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1

Reconciliation of Values for Bohr Radius and Empirical Radius of H-Atom Using Nuclear Vibration Factor  

NASA Astrophysics Data System (ADS)

The value for the calculated H-atom radius, the Bohr value, is 5.29x10-11 m and the empirical value for that radius has been found to be 2.5x10-11 m. Since the nucleus is vibrating, the distance relation, d, between the nucleus and the electron is r + Acos2?ft = d, due to a slight lag time between nuclear vibration and orbiting electron repeatedly changing the distance between the vibrating nucleus and the electron. Therefore, the distance between vibrating nucleus and orbiting electron must only be an average distance. The average value for the cosine is the RMS value of 0.707. Substituting the calculated distance for r and the empirical distance for d, the equation becomes 5.29x10-11 m + (0.707)A = 2.5x10-11 m. Solving for the average amplitude of nuclear vibration, A, A=3.95x 10-11 m.

Brekke, Stewart

2011-10-01

2

Lateral induced dipole moment and polarizability of excitons in a ZnO single quantum disk  

NASA Astrophysics Data System (ADS)

The lateral Stark shift of an exciton confined in a single ZnO quantum thin disk of radius R was calculated using a variational approach within the two bands effective mass approximation. It is shown that the exciton has a non negligible induced dipole moment when an external electric field is applied mainly for electron-hole separation below to the 3D excitonic Bohr radius. The behavior of the exciton lateral Stark shift proves the existence of an important correlation between the polarizability and the induced dipole moment.

Dujardin, F.; Feddi, E.; Oukerroum, A.; Bosch Bailach, J.; Martínez-Pastor, J.; Assaid, E.

2013-02-01

3

Exciton binding energy in semiconductor quantum dots  

SciTech Connect

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

4

Effect of simultaneous excitation of singlet and triplet excitons on the operation of organic solar cells  

NASA Astrophysics Data System (ADS)

Primary steps of exciton formation in organic solar cells are presented here. The rates of absorption of photons to excite singlet and triplet excitons are derived using exciton-photon and exciton-spin-orbit-photon-interaction, respectively, as perturbation operators. In both singlet and triplet absorptions, the rates are found to depend on the absorption energy, excitonic Bohr radius, and the dielectric constant of the donor organic material. Incorporation of heavy metal atoms enhances the exciton-spin-orbit-photon interaction and hence the rate of excitation of triplet excitons because it depends on the square of the heaviest atomic number. The new exciton-spin-orbit-photon interaction operator flips the spin to a singlet form leading to faster dissociation into charge carriers and resulting in higher photon to electron-hole pair conversion efficiency in organic solar cells.

Narayan, Monishka Rita; Singh, Jai

2013-10-01

5

Time-resolved spectra and kinetics of the exciton photoluminescence in different types of GaAs\\/AlAs superlattices  

Microsoft Academic Search

The temperature dependence of the recombination dynamics of excitons is investigated by time-resolved photoluminescence spectroscopy in(GaAs)n\\/(AlAs)n superlattices, where n denotes the layer thickness in monolayers, for different types of band structures. In direct-gap superlattices with a layer thickness of the order or larger than the exciton Bohr radius, the carrier dynamics is dominated by the transfer from light-hole to heavy-hole

S. Krylyuk; V. G. Litovchenko; D. V. Korbutyak; H. T. Grahn; K. H. Ploog

2001-01-01

6

Nature of one-dimensional excitons in polysilanes  

NASA Astrophysics Data System (ADS)

One-dimensional (1D) exciton states have been studied by nonlinear optical spectroscopy on organosilicon polymer polysilanes (PS's). From systematic variation of the linear and nonlinear optical spectra upon changing the backbone conformation, the following two characteristic exciton parameters have been evaluated; the ratio \\|?(3)\\|/? of the modulus of third-order nonlinear optical susceptibility \\|?(3)\\| to the absorption coefficient ? and the energy difference ?E12 between the lowest and second lowest exciton energies E1 and E2, which are theoretically related, respectively, to the Bohr radius and to the binding energy of excitons. It was found that \\|?(3)\\|/? increases, whereas ?E12 remains almost unchanged, with increasing valence and conduction-band widths. These features cannot be accounted for either by the Wannier- or Frenkel-type exciton model. From analysis by a unified 1D exciton model, it has been concluded that the 1D excitons in PS's represent a unique system having an intermediate character in between the Frenkel- and Wannier-exciton regimes, where the on-site Coulomb (or exchange) interactions play a crucial role in determining the characteristic exciton structures as observed.

Hasegawa, T.; Iwasa, Y.; Koda, T.; Kishida, H.; Tokura, Y.; Wada, S.; Tashiro, H.; Tachibana, H.; Matsumoto, M.

1996-10-01

7

Tribute to Niels Bohr.  

National Technical Information Service (NTIS)

This report is organized around the contribution of A. Pais, 'Niels Bohr and the development of physics', presented at CERN on the occasion of a special colloquium in the honour of Niels Bohr. It gives a broad survey of Bohr's life, the revolution in phys...

1985-01-01

8

Bohr's power series theorem in several variables  

Microsoft Academic Search

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

"Bohr's Atomic Model."  

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

10

Influence of the image charge effect on excitonic energy structure in organic-inorganic multiple quantum well crystals  

NASA Astrophysics Data System (ADS)

We have experimentally compared the excitonic properties of hybrid multiple quantum wells, (C6H5-C2H4NH3)2PbBr4 and (C4H9NH3)2PbBr4, using photoluminescence, reflection, and photoluminescence excitation measurements. We focused on the contribution of the image charge effect (ICE) to the excitonic energy structure in these materials which have different dielectric constants of the barrier layers. We have found that the binding energies of the 2s and 3s excitons are considerably enhanced by ICE, while the contribution of ICE to the 1s excitons is smaller because of the small Bohr radius, which is comparable to the well width.

Takagi, Hidetsugu; Kunugita, Hideyuki; Ema, Kazuhiro

2013-03-01

11

Nondispersing Bohr Wave Packets  

SciTech Connect

Long-lived, nondispersing circular, or Bohr, wave packets are produced starting from Li Rydberg atoms by exposing them first to a linearly polarized microwave field at the orbital frequency, 17.6 GHz at principal quantum number n=72, which locks the electron's motion into an approximately linear orbit in which the electron oscillates in phase with the microwave field. The microwave polarization is changed to circular polarization slowly compared to the orbital frequency, and the electron's motion follows, resulting in a nondispersing Bohr wave packet.

Maeda, H. [Department of Physics, University of Virginia, Charlottesville, Virginia 22904-0714 (United States); PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012 (Japan); Gurian, J. H.; Gallagher, T. F. [Department of Physics, University of Virginia, Charlottesville, Virginia 22904-0714 (United States)

2009-03-13

12

On solvable Bohr Hamiltonians  

Microsoft Academic Search

The sextic oscillator is discussed as a two-parameter potential for which the Bohr Hamiltonian can be solved analytically in a number of situations. In the simplest case it can be considered as a gamma-independent potential, which can have a minimum at beta = 0 and\\/or at beta > 0. Preliminary results are presented from a study in which the potential

G. Lévai

2004-01-01

13

Bohr Sommerfeld star products  

NASA Astrophysics Data System (ADS)

We relate the Bohr Sommerfeld conditions to formal deformation quantization of symplectic manifolds by classifying star products adapted to some Lagrangian submanifold L, i.e. products preserving the classical vanishing ideal IL of L up to IL-preserving equivalences.

Carl, Michael

2008-05-01

14

THE CENTENARY OF NIELS BOHR: Niels Bohr and quantum physics  

Microsoft Academic Search

The way of thinking and scientific style of Niels Bohr are discussed in connection with developments of his emotional and spiritual life. Analysis of the papers of Bohr, his predecessors, and his contemporaries reveals that he was a philosopher of physics who had an incomparable influence upon the creation and development of quantum mechanics. His struggle against nuclear weapons is

A. B. Migdal

1985-01-01

15

Niels Bohr and quantum physics  

Microsoft Academic Search

The way of thinking and scientific style of Niels Bohr are discussed in connection with developments of his emotional and spiritual life. Analysis of the papers of Bohr, his predecessors, and his contemporaries reveals that he was a philosopher of physics who had an incomparable influence upon the creation and development of quantum mechanics. His struggle against nuclear weapons is

A B Migdal

1985-01-01

16

Bohr as a Phenomenological Realist  

Microsoft Academic Search

There is confusion among scholars of Bohr as to whether he should be categorized as an instrumentalist (see Faye 1991) or a realist (see Folse 1985). I argue that Bohr is a realist, and that the confusion is due to the fact that he holds a very special view of realism,\\u000a which did not coincide with the philosophers’ views. His

Towfic Shomar

2008-01-01

17

New criteria for bosonic behavior of excitons  

Microsoft Academic Search

Dimensional arguments lead to say that N excitons in a sample of volume V behave as bosons for lambdaNax3\\/V ll 1, ax being the exciton radius and lambda a dimensionless factor. The Mott criterion, which is based on the disappearance of all exciton bound states because of screening, gives lambda simeq 1. Since excitons feel each other through both Coulomb

Monique Combescot; Christian Tanguy

2001-01-01

18

Excitonic spectrum of the ZnO/ZnMgO quantum wells  

SciTech Connect

Excitonic spectrum of the wurtzite ZnO/Zn{sub 1-x}Mg{sub x}O quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schroedinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.

Bobrov, M. A., E-mail: largaseal@gmail.com; Toropov, A. A.; Ivanov, S. V. [Russian Academy of Sciences, Ioffe Physical Technical Institute (Russian Federation); El-Shaer, A.; Bakin, A.; Waag, A. [TU Braunschweig, Institute of Semiconductor Technology (Germany)

2011-06-15

19

Magnetoabsorption of the lowest exciton in perovskite-type compound (CH3NH3)PbI3  

NASA Astrophysics Data System (ADS)

Magnetoabsorption spectra of (CH3NH3)PbI3 are investigated in the Faraday configuration up to 40 T at 4.2 K. The compound consists of three-dimensional networks of corner-sharing octahedra [PbI6]4-. The Zeeman splitting and the diamagnetic shift are observed, and the effective g-factor and the diamagnetic coefficient are 1.2 +/- 0.1 and (2.7 +/- 0.1) × 10-6 eV/T2, respectively. From the diamagnetic shift, the Bohr radius, the binding energy and the reduced mass of the exciton are estimated to be 28 Å, 37 meV and 0.12m0, respectively. The exciton in (CH3NH3)PbI3 is one of typical Wannier-type excitons with large radius, which is in contrast with the exciton in (C10H21NH3)2PbI4, consisting of two-dimensional networks of [PbI6]4-.

Hirasawa, M.; Ishihara, T.; Goto, T.; Uchida, K.; Miura, N.

1994-07-01

20

Electric Field of Bohr's Atom.  

National Technical Information Service (NTIS)

On the basis of the Lienard-Wiechert potential, it is shown that the electric field of the simplest Bohr atom oscillates though the mean (per period) value is equal to zero. The calculation with the use of the generally accepted presentation of the moving...

V. N. Strel'tsov

1994-01-01

21

A Simple Relativistic Bohr Atom  

ERIC Educational Resources Information Center

|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…

Terzis, Andreas F.

2008-01-01

22

The BOHR Effect before Perutz  

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

23

Atempts to link Quanta & Atoms before the Bohr Atom model  

NASA Astrophysics Data System (ADS)

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 electrons were distributed.He obtained expressions for the Rydberg constant and what is known today as the Bohr radius by balancing centrifugal energy with Coulomb energy and quantizing it with Planck's relation E=h?. We point out that Haas would have arrived at better estimates of these constants had he used the virial theorem apart from the fact that the fundamental constants were not well known. The crux of Haas's physical picture was to derive Planck's constant h from charge quantum e , mass of electron m and atomic radius. Haas faced severe criticism for applying thermodynamic concepts like Planck distribution to microscopic phenomena. We will try to give a flavor for how quantum phenomena were viewed at that time. It is of interest to note that the driving force behind Haas's work was to present a paper that would secure him a position as a Privatdozent in History of Physics. We end with comments by Bohr and Sommerfeld on Haas's work and with some brief biographical remarks.

Venkatesan, A.; Lieber, M.

2005-03-01

24

Magneto-optical effects of excitons in the layered perovskite-type material (C6H13NH3)2(CH3NH3)Pb2I7  

NASA Astrophysics Data System (ADS)

Magnetoabsorption spectra of (C6H13NH3)2(CH3NH3)Pb2I7, a layered perovskite-type material with ``bilayer'' [PbI6]-octahedra planes, measured under pulsed magnetic fields up to 41 T at 4.2 K show that its lowest-exciton Bohr radius is 6.9 Å. This is 35% longer than that of (C6H13NH3)2PbI4, a monolayer-well counterpart, and demonstrates the effect of the well-layer thickness.

Kataoka, Takeshi; Kondo, Takashi; Ito, Ryoichi; Sasaki, Satoshi; Uchida, Kazuhito; Miura, Noboru

1994-07-01

25

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

26

Exciton spectra of an AlN epitaxial film on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy  

NASA Astrophysics Data System (ADS)

Exciton resonance energies in an AlN epilayer on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral line shape was fitted considering A (Gamma7vu[right arrow]Gamma7c) and BC (Gamma]9v,[Gamma7vl[right arrow]Gamma7c) exciton transitions. The fitting gave the values of them at 0 K to be 6.211 and 6.266 eV, giving the crystal- field splitting (Deltacr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature ThetaE was estimated to be 580 K.

Onuma, T.; Chichibu, S. F.; Sota, T.; Asai, K.; Sumiya, S.; Shibata, T.; Tanaka, M.

2002-07-01

27

Crossover from strong to weak exciton confinement and third-harmonic generation on one-dimensional quantum dots  

NASA Astrophysics Data System (ADS)

We study the effects of exciton confinement on the nonlinear optical susceptibility of one-dimensional quantum dots. We use a direct numerical diagonalization to obtain the eigenenergies and eigenstates of the discretized Hamiltonian representing an electron-hole pair confined by a semiparabolic potential and interacting with each other via a Coulomb potential. Density matrix perturbation theory is used to compute the nonlinear optical susceptibilities due to third-harmonic generation and the corresponding nonlinear corrections to the refractive index and absorption coefficient. These quantities are analyzed as a function of ratio between the confinement length L and the exciton Bohr radius a0. The Coulomb potential degrades the uniformity of the level separation. We show that this effect promotes the emergence of multiple resonance peaks in the third-harmonic generation spectrum. In the weak confinement regime ? = L/a0 ? 1, the third-order susceptibility is shown to decay as 1/?8 due to the prevalence of the hydrogenoid character of the exciton eigenstates.

Bautista, Jessica E. Q.; Lyra, Marcelo L.; Lima, R. P. A.

2013-02-01

28

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

29

Magnetization Saturation of Exciton Magnetic Polarons in ZnMnSe/ZnSe Magnetic Quantum Wells in Small Magnetic Fields  

NASA Astrophysics Data System (ADS)

Exciton magnetic polarons (EMP) are excitons dressed by the local magnetization of the magnetic impurities. The EMP lower their energy by spontaneously aligning the magnetic impurities within the Bohr radius. In 14% ZnMnSe/ZnSe multiple quantum wells, the hole becomes confined to the ZnMnSe ``barriers'' while the electron remains in the ZnSe ``wells.'' This weak electron-hole overlap results in a 1000-fold increase in the exiton lifetime (10 ns). We use time-resolved photoluminescence spectroscopy to study the magnetization of spatially-indirect EMP in small magnetic fields. We find that the EMP polarization approaches 100% in fields as small as 200 mT, consistent with their extremely long recombination lifetime. This is in contrast with previous measurements which show that the polarization for short-lived spatially direct EMP never completely saturate.(I.A. Merkulov, D.R. Yakovlev, K.V. Kavokin, G. Mackh, W. Ossau, A. Waag, and G. Landwehr. JETP Letters 62 (4), 335 (1995).)

Robinson, L. M.; Smith, L. M.; Sun, T.-B.; Jonker, B. T.

1998-03-01

30

Realization of Localized Bohr-like Wavepackets  

SciTech Connect

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

31

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.

32

The boundary conditions for Bohr’s law: when is reacting faster than acting?  

Microsoft Academic Search

In gunfights in Western movies, the hero typically wins, even though the villain draws first. Niels Bohr (Gamow, The great physicists from Galileo to Einstein. Chapter: The law of quantum, 1988) suggested that this reflected a psychophysical law, rather than a dramatic conceit. He hypothesized that reacting is faster than acting. Welchman, Stanley, Schomers, Miall, and Bülthoff (Proceedings of the

Yaïr Pinto; Marte Otten; Michael A. Cohen; Jeremy M. Wolfe; Todd S. Horowitz

2011-01-01

33

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

34

Electrostatic conveyer for excitons  

NASA Astrophysics Data System (ADS)

We report on the realization of electrostatic conveyers for indirect excitons in GaAs/AlGaAs coupled quantum wells. The conveyer is a laterally moving lattice potential for excitons. Its amplitude and speed can be controlled by laterally modulated oscillating electrode voltages. We observed the exciton transport via the conveyer over several tens of microns and studied the transport as a function of the conveyer amplitude, exciton density, and exciton lifetime. We observed a dynamical localization-delocalization transition for the excitons in the conveyer with varying exciton density and conveyer amplitude: In the localization regime, excitons are moved by the conveyer, following the moving lattice potential, while in the delocalized regime, excitons do not follow the conveyer motion.

Winbow, A. G.; Leonard, J. R.; Remeika, M.; High, A. A.; Green, E.; Hammack, A. T.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2010-03-01

35

Are excitons really bosons?  

PubMed

Non-bosonic commutation relations of exciton operators discovered by Keldysh and Kozlov (1968 Zh. Eksp. Teor. Fiz. 54 978; 1968 Sov. Phys.-JETP 27 521 (Engl. Transl.)) led to a long-standing question about the possible difference between excitons and bosons in spite of the integer spin of excitons. In this paper the problem is analysed and it is shown that, in spite of the difference between separate excitons and bosons, the exciton gas is a Bose gas. Bosons comprising this gas are mixtures of separate excitons, and the non-bosonic nature of excitons leads only to a renormalization of the interaction between them. Features of the exciton luminescence coming from the interaction and non-bosonic nature of excitons are considered. PMID:21483066

Laikhtman, B

2007-06-11

36

Volume Spectrum from Bohr-Sommerfeld Quantization  

NASA Astrophysics Data System (ADS)

As first observed by Roger Penrose, angular momentum vectors can be used to describe geometrical objects, such as convex polyhedra. A remarkable outgrowth of this idea is that spaces of geometrical shapes can be endowed with a phase space structure. This allows all the tools of dynamical systems and semiclassical mechanics to be explored in the context of geometrical shapes. From the perspective of discrete approaches to gravity, such as the Regge Calculus where space is chopped into tetrahedral pieces, this opens fascinating new prospects. Here we present a discrete spectrum for the volume of a tetrahedron as obtained from Bohr-Sommerfeld quantization. We discuss connections with Loop Quantum Gravity and the implications for approximate treatments of more complex geometries.

Haggard, Hal; Bianchi, Eugenio

2011-04-01

37

Excitons in nanoscale systems.  

PubMed

Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes. PMID:16946728

Scholes, Gregory D; Rumbles, Garry

2006-09-01

38

Generalizations of Bohr inequality for Hilbert space operators  

Microsoft Academic Search

Let B(H) be the space of all bounded linear operators on a complex separable Hilbert space H. Bohr inequality for Hilbert space operators asserts that for A,B?B(H) and p,q>1 real numbers such that 1\\/p+1\\/q=1,|A+B|2?p|A|2+q|B|2 with equality if and only if B=(p?1)A. In this paper, a number of generalizations of Bohr inequality for operators in B(H) are established. Moreover, Bohr inequalities

P. Chansangiam; P. Hemchote; P. Pantaragphong

2009-01-01

39

Control of exciton fluxes  

NASA Astrophysics Data System (ADS)

We present proof of principle for control of excitonic fluxes by gate in mesoscopic devices. Since excitons are bosonic particles, control of exciton fluxes can extend mesoscopics, the field which electron transport in potential reliefs, to bosons. Also, as are coupled to light, the control of exciton fluxes may lead to development of new optoelectronic devices. The demonstrated devices as a directional switch, star switch, and flux merger.

High, Alex; Hammack, Aaron; Novitskaya, E. E.; Butov, Leonid; Hanson, Micah; Gossard, Arthur

2008-03-01

40

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

41

Studies on the Bohr Effect of Sheep Hemoglobin  

PubMed Central

A simple method is described for the measurement of pH changes in hemoglobin solutions on oxygenation and reduction. Data are presented establishing the absence of a Bohr effect in p-chloromercuribenzoate [PCMB] treated hemoglobin. The influence of a number of sulfhydryl inhibitors on the Bohr effect of a hemoglobin solution is reported and an interpretation based on steric factors in the protein is proposed.

Smyth, Derek G.; Battaglia, Frederick C.; Meschia, Giacomo

1961-01-01

42

Excitons in electrostatic traps  

NASA Astrophysics Data System (ADS)

We consider in-plane electrostatic traps for indirect excitons in coupled quantum wells, where the traps are formed by a laterally modulated gate voltage. An intrinsic obstacle for exciton confinement in electrostatic traps is an in-plane electric field that can lead to exciton dissociation. We propose a design to suppress the in-plane electric field and, at the same time, to effectively confine excitons in the electrostatic traps. We present calculations for various classes of electrostatic traps and experimental proof of principle for trapping of indirect excitons in electrostatic traps.

Hammack, A. T.; Gippius, N. A.; Yang, Sen; Andreev, G. O.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2006-03-01

43

Exciton states in quasi-zero-dimensional semiconductor nanosystems  

SciTech Connect

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

44

Electrostatic Conveyer for Excitons  

NASA Astrophysics Data System (ADS)

We report on the study of indirect excitons in moving lattices—conveyers created by a set of ac voltages applied to the electrodes on the sample surface. The wavelength of this moving lattice is set by the electrode periodicity, the amplitude is controlled by the applied voltage, and the velocity is controlled by the ac frequency. We found the dynamical localization-delocalization transition for excitons in the conveyers and determined its dependence on exciton density and conveyer amplitude and velocity.

Winbow, A. G.; Leonard, J. R.; Remeika, M.; Kuznetsova, Y. Y.; High, A. A.; Hammack, A. T.; Butov, L. V.; Wilkes, J.; Guenther, A. A.; Ivanov, A. L.; Hanson, M.; Gossard, A. C.

2011-05-01

45

Excitons in electrostatic traps  

Microsoft Academic Search

We consider in-plane electrostatic traps for indirect excitons in coupled quantum wells, where the traps are formed by a laterally modulated gate voltage. An intrinsic obstacle for exciton confinement in electrostatic traps is an in-plane electric field that can lead to exciton dissociation. We propose a design to suppress the in-plane electric field and, at the same time, to effectively

A. T. Hammack; N. A. Gippius; Sen Yang; G. O. Andreev; L. V. Butov; M. Hanson; A. C. Gossard

2006-01-01

46

Excitons in moving lattices  

NASA Astrophysics Data System (ADS)

We report on the study of indirect excitons in moving lattices--conveyers--created by a set of AC voltages applied to the electrodes on the sample surface. The wavelength of this moving lattice is set by the electrode periodicity, the amplitude is controlled by the applied voltage and the speed is controlled by the AC frequency. We probed the conveyer speeds from well below to well above the sound velocity. We observed the dynamical localization-delocalization transitions for excitons in the conveyers and measured its dependence on the exciton density and conveyer speed and amplitude. We also developed the theory of exciton transport via conveyers.

Leonard, Jason; Winbow, Alexander; Remeika, Mikas; Kuznetsova, Yuliya; High, Alexander; Hammack, Aaron; Butov, Leonid; Wilkes, Joseph; Guenther, Alrun; Ivanov, Alexander; Hanson, Micah; Gossard, Arthur

2011-03-01

47

Confined excitons, phonons and their interactions in Ge nanocrystals embedded in SiO2  

SciTech Connect

The authors report the resonant Raman scattering of the optical phonon in Ge nanocrystals with radius ranging from 2 to 5 nm. They have observed the effect of quantum confinement on both the optical phonon and the E{sub 1} exciton. The confinement energy of the E{sub 1} exciton has been explained within the effective mass approximation.

Teo, K.L.; Kwok, S.H.; Yu, P.Y.

2000-12-31

48

Steering quantum states toward classical Bohr-like orbits  

NASA Astrophysics Data System (ADS)

We describe how, almost 100 years after the introduction of the Bohr model of the atom, it is now possible using pulsed electric fields to create localized wavepackets in high n (n~300) Rydberg atoms that travel in near-circular Bohr-like orbits mimicking the behavior of a classical electron. The protocols employed are explained with the aid of quantum and classical dynamics. Although many aspects of the underlying behavior can be described using classical arguments, purely quantum effects such as revivals can be seen even for n~300.

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

2010-08-01

49

Simple derivation of the Bohr-Wheeler spontaneous fission limit  

NASA Astrophysics Data System (ADS)

A simple model for establishing the Bohr and Wheeler limit for Z2/A against spontaneous fission is developed. The physics involved is sufficiently simple to make the model suitable for sophomore students. Beyond showing how fundamental energy considerations lead to a limit for Z2/A, the model helps to make clear the physical origin of the numerical values of the surface and Coulomb terms in the semiempirical mass formula. The resulting limit against spontaneous fission, Z2/A~60, is in fair agreement with the original Bohr and Wheeler value of ~48.

Reed, B. Cameron

2003-03-01

50

Exciton optoelectronic transistor.  

PubMed

We demonstrate experimental proof of principle for an optoelectronic transistor based on the modulation of exciton flux via gate voltage. The exciton optoelectronic transistor (EXOT) implements electronic operation on photons by using excitons as intermediate media; the intensity of light emitted at the optical output is proportional to the intensity of light at the optical input and is controlled electronically by the gate. We demonstrate a contrast ratio of 30 between an on state and an off state of the EXOT and its operation at speeds greater than 1 GHz. Our studies also demonstrate high-speed control of both the flux and the potential energy of excitons on a time scale much shorter than the exciton lifetime. PMID:17767273

High, A A; Hammack, A T; Butov, L V; Hanson, M; Gossard, A C

2007-09-01

51

Optically programmable excitonic traps  

PubMed Central

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.

Alloing, Mathieu; Lemaitre, Aristide; Galopin, Elisabeth; Dubin, Francois

2013-01-01

52

Optically programmable excitonic traps  

NASA Astrophysics Data System (ADS)

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.

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

2013-04-01

53

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

SciTech Connect

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

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

1988-07-01

54

All-optical excitonic transistor.  

PubMed

We demonstrate experimental proof of principle for all-optical excitonic transistors where light controls light by using excitons as an intermediate medium. The principle of operation of all-optical excitonic transistors is based on the control of exciton fluxes by light. PMID:20479817

Kuznetsova, Y Y; Remeika, M; High, A A; Hammack, A T; Butov, L V; Hanson, M; Gossard, A C

2010-05-15

55

Exciton Photophysics of Carbon Nanotubes  

Microsoft Academic Search

The goal of this chapter is to review the importance of excitons to single-wall carbon nanotube (SWNT) optics. We have developed the presentation for both researchers in the SWNT field who want to learn more about the unusual aspects of SWNT exciton photophysics and researchers more knowledgeable about the physics of excitons, but not about SWNT physics. Excitons in SWNTs

Mildred S. Dresselhaus; Gene Dresselhaus; Riichiro Saito; Ado Jorio

2007-01-01

56

Steering Quantum States Towards Classical Bohr-Like Orbits  

Microsoft Academic Search

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

F. B. Dunning; Carlos O Reinhold; S. Yoshida; J. Burgdorfer

2010-01-01

57

"Bohr and Einstein": A Course for Nonscience Students  

ERIC Educational Resources Information Center

|A study of the concepts of relativity and quantum physics through the work of Bohr and Einstein is the basis for this upper level course for nonscience students. Along with their scientific philosophies, the political and moral theories of the scientists are studied. (CP)|

Schlegel, Richard

1976-01-01

58

Indirect Excitons in Elevated Traps  

NASA Astrophysics Data System (ADS)

We introduced an elevated trap technique and exploited it for lowering the effective temperature of indirect excitons. We observed narrow photoluminescence lines which correspond to the emission of individual states of indirect excitons in a disorder potential. We studied the effect of exciton-exciton interaction on the localized and delocalized exciton states and found that the homogeneous line broadening increases with density and dominates the linewidth at high densities.

High, A. A.; Hammack, A. T.; Butov, L. V.; Mouchliadis, L.; Ivanov, A. L.; Hanson, M.; Gossard, A. C.

2009-05-01

59

Electrostatic conveyer for excitons.  

PubMed

We report on the study of indirect excitons in moving lattices-conveyers created by a set of ac voltages applied to the electrodes on the sample surface. The wavelength of this moving lattice is set by the electrode periodicity, the amplitude is controlled by the applied voltage, and the velocity is controlled by the ac frequency. We found the dynamical localization-delocalization transition for excitons in the conveyers and determined its dependence on exciton density and conveyer amplitude and velocity. PMID:21668190

Winbow, A G; Leonard, J R; Remeika, M; Kuznetsova, Y Y; High, A A; Hammack, A T; Butov, L V; Wilkes, J; Guenther, A A; Ivanov, A L; Hanson, M; Gossard, A C

2011-05-12

60

Photovoltaics: Separating Multiple Excitons  

SciTech Connect

Scientists have demonstrated an efficient process for generating multiple excitons in adjacent silicon nanocrystals from a single high-energy photon. Their findings could prove useful for a wide range of photovoltaic applications.

Nozik, A. J.

2012-05-01

61

The many relationships between the IBM and the Bohr model  

NASA Astrophysics Data System (ADS)

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 the IBM into the Bohr model gives a contraction appropriate for the U(5) dynamical symmetry limit. A new map is given consistent with a contraction of the O(6) dynamical symmetry of the IBM. This map gives an explicit identification of IBM states in an O(6) ? O(5) basis with states of the Wilets Jean model and makes it possible to benefit from the different but complementary perspectives of the two models. For example, it leads to explicit expressions for the matrix elements of an IBM O(6) irrep in terms of the known matrix elements of the corresponding Wilets Jean model. The relationship also shows how to obtain the familiar rotor plus beta- and gamma-vibrational bands of the Bohr Mottelson model, in the IBM, by the addition to an O(6) Hamiltonian of a scalar cubic in the quadrupole moment operators of the type considered recently by Van Isacker. The establishment of close relationships between the two models enables one to benefit from the different and complementary perspectives they afford. For example, recent developments of an algebraic version of the collective model has shown that the Bohr model has an SU(1,1)×O(5) dynamical group with representations ranging from those of a spherical vibrator to a beta-vibrational soft-gamma rotor of the Wilets Jean limit and, with the addition of an interaction, to a rotor of the standard Bohr Mottelson type with beta- and gamma-vibrational bands. A parallel range of results can now be obtained in the IBM, albeit with some differences, within the framework of an SU(1,1)×O(5) dynamical group.

Rowe, D. J.; Thiamova, G.

2005-10-01

62

Excitonic switches operating at around 100 K  

Microsoft Academic Search

We report on experimental proof of principle for the operation of excitonic switches at temperatures around 100 K. The devices include the exciton optoelectronic transistor, the excitonic bridge modulator, and the excitonic pinch-off modulator.

G. Grosso; J. C. Graves; A. T. Hammack; A. A. High; L. V. Butov; M. Hanson; A. C. Gossard

2010-01-01

63

Steering Quantum States Towards Classical Bohr-Like Orbits  

SciTech Connect

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

64

Analytical solutions of the Bohr Hamiltonian with the Morse potential  

SciTech Connect

Analytical solutions of the Bohr Hamiltonian are obtained in the {gamma}-unstable case, as well as in an exactly separable rotational case with {gamma}{approx_equal}0, called the exactly separable Morse (ES-M) solution. Closed expressions for the energy eigenvalues are obtained through the asymptotic iteration method (AIM), the effectiveness of which is demonstrated by solving the relevant Bohr equations for the Davidson and Kratzer potentials. All medium mass and heavy nuclei with known {beta}{sub 1} and {gamma}{sub 1} bandheads have been fitted by using the two-parameter {gamma}-unstable solution for transitional nuclei and the three-parameter ES-M for rotational ones. It is shown that bandheads and energy spacings within the bands are well reproduced for more than 50 nuclei in each case.

Boztosun, I.; Inci, I. [Department of Physics, Erciyes University, Kayseri (Turkey); Bonatsos, D. [Institute of Nuclear Physics, N. C. S. R. 'Demokritos', GR-15310 Aghia Paraskevi, Attiki (Greece)

2008-04-15

65

Multidimensional analogues of Bohr's theorem on power series  

Microsoft Academic Search

Generalizing the classical result of Bohr, we show that if an n-variable power series converges in an n-circular bounded complete domain D and its sum has modulus less than 1, then the sum of the maximum of the moduli of the terms is less than 1 in the homothetic domain r*D, where r = 1 - (2\\/3)^(1\\/n). This constant is

Lev Aizenberg

1998-01-01

66

Microscopic Uni-axial Bohr-Mottelson Rotational Model  

SciTech Connect

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

67

Creation of Non-dispersive Bohr-like Wavepackets  

SciTech Connect

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

68

Bound Exciton Complexes  

NASA Astrophysics Data System (ADS)

In the preceding chapter, we concentrated on the properties of free excitons. These free excitons may move through the sample and hit a trap, a nonradiative or a radiative recombination center. At low temperatures, the latter case gives rise to either deep center luminescence, mentioned in Sect. 7.1 and discussed in detail in Chap. 9, or to the luminescence of bound exciton complexes (BE or BEC). The chapter continues with the most prominent of these BECs, namely A-excitons bound to neutral donors. The next aspects are the more weakly BEs at ionized donors. The Sect. 7.4 treats the binding or localization energies of BEC from a theoretical point of view, while Sect. 7.5 is dedicated to excited states of BECs, which contain either holes from deeper valence bands or an envelope function with higher quantum numbers. The last section is devoted to donor-acceptor pair transitions. There is no section devoted specifically to excitons bound to neutral acceptors, because this topic is still partly controversially discussed. Instead, information on these A0X complexes is scattered over the whole chapter, however, with some special emphasis seen in Sects. 7.1, 7.4, and 7.5.

Meyer, B. K.

69

Effective Cleaning Radius Studies  

SciTech Connect

This report discusses results of testing done in the Savannah River Laboratory half tank and full tank mockup facilities using kaolin clay slurries and the relationship between cleaning radius and pump and slurry characteristics.

Churnetski, B.V.

2001-10-15

70

Exciton Multiplication from First Principles.  

PubMed

Third-generation photovolatics require demanding cost and power conversion efficiency standards, which may be achieved through efficient exciton multiplication. Therefore, generating more than one electron-hole pair from the absorption of a single photon has vast ramifications on solar power conversion technology. Unlike their bulk counterparts, irradiated semiconductor quantum dots exhibit efficient exciton multiplication, due to confinement-enhanced Coulomb interactions and slower nonradiative losses. The exact characterization of the complicated photoexcited processes within quantum-dot photovoltaics is a work in progress. In this Account, we focus on the photophysics of nanocrystals and investigate three constituent processes of exciton multiplication, including photoexcitation, phonon-induced dephasing, and impact ionization. We quantify the role of each process in exciton multiplication through ab initio computation and analysis of many-electron wave functions. The probability of observing a multiple exciton in a photoexcited state is proportional to the magnitude of electron correlation, where correlated electrons can be simultaneously promoted across the band gap. Energies of multiple excitons are determined directly from the excited state wave functions, defining the threshold for multiple exciton generation. This threshold is strongly perturbed in the presence of surface defects, dopants, and ionization. Within a few femtoseconds following photoexcitation, the quantum state loses coherence through interactions with the vibrating atomic lattice. The phase relationship between single excitons and multiple excitons dissipates first, followed by multiple exciton fission. Single excitons are coupled to multiple excitons through Coulomb and electron-phonon interactions, and as a consequence, single excitons convert to multiple excitons and vice versa. Here, exciton multiplication depends on the initial energy and coupling magnitude and competes with electron-phonon energy relaxation. Multiple excitons are generated through impact ionization within picoseconds. The basis of exciton multiplication in quantum dots is the collective result of photoexcitation, dephasing, and nonadiabatic evolution. Each process is characterized by a distinct time-scale, and the overall multiple exciton generation dynamics is complete by about 10 ps. Without relying on semiempirical parameters, we computed quantum mechanical probabilities of multiple excitons for small model systems. Because exciton correlations and coherences are microscopic, quantum properties, results for small model systems can be extrapolated to larger, realistic quantum dots. PMID:23459543

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

2013-03-01

71

Challenges to Bohr's Wave-Particle Complementarity Principle  

NASA Astrophysics Data System (ADS)

Contrary to Bohr's complementarity principle, in 1995 Rabinowitz proposed that by using entangled particles from the source it would be possible to determine which slit a particle goes through while still preserving the interference pattern in the Young's two slit experiment. In 2000, Kim et al. used spontaneous parametric down conversion to prepare entangled photons as their source, and almost achieved this. In 2012, Menzel et al. experimentally succeeded in doing this. When the source emits entangled particle pairs, the traversed slit is inferred from measurement of the entangled particle's location by using triangulation. The violation of complementarity breaches the prevailing probabilistic interpretation of quantum mechanics, and benefits Bohm's pilot-wave theory.

Rabinowitz, Mario

2013-02-01

72

Challenges to Bohr's Wave-Particle Complementarity Principle  

NASA Astrophysics Data System (ADS)

Contrary to Bohr's complementarity principle, in 1995 Rabinowitz proposed that by using entangled particles from the source it would be possible to determine which slit a particle goes through while still preserving the interference pattern in the Young's two slit experiment. In 2000, Kim et al. used spontaneous parametric down conversion to prepare entangled photons as their source, and almost achieved this. In 2012, Menzel et al. experimentally succeeded in doing this. When the source emits entangled particle pairs, the traversed slit is inferred from measurement of the entangled particle's location by using triangulation. The violation of complementarity breaches the prevailing probabilistic interpretation of quantum mechanics, and benefits Bohm's pilot-wave theory.

Rabinowitz, Mario

2012-10-01

73

Bohr Hamiltonian with Davidson potential for triaxial nuclei  

SciTech Connect

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}={pi}/6, is developed. Analytical expressions for spectra andB(E2) transition rates ranging from a triaxial vibrator to the rigid triaxial rotator are obtained and compared to experimental results. Using a variational procedure, we point 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.

Yigitoglu, I. [Faculty of Science and Arts, Department of Physics, Gaziosmanpasa University, TR-60240 Tokat (Turkey); Bonatsos, Dennis [Institute of Nuclear Physics, National Centre for Scientific Research 'Demokritos', GR-15310 Aghia Paraskevi, Attiki (Greece)

2011-01-15

74

Splitting of the excitonic peak in quantum wells with interfacial roughness  

NASA Astrophysics Data System (ADS)

Excitons in a quantum well depend on the interfacial roughness resulting from its growth. The interface is characterized by islands of size ? separated by one monolayer steps across which the confining potential decreases by V0 for wider wells. A natural length is the localization length ?0=??/2MV0 characterizing the minimum size island to confine an exciton. For small islands (?exciton peak. As the island size ? exceeds the localization length ?0, the peak gradually splits into a doublet. Generally the spectra exhibit the following features: (1) the shape is very sensitive to ?/?0 and depends only weakly on the ratio of island size to exciton radius; (2) in the small island regime ?<exciton peak is correctly described by a model of white-noise potential, except for the position of the peak which still depends on the correlation length of the disorder.

Castella, Hervé; Wilkins, John W.

1998-12-01

75

Distal Radius Fracture (Colles' Fracture)  

MedlinePLUS

... 2013 American Academy of Orthopaedic Surgeons Distal Radius Fractures (Broken Wrist) The radius is the larger of ... the wrist is called the distal end. A fracture of the distal radius occurs when the area ...

76

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

77

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

Microsoft Academic Search

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

78

Effect of shell thickness on exciton and biexciton binding energies in ZnSe\\/ZnS core\\/shell quantum dot  

Microsoft Academic Search

The exciton and biexciton binding energies are calculated for a ZnSe\\/ZnS core\\/shell quantum dot incorporating WKB approximation. With the variations in shell thickness, the exciton binding energy shows nonlinear behavior. A maximum value of exciton binding energy is expected to occur for shell thickness equals to the core radius. Similar results are reported for biexcitons where for thicker shells the

Saikat Chattopadhyay; J. T. Andrews; P. Sen

2009-01-01

79

Lightcurves for 1560 Strattonia, 1928 Summa, 2763 Jeans, 3478 Fanale, 3948 Bohr, 5275 Zdislava, and 5369 Virgiugum  

NASA Astrophysics Data System (ADS)

Lightcurve observations have yielded period determinations for the following asteroids: 1560 Strattonia, 1928 Summa, 2763 Jeans, 3478 Fanale, 3948 Bohr, 5275 Zdislava, and 5369 Viriugum. In addition, HG values were found for 3948 Bohr and 5369 Viriugum.

Owings, Larry E.

2013-04-01

80

Electrostatic traps for dipolar excitons  

Microsoft Academic Search

We consider the design of two-dimensional electrostatic traps for dipolar indirect excitons. We show that the exciton dipole-dipole interaction, combined with the in-plane electric fields that arise due to the trap geometry, constrains the maximal density and lifetime of trapped excitons. We derive an analytic estimate of these values and determine their dependence on the trap geometry, thus suggesting the

Ronen Rapaport; Gang Chen; Steven Simon; Oleg Mitrofanov; Loren Pfeiffer; P. M. Platzman

2005-01-01

81

Variable Atomic Radius of Hydrogen Due to Vibrating Nucleus  

NASA Astrophysics Data System (ADS)

The H-atomic radius is variable because the H-nucleus is vibrating and the electric force field upon the electron is repeatedly changing due to the changing distance from the positive nucleus to the negatively charged electron. If the the distance from the nucleus to the electron is d=r + Acos2?ft where r=5.29x10-11m, the calculated Bohr radius, and d=2.5x10-11m, the measured atomic radius of the H-atom, then the equation for the variable atomic radius of the H-atom is 5.29x10-11m + Acos2?ft= 2.5x10-11m. If the RMS value for the average cosine is 0.707, solving for A, the average amplitude of nuclear vibration, A=3.95x10-11m. Therefore, the oscillating orbit of the electron in an H-atom has an average amplitude of A=3.95x10-11.

Brekke, Stewart

2012-02-01

82

Phase, shape, and architecture of SF6 and SF6\\/CO2 aerosol particles: Infrared spectra and modeling of vibrational excitons  

Microsoft Academic Search

Information on the phase, shape, and architecture of pure SF6 and mixed SF6\\/CO2 aerosol particles is extracted from experimental infrared spectra by comparison with predictions from quantum mechanical exciton calculations. The radius of the particles lies around 50 nm. The following extensions to our previous vibrational exciton model are included: (i) To account for the many degrees of freedom of

George Firanescu; David Luckhaus; Ruth Signorell

2008-01-01

83

Effective cleaning radius studies  

SciTech Connect

Results of testing done in the Savannah River Laboratory half tank and full tank mockup facilities using kaolin clay slurries indicate a relationship between cleaning radius and pump and slurry characteristics of the form: ECR ..cap alpha.. DV/sub 0/ (rho/tau/sub 0/) sup 1/2. This can be used to predict the slurry pump requirements for sludge removal during waste tank retirement.

Churnetski, B.V.

1981-02-19

84

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

85

Effects of gallium alloy content and the geometrical confinement on effective exciton g-factor in a III-V semiconductor quantum dot  

NASA Astrophysics Data System (ADS)

Effective exciton-g factor as functions of dot radius and Ga alloy content in a GaxIn1-xAs/GaAs quantum dot is computed. The computations are included with the effect of mass anisotropy and the non-parabolicity of the conduction band. The dependence of excitonic binding energy with respect to the dot radius is investigated for various composition of Ga alloy content.

Senthilkumar, N. R.; Peter, A. John

2013-02-01

86

Indirect excitons in elevated traps.  

PubMed

We report on the study of indirect excitons in elevated traps. The transition from a normal to elevated trap results in the appearance of narrow lines in the emission spectrum. The density, temperature, and voltage dependences indicate that these lines correspond to the emission of individual states of indirect excitons in a disorder potential in the elevated trap. PMID:19382780

High, A A; Hammack, A T; Butov, L V; Mouchliadis, L; Ivanov, A L; Hanson, M; Gossard, A C

2009-05-01

87

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

88

Noninvasive measurement of mean alveolar carbon dioxide tension and Bohr's dead space during tidal breathing  

Microsoft Academic Search

The lack of methodology for measuring the alveolar carbon dioxide tension (PA,CO2) has forced investigators to make several assumptions, such as that PA,CO2 is equal to end-tidal (PET,CO2) and arterial CO2 tension (Pa,CO2). The present study measured the mean PA,CO2 and Bohr9s dead space ratio (Bohr9s dead space\\/tidal volume (VD,Bohr\\/VT)) during tidal breathing. The method used is a new, simple

N. G. Koulouris; P. Latsi; J. Dimitroulis; B. Jordanoglou; M. Gaga; J. Jordanoglou

2001-01-01

89

Kinematics of Cold Excitons in the Laser Induced Exciton Trap  

NASA Astrophysics Data System (ADS)

We have recently demonstrated laser induced trapping of indirect excitons in coupled quantum wells [1]. An important advantage of the laser induced exciton trapping is the possibility of controlling the trap in-situ by varying the laser intensity in space and time. Moreover, the excitons at the trap center are cold since they are far from the hot area of the laser excitation. Here, we report the studies of exciton kinetics in the laser induced traps. In particular, the kinetics of the trap formation and the exciton collection to the trap center when the excitation is switched on, as well as the kinetics of the exciton cloud when the excitation and trap are switched off have been measured by ultrafast imaging spectroscopy. The dynamics of the degenerate Bose gas of excitons in the trap was measured and analyzed for the first time. [1] A.T. Hammack, M. Griswold, L.V. Butov, L.E. Smallwood, A.L. Ivanov, and A.C. Gossard, Phys. Rev. Lett. 96, 227402 (2006).

Hammack, A. T.; Butov, L. V.; Smallwood, L. E.; Mouchliadis, L.; Ivanov, A. L.; Gossard, A. C.

2007-03-01

90

Binding energy calculation of excitonic trions in spherical quantum dots with the quantum adiabatic theorem  

Microsoft Academic Search

The stability of the charged excitons X+ and X? as a function of the spherical quantum dot radius R and also as a function of the effective mass ratio ? is studied with a formulation of the Hartree–Fock approximation using a calculation method which is based on the quantum adiabatic theorem. Our results indicate that the binding energies of X+

S. Baskoutas; A. F. Terzis

2005-01-01

91

Stability of an Exciton Bound to an Ionized Acceptor in Quantum Dots  

Microsoft Academic Search

Binding energy for an exciton (X) bound in a parabolic two-dimensional quantum dot by an acceptor impurity A- located on the z-axis at a distance d from the dot plane, are calculated using the Hartree formalism with a recently developed numerical method (PMM) for the solution of the Schrödinger equation. As our analysis indicates there is a critical dot radius

S. Baskoutas; A. F. Terzis; C. Politis

2003-01-01

92

Van der Waals Interactions and Exciton Condensation  

PubMed Central

It is shown that the van der Waals interaction can lead at low temperatures to a condensed state of excitons with properties in qualitative agreement with the observations of exciton droplets. Our calculation gives a binding energy of the correct sign and magnitude for the exciton condensate. In a diclectric medium, the strong enhancement of the exciton polarizability leads to a giant van der Waals interaction, and this interaction appears to make possible a condensed exciton phase.

Handel, P. H.; Kittel, C.

1971-01-01

93

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

94

Lightcurve Analysis of 3948 Bohr and 4874 Burke: An International Collaboration  

NASA Astrophysics Data System (ADS)

An international collaboration provided complete coverage of the 24.884 ± 0.002 h period for 3948 Bohr and a confirmation of a 3.657 ± 0.001 h period for 4874 Burke. The amplitudes were A = 0.89 ± 0.10 mag for 3948 Bohr and A = 0.22 ± 0.07 mag for 4874 Burke.

Klinglesmith, Daniel A., III; Risley, Ethan; Turk, Janek; Vargas, Angelica; Warren, Curtis; Ferrero, Andrea

2013-01-01

95

Exciton spectra of semiconductor clusters  

SciTech Connect

The band gaps and exciton energies of CdS and GaP clusters are calculated for the first time using pseudopotentials. The calculated exciton energies of CdS over a wide range of cluster sizes are in excellent agreement with experiment. Furthermore, the exciton states of clusters with zinc-blende and hexagonal lattices are similar in large clusters, but differ dramatically in small clusters. Finally, the spectra of small GaP clusters shift to red, instead of to the blue, with decreasing cluster size. These effects provide novel ways of tuning the optical properties of clusters.

Rama Krishna, M.V.; Friesner, R.A. (Department of Chemistry, Columbia University, New York, New York (USA))

1991-07-29

96

Exciton-exciton interaction and heterobiexcitons in GaN  

Microsoft Academic Search

The formation of not only A biexcitons (XXAA) but also heterobiexcitons that consist of A and B excitons (XXAB) in a free-standing bulk GaN is identified by polarization-sensitive spectrally resolved FWM measurements. The FWM spectra and delay-time dependence show that the interaction between A and B exciton gives rise to the energy shifts of the spectra and the phase shifts

S. Adachi; S. Muto; K. Hazu; T. Sota; K. Suzuki; S. F. Chichibu; T. Mukai

2003-01-01

97

Corrections of Enghoff's dead space formula for shunt effects still overestimate Bohr's dead space.  

PubMed

Dead space ratio is determined using Enghoff's modification (VdB-E/Vt) of Bohr's formula (VdBohr/Vt) in which arterial is used as a surrogate of alveolar PCO2. In presence of intrapulmonary shunt Enghoff's approach overestimates dead space. In 40 lung-lavaged pigs we evaluated the Kuwabara's and Niklason's algorithms to correct for shunt effects and hypothesized that corrected VdB-E/Vt should provide similar values as VdBohr/Vt. We analyzed 396 volumetric capnograms and arterial and mixed-venous blood samples to calculate VdBohr/Vt and VdB-E/Vt. Thereafter, we corrected the latter for shunt effects using Kuwabara's (K) VdB-E/Vt and Niklason's (N) VdB-E/Vt algorithms. Uncorrected VdB-E/Vt (mean±SD of 0.70±0.10) overestimated VdBohr/Vt (0.59±0.12) (p<0.05), over the entire range of shunts. Mean (K) VdB-E/Vt was significantly higher than VdBohr/Vt (0.67±0.08, bias -0.085, limits of agreement -0.232 to 0.085; p<0.05) whereas (N)VdB-E/Vt showed a better correction for shunt effects (0.64±0.09, bias 0.048, limits of agreement -0.168 to 0.072; p<0.05). Neither Kuwabara's nor Niklason's algorithms were able to correct Enghoff's dead space formula for shunt effects. PMID:23827851

Suarez-Sipmann, Fernando; Santos, Arnoldo; Böhm, Stephan H; Borges, Joao Batista; Hedenstierna, Göran; Tusman, Gerardo

2013-07-01

98

DNA-Controlled Excitonic Switches  

PubMed Central

Fluorescence resonance energy transfer (FRET) is a promising means of enabling information processing in nanoscale devices, but dynamic control over exciton pathways is required. Here, we demonstrate the operation of two complementary switches consisting of diffusive FRET transmission lines in which exciton flow is controlled by DNA. Repeatable switching is accomplished by the removal or addition of fluorophores through toehold-mediated strand invasion. In principle, these switches can be networked to implement any Boolean function.

2012-01-01

99

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

100

Biexciton formation and exciton exciton correlation effects in bulk ZnO  

Microsoft Academic Search

We demonstrate that exciton-exciton correlations strongly affect FWM signals. In particular, A exciton-B exciton correlation (bound and unbound AB-biexcitons), which can be generated even in co-circular polarizations, has a significant qualitative impact on the optical response of bulk ZnO.

Satoru Adachi; Kouji Hazu; Takayuki Sota; Shigefusa F. Chichibu; Gene Cantwell; David B. Eason; Donald C. Reynolds; Cole W. Litton

2004-01-01

101

Pairing Interaction Effects in Exciton Level Densities.  

National Technical Information Service (NTIS)

Recent progress in pairing corrections for exciton state-density formulas used in pre-compound nuclear reaction theories is reviewed. These correction factors are, strictly speaking, dependent on the nuclear excitation energy U and the exciton number n. A...

C. Y. Fu

1989-01-01

102

Exciton dynamicsstudied via internal THz transitions  

SciTech Connect

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

103

BOSE-EINSTEIN CONDENSATION OF EXCITONS  

Microsoft Academic Search

Whether quasi-particles, such as excitons, i.e., nonlocalized excited ; states of solids, can fulfill necessary conditions for a Bose-Einstein ; condensation and whether such condensation can be observed are discussed. ; Although uncertainties of data on excitons preclude precise numerical ; predictions, it is concluded that under certain experimentally attainable ; circumstances excitons fulfill the necessary conditions, i.e., condensation is

John Blatt; K. W. Boeer; Werner Brandt

1962-01-01

104

Excitons in rare-gas crystals  

Microsoft Academic Search

The paper is a survey of experimental and theoretical results on exciton states in crystals of xenon, krypton, argon and neon. The absorption and luminescence spectra of the crystals and the band structure of electron and exciton states are analysed. Some features of the exciton-phonon interaction in pure and doped crystals are discussed. In contrast with other types of molecular

Irene Ya Fugol

1978-01-01

105

Crystallization of an exciton superfluid  

SciTech Connect

Indirect excitons--pairs of electrons and holes spatially separated in semiconductor bilayers or quantum wells--are known to undergo Bose-Einstein condensation and to form a quantum fluid. Here we show that this superfluid may crystallize upon compression. However, further compression results in quantum melting back to a superfluid. This unusual behavior is explained by the effective interaction potential between indirect excitons, which strongly deviates from a dipole potential at small distances due to many-particle and quantum effects. Based on first-principles path-integral Monte Carlo simulations, we compute the complete phase diagram of this system and predict the relevant parameters necessary to experimentally observe exciton crystallization in semiconductor quantum wells.

Boening, J.; Filinov, A.; Bonitz, M. [Institut fuer Theoretische Physik und Astrophysik, Christian-Albrechts-Universitaet, Leibnizstrasse 15, D-24098 Kiel (Germany)

2011-08-15

106

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

SciTech Connect

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

107

Excitons and charged excitons in InAs nanorods  

Microsoft Academic Search

Using a novel formulation of the Hartree–Fock approximation with the flexible potential morphing method we calculate the effective band gap of InAs nanorods as well as the binding and correlation energies of neutral and charged excitons confined in such nanostructures. Our results indicate very good agreement with recent experimental data as regards the behavior of the effective band gap with

S. Baskoutas

2005-01-01

108

Le cosí dette « orbite » di Bohr e di Bohr-Sommerfeld della quantistica classica e le onde associate agli elettroni nei varii livelli energetici  

Microsoft Academic Search

Sunto  Già l’A. aveva trovato, che a meno di un coefficiente di costipazione massima, « i raggi atomici sono multipli della lunghezza\\u000a d’ onda delle onde stazionarie di De Broglie associate all’elettrone in riposo ». Qui sono mostrate altre coincidenze, che\\u000a si riferiscono all’elettrone considerato, secondo la vecchia teoria dei quanta, in moto sulle cosiddette « orbite » stazionarie\\u000a di Bohr:

Mariaho Pierucci

1942-01-01

109

Spin Texture in a Cold Exciton Gas  

NASA Astrophysics Data System (ADS)

We report on the observation of a spin texture in a cold exciton gas in a GaAs/AlGaAs coupled quantum well structure. The spin texture is observed around the rings in the exciton emission pattern. The observed phenomena include: a ring of linear polarization, a vortex of linear polarization with polarization perpendicular to the radial direction, an anisotropy in the exciton flux, a skew of the exciton fluxes in orthogonal circular polarizations and a corresponding four-leaf pattern of circular polarization, and a periodic spin texture. These phenomena emerge when the exciton gas is cooled below a few Kelvin.

High, Alexander; Hammack, Aaron; Leonard, Jason; Yang, Sen; Butov, Leonid; Ostatnicky, Tomas; Kavokin, Alexei; Gossard, Arthur

2011-03-01

110

Distal radius fractures: current concepts.  

PubMed

Despite the frequency of distal radius fractures, the optimal treatment remains without consensus opinion. A trend toward increased distal radius fracture open reduction and internal fixation has been identified, with biomechanical and clinical studies suggesting treatment advantages of certain fixation methods over others. Well-controlled patient trials are still missing to lend objective findings to management algorithms. This article reviews the literature over the past 5 years to guide our management regarding this common upper-extremity injury. PMID:22763062

Schneppendahl, Johannes; Windolf, Joachim; Kaufmann, Robert A

2012-07-03

111

Conjugated “Molecular Wire” for Excitons  

SciTech Connect

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

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

2010-05-06

112

Exciton-polariton lasing and amplification based on exciton-exciton scattering in CdTe microcavity quantum wells  

NASA Astrophysics Data System (ADS)

We describe experiments demonstrating an exciton-polariton laser and amplifier based on an incoherent exciton-polariton reservoir in CdTe microcavity quantum wells. The gain mechanism is real excited exciton-exciton scattering, in which excitons created at large in-plane wave vectors are thermalized and accumulate at the bottleneck lower polariton states at smaller in-plane wave vectors. Because the exciton-exciton scattering rate for CdTe at the saturation density is higher than that for GaAs, the threshold for spontaneous polariton lasing is more easily reached in the case of CdTe with respect to GaAs. We demonstrate a high-gain amplification of bottleneck lower polaritons close to the lasing threshold. By performing a pulsed pump and probe experiment, we observe unambiguous evidence of real excited exciton-exciton scattering gain in the form of exp(const N2exc), where Nexc is the exciton-polariton reservoir population. This result is in sharp contrast to the recently demonstrated parametric polariton amplifier based on virtual coherent four wave mixing, in which gain is proportional to exp(const Nexc). [P.G. Savvidis et al., Phys. Rev. Lett. 84, 1547 (2000)].

Huang, R.; Yamamoto, Y.; André, R.; Bleuse, J.; Muller, M.; Ulmer-Tuffigo, H.

2002-04-01

113

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

114

Bohr's Correspondence Principle and the Area Spectrum of Quantum Black Holes  

Microsoft Academic Search

During the last twenty-five years evidence has been mounting that a black-hole surface area has a discrete spectrum. Moreover, it is widely believed that area eigenvalues are uniformly spaced. There is, however, no general agreement on the spacing of the levels. In this Letter we use Bohr's correspondence principle to provide this missing link. We conclude that the area spacing

Shahar Hod

1998-01-01

115

Bohr Hamiltonian, mass coefficients, and the structure of well deformed axially symmetric nuclei  

SciTech Connect

It is shown that in the Bohr Hamiltonian the use of three different mass coefficients for the two vibrational and the rotational modes is very important for the correct description of the properties of the well deformed axially symmetric nuclei. Four parameters per nucleus are needed to describe the relative values of energies and B(E2)'s.

Jolos, R. V. [Joint Institute for Nuclear Research, RU-141980 Dubna (Russian Federation); Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany); Brentano, P. von [Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany)

2008-12-15

116

Identification of Residues responsible for the Alkaline Bohr Effect in Haemoglobin  

Microsoft Academic Search

The imidazole groups of the C-terminal histidines of the beta-chains, together with the alpha-amino groups of the alpha-chains, are responsible for most of the Bohr effect. In oxyhaemoglobin these groups are free, while in deoxyhaemoglobin their pKs are raised, probably by linkage to carboxyl groups.

M. F. Perutz; Hilary Muirhead; L. Mazzarella; R. A. Crowther; J. Greer; J. V. Kilmartin

1969-01-01

117

An immanent criticism of Lakatos' account of the ‘degenerating phase’ of Bohr's atomic theory  

Microsoft Academic Search

Summary This paper presents an immanent criticism of Lakatos' reconstruction of the degenerating phase of Bohr's atomic theory. That is to say, the historiographical methods used are exclusively of a Lakatosian kind. Such a closer Lakatosian look at the historical episode in question shows that Lakatos' own reconstruction is incorrect on three essential points. These are the role of the

Hans Radder

1982-01-01

118

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…

Niaz, Mansoor; Cardellini, Liberato

2011-01-01

119

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…

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

2008-01-01

120

Exact diagonalization of the Bohr Hamiltonian for rotational nuclei: Dynamical {gamma} softness and triaxiality  

SciTech Connect

Detailed quantitative predictions are obtained for phonon and multiphonon excitations in well-deformed rotor nuclei within the geometric framework, by exact numerical diagonalization of the Bohr Hamiltonian in an SO(5) basis. Dynamical {gamma} deformation is found to significantly influence the predictions through its coupling to the rotational motion. Basic signatures for the onset of rigid triaxial deformation are also obtained.

Caprio, M. A. [Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556-5670 (United States)

2011-06-15

121

Exciton-exciton scattering dynamics in a semiconductor microcavity and stimulated scattering into polaritons  

NASA Astrophysics Data System (ADS)

We study the polariton dynamics in a semiconductor microcavity for small temperatures and zero exciton-cavity detuning, including the exciton-exciton and exciton-phonon scatterings. A bottleneck in the relaxation of excitons into lower polariton is found, which persists up to high densities. We then consider injection of large populations of lower polaritons with an external pump. All scatterings featuring the lower polariton as the final state become stimulated. In particular, scattering of two excitons into both lower and upper polaritons shows direct evidence of stimulation. Within the rate equation approach, we also predict sizable saturation effects due to the stimulated emission.

Tassone, F.; Yamamoto, Y.

1999-04-01

122

Excitonic processes in aromatic molecular crystals of strong exciton-phonon coupling  

NASA Astrophysics Data System (ADS)

We overview our experimental work on exciton self-trapping in simple organic molecular crystals. First we show pressure-induced change in the exciton-phonon interaction, indicating that the interaction depends strongly on the intermolecular distance, and then we demonstrate the experimental results and compare them with theoretical results on the exciton-phonon coupling constants, presence of the self-trapping barrier, intermediate self-trapped exciton states, the radiative annihilation of excitons at the exit of the self-trapping path. Most of this paper is devoted to show exciton relaxation processes visualized in time-resolved luminescence in Pyrene.

Matsui, Atsuo H.

1995-02-01

123

Berry Phase Effect on the Exciton Transport and on the Exciton Bose-Einstein Condensate  

SciTech Connect

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.

Yao Wang; Niu Qian [Department of Physics, University of Texas, Austin, Texas 78712 (United States)

2008-09-05

124

Excitonic switches operating at around 100 K  

NASA Astrophysics Data System (ADS)

Photonic and optoelectronic devices may offer the opportunity to realize efficient signal processing at speeds higher than in conventional electronic devices. Switches form the building blocks for circuits, and fast photonic switches have been realized. Recently, a proof of principle demonstration of exciton optoelectronic devices was reported. The potential advantages of excitonic devices include high operation and interconnection speed, small dimensions and the opportunity to combine many elements into integrated circuits. Here, we demonstrate experimental proof of principle for the operation of excitonic switching devices at temperatures around 100 K. The devices are based on an AlAs/GaAs coupled quantum well structure and include the exciton optoelectronic transistor (EXOT), the excitonic bridge modulator (EXBM), and the excitonic pinch-off modulator (EXPOM). A two orders of magnitude increase in the operation temperature compared to earlier devices (1.5 K refs 7,8) is achieved.

Grosso, G.; Graves, J.; Hammack, A. T.; High, A. A.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2009-10-01

125

Strong biexcitonic effects and exciton-exciton correlations in ZnO  

Microsoft Academic Search

We have studied experimentally biexcitonic states of ZnO of rather high quality in the low exciton density limit by means of time-integrated and spectrally-resolved four-wave mixing (FWM). FWM emission signals due to biexcitons consisting of two A-hole excitons (AA biexcitons), an A-hole and a B-hole exciton (AB biexcitons), and two B-hole excitons (BB biexcitons) have been clearly observed according to

K. Hazu; T. Sota; K. Suzuki; S. Adachi; Sf. Chichibu; G. Cantwell; D. B. Eason; D. C. Reynolds; C. W. Litton

2003-01-01

126

Relaxation kinetics of excitons in cuprous oxide  

NASA Astrophysics Data System (ADS)

Cuprous oxide is a thoroughly studied semiconductor with long-lived, mobile excitons. The kinetic energy distribution of the excitons is reproduced in phonon-assisted luminescence spectra. When non-equilibrium excitons are produced by a laser pulse, nanosecond-time-resolved luminescence reveals the relaxation of their kinetic energy distribution. It has been known for several years that, when the excitons are produced with the highest attainable densities, the time-sequence of exciton kinetic energy distributions closely resembles a sequence of Bose-Einstein distributions, with apparent temperatures greater than that of the lattice. A widely used hypothesis is that the excitons exchange energy through elastic collisions quickly enough to establish a quasi-equilibrium, with the exciton gas having a temperature distinct from that of the lattice. The transient kinetic energy distributions have thus been interpreted simply as quasi-equilibrium distributions, with the gas density near enough the quantum density that the Bose statistics of the excitons is noticeable. This work is an experimental and theoretical study of such luminescence spectra. The absolute brightness of the photoluminescence, compared with that of a known number of excitons, indicates that the density of excitons is at all times too low to reveal their Bose statistics---assuming they act as an ideal gas. The assumption of a quasi-equilibrium among the excitons is abandoned, and the relaxation problem is treated through a Boltzmann equation. Only relaxation processes which have been measured experimentally are included: acoustic phonon emission and absorption, interconversion between spin states, and non-radiative two-exciton decay. Numerical modeling reproduces most of the experimental observations, without invoking Bose statistics.

O'Hara, Keith E.

127

Spontaneous coherence in a cold exciton gas.  

PubMed

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

2012-03-21

128

Excitons in the rare gas solids  

SciTech Connect

Excitons play a prominent role in the chemistry and physics of condensed matter. Excitons in the rare gas solids, the prototypical van der Waals insulators, will be the focus of the remainder of this report. The goal here is to investigate the controversies surrounding the description of excitons in insulators and, therefore the simplest class of these solids, namely the rare gas solids, is chosen as the exemplary system. Specific problems associated with molecular crystals are, therefore, avoided and only the salient features of excitons are thus considered. 47 refs., 9 figs., 4 tabs.

Not Available

1988-01-01

129

Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies  

PubMed Central

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4? complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra.

Baranov, P. G.; Poluektov, O. G.; Schmidt, J.

2010-01-01

130

Verification of Bohr's frequency condition and Moseley's law: An undergraduate laboratory experiment  

Microsoft Academic Search

We describe an undergraduate laboratory experiment to verify Bohr's frequency condition and Moseley's law using a thin NaI(Tl) detector spectrometer and a weak 57Co source. The slope of the plot of Kalpha x-ray energy versus (Z-1)2 yields a value for the Rydberg constant, R=(1.19+\\/-0.01)×107 m-1, which is in fair agreement with the best literature value, R=10 973 731.534(13) m-1.

S. B. Gudennavar; N. M. Badiger; S. R. Thontadarya; B. Hanumaiah

2003-01-01

131

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

NASA Astrophysics Data System (ADS)

It is shown that solutions of the second-order decoupled radial Dirac equations satisfy Ermakov-type invariants. These invariants lead to amplitude-phase-type representations of the radial spinor solutions, with exact relations between their amplitudes and phases. Implications leading to a Bohr-Sommerfeld quantization condition for bound states, and a few particular atomic/ionic and nuclear/hadronic bound-state situations are discussed.

Thylwe, Karl-Erik; McCabe, Patrick

2013-05-01

132

Observation of a Rydberg Series in H{sup +}H{sup -}: A Heavy Bohr Atom  

SciTech Connect

We report on the realization of a heavy 'Bohr atom', through the spectroscopic observation of a Rydberg series of bound quantum states at principal quantum numbers n=140 to 230. The system is made heavy by replacing an electron inside a hydrogen atom by a composite H{sup -} particle, thus forming a H{sup +}H{sup -} Coulombically bound system obeying the physical laws of a generalized atom with appropriate mass scaling.

Vieitez, M. O.; Ivanov, T. I.; Lange, C. A. de; Ubachs, W. [Laser Centre, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam (Netherlands); Reinhold, E. [Laser Centre, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam (Netherlands); FOM-institute NIKHEF, Kruislaan 409, 1098 SJ Amsterdam (Netherlands)

2008-10-17

133

Electric quadrupole transitions of the Bohr Hamiltonian with the Morse potential  

SciTech Connect

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

134

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 Schrödinger models. We examine student descriptions of atoms on final exams in transformed modern physics classes using various versions of this curriculum. We find that if the curriculum does not include sufficient connections between different models, many students still have a Bohr-like view of atoms rather than a more accurate Schrödinger model. However, with an improved curriculum designed to develop model-building skills and with better integration between different models, it is possible to get most students to describe atoms using the Schrödinger model. In comparing our results with previous research, we find that comparing and contrasting different models is a key feature of a curriculum that helps students move beyond the Bohr model and adopt Schrödingerâs 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

135

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

PubMed

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

136

Cascade-exciton model of nuclear reactions  

Microsoft Academic Search

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

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

1983-01-01

137

Excitonic superconductivity in copper oxides  

SciTech Connect

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

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

1988-01-01

138

New technique to determine exciton bandwidths of the lowest exciton band in aromatic molecular crystals  

NASA Astrophysics Data System (ADS)

A new technique to determine the exciton bandwidths of the lowest exciton bands in aromatic molecular crystals has been developed. This technique is based on the fact that in microcrystallites k is not a good quantum number, allowing the optical transition in whole exciton band states. In anthracene microcrystallites, the exciton band width increases with microcrystallite diameter, reaching its maximum value of 340 cm-1. For microcrystallites larger than 65 A in diameter, the observed bandwidth decreases asymptotically to 300 cm-1, which is interpreted to be the bandwidth of bulk crystal. For the above figures 340 cm-1 and 300-1 strong exciton scattering at microcrystallite surfaces is suggested. The same technique is applied to pyrene microcrystallites. The exciton bandwidth for bulk crystal is estimated to be at most 330 cm-1. With the present results, the excitonic state and exciton relaxation processes in pyrene crystals are understood quite successfully. A brief discussion on the exciton bandwidth of the lowest exciton band in coronene is also given.

Matsui, Atsuo H.; Nishi, Osamu; Matsushima, Yoshihiro; Mizuno, Ken-Ichi; Takeshima, Masumi; Oeda, Yoshitaka; Goto, Takenari

1996-03-01

139

Exciton Dephasing in Semiconducting Single-Walled Carbon Nanotubes  

NASA Astrophysics Data System (ADS)

Two-pulse four-wave mixing experiments at various excitation intensities and temperatures enable the contributions of exciton-exciton and exciton-phonon scattering to exciton dephasing to be separated. We identify the dominant phonon mode, and estimate the homogeneous linewidth

Ma, Y.-Z.; Graham, M. W.; Green, A. A.; Stupp, S. I.; Hersam, M. C.; Fleming, G. R.

140

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

SciTech Connect

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

141

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

NASA Astrophysics Data System (ADS)

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.

González-Santander, C.; Domínguez-Adame, F.; Römer, R. A.

2011-12-01

142

Bohr-effect and pH-dependence of electron spin resonance spectra of a cobalt-substituted monomeric insect haemoglobin  

Microsoft Academic Search

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

143

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 Rgexciton localization along d , and the value of meff becomes very large. This enhancement in the effective mass maybe used to quantify localization. The variations of electron density (?) with d , i.e., the electron density profiles (EDPs) of the films extracted from x-ray reflectivity studies, indicate formation of layers with period “ b ” (in Å), Rg0 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

144

Spin Currents in a Coherent Exciton Gas  

NASA Astrophysics Data System (ADS)

We report the observation of spin currents in a coherent gas of indirect excitons. The realized long-range spin currents originate from the formation of a coherent gas of bosonic pairs—a new mechanism to suppress the spin relaxation. The spin currents result in the appearance of a variety of polarization patterns, including helical patterns, four-leaf patterns, spiral patterns, bell patterns, and periodic patterns. We demonstrate control of the spin currents by a magnetic field. We also present a theory of coherent exciton spin transport that describes the observed exciton polarization patterns and indicates the trajectories of the spin currents.

High, A. A.; Hammack, A. T.; Leonard, J. R.; Yang, Sen; Butov, L. V.; Ostatnický, T.; Vladimirova, M.; Kavokin, A. V.; Liew, T. C. H.; Campman, K. L.; Gossard, A. C.

2013-06-01

145

Excitons in resonant coupling of quantum wells  

NASA Astrophysics Data System (ADS)

We examine, experimentally and theoretically, the effects of excitons in optical measurements of resonant coupling of quantum wells. We find that the exciton line splittings do not correspond directly to the underlying electron levels, which leads to differences in the bias fields for resonance of, e.g., ~10%. We construct a variational model of excitons in coupled wells and successfully compare this model with measured splittings near the resonance between the first and second electron levels of adjacent wells, deducing the actual conditions for coupling of the ``bare'' electron levels.

Fox, A. M.; Miller, D. A. B.; Livescu, G.; Cunningham, J. E.; Henry, J. E.; Jan, W. Y.

1990-07-01

146

Photoluminescence due to inelastic exciton-exciton scattering in ZnMgO-alloy thin film  

SciTech Connect

We studied the photoluminescence of ZnMgO thin film, grown by the radiofrequency sputtering method, as a function of excitation intensity and temperature. As the excitation intensity increases, a nonlinear emission band caused by the radiative recombination of the inelastic exciton-exciton scattering was detected at low temperature. We found that the inelastic exciton-exciton scattering process can only persist up to T {approx} 260 K. The nonlinear emission band observed at room temperature is due to the radiative recombination of the electron-hole plasma.

Chia, C. H.; Chen, J. N.; Hu, Y. M. [Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan (China)

2011-09-26

147

Multiple Exciton Generation Solar Cells  

SciTech Connect

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

148

Excitonic phases from Weyl semimetals.  

PubMed

Systems with strong spin-orbit coupling, which competes with other interactions and energy scales, offer a fertile playground to explore new correlated phases of matter. Weyl semimetals are an example where the phenomenon leads to a low-energy effective theory in terms of massless linearly dispersing fermions in three dimensions. In the absence of interactions chirality is a conserved quantum number, protecting the semimetallic physics against perturbations that are translationally invariant. In this Letter we show that the interplay between interaction and topology yields a novel chiral excitonic insulator. The state is characterized by a complex vectorial order parameter leading to a gapping out of the Weyl nodes. A striking feature is that it is ferromagnetic, with the phase of the order parameter determining the direction of the induced magnetic moment. PMID:23215410

Wei, Huazhou; Chao, Sung-Po; Aji, Vivek

2012-11-09

149

Two novel patients with Bohring-Opitz syndrome caused by de novo ASXL1 mutations.  

PubMed

Bohring-Opitz syndrome (BOS) is a rare condition characterized by facial anomalies, multiple malformations, failure to thrive and severe intellectual disabilities. Recently, the cause was identified on the basis of de novo heterozygous mutations in the ASXL1 gene. We report on two novel cases carrying two previously undescribed mutations (c.2407_2411del5 [p.Q803TfsX17] and c.2893C>T [p.R965X]). These new data further support ASXL1 as cause of BOS and may contribute to a more precise definition of the phenotype caused by the disruption of this gene. PMID:22419483

Magini, Pamela; Della Monica, Matteo; Uzielli, Maria Luisa Giovannucci; Mongelli, Patrizia; Scarselli, Gloria; Gambineri, Eleonora; Scarano, Gioacchino; Seri, Marco

2012-03-14

150

The Schwarzschild black hole's remnant via the Bohr-Sommerfeld quantization rule  

NASA Astrophysics Data System (ADS)

A d-dimensional Schwarzschild black hole is quantized by the action variable and the Bohr-Sommerfeld quantization rule in this paper. We find that the spectra of the horizon area and the entropy are evenly spaced. The black hole mass is also quantized and it's spectrum spacing is proportional inversely to the mass. The ground state appears and has a constant entropy ? k_B. The ground state mass is shown to be the black hole remnant predicted by the generalized uncertainty principle and may be a candidate of dark matter.

Chen, Deyou; Zeng, Xiaoxiong

2013-03-01

151

Niels Bohr collected works, volume 8: The penetration of charged particles through matter (1912-1954)  

SciTech Connect

This volume is devoted to the subject of penetration of charged particles through matter. Bohr's first acquaintance with this subject was as early as in 1912 when he treated the absorption of a and v rays on the basis of Rutherford's atomic model and from then on he kept a lifelong interest in the subject, often using it as an important test of the methods of atomic mechanics. His last paper on penetration, written together with Hens Lindhard, dealt with electron capture and loss and was published in 1954.

Thorsen, J.

1987-01-01

152

Characteristics of excitonic emission in diamond  

NASA Astrophysics Data System (ADS)

The Editor's Choice [1] is a theoretical study of the excitonic emission properties in diamond, in particular with regard to possible Bose-Einstein condensation (BEC). The cover picture shows a simple phase diagram for electron-hole pairs in dependence on their density and temperature. It is predicted that exciton BEC does occur for highly excited exciton densities, e.g. on the order of 1018-1019 cm-3, at low temperatures.The first author, Hideyo Okushi, started to study homoepitaxial CVD diamond films for electronic devices in 1995, now at AIST. Since 2001, he has been the leader of the CREST project Ultraviolet nanodevices using high-density exciton in diamond.This issue contains papers presented at SBDD X. This year saw the 10th anniversary of the series of International Workshops on Surface and Bulk Defects in CVD Diamond Films, which are traditionally held in Diepenbeek-Hasselt, Belgium.

Okushi, Hideyo; Watanabe, Hideyuki; Kanno, Shokichi

2005-09-01

153

Exciton Cascade Model for Fast Neutron Reactions.  

National Technical Information Service (NTIS)

A more sophisticated version of the exciton cascade model, treating equilibrium and pre-equilibrium particle emissions in a unique way has been developed and applied to the description of neutron induced reactions, using realistic input data. The master e...

Z. Schram G. Kluge K. Sailer

1987-01-01

154

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

155

Inflating Antenna: New Mechanism of Exciton Transients  

NASA Astrophysics Data System (ADS)

We investigate effect of the exciton continuum on exciton transients as applied to the Free Induction (FI) signal. Low- and moderate-intensity regime is considered, hence, bound excitons and biexcitons can be considered as noninteracting entities. Exciton coupled to an impurity is used as a model system. For it the FI signal can be found as a linear response. Results for biexcitons are actually the same. The theory results in an exact expression for the photon scattering amplitude A(t) in terms of Im hat T(?) where hat T(?) is the scattering operator for the exciton+impurity problem. For large values of the time t, t agt \\varepsilon_b-1, the FI intensity follows the law I(t)? |A(t)|^2 ? t^2 + bt \\cos(\\varepsilon_bt + ?) where \\varepsilonb is the exciton-to-impurity binding energy. The monotonic and oscillatory parts of the FI grow (rather than decay!) with t. It is because the wave packet created by the exciting pulse expands as t increases and forms an inflating antenna. FI signal from the antenna grows with t since the zero-momentum component of the packet grows. Exciton continuum and the impurity potential critically contribute to this behavior. Fast increase in I(t) results in the decrease in the effective radiative time ?_R^*(t). Finally, decreasing ?_R^*(t) restricts the signal growth and results in a short FI pulse. If the exciting-pulse length is about \\varepsilon_b-1, the FI pulse ceases in the time of about ?_R^* ~ (?_R/\\varepsilon_b^2)^1/3, where ?R is the bound-exciton radiative time.

Rashba, Emmanuel

1997-03-01

156

Diffusion of a Plasmon-Exciton Polaron  

NASA Astrophysics Data System (ADS)

We consider the motion of an exciton constrained to a quasi-one-dimensional geometry in the vicinity of a metal interface. For weak coupling, the metal causes damping of the center of mass motion, leading to a decrease in the exciton diffusion constant. This can be modeled as non-contact dielectric friction between an oscillating dipole and a substrate, where the frictional force is related to the response of the metal through the fluctuation dissipation theorem [1]. When the exciton frequency is in the neighborhood of the plasma resonance, the interaction can no longer be described by linear response theory, for the exciton and plasmon form a quasiparticle, an exciton-plasmon polaron. We calculate the transmission and reflection coefficients for the exciton-plasmon polaron in the neighborhood of a metal interface, as well as the diffusion rate and radiative lifetime versus coupling strength.[4pt] [1] Seppe Kuehn, John A. Marohn, and Roger F. Loring, 110(30) J. Phys. Chem. B, (2006) 1425

Cherqui, Charles; Dunlap, David; Piryatinski, Andrei

2012-02-01

157

Optimization of exciton currents in photosynthetic systems.  

PubMed

In an approach analogous to that used to treat electronic currents in semiconductor quantum dots, we investigate the exciton current in a pigment network that is sandwiched between two exciton reservoirs, also known as the emitter and the acceptor. Employing the master equation for the reduced density matrix, the exciton current is obtained analytically for a two-site model, and numerically for an eight-site Fenna-Matthews-Olson (FMO) subunit model. It is found that, to maximize the exciton current with a specific network configuration, there exist optimal emitter temperatures and exciton transfer rates between the network and the reservoirs. The steady state current in the FMO model is consistent with the trapping time calculated by network optimization in the one-exciton picture. The current optimization with respect to various control parameters is discussed for the FMO model. At and below the biologically relevant transfer rate 1 ps(-1), the FMO network is more efficient for excitation energy transfer than the two-site model. Beyond this scale, the FMO network shows robustness with respect to the interplay with the reservoirs. PMID:23534666

Guan, Chengbo; Wu, Ning; Zhao, Yang

2013-03-21

158

Pattern formation in the exciton inner ring  

NASA Astrophysics Data System (ADS)

We report on the two-beam study of indirect excitons in the inner ring in the exciton emission pattern. One laser beam generates the inner ring and a second weaker beam is positioned in the inner ring. The beam positioned in the inner ring is found to locally suppress the exciton emission intensity. We also report on the inner ring fragmentation and formation of multiple rings in the inner ring region. These features are found to originate from a weak spatial modulation of the excitation beam intensity in the inner ring region. The modulation of exciton emission intensity anticorrelates with the modulation of the laser excitation intensity. The three phenomena—inner ring fragmentation, formation of multiple rings in the inner ring region, and emission suppression by a weak laser beam in the inner ring—have a common feature: a reduction of exciton emission intensity in the region of enhanced laser excitation. This effect is explained in terms of exciton transport and thermalization.

Remeika, M.; Hammack, A. T.; Poltavtsev, S. V.; Butov, L. V.; Wilkes, J.; Ivanov, A. L.; Campman, K. L.; Hanson, M.; Gossard, A. C.

2013-09-01

159

Surface Exciton-Plasmons in Carbon Nanotubes  

NASA Astrophysics Data System (ADS)

We study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. We use our previously developed Green's function formalism to quantize an electromagnetic field in the presence of quasi-1D absorbing bodies [1]. We show that these interactions result in the exciton-plasmon coupling that is significant in its strength due to the presence of weakly-dispersive low-energy (˜0.5-2eV) interband surface plasmon modes [2] and large exciton excitation energies ˜1eV in small-diameter nanotubes [3]. We estimate the exciton-plasmon Rabi splitting to be ˜0.01-0.1eV which is close to that observed in organic semiconductors [4] and much larger than that reported for hybrid semiconductor-metal nanoparticle molecules [5]. We calculate the exciton absorption lineshape and demonstrate a clear line splitting effect as the exciton energy is tuned to the closest interband surface plasmon resonance. [1] I.V.Bondarev and Ph.Lambin, Phys. Rev. B72, 035451 (2005). [2] T.Pichler, et al., Phys. Rev. Lett. 80, 4729 (1998). [3] D.Spataru, et al., Phys. Rev. Lett. 95, 247402 (2005). [4] J.Belessa, et al., Phys. Rev. Lett. 93, 036404 (2004). [5] W.Zhang, A.O.Govorov, G.W.Bryant, Phys. Rev. Lett. 97, 146804 (2006).

Bondarev, Igor; Tatur, Kevin; Woods, Lilia

2008-03-01

160

Orientation of luminescent excitons in layered nanomaterials  

NASA Astrophysics Data System (ADS)

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling--MoS2 and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS2 mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.

Schuller, Jon A.; Karaveli, Sinan; Schiros, Theanne; He, Keliang; Yang, Shyuan; Kymissis, Ioannis; Shan, Jie; Zia, Rashid

2013-04-01

161

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

162

Excitons Bound to Ionized Impurities: Calculation of the Binding Energies of Exciton-Ionized-Donor Complexes  

Microsoft Academic Search

The method given by Pekeris for the helium atom and generalized recently by Frost for the three-particle system has been developed extensively to apply to complexes of excitons bound to ionized impurities in semiconductors. Haken's exciton potential is generalized for the complex, and the dielectric constant between the different particles is a function of the interparticle distances. This potential is

S. G. Elkomoss

1971-01-01

163

Coherence Length of Cold Exciton Gases in Coupled Quantum Wells  

NASA Astrophysics Data System (ADS)

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.

Yang, Sen; Hammack, A. T.; Fogler, M. M.; Butov, L. V.; Gossard, A. C.

2006-11-01

164

Coherence length of cold exciton gases in coupled quantum wells.  

PubMed

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas. PMID:17155574

Yang, Sen; Hammack, A T; Fogler, M M; Butov, L V; Gossard, A C

2006-10-31

165

Distinguishing excitonic from vibronic oscillations in ultrafast spectroscopy  

NASA Astrophysics Data System (ADS)

Ultrafast experiments on photosynthetic and conjugated organic systems have indicated that coherent delocalization of exciton states significantly contributes to exciton transport, even up to room temperature. Oscillations in 2D spectra due to excitonic delocalization can be similar to those from vibronic oscillations, which are not important for exciton transport. We describe a straightforward experiment -- broadband pump-probe -- to distinguish between ultrafast oscillations from excitonic or vibronic-only sources. We consider anharmonic molecular oscillators and consider the requirements for how broadband (short in time) the experimental pulses must be to distinguish excitonic from vibronic oscillations.

Krich, Jacob; Yuen, Joel; Johnson, Allan; Goodknight, Joseph; Aspuru-Guzik, Alán

2013-03-01

166

Electric field induced nonlinear optical properties of a confined exciton in a ZnO/Zn1-xMgxO strained quantum dot  

NASA Astrophysics Data System (ADS)

Electric field induced exciton binding energy as a function of dot radius in a ZnO/Zn1-xMgxO quantum dot is investigated. The interband emission as a function of dot radius is obtained in the presence of electric field strength. The Stark effect on the exciton as a function of the dot radius is discussed. The effects of strain, including the hydrostatic and the biaxial strain and the internal electric field, induced by spontaneous and piezoelectric polarization are taken into consideration in all the calculations. Numerical calculations are performed using variational procedure within the single band effective mass approximation. Some nonlinear optical properties are investigated for various electric field strengths in a ZnO/Zn1-xMgxO quantum dot taking into account the strain-induced piezoelectric effects. Our results show that the nonlinear optical properties strongly depend on the effects of electric field strength and the geometrical confinement.

Minimala, N. S.; Peter, A. John; Lee, Chang Woo

2013-02-01

167

Inhibition of CO2 Combination and Reduction of the Bohr Effect in Haemoglobin chemically modified at its alpha-Amino Groups  

Microsoft Academic Search

Blocking of the alpha-amino groups by cyanate inhibits the uptake of CO2 by haemoglobin. It also inhibits the influence which changes in pCO2 at constant pH normally have on the oxygen affinity and on the Bohr effect. Blocking the alpha-amino groups of the alpha chain reduces the alkaline Bohr effect by 25 per cent.

J. V. Kilmartin; L. ROSSI-BERNARDI

1969-01-01

168

Excitons in the optical properties of nanotubes  

NASA Astrophysics Data System (ADS)

We present ab initio calculation of self-energy and electron-hole interaction (excitonic) effects on the optical spectra of single-walled carbon and BN nanotubes. We employed a many-electron Green's function approach that determines both the quasiparticle and optical excitations from first principles. We found important many-electron effects that explain many of the puzzling experimental findings in the optical spectrum of these quasi-one dimensional systems, and the calculated spectra are in excellent quantitative agreement with measurements. In carbon nanotubes, excitons can bind by as much as one eV in semiconducting nanotubes^a). We discovered that bound excitons also exist in metallic carbon nanotubes with binding energy of many tens of meVs^a). Excitonic effects are shown to be even more inportant in BN nanotubes than in carbon nanotubes. Unlike the carbon nanotubes, theory predicts that excitons in some BN nanotubes are comprised of coherent superposition of transitions from several different subband pairs^b). We have also calculated the radiative lifetime of excitons in semiconducting carbon nanotubes. Assuming a thermal occupation of bright and dark exciton bands, we find an effective radiative lifetime of the order of 10 ns at room temperature, in good accord with recent experiments^c). a) C.D. Spataru, S. Ismail-Beigi, L.X. Benedict and S.G. Louie, Phys. Rev. Lett. 92, 077402 (2004). b) C.-H. Park, C.D. Spataru and S.G. Louie, to be published. c) C.D. Spataru, S. Ismail-Beigi, R.B. Capaz and S.G. Louie, in press Phys. Rev. Lett.

Spataru, Catalin

2006-03-01

169

F4TCNQ-Induced Exciton Quenching Studied by Using in-situ Photoluminescence Measurements  

NASA Astrophysics Data System (ADS)

The role of F4TCNQ as an exciton quenching material in thin organic light-emitting films is investigated by means of in situ photoluminescence measurements. C60 was used as another quenching material in the experiment for comparison, with Alq3 as a common organic light-emitting material. The effect of the growth sequence of the materials on quenching was also examined. It is found that the radius of Förster energy transfer between F4TCNQ and Alq3 is close to 0 nm and Dexter energy transfer dominates in the quenching process.

Zhu, Jian; Lu, Min; Wu, Bo; Hou, Xiao-Yuan

2012-09-01

170

Short radius drilling system improves directional control  

SciTech Connect

Horizontal drilling capabilities and applications have been dramatically increased with development of Becfield Drilling Services` Short Radius Horizontal Drilling System utilizing the Articulated Downhole Drilling Motor (ADM). The system gives precise directional control, predictability , and reliability not previously available in short-radius operations. Because of the unique, patented design of the ADM, the short-radius system can be rotated during lateral drilling operations. This is a significant development in short-radius horizontal drilling technology. This paper reviews the design and operation of this equipment.

Leazer, C. [Becfield Drilling Services, Houston, TX (United States)

1995-08-01

171

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

172

Exciton annihilation studies in poly(p-phenylene vinylene)  

SciTech Connect

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

173

Diamagnetic Exciton Properties in Quantum Dot Molecules  

NASA Astrophysics Data System (ADS)

The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots (artificial molecules) has become subject of interest. The diamagnetic shift of confined excitons complexes has been used as a measured of the wave function spatial extent in semiconductor nanostructures. In weak magnetic field, the diamagnetic shift is expected to exhibit quadratic dependence. However, for exciton complexes the diamagnetic behavior is expected to exhibit more complicated features related to electron-hole asymmetry effects on Coulomb interactions. In this work we study the magnetic response of neutral and charged excitons in InAs/GaAs asymmetric artificial molecules By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. These predicted regions, which show coexistence of crossing and anticrossing exciton states, because of allowing control of charge localization and polarization of emitted photons. .

Ricardo, Fino Puerto Nelson; Hanz Yecid, Ramirez; S, Camacho Angela

2012-02-01

174

Ultrafast Exciton Dephasing in Semiconducting Single-Walled Carbon Nanotubes  

Microsoft Academic Search

Femtosecond two-pulse degenerate four-wave mixing spectroscopy was applied to study the exciton dephasing in a broad range of excitation intensities and lattice temperatures. We find that both exciton-exciton and exciton-phonon scattering have profound effects on the dephasing process. The dominant phonon mode involved in the dephasing is identified as the out-of-plane, transverse optical mode with a frequency of 847cm-1. The

Ying-Zhong Ma; Matthew W. Graham; Graham R. Fleming; Alexander A. Green; Mark C. Hersam

2008-01-01

175

The Bohr Hamiltonian Solution with the Morse Potential for the {gamma}-unstable and the Rotational Cases  

SciTech Connect

Analytical solutions of the collective Bohr Hamiltonian with the Morse potential have been obtained for the U(5)-O(6) and U(5)-SU(3) transition regions through the Asymptotic Iteration Method (AIM). The obtained energy eigenvalue equations have been used to get the experimental excitation energy spectrum of Xe and Yb isotopes. The results are in good agreement with experimental data.

Inci, I.; Boztosun, I. [Department of Physics, Erciyes University, 38039, Kayseri (Turkey); Bonatsos, D. [Insitue of Nuclear Physics, N.C.S.R. 'Demokritos' GR-15310 Aghia Paraskevi, Attiki (Greece)

2008-11-11

176

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

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

177

Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires  

SciTech Connect

Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.

Ma, Yingzhong [ORNL; Xiao, Kai [ORNL; Shaw, Robert W [ORNL

2012-01-01

178

Dark excitons and Bose-Einstein condensation in strain-trapped excitons in coupled quantum wells  

NASA Astrophysics Data System (ADS)

It is possible to make excitons in coupled quantum wells with very long lifetime compared to their thermalization, allowing us to study equilibrium behavior. Using a localized stress to create traps for excitons in coupled quantum wells, we have demonstrated that at low temperature, high density, and large stress, the spatial pattern of photoluminescence (PL) from interwell excitons transitions to one with a dimmed center. This pattern emerges despite the center remaining the region of highest exciton density. This darkening is related to a strain-induced interaction between the light hole and heavy hole states. However, while this explanation provides a mechanism to explain many of the features, a few important predictions of this theory are not borne out by experiments. An alternate explanation is possible, utilizing an increasing population of dark (J=2) excitons and a separation of the dark and bright species. It has been proposed that a Bose-Einstein Condensate in this system would occur in a dark state, and this transition is consistent with the onset criteria of the pattern formation and explains how a slight bright/dark energy difference could lead to spatial separation of the species. Experiments employing a magnetic field to turn `dark' excitons slightly `bright' should allow the disambiguation of the role of dark excitons in this system. I will review this pattern formation and discuss data from experiments employing a magnetic field.

Sinclair, Nicholas; Wuenschell, Jeff; West, Kenneth; Pfeiffer, Loren; Snoke, David

2012-02-01

179

The full five-dimensional Bohr Hamiltonian from the generator coordinate method  

NASA Astrophysics Data System (ADS)

The five-dimensional generator coordinate method with the five generator coordinates being the components of the quadrupole tensor is used to generate the nuclear collective quadrupole excitations by the vibrations and rotations of the intrinsic deformed ground state of the mean-field Hamiltonian. The Gaussian overlap approximation leads to a differential eigenvalue equation with the five-dimensional Bohr Hamiltonian. The resulting collective Hamiltonian is presented. It can be calculated only approximately. However, there is a clear algorithm how to improve the approximation. Formulae for the inverse vibrational inertial functions, the moments of inertia and the potential with the vibrational and rotational zero-point energy corrections are given in the lowest-order approximation.

Rohozi?ski, Stanis?aw G.

2013-05-01

180

Mass tensor in the Bohr Hamiltonian from the nondiagonal energy weighted sum rules  

SciTech Connect

Relations are derived in the framework of the Bohr Hamiltonian that express the matrix elements of the deformation-dependent components of the mass tensor through the experimental data on the energies and the E2 transitions relating the low-lying collective states. These relations extend the previously obtained results for the intrinsic mass coefficients of the well-deformed axially symmetric nuclei on nuclei of arbitrary shape. The expression for the mass tensor is suggested, which is sufficient to satisfy the existing experimental data on the energy weighted sum rules for the E2 transitions for the low-lying collective quadrupole excitations. The mass tensor is determined for {sup 106,108}Pd, {sup 108-112}Cd, {sup 134}Ba, {sup 150}Nd, {sup 150-154}Sm, {sup 154-160}Gd, {sup 164}Dy, {sup 172}Yb, {sup 178}Hf, {sup 188-192}Os, and {sup 194-196}Pt.

Jolos, R. V. [Joint Institute for Nuclear Research, RU-141980 Dubna (Russian Federation); Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany); Brentano, P. von [Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany)

2009-04-15

181

Bohr Hamiltonian for collective low-lying vibrational states of well deformed nuclei  

SciTech Connect

It is shown that the Bohr Hamiltonian in the intrinsic frame used for the description of the low-lying vibrational states in the well-deformed nuclei can be presented in two different forms. In the first form the Hamiltonian has three different mass coefficients but the quadrupole transition operator has a standard form with one parameter only. The second form of the Hamiltonian can be derived by a transformation from {beta} and {gamma} to the new variables. In this form the Hamiltonian contains only one mass coefficient but the quadrupole operator takes a different form with three parameters. It is shown also that this Hamiltonian can describe a situation when a collectivity of the vibrational states is rather low, but their excitation energies are relatively small, while the E2 transitions inside the bands are very strong.

Jolos, R. V. [Joint Institute for Nuclear Research, RU-141980 Dubna (Russian Federation); Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany); Brentano, P. von [Institut fuer Kernphysik der Universitaet zu Koeln, D-50937 Koeln (Germany)

2008-06-15

182

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

SciTech Connect

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

183

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

184

Contribution of cooperativity and the Bohr effect to efficient oxygen transport by hemoglobins from five mammalian species.  

PubMed

By using published experimental values of the standard oxygen (O2) equilibrium curve and the in vivo arterial and venous O2 pressure (PO2) of fetal and maternal blood in five mammalian species (human, cow, pig, sheep, and horse), we investigated the relationship between the efficiency of O2 delivery and the effectiveness of the Bohr shift, and discussed the significance of cooperativity for mammalian Hb. The O2 delivery of fetal blood was more efficient than that of maternal blood, and the effectiveness of the Bohr shift at both O2 loading and release sites of fetal blood was high. A linear relationship was observed between the efficiency of O2 delivery and the effectiveness of the Bohr shift at O2 loading sites of the five mammalian species. In both fetal and maternal blood, the theoretically obtained optimal P50 value for O2 delivery (optP50(OD)) was nearly equal to the optimal P50 value for the effectiveness of the Bohr shift at the O2 loading site (optP50(BS)(loading)). This phenomenon was favorable for fetal blood to uptake O2 from maternal blood with the aid of the Bohr shift and to deliver a large amount of O2 to the tissues. The optP50s for the effectiveness of the Bohr shift at given arterial PO2 (PaO2) and venous PO2 (PvO2) were derived as follows: optP50(BS)(loading) = PaO2((n+1)/(n-1))(1/n), and optP50(BS)(release) = PvO2((n+1)/(n-1))(1/n). The relationship between in vivo PO2s and n, PaO2/PvO2 = ((n+1)/(n-1))(2/n), was derived by letting optP50 for the efficiency of O2 delivery be equal to that for the effectiveness of the Bohr shift. PMID:16547405

Zhang, Yan; Kobayashi, Keiko; Kitazawa, Kazuki; Imai, Kiyohiro; Kobayashi, Michiyori

2006-01-01

185

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

186

Chemical equilibrium between excitons, electrons, and negatively charged excitons in semiconductor quantum wells  

NASA Astrophysics Data System (ADS)

The principle of chemical equilibrium is found to govern formation of negatively charged excitons (X-) by excitons (X) and free electrons (e-). An optical spectroscopy study of undoped CdTe/Cd0.88Mn0.12Te double quantum wells (QW's) revealed formation of a negatively charged exciton state in the thicker QW due to the interwell tunneling of photoexcited electrons. A peculiar, nonmonotonic behavior of the ratio of X to X- densities as a function of the excitation density evidenced that the chemical law of action of masses corresponding to the reaction X+e-<==>X- is fulfilled.

Siviniant, J.; Scalbert, D.; Kavokin, A. V.; Coquillat, D.; Lascaray, J.-P.

1999-01-01

187

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

NSDL National Science Digital Library

The year 2010 is the centennial of the publication of the ÃÂSeven Little Devilsàin the predecessor of Acta Physiologica. In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity substitute to secretion. The subjects of the present reevaluation of this controversy are Christian Bohr, Professor and Doctor of Medicine (1855ÃÂ1911), nominated three times for the Nobel Prize; August Krogh, Doctor of Philosophy (1874ÃÂ1949), Christian Bohr's assistant and later Nobel Prize laureate (1920); and Marie Krogh, née Jørgensen, Doctor of Medicine and wife of August Krogh (1874ÃÂ1943). The controversy concerned is the transport of oxygen from the lungs into the bloodstream: are passive transport and diffusion capacity together sufficient to secure the oxygen supply in all circumstances or is there an additional specific (ÃÂenergy consumingàor ÃÂactiveÃÂ) mechanism responsible for the transport of oxygen from the alveoli into the bloodstream? The present discussion purports to show that the contestants' views were closer than the parties themselves and posterity recognized. Posterity has judged the dispute unilaterally from the Nobel laureate's point of view, but it is evident that August Krogh's Nobel Prize was awarded for the discovery of a cellular activity (Christian Bohr's expression), represented by Krogh's discovery of capillary recruitment. Christian Bohr appears to have been correct in the narrower sense that the diffusion capacity at rest is not great enough to explain the transport during work; a special mechanism intervenes and optimizes the conditions under which diffusion acts. August Krogh, of course, was right in the wider sense that the transport mechanism itself is always entirely passive.

2010-10-01

188

Excitonic Dynamical Franz-Keldysh Effect  

NASA Astrophysics Data System (ADS)

The dynamical Franz-Keldysh effect is exposed by exploring near-band-gap absorption in the presence of intense THz electric fields. It bridges the gap between the dc Franz-Keldysh effect and multiphoton absorption and competes with the THz ac Stark effect in shifting the energy of the excitonic resonance. A theoretical model which includes the strong THz field nonperturbatively via a nonequilibrium Green functions technique is able to describe the dynamical Franz-Keldysh effect in the presence of excitonic absorption.

Nordstrom, K. B.; Johnsen, K.; Allen, S. J.; Jauho, A.-P.; Birnir, B.; Kono, J.; Noda, T.; Akiyama, H.; Sakaki, H.

1998-07-01

189

Bright and dark excitons in semiconductor carbon nanotubes  

SciTech Connect

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

190

Excitonic emission of electrons from aromatic hydrocarbon crystals  

NASA Astrophysics Data System (ADS)

Electron emission has been observed with crystals of pyrene, (alpha) -perylene and anthracene excited with photons whose energies are less than the threshold for conventional monophotonic emission. It has been established that exciton fusion is responsible for this emission. In the case of pyrene, the lifetime of excitons observed through the excitonic photoemission is the same with that obtained from the fluorescence decay, whereas in (alpha) -perylene and anthracene the lifetime obtained from photoemission experiments is shorter compared to the fluorescence lifetime. It is considered that fluorescence measures the lifetime of excitons in the bulk of a crystal, whereas electron emission of this type probes excitons at the surface.

Ono, Masaki; Katoh, Ryuzi; Kotani, Masahiro

1995-02-01

191

Three-photon magnetoabsorption of excitons in alkali halides  

NASA Astrophysics Data System (ADS)

Three-photon magnetoabsorption measurements in fields up to 9 T are presented for the first time. A narrow exciton line of ?-5 symmetry is resolved in CsI at 5.8076 eV. A g value of 1.17 is deduced from the Zeeman splitting of this exciton. At high magnetic fields an exciton of ?-3 symmetry is detected. The analysis of the field-dependent mixing of the ?-3 and ?-5 excitons yields a zero-field shift of the ?-3 exciton to lower energies of 0.54 meV and an effective g value for the admixture of 1.20.

Beerwerth, F.; Fröhlich, D.

1986-09-01

192

Coherence Length of Cold Exciton Gases in Coupled Quantum  

NASA Astrophysics Data System (ADS)

We report on emergence of spontaneous coherence of excitons at low temperatures. A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length reaches about 2-3 microns at the lowest temperature(1.5K), this corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.

Yang, Sen; Hammack, A. T.; Fogler, M. M.; Butov, L. V.; Gossard, A. C.

2007-03-01

193

Temperature dependence of free excitons in GaN  

NASA Astrophysics Data System (ADS)

The excitons involved in this study are the longitudinal and the Gamma5 and Gamma6 free excitons, as well as the donor bound exciton (D0),X. The temperature dependence of the energy positions of the Gamma5, Gamma6, and (D0),X excitons are well accounted for by the Varshni equation (Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)). In the same temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation. The separation between the longitudinal- and transverse-mode free excitons has been previously reported. One component of this separation is the polarizability, which has a temperature dependence. The longitudinal exciton therefore has a band-gap temperature dependence, predicted by the Varshni equation, as well as an additional dependence due to polarizability. This temperature dependence has been accounted for by the Varshni equation, plus an additional linear and a quadratic temperature dependent term.

Reynolds, D. C.; Hoelscher, J.; Litton, C. W.; Collins, T. C.

2002-11-01

194

(Gene sequencing by scanning molecular exciton microscopy)  

SciTech Connect

This report details progress made in setting up a laboratory for optical microscopy of genes. The apparatus including a fluorescence microscope, a scanning optical microscope, various spectrometers, and supporting computers is described. Results in developing photon and exciton tips, and in preparing samples are presented. (GHH)

Not Available

1991-01-01

195

Sun as many-particle spherical exciton  

Microsoft Academic Search

A theory is presented which provides a description of the Sun as a quantum object with the use of the spherical exciton concept. Most of the results can also be applied to many other plasma objects such as stars of different types, spherical lightning, electron-hole drops in semiconductors, etc. The author calculates the minimum number of particles needed for stability

G. K. Vlasov

1997-01-01

196

Trapping excitons in a two-dimensional in-plane harmonic potential: experimental evidence for equilibration of indirect excitons.  

PubMed

We have trapped a gas of long-lifetime, high-mobility excitons in an in-plane harmonic potential. Trapping is an important step toward the goal of a controlled Bose-Einstein condensate of excitons. We show that the repulsive interaction between the excitons plays a dominant role in the behavior of the excitons, in contrast with the weak interactions in atomic gases. We show that under proper conditions the excitons thermalize in the trap to a well-defined equilibrium spatial distribution. PMID:16907396

Vörös, Z; Snoke, D W; Pfeiffer, L; West, K

2006-07-07

197

Nuclear charge radius of 12Be.  

PubMed

The nuclear charge radius of (12)Be was precisely determined using the technique of collinear laser spectroscopy on the 2s(1/2)?2p(1/2,3/2) transition in the Be(+) ion. The mean square charge radius increases from (10)Be to (12)Be by ?(10,12)=0.69(5) fm(2) compared to ?(10,11)=0.49(5) fm(2) for the one-neutron halo isotope ^{11}Be. Calculations in the fermionic molecular dynamics approach show a strong sensitivity of the charge radius to the structure of ^{12}Be. The experimental charge radius is consistent with a breakdown of the N=8 shell closure. PMID:22540787

Krieger, A; Blaum, K; Bissell, M L; Frömmgen, N; Geppert, Ch; Hammen, M; Kreim, K; Kowalska, M; Krämer, J; Neff, T; Neugart, R; Neyens, G; Nörtershäuser, W; Novotny, Ch; Sánchez, R; Yordanov, D T

2012-04-03

198

Truck Paths on Short Radius Turns.  

National Technical Information Service (NTIS)

Transparent templates of turning track widths of trucks on short radius turns were developed for use by highway designers. Templates were developed using a model known as a Tractrix Integrator. These were compared with offtracking results obtained from ac...

C. W. Otte

1972-01-01

199

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

200

The Radius Cutoff for Relativistic Clusters  

NASA Astrophysics Data System (ADS)

In this work we consider an isothermic relativistic kinetic gas under relativistic conditions, as a model for high center density galaxies. We apply the radius cutoff of the Emden type, in order to remove infinities. Numerically we find some theoretical configurations with very high values of masses concentrated in small volumes. Profiles of some thermodynamic quantities such as density, presure, will be independent from the choice of radius cutoff, whereas temperature and gravitational potential depend essentially on the cutoff value.

Hafizi, M.

2000-07-01

201

Mass-Radius Relationships for Solid Exoplanets  

Microsoft Academic Search

We use new interior models of cold planets to investigate the mass-radius relationships of solid exoplanets, considering planets made primarily of iron, silicates, water, and carbon compounds. We find that the mass-radius relationships for cold terrestrial mass planets of all compositions we considered follow a generic functional form that is not a simple power law: log10Rs=k1+1\\/3log10(Ms)-k2Mk3s for up to Mp~20

S. Seager; M. Kuchner; C. A. Hier-Majumder; B. Militzer

2007-01-01

202

Exciton states in CdSe/ZnS core–shell quantum dots under applied electric fields  

NASA Astrophysics Data System (ADS)

Within the effective-mass and parabolic-band approximations, the effect of the applied electric field on the exciton states in CdSe/ZnS core–shell quantum dots is investigated. Electron and hole single-particle energies as functions of the electric field are obtained by using the finite element method. The Coulomb interaction is calculated in the first-order perturbation theory, and optical absorption wavelengths of the nanocrystals with a fixed outer radius and different shell thicknesses are compared. We found that the spatial distribution of the carrier wave functions inside the heterostructure and emission spectra of the excitons can be tuned by varying the core size and the electric field strength.

Niculescu, E. C.; Cristea, M.; Spandonide, A.

2013-11-01

203

A Novel System for Transcutaneous Application of Carbon Dioxide Causing an ``Artificial Bohr Effect'' in the Human Body  

Microsoft Academic Search

BackgroundCarbon dioxide (CO2) therapy refers to the transcutaneous administration of CO2 for therapeutic purposes. This effect has been explained by an increase in the pressure of O2 in tissues known as the Bohr effect. However, there have been no reports investigating the oxygen dissociation of haemoglobin (Hb) during transcutaneous application of CO2in vivo. In this study, we investigate whether the

Yoshitada Sakai; Masahiko Miwa; Keisuke Oe; Takeshi Ueha; Akihiro Koh; Takahiro Niikura; Takashi Iwakura; Sang Yang Lee; Masaya Tanaka; Masahiro Kurosaka; Mauricio Rojas

2011-01-01

204

Excess equimolar radius of liquid drops.  

PubMed

The curvature dependence of the surface tension is related to the excess equimolar radius of liquid drops, i.e., the deviation of the equimolar radius from the radius defined by the macroscopic capillarity approximation. Based on the Tolman [J. Chem. Phys. 17, 333 (1949)] approach and its interpretation by Nijmeijer et al. [J. Chem. Phys. 96, 565 (1991)], the surface tension of spherical interfaces is analyzed in terms of the pressure difference due to curvature. In the present study, the excess equimolar radius, which can be obtained directly from the density profile, is used instead of the Tolman length. Liquid drops of the truncated and shifted Lennard-Jones fluid are investigated by molecular dynamics simulation in the canonical ensemble, with equimolar radii ranging from 4 to 33 times the Lennard-Jones size parameter ?. In these simulations, the magnitude of the excess equimolar radius is shown to be smaller than ?/2. This suggests that the surface tension of liquid drops at the nanometer length scale is much closer to that of the planar vapor-liquid interface than reported in studies based on the mechanical route. PMID:22587106

Horsch, Martin; Hasse, Hans; Shchekin, Alexander K; Agarwal, Animesh; Eckelsbach, Stefan; Vrabec, Jadran; Müller, Erich A; Jackson, George

2012-03-26

205

Electric field controlled exciton transport in a conjugated polymer chain  

NASA Astrophysics Data System (ADS)

The control of exciton transport in conjugated polymers is of fundamental importance for luminescence and photovoltaic properties of polymeric optoelectronic devices. We have investigated the elastic scattering processes of a negative polaron and a neutral exciton in a conjugated polymer chain in the presence of an external electric field. We demonstrate that the exciton can be pushed or pulled to transport in a given direction by the polaron, or migrate by exchanging positions with the polaron in particular range of electric field strength. The exciton can preserve quantum coherence in these field induced transfer processes. The manner of exciton motion depends on the spin configuration of the exciton and the polaron as well as the electric field strength. This knowledge will serve to understand the dynamics of intrachain energy transport in conjugated polymers.

Qiu, Yu; Wang, Xiao-Lei; Tong, Guo-Ping

2012-06-01

206

Energy levels of exciton in a gapped graphene sheet  

NASA Astrophysics Data System (ADS)

A theory is presented for exciton formation in a graphene sheet using the center-of-mass approximation. The energy levels and wavefunctions of exciton are calculated analytically which show that the exciton can form if the band gap of graphene is not zero. We show that the energy gap of graphene plays the role of the mass which if not zero, leads to formation of the excitons. It is shown that the main quantum number of the exciton ground state changes with the graphene dielectric constant. Also, all of the states are found to be four-fold degenerate. The binding energy of exciton can reach as high as 1/4 of the energy gap of graphene which is notable among the conventional quasi-2D systems. This result can play an important rule in the photonics of graphene.

Fallah, Farhang; Esmaeilzadeh, Mahdi

2013-08-01

207

Exciton localization in films of ZnO nanoparticles  

NASA Astrophysics Data System (ADS)

Temperature dependent photoluminescence (PL) spectroscopy has been employed to study the exciton localization in ZnO nanostructures of various sizes. Low temperature (LT) near-band-edge UV emission from ZnO nanostructures of size less than 100 nm show signature of free exciton (FE), bound exciton and longitudinal-optical phonon replica of FE lines. Free and bound excitonic emission deviates from the Varshni equation at LT and exhibits S-shaped temperature dependence due to the exciton localization effect. Deviations of the measured data from the Varshni equation directly provide the exciton localization energies, ELoc. It was found that ELoc increases and dELocalization due to thermal activation occurs at higher temperature for smaller nanoparticles.

Ghosh, Manoranjan; Ghosh, Karabi; Patra, G. D.; Tyagi, M.; Sen, S.; Gadkari, S. C.

2012-06-01

208

Nanotransformation and current fluctuations in exciton condensate junctions.  

PubMed

We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale. PMID:22587267

Soller, H; Dolcini, F; Komnik, A

2012-04-10

209

The Allender-Bray-Bardeen Model of Excitonic Superconductivity Revisited  

NASA Astrophysics Data System (ADS)

The Allender-Bray-Bardeen (ABB) model of excitonic superconductivity is re-formulated in a non-perturbative approach. The exciton in the new formulation is taken to be real and stable instead of the virtual ones assumed in the ABB model. A many-body variational technique is used to calculate the ground-state energy of two electrons plus an exciton. The result gives attractive interaction to the two electrons. The difficulties of the original ABB model are shown linked to the virtual excitons. The band-structure for the existence of excitonic superconductivity is predicted as our result. Comparison with cuprate, boride and field-doped C-60 high critical temperature superconductors suggests an excitonic origin of their superconductivity.

Luo, Nie

2002-03-01

210

Exciton binding energies in chalcopyrite semiconductors  

NASA Astrophysics Data System (ADS)

The optical spectra of Cu-III-VI2 chalcopyrite compounds display rich excitonic features in the fundamental direct bandgap energy region. The energy structure of excited excitonic states reported in the literature are reexamined using a calculation of the eigenstates of the hydrogenic problem in the context of the anisotropic band structure and the anisotropy of the dielectric constant. We find some remarkable agreements as well as inconsistencies in the literature that we attribute to the following reasons: (i) the difficulty to interpret fine structure-splitting data in noncubic semiconductors, and (ii) the more severe difficulty growing these materials with high enough quality. We finally propose some values that match very well with recent proposals and integrate the trend between Rydberg energies and bandgap values for the binary inorganic zincblende and wurtzite semiconductors.

Gil, Bernard; Felbacq, Didier; Chichibu, Shigefusa F.

2012-02-01

211

Directional exciton transport in supramolecular nanostructured assemblies.  

PubMed

Supramolecular nanostructured host-guest compounds provide an intriguing strategy for improved materials in optoelectronic devices. Here, conjugated organic guest molecules are embedded in channel-forming organic or inorganic hosts, like perhydrotriphenylene (PHTP) and zeolites, leading to weakly coupled, highly ordered, and brightly emissive materials. In-depth material characterization allows for a full understanding of the exciton transport mechanism in the weak coupling regime through steady-state time-resolved fluorescence studies combined with quantum-chemical based Monte-Carlo simulations without adjustable parameters. Despite weak chromophore coupling and dilution by the host, the exciton diffusion length exceeds 50 nm similar to molecular crystals, and might be tuned to the ?m range by the proper choice of host and guest materials. PMID:22941317

Gierschner, Johannes

2012-10-14

212

Excitons in T-shaped quantum wires  

Microsoft Academic Search

We calculate energies, oscillator strengths for radiative recombination, and\\u000atwo-particle wave functions for the ground state exciton and around 100 excited\\u000astates in a T-shaped quantum wire. We include the single-particle potential and\\u000athe Coulomb interaction between the electron and hole on an equal footing, and\\u000aperform exact diagonalisation of the two-particle problem within a finite basis\\u000aset. We calculate

M. H. Szymanska; P. B. Littlewood; R. J. Needs

2001-01-01

213

Effective scatterings between electrons, excitons and trions  

Microsoft Academic Search

.  \\u000a The final goal of this paper is to derive the effective scattering ruling the time evolution of two semiconductor trions using\\u000a the many-body formalism for composite fermions we have just proposed. However, to understand the importance of the particle\\u000a composite nature, their bosonic\\/fermionic character and their overall charge, we also report on scatterings between free electrons,\\u000a excitons and trions.

M. Combescot; O. Betbeder-Matibet

2011-01-01

214

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

215

Nuclear Charge Radius of {sup 8}He  

SciTech Connect

The root-mean-square (rms) nuclear charge radius of {sup 8}He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of {sup 6}He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from {sup 6}He to {sup 8}He is an indication of the change in the correlations of the excess neutrons and is consistent with the {sup 8}He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.

Mueller, P.; Bailey, K.; Holt, R. J.; Janssens, R. V. F.; O'Connor, T. P. [Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Sulai, I. A.; Lu, Z.-T. [Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Department of Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 (United States); Villari, A. C. C.; Alcantara-Nunez, J. A.; Alves-Conde, R.; Dubois, M.; Eleon, C.; Gaubert, G.; Lecesne, N.; Saint-Laurent, M.-G.; Thomas, J.-C. [GANIL (IN2P3/CNRS-DSM/CEA), B.P. 55027 F-14076 Caen Cedex 5 (France); Drake, G. W. F. [Physics Department, University of Windsor, Windsor, Ontario, N9B 3P4 (Canada); Wang, L.-B. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2007-12-21

216

Nuclear charge radius of 8He.  

PubMed

The root-mean-square (rms) nuclear charge radius of 8He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of 6He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from 6He to 8He is an indication of the change in the correlations of the excess neutrons and is consistent with the 8He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations. PMID:18233516

Mueller, P; Sulai, I A; Villari, A C C; Alcántara-Núñez, J A; Alves-Condé, R; Bailey, K; Drake, G W F; Dubois, M; Eléon, C; Gaubert, G; Holt, R J; Janssens, R V F; Lecesne, N; Lu, Z-T; O'Connor, T P; Saint-Laurent, M-G; Thomas, J-C; Wang, L-B

2007-12-21

217

Quantum Simulation of Multiple-Exciton Generation in a Nanocrystal by a Single Photon.  

National Technical Information Service (NTIS)

We have shown theoretically that efficient multiple exciton generation (MEG) by a single photon can be observed in small nanocrystals (NCs). Our quantum simulations that include hundreds of thousands of exciton and multi-exciton states demonstrate that th...

A. Shabaev A. L. Efros C. S. Hellberg V. L. Jacobs W. M. Witzel

2010-01-01

218

Boron acceptor concentration in diamond from excitonic recombination intensities  

Microsoft Academic Search

Excitonic recombinations are investigated by cathodoluminescence in a series of homoepitaxial diamond layers doped with boron in the range (2×1016)-(5×1018)atcm-3. As opposed to earlier observations made on polycrystalline boron-doped diamond, we show that the ratio between the neutral-boron bound exciton and the free-exciton recombination intensities is proportional to the boron content up to 6×1017cm-3 and starts to saturate above this

J. Barjon; T. Tillocher; N. Habka; O. Brinza; J. Achard; R. Issaoui; F. Silva; C. Mer; P. Bergonzo

2011-01-01

219

Bose-Einstein Condensation of Excitons in Cuprous Oxide  

Microsoft Academic Search

Free excitons provide the only experimental system other than helium in which the behavior of particles with mass is known to follow Bose-Einstein statistics. I present experimental observations of the kinetic energy distribution of excitons in the direct-gap semiconductor Cu _2O, both the triplet orthoexciton state and the singlet paraexciton state. The density and temperature of the exciton gas closely

David Wayne Snoke

1990-01-01

220

Three-photon magnetoabsorption of excitons in alkali halides  

Microsoft Academic Search

Three-photon magnetoabsorption measurements in fields up to 9 T are presented for the first time. A narrow exciton line of Gamma-5 symmetry is resolved in CsI at 5.8076 eV. A g value of 1.17 is deduced from the Zeeman splitting of this exciton. At high magnetic fields an exciton of Gamma-3 symmetry is detected. The analysis of the field-dependent mixing

F. Beerwerth; D. Fröhlich

1986-01-01

221

Enhancement of spin propagation due to interlayer exciton condensation  

NASA Astrophysics Data System (ADS)

We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic x-ray scattering. The bandwidth of the triplon scales with the exciton superfluid density, but only in the limit of strong correlations. As such the triplon bandwidth acts as a probe of exciton-spin interactions in the condensate.

Rademaker, Louk; van den Brink, Jeroen; Hilgenkamp, Hans; Zaanen, Jan

2013-09-01

222

Excitons in intact cells of photosynthetic bacteria.  

PubMed

Live cells and regular crystals seem fundamentally incompatible. Still, effects characteristic to ideal crystals, such as coherent sharing of excitation, have been recently used in many studies to explain the behavior of several photosynthetic complexes, especially the inner workings of the light-harvesting apparatus of the oldest known photosynthetic organisms, the purple bacteria. To this date, there has been no concrete evidence that the same effects are instrumental in real living cells, leaving a possibility that this is an artifact of unnatural study conditions, not a real effect relevant to the biological operation of bacteria. Hereby, we demonstrate survival of collective coherent excitations (excitons) in intact cells of photosynthetic purple bacteria. This is done by using excitation anisotropy spectroscopy for tracking the temperature-dependent evolution of exciton bands in light-harvesting systems of increasing structural complexity. The temperature was gradually raised from 4.5 K to ambient temperature, and the complexity of the systems ranged from detergent-isolated complexes to complete bacterial cells. The results provide conclusive evidence that excitons are indeed one of the key elements contributing to the energetic and dynamic properties of photosynthetic organisms. PMID:23379598

Freiberg, Arvi; Pajusalu, Mihkel; Rätsep, Margus

2013-02-18

223

Microcavity controlled coupling of excitonic qubits.  

PubMed

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

224

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

SciTech Connect

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

225

Ultrafast exciton dynamics in ZnO: Excitonic versus electron-hole plasma lasing  

NASA Astrophysics Data System (ADS)

The use of ZnO bulk and especially nanolayer and nanowire structures for novel device applications has led to a renewal of interest in high-electron-density processes in ZnO, such as those occurring during lasing in ZnO. Using a pump-probe reflectometry technique, we investigate the ultrafast exciton dynamics of bulk ZnO under femtosecond laser excitation close to lasing conditions. Under intense excitation by 266-nm femtosecond (fs) pump pulses, the exciton resonance becomes highly damped and does not recover for several picoseconds. This slow recovery indicates a significant screening of the Coulomb interaction. Even below the lasing thresholds typically found for ZnO nanolayers and nanowires, we observe damping of the exciton resonance for several picoseconds, which indicates that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is electron-hole plasma recombination.

Shih, T.; Mazur, E.; Richters, J.-P.; Gutowski, J.; Voss, T.

2011-02-01

226

The charge radius of the proton  

NASA Astrophysics Data System (ADS)

Recently, the charge radius of the proton was extracted for the first time from muonic hydrogen. The value was five standard deviations away from similar measurement of regular hydrogen. This talk discusses work done in collaboration with Richard J. Hill, to address this discrepancy. First, we have studied the extraction of the charge radius of the proton from electron-proton scattering data in a model-independent way. Second, we have looked at a model-independent analysis of proton structure effects for hydrogen-like bound states, using the tool of an effective field theory, namely NRQED.

Paz, Gil

2012-04-01

227

Management of malunions of the distal radius.  

PubMed

Despite encouraging results from small case series, correction of distal radius malunion remains a challenging procedure with uncertain outcomes. The most appropriate treatment for a distal radius malunion is prevention. If a symptomatic malunion is discovered, correction should be undertaken as early as possible. It is recommended that action be taken within six months of the primary injury to decrease the negative impact of soft-tissue contracture on the eventual reconstruction. Although some patients complain about residual problems after malunion surgery, corrective surgery has been shown to improve both radiographic and functional outcomes, and may prevent future secondary problems. PMID:22554664

Haase, Steven C; Chung, Kevin C

2012-04-13

228

Localization - Delocalization Transition of Indirect Excitons in Lateral Electrostatic Lattices  

NASA Astrophysics Data System (ADS)

We report on a study of transport of indirect excitons in GaAs/AlGaAs coupled quantum wells in linear lateral lattices created by laterally modulated gate voltage. The localization-delocalization transition for the excitons was observed with increasing density in the directions along and across the lattice. At high lattice amplitudes, the density corresponding to the transition across the lattice was found to linearly depend on the lattice amplitude. Screening of the potential by repulsive exciton-exciton interaction was demonstrated.

Remeika, M.; Graves, J.; Hammack, A. T.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2009-03-01

229

Ultrafast exciton dephasing in semiconducting single-walled carbon nanotubes.  

PubMed

Femtosecond two-pulse degenerate four-wave mixing spectroscopy was applied to study the exciton dephasing in a broad range of excitation intensities and lattice temperatures. We find that both exciton-exciton and exciton-phonon scattering have profound effects on the dephasing process. The dominant phonon mode involved in the dephasing is identified as the out-of-plane, transverse optical mode with a frequency of 847 cm(-1). The extracted homogeneous linewidths at all measured temperatures are in excellent agreement with the results of a single-tube photoluminescence experiment. PMID:19113450

Ma, Ying-Zhong; Graham, Matthew W; Fleming, Graham R; Green, Alexander A; Hersam, Mark C

2008-11-18

230

Ultrafast Exciton Dephasing in Semiconducting Single-Walled Carbon Nanotubes  

NASA Astrophysics Data System (ADS)

Femtosecond two-pulse degenerate four-wave mixing spectroscopy was applied to study the exciton dephasing in a broad range of excitation intensities and lattice temperatures. We find that both exciton-exciton and exciton-phonon scattering have profound effects on the dephasing process. The dominant phonon mode involved in the dephasing is identified as the out-of-plane, transverse optical mode with a frequency of 847cm-1. The extracted homogeneous linewidths at all measured temperatures are in excellent agreement with the results of a single-tube photoluminescence experiment.

Ma, Ying-Zhong; Graham, Matthew W.; Fleming, Graham R.; Green, Alexander A.; Hersam, Mark C.

2008-11-01

231

First-principles simulations of exciton diffusion in organic semiconductors  

NASA Astrophysics Data System (ADS)

Exciton diffusion is of great importance to the performance of organic optoelectronic devices, including organic photovoltaics and solid-state lighting. The ability to control exciton diffusion in organic semiconductors is crucial to the design of efficient optoelectronic devices. However, such ability can only be achieved through a fundamental understanding of exciton diffusion mechanism. We have proposed a first-principles based frame work that can predict exciton dynamics in organic semiconductors.The framework is based on time-dependent density functional theory to provide the energy and many-body wave functions of excitons. Nonadiabatic ab initio molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, and the results agree very well with corresponding experimental values. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons; a widely speculated diffusion mechanism has been confirmed and elucidatedby the simulations. Some general guidelines for designing more efficient organic solar cells can be gleaned from the simulation results

Zhang, Xu; Li, Zi; Lu, Gang

2013-03-01

232

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

233

Theory of Excitonic Spectra and Entanglement Engineering in Dot Molecules  

NASA Astrophysics Data System (ADS)

We present results of correlated pseudopotential calculations of an exciton in a pair of vertically stacked InGaAs/GaAs dots. Competing effects of strain, geometry, and band mixing lead to many unexpected features missing in contemporary models. The first four excitonic states are all optically active at small interdot separation, due to the broken symmetry of the single-particle states. We quantify the degree of entanglement of the exciton wave functions and show its sensitivity to interdot separation. We suggest ways to spectroscopically identify and maximize the entanglement of exciton states.

Bester, Gabriel; Shumway, J.; Zunger, Alex

2004-07-01

234

Multiple Exciton Generation and Recombination in Carbon Nanotubes and Nanocrystals.  

PubMed

Semiconducting nanomaterials such as single-walled carbon nanotubes (SWCNTs) and nanocrystals (NCs) exhibit unique size-dependent quantum properties. They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications. SWCNTs and NCs also provide an excellent new stage for experimental studies of many-body effects of electrons and excitons on optical processes in nanomaterials. In this Account, we discuss multiple exciton generation and recombination in SWCNTs and NCs for next-generation photovoltaics. Strongly correlated ensembles of conduction-band electrons and valence-band holes in semiconductors are complex quantum systems that exhibit unique optical phenomena. In bulk crystals, the carrier recombination dynamics can be described by a simple model, which includes the nonradiative single-carrier trapping rate, the radiative two-carrier recombination rate, and the nonradiative three-carrier Auger recombination rate. The nonradiative Auger recombination rate determines the carrier recombination dynamics at high carrier density and depends on the spatial localization of carriers in two-dimensional quantum wells. The Auger recombination and multiple exciton generation rates can be advantageously manipulated by nanomaterials with designated energy structures. In addition, SWCNTs and NCs show quantized recombination dynamics of multiple excitons and carriers. In one-dimensional SWCNTs, excitons have large binding energies and are very stable at room temperature. The extremely rapid Auger recombination between excitons determines the photoluminescence (PL) intensity, the PL linewidth, and the PL lifetime. SWCNTs can undergo multiple exciton generation, while strong exciton-exciton interactions and complicated exciton structures affect the quantized Auger rate and the multiple exciton generation efficiency. Interestingly, in zero-dimensional NC quantum dots, quantized Auger recombination causes unique optical phenomena. The breakdown of the k-conversion rule and strong Coulomb interactions between carriers in NCs enhance Auger recombination rate and decrease the energy threshold for multiple exciton generation. We discuss this impact of the k-conservation rule on two-carrier radiative recombination and the three-carrier Auger recombination processes in indirect-gap semiconductor Si NCs. In NCs and SWCNTs, multiple exciton generation competes with Auger recombination, surface trapping of excitons, and cooling of hot electrons or excitons. In addition, we explore heterostructured NCs and impurity-doped NCs in the context of the optimization of charge carrier extraction from excitons in NCs. PMID:23421584

Kanemitsu, Yoshihiko

2013-02-19

235

Mass radius scaling near the Chandrasekhar Limit  

Microsoft Academic Search

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

Sayan Chakraborti

2007-01-01

236

Prestress Tendons On Short Radius Curves.  

National Technical Information Service (NTIS)

This report discusses the friction factor and ultimate tensile strength achieved by multi-strand tendons stressed around short radius curves. It was found that a pre-fabricated tendon of as many as 33 one-half-inch diameter strands, stressed through 1.7 r...

T. J. Bezouska

1975-01-01

237

Solar Radius Measurements at Mount Wilson Observatory  

Microsoft Academic Search

Possible temporal variations of the solar radius are important as an indicator of internal energy storage and as a mechanism for changes in the total solar irradiance. Variations in the total solar irradiance with an amplitude of 0.1% have been observed from space for more than two decades. Although the variability of this solar output has been definitely established, the

S. Lefebvre; L. Bertello; R. K. Ulrich; J. E. Boyden; J. P. Rozelot

2006-01-01

238

MASS-RADIUS RELATIONSHIPS FOR EXOPLANETS  

SciTech Connect

For planets other than Earth, particularly exoplanets, interpretation of the composition and structure depends largely on comparing the mass and radius with the composition expected given their distance from the parent star. The composition implies a mass-radius relation which relies heavily on equations of state calculated from electronic structure theory and measured experimentally on Earth. We lay out a method for deriving and testing equations of state, and deduce mass-radius and mass-pressure relations for key, relevant materials whose equation of state (EOS) is reasonably well established, and for differentiated Fe/rock. We find that variations in the EOS, such as may arise when extrapolating from low-pressure data, can have significant effects on predicted mass-radius relations and on planetary pressure profiles. The relations are compared with the observed masses and radii of planets and exoplanets, broadly supporting recent inferences about exoplanet structures. Kepler-10b is apparently 'Earth-like', likely with a proportionately larger core than Earth's, nominally 2/3 of the mass of the planet. CoRoT-7b is consistent with a rocky mantle over an Fe-based core which is likely to be proportionately smaller than Earth's. GJ 1214b lies between the mass-radius curves for H{sub 2}O and CH{sub 4}, suggesting an 'icy' composition with a relatively large core or a relatively large proportion of H{sub 2}O. CoRoT-2b is less dense than the hydrogen relation, which could be explained by an anomalously high degree of heating or by higher than assumed atmospheric opacity. HAT-P-2b is slightly denser than the mass-radius relation for hydrogen, suggesting the presence of a significant amount of matter of higher atomic number. CoRoT-3b lies close to the hydrogen relation. The pressure at the center of Kepler-10b is 1.5{sup +1.2}{sub -1.0} TPa. The central pressure in CoRoT-7b is probably close to 0.8 TPa, though may be up to 2 TPa. These pressures are accessible by planar shock and ramp-loading experiments at large laser facilities. The center of HAT-P-2b is probably around 210 TPa, in the range of planned National Ignition Facility experiments, and that of CoRoT-3b around 1900 TPa.

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

2012-01-01

239

Quantum decoherence in finite size exciton-phonon systems  

NASA Astrophysics Data System (ADS)

Based on the operatorial formulation of the perturbation theory, the properties of a confined exciton coupled with phonons in thermal equilibrium is revisited. Within this method, the dynamics is governed by an effective Hamiltonian which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. Special attention is thus paid for describing the time evolution of the excitonic coherences at finite temperature. As in an infinite lattice, temperature-enhanced quantum decoherence takes place. However, it is shown that the confinement softens the decoherence. The coherences are very sensitive to the excitonic states so that the closer to the band center the state is located, the slower the coherence decays. In particular, for odd lattice sizes, the coherence between the vacuum state and the one-exciton state exactly located at the band center survives over an extremely long time scale. A superimposition involving the vacuum and this specific one-exciton state behaves as an ideal qubit insensitive to its environment.

Pouthier, Vincent

2011-03-01

240

Quantum decoherence in finite size exciton-phonon systems.  

PubMed

Based on the operatorial formulation of the perturbation theory, the properties of a confined exciton coupled with phonons in thermal equilibrium is revisited. Within this method, the dynamics is governed by an effective Hamiltonian which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. Special attention is thus paid for describing the time evolution of the excitonic coherences at finite temperature. As in an infinite lattice, temperature-enhanced quantum decoherence takes place. However, it is shown that the confinement softens the decoherence. The coherences are very sensitive to the excitonic states so that the closer to the band center the state is located, the slower the coherence decays. In particular, for odd lattice sizes, the coherence between the vacuum state and the one-exciton state exactly located at the band center survives over an extremely long time scale. A superimposition involving the vacuum and this specific one-exciton state behaves as an ideal qubit insensitive to its environment. PMID:21428641

Pouthier, Vincent

2011-03-21

241

Enhanced Resonant Backscattering of Excitons in Disordered Quantum Wells  

Microsoft Academic Search

A clear signature of enhanced backscattering of excitons is observed in the directional resonant Rayleigh scattering of light from localized two-dimensional excitons in disordered quantum wells. Its spectral dependence and time dynamics are measured and theoretically predicted in a quantitative way. The intensity enhancement has a large momentum span extending beyond the external light emission cone. This is a consequence

W. Langbein; E. Runge; V. Savona; R. Zimmermann

2002-01-01

242

Correlated exciton fluctuations in cylindrical molecular aggregates.  

PubMed

Femtosecond electronic relaxation dynamics of a cylindrical molecular aggregate are measured with transient grating (TG) and two-dimensional Fourier transform photon echo (PE) spectroscopies. The aggregates are double-walled cylindrical structures formed by self-assembly of amphiphilic cyanine dye molecules in water. The diameters of the inner and outer cylinders are approximately 6 and 10 nm. The linear absorption spectrum of the aggregate exhibits four spectrally resolved single exciton transitions corresponding to excited states localized on particular regions of the structure: (1) an excited state localized on the inner cylinder corresponds to the lowest energy transition at 16670 cm(-1); (2) a transition at 17150 cm(-1) represents a state localized on the outer cylinder, (3) whereas an overlapping peak found at 17330 cm(-1) is more closely associated with the inner cylinder; (4) an excited state delocalized between the inner and outer cylinder is assigned to a transition in the linear absorption spectrum at 17860 cm(-1). TG spectra show a series of resonances reflecting the electronic structure of both the single and double exciton manifolds. In addition, PE spectra reveal coherent modulation of both diagonal and cross-peak amplitudes persisting for 100 fs, where the coherence frequency matches the energy gap between transitions 1 and 4 in the linear absorption spectrum. PE line shapes suggest correlated energy level fluctuations for the exciton states associated with these two transitions, which is consistent with this fairly long-lasting coherence at room temperature in aqueous solution. The impact of these correlations on Forster energy transfer efficiency is discussed. The observations imply fairly long-range correlations between the molecular sites (>0.6 nm), which in turn reflects the length scale of the environmental motion inducing the fluctuations. We suggest that this environmental motion is most likely associated with water confined inside the cylinder and/or fluctuations of the dye's aliphatic functional groups. PMID:19382770

Womick, Jordan M; Miller, Stephen A; Moran, Andrew M

2009-05-14

243

The Kopenhagen operation of the Soviet KGB. The Kopenahagen interview of Niels Bohr by a Soviet scientist and KGB  

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.

244

Electron teleportation between quantum dots using virtual dark exciton  

NASA Astrophysics Data System (ADS)

We here propose a mechanism to teleport electrons between quantum dots through the transformation of a virtual bright exciton into a dark exciton. This mechanism relies on the interactions of two composite bosons: a pair of electrons with opposite spins, trapped in two dots and an electron-hole pair in a free exciton coupled to an unabsorbed pump pulse, which makes it "bright" but virtual. This bright exciton first turns "dark" by dropping its electron and stealing the trapped electron with opposite spin through an exchange Coulomb process with the trapped pair. In a second step, the dark exciton "flies" with its electron to the other dot where it turns bright again, by the inverse process. The "Shiva diagrams" for composite-boson many-body effects that we have recently introduced, enlighten this understanding.

Combescot, M.; Betbeder-Matibet, O.; Voliotis, V.

2006-06-01

245

"Gray" BCS condensate of excitons and internal Josephson effect.  

PubMed

It has been recently suggested that the Bose-Einstein condensate formed by excitons in the dilute limit must be dark, i.e., not coupled to photons. Here, we show that, under a density increase, the dark exciton condensate must acquire a bright component due to carrier exchange in which dark excitons turn bright. This, however, requires a density larger than a threshold which seems to fall in the forbidden region of the phase separation between a dilute exciton gas and a dense electron-hole plasma. The BCS-like condensation which is likely to take place on the dense side, must then have a dark and a bright component--which makes it "gray." It should be possible to induce an internal Josephson effect between these two coherent components, with oscillations of the photoluminescence as a strong proof of the existence for this "gray" BCS-like exciton condensate. PMID:23030185

Combescot, Roland; Combescot, Monique

2012-07-09

246

Exciton Scattering Rate by Optical Phonons in a Quantum Well  

NASA Astrophysics Data System (ADS)

By using the envelope function approximation in a narrow quantum well (QW), the exciton scattering rate by confined phonons and interface modes is calculated. The exciton motion is considered using two-dimensional in-plane exciton wave-functions and free electron-hole pair wave-functions along the QW growth direction. The long-wavelength polar optical phonons have been included using the framework of a phenomenological approach in which the mechanical and electrostatic matching boundary conditions are fulfilled at QW interfaces. The contributions of confined phonons and interface modes to the total exciton scattering rate are discussed in terms of the in-plane exciton kinetic energy and quantum well width. *Permanent Address: Department of Theoretical Physics, Havana University, 10400 Havana, Cuba.

Lu, Meng; Birman, Joseph L.; Trallero-Giner, Carlos

1998-03-01

247

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

248

Cross-polarized excitons in double-wall carbon nanotubes  

NASA Astrophysics Data System (ADS)

Optical absorption in double-wall carbon nanotubes for light polarized perpendicular to the tube axis is studied by taking into account exciton effects and depolarization effects within an effective-mass theory. The Coulomb interaction is suppressed by not only intrawall screening effects but also interwall screening, leading to the reduction of exciton binding energies and band gaps. When two tubes are both semiconducting, a clear exciton peak still survives even under depolarization effects for the outer tube, but the exciton peak of the inner tube has an asymmetric Fano line shape due to the coupling with continuum states of the outer tube. When a double-wall nanotube contains a metallic tube, either inner or outer, the exciton of the semiconducting tube loses its peak structure under depolarization effects.

Tomio, Yuh; Suzuura, Hidekatsu; Ando, Tsuneya

2012-12-01

249

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

250

The Epidemiology of Distal Radius Fractures  

PubMed Central

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.

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

2012-01-01

251

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

252

Displaced paediatric fractures of the distal radius  

Microsoft Academic Search

Introduction  The purpose of this study was to review, over a fixed period of time and in a geographically defined area, all paediatric\\u000a fractures of the distal radius that were initially reduced. We wished to assess the fracture pattern at different ages, the\\u000a seasonal variation, the severity of the initial angulation, and the radiological outcome in terms of reangulation, remodelling,\\u000a refracture,

Leiv M. Hove; Christina Brudvik

2008-01-01

253

Optical absorption and refraction index change of a confined exciton in a spherical quantum dot nanostructure  

NASA Astrophysics Data System (ADS)

Electronic energies of an exciton confined in a strained Zn1- x Cd x Se/ZnSe quantum dot have been computed as a function of dot radius with various Cd content. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption coefficients and the refractive index changes between the ground state ( L = 0) and the first excited state ( L = 1) are investigated. It is found that the optical properties in the strained ZnCdSe/ZnSe quantum dot are strongly affected by the confinement potentials and the dot radii. The intensity of the total absorption spectra increases for the transition between higher levels. The obtained optical nonlinearity brings out the fact that it should be considered in calculating the optical properties in low dimensional semiconductors especially in quantum dots.

Mathan Kumar, K.; John Peter, A.; Lee, C. W.

2011-12-01

254

Open universes from finite radius bubbles  

SciTech Connect

The interior of a vacuum bubble in de Sitter space may give an open universe with sufficient homogeneity to agree with observations. Here, previous work by Bucher, Goldhaber, and Turok is extended to describe a thin bubble wall with a nonzero radius and energy difference across the wall. The vacuum modes present before the formation of the bubble propagate into the interior of the open universe and the power spectrum of the resulting gauge-invariant gravitational potential is calculated. It appears to become scale invariant on small scales, with the onset at about the same scale as that found in the zero radius case. There is sensitivity to the radius and energy difference at large scales, but it is expected that they cannot be strongly constrained because of cosmic variance. The prediction of a scale invariant spectrum seems to be robust with respect to variation of these parameters at small scales, and apparently is a generic feature of the contribution of these modes for these thin wall models. {copyright} {ital 1996 The American Physical Society.}

Cohn, J.D. [Department of Physics]|[Theoretical Physics Group, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 (United States)

1996-12-01

255

Open universes from finite radius bubbles  

NASA Astrophysics Data System (ADS)

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

Cohn, J. D.

1996-12-01

256

Exciton dynamics in semiconducting carbon nanotubes.  

PubMed

We report a femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E(11)) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E(22)) is excited than that measured by exciting its first transition-allowed state (E(11)). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E(22) excitation, whereas the corresponding amplitude scales linearly with the square root of the E(11) excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E(22) and E(11) excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model. PMID:21090793

Graham, M W; Chmeliov, J; Ma, Y-Z; Shinohara, H; Green, A A; Hersam, M C; Valkunas, L; Fleming, G R

2010-11-23

257

Exciton Dynamics in Semiconducting Carbon Nanotubes  

SciTech Connect

We report femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E11) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E22) is excited than that measured by exciting its first transition-allowed state (E11). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E22 excitation, whereas the corresponding amplitude scales linearly with the square root of the E11 excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E22 and E11 excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model.

Graham, Matt [University of California, Berkeley; Chmeliov, Javgenij [Vilnius University, Lithuania; Ma, Yingzhong [ORNL; Shinohara, Nori [Nagoya University, Japan; Green, Alexander A. [Northwestern University, Evanston; Hersam, Mark C. [Northwestern University, Evanston; Valkunas, Leonas [Vilnius University, Lithuania; Fleming, Graham [University of California, Berkeley

2010-01-01

258

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

259

Theoretical and computational studies of excitons in conjugated polymers  

NASA Astrophysics Data System (ADS)

We present a theoretical and computational analysis of excitons in conjugated polymers. We use a tight-binding model of ?-conjugated electrons, with 1/r interactions for large r. In both the weak-coupling limit (defined by W>>U) and the strong-coupling limit (defined by W<excitons, i.e., conduction-band electrons bound to valence-band holes. Singlet and triplet excitons whose relative wave functions are odd under a reflection of the relative coordinate are degenerate. Thus, the 2 1A+g and 1 3A-g states are degenerate in this limit. (2) In the strong-coupling limit the bound states are Mott-Hubbard excitons, i.e., particles in the upper Hubbard band bound to holes in the lower Hubbard band. These bound states occur in doublets of even and odd parity excitons. Triplet excitons are magnons bound to the singlet excitons, and hence are degenerate with their singlet counterparts. (3) In the intermediate-coupling regime Mott-Wannier excitons are the more appropriate description for large dimerization, while for the undimerized chain Mott-Hubbard excitons are the correct description. For dimerizations relevant to polyacetylene and polydiacetylene both Mott-Hubbard and Mott-Wannier excitons are present. (4) For all coupling strengths an infinite number of bound states exist for 1/r interactions for an infinite polymer. As a result of the discreteness of the lattice and the restrictions on the exciton wave functions in one dimension, the progression of states does not follow the Rydberg series. In practice, excitons whose particle-hole separation exceeds the length of the polymer can be considered unbound. (5) The DMRG calculated exciton excitation energies scale as the inverse of the chain length for short chains and the inverse of the square of the chain length for long chains. This fits the effective-particle-in-a-box model.

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

2002-09-01

260

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

NASA Astrophysics Data System (ADS)

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

Plotnitsky, Arkady

2012-12-01

261

Dynamical fermion mass generation and exciton spectra in graphene  

NASA Astrophysics Data System (ADS)

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, Chun-Xu; Liu, Guo-Zhu; Huang, Ming-Qiu

2011-03-01

262

Exciton Mott transition in Si revealed by terahertz spectroscopy.  

PubMed

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

2012-07-26

263

Photonics meets excitonics: natural and artificial molecular aggregates  

NASA Astrophysics Data System (ADS)

Organic molecules store the energy of absorbed light in the form of charge-neutral molecular excitations - Frenkel excitons. Usually, in amorphous organic materials, excitons are viewed as quasiparticles, localized on single molecules, which diffuse randomly through the structure. However, the picture of incoherent hopping is not applicable to some classes of molecular aggregates - assemblies of molecules that have strong near-field interaction between electronic excitations in the individual subunits. Molecular aggregates can be found in nature, in photosynthetic complexes of plants and bacteria, and they can also be produced artificially in various forms including quasi-one dimensional chains, two-dimensional films, tubes, etc. In these structures light is absorbed collectively by many molecules and the following dynamics of molecular excitation possesses coherent properties. This energy transfer mechanism, mediated by the coherent exciton dynamics, resembles the propagation of electromagnetic waves through a structured medium on the nanometer scale. The absorbed energy can be transferred resonantly over distances of hundreds of nanometers before exciton relaxation occurs. Furthermore, the spatial and energetic landscape of molecular aggregates can enable the funneling of the exciton energy to a small number of molecules either within or outside the aggregate. In this review we establish a bridge between the fields of photonics and excitonics by describing the present understanding of exciton dynamics in molecular aggregates.

Saikin, Semion K.; Eisfeld, Alexander; Valleau, Stéphanie; Aspuru-Guzik, Alán

2013-02-01

264

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

SciTech Connect

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

265

Super-elite plasma rings and the orbits of planets and satellites isomorphic to the orbits of electrons in the Bohr's model of the hydrogen atom  

NASA Astrophysics Data System (ADS)

This paper continues the series of papers [1 5] and generalizes the previous results to a proto-ring of magnetized plasma whose density decreases in the radial direction. The problem of quantization of the sector and orbital velocities, and of the radii and periods of revolution of elite plasma rings is considered. A new concept of super-elite rings is introduced. Their isomorphism with the orbits of the planets and planetary satellites in the Solar System is proved. This isomorphism also extends to the orbits of electrons in the Bohr’s model of the hydrogen atom.

Rabinovich, B. I.

2007-10-01

266

Giant cell tumour of the proximal radius.  

PubMed

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

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

2009-11-01

267

Free and self-trapped excitons in rare-gas cryocrystals: coexistence and mixing of states  

Microsoft Academic Search

The absorption spectra for rare-gas cryocrystals are calculated taking into account one-phonon scattering and multiphonon interaction of free excitons with local lattice vibrations. The possibility of coexistence of free and self-trapped excitons in nonequilibrium states above the bottom of the exciton band is discussed on the basis of a comparison of the free exciton energy with exciton damping in the

I. Ya. Fugol; E. I. Tarasova

1997-01-01

268

Excitonic and vibronic spectra of Frenkel excitons in a two-dimensional simple lattice  

NASA Astrophysics Data System (ADS)

Excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional (2D) lattice with one molecule per unit cell have been studied and their manifestation in the linear absorption is simulated. We use the Green function formalism, the vibronic approach, see Lalov and Zhelyazkov [I.J. Lalov, I. Zhelyazkov, Phys. Rev. B 75 (2007) 245435], and the nearest-neighbor approximation to find expressions of the linear absorption lineshape in closed form (in terms of the elliptic integrals) for the following 2D models: (a) vibronic spectra of polyacenes (naphthalene, anthracene, tetracene); (b) vibronic spectra of a simple hexagonal lattice. The two 2D models include both linear and quadratic FE-phonon coupling. Our simulations concern the excitonic density of state (DOS), and also the position and lineshape of vibronic spectra (FE plus one phonon, FE plus two phonons). The positions of many-particle (MP-unbound) FE-phonon states, as well as the impact of the Van Hove singularities on the linear absorption have been established by using typical values of the excitonic and vibrational parameters. In the case of a simple hexagonal lattice the following types of FEs have been considered: (i) non-degenerate FEs whose transition dipole moment is perpendicular to the plane of the lattice, and (ii) degenerate FEs with transition dipole moments parallel to the layer. We found a cumulative impact of the linear and quadratic FE-phonon coupling on the positions of vibronic maxima in the case (ii), and a compensating impact in the case (i).

Lalov, I. J.; Zhelyazkov, I.

2013-01-01

269

Exciton-exciton correlation effects on FWM in GaN  

Microsoft Academic Search

Bound and unbound biexcitons in a free-standing bulk GaN are investigated by time-integrated and spectrally-resolved four-wave mixing measurements, where the formation of hetero-biexcitons that consist of A and B excitons (XXAB) as well as A biexcitons and their unbound biexciton are clearly observed. Through the FWM spectra and delay-time dependence, it is found that the interaction between A and B

Satoru Adachi; Hirotaka Sasakura; Shunichi Muto; Kouji Hazu; Takayuki Sota; Shigefusa F. Chichibu; Takashi Mukai

2003-01-01

270

Polarization selection rules in exciton-based terahertz lasers  

NASA Astrophysics Data System (ADS)

Optical pumping of excited exciton states in semiconductor quantum wells is a tool for the realization of ultracompact terahertz (THz) lasers based on stimulated optical transition between excited (2p) and ground (1s) exciton states. We show that the probability of two-photon absorption by a 2p exciton is strongly dependent on the polarization of both photons. Variation of the threshold power for THz lasing by a factor of 5 is predicted by switching from linear to circular pumping. We calculate the polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency.

Slavcheva, G.; Kavokin, A. V.

2013-08-01

271

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

272

Josephson oscillations between exciton condensates in electrostatic traps  

SciTech Connect

Technological advances allow for tunable lateral confinement of cold dipolar excitons in coupled quantum wells. We consider theoretically the Josephson effect between exciton condensates in two traps separated by a weak link. The flow of the exciton supercurrent is driven by the dipole-energy difference between the traps. The Josephson oscillations may be observed after ensemble average of the time correlation of photons separately emitted from the two traps. The fringe visibility is controlled by the trap coupling and is robust against quantum and thermal fluctuations.

Rontani, Massimo [CNR-INFM Research Center on nanoStructures and bioSystems at Surfaces (S3), Via Campi 213/A, 41125 Modena (Italy); Sham, L. J. [Department of Physics, University of California San Diego, La Jolla, California 92093-0319 (United States)

2009-08-15

273

Mechanism for radiative recombination in ZnCdO alloys  

SciTech Connect

Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content.

Buyanova, I. A.; Bergman, J. P.; Pozina, G.; Chen, W. M.; Rawal, S.; Norton, D. P.; Pearton, S. J.; Osinsky, A.; Dong, J. W. [Department of Physics, Chemistry and Biology, Linkoeping University, S-581 83 Linkoeping (Sweden); Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (United States); SVT Associates, Eden Prairie, Minnesota 55344 (United States)

2007-06-25

274

Forearm and distal radius fractures in children.  

PubMed

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

275

Nuclear Charge Radius of Lithium-11  

SciTech Connect

We have determined the nuclear charge radius of 11Li by high-precision laser spectroscopy. The experiment was performed at the TRIUMF-ISAC facility where the 7Li-11Li isotope shift was measured in the 2s to 3s electronic transition using Doppler-free two-photon spectroscopy with a relative accuracy better than 10 5. The accuracy reached in previous experiments on the other lithium isotopes was improved. Most of the isotope shifts measured in the experiment are due to difference in the mass of the nuclei but small contributions are produced by the change in proton distribution, QED and relativistic effects have to be taken into account as well. By comparing the experimental results with sophisticated atomic calculations of the mass dependent effect the nuclear charge radii of the lithium isotopes are found to decrease monotonically from 6Li to 9Li while the nuclear charge radius of 11Li is about 11% larger than that of 9Li.

Sanchez, Rodolfo; Nortershauser, W.; Dax, A. ..; Ewald, G.; Gotte, S.; Kirchner, R. G.; Kluge, H. J.; Kuhl, T. H.; Wojtaszek, A.; Bushaw, Bruce A.; Drake, Gordon W. F.; Yan, Z. C.; Zimmermann, Claus; Albers, Daniel; Behr, John; Bricault, Pierre; Dilling, Jens; Dombsky, Marik; Lassen, J.; Levy, C.D. P.; Pearson, Matthew; Prime, Erika; Ryjkov, Vladimir L.

2006-07-01

276

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

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

277

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

PubMed

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

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

2013-10-08

278

Hot exciton dissociation in polymer solar cells  

NASA Astrophysics Data System (ADS)

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC60BM system. Exciton splitting occurs within the first 50?fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200?fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.

2013-01-01

279

Exciton-exciton interactions in CdWO{sub 4} irradiated by intense femtosecond vacuum ultraviolet pulses  

SciTech Connect

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

280

Ultrafast exciton dynamics in colloidal aluminum phosphide nanocrystals  

NASA Astrophysics Data System (ADS)

The primary exciton dynamics of colloidal indirect bandgap AlP nanocrystals are characterized with ultrafast transient absorption spectroscopy. A 400-nm excitation results in a high yield formation of an emissive exciton with a ˜1-ns lifetime and a 50-nm bandwidth red-shifted emission. Multi-wavelength target analysis is used to decompose the measured signals into sequential and parallel models, which interpret the measured data as an emissive exciton with a 1.2-ns decay time and a dark exciton which is attributed to surface trapping. Reconstructed nonlinear absorption spectra resolve a broad optical gain persisting for >1 ns. The 15% emission yield demonstrates that colloidal AlP nanocrystals are useful as a potential broadband, high efficiency optoelectronics material.

Busby, Erik; Thibert, Arthur; Fuzell, Jack; Arrington, Deisy C.; Jawaid, Ali M.; Snee, Preston T.; Larsen, Delmar S.

2013-02-01

281

Hopping approach towards exciton dissociation in conjugated polymers  

SciTech Connect

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

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

2008-06-14

282

Singlet exciton fission-sensitized infrared quantum dot solar cells.  

PubMed

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

2012-01-23

283

Picosecond kinetics of strongly coupled excitons and surface plasmon polaritons.  

PubMed

Coupling between excitons of CdSe nanocrystal quantum dots (NQDs) and surface plasmon polaritons (SPPs) of an Ag film attached to a prism have been studied by steady-state and transient reflectivity measurements in the Kretschmann geometry. In these experiments, the angle of incidence of the probe beam selects hybrid exciton/SPP states with different wavevectors and exciton/SPP compositions. The dynamics measured in the transient reflectivity experiments are sensitive to the composition of the hybrid states. Specifically, fast dynamics are observed at probe wavevectors where the lower hybrid state has predominant SPP character. In contrast, at probe wavevectors where the lower hybrid state is predominantly excitonic, the dynamics are similar to that measured for CdSe NQDs on glass. PMID:23025634

Gómez, Daniel E; Lo, Shun Shang; Davis, Timothy J; Hartland, Gregory V

2012-10-11

284

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

285

A max version of the generalized spectral radius theorem  

Microsoft Academic Search

Let ? be a bounded set of n×n nonnegative matrices in max algebra. In this paper we propose the notions of the max algebra version of the generalized spectral radius ?(?) of ?, and the max algebra version of the joint spectral radius ?(?) of ?. The max algebra version of the generalized spectral radius theorem ?(?)=?(?) is established. We

Yung-Yih Lur

2006-01-01

286

The minimal spectral radius of graphs with a given diameter  

Microsoft Academic Search

The spectral radius of a graph (i.e., the largest eigenvalue of its corresponding adjacency matrix) plays an important role in modeling virus propagation in networks. In fact, the smaller the spectral radius, the larger the robustness of a network against the spread of viruses. Among all connected graphs on n nodes the path Pn has minimal spectral radius. However, its

E. R. van Dam; R. E. Kooij

2007-01-01

287

Improving optical bench radius measurements using stage error motion data  

Microsoft Academic Search

We describe the application of a vector-based radius approach to optical bench radius measurements in the presence of imperfect stage motions. In this approach, the radius is defined using a vector equation and homogeneous transformation matrix formulism. This is in contrast to the typical technique, where the displacement between the confocal and cat's eye null positions alone is used to

Tony L. Schmitz; Neil Gardner; Matthew Vaughn; Kate Medicus; Angela Davies

2008-01-01

288

Traceable radius of curvature measurements on a micro-interferometer  

Microsoft Academic Search

Radius of curvature is a critical parameter to measure in the manufacturing of micro-refractive elements. It defines the power of the surface and provides important information about the stability and uniformity of the manufacturing process. The radius of curvature of an optical surface can be measured using an interferometer and radius slide where the distance is measured as the surface

Devendra Karodkar; Neil Gardner; Brent C. Bergner; Angela Davies

2004-01-01

289

Excitons in organic molecular nanostructures: Physics and applications  

NASA Astrophysics Data System (ADS)

Excitonic processes dominate the electric and optical properties of organic materials. From delocalized charge-transfer (CT) excitons in very closely packed organic molecular crystals (OMCs) to localized Frenkel excitons in loosely packed amorphous organic solids, they determine material characteristics such as absorption, photo-conduction and luminescence. Recently, organic light emitting displays (OLEDs) and organic thin film transistors whose functionality partially depends on fundamental excitations have attracted substantial interest due to their unique properties unattainable with conventional semiconductors. To optimize the device performance such as shifting absorption peak wavelength, enhancing current injection, controlling channel conduction, maximizing electroluminescence (EL) efficiency, and obtaining saturated red, green and blue emission colors, it is essential to understand how excitonic processes is modified in organic nanostructures. In this work, we examine CT and Frenkel excitons in OMCs and amorphous organic materials. A quantum mechanical model is developed to study electrooptical properties of delocalized CT excitons in closely packed OMC nanostructures. Based on this model, we analyze the electroabsorption (EA) spectrum in bulk PTCDA and the absorption spectral shifts in PTCDA/NTCDA multilayers, and obtain consistent values of effective masses and exciton radii along difference crystalline axes. The same treatment is extended to fit the GaAs EA spectrum, suggesting a common physical origin for both CT and Wannier excitons. We also examine the Frenkel excitons in more decoupled amorphous organic solids used for OLEDs. Assuming an exponentially distributed trap states in the lowest unoccupied molecular orbital and the highest occupied molecular orbital gap, we are able to explain current-voltage characteristics and EL efficiencies observed under various temperature and charge injection conditions. We infer that the traps are due to molecular polarons, which also determine the energy distribution of excitons, and hence the EL emission spectrum. To utilize some unique properties of OMCs, we demonstrate a vertically stacked, three color OLED which allows for independent tuning of color, gray scale and intensity. The radiative recombination of Frenkel excitons is influenced by the heterogeneous multilayer structure via microcavity interference. Controlling the recombination environment by varying the layer thickness, and emissive layer positions, color saturation and EL efficiency can be optimized.

Shen, Zilan

290

Possibilities for exciton condensation in semiconductor quantum-well structures  

Microsoft Academic Search

Recent progress in understanding the behavior of condensed excitonic systems is reviewed. The Bose-Einstein condensed excitonic insulator is studied numerically within the mean-field pairing approximation. In particular, we examine in some detail a system of spatially separated, two-dimensional (2D) electrons and holes, as a candidate system for study in semiconductor heterostructures. Based on these calculations, we suggest that it may

P. B. Littlewood; Xuejun Zhu

1996-01-01

291

Dynamical reconstruction of the valence exciton in LiF  

Microsoft Academic Search

We have used inelastic x-ray scattering, coupled with recently developed inversion techniques, to reconstruct the structure and dynamics of the valence exciton in the prototype alkali halide LiF. Our inversions, which yield resolutions deltax = 0.533 å and deltat = 20.67 as (2.067 x10-17 s), reveal that the exciton forms in less than 50 as, oscillates with a period of

Peter Abbamonte; Wei Ku; Tim Graber; James Reed; Serban Smadici; Abhay Shukla; Jean-Pascal Rueff

2008-01-01

292

Quantum condensation from a tailored exciton population in a microcavity  

SciTech Connect

An experiment is proposed on the coherent quantum dynamics of a semiconductor microcavity containing quantum dots. Modeling the experiment using a generalized Dicke model, we show that a tailored excitation pulse can create an energy-dependent population of excitons, which subsequently evolves to a quantum condensate of excitons and photons. The population is created by a generalization of adiabatic rapid passage and then condenses due to a dynamical analog of the BCS instability.

Eastham, P. R. [Department of Physics, Imperial College London, London SW7 2BW (United Kingdom); Phillips, R. T. [Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE (United Kingdom)

2009-04-15

293

Optical Properties of Strongly Coupled Plasmon-Exciton Hybrid Nanostructures  

NASA Astrophysics Data System (ADS)

Strongly coupled plasmon-exciton hybrid nanostructures are fabricated and their optical properties are studied. The plasmonic and excitonic systems are gold nanoshells and J-aggregates, respectively. Gold nanoshells are tunable plasmonic core-shell nanoparticles which can sustain distinct dipole and quadrupole plasmons with resonant energies dependent on core-size/shell-thickness ratio. J-aggregates are organic semiconducting material with excitons that possess very high oscillator strength making them suitable for coherent interaction with other kinds of excitations. The J-aggregates are formed on the surface of the nanoshells when a water/ethanol (50:50) solution of the dye molecules (2,2'-dimethyl-8-phenyl-5,6,5',6'-dibenzothiacarbocyanine chloride) is added to an aqueous solution of nanoshells. These nanoshell-J-aggregate complexes exhibit coherent coupling between localized plasmons of the nanoshell and excitons of the molecular J-aggregates. Coherent coupling strengths of 120 meV and 100 meV have been measured for dipole and quadrupole plasmon interactions with excitons, respectively. Femtosecond time-resolved transmission spectroscopy studies are carried out in order to understand the possible sources of optical nonlinearities in the nanoshell-J-aggregate hybrid. Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. An additional, transient Fano-shaped modulation within the Fano dip is also observable. The transient behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon.

Fofang, Nche Tumasang

294

Phonon scattering of excitons and biexcitons in ZnO  

Microsoft Academic Search

The phase relaxation time of biexcitons T2bi and that of excitons T2ex in a bulk ZnO have been measured by the use of femtosecond four-wave mixing as functions of excitation wavelength, excitation power, and temperature. The biexciton-acoustic phonon interaction coefficient has been determined to be betaacbi~53 mueV\\/K as an average value. For A excitons, the interaction coefficients with acoustic and

K. Hazu; T. Sota; S. Adachi; Sf. Chichibu; G. Cantwell; D. C. Reynolds; C. W. Litton

2004-01-01

295

Magnetic Edge-State Excitons in Zigzag Graphene Nanoribbons  

Microsoft Academic Search

We present first-principles calculations of the optical properties of zigzag-edged graphene nanoribbons (ZGNRs) employing the GW-Bethe-Salpeter equation approach with the spin interaction included. Optical response of the ZGNRs is found to be dominated by magnetic edge-state-derived excitons with large binding energy. The absorption spectrum is composed of a characteristic series of exciton states, providing a possible signature for identifying the

Li Yang; Marvin L. Cohen; Steven G. Louie

2008-01-01

296

Free-exciton recombination in GaAs  

SciTech Connect

We have studied the kinetics of free-exciton recombination in ideal'' GaAs structures. Our studies include detailed photoluminescence kinetics measurements versus laser excitation power, temperature, sample structure, and surface treatment. The primary goal in these studies is to understand the mechanism precipitating free-exciton recombination, and the role, if any, that polaritons play. Additionally, we have assessed the effects free-carriers and a variety of different sample structures and surface treatments have on these kinetics.

Wolford, D.J.; Gilliland, G.D.; Kuech, T.F.; Bradley, J.A. (Thomas J. Watson Research Center, Yorktown Heights, NY (United States)); Hjalmarson, H.P. (Sandia National Labs., Albuquerque, NM (United States))

1992-01-01

297

Free-exciton recombination in GaAs  

SciTech Connect

We have studied the kinetics of free-exciton recombination in ``ideal`` GaAs structures. Our studies include detailed photoluminescence kinetics measurements versus laser excitation power, temperature, sample structure, and surface treatment. The primary goal in these studies is to understand the mechanism precipitating free-exciton recombination, and the role, if any, that polaritons play. Additionally, we have assessed the effects free-carriers and a variety of different sample structures and surface treatments have on these kinetics.

Wolford, D.J.; Gilliland, G.D.; Kuech, T.F.; Bradley, J.A. [Thomas J. Watson Research Center, Yorktown Heights, NY (United States); Hjalmarson, H.P. [Sandia National Labs., Albuquerque, NM (United States)

1992-12-31

298

Magnetism in matter before the discovery of quantum spin: Bohr's less well-known contribution to the transition from classical to quantum physics  

Microsoft Academic Search

How does one explain magnetic effects in matter when one views matter as a collection of classical charges in motion? The answer is: not at all! This is one of the points that Niels Bohr made in his doctoral dissertation in 1911, two years before addressing the issue of the stability of the hydrogen atom. The result, later rediscovered by

Jean-Francois van Huele

2008-01-01

299

Gravitational energy, solar radius and solar cycle  

NASA Astrophysics Data System (ADS)

A self-consistent approach is used. From the change (1.2 W/m2) in the solar constant (1367 W/m2) during a solar cycle we deduced a relation between the change in solar radius ?R and the depth d = (1-?)R in the convection zone where the expansion starts. A second relation is obtained by equating the gravitational energy required for the expansion and the decrease in luminosity during half a solar cycle. This yields values for ?R ? 8 km, d ? 0.96R (super-granular region) and for the change in gravitational energy ?E ? 1032J. Similar considerations are made for the Maunder Minimum yielding ?R ? 60 km, d ? 0.94R and ?E ? 1033J. There is some change, say 40 per cent, if we use a quadratic expansion instead of a linear one. Moreover this theory suits a qualitative explanation why the Sun expands during a minimum of the magnetic activity.

Callebaut, Dirk K.; Makarov, Valentine I.; Tlatov, Andrej G.

2002-03-01

300

Analogs of Bohr-Sommerfeld-Maslov quantization conditions on Riemann surfaces and spectral series of nonself-adjoint operators  

NASA Astrophysics Data System (ADS)

In the paper, the semiclassical asymptotic behavior of the eigenvalues of some nonself-adjoint operators important for applications is studied (for the Sturm-Liouville operator with complex potential and the operator of induction). It turns out that the asymptotic behavior can be calculated using the quantization conditions, which can be represented as the condition that the integrals of a holomorphic form over the cycles on the corresponding complex Lagrangian manifold, which is a Riemann surface of constant energy, are integers. In contrast to the real case (the Bohr-Sommerfeld-Maslov formulas), to calculate a chosen spectral series, it is sufficient to assume that the integral over only one of the cycles takes integer values, and different cycles determine different series.

Esina, A. I.; Shafarevich, A. I.

2013-04-01

301

Assessing Results After Distal Radius Fracture Treatment  

PubMed Central

Objectives: Functional outcomes following distal radius fractures are directly influenced by the choice of outcome assessment instruments used. Our objective was to compare scoring systems in measuring patient functional outcomes and to determine which scoring system compared most favorably with the widely used Disabilities of the Arm, Shoulder, and Hand (DASH) questionnaire. Methods: In all, 108 patients between May 2004 and November 2006 were treated operatively following distal radius fractures. Follow-up was at 3 months, 6 months, 1 year, and 2 years postsurgery, during which anatomical and functional assessments were performed. Patient outcomes were recorded using DASH, Green and O’Brien system, Gartland and Werley system, and Sarmiento radiological scoring system. Results: There was a stronger correlation between the Green and O’Brien scoring system and DASH (r = ?.54) than Gartland and Werley and DASH (r = .44). The Green and O’Brien scoring system was more demanding so patients rated “excellent” or “good” had better functional outcome than those bearing the same grade in the Gartland and Werley system. Nonetheless, the Green and O’Brien score and Gartland and Werley score showed good correlation with each other (r = .66). The Sarmiento radiological score had no significant correlation with any of the other scoring systems. Significant predictors of the DASH score were function (r = .42), power grip (r = .41), pain (r = .37), and range of motion (r = .28). Conclusion: The Green and O’Brien scoring system correlated most strongly with the DASH score. Radiological scoring (reflecting anatomical deformity) was not significantly correlated with functional outcome. While subjective parameters “pain” and “function” are influenced by psychosocial factors and thus highly variable, it is paramount to include subjective tools in outcome assessment in future studies on wrist fractures.

Kwok, Iris H. Y.; Leung, Frankie; Yuen, Grace

2011-01-01

302

Femtosecond THz Studies of Intra-Excitonic Transitions  

SciTech Connect

Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.

Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.

2007-10-02

303

Exciton-plasmon coupling in monolayer molybdenum disulfide  

NASA Astrophysics Data System (ADS)

Two-dimensional materials such as monolayer molybdenum disulfide (MoS2) represent a unique platform for investigating the dynamics of exciton-plasmon coupling. We report on the generation and modulation of coherent and incoherent coupled states between excitons in monolayer MoS2 and plasmons in an array of gold nanoparticle deposited onto the surface of MoS2. We study the behavior of these coherent states, termed plexcitons using a combination of photoluminescence, extinction and ultrafast spectroscopies. The close proximity of the two characteristic exciton bands of MoS2 presents multiple coherent coupling configurations, including A-or-B exciton-plasmon, and A-and-B exciton-plasmon interactions. These configurations of plexciton formation that are shown to modulate both the extinction and photoluminescence spectra of the hybrid system. This includes broadband photoluminescence and Fano-type resonances. This behavior is distinct from the spectral response of the MoS2 and plasmonic components of the system. Incoherent exciton-plasmon coupling, achieved by detuning from the plasmon extinction peaks, enhances the interaction of MoS2 with light by focusing the plasmon energy. Depending on which coupling configuration is chosen, our results show that the MoS2/plasmon hybrid systems can act as high efficiency light harvesters, broadband emitters and as tunable visible and NIR photodetectors.

Ziegler, Jed; Newaz, A. K. M.; Bolotin, Kirill; Haglund, Richard

2013-03-01

304

Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy  

SciTech Connect

We report the density-dependent crossover of a resonantly photoexcited exciton gas from insulating to conducting phases. Broadband terahertz spectroscopy gives direct access to the exciton binding energy via intra-excitonic 1s-2p transitions. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with decreasing inter-particle distance. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite free-carrier and exciton terahertz conductivity. Comparison with near-infrared absorption changes illustrates a significantly enhanced energy shift and broadening of the intra-excitonic resonance.

Huber, Rupert; Kaindl, Robert A.; Schmid, Ben A.; Chemla, Daniel S.

2005-06-25

305

Studies of transport and thermalization of excitons and the development of techniques for in-situ manipulation of excitons in coupled quantum wells  

NASA Astrophysics Data System (ADS)

The study of excitons in semiconductors is a rich field that has seen numerous beautiful developments during the seven decades since they were proposed by Frenkel [1931]. The exciton states in dielectric materials and molecular complexes are fundamental to understanding the full electronic structure of materials and the complex interactions between light and matter. Of particular interest is the fact that excitons, bound electron-hole pairs, form composite bosons with very low effective masses. This opens up an incredibly rich opportunity to study the phase space of quantum degenerate Bose gases at relatively high temperatures. An overview of many of the rich features that have been observed in exciton gases is presented. The system of indirect excitons in coupled quantum wells (CQWs) provides for drastically increased and tunable exciton lifetimes, leading to improved thermalization. The present experimental study presents a full steady state and dynamic model for the transport, generation, recombination, and thermalization of indirect excitons in CQWs, as well as methods for in-situ manipulation of indirect excitons by both optical and electrical methods. Both the optical and electrical methods for exciton control provide for dynamic manipulation of excitons on timescales significantly shorter than the lifetimes of indirect excitons.

Hammack, Aaron Tynes

306

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

307

PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS  

SciTech Connect

On 2008 August 24 the new magnetar SGR 0501+4516 (discovered by Swift) emitted a bright burst with a pronounced double-peaked structure in hard X-rays, reminiscent of the double-peaked temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However, for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24 burst.

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

2010-08-10

308

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

PubMed

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

2013-09-03

309

Kinetics of indirect excitons in an optically induced trap in GaAs quantum wells  

NASA Astrophysics Data System (ADS)

We report on the kinetics of a low-temperature gas of indirect excitons in an optically induced exciton trap. The excitons in the region of laser excitation are found to rapidly—within 4ns —cool to the lattice temperature T=1.4K , while the excitons at the trap center are found to be cold—essentially at the lattice temperature—even during the excitation pulse. The loading time of excitons to the trap center is about 40ns , longer than the cooling time yet shorter than the lifetime of the indirect excitons. The observed time hierarchy is favorable for creating a dense and cold exciton gas in optically induced traps and for in situ control of the gas by varying the excitation profile in space and time before the excitons recombine.

Hammack, A. T.; Butov, L. V.; Mouchliadis, L.; Ivanov, A. L.; Gossard, A. C.

2007-11-01

310

Nonmonotonic energy harvesting efficiency in biased exciton chains  

NASA Astrophysics Data System (ADS)

We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well.

Vlaming, S. M.; Malyshev, V. A.; Knoester, J.

2007-10-01

311

Localization length and intraband scattering of excitons in linear aggregates  

NASA Astrophysics Data System (ADS)

A theoretical model to describe the intraband scattering of excitons in linear aggregates of finite size which exhibit strong intermolecular interactions is presented. From the calculation of the aggregate eigenstates, the localization length of excitons is evaluated for various configurations featuring physical situations like trapping, edge effects, inclusion of diagonal and/or orientational disorders. The intraband scattering is studied by considering the exciton-phonon stochastic coupling induced by the thermal bath. This coupling creates local dynamical fluctuations in the site energies which are characterized by their amplitude ( ?) and their correlation time ( ?c). Expressions of scattering rates are provided and used in a Pauli master equation to calculate the time dependence of the eigenstates populations after initial excitation of the quasi exciton-band. It is shown that the time evolution of the lowest state population as well as the Stokes shift strongly depend on ?c. Comparison of the theoretical results to time-resolved experiments performed on triaryl pyrylium salts allows us to interpret the observed Stokes shift and to derive an average value of the exciton-phonon correlation time.

Lemaistre, J. P.

1999-07-01

312

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

313

Contribution to the cloud droplet effective radius parameterization  

SciTech Connect

An analytic cloud droplet effective radius expression is derived and validated by using field experiment microphysical data. This expression shows that the effective radius depends simultaneously upon the cloud liquid water content, droplet concentration and droplet spectral dispersion. It further suggests that the variability in these parameters present at all scales, due to turbulent mixing and secondary droplet activation, could limit the accuracy of the effective radius parameterizations used in climate models. 12 refs.

Pontikis, C.; Hicks, E. (Univ. des Antilles et de la Guyane, Pointe-a-Pitre (Guadeloupe))

1992-11-01

314

Raman dispersion spectroscopy probes heme distortions in deoxyHb-trout IV involved in its T-state Bohr effect  

PubMed Central

The depolarization ratios of heme protein Raman lines arising from vibrations of the heme group exhibit significant dependence on the excitation wavelength. From the analysis of this depolarization ratio dispersion, one obtains information about symmetry-lowering distortions ?Q? of the heme group that can be classified in terms of the symmetry races ? = A1g, B1g, B2g, and A2g in D4h symmetry. The heme-protein interaction can be changed by the protonation of distinct amino acid side chains (i.e., for instance the Bohr groups in hemoglobin derivates), which gives rise to specific static heme distortions for each protonation state. From the Raman dispersion data, it is possible to obtain parameters by fitting to a theoretical expression of the Raman tensor, which provide information on these static distortions and also about the pK values of the involved titrable side chains. We have applied this method to the ?4 (1,355 cm-1) and ?10 (1,620 cm-1) lines of deoxygenated hemoglobin of the fourth component of trout and have measured their depolarization ratio dispersion as a function of pH between 6 and 9. From the pH dependence of the thus derived parameters, we obtain pK values identical to those of the Bohr groups, which were earlier derived from the corresponding O2-binding isotherms. These are pK?1 = pK?2 = 8.5 for the ? and pK?1 = 7.5, pK?2 = 7.4 for the ? chains. We also obtain the specific distortion parameters for each protonation state. As shown in earlier studies, the ?4 mode mainly probes distortions from interactions between the proximal histidine and atoms of the heme core (i.e., the nitrogens and the C? atoms of the pyrroles). Group theoretical argumentation allows us to relate specific changes of the imidazole geometry as determined by its tilt and azimuthal angle and the iron-out-of-plane displacement to distinct variations of the normal distortions ?Q? derived from the Raman dispersion data. Thus, we found that the pH dependence of the heme distortions ?QA1g (totally symmetric) and ?QB1g (asymmetric) is caused by variations of the azimuthal rather than the tilt angle of the Fe-His (F8) bond. In contrast to this, the ?10 line mainly monitors changes resulting from the interaction between peripheral substituents of the porphyrin macrocycle (vinyl). From the pH dependence of the parameters, it is possible to separately identify distortions ?Q? affecting the hemes in the ? and ? chains, respectively. From this, we find that in the ? subunit structural changes induced on protonation of the corresponding Bohr groups are mainly transferred via the Fe—N? bond and give rise to changes in the azimuthal angle. In the ? subunit, however, in addition, structural changes of the heme pocket arise, which most probably result from protonation of the imidazole of the COOH-terminal His (HC3 ?). This rearranges the net of H bonds between His HC3 ?, Ser (F9 ?), and Glu (F7 ?).

Schweitzer-Stenner, Reinhard; Bosenbeck, Michael; Dreybrodt, Wolfgang

1993-01-01

315

Anterior corneal asphericity calculated by the tangential radius of curvature.  

PubMed

We propose a method of calculating the corneal asphericity (Q) and analyze the characteristics of the anterior corneal shape using the tangential radius. Fifty-eight right eyes of 58 subjects were evaluated using the Orbscan II corneal topographer. The Q-values of the flat principal semi-meridians calculated by the sagittal radius were compared to those by the tangential radius. Variation in the Q-value with semi-meridian in the nasal and temporal cornea calculated by the tangential radius was analyzed. There were significant differences in Q-values (P<0.001) between the two methods. The mean Q-values of the flat principal semi-meridians calculated by tangential radius with -0.33 ± 0.10 in the nasal and -0.22 ± 0.12 in the temporal showed more negative than the corresponding Q-values calculated by the sagittal radius. The Q-values calculated by tangential radius became less negative gradually from horizontal semi-meridians to oblique semi-meridians in both nasal and temporal cornea. Variation in Q-value with semi-meridian was more obvious in the nasal cornea. The method of calculating corneal Q using the tangential radius could provide more reasonable and complete Q-value than that by the sagittal radius. The model of a whole anterior corneal surface could be reconstructed on the basis of the above method. PMID:22894477

Ying, Jinglu; Wang, Bo; Shi, Mingguang

2012-07-01

316

Anterior corneal asphericity calculated by the tangential radius of curvature  

NASA Astrophysics Data System (ADS)

We propose a method of calculating the corneal asphericity (Q) and analyze the characteristics of the anterior corneal shape using the tangential radius. Fifty-eight right eyes of 58 subjects were evaluated using the Orbscan II corneal topographer. The Q-values of the flat principal semi-meridians calculated by the sagittal radius were compared to those by the tangential radius. Variation in the Q-value with semi-meridian in the nasal and temporal cornea calculated by the tangential radius was analyzed. There were significant differences in Q-values (P<0.001) between the two methods. The mean Q-values of the flat principal semi-meridians calculated by tangential radius with -0.33±0.10 in the nasal and -0.22±0.12 in the temporal showed more negative than the corresponding Q-values calculated by the sagittal radius. The Q-values calculated by tangential radius became less negative gradually from horizontal semi-meridians to oblique semi-meridians in both nasal and temporal cornea. Variation in Q-value with semi-meridian was more obvious in the nasal cornea. The method of calculating corneal Q using the tangential radius could provide more reasonable and complete Q-value than that by the sagittal radius. The model of a whole anterior corneal surface could be reconstructed on the basis of the above method.

Ying, Jinglu; Wang, Bo; Shi, Mingguang

2012-07-01

317

Turbulence in a Bose-Einstein condensate of dipolar excitons in coupled quantum wells  

NASA Astrophysics Data System (ADS)

The nonlinear dynamics of a Bose-Einstein condensate (BEC) of dipolar excitons trapped in an external confining potential in coupled quantum wells is analyzed. It is demonstrated that under typical experimental conditions the dipolar exciton BEC can be described by a generalized Gross-Pitaevskii equation with the local interaction between the excitons, which depends on the exciton distribution function. It is shown that, if the system is pumped at sufficiently high frequencies, a steady turbulent state can be formed.

Berman, O. L.; Kezerashvili, R. Ya.; Kolmakov, G. V.; Lozovik, Yu. E.

2012-07-01

318

Mapping the spatial overlap of excitons in a photosynthetic complex via coherent nonlinear frequency generation  

NASA Astrophysics Data System (ADS)

We experimentally demonstrate a nonlinear spectroscopic method that is sensitive to exciton-exciton interactions in a Frenkel exciton system. Spatial overlap of one-exciton wavefunctions leads to coupling between them, resulting in two-exciton eigenstates that have the character of many single-exciton pairs. The mixed character of the two-exciton wavefunctions gives rise to a four-wave-mixing nonlinear frequency generation signal. When only part of the linear excitation spectrum of the complex is excited with three spectrally tailored pulses with separate spatial directions, a frequency-shifted third-order nonlinear signal emerges in the phase-matched direction. We employ the nonlinear response function formalism to show that the emergence of the signal is mediated by and carries information about the two-exciton eigenstates of the system. We report experimental results for nonlinear frequency generation in the Fenna-Matthews-Olson (FMO) photosynthetic pigment-protein complex. Our theoretical analysis of the signal from FMO confirms that the emergence of the frequency-shifted signal is due to the interaction of spatially overlapped excitons. In this method, the signal intensity is directly measured in the frequency domain and does not require scanning of pulse delays or signal phase retrieval. The wavefunctions of the two-exciton states contain information about the spatial overlap of excitons and can be helpful in identifying coupling strengths and relaxation pathways. We propose this method as a facile experimental means of studying exciton correlations in systems with complicated electronic structures.

Dawlaty, Jahan M.; Bennett, Doran I. G.; Huxter, Vanessa M.; Fleming, Graham R.

2011-07-01

319

Phosphorescence as a probe of exciton formation and energy transfer in organic light emitting diodes  

Microsoft Academic Search

The development of highly efficient phosphorescent molecules has approximately quadrupled the quantum efficiency of organic light emitting devices (OLEDs). By harnessing triplet as well as singlet excitons, efficient molecular phosphorescence has also enabled novel studies of exciton physics in organic semiconductors. In this review, we will summarize recent progress in understanding exciton formation and energy transfer using phosphorescent molecular probes.

Marc Baldo; Michaela Segal

2004-01-01

320

Exciton dynamics of a single quantum dot embedded in a nanowire  

Microsoft Academic Search

We have carried out a detailed optical characterization of a single CdSe quantum dot embedded in a ZnSe nanowire. Exciton, biexciton, and charged exciton lines have been identified unambiguously using photon correlation spectroscopy. This technique has provided a detailed picture of the dynamics of this system. It has been found that the dark exciton has a strong influence on the

G. Sallen; A. Tribu; T. Aichele; R. André; L. Besombes; C. Bougerol; S. Tatarenko; K. Kheng; J. Ph. Poizat

2009-01-01

321

Novel Quantum Condensates in Excitonic Matter  

NASA Astrophysics Data System (ADS)

These lectures interleave discussion of a novel physical problem of a new kind of condensate with teaching of the fundamental theoretical tools of quantum condensed matter field theory. Polaritons and excitons are light mass composite bosons that can be made inside solids in a number of different ways. As bosonic particles, they are liable to make a phase coherent ground state-generically called a Bose-Einstein condensate (BEC)-and these lectures present some models to describe that problem, as well as general approaches to the theory. The focus is very much to explain how mean-field-like approximations that are often presented heuristically can be derived in a systematic fashion by path integral methods. Going beyond the mean field theory then produces a systematic approach to calculation of the excitation energies, and the derivation of effective low energy theories that can be generalised to more complex dynamical and spatial situations than is practicable for the full theory, as well as to study statistical properties beyond the semi-classical regime. in particular, for the polariton problem, it allows one to connect the regimes of equilibrium BEC and non-equilibrium laser. The lectures are self-sufficient, but not highly detailed. The methodological aspects are covered in standard quantum field theory texts and the presentation here is deliberately cursory: the approach will be closest to the book of Altland and Simons [1]. Since these lectures concern a particular type of condensate, reference should also be made to texts on BEC, for example by Pitaevskii and Stringari [2]. A recent theoretically focussed review of polariton systems is [3] covers many of the technical issues associated with the polariton problem in greater depth and provides many further references.

Littlewood, P. B.; Keeling, J. M. J.; Simons, B. D.; Eastham, P. R.; Marchetti, F. M.; Szyma?ska, M. H.

2009-08-01

322

Laser induced trapping of excitons in coupled quantum wells  

NASA Astrophysics Data System (ADS)

Optical trapping and manipulation of neutral particles plays a major role in single particle studies in physics, chemistry, and biology [1]. An exciting recent outgrowth of the technique has been the experimental implementation of atom Bose- Einstein Condensation [2,3]. In this contribution, we report proposal and demonstration of laser induced trapping for a new system - a cold gas of excitons in coupled quantum wells. We report trapping a cold gas of excitons in laser induced traps and on the formation of a highly degenerate Bose gas of excitons in the trap. [1] A. Ashkin, IEEE Journal on Selected Items in Quantum Electronics/ 6, 841 (2000). [2] E. A. Cornell, C. E. Wieman, Rev. Mod. Phys./ 74, 875 (2002). [3] W. Ketterle, Rev. Mod. Phys./ 74, 1131 (2002).

Hammack, A. T.; Griswold, M.; Butov, L. V.; Ivanov, A. L.; Smallwood, L. E.; Gossard, A. C.

2006-03-01

323

Huge excitonic effects in layered hexagonal boron nitride.  

PubMed

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

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

2006-01-18

324

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

325

Multichromophore excitons and resonance energy transfer: Molecular quantum electrodynamics  

NASA Astrophysics Data System (ADS)

Resonance energy transfer in multichromophore arrays, such as light-harvesting complexes and dendrimers, is well documented. The theory involved in the migration of energy to an acceptor from one excited donor, or concertedly from two of three such donors, has also been thoroughly investigated. In cases where the initial excitations form a delocalized exciton amongst the donors the corresponding theory describing transfer to an acceptor is less well-developed. By considering a model dendrimeric system we analyze the configuration and energy transfer properties of excitonic states formed by the absorption of one and two photons. Using molecular quantum electrodynamics and interaction-pair notation we quantify these effects in terms of quantum amplitudes. New insights result from our description in terms of state sequences. In particular it is possible for the first time to identify quantum interconnections between different exciton relaxation routes.

Jenkins, Robert D.; Andrews, David L.

2003-02-01

326

Long-range exciton dissociation in organic solar cells.  

PubMed

It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range exciton dissociation is likely (using a kinetic model and a microscopic model for the calculation of the long-range electron transfer rate). Then, defining a rather general model Hamiltonian for the donor material, we show that the phenomenon is extremely common in the majority of polymer:fullerene bulk heterojunction solar cells. PMID:22869702

Caruso, Domenico; Troisi, Alessandro

2012-08-06

327

Diamond C 1s core-level excitons: Surface sensitivity  

NASA Astrophysics Data System (ADS)

The effect of surface termination on C 1s core-level bulk excitons found near the surface in single-crystal diamond is reported. By simultaneously recording bulk (total electron yield) and surface (partial electron yield) near-edge x-ray absorption fine-structure (NEXAFS) spectra with a range of single-crystal surface terminations, variable bulk exciton energy blueshifts and linewidth broadening were observed, most notably in the reconstructed single-crystal diamond. Investigation of polycrystalline diamond and ultrananocrystalline diamond films, using the same technique, allowed for a comparison between the reconstructed single-crystal diamond surface and nanodiamond bulk excitons. These findings suggest that surface-related effects can be misinterpreted as quantum confinement in some nanodiamond NEXAFS studies. Band bending is suggested as a possible contributor to these results.

Stacey, Alastair; Cowie, Bruce C. C.; Orwa, Julius; Prawer, Steven; Hoffman, Alon

2010-09-01

328

Surface plasmon mediated energy transfer of electrically-pumped excitons  

SciTech Connect

We report strong surface plasmon polariton mediated transfer of energy between molecular excitons across the metallic cathode of an electrically-pumped organic heterostructure. The donor molecular excitons at the organic heterojunction resonantly excite surface plasmon modes on both sides of the optically thick metal electrode, which evanescently couple to dye molecules near the electrode’s exterior surface. Dye fluorescence in the capping layer on the exterior of the device shows a 6.5-fold increase in intensity due to this effect, far exceeding any enhancement attributable to Purcell or optical microcavity effects. Demonstration of this energy transfer mechanism for electrically-pumped excitons suggests new sensing and imaging applications with high signal to noise ratio and new routes for performance improvement in energy harvesting devices, plasmonic devices, and organic LEDs (including white light emission).

An, Kwang Hyup; Shtein, Max; Pipe, Kevin P.

2010-01-01

329

Collective exciton magnetic polarons in quantum wells with semimagnetic barriers  

NASA Astrophysics Data System (ADS)

A mechanism for exciton Bose condensation is suggested. We show that the interaction of localized exciton magnetic polarons (EMP's) can lead to the formation of a free collective exciton magnetic polaron (CEMP) able to move as a whole particle. Critical conditions for this transition and characteristics of the CEMP are found. We interpret the recent data on giant magnetic polaron mobility in ZnSe/ZnMnxSe1-x quantum wells [G. A. Balchin, C. D. Poweleit, and L. M. Smith, in Proceedings of the 23rd International Conference on the Physics of Semiconductors, edited by M. Scheffler and R. Zimmermann (World Scientific, Singapore 1996), p. 2055] as a manifestation of the free CEMP formation due to clusterization of long-living indirect EMP's.

Kavokin, Alexey; Gil, Bernard; Bigenwald, Pierre

1998-02-01

330

Exciton effects in boron-nitride (BN) nanotubes  

NASA Astrophysics Data System (ADS)

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6eV at the onsite interaction U=2t with the hopping integral t=1.1eV. The binding energy of the exciton is 0.50eV 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 nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiralities.

Harigaya, Kikuo

2012-12-01

331

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

PubMed

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

332

Exciton absorption of entangled photons in semiconductor quantum wells  

NASA Astrophysics Data System (ADS)

The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers.

Rodriguez, Ferney; Guzman, David; Salazar, Luis; Quiroga, Luis

2013-03-01

333

Electrodynamic and excitonic intertube interactions in semiconducting carbon nanotube aggregates.  

PubMed

The optical properties of selectively aggregated, nearly single chirality single-wall carbon nanotubes were investigated by both continuous-wave and time-resolved spectroscopies. With reduced sample heterogeneities, we have resolved aggregation-dependent reductions of the excitation energy of the S(1) exciton and enhanced electron-hole pair absorption. Photoluminescence spectra revealed a spectral splitting of S(1) and simultaneous reductions of the emission efficiencies and nonradiative decay rates. The observed strong deviations from isolated tube behavior are accounted for by enhanced screening of the intratube Coulomb interactions, intertube exciton tunneling, and diffusion-driven exciton quenching. We also provide evidence that density gradient ultracentrifugation can be used to structurally sort single-wall carbon nanotubes by aggregate size as evident by a monotonic dependence of the aforementioned optical properties on buoyant density. PMID:21391554

Crochet, Jared J; Sau, Jay D; Duque, Juan G; Doorn, Stephen K; Cohen, Marvin L

2011-03-10

334

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

335

Long-range exciton dissociation in organic solar cells  

PubMed Central

It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range exciton dissociation is likely (using a kinetic model and a microscopic model for the calculation of the long-range electron transfer rate). Then, defining a rather general model Hamiltonian for the donor material, we show that the phenomenon is extremely common in the majority of polymer:fullerene bulk heterojunction solar cells.

Caruso, Domenico; Troisi, Alessandro

2012-01-01

336

Bohr's indeterminacy principle in quantum holography, adaptive neural networks, cortical self-organization, molecular computers, magnetic resonance imaging, and solitonic nanotechnology  

Microsoft Academic Search

A rigorous proof of quantum parallelism cannot be based on the Heisenberg inequality because the standard deviation of self-adjoint operators in complex Hilbert space is insensitive to fine structures of the collective stationary interference distribution generated by a Mach-Zehnder interferometer from a coherent primary beam. Actually Niels Bohr's indeterminacy principle of spatio-temporal quantum electrodynamics cannot be based on any of

Walter Schempp

1992-01-01

337

Excitonic bandgap dependence on stacking configuration in four layer graphene  

NASA Astrophysics Data System (ADS)

Different crystallographic stacking configurations in graphene provide an additional degree of freedom in the electronic structure. We have conducted systematic investigations of the transport properties of ABAB- and ABCA-stacked four-layer graphene. Our results reveal that ABAB and ABCA graphene exhibit markedly different properties as functions of both temperature and magnetic field. The temperature-dependant resistance measurement reveals that the excitonic gap of ABCA stacked graphene increases as a function of temperature, while for ABAB, a shrinking excitonic gap configuration is observed.

Liu, Y. P.; Goolaup, S.; Lew, W. S.; Purnama, I.; Chandra Sekhar, M.; Zhou, T. J.; Wong, S. K.

2013-10-01

338

Bound excitons in Sr2CuO3.  

PubMed

We investigated temperature dependent optical spectra of the one-dimensional chain compound Sr2CuO3. The charge transfer transition polarized along the chain direction shows a strongly asymmetric line shape as expected in one-dimensional extended Hubbard model. At low temperature, the charge transfer peak shows a large blueshift and reveals additional sharp peaks at the gap. Even though many spectroscopic studies suggest that this material cannot have a bound exciton based on the one-dimensional extended Hubbard model, we attribute the additional sharp peaks to excitons, which come to exist due to the long-range Coulomb interaction. PMID:18999786

Kim, K W; Gu, G D; Homes, C C; Noh, T W

2008-10-24

339

Exciton supersolidity in hybrid Bose-Fermi systems.  

PubMed

We investigate the ground states of a Bose-Einstein condensate of indirect excitons coupled to an electron gas. We show that in a properly designed system the crossing of a roton minimum into the negative energy domain can result in the appearance of the supersolid phase, characterized by periodicity in both real and reciprocal space. Accounting for the spin-dependent exchange interaction of excitons we obtain ferromagnetic supersolid domains. The Fourier spectra of excitations of weakly perturbed supersolids show pronounced diffraction maxima which may be detected experimentally. PMID:22401039

Matuszewski, Micha?; Taylor, Thomas; Kavokin, Alexey V

2012-02-09

340

Terahertz magnetoconductivity of excitons and electrons in quantum cascade structures  

NASA Astrophysics Data System (ADS)

We examined the quasiparticles formed by the photoexcitation of GaAs/AlGaAs terahertz quantum cascade structures using terahertz time-domain spectroscopy. At low temperature and excitation density the measured conductivity was excitonic, with a 1s-2p transition energy indicative of three-dimensional excitons correlated across the quantum well barriers. Free electrons increasingly dominated the conductive response at higher lattice temperatures and excitation densities. Under an external magnetic field transitions from the 1s level into 2p states with different magnetic quantum number were observed, while at high excitation densities the electron cyclotron resonance became more prominent.

Lloyd-Hughes, J.; Beere, H. E.; Ritchie, D. A.; Johnston, M. B.

2008-03-01

341

Charged excitons and biexcitons bound to isoelectronic centers  

NASA Astrophysics Data System (ADS)

We demonstrate that the singular binding mechanism characterizing isoelectronic centers formed from two isoelectronic traps can also bind, in addition to the well-studied excitons, various number of charges. Using the emission fine structure of Te dyads in ZnSe and N dyads in GaAs, we establish that these pseudodonors and pseudoacceptors can bind positively and negatively charged excitons, respectively, and that both can bind biexcitons. This ability to bind various charge configurations, in addition to their very low inhomogeneous broadenings and perfectly defined symmetries, further establishes isoelectronic centers as an interesting alternative to epitaxial quantum dots for a number of applications.

Marcet, S.; Ouellet-Plamondon, C.; Éthier-Majcher, G.; Saint-Jean, P.; André, R.; Klem, J. F.; Francoeur, S.

2010-12-01

342

Coherent exciton delocalization in strongly coupled quantum dot arrays.  

PubMed

Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical transitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron-hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter. PMID:24041088

Crisp, Ryan W; Schrauben, Joel N; Beard, Matthew C; Luther, Joseph M; Johnson, Justin C

2013-09-16

343

Search for E(5) behavior: IBM and Bohr Mottelson model with Davidson potential calculations of some even even Xe isotopes  

NASA Astrophysics Data System (ADS)

The interacting boson model (IBM) has been widely used for describing the quadrupole collective states of the medium-heavy nuclei and no distinction is made between proton and neutron variables when the first version of the model is applied. However, the neutrons' and protons' degrees of freedom are described explicitly in the second version of the model (IBM-2). Moreover, the microscopic foundations certainly state that it is very important to describe the proton and neutron variables explicitly and this is also the generalized definition of the second version of the IBA model (IBM-2 model). So, triaxiality can be described explicitly through the introduction of cubic terms in the boson operators. Using the best-fitted values of parameters in the Hamiltonian of the IBM-2, we have calculated energy levels and B(E2) values for a number of transitions in 122,124,126,128,130,132,134Xe. The results were compared with the previous experimental and theoretical data and it has been observed that they are in good agreement. Many B(E2) values that are still not known so far are stated and the set of parameters used in these calculations is the best approximation that has been carried out so far. It has also turned out that the IBA and Bohr-Mottelson Hamiltonian with Davidson potential are fairly reliable models for the calculation of spectra in the entire set of 122,124,126,128,130,132,134Xe isotopes.

Turkan, Nureddin

2007-11-01

344

Revealing the structural origin of the redox-Bohr effect: the first solution structure of a cytochrome from Geobacter sulfurreducens.  

PubMed

Gs (Geobacter sulfurreducens) can transfer electrons to the exterior of its cells, a property that makes it a preferential candidate for the development of biotechnological applications. Its genome encodes over 100 cytochromes and, despite their abundance and key functional roles, to date there is no structural information for these proteins in solution. The trihaem cytochrome PpcA might have a crucial role in the conversion of electronic energy into protonmotive force, a fundamental step for ATP synthesis in the presence of extracellular electron acceptors. In the present study, 15N-labelled PpcA was produced and NMR spectroscopy was used to determine its solution structure in the fully reduced state, its backbone dynamics and the pH-dependent conformational changes. The structure obtained is well defined, with an average pairwise rmsd (root mean square deviation) of 0.25 Å (1 Å=0.1 nm) for the backbone atoms and 0.99 Å for all heavy atoms, and constitutes the first solution structure of a Gs cytochrome. The redox-Bohr centre responsible for controlling the electron/proton transfer was identified, as well as the putative interacting regions between PpcA and its redox partners. The solution structure of PpcA will constitute the foundation for studies aimed at mapping out in detail these interacting regions. PMID:21861844

Morgado, Leonor; Paixão, Vítor B; Schiffer, Marianne; Pokkuluri, P Raj; Bruix, Marta; Salgueiro, Carlos A

2012-01-01

345

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

346

Modeling the interferometric radius measurement using Gaussian beam propagation  

SciTech Connect

We model the interferometric radius measurement using Gaussian beam propagation to identify biases in the measurement due to using a simple geometric ray-trace model instead of the more complex Gaussian model. The radius measurement is based on using an interferometer to identify the test part's position when it is at two null locations, and the distance between the positions is an estimate of the part's radius. The null condition is observed when there is no difference in curvature between the reflected reference and the test wavefronts, and a Gaussian model will provide a first-order estimate of curvature changes due to wave propagation and therefore changes to the radius measurement. We show that the geometric ray assumption leads to radius biases (errors) that are a strong function of the test part radius and increase as the radius of the part decreases. We tested for a bias for both microscaled(<1 mm) and macroscaled parts. The bias is of the order of parts in 105 for micro-optics with radii a small fraction of a millimeter and much smaller for macroscaled optics. The amount of bias depends on the interferometer configuration (numerical aperture, etc.), the nominal radius of the test part, and the distances in the interferometer.

Medicus, Katherine M.; Snyder, James J.; Davies, Angela

2006-12-01

347

Traceable radius of curvature measurements on a micro-interferometer  

NASA Astrophysics Data System (ADS)

Radius of curvature is a critical parameter to measure in the manufacturing of micro-refractive elements. It defines the power of the surface and provides important information about the stability and uniformity of the manufacturing process. The radius of curvature of an optical surface can be measured using an interferometer and radius slide where the distance is measured as the surface is moved between the confocal and cat"s eye positions. However, the radius of curvature for micro-refractive elements can be on the order of a few hundred microns and the uncertainty in the measurement due to stage error motions can become a significant portion of the tolerance. Typically the radius slide is calibrated using an artifact, but the radius of the artifact must be traceable to the base unit of length and the calibration is subject to misalignment errors. Alternatively, the stage error motions can be measured with standard machine tool metrology techniques and used to correct the errors in the radius of curvature measurement. This paper details the implementation of a directly traceable radius of curvature measurement on a micro-interferometer, including alignment procedures, measurement of stage error motions, displacement gauge calibration, and data analysis strategies.

Karodkar, Devendra; Gardner, Neil; Bergner, Brent C.; Davies, Angela

2004-01-01

348

Minimum mass radius ratio for charged gravitational objects  

NASA Astrophysics Data System (ADS)

We rigorously prove that for compact charged general relativistic objects there is a lower bound for the mass radius ratio. This result follows from the same Buchdahl type inequality for charged objects, which has been extensively used for the proof of the existence of an upper bound for the mass radius ratio. The effect of the vacuum energy (a cosmological constant) on the minimum mass is also taken into account. Several bounds on the total charge, mass and the vacuum energy for compact charged objects are obtained from the study of the Ricci scalar invariants. The total energy (including the gravitational one) and the stability of the objects with minimum mass radius ratio is also considered, leading to a representation of the mass and radius of the charged objects with minimum mass radius ratio in terms of the charge and vacuum energy only.

Böhmer, C. G.; Harko, T.

2007-06-01

349

Excitons and excess electrons in nanometer size molecular polyoxotitanate clusters: electronic spectra, exciton dynamics, and surface states.  

PubMed

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

2012-11-12

350

Localization-Delocalization Transition of Indirect Excitons in Lateral Electrostatic Lattices  

NASA Astrophysics Data System (ADS)

We study transport of indirect excitons in GaAs/AlGaAs coupled quantum wells in linear lattices created by laterally modulated gate voltage. The localization-delocalization transition for transport across the lattice was observed with reducing lattice amplitude or increasing exciton density. The exciton interaction energy at the transition is close to the lattice amplitude. These results are consistent with the model, which attributes the localization-delocalization transition to the interaction-induced percolation of the exciton gas through the external potential. We also discuss applications of the lattice potentials for estimating the strength of disorder and exciton interaction.

Remeika, M.; Graves, J. C.; Hammack, A. T.; Meyertholen, A. D.; Fogler, M. M.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2009-05-01

351

Excitonic resonances in the 2D extended Falicov-Kimball model  

NASA Astrophysics Data System (ADS)

Using the projector-based renormalization method we investigate the formation of the excitonic insulator phase in the two-dimensional (2D) spinless Falicov-Kimball model with dispersive f electrons and address the existence of excitonic bound states at high temperatures on the semiconductor side of the semimetal-semiconductor transition. To this end we calculate the imaginary part of the dynamical electron-hole pair susceptibility and analyze the wave vector and energy dependence of excitonic resonances emerging in the band gap. We thereby confirm the existence of the exciton insulator and its exciton environment within a generic two-band lattice model with local Coulomb attraction.

Phan, V.-N.; Fehske, H.; Becker, K. W.

2011-07-01

352

Free versus localized exciton in GaAs V-shaped quantum wires  

NASA Astrophysics Data System (ADS)

We present a systematic investigation of continuous wave and transient photoluminescence in GaAs V-shaped quantum wires as a function of temperature, aimed to the understanding of the radiative recombination mechanism of the ground level (localized versus free-exciton recombination). Exciton localization is observed at low temperatures. Free-exciton polariton transitions are monitored at intermediate temperatures through the square root temperature dependence of the decay time. Exciton localization energy, density of localization centers and exciton intrinsic lifetime have been determined from the theoretical analysis of the transient photoluminescence, thus providing a simple quantitative method for the assessment of sample quality.

Lomascolo, M.; Ciccarese, P.; Cingolani, R.; Rinaldi, R.; Reinhart, F. K.

1998-01-01

353

Exciton spin relaxation time in quantum dots measured by continuous-wave photoluminescence spectroscopy  

NASA Astrophysics Data System (ADS)

We demonstrate a method of measuring the exciton spin relaxation time in semiconductor nanostructures by continuous-wave photoluminescence. We find that for self-assembled CdTe quantum dots (QDs) the degree of circular polarization of emission is larger when exciting polarized excitons into the lower energy spin state (?--polarized) than in the case when the excitons are excited into the higher energy spin state (?+-polarized). A simple rate equation model gives the exciton spin relaxation time in CdTe QDs equal to ?S=4.8+/-0.3 ns, significantly longer than the QD exciton recombination time ?R=300 ps.

Mackowski, S.; Nguyen, T. A.; Jackson, H. E.; Smith, L. M.; Kossut, J.; Karczewski, G.

2003-12-01

354

Trapping Indirect Excitons in a GaAs Quantum-Well Structure with a Diamond-Shaped Electrostatic Trap  

NASA Astrophysics Data System (ADS)

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.

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.

2009-08-01

355

Excitation power and temperature dependence of excitons in CuInSe2  

NASA Astrophysics Data System (ADS)

Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4 meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low.

Luckert, F.; Yakushev, M. V.; Faugeras, C.; Karotki, A. V.; Mudryi, A. V.; Martin, R. W.

2012-05-01

356

Mechanism for controlling the exciton fine structure in quantum dots using electric fields: Manipulation of exciton orientation and exchange splitting at the atomic scale  

NASA Astrophysics Data System (ADS)

Using atomistic tight-binding theory with a configuration interaction description of Coulomb and exchange effects, we describe excitons in symmetric quantum dots in a vertical electric field to explain how field-induced manipulation of exciton orientation and phase can eliminate the anisotropic exchange, leading to a drastic reduction of the fine-structure splitting and a 90 degree rotation of polarization, similar to experiment. This reorientation and rephasing of the exciton is done plane-by-plane inside the dot without significant squeezing of the exciton.

Bryant, Garnett W.; Malkova, Natalia; Sims, James

2013-10-01

357

Wavelength modulated excitonic spectra of Cu2O thin films sandwiched by MgO plates  

NASA Astrophysics Data System (ADS)

We have investigated stress effects on yellow exciton states in a Cu2O thin film sandwiched by MgO plates by measuring wavelength modulated (WM) absorption spectra and their thermal variations. In the WM absorption spectra, dispersive spectral structures owing to 2P~4P states in the yellow excitons are clearly resolved in the thin film sample. A stress due to the lattice mismatch between Cu2O and MgO provides a large red shift of the band gap in the green excitonic system. However, in the yellow excitonic transitions, it is found that a red-shift of the 2P excitonic state is much smaller than that in the green excitonic system. This result suggests that a shallow potential minimum for the yellow excitons is built up in the Cu2O thin film sandwiched by MgO plates.

Iwamitsu, Kazunori; Aihara, Shingo; Shimamoto, Tomoshige; Fujii, Atsuhiro; Akai, Ichiro

2013-05-01

358

Excitons in the LH3 Complexes from Purple Bacteria.  

PubMed

The noncovalently bound and structurally identical bacteriochlorophyll a chromophores in the peripheral light-harvesting complexes LH2 (B800-850) and LH3 (B800-820) from photosynthetic purple bacteria ensure the variability of the exciton spectra in the near-infrared (820-850 nm) wavelength region. As a result, the spectroscopic properties of the antenna complexes, such as positions of the maxima in the exciton absorption spectra, give rise to very efficient excitation transfer toward the reaction center. In this work, we investigated the possible molecular origin of the excitonically coupled B820 bacteriochlorophylls in LH3 using femtosecond transient absorption spectroscopy, deconvolution of steady-state absorption spectra, and modeling of the electrostatic intermolecular interactions using a charge density coupling approach. Compared to LH2, the upper excitonic level is red-shifted from 755 to 790 nm and is associated with an approximate 2-fold decrease of B820 intrapigment coupling. The absorption properties of LH3 cannot be reproduced by only changing the B850 site energy but also require a different scaling factor to be used to calculate interpigment couplings and a change of histidine protonation state. Several protonation patterns for distinct amino acid groups are presented, giving values of 162-173 cm(-1) at 100 K for the intradimer resonance interaction in the B820 ring. PMID:23570515

Chmeliov, Jevgenij; Songaila, Egidijus; Rancova, Olga; Gall, Andrew; Robert, Bruno; Abramavicius, Darius; Valkunas, Leonas

2013-04-30

359

Many-body correlations of electrostatically trapped dipolar excitons  

NASA Astrophysics Data System (ADS)

We study the photoluminescence (PL) of a two-dimensional liquid of oriented dipolar excitons in InxGa1-xAs coupled double quantum wells confined to a microtrap. Generating excitons outside the trap and transferring them at lattice temperatures down to T=240 mK into the trap we create cold quasiequilibrium bosonic ensembles of some 1000 excitons with thermal de Broglie wavelengths exceeding the excitonic separation. With decreasing temperature and increasing density n?5×1010(1)/(cm2) we find an increasingly asymmetric PL line shape with a sharpening blue edge and a broad red tail which we interpret to reflect correlated behavior mediated by dipolar interactions. From the PL intensity I(E) below the PL maximum at E0 we extract at T<5 K a distinct power law I(E)˜(E0-E)-|?| with -|?|?-0.8 in the range E0-E of 1.5-4 meV, comparable to the dipolar interaction energy.

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

2013-05-01

360

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

361

Connecting molecular structure and exciton diffusion length in rubrene derivatives.  

PubMed

Connecting molecular structure and exciton diffusion length in rubrene derivatives demonstrates how the diffusion length of rubrene can be enhanced through targeted functionalization aiming to enhance self-Förster energy transfer. Functionalization adds steric bulk, forcing the molecules farther apart on average, and leading to increased photoluminescence efficiency. A diffusion length enhancement greater than 50% is realized over unsubstituted rubrene. PMID:23754475

Mullenbach, Tyler K; McGarry, Kathryn A; Luhman, Wade A; Douglas, Christopher J; Holmes, Russell J

2013-06-10

362

TRIPLET EXCITONS AND DELAYED FLUORESCENCE IN ANTHRACENE CRYSTALS  

Microsoft Academic Search

Delayed fluorescence was detected in laser-induced fluorescence in ; anthracene single crystals after the laser pulse ended. The rate of decay of ; this fluorescence and other experimental observations was explained by assuming ; that the singlet state giving rise to the fluorescence is produced by a ; bimolecular annihilation of triplet excitons generated directly by the laser ; light.

R. G. Kepler; J. C. Caris; P. Avakian; E. Abramson

1963-01-01

363

PREFACE: International Conference on Optics of Excitons in Confined Systems  

Microsoft Academic Search

The OECS11 (International Conference on Optics of Excitons in Confined Systems) was the eleventh of a very successful series of conferences that started in 1987 in Rome (Italy). Afterwards the conference was held at Naxos (Sicily, Italy, 1991), Montpellier (France, 1993), Cortona (Italy, 1995), Göttingen (Germany, 1997), Ascona (Switzerland, 1999), Montpellier (France, 2001), Lecce (Italy, 2003), Southampton (UK, 2005) and

Luis Viña; Carlos Tejedor; José M. Calleja

2010-01-01

364

Multichromophore excitons and resonance energy transfer: Molecular quantum electrodynamics  

Microsoft Academic Search

Resonance energy transfer in multichromophore arrays, such as light-harvesting complexes and dendrimers, is well documented. The theory involved in the migration of energy to an acceptor from one excited donor, or concertedly from two of three such donors, has also been thoroughly investigated. In cases where the initial excitations form a delocalized exciton amongst the donors the corresponding theory describing

Robert D. Jenkins; David L. Andrews

2003-01-01

365

Magneto-exciton transitions in laterally coupled quantum dots  

NASA Astrophysics Data System (ADS)

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

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

2008-03-01

366

Charged excitons in doped extended Hubbard model systems  

Microsoft Academic Search

We show that the charge transfer excitons in a Hubbard model system including nearest-neighbor Coulomb interactions effectively attain some charge in doped systems and become visible in photoelectron and inverse photoelectron spectroscopies. This shows that the description of a doped system by an extended Hubbard model differs substantially from that of a simple Hubbard model. Longer-range Coulomb interactions cause satellites

J. van den Brink; R. Eder; G. A. Sawatzky

1997-01-01

367

Strong exciton-photon coupling in organic semiconductor microcavities  

Microsoft Academic Search

Organic optoelectronic devices have already found their way into consumer electronics. This is remarkable, considering how little we understand about the microscopic processes involving charge carriers and excitons (bound electron-hole pairs), the protagonists in such devices. To understand the many-body physics of organic films (which are composed of ˜ 1021\\/cm 3 molecules), the quasiparticle concept is often introduced. As a

Stephane Kena-Cohen

2010-01-01

368

Surface Plasmon Generation by Excitons in Carbon Nanotubes  

NASA Astrophysics Data System (ADS)

Optical properties of semiconducting carbon nanotubes (CNs) originate from excitons and may be tuned by either electrostatic doping [1], or via the quantum confined Stark effect (QCSE) by means of an electrostatic field applied perpendicular to the CN axis[2]. In both cases exciton properties are mediated by surface plasmon excitations [2,3]. We have shown recently that the QCSE allows one to control the exciton-interband-plasmon coupling in individual CNs and their optical absorption, accordingly [2]. Here, we extend our studies to demonstrate the possibility of low-energy localized surface plasmon generation by optically excited excitons in small-diameter (˜1nm) CNs. The stimulated character of such an energy transfer causes the buildup of the macroscopic population numbers of coherent localized surface plasmons and, as a consequence, high-intensity coherent optical-frequency fields localized at nanoscale, which can be used for various applications, such as near-field nonlinear-optical probing, sensing, or materials nanoscale modification. [1] M.Steiner, et al., NL9,3477. [2] I.V.Bondarev, et al., PRB80,085407. [3] C.D.Spataru and F.Leonard, PRL104,177402.

Bondarev, Igor; Antonijevic, Todor

2011-03-01

369

Nonradiative exciton energy transfer in hybrid organic-inorganic heterostructures  

Microsoft Academic Search

Nonradiative energy transfer from a GaAs quantum well to a thin overlayer of an infrared organic semiconductor dye is unambiguously demonstrated. The dynamics of exciton transfer are studied in the time domain by using pump-probe spectroscopy at the donor site and fluorescence spectroscopy at the acceptor site. The effect is observed as simultaneous increase in the population decay rate at

S. Chanyawadee; P. G. Lagoudakis; R. T. Harley; D. G. Lidzey; M. Henini

2008-01-01

370

Trapping effects in exciton motion in the framework of CTRW.  

National Technical Information Service (NTIS)

Influence of a trap (sink) on an exciton transfer in molecular aggregates is investigated. Pausing time distribution functions phi(sup p)(sub m)(t) and probability densities Q(sub mn)(t) on the continuous time random walk theory are calculated from memory...

I. Barvik P. Herman

1990-01-01

371

Optical nonlinearities of excitons in ZnO single crystal  

Microsoft Academic Search

Time-integrated and spectrally-resolved four-wave mixing (FWM) has been used to study dephasing dynamics of excitons in a free-standing bulk ZnO. Clear FWM signals due to A>Gamma5- and BGamma5-excitions have been observed. We discuss the dephasing dynamics based on the polariton dispersion and four-particle Coulomb correlations.

Kouji Hazu; Takayuki Sota; Katsuo Suzuki; Satoru Adachi; Shigefusa F. Chichibu

2003-01-01

372

Slow exciton trapping in Photosystem II: A possible physiological role  

Microsoft Academic Search

Photosystem II, which has a primary photochemical charge separation time of about 300 ps, is the slowest trapping of all photosystems. On the basis of an analysis of data from the literature this is shown to be due to a number of partly independent factors: a shallow energy funnel in the antenna, an energetically shallow trap, exciton dynamics which are

Robert C. Jennings; Flavio M. Garlaschi; Laura Finzi; Giuseppe Zucchelli

1996-01-01

373

Theory of Self-Trapped Exciton Luminescence in Halide Crystals  

Microsoft Academic Search

A detailed phenomenological theory of the triplet luminescence from self-trapped excitons in halide crystals is developed. Energy levels and wave functions are obtained by constructing and diagonalizing a Hamiltonian matrix. Coupled rate equations for the decay of the three lowest triplet levels are solved for the general case in which the levels are not in thermal equilibrium, and circular polarizations,

W. Beall Fowler; M. J. Marrone; M. N. Kabler

1973-01-01

374

Excitonic Properties of Novel ?-conjugated Polymers for Organic Electronics  

NASA Astrophysics Data System (ADS)

We compare the photophysics of different derivatives of the ?-conjugated polymer Poly(thienylenevinylene) (PTV) by photoluminescence (PL) and electro-absorption (EA) spectroscopy. The binding energy of the primary excitonic excited state is obtained from EA and is found to be related to the quantum efficiency for PL. In particular, both quantities are determined by the energies of the first optically allowed state and the first optically forbidden state above the ground state. In most PTV derivatives, the optically forbidden state lies below the optically allowed state and the PL is efficiently quenched by internal conversion. When the order of excited states is reversed, PL is observable with an efficiency that scales with the binding energy of the exciton as determined by EA. Thus, the chemical structure governs the interplay between ?-conjugation delocalization and electron correlation that determines the ordering of excitonic states. This ordering then in turn dictates the effectiveness of a ?-conjugated polymer for both emission and exciton dissociation and therefore dictates a material's suitability for either Organic Light-emitting Diodes or Organic Photovoltaic devices. This information then may be useful in the design of novel materials for application in these devices.

Lafalce, Evan; Jiang, Xiaomei; Zhang, Cheng

2013-03-01

375

Optical generation of excitonic valley coherence in monolayer WSe2.  

PubMed

As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges ('valleys') to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics. PMID:23934096

Jones, Aaron M; Yu, Hongyi; Ghimire, Nirmal J; Wu, Sanfeng; Aivazian, Grant; Ross, Jason S; Zhao, Bo; Yan, Jiaqiang; Mandrus, David G; Xiao, Di; Yao, Wang; Xu, Xiaodong

2013-08-11

376

Is there an excitonic interaction or antenna system in bacteriorhodopsin  

SciTech Connect

The presence of a biphasic circular dichroism (CD) observed in the visible absorption spectrum of retinal in bacteriorhodopsin (bR) has been believed for many years to be due to excitonic-type interaction within the trimeric structure of the retinal in the protein membrane. In the present work, the authors present data and discuss previous observations that strongly suggest the absence of such an excitonic interaction. The magnetic CD spectrum of the trimer is found to be similar to that of the monomer and shows no sign of absorption to the double degenerate state predicted to be present from the exciton theory. This, together with the previous observations on the CD spectra of the photocycle intermediates of bR as well as the linear polarization studies of the fluorescence and the daughter absorption, sheds doubt on the presence of exciton interaction and thus suggests the presence of neither an antenna system nor a viable special reaction center in bR. Possible explanation for the observed biphasic nature of the CD spectrum is given in terms of having more than one type of bR with different protein conformations around the retinals in the trimer giving each similar absorption maximum but opposite signs for its rotary dispersion power.

El-Sayed, M.A.; Lin, C.T.; Mason, W.R. (Univ. of California, Los Angeles (USA))

1989-07-01

377

Heavy ion collisions and the pre-equilibrium exciton model  

SciTech Connect

We present a feasible way to apply the pre-equilibrium exciton model in its masterequation formulation to heavy-ion induced reactions including spin variables. Emission of nucleons, {gamma}'s and also light clusters is included in our model.

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

2012-10-20

378

Different responses of localized and extended excitons to exciton{endash}exciton scattering manifested in excitation density-dependent photoluminescence excitation spectra  

SciTech Connect

Excitation density-dependent photoluminescence excitation (PLE) measurements are performed for a disordered two-dimensional (2D) In{sub 0.40}Ga{sub 0.60}As/GaAs(311)B quantum dot superlattice (QDSL) containing localized and extended states separated by a mobility edge. For low excitation densities, the PLE spectrum is independent of excitation density. In the extended state region, it takes the shape of the absorption spectrum of the 2D QDSL. With increasing excitation density, the PLE spectral shape is dramatically changed. For excitation densities at which the state filling can be excluded, it is found that the different responses of localized and extended excitons to exciton{endash}exciton scattering are responsible for the observed change in the PLE spectrum. A qualitative explanation for the evolution of the PLE spectrum is presented based on the excitation density-dependent capture, relaxation and recombination times obtained previously in time-resolved photoluminescence (TRPL) experiments. The mobility edge extracted from the modification of the PLE spectrum is in good agreement with that determined by TRPL measurements. To show the effect of coupling strength, a comparison of the excitation density-dependent PLE spectra is made between the 2D QDSL and a weekly coupled quantum dot array. It further confirms the existence of highly extended states in the 2D QDSL. {copyright} 2001 American Institute of Physics.

Lan, Sheng; Nishikawa, Satoshi; Wada, Osamu; Akahane, Kouichi; Okada, Yoshitaka; Kawabe, Mitsuo

2001-06-01

379

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

SciTech Connect

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

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

2006-11-01

380

An Asian perspective on the management of distal radius fractures.  

PubMed

There is limited data regarding the epidemiology, pathology, and management of distal radius fractures from centers in Asia. The advanced economies in Asia include Hong Kong, Japan, Korea, Singapore, and Taiwan, whereas the prominent emerging economies are China, India, Malaysia, Philippines, and Thailand. This article examines the available epidemiological data from Asia, compares the management of distal radius fractures in the advanced and emerging Asian economies and how they compare with the current management in the west. It concludes by offering solutions for improving outcomes of distal radius fractures in Asia. PMID:22554658

Sebastin, Sandeep J; Chung, Kevin C

2012-05-01

381

An Asian Perspective on the Management of Distal Radius Fractures  

PubMed Central

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

Sebastin, Sandeep J.; Chung, Kevin C.

2012-01-01

382

Stress fractures of the distal radius in adolescent gymnasts  

Microsoft Academic Search

Adolescent girl gymnasts sustained stress fractures of the distal end of the radius in the wrist on which a rotational vault was performed. The history and clinical progress were typical of stress fractures.

M. T. F. Read

1981-01-01

383

Fixed-Radius Point Counts in Forests: Factors Influencing ...  

Treesearch

... Petit, Lisa J.; Saab,Victoria A.; Martin, Thomas E. Subject: Uncontrolled Keywords: ... A single visit detected approximately 70 percent, and two visits 90 percent, ... Moreover, fixed-radius plots may allow for greater flexibility in study design ...

384

Optical Study of Exciton Localization Phenomena in Semimagnetic Semiconductors and Their Multiple Quantum Wells.  

NASA Astrophysics Data System (ADS)

The results of picosecond photomodulation and photoluminescence spectroscopies in novel II-VI semimagnetic semiconductors Cd(,1-x)Mn(,x)Te (x < 0.50) bulk and multiple quantum well (MQW) samples are presented. By studying excitonic emission near the bandgap of semiconductors, it is found that excitons can be confined or localized by alloy potential fluctuations, quantum well confinements, local strain of heterointerfaces and energy self-trapping. Steady-state photoluminescence in undoped CdTe/Cd(,1 -x)Mn(,x)Te MQW samples at low temperature shows intense excitonic emission where their radiative quantum efficiencies are two or three orders of magnitude larger than that of the high quality CdTe bulk samples. Time-resolved photoluminescence shows that the excitons have relatively short lifetime (500 picosecond). High quantum efficiency and short exciton lifetime suggest that the radiative recombination is a dominating factor in the excitonic-decay processes in the MQW samples. In general, excitonic emission energies in CdMnTe MQW samples are lower than the free exciton energies (typically 20-40 meV lower as noted from the reflectance spectra). The behavior of these emissions under an external magnetic field (up to 36 tesla) shows that excitons prefer to be localized at the heterointerfaces rather than at the center of the wells in MQW samples. The kinetics of the free and the heterointerface localized excitons in the Cd(,1-x)Mn(,x)Te/Cd(,1-y)Mn(,y)Te MQW samples have been studied by using a transient photoluminescence technique. Exciton lifetimes have been measured in several samples with various quantum well widths. The trapping time of the free exciton localized at the interface has been observed in the wide quantum well samples. The average energy loss rate of localized excitons has been calculated. The resonance excitation spectra of steady-state and transient luminescence show that the exciton spectra are spatially inhomogeneously broadened. An external magnetic field accelerates the localized exciton recombination processes. The dynamics of the excitons in CdMnTe MQW samples suggests that quasi-2D bound magnetic polarons (BMP) exist. A discussion of this quasi-2D BMP and the influence of an external magnetic field on exciton lifetime and exciton dynamics in Cd(,1 -x)Mn(,x)Te MQW is included.

Zhang, Xi-Cheng

1986-12-01

385

Radius scaling of titanium wire arrays on the Z accelerator  

Microsoft Academic Search

Summary form only given. The 20 MA Z accelerator has made possible the generation of substantial radiation (>100 kJ) at higher photon energies (4.8 keV) through the use of titanium wire arrays. In this paper, the results of experiments designed to study the effects of initial load radius variations of nickel-clad titanium wire arrays will be presented. The load radius

C. A. Coverdale; C. Deeney; R. B. Spielman; M. R. Douglas; T. J. Nash; K. G. Whitney; W. Thornhill; J. P. Apruzese; R. C. Clark; J. Davis; D. L. Peterson; F. N. Beg; J. Ruiz-Camacho; R. Schneider

1999-01-01

386

Bose-Einstein statistics in thermalization and photoluminescence of quantum-well excitons  

NASA Astrophysics Data System (ADS)

Quasiequilibrium relaxational thermodynamics is developed to understand LA-phonon-assisted thermalization of Bose-Einstein distributed excitons in quantum wells. We study quantum-statistical effects in the relaxational dynamics of the effective temperature of excitons T=T(t). When T is less than the degeneracy temperature T0, well-developed Bose-Einstein statistics of quantum-well excitons leads to nonexponential and density-dependent thermalization. At low bath temperatures Tb-->0, the thermalization of quantum statistically degenerate excitons effectively slows down and T(t)~1/ln t. We also analyze the optical decay of Bose-Einstein distributed excitons in perfect quantum wells, and show how nonclassical statistics influences the effective lifetime ?opt. In particular, ?opt of a strongly degenerate gas of excitons is given by 2?R, where ?R is the intrinsic radiative lifetime of quasi-two-dimensional excitons. Kinetics of resonant photoluminescence of quantum-well excitons during their thermalization is studied within the thermodynamic approach and taking into account Bose-Einstein statistics. We find density-dependent photoluminescence dynamics of statistically degenerate excitons. Numerical modeling of the thermalization and photoluminescence kinetics of quasi-two-dimensional excitons are given for GaAs/AlxGa1-xAs quantum wells.

Ivanov, A. L.; Littlewood, P. B.; Haug, H.

1999-02-01

387

Isolated Diaphyseal Fractures of the Radius in Skeletally Immature Patients  

PubMed Central

Diaphyseal radius fractures without associated ulna fracture or radioulnar dislocation (isolated fracture of the radius) are recognized in adults but are rarely described in skeletally immature patients. A search of our database (1974–2002) identified 17 pediatric patients that had an isolated fracture of the radius. Among the 13 patients with at least 1 year follow-up, ten were treated with manipulative reduction and immobilization in an above elbow cast and three had initial operative treatment with plate and screw fixation. These 13 patients were evaluated for an average of 18 months (range, 12 to 45 months) after injury using the system of Price and colleagues. The incidence of isolated diaphyseal radius fractures in skeletally immature patients was 0.56 per year in our database and represented 27% of the 63 patients with a diaphyseal forearm fracture. All 13 patients, with at least 1 year follow-up, regained full elbow flexion and extension and full forearm rotation. According to the classification system of Price, all 13 patients (100%) had an excellent result. As in adults, isolated radius fractures seem to occur in children more frequently than previously appreciated. Treatment of isolated radius fractures in skeletally immature patients has a low complication rate, and excellent functional outcomes are the rule.

Guitton, Thierry G.; Van Dijk, Niek C.; Raaymakers, Ernst L.

2009-01-01

388

The seismic radius of the Sun, and structure inversions  

NASA Astrophysics Data System (ADS)

It is known (Schou et al., 1997; Antia, 1998) that the effective radius of the Sun determined by f-mode frequencies is different by a few hundredths per cent from the photospheric radius determined by direct photometric measurement (Brown and Christensen-Dalsgaard 1998). It is fair to say that we still do not fully comprehend the implications of the difference, save that the two radii are rather different quantities: the radius inferred from f-mode frequencies is determined by the location of the maximum in the f-mode energy (Gough, 1993), whereas the photospheric radius is determined by extrapolation to some prescribed optical depth from a fiducial point in the limb-darkening function using a theoretical solar model. Both depend in particular on the structure of the upper superadiabatic convective boundary layer, the physics of which is not well understood. In this report we attempt to shed some light on the difference by determining a seismic radius from p-mode frequencies; the outcome depends predominantly on the variation of sound speed, and it is consistent with the f-mode value (Takata and Gough 2001). By considering the mathematical structure of an inversion process that does not explicitly distinguish f modes from p modes, we offer an interpretation of the seismic radius. This interpretation has led us to revise the method by which we carry out structure inversions.

Takata, M.; Gough, D. O.

2003-02-01

389

Coherent dynamics of excitons and continuum excitations in indium phosphide  

NASA Astrophysics Data System (ADS)

The dynamics of optically generated, coherent electronic excitations in Indium Phosphide at a temperature of 5 K are investigated over a 140 meV energy range near the band gap, using the two-pulse, self-diffracted, degenerate four-wave mixing technique with 30-50 fs near-infra-red pulses and spectrally resolved detection. The coherent dynamics of excitons associated with the fundamental energy gap are investigated under conditions of simultaneous excitation of continuum transitions. Exciton-carrier scattering is shown to produce the dominant source of diffraction for excitons due to excitation-induced dephasing. The dephasing rate of continuum excitations is found to increase abruptly at the threshold energy for LO phonon emission by electrons. Below this threshold energy the dephasing rate is determined to be at least 8 pssp{-1}, whereas above the threshold energy, the dephasing rate is at least 13 pssp{-1}. Near the spin-orbit split-off valence to conduction band transition energy the diffracted spectrum is strongly affected by an interaction between split-off excitons and continuum excitations of heavy and light hole bands. A Fano-like spectral profile is observed in the coherent emission spectrum at the split-off transition and the coherent dynamics is discussed in terms of coupled exciton-continuum excitations. The Fano coupling parameter q, which characterizes the spectral profile, is found to decrease as excitation density increases. The interaction matrix element between split-off excitons and continuum excitations is determined to be approximately 5 meV, which corresponds to a 130 fs lifetime, over the density range investigated. Observation of the Fano interference phenomenon by four-wave mixing is found to depend on the relative orientation of the electric field polarization vectors. The Fano-like spectral profile is much less prominent for cross-polarized pulses than for co-polarized pulses. Evidence for Fano interference is also observed by spectrally integrated four-wave mixing; there is no evidence of the interference in the cross-polarized case. At present, the polarization dependence is not understood; therefore, the interaction mechanism is not identified but possible interaction mechanisms that could give rise to Fano interference are discussed.

Allan, Gary Richard

390

Study of temperature and indium concentration-dependent dielectric constant and electron affinity effects on the exciton optical transition and binding energy in spherical GaSb – Ga 1 ? x In x As y Sb 1 ? y – GaSb quantum dots  

Microsoft Academic Search

We have study the heavy-hole exciton states in GaSb–GaInAsSb–GaSb type-I spherical Quantum Dots, using temperature-dependent static dielectric constant and electron affinity, with a finite height potential barrier, as a function of the quantum dot radius for several values of Indium concentration. Our calculations have been worked out using interpolating methods to find the temperature and Indium concentration dependence of both

R. Sánchez-Cano; N. Porras-Montenegro

2010-01-01

391

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

NASA Astrophysics Data System (ADS)

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.

Williams, R. T.; Grim, Joel Q.; Li, Qi; Ucer, K. B.; Bizarri, G. A.; Kerisit, S.; Gao, Fei; Bhattacharya, P.; Tupitsyn, E.; Rowe, E.; Buliga, V. M.; Burger, A.

2013-09-01

392

Target Z dependence of Xe L x-ray emission in heavy ion-atom collision near the Bohr velocity: influence of level matching  

NASA Astrophysics Data System (ADS)

X-ray yields for the projectile L-shell have been measured for collisions between Xe20+ and thick solid targets throughout the periodic table with incident energies near the Bohr velocity. The yields show a very pronounced cyclic dependence on the target atomic number. This result indicates that Xe L x-ray emission intensity is greatly enhanced either in near-symmetric collisions or if the binding energy of the Xe M-shell matches the L- or N-shell binding energy of the target.

Ren, Jieru; Zhao, Yongtao; Zhou, Xianming; Cheng, Rui; Lei, Yu; Sun, Yuanbo; Wang, Xing; Xu, Ge; Wang, Yuyu; Liu, Shidong; Yu, Yang; Li, Yongfeng; Zhang, Xiaoan; Xu, Zhongfeng; Xiao, Guoqing

2013-09-01

393

Z(4): {gamma}-rigid Solution of the Bohr Hamiltonian for {gamma} = 30 deg. Compared to the E(5) Critical Point Symmetry  

SciTech Connect

A {gamma}-rigid solution of the Bohr Hamiltonian for {gamma} = 30 deg. is derived, its {beta}-part being related to the second order Casimir operator of the Euclidean algebra E(4). The solution is called Z(4), since it 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 experimental data in the Xe region around A = 130.

Bonatsos, D.; Lenis, D.; Petrellis, D. [Institute of Nuclear Physics, N.C.S.R. '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. [Institute of Nuclear Physics, N.C.S.R. 'Demokritos', GR-15310 Aghia Paraskevi, Attiki (Greece); Hasan Ali Yucel Faculty of Education, Istanbul University, TR-34470 Beyazit, Istanbul (Turkey)

2006-04-26

394

Optically Induced Rotation of an Exciton Spin in a Semiconductor Quantum Dot  

NASA Astrophysics Data System (ADS)

We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton’s spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.

Poem, E.; Kenneth, O.; Kodriano, Y.; Benny, Y.; Khatsevich, S.; Avron, J. E.; Gershoni, D.

2011-08-01

395

Exciton Confinement in Traps Formed by a Laterally Modulated Gate Voltage  

NASA Astrophysics Data System (ADS)

In semiconductor materials, cold gases of bosons can be realized in the system of indirect excitons in coupled quantum well structures. Boson confinement in potential traps improves the critical temperature for Bose-Einstein condensation and allows manipulation of the bosons by varying the trap potential [1,2]. Here, we present in-plane potential traps for indirect excitons, where the traps are formed by a laterally modulated gate voltage. The calculated trap design allows effective exciton confinement as well as in situ manipulation of excitons by the gate electrodes that control the confining potential. The design also ensures that the electric fields caused by the confining potential are well below the threshold for the exciton dissociation. Experiments with indirect excitons in the traps are presented as well. 1. E.A. Cornell, C.E. Wieman, Rev. Mod. Phys. 74, 875 (2002). 2. W. Ketterle, Rev. Mod. Phys. 74, 1131 (2002).

Hammack, A. T.; Hanson, M.

2005-03-01

396

Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys  

SciTech Connect

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

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

2000-01-27

397

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

NASA Astrophysics Data System (ADS)

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

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

2010-11-01

398

Jointly tuned plasmonic-excitonic photovoltaics using nanoshells.  

PubMed

Recent advances in spectrally tuned, solution-processed plasmonic nanoparticles have provided unprecedented control over light's propagation and absorption via engineering at the nanoscale. Simultaneous parallel progress in colloidal quantum dot photovoltaics offers the potential for low-cost, large-area solar power; however, these devices suffer from poor quantum efficiency in the more weakly absorbed infrared portion of the sun's spectrum. Here, we report a plasmonic-excitonic solar cell that combines two classes of solution-processed infrared materials that we tune jointly. We show through experiment and theory that a plasmonic-excitonic design using gold nanoshells with optimized single particle scattering-to-absorption cross-section ratios leads to a strong enhancement in near-field absorption and a resultant 35% enhancement in photocurrent in the performance-limiting near-infrared spectral region. PMID:23444829

Paz-Soldan, Daniel; Lee, Anna; Thon, Susanna M; Adachi, Michael M; Dong, Haopeng; Maraghechi, Pouya; Yuan, Mingjian; Labelle, André J; Hoogland, Sjoerd; Liu, Kun; Kumacheva, Eugenia; Sargent, Edward H

2013-03-06

399

Spectrally-tuned plasmonic-excitonic photovoltaics using gold nanoshells  

NASA Astrophysics Data System (ADS)

Recent advances in spectrally-tunable solution-processed metal nanoparticles have provided unprecedented control over light at the nanoscale. Parallel progress in colloidal quantum dot photovoltaics offers the potential for low-cost, large-area solar power; however, these devices suffer from poor quantum efficiency in the more weakly-absorbed near infrared portion of the sun's spectrum. Here, we report a plasmonic-excitonic solar cell that combines two jointly-tuned solution processed infrared materials. We show through experiment and theory that a plasmonic-excitonic design using gold nanoshells with optimized single-particle scattering-to-absorption cross section ratios leads to a strong enhancement in near-field absorption and resultant photocurrent in the performance-limiting near infrared spectral region.

Lee, Anna; Paz-Soldan, Daniel; Thon, Susanna; Adachi, Michael; Dong, Haopeng; Maraghechi, Pouya; Yuan, Mingjian; Labelle, André; Hoogland, Sjoerd; Liu, Kun; Kumacheva, Eugenia; Sargent, Edward

2013-04-01

400

Improved Cascade-Exciton Model of Nuclear Reactions  

Microsoft Academic Search

Recent improvements to the Cascade-Exciton Model (CEM) of nuclear reactions\\u000aare briefly described. They concern mainly the cascade stage of reactions and a\\u000abetter description of nuclei during the preequilibrium and evaporation stages\\u000aof reactions. The development of the CEM concerning fission is given in a\\u000aseparate talk at this conference. The increased accuracy and predictive power\\u000aof the CEM

Stepan G. Mashnik; Arnold J. Sierk

1998-01-01

401

Optically induced entanglement of excitons in a single quantum Dot  

PubMed

Optically induced entanglement is identified by the spectrum of the phase-sensitive homodyne-detected coherent nonlinear optical response in a single gallium arsenide quantum dot. The electron-hole entanglement involves two magneto-excitonic states differing in transition energy and polarization. The strong coupling needed for entanglement is provided through the Coulomb interaction involving the electrons and holes. The result presents a first step toward the optical realization of quantum logic operations using two or more quantum dots. PMID:10988065

Chen; Bonadeo; Steel; Gammon; Katzer; Park; Sham

2000-09-15

402

Increased exciton harvesting in organic thin film solar cells  

NASA Astrophysics Data System (ADS)

The optimization of organic solar cells involves a fundamental tradeoff between optical absorption length, mobility, and exciton diffusion length (LD). Organic semiconductors possess LD that are at least one order of magnitude less than their respective absorption lengths, meaning that many excitons decay before reaching a dissociating interface. The bulk heterojunction concept, whereby one mixes donor and acceptor components into a single layer, is an effective way to avoid this bottleneck. However, because mixed layers tend to have lower mobilities compared with pure films, carrier transport in devices composed of mixed layers thick enough to absorb a significant amount of light is poor, producing an inefficient device. In this talk, we explore two promising approaches to solve these challenges. In one, we investigate the possibility of increasing LD of a given material. By employing a properly chosen phosphorescent dopant, we are able to sensitize a population of long-lived triplet excitons in a normally fluorescent material, increasing the diffusion length by more than a factor of 2. In another approach, we look into the possibility of exploiting surface plasmon resonances of metal nanoparticles. These surface plasmon resonances lead to strongly enhanced near fields, increasing absorption of nearby chromophores. With this approach, therefore, the thickness of organic semiconductor layers containing metal nanoparticles could be reduced without compromising absorption. Here, we investigate exciton-plasmon interactions through photoluminescence and absorption measurements of thin-films consisting of organic semiconductors and metal nanoparticles, as a function of film thickness with and without the presence of spacer layers between the nanoparticles and absorbers. From this knowledge, we assess the prospect of using plasmonic effects in thin film organic solar cells.

Rand, Barry P.

2010-03-01

403

Exciton–vibrational coupling in pinwheel aggregates of ?-conjugated molecules  

Microsoft Academic Search

Absorption spectra of para-distyrylbenzene (DSB) aggregates are calculated based on a Hamiltonian which includes excitonic coupling between molecules, each containing a single electronic transition coupled to an intramolecular vibration (with frequency ?0 and Huang–Rhys factor ?2). Aggregates are based on the four-molecule pinwheel unit which maximizes the favorable edge–face interactions. Pinwheel aggregates of DSB lie within the intermediate coupling regime,

F. C. Spano; S. Siddiqui

1999-01-01

404

Functional formulation of microscopic theory of exciton polaritons  

Microsoft Academic Search

Exact results obtained from quantum field theory in its functional formulation are used to construct a microscopic theory of exciton polaritons in semiconductors. It is shown that the polariton spectrum is determined by the poles of the two-particle Green's function that describes both the direct Coulomb electron-hole interaction as well as the exchange electron-hole interaction. By means of higher Legendre

G. F. Glinskii; Zl. Koinov

1987-01-01

405

Nonradiative exciton energy transfer in hybrid organic-inorganic heterostructures  

Microsoft Academic Search

Non-radiative optical energy transfer from a GaAs quantum well to a thin\\u000aoverlayer of an infrared organic semiconductor dye is unambiguously\\u000ademonstrated. The dynamics of exciton transfer are studied in the time-domain\\u000ausing pump-probe spectroscopy at the donor site and fluorescence spectroscopy\\u000aat the acceptor site. The effect is observed as simultaneous increase of the\\u000apopulation decay rate at the

S. Chanyawadee; P. G. Lagoudakis; R. T. Harley; D. G. Lidzey; M. Henini

2008-01-01

406

Signatures in the Conductance for Phase Transitions in Excitonic Systems  

NASA Astrophysics Data System (ADS)

In this paper, we analyze two phase transitions in exciton bilayer systems: a topological phase transition to a phase which hosts Majorana fermions and a phase transition to a Wigner crystal. Using generic simple models for different phases, we discuss the conductance properties of the latter when contacted to metallic leads and demonstrate the possibility to observe the different phase transitions by simple conductance measurements.

Soller, H.; Breyel, D.

2013-10-01

407

Bound Exciton Luminescence of the Intercalated Tungsten Dichalcogenide Layer Compounds  

Microsoft Academic Search

The radiative recombination in the layered transition metal dichalcogenide compounds 2H-WS 2 and 2H-WSe 2 has been investigated. It is shown that the strong photoluminescence (PL) of these indirect band gap semiconductors is caused by recombination of excitons bound to neutral centers formed due to the intercalation of halogen molecules Br 2 and I 2 in the well defined sites

L. Kulyuk; E. Bucher; L. Charron; E. Fortin; A. Nateprov; O. Schenker

2002-01-01

408

Exciton delocalization and energy transport mechanisms in R-phycoerythrin.  

PubMed

Energy transport mechanisms in R-Phycoerythrin (RPE), a light harvesting protein located at the top of the phycobilisome antenna in red algae, are investigated using nonlinear optical spectroscopies and theoretical models. The RPE hexamer possesses a total of 30 bilin pigments, which can be subdivided into three classes based on their molecular structures and electronic resonance frequencies. Of particular interest to this study is the influence of exciton delocalization on the real-space paths traversed by photoexcitations as they concentrate on the lowest energy pigment sites. Transient grating measurements show that significant nuclear relaxation occurs at delay times less than 100 fs, whereas energy transport spans a wide range of time scales depending on the proximity of the initial and final states involved in the process. The fastest energy transport dynamics within the RPE complex are close to 1 ps; however, evidence for sub-100 fs exciton self-trapping is also obtained. In addition, photon echo experiments reveal vibronic interactions with overdamped and underdamped nuclear modes. To establish signatures of exciton delocalization, energy transport is simulated using both modified Redfield and Fo?rster theories, which respectively employ delocalized and localized basis states. We conclude that exciton delocalization occurs between six pairs of phycoerythrobilin pigments (i.e., dimers) within the protein hexamer. It is interesting that these dimers are bound in locations analogous to the well-studied phycocyanobilin dimers of cyanobacterial allophycocyanin and c-phycocyanin in which wave function delocalization is also known to take hold. Strong conclusions regarding the electronic structures of the remaining pigments cannot be drawn based on the present experiments and simulations due to overlapping resonances and broad spectroscopic line widths, which prevent the resolution of dynamics at particular pigment sites. PMID:21381708

Womick, Jordan M; Liu, Haoming; Moran, Andrew M

2011-03-07

409

Quantum entanglement of excitons in coupled quantum dots  

Microsoft Academic Search

Optically controlled exciton dynamics in coupled quantum dots is studied. We show that the maximally entangled Bell states and Greenberger-Horne-Zeilinger (GHZ) states can be robustly generated by manipulating the system parameters to be at the avoided crossings in the eigenenergy spectrum. The analysis of population transfer is systematically carried out by using a dressed-state picture. In addition to the quantum

Ping Zhang; C. K. Chan; Qi-Kun Xue; Xian-Geng Zhao

2003-01-01

410

Effect of Magnetic Field on Broadening of Excitonic Spectra in Superlattices  

Microsoft Academic Search

We present a comprehensive study of optical properties of GaAs\\/AlGaAs superlattices (SLs) with different tunneling coupling between wells in magnetic fields in a wide range of excitation levels. Photoluminescence (PL) and PL excitation spectra demonstrate significant broadening of the exciton lines with increasing excitation power. Several features indicate that the exciton line widths are mainly determined by excitonic scattering. In

Yuri Khavin; Nikolai Sibeldin; Mikhail Skorikov; Vitaliy Tsvetkov; Daniel Oberli; Alok Rudra; Romain Carron; Eli Kapon

2009-01-01

411

Excitonic gain and laser emission in ZnSe-based quantum wells  

Microsoft Academic Search

We show spectroscopically that the origin of optical gain and laser emission in (Zn,Cd)Se\\/ZnSe quantum wells at blue-green wavelengths is of excitonic nature. This circumstance derives from the large enhancement in the exciton binding and its oscillator strength which occurs in the quasi-2D case, so that an exciton gas is stable against ionization by optical phonons up to room temperature

J. Ding; H. Jeon; T. Ishihara; M. Hagerott; A. V. Nurmikko; H. Luo; N. Samarth; J. Furdyna

1992-01-01

412

Optical-electrical simulation of organic solar cells: excitonic modeling parameter influence on electrical characteristics  

NASA Astrophysics Data System (ADS)

This paper deals with Organic Solar Cells (OSCs) simulation using finite element method. Optical modeling is performed via Finite Difference Time Domain method whereas the continuity and Poisson's equations are solved to obtain electrical characteristics of the OSC. In this work, simulation results point out the influence of physical parameters such as the exciton diffusion coefficient or the exciton lifetime on OSC performances. The comparison of modeling results and experimental measurement allows the exciton recombination, dissociation rate and lifetime to be determinated.

Vervisch, Wilfried; Biondo, Stéphane; Rivière, Guillaume; Duché, David; Escoubas, Ludovic; Torchio, Philippe; Simon, Jean-Jacques; Le Rouzo, Judikaël

2011-06-01

413

Bose-Einstein condensation of excitons in Cu sub 2 O  

Microsoft Academic Search

Free excitons provide the only experimental system other than helium in which the behavior of particles with mass is known to follow Bose-Einstein statistics. Experimental observations are presented of the kinetic energy distribution of excitons in the direct-gap semiconductor CuâO, both the triplet orthoexciton state and the singlet paraexciton state. The density and temperature of the exciton gas closely follow

Snoke

1990-01-01

414

Bose-Einstein statistics in thermalization and photoluminescence of quantum-well excitons  

Microsoft Academic Search

Quasiequilibrium relaxational thermodynamics is developed to understand LA-phonon-assisted thermalization of Bose-Einstein distributed excitons in quantum wells. We study quantum-statistical effects in the relaxational dynamics of the effective temperature of excitons T=T(t). When T is less than the degeneracy temperature T0, well-developed Bose-Einstein statistics of quantum-well excitons leads to nonexponential and density-dependent thermalization. At low bath temperatures Tb-->0, the thermalization of

A. L. Ivanov; P. B. Littlewood; H. Haug

1999-01-01

415

Exciton-Mediated Raman Scattering in One-Dimensional Quantum Dots  

NASA Astrophysics Data System (ADS)

The differential cross section (DCS) for exciton-mediated Raman scattering (EMRS) in one-dimensional semiconductor quantum dots is presented. The exciton states are considered as intermediate states in the Raman scattering process. The selection rules for the EMRS process are studied. The numerical results show that the contribution to DCS indicated by exciton is larger than that by electron. DCS of EMRS is larger when there is a bigger confinement potential frequency.

Sun, Hai-Chao; Liu, Cui-Hong

416

Enhanced photovoltaic response of organic solar cell by singlet-to-triplet exciton conversion  

Microsoft Academic Search

Ir complex was doped to conjugated polymers, and the photoinduced absorption of triplet excitons in host materials was examined. A greatly enhanced intersystem crossing rate was observed, despite the decrease in triplet exciton lifetime. The authors find that the steady-state triplet exciton population in host polymer would increase by an order of magnitude. Conjugated polymer\\/colloidal CdSe nanocrystal hybrid solar cells

Chia-Ming Yang; Chi-Hui Wu; Hua-Hsin Liao; Kuei-Yuan Lai; Hong-Ping Cheng; Sheng-Fu Horng; Hsin-Fei Meng; Jow-Tsong Shy

2007-01-01

417

Exciton polaritons in a cylindrical microcavity with an embedded quantum wire  

Microsoft Academic Search

Exciton-light coupling in cylindrical microcavities containing quantum wires has been treated by means of classical electrodynamics within the nonlocal dielectric response model. A typical anticrossing behavior of quasi-one-dimensional exciton-polariton modes has been obtained, as well as the weak-coupling-strong-coupling threshold. Effects of the nonradiative damping of the exciton resonance in the quantum wire on the optical response of the microcavity structure

M. A. Kaliteevski; S. Brand; R. A. Abram; V. V. Nikolaev; M. V. Maximov; N. N. Ledentsov; C. M. Sotomayor Torres; A. V. Kavokin

2000-01-01

418

Fluctuating exciton localization in giant ?-conjugated spoked-wheel macrocycles.  

PubMed

Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended ? conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment. PMID:24153376

Aggarwal, A Vikas; Thiessen, Alexander; Idelson, Alissa; Kalle, Daniel; Würsch, Dominik; Stangl, Thomas; Steiner, Florian; Jester, Stefan-S; Vogelsang, Jan; Höger, Sigurd; Lupton, John M

2013-09-29

419

Undoped GaAs bilayers for exciton condensation experiments  

NASA Astrophysics Data System (ADS)

Experimental progress in transport studies of exciton condensation of in electron and hole bilayers at high magnetic fields [1,2] has shown this novel physics can be observed. Fabrication of the bipolar electron-hole bilayers for zero field studies of exciton condensation still remains elusive. We describe a series of experiments on undoped GaAs/AlGaAs heterostructures with the motivation of making electron-hole bilayers. In these undoped devices, external electric fields induce carriers rather than the traditional doping techniques. Single layer electron (or hole) devices demonstrate a high mobility over a wide range of density. More recently, fully undoped bilayers have been made where the density in each layer is independently controlled with gates on the top and bottom of the bilayer. In this talk we present high field transport of undoped electron-electron bilayers, and describe recent progress towards extending the fabrication techniques to creating electron-hole bilayers for exciton condensation studies at zero magnetic field. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. 1. M. Kellogg, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 93 036801 (2004). 2. E. Tutoc, M. Shayegan, and D. A. Huse, Phys. Rev. Lett. 93, 036802 (2004).

Lilly, M. P.

2005-03-01

420

Excitons imaging in hybrid organic-inorganic films  

NASA Astrophysics Data System (ADS)

In this work we investigate the excitonic properties of (4-FC6H4C2H4NH3)2PbI4 hybrid organic/inorganic thin films. We first use a standard point-by-point photoluminescence mapping. The maps formed using the photoluminescence intensity, line width, and broadening reveal the presence of structural defects. Using a statistical treatment of the data we found that the spatial fluctuations of the photoluminescence peak wavelengths are rather small compared to the photoluminescence line width. Moreover, we report the first direct observation of spatially resolved excitonic photoluminescence in this type of materials using dark-field imaging of white-light pumped photoluminescence. Owing to the rapid acquisition time of the dark-field images, their temperature dependence was studied, and the thermal behavior of the photoluminescence was investigated using this technique. We show that photoluminescence mapping combined with dark-field imaging and spectroscopy provides valuable information on the excitonic properties of hybrid organic/inorganic thin films.

Trigui, Amani; Mlayah, Adnen; Abid, Younes; Zwick, Antoine; Boughzala, Habib

2012-11-01

421

Accuracy of distal radius positioning using an anatomical plate.  

PubMed

Over the past decade, several anatomical plates have been introduced to improve the result of open reduction and internal fixation of the distal radius. Using 3-dimensional imaging techniques, the authors studied the accuracy and reproducibility of distal radius positioning using anatomical plates.Distal radius fractures and the correction of these fractures were simulated with plastic bone models of radii. The authors simulated a defect by removing an arbitrary wedge shape from the artificial radii. Two surgeons corrected these fractures by placing 2 anatomical plate types according to the plate manufacturers' instructions. The residual positioning errors of the distal segment in relation to the unaffected radii were determined using 3-dimensional imaging and were compared with naturally occurring bilateral radius differences in healthy individuals. In many cases, positioning does not agree with differences based on bilateral asymmetry in healthy patients.This study indicated the accuracy of anatomical plates. Positioning an anatomical plate may lead to considerable residual errors in individual patients. Volar distal radius plate shapes differ among plate manufacturers. Therefore, one plate may perform better than another in an individual. PMID:23590785

Vroemen, Joy C; Dobbe, Johannes G G; Sierevelt, Inger N; Strackee, Simon D; Streekstra, Geert J

2013-04-01

422

Spontaneous First-order Optical Coherence in Cold Exciton Gases in Coupled Quantum Wells  

NASA Astrophysics Data System (ADS)

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells[1]. A strong enhancement of spontaneous first-order optical coherence was observed at low temperatures below a few Kelvin where the thermal de Broglie wavelength becomes comparable to the interparticle separation and the exciton gas becomes nonclassical. The onset of spontaneous first-order optical coherence was found to be correlated with macroscopic spatial ordering in the exciton system.[1] L.V. Butov, A.C. Gossard, D.S. Chemla, Nature 418, 751 (2002).

Yang, Sen; Hammack, A. T.; Butov, L. V.; Gossard, A. C.

2006-03-01

423

Resonant manifestations of chiral excitons in Faraday and Kerr effects in a topological insulator film  

NASA Astrophysics Data System (ADS)

Manifestations of chiral excitons on a magnetically gapped surfaces of a topological insulator thin film in Kerr and Faraday effects are analyzed. Excitonic contribution to a surface optical conductivity tensor is calculated. Chiral excitons contrary to conventional ones resonantly contribute to Hall conductivity due to the lack of symmetry between the states with opposite angular momentum. They can lead to the considerable enhancement of Faraday angle and ellipticity of transmitted electromagnetic wave. Chiral excitons cause a decrease of Kerr angle and prominent signatures in ellipticity of reflected electromagnetic wave. Conditions for experimental observation of the described effects are discussed.

Efimkin, D. K.; Lozovik, Yu. E.

2013-06-01

424

Two-photon excitation of excitons in CuCl in total reflection geometry.  

PubMed

We have observed emission spectra of a CuCl film on a TiO2 prism surface(110) associated with the two-photon excitation of the exciton system in total reflection geometry. The I1 bound exciton emission, which resonantly appeared at the two-photon excitation of the Z3-longitudinal excitons and Z1,2-exciton band, was observed. The dependence of the emission intensities on the polarization of the excitation light was explained from the field intensity and the penetration depth of the evanescent light in CuCl accompanying the totally reflected light at the TiO2/CuCl interface. PMID:12641758

Hasuo, M; Shimamoto, A; Fujimoto, T

2003-03-01

425

Electronic structure and absorption spectrum of biexciton obtained by using exciton basis  

NASA Astrophysics Data System (ADS)

We approach the biexciton Schrödinger equation not through the free-carrier basis as usually done, but through the free-exciton basis, exciton–exciton interactions being treated according to the recently developed composite boson many-body formalism which allows an exact handling of carrier exchange between excitons, as induced by the Pauli exclusion principle. We numerically solve the resulting biexciton Schrödinger equation with the exciton levels restricted to the ground state and we derive the biexciton ground state as well as the bound and unbound excited states as a function of hole-to-electron mass ratio. The biexciton ground-state energy we find, agrees reasonably well with variational results. Next, we use the obtained biexciton wave functions to calculate optical absorption in the presence of a dilute exciton gas in quantum well. We find an asymmetric peak with a characteristic low-energy tail, identified with the biexciton ground state, and a set of Lorentzian-like peaks associated with biexciton unbound states, i.e., exciton–exciton scattering states. Last, we propose a pump–probe experiment to probe the momentum distribution of the exciton condensate.

Shiau, Shiue-Yuan; Combescot, Monique; Chang, Yia-Chung

2013-09-01

426

Ultrafast dynamics of excitons in delafossite CuScO2 thin films  

NASA Astrophysics Data System (ADS)

Ultrafast carrier dynamics were investigated in a delafossite CuScO2, a material with a remarkably large binding energy of exciton (~0.4 eV), using femtosecond transient transmission spectroscopy. Differential transmission spectra showed dispersive structures in the excitonic resonance energies. We have observed a delayed rise-up on a time scale of 10 ps, suggesting slow carrier cooling. It is followed by a slower decay, time constant of which corresponds to the lifetime of exciton (approximately 0.75 ns). These results were analyzed in terms of the generalized many-body Elliott model, accounting for a screening effect of excitons.

Liu, Fucai; Makino, T.; Hiraga, H.; Fukumura, T.; Kong, Yongfa; Kawasaki, M.

2010-05-01

427

Epithelioid hemangioendothelioma of the forearm with radius involvement. Case report  

PubMed Central

Epithelioid hemangioendothelioma (EHE) is a rare, well-differentiated endothelial tumor with intermediate malignancy which develops more frequently from the peripheral veins, generally in the lower limb. Bone EHE comprises less than 1% of the bone neoplasms. We present the case of a young man, 24-year-old, with EHE of the forearm with secondary involvement of the distal radius. The location and the extension of the tumor allowed a wide excision, without the reconstruction of radius, followed by adjuvant radiotherapy, with a subsequent favorable evolution. Based on the clinical, radiographic, and pathological features of the EHE review, we concluded that it is difficult to adopt a standardized therapeutic approach due to the extremely low incidence of the bone involvement in EHE and the variable tendencies towards malignancy of this tumor. To our knowledge this is the third case of EHE with the involvement of the radius.

2011-01-01

428

Radius dependence of solute concentration estimates of simulated ultrafine precipitates.  

PubMed

Estimates of the radii and solute concentrations of simulated microstructures containing ultrafine spherical precipitates were determined from isoconcentration surfaces and proximity histograms. The accuracy of the estimates of the solute concentrations and the radii of precipitates was found to depend on the size of precipitates. Optimized parameters for analyzing 0.5- to 2-nm-radius precipitates are proposed. The solute content of 0.5-nm-radius precipitates was not estimated correctly by this method. The accuracy of the estimates of the solute concentration and the radius of precipitates were primarily influenced by the solute concentrations of the precipitates. The ranges of error of the solute concentration in the precipitates, which are associated with the analytical limitations of the ultrafine precipitates, were determined, and the results indicated a limitation of the estimates. Microsc. Res. Tech. 76:1196-1203, 2013. © 2013 Wiley Periodicals, Inc. PMID:24009187

Serizawa, Ai; Miller, Michael K

2013-09-05

429

Closed reduction of radius refracture: A case report  

PubMed Central

INTRODUCTION Refractures of the radius and ulna in the paediatric patient with flexible intramedullary nails in situ are known to occur. There are no formal guidelines currently in the literature to guide the management of such fractures. PRESENTATION OF CASE A 10-year-old Caucasian girl, sustained a closed refracture of the radius at the same level, with the flexible intramedullary nails in situ to treat her recent ulna and radius fractures. DISCUSSION We proposed a new non-invasive way of reducing and maintaining such fractures without removal of the bent nail completely. CONCLUSION This method does not significantly reduce the mechanical strength of the nail, as we do not advocate applying an external lateral force, which would compromise nail mechanical strength and eventually lead to breakage of the nail in situ.

Shahid, Mohammad; Yeo, Mildrid; Smibert, John Graham

2011-01-01

430

Coulomb corrections in the extraction of the proton radius  

NASA Astrophysics Data System (ADS)

Multi-photon exchange contributions are important in extracting the proton charge radius from elastic electron–proton scattering. So far, only diagrams associated with the exchange of a second photon have been evaluated. At the low Q2 values relevant to the radius extraction, and especially the very low Q2 region to be probed by proposed measurements, higher order contributions may become important. We evaluate these corrections in the Effective Momentum Approximation, which includes the Coulomb interaction to all orders, and find small corrections with a strong Q2 dependence at low Q2 and large scattering angles. This suggests that the higher order terms may be important in the evaluation of the proton magnetic radius.

>John Arrington,

2013-11-01

431

Thickness dependence of photoluminescence-decay profiles of exciton-exciton scattering in ZnO thin films  

NASA Astrophysics Data System (ADS)

We have investigated the photoluminescence (PL) dynamics of ZnO thin films under intense excitation conditions using an optical-Kerr-gating method. The PL bands originating from exciton-exciton scattering (P emission) and biexciton (M emission) have been observed at 10 K. The ultrashort gating time of 0.6 ps has enabled us to obtain precise information of the temporal profiles of the peak energies and the intensities of the P- and M-PL bands. We have found that the decay time of the P emission becomes longer with increasing film thickness, while that of the M emission is independent of the film thickness. Although the decay time of the P emission is an increasing function of the film thickness, the relation is not in proportion, which is contrary to the predicted proportionality based on a simple model of photon-like polariton propagation. Contribution to the Topical Issue "Excitonic Processes in Condensed Matter, Nanostructured and Molecular Materials", edited by Maria Antonietta Loi, Jasper Knoester and Paul H.M. van Loosdrecht.

Wakaiki, Shuji; Ichida, Hideki; Kawase, Toshiki; Mizoguchi, Kohji; Kim, DaeGwi; Nakayama, Masaaki; Kanematsu, Yasuo

2013-09-01

432

Confocal microscope method for curvature radius measurement of small lens  

NASA Astrophysics Data System (ADS)

With the minimization of optical devices, the size of lens becomes much smaller. As a crucial parameter of spherical lens, the measurement of curvature radius is valuable in small lens manufacturing. In this paper, confocal microscope method is proposed to measure curvature radius of spherical lens, especially small spherical lens. The setup of this method consists of a confocal microscope and a grating bar. The detector of confocal microscope gives maximum output when the spherical surface is placed at "cat's eye" position or confocal position. The distance between cat's eye and confocal position is the curvature radius of spherical surface. This method is suitable for both concave and convex surface. The response of detector to axial and traversal displacement is analyzed and simulated. And the measurement error is estimated with the simulated results. The setup employed a 10x microscope objective of 0.2 NA and pinhole of 0.04mm diameter, and experiment was conducted to measure the radius of a standard optical ball of 3mm diameter. The error of result is less than 0.01mm. It's concluded that the error could be reduced less than 0.001mm with higher numerical aperture and more precise movement stages.

Liu, Qian; Yang, Weichuan; Wu, Pengyue; Yuan, Daocheng

2012-10-01

433

Proton-electron elastic scattering and the proton charge radius  

NASA Astrophysics Data System (ADS)

It is suggested that proton elastic scattering on atomic electrons allows a precise measurement of the proton charge radius. The main advantage is that inverse kinematics allows one to access with a huge cross section very small values of transferred momenta, up to four orders of magnitude smaller than the ones presently achieved.

Gakh, G. I.; Dbeyssi, A.; Tomasi-Gustafsson, E.; Marchand, D.; Bytev, V. V.

2013-09-01

434

Outcomes of percutaneous pinning in treatment of distal radius fractures.  

PubMed

Distal radius fractures are among the most common bone fractures all over the world. Close reduction and fixation by percutaneous pinning is a less invasive method comparing with other open surgeries. This study aims at evaluating the functional outcome of this treatment in distal radius fractures. In this prospective study, 50 patients with distal radius fractures underwent percutaneous pinning during a 16 month period in Tabriz Shohada Hospital. Follow-up for 3 months was possible in 48 (96%) patients. Possible complications (such as infection, nonunion, pin loosening and pain), functional outcomes (including range of motion, grip and pinch strengths) and radiological outcome were documented in intervals during follow-up. Forty eight patient, 22 males and 27 females with a mean age of 47.1 +/- 13.7 (19-80) years were enrolled. Local pain, infection and pin loosening were documented in 14.6, 16.7 and 2.1% of cases, respectively. Based on radiological reports, unacceptable palmar tilt, articular step, radial inclination and radial shortening were present in 6.3, 0, 2.1 and 6.3% of cases, respectively three months post-operation. The clinical outcomes according to Cooney's modification of the Green and O'Brien scheme were as excellent, good, fair and poor in 52.1, 31.3, 10.4 and 6.3% of cases, respectively. According to these results, closed reduction and percutaneous pinning of distal radius fractures is a rather successful method with minor complications. PMID:21848063

Sadighi, A; Bazavar, M; Moradi, A; Eftekharsadat, B

2010-07-15

435

Dynamic splinting in wrist extension following distal radius fractures  

Microsoft Academic Search

BACKGROUND: Wrist flexion contracture is a common pathology which presents secondary to distal radius fractures. Joint stability, restoration and early mobilization are frequently achieved through surgical treatment after such an injury. The purpose of this retrospective study was to evaluate the initial effect of dynamic splinting on wrist extension (active range of motion), in both surgical and non-surgical patients following

Stacey H Berner; F Buck Willis

2010-01-01

436

Acceleration of beam ions during major radius compression in TFTR  

SciTech Connect

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

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

1985-09-01

437

Image registration using radial basis functions with adaptive radius  

PubMed Central

Purpose: Deformable registration of medical images often requires initial rigid alignment. Because of variations in the articulation of bony structures, rigid alignment can capture only limited regions of the image. We propose a method that allows us to compensate for misalignment of mobile parts, which leads to improved accuracy of deformable registration. The method is based on matching landmarks using radial basis functions (RBF) with adaptive radius. Methods: Based on the assumption that the compactly positioned landmarks likely delineate an anatomic structure whose position needs to be corrected, the algorithm incorporates unsupervised clustering of landmarks based on their positions within the reference image. It calculates an appropriate RBF radius based on the set of pairwise distances between landmarks within the cluster. The algorithm distinguishes between clusters of different size and between clusters of spherical and elongated shape, and assigns the optimal RBF radius for each cluster in order to restrict the deformation field to the closest vicinity of the structure of interest. Results: Experiments with synthetic images demonstrate sensitivity of registration results to the choice of the radius of RBF support. We have statistically validated the methods on a large set of pulmonary landmarks. We also tested the method on medical use cases that show that it is potentially advantageous for initial registration of images with large spatial dislocations. Conclusions: The results of registration of CT images demonstrate that an automated selection of the RBF radius simplifies the registration routine and improves the registration quality. The selection is based on two criteria of preserving diffeomorphism of deformation and localization of the deformation within a desired area of the image.

Shusharina, Nadezhda; Sharp, Gregory

2012-01-01

438

Wave-vector dependence of magnetic properties of excitons in ZnTe  

NASA Astrophysics Data System (ADS)

The magnetic properties of heavy-hole excitons in wide quantum wells of ZnTe with ZnxMg1-xTe barriers have been studied with photoluminescence and reflectivity measurements. The exciton magnetic moments (as characterized by the g values) and the diamagnetic shifts of the exciton transitions are found to depend strongly on the wave-vector component Kz associated with translational motion of the exciton normal to the plane of the quantum well. The case of ZnTe differs from examples of this behavior previously reported for GaS, CdTe, and ZnSe since the ZnTe is under tensile biaxial strain, so that the heavy-hole exciton states lie higher in energy than the corresponding states of the light-hole excitons. The dependence of the magnetic properties on Kz is nevertheless still in excellent agreement with the predictions of a model proposed by Smith [Phys. Rev. BPRLTAO1098-012110.1103/PhysRevB.78.085204 78, 085204 (2008)], in which mixing of the heavy-hole 1S exciton state with light-hole nP states is found to be responsible for motion-induced changes in the internal structure of the exciton.

Smith, L. C.; Davies, J. J.; Wolverson, D.; Boukari, H.; Mariette, H.; Kochereshko, V. P.; Phillips, R. T.

2011-04-01

439

Bulk excitonic currents in a bilayer quantum Hall system and Andreev reflection  

Microsoft Academic Search

Bilayer 2D electron systems in the quantum Hall regime can support a novel interlayer coherent phase which may be viewed as a Bose condensate of interlayer excitons. While numerous experiments over the past decade have revealed a host of remarkable properties of this strongly correlated quantum fluid, heretofore none have directly demonstrated the transport of excitons across the electrically insulating

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

2011-01-01

440

Generation of new bound exciton lines by lithium implantation of CdS  

Microsoft Academic Search

Sharp new bound exciton optical absorption lines are observed following very low fluence (?<5×1012 cm?2 room-temperature 7Li implantation of high quality cadmium sulfide platelets. A substantial broadening of the free exciton resonance by the lithium neutral donor impurities is observed. A model of the line shape is proposed.

M. Dagenais; B. S. Elman; W. F. Sharfin

1985-01-01

441

Intrinsic origin of visible light emission from silicon quantum wires: Electronic structure and geometrically restricted exciton  

Microsoft Academic Search

We theoretically investigate excitonic effects on the optical properties of silicon quantum wires, based on ab initio electronic structure calculations. The Si wires have a direct, allowed band gap in the visible energy range and exhibit a strong optical anisotropy. Taking into account the electron-hole Coulomb interaction, the geometrical restriction of excitons dramatically enhances the oscillator strength of the optical

Takahisa Ohno; Kenji Shiraishi; Tetsuo Ogawa

1992-01-01

442

Trapping long-lifetime excitons in a two-dimensional harmonic potential  

Microsoft Academic Search

We report an important step forward for the goal of unambiguous observation of Bose–Einstein condensation of excitons in semiconductors. We have demonstrated a system in which excitons live for microseconds, much longer than their thermalization time, move over distances of hundreds of microns, and can be trapped in a harmonic potential exactly analogous to the traps for atomic condensates. We

D. W. Snoke; Y. Liu; Z. Vörös; L. Pfeiffer

2005-01-01

443

Dynamical reconstruction of the exciton in LiF with inelastic x-ray scattering  

Microsoft Academic Search

The absorption of light by materials proceeds through the formation of excitons, which are states in which an excited electron is bound to the valence hole it vacated. Understanding the structure and dynamics of excitons is important, for example, for developing technologies for light-emitting diodes or solar energy conversion. However, there has never been an experimental means to study the

Peter Abbamonte; Tim Graber; James P. Reed; Serban Smadici; Chen-Lin Yeh; Abhay Shukla; Jean-Pascal Rueff; Wei Ku

2008-01-01

444

Quantum-well exciton dipolar interaction: Polarization-dependence and Z-LT splitting.  

National Technical Information Service (NTIS)

We calculate the exciton dipolar interaction in a semiconductor quantum well. The explicit polarization-dependence, i.e, the dependence on both the exciton dipole moment (mu)-vector and its inplane wavevector k-vector is derived. The obtained results for ...

Nguyen Ba An

1996-01-01

445

Exciton self-trapping in ZnSe-ZnTe alloys  

Microsoft Academic Search

The low-intensity luminescence of ZnSe1-xTex with x=1% exhibits several unusual features which are attributed to the occurrence of efficient exciton self-trapping induced by Te atoms. This type of exciton localization is strikingly different from that observed in other II-VI or III-V mixed crystals.

D. Lee; A. Mysyrowicz; A. V. Nurmikko; B. J. Fitzpatrick

1987-01-01

446

Exciton Diffusion in Air-Suspended Single-Walled Carbon Nanotubes  

Microsoft Academic Search

Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in

S. Moritsubo; T. Murai; T. Shimada; Y. Murakami; S. Chiashi; S. Maruyama; Y. K. Kato

2010-01-01

447

The effect of optical phonons on the exciton in a type-II heterostructure quantum well  

Microsoft Academic Search

The exciton-phonon system of a type-II heterostructure quantum well in which the electron and the hole are spatially separated is studied with both longitudinal optical (LO) and surface optical (SO) phonons taken into account. Numerical results indicate that SO phonons play a unique role in the induced potential which determines the change of the energy difference of the exciton in

Meng Lu; Shi-Wei Gu

1993-01-01

448

Energy transfer by singlet and triplet excitons in carbazole-containing polymers  

NASA Astrophysics Data System (ADS)

Quantum yields of pyrene fluorescence and bis[2-(2'-benzothienyl)-pyridinato-N,C3'](acetylacetonate)iridium [Btp2Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole-dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the "tail" energy states, dissociation of singlet excitons into geminal electron-hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.

Skryshevski, Yu. A.

2012-09-01

449

The dependence of exciton transport efficiency on spatial patterns of correlation within the spectral bath  

NASA Astrophysics Data System (ADS)

Spatial correlations in spectral bath motions have been proposed to explain long-lived coherence in exciton transport. Systems of interest, ranging from photosynthetic complexes to organic photovoltaics, contain inhomogeneous environments. We consider the possibility that the degree of spatial correlation varies throughout an exciton transport system. We model exciton transport in the Fenna-Matthews-Olson complex (FMO), a photosynthetic light-harvesting complex. Although it remains unclear whether significant spatial correlations exist in FMO, its very high exciton transport efficiency makes it an interesting case for studies of exciton transport. We also simulate a highly symmetric ten-site model system. We use an extension of the environment-assisted quantum transport model to simulate transport, allowing the spatial correlation function to vary throughout the system. We demonstrate both via analysis and via simulation that exciton transport efficiency is most sensitive to changes in correlation between the site coupled to the trap and its neighboring sites. This asymmetry in sensitivity is highly robust and appears irrespective of changes in parameters such as transition dipole orientations and initial conditions. Our results suggest that in the design of exciton transport systems, efforts to increase efficiency by controlling spatial correlation should be focused on the region near the site of exciton trapping.

Pelzer, Kenley M.; Fidler, Andrew F.; Griffin, Graham B.; Gray, Stephen K.; Engel, Gregory S.

2013-09-01

450

Entanglement evolution for excitons of two separate quantum dots in a cavity driven by magnetic field  

Microsoft Academic Search

The time evolution of entanglement for excitons in two quantum dots embedded in a single mode cavity is studied in a ``spin-boson'' regime. It is found that although with the dissipation from the boson mode, the excitons in the two quantum dots can be entangled by only modulating their energy bias $\\\\epsilon$ under the influence of external driving magnetic field.

Jun Jing; Z. G. Lü; H. R. Ma

2006-01-01

451

Radiation effects from first principles : the role of excitons in electronic-excited processes.  

SciTech Connect

Electron-hole pairs, or excitons, are created within materials upon optical excitation or irradiation with X-rays/charged particles. The ability to control and predict the role of excitons in these energetically-induced processes would have a tremendous impact on understanding the effects of radiation on materials. In this report, the excitonic effects in large cycloparaphenylene carbon structures are investigated using various first-principles methods. These structures are particularly interesting since they allow a study of size-scaling properties of excitons in a prototypical semi-conducting material. In order to understand these properties, electron-hole transition density matrices and exciton binding energies were analyzed as a function of size. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in these structures. Based on overall trends in exciton binding energies and their spatial delocalization, we find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these systems.

Wong, Bryan Matthew

2009-09-01

452

Exciton states of silicon nanocrystals studied by magneto-optical spectral hole burning  

Microsoft Academic Search

The absorbing and emitting states of excitons, confined in silicon nanocrystals, are separately investigated by magneto-optical spectral hole burning. Due to Zeeman splitting of the exciton ground level, the whole persistent spectral hole shifts linearly towards lower energies with rising magnetic field. Contrarily, the external magnetic field applied (up to 10 T) does not influence the absorbing state. This strongly

J. Diener; D. Kovalev; H. Heckler; G. Polisski; F. Koch

2001-01-01

453

Redistribution of localised excitons in CdSe\\/ZnSe quantum dot structures  

Microsoft Academic Search

Population processes and recombination mechanisms of excitons localised in CdSe\\/ZnSe quantum dot structures are investigated. The photoluminescence (PL) properties are governed by lateral energy transfer within a dense ensemble of quantum dots, which differ in size and Cd concentration, providing for a complex potential landscape with localisation sites of widely varying depth for excitons. At low temperatures, lateral transfer by

M Strassburg; M Dworzak; R Heitz; A Hoffmann; J Christen; D Schikora

2002-01-01

454

Study of the nature of light hole excitonic transitions in InGaAs\\/GaAs quantum well  

Microsoft Academic Search

Photoreflectance (PR) spectra of strained In GaAs\\/GaAs MQWs were measured. Heavy and light hole excitonic transitions were observed. Applying results of theoretical calculations which include excitonic and strain effects, we obtained that the existence of type II light hole exciton in our type I MQW system should be considered.

G S?k; J Misiewicz; D Radziewicz; M T?acza?a; M Panek; R Korbutowicz

1998-01-01

455

NONLINEAR OPTICAL PHENOMENA: Interaction of an ultrashort light pulse with thin semiconductor films in the exciton part of the spectrum  

Microsoft Academic Search

The problem of transmission of an ultrashort laser pulse by a thin semiconductor film in the exciton part of the spectrum is solved for low excitation levels. Analytic solutions of the equations describing the dynamics of the exciton---photon interaction are obtained for homogeneous thin films and for films with an inhomogeneously broadened exciton system.

P. I. Khadzhi; S. L. Gaivan

1996-01-01

456

Temperature dependence of excitonic transitions in AlxGa1-xAs\\/GaAs quantum wells  

Microsoft Academic Search

The temperature dependence of excitonic transitions in double quantum well heterostructures in the temperature range of 2-300 K were investigated. A crossing between excitonic transition experimental curves as a function of temperature in quantum wells of the same thickness and different barrier height is observed. The influence of the barrier height on the temperature dependence of excitonic states in the

S. A. Lourenço; I. F. L. Dias; J. L. Duarte; E. Laureto; H. Iwamoto; E. A. Meneses; J. R. Leite

2001-01-01

457

Radiative behavior of negatively charged excitons in CdTe-based quantum wells: A spectral and temporal analysis  

Microsoft Academic Search

Using reflectivity and picosecond time-resolved photoluminescence, we have studied the intrinsic optical properties of negatively charged excitons in modulation doped CdTe quantum wells. In emission, we observe simultaneously a low energy exponential tail in the charged exciton spectral line and a linear increase of its radiative lifetime with temperature. In absorption, we find a consistent decrease of the charged exciton

V. Ciulin; P. Kossacki; S. Haacke; J.-D. Ganière; B. Deveaud; A. Esser; M. Kutrowski; T. Wojtowicz

2000-01-01

458

Polarons and excitons in insulators: insight from computer simulations  

NASA Astrophysics Data System (ADS)

Localization of electrons and holes as well as excitons in insulators is a ubiquitous phenomenon which controls carrier mobility, luminescence and radiation damage of many materials. When such localization takes place in a perfect lattice it is called self-trapping, however in many cases it is facilitated by perturbation induced by intrinsic defects and impurities. Whatever the mechanism, it is hard to prove experimentally and especially theoretically. I will first review briefly the established models of self-trapped polarons and excitons (STE) in alkali halides and cubic oxides and will demonstrate how they are linked to the mechanisms of photo-induced desorption of these materials [1]. I will then discuss the results of our modeling, which extend these models further to more complex oxides forming so called electrides -- materials where electrons serve as anions [2], and to a qualitatively new type of electron trapping at grain boundaries in polycrystalline materials with negative electron affinity [3]. Combining periodic and embedded cluster methods we can explain and sometimes predict the properties of polarons and excitons in a range of insulators, such as amorphous SiO2 [4], and polycrystalline HfO2 [5] and HfSiO4. I will discuss the applicability of different techniques to studying localization problems in insulators and will compare the predictions of periodic plane wave and embedded cluster DFT calculations. [4pt] [1] W. P. Hess, et al. J. Phys. Chem. B, 109, 19563 (2005) [0pt] [2] P. V. Sushko et al. J. Amer. Chem. Soc., 129, 942 (2007) [0pt] [3] K. P. McKenna and A. L. Shluger, Nature Materials, 7, 859 (2008) [0pt] [4] A. V. Kimmel, et al. J. Non-Cryst. Sol., 353, 599 (2007) [0pt] [5] D. Munoz Ramo, et al. Phys. Rev. Lett. 99, 155504 (2007)

Shluger, Alexander

2010-03-01

459

Effect of spatial resolution on the estimates of the coherence length of excitons in quantum wells  

NASA Astrophysics Data System (ADS)

We evaluate the effect of diffraction-limited resolution of the optical system on the estimates of the coherence length of two-dimensional excitons deduced from the interferometric study of the exciton emission. The results are applied for refining our earlier estimates of the coherence length of a cold gas of indirect excitons in coupled quantum wells [S. Yang , Phys. Rev. Lett. 97, 187402 (2006)]. We show that the apparent coherence length is well approximated by the quadratic sum of the actual exciton coherence length and the diffraction correction given by the conventional Abbe limit divided by ? . In practice, accounting for diffraction is necessary only when the coherence length is smaller than about one wavelength. The earlier conclusions regarding the strong enhancement of the exciton coherence length at low temperatures remain intact.

Fogler, M. M.; Yang, Sen; Hammack, A. T.; Butov, L. V.; Gossard, A. C.

2008-07-01

460

Magnetic field effect on free and bound excitons in chalcopyrite CuInS2  

NASA Astrophysics Data System (ADS)

The effect of a strong magnetic field (induction up to 10 T) on free and bound excitons in CuInS2 single crystals is studied. A diamagnetic shift to higher energies is observed in the luminescence and reflectance spectra for free-exciton lines ALPB ? 1.5348 eV, AUPB ? 1.5361 eV, and BC ? 1.557 eV. The diamagnetic shifts of free-exciton lines ALPB, AUPB, and BC provide a basis for estimating the exciton reduced masses ? _{A_{LPB} } = 0.131m0, ? _{A_{UPB} } = 0.13 4m0, and ?BC = 0.111m0, respectively. Bound-exciton lines in luminescence spectra are split under the influence of the magnetic field. The magnitude of the Zeeman effect (g-factor of the magnetic splitting) is estimated.

Mudryi, A. V.; Ivanyukovich, A. V.; Yakushev, M. V.; Martin, R.; Saad, A.

2007-05-01

461

Lasing threshold in traps for Bose-condensation of dipolar excitons  

NASA Astrophysics Data System (ADS)

We consider exciton recombination lasing in heterostructure traps for Bose-Einstein condensation of dipolar excitons. We show that such structures suit well for class D lasers where cavity decay strongly exceeds polarization decay. We evaluate lasing threshold taking into account specific inhomogeneous broadening of the exciton spectral line owing to Bose-Einstein condensation phenomenon under quasi-equilibrium conditions.It is found that narrowing of the exciton momentum distribution just before the condensation onset considerably lowers lasing threshold. At the same time, it is pointed out that a subsequent formation of condensate itself does not help lasing much. We conclude that it is possible to achieve lasing on polariton modes in nowadays experiments aimed on Bose-Einstein condensation of excitons.

Kalinin, Petr A.; Kocharovsky, Vitaly V.; Kocharovsky, Vladimir V.

2012-06-01

462

Two-dimensional dynamical reconstruction of the valence exciton in LiF  

NASA Astrophysics Data System (ADS)

The structure and dynamics of excitons are interesting because excitons are model many-body excitations with technological relevance, e.g. to the behavior of photocells. In a previous study, we used inelastic X-ray scattering, together with inversion techniques, to reconstruct one-dimensional projection images of