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We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a(B) ? 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement. PMID:22881597

Ulbricht, Ronald; Pijpers, Joep J H; Groeneveld, Esther; Koole, Rolf; Donega, Celso de Mello; Vanmaekelbergh, Daniel; Delerue, Christophe; Allan, Guy; Bonn, Mischa

2012-09-12

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condensation of bosons is expected to occur in coupled quantum wells CQW's . The critical con- ditions, where aB is the exciton Bohr radius, n is the e-h density, and d is the dimensionality the excitons can to Cooper pairs, and the exciton condensate, called the excitonic insulator, is analogous to the BCS

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NASA Astrophysics Data System (ADS)

Recently, the ground state Bohr radius (aB) of hydrogen was shown to be divided into two Golden sections, aB,p = aB/ø2 and aB,e = aB/ø at the point of electrical neutrality, where ø = 1.618 is the Golden ratio (R. Heyrovska, Molecular Physics 103: 877-882, and the literature cited therein). The origin of the difference of two energy terms in the Rydberg equation was thus shown to be in the ground state energy itself, as shown below: EH = (1/2)e2/(?aB) = (1/2)(e2/?) [(1/aB,p - (1/aB,e)] (1). This work brings some new results that 1) a unit charge in vacuum has a magnetic moment, 2) (e2/2?) in eq. (1) is an electromagnetic condenser constant, 3) the de Broglie wavelengths of the proton and electron correspond to the Golden arcs of a circle with the Bohr radius, 4) the fine structure constant (?) is the ratio of the Planck's constants without and with the interaction of light with matter, 5) the g-factors of the electron and proton, ge/2 and gp/2 divide the Bohr radius at the magnetic center and 6) the ``mysterious'' value (137.036) of ? -1 = (360/ø2) - (2/ø3), where (2/ø3) arises from the difference, (gp - ge).

Heyrovska, R.; Narayan, S.

2005-10-01

4

Exciton binding energy in semiconductor quantum dots

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

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

2010-04-15

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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

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Gain dynamics and excitonic transition in CdSe colloidal quantum dots

NASA Astrophysics Data System (ADS)

Dynamics of the photo-excited excitons in CdSe colloidal quantum dots was investigated by degenerate pump-probe measurement. Under resonant excitation, the lowest energy state of an electron-hole pair dominates two-step decay. The fast initial decay and long-term decay were attributed to the intrinsic Auger recombination and radiative recombination, respectively. As increasing the excited exciton density, a significant change of the later decay was seen before the absorption bleaching. As our nanocrystal is the case of strong confinement, in which an excitonic Bohr radius is smaller than the dot size, the biexciton-involved recombination is suggested to dominate at high carrier densities. Optical gain was measured by the variable stripe length method and broad gain spectrum was obtained due to the recombination of many excited levels between biexciton and exciton states. This result also supports biexcitons play an important role in the strong confinement regime at high carrier densities.

Kyhm, K.; Kim, J. H.; Kim, S. M.; Yang, Ho-soon

2007-09-01

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Multiple exciton generation in colloidal silicon nanocrystals.

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity. PMID:17645368

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

2007-08-01

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Multiple Exciton Generation in Colloidal Silicon Nanocrystals

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap {identical_to} Eg = 1.20 eV) to be 2.4 {+-} 0.1E{sub g} and find an exciton-production quantum yield of 2.6 {+-} 0.2 excitons per absorbed photon at 3.4E{sub g}. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. M.

2007-01-01

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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

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The Aharonov-Bohm effect for an exciton

NASA Astrophysics Data System (ADS)

We study theoretically the exciton absorption (luminescence) of a ring-like quantum dot shreded by a magnetic flux. We consider the limit when the width of the ring is smaller than the excitonic Bohr radius a_B. We demonstrate that, despite the electrical neutrality of the exciton, both the spectral position of the exciton peak in the absorption (luminescence), and the corresponding oscillator strength oscillate with magnetic flux with a period ?0 --- the universal flux quantum. Assuming that the attraction between electron and hole is short-ranged we find analytically the functional form of these oscillations for both quantities.^1 This enables us to trace the magnitude of the effect with changing the ratio 2 ? R/aB where R is the radius of the ring. Physically, the origin of the oscillations is the finite probability for electron and hole, created by a photon at the same point, to tunnel in the opposite directions and meet each other on the opposite side of the ring. Possible candidates for the experimental observation of the effect are recently discovered self-assembled quantum ring-like structures of InAs embedded in GaAs.^2,3 ^1R.A. Römer and M.E. Raikh, preprint cond-mat/9906314. ^2A. Lorke et al., Microelectronic Engeneering 47, 95 (1999). ^3H. Petterson et al., Proceedings of EP2DS-13, to be published in Physica E, (1999).

Römer, R. A.; Raikh, M. E.

2000-03-01

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

Arne Schirrmacher; A. Shimony

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Investigation of exciton ground state in quantum dots via Hamiltonian diagonalization method

NASA Astrophysics Data System (ADS)

We analyze the electron-hole (exciton) ground state associated with the first peak in the optical absorption spectra of semiconductor quantum dots. We assume the effective mass approximation and a dot radius R on the order of the exciton Bohr radius aB. A Hamiltonian diagonalization method which accounts for the exciton's kinetic, direct Coulomb, and surface polarization energies is used. We obtain a representation of the exciton ground-state wavefunction and a value for its energy using a basis set consisting of only three composite infinite spherical well wavefunctions. We discuss the precision obtained by this basis set by comparing with results from a much more extended basis set. Our results are used to predict the radius-dependent energy of the first peak in visible-light absorption spectra for CdSe quantum dots. Our analysis accurately describes the experimental data for dots with radii in the range aB

Schultz, Zachary M.; Essick, John M.

2008-03-01

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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

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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

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NASA Astrophysics Data System (ADS)

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

Bagheri, Mehran

2010-07-01

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NASA Astrophysics Data System (ADS)

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

Jiang, Jian-Hua; John, Sajeev

2014-07-01

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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

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A Simple Relativistic Bohr Atom

ERIC Educational Resources Information Center

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

Terzis, Andreas F.

2008-01-01

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Bohr's 1913 molecular model revisited

It is generally believed that the old quantum theory, as presented by Niels Bohr in 1913, fails when applied to few electron systems, such as the H2 molecule. Here, we find previously undescribed solutions within the Bohr theory that describe the potential energy curve for the lowest singlet and triplet states of H2 about as well as the early wave mechanical treatment of Heitler and London. We also develop an interpolation scheme that substantially improves the agreement with the exact ground-state potential curve of H2 and provides a good description of more complicated molecules such as LiH, Li2, BeH, and He2. PMID:16103360

Svidzinsky, Anatoly A.; Scully, Marlan O.; Herschbach, Dudley R.

2005-01-01

20

Bohr quantum theory and (Tesla) magnetic monopoles

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

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

2010-07-02

21

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

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Rutherford and Bohr describe atomic structure 1913

NSDL National Science Digital Library

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

1998-01-01

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NASA Astrophysics Data System (ADS)

The size of Pluto has been difficult to measure. Stellar occultations by Pluto have not yet probed altitudes lower than 1198 km, assuming the clear atmosphere model of Elliot, Person and Qu (2003). Differential refraction by Pluto's atmosphere attenuates the light from an occulted star to a level that is indistinguishable from the zero-level baseline long before Pluto's solid surface is a factor. Since Charon has no detectable atmosphere, its radius was well determined from a stellar occultation in 2005 (Gulbis et al. 2006, Sicardy et al. 2006). Combined with the mutual event photometry (Charon transited Pluto every 6.38 days between 1986 through 1992) - for which differential refraction is a negligible effect - the well-known radius of Charon translates into a more accurate radius for Pluto's solid surface. Our preliminary solid radius estimate for Pluto is 1161 km. We will discuss error bars and the correlations of this determination with Pluto albedo maps. We will also discuss the implications for Pluto's thermal profile, surface temperature and pressure, and constraints on the presence of a haze layer. This work is funded by NASA's Planetary Astronomy program. References Elliot, J.L., Person, M.J., & Qu, S. 2003, "Analysis of Stellar Occultation Data. II. Inversion, with Application to Pluto and Triton." AJ, 126, 1041. Gulbis, A.A.S. et al. 2006, "Charon's radius and atmospheric constraints from observations of a stellar occultation." Nature, 49, 48. Sicardy, B. et al. 2006, "Charon's size and an upper limit on its atmosphere from a stellar occultation." Nature, 49, 52.

Young, Eliot F.; Young, L. A.; Buie, M.

2007-10-01

24

Mott transition of excitons in coupled quantum wells.

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition. PMID:18643682

Stern, M; Garmider, V; Umansky, V; Bar-Joseph, I

2008-06-27

25

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

26

How good is Niels Bohr's atomic model?

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

R. Stephen Berry

1989-01-01

27

Analytical Special Solutions of the Bohr Hamiltonian

The following special solutions of the Bohr Hamiltonian are briefly described: 1) Z(5) (approximately separable solution in five dimensions with gamma close to 30 degrees), 2) Z(4) (exactly separable gamma-rigid solution in four dimensions with gamma = 30 degrees), 3) X(3) (exactly separable gamma-rigid solution in three dimensions with gamma =0). The analytical solutions obtained using Davidson potentials in the E(5), X(5), Z(5), and Z(4) frameworks are also mentioned.

D. Bonatsos; D. Lenis; D. Petrellis; P. A. Terziev; I. Yigitoglu

2005-12-13

28

Spreading the gospel: The Bohr atom popularised

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

Kragh, Helge

2011-01-01

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

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.

31

Exciton diffusion and relaxation in methyl-substituted polyparaphenylene polymer films

NASA Astrophysics Data System (ADS)

Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units.

Gulbinas, V.; Minevi?iut?, I.; Hertel, D.; Wellander, R.; Yartsev, A.; Sundström, V.

2007-10-01

32

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

33

Bohr's Creation of his Quantum Atom

NASA Astrophysics Data System (ADS)

Fresh letters throw new light on the content and state of Bohr's mind before and during his creation of the quantum atom. His mental furniture then included the atomic models of the English school, the quantum puzzles of Continental theorists, and the results of his own studies of the electron theory of metals. It also included the poetry of Goethe, plays of Ibsen and Shakespeare, novels of Dickens, and rhapsodies of Kierkegaard and Carlyle. The mind that held these diverse ingredients together oscillated between enthusiasm and dejection during the year in which Bohr took up the problem of atomic structure. He spent most of that year in England, which separated him for extended periods from his close-knit family and friends. Correspondence with his fianc'ee, Margrethe Nørlund, soon to be published, reports his ups and downs as he adjusted to J.J. Thomson, Ernest Rutherford, the English language, and the uneven course of his work. In helping to smooth out his moods, Margrethe played an important and perhaps an enabling role in his creative process.

Heilbron, John

2013-04-01

34

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

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

Schwarz, W H Eugen

2013-11-18

35

Bohr model and dimensional scaling analysis of atoms and molecules

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

Kerim Urtekin

2006-01-01

36

Davidson potential and SUSYQM in the Bohr Hamiltonian

The Bohr Hamiltonian is modified through the Shape Invariance principle of SUper-SYmmetric Quantum Mechanics for the Davidson potential. The modification is equivalent to a conformal transformation of Bohr's metric, generating a different {beta}-dependence of the moments of inertia.

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

2013-06-10

37

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

38

Volume Spectrum from Bohr-Sommerfeld Quantization

NASA Astrophysics Data System (ADS)

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

Haggard, Hal; Bianchi, Eugenio

2011-04-01

39

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

NASA Astrophysics Data System (ADS)

In May 1918 Paul Ehrenfest received a monograph from Niels Bohr in which Bohr had used Ehrenfest's adiabatic principle as an essential assumption for understanding atomic structure. Ehrenfest responded by inviting Bohr, whom he had never met, to give a talk at a meeting in Leiden in late April 1919, which Bohr accepted; he lived with Ehrenfest, his mathematician wife Tatyana, and their young family for two weeks. Albert Einstein was unable to attend this meeting, but in October 1919 he visited his old friend Ehrenfest and his family in Leiden, where Ehrenfest told him how much he had enjoyed and profited from Bohr's visit. Einstein first met Bohr when Bohr gave a lecture in Berlin at the end of April 1920, and the two immediately proclaimed unbounded admiration for each other as physicists and as human beings. Ehrenfest hoped that he and they would meet at the Third Solvay Conference in Brussels in early April 1921, but his hope was unfulfilled. Einstein, the only physicist from Germany who was invited to it in this bitter postwar atmosphere, decided instead to accompany Chaim Weizmann on a trip to the United States to help raise money for the new Hebrew University in Jerusalem. Bohr became so overworked with the planning and construction of his new Institute for Theoretical Physics in Copenhagen that he could only draft the first part of his Solvay report and ask Ehrenfest to present it, which Ehrenfest agreed to do following the presentation of his own report. After recovering his strength, Bohr invited Ehrenfest to give a lecture in Copenhagen that fall, and Ehrenfest, battling his deep-seated self-doubts, spent three weeks in Copenhagen in December 1921 accompanied by his daughter Tanya and her future husband, the two Ehrenfests staying with the Bohrs in their apartment in Bohr's new Institute for Theoretical Physics. Immediately after leaving Copenhagen, Ehrenfest wrote to Einstein, telling him once again that Bohr was a prodigious physicist, and again expressing the hope that he soon would see both of them in Leiden.

Klein, Martin J.

2010-09-01

40

When champions meet: Rethinking the Bohr--Einstein debate

Einstein's philosophy of physics (as clarified by Fine, Howard, and Held) was predicated on his Trennungsprinzip, a combination of separability and locality, without which he believed objectification, and thereby "physical thought" and "physical laws", to be impossible. Bohr's philosophy (as elucidated by Hooker, Scheibe, Folse, Howard, Held, and others), on the other hand, was grounded in a seemingly different doctrine about the possibility of objective knowledge, namely the necessity of classical concepts. In fact, it follows from Raggio's Theorem in algebraic quantum theory that - within an appropriate class of physical theories - suitable mathematical translations of the doctrines of Bohr and Einstein are equivalent. Thus - upon our specific formalization - quantum mechanics accommodates Einstein's Trennungsprinzip if and only if it is interpreted a la Bohr through classical physics. Unfortunately, the protagonists themselves failed to discuss their differences in this constructive way, since their debate was dominated by Einstein's ingenious but ultimately flawed attempts to establish the "incompleteness" of quantum mechanics. This aspect of their debate may still be understood and appreciated, however, as reflecting a much deeper and insurmountable disagreement between Bohr and Einstein on the knowability of Nature. Using the theological controversy on the knowability of God as a analogy, Einstein was a Spinozist, whereas Bohr could be said to be on the side of Maimonides. Thus Einstein's off-the-cuff characterization of Bohr as a 'Talmudic philosopher' was spot-on.

N. P. Landsman

2005-07-22

41

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

42

Solutions of the Bohr hamiltonian, a compendium

The Bohr hamiltonian, also called collective hamiltonian, is one of the cornerstone 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 $V(\\beta,\\gamma)$ 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 $\\gamma-$unstable and $\\gamma-$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, and an appendix on the analysis of a few interesting numerical sequences appearing in this context. The quasi-dynamical SO(2) symmetry is proposed in connection with the labeling of bands in triaxial nuclei.

Lorenzo Fortunato

2004-11-22

43

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

Howard, Don

44

NASA Technical Reports Server (NTRS)

Resonator configurations permitting operation with large mode radius while maintaining good transverse mode discrimination are considered. Stable resonators incorporating an intracavity telescope and unstable resonator geometries utilizing an output coupler with a Gaussian reflectivity profile are shown to enable large radius single mode laser operation. Results of heterodyne studies of pulsed CO2 lasers with large (11mm e sup-2 radius) fundamental mode sizes are presented demonstrating minimal frequency sweeping in accordance with the theory of laser-induced medium perturbations.

Harris, Michael R.

1987-01-01

45

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

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

Zheng, Guishan; Schaefer, Michael; Karplus, Martin

2013-11-26

46

The one-radius triplet is an optical system that consists of three lenses. Either the radii of curvature of the lenses have the same absolute value or one radius of curvature has an infinitely large value. The advantage of such optical systems is that their production cost is less than that of systems with ordinary triplets. Here a theory of one-radius triplets is described and tables of parameters for their modification are provided. Residual aberrations are given for several selected triplets. One-radius triplets are suitable for use in laser technology and metrology. PMID:11900004

Miks, Antonin

2002-03-01

47

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

48

Solution of the Bohr hamiltonian for soft triaxial nuclei

The Bohr-Mottelson model is solved for a generic soft triaxial nucleus, separating the Bohr hamiltonian exactly and using a number of different model-potentials: a displaced harmonic oscillator in $\\gamma$, which is solved with an approximated algebraic technique, and Coulomb/Kratzer, harmonic/Davidson and infinite square well potentials in $\\beta$, which are solved exactly. In each case we derive analytic expressions for the eigenenergies which are then used to calculate energy spectra. Here we study the chain of osmium isotopes and we compare our results with experimental information and previous calculations.

L. Fortunato; S. De Baerdemacker; K. Heyde

2006-07-24

49

NASA Astrophysics Data System (ADS)

Recently a new type of system exhibiting spontaneous coherence has emerged--the exciton-polariton condensate. Exciton-polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The polaritons have a lifetime that is typically comparable to or shorter than thermalization times, giving them an inherently non-equilibrium nature. Nevertheless, they exhibit many of the features that would be expected of equilibrium Bose-Einstein condensates (BECs). The non-equilibrium nature of the system raises fundamental questions as to what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus on several physical phenomena exhibited by exciton-polariton condensates. In particular, we examine topics such as the difference between a polariton BEC, a polariton laser and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, and Berezinskii-Kosterlitz-Thouless and Bardeen-Cooper-Schrieffer physics. We also discuss the physics and applications of engineered polariton structures.

Byrnes, Tim; Kim, Na Young; Yamamoto, Yoshihisa

2014-11-01

50

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

51

Critical radius of endotherms.

The critical radius effect for insulation, well known in the engineering literature, was used by other authors to explain the lack of insulation on newborn endotherms. If that effect existed in small animals, they would lose less heat if nude than if fur or feathers were present. We show 1) that the previous analysis, although incomplete, yields the same result as a solid insulation model with the required sophistication and 2) that a proper model of fur is a porous media model. Neither of two porous media versions yield a critical radius effect. No critical radius effect occurs because simultaneous heat transfer by conduction and radiation makes it impossible to obtain the required logarithmic increase in thermal resistance with increasing insulation radius in a porous medium. PMID:3963238

Porter, W P; Parkhurst, D F; McClure, P A

1986-04-01

52

Effective Cleaning Radius Studies

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

Churnetski, B.V.

2001-10-15

53

Bohr Hamiltonian with deformation-dependent mass term

The Bohr Hamiltonian describing the collective motion of atomic nuclei is modified by allowing the mass to depend on the nuclear deformation. Exact analytical expressions are derived for spectra and wave functions in the case of a gamma-unstable Davidson potential, using techniques of supersymmetric quantum mechanics. Numerical results in the Xe-Ba region are discussed.

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

2009-12-18

54

Bohr Hamiltonian with deformation-dependent mass term

The Bohr Hamiltonian describing the collective motion of atomic nuclei is modified by allowing the mass to depend on the nuclear deformation. Exact analytical expressions are derived for spectra and wave functions in the case of a gamma-unstable Davidson potential, using techniques of supersymmetric quantum mechanics. Numerical results in the Xe-Ba region are discussed.

Bonatsos, Dennis; Lenis, D; Minkov, N; Quesne, C

2009-01-01

55

"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

56

When champions meet: Rethinking the Bohr–Einstein debate

Einstein's philosophy of physics (as clarified by Fine, Howard, and Held) was predicated on his Trennungsprinzip, a combination of separability and locality, without which he believed objectification, and thereby “physical thought” and “physical laws’, to be impossible. Bohr's philosophy (as elucidated by Hooker, Scheibe, Folse, Howard, Held, and others), on the other hand, was grounded in a seemingly different doctrine

N. P. Landsman

2006-01-01

57

Whispering-gallery exciton polaritons in submicron spheres

NASA Astrophysics Data System (ADS)

We show that a semiconductor sphere with a radius comparable with the wavelength of light at the exciton resonance frequency can behave as a high-quality three-dimensional microcavity or “polariton dot.” We obtain numerical results for gallium arsenide submicron spheres and demonstrate that, in contrast to a larger sphere or planar microcavity, they can simultaneously possess both large quality factor and high finesse. In the strong-coupling regime the Rabi splitting approaches the bulk polariton splitting.

Platts, C. E.; Kaliteevski, M. A.; Brand, S.; Abram, R. A.; Iorsh, I. V.; Kavokin, A. V.

2009-06-01

58

Quantum theory of exciton polaritons in cylindrical semiconductor microcavities

A quantum-mechanical formalism is developed in order to study the interaction between a quantum-well exciton and the electromagnetic (e.m.) field inside a cylindrical microcavity. The cavity modes are evaluated as a function of the radius according to a self-consistent procedure, and are found to be in good agreement with the experiment [J.M. Gérard, D. Barrier, J. Y. Marzin, R. Kuszelewicz,

Giovanna Panzarini; Lucio Claudio Andreani

1999-01-01

59

NASA Astrophysics Data System (ADS)

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

Han, Zhenyu

2014-10-01

60

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

Zhenyu Han

2014-02-06

61

Bohr model and dimensional scaling analysis of atoms and molecules

NASA Astrophysics Data System (ADS)

It is generally believed that the old quantum theory, as presented by Niels Bohr in 1913, fails when applied to many-electron systems, such as molecules, and nonhydrogenic atoms. It is the central theme of this dissertation to display with examples and applications the implementation of a simple and successful extension of Bohr's planetary model of the hydrogenic atom, which has recently been developed by an atomic and molecular theory group from Texas A&M University. This "extended" Bohr model, which can be derived from quantum mechanics using the well-known dimentional scaling technique is used to yield potential energy curves of H2 and several more complicated molecules, such as LiH, Li2, BeH, He2 and H3, with accuracies strikingly comparable to those obtained from the more lengthy and rigorous "ab initio" computations, and the added advantage that it provides a rather insightful and pictorial description of how electrons behave to form chemical bonds, a theme not central to "ab initio" quantum chemistry. Further investigation directed to CH, and the four-atom system H4 (with both linear and square configurations), via the interpolated Bohr model, and the constrained Bohr model (with an effective potential), respectively, is reported. The extended model is also used to calculate correlation energies. The model is readily applicable to the study of molecular species in the presence of strong magnetic fields, as is the case in the vicinities of white dwarfs and neutron stars. We find that magnetic field increases the binding energy and decreases the bond length. Finally, an elaborative review of doubly coupled quantum dots for a derivation of the electron exchange energy, a straightforward application of Heitler-London method of quantum molecular chemistry, concludes the dissertation. The highlights of the research are (1) a bridging together of the pre- and post quantum mechanical descriptions of the chemical bond (Bohr-Sommerfeld vs. Heisenberg-Schrodinger), and (2) the reporting of the appearance of new bound states of H2 in the presence of very strong magnetic fields. The new states emerge above the critical value of 5 x 107 G, and hence cannot be obtained perturbatively.

Urtekin, Kerim

62

Distal radius triplane fracture.

A triplane fracture is so named because of the three planes traversed by the fracture line. These are physeal fractures that result from injury during the final phase of maturation and cessation of growth. This fracture pattern typically involves the distal tibia. We present a rare case of a triplane fracture involving the distal radius. PMID:25350166

Parkar, Aah; Marya, S; Auplish, S

2014-11-01

63

Microscopic Uni-axial Bohr-Mottelson Rotational Model

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

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

2010-08-04

64

to elastic disorder scattering (third diagram). The total self energy enters Dyson's equation (third line direction. Thus, equilibration at low temperatures may be within reach [1]. In addi- tion, a lateral for multiple exciton-exciton (XX) scattering [4], and add disorder effects in lowest order. The aim

Zimmermann, Roland

65

Detonation Shock Radius Experiments.

NASA Astrophysics Data System (ADS)

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

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

2007-06-01

66

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

67

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

68

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

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

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

2014-10-01

69

Transient excitons at metal surfaces

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

70

Validation of Bohr dead space measured by volumetric capnography

Purpose Bohr’s dead space (VDBohr) is commonly calculated using end-tidal CO2 instead of the true alveolar partial pressure of CO2 (PACO2). The aim of this work was to validate VDBohr using PACO2 derived from volumetric capnography (VC) against VDBohr with PACO2 values obtained from the standard alveolar air formula.\\u000a \\u000a \\u000a \\u000a \\u000a Methods Expired gases of seven lung-lavaged pigs were analyzed at different lung conditions

Gerardo Tusman; Fernando Suarez Sipmann; Joao B. Borges; Göran Hedenstierna; Stephan H. Bohm

2011-01-01

71

An Increased Bohr Effect in Sickle Cell Anemia

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

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

1979-01-01

72

Bohr Hamiltonian with Davidson potential for triaxial nuclei

A solution of the Bohr Hamiltonian appropriate for triaxial shapes, involving a Davidson potential in {beta} and a steep harmonic oscillator in {gamma}, centered around {gamma}={pi}/6, is developed. Analytical expressions for spectra andB(E2) transition rates ranging from a triaxial vibrator to the rigid triaxial rotator are obtained and compared to experimental results. Using a variational procedure, we point out that the Z(5) solution, in which an infinite square well potential in {beta} is used, corresponds to the critical point of the shape phase transition from a triaxial vibrator to the rigid triaxial rotator.

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

2011-01-15

73

NSDL National Science Digital Library

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

2011-01-01

74

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

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

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

2010-07-02

75

Molecular Basis of the Bohr Effect in Arthropod Hemocyanin

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

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

2008-01-01

76

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

A de Broglie-Bohr Model for the Hydrogen Atom Frank Rioux Chemistry Department CSB|SJU The 1913 Bohr model of the hydrogen atom was replaced by Schrodinger's wave mechanical model in 1926. However (accelerating) charge radiates energy and will collapse into the oppositely charge nucleus. In 1924 de Broglie

Rioux, Frank

77

Moving gapless indirect excitons in monolayer graphene.

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

Mahmoodian, Mahmood; Entin, Matvey

2012-01-01

78

Singlet exciton fission in solution

(Fig. 2c). These triplet features grow, and the singlet decays, with a characteristic time of 530 ps (Supplementary Fig. S7). This kinetic is equivalent to a triplet formation rate of 1.7 x 10-6 mol L-1 ps-1, with a bimolecular rate constant of 2.2 x... . & Nozik, A. J. Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers. Journal of Applied Physics 100, 074510 (2006). 13. Jadhav, P. J. et al. Triplet exciton dissociation in singlet exciton fission...

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

2013-11-17

79

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

80

Conjugated “Molecular Wire” for Excitons

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

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

2010-05-06

81

All conjugated copolymer excitonic multiferroics.

A substantial magnetoelectric coupling effect of an excitonic all-conjugated block copolymer multiferroics consisting of electronically distinct polythiophene derivatives is reported. The observations open new avenues for the multifunctional all-conjugated block copolymer synthesis and electric field tunable multiferroic devices. PMID:23172730

Lohrman, Jessica; Liu, Yueying; Duan, Shaofeng; Zhao, Xiaoyong; Wuttig, Manfred; Ren, Shenqiang

2013-02-01

82

Distal Radius Fracture (Broken Wrist)

... the distal radius can break include: • Intra-articular fracture. A fracture that extends into the wrist joint. (“Articular” means “joint.”) • Extra-articular fracture. A fracture that does not extend into the ...

83

Exciton Seebeck effect in molecular systems.

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

Yan, Yun-An; Cai, Shaohong

2014-08-01

84

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

We study theoretically the optical properties of an exciton in a two-dimensional ring threaded by a magnetic flux. We model the quantum ring by a confining potential that can be continuously tuned from strictly one-dimensional to truly two-dimensional with finite radius-to-width ratio. We present an analytic solution of the problem when the electron-hole interaction is short ranged. The oscillatory dependence

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

2011-01-01

85

Exciton trapping at heterojunctions in polymer blends.

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency. PMID:16035815

Morteani, Arne C; Friend, Richard H; Silva, Carlos

2005-06-22

86

Cross-polarized excitons in carbon nanotubes

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

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

2008-01-01

87

Exciton liquid in coupled quantum wells.

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

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

2014-01-01

88

Spontaneous coherence in a cold exciton gas.

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

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

2012-03-29

89

Exciton circular dichroism in channelrhodopsin.

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

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

2014-10-16

90

Derivation of microscopic unified Bohr-Mottelson rotational model

NASA Astrophysics Data System (ADS)

In a previous article, we derived a microscopic version of the phenomenological Bohr-Mottelson unified rotational model for rotation about a single axis. In this article, we generalize the derivation to that for rotation about all the three axes. As in the previous derivation, we apply the nuclear Hamiltonian directly to the rotational-model wavefunction instead of using the usual canonical transformation. In this way, we avoid using redundant coordinates or imposing any constraints on the rotationally-invariant rotational-model intrinsic wavefunction. We show that, in the transformed nuclear Schrödinger equation, the Coriolis coupling term vanishes exactly only for a choice of the rotational-model Euler angles that is consistent with angle-angular momentum commutation relation and rotational invariance of the intrinsic wavefunction. For this choice of the Euler angles, the kinematic moment-of-inertia tensor, collective-rotation velocity field, and flow vorticity have the rigid-flow characteristics. This quantum rigid flow reduces to irrotational free-vortex flow in the limit of a single particle. We derive a microscopic effective rotation-intrinsic unified Schrödinger equation for the states of a rotational band that reduces to the phenomenological, unified, tri-axial quantum rigid-rotor model in the limit that the off-diagonal elements of the kinematic inertia tensor operator can be neglected. The model derivation shows that a multi-fermion system with unpaired or paired (quasi) particles rotates rigidly and a single-particle system rotates irrotationally if the intrinsic system is rotationally invariant.

Gulshani, P.

2011-02-01

91

Exciton-Exciton Interaction in Semiconductor Quantum Wells

Structures 7 PFIF untum ells F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F U PFPF he ingleEixiton rolem F F F F F F F F F F F F F F F F F F F F F F F IH PFQF wnyErtile sntertions F F F F F F F F F F F F F F F F F F F F F F F F IQ 3. Treatment of Interacting Excitons 17 QFIF wnyEixiton rmiltonin F F F F

Zimmermann, Roland

92

Effects of excitons on solar cells

NASA Astrophysics Data System (ADS)

We have studied the effects of excitons on the two key parameters of a Si solar cell: the dark-saturation current and short-circuit current. We have found that the effect of excitons on the dark-saturation current is very sensitive to the boundary condition for excess excitons at the edge of the depletion region. With the assumption of near equilibrium between the electrons and excitons, we find that the exciton effect is rather small, which is contrary to the conclusion of significant reduction in the dark-saturation current made in previous work with the assumption of no excess excitons at the edge [J. Appl. Phys. 79, 195 (1996)]. The results for the short-circuit current are very similar to the previous work. However, the analytical results for the carrier concentrations and the corresponding currents are now presented in a simple way in which the physical meaning of each individual term is elucidated or revealed. Furthermore, we have found, for practical purposes, very accurate approximate solutions for the carrier concentrations and corresponding currents. Our conclusion is that the major effect of excitons on the Si solar cell performance relies on whether the effective diffusion length (L1) of the coupled electron-exciton system is significantly greater than that of the electron itself (Le).

Zhang, Yong; Mascarenhas, Angelo; Deb, Satyen

1998-10-01

93

Excitonic properties of graphene-based materials.

First-principle density functional theory (DFT) calculations with quasiparticle corrections and many body effects are performed to study the electronic and optical properties of graphene-based materials. This review summarizes the excitonic properties including optical transition spectra and the distribution of exciton wavefunctions, thus providing the theoretical knowledge and predictions for promising optical applications of graphene materials. PMID:21960114

Wang, Min; Li, Chang Ming

2012-02-21

94

This paper describes the enhancement of the quantum yield of photoluminescence (PL) of CdSe quantum dots (QDs) upon the adsorption of an exciton-delocalizing ligand, phenyldithiocarbamate. Increasing the apparent excitonic radius by only 10% increases the value of the radiative rate constant by a factor of 1.8 and the PL quantum yield by a factor of 2.4. Ligand exchange therefore simultaneously perturbs the confinement energy of charge carriers and enhances the probability of band-edge transitions. PMID:25167466

Jin, Shengye; Harris, Rachel D; Lau, Bryan; Aruda, Kenneth O; Amin, Victor A; Weiss, Emily A

2014-09-10

95

Tightly Bound Excitons in Monolayer WSe2

NASA Astrophysics Data System (ADS)

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.

He, Keliang; Kumar, Nardeep; Zhao, Liang; Wang, Zefang; Mak, Kin Fai; Zhao, Hui; Shan, Jie

2014-07-01

96

The Structure and Dynamics of Molecular Excitons

NASA Astrophysics Data System (ADS)

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

Bardeen, Christopher J.

2014-04-01

97

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4? complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra. PMID:21151483

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

2010-01-01

98

Excitonic effects in intraband quantum dot spectroscopy: Formation of bound continuum excitons

NASA Astrophysics Data System (ADS)

We study intraband spectra, resulting from intraband transitions between the bound ground state of a semiconductor quantum dot and unbound continuum states in the host medium. The influence of excitonic coupling on these intraband absorption spectra and the formation of special excitons are discussed. Involving Coulomb coupled bound-to-continuum transitions, the spectra show the formation of bound excitons at the continuum edge and spectral shifts. The analyzed intraband absorption spectra exhibit different signatures for different initial carrier configurations.

Kuhn, Sandra C.; Richter, Marten

2014-09-01

99

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

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

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

2011-12-15

100

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

101

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

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

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

2011-09-26

102

Excitonic Phases from Weyl Semimetals

NASA Astrophysics Data System (ADS)

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

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

2012-11-01

103

Excitonic phases from Weyl semimetals.

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

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

2012-11-01

104

Special Solutions of the Bohr Hamiltonian Related to Shape Phase Transitions in Nuclei

Nuclei exhibit quantum phase transitions (earlier called ground state phase transitions) between different shapes as the number of nucleons is modified, resulting in changes in the ground and low lying nuclear states. Special solutions of the Bohr Hamiltonian appropriate for the critical point of such shape phase transitions, as well as other special solutions applicable to relevant nuclear regions are described.

Dennis Bonatsos; D. Lenis; D. Petrellis

2007-01-19

105

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

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

Helge Kragh; Kristian Hvidtfelt Nielsen

2012-01-01

106

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

ERIC Educational Resources Information Center

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

Niaz, Mansoor; Cardellini, Liberato

2011-01-01

107

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

108

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

Gurmukh Singh; Amitabha Mukhopadyay

2010-01-01

109

Correlated exciton dynamics in semiconductor nanostructures

The absorption and dissipation of energy in semiconductor nanostructures are often determined by excited electron dynamics. In semiconductors, one fundamentally important electronic state is an exciton, an excited electron ...

Wen, Patrick, Ph. D. Massachusetts Institute of Technology

2013-01-01

110

Topologically protected excitons in porphyrin thin films

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

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

2014-06-05

111

Triplet excitons: Bringing dark states to light

NASA Astrophysics Data System (ADS)

Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.

Bardeen, Christopher J.

2014-11-01

112

Characteristics of excitonic emission in diamond

NASA Astrophysics Data System (ADS)

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

Okushi, Hideyo; Watanabe, Hideyuki; Kanno, Shokichi

2005-09-01

113

Giant Rydberg Excitons in Cuprous Oxide

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

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

2014-01-01

114

Phase diagram of the excitonic insulator

Motivated by recent experiments, which give strong evidence for an excitonic insulating phase in TmSe0.45Te0.55, we developed a scheme to quantitatively construct, for generic two-band models, the phase diagram of an excitonic insulator. As a first application of our approach, we calculated the phase diagram for an effective mass two-band model with long-range Coulomb interaction. The shielded potential approximation is

B. Hülsen; F. X. Bronold; H. Fehske; K. Yonemitsu

2006-01-01

115

Bounded coherent states and excitonic systems

NASA Astrophysics Data System (ADS)

New bounded coherent state construction, based in a Keldysh conjecture, is presented. The particular group structure arising from the model leads a new symmetry transformations for the coherent state system. The emergent new symmetry transformations are reminiscent of the Bogoliubov ones. This construction is applied to describe an excitonic system. We discuss how the symmetry of these transformations is intrinsically related with the stability and the behavior of the physical systems as in the excitonic case.

Cirilo-Lombardo, Diego Julio

2014-07-01

116

Exciton energy transfer between nanoparticles and nanowires

In this paper, we study the processes of exciton transfer in coupled nanoparticles and nanowires, and obtain convenient equations and numerical results for energy-transfer rates. For large distances, the energy-transfer rates are proportional to 1\\/d5 , where d is the nanoparticle-nanowire distance. Our models incorporate both semiconductor and metal material systems. In this paper, we show that excitonic energy generated

Pedro L. Hernández-Martínez; Alexander O. Govorov

2008-01-01

117

Charge transfer excitons in codeposited organic films

NASA Astrophysics Data System (ADS)

Charge transfer (CT) excitons play an important role in solar cells and organic light emitting devices. Such excitons are usually formed at the hetero-interfaces with an electron and hole residing in different materials. We studied the formation and recombination dynamics of CT excitons in co-deposited Alq3/PTCDA, TPD/Alq3, and TPD/PTCDA thin films that are grown by organic molecular beam deposition with various co-deposition ratios. (Alq3: Tris(8-hydroxyquinolinato)aluminium, PTCDA: perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride, TPD: N,N'-Bis(3-methylphenyl)- N,N'-diphenylbenzidine.) The formation of CT excitons is associated with a quenching of the photoluminescence (PL) of the individual material. The PL spectrum shows a new emission peak at ˜750 nm for Alq3/PTCDA films and ˜700 nm for TPD/Alq3 which we tentatively attribute to the radiative recombination of CT excitons. The formation and decay time of the CT excitons is studied by time-resolved PL measurements using the technique of time-correlated single photon counting.

Wang, Xiaosheng; Milhan, Ajward; Wagner, Hans Peter

2010-03-01

118

Molecular excitons in a copper azadipyrrin complex.

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

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

2014-11-12

119

Localization of Excitons by Molecular Layer Formation in a Polymer Film

Atactic polystyrene of two different molecular weights (560900 g.mol-1 and 212400 g.mol-1) have been studied as spin-coated films of thickness varying from ~30 nm to ~250 nm, i.e., ~2Rg to ~12Rg where Rg is the unperturbed radius of gyration of polystyrene, using x-ray reflectivity and transmission UV-spectroscopy. Electron density profiles along depth of the films show formation of layers parallel to the substrate surface, when the film thickness is below 4Rg, with layer spacing ~2Rg. The pure electronic singlet 1A1g -> 1E1u indicates exciton interaction throughout film thickness, as evidenced by the lineshape, drop in extinction coefficient and size-dependent blue shift of the corresponding UV peak. Analysis of the total energy of the exciton as a function of film thickness shows that above the thickness of 4Rg the exciton is delocalized over the film thickness through linear, molecular J-aggregates. The separations between these molecules in the two polystyrene samples match with the respective layer spacing obtained from x-ray reflectivity. Transition dipole moments of the molecules in this aggregate are parallel and each dipole is arranged at an angle of ~43 degrees with the axis of the linear chain. Below a film thickness of 4Rg, as the molecular layers are formed, the exciton becomes localized within these layers. This localization is clearly indicated by the lineshape becoming comparable to isolated molecular bands, and by both the extinction coefficient and blue shift reaching maximum values and becoming size independent. Localization of excitons due to layer formation has been explained as caused by decrease of cohesion between layers.

Sudeshna Chattopadhyay; Alokmay Datta

2005-01-20

120

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

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

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

2013-02-15

121

Exciton transport and dissociation at organic interfaces

NASA Astrophysics Data System (ADS)

This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length LD in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger LD values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules (>50nm) to values actually measured in CPs (<10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how these interactions in turn affect the energy landscape around the interface and therefore also the energy barrier for exciton dissociation into free carriers.

Beljonne, David

2011-03-01

122

Coherence Length of Excitons in a Semiconductor Quantum Well

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

Zhao, Hui; Moehl, Sebastian; Kalt, Heinz

2002-08-01

123

Investigating exciton correlations using coherent multidimensional optical spectroscopy

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

Turner, Daniel Burton

2010-01-01

124

Hot exciton transport in ZnSe quantum wells

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

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

2002-02-01

125

Macroscopically ordered state in an exciton system.

There is a rich variety of quantum liquids -- such as superconductors, liquid helium and atom Bose-Einstein condensates -- that exhibit macroscopic coherence in the form of ordered arrays of vortices. Experimental observation of a macroscopically ordered electronic state in semiconductors has, however, remained a challenging and relatively unexplored problem. A promising approach for the realization of such a state is to use excitons, bound pairs of electrons and holes that can form in semiconductor systems. At low densities, excitons are Bose-particles, and at low temperatures, of the order of a few kelvin, excitons can form a quantum liquid -- that is, a statistically degenerate Bose gas or even a Bose-Einstein condensate. Here we report photoluminescence measurements of a quasi-two-dimensional exciton gas in GaAs/AlGaAs coupled quantum wells and the observation of a macroscopically ordered exciton state. Our spatially resolved measurements reveal fragmentation of the ring-shaped emission pattern into circular structures that form periodic arrays over lengths up to 1 mm. PMID:12181559

Butov, L V; Gossard, A C; Chemla, D S

2002-08-15

126

put forward a new model for the hydrogen atom, combining Ernest Rutherford's idea of a positively to the time of Bohr and Rutherford, the field of atomic physics only really became recognizable as a separate

Loss, Daniel

127

Synergetics in multiple exciton generation effect in quantum dots

NASA Astrophysics Data System (ADS)

We present detailed analysis of the non-Poissonian population of excitons produced by multiple exciton generation (MEG) effect in quantum dots on the base of statistic theory of MEG and synergetic approach for chemical reactions. From the analysis we can conclude that a non-Poissonian distribution of exciton population is evidence of nonlinear and nonequilibrium character of the process of multiple generation of excitons in quantum dots at a single photon absorption.

Turaeva, N. N.; Oksengendler, B. L.; Uralov, I.

2011-06-01

128

Disorder-enhanced exciton delocalization in an extended dendrimer

NASA Astrophysics Data System (ADS)

The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory.

Pouthier, Vincent

2014-08-01

129

Narrowed bandgaps and stronger excitonic effects from small boron nitride nanotubes

NASA Astrophysics Data System (ADS)

The bandgap of boron nitride nanotubes (BNNTs) is generally considered to be independent on tube radius and chirality. However, we have observed that the bandgaps of BNNTs do depend on the tube size. Photoluminescence excitation spectroscopy with variable photon energies in vacuum ultraviolet (VUV) range revealed that the bandgap becomes smaller when the tube diameter decreases. This is consistent with red-shifted luminescent emissions. The strong interactions between excitons and phonons are possibly responsible for the bandgap narrowing as the function of nanotube size.

Yu, Jun; Yu, Dehong; Chen, Ying; Chen, Hua; Lin, Meng-Yeh; Cheng, Bing-Ming; Li, Jia; Duan, Wenhui

2009-07-01

130

Quantum Humor: The Playful Side of Physics at Bohr's Institute for Theoretical Physics

NASA Astrophysics Data System (ADS)

From the 1930s to the 1950s, a period of pivotal developments in quantum, nuclear, and particle physics, physicists at Niels Bohr's Institute for Theoretical Physics in Copenhagen took time off from their research to write humorous articles, letters, and other works. Best known is the Blegdamsvej Faust, performed in April 1932 at the close of one of the Institute's annual conferences. I also focus on the Journal of Jocular Physics, a humorous tribute to Bohr published on the occasions of his 50th, 60th, and 70th birthdays in 1935, 1945, and 1955. Contributors included Léon Rosenfeld, Victor Weisskopf, George Gamow, Oskar Klein, and Hendrik Casimir. I examine their contributions along with letters and other writings to show that they offer a window into some issues in physics at the time, such as the interpretation of complementarity and the nature of the neutrino, as well as the politics of the period.

Halpern, Paul

2012-09-01

131

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

NASA Astrophysics Data System (ADS)

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

Kragh, Helge

2011-11-01

132

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

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

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

2012-01-01

133

NASA Astrophysics Data System (ADS)

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

Lee, J. H.

2013-11-01

134

Electric quadrupole transitions of the Bohr Hamiltonian with the Morse potential

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

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

2011-08-15

135

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

NSDL National Science Digital Library

Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits studentsâ ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary school to graduate school. We present results from a study designed to test this claim by developing a curriculum on models of the atom, including the Bohr and SchrÃ¶dinger models. We examine student descriptions of atoms on final exams in transformed modern physics classes using various versions of this curriculum. We find that if the curriculum does not include sufficient connections between different models, many students still have a Bohr-like view of atoms rather than a more accurate SchrÃ¶dinger model. However, with an improved curriculum designed to develop model-building skills and with better integration between different models, it is possible to get most students to describe atoms using the SchrÃ¶dinger model. In comparing our results with previous research, we find that comparing and contrasting different models is a key feature of a curriculum that helps students move beyond the Bohr model and adopt SchrÃ¶dingerâs view of the atom. We find that understanding the reasons for the development of models is much more difficult for students than understanding the features of the models. We also present interactive computer simulations designed to help students build models of the atom more effectively.

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

2008-09-24

136

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

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

2013-05-15

137

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

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

1989-06-13

138

Exciton transport and quenching in phenylenevinylenes

We report measurements of the photoluminescence decay dynamics in phenylenevinylene polymers. These show that incorporation of aldehyde defects which can result from photochemical degradation or during thermal conversion of a precursor polymer leads to quenching of the photoluminescence. The quenching mechanism is investigated by transient photoluminescence studies on model oligomers and shown to be charge transfer quenching. A model for exciton diffusion to these quench centers can quantitatively explain the decay dynamics. Temperature dependent studies of the luminescence decay dynamics enable us to extract exciton diffusion constants and radii as a function of temperature in polyparaphenylenevinylene.

Rothberg, L.J.; Galvin, M.E. [Lucent Technologies, Murray Hill, NJ (United States); Yan, M. [Lawrence Livermore National Labs., CA (United States)] [and others

1996-12-31

139

Bright and dark excitons in semiconductor carbon nanotubes

We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.

Tretiak, Sergei [Los Alamos National Laboratory

2008-01-01

140

Exciton-plasmaritons in graphene/semiconductor structures

NASA Astrophysics Data System (ADS)

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

Velizhanin, Kirill A.; Shahbazyan, Tigran V.

2014-08-01

141

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

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

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

2010-09-15

142

Bending losses of optically anisotropic exciton polaritons in organic molecular-crystal nanofibers.

We theoretically examine the bending loss of organic molecular-crystal nanofibers for which the light propagation is carried out by optically anisotropic exciton polaritons. Previous experimental studies showed that the leakage of light for bent thiacyanine nanofibers was negligibly small even for the radius of curvature of several microns. We formulate a finite-difference frequency-domain method stabilized by a conformal transformation to calculate the bending loss as a function of the radius of curvature and the propagation frequency. The present method is applied to the thiacyanine nanofiber and numerical results that support the previous experimental observation are obtained. The present study clearly shows that the polariton nanofiber gives a novel possibility for bent waveguides to fabricate optical microcircuits and interconnection that cannot be attained by the conventional waveguides based on the index guiding. PMID:24514716

Takeda, Hiroyuki; Sakoda, Kazuaki

2013-12-16

143

NASA Astrophysics Data System (ADS)

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

Ishii, Ryota; Funato, Mitsuru; Kawakami, Yoichi

2014-09-01

144

Solar Radius at Minimum of Cycle 23

NASA Astrophysics Data System (ADS)

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

Sigismondi, Costantino

2008-09-01

145

Rotational Frenkel excitons in optical lattices with polar molecules

NASA Astrophysics Data System (ADS)

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

Litinskaya, Marina

2011-06-01

146

Taming excitons in II-VI semiconductor nanowires and nanobelts

NASA Astrophysics Data System (ADS)

Excitons are one of the most important fundamental quasi-particles, and are involved in a variety of processes forming the basis of a wide range of opto-electronic and photonic devices based on II-VI semiconductor nanowires and nanobelts, such as light-emitting diodes, photovoltaic cells, photodetectors and nanolasers. A clear understanding of their properties and unveiling the potential engineering for excitons is of particular importance for the design and optimization of nanoscale opto-electronic and photonic devices. Herein, we present a comprehensive review on discussing the fundamental behaviours of the excitons in one-dimensional (1D) II-VI semiconductor nanomaterials (nanowires and nanobelts). We will start with a focus on the unique properties (origin, generation, etc) and dynamics of excitons and exciton complexes in the II-VI semiconductor nanowires and nanobelts. Then we move to the recent progress on the excitonic response in 1D nanomaterials and focus on the tailoring and engineering of excitonic properties through rational controlling of the physical parameters and conditions, intrinsically and extrinsically. These include (1) exciton-exciton interaction, which is important for 1D nanomaterial nanolasing; (2) exciton-phonon interaction, which has interesting applications for laser cooling; and (3) exciton-plasmon interaction, which is the cornerstone towards the realization of plasmonic lasers. The potential of electric field, morphology and size control for excitonic properties is also discussed. Unveiling and controlling excitonic properties in II-VI semiconductor nanowires and nanobelts would promote the development of 1D nanoscience and nanotechnology.

Xu, Xinlong; Zhang, Qing; Zhang, Jun; Zhou, Yixuan; Xiong, Qihua

2014-10-01

147

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

148

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

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

Lenis, D; Bonatsos, Dennis

2006-01-01

149

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

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

D. Lenis; Dennis Bonatsos

2005-12-06

150

Low-speed limit of Bohr{close_quote}s stopping-power formula

Bohr{close_quote}s classical (nonrelativistic) model of charged-particle stopping is evaluated explicitly for arbitrary values of projectile charge and speed. This removes the logarithmic cutoff from the original expression and generates a stopping formula which can be utilized also to extend the range of practical applicability of Bloch{close_quote}s theory. As expected such a formulation appears to be a better starting point than the Born approximation for estimating stopping powers of heavy ions at velocities {ital v}{lt}{ital Z}{sub 1}{ital e}{sup 2}/{h_bar}. {copyright} {ital 1996 The American Physical Society.}

Sigmund, P. [Physics Department, Odense University, DK-5230 Odense M (Denmark)] [Physics Department, Odense University, DK-5230 Odense M (Denmark)

1996-10-01

151

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

152

Evidence for a Bose-Einstein condensate of excitons

NASA Astrophysics Data System (ADS)

We report compelling evidence for a “gray” condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas (\\sim 10^{10}\\ \\text{cm}^{-2}) that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the “gray” condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.

Alloing, Mathieu; Beian, Mussie; Lewenstein, Maciej; Fuster, David; González, Yolanda; González, Luisa; Combescot, Roland; Combescot, Monique; Dubin, François

2014-07-01

153

Exciton states and optical properties of carbon nanotubes.

Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k·p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis. PMID:23139202

Ajiki, Hiroshi

2012-12-01

154

Exciton states and optical properties of carbon nanotubes

NASA Astrophysics Data System (ADS)

Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k ?p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis.

Ajiki, Hiroshi

2012-12-01

155

Novel dynamical investigations of excitons and their structure in semiconductors

NASA Astrophysics Data System (ADS)

Two spectroscopic investigations in the time dependent behavior of excitons in selected semiconductors are discussed. First, as a continuation of previous grant work, issues of energy dependent exciton-exciton scattering in a model bulk semiconductor Cu2O have been studied. Picosecond photomodulation methods have yielded for the first time direct kinetic information about scattering involving the 1S and 2P exciton states. Second, exciton formation and localization kinetics have been investigated through time-resolved photoluminescence in a new wide bandgap semiconductor quantum well structure, based on the ZnSe/(Zn,Mn)Se heterojunction. In this case exciton formation and recombination rates have been shown to also depend on an additional energy relaxation channel into the Mn-ion d-electron excitations.

Nurmikko, A. V.

1987-08-01

156

Photon statistics dispersion in excitonic composites

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

G. Ya. Slepyan; S. A. Maksimenko

2006-05-22

157

Directional exciton transport in supramolecular nanostructured assemblies.

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

Gierschner, Johannes

2012-10-14

158

Can Natural Sunlight Induce Coherent Exciton Dynamics?

Excitation of a model photosynthetic molecular aggregate by incoherent sunlight is systematically examined. For a closed system, the excited state coherence induced by the sunlight oscillates with an average amplitude that is inversely proportional to the excitonic gap, and reaches a stationary amplitude that depends on the temperature and coherence time of the radiation field. For an open system, the light-induced dynamical coherence relaxes to a static coherence determined by the non-canonical thermal distribution resulting from the entanglement with the phonon bath. The decay of the excited state population to the common ground state establishes a non-equilibrium steady-state flux driven by the sunlight, and it defines a time window to observe the transition from dynamical to static coherence. For the parameters relevant to photosynthetic systems, the exciton dynamics initiated by the sunlight exhibits a non-negligible amount of dynamical coherence (quantum beats) on the sub-picosecond timescale; however, ...

Olšina, Jan; Wang, Chen; Cao, Jianshu

2014-01-01

159

Study of exciton transfer in dense quantum dot nanocomposites

NASA Astrophysics Data System (ADS)

Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ~70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies.Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ~70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies. Electronic supplementary information (ESI) available: Information about the functionalized polymer structure, TEM of the QDs, XPS analysis of the nanocomposites, derivation of the energy transfer models and temperature dependent steady state PLs. See DOI: 10.1039/c4nr03456b

Guzelturk, Burak; Hernandez-Martinez, Pedro Ludwig; Sharma, Vijay Kumar; Coskun, Yasemin; Ibrahimova, Vusala; Tuncel, Donus; Govorov, Alexander O.; Sun, Xiao Wei; Xiong, Qihua; Demir, Hilmi Volkan

2014-09-01

160

Excitons in anisotropic two-dimensional semiconducting crystals

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

161

Mass-Radius Relationships for Solid Exoplanets

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

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

2007-01-01

162

Exactly separable version of the Bohr Hamiltonian with the Davidson potential

An exactly separable version of the Bohr Hamiltonian is developed using a potential of the form u(beta)+u(gamma)/beta^2, with the Davidson potential u(beta)= beta^2 + beta_0^4/beta^2 (where beta_0 is the position of the minimum) and a stiff harmonic oscillator for u(gamma) centered at gamma=0. In the resulting solution, called exactly separable Davidson (ES-D), the ground state band, gamma band and 0_2^+ band are all treated on an equal footing. The bandheads, energy spacings within bands, and a number of interband and intraband B(E2) transition rates are well reproduced for almost all well-deformed rare earth and actinide nuclei using two parameters (beta_0, gamma stiffness). Insights regarding the recently found correlation between gamma stiffness and the gamma-bandhead energy, as well as the long standing problem of producing a level scheme with Interacting Boson Approximation SU(3) degeneracies from the Bohr Hamiltonian, are also obtained.

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

2008-07-28

163

Exactly separable version of the Bohr Hamiltonian with the Davidson potential

An exactly separable version of the Bohr Hamiltonian is developed using a potential of the form u(beta)+u(gamma)/beta^2, with the Davidson potential u(beta)= beta^2 + beta_0^4/beta^2 (where beta_0 is the position of the minimum) and a stiff harmonic oscillator for u(gamma) centered at gamma=0. In the resulting solution, called exactly separable Davidson (ES-D), the ground state band, gamma band and 0_2^+ band are all treated on an equal footing. The bandheads, energy spacings within bands, and a number of interband and intraband B(E2) transition rates are well reproduced for almost all well-deformed rare earth and actinide nuclei using two parameters (beta_0, gamma stiffness). Insights regarding the recently found correlation between gamma stiffness and the gamma-bandhead energy, as well as the long standing problem of producing a level scheme with Interacting Boson Approximation SU(3) degeneracies from the Bohr Hamiltonian, are also obtained.

Bonatsos, Dennis; Minkov, N; Casten, R F; Yotov, P; Lenis, D; Petrellis, D; Yigitoglu, I

2007-01-01

164

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

In gunfights in Western movies, the hero typically wins, even though the villain draws first. Niels Bohr (Gamow, The great physicists from Galileo to Einstein. Chapter: The law of quantum, 1988) suggested that this reflected a psychophysical law, rather than a dramatic conceit. He hypothesized that reacting is faster than acting. Welchman, Stanley, Schomers, Miall, and Bülthoff (Proceedings of the Royal Society of London B: Biological Sciences, 277, 1667-1674, 2010) provided empirical evidence supporting "Bohr's law," showing that the time to complete simple manual actions was shorter when reacting than when initiating an action. Here we probe the limits of this effect. In three experiments, participants performed a simple manual action, which could either be self-initiated or executed following an external visual trigger. Inter-button time was reliably faster when the action was externally triggered. However, the effect disappeared for the second step in a two-step action. Furthermore, the effect reversed when a choice between two actions had to be made. Reacting is faster than acting, but only for simple, ballistic actions. PMID:21264708

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

2011-02-01

165

Excitonic effects in oxyhalide scintillating host compounds

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

166

Microcavity controlled coupling of excitonic qubits

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288

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

2013-01-01

167

Microcavity controlled coupling of excitonic qubits.

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits--like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots--is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288

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

2013-01-01

168

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

169

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

170

Multiple exciton generation and recombination in carbon nanotubes and nanocrystals.

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

Kanemitsu, Yoshihiko

2013-06-18

171

Dependence of nonproportionality in scintillators on diffusion of excitons and charge carriers

NASA Astrophysics Data System (ADS)

The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval ? when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (Deff?)1/2 where Deff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of Deff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K2 and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r0 from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K2/Deff and does not depend on r0 in lowest order; however, the absolute rates of both processes decrease with increasing r0.

Williams, R. T.; Li, Qi; Grim, Joel Q.; Ucer, K. B.

2011-09-01

172

NASA Astrophysics Data System (ADS)

In the approximation of effective masses for electronic and phononic - dielectric continuum - systems, the influence of spatial bounding, self-polarization, and exciton-phonon interactions on the exciton state in a flat double nanoheterostructure (a nanofilm) - lead iodide in a polymer matrix -is theoretically investigated for the model of a single infinitely deep quantum well. It is demonstrated that the dominating factor determining the energy of the bottom of the ground exciton band and its binding energy is spatial bounding. The relationship between two other effects depends on the nanofilm thickness, namely, the influence of the self-polarization effect in ultrathin films significantly exceeds that of exciton-phonon interaction.

Kramar, V. M.; Pugantseva, O. V.

2014-08-01

173

NASA Astrophysics Data System (ADS)

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.

Elgawadi, Amal; Gainer, Gordon; Krasinski, Jerzy

2013-08-01

174

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

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

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

2014-07-10

175

Observationally Cooperative Multithreading Christopher A. Stone Melissa E. O'Neill Sonja A. Bohr lead to subtle, error-prone code. Observationally Cooperative Multithreading (OCM) is a new approach- preemptive) multithreading model for uniprocessors. OCM then allows threads to run in parallel, so long

Stone, Christopher A.

176

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

K. -H. Schmidt

2008-08-11

177

Nuclear Charge Radius of {sup 8}He

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

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

2007-12-21

178

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

179

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

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

2007-12-21

180

Mass-Radius Relationships for Solid Exoplanets

We use new interior models of cold planets to investigate the mass-radius relationships of solid exoplanets, considering planets made primarily of iron, silicates, water, and carbon compounds. We find that the mass-radius relationships for cold terrestrial-mass planets of all compositions we considered follow a generic functional form that is not a simple power law: $\\log_{10} R_s = k_1 + 1/3 \\log_{10}(M_s) - k_2 M_s^{k_3}$ for up to $M_p \\approx 20 M_{\\oplus}$, where $M_s$ and $R_s$ are scaled mass and radius values. This functional form arises because the common building blocks of solid planets all have equations of state that are well approximated by a modified polytrope of the form $\\rho = \\rho_0 + c P^n$. We find that highly detailed planet interior models, including temperature structure and phase changes, are not necessary to derive solid exoplanet bulk composition from mass and radius measurements. For solid exoplanets with no substantial atmosphere we have also found that: with 5% fractional uncertainty in planet mass and radius it is possible to distinguish among planets composed predominantly of iron or silicates or water ice but not more detailed compositions; with $\\sim$~5% uncertainty water ice planets with $\\gtrsim 25%$ water by mass may be identified; the minimum plausible planet size for a given mass is that of a pure iron planet; and carbon planet mass-radius relationships overlap with those of silicate and water planets due to similar zero-pressure densities and equations of state. We propose a definition of "super Earths'' based on the clear distinction in radii between planets with significant gas envelopes and those without.

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

2007-07-19

181

Tunable exciton Aharonov-Bohm effect in a quantum ring

NASA Astrophysics Data System (ADS)

We studied the optical Aharonov-Bohm effect for an exciton in a semiconductor quantum ring. A perpendicular electric field applied to a quantum ring with large height, is able to tune the exciton ground state energy such that it exhibits a weak observable Aharonov-Bohm oscillations. This Aharonov-Bohm effect is tunable in strength and period.

Li, Bin; Magnus, W.; Peeters, F. M.

2010-02-01

182

Tunable exciton Aharonov-Bohm effect in a quantum ring

We studied the optical Aharonov-Bohm effect for an exciton in a semiconductor quantum ring. A perpendicular electric field applied to a quantum ring with large height, is able to tune the exciton ground state energy such that it exhibits a weak observable Aharonov-Bohm oscillations. This Aharonov-Bohm effect is tunable in strength and period.

Bin Li; W. Magnus; F. M. Peeters

2010-01-01

183

Polarized superradiance from delocalized exciton transitions in tetracene single crystals

Polarized superradiant emission and exciton delocalization in tetracene single crystals are reported. Polarization-, time-, and temperature-resolved spectroscopies evidence the complete polarization of the zero-phonon line of the intrinsic tetracene emission from both the lower ( F state) and the upper (thermally activated) Davydov excitons. The superradiance of the F emission is substantiated by a nearly linear decrease in the radiative

A. Camposeo; M. Polo; S. Tavazzi; L. Silvestri; P. Spearman; R. Cingolani; D. Pisignano

2010-01-01

184

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

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

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

2011-03-30

185

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

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

Bonatsos, Dennis; Lenis, D; Minkov, N; Quesne, C

2011-01-01

186

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

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

Sascha Trippe

2014-05-06

187

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

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

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

2011-04-15

188

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

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

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

2009-04-15

189

Charge-transfer excitons at organic semiconductor surfaces and interfaces.

When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT(1s) as expected from energy resonance arguments. We suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface. PMID:19378979

Zhu, X-Y; Yang, Q; Muntwiler, M

2009-11-17

190

Nonlinear photoluminescence spectroscopy of carbon nanotubes with localized exciton States.

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

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

2014-11-25

191

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

Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, kon, rather than changes in koff. In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure (Blackburn, N. J., Strange, R. W., Reedijk, J., Volbeda, A., Farooq, A., Karlin, K. D., and Zubieta, J. (1989) Inorg. Chem.,28 ,1349 -1357). A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s ? 4pz transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs. PMID:18725416

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

2008-01-01

192

The year 2010 is the centennial of the publication of the "Seven Little Devils" in the predecessor of Acta Physiologica. In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity substitute to secretion. The subjects of the present reevaluation of this controversy are Christian Bohr, Professor and Doctor of Medicine (1855-1911), nominated three times for the Nobel Prize; August Krogh, Doctor of Philosophy (1874-1949), Christian Bohr's assistant and later Nobel Prize laureate (1920); and Marie Krogh, née Jørgensen, Doctor of Medicine and wife of August Krogh (1874-1943). The controversy concerned is the transport of oxygen from the lungs into the bloodstream: are passive transport and diffusion capacity together sufficient to secure the oxygen supply in all circumstances or is there an additional specific ("energy consuming" or "active") mechanism responsible for the transport of oxygen from the alveoli into the bloodstream? The present discussion purports to show that the contestants' views were closer than the parties themselves and posterity recognized. Posterity has judged the dispute unilaterally from the Nobel laureate's point of view, but it is evident that August Krogh's Nobel Prize was awarded for the discovery of a cellular activity (Christian Bohr's expression), represented by Krogh's discovery of capillary recruitment. Christian Bohr appears to have been correct in the narrower sense that the diffusion capacity at rest is not great enough to explain the transport during work; a special mechanism intervenes and optimizes the conditions under which diffusion acts. August Krogh, of course, was right in the wider sense that the transport mechanism itself is always entirely passive. PMID:21098384

Gjedde, Albert

2010-12-01

193

Excitonic resonances in WS2 nanotubes

NASA Astrophysics Data System (ADS)

Resonance Raman profiles of WS2 nanotubes of different diameter are presented. We show that the A excitonic transition energy lies below the bulk value and is increasingly redshifted with decreasing diameter of the nanotubes. The findings are attributed to strain effects associated with the curvature of the nanotube walls. A silent Raman mode, the B1u mode, is disorder enhanced in the Raman spectra of WS2 nanomaterials. We discuss the development of the relative intensities of the B1u mode and the nearby A1g mode with nanotube diameter, excitation energy, and hydrostatic pressure in terms of a slight difference in resonance condition.

Staiger, Matthias; Rafailov, Peter; Gartsman, Konstantin; Telg, Hagen; Krause, Matthias; Radovsky, Gal; Zak, Alla; Thomsen, Christian

2012-10-01

194

Probing excitonic dark states in single-layer tungsten disulphide

NASA Astrophysics Data System (ADS)

Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing.

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

2014-09-01

195

Numerical radius preserving operators on C * -algebras

Let A be a unital C*-algebra. An element u of A is unitary and belongs to the centre of A if and only if $ |\\\\varphi (u)| = 1 $ for every pure state $ \\\\varphi $. Using this fact we show that a numerical radius preserving linear isomorphism on A is a C*-isomorphism multiplied by a fixed unitary element

Jor-Ting Chan

1998-01-01

196

On Solar Radius Variations Observed with Astrolabes

Ground-based results on cyclic variations of the apparent solar radius are so far controversial and inconsistent. This is blamed to atmospheric noise which effects can be so severe that even in cases in which the observations are made with similar instruments, the results show strong disagreements (Li et al.: 2003, Astrophys. J.591, 1267). Such claim concerns the results of Danjon

F. Noël

2005-01-01

197

Submitted to J. Chem. Phys. Quantum decoherence in finite size exciton-phonon systems

in under- standing many phenomena: exciton dynamics in photo- synthetic antenna and polymers7Â11 , vibronSubmitted to J. Chem. Phys. Quantum decoherence in finite size exciton-phonon systems Vincent energies and they tend to break the excitonic coherence. The exciton-phonon system is a prototype

Paris-Sud XI, UniversitÃ© de

198

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

199

Excitons and charges at organic semiconductor heterojunctions.

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

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

2012-01-01

200

Can Natural Sunlight Induce Coherent Exciton Dynamics?

Excitation of a model photosynthetic molecular aggregate by incoherent sunlight is systematically examined. For a closed system, the excited state coherence induced by the sunlight oscillates with an average amplitude that is inversely proportional to the excitonic gap, and reaches a stationary amplitude that depends on the temperature and coherence time of the radiation field. For an open system, the light-induced dynamical coherence relaxes to a static coherence determined by the non-canonical thermal distribution resulting from the entanglement with the phonon bath. The decay of the excited state population to the common ground state establishes a non-equilibrium steady-state flux driven by the sunlight, and it defines a time window to observe the transition from dynamical to static coherence. For the parameters relevant to photosynthetic systems, the exciton dynamics initiated by the sunlight exhibits a non-negligible amount of dynamical coherence (quantum beats) on the sub-picosecond timescale; however, this sub-picosecond time-scale is long enough for light-harvesting systems to establish static coherence, which plays a crucial role in efficient energy transfer. Further, a relationship is established between the non-equilibrium steady-state induced by the sunlight and the coherent dynamics initiated from the ground state by a laser $\\delta$-pulse, thereby making a direct connection between incoherent sunlight excitation and ultrafast spectroscopy.

Jan Olšina; Arend G. Dijkstra; Chen Wang; Jianshu Cao

2014-08-21

201

Nanoscale Charge Transport in Excitonic Solar Cells

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

Venkat Bommisetty, South Dakota State University

2011-06-23

202

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

203

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

204

Exciton Mott transition in Si revealed by terahertz spectroscopy.

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

Suzuki, Takeshi; Shimano, Ryo

2012-07-27

205

Dynamical fermion mass generation and exciton spectra in graphene

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

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

2011-03-15

206

Giant Rydberg excitons in the copper oxide Cu2O.

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

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

2014-10-16

207

Nearly degenerate time-resolved Faraday rotation in an interacting exciton system

NASA Astrophysics Data System (ADS)

We report experimental studies of nearly degenerate time-resolved Faraday rotation (TRFR) of electron spin precession in an interacting exciton system. We show that many-body interactions between excitons strongly modify the TRFR response through the coupling of the spin coherence to two-exciton states. The striking difference between TRFR and transient differential absorption (DA) further reveals that exciton-exciton interactions play crucial but qualitatively different roles in TRFR and DA.

Shen, Yumin; Goebel, A. M.; Khitrova, G.; Gibbs, H. M.; Wang, Hailin

2005-12-01

208

Solar Radius Variations Measured in Central Eclipses

NASA Astrophysics Data System (ADS)

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

Sigismondi, Costantino

2008-01-01

209

Radius measurement by laser confocal technology.

A laser confocal radius measurement (LCRM) method is proposed for high-accuracy measurement of the radius of curvature (ROC). The LCRM uses the peak points of confocal response curves to identify the cat eye and confocal positions precisely. It then accurately measures the distance between these two positions to determine the ROC. The LCRM also uses conic fitting, which significantly enhances measurement accuracy by restraining the influences of environmental disturbance and system noise on the measurement results. The experimental results indicate that LCRM has a relative expanded uncertainty of less than 10 ppm for both convex and concave spheres. Thus, LCRM is a feasible method for ROC measurements with high accuracy and concise structures. PMID:24921872

Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Zhang, Xin; Wang, Xu

2014-05-01

210

Surgical approaches to the distal radius

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

Asif M. Ilyas

2011-01-01

211

Z -scan technique through beam radius measurements

. A modification to the well-known z-scan technique for measuring optical non-linearities is introduced. It is based on directly\\u000a measuring the beam radius in the far field instead of the transmittance of the irradiance through an aperture, as in the original\\u000a version. It has the advantage of being insensitive to beam pointing instability and is almost insensitive to power fluctuations.

G. Tsigaridas; M. Fakis; I. Polyzos; P. Persephonis; V. Giannetas

2003-01-01

212

Excitonic properties of hydrogen saturation-edged armchair graphene nanoribbons.

First-principle density functional theory calculations with quasiparticle corrections and many body effects are performed to study the electronic and optical properties of armchair graphene nanoribbons (AGNRs) with variant edges saturated by hydrogen atoms. The "effective width" method associated with the reported AGNR family effect is introduced to understand the electronic structures. The method is further confirmed by analyses of the optical transition spectra and the exciton wavefunctions. The optical properties, including the optical transition spectra, exciton binding energies and the distribution of exciton wavefunctions, can be tuned with the hydrogen saturation edge, thus providing an effective way to control the features of the AGNRs. PMID:21503364

Wang, Min; Li, Chang Ming

2011-05-01

213

Intermediate valence and the possibility of a magnetic excitonic insulator

NASA Astrophysics Data System (ADS)

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

Wachter, Peter; Jung, Anatol

1994-03-01

214

On Solar Radius Variations Observed with Astrolabes

NASA Astrophysics Data System (ADS)

Ground-based results on cyclic variations of the apparent solar radius are so far controversial and inconsistent. This is blamed to atmospheric noise which effects can be so severe that even in cases in which the observations are made with similar instruments, the results show strong disagreements (Li et al.: 2003, Astrophys. J. 591, 1267). Such claim concerns the results of Danjon astrolabes which during the last decades have been used widely at several sites for solar metrology. The long-term series with thousands of radius measurements made with astrolabe at Calern, France, and at Santiago, Chile, is a case in which the results of radius variations in time are strongly discrepant in spite that the observations were made simultaneously, in quite similar conditions and with almost identical instruments (Noël: 2004, Astron. Astrophys. 413, 725). However, we show here that most of astrolabe discrepancies may be due to data analysis biased by theoretical preconceptions, by empirical results which without scientific arguments are considered as canonical references and by over interpretations of casual agreements between visual and CCD astrolabe results.

Noël, F.

2005-11-01

215

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

216

Problem Set 1 (due 17th September) (1) A planet of radius Rp orbits a star of radius R) Suppose that the planet, of mass Mp, orbital radius ap, and orbital period P, is itself orbited by a moon the presence of a moon similar to Jupiter's Ganymede orbiting a Jupiter mass planet at 0.1 AU about a Solar

Armitage, Phil

217

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

Baym, Gordon; Ozawa, Tomoki

2009-01-01

218

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

219

Excitons in a surface quantum well

NASA Astrophysics Data System (ADS)

Binding energies of excitons in a Surface Quantum Well (SQW) composed of vacuum/GaAs/AlxGa1-xAs as a function of wellwidth are calculated. The effect of non-parabolicity is considered by using an energy dependent effective mass. The effect of mass anisotropy and the effect of image charges which arise due to the large dielectric discontinuity at the vacuum/GaAs interface are also considered. The average distances of the electron

Arulmozhi, M.; Anitha, A.

2014-11-01

220

A gamma-rigid solution of the Bohr Hamiltonian for gamma = 30 degrees is derived, its ground state band being related to the second order Casimir operator of the Euclidean algebra E(4). Parameter-free (up to overall scale factors) predictions for spectra and B(E2) transition rates are in close agreement to the E(5) critical point symmetry, as well as to experimental data in the Xe region around A=130.

Dennis Bonatsos; D. Lenis; D. Petrellis; P. A. Terziev; I. Yigitoglu

2005-07-08

221

NASA Astrophysics Data System (ADS)

The Fisher-Shannon information and a statistical measure of complexity are calculated in position and momentum spaces for the wavefunctions of the quantum isotropic harmonic oscillator. We show that these quantities are independent of the strength of the harmonic potential. Moreover, for each level of energy, it is found that these two indicators take their minimum values on the orbitals that correspond to the classical (circular) orbits in the Bohr-like quantum image, just those with the highest orbital angular momentum.

Sañudo, Jaime; López-Ruiz, Ricardo

2008-07-01

222

Dynamics of exciton dissociation in donor-acceptor polymer heterojunctions

NASA Astrophysics Data System (ADS)

Exciton dissociation in a donor-accepter polymer heterojunction has been simulated using a nonadiabatic molecular dynamics approach, which allows for the coupled evolution of the nuclear degrees of freedom and the electronic degrees of freedom described by multiconfigurational electronic wavefunctions. The simulations reveal important details of the charge separation process: the exciton in the donor polymer first dissociates into a ``hot'' charge transfer state, which is best described as a polaron pair. The polaron pair can be separated into free polaron charge carriers if a sufficiently strong external electric field is applied. We have also studied the effects of inter-chain interaction, temperature, and the external electric field strength. Increasing inter-chain interactions makes it easier for the exciton to dissociate into a polaron pair state, but more difficult for the polaron pair to dissociate into free charge carriers. Higher temperature and higher electric field strength both favor exciton dissociation as well as the formation of free charge carriers.

Sun, Zhen; Stafström, Sven

2013-04-01

223

Topologically protected excitons in porphyrin thin films.

The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials. PMID:25242533

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

2014-11-01

224

Singlet exciton fission : applications to solar energy harvesting

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

Thompson, Nicholas John

2014-01-01

225

ULTRAFAST STUDIES OF EXCITON DYNAMICS IN LIGHT HARVESTING DIMERS

, Rb Sphaeroides. 1. INTRODUCTION The first events in bacterial photosynthesis involve the absorption. Keywords: photosynthesis, femtosecond, anisotropy, exciton, B820, photosynthetic reaction center, Rs rubrum of the energy transfer and electron transfer "reactions" within bacterial Current Address: Pacific Northwest

Scherer, Norbert F.

226

Topologically protected excitons in porphyrin thin films

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

227

Exciton dynamics in alternating polyfluorene/fullerene blends

NASA Astrophysics Data System (ADS)

Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton-exciton energy transfer in these blends at low acceptor concentrations.

De, Swati; Kesti, Tero; Maiti, Manisankar; Zhang, Fengling; Inganäs, Olle; Yartsev, Arkady; Pascher, Torbjörn; Sundström, Villy

2008-06-01

228

Exciton/Charge-transfer Electronic Couplings in Organic Semiconductors

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

Difley, Seth

229

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

230

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

231

Aharonov-Bohm effect of excitons in nanorings

The magnetic field effects on excitons in an InAs nanoring are studied theoretically. By numerically diagonalizing the effective-mass Hamiltonian of the problem that can be separated into terms in center-of-mass and relative coordinates, we calculate the low-lying excitonic energy levels and oscillator strengths as a function of the ring width and the strength of an external magnetic field. It is

Hui Hu; Jia-Lin Zhu; Dai-Jun Li; Jia-Jiong Xiong

2001-01-01

232

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

233

ULTRASONIC ASSESSMENT OF THE RADIUS IN VITRO

The overall objective of this research is to develop an ultrasonic system for non-invasive assessment of the distal radius. The specific objective of this study was to examine the relationship between geometrical features of cortical bone and ultrasound measurements in vitro. Nineteen radii were measured in through transmission in a water bath. A 3.5 MHz rectangular (1 cm × 4.8 cm) single element transducer served as the source and a 3.5 MHz rectangular (1 cm × 4.8 cm) linear array transducer served as the receiver. The linear array consisted of 64 elements with a pitch of 0.75 mm. Ultrasound measurements were carried out at a location that was 1/3 of the length from the distal end of each radius, and two net time delay parameters, ?NetDW and ?NetCW, associated with a direct wave (DW) and a circumferential wave (CW), respectively, were evaluated. The cortical thickness (CT), medullar thickness (MT) and cross-sectional area (CSA) of each radius was also evaluated based on a digital image of the cross-section at the “1/3” location. The linear correlations between CT and ?NetDW was r = 0.91 (p<0.001) and between MT and ?NetDW - ?NetCW was r = 0.63 (p<0.05). The linear correlation between CSA and a non-linear combination of the two net time delays, ?NetDW and ?NetCW, was r = 0.95 (p<0.001). The study shows that ultrasound measurements can be used to non-invasively assess cortical bone geometrical features in vitro as represented by cortical thickness, medullar thickness and cross-sectional area. PMID:18692295

Le Floch, Vincent; Luo, Gangming; Kaufman, Jonathan J.; Siffert, Robert S.

2008-01-01

234

Taking Exomoons to the (Radius) Limit

NASA Astrophysics Data System (ADS)

Moons around extrasolar planets are the next up-and-coming objects to be not only observed, but characterized for possible habitability. As with planets orbiting stars, exomoons have a limiting radius at which they are gravitationally bound, or the Hill radius. At a particular distance, they may also become tidally locked and therefore be in a synchronous rotation with the host-planet. We have examined the flux phase profile of a simulated, hypothetical moon orbiting at a distant radius around the confirmed exoplanets ? Ara b, HD 28185 b, BD +14 4559 b, and HD 73534 b. The irradiated flux on an exomoon at it's furthest, stable distance from the planet achieves its largest flux gradient, which places a limit on the flux ranges expected for subsequent (observed) moons closer in distance. We have also analyzed the effect of planetary eccentricity and time spent within the habitable zone on the flux on the moon. From stellar contributions alone, we find moons with host-planets fully within the habitable zone experience thermal equilibrium temperatures above the runaway greenhouse limit, requiring a small heat redistribution efficiency. In contrast, exomoons orbiting planets that only spend a fraction of their time within the habitable zone require a heat redistribution efficiency near 100% in order to achieve temperatures suitable for habitability. In other words, a planet might not need to spend its entire orbit within the habitable zone in order for the exomoon to be potentially habitable. In this way our findings separate exomoons and exoplanets, but it broadens the scope of worlds that may occupy a habitable zone.

Hinkel, Natalie R.; Kane, S. R.

2014-01-01

235

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...

236

Giant cell tumour of the proximal radius.

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

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

2009-11-01

237

Molecular packing determines singlet exciton fission in organic semiconductors.

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission. PMID:24957197

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

2014-07-22

238

Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy

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

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

2005-06-25

239

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

NASA Astrophysics Data System (ADS)

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

Snoke, David; Kavoulakis, G. M.

2014-11-01

240

EGS Abstract for MEAN RADIUS, MASS AND INERTIA FOR REFERENCE

EGS Abstract for Nice, 2000 MEAN RADIUS, MASS AND INERTIA FOR REFERENCE EARTH'S MODELS. F. CHAMBAT Earth density models : radius, mass and inertia. We recall how these quantities are measured and give

241

Thiol-Capped Germanium Nanocrystals: Preparation and Evidence for Quantum Size Effects

confinement effects are activated in larger Ge NCs relative to Si due to the larger excitonic Bohr radius-capped Ge NCs suffer from incomplete surface passivation. Surface ligands play a critical role in determining the properties of NCs. While their insulating characteristics are considered detrimental

Osterloh, Frank

242

Nanocrystalline ceria imparts better high-temperature protection

Nanomaterials have attracted considerable interest with numerous technological developments in the last decade. Nanomaterials exhibit different physicochemical properties compared with their bulk counterparts because the diameters of the nanoparticles are less than the Bohr exciton radius. Cerium oxide based materials have been extensively studied for various technological applications. In the present study, the application of nanocrystalline cerium oxide for improvement

S. Patil; S. C. Kuiry; S. Seal

2004-01-01

243

Asteroseismic age and radius of Kepler stars

The Kepler mission's primary goal is the detection and characterization of Earth-like planets by observing continuously a region of sky for a nominal period of three-and-a-half years. Over 100,000 stars will be monitored, with a small subset of these having a cadence of 1 minute, making asteroseismic studies for many stars possible. The subset of targets will consist of mainly solar-type and planet-hosting stars, and these will be observed for a minimum period of 1 month and a maximum depending on the scientific yield of the individual target. Many oscillation frequencies will be detected in these data, and these will be used to constrain the star's fundamental parameters. I investigate the effect that an increase in a) the length of observation and b) the signal quality, has on the final determination of some stellar global parameters, such as the radius and the age.

Orlagh L. Creevey

2008-10-14

244

New charge radius relations for atomic nuclei

We show that the charge radii of neighboring atomic nuclei, independent of atomic number and charge, follow remarkably very simple relations, despite the fact that atomic nuclei are complex finite many-body systems governed by the laws of quantum mechanics. These relations can be understood within the picture of independent-particle motion and by assuming neighboring nuclei having similar pattern in the charge density distribution. A root-mean-square (rms) deviation of 0.0078 fm is obtained between the predictions in these relations and the experimental values, i.e., a comparable precision as modern experimental techniques. Such high accuracy relations are very useful to check the consistence of nuclear charge radius surface and moreover to predict unknown nuclear charge radii, while large deviations from experimental data is seen to reveal the appearance of nuclear shape transition or coexsitence.

B. H. Sun; Y. Lu; J. P. Peng; C. Y. Liu; Y. M. Zhao

2014-08-29

245

New charge radius relations for atomic nuclei

We show that the charge radii of neighboring atomic nuclei, independent of atomic number and charge, follow remarkably very simple relations, despite the fact that atomic nuclei are complex finite many-body systems governed by the laws of quantum mechanics. These relations can be understood within the picture of independent-particle motion and by assuming neighboring nuclei having similar pattern in the charge density distribution. A root-mean-square (rms) deviation of 0.0078 fm is obtained between the predictions in these relations and the experimental values, i.e., a comparable precision as modern experimental techniques. Such high accuracy relations are very useful to check the consistence of nuclear charge radius surface and moreover to predict unknown nuclear charge radii, while large deviations from experimental data is seen to reveal the appearance of nuclear shape transition or coexsitence.

Peng, J J; Sun, B H; Zhao, Y M

2014-01-01

246

Photosynthetic light harvesting: excitons and coherence

Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques. PMID:24352671

Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C.; Scholes, Gregory D.

2014-01-01

247

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

Mendoza, Hiroshi Antonio

2012-01-01

248

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

249

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

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

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

2013-10-21

250

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

251

Incoherent exciton trapping in self-similar aperiodic lattices

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

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

1995-01-01

252

Strongly bound excitons in gapless two-dimensional structures

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

253

Localization length and intraband scattering of excitons in linear aggregates

NASA Astrophysics Data System (ADS)

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

Lemaistre, J. P.

1999-07-01

254

The exciton states in strained (In,Ga)As nanorings embedded in a GaAs matrix\\u000aare computed. The strain distribution is extracted from the continuum\\u000amechanical model, and the exact diagonalization approach is employed to compute\\u000athe exciton states. Weak oscillations of the ground exciton state energy with\\u000athe magnetic field normal to the ring are an expression of the excitonic\\u000aAharonov-Bohm effect.

M. Tadi?; N. ?ukari?; V. Arsoski; F. M. Peeters

2010-01-01

255

The exciton states in strained (In,Ga)As nanorings embedded in a GaAs matrix are computed. The strain distribution is extracted from the continuum mechanical model, and the exact diagonalization approach is employed to compute the exciton states. Weak oscillations of the ground exciton state energy with the magnetic field normal to the ring are an expression of the excitonic Aharonov-Bohm effect.

M. Tadic; N. Cukaric; V. Arsoski; F. M. Peeters

2010-01-01

256

Coupled exciton model with off-diagonal disorder for optical excitations in extended dendrimers

A phenomenological coupled exciton model is proposed in order to characterize optical excitations in extended dendrimers. An onsite exciton state is assigned at each phenyl rings and a nearest neighbor hopping integral which obeys the Gaussian distribution is considered between the exciton states. The decreasing optical excitation energy with respect to the dendrimer size indicates the presence of exciton funnels along the backbone of the dendrimers. Therefore, the extended dendrimers can work as artificial fractal antenna systems which capture energy of light.

Kikuo Harigaya

1998-10-06

257

We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both. PMID:25166663

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

258

Aharonov-Bohm effects on bright and dark excitons in carbon nanotubes

A short-range part of the Coulomb interaction causes splitting and shift of excitons due to exchange interaction and mixing between different valleys in semiconducting carbon nanotubes. In the absence of a magnetic flux only a single exciton is optically active (bright) and all others are inactive (dark). Two bright excitons appear in the presence of an Aharonov- Bohm magnetic flux.

Tsuneya Ando

2006-01-01

259

PRB/LA13144B Aharonov-Bohm Exciton Absorption Splitting in Chiral Specific

PRB/LA13144B Aharonov-Bohm Exciton Absorption Splitting in Chiral Specific Single-Walled Carbon-Bohm effect was observed in chiral specific SWNTs by the magneto-absorption measure- ments conducted of the E11 optical excitonic transitions. The parameters of both the dark-bright exciton energy splitting

Maruyama, Shigeo

260

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

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

Dhinojwala, Ali

261

Photonics meets excitonics: natural and artificial molecular aggregates

Organic molecules store the energy of absorbed light in the form of charge-neutral molecular excitations -- Frenkel excitons. Usually, in amorphous organic materials, excitons are viewed as quasiparticles, localized on single molecules, which diffuse randomly through the structure. However, the picture of incoherent hopping is not applicable to some classes of molecular aggregates -- assemblies of molecules that have strong near field interaction between electronic excitations in the individual subunits. Molecular aggregates can be found in nature, in photosynthetic complexes of plants and bacteria, and they can also be produced artificially in various forms including quasi-one dimensional chains, two-dimensional films, tubes, etc. In these structures light is absorbed collectively by many molecules and the following dynamics of molecular excitation possesses coherent properties. This energy transfer mechanism, mediated by the coherent exciton dynamics, resembles the propagation of electromagnetic waves throu...

Saikin, Semion K; Valleau, Stéphanie; Aspuru-Guzik, Alán; 10.1515/nanoph-2012-0025

2013-01-01

262

Electrodynamic and excitonic intertube interactions in semiconducting carbon nanotube aggregates.

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

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

2011-04-26

263

Biexcitonic molecules survive excitons at the Mott transition.

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

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

2014-01-01

264

Biexcitonic molecules survive excitons at the Mott transition

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

265

Surface plasmon mediated energy transfer of electrically-pumped excitons

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

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

2010-01-01

266

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

267

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

268

Novel Quantum Condensates in Excitonic Matter

NASA Astrophysics Data System (ADS)

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

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

2009-08-01

269

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

270

Exciton Autoionization in Ion-Induced Electron Emission

NASA Astrophysics Data System (ADS)

We report on measurements of electron emission spectra from surfaces of highly oriented pyrolytic graphite (HOPG) excited by 1 5 keV He+ and Li+ which, for He+, exhibit a previously unreported high-energy structure. Through a full quantum dynamic description that allows for the calculation of neutralization and electron-hole pair excitation, we show that these high-energy electrons can arise from autoionization of excitons formed by electron promotion to conduction band states close to the vacuum level. The same calculation explains the observed absence of high-energy excitons for Li+ on HOPG.

Bajales, N.; Cristina, L.; Mendoza, S.; Baragiola, R. A.; Goldberg, E. C.; Ferrón, J.

2008-06-01

271

Crossed excitons in a semiconductor nanostructure of mixed dimensionality

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

272

Evidence of Hybrid Excitons in Weakly Interacting Nanopeapods

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

2013-01-01

273

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

274

Exciton localization-delocalization transition in an extended dendrimer

Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G{sub c} ? 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.

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

2013-12-21

275

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

276

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

Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (?80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit. PMID:23656886

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

2013-06-18

277

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

278

Precessing Asteroids ftrom Radius Vector Models?

NASA Astrophysics Data System (ADS)

Examining a sample of asteroids (the first 99) for which radius vector models have been constructed from mostly lightcurves, located on a web site where such models are listed (http://astro.troja.mff.cuni.cz/projects/damit ; see Durech et al. (2010), DAMIT: a database of asteroid models, A&A, 513, A46), we fit their surfaces as triaxial ellipsoids and provide their three dimensions. In the process we also derive an Euler angular offset ? between each model's spin axis and its axis of maximum moment of inertia assuming a uniform distribution of mass. Most ?'s conform to a chi-squared distribution having a maximum at 3° and a mean at 5°, and with the square root of the variance being 3°. However, seven models produce ?>20°, which we interpret as indicating possibly strong precessors, tumblers, or due to incorrect models: asteroids (68), (89), (125), (162), (167), (222), and (230). Nine others produce an excess over the distribution at 12°

Drummond, Jack D.

2014-11-01

279

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

280

An iterative algorithm for calculating stylus radius unambiguously

NASA Astrophysics Data System (ADS)

The stylus radius is an important specification for stylus instruments and is commonly provided by instrument manufacturers. However, it is difficult to measure the stylus radius unambiguously. Accurate profiles of the stylus tip may be obtained by profiling over an object sharper than itself, such as a razor blade. However, the stylus profile thus obtained is a partial arc, and unless the shape of the stylus tip is a perfect sphere or circle, the effective value of the radius depends on the length of the tip profile over which the radius is determined. We have developed an iterative, least squares algorithm aimed to determine the effective least squares stylus radius unambiguously. So far, the algorithm converges to reasonable results for the least squares stylus radius. We suggest that the algorithm be considered for adoption in documentary standards describing the properties of stylus instruments.

Vorburger, T. V.; Zheng, A.; Renegar, T. B.; Song, J.-F.; Ma, L.

2011-08-01

281

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

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

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

2003-07-28

282

NASA Astrophysics Data System (ADS)

Theoretical investigation of the spatial confinement, self-polarization and exciton-phonon interaction influence on the exciton state in plane double nanoheterostructure (nanofilm)-lead iodide in polymeric matrix is performed within the effective mass approximation for the electron and dielectric continuum for the phonons in the framework of infinitely deep single quantum well. It is shown that spatial confinement is the dominating feature determining the energy of the bottom of exciton ground band and its binding energy. The relationship of two others depends on nanofilm thickness: in ultrathin films the influence of self-polarization effect is essentially bigger than the role of exciton-phonon interaction.

Kramar, V. M.; Pugantseva, O. V.; Derevyanchuk, A. V.

2014-08-01

283

Organic photovoltaics (OPVs) offer the opportunity for cheap, lightweight and mass-producible devices. However, an incomplete understanding of the charge generation process, in particular the timescale of dynamics and role of exciton diffusion, has slowed further progress in the field. We report a new Kinetic Monte Carlo model for the exciton dissociation mechanism in OPVs that addresses the origin of ultra-fast (<1?ps) dissociation by incorporating exciton delocalization. The model reproduces experimental results, such as the diminished rapid dissociation with increasing domain size, and also lends insight into the interplay between mixed domains, domain geometry, and exciton delocalization. Additionally, the model addresses the recent dispute on the origin of ultra-fast exciton dissociation by comparing the effects of exciton delocalization and impure domains on the photo-dynamics.This model provides insight into exciton dynamics that can advance our understanding of OPV structure-function relationships. PMID:24829165

Heitzer, Henry M; Savoie, Brett M; Marks, Tobin J; Ratner, Mark A

2014-07-14

284

Exciton magnetic polaron in semimagnetic semiconductor nanocrystals

NASA Astrophysics Data System (ADS)

We present a theoretical study of the magnetic polaron associated with an electron-hole pair in a diluted magnetic semiconductor quantum dot. It is based on the effective-mass approximation in the strong confinement regime, which incorporates the coupling between the light- and heavy-hole bands. The magnetic polaron, arising from the sp-d exchange interaction between the confined carriers and the magnetic ions, is treated in a self-consistent mean-field approach that leads to coupled nonlinear Schröautdinger equations for the electron and the hole. The local response to the effective field is modeled by the experimental high-field magnetization curve in the bulk. The electron-hole Coulomb interaction is taken into account. An exact numerical solution of the three coupled equations is used to calculate the equilibrium polaron size, binding energy (Ep), and spin (Sp). Results are first presented for Cd1-xMnxTe nanocrystals with x=0.11. Ep decreases and the orbital contraction increases with an increasing quantum dot radius (a). In small dots, approaching saturation in the core region, Ep decreases slowly as the temperature (T) increases. In large dots Ep(T) decreases rapidly towards the fluctuation regime, where Ep~a-3. A similar temperature dependence is obtained for Sp; the fluctuation-regime value is, however, size independent. The light-induced magnetization enhancement due to polaron formation is considered and an optimal quantum dot radius is predicted to be ~30 Å. We have also calculated Ep as a function of an applied magnetic field, which shows a decreasing behavior that depends on a and T. Theoretical results for Cd1-xMnxSe nanocrystals show a good agreement with recently reported experimental data on the photoluminescence Stokes shift versus magnetic field.

Bhattacharjee, A. K.; Benoit à la Guillaume, C.

1997-04-01

285

Many-body correlations of electrostatically trapped dipolar excitons

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

286

Transient Grating Measurements of Excitonic Dynamics in Single-Walled

. Because of their small diameter (1 nm) and extended dimension along the tube axis, single-walled carbon nano- tubes (SWNTs) are characterized as quasi-one-dimensional charge carriers,1 with very lowTransient Grating Measurements of Excitonic Dynamics in Single-Walled Carbon Nanotubes: The Dark

Apkarian, V. Ara

287

Binding energy and exciton spectrum in double cylindrical quantum dot

The spectral characteristics of exciton in combined nanoheterosystem consisting of semiconductor cylindrical quantum wire containing two quantum dots separated by thin barrier-shell are investigated. It is shown that the binding energy non monotonously depends on the geometrical characteristics of nanoheterosystem approaching several minimum and maximum magnitudes.

O. M. Makhanets; M. M. Dovganiuk; J. O. Seti

2008-01-01

288

EXCITON BEHAVIOR IN CARBON NANOTUBES: DIELECTRIC SCREENING AND DECAY DYNAMICS

BEHAVIOR IN CARBON NANOTUBES: DIELECTRIC SCREENING AND DECAY DYNAMICS by ANDREW GERALD WALSH B. S., Cornell DYNAMICS (Order No. ) ANDREW GERALD WALSH Boston University Graduate School of Arts and Sciences, 2009' & $ % EXCITON BEHAVIOR IN CARBON NANOTUBES: DIELECTRIC SCREENING AND DECAY DYNAMICS ANDREW GERALD

289

Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics

Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics-radiative pathways. The long- range order of P3HT is disrupted when spin-cast on rough TiO2 nanoparticles electronics 1. INTRODUCTION Heterojunctions of polythiophenes with a variety of electron-transporters (such

Carter, Sue

290

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

291

Femtosecond probing of exciton relaxation and transport dynamics in polybithiophene

NASA Astrophysics Data System (ADS)

The relaxation and transport dynamics of singlet excitons in 100 nm thick, electrochemically prepared polybithiophene films were investigated by monitoring the time evolution of photoinduced bleaching of the S0-S1 absorption and photoinduced absorption (S1-Sn) with femtosecond resolved transient absorption spectroscopy. The decay dynamics of both photoinduced bleaching and photoinduced absorption in the spectral range from 500 to 800 nm are observed to be independent of the pump pulse wavelength and can be fitted by a double exponential, the time constants of which are ?1=120±20 fs and ?2=2±0.3 ps. The fast decay process with ˜?1=120 fs was ascribed to the transport dynamics of the initially generated free excitons migrating over parallel aligned polymer segments to structural defects acting as traps. The competing process, occurring predominantly in structurally disordered regions, is structural relaxation of the initially generated free excitons to self-trapped excitons decaying with a lifetime of about 2 ps.

Bock, Alexander M.; Schmid, Dankward; Kryschi, Carola

1999-07-01

292

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

293

Diamagnetic Shift and Localization of Excitons in Disordered Quantum Wells

.grochol@physik.hu-berlin.de (M. Grochol). URL: www-semic.physik.hu-berlin.de (M. Grochol). exciton diamagnetic shift (XDS.3/GaAs QWs, all the measured and analysed states (in total, 9) showed variations in the XDS [6 realization are presented for a series of MFs. The eigenfunctions are studied in relation to the XDS

Zimmermann, Roland

294

8. Ueta, M. et al. in Excitonic Processes in Solids Ch. 3 (Springer, Berlin, 1986). 9. Itoh, T. Soc. Jpn 45, 1939Â1948 (1978). 10. HoÂ¨nerlage, B. et al. The dispersion of excitons, polaritons pairs from the optical decay of biexcitons. Solid State Commun. 111, 495Â500 (1999). 13. Shimano, R

Haile, Sossina M.

295

Precise root-mean-square radius of He

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

Sick; Ingo

2008-01-01

296

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

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

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

2008-04-15

297

Reduction of the proton radius discrepancy by 3?

NASA Astrophysics Data System (ADS)

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

Lorenz, I. T.; Meißner, Ulf-G.

2014-10-01

298

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

299

Optimal Transmission Radius for Flooding in Large Scale Sensor Networks

1 Optimal Transmission Radius for Flooding in Large Scale Sensor Networks Marco ZÂ´u~niga Z, applications such as query propagation rely regularly on network-wide flooding for information dissemination. If the transmission radius is not set optimally, the flooded packet may be holding the transmission medium for longer

Krishnamachari, Bhaskar

300

Neighborhood radius estimation for Variable-neighborhood Random Fields

bounds for the probability of wrong estimation of the radius of the context. Key words: Gibbs measuresNeighborhood radius estimation for Variable-neighborhood Random Fields Eva LÂ¨ocherbach, Enza probabilities depending on a neighborhood of the region which changes with the boundary conditions. To pre- dict

Orlandi, Enza

301

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

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

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

2013-11-07

302

The effect of large Larmor radius on nonlinear Alfven waves

The effect of large Larmor radius on the nonlinear behavior of Alfven waves propagating parallel to a uniform magnetic field in a compressible fluid is investigated with the aid of LLR-MHD equations. It is shown that asymptotic evolution of these waves is governed by the modified nonlinear Schroedinger equation. The dispersion is provided by the large Larmor radius effect in

Nagendra Kumar; Krishna M. Srivastava

1991-01-01

303

The effect of large Larmor radius on nonlinear Alfvén waves

The effect of large Larmor radius on the nonlinear behaviour of Alfvén waves propagating parallel to a uniform magnetic field in a compressible fluid is investigated with the aid of LLR-MHD equations. It is shown that asymptotic evolution of these waves is governed by the modified nonlinear Schrödinger equation. The dispersion is provided by the large Larmor radius effect in

Nagendra Kumar; Krishna M. Srivastava

1991-01-01

304

bamr: Bayesian analysis of mass and radius observations

NASA Astrophysics Data System (ADS)

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

Steiner, Andrew W.

2014-08-01

305

NASA Astrophysics Data System (ADS)

The interaction of electron and hole spins with the spins of paramagnetic centers and the atomic nuclei is usually negligibly weak in bulk semiconductors because it is a contact interaction. However, in nanosize quantum dots this interaction, proportional to the square of the electron and hole wave functions at the nuclear or paramagnetic center position, grows dramatically, as the inverse of the volume of the quantum dot. As a result fluctuations of the effective random magnetic field of the nuclei acting on the electron spin can reach several Tesla, and in a crystal with a paramagnetic center this field can be several hundred Tesla. This field may be responsible for the radiative recombination of the optically forbidden Dark Exciton and for the temperature dependent Stokes shift observed in CdSe quantum dots in the 1.4 to 15 K region. Conversely, the electron and hole spins strongly affect the spins of the nuclei and paramagnetic defects as the inverse cube of the dot radius. This should lead to their coalignment and to a large NMR Knight shift in optically excited quantum dots.

Efros, Al. L.; Rosen, M.; Nirmal, M.; Bawendi, M. G.

1996-03-01

306

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

307

Direct electrical tunneling into excitons and exciton polaritons is not only interesting from fundamental viewpoint but can also be applied to optoelectronic devices. In the first part of this thesis, theoretical analysis of such a pumping scheme is presented, a scheme which can provide faster speed, higher efficiency, and better selectivity of pumping in comparison with conventional indirect electrical pumping

Gleb Vladimirovich Klimovitch

1999-01-01

308

Magnetic Brightening of Dark Excitons in Carbon Nanotubes

NASA Astrophysics Data System (ADS)

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

Kono, Junichiro

2007-03-01

309

Nonlinear Nano-Optics: Probing One Exciton at a Time

NASA Astrophysics Data System (ADS)

Optical studies in single quantum dots (QD's) have recently been possible with the use of high spatial resolution techniques (K. Brunner,et al., Phys. Rev. Lett. 69, 3216 (1992).)^,(H. F. Hess,et al., Science 264, 1740 (1994).)^,(D. Gammon, et al., Phys. Rev. Lett. 76, 3005 (1996).). The various approaches remove the spectral blurring caused by inhomogeneous broadening in ensemble measurements revealing extremely sharp resonances that result from the complete energy quantization of the zero-dimensional exciton. Previous experiments in single QD's have been based uniquely in photoluminescence (PL) detection and were mainly performed in the frequency domain. In this work, we present data from two different experimental approaches that go beyond these limitations and open up a new direction of research for direct measurements of exciton dynamics, coherent transients and optical nonlinearities in QD's. The first set of experiments combines the elegance and power of CW coherent nonlinear optical spectroscopy with the breakthrough of single QD probing producing the first nonlinear measurement in a single QD. The nonlinear measurements allow us to identify an incoherent and coherent contribution to the resonant electronic response, extract the excitation decoherence time and energy relaxation rate, as well as demonstrate a behavior similar to two beam coupling. In the second set of experiments, using two phase-locked lasers pulses, we show the possibility to control the population(A. P. Heberly, J.J. Baumberg, and Kohler, Phys. Rev. Lett. 74, 3596 (1995))of a single QD in times shorter than the excitonic lifetime and taking thus, coherent-control to the ultimate quantum limit of a single exciton per control box. In addition, we performed a series of transient experiments that includes the first direct measurement of the decoherence time in single QD. The measurements are performed at T=6K in a narrow (42 Åsingle MBE grown GaAs quantum well with 250 ÅAl _0.3Ga_0.7As barriers with large monolayer-high islands which localize excitons in QD potentials. Excitons in isolated QD's were probed by exciting through a submicron sized aperture in a 100-nm-thick Al mask deposited directly on the sample surface obtaining the required spatial resolution.

Bonadeo, Nicolas H.

1998-03-01

310

NASA Astrophysics Data System (ADS)

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

Ji, Haojie; Dhomkar, Siddharth; Roy, Bidisha; Shuvayev, Vladimir; Deligiannakis, Vasilios; Tamargo, Maria C.; Ludwig, Jonathan; Smirnov, Dmitry; Wang, Alice; Kuskovsky, Igor L.

2014-10-01

311

Thermoconvective vortices in a cylindrical annulus with varying inner radius

NASA Astrophysics Data System (ADS)

This paper shows the influence of the inner radius on the stability and intensity of vertical vortices, qualitatively similar to dust devils and cyclones, generated in a cylindrical annulus non-homogeneously heated from below. Little relation is found between the intensity of the vortex and the magnitude of the inner radius. Strong stable vortices can be found for both small and large values of the inner radius. The Rankine combined vortex structure, that characterizes the tangential velocity in dust devils, is clearly observed when small values of the inner radius and large values of the ratio between the horizontal and vertical temperature differences are considered. A contraction on the radius of maximum azimuthal velocity is observed when the vortex is intensified by thermal mechanisms. This radius becomes then nearly stationary when frictional force balances the radial inflow generated by the pressure drop in the center, despite the vortex keeps intensifying. These results connect with the behavior of the radius of the maximum tangential wind associated with a hurricane.

Castaño, D.; Navarro, M. C.; Herrero, H.

2014-12-01

312

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

313

Bleaching in the region of exciton resonance of layered GaSe crystals

NASA Astrophysics Data System (ADS)

Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd3+ laser emitting 30-ps light pulses and a dye laser with a pulse width of ˜3 ns tunable within the range 594-643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.

Kyazym-zade, A. G.; Salmanov, V. M.; Guseinov, A. G.; Salmanova, A. A.; Mamedov, R. M.; Dzhavadzade, A. A.

2014-09-01

314

Dielectric Screening of Excitons and Trions in Single-Layer MoS2.

Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics. PMID:25216267

Lin, Yuxuan; Ling, Xi; Yu, Lili; Huang, Shengxi; Hsu, Allen L; Lee, Yi-Hsien; Kong, Jing; Dresselhaus, Mildred S; Palacios, Tomás

2014-10-01

315

In-plane radiative recombination channel of a dark exciton in self-assembled quantum dots

NASA Astrophysics Data System (ADS)

We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization-resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole g factor enabled us to show that the radiative recombination is a dominant decay channel of the dark excitons in CdTe/ZnTe quantum dots.

Smole?ski, T.; Kazimierczuk, T.; Goryca, M.; Jakubczyk, T.; K?opotowski, ?.; Cywi?ski, ?.; Wojnar, P.; Golnik, A.; Kossacki, P.

2012-12-01

316

An Asian Perspective on the Management of Distal Radius Fractures

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

Sebastin, Sandeep J.; Chung, Kevin C.

2012-01-01

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. PMID:22554653

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

2012-01-01

318

Exciton-polariton Josephson interferometer in a semiconductor microcavity

NASA Astrophysics Data System (ADS)

We propose to construct an exciton-polariton Josephson interferometer in a planar semiconductor microcavity. For a single excitation laser, the time-independent Josephson currents appear, regardless of the excitation laser, energy differences, and interactions. Exciton polaritons can exist in the trap far away from the excitation area (several micrometres), although their lifetime is very short (several picoseconds). If there are two excitation sources, the constructive and destructive interferences can be modulated, and they are related to the condensate energies, densities, interactions, and hopping energy in the dc Josephson effect. The 0\\text-? transition is obtained by altering the condensate energies. A synchronized phase exists in the ac Josephson effect and keeps the system oscillating in the steady state. These results are useful to interpret the coherence in the driven-dissipative Josephson interferometer.

Zhang, Chuanyi; Zhang, Weifeng

2014-10-01

319

Exciton scattering on symmetric branching centers in conjugated molecules.

The capability of the exciton scattering approach, an efficient methodology for excited states in branched conjugated molecules, is extended to include symmetric triple and quadruple joints that connect linear segments on the basis of the phenylacetylene backbone. The obtained scattering matrices that characterize these vertices are used in application of our approach to several test structures, where we find excellent agreement with the transition energies computed by the reference quantum chemistry. We introduce topological charges, associated with the scattering matrices, which help to formulate useful relations between the number of excitations in the exciton band and the number of repeat units. The obtained features of the scattering phases are analyzed in terms of the observed excited state electronic structure. PMID:21194223

Li, Hao; Wu, Chao; Malinin, Sergey V; Tretiak, Sergei; Chernyak, Vladimir Y

2011-05-12

320

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

321

Observation of Dressed Excitonic States in a Single Quantum Dot

NASA Astrophysics Data System (ADS)

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.

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

2008-05-01

322

Exciton-polariton mediated light propagation in anisotropic waveguides

NASA Astrophysics Data System (ADS)

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

Takeda, Hiroyuki; Sakoda, Kazuaki

2012-11-01

323

Collective oscillations in spatially modulated exciton-polariton condensate arrays

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

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

2014-07-25

324

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

325

Anomalous Magnetic-Field Dependence and Fluctuations of Indirect Exciton Luminescence

Indirect excitons in Coupled Quantum Wells (CQW) form a two dimensional exciton gas with long lifetime (100 ns) which are good candidates for Bose condensation. In a two-dimensional system, Bose condensation is possible in a confining potential. Recently, we have observed a strong red shift of indirect excitons (60 meV\\/T) at low magnetic field, in contrast to previous measurements which

Viorel Negoita; Douglas Hackworth; David Snoke; Karl Eberl

2000-01-01

326

We investigate theoretically the effects of electron-electron interactions on excitonic absorption in charged, lens-shaped, self-assembled quantum dots (SAD). The electronic shells of SAD are filled with N electrons according to generalized Hund's rules. In absorption, an exciton is added to the electronic system. The electronic part of the exciton couples in a nontrivial way to resonant electronic configurations of SAD

Gustavo A. Narvaez; Pawel Hawrylak

2000-01-01

327

Self-consistent approach for excitons in quantum wells

We introduce a computationally efficient approach to calculating the\\u000acharacteristics of excitons in quantum wells. In this approach we derive a\\u000asystem of self-consistent equations describing the motion of an electron-hole\\u000apair. The motion in the growth direction of the quantum well in this approach\\u000ais separated from the in-plane motion, but each of them occurs in modified\\u000apotentials found

I. V. Ponomarev; L. I. Deych; A. A. Lisyansky

2003-01-01

328

Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics

We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT

Stephanie V. Chasteen; Sue A. Carter; Garry Rumbles

2005-01-01

329

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

330

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

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

331

Exciton valley dynamics probed by Kerr rotation in WSe2 monolayers

NASA Astrophysics Data System (ADS)

We have experimentally studied the pump-probe Kerr rotation dynamics in WSe2 monolayers. This yields a direct measurement of the exciton valley depolarization time ?v. At T =4K, we find ?v?6 ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases, with ?v being as short as 1.5 ps at 125 K. The temperature dependence is well explained by the developed theory, taking into account the exchange interaction and fast exciton scattering time on the short-range potential.

Zhu, C. R.; Zhang, K.; Glazov, M.; Urbaszek, B.; Amand, T.; Ji, Z. W.; Liu, B. L.; Marie, X.

2014-10-01

332

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

333

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

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

2014-01-01

334

Effect of Ag-doped bathocuproine on the recombination properties of exciton in fullerene

NASA Astrophysics Data System (ADS)

The recombination properties of exciton in fullerene (C60) deposited onto Ag-doped BCP layer with different doping concentrations were studied by photoluminescence (PL). The results show that low doping does not affect the exciton quenching of C60. On the contrary, the exciton quenching of C60 can be observed when the molar ratio of Ag to BCP is more than 1:1. The exciton quenching may be caused by the increase of metal-induced nonradiation centers at the interface since the existence of Ag cluster was confirmed in this case.

Wang, Shenghao; Sakurai, Takeaki; Komatsu, Keiichirou; Akimoto, Katsuhiro

2013-09-01

335

Graded conforming Delaunay tetrahedralization with bounded radius-edge ratio

We propose an algorithm to compute a conforming Delaunay mesh of a polyhedral domain. Arbitrarily small input angles are allowed. The output mesh is graded and has bounded radius-edge ratio everywhere.

Siu-Wing Cheng; Sheung-Hung Poon

2003-01-01

336

Stress fractures of the distal radius in adolescent gymnasts

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

M. T. F. Read

1981-01-01

337

Squat exercise biomechanics during short-radius centrifugation

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

Duda, Kevin R., 1979-

2007-01-01

338

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

339

Radius Constants for Analytic Functions with Fixed Second Coefficient

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

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

2014-01-01

340

Radius of Curvature of Off-Axis Paraboloids

NASA Technical Reports Server (NTRS)

We present several methods for measuring the vertex radius of curvature of off-axis paraboloidal mirrors. One is based on least-squares fitting of interferometer output, one on comparison of sagittal and tangential radii of curvature, and another on measurement of displacement of the nulled test article from the ideal reference wave. Each method defines radius of curvature differently and, as a consequence, produces its own sort of errors.

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

2002-01-01

341

Solar cycle dependence of the apparent radius of the Sun

NASA Astrophysics Data System (ADS)

Visual astrometric observations of the Sun covering the second and first part of solar cycles 22 and 23 respectively, have been carried out during the last 13 years with the Danjon astrolabe of Santiago, Chile. These observations give, among other solar parameters, an absolute value of the Sun's apparent radius. We report here the results obtained from 4092 homogeneous radius measurements at 30o and 60o zenith distances. The data set shows at both zenith distances a significant radius variation in phase with magnetic activity. Moreover, the observations at 30o, which are less affected by atmospheric noise, give a significantly higher correlation coefficient between radius variation and sunspot numbers. Other investigations of solar radius variations during the last decades based on different observing techniques, as well as two analyses of historical data, are commented. Most of them show also positive correlations between radius variation and solar activity. With the noted exception of Calern, France, the results obtained at other astrolabe stations during recent years are in agreement with Santiago. The discrepancy between Calern and Santiago and its probable cause are discussed.

Noël, F.

2004-01-01

342

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 ?). PMID:19431886

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

1993-01-01

343

NASA Astrophysics Data System (ADS)

The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.

Ding, Baofu; Alameh, Kamal

2014-07-01

344

/www//htdocs/physics/personnel/email.ps on bohr FACULTY Bruce.Barrett bbarrett William.Bickel bickel Elliott.Cheu elliott Alex.Cronin cronin Keith APPOINTMENTS Michael.Brown mfbrown Robert.Maier rsm Feryal.Ozel fozel Philip.Pinto ppinto Dimitrios.Kirby - matthewkirby Jacob.Kinnun kinnun Nikita.Kirnosov kirnosov Jeffrey.Kost jkost Anirban.Kundu kunduan Robert

Arizona, University of

345

ON THE CONSTANCY OF THE SOLAR RADIUS. III

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

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

2010-06-20

346

SILICON MULTIPLE EXCITON GENERATION/PN JUNCTION HYBRID SOLAR CELL Sean Jacobs

SILICON MULTIPLE EXCITON GENERATION/PN JUNCTION HYBRID SOLAR CELL Sean Jacobs 1* , Michael Levy 1 , Elton Marchena 1 , Christiana B. Honsberg 1 1 Department of Electrical and Computer Engineering-surface/pn junction hybrid solar cell has been modeled to incorporate multiple excitons generation effects. Using

Honsberg, Christiana

347

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

to 300 K. The excitonic transition and coupling strength of exciton-longitudinal optical phonon were-efficiency light-emitting devices and lasers in the UV region. Finally, the carrier dynamics of the ZnS NWs were absorption band of most organics and biomolecules,10,11 thus it is envisaged that ZnS NWs based devices may

Xiong, Qihua

348

Excitons in Carbon Nanotubes Revisited: Dependence on Diameter, Aharonov-Bohm Flux, and Strain

Optical spectra are calculated in semiconducting carbon nanotubes in the presence of magnetic flux and strain within a k{\\\\cdot}p scheme. The exciton absorption energies show an extra logarithmic dependence on the diameter after being scaled by the inverse diameter, coming from that of the enhancement of the band gaps. The exciton binding energy remains almost independent of the extra dependence.

Tsuneya Ando

2004-01-01

349

Exciton Storage in a Nanoscale Aharonov-Bohm Ring with Electric Field Tuning

We study analytically the optical properties of a simple model for an electron-hole pair on a ring subjected to perpendicular magnetic flux and in-plane electric field. We show how to tune this excitonic system from optically active to optically dark as a function of these external fields. Our results offer a simple mechanism for exciton storage and readout.

Fischer, Andrea M.; Roemer, Rudolf A. [Department of Physics and Centre for Scientific Computing, University of Warwick, Coventry, CV4 7AL (United Kingdom); Campo, Vivaldo L. Jr. [Departmento de Fisica, Universidade Federal de Sao Carlos-UFSCar, 13565-905 Sao Carlos, SP (Brazil); Portnoi, Mikhail E. [School of Physics, University of Exeter, Exeter EX4 4QL (United Kingdom)

2009-03-06

350

Steady-state propagation of ultrashort optical pulses in semiconductor excitonic medium

NASA Astrophysics Data System (ADS)

The form-invariant coherent pulse propagation in semiconductors excited at the 1 s -exciton resonance is studied analytically with the reduced semiconductor Maxwell-Bloch equations. The sech-shaped pulse solution for the electric field is presented. The effects of Coulomb interactions between excitons and spatial dispersion are discussed. Applications to CdS and CdSe crystals are made.

Talanina, Irina B.

1996-06-01

351

The Role of Length and Defects on Optical Quantum Efficiency and Exciton

by excitons and perform Monte Carlo simulations of these random walks to model the fluorescence from nanotubes using an exciton random-walk and defect-induced quenching model. When nonradiative decay is due solely clearly also be utilized in sensor applications, such as biological sen- sors, ideally capable

352

Bright and dark excitons in semiconductor carbon nanotubes: insights from electronic structure Article on the web 23rd March 2009 DOI: 10.1039/b818473a We review electronic structure calculations (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes

Tretiak, Sergei

353

Rabi splitting and ac-Stark shift of a charged exciton M. Kroner,a

Rabi splitting and ac-Stark shift of a charged exciton M. Kroner,a C. Lux, S. Seidl, A. W online 23 January 2008 The Rabi splitting of the negatively charged exciton in a single InGaAs quantum dispersion, with a power dependent Rabi splitting on resonance, both signatures of a strongly coupled two

Ludwig-Maximilians-UniversitÃ¤t, MÃ¼nchen

354

Contribution E.49 -Thursday, June 22 Dependence of exciton transition energy

Contribution E.49 - Thursday, June 22 Dependence of exciton transition energy of single Univ., PRESTO/Japan Science and Technology Agency 2 Dept. of Quantum Eng., Nagoya Univ. 3 Dept investigated the dependence of exciton transition ener- gy of single-walled carbon nanotubes (SWNTs

Maruyama, Shigeo

355

Superradiant and dark exciton states in an optical lattice within a cavity

We study ultracold atoms trapped in a one-dimensional optical lattice prepared in a Mott insulator state of finite extend and collectively coupled to a single cavity mode. Due to resonant dipole-dipole interactions among the atoms, electronic excitations get delocalized and form excitons. These excitons can be explicitly calculated and divided into two groups: antisymmetric modes which decouple from the cavity

Hashem Zoubi; Helmut Ritsch

2009-01-01

356

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

357

NASA Astrophysics Data System (ADS)

So far, excitonic luminescence in metal halide (MH) has extensively been studied by many workers, with the interest not only in physics of excitons but also in its possible application in optoelectronics. In the actual MHs, however, excitonic luminescence is so weak that it is beyond the scope of applications. Here we report the rediscovery that inherent excitonic luminescence in MH is dominated by a dramatically high light-emitting mechanism. This is demonstrated using improved films of MH yielded by two simple, independent methods, both applicable to several MHs. For example, improved CsPbCl3 and CuBr films exhibit more than 103-104 times stronger excitonic luminescence than conventional films. Inherent excitonic luminescence in MH is promising for applications in exciton-based light-emitting devices, particularly in shorter-wavelength devices than the traditional, nonexcitonic ones because it is MH and only MH that includes many compounds with large band gap and large exciton binding energies.

Kondo, S.; Ohsawa, H.; Asada, H.; Saito, T.

2010-05-01

358

Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor of the nanoring)10ï¿½15 have demonstrated that the optical exciton Aharonov-Bohm effect (X-ABE), i. e-Universitï¿½at zu Berlin, Newtonstr. 15, 12489 Berlin, Germany (Dated: May 11, 2006) The optical exciton Aharonov-Bohm

Zimmermann, Roland

359

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 quantum-collective phenomena. The meeting has included invited lectures, contributed oral presentations and posters, covering the following general topics: low-dimensional heterostructures: quantum wells, quantum wires and quantum dots polaritons quantum optics with excitons and polaritons many-body effects under coherent and incoherent excitation coherent optical spectroscopy quantum coherence and quantum-phase manipulation Bose-Einstein condensation and other collective phenomena excitons in novel materials The OECS 11 was held at the campus of the Universidad Autónoma de Madrid in Cantoblanco. The scientific program was composed of more than 200 contributions divided into 16 invited talks, 44 oral contributions and 3 poster sessions with a total of 150 presentations. The scientific level of the presentations was guaranteed by a selection process where each contribution was rated by three members of the Program Committee. The Conference has gathered 238 participants from 21 different countries, with the following distribution: Germany (43), France (41), Spain (33), UK (24), Switzerland (21), Italy (14), The Netherlands (12), USA (11), other (23). The conference was made possible by generous sponsors, whom we thank earnestly: Universidad Autónoma de Madrid, Spanish Ministry of 'Educación y Ciencia', Consejo Superior de Investigaciones Científicas, European Union (ITN- 235114), Europhysics Letters, Semiconductor Science and Technology, Consolider Research Project 'Quantum Optical Information Technology', Lasing S A, Newport, Innova Scientific, Foundation Madrid-2016 and European Physical Society. We would like to acknowledge the members of the Organizing and Program Committees, who are responsible for the success of the Conference (names are listed below). Finally, the authors are thanked for the quality of their contributions. Luis Viña Carlos Tejedor José M Calleja EDITORS Organizing Committee Luis Viña-Chair, Universidad Autónoma de Madrid María D Martín-Scientific Secretary, Universidad Autónoma de Madrid José M Calleja, Universidad Autónoma de Madrid Luisa González, Instituto de Microelectrónica de Madrid Herko van der Meulen, Universidad Autónoma de Madrid Enrique Calleja, Instituto de Sistemas Optoelectrónicos y Microtecnología Madrid Daniele Sanvitto, Universidad Autónoma de Madrid Program Committee Carlos Tejedor-Chair, Universidad Autónoma de Madrid Israel Bar-Joseph, Weizmann Institute of Science Jeremy J Baumberg, Cambridge University Manfred Bayer, Universität Dortmund Jacqueline Bloch, Laboratoire de Photonique et de Nanostructures - CNRS Wolfgang Langbein, Cardiff University Marek Potemski, Grenoble High Magnetic Field Laboratory Antonio Quattropani, Ecole Polytechnique Fédérale de Lausanne Salvatore Savasta, Università di Messina Vincenzo Savona, Ecole Polytechnique Fédérale de Lausanne David Snoke, University of Pittsburgh Jerome Tignon, Ecole Normale Superieure Paris

Viña, Luis; Tejedor, Carlos; Calleja, José M.

2010-01-01

360

SATELLITE FORMATION :spreading of rings beyond the Roche radius

NASA Astrophysics Data System (ADS)

When a disk of solid particles (like Saturn's rings) spreads beyond the Roche radius (inside which planetary tides prevent aggregation), satellites form and migrate away. After a quick derivation of the Roche radius, here we describe analytically this process. When the spreading is fast, only one large satellite forms, as was the case for Pluto and Earth. When the spreading is slow, a retinue of satellites appear with masses increasing with distance to the Roche radius, in excellent agreement with Saturn's, Uranus', and Neptune's satellite systems. This suggests that Uranus and Neptune used to have massive rings that disappeared to give birth to their regular satellites. Thus, we argue in Crida & Charnoz (2012) that most regular satellites in the Solar System probably formed in this way.

Crida, A.; Charnoz, S.

2013-11-01

361

Effect of Monolayer Thickness Fluctuations on Coherent Exciton Coupling in Single Quantum Wells

Monolayer fluctuations in the thickness of a semiconductor quantum well (QW) lead to three types of excitons, located in the narrower, average and thicker regions of the QW, which are clearly resolved in optical spectra. Whether or not these excitons are coherently coupled via Coulomb interactions is a long-standing debate. We demonstrate that different types of disorder in QWs distinctly affects the coherent coupling and that the coupling strength can be quantitatively measured using optical two-dimensional Fourier transform spectroscopy. We prove experimentally and theoretically that in narrow quantum wells the coherent coupling occurs predominantly between excitons residing in the disorder-free areas of the QWs and those residing in the plateau-type disorder. In contrast, excitons localized in the fault-type disorder potentials do not coherently couple to other excitons.

Yuri D. Glinka; Zheng Sun; Mikhail Erementchouk; Michael N. Leuenberger; Alan D. Bristow; Steven T. Cundiff; Allan S. Bracker; Xiaoqin Li

2012-10-04

362

The electronic structure and size-scaling of optoelectronic properties in cycloparaphenylene carbon nanorings are investigated using time-dependent density functional theory (TDDFT). The TDDFT calculations on these molecular nanostructures indicate that the lowest excitation energy surprisingly becomes larger as the carbon nanoring size is increased, in contradiction with typical quantum confinement effects. In order to understand their unusual electronic properties, I performed an extensive investigation of excitonic effects by analyzing electron-hole transition density matrices and exciton binding energies as a function of size in these nanoring systems. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in the nanorings. Based on overall trends in exciton binding energies and their spatial delocalization, I find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these carbon nanoring systems. PMID:22481999

2009-01-01

363

Rapid calculation method for Frenkel-type two-exciton states in one to three dimensions.

Biexciton and two-exciton dissociated states of Frenkel-type excitons are well described by a tight-binding model with a nearest-neighbor approximation. Such two-exciton states in a finite-size lattice are usually calculated by numerical diagonalization of the Hamiltonian, which requires an increasing amount of computational time and memory as the lattice size increases. I develop here a rapid, memory-saving method to calculate the energies and wave functions of two-exciton states by employing a bisection method. In addition, an attractive interaction between two excitons in the tight-binding model can be obtained directly so that the biexciton energy agrees with the observed energy, without the need for the trial-and-error procedure implemented in the numerical diagonalization method. PMID:25053304

Ajiki, Hiroshi

2014-07-21

364

Self-trapped exciton and core-valence luminescence in BaF{sub 2} nanoparticles

The influence of the BaF{sub 2} nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (h? ? E{sub g}) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

Vistovskyy, V. V., E-mail: vistvv@gmail.com; Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Voloshinovskii, A. S. [Ivan Franko National University of Lviv, 8 Kyryla i Mefodiya, 79005 Lviv (Ukraine); Myagkota, O. S. [Lviv Polytechnic National University, 12S. Bandera, 79013 Lviv (Ukraine); Gloskovskii, A. [Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg (Germany); Gektin, A. V. [Institute for Scintillation Materials, NAS of Ukraine 60 Lenina Ave, 61001 Kharkiv (Ukraine); Vasil'ev, A. N. [Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119991 Moscow (Russian Federation); Rodnyi, P. A. [Saint-Petersburg State Polytechnical University, 29, Polytekhnicheskaya, 195251 Saint-Petersburg (Russian Federation)

2013-11-21

365

Long-lived exciton coherences have been recently observed in photosynthetic complexes via ultrafast spectroscopy, opening exciting possibilities for the study and design of coherent exciton transport. Yet, ambiguity in the spectroscopic signals has led to arguments against interpreting them in terms of exciton dynamics, demanding more stringent tests. We propose a novel strategy, quantum process tomography (QPT), for ultrafast spectroscopy and apply it to reconstruct the evolving quantum state of excitons in double-walled supramolecular light-harvesting nanotubes at room temperature from eight narrowband transient grating experiments. Our analysis reveals the absence of nonsecular processes, unidirectional energy transfer from the outer to the inner wall exciton states, and coherence between those states lasting about 150 fs, indicating weak electronic coupling between the walls. Our work constitutes the first experimental QPT in a "warm" and complex system and provides an elegant scheme to maximize information from ultrafast spectroscopy experiments. PMID:24724614

Yuen-Zhou, Joel; Arias, Dylan H; Eisele, Dorthe M; Steiner, Colby P; Krich, Jacob J; Bawendi, Moungi G; Nelson, Keith A; Aspuru-Guzik, Alán

2014-06-24

366

NASA Astrophysics Data System (ADS)

A generalized master equation (GME) governing quantum evolution of modular exciton density (MED) is derived for large scale light harvesting systems composed of weakly interacting modules of multiple chromophores. The GME-MED offers a practical framework to incorporate real time coherent quantum dynamics calculations of small length scales into dynamics over large length scales, and also provides a non-Markovian generalization and rigorous derivation of the Pauli master equation employing multichromophoric Förster resonance energy transfer rates. A test of the GME-MED for four sites of the Fenna-Matthews-Olson complex demonstrates how coherent dynamics of excitonic populations over coupled chromophores can be accurately described by transitions between subgroups (modules) of delocalized excitons. Application of the GME-MED to the exciton dynamics between a pair of light harvesting complexes in purple bacteria demonstrates its promise as a computationally efficient tool to investigate large scale exciton dynamics in complex environments.

Jang, Seogjoo; Hoyer, Stephan; Fleming, Graham; Whaley, K. Birgitta

2014-10-01

367

Excitons versus free charges in organo-lead tri-halide perovskites.

Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50?meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications. PMID:24710005

D'Innocenzo, Valerio; Grancini, Giulia; Alcocer, Marcelo J P; Kandada, Ajay Ram Srimath; Stranks, Samuel D; Lee, Michael M; Lanzani, Guglielmo; Snaith, Henry J; Petrozza, Annamaria

2014-01-01

368

Revealing the dark side of a bright exciton-polariton condensate.

Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light-matter-coupled excitations, such as exciton-polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates. PMID:25115964

Ménard, J-M; Poellmann, C; Porer, M; Leierseder, U; Galopin, E; Lemaître, A; Amo, A; Bloch, J; Huber, R

2014-01-01

369

Rapid calculation method for Frenkel-type two-exciton states in one to three dimensions

NASA Astrophysics Data System (ADS)

Biexciton and two-exciton dissociated states of Frenkel-type excitons are well described by a tight-binding model with a nearest-neighbor approximation. Such two-exciton states in a finite-size lattice are usually calculated by numerical diagonalization of the Hamiltonian, which requires an increasing amount of computational time and memory as the lattice size increases. I develop here a rapid, memory-saving method to calculate the energies and wave functions of two-exciton states by employing a bisection method. In addition, an attractive interaction between two excitons in the tight-binding model can be obtained directly so that the biexciton energy agrees with the observed energy, without the need for the trial-and-error procedure implemented in the numerical diagonalization method.

Ajiki, Hiroshi

2014-07-01

370

Screening effect on the binding energy of the exciton in quantum wires

NASA Astrophysics Data System (ADS)

This paper describes using the finite difference method to study the screening effect on the binding energy of excitons in quantum wires (QWRs). We have derived an analytical formula for the effective screened interaction potential between an electron and a hole in an exciton. The exciton binding energy as functions of the screening length, carrier density, and QWR width is calculated and is found to be sensitive to the screen length when the length is less than 1 × 103 Å. Furthermore, when the screen length is less than 3 × 102 Å, the exciton binding energy decreases rapidly with decreasing screen length. It is also found that the wider the QWR is, the more pronounced the screening effect will be. The transition from a dilute exciton gas to a dense electron-hole plasma in a QWR is clearly shown on a phase diagram for the carrier density-temperature plane.

Zhai, Li-Xue; Wang, Yan; Liu, Jian-Jun

2012-08-01

371

Excitons versus free charges in organo-lead tri-halide perovskites

NASA Astrophysics Data System (ADS)

Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50?meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.

D'Innocenzo, Valerio; Grancini, Giulia; Alcocer, Marcelo J. P.; Kandada, Ajay Ram Srimath; Stranks, Samuel D.; Lee, Michael M.; Lanzani, Guglielmo; Snaith, Henry J.; Petrozza, Annamaria

2014-04-01

372

Formation of the self-trapped exciton via thermally induced defect reactions in alkali halides

NASA Astrophysics Data System (ADS)

Low-temperature thermoluminescence has been studied in KBr, NaCl, and KI by means of emission-spectrum measurements of glow peaks and of detailed analysis as to the quantitative correlation between emission intensities and the amount of reacted defects. It is found that the luminescence center is the self-trapped exciton which results in both ? and ? emissions in these salts below 100 K and that the self-trapped exciton is produced at the temperature where the annihilation of the F center takes place. It is, however, shown that the recombination between F and H centers can excite only the lowest triplet exciton in KBr. The quantum yield of the exciton formation by F-H recombination is estimated less than 0.03. The energetic correlation between the exciton and the F-H pair state in the defect-annihilation process in KBr is discussed.

Tanimura, K.; Okada, T.

1980-02-01

373

Multiple exciton generation and dissociation in PbS quantum dot-electron acceptor complexes.

Multiple exciton generation (MEG) in quantum dots (QDs), a process by which one absorbed photon generates multiple electron-hole pairs, has provided exciting possibilities for improving the energy conversion efficiency of photovoltaic and photocatalytic devices. However, implementing MEG in practical devices requires the extraction of multiple charge carriers before exciton-exciton annihilation and the development of materials with improved MEG efficiency. In this report, using PbS QD/methylene blue complexes as a QD/electron acceptor model system, we demonstrate that the presence of electron acceptors does not affect the MEG efficiency of QDs and all generated excitons can be dissociated by electron transfer to the acceptor, achieving MEG and multiple exciton dissociation efficiencies of 112%. We further demonstrate that these efficiencies are not affected by the charging of QDs. PMID:22757981

Yang, Ye; Rodríguez-Córdoba, William; Lian, Tianquan

2012-08-01

374

Solving the discrepancy between the seismic and photospheric solar radius

Two methods are used to observationally determine the solar radius: One is the observation of the intensity profile at the limb, the other one uses f-mode frequencies to derive a 'seismic' solar radius which is then corrected to optical depth unity. The two methods are inconsistent and lead to a difference in the solar radius of $\\sim$0.3 Mm. Because of the geometrical extention of the solar photosphere and the increased path lengths of tangential rays the Sun appears to be larger to an observer who measures the extent of the solar disk. Based on radiative transfer calculations we show that this discrepancy can be explained by the difference between the height at disk center where $\\tau_{\\mathrm{5000}}=1$ ($\\tau_{\\mathrm{Ross}}=2/3$) and the inflection point of the intensity profile on the limb. We calculate the intensity profile of the limb for the MDI continuum and the continuum at 5000 {\\AA} for two atmosphere structures and compare the position of the inflection points with the radius at $\\tau_{\\mathrm{5000}}=1$ ($\\tau_{\\mathrm{Ross}}=2/3$). The calculated difference between the 'seismic' radius and the inflection point is $0.347\\pm 0.06$ Mm with respect to $\\tau_{\\mathrm{5000}}=1$ and $0.333\\pm 0.08$ Mm with respect to $\\tau_{\\mathrm{Ross}}=2/3$. We conclude that the standard solar radius in evolutionary models has to be lowered by $0.333\\pm 0.08$ Mm and is 695.66 Mm. Furthermore, this correction reconciles inflection point measurements and the 'seismic' radii within the uncertainty.

Margit Haberreiter; Alexander G. Kosovichev; Werner Schmutz

2007-11-15

375

Coherent Oscillations in an Exciton-Polariton Josephson Junction

NASA Astrophysics Data System (ADS)

We report on the observation of spontaneous coherent oscillations in a microcavity polariton bosonic Josephson junction. Condensation of exciton polaritons here takes place under incoherent excitation in a double potential well naturally formed in the disorder. Coherent oscillations set on at an excitation power well above the condensation threshold. The time resolved population and phase dynamics reveal the analogy with the ac Josephson effect. A theoretical two-mode model describes the observed effects, explaining how the different realizations of the pulsed experiment can be in phase.

Lagoudakis, K. G.; Pietka, B.; Wouters, M.; André, R.; Deveaud-Plédran, B.

2010-09-01

376

A measurable force driven by an excitonic condensate

NASA Astrophysics Data System (ADS)

Free energy signatures related to the measurement of an emergent force (?10-9N) due to the exciton condensate (EC) in Double Quantum Wells are predicted and experiments are proposed to measure the effects. The EC-force is attractive and reminiscent of the Casimir force between two perfect metallic plates, but also distinctively different from it by its driving mechanism and dependence on the parameters of the condensate. The proposed experiments are based on a recent experimental work on a driven micromechanical oscillator. Conclusive observations of EC in recent experiments also provide a strong promise for the observation of the EC-force.

Hakio?lu, T.; Ã-zgün, Ege; Günay, Mehmet

2014-04-01

377

Phonons in quantum dots and their role in exciton dephasing

) of carriers or excitons can be achieved by embedding a small band gap material like InAs into a material with a larger band gap like GaAs. Since the band gap decreases with increasing of the equilibrium lattice for usability in optical setups. 4.5 5.0 5.5 6.0 6.5 0 50 100 150 200 250 300 C[GPa] a [ ]Ã? AlN GaN InN GaP Al

Zimmermann, Roland

378

Hybrid exciton recombination dynamics in inorganic-organic materials

NASA Astrophysics Data System (ADS)

A systematic analysis of hybrid Frenkel-Wannier-Mott excitons recombination dynamics in nanocomposite material (organic-inorganic) is performed. A theoretical model based on the rate equation is used in the calculation of the light intensity and relative quantum efficiency. Numerical results have been presented for low and high concentration of quantum dots (Qds). Our results show that the light emission and relative quantum efficiency are significantly enhanced by incorporation of Qds in polymer matrix. Moreover our calculations were found to be in good agreement with the experimental data.

Mastour, N.; Bouchriha, H.

2013-12-01

379

Exciton dynamics in chromophore aggregates with correlated environment fluctuations.

We study the effects of correlated molecular transition energy fluctuations in molecular aggregates on the density matrix dynamics, and their signatures in the optical response. Correlated fluctuations do not affect single-exciton dynamics and can be described as a nonlocal contribution to the spectral broadening, which appears as a multiplicative factor in the time-domain response function. Intraband coherences are damped only by uncorrelated transition energy fluctuations. The signal can then be expressed as a spectral convolution of a local contribution of the uncorrelated fluctuations and the nonlocal contribution of the correlated fluctuations. PMID:21548696

Abramavicius, Darius; Mukamel, Shaul

2011-05-01

380

Distal plate placement for distal radius fractures limits wrist motion

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

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

381

Measurements of the Solar Radius in Antalya between 2001 2003

NASA Astrophysics Data System (ADS)

The results of the solar radius measurements from February 2001 to November 2003 with the solar astrolabe at the TUBITAK National Observatory are presented. The mean semi-diameter for the period, corrected for systematic effects such as the Fried parameter and the zenith distance, is found to be 959.29 ± 0.01 arc sec. A comparison of the monthly averages of the solar radius with the monthly means of sunspot numbers shows that the semi-diameter of the Sun increases with an amplitude of 0.017 arc sec per year in opposite phase with solar cycle 23.

Kilic, H.; Golbasi, O.; Chollet, F.

2005-06-01

382

NASA Astrophysics Data System (ADS)

Neutral excitons in axially symmetric GaAs nanodots embedded in an (Al,Ga)As matrix, which are formed by the droplet epitaxy technique, are investigated theoretically. An electric field perpendicular to the nanodot base results in both a vertical and an in-plane exciton polarization, which is beneficial for the appearance of the excitonic Aharonov-Bohm effect. In the range of low magnetic fields (below 5 Tesla), we found that the bright and dark exciton states can cross twice. This results in oscillations of the photoluminescence intensity with magnetic field, which are a striking manifestation of the optical excitonic Aharonov-Bohm effect.

Arsoski, V.; Tadi?, M.; Peeters, F. M.

2013-11-01

383

Enlarged magnetic focusing radius of photoinduced ballistic currents

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

Ludwig-Maximilians-UniversitÃ¤t, MÃ¼nchen

384

Decoding by Embedding: Correct Decoding Radius and DMT Optimality

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

Paris-Sud XI, UniversitÃ© de

385

Advanced nucleon electromagnetic structure model and charge proton rms radius

NASA Astrophysics Data System (ADS)

Description of the nucleon electromagnetic structure by advanced Unitary and Analytic approach, respecting SU(3) symmetry and analyzing also the recent Mainz MAMI data with impact on the proton charge radius to be compared with the results from the muon hydrogen atom spectroscopy.

Adamuš?ín, C.; Bartoš, E.; Dubni?ka, S.; Dubni?ková, A. Z.

2013-12-01

386

Correlation radius of polarization fluctuations in the disordered ferroelectrics

We calculate correlation radius of polarization fluctuations in the disordered ferroelectrics. The consideration is performed in the model suitable for relaxor ferroelectrics. We consider reference Burns phase in a mean field approximation subjected to random electric field E. The ``entire relaxor'' is then obtained by averaging of characteristics of the reference phase with distribution function of the random field. For

M. D. Glinchuk; E. A. Eliseev; V. A. Stephanovich; E. V. Kirichenko; L. Jastrabik

2002-01-01

387

Correlation radius of polarization fluctuations in the disordered ferroelectrics

We calculate correlation radius of polarization fluctuations in the disordered ferroelectrics. The consideration is performed in the model suitable for relaxor ferroelectrics. We consider reference Burns phase in a mean field approximation subjected to random electric field E. The “entire relaxor” is then obtained by averaging of characteristics of the reference phase with distribution function of the random field. For

M. D. Glinchuk; E. A. Eliseev; V. A. Stephanovich; E. V. Kirichenko; L. Jastrabik

2002-01-01

388

'Wristwatch fracture' of the distal radius: a new diagnosis?

Segmental forearm fractures are relatively uncommon in children, and their optimal management is unclear. We present an unusual segmental fracture of the distal radius in a 14 year old that appears to be related to a large metal wristwatch worn by the patient. It was successfully managed by dosed reduction and immobilisation in a plaster cast. PMID:19202823

Yewlett, A D; Khan, R; Clasper, J

2008-09-01

389

Osteoid osteoma after a fracture of the distal radius.

We report a case of osteoid osteoma occurring at the site of a previous fracture of the radius treated by internal fixation with insertion of a rod. The fracture may have acted as a trigger for the formation of an osteoid osteoma. PMID:8659359

Adil, A; Hoeffel, C; Fikry, T

1996-07-01

390

Radius Stabilization and Anomaly-Mediated Supersymmetry Breaking

We analyze in detail a specific 5-dimensional realization of a "brane-universe" scenario where the visible and hidden sectors are localized on spatially separated 3-branes coupled only by supergravity, with supersymmetry breaking originating in the hidden sector. Although general power counting allows order 1/M_{Planck}^2 contact terms between the two sectors in the 4-dimensional theory from exchange of supergravity Kaluza-Klein modes, we show that they are not present by carefully matching to the 5-dimensional theory. We also find that the radius modulus corresponding to the size of the compactified dimension must be stabilized by additional dynamics in order to avoid run-away behavior after supersymmetry breaking and to understand the communication of supersymmetry breaking. We stabilize the radius by adding two pure Yang--Mills sectors, one in the bulk and the other localized on a brane. Gaugino condensation in the 4-dimensional effective theory generates a superpotential that can naturally fix the radius at a sufficiently large value that supersymmetry breaking is communicated dominantly by the recently-discovered mechanism of anomaly mediation. The mass of the radius modulus is large compared to m_{3/2}. The stabilization mechanism requires only parameters of order one at the fundamental scale, with no fine-tuning except for the cosmological constant.

Markus A. Luty; Raman Sundrum

1999-10-26

391

Computational Analysis of Dual Radius Circulation Control Airfoils

NASA Technical Reports Server (NTRS)

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

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

2006-01-01

392

Optimal Transmission Radius for Flooding in Large Scale Sensor Networks

1 Optimal Transmission Radius for Flooding in Large Scale Sensor Networks Marco ZÂ´u~niga Z, applications such as query propagation rely regularly on network-wide flooding. Besides consuming energy and bandwidth resources, the flooded packet may keep the transmission medium within the network busy for too

Krishnamachari, Bhaskar

393

Nonlinear buckling analyses of a small-radius carbon nanotube

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

394

Form factor and radius of the negative pion

The form factor and the radius of the negative pion are calculated by means of the Chou-Yang geometrical model based on the published data for 50-, 100-, and 200-GeV/c ..pi../sup -/ p elastic scattering.

Bellandi Fo, J.; Brunetto, S.Q.; Covolan, R.J.M.; Menon, M.J.; Pimentel, B.M.; de Padua, A.B.; Paes, J.T.S.

1986-11-01

395

Quantum Process Tomography Quantifies Coherence Transfer Dynamics in Vibrational Exciton

Quantum coherence has been a subject of great interest in many scientific disciplines. However, detailed characterization of the quantum coherence in molecular systems, especially its transfer and relaxation mechanisms, still remains a major challenge. The difficulties arise in part because the spectroscopic signatures of the coherence transfer are typically overwhelmed by other excitation relaxation processes. We use quantum process tomography (QPT) via two-dimensional infrared spectroscopy to quantify the rate of the elusive coherence transfer between two vibrational exciton states. QPT retrieves the dynamics of the dissipative quantum system directly from the experimental observables. It thus serves as an experimental alternative to theoretical models of the system-bath interaction, and can be used to validate these theories. Our results for coupled carbonyl groups of a diketone molecule in chloroform, used as a benchmark system, reveal the non-secular nature of the interaction between the exciton and the Markovian bath and open the door for the systematic studies of the dissipative quantum systems dynamics in detail. PMID:24079417

Chuntonov, Lev; Ma, Jianqiang

2013-01-01

396

Quantum Dot Solar Cells: High Efficiency through Multiple Exciton Generation

Impact ionization is a process in which absorbed photons in semiconductors that are at least twice the bandgap can produce multiple electron-hole pairs. For single-bandgap photovoltaic devices, this effect produces greatly enhanced theoretical thermodynamic conversion efficiencies that range from 45-85%, depending upon solar concentration, the cell temperature, and the number of electron-hole pairs produced per photon. For quantum dots (QDs), electron-hole pairs exist as excitons. We have observed astoundingly efficient multiple exciton generation (MEG) in QDs of PbSe (bulk Eg = 0.28 eV), ranging in diameter from 3.9 to 5.7nm (Eg = 0.73, 0.82, and 0.91 eV, respectively). The effective masses of electron and holes are about equal in PbSe, and the onset for efficient MEG occurs at about three times the QD HOMO-LUMO transition (its ''bandgap''). The quantum yield rises quickly after the onset and reaches 300% at 4 x Eg (3.64 eV) for the smallest QD; this means that every QD in the sample produces three electron-hole pairs/photon.

Hanna, M. C.; Ellingson, R. J.; Beard, M.; Yu, P.; Micic, O. I.; Nozik, A. J.; c.

2005-01-01

397

Temporal long-range order in exciton-polariton condensates

NASA Astrophysics Data System (ADS)

We demonstrate temporal long-range off-diagonal order in microcavity exciton-polariton dynamic condensation, by interference between two separate condensates, generated at different times and with different momenta. In our pulsed resonantly-injected condensates, stimulated polariton-polariton scattering results in spectral narrowing of the dynamic condensates and thus in longer coherence times. We study the temporal decay of the long-range order by monitoring the interference visibility between the condensates. We show that it strongly depends on the excitonic fraction of the polaritons and the corresponding polariton-polariton interaction strength, as well as on the temperature and pump intensity. Moreover, polariton interaction yields a blue shift of the condensate energy, which appears as a time-dependent shift in the interference pattern. These results show a direct evidence of temporal long-range order in dynamic condensates as well as demonstrate a new method for probing their ultrafast dynamics, opening new directions in the fundamental study of coherence in matter.

Hayat, Alex; Lange, Christoph; Rozema, Lee; Chang, Rockson; Potnis, Shreyas; van Driel, Henry; Steinberg, Aephraim; Steger, Mark; Snoke, David; Pfeiffer, Loren; West, Kenneth

2013-03-01

398

Boosting the performance of red PHOLEDs by exciton harvesting

NASA Astrophysics Data System (ADS)

Significant development has been made on phosphorescent organic light emitting diodes (PHOLEDs) over the past decade, which eventually resulted in the commercialization of widely distributed active-matrix organic light emitting diode displays for mobile phones. However, higher efficiency PHOLEDs are still needed to further reduce the cost and lower the power consumption for general lighting and LED backlight applications. In particular, red PHOLEDs currently have in general the lowest efficiencies among the three primary colors, due most likely to the energy-gap law. Therefore, a number of groups have of made use of various device configurations, including insertion of a carrier blocking or exciton confining layer, doping the transport layers, as well as employing multiple emissive zone structures to improve the device efficiency. However, these approaches are rather inconvenient for commercial applications. In this work, we have developed a simpler way to boost the performance of red PHOLEDs by incorporating an exciton harvesting green emitter, which transfers a large portion of the energy to the co-deposited red emitter. A high external quantum efficiency (EQE) of 20.6% was achieved, which is among the best performances for red PHOLEDs.

Chang, Y.-L.; Wang, Z. B.; Helander, M. G.; Qiu, J.; Lu, Z. H.

2012-09-01

399

Lindblad theory of dynamical decoherence of quantum-dot excitons

NASA Astrophysics Data System (ADS)

We use the Bloch-Redfield-Wangsness theory to calculate the effects of acoustic phonons in coherent control experiments where quantum-dot excitons are driven by shaped laser pulses. This theory yields a generalized Lindblad equation for the density operator of the dot, with time-dependent damping and decoherence due to phonon transitions between the instantaneous dressed states. It captures similar physics to the form recently applied to Rabi oscillation experiments [Ramsay , Phys. Rev. Lett.0031-900710.1103/PhysRevLett.104.017402 104, 017402 (2010)] but guarantees positivity of the density operator. At sufficiently low temperatures, it gives results equivalent to those of fully non-Markovian approaches [Lüker , Phys. Rev. B1098-012110.1103/PhysRevB.85.121302 85, 121302 (2012)] but is significantly simpler to simulate. Several applications of this theory are discussed. We apply it to adiabatic rapid passage experiments and show how the pulses can be shaped to maximize the probability of creating a single exciton using a frequency-swept laser pulse. We also use this theory to propose and analyze methods to determine the phonon density of states experimentally, i.e., phonon spectroscopy, by exploring the dependence of the effective damping rates on the driving field.

Eastham, P. R.; Spracklen, A. O.; Keeling, J.

2013-05-01

400

NASA Astrophysics Data System (ADS)

Ultracold polar molecules trapped on an optical lattice is a many-body system that, under appropriate conditions, may support collective excitations reminiscent of excitons in solid state crystals. Here, we discuss the rotational excitations of molecules on an optical lattice leading to rotational Frenkel excitons. Apart from solid hydrogen, there is no other natural system that exhibits rotational excitons. The rotational excitons have unique properties that can be exploited for tuning exciton-exciton interactions and exciton-impurity scattering by applying an external electric field. We show that this can be used to explore the competing role of the dynamical and kinematic exciton-exciton interactions in excitonic energy transfer and to study quantum localization in a dynamically tunable disordered potential. The rotational excitons can also be used as a basis for quantum simulation of condensed matter models that cannot be realized with ultracold atoms. As an example, we discuss the possibility of engineering the Holstein model with polar molecules on an optical lattice.

Litinskaya, Marina; Krems, Roman V.

2013-12-01

401

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

402

Distal radius malunion and forearm rotation: a cadaveric study.

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

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

2014-02-01

403

NASA Astrophysics Data System (ADS)

Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL) method in a coherently coupled exciton-biexciton system in a single quantum dot (QD). PL and photoluminescence excitation spectroscopy (PLE) are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics.

Gotoh, Hideki; Sanada, Haruki; Yamaguchi, Hiroshi; Sogawa, Tetsuomi

2014-10-01

404

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

The absorption of light by materials proceeds through the formation of excitons, which are states in which an excited electron is bound to the valence hole it vacated. Understanding the structure and dynamics of excitons is important, for example, for developing technologies for light-emitting diodes or solar energy conversion. However, there has never been an experimental means to study the time-dependent structure of excitons directly. Here, we use causality-inverted inelastic x-ray scattering (IXS) to image the charge-transfer exciton in the prototype insulator LiF, with resolutions {Delta}t = 20.67 as (2.067 x 10{sup -17} s) in time and {Delta}x = 0.533 {angstrom} (5.33 x 10{sup -11} m) in space. Our results show that the exciton has a modulated internal structure and is coherently delocalized over two unit cells of the LiF crystal ({approx}8 {angstrom}). This structure changes only modestly during the course of its life, which establishes it unambiguously as a Frenkel exciton and thus amenable to a simplified theoretical description. Our results resolve an old controversy about excitons in the alkali halides and demonstrate the utility of IXS for imaging attosecond electron dynamics in condensed matter.

Abbamonte, Peter; Graber, Tim; Reed, James P.; Smadici, Serban; Yeh, Chen-Lin; Shukla, Abhay; Rueff, Jean-Pascal; Ku, Wei (UIUC); (Tamkang); (UPMC); (SOLEIL); (UC); (BNL)

2008-11-03

405

Shape Dependence of Band-Edge Exciton Fine Structure in CdSe Nanocrystals

The band-edge exciton fine structure of wurtzite CdSe nanocrystals is investigated by a plane-wave pseudopotential method that includes spin-orbit coupling, screened electron-hole Coulomb interactions, and exchange interactions. Large-scale, systematic simulations have been carried out on quantum dots, nanorods, nanowires, and nanodisks. The size and shape dependence of the exciton fine structure is explored over the whole diameter-length configuration space and is explained by the interplay of quantum confinement, intrinsic crystal-field splitting, and electron-hole exchange interactions. Our results show that the band-edge exciton fine structure of CdSe nanocrystals is determined by the origin of their valence-band single-particle wave functions. Nanocrystals where the valence-band maximum originates from the bulk A band have a 'dark' ground-state exciton. Nanocrystals where the valence-band maximum is derived from the bulk B band have a 'quasi-bright' ground-state exciton. Thus, the diameter-length configuration map can be divided into two regions, corresponding to dark and quasi-bright ground-state excitons. We find that the dark/quasi-bright ground-state exciton crossover is not only diameter-dependent but also length-dependent, and it is characterized by a curve in the two-parameter space of diameter and length.

Zhao, Q.; Graf, P. A.; Jones, W. B.; Franceschetti, A.; Li, J.; Wang, L. W.; Kim, K.

2007-01-01

406

Shape dependence of band-edge exciton fine structure in CdSe nanocrystals.

The band-edge exciton fine structure of wurtzite CdSe nanocrystals is investigated by a plane-wave pseudopotential method that includes spin-orbit coupling, screened electron-hole Coulomb interactions, and exchange interactions. Large-scale, systematic simulations have been carried out on quantum dots, nanorods, nanowires, and nanodisks. The size and shape dependence of the exciton fine structure is explored over the whole diameter-length configuration space and is explained by the interplay of quantum confinement, intrinsic crystal-field splitting, and electron-hole exchange interactions. Our results show that the band-edge exciton fine structure of CdSe nanocrystals is determined by the origin of their valence-band single-particle wave functions. Nanocrystals where the valence-band maximum originates from the bulk A band have a "dark" ground-state exciton. Nanocrystals where the valence-band maximum is derived from the bulk B band have a "quasi-bright" ground-state exciton. Thus, the diameter-length configuration map can be divided into two regions, corresponding to dark and quasi-bright ground-state excitons. We find that the dark/quasi-bright ground-state exciton crossover is not only diameter-dependent but also length-dependent, and it is characterized by a curve in the two-parameter space of diameter and length. PMID:17900160

Zhao, Qingzhong; Graf, Peter A; Jones, Wesley B; Franceschetti, Alberto; Li, Jingbo; Wang, Lin-Wang; Kim, Kwiseon

2007-11-01

407

The steady-state transmission (reflection) of laser radiation by a thin semiconductor film was investigated taking into account the exciton - photon and elastic exciton - exciton interactions. Under certain conditions the resultant dependences of the transmission by a thin film on the amplitude of the incident radiation and on the detuning from a resonance exhibit a hysteresis, which can be explained by a shift of the exciton level when the exciton density is increased. (nonlinear optical phenomena)

Khadzhi, P I [Department of Physics and Mathematics, T.G. Shevchenko Pridnestrie State University (Moldova, Republic of); Tkachenko, D V [T.G. Shevchenko Dnester State University, Tiraspol (Moldova, Republic of); Gaivan, S L [Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev (Moldova, Republic of)

1999-06-30

408

NASA Astrophysics Data System (ADS)

The yellow- and red-emitting CdSeS nanocrystals (NCs) synthesized through one-step organometallic synthesis method are uniformly assembled in polymethyl methacrylate (PMMA). A higher-energy emission band originates from band-edge excitonic state appeared at low temperature. With the Se dopant concentration increasing, the luminescent spectra of CdSeS NCs have a red-shifted emission peak and a shorter luminescent lifetime, which is attributed to the existence of trapping state caused by surface defect and Se dopant. CdSeS NC shows a shorter luminescence lifetime and higher energy emission peak in PMMA matrix than that in toluene, indicating that the former is more favorable to transfer energy through exciton-phonon coupling. The upconversion luminescence (UCL) is observed using 800 nm femtosecond laser excitation. The pump power dependence demonstrated UCL spectra of yellow-emitting CdSeS NCs has a slope of 2.2, while that of red-emitting CdSeS NCs has a slope of 1.4. The results demonstrate that the two-photon absorption plays a dominating role when Se concentration of CdSeS NCs is lower, while phonon-assisted UCL by one-photon excitation gradually takes place with the amount of Se dopants increasing.

Wu, Wenzhi; Yu, Dongqi; Ye, Hong-an; Gao, Yachen; Chang, Qing

2012-06-01

409

The yellow- and red-emitting CdSeS nanocrystals (NCs) synthesized through one-step organometallic synthesis method are uniformly assembled in polymethyl methacrylate (PMMA). A higher-energy emission band originates from band-edge excitonic state appeared at low temperature. With the Se dopant concentration increasing, the luminescent spectra of CdSeS NCs have a red-shifted emission peak and a shorter luminescent lifetime, which is attributed to the existence of trapping state caused by surface defect and Se dopant. CdSeS NC shows a shorter luminescence lifetime and higher energy emission peak in PMMA matrix than that in toluene, indicating that the former is more favorable to transfer energy through exciton-phonon coupling. The upconversion luminescence (UCL) is observed using 800?nm femtosecond laser excitation. The pump power dependence demonstrated UCL spectra of yellow-emitting CdSeS NCs has a slope of 2.2, while that of red-emitting CdSeS NCs has a slope of 1.4. The results demonstrate that the two-photon absorption plays a dominating role when Se concentration of CdSeS NCs is lower, while phonon-assisted UCL by one-photon excitation gradually takes place with the amount of Se dopants increasing. PMID:22682098

2012-01-01

410

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

411

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C{sub 12}H{sub 25}NH{sub 3}){sub 2}PbI{sub 4(1?y)}Br{sub 4y} (y?=?0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510?nm to 350?nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices.

Ahmad, Shahab; Vijaya Prakash, G., E-mail: prakash@physics.iitd.ac.in [Nanophotonics Lab, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016 (India); Baumberg, Jeremy J. [Nanophotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE (United Kingdom)

2013-12-21

412

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. PMID:19625542

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

2009-01-01

413

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

NASA Astrophysics Data System (ADS)

Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.

Thompson, Nicholas J.; Congreve, Daniel N.; Goldberg, David; Menon, Vinod M.; Baldo, Marc A.

2013-12-01

414

Exciton polarization, fine structure splitting and quantum dot asymmetry under uniaxial stress

We derive a general relation between the fine structure splitting (FSS) and the exciton polarization angle of self-assembled quantum dots (QDs) under uniaxial stress. We show that the FSS lower bound under external stress can be predicted by the exciton polarization angle and FSS under zero stress. The critical stress can also be determined by monitoring the change in exciton polarization angle. We confirm the theory by performing atomistic pseudopotential calculations for the InAs/GaAs QDs. The work provides a deep insight into the dots asymmetry and their optical properties, and a useful guide in selecting QDs with smallest FSS which are crucial in entangled photon sources applications.

Gong, Ming; Guo, Guang-Can; He, Lixin

2011-01-01

415

Exciton polarization, fine structure splitting and quantum dot asymmetry under uniaxial stress

We derive a general relation between the fine structure splitting (FSS) and the exciton polarization angle of self-assembled quantum dots (QDs) under uniaxial stress. We show that the FSS lower bound under external stress can be predicted by the exciton polarization angle and FSS under zero stress. The critical stress can also be determined by monitoring the change in exciton polarization angle. We confirm the theory by performing atomistic pseudopotential calculations for the InAs/GaAs QDs. The work provides a deep insight into the dots asymmetry and their optical properties, and a useful guide in selecting QDs with smallest FSS which are crucial in entangled photon sources applications.

Ming Gong; Weiwei Zhang; Guang-Can Guo; Lixin He

2010-12-13

416

Exciton recombination in ZnO nanorods grown on GaN/sapphire template

NASA Astrophysics Data System (ADS)

The authors have employed variable temperature photoluminescence (PL) and time-resolved PL spectroscopy to probe the exciton recombination in high density and vertically aligned ZnO nanorods grown on p-type GaN/sapphire template. The low-temperature PL characterizes the dominant near-band-edge excitonic emissions from such nanorod arrays. At 4.3 K, a PL decay time of 432 ps reveals improved crystalline quality. The PL decay time shows irregular behavior due to different types of excitonic transitions dominating the PL spectra at different temperatures and a competitive effect of radiative recombination and nonradiative relaxation processes.

Mohanta, S. K.; Tripathy, S.; Zhang, X. H.; Kim, D. C.; Soh, C. B.; Yong, A. M.; Liu, W.; Cho, H. K.

2009-01-01

417

Gate controlled Aharonov-Bohm-type oscillations from single neutral excitons in quantum rings

NASA Astrophysics Data System (ADS)

We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr3 in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.

Ding, F.; Akopian, N.; Li, B.; Perinetti, U.; Govorov, A.; Peeters, F. M.; Bof Bufon, C. C.; Deneke, C.; Chen, Y. H.; Rastelli, A.; Schmidt, O. G.; Zwiller, V.

2010-08-01

418

Stimulated Terahertz Emission from Intra-Excitonic Transitions inCu2O

We report the first observation of stimulated emission of terahertz radiation from internal transitions of excitons. The far-infrared electromagnetic response of Cu{sub 2}O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of {approx} 10{sup 14} cm{sup 2}. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency.

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

2005-06-16

419

Quantum-dot gain without inversion: Effects of dark plasmon-exciton hybridization

NASA Astrophysics Data System (ADS)

We propose an initial-state-dependent quantum-dot gain without population inversion in the vicinity of a resonant metallic nanoparticle. The gain originates from the hybridization of a dark plasmon-exciton and is accompanied by efficient energy transfer from the nanoparticle to the quantum dot. This hybridization of the dark plasmon-exciton, attached to the hybridization of the bright plasmon-exciton, strengthens nonlinear light-quantum emitter interactions at the nanoscale, thus the spectral overlap between the dark and the bright plasmons enhances the gain effect. This hybrid system has potential applications in ultracompact tunable quantum devices.

Zhao, Dongxing; Gu, Ying; Wu, Jiarui; Zhang, Junxiang; Zhang, Tiancai; Gerardot, Brian D.; Gong, Qihuang

2014-06-01

420

NASA Astrophysics Data System (ADS)

Optical pumping of excited exciton states in a semiconductor quantum well embedded in a microcavity is a tool for realisation of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited (2p) and ground (1s) exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarisation of both pumping photons. Five-fold variation of the threshold power for terahertz lasing by switching from circular to co-linear pumping is predicted. We identify photon polarisation configurations for achieving maximum THz photon generation quantum efficiency.

Slavcheva, G.; Kavokin, A. V.

2014-11-01

421

Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature

NASA Astrophysics Data System (ADS)

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on Ta2NiSe5 across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that Ta2NiSe5 falls in the Bose-Einstein condensation regime of the excitonic insulator state.

Seki, K.; Wakisaka, Y.; Kaneko, T.; Toriyama, T.; Konishi, T.; Sudayama, T.; Saini, N. L.; Arita, M.; Namatame, H.; Taniguchi, M.; Katayama, N.; Nohara, M.; Takagi, H.; Mizokawa, T.; Ohta, Y.

2014-10-01

422

We calculate excitonic absorption spectra in several types of semiconductor nanostructures by using an original formalism involving fractional-dimensional spaces. This compact, analytical formulation predicts a surprising one-to-one mapping between the enhanced exciton Rydberg and the shape of the spectrum, whatever the physical origin of the confinement. By all-numerical calculation of the optical susceptibility we check that different systems with identical exciton Rydbergs exhibit nearly identical spectra, strikingly close to those given by the fractional-dimensional approach.

Lefebvre, P.; Christol, P.; Mathieu, H. [Groupe d`Etude des Semiconducteurs, CNRS, Universite Montpellier II, Case courrier 074, 34095 Montpellier cedex 5 (France)] [Groupe d`Etude des Semiconducteurs, CNRS, Universite Montpellier II, Case courrier 074, 34095 Montpellier cedex 5 (France); Glutsch, S. [Lawrence Berkeley Laboratory, 1 Cyclotron Road, Mail Stop 2-300, Berkeley, California 94720 (United States)] [Lawrence Berkeley Laboratory, 1 Cyclotron Road, Mail Stop 2-300, Berkeley, California 94720 (United States)

1995-08-15

423

Radius of B-8 halo from the asymptotic normalization coefficient

functional form of the potential, we search on geo- metrical parameters in such a way that the quantity 2 RADIUS OF 8B HALO FROM THE ASYMPTOTIC . . . PHYSICAL REVIEW C 63 054310 x25 ( r5RN 40 fm S Sg1/2c~ri!2Ce f f W~2kBri!ri D 2 ~10! becomes minimum... functional form of the potential, we search on geo- metrical parameters in such a way that the quantity 2 RADIUS OF 8B HALO FROM THE ASYMPTOTIC . . . PHYSICAL REVIEW C 63 054310 x25 ( r5RN 40 fm S Sg1/2c~ri!2Ce f f W~2kBri!ri D 2 ~10! becomes minimum...

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

2001-01-01

424

Charge radius and dipole response of {sup 11}Li

We investigate the consistency of the measured charge radius and dipole response of {sup 11}Li within a three-body model. We show how these observables are related to the mean-square distance between the {sup 9}Li core and the center of mass of the two valence neutrons. In this representation we find by considering the effect of smaller corrections that the discrepancy between the results of the two measurements is of the order of 1.5{sigma}. We also investigate the sensitivity to the three-body structure of {sup 11}Li and find that the charge radius measurement favors a model with a 50% s-wave component in the ground state of the two-neutron halo, whereas the dipole response is consistent with a smaller s-wave component of about 25% value.

Esbensen, H.; Mueller, P. [Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Hagino, K. [Department of Physics, Tohoku University, Sendai 980-8578 (Japan); Sagawa, H. [Center for Mathematical Sciences, University of Aizu, Aizu-Wakamatsu, Fukushima 965-8560 (Japan)

2007-08-15

425

Structural variability of the Sun. [radius and luminosity

NASA Technical Reports Server (NTRS)

Changes in solar radius and luminosity are discussed as a function of age. Surface and satellite observations showing departures from mean solar irradiance on a time scale of a few days are summarized. The most prominent depressions in these data appear to be correlated with sunspot number, indicating that the activity modulation of solar luminosity may be larger than expected. High activity is correlated with low luminosity, but during the Maunder minimum the mini ice age (presumably low luminosity) was accompanied by a near absence of solar activity. Pulsations driven by sources deep within the Sun are thought to be an unlikely cause of luminosity changes, but causes may be sought in the unstable convective zone. Experiments designed to measure the solar radius are mentioned, along with modelling efforts.

Sofia, S.

1980-01-01

426

The root-mean-square radius of the deuteron

Re-examines the determination of the root-mean square radius of the deuteron (rd) from the appropriate experimentally determined electric form factors, GEd, GEp, GEd\\/GEp and GEn. With a particular extrapolation of results from realistic NN interactions and a specific assumption regarding the shape of GEn for small q2 consistent with a recent quark-model calculation the authors obtain rd=1.952+or-0.004 fm. Their model-independent

L. J. Allen; J. P. McTavish; M. W. Kermode; A. McKerrell

1981-01-01

427

THE SIZE-VIRIAL RADIUS RELATION OF GALAXIES

I use the abundance matching ansatz, which has proven to be successful in reproducing galaxy clustering and other statistics, to derive estimates of the virial radius, R{sub 200}, for galaxies of different morphological types and a wide range of stellar masses. I show that over eight orders of magnitude in stellar mass galaxies of all morphological types follow an approximately linear relation between half-mass radius of their stellar distribution, r{sub 1/2}, and virial radius, r{sub 1/2} Almost-Equal-To 0.015 R{sub 200}, with scatter of Almost-Equal-To 0.2 dex. Such scaling is in remarkable agreement with the expectation of models that assume that galaxy sizes are controlled by halo angular momentum, r{sub 1/2}{proportional_to}{lambda}R{sub 200}, where {lambda} is the spin of galaxy parent halo. The scatter about the relation is comparable with the scatter expected from the distribution of {lambda}. Moreover, I show that when the stellar and gas surface density profiles of galaxies of different morphological types are rescaled by the radius r{sub n} = 0.015 R{sub 200}, the rescaled profiles follow approximately universal exponential (for late types) and de Vaucouleurs (for early types) form with scatter of only Almost-Equal-To 30%-50% at R Almost-Equal-To 1-3r{sub n} . Remarkably, both late- and early-type galaxies have similar mean stellar surface density profiles at R {approx}> 1r{sub n} . The main difference between their stellar distributions is thus at R < r{sub n} . The results of this study imply that galaxy sizes and radial distribution of baryons are shaped primarily by properties of their parent halos and that the sizes of both late-type disks and early-type spheroids are controlled by halo angular momentum.

Kravtsov, Andrey V., E-mail: andrey@oddjob.uchicago.edu [Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637 (United States)

2013-02-20

428

Osteosarcoma of the distal radius treated by intraoperative extracorporeal irradiation

Osteoblastic osteosarcoma in the distal radius of a 50-year-old woman was treated by en bloc resection, intraoperative extracorporeal irradiation, and reimplantation. Twenty-three months after surgery finger function was good. No tumor recurrence or distant metastasis was found at the final follow-up examination. Intraoperative extracorporeal irradiation and reimplantation is an easy and convenient reconstructive surgical procedure after en bloc excision of

Tetsuji Yamamoto; Toshihiro Akisue; Takashi Marui; Keiko Nagira; Masahiro Kurosaka

2002-01-01

429

Visual and CCD astrolabe observations of the solar radius

NASA Astrophysics Data System (ADS)

Here we discuss and compare two sets of solar semidiameter measurements made with Danjon astrolabes during the period 1997-1999. One is based on 10 522 CCD observations made at Rio de Janeiro, Brazil; the other one is based on 956 visual observations made at Santiago, Chile. Linear fits applied to both data sets give a standard deviation of +/-0farcs365 and +/-0farcs360 for each daily mean value of the solar radius for Rio de Janeiro and Santiago respectively. During the period of this analysis, the apparent solar radius had an annual variation of -0farcs060 +/-0farcs013 according to the CCD observations of Rio de Janeiro and of +0farcs213 +/-0farcs018 according to the visual observations of Santiago. We show here that this discrepancy could be due to systematic effects, probably of instrumental origin, that produce significant internal inconsistencies in the results of Rio de Janeiro. The drift of the apparent solar radius observed with the astrolabe of Santiago during 1997-1999 is consistent with former results obtained with this instrument since 1990. These results show that the apparent semidiameter of the Sun varies in phase with solar activity (Noël 1997, 2001).

Noël, F.

2001-08-01

430

Stochastic Liouville Equations for Coherent Multidimensional Spectroscopy of Excitons

Signatures of chemical exchange and spectral diffusion in 2D photon-echo lineshapes of molecular aggregates are studied using model calculations for a dimer whose Hamiltonian parameters are stochastically modulated. Cross peaks induced by chemical exchange and by exciton transport has different dynamics, and and distinguish two models which have the same absorption spectrum (a two-state-jump bath modulation model of a dimer and a four state jump bath model of a single chromophore). Slow Gaussian-Markovian spectral diffusion of a symmetric dimer induces new peaks which are damped as the dipole moment is equilibrated. These effects require an explicit treatment of the bath and may not be described by lower-level theories such as the Redfield equations, which eliminate the bath. PMID:18925774

Sanda, Frantisek; Mukamel, Shaul

2010-01-01

431

Quantum Dot Solar Cells with Multiple Exciton Generation

We have measured the quantum yield of the multiple exciton generation (MEG) process in quantum dots (QDs) of the lead-salt semiconductor family (PbSe, PbTe, and PbS) using fs pump-probe transient absorption measurements. Very high quantum yields (up to 300%) for charge carrier generation from MEG have been measured in all of the Pb-VI QDs. We have calculated the potential maximum performance of various MEG QD solar cells in the detailed balance limit. We examined a two-cell tandem PV device with singlet fission (SF), QD, and normal dye (N) absorbers in the nine possible series-connected combinations to compare the tandem combinations and identify the combinations with the highest theoretical efficiency. We also calculated the maximum efficiency of an idealized single-gap MEG QD solar cell with M multiplications and its performance under solar concentration.

Hanna, M. C.; Beard, M. C.; Johnson, J. C.; Murphy, J.; Ellingson, R. J.; Nozik, A. J.

2005-11-01

432

Strong exciton-photon coupling in open semiconductor microcavities

We present a method to implement 3-dimensional polariton confinement with in-situ spectral tuning of the cavity mode. Our tunable microcavity is a hybrid system consisting of a bottom semiconductor distributed Bragg reflector (DBR) with a cavity containing quantum wells (QWs) grown on top and a dielectric concave DBR separated by a micrometer sized gap. Nanopositioners allow independent positioning of the two mirrors and the cavity mode energy can be tuned by controlling the distance between them. When close to resonance we observe a characteristic anticrossing between the cavity modes and the QW exciton demonstrating strong coupling. For the smallest radii of curvature concave mirrors of 5.6 $\\mu$m and 7.5 $\\mu$m real-space polariton imaging reveals submicron polariton confinement due to the hemispherical cavity geometry.

Dufferwiel, S; Trichet, A; Walker, P M; Li, F; Giriunas, L; Makhonin, M N; Wilson, L R; Smith, J M; Clarke, E; Skolnick, M S; Krizhanovskii, D N

2014-01-01

433

Strong exciton-photon coupling in open semiconductor microcavities

NASA Astrophysics Data System (ADS)

We present a method to implement 3-dimensional polariton confinement with in-situ spectral tuning of the cavity mode. Our tunable microcavity is a hybrid system consisting of a bottom semiconductor distributed Bragg reflector (DBR) with a cavity containing quantum wells (QWs) grown on top and a dielectric concave DBR separated by a micrometer sized gap. Nanopositioners allow independent positioning of the two mirrors and the cavity mode energy can be tuned by controlling the distance between them. When close to resonance, we observe a characteristic anticrossing between the cavity modes and the QW exciton demonstrating strong coupling. For the smallest radii of curvature concave mirrors of 5.6 ?m and 7.5 ?m, real-space polariton imaging reveals submicron polariton confinement due to the hemispherical cavity geometry.

Dufferwiel, S.; Fras, F.; Trichet, A.; Walker, P. M.; Li, F.; Giriunas, L.; Makhonin, M. N.; Wilson, L. R.; Smith, J. M.; Clarke, E.; Skolnick, M. S.; Krizhanovskii, D. N.

2014-05-01

434

Perspective: Detecting and measuring exciton delocalization in photosynthetic light harvesting.

Photosynthetic units perform energy transfer remarkably well under a diverse range of demanding conditions. However, the mechanism of energy transfer, from excitation to conversion, is still not fully understood. Of particular interest is the possible role that coherence plays in this process. In this perspective, we overview photosynthetic light harvesting and discuss consequences of excitons for energy transfer and how delocalization can be assessed. We focus on challenges such as decoherence and nuclear-coordinate dependent delocalization. These approaches complement conventional spectroscopy and delocalization measurement techniques. New broadband transient absorption data may help uncover the difference between electronic and vibrational coherences present in two-dimensional electronic spectroscopy data. We describe how multipartite entanglement from quantum information theory allows us to formulate measures that elucidate the delocalization length of excitation and the details of that delocalization even from highly averaged information such as the density matrix. PMID:24655162

Scholes, Gregory D; Smyth, Cathal

2014-03-21

435

Complete quantum control of exciton qubits bound to isoelectronic centres

NASA Astrophysics Data System (ADS)

In recent years, impressive demonstrations related to quantum information processing have been realized. The scalability of quantum interactions between arbitrary qubits within an array remains however a significant hurdle to the practical realization of a quantum computer. Among the proposed ideas to achieve fully scalable quantum processing, the use of photons is appealing because they can mediate long-range quantum interactions and could serve as buses to build quantum networks. Quantum dots or nitrogen-vacancy centres in diamond can be coupled to light, but the former system lacks optical homogeneity while the latter suffers from a low dipole moment, rendering their large-scale interconnection challenging. Here, through the complete quantum control of exciton qubits, we demonstrate that nitrogen isoelectronic centres in GaAs combine both the uniformity and predictability of atomic defects and the dipole moment of semiconductor quantum dots. This establishes isoelectronic centres as a promising platform for quantum information processing.

Éthier-Majcher, G.; St-Jean, P.; Boso, G.; Tosi, A.; Klem, J. F.; Francoeur, S.

2014-05-01

436

Excitonic quasiparticles in a spin-orbit Mott insulator.

In condensed matter systems, out of a large number of interacting degrees of freedom emerge weakly coupled quasiparticles (QPs), in terms of which most physical properties are described. The lack of identification of such QPs is a major barrier for understanding myriad exotic properties of correlated electrons, such as unconventional superconductivity and non-Fermi liquid behaviours. Here we report the observation of a composite particle in a quasi-two-dimensional spin-1/2 antiferromagnet Sr2IrO4--an exciton dressed with magnons--that propagates with the canonical characteristics of a QP: a finite QP residue and a lifetime longer than the hopping time scale. The dynamics of this charge-neutral excitation mirrors the fundamental process of the analogous one-hole propagation in the background of spins-1/2, and reveals the same intrinsic dynamics that is obscured for a single, charged-hole doped into two-dimensional cuprates. PMID:25029968

Kim, Jungho; Daghofer, M; Said, A H; Gog, T; van den Brink, J; Khaliullin, G; Kim, B J

2014-01-01

437

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

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

2006-04-26

438

Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics

NASA Astrophysics Data System (ADS)

We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO2 solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO2 nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO2 solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO2.

Chasteen, Stephanie V.; Carter, Sue A.; Rumbles, Garry

2005-10-01

439

Methane planets and the mass-radius diagram

NASA Astrophysics Data System (ADS)

The multitude of newly discovered exoplanets are too far away to be studied in the same detail as the planets of our own solar system. Many planets have measured masses and radii, and their mean densities can be compared to those expected for different simple compositions (see, e.g. Seager et al. 2007). Clearly, different mixtures of materials can give similar density distributions and as a result, the mass and radius of a planet do not give a unique composition. It turns out that even if we limit the composition to one species, the mass-radius relation can show complex structure. To illustrate this, we consider planets composed of pure CH4. The complications arise because CH4 is expected to undergo dissociation at high pressure. Ab initio calculations (Gao et al. 2010) suggest that CH4 dissociates to C2H6, C4H10, and finally carbon + hydrogen at progressively higher pressures. We have modeled isothermal planets composed initially of pure CH4. We assume that if the planet is massive enough so that the central pressure exceeds the dissociation pressure of CH4, a diamond core is formed and the hydrogen released diffuses through the intermediate CH4 shell to form an H2 atmosphere. This leads to a sharp discontinuity in the mass-radius relation for such planets. A further complication arises from the fact that within a narrow range around the transition mass there can be multiple solutions ranging from a pure CH4 planet to those with diamond cores, CH4 shells, and hydrogen atmospheres of different masses. Methane planets thus provide an example of the instability first noted by Ramsey (1950) and Lighthill (1950). As a result, even for a given composition the mass-radius diagram is non-unique, making the characterization of extrasolar planets even more challenging. REFERENCES Gao, G., Oganov, A. R., Wang, H., Li, P., Ma, Y., Cui, T., and Zou, G., 2010. Dissociation of methane under high pressure. J. Chem. Phys., 133:144,508-1 - 144,508-5. Lighthill, M. J., 1950. On the instability of small planetary cores (II). Mon. Not. RAS, 110:339. Ramsey, W. H., 1950. On the instability of small planetary cores (I). Mon. Not. RAS, 110:325. Seager, S., Kuchner, M., Hier-Majumder, C. A., and Militzer, B., 2007. Mass-radius relationships for solid exoplanets. Astrophys. J., 669:1279-1297.

Podolak, Morris; Helled, Ravit; Levi, Amit

2014-05-01

440

Exciton size and mobility in (6,5) single-walled carbon nanotubes

NASA Astrophysics Data System (ADS)

Using femtosecond pump probe spectroscopy with sub-20 fs resolution, we probe fundamental properties of the E11 exciton in (6,5) single walled carbon nanotubes, prepared by density gradient ultracentrifugation. From the initial photobleaching signal, measured faster than any relaxation process, we obtain the one-dimensional size of the excitonic wavefunction along the nanotube. Exciton decay is found pump-intensity dependent only at elevated pump intensities. Numerical modelling of decay kinetics yields an exciton diffusion coefficient of about 0.1 cm^2/s. Anisotropy measurements in highly purified samples show that there is virtually no depolarisation of the E11 bleach over 40 ps. A photoinduced absorption (PA) band, blueshifted against the E11 bleach, shows only weak anisotropy.

Lüer, Larry; Hoseinkhani, Sajjad; Polli, Dario; Crochet, Jared; Hertel, Tobias; Lanzani, Guglielmo

2008-04-01

441

Plasmon-exciton hybridization in ZnO quantum well-Al nanodisc heterostructures

We demonstrate the formation of a hybridized plasmon-exciton state exhibiting strong exciton-plasmon coupling in ZnO/Zn0.85Mg0.15O single quantum-wells capped with arrays of Al nanodiscs. Tuning the quantum-well width and the diameter and pitch of the Al nanodisc arrays facilitates a transition from the weak-coupling regime into the strong coupling regime. Finite-difference time-domain simulations substantiate the localization of the plasmonic quadrupole moment within the ZnO quantum-well layer, resulting in a hybridized plasmon exciton state demonstrating a Rabi splitting of roughly 15 meV in heterostructures that exhibit a prominent plasmon quadrupole mode. The significant tunability offered by quantum-well heterostructures like those discussed here provides a flexible system for controlling exciton plasmon coupling in a device-compatible thin-film architecture.

Lawrie, Benjamin J [ORNL; Kim, Kyeong-won [University of Florida; Norton, David P. [University of Florida; HaglundJr., Richard F [Vanderbilt University

2012-01-01

442

Exciton localization mechanisms in wurtzite/zinc-blende GaAs nanowires

We investigate the emission properties of excitons in GaAs nanowires containing quantum disks formed by structural alternation between the zinc-blende and wurtzite phases, by means of temperature-dependent photoluminescence. At 10 K the emission...

Graham, Alexandra; Corfdir, Pierre; Heiss, Martin; Conesa-Boj, Sonia; Uccelli, Emanuele; Fontcuberta i Morral, Anna; Phillips, Richard

443

Reducing exciton-polaron annihilation in organic planar heterojunction solar cells

NASA Astrophysics Data System (ADS)

We investigate the relationship between charge concentration, exciton concentration, and photocurrent generation in fullerene-containing heterojunction diodes. Impedance measurements on C60 diodes reveal a charge buildup at the C60/bathocuproine (BCP) interface that can be swept out under reverse bias. In solar cell structures, a similar charge buildup is observed in dark conditions, and increases as a function of incident light intensity. Photoluminescence measurements reveal that the C60 exciton concentration is voltage dependent, explained via the process of exciton-polaron annihilation. This process has a negative impact on the generated photocurrent of the solar cells and thereby decreases the fill factor. A combination of electroabsorption, photoluminescence, and impedance measurements reveal a decrease in charge buildup and the associated exciton-polaron annihilation through the use of a BCP/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag cathode.

Verreet, Bregt; Bhoolokam, Ajay; Brigeman, Alyssa; Dhanker, Rijul; Cheyns, David; Heremans, Paul; Stesmans, Andre; Giebink, Noel C.; Rand, Barry P.

2014-09-01

444

Influence of intra-pigment vibrations on dynamics of photosynthetic exciton.

We have numerically investigated the effect of an underdamped intra-pigment vibrational mode on an exciton's quantum coherence and energy transfer efficiency. Our model describes a bacteriochlorophyll a pigment-protein dimer under the conditions at which photosynthetic energy transfer occurs. The dimer is modeled using a theoretical treatment of a vibronic exciton, and its dynamics are numerically analyzed using a non-Markovian and non-perturbative method. We examined the system's response to various values of the Huang-Rhys factor, site energy difference, reorganization energy, and reorganization energy difference. We found that the inclusion of the intra-pigment vibronic mode allows for long-lived oscillatory quantum coherences to occur. This excitonic coherence is robust against static site-energy disorder. The vibrational mode also promotes exciton transfer along the site-energy landscape thus improving the overall energy transfer efficiency. PMID:25399162

Sato, Yoshihiro; Doolittle, Brian

2014-11-14

445

Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals.

The efficient transfer of energy between organic and inorganic semiconductors is a widely sought after property, but has so far been limited to the transfer of spin-singlet excitons. Here we report efficient resonant-energy transfer of molecular spin-triplet excitons from organic semiconductors to inorganic semiconductors. We use ultrafast optical absorption spectroscopy to track the dynamics of triplets, generated in pentacene through singlet exciton fission, at the interface with lead selenide (PbSe) nanocrystals. We show that triplets transfer to PbSe rapidly (<1 ps) and efficiently, with 1.9 triplets transferred for every photon absorbed in pentacene, but only when the bandgap of the nanocrystals is close to resonance (±0.2 eV) with the triplet energy. Following triplet transfer, the excitation can undergo either charge separation, allowing photovoltaic operation, or radiative recombination in the nanocrystal, enabling luminescent harvesting of triplet exciton energy in light-emitting structures. PMID:25282509

Tabachnyk, Maxim; Ehrler, Bruno; Gélinas, Simon; Böhm, Marcus L; Walker, Brian J; Musselman, Kevin P; Greenham, Neil C; Friend, Richard H; Rao, Akshay

2014-11-01

446

Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals

NASA Astrophysics Data System (ADS)

The efficient transfer of energy between organic and inorganic semiconductors is a widely sought after property, but has so far been limited to the transfer of spin-singlet excitons. Here we report efficient resonant-energy transfer of molecular spin-triplet excitons from organic semiconductors to inorganic semiconductors. We use ultrafast optical absorption spectroscopy to track the dynamics of triplets, generated in pentacene through singlet exciton fission, at the interface with lead selenide (PbSe) nanocrystals. We show that triplets transfer to PbSe rapidly (<1 ps) and efficiently, with 1.9 triplets transferred for every photon absorbed in pentacene, but only when the bandgap of the nanocrystals is close to resonance (±0.2 eV) with the triplet energy. Following triplet transfer, the excitation can undergo either charge separation, allowing photovoltaic operation, or radiative recombination in the nanocrystal, enabling luminescent harvesting of triplet exciton energy in light-emitting structures.

Tabachnyk, Maxim; Ehrler, Bruno; Gélinas, Simon; Böhm, Marcus L.; Walker, Brian J.; Musselman, Kevin P.; Greenham, Neil C.; Friend, Richard H.; Rao, Akshay

2014-11-01

447

Zero-dimensional excitons in CdTe/ZnTe nanostructures

NASA Astrophysics Data System (ADS)

Thin CdTe layers embedded in ZnTe matrix grown by atomic layer epitaxy have been studied by time resolved spectroscopy and spatially resolved spectroscopy. The presence of Cd-rich dotlike islands in these CdTe nanostructures is shown by both atomic force microscopy and high resolution transmission electron microscopy. Zero-dimensional nature of excitons is shown both by the temperature dependence of the decay time and observation of sharp exciton lines in microphotoluminescence spectra. Zero-dimensional excitons probed by microphotoluminescence present a doublet structure linearly polarized along two orthogonal directions. This doublet structure is attributed to bright heavy-hole exciton states split by the local asymmetry of the localization potential. Reversible spectral shifts in the emission of some single quantum dots are observed on a time scale of hundreds of milliseconds. These small shifts can be attributed to the Stark effect caused by fluctuating electric fields and can significantly affect time-integrated transition linewidths.

Marsal, L.; Besombes, L.; Tinjod, F.; Kheng, K.; Wasiela, A.; Gilles, B.; Rouvière, J.-L.; Mariette, H.

2002-04-01

448

Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells

The recent advent of two-dimensional monolayer materials with tunable optical properties and high carrier mobility offers renewed opportunities for efficient, ultrathin excitonic solar cells alternative to those based on ...

Bernardi, Marco

449

Using two-dimensional spectroscopy, we resolve multi-polariton coherences in quantum wells embedded inside a semiconductor microcavity and elucidate how multi-exciton correlations mediate polariton nonlinear dynamics. We ...

Wen, P.

450

Optical spectra of biological polymers contain important information about their structure and function in living organisms. This information can be accessed by extracting an optical interaction of monomers, i.e., their exciton coupling, from experimental data. This coupling is sensitive to molecular structure, geometry, and conformation and can be used to characterize them. However, the accurate determination of exciton coupling in important biological molecules is difficult because inhomogeneous broadening smears out the monomer interaction. We suggest a way to overcome this problem by applying exact sum rules. These sum rules are derived by establishing a straightforward relationship between integral characteristics of absorption and circular dicroism spectra, and exciton coupling. Exciton coupling between AT pairs in native DNA conformation is estimated by applying these sum rules to DNA hairpin optical spectra as V0 ? 0.035 eV in agreement with the earlier numerical calculations. PMID:19164590

Burin, Alexander L.; Armbruster, Michael E.; Hariharan, Mahesh; Lewis, Frederick D.

2009-01-01

451

BCS wave-function approach to the BEC-BCS crossover of exciton-polariton condensates.

The crossover between low and high density regimes of exciton-polariton condensates is examined using a BCS wave-function approach. Our approach is an extension of the BEC-BCS crossover theory for excitons, but includes a cavity photon field. The approach can describe both the low density limit, where the system can be described as a Bose-Einstein condensate (BEC) of exciton-polaritons, and the high density limit, where the system enters a photon-dominated regime. In contrast to the exciton BEC-BCS crossover where the system approaches an electron-hole plasma, the polariton high density limit has strongly correlated electron-hole pairs. At intermediate densities, there is a regime with BCS-like properties, with a peak at nonzero momentum of the singlet pair function. We calculate the expected photoluminescence and give several experimental signatures of the crossover. PMID:21231119

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

2010-10-29

452

2D Excitons as Primary Energy Carriers in Organic Crystals: The Case of Oligoacenes

A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intralayer 2D ...

Emelianova, E.V.

453

Quantum effects in biology: master equation studies of exciton motion in photosynthetic systems

The present review is devoted to our recent studies on the excitonic motion in photosynthetic systems. In photosynthesis, the light photon is absorbed to create an exciton in the antenna complex of the photosynthetic pigments. This exciton then migrates along the chain-biomolecules, like FMO complex, to the reaction centre where it initiates the chemical reactions leading to biomass generation. Recently, it has been experimentally observed that the exciton motion is highly quantum mechanical in nature i.e., it involve long time ($\\sim 600$ femto sec) quantum coherence effects. Traditional semiclassical theories like Forrester's and second Born master equations cannot be applied. We point out why the 2nd Born non-Markovian master equation and its Markovian limit (also called the Redfield master equation) cannot be used to explain the observed long coherences. Briefly, the reason is that these approaches are perturbative in nature and in real light harvesting systems various couplings (system-system and system-...

Singh, Navinder

2012-01-01

454

Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence ...

Tongay, Sefaattin

455

Relaxation explosion of a quantum degenerate exciton gas in Cu2O

NASA Astrophysics Data System (ADS)

We present our recent experimental studies on anomalous luminescence and its connection to Bose-Einstein condensation (BEC) transition of dark excitons in a bulk semiconductor. Our sensitive and quantitative detection of this nonluminous quasi-particle using hydrogen-like internal transitions allows obtaining continuous spectra of dark excitons using a quantum cascade laser. According to quantitative measurements on the two-body inelastic collision cross section of excitons, the system needs to be cooled to sub-Kelvin temperatures. We discuss in detail our recent observation of an explosive phenomenon when the BEC criterion is satisfied (Yoshioka et al 2011 Nature Commun. 2 328) for trapped excitons using a helium-3 refrigerator, and outline a plausible scenario when the BEC transition occurs in an inelastic environment. We also discuss how to increase the condensate fraction in order to study the unique ground state of many-body electric excitations in solids.

Yoshioka, Kosuke; Kuwata-Gonokami, Makoto

2012-05-01

456

Jahn-Teller effect on exciton states in hexagonal boron nitride single crystal

NASA Astrophysics Data System (ADS)

Optical properties near the band edge of hexagonal boron nitride were studied at 8 K. The photoluminescence spectrum shows two series of bands, namely, sharp (S) and diffuse (D), which are also distinguished by their fast (0.6 ns) for S and slow (5 ns) for D radiative decay time. Each series is composed of four bands with large Stokes shifts that are attributed to self-trapped excitons by the strong exciton-phonon interaction. The respective four luminescence bands of the two series originate from the four free-exciton levels in which the doubly degenerated dark and bright exciton levels theoretically predicted are resolved with the Jahn-Teller distortion in the excited states.

Watanabe, Kenji; Taniguchi, Takashi

2009-05-01

457

LOCALIZED OPTICAL EXCITATIONS AND TWO-EXCITON SPECTROSCOPY OF PHENYLACETYLENE DENDRIMERS

, New York 14627 ABSTRACT The one- and two-exciton manifolds of conjugated dendrimers possessing fractal antenna systems that mimic biological energy transfer processes [1, 4-8]. The first five members

Tretiak, Sergei

458

Microcavity effects on the generation,fluorescence, and diffusion of excitons in organic solar cells

We compute the short-circuit diffusion current of excitons in an organic solar cell, with special emphasis on fluorescence losses. The exciton diffusion length is not uniform but varies with its position within the device, even with moderate fluorescence quantum efficiency. With large quantum efficiencies, the rate of fluorescence can be strongly reduced with proper choices of the geometrical and dielectric parameters. In this way, the diffusion length can be increased and the device performance significantly improved.

Kozyreff, G; Vuong, L T; Silleras, O Nieto; Martorell, J

2013-01-01

459

Exciton-stimulated molecular transformation in organic light-emitting diodes.

An exciton-stimulated molecular transformation in an organic light-emitting diode (OLED) on a time scale of a few seconds under electrical bias is shown to reach nearly 100% under standard operating conditions, leading to color switching. It is reversible in both a thin film and an OLED when sufficient thermal energy is supplied. Such an exciton-stimulated molecular transformation suggests a new process which may be exploited for applications such as electrochromic and memory devices. PMID:25146980

Chang, Yi-Lu; Rao, Ying-Li; Gong, Shaolong; Ingram, Grayson L; Wang, Suning; Lu, Zheng-Hong

2014-10-01

460

Determination of Optical Gain of Self-Trapped Exciton Luminescence in CdI2

Luminescence from self-trapped excitons in high-purity CdI2 has been studied at liquid nitrogen temperature under intense excitation by a pulsed nitrogen laser. Luminescence intensity was carefully measured by changing excitation density and length of the excited region on a crystal surface. Obtained results are well explained by introducing a stimulated emission process into radiative decay channels of the self-trapped excitons.

Akimasa Ohnishi; Mamoru Kitaura; Hideyuki Nakagawa

1994-01-01

461

Synthesis of titanium dioxide and zinc oxide nanowires for excitonic solar cells

Dye-sensitized, quantum-dot sensitized, quantum-dot, and hybrid organic\\/inorganic solar cells are promising excitonic photovoltaic devices for the generation of low cost, carbon free energy. Wide-band gap semiconductor nanowire photoanodes have the potential to increase the efficiencies of these excitonic solar cells. Controlling and tailoring the dimensions of the nanowires (i.e. nanowire height, diameter, and planar number density) for each solar cell

Janice Elaine Boercker

2009-01-01

462

Utility of the Cortical Thickness of the Distal Radius as a Predictor of Distal-Radius Bone Density

Background Bone density is an important factor in the management of fractures of the distal radius. Objectives The aim of this study was to establish whether standard anteroposterior (AP) radiographs would provide the attending physician with a prediction of bone density. Patients and Methods Six pairs of human cadaveric radii were harvested. The mean donor age was 74 years. Standardized AP radiographs were taken of the radii. The outside diameter and the inside diameter of the cortical shell at the metaphyseal / diaphyseal junction were measured and their ratio was calculated. Dual-energy x-ray absorptiometry (DXA) was used to obtain the bone mineral density (BMD) of the distal parts of the radii. The correlation of the BMD values with these ratios was studied. Results The mean BMD was 0.559 (SD = 0.236) g / cm2. The mean outside diameter/inside diameter ratio was 1.24 (SD = 0.013); the ratio significantly correlated with the total BMD (P = 0.001; R2 = 0.710). In the BMD subregions, the correlation was also significant. Conclusions The outside diameter/inside diameter ratio at the metaphyseal/diaphyseal junction of the distal radius on AP radiographs is suitable for use as a predictor of distal-radius bone density. Further studies should be performed, and clinical utility evaluated. PMID:24396783

Rausch, Sascha; Klos, Kajetan; Gras, Florian; Skulev, Hristo Kostov; Popp, Albrecht; Hofmann, Gunther Olaf; Mückley, Thomas

2013-01-01

463

Effective attractive polariton-polariton interaction mediated by an exciton reservoir

NASA Astrophysics Data System (ADS)

We present a mechanism to endow the polariton gas with attractive interactions. The scheme relies on an exciton reservoir, which can be formed even if the excitons lie outside of the light cone. Such an attractive interaction should open different routes in the physics of quantum fluids, which we illustrate with a simple but nontrivial application whereby the polariton condensate enters the self-oscillation regime powered by the attraction from the reservoir. This results in pulsed emission in the GHz regime.

Vishnevsky, D. V.; Laussy, F.

2014-07-01

464

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that

Randy J. Ellingson; Matthew C. Beard; Justin C. Johnson; Pingrong Yu; Olga I. Micic; Arthur J. Nozik; Andrew Shabaev; Alexander L. Efros

2005-01-01

465

Dynamics of exciton creation and decay processes in composition - disordered InGaN thin films

NASA Astrophysics Data System (ADS)

In the GaN-based ternary alloys, InGaN crystals have been recognized as key materials for e-h plasmas-exciton dynamics, because of large exciton binding energies (24.8 meV in GaN). We report investigations of creating and recombination dynamics of excitons in commercially important InxGa1-xN composition range from x = 0.1 to 0.18 in which nanoscale indium composition fluctuation occurs and formation of indium rich clusters acting as quantum dots (QD) can be expected. Three MOCVD grown samples having x = 0.1; 0.14 and 0.18 have been investigated using 3D picosecond transient PL spectroscopy. It has been found that the band to band photo excitation at 15 K in whole composition range results in creating of complex luminescence band represented by three close overlapping Gaussian shape single exciton bands. All three exciton bands show fast decay time constants in a picosecond range. For all the samples the PL intensity dependence on excitation pulse power for each exciton band is different. An increase of the excitation pulse power density results in the linear growth of the band at the higher photon energy side. The next energy bands with lower photon energy show correspondingly quadratic and cubic dependences on laser pulse energy and are caused by the formation of biexcitons and triexcitons. Fast decay kinetics and excitation of multi-excitonic complexes are evident for important role of quantum dots by quantum capture of excitons in the considered composition range.

Dimitrocenko, L.; Kulis, P.; Sarakovskis, A.; Tale, I.; Voitkans, A.

2011-06-01

466

Two-site self-trapping of bulk and surface wide band excitons

The self-trapping of bulk and surface exc