407 research outputs found
BCS-BEC crossover on the two-dimensional honeycomb lattice
The attractive Hubbard model on the honeycomb lattice exhibits, at
half-filling, a quantum critical point (QCP) between a semimetal with massless
Dirac fermions and an s-wave superconductor (SC). We study the BCS-BEC
crossover in this model away from half-filling at zero temperature and show
that the appropriately defined crossover line (in the interaction-density
plane) passes through the QCP at half-filling. For a range of densities around
half-filling, the ``underlying Fermi surface'' of the SC, defined as the
momentum space locus of minimum energy quasiparticle excitations, encloses an
area which evolves nonmonotonically with interactions. We also study
fluctuations in the SC and the semimetal, and show the emergence of an undamped
Leggett mode deep in the SC. We consider possible implications for experiments
on ultracold atoms and high temperature SCs.Comment: Revised - added section on the Fermi surface evolution, corrected
error in superfluid density, added possible implications for cuprate
Short-range correlations in dilute atomic Fermi gases with spin-orbit coupling
We study the short-range correlation strength of three dimensional spin half
dilute atomic Fermi gases with spin-orbit coupling. The interatomic interaction
is modeled by the contact pseudopotential. In the high temperature limit, we
derive the expression for the second order virial expansion of the
thermodynamic potential via the ladder diagrams. We further evaluate the second
order virial expansion in the limit that the spin-orbit coupling constants are
small, and find that the correlation strength between the fermions increases as
the forth power of the spin-orbit coupling constants. At zero temperature, we
consider the cases in which there are symmetric spin-orbit couplings in two or
three directions. In such cases, there is always a two-body bound state of zero
net momentum. In the limit that the average interparticle distance is much
larger than the dimension of the two-body bound state, the system primarily
consists of condensed bosonic molecules that fermions pair to form; we find
that the correlation strength also becomes bigger compared to that in the
absence of spin-orbit coupling. Our results indicate that generic spin-orbit
coupling enhances the short-range correlations of the Fermi gases. Measurement
of such enhancement by photoassociation experiment is also discussed.Comment: 7 pages, 4 figure
Collective oscillations of a trapped Fermi gas near a Feshbach resonance
The frequencies of the collective oscillations of a harmonically trapped
Fermi gas interacting with large scattering lengths are calculated at zero
temperature using hydrodynamic theory. Different regimes are considered,
including the molecular Bose-Einstein condensate and the unitarity limit for
collisions. We show that the frequency of the radial compressional mode in an
elongated trap exhibits a pronounced non monotonous dependence on the
scattering length, reflecting the role of the interactions in the equation of
state.Comment: 3 pages, including 1 figur
Shear viscosity and damping for a Fermi gas in the unitarity limit
The shear viscosity of a two-component Fermi gas in the normal phase is
calculated as a function of temperature in the unitarity limit, taking into
account strong-coupling effects that give rise to a pseudogap in the spectral
density for single-particle excitations. The results indicate that recent
measurements of the damping of collective modes in trapped atomic clouds can be
understood in terms of hydrodynamics, with a decay rate given by the viscosity
integrated over an effective volume of the cloud.Comment: 7 pages, 3 figures. Discussion significantly extended. Appendix
added. To appear in PR
High Tc Superconductors -- A Variational Theory of the Superconducting State
We use a variational approach to gain insight into the strongly correlated
d-wave superconducting state of the high Tc cuprates at T=0. We show that
strong correlations lead to qualitatively different trends in pairing and phase
coherence: the pairing scale decreases monotonically with hole doping while the
SC order parameter shows a non-monotonic dome. We obtain detailed results for
the doping-dependence of a large number of experimentally observable
quantities, including the chemical potential, coherence length, momentum
distribution, nodal quasiparticle weight and dispersion, incoherent features in
photoemission spectra, optical spectral weight and superfluid density. Most of
our results are in remarkable quantitative agreement with existing data and
some of our predictions, first reported in Phys. Rev. Lett. {\bf 87}, 217002
(2001), have been recently verified.Comment: (Minor revisions, 1 figure added, version to appear in PRB) 23 RevTeX
pages, 11 eps figs, long version of cond-mat/0101121, contains detailed
comparisons with experiments, analytical insights, technical aspects of the
calculation, and comparison with slave boson MF
Ground State Properties of Fermi Gases in the Strongly Interacting Regime
The ground state energies and pairing gaps in dilute superfluid Fermi gases
have now been calculated with the quantum Monte Carlo method without detailed
knowledge of their wave functions. However, such knowledge is essential to
predict other properties of these gases such as density matrices and pair
distribution functions. We present a new and simple method to optimize the wave
functions of quantum fluids using Green's function Monte Carlo method. It is
used to calculate the pair distribution functions and potential energies of
Fermi gases over the entire regime from atomic Bardeen-Cooper-Schrieffer
superfluid to molecular Bose-Einstein condensation, spanned as the interaction
strength is varied.Comment: 4 pages, 4 figure
Topology- and symmetry-protected domain wall conduction in quantum Hall nematics
We consider domain walls in nematic quantum Hall ferromagnets predicted to
form in multivalley semiconductors, recently probed by scanning tunnelling
microscopy experiments on Bi(111) surfaces. We show that the domain wall
properties depend sensitively on the filling factor of the underlying
(integer) quantum Hall states. For and in the absence of impurity
scattering we argue that the wall hosts a single-channel Luttinger liquid whose
gaplessness is a consequence of valley and charge conservation. For , it
supports a two-channel Luttinger liquid, which for sufficiently strong
interactions enters a symmetry-preserving thermal metal phase with a charge gap
coexisting with gapless neutral intervalley modes. The domain wall physics in
this state is identical to that of a bosonic topological insulator protected by
symmetry, and we provide a formal mapping between these
problems. We discuss other unusual properties and experimental signatures of
these `anomalous' one-dimensional systems.Comment: 11 pages, 3 figures, published versio
Photoelectron Escape Depth and Inelastic Secondaries in High Temperature Superconductors
We calculate the photoelectron escape depth in the high temperature
superconductor Bi2212 by use of electron energy-loss spectroscopy data. We find
that the escape depth is only 3 Ang. for photon energies typically used in
angle resolved photoemission measurements. We then use this to estimate the
number of inelastic secondaries, and find this to be quite small near the Fermi
energy. This implies that the large background seen near the Fermi energy in
photoemission measurements is of some other origin.Comment: 2 pages, revtex, 3 encapsulated postscript figure
Photoassociation dynamics in a Bose-Einstein condensate
A dynamical many body theory of single color photoassociation in a
Bose-Einstein condensate is presented. The theory describes the time evolution
of a condensed atomic ensemble under the influence of an arbitrarily varying
near resonant laser pulse, which strongly modifies the binary scattering
properties. In particular, when considering situations with rapid variations
and high light intensities the approach described in this article leads, in a
consistent way, beyond standard mean field techniques. This allows to address
the question of limits to the photoassociation rate due to many body effects
which has caused extensive discussions in the recent past. Both, the possible
loss rate of condensate atoms and the amount of stable ground state molecules
achievable within a certain time are found to be stronger limited than
according to mean field theory. By systematically treating the dynamics of the
connected Green's function for pair correlations the resonantly driven
population of the excited molecular state as well as scattering into the
continuum of non-condensed atomic states are taken into account. A detailed
analysis of the low energy stationary scattering properties of two atoms
modified by the near resonant photoassociation laser, in particular of the
dressed state spectrum of the relative motion prepares for the analysis of the
many body dynamics. The consequences of the finite lifetime of the resonantly
coupled bound state are discussed in the two body as well as in the many body
context. Extending the two body description to scattering in a tight trap
reveals the modifications to the near resonant adiabatic dressed levels caused
by the decay of the excited molecular state.Comment: 27 pages revtex, 16 figure
- …