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1

Large radius excitons in single-walled carbon nanotubes

The spectrum of large-radius excitons in an individual semiconducting single-walled carbon nanotube (SWCNT) is described within the framework of an elementary potential model, in which an exciton is modelled as a bound state of two oppositely charged quasi-particles confined on the tube surface. Due to the parity of the interaction potential the exciton states split into the odd and even

Vadym M. AdamyanOleksii; Oleksii A. Smyrnov

2007-01-01

2

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

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

Selvakumar V. Nair; Toshihide Takagahara

1997-01-01

3

ERIC Educational Resources Information Center

Discusses the importance of teaching the Bohr atom at both freshman and advanced levels. Focuses on the development of Bohr's ideas, derivation of the energies of the stationary states, and the Bohr atom in the chemistry curriculum. (SK)

Haendler, Blanca L.

1982-01-01

4

Two-dimensional excitons in three-dimensional hexagonal boron nitride

The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8?Å and 740?meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54m{sub o}, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

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

2013-11-04

5

Revisiting the Bohr Atom 100 Years Later

NASA Astrophysics Data System (ADS)

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

Wall, Ernst

2013-03-01

6

NASA Astrophysics Data System (ADS)

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

Georgoudis, P. E.

2014-04-01

7

ERIC Educational Resources Information Center

Discusses some lesser known examples of atomic phenomena to illustrate to students that the old quantum theory in its simplest (Bohr) form is not an antiquity but can still make an important contribution to understanding such phenomena. Topics include hydrogenic/non-hydrogenic spectra and atoms in strong electric and magnetic fields. (Author/JN)

Latimer, Colin J.

1983-01-01

8

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

9

NASA Astrophysics Data System (ADS)

The final form of quantum physics, in the particular case of wave mechanics, was established in the years 1925-1927 by Heisenberg, Schrödinger, Born and others, but the synthesis was the work of Bohr who gave an epistemological interpretation of all the technicalities built up over those years; this interpretation will be examined briefly in Chapter 10. Although Einstein acknowledged the success of quantum mechanics in atomic, molecular and solid state physics, he disagreed deeply with Bohr's interpretation. For many years, he tried to find flaws in the formulation of quantum theory as it had been more or less accepted by a large majority of physicists, but his objections were brushed away by Bohr. However, in an article published in 1935 with Podolsky and Rosen, universally known under the acronym EPR, Einstein thought he had identified a difficulty in the by then standard interpretation. Bohr's obscure, and in part beyond the point, answer showed that Einstein had hit a sensitive target. Nevertheless, until 1964, the so-called Bohr-Einstein debate stayed uniquely on a philosophical level, and it was actually forgotten by most physicists, as the few of them aware of it thought it had no practical implication. In 1964, the Northern Irish physicist John Bell realized that the assumptions contained in the EPR article could be tested experimentally. These assumptions led to inequalities, the Bell inequalities, which were in contradiction with quantum mechanical predictions: as we shall see later on, it is extremely likely that the assumptions of the EPR article are not consistent with experiment, which, on the contrary, vindicates the predictions of quantum physics. In Section 3.2, the origin of Bell's inequalities will be explained with an intuitive example, then they will be compared with the predictions of quantum theory in Section 3.3, and finally their experimental status will be reviewed in Section 3.4. The debate between Bohr and Einstein goes much beyond a simple controversy, which is after all almost eighty years old and has been settled today. In fact, the concept introduced in this debate, that of entanglement, lies at the heart of many very important developments of modern quantum physics, in particular all those linked to quantum information (Chapter 8). Moreover, we shall see that the phenomenon of non-local correlations compels us to revise in depth our space-time representation of quantum processes. These are the two reasons why a whole chapter is devoted to this debate.

Bellac, Michel Le

2014-11-01

10

NASA Astrophysics Data System (ADS)

Bohr used to introduce his attempts to explain clearly the principles of the quantum theory of the atom with an historical sketch, beginning invariably with the nuclear model proposed by Rutherford. That was sound pedagogy but bad history. The Rutherford-Bohr atom stands in the middle of a line of work initiated by J.J. Thomson and concluded by the invention of quantum mechanics. Thompson's program derived its inspiration from the peculiar emphasis on models characteristic of British physics of the 19th century. Rutherford's atom was a late product of the goals and conceptions of Victorian science. Bohr's modifications, although ultimately fatal to Thomson's program, initially gave further impetus to it. In the early 1920s the most promising approach to an adequate theory of the atom appeared to be the literal and detailed elaboration of the classical mechanics of multiply periodic orbits. The approach succeeded, demonstrating in an unexpected way the force of an argument often advanced by Thomson: because a mechanical model is richer in implications than the considerations for which it was advanced, it can suggest new directions of research that may lead to important discoveries.

Heilbron, J. L.

1981-03-01

11

Revisiting Bohr's semiclassical quantum theory.

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

Ben-Amotz, Dor

2006-10-12

12

Excitonic spectrum of the ZnO/ZnMgO quantum wells

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

13

NSDL National Science Digital Library

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

Niederriter, Chuck; Belloni, Mario

2008-11-08

14

The Bohr effect before Perutz.

Before the outbreak of World War II, Jeffries Wyman postulated that the Bohr effect in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on the chemistry and structure of the protein was essentially nil. The magnetochemical properties of hemoglobin led Linus Pauling to hypothesize that the (so called) Bohr histidines were coordinated to the heme iron in the fifth and sixth positions; and Wyman shared this opinion. However, this structural hypothesis was abandoned in 1951 when J. Wyman and D. W. Allen proposed the pK shift of the oxygen linked histidines to be the result of "...a change of configuration of the hemoglobin molecule as a whole accompanying oxygenation." This shift in paradigm, that was published well before the 3D structure of hemoglobin was solved by M.F. Perutz, paved the way to the concept of allostery. After 1960 the availability of the crystallographic structure opened new horizons to the interpretation of the allosteric properties of hemoglobin. PMID:22987550

Brunori, Maurizio

2012-01-01

15

The Influence of Bohr on Delbruck

NASA Astrophysics Data System (ADS)

The book by Robert Lagemann on the history of physics and astronomy at Vanderbilt University contains a chapter on Max Delbruck, a member of the Vanderbilt physics department from 1940 - 1947, where he did seminal work in establishing microbial genetics, for which he received the Nobel prize in physiology in 1969. Delbruck, a Ph.D. in physics for work with Max Born in Gottingen, had been inspired by Niels Bohr's suggestion of a complementary relation between biology and atomic physics to work in biology. We will explore exactly what Bohr said in this connection and argue that Delbruck's own work leads to a conclusion in opposition to Bohr's suggestion, namely that the existence of life is reducible to molecular physics, through the remarkable properties of DNA. The lesson for scientific methodology to be learned from this example is that science can lead to truth even if motivated by an ideology pushing in the opposite direction.

Holladay, Wendell

2000-11-01

16

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

17

Niels Bohr and the Third Quantum Revolution

NASA Astrophysics Data System (ADS)

In the history of science few developments can rival the discovery of quantum mechanics, with its series of abrupt leaps in unexpected directions stretching over a quarter century. The result was a new world, even more strange than any previously imagined subterranean (or in this case submicroscopic) kingdom. Niels Bohr made the third of these leaps (following Planck and Einstein) when he realized that still-new quantum ideas were essential to account for atomic structure: Rutherford had deduced, using entirely classical-physics principles, that the positive charge in an atom is contained in a very small kernel or nucleus. This made the atom an analogue to the solar system. Classical physics implied that negatively charged electrons losing energy to electromagnetic radiation would ``dive in'' to the nucleus in a very short time. The chemistry of such tiny atoms would be trivial, and the sizes of solids made from these atoms would be much too small. Bohr initially got out of this dilemma by postulating that the angular momentum of an electron orbiting about the nucleus is quantized in integer multiples of the reduced quantum constant = h/2?. Solving for the energy of such an orbit in equilibrium immediately produces the famous Balmer formula for the frequencies of visible light radiated from hydrogen as an electron jumps from any particular orbit to another of lower energy. There remained mysteries requiring explanation or at least exploration, including two to be discussed here: 1. Rutherford used classical mechanics to compute the trajectory and hence the scattering angle of an ? particle impinging on a small positively charged target. How could this be consistent with Bohr's quantization of particle orbits about the nucleus? 2. Bohr excluded for his integer multiples of the value 0. How can one justify this exclusion, necessary to bar tiny atoms of the type mentioned earlier?

Scharff Goldhaber, Alfred

2013-04-01

18

Bohr-Sommerfeld quantization of pseudospin Hamiltonians

It is shown here how to map the problem with pseudospin J into an equivalent one in which 1/J plays the role of h and canonical variables exist at the classical level. Bohr-Sommerfeld quantization of the equivalent theory is found to produce a spectrum in very good agreement with the exact results for the Lipkin-Meshkov-Glick model at J=15 and 25. The method readily extends to the SU(n) case.

Shankar, R.

1980-09-29

19

NSDL National Science Digital Library

This is a set of animations that demonstrates properties of the spherical holes formed when uniform spheres are packed. Cubic, octahedral and tetrahedral packing arrangements may be examined without anything in the holes,and with the repective holes filled. The sizes of the various holes relative to the spheres being packed are shown, which can lead students into an exploration of the radius ratio concept. An example is given of computing the relative size of an octahedral hole.

20

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

We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.

LYO,SUNGKWUN K.

1999-12-06

21

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.

22

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

23

Bohr's Principle of Complementarity and Beyond

NASA Astrophysics Data System (ADS)

All knowledge is of an approximate character and always will be (Russell, Human Knowledge, 1948, pg 497,507). The laws of nature are not unique (Smolin, Three Roads to Quantum Gravity, 2001, pg 195). There may be a number of different sets of equations which describe our data just as well as the present known laws do (Mitchell, Machine Learning, 1997, pg 65-66 and Cooper, Machine Learning, Vol. 9, 1992, pg 319) In the future every field of intellectual study will possess multiple theories of its domain and scientific work and engineering will be performed based on the ensemble predictions of ALL of these. In some cases the theories may be quite divergent, differing greatly one from the other. The idea can be considered an extension of Bohr's notions of complementarity, "...different experimental arrangements.. described by different physical concepts...together and only together exhaust the definable information we can obtain about the object" (Folse, The Philosophy of Niels Bohr, 1985, pg 238). This idea is not postmodernism. Witchdoctor's theories will not form a part of medical science. Objective data, not human opinion, will decide which theories we use and how we weight their predictions.

Jones, R.

2004-05-01

24

Resisting the Bohr Atom: The Early British Opposition

NASA Astrophysics Data System (ADS)

When Niels Bohr's theory of atomic structure appeared in the summer and fall of 1913, it quickly attracted attention among British physicists. While some of the attention was supportive, others was critical. I consider the opposition to Bohr's theory from 1913 to about 1915, including attempts to construct atomic theories on a classical basis as alternatives to Bohr's. I give particular attention to the astrophysicist John W. Nicholson, who was Bohr's most formidable and persistent opponent in the early years. Although in the long run Nicholson's objections were inconsequential, for a short period of time his atomic theory was considered to be a serious rival to Bohr's. Moreover, Nicholson's theory is of interest in its own right.

Kragh, Helge

2011-03-01

25

Strong charge-transfer excitonic effects and the Bose-Einstein exciton condensate in graphane.

Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally. PMID:20867194

Cudazzo, Pierluigi; Attaccalite, Claudio; Tokatly, Ilya V; Rubio, Angel

2010-06-01

26

Ultrafast Optical Studies of Multiple Exciton Generation in Lead Chalcogenide Quantum Dots

NASA Astrophysics Data System (ADS)

Providing affordable, clean energy is one of the major challenges facing society today, and one of the promising solutions is third generation solar energy conversion. Present day, first and second-generation solar cells can at most convert each absorbed photon into a single electron hole pair, thereby establishing a theoretical limit to the power conversion efficiency. The process of multiple exciton generation (MEG) in semiconductor quantum dots increases that theoretical efficiency from 33% to 42% by utilizing the excess energy of high energy photons that is otherwise wasted as heat to excite a second electron-hole pair, thereby boosting the potential photocurrent. This thesis explores the benefits of MEG in quantum confined systems and shows that quantum dots are more efficient at generating multiple excitons from a single photon than bulk semiconductors. The variations in optical measurements of MEG have raised skepticism and brought into question the validity of these experiments. The two important questions that this thesis attempts to address are (1) what are the enhanced QYs in isolated PbSe QDs and (2) does quantum confinement enhance MEG over bulk semiconductors. Experimental variations in the enhanced QYs are partially explained by the production of a long-lived photocharged state that increases the apparent photon-to-exciton QYs. A procedure is detailed that decreases the possibility of producing this charged state. By studying the production of these states, conditions are found that minimize their effect and produce less variation in the reported QYs. Variations in the MEG efficiency were studied in films of chemically treated PbSe quantum dots where a different mechanism was responsible for an apparent decrease of the measured QYs. Finally, for the first time, a quantum dot size-dependence in the MEG efficiency was found in colloidal PbSe, PbS, and PbSxSe1-x quantum dot solutions and is attributed to the increased Coulomb interaction in materials with a larger Bohr exciton radius. These results will allow a better understanding of MEG and how this important process may be used to enhance solar energy conversion.

Midgett, Aaron G.

27

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

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

2013-11-15

28

Solutions of the Bohr Hamiltonian, a compendium

NASA Astrophysics Data System (ADS)

The Bohr Hamiltonian, also called collective Hamiltonian, is one of the cornerstones of nuclear physics and a wealth of solutions (analytic or approximated) of the associated eigenvalue equation have been proposed over more than half a century (confining ourselves to the quadrupole degree of freedom). Each particular solution is associated with a peculiar form for the V(?,?) potential. The large number and the different details of the mathematical derivation of these solutions, as well as their increased and renewed importance for nuclear structure and spectroscopy, demand a thorough discussion. It is the aim of the present monograph to present in detail all the known solutions in ?-unstable and ?-stable cases, in a taxonomic and didactical way. In pursuing this task we especially stressed the mathematical side leaving the discussion of the physics to already published comprehensive material. The paper contains also a new approximate solution for the linear potential, and a new solution for prolate and oblate soft axial rotors, as well as some new formulae and comments. The quasi-dynamical SO(2) symmetry is proposed in connection with the labeling of bands in triaxial nuclei.

Fortunato, L.

2005-10-01

29

Exciton states in quasi-zero-dimensional semiconductor nanosystems

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

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

2012-02-15

30

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

31

Collisional dephasing of excitons

NASA Astrophysics Data System (ADS)

We studied experimentally and theoretically the dependence of optical exciton dephasing on exciton concentration. The measurements were made with the help of recently developed technique^1 in GaAs/AlGaAs quantum wells in the region of concentrations up to 3×10^10 cm-2. The measured dephasing rate of circularly polarized exciton is nearly concentration independent while the dephasing rate of linearly polarized excitons grows linearly with the concentration. The different behavior of circular and linear excitons is explained by pair exciton interactions where total excion spin is conserved. The exchange interaction of excitons with only +1 or only -1 circular polarization does not change their phase and leads only only -1 circular polarization does not change their phase and leads only to a renormalization of their energy. The interaction of linear excitons (i.e. coherent superposition of bright +1 and -1 excitons ) leads to creation of dark excitons with spins +2 and -2. While the sum of the phases of the two dark excitons is fixed by the phase of bright excitons the difference of their phases is arbitrary. This and the inverse process lead to the dephasing of bright excitons. Theoretical estimate of the dephasing time based on the recently calculated interaction matrix elements^2 gives a good agreement with the experiment. 1. X.Marie, P.Le Jeune, T.Amand, M.Brousseau, J.Barrau, M.Paillard, and R.Planel, Phys.Rev.Lett. 79, 3222 (1997). 2. S. Ben-Tabou de-Leon, B. Laikhtman, cond-mat/0010247.

Laikhtman, Boris; Marie, Xavier; Amand, Thierry

2001-03-01

32

"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

33

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

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

34

Effect of the dielectric constant of mesoscopic particle on the exciton binding energy.

National Technical Information Service (NTIS)

For materials with big exciton reduced mass and big dielectric constant, such as TiO(sub 2), the variation of dielectric constant with the radius of an ultrafine particle (UFP) is important for determining the exciton binding energy. For the first time a ...

Lai Zuyou Gu Shiwei

1991-01-01

35

Kinetic theory of exciton-exciton annihilation

NASA Astrophysics Data System (ADS)

Weakly excited states of dye aggregates and supramolecular complexes can be characterized by single or two exciton states. Stronger excitation results in the presence of multiple excited molecules, and complex processes of internal energy transfer dynamics take place. The direct consideration of all excited states is limited to systems with a few molecules only. Therefore, an approach is used based on transition operators among the molecular states of interest and resulting in a dynamic theory for excitation energy transfer in strongly excited molecular systems. As a first application of this theory a detailed description of exciton-exciton annihilation is given. The obtained novel nonlinear theory is related to the standard description. Possible further approximation schemes in the offered theoretical framework are discussed.

May, Volkhard

2014-02-01

36

Optically programmable excitonic traps

With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits.

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

2013-01-01

37

Theory of exciton transport with quadratic exciton-phonon coupling

The coupling of excitons with phonons is an important process determining the mechanism of exciton transport in molecular crystals. Although linear exciton–phonon coupling has been extensively treated before, there has been little work on quadratic coupling. In this paper the theory of exciton transport is extended to take weak quadratic coupling into account, and the diffusion constant computed for a

R. W. Munn; R. Silbey

1978-01-01

38

Creation of non-dispersive Bohr-like wave packets

NASA Astrophysics Data System (ADS)

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

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

2009-05-01

39

Neutral and positively charged excitons in narrow quantum ring

NASA Astrophysics Data System (ADS)

We study theoretically quantized states of a neutral and a positively charged exciton (trion X+) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.

Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.

2014-05-01

40

Nekrasov functions and exact Bohr-Sommerfeld integrals

In the case of SU(2), associated by the AGT relation to the 2d Liouville theory, the Seiberg-Witten prepotential is constructed from the Bohr-Sommerfeld periods of 1d sine-Gordon model. If the same construction is literally applied to monodromies of exact wave functions, the prepotential\\u000a turns into the one-parametric Nekrasov prepotential $$\\\\mathcal{F} (a, \\\\epsilon_{1}) $$ with the other epsilon parameter vanishing, ?2

A. Mironovand; A. Morozov

2010-01-01

41

Analytical Solutions of Bohr Collective Hamiltonian with ?-INSTABILITY

NASA Astrophysics Data System (ADS)

The main aim of this contribution is to discuss the analytical solution of the collective Bohr equation with a Coulomb-like and a Kratzer-like ?-unstable potentials in quadrupole deformation space. Eigenvalues and eigenfunctions are given in closed form and transition rates are calculated for the two cases. The corresponding SO(2,1) × SO(5) algebraic structure is discussed. A few remarks concerning the possibility to study an approximate solution in the ?-stable case with these potential are treated briefly.

Fortunato, L.; Vitturi, A.

2004-04-01

42

Control of exciton fluxes in an excitonic integrated circuit.

Efficient signal communication uses photons. Signal processing, however, uses an optically inactive medium, electrons. Therefore, an interconnection between electronic signal processing and optical communication is required at the integrated circuit level. We demonstrated control of exciton fluxes in an excitonic integrated circuit. The circuit consists of three exciton optoelectronic transistors and performs operations with exciton fluxes, such as directional switching and merging. Photons transform into excitons at the circuit input, and the excitons transform into photons at the circuit output. The exciton flux from the input to the output is controlled by a pattern of the electrode voltages. The direct coupling of photons, used in communication, to excitons, used as the device-operation medium, may lead to the development of efficient exciton-based optoelectronic devices. PMID:18566248

High, Alex A; Novitskaya, Ekaterina E; Butov, Leonid V; Hanson, Micah; Gossard, Arthur C

2008-07-11

43

Photovoltaics: Separating Multiple Excitons

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

44

Excitonic Nonlinear Optical Properties.

National Technical Information Service (NTIS)

Exciton and biexciton binding and excited state energies, and wave functions are calculated numerically via a finite element method in an effective mass approximation for GaAs quantum well disks and dots (including pyramid shaped structures) surrounded by...

R. O. Klepfer F. L. Madarasz

1996-01-01

45

Holographic radius test plates

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

46

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

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

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

1996-01-01

47

Quasiparticle Gaps and Exciton Coulomb Energies in Si Nanoshells

Quasiparticle gaps and exciton Coulomb energies of H-passivated spherical Si nanoshells are computed using rst principles SCF and GW methods. We nd that the quasiparticle gap of a nanoshell depends on both its inner radius R1 (weakly) and outer radius R2 (strongly). These dependences on R1 and R2 are mostly consistent with electrostatics of a metallic shell. We also nd that the unscreened Coulomb energy ECoul in Si nanoshells has a somewhat unexpected size dependence at xed outer radius R2: ECoul decreases as the nanoshell becomes more conning, contrary to what one would expect from quantum connement eects. We show that this is a consequence of an increase in the average electron-hole distance, giving rise to reduced exciton Coulomb energies in spite of the reduction in the conning nanoshell volume.

Frey, K. [University of Illinois, Chicago; Idrobo Tapia, Juan C [ORNL; Tiago, Murilo L [ORNL; Reboredo, Fernando A [ORNL; Ogut, Serdar [University of Illinois, Chicago

2009-01-01

48

Revisiting Bohr's principle of complementarity with a quantum device

NASA Astrophysics Data System (ADS)

Bohr's principle of complementarity (BPC) is the cornerstone of quantum mechanics. According to this principle, the total wavelike and particlelike information of a particle is limited by the Englert-Greenberger (EG) duality relation. Here, by introducing a quantum detecting device into the experiment, we find that the limit of the EG duality relation is exceeded because of the interference between the wave and particle properties of the photon. A generalized EG duality relation is further developed. Our work provides a generalization of BPC and gives new insights into quantum mechanics.

Tang, Jian-Shun; Li, Yu-Long; Li, Chuan-Feng; Guo, Guang-Can

2013-07-01

49

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.

50

Excitons in double quantum wells

Excitons in a double-quantum-well structure, consisting of two wells separated by a thin barrier, are studied. Binding energies and oscillator strengths of the exciton ground state are calculated as a function of the barrier width and the well width. It is shown that the barrier width and the barrier height give the significant influence on the dimensional character of excitons

Tsuneo Kamizato; Mitsuru Matsuura

1989-01-01

51

Exciton-exciton interaction and biexciton formation in bilayer systems

NASA Astrophysics Data System (ADS)

We report quantum Monte Carlo calculations of biexciton binding energies in ideal two-dimensional bilayer systems with isotropic electron and hole masses. We have also calculated exciton-exciton interaction potentials and pair-distribution functions for electrons and holes in bound biexcitons. Comparing our data with results obtained in a recent study using a model exciton-exciton potential [Schindler and Zimmermann, Phys. Rev. B 78, 045313 (2008)], we find a somewhat larger range of layer separations at which biexcitons are stable. We find that individual excitons retain their identity in bound biexcitons for large layer separations.

Lee, R. M.; Drummond, N. D.; Needs, R. J.

2009-03-01

52

Variable Radius Nacelle Studies

NASA Technical Reports Server (NTRS)

An overview of the active shape control for a variable radius nacelle leading edge program is presented. The current technical plan and schedule will be discussed. Results from the structural shape change of curved plates demonstration will be presented, as well as the NASA LaRC concept for a variable radius nacelle leading edge. Results of a Boeing systems integration study of this concept will be discussed briefly. The status of the sensors, actuators, and computational design tools tasks will also be presented.

McGowan, David M.

2001-01-01

53

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,

Gregory D. Scholes; Garry Rumbles

2006-01-01

54

Oral History Interviews at the Niels Bohr Library & Archives

NSDL National Science Digital Library

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

55

Semiclassical quantization of Bohr orbits in the helium atom

NASA Astrophysics Data System (ADS)

We use the complex WKB-Maslov method to construct the semiclassical spectral series corresponding to the resonance Bohr orbits in the helium atom. The semiclassical energy levels represented as the Rydberg tetra series correspond to the doubly symmetrically excited states of helium-like atoms. This level series contains the Rydberg triple series reported by Richter and Wintgen in 1991, which corresponds to the Z2+e-e- configuration of electrons observed by Eichmann and his collaborators in experiments on the laser excitation of the barium atom in 1992. The lower-level extrapolation of the formula obtained for the semiclassical spectrum gives the value of the ground state energy, which differs by 6% from the experimental value obtained by Bergeson and his collaborators in 1998. We also calculate the fine structure of the semiclassical spectrum due to the spin-orbit and spin-spin interactions of electrons.

Belov, V. V.; Maksimov, V. A.

2007-05-01

56

Nekrasov functions and exact Bohr-Sommerfeld integrals

NASA Astrophysics Data System (ADS)

In the case of SU(2), associated by the AGT relation to the 2 d Liouville theory, the Seiberg-Witten prepotential is constructed from the Bohr-Sommerfeld periods of 1 d sine-Gordon model. If the same construction is literally applied to monodromies of exact wave functions, the prepotential turns into the one-parametric Nekrasov prepotential mathcal{F} (a, ?1) with the other epsilon parameter vanishing, ?2 = 0, and ?1 playing the role of the Planck constant in the sine-Gordon Shrödinger equation, ? = ?1. This seems to be in accordance with the recent claim in [1] and poses a problem of describing the full Nekrasov function as a seemingly straightforward double-parametric quantization of sine-Gordon model. This also provides a new link between the Liouville and sine-Gordon theories.

Mironov, A.; Morozov, A.

2010-04-01

57

Effect of dephasing on exciton superradiance and exciton cavity polaritons

NASA Astrophysics Data System (ADS)

Quantum-well exciton superradiance and exciton cavity polarization formation both require a substantial spatial coherence area in order to become the dominant exciton field interaction. In both interaction processes the quantum-well (in-plane) momentum is conserved. Momentum scattering, which is due, e.g., to interaction with a phonon reservoir, quickly localizes an initially delocalized exciton and randomizes the excitation momentum. We take a close look at how momentum scattering influences measurements of excitonic superradiance and of exciton cavity polariton splitting. An important conclusion is that, in general, measurements of the emitted light do not correspond to the evolution of the system as a whole. Therefore, e.g., the measured decay rates do not correspond to the true decay rate of the system.

Björk, Gunnar; Pau, Stanley; Jacobson, Joseph M.; Cao, Hui; Yamamoto, Yoshihisa

1996-05-01

58

Exciton-exciton annihilation in MoSe2 monolayers

NASA Astrophysics Data System (ADS)

We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by measuring a differential reflection of a probe pulse tuned in the range 790-820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in a bulk sample under comparable conditions. We also observe the saturated absorption induced by excitons in both monolayers and the bulk in the differential reflection measurements, which indicates their potential applications as saturable absorbers.

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

2014-03-01

59

Electrostatic traps for dipolar excitons

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

60

Distal radius fractures are a common injury, particularly in the elderly population. Severity of these fractures is directly related to the bone mineral density of the patient, and clinical results are dependent on this parameter as well. In terms of treatment, several options exist. Nonoperative management consists of closed treatment with casting. Operative treatment options include intrafocal pinning, nonbridging and bridging external fixation, arthroscopic-assisted external fixation, and various methods of open reduction internal fixation. When operative intervention is indicated, considerations include the characteristics of the fracture and the experience of the surgeon with the treatment modalities. PMID:19858984

Liporace, Frank A; Adams, Mark R; Capo, John T; Koval, Kenneth J

2009-01-01

61

Exciton–light coupled modes in a spherical crystal are studied as a function of radius. The coupled mode has complex eigenenergy, and its imaginary part corresponds to the radiative width in optical spectra. The radiative width of each mode shows resonant enhancement as a function of crystal size due to the cavity effect. The maximum radiative width increases with radius, and

Hiroshi Ajiki

2001-01-01

62

Going Beyond Bohr: Computer Experiments to Study Quantum Reality

NASA Astrophysics Data System (ADS)

As is well known (See,for example,Nick Herbert (1985) Quantum Reality.Anchor Doubleday, New York),there are several theories of quantum reality.Many of these are close to the principal one, the 'Copenhagen Interpretation',advanced by Bohr and others, which says that there is no deep reality and particles like electrons have no dynamic attributes.In contrast to these, the author has put forward a new theory in the last couple of years, according to which there is a deep reality which is deterministic (agreeing with Einstein), and which consists entirely of mathematical objects. Consciousness interprets some of these objects as being physical.The present paper considers the problem of testing the author's theory vis-a-vis others. So far, experiments in Physics are constrained to be in our 3-D universe.But, this paper lays the foundation of the subject of "Physics experiments inside the computer" by considering, for example, the possiblity of creating higher dimensional spaces inside the computer.

Srivastava, Jagdish

2002-10-01

63

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

64

Superconductivity and Excitonic Charge Order

NASA Astrophysics Data System (ADS)

It was suggested four decades ago that excitons as well as phonons could mediate superconductivity and that the temperature limits usually imposed by phonons could thus be avoided. In practice this form of excitonic pairing turned out to be elusive, because phonon softening typically causes a structural instability to occur before excitonic superconductivity has a chance to arise. Upon suppression of this CDW order however, superconductivity once again has an opportunity to materialise, as has recently been observed in for example pure TiSe2 under pressure. It is unclear what role is played by the excitons in such an environment of critical structural fluctuations, and whether they can have any effect on the pairing or indeed TC. Here we introduce a theoretical model to study the ways in which SC, CDW and excitonic order compete, coexist and even cooperate. Applying the model to TiSe2, we show that the hitherto elusive mechanism driving its CDW transition is a combination of excitonic and Jahn-Teller effects, and that under pressure it is likely to display an unusual type of superconductivity mediated by combinations of excitons and phonons.

van Wezel, Jasper; Nahai-Williamson, Paul; Saxena, Siddarth

2010-03-01

65

Exciton Dynamics in Semiconductor Nanocrystals.

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically. PMID:23625792

Wheeler, Damon A; Zhang, Jin Z

2013-04-26

66

Momentum dependence of the excitons in pentacene

We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.

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

2012-05-28

67

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

68

Why has the bohr-sommerfeld model of the atom been ignoredby general chemistry textbooks?

Bohr's model of the atom is considered to be important by general chemistry textbooks. A major shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. In order to increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study has the following objectives: 1) Formulation of criteria based on a history and philosophy of science framework; and 2) Evaluation of university-level general chemistry textbooks based on the criteria, published in Italy and U.S.A. Presentation of a textbook was considered to be "satisfactory" if it included a description of the Bohr-Sommerfeld model along with diagrams of the elliptical orbits. Of the 28 textbooks published in Italy that were analyzed, only five were classified as "satisfactory". Of the 46 textbooks published in U.S.A., only three were classified as "satisfactory". This study has the following educational implications: a) Sommerfeld's innovation (auxiliary hypothesis) by introducing elliptical orbits, helped to restore the viability of Bohr's model; b) Bohr-Sommerfeld's model went no further than the alkali metals, which led scientists to look for other models; c) This clearly shows that scientific models are tentative in nature; d) Textbook authors and chemistry teachers do not consider the tentative nature of scientific knowledge to be important; e) Inclusion of the Bohr-Sommerfeld model in textbooks can help our students to understand how science progresses. PMID:24061142

Niaz, Mansoor; Cardellini, Liberato

2011-12-01

69

Singlet exciton diffusion in MEH-PPV films studied by exciton–exciton annihilation

We report time-resolved photoluminescence studies in MEH-PPV films for a wide range of excitation intensities. The results fit well to a diffusion-limited exciton–exciton annihilation model with an annihilation constant ?=(2.8±0.5)×10?8cm3s?1. This enables us to estimate the exciton diffusion coefficient to be D=(3±1)×10?3cm2s?1. This corresponds to a diffusion length between 5 and 8nm for one-dimensional diffusion.

A. J. Lewis; A. Ruseckas; O. P. M. Gaudin; G. R. Webster; P. L. Burn; I. D. W. Samuel

2006-01-01

70

Exciton photophysics of carbon nanotubes.

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 are special because graphite has two energy bands at the Fermi energy related to time-reversal symmetry and because SWNTs are actually one dimensional. This review discusses both theoretical and experimental points of view, thus aiming to provide a summary of the most important work in the field, as well as to identify open questions. PMID:17201684

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

2007-01-01

71

DNA-controlled excitonic switches.

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. PMID:22401838

Graugnard, Elton; Kellis, Donald L; Bui, Hieu; Barnes, Stephanie; Kuang, Wan; Lee, Jeunghoon; Hughes, William L; Knowlton, William B; Yurke, Bernard

2012-04-11

72

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

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

73

Discussion Between Einstein and Bohr About the Essence of Quantum Mechanics.

National Technical Information Service (NTIS)

The so-called Einstein, Podolski and Rozen paradox in all its variants is shown not to exist: quantum mechanics doesn't allow both joint direct measurement of non-permutable (complementary according to Bohr) observables and separate indirect measurements ...

V. M. Morozov

1985-01-01

74

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

75

Exciton dynamicsstudied via internal THz transitions

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

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

2003-02-26

76

Binding energy of the barbell exciton

The exciton binding energy in asymmetric coupled double quantum wells is calculated. As the system is electrically tuned from type I to type II, the exciton binding energy decreases from that of a two-dimensional exciton to the binding energy of a spatially separated electron-hole pair, i.e., the barbell exciton.$-- We compare our theoretical results with a recent experiment and find

F. M. Peeters; J. E. Golub

1991-01-01

77

Binding energy of the barbell exciton

NASA Astrophysics Data System (ADS)

The exciton binding energy in asymmetric coupled double quantum wells is calculated. As the system is electrically tuned from type I to type II, the exciton binding energy decreases from that of a two-dimensional exciton to the binding energy of a spatially separated electron-hole pair, i.e., the barbell exciton.$-- We compare our theoretical results with a recent experiment and find good agreement.

Peeters, F. M.; Golub, J. E.

1991-02-01

78

Nonlinear optical probe of indirect excitons

NASA Astrophysics Data System (ADS)

We propose the application of nonlinear optics for studies of spatially indirect excitons in coupled quantum wells. We demonstrate that despite their vanishing oscillator strength, indirect excitons can strongly contribute to the photoinduced reflectivity and Kerr rotation. This phenomenon is governed by the interaction between direct and indirect excitons. Both dark and bright states of indirect excitons can be probed by these nonlinear optical techniques.

Nalitov, A. V.; Vladimirova, M.; Kavokin, A. V.; Butov, L. V.; Gippius, N. A.

2014-04-01

79

Crystallization of an exciton superfluid

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

80

Excitonic polaritons in Fibonacci quasicrystals.

The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission. PMID:18825174

Hendrickson, J; Richards, B C; Sweet, J; Khitrova, G; Poddubny, A N; Ivchenko, E L; Wegener, M; Gibbs, H M

2008-09-29

81

NASA Astrophysics Data System (ADS)

The excitonic nonlinear optical responses due to exciton-phonon interactions in strongly coupled exciton-phonon systems are investigated theoretically. It is shown that the influence of exciton-phonon interactions on the nonlinear optical absorptions and Kerr nonlinear coefficients is significant as the signal field frequency detuning from the exciton frequency approaches to the optical phonon frequency. How to manipulate the nonlinear optical responses by using the control fields is also presented.

Zhu, K.-D.; Yuan, X.-Z.; Dong, J.-J.; Li, W.-S.

2004-07-01

82

Subdiffusive exciton transport in quantum dot solids.

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

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

2014-06-11

83

Intramolecular radiationless transitions dominate exciton relaxation dynamics

NASA Astrophysics Data System (ADS)

Reports of long-lived exciton coherences have lead researchers to expect that model dimer systems inevitably generate exciton superposition states observable by two-dimensional electronic spectroscopy. Here we report a careful photophysical characterization of a model dimer system, a diacetylene-linked perylenediimide dimer to examine that issue. The absorption spectrum of the dimer shows molecular exciton splitting, indicating that excitation is delocalized. The assignment of exciton states was supported by other photophysical measurements as well as theoretical calculations. Ultrafast two-dimensional electronic spectroscopy was employed to identify and characterize excitonic and vibrational features, as they evolve over time. Population transfer between the two exciton states is found to happen in <50 fs, thus preventing the sustainment of exciton coherences. We show that such fast radiationless relaxation cannot be explained by coupling to a solvent spectral density and is therefore missed by standard approaches such as Redfield theory and the hierarchical equations of motion.

Jumper, Chanelle C.; Anna, Jessica M.; Stradomska, Anna; Schins, Juleon; Myahkostupov, Mykhaylo; Prusakova, Valentina; Oblinsky, Daniel G.; Castellano, Felix N.; Knoester, Jasper; Scholes, Gregory D.

2014-04-01

84

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

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

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

2008-09-05

85

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

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

Yao, Wang; Niu, Qian

2008-09-01

86

Spatially indirect excitons in coupled quantum wells

NASA Astrophysics Data System (ADS)

Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under a cross-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time---a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect exciton such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 mum)2 was observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

Lai, Chih-Wei Eddy

87

Excitonic Polarization and Polarization Scattering in a Highly Excited Semiconductor

Interaction between free carriers and excitons in a highly excited semiconductor is considered in terms of the polarization on the part of excitons caused by the elecric field due to the free carriers. When the distribution of excitons is uniform, one may have the chance of getting an orderly coupling between carriers and excitons which we call ``excitonic polaron''. On

Eizo Otsuka

1975-01-01

88

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

89

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

NASA Astrophysics Data System (ADS)

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

Eckert, Michael

2014-04-01

90

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

A major challenge for any theory of quantum gravity is to quantize general relativity while retaining some part of its geometrical character. We present new evidence for the idea that this can be achieved by directly quantizing space itself. We compute the Bohr-Sommerfeld volume spectrum of a tetrahedron and show that it reproduces the quantization of a grain of space found in loop gravity. PMID:21797533

Bianchi, Eugenio; Haggard, Hal M

2011-07-01

91

Isoelectric focusing on polyacrylamide gel in the absence of haem ligands represents a useful, convenient and rapid procedure to isolate silent Hb variants in their native forms, provided that they exhibit an abnormal Bohr effect. The amount of material which is eluted is sufficient for both a limited functional study and a structural determination using microscale high-performance liquid chromatography. This is exemplified by the isolation and the study of Hb Rainier. PMID:3793825

Rochette, J; Baudin, V; Bohn, B; Poyart, C; Wajcman, H

1986-11-01

92

Exciton localization in disordered poly(3-hexylthiophene).

Singlet exciton localization in conformationally disordered poly(3-hexylthiophene) (P3HT) is investigated via configuration interaction (singles) calculations of the Pariser-Parr-Pople model. The P3HT structures are generated by molecular dynamics simulations. The lowest-lying excitons are spatially localized, space filling, and nonoverlapping. These define spectroscopic segments or chromophores. The strong conformational disorder in P3HT causes breaks in the pi-conjugation. Depending on the relative values of the disorder-induced localization length and the distances between the pi-conjugation breaks, these breaks sometimes serve to pin the low-lying localized excitons. The exciton confinement also causes a local spectrum of low-lying exciton states. Coulomb-induced intra- or interchain interactions between spectroscopic segments in close spatial proximity can delocalize an exciton across these segments, in principle causing phase coherent transition dipole moments. PMID:20687660

Barford, William; Lidzey, David G; Makhov, Dmitry V; Meijer, Anthony J H

2010-07-28

93

Generating Excitonic Supercurrent in Quantum Hall Bilayers

NASA Astrophysics Data System (ADS)

Among the many examples of Bose condensation considered in physics, exciton condensation has maintained special interest because of controversy about condensate properties. Although ideal condensates can support an exciton supercurrent, it has not been clear how such a current could be induced or detected. We discuss the circuit conditions required to induce a steady-state counterflow superfluid. In addition, we will discuss interpretations of tunnel, drag and counterflow experiments in quantum Hall exciton condensates.

Su, Jung-Jung; Pereg-Barnea, Tami; MacDonald, Allan H.

2009-03-01

94

Nonboson treatment of excitonic nonlinearity in optically excited media.

National Technical Information Service (NTIS)

The present article shortly reviews some recent results in the study of excitonic nonlinearity in optically excited media using a nonboson treatment for many-exciton systems. After a brief discussion of the exciton nonbosonity the closed commutation relat...

Nguyen Ba An

1990-01-01

95

Magnetic-field modulated exciton-exciton interaction in semiconductor microcavities

We study the quantum-well magnetoexcitons by the variational method in the strong coupling regime of excitons with microcavity photons. In such strong coupling regime, we find that the coupling between the exciton internal state and center of mass motion can be neglected. Through the calculations, we find that the magnetic field can reduce the exciton exchange interaction to 30% as

Yongyou Zhang; Li-Jie Shi; Guojun Jin; Bingsuo Zou

2010-01-01

96

Josephson effects in condensates of excitons and exciton polaritons

We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.

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

2008-07-15

97

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

98

Bulk excitons in solid neon: Theory

In a recent paper Saile and Koch present a detailed experimental reinvestigation of the excitonic spectrum of solid neon. With respect to a previous work, relevant corrections are introduced in the case of the n=1 excitonic transitions. Saile and Koch compare their results with theoretical studies which were, however, based on the previous data. Other relevant theoretical works have been

L. Resca; R. Resta

1980-01-01

99

NASA Astrophysics Data System (ADS)

Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons --- a bound electron--hole pair --- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated: Photon storage --- an optical memory --- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization--delocalization transitions for the excitons in the conveyer: In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton density, excitons do not follow the conveyer motion. We explore conveyer velocities both slower and faster than phonon velocities. Realizing subnanosecond manipulations of exciton energy and lifetime required versatile control of pulsed and multiple AC RF electrical signals in optical, liquid helium cryogenic systems. Considerable detail is presented of design, construction, and test of flexible experimental apparatus.

Winbow, Alexander Graham

100

Exciton polaritons in semiconductor waveguides

NASA Astrophysics Data System (ADS)

We report the observation of strong coupling between quantum well excitons and a guided mode of a semiconductor planar waveguide by observation of anticrossing in the dispersion. Strong spatial confinement of the optical mode allows a splitting between the two polariton modes of 5-6 meV for a single quantum well. Polaritons on resonance are shown to propagate with a characteristic decay length of 280 ?m and a group velocity of 26 ?m ps-1. This is a promising first step towards developing an alternative to microcavities for the study of rapidly propagating polaritons, which is particularly well suited to prospective on-chip polaritonic circuit applications.

Walker, P. M.; Tinkler, L.; Durska, M.; Whittaker, D. M.; Luxmoore, I. J.; Royall, B.; Krizhanovskii, D. N.; Skolnick, M. S.; Farrer, I.; Ritchie, D. A.

2013-01-01

101

The structure and dynamics of molecular excitons.

The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet ? triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion. PMID:24313684

Bardeen, Christopher J

2014-04-01

102

Intrinsic frame inverse mass tensor as a function of {beta} and {gamma} in the Bohr Hamiltonian

Analytical expressions are derived for the components of the intrinsic frame inverse mass tensor of the Bohr Hamiltonian. These expressions contain parameters which are determined by the experimental data on the B(E2)'s and the excitation energies of the low-lying collective states. It is shown that the nondiagonal component of the intrinsic frame mass tensor has a small effect on the collective motion. It is shown also that the values of the B{sub {beta}{beta}}, B{sub {gamma}{gamma}} and the rotational mass coefficientB{sub 1} differ in the well-deformed nuclei by factor 3 or more.

Jolos, R. V., E-mail: jolos@theor.jinr.ru [JINR, Bogoliubov Laboratory of Theoretical Physics (Russian Federation); Brentano, P. von, E-mail: brentano@ikp.uni-koeln.de [Institut fuer Kernphysik der Universitaet zu Koeln (Germany)

2012-04-15

103

The effective neutrino charge radius

NASA Astrophysics Data System (ADS)

It is shown that at one-loop order a neutrino charge radius (NCR) may be defined, which is ultraviolet finite, does not depend on the gauge-fixing parameter, nor on properties of the target other than its electric charge. This is accomplished through the systematic decomposition of physical amplitudes into effective self-energies, vertices, and boxes, which separately respect electroweak gauge invariance. In this way the NCR stems solely from an effective proper photon-neutrino one-loop vertex, which satisfies a naive, QED-like Ward identity. The NCR so defined may be extracted from experiment, at least in principle, by expressing a set of experimental electron-neutrino cross-sections in terms of the finite NCR and two additional gauge- and renormalization-group-invariant quantities, corresponding to the electroweak effective charge and mixing angle. PACS: 13.15.+g Neutrino interactions - 13.40.Gp Electromagnetic form factors

Papavassiliou, J.; Bernabéu, J.; Binosi, D.; Vidal, J.

104

Interlayer excitonic superfluidity in graphene

NASA Astrophysics Data System (ADS)

We discuss the conditions under which the predicted (but not yet observed) zero-field inter-layer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Crucially, disorder arising from charged impurities and corrugation in the lattice structure—invariably present in all real samples—affects the formation of the condensate via the induced charge inhomogeneity. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the effect these realistic fluctuations have on the formation of the condensate. To make this estimate, we calculate the critical temperature for the interlayer excitonic superfluid transition within the mean-field BCS theory for both optimistic (unscreened) and conservative (statically screened) approximations for the screening of the interlayer Coulomb interaction. We also estimate the effect of allowing dynamic contributions to the interlayer screening. We then conduct similar calculations for double quadratic bilayer graphene, showing that the quadratic nature of the low-energy bands produces pairing with critical temperature of the same order of magnitude as the linear bands of double monolayer graphene.

Abergel, D. S. L.; Rodriguez-Vega, M.; Rossi, Enrico; Das Sarma, S.

2013-12-01

105

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.

106

A Bohr-type model of a composite particle using gravity as the attractive force

NASA Astrophysics Data System (ADS)

We formulate a Bohr-type rotating particle model for three light particles of rest mass mo each, forming a bound rotational state under the influence of their gravitational attraction, in the same way that electrostatic attraction leads to the formation of a bound proton-electron state in the classical Bohr model of the H atom. By using special relativity, the equivalence principle and the de Broglie wavelength equation, we find that when each of the three rotating particles has the same rest mass as the rest mass of a neutrino or an antineutrino (?0.05 eV/c2) then surprisingly the composite rotating state has the rest mass of the stable baryons, i.e. of the proton and the neutron (?1 GeV/c2). This rest mass is due almost exclusively to the kinetic energy of the rotating particles. The results are found to be consistent with the theory of general relativity. The model contains no unknown parameters, describes both asymptotic freedom and confinement and also provides good agreement with QCD regarding the QCD condensation temperature. Predictions for the thermodynamic and other physical properties of these bound rotational states are compared with experimental values.

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

2014-07-01

107

The Redox-Bohr group associated with iron-sulfur cluster N2 of complex I.

Proton pumping respiratory complex I (NADH:ubiquinone oxidoreductase) is a major component of the oxidative phosphorylation system in mitochondria and many bacteria. In mammalian cells it provides 40% of the proton motive force needed to make ATP. Defects in this giant and most complicated membrane-bound enzyme cause numerous human disorders. Yet the mechanism of complex I is still elusive. A group exhibiting redox-linked protonation that is associated with iron-sulfur cluster N2 of complex I has been proposed to act as a central component of the proton pumping machinery. Here we show that a histidine in the 49-kDa subunit that resides near iron-sulfur cluster N2 confers this redox-Bohr effect. Mutating this residue to methionine in complex I from Yarrowia lipolytica resulted in a marked shift of the redox midpoint potential of iron-sulfur cluster N2 to the negative and abolished the redox-Bohr effect. However, the mutation did not significantly affect the catalytic activity of complex I and protons were pumped with an unchanged stoichiometry of 4 H(+)/2e(-). This finding has significant implications on the discussion about possible proton pumping mechanism for complex I. PMID:16760472

Zwicker, Klaus; Galkin, Alexander; Dröse, Stefan; Grgic, Ljuban; Kerscher, Stefan; Brandt, Ulrich

2006-08-11

108

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

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

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

2011-02-15

109

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

NASA Astrophysics Data System (ADS)

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

Bölükdemir, M. H.; Tel, E.; Ayd?n, A.; Okuducu, S.; Kaplan, A.

2011-02-01

110

National Technical Information Service (NTIS)

From literature searches it can be determined that considerable research is being done in a number of countries, on both theory and experiment relative to excition phenomena. At least one author points this out with the fact that a large number of discipl...

J. E. Brown

1974-01-01

111

Excitons in the Fractional Quantum Hall Effect

Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.

Laughlin, R.B.

1984-09-01

112

Multiple Exciton Generation Solar Cells

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

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

2012-01-01

113

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

NASA Astrophysics Data System (ADS)

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 ~ 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.

2011-09-01

114

Supramolecular exciton chirality of carotenoid aggregates.

The conventional organic chemistry concept of chirality relates to single molecules. This article deals with cases in which exciton chirality is generated by the interaction of associated carotenoids. The handed property responsible for exciton signals in these systems is due to the alignment of neighboring molecules held together by secondary chemical forces. Their mutual positions are characterized by the overlay angle. Experimental manifestation is obtained by spectroscopic studies on carotenoid aggregates. Compared to molecular spectra, both UV/visible and circular dichroism spectroscopic observations reveal modified absorption bands and induced Cotton effects of opposite sign (exciton couplets), respectively. A new term, "supramolecular exciton chirality," is suggested for these phenomena, allowing the detection of weak chemical interactions not readily accessible for experimental studies, although highly important in the mechanism of biological processes. PMID:12923806

Simonyi, Miklós; Bikádi, Zsolt; Zsila, Ferenc; Deli, József

2003-10-01

115

Intermediate Exciton-Phonon Coupling in Tetracene

NASA Astrophysics Data System (ADS)

Polarized absorption spectra of tetracene (naphthacene) single crystals have been investigated between 300 and 13 K, with particular emphasis on the b-polarized spectrum. The low energy tails of the a- and b-polarized first absorption bands are expressed by the Urbach rule with the steepness coefficients ?a'{=}1.37± 0.12, ?b'{=}1.58± 0.13, with which possibility of self-trapping of excitons is discussed along with the dimensionality of tetracene crystals. The exciton-phonon coupling constants are found to be ga{=}1.09± 0.1 and gb{=}0.95± 0.08, consistent with luminescence experiments which show that excitons in tetracene are self-trapped very shallowly. The line halfwidth, the oscillator strength of the first and the second absorption bands, refractive indices, and surface exciton structure are briefly discussed.

Mizuno, Kenichi; Matsui, Atsuo; Sloan, G. J.

1984-08-01

116

Atomic lattice excitons: from condensates to crystals

NASA Astrophysics Data System (ADS)

We discuss atomic lattice excitons (ALEs), bound particle hole pairs formed by fermionic atoms in two bands of an optical lattice. Such a system provides a clean set-up, with tunable masses and interactions, to study fundamental properties of excitons including exciton condensation. We also find that for a large effective mass ratio between particles and holes, effective long-range interactions can mediate the formation of an exciton crystal, for which superfluidity is suppressed. Using a combination of mean-field treatments, bosonized theory based on a Born Oppenheimer approximation, and one-dimensional (1D) numerical computation, we discuss the properties of ALEs under varying conditions, and discuss in particular their preparation and measurement.

Kantian, A.; Daley, A. J.; Törmä, P.; Zoller, P.

2007-11-01

117

Quantum Hall signatures of dipolar Mahan excitons

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

118

NASA Astrophysics Data System (ADS)

Resonant hyper-Raman-scattering spectroscopy was applied for studies of the exciton-LO-phonon interaction in spherical CuCl nanocrystals of different sizes embedded in a glass matrix. Both the LO and 2LO phonon bands have shown a prominent resonance with the lowest energy 1S confined exciton state. The ratio of the integral intensities of the LO and 2LO bands ? was found to increase with increase of the incident photon energy, or decrease of the nanocrystal radius. The Huang-Rhys factor S has been calculated as a function of the nanocrystal size from the experimentally measured values of ? on the offset harmonic-oscillator model of electron-vibrational coupling. It has been found that S increases monotonically from 0.22 to 0.7, with the nanocrystal radius decreasing from 3.6 to 1.6 nm.

Baranov, A. V.; Yamauchi, S.; Masumoto, Y.

1997-10-01

119

Optimization of exciton currents in photosynthetic systems.

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

120

Exciton Dynamics in Gallium Arsenide Quantum Wells

NASA Astrophysics Data System (ADS)

The dynamics of excitons is studied using time -resolved photoluminescence. We deduce experimentally, for the first time, the intrinsic homogeneous linewidth of free excitons in GaAs quantum wells. The scattering rate of excitons is directly estimated from temperature dependent linewidth measurements. A theoretical model is presented that correlates the observed photoluminescence linewidth, and hence the scattering rate, to the measured spin relaxation rate of excitons. We show that quantum confinement enhances the spin relaxation rate of weakly scattered electrons by more than two orders of magnitude over that of the bulk. The spin exchange energy is experimentally determined for the first time for GaAs quantum wells. The radiative recombination rate of excitons in GaAs quantum wells is very large due to the macroscopic polarization of the exciton-polariton. Thermalization and scattering reduce the coherence volume of this polarization, resulting in a temperature and intensity dependent photoluminescence lifetime. We derive a theoretical model which is shown to explain the experimental results on the radiative lifetimes.

Srinivas, Vivek

121

Optimization of exciton currents in photosynthetic systems

NASA Astrophysics Data System (ADS)

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.

Guan, Chengbo; Wu, Ning; Zhao, Yang

2013-03-01

122

Physical theory of excitons in conducting polymers.

In this tutorial review, we cover the solid state physics approach to electronic and optical properties of conducting polymers. We attempt to bring together languages and advantages of the solid state theory for polymers and of the quantum chemistry for monomers. We consider polymers as generic one-dimensional semiconductors with features of strongly correlated electronic systems. Our model combines the long range electron-hole Coulomb attraction with a specific effect of strong intra-monomer electronic correlations, which results in effective intra-monomer electron-hole repulsion. Our approach allows to go beyond the single-chain picture and to compare excitons for polymers in solutions and in films. The approach helps connecting such different questions as shallow singlet and deep triplet excitons, stronger binding of interchain excitons in films, crossings of excitons' branches, 1/N energies shifts in oligomers. We describe a strong suppression of the luminescence from free charge carriers by long-range Coulomb interactions. Main attention is devoted to the most requested in applications phenyl based polymers. The specifics of the benzene ring monomer give rise to existence of three possible types of excitons: Wannier-Mott, Frenkel and intermediate ones. We discuss experimental manifestations of various excitons and of their transformations. We touch effects of the time-resolved self-trapping by libron modes leading to formation of torsion polarons. PMID:20517580

Brazovskii, Serguei; Kirova, Natasha

2010-07-01

123

Large boson number IBM calculations and their relationship to the Bohr model

NASA Astrophysics Data System (ADS)

Recently, the SO(5) Clebsch-Gordan (CG) coefficients up to the seniority v max = 40 were computed in floating point arithmetic (T.A. Welsh, unpublished (2008)); and, in exact arithmetic, as square roots of rational numbers (M.A. Caprio et al., to be published in Comput. Phys. Commun.). It is shown in this paper that extending the QQQ model calculations set up in the work by D.J. Rowe and G. Thiamova (Nucl. Phys. A 760, 59 (2005)) to N = v max = 40 is sufficient to obtain the IBM results converged to its Bohr contraction limit. This will be done by comparing some important matrix elements in both models, by looking at the seniority decomposition of low-lying states and at the behavior of the energy and B( E2) transition strengths ratios with increasing seniority.

Thiamova, G.; Rowe, D. J.

2009-08-01

124

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

125

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

126

Exciton dynamics in disordered poly(p-phenylenevinylene). 2. Exciton diffusion.

We present a first principles theory of exciton diffusion in conformationally disordered conjugated polymers. Central to our theory is that exciton transfer occurs from vibrationally relaxed states (VRSs) to local exciton ground states (LEGSs). LEGSs are determined by the diagonal and off-diagonal disorder induced by static density and torsional fluctuations, and VRSs are further localized by exciton-phonon coupling. The theory is implemented using the Frenkel-Holstein model to calculate the wave functions and energies of the LEGSs and VRSs. The coupling of VRSs and LEGSs via long-range dipole-dipole interactions leads to the familiar line-dipole approximation for the exciton transfer integral. The exciton transfer rates are derived from the Fermi Golden rule. The theory is applied to an ensemble of conformationally disordered poly(p-phenylenevinylene) chains using a kinetic Monte Carlo algorithm. The following are shown: (i) Torsional disorder and trans-cis defects reduce the exciton diffusion length. (ii) Radiative recombination occurs from VRSs in the tail of their density of states. (iii) Torsional disorder increases the band gap, the line width of the density of states, and the Stokes shift. As a consequence, it causes a blue shift in the vertical absorption, but a red shift in the emission. (iv) The energy of the radiated photon decreases as -log t, with a gradient that increases with torsional disorder. The predicted exciton diffusion lengths of ~8-11 nm are in good agreement with experimental values. PMID:23030335

Barford, William; Bittner, Eric R; Ward, Alec

2012-10-25

127

Exciton-exciton interaction in MoSe2 during laser irradiation

Three regions are found in the photoconductivity of MoSe2 single crystals as a function of light intensity. At the highest intensities and low temperatures, the response is non linear with intensity, and is due to exciton-exciton annihilation.

J. M. Doviak; A. D. Yoffe

1972-01-01

128

Theoretical study on exciton dynamics in dendritic systems: exciton recurrence and migration.

The optical functionalities such as exciton recurrence and migration for dendritic systems, e.g., dendrimers, are investigated using the quantum master equation (QME) approach based on the ab initio molecular orbital configuration interaction (MOCI) method, which can treat both the coherent and incoherent exciton dynamics at the first principle level. Two types of phenylacetylene dendrimers, Cayley-tree dendrimer and nanostar dendrimer with anthracene core, are examined to elucidate the features of excion recurrence and migration motions in relation to their structural dependences. It is found that the nanostar dendrimer exhibits faster exciton migration from the periphery to the core than Cayley-tree dendrimer, which alternatively exhibits exciton recurrence motion among dendron parts in case of small relaxation parameters. Such strong structural dependence of exciton dynamics demonstrates the advantage of dendritic molecular systems for future applications in nano-optical and light-harvesting devices. PMID:19783953

Nakano, Masayoshi; Kishi, Ryohei; Minami, Takuya; Yoneda, Kyohei

2009-01-01

129

Triplane fracture of the distal radius.

We report the case of a 14-year-old boy who sustained a displaced triplane fracture of the distal radius. This was treated with closed reduction and application of a cast with good clinical and radiological results. We discuss (for the first time) the reasons for the rarity of this fracture at the distal radius compared to the distal tibia. PMID:24765336

Pearce, Christopher; Chung, Raymond

2011-07-01

130

[Osteosarcoma of the radius after intramedullary osteosynthesis].

Osteosarcoma is a malignant neoplasm characterized by local lytic or plastic processes. We observed a case of radius osteosarcoma in a 10-year-old boy 3 months after intramedullary osteosynthesis in the ipsilateral distal radius. Treatment consisted of amputation of the distal humerus and adjuvant chemotherapy; no complications were observed. PMID:24290893

Raherinantenaina, F; Andriamampionona, R F; Raherison, A R; Rakotosamimanana, J; Hunald, F A; Andriamanarivo, M L; Randrianjafisamindrakotroka, N S; Rakoto Ratsimba, H N

2014-01-01

131

Mass-radius relationships for exoplanets

For planets other than Earth, 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

Damian C. Swift; Jon Eggert; Damien G. Hicks; Sebastien Hamel; Kyle Caspersen; Eric Schwegler; Gilbert W. Collins; Nadine Nettelmann; Graeme Ackland

2010-01-01

132

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

133

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

134

Pairing interaction effects in exciton level densities

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 simple formula for (U,n)-dependent pairing corrections has been derived, based on the BCS pairing equations for constant single-particle spacing, for the exciton state-density formula for one kind of Fermion. It has been shown that the constant-pairing-energy correction used in standard state-density formulas, such U{sub 0} in Gilbert and Cameron, is a limiting case of the general (U,n)-dependent results. Spin cutoff factors with pairing effects were also obtained using the same theory and parameterized into an explicit (U,n)-dependent function, thereby defining a simple exciton level-density formula for applications in quantum mechanical precompound theories. Preliminary results from extending such simple pairing-interaction representations to level-density formulas for two kinds of Fermions are summarized. The results show that the ratios in the exciton level densities in the one-Fermion and two-Fermion approaches vary with both U and n, thus likely leading to differences in calculated compound to precompound ratios. However, the differences in the spin cutoff factors in the two cases are found to be rather small. 12 refs., 3 figs.

Fu, C.Y.

1989-01-01

135

Visualization of exciton transport in ordered and disordered molecular solids

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

136

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

137

Bose-Einstein condensation of excitons in Cu2O

We present a parameter-free model that estimates the density of excitons in Cu2O, related to experiments that have tried to create an excitonic Bose-Einstein condensate. Our study demonstrates that the triplet-state excitons move along adiabats and obey classical statistics, while the singlet-state excitons are a possible candidate for forming a Bose-Einstein condensate. Finally we show that the results of this

G. M. Kavoulakis

2002-01-01

138

NASA Astrophysics Data System (ADS)

We analyze possible nonlinear exciton-exciton correlation effects in the optical response of semiconductors by using a time-dependent density-functional theory (TDDFT) approach. For this purpose we derive the nonlinear (third-order) TDDFT equation for the excitonic polarization. In this equation the nonlinear time-dependent effects are described by the time-dependent (nonadiabatic) part of the effective exciton-exciton interaction, which depends on the exchange-correlation (XC) kernel. We apply the approach to study the nonlinear optical response of a GaAs quantum well. In particular, we calculate the two-dimensional Fourier spectrum (2DFS) of the system and compare it with experimental data. We find that the memory effects play a crucial role in this response, and in particular that it is necessary to use a nonadiabatic XC kernel to describe excitonic bound states, biexcitons, which are formed due to the retarded TDDFT exciton-exciton interaction.

Turkowski, Volodymyr; Leuenberger, Michael N.

2014-02-01

139

We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, {ge}50 mK . The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result{emdash}a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons{emdash}reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.

Butov, L. V.; Ivanov, A. L.; Imamoglu, A.; Littlewood, P. B.; Shashkin, A. A.; Dolgopolov, V. T.; Campman, K. L.; Gossard, A. C.

2001-06-11

140

Spin injection effects on exciton distributions in conjugated organic semiconductors

Conjugated organic semiconductors are under rapid development as the active material in organic light emitting diodes (OLEDs). Electrons and holes injected into the organic semiconductor form bound singlet or triplet excitons. Singlet excitons may recombine radiatively giving rise to light emission whereas triplet excitons do not recombine radiatively. Thus the quantum efficiency of OLEDs is limited by the fraction of

Mohammad Yunus; P. Paul Ruden; Darryl Smith

2008-01-01

141

Exciton–polariton light–semiconductor coupling effects

The integrated absorption of an excitonic resonance is a measure of a semiconductor's coupling to an optical field. The concept of an exciton–polariton expresses the non-perturbative coupling between the electromagnetic field and the optically induced matter polarization. Ways to alter this coupling include confining the light in optical cavities and localizing the excitonic wavefunction in quantum wells and dots, which

G. Khitrova; S. W. Koch; H. M. Gibbs

2011-01-01

142

Nuclear charge radius of 12Be.

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 ?

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

2012-04-01

143

Detecting an exciton crystal by statistical means

NASA Astrophysics Data System (ADS)

We investigate an ensemble of excitons in a coupled quantum well excited via an applied laser field. Using an effective disordered quantum Ising model, we perform a numerical simulation of the experimental procedure and calculate the probability distribution function P(M) to create M excitons as well as their correlation function. It shows clear evidence of the existence of two phases corresponding to a liquid and a crystal phase. We demonstrate that not only the correlation function but also the distribution P(M) is very well suited to monitor this transition.

Breyel, D.; Soller, H.; Schmidt, T. L.; Komnik, A.

2014-05-01

144

Half-quantum vortices in exciton-polariton condensates in applied magnetic field

NASA Astrophysics Data System (ADS)

We study the behavior of half-quantum vortices (HQVs) in exciton-polariton condensates in planar semiconductor microcavities in applied magnetic field. Below the critical magnetic field, that is defined by the polariton-polariton interaction constant, the condensate is elliptically polarized and there are two types of HQVs, deep and shallow. They correspond to singularities in majority and minority circular components of the condensate wave function, respectively. The core radius (healing length) of the deep HQVs decreases and the core radius of the shallow HQVs increases with increase of magnetic field. The shallow HQVs disappear and the deep HQVs transform into the integer vortices in the circularly polarized condensate when the applied magnetic field exceeds the critical one.

Toledo Solano, M.; Rubo, Yuri G.

2011-03-01

145

Especially the (alkaline) proton Bohr effect seems to provide an important self regulating mechanism of the organism to deliver specifically oxygen into tissues suffering from O2 deficit. In this way these tissues switch from aerobic to anaerobic metabolism, get lactacid, thereby shifting oxygen hemoglobin binding curve to the right and thus facilitating the oxygen release. The higher the absolute value of the proton Bohr factor (: delta logP50/ delta pH) is the better this mechanism works. To get one characteristic number the proton Bohr factor at pH 7.1 is taken. This pH in blood is about a lower limit for organism and human blood has at this pH its maximum proton Bohr factor which is about -0.5. When designing a hemoglobin based artificial oxygen carrier such a high or even a higher proton Bohr factor should be aimed at. But bringing human hemoglobin into extracellular milieu decreases the said proton Bohr effect down to -0.31; about the same values have bovine and porcine hemoglobin under these conditions. Before native hemoglobin can be used as an artificial oxygen carrier outside the red blood cells, they must be crosslinked; otherwise they are cleared quickly by the kidneys. Reaction of human and bovine hemoglobin with the crosslinkers DIBS (2,5- diisothiocyanatobenzenesulfonate) and DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) decreases the proton Bohr effect once again substantially down to about -0.1 irrespective of the degree of polymerization (monomer and hyperpolymer). Proton Bohr factors of reaction products from various hemoglobins and different crosslinkers evaluated from measurements of other investigators largely confirm the findings of this study.(ABSTRACT TRUNCATED AT 250 WORDS) PMID:7597959

Barnikol, W K

1994-01-01

146

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

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

2014-06-01

147

An investigation of the nature of Bohr, Root, and Haldane effects in Octopus dofleini hemocyanin.

1. The pH dependence of Octopus dofleini hemocyanin oxygenation is so great that below pH 7.0 the molecule does not become fully oxygenated, even in pure O2 at 1 atm pressure. However, the curves describing percent oxygenation as a function of PO2 appear to be gradually increasing in oxygen saturation, rather than leveling out at less than full saturation. Hill plots indicate that at pH 6.6 and below the molecule is stabilized in its low affinity conformation. Thus, the low saturation of this hemocyanin in air is due to the very large Bohr shift, and not to the disabling of one or more functionally distinct O2 binding sites on the native molecule. 2. Experiments in which pH was monitored continuously while oxygenation was manipulated in the presence of CO2 provide no evidence of O2 linked binding of CO2. While CO2 does influence O2 affinity independently of pH, its effect may be due to high levels of HCO3- and CO3-, rather than molecular CO2, and it may entail a lowering of the activities of the allosteric effectors Mg2+ and Ca2+. PMID:3150406

Miller, K I; Mangum, C P

1988-01-01

148

Generation of localized ``Bohr-like'' wavepackets in near-circular orbit about the nucleus

NASA Astrophysics Data System (ADS)

Atoms in high-lying (n ˜ 300) Rydberg states provide a valuable laboratory in which to explore the engineering of electronic wavefunctions using carefully-tailored sequences of short electric field pulses whose characteristic times (duration and/or rise/fall times) are less than the classical electron orbital period. The level of control that can be exercised is illustrated with reference to the generation of localized wavepackets in ``Bohr-like'' near circular orbits. While such wavepackets slowly dephase and undergo dispersion, their localization can be maintained for extended periods (many hundreds of orbits) through external driving using a periodic train of pulses. The wavepackets can be further manipulated by slowly varying, or ``chirping,'' the pulse repetition frequency. The physics underlying these control protocols is explained using classical trajectory Monte Carlo simulations. Even in the absence of external driving, however, wavepacket relocalization is expected at late times due to quantum revivals. The observation of such relocalization is described and demonstrates that quantum phenomena can be seen even in mesoscopic very-high-n atoms. Research undertaken in collaboration with J. J. Mestayer, B. Wyker, C. O. Reinhold, S. Yoshida and J. Burgdörfer.

Dunning, F. Barry

2009-05-01

149

Foundations of quantum mechanics (II): equilibrium, Bohr-Sommerfeld rules and duality

NASA Astrophysics Data System (ADS)

The present paper is divided into two parts. In the first part we present a new axiomatic derivation of the Schrödinger equation from three basic postulates. This new derivation sheds some light on the statistical character of the quantum formalism. We also show the formal connection between this derivation and the one done by other means in paper I of this series. The role of the entropy in all these developments is discussed at length. A discussion about the wave-particle duality problem is also presented. It is shown that the present approach avoids this problem. In the second part we show how to derive the Bohr-Sommerfeld quantization rules from the infinitesimal Wigner-Moyal transformation of paper I. We will thus show to what extent these rules are constituent of the modern quantum formalism and interpretation. The Feynman path integral formalism will also be derived from some slight modification of this approach, thus providing a high level of formal unification for our developments. These results will then be applied to provide fully corpuscular explanations of diffraction and interference experiments. When connecting these applications to the developments of the first part we will be able to exemplify quite clearly in what sense the duality problem is avoided by the present approach.

Olavo, L. S. F.

1999-09-01

150

Transporting near-circular Bohr-like wave packets using chirped pulse trains

NASA Astrophysics Data System (ADS)

Protocols for driving localized high-n (n 300) wave packets in near-circular Bohr-like orbits to higher n states using chirped sine waves are described. While Rydberg wave packets involving several n levels are known to be dispersive, circular wave packets can be stabilized by the application of a circularly- or linearly- polarized resonant sine wave. A similar stabilization mechanism is known for the formation of the Trojan asteroids in celestial mechanics. We demonstrate that by slowly chirping the drive frequency parent high-n wave packets can be transported to a narrow distribution of much higher n states because the motion of the wave packet remains locked to the sine wave during the chirping. Use of a chirped HCP train instead of a sine wave allows similar transport but also provides some control of the orbit eccentricity. Research supported by the NSF, the Robert A. Welch Foundation, the OBES US DoE to ORNL, and by the FWF (Austria)

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

2011-06-01

151

Frenkel versus charge-transfer exciton dispersion in molecular crystals

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

152

Exciton–Exciton Interaction in CdWO Under Resonant Excitation by Intense Femtosecond Laser Pulses

Using time-resolved luminescence spectroscopy, excitation density effects were investigated under conditions of resonant creation of excitons by femtosecond laser pulses in CdWO4 scintillator at room temperature. It was revealed that the decay kinetics of intrinsic emission becomes accelerated and deviates from the exponential law in the initial stage due to the Fo?rster dipole-dipole interaction of self-trapped excitons. It is shown

V. Nagirnyi; S. Dolgov; R. Grigonis; M. Kirm; L. L. Nagornaya; F. Savikhin; V. Sirutkaitis; S. Vielhauer; A. Vasil'ev

2010-01-01

153

Exciton Dynamics in soluble Poly( p-phenylene-vinylene): Towards an Ultrafast Excitonic Switch

NASA Astrophysics Data System (ADS)

We have applied a variety of ps transient and cw optical techniques to elucidate the dynamics, absorption, and emission properties of excitons in soluble derivatives of poly( p-phenylene vinylene) neat films and dilute solutions. We found that the photogenerated singlet excitons in both films and solutions are characterized by strong stimulated emission and two photoinduced absorption bands. We demonstrate that these bands can be used to form an ultrafast optical switch in the near IR spectral range with variable switching times.

Frolov, S. V.; Liess, M.; Lane, P. A.; Gellermann, W.; Vardeny, Z. V.; Ozaki, M.; Yoshino, K.

1997-06-01

154

Exciton transfer integrals between polymer chains.

The line-dipole approximation for the evaluation of the exciton transfer integral J between conjugated polymer chains is rigorously justified. Using this approximation, as well as the plane-wave approximation for the exciton center-of-mass wave function, it is shown analytically that J approximately L when the chain lengths are smaller than the separation between them, or J approximately L-1 when the chain lengths are larger than their separation, where L is the chain length. Scaling relations are also obtained numerically for the more realistic standing wave approximation for the exciton center-of-mass wave function, where it is found that for chain lengths larger than their separation J approximately L-1.8 or J approximately L-2, for parallel or collinear chains, respectively. These results have important implications for the photophysics of conjugated polymers and self-assembled molecular systems, as the Davydov splitting in aggregates and the Forster transfer rate for exciton migration decrease with chain lengths larger than their separation. This latter result has obvious deleterious consequences for the performance of polymer photovoltaic devices. PMID:17430065

Barford, William

2007-04-01

155

Exciton transfer integrals between polymer chains

NASA Astrophysics Data System (ADS)

The line-dipole approximation for the evaluation of the exciton transfer integral J between conjugated polymer chains is rigorously justified. Using this approximation, as well as the plane-wave approximation for the exciton center-of-mass wave function, it is shown analytically that J~L when the chain lengths are smaller than the separation between them, or J~L-1 when the chain lengths are larger than their separation, where L is the chain length. Scaling relations are also obtained numerically for the more realistic standing wave approximation for the exciton center-of-mass wave function, where it is found that for chain lengths larger than their separation J~L-1.8 or J~L-2, for parallel or collinear chains, respectively. These results have important implications for the photophysics of conjugated polymers and self-assembled molecular systems, as the Davydov splitting in aggregates and the Förster transfer rate for exciton migration decrease with chain lengths larger than their separation. This latter result has obvious deleterious consequences for the performance of polymer photovoltaic devices.

Barford, William

2007-04-01

156

Magnetic field effects on quantum ring excitons

We study the effect of magnetic field and geometric confinement on excitons confined to a quantum ring. We use analytical matrix elements of the Coulomb interaction and diagonalize numerically the effective-mass Hamiltonian of the problem. To explore the role of different boundary conditions, we investigate the quantum ring structure with a parabolic confinement potential, which allows the wave functions to be expressed in terms of center of mass and relative degrees of freedom of the exciton. On the other hand, wave functions expressed in terms of Bessel functions for electron and hole are used for a hard-wall confinement potential. The binding energy and electron--hole separation of the exciton are calculated as function of the width of the ring and the strength of an external magnetic field. The linear optical susceptibility as a function of magnetic fields is also discussed. We explore the Coulomb electron--hole correlation and magnetic confinement for several ring width and size combinations. The Aharanov--Bohm oscillations of exciton characteristics predicted for one-dimensional rings are found to not be present in these finite-width systems.

Song, Jakyoung; Ulloa, Sergio E.

2001-03-15

157

(Gene sequencing by scanning molecular exciton microscopy)

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

158

Exciton-polaritons in quantum wells

NASA Astrophysics Data System (ADS)

An overview of the properties of excitons in quantum wells is presented, together with a discussion of polariton effects. Exciton binding energies are calculated including valence band mixing, Coulomb coupling between different subbands, nonparabolicity, and the dielectric mismatch: all these effects are found to be of a comparable size. Theoretical results agree within a few tenths of a meV with photoluminescence excitation experiments. Valance band mixing gives a finite oscillator strength to some excitons not in s-states, but does not change the selection rules based on parity. Inclusion of the interaction with radiation gives rise to polariton states, which, however, are surface modes and are not observed in usual optical experiments. Free quantum-well excitons are shown to have a finite radiative lifetime, unlike in bulk crystals, due to the lack of translational invariance along the growth direction. Due to thermalization processes, the radiative decay width must be averaged over the thermal distribution, resulting in an effective radiative lifetime which rises linearly with temperature. This behavior, as well as the calculated lifetimes, is in qualitative agreement with experiment.

Andreani, L. C.

1991-01-01

159

First-principles simulations of exciton diffusion in organic semiconductors

NASA Astrophysics Data System (ADS)

Exciton diffusion is crucial for the performance of organic semiconductors in photovoltaic and solid state lighting applications. We propose a first-principles approach that can predict exciton dynamics in organic semiconductors. The method is based on time-dependent density functional theory to describe 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 the exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, which agree very well with the experiments. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons. A widely speculated diffusion mechanism, namely an initial downhill migration followed by thermally activated migration, is confirmed and elucidated by the simulations. Some general guidelines for designing more efficient organic solar cells are obtained from the simulations.

Zhang, Xu; Li, Zi; Lu, Gang

2011-12-01

160

Spin dynamics of isoelectronic bound excitons in ZnO

NASA Astrophysics Data System (ADS)

Time-resolved optical spin orientation is employed to study spin dynamics of I* and I1* excitons bound to isoelectronic centers in bulk ZnO. It is found that spin orientation at the exciton ground state can be generated using resonant excitation via a higher lying exciton state located at about 4 meV from the ground state. Based on the performed rate equation analysis of the measured spin dynamics, characteristic times of subsequent hole, electron, and direct exciton spin flips in the exciton ground state are determined as being ?hs = 0.4 ns, ?es? 15 ns, and ?ehs? 15 ns, respectively. This relatively slow spin relaxation of the isoelectronic bound excitons is attributed to combined effects of (i) weak e-h exchange interaction, (ii) restriction of the exciton movement due to its binding at the isoelectronic center, and (iii) suppressed spin-orbit coupling for the tightly bound hole.

Chen, S. L.; Chen, W. M.; Buyanova, I. A.

2014-06-01

161

Exciton condensation and its influence on the specific heat

NASA Astrophysics Data System (ADS)

In rare earth compounds with localized 4f states the observation of bound 4f-hole-5d-electron states, excitons, is questionable. On the other hand the same compounds exhibit p-d excitons, which are derived from itinerant bands. In rare earth compounds, which exhibit intermediate valence, 4f-5d hybridization produces a narrow, several 10 meV wide 4f band. Now 4f-5d excitons are possible and have been observed in TmSe0.45Te0.55 and similar compositions. The special band structure of these materials permits an enormous amount of excitons (?1021 cm-3), which condense in a first order transition at low temperatures and high pressure. This static and immense concentration of excitons dominates the heat conductivity and the thermal diffusivity and even exhibits the phenomenon of superfluidity in a solid. The measured specific heat shows that phonons couple to these excitons forming exciton polarons.

Wachter, P.; Bucher, B.

2013-01-01

162

Exciton Resonance of Second Harmonic Generation in ZnSe

NASA Astrophysics Data System (ADS)

We have shown that the normally forbidden second harmonic generation signal is enhanced drastically when twice the incident photon energy is resonant with the excitation energy of a 2P exciton in ZnSe/GaAs thin films. Using this resonant effect and a novel spectroscopic technique which utilizes a phase-locked laser pulse pair, we have investigated the dynamical behavior of 2P excitons. The interferogram of exciton polarization has a tail of a few picoseconds. Beating structure was observed in the decay profile. This structure can be explained by polarization interference between the heavy hole and light hole excitons split by strain. Further, we have studied the fine structure of the 2P exciton in ZnSe films grown on GaAs substrates using this resonance phenomenon. The 2P exciton line is found to show distinct fine structure caused by envelope-hole coupling, in addition to splitting of the exciton state due to strain effects.

Minami, F.; Mitsumori, Y.; Matsushita, S.

1997-08-01

163

Scattering process between polaron and exciton in conjugated polymers.

Scattering process between a negative polaron and an exciton in a polymer chain is investigated by using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term, and an external electric field. It is found that the scattering process is spin dependent. If the polaron and the exciton have parallel spins, the polaron can easily pass through the exciton as if it "do not see" the exciton. If the polaron and the exciton have antiparallel spins, there exist strong repulsion between them. The polaron may be bounced back, be dissociated or pass through the exciton depending on the strength of the external electric field. In any of these cases, the polaron cannot break the exciton. PMID:21280797

Sun, Zhen; Liu, Desheng; Stafström, Sven; An, Zhong

2011-01-28

164

Scattering process between polaron and exciton in conjugated polymers

NASA Astrophysics Data System (ADS)

Scattering process between a negative polaron and an exciton in a polymer chain is investigated by using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term, and an external electric field. It is found that the scattering process is spin dependent. If the polaron and the exciton have parallel spins, the polaron can easily pass through the exciton as if it ``do not see'' the exciton. If the polaron and the exciton have antiparallel spins, there exist strong repulsion between them. The polaron may be bounced back, be dissociated or pass through the exciton depending on the strength of the external electric field. In any of these cases, the polaron cannot break the exciton.

Sun, Zhen; Liu, Desheng; Stafström, Sven; An, Zhong

2011-01-01

165

Exciton diffusion in monolayer and bulk MoSe2

NASA Astrophysics Data System (ADS)

The exciton dynamics in monolayer and bulk MoSe2 samples are studied by transient absorption microscopy with a high spatiotemporal resolution. Excitons are injected with a point-like spatial distribution using a tightly focused femtosecond pulse. The spatiotemporal dynamics of these excitons are monitored by measuring transient absorption of a time-delayed and spatially scanned probe pulse. We obtain the exciton diffusion coefficients of 12 +/- 3 and 19 +/- 2 cm2 s-1 and exciton lifetimes of 130 +/- 20 and 210 +/- 10 ps in the monolayer and bulk samples, respectively. These values are useful for understanding excitons and their interactions with the environment in these structures and potential applications of MoSe2 in optoelectronics and electronics.The exciton dynamics in monolayer and bulk MoSe2 samples are studied by transient absorption microscopy with a high spatiotemporal resolution. Excitons are injected with a point-like spatial distribution using a tightly focused femtosecond pulse. The spatiotemporal dynamics of these excitons are monitored by measuring transient absorption of a time-delayed and spatially scanned probe pulse. We obtain the exciton diffusion coefficients of 12 +/- 3 and 19 +/- 2 cm2 s-1 and exciton lifetimes of 130 +/- 20 and 210 +/- 10 ps in the monolayer and bulk samples, respectively. These values are useful for understanding excitons and their interactions with the environment in these structures and potential applications of MoSe2 in optoelectronics and electronics. Electronic supplementary information (ESI) available: Relationship between the differential reflection, the differential absorption, and the exciton density; drift diffusion model and fits to the exciton density profiles. See DOI: 10.1039/c3nr06863c

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

2014-04-01

166

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions. PMID:16375462

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

2005-12-01

167

NASA Astrophysics Data System (ADS)

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

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

2005-12-01

168

The distribution of SNRs with galactocentric radius

NASA Astrophysics Data System (ADS)

In order to determine the Galactic distribution of supernova remnants (SNRs) there are two main difficulties: (i) there are selection effects which mean that catalogues of SNRs are not complete, and (ii) distances are not available for most SNRs, so distance estimates from the `?-D' relation are used. Here I compare the observed distribution of 69 `bright' SNRs with Galactic longitude with that expected from the projection of various model Galactocentric radius distributions. This does not require distances from the `?-D' relation, and selecting only `bright' remnants aims to avoid major issues with the selection effects. Although this method does not provide a direct inversion to the 3-D distribution of SNRs in the Galaxy, it does provide useful constraints on the Galactocentric radius distribution. For a combined power-law/exponential model for SNR surface density variation with Galactocentric radius, the best fitted distributions are more concentrated towards lower radii than the distribution derived by Case & Bhattacharya [1].

Green, D. A.

2012-12-01

169

Mass and radius of cosmic balloons

NASA Technical Reports Server (NTRS)

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

Wang, Yun

1994-01-01

170

Operative treatment of distal radius fractures.

The incidence of distal radius fractures is increasing together with the average age of population. Intra-articular incongruity is the most probable cause of unsatisfactory outcome of distal radius fractures in younger and more active patients. Thus, the main goal in the treatment of distal radius fractures should be restoration of articular congruence. A computed tomography (CT) is recommended to help surgeon in preoperative planning in the treatment of comminuted intra-articular fractures. New implants have been designed to provide stable enough fixation for early mobilisation after surgery and to lower rather high complication rates related to conventional fixation methods such as external fixation and dorsal plating. The most common complications related to volar fixed angle plating such as flexor and extensor tendon problems, median nerve neuropathy, and screw diplacement into the radiocarpal joint are surgeon related and are avoidable with proper education. More randomized prospective studies are needed to prove superiority of any fixation method to another. PMID:19211382

Vasenius, J

2008-01-01

171

Muonic Hydrogen and the Proton Radius Puzzle

NASA Astrophysics Data System (ADS)

The extremely precise extraction of the proton radius obtained by Pohl et al. from the measured energy difference between the 2P and 2S states of muonic hydrogen disagrees significantly with that extracted from electronic hydrogen or elastic electron-proton scattering. This discrepancy is the proton radius puzzle. In this review, we explain the origins of the puzzle and the reasons for believing it to be very significant. We identify various possible solutions of the puzzle and discuss future research needed to resolve the puzzle.

Pohl, Randolf; Gilman, Ronald; Miller, Gerald A.; Pachucki, Krzysztof

2013-10-01

172

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

173

Finite Larmor Radius Effects in Ponderomotive Force

The Ponderomotive Force (PF) due to a high frequency electromagnetic wave in a magnetized plasma is derived using a hybrid model that involves both the fluid and the kinetic approaches. We obtain an analytical expression for the PF using the fluid model and the hot plasma dielectric tensor [1] elements where the contribution of finite Larmor radius (FLR) are appropriately

M. N. Nishino; T. Terasawa; M. Hoshino; M. Fujimoto; T. Mukai; Y. Kasaba; H. Kojima; B. Dasgupta; B. T. Tsurutani; M. S. Janaki

2002-01-01

174

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

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

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

2011-07-01

175

Fluorescence spectroscopy, exciton dynamics, and photochemistry of single allophycocyanin trimers

The authors report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasi-independent pairs of strong interacting {alpha}84 and {beta}84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.

Ying, L.; Sie, X.S. [Pacific Northwest National Lab., Richland, WA (United States). William R. Wiley Environmental Molecular Sciences Lab.] [Pacific Northwest National Lab., Richland, WA (United States). William R. Wiley Environmental Molecular Sciences Lab.

1998-12-10

176

Mechanism of excitonic dephasing in layered InSe crystals

NASA Astrophysics Data System (ADS)

The dephasing and lifetime of excitons in InSe layered crystals are carefully measured using three-pulse, four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. We obtain a detailed picture of the mechanism of excitonic dephasing in this layered material. The 2DFT spectra reveal contributions from the coherent excitation of continuum states, whereas the temperature dependence provides a detailed description of the phonon-exciton interactions and the low temperature limit of the homogeneous linewidth. The excitation density dependence reveals excitation-induced dephasing due to exciton-exciton scattering. The temperature-dependent homogeneous linewidth is dominated by the interaction of excitons with the ˜113 cm-1 out-of-plane phonon mode.

Dey, P.; Paul, J.; Glikin, N.; Kovalyuk, Z. D.; Kudrynskyi, Z. R.; Romero, A. H.; Karaiskaj, D.

2014-03-01

177

Nanotransformation and Current Fluctuations in Exciton Condensate Junctions

NASA Astrophysics Data System (ADS)

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.

Soller, H.; Dolcini, F.; Komnik, A.

2012-04-01

178

Exciton states in solid rare gases

The problem of the exciton states in solid rare gases is studied within the framework of the integral-equation approach appropriate for intermediate-coupling interaction. The different contributions appearing in the integral equation are investigated and related to physical quantities by means of an analysis of their behavior in the two limiting situations in which the effective-mass approximation (EMA) or the atomic

L. Resca; S. Rodriguez

1978-01-01

179

Spin Polarizing Neutral Excitons In Quantum Dots

NASA Astrophysics Data System (ADS)

A high degree of spin polarization for the neutral exciton in individual InAs quantum dots, without any external magnetic field applied, is demonstrated. The polarization mechanism is shown to be due to the difference in capture time into the QD for the electrons and holes after photo excitation in the wetting layer. This leads to optical pumping of the QD nuclei by spin polarized electrons and hence suppression of the anisotropic electron--hole exchange interaction.

Larsson, L. A.; Moskalenko, E. S.; Holtz, P. O.

2011-12-01

180

Exciton transfer dynamics in a trimer system

NASA Astrophysics Data System (ADS)

We study exciton transfer dynamics in a trimer system by investigating excitation transfer probability (ETP). We calculate ETP in the zero-temperature limit and theoretically predict the environment-assisted quantum critical effect, in which ETP exhibits a sudden change at the critical point of quantum phase transition for the trimer. In particular, we find that the steady-state ETP can be observed in the presence of the environment interaction.

Tan, Qing-Shou; Xu, Lan; Kuang, Le-Man

2013-01-01

181

Background Carbon dioxide (CO2) therapy refers to the transcutaneous administration of CO2 for therapeutic purposes. This effect has been explained by an increase in the pressure of O2 in tissues known as the Bohr effect. However, there have been no reports investigating the oxygen dissociation of haemoglobin (Hb) during transcutaneous application of CO2 in vivo. In this study, we investigate whether the Bohr effect is caused by transcutaneous application of CO2 in human living body. Methods We used a novel system for transcutaneous application of CO2 using pure CO2 gas, hydrogel, and a plastic adaptor. The validity of the CO2 hydrogel was confirmed in vitro using a measuring device for transcutaneous CO2 absorption using rat skin. Next, we measured the pH change in the human triceps surae muscle during transcutaneous application of CO2 using phosphorus-31 magnetic resonance spectroscopy (31P-MRS) in vivo. In addition, oxy- and deoxy-Hb concentrations were measured with near-infrared spectroscopy in the human arm with occulted blood flow to investigate O2 dissociation from Hb caused by transcutaneous application of CO2. Results The rat skin experiment showed that CO2 hydrogel enhanced CO2 gas permeation through the rat skin. The intracellular pH of the triceps surae muscle decreased significantly 10 min. after transcutaneous application of CO2. The NIRS data show the oxy-Hb concentration decreased significantly 4 min. after CO2 application, and deoxy-Hb concentration increased significantly 2 min. after CO2 application in the CO2-applied group compared to the control group. Oxy-Hb concentration significantly decreased while deoxy-Hb concentration significantly increased after transcutaneous CO2 application. Conclusions Our novel transcutaneous CO2 application facilitated an O2 dissociation from Hb in the human body, thus providing evidence of the Bohr effect in vivo.

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

2011-01-01

182

Microcavity controlled coupling of excitonic qubits

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

Albert, F.; Sivalertporn, K.; Kasprzak, J.; Strauss, M.; Schneider, C.; Hofling, S.; Kamp, M.; Forchel, A.; Reitzenstein, S.; Muljarov, E.A.; Langbein, W.

2013-01-01

183

Exciton Transport in a Bilayer Quantum Hall Superfluid

NASA Astrophysics Data System (ADS)

Bilayer quantum Hall systems at vT = 1 support an excitonic ground state. In addition to the usual charged quasiparticles, this system possesses a condensate degree of freedom: exciton transport. Detection of this neutral transport mode is facilitated by the use of the Corbino multiply-connected geometry in which charge transport is suppressed. We here summarize our recent experiments on Corbino devices which directly demonstrate exciton transport across the bulk of the incompressible vT = 1 quantum Hall state.

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

2013-08-01

184

Piezoelectric exciton acoustic-phonon coupling in single quantum dots

Microphotoluminescence spectroscopy at variable temperature, excitation intensity, and energy was performed on a single InAs\\/AlAs self-assembled quantum dot. The exciton emission line [zero-phonon line (ZPL)] exhibits a broad sideband due to exciton acoustic-phonon coupling by the deformation-potential mechanism. Additionally, narrow low-energy sidebands at about 0.25 meV of the ZPL are attributed to exciton acoustic-phonon piezoelectric coupling. In lowering the excitation

D. Sarkar; H. P. van der Meulen; J. M. Calleja; J. M. Meyer; R. J. Haug; K. Pierz

2008-01-01

185

Interaction between excitons and rare-earth ions

NASA Astrophysics Data System (ADS)

Energy transfer between rare-earth (RE) ions and excitons in semiconductors has already been known for some time. In this paper we would like to direct your attention to the interaction between excitons and RE ions in ionic crystals as used, e.g. for laser applications. A suitable model substance to study these effects is RE-doped CsCdBr 3. The reasons are: (i) the low phonon frequencies due to the heavy ion masses ( ??max<200 cm -1); (ii) metastable excitonic states in the visible spectral range; (iii) strong exciton-ion interaction due to covalent overlap of the wavefunctions. Due to the low phonon frequencies, multiphonon processes of the RE ions are reduced drastically and ion-exciton processes can be studied in more detail. The following processes were found: (i) exciton-mediated nonradiative relaxations. These processes can bridge much larger energy gaps than direct multiphonon relaxation and are more efficient. The reason is that the nonradiative multiphonon relaxation of the RE ion is dominated by a fast multistep one-phonon relaxation of the exciton by energy transfer; (ii) exciton-mediated quantum upconversion. This effect is based on a cooperative energy transfer from two excited RE ions to an exciton and a subsequent back transfer to a single ion. This process is much faster than upconversion by phonon-assisted cross relaxation between two excited RE ions; (iii) exciton-induced changes in the crystal-field splitting of RE ions. Energy levels of RE ions in resonance with the excitons show crystal-field splittings which cannot be described by the parameters suitable for the other levels. We propose an increased covalent overlap between the wavefunction of the RE ion and of the exciton-forming ligands due to hybridization as an explanation for this effect.

Heber, J.; Neukum, J.; Altwein, M.; Demirbilek, R.; Bodenschatz, N.

1998-09-01

186

Exciton binding energy in poly(phenylene vinylene)

The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene) (PPV) have been determined by photoconductivity excitation profile spectroscopy as a function of light polarization, applied electric field, and temperature. The exciton binding energy, Eb?60meV, is obtained from the energy of the exciton peak relative to the band edge, and independently from analysis of the dependence

D. Moses; J. Wang; A. J. Heeger; N. Kirova; S. Brazovski

2001-01-01

187

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.

188

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

189

NASA Astrophysics Data System (ADS)

Phonon-induced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) are investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we propose that MEF occurs by phonon-induced dephasing and, for the first time, estimate its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times are all sub-10 fs. Generally, dephasing is faster for higher-energy and higher-order excitons and increased temperatures. MEF is slow because it is facilitated only by low-frequency acoustic modes. Luminescence and MEG couple to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advances understanding of exciton dynamics in QDs and other nanoscale materials.

Madrid, Angeline; Kim, Hyeon-Deuk; Habenicht, Bradley; Prezhdo, Oleg

2010-03-01

190

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

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

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

2012-04-26

191

MASS-RADIUS RELATIONSHIPS FOR EXOPLANETS

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

192

Mass-Radius Relationships for Exoplanets

NASA Astrophysics Data System (ADS)

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 H2O and CH4, suggesting an "icy" composition with a relatively large core or a relatively large proportion of H2O. 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+1.2 - 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.; Nettelmann, N.; Ackland, G. J.

2012-01-01

193

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

194

Exciton management in organic photovoltaic multidonor energy cascades.

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

Griffith, Olga L; Forrest, Stephen R

2014-05-14

195

Excitonic Properties of ZnO Films and Nanorods

NASA Astrophysics Data System (ADS)

We report on the comparative studies of linearly polarized photoluminescence (PL) in a ZnO epitaxial film and ZnO nanorods. At low temperatures the PL spectrum of both samples included a number of narrow lines attributed to donor-bound excitons and a peak of free A excitons. An additional line observed in the nanorod sample was assigned to the excitons bound to some defects introduced during the sample post-growth history and located near the nanorod tips. The emission of mixed longitudinal - transverse exciton polariton modes was observed at elevated temperatures in both samples.

Toropov, A. A.; Nekrutkina, O. V.; Shubina, T. V.; Ivanov, S. V.; Gruber, Th.; Kling, R.; Reuss, F.; Kirchner, C.; Waag, A.; Karlsson, K. F.; Bergman, J. P.; Monemar, B.

2005-06-01

196

Bandwidth of excitons in LH2 bacterial antenna chromoproteins

NASA Astrophysics Data System (ADS)

The bandwidth of the exciton manifold in LH2 peripheral antenna complexes from the photosynthetic bacterium Rhodobacter sphaeroides has been determined using polarized fluorescence excitation spectroscopy and model simulations. It is shown that the fluorescence anisotropy spectra reveal a hidden structure of the exciton band that is correlated with its boundaries. The estimated exciton coupling energy and exciton bandwidth in the native complex is ˜360 and ˜1620 cm -1, respectively. The corresponding numbers in a mutant with the B800 bacteriochlorophyll molecules absent are somewhat larger (˜420 and ˜1890 cm -1), probably due to tightening of the protein structure.

Timpmann, Kõu; Trinkunas, Gediminas; Olsen, John D.; Neil Hunter, C.; Freiberg, Arvi

2004-11-01

197

Surface photovoltage in exciton absorption range in CdS

NASA Technical Reports Server (NTRS)

The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120-160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.

Morawski, A.; Banisch, R.; Lagowski, J.

1977-01-01

198

Nonthermally activated exciton transport in crystalline organic semiconductor thin films

NASA Astrophysics Data System (ADS)

The temperature dependent exciton transport in the prototypical organic semiconductor di-indeno-perylene (DIP) is investigated by photoluminescence quenching. Analysis by an advanced diffusion model including interference and morphological aspects reveals an exciton diffusion length of about 60 nm at room temperature, which relates to the long-range order induced by the DIP molecular shape. Above 80 K, singlet exciton transport is thermally activated with an energy of 10 to 20 meV. Below 80 K, exciton motion becomes temperature independent and is supported by the crystalline structure of the transport layer in combination with the reduced phonon interaction.

Topczak, Anna K.; Roller, Tobias; Engels, Bernd; Brütting, Wolfgang; Pflaum, Jens

2014-05-01

199

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

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

2009-09-15

200

The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry.

Abramavicius, Darius; Nemeth, Alexandra; Milota, Franz; Sperling, Jaroslaw; Mukamel, Shaul; Kauffmann, Harald F.

2013-01-01

201

Surgical exposures of the radius and ulna.

The forearm contains many muscles, nerves, and vascular structures that change position on forearm rotation. Exposure of the radial shaft is best achieved with the Henry (volar) or Thompson (dorsal) approach. The volar flexor carpi radialis approaches are used increasingly for exposure of the distal radius. Although the dorsal approach is a safe utilitarian option with many applications, its use for managing fracture of the distal radius has waned. Potential complications associated with radial exposure include injury to the superficial branch of the radial nerve, the lateral antebrachial cutaneous nerve, and the cephalic vein. Dorsal and ulnar proximal radial exposures are associated with increased risk of injury to the posterior interosseous nerve. With surgical exposure of the ulna, care is required to avoid injuring the dorsal cutaneous branch of the ulnar nerve. PMID:21724922

Catalano, Louis W; Zlotolow, Dan A; Hitchcock, Phillip B; Shah, Suparna N; Barron, O Alton

2011-07-01

202

Proton Radius Puzzle and Large Extra Dimensions

NASA Astrophysics Data System (ADS)

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

Wang, Li-Bang; Ni, Wei-Tou

2013-06-01

203

We report a photoluminescence observation of robust excitonic polarons due to strong coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, the strong coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the

Shi-Jie Xu; Shi-Jie Xiong; Shen-Lei Shi

2005-01-01

204

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

205

The effective pore radius of screen wicks

The effective pore radius in screen-wick heat pipes was investigated, which is very important for the prediction of maximum heat transfer rates due to capillary limitation. An equation for the effective pore radius of the screen wicks was derived based on the model of the screen geometry. The capillary height for stainless steel and phosphor bronze screens was measured using water, ethyl alcohol, and Freon 113 as the test liquids. The effect of surface treatment (acid cleaning and oxidation) on the capillary height was also examined. From the comparison of the experimental data for water and ethyl alcohol with those for Freon 113, it was indicated that the contact angle was 24.2{degree} for water and 16.9{degree} for ethyl alcohol. Consequently, it was found that the effective pore radius of the screen wicks could be predicted fairly well from the expression presented in this study, and that the contact angle should be taken into consideration to evaluate the maximum capillary pressure accurately.

Imura, Hideaki [Kumamoto Univ. (Japan); Kozai, Hiroaki [Kyushu Tokai Univ., Kumamoto (Japan); Ikeda, Yuji [Kyocera Corp., Sendai (Japan)

1994-10-01

206

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

207

Myocardial heating by transcatheter delivery of radiofrequency (RF) energy has been proposed as an effective means of arrhythmia ablation. A thermodynamic model describing the radial temperature gradient at steady state during RF-induced heating is proposed. If one assumes that RF power output is adjusted to maintain a constant electrode-tissue interface temperature at all times, then this thermodynamic model predicts that the radius of the RF-induced lesion will be directly proportional to the electrode radius. A total of 76 RF-induced lesions were created in a model of isolated canine right ventricular free wall perfused and superfused with oxygenated Krebs-Henseleit buffer. Electrode radius was varied between 0.75 and 2.25 mm. RF energy (500 kHz) was delivered for 90 seconds, and the power output was adjusted to maintain a constant electrode-tissue interface temperature of 60 degrees C. A strong linear correlation was observed between electrode radius and lesion radius in two dimensions: transverse (p = 0.0001, r = 0.85) and transmural (p = 0.0001, r = 0.89). With these data, the temperature correlation with irreversible myocardial injury in this model was calculated at 46.6-48.8 degrees C. Therefore, the proposed thermodynamic model closely predicts the observed relation between electrode radius and lesion size during RF myocardial heating.

Haines, D.E.; Watson, D.D.; Verow, A.F. (Univ. of Virginia, School of Medicine, Charlottesville (USA))

1990-07-01

208

The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ? c) or perpendicular (K ? c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton ?5 in the K?c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ? c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton-photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers. PMID:23880965

Elgawadi, Amal; Gainer, Gordon; Krasinski, Jerzy

2013-08-21

209

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

210

Several of my recent papers in various areas of scientific and engineering investigations have been found to validate famous atomic theory of Bohr and chemical bonding theory of Pauling. In view of this, I thought of composing a paper to go alongside original Bohr and prior Rutherford work of attempting to launch an atomic (theory) picture using a special technique

Das Ashis

2004-01-01

211

Nonstructural theory of the exciton states in solid rare gases

We develop a simple nonstructural theory of the exciton states in solid rare gases. The potential energy in the Schrödinger equation of the free rare-gas atom is parametrized in the simplest form which reproduces the exact experimental excited atomic levels. The same equation is modified to describe the exciton states in the solid phase simply by introducing the effective mass

L. Resca; R. Resta; S. Rodriguez

1978-01-01

212

Electron-phonon and exciton-phonon bound states

A review is given of the theoretical and experimental work which has shown the possibility of forming bound states of an electron or an exciton with an optical phonon. Most attention is devoted to wide-band systems, in which the width of the electron (or exciton) band is larger than the phonon frequency; this is the normal situation for semiconductors and

Y. B. Levinson; E. I. Rashba

1973-01-01

213

Quantum decoherence in finite size exciton-phonon systems.

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

214

Observation of Dressed Excitonic States in a Single Quantum Dot

We report the observation of dressed states of a quantum dot. The optically excited exciton and biexciton states of the quantum dot are coupled by a strong laser field and the resulting spectral signatures are measured using differential transmission of a probe field. We demonstrate that the anisotropic electron-hole exchange interaction induced splitting between the x- and y-polarized excitonic states

Gregor Jundt; Lucio Robledo; Alexander Högele; Stefan Fält; Atac Imamoglu

2008-01-01

215

Observation of Dressed Excitonic States in a Single Quantum Dot

We report the observation of dressed states of a quantum dot. The optically excited exciton and biexciton states of the quantum dot are coupled by a strong laser field and the resulting spectral signatures are measured using differential transmission of a probe field. We demonstrate that the anisotropic electron-hole exchange interaction induced splitting between the x- and y-polarized excitonic states

Gregor Jundt; Lucio Robledo; Alexander Hogele; Stefan Falt; Atac Imamoùglu

216

Exciton dressing and capture by a photonic band edge

NASA Astrophysics Data System (ADS)

We demonstrate electromagnetically induced anomalous quantum dynamics of an exciton in a photonic band gap (PBG)-quantum well (QW) heterostructure. Within the engineered electromagnetic vacuum of the PBG material, the exciton can propagate through the QW by the emission and reabsorption of virtual photons in addition to the conventional electronic hopping mechanism. When the exciton wave vector and recombination energy nearly coincide with a photonic band edge, the exciton kinetic energy is lowered by 1-10meV through coherent radiative hopping. This capture of the exciton by the photonic band edge is accompanied by strong electromagnetic dressing in which exciton’s renormalized effective mass is 4-5 orders of magnitude smaller than in the absence of the PBG environment. This dressed exciton exhibits a long radiative lifetime characteristic of a photon-atom bound state and is robust to phonon-assisted recombinative decay. By inheriting properties of the PBG electromagnetic vacuum, the bound electron-hole pair becomes a stable, ultramobile quantum excitation.

Yang, Shengjun; John, Sajeev

2007-06-01

217

Exciton Sounding of ?-Particle Induced Radiation Defects in Anthracene

Anthracene crystals have been irradiated ? to ? by ?-particles at 28.3, 30.00 and 32.26 MeV. A laser beam focused to a line ? to the ? direction is used to generate triplet excitons at 50 micron intervals in this direction. The spatial distribution of residual damage is determined from the variations in the delayed fluorescence and triplet exciton lifetime.

S. Arnold; H. T. Hu; M. Pope

1976-01-01

218

Feasibility study of a nuclear exciton laser

NASA Astrophysics Data System (ADS)

Nuclear excitons known from Mössbauer spectroscopy describe coherent excitations of a large number of nuclei—analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free-electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology; but our design should be feasible in the UV regime, for example.

ten Brinke, Nicolai; Schützhold, Ralf; Habs, Dietrich

2013-05-01

219

NASA Astrophysics Data System (ADS)

In the first chapters we give some short comments on the history of ZnO research and on growth, doping, transport, and deep centers, topics which will be covered in more detail by other contributions to this symposium "ZnO-rediscovered". Then we concentrate in a first main chapter on the band-structure, excitons, and polaritons in the regime of linear optics. The second main chapter deals with many particle effects and lasing. It is followed by a short conclusion and outlook.

Klingshirn, C.; Priller, H.; Decker, M.; Brückner, J.; Kalt, H.; Hauschild, R.; Zeller, J.; Waag, A.; Bakin, A.; Thonke, H. Wehmann K.; Sauer, R.; Kling, R.; Reuss, F.; Kirchner, Ch.

220

Optical diode based on exciton-polaritons

NASA Astrophysics Data System (ADS)

We propose theoretically an optical diode based on exciton-polaritons in semiconductor microcavities. A flow of polaritons in the bistable regime is used to send signals through an asymmetric fixed potential that favours the bridging of particles in one direction. Through dynamic modelling of the coherent polariton field, we demonstrate the characteristics of an ideal diode, namely, that the forward signal is fully transmitted while the transmission in the reverse direction tends to zero, without any additional external control. Moreover, the system proves to be robust to the presence of disorder, intrinsic to microcavities, and can function at gigahertz repetition rates.

Espinosa-Ortega, T.; Liew, T. C. H.; Shelykh, I. A.

2013-11-01

221

NSDL National Science Digital Library

This is a task from the Illustrative Mathematics website that is one part of a complete illustration of the standard to which it is aligned. Each task has at least one solution and some commentary that addresses important asects of the task and its potential use. Here are the first few lines of the commentary for this task: Given the height $h$ and volume $V$ of a certain cylinder, Jill uses the formula r=\\sqrt{\\frac{V}{\\pi h}} to compute its radius to be 20 meters. If a s...

222

Exciton Dynamics in Semiconducting Carbon Nanotubes

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

223

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

224

Long-Distance Diffusion of Excitons in Double Quantum Well Structures

In this Letter we report on lateral diffusion measurements of excitons at low temperature in double quantum wells of various widths. The structure is designed so that excitons live up to 30 mus and diffuse up to 500 mum. Particular attention is given to establishing that the transport occurs by exciton motion. The deduced exciton diffusion coefficients have a very

Z. Vörös; R. Balili; D. W. Snoke; L. Pfeiffer

2005-01-01

225

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

226

Experimental methods and analysis of cold and dense dipolar exciton fluids.

We review various aspects of our recent work on dipolar excitons in double quantum well systems. We describe and analyse different possible avenues for obtaining high density and cold dipolar exciton fluids that may enable an observation of quantum phase transitions in excitonic systems. These avenues include free dipolar exciton fluids, dipolar exciton fluids in electrostatic traps and in excitonic rings. We present our experimental and modelling work on the exciton dynamics in such systems, and discuss our current view of the advances made and the challenges that remain in this fast evolving and promising field of research. PMID:21483059

Rapaport, Ronen; Chen, Gang

2007-07-25

227

Mass-Radius Relationships of Rocky Exoplanets

NASA Astrophysics Data System (ADS)

Mass and radius of planets transiting their host stars are provided by radial velocity and photometric observations. Structural models of solid exoplanet interiors are then constructed by using equations of state for the radial density distribution, which are compliant with the thermodynamics of the high-pressure limit. However, to some extent those structural models suffer from inherent degeneracy or non-uniqueness problems owing to a principal lack of knowledge of the internal differentiation state and/or the possible presence of an optically thick atmosphere. We here discuss the role of corresponding measurement errors, which adversely affect determinations of a planet's mean density and bulk chemical composition. Precise measurements of planet radii will become increasingly important as key observational constraints for radial density models of individual solid low-mass exoplanets or super-Earths.

Sohl, Frank; Wagner, Frank W.; Rauer, Heike

2014-04-01

228

Quantum and nanoscale modelling of exciton dynamics in polymeric systems.

One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as photovoltaic solar cells and light emitting diodes, is the exciton diffusion within the polymeric network. Due to the amorphous nature the of polymeric materials, the diffusion of excitons is limited by the energetic and spatial disorder in such systems, which is a consequence not only of the chemical structure of the polymer but also from its morphology at nanoscale. To get a deep understanding on how such effects influence exciton dynamics we performed a quantum molecular dynamics simulations to determine the energetic disorder within the polymer system, and Monte Carlo simulations to study exciton diffusion in three-dimensional (3D) polymer networks that present both spatial and energetic disorder at nanometre scale. Our results show clearly that exciton diffusion in poly(p-phenylenevinylene) (PPV) occurs preferentially in the direction parallel to the electrodes surface for a polymer-based optoelectronic devices with the orientation of the conjugated strands similar to those obtained by the spin-coating technique and the decay of such excitons occurs preferentially in longer strands which allow us to get insight on exciton behaviour in polymeric systems that are not possible to be obtained directly from the experiments. PMID:20352770

Barbosa, H M C; Correia, H M G; Ramos, M M D

2010-02-01

229

Optimizing hydraulic radius and acoustic field of the thermoacoustic engine

It is well known that the acoustic field and the hydraulic radius of the regenerator play key roles in thermoacoustic processes. The optimization of hydraulic radius strongly depends on the acoustic field in the regenerator. This paper investigates the synthetical optimization of the hydraulic radius and the acoustic field which is characterized by the ratio of the traveling wave component

Huifang Kang; Qing Li; Gang Zhou

2009-01-01

230

Role of phonons in Josephson oscillations of excitonic and polaritonic condensates

We analyze theoretically the role of the exciton-phonon interactions in phenomena related to the Josephson effect between two spatially separated exciton and exciton-polariton condensates. We consider the role of the dephasing introduced by phonons in such phenomena as Josephson tunneling, self-trapping and spontaneous polarization separation. In the regime of cw pumping we find a remarkable bistability effect arising from exciton-exciton interactions as well as regimes of self-sustained regular and chaotic oscillations.

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

2010-11-15

231

Systematic investigation of terahertz-induced excitonic Rabi splitting

NASA Astrophysics Data System (ADS)

Weak near-infrared and strong terahertz excitation are applied to study excitonic Rabi splitting in (GaIn)As/GaAs quantum wells. Pronounced anticrossing behavior of the split peaks is observed for different terahertz intensities and detunings relative to the intra-excitonic heavy-hole 1s-2p transition. At intermediate to high electric fields the splitting becomes highly asymmetric and exhibits significant broadening. A fully microscopic theory is needed to explain the experimental results. Comparisons with a two-level model reveal the increasing importance of higher excitonic states at elevated excitation levels.

Teich, M.; Wagner, M.; Stehr, D.; Schneider, H.; Helm, M.; Böttge, C. N.; Klettke, A. C.; Chatterjee, S.; Kira, M.; Koch, S. W.; Khitrova, G.; Gibbs, H. M.

2014-03-01

232

Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals

NASA Astrophysics Data System (ADS)

This work discusses the photocurrent and photoluminescence that can be induced by short-pulse illumination in rubrene single crystals. The pulsed illumination excites a rubrene molecule from the ground state to its first optically accessible excited state, resulting in a singlet exciton state. In rubrene, a singlet exciton can transform into two triplet excitons - which together have a spin of zero - by an efficient spin-conserved fission process. On the other hand, two triplet excitons can interact to again form a singlet exciton by a fusion process. Quantitative modeling of the transformation of singlet excitons into triplet excitons and vice-versa shows that both photoconductivity dynamics and photocurrent dynamics after pulsed excitations can be understood within the same framework. The photoluminescence observed after pulsed excitation is only emitted upon radiative recombination of singlet excitons. A simple model of fission and fusion based on rate equations leads to a qualitatively different photoluminescence dynamics depending on the time scale. In particular, it predicts a fast exponential decay corresponding to the initial fission process, later a power-law (quadratic) decay corresponding to a regime when triplet-triplet interaction is dominant, and a final exponential decay with a time-constant which is half the triplet exciton lifetime. This last exponential decay corresponds to the case when only a lower density of triplet excitons is left. The same model can be used to predict the photocurrent dynamics after pulsed excitation. Experimental observations after pulsed illumination show that, for low excitation pulse energies, a large photocurrent grows exponentially with a time constant of the order of 100 microseconds. This photocurrent build-up time then becomes shorter at higher excitation energies, with the peak photocurrent also saturating. One finds that the observed photocurrent dynamics can be reproduced with the same model based on exciton fission and fusion that successfully explained photoluminescence dynamics. The only additional assumption that is required to do so is that triplet excitons be able dissociate and release free holes by direct interaction with a defect state. The 100 microsecond build-up time of the impulsively induced photocurrent then corresponds to the triplet lifetime.

Lyu, ByungGook

233

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

234

Excitons in conjugated oligomer aggregates, films, and crystals.

Recent experimental and theoretical investigations of excitons in conjugated oligomer nanoaggregates, thin films, and crystals are reviewed. The review focuses on the technologically important unsubstituted oligo-phenylene vinylenes (OPVn) and oligo-thiophenes (OTn), which exhibit side-by-side herringbone crystal packing. Many of the salient photophysical properties displayed by OPVn and OTn solid phases, including the large Davydov splitting, the rich variety of peaks due to vibronic coupling in both absorption and emission, and the unusual behavior of the emission origin, are accounted for in a model including excitonic coupling between molecules, linear exciton-phonon coupling, and disorder. PMID:16599810

Spano, Frank C

2006-01-01

235

Exciton scattering from impurities and acoustic phonons in carbon nanotubes

NASA Astrophysics Data System (ADS)

Exciton scattering from impurities and acoustic phonons in semiconducting carbon nanotubes is theoretically studied in an effective-mass approximation. A selection rule for transitions resulting from the scattering is obtained, showing that the ground-state exciton for the lowest-exciton band is never scattered from long-range impurities and deformation potentials near the band edge. This arises from the symmetry of nanotubes for the successive transformations of the charge conjugation and parity and for the reflection of the coordinate along the nanotube axis. Agreement of numerical results with recent experiments supports the validity of this selection rule.

Fujimoto, Yuta; Uryu, Seiji

2013-12-01

236

Piezoelectric exciton acoustic-phonon coupling in single quantum dots

Micro-photoluminescence spectroscopy at variable temperature, excitation\\u000aintensity and energy was performed on a single InAs\\/AlAs self-assembled quantum\\u000adot. The exciton emission line (zero-phonon line, ZPL) exhibits a broad\\u000asideband due to exciton-acoustic phonon coupling by the deformation potential\\u000amechanism. Additionally, narrow low-energy sidebands at about 0.25 meV of the\\u000aZPL are attributed to exciton-acoustic phonon piezoelectric coupling. In\\u000alowering the

D. Sarkar; H. P. van der Meulen; J. M. Calleja; J. M. Meyer; R. J. Haug; K. Pierz

2008-01-01

237

Electromagnetically Induced Exciton Mobility in a Photonic Band Gap

NASA Astrophysics Data System (ADS)

It is suggested that an exciton in the engineered vacuum of a photonic-band-gap quantum-well heterostructure exhibits electromagnetically induced anomalous quantum dynamics. The exciton is dressed by coherent emission and reabsorption of virtual photons near the photonic band edge and captured in momentum space, lowering its energy by 1 10 meV and lowering its effective mass by 4 5 orders of magnitude. The photonic band gap simultaneously enables strong coupling to confined optical modes and long exciton lifetime.

John, Sajeev; Yang, Shengjun

2007-07-01

238

Hopping process of bound excitons under an energy gradient

NASA Astrophysics Data System (ADS)

We report on the mechanism of hopping for bound excitons under an energy gradient. By means of a Monte-Carlo simulation, we show that this mechanism explains the movement of bound excitons observed experimentally. We show that the speed of the excitons decreases quickly with temperature. Thanks to an effective medium approximation, we deduce an analytical model to estimate the average speed at T = 0 K. Finally, we compare our simulations results to the speed observed in bent ZnO wires and find a good agreement between theory and experiments.

Jacopin, Gwénolé; Shahmohammadi, Mehran; Ganière, Jean-Daniel; Deveaud, Benoêt.

2014-01-01

239

Bose-Einstein condensation of excitons in Cu2O

We present a parameter-free model which estimates the density of excitons in\\u000aCu$_2$O, related to experiments that have tried to create an excitonic\\u000aBose-Einstein condensate. Our study demonstrates that the triplet-state\\u000aexcitons move along adiabats and obey classical statistics, while the\\u000asinglet-state excitons are a possible candidate for forming a Bose-Einstein\\u000acondensate. Finally we show that the results of this

G. M. Kavoulakis

2001-01-01

240

NASA Astrophysics Data System (ADS)

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

Brüggemann, B.; May, V.

2004-02-01

241

Excitonic processes in pure and doped ?

The optical properties of nominally pure 0953-8984\\/11\\/15\\/016\\/img9 crystals as well as of those of pure and 0953-8984\\/11\\/15\\/016\\/img10 and 0953-8984\\/11\\/15\\/016\\/img11-doped 0953-8984\\/11\\/15\\/016\\/img9 powders are compared. The emission band peaking at 3.9 eV and the excitation bands in the region 10-10.6 eV observed for 0953-8984\\/11\\/15\\/016\\/img13 and 0953-8984\\/11\\/15\\/016\\/img14 powders are ascribed to the radiative decay of a self-trapped exciton perturbed by an anion vacancy

V. Denks; A. Maaroos; V. Nagirnyi; T. Savikhina; V. Vassiltsenko

1999-01-01

242

Plasmon-exciton self-induced transparency

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

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

2011-04-15

243

Impurity trapped excitons under high hydrostatic pressure

NASA Astrophysics Data System (ADS)

Paper summarizes the results on pressure effect on energies of the 4fn ? 4fn and 4fn-15d1 ? 4fn transitions as well as influence of pressure on anomalous luminescence in Ln?+ doped oxides and fluorides. A model of impurity trapped exciton (ITE) was developed. Two types of ITE were considered. The first where a hole is localized at the Ln?+ ion (creation of Ln(?+1)+) and an electron is attracted by Coulomb potential at Rydberg-like states and the second where an electron captured at the Ln?+ ion (creation of Ln(?-1)+) and a hole is attracted by Coulomb potential at Rydberg-like states. Paper presents detailed analysis of nonlinear changes of energy of anomalous luminescence of BaxSr1-xF2:Eu2+ (x > 0.3) and LiBaF3:Eu2+, and relate them to ITE-4f65d1 states mixing.

Grinberg, Marek

2013-09-01

244

Exciton-exciton recombination in isolated semiconducting single-walled carbon nanotubes was studied using femtosecond transient absorption. Under sufficient excitation to saturate the optical absorption, we observed an abrupt transition between reaction- and diffusion-limited kinetics, arising from reactions between incoherent localized excitons with a finite probability of ~0.2 per encounter. This represents the first experimental observation of a crossover between classical and critical kinetics in a 1D coalescing random walk, which is a paradigm for the study of nonequilibrium systems. PMID:24266488

Allam, J; Sajjad, M T; Sutton, R; Litvinenko, K; Wang, Z; Siddique, S; Yang, Q-H; Loh, W H; Brown, T

2013-11-01

245

Exciton separation at the interface between pentacene and dipolar chromophore

NASA Astrophysics Data System (ADS)

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

Park, Byoungnam; Park, Jonghoo

2013-11-01

246

Enhanced multiple exciton generation in quasi-one-dimensional semiconductors.

The creation of a single electron-hole pair (i.e., exciton) per incident photon is a fundamental limitation for current optoelectronic devices including photodetectors and photovoltaic cells. The prospect of multiple exciton generation per incident photon is of great interest to fundamental science and the improvement of solar cell technology. Multiple exciton generation is known to occur in semiconductor nanostructures with increased efficiency and reduced threshold energy compared to their bulk counterparts. Here we report a significant enhancement of multiple exciton generation in PbSe quasi-one-dimensional semiconductors (nanorods) over zero-dimensional nanostructures (nanocrystals), characterized by a 2-fold increase in efficiency and reduction of the threshold energy to (2.23 ± 0.03)E(g), which approaches the theoretical limit of 2E(g). Photovoltaic cells based on PbSe nanorods are capable of improved power conversion efficiencies, in particular when operated in conjunction with solar concentrators. PMID:21766838

Cunningham, Paul D; Boercker, Janice E; Foos, Edward E; Lumb, Matthew P; Smith, Anthony R; Tischler, Joseph G; Melinger, Joseph S

2011-08-10

247

Singlet exciton binding energy in poly(phenylene vinylene)

The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band ?-?* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with Eb ? 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.

Moses, D.; Wang, J.; Heeger, A. J.; Kirova, N.; Brazovski, S.

2001-01-01

248

Picosecond kinetics of strongly coupled excitons and surface plasmon polaritons.

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

2013-04-25

249

Hopping approach towards exciton dissociation in conjugated polymers

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

250

Plexcitonics: Coupled and Plasmon-Exciton Systems with Tailorable Properties.

National Technical Information Service (NTIS)

This project focuses on the interactions between plasmon-resonant- 'plasmonic'- materials and structures and molecules, and how the coupling between plasmonic media can modify either molecular-'excitonic' properties. In this three year project we have dem...

N. Halas S. Lal

2013-01-01

251

Quasibosonic exciton dynamics near the semiconductor band edge

NASA Astrophysics Data System (ADS)

We develop a heirarchy of equations that describes the electrodynamics of the semiconductor band edge. An exciton basis that incorporates intrapair Coulomb interaction is used. The excitons are treated as quasibosons that satisfy commutation relations intermediate between bosonic and fermionic. Their nonlinear dynamics are a consequence of phase-space filling and an effective exciton-exciton interaction that depends only on their center-of-mass wave vectors and internal quantum numbers. The resulting expansion in powers of the optical field is analogous to Axt and Stahl's dynamically controlled truncation, but takes a simpler form. If damping is omitted or if all interband and intraband dephasing time constants are the same, we find a set of relationships among the interband and intraband correlation functions which in the free pair basis give the semiconductor Bloch equations.

Hawton, Margaret; Nelson, Delene

1998-02-01

252

Efficient Multi-Exciton Emission from Quantum Dots.

National Technical Information Service (NTIS)

The fundamental spontaneous emission rate an emitter can be modified by its photonic environment. By enhancing the spontaneous emission rate, there is a possibility of extracting multi-exciton energies through radiative decay. In this report, we explore u...

T. S. Luk

2010-01-01

253

PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS

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

254

Experimental study of finite Larmor radius effects

Linear Z-pinches in Ar, Kr, Xe, N/sub 2/, and He are experimentally studied in regimes where strong finite Larmor radius effects could provide a significant stabilizing effect. Scaling arguments show that for deuterium such a pinch has an electron line density of order 2 x 10/sup 15//cm. For higher Z plasmas a higher line density is allowed, the exact value of which depends on the average ion charge. The pinch is formed by puffing gas axially through the cathode towards the anode of an evacuated pinch chamber. When the gas reaches the anode, the pinch bank is fired. The pinch current rises in 2 to 3 ..mu..sec to a maximum of 100 to 200 kA. The pinch bank capacitance is 900 ..mu..F, and the external inductance is 100 nH. Additionally, the bank is fused to increase dI/dt. The primary diagnostics are a framing camera, a spatially resolved Mach-Zehnder interferometer, and X-ray absorption.

Struve, K.W.

1980-08-01

255

New Physics and the Proton Radius Problem

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 moments 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.

Carl E. Carlson, Benjamin C. Rislow

2012-08-01

256

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

257

Optical Spectroscopy on Single Quantum Dots: Charged Excitons

Single self-assembled quantum dots are among the most widely studied systems in the field of modern solid state spectroscopy.\\u000a Neutral multi-exciton complexes have been investigated in quantum dots by power dependent photoluminescence spectroscopy.\\u000a The experimental preparation of specific charged exciton states in a single quantum dot can be realized by bias controlled\\u000a single electron charging and can be probed by

Frank Findeis; Martin Baier; Eric Duijs; Evelin Beham; Max Bichler; Artur Zrenner; Ulrich Hohenester; Elisa Molinari

2001-01-01

258

Phonon-assisted exciton-polariton emission in a microcavity

NASA Astrophysics Data System (ADS)

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

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

1996-05-01

259

Monte Carlo Simulation of Exciton Dynamics in Supramolecular Semiconductor Architectures

Supramolecular chemistry is useful to construct molecular architectures with functional semiconductor properties. To explore the consequences of this approach in molecular electronics, we have carried out ultrafast measurements of exciton dynamics in supramolecular assemblies of an oligo-p-phenyl-ene-vinyl-ene derivative functionalized to form chiral stacks in dodecane solution in a thermotropically reversible manner. We apply a model of incoherent exciton hopping within

Carlos Silva; David Beljonne; Laura Herz; Freek Hoeben

2005-01-01

260

Excitons in single-walled carbon nanotubes: environmental effect

The properties of excitons in semiconducting single-walled carbon nanotubes\\u000a(SWCNTs), isolated in vacuum or a medium, and their contributions to the\\u000aoptical spectra of nanotubes are studied within the elementary potential model,\\u000ain which an exciton is represented as a bound state of two oppositely charged\\u000aquasi-particles confined to the nanotube surface. The emphasis is given on the\\u000ainfluence of

Oleksii A. Smyrnov; Odessa I. I. Mechnikov

2009-01-01

261

Exciton-assisted tunneling transport in heterojunction microstructures

NASA Astrophysics Data System (ADS)

Longitudinal tunneling transport in the low-dimensional heterojunction structures induced by the excitonic Coulomb interaction has been formulated and discussed in the framework of Fermi's golden rule. We have investigated the tunneling transition of free carriers to quantum-well Wannier-Mott excitons incorporated in the sequential tunneling Hamiltonian. The modeling is evaluated by a set of coupled rate equations involving subband states of electron, hole and exciton. The exciton-assisted tunneling (EAT) phenomenon has its characteristic fingerprint causing tunneling current prior to the resonance electric fields, and a significant modulation of the I-V characteristics. It is also found that the bias offset and the FWHM of the EAT current spectrum can be comparable to that of resonant tunneling (RT) current, depending both on the 2D hole density of the confined subband and the excitonic properties in the active region. The EAT effect has a different I-V spectral line shape, compared to that of the RT or its replica, tailing off in the resonance regime induced by the exciton binding energy.

Cao, S. M.; Willander, M.

1997-12-01

262

Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I=2 for excitons inside the light cone, that is, bright excitons. Under moderate strain, the I=2 Dirac cone splits into two degenerate I=1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton’s optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin.

Yu, Hongyi; Liu, Gui-Bin; Gong, Pu; Xu, Xiaodong; Yao, Wang

2014-05-01

263

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

264

Exciton-dominant electroluminescence from a diode of monolayer MoS2

NASA Astrophysics Data System (ADS)

In two-dimensional monolayer MoS2, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS2 fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of a higher energy exciton peak of 2.255 eV in the monolayer MoS2 diode, attributed to the excited exciton state of a direct-exciton transition.

Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Zhao, Mervin; Wang, Yuan; Yin, Xiaobo; Zhang, Xiang

2014-05-01

265

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

266

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 degrees 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 for 25 degrees C are: inflection point of the Bohr-effect curve at pH 7.1, number of Bohr protons -- deltalog p1/2 (O2)/deltapH = 0.36 mol H+/mol O2 and amplitude of the Bohr-effect curve deltalogp1/2 (O2) = 0.84. The substitution of protoporphyrin for mesoporphyrin causes a 10 nm blue-shift of the visible absorption maxima in both, the native and the cobalt-substituted forms of CTT IV. Furthermore, the replacement of vinyl groups by ethyl groups at position 2 and 4 of the porphyrin system leads to an increase of O2 affinities at 25 degrees C which follows the order: proto less than meso less than deutero for iron and cobalt CTT IV, respectively. Again, the Bohr-effect is not affected by the replacement of protoporphyrin for mesoporphyrin or deuteroporphyrin. The electron spin resonance (ESR) spectra of both, deoxy cobalt proto- and deoxy cobalt meso-CTT IV, are independent of pH. The stronger electron-withdrawing effect by protoporphyrin is reflected by the decrease of the cobalt hyperfine constants coinciding with gparallel = 2.035 and by the low-field shift of gparallel. The ESR spectra of oxy cobalt proto- and oxy cobalt meso-CTT IV are dependent of pH. The cobalt hyperfine constants coinciding with gparallel - 2.078 increase during transition from low to high pH. The pH-induced ESR spectral changes correlate with the alkaline Bohr-effect. Therefore, the two O2 affinity states can be assigned to the low-pH and high-pH ESR spectral species. The low-pH form (low-affinity state) is characterized by a smaller, the high-pH form (high-affinity state) by a larger cobalt hyperfine constant in gparallel. The correlation of the cobalt hyperfine constants of the oxy forms with the O2 affinities is discussed for several monomeric haemoglobins. The Co-O-O bond angle in cobalt oxy CTT IV is characterized by an ozonoid type of binding geometry and varies little during the pH-induced conformation transition. Due to the lack of the distal histidine in CTT IV no additional interaction via hydrogen-bonding with dioxygen is possible; this is reflected by the cobalt hyperfine constants. PMID:6284268

Gersonde, K; Twilfer, H; Overkamp, M

1982-01-01

267

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

268

Discharge coefficients of cooling holes with radiused and chamfered inlets

NASA Astrophysics Data System (ADS)

The flow of cooling air within the internal passages of gas turbines is controlled and metered using holes in disks and casings. The effects of inlet radiusing and chamfering of these holes on the discharge coefficient are discussed. Experimental results for a range of radiusing and chamfering ratios for holes of different length to diameter ratios are presented, covering the range of pressure ratios of practical interest. The results indicate that radiusing and chamfering are both beneficial in increasing the discharge coefficient. Increases of 10-30 percent are possible. Chamfered holes give the more desirable performance characteristics in addition to being easier to produce than radiused holes.

Hay, N.; Spencer, A.

1991-06-01

269

Improving optical bench radius measurements using stage error motion data

We describe the application of a vector-based radius approach to optical bench radius measurements in the presence of imperfect stage motions. In this approach, the radius is defined using a vector equation and homogeneous transformation matrix formulism. This is in contrast to the typical technique, where the displacement between the confocal and cat's eye null positions alone is used to determine the test optic radius. An important aspect of the vector-based radius definition is the intrinsic correction for measurement biases, such as straightness errors in the stage motion and cosine misalignment between the stage and displacement gauge axis, which lead to an artificially small radius value if the traditional approach is employed. Measurement techniques and results are provided for the stage error motions, which are then combined with the setup geometry through the analysis to determine the radius of curvature for a spherical artifact. Comparisons are shown between the new vector-based radius calculation, traditional radius computation, and a low uncertainty mechanical measurement. Additionally, the measurement uncertainty for the vector-based approach is determined using Monte Carlo simulation and compared to experimental results.

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

2008-12-20

270

Centrifugal—Barrier Effects and Determination of Interaction Radius

NASA Astrophysics Data System (ADS)

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

Wu, Ning

2014-01-01

271

Electrons, Phonons and Excitons at Semiconductor Surfaces

NASA Astrophysics Data System (ADS)

We briefly address 'state-of-the-art' ab-initio calculations of basic properties of semiconductor surfaces such as their atomic configuration, electronic structure and surface vibrations, as well as, their optical properties and compare exemplary results with experimental data. The surface structure and the electronic ground state are described within the local density approximation (LDA) of density functional theory (DFT). The description of excited electronic states requires to take dynamical correlations in the many electron system into account, which is achieved to a considerable extent within the GW approximation (GWA) leading to the concept of the quasiparticle bandstructure. Surface phonons are treated from first principles within density functional perturbation theory (DFPT). The theory of optical surface properties and surface excitons, in particular, requires to account for the electron-hole Coulomb correlation which is done in the framework of the Bethe-Salpeter equation (BSE). These methods yield results which are in good agreement with experiment and can significantly contribute to an interpretation of experimental data from high-resolution surface microscopy and spectroscopy thus enhancing our current understanding of semiconductor surfaces.

Pollmann, Johannes; Krueger, Peter; Mazur, Albert; Rohlfing, Michael

272

Bose-Einstein condensation of exciton polaritons.

Phase transitions to quantum condensed phases--such as Bose-Einstein condensation (BEC), superfluidity, and superconductivity--have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 10(9) times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase. PMID:17006506

Kasprzak, J; Richard, M; Kundermann, S; Baas, A; Jeambrun, P; Keeling, J M J; Marchetti, F M; Szyma?ska, M H; André, R; Staehli, J L; Savona, V; Littlewood, P B; Deveaud, B; Dang, Le Si

2006-09-28

273

Exciton condensation and perfect Coulomb drag.

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

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

2012-08-23

274

Transport of superradiant excitons in GaAs single quantum wells

NASA Astrophysics Data System (ADS)

The dynamics of resonantly excited 1s-HH excitons in GaAs single quantum wells is studied by picosecond transient reflection and photon echo. The energy relaxation and transport properties of the excitons are probed, making use of the surface electric field. At low exciton densities and 5 K, transport appears to be absent, and superradiant decay takes the place of free excitons with characteristic times ranging from 5 to 13 ps, depending on the well width. The polarization decay is governed by energy relaxation, and a coherence length is found for these free excitons of ~200 nm. Low in the exciton resonance, we measure a much slower radiative decay and smaller coherence lengths, due to localization. Here both radiative decay and pure dephasing contribute to the polarization dephasing rate. At higher exciton densities and temperature, transport is observed, induced by exciton-exciton and exciton-phonon collisions. For free excitons it is demonstrated that the increased polarization dephasing rate is exclusively due to exciton transport, and not to pure dephasing. For localized excitons, pure dephasing processes prevail in T2, and thermally activated transport is found.

Vledder, P.; Akimov, A. V.; Dijkhuis, J. I.; Kusano, J.; Aoyagi, Y.; Sugano, T.

1997-12-01

275

Exciton diffusion in small molecules 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione [DPP(TBFu)2] is studied using first-principles simulations. We have examined dependence of exciton diffusion on structure disorder, temperature and exciton energy. We find that exciton diffusion length and diffusivity increase with structural order, temperature and the initial exciton energy. Compared to conjugated polymer poly(3-hexylthiophene) (P3HT), DPP(TBFu)2 small molecules exhibit a much higher exciton diffusivity, but a shorter lifetime. The exciton diffusion length in DPP(TBFu)2 is 50% longer than that in P3HT, yielding a higher exciton harvesting efficiency; the physical origin behind these differences is discussed. The time evolutions of exciton energy, electron-hole distance, and exciton localization are explored, and the widely speculated exciton diffusion mechanism is confirmed theoretically. The connection between exciton diffusion and carrier mobilities is also studied. Finally we point out the possibility to estimate exciton diffusivity by measuring carrier mobilities under AC electric fields. PMID:24759042

Li, Z; Zhang, X; Lu, G

2014-05-01

276

NASA Astrophysics Data System (ADS)

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

Li, Z.; Zhang, X.; Lu, G.

2014-05-01

277

Exciton-plasmon interactions and energy transfer in nanoparticles

NASA Astrophysics Data System (ADS)

Energy transfer between optically-excited nanocrystals coupled by the Coulomb interaction can be very efficient. The interaction of excitons and plasmons in nanocrystals leads to several effects: energy transfer between nanoparticles (NPs), electromagnetic enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, and interference and non-linear phenomena [1-3]. Using kinetic equations for excitons, we model exciton transport in a NW and explain the origin of the blue shift of exciton emission observed in the recent experiments on hybrid NW-NP assemblies [2]. We also model artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. Using superior optical properties of metal and semiconductor NPs, one can strongly enhance the efficiency of light harvesting [3]. An interaction between a discrete state of exciton and a continuum of plasmonic states can give rise to interference effects (Fano-like asymmetric resonances). These interference effects greatly enhance visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticle and molecules. In the nonlinear regime, the Fano effect becomes strongly amplified [4]. In conclusion, our theory explains present experimental results and also provides motivation for future experiments and applications. Potential applications of dynamical exciton-plasmon systems include sensors and light-harvesting. The above theoretical studies were performed in collaboration with several groups [1-4]. [4pt] [1] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters 6, 984 (2006).[0pt] [2] J. Lee, P. Hernandez, J. Lee, A. Govorov, and N. Kotov, Nature Materials 6, 291 (2007).[0pt] [3] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007); S. Mackowski, S. W"ormke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Br"auchle, Nano Lett. 8, 558 (2008). [0pt] [4] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R.Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature 451, 311 (2008).

Govorov, Alexander

2009-03-01

278

Effects of errors in the solar radius on helioseismic inferences

Frequencies of intermediate-degree f modes of the Sun seem to indicate that the solar radius is smaller than what is normally used in constructing solar models. We investigate the possible consequences of an error in radius on results for solar structure obtained using helioseismic inversions. It is shown that solar sound speed will be overestimated if oscillation frequencies are inverted

Sarbani Basu

1998-01-01

279

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

280

Tool nose radius effects on finish hard turning

Tool nose radius effects on finish turning of hardened AISI 52100 steels have been investigated. Surface finish, tool wear, cutting forces, and, particularly, white layer (phase transformation structures) were evaluated at different machining conditions. Results show that large tool nose radii only give finer surface finish, but comparable tool wear compared to small nose radius tools. Specific cutting energy slightly

Y. Kevin Chou; Hui Song

2004-01-01

281

Equatorial Radius of the Earth a Dynamical Determination.

National Technical Information Service (NTIS)

An interesting variation on the familiar method of determi-ning the earth's equatorial radius a sub e, from a knowledge of the earth's equatorial gravity is suggested. The value of equatorial radius thus found is 6378,142 + or - 5 meters. The associated p...

M. A. Khan

1972-01-01

282

Exciton Transport Simulations in Phenyl Cored Thiophene Dendrimers

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

283

Density functional study of tetraphenylporphyrin long-range exciton coupling.

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

Moore, Barry; Autschbach, Jochen

2012-08-01

284

Density Functional Study of Tetraphenylporphyrin Long-Range Exciton Coupling

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

Moore, Barry; Autschbach, Jochen

2012-01-01

285

The discovery of the S-shaped O2 equilibrium curve and the Bohr effect in 1904 stimulated a fertile and continued research into respiratory functions of blood and allosteric mechanisms in haemoglobin (Hb). The Bohr effect (influence of pH/CO2 on Hb O2 affinity) and the reciprocal Haldane effect (influence of HbO2 saturation on H+/CO2 binding) originate in the Hb oxy-deoxy conformational change and allosteric interactions between O2 and H+/CO2 binding sites. In steady state, H+ is passively distributed across the vertebrate red blood cell (RBC) membrane, and intracellular pH (pHi) changes are related to changes in extracellular pH, Hb-O2 saturation and RBC organic phosphate content. As the Hb molecule shifts between the oxy and deoxy conformation in arterial-venous gas transport, it delivers O2 and takes up CO2 and H+ in tissue capillaries (elegantly aided by the Bohr effect). Concomitantly, the RBC may sense local O2 demand via the degree of Hb deoxygenation and release vasodilatory agents to match local blood flow with requirements. Three recent hypotheses suggest (1) release of NO from S-nitroso-Hb upon deoxygenation, (2) reduction of nitrite to vasoactive NO by deoxy haems, and (3) release of ATP. Inside RBCs, carbonic anhydrase (CA) provides fast hydration of metabolic CO2 and ensures that the Bohr shift occurs during capillary transit. The formed H+ is bound to Hb (Haldane effect) while HCO3- is shifted to plasma via the anion exchanger (AE1). The magnitude of the oxylabile H+ binding shows characteristic differences among vertebrates. Alternative strategies for CO2 transport include direct HCO3- binding to deoxyHb in crocodilians, and high intracellular free [HCO3-] (due to high pHi) in lampreys. At the RBC membrane, CA, AE1 and other proteins may associate into what appears to be an integrated gas exchange metabolon. Oxygenation-linked binding of Hb to the membrane may regulate glycolysis in mammals and perhaps also oxygen-sensitive ion transport involved in RBC volume and pHi regulation. Blood O2 transport shows several adaptive changes during exposure to environmental hypoxia. The Bohr effect is involved via the respiratory alkalosis induced by hyperventilation, and also via the pHi change that results from modulation of RBC organic phosphate content. In teleost fish, beta-adrenergic activation of Na+/H+ exchange rapidly elevates pHi and O2 affinity, particularly under low O2 conditions. PMID:15491402

Jensen, F B

2004-11-01

286

In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I=2 for excitons inside the light cone, that is, bright excitons. Under moderate strain, the I=2 Dirac cone splits into two degenerate I=1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton's optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin. PMID:24821438

Yu, Hongyi; Liu, Gui-Bin; Gong, Pu; Xu, Xiaodong; Yao, Wang

2014-01-01

287

Scalable interconnections for remote indirect exciton systems based on acoustic transport

NASA Astrophysics Data System (ADS)

Excitons, quasiparticles consisting of electron-hole pairs bound by the Coulomb interaction, are a potential medium for the processing of photonic information in the solid state. Information processing via excitons requires efficient techniques for the transport and manipulation of these uncharged particles. We have carried out a detailed investigation of the transport of excitons in GaAs quantum wells by surface acoustic waves. Based on these results, we introduce here a concept for the interconnection of multiple remote exciton systems based on the long-range transport of dipolar excitons by a network of configurable interconnects driven by acoustic wave beams. By combining this network with electrostatic gates, we demonstrate an integrated exciton multiplexer capable of interconnecting, gating, and routing exciton systems separated by millimeter distances. The multiplexer provides a scalable platform for the manipulation of exciton fluids with potential applications in information processing.

Lazi?, S.; Violante, A.; Cohen, K.; Hey, R.; Rapaport, R.; Santos, P. V.

2014-02-01

288

Strongly coupled surface plasmon-exciton excitations in small-diameter carbon nanotubes

We study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. 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 (~0.5-2 eV) interband surface plasmon modes and large exciton excitation energies ~1 eV in small-diameter nanotubes. We estimate the exciton-plasmon

I. V. Bondarev; K. Tatur; L. M. Woods

2008-01-01

289

Motion of neutral and negatively charged excitons in high magnetic fields

NASA Astrophysics Data System (ADS)

The in-plane motion of excitons (X) and negatively charged excitons (X -) in a doped CdTe/CdMgTe quantum well has been measured at 4.2 K in magnetic fields up to 12 T, using spatially resolved photoluminescence (PL) spectroscopy. The populations of X and X - have been found to be in local dynamical equilibrium. The measured PL patterns in real space could therefore be described by considering free and mobile excitons in equilibrium with localised charged excitons.

Pulizzi, F.; Thijssen, W. H. A.; Christianen, P. C. M.; Maan, J. C.; Yakovlev, D. R.; Ossau, W.; Wojtowicz, T.; Karczewski, G.; Kossut, J.

2001-04-01

290

Dephasing of Excitonic Polaritons Confined in GaAs Thin Films

We report the dephasing of excitonic polaritons due to propagation in GaAs thin films measured using a reflection-type pump--probe technique. The rise time of the transient signal due to the propagation of excitonic polaritons decreases with an increase in temperature. This decrease originates from the dephasing of excitonic polaritons. From the analysis based on exciton--phonon scattering, the interaction with optical

Osamu Kojima; Shingo Watanabe; Takashi Kita; Osamu Wada; Toshiro Isu

2011-01-01

291

Exciton Condensation Driving the Periodic Lattice Distortion of 1T-TiSe2

We address the lattice deformation of 1T-TiSe2 within the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in

C. Monney; C. Battaglia; H. Cercellier; P. Aebi; H. Beck

2011-01-01

292

Novel Quantum Condensates in Excitonic Matter

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

Littlewood, P. B.; Keeling, J. M. J.; Simons, B. D. [Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE (United Kingdom); Eastham, P. R. [Blackett Laboratory, Imperial College London, London SW7 2AZ (United Kingdom); Marchetti, F. M. [Departamento de Fisica Teorica de la Materia Condensada, Universidad Autonoma de Madrid, Madrid 28049 (Spain); Szymanska, M. H. [Department of Physics, University of Warwick, Coventry, CV4 7AL (United Kingdom)

2009-08-20

293

Long-range exciton dissociation in organic solar cells

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

294

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

295

Non-Markovian quantum jumps in excitonic energy transfer

NASA Astrophysics Data System (ADS)

We utilize the novel non-Markovian quantum jump (NMQJ) approach to stochastically simulate exciton dynamics derived from a time-convolutionless master equation. For relevant parameters and time scales, the time-dependent, oscillatory decoherence rates can have negative regions, a signature of non-Markovian behavior and of the revival of coherences. This can lead to non-Markovian population beatings for a dimer system at room temperature. We show that strong exciton-phonon coupling to low frequency modes can considerably modify transport properties. We observe increased exciton transport, which can be seen as an extension of recent environment-assisted quantum transport concepts to the non-Markovian regime. Within the NMQJ method, the Fenna-Matthew-Olson protein is investigated as a prototype for larger photosynthetic complexes.

Rebentrost, Patrick; Chakraborty, Rupak; Aspuru-Guzik, Alán

2009-11-01

296

Quantum confinement-induced tunable exciton states in graphene oxide

NASA Astrophysics Data System (ADS)

Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology.

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

2013-07-01

297

Multisubband infrared dressing of excitons in quantum wells

NASA Astrophysics Data System (ADS)

A formalism is described that self-consistently treats the effects of infrared (ir) coupling fields on the interband quantum-well absorption spectrum when both the e1-e2 and e2-e3 conduction subband transitions are nearly resonant with the ir frequency. For the 8.5-nm-wide GaAs quantum-well samples typically considered for ir dressing studies, we show that the simultaneous two-photon coupling of the e1-hh1 and e3-hh1 excitons and the one-photon coupling of all three excitons produces significantly different results than those obtained when the e2-e3 coupling is neglected. In particular we find that the position and shape of the dominant e1-hh1 exciton feature each change nonmonotonically, and saturate with increasing ir intensity. This is in better agreement with existing experimental results than previous calculations based on simple e1-e2 coupling.

Sadeghi, S. M.; Young, J. F.; Meyer, J.

1997-12-01

298

Quantum confinement-induced tunable exciton states in graphene oxide

Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology.

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

2013-01-01

299

Dual Aharonov-Casher effect in singlet-exciton systems

NASA Astrophysics Data System (ADS)

Dual Aharonov-Casher (DAC) phase (or He-McKellar-Wilkens phase), which is defined as a quantum topological phase acquired by a neutral particle only with an electric dipole moment ?E being taken on a closed path around a magnetic monopole wire, is theoretically investigated in singlet-exciton systems. In the Sangster's interference scheme, a moving 2s exciton in magnetic fields feels an effective electric field in co-moving frame, which causes a superposition of opposite parity states, i.e., |2s> and |2p>, due to the motional Stark effect. The superposition gives rise to a nonvanishing electric dipole moment required for the DAC effect to the exciton. The accumulated phase is determined by detecting photon emissions from 2p states as a function of applied magnetic fields.

Hatakenaka, Noriyuki; Nishida, Munehiro; Kumagai, Masami; Takayanagi, Hideaki

2003-05-01

300

Piezoelectric exciton acoustic-phonon coupling in single quantum dots

NASA Astrophysics Data System (ADS)

Microphotoluminescence spectroscopy at variable temperature, excitation intensity, and energy was performed on a single InAs/AlAs self-assembled quantum dot. The exciton emission line [zero-phonon line (ZPL)] exhibits a broad sideband due to exciton acoustic-phonon coupling by the deformation-potential mechanism. Additionally, narrow low-energy sidebands at about 0.25 meV of the ZPL are attributed to exciton acoustic-phonon piezoelectric coupling. In lowering the excitation energy or intensity these bands gradually dominate the emission spectrum of the quantum dot while the ZPL disappears. At high excitation intensity the sidebands due to piezoelectric coupling decrease strongly and the ZPL dominates the spectrum as a consequence of screening of the piezoelectric coupling by the photocreated free carriers.

Sarkar, D.; van der Meulen, H. P.; Calleja, J. M.; Meyer, J. M.; Haug, R. J.; Pierz, K.

2008-12-01

301

Exciton-assisted optomechanics with suspended carbon nanotubes

NASA Astrophysics Data System (ADS)

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

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

2012-11-01

302

Exciton coupling induces vibronic hyperchromism in light-harvesting complexes

NASA Astrophysics Data System (ADS)

The recently suggested possibility that weak vibronic transitions can be excitonically enhanced in light-harvesting complexes is studied in detail. A vibronic exciton dimer model that includes ground-state vibrations is investigated using the multi-configuration time-dependent Hartree method with a parameter set typical to photosynthetic light-harvesting complexes. The absorption spectra are discussed based on the Coulomb coupling, the detuning of the site energies, and the number of vibrational modes. Fluorescence spectra calculations show that the spectral densities obtained from the low-temperature fluorescence line-narrowing measurements of light-harvesting systems need to be corrected for the effects of excitons. For the J-aggregate configuration, as in most light-harvesting complexes, the true spectral density has a larger amplitude than that obtained from the measurement.

Schulze, Jan; Torbjörnsson, Magne; Kühn, Oliver; Pullerits, Tõnu

2014-04-01

303

Bose condensation of two-dimensional dipolar excitons: Simulation by the quantum Monte Carlo method

The Bose condensation of two-dimensional dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system.

Lozovik, Yu. E.; Kurbakov, I. L. [Russian Academy of Sciences, Institute of Spectroscopy (Russian Federation); Astrakharchik, G. E. [Polytechnic University of Catalonia E-08034 (Spain); Willander, M. [Linkoeping University SE-581 83, Institute of Science and Technology (ITN) (Sweden)], E-mail: lozovik@isan.troitsk.ru

2008-02-15

304

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

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

1998-03-01

305

Excitonic splittings in jet-cooled molecular dimers.

In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced. PMID:21678767

Ottiger, Philipp; Leutwyler, Samuel

2011-01-01

306

Isotope effects on vibrational excitons in carbon dioxide particles

NASA Astrophysics Data System (ADS)

The vibrational exciton approach is used to answer a long-standing question concerning the infrared spectra of CO 2 particles: What causes the characteristic structure of the infrared absorption bands? The CO 2 particles which are generated in a collisional cooling cell at 78 K consist of many thousands of molecules. Isotopically pure ( 12CO2 or 13CO2) particles as well as isotopically mixed ( 12CO2/ 13CO2) particles are investigated. Combining the experimental observations with exciton calculations, the band structure in the infrared spectra can be traced back to the shape of the particles.

Signorell, Ruth; Kunzmann, Marc K.

2003-04-01

307

Evidence of Hybrid Excitons in Weakly Interacting Nanopeapods

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

2013-01-01

308

Excitonic collapse in semiconducting transition-metal dichalcogenides

NASA Astrophysics Data System (ADS)

Semiconducting transition metal dichalcogenides (STMDC) are two-dimensional (2D) crystals characterized by electron-volt-size band gaps, spin-orbit coupling (SOC), and d-orbital character of its valence and conduction bands. We show that these materials carry unique exciton quasiparticles (electron-hole bound states) with energy within the gap but which can “collapse” in the strong coupling regime by merging into the band structure continuum. The exciton collapse seems to be a generic effect in these 2D crystals.

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

2013-11-01

309

An exciton-polariton mediated all-optical router

NASA Astrophysics Data System (ADS)

We propose an all-optical nonlinear router based on a double barrier gate connected to periodically modulated guides. A semiconductor microcavity is driven nonresonantly in-between the barriers to form an exciton-polariton condensate on a discrete state that is subject to the exciton blueshift. The subsequent coherent optical signal is allowed to propagate through a guide provided that the condensate energy is resonant with a miniband or is blocked if it faces a gap. While a symmetric sample operates as an optical switch, its asymmetric counterpart embodies a router turned to be polarization selective under applied magnetic field.

Flayac, H.; Savenko, I. G.

2013-11-01

310

Exciton States in a Quantum Dot with Parabolic Confinement

NASA Astrophysics Data System (ADS)

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

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

311

Plasmon and exciton superconductivity mechanisms in layered structures

NASA Technical Reports Server (NTRS)

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

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

1977-01-01

312

Scaling and universality in the optics of disordered exciton chains.

The joint probability distribution of exciton energies and transition dipole moments determines a variety of optical observables in disordered exciton systems. We demonstrate numerically that this distribution obeys a one-parameter scaling, originating from the fact that both the energy and the dipole moment are determined by the number of coherently bound molecules. A universal underlying distribution is found, which is identical for uncorrelated Gaussian disorder in the molecular transition energies or in the intermolecular transfer interactions. The universality breaks down for disorder in the transfer interactions resulting from variations in the molecular positions. We suggest the possibility to probe the joint distribution by means of single-molecule spectroscopy. PMID:18518623

Klugkist, J A; Malyshev, V A; Knoester, J

2008-05-30

313

Dynamics of Photogenerated Polaron-Excitons in Organic Semiconductors

NASA Astrophysics Data System (ADS)

In this work we performed numerical simulations of one ?-conjugated polymer chain subjected to photogeneration. Within the SSH model modi?ed to include the Brazoviskii-Kirova symmetry breaking term, we investigate the dynamics of photoexcitations to address the generation mechanism of polaron-excitons using the unrestricted Hartree-Fock approximation. It was obtained that after the photoexcitation the system relaxes spontaneously into a polaron-exciton in a transient state in a range of 200 fs. Our results also show that charged polarons are generated directly after this transient state.

Junior, Luiz A. Ribeiro; Neto, Pedro H. Oliveira; da Cunha, Wiliam F.; Silva, Geraldo M. e.

314

Coherent exciton delocalization in strongly coupled quantum dot arrays.

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

2013-10-01

315

Resonance Raman spectroscopy has been employed to detect the iron-proximal histidine stretching mode in deoxyhemoglobins from insect larvae of Chironomus thummi thummi (CTT). With the excitation of 413.1 nm, we observe a sharp and intense line in the 220-224 cm-1 region. The assignment of this line to the Fe-N epsilon (His) stretching mode was made on the basis of a 3-cm-1 shift upon 57Fe/54Fe isotope substitution. The Fe-N epsilon (His) vibration is used to monitor the possible changes in the Fe-N epsilon (His) bond strength (hence bone length) in the deoxy state of the monomeric (CTT I, III, and IV) and dimeric (CTT II) insect hemoglobins. As these hemoglobins differ in O2 affinity, off-rate and on-rate constants, and in the Bohr effect, they are excellent model systems for investigating the mechanism of protein control of the heme reactivity. Some of these hemoglobins (CTT III, IV, and II) are allosteric, exhibiting two interconvertible conformational states with high and low O2 affinity at high and low pH, respectively. The Fe-N epsilon (His) stretching frequency does not correlate with the O2 affinity, the on-rate and the off-rate constants for different hemoglobins, for different conformational states, and for modified hemoglobins with different heme peripheral groups. This vibrational mode is insensitive to deuteration of the heme vinyl groups. It is important to note that the Fe-N epsilon (His) bonds in the high pH (high-affinity) and the low pH (low-affinity) states are identical. This implies that the O2 molecule, prior to binding, "sees" identical binding sites. Thus, the difference in free energy changes upon O2 binding is manifested only in the oxy form. PMID:4044602

Kerr, E A; Yu, N T; Gersonde, K; Parish, D W; Smith, K M

1985-10-15

316

We demonstrate the strong coupling of surface lattice resonances (SLRs)--hybridized plasmonic/photonic modes in metallic nanoparticle arrays--to excitons in Rhodamine 6G molecules. We investigate experimentally angle-dependent extinction spectra of silver nanorod arrays with different lattice constants, with and without the Rhodamine 6G molecules. The properties of the coupled modes are elucidated with simple Hamiltonian models. At low momenta, plasmon-exciton-polaritons--the mixed SLR/exciton states--behave as free-quasiparticles with an effective mass, lifetime, and composition tunable via the periodicity of the array. The results are relevant for the design of plasmonic systems aimed at reaching the quantum degeneracy threshold, wherein a single quantum state becomes macroscopically populated. PMID:24216963

Rodriguez, S R K; Rivas, J Gómez

2013-11-01

317

The Evolution of Distal Radius Fracture Management - A Historical Treatise

Distal radius fractures have been a common affliction for millennia, but their treatment is a more recent development as a result of human erudition. While immobilization has served as the only available treatment for most of our history, many advances have been made in the management of distal radius fractures over the last century as the field of orthopedics has grown. Yet, the topic remains hotly contested in the literature, and research continues to focus upon it given the frequency of the injury. In this article, we chronicle the evolution of distal radius fracture treatment in hopes of providing context for the future that lies ahead.

Diaz-Garcia, Rafael J.; Chung, Kevin C.

2012-01-01

318

Study of a non-intrusive electron beam radius diagnostic

The authors have evaluated the usefulness and limitation of a non-intrusive beam radius diagnostic which is based on the measurement of the magnetic moment of a high-current electron beam in an axisymmetric focusing magnetic field, and relates the beam root-mean-square (RMS) radius to the change in magnetic flux through a diamagnetic loop encircling the beam. An analytic formula that gives the RMS radius of the electron beam at a given axial position and a given time is derived and compared with results from a 2-D particle-in-cell code. The study has established criteria for its validity and optimal applications.

Kwan, T.J.T.; DeVolder, B.G.; Goldstein, J.C.; Snell, C.M.

1997-12-01

319

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS2. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and k .p perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.

Glazov, M. M.; Amand, T.; Marie, X.; Lagarde, D.; Bouet, L.; Urbaszek, B.

2014-05-01

320

Nanosecond exciton recombination dynamics in colloidal CdSe quantum dots under ambient conditions

NASA Astrophysics Data System (ADS)

In solution, CdSe quantum dot exciton recombination is composed of an intrinsic band edge exciton decay and a photoinduced charged exciton, which produces the observation of biexponential decay dynamics. The nearly identical radiative lifetimes of both intrinsic and extrinsic decays scale with the cube of the size, and the nonradiative rate for the intrinsic decay follows the energy gap law, while the charged exciton exhibits strong lattice perturbations arising from Frölich coupling to optical phonons. The charged exciton, which arises from recombination in a particle that has a photo-induced trapped carrier has a temporally resolved Stark shift of ~20 meV.

Javier, Artjay; Magana, Donny; Jennings, Travis; Strouse, Geoffrey F.

2003-08-01

321

Exciton condensation driving the periodic lattice distortion of 1T-TiSe2.

We address the lattice deformation of 1T-TiSe2 within the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in experiment. This is thus the first quantitative estimation of the amplitude of the periodic lattice distortion observed in 1T-TiSe2 as a consequence of the exciton condensate phase. PMID:21469817

Monney, C; Battaglia, C; Cercellier, H; Aebi, P; Beck, H

2011-03-11

322

Exciton Condensation Driving the Periodic Lattice Distortion of 1T-TiSe2

NASA Astrophysics Data System (ADS)

We address the lattice deformation of 1T-TiSe2 within the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in experiment. This is thus the first quantitative estimation of the amplitude of the periodic lattice distortion observed in 1T-TiSe2 as a consequence of the exciton condensate phase.

Monney, C.; Battaglia, C.; Cercellier, H.; Aebi, P.; Beck, H.

2011-03-01

323

Gaussian beam modeling of the radius of curvature

NASA Astrophysics Data System (ADS)

A Gaussian model of the radius measurement of micro-optics has been developed and tested using simulations. The model is based on the propagation distances in the interferometer, a heretofore uninvestigated effect. The goal of the model is to determine the bias error in the radius due to the Gaussian Beam propagation model. After testing the model with varying conditions, we have concluded the following: the measured part is smaller than the input, the cat's eye and confocal positions have approximately the same error, radius error increases with smaller test parts, decreasing the numerical aperture increases the errors, and the propagation distances do not affect the radius. The outline of the experimental plan to be used to verify the results is given.

Medicus, Kate M.; Snyder, James; Davies, Angela D.

2005-08-01

324

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

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

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

2011-05-23

325

Split radius-form blocks for tube benders

NASA Technical Reports Server (NTRS)

Two-piece, radius-form block permits accurate forming and removing of parts with more than a 180 degree bend. Tube bender can shape flexible metal tubing in applications dealing with plumbing, heating, and pressure transmission lines.

Lange, D. R.; Seiple, C. W.

1970-01-01

326

Influence of the Finite Larmor Radius Effect on Plasma Equilibrium.

National Technical Information Service (NTIS)

Equilibria of a small beta plasma immersed in a magnetic field with parallel straight lines of force are considered. It is shown that finite ion Larmor radius effects (in hydrodynamical description of plasma these effects correspond to accounting for coll...

G. V. Stupakov

1984-01-01

327

Recovering lost excitons in organic photovoltaics using a transparent dissociation layer

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

328

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

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

2009-08-21

329

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

330

Exciton condensation in microcavities under three-dimensional quantization conditions

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

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

2013-11-15

331

Excitonic absorption and Urbach's tail in bismuth sulfide single crystals

NASA Astrophysics Data System (ADS)

The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from 29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds to a strong exciton-phonon coupling. The exciton binding energy is estimated (28±3 meV) and then the energy gap at 29 K is obtained (E g =1.523±0.003 eV). The fundamental electronic transition has been found to be a strongly anisotropic allowed direct transition. From reflectivity measurements a localized level at 1.361 eV at 29 K has been found. The change of the gap with temperature is interpreted through an electron-phonon mechanism.

Cantarero, A.; Martinez-Pastor, J.; Segura, A.; Chevy, A.

1988-08-01

332

Organic photosensitive optoelectronic device having a phenanthroline exciton blocking layer

An organic photosensitive optoelectronic device, having an anode, a cathode, and an organic blocking layer between the anode and the cathode is described, wherein the blocking layer comprises a phenanthroline derivative, and at least partially blocks at least one of excitons, electrons, and holes.

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

2011-02-22

333

Electric field control of exciton states in quantum dot molecules

Semiconductor nanostructures have attracted considerable interest during the recent years in view of the potential application in quantum information processing. In particular, quantum dots have been suggested to fulfill an essential requirement for quantum computation: controllable interaction that couples two quantum dot qubits. Previous experiments on two vertically aligned quantum dots have demonstrated the formation of coupled exciton states. We

G. Ortner; M. Bayer; A. Kress; A. Forchel; Y. B Lyanda-Geller; T. L Reinecke

2004-01-01

334

Ultrafast exciton dissociation at donor/acceptor interfaces

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

335

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

336

Temperature Dependence of Exciton Reflection Spectra in Monoclinic Zinc Diphosphide

Reflection spectra near the fundamental edge of monoclinic ZnP2 crystal have been measured at various temperatures from 2 K to 200 K. As the temperature is raised from 2 to 80 K, minimum value of the reflectivity Rmin due to the lowest 1s exciton is found to decrease. The Rmin turns to increase above 80 K. The model calculations of

Osamu Arimoto; Minoru Tachiki; Kaizo Nakamura

1991-01-01

337

Optical generation of excitonic valley coherence in monolayer WSe2

NASA Astrophysics Data System (ADS)

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.

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

2013-09-01

338

Optical generation of excitonic valley coherence in monolayer WSe2.

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

2013-09-01

339

Time-domain evidence for an excitonic insulator

NASA Astrophysics Data System (ADS)

Time- and angle-resolved photoemission spectroscopy with a high-harmonic- generation source is used to classify the potential excitonic insulator 1T-TiSe2 and the reference Peierls-Mott insulator 1T-TaS2 on the basis of the melting times of "spectroscopic order parameters".

Hellmann, S.; Rohwer, T.; Kalläne, M.; Hanff, K.; Carr, A.; Murnane, M. M.; Kapteyn, H. C.; Kipp, L.; Bauer, M.; Rossnagel, K.

2013-03-01

340

Advanced theory of multiple exciton generation effect in quantum dots

NASA Astrophysics Data System (ADS)

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

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

2012-06-01

341

Fine structure of excitons and polariton dispersion in quantum wells

NASA Astrophysics Data System (ADS)

The fine structure of excitons, caused by the electron-hole exchange interaction, is modified in quantum wells due to the reduced symmetry and increased Coulomb interaction. We present a systematic formulation of the exchange interaction for quantum-well excitons and perform numerical calculations of the fine-structure splitting and dispersion for Al0.3Ga0.7As/GaAs quantum wells. The coupling of quantum-well excitons with the electromagnetic field is formulated in second quantization. By solving the coupled equations of motion for excitons and photons we calculate the dispersion relations for localized and resonant polaritons as well as the radiative lifetime for the resonant modes. Our results agree with those obtained by Andreani and Bassani [Phys. Rev. B 41, 7536 (1990)] and Tassone, Bassani, and Andreani [II Nuovo Cimento D 12, 1673 (1990)], who used Maxwell's equations instead of the quantum field formulation. We draw some conclusions on the fine-structure splitting expected for excitions in quantum wells of II-VI semiconductors.

Jorda, S.; Rössler, U.; Broido, D.

1993-07-01

342

Light-exciton coupling in semiconductor micro- and nano- structures

NASA Astrophysics Data System (ADS)

The optical properties of planar semiconductor microstructures and three-dimensional nanostructures containing narrow linewidth In0.04Ga0.96As quantum wells are studied in this dissertation. The interaction of quantum- well excitons with light in environments different from free space gives a pronounced effect on the optical response. N periodically arranged quantum wells are coupled to each other by light leading to N exciton- polariton eigenmodes. Each eigenmode is characterized by a distinct energy and radiative lifetime depending on the periodicity of the quantum wells. For a period of about half the excitonic transition wavelength, linear measurements of reflection, transmission, and absorption show significant features of the light-coupled eigenmodes. At Bragg periodicity, the oscillator strengths of all quantum well excitons are concentrated into one superradiant mode resulting in an N times increased radiative decay rate. The slope of the reflectivity linewidths versus N gives the radiative linewidth of the quantum well exciton. For off-Bragg periodicity, however, other eigenmodes become optically active and show their features in reflection and absorption spectra. Oxide-aperture three-dimensional nanocavities containing a single quantum well are investigated. The discrete transverse modes due to the lateral confinement of the optical field are observed in empty cavities with various aperture sizes. The linewidth measurements of the cavity modes show quality-factor values around 2000 for aperture diameters down to 2 ?m. This is high enough to give a strong light-coupling effect in the nonperturbative regime, named normal mode coupling or vacuum Rabi splitting. The anti-crossing behavior of exciton and cavity modes for a 2 ?m diameter aperture cavity is measured in transmission by temperature tuning of the exciton resonance through the lowest transverse cavity mode. A minimum splitting value of 3.9 meV and a splitting-to-linewidth ratio of 4.9 are obtained. Then, nonlinear pump-probe measurements on nanocavities with several aperture sizes are performed. The transition from the nonperturbative regime to the weak coupling regime is observed as the pump power increases. From the measured saturation powers for various aperture diameters, a photon density of 90 photons/?m 2 is found necessary to saturate the normal-mode peaks. The effect of quantum fluctuations of the light field in the nonperturbative regime of planar semiconductor microcavities containing quantum wells is studied. A pronounced third transmission peak lying spectrally between the two normal modes is observed in resonant single-beam-transmission and pump-probe measurements. Measurements on three-dimensional nanocavities confirm the important role of guided modes for this intriguing effect.

Lee, Eun Seong

2001-10-01

343

The effective Bohr magneton number {micro}{sub eff} of Cu(II) adsorbed at the boehmite/water interface is 14% to 21% less than anticipated for a uniform dispersal of Cu(II) ions throughout the interface. Insufficient evidence for exchange coupling indicates the lost electron paramagnetic resonance signal intensity must largely result from spin-spin relaxation. Polynuclear Cu(II) clusters at the silica/water interface are superparamagnetic with a blocking temperature between 5 K and 7 K. Superparamagnetic Cu(II) clusters at the silica/water interface do not saturate at magnetic fields as high as 10,000 G.

Xia, K.; Bleam, W.F.; Helmke, P.A. [Univ. of Wisconsin, Madison, WI (United States). Dept. of Soil Science] [Univ. of Wisconsin, Madison, WI (United States). Dept. of Soil Science; Taylor, R.W. [Alabama A and M Univ., Normal, AL (United States). Plant and Soil Science Dept.] [Alabama A and M Univ., Normal, AL (United States). Plant and Soil Science Dept.

1998-03-01

344

Imaging heterogeneous ultrafast exciton dynamics in organic semiconducting thin films

NASA Astrophysics Data System (ADS)

In solid state semiconducting molecular materials used in electro-optical applications, relatively long exciton diffusion lengths hold the promise to boost device performance by relaxing proximity constraints on the locations for light absorption and interfacial charge separation. The architecture of such materials determines their optical and electronic properties as a result of spacing- and orientation-dependent Coulomb couplings between adjacent molecules. Exciton character and dynamics are generally inferred from bulk optical measurements, which can present a severe limitation on our understanding of these films because their constituent molecules are not perfectly ordered. Rather, films of small organic molecules are composed of multiple microcrystalline domains, and this deposition-dependent microstructure can have profound impacts on transport properties. Using ultrafast transient absorption microscopy, we track the time evolution of excitons, domain by domain, in solid state thin films of TIPS-pentacene, a small soluble molecule that has recently been used in organic semiconducting devices because of its high hole mobility. The results from this spatially-resolved nonlinear optical spectroscopy support our hypothesis that bulk optical measurements deleteriously average over heterogeneities in both spatial and electronic structure; we have revealed significant inhomogeneity in exciton dynamics. Domains that appear homogeneous in linear optical microscopy are shown to have spatial variation and defects, and notable differences in exciton character and behavior are observed at domain boundaries. To interpret the contrast we observe with ultrafast dynamics, we correlate our data to local linear absorption, polarization analysis, profilometry, and atomic force microscopy. With this combined approach, we aim to ultimately understand fundamental structure-function relationship in molecular materials to provide predictive power to material development and device efficiency.

Ginsberg, Naomi S.

2013-03-01

345

Excitons and magnetic fields in bidimensional quantum rings

NASA Astrophysics Data System (ADS)

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

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

2003-03-01

346

Temperature-insensitive optical alignment of the exciton in nanowire-embedded GaN quantum dots

NASA Astrophysics Data System (ADS)

We report on the exciton spin dynamics of nanowire-embedded GaN/AlN quantum dots (QDs) investigated by time-resolved photoluminescence spectroscopy. Under a linearly polarized quasiresonant excitation, we evidence the quenching of the exciton spin relaxation and a temperature-insensitive degree of the exciton linear polarization, demonstrating the robustness of the optical alignment of the exciton spin in these nanowire-embedded QDs. A detailed examination of the luminescence polarization angular dependence shows orthogonal linear exciton eigenstates.

Balocchi, A.; Renard, J.; Nguyen, C. T.; Gayral, B.; Amand, T.; Mariette, H.; Daudin, B.; Tourbot, G.; Marie, X.

2011-12-01

347

NASA Astrophysics Data System (ADS)

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

Malkova, Natalia; Bryant, Garnett W.

2010-10-01

348

Effects of errors in the solar radius on helioseismic inferences

NASA Astrophysics Data System (ADS)

Frequencies of intermediate-degree f modes of the Sun seem to indicate that the solar radius is smaller than what is normally used in constructing solar models. We investigate the possible consequences of an error in radius on results for solar structure obtained using helioseismic inversions. It is shown that solar sound speed will be overestimated if oscillation frequencies are inverted using reference models with a larger radius. Using solar models with a radius of 695.78 Mm and new data sets, the base of the solar convection zone is estimated to be at a radial distance of 0.7135+/-0.0005 of the solar radius. The helium abundance in the convection zone as determined using models with an OPAL equation of state is 0.248+/-0.001, where the errors reflect the estimated systematic errors in the calculation, the statistical errors being much smaller. Assuming that the OPAL opacities used in the construction of the solar models are correct, the surface Z/X is estimated to be 0.0245+/-0.0006.

Basu, Sarbani

1998-08-01

349

Enhanced multiple exciton dissociation from CdSe quantum rods: the effect of nanocrystal shape.

A unique ability of semiconductor nanocrystals (NCs) is the generation and accommodation of multiple excitons through either optical or electric current pumping. The development and improvement of NC-based optoelectronic devices that utilize multiple excitons requires the understanding of multiple exciton dynamics and their efficient conversion to emitted photons or external charges prior to exciton-exciton annihilation. Here, we demonstrate that significantly enhanced multiexciton dissociation efficiency can be achieved in CdSe quantum rods (QRs) compared to CdSe quantum dots (QDs). Using transient absorption spectroscopy, we reveal the formation of bound one-dimensional exciton states in CdSe QRs and that multiple exciton Auger recombination occurs primarily via exciton-exciton collision. Furthermore, quantum confinement in the QR radial direction facilitates ultrafast exciton dissociation by interfacial electron transfer to adsorbed acceptors. Under high excitation intensity, more than 21 electrons can be transferred from one CdSe QR to adsorbed methylviologen molecules, greatly exceeding the multiexciton dissociation efficiency of CdSe QDs. PMID:22702343

Zhu, Haiming; Lian, Tianquan

2012-07-11

350

NASA Astrophysics Data System (ADS)

We show through a novel general field theory argument that for the very same reason that excitons are bright, i.e. emitting photons, they have a higher energy than dark excitons, whatever the carrier spatial configurations is, i.e., even in stressed geometry or for electrons well separated from holes as in a double quantum well structure. Indeed, the same channel which produces the necessary finite electron-hole effective overlap to make them bright, allows for Coulomb interband exchange processes, which are nothing but a sequence of virtual recombination and creation of one electron-hole pair, a fact known in relativistic quantum field theory but never extended to semiconductor physics. The repulsive electron-hole Coulomb exchange interaction, which exists for bright excitons, but not for dark excitons, pushes the bright exciton energy up. If we now remember that dark excitons with spins ±2 are formed in a natural way through carrier exchange between opposite spin bright excitons, we here predict that in a double quantum well sample with one parabolic trap-a configuration quite appropriate to get a high density-exciton Bose-Einstein condensation should appear, when cooling down the sample, as a dark spot made of (±2) excitons at the center of the trap. In this paper, we also suggest a possible link between the observed ring structure in a double quantum well and the formation of dark exciton condensate.

Combescot, Monique; Leuenberger, Michael N.

2009-04-01

351

Nonradiative Relaxation Processes of Excitons in ?-ZnP 2 Studied by Photocalorimetric Spectroscopy

NASA Astrophysics Data System (ADS)

Nonradiative relaxation processes of excitons have been investigated by means of photocalorimetric (PC) spectroscopy in the exciton resonance region of ?-ZnP2 by detecting temperature increase of the crystal under excitation with a Ti:sapphire laser at 4.5 K. For singlet excitons, power of the light absorbed by the crystal is converted to that of the thermal heating of the crystal almost at a constant rate as high as about 80% in the energy range from the 1s longitudinal exciton energy E L to the band gap energy E g. The heat generation, however, is largely suppressed in the 1s reflection band, where the exciton luminescence is also decreased in intensity, which suggests the presence of other optical process in this region such as resonant Rayleigh scattering. From comparison of the PC spectrum with excitation spectrum for the singlet exciton luminescence, it is found that a ratio of the PC signal to the luminescence intensity increases at the exciton resonances n=1, 2, 3 and 4. This fact indicates that the rate of the nonradiative decay of excitons relative to the emission of the luminescence is high at the exciton resonances. The PC spectrum for triplet excitons is also presented.

Arimoto, Osamu; Umemoto, Shigeo; Nakamura, Kaizo

2000-11-01

352

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

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. -- Highlights: •New composite boson many-body theory is used to derive exactly the biexciton Schrödinger equation using the exciton basis. •We solved the 2D and 3D biexciton ground- and excited-state binding energies for various electron-to-hole mass ratios. •The absorption spectrum shows an asymmetric low-energy peak identified with the biexciton ground state. •High-energy Lorentzian-like peaks in the absorption spectrum are associated with the exciton–exciton scattering states. •The exciton gas momentum distribution can be determined by the absorption spectrum via the biexciton wave functions.

Shiau, Shiue-Yuan, E-mail: shiau.sean@gmail.com [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China) [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China); Department of Physics, National Cheng Kung University, Tainan, 701, Taiwan (China); Combescot, Monique [Institut des NanoSciences de Paris, Université Pierre et Marie Curie, CNRS, 4 place Jussieu, 75005 Paris (France)] [Institut des NanoSciences de Paris, Université Pierre et Marie Curie, CNRS, 4 place Jussieu, 75005 Paris (France); Chang, Yia-Chung, E-mail: yiachang@gate.sinica.edu.tw [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China)] [Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan (China)

2013-09-15

353

Dorsal locked plate fixation of distal radius fractures.

Distal radius fractures are common, and internal fixation for operative management of these injuries is widely accepted. Although use of the volar approach for plate fixation has become more popular, benefits of the dorsal surgical approach include the potential for direct reduction and assessment of articular alignment, evaluation and management of concomitant intrinsic intercarpal ligament injury, and initiation of early range of motion. For certain fracture patterns, dorsal plate fixation is the preferred surgical technique. Improvements in implant design, in particular the use of low-profile dorsal plates, has decreased the rate of complications seen previously with this technique. Here, we provide an overview of the evaluation of patients with distal radius fractures, as well as the surgical indications and contraindications, techniques, and complications after dorsal locked plate fixation of intra-articular distal radius fractures. PMID:23751326

Lutsky, Kevin; Boyer, Martin; Goldfarb, Charles

2013-07-01

354

Distal radius fractures: strategic alternatives to volar plate fixation.

Volar locking plates have provided surgeons with enhanced capability to reliably repair both simple and complex fractures and avoid the hardware-related complications associated with dorsal plating. However, there have been an increasing number of published reports on the frequency and types of complications and failures associated with volar locked plating of distal radius fractures. An informed, critical assessment of distal radius fracture characteristics will allow surgeons to select an individualized treatment strategy that maximizes the likelihood of a successful outcome. Knowledge of the anatomy, patterns, and characteristics of the diverse types of distal radius fractures and the complications and failures associated with volar locked plating will be helpful to orthopaedic surgeons who treat patients with these injuries. PMID:24720291

Dy, Christopher J; Wolfe, Scott W; Jupiter, Jesse B; Blazar, Philip E; Ruch, David S; Hanel, Douglas P

2014-01-01

355

Epithelioid hemangioendothelioma of the forearm with radius involvement. Case report

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

356

Stability of a Wheel with Various Radius Rim

NASA Astrophysics Data System (ADS)

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

Kinugasa, Tetsuya; Yoshida, Koji

357

21 CFR 886.1430 - Ophthalmic contact lens radius measuring device.

Code of Federal Regulations, 2010 CFR

...2009-04-01 false Ophthalmic contact lens radius measuring device. 886...Devices Â§ 886.1430 Ophthalmic contact lens radius measuring device. (a) Identification. An ophthalmic contact lens radius measuring device is...

2009-04-01

358

21 CFR 886.1430 - Ophthalmic contact lens radius measuring device.

Code of Federal Regulations, 2013 CFR

...2013-04-01 false Ophthalmic contact lens radius measuring device. 886...Devices Â§ 886.1430 Ophthalmic contact lens radius measuring device. (a) Identification. An ophthalmic contact lens radius measuring device is...

2013-04-01

359

NASA Astrophysics Data System (ADS)

Si nanowires have attracted considerable attention as promising candidates for electronic, thermoelectric, photonic, and photovoltaic devices, yet there appears to be only limited understanding of the underlying electronic and excitonic structures on all pertinent energy scales. Using atomistic pseudopotential calculations of single-particle as well as many-body states, we have identified remarkable properties of Si nanowires in three energy scales: (i) In the “high-energy” ˜1-eV scale, we find an unusual electronic state crossover whereby the nature of the lowest unoccupied molecular orbital (LUMO) state changes its symmetry with wire diameters for [001]-oriented wires but not for [011]-oriented wires. This change leads to orbitally allowed transitions becoming orbitally forbidden below a certain critical diameter for [001] wires. (ii) In the “intermediate-energy” ˜10-1-eV scale, we describe the excitonic binding, finding that in [001] wires the diameter (D) dependence of excitonic gap scales as 1/D1.9, not as 1/D1 as expected. The exciton binding energy increases from 52 meV at D=7.6 nm to 85 meV at D=3.3 nm and 128 meV at D=2.2 nm. (iii) In the “low-energy” ˜10-3-eV scale, we describe dark/bright excitonic states and predict how orbitally allowed transitions [in scale (i)] become spin-forbidden due to the electron-hole exchange interaction, whereas the spin-allowed states in the orbitally forbidden diameter region remain dark. The diameter dependence of the fine-structure splitting of excitonic states scales as 1/D2.3 in [001] wires and as 1/D2.6 in [011] wires. Surface-polarization effects are found to significantly enhance electron-hole Coulomb interaction, but have a small effect on the exchange fine-structure splitting. The present work provides a road map for a variety of electronic and optical effects in Si nanowires that can guide spectroscopic studies.

Zhang, Lijun; Luo, Jun-Wei; Franceschetti, Alberto; Zunger, Alex

2011-08-01

360

Scaling of excitons in graphene nanoribbons with armchair shaped edges.

The scaling behavior of band gaps and fundamental quantities of exciton, i.e., reduced mass, size, and binding strength, in three families of quasi one-dimensional graphene nanoribbons with hydrogen passivated armchair shaped edge (AGNRs) are comprehensively investigated by density functional theory with quasi-particle corrections and many body, i.e., electron-hole, interactions. Compared with single-walled carbon nanotubes (SWCNTs) where the scaling character features a single exponent, each family of AGNRs has its own single exponent, due to its intrinsic zero curvature, which also accounts for the absent "family spreading" of optical transition energies in the smaller width region in the Kataura plots of AGNRs as compared to those of SWCNTs. Moreover, the scaling relation between exciton binding strength and the geometric parameter is established. PMID:21939213

Zhu, Xi; Su, Haibin

2011-11-01

361

Exciton-polariton patterns in coherently pumped semiconductor microcavities

NASA Astrophysics Data System (ADS)

We theoretically study spatially periodic exciton-polariton patterns in a semiconductor microcavity coherently driven by an optical pump. Patterns grow spontaneously in a uniformly pumped cavity above a certain threshold (Turing instability). By means of a perturbation analysis in the vicinity of this bifurcation point, we determine the conditions for pattern formation and predict their properties. The shape of those patterns depends strongly on the detuning of the pump frequency from the system's resonances. For instance, near the bottom of the lower branch of the polariton dispersion relation, a hexagonal pattern is the most favorable periodic solution similar to pattern formation in optical cavities endowed with a defocusing Kerr nonlinearity. Towards the excitonic resonance, the influence of the upper polariton branch becomes important giving rise to a diversity of other solutions, such as roll, labyrinthine, and honeycomb patterns. Our numerical simulations reveal patterns with point and line defects and the formation of disordered patterns of a soliton gas.

Werner, Albrecht; Egorov, Oleg A.; Lederer, Falk

2014-06-01

362

Exciton spectra in two-dimensional graphene derivatives

NASA Astrophysics Data System (ADS)

The energy spectra and wave functions of bound excitons in important two-dimensional (2D) graphene derivatives, i.e., graphyne and graphane, are found to be strongly modified by quantum confinement, making them qualitatively different from the usual Rydberg series. However, their parity and optical selection rules are preserved. Thus a one-parameter modified hydrogenic model is applied to quantitatively explain the ab initio exciton spectra, and allows one to extrapolate the electron-hole binding energy from optical spectroscopies of 2D semiconductors without costly simulations. Meanwhile, our calculated optical absorption spectrum and enhanced spin singlet-triplet splitting project graphyne, an allotrope of graphene, as a candidate for intriguing energy and biomedical applications.

Huang, Shouting; Liang, Yufeng; Yang, Li

2013-08-01

363

Observation of dressed excitonic states in a single quantum dot.

We report the observation of dressed states of a quantum dot. The optically excited exciton and biexciton states of the quantum dot are coupled by a strong laser field and the resulting spectral signatures are measured using differential transmission of a probe field. We demonstrate that the anisotropic electron-hole exchange interaction induced splitting between the x- and y-polarized excitonic states can be completely erased by using the ac-Stark effect induced by the coupling field, without causing any appreciable broadening of the spectral lines. We also show that by varying the polarization and strength of a resonant coupling field, we can effectively change the polarization axis of the quantum dot. PMID:18518335

Jundt, Gregor; Robledo, Lucio; Högele, Alexander; Fält, Stefan; Imamo?lu, Atac

2008-05-01

364

Operation speed of polariton condensate switches gated by excitons

NASA Astrophysics Data System (ADS)

We present a time-resolved photoluminescence (PL) study in real and momentum space of a polariton condensate switch in a quasi-one-dimensional semiconductor microcavity. The polariton flow across the ridge is gated by excitons inducing a barrier potential due to repulsive interactions. A study of the device operation dependence on the power of the pulsed gate beam obtains a satisfactory compromise for the on-off signal ratio and switching time of the order of 0.3 and ˜50 ps, respectively. The opposite transition is governed by the long-lived gate excitons, consequently, the off-on switching time is ˜200 ps, limiting the overall operation speed of the device to ˜3 GHz. The experimental results are compared to numerical simulations based on a generalized Gross-Pitaevskii equation, taking into account incoherent pumping, decay, and energy relaxation within the condensate.

Antón, C.; Liew, T. C. H.; Sarkar, D.; Martín, M. D.; Hatzopoulos, Z.; Eldridge, P. S.; Savvidis, P. G.; Viña, L.

2014-06-01

365

Quantum Stark confined strongly correlated indirect excitons in quantum wells

NASA Astrophysics Data System (ADS)

We consider small ensembles of optically excited indirect excitons (IE) in a quantum well. Using path integral Monte Carlo simulations we compute from first principles the spatial separation of electrons (e) and holes (h) and the lateral quantum Stark confinement in the quantum well due to a strong electric field from a tip electrode [1]. Electrons and holes are shown to form permanent dipoles with a strong repulsion giving rise to interesting correlation and quantum effects [2,3]. By changing the field strength, tip to sample distance and excitation intensity (IE number) we predict the parameter range where exciton crystallization is expected to be observable in experiments on ZnSe based quantum wells. [1] P. Ludwig et al., phys. Stat. Sol. (b) 243, 2363 (2006) [2] A. Filinov et al., phys. Stat. Sol. (c) 3, 2457 (2006) [3] A. Filinov et al., J. Phys. Conf. Ser. 35, 197 (2006)

Ludwig, P.; Filinov, A.; Bonitz, M.; Stolz, H.; Lozovik, Yu. E.

2007-03-01

366

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

367

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

2013-10-01

368

Exciton and biexciton fine structure in single elongated islands grown on a vicinal surface

We report a microphotoluminescence study of the exciton and the biexciton localized in very elongated islands formed by well-width fluctuations in a thin CdTe/CdMgTe quantum well grown on a vicinal surface. The electron-hole exchange interaction in a local reduced symmetry splits the exciton states. The resulting transitions are linearly polarized along the two orthogonal principal axes of the island. The valence band mixing induced by the elongated shape of the potential leads to a strong polarization anisotropy and to the observation of dark exciton states under magnetic field. The biexciton-exciton transition reproduces all the fine structure of the exciton state including the transition of the biexciton to the dark exciton state. PMID:10991299

Besombes; Kheng; Martrou

2000-07-10

369

Exciton and Biexciton Fine Structure in Single Elongated Islands Grown on a Vicinal Surface

NASA Astrophysics Data System (ADS)

We report a microphotoluminescence study of the exciton and the biexciton localized in very elongated islands formed by well-width fluctuations in a thin CdTe/CdMgTe quantum well grown on a vicinal surface. The electron-hole exchange interaction in a local reduced symmetry splits the exciton states. The resulting transitions are linearly polarized along the two orthogonal principal axes of the island. The valence band mixing induced by the elongated shape of the potential leads to a strong polarization anisotropy and to the observation of dark exciton states under magnetic field. The biexciton-exciton transition reproduces all the fine structure of the exciton state including the transition of the biexciton to the dark exciton state.

Besombes, L.; Kheng, K.; Martrou, D.

2000-07-01

370

NASA Astrophysics Data System (ADS)

A coherent emission of excitons in semiconductor multiple quantum wells was investigated by using transient Brewster reflection spectroscopy. The method reduced nonresonant background reflection greatly and allowed us to observe the coherent dynamics of the excitons clearly. The coherent signal showed not only the quantum beats of the heavy- and light-hole excitons but also the oscillatory modulation structures that resulted from the radiative coupling of exciton polaritons. In the linear regime, these observations were compared with results calculated from the transfer-matrix method involving multiple-reflection effects, and good agreement was obtained. As the excitation density of the incident pulses was raised, the decay rate of the coherent signal increased due to the exciton-exciton scattering.

Hyun, Byung-Ryool; Mishina, Tomobumi; Masumoto, Yasuaki; Nakayama, Masaaki

1997-11-01

371

Temperature-Dependent Exciton Hopping in an Array of Inhomogeneously Broadened Quantum Dots

NASA Astrophysics Data System (ADS)

The dynamics of exciton hopping in an array of inhomogeneously broadened CdSe/ZnS quantum dot (QD) ensembles is examined by measuring time- and spectrally resolved fluorescence intensities. We have found a decrease in the fluorescence decay time as well as a dynamic redshift of the fluorescence spectrum originating from exciton transfer. Both show the characteristic temperature dependence reflecting the peculiar exciton dynamics in the QD ensemble. We propose a model of coupled QD arrays where inhomogeneous distribution and dark QDs that are related to a long-lasting off-state of blinking QDs are taken into account. Experimental results together with numerical calculations based on this model suggest that at low temperatures, an exciton transfers to a local low-energy site and tends to be trapped, whereas at high temperatures, thermally activated hopping of the exciton occurs repeatedly. Furthermore, we show that the decay time decrease of the QD array is attributable to exciton hopping to dark QDs.

Miyazaki, Jun; Kinoshita, Shuichi

2012-07-01

372

Understanding molecular structure dependence of exciton diffusion in conjugated small molecules

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

373

Intrinsic linewidths and radiative lifetimes of free excitons in GaAs quantum wells

NASA Astrophysics Data System (ADS)

The giant oscillator strength for radiative transitions of excitons in quantum wells is largely due to the macroscopic polarization of a two-dimensional system. At low temperatures the large oscillator strength leads to a short radiative lifetime of free excitons. We investigate the photoluminescence linewidths and lifetimes of free excitons in a series of extremely high-quality GaAs quantum wells as a function of lattice temperature, excitation intensity, and quantum-well width. With only negligible defect states in our quantum-well sample, we are able to correlate the time-resolved data with temperature-dependent linewidth measurements on the series of quantum wells to estimate the homogeneous linewidth and acoustic-phonon scattering rate of free excitons. Our studies show that thermalization of the excitonic states, ionization into free carriers, and a reduction in the coherence volume of the exciton polarization due to defect scattering, lead to a decrease in the net radiative recombination rate.

Srinivas, Vivek; Hryniewicz, John; Chen, Yung Jui; Wood, Colin E. C.

1992-10-01

374

Narrowing of the exciton lines using WAHUHA method of solid state NMR spectroscopy

NASA Astrophysics Data System (ADS)

A method for allocating of two thermodynamic reservoirs which are not related to each other in high density exciton gas in semiconductors using the Schwinger representation for the projection operators of the effective spin (pseudospin) of the excitons is proposed. It is shown that the method of narrowing the NMR resonance lines in solids under the influence of a sequence of four electromagnetic pulses of the WAHUHA type can be applied for the line narrowing of the recombination emission and absorption of light by excitons in optical spectroscopy. We found out that, in contrast to the averaging to zero of the magnetic dipole-dipole interaction between nuclei, the exciton-exciton interaction is averaged only partially. This analysis for the excitons can be extended to other quasiparticles of boson type, i.e. biexcitons, polaritons and magnons, in solids.

Geru, I. I.

2011-10-01

375

NASA Astrophysics Data System (ADS)

Although the excitonic nature of the primary photoexcitation has been firmly established in semiconducting single-walled carbon nanotubes (S-SWCNTs), the magnitude of the exciton binding energy is still being debated. Recent photoluminescence excitation experiments have detected excitons above the threshold of the continuum band predicted from two-photon absorption measurements in the (10,6) S-SWCNT footnotetextJ. Lefebvre and P. Finnie, Nano Lett. 8, 1890 (2008). One interpretation of this experiment is that the exciton binding energy is much larger than previous estimates footnotetextJ. Deslippe et al., Nano Lett. 9, 1330 (2009). We have performed configuration interaction calculations for the (10,6) S-SWCNT within the molecular PPP model that quantitatively reproduces the earlier estimate for the exciton binding energy and also finds excitons deep inside the continuum. A similar observation has previously been made for the conjugated polymer PPV.

Zhao, Hongbo; Mazumdar, Sumit

2010-03-01

376

Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys

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

377

Hybrid states of Tamm plasmons and exciton polaritons

Channeling of exciton polaritons in the plane of semiconductor microcavities can be achieved by the deposition of metallic mesas on the top of the semiconductor structure. We show theoretically that the regime of strong coupling between cavity polaritons and Tamm surface plasmons is possible in such structures. The effect is favorable for the spatial confinement of polaritons and the formation of hybrid one-dimensional plasmon-polariton modes.

Kaliteevski, M.; Brand, S.; Abram, R. A. [Department of Physics, Durham University, Durham DH1 3LE (United Kingdom); Iorsh, I. [Ioffe Institute, Polytechnicheskaya 26, 194021 St. Petersburg (Russian Federation); Kavokin, A. V. [Facolta di Fisica, Universita di Roma II Tor Vergata, 1, via della Ricerca Scientifica, 00133 Roma (Italy); Physics and Astronomy School, University of Southampton, Highfield, Southampton SO17 1BJ (United Kingdom); Shelykh, I. A. [Science Institute, University of Iceland, Dunhagi-3, IS-107 Reykjavik (Iceland)

2009-12-21

378

Efficient multi-exciton emission from quantum dots.

The fundamental spontaneous emission rate an emitter can be modified by its photonic environment. By enhancing the spontaneous emission rate, there is a possibility of extracting multi-exciton energies through radiative decay. In this report, we explore using high Q and small volume cavities to enhance the spontaneous emission rate. We observed greater than 50 folds enhancement in the spontaneous emission from photonic crystal waveguide or microcavity using close-packed monolayer of PbS quantum dot emitters.

Luk, Ting Shan

2010-09-01

379

Core excitons in solid rare gases: Nonstructural theory

We study transitions from inner shells in solid rare gases using a nonstructural theory in direct space which provides the proper behavior in any intermediate situation between the effective-mass approximation (EMA) and the atomic limit. This theory has been successful in describing all the valence excitons in the whole solid-rare-gases series. We find as well that it gives a proper

L. Resca; R. Resta; S. Rodriguez

1978-01-01

380

Exciton dynamics and energy transfer processes in semiconductor nanocrystals

Exciton dynamics provide unique information on both the nature of optical transitions and the local environment of an optically\\u000a active species. However, experimental facilities for measuring (fast) luminescence decay dynamics have been developed long\\u000a after techniques for time-averaged optical spectroscopy (absorption, excitation and emission spectroscopy). Studies on the\\u000a dynamics of excited states therefore lag behind of steady state spectroscopy. This

Andries Meijerink

381

Dynamics of microcavity exciton polaritons in a Josephson double dimer

NASA Astrophysics Data System (ADS)

We study the dynamics of exciton polaritons in a double-well configuration. The system consists of two weakly coupled Bose-Josephson junctions, each corresponding to a different circular polarization of the polaritons, forming a Josephson double dimer. We show that the Josephson oscillation between the wells is strongly coupled to the polarization rotation and that, consequently, Josephson excitation is periodically exchanged between the two polarizations. Linearized analysis agrees well with numerical simulations using typical experimental parameters.

Khripkov, Christine; Piermarocchi, Carlo; Vardi, Amichay

2013-12-01

382

Exciton delocalization and energy transport mechanisms in R-phycoerythrin.

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

2011-03-31

383

Exciton dephasing in single InGaAs quantum dots

Summary form only given. The homogeneous linewidth of excitonic transitions is a parameter of fundamental physical importance. In self-assembled quantum dot systems, a strong inhomogeneous broadening due to dot size fluctuations masks the homogeneous linewidth associated with transitions between individual states. The homogeneous and inhomogeneous broadening of InGaAs quantum dot luminescence is of central importance for the potential application of

K. Leosson; J. Erland; J. R. Jensen; J. M. Hvam

2000-01-01

384

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

385

Quantum theory of excitonic polaritons in quantum wells

NASA Astrophysics Data System (ADS)

We derive the Hamiltonian which describes in second quantization the interaction of quantum-well excitons with photons and obtain by solving the corresponding quantum-mechanical equations of motion the dispersion relations for localized and resonant polaritons as well as the lifetime for the resonant modes. The results are compared with those obtained by Andreani et al. and Tassone et al. who instead solved Maxwell's equations.

Jorda, S.; Rössler, U.; Broido, D.

386

Variational discrete variable representation for excitons on a lattice

NASA Astrophysics Data System (ADS)

We construct numerical basis function sets on a lattice, whose spatial extension is scalable from single lattice sites to the continuum limit. They allow us to compute small and large bound states with comparable, moderate effort. Adopting concepts of discrete variable representations, a diagonal form of the potential term is achieved through a unitary transformation to Gaussian quadrature points. Thereby the computational effort in three dimensions scales as the fourth instead of the sixth power of the number of basis functions along each axis, such that it is reduced by two orders of magnitude in realistic examples. As an improvement over standard discrete variable representations, our construction preserves the variational principle. It allows for the calculation of binding energies, wave functions, and excitation spectra. We use this technique to study central-cell corrections for excitons beyond the continuum approximation. A discussion of the mass and spectrum of the yellow exciton series in the cuprous oxide, which does not follow the hydrogenic Rydberg series of Mott-Wannier excitons, is given on the basis of a simple lattice model.

Alvermann, A.; Littlewood, P. B.; Fehske, H.

2011-07-01

387

Excitonic instabilities and insulating states in bilayer graphene

NASA Astrophysics Data System (ADS)

The competing ground states of bilayer graphene are studied by applying renormalization group techniques to a bilayer honeycomb lattice with nearest neighbor hopping. In the absence of interactions, the Fermi surface of this model at half-filling consists of two nodal points with momenta K, K', where the conduction band and valence band touch each other, yielding a semimetal. Since near these two points the energy dispersion is quadratic with perfect particle-hole symmetry, excitonic instabilities are inevitable if interband interactions are present. Using a perturbative renormalization group analysis up to the one-loop level, we find different competing ordered ground states, including ferromagnetism, superconductivity, spin and charge density wave states with ordering vector Q=K-K', and excitonic insulator states. In addition, two states with valley symmetry breaking are found in the excitonic insulating and ferromagnetic phases. This analysis strongly suggests that the ground state of bilayer graphene should be gapped, and with the exception of superconductivity, all other possible ground states are insulating.

Song, Kok Wee; Liang, Yung-Ching; Haas, Stephan

2012-11-01

388

Excitons in conjugated polymers: a tale of two particles.

Since the discovery of electroluminescence in the phenyl-based conjugated polymers in 1990, the field of polymer optoelectronics has matured to the extent that presently a wide class of devices have been commercialized. These range from both miniature and wide-area light emitting devices to hybrid photovoltaic devices. Similarly, our understanding of the fundamental processes that determine these optoelectronic properties has also progressed. In particular, owing to insights from both experimental and theoretical investigations, the role of the primary excited states, i.e., excitons, is now considerably clearer. This review discusses these primary excited states and explains how the three key roles of electron-electron interactions, electron-nuclear coupling, and disorder determine their properties. We show that the properties of an exciton are more readily understood by decomposing it into two effective particles. First, a relative particle that describes the size and binding energy of the electron-hole pair. Second, a center-of-mass particle that describes the extent of the delocalization of the electron-hole pair. Disorder and coupling to the normal modes localizes the center-of-mass particle and provides a quantitative definition of chromophores in conjugated polymers, paving the way for a first-principles theory of exciton diffusion in these systems. PMID:23427996

Barford, William

2013-04-01

389

The nature of singlet excitons in oligoacene molecular crystals.

A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport. PMID:21639463

Yamagata, H; Norton, J; Hontz, E; Olivier, Y; Beljonne, D; Brédas, J L; Silbey, R J; Spano, F C

2011-05-28

390

A light-hole exciton in a quantum dot

NASA Astrophysics Data System (ADS)

A light-hole exciton is a quasiparticle formed from a single electron bound to a single light hole. This type of fundamental excitation, if confined inside a semiconductor quantum dot, could be advantageous in quantum information science and technology. However, it has been difficult to access it so far, because confinement and strain in conventional quantum dots favour a ground-state single-particle hole with a predominantly heavy-hole character. Here we demonstrate the creation of a light-hole exciton ground state by applying elastic stress to an initially unstrained quantum dot. Its signature is clearly distinct from that of the well-known heavy-hole exciton and consists of three orthogonally polarized bright optical transitions and a fine-structure splitting of hundreds of microelectronvolts between in-plane and out-of-plane components. This work paves the way for the exploration of the fundamental properties and of the potential relevance of three-dimensionally confined light-hole states in quantum technologies.

Huo, Y. H.; Witek, B. J.; Kumar, S.; Cardenas, J. R.; Zhang, J. X.; Akopian, N.; Singh, R.; Zallo, E.; Grifone, R.; Kriegner, D.; Trotta, R.; Ding, F.; Stangl, J.; Zwiller, V.; Bester, G.; Rastelli, A.; Schmidt, O. G.

2014-01-01

391

Photoluminescence of localized excitons in InGan quantum dots

Photoluminescence spectra of samples with ultrathin InGaN layers embedded in AlGaN and GaN matrices are studied experimentally in the temperature range of 80 to 300 K. It is shown that the temperature dependences can be understood in the context of Eliseev's model and that, in the active region of the structures under study, the dispersion {sigma} of the exciton-localization energy depends on the average In content in InGaN-alloy layers. Furthermore, the Urbach energy E{sub U}, which characterizes the localization energy of excitons in the tails of the density of states, was determined from an analysis of the shape of the low-energy slope of the spectrum. It is shown that {sigma} and E{sub U}, quantities representing the scale of the exciton-localization effects, vary linearly with the photoluminescence-peak wavelength in the range from the ultraviolet to the green region of the spectrum.

Usov, S. O., E-mail: S.Usov@mail.ioffe.ru; Tsatsul'nikov, A. F.; Lundin, V. V.; Sakharov, A. V.; Zavarin, E. E.; Ledentsov, N. N. [Russian Academy of Sciences, Ioffe Physicotechnical Institute (Russian Federation)

2008-02-15

392

Transient optical alignment and relaxation of excitons in GaAs\\/AlGaAs quantum wells

The dynamical behaviour of excitons in quantum well structures is studied by picosecond time-resolved resonance fluorescence. Transient optical alignment of 2d-excitons is observed which is destroyed by fast depolarization scattering in some ten picoseconds. The analysis of the time-dependent spectra within the density matrix formalism allows to determine the energy and cross relaxation times between degenerate exciton levels separately. To

H. Stolz; D. Schwarze; W. von der Osten; G. Weimann

1989-01-01

393

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

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

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

1991-01-01

394

Infrared photocurrent response of charge-transfer exciton in polymer bulk heterojunction

We study the charge-transfer exciton absorption and photocurrent response in solution-processed bulk heterojunction based on poly(3-hexylthiophene) donor and (6,6)-phenyl-C61-butyric acid methyl ester acceptor in the near-infrared wavelength region. While the exciton absorption exists only for wavelength below 650 nm, direct generation of charge-transfer exciton formed between the donor and acceptor extends the absorption wavelength to 950 nm. For films with

Chia-Ming Yang; Pei-Yu Tsai; Sheng-Fu Horng; Kuan-Chen Lee; Shin-Rong Tzeng; Hsin-Fei Meng; Jow-Tsong Shy; Ching-Fong Shu

2008-01-01

395

Geminate and Nongeminate Recombination of Triplet Excitons Formed by Singlet Fission

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

396

Bound exciton induced photoluminescence linewidth broadening in GaAs quantum wells

NASA Astrophysics Data System (ADS)

The linewidth of the free exciton photoluminescence peak has been observed to broaden when the sample is excited by light whose energy exceeds the bandgap of the barrier. The net photoluminescence efficiency as well as the width of the free exciton line is observed to depend on the lattice temperature as well as the excitation wavelength and intensity. This is attributed to the scattering of free excitons by the charges trapped in the Al xGa 1- xAs barrier.

Srinivas, Vivek; Chen, Yung Jui; Wood, Colin E. C.

1994-02-01

397

Image states and excitons at insulator surfaces with negative electron affinity

We report on a novel type of surface exciton occuring at insulator surfaces with negative electron affinity. At a usual semiconductor or insulator surface, the one-particle states near top and bottom of the surface gap, giving rise to surface excitons, are both localized within the topmost layers. At LiF(001), the respective surface exciton results at 12.3 eV. This excitation energy,

Johannes Pollmann; Rohlfing; Neng-Ping Wang; Peter Krüger

2004-01-01

398

Although the excitonic nature of the primary photoexcitation has been firmly established in semiconducting single-walled carbon nanotubes (S-SWCNTs), the magnitude of the exciton binding energy is still being debated. Recent photoluminescence excitation experiments have detected excitons above the threshold of the continuum band predicted from two-photon absorption measurements in the (10,6) S-SWCNT footnotetextJ. Lefebvre and P. Finnie, Nano Lett. 8,

Hongbo Zhao; Sumit Mazumdar

2010-01-01

399

Non-local coherent coupling between excitons in a disordered quantum well

NASA Astrophysics Data System (ADS)

We investigated coherent coupling among multiple exciton resonances formed in a single disordered quantum well using the powerful electronic two-dimensional Fourier transform spectroscopy. Our experiment revealed clear signatures of non-local coherent coupling between the heavy-hole and light-hole excitons residing in regions that differ in thickness by one atomic layer. The experimental observation is qualitatively explained by spatial overlap between exciton linear response functions calculated within a single defect model.

Glinka, Yuri D.; Erementchouk, Mikhail; Dass, Chandriker K.; Leuenberger, Michael N.; Bracker, Allan S.; Li, Xiaoqin

2013-07-01

400

21 CFR 886.1450 - Corneal radius measuring device.

Code of Federal Regulations, 2010 CFR

A corneal radius measuring device is an AC-powered device intended to measure corneal size by superimposing the image of the cornea on a scale at the focal length of the lens of a small, hand held, single tube penscope or eye gauge magnifier. (b) Classification. Class I (general...

2010-04-01

401

21 CFR 886.1450 - Corneal radius measuring device.

Code of Federal Regulations, 2010 CFR

A corneal radius measuring device is an AC-powered device intended to measure corneal size by superimposing the image of the cornea on a scale at the focal length of the lens of a small, hand held, single tube penscope or eye gauge magnifier. (b) Classification. Class I (general...

2009-04-01

402

Stable orbit rendezvous for a small radius translunar halo orbit

A two spacecraft terminal phase rendezvous targeting law, which is valid for the three-body problem, is given. The relevant equations of motion are derived and the targeting law developed assuming the traditional target\\/chaser vehicle relationship. The targeting law is demonstrated using a small radius translunar halo orbit. Nonlinear simulation results verify that acceptable performance is obtained. Using these results, the

Brian L. Jones; Robert H. Bishop

1993-01-01

403

Effect of limiter end loss in finite Larmor radius theory

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

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

1993-08-01

404

Comparison of internal and external fixation of distal radius fractures

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

2013-01-01

405

Intraosseous neurilemmoma of the radius: a case report.

Neurilemmomas are benign tumors that arise from Schwann cells. They are rarely found on bone. We describe a neurilemmoma in a 45-year-old patient that affected the distal metaphysis of the radius. Only 1 previous case has been described in the literature. We discuss the clinical presentation, the radiographic aspect, particularly the magnetic resonance imaging characteristics, and the microscopic findings. PMID:10722831

Giné, J; Calmet, J; Sirvent, J J; Domènech, S

2000-03-01

406

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

407

Acceleration of beam ions during major radius compression in TFTR

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

408

Small Larmor radius limit for a warm relativistic electron fluid

A fluid model for a relativistic warm plasma is obtained using a closure approach that allows for a nonvanishing heat tensor. A small Larmor radius limit is applied to the model. Since the model allows for a nonvanishing heat tensor, the equations of relativistic anisotropic magnetohydrodynamics with 'higher-order' state equations can be obtained that take into account the heat tensor

A. M. Anile; O. Muscato

1989-01-01

409

Observational Constraints on the White Dwarf Mass-Radius Relation

NASA Astrophysics Data System (ADS)

We propose to measure gravitational redshifts for white dwarf stars that have distant, non-interacting main-sequence companions. With independent radius constraints obtained from parallaxes and surface gravity determinations obtained by fitting the Balmer series from our spectra, we will make improved estimates of white dwarf masses and radii that can be critically compared with theoretical mass-radius relations specific to each star. These observations will allow us to examine serious discrepancies between the theoretical and empirical measurements of the white dwarf mass-radius relation and extend the range of masses over which it has been tested, spanning 0.5-1.2 Msun. Currently, the measured radius for only a single WD matches its predicted value within 5%. With the expected precision of ?5% for over half the sample, we will also distinguish whether the white dwarfs have ``thick'' or ``thin'' H envelopes. Using the same spectra, we will also estimate the metallicity of the main-sequence companion and examine how the initial-final-mass ratio for WDs depends on metallicity. Thus, this project will put robust constraints on two fundamental relations that govern our understanding of white dwarfs: the mass-ratio and the initial-final-mass relations.

Dhital, Saurav; Oswalt, Terry D.; Holberg, J. B.; Zhao, Jingkun

2014-08-01

410

Finite Larmor radius flute mode theory with end loss

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

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

1993-08-01

411

Finite-Larmor-Radius Stability Theory of EBT Plasmas.

National Technical Information Service (NTIS)

An eikonal ballooning-mode formalism is developed to describe curvature-driven modes of hot electron plasmas in bumpy tori. The formalism treats frequencies comparable to the ion-cyclotron frequency, as well as arbitrary finite Larmor radius and field pol...

H. L. Berk C. Z. Cheng M. N. Rosenbluth J. W. Van Dam

1982-01-01

412

Line shape of the low-energy tail of exciton absorption in molecular crystals

NASA Astrophysics Data System (ADS)

The influence of crystal defects and phonons on the line shape of the low-energy tail of exciton absorption is calculated for molecular crystals with the lower limit of their exciton band at k-->=0. The exciton Hamiltonian for a crystal with lattice defects is derived from that for a perfect crystal with lattice vibrations. Calculation of the line shape is done assuming that the individual lattice defects have independent effects on the unperturbed exciton band. Three examples of the molecular exciton energy bands are considered: (1) one-dimensional tight-binding exciton band, (2) the exciton bands with Hubbard's model of density of states, and (3) exciton bands with density of states independent of the exciton energies. At very low temperatures where the influence of the defects is dominant, we have found the asymmetric low-energy tail due to defects, acting as shallow traps, in all the three examples. The observed asymmetric line shape in 1,2-dibromonaphthalene single crystals is, therefore, attributed to such shallow traps and analogous to the Urbach-Martienssen tail observed in alkali halides. As the temperature increases and phonons become more active, the line shape becomes symmetric due to the dominant influence of thermal broadening.

Singh, Jai

1981-02-01

413

Exciton mediated self-organization in glass driven by ultrashort light pulses

NASA Astrophysics Data System (ADS)

We propose an exciton-polariton-mediated self-organization effect in transparent SiO2 glass under intense femtosecond light irradiation. Interference and dipole-dipole interaction of polaritons causes formation of gratings of dielectric polarization. Due to an ultrafast exciton self-localization into a quasicrystal structure, the polariton gratings remain frozen in glass and a permanent three-dimensional image of exciton-polariton gas is created. We show that coherent effects in propagation of exciton-polaritons can serve as a tool for nanostructuring and fabrication of 5-dimensional optical memories in glass, opening new horizons for polaritronics.

Beresna, Martynas; Gecevi?ius, Mindaugas; Kazansky, Peter G.; Taylor, Thomas; Kavokin, Alexey V.

2012-07-01

414

Experimental Evidence for Exciton Scaling Effects in Self-Assembled Molecular Wires

NASA Astrophysics Data System (ADS)

Resonant Rayleigh scattering from self-assembled one-dimensional molecular J-aggregate wires reveals a distinct dependence of the exciton energy on the width of lateral extension. For the J aggregates used in this study, strong in-line dipole coupling leads to a delocalization of the exciton wave function over several molecular units. Polarization dependent measurements of resonantly scattered light from the wires show that the exciton dipole moment is oriented perpendicular to the long axis. The experimental observations can be described by applying a quantization condition to the center of mass motion of the J-band exciton in the wires.

Lagoudakis, P. G.; de Souza, M. M.; Schindler, F.; Lupton, J. M.; Feldmann, J.; Wenus, J.; Lidzey, D. G.

2004-12-01

415

Experimental evidence for exciton scaling effects in self-assembled molecular wires.

Resonant Rayleigh scattering from self-assembled one-dimensional molecular J-aggregate wires reveals a distinct dependence of the exciton energy on the width of lateral extension. For the J aggregates used in this study, strong in-line dipole coupling leads to a delocalization of the exciton wave function over several molecular units. Polarization dependent measurements of resonantly scattered light from the wires show that the exciton dipole moment is oriented perpendicular to the long axis. The experimental observations can be described by applying a quantization condition to the center of mass motion of the J-band exciton in the wires. PMID:15697939

Lagoudakis, P G; de Souza, M M; Schindler, F; Lupton, J M; Feldmann, J; Wenus, J; Lidzey, D G

2004-12-17

416

Decay process of the Mg 2p core exciton in magnesium halides studied by photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

The decay of the Mg 2p core exciton in MgF2, MgCl2, and MgBr2 was investigated by use of the technique of resonant photoemission. The constant-initial-state (CIS) spectrum with the initial state at the peak of the valence band and the constant-final-state (CFS) spectrum with the final state corresponding to the kinetic energy of the Auger electron were measured at photon energies around the Mg 2p core-exciton excitation in these substances. The resonant enhancements were observed in the CIS and CFS spectra of MgCl2 and MgBr2, whereas in MgF2 only the Auger-electron peak shows resonant behavior. These resonant enhancements are interpreted in terms of the nonradiative decay of the Mg 2p core exciton through the L2,3(exciton)-V and the L2,3(exciton)-VV processes, respectively, where L(exciton) denotes the initial state with the L hole accompanied by a bound electron (L core exciton) and V denotes the final state with a hole in the valence band. The shapes of the valence-band energy distribution curves obtained at the photon energies of the L2 and the L3 core-exciton excitations are different in MgCl2 and MgBr2. This is attributed to the difference in the relaxation of the L2 and the L3 core excitons.

Aita, Osamu; Ichikawa, Kouichi; Tsutsumi, Kenjiro

1989-05-01

417

Electric and magnetic field manipulation and storage of charge-tunable excitons

NASA Astrophysics Data System (ADS)

The excitonic spectrum of radially polarized semiconductor rings has been analyzed theoretically, in the presence of an in-plane electric field and a perpendicular magnetic field. Based on the numerically exact solution, a regime has been found where the exciton behaves as a single carrier or quasiparticle, with an effective and tunable electric charge determined by the ring geometry. A protocol is proposed for the storage of excitons without destroying them, consisting in converting them from "bright" to "dark," by performing a sequence of well-defined steps. Accurate analytical approximations are provided for each of the exciton regimens found: quasifree, locked, and broken.

Simonin, J.; Proetto, C. R.; Pacheco, M.; Barticevic, Z.

2014-02-01

418

Optical bistability and nonlinearity of coherently coupled exciton-plasmon systems.

We theoretically investigated optical third-order nonlinearity of a coherently coupled exciton-plasmon hybrid system under a strong control field with a weak probe field. The analytic formulas of exciton population and effective third-order optical susceptibility of the hybrid of a metal nanoparticle (MNP) and a semiconductor quantum dot (SQD) were deduced. The bistable exciton population and the induced bistable nonlinear absorption and refraction response were revealed. The bistability region can be tuned by adjusting the size of metal nanoparticle, interparticle distance and intensity of control field. Our results have perspective applications in optical information processing based on resonant coupling of exciton-plasmon. PMID:22274530

Li, Jian-Bo; Kim, Nam-Chol; Cheng, Mu-Tian; Zhou, Li; Hao, Zhong-Hua; Wang, Qu-Quan

2012-01-16

419

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

420

Time resolved dynamics of the excitonic transfer in double quantum well structures

NASA Astrophysics Data System (ADS)

In biased asymmetric double quantum wells a photocreated excitation can tunnel from one well to the other before recombination. By tuning the external electric field, the fundamental conduction level and the fundamental valence level become localized in different wells and a crossed exciton appears. The excitonic transfer from a direct to a crossed exciton is studied by means of time resolved photoluminescence. The formation of the crossed exciton also induces a dipolar electric field that leads to a non linear behaviour of the transfer and recombination mechanisms.

Rolland, P.; Ferreira, R.; Roy, N.; Roussignol, Ph.; Vinattieri, A.; Carraresi, L.; Palmier, J. F.; Etienne, B.; Sermage, B.; Delalande, C.

1992-02-01

421

We report on a general theoretical approach to study exciton transport and emission in a single-walled carbon nanotube (SWNT) in the presence of a localized surface-plasmon (SP) mode within a metal nanoparticle interacting via near-field coupling. We derive a set of quantum mechanical equations of motion and approximate rate equations that account for the exciton, SP, and the environmental degrees of freedom. The material equations are complemented by an expression for the radiated power that depends on the exciton and SP populations and coherences, allowing for an examination of the angular distribution of the emitted radiation that would be measured in experiment. Numerical simulations for a (6,5) SWNT and cone-shaped Ag metal tip (MT) have been performed using this methodology. Comparison with physical parameters shows that the near-field interaction between the exciton-SP occurs in a weak coupling regime, with the diffusion processes being much faster than the exciton-SP population exchange. In such a case, the effect of the exciton population transfer to the MT with its subsequent dissipation (i.e., the Förster energy transfer) is to modify the exciton steady state distribution while reducing the equilibration time for excitons to reach a steady sate distribution. We find that the radiation distribution is dominated by SP emission for a SWNT-MT separation of a few tens of nanometers due to the fast SP emission rate, whereas the exciton-SP coherences can cause its rotation. PMID:24666158

Roslyak, Oleksiy; Cherqui, Charles; Dunlap, David H; Piryatinski, Andrei

2014-07-17

422

Exciton-phonon coupling in single quantum dots with different barriers

NASA Astrophysics Data System (ADS)

The coupling between longitudinal-optical (LO) phonons and neutral excitons in two different kinds of InGaAs pyramidal quantum dots (QDs) embedded in either AlGaAs or GaAs barriers is experimentally examined. We find a slightly weaker exciton-LO-phonon coupling and increased linewidth of the phonon replicas for the QDs with GaAs barriers compared to the ones with AlGaAs barriers. These results, combined with the fact that the LO-phonon energy of the exciton is the same for both kinds of dots, are taken as evidence that the excitons mainly couple to LO-phonons within the QDs.

Dufa?Ker, D.; Mereni, L. O.; Karlsson, K. F.; Dimastrodonato, V.; Juska, G.; Holtz, P. O.; Pelucchi, E.

2011-06-01

423

OBSERVATIONAL CONSTRAINTS ON THE DEGENERATE MASS-RADIUS RELATION

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

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

2012-03-15

424

Optimal Taylor–Couette flow: radius ratio dependence

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

425

In this Rapid Communication we propose to use GaN-based quantum dots as building blocks for solid-state quantum-computing devices. The existence of a strong built-in electric field induced by the spontaneous polarization and by the piezoelectricity is exploited to generate entangled few-exciton states in coupled quantum dots without resorting to external fields. More specifically, we shall show how the built-in field

S. de Rinaldis; I. D'Amico; E. Biolatti; R. Rinaldi; R. Cingolani; F. Rossi

2002-01-01

426

Variable radius cartography - History and perspectives of a new discipline

NASA Astrophysics Data System (ADS)

The map that Toscanelli sent to Columbus was an unconscious application of cartography at a smaller radius than the real. The first really conscious attempts to represent the geography of Earth on globes of radius less than the current one occurred after the formulation of the concept of expanding Earth through geological time. The American chemist and geologist Richard Owen (1810-1890) in his book Key to the geology of the globe (1857) described the principles of what he himself called Anatomical Geology, with the Earth growing as a biological organism. The book contained a global paleogeographic map of the Earth that would have had a radius of about 4000 kilometers. In 1928 J.A.H. Kerkhoff (under the pseudonym Aero-dilettant) published a series of paleogeographic globes on which the modern oceans disappeared. With the same artisan methods of transfer continental outlines from a sphere to a smaller one, in 1933 O.C. Hilgenberg represented three different geological epochs, and, later, for the first time mapped paleopoles with their site-pole segments of meridian. Even today the traditional method of Hilgenberg is followed by senior researchers (Klaus Vogel, 2003) and younger geologists (James Maxlow). In England Hugh Owen applied the methods of traditional cartography to the variable radius one. His Atlas of Continental Displacement was in the 70s and 80s, for this discipline, a real milestone. While in the field of constant radius paleogeography the adherents to plate tectonics created many computer codes of automatic mapping (Bullard et al., 1965; Smith & Hallam, 1970; Scotese et al., 1979; and many others), in the variable radius field few tried to reach the same task. In 1972 in United States a first very simple attempt (but was not further developed) came from a private, R.B. Perry, followed by the still not-computerized Atlas of Owen, and both them constituted inspiration for the construction of a FORTRAN variable radius mapping code at INGV, with which it is now possible to represent paleopoles, their uncertainty ellipses, and site-pole segments of meridian (Scalera, 1988, 1990). In all paleogeographic reconstructions of the different authors, variable radius cartography is used in a way more or less complex, more or less intertwined with other disciplines and databases, not as pure representation or in the spirit of the simple fits that supported plate tectonics, but as experiments of greater complexity with a value of proof in favor of the planet expansion. Today a common feeling is that is now necessary to develop an interactive and user friendly program code, which could be distributed or used in the web. The use of variable radius mapping would be a profitable tool in the field of geodesy, where a full treatment without subtle vicious loops of an expanding globe has yet to be developed.

Scalera, Giancarlo

2014-05-01

427

Motion Perception During Variable-Radius Swing Motion in Darkness

Using a variable-radius roll swing motion paradigm, we examined the influence of interaural (y-axis) and dorsoventral (z-axis) force modulation on perceived tilt and translation by measuring perception of horizontal translation, roll tilt, and distance from center of rotation (radius) at 0.45 and 0.8 Hz using standard magnitude estimation techniques (primarily verbal reports) in darkness. Results show that motion perception was significantly influenced by both y- and z-axis forces. During constant radius trials, subjects' perceptions of tilt and translation were generally almost veridical. By selectively pairing radius (1.22 and 0.38 m) and frequency (0.45 and 0.8 Hz, respectively), the y-axis acceleration could be tailored in opposition to gravity so that the combined y-axis gravitoinertial force (GIF) variation at the subject's ears was reduced to ?0.035 m/s2 – in effect, the y-axis GIF was “nulled” below putative perceptual threshold levels. With y-axis force nulling, subjects overestimated their tilt angle and underestimated their horizontal translation and radius. For some y-axis nulling trials, a radial linear acceleration at twice the tilt frequency (0.25 m/s2 at 0.9 Hz, 0.13 m/s2 at 1.6 Hz) was simultaneously applied to reduce the z-axis force variations caused by centripetal acceleration and by changes in the z-axis component of gravity during tilt. For other trials, the phase of this radial linear acceleration was altered to double the magnitude of the z-axis force variations. z-axis force nulling further increased the perceived tilt angle and further decreased perceived horizontal translation and radius relative to the y-axis nulling trials, while z-axis force doubling had the opposite effect. Subject reports were remarkably geometrically consistent; an observer model-based analysis suggests that perception was influenced by knowledge of swing geometry.

Rader, A. A.; Oman, C. M.; Merfeld, D. M.

2009-01-01

428

Composition Effect on Mass-Radius Relation of Exoplanets

NASA Astrophysics Data System (ADS)

In order to characterize the many hundreds of exoplanets that have been discovered, it is necessary to determine their internal structure and composition. For many exoplanets, masses and radii are available. The radius-mass relation can be substantially affected by the composition of the high-Z material, its mass fraction and its distribution within the planet. Adding high-Z material to a planet changes the density, energy and entropy profiles inside the planet, and contributes to the opacity as well. Therefore, in certain conditions the heat transfer and luminosity (and therefore contraction timescale) are also determined by the composition. Here we model the evolution of giant and intermediate mass planets with various compositions and structures. We investigate the effects of the composition and its depth dependence on the long-term evolution of the planets. We study the influences of the heavy components, like rocks and ices, on the evolution of the planets by considering their contribution to the energy and also to the opacity. We calculate the EOS for a mixture of hydrogen and helium with heavy element for each of the planetary layers, and tie the grain opacity to the atmospheric metallicity in a self-consistent manner. The effect of stellar irradiation is also considered. We find that the most important effect is that of the grain opacity due to the additional high-Z material in the envelope. This has the potential of increasing the computed radius of the planet by several tens of percent. The changes in radius due to various compositions and stellar irradiation also affect the planetary contraction but are found to be less important than the opacity effects. We suggest that the mass-radius relationship used for characterization of observed exoplanets should be taken with great caution since different physical conditions can result in very different mass-radius relationships. Moreover, assuming constant composition distribution inside the planet in time is not necessarily correct. We calculate mass redistribution processes, like core erosion by convection, during the planetary evolution, for different initial assumptions. We show effects of mass redistribution on the inner structure and mass-radius relationship of exoplanets.

Vazan, Allona; Kovetz, Attay; Podolak, Morris; Helled, Ravit

2014-05-01

429

Human carbonmonoxy- and deoxy-haemoglobins were incubated at 37 degrees C in 3H2O at various pH values to measure the pH-dependent hydrogen--tritium exchange at the C-2 position of the imidazole ring of histidine-122alpha. To obtain the pseudo-first-order rate constants for the exchange, k, the two peptides containing histidine-122alpha were isolated and the amounts of tritium incorporated were determined. The rate constants gave pK values for the histidine of 6.1 in carbonmonoxyhaemoglobin and 6.6 in deoxyhaemoglobin, showing that it contributes about 20% to the total alkaline Bohr effect and about 10% at pH7.4.

Nishikura, K

1978-01-01

430

Proton Radius, Darwin-Foldy Term and Radiative Corrections

NASA Astrophysics Data System (ADS)

It is not an easy task to define the proton charge radius. Namely, by definition, the proton radius is the slope of the GESachs form factor of the proton at zero momentum transfer, provided one has subtracted from the scattering cross sections, all effects due to QED. That means that radiative corrections must be subtracted; these otherwise ``mask'' the proton structure from the surroundings. On the other hand, the self-energy of the proton (not of the electron or of the muon) also influence the spectrum of atomic hydrogen, or muonic hydrogen, respectively. In the talk, we shall review the difficulties faced by a consistent definition, offer a way to resolve them, and review the current status of Lamb shift predictions in muonic hydrogen, with a special reference to the current experimental-theoretical discrepancy, as reported by the CREMA collaboration.

Jentschura, Ulrich

2013-04-01

431

A new Period-Radius relation for Galactic Classical Cepheids

NASA Astrophysics Data System (ADS)

We discuss a new Period-Radius (PR) relation for Galactic Classical Cepheids, obtained by means of a new version of the CORS method which has been modified in order to be run with the Strömgren photometric system. The major change consists in the calibration of the Surface Brightness as a function of the two ``reddening free'' colour indexes [c1] and [m1], by means of the model atmospheres by Castelli et al. (1997). In this contribution we first briefly discuss some numerical experiments performed on the basis of synthetic Cepheid light curves to test the accuracy of the method, and then report the Period-Radius relation for Classical Cepheids obtained by applying the the new method to a sample of Galactic Cepheids.

Ruoppo, A.; Ripepi, V.; Marconi, M.; Russo, G.

432

Precise Determination of the Proton Charge Radius Experiment (rpex)

NASA Astrophysics Data System (ADS)

At the HUGS 2002 student seminars, an introduction to RPEX (Pr. 00-02, spokespersons: H. Gao, J. Calarco) will be given. The proton rms charge radius is a fundamental quantity, and its precise determination will have significant impacts on tests of QCD and QED. It will be measured with sub 1% precision and will run at the Bates Linear Accelerator Center located in Middleton, Massachusetts. This experiment will utilize the symmetric sectors of the BLAST detector, and the super-ratio technique to extract both the proton elastic form factor ratio at low Q2 and the proton charge radius. A laser driven polarized H/D target is being developed for use in this experiment.

Clasie, B.

2004-03-01

433

Artificial gravity: head movements during short-radius centrifugation

NASA Astrophysics Data System (ADS)

Short-radius centrifugation is a potential countermeasure to long-term weightlessness. Unfortunately, head movements in a rotating environment induce serious discomfort, non-compensatory vestibulo-ocular reflexes, and subjective illusions of body tilt. In two experiments we investigated the effects of pitch and yaw head movements in participants placed supine on a rotating bed with their head at the center of rotation, feel at the rim. The vast majority of participants experienced motion sickness, inappropriate vertical nystagmus and illusory tilt and roll as predicted by a semicircular canal model. However, a small but significant number of the 28 participants experienced tilt in the predicted plane but in the opposite direction. Heart rate was elevated following one-second duration head turns. Significant adaptation occurred following a series of head turns in the light. Vertical nystagmus, motion sickness and illusory tilt all decreased with adaptation. Consequences for artificial gravity produced by short-radius centrifuges as a countermeasure are discussed.

Young, Laurence R.; Hecht, Heiko; Lyne, Lisette E.; Sienko, Kathleen H.; Cheung, Carol C.; Kavelaars, Jessica

2001-08-01

434

Flute waves at the ion Larmor radius scales

The theory of the magnetic Rayleigh-Taylor instability (RTI) is discussed. Modified linear kinetic theory allows us to investigate RTI and flute waves with arbitrary perpendicular spatial scales compared to the ion Larmor radius. It is shown that in the linear limit a Fourier transform of these equations yields the dispersion relation which in the so-called Pade approximation corresponds to results of the kinetic theory. This analysis represents an extension of the previous study of the magnetic RTI obtained in the large wave scale approximation. It is shown that incorporation of the effects associated with wave scales of the order of the ion Larmor radius leads to a broader wave number range of the magnetic RTI.

Onishchenko, O. G. [Institute of Physics of the Earth, 10 B. Gruzinskaya, 123995 Moscow (Russian Federation)

2010-12-14

435

The radius of baryonic collapse in disc galaxy formation

NASA Astrophysics Data System (ADS)

In the standard picture of disc galaxy formation, baryons and dark matter receive the same tidal torques, and therefore approximately the same initial specific angular momentum. However, observations indicate that disc galaxies typically have only about half as much specific angular momentum as their dark matter haloes. We argue this does not necessarily imply that baryons lose this much specific angular momentum as they form galaxies. It may instead indicate that galaxies are most directly related to the inner regions of their host haloes, as may be expected in a scenario where baryons in the inner parts of haloes collapse first. A limiting case is examined under the idealized assumption of perfect angular momentum conservation. Namely, we determine the density contrast ?, with respect to the critical density of the Universe, by which dark matter haloes need to be defined in order to have the same average specific angular momentum as the galaxies they host. Under the assumption that galaxies are related to haloes via their characteristic rotation velocities, the necessary ? is ˜600. This ? corresponds to an average halo radius and mass which are ˜60 per cent and ˜75 per cent, respectively, of the virial values (i.e. for ?= 200). We refer to this radius as the radius of baryonic collapse RBC, since if specific angular momentum is conserved perfectly, baryons would come from within it. It is not likely a simple step function due to the complex gastrophysics involved; therefore, we regard it as an effective radius. In summary, the difference between the predicted initial and the observed final specific angular momentum of galaxies, which is conventionally attributed solely to angular momentum loss, can more naturally be explained by a preference for collapse of baryons within RBC, with possibly some later angular momentum transfer.

Kassin, Susan A.; Devriendt, Julien; Fall, S. Michael; de Jong, Roelof S.; Allgood, Brandon; Primack, Joel R.

2012-07-01

436

Biomechanical Evaluation of Volar Locking Plates for Distal Radius Fractures

Purpose Fixed-angle devices have been a major advancement in orthopedic fracture care and have become an attractive option for fixation\\u000a of distal radius fractures. Several volar locking plates exist, but there is insufficient literature comparing the strengths\\u000a of these plates. This study compares the biomechanical strength of two popular volar locking plate systems (Synthes LCP and\\u000a Hand Innovations DVR-A) along with

Scott M. Levin; Cory O. Nelson; Jonathan D. Botts; Glenn A. Teplitz; Yong Kwon; Fred Serra-Hsu

2008-01-01

437

Pionic fluctuations of constituent quarks and the neutron charge radius

Pion loop fluctuations of the constituent u and d quarks are shown to give only a minute contribution to the intrinsic charge radius of the neutron, under the assumption that the pion-quark coupling constant has its conventional value, with a cut-off scale of the order of 1.0 GeV. The contribution from the pion loops to the anomalous magnetic moment of

L. Ya. Glozman; D. O. Riska

1999-01-01

438

Determining the Radius of Mercury from Messenger Flyby Occultations

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) space-craft has completed three flybys of Mercury. During the first (M1) and third (M3) flybys, MES-SENGER passed behind Mercury from the perspective of Earth, occulting the radio-frequency (RF) transmissions. Combined with an accurate ephemeris, the times of the RF occultations provide estimates of the planet radius at the surface location tangent

Mark Perry; Daniel Kahan; Carolyn Ernst; Sean C. Solomon; Maria T. Zuber; David E. Smith; Roger J. Phillips; Juergen Oberst; Sami Asmar

2010-01-01

439

Relation Between Viscosity and Radius of Gyration in Polymer Solution

NASA Astrophysics Data System (ADS)

The problem of a unified treatment of static and dynamic properties of dilute and semidilute solutions of high polymers has received the attention of many researchers, and has been the goal of many theoretical and experimental studies. In this work we present a molecular dynamics study of the relation between newtonian viscosity and radius of gyration. We have studied chains of 100 soft spheres bonded by harmonic potentials in concentrations belonging to the dilute and semidilute regimes. As a first approximation, the chains were simulated in vacuo. All runs were done in a square box with periodic boundary conditions and coupled to a heat bath . Viscosity was computed through time correlation functions (Kubo relations). We report data on newtonian viscosity, diffussion coefficient and radius of gyration as a function of concentration, and focus on the relation between viscosity and radius of gyration. We compare our results with theoretical predictions, in particular the power law predicted by simple scaling arguments together with blob and reptation models.

Grigera, Tomas S.; Grigera, J. Raul

1997-03-01

440

Uncertainties in interferometric measurements of radius of curvature

NASA Astrophysics Data System (ADS)

The radius of curvature of spherical surfaces may be determined using the well-known radius, or optical, bench. In this method, a figure measuring interferometer is employed to identify the null positions at the center of curvature (confocal) and surface (cat's eye) of the test optic. A linear slide provides motion between these positions and one or more displacement transducers is used to record the displacement between the cat's eye and confocal positions and, hence, the radius of curvature. Measurements of a polished Zerodur sphere have been completed on the X-ray Optics Calibration Interferometer (XCALIBIR) using both Twyman-Green and Fizeau configurations. Mechanical measurements of the spherical artifact have also been completed using a coordinate measuring machine (CMM). Recorded disagreement between the individual transmission sphere measurements and CMM measurements under well-controlled environmental conditions is larger than the limits predicted from a traditional uncertainty analysis based on a geometric measurement model. Additional uncertainty sources for the geometric model, as well as a physical optics model of the propagation of light, are therefore suggested. The expanded uncertainty analysis is described.

Schmitz, Tony L.; Davies, Angela D.; Evans, Christopher J.

2001-12-01

441

Stokes radius determination of radioiodinated polypeptide hormones by gel filtration

A simple technique for determination of the molecular (Stokes) radius of radioiodinated proteins was developed using the same column and chromatographic conditions employed in routine radioimmunoassay tracer purification. The calibration curve for five radioiodinated standard proteins presented a highly significant correlation (r = -0.996; P less than 0.001) and allowed precise molecular radius determination for labeled human growth hormone (hGH), luteotropin (hLH), follicle-stimulating hormone (hFSH), thyrotropin (hTSH), prolactin (hPRL), and corticotropin (hACTH), enabling detection of differences of the order of +/- 3%. The validity of the method was verified by determining the molecular radius of hGH in both ''cold'' (unlabeled standards and unknowns) and ''hot'' (radioiodinated standards and unknowns) systems. The technique can be applied in a very simple manner, requiring just one simple additional calibration run before Sephadex G-100 tracer purification. Furthermore, it can be applied to any protein, even when only extremely limited amounts are available. Since the standards and unknowns are labeled and chromatographed under identical conditions, potential common alterations of the molecule due to oxidation, iodine incorporation, tracer-carrier interactions, etc., are automatically corrected for.

Ribela, M.T.; Bartolini, P.

1988-11-01

442

Adaptation to head movements during short radius centrifugation

NASA Astrophysics Data System (ADS)

A fundamental lower limit to the radius of an artificial gravity centrifuge for long duration human space flights is the high rotation rate needed to achieve the acceleration needed at the rim. At high angular speeds, head movements out of the plane of rotation produce unexpected and disturbing Coriolis forces and cross-coupled angular accelerations, which lead to motion-sickness and disorientation. Previous studies in slowly rotating rooms set the practical upper speed limit at 6 rpm. Our research on intermittent repeated exposure to a short (2 m radius) high speed (23rpm,138?/s) centrifuge has shown that most subjects can adapt to such head movements in a short time, and retain adaptation over days or weeks. The practical implications of this include the use of intermittent short radius centrifugation for long duration flights and the possibility of adapting astronauts to the unusual stimulus before flight. Theoretical issues include the possible development of a generalized internal model to describe the effects of Coriolis forces, as opposed to the development of families of stimulus-response relationships.

Jarchow, Thomas; Young, Laurence R.

2007-11-01

443

PREFACE: International Conference on Optics of Excitons in Confined Systems

NASA Astrophysics Data System (ADS)

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 Patti (Sicily, Italy, 2007). It is addressed to scientists who lead fundamental and applied research on the optical properties of excitons in novel condensed-matter nanostructures. The 2009 meeting (7-11 September 2009) has brought together a large representation of the world leading actors in this domain, with the aim of stimulating the exchange of ideas, promoting international collaborations, and coordinating research on the newest exciton-related issues such as quantum information science and exciton