296 research outputs found
On Critical Velocities in Exciton Superfluidity
The presence of exciton phonon interactions is shown to play a key role in
the exciton superfluidity. We apply the Landau criterion for an exciton-phonon
condensate moving uniformly at zero temperature. It turns out that there are
essentially two critical velocities in the theory. Within the range of these
velocities the condensate can exist only as a bright soliton. The excitation
spectrum and differential equations for the wave function of this condensate
are derived.Comment: 7 pages, Latex; to be published in Phys.Rev.Lett (1997
Semiconductor-cavity QED in high-Q regimes: Detuning effect
The non-resonant interaction between the high-density excitons in a quantum
well and a single mode cavity field is investigated. An analytical expression
for the physical spectrum of the excitons is obtained. The spectral properties
of the excitons, which are initially prepared in the number states or the
superposed states of the two different number states by the resonant
femtosecond pulse pumping experiment, are studied. Numerical study of the
physical spectrum is carried out and a discussion of the detuning effect is
presented.Comment: 7 pages, 8 figure
Algebraic varieties with automorphism groups of maximal rank
We confirm, to some extent, the belief that a projective variety X has the
largest number (relative to the dimension of X) of independent commuting
automorphisms of positive entropy only when X is birational to a complex torus
or a quotient of a torus. We also include an addendum to an early paper though
it is not used in the present paper.Comment: Mathematische Annalen (to appear
Nonlinear optical spectroscopy of single, few, and many molecules; nonequilibrium Green's function QED approach
Nonlinear optical signals from an assembly of N noninteracting particles
consist of an incoherent and a coherent component, whose magnitudes scale \sim
N and \sim N(N-1), respectively. A unified microscopic description of both
types of signals is developed using a quantum electrodynamical (QED) treatment
of the optical fields. Closed nonequilibrium Green's function expressions are
derived that incorporate both stimulated and spontaneous processes. General
(n+1)-wave mixing experiments are discussed as an example of spontaneously
generated signals. When performed on a single particle, such signals cannot be
expressed in terms of the nth order polarization, as predicted by the
semiclassical theory. Stimulated processes are shown to be purely incoherent in
nature. Within the QED framework, heterodyne-detected wave mixing signals are
simply viewed as incoherent stimulated emission, whereas homodyne signals are
generated by coherent spontaneous emission.Comment: article: 33 pages (preprint format!) ''paper.tex'' figures: 17
figures (.eps) in folder ``figures'
Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators
We investigate the optical properties of one-dimensional (1D) dimerized Mott
insulators using the 1D dimerized extended Hubbard model. Numerical
calculations and a perturbative analysis from the decoupled-dimer limit clarify
that there are three relevant classes of charge-transfer (CT) states generated
by photoexcitation: interdimer CT unbound states, interdimer CT exciton states,
and intradimer CT exciton states. This classification is applied to
understanding the optical properties of an organic molecular material,
1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its
photoinduced transition from the dimerized spin-singlet phase to the regular
paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is
the interdimer CT exciton state and the second lowest state is the intradimer
CT exciton state.Comment: 6 pages, 6 figures, to be published in J. Phys. Soc. Jp
Decay rate and renormalized frequency shift of a quantum wire Wannier exciton in a planar microcavity
The superradiant decay rate and frequency shift of a Wannier exciton in a
one-dimensional quantum wire are studied. It is shown that the dark mode
exciton can be examined experimentally when the quantum wire is embedded in a
planar microcavity. It is also found that the decay rate is greatly enhanced as
the cavity length is equal to the multiple wavelength of the emitted
photon. Similar to its decay rate counterpart, the frequency shift also shows
discontinuities at resonant modes.Comment: 12 pages, 2 figures. To appear in P. R. B. September 200
Bosons in a Lattice: Exciton-Phonon Condensate in Cu2O
We explore a nonlinear field model to describe the interplay between the
ability of excitons to be Bose-condensed and their interaction with other modes
of a crystal. We apply our consideration to the long-living para-excitons in
Cu2O. Taking into account the exciton-phonon interaction and introducing a
coherent phonon part of the moving condensate, we derive the dynamic equations
for the exciton-phonon condensate. These equations can support localized
solutions, and we discuss the conditions for the moving inhomogeneous
condensate to appear in the crystal. We calculate the condensate wave function
and energy, and a collective excitation spectrum in the semiclassical
approximation; the inside-excitations were found to follow the asymptotic
behavior of the macroscopic wave function exactly. The stability conditions of
the moving condensate are analyzed by use of Landau arguments, and Landau
critical parameters appear in the theory. Finally, we apply our model to
describe the recently observed interference and strong nonlinear interaction
between two coherent exciton-phonon packets in Cu2O.Comment: 34 pages, LaTeX, four figures (.ps) are incorporated by epsf.
Submitted to Phys. Rev.
Many-body theory of pump-probe spectra for highly excited semiconductors
We present a unified theory for pump-probe spectra in highly excited
semiconductors, which is applicable throughout the whole density regime
including the high-density electron-hole BCS state and the low-density
excitonic Bose-Einstein condensate (BEC). The analysis is based on the BCS-like
pairing theory combined with the Bethe-Salpeter (BS) equation, which first
enables us to incorporate the state-filling effect, the band-gap
renormalization and the strong/weak electron-hole pair correlations in a
unified manner. We show that the electron-hole BCS state is distinctly
stabilized by the intense pump-light, and this result strongly suggests that
the macroscopic quantum state can be observed under the strong photoexcitation.
The calculated spectra considerably deviate from results given by the BCS-like
mean field theory and the simple BS equation without electron-hole pair
correlation especially in the intermediate density states between the
electron-hole BCS state and the excitonic BEC state. In particular, we find the
sharp stimulated emission and absorption lines which originate from the optical
transition accompanied by the collective phase fluctuation mode in the
electron-hole BCS state. From the pump-probe spectral viewpoint, we show that
this fluctuation mode changes to the exciton mode with decreasing carrier
densityComment: RevTeX 11 pages, 10 figures. To appear in Phys.Rev.B1
A new method for detection of exciton Bose condensation using stimulated two-photon emission
Stimulated two-photon emission by Bose-condensed excitons accompanied by a
coherent two-exciton recombination, i.e., by simultaneous recombination of two
excitons with opposite momenta leaving unchanged the occupation numbers of
excitonic states with nonzero momenta, is investigated. Raman light scattering
accompanied by a similar two-exciton recombination (or generation of two
excitons) is also analyzed. The processes under consideration can occur only if
a system contains Bose condensate, therefore, their detection can be used as a
new method to reveal Bose condensation of excitons. The recoil momentum, which
corresponds to a change in the momentum of the electromagnetic field in the
processes, is transferred to phonons or impurities. If the recoil momentum is
transmitted to optical phonons with frequency , the stimulated
two-photon emission with the coherent two-exciton recombination leads to the
appearance of a line at , where
and is the light frequency corresponding to the recombination of an
exciton with zero momentum. Formulas for the cross sections at finite
temperatures are obtained for the processes under consideration. Our estimates
indicate that a spectral line, corresponding to the stimulated two-photon
emission accompanied by the coherent optical phonon-assisted two-exciton
recombination can be experimentally detected in CuO.Comment: 28 pages, 3 Postscript figure
Evaluation of local eggplant cultivars in terms of the suitability as materials for 'Yakuzen' dishes
"Yakuzen" is a form of medicinal cooking based on the theories of Oriental medicine. To prepare Yakuzen dishes, in-season materials with appropriate properties and tastes, "Sei-Mi," are selected according to the health status and constitution of each person. In this study, the suitability of eggplant (Solanum melongena L.) cultivars for Yakuzen was evaluated by sensory tests and by analysis of the functional constituents considered to be closely related to the taste and functionality of Yakuzen dishes. Twenty-two eggplant cultivars including 21 Japanese and 1 Italian cultivar, and a Thai species (Solanum xanthocarpum Schrad & Wendl.) were evaluated. Principal component analysis (PCA) was used for comprehensive evaluation among the cultivars. From the PCA, many of the cultivars with round or oval fruit were characterized as juicy and sweet and considered easy to eat; most of the long-fruit cultivars were characterized as having higher specific amino acid contents. The small and round fruit cultivar Dewako and the Thai species (Makhuea pro) were considered to contain many functional ingredients, such as ash, polyphenols, and specific amino acids, and to have higher suitability for Yakuzen dishes
- …