22,740 research outputs found
Scattering coefficients and gray-body factor for 1D BEC acoustic black holes: exact results
A complete set of exact analytic solutions to the mode equation is found in
the region exterior to the acoustic horizon for a class of 1D Bose-Einstein
condensate (BEC) acoustic black holes. From these, analytic expressions for the
scattering coefficients and gray-body factor are obtained. The results are used
to verify previous predictions regarding the behaviors of the scattering
coefficients and gray-body factor in the low frequency limit.Comment: 13 pages, 1 figure, Final version, to appear in Phys. Rev.
Low frequency gray-body factors and infrared divergences: rigorous results
Formal solutions to the mode equations for both spherically symmetric black
holes and Bose-Einstein condensate acoustic black holes are obtained by writing
the spatial part of the mode equation as a linear Volterra integral equation of
the second kind. The solutions work for a massless minimally coupled scalar
field in the s-wave or zero angular momentum sector for a spherically symmetric
black hole and in the longitudinal sector of a 1D Bose-Einstein condensate
acoustic black hole. These solutions are used to obtain in a rigorous way
analytic expressions for the scattering coefficients and gray-body factors in
the zero frequency limit. They are also used to study the infrared behaviors of
the symmetric two-point function and two functions derived from it: the
point-split stress-energy tensor for the massless minimally coupled scalar
field in Schwarzschild-de Sitter spacetime and the density-density correlation
function for a Bose-Einstein condensate acoustic black hole.Comment: 41 pages, 5 figure
Collective excitations and low temperature transport properties of bismuth
We examine the influence of collective excitations on the transport
properties (resistivity, magneto- optical conductivity) for semimetals,
focusing on the case of bismuth. We show, using an RPA approximation, that the
properties of the system are drastically affected by the presence of an
acoustic plasmon mode, consequence of the presence of two types of carriers
(electrons and holes) in this system. We found a crossover temperature T*
separating two different regimes of transport. At high temperatures T > T* we
show that Baber scattering explains quantitatively the DC resistivity
experiments, while at low temperatures T < T* interactions of the carriers with
this collective mode lead to a T^5 behavior of the resistivity. We examine
other consequences of the presence of this mode, and in particular predict a
two plasmon edge feature in the magneto-optical conductivity. We compare our
results with the experimental findings on bismuth. We discuss the limitations
and extensions of our results beyond the RPA approximation, and examine the
case of other semimetals such as graphite or 1T-TiSe_2
Acoustic white holes in flowing atomic Bose-Einstein condensates
We study acoustic white holes in a steadily flowing atomic Bose-Einstein
condensate. A white hole configuration is obtained when the flow velocity goes
from a super-sonic value in the upstream region to a sub-sonic one in the
downstream region. The scattering of phonon wavepackets on a white hole horizon
is numerically studied in terms of the Gross-Pitaevskii equation of mean-field
theory: dynamical stability of the acoustic white hole is found, as well as a
signature of a nonlinear back-action of the incident phonon wavepacket onto the
horizon. The correlation pattern of density fluctuations is numerically studied
by means of the truncated-Wigner method which includes quantum fluctuations.
Signatures of the white hole radiation of correlated phonon pairs by the
horizon are characterized; analogies and differences with Hawking radiation
from acoustic black holes are discussed. In particular, a short wavelength
feature is identified in the density correlation function, whose amplitude
steadily grows in time since the formation of the horizon. The numerical
observations are quantitatively interpreted by means of an analytical
Bogoliubov theory of quantum fluctuations for a white hole configuration within
the step-like horizon approximation
Superradiant scattering in fluids of light
We theoretically investigate the scattering process of Bogoliubov excitations
on a rotating photon-fluid. Using the language of Noether currents we
demonstrate the occurrence of a resonant amplification phenomenon, which
reduces to the standard superradiance in the hydrodynamic limit. We make use of
a time-domain formulation where superradiance emerges as a transient effect
encoded in the amplitudes and phases of propagating localised wavepackets. Our
findings generalize previous studies in quantum fluids to the case of a
non-negligible quantum pressure and can be readily applied also to other
physical systems, in particular atomic Bose-Einstein condensates. Finally we
discuss ongoing experiments to observe superradiance in photon fluids, and how
our time domain analysis can be used to characterise superradiant scattering in
non-ideal experimental conditions.Comment: 11 pages, 6 figures Version 2: Updated first author affiliation,
fixed grammatical typo
Spin dynamics of low-dimensional excitons due to acoustic phonons
We investigate the spin dynamics of excitons interacting with acoustic
phonons in quantum wells, quantum wires and quantum disks by employing a
multiband model based on the Luttinger Hamiltonian. We also use the
Bir-Pikus Hamiltonian to model the coupling of excitons to both longitudinal
acoustic phonons and transverse acoustic phonons, thereby providing us with a
realistic framework in which to determine details of the spin dynamics of
excitons. We use a fractional dimensional formulation to model the excitonic
wavefunctions and we demonstrate explicitly the decrease of spin relaxation
time with dimensionality. Our numerical results are consistent with
experimental results of spin relaxation times for various configurations of the
GaAs/AlGaAs material system. We find that longitudinal and
transverse acoustic phonons are equally significant in processes of exciton
spin relaxations involving acoustic phonons.Comment: 24 pages, 3 figure
Hole Transport in p-Type ZnO
A two-band model involving the A- and B-valence bands was adopted to analyze
the temperature dependent Hall effect measured on N-doped \textit{p}-type ZnO.
The hole transport characteristics (mobilities, and effective Hall factor) are
calculated using the ``relaxation time approximation'' as a function of
temperature. It is shown that the lattice scattering by the acoustic
deformation potential is dominant. In the calculation of the scattering rate
for ionized impurity mechanism, the activation energy of 100 or 170 meV is used
at different compensation ratios between donor and acceptor concentrations. The
theoretical Hall mobility at acceptor concentration of
cm is about 70 cmVs with the activation energy of 100 meV
and the compensation ratio of 0.8 at 300 K. We also found that the compensation
ratios conspicuously affected the Hall mobilities.Comment: 5page, 5 figures, accepted for publication in Jpn. J. Appl. Phy
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