2,843 research outputs found
Density and spin response function of a normal Fermi gas at unitarity
Using Landau theory of Fermi liquids we calculate the dynamic response of
both a polarized and unpolarized normal Fermi gas at zero temperature in the
strongly interacting regime of large scattering length. We show that at small
excitation energies the {\it in phase} (density) response is enhanced with
respect to the ideal gas prediction due to the increased compressibility.
Viceversa, the {\it out of phase} (spin) response is quenched as a consequence
of the tendency of the system to pair opposite spins. The long wavelength
behavior of the static structure factor is explicitly calculated. The results
are compared with the predictions in the collisional and superfluid regimes.
The emergence of a spin zero sound solution in the unpolarized normal phase is
explicitly discussed.Comment: 4 pages, 2 figure
Self-consistent calculation of particle-hole diagrams on the Matsubara frequency: FLEX approximation
We implement the numerical method of summing Green function diagrams on the
Matsubara frequency axis for the fluctuation exchange (FLEX) approximation. Our
method has previously been applied to the attractive Hubbard model for low
density. Here we apply our numerical algorithm to the Hubbard model close to
half filling (), and for , in order to study the
dynamics of one- and two-particle Green functions. For the values of the chosen
parameters we see the formation of three branches which we associate with the a
two-peak structure in the imaginary part of the self-energy. From the imaginary
part of the self-energy we conclude that our system is a Fermi liquid (for the
temperature investigated here), since Im
around the chemical potential. We have compared our fully self-consistent FLEX
solutions with a lower order approximation where the internal Green functions
are approximated by free Green functions. These two approches, i.e., the fully
selfconsistent and the non-selfconsistent ones give different results for the
parameters considered here. However, they have similar global results for small
densities.Comment: seven pages, nine figures as ps files. Accepted in Int. J. Modern
Phys. C (1997
Partially suppressed long-range order in the Bose-Einstein condensation of polaritons
We adopt a kinetic theory of polariton non-equilibrium Bose-Einstein
condensation, to describe the formation of off-diagonal long-range order. The
theory accounts properly for the dominant role of quantum fluctuations in the
condensate. In realistic situations with optical excitation at high energy, it
predicts a significant depletion of the condensate caused by long-wavelength
fluctuations. As a consequence, the one-body density matrix in space displays a
partially suppressed long-range order and a pronounced dependence on the finite
size of the system
Valley dependent many-body effects in 2D semiconductors
We calculate the valley degeneracy () dependence of the many-body
renormalization of quasiparticle properties in multivalley 2D semiconductor
structures due to the Coulomb interaction between the carriers. Quite
unexpectedly, the dependence of many-body effects is nontrivial and
non-generic, and depends qualitatively on the specific Fermi liquid property
under consideration. While the interacting 2D compressibility manifests
monotonically increasing many-body renormalization with increasing , the
2D spin susceptibility exhibits an interesting non-monotonic dependence
with the susceptibility increasing (decreasing) with for smaller (larger)
values of with the renormalization effect peaking around .
Our theoretical results provide a clear conceptual understanding of recent
valley-dependent 2D susceptibility measurements in AlAs quantum wells.Comment: 5 pages, 3 figure
Dipole and monopole modes in the Bose-Hubbard model in a trap
The lowest-lying collective modes of a trapped Bose gas in an optical lattice
are studied in the Bose-Hubbard model. An exact diagonalization of the
Hamiltonian is performed in a one-dimensional five-particle system in order to
find the lowest few eigenstates. Dipole and breathing character of the
eigenstates is confirmed in the limit where the tunneling dominates the
dynamics, but under Mott-like conditions the excitations do not correspond to
oscillatory modes.Comment: 19 pages, 11 figures; submitted to Phys. Rev.
Boson Core Compressibility
Strongly interacting atoms trapped in optical lattices can be used to explore
phase diagrams of Hubbard models. Spatial inhomogeneity due to trapping
typically obscures distinguishing observables. We propose that measures using
boson double occupancy avoid trapping effects to reveal key correlation
functions. We define a boson core compressibility and core superfluid stiffness
in terms of double occupancy. We use quantum Monte Carlo on the Bose-Hubbard
model to empirically show that these quantities intrinsically eliminate edge
effects to reveal correlations near the trap center. The boson core
compressibility offers a generally applicable tool that can be used to
experimentally map out phase transitions between compressible and
incompressible states.Comment: 11 pages, 11 figure
Umklapp collisions and center of mass oscillation of a trapped Fermi gas
Starting from the the Boltzmann equation, we study the center of mass
oscillation of a harmonically trapped normal Fermi gas in the presence of a
one-dimensional periodic potential. We show that for values of the the Fermi
energy above the first Bloch band the center of mass motion is strongly damped
in the collisional regime due to umklapp processes. This should be contrasted
with the behaviour of a superfluid where one instead expects the occurrence of
persistent Josephson-like oscillations.Comment: 11 pages, 3 figures, corrected typo
Tunable Charge and Spin Seebeck Effects in Magnetic Molecular Junctions
We study the charge and spin Seebeck effects in a spin-1 molecular junction
as a function of temperature (T), applied magnetic field (H), and magnetic
anisotropy (D) using Wilson's numerical renormalization group. A hard-axis
magnetic anisotropy produces a large enhancement of the charge Seebeck
coefficient Sc (\sim k_B/|e|) whose value only depends on the residual
interaction between quasiparticles in the low temperature Fermi-liquid regime.
In the underscreened spin-1 Kondo regime, the high sensitivity of the system to
magnetic fields makes it possible to observe a sizable value for the spin
Seebeck coefficient even for magnetic fields much smaller than the Kondo
temperature. Similar effects can be obtain in C60 junctions where the control
parameter is the gap between a singlet and a triplet molecular state.Comment: 5 pages, 4 figure
Single-particle and collective excitations in quantum wires made up of vertically stacked quantum dots: Zero magnetic field
We report on the theoretical investigation of the elementary electronic
excitations in a quantum wire made up of vertically stacked self-assembled
InAs/GaAs quantum dots. The length scales (of a few nanometers) involved in the
experimental setups prompt us to consider an infinitely periodic system of
two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers.
The the Bloch functions and the Hermite functions together characterize the
whole system. We then make use of the Bohm-Pines' (full) random-phase
approximation in order to derive a general nonlocal, dynamic dielectric
function. Thus developed theoretical framework is then specified to work within
a (lowest miniband and) two-subband model that enables us to scrutinize the
single-particle as well as collective responses of the system. We compute and
discuss the behavior of the eigenfunctions, band-widths, density of states,
Fermi energy, single-particle and collective excitations, and finally size up
the importance of studying the inverse dielectric function in relation with the
quantum transport phenomena. It is remarkable to notice how the variation in
the barrier- and well-widths can allow us to tailor the excitation spectrum in
the desired energy range. Given the advantage of the vertically stacked quantum
dots over the planar ones and the foreseen applications in the single-electron
devices and in the quantum computation, it is quite interesting and important
to explore the electronic, optical, and transport phenomena in such systems
Quasiparticle Self-Consistent GW Theory
In past decades the scientific community has been looking for a reliable
first-principles method to predict the electronic structure of solids with high
accuracy. Here we present an approach which we call the quasiparticle
self-consistent GW approximation (QpscGW). It is based on a kind of
self-consistent perturbation theory, where the self-consistency is constructed
to minimize the perturbation. We apply it to selections from different classes
of materials, including alkali metals, semiconductors, wide band gap
insulators, transition metals, transition metal oxides, magnetic insulators,
and rare earth compounds. Apart some mild exceptions, the properties are very
well described, particularly in weakly correlated cases. Self-consistency
dramatically improves agreement with experiment, and is sometimes essential.
Discrepancies with experiment are systematic, and can be explained in terms of
approximations made.Comment: 12 pages, 3 figure
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