By rigorously formalizing the Einstein–Podolsky–Rosen (EPR) argument, and Bohr’s reply, one can appreciate that both arguments were technically correct. Their opposed conclusions about the completeness of quantum mechanics hinged upon an explicit difference in their criteria for when a measurement on Alice’s system can be regarded as not disturbing Bob’s system. The EPR criteria allow their conclusion–incompleteness–to be reached by establishing the physical reality of just a single observable q (not of both q and its conjugate observable p), but I show that Bohr’s definition of disturbance prevents the EPR chain of reasoning from establishing even this. Moreover, I show that Bohr’s definition is intimately related to the asymmetric concept of quantum discord from quantum information theory: if and only if the joint state has no Alice-discord, she can measure any observable without disturbing (in Bohr’s sense) Bob’s system. Discord can be present even when systems are unentangled, and this has implications for our understanding of the historical development of notions of quantum nonlocality. -- Highlights: •Both the EPR argument, and Bohr’s reply, were technically correct. •Their opposed conclusions came from different criteria for disturbance. •Bohr’s criterion works against even the simplified (one-variable) EPR argument. •Bohr’s criterion for disturbance is intimately related to quantum discord. •This illuminates the historical development of notions of quantum nonlocality.

  12. Elastic exciton-exciton scattering in photoexcited carbon nanotubes.

    PubMed

    Nguyen, D T; Voisin, C; Roussignol, Ph; Roquelet, C; Lauret, J S; Cassabois, G

    2011-09-16

    We report on original nonlinear spectral hole-burning experiments in single wall carbon nanotubes that bring evidence of pure dephasing induced by exciton-exciton scattering. We show that the collision-induced broadening in carbon nanotubes is controlled by exciton-exciton scattering as for Wannier excitons in inorganic semiconductors, while the population relaxation is driven by exciton-exciton annihilation as for Frenkel excitons in organic materials. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. PMID:22026798

  13. 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.

  14. Excitons in Electrostatic Lattices

    NASA Astrophysics Data System (ADS)

    Remeika, Mikas

    An indirect exciton is a bound state of an electron and a hole, with electrons and holes confined to separate quantum wells. Indirect excitons have a number of distinctive properties that make them an excellent basis for novel optoelectronic devices and a model system for studies of condensed matter phenomena. These properties include in-situ tunable energy, in-situ density control, long recombination times, fast thermalization, and wide range of tunable parameters such as effective mass, lifetime etc. This dissertation presents a study of excitons in a periodic potential created by patterned electrodes -- an electrostatic lattice. The study of excitons in electrostatic lattices has a twofold aim -- to develop tools to emulate behavior of quantum particles in a periodic potential and to use the periodic potential to accurately characterize properties of exciton gasses. Large number of tunable parameters of excitons combined with adjustable parameters of electrostatic lattices make then an excellent system for creating potential landscapes that emulate crystal potentials seen by electrons in an atomic lattice and can help to answer questions about origins of material properties. Periodically repeating nature of an electrostatic lattice makes it an excellent system for precision measurements, making it possible to average a measurement over a large number of lattice sites. Modeling of exciton behavior in a periodic potential is more accessible from a theoretical standpoint, making it possible to compare accurate models to high-precision measurements, leading to more quantitative understanding of exciton properties. We present electrode designs that make it possibles to create tunable lattices for indirect excitons. Linear electrostatic lattices for indirect excitons are implemented by interdigitated electrodes. 2-dimensional electrostatic lattices, including square, triangular, and honeycomb geometries, are realized by a special design of patterned interdigitated electrodes. We present transport measurements of excitons in linear and square electrostatic lattices. Exciton localization-delocalization transitions is observed in a linear lattice and interpreted in terms of interaction-induced percolation of exciton gas through the external potential. We also present a method to characterize the degree of correlation in an excitons gas by measuring exciton energy shift and emission modulation in an electrostatic lattice.

  15. 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.

  16. Stabilizing effect of aromatic impurities on radical formation by the exciton mechanism

    SciTech Connect

    Grigor'ev, E.I.; Slavinskaya, N.A.; Pshezhetskii, S.Ya.; Trakhtenberg, L.I.

    1988-07-01

    The protection mechanism by an aromatic impurity during irradiation of xenon matrix containing methane by /gamma/-rays and metastable xenon atoms is studied. It is demonstrated that non-coherent triplet excitons of xenon are responsible for the formation of methyl radicals in matrices containing methane and benzene. Benzene molecules are found to be effective exciton acceptors. The capture radius of excitons by benzene molecules is estimated as /approximately/1.0 nm, and the diffusion coefficient for excitons in solid xenon is /approximately/0.1 cm/sup 2//centered dot/sec/sup /minus/1/.

  17. Neutral and positively charged excitons in narrow quantum ring

    SciTech Connect

    Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D. [Escuela de Física, Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)

    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.

  18. Optically programmable excitonic traps.

    PubMed

    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

  19. Large mode radius resonators

    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.

  20. Radius Indiana Thomas Cooley

    E-print Network

    Radius Indiana Thomas Cooley 300 HWY 361 Crane, IN 47522 Ph: 812-854-8245 email: info@radiusindiana.com http://www.radiusindiana.com/ Radius Indiana is a 501(c) 3 corporation created to give businesses one-stop access to economic development in South Central Indiana. With a vision to create a dynamic network

  1. VOLUME 84, NUMBER 9 P H Y S I C A L R E V I E W L E T T E R S 28 FEBRUARY 2000 Many-Body Renormalization of Semiconductor Quantum Wire Excitons

    E-print Network

    Wang, Daw-Wei

    as a function of the photoex- cited electron-hole density [7­9]. This striking lack of any dependence a function of carrier density) in 1D quantum wires due to a near exact cancellation between the redshift aris-exciton Wannier equa- tion, several commonly used excitonic concepts, such as exciton radius, binding energy

  2. Optical switching: Excitonic interconnects

    SciTech Connect

    Stojanovic, V.; Baldo, Marc

    2009-01-01

    Interconnects and switches relying on excitons — quasiparticles consisting of bound electron–hole pairs — may offer a promising energy-efficient alternative to electrons in wires for future electronic circuitry.

  3. Triplet exciton dissociation in singlet exciton fission photovoltaics.

    PubMed

    Jadhav, Priya J; Brown, Patrick R; Thompson, Nicholas; Wunsch, Benjamin; Mohanty, Aseema; Yost, Shane R; Hontz, Eric; Van Voorhis, Troy; Bawendi, Moungi G; Bulovi?, 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

  4. Microscopic Uni-axial Bohr-Mottelson Rotational Model

    SciTech Connect

    Gulshani, P. [NUTECH Services, 3313 Fenwick Cres., Mississauga, Ontario, L5L 5N1 (Canada)

    2010-08-04

    A microscopic version of the phenomenological Bohr-Mottelson unified adiabatic rotational model is derived using only space-fixed particle coordinates, and without imposing any constraints on the particle coordinates or the intrinsic wavefunction. It is shown that this can done only for rigid flow. A collective-rotation velocity field is defined and is used to show that, although their Hamiltonians are closely related, the flows in a multi-fermion and single-particle system are inherently different.

  5. Bohr--Sommerfeld Lagrangians of moduli spaces of Higgs bundles

    E-print Network

    Indranil Biswas; Niels Leth Gammelgaard; Marina Logares

    2015-04-03

    Let $X$ be a compact connected Riemann surface of genus at least two. Let $M_H(r,d)$ 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 $M_H(r,d)$ are precisely the irreducible components of the nilpotent cone in $M_H(r,d)$. This generalizes to Higgs $G$-bundles and also to the parabolic Higgs bundles.

  6. Exciton Transport in Nanostructured Solids

    NASA Astrophysics Data System (ADS)

    Bulovic, Vladimir

    2015-03-01

    Transport of nanoscale energy in the form of excitons is at the core of operation of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. The talk will present recent advancements in directly visualizing exciton transport, with spatial, temporal and spectral evolution recorded for molecular crystals, disordered thin films, and colloidal quantum dot solids. Our measurements demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and the design of nanoscale building blocks. In addition, the talk will show that the excitonic energy landscape can be directly manipulated in solid-state thin films using dipole -dipole interactions, which can be increased under mechanical pressure, or molecular doping with polar molecules, leading to dramatic shifts in the exciton energy structure.

  7. Triplet exciton caging in two dimensions

    Microsoft Academic Search

    S. Arnold; R. R. Alfano; M. Pope; W. Yu; P. Ho; R. Selsby; J. Tharrats; C. E. Swenberg

    1976-01-01

    It is proposed that the bimolecular process of triplet exciton fusion to form singlet excitons can be enhanced by reducing the size of the domain in which the triplet exciton pair is free to move. These small domains, or exciton cages, are much more effective when the host material is highly anisotropic, and the triplet excitons are constrained to move

  8. Radius of neutron stars

    SciTech Connect

    Meszaros, P.; Riffert, H.

    1987-12-01

    Consideration is given to recent calculations of general relativistic effects in the beaming, spectrum, and pulse properties of accreting neutron stars. Some possible models for X-ray pulsars and QPOs are analyzed, which indicate that current observational and theoretical requirements can be explained with a value of the radius smaller than about two Schwarzschild radii. Concurrent information and calculations on several X-ray burster sources are compatible with this conclusion. 33 references.

  9. Excitons in nanoscale systems

    Microsoft Academic Search

    Gregory D. Scholes; Garry Rumbles

    2006-01-01

    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,

  10. Effect of haemoglobin oxygenation on Bohr proton release and CO 2 excretion in the rainbow trout

    Microsoft Academic Search

    C. J. Brauner; K. M. Gilmour; S. F. Perry

    1996-01-01

    CO2 excretion in trout is dependent upon the Haldane effect and the release of protons (Bohr protons) from haemoglobin during oxygenation of the blood. Oxygenation of whole blood from trout in vitro, resulted in a non-linear release of Bohr protons over the haemoglobin-oxygen (Hb?O2) equilibrium curve, where the majority of Bohr protons were released between 60 and 100% of Hb

  11. Exciton-polariton transition induced by elastic exciton-exciton collisions in ultrahigh quality AIGaAs alloys

    SciTech Connect

    Shamirzaev, T. S., E-mail: timur@thermo.isp.nsc.ru; Toropov, A. I.; Bakarov, A. K.; Zhuravlev, K. S. [Institute of Semiconductor Physics (Russian Federation); Kobitski, A. Yu.; Wagner, H. P.; Zahn, D. R. T. [Institut fuer Physik, TU Chemnitz (Germany)

    2006-05-15

    The stationary and time-resolved polariton radiation in ultrahigh quality AIGaAs layers have been studied. It has been found that elastic exciton-exciton collisions lead to the appearance of a low-energy line of polariton radiation. We show that the rate of exciton-to-polariton transitions caused by elastic exciton-exciton collisions is determined not only by the density of the excitonic gas, but also by its temperature; this is in accordance with existing theoretical predictions.

  12. Exciton linewidth due to fracton-exciton interaction

    NASA Astrophysics Data System (ADS)

    Wang, Xiao-Bing; Jiang, Qing; Tian, De-Cheng

    1994-08-01

    The contribution of the fracton-exciton interaction to the exciton linewidth is calculated. It is found that the linewidth is significantly dependent on the fractal dimension and the absorption process of a fracton by an exciton at the bottom of the band is forbidden for some dimensions. In the high temperature limit, the linewidth increases linearly with temperature. Possible relevance of the calculation to experiments is also discussed.

  13. Singlet exciton fission photovoltaics.

    PubMed

    Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

    2013-06-18

    Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses 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

  14. Bohr and Besicovitch almost periodic discrete sets and quasicrystals

    E-print Network

    Favorov, Sergey

    2010-01-01

    A discrete set $A$ in the Euclidian space is almost periodic if the measure with the unite masses at points of the set is almost periodic in the weak sense. We investigate properties of such sets in the case when $A-A$ is discrete. In particular, if $A$ is a Bohr almost periodic set, we prove that $A$ is a union of a finite number of translates of a certain full--rank lattice. If $A$ is a Besicovitch almost periodic set, then there exists a full-rank lattice such that in most cases a nonempty intersection of its translate with $A$ is large.

  15. An Increased Bohr Effect in Sickle Cell Anemia

    Microsoft Academic Search

    Yoshihiro Ueda; Ronald L. Nagel; Robert M. Bookchin

    1979-01-01

    Recent findings that hemoglobin S gelation was greatly increased only between blood and sickling are pH-dependent and also pH 7.4 and 7.2 (cell pH 7.2 and 7.0. a shift influence oxygen affinity suggested that that strongly affects gelation). with Llog the red cells containing this hemoglobin p50\\/ LpH -0.92 to -0.99 (normal variant might show an abnormal Bohr 0.42 to

  16. Bohr - Planck quantum theory, (Tesla) magnetic monopoles and fine structure constant

    E-print Network

    Vladan Pankovic; Darko V. Kapor; Stevica Djurovic; Miodrag Krmar

    2014-10-17

    In this work we apply Bohr-Planck (Old quantum atomic and radiation) theory, i.e. and quasi-classical methods for analysis of the magnetic monopoles and other problems. We reproduce exactly some basic elements of the Dirac magnetic monopoles theory, especially Dirac electric/magnetic charge quantization condition. Also, we suggest a new, effective, simply called Tesla model (for analogy with positions of the solenoids by Tesla inductive motor) of the magnetic monopole instead of usual effective Dirac model (half-infinite, very tinny solenoid) of the magnetic monopole. In our, i.e. Tesla model we use three equivalent tiny solenoids connected in series with a voltage source. One end of any solenoid is placed at the circumference of a circle and solenoids are directed radial toward circle center. Length of any solenoid is a bit smaller than finite circle radius so that other end of any solenoid is very close to the circle center. Angles between neighboring solenoids equal $120^{\\circ}$. All this implies that, practically, there is no magnetic field, or, magnetic pole, e.g. $S$, in the circle center, and that whole system holds only other, $N$ magnetic pole, at the ends of the solenoids at circle circumference. Finally, we reproduce relatively satisfactory value of the fine structure constant using Planck, i.e. Bose-Einstein statistics and Wien displacement law.

  17. Proton Radius Puzzle 1 Muonic hydrogen and the proton radius

    E-print Network

    Pachucki, Krzysztof

    Proton Radius Puzzle 1 Muonic hydrogen and the proton radius puzzle Randolf Pohl Max-681 Warsaw, Poland Key Words Laser Spectroscopy, Atomic Physics, Proton Structure, Exotic Atoms, Nuclear extraction of the proton radius by Pohl et al. from the mea- #12;Annu. Rev. Nucl. Part. Sci. 2013, Vol. 63

  18. Experimental Observation of Bohr's Nonlinear Fluidic Surface Oscillation

    E-print Network

    Songky Moon; Younghoon Shin; Hojeong Kwak; Juhee Yang; Sang-Bum Lee; Soyun Kim; Kyungwon An

    2015-04-20

    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 $0.41\\dot{6}\\eta^2$ for an octapolar component, nonlinearly induced by a quadrupolar one with a magnitude of $\\eta$ 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 $0.42\\pm0.08$ 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 $0.415\\pm0.010$. Our measurements establish the first experimental observation of Bohr's hydrodynamic theory.

  19. ChemTeacher: Atomic Radius

    NSDL National Science Digital Library

    2011-01-01

    ChemTeacher compiles background information, videos, articles, demonstrations, worksheets and activities for high school teachers to use in their classrooms. The Atomic Radius page includes resources for teaching students about trends in atomic radius.

  20. Prediction of the nature of hafnium from chemistry, Bohr's theory and quantum theory

    Microsoft Academic Search

    Eric R. Scerri

    1994-01-01

    The chemical nature of element 72, subsequently named hafnium, is generally regarded as a prediction from Bohr's theory of the periodic system and hence as a prediction from quantum theory. It is argued that both of these views and in particular the latter are mistaken. The claim in favour of Bohr's theory is weakened by his accommodation of independent chemical

  1. Exciton mechanism of radical formation in irradiated xenon matrixes containing methane

    SciTech Connect

    Grigor'ev, E.I.; Pshezhetskii, S.Ya.; Slavinskaya, N.A.; Trakhtenberg, L.I.

    1988-05-01

    A study of the laws governing the formation of methyl radicals from methane present in xenon matrixes at 77 K has been carried out for the cases of /gamma/-irradiation and bombardment of the matrix surface with metastable xenon atoms.The radicals are shown to be formed by the interaction of triplet xenon excitons with methane molecules. The exciton capture radius of the methane moleucle is 0.08 nm. The exciton lifetime in the surface layer is 2.3/centered dot/10/sup /minus/13/ sec, while within the matrix it is 5/centered dot/10/sup /minus/12/ sec. The radiation yield of xenon excitons is 0.8.

  2. 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.

  3. Experimental Observation of Bohr's Nonlinear Fluidic Surface Oscillation

    E-print Network

    Moon, Songky; Kwak, Hojeong; Yang, Juhee; Lee, Sang-Bum; Kim, Soyun; An, Kyungwon

    2015-01-01

    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 $0.41\\dot{6}\\eta^2$ for an octapolar component, nonlinearly induced by a quadrupolar one with a magnitude of $\\eta$ 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...

  4. Exciton mechanism for radical formation in a xenon matrix during bombardment of the surface with metastable xenon and argon atoms

    SciTech Connect

    Grigor'ev, E.I.; Pshezhetskii, S.Y.; Trakhtenberg, L.I.

    1985-07-01

    This article describes how the bombardment at 77 degrees K of the surface of a xenon matrix containing 1-10 mole % methane or propane with metastable argon or xenon atoms leads to the formation of methyl or propyl radicals. The results are explained on the basis of an exciton mechanism of radical formation. It was calculated that the radius for the capture of a triplet xenon exciton by a methane molecule is 0.25 nm.

  5. Momentum dependence of the excitons in pentacene

    SciTech Connect

    Roth, Friedrich; Schuster, Roman; Koenig, Andreas; Knupfer, Martin [IFW Dresden, P.O. Box 270116, D-01171 Dresden (Germany); Berger, Helmuth [Institute of Physics of Complex Matter, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne (Switzerland)

    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.

  6. Nomenclature R mean rotor radius

    E-print Network

    Belta, Calin A.

    Nomenclature R mean rotor radius A compressor duct area ; blade speed at mean radius a speed of sound B "(;/2a )(blade passage m exit duct length factor l # , l ' , l 2). Automatica 37 (2001) 921}931 Brief Paper Rotating stall control for axial #ow compressors Calin Belta

  7. Complementarity in the Einstein-Bohr Photon Box

    E-print Network

    Dennis Dieks; Sander Lam

    2007-05-18

    The photon box thought experiment can be considered a forerunner of the EPR-experiment: by performing suitable measurements on the box it is possible to ``prepare'' the photon, long after it has escaped, in either of two complementary states. Consistency requires that the corresponding box measurements be complementary as well. At first sight it seems, however, that these measurements can be jointly performed with arbitrary precision: they pertain to different systems (the center of mass of the box and an internal clock, respectively). But this is deceptive. As we show by explicit calculation, although the relevant quantities are simultaneously measurable, they develop non-vanishing commutators when calculated back to the time of escape of the photon. This justifies Bohr's qualitative arguments in a precise way; and it illustrates how the details of the dynamics conspire to guarantee the requirements of complementarity. In addition, our calculations exhibit a ``fine structure'' in the distribution of the uncertainties over the complementary quantities: depending on when the box measurement is performed, the resulting quantum description of the photon differs. This brings us close to the argumentation of the later EPR thought experiment.

  8. Complementarity in the Bohr-Einstein Photon Box

    E-print Network

    Dennis Dieks; Sander Lam

    2007-05-18

    The photon box thought experiment can be considered a forerunner of the EPR-experiment: by performing suitable measurements on the box it is possible to ``prepare'' the photon, long after it has escaped, in either of two complementary states. Consistency requires that the corresponding box measurements be complementary as well. At first sight it seems, however, that these measurements can be jointly performed with arbitrary precision: they pertain to different systems (the center of mass of the box and an internal clock, respectively). But this is deceptive. As we show by explicit calculation, although the relevant quantities are simultaneously measurable, they develop non-vanishing commutators when calculated back to the time of escape of the photon. This justifies Bohr's qualitative arguments in a precise way; and it illustrates how the details of the dynamics conspire to guarantee the requirements of complementarity. In addition, our calculations exhibit a ``fine structure'' in the distribution of the uncertainties over the complementary quantities: depending on \\textit{when} the box measurement is performed, the resulting quantum description of the photon differs. This brings us close to the argumentation of the later EPR thought experiment.

  9. Moving gapless indirect excitons in monolayer graphene

    PubMed Central

    2012-01-01

    The existence of moving indirect excitons in monolayer graphene is theoretically evidenced in the envelope-function approximation. The excitons are formed from electrons and holes near the opposite conic points. The electron-hole binding is conditioned by the trigonal warping of the electron spectrum. It is stated that the exciton exists in some sectors of the exciton momentum space and has the strong trigonal warping of the spectrum. PMID:23110813

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

    PubMed Central

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

    2015-01-01

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979

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

    PubMed

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

    2015-01-01

    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

  12. Excitonic ferromagnetism in the hexaborides

    NASA Astrophysics Data System (ADS)

    Zhitomirsky, Michael

    2000-03-01

    The unusual high-temperature weak ferromagnetism in doped hexaborides is explained as a spontaneous spin polarization in a novel ground state of a low-density electron-hole plasma -- a triplet excitonic insulator.footnote M. E. Zhitomirsky, T. M. Rice, and V. I. Anisimov, Nature 402, 251 (1999). The excitonic insulating state is a condensate of bound electron-hole pairs formed in a narrow-gap semiconductor or a weak semimetal. Homogeneous excitonic insulator polarizes under doping because of a coherent interaction of doped electrons with the condensate. The origin of small ferromagnetic moments is attributed to a time-reversal symmetry breaking transition into a nonunitary triplet state. Two mechanisms which can lead to the broken time-reversal symmetry and to ferromagnetism even in undoped excitonic insulator are also proposed. A possibility of gas-liquid type phase transition between a dilute excitonic gas and a dense electron-hole plasma for varying bandgap (band overlap) is investigated.footnote M. E. Zhitomirsky and T. M. Rice,

  13. Exciton formation and stability in semiconductor heterostructures

    Microsoft Academic Search

    S. Siggelkow; W. Hoyer; M. Kira; S. W. Koch

    2004-01-01

    The formation and stability of excitons in semiconductors is studied on the basis of a microscopic model that includes Coulomb interacting fermionic electrons and holes as well as phonons. Whereas quasiequilibrium calculations predict substantial exciton fractions coexisting with an electron-hole plasma at low temperatures and densities, dynamic calculations reveal that the exciton formation times under these conditions exceed the characteristic

  14. Aharonov-Bohm effect for an exciton

    Microsoft Academic Search

    R. A. Römer; M. E. Raikh

    2000-01-01

    We theoretically study exciton absorption on a ring threaded by a magnetic flux. For the case when the attraction between an electron and a hole is short ranged, we obtain an exact solution of the problem. We demonstrate that, despite the electrical neutrality of the exciton, both the spectral position of the exciton peak in the absorption and the corresponding

  15. Scaling of Excitons in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Perebeinos, Vasili; Tersoff, J.; Avouris, Phaedon

    2004-06-01

    Light emission from carbon nanotubes is expected to be dominated by excitonic recombination. Here we calculate the properties of excitons in nanotubes embedded in a dielectric, for a wide range of tube radii and dielectric environments. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength.

  16. 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.

  17. 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.

  18. Exciton mechanism for radical formation in a xenon matrix during bombardment of the surface with metastable xenon and argon atoms

    Microsoft Academic Search

    E. I. Grigorev; S. Y. Pshezhetskii; L. I. Trakhtenberg

    1985-01-01

    This article describes how the bombardment at 77 degrees K of the surface of a xenon matrix containing 1-10 mole % methane or propane with metastable argon or xenon atoms leads to the formation of methyl or propyl radicals. The results are explained on the basis of an exciton mechanism of radical formation. It was calculated that the radius for

  19. [Fractures of the distal radius].

    PubMed

    Rueger, J M; Hartel, M J; Ruecker, A H; Hoffmann, M

    2014-11-01

    The most prevalent fractures managed by trauma surgeons are those involving the distal radius. The injury occurs in two peaks of prevalence: the first peak around the age of 10 years and the second peak around the age of 60 years. Distal radius fracture management requires sensitive diagnostics and classification. The objectives of treatment are the reconstruction of a pain-free unlimited durable functioning of the wrist and avoidance of typical fracture complications. Non-operative conservative management is generally employed for stable non-displaced fractures of the distal radius with the expectation of a good functional outcome. Unstable comminuted fractures with intra-articular and extra-articular fragment zones are initially set in a closed operation and finally by osteosynthesis. An armament of surgical implants is available for instable fractures requiring fixation. Palmar locked plate osteosynthesis has been established in recent years as the gold standard for operative management of distal radius fractures. Complex Working Group on Osteosynthesis (AO) classification type 3 fractures require extensive preoperative diagnostics to identify and treat typical associated injuries around the wrist. PMID:25398510

  20. Conjugated “Molecular Wire” for Excitons

    SciTech Connect

    Shibano, Y.; Miller, J.; Imahori, H.; Sreearunothai, P.; Cook, A.R.

    2010-05-06

    We have synthesized new conjugated, rigid rod oligomers of fluorene, F{sub n}(C{sub 60}){sub 2}, n = 4, 8, 12, and 16. These pure compounds have F{sub n} chains up to 140 {angstrom} long. The C{sub 60} groups covalently attached at both ends serve as traps for excitons created in the F{sub n} chains. Excitons created in the chains by photoexcitation reacted rapidly with the C{sub 60} groups with decays described well by the sum of two exponentials. Mean reaction times were 2.3, 5.5, and 10.4 ps for n = 8, 12, and 16. In F{sub 16}(C{sub 60}){sub 2}, the 10.4 ps reaction time was 40 times faster than that found in earlier reports on molecules of slightly longer length. The simplest possible model, that of one-dimensional diffusion of excitonic polarons that react whenever they encounter the end of a chain, fits the results to obtain diffusion coefficients. Deviations of those fits from the data may point to the need for alternative pictures or may just indicate that diffusion is not ideal. The definite lengths of these molecules enable a stringent test for theories. These results reveal that exciton transport can be much faster than previously believed, a finding that could, along with appropriate nanoassembly, enable new kinds of high-efficiency organic photovoltaics.

  1. Hybridized quadrupole-dipole exciton effects in a Cu2O -organic heterostructure

    NASA Astrophysics Data System (ADS)

    Roslyak, Oleksiy; Birman, Joseph L.

    2007-06-01

    In the present work, we discuss resonant hybridization of the 1S quadrupole Wannier-Mott exciton (WE) in a Cu2O quantum well with the Frenkel dipole exciton in an adjacent layer of organic DCM2:CA:PA. The coupling between excitons is due to interaction between the gradient of electric field induced by the DCM2 Frenkel exciton (FE) and the quadrupole moment of the 1S transition in the cuprous oxide. The specific choice of the organic allows us to use the mechanism of “solid state solvation” [C. Madigan and V. Bulovic, Phys. Rev. Lett. 91, 247403 (2003)] to dynamically tune the WE and FE into resonance for ?3.3ns (comparable with the big lifetime of the WE) of the “slow” phase of the solvation. The quadrupole-dipole hybrid utilizes the big oscillator strength of the FE, along with the big lifetime of the quadrupole exciton, unlike dipole-dipole hybrid exciton which utilizes the big oscillator strength of the FE and big radius of the dipole allowed WE. Due to the strong spatial dispersion and big mass of the quadrupole WE, the hybridization is not masked by the kinetic energy or the radiative broadening. The lower branch of the hybrid dispersion exhibits a pronounced minimum and may be used in applications. Also, we investigate and report noticeable change in the coupling due to an induced “Stark effect” from the strong local electric field of the FE. We investigated the fine energy structure of the quantum well confined ortho and para excitons in cuprous oxide.

  2. EPR before EPR: a 1930 Einstein-Bohr thought experiment revisited

    E-print Network

    Nikolic, H

    2012-01-01

    In 1930 Einstein argued against consistency of the time-energy uncertainty relation by discussing a thought experiment involving a measurement of mass of the box which emitted a photon. Bohr seemingly triumphed over Einstein by arguing that the Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisit this thought experiment from a modern point of view and find that neither Einstein nor Bohr was right. Instead, this thought experiment should be thought of as an early example of a system demonstrating nonlocal "EPR" quantum correlations, five years before the famous Einstein-Podolsky-Rosen paper.

  3. Spatially indirect excitons in coupled quantum wells

    SciTech Connect

    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.

  4. Exciton-exciton scattering: Composite boson versus elementary boson

    NASA Astrophysics Data System (ADS)

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

    2007-05-01

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

  5. Electric arc radius and characteristics

    SciTech Connect

    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.

  6. 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…

  7. Why we should teach the Bohr model and how to teach it effectively

    NASA Astrophysics Data System (ADS)

    McKagan, S. B.; Perkins, K. K.; Wieman, C. E.

    2008-06-01

    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 to graduate school. We present results from a study designed to test this claim by developing a curriculum on models of the atom, including the Bohr and Schrödinger models. We examine student descriptions of atoms on final exams in transformed modern physics classes using various versions of this curriculum. We find that if the curriculum does not include sufficient connections between different models, many students still have a Bohr-like view of atoms rather than a more accurate Schrödinger model. However, with an improved curriculum designed to develop model-building skills and with better integration between different models, it is possible to get most students to describe atoms using the Schrödinger model. In comparing our results with previous research, we find that comparing and contrasting different models is a key feature of a curriculum that helps students move beyond the Bohr model and adopt Schrödinger’s view of the atom. We find that understanding the reasons for the development of models is much more difficult for students than understanding the features of the models. We also present interactive computer simulations designed to help students build models of the atom more effectively.

  8. Exciton-polariton emission from organic semiconductor optical waveguides

    NASA Astrophysics Data System (ADS)

    Ellenbogen, Tal; Crozier, Kenneth B.

    2011-10-01

    We photoexcite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Nonresonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symmetry. These leak to cones of light with radial and azimuthal polarizations.

  9. PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation.

    PubMed

    Murphy, James E; Beard, Matthew C; Norman, Andrew G; Ahrenkiel, S Phillip; Johnson, Justin C; Yu, Pingrong; Mi?i?, Olga I; Ellingson, Randy J; Nozik, Arthur J

    2006-03-15

    We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts. PMID:16522105

  10. Exciton-phonon interactions and exciton dephasing in semiconductor quantum-well heterostructures

    E-print Network

    Nabben, Reinhard

    Exciton-phonon interactions and exciton dephasing in semiconductor quantum-well heterostructures I semiconductor heterostructures have at- tracted considerable attention of experimentalists due to their unique and related exciton dephasing processes in monolayer semi- conductor heterostructures with localized quasi two

  11. Fission of singlet excitons into triplet-exciton pairs in molecular crystals

    Microsoft Academic Search

    M. Chabr; D. F. Williams

    1977-01-01

    A kinetic model of singlet-exciton fission into pairs of triplet excitons in aromatic hydrocarbon crystals is presented. The model is based on Suna's hopping model for triplet-exciton fusion. The assumptions made in this description of fission and the relationship between fusion and fission are analyzed in detail. According to this theoretical model, the magnetic field modulation of anthracene hot singlet

  12. Exciton Binding Energy of Monolayer WS2

    NASA Astrophysics Data System (ADS)

    Zhu, Bairen; Chen, Xi; Cui, Xiaodong

    2015-03-01

    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.

  13. Aharonov-Bohm effect for an exciton

    Microsoft Academic Search

    R. A. Romer; M. E. Raikh

    2000-01-01

    We study theoretically the exciton absorption on a ring shreded by a magnetic\\u000aflux. For the case when the attraction between electron and hole is\\u000ashort-ranged we get an exact solution of the problem. We demonstrate that,\\u000adespite the electrical neutrality of the exciton, both the spectral position of\\u000athe exciton peak in the absorption, and the corresponding oscillator strength

  14. Exciton Binding Energy of Monolayer WS2

    PubMed Central

    Zhu, Bairen; Chen, Xi; Cui, Xiaodong

    2015-01-01

    The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01?eV around K valley in the Brillouin zone. PMID:25783023

  15. Exciton dynamics and annihilation in WS2 2D semiconductors

    NASA Astrophysics Data System (ADS)

    Yuan, Long; Huang, Libai

    2015-04-01

    We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 +/- 37 ps, 401 +/- 25 ps, and 332 +/- 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 +/- 0.02, (6.00 +/- 1.09) × 10-3 and (1.88 +/- 0.47) × 10-3 cm2 s-1, respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons.We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 +/- 37 ps, 401 +/- 25 ps, and 332 +/- 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 +/- 0.02, (6.00 +/- 1.09) × 10-3 and (1.88 +/- 0.47) × 10-3 cm2 s-1, respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr00383k

  16. Extraordinary Exciton Conductance Induced by Strong Coupling

    NASA Astrophysics Data System (ADS)

    Feist, Johannes; Garcia-Vidal, Francisco J.

    2015-05-01

    We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role.

  17. 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.)).

  18. Extraordinary exciton conductance induced by strong coupling.

    PubMed

    Feist, Johannes; Garcia-Vidal, Francisco J

    2015-05-15

    We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role. PMID:26024185

  19. Thrombocytopenia-absent radius syndrome.

    PubMed

    Toriello, Helga V

    2011-09-01

    Thrombocytopenia-absent radius (TAR) syndrome is a relatively uncommon condition characterized by absent radii with the presence of thumbs and congenital or early-onset thrombocytopenia that tends to resolve in childhood. The precise cause of this condition is unknown, although recently a microdeletion of chromosome 1q21.1 has been found in all investigated individuals. However, this microdeletion alone is not sufficient to cause TAR syndrome, and another, uncharacterized genetic alteration is thought to be involved in the pathogenesis. PMID:22102274

  20. Diffusion of Singlet Excitons in Tetracene Crystals

    Microsoft Academic Search

    G. Vaubel; H. Baessler

    1970-01-01

    The diffusion length ls of singlet excitons in crystalline tetracene was measured in the temperature range 293 to 160 °K utilizing the quenching effect exerted on the crystal fluorescence by exciton traps located at the crystal surface. It is found that ls = 120 ± 10 Å at room temperature. It increases exponentially with decreasing temperature until it approaches a

  1. Cascade-exciton model of nuclear reactions

    Microsoft Academic Search

    K. K. Gudima; S. G. Mashnik; V. D. Toneev

    1983-01-01

    An approach combining essential features of the exciton and intranuclear cascade models is developed. The cascade-exciton model predictions for the energy spectra, angular distributions and double differential cross sections of nucleons and complex particles as well as for the excitation functions are analyzed at incident nucleon energies T0 <~ 100 MeV and in a large range of nuclear target masses.

  2. Josephson effects in condensates of excitons and exciton polaritons

    SciTech Connect

    Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G. [Physics Department, University of Iceland, Dunhaga-3, Reykjavik IS-107 (Iceland) and St. Petersburg State Polytechnical University, Polytechnicheskaya 29, St. Petersburg 195251 (Russian Federation); LASMEA, UMR CNRS-Universite Blaise Pascal 6602, 24 Avenue des Landais, Aubiere 63177 Cedex (France)

    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.

  3. Nanocrystal growth in alkali halides observed by exciton spectroscopy

    NASA Astrophysics Data System (ADS)

    Haselhoff, M.; Weber, H.-J.

    1998-08-01

    We have developed a method to study the growth of CuCl and CuBr nanocrystals in alkali halides by in situ absorption spectroscopy. Exciton lines are used as a signature of crystallinity and a broad absorption peak measures the number of Cu+ ions. The mean radius R and the concentration xcr of nanocrystals are determined simultaneously, which in turn enables the determination of the particle density N of crystals as a function of time. Neither the size-distribution function nor R(t) follow the prediction of the classical growth theory. xcr(t) reveals two different growth mechanisms, one for high and one for low growth temperatures. N(t) demonstrates that the growth of big crystals at the expense of small crystals happens from the beginning of the growth process. This result contradicts the traditional view of Ostwald ripening as a final stage of precipitation and stresses the dynamical character of cluster growth. The experimental data are described by a set of empirical parameters that can be used as a guide for controlled growth of CuCl nanocrystals in NaCl. The smallest nanocrystals detected by exciton spectroscopy consist of 50 unit cells. The kinetic properties of KCl and NaBr crystals doped with CuCl and CuBr are very similar to those observed in NaCl doped with CuCl.

  4. Nonlocality and the radius of the deuteron

    Microsoft Academic Search

    M. W. Kermode; S. A. Moszkowski; M. M. Mustafa; W. van Dijk

    1991-01-01

    We consider the effects of nonlocality on the determination of the deuteron radius for model potentials. We show that the relation between the model values for the radius and the scattering length which holds for local potentials is not necessarily valid for nonlocal potentials. It is possible for nonlocal potential models to produce a deuteron of smaller radius as required

  5. Thermodynamic efficiency limit of excitonic solar cells

    SciTech Connect

    Giebink, Noel C. [Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL (United States); Wiederrecht, Gary P. [Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL (United States); Wasielewski, Michael R. [Northwestern Univ., Evanston, IL (United States); Forrest, Stephen R. [Univ. of Michigan, Ann Arbor, MI (United States)

    2011-05-01

    Excitonic solar cells, comprised of materials such as organic semiconductors, inorganic colloidal quantum dots, and carbon nanotubes, are fundamentally different than crystalline, inorganic solar cells in that photogeneration of free charge occurs through intermediate, bound exciton states. Here, we show that the Second Law of Thermodynamics limits the maximum efficiency of excitonic solar cells below the maximum of 31% established by Shockley and Queisser [J. Appl. Phys. 32, 510 (1961)] for inorganic solar cells (whose exciton-binding energy is small). In the case of ideal heterojunction excitonic cells, the free energy for charge transfer at the interface, ?G, places an additional constraint on the limiting efficiency due to a fundamental increase in the recombination rate, with typical ??G in the range 0.3 to 0.5 eV decreasing the maximum efficiency to 27% and 22%, respectively.

  6. The structure and dynamics of molecular excitons.

    PubMed

    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

  7. 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.

  8. Exciton dynamics and annihilation in WS2 2D semiconductors.

    PubMed

    Yuan, Long; Huang, Libai

    2015-04-01

    We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10(-3) and (1.88 ± 0.47) × 10(-3) cm(2) s(-1), respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons. PMID:25826397

  9. High efficiency organic multilayer photodetectors based on singlet exciton fission

    Microsoft Academic Search

    J. Lee; P. Jadhav; M. A. Baldo

    2009-01-01

    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

  10. Long-range excitons in conjugated polymers with ring torsions

    E-print Network

    Harigaya, Kikuo

    Long-range excitons in conjugated polymers with ring torsions Kikuo Harigaya Physical Science) are investigated by the intermediate exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We nd that ring torsions a ect the long-range excitons

  11. Bohr's complementarity relation and the violation of the CP symmetry in high energy physics

    E-print Network

    Beatrix C. Hiesmayr; Marcus Huber

    2007-11-08

    We test Bohr's complementary relation, which captures the most counterintuitive difference of a classical and a quantum world, for single and bipartite neutral kaons. They present a system that is naturally interfering, oscillating and decaying. Moreover, kaons break the CP symmetry (C...charge conjugation, P...parity). In detail we discuss the effect of the CP violation on Bohr's relation, i.e. the effect on the "particle-like" information and the "wave-like" information. Further we show that the quantity that complements the single partite information for bipartite kaons is indeed concurrence, a measure of entanglement, strengthening our concept of entanglement. We find that the defined entanglement measure is independent of CP violation while it has been shown that nonlocality is sensitive to CP violation.

  12. Absorption Spectra and Refractive Index Changes of an Exciton in a Core/Shell Quantum Dot

    NASA Astrophysics Data System (ADS)

    Xie, Wen-Fang

    2015-05-01

    The absorption spectra and the refractive index changes are calculated theoretically for an exciton in a core/shell quantum dot. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The dimensionality effect of exciton quantum dots on the optical absorptions has been studied. It has been found that in the same regime, the optical absorption intensities of excitons are much smaller for the core/shell quantum dots than for the two-dimensional quantum rings. The linear and the nonlinear optical absorption coefficients and refractive index changes have been examined with the change of the confinement potential. The results show that the optical absorptions and the refractive index changes are strongly affected by the repulsive core of core/shell quantum dots. Moreover, the calculated results also reveal that as the inner radius increases, the peak values of the absorption coefficients and the refractive index changes of an exciton will show the optical Aharonov-Bohm oscillation in core/shell quantum dots.

  13. Excitonic superconductivity in copper oxides

    SciTech Connect

    Tesanovic, Z.; Bishop, A.R.; Martin, R.L.; Harris, C.

    1988-01-01

    We discuss the possibility of excitonic superconductivity in high T/sub c/ copper oxides. The Hamiltonians describing CuO/sub 2/ planes supports both antiferromagnetism and low-lying Cu /longleftrightarrow/ O intra- and interband charge fluctuations. One crosses from one regime to another as the number of holes per unit cell increases. The high T/sub c/ superconductivity takes place at hole concentrations most favorable for intraband charge transfer excitations. The dynamic polarizability of the environment surrounding CuO/sub 2/ planes plays an important role in enhancing T/sub c/. 15 refs., 4 figs.

  14. Bohr-Sommerfeld quantization condition for Dirac states derived from an Ermakov-type invariant

    SciTech Connect

    Thylwe, Karl-Erik [KTH-Mechanics, Royal Institute of Technology, S-10044 Stockholm (Sweden)] [KTH-Mechanics, Royal Institute of Technology, S-10044 Stockholm (Sweden); McCabe, Patrick [CCDC, 12 Union Road, CB2 1EZ Cambridge (United Kingdom)] [CCDC, 12 Union Road, CB2 1EZ Cambridge (United Kingdom)

    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.

  15. Discreteness of the volume of space from Bohr-Sommerfeld quantization.

    PubMed

    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

  16. Bohr Hamiltonian with a deformation-dependent mass term: physical meaning of the free parameter

    E-print Network

    Bonatsos, Dennis; Petrellis, D

    2015-01-01

    Embedding of the 5-dimensional (5D) space of the Bohr Hamiltonian with a deformation-dependent mass (DDM) into a 6-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) (gamma-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 ...

  17. Bohr Hamiltonian with a deformation-dependent mass term: physical meaning of the free parameter

    E-print Network

    Dennis Bonatsos; N. Minkov; D. Petrellis

    2015-06-24

    Embedding of the 5-dimensional (5D) space of the Bohr Hamiltonian with a deformation-dependent mass (DDM) into a 6-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) (gamma-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 deformation-dependent mass.

  18. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system.

    PubMed

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong

    2015-05-01

    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

  19. Aharonov-Bohm effect for an exciton

    NASA Astrophysics Data System (ADS)

    Römer, R. A.; Raikh, M. E.

    2000-09-01

    We theoretically study exciton absorption on a ring threaded by a magnetic flux. For the case when the attraction between an electron and a hole is short ranged, we obtain an exact solution of the problem. We demonstrate that, despite the electrical neutrality of the exciton, both the spectral position of the exciton peak in the absorption and the corresponding oscillator strength oscillate with magnetic flux with a period ?0-the universal flux quantum. The origin of the effect 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.

  20. Spontaneous Exciton Dissociation in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Kumamoto, Y.; Yoshida, M.; Ishii, A.; Yokoyama, A.; Shimada, T.; Kato, Y. K.

    2014-03-01

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies.

  1. Spontaneous exciton dissociation in carbon nanotubes.

    PubMed

    Kumamoto, Y; Yoshida, M; Ishii, A; Yokoyama, A; Shimada, T; Kato, Y K

    2014-03-21

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies. PMID:24702413

  2. Polynomial fits and the proton radius puzzle

    E-print Network

    E. Kraus; K. E. Mesick; A. White; R. Gilman; S. Strauch

    2014-10-27

    The Proton Radius Puzzle refers to the ~7{\\sigma} discrepancy that exists between the proton charge radius determined from muonic hydrogen and that determined from electronic hydrogen spectroscopy and electron-proton scattering. One possible partial resolution to the puzzle includes errors in the extraction of the proton radius from ep elastic scattering data. This possibility is made plausible by certain fits which extract a smaller proton radius from the scattering data consistent with that determined from muonic hydrogen. The reliability of some of these fits that yield a smaller proton radius was studied. We found that fits of form factor data with a truncated polynomial fit are unreliable and systematically give values for the proton radius that are too small. Additionally, a polynomial fit with a \\chi^2_{reduced} ~ 1 is not a sufficient indication for a reliable result.

  3. Exciton-exciton and exciton-phonon interactions in an interfacial GaAs quantum dot ensemble

    NASA Astrophysics Data System (ADS)

    Moody, G.; Siemens, M. E.; Bristow, A. D.; Dai, X.; Karaiskaj, D.; Bracker, A. S.; Gammon, D.; Cundiff, S. T.

    2011-03-01

    Using optical two-dimensional Fourier transform spectroscopy, we report temperature- and excitation-density-dependent measurements of the homogeneous linewidth of the exciton ground-state transition in a single layer of interfacial GaAs quantum dots (QDs). We show that the homogeneous linewidth increases nonlinearly with temperature from 6 to 50 K and that the thermal broadening is well described by an activation term and offset. The absence of a phonon-activation peak in the two-dimensional spectra reveals that elastic scattering of excitons with acoustic phonons via virtual transitions between the ground and excited states significantly contributes to the thermal broadening. We find that the combination of increasing virtual activation energy and exciton-phonon coupling strength with decreasing QD size results in greater thermal broadening for excitons localized in smaller QDs. The homogeneous linewidth also exhibits a strong excitation-density dependence and is shown to increase linearly as the photon density increases from 2×1011 to 1×1012 photons pulse-1 cm-2 at 6 K. This trend is attributed to strong coupling of excitons within the same QD and is independent of the quantum-well exciton population density.

  4. Dynamics of excitons and trions in semiconducting carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Okano, Makoto; Nishihara, Taishi; Yamada, Yasuhiro; Kanemitsu, Yoshihiko

    2013-03-01

    We report the optical properties and exciton dynamics of undoped and hole-doped single-walled carbon nanotubes (SWCNTs). In the one-dimensional structures of SWCNTs, an electron and a hole form an exciton with a huge exciton binding energy. Stable excitons govern the optical responses of SWCNTs even at room temperature. With hole doping of SWCNTs, new peaks due to positive trions (positively charged excitons) appear below the E11 exciton peaks in the absorption and photoluminescence spectra. Trions are also stable at room temperature. Using femtosecond pump-probe transient absorption spectroscopy, we revealed that the exciton decay dynamics depends on the number of holes in SWCNTs. The exciton lifetime of hole-doped SWCNTs is much shorter than that of undoped SWCNTs. Fast decay components with lifetimes of a few picoseconds are attributed to trion formation and exciton-hole scattering in holedoped SWCNTs.

  5. Excitonic Exchange Splitting and Radiative Lifetime in PbSe Quantum Dots

    SciTech Connect

    An, J. M.; Franceschetti, A.; Zunger, A.

    2007-01-01

    An exciton evolving from an m-fold degenerate hole level and an n-fold degenerate electron level has a nominal m x n degeneracy, which is often removed by electron-hole interactions. In PbSe quantum dots, the degeneracy of the lowest-energy exciton is m x n = 64 because both the valence-band maximum and the conduction-band minimum originate from the 4-fold degenerate (8-fold including spin) L valleys in the Brillouin zone of bulk PbSe. Using a many-particle configuration-interaction approach based on atomistic single-particle wave functions, we have computed the fine structure of the lowest-energy excitonic manifold of two nearly spherical PbSe quantum dots of radius R = 15.3 and 30.6 {angstrom}. We identify two main energy splittings, both of which are accessible to experimental probe: (i) The intervalley splitting is the energy difference between the two near-edge peaks of the absorption spectrum. We find {delta} = 80 meV for R = 15.3 {angstrom} and {delta} = 18 meV for R = 30.6 {angstrom}. (ii) The exchange splitting {Delta}{sub x} is the energy difference between the lowest-energy optically dark exciton state and the first optically bright exciton state. We find that {Delta}{sub x} ranges between 17 meV for R = 15.3 {angstrom}, and 2 meV for R = 30.6 {angstrom}. We also find that the room-temperature radiative lifetime is {tau}{sub R} {approx} 100 ns, considerably longer than the {approx}10 ns radiative lifetime of CdSe dots, in quantitative agreement with experiment.

  6. 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.

  7. 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.

  8. Cavity-enhanced transport of excitons.

    PubMed

    Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido

    2015-05-15

    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. PMID:26024186

  9. Excitonic Coherent States: Symmetries and Thermalization

    NASA Astrophysics Data System (ADS)

    Cirilo-Lombardo, Diego Julio

    2015-04-01

    In this paper we considered the theoretical treatment of a physical system of excitons and its behaviour under temperature by means of a new coherent state construction of bounded states in a quantum field theoretical context.

  10. Singlet exciton fission in solution.

    PubMed

    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

  11. 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.

  12. Anisotropic exciton Stark shift in black phosphorus

    NASA Astrophysics Data System (ADS)

    Chaves, A.; Low, Tony; Avouris, P.; ?ak?r, D.; Peeters, F. M.

    2015-04-01

    We calculate the excitonic spectrum of few-layer black phosphorus by direct diagonalization of the effective mass Hamiltonian in the presence of an applied in-plane electric field. The strong attractive interaction between electrons and holes in this system allows one to investigate the Stark effect up to very high ionizing fields, including also the excited states. Our results show that the band anisotropy in black phosphorus becomes evident in the direction-dependent field-induced polarizability of the exciton.

  13. 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.

  14. Treatment of osteoporotic distal radius fractures

    Microsoft Academic Search

    David Ring; Jesse B. Jupiter

    2005-01-01

    Fracture of distal radius is most commonly an injury of the fit osteoporotic patient. As the population and health of older individuals continue to expand, osteoporotic distal radius fractures will become increasingly common. While many older patients have limited functional demands and can accept some deformity and wrist dysfunction, others remain very active into older age and desire optimal wrist

  15. Solar Radius at Minimum of Cycle 23

    NASA Astrophysics Data System (ADS)

    Sigismondi, Costantino

    2008-09-01

    Observations of Baily beads in French Guyana, during 2006 September 22 annular eclipse, have been made to measure solar radius around solar minimum activity of cycle 23. The correction to standard solar radius at unit distance (1 AU) 959.63" to fit observations is ?R? = -0.01" ± 0.17". Sources of errors are outlined in view of relativistic accuracies.

  16. Orientable Strong Radius and Diameter of Hypercube

    Microsoft Academic Search

    Yung-Ling Lai; Jen-Hao Hsiao; Feng-Hsu Chiang

    For two vertices u, v in a strong digraph D, the strong distance sd(u, v) between u and v is the minimum size of a strong subdigraph of D containing u and v. The upper (lower) orientable strong radius SRAD(G) of a graph G is the maximum (minimum) strong radius over all strong orientations of G. The upper (lower) orientable

  17. Exciton-phonon interaction in CsPbCl3(1-x)I3x (x=0...1) crystals (in Ukrainian)

    NASA Astrophysics Data System (ADS)

    Kutsyk, M. V.; Myagkota, S. V.; Pidzyrailo, M. S.

    The reflection spectra near the absorption edge and the photoluminescence spectra of CsPbCl_{3(1-x)}I_{3x} solid solutions (x=0ldots1) in the temperature range of 4.2-80 K have been measured. There are exciton series of reflection bands for the crystals with x =0; 0.33 and 0.5 at 4.2 K (2.981; 2.987 and 2.989 eV for n=1 respectively). For the rest of the crystals (x =0.66; 1) the doublet structure takes place as a result of spin - orbit splitting (? E?35 meV). The exciton binding energies and the exciton radii as well as also the band gap energies and the dielectric constants of the crystals have been determined. The exciton radius tends to diminish when the iodine ion concentration increases (10.8 Å for CsPbCl_3 and 8.0 Å for CsPbI_3). As the exciton-phonon interaction becomes stronger, the electron excitation relaxation of the investigated solid solutions leads to the emission of free exciton as well as of the bound and self-trapped one. A nearly ideal regularity of localization of the Cl^- and I^- ions for CsPbCl_2I and CsPbClI_2 crystals was found.

  18. Interaction of Dirac Fermion excitons and biexciton-exciton cascade in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel

    2015-03-01

    We present a microscopic theory of interacting Dirac quasi-electrons and quasi-holes confined in graphene quantum dots. The single particle states of quantum dots are described using a tight binding model and screened direct, exchange, and scattering Coulomb matrix elements are computed using Slater pz orbitals. The many-body ground and excited states are expanded in a finite number of electron-hole pair excitations from the Hartree-Fock ground state and computed using exact diagonalization techniques. The resulting exciton and bi-exciton spectrum reflects the degeneracy of the top of the valence and bottom of the conduction band characteristic of graphene quantum dots with C3 symmetry. We study the interaction of multi-electron and hole complexes as a function of quantum dot size, shape and strength of Coulomb interactions. We identify two degenerate bright exciton (X) states and a corresponding biexciton (XX) state as XX-X cascade candidates, a source of entangled photon pairs. We next calculate the exciton to bi-exciton transitions detected in transient absorption experiments to extract the strength of exciton-exciton interactions and biexciton binding energies. We further explore the possibility of excitonic instability.

  19. Communicating the Heisenberg uncertainty relations: Niels Bohr, Complementarity and the Einstein-Rupp experiments

    E-print Network

    van Dongen, Jeroen

    2015-01-01

    The Einstein-Rupp experiments have been unduly neglected in the history of quantum mechanics. While this is to be explained by the fact that Emil Rupp was later exposed as a fraud and had fabricated the results, it is not justified, due to the importance attached to the experiments at the time. This paper discusses Rupp's fraud, the relation between Albert Einstein and Rupp, and the Einstein-Rupp experiments, and argues that these experiments were an influence on Niels Bohr's development of complementarity and Werner Heisenberg's formulation of the uncertainty relations.

  20. 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.

  1. Communicating the Heisenberg uncertainty relations: Niels Bohr, Complementarity and the Einstein-Rupp experiments

    E-print Network

    Jeroen van Dongen

    2015-05-29

    The Einstein-Rupp experiments have been unduly neglected in the history of quantum mechanics. While this is to be explained by the fact that Emil Rupp was later exposed as a fraud and had fabricated the results, it is not justified, due to the importance attached to the experiments at the time. This paper discusses Rupp's fraud, the relation between Albert Einstein and Rupp, and the Einstein-Rupp experiments, and argues that these experiments were an influence on Niels Bohr's development of complementarity and Werner Heisenberg's formulation of the uncertainty relations.

  2. 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.

  3. Preservers of spectral radius, numerical radius, or spectral norm of the sum on nonnegative matrices

    E-print Network

    Li, Chi-Kwong

    Preservers of spectral radius, numerical radius, or spectral norm of the sum on nonnegative, or the spectral norm. In the case of the numerical radius, a full description of preservers of the sum is also obtained, but in this case it turns out that the standard forms do not describe all such preservers. Key

  4. Intracavity optical pumping of J-aggregate microcavity exciton polaritons

    E-print Network

    Bulovic, Vladimir

    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 ...

  5. Exciton annihilation studies in poly(p-phenylene vinylene)

    SciTech Connect

    Valencia, V.S.; Kepler, R.G.; Jacobs, S.J.; Beeson, P.M. [Sandia National Labs., Albuquerque, NM (United States); Allemond, P.M. [Donnelly Corp., Tucson, AZ (United States)

    1995-08-01

    To verify the excitonic nature of the light-emitting state in PPV, fluorescence intensities and decay lifetimes were investigated as a function of excitation intensity. The results agree with the behavior predicted by the molecular exciton model. In particular, exciton-exciton annihilation causes the fluorescence intensity to saturate and the fluorescence lifetime to shorten at high exciton densities. In addition, the exciton annihilation, and thus diffusion, coefficients are found to be relatively large, even at low temperatures, indicating that exciton migration is important in PPV. These results indicate that the fluorescent (photoluminescent) state in PPV is excitonic in nature. The results argue against the band model where high mobility at reduced temperatures is not expected because the light-emitting species, neutral bipolarons, are associated with large lattice distortions.

  6. The theory of the Bohr-Weisskopf effect in the hyperfine structure

    E-print Network

    F. F. Karpeshin; M. B. Trzhaskovskaya

    2015-03-21

    For twenty years research into the anomalies in the HF spectra was going in a wrong direction by fighting the related Bohr-Weisskopf effect. As a way out, the model-independent way is proposed of estimating the nuclear radii from the hyper-fine splitting. 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. This makes transparent It is shown that the parameters which can be extracted from the data are the even nuclear moments of the magnetization distribution. The radii $R_2$ and (for the first time) $R_4$ are thus obtained by analysis of the experimental HFS for the H- and Li-like ions of $^{209}$Bi. The critical prediction of the HFS for the $2p_{1/2}$ state is discussed. The moments may be determined in this way only if the higher QED effects are properly taken into account. Therefore, this set of the parameters form a basis of a strict QED test. Experimental prospects are discussed, aimed at retrieving data on the HFS values for a set of a few-electron configurations of the atom.

  7. Can Equivalence Principle be consistent with the Bohr-Somerfeld-Hansson Theory of the Newtonian Gravity

    E-print Network

    Vladan Pankovic; Darko V. Kapor

    2010-09-10

    In this work we consider some consequences of the Bohr-Sommerfeld-Hansson (Old or quasi-classical) quantum theory of the Newtonian gravity, i.e. of the "gravitational atom". We prove that in this case (for gravitational central force and quantized angular momentum) centrifugal acceleration becomes formally-theoretically dependent (proportional to fourth degree) of the mass of "gravitational electron" rotating around "gravitational nucleus" for any quantum number (state). It seemingly leads toward a paradoxical breaking of the relativistic equivalence principle which contradicts to real experimental data. We demonstrate that this equivalence principle breaking does not really appear in the (quasi classical) quantum theory, but that it necessary appears only in a hypothetical extension of the quantum theory that needs a classical like interpretation of the Bohr-Sommerfeld angular momentum quantization postulate. It is, in some sense, similar to Bell-Aspect analysis that points out that a hypothetical deterministic extension of the quantum mechanics, in distinction to usual quantum mechanics, can reproduce experimental data if and only if it is non-local (superluminal) in contradiction with relativistic locality (luminality) principle.

  8. Exactly separable version of the Bohr Hamiltonian with the Davidson potential

    SciTech Connect

    Bonatsos, Dennis; Lenis, D.; Petrellis, D. [Institute of Nuclear Physics, N.C.S.R. 'Demokritos', GR-15310 Aghia Paraskevi, Attiki (Greece); McCutchan, E. A.; Casten, R. F. [Wright Nuclear Structure Laboratory, Yale University, New Haven, Connecticut 06520-8124 (United States); Minkov, N.; Yotov, P. [Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigrad Road, 1784 Sofia (Bulgaria); Yigitoglu, I. [Hasan Ali Yucel Faculty of Education, Istanbul University, TR-34470 Beyazit, Istanbul (Turkey)

    2007-12-15

    An exactly separable version of the Bohr Hamiltonian is developed using a potential of the form u({beta})+u({gamma})/{beta}{sup 2}, with the Davidson potential u({beta})={beta}{sup 2}+{beta}{sub 0}{sup 4}/{beta}{sup 2} (where {beta}{sub 0} is the position of the minimum) and a stiff harmonic oscillator for u({gamma}) centered at {gamma}=0 deg. In the resulting solution, called the exactly separable Davidson (ES-D) solution, the ground-state, {gamma}, and 0{sub 2}{sup +} bands are all treated on an equal footing. The bandheads, energy spacings within bands, and a number of interband and intraband B(E2) transition rates are well reproduced for almost all well-deformed rare-earth and actinide nuclei using two parameters ({beta}{sub 0},{gamma} stiffness). Insights are also obtained regarding the recently found correlation between {gamma} stiffness and the {gamma}-bandhead energy, as well as the long-standing problem of producing a level scheme with interacting boson approximation SU(3) degeneracies from the Bohr Hamiltonian.

  9. 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.

  10. Excitons in organic molecular nanostructures: Physics and applications

    Microsoft Academic Search

    Zilan Shen

    1997-01-01

    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

  11. The role of excitons and interfaces in molecular organic devices

    Microsoft Academic Search

    Vladimir Bulovic

    1998-01-01

    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

  12. Hybrid excitons in organic-inorganic semiconducting quantum dots

    Microsoft Academic Search

    A. Engelmann; V. I. Yudson; P. Reineker

    1998-01-01

    We investigate the nonlinear optical susceptibility and the radiative decay rate of a new excitonic state in organic-inorganic heterostructures. These hybrid excitons could appear due to resonant mixing of Wannier-Mott and Frenkel excitons. The aim of our investigation is to examine the nonlinear optical properties of this new hybrid state for the quantum dot geometry. We will show that the

  13. Singlet exciton fission in pure and doped anthracene

    Microsoft Academic Search

    K. von Burg; I. Zschokke-Gränacher

    1979-01-01

    On the basis of the kinetic model by Merrifield for triplet exciton fusion in molecular crystals, we have derived an expression which describes the reciprocal process, i.e., the anisotropy of the prompt fluorescence in the presence of singlet exciton fission in a magnetic field. For the first time a thorough comparison of Merrifield's theory with singlet exciton fission experiments is

  14. Singlet exciton fission in pure and doped anthracene

    Microsoft Academic Search

    K. von Burg

    1979-01-01

    On the basis of the kinetic model by Merrifield for triplet exciton fusion in molecular crystals, we have derived an expression which describes the reciprocal process, i.e., the anisotropy of the prompt fluorescence in the presence of singlet exciton fission in a magnetic field. For the first time a thorough comparison of Merrifield’s theory with singlet exciton fission experiments is

  15. Mass-radius relationships in icy satellites

    NASA Technical Reports Server (NTRS)

    Lupo, M. J.; Lewis, J. S.

    1979-01-01

    Using published laboratory data for H2O ice, a modeling technique was developed by which the bulk density, density and temperature profile, rotational moment of inertia, central pressure, and location of the rock-ice interface can all be obtained as a function of the radius, the heliocentric distance, and the silicate composition. Models of the interiors of Callisto, Ganymede, Europa, Rhea, and Titan are given, consistent with present mass and radius data. The radius and mass of spheres of ice under self-gravitation for two different temperature classes are given (103 and 77 deg K). Measurements of mass, radius and I/MR2 by spacecraft can be interpreted by this model to yield substantial information about the internal structure and the ice/rock ratio of the icy satellites of Jupiter and Saturn.

  16. The escape of pedestrians with view radius

    NASA Astrophysics Data System (ADS)

    Ma, Peijie; Wang, Binghong

    2013-01-01

    In this brief letter, we modify the classic social force model of Helbing which is applied to simulate how a pedestrian gets outside a hall full of smoke. As the Vicsek model does, the view radius is introduced to describe the range the pedestrian can see. The relation between the evacuation time and the view radius is studied with different numbers of pedestrians. The results show that the shorter the view radius is, the more time walkers will spend escaping, and even fail to escape. And the relation between the number of remaining walkers and the view radius shows non-monotonicity, if the number of pedestrians is larger than 600. And lastly, we propose to enlarge the width of the exit or to add two small exits in the corners, which may decrease the evacuation time greatly and obviously reduce the number of remaining walkers.

  17. 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 ? ? 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.

  18. Luminescence properties of II\\/VI semiconductor colloidal nanocrystals at collective and single scales

    Microsoft Academic Search

    Céline Vion; Carlos Barthou; Laurent Coolen; Paul Bennaloul; Vu Duc Chinh; Pham Thuy Linh; Vu Thi Bich; Pham Thu Nga; Agnès Maître

    2009-01-01

    Colloidal nanocrystals are crystalline spheres of semiconductors of a few nanometers, obtained by chemical synthesis. At this size scale, lower than Bohr radius of the exciton, emission properties are dominated by quantum confinement effects and depend crucially on the nanocrystal radius, which can be controlled by adjusting the synthesis parameters. Nanocrystals present high photostability and good quantum efficiency, even at

  19. Inside the Bondi radius of M87

    E-print Network

    Russell, H R; McNamara, B R; Broderick, A E

    2015-01-01

    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, 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 PSF, 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 keV 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 $\\rho\\propto r^{-1}$. 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...

  20. Excitonic effects in the Casimir force: A-exciton in CdS

    NASA Astrophysics Data System (ADS)

    Hernández de la Luz, A. D.; Alvarado-García, A. F.; Cocoletzi, Gregorio H.; Esquivel-Sirvent, R.

    2004-12-01

    Calculations of the Casimir force are performed between parallel nonlocal excitonic semiconductor slabs. Using a frequency and wave vector dependent dielectric response we describe the An=1 excitonic transition in CdS and calculate the electromagnetic waves reflectivity in the free space between the slabs. The results of the normal incidence reflectivity coefficients are then employed to determine the Casimir forces, which are investigated as functions of the layer thickness and the vacuum gap width between slabs. Comparisons with the local case show that the excitons are important as they produce a change of the Casimir force within current experimental precision.

  1. 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.

  2. Excitons confined by split-gate potentials

    NASA Astrophysics Data System (ADS)

    Cocoletzi, Gregorio H.; Ulloa, Sergio E.

    1994-03-01

    Quasi-one-dimensional excitons in a GaAs-AlxGa1-xAs quantum well are studied; they are produced by an applied twin-split-gate potential which confines the particles laterally and allows free motion in one dimension. A variational approach is used to calculate the binding energies Eex and oscillator strength fex of these excitonic transitions as functions of the applied voltage and width of the induced potential wells. In the limit of high electrostatic confinement the excitons are strongly polarized and the system resembles a type II structure where electron and hole are spatially separated. The resulting Eex and fex show a strong dependence on applied voltage and structure width. Strong oscillations are found, which should be observed experimentally, as a consequence of subtle competition between confinement and Coulomb attraction.

  3. Spontaneous exciton dissociation in carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Yoshida, Masahiro; Kumamoto, Yusuke; Ishii, Akihiro; Yokoyama, Akio; Shimada, Takashi; Kato, Yuichiro K.

    2014-03-01

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs.[2] A simple model is constructed to consistently describe the excitation power and voltage dependence of the photoluminescence and photocurrent. Using this model, we find that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons for photoconductivity measurements in single-walled carbon nanotubes being straightforward despite the large exciton binding energies. Work supported by KAKENHI, SCOPE, KDDI Foundation, and the Photon Frontier Network Program of MEXT, Japan.

  4. Resonant Transfer of Excitons and Quantum Computation

    E-print Network

    Lovett, B; Nazir, A; Kothari, B; Briggs, A; Lovett, Brendon; Reina, John H.; Nazir, Ahsan; Kothari, Beeneet; Briggs, Andrew

    2003-01-01

    The excitation-energy transfer--the so-called Forster resonant energy transfer--plays a key role in light harvesting processes in photosynthetic organisms in nature. Here we give two methods for performing quantum logic operations by tailoring this interaction. The first implementation uses a coupled quantum dot molecule where the exciton-exciton interaction and the Forster coupling are controlled by means of the dot size, interdot separation, material composition, confinement potential and applied electric field to obtain high fidelity logic. The second proposes the use of biological systems for embodying qubits where, as a result of a stronger Forster interaction, extended exciton states are expected. These states are likely to be more immune to decoherence.

  5. Bright and dark excitons in semiconductor carbon nanotubes

    SciTech Connect

    Tretiak, Sergei [Los Alamos National Laboratory

    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.

  6. 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.

  7. 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 ?s, which explains the delayed buildup of a large photocurrent that has been reported earlier for low excitation densities.

  8. Heat pumping with optically driven excitons

    E-print Network

    Erik M. Gauger; Joachim Wabnig

    2010-06-07

    We present a theoretical study showing that an optically driven excitonic two-level system in a solid state environment acts as a heat pump by means of repeated phonon emission or absorption events. We derive a master equation for the combined phonon bath and two-level system dynamics and analyze the direction and rate of energy transfer as a function of the externally accessible driving parameters. We discover that if the driving laser is detuned from the exciton transition, cooling the phonon environment becomes possible.

  9. Effect of fractons on the exciton dynamics in dilute magnets

    NASA Astrophysics Data System (ADS)

    Wang, Xiao-Bing; Tian, De-Cheng; Jiang, Qing; Zhang, Zhe-Hua

    1995-02-01

    The effect of fractons on the exciton dynamics in dilute magnets is investigated theoretically. It is predicted that the total intensity of the hot exciton-fracton absorption band varies with temperature as Td¯+2d¯/D, where d¯ is the fracton dimension and D the fractal dimension. Also, the fracton-exciton interaction results in a quadratic temperature dependence of the exciton intersublattice relaxation rate in dilute antiferromagnets, substantially different from that (cubic temperature dependence) of magnon-exciton scattering. Implication of the results for experimental study of the magnon-fracton crossover in dilute magnets is demonstrated.

  10. Diffusive Insights: On the Disagreement of Christian Bohr and August Krogh at the Centennial of the Seven Little Devils

    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…

  11. Excitons in Cuprous Oxide: Photoionization and Other Multiphoton Processes

    NASA Astrophysics Data System (ADS)

    Frazer, Nicholas Laszlo

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

  12. Promontory of radius: a new anatomical description on the distal radius

    Microsoft Academic Search

    G. Windisch; H. Clement; K. Tanzer; G. Feigl; W. Grechenig; F. Anderhuber; W. Pichler

    2007-01-01

    Surgical treatment of distal radius fractures with palmar plates has gained popularity as the preferred approach to achieve\\u000a anatomical fracture reposition. One hundred and thirty four radii of human cadavers were examined to elucidate the anatomy\\u000a of the distal radius, especially the transition of the anterior into the lateral surface and a new term was given: promontory\\u000a of radius. The

  13. Spatiotemporal dynamics of excitons in monolayer and bulk WS2

    NASA Astrophysics Data System (ADS)

    He, Jiaqi; He, Dawei; Wang, Yongsheng; Cui, Qiannan; Ceballos, Frank; Zhao, Hui

    2015-05-01

    Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 +/- 1 and 110 +/- 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 +/- 20 and 3.5 +/- 0.5 cm2 s-1 are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications.

  14. 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.

  15. Dynamics of the excitonic coupling in organic crystals.

    PubMed

    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

  16. Spatiotemporal dynamics of excitons in monolayer and bulk WS2.

    PubMed

    He, Jiaqi; He, Dawei; Wang, Yongsheng; Cui, Qiannan; Ceballos, Frank; Zhao, Hui

    2015-05-21

    Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 ± 1 and 110 ± 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 ± 20 and 3.5 ± 0.5 cm(2) s(-1) are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications. PMID:25947347

  17. Vibrational and rotational excited states within a Bohr Hamiltonian with a deformation-dependent mass formalism

    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.

  18. Multiple Exciton Interaction in Condensates of Excitons at a Stacking Fault Interface in Bii 3

    Microsoft Academic Search

    A. Tanji; I. Akai; T. Karasawa

    2002-01-01

    Photoinduced high-density phase of exciton masses excited at a two-dimensional stacking fault interface in BiI 3 has been studied by measuring the energy spectra of the degenerate four-wave-mixing (DFWM) signals and their excitation spectra using a nano-second laser with a very narrow line width. An energy splitting structure on the'DFWM spectra shows a repulsive interaction between excitons. Furthermore, multiple fine

  19. Image excitons and plasmon-exciton strong coupling in two-dimensional perovskite semiconductors

    E-print Network

    Niu, Wendy; Ibbotson, Lindsey A.; Leipold, David; Runge, Erich; Prakash, G. Vijaya; Baumberg, Jeremy J.

    2015-04-10

    in ethylene tetrafluoroethylene (ETFE) from nanopatterned silicon stamps using nanoimprinting. An optically opaque Ag layer (?120 nm thick) is deposited onto the polymer to form metal gratings. Chemically synthe- sised CHPI powder12 is dissolved... ) and grey dashed lines (‘plasmonic’ grating modes) are guides, and exciton modes indicated by black arrows. The exciton splitting increases with ?. where km is the wavevector of the measured grating mode, and ki is the wavevector of the incident light taking...

  20. Molecular Basis of the Bohr Effect in Arthropod Hemocyanin*S?

    PubMed Central

    Hirota, Shun; Kawahara, Takumi; Beltramini, Mariano; Di Muro, Paolo; Magliozzo, Richard S.; Peisach, Jack; Powers, Linda S.; Tanaka, Naoki; Nagao, Satoshi; Bubacco, Luigi

    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 (Blackburn, N. J., Strange, R. W., Reedijk, J., Volbeda, A., Farooq, A., Karlin, K. D., and Zubieta, J. (1989) Inorg. Chem.,28 ,1349 -1357). 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. PMID:18725416

  1. Diffusive insights: on the disagreement of Christian Bohr and August Krogh at the Centennial of the Seven Little Devils

    NSDL National Science Digital Library

    2010-10-01

    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 substitute to secretion. The subjects of the present reevaluation of this controversy are Christian Bohr, Professor and Doctor of Medicine (1855�1911), nominated three times for the Nobel Prize; August Krogh, Doctor of Philosophy (1874�1949), Christian Bohr's assistant and later Nobel Prize laureate (1920); and Marie Krogh, née Jørgensen, Doctor of Medicine and wife of August Krogh (1874�1943). The controversy concerned is the transport of oxygen from the lungs into the bloodstream: are passive transport and diffusion capacity together sufficient to secure the oxygen supply in all circumstances or is there an additional specific (�energy consuming� or �active�) mechanism responsible for the transport of oxygen from the alveoli into the bloodstream? The present discussion purports to show that the contestants' views were closer than the parties themselves and posterity recognized. Posterity has judged the dispute unilaterally from the Nobel laureate's point of view, but it is evident that August Krogh's Nobel Prize was awarded for the discovery of a cellular activity (Christian Bohr's expression), represented by Krogh's discovery of capillary recruitment. Christian Bohr appears to have been correct in the narrower sense that the diffusion capacity at rest is not great enough to explain the transport during work; a special mechanism intervenes and optimizes the conditions under which diffusion acts. August Krogh, of course, was right in the wider sense that the transport mechanism itself is always entirely passive.

  2. Tailoring quantum dot assemblies to extend exciton coherence times and improve exciton transport

    NASA Astrophysics Data System (ADS)

    Seward, Kenton T.

    Electron energy transfer (EET) through nanostructured assemblies plays a crucial role in a wide range of emerging technologies such as quantum dot solar cells, quantum computing, molecular electronics, excitonic transistors, and light emitting diodes. These technologies are very dependent on excitonic lifetimes which are short on the order of a nanosecond. In order to efficiently use this short time scale, EET needs to be as fast as possible. This leads to an interest in the application of coherent exciton transfer. To examine the possibility of coherent transfer, we ask a simple question: How rapidly do coherent superpositions of excitonic states dephase between quantum dots?. We assume that the major source of decoherence at room temperature is from the internal phonon modes of silicon quantum dots. The question is then addressed using a combination of ab initio calculations and a master equation formulation for the evolution of the electronic density operator for a dimer of interacting two-level systems coupled to a shared bath of harmonic oscillators. A combination of density functional theory (DFT) and frozen phonon method (FPM) analysis was used to obtain exciton-phonon coupling in various sizes of silicon quantum dots. As expected, coherent EET is faster in comparison to incoherent EET in assemblies of identical nanostructures. In cases of non-identical assemblies of nanostructures, the low energy regions act as trap states for the exciton. In this case, a combination of coherent and incoherent transport leads to the fastest transport rate.

  3. A Maximum Radius for Habitable Planets.

    PubMed

    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

  4. Singlet exciton fission in a hexacene derivative.

    PubMed

    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

  5. Distal radius-ulna fractures in children.

    PubMed

    Pannu, Gurpal S; Herman, Marty

    2015-04-01

    Fractures involving the distal radius and ulna are commonly seen in children and adolescents. Management of these injuries in pediatric patients should include assessment of the neurovascular status of the extremity, associated soft-tissue injury, and, most importantly, possible involvement of the physes of the radius and ulna. Treatment of these injuries may vary from simple casting and radiographic follow-up to urgent reduction and surgical fixation. Regardless of the initial treatment plan, the treating surgeon must remain aware of the potential for both early and late complications that may affect outcomes. PMID:25771318

  6. Many body exciton due to Fano resonance in graphene

    NASA Astrophysics Data System (ADS)

    Yadav, Premlata; Ghosh, Subhasis

    2015-06-01

    The excitonic effect is thought to be generally unimportant in zero gap systems (at K point of the Brillouin zone) like monolayer graphene, but excitonic transition in graphene at the saddle point (M) of Brillouin zone has received increasing attentions. There are two important issues with excitons in graphene. Firstly: in contrast to excitonic transitions in semiconductors the line shape of excitonic peaks in graphene is asymmetric which is due to Fano resonance, a many body coupling between discrete excitonic state and continuous band states, Secondly due to many body effects the excitonic peak is sensitive to dielectric environment. Hence it is desirable to vary the dielectric environment of graphene without varying carrier concentration. To investigate this completely new method for obtaining graphene monolayer has been developed using chemical exfoliation technique. We show that there is shift in excitonic peak position with change in the dielectric environment of graphene and this has been achieved by varying Fermi velocity without varying the carrier concentration. The observed distinctive effect is decrease in exciton binding energy with increase in dielectric value of exfoliating solvents, resulting into a scaling relation between the dielectric environment and the exciton binding energy of graphene.

  7. Relaxation of excitons in semimagnetic asymmetric double quantum wells

    SciTech Connect

    Zaitsev, S. V., E-mail: szaitsev@issp.ac.ru; Brichkin, A. S.; Dorozhkin, P. S. [Russian Academy of Sciences, Institute of Solid State Physics (Russian Federation); Bacher, G. [Universitaet Duisburg-Essen, Lehrstuhl Werkstoffe der Electrotechnik (Germany)

    2008-07-15

    The steady-state circular-polarized photoluminescence in semimagnetic asymmetric double quantum wells based on Cd(Mn,Mg)Te is studied thoroughly in relation to the polarization of intrawell nonresonance photoexcitation in magnetic fields Bup to 9 T. In low fields B, in which the exciton in the magnetic well is higher in energy than the exciton in the nonmagnetic well, the complete interwell relaxation of excitons is observed. In fields higher than B{sub c} = 3-6 T, at which the exciton level in the magnetic well crosses the field-independent exciton level in the nonmagnetic well, the magnetic-field-induced red shift of the exciton in the magnetic well is accompanied by the establishment of a nonequilibrium distribution of excitons. This suggests that spin relaxation plays an important part in the interwell separation of excitons in the spin-dependent potential of the heterostructure. The efficiency of spin relaxation is controlled by mixing of valence band states in the nonmagnetic well and by splitting of heavy and light holes {delta}{sub hh-lh}. Different modes of interwell tunneling are observed in different field regions separated by the field B{sub c}* > B{sub c} corresponding to the crossing of the localized excitons in the nonmagnetic well and free excitons in the magnetic well. Possible mechanisms of interwell tunnel relaxation are discussed.

  8. 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.

  9. Mass radius scaling near the Chandrasekhar Limit

    Microsoft Academic Search

    Sayan Chakraborti

    2007-01-01

    The mass radius relationship of white dwarfs, near the Chandrasekhar Limit, is derived for a toy model of uniform density, using the variational principle. A power law scaling, reminiscent of those found in 2nd order phase transitions, is obtained. The derived exponent is shown to explain the relationship obtained by numerically integrating the TOV equations with the equation of state

  10. Distal Radius Fractures: Choice of Treatment Procedures

    Microsoft Academic Search

    S. Pechlaner

    2003-01-01

    Summary BACKGROUND: Distal radius fractures are no longer injuries typical of elderly patients with osteoporosis. Driving at high speed and high-speed sports activities are responsible for causing such injuries in an increasing number of younger people. Although the kind of fracture suffered, bone density, and personal needs and requirements of patients demand an individualized treatment concept, a surgical approach for

  11. Functional outcome after centralization for radius dysplasia

    Microsoft Academic Search

    Charles A. Goldfarb; Steven J. Klepps; Loray A. Dailey; Paul R. Manske

    2002-01-01

    Centralization for radius dysplasia purportedly offers a more normal appearance, provides length to a shortened forearm, and improves upper-extremity function. Limited objective outcome data, however, exist to substantiate its use. To better define functional status after centralization, the Jebsen-Taylor hand test and the Disabilities of the Arm, Shoulder, and Hand questionnaire (DASH) were administered to 21 patients (25 wrists) at

  12. On solar radius measurements with PICARD

    NASA Astrophysics Data System (ADS)

    Meftah, M.; Irbah, A.; Hauchecorne, A.; Corbard, T.; Hochedez, J. F.

    2014-12-01

    Solar diameter measurements performed from the ground for several decades seem to indicate a relation between the solar diameter and the solar activity. If this relationship is confirmed, it would be possible to use measurements of solar diameter as a proxy of solar activity in the past since the 1715 solar eclipses, and to use this input for the reconstruction of solar irradiance in climate models. However the interpretation of ground observations is controversial, ground-based measurements being affected by refraction, by atmospheric turbulence, and perhaps by atmospheric aerosols scattering. The only way to be free from atmospheric effects is to measure from space. This is the reason why, since the beginning, the PICARD program included a space and a ground component set up at the Calern site of the Observatoire de la Côte dAzur. During the last 4 years, the PICARD space mission has been used for observing the apparent solar diameter. First results of the astrometry program include a study of the June 2012 Venus transit for solar diameter determination. From this, the value of the solar radius from one astronomical unit was found to be equal to 959.86 arc-seconds at 607.1 nm. However, concerning observed variations in time of the solar radius, instrumental effects affect the results. Space is known to represent a harsh environment for optical instruments. Nevertheless, we can use the PICARD data to monitor the solar radius variation. PICARD aims to perpetuate historical series of the solar radius measurements, in particular during the solar cycle 24. This paper presents solar radius measurements obtained with PICARD.

  13. Optical response of localized excitons in layered systems.

    NASA Astrophysics Data System (ADS)

    Cocoletzi, Gregorio H.; Coyotecatl, Honorato A.

    1998-03-01

    Theoretical studies are presented of exciton bound states effects on the reflectivity of semiconductor superlattices and thin films. Semiconductors of direct gap are considered and model with extrinsic potentials near surfaces to account for the exciton interactions. Using a multistep method and the N× N transfer matrix approach, with N being the number of propagating modes, we solve analytically the polariton exciton equations. Results are presented for the A_n=1 excitonic transition of CdS and interpreted in terms of exciton bound states and Fabry-Perot resonances of the transverse and longitudinal modes in the excitonic layers. It is found that for the superlattice, the reflectivity peaks of the bound states are notably enhanced as compared with the corresponding of the single isolated film.

  14. Two-exciton states and spectroscopy of phenylacetylene dendrimers

    NASA Astrophysics Data System (ADS)

    Chernyak, Vladimir; Poliakov, Evgeni Y.; Tretiak, Sergei; Mukamel, Shaul

    1999-09-01

    The two-exciton wave functions of conjugated dendrimers with fractal geometries are calculated using the Frenkel-exciton model. Self-similarity and the high degree of symmetry make it possible to express the two-photon spectra of these chromophore aggregates in a compact form using irreducible representations of optical excitations, single-exciton states, and an effective two-exciton transition dipole moment. The explicit calculation of the complete manifold of two-exciton states which involves an expensive l3×l3 diagonalization, l being number of generations, is totally avoided. A real space analysis shows that the two-exciton states and resonances are dominated by periphery chromophores due to their exponentially large number.

  15. Singlet Exciton Fission in Nanostructured Organic Solar Cells

    SciTech Connect

    Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc

    2011-01-01

    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.

  16. 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.

  17. Ultrafast dynamics of excitons in tetracene single crystals.

    PubMed

    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

  18. Ultrafast dynamics of excitons in tetracene single crystals

    SciTech Connect

    Birech, Zephania; Schwoerer, Heinrich, E-mail: heso@sun.ac.za [Laser Research Institute, Stellenbosch University, Stellenbosch 7600 (South Africa)] [Laser Research Institute, Stellenbosch University, Stellenbosch 7600 (South Africa); Schwoerer, Markus [Department of Physics, University of Bayreuth, Bayreuth (Germany)] [Department of Physics, University of Bayreuth, Bayreuth (Germany); Schmeiler, Teresa; Pflaum, Jens [Experimental Physics VI, University of Würzburg and Bavarian Center for Applied Energy Research, Würzburg (Germany)] [Experimental Physics VI, University of Würzburg and Bavarian Center for Applied Energy Research, Würzburg (Germany)

    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.

  19. 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 ? m away from the excitation spot.

  20. Localization of excitons by molecular layer formation in a polymer film

    SciTech Connect

    Chattopadhyay, S.; Datta, A. [Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064 (India)

    2005-10-15

    Spin coated films of atactic polystyrene of two different molecular weights have been studied with uv spectroscopy and x-ray reflectivity, the film thickness (d) varying from {approx}2R{sub g} to {approx}12R{sub g} where R{sub g} is the unperturbed radius of gyration of the polymer. uv extinction due to the pure electronic singlet {sup 1}A{sub 1g}{yields}{sup 1}E{sub 1u} is seen to increase with d{sup -1} for 4R{sub g}{<=}d{<=}12R{sub g} (region 1). This suggests excitonic interaction along d. The variation of total exciton energy (E) of the A{sub 1g}{yields}E{sub 1u} singlet with d in region 1 can be well explained by formation of linear J-aggregates of polystyrene molecules, in a lattice with spacing 'a' (in A) R{sub g}exciton is also determined. For R{sub g}exciton localization along d, and the value of m{sub eff} becomes very large. This enhancement in the effective mass maybe used to quantify localization. The variations of electron density ({rho}) with d, i.e., the electron density profiles (EDPs) of the films extracted from x-ray reflectivity studies, indicate formation of layers with period 'b' (in A), R{sub g}0 at d{approx_equal}4R{sub g}, where the exciton is still delocalized. Layering reduces the Hamaker constant (A{sub H}), deciding the cohesive force, between the layers and this reduction, {delta}A{sub H}, is found to be less than A{sub e} at d{>=}4R{sub g}, where iA{sub e}/({Dirac_h}/2{pi}) is the amplitude for exciton transfer between neighboring molecules in the excitonic lattice of region 1. On the other hand, {delta}A{sub H} in region 2 starts from a value larger than A{sub e}. This indicates that {delta}A{sub H} acts as a barrier between the layer, which localizes the exciton within the layers.

  1. MASS-RADIUS RELATIONSHIPS FOR EXOPLANETS

    SciTech Connect

    Swift, D. C.; Eggert, J. H.; Hicks, D. G.; Hamel, S.; Caspersen, K.; Schwegler, E.; Collins, G. W. [Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California, CA 94550 (United States); Nettelmann, N. [Institut fuer Physik, Universitaet Rostock, D-18051 Rostock (Germany); Ackland, G. J. [Centre for Science at Extreme Conditions, School of Physics, University of Edinburgh, Edinburgh EH9 3JZ (United Kingdom)

    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.

  2. Exciton Polarons of Molecular Crystal Model. II. Optical Spectra

    Microsoft Academic Search

    Hitoshi Sumi

    1975-01-01

    Absorption and emission spectra of excitons interacting with optical or intramolecular vibrations are calculated with the dynamical coherent-potential approximation proposed previously. Investigations are made for various situations to which limiting concepts of nearly-free, self-trapped and vibronic excitons are not applicable straightforwardly. An interpretation is proposed that the absorption structure of alkali halides around the exciton peak originates from a situation

  3. Exciton luminescence polarization decay in type II semiconductor heterostructures

    Microsoft Academic Search

    Erasmo A. de Andrada e Silva; Giuseppe C. La Rocca

    1998-01-01

    In type II semiconductor heterostructures, one can reduce the electron–hole overlap, without changing much the oscillator strength, so that the space indirect exciton luminescence polarization will decay according to the exciton-bound single-particle spin flip. The exciton spin relaxation rate is then limited by the slower single-particle spin-flip rate which is typically the electron one. We discuss the microscopic theory of

  4. Exciton luminescence polarization decay in type II semiconductor heterostructures

    Microsoft Academic Search

    Erasmo A. de Andrada e Silva; Giuseppe C. La Rocca

    1998-01-01

    In type II semiconductor heterostructures, one can reduce the electron-hole overlap, without changing much the oscillator strength, so that the space indirect exciton luminescence polarization will decay according to the exciton-bound single-particle spin flip. The exciton spin relaxation rate is then limited by the slower single-particle spin-flip rate which is typically the electron one. We discuss the microscopic theory of

  5. Exciton fractional dimension in semiconductor heterostructures arising from variational principle

    Microsoft Academic Search

    R. A. Escorcia; J. Sierra-Ortega; I. D. Mikhailov; F. J. Betancur

    2005-01-01

    We present a simple method for calculating the ground-state energy of an exciton in quantum confined structures. We express the exciton wave function as a product of the electron and hole one-particle wave functions with a variationally determined envelope function which describes the exciton intrinsic properties. Starting from the variational principle, we derive an one-dimensional wave equation for this envelope

  6. The Epidemiology of Distal Radius Fractures

    PubMed Central

    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

  7. Proton radius puzzle and large extra dimensions

    E-print Network

    Li-Bang Wang; Wei-Tou Ni

    2013-05-23

    We propose a theoretical scenario to solve the proton radius puzzle which recently arises from the muonic hydrogen experiment. In this framework, 4 + n dimensional theory is incorporated with modified gravity. The extra gravitational interaction between the proton and muon at very short range provides an energy shift which accounts for the discrepancy between spectroscopic results from muonic and electronic hydrogen experiments. Assuming the modified gravity is a small perturbation to the existing electromagnetic interaction, we find the puzzle can be solved with stringent constraint on the range of the new force. Our result not only provides a possible solution to the proton radius puzzle but also suggest a direction to test new physics at very small length scale.

  8. Solar Radius Variations Measured in Central Eclipses

    NASA Astrophysics Data System (ADS)

    Sigismondi, Costantino

    2008-01-01

    Data on Baily beads observed in total eclipse of March 29, 2006 (Egypt) and those of annular eclipses of September 22, 2006 (French Guyana) and October 3, 2005 (Spain) are used to evaluate the variations of solar radius with respect to its standard value during a whole draconitic year. A portable observatory has to be set on the shadow limit of central eclipses, where lunar limb is grazing to the solar one and the number of beads is large. The observation of solar corona during Egyptian eclipse for several minutes during maximum eclipse on shadow's limits is studied in parallel with the eclipse observed by Clavius in 1567. From fall 2005 to fall 2006 the solar radius does not show significant changes (0.00 to -0.01 arcsecs) with respect to its standard value of 959.63 arcsec within errorbars of 0.17 arcsecs. This is its value at minimum of cycle 23 of solar activity.

  9. Mass radius scaling near the Chandrasekhar Limit

    E-print Network

    Chakraborty, S

    2007-01-01

    The mass radius relationship of white dwarfs, near the Chandrasekhar Limit, is derived for a toy model of uniform density, using the variational principle. A power law scaling, reminiscent of those found in 2nd order phase transitions, is obtained. The derived exponent is shown to explain the relationship obtained by numerically integrating the TOV equations with the equation of state for a relativistic Fermi gas of electrons.

  10. Mass radius scaling near the Chandrasekhar Limit

    E-print Network

    Sayan Chakraborti

    2007-03-22

    The mass radius relationship of white dwarfs, near the Chandrasekhar Limit, is derived for a toy model of uniform density, using the variational principle. A power law scaling, reminiscent of those found in 2nd order phase transitions, is obtained. The derived exponent is shown to explain the relationship obtained by numerically integrating the TOV equations with the equation of state for a relativistic Fermi gas of electrons.

  11. Surgical approaches to the distal radius

    Microsoft Academic Search

    Asif M. Ilyas

    2011-01-01

    Introduction  Fractures of the distal radius are among the most common fractures seen. They encompass a myriad of presentations and fracture\\u000a patterns that often benefit from various open reduction and internal fixation techniques—including volar plating, dorsal plating,\\u000a radial plating, intramedullary nailing, and fragment-specific fixation. In order to obtain optimal reduction of these fractures,\\u000a surgeons require a thorough understanding of the anatomy

  12. Fractures of Distal Radius: An Overview

    PubMed Central

    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

  13. Photon statistics dispersion in excitonic composites

    E-print Network

    G. Ya. Slepyan; S. A. Maksimenko

    2006-05-22

    Linear media are predicted to exist whose relative permiability is an operator in the space of quantum states of light. Such media are characterized by a photon statistics--dependent refractive index. This indicates a new type of optical dispersion -- the photon statistics dispersion. Interaction of quantum light with such media modifies the photon number distribution and, in particular, the degree of coherence of light. An excitonic composite -- a collection of noninteracting quantum dots -- is considered as a realization of the medium with the photon statistics dispersion. Expressions are derived for generalized plane waves in an excitonic composite and input--output relations for a planar layer of the material. Transformation rules for different photon initial states are analyzed. Utilization of the photon statistics dispersion in potential quantum--optical devices is discussed.

  14. Exciton management in organic photovoltaic multidonor energy cascades.

    PubMed

    Griffith, Olga L; Forrest, Stephen R

    2014-05-14

    Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. PMID:24702468

  15. Polarization-dependent exciton dynamics in tetracene single crystals.

    PubMed

    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

  16. Polarization-dependent exciton dynamics in tetracene single crystals

    E-print Network

    Zhang, Bo; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min

    2014-01-01

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission 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.

  17. 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.

  18. 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%.

  19. 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.

  20. 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.

  1. Comparison of transient state and steady state exciton–exciton annihilation rates based on Förster-type energy transfer

    NASA Astrophysics Data System (ADS)

    Yonehara, Toshiya; Goushi, Kenichi; Sawabe, Tomoaki; Takasu, Isao; Adachi, Chihaya

    2015-07-01

    We investigated differences between the transient state and steady state exciton–exciton annihilation rates based on Förster-type energy transfer. The exciton–exciton annihilation rate of an organic semiconductor is usually determined by transient state photoluminescence measurements using a pulsed laser or steady state photoluminescence measurements using a continuous wave laser. However, it is unclear that the respective annihilation rates determined by their rate equations are the same. In calculations with platinum-octaethylporphyrin (PtOEP) parameters, Monte Carlo simulations gave two different annihilation rates for the transient state and the steady state. The analytical models based on Förster-type energy transfer also showed the same result. These results indicate that the exciton–exciton annihilation rates in transient state and steady state are distinguished.

  2. Open universes from finite radius bubbles

    NASA Astrophysics Data System (ADS)

    Cohn, J. D.

    1996-12-01

    The interior of a vacuum bubble in de Sitter space may give an open universe with sufficient homogeneity to agree with observations. Here, previous work by Bucher, Goldhaber, and Turok is extended to describe a thin bubble wall with a nonzero radius and energy difference across the wall. The vacuum modes present before the formation of the bubble propagate into the interior of the open universe and the power spectrum of the resulting gauge-invariant gravitational potential is calculated. It appears to become scale invariant on small scales, with the onset at about the same scale as that found in the zero radius case. There is sensitivity to the radius and energy difference at large scales, but it is expected that they cannot be strongly constrained because of cosmic variance. The prediction of a scale invariant spectrum seems to be robust with respect to variation of these parameters at small scales, and apparently is a generic feature of the contribution of these modes for these thin wall models.

  3. Niels Bohr's discussions with Albert Einstein, Werner Heisenberg, and Erwin Schrödinger: The origins of the principles of uncertainty and complementarity

    Microsoft Academic Search

    Jagdish Mehra

    1987-01-01

    In this paper, the main outlines of the discussions between Niels Bohr with Albert Einstein, Werner Heisenberg, and Erwin Schrödinger during 1920–1927 are treated. From the formulation of quantum mechanics in 1925–1926 and wave mechanics in 1926, there emerged Born's statistical interpretation of the wave function in summer 1926, and on the basis of the quantum mechanical transformation theory—formulated in

  4. Microcavity controlled coupling of excitonic qubits

    PubMed Central

    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

  5. Influence of Effects of Self-Polarization and Exciton-Phonon Interactions on the Exciton Energy in Lead Iodide Nanofilms

    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.

  6. Exciton-exciton correlations revealed by two-quantum, two-dimensional fourier transform optical spectroscopy.

    PubMed

    Stone, Katherine W; Turner, Daniel B; Gundogdu, Kenan; Cundiff, Steven T; Nelson, Keith A

    2009-09-15

    The Coulomb correlations between photoexcited charged particles in materials such as photosynthetic complexes, conjugated polymer systems, J-aggregates, and bulk or nanostructured semiconductors produce a hierarchy of collective electronic excitations, for example, excitons, and biexcitons, which may be harnessed for applications in quantum optics, light-harvesting, or quantum information technologies. These excitations represent correlations among successively greater numbers of electrons and holes, and their associated multiple-quantum coherences could reveal detailed information about complex many-body interactions and dynamics. However, unlike single-quantum coherences involving excitons, multiple-quantum coherences do not radiate; consequently, they have largely eluded direct observation and characterization. In this Account, we present a novel optical technique, two-quantum, two-dimensional Fourier transform optical spectroscopy (2Q 2D FTOPT), which allows direct observation of the dynamics of multiple exciton states that reflect the correlations of their constituent electrons and holes. The approach is based on closely analogous methods in NMR, in which multiple phase-coherent fields are used to drive successive transitions such that multiple-quantum coherences can be accessed and probed. In 2Q 2D FTOPT, a spatiotemporal femtosecond pulse-shaping technique has been used to overcome the challenge of control over multiple, noncollinear, phase-coherent optical fields in experimental geometries used to isolate selected signal contributions through wavevector matching. We present results from a prototype GaAs quantum well system, which reveal distinct coherences of biexcitons that are formed from two identical excitons or from two excitons that have holes in different spin sublevels ("heavy-hole" and "light-hole" excitons). The biexciton binding energies and dephasing dynamics are determined, and changes in the dephasing rates as a function of the excitation density are observed, revealing still higher order correlations due to exciton-biexciton interactions. Two-quantum coherences due to four-particle correlations that do not involve bound biexciton states but that influence the exciton properties are also observed and characterized. The 2Q 2D FTOPT technique allows many-body interactions that cannot be treated with a mean-field approximation to be studied in detail; the pulse-shaping approach simplifies greatly what would have otherwise been daunting measurements. This spectroscopic tool might soon offer insight into specific applications, for example, in detailing the interactions that affect how electronic energy moves within the strata of organic photovoltaic cells. PMID:19691277

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

    PubMed

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

    2015-04-28

    Exciton transport in organic semiconductors is a critical, mediating process in many optoelectronic devices. Often, the diffusive and subdiffusive nature of excitons in these systems can limit device performance, motivating the development of strategies to direct exciton transport. In this work, directed exciton transport is achieved with the incorporation of exciton permeable interfaces. These interfaces introduce a symmetry-breaking imbalance in exciton energy transfer, leading to directed motion. Despite their obvious utility for enhanced exciton harvesting in organic photovoltaic cells (OPVs), the emergent properties of these interfaces are as yet uncharacterized. Here, directed exciton transport is conclusively demonstrated in both dilute donor and energy-cascade OPVs where judicious optimization of the interface allows exciton transport to the donor-acceptor heterojunction to occur considerably faster than when relying on simple diffusion. Generalized systems incorporating multiple exciton permeable interfaces are also explored, demonstrating the ability to further harness this phenomenon and expeditiously direct exciton motion, overcoming the diffusive limit. PMID:25798712

  8. Giant cell tumour of the proximal radius.

    PubMed

    Singh, A P; Mahajan, S; Singh, A P

    2009-11-01

    A 52-year-old Indian woman presented with a progressively increasing swelling and pain in the right elbow for the past eight months, which was not associated with trauma or constitutional symptoms. The patient was diagnosed to have Campanacci grade III giant cell tumour of the proximal radius, and was treated with above elbow amputation. The patient has not shown any recurrence after five years of follow-up. The case was reported because of its rarity and the unusual site of occurrence of the tumour. PMID:19960152

  9. Neutron charge radius and the Dirac equation

    E-print Network

    M. Bawin; S. A. Coon

    1999-06-05

    We consider the Dirac equation for a finite-size neutron in an external electric field. We explicitly incorporate Dirac-Pauli form factors into the Dirac equation. After a non-relativistic reduction, the Darwin-Foldy term is cancelled by a contribution from the Dirac form factor, so that the only coefficient of the external field charge density is $e/6 r^2_{En}$, i. e. the root mean square radius associated with the electric Sachs form factor . Our result is similar to a recent result of Isgur, and reconciles two apparently conflicting viewpoints about the use of the Dirac equation for the description of nucleons.

  10. Multi-Exciton Generation in Nanostructured Solar Cells

    E-print Network

    Multi-Exciton Generation in Nanostructured Solar Cells 1 G.T. Zimanyi UC Davis The energy is challenging, philosophically satisfying and fun #12;Multi-Exciton Generation in Nanostructured Solar Cells 2. Science: - GaAs: Alta Devices: 28% lab, 23.5% NREL verified - Organic solar cells: Sumitomo 10

  11. Mapping the exciton diffusion in semiconductor nanocrystal solids.

    PubMed

    Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail

    2015-03-24

    Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling. PMID:25682881

  12. Visualization of Exciton Transport in Molecular and Quantum Dot Solids

    NASA Astrophysics Data System (ADS)

    Akselrod, Gleb; Deotare, Parag; Prins, Ferry; Thompson, Nicholas; Poulikakos, Lisa; Lee, Elizabeth; Weidman, Mark; Mork, Jolene; Lee, Jiye; Willard, Adam; Baldo, Marc; Menon, Vinod; Tisdale, William; Bulovic, Vladimir

    2015-03-01

    Transport of nanoscale energy in the form excitons is at the core of the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light emitting diodes and excitonic transistors. Particularly important is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal, and spectral visualization of exciton transport in molecular crystals and quantum dot solids. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of tetracene, we show that the transition to subdiffusive transport occurs at earlier times as disorder is increased. In colloidal quantum dot films, we show that diffusion does not occur by a random-walk process; instead, energetic disorder causes the exciton diffusivity to decrease over time. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and disorder.

  13. Configurational coordinates model in the alkali-halide exciton laser

    Microsoft Academic Search

    JOSE M. GUERRA; JOSB LUIS ESCUDERO; JESUS SANCHO

    1974-01-01

    A configurational coordinates model in the fully quantum theoretical treatment of the alkali-halide exciton laser is used. The theory predicts a correlation between threshold pumping power and the Stokes shift. The predictions of the theory are compared with the observed characteristics of laser action on the first exciton peak of KBr. The observed spectral narrowing is in good agreement with

  14. Detailed balance theory of excitonic and bulk heterojunction solar cells

    Microsoft Academic Search

    Thomas Kirchartz; Julian Mattheis; Uwe Rau

    2008-01-01

    A generalized solar cell model for excitonic and classical bipolar solar cells describes the combined transport and interaction of electrons, holes, and excitons in accordance with the principle of detailed balance. Conventional inorganic solar cells, single-phase organic solar cells and bulk heterojunction solar cells, i.e., nanoscale mixtures of two organic materials, are special cases of this model. For high mobilities,

  15. Plasmon - exciton coupling effects in light absorption and scattering by metal/J-aggregate bilayer nanoparticles

    SciTech Connect

    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)

  16. Singlet Exciton Fission and Tri plet-Tri plet Exciton Fusion in Crystalline Tet racene

    Microsoft Academic Search

    Martin Pope; Nicholas E. Geacintov; Frank Vogel

    1969-01-01

    The dominant radiationles decay channel in crystalline tetracene at 300 °K is a fission of an excited singlet into two triplet excitons with a rate constant ?S = 1.5 × 10 ± 5% cm -sec. The efficiency of this process at room temperature is estimated as 95%and constitutesan efficient intersystem crossing mechanism. At light intensities I ? 10 quanta-cm sec

  17. Optical response of localized excitons near surfaces of multilayer systems

    NASA Astrophysics Data System (ADS)

    Coyotécatl, H. A.; Cocoletzi, G. H.

    1998-01-01

    We study theoretically the effects of exciton bound states on the reflectivity of semiconductor superlattices and thin films. We consider direct gap semiconductors and assume extrinsic potentials near surfaces to model the exciton interactions. Using a multistep method and the 0953-8984/10/1/009/img5 transfer matrix approach, we solve analytically the polariton exciton equations for s-polarized light. Results are presented for the 0953-8984/10/1/009/img6 excitonic transition of CdS and interpreted in terms of exciton bound states and Fabry-Pérot resonances of the transverse modes. We find that for the superlattice the reflectivity peaks of the high-energy bound states are notably enhanced as compared with the corresponding ones of the single isolated film.

  18. Exciton complexes in low dimensional transition metal dichalcogenides

    SciTech Connect

    Thilagam, A., E-mail: thilaphys@gmail.com [Information Technology, Engineering and Environment, University of South Australia, Adelaide 5095 (Australia)

    2014-08-07

    We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter ?, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of ? is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.

  19. Activated singlet exciton fission in a semiconducting polymer.

    PubMed

    Musser, Andrew J; Al-Hashimi, Mohammed; Maiuri, Margherita; Brida, Daniele; Heeney, Martin; Cerullo, Giulio; Friend, Richard H; Clark, Jenny

    2013-08-28

    Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1Bu exciton, contrary to current models that involve the lower-lying 2Ag exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2Ag state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends. PMID:23883167

  20. Exciton-mediated quantum search on a star graph

    NASA Astrophysics Data System (ADS)

    Pouthier, Vincent

    2015-06-01

    A fast and efficient quantum search algorithm is established by using the ability of an exciton to propagate along a star graph that exhibits two identical energetic defects. The first defect lies on the well-defined input site where the exciton is initially created, whereas the second defect occupies the target site whose unknown position must be determined. It is shown that when the energetic defects are judiciously chosen, specific quantum interferences arise so that the probability to observe the exciton on the target site becomes close to unity at a very short time t^{*} . Consequently, a measurement of the exciton quantum state at time t^{*} will reveal the identity of the position of the target site. The key point is that t^{*} is the shortest time independent on the size of the graph that is physically accessible to the exciton to tunnel.

  1. Nonlinear photoluminescence spectroscopy of carbon nanotubes with localized exciton states.

    PubMed

    Iwamura, Munechiyo; Akizuki, Naoto; Miyauchi, Yuhei; Mouri, Shinichiro; Shaver, Jonah; Gao, Zhenghong; Cognet, Laurent; Lounis, Brahim; Matsuda, Kazunari

    2014-11-25

    We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong nonlinear behavior was observed regardless of the origin of the local states and found to be nearly independent of the local state density. These results indicate that the strong PL nonlinearity arises from a universal mechanism to SWNTs with sparse local states. The significant nonlinear PL is attributed to rapid ground-state depletion of the local states caused by an efficient accumulation of photogenerated free excitons into the sparse local states through one-dimensional diffusional migration of excitons along the nanotube axis; this mechanism is verified by Monte Carlo simulations of exciton diffusion dynamics. PMID:25331628

  2. Radiative properties of multicarrier bound excitons in GaAs

    NASA Astrophysics Data System (ADS)

    Karin, Todd; Barbour, Russell J.; Santori, Charles; Yamamoto, Yoshihisa; Hirayama, Yoshiro; Fu, Kai-Mei C.

    2015-04-01

    Excitons in semiconductors can have multiple lifetimes due to spin-dependent oscillator strengths and interference between different recombination pathways. In addition, strain and symmetry effects can further modify lifetimes via the removal of degeneracies. We present a convenient formalism for predicting the optical properties of k =0 excitons with an arbitrary number of charge carriers in different symmetry environments. Using this formalism, we predict three distinct lifetimes for the neutral acceptor bound exciton in GaAs, and confirm this prediction through polarization dependent and time-resolved photoluminescence experiments. We find the acceptor bound-exciton lifetimes to be To×(1 ,3 ,3/4 ) , where To=(0.61 ±0.12 ) ns . Furthermore, we provide an estimate of the intralevel and interlevel exciton spin-relaxation rates.

  3. 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 ?. The Mott crossover density region in Ge was elucidated from the density dependence of ? 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.

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

    NASA Astrophysics Data System (ADS)

    Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.

    2015-03-01

    The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be 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.

  5. Probing excitonic dark states in single-layer tungsten disulphide.

    PubMed

    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

  6. Towards r-space Bose-Einstein condensation of photonic crystal exciton polaritons

    Microsoft Academic Search

    D. L. Boiko

    2008-01-01

    Coupled states of semiconductor quantum well (QW) excitons and photons in a two dimensional (2D) periodic lattice of microcavities are analyzed theoretically, revealing allowed bands and forbidden gaps in the energy spectrum of exciton polaritons. Photonic crystal exciton polaritons have spatially uniform excitonic constituent set by flat QWs, but exhibit periodic Bloch oscillations in the plane of QWs due to

  7. 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.

  8. Theory of exciton transfer and diffusion in conjugated polymers

    SciTech Connect

    Barford, William, E-mail: william.barford@chem.ox.ac.uk [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); Tozer, Oliver Robert [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); University College, University of Oxford, Oxford OX1 4BH (United Kingdom)

    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 ?? < 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.

  9. Theory of exciton transfer and diffusion in conjugated polymers.

    PubMed

    Barford, William; Tozer, Oliver Robert

    2014-10-28

    We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < 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. PMID:25362268

  10. Chiral Inflation of the Pion Radius

    E-print Network

    I. A. Perevalova; M. V. Polyakov; A. N. Vall; A. A. Vladimirov

    2011-05-25

    We derive expression for the large b_perp asymptotic of the 3D parton distributions q(x,b_perp) in the pion. The asymptotic depends exclusively on the mass scales F_pi and m_pi. Therefore it provides us with a nice example of a strict non-perturbative result for the partonic structure of Nambu-Goldstone bosons in QCD. Analyzing the x-dependent pion transverse radius we reveal a new phenomenon of "chiral inflation"-- in the parametrically wide region of Bjorken x (m_pi^2/(4 pi F_pi)^2 inflation is at variance with the Gribov diffusion, because of long-range interaction of the Nambu-Goldstone bosons.

  11. The Proton Radius from Bayesian Inference

    E-print Network

    Krzysztof M. Graczyk; Cezary Juszczak

    2015-05-14

    The methods of Bayesian statistics are used to extract the value of the proton radius from the elastic $ep$ scattering data in a model independent way. To achieve that goal a large number of parametrizations (equivalent to neural network schemes) are considered and ranked by their conditional probability $P(\\mathrm{parametrization}\\,|\\,\\mathrm{data})$ instead of using the minimal error criterion. As a result the most probable proton radii values ($r_E^p=0.899\\pm 0.003$ fm, $r_M^p=0.879\\pm 0.007$ fm) are obtained and systematic error due to freedom in the choice of parametrization is estimated. Correcting the data for the two photon exchange effect turns out to be important to obtain the agreement between the $r_E^p$ and $r_M^p$ values. The results disagree with recent muonic atom measurements.

  12. Carpal Canal Pressures after Volar Plating of Distal Radius Fractures

    Microsoft Academic Search

    D. A. FULLER; M. BARRETT; R. K. MARBURGER; R. HIRSCH

    2006-01-01

    Elevated pressures within the carpal canal are known to occur after distal radius fractures. Controversy exists regarding prophylactic carpal tunnel release after open reduction with internal fixation of distal radius fractures. The purpose of this study was to determine the tissue pressures within the carpal canal after volar plating of distal radius fractures. This study was a prospective, observational, IRB

  13. Early active rehabilitation for operatively stabilized distal radius fractures

    Microsoft Academic Search

    Mark H Henry; K BROU

    2004-01-01

    From the young to the elderly, distal radius fractures are very common. Extensive literature has been written regarding surgical management of distal radius fractures, but the same degree of attention has not been given to the critical rehabilitation that follows. Successful functional outcomes for distal radius fractures are a result of appropriate surgical treatment as well as timely and specific

  14. Understanding the Mass-Radius Relation for Sub-neptunes: Radius as a Proxy for Composition

    NASA Astrophysics Data System (ADS)

    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 ?, 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 ? 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.

  15. Role of phonons in Josephson oscillations of excitonic and polaritonic condensates

    SciTech Connect

    Magnusson, E. B.; Flayac, H.; Malpuech, G.; Shelykh, I. A. [Science Institute, University of Iceland, Dunhagi-3, IS-107 Reykjavik (Iceland); LASMEA, Clermont Universite-Universite Blaise Pascal, BP 10448, 63000 Clermont-Ferrand (France) and LASMEA, UMR 6602, CNRS, 63177 Aubiere (France); Science Institute, University of Iceland, Dunhagi-3, IS-107 Reykjavik (Iceland) and International Institute for Physics, Av. Odilon Gomes de Lima, 1722, CEP 59078-400 Capim Macio Natal, RN (Brazil)

    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.

  16. JOURNALDE PHYSIQUEIV ColloqueC5,supplkmentau Journalde Physique11, Volume 3, octobre 1993

    E-print Network

    Boyer, Edmond

    , 11 Universitd di Roma, via E. Camevale, 00173Roma, Italy Abstract:Variational envelope function) as a function of lateral dimension. The techniques widely used to define QWWs (electron-beam lithography, while L, > 3aBis the lateral dimension, and ag is the Bohr radius. Modelling the Wannier exciton

  17. Nexus between coherent longitudinal-optical phonons and electronic diffusions in low-gap semiconductors

    SciTech Connect

    Min, Kyung-Gu; Jho, Young-Dahl, E-mail: jho@gist.ac.kr [School of Info. and Comm., Gwangju Institute of Science and Technology, Gwangju 500-712 (Korea, Republic of); Yee, Ki-Ju [Department of Physics, Chungnam National University, Daejeon 305-764 (Korea, Republic of); Stanton, C. J. [Department of Physics, University of Florida, Gainesville, Florida 32611-8440 (United States); Song, Jin-Dong [Nano-Photonics Research Center, Korea Institute of Science and Technology, Seoul 136-791 (Korea, Republic of)

    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.

  18. Bohring–Opitz (Oberklaid–Danks) syndrome: clinical study, review of the literature, and discussion of possible pathogenesis

    PubMed Central

    Hastings, Rob; Cobben, Jan-Maarten; Gillessen-Kaesbach, Gabriele; Goodship, Judith; Hove, Hanne; Kjaergaard, Susanne; Kemp, Helena; Kingston, Helen; Lunt, Peter; Mansour, Sahar; McGowan, Ruth; Metcalfe, Kay; Murdoch-Davis, Catherine; Ray, Mary; Rio, Marlène; Smithson, Sarah; Tolmie, John; Turnpenny, Peter; van Bon, Bregje; Wieczorek, Dagmar; Newbury-Ecob, Ruth

    2011-01-01

    Bohring–Opitz syndrome (BOS) is a rare congenital disorder of unknown etiology diagnosed on the basis of distinctive clinical features. We suggest diagnostic criteria for this condition, describe ten previously unreported patients, and update the natural history of four previously reported patients. This is the largest series reported to date, providing a unique opportunity to document the key clinical features and course through childhood. Investigations undertaken to try and elucidate the underlying pathogenesis of BOS using array comparative genomic hybridization and tandem mass spectrometry of cholesterol precursors did not show any pathogenic changes responsible. PMID:21368916

  19. Robust excitons inhabit soft supramolecular nanotubes

    PubMed Central

    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

  20. Nanoscale Charge Transport in Excitonic Solar Cells

    SciTech Connect

    Venkat Bommisetty, South Dakota State University

    2011-06-23

    Excitonic solar cells, including all-organic, hybrid organic-inorganic and dye-sensitized solar cells (DSSCs), offer strong potential for inexpensive and large-area solar energy conversion. Unlike traditional inorganic semiconductor solar cells, where all the charge generation and collection processes are well understood, these excitonic solar cells contain extremely disordered structures with complex interfaces which results in large variations in nanoscale electronic properties and has a strong influence on carrier generation, transport, dissociation and collection. Detailed understanding of these processes is important for fabrication of highly efficient solar cells. Efforts to improve efficiency are underway at a large number of research groups throughout the world focused on inorganic and organic semiconductors, photonics, photophysics, charge transport, nanoscience, ultrafast spectroscopy, photonics, semiconductor processing, device physics, device structures, interface structure etc. Rapid progress in this multidisciplinary area requires strong synergetic efforts among researchers from diverse backgrounds. Such effort can lead to novel methods for development of new materials with improved photon harvesting and interfacial treatments for improved carrier transport, process optimization to yield ordered nanoscale morphologies with well defined electronic structures.

  1. CCMR: Exciton Blocking Layers in Organic Photovoltaics

    NSDL National Science Digital Library

    Becker, Katherine

    2007-08-29

    Organic solar cells have been attracting increased attention recently as an alternative to more costly silicon cells. Organic photovoltaics can be lightweight, processed on flexible substrates, and use less energy to produce [1]. However, significant barriers in efficiency and longevity must be overcome before such cells are commercially viable. Current organic cells can reach only up to 6% efficiency in the lab, while commercially available silicon cells average 15% efficiency [2, 3]. Cell longevity is also severely restricted by the tendency of C60 to increase in resistance when exposed to oxygen. Exciton blocking layers (EBLs) have been exploited to increase longevity [4], mechanically protect the donor and acceptor layers [5] and increase efficiency [6]. Until recently, primarily bathocuproine (BCP) has been used, with some investigation into tris-8-hydroxyquinolinato aluminum (Alq3) [5] and bathophenanthroline (BPhen) [6]. There has also been investigation into the positive effects of annealing on efficiency [7,8,9]. This research seeks to evaluate the effects of thermal annealing on 4 different exciton blocking layer types: BCP, Alq3, BPhen, and NBPhen, a close relative of BPhen.

  2. PbSe nanocrystal excitonic solar cells.

    PubMed

    Choi, Joshua J; Lim, Yee-Fun; Santiago-Berrios, Mitk'el B; Oh, Matthew; Hyun, Byung-Ryool; Sun, Liangfeng; Bartnik, Adam C; Goedhart, Augusta; Malliaras, George G; Abruña, Héctor D; Wise, Frank W; Hanrath, Tobias

    2009-11-01

    We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date. PMID:19719095

  3. Giant Rydberg excitons in the copper oxide Cu2O.

    PubMed

    Kazimierczuk, T; Fröhlich, D; Scheel, S; Stolz, H; Bayer, M

    2014-10-16

    A highly excited atom having an electron that has moved into a level with large principal quantum number is a hydrogen-like object, termed a Rydberg atom. The giant size of Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided insights into atomic and molecular physics on the single-quantum level. Excitons--the fundamental optical excitations in semiconductors, consisting of an electron and a positively charged hole--are the condensed-matter analogues of hydrogen. Highly excited excitons with extensions similar to those of Rydberg atoms are of interest because they can be placed and moved in a crystal with high precision using microscopic energy potential landscapes. The interaction of such Rydberg excitons may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level. Here we demonstrate the existence of Rydberg excitons in the copper oxide Cu2O, with principal quantum numbers as large as n = 25. These states have giant wavefunction extensions (that is, the average distance between the electron and the hole) of more than two micrometres, compared to about a nanometre for the ground state. The strong dipole-dipole interaction between such excitons is indicated by a blockade effect in which the presence of one exciton prevents the excitation of another in its vicinity. PMID:25318523

  4. First-principles method of propagation of tightly bound excitons: verifying the exciton band structure of LiF with inelastic x-ray scattering.

    PubMed

    Lee, Chi-Cheng; Chen, Xiaoqian M; Gan, Yu; Yeh, Chen-Lin; Hsueh, H C; Abbamonte, Peter; Ku, Wei

    2013-10-11

    We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures. PMID:24160627

  5. First-Principles Method of Propagation of Tightly Bound Excitons: Verifying the Exciton Band Structure of LiF with Inelastic x-Ray Scattering

    NASA Astrophysics Data System (ADS)

    Lee, Chi-Cheng; Chen, Xiaoqian M.; Gan, Yu; Yeh, Chen-Lin; Hsueh, H. C.; Abbamonte, Peter; Ku, Wei

    2013-10-01

    We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures.

  6. 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.

  7. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    NASA Astrophysics Data System (ADS)

    Repp, J.; Schinner, G. J.; Schubert, E.; Rai, A. K.; Reuter, D.; Wieck, A. D.; Wurstbauer, U.; Kotthaus, J. P.; Holleitner, A. W.

    2014-12-01

    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.

  8. Intermediate valence and the possibility of a magnetic excitonic insulator

    NASA Astrophysics Data System (ADS)

    Wachter, Peter; Jung, Anatol

    1994-03-01

    The long predicted existence of an excitonic condensed phase - i.e. the excitonic insulator - has first been verified experimentally under pressure and low temperature on the intermediate valent system TmSe(0.45)Te(0.55) and TmSe(0.32)Te(0.68) by Neuenschwander and Wachter and Bucher, Steiner and Wachter. Now additional compounds such as Sm(0.75)La(0.25)S and YbO and YbS have been shown to fall under pressure into the category of excitonic insulators. The problem whether magnetism plays a role in the condensation will be discussed.

  9. 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.

  10. Excitonic AND Logic Gates on DNA Brick Nanobreadboards

    PubMed Central

    2015-01-01

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

  11. 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.

  12. "It's best not to think about it at all-like the new taxes": Reality, observer, and complementarity in Bohr and Pauli

    NASA Astrophysics Data System (ADS)

    Plotnitsky, Arkady

    2012-12-01

    This article considers the concepts of reality, observer, and complementarity in Pauli and Bohr, and the similarities and, especially, differences in their understanding of these concepts, differences defined most essentially by their respective views of the role of the human observer in quantum measurement. These differences are significant even in the case of their respective interpretations of quantum phenomena and quantum mechanics, where the influence of Bohr's ideas on Pauli's understanding of quantum physics is particularly strong. They become especially strong and even radical in the case of their overall philosophical visions, where the impact of Jungean psychology, coupled to that of the earlier archetypal thinking of such figures as Kepler and Fludd, drives Pauli's thinking ever further away from that of Bohr.

  13. Ultrafast exciton dynamics of J-aggregates in room temperature solution studied by third-order nonlinear optical spectroscopy and numerical simulation based on exciton theory

    Microsoft Academic Search

    Kaoru Ohta; Mino Yang; Graham R. Fleming

    2001-01-01

    We report a study of the exciton dynamics in 1,1'-diethyl-3,3'-bis(sulforpropyl)-5,5',6,6' -tetrachlorobenzimidacarbocyanine (BIC) J-aggregates in water solution at room temperature by third-order nonlinear optical spectroscopy and numerical simulations based on exciton theory. The temporal profiles of the transient grating signals depend strongly on the excitation intensity as a result of exciton-exciton annihilation. On the other hand, the peak shift measurement gives

  14. Experimental study of finite Larmor radius effects

    SciTech Connect

    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.

  15. PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS

    SciTech Connect

    Watts, Anna L.; Van der Klis, Michiel; Wijers, Ralph A. M. J. [Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Postbus 94249, 1090 GE Amsterdam (Netherlands); Kouveliotou, Chryssa [Space Science Office, VP62, NASA Marshall Space Flight Center, Huntsville, AL 35812 (United States); Van der Horst, Alexander J. [NASA Marshall Space Flight Center, Huntsville, AL 35805 (United States); Goegues, Ersin; Kaneko, Yuki [Sabanci University, Orhanli-Tuzla, Istanbul 34956 (Turkey); Harding, Alice K. [NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States); Baring, Matthew G., E-mail: A.L.Watts@uva.n [Department of Physics and Astronomy, MS-108, Rice University, P.O. Box 1892, Houston, TX 77251-1892 (United States)

    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.

  16. Anomalous transport and luminescence of indirect excitons in AlAs/GaAs coupled quantum wells as evidence for exciton condensation

    E-print Network

    Anomalous transport and luminescence of indirect excitons in AlAs/GaAs coupled quantum wells excitons in AlAs/GaAs CQW's at low temperatures (T 350 mK and high magnetic fields (B 14 T . Strong radiative decay rate and a huge noise in the integrated exciton PL intensity. An interpretation

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

    SciTech Connect

    Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Wang, Yuan [NSF Nanoscale Science and Engineering Center, University of California, 3112 Etcheverry Hall, Berkeley, California 94720 (United States); Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang, E-mail: xiang@berkeley.edu [NSF Nanoscale Science and Engineering Center, University of California, 3112 Etcheverry Hall, Berkeley, California 94720 (United States); Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720 (United States)

    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.

  18. Energy accumulation in relativistic sub-Bohr orbitals, Franklin's relativistic rotation of quarks and gravitational field bounceback as processes relevant to explosion of supernovae

    NASA Astrophysics Data System (ADS)

    Ben-Amots, N.

    2015-05-01

    We discuss exploding stars exceeding 8 MSun. Supernova processes start with implosion before the explosion. The processes causing implosion are relatively well understood. Not so the processes that reverse the implosion into explosion. We propose and discuss three possible processes that can reverse implosion of a star into an explosion, causing supernova: a) energy accumulation in sub-Bohr electron orbitals populated by an electron moving at high relativistic velocities [1], b) Franklin's relativistic rotation of quarks [2], [3] and c) Einstein's gravitational field bounceback[4]. The relativistic sub-Bohr orbitals are derived by using an analogy to exponential gravitation (recent refs [1], [4], [5], [6], [7] and [8]).

  19. Comparison of the E(5) Critical Point Symmetry to the {gamma}-Rigid Solution of The Bohr Hamiltonian for {gamma} =30 deg

    SciTech Connect

    Bonatsos, D.; Lenis, D.; Petrellis, D. [Institute of Nuclear Physics, National Center for Scientific Research 'Demokritos', GR-15310 Aghia Paraskevi, Attiki (Greece); Terziev, P. A. [Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigrad Road, BG-1784 Sofia (Bulgaria); Yigitoglu, I. [Hasan Ali Yucel Faculty of Education, Istanbul University, TR-34470 Beyazit, Istanbul (Turkey)

    2007-04-23

    A {gamma}-rigid solution of the Bohr Hamiltonian for {gamma}=30 deg. is derived. Bohr Hamiltonians {beta}-part being related to the second order Casimir operator of the Euclidean algebra E(4). The solution is called Z(4) since it is corresponds to the Z(5) model with the {gamma} variable ''frozen''. Parameter-free (up to overall scale factors) predictions for spectra and B(E2) transition rates are in close agreement to the E(5) critical point symmetry as well as to the experimental data in the Xe region around A=130.

  20. Singlet exciton fission-sensitized infrared quantum dot solar cells.

    PubMed

    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

  1. Hopping approach towards exciton dissociation in conjugated polymers

    SciTech Connect

    Emelianova, E. V.; Auweraer, M. van der [Laboratory for Molecular Dynamics and Spectroscopy, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee-Leuven (Belgium); Institute for Nanoscale Physics and Chemistry (INPAC), Celestijnenlaan 200D, B-3001 Leuven (Belgium); Baessler, H. [Institute of Physical, Macromolecular and Nuclear Chemistry and Materials Science Center, Philipps University, D-35032 Marburg (Germany)

    2008-06-14

    By employing random walk an analytic theory for the dissociation of singlet excitons in a random organic solid, for instance, a conjugated polymer, has been developed. At variance of conventional three-dimensional Onsager theory, it is assumed that an exciton with finite lifetime can first transfer endothermically an electron to an adjacent site, thereby generating a charge transfer state whose energy is above the energy of that of the initial exciton. In a second step the latter can fully dissociate in accordance with Onsager's concept Brownian motion. The results indicate that, depending of the energy required for the first jump, the first jump contributes significantly to the field dependence of the dissociation yield. Disorder weakens the temperature dependence of the yield dramatically and precludes extracting information on the exciton binding energy from it.

  2. Excitonic condensation in spatially separated one-dimensional systems

    NASA Astrophysics Data System (ADS)

    Abergel, D. S. L.

    2015-05-01

    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.

  3. Spatio-temporal dynamics of quantum-well excitons

    E-print Network

    Zhao, Hui; Dal Don, B.; Moehl, Sebastian; Kalt, Heinz; Ohkawa, K.; Hommel, D.

    2003-01-01

    We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm ...

  4. Charge Transfer Excitons at van der Waals Interfaces.

    PubMed

    Zhu, Xiaoyang; Monahan, Nicholas R; Gong, Zizhou; Zhu, Haiming; Williams, Kristopher W; Nelson, Cory A

    2015-07-01

    The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation. PMID:26001297

  5. Topologically protected excitons in porphyrin thin films

    E-print Network

    Saikin, Semion K.

    he control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. ...

  6. How bilayer excitons can greatly enhance thermoelectric efficiency

    NASA Astrophysics Data System (ADS)

    Wu, Kai; Rademaker, Louk; Zaanen, Jan

    2015-03-01

    Presently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer exciton thermocouple. Based on current experimental parameters, a bilayer exciton heterostructures of p- and n-doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3. Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9.

  7. Coherence Length of Excitons in a Semiconductor Quantum Well

    E-print Network

    Zhao, Hui; Moehl, Sebastian; Kalt, Heinz

    2002-08-01

    We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length ...

  8. Plasmon-exciton self-induced transparency

    SciTech Connect

    Zabolotskii, A. A., E-mail: zabolotskii@iae.nsk.su [Russian Academy of Sciences, Institute of Automatics and Electrometry, Siberian Branch (Russian Federation)

    2011-04-15

    The possibility of forming stable bound plasmon-polariton states in an extended metallic cylinder surrounded by a two-level medium has been investigated. The dynamics of plasmons is described in the hydrodynamic approximation. It has been shown that the equations of motion of charge-density bunches and the Bloch equations for the two-level medium are reduced in certain approximations to integrable equations for both transverse and longitudinal plasmons. In the former case, the initial system of equations after the application of the slow-envelope approximation is reduced to equations equivalent to the Maxwell-Bloch equations. In the latter case, the equations describe wave dynamics beyond the slow-envelope approximation. In the approximation of unidirectional wave propagation, the initial system of equations is reduced to equations related to the reduced Maxwell-Bloch equations. Soliton and breather-like solutions of the derived equations describe plasmon-exciton self-induced transparency.

  9. 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 and the hole from the vacuum/GaAs interface, with and without image charges and the integrated probability of finding an electron and a hole inside the well are also calculated. The results agree well with the available experimental data.

  10. Acoustically Induced Stark Effect for Excitons in Intrinsic Semiconductors

    Microsoft Academic Search

    A. L. Ivanov; P. B. Littlewood

    2001-01-01

    A Stark effect for excitons parametrically driven by coherent acoustic phonons is proposed. Our scheme refers to a low-temperature intrinsic semiconductor or semiconductor nanostructure pumped by an acoustic wave (frequency band nuac~=1-40 GHz and intensity range Iac~=10-2-102 W\\/cm2) and probed by low-intensity light. Tunable optical band gaps, which strongly change the spectral shape of the exciton line, are induced in

  11. Exciton relaxation by phonon scattering in Cu 2O

    Microsoft Academic Search

    H. Haug

    1999-01-01

    The Boltzmann relaxation kinetics of a non-equilibrium distribution of a dense gas of excitons by scattering with thermal acoustic phonons is reviewed. The relaxation kinetics of the optically forbidden para-excitons can result in a Bose–Einstein condensation although the filling of the condensate is slowed down because only a few states are coupled directly to the lowest momentum state. An analogy

  12. Quantum confinement of excitons in wurtzite InP nanowires

    NASA Astrophysics Data System (ADS)

    Pemasiri, K.; Jackson, H. E.; Smith, L. M.; Wong, B. M.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.

    2015-05-01

    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.

  13. New polaronic excitons in ferroelectric oxides: phenomenological theory

    Microsoft Academic Search

    V. S. Vikhnin

    2003-01-01

    It is shown that three types of new polaronic excitons exist in ferroelectric oxides–charge transfer vibronic excitons (CTVEs). The electron-hole bipolaron pairs are common in all these cases, the difference having to do with the origin of charge transfer–lattice instabilities. These are the cases of the charge transfer and lattice distortions and their vibronic interactions induced in the harmonic (i)

  14. Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells

    Microsoft Academic Search

    George F. Burkhard; Eric T. Hoke; Shawn R. Scully; Michael D. McGehee

    2009-01-01

    We investigate the internal quantum efficiencies (IQEs) of high efficiency poly-3-hexylthiophene:(6,6)-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells and find them to be lower at wavelengths where the PCBM absorbs. Because the exciton diffusion length in PCBM is too small, excitons generated in PCBM decay before reaching the donor-acceptor interface. This result has implications for most state of the art organic

  15. Mechanisms of exciton energy transfer in Scheibe aggregates

    NASA Astrophysics Data System (ADS)

    Tuszy?ski, J. A.; Jørgensen, M. F.; Möbius, D.

    1999-04-01

    In this paper we provide a critical reassessment of the most important physical mechanisms in Langmuir-Blodgett Scheibe aggregates. Specific issues discussed involve the hierarchy of time and energy scales, estimates of model parameters, and their consequences in terms of physical behavior. In particular, we address the issue of determining the strength of the exciton-phonon interaction and its effect on the formation and dynamics of a coherent exciton domain.

  16. Singlet fission in pentacene through multi-exciton quantum states

    Microsoft Academic Search

    Paul M. Zimmerman; Zhiyong Zhang; Charles B. Musgrave

    2010-01-01

    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

  17. Singlet fission in pentacene through multi-exciton quantum states

    Microsoft Academic Search

    Paul M. Zimmerman; Zhiyong Zhang; Charles B. Musgrave

    2010-01-01

    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

  18. Size and composition dependent multiple exciton generation efficiency in PbS, PbSe, and PbS(x)Se(1-x) alloyed quantum dots.

    PubMed

    Midgett, Aaron G; Luther, Joseph M; Stewart, John T; Smith, Danielle K; Padilha, Lazaro A; Klimov, Victor I; Nozik, Arthur J; Beard, Matthew C

    2013-07-10

    Using ultrafast transient absorption and time-resolved photoluminescence spectroscopies, we studied multiple exciton generation (MEG) in quantum dots (QDs) consisting of either PbSe, PbS, or a PbSxSe1-x alloy for various QD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine the MEG efficiency, ?MEG, defined in terms of the electron-hole pair creation energy (?eh) such that ?MEG = Eg/?eh. In previous reports, we found that ?MEG is about two times greater in PbSe QDs compared to bulk PbSe, however, little could be said about the QD-size dependence of MEG. In this study, we find for both PbS and PbSxSe1-x alloyed QDs that ?MEG decreases lineally with increasing QD diameter within the strong confinement regime. When the QD radius is normalized by a material-dependent characteristic radius, defined as the radius at which the electron-hole Coulomb and confinement energies are equivalent, PbSe, PbS, and PbSxSe1-x exhibit similar MEG behaviors. Our results suggest that MEG increases with quantum confinement, and we discuss the interplay between a size-dependent MEG rate versus hot exciton cooling. PMID:23750998

  19. Trapping and transport of indirect excitons in coupled quantum wells

    NASA Astrophysics Data System (ADS)

    Wuenschell, Jeffrey K.

    Spatially indirect excitons are optically generated composite bosons with a radiative lifetime sufficient to reach thermal equilibrium. This work explores the physics of indirect excitons in coupled quantum wells in the GaAs/AlGaAs system, specifically in the low-temperature, high-density regime. Particular attention is paid to a technique whereby a spatially inhomogeneous strain field is used as a trapping potential. In the process of modeling the trapping profile in wide quantum wells, dramatic effects due to intersubband coupling were observed at high strain. Experimentally, this regime coincides with the abrupt appearance of a dark population of indirect excitons at trap center, an effect originally suspected to be related to Bose-Einstein condensation. Here, the role of band mixing due to the strain-induced distortion of the crystal symmetry will be explored in detail in the context of this effect. Experimental studies presented here and in the literature suggest that Bose-Einstein condensation in indirect exciton systems may be difficult to detect with optical means (e.g., coherence measurements, momentum-space narrowing), possibly due to the strong dipole interaction between indirect excitons. Due to similarities between this system and liquid helium, it may be more fruitful to look for transport-related signatures of condensation, such as super fluidity. Here, a method for performing transport measurements on optically generated indirect excitons is also outlined and preliminary results are presented.

  20. Molecular packing determines singlet exciton fission in organic semiconductors.

    PubMed

    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

  1. Femtosecond THz Studies of Intra-Excitonic Transitions

    SciTech Connect

    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.

  2. Polaron and Exciton-Phonon Complexes in CuCl Nanocrystals

    NASA Astrophysics Data System (ADS)

    Itoh, T.; Nishijima, M.; Ekimov, A. I.; Gourdon, C.; Efros, Al. L.; Rosen, M.

    1995-02-01

    We report the first evidence of an excitonic polaron and exciton-phonon complexes in nanosize semiconductor crystals. These are observed in CuCl nanocrystal excitonic luminescence spectra under resonant size-selective excitation. The theory of exciton coupling with spherical phonons through the Fröhlich and deformation potentials is developed and describes both the size dependence of the polaron binding energy and the degree of the exciton-phonon hybridization near resonance, when the 1P and 1S exciton level spacing coincides with the one- or two-phonon energy.

  3. Influence of dielectric environment on exciton and bi-exciton properties in colloidal, type II quantum dots

    NASA Astrophysics Data System (ADS)

    Miloszewski, Jacek M.; Walsh, Thomas; Tomi?, Stanko

    2015-05-01

    We present theoretical calculations of type II CdSe/CdTe quantum dots systems. We use an 8-band k.p Hamiltonian that includes spin-orbit interaction, strain, and first order piezoelectric effects. Exciton and bi-exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of the Coulomb interaction, as well as exchange and correlation effects between many-electron configurations. We study convergence of the CI Hamiltonian with respect to the number of single particle states used in creation of the Hamiltonian. We show that there is a very strong correlation between the dielectric constant of the environment and exciton and bi-exciton energies.

  4. Centrifugal-Barrier Effects and Determination of the Interaction Radius

    E-print Network

    Ning Wu

    2013-05-21

    The interaction radius of a resonance is an important physical quantity to describe the structure of a resonance. But, for a long time, physicists do not find a reliable way to measure the magnitude of the interaction radius of a resonance. In this paper, a method is proposed to measure the interaction radius in physics analysis. It is found that the centrifugal barrier effects have great influence to physical results obtained in the PWA fit, and the interaction radius of some resonances can be well measured in the fit.

  5. Excitons: from excitations at surfaces to confinement in nanostructures

    NASA Astrophysics Data System (ADS)

    Cocoletzi, Gregorio H.; Mochán, W. Luis

    2005-04-01

    We present a review of optical properties of semiconductors capable of supporting excitons, that is, electron-hole pairs that form states bound by their mutual screened Coulomb attraction. We consider the interaction of light with surfaces, thin films, multilayered systems, small particles and rough surfaces accounting for excitonic transitions. The first part of the paper is devoted to studies done using classical electrodynamics within the nonlocal dielectric response theory. For the dielectric function we take the Hopfield and Thomas coupled harmonic oscillator model, which yields excitonic modes beyond the usual optical waves. Therefore, studies of the coupling of light to exciton-polaritons in the presence of surfaces require additional boundary conditions (ABCs) to determine the reflected and transmitted electromagnetic field amplitudes within models of abruptly terminated semiconductors. An alternative consisting in solving Maxwell's equations for the electromagnetic field together with an equation for the excitonic polarization derived from the quantum mechanical dynamics of electrons and holes, including a surface potential that accounts for the interaction of excitons with the surface is explored in the latter part of the paper. The surface potential may be modeled by an infinite barrier, or by smooth repulsive exponential potentials and Morse-type potential wells. Surface potential wells may produce entrapped excitonic states, which are explored at surfaces, films and superlattices. Scattering of light from non-ideal rough surfaces is also discussed. Comparison between theory and experiment is emphasized all along the paper. The final section is devoted to a microscopic theory which is ABC independent and explains the experimental measurements of transmission in thin films.

  6. Singlet fission of hot excitons in ?-conjugated polymers.

    PubMed

    Zhai, Yaxin; Sheng, Chuanxiang; Vardeny, Z Valy

    2015-06-28

    We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(?)) 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 ?-conjugated polymers. From the high energy step in the triplet EXPA(?) 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(?) 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 ?-conjugated polymers is a much more general process than thought previously. PMID:25987576

  7. What is the hydrodynamic radius (RH)? Chapter: Dynamic Light Scattering

    E-print Network

    Economou, Tassos

    What is the hydrodynamic radius (RH)? Chapter: Dynamic Light Scattering Key Words: hydrodynamic, shape effects, radius of gyration The measured data in a dynamic light scattering (DLS) experimentT f kT D == Figure 1: Representative correlation function measured during a dynamic light scattering

  8. Reduction of the proton radius discrepancy by 3 sigma

    E-print Network

    I. T. Lorenz; U. -G. Meißner

    2014-08-25

    We show that in previous analyses of electron-proton scattering, the uncertainties in the statistical procedure to extract the proton charge radius are underestimated. Using a fit function based on a conformal mapping, we can describe the scattering data with high precision and extract a radius value in agreement with the one obtained from muonic hydrogen.

  9. Calculating the Atomic Radius of Gold Frank Rioux

    E-print Network

    Rioux, Frank

    Calculating the Atomic Radius of Gold Frank Rioux Three experimental facts are required to determine the atomic radius of a metallic element such as gold: density, or basic repeating unit, shows that it contains four gold atoms and that the gold atoms touch along

  10. Screw prominences related to palmar locking plating of distal radius

    Microsoft Academic Search

    T. S. Sügün; N. Karabay; Y. Gürbüz; K. Özaksar; T. Toros; M. Kayalar

    2011-01-01

    Fixation of unstable distal radius fractures with palmar locking plates provides a stable reduction and early return of function, but complications arising from unrecognized dorsally prominent screws penetrating the extensor compartments are increasingly reported. Standard radiographs and fluoroscopy may not adequately visualize screw lengths, owing to the complex shape of the dorsum of the distal radius. We examined 46 distal

  11. Optimal Transmission Radius for Energy Efficient Broadcasting Protocols in

    E-print Network

    Stojmenovic, Ivan

    of minimum energy broadcasting in ad hoc networks where nodes have capability to adjust their transmission networks. We present two localized broadcasting protocols, based on derived "target" radius, that remain competitive for all network densities. The first one, TR-LBOP, computes the minimal radius needed

  12. Optimal Transmission Radius for Energy Efficient Broadcasting Protocols

    E-print Network

    Paris-Sud XI, Université de

    of minimum en- ergy broadcasting in ad hoc networks where nodes have capability to adjust their transmission networks. We present two localized broadcasting protocols, based on derived `target' radius, that remain competitive for all network densities. The first one, TR-LBOP, computes the minimal radius needed

  13. Studying the proton 'radius' puzzle with ?p elastic scattering

    SciTech Connect

    Gilman, R. [Rutgers, The State University of New Jersey, Piscataway, NJ 08854 (United States)

    2013-11-07

    The disagreement between the proton radius determined from muonic hydrogen and from electronic measurements is called the proton radius puzzle. The resolution of the puzzle remains unclear and appears to require new experimental results. An experiment to measure muon-proton elastic scattering is presented here.

  14. 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…

  15. Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon

    E-print Network

    Thompson, Nicholas J.

    Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate ...

  16. Design and construction of instruments for exciton diffusion characterization and for patterning of thin films

    E-print Network

    Mendoza, Hiroshi Antonio

    2012-01-01

    In this thesis the instruments explore two main aspects of organic optoelectronic devices. One instrument characterizes exciton diffusion and the other patterns organic thin films. Exciton diffusion characteristics are ...

  17. Coherent Exciton Dynamics in Supramolecular Light-Harvesting Nanotubes Revealed by Ultrafast Quantum Process Tomography

    E-print Network

    Arias, Dylan H.

    Long-lived exciton coherences have been recently observed in photosynthetic complexes via ultrafast spectroscopy, opening exciting possibilities for the study and design of coherent exciton transport. Yet, ambiguity in the ...

  18. Tuneable Singlet Exciton Fission and Triplet-Triplet Annihilation in an Orthogonal Pentacene Dimer

    E-print Network

    Lukman, Steven; Musser, Andrew J.; Chen, Kai; Athanasopoulos, Stavros; Yong, Chaw K.; Zeng, Zebing; Ye, Qun; Chi, Chunyan; Hodgkiss, Justin M.; Wu, Jishan; Friend, Richard H.; Greenham, Neil C.

    2015-01-01

    We report fast and highly efficient intramolecular singlet exciton fission in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units. Fission to triplet excitons from this ground state geometry occurs within 1 ps...

  19. Echo peak-shift spectroscopy of non-Markovian exciton dynamics in quantum wells

    NASA Astrophysics Data System (ADS)

    Carter, S. G.; Chen, Z.; Cundiff, S. T.

    2007-09-01

    Three-pulse four-wave mixing is used to observe the loss of system memory for excitons confined in quantum wells. By measuring the photon echo peak shift with pulse delay, we obtain the two-time exciton frequency fluctuation correlation function. Spectral diffusion of localized excitons gives rise to exponential decay of the correlation function, while static inhomogeneity gives an offset. Nonmonotonic behavior during the first few picoseconds indicates complex memory effects due to the interaction of excitons with acoustic phonons.

  20. Aharonov-Bohm effect for excitons in a semiconductor quantum ring dressed by circularly polarized light

    NASA Astrophysics Data System (ADS)

    Kibis, O. V.; Sigurdsson, H.; Shelykh, I. A.

    2015-06-01

    We show theoretically that the strong coupling of circularly polarized photons to an exciton in ringlike semiconductor nanostructures results in physical nonequivalence of clockwise and counterclockwise exciton rotations in the ring. As a consequence, the stationary energy splitting of exciton states corresponding to these mutually opposite rotations appears. This excitonic Aharonov-Bohm effect depends on the intensity and frequency of the circularly polarized field and can be detected in state-of-the-art optical experiments.

  1. Hybridized quadrupole-dipole exciton effects in a Cu2O -organic heterostructure

    Microsoft Academic Search

    Oleksiy Roslyak; Joseph L. Birman

    2007-01-01

    In the present work, we discuss resonant hybridization of the 1S quadrupole Wannier-Mott exciton (WE) in a Cu2O quantum well with the Frenkel dipole exciton in an adjacent layer of organic DCM2:CA:PA. The coupling between excitons is due to interaction between the gradient of electric field induced by the DCM2 Frenkel exciton (FE) and the quadrupole moment of the 1S

  2. Excitons in charged Ge\\/Si type-II quantum dots

    Microsoft Academic Search

    A. I. Yakimov; N. P. Stepina; A. V. Dvurechenskii; A. I. Nikiforov; A. V. Nenashev

    2000-01-01

    Using electron-filling modulation absorption spectroscopy, we study the effect of quantum dot (QD) charging on the interband excitonic transitions in type-II Ge\\/Si heterostructures containing pyramidal Ge nanocrystals. In contrast to type-I systems, the ground-state absorption is found to be blueshifted when exciton-hole and exciton-exciton complexes are formed. We argue that this is the consequence of dominance of the hole-hole and

  3. Exciton-plasmon and spin-plasmon interactions in hybrid semiconductor-metal nanostructures

    Microsoft Academic Search

    Alexander Govorov

    2011-01-01

    Coulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several effects: energy transfer between nanoparticles, plasmon enhancement, Lamb shifts of exciton lines, Fano interference. In a complex composed of semiconductor quantum dot and metal nanoparticle, plasmons interact with spin-polarized excitons. This interaction leads to the formation of coupled spin-plasmon excitations and to spin-dependent Fano resonances. If an exciton-plasmon

  4. Towards R-space Bose-Einstein Condensation of Photonic Crystal Exciton Polaritons

    Microsoft Academic Search

    Dmitri L. Boiko

    2008-01-01

    Coupled states of semiconductor quantum well (QW) excitons and photons in a\\u000atwo dimensional (2D) periodic lattice of microcavities are analyzed\\u000atheoretically, revealing allowed bands and forbidden gaps in the energy\\u000aspectrum of exciton polaritons. Photonic crystal exciton polaritons have\\u000aspatially uniform excitonic constituent set by flat QWs, but exhibit periodic\\u000aBloch oscillations in the plane of QWs due to

  5. Incoherent exciton trapping in self-similar aperiodic lattices

    SciTech Connect

    Dominguez-Adame, F.; Macia, E. (Departamento de Fisica de Materiales, Facultad de Fisicas, Universidad Complutense, E-28040 Madrid (Spain)); Sanchez, A. (Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States) Escuela Politecnica Superior, Universidad Carlos III de Madrid, C./Butarque 15, E-28911 Leganes, Madrid (Spain))

    1995-01-01

    Incoherent exciton dynamics in one-dimensional perfect lattices with traps at sites arranged according to aperiodic deterministic sequences is studied. We focus our attention on Thue-Morse and Fibonacci systems as canonical examples of self-similar aperiodic systems. Solving numerically the corresponding master equation we evaluate the survival probability and the mean-square displacement of an exciton initially created at a single site. Results are compared to systems of the same size with the same concentration of traps randomly as well as periodically distributed over the whole lattice. Excitons progressively extend over the lattice on increasing time and, in this sense, they act as a probe of the particular arrangements of traps in each system considered. The analysis of the characteristic features of their time decay indicates that exciton dynamics in self-similar aperiodic arrangements of traps is quite close to that observed in periodic ones, but differs significantly from that corresponding to random lattices. We also report on characteristic features of exciton motion suggesting that Fibonacci and Thue-Morse orderings might be clearly observed by appropriate experimental measurements. In the conclusions we comment on the implications of our work on the way towards a unified theory of the ordering of matter.

  6. Quantitative interpretation of the excitonic splittings in aluminum nitride

    NASA Astrophysics Data System (ADS)

    Gil, B.; Guizal, B.; Felbacq, D.; Bouchitté, G.

    2011-02-01

    We address the interpretation of the splitting between the ground state excitonic transition which indicates the energy of the lowest direct band gap in AlN bulk films and epilayers, and a 36-38 meV higher energy companion. We demonstrate that this splitting is consistent with the initial interpretation in terms of 1s-2s excitonic splitting by using a calculation of the exciton binding energy which includes mass anisotropy and anisotropy of the dielectric constant. Analytical expressions are proposed to compute the evolution of 1s and 2s excitonic energies using an anisotropy parameter. We show that the values of the dielectric constant that are required to fit the data are \\varepsilon bot ? 8.7 and \\varepsilon allel ? 10, values different from the couple of values \\varepsilon bot ? 7.33 and \\varepsilon allel ? 8.45 erroneously obtained after a fitting procedure using a spherical description of the long range Coulomb interaction and the classical textbook n-2 spectrum of the excitonic eigenstates. Starting from now, our values are the recommended ones.

  7. Thrombocytopenia with Unilateral Dysplastic Radius- Is it Thrombocytopenia - Absent Radius (TAR) Syndrome?

    PubMed Central

    Chaudhary, Indradeo Prasad; Ranjan, Ram Bilas; Kumar, Prashant

    2015-01-01

    Thrombocytopenia - absent radii (TAR) syndrome is an autosomal recessive genetic rare disorder with hypomegakaryocytic thrombocytopenia and bilateral absent radius that may have additional anomalies. This disorder is characterized by thrombocytopenia resulting in potentially severe bleeding episodes primarily during infancy. We report the case of a 7-day-old term appropriate for gestational age (AGA) male baby, product of non consanguineous marriage presented with bloody loose stool, right sided upper limb deformity and paleness of the body, was diagnosed as TAR syndrome with some atypical presentation. Such type of atypical presentation has not been previously reported in a case with TAR Syndrome.Patient was managed in our hospital with packed cell transfusion and two units platelets concentrates transfusion, Intra-venous antimicrobials, and other supportive treatment. He gradually improved and was discharged after seven days of hospital stay with advice to consult orthopedic surgeon for opinion regarding limb reconstruction. PMID:25954675

  8. Exciton-Polariton Coupling with Acoustic Phonons

    NASA Astrophysics Data System (ADS)

    Cerda-Méndez, Edgar; Krizhanovskii, Dmitryi; Wouters, Michiel; Biermann, Klaus; Hey, Rudolf; Skolnick, Maurice S.; Santos, Paulo V.

    Exciton-polariton are solid-state composite bosons with a high photonic character and low effective mass, which have been proven to undergo a thermodynamic phase transition to a macroscopically occupied state—a condensate—above a characteristic threshold density (Kasprzak et al. Nature 443:409, 2006; Balili et al. Science 316:1007, 2007). Full exploitation of the unique polariton properties requires dynamic processes for the dynamic confinement and control of the interaction between condensates. Here, we demonstrate a novel approach for the formation of arrays of interacting polariton condensates based on the spatial and temporal modulation by a coherent acoustic phonon. Analogous to the confinement of atomic Bose-Einstein condensates by optical lattices, the acoustic spatial modulation forms an array of polariton wires aligned with the phonon wavefronts. We show that the moving acoustic modulation controls both the energetic configuration and the spatial coherence length of the polariton condensates. Furthermore, the confinement potential moves with the acoustic velocity, thereby transporting the polariton wires. These moving acoustic confinement potentials provide, therefore, a powerful framework for manipulation and transport of solid-state condensates.

  9. Long-range transport in excitonic dark states in coupled quantum wells

    E-print Network

    Snoke, David

    Long-range transport in excitonic dark states in coupled quantum wells D. Snoke1 , S. Denev1 , Y, NJ 07974-0636 In the past ten tears, the system of coupled quantum wells has emerged as a promising is the possibility of long-range coherent transport of excitons. Excitons in quantum wells typically diffuse only

  10. Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

    E-print Network

    Rossi, Enrico

    Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots manipulation of excitonic qubits in self-assembled quantum dots. Rabi oscillations were recorded by varying of excitonic states,4­6 the hallmark for optical manipulation of qubits in QDs. However, all found that ROs

  11. Lattice exciton-polaron problem by quantum Monte Carlo simulations Martin Hohenadler,1,

    E-print Network

    Fehske, Holger

    Lattice exciton-polaron problem by quantum Monte Carlo simulations Martin Hohenadler,1, * Peter B Greifswald, Germany Received 21 September 2007; published 15 November 2007 Exciton-polaron formation in one simulations. Depending on the relative sign of electron and hole-phonon coupling, the exciton-polaron size

  12. Exciton-charge carrier interactions in the electroluminescence of crystalline anthracene

    Microsoft Academic Search

    M. Wittmer; I. Zschokke-Gränacher

    1975-01-01

    The interactions of triplet excitons with trapped charge carriers are studied in anthracene single crystals. The triplet excitons are generated by the recombination of charge carriers through double injection. The lifetime of the triplet excitons is studied from the decay of the phosphorescence intensity in a transient measurement. The interaction rate constant for the trapped charge carriers is of the

  13. Theory of the lifetime of an exciton incoherently created below its resonance frequency by inelastic scattering

    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.

  14. Charge separation in excitonic and bipolar solar cells — A detailed balance approach

    Microsoft Academic Search

    Thomas Kirchartz; Uwe Rau

    2008-01-01

    A generalized solar cell model for excitonic and classical, bipolar solar cells is developed that describes the combined transport and interaction of electrons, holes and excitons. Both, conventional inorganic solar cells as well as organic solar cells, where excitons play a dominant role for energy transport, turn out to be special cases of this model. Due to the inclusion of

  15. Effet d'une microonde sur la fission des excitons singulets dans le ttracne cristallin

    E-print Network

    Paris-Sud XI, Université de

    of fission singlet exciton processes by high power microwaves in crystalline tetracene. The observed effects indicate transitions between pair states of triplets produced by singlet exciton fission. The study643 Effet d'une microonde sur la fission des excitons singulets dans le tétracène cristallin J

  16. MAGNETIC FIELD DEPENDENCE OF SINGLET EXCITON FISSION AND FLUORESCENCE IN CRYSTALLINE TETRACENE AT 300 K

    E-print Network

    Boyer, Edmond

    257 MAGNETIC FIELD DEPENDENCE OF SINGLET EXCITON FISSION AND FLUORESCENCE IN CRYSTALLINE TETRACENE'excitons singulet et triplet. Abstract. 2014 The magnetic field dependence of singlet exciton fission by the zero-field splitting tensor as obtained by EPR, indicating that the outcome of the singlet fission

  17. Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

    E-print Network

    singlet- triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel behavior in an organic semiconductor by effectively turning singlet- triplet exciton fission decay channelsMorphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic

  18. Two-Dimensional Correlation Spectroscopy of Two-Exciton Resonances in Semiconductor Quantum Wells

    E-print Network

    Mukamel, Shaul

    Two-Dimensional Correlation Spectroscopy of Two-Exciton Resonances in Semiconductor Quantum Wells the correlations of multiple excitons in semiconductors is a challenging many-body problem that had drawn)] or a blueshift [two-exciton scattering energy (TSE)]. In GaAs semicon- ductor quantum wells both couplings

  19. Enhanced ExcitonPhonon Coupling in Spherical GaAs Nanocrystals Studied

    E-print Network

    Atwater, Harry

    Enhanced Exciton­Phonon Coupling in Spherical GaAs Nanocrystals Studied by Selective Excitation-assisted exciton transitions in spherical GaAs nanocrystals embedded in SiO2 matrices have been studied by means the luminescence properties of GaAs nanocrystals. The coupling between excitons and phonons in spherical Ga

  20. Long-range excitons in conjugated polymers with ring torsions: poly(para-phenylene) and polyaniline

    E-print Network

    Harigaya, Kikuo

    (Running head: Long-range excitons in conjugated polymers) Kikuo Harigaya Physical Science DivisionLong-range excitons in conjugated polymers with ring torsions: poly(para-phenylene) and polyaniline formalism. Long-range excitons are charac- terized, and the long-range component of the oscillator strengths

  1. Thermoconvective vortices in a cylindrical annulus with varying inner radius.

    PubMed

    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

  2. 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.

  3. An Asian Perspective on the Management of Distal Radius Fractures

    PubMed Central

    Sebastin, Sandeep J.; Chung, Kevin C.

    2012-01-01

    Synopsis There is little data with regards to the epidemiology, pathology, or management of distal radius fractures from centers in Asia. Asia includes five advanced economies, namely Hong Kong SAR, Japan, Korea, Singapore, and Taiwan and a number of emerging economies prominent among which are China, India, Malaysia, Philippines, and Thailand. This article examines the available epidemiological data from Asia, and compares the management of distal radius fractures in the advanced and emerging Asian economies and how they match up to the current management in the west. It concludes by offering solutions for improving outcomes of distal radius fractures in both the advanced and emerging economies of Asia. PMID:22554658

  4. Finite Larmor radius modification of the Mercier criterion

    SciTech Connect

    Connor, J.W.; Tang, W.M.; Allen, L.

    1984-04-01

    The finite Larmor radius modification of the Suydam criterion involves a competition between stabilizing finite Larmor radius effects and destabilizing curvature. In the case of the toroidal calculation, corresponding to the Mercier criterion, ballooning effects from regions of unfavorable curvature must be taken into account. In the case of a model equilibrium, valid near the magnetic axis, a complete solution is obtained. Results indicate that the amount of finite Larmor radius stabilization needed to overcome the effects of unfavorable average curvature increases as a function of the toroidal ballooning parameter.

  5. Effects of fermion exchange on the polarization of exciton condensates.

    PubMed

    Combescot, Monique; Combescot, Roland; Alloing, Mathieu; Dubin, François

    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

  6. Resonant scattering of light by excitonic rough surfaces

    NASA Astrophysics Data System (ADS)

    Cocoletzi, G. H.; Wang, S.

    1993-12-01

    The Rayleigh-Fano approach is used to study the scattering of p-polarized light from the deterministically shaped rough surface of a semiconductor in the vicinity of an isolated excitonic transition. Nonlocal effects are accounted by the Hopfield and Thomas dielectric function. A perturbative formalism is developed to calculate the amplitudes of the scattered fields up to second order on the roughness height. Numerical results are presented for the An=1 excitonic transition of CdS using different choices of additional boundary conditions. Comparisons are made with the local model calculations and discussed in terms of resonant elastic scattering. The differential reflectance spectra peaks are also shown and interpreted as surface-exciton-polariton coupling.

  7. Quasi-1D excitons in lateral induced superlattices.

    NASA Astrophysics Data System (ADS)

    Cocoletzi, Gregorio H.; Hernández de La Luz, Alvaro D.; Ulloa, Sergio E.

    1998-03-01

    Effects of electrostatic-potential-induced lateral superlattices on the optical properties of quasi-1D excitons in GaAs-AlGaAs quantum wells are investigated. Variational estimations are presented, in the tight binding limit, of minibands, binding energies E_ex and absorption coefficients ?_ex of the ground state and first excited state of heavy-hole excitonic transitions as functions of the applied electrostatic potentials and the period of induced superlattices. The limit of small periods and high electrostatic potential produces strong confinement, and consequently, this polarizes the excitons, resembling a type II superlattice where electrons and holes are spatially separated in different potential wells. The variations of the period induce strongh oscillations on E_ex and ?_ex as a result of a competition between quantum structural confinement and Coulomb interactions.

  8. Quantum confinement-induced tunable exciton states in graphene oxide

    PubMed Central

    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

  9. Excitons recombination engineering in self assembled quantum dots

    NASA Astrophysics Data System (ADS)

    Petroff, Pierre

    2000-03-01

    We demonstrate using band gap engineering in self assembled quantum dot structures that excitons can be stored for several seconds . The optically generated excitons are dissociated and stored as separated electron hole pairs into coupled quantum dot pairs. A bias voltage restores the excitons which recombine radiatively to provide a read out optical signal. The localization of the spatially separated electron hole pair in quantum dots is responsible for the ultra long storage times which are on the order of several seconds. The present limits of this optical storage medium as a function of time and temperature are discussed. Acknowledgements: This research was done in collaboration with T.Lundstrom, W.Schoenfeld, H.Lee and was financed by DARPA, ARO and QUEST the NSF-STC center at UCSB.

  10. 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.

  11. Controlled Spin Transport in Planar Systems Through Topological Exciton

    E-print Network

    Abhinav, Kumar

    2015-01-01

    It is shown that a charge-neutral spin-1 exciton, possibly realizable only in planar systems like graphene and topological insulators, can be effectively used for controlled spin transport in such media. The effect of quantum and thermal fluctuations yield a parametric excitation threshold for its realization. This planar exciton differs from the conventional ones, as it owes its existence to the topological Chern-Simons (CS) term. The parity and time-reversal violating CS term can arise from quantum effects in systems with parity-breaking mass-gap. The spinning exciton naturally couples to magnetic field, leading to the possibility of controlled spin transport. Being neutral, it is immune to a host of effect, which afflicts spin transport through charged fermions.

  12. Huge Excitonic Effects in Layered Hexagonal Boron Nitride

    NASA Astrophysics Data System (ADS)

    Arnaud, B.; Lebègue, S.; Rabiller, P.; Alouani, M.

    2006-01-01

    The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).1476-112210.1038/nmat1134] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.

  13. Impurity effects on polaron-exciton formation in conjugated polymers

    SciTech Connect

    Ribeiro, Luiz Antonio, E-mail: ribeirojr@fis.unb.br; Ferreira da Cunha, Wiliam; Neto de Oliveira, Pedro Henrique; Gargano, Ricardo; Magela e Silva, Geraldo [Institute of Physics, University of Brasilia, 70.919-970 Brasilia (Brazil)] [Institute of Physics, University of Brasilia, 70.919-970 Brasilia (Brazil)

    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.

  14. Measurement and modification of biexciton-exciton time correlations.

    PubMed

    Huber, Tobias; Predojevi?, Ana; Zoubi, Hashem; Jayakumar, Harishankar; Solomon, Glenn S; Weihs, Gregor

    2013-04-22

    Photons which are generated in a two-photon cascade process have an underlying time correlation since the spontaneous emission of the upper level populates the intermediate state. This correlation leads to a reduction of the purity of the photon emitted from the intermediate state. Here we characterize this time correlation for the biexciton-exciton cascade of an InAs/GaAs quantum dot. We show that the correlation can be reduced by tuning the biexciton transition in resonance to a planar distributed Bragg reflector cavity. The enhanced and inhibited emission into the cavity accelerates the biexciton emission and slows down the exciton emission thus reduces the correlation and increases the purity of the exciton photon. This is essential for schemes like creating time-bin entangled photon pairs from quantum dot systems. PMID:23609694

  15. Robust Excitons and Trions in Monolayer MoTe2.

    PubMed

    Yang, Jiong; Lü, Tieyu; Myint, Ye Win; Pei, Jiajie; Macdonald, Daniel; Zheng, Jin-Cheng; Lu, Yuerui

    2015-06-23

    Molybdenum telluride (MoTe2) has emerged as a special member in the family of two-dimensional transition metal dichalcogenide semiconductors, owing to the strong spin-orbit coupling and relatively small energy gap, which offers new applications in valleytronic and excitonic devices. Here we successfully demonstrated the electrical modulation of negatively charged (X(-)), neutral (X(0)), and positively charged (X(+)) excitons in monolayer MoTe2 via photoluminescence spectroscopy. The binding energies of X(+) and X(-) were measured to be ?24 and ?27 meV, respectively.The exciton binding energy of monolayer MoTe2 was measured to be 0.58 ± 0.08 eV via photoluminescence excitation spectroscopy, which matches well with our calculated value of 0.64 eV. PMID:26039551

  16. Avalanche amplification of a single exciton in a semiconductor nanowire

    NASA Astrophysics Data System (ADS)

    Bulgarini, Gabriele; Reimer, Michael E.; Hocevar, Moïra; Bakkers, Erik P. A. M.; Kouwenhoven, Leo P.; Zwiller, Val

    2012-07-01

    Interfacing single photons and electrons is a crucial element in sharing quantum information between remote solid-state qubits. Semiconductor nanowires offer the unique possibility of combining optical quantum dots with avalanche photodiodes, thus enabling the conversion of an incoming single photon into a macroscopic current for efficient electrical detection. Currently, millions of excitation events are required to perform electrical readout of an exciton qubit state. Here, we demonstrate multiplication of carriers from only a single exciton generated in a quantum dot after tunnelling into a nanowire avalanche photodiode. Owing to the large amplification of both electrons and holes (>104), we reduce by four orders of magnitude the number of excitation events required to electrically detect a single exciton generated in a quantum dot. This work represents a significant step towards achieving single-shot electrical readout and offers a new functionality for on-chip quantum information circuits.

  17. Huge excitonic effects in layered hexagonal boron nitride.

    PubMed

    Arnaud, B; Lebègue, S; Rabiller, P; Alouani, M

    2006-01-20

    The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor. PMID:16486604

  18. Optical control of charged exciton states in tungsten disulfide

    NASA Astrophysics Data System (ADS)

    Currie, M.; Hanbicki, A. T.; Kioseoglou, G.; Jonker, B. T.

    2015-05-01

    A method is presented for optically preparing WS2 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 A1g intensity and an enhanced feature on the low energy side of the E12g 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.

  19. 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.

  20. Microscopic theory of singlet exciton fission. I. General formulation.

    PubMed

    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

  1. Controlled Spin Transport in Planar Systems Through Topological Exciton

    E-print Network

    Kumar Abhinav; Prasanta K. Panigrahi

    2015-04-29

    It is shown that a charge-neutral spin-1 exciton, possibly realizable only in planar systems like graphene and topological insulators, can be effectively used for controlled spin transport in such media. The effect of quantum and thermal fluctuations yield a parametric excitation threshold for its realization. This planar exciton differs from the conventional ones, as it owes its existence to the topological Chern-Simons (CS) term. The parity and time-reversal violating CS term can arise from quantum effects in systems with parity-breaking mass-gap. The spinning exciton naturally couples to magnetic field, leading to the possibility of controlled spin transport. Being neutral, it is immune to a host of effect, which afflicts spin transport through charged fermions.

  2. Exciton condensation due to electron-phonon interaction

    NASA Astrophysics Data System (ADS)

    Phan, Van-Nham; Becker, Klaus W.; Fehske, Holger

    2013-11-01

    We show that coupling to vibrational degrees of freedom can drive a semimetal excitonic-insulator quantum phase transition in a one-dimensional two-band f-c-electron system at zero temperature. The insulating state typifies an excitonic condensate accompanied by a finite lattice distortion. Using the projector-based renormalization method we analyze the ground-state and spectral properties of the interacting electron-phonon model at half filling. In particular we calculate the momentum dependence of the excitonic order-parameter function and determine the finite critical interaction strength for the metal-insulator transition to appear. The electron spectral function reveals the strong hybridization of f- and c-electron states and the opening of a single-particle excitation gap. The phonon spectral function indicates that the phonon mode involved in the transition softens (hardens) in the adiabatic (nonadiabatic and extreme antiadiabatic) phonon frequency regime.

  3. Effects of Fermion Exchange on the Polarization of Exciton Condensates

    NASA Astrophysics Data System (ADS)

    Combescot, Monique; Combescot, Roland; Alloing, Mathieu; Dubin, François

    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.

  4. 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.

  5. Diffusion-mediated exciton processes in molecular crystals: Comment on the magnetic field dependence of fission and fusion

    Microsoft Academic Search

    Norman F. Berk

    1978-01-01

    It is shown in a kinematic analysis based on Suna's theory of diffusion-mediated triplet exciton-exciton annihilation that the rate constants for triplet exciton fusion (annihilation) and singlet exciton fission have the same magnetic field dependence. The relation between Suna's theory and Noyes' stochastic theory of diffusion-mediated chemical reactions is briefly discussed.

  6. Bi-layer excitons in two-dimensional layered materials

    NASA Astrophysics Data System (ADS)

    Neupane, Mahesh; Yin, Gen; Wickramaratne, Darshana; Lake, Roger

    2013-03-01

    Following the prediction of exciton condensation in closely spaced two-dimensional electron-hole bilayer systems, there has been a sustained theoretical and experimental investigation of this condensation phase in coupled quantum well material systems. The electron-hole pairs are bound by the interlayer Coulomb interaction, which is tuned by electrostatic gating of the charge density. The magnitude of this interaction is determined by the binding energy between the electron and the hole. Improvements in the exciton binding energy can be achieved by an appropriate choice of materials. The family of van der Walle materials is considered in this study, and the effect of material choice and insulating layer thickness on the excitonic properties will be discussed and compared to experimental investigations using traditional GaAs-AlGaAs coupled quantum wells. Following the prediction of exciton condensation in closely spaced two-dimensional electron-hole bilayer systems, there has been a sustained theoretical and experimental investigation of this condensation phase in coupled quantum well material systems. The electron-hole pairs are bound by the interlayer Coulomb interaction, which is tuned by electrostatic gating of the charge density. The magnitude of this interaction is determined by the binding energy between the electron and the hole. Improvements in the exciton binding energy can be achieved by an appropriate choice of materials. The family of van der Walle materials is considered in this study, and the effect of material choice and insulating layer thickness on the excitonic properties will be discussed and compared to experimental investigations using traditional GaAs-AlGaAs coupled quantum wells. Microelectronics Advanced Research Corporation Focus Center on Nano Materials (FENA)

  7. Optical properties of excitons in semiconductor superlattices and microcavities

    NASA Astrophysics Data System (ADS)

    Coyotécatl, H. A.; Palomino-Ovando, M.; Cocoletzi, Gregorio H.

    1999-07-01

    The optical response of Mott-Wannier excitons is investigated in semiconductor superlattices and microcavities. p-Polarized light is considered to calculate the reflectivity Rpand dispersion relation of the collective normal modes in superlattices accounting for extrinsic Morse potential wells, and Rp in microcavities. Results of Rpexhibit well-defined peaks of the exciton bound states in the Morse potentials for both transverse and longitudinal modes. Comparisons of Rp with experimental reflectivity data of light for semiconductor microcavities exhibit good qualitative agreement as well as Rabi splitting.

  8. Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.

    PubMed

    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

  9. Frequency Combs with Weakly Lasing Exciton-Polariton Condensates

    NASA Astrophysics Data System (ADS)

    Rayanov, K.; Altshuler, B. L.; Rubo, Y. G.; Flach, S.

    2015-05-01

    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.

  10. Excitonic splitting, delocalization, and vibronic quenching in the benzonitrile dimer.

    PubMed

    Balmer, Franziska A; Ottiger, Philipp; Leutwyler, Samuel

    2014-11-26

    The excitonic S1/S2 state splitting and the localization/delocalization of the S1 and S2 electronic states are investigated in the benzonitrile dimer (BN)2 and its (13)C and d5 isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h5)2 and (BN-d5)2 dimers are C2h symmetric with equivalent BN moieties. Only the S0 ? S2 electronic origin is observed, while the S0 ? S1 excitonic component is electric-dipole forbidden. A single (12)C/(13)C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 – h5(13)C), (BN)2-(h5 – d5), and (BN)2-(h5 – h5(13)C) exhibit both S0 ? S1 and S0 ? S2 origins. Isotope-dependent contributions ?iso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from ?iso((12)C/(13)C) = 3.3 cm(–1) to ?iso(h5/d5) = 155.6 cm(–)1. The analysis of the experimental S1/S2 splittings of six different isotopomeric dimers yields the S1/S2 exciton splitting ?exc = 2.1 ± 0.1 cm(–1). Since ?iso(h5/d5) ? ?exc and ?iso((12)C/(13)C) > ?exc, complete and near-complete exciton localization occurs upon (12)C/(13)C and h5/d5 substitutions, respectively, as diagnosed by the relative S0 ? S1 and S0 ? S2 origin band intensities. The S1/S2 electronic energy gap of (BN)2 calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is ?el(calc) = 10 cm(–1). This electronic splitting is reduced by the vibronic quenching factor ?. The vibronically quenched exciton splitting ?el(calc)·? = ?vibron(calc) = 2.13 cm(–1) is in excellent agreement with the observed splitting ?exc = 2.1 cm(–1). The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the (BN)2(h5 – d5) heterodimer. PMID:25353324

  11. Coherent exciton delocalization in strongly coupled quantum dot arrays.

    PubMed

    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

  12. Exciton States in a Quantum Dot with Parabolic Confinement

    NASA Astrophysics Data System (ADS)

    Do?an, Ü.; Sakiro?lu, S.; Yildiz, A.; Akgüngör, K.; Epik, H.; Sökmen, I.; Sari, H.; Ergün, Y.

    In this study the electronic eigenstructure of an exciton in a parabolic quantum dot (QD) has been calculated with a high accuracy by using Finite element method (FEM). We have converted the coordinates of electron-light-hole system to relative and center of mass coordinate, then placed the Spherical Harmonics into Schrödinger equation analytically and obtained the Schrödinger equation which depends only on the radial variable. Finally we used FEM with only radial variable in order to get the accurate numerical results. We also showed first 21 energy level spectra of exciton depending on confinement and Coulomb interaction parameters.

  13. Evidence of Hybrid Excitons in Weakly Interacting Nanopeapods.

    PubMed

    Milko, Matus; Puschnig, Peter; Blondeau, Pascal; Menna, Enzo; Gao, Jia; Loi, Maria Antonietta; Draxl, Claudia

    2013-08-15

    Nanopeapods, consisting of optically active ?-conjugated molecules encapsulated inside of the cavity of carbon nanotubes, exhibit efficient photon emission in the visible spectral range. Combining optical experiments with ab initio theory, we show that the puzzling features observed in photoluminescence spectra are of excitonic nature. The subunits though being van der Waals bound are demonstrated to interact in the excited state, giving rise to the formation of hybrid excitons. We rationalize why this many-body effect makes such nanohybrids useful for optoelectronic devices. PMID:23991266

  14. Exciton-Plasmon Interaction Effects in Individual Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Bondarev, Igor; Meliksetyan, Areg

    2013-03-01

    We have recently developed a theory for the electrostatically controlled coupling between excitons and low-energy inter-band plasmons in individual semiconducting carbon nanotubes. Here, we report on our studies towards the applications of this effect of both applied and fundamental interest. One practical application is the electromagnetic absorption/photoluminescence control for individual nanotubes. Another, fundamental one, comes from the fact that the coupling of the excitons to the same inter-band plasmon resonance results in their entanglement, a pre-requisite for strong quantum correlations/quantum phase transitions in many-particle systems. Our coupled exciton-plasmon excitation is a quasi-1D Bose system and could possibly be Bose-condensed in an individual carbon nanotube under appropriately created external conditions -- despite the mathematical statements of the BEC impossibility in ideal 1D and 2D quantum systems and previously reported evidence for no free-exciton BEC in carbon nanotubes. We have recently developed a theory for the electrostatically controlled coupling between excitons and low-energy inter-band plasmons in individual semiconducting carbon nanotubes. Here, we report on our studies towards the applications of this effect of both applied and fundamental interest. One practical application is the electromagnetic absorption/photoluminescence control for individual nanotubes. Another, fundamental one, comes from the fact that the coupling of the excitons to the same inter-band plasmon resonance results in their entanglement, a pre-requisite for strong quantum correlations/quantum phase transitions in many-particle systems. Our coupled exciton-plasmon excitation is a quasi-1D Bose system and could possibly be Bose-condensed in an individual carbon nanotube under appropriately created external conditions -- despite the mathematical statements of the BEC impossibility in ideal 1D and 2D quantum systems and previously reported evidence for no free-exciton BEC in carbon nanotubes. DOE-DE-SC0007117 (I.B.), ARO-W911NF-11-1-0189 (A.M.)

  15. Frequency combs with weakly lasing exciton-polariton condensates.

    PubMed

    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

  16. Exciton localization-delocalization transition in an extended dendrimer

    SciTech Connect

    Pouthier, Vincent, E-mail: vincent.pouthier@univ-fcomte.fr [Institut UTINAM, Université de Franche-Comté, CNRS UMR 6213, 25030 Besançon Cedex (France)] [Institut UTINAM, Université de Franche-Comté, CNRS UMR 6213, 25030 Besançon Cedex (France)

    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.

  17. Exciton Lifetime Paradoxically Enhanced by Dissipation and Decoherence - Toward Efficient Energy Conversion of Solar Cell

    E-print Network

    Yamada, Yasuhiro; Imada, Masatoshi

    2015-01-01

    Energy dissipation and decoherence are at first glance harmful to acquiring 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 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 carbon nanotube, the exciton lifetime increases by nearly two orders of magnitude when the crossover triggers stable high population in the dark exciton.

  18. Exciton Lifetime Paradoxically Enhanced by Dissipation and Decoherence - Toward Efficient Energy Conversion of Solar Cell

    E-print Network

    Yasuhiro Yamada; Youhei Yamaji; Masatoshi Imada

    2015-02-25

    Energy dissipation and decoherence are at first glance harmful to acquiring 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 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 carbon nanotube, the exciton lifetime increases by nearly two orders of magnitude when the crossover triggers stable high population in the dark exciton.

  19. All-optical depletion of dark excitons from a semiconductor quantum dot

    NASA Astrophysics Data System (ADS)

    Schmidgall, E. R.; Schwartz, I.; Cogan, D.; Gantz, L.; Heindel, T.; Reitzenstein, S.; Gershoni, D.

    2015-05-01

    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.

  20. 21 CFR 886.1450 - Corneal radius measuring device.

    Code of Federal Regulations, 2013 CFR

    2013-04-01

    ...2013-04-01 false Corneal radius measuring device. 886.1450 Section 886.1450 Food and...DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES OPHTHALMIC DEVICES Diagnostic Devices § 886.1450 Corneal...

  1. 21 CFR 886.1450 - Corneal radius measuring device.

    Code of Federal Regulations, 2012 CFR

    2012-04-01

    ...2012-04-01 false Corneal radius measuring device. 886.1450 Section 886.1450 Food and...DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES OPHTHALMIC DEVICES Diagnostic Devices § 886.1450 Corneal...

  2. The scalar form factor and radius of the {sigma} resonance

    SciTech Connect

    Albaladejo, M.; Oller, J. A. [Departamento de Fisica, Univerisidad de Murcia, E-30071, Murcia (Spain)

    2011-05-23

    We calculate the scalar form factor and the quadratic scalar radius of the {sigma} meson from Chiral Lagrangians. We also study the behavior of the Feynman-Hellman theorem for the mass of the {sigma}.

  3. Radius of B-8 halo from the asymptotic normalization coefficient 

    E-print Network

    Carstoiu, F.; Trache, L.; Gagliardi, Carl A.; Tribble, Robert E.; Mukhamedzhanov, AM.

    2001-01-01

    The experimental asymptotic normalization coefficient determined from peripheral transfer reactions is used to obtain the root-mean-square radius of the wave function for the loosely bound proton in SB. It is shown that the asymptotic region...

  4. Squat exercise biomechanics during short-radius centrifugation

    E-print Network

    Duda, Kevin R., 1979-

    2007-01-01

    Artificial gravity (AG) created by short-radius centrifugation is a promising countermeasure to the physiological de-conditioning that results from long-duration spaceflight. However, as on Earth, gravity alone does not ...

  5. Fast Methods for Computing the $p$-Radius of Matrices

    E-print Network

    Jungers, Raphael M.

    The $p$-radius characterizes the average rate of growth of norms of matrices in a multiplicative semigroup. This quantity has found several applications in recent years. We raise the question of its computability. We prove ...

  6. 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.

  7. Anatomical and radiological study applied to distal radius surgery

    Microsoft Academic Search

    N. Gasse; D. Lepage; R. Pem; C. Bernard; J. M. Lerais; P. Garbuio; L. Obert

    Purpose  The purpose of this study was to determine, during anterior plating of the distal radius, the length of a screw above which\\u000a there is a risk for the extensor tendons and the optimal shape of an anterior plate. It was also to determine the projection\\u000a of the axis of the distal third of the radius on the distal articular surface

  8. LONG-TERM PERIODICITY VARIATIONS OF THE SOLAR RADIUS

    SciTech Connect

    Qu, Z. N.; Xie, J. L. [National Astronomical Observatories/Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming, Yunnan 650011 (China)] [National Astronomical Observatories/Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming, Yunnan 650011 (China)

    2013-01-01

    In order to study the long-term periodicity variations of the solar radius, daily solar radius data from 1978 February to 2000 September at the Calern Observatory are used. Continuous observations of the solar radius are difficult due to the weather, seasonal effects, and instrument characteristics. Thus, to analyze these data, we first use the Dixon criterion to reject suspect values, then we measure the cross-correlation between the solar radius and sunspot numbers. The result indicates that the solar radius is in complete antiphase with the sunspot numbers and shows lead times of 74 months relative to the sunspot numbers. The Lomb-Scargle and date compensated discrete Fourier transform methods are also used to investigate the periodicity of the solar radius. Both methods yield similar significance periodicities around {approx}1 yr, {approx}2.6 yr, {approx}3.6 yr, and {approx}11 yr. Possible mechanisms for these periods are discussed. The possible physical cause of the {approx}11 yr period is the cyclic variation of the magnetic pressure of the concentrated flux tubes at the bottom of the solar convection zone.

  9. Spin-Flip Limited Exciton Dephasing in CdSe/ZnS Colloidal Quantum Dots

    NASA Astrophysics Data System (ADS)

    Masia, Francesco; Accanto, Nicolò; Langbein, Wolfgang; Borri, Paola

    2012-02-01

    The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ˜10ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.

  10. Carrier-collision-induced formation of charged excitons and ultrafast dynamics fluorescence spectra.

    PubMed

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

    2012-12-13

    After a hole injection layer is inserted into a polymer light-emitting material, the injection of positive charge not only easily causes distortion in the conjugated polymer chain but also produces positive polarons. The ultrafast dynamics shows that, when the positive polaron approaches and collides with the triplet exciton, that exciton will become charged, whereby the non-emissive triplet exciton becomes radiative and emits light. Furthermore, the lifetime of the charged triplet exciton is longer than the singlet exciton. This paper explicitly depicts the dynamic fluorescence spectra of the radiative transition of the charged triplet exciton occurring during the decay of the charged exciton, and also exhibits the difference between traditional adiabatic dynamics and non-adiabatic dynamics. PMID:23186537

  11. 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.

  12. Spatial confinement, self-polarization and exciton-phonon interaction effect on the location of exciton line in lead iodide nanofilms

    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.

  13. Direct evidence for self-trapping of excitons by indium nanowires at In/Si(111) surface

    SciTech Connect

    Xu, Maojie, E-mail: mjxu@sjtu.edu.cn [Institute of Applied Physics, CREST-JST, University of Tsukuba, 305-8573 Tsukuba (Japan) [Institute of Applied Physics, CREST-JST, University of Tsukuba, 305-8573 Tsukuba (Japan); Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240 (China); Zhang, Yafei [Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240 (China)] [Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240 (China)

    2013-11-04

    We report on the real-space observation of self-trapped excitons using scanning tunneling microscope. Electrons of In nanowires transfer to the Si substrate, yielding charge-transfer excitons at In/Si interface. The strong coupling between excitons and lattice vibrations leads to the exciton localization at low carrier density and 80.0?K. Exciton condensation was observed at the proper carrier density and its microscopic origin is discussed.

  14. Cathodoluminescence of stacking fault bound excitons for local probing of the exciton diffusion length in single GaN nanowires

    SciTech Connect

    Nogues, Gilles, E-mail: gilles.nogues@neel.cnrs.fr; Den Hertog, Martien [Inst. NEEL, Univ. Grenoble Alpes, F-38042 Grenoble (France); Inst. NEEL, CNRS, F-38042 Grenoble (France); Auzelle, Thomas; Gayral, Bruno; Daudin, Bruno [INAC, CEA, F-38054 Grenoble (France)

    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.

  15. 8. Ueta, M. et al. in Excitonic Processes in Solids Ch. 3 (Springer, Berlin, 1986). 9. Itoh, T. & Suzuki, T. Excitonic polariton-polariton resonance scattering via excitonic molecules in

    E-print Network

    Haile, Sossina M.

    8. Ueta, M. et al. in Excitonic Processes in Solids Ch. 3 (Springer, Berlin, 1986). 9. Itoh, T. Soc. Jpn 45, 1939­1948 (1978). 10. Ho¨nerlage, B. et al. The dispersion of excitons, polaritons by a coherent biexciton wave. Phys. Rev. Lett. 89, 233601 (2002). 14. Saba, M. et al. High-temperature ultrafast

  16. Excitonic absorption in gate-controlled graphene quantum dots

    Microsoft Academic Search

    A. D. Güçlü; P. Potasz; P. Hawrylak

    2010-01-01

    We present a theory of excitonic processes in gate controlled graphene quantum dots. The dependence of the energy gap on shape, size, and edge for graphene quantum dots with up to a million atoms is predicted. Using a combination of tight-binding, Hartree-Fock and configuration interaction methods, we show that triangular graphene quantum dots with zigzag edges exhibit optical transitions simultaneously

  17. High quantum efficiency photoluminescence from localized excitons in Si, -,Ge,

    E-print Network

    measured an external photoluminescence quantum efficiency of 11.5 *2%. Recent progress in the epitaxialHigh quantum efficiency photoluminescence from localized excitons in Si, -,Ge, L. C. Lenchyshyn-*), very long decay times ( > 1 ms), and high quantum efficiency at low excitation. We have directly

  18. Heavy ion collisions and the pre-equilibrium exciton model

    SciTech Connect

    Betak, E. [Institute of Physics SAS, 84511 Bratislava (Slovakia); Faculty of Philosophy and Science, Silesian Univ., 74601 Opava (Czech Republic)

    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.

  19. EXCITON BEHAVIOR IN CARBON NANOTUBES: DIELECTRIC SCREENING AND DECAY DYNAMICS

    E-print Network

    ' & $ % EXCITON BEHAVIOR IN CARBON NANOTUBES: DIELECTRIC SCREENING AND DECAY DYNAMICS ANDREW GERALD BEHAVIOR IN CARBON NANOTUBES: DIELECTRIC SCREENING AND DECAY DYNAMICS by ANDREW GERALD WALSH B. S., Cornell place to work. To Mark, Craig, and Michele, thank you for all your support on the nanotube project. I

  20. Exciton Polarons of Molecular Crystal Model. I. Dynamical CPA

    Microsoft Academic Search

    Hitoshi Sumi

    1974-01-01

    Energy spectrum of exciton polaron is studied with the dynamical CPA (coherent potential approximation), which is introduced for inelastic scattering by Einstein phonons at every lattice site. The coherent potential at energy E is determined by the potentials at energies apart from E by integral times the phonon energy. We determine applicability ranges of the concepts used in the limiting

  1. Exciton Radiative Lifetimes in Layered Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Palummo, Maurizia; Benardi, Marco; Grossman, Jeffrey C.

    2015-03-01

    Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with 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. In this talk, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with chemical formula MX2 (M=Mo,W and X=S,Se), in bilayer and bulk MoS2, and in two MX2 hetero-bilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs, which have been the subjects of recent intense investigation. We find radiative lifetimes of a few ps at low temperature and a few ns at room temperature in the monolayers, and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 hetero-bilayers exhibit long-lived (~30 ns at room temperature) inter-layer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer; this finding agrees with recent ultrafast spectroscopy experiments. We discuss how the radiative lifetime tunability can be employed to manipulate excitons in 2D-TMDs.

  2. Coherent control of multipartite excitonic entanglement in quantum dot arrays

    NASA Astrophysics Data System (ADS)

    Rolon, Juan E.; Drut, Joaquin E.

    2015-03-01

    We propose a coherent control scheme for multipartite entanglement of exciton states in optically driven quantum dot arrays (QDAs) coupled by charge tunneling and resonant energy transfer (RET) processes. An adiabatic manipulation of the entanglement dynamics is devised by pulse shaping and time-dependent electric field sweeps. By varying the inter-dot distance and number of quantum dots (QDs) comprising the QDA, the excitonic qubit manifolds are obtained by a Feshbach projection over the resulting multilevel exciton configurations. We identify regimes in which the dynamics is confined to decoherence-free excitonic qubit manifolds taking into account spontaneous recombination and non-Markovian effects introduced by a phonon bath. We present results for entanglement monotones and measures such as the entanglement of formation and entanglement entropy for different QDA geometries and carrier injection conditions. Our results indicate that in spite of the effects of phonon-assisted relaxation, entanglement can be optimized and transferred between QDs by the controlled interplay of system geometry, pulse shaping, RET and carrier tunneling.

  3. Highly anisotropic and robust excitons in monolayer black phosphorus

    NASA Astrophysics Data System (ADS)

    Wang, Xiaomu; Jones, Aaron M.; Seyler, Kyle L.; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian

    2015-06-01

    Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3?eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2?eV, from which we estimate an exciton binding energy of ?0.9?eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices.

  4. Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus

    NASA Astrophysics Data System (ADS)

    Wang, Xiaomu; Jones, Aaron M.; Seyler, Kyle L.; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian

    2015-03-01

    Recently, black phosphorus emerged as a promising new 2D material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form, and its unique properties at the truly 2D quantum confinement have yet to be demonstrated. Here, we reveal highly anisotropic and tightly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centers around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. In addition, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of around 0.9 eV, consistent with theoretical results based on first-principles. The experimental observation of highly anisotropic, bright excitons with exceedingly large binding energy not only opens avenues for the future explorations of many-electron effects in this unusual 2D material, but also suggests a promising future in optoelectronic devices such as on-chip infrared light sources.

  5. 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.

  6. Singlet fission in pentacene through multiple exciton quantum states

    Microsoft Academic Search

    Zhiyong Zhang; Paul Zimmerman; Charles Musgrave

    2010-01-01

    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

  7. Light-hole exciton spin relaxation in quantum dots

    NASA Astrophysics Data System (ADS)

    Tsitsishvili, E.

    2015-04-01

    The phonon-induced flip of the exciton spin in single flat semiconductor quantum dots with a light-hole exciton ground state is studied. The corresponding quartet, split by the exchange interaction, consists of three bright states and a dark state located energetically below the bright exciton. The two in-plane polarized bright states contribute to single-phonon transitions to the dark state and also to the upper bright state polarized in the z growth direction of the dot. For these processes, the presented analytical results are calculated for the relaxation driven by the spin-orbit interaction in the conduction and the light-hole valence subbands. The estimated spin-relaxation times at low temperature are (at least) one order of magnitude lower than the bright exciton lifetime. Two other possible transitions, within the in-plane polarized doublet and between the z -polarized bright and dark states as well, proceed via intermediate states with a contribution from two acoustic phonons. These processes are strongly suppressed at low temperature, whereas they appear to be of the same intensity as single-phonon transitions at high enough temperatures.

  8. Modeling Fission in the Cascade-Exciton Model

    Microsoft Academic Search

    Arnold J. Sierk; Stepan G. Mashnik

    1998-01-01

    Recent developments of the Cascade-Exciton Model (CEM) of nuclear reactions\\u000ato describe high energy particle induced fission are briefly described. The\\u000aincreased accuracy and predictive power of the CEM are shown by several\\u000aexamples. Further necessary work is outlined.

  9. Modeling Fission in the Cascade-Exciton Model

    E-print Network

    Arnold J. Sierk; Stepan G. Mashnik

    1998-12-25

    Recent developments of the Cascade-Exciton Model (CEM) of nuclear reactions to describe high energy particle induced fission are briefly described. The increased accuracy and predictive power of the CEM are shown by several examples. Further necessary work is outlined.

  10. Exciton Condensation and Charge Fractionalization in a Topological Insulator Film

    Microsoft Academic Search

    B. Seradjeh; J. E. Moore; M. Franz

    2009-01-01

    An odd number of gapless Dirac fermions is guaranteed to exist at a surface of a strong topological insulator. We show that in a thin-film geometry and under external bias, electron-hole pairs that reside in these surface states can condense to form a novel exotic quantum state which we propose to call ``topological exciton condensate'' (TEC). This TEC is similar

  11. Highly anisotropic and robust excitons in monolayer black phosphorus.

    PubMed

    Wang, Xiaomu; Jones, Aaron M; Seyler, Kyle L; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian

    2015-06-01

    Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3?eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2?eV, from which we estimate an exciton binding energy of ?0.9?eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices. PMID:25915195

  12. Organic photosensitive optoelectronic device having a phenanthroline exciton blocking layer

    DOEpatents

    Thompson, Mark E. (Anaheim Hills, CA); Li, Jian (Los Angeles, CA); Forrest, Stephen (Princeton, NJ); Rand, Barry (Princeton, NJ)

    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.

  13. Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides.

    PubMed

    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

  14. Advanced theory of multiple exciton generation effect in quantum dots

    NASA Astrophysics Data System (ADS)

    Oksengendler, B. L.; Turaeva, N. N.; Rashidova, S. S.

    2012-06-01

    The theoretical aspects of the effect of multiple exciton generation (MEG) in quantum dots (QDs) have been analysed in this work. The statistical theory of MEG in QDs based on Fermi's approach is presented, taking into account the momentum conservation law. According to Fermi this approach should give the ultimate quantum efficiencies of multiple particle generation. The microscopic mechanism of this effect is based on the theory of electronic "shaking". According to this approach, the wave function of "shaking" electrons can be selected as Plato's functions with effective charges depending on the number of generated excitons. From the theory it is known increasing the number of excitons leads to enhancement of the Auger recombination of electrons which results in reduced quantum yields of excitons. The deviation of the averaged multiplicity of the MEG effect from the Poisson law of fluctuations has been investigated on the basis of synergetics approaches. In addition the role of interface electronic states of QDs and ligands has been considered by means of quantum mechanical approaches. The size optimisation of QDs has been performed to maximise the multiplicity of the MEG effect.

  15. Synergy between thermal spike and exciton decay mechanisms for ion damage and amorphization by electronic excitation

    SciTech Connect

    Agullo-Lopez, F.; Mendez, A. [Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid (Spain); Departamento de Fisica de Materiales, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid (Spain); Garcia, G. [Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid (Spain); Olivares, J. [Instituto de Optica 'Daza de Valdes', CSIC, 28004 Madrid (Spain); Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid (Spain); Cabrera, J. M. [Departamento de Fisica de Materiales, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid (Spain)

    2006-11-01

    A theoretical model is proposed to account for the damage and amorphization induced in LiNbO{sub 3} by ion bombardment in the electronic energy-loss regime. It relies on the synergy between the thermal spike generated by electron-phonon interaction and the nonradiative decay of localized (self-trapped) excitons. Calculations have been carried out to describe the effect of single impact as well as multiple impact (high fluence) irradiations. In the first case, the defect concentration profile and the radius of the amorphous tracks have been theoretically predicted and they are in good accordance with those experimentally determined. For high fluence irradiations ({>=}10{sup 13} cm{sup -2}) the model predicts the formation of homogeneous amorphous surface layers whose thickness increases with fluence. The propagation of the crystalline-amorphous boundary has been determined as a function of irradiation fluence. Theoretical predictions are also in good agreement with experimental data on Si-irradiated (7.5 and 5 MeV) LiNbO{sub 3} outside the region of nuclear collision damage.

  16. Determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer quantum dots via spectral analysis of optical signature of the Aharanov-Bohm excitons

    SciTech Connect

    Ji, Haojie; Dhomkar, Siddharth; Roy, Bidisha; Kuskovsky, Igor L. [Department of Physics, Queens College of CUNY, Queens, New York 11367 (United States); The Graduate Center of CUNY, New York, New York 10016 (United States); Shuvayev, Vladimir [Department of Physics, Queens College of CUNY, Queens, New York 11367 (United States); Deligiannakis, Vasilios; Tamargo, Maria C. [The Graduate Center of CUNY, New York, New York 10016 (United States); Department of Chemistry, City College of CUNY, New York, New York 10031 (United States); Ludwig, Jonathan [National High Magnetic Field Laboratory, Tallahassee, Florida 32310 (United States); Department of Physics, Florida State University, Tallahassee, Florida 32306 (United States); Smirnov, Dmitry [National High Magnetic Field Laboratory, Tallahassee, Florida 32310 (United States); Wang, Alice [Evans Analytical Group, Sunnyvale, California 94086 (United States)

    2014-10-28

    For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.

  17. Excitons and magnetic fields in bidimensional quantum rings

    NASA Astrophysics Data System (ADS)

    Sierra-Ortega, J.; Mikhailov, I. D.; Ulloa, S. E.; Govorov, A. O.

    2003-03-01

    We present calculations of structural features and magnetic field effects on the properties of an exciton localized in nanometer-size bidimensional quantum rings and disks. The recently developed fractal dimension method [1] is used, where the exciton trial wave function is taken as a product of the unbound electron and hole wave functions in the quantum system, with an arbitrary correlation function that depends only on electron-hole separation. In particular, we consider a realistic confinement potential shape that models quantum rings used in recent experiments. The binding energy, electron-hole separation, and the oscillator strength of the exciton are calculated as function of the width of the ring and magnetic field strength. In addition, we evaluate the magnitude of the in-plane polarization of the exciton for different structure parameters. This in-plane dipole, which arises from the built-in asymmetry of the potential confinement for the electron and hole in the system, produces a strong quantum phase accumulation effect that is reflected in the optical properties of the ring. This Aharonov-Bohm effect for the exciton dipole is similar to the results obtained for type-II quantum dot systems (where the electron and hole lie in different materials and are also polarized in-plane), and from other models of type-I structures [2]. Supported by US-DOE and OU-CMSS [1] I. D. Mikhailov, F. J. Betancur, R. Escorcia and J. Sierra-Ortega, Phys. Stat. Sol., 234(b), 590 (2002) [2] A. O. Govorov, S. E. Ulloa, K. Karrai and R. Warburton, Phys. Rev. B 66, 081309 (2002)

  18. The Radius Distribution of Planets around Cool Stars

    NASA Astrophysics Data System (ADS)

    Morton, Timothy D.; Swift, Jonathan

    2014-08-01

    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 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 ?, 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 ? implies that a large number of planets await discovery around cool dwarfs as the sensitivities of ground-based transit surveys increase.

  19. ON THE CONSTANCY OF THE SOLAR RADIUS. III

    SciTech Connect

    Bush, R. I. [Stanford University, Stanford, CA 94305 (United States); Emilio, M. [Observatorio Astronomico-Departamento de Geociencias Universidade Estadual de Ponta Grossa, Parana (Brazil); Kuhn, J. R., E-mail: rock@sun.stanford.ed, E-mail: memilio@uepg.b, E-mail: kuhn@ifa.hawaii.ed [Institute for Astronomy, University of Hawaii, Woodlawn Dr., HI 96822 (United States)

    2010-06-20

    The Michelson Doppler Imager on board the Solar and Heliospheric Observatory satellite has operated for over a sunspot cycle. This instrument is now relatively well understood and provides a nearly continuous record of the solar radius in combination with previously developed algorithms. Because these data are obtained from above Earth's atmosphere, they are uniquely sensitive to possible long-term changes of the Sun's size. We report here on the first homogeneous, highly precise, and complete solar-cycle measurement of the Sun's radius variability. Our results show that any intrinsic changes in the solar radius that are synchronous with the sunspot cycle must be smaller than 23 mas peak to peak. In addition, we find that the average solar radius must not be changing (on average) by more than 1.2 mas yr{sup -1}. If ground- and space-based measurements are both correct, the pervasive difference between the constancy of the solar radius seen from space and the apparent ground-based solar astrometric variability can only be accounted for by long-term changes in the terrestrial atmosphere.

  20. Negative-band-gap quantum dots: Gap collapse, intrinsic surface states, excitonic response, and excitonic insulator phase

    NASA Astrophysics Data System (ADS)

    Malkova, Natalia; Bryant, Garnett W.

    2010-10-01

    The spectrum of quantum dots made from semiconductors such as HgTe and HgS changes from negative gap to positive gap with decreasing size. Furthermore, intrinsic surface states, which are not related to dangling bonds, appear in the negative-gap regime. We investigate theoretically the evolution of the spectrum of HgS quantum dots with decreasing size and show how states evolve from a negative gap to a positive gap as confinement is increased. The lowest confined electron level evolves into an intrinsic surface state with increasing size and, thus, is not derived directly from a bulk HgS band. Due to strong band mixing in narrow-gap semiconductors, spacing between confined levels decreases more slowly with increasing size than for quantum dots made from wide-gap semiconductors. Moreover, dielectric screening becomes nearly metallic as the gap closes. As a consequence, confinement energies dominate exciton binding energies for all dot sizes up to the gap closure. Excitons remain in the strong confinement limit as size increases until the gap closes. Nonetheless, the exciton binding exceeds the single-particle gap for sizes near gap closure, opening up the possibility of an excitonic insulator phase in quantum dots not possible in positive-gap quantum dots. Signatures in the quantum-dot optical response for gap collapse and surface states are identified.

  1. Size effect on the electronic and optical band gap of CdSe QD

    SciTech Connect

    Sisodia, Namita, E-mail: namitasisodiya@gmail.com [Department of Physics, Holkar Science College, Indore-45200 (India)

    2014-04-24

    Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed.

  2. Mass-radius relations for massive white dwarf stars

    E-print Network

    L. G. Althaus; E. García--Berro; J. Isern; A. H. Córsico

    2005-07-25

    We present detailed theoretical mass-radius relations for massive white dwarf stars with oxygen-neon cores. This work is motivated by recent observational evidence about the existence of white dwarf stars with very high surface gravities. Our results are based on evolutionary calculations that take into account the chemical composition expected from the evolutionary history of massive white dwarf progenitors. We present theoretical mass-radius relations for stellar mass values ranging from 1.06 to 1.30 Mo with a step of 0.02 Mo and effective temperatures from 150000 K to approx. 5,000 K. A novel aspect predicted by our calculations is that the mass-radius relation for the most massive white dwarfs exhibits a marked dependence on the neutrino luminosity. Extensive tabulations for massive white dwarfs, accessible from our web site, are presented as well.

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

    NASA Astrophysics Data System (ADS)

    Alam, Muhammad

    2014-03-01

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

  4. Distal plate placement for distal radius fractures limits wrist motion

    Microsoft Academic Search

    Shingo Komura; Hiroyuki Tanahashi; Yoshihisa Yamada; Tatsuo Yokoi; Hidehiko Nonomura; Yasushi Suzuki

    Purpose  To investigate the influence of distal plate placement for distal radius fractures on risk of flexor tendon complications\\u000a and wrist motion by performing hardware removal.\\u000a \\u000a \\u000a \\u000a \\u000a Materials and methods  Fourteen patients treated with a Synthes 2.4 mm juxta-articular LCP Distal Radius Plate underwent hardware removal at ?6 months\\u000a postoperatively when improvement of range of motion and grip strength were obtained. Location of plate placement,

  5. Stability of a Wheel with Various Radius Rim

    NASA Astrophysics Data System (ADS)

    Kinugasa, Tetsuya; Yoshida, Koji

    This paper describes the dynamics and impact model of a wheel with various radius rim. The dynamics is expressed by a rst order linear ordinary dierential equation with respect to the absolute orientation of the wheel, and an analytic solution is derived. Poincaré map is also derived analytically. Stability and basin of attraction (BoA) of the Poincaré map are discussed. Finally, the analysis is validated through some numerical simulations. As a result, the rim radius aects the stability and broadens its BoA. The analysis helps understanding of not only a geometric tracking control but also many underactuated control methods for bipeds.

  6. The PRad experiment and the proton radius puzzle

    SciTech Connect

    Gasparian, Ashot H. [North Carolina Ag. and Tech. St. Univ.

    2014-06-01

    New results from the recent muonic hydrogen experiments seriously questioned our knowledge of the proton charge radius, r_p. The new value, with its unprecedented less than sub-percent precision, is currently up to eight standard deviation smaller than the average value from all previous experiments, triggering the well-known "proton charge radius puzzle" in nuclear and atomic physics. The PRad collaboration is currently preparing a novel, magnetic-spectrometer-free ep scattering experiment in Hall B at JLab for a new independent r_p measurement to address this growing "puzzle" in physics.

  7. Experimental and computational results on exciton/free-carrier ratio, hot/thermalized carrier diffusion, and linear/nonlinear rate constants affecting scintillator proportionality

    SciTech Connect

    Williams, Richard; Grim, Joel; Li, Qi; Ucer, K. B.; Bizarri, G. A.; Kerisit, Sebastien N.; Gao, Fei; Bhattacharya, Pijush; Tupitsyn, Eugene; Rowe, Emmanuel; Buliga, Vladimir M.; Burger, Arnold

    2013-10-01

    Models of nonproportional response in scintillators have highlighted the importance of parameters such as branching ratios, carrier thermalization times, diffusion, kinetic order of quenching, associated rate constants, and radius of the electron track. For example, the fraction ?eh of excitations that are free carriers versus excitons was shown by Payne and coworkers to have strong correlation with the shape of electron energy response curves from Compton-coincidence studies. Rate constants for nonlinear quenching are implicit in almost all models of nonproportionality, and some assumption about track radius must invariably be made if one is to relate linear energy deposition dE/dx to volume-based excitation density n (eh/cm3) in terms of which the rates are defined. Diffusion, affecting time-dependent track radius and thus density of excitations, has been implicated as an important factor in nonlinear light yield. Several groups have recently highlighted diffusion of hot electrons in addition to thermalized carriers and excitons in scintillators. However, experimental determination of many of these parameters in the insulating crystals used as scintillators has seemed difficult. Subpicosecond laser techniques including interband z scan light yield, fluence-dependent decay time, and transient optical absorption are now yielding experimental values for some of the missing rates and ratios needed for modeling scintillator response. First principles calculations and Monte Carlo simulations can fill in additional parameters still unavailable from experiment. As a result, quantitative modeling of scintillator electron energy response from independently determined material parameters is becoming possible on an increasingly firmer data base. This paper describes recent laser experiments, calculations, and numerical modeling of scintillator response.

  8. Magnetic Brightening of Dark Excitons in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Kono, Junichiro

    2007-03-01

    To gain insight into the internal energy structure and radiative properties of excitons in single-walled carbon nanotubes (SWNTs), we have studied photoluminescence (PL) from individualized HiPco and CoMoCAT samples as a function of magnetic field (B) and temperature (T). The PL intensity increased, or ``brightened,'' with B applied along the tube axis and the amount of brightening increased with decreasing T. These results are consistent with the existence of a dark state below the first bright state [1]. In the presence of time reversal symmetry, exchange-interaction-induced mixing between excitons in two equivalent valleys (the K and K' valleys) is expected to result in a set of exciton states, only one of which is optically active. This predicted bright state, however, is not the lowest in energy. Excitons would be trapped in the dark, lowest-energy state without a radiative recombination path. When a tube-threading B is applied, addition of an Aharonov-Bohm phase modifies the circumferential boundary conditions on the wave functions and lifts time reversal symmetry [2,3]. This symmetry breaking splits the K and K' valley transitions, lessening the intervalley mixing and causing the recovery of the unmixed direct K and K' excitons, which are both optically active. We have calculated PL spectra through B-dependent effective masses, populations of finite-k states, and acoustic phonon scattering, which quantitatively agree with the observations. These results demonstrate the existence of dark excitons, their influence on the PL quantum yield, and their elimination through symmetry manipulation by a B. This work was performed in collaboration with J. Shaver, S. Zaric, O. Portugall, V. Krstic, G. L. J. A. Rikken, X. Wei, S. A. Crooker, Y. Miyauchi, S. Maruyama, and V. Perebeinos and supported by the Robert A. Welch Foundation, the NSF, and EuroMagNET. [1] V. Perebeinos et al., Phys. Rev. Lett. 92, 257402 (2004); H. Zhao and S. Mazumdar, Phys. Rev. Lett. 93, 157402 (2004); V. Perebeinos et al., Nano Lett. 5, 2495 (2005); C. D. Spataru et al., Phys. Rev. Lett. 95, 247402 (2005). [2] T. Ando, J. Phys. Soc. Jpn. 75, 024707 (2006). [3] S. Zaric et al., Science 304, 1129 (2004); Phys. Rev. Lett. 96, 016406 (2006).

  9. Exciton Structure and Dynamics in Solution Aggregates of a Low-Bandgap Copolymer.

    PubMed

    Guo, Zhi; Lee, Doyun; Gao, Haifeng; Huang, Libai

    2015-06-18

    In this work, we elucidate exciton structure, dynamics, and charge generation in the solution phase aggregates of a low-bandgap donor-acceptor polymer, poly(4,8-bis-alkyloxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-(alkylthieno[3,4-b]thiophene-2carboxylate)-2,6-diyl (PBDTTT). The polymer aggregates in the solution phase serve as precursors for thin film morphologies. We have identified intrachain and interchain exciton transitions and resolved their relaxation pathways by comparing excitons in solution aggregates to those in isolated polymer chains. Hot intrachain excitons have led to the generation of stabilized interchain charge-separated states in solution aggregates, which could serve as the intermediate state to the hot exciton charge separation in bulk heterojunctions (BHJs). These results have important implications for controlling morphology dependent exciton dynamics in solution processed BHJs. PMID:25666173

  10. Exciton localization in inhomogeneously broadened ZnO/MgxZn1-xO quantum wells

    NASA Astrophysics Data System (ADS)

    Ashrafi, Almamun

    2010-06-01

    Exciton localization in ZnO/MgxZn1-xO quantum wells (QWs) has been investigated systematically with various ZnO well widths for the fixed Mg0.23Zn0.77O barrier height. A strong exciton confinement is observed with an implicit dependence on the built-in electric field which is calculated to be 0.37 MV/cm. The exciton-phonon coupling strength varied significantly depending upon the degrees of exciton localization with the activation energy of 18-29 meV. The relaxation mechanism in ZnO/Mg0.23Zn0.77O QWs starts to dominate when the exciton localization energy is above the thermal energy, kBT. The band characteristics and strong exciton localization in ZnO/Mg0.23Zn0.77O QWs are attributed to the potential fluctuations associated with the inhomogeneous broadening, represented by the schematics.

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

    PubMed Central

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

    2015-01-01

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

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

    PubMed

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

    2015-01-01

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

  13. Observation of long-range exciton diffusion in highly ordered organic semiconductors

    NASA Astrophysics Data System (ADS)

    Najafov, H.; Lee, B.; Zhou, Q.; Feldman, L. C.; Podzorov, V.

    2010-11-01

    Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8?m), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.

  14. Enhanced external quantum efficiency in an organic photovoltaic cell via singlet fission exciton sensitizer

    NASA Astrophysics Data System (ADS)

    Reusswig, P. D.; Congreve, D. N.; Thompson, N. J.; Baldo, M. A.

    2012-09-01

    We demonstrate bilayer organic photovoltaic cells that incorporate a singlet exciton fission sensitizer layer to increase the external quantum efficiency (EQE). This solar cell architecture is realized by pairing the singlet exciton donor layer tris[4-(5-phenylthiophen-2-yl)phenyl]amine (TPTPA) with the singlet exciton fission layer 5,6,11,12-tetraphenylnaphthacene (rubrene). The presence of the rubrene layer at the donor-acceptor interface allows for a singlet generated in TPTPA to undergo singlet exciton fission with a corresponding doubling in the TPTPA EQE from 12.8% to 27.6%. This scheme de-couples singlet exciton fission from photon absorption, exciton diffusion, and charge transport for very high EQE organic photovoltaic cells.

  15. Laser Emission from GaAs Quantum Well Microcavities during the Exciton to Free Carrier Transition

    NASA Astrophysics Data System (ADS)

    Fan, Xudong; Wang, Hailin; Hou, H. Q.; Hammons, B. E.

    1998-03-01

    Laser emission from an excitonic system has been a subject of considerable interest and debate in recent years. By tuning the cavity resonance to a few meV below the exciton absorption line center, we have observed laser emission at an excitation level well below the exciton Mott density in a high-Q GaAs quantum well microcavity structure. The exciton Mott density (or more precisely, the exciton saturation density) is determined from the collapse of polariton splitting of the microcavity structure. In addition, qualitative changes in spectral line shapes of laser emission and especially an abrupt increase in the lasing threshold are observed when the excitons undergo a transition to free-carriers. Laser emissions in this transition region provide a unique probe of manybody optical processes in a quasi-two dimensional system.

  16. Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys

    SciTech Connect

    JONES,ERIC D.; ALLERMAN,ANDREW A.; KURTZ,STEVEN R.; MODINE,NORMAND A.; BAJAJ,K.K.; TOZER,S.T.; WEI,XING

    2000-01-27

    The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.

  17. Response of a Bose-Einstein condensate of dipole excitons to static and dynamic perturbations

    NASA Astrophysics Data System (ADS)

    Batyev, E. G.; Kovalev, V. M.; Chaplik, A. V.

    2014-07-01

    Studies of the interaction of a Bose-Einstein condensate of two-dimensional spatially indirect excitons with the static fields of impurities, surface acoustic waves, and elementary excitations of a degenerate electron gas have been reviewed. The effects of screening of charged impurities and absorption of a Bleustein-Gulyaev surface acoustic wave by an exciton condensate have been considered. Friedel oscillations of the exciton density in a hybrid electron-exciton system, which consists of spatially separated layers of condensed exciton gas and degenerate electron gas, have been studied. The lifetimes of quasiparticle excitations (electrons, plasmons, and bogolons) in the hybrid system have been calculated. The contributions to the effects under study from condensate and above-condensate particles have been determined. The properties of an excitonic insulator have been analyzed within the Bardeen-Cooper-Schrieffer model with a built-in dissipation-free current.

  18. Observation of Excitonic Rydberg States in Monolayer MoS2 and WS2 by Photoluminescence Excitation Spectroscopy.

    PubMed

    Hill, Heather M; Rigosi, Albert F; Roquelet, Cyrielle; Chernikov, Alexey; Berkelbach, Timothy C; Reichman, David R; Hybertsen, Mark S; Brus, Louis E; Heinz, Tony F

    2015-05-13

    We have identified excited exciton states in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation spectroscopy. In monolayer WS2, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS2, excited exciton transitions are observed at energies of 2.24 and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV. PMID:25816155

  19. Acceleration of beam ions during major radius compression in TFTR

    SciTech Connect

    Wong, K.L.; Bitter, M.; Hammett, G.W.; Heidbrink, W.; Hendel, H.; Kaita, R.; Scott, S.; Strachan, J.D.; Tait, G.; Bell, M.G.

    1985-09-01

    Tangentially co-injected deuterium beam ions were accelerated from 82 keV up to 150 keV during a major radius compression experiment in TFTR. The ion energy spectra and the variation in fusion yield were in good agreement with Fokker-Planck code simulations. In addition, the plasma rotation velocity was observed to rise during compression.

  20. Decoding by Embedding: Correct Decoding Radius and DMT Optimality

    E-print Network

    Paris-Sud XI, Université de

    Decoding by Embedding: Correct Decoding Radius and DMT Optimality Cong Ling and Shuiyin Liu gain tradeoff (DMT). I. INTRODUCTION Lattice decoding for the linear multiple-input multiple- output and multiplexing tradeoff (DMT) [4]. However, lattice-reduction-aided decoding exhibits a widen- ing gap

  1. A Smaller Radius for the Transiting Exoplanet WASP-10b

    E-print Network

    John A. Johnson; Joshua N. Winn; Nicole E. Cabrera; Joshua A. Carter

    2008-11-29

    We present photometry of WASP-10 during the transit of its short-period Jovian planet. We employed the novel PSF-shaping capabilities the OPTIC camera mounted on the UH 2.2m telescope to achieve a photometric precision of 4.7e-4 per 1.3 min sample. With this new light curve, in conjunction with stellar evolutionary models, we improve on existing measurements of the planetary, stellar and orbital parameters. We find a stellar radius Rstar = 0.698 +/- 0.012 Rsun and a planetary radius Rp = 1.080 +/- 0.020 Rjup. The quoted errors do not include any possible systematic errors in the stellar evolutionary models. Our measurement improves the precision of the planet's radius by a factor of 4, and revises the previous estimate downward by 16% (2.5sigma, where sigma is the quadrature sum of the respective confidence limits). Our measured radius of WASP-10b is consistent with previously published theoretical radii for irradiated Jovian planets.

  2. Optimal Transmission Radius for Flooding in Large Scale Sensor Networks

    E-print Network

    Krishnamachari, Bhaskar

    [19] that caused us to further analyze the problem in depth. The optimization of the flooding event1 Optimal Transmission Radius for Flooding in Large Scale Sensor Networks Marco Z´u~niga Z, applications such as query propagation rely regularly on network-wide flooding for information dissemination

  3. Modeling conifer tree crown radius and estimating canopy cover

    Microsoft Academic Search

    Samantha J Gill; Gregory S Biging; Edward C Murphy

    2000-01-01

    Models of tree crown radius were developed for several conifer species of California. Typical forest inventory variables (DBH, height, height-to-crown base, crown class, basal area per hectare, and trees per hectare) were considered as independent variables in model development. Models were fitted using both ordinary and weighted least squares methods. It was found that for the species studied, an ordinary

  4. A Fast, Small-Radius GPU Median Filter

    E-print Network

    Burgstaller, Bernd

    , Small-Radius GPU Median Filter · Prof. Morgan McGuire, Williams College · the chapter of book "Shader@Yonsei Univ.10 #12;Proposed technique of Prof. Mcguire Jongtae Park@Yonsei Univ.11 · No use of "if" statement · starting with a subset of size 6, remove the min and max each time #12;Proposed technique of Prof. Mcguire

  5. Nonlinear buckling analyses of a small-radius carbon nanotube

    NASA Astrophysics Data System (ADS)

    Liu, Ning; Wang, Yong-Gang; Li, Min; Jia, Jiao

    2014-04-01

    Carbon nanotube (CNT) was first discovered by Sumio Iijima. It has aroused extensive attentions of scholars from all over the world. Over the past two decades, we have acquired a lot of methods to synthesize carbon nanotubes and learn their many incredible mechanical properties such as experimental methods, theoretical analyses, and computer simulations. However, the studies of experiments need lots of financial, material, and labor resources. The calculations will become difficult and time-consuming, and the calculations may be even beyond the realm of possibility when the scale of simulations is large, as for computer simulations. Therefore, it is necessary for us to explore a reasonable continuum model, which can be applied into nano-scale. This paper attempts to develop a mathematical model of a small-radius carbon nanotube based on continuum theory. An Isotropic circular cross-section, Timoshenko beam model is used as a simplified mechanical model for the small-radius carbon nanotube. Theoretical part is mainly based on modified couple stress theory to obtain the numerical solutions of buckling deformation. Meanwhile, the buckling behavior of the small radius carbon nanotube is simulated by Molecular Dynamics method. By comparing with the numerical results based on modified couple stress theory, the dependence of the small-radius carbon nanotube mechanical behaviors on its elasticity constants, small-size effect, geometric nonlinearity, and shear effect is further studied, and an estimation of the small-scale parameter of a CNT (5, 5) is obtained.

  6. Finite Larmor radius flute mode theory with end loss

    SciTech Connect

    Kotelnikov, I.A. [AN SSSR, Novosibirsk (Russian Federation). Inst. Yadernoj Fiziki; Berk, H.L. [Texas Univ., Austin, TX (United States). Inst. for Fusion Studies

    1993-08-01

    The theory of flute mode stability is developed for a two-energy- component plasma partially terminated by a conducting limiter. The formalism is developed as a preliminary study of the effect of end-loss in open-ended mirror machines where large Larmor radius effects are important.

  7. Open reduction and internal fixation of the distal radius

    Microsoft Academic Search

    David S. Ruch; T. Adam Ginn

    2003-01-01

    Fractures of the distal radius are one of the most common problems treated by orthopaedic surgeons. The managementof unstable fractures is now almost routinely surgical, and multiple techniques have been developed to accomplish this including pins and plaster, external fixation, and internal fixation. Recent studies and classification systems have stressed the importance of identification of specific fracture fragments. Given that

  8. Functional results and complications of locked distal radius volar plates

    Microsoft Academic Search

    M. R. Sánchez-Crespo; F. del Canto-Álvarez; F. Peñas-Díaz; V. de Diego-Gutiérrez; M. Gutiérrez-Santiago; P. Sánchez-Juan

    2009-01-01

    Introduction and purposeIn the last few years, there has been increased aggressiveness in the treatment of distal radius fractures by means of internal fixation. Locked plates prevent metaphyseal collapse even in the presence of osteoporotic or comminuted bone; they also maintain reduction and allow early motion. Nevertheless, there is as yet not enough evidence as to what may be the

  9. The strength of polyaxial locking interfaces of distal radius plates

    Microsoft Academic Search

    Konrad L. Hoffmeier; Gunther O. Hofmann; Thomas Mückley

    2009-01-01

    BackgroundCurrently available polyaxial locking plates represent the consequent enhancement of fixed-angle, first-generation locking plates. In contrast to fixed-angle locking plates which are sufficiently investigated, the strength of the new polyaxial locking options has not yet been evaluated biomechanically. This study investigates the mechanical strength of single polyaxial interfaces of different volar radius plates.

  10. Computational Analysis of Dual Radius Circulation Control Airfoils

    NASA Technical Reports Server (NTRS)

    Lee-Rausch, E. M.; Vatsa, V. N.; Rumsey, C. L.

    2006-01-01

    The goal of the work is to use multiple codes and multiple configurations to provide an assessment of the capability of RANS solvers to predict circulation control dual radius airfoil performance and also to identify key issues associated with the computational predictions of these configurations that can result in discrepancies in the predicted solutions. Solutions were obtained for the Georgia Tech Research Institute (GTRI) dual radius circulation control airfoil and the General Aviation Circulation Control (GACC) dual radius airfoil. For the GTRI-DR airfoil, two-dimensional structured and unstructured grid computations predicted the experimental trend in sectional lift variation with blowing coefficient very well. Good code to code comparisons between the chordwise surface pressure coefficients and the solution streamtraces also indicated that the detailed flow characteristics were matched between the computations. For the GACC-DR airfoil, two-dimensional structured and unstructured grid computations predicted the sectional lift and chordwise pressure distributions accurately at the no blowing condition. However at a moderate blowing coefficient, although the code to code variation was small, the differences between the computations and experiment were significant. Computations were made to investigate the sensitivity of the sectional lift and pressure distributions to some of the experimental and computational parameters, but none of these could entirely account for the differences in the experimental and computational results. Thus, CFD may indeed be adequate as a prediction tool for dual radius CC flows, but limited and difficult to obtain two-dimensional experimental data prevents a confident assessment at this time.

  11. Fast and precise approximations of the joint spectral radius

    Microsoft Academic Search

    VINCENT D. BLONDEL; YURII NESTEROV

    2003-01-01

    In this paper, we introduce a procedure for approximating the joint spectral radius of a finite set of matrices with arbitrary precision. Our approximation procedure is based on semidefinite liftings and can be implemented in a recursive way. For two matrices even the first step of the procedure gives an approximation, whose relative quality is at least 1\\/sq.2, that is,

  12. Shell corrections to the proton mean square radius

    SciTech Connect

    Jensen, A.S.; Miranda, A.

    1987-12-10

    The shell correction to the charge mean square radius is calculated in a simple model. For a spherical double magic nucleus we find about -0.1 fm/sup 2/ for any nucleon number. The deformation dependence is investigated and it is calculated that any model correct to a better accuracy must include pairing effects in some approximation.

  13. Hardness of the Covering Radius Problem on Lattices

    Microsoft Academic Search

    Ishay Haviv; Oded Regev

    2006-01-01

    We provide the first hardness result for the Covering Radius P roblem on lattices (CRP). Namely, we show that for any large enough p ? 1 there exists a constant cp > 1 such that CRP in thep norm is ?2-hard to approximate to within any constant less than cp. In particular, for the case p = 1, we obtain

  14. Wear at the die radius in sheet metal stamping

    Microsoft Academic Search

    Michael P. Pereira; Wenyi Yan; Bernard F. Rolfe

    In sheet metal stamping, it is known that wear is unevenly distributed over the die radius and that multiple wear mechanisms may occur simultaneously. However, there has been little or no work that details the types of wear mechanisms, and quantifies the locations at which they occur. Furthermore, the link between recently identified time-dependent contact conditions and the wear response

  15. Effect of limiter end loss in finite Larmor radius theory

    SciTech Connect

    Berk, H.L. [Texas Univ., Austin, TX (United States). Inst. for Fusion Studies; Kotelnikov, I.A. [AN SSSR, Novosibirsk (Russian Federation). Inst. Yadernoj Fiziki

    1993-08-01

    We have examined the effect of incomplete line tying on the MHD flute mode with FLR (finite Larmor radius) effects. We show that the combination of line tying and FLR effects can slow down MHD instability, but cannot produce complete stabilization.

  16. Enlarged magnetic focusing radius of photoinduced ballistic currents

    E-print Network

    Ludwig-Maximilians-Universität, München

    1 Enlarged magnetic focusing radius of photoinduced ballistic currents Markus Stallhofer,1 as mesoscopic detectors to analyze the ballistic flow of photogenerated electrons in a two-dimensional electron as the underlying reason. PACS: 73.23.Ad, 42.82.Fv, 81.07.St, 85.35.Be, 73.63.-b, 72.20.Dp KEYWORDS. Ballistic

  17. Exciton-scaling and optical excitations of self-similar phenylacetylene dendrimers

    NASA Astrophysics Data System (ADS)

    Poliakov, Evgeni Y.; Chernyak, Vladimir; Tretiak, Sergei; Mukamel, Shaul

    1999-04-01

    The collective electronic oscillators method is used to construct an effective Frenkel exciton Hamiltonian for conjugated dendrimers with fractal geometry. Self-similarity and the high degree of symmetry utilized by decomposing the space of optical excitations into irreducible representations make it possible to compute the one-exciton states and the linear optical response with reduced numerical effort that scales linearly rather than exponentially with the number of generations. The linear optical response is dominated by localized excitons belonging to the periphery.

  18. Optical properties of hybrid nanostructures: exciton-plasmon interaction, Fano effect, and plasmon-induced chirality

    Microsoft Academic Search

    Alexander Govorov

    2011-01-01

    Coulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several prominent effects: Energy transfer between nanoparticles, plasmon enhancement, exciton energy shifts, Fano interference and non-linear effects, and new mechanisms of optical chirality [1–4]. An interaction between a discrete state of exciton and a continuum of plasmonic states gives rise to interference effects (Fano-like asymmetric resonances and anti-resonances) [2,3].

  19. Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission

    Microsoft Academic Search

    Jonathan J. Burdett; Astrid M. Müller; David Gosztola; Christopher J. Bardeen

    2010-01-01

    The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence

  20. Magnetic field effects on triplet exciton fission and fusion in a polydiacetylene

    Microsoft Academic Search

    R. H. Austin; G. L. Baker; S. Etemad; R. Thompson

    1989-01-01

    We have studied the origin and decay dynamics of triplet excitons in the conjugated polymer poly(4BCMU) in its sol(yellow) and gel(red) phases. Wavelength and intensity dependencies of the triplet yield show that the triplet exciton cannot be produced by excitation into the singlet exciton edge but only from higher lying states. The observed lifetime of the triplet state, coupled with

  1. Geminate and Nongeminate Recombination of Triplet Excitons Formed by Singlet Fission

    NASA Astrophysics Data System (ADS)

    Bayliss, Sam L.; Chepelianskii, Alexei D.; Sepe, Alessandro; Walker, Brian J.; Ehrler, Bruno; Bruzek, Matthew J.; Anthony, John E.; Greenham, Neil C.

    2014-06-01

    We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV.

  2. Optically detected microwave-induced impact ionization of ytterbium bound excitons in InP

    Microsoft Academic Search

    B. J. Heijmink Liesert; M. Godlewski; A. Stapor; T. Gregorkiewicz; C. A. J. Ammerlaan; J. Weber; M. Moser; F. Scholz

    1991-01-01

    Optically detected microwave-induced impact ionization of excitons and shallow donors is studied in Yb-doped InP grown by metalorganic chemical vapor deposition. The experimental results directly confirm that Yb3+ intrashell emission is induced by nonradiative recombination of Yb bound excitons due to an impurity Auger effect. Yb3+ ions in InP are found to bind excitons with the electron being localized first,

  3. OBSERVATIONAL CONSTRAINTS ON THE DEGENERATE MASS-RADIUS RELATION

    SciTech Connect

    Holberg, J. B. [Lunar and Planetary Laboratory, 1541 East University Boulevard, Sonett Space Sciences Building, University of Arizona, Tucson, AZ 85721 (United States); Oswalt, T. D. [Florida Institute of Technology, Melbourne, FL 32901 (United States); Barstow, M. A., E-mail: holberg@argus.lpl.arizona.edu, E-mail: toswalt@fit.edu, E-mail: mab@le.ac.uk [Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH (United Kingdom)

    2012-03-15

    The white dwarf mass-radius relationship is fundamental to modern astrophysics. It is central to routine estimation of DA white dwarf masses derived from spectroscopic temperatures and gravities. It is also the basis for observational determinations of the white dwarf initial-final-mass relation. Nevertheless, definitive and detailed observational confirmations of the mass-radius relation (MRR) remain elusive owing to a lack of sufficiently accurate white dwarf masses and radii. Current best estimates of masses and radii allow only broad conclusions about the expected inverse relation between masses and radii in degenerate stars. In this paper, we examine a restricted set of 12 DA white dwarf binary systems for which accurate (1) trigonometric parallaxes, (2) spectroscopic effective temperatures and gravities, and (3) gravitational redshifts are available. We consider these three independent constraints on mass and radius in comparison with an appropriate evolved MRR for each star. For the best-determined systems it is found that the DA white dwarfs conform to evolve theoretical MRRs at the 1{sigma} to 2{sigma} level. For the white dwarf 40 Eri B (WD 0413-077) we find strong evidence for the existence of a 'thin' hydrogen envelope. For other stars improved parallaxes will be necessary before meaningful comparisons are possible. For several systems current parallaxes approach the precision required for the state-of-the-art mass and radius determinations that will be obtained routinely from the Gaia mission. It is demonstrated here how these anticipated results can be used to firmly constrain details of theoretical mass-radius determinations.

  4. Distal radius malunion and forearm rotation: a cadaveric study.

    PubMed

    Bronstein, Andrew; Heaton, Dennis; Tencer, Allan F; Trumble, Thomas E

    2014-02-01

    Background?Malunions following distal radius fractures are common, with shortening, translation, and rotation occurring. The patients frequently lose forearm rotation, but there is no data to indicate whether this is due to mechanical misalignment between the radius and the ulna or to contracture of the soft tissues. Material and Methods?Seven fresh cadaveric specimens were used to determine the loss of forearm rotation with varying simulated distal radius fracture malalignment patterns. Uniplanar malunion patterns consisting of dorsal tilt, radioulnar translation, or radial shortening were simulated by creating an osteotomy at the distal end of the radius. Description of Technique?By orienting the distal fragment position using an external fixator and maintaining the position with wedges and a T-plate, varying degrees of malunion of the distal radius could be simulated. Rotation of the forearm was produced by fixing the elbow in a flexed position and applying a constant torque to the forearm using deadweights. Forearm rotation was measured with a protractor. Results?Dorsal tilt to 30° and radial translation to 10?mm led to no significant restriction in forearm pronation or supination ranges of motion. A 5-mm ulnar translation deformity resulted in a mean 23% loss of pronation range of motion. Radial shortening of 10?mm reduced forearm pronation by 47% and supination by 29%. Conclusion?Because a severe osseous misalignment was required to produce a significant loss in rotation, contracture of the soft tissues is most likely the cause of the loss of rotation in most cases. PMID:24533239

  5. Optimal Taylor-Couette flow: radius ratio dependence

    NASA Astrophysics Data System (ADS)

    Ostilla-Mónico, Rodolfo; Huisman, Sander G.; Jannink, Tim J. G.; Van Gils, Dennis P. M.; Verzicco, Roberto; Grossmann, Siegfried; Sun, Chao; Lohse, Detlef

    2014-05-01

    Taylor-Couette flow with independently rotating inner (i) and outer (o) cylinders is explored numerically and experimentally to determine the effects of the radius ratio {\\eta} on the system response. Numerical simulations reach Reynolds numbers of up to Re_i=9.5 x 10^3 and Re_o=5x10^3, corresponding to Taylor numbers of up to Ta=10^8 for four different radius ratios {\\eta}=r_i/r_o between 0.5 and 0.909. The experiments, performed in the Twente Turbulent Taylor-Couette (T^3C) setup, reach Reynolds numbers of up to Re_i=2x10^6$ and Re_o=1.5x10^6, corresponding to Ta=5x10^{12} for {\\eta}=0.714-0.909. Effective scaling laws for the torque J^{\\omega}(Ta) are found, which for sufficiently large driving Ta are independent of the radius ratio {\\eta}. As previously reported for {\\eta}=0.714, optimum transport at a non-zero Rossby number Ro=r_i|{\\omega}_i-{\\omega}_o|/[2(r_o-r_i){\\omega}_o] is found in both experiments and numerics. Ro_opt is found to depend on the radius ratio and the driving of the system. At a driving in the range between {Ta\\sim3\\cdot10^8} and {Ta\\sim10^{10}}, Ro_opt saturates to an asymptotic {\\eta}-dependent value. Theoretical predictions for the asymptotic value of Ro_{opt} are compared to the experimental results, and found to differ notably. Furthermore, the local angular velocity profiles from experiments and numerics are compared, and a link between a flat bulk profile and optimum transport for all radius ratios is reported.

  6. Exciton-polariton confinement in Fibonacci quasiperiodic superlattice

    NASA Astrophysics Data System (ADS)

    de Medeiros, F. F.; Albuquerque, E. L.; Vasconcelos, M. S.; Farias, G. A.

    2006-09-01

    We investigate the propagation of exciton-polaritons (bulk and surface modes) in quasiperiodic superlattices of Fibonacci type, truncated at z = 0, where z is defined as the growth axis. For our purposes, the Fibonacci structure can be realized experimentally by juxtaposing two basic building blocks A and B, following a Fibonacci sequential rule. Here A is the spatially dispersive medium modelled by a semiconductor from the nitride family (GaN, for instance), which alternates with a common dielectric medium B (sapphire). Our main aim is the investigation of the modified optical properties of the exciton polariton modes for a better understanding of the dynamics of the excitation in confined systems. The dispersion relation shows a bottleneck profile for the superlattice modes, whose behavior is similar to those found in the bulk crystal. The Cantor-like energy spectrum is also investigated, and scaling properties are shown for the Fibonacci quasiperiodic structure.

  7. Exciton band structure of monolayer MoS2

    NASA Astrophysics Data System (ADS)

    Wu, Fengcheng; Qu, Fanyao; MacDonald, Allan

    2015-03-01

    We describe a theory of the momentum-dependent exciton spectrum of monolayer molybdenum disulfide. Low-energy excitons occur both at the Brillouin zone center and at the Brillouin-zone corners. We find that binding energies at the Brillouin-zone center deviate qualitatively from the (n - 1 / 2) - 2 pattern of the two-dimensional hydrogenic model. Moreover, the four 2 p states of A series are lower in energy than the corresponding 2 s states and not degenerate. The two-fold ground-state valley degeneracy is lifted linearly at small momenta by electron-hole exchange processes that establish inter valley coherence. We conclude that atlhough monolayer MoS2 is a direct-gap semiconductor when classified by its quasiparticle band structure it may well be an indirect gap material when classified by its excitation spectra, and speculate on the role of this property in luminescence characteristics.

  8. Exciton-polariton mediated light propagation in anisotropic waveguides

    NASA Astrophysics Data System (ADS)

    Takeda, Hiroyuki; Sakoda, Kazuaki

    2012-11-01

    To analyze the exciton-polariton dispersion relation of highly anisotropic thiacyanine films and nanofibers, we formulated a plane-wave expansion method by which we could obtain the eigenfrequencies of polaritons as eigenvalues of a non-Hermitian and frequency-independent matrix. The group refractive index calculated from the slope of the dispersion curve agreed quite well with the Fabry-Perot interference patterns found in both the calculated and observed transmission spectra. We found that the dispersion relation of the anisotropic polariton was quite different from the isotropic case and depended strongly on the tilt angle of the optical transition dipole moment of the constituent molecules to the propagation direction. Material parameters such as the transverse and longitudinal exciton frequencies are also discussed.

  9. Structure-Directed Exciton Dynamics in Templated Molecular Nanorings

    PubMed Central

    2015-01-01

    Conjugated polymers with cyclic structures are interesting because their symmetry leads to unique electronic properties. Recent advances in Vernier templating now allow large shape-persistent fully conjugated porphyrin nanorings to be synthesized, exhibiting unique electronic properties. We examine the impact of different conformations on exciton delocalization and emission depolarization in a range of different porphyrin nanoring topologies with comparable spatial extent. Low photoluminescence anisotropy values are found to occur within the first few hundred femtoseconds after pulsed excitation, suggesting ultrafast delocalization of excitons across the nanoring structures. Molecular dynamics simulations show that further polarization memory loss is caused by out-of-plane distortions associated with twisting and bending of the templated nanoring topologies. PMID:25960822

  10. One-dimensional exciton diffusion in a conjugated polymer

    NASA Astrophysics Data System (ADS)

    Rughooputh, S. D. D. V.; Bloor, D.; Phillips, D.; Movaghar, B.

    1987-05-01

    One of the most interesting questions in modern transport theory has been and still is the following: What is the influence of dimensionality (including fractal) on the carrier and exciton dynamics and decay kinetics in quasiordered and disordered systems. An enormous amount of theoretical work has been done in this field, extended now to include direct computer simulations. Experimental data have been more difficult to find and are consequently sparse and not always convincing. Certain types of organic compounds and, in particular polydiacetylenes, constitute unique classes of nearly one-dimensional systems. Studies including photoconductive decays have been extended to include picosecond luminescence decay on PDA-1OH [poly(diacetylene-1-hydroxy-hexadyine-diol)], a continuous highly oriented fibrous polymer. The decay laws are compared with exact results in dimensionalities d=1 and near 1. The present data together with the recent work on doped tetramethylammonium manganese trichloride may be evidence for the first time of one-dimensional exciton motion.

  11. Diffusive Propagation of Exciton-Polaritons through Thin Crystal Slabs

    PubMed Central

    Zaitsev, D. A.; Il’ynskaya, N. D.; Koudinov, A. V.; Poletaev, N. K.; Nikitina, E. V.; Egorov, A. Yu.; Kavokin, A. V.; Seisyan, R. P.

    2015-01-01

    If light beam propagates through matter containing point impurity centers, the amount of energy absorbed by the media is expected to be either independent of the impurity concentration N or proportional to N, corresponding to the intrinsic absorption or impurity absorption, respectively. Comparative studies of the resonant transmission of light in the vicinity of exciton resonances measured for 15 few-micron GaAs crystal slabs with different values of N, reveal a surprising tendency. While N spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as N1/6. We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in a semiconductor. PMID:26088555

  12. Microscopic theory of singlet exciton fission. I. General formulation

    E-print Network

    Berkelbach, Timothy C; Reichman, David R

    2012-01-01

    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 adiabatic 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 t...

  13. Two dimensional excitons in thin films of thiophene oligomers

    NASA Astrophysics Data System (ADS)

    Lanzani, G.; Rossi, L.; Stagira, S.; De Silvestri, S.; Garnier, F.

    1998-01-01

    Transient and quasi-steady state photomodulation spectroscopy has been applied to films of substituted sexithiophene, namely ?, ?dihexylsexithiophene (?, ?DHT 6), which is prototype two dimensional organic semiconductor. Two photoexcited states are seen in the ultrafast time domain characterized by photoinduced absorption bands at 1.7 eV and 1.5 eV respectively and different dynamics. We assign them to Frenkel excitons and inter-layer charge transfer excitons. The photoinduced absorption spectrum in the ms time domain shows a very narrow line at 0.7 eV and a broad structure around 1 eV which are assigned to doubly occupied states, ?-dimers and bipolarons respectively.

  14. Supercritical Coulomb center and excitonic instability in graphene

    E-print Network

    O. V. Gamayun; E. V. Gorbar; V. P. Gusynin

    2009-10-31

    It is well known that there are resonant states with complex energy for the supercritical Coulomb impurity in graphene. We show that opening of a quasiparticle gap decreases the imaginary part of energy, |ImE|, of these states and stabilizes the system. For gapless quasiparticles with strong Coulomb interaction in graphene, we solve the Bethe-Salpeter equation for the electron - hole bound state and show that it has a tachyonic solution for strong enough coupling \\alpha=e^2/\\kappa\\hbar v_F leading to instability of the system. In the random-phase approximation, the critical coupling is estimated to be \\alpha_c =1.62 and is an analogue of the critical charge in the Coulomb center problem. We argue that the excitonic instability should be resolved through the formation of an excitonic condensate and gap generation in the quasiparticle spectrum.

  15. Two-Dimensional Superfluidity of Exciton Polaritons Requires Strong Anisotropy

    NASA Astrophysics Data System (ADS)

    Altman, Ehud; Sieberer, Lukas M.; Chen, Leiming; Diehl, Sebastian; Toner, John

    2015-01-01

    Fluids of exciton polaritons, excitations of two-dimensional quantum wells in optical cavities, show collective phenomena akin to Bose condensation. However, a fundamental difference from standard condensates stems from the finite lifetime of these excitations, which necessitates continuous driving to maintain a steady state. A basic question is whether a two-dimensional condensate with long-range algebraic correlations can exist under these nonequilibrium conditions. Here, we show that such driven two-dimensional Bose systems cannot exhibit algebraic superfluid order except in low-symmetry, strongly anisotropic systems. Our result implies, in particular, that recent apparent evidence for Bose condensation of exciton polaritons must be an intermediate-scale crossover phenomenon, while the true long-distance correlations fall off exponentially. We obtain these results through a mapping of the long-wavelength condensate dynamics onto the anisotropic Kardar-Parisi-Zhang equation.

  16. Explicit Correlated Exciton-Vibrational Dynamics of the FMO Complex.

    PubMed

    Schulze, J; Kühn, O

    2015-05-21

    The coupled exciton-vibrational dynamics of a three-site model of the Fenna-Matthews-Olson complex is investigated using the numerically exact multilayer multiconfiguration time-dependent Hartree approach. Thereby the specific coupling of the vibrational modes to local electronic transitions is adapted from a discretized experimental spectral density. The solution of the resulting time-dependent Schrödinger equation including three electronic and 450 vibrational degrees of freedom is analyzed in terms of excitonic populations and coherences. Emphasis is put onto the role of specific ranges of vibrational frequencies. It is observed that modes between 160 and 300 cm(-1) are responsible for the sub-picosecond population and coherence decay. Further, it is found that a mean-field approach with respect to the vibrational degrees of freedom is not applicable. PMID:25927682

  17. Organic photovoltaic cell incorporating electron conducting exciton blocking layers

    DOEpatents

    Forrest, Stephen R.; Lassiter, Brian E.

    2014-08-26

    The present disclosure relates to photosensitive optoelectronic devices including a compound blocking layer located between an acceptor material and a cathode, the compound blocking layer including: at least one electron conducting material, and at least one wide-gap electron conducting exciton blocking layer. For example, 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) and 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) function as electron conducting and exciton blocking layers when interposed between the acceptor layer and cathode. Both materials serve as efficient electron conductors, leading to a fill factor as high as 0.70. By using an NTCDA/PTCBI compound blocking layer structure increased power conversion efficiency is achieved, compared to an analogous device using a conventional blocking layers shown to conduct electrons via damage-induced midgap states.

  18. Diffusive Propagation of Exciton-Polaritons through Thin Crystal Slabs.

    PubMed

    Zaitsev, D A; Il'ynskaya, N D; Koudinov, A V; Poletaev, N K; Nikitina, E V; Egorov, A Yu; Kavokin, A V; Seisyan, R P

    2015-01-01

    If light beam propagates through matter containing point impurity centers, the amount of energy absorbed by the media is expected to be either independent of the impurity concentration N or proportional to N, corresponding to the intrinsic absorption or impurity absorption, respectively. Comparative studies of the resonant transmission of light in the vicinity of exciton resonances measured for 15 few-micron GaAs crystal slabs with different values of N, reveal a surprising tendency. While N spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as N(1/6). We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in a semiconductor. PMID:26088555

  19. Collective oscillations in spatially modulated exciton-polariton condensate arrays

    E-print Network

    A. A. Tikhomirov; O. I. Kanakov; B. L. Altshuler; M. V. Ivanchenko

    2014-07-25

    We study collective dynamics of interacting centers of exciton-polariton condensation in presence of spatial inhomogeneity, as modeled by diatomic active oscillator lattices. The mode formalism is developed and employed to derive existence and stability criteria of plane wave solutions. It is demonstrated that $k_0=0$ wave number mode with the binary elementary cell on a diatomic lattice possesses superior existence and stability properties. Decreasing net on-site losses (balance of dissipation and pumping) or conservative nonlinearity favors multistability of modes, while increasing frequency mismatch between adjacent oscillators detriments it. On the other hand, spatial inhomogeneity may recover stability of modes at high nonlinearities. Entering the region where all single-mode solutions are unstable we discover subsequent transitions between localized quasiperiodic, chaotic and global chaotic dynamics in the mode space, as nonlinearity increases. Importantly, the last transition evokes the loss of synchronization. These effects may determine lasing dynamics of interacting exciton-polariton condensation centers.

  20. Surface current effects on the optical response of a multilayer excitonic system

    NASA Astrophysics Data System (ADS)

    López-Bolaños, R.; Cocoletzi, G. H.; Wang, S.

    1999-01-01

    Optical response of an insulator-excitonic semiconductor multilayer system is studied in the presence of surface currents at the interfaces of the inert layer-excitonic active layer. p-polarized light is considered to obtain the reflectance of a semi-infinite superlattice and the dispersion relation of the electromagnetic normal modes of an infinite superlattice. A blueshift of the resonance frequencies is manifested due to the existence of currents. The interpretation of the optical spectra is given in comparison with the case of a single-excitonic film, and in terms of transverse and longitudinal modes resonances in the excitonic layers.

  1. Plasmonic Structure Enhanced Exciton Generation at the Interface between the Perovskite Absorber and Copper Nanoparticles

    PubMed Central

    Lin, Kuen-Feng; Chiang, Chien-Hung; Wu, Chun-Guey

    2014-01-01

    The refractive index and extinction coefficient of a triiodide perovskite absorber (TPA) were obtained by fitting the transmittance spectra of TPA/PEDOT:PSS/ITO/glass using the transfer matrix method. Cu nanoplasmonic structures were designed to enhance the exciton generation in the TPA and to simultaneously reduce the film thickness of the TPA. Excitons were effectively generated at the interface between TPA and Cu nanoparticles, as observed through the 3D finite-difference time-domain method. The exciton distribution is advantageous for the exciton dissociation and carrier transport. PMID:25295290

  2. Polarons, localization, and excitonic coherence in superradiance of biological antenna complexes

    E-print Network

    Mukamel, Shaul

    Polarons, localization, and excitonic coherence in superradiance of biological antenna complexes T, and polaron formation are explored. The theory is applied for the interpretation of recent measurements

  3. Anisotropic turbulence of a dipolar exciton Bose-Einstein condensate in coupled quantum wells

    NASA Astrophysics Data System (ADS)

    Tishinski, George; Berman, Oleg; Kolmakov, German

    2014-03-01

    We report formation of turbulence in a non-equilibrium Bose-Einstein condensate (BEC) of dipolar excitons trapped in a confining parabolic potential. This turbulent state is characterized by strong non-equilibrium oscillations of the occupation numbers for the excitons states in the presence of the energy source and exciton decay. By considering the potentials where a spring constant in one direction differs from that in another direction, we study a transition from isotropic two-dimensional turbulence to anisotropic turbulence and then, to quasi-one dimensional turbulence. We discuss application of the theory of wave turbulence to the description of turbulent states in an exciton BEC.

  4. Hybrid pentacene/a-silicon solar cells utilizing multiple carrier generation via singlet exciton fission

    NASA Astrophysics Data System (ADS)

    Ehrler, Bruno; Musselman, Kevin P.; Böhm, Marcus L.; Friend, Richard H.; Greenham, Neil C.

    2012-10-01

    Silicon dominates the solar cell market because of its abundance, mature production processes, and high efficiencies, with the best solar cells approaching the Shockley-Queisser limit. Multiple exciton photogeneration provides a route to solar cells that surpass the Shockley-Queisser limit, and we report the use of pentacene, for which photogenerated singlet excitons rapidly convert into two lower-energy spin-triplet excitons. We report solar cells that couple amorphous silicon to pentacene. We show that a thin layer of nanocrystals between silicon and pentacene allows simultaneously harnessing low-energy photons absorbed in silicon and high-energy photons absorbed in pentacene, generating two excitons via singlet fission.

  5. Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells

    NASA Astrophysics Data System (ADS)

    Fedichkin, F.; Andreakou, P.; Jouault, B.; Vladimirova, M.; Guillet, T.; Brimont, C.; Valvin, P.; Bretagnon, T.; Dussaigne, A.; Grandjean, N.; Lefebvre, P.

    2015-05-01

    We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 ? m is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 ? m . The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

  6. Nonlinear dynamics and inner-ring photoluminescence pattern of indirect excitons

    NASA Astrophysics Data System (ADS)

    Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François

    2012-06-01

    We study the photoluminescence dynamics of ultracold indirect excitons optically created in a double-quantum-well heterostructure in the regime in which the inner photoluminescence ring is formed. We show that the spectrally resolved dynamics is in agreement with an excitonic origin for the inner ring which appears due to the local heating of indirect excitons by laser excitation. To confirm this interpretation and exclude the ionization of indirect excitons, we evaluate the excitonic density that is extracted from the energy of the photoluminescence emission. It is shown that optically injected carriers play a crucial role in that context as these are trapped in our field-effect device and then vary the electrostatic potential controlling the confinement of indirect excitons. This disruptive effect blurs the estimation of the exciton concentration. However, it is suppressed by placing the double quantum well behind a superlattice where a fraction of photoinjected carriers remains trapped and then screens fluctuations of the electrostatic potential at the gate electrodes. In this improved geometry, we then estimate that the exciton density remains one order of magnitude smaller than the critical density for the ionization of indirect excitons (or Mott transition) in the regime where the inner ring is formed.

  7. Spatially Resolved Thermodynamics of the Partially Ionized Exciton Gas in GaAs

    NASA Astrophysics Data System (ADS)

    Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.

    2015-06-01

    We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.

  8. Localization of coherent exciton transport in phase space.

    PubMed

    Mülken, Oliver; Bierbaum, Veronika; Blumen, Alexander

    2007-03-01

    We study numerically the dynamics of excitons on discrete rings in the presence of static disorder. Based on continuous-time quantum walks we compute the time evolution of the Wigner function (WF) both for pure diagonal (site) disorder, as well as for diagonal and off-diagonal (site and transfer) disorder. In both cases, large disorder leads to localization and destroys the characteristic phase space patterns of the WF found in the absence of disorder. PMID:17500682

  9. Wannier excitons signalling strong Coulomb coupling in graphene

    NASA Astrophysics Data System (ADS)

    Grönqvist, J. H.; Stroucken, T.; Lindberg, M.; Koch, S. W.

    2012-12-01

    The Wannier equation for quasiparticles with a linear dispersion is investigated. It is shown that Coulomb bound compounds of mass less electron-hole pairs can only exist if the effective coupling strength exceeds the critical value of approximately 0.46. At the critical value, a second-order transition to a weak Coulomb regime is found. It is shown that the existence of bound excitons indicates an instability of the noninteracting ground state.

  10. Exciton delocalization and energy transport mechanisms in R-phycoerythrin.

    PubMed

    Womick, Jordan M; Liu, Haoming; Moran, Andrew M

    2011-03-31

    Energy transport mechanisms in R-Phycoerythrin (RPE), a light harvesting protein located at the top of the phycobilisome antenna in red algae, are investigated using nonlinear optical spectroscopies and theoretical models. The RPE hexamer possesses a total of 30 bilin pigments, which can be subdivided into three classes based on their molecular structures and electronic resonance frequencies. Of particular interest to this study is the influence of exciton delocalization on the real-space paths traversed by photoexcitations as they concentrate on the lowest energy pigment sites. Transient grating measurements show that significant nuclear relaxation occurs at delay times less than 100 fs, whereas energy transport spans a wide range of time scales depending on the proximity of the initial and final states involved in the process. The fastest energy transport dynamics within the RPE complex are close to 1 ps; however, evidence for sub-100 fs exciton self-trapping is also obtained. In addition, photon echo experiments reveal vibronic interactions with overdamped and underdamped nuclear modes. To establish signatures of exciton delocalization, energy transport is simulated using both modified Redfield and Fo?rster theories, which respectively employ delocalized and localized basis states. We conclude that exciton delocalization occurs between six pairs of phycoerythrobilin pigments (i.e., dimers) within the protein hexamer. It is interesting that these dimers are bound in locations analogous to the well-studied phycocyanobilin dimers of cyanobacterial allophycocyanin and c-phycocyanin in which wave function delocalization is also known to take hold. Strong conclusions regarding the electronic structures of the remaining pigments cannot be drawn based on the present experiments and simulations due to overlapping resonances and broad spectroscopic line widths, which prevent the resolution of dynamics at particular pigment sites. PMID:21381708

  11. Diffusion of excitons in materials for optoelectronic device applications

    NASA Astrophysics Data System (ADS)

    Singh, Jai; Narayan, Monishka Rita; Ompong, David

    2015-06-01

    The diffusion of singlet excitonsis known to occur through the Förster resonance energy transfer (FRET) mechanism and that of singlet and triplet excitonscan occur through the Dexter carrier transfer mechanism. It is shown here that if a material possesses the strong exciton-spin-orbit-photon interaction then triplet excitonscan also be transported /diffused through a mechanism like FRET. The theory is applicable to the diffusion of excitonsin optoelectronic devices like organic solar cells, organic light emitting devices and inorganic scintillators.

  12. Exciton dynamics and energy transfer processes in semiconductor nanocrystals

    Microsoft Academic Search

    Andries Meijerink

    Exciton dynamics provide unique information on both the nature of optical transitions and the local environment of an optically\\u000a active species. However, experimental facilities for measuring (fast) luminescence decay dynamics have been developed long\\u000a after techniques for time-averaged optical spectroscopy (absorption, excitation and emission spectroscopy). Studies on the\\u000a dynamics of excited states therefore lag behind of steady state spectroscopy. This

  13. The Excitonic Effect in the Optical Spectrum of Semiconductors

    Microsoft Academic Search

    Marcelo del Castillo Mussot

    1984-01-01

    The electron-hole interaction affects the optical spectrum of many semiconductors, not only in the bound exciton region in the energy gap, but also throughout the continuum. In particular, it controls the strength of the E(,1) and E(,2) peaks in group IV, III-V and II-VI semiconductors, for which the calculated spectrum using non-interacting electron-hole pairs yields an E(,1) peak with less

  14. Quantum Tunneling, Field Induced Injecting Contact, and Excitons

    Microsoft Academic Search

    Yixin Liu

    1995-01-01

    This thesis consists of three parts: Quantum tunneling simulation, Schottky barrier induced injecting contact on wide band gap II-VI materials, and excitons in semiconductor heterostructures. Part I presents a new method for quantum transport calculations in semiconductor tunnel structures using multiband {bf k}cdot{bf p} theory. This method circumvents the numerical instability problems that arise in the standard transfer -matrix method.

  15. Optical and excitonic properties of ZnO films

    Microsoft Academic Search

    M. Mihailovic; A.-L. Henneghien; S. Faure; P. Disseix; J. Leymarie; A. Vasson; D. A. Buell; F. Semond; C. Morhain; J. Zùñiga Pérez

    2009-01-01

    Optical and excitonic properties of ZnO heterostructures are studied in order to observe the strong light-matter coupling in this material as it has been recently demonstrated in GaN. The optical index of ZnO is first studied as a function of wavelength through spectroscopic ellipsometric and reflectivity experiments on ZnO layers grown by molecular beam epitaxy and deposited on two different

  16. Exciton Rydberg in T-shaped quantum wires

    NASA Astrophysics Data System (ADS)

    Brinkmann, Dirk; Fishman, Guy

    1997-12-01

    We have calculated the electronic structure of the two series of T-shaped wires experimentally studied by T. Someya et al. [Phys. Rev. Lett. 76, 2965 (1996)]. The finite depth of the wells and the cubic symmetry of the valence band are explicitly taken into account. We obtain the theoretical value of the exciton Rydberg RthX and the exciton transition energy EthX. The results derived from our theory, which has no adjustable parameters, are compared with the experimental value EexX and the estimated value ResX obtained for S1 series (GaAs-Ga0.7Al0.3 As) and S2 series (GaAs-AlAs). In the S1 sample, where the thicknesses of the two quantum wells, which define a T wire, are identical (balanced T wire), EthX=1590 meV compared to EexX=1591 and RthX=15 meV while ResX=17+/-3 meV. In the balanced S2 sample we obtain EthX=1626 meV compared to EexX=1621 meV and RthX=18 meV while ResX=27+/-3 meV. The large width of the photoluminescence line for the samples of the S2 series and the discrepancy between the experimental and calculated values suggest that the luminescence is due to excitons bound to interfaces. The values of the exciton Rydberg deduced from the experiments is therefore overestimated.

  17. Coexistence of Exciton Fission and Fusion in Tetracene Crystals

    Microsoft Academic Search

    R. P. Groff; P. Avakian; R. E. Merrifield

    1970-01-01

    Triplet exciton fusion and singlet excition fission have been observed in tetracene crystals, and the magnetic field dependence and the rate constants for these processes have been measured. At 0°C the singlet fission rate constant and the triplet-triplet fusion rate constant (leading to excited singlets) are (6.3+\\/-0.7) ×108 sec-1 and 9×10-10 cm3 sec-1 (within an order of magnitude), respectively.

  18. Quantum Dot Solar Cells with Multiple Exciton Generation

    Microsoft Academic Search

    M. C. Hanna; M. C. Beard; J. C. Johnson; J. Murphy; R. J. Ellingson; A. J. Nozik

    2005-01-01

    We have measured the quantum yield of the multiple exciton generation (MEG) process in quantum dots (QDs) of the lead-salt semiconductor family (PbSe, PbTe, and PbS) using fs pump-probe transient absorption measurements. Very high quantum yields (up to 300%) for charge carrier generation from MEG have been measured in all of the Pb-VI QDs. We have calculated the potential maximum

  19. Improved Cascade-Exciton Model of Nuclear Reactions

    Microsoft Academic Search

    Stepan G. Mashnik; Arnold J. Sierk

    1998-01-01

    Recent improvements to the Cascade-Exciton Model (CEM) of nuclear reactions\\u000aare briefly described. They concern mainly the cascade stage of reactions and a\\u000abetter description of nuclei during the preequilibrium and evaporation stages\\u000aof reactions. The development of the CEM concerning fission is given in a\\u000aseparate talk at this conference. The increased accuracy and predictive power\\u000aof the CEM

  20. Improved Cascade-Exciton Model of Nuclear Reactions

    E-print Network

    Stepan G. Mashnik; Arnold J. Sierk

    1998-12-24

    Recent improvements to the Cascade-Exciton Model (CEM) of nuclear reactions are briefly described. They concern mainly the cascade stage of reactions and a better description of nuclei during the preequilibrium and evaporation stages of reactions. The development of the CEM concerning fission is given in a separate talk at this conference. The increased accuracy and predictive power of the CEM are shown by several examples. Possible further improvements to the CEM and other models are discussed.