5,859 research outputs found
Tuning Rashba and Dresselhaus spin-orbit couplings: Effects on singlet and triplet condensation with Fermi atoms
We investigate the pair condensation of a two-spin-component Fermi gas in the
presence of both Rashba and Dresselhaus spin-orbit couplings. We calculate the
condensate fraction in the BCS-BEC crossover both in two and in three
dimensions by taking into account singlet and triplet pairings. These
quantities are studied by varying the spin-orbit interaction from the case with
the only Rashba to the equal-Rashba-Dresselhaus one. We find that, by mixing
the two couplings, the singlet pairing decreases while the triplet pairing is
suppressed in the BCS regime and increased in the BEC regime, both in two and
three dimensions. At fixed spin-orbital strength, the greatest total condensate
fraction is obtained when only one coupling (only Rashba or only Dresselhaus)
is present.Comment: 9 pages, 6 figures, final versio
On Koopman-von Neumann Waves II
In this paper we continue the study, started in [1], of the operatorial
formulation of classical mechanics given by Koopman and von Neumann (KvN) in
the Thirties. In particular we show that the introduction of the KvN Hilbert
space of complex and square integrable "wave functions" requires an enlargement
of the set of the observables of ordinary classical mechanics. The possible
role and the meaning of these extra observables is briefly indicated in this
work. We also analyze the similarities and differences between non selective
measurements and two-slit experiments in classical and quantum mechanics.Comment: 18+1 pages, 1 figure, misprints fixe
Spin 1/2 Fermions in the Unitary Regime: A Superfluid of a New Type
We have studied, in a fully non-perturbative calculation, a dilute system of
spin 1/2 interacting fermions, characterized by an infinite scattering length
at finite temperatures. Various thermodynamic properties and the condensate
fraction were calculated and we have also determined the critical temperature
for the superfluid-normal phase transition in this regime. The thermodynamic
behavior appears as a rather surprising and unexpected melange of fermionic and
bosonic features. The thermal response of a spin 1/2 fermion at the BCS-BEC
crossover should be classified as that of a new type of superfluid.Comment: 4 pages, 1 figure, published versio
Toward an AdS/cold atoms correspondence: a geometric realization of the Schroedinger symmetry
We discuss a realization of the nonrelativistic conformal group (the
Schroedinger group) as the symmetry of a spacetime. We write down a toy model
in which this geometry is a solution to field equations. We discuss various
issues related to nonrelativistic holography. In particular, we argue that free
fermions and fermions at unitarity correspond to the same bulk theory with
different choices for the near-boundary asymptotics corresponding to the source
and the expectation value of one operator. We describe an extended version of
nonrelativistic general coordinate invariance which is realized
holographically.Comment: 14 pages; v2: typos fixed, published versio
Coherent population trapping in the stochastic limit
A 2-level atom with degenerate ground state interacting with a quantum field
is investigated. We show, that the field drives the state of the atom to a
stationary state, which is non-unique, but depends on the initial state of the
system through some conserved quantities. This non-uniqueness follows from the
degeneracy of the ground state of the atom, and when the ground subspace is
two-dimensional, the family of stationary states will depend on a
one-dimensional parameter. Only one of the stationary states in this family is
a pure state, and this state coincides with the known non-coupled population
trapped state (zero population in the excited level. Another one stationary
state corresponds to an equal weight mixture of the excited level and of the
coupled state.Comment: 13 pages, LaTe
Tunnelling of condensate magnetization in a double-well potential
We study quantum dynamical properties of a spin-1 atomic Bose-Einstein
condensate in a double-well potential. Adopting a mean field theory and single
spatial mode approximation, we characterize our model system as two coupled
spins. For certain initial states, we find full magnetization oscillations
between wells not accompanied by mass (or atom numbers) exchange. We identify
dynamic regimes of collective spin variables arising from nonlinear
self-interactions that are different from the usual Josephson oscillations. We
also discuss magnetization beats and incomplete oscillations of collective spin
variables other than the magnetization. Our study points to an alternative
approach to observe coherent tunnelling of a condensate through a (spatial)
potential barrier.Comment: 5 pages, 5 figures, submitted to Physical Review
BCS-BEC Crossover in Atomic Fermi Gases with a Narrow Resonance
We determine the effects on the BCS-BEC crossover of the energy dependence of
the effective two-body interaction, which at low energies is determined by the
effective range. To describe interactions with an effective range of either
sign, we consider a single-channel model with a two-body interaction having an
attractive square well and a repulsive square barrier. We investigate the
two-body scattering properties of the model, and then solve the Eagles-Leggett
equations for the zero temperature crossover, determining the momentum
dependent gap and the chemical potential self-consistently. From this we
investigate the dependence of the crossover on the effective range of the
interaction.Comment: 12 pages, 14 figure
BCS - BEC crossover at T=0: A Dynamical Mean Field Theory Approach
We study the T=0 crossover from the BCS superconductivity to Bose-Einstein
condensation in the attractive Hubbard Model within dynamical mean field
theory(DMFT) in order to examine the validity of Hartree-Fock-Bogoliubov (HFB)
mean field theory, usually used to describe this crossover, and to explore
physics beyond it. Quantum fluctuations are incorporated using iterated
perturbation theory as the DMFT impurity solver. We find that these
fluctuations lead to large quantitative effects in the intermediate coupling
regime leading to a reduction of both the superconducting order parameter and
the energy gap relative to the HFB results. A qualitative change is found in
the single-electron spectral function, which now shows incoherent spectral
weight for energies larger than three times the gap, in addition to the usual
Bogoliubov quasiparticle peaks.Comment: 11 pages,12 figures, Published versio
Equation of state of a Fermi gas in the BEC-BCS crossover: a quantum Monte Carlo study
We calculate the equation of state of a two-component Fermi gas with
attractive short-range interspecies interactions using the fixed-node diffusion
Monte Carlo method. The interaction strength is varied over a wide range by
tuning the value of the s-wave scattering length of the two-body potential.
For and smaller than the inverse Fermi wavevector our results show a
molecular regime with repulsive interactions well described by the dimer-dimer
scattering length . The pair correlation functions of parallel and
opposite spins are also discussed as a function of the interaction strength.Comment: 4 pages, 3 figures. Version accepted for publication in Phys. Rev.
Lett.. Figure 3 removed. Expanded discussion of correlation functions. New
figure 4. Calculation of pair correlation functions improved: more statistics
and extrapolation technique to remove residual dependences on the trial wave
function. Added comparison with Bogoliubov theory. References adde
On the specific heat of a fermionic atomic cloud in the unitary regime
In the unitary regime, when the scattering amplitude greatly exceeds in
magnitude the average inter-particle separation, and below the critical
temperature thermal properties of an atomic fermionic cloud are governed by the
collective modes, specifically the Bogoliubov-Anderson sound modes. The
specific heat of an atomic cloud in a elongated trap in particular has a rather
compex temperature dependence, which changes from an exponential behavior at
very low temperatures (), to for
and then continuosly to at temperatures just below the critical temperature, when the surface
modes play a dominant role. Only the low () and high () temperature
power laws are well defined. For the intermediate temperatures one can
introduce at most a gradually increasing with temperature exponent.Comment: 4 page
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