For comprehensive and current results, perform a real-time search at Science.gov.

1

Theory of exciton pair states and their nonlinear optical properties in semiconductor quantum dots

The exciton and two-exciton states in semiconductor quantum dots much larger in size than the exciton Bohr radius are investigated, and the energies and oscillator strengths of several exciton and biexciton states are calculated. The presence of weakly correlated exciton-pair states are identified and these have a large oscillator strength increasing proportional to the volume of the quantum dot. These

Selvakumar V. Nair; Toshihide Takagahara

1997-01-01

2

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

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

David C. Lush

2007-01-01

3

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

The dynamics of the spin-orbit interaction in atomic hydrogen are studied in\\u000aa classical electrodynamics-like setting. A Rutherfordian atomic model is used\\u000aassuming a circular electron orbit, without the quantum principle as imposed\\u000aarbitrarily in the Bohr model, but with an ad hoc incorporation in the electron\\u000aof intrinsic spin and associated magnetic dipole moment. Analyzing the motions\\u000aof the

David C. Lush

2007-01-01

4

Enhancement of the Longitudinal Magnetic Moment of the Exciton due to its Motion

NASA Astrophysics Data System (ADS)

A new physical phenomenon has been found, namely a giant enhancement of the exciton magnetic moment due to its motion. This phenomenon was observed in GaAs, CdTe and ZnSe based quantum wells with QW widths much larger than the exciton Bohr radius. Consequently, it is relevant to any crystal with zinc blende structure.

Kochereshko, V. P.; Platonov, A. V.; Mikhailov, G. V.; Davies, J. J.; Wolverson, D.; Cox, R. T.; Cibert, J.; Mariette, H.; Loginov, D. K.; Ubyivovk, E. V.; Efimov, Yu. P.; Eliseev, S. A.

5

Wave functions of hot excitons in semiconductors with degenerate bands

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)

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

2008-06-15

6

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass {mu} = {mu}(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E{sub ex}(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a{sub ex}, the exciton binding energy E{sub ex}(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.

Pokutnii, S. I., E-mail: Pokutnyi_Sergey@inbox.ru [National Academy of Sciences of Ukraine, G.V. Kurdjumov Institute for Metal Physics (Ukraine)

2010-04-15

7

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

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

2011-05-12

8

Bohr's power series theorem in several variables

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}).

Harold P. Boas; Dmitry Khavinson

1996-01-01

9

Revisiting the Bohr Atom 100 Years Later

NASA Astrophysics Data System (ADS)

We use a novel electron model wherein the electron is modeled as a point charge behaving as a trapped photon revolving in a Compton wavelength orbit at light speed. The revolving point charge gives rise to spiraling Compton wavelets around the electron, which give rise to de Broglie waves. When applied to the Bohr model, the orbital radius of the electron scales to the first Bohr orbit's radius via the fine structure constant. The orbiting electron's orbital velocity, Vb, scales to that of the electron's charge's internal velocity (the velocity of light, c) via the fine structure constant. The Compton wavelets, if they reflect off the nucleus, have a round trip time just long enough to allow the electron to move one of its diameters in distance in the first Bohr orbit. The ratio of the electron's rotational frequency, fe, to its rotational frequency in the Bohr orbit fb, is fe/fb = 1/2?, which is also the number of electron rotations in single orbit. If we scale the electron's rotational energy (h*fe) to that of the orbit using this, the orbital energy value (h*fb) would be 27.2114 eV. However, the virial theorem reduces it to 13.6057, the ground state energy of the first Bohr orbit. Ref: www.tachyonmodel.com.

Wall, Ernst

2013-03-01

10

Bohr's atomic model, its relationship to the radiation spectrum of the hydrogen atom and the inherent hypotheses are revisited. It is argued that Bohr could have adopted a different approach, focusing his analyzes on the stationary orbit of the electron and its decomposition on two harmonic oscillators and then imposing, as actually he did, Planck's quantization for the oscillators' energies.

Francisco Caruso; Vitor Oguri

2009-01-01

11

Bohr used to introduce his attempts to explain clearly the principles of the quantum theory of the atom with an historical sketch, beginning invariably with the nuclear model proposed by Rutherford. That was sound pedagogy but bad history. The Rutherford-Bohr atom stands in the middle of a line of work initiated by J.J. Thomson and concluded by the invention of

J. L. Heilbron

1981-01-01

12

ERIC Educational Resources Information Center

"Bohr's Atomic Model" is a small interactive multimedia program that introduces the viewer to a simplified model of the atom. This interactive simulation lets students build an atom using an atomic construction set. The underlying design methodology for "Bohr's Atomic Model" is model-centered instruction, which means the central model of the…

Willden, Jeff

2001-01-01

13

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.

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

14

Bohr's atomic model, its relationship to the radiation spectrum of the hydrogen atom and the inherent hypotheses are revisited. It is argued that Bohr could have adopted a different approach, focusing his analyzes on the stationary orbit of the electron and its decomposition on two harmonic oscillators and then imposing, as actually he did, Planck's quantization for the oscillators' energies. Some consequences of this procedure are examined.

Caruso, Francisco

2008-01-01

15

Bohr's atomic model, its relationship to the radiation spectrum of the hydrogen atom and the inherent hypotheses are revisited. It is argued that Bohr could have adopted a different approach, focusing his analyzes on the stationary orbit of the electron and its decomposition on two harmonic oscillators and then imposing, as actually he did, Planck's quantization for the oscillators' energies. Some consequences of this procedure are examined.

Francisco Caruso; Vitor Oguri

2008-06-03

16

The model of Niels Bohr (1885–1962) for the atom is since long just the one and only conception for atoms of the vast majority\\u000a of educated people. The picture of ? electrons revolving round a nucleus on select avenues has become the icon of the atomic\\u000a age. In stark contrast to this omnipresence, historically, the Bohr atom may be identified

Arne Schirrmacher; A. Shimony

17

A conformal factor in the Bohr model embeds Bohr space in six dimensions, revealing the $O(6)$ symmetry and its contraction to the $E(5)$ at infinity. Phenomenological consequences are discussed after the re-formulation of the Bohr Hamiltonian in six dimensions on a five sphere.

P. E. Georgoudis

2014-03-18

18

NASA Astrophysics Data System (ADS)

A conformal factor in the Bohr model embeds Bohr space in six dimensions, revealing the O(6) symmetry and its contraction to the E(5) at infinity. Phenomenological consequences are discussed after the re-formulation of the Bohr Hamiltonian in six dimensions on a five sphere.

Georgoudis, P. E.

2014-04-01

19

The methodological lesson of complementarity: Bohr’s naturalistic epistemology

NASA Astrophysics Data System (ADS)

Bohr’s intellectual journey began with the recognition that empirical phenomena implied the breakdown of classical mechanics in the atomic domain; this, in turn, led to his adoption of the ‘quantum postulate’ that justifies the ‘stationary states’ of his atomic model of 1913. His endeavor to develop a wider conceptual framework harmonizing both classical and quantum descriptions led to his proposal of the new methodological goals and standards of complementarity. Bohr’s claim that an empirical discovery can demand methodological revision justifies regarding his epistemological lesson as supporting a naturalistic epistemology.

Folse, H. J.

2014-12-01

20

\\u000a We come now to a new aspect of atoms: the existence of discrete energy states. Niels Bohr’s idea that atoms can possess only\\u000a certain well–defined amounts of energy was a major development in our understanding of atoms. In 1911 Bohr, a young Dane who\\u000a had just received his Ph.D. in physics from the University in Copenhagen, came to England to

Charles H. Holbrow; James N. Lloyd; Joseph C. Amato; Enrique Galvez; M. Elizabeth Parks

21

Exciton-Phonon Interaction Effects in II-Vi Compound Semiconductor Quantum Wells

NASA Astrophysics Data System (ADS)

In this thesis, we report on two specific examples of exciton-LO phonon Frohlich interaction effects, namely, hot carrier relaxation and temperature dependent exciton linewidth broadening. These phenomena are considered in the context of quasi-two dimensional excitons in strongly polar II-VI semiconductor quantum wells. Hot-exciton luminescence phenomena are investigated in a single quantum well of ZnTe/MnTe where tunneling through thin MnTe barrier layers suppresses the formation of thermalized luminescence. For near resonant photoexcitation, the secondary emission spectrum is modulated by distinct LO-phonon peaks, which, for sufficiently high order of scattering ( >=4), behave like hot luminescence (HPL) as opposed to resonant Raman scattering. This is confirmed by time-resolved spectroscopy as well as by steady-state characteristics such as linewidth broadening and lack of polarization memory. Several novel observations are made: (1) The LO-phonon intermediated energy relaxation involves Coulomb-correlated pairs, i.e. hot excitons, as opposed to independently-relaxing free electrons and holes. (2) The additional weak disorder originating from QW thickness fluctuations plays a major role in the details of the HPL spectra. The major contribution to the ground state exciton linewidth at room temperature originates from LO phonon -intermediated exciton scattering to higher exciton states. A measure of the effect is given by the parameter Gamma_{LO} which increases with the polarity of the material and is independent of dimensionality provided that the LO phonon energy is greater than the exciton binding energy. Measurements of Gamma_{LO} are performed in two quantum well systems: CdTe/MnTe and (Zn,Cd)Se/ZnSe. In the latter system, a strong reduction of Gamma _{LO} is observed as the quantum well width becomes comparable to the three-dimensional exciton Bohr radius. This is explained in terms of a model where quasi-2D confinement effects increase the exciton binding energy to a value greater than the LO phonon energy and hence reduce the available phase space for the exciton -LO phonon scattering process. Direct confirmation of our interpretation is found in magneto-transmission experiments.

Pelekanos, Nikolaos Themelis

1992-01-01

22

NASA Astrophysics Data System (ADS)

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.

Jiang, Jian-Hua; John, Sajeev

2014-07-01

23

Revisiting Bohr's semiclassical quantum theory.

Bohr's atomic theory is widely viewed as remarkable, both for its accuracy in predicting the observed optical transitions of one-electron atoms and for its failure to fully correspond with current electronic structure theory. What is not generally appreciated is that Bohr's original semiclassical conception differed significantly from the Bohr-Sommerfeld theory and offers an alternative semiclassical approximation scheme with remarkable attributes. More specifically, Bohr's original method did not impose action quantization constraints but rather obtained these as predictions by simply matching photon and classical orbital frequencies. In other words, the hydrogen atom was treated entirely classically and orbital quantized emerged directly from the Planck-Einstein photon quantization condition, E = h nu. Here, we revisit this early history of quantum theory and demonstrate the application of Bohr's original strategy to the three quintessential quantum systems: an electron in a box, an electron in a ring, and a dipolar harmonic oscillator. The usual energy-level spectra, and optical selection rules, emerge by solving an algebraic (quadratic) equation, rather than a Bohr-Sommerfeld integral (or Schroedinger) equation. However, the new predictions include a frozen (zero-kinetic-energy) state which in some (but not all) cases lies below the usual zero-point energy. In addition to raising provocative questions concerning the origin of quantum-chemical phenomena, the results may prove to be of pedagogical value in introducing students to quantum mechanics. PMID:17020371

Ben-Amotz, Dor

2006-10-12

24

ERIC Educational Resources Information Center

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…

Brunori, Maurizio

2012-01-01

25

Bohr model without quantum jumps

Omission of Bohr's second postulate permits a derivation of spectral intensity. The transition amplitudes serve as upper bounds to quantum mechanical matrix elements. They also provide insight into the latter in terms of Sommerfeld ellipses and transition trajectories. The speed of a nascent photon in the region of the electron transiton is addressed and the orbit concept is reinterpreted.

Manfred Bucher

2006-01-01

26

A simple relativistic Bohr atom

A simple concise relativistic modification of the standard Bohr model for hydrogen-like atoms with circular orbits is presented. As the derivation requires basic knowledge of classical and relativistic mechanics, it can be taught in standard courses in modern physics and introductory quantum mechanics. In addition, it can be shown in a class that one straightforward prediction of this relativistic version

Andreas F. Terzis

2008-01-01

27

Is Bohr challenge still relevant?

We argue that not all the theoretical content of the Bohr model has been captured by the definitive quantum formalism currently in use. In particular, the notion of quantum leap seems to refer to non-dynamic features, closely related to non-locality, which have not yet been formalized in a satisfactory way.

Leonardo Chiatti

2014-12-11

28

NSDL National Science Digital Library

This applet demonstrates light absorption in the Bohr model. An electron is shown orbiting a proton in the Bohr model for hydrogen in level 2. Full spectrum light is incident upon the atom. The user may change the final level of the electron between 3 and 6. After a short time, a photon of the appropriate wavelength is absorbed and the electron "jumps" to the chosen level. The absorption spectrum is shown. The user is asked to determine how the wavelength of the absorbed photon depends on the starting and ending orbital levels. This applet is based on Physlets, developed at Davidson College. It is part of a collection of similar applet-based exercises covering the topics in a standard introductory physics textbook.

Niederriter, Chuck; Belloni, Mario

2008-11-08

29

NASA Astrophysics Data System (ADS)

Exciton condensation may occur not only in semiconductors, but also in mixed-valent compounds(H. J. Leder, Solid State Commun. 27), 579 (1978). Whereas the excitonic condensate in a semiconductor must be created artificially using optical pumping, the excitonic condensate in a mixed-valent compound is already built in by nature. This makes mixed-valent compounds ideal systems for studying the optical properties of excitonic condensates(V. Yu. Irkin and M. I. Katsnel'son, Zh. Eksp. Teor. Fiz. 90), 1080 (1986) [Sov. Phys. JETP 63, 631 (1986)]. Three surprising manifestations of exciton condensation in mixed-valent compounds are discussed in this talk: 1) electronic ferroelectricity, 2) ferroelectric resonance, and 3) a nonvanishing susceptibility for second-harmonic generation. A number of experiments are proposed which can give evidence for exciton condensation in mixed-valent compounds.

Portengen, T.

1998-03-01

30

Rutherford-Bohr Model of the Atom

\\u000a This chapter is devoted to a discussion of the Rutherford—Bohr model of the atom. The two giants of modern physics, Rutherford\\u000a and Bohr, have not collaborated on the model; however, they both made a major contribution to it; Rutherford by introducing\\u000a the concept of the atomic nucleus with electrons revolving about the nucleus in a cloud and Bohr by introducing

Ervin B. Podgoršak

31

The de Broglie-Bohr Model for the Hydrogen Atom = de Broglie's hypothesis that

The de Broglie-Bohr Model for the Hydrogen Atom h m v = de Broglie's hypothesis that matter has -= Below the ground state energy and orbit radius of the electron in the hydrogen atom is found by plotting 52.9 r pm This figure shows that atomic stability involves a balance between potential and kinetic

Rioux, Frank

32

The Bohr effect before Perutz.

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

Brunori, Maurizio

2012-01-01

33

Bohr model as an algebraic collective model

Developments and applications are presented of an algebraic version of Bohr's collective model. Illustrative examples show that fully converged calculations can be performed quickly and easily for a large range of Hamiltonians. As a result, the Bohr model becomes an effective tool in the analysis of experimental data. The examples are chosen both to confirm the reliability of the algebraic

D. J. Rowe; T. A. Welsh; M. A. Caprio

2009-01-01

34

Vibrating Wire Loop and the Bohr Model

PASCO scientific1 has designed a vibration apparatus that includes a mechanical driver used to excite vibrations in various systems. One of these is a circular wire loop, which PASCO suggests can be used as a visual representation of the Bohr model of the hydrogen atom. In this paper we investigate the similarity between the wire loop and the Bohr atom.

Danning Bloom; Dan W. Bloom

2003-01-01

35

Rutherford and Bohr describe atomic structure 1913

NSDL National Science Digital Library

This page is from PBS A Science Odyssey: People and Discoveries. It describes Bohr's contribution to theories of the atom and its relationship to others' such as Rutherford's. Links to relevant pages are provided. An image of Bohr's conception drawn in his notebook is included.

1998-01-01

36

Bohr model as an algebraic collective model

Developments and applications are presented of an algebraic version of Bohr's collective model. Illustrative examples show that fully converged calculations can be performed quickly and easily for a large range of Hamiltonians. As a result, the Bohr model becomes an effective tool in the analysis of experimental data. The examples are chosen both to confirm the reliability of the algebraic collective model and to show the diversity of results that can be obtained by its use. The focus of the paper is to facilitate identification of the limitations of the Bohr model with a view to developing more realistic, computationally tractable models.

Rowe, D. J.; Welsh, T. A.; Caprio, M. A. [Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7 (Canada); Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556-5670 (United States)

2009-05-15

37

A deBroglie-Bohr Model for Positronium Frank Rioux

A deBroglie-Bohr Model for Positronium Frank Rioux Positronium is a metastable bound state, to be determined. The purpose of this tutorial is to calculate this value using the Bohr model for positronium

Rioux, Frank

38

Davidson potential and SUSYQM in the Bohr Hamiltonian

NASA Astrophysics Data System (ADS)

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 ?-dependence of the moments of inertia.

Georgoudis, P. E.

2013-06-01

39

Should we teach the Bohr model?

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 wave nature of electrons in atoms. Although the evidence for this claim is weak, many in the physics education research community have accepted it. This claim has implications for how we present atoms in classes

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

2007-01-01

40

IBA and the bohr-mottelson model

The Bohr-Mottelson Model (BMM) and the Interacting-Boson Approximation (IBA) of Arima and Iachello are compared with each other on the phenomene logical and the microscopic level and on the level of selected data. The aim is here not to demonstrate the great successes of both models. But on the phenomenological level we discuss how far the models are identical and

Amand Faessler

1983-01-01

41

Mathematical analysis of a Bohr atom model

Bohr proposed in 1913 a model for atoms and molecules by synthesizing Planck’s quantum hypothesis with classical mechanics. When the atom number Z is small, his model provides good accuracy for the ground-state energy. When Z is large, his model is not as accurate in comparison with the experimental data but still provides a good trend agreeing with the experimental

Goong Chen; Zhonghai Ding; Sze-Bi Hsu; Moochan Kim; Jianxin Zhou

2006-01-01

42

How good is Niels Bohr's atomic model?

Since 1913, Bohr's planetary model for atomic structure has been predominant, but recent analyses suggest that collective quantization of atomic valence shells containing two electrons may be more valid. The history and evidence for these two extreme paradigms are presented and some of the potentially observable consequences are discussed briefly.

R. Stephen Berry

1989-01-01

43

Generalized Bohr compactication and model-theoretic connected components

Generalized Bohr compactication and model-theoretic connected components Krzysztof Krupi: [externally] denable Bohr compactication, model-theoretic connected components, denable strong amenability 1-called generalized Bohr compactication from [11]. As mentioned in the abstract, our model-theoretic set-up subsumes

KrupiÃ±ski, Krzysztof

44

Bohr Model Calculations for Atoms and Ions Frank Rioux

Bohr Model Calculations for Atoms and Ions Frank RiouxBroglie Bohr model is described that can be used to calculate the electronic energies of atoms or ions extending the Bohr model to two and threeelectron atoms and ions could be used to enhance student

Rioux, Frank

45

The major chemical trends in the binding energies of intrinsic and extrinsic core excitons are predicted for zinc-blende semiconductors using an empirical tight-binding theory and localized empirical core-hole potentials. A transition from a shallow Wannier exciton to a deep Frenkel exciton is predicted for an exciton at a core-exciton absorption edge, depending on the chemical structure of the excited atom

Harold P. Hjalmarson; Helmut Büttner; John D. Dow

1981-01-01

46

Quadrupole collective states within the Bohr collective Hamiltonian

The article reviews the general version of the Bohr collective model for the description of quadrupole collective states, including a detailed study the model's kinematics. The general form of the classical and quantum Bohr Hamiltonian is presented together with a discussion of the tensor structure of the collective wave functions and with a short review of various methods of solving the Bohr Hamiltonian eigenvalue equation.The methods of derivation of the classical and quantum Bohr Hamiltonian from the microscopic many-body theory are recalled and the microscopic approach to the Bohr Hamiltonian is applied to interpret collective properties of 12 heavy even-even nuclei in the Hf-Hg region.

L. Prochniak; S. G. Rohozinski

2009-11-02

47

Microscopic Uniaxial Bohr-Mottelson Rotational Model

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,

P. Gulshani

2010-01-01

48

Spreading the gospel: The Bohr atom popularised

The emergence of quantum theory in the early decades of the twentieth century was accompanied by a wide range of popular science books, all of which presented in words and in images new scientific ideas about the structure of the atom. The work of physicists such as Ernest Rutherford and Niels Bohr, among others, was pivotal to the so-called planetary model of the atom, which, still today, is used in popular accounts and in science textbooks. In an attempt to add to our knowledge about the popular trajectory of the new atomic physics, this paper examines one book in particular, coauthored by Danish science writer Helge Holst and Dutch physicist and close collaborator of Niels Bohr, Hendrik A. Kramers. Translated from Danish into four European languages, the book not only presented contemporary ideas about the quantum atom, but also went into rather lengthy discussions about unresolved problems. Moreover, the book was quite explicit in identifying the quantum atom with the atom as described by Bohr's theory. W...

Kragh, Helge

2011-01-01

49

The operation : The interview of Niels Bohr

NASA Astrophysics Data System (ADS)

The article represents a stenogramme of the recollections of professor Yakov Petrovich Terletskij (1912, St. Petersburg-1993, Moscow) concerning his meeting with professor Niels Bohr in Kopenhagen from 24 september 1945 till 22 november 1945 under the task of Soviet KGB and personally Lavrentii Berija. Some additional informations concerning previous preparations of the trip, the final stage of the mission, as well as general considerations about the development of Physics, persons of physicists and nuclear technologies has been given by the author of recollections throughout the text

Terletskij, Yakov P.

50

Realization of Localized Bohr-like Wavepackets

We demonstrate a protocol to create localized wavepackets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wavepackets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wavepackets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in circular classical orbit around the nucleus. Possible extension of the approach to create so far elusive "planetary atoms" in highly correlated stable multiply-excited states is discussed.

Mestayer, J. J. [Rice University; Wyker, B. [Rice University; Lancaster, J. C. [Rice University; Dunning, F. B. [Rice University; Reinhold, Carlos O [ORNL; Yoshida, S. [Vienna University of Technology, Austria; Burgdorfer, J. [Vienna University of Technology, Austria

2008-01-01

51

Exciton annihilation and diffusion in semiconducting polymers

NASA Astrophysics Data System (ADS)

We show that time-resolved luminescence measurements at high excitation densities can be used to study exciton annihilation and diffusion, and report the results of such measurements on films of P3HT and MEH-PPV. The results fit to an exciton-exciton annihilation model with a time independent annihilation rate ?, which was measured to be ? = (2.8+/-0.5)×10 -8 cm 3s -1 in MEH-PPV and ? = (5.2+/-1)×10 -10 cm 3s -1 in P3HT. This implies much faster diffusion in MEHPPV. Assuming a value of 1 nm for the annihilation radius we evaluated the diffusion length for pristine P3HT in one direction to be 3.2 nm. Annealing of P3HT was found to increase the annihilation rate to (1.1+/-0.2)×10 -9 cm 3s -1 and the diffusion length to 4.7 nm.

Shaw, P. E.; Lewis, A. J.; Ruseckas, A.; Samuel, I. D. W.

2006-08-01

52

Exciton states in quasi-zero-dimensional semiconductor nanosystems

The variational method in the context of the modified effective mass approximation is used to calculate the dependence of exciton ground-state energy for a quantum dot embedded in a borosilicate glassy matrix on the quantum dot radius. It is shown that the peaks in the absorption and low-temperature luminescence spectra of such a nanosystem are shifted to shorter wavelengths due to size quantization of the exciton ground-state energy in the quantum dot.

Pokutnyi, S. I., E-mail: Pokutnyi_Sergey@inbox.ru [National Academy of Sciences of Ukraine, Kurdyumov Institute for Metal Physics (Ukraine)

2012-02-15

53

Bohr's Creation of his Quantum Atom

NASA Astrophysics Data System (ADS)

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.

Heilbron, John

2013-04-01

54

Bohr model and dimensional scaling analysis of atoms and molecules

It is generally believed that the old quantum theory, as presented by Niels Bohr in 1913, fails when applied to few electron systems, such as the H2 molecule. Here we review recent developments of the Bohr model that connect it with dimensional scaling procedures adapted from quantum chromodynamics. This approach treats electrons as point particles whose positions are determined by

Anatoly Svidzinsky; Goong Chen; Siu Chin; Moochan Kim; Dongxia Ma; Robert Murawski; Alexei Sergeev; Marlan Scully; Dudley Herschbach

2008-01-01

55

The many relationships between the IBM and the Bohr model

Relationships between the IBM-1 and the Bohr collective model are explored in which the states of the IBM in its various dynamical symmetry limits are identified with subsets of Bohr model states of corresponding dynamical symmetries. The maps of interest are ones which give the contractions of the IBM in the limit of large boson number. The known map from

D. J. Rowe; G. Thiamova

2005-01-01

56

Bohr model and dimensional scaling analysis of atoms and molecules

It is generally believed that the old quantum theory, as presented by Niels Bohr in 1913, fails when applied to many-electron systems, such as molecules, and nonhydrogenic atoms. It is the central theme of this dissertation to display with examples and applications the implementation of a simple and successful extension of Bohr's planetary model of the hydrogenic atom, which has

Kerim Urtekin

2006-01-01

57

100th anniversary of Bohr's model of the atom.

In the fall of 1913 Niels Bohr formulated his atomic models at the age of 27. This Essay traces Bohr's fundamental reasoning regarding atomic structure and spectra, the periodic table of the elements, and chemical bonding. His enduring insights and superseded suppositions are also discussed. PMID:24123759

Schwarz, W H Eugen

2013-11-18

58

Physics Today Bohr's molecular model, a century later

Physics Today Bohr's molecular model, a century later Anatoly Svidzinsky, Marlan Scully, and Dudley Herschbach Citation: Physics Today 67(1), 33 (2014); doi: 10.1063/PT.3.2243 View online: http://dx.doi.org/10. Even so, today the Bohr model has valid roles in describ- ing highly excited Rydberg atoms,5 cavity

Allen, Jont

59

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

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

2011-11-01

60

Stacking in colloidal nanoplatelets: tuning excitonic properties.

Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Förster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Förster radius of ?13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening. PMID:25469555

Guzelturk, Burak; Erdem, Onur; Olutas, Murat; Kelestemur, Yusuf; Demir, Hilmi Volkan

2014-12-23

61

The many faces of the Bohr atom

The atomic model that Niels Bohr suggested in 1913 celebrated its greatest victories in connection with one-electron atoms. Among them were the isotopic spectral effect and what became known as Rydberg atoms, insights that were fully recognized only many years later. He considered the original ring model a first step towards an understanding of atomic structure, and during the following years he developed it into more ambitious models that, he hoped, would also describe many-electron atoms. His theory of the periodic system marked the culmination of the orbital atom within the framework of the old quantum theory. However, the theory would soon be replaced by more symbolic models that heralded the coming of the quantum-mechanical atom.

Kragh, Helge

2013-01-01

62

Paul Ehrenfest, Niels Bohr, and Albert Einstein: Colleagues and Friends

NASA Astrophysics Data System (ADS)

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.

Klein, Martin J.

2010-09-01

63

Relativistic Corrections to the Bohr Model of the Atom

ERIC Educational Resources Information Center

Presents a simple means for extending the Bohr model to include relativistic corrections using a derivation similar to that for the non-relativistic case, except that the relativistic expressions for mass and kinetic energy are employed. (Author/GS)

Kraft, David W.

1974-01-01

64

A Bohr's Semiclassical Model of the Black Hole Thermodynamics

We propose a simple procedure for evaluating the main thermodynamical attributes of a Schwarzschild's black hole: Bekenstein-Hawking entropy, Hawking's temperature and Bekenstein's quantization of the surface area. We make use of the condition that the circumference of a great circle on the black hole horizon contains finite number of the corresponding reduced Compton's wavelength. It is essentially analogous to Bohr's quantization postulate in Bohr's atomic model interpreted by de Broglie's relation. We present black hole radiation in the form conceptually analogous to Bohr's postulate on the photon emission by discrete quantum jump of the electron within the Old quantum theory. It enables us, in accordance with Heisenberg's uncertainty relation and Bohr's correspondence principle, to make a rough estimate of the time interval for black hole evaporation, which turns out very close to time interval predicted by the standard Hawking's theory. Our calculations confirm Bekenstein's semiclassical result for the e...

Pankovic, V; Grujic, P

2007-01-01

65

Optically programmable excitonic traps.

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

Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François

2013-01-01

66

Neutral and positively charged excitons in narrow quantum ring

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.

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

67

Anomalous magnetization of a carbon nanotube as an excitonic insulator

NASA Astrophysics Data System (ADS)

We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.

Rontani, Massimo

2014-11-01

68

Holographic radius test plates

NASA Astrophysics Data System (ADS)

We evaluate a method for testing the radius of a spherical surface with a hologram that consists of a pair of nested Fresnel zone lenses. The hologram is positioned in the collimated test beam of a Fizeau interferometer. The inner zone lens generates a focus at the test part surface, whereas the wavefront of the first diffraction order of the outer zone lens is confocal with the test part. When the test part radius is equal to the nominal radius, the fringes in both zone lens areas are nulled at the same distance of the test sphere from the zone lens. The radius error of the spherical surface can be calculated from the test sphere displacement between interferometer null positions for the inner and outer zone lenses, or from the defocus term of the outer (confocal) lens at the position of zero defocus of the inner (cat's-eye) zone lens. The primary benefits of the nested zone lens method are its ease of use, and that it enables radius measurements of spherical surfaces with large radii. We describe the radius measurement of a precise convex sphere with a nominal radius of 80mm.

Wang, Quandou; Soons, Johannes A.; Griesmann, Ulf

2013-09-01

69

in the history of twentieth-century physics, and rightly so, Albert Einstein and Niels Bohr being the figuresRevisiting the Einstein-Bohr Dialogue Don Howard Einstein and Bohr Â No names loom larger identified complementarity as the chief novelty in the quantum description of nature, Einstein for having

Howard, Don

70

Hemoglobin Bohr effects: atomic origin of the histidine residue contributions.

The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resolution structures, the residue pKa values corresponding to the two structures are calculated. The method is based on determining the electrostatic interactions between residues in the protein, relative to those of the residue in solution, by use of the linearized finite difference Poisson-Boltzmann equation and Monte Carlo sampling of protonation states. Given that good agreement is obtained with the available experimental values for the contribution of His residues in HbA to the Bohr effect, the calculated results are used to determine the atomic origin of the pKa shift between deoxy and carbonmonoxy HbA. The contributions to the pKa shift calculated by means of the linear response approximation show that the salt bridge involving His146 plays an important role in the alkaline Bohr effect, as suggested by Perutz but that other interactions are significant as well. A corresponding analysis is made for the contribution of His143 to the acid Bohr effect for which there is no proposed explanation. The method used is summarized and the program by which it is implemented is described in the Appendix . PMID:24224786

Zheng, Guishan; Schaefer, Michael; Karplus, Martin

2013-11-26

71

Jet radiation patterns are indispensable for the purpose of discriminating partons' with different quantum numbers. However, they are also vulnerable to various contaminations from the underlying event, pileup, and radiation of adjacent jets. In order to maximize the discrimination power, it is essential to optimize the jet radius used when analyzing the radiation patterns. We introduce the concept of jet radiation radius which quantifies how the jet radiation is distributed around the jet axes. We study the color and momentum dependence of the jet radiation radius, and discuss two applications: quark-gluon discrimination and $W$ jet tagging. In both cases, smaller (sub)jet radii are preferred for jets with higher PTs, albeit due to different mechanisms: the running of the QCD coupling constant and the boost to a color singlet system. A shrinking cone W jet tagging algorithm is proposed to achieve better discrimination than previous methods.

Zhenyu Han

2014-02-06

72

Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies exciton and electron-hole plasma contributions. For low temperatures and low densities, the excitonic emission is extremely sensitive to details of the electron-hole-pair population making it possible to identify even minute fractions of optically active excitons.

S. Chatterjee; C. Ell; S. Mosor; G. Khitrova; H. M. Gibbs; W. Hoyer; M. Kira; S. W. Koch; J. P. Prineas; H. Stolz

2004-01-01

73

Jocular Physics: A Tribute to Bohr in Humor

NASA Astrophysics Data System (ADS)

Copenhagen, starting in the 1920s and 1930s and continuing after the Second World War, was not just a center for extraordinary developments in quantum and nuclear physics; it also provided a perfect stage for physicists' abundant humor. We will examine the Journal of Jocular Physics, a humorous tribute to Bohr published on the occasions of his 50^th, 60^th and 70^th birthdays. We will discuss how the articles in the journal reflected attempts by the contributors, such as L'eon Rosenfeld, Victor Weisskopf and others, to interpret and explain aspects of Bohr's philosophy, such as complementarity and the abandonment of pure Laplacian determinism.

Halpern, Paul

2011-04-01

74

Excitonic optical nonlinearity and exciton dynamics in semiconductor quantum dots

Two salient features of the excitonic state in semiconductor quantum dots are theoretically clarified. One is the enhanced excitonic optical nonlinearity arising from the state filling of discrete levels due to the quantum size effect. The calculated third-order nonlinear susceptibility explains successfully the recent experimental results. The other feature is the exciton dynamics in semiconductor quantum dots. A comprehensive interpretation

T. Takagahara

1987-01-01

75

Triplet exciton dissociation in singlet exciton fission photovoltaics.

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

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

76

phys. stat. sol. (b) 216, 187 (1999) Subject classification: 78.55.Cr; 78.66.Fd; S7.14

by the exciton. The main parameters which enter into the model calculation are the exci- ton volume and its Bohr and cathodoluminescence (CL) measurements with a calculation based on a model by Schubert et al. [6]. The broadening radius. The exciton volume is calculated from the Bohr radius for which we took 28 #e in GaN and 17 #e

Nabben, Reinhard

77

The Bohr Model of the EarthSun System* Frank Rioux

The Bohr Model of the EarthSun System* Frank Rioux Assuming the earth executes a circular orbit with the Bohr model for the earthsun system. Is this a legitimate example of the correspondence principle? E G

Rioux, Frank

78

Lamprey Hemoglobin STRUCTURAL BASIS OF THE BOHR EFFECT*

Lamprey Hemoglobin STRUCTURAL BASIS OF THE BOHR EFFECT* Received for publication, July 23, 1999-1892, and the Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712-1167 Lampreys, among. Monomeric, ligated lamprey Hb self-associates to dimers and tetramers upon deoxygenation. Dissocia- tion

Riggs, Austen

79

Bohr Topologies and Partition Theorems for Vector Spaces \\Lambda

Bohr Topologies and Partition Theorems for Vector Spaces \\Lambda Kenneth Kunen y University in spirit to the work of K. P. Hart and J. van Mill [6]. In fact, it is clear from [6] that one should try # . Independently of us, S. Watson [8] discovered a related nonÂhomeomorphism result, also by following the Hart

Kunen, Ken

80

Relativistic Bohr Model with Finite-Mass Nucleus

An elementary but rigorous treatment is given of the Bohr model of hydrogenlike atoms in which first-order corrections are made for both the finite mass of the nucleus and for the relativistic motion of the electron. Particular attention is paid to the approximations necessary to obtain the energy levels first given by Sommerfeld.

James T. Cushing

1970-01-01

81

Microscopic aspects of the Bohr-Mottelson model

A generalized description of the variation of the mean field expressed in a coherent-state basis is presented. An appropriate form of the coherent state is used such that the parameters corresponding to creation operators of particle-hole pairs varying from -? to +? can be taken as the variables of of the Bohr-Mottelson model. The generators in the coherent state are

Xu Gong-Ou; Li Fu-Li; Fu de-Ji

1991-01-01

82

On a possible consistency check for the bohr vibrational model

From an interpretation of what is meant by a vibrational band in a nucleus, fully consistent with the Bohr vibrational model, and indeed implied by this model, a dispersion formula is obtained. This establishes a relationship between certain measurable quantities. When the available relevant information for Pb and Hg isotopes is inserted into this formula, it is nearly satisfied in

A. Miranda

1970-01-01

83

Relativistic Corrections to the Bohr Model of the Atom

A simple means for extending the Bohr model of the atom to include relativistic corrections is presented. The derivation, which assumes circular orbits and a stationary nucleus, is similar to that for the non-relativistic case, except that the relativistic expressions for mass and kinetic energy are employed. Corrections, consistent with those of Sommerfeld, can thus be obtained to the radii

David W. Kraft

1974-01-01

84

Bohr's Model of the Hydrogen Molecules and their Magnetic Susceptibility

BOHR'S model of the molecules of hydrogen explains very satisfactorily the light dispersion of hydrogen,1 and gives the same value for the moment of inertia as that deduced from the specific heat;2 but it is generally believed that the model does not explain the diamagnetic property of the gas.3 For, according to P. Langevin's theory,4 the hydrogen molecules must have

Kôtarô Honda

1922-01-01

85

Derivation of microscopic unified Bohr–Mottelson rotational model

In a previous article, we derived a microscopic version of the phenomenological Bohr–Mottelson unified rotational model for rotation about a single axis. In this article, we generalize the derivation to that for rotation about all the three axes. As in the previous derivation, we apply the nuclear Hamiltonian directly to the rotational-model wavefunction instead of using the usual canonical transformation.

P. Gulshani

2011-01-01

86

Revisiting Bohr's quantization hypothesis for the atomic orbitals

We deduce the quantization of the atomic orbit for the hydrogen's atom model proposed by Bohr without using his hypothesis of angular momentum quantization. We show that his hypothesis can be deduced from and is a consequence of the Planck's energy quantization.

J. H. O. Sales; A. T. Suzuki; D. S. Bonafe

2006-01-01

87

True\\/untrue explanations in Physics: the Bohr's atom model

In this note, and inspired by an article appeared recently in this journal, we discuss an example related to the theory of the Bohr's atom, who illustrates the fact that erroneous expositions in the learning of Physics exist, considering that this concept includes the use of well internationally recognized and commonly used educational text books.

E. Marín

2008-01-01

88

Revisiting Bohr's quantization hypothesis for the atomic orbitals

We deduce the quantization of the atomic orbit for the hydrogen's atom model\\u000aproposed by Bohr without using his hypothesis of angular momentum quantization.\\u000aWe show that his hypothesis can be deduced from and is a consequence of the\\u000aPlanck's energy quantization.

J. H. O. Sales; A. T. Suzuki; D. S. Bonafe

2006-01-01

89

Atempts to link Quanta & Atoms before the Bohr Atom model

Attempts to quantize atomic phenomena before Bohr are hardly ever mentioned in elementary textbooks.This presentation will elucidate the contributions of A.Haas around 1910. Haas tried to quantize the Thomson atom model as an optical resonator made of positive and negative charges. The inherent ambiguity of charge distribution in the model made him choose a positive spherical distribution around which the

A. Venkatesan; M. Lieber

2005-01-01

90

A Bohr's Semiclassical Model of the Black Hole Thermodynamics

We propose a simple procedure for evaluating the main thermodynamical attributes of a Schwarzschild's black hole: Bekenstein-Hawking entropy, Hawking's temperature and Bekenstein's quantization of the surface area. We make use of the condition that the circumference of a great circle on the black hole horizon contains finite number of the corresponding reduced Compton's wavelength. It is essentially analogous to Bohr's quantization postulate in Bohr's atomic model interpreted by de Broglie's relation. We present black hole radiation in the form conceptually analogous to Bohr's postulate on the photon emission by discrete quantum jump of the electron within the Old quantum theory. It enables us, in accordance with Heisenberg's uncertainty relation and Bohr's correspondence principle, to make a rough estimate of the time interval for black hole evaporation, which turns out very close to time interval predicted by the standard Hawking's theory. Our calculations confirm Bekenstein's semiclassical result for the energy quantization, in variance with Frasca's (2005) calculations. Finally we speculate about the possible source-energy distribution within the black hole horizon.

V. Pankovic; M. Predojevic; P. Grujic

2007-09-12

91

Exciton-exciton interaction under elliptically polarized light excitation

NASA Astrophysics Data System (ADS)

We illustrate how interesting it is to use an elliptically polarized laser beam for the investigation of the polarization properties of dense excitonic systems in semiconductor quantum wells. We investigate the spectral splitting of the exciton luminescence, observed in GaAs multiple-quantum-well structures at high exciton density, when the polarization of the resonant laser beam is progressively varied from circular determination of the strengths of both repulsive and attractive parts of the interaction between the excitons. We investigate the stability of the polarized excitonic phase as a function of both the ellipticity and the intensity of the photogenerating picosecond laser beam and measure the longitudinal and the transverse spin observations illustrate the driving role of the exciton-exciton exchange interaction in the spin relaxation mechanism at high density.

Robart, D.; Amand, T.; Marie, X.; Brousseau, M.; Barrau, J.; Bacquet, G.

1996-05-01

92

Extracting Atomic and Molecular Parameters From the de BroglieBohr Model of the Atom

Extracting Atomic and Molecular Parameters From the de BroglieBohr Model of the Atom Frank Rioux The 1913 Bohr model of the hydrogen atom was replaced by SchrÃ¶dingers wave mechanical model in 1926. In his planetary model of the hydrogen atom (see figure below) Bohr began with a Newtonian analysis

Rioux, Frank

93

Chem. Educator 2007, 12, 243245 243 Bohr Model Calculations for Atoms and Ions

Chem. Educator 2007, 12, 243Â245 243 Bohr Model Calculations for Atoms and Ions Frank Rioux-electron atoms and ions could be used to enhance student understanding of atomic structure. A Bohr model, more correctly a deBroglieÂBohr model, is used here to calculate the total electronic energies of atoms and ions

Rioux, Frank

94

A de Broglie-Bohr Model for the Hydrogen Atom Frank Rioux

A de Broglie-Bohr Model for the Hydrogen Atom Frank Rioux Chemistry Department CSB|SJU The 1913 Bohr model of the hydrogen atom was replaced by Schrodinger's wave mechanical model in 1926. However to the nucleus. The de Broglie-Bohr model of the hydrogen atom presented here treats the electron as a particle

Rioux, Frank

95

Detonation Shock Radius Experiments.

NASA Astrophysics Data System (ADS)

A previous passover experiment [1] was designed to create a complex detonation transient used in validating a reduced, asymptotically derived description of detonation shock dynamics (DSD). An underlying question remained on determining the location of the initial detonation shock radius to start the DSD simulation with respect to the dynamical response of the initiation system coupling's to the main charge. This paper concentrates on determining the initial shock radius required of such DSD governed problems. `Cut-back' experiments of PBX-9501 were conducted using an initiation system that sought to optimize the transferred detonation to the desired constant radius, hemispherical shape. Streak camera techniques captured the breakout on three of the prism's surfaces for time-of-arrival data. The paper includes comparisons to simulations using constant volume explosion and high pressure hot spots. The results of the experiments and simulation efforts provide fundamental design considerations for actual explosive systems and verify necessary conditions from which the asymptotic theory of DSD may apply. [1] Lambert, D., Stewart, D. Scott and Yoo, S. and Wescott, B., ``Experimental Validation of Detonation Shock Dynamics in Condensed Explosives. J. of Fluid Mechs., Vol. 546, pp.227-253 (2006).

Lambert, David; Debes, Joshua; Stewart, Scott; Yoo, Sunhee

2007-06-01

96

Daldorph for your help and support! To my radius: Carrie & Asmund & my mothers, Susan & Sondra 1 My sister and I climb the trunk to begin with a tree for this world. I spread out leaves of paper. She says.... Trap a frog in a jar as an experiment. 8. Imagine a robin perched on a tree limb. 9. In the rain, orange heart beating. 10. The tree limb as a guarding hand. 11. Make the sun orange. 12. Make trees. 13. Make robin hearts beat...

Etzel Jr., Dennis

2010-04-16

97

Triplet exciton caging in two dimensions

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

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

1976-01-01

98

What is complementarity?: Niels Bohr and the architecture of quantum theory

NASA Astrophysics Data System (ADS)

This article explores Bohr’s argument, advanced under the heading of ‘complementarity,’ concerning quantum phenomena and quantum mechanics, and its physical and philosophical implications. In Bohr, the term complementarity designates both a particular concept and an overall interpretation of quantum phenomena and quantum mechanics, in part grounded in this concept. While the argument of this article is primarily philosophical, it will also address, historically, the development and transformations of Bohr’s thinking, under the impact of the development of quantum theory and Bohr’s confrontation with Einstein, especially their exchange concerning the EPR experiment, proposed by Einstein, Podolsky and Rosen in 1935. Bohr’s interpretation was progressively characterized by a more radical epistemology, in its ultimate form, which was developed in the 1930s and with which I shall be especially concerned here, defined by his new concepts of phenomenon and atomicity. According to this epistemology, quantum objects are seen as indescribable and possibly even as inconceivable, and as manifesting their existence only in the effects of their interactions with measuring instruments upon those instruments, effects that define phenomena in Bohr’s sense. The absence of causality is an automatic consequence of this epistemology. I shall also consider how probability and statistics work under these epistemological conditions.

Plotnitsky, Arkady

2014-12-01

99

Singlet exciton fission photovoltaics.

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

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

100

Creation of non-dispersive Bohr-like wave packets

NASA Astrophysics Data System (ADS)

We demonstrate the use of a periodic train of half-cycle pulses to create strongly-localized non-dispersive wave packets in very-high-n (n ˜ 300) Rydberg atoms that travel in near-circular orbits about the nucleus. This motion can be maintained for hundreds of orbital periods mimicking the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about the nucleus. The conditions for formation of non-dispersive Bohr-like wave packets are discussed with the aid of Classical Trajectory Monte Carlo (CTMC) simulations and demonstrated through experiment. Research supported by the NSF, the Robert A. Welch Foundation, the OBES, U.S. DoE to ORNL, and by the FWF (Austria).

Mestayer, Jeff; Wyker, B.; Dunning, F. B.; Reinhold, C. O.; Yoshida, S.; Burgdörfer, J.

2009-05-01

101

Steering Quantum States Towards Classical Bohr-Like Orbits

This article furnishes an introduction to the properties of time-dependent electronic wavefunctions in atoms and to physics at the interface between the quantum and classical worlds. We describe how, almost 100 years after the introduction of the Bohr model of the atom, it is now possible using pulsed electric fields to create in the laboratory localized wavepackets in high-n (n ~ 300) Rydberg atoms that travel in near-circular Bohr-like orbits mimicking the behavior of a classical electron. The control protocols employed are explained with the aid of quantum and classical dynamics. Remarkably, while many aspects of the underlying behavior can be described using classical arguments, even at n ~ 300 purely quantum effects such as revivals can be seen.

Dunning, F. B. [Rice University; Reinhold, Carlos O [ORNL; Yoshida, S. [Vienna University of Technology, Austria; Burgdorfer, J. [Vienna University of Technology, Austria

2010-01-01

102

Boson quasispin in the generalised Bohr-Mottelson model

It is shown that the GBMM (an outgrowth of the Bohr-Mottelson model to encompass isovector vibrations) is endowed with an Sp(20, R) algebraic structure. This is connected with the fact that independent proton and neutron harmonic oscillations, which are described by U(10), are the main part of this model. The interaction between the different vibrations are described by the non-compact

Amand Faessler; G. Kyrchev; R. Nojarov

1988-01-01

103

Microscopic Uni-axial Bohr-Mottelson Rotational Model

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.

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

2010-08-04

104

Creation of Non-dispersive Bohr-like Wavepackets

We demonstrate the use of a periodic train of half-cycle pulses to maintain strongly-localized wavepackets in very-high-n (n~300) Rydberg atoms that travel in near circular orbits about the nucleus. This motion can be followed for hundreds of orbital periods and mimics the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about the nucleus.

Mestayer, J. J. [Rice University; Wyker, B. [Rice University; Dunning, F. B. [Rice University; Yoshida, S. [Vienna University of Technology, Austria; Reinhold, Carlos O [ORNL; Burgdorfer, J. [Vienna University of Technology, Austria

2009-01-01

105

Ericson fluctuations and the bohr model in hadron physics

Summary Analogies between the statistical bootstrap model for hadrons and the familiar statistical model for nuclei are pointed out,\\u000a and used as a guide for suggesting new statistical treatments of hadron reactions: i) the Fermi statistical model is modernized\\u000a by including the full Hagedorn spectrum of resonances, and brought into correspondence with the Bohr model by assuming that\\u000a the reaction proceeds

S. Frautschi

1972-01-01

106

Bohr--Sommerfeld Lagrangians of moduli spaces of Higgs bundles

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.

Indranil Biswas; Niels Leth Gammelgaard; Marina Logares

2014-09-24

107

NSDL National Science Digital Library

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.

2012-07-20

108

Exciton dispersion from first principles

NASA Astrophysics Data System (ADS)

We present a scheme to calculate exciton dispersions in real materials that is based on the first-principles many-body Bethe-Salpeter equation. We assess its high level of accuracy by comparing our results for LiF with recent inelastic x-ray scattering experimental data on a wide range of energy and momentum transfer. We show its great analysis power by investigating the role of the different electron-hole interactions that determine the exciton band structure and the peculiar “exciton revival” at large momentum transfer. Our calculations for solid argon are a prediction and a suggestion for future experiments. These results demonstrate that the first-principles Bethe-Salpeter equation is able to describe the dispersion of localized and delocalized excitons on equal footing and represent a key step for the ab initio study of the exciton mobility.

Gatti, Matteo; Sottile, Francesco

2013-10-01

109

Bohr Hamiltonian with Davidson potential for triaxial nuclei

A solution of the Bohr Hamiltonian appropriate for triaxial shapes, involving a Davidson potential in beta and a steep harmonic oscillator in gamma, centered around gamma=30 degrees, is developed. Analytical expressions for spectra and B(E2) transition rates ranging from a triaxial vibrator to the rigid triaxial rotator are obtained and compared to experiment. Using a variational procedure it is pointed out that the Z(5) solution, in which an infinite square well potential in beta is used, corresponds to the critical point of the shape phase transition from a triaxial vibrator to the rigid triaxial rotator.

I. Yigitoglu; Dennis Bonatsos

2010-12-16

110

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

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.

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

2014-10-17

111

Molecular Basis of the Bohr Effect in Arthropod Hemocyanin

Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, kon, rather than changes in koff. In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s ? 4pz transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs.

Hirota, S.; Kawahara, T; Beltramini, M; Di Muro, P; Magliozzo, R; Peisach, J; Powers, L; Tanaka, N; Nagao, S; Bubacco, L

2008-01-01

112

Momentum dependence of the excitons in pentacene.

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. PMID:22667582

Roth, Friedrich; Schuster, Roman; König, Andreas; Knupfer, Martin; Berger, Helmuth

2012-05-28

113

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

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

Eric R. Scerri

1994-01-01

114

Confrontations between the interacting boson approximation and the Bohr-Mottelson model

In the past year several papers were published discussing, from very different points of view, the relation between the interacting boson approximation and the Bohr-Mottelson model. We show that, under certain assumptions about the measure in the Hilbert space, these approaches are equivalent. NUCLEAR STRUCTURE Nuclear collective models, interacting boson approximation, Bohr-Mottelson model.

O. Castaños; A. Frank; P. Hess; M. Moshinsky

1981-01-01

115

A rigorous proof of the Bohr-van Leeuwen theorem in the semiclassical limit.

A rigorous proof of the Bohr-van Leeuwen theorem in the semiclassical limit. March 11, 2014 Baptiste Savoie Abstract The original formulation of the Bohr-van Leeuwen (BvL) theorem states that Drude-Lorentz model vanishes identically. This stems from classical statistics which assign

Paris-Sud XI, UniversitÃ© de

116

Anatomy of a BathtubVortex A. Andersen,1,2,* T. Bohr,1

Anatomy of a BathtubVortex A. Andersen,1,2,* T. Bohr,1 B. Stenum,2 J. Juul Rasmussen,2 and B National Laboratory, Optics and Fluid Dynamics Department, DK-4000 Roskilde, Denmark 3 The Niels Bohr. This flow structure leads us to a theoretical model similar to one obtained earlier by Lundgren [J. Fluid

Lautrup, Benny

117

The General Principle of Relativity Applied to the Rutherford-Bohr Atom-Model

Application of the general principle of relativity to the Rutherford-Bohr atom.-In this atom the possible electron orbits are determined according to mechanical principles, then the actual orbits are selected by the quantum conditions. In computing the possible orbits, Bohr assumed classical mechanics, Sommerfeld applied the special theory of relativity with marked success, and now his assumptions are further generalized. In

Th. Wereide

1923-01-01

118

The Bohr-Wheeler spontaneous fission limit: an undergraduate-level derivation

NASA Astrophysics Data System (ADS)

An upper-undergraduate level derivation of the Z2/A ~ 50 limit against spontaneous fission first published by Bohr and Wheeler (1939 Phys. Rev. 56 426) is provided. The purpose in offering this derivation is that most textbooks give no details of it and sometimes fail to make clear key assumptions and mathematical manipulations underlying the Bohr and Wheeler model.

Reed, B. Cameron

2009-07-01

119

I develop a new analysis of Niels Bohr's Copenhagen interpretation of quantum mechanics by examining the development of his views from his earlier use of the correspondence principle in the so-called 'old quantum theory' to his articulation of the idea of complementarity in the context of the novel mathematical formalism of quantum mechanics. I argue that Bohr was motivated not

Scott Daniel Tanona

2002-01-01

120

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

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature. PMID:25171389

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

2014-10-01

121

Constitution and model. The quantum theory of Bohr and imagining the atom

The quantum theory of Bohr has roots in the theories of Rutherford and J. J. Thomson on the one hand, and that of Nicholson on the other. We note that Bohr neither presented the theories of Rutherford and Thomson faithfully, nor did he refer to the theory of Nicholson in its own terms. The contrasting attitudes towards these antecedent theories is telling and reveals the philosophical disposition of Bohr. We argue that Bohr intentionally avoided the concept of model as inappropriate for describing his proposed theory. Bohr had no problem in referring to the works of others as 'models', thus separating his theory from previous theories. He was interested in uncovering 'a little piece of reality'.

Hon, Giora

2014-01-01

122

Singlet exciton fission in solution

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...

Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.

2013-11-17

123

Transient excitons at metal surfaces

NASA Astrophysics Data System (ADS)

Excitons, electron-hole pairs bound by the Coulomb potential, are the fundamental quasiparticles of coherent light-matter interaction relevant for processes such as photosynthesis and optoelectronics. The existence of excitons in semiconductors is well established. For metals, however, although implied by the quantum theory of the optical response, experimental manifestations of excitons are tenuous owing to screening of the Coulomb interaction taking place on timescales of a few femtoseconds. Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full onset of screening of the Coulomb interaction, in the course of a three-photon photoemission process with ~15 fs laser pulses. During this transient regime, electron-hole pair Coulomb interactions introduce coherent quasiparticle correlations beyond the single-particle description of the optics of metals that dominate the multi-photon photoemission process on the timescale of screening at a Ag(111) surface.

Cui, Xuefeng; Wang, Cong; Argondizzo, Adam; Garrett-Roe, Sean; Gumhalter, Branko; Petek, Hrvoje

2014-07-01

124

Oral History Interviews at the Niels Bohr Library & Archives

NSDL National Science Digital Library

Housed at the American Institute of Physics, the Niels Bohr Library & Archives is a very useful place for historians of science and others with an interest in the lives and personalities of modern scientists. Here, visitors will find their rather unique collection of oral history interviews. They happen to have more than a thousand, and this site contains transcriptions of several hundred of these interviews, along with a selection of audio clips. The subjects covered in these interviews are rather diverse, and they include conversations about laser science, science education, solid state physics, and science institutions. Visitors can click on to the voice clips to hear such notable personages as Felix Bock, H.R. Crane, and Werner Heisenberg talk about their work. Moving on, visitors can click on the "Read Oral History Transcripts Online" section to browse an alphabetical list of scientists. Finally, visitors can click on the feedback form to ask questions of staff members at the Library.

125

Exciton dynamicsstudied via internal THz transitions

We employ a novel, ultrafast terahertz probe to investigatethe dynamical interplay of optically-induced excitons and unboundelectron-hole pairs in GaAs quantum wells. Resonant creation ofheavy-hole excitons induces a new low-energy oscillator linked totransitions between the internal exciton degrees of freedom. The timeresolved terahertz optical conductivity is found to be a probe wellsuited for studies of fundamental processes such as formation, relaxationand ionization of excitons.

Kaindl, R.A.; Hagele, D.; Carnahan, M.A.; Lovenich, R.; Chemla,D.S.

2003-02-26

126

Excitonic absorption in cerium phosphate glasses

In a number of ionic crystals an absorption peak or peaks just beyond the sharp absorption edge and due to excitons is frequently observed. The shape of the curve of exciton absorption has been well accounted for by Toyozawa [1]. Mort and Davis [2] have discussed the possibility of the existence of excitons in glassy materials. Originally it was suggested

Ravishankar Harani; C. A. Hogarth

1989-01-01

127

Exciton coupling in molecular crystals

NASA Technical Reports Server (NTRS)

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.

Ake, R. L.

1976-01-01

128

Memories of Crisis: Bohr, Kuhn, and the Quantum Mechanical ``Revolution''

NASA Astrophysics Data System (ADS)

``The history of science, to my knowledge,'' wrote Thomas Kuhn, describing the years just prior to the development of matrix and wave mechanics, ``offers no equally clear, detailed, and cogent example of the creative functions of normal science and crisis.'' By 1924, most quantum theorists shared a sense that there was much wrong with all extant atomic models. Yet not all shared equally in the sense that the failure was either terribly surprising or particularly demoralizing. Not all agreed, that is, that a crisis for Bohr-like models was a crisis for quantum theory. This paper attempts to answer four questions: two about history, two about memory. First, which sub-groups of the quantum theoretical community saw themselves and their field in a state of crisis in the early 1920s? Second, why did they do so, and how was a sense of crisis related to their theoretical practices in physics? Third, do we regard the years before 1925 as a crisis because they were followed by the quantum mechanical revolution? And fourth, to reverse the last question, were we to call into the question the existence of a crisis (for some at least) does that make a subsequent revolution less revolutionary?

Seth, Suman

2013-04-01

129

Some Conspicuous Successes of the Bohr Atom and a Serious Difficulty

Some consequences to the Bohr theory of extending the formulas for x-ray doublets to optical spectra.-The extension of the laws for x-ray doublets to ultra-violet spectra has given values of the screening constant in agreement with the Moseley-Bohr formula. This extension means a considerable simplification but it implies the identification of the LI, LII, LIII x-ray levels with the s,

R. A. Millikan; I. S. Bowen

1924-01-01

130

Oxygen affinity and Bohr effect responses to 2,3-diphosphoglycerate in equine and human blood

The dependence of blood oxygen affinity and the Bohr effect on the concentration of 2,3-diphosphoglycerate (DPG) in erythrocytes was investigated in 24 trotter horses and 24 healthy men. The oxygen tension at half saturation and standard conditions (P50st at pH 7·4 PCO240 mniHg and 37°C) and the carbon dioxide or fixed-acid-induced Bohr effect (dlogP50\\/dpH) were determined. Samples of fresh blood

G Di Bella; G Scandariato; O Suriano; A Rizzo

1996-01-01

131

PAN ET AL. VOL. 7 ' NO. 11 ' 1015810166 ' 2013 www.acsnano.org

the semiconductor's large Bohr exciton radius enables quantum- size-effect tuning over the broad solar spec- trum. We use a kinetic model to explain and optimize the nucleation and growth processes in the reactor

132

Subdiffusive exciton transport in quantum dot solids.

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications. PMID:24807586

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

2014-06-11

133

Gravitational excitons from extra dimensions

We study inhomogeneous multidimensional cosmological models with a higher dimensional space-time manifold under dimensional reduction and show that small inhomogeneous excitations of the scale factors of the internal spaces near minima of effective potentials should be observable as massive scalar particles (gravitational excitons) in the external space-time.

U. Guenther; A. Zhuk

1997-10-17

134

Stirring potential for indirect excitons

NASA Astrophysics Data System (ADS)

We demonstrate experimental proof of principle for a stirring potential for indirect excitons. The azimuthal wavelength of this stirring potential is set by the electrode periodicity, the amplitude is controlled by the applied AC voltage, and the angular velocity is controlled by the AC frequency.

Hasling, M. W.; Kuznetsova, Y. Y.; Andreakou, P.; Leonard, J. R.; Calman, E. V.; Dorow, C. J.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2015-01-01

135

Berry phase effect on the exciton transport and on the exciton Bose-Einstein condensate.

With the exciton lifetime much extended in semiconductor quantum-well structures, the exciton transport and Bose-Einstein condensation have become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict a spin-dependent transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and instability in an exciton condensate in favor of vortex formation. PMID:18851231

Yao, Wang; Niu, Qian

2008-09-01

136

We investigate the excitonic optical properties in thin quantum boxes in the intermediate regime between the two-dimensional (2D) and zero-dimensional (0D) with a theoretical analysis that rigorously treats excitonic confinement effects. It is found that the exciton binding energy is substantially enhanced and that the oscillator strength concentrates to the lowest excitonic transition, even in a thin box whose lateral

Hideki Gotoh; Hiroaki Ando

1997-01-01

137

Exciton Seebeck effect in molecular systems.

We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect. PMID:25106568

Yan, Yun-An; Cai, Shaohong

2014-08-01

138

Bohr's Electron was Problematic for Einstein: String Theory Solved the Problem

NASA Astrophysics Data System (ADS)

Neils Bohr's 1913 model of the hydrogen electron was problematic for Albert Einstein. Bohr's electron rotates with positive kinetic energies +K but has addition negative potential energies - 2K. The total net energy is thus always negative with value - K. Einstein's special relativity requires energies to be positive. There's a Bohr negative energy conflict with Einstein's positive energy requirement. The two men debated the problem. Both would have preferred a different electron model having only positive energies. Bohr and Einstein couldn't find such a model. But Murray Gell-Mann did! In the 1960's, Gell-Mann introduced his loop-shaped string-like electron. Now, analysis with string theory shows that the hydrogen electron is a loop of string-like material with a length equal to the circumference of the circular orbit it occupies. It rotates like a lariat around its centered proton. This loop-shape has no negative potential energies: only positive +K relativistic kinetic energies. Waves induced on loop-shaped electrons propagate their energy at a speed matching the tangential speed of rotation. With matching wave speed and only positive kinetic energies, this loop-shaped electron model is uniquely suited to be governed by the Einstein relativistic equation for total mass-energy. Its calculated photon emissions are all in excellent agreement with experimental data and, of course, in agreement with those -K calculations by Neils Bohr 100 years ago. Problem solved!

Webb, William

2013-04-01

139

On Solar Radius Variation with Magnetic Field

In response to the claim by Dziembowski et al. (2001) that the solar radius decreases with magnetic activity at the rate of 1.5 km/yr, we consider the theoretical question whether a radius variation is expected with the solar cycle. If the radius variation is caused by the magnetic pressure of toroidal flux tubes at the bottom of the convection zone, then the dynamo model of Nandy and Choudhuri (2002) would suggest a radius decrease with magnetic activity, in contrast to other dynamo models which would suggest a radius increase. However, the radius decrease is estimated to be only of the order of hundreds of metres.

Arnab Rai Choudhuri; Piyali Chatterjee

2003-11-03

140

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

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

M. Chabr; D. F. Williams

1977-01-01

141

Excitonic dephasing in semimagnetic semiconductors

NASA Astrophysics Data System (ADS)

The presence of magnetic ions in a dilute semiconductor quantum well is shown to decrease dramatically the low-temperature excitonic dephasing time measured by transient four-wave mixing. The decrease is associated with the magnetic nature of the material. Surprisingly, the application of a magnetic field does not significantly change the dephasing time, which indicates that there must exist a fast dephasing mechanism in addition to the spin-spin interactions responsible for magnetic polaron formation.

Cundiff, S. T.; Hellmann, R.; Koch, M.; Mackh, G.; Waag, A.; Landwehr, G.; Knox, W. H.; Göbel, E. O.

1996-06-01

142

Spontaneous coherence in a cold exciton gas.

If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose-Einstein condensation. Spontaneous coherence is the key characteristic of condensation in momentum space. Excitons--bound pairs of electrons and holes--form a model system to explore the quantum physics of cold bosons in solids. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin. PMID:22437498

High, A A; Leonard, J R; Hammack, A T; Fogler, M M; Butov, L V; Kavokin, A V; Campman, K L; Gossard, A C

2012-03-29

143

Extraordinary exciton conductance induced by strong coupling

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.

Johannes Feist; Francisco J. Garcia-Vidal

2014-09-08

144

Cross-polarized excitons in carbon nanotubes

Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293

Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K.; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R.

2008-01-01

145

Exciton liquid in coupled quantum wells.

Excitons in semiconductors may form correlated phases at low temperatures. We report the observation of an exciton liquid in gallium arsenide/aluminum gallium arsenide-coupled quantum wells. Above a critical density and below a critical temperature, the photogenerated electrons and holes separate into two phases: an electron-hole plasma and an exciton liquid, with a clear sharp boundary between them. The two phases are characterized by distinct photoluminescence spectra and by different electrical conductance. The liquid phase is formed by the repulsive interaction between the dipolar excitons and exhibits a short-range order, which is manifested in the photoluminescence line shape. PMID:24385625

Stern, Michael; Umansky, Vladimir; Bar-Joseph, Israel

2014-01-01

146

Photoexcitation of the triplet exciton in single wall carbon nanotubes

The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), ...

Santos, Tiffany S.

147

Proton radius from Bayesian inference

NASA Astrophysics Data System (ADS)

The methods of Bayesian statistics are used to extract the value of the proton radius from the elastic e p 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 (parametrization |data ) instead of using the minimal error criterion. As a result the most probable proton radii values (rEp=0.899 ±0.003 fm, rMp=0.879 ±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 leads to smaller differences between the extracted values of rEp and rMp. The results disagree with recent muonic atom measurements.

Graczyk, Krzysztof M.; Juszczak, Cezary

2014-11-01

148

Exciton circular dichroism in channelrhodopsin.

Channelrhodopsins (ChRs) are of great interest currently because of their important applications in optogenetics, the photostimulation of neurons. The absorption and circular dichroism (CD) spectra of C1C2, a chimera of ChR1 and ChR2 of Chlamydomonas reinhardtii, have been studied experimentally and theoretically. The visible absorption spectrum of C1C2 shows vibronic fine structure in the 470 nm band, consistent with the relatively nonpolar binding site. The CD spectrum has a negative band at 492 nm (??(max) = -6.17 M(-1) cm(-1)) and a positive band at 434 nm (??(max) = +6.65 M(-1) cm(-1)), indicating exciton coupling within the C1C2 dimer. Time-dependent density functional theory (TDDFT) calculations are reported for three models of the C1C2 chromophore: (1) the isolated protonated retinal Schiff base (retPSB); (2) an ion pair, including the retPSB chromophore, two carboxylate side chains (Asp 292, Glu 162), modeled by acetate, and a water molecule; and (3) a hybrid quantum mechanical/molecular mechanical (QM/MM) model depicting the binding pocket, in which the QM part consists of the same ion pair as that in (2) and the MM part consists of the protein residues surrounding the ion pair within 10 Å. For each of these models, the CD of both the monomer and the dimer was calculated with TDDFT. For the dimer, DeVoe polarizability theory and exciton calculations were also performed. The exciton calculations were supplemented by calculations of the coupling of the retinal transition with aromatic and peptide group transitions. For the dimer, all three methods and three models give a long-wavelength C2-axis-polarized band, negative in CD, and a short-wavelength band polarized perpendicular to the C2 axis with positive CD, differing in wavelength by 1-5 nm. Only the retPSB model gives an exciton couplet that agrees qualitatively with experiment. The other two models give a predominantly or solely positive band. We further analyze an N-terminal truncated mutant because it was assumed that the N-terminal domain has a crucial role in the dimerization of ChRs. However, the CD spectrum of this mutant has an exciton couplet comparable to that of the wild-type, demonstrating that it is dimeric. Patch-clamp experiments suggest that the N-terminal domain is involved in protein stabilization and channel kinetics rather than dimerization or channel activity. PMID:25247388

Pescitelli, Gennaro; Kato, Hideaki E; Oishi, Satomi; Ito, Jumpei; Maturana, Andrés Daniel; Nureki, Osamu; Woody, Robert W

2014-10-16

149

Generation of Quasiclassical Bohr-Like Wave Packets Using Half-Cycle Pulses

We demonstrate the experimental realization of Bohr-like atoms by applying a pulsed unidirectional field, termed a half-cycle pulse (HCP), to atoms in quasi-two-dimensional near-circular states. This leads to creation of localized wave packets that travel in near-circular orbits and mimic the dynamics of an electron in the original Bohr model of the hydrogen atom. This motion can be followed for several orbital periods before the localization of the wave packet is lost due to dephasing. We show, however, that localization can be recovered by application of further HCPs.

Mestayer, J. J. [Rice University; Wyker, B. [Rice University; Dunning, F. B. [Rice University; Reinhold, Carlos O [ORNL; Yoshida, S. [Institute for Theoretical Physics, Vienna University of Technology, Austria; Burgdorfer, J. [Institute for Theoretical Physics, Vienna University of Technology, Austria

2008-08-01

150

Dual effect of 2,3-diphosphoglycerate on the Bohr effects of human blood

The influence of the red cell concentration of 2,3-diphosphoglycerate (2,3-DPG, 0.5–26 µmoles\\/g erythrocytes) on the “CO2-Bohr effect” (pH varied by CO2 at constant base excess) and the “fixed acid-Bohr effect” (pH varied by fixed acid or base at constantPCO2) was studied in human blood at plasma pH values ranging between pH 7.2 and pH 7.6.

Jochen Duhm

1976-01-01

151

Josephson effects in condensates of excitons and exciton polaritons

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.

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

152

Excitonic Polarons in Semiconductor Quantum Dots

The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas

O. Verzelen; R. Ferreira; G. Bastard

2002-01-01

153

Four-Particle Anyon Exciton:. Boson Approximation

A theory of anyon excitons consisting of a valence hole and three quasielectrons with electric charges -e\\/3 is presented. A full symmetry classification of the k = 0 states is given, where k is the exciton momentum. The energy levels of these states are expressed by quadratures of confluent hypergeometric functions. It is shown that the angular momentum L of

M. E. Portnoi; E. I. Rashba

1995-01-01

154

Diffusion of Singlet Excitons in Tetracene Crystals

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

G. Vaubel; H. Baessler

1970-01-01

155

Exciton-polaritons condensate in a microwire

NASA Astrophysics Data System (ADS)

Recently, polariton condensation has been claimed in microwires. Numerical solutions of the time-dependent Gross-Pitaevskii equation that describes the behavior of the condensate in a trap and exciton-polariton interaction, have been obtained. In this work we study theoretically exciton-polariton one dimensional condensation in several quantized states.

Kamoun, O.; Jaziri, S.

2013-12-01

156

Excitons and fundamental absorption in quantum wells

We present an accurate theory for the excitonic absorption in quantum-well structures which yields very good agreement with a wide range of experimental data. Our approach is based on an expansion of the exciton wave function in terms of multicomponent envelope functions of electron and hole states. It is rather general with respect to band-structure effects and potential profiles. In

R. Winkler

1995-01-01

157

The structure and dynamics of molecular excitons.

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

Bardeen, Christopher J

2014-01-01

158

The Structure and Dynamics of Molecular Excitons

NASA Astrophysics Data System (ADS)

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.

Bardeen, Christopher J.

2014-04-01

159

Excitonic phase diagram in unstressed Ge

NASA Astrophysics Data System (ADS)

Evidence is presented for a second condensed phase of excitons in unstressed, high-purity Ge, based on a simple plasma line-shape analysis. Above the critical temperature for electron-hole liquid condensation (Tc~=7 K), time-resolved photoluminescence spectra can be fit with an electron-hole plasma component with nondecaying density, implying spatial condensation into droplets. Shortly after the excitation source is switched off, all successive luminescence spectra are fit with a constant-density electron-hole plasma line shape plus contributions from free excitons and excitonic complexes. The spectral analysis is based on separate characterizations of electron-hole plasma and excitonic complexes from published experimental and theoretical work. The presence of a second condensed phase of excitons in unstressed Ge would be consistent with recent findings of a similar nature in stressed germanium and unstressed silicon. A possible phase diagram is suggested based on the present findings.

Simon, A. H.; Kirch, S. J.; Wolfe, J. P.

1992-10-01

160

EPR before EPR: A 1930 Einstein-Bohr thought Experiment Revisited

ERIC Educational Resources Information Center

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…

Nikolic, Hrvoje

2012-01-01

161

Derivation of microscopic uni-axial unified adiabatic Bohr–Mottelson rotational model

For a rotational motion about a single axis, we derive a microscopic, quantum version of the phenomenological Bohr–Mottelson unified adiabatic rotational model without using redundant coordinates, imposing constraints on the intrinsic state or the particle coordinates, or assuming explicitly a deformed intrinsic state. The model Schrödinger equation is derived from the direct action of the multi-particle Hamiltonian operator on the

P. Gulshani

2010-01-01

162

Exact and analytic solutions of the generalized bohr-mottelson model for spherical nuclei

The generalized Bohr-Mottelson model (GBMM), which encompasses isoscalar and isovector collective vibrations, is solved exactly and analytically in the harmonic approximation for proton and neutron vibrations, coupled via the symmetry energy. To this end we utilize the underlying group structure (which is Sp(4,R) in this case) to construct a unitary transformation that diagonalizes the hamiltonian. Closed expressions for the eigenvalues

F. G. Scholtz; G. Kyrchev; Amand Faessler

1989-01-01

163

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

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

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

2008-01-01

164

Spreading the Gospel: A Popular Book on the Bohr Atom in its Historical Context

The emergence of quantum theory in the early decades of the twentieth century was accompanied by a wide range of popular science books, all of which presented in words, and a few in images, new scientific ideas about the structure of the atom. The work of physicists such as Ernest Rutherford and Niels Bohr, among others, was pivotal to the

Helge Kragh; Kristian Hvidtfelt Nielsen

2012-01-01

165

A Possible Reconciliation of the Atomic Models of Bohr and of Lewis and Langmuir

BROADLY speaking, the merits of Bohr's atomic model lie in its very accurate explanation of the reaction of atoms and molecules with radiation, while those of the Lewis-Langmuir model lie in its very satisfactory representation of the mechanism of chemical combination, but the merits of either model are lacking in the other. Both must therefore possess properties which are accurate

W. Hughes

1922-01-01

166

Bohr-Neugebauer type theorem for some partial neutral functional differential

Bohr-Neugebauer type theorem for some partial neutral functional differential equations Mostafa atomic at zero, which means that there exists a continuous nondecreasing function : [0, r] [0-difference equations defined on the unit circle S, which is a model for a continuous circular array of resistively

Boyer, Edmond

167

This article is basically targeted for the advanced university and college undergraduate and graduate students of physics and chemistry education, computational physics and chemistry, and computer science. Here, we employed Microsoft Excel software system to perform computer simulations for modeling Bohr's theory of hydrogen atom in college and university classroom setting. We developed necessary computer algorithm to compute discrete values

Gurmukh Singh; Amitabha Mukhopadyay

2010-01-01

168

Why We Should Teach the Bohr Model and How to Teach it Effectively

ERIC Educational Resources Information Center

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…

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

2008-01-01

169

What Can the Bohr-Sommerfeld Model Show Students of Chemistry in the 21st Century?

ERIC Educational Resources Information Center

Bohr's model of the atom is considered to be important by general chemistry textbooks. A shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. To increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study aims to elaborate a framework…

Niaz, Mansoor; Cardellini, Liberato

2011-01-01

170

Poole-Frenkel ionization of excitons in solid Kr

During vacuum-UV excitation of the excitons in solid Kr photoconductivity was observed. For applied voltages below 50 V the photocurrent spectrum reflected the number of excitons produced in the bulk, whereas at higher voltages the contribution of excitons in a thin surface layer, including longitudinal and surface excitons, dominated the spectrum. These findings can be explained under the assumption of

J. Kraft; A. Schrimpf; B. Schweitzer; K. Ibbeken; H.-J. Stöckmann

1997-01-01

171

High efficiency organic multilayer photodetectors based on singlet exciton fission

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

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

2009-01-01

172

The effect of the initial exciton numbers on {sup 54,56}Fe(p, xp) Pre-Equilibrium Reactions

In pre-equilibrium nuclear reactions, the geometry-dependent hybrid model is applied with the use of the neutron and proton densities to investigate the effect of initial exciton numbers on the nucleon emission spectra. The initial exciton numbers calculated with the theoretical neutron and proton densities have been obtained within the Skryme-Hartree-Fock method with SKM* and SLy4 forces on target nuclei in the {sup 54,56}Fe(p, xp) reaction at 61.5-MeV incident proton energy by using a new calculationmethod of Tel et al. Also, the differences between the initial exciton numbers for protons and neutrons as a function of nuclear radius, focusing on systematic discrepancies correlated to differences in the proton and neutron densities have been investigated.

Boeluekdemir, M. H., E-mail: bolukdemir@gmail.com [Gazi University, Faculty of Arts and Sciences, Department of Physics (Turkey); Tel, E. [Osmaniye Korkut Ata University, Faculty of Arts and Sciences, Department of Physics (Turkey); Ayd Latin-Small-Letter-Dotless-I n, A. [Kirikkale University, Faculty of Arts and Sciences, Department of Physics (Turkey); Okuducu, S. [Gazi University, Faculty of Arts and Sciences, Department of Physics (Turkey); Kaplan, A. [Sueleyman Demirel University, Faculty of Arts and Sciences, Department of Physics (Turkey)

2011-02-15

173

Excitonic Aharonov-Bohm effect in a two-dimensional quantum ring

We study theoretically the optical properties of an exciton in a two-dimensional ring threaded by a magnetic flux. We model the quantum ring by a confining potential that can be continuously tuned from strictly one-dimensional to truly two-dimensional with finite radius-to-width ratio. We present an analytic solution of the problem when the electron-hole interaction is short ranged. The oscillatory dependence of the oscillator strength as a function of the magnetic flux is attributed to the Aharonov-Bohm effect. The amplitude of the oscillations changes upon increasing the width of the quantum ring. We find that the Aharonov-Bohm oscillations of the ground state of the exciton decrease with increasing the width, but, remarkably, the amplitude remains finite down to radius-to-width ratios less than unity. We attribute this resilience of the excitonic oscillations to the nonsimple connectedness of our chosen confinement potential with its centrifugal core at the origin.

Gonzalez-Santander, C.; Dominguez-Adame, F. [Departamento de Fisica de Materiales, Universidad Complutense de Madrid, ES-28040 (Spain); Roemer, R. A. [Department of Physics and Centre for Scientific Computing, University of Warwick, Coventry, CV4 7AL (United Kingdom)

2011-12-15

174

Thermalization of free excitons in ZnSe quantum wells

The thermalization dynamics of hot free excitons in ZnSe quantum wells is studied by photoluminescence excitation (PLE), steady-state (cw-PL) and time-resolved photoluminescence (TRPL). Formation of excitons by emission of LO phonons is very efficient and leads to the generation of narrow hot-exciton distributions. The LO phonon-assisted recombination of the excitons is a direct monitor of the hot-exciton dynamics. A thermalized

H. Kalt; M. Umlauff; J. Hoffmann; W. Langbein; J. M. Hvam; M. Scholl; J. Söllner; M. Heuken; B. Jobst; D. Hommel

1998-01-01

175

Exciton localization and drift in tailored-potential quantum nanowires

Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm{sup 2}/(eVs).

Szeszko, J., E-mail: justyna.szeszko@epfl.ch; Rudra, A.; Kapon, E. [Laboratory of Physics of Nanostructures, École Polytechnique Fédérale de Lausanne, 1015 Lausanne (Switzerland); Belykh, V. V.; Sibeldin, N. N. [P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow 119991 (Russian Federation)

2014-06-30

176

Localization of excitons by molecular layer formation in a polymer film

NASA Astrophysics Data System (ADS)

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 ˜2Rg to ˜12Rg where Rg is the unperturbed radius of gyration of the polymer. uv extinction due to the pure electronic singlet A1g1?E1u1 is seen to increase with d-1 for 4Rg?d?12Rg (region 1). This suggests excitonic interaction along d . The variation of total exciton energy (E) of the A1g?E1u 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 Å) Rgexciton is also determined. For Rg**0 at d?4Rg , where the exciton is still delocalized. Layering reduces the Hamaker constant (AH) , deciding the cohesive force, between the layers and this reduction, ?AH , is found to be less than Ae at d?4Rg , where iAe/? is the amplitude for exciton transfer between neighboring molecules in the excitonic lattice of region 1. On the other hand, ?AH in region 2 starts from a value larger than Ae . This indicates that ?AH acts as a barrier between the layer, which localizes the exciton within the layers.**

Chattopadhyay, S.; Datta, A.

2005-10-01

177

NASA Technical Reports Server (NTRS)

The solar radius at 35 GHz has been determined from solar radio maps made with a pencil beam antenna of half-power beam width 2.8 arcmin at the La Posta Astrogeophysical Observatory during 1973 and 1974. The 35 GHz radius was found to be 2.57% plus or minus 0.88% larger than the photospheric radius. The sensitivity of the result to the method of determination is discussed.

Wefer, F. L.; Bleiweiss, M. P.

1976-01-01

178

Two Exciton States in Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

In a semiconductor quantum dot (QD), the conventional biexciton is formed by the coulomb attraction between two excitons. The interaction of two excitons can also create two-exciton states in a QD that in higher dimensional systems are unbound and part of the continuum. However, the confinement potential of the QD binds these excitons into an interaction that creates a biexciton whose energy is greater than the non-interacting two-exciton energy. We report here a nonlinear spectral signature consistent with the creation of an interdot ``anti-bound'' biexciton in GaAs interface fluctuation QDs obtained by high resolution nonlinear spectroscopy. These measurements show evidence of a coupled excitonic system distinct from both the conventional and the intradot ``anti-bound'' biexcitons [1,2] but similar to two interacting separately localized excitons. This work was supported by the ARO, AFOSR and ONR. [1] Y.Z. Hu, et al. Phys. Rev. Lett. 64, 1805 (1990). [2] H. Kamada, et al. Phys. Rev. B 58, 16243 (1998).

Batteh, E. T.; Chen, Gang; Cheng, Jun; Steel, D. G.; Gammon, D.; Katzer, D. S.; Park, D.; Chen, Pochung; Piermarocchi, C.; Sham, L. J.

2002-03-01

179

Optical nutation in the exciton range of spectrum

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

Khadzhi, P. I. [Academy of Sciences of Moldova, Institute of Applied Physics (Moldova, Republic of)] [Academy of Sciences of Moldova, Institute of Applied Physics (Moldova, Republic of); Vasiliev, V. V., E-mail: vasscorp@mail.ru [Taras Shevchenko Transnistria State University (Moldova, Republic of)

2013-08-15

180

Switching Exciton Pulses Through Conical Intersections

NASA Astrophysics Data System (ADS)

Exciton pulses transport excitation and entanglement adiabatically through Rydberg aggregates, assemblies of highly excited light atoms, which are set into directed motion by resonant dipole-dipole interaction. Here, we demonstrate the coherent splitting of such pulses as well as the spatial segregation of electronic excitation and atomic motion. Both mechanisms exploit local nonadiabatic effects at a conical intersection, turning them from a decoherence source into an asset. The intersection provides a sensitive knob controlling the propagation direction and coherence properties of exciton pulses. The fundamental ideas discussed here have general implications for excitons on a dynamic network.

Leonhardt, K.; Wüster, S.; Rost, J. M.

2014-11-01

181

Spontaneous Exciton Dissociation in Carbon Nanotubes

NASA Astrophysics Data System (ADS)

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.

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

2014-03-01

182

NASA Astrophysics Data System (ADS)

We studied multiexciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.

Mouri, Shinichiro; Miyauchi, Yuhei; Toh, Minglin; Zhao, Weijie; Eda, Goki; Matsuda, Kazunari

2014-10-01

183

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

NASA Astrophysics Data System (ADS)

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

Seward, Kenton; Lin, Zhibin; Lusk, Mark

2012-02-01

184

Solar Radius at Minimum of Cycle 23

NASA Astrophysics Data System (ADS)

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.

Sigismondi, Costantino

2008-09-01

185

Orientable Strong Radius and Diameter of Hypercube

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

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

186

Multiple Exciton Generation Solar Cells

Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.

Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.

2012-01-01

187

Gravitational Excitons as Dark Matter

In earlier work it was pointed out that for warped product spacetimes the conformal (geometrical moduli) excitations of the internal compactified factor spaces should be observable as massive scalar fields in the external spacetime. Here we show that these scalar fields (gravitational excitons) describe weakly interacting particles and can be considered as dark matter component. Masses of the gravexcitons are defined by the form of the effective potential of the theory and the stabilization scales of the internal space. This implies that different stabilization scales result in different types of DM. An essential role is played by the effective potential. On the one hand, its minima fix possible stabilization scales of the internal spaces; on the other hand, they provide possible values for the effective cosmological constant.

U. Guenther; A. Zhuk

2000-11-01

188

Singlet exciton fission in solution.

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

Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H

2013-12-01

189

Singlet exciton fission in solution

NASA Astrophysics Data System (ADS)

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.

Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.

2013-12-01

190

Giant Rydberg Excitons in Cuprous Oxide

Highly excited atoms with an electron moved into a level with large principal quantum number are fascinating hydrogen-like objects. The giant extension of these Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided novel insights into molecular and condensed matter physics problems on a single quantum level. Excitons, the fundamental optical excitations in semiconductors consisting of a negatively charged electron and a positively charged hole, are the condensed matter analogues of hydrogen. Highly excited excitons with extensions similar to Rydberg atoms are attractive because they may be placed and moved in a crystal with high precision using microscopic potential landscapes. Their interaction may allow formation of ordered exciton phases or sensing of elementary excitations in the surrounding, also on a quantum level. Here we demonstrate the existence of Rydberg excitons in cuprous oxide, Cu2O, with principal quantum numbers as large as n=25 . These states have giant w...

Kazimierczuk, Tomasz; Scheel, Stefan; Stolz, Heinrich; Bayer, Manfred

2014-01-01

191

Exciton spin noise in quantum wells

NASA Astrophysics Data System (ADS)

A theory of spin fluctuations of excitons in quantum wells in the presence of nonresonant excitation has been developed. Both bright and dark excitonic states have been taken into account. The effect of a magnetic field applied in a quantum-well plane has been analyzed in detail. We demonstrate that in relatively small fields, the spin-noise spectrum consists of a single peak centered at a zero frequency, while an increase of magnetic field results in the formation of the second peak in the spectrum, owing to an interplay of the Larmor effect of the magnetic field and the exchange interaction between electrons and holes forming excitons. Experimental possibilities to observe the exciton spin noise are discussed, particularly by means of ultrafast spin-noise spectroscopy. We show that the fluctuation spectra contain, in addition to individual contributions of electrons and holes, information about the correlation of their spins.

Smirnov, D. S.; Glazov, M. M.

2014-08-01

192

Triplet excitons: Bringing dark states to light

NASA Astrophysics Data System (ADS)

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.

Bardeen, Christopher J.

2014-11-01

193

Topologically protected excitons in porphyrin thin films

The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyze a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogenous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.

Joel Yuen-Zhou; Semion S. Saikin; Norman Y. Yao; Alán Aspuru-Guzik

2014-06-05

194

Exciton properties of selected aromatic hydrocarbon systems

NASA Astrophysics Data System (ADS)

We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.

Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin

2013-02-01

195

NASA Astrophysics Data System (ADS)

Recent developments and applications of an algebraic version of Bohr's collective model, known as the algebraic collective model (ACM), have shown that fully converged calculations can be performed for a large range of Hamiltonians. Examining the algebraic structure underlying the Bohr model (BM) has also clarified its relationship with the interacting boson model (IBM), with which it has related solvable limits and corresponding dynamical symmetries. In particular, the algebraic structure of the IBM is obtained as a compactification of the BM and conversely the BM is regained in various contraction limits of the IBM. In a previous paper, corresponding contractions were identified and confirmed numerically for axially-symmetric states of relatively small deformation. In this paper, we extend the comparisons to realistic deformations and compare results of the two models in the rotor-vibrator limit. These models describe rotations and vibrations about an axially symmetric prolate or oblate rotor, and rotations and vibrations of a triaxial rotor. It is determined that most of the standard results of the BM can be obtained as contraction limits of the IBM in its U(5)-SO(6) dynamical symmetries.

Thiamova, G.; Rowe, D. J.; Caprio, M. A.

2012-12-01

196

Conceptual objections to the Bohr atomic theory — do electrons have a "free will" ?

NASA Astrophysics Data System (ADS)

The atomic model introduced by Bohr in 1913 dominated the development of the old quantum theory. Its main features, such as the radiationless stationary states and the discontinuous quantum jumps between the states, were hard to swallow for contemporary physicists. While acknowledging the empirical power of the theory, many scientists criticized its foundation or looked for ways to reconcile it with classical physics. Among the chief critics were A. Crehore, J.J. Thomson, E. Gehrcke and J. Stark. This paper examines from a historical perspective the conceptual objections to Bohr's atom, in particular the stationary states (where electrodynamics was annulled by fiat) and the mysterious, apparently teleological quantum jumps. Although few of the critics played a constructive role in the development of the old quantum theory, a history neglecting their presence would be incomplete and distorted.

Kragh, Helge

2011-11-01

197

A Bohr-type model with gravity as the attractive force

We formulate a Bohr-type rotating particle model for three light particles of the same rest mass, forming a bound rotational state under the influence of their gravitational attraction, in the same way that electrostatic attraction leads to the formation of a bound proton-electron state in the classical Bohr model of the H atom. By using special relativity, the equivalence principle and the de Broglie wavelength equation, we find that when the three rotating particles have the rest masses of neutrinos or antineutrinos then surprisingly the rest mass of the rotating state has the rest mass of the stable baryons, i.e. of the proton and the neutron. This rest mass is due almost exclusively to the kinetic energy of the rotating neutrinos. The results are found to be consistent with the theory of general relativity. Predictions for the properties of these bound rotational states are compared with experimental values.

C. G. Vayenas; S. Souentie; A. Fokas

2014-12-23

198

Molecular excitons in a copper azadipyrrin complex.

Exciton coupling is investigated in a copper azadipyrrin complex, Cu(L-aza)2. Exciton coupling in Cu(L-aza)2 assuming a single ?-?* state on the L-aza ligand fails to account for the electronic structure of Cu(L-aza)2, which displays two almost equal intensity transitions at 15?600 cm(-1) and 17?690 cm(-1). TD-UB3LYP/6-31G(d) calculations suggest multiple ?-?* transitions for the L-aza ligands and simple vector addition of the transition dipoles predicts two nearly orthogonal co-planar excitonic transitions that correctly reproduce the absorption band profile. Empirical modelling of absolute resonance Raman intensities using wavepacket dynamics confirms Cu(L-aza)2 has two equal intensity orthogonal exciton transitions. The phenyl substituents at the ?- and ?-positions of the pyrrole rings play a central role in determining the orientation of the transition dipoles. Consequently the ?-?* transitions for the L-aza ligands are oriented towards the substituent groups and are not in the plane of the pyrrole rings. Mode displacements in the Franck-Condon (FC) region obtained from the wavepacket model suggest that pyrrole ring and phenyl modes control the exciton FC dynamics. Our results suggest that Cu(L-aza)2 is an ideal model for theoretical, computational and experimental investigations of molecular excitons in molecular systems. PMID:25025874

McLean, T M; Telfer, S G; Elliott, A B S; Gordon, K C; Lein, M; Waterland, M R

2014-12-21

199

Unexpectedly slow two particle decay of ultra-dense excitons in cuprous oxide

NASA Astrophysics Data System (ADS)

For an ultra-dense exciton gas in cuprous oxide (Cu2O), exciton-exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

Laszlo Frazer, N.; Schaller, Richard D.; Ketterson, J. B.

2013-09-01

200

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

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

Bianchi, Eugenio; Haggard, Hal M

2011-07-01

201

Electric quadrupole transitions of the Bohr Hamiltonian with the Morse potential

Eigenfunctions of the collective Bohr Hamiltonian with the Morse potential have been obtained by using the asymptotic iteration method (AIM) for both {gamma}-unstable and rotational structures. B(E2) transition rates have been calculated and compared to experimental data. Overall good agreement is obtained for transitions within the ground-state band, while some interband transitions appear to be systematically underpredicted in {gamma}-unstable nuclei and overpredicted in rotational nuclei.

Inci, I. [I.N.F.N. and Dipartimento di Fisica ''Galileo Galilei,'' Universita di Padova, I-35131 Padova (Italy); Department of Physics, Akdeniz University, TR-07058 Antalya (Turkey); Bonatsos, D. [Institute of Nuclear Physics, National Centre for Scientific Research ''Demokritos,'' GR-153 10 Aghia Paraskevi, Attiki (Greece); Boztosun, I. [Department of Physics, Akdeniz University, TR-07058 Antalya (Turkey)

2011-08-15

202

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

NSDL National Science Digital Library

Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits studentsâ 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 SchrÃ¶dinger 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 SchrÃ¶dinger 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 SchrÃ¶dinger 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 SchrÃ¶dingerâs 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.

Mckagan, Sam B.; Perkins, Katherine K.; Wieman, Carl E.

2008-09-24

203

Angular Distribution of Fission Fragments at Threshold According to the Bohr Model

The sideways-peaked angular distribution of fragments from neutron-induced fission of Th232 near threshold, measured by Brolley and Henkel, is found to be consistent with the model of A. Bohr. It is assumed that the saddle-point states through which the fissioning nucleus passes include primarily only a few members of a particular rotational band. Indications are adduced that this band has

Lawrence Wilets; David M. Chase

1956-01-01

204

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

Some education researchers have claimed that we should not teach the Bohr\\u000amodel of the atom because it inhibits students' ability to learn the true\\u000aquantum nature of electrons in atoms. Although the evidence for this claim is\\u000aweak, many have accepted it. This claim has implications for how to present\\u000aatoms in classes ranging from elementary school to graduate

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

2008-01-01

205

How Sommerfeld extended Bohr's model of the atom (1913-1916)

NASA Astrophysics Data System (ADS)

Sommerfeld's extension of Bohr's atomic model was motivated by the quest for a theory of the Zeeman and Stark effects. The crucial idea was that a spectral line is made up of coinciding frequencies which are decomposed in an applied field. In October 1914 Johannes Stark had published the results of his experimental investigation on the splitting of spectral lines in hydrogen (Balmer lines) in electric fields, which showed that the frequency of each Balmer line becomes decomposed into a multiplet of frequencies. The number of lines in such a decomposition grows with the index of the line in the Balmer series. Sommerfeld concluded from this observation that the quantization in Bohr's model had to be altered in order to allow for such decompositions. He outlined this idea in a lecture in winter 1914/15, but did not publish it. The First World War further delayed its elaboration. When Bohr published new results in autumn 1915, Sommerfeld finally developed his theory in a provisional form in two memoirs which he presented in December 1915 and January 1916 to the Bavarian Academy of Science. In July 1916 he published the refined version in the Annalen der Physik. The focus here is on the preliminary Academy memoirs whose rudimentary form is better suited for a historical approach to Sommerfeld's atomic theory than the finished Annalen-paper. This introductory essay reconstructs the historical context (mainly based on Sommerfeld's correspondence). It will become clear that the extension of Bohr's model did not emerge in a singular stroke of genius but resulted from an evolving process.

Eckert, Michael

2014-04-01

206

Statistical complexity, Fisher-Shannon information, and Bohr orbits in the H-atom

The Fisher-Shannon information and a statistical measure of complexity are calculated in the position and momentum spaces for the wave functions of the H-atom. For each level of energy, it is found that these two indicators take their minimum values on the orbitals that correspond to the classical (circular) orbits in the Bohr atomic model, just those with the highest orbital angular momentum.

Sanudo, Jaime

2008-01-01

207

Statistical complexity, Fisher-Shannon information, and Bohr orbits in the H-atom

The Fisher-Shannon information and a statistical measure of complexity are\\u000acalculated in the position and momentum spaces for the wave functions of the\\u000aH-atom. For each level of energy, it is found that these two indicators take\\u000atheir minimum values on the orbitals that correspond to the classical\\u000a(circular) orbits in the Bohr atomic model, just those with the highest

Jaime Sanudo; Ricardo Lopez-Ruiz

2008-01-01

208

Model of molecular bonding based on the Bohr-Sommerfeld picture of atoms

We develop a model of molecular binding based on the Bohr-Sommerfeld description of atoms together with a constraint taken from conventional quantum mechanics. The model can describe the binding energy curves of H2, H3 and other molecules with striking accuracy. Our approach treats electrons as point particles with positions determined by extrema of an algebraic energy function. Our constrained model provides a physically appealing, accurate description of multi-electron chemical bonds.

Svidzinsky, A A; Scully, M O; Svidzinsky, Anatoly A.; Chin, Siu A.; Scully, Marlan O.

2006-01-01

209

High-resolution proton nuclear magnetic resonance spectroscopy has been used to investigate the molecular mechanism of the Bohr effect of human normal adult hemoglobin in the presence of two allosteric effectors, i.e., chloride and inorganic phosphate ions. The individual hydrogen ion equilibria of 22-26 histidyl residues of hemoglobin have been measured in anion-free 0.1 M HEPES buffer and in the presence of 0.18 M chloride or 0.1 M inorganic phosphate ions in both deoxy and carbonmonoxy forms. The results indicate that the {beta}2-histidyl residues are strong binding sites for chloride and inorganic phosphate ions in hemoglobin. The affinity of the {beta}2-histidyl residues for these anions is larger in the deoxy than in the carbonmonoxy form. Nevertheless, the contribution of these histidyl residues to the anion Bohr effect is small due to their low pK value in deoxyhemoglobin in anion-free solvents. The interactions of chloride and inorganic phosphate ions with the hemoglobin molecule also result in lower pK values and/or changes in the shapes of the hydrogen ion binding curves for several other surface histidyl residues. These results suggest that long-range electrostatic interactions between individual ionizable sites in hemoglobin could play an important role in the molecular mechanism of the anion Bohr effect.

Russu, I.M.; Wu, Shingshing; Ho, N.T.; Kellogg, G.W.; Ho, Chien (Carnegie Mellon Univ., Pittsburgh, PA (USA))

1989-06-13

210

NASA Astrophysics Data System (ADS)

The triaxial shape in the vicinity of ? = ?/6 in nuclei has been studied making use of a potential with the exactly separable form u( ?) + v( ?)/ ? 2 in the Bohr collective Hamiltonian. The Davidson potential for u( ?) and a harmonic potential centered at ? = ?/6 for v( ?) are assumed to solve the Bohr Hamiltonian. We present analytic expressions for the energy spectra and the B( E2) transition rates, and compare theoretical results with available experimental data for 192Pt.

Lee, J. H.

2013-11-01

211

Exciton-Exciton Interactions in Tensile-Strained GaN

NASA Astrophysics Data System (ADS)

We report on the four-wave mixing (FWM) spectroscopy of excitons in tensile-strained GaN films. Three excitonic resonances (A-, B-, and C-excitons) were clearly observed with energy separations of ?AB˜4.7 meV and ?BC˜14.5 meV. Owing to tensile strain, these values are much smaller than those of free-standing GaN, allowing the simultaneous excitations of A-B and B-C excitons pairs. The time evolutions of the FWM in different polarization configurations resolved the differences in the simultaneous excitonic transitions; the excitation of A-B excitons showed an oscillation with a critical ?-phase shift in the orthogonal polarization configuration as expected in the quantum beat, and the oscillation of B-C excitons pair is independent of the polarization configuration, indicating polarization interference. The polarization-dependent FWM spectra of A-B excitons revealed peaks associated with biexciton transitions with binding energies of EAA = 4.6 meV, EAB = 2.1 meV, and EBB = 1.7 meV.

Toda, Yasunori; Shigematsu, Kyohhei; Hasegawa, Akira; Adachi, Satoru

2013-08-01

212

Exciton correlations and input-output relations in non-equilibrium exciton superfluids

The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron-hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg-Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input-output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton-polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron-electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor {nu}{sub T}=1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: Black-Right-Pointing-Pointer Establish the relations between photoluminescence and transport measurements. Black-Right-Pointing-Pointer Stress the violations of the fluctuation-dissipation theorem in non-equilibrium systems. Black-Right-Pointing-Pointer Explore the input-output relations between excitons and photons. Black-Right-Pointing-Pointer Discuss differences between non-equilibrium systems and dissipative ones.

Ye, Jinwu, E-mail: jy306@ccs.msstate.edu [Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Department of Physics, Capital Normal University, Beijing 100048 (China) [Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Department of Physics, Capital Normal University, Beijing 100048 (China); Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States); Sun, Fadi; Yu, Yi-Xiang [Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States) [Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States); Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China); Liu, Wuming [Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China)] [Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China)

2013-02-15

213

Microscopic theories of excitons and their dynamics

NASA Astrophysics Data System (ADS)

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.

Berkelbach, Timothy C.

214

The energetics and dynamics of multiply excited states in single material colloidal quantum dots have already been shown to exhibit universal trends. Here we attempt to identify similar trends in exciton-exciton interactions within compound colloidal quantum dots. For this end, we thoroughly review previously available data and also present experimental data on several newly synthesized systems, focusing on core/shell nanocrystals with a type-II band alignment. A universal condition for the transition from binding to repulsion of the biexciton (type-I-type-II transition) is established in terms of the change in the exciton radiative lifetime. A scaling rule is also presented for the magnitude of exciton-exciton repulsion. In contrast, we do not identify a clear universal scaling of the non-radiative Auger recombination lifetime of the biexciton state. Finally, a perspective on future applications of engineered multiexcitonic states is presented. PMID:21229166

Deutsch, Zvicka; Avidan, Assaf; Pinkas, Iddo; Oron, Dan

2011-02-28

215

Laser differential confocal paraboloidal vertex radius measurement.

This Letter proposes a laser differential confocal paraboloidal vertex radius measurement (DCPRM) method for the high-accuracy measurement of the paraboloidal vertex radius of curvature. DCPRM constructs an autocollimation vertex radius measurement light path for the paraboloid by placing a reflector in the incidence light path. This technique is based on the principle that a paraboloid can aim a parallel beam at its focus without aberration and uses differential confocal positioning technology to identify the paraboloid focus and vertex accurately. Measurement of the precise distance between these two positions is achieved to determine the paraboloid vertex radius. Preliminary experimental results indicate that DCPRM has a relative expanded uncertainty of less than 0.001%. PMID:24562218

Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Shen, Yang; Jiang, Hongwei

2014-02-15

216

Hot exciton transport in ZnSe quantum wells

The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Froehlich coupling, the initial kinetic energy of the excitons is well...

Zhao, Hui; Moehl, Sebastian; Wachter, Sven; Kalt, Heinz

2002-02-01

217

Investigating exciton correlations using coherent multidimensional optical spectroscopy

The optical measurements described in this thesis reveal interactions among bound electron-hole pairs known as excitons in a semiconductor nanostructure. Excitons are quasiparticles that can form when light is absorbed by ...

Turner, Daniel Burton

2010-01-01

218

Exciton annihilation studies in poly(p-phenylene vinylene)

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.

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

219

Photoexcitation and Exciton Transport in Molecular Crystals

NASA Astrophysics Data System (ADS)

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.

Irkhin, Pavel

220

Exciton redshift for coherent pumping near the absorption edge

The excitons are studied for a direct-gap semiconductor irradiated with a strong monochromatic laser beam near the exciton resonance. Starting from the Bethe-Salpeter equation in the screened-ladder approximation for the system polarization function, we study the pump-field influence on the excitons in the collision-free limit. Far from the absorption edge the results for the excitonic optical Stark effect are confirmed.

S. Glutsch; F. Bechstedt

1991-01-01

221

Biexciton versus Exciton Lifetime in a Single Semiconductor Quantum Dot

The decay characteristics of excitons and biexcitons in one single semiconductor quantum dot (QD) are directly monitored using time- and spatially resolved photoluminescence spectroscopy. The experiments are performed on a CdSe\\/ZnSe QD, occupied by either one or two excitons at a time, allowing a direct comparison between the radiative lifetime of a biexciton and an exciton confined in the same

G. Bacher; R. Weigand; J. Seufert; V. D. Kulakovskii; N. A. Gippius; A. Forchel; K. Leonardi; D. Hommel

1999-01-01

222

BoseEinstein condensation of excitons in bilayer electron systems

BoseÂEinstein condensation of excitons in bilayer electron systems J. P. Eisenstein1 * & A. H. Mac'. An ordered electronic state in which excitons condense into a single quantum state was proposed clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime

Eisenstein, Jim

223

Singlet exciton fission in pure and doped anthracene

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

K. von Burg

1979-01-01

224

Singlet exciton fission in pure and doped anthracene

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

K. von Burg; I. Zschokke-Gränacher

1979-01-01

225

Triplet exciton formation due to interaction between singlet excitons in polydiacetylene

The time-resolved spectroscopy in polydiacetylene single crystals (PDA-MADF {poly-1-[3-(methylamino)phenyl]-4-[3,5-bis(trifluoromethyl)phenyl]-1, 3-butadiyne}) on femtosecond and picosecond time scales was performed. The time dependence of the photoinduced reflectance change in PDA-MADF exhibits three components with lifetimes ?200 fs, 1–2 ps, and ≫200 ps. These components are assigned to the free 1Bu excitons, self-trapped excitons, and triplet excitons. From the excitation intensity dependence of

K. Ichimura; M. Yoshizawa; H. Matsuda; S. Okada; M. M. Ohsugi; H. Nakanishi; T. Kobayashi

1993-01-01

226

NASA Astrophysics Data System (ADS)

Using the derived expression for the light yield in a scintillator, the influence of linear radiative and non-radiative (quenching) rates on the nonproportionality in light yield is studied. It is found that if the excitation created within the electron track initiated by a ?-photon incident on a scintillator remains mainly excitonic, then nonproportionality can be minimised by inventing a scintillator material with linear radiative rate >107 s-1, linear quenching rate <106 s-1 and track radius ?70 nm along with maintaining the rates of other nonlinear processes as discovered earlier. If one can increase the linear radiative rate to 109 s-1, then the nonproportionality can be eliminated at a track radius >20 nm.

Singh, Jai; Koblov, Alexander

2013-02-01

227

Four-Particle Anyon Exciton:. Boson Approximation

NASA Astrophysics Data System (ADS)

A theory of anyon excitons consisting of a valence hole and three quasielectrons with electric charges -e/3 is presented. A full symmetry classification of the k = 0 states is given, where k is the exciton momentum. The energy levels of these states are expressed by quadratures of confluent hypergeometric functions. It is shown that the angular momentum L of the exciton ground state depends on the distance between the electron and hole confinement planes and takes the values L = 3n, where n is an integer. With increasing k the electron density shows a spectacular splitting on bundles. At first a single anyon splits off of the two-anyon core, and finally all anyons become separated.

Portnoi, M. E.; Rashba, E. I.

228

Förster coupling in nanoparticle excitonic circuits.

Exciton transport in semiconductor nanoparticles underlies recent experiments on electrically controlled nanostructures and proposals for new artificial light-harvesting systems. In this work, we develop a novel method for the numerical evaluation of the Forster matrix element, based on a three-dimensional real space grid and the self-consistent solution of the mesoscopic exciton in a macroscopic dielectric environment. This method enables the study of the role of the nanoparticle shape, spatially varying dielectric environments, and externally applied electric fields. Depending on the orientation of the transition dipole, the Forster coupling is shown to be either increased or decreased as a function of the nanoparticle shape and of the properties of the dielectric environment. In the presence of an electric field, we investigate the relation between excitonic binding and confinement effects. We also study a type II core-shell quantum dot where electron and hole are spatially separated due to a particular configuration of the bandstructure. PMID:20698598

Rebentrost, Patrick; Stopa, Michael; Aspuru-Guzik, Alán

2010-08-11

229

Topological Excitonic Superfluids in Three Dimensions

NASA Astrophysics Data System (ADS)

We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current.

Gilbert, Matthew; Hankiewicz, Ewelina; Kim, Youngseok

2013-03-01

230

The monomeric haemoglobin IV from Chironomus thummi thummi (CTT IV) exhibits an alkaline Bohr-effect and therefore it is an allosteric protein. By substitution of the haem iron for cobalt the O2 half-saturation pressure, measured at 25‡ C, increases 250-fold. The Bohr-effect is not affected by the replacement of the central atom. The parameters of the Bohr-effect of cobalt CTT IV

K. Gersonde; H. Twilfer; M. Overkamp

1982-01-01

231

Holographic radius test plates for spherical surfaces with large radius of curvature.

We describe a novel interferometric method, based on nested Fresnel zone lenses or photon sieves, for testing and measuring the radius of curvature of precision spherical surfaces that have radii in a range between several meters and a few hundred meters. We illustrate the measurement concept with radius measurements of a spherical mirror with a radius of about 10 m. The measured radius is 9877??mm±10??mm for a coverage factor k=2. Our measurements also demonstrate, for the first time to the best of our knowledge, the utility of photon sieves for precision surface metrology because they diffuse higher diffraction orders of computer generated holograms, which reduces coherent noise. PMID:25090075

Wang, Quandou; Griesmann, Ulf; Soons, Johannes A

2014-07-10

232

Bright and dark excitons in semiconductor carbon nanotubes

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.

Tretiak, Sergei [Los Alamos National Laboratory

2008-01-01

233

Triplet exciton dynamics in rubrene single crystals

NASA Astrophysics Data System (ADS)

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.

Ryasnyanskiy, Aleksandr; Biaggio, Ivan

2011-11-01

234

Mass and radius of cosmic balloons

NASA Technical Reports Server (NTRS)

Cosmic balloons are spherical domain walls with relativistic particles trapped inside. We derive the exact mass and radius relations for a static cosmic balloon using Gauss-Codazzi equations. The cosmic balloon mass as a function of its radius, M(R), is found to have a functional form similar to that of fermion soliton stars, with a fixed point at 2GM(R)/R approximately or equal to 0.486 which corresponds to the limit of infinite central density. We derive a simple analytical approximation for the mass density of a spherically symmetric relativistic gas star. When applied to the computation of the mass and radius of a cosmic balloon, the analytical approximation yields fairly good agreement with the exact numerical solutions.

Wang, Yun

1994-01-01

235

Mass and radius of cosmic balloons

Cosmic balloons are spherical domain walls with relativistic particles trapped inside. We derive the exact mass and radius relations for a static cosmic balloon using Gauss-Codazzi equations. The cosmic balloon mass as a function of its radius, [ital M]([ital R]), is found to have a functional form similar to that of fermion soliton stars, with a fixed point at 2[ital GM]([ital R])/[ital R][congruent]0.486, which corresponds to the limit of infinite central density. We derive a simple analytical approximation for the mass density of a spherically symmetric relativistic gas star. When applied to the computation of the mass and radius of a cosmic balloon, the analytical approximation yields fairly good agreement with the exact numerical solutions.

Wang, Y. (NASA/Fermilab Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, Illinois 60510-0500 (United States))

1994-05-15

236

The Two Electron Molecular Bond Revisited: From Bohr Orbits to Two-Center Orbitals

In this review we first discuss extension of Bohr's 1913 molecular model and\\u000ashow that it corresponds to the large-D limit of a dimensional scaling\\u000a(D-scaling) analysis, as developed by Herschbach and coworkers.\\u000a In a separate but synergetic approach to the two-electron problem, we\\u000asummarize recent advances in constructing analytical models for describing the\\u000atwo-electron bond. The emphasis here is

Goong Chen; Siu A. Chin; Yusheng Dou; Kishore T. Kapale; Moochan Kim; Anatoly A. Svidzinsky; Kerim Urtekin; Han Xiong; Marlan O. Scully

2005-01-01

237

Exactly separable Bohr Hamiltonian with the Morse potential for triaxial nuclei

NASA Astrophysics Data System (ADS)

In this paper, the Morse potential is used in the ?-part of the collective Bohr Hamiltonian for triaxial nuclei. Energy eigenvalues and eigenfunctions are obtained in a closed form through exactly separating the Hamiltonian into its variables by using an appropriate form of the potential. The results are applied to generate the nuclear spectrum of 192Pt, 194Pt and 196Pt isotopes which are known to be the best candidate exhibiting triaxiality. Electric quadrupole transition ratios are calculated and then compared with the experimental data and the Z(5) model results.

Inci, I.

2014-08-01

238

Parameter-Free Solution of the Bohr Hamiltonian for Actinides Critical in the Octupole Mode

An analytic, parameter-free (up to overall scale factors) solution of the Bohr Hamiltonian involving axially symmetric quadrupole and octupole deformations, as well as an infinite well potential, is obtained, after separating variables in a way reminiscent of the Variable Moment of Inertia (VMI) concept. Normalized spectra and B(EL) ratios are found to agree with experimental data for 226-Ra and 226-Th, the nuclei known to lie closest to the border between octupole deformation and octupole vibrations in the light actinide region.

D. Lenis; Dennis Bonatsos

2005-12-06

239

High efficiency organic multilayer photodetectors based on singlet exciton fission

NASA Astrophysics Data System (ADS)

We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.

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

2009-07-01

240

Exciton-plasmaritons in graphene/semiconductor structures

NASA Astrophysics Data System (ADS)

We study strong coupling between plasmons in monolayer charge-doped graphene and excitons in a narrow gap semiconductor quantum well separated from graphene by a potential barrier. We show that the Coulomb interaction between excitons and plasmons results in mixed states described by a Hamiltonian similar to that for exciton-polaritons and derive the exciton-plasmon coupling constant that depends on system parameters. We calculate numerically the Rabi splitting of exciton-plasmariton dispersion branches for several semiconductor materials and find that it can reach values of up to 50-100 meV.

Velizhanin, Kirill A.; Shahbazyan, Tigran V.

2014-08-01

241

Strong exciton-plasmon coupling in graphene-semiconductor structures

NASA Astrophysics Data System (ADS)

We study strong coupling between plasmons in monolayer charge-doped graphene and excitons in a narrow gap semiconductor quantum well separated from graphene by a potential barrier. We show that the Coulomb interaction between excitons and plasmons result in mixed states described by a Hamiltonian similar to that for exciton-polaritons and derive the exciton-plasmon coupling constant that depends on system parameters. We calculate numerically the Rabi splitting of exciton-plasmariton dispersion branches for several semiconductor materials and find that it can reach values of up to 50 - 100 meV.

Velizhanin, Kirill A.; Shahbazyan, Tigran V.

2014-09-01

242

Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.

Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs. PMID:20735067

Rao, Akshay; Wilson, Mark W B; Hodgkiss, Justin M; Albert-Seifried, Sebastian; Bässler, Heinz; Friend, Richard H

2010-09-15

243

21 CFR 886.1450 - Corneal radius measuring device.

Code of Federal Regulations, 2010 CFR

...2010-04-01 2010-04-01 false Corneal radius measuring device. 886.1450... Diagnostic Devices § 886.1450 Corneal radius measuring device. (a) Identification. A corneal radius measuring device is an...

2010-04-01

244

Discrete hopping model of exciton transport in disordered media

NASA Astrophysics Data System (ADS)

A model of dispersive exciton transport has been developed for a medium with exciton energy levels randomly distributed in both space and energy scale. For a boxcar density of states of excitons an analytical solution is given describing the exciton density as a function of time and the proximity to the exciton quenching interfaces. The model parameters, such as exciton lifetime, effective number of exciton energy levels within inhomogeneously broadened density of states, and hopping distance and hopping rate constants, could be determined using time-resolved photoluminescence data. The developed model is verified via comparison with experimental data on the time-dependent photoluminescence decay of the conjugated polymer MEH-PPV and on the spectrum of internal quantum efficiency of the heterojunction photovoltaic device based on the MEH-PPV/TiO2 nanostructure.

Burlakov, V. M.; Kawata, K.; Assender, H. E.; Briggs, G. A. D.; Ruseckas, A.; Samuel, I. D. W.

2005-08-01

245

Dynamics of the excitonic coupling in organic crystals.

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

Aragó, Juan; Troisi, Alessandro

2015-01-16

246

Dynamics of the Excitonic Coupling in Organic Crystals

NASA Astrophysics Data System (ADS)

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.

Aragó, Juan; Troisi, Alessandro

2015-01-01

247

Ultrafast dynamics of exciton fission in polycrystalline pentacene.

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ?80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. PMID:21755937

Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H

2011-08-10

248

Resonant exciton trapping at impurity states in silver bromide

NASA Astrophysics Data System (ADS)

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(?), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd 2+ and Ca 2+, respectively.

Sliwczuk, U.; Nakamura, K.; von der Osten, W.

1983-03-01

249

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

Filippo Troiani; Ulrich Hohenester; Elisa Molinari

2000-01-01

250

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

Filippo Troiani; Ulrich Hohenester; Elisa Molinari

2000-05-17

251

Singlet exciton fission in a hexacene derivative.

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

Lee, Jiye; Bruzek, Matthew J; Thompson, Nicholas J; Sfeir, Matthew Y; Anthony, John E; Baldo, Marc A

2013-03-13

252

Excitonic electroabsorption in type II superlattices

Electric field dependence of the exciton absorption in the ZnSe0.35Te0.65-ZnTe type II superlattice has been studied theoretically. Large effective blue shift of the absorption edge and strong modulation of the absorption prove that type II structures can be used in visible spatial light modulators and switches.

Shaozhong Li; Jacob B. Khurgin

1992-01-01

253

Exactly separable version of the Bohr Hamiltonian with the Davidson potential

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.

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

254

ZM theory III: Classical oscillators and semi-classical Bohr-Sommerfeld quantization

We consider the description of classical oscillatory motion in ZM theory, and explore the relationship of ZM theory to semi-classical Bohr-Sommerfeld quantization. The treatment illustrates some features of ZM theory, especially the inadequacies of classical and semi-classical treatments due to non-analyticity of the mapping of classical trajectories onto the ZM clock field. While the more complete ZM formalism is not developed here, the non-analyticities in the classical treatment resemble issues in the comparison of classical and quantum formalisms. We also show that semi-classical quantization is valid for a periodic manifold in ZM theory, though the quantum number $n=0$ is allowed, as it would be in quantum mechanics for a periodic manifold. Still, this suggests a connection to the first-order success of Bohr theory in describing the phenomenology of atomic quantum states. The approximate nature of the semi-classical treatment of three dimensional atomic orbits is, however, also apparent in relation to ZM theory. These observations are preliminary to a discussion of ZM theory in relation to quantum mechanics and quantum field theory in subsequent papers.

Yaneer Bar-Yam

2006-04-30

255

Exactly separable version of the Bohr Hamiltonian with the Davidson potential

An exactly separable version of the Bohr Hamiltonian is developed using a potential of the form u(beta)+u(gamma)/beta^2, with the Davidson potential u(beta)= beta^2 + beta_0^4/beta^2 (where beta_0 is the position of the minimum) and a stiff harmonic oscillator for u(gamma) centered at gamma=0. In the resulting solution, called exactly separable Davidson (ES-D), the ground state band, gamma band and 0_2^+ band 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_0, gamma stiffness). Insights 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, are also obtained.

Dennis Bonatsos; E. A. McCutchan; N. Minkov; R. F. Casten; P. Yotov; D. Lenis; D. Petrellis; I. Yigitoglu

2008-07-28

256

NASA Astrophysics Data System (ADS)

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.

Xu, S. J.; Xiong, Shi-Jie; Shi, S. L.

2005-12-01

257

NASA Astrophysics Data System (ADS)

In this study, the exciton binding energies of AlN are calculated by the anisotropic effective mass theory, which considers the strong electron-hole exchange and Fröhlich-type exciton-phonon interactions. Our calculated results are in good agreement with recent experimental results, indicating that the electron-hole exchange and exciton-phonon interactions play essential roles in describing the excitonic structure of AlN. We estimate that the exciton binding energies of AlN are 53.7 and 67.3 meV, for the spin-singlet and the spin-triplet states, respectively. In addition, a universal correlation has been found in compound semiconductors including AlN, where the effect of the electron-hole exchange and Fröhlich-type exciton-phonon interactions almost compensates each other on the binding energy of a spin-singlet exciton.

Ishii, Ryota; Funato, Mitsuru; Kawakami, Yoichi

2014-09-01

258

Characterization of Macroscopic Ordering in Exciton Rings

NASA Astrophysics Data System (ADS)

Recently observed complex PL patterns in 2D QW structures exhibit the inner [1,3] and the outer [1-4] exciton rings, localized bright spots [1,3], and the macroscopically ordered exciton state (MOES) [1,3]. The latter appears at the outer ring via its fragmentation into a periodic array of aggregates. While the gross features have been explained within classical framework, attributing the inner rings to nonradiative exciton transport and cooling [1], and the outermost rings and the bright spots to macroscopic charge separation [3,4], the origin of the MOES remains unidentified [5]. Here, for the first time, we report experiments demonstrating the exciton energy modulation over the MOES as well as the phase diagram of MOES in exciton density and temperature coordinates. The experiments shed new light on the dynamical origin of MOES. Besides, we present the studies of dynamical processes within MOES including the observation of aggregate instabilities and bifurcations that point to the spontaneous character of the instability.[1] L.V. Butov, A.C. Gossard, D.S. Chemla, Nature 418, 751 (2002). [2] D. Snoke, S. Denev, Y. Liu, L. Pfeiffer, K. West, Nature 418, 754 (2002). [3] L.V. Butov, L.S. Levitov, A.V. Mintsev, B.D. Simons, A.C. Gossard, D.S. Chemla PRL 92, 117404 (2004). [4] R. Rapaport, G. Chen, D. Snoke, S.H. Simon, L. Pfeiffer, K. West, Y. Liu, S. Denev PRL 92, 117405 (2004). [5] L.S. Levitov, B.D. Simons, L.V. Butov, cond-mat/0403377.

Yang, Sen; Levitov, L. S.; Simons, B. D.; Gossard, A. C.

2005-03-01

259

Relaxation of excitons in semimagnetic asymmetric double quantum wells

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.

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

260

Rotational Frenkel excitons in optical lattices with polar molecules

NASA Astrophysics Data System (ADS)

Ultracold polar molecules trapped in an optical lattice may form crystal-like structures with unique properties. Here, I will discuss a Mott insulator phase of ultracold molecules with one molecule per lattice site under conditions that can be realized in ongoing experiments with optical lattices. I will show that dipole-dipole interactions between molecules in different lattice sites give rise to collective excitations, such as Frenkel excitons, characteristic of solid-state molecular crystals. Due to the perturbative nature of the intermolecular interactions, the collective excitations in this system can be controlled by an external electric field. This can be used to realize Frenkel excitons in the presence of dynamically tunable disorder or an ensemble of Frenkel excitons with tunable exciton - exciton interactions. An external electric field can thus be used to induce strong localization or delocalization of Frenkel excitons as well as bi-exciton annihilation. The latter can be used to produce dark exciton states and entangled exciton pairs. The complicated quantum statistics of excitons leads to kinematic interactions. Our results show that the kinematic interactions can be eliminated by varying an external electric field, effectively leading to a system of quantum quasi-particles with tunable quantum statistics. [4pt] [1] ``Tunable disorder in a crystal of cold polar molecules,'' Felipe Herrera, Marina Litinskaya, Roman V. Krems, Phys. Rev. A 82, 033428 (2010).

Litinskaya, Marina

2011-06-01

261

The Bohr Model of the Earth-Sun System* Data for the earth-sun system assuming a circular earth with the Bohr model for the earth-sun system. Is this a legitimate example of the correspondence principle? E G

Rioux, Frank

262

Solar Radius Variations Measured in Central Eclipses

NASA Astrophysics Data System (ADS)

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.

Sigismondi, Costantino

2008-01-01

263

The Epidemiology of Distal Radius Fractures

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

Nellans, Kate W.; Kowalski, Evan; Chung, Kevin C.

2012-01-01

264

Proton Radius Puzzle and Large Extra Dimensions

NASA Astrophysics Data System (ADS)

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 suggests a direction to test new physics at very small length scale.

Wang, Li-Bang; Ni, Wei-Tou

2013-06-01

265

Neutron charge radius and the Dirac equation

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 nonrelativistic 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)r2En, i.e., the root mean square radius

M. Bawin; S. A. Coon

1999-01-01

266

Circle Problems - Radius, Diameter, Circumference and Area

NSDL National Science Digital Library

This "fill-in-the-blank" worksheet allows students to generate radius, diameter, circumference, and area for circles given only one of those measurements. There are eight problems and students may check their problems at the end or get a hint when they are stuck. There is also linksin the instructions which review the formulae for the area and circumference of a circle that they can consult as they solve.

2006-01-01

267

Fractures of Distal Radius: An Overview

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.

Meena, Sanjay; Sharma, Pankaj; Sambharia, Abhishek Kumar; Dawar, Ashok

2014-01-01

268

Measurement of the Radius of Neutron Stars

NASA Astrophysics Data System (ADS)

A physical understanding of the behavior of cold ultra dense matter - at and above nuclear density - can only be achieved by the study of neutron stars. The recent 1.97 ± 0.04 M? measurement for PSR 1614-2230 suggests that strange quark matter and hyperons/kaons condensate equations of state (EoSs) are disfavored, in favor of hadronic EoSs. Over much of the neutron star mass-radius parameter space, the latter EoSs produce lines of nearly constant radii (within about 10%). We present a simultaneous spectral analysis of several globular cluster quiescent low-mass X-ray binaries where we require the radius to be the same among all neutron stars analyzed. Our (preliminary) results suggest a neutron star radius much smaller than previously reported, in the range 7.5-10 km (90% confidence). The Markov-Chain Monte-Carlo method and the Bayesian approach developed in this analysis permits including uncertainties in the distance, in the hydrogen column density, and possible contributions to the spectra due to unmodeled spectrally hard components.

Guillot, Sebastien; Rutledge, R. E.; Servillat, M.; Webb, N.

2013-01-01

269

Measurement of the Radius of Neutron Stars

NASA Astrophysics Data System (ADS)

A physical understanding of the behavior of cold ultra-dense matter -- at and above nuclear density -- can only be achieved by the study of neutron stars. The recent 1.97+/-0.04 Msun measurement for PSR 1614-2230 suggests that strange quark matter and hyperons/kaons condensate equations of state (EoSs) are disfavored, in favor of hadronic EoSs. Over much of the neutron star mass-radius parameter space, the latter EoSs produce lines of nearly constant radii (within about 10%). We present a simultaneous spectral analysis of several globular cluster quiescent low-mass x-ray binaries where we require the radius to be the same among all neutron stars analyzed. Our (preliminary) results suggest a neutron star radius much smaller than previously reported, in the range 7.5-10 km (90% confidence). The Markov-Chain Monte-Carlo method and the Bayesian approach developed in this analysis permits including uncertainties in the distance, in the hydrogen column density, and possible contributions to the spectra due to unmodelled spectrally hard components.

Guillot, Sebastien

2012-07-01

270

Excition states in semiconductor quantum dots in the modified effective mass approximation

A new modified effective mass approximation is suggested to describe the excitonic energy spectrum of quantum dots of radii a comparable to the exciton Bohr radius a{sub ex}{sup 0}. It is shown that, for quantum dots simulated by infinitely deep potential wells, the effective mass approximation is appropriate for describing excitons in quantum dots of radii a {approx} a{sub ex}{sup 0}, if the reduced effective mass of the excitons, {mu}, is considered as a function of the radius of the quantum dot a, {mu} = {mu}(a)

Pokutnyi, S. I. [National Academy of Sciences of Ukraine, Kurdyumov Institute of the Physics of Metals (Ukraine)], E-mail: Pokutnyi_Sergey@inbox.ru

2007-11-15

271

Ultrafast dynamics of excitons in tetracene single crystals

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.

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

272

Photocurrent measurement on donor bound excitons in Si

NASA Astrophysics Data System (ADS)

Donor bound excitons are formed when free excitons are captured by neutral donor impurities. Due to the spatial localization of exciton at the impurity site, the decay process of donor bound exciton state to neutral donor state features extremely narrow linewidth in energy. Utilizing this inherent feature, it would be feasible to identify nuclear spin states of the donor impurity resulting from the hyperfine interaction between phosphorus nucleus spin and electron spin. We study ensembles of phosphorus donor bound excitons in Si via photocurrent measurements at low temperatures since Auger non-radiative decay process is primarily dominant in an indirect band-gap semiconductor such as Si. We report electric and magnetic field effects on photocurrent signals of phosphorus donor bound excitons.

Kim, Na Young; Sleiter, Darin; Ladd, Thaddeus; Nozawa, Katsuya; Yamamoto, Yoshihisa

2009-03-01

273

Localized excitons mediate defect emission in ZnO powders

NASA Astrophysics Data System (ADS)

A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.

Foreman, J. V.; Simmons, J. G.; Baughman, W. E.; Liu, J.; Everitt, H. O.

2013-04-01

274

Singlet exciton fission in nanostructured organic solar cells.

Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale. PMID:21355536

Jadhav, Priya J; Mohanty, Aseema; Sussman, Jason; Lee, Jiye; Baldo, Marc A

2011-04-13

275

Ultrafast dynamics of excitons in tetracene single crystals.

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

Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich

2014-03-21

276

Singlet Exciton Fission in Nanostructured Organic Solar Cells

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.

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

2011-01-01

277

Ultrafast dynamics of excitons in tetracene single crystals

NASA Astrophysics Data System (ADS)

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.

Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich

2014-03-01

278

Excitonic ring formation in ultrapure bulk GaAs

NASA Astrophysics Data System (ADS)

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.

Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.

2014-11-01

279

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.

Neumann, M. D.; Cobet, C.; Esser, N. [Leibniz-Institut fuer Analytische Wissenschaften - ISAS - e.V., Albert-Einstein-Str. 9, 12489 Berlin (Germany); Laumer, B. [I. Physikalisches Institut, Justus-Liebig-Universitaet Giessen, Heinrich-Buff-Ring 16, 35392 Giessen (Germany); Walter Schottky Institut, Technische Universitaet Muenchen, Am Coulombwall 3, 85748 Garching (Germany); Wassner, T. A. [Walter Schottky Institut, Technische Universitaet Muenchen, Am Coulombwall 3, 85748 Garching (Germany); Eickhoff, M. [I. Physikalisches Institut, Justus-Liebig-Universitaet Giessen, Heinrich-Buff-Ring 16, 35392 Giessen (Germany); Feneberg, M.; Goldhahn, R. [Institut fuer Experimentelle Physik, Otto-von-Guericke-Universitaet Magdeburg, 39106 Magdeburg (Germany)

2011-07-01

280

Problem Set 1 (due 17th September) (1) A planet of radius Rp orbits a star of radius R in a circular orbit at radius a. If the orbit plane is randomly distributed relative to the observer's line of sight, show that the probability that transits will be observed is, ptrans = R + Rp a . If the orbital

Armitage, Phil

281

The problem of an apparent inconsistency between the fission rates derived on the basis of Bohr-Wheeler's transition-state method and Kramers' dynamical model of nuclear fission, first pointed out by Strutinsky in 1973, is revisited. The study is based on studying the features of individual trajectories on the fission path.

K. -H. Schmidt

2008-08-11

282

An intrinsic state for the interacting boson model and its relationship to the Bohr-Mottelson model

Ah&act: Within the interacting boson model, an intrinsic state is defined, in terms of quadrupole shape variabtes, from which ali eigenstates of the interacting boson model can be projected by appro- priately averaging over these shape variables and the orientations of the intrinsic state. With this intrinsic state, an energy surface and a Bohr hamiltonian can be derived for any

J. N. GINOCCHIO; M. W. KIRSON

1980-01-01

283

An intrinsic state for the interacting boson model and its relationship to the Bohr-Mottelson model

Within the interacting boson model, an intrinsic state is defined, in terms of quadrupole shape variables, from which all eigenstates of the interacting boson model can be projected by appropriately averaging over these shape variables and the orientations of the intrinsic state. With this intrinsic state, an energy surface and a Bohr hamiltonian can be derived for any boson hamiltonian

J. N. Ginocchio; M. W. Kirson

1980-01-01

284

ERIC Educational Resources Information Center

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…

Gjedde, Albert

2010-01-01

285

Bohr Hamiltonian with a deformation-dependent mass term for the Davidson potential

Analytical expressions for spectra and wave functions are derived for a Bohr Hamiltonian, describing the collective motion of deformed nuclei, in which the mass is allowed to depend on the nuclear deformation. Solutions are obtained for separable potentials consisting of a Davidson potential in the {beta} variable, in the cases of {gamma}-unstable nuclei, axially symmetric prolate deformed nuclei, and triaxial nuclei, implementing the usual approximations in each case. The solution, called the deformation-dependent mass (DDM) Davidson model, is achieved by using techniques of supersymmetric quantum mechanics (SUSYQM), involving a deformed shape invariance condition. Spectra and B(E2) transition rates are compared to experimental data. The dependence of the mass on the deformation, dictated by SUSYQM for the potential used, reduces the rate of increase of the moment of inertia with deformation, removing a main drawback of the model.

Bonatsos, Dennis; Georgoudis, P. E.; Lenis, D. [Institute of Nuclear Physics, National Centre for Scientific Research 'Demokritos', GR-15310 Aghia Paraskevi, Attiki (Greece); Minkov, N. [Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigrad Road, 1784 Sofia (Bulgaria); Quesne, C. [Physique Nucleaire Theorique et Physique Mathematique, Universite Libre de Bruxelles, Campus de la Plaine CP229, Boulevard du Triomphe, B-1050 Brussels (Belgium)

2011-04-15

286

Bohr Hamiltonian with a deformation-dependent mass term for the Kratzer potential

NASA Astrophysics Data System (ADS)

The deformation-dependent mass Kratzer model is constructed by considering the Kratzer potential in a Bohr Hamiltonian, in which the mass is allowed to depend on the nuclear deformation, and solving it by using techniques of supersymmetric quantum mechanics (SUSYQM), involving a deformed shape invariance condition. Analytical expressions for spectra and wave functions are derived for separable potentials in the cases of ?-unstable nuclei, axially symmetric prolate deformed nuclei, and triaxial nuclei, implementing the usual approximations in each case. Spectra and B(E2) transition rates are compared to experimental data. The dependence of the mass on the deformation, dictated by SUSYQM for the potential used, moderates the increase of the moment of inertia with deformation, removing a main drawback of the model.

Bonatsos, Dennis; Georgoudis, P. E.; Minkov, N.; Petrellis, D.; Quesne, C.

2013-09-01

287

The "Graviton Picture": a Bohr Model for Gravitation on Galactic Scales?

Modified Newtonian Dynamics (MOND) provides a successful description of stellar and galactic dynamics on almost all astronomical scales. A key feature of MOND is the transition function from Newtonian to modified dynamics which corresponds to the empirical mass discrepancy--acceleration (MDA) relation. However, the functional form of the MDA relation does not follow from theory in a straightforward manner; in general, empirical MDA relations are inserted ad hoc into analyses of stellar dynamics. I revisit the possibility of gravity being mediated by massive virtual particles, gravitons. Under certain reasonable assumptions, the resulting "graviton picture" implies a MDA relation that is equivalent to the - empirical - "simple mu" function of MOND which is in very good agreement with observations. I conclude that the "graviton picture" offers a simple description of gravitation on galactic scales, potentially playing a role for gravitation analogous to the role played by Bohr's model for atomic physics.

Sascha Trippe

2014-05-06

288

Bohr Hamiltonian with deformation-dependent mass term for the Davidson potential

Analytical expressions for spectra and wave functions are derived for a Bohr Hamiltonian, describing the collective motion of deformed nuclei, in which the mass is allowed to depend on the nuclear deformation. Solutions are obtained for separable potentials consisting of a Davidson potential in the beta variable, in the cases of gamma-unstable nuclei, axially symmetric prolate deformed nuclei, and triaxial nuclei, implementing the usual approximations in each case. The solution, called the Deformation Dependent Mass (DDM) Davidson model, is achieved by using techniques of supersymmetric quantum mechanics (SUSYQM), involving a deformed shape invariance condition. Spectra and B(E2) transition rates are compared to experimental data. The dependence of the mass on the deformation, dictated by SUSYQM for the potential used, reduces the rate of increase of the moment of inertia with deformation, removing a main drawback of the model.

Dennis Bonatsos; P. E. Georgoudis; D. Lenis; N. Minkov; C. Quesne

2011-03-30

289

Spin relaxation and spin-dependent exciton interactions in ZnSe quantum wells

We demonstrate the mutual influence of the dynamics in the real-space and the spin parts of the exciton wavefunction evoked by the exchange interaction. In the regime of high exciton density, the exciton–exciton interaction in ZnSe\\/ZnMgSSe quantum wells is found to be strongly affected by the exciton spin orientation and the longitudinal spin relaxation via single-fermion spin flips. The exciton's

H. Kalt; J Hoffmann; D Tröndle; S Wachter; D Lüerßen; D Hägele; W. W Rühle; M Oestreich; I. J Blewett; I Galbraith

2000-01-01

290

Excitons in anisotropic two-dimensional semiconducting crystals

NASA Astrophysics Data System (ADS)

The excitonic behavior of anisotropic two-dimensional crystals is investigated using numerical methods. We employ a screened potential arising due to the system polarizability to solve the central-potential problem using the Numerov approach. The dependence of the exciton energies on the interaction strength and mass anisotropy is demonstrated. We use our results to obtain the exciton binding energy in phosphorene as a function of the substrate dielectric constant.

Rodin, A. S.; Carvalho, A.; Castro Neto, A. H.

2014-08-01

291

Tailoring exciton dynamics by elastic strain-gradient in semiconductors.

In purely bent ZnO microwires, the excitons can be effectively driven and concentrated by the elastic strain-gradient towards the tensile outer side of the purely bent wire. Experimental and theoretical approaches are combined to investigate the dynamics of excitons in an inhomogeneous strain field with a uniform elastic strain-gradient. Cathodoluminescence spectroscopy analysis on purely bent ZnO microwires verifies that excitons can be effectively driven and concentrated along the elastic strain-gradient. PMID:24470383

Fu, Xuewen; Su, Cong; Fu, Qiang; Zhu, Xinli; Zhu, Rui; Liu, Chuanpu; Liao, Zhimin; Xu, Jun; Guo, Wanlin; Feng, Ji; Li, Ju; Yu, Dapeng

2014-04-23

292

Excitonic photoconductivity of 4BCMU polydiacetylene single crystals

Thermal annealing of 4BCMU single crystals increases the transition energy of excitons from 2.0 to 2.3eV and a similar increase is observed for the free carrier gap resolved by electroabsorption spectra. This modification leads to a significant change of the photocurrent threshold which after the transformation shifts to the excitonic absorption edge. Since the exciton binding energy of 0.5eV is

S Möller; G Weiser; C Lapersonne-Meyer

2001-01-01

293

Photon statistics dispersion in excitonic composites

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.

G. Ya. Slepyan; S. A. Maksimenko

2006-05-22

294

Exciton-polariton oscillations in real space

NASA Astrophysics Data System (ADS)

We introduce and model spin-Rabi oscillations based on exciton-polaritons in semiconductor microcavities. The phase and polarization of oscillations can be controlled by resonant coherent pulses and the propagation of oscillating domains gives rise to phase-dependent interference patterns in real space. We show that interbranch polariton-polariton scattering controls the propagation of oscillating domains, which can be used to realize logic gates based on an analog variable phase.

Liew, T. C. H.; Rubo, Y. G.; Kavokin, A. V.

2014-12-01

295

Multiple exciton generation in semiconductor quantum dots

A review is presented of recent work on (1) the origin of the concept of enhanced multiple electron–hole pair (i.e. exciton) production in semiconductor quantum dots (QDs), (2) various experiments based on time-resolved fs to ns spectroscopy (transient IR absorption, transient visible to near-IR bleaching due to state filling, terahertz spectroscopy, and time-resolved photoluminescence) that support the occurrence of highly

Arthur J. Nozik

2008-01-01

296

Exciton Transport and Perfect Coulomb Drag

NASA Astrophysics Data System (ADS)

Exciton condensation is realized in closely-spaced bilayer quantum Hall systems at ?T=1 when the total density in the two 2D electron layers matches the Landau level degeneracy. In this state, electrons in one layer become tightly bound to holes in the other layer, forming a condensate similar to the Cooper pairs in a superconductor. Being charge neutral, these excitons ought to be free to move throughout the bulk of the quantum Hall fluid. One therefore expects that electron current driven in one layer would spontaneously generate a ``hole'' current in the other layer, even in the otherwise insulating bulk of the 2D system. We demonstrate precisely this effect, using a Corbino geometry to defeat edge state transport. Our sample contains two essentially identical two-dimensional electron systems (2DES) in GaAs quantum wells separated by a thin AlGaAs barrier. It is patterned into an annulus with arms protruding from each rim that provide contact to each 2DES separately. A current drag geometry is realized by applying a drive voltage between the outer and inner rim on one 2DES layer while the two rims on the opposite layer are connected together in a closed loop. There is no direct electrical connection between the two layers. At ?T=1 the bulk of the Corbino annulus becomes insulating owing to the quantum Hall gap and net charge transport across the bulk is suppressed. Nevertheless, we find that in the drag geometry appreciable currents do flow in each layer. These currents are almost exactly equal magnitude but, crucially, flow in opposite directions. This phenomenon reflects exciton transport within the ?T=1 condensate, rather than its quasiparticle excitations. We find that quasiparticle transport competes with exciton transport at elevated temperatures, drive levels, and layer separations. This work represents a collaboration with A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer and K.W. West.

Nandi, Debaleena

2013-03-01

297

Electroabsorption in Semiconductors: The Excitonic Absorption Edge

Numerical calculations of the optical-absorption coefficient for direct, excitonic transitions in a uniform applied electric field are presented. The electron-hole scattering is treated within the effective-mass approximation and leads to an absorption coefficient which differs markedly in size and shape from the Franz-Keldysh absorption spectrum. A detailed numerical study of the shape of the absorption-edge spectrum at photon energies somewhat

John D. Dow; David Redfield

1970-01-01

298

Why we should teach the Bohr model and how to teach it effectively S. B. McKagan,1 K. K. Perkins,2 2008 Some education researchers have claimed that we should not teach the Bohr model of the atom designed to test this claim by developing a curriculum on models of the atom, including the Bohr

Colorado at Boulder, University of

299

Exciton management in organic photovoltaic multidonor energy cascades.

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

Griffith, Olga L; Forrest, Stephen R

2014-05-14

300

Triplet diffusion in singlet exciton fission sensitized pentacene solar cells

NASA Astrophysics Data System (ADS)

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%.

Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.

2013-10-01

301

Polarization-dependent exciton dynamics in tetracene single crystals.

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

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

2014-12-28

302

Observation of a C-1s core exciton in diamond

A well-resolved core exciton at the bulk diamond C-1s absorption edge has been observed using high-resolution partial-yield spectroscopy with synchrotron radiation. The obtained excitonic binding energy, 0.19 +- 0.015 eV, agrees well with a first-principles effective-mass approximation (EMA). This is in sharp contrast to other semiconductors (Si, Ge, and GaAs) where reported excitonic shifts far exceed EMA estimates. In light of these results, one must question whether previous measurements overestimate the core-hole interaction or if they indicate a breakdown of the EMA for core excitons.

Morar, J.F.; Himpsel, F.J.; Hollinger, G.; Hughes, G.; Jordan, a.J.L.

1985-04-29

303

Polarization-dependent exciton dynamics in tetracene single crystals

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

304

Optimal Driving Force for Converting Excitons into Free Carriers in Excitonic Solar Cells

A general but limiting characteristic in excitonic photovoltaics is that a portion of the incident photon energy appears necessary for converting excitons into electrical charges, resulting in a loss of efficiency. Currently, the mechanism underlying this process is unclear. Here, we describe the development of an experimental method for measuring charge creation yields in organic solar cell materials. We use this method to examine a series of conjugated polymer:fullerene blend films and observe two unexpected features: the existence of an optimal driving force and a loss in conversion efficiency if this force is exceeded. These observations have implications for the design of excitonic photovoltaic devices and can be explained by a simple Marcus formulation that introduces the importance of reorganization energy.

Coffey, D. C.; Larson, B. W.; Hains, A. W.; Whitaker, J. B.; Kopidakis, N.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G.

2012-04-26

305

Exciton-exciton scattering in vapor phase ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Photoluminescence (PL) properties of suspended ZnO nanoparticles formed in vapor phase due to the condensation of the Nd:YAG laser ablated ZnO plasma species are investigated by varying both ablating and excitation intensity at different time delays with respect to the ablating pulse and at different axial distances from the target surface. Emission due to inelastic exciton-exciton (X-X) scattering is observed and is found to be dependent on the size of the vapor phase ZnO nanoparticles. The PL intensity shows nonlinear behavior with increasing ablating intensity, indicating generation and participation of more excitons in X-X scattering process in lager size ZnO nanoparticles.

Mohanta, Antaryami; Kung, Patrick; Thareja, Raj K.

2015-01-01

306

Excitonic effects in oxyhalide scintillating host compounds

NASA Astrophysics Data System (ADS)

Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

Shwetha, G.; Kanchana, V.; Valsakumar, M. C.

2014-10-01

307

Excitonic effects in oxyhalide scintillating host compounds

Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X?=?F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

Shwetha, G.; Kanchana, V., E-mail: kanchana@iith.ac.in [Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Telangana (India); Valsakumar, M. C. [School of Engineering Sciences and Technology (SEST), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana (India)

2014-10-07

308

Microcavity controlled coupling of excitonic qubits

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

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

309

Forearm and distal radius fractures in children.

Pediatric forearm and distal radius fractures are common injuries. Resultant deformities are usually a product of indirect trauma involving angular loading combined with rotational displacement. Fractures are classified by location, completeness, angular and rotational deformity, and fragment displacement. Successful outcomes are based on restoration of adequate pronation and supination and, to a lesser degree, acceptable cosmesis. When several important concepts are kept in mind, these goals are usually met with conservative treatment by reduction and immobilization. Greenstick fractures are reduced by rotating the forearm such that the palm is directed toward the fracture apex. Complete fractures are manipulated and reduced with traction and rotation; extremities are then immobilized in well-molded plaster casts until healing, which usually takes about 6 weeks. Radiographs should be obtained between 1 and 2 weeks after initial reduction to detect early angulation. In fractures in any level in children less than 9 years of age, complete displacement, 15 degrees of angulation, and 45 degrees of malrotation are acceptable. In children 9 years of age or older, 30 degrees of malrotation is acceptable, with 10 degrees of angulation for proximal fractures and 15 degrees for more distal fractures. Complete bayonet apposition is acceptable, especially for distal radius fractures, as long as angulation does not exceed 20 degrees and 2 years of growth remains. Operative intervention is used when the fracture is open and when acceptable alignment cannot be achieved or maintained. Single-bone intramedullary fixation has proven useful. PMID:9689186

Noonan, K J; Price, C T

1998-01-01

310

Study of exciton transfer in dense quantum dot nanocomposites

Study of exciton transfer in dense quantum dot nanocomposites Burak Guzelturk,ab Pedro Ludwig. Govorov,c Xiao Wei Sun,b Qihua Xiongb and Hilmi Volkan Demir*ab Nanocomposites of colloidal quantum dots, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites

Demir, Hilmi Volkan

311

Multi-Exciton Generation in Nanostructured Solar Cells

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 mode 4. politically safest, conflict-free 5. non-man-made price volatility is minimal #12;The Solar

312

Coulomb Memory Signatures in the Excitonic Optical Stark Effect

NASA Astrophysics Data System (ADS)

Differential absorption spectra of high-quality InGaAs quantum wells are presented for various pump detunings and polarization configurations. For low intensity pump pulses tuned well below the exciton a redshift is observed for opposite circularly polarized probe pulses. Microscopic calculations show that this redshift originates from memory effects in the Coulomb-induced excitonic correlations.

Sieh, C.; Meier, T.; Jahnke, F.; Knorr, A.; Koch, S. W.; Brick, P.; Hübner, M.; Ell, C.; Prineas, J.; Khitrova, G.; Gibbs, H. M.

1999-04-01

313

Detailed balance theory of excitonic and bulk heterojunction solar cells

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,

Thomas Kirchartz; Julian Mattheis; Uwe Rau

2008-01-01

314

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

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

Martin Pope; Nicholas E. Geacintov; Frank Vogel

1969-01-01

315

Chiral Inflation of the Pion Radius

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.

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

2011-05-25

316

Dynamics of a single exciton in strongly correlated bilayers

NASA Astrophysics Data System (ADS)

We formulated an effective theory for a single interlayer exciton in a bilayer quantum antiferromagnet, in the limit when the holon and doublon are strongly bound onto one interlayer rung by the Coulomb force. Upon using a rung linear spin-wave approximation of the bilayer Heisenberg model, we calculated the spectral function of the exciton for a wide range of the interlayer Heisenberg coupling ? = J?/Jz. In the disordered phase at large ?, a coherent quasi-particle peak appears, representing free motion of the exciton in a spin singlet background. In the Néel phase, which applies to more realistic model parameters, a ladder spectrum arises due to Ising confinement of the exciton. The exciton spectrum is visible in measurements of the dielectric function, such as c-axis optical conductivity measurements.

Rademaker, Louk; Wu, Kai; Zaanen, Jan

2012-08-01

317

A solvable model for excitonic complexes in one dimension

NASA Astrophysics Data System (ADS)

It is known experimentally that stable few-body clusters containing negatively-charged electrons (e) and positively-charged holes (h) can exist in low-dimensional semiconductor nanostructures. In addition to the familiar exciton (e+h), three-body "charged excitons" (2e+h and 2h+e) have also been observed. Much less is known about the properties of such charged excitons since three-body problems are generally very difficult to solve, even numerically. Here we introduce a simple model, which can be considered as an extended Calogero model, to calculate analytically the energy spectra for both a charged exciton and a neutral exciton in a one-dimensional nanostructure, such as a finite-length quantum wire. Apart from its physical motivation, the model is of mathematical interest in that it can be related to the Heun (or Heine) equation and, as shown explicitly, highly accurate, closed form solutions can be obtained.

Markvardsen, Anders J.; Johnson, Neil F.

1997-08-01

318

Exciton complexes in low dimensional transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

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.

Thilagam, A.

2014-08-01

319

Multiple population-period transient spectroscopy (MUPPETS) in excitonic systems.

Time-resolved experiments with more than one period of incoherent time evolution are becoming increasingly accessible. When applied to a two-level system, these experiments separate homogeneous and heterogeneous contributions to kinetic dispersion, i.e., to nonexponential relaxation. Here, the theory of two-dimensional (2D) multiple population-period transient spectroscopy (MUPPETS) is extended to multilevel, excitonic systems. A nonorthogonal basis set is introduced to simplify pathway calculations in multilevel systems. Because the exciton and biexciton signals have different signs, 2D MUPPETS cleanly separates the exciton and biexciton decays. In addition to separating homogeneous and heterogeneous dispersion of the exciton, correlations between the exciton and biexciton decays are measurable. Such correlations indicate shared features in the two relaxation mechanisms. Examples are calculated as both 2D time decays and as 2D rate spectra. The effect of solvent heating (i.e., thermal gratings) is also calculated in multidimensional experiments on multilevel systems. PMID:23343269

Wu, Haorui; Berg, Mark A

2013-01-21

320

Activated singlet exciton fission in a semiconducting polymer.

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

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

2013-08-28

321

Exciton states and interband absorption of cylindrical quantum dot with Morse confining potential

NASA Astrophysics Data System (ADS)

In this paper the exciton and electron sates in cylindrical quantum dot with Morse potential made of GaAs are studied. For the regime of strong size quantization, energy spectrum with the parabolic approximation case are compared. For strong and weak size quantization regimes analytic expressions for the particle energy spectrum, absorption coefficient and dependencies of effective threshold frequencies of absorption on the geometrical parameters quantum dot are obtained. For the intermediate size quantization regime the problem solved in the framework of variation method. The selection rules corresponding to different transitions between quantum levels are found. The size dispersion distribution of growing quantum dots by the radius and height by two experimentally realizing distribution functions have been taken into account. Distribution functions of Gauss, Lifshits-Slezov have been considered.

Hayrapetyan, D. B.; Kazaryan, E. M.; Kotanjyan, T. V.; Tevosyan, H. Kh.

2015-02-01

322

Probing excitonic dark states in single-layer tungsten disulphide

NASA Astrophysics Data System (ADS)

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.

Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang

2014-09-01

323

Probing excitonic dark states in single-layer tungsten disulphide.

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

Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang

2014-09-11

324

NASA Astrophysics Data System (ADS)

Excitons in high-purity crystals of Cu2O undergo a density-dependent lifetime that opposes Bose–Einstein condensation (BEC). This rapid decay rate of excitons at a density n has generally been attributed to Auger recombination having the form dn/dt=-A{{n}2}, where A is an exciton-Auger constant. Various measurements of A, however, have reported values that are orders-of-magnitude larger than the existing theory. In response to this conundrum, recent work has suggested that excitons bind into excitonic molecules, or biexcitons, which are short-lived and expected to be optically inactive. Of particular interest is the case of excitons confined to a parabolic strain well—a method that has recently achieved exciton densities approaching BEC. In this paper we report time- and space-resolved luminescence data that supports the existence of short-lived biexcitons in a strain well, implying an exciton loss rate of the form dn/dt=-2C{{n}2} with a biexciton capture coefficient C(T) proportional to 1/T, as predicted by basic thermodynamics. This alternate theory will be considered in relation to recent experiments on the subject.

Wolfe, James P.; Jang, Joon I.

2014-12-01

325

Journal of Luminescence 111 (2005) 343347 Exciton annihilation on dendrimeric trees

Journal of Luminescence 111 (2005) 343Â347 Exciton annihilation on dendrimeric trees Subhadip-2025, USA Available online 1 February 2005 Abstract ExcitonÂexciton annihilation on Cayley tree likeÂdiffusion systems; Exciton annihilation; Cayley tree 1. Introduction Extended dendrimers (Fig. 1) are large nanos

Mukamel, Shaul

326

SPECTROSCOPIE DE MODULATION LES EFFETS D'EXCITONS DANS LES SPECTRES LECTRO-OPTIQUES

les excitons (effet Stark quadratique sur le mouvement relatif de l'exciton). Le comportement des Stark quadratique des excitons, on observe un effet Stark sur les bandes du cristal (linÃ©aire dans CuSPECTROSCOPIE DE MODULATION LES EFFETS D'EXCITONS DANS LES SPECTRES Ã?LECTRO-OPTIQUES A. DAUNOIS et

Paris-Sud XI, UniversitÃ© de

327

We analyze Niels Bohr's proposed two-slit interference experiment with highly charged particles which argues that the consistency of elementary quantum mechanics requires that the electromagnetic field must be quantized. In the experiment a particle's path through the slits is determined by measuring the Coulomb field that it produces at large distances; under these conditions the interference pattern must be suppressed. The key is that, as the particle's trajectory is bent in diffraction by the slits, it must radiate and the radiation must carry away phase information. Thus, the radiation field must be a quantized dynamical degree of freedom. However, if one similarly tries to determine the path of a massive particle through an inferometer by measuring the Newtonian gravitational potential the particle produces, the interference pattern would have to be finer than the Planck length and thus indiscernible. Unlike for the electromagnetic field, Bohr's argument does not imply that the gravitational field must be quantized. PMID:19218440

Baym, Gordon; Ozawa, Tomoki

2009-01-01

328

Nuclear structure of â¶â¹ â·Â¹Ge in the semimicroscopic model. [Alaga and Bohr-Mottelson models

The properties of negative-parity states of â¶â¹,â·Â¹Ge are interpreted in the framework of the Alaga and the Bohr-Mottelson semimicroscopic models, respectively. The situation for positive-parity states is more complicated, but gives evidence for the correlations of the Alaga model. Both the calculations and the generalized vibrational intensity and selection rules discussion of low-lying spectra are presented. (AIP)

V. Paar; U. Eberth; J. Eberth

1976-01-01

329

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

Jagdish Mehra

1987-01-01

330

A rigorous proof of the Bohr-van Leeuwen theorem in the semiclassical limit

The original formulation of the Bohr-van Leeuwen (BvL) theorem states that, in a uniform magnetic field and in thermal equilibrium, the magnetization of an electron gas in the classical Drude-Lorentz model vanishes identically. This stems from classical statistics which assign the canonical momenta all values ranging from $-\\infty$ to $\\infty$ what makes the free energy density magnetic-field-independent. When considering the classical Maxwell-Boltzmann electron gas, it is often admitted that the BvL theorem holds on condition that the potentials modeling the interactions are particle-velocities-independent and do not cause the system to rotate after turning on the magnetic field. From a rigorous viewpoint, when treating large macroscopic systems one expects the BvL theorem to hold provided the thermodynamic limit of the free energy density exists (and the equivalence of ensemble holds). This requires suitable assumptions on the many-body interactions potential and on the possible external potentials to prevent the system from collapsing or flying apart. Starting from quantum statistical mechanics, the purpose of this article is to give within the linear-response theory a proof of the BvL theorem in the semiclassical limit when considering a dilute electron gas subjected to a class of translational invariant external potentials.

Baptiste Savoie

2014-03-12

331

Third-order optical response of intermediate excitons with fractional nonlinear statistics

NASA Astrophysics Data System (ADS)

The interplay of exciton statistics and Coulomb interactions in the optical response of semiconductors is studied by derivation of an effective Hamiltonian written in terms of exciton operators, starting with a two-band model. Statistical effects are incorporated through the nonboson commutation relations of exciton operators, which contain an exciton-size-dependent parameter q that may vary from q=0 (boson statistics) through q<<1 (Wannier excitons) to q=2 (Frenkel excitons or Pauli statistics). A unified Green's function expression for chi (3) that applies to excitons of an arbitrary nature is derived.

Chernyak, Vladimir; Mukamel, Shaul

1996-06-01

332

We have measured the affinity of the CysF9[93]? sulfhydryl group of human deoxyhemoglobin and oxyhemoglobin for 5,5'-dithiobis(2-nitrobenzoate), DTNB, between pH ?5.6 and 9 in order to understand the basis of the reported reduction of the Bohr effect induced by chemical modification of the sulfhydryl. We analyzed the results quantitatively on the basis of published data indicating that the sulfhydryl exists in two conformations that are coupled to the transition between two tertiary structures of hemoglobin in dynamic equilibrium. Our analyses show that the ionizable groups linked to the DTNB reaction have lower pKas of ionization in deoxyhemoglobin compared to oxyhemoglobin. So these ionizable groups should make negative contributions to the Bohr effect. We identify these groups as HisNA2[2]?, HisEF1[77]? and HisH21[143]?. We provide explanations for the finding that hemoglobin, chemically modified at CysF9[93]?, has a lower Bohr effect and a higher oxygen affinity than unmodified hemoglobin. PMID:24824171

Okonjo, Kehinde O; Olatunde, Abimbola M; Fodeke, Adedayo A; Babalola, J Oyebamiji

2014-06-01

333

. & Suzuki, T. Excitonic polariton-polariton resonance scattering via excitonic molecules in CuCl. J. Phys and biexcitons in direct-gap semiconductors. Phys. Rep. 124, 161Â253 (1985). 11. Kuwata, M., Mita, T. & Nagasawa

Haile, Sossina M.

334

Robust excitons inhabit soft supramolecular nanotubes

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

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

335

Nanoscale Charge Transport in Excitonic Solar Cells

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.

Venkat Bommisetty, South Dakota State University

2011-06-23

336

Robust excitons inhabit soft supramolecular nanotubes.

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

Eisele, Dörthe M; Arias, Dylan H; Fu, Xiaofeng; Bloemsma, Erik A; Steiner, Colby P; Jensen, Russell A; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G

2014-08-19

337

Excitons and charges at organic semiconductor heterojunctions.

All-organic heterojunction solar cells now provide very high quantum efficiencies for charge generation and rapidly-improving power conversion efficiencies. Charge generation and separation however, must overcome the strong Coulomb interactions between electrons and holes in these materials that is manifest also through the large exchange energies usually observed. We show for a polymer-polymer system with low charge generation efficiency that this arises through intersystem crossing from the photogenerated charge-transfer state to a lower lying triplet state, mediated by the proton hyperfine interaction, and that the activation barrier for full separation of electrons and holes is of the order of 250 meV. We observe, using transient optical spectroscopy, the processes of charge separation, recombination and sweep-out in efficient polymer-fullerene devices. We report also on the process of singlet exciton fission to form a pair of triplet excitons in pentacene that can later be dissociated against a heterojunction formed with C60. PMID:22470984

Friend, Richard H; Phillips, Matthew; Rao, Akshay; Wilson, Mark W B; Li, Zhe; McNeill, Christopher R

2012-01-01

338

CCMR: Exciton Blocking Layers in Organic Photovoltaics

NSDL National Science Digital Library

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.

Becker, Katherine

2007-08-29

339

Mathematical Analysis of a Bohr Atom Model Goong Chen1,2

of the electron on a circular orbit with radius r and velocity vvv is E = kinetic energy + potential energy = mev2 that the ratio of the total kinetic energy of the electron to its orbital frequency be an integral multiple E is the energy difference between the two orbits where the transition occurs. From now on, vectors

Hsu, Sze-Bi

340

Role of phonons in Josephson oscillations of excitonic and polaritonic condensates

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.

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

341

Experimental study of finite Larmor radius effects

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.

Struve, K.W.

1980-08-01

342

New physics and the proton radius problem

NASA Astrophysics Data System (ADS)

Background: The recent disagreement between the proton charge radius extracted from Lamb shift measurements of muonic and electronic hydrogen invites speculation that new physics may be to blame. Several proposals have been made for new particles that account for both the Lamb shift and the muon anomalous moment discrepancies. Purpose: We explore the possibility that new particles’ couplings to the muon can be fine-tuned to account for all experimental constraints. Method: We consider two fine-tuned models, the first involving new particles with scalar and pseudoscalar couplings, and the second involving new particles with vector and axial couplings. The couplings are constrained by the Lamb shift and muon magnetic moment measurements while mass constraints are obtained by kaon decay rate data. Results: For the scalar-pseudoscalar model, masses between 100 to 200 MeV are not allowed. For the vector model, masses below about 200 MeV are not allowed. The strength of the couplings for both models approach that of electrodynamics for particle masses of about 2 GeV. Conclusions: New physics with fine-tuned couplings may be entertained as a possible explanation for the Lamb shift discrepancy.

Carlson, Carl E.; Rislow, Benjamin C.

2012-08-01

343

Exciton Mott transition in Si revealed by terahertz spectroscopy.

We investigate the exciton Mott transition in Si by using optical pump and terahertz probe spectroscopy. The density-dependent exciton ionization ratio ? is quantitatively evaluated from the analysis of dielectric function and conductivity spectra. The Mott density is clearly determined by the rapid increase in ? as a function of electron-hole (e-h) pair density, which agrees well with the value expected from the random phase approximation theory. However, exciton is sustained in the high-density metallic region above the Mott density as manifested by the 1s-2p excitonic resonance that remains intact across the Mott density. Moreover, the charge carrier scattering rate is strongly enhanced slightly above the Mott density due to nonvanishing excitons, indicating the emergence of highly correlated metallic phase in the photoexcited e-h system. Concomitantly, the loss function spectra exhibit the signature of plasmon-exciton coupling, i.e., the existence of a new collective mode of charge density excitation combined with the excitonic polarization at the proximity of Mott density. PMID:23006100

Suzuki, Takeshi; Shimano, Ryo

2012-07-27

344

Dynamical fermion mass generation and exciton spectra in graphene

The Coulomb interaction between massless Dirac fermions may induce dynamical chiral symmetry breaking by forming excitonic pairs in clean graphene, leading to semimetal-insulator transition. If the Dirac fermions have zero bare mass, an exact continuous chiral symmetry is dynamically broken and thus there are massless Goldstone excitons. If the Dirac fermions have a small bare mass, an approximate continuous chiral symmetry is dynamically broken and the resultant Goldstone-type excitons become massive, which is analogous to what happens in QCD. In this paper, after solving the Dyson-Schwinger gap equation in the presence of a small bare fermion mass, we found a remarkable reduction of the critical Coulomb interaction strength for excitonic pair formation and a strong enhancement of dynamical fermion mass. We then calculate the masses of Goldstone-type excitons using the Shifman-Vainshtein-Zakharov sum-rule method and operator product expansion technique developed in QCD and find that the exciton masses are much larger than bare fermion mass but smaller than the width of dynamical fermion mass gap. We also study the spin susceptibilities and estimate the masses of non-Goldstone-type excitons using the same tools.

Zhang Chunxu [Department of Physics, National University of Defense Technology, Changsha, Hunan 410073 (China); Interdisciplinary Center for Theoretical Study, University of Science and Technology of China, Hefei, Anhui 230026 (China); Liu Guozhu [Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026 (China); Institut fuer Theoretische Physik, Freie Universitaet Berlin, Arnimallee 14, D-14195 Berlin (Germany); Huang Mingqiu [Department of Physics, National University of Defense Technology, Changsha, Hunan 410073 (China)

2011-03-15

345

Confocal shift interferometry of coherent emission from trapped dipolar excitons

NASA Astrophysics Data System (ADS)

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.

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

346

Exciton coupling of surface complexes on a nanocrystal surface.

Exciton coupling may arise when chromophores are brought into close spatial proximity. Herein the intra-nanocrystal exciton coupling of the surface complexes formed by coordination of 8-hydroxyquinoline to ZnS nanocrystals (NCs) is reported. It is studied by absorption, photoluminescence (PL), PL excitation (PLE), and PL lifetime measurements. The exciton coupling of the surface complexes tunes the PL color and broadens the absorption and PLE windows of the NCs, and thus is a potential strategy for improving the light-harvesting efficiency of NC solar cells and photocatalysts. PMID:24863364

Xu, Xiangxing; Ji, Jianwei; Wang, Guan; You, Xiaozeng

2014-08-25

347

Excitons and biexcitons in symmetric electron-hole bilayers

NASA Astrophysics Data System (ADS)

Symmetric electron-hole bilayer systems have been studied at zero temperature using the diffusion quantum Monte Carlo method. A flexible trial wave function is used that can describe fluid, excitonic, and biexcitonic phases. We calculate condensate fractions and pair correlation functions for a large number of densities rs and layer separations d. At small d we find a one-component fluid phase, an excitonic fluid phase, and a biexcitonic fluid phase, and the transitions among them appear to be continuous. At d=0, excitons appear to survive down to about rs=0.5a.u., and biexcitons form at rs>2.5a.u.

Maezono, Ryo; López Ríos, Pablo; Ogawa, Tetsuo; Needs, Richard J.

2013-05-01

348

Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals

NASA Astrophysics Data System (ADS)

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.

Lyu, ByungGook

349

Evidence for a Surface Exciton in KBr via Laser Desorption

We demonstrate that direct photoexcitation of the KBr surface exciton leads to desorption of hyperthermal neutral bromine atoms. We have for the first time produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We further demonstrate that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provides strong support to a theoretical emission model previously described in the literature.

Beck, Kenneth M. (BATTELLE (PACIFIC NW LAB)); Joly, Alan G. (BATTELLE (PACIFIC NW LAB)); Hess, Wayne P. (BATTELLE (PACIFIC NW LAB))

2001-01-01

350

Evidence for a surface exciton in KBr via laser desorption

We demonstrate that direct photoexcitation of the single crystal KBr surface leads to desorption of hyperthermal neutral bromine atoms. We have produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We argue that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provide strong support to a theoretical excitonic emission model previously described in the literature.

Beck, Kenneth M.; Joly, Alan G.; Hess, Wayne P.

2001-03-15

351

Photoluminescence of high-density exciton-polariton condensates

NASA Astrophysics Data System (ADS)

We examine the photoluminescence of highly excited exciton-polariton condensates in semiconductor microcavities. Under strong pumping, exciton-polariton condensates have been observed to undergo a lasing transition where the strong coupling between the excitons and photons is lost. We discuss an alternative high-density scenario, where the strong coupling is maintained. We find that the photoluminescence smoothly transitions between the lower polariton energy to the cavity photon energy. An intuitive understanding of the change in spectral characteristics is given, as well as differences to the photoluminescence characteristics of the lasing case.

Ishida, Natsuko; Byrnes, Tim; Horikiri, Tomoyuki; Nori, Franco; Yamamoto, Yoshihisa

2014-12-01

352

The Stark effect in the Bohr-Sommerfeld theory and in Schrödinger's wave mechanics

The explanation of the first-order Stark effect in hydrogen by Epstein and Schwarzschild in 1916 was seen as a great success for the old quantum theory. Yet, it also revealed some serious limitations of the theory. To recover the experimentally found line splittings, one had to make some arbitrary assumptions in addition to the basic quantum conditions to rule out certain orbits. The calculation of intensities of lines on the basis of Bohr's correspondence principle likewise required arbitrary additional assumptions. Finally, the actual orbits predicted by the old quantum theory depend on the coordinates chosen to impose the quantum conditions. Both Sommerfeld and Epstein recognized this problem but offered no solution for it. All these problems were solved in 1926 when Schr\\"odinger and Epstein explained the Stark effect on the basis of the new wave mechanics. The calculations in the two theories are similar. In particular, both the Schr\\"odinger equation in the new theory and the Hamilton-Jacobi equation in the old theory are separated in parabolic coordinates. The new quantum mechanics determines all allowed states and transitions without any additional assumptions. It also replaced the ambiguous guidelines based on the correspondence principle for calculating intensities by the straightforward prescription that intensities are given by the squares of the matrix elements of position, leading to results that agreed much better with the experimental data. Finally, the embarrassing non-uniqueness of orbits in the old quantum theory turned into the innocuous non-uniqueness of bases of eigenfunctions in wave mechanics. To this day, the Stark effect is remembered as one of the few qualified successes of the old quantum theory. We suspect that this is largely because after 1926 it became just one of the many unqualified successes of the new quantum theory.

Anthony Duncan; Michel Janssen

2014-04-21

353

Exciton-exciton interaction is experimentally revealed and quantitatively analyzed in a wide band-gap scintillator material CdWO{sub 4}. Under high-intensity femtosecond vacuum ultraviolet excitation, the CdWO{sub 4} luminescence is quenched, while its decay becomes essentially nonexponential. We propose an analytical model, which successfully reproduces the decay kinetics recorded in a wide range of excitation densities. The dipole-dipole interaction between excitons leading to their nonradiative decay is shown to be the main cause of a nonproportional response common for many scintillators.

Kirm, M.; Nagirnyi, V.; Feldbach, E.; De Grazia, M.; Carre, B.; Merdji, H.; Guizard, S.; Geoffroy, G.; Gaudin, J.; Fedorov, N.; Martin, P.; Vasil'ev, A.; Belsky, A. [Institute of Physics, University of Tartu, Riia 142, 51014 Tartu (Estonia); Service des Photons Atomes et Molecules, CEA/Iramis, 91191 Gif sur Yvette (France); Laboratoire des Solides Irradies, CEA/Iramis, CNRS and Ecole Polytechnique, 91128 Palaiseau (France); European XFEL, Notkestrasse 85, 22607 Hamburg (Germany); Laboratoire CELIA, CNRS, CEA et Universite de Bordeaux I, 33400 Talence (France); Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119991 Moscow (Russian Federation); Laboratoire de Physico-Chimie des Materiaux Luminescents, CNRS, UMR5620, Universite Lyon 1, Villeurbanne F-69622 (France)

2009-06-15

354

as evidence for exciton condensation L. V. Butov and A. I. Filin Institute of Solid State Physics, Russian to the long lifetime of indirect interwell excitons, exciton condensation analogous to the Bose- Einstein condensation of bosons is expected to occur in coupled quantum wells CQW's . The critical con- ditions

355

Cerium-, praseodymium- and terbium-trapped excitons in oxides

NASA Astrophysics Data System (ADS)

Spectroscopic study of CaNb 2O 6 single crystals doped with Ce 3+, Pr 3+ or Tb 3+ ions shows existence of a broad excitation band attributed to a rare-earth-trapped exciton, with the hole trapped at the rare earth ion and the electron delocalized over the surrounding of the next-neighbor Nb 5+ cations. Their energies are in agreement with the Dorenbos' model determining the energy level location of trivalent lanthanides in oxides. The rare-earth-trapped excitons occurred in the Pr 3+ or Tb 3+ doped CaNb 2O 6 crystals undergo non-radiative relaxation to the 4f n states. In contrast, the exciton trapped on Ce 3+ ion was shown to exhibit luminescence from the excitonic state that is the lowest excited state of the system.

Pedrini, C.; Belsky, A.; Ivanovskikh, K. V.; Petrosyan, A. G.; Sargsyan, R. V.; Kamenskikh, I.

2011-10-01

356

Excitonic absorption in ZnI2 films

NASA Astrophysics Data System (ADS)

Optical absorption measurements were carried out at room temperature on stoichiometric and well characterized vacuum evaporated ZnI2 films as function of film thickness and heat-treatment temperatures. The films were transparent and tetragonally structured. X-ray diffraction analysis reveals residual strain on the crystal planes parallel to film thickness. Optical absorption shows three distinct peaklike structures at 3, 3.5 (attributed to be excitonic), and 4.1eV. The residual strains as well as all the absorption structures vary with film thickness and treatment temperatures. Combining all the data from the two experiments, we find that the two excitonic peaks blueshift with residual strain of (104). The strain-free values for all the above quantities have been determined by the extrapolation of their linear dependence on residual strain. The excitonic absorption observed at room temperature shows a large excitonic binding energy of 150meV.

Vedeshwar, Agnikumar G.; Tyagi, Pankaj

2006-10-01

357

Exciton separation at the interface between pentacene and dipolar chromophore

NASA Astrophysics Data System (ADS)

We demonstrate photo-induced charge transfer between pentacene and dipolar chromophore, disperse red 19 (DR19), which can be applied in fabricating optoelectronic devices with abundant flexibility due to the functionality of the dipolar chromophore at the interface. Photo-induced charge transfer phenomenon between pentacene and DR19 is explored through pentacene thickness-dependent threshold voltage measurements using pentacene/DR19 bilayer field effect transistors under illumination. Threshold voltage increases as pentacene thickness increases up to a certain thickness followed by a decrease, resulting in a peak threshold voltage. The presence of the peak explains competition between optical absorption in the pentacene layer and exciton diffusion followed by exciton separation at the pentacene/DR19 interface. The exciton diffusion length in pentacene is estimated using an exciton diffusion-dissociation model.

Park, Byoungnam; Park, Jonghoo

2013-11-01

358

Fate of the excitonic insulator in the presence of phonons

NASA Astrophysics Data System (ADS)

The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on Ta2NiSe5 ,1 T -TiSe2, and TmSe0.45Te0.55 , being candidates for realizing the excitonic-insulator state, suggest, however, that the underlying lattice plays a significant role. Employing the Kadanoff-Baym approach we address this issue theoretically. We show that owing to the electron-phonon coupling a static lattice distortion may arise at the excitonic instability. Most importantly such a distortion will destroy the acoustic phase mode being present if the electron-hole pairing and condensation is exclusively driven by the Coulomb interaction. The absence of off-diagonal long-range order, when lattice degrees of freedom are involved, challenges that excitons in these materials form a superfluid condensate of Bose particles or Cooper pairs composed of electrons and holes.

Zenker, B.; Fehske, H.; Beck, H.

2014-11-01

359

Exciton/Charge-transfer Electronic Couplings in Organic Semiconductors

Charge transfer (CT) states and excitons are important in energy conversion processes that occur in organic light emitting devices (OLEDS) and organic solar cells. An ab initio density functional theory (DFT) method for ...

Difley, Seth

360

Dynamics of dissipative multiple exciton generation in semiconductor nanostructures

NASA Astrophysics Data System (ADS)

The population dynamics of single exciton and biexciton states in a simple model of a spherical semiconductor nanostructure is modeled numerically in the presence of Coulomb coupling between single and two exciton states and a dissipation channel in order to study the transient biexciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton number (multiple exciton generation process) is fast (on picosecond time scale) and the following decay (Auger process) is much slower (hundreds of picoseconds). In some cases, the maximum occupation of the biexciton state increases when dissipation is included. The dynamics of an ensemble of nanostructures with a certain size dispersion is studied by averaging over the energy of the biexciton state which can be different for each single nanostructure. The validity of Markov and secular approximation is also verified.

Azizi, Maryam; Machnikowski, Pawe?

2013-09-01

361

Singlet exciton fission-sensitized infrared quantum dot solar cells.

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

Ehrler, Bruno; Wilson, Mark W B; Rao, Akshay; Friend, Richard H; Greenham, Neil C

2012-02-01

362

Topologically protected excitons in porphyrin thin films

NASA Astrophysics Data System (ADS)

The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov–Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.

Yuen-Zhou, Joel; Saikin, Semion K.; Yao, Norman Y.; Aspuru-Guzik, Alán

2014-11-01

363

Singlet exciton fission : applications to solar energy harvesting

Singlet exciton fission transforms a single molecular excited state into two excited states of half the energy. When used in solar cells it can double the photocurrent from high energy photons increasing the maximum ...

Thompson, Nicholas John

2014-01-01

364

Exciton front propagation in photoexcited GaAs quantum wells

We report on the study of spatiotemporal self-organization of carriers in photoexcited GaAs quantum wells. Propagating interfaces between electron-rich and hole-rich regions are seen as expanding and collapsing exciton ...

Yang, Sen

365

The theory of simultaneous excitonic-superconductivity condensation

NASA Astrophysics Data System (ADS)

The theory of simultaneous excitonic-superconductivity condesation also known as the excitonic enhancement model (EEM) for high Tc superconductivity is presented in two different approaches: (a) Based on the assumption of a strong Coulomb binding between electron and hole carriers in a two-band model, we obtained the total excitation gap in terms of a Pythagorean sum of an excitonic and a BCS gap component in a mean-field approximation. The importance of near two-dimensionality in the excitonic component was emphasized. (b) The off-diagonal-long-range-ordering [ODLRO] of the superconducting quasi-particle was assumed to be a linear combination of electron and hole states due to a strong electron-hole correlation. The result gives a charge reduction in the quantized flux. An estimation to this reduction was given in the mean-field isotropic limit.

Wong, K. W.; Ching, W. Y.

1989-04-01

366

The pion charge radius from charged pion electroproduction

We analyze a low-energy theorem of threshold pion electroproduction which allows one to determine the charge radius of the pion. We show that at the same order where the radius appears, pion loops induce a correction to the momentum dependence of the longitudinal dipole amplitude $L_{0+}^{(-)}$. This model-independent correction amounts to an increase of the pion charge radius squared from the electroproduction data by about 0.26~fm$^2$. It sheds light on the apparent discrepancy between the recent determination of the pion radius from electroproduction data and the one based on pion-electron scattering.

Véronique Bernard; Norbert Kaiser; Ulf-G. Meißner

2000-03-28

367

Centrifugal-Barrier Effects and Determination of the Interaction Radius

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.

Ning Wu

2013-05-21

368

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

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.

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

369

Exciton dynamics in atomically thin MoS2: Interexcitonic interaction and broadening kinetics

NASA Astrophysics Data System (ADS)

We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS2. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons (A and B) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the B-exciton linewidth in turn lowers the peak spectral amplitude of the A exciton. Resonant excitation at the B-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.

Sim, Sangwan; Park, Jusang; Song, Jeong-Gyu; In, Chihun; Lee, Yun-Shik; Kim, Hyungjun; Choi, Hyunyong

2013-08-01

370

NASA Astrophysics Data System (ADS)

We report a photoluminescence (PL) and transient reflection spectroscopy study of the exciton dynamics in monolayer transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs at an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that nonlinear exciton-exciton annihilation is the dominant exciton decay process in monolayer MoSe2 in contrast to the previously-reported linear exciton decay in monolayer MoS2. In addition, the exciton lifetime in MoSe2, which is greater than 125 ps, is more than an order of magnitude longer than the several-ps exciton lifetime in MoS2. We find that the dramatically different exciton decay mechanisms and PL saturation behaviors of the MoSe2 and the MoS2 monolayers can be explained in terms of the difference in their exciton lifetimes.

Shin, Min Ju; Kim, Dong Hak; Lim, D.

2014-12-01

371

Phonon-assisted exciton-polariton emission in a microcavity

NASA Astrophysics Data System (ADS)

Quantum-well excitons in a semiconductor microcavity are studied by photoluminescence and input-output experiments. Emission peaks at both longer and shorter wavelengths are observed, indicating the importance of exciton-acoustic-phonon interaction at low temperature. Pump-probe experiments show that the probe beam can be either amplified or attenuated, depending on the energies of the pump and the probe. The dynamics of this process are described by a simple rate-equation model. 42.50.Ne.

Pau, Stanley; Jacobson, Joseph; Björk, Gunnar; Yamamoto, Yoshihisa

1996-05-01

372

Superfluidity of excitons in type-II semiconductor quantum well

The order formation and superfluidity are theoretically investigated in a quasi-two-dimensional Bose gas of excitons in a type-II quantum well. In previous work, we have studied the stationary state of the condensed excitons irradiated by a weak laser light at T=0K, and showed that under the control of the external current, there appears a vortex lattice with a net supercurrent.

T. Iida; M. Tsubota

2000-01-01

373

Singlet fission in pentacene through multi-exciton quantum states

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

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

2010-01-01

374

Singlet fission in pentacene through multi-exciton quantum states

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

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

2010-01-01

375

Universality in nonequilibrium condensation of exciton-polaritons

NASA Astrophysics Data System (ADS)

We investigate the process of condensation of exciton-polaritons in a one-dimensional nanowire, predicting spontaneous formation of domains of uncondensed excitons and condensed polaritons. We find that the system does not follow the standard Kibble-?urek scenario of defect formation. Nevertheless, the universal scaling laws are still present if the critical exponents are replaced by their "nonequilibrium" counterparts related to the imaginary part of the time-dependent Bogoliubov spectrum.

Matuszewski, Micha?; Witkowska, Emilia

2014-04-01

376

Multiphonon excitonic absorption in semiconductors and quantum wells

In this article, we present a straightforward procedure based on the cumulant expansion for computing the contribution of exciton-phonon coupling to the exciton line-shape function and thus absorption in bulk and quantum well semiconductors. Using the functional integral technique we derive the effective equation of motion for the two-particle Green's function in the presence of interaction with phonons. We show

Witold Bardyszewski; David Yevick

2001-01-01

377

Monte Carlo Simulation of Exciton Dynamics in Supramolecular Semiconductor Architectures

NASA Astrophysics Data System (ADS)

Supramolecular chemistry is useful to construct molecular architectures with functional semiconductor properties. To explore the consequences of this approach in molecular electronics, we have carried out ultrafast measurements of exciton dynamics in supramolecular assemblies of an oligo-p-phenyl-ene-vinyl-ene derivative functionalized to form chiral stacks in dodecane solution in a thermotropically reversible manner. We apply a model of incoherent exciton hopping within a Monte Carlo scheme to extract microscopic physical quantities. The simulation first builds the chiral stacks with a Gaussian disorder of site energies and then simulates exciton hopping on the structure and exciton-exciton annihilation to reproduce ensemble-averaged experimental data. The exciton transfer rates are calculated beyond the point-dipole approximation using the so-called line-dipole approach in combination with the Förster expression. The model of incoherent hopping successfully reproduces the data and we extract a high diffusion coefficient illustrating the polymeric properties of such supramolecular assemblies. The scope and limitations of the line-dipole approximation as well as the resonance energy transfer concept in this system are discussed.

Silva, Carlos; Beljonne, David; Herz, Laura; Hoeben, Freek

2005-03-01

378

Singlet fission in pentacene through multi-exciton quantum states.

Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here. PMID:20651727

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

2010-08-01

379

Molecular packing determines singlet exciton fission in organic semiconductors.

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

Kolata, Kolja; Breuer, Tobias; Witte, Gregor; Chatterjee, Sangam

2014-07-22

380

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.

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

381

Fano Interference S. Bar-Ad, P. Kner, M. V. Marquezini, S. Mukamel,* and D. S. Chemla Department of that phenomenon. In the optical domain the exciton Bohr radius ao is usu- ally the relevant length scale L . 15ao that are unusually large for the optical domain. Quantum mechanical interference between

Mukamel, Shaul

382

the theoretical model from Refs. [1-3]. The model allows us to takes into account the strain and piezoelectricN/AlN QDs is less than the exciton Bohr radius of bulk GaN (~ 2 nm), we first calculate separate electron

Fonoberov, Vladimir

383

Bose-Einstein condensation of excitons in Cu2O: progress over 30 years

NASA Astrophysics Data System (ADS)

Experiments on Bose-Einstein condensation (BEC) of excitons in the semiconductor Cu2O started over 30 years ago, as one of the first serious attempts at exciton BEC. Early claims were based on spectroscopic signatures and transport data which have since been reinterpreted, in large part because the Auger recombination process for excitons was not well understood. Understanding of the Auger process has advanced, and recent experiments have made significant progress toward exciton BEC. We review the history of experiments on exciton BEC in Cu2O, the Auger recombination process, and the prospects for observing exciton BEC in this system in the near future.

Snoke, David; Kavoulakis, G. M.

2014-11-01

384

NASA Astrophysics Data System (ADS)

The authors describes the Soveit KGB operation of interviewing Niels Bohr by soviet scientist Yakov. P. Terletskii(1912-1993) and KGB kolonel Lev Petrovich Vasilevskii (b. 1903) on 24 september 1945-20 november 1945 concerning the American Nuclear weapons (Manhattan project)undertaken under the project of the Soviet KGB Lieder Lavrentij P. Berija and supervised by Soviet KGB generals Pavel A. Sudoplatov (b. 1907) and Nikolay S. Sazykin (1910-1985) after the detailed magnetophone interview of Professor Ya. P. Terletskij before his die in Moscow.

Andreev, A. V.; Kozhevnikov, A. B.; Yavelov, Boris E.

385

NASA Astrophysics Data System (ADS)

We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both.

Schmidt, L. Ph. H.; Lower, J.; Jahnke, T.; Schößler, S.; Schöffler, M. S.; Menssen, A.; Lévêque, C.; Sisourat, N.; Taïeb, R.; Schmidt-Böcking, H.; Dörner, R.

2013-09-01

386

Calculating the Atomic Radius of Polonium Frank Rioux

Calculating the Atomic Radius of Polonium Frank Rioux are required to determine the atomic radius of a metallic element such as polonium: density, molar mass and crystal structure. The crystal structure of room temperature polonium is simple cubic, the only metallic

Rioux, Frank

387

bamr: Bayesian analysis of mass and radius observations

NASA Astrophysics Data System (ADS)

bamr is an MPI implementation of a Bayesian analysis of neutron star mass and radius data that determines the mass versus radius curve and the equation of state of dense matter. Written in C++, bamr provides some EOS models. This code requires O2scl (ascl:1408.019) be installed before compilation.

Steiner, Andrew W.

2014-08-01

388

Precise root-mean-square radius of {sup 4}He

We study the world data on elastic electron-helium scattering to determine the {sup 4}He charge root-mean-square radius. A precise value for this radius is needed as a reference for a number of ongoing studies in nuclear and atomic physics.

Sick, Ingo [Dept. fuer Physik, Universitaet Basel, CH4056 Basel (Switzerland)

2008-04-15

389

The Origin of the Ionic-Radius Ratio Rules

ERIC Educational Resources Information Center

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…

Jensen, William B.

2010-01-01

390

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

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.

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

2013-11-07

391

Return Radius and volume of recrystallized material in Ostwald Ripening

Within the framework of the LSW theory of Ostwald ripening the amount of volume of the second (solid) phase that is newly formed by recrystallization is investigated. It is shown, that in the late stage, the portion of the newly generated volume formed within an interval from time $t_0$ to $t$ is a certain function of $t/t_0$ and an explicit expression of this volume is given. To achieve this, we introduce the notion of the {\\it return radius} $r(t,t_0)$, which is the unique radius of a particle at time $t_0$ such that this particle has -- after growing and shrinking -- the same radius at time $t$. We derive a formula for the return radius which later on is used to obtain the newly formed volume. Moreover, formulas for the growth rate of the return radius and the recrystallized material at time $t_0$ are derived.

Hausser, Frank

2012-01-01

392

Return Radius and volume of recrystallized material in Ostwald Ripening

Within the framework of the LSW theory of Ostwald ripening the amount of volume of the second (solid) phase that is newly formed by recrystallization is investigated. It is shown, that in the late stage, the portion of the newly generated volume formed within an interval from time $t_0$ to $t$ is a certain function of $t/t_0$ and an explicit expression of this volume is given. To achieve this, we introduce the notion of the {\\it return radius} $r(t,t_0)$, which is the unique radius of a particle at time $t_0$ such that this particle has -- after growing and shrinking -- the same radius at time $t$. We derive a formula for the return radius which later on is used to obtain the newly formed volume. Moreover, formulas for the growth rate of the return radius and the recrystallized material at time $t_0$ are derived.

Frank Hausser; Evgeny Lakshtanov

2012-01-21

393

Excitonic and vibronic structure of absorption spectra of Me-PTCDI and PTCDA crystals

NASA Astrophysics Data System (ADS)

The excitonic and vibronic spectra (exciton + one quantum of intramolecular vibration) of Me-PTCDI and PTCDA crystals are studied in the case of strong mixing of a Frenkel exciton (FE) and charge-transfer excitons (CTEs). The linear optical susceptibility is calculated in the framework of dynamical theory of vibronic spectra. The absorption spectra of both crystals have been modelled. The positions, maximal values and integral intensity of the absorption peaks which correspond to the bound exciton-phonon states and to unbound (many-particle) states have been calculated using FE and CTEs's parameters of Me-PTCDI and PTCDA crystals. The calculated spectra show: (i) the possible recovering of excitonic and vibronic regions; (ii) bigger integral intensity of many-particle states in the case of weak exciton-phonon coupling; (iii) the dominant role of the bound states in the case of intermediate and strong exciton-phonon coupling.

Lalov, I. J.; Zhelyazkov, I.

2006-01-01

394

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

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 ...

Thompson, Nicholas J.

395

Excitonic effects on coherent phonon dynamics in single-wall carbon nanotubes

We discuss how excitons can affect the generation of coherent radial breathing modes in the ultrafast spectroscopy of single-wall carbon nanotubes. Photoexcited excitons can be localized spatially and give rise to a spatially ...

Nugraha, A. R. T.

396

Studying the Proton "Radius" Puzzle with ?p Elastic Scattering

The Proton Radius Puzzle is the inconsistency between the proton radius determined from muonic hydrogen and the proton radius determined from atomic hydrogen level transitions and ep elastic scattering. No generally accepted resolution to the Puzzle has been found. Possible solutions generally fall into one of three categories: the two radii are different due to novel beyond-standard-model physics, the two radii are different due to novel aspects of nucleon structure, and the two radii are the same, but there are underestimated uncertainties or other issues in the ep experiments. The MUon proton Scattering Experiment (MUSE) at the Paul Scherrer Institut is a simultaneous measurement of \\mu^+ p and e^+ p elastic scattering, as well as \\mu^- p and e^- p elastic scattering, which will allow a determination of the consistency of the \\mu p and the ep interactions. The differences between + and - charge scattering are sensitive to two-photon exchange effects, higher-order corrections to the scattering process. The slopes of the cross sections as Q^2 -> 0 determine the proton "radius". We plan to measure relative cross sections at a typical level of a few tenths of a percent, which should allow the proton radius to be determined at the level of ~0.01 fm, similar to previous ep measurements. The measurements will test several possible explanations of the proton radius puzzle, including some models of beyond-standard-model physics, some models of novel hadronic physics, and some issues in the radius extraction from scattering data.

R. Gilman; E. J. Downie; G. Ron; A. Afanasev; J. Arrington; O. Ates; F. Benmokhtar; J. Bernauer; E. Brash; W. J. Briscoe; K. Deiters; J. Diefenbach; C. Djalali; B. Dongwi; L. El Fassi; S. Gilad; K. Gnanvo; R. Gothe; D. Higinbotham; R. Holt; Y. Ilieva; H. Jiang; M. Kohl; G. Kumbartzki; J. Lichtenstadt; A. Liyanage; N. Liyanage; M. Meziane; Z. -E. Meziani; D. G. Middleton; P. Monaghan; K. E. Myers; C. Perdrisat; E. Piasetzsky; V. Punjabi; R. Ransome; D. Reggiani; P. Reimer; A. Richter; A. Sarty; E. Schulte; Y. Shamai; N. Sparveris; S. Strauch; V. Sulkosky; A. S. Tadepalli; M. Taragin; L. Weinstein

2013-03-09

397

On the Variation of Solar Radius in Rotation Cycles

NASA Astrophysics Data System (ADS)

The Date Compensated Discrete Fourier Transform and CLEANest algorithm are used to study the temporal variations of the solar radius observed at Rio de Janeiro Observatory from 1998 March 2 to 2009 November 6. The CLEANest spectra show several significant periodicities around 400, 312, 93.5, 86.2, 79.4, 70.9, 53.2, and 26.3 days. Then, combining the data on the daily solar radius measured at Calern Observatory and Rio de Janeiro Observatory and the corresponding daily sunspot areas, we study the short-term periodicity of the solar radius and the role of magnetic field in the variation of the solar radius. The rotation period of the daily solar radius is determined to be statistically significant. Moreover, its temporal evolution is anti-phase with that of sunspot activity, and it is found anti-phase with solar activity. Generally, the stronger solar activity is, the more obvious is the anti-phase relation of radius with solar activity. This indicates that strong magnetic fields have a greater inhibitive effect than weak magnetic fields on the variation of the radius.

Qu, Z. N.; Kong, D. F.; Xiang, N. B.; Feng, W.

2015-01-01

398

Thermoconvective vortices in a cylindrical annulus with varying inner radius.

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

Castaño, D; Navarro, M C; Herrero, H

2014-12-01

399

Exciton condensation and perfect Coulomb drag.

Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, double quantum well systems can support exciton condensates, which consist of electrons in one well tightly bound to holes in the other. 'Perfect' drag is therefore expected; a steady transport current of electrons driven through one quantum well should be accompanied by an equal current of holes in the other. Here we demonstrate this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunnelling of charge between the quantum wells, which can readily compromise drag measurements, is negligible. We note that, from an electrical engineering perspective, perfect Coulomb drag is analogous to an electrical transformer that functions at zero frequency. PMID:22914164

Nandi, D; Finck, A D K; Eisenstein, J P; Pfeiffer, L N; West, K W

2012-08-23

400

Excitonic and vibronic structure of absorption spectra of Me-PTCDI and PTCDA crystals

The excitonic and vibronic spectra (exciton+one quantum of intramolecular vibration) of Me-PTCDI and PTCDA crystals are studied in the case of strong mixing of a Frenkel exciton (FE) and charge-transfer excitons (CTEs). The linear optical susceptibility is calculated in the framework of dynamical theory of vibronic spectra. The absorption spectra of both crystals have been modelled. The positions, maximal values

I. J. Lalov; I. Zhelyazkov

2006-01-01

401

Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon.

We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time. PMID:21230809

Witzel, Wayne M; Shabaev, Andrew; Hellberg, C Stephen; Jacobs, Verne L; Efros, Alexander L

2010-09-24

402

The nonparabolicity of exciton dispersion due to the mixing of the ground and excited states of an exciton in an external magnetic field perpendicular to the direction of its motion is considered. A model describing this effect is proposed and the nonparabolicity for an exciton in a CdTe crystal is calculated. The magnetic-field induced exciton nonparabolicity is compared with the effect caused by the nonparabolicity of the electron energy dispersion in the conduction band.

Loginov, D. K., E-mail: loginov999@gmail.com; Chegodaev, A. D. [St. Petersburg State University (Russian Federation)

2011-09-15

403

Collective effects in emission of localized excitons strongly coupled to a microcavity photon

NASA Astrophysics Data System (ADS)

A theory of nonlinear emission of localized excitons coupled to the optical mode of the microcavity is presented. Numerical results are compared with analytical ones. The effects of exciton-exciton interaction within the quantum dots and with the reservoir formed by non-resonant pumping are considered. It is demonstrated that the nonlinearity due to the interaction strongly affects the shape of the emission spectra. The collective superradiant mode of the excitons is shown to be stable against the nonlinear effects.

Poddubny, A. N.; Glazov, M. M.; Averkiev, N. S.

2013-02-01

404

Multiple exciton generation and ultrafast exciton dynamics in HgTe colloidal quantum dots.

The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model. PMID:23999734

Al-Otaify, Ali; Kershaw, Stephen V; Gupta, Shuchi; Rogach, Andrey L; Allan, Guy; Delerue, Christophe; Binks, David J

2013-10-21

405

Hybrid quadrupole excitons and polaritons in cuprous oxide

NASA Astrophysics Data System (ADS)

In this thesis I consider novel type of materials such as hybrid organic/inorganic heteoro-structures and polystyrene micro-spheres/inorganic composites. The organic/inorganic compound is presented by DCM2:CA:PS/cuprous oxide material. Using "solid state solvent" mechanism I propose to bring the Frenkel exciton (FE) of the DCM2 into resonance with 1S quadrupole Wanier-Mott exciton (WE) in cuprous oxide. This two types of the excitons form new type of quadrupole-dipole hybrid exciton. This hybrid is characterized by long lifetime and big oscillator strength inherited from the organic FE. In the part I of the thesis I investigate the enhancement of the quadrupole properties generic to cuprous oxide exciton by means of such resonant hybridization. I consider enhancement of photo-thermal bi-stability and second harmonic generation. The second part is devoted to the problems of light-matter interaction in cuprous oxide crystals such as weak interaction with LA phonons and whispering gallery modes (WGM) in adjacent layer of polystyrene micro-spheres. While the first effect is likely to impeded BEC of the polaritons, the second mechanism provides necessary temporal coherence. It is possible by trapping the light part of the polariton into resonant WGM through big gradient of the evanescent tail which provides big lifetime of such evanescent polariton. Due to big gradient of the evanescent field it couples "naturally" to the quadrupole WE in cuprous oxide.

Roslyak, Oleksiy

406

Incoherent exciton trapping in self-similar aperiodic lattices

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.

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

407

Strongly bound excitons in gapless two-dimensional structures

NASA Astrophysics Data System (ADS)

Common wisdom asserts that bound excitons cannot form in high-dimensional (d>1) metallic structures because of their overwhelming screening and the unavoidable resonance with nearby continuous bands. Strikingly we illustrate that this prevalent assumption is not quite true. A key ingredient has been overlooked: Destructive coherent effects are capable of thwarting the formation of resonance. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure that exhibits metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.

Liang, Yufeng; Soklaski, Ryan; Huang, Shouting; Graham, Matthew W.; Havener, Robin; Park, Jiwoong; Yang, Li

2014-09-01

408

Density Functional Study of Tetraphenylporphyrin Long-Range Exciton Coupling

The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intensity is consistent with TPP separations of over 40 Å. It is found that a hybrid functional with fully long-range corrected range-separated exchange performs best for full TDDFT calculations of the dimer. The range-separation parameter is optimally tuned for TPP, resulting in a good quality TPP absorption spectrum and small DFT delocalization error (measured by the curvature of the energy calculated as a function of fractional electron numbers). Calculated TDDFT data for the absorption spectra of TPP are also used as input for a ‘matrix method’ (MM) model of the exciton CD. For long-range exciton CD, comparison of MM spectra with full TDDFT CD spectra for the dimer shows that the matrix method is capable of producing very accurate results. A MM spectrum obtained from TPP absorption data calculated with the nonhybrid Becke88–Perdew86 (BP) functional is shown to match the experimental brevetoxin spectrum ‘best’, but for the wrong reasons. PMID:24551508

Moore, Barry; Autschbach, Jochen

2012-01-01

409

Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes

Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.

Harigaya, Kikuo

2007-01-01

410

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.

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

411

Origin of the Variation of Exciton Binding Energy in Semiconductors Marc Dvorak,1

Origin of the Variation of Exciton Binding Energy in Semiconductors Marc Dvorak,1 Su-Huai Wei,2, and the exciton binding energy Eb in technologically important semiconductors varies from merely a few me between the electronic band structures and exciton binding energies in semiconductors, employing first

Wu, Zhigang

412

Dynamic Monte Carlo modeling of exciton dissociation in organic donor-acceptor solar cells

Dynamic Monte Carlo modeling of exciton dissociation in organic donor- acceptor solar cells Michael://jcp.aip.org/about/rights_and_permissions #12;THE JOURNAL OF CHEMICAL PHYSICS 137, 014903 (2012) Dynamic Monte Carlo modeling of exciton 2012; published online 6 July 2012) A general dynamic Monte Carlo model for exciton dissociation

Dhinojwala, Ali

413

Optical and Excitonic Properties of Crystalline ZnS Nanowires: Toward Efficient Ultraviolet

semiconductor, has been most widely used for phosphor host,5 optical coating, and solar cells.6 Due to the wide transition from free exciton A, free exciton B, and shallow level emission are observed and analyzed through indicate that free excitons in ZnS nanowires are very stable, suggesting a great promise for high

Xiong, Qihua

414

Vibrational excitons in -helical polypeptides: Multiexciton self-trapping and related infrared on the multiexciton expansion of a model Hamiltonian, an accurate quantum-dynamical description of vibrational states propagation of the exciton-chain vibrational wave function. The formation of self-trapped exciton states

RÃ¶der, Beate

415

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

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 tÃ©tracÃ¨ne cristallin J

Paris-Sud XI, UniversitÃ© de

416

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

Boyer, Edmond

417

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

418

Study of exciton dynamics in garnets by low temperature thermo-luminescence

Study of exciton dynamics in garnets by low temperature thermo- luminescence D. T. Mackay, C. R://jap.aip.org/about/rights_and_permissions #12;Study of exciton dynamics in garnets by low temperature thermo-luminescence D. T. Mackay, C. R July 2012) Shallow traps that affect exciton dynamics in undoped and Ce doped yttrium aluminum garnet

Collins, Gary S.

419

The femtosecond transient absorption of phenylene-based polymers at low intensities is dominated by singlet excitons. In films, inter-chain excitons are created at high intensities via two-exciton states. In solution, the authors observe signatures of stable biexcitons.

McBranch, D.; Klimov, V.; Kraabel, B.

1998-03-01

420

Experimental Test of Theoretical Models for Urbach's Rule at Excitonic Absorption Edges

It is shown that electroabsorption measurements provide an elegant test to decide between the two most favored theories of Urbach's rule at excitonic absorption edges, namely the model of the field-ionized exciton by Dow and Redfield and the model of the momentarily trapped exciton by Sumi and Toyozawa. Experiments performed at the intrinsic Urbach tails of the ionic insulators CuCl

E. Mohler; B. Thomas

1980-01-01

421

21 CFR 886.1450 - Corneal radius measuring device.

Code of Federal Regulations, 2012 CFR

...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...

2012-04-01

422

21 CFR 886.1450 - Corneal radius measuring device.

Code of Federal Regulations, 2011 CFR

...2011-04-01 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...

2011-04-01

423

21 CFR 886.1450 - Corneal radius measuring device.

Code of Federal Regulations, 2013 CFR

...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...

2013-04-01

424

Radius Constants for Analytic Functions with Fixed Second Coefficient

Let f(z) = z + ?n=2?anzn be analytic in the unit disk with the second coefficient a2 satisfying |a2 | = 2b, 0 ? b ? 1. Sharp radius of Janowski starlikeness is obtained for functions f whose nth coefficient satisfies |an | ? cn + d??(c, d ? 0) or |an | ? c/n??(c > 0??and??n ? 3). Other radius constants are also obtained for these functions, and connections with earlier results are made. PMID:25101327

Nargesi, Mahnaz M.; Ali, Rosihan M.; Ravichandran, V.

2014-01-01

425

Radius of Curvature of Off-Axis Paraboloids

NASA Technical Reports Server (NTRS)

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.

Robinson, Brian; Reardon, Patrick; Hadaway, James; Geary, Joseph; Russell, Kevin (Technical Monitor)

2002-01-01

426

Striated nozzle flow with small radius of curvature ratio throats

NASA Technical Reports Server (NTRS)

A theory is presented that makes it possible to apply one-dimensional techniques to compressible striated nozzle flow with small radius of curvature ratio throats. The throat plane solution is shown to approximate the two-dimensional axisymmetric flowfield and can be used for analyzing the throat plane in converging-diverging type nozzles with striated flow for radius of curvature throat ratios down to 0.25.

Norton, D. J.; White, R. E.

1973-01-01

427

Biexcitonic molecules survive excitons at the Mott transition.

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton. PMID:25341721

Shahmohammadi, Mehran; Jacopin, Gwénolé; Rossbach, Georg; Levrat, Jacques; Feltin, Eric; Carlin, Jean-François; Ganière, Jean-Daniel; Butté, Raphaël; Grandjean, Nicolas; Deveaud, Benoit

2014-01-01

428

Impurity effects on polaron-exciton formation in conjugated polymers

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.

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

429

Tuning of exciton states in a magnetic quantum ring

NASA Astrophysics Data System (ADS)

The exciton states in a CdTe quantum ring subjected to an external magnetic field containing a single magnetic impurity are investigated. We have used the multiband approximation which includes the heavy hole-light hole coupling effects. The electron-hole spin interactions and the s, p-d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin interactions it is possible to change the bright exciton state into the dark state and vice versa with the help of a magnetic field. We propose a new route to experimentally estimate the s, p-d spin interaction constants.

Ghazaryan, Areg; Manaselyan, Aram; Chakraborty, Tapash

2015-02-01

430

Probing Excitonic Dark States in Single-layer Tungsten Disulfide

Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an important two-dimensional semiconductor with promising potentials for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDC has a sizable band gap. More interestingly, when thinned down to a monolayer, TMDC transforms from an indirect bandgap to a direct bandgap semiconductor, 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. The observed optical resonance was initially considered to be band-to-band transitions. In contrast, first-principle calculations predicted a much larger quasiparticle band gap size and an optical response that is dominated by excitonic effects. Here, we report experimental evidence of the exciton dominance mechanism by discovering a series of exc...

Ye, Ziliang; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang

2014-01-01

431

Biexcitonic molecules survive excitons at the Mott transition

NASA Astrophysics Data System (ADS)

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron–hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton.

Shahmohammadi, Mehran; Jacopin, Gwénolé; Rossbach, Georg; Levrat, Jacques; Feltin, Eric; Carlin, Jean-François; Ganière, Jean-Daniel; Butté, Raphaël; Grandjean, Nicolas; Deveaud, Benoit

2014-10-01

432

Quantum confinement-induced tunable exciton states in graphene oxide.

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

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

433

Quantum confinement-induced tunable exciton states in graphene oxide

NASA Astrophysics Data System (ADS)

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.

Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin

2013-07-01

434

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

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

Berkelbach, Timothy C; Hybertsen, Mark S; Reichman, David R

2013-03-21

435

Microscopic theory of singlet exciton fission. I. General formulation

NASA Astrophysics Data System (ADS)

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.

Berkelbach, Timothy C.; Hybertsen, Mark S.; Reichman, David R.

2013-03-01

436

Profluorescent protease substrates: intramolecular dimers described by the exciton model.

Xanthene dyes are known to form dimers with spectral characteristics that have been interpreted in terms of exciton theory. A unique aspect of H-type dimers is the fluorescence quenching that accompanies their formation. Using the principles of exciton theory as a guide, a series of protease substrates was synthesized with a xanthene dye on each side of the cleavage site. To bring the attached dyes into spatial proximity to form a dimer, the molecular design included structure determinant regions in the amino acid sequence. In addition, chromophores were chosen such that changes in absorption spectra indicative of exciton splitting were anticipated. Cleavage of the peptides by a protease resulted in disruption of the dimers and indeed significant absorption spectral changes were observed. Furthermore, substrate cleavage was accompanied by at least an order of magnitude increase in fluorescence intensity. This has allowed determination of intracellular elastase activity using a fluorescence microscope equipped with standard optics. Images Fig. 3 Fig. 6 PMID:8876189

Packard, B Z; Toptygin, D D; Komoriya, A; Brand, L

1996-01-01

437

Measurement and modification of biexciton-exciton time correlations.

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

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

2013-04-22

438

Exciton-assisted optomechanics with suspended carbon nanotubes

NASA Astrophysics Data System (ADS)

We propose a framework for inducing strong optomechanical effects in a suspended carbon nanotube based on deformation potential (DP) exciton-phonon coupling. The excitons are confined using an inhomogeneous axial electric field which generates optically active quantum dots with a level spacing in the milli-electronvolt range and a characteristic size in the 10 nm range. A transverse field induces a tunable parametric coupling between the quantum dot and the flexural modes of the nanotube mediated by electron-phonon interactions. We derive the corresponding excitonic DPs and show that this interaction enables efficient optical ground-state cooling of the fundamental mode and could allow us to realize the strong and ultra-strong coupling regimes of the Jaynes-Cummings and Rabi models.

Wilson-Rae, I.; Galland, C.; Zwerger, W.; Imamo?lu, A.

2012-11-01

439

Excitons recombination engineering in self assembled quantum dots

NASA Astrophysics Data System (ADS)

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.

Petroff, Pierre

2000-03-01

440

Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics

NASA Astrophysics Data System (ADS)

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.

Scully, Shawn Ryan

441

Characteristics of exciton polaritons in a ZnO microcavity

NASA Astrophysics Data System (ADS)

We have investigated the characteristics of exciton polaritons in a ZnO microcavity with HfO 2/SiO 2 distributed Bragg reflectors. The results of the angle-resolved reflectance spectra were analyzed by calculating the cavity polariton dispersions with a phenomenological Hamiltonian for the coupling between the cavity photon and three kinds of excitons labeled A, B, and C peculiar to ZnO. The vacuum-Rabi-splitting energy is estimated to be ?80 meV. The reflectance dips originating from the cavity polaritons shift to lower energy side with an increase in temperature. We discuss the temperature dependence of the cavity-polariton energies of the ZnO microcavity on the basis of the phenomenological Hamiltonian taking account of the temperature dependence of the exciton energies with Varshni’s law.

Kawase, Toshiki; Komura, Shingo; Miyazaki, Kenichi; Kim, DaeGwi; Nakayama, Masaaki

2010-09-01

442

Long-term Periodicity Variations of the Solar Radius

NASA Astrophysics Data System (ADS)

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 ~1 yr, ~2.6 yr, ~3.6 yr, and ~11 yr. Possible mechanisms for these periods are discussed. The possible physical cause of the ~11 yr period is the cyclic variation of the magnetic pressure of the concentrated flux tubes at the bottom of the solar convection zone.

Qu, Z. N.; Xie, J. L.

2013-01-01

443

LONG-TERM PERIODICITY VARIATIONS OF THE SOLAR RADIUS

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.

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

444

Relaxation of exciton and photoinduced dimerization in crystalline C60

NASA Astrophysics Data System (ADS)

We numerically investigate the lattice relaxation of photogenerated exciton in crystalline C60 so as to clarify the mechanism of the photoinduced dimerization processes in this material. In our theory, we deal with the ? electrons together with the interatomic effective potentials. Calculations are mainly based on the mean-field theory for interelectron interactions but are also reinforced by taking the electron-hole correlation into account, so that we can obtain the exciton effect. Using a cluster model, we calculate the adiabatic potential energy surfaces of the excitons relevant to the photoinduced dimerization processes occurring in a face-centered-cubic crystal of C60. The potential surfaces of the Frenkel excitons turned out to be quite uneven with several energy minimum points during the structural changes from the Franck-Condon state to the dimerized state. This leads to the conclusion that various structural defects exist at low temperatures even in the single crystal, as an intrinsic property of this molecular crystal with a complicated intermolecular interaction. From the analysis of the potential surfaces of the charge-transfer (CT) excitons, it is confirmed that the CT exciton relaxes down to its self-trapped state, wherein the adjacent two molecules get close together. This implies that the CT between adjacent two molecules is one of mechanisms that triggers the photodimerization or the photopolymerization. The oscillator strength distributions are also calculated for various intermediate structures along the lattice relaxation path. As the dimerization reaction proceeds, the oscillator strength grows in the energy region below the fundamental absorption edge, and the lowest-energy peak, originally at about 1.9 eV, finally shifts down to about 1.7 eV in the final dimerized structure. These results clarify the electronic origins of the luminescence observed in the C60 single crystal. Moreover, the origins of the photoinduced absorption spectra observed by Bazhenov, Gorbunov, and Volkodav are elucidated by characteristics of the adiabatic potential energy surfaces obtained here.

Suzuki, Masato; Iida, Takeshi; Nasu, Keiichiro

2000-01-01

445

Evidence of Hybrid Excitons in Weakly Interacting Nanopeapods

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

2013-01-01

446

Parametric relaxation in whispering gallery mode exciton-polariton condensates

NASA Astrophysics Data System (ADS)

Polariton condensation of one-dimensional multimode whispering gallery mode exciton polaritons is investigated in hexagonal ZnO microwires at cryogenic temperatures. At threshold, stimulated scattering is fed by the resonant emission from highly populated defect-bound excitons. With further increasing excitation power, condensates relax within the multimode whispering gallery mode polariton ladder, leading to their effective evaporative cooling. The relaxation is a parametric polariton-polariton scattering process evidenced by its intensity, energy, and momentum evolution. The experimental observations are in good agreement with numerical simulations based on a model for polariton-polariton scattering extended to the multimodal whispering gallery mode system.

Dietrich, C. P.; Johne, R.; Michalsky, T.; Sturm, C.; Eastham, P.; Franke, H.; Lange, M.; Grundmann, M.; Schmidt-Grund, R.

2015-01-01

447

Plasmon and exciton superconductivity mechanisms in layered structures

NASA Technical Reports Server (NTRS)

Plasmon and exciton superconductivity mechanisms are discussed. Superconductivity in a three layer metal semiconductor metal and insulator semimetal insulator sandwich structure was described in terms of the temperature dependent Green function of the longitudinal (Coulomb) field. The dependences of the superconducting transition temperature on structure parameters were obtained. In a semiconducting film, as a result of interactions of degenerate free carriers with excitons, superconductivity exists only in a certain range of parameter values, and the corresponding critical temperature is much lower than in the plasmon mechanism of superconductivity.

Gabovich, A. M.; Pashitskiy, E. A.; Uvarova, S. K.

1977-01-01

448

Resolving multi-exciton generation by attosecond spectroscopy.

We propose an experimentally viable attosecond transient absorption spectroscopy scheme to resolve controversies regarding multiexciton (ME) generation in nanoscale systems. Absence of oscillations indicates that light excites single excitons, and MEs are created by incoherent impact ionization. An oscillation indicates the coherent mechanism, involving excitation of superpositions of single and MEs. The oscillation decay, ranging from 5 fs at ambient temperature to 20 fs at 100 K, gives the elastic exciton-phonon scattering time. The signal is best observed with multiple-cycle pump pulses. PMID:25401661

Neukirch, A J; Neumark, D M; Kling, M F; Prezhdo, O V

2014-10-20

449

Crossed excitons in a semiconductor nanostructure of mixed dimensionality

NASA Astrophysics Data System (ADS)

Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of "crossed" excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.

Owschimikow, Nina; Kolarczik, Mirco; Kaptan, Yücel I.; Grosse, Nicolai B.; Woggon, Ulrike

2014-09-01

450

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

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

Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H

2013-06-18

451

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.

Norman F. Berk

1978-01-01

452

Using a modified version of the Su-Schrieffer-Heeger (SSH) model combined with the extended Hubbard model (EHB), the recombination between a singlet exciton pair is investigated under the influence of an external electric field, electron-electron interactions, and temperature effects in the scope of a nonadiabatic evolution method. The excitons are positioned very close to each other in a way to mimic a high-density region in monomolecular conjugated polymer systems. Results show that there are mainly three possible channels resulting from singlet-singlet exciton recombination: (1) forming an excited negative polaron and an excited positive bipolaron, (2) forming two free and excited oppositely charged polarons, and (3) forming a biexciton. These results suggest that the recombination processes critically depends on the condition imposed to the system. The description of this dependence, as carried out in the present work, may provide guidance to improve the generation of free charge carriers in organic optoelectronic devices. PMID:24754540

Ribeiro Junior, Luiz Antonio; da Cunha, Wiliam Ferreira; E Silva, Geraldo Magela

2014-05-15

453

Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes.

Stable Au nanoshell-J-aggregate complexes are formed that exhibit coherent coupling between the localized plasmons of a nanoshell and the excitons of molecular J-aggregates adsorbed on its surface. By tuning the nanoshell plasmon energies across the exciton line of the J-aggregate, plasmon-exciton coupling energies for these complexes are obtained. The strength of this interaction is dependent on the specific plasmon mode of the nanoparticle coupled to the J-aggregate exciton. From a model based on Gans theory, we obtain an expression for the plasmon-exciton hybridized states of the complex. PMID:18729410

Fofang, Nche T; Park, Tae-Ho; Neumann, Oara; Mirin, Nikolay A; Nordlander, Peter; Halas, Naomi J

2008-10-01

454

We report on the principle and realization of a new trap for excitons--the diamond electrostatic trap--which uses a single electrode to create a confining potential for excitons. We also create elevated diamond traps which permit evaporative cooling of the exciton gas. We observe the collection of excitons towards the trap center with increasing exciton density. This effect is due to screening of disorder in the trap by the excitons. As a result, the diamond trap behaves as a smooth parabolic potential which realizes a cold and dense exciton gas at the trap center. PMID:19792761

High, A A; Thomas, A K; Grosso, G; Remeika, M; Hammack, A T; Meyertholen, A D; Fogler, M M; Butov, L V; Hanson, M; Gossard, A C

2009-08-21

455

The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti17(?4-O)4(?3-O)16(?2-O)4(OPr(i))20 (in short Ti17) was studied using femtosecond pump-probe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17(•-), are blue-shifted in comparison with bulk TiO2 and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17* and Ti17(•-) indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the ?1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to play a special role in this regard. Solvent polarity has only a minor influence on the spectral behavior of Ti17*. Exciton recombination in Ti17 is faster than in anatase nanoparticles or mesoporous films. The kinetics exhibits three components, ranging from less than 1 ps to 100 ps, which are tentatively assigned to the geminate recombination within the core of the cluster and to the decay of the surface stabilized charge transfer exciton. A persistent long-lived component with ? > 300 ps may indicate the involvement of intraband dark states, i.e., triplet excitons (3)Ti17*. PMID:23113586

Bao, Jianhua; Yu, Zhihao; Gundlach, Lars; Benedict, Jason B; Coppens, Philip; Chen, Hung Cheng; Miller, John R; Piotrowiak, Piotr

2013-04-25

456

A model of a second-order shape phase transition is investigated in the Bohr collective model. The model contains two variable parameters, a mass parameter M and a control parameter ?, and is such that when ?=0 the Hamiltonian is that of a harmonic spherical vibrator and when ? is large it approaches that of an adiabatically decoupled rotor-vibrator. The results

P. S. Turner; D. J. Rowe

2005-01-01

457

Exciton spin relaxation in resonantly excited CdTe/ZnTe self-assembled quantum dots

We study the exciton spin relaxation in CdTe self-assembled quantum dots by using polarized photoluminescence spectroscopy in magnetic field. The experiments on single CdTe quantum dots and on large quantum dot ensembles show that by combining phonon-assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of quantum dots. We find that for single symmetric quantum dots at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by quantum dot shape asymmetry. Similar behavior is found in a large ensemble of CdTe quantum dots. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a quantum dot become non-degenerate. Finally, due to strong electronic confinement in CdTe quantum dots, the large spin polarization of excitons shows no dependence on the number of phonons emitted between the virtual state and the exciton ground state during the excitation.

S. Mackowski; T. A. Nguyen; T. Gurung; K. Hewaparkarama; H. E. Jackson; L. M. Smith; J. Wrobel; K. Fronc; J. Kossut; G. Karczewski

2003-07-28

458

Recovering lost excitons in organic photovoltaics using a transparent dissociation layer

NASA Astrophysics Data System (ADS)

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that "reflect" excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

Barito, A.; Sykes, M. E.; Bilby, D.; Amonoo, J.; Jin, Y.; Morris, S. E.; Green, P. F.; Kim, J.; Shtein, M.

2013-05-01

459

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.

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

460

Coherent optical writing and reading of the exciton spin state in single quantum dots.

We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one-to-one mapping between any point on the Poincaré sphere of the light polarization to a point on the Bloch sphere of the exciton spin. PMID:21405314

Benny, Y; Khatsevich, S; Kodriano, Y; Poem, E; Presman, R; Galushko, D; Petroff, P M; Gershoni, D

2011-01-28

461

Excitonic Side Bands of Inner-Shell Excitations in Rare Gas Solids

NASA Astrophysics Data System (ADS)

Valence-exciton luminescence under inner-shell excitation of the rare gas solids Xe, Kr, and Ar has been measured using time-resolved photoluminescence. Two different processes for exciton creation can be distinguished: creation of ``prompt'' excitons immediately after excitation (within the experimental time resolution), and creation of ``delayed'' excitons through electron-hole recombination. The decay structure of the exciton emission in the range of inner-shell excitation is characterized by the coexistence of the two processes. Time-resolved excitation spectra near the 2p edge in Ar, the 3d edge in Kr, and the 4d edge in Xe are discussed. The process of prompt exciton creation is strongly enhanced above an excitation threshold at the energy position of the ionization limit of the core state plus the energy of the valence free exciton.

Vielhauer, S.; Kirm, M.; Kisand, V.; Negodin, E.; Sombrowski, E.; Steeg, B.; Zimmerer, G.

462

Photophysics of pentacene thin films: The role of exciton fission and heating effects

NASA Astrophysics Data System (ADS)

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.

Rao, Akshay; Wilson, Mark W. B.; Albert-Seifried, Sebastian; di Pietro, Riccardo; Friend, Richard H.

2011-11-01

463

ON THE CONSTANCY OF THE SOLAR RADIUS. III

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.

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

464

The Radius Distribution of Planets around Cool Stars

NASA Astrophysics Data System (ADS)

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.

Morton, Timothy D.; Swift, Jonathan

2014-08-01

465

The Excitonic Effect in the Optical Spectrum of Semiconductors

NASA Astrophysics Data System (ADS)

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 strength than measured, but gives an E(,2) peak with more strength than the measured one. We investigate the excitonic effect in the optical spectrum using a simple model of multi-plane wave representation, which contains the relevant band structure features of this entire class of semiconductors. Solution of the Bethe -Salpeter equation yields the absorption spectra in silicon and germanium which accounts for the discrepancy between the measured strength using the one-electron band structure. The electron-hole states which contribute to E(,2) also form a bound state below the E(,2) edge, which is broadened by interaction with the states from the E(,1) continuum. Based on our results for Si and Ge, we discuss how the excitonic effect shifts the strength of the spectrum from the E(,2) peak to the E(,1) peak. Our conclusions regarding the excitonic effects on these peaks can be extended to the III-V and II-VI compounds.

del Castillo Mussot, Marcelo

466

Spatio-temporal dynamics of quantum-well excitons

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 and 5 ps, respectively...

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

2003-01-01

467

Advanced theory of multiple exciton generation effect in quantum dots

NASA Astrophysics Data System (ADS)

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.

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

2012-06-01

468

Organic photosensitive optoelectronic device having a phenanthroline exciton blocking layer

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.

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

2011-02-22

469

Superradiant exciton-light coupling in semiconductor heterostructures—Theory

NASA Astrophysics Data System (ADS)

Superradiant coupling of excitons in multiple quantum well Bragg structures is investigated. The superradiance leads to characteristic signatures in the spectral and time-resolved optical response. It is shown, that the superradiant mode gradually vanishes for increasing excitation intensity and for larger static disorder due to excitation induced dephasing or dephasing trough static disorder, respectively.

Stroucken, T.; Haas, S.; Grote, B.; Koch, S. W.; Hübner, M.; Ammerlann, D.; Kuhl, J.

470

Exciton condensation in microcavities under three-dimensional quantization conditions

The dependence of the spectra of the polarized photoluminescence of excitons in microcavities under conditions of three-dimensional quantization on the optical-excitation intensity is investigated. The cascade relaxation of polaritons between quantized states of a polariton Bose condensate is observed.

Kochereshko, V. P., E-mail: Vladimir.Kochereshko@mail.ioffe.ru; Platonov, A. V. [Russian Academy of Sciences, Ioffe Physical-Technical Institute (Russian Federation)] [Russian Academy of Sciences, Ioffe Physical-Technical Institute (Russian Federation); Savvidis, P.; Kavokin, A. V. [St. Petersburg State University, Spin-Optics Laboratory (Russian Federation)] [St. Petersburg State University, Spin-Optics Laboratory (Russian Federation); Bleuse, J.; Mariette, H. [CNRS, Institut Neel (France)] [CNRS, Institut Neel (France)

2013-11-15

471

Effect of strain on GaN exciton spectra

NASA Astrophysics Data System (ADS)

The large majority of GaN epitaxial films have been grown on substrates which show both lattice and thermal expansion mismatch and, as a result, are significantly strained. The effect is most readily monitored by measuring exciton energies in photoluminescence spectra, and this paper compares published data on free exciton energies in a homoepitaxial film with a range of heteroepitaxial samples grown on sapphire substrates, all the films having the hexagonal (wurtzite) structure. We observe that strain results in a marked increase in separation of the B and C excitons while leaving the A, B separation relatively unaffected, and interpret this in terms of the quasi-cubic band structure model developed by Hopfield for the very similar case of CdS and other II - VI compounds. Within the limits of this model, there is an ambiguity in the interpretation of exciton energies which depends on the fact that the spin - orbit and crystal field splitting parameters are interchangeable. Consideration of strain effects removes this ambiguity and allows us to derive values of 0268-1242/11/7/009/img1 meV and 0268-1242/11/7/009/img2 meV for the unstrained homoepitaxial sample, consistent with the earlier estimates of Dingle et al based on relative intensity measurements.

Orton, J. W.

1996-07-01

472

Singlet fission in pentacene through multiple exciton quantum states

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

Zhiyong Zhang; Paul Zimmerman; Charles Musgrave

2010-01-01

473

Singlet Fission and Multi-Exciton Generation in Organic Systems

NASA Astrophysics Data System (ADS)

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.

Musgrave, Charles

2012-02-01

474

Singlet fission in pentacene through multiple exciton quantum states

NASA Astrophysics Data System (ADS)

Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states have single-exciton character, singlet fission to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple electron-hole pairs. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through a dark state (D) of ME character that lies just below S1, satisfies the fission energy requirement (ED>2ET0), and splits into two triplets (2xT0). In tetracene, D lies just above S1, consistent with the observation that singlet fission is thermally activated in tetracene. Rational design of photovoltaic systems that exploit singlet fission will require ab initio analysis of ME states such as D.

Zhang, Zhiyong; Zimmerman, Paul; Musgrave, Charles

2010-03-01

475

Ultrafast exciton dissociation at donor/acceptor interfaces

NASA Astrophysics Data System (ADS)

Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV) community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very intriguing their full understanding. In particular charge generation is believed to occur in < 200 fs, but no clear picture emerged so far. In this work we reveal for the first time the actual charge generation mechanism following in real time the exciton dissociation mechanism by means of sub-22 fs pump-probe spectroscopy. We study a low-band-gap polymer: fullerene interface as an ideal system for OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50 fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher fraction of free charges.

Grancini, G.; Fazzi, D.; Binda, M.; Maiuri, M.; Petrozza, A.; Criante, L.; Perissinotto, S.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.

2013-09-01

476

Electric Field Effects on Charged Excitons in Semiconductors

Effects of a uniform electric field on excitonic complexes X - and X+2 in semiconductors are studied by using a pure variational method. Wannier-Stark levels of the centre of mass motion are then obtained and ground state energies for both complexes X - and X+2 are computed by using a (3 × 34) term trial wave function. Our results show

E. Feddi; F. Dujardin; J. Diouri; A. Elhassani; M. Katih; B. Stebe

1997-01-01

477

Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar

Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells George F. Burkhard-acceptor interface. This result has implications for most state of the art organic solar cells, since all of the most efficient devices use fullerenes as electron acceptors. Since their inception, organic photovoltaics (OPVs

McGehee, Michael

478

Mass-radius relations for massive white dwarf stars

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.

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

2005-07-25

479

Attractor radius, a new determination criterion of predictability limit

NASA Astrophysics Data System (ADS)

Firstly, the definition of the attractor radius was given and then the property of that the attractor radius (AR) in a given n-dimensional attractor A is a constant was proved in theory. Secondly, the SV of the square of the RMS difference was separated into two components - the systematic error and the attractor radius, and it was proved that the observed global climatological RMS (OCR) difference is not equal to 71% of the SV of the RMS difference when the systematic error is existed, however, it is always equal to 71% of the AR. Then the physical understanding of the AR and also the predictability limit determinated by it were discussed. Finally, the spatial distributions of the predictability limit calculated from CFSv2 data by different criterions were compared.

Liu, Deqiang; Ding, Ruiqiang; Li, Jianping; Feng, Jie

2014-05-01

480

Hominid radius from the middle Pliocene of Lake Turkana, Kenya.

A nearly complete left radius, KNM-ER 20419, was recovered from middle Pliocene sediments east of Lake Turkana, Kenya in 1988. Ape-like characteristics of the fossil include an eccentrically positioned articular fovea, relatively long radial neck, wide distal metaphysis, and large brachioradialis crest. The robustness of the radial neck in proportion to the radial head, and the semilunar shape of the distal diaphysis, however, clearly distinguish KNM-ER 20419 as hominid. The distal articular surface possesses a larger area for radius-lunate articulation than for radius and scaphoid, a radiocarpal arrangement that is associated with increased wrist adduction among quadrumanous climbers. Since this morphology is also found in hylobatids, Pongo, and other early australopithecines, it is argued to be plesiomorphic for hominoids. This further supports the argument that vertical climbing was an important locomotor behavior among both early hominoids and our more immediate prebipedal ancestors. PMID:8273826

Heinrich, R E; Rose, M D; Leakey, R E; Walker, A C

1993-10-01

481

Stability of a Wheel with Various Radius Rim

NASA Astrophysics Data System (ADS)

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.

Kinugasa, Tetsuya; Yoshida, Koji

482

The PRad experiment and the proton radius puzzle

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.

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

2014-06-01

483

ELECTROMAGNETIC CONSTRUCTION OF A 1 KM-RADIUS RADIATION SHIELD

1 ELECTROMAGNETIC CONSTRUCTION OF A 1 KM-RADIUS RADIATION SHIELD Ganesh, B.A., Wanis, S A fundamental obstacle to building human settlements in orbit is the construction of the massive outer radiation shield. This problem is used to illustrate the relevance of a comprehensive plan in developing a Space

484

Refining the Radius-Luminosity Relationship for Active Galactic Nuclei

We have measured the host-galaxy starlight contribution to four lower-luminosity AGNs (NGC 3516, NGC 4593, IC 4329A, and NGC 7469). We include these objects with new broad line region measurements for NGC 4151 and NGC 4593 to present a revised version of the radius-luminosity relationship for AGNs.

Misty C. Bentz; Kelly D. Denney; Bradley M. Peterson; Richard W. Pogge

2007-02-23

485

Comparison of internal and external fixation of distal radius fractures

Background and purpose There is no consensus on the difference in effects of internal fixation (IF) and external fixation (EF) on outcomes for the treatment of distal radius fractures. We performed a meta-analysis of randomized clinical studies. Methods We searched the literature and included studies that compared the effects of IF and EF on the treatment of distal radius fractures. Statistically, we pooled patient data using standard meta-analytic methods. For the continuous variables, the weighted mean difference (WMD) was used. For dichotomous data, the relative risk (RR) was calculated. Results 10 studies were eligible for data extraction. The pooled data showed that compared with EF, IF led to statistically significantly better Disabilities of the Arm, Shoulder, and Hand (DASH) scores at 12 months postoperatively, recovery of forearm supination at 3 months, and restoration of volar tilt and radial inclination. IF using volar locking plates resulted in better DASH scores than EF at 3 and 6 months, but the trend diminished over time; at 12 months postoperatively, the scores were not statistically significant. Compared with EF, IF led to fewer minor surgical complications. Interpretation For surgical treatment of distal radius fractures, IF yields better functional outcomes, forearm supination, restoration of anatomic volar tilt and radial inclination, and fewer minor complications. The patients who received IF using volar locking plates for the treatment of distal radius recovered more quickly than did patients who received EF. PMID:23594247

2013-01-01

486

Finite Larmor radius flute mode theory with end loss

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.

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

487

A 4-Sphere With Noncentral Radius and its Instanton Sheaf

NASA Astrophysics Data System (ADS)

We build an SU(2)-Hopf bundle over a quantum toric four-sphere whose radius is noncentral. The construction is carried out using local methods in terms of sheaves of Hopf-Galois extensions. The associated instanton bundle is presented and endowed with a connection with anti-self-dual curvature.

Cirio, Lucio Simone; Pagani, Chiara

2015-02-01

488

Failure of dual radius hydroxyapatite-coated acetabular cups

INTRODUCTION: Many kind of hydroxyapatite-coated cups were used, with favorable results in short term studies; it was supposed that its use could improve osteointegration of the cup, enhancing thus stability and survivorship. The purpose of this study is to analyze the long term behavior of the hemispheric HA coated, Dual Radius Osteonics cup and to discuss the way of failure

Fabio D'Angelo; Mauro Molina; Giacomo Riva; Giovanni Zatti; Paolo Cherubino

2008-01-01

489

A Smaller Radius for the Transiting Exoplanet WASP-10b

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.

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

2008-11-29

490

The strength of polyaxial locking interfaces of distal radius plates

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.

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

2009-01-01

491

Nonlinear buckling analyses of a small-radius carbon nanotube

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.

Liu, Ning, E-mail: liuxiao@ase.buaa.edu.cn; Li, Min; Jia, Jiao [School of Aeronautic Science and Engineering, Beihang University, Beijing 100091 (China); Wang, Yong-Gang [Department of Applied Mechanics, China Agricultural University, Beijing 100083 (China)

2014-04-21

492

Computational Analysis of Dual Radius Circulation Control Airfoils

NASA Technical Reports Server (NTRS)

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.

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

2006-01-01

493

OBSERVATIONAL CONSTRAINTS ON THE DEGENERATE MASS-RADIUS RELATION

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.

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

494

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.