463 research outputs found
Exciton-exciton scattering: Composite boson versus elementary boson
This paper introduces a new quantum object, the ``coboson'', for composite
particles, like the excitons, which are made of two fermions. Although commonly
dealed with as elementary bosons, these composite bosons -- ``cobosons'' in
short -- differ from them due to their composite nature which makes the
handling of their many-body effects quite different from the existing
treatments valid for elementary bosons. Due to this composite nature, it is not
possible to correctly describe the interaction between cobosons as a potential
. Consequently, the standard Fermi golden rule, written in terms of ,
cannot be used to obtain the transition rates between exciton states. Through
an unconventional expression for this Fermi golden rule, which is here given in
terms of the Hamiltonian only, we here give a detailed calculation of the time
evolution of two excitons. We compare the results of this exact approach with
the ones obtained by using an effective bosonic exciton Hamiltonian. We show
that the relation between the inverse lifetime and the sum of transition rates
for elementary bosons differs from the one of composite bosons by a factor of
1/2, whatever the mapping from composite bosons to elementary bosons is. The
present paper thus constitutes a strong mathematical proof that, in spite of a
widely spread belief, we cannot forget the composite nature of these cobosons,
even in the extremely low density limit of just two excitons. This paper also
shows the (unexpected) cancellation, in the Born approximation, of the
two-exciton transition rate for a finite value of the momentum transfer
Dimensionality dependence of optical nonlinearity and relaxation dynamics in cuprates
Femtosecond pump-probe measurements find pronounced dimensionality dependence
of the optical nonlinearity in cuprates. Although the coherent two-photon
absorption (TPA) and linear absorption bands nearly overlap in both quasi-one
and two-dimensional (1D and 2D) cuprates, the TPA coefficient is one order of
magnitude smaller in 2D than in 1D. Furthermore, picosecond recovery of optical
transparency is observed in 1D cuprates, while the recovery in 2D involves
relaxation channels with a time scales of tens of picoseconds. The experimental
results are interpreted within the two-band extended Hubbard model.Comment: 10 pages, 4 figure
A Cooper pair light emitting diode
We demonstrate Cooper-pair's drastic enhancement effect on band-to-band
radiative recombination in a semiconductor. Electron Cooper pairs injected from
a superconducting electrode into an active layer by the proximity effect
recombine with holes injected from a p-type electrode and dramatically
accelerate the photon generation rates of a light emitting diode in the
optical-fiber communication band. Cooper pairs are the condensation of
electrons at a spin-singlet quantum state and this condensation leads to the
observed enhancement of the electric-dipole transitions. Our results indicate
the possibility to open up new interdisciplinary fields between
superconductivity and optoelectronics.Comment: 5 pages (4 figures
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
Resonant inelastic x-ray scattering study of charge excitations in La2CuO4
We report a resonant inelastic x-ray scattering study of the dispersion
relations of charge transfer excitations in insulating LaCuO. These
data reveal two peaks, both of which show two-dimensional characteristics. The
lowest energy excitation has a gap energy of eV at the zone center,
and a dispersion of eV. The spectral weight of this mode becomes
dramatically smaller around (, ). The second peak shows a smaller
dispersion ( eV) with a zone-center energy of eV. We argue
that these are both highly dispersive exciton modes damped by the presence of
the electron-hole continuum.Comment: 5 pages, 3 figure
Anisotropic Hubbard model on a triangular lattice -- spin dynamics in Ho Mn O_3
The recent neutron-scattering data for spin-wave dispersion in are well described by an anisotropic Hubbard model on a triangular lattice
with a planar (XY) spin anisotropy. Best fit indicates that magnetic
excitations in correspond to the strong-coupling limit , with planar exchange energy meV and planar
anisotropy meV.Comment: 4 pages, 3 figure
CORRELATION BETWEEN THE CHANGES IN STRUCTURE AND ELECTRICAL RESISTIVITY OF AMORPHOUS METALS
Using electron microscopy and electrical resistivity measurements, structural changes in amorphous alloys, which occurred during heating, were studied over a wide temperature range. Ribbons of Fe-B-Si alloy were prepared by quenching liquid alloy on to cold twin rollers rotating at high speed ; several different rotation speeds were used. Amorphous ribbons of Ni_ (P_x B_)_ alloys, where X=0.25, 0.5, 0.75, were made also using one roll speed. Among these ribbons, differences were found in electrical resistivity and its annealing behavior, which depended on the quenching conditions and on the ratio of the contents of two component metalloids. In the system Ni_ (P_x B_)_, it was found that the electrical resistivity of the Ni_ P_ B_ alloy specimen had the highest value. It was also found that the electrical resistivity was increased by the cyclic deformation of the liquid-quenched Co-B-Si amorphous alloy. Differences in thermal and mechanical stability of these alloys suggest that so-called amorphous alloys do not have one definite atomic arrangement but have a variety of arrangements, depending on the conditions of preparation
Superradiance of low density Frenkel excitons in a crystal slab of three-level atoms: Quantum interference effect
We systematically study the fluorescence of low density Frenkel excitons in a
crystal slab containing V-type three-level atoms. Based on symmetric
quasi-spin realization of SU(3) in large limit, the two-mode exciton
operators are invoked to depict various collective excitations of the
collection of these V-type atoms starting from their ground state. By making
use of the rotating wave approximation, the light intensity of radiation for
the single lattice layer is investigated in detail. As a quantum coherence
effect, the quantum beat phenomenon is discussed in detail for different
initial excitonic states. We also test the above results analytically without
the consideration of the rotating wave approximation and the self-interaction
of radiance field is also included.Comment: 18pages, 17 figures. Resubmit to Phys. Rev.
Electrical reduction of perovskite electrodes for accelerating exsolution of nanoparticles
This work was supported through the Leading Graduate School Program: Academy for Co-creative Education of Environment and Energy Science (ACEEES) funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan).Growth of finely dispersed nanocatalysts by exsolution of metal nanoparticles from perovskite oxides under reducing conditions at elevated temperature is a promising approach of producing highly active catalytic materials. An alternative method of exsolution using an applied potential has been recently shown to potentially accelerate the exsolution process of nanoparticles that can be achieved in minutes rather than the hours required in chemical reduction. In the present study, we investigate exsolution of nanoparticles from perovskite oxides of La0.43Ca0.37Ni0.06Ti0.94O3-γ (LCTNi) and La0.43Ca0.37Ni0.03Fe0.03Ti0.94O3-γ (LCTNi-Fe) under applied potentials in carbon dioxide atmosphere. The impedance spectra of single cells measured before and after electrochemical poling at varying voltages showed that the onset of exsolution process occurred at 2 V of potential reduction. An average particle size of the exsolved nanoparticles observed after testing using a scanning electron microscopy was about 30–100 nm. The cells with the reduced electrodes exhibited desirable electrochemical performances not only in pure carbon dioxide (current density of 0.37 A cm−2 for LCTNi and 0.48 A cm−2 for LCTNi-Fe at 1.5 V) but also in dry hydrogen (0.36 W cm−2 for LCTNi and 0.43 W cm−2 for LCTNi-Fe).PostprintPeer reviewe
Approach to the semiconductor cavity QED in high-Q regimes with q-deformed boson
The high density Frenkel exciton which interacts with a single mode
microcavity field is dealed with in the framework of the q-deformed boson. It
is shown that the q-defomation of bosonic commutation relations is satisfied
naturally by the exciton operators when the low density limit is deviated. An
analytical expression of the physical spectrum for the exciton is given by
using of the dressed states of the cavity field and the exciton. We also give
the numerical study and compare the theoretical results with the experimental
resultsComment: 6 pages, 2 figure
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