Spin 1/2 Fermions in the Unitary Regime: A Superfluid of a New Type (original) (raw)

SPIN 1/2 FERMIONS IN THE UNITARY REGIME AT FINITE TEMPERATURE

International Journal of Modern Physics B, 2006

We have performed a fully non-perturbative calculation of the thermal properties of a system of spin 1/2 fermions in 3D in the unitary regime. We have determined the critical temperature for the superfluid-normal phase transition. The thermodynamic behavior of this system presents a number of unexpected features, and we conclude that spin 1/2 fermions in the BCS-BEC crossover should be classified as a new type of superfluid.

Asymmetric two-component fermion systems in strong coupling

2005

We study the phase structure of a dilute two-component Fermi system with attractive interactions as a function of the coupling and the polarization or number difference between the two components. In weak coupling, a finite number asymmetry results in phase separation. A mixed phase containing symmetric superfluid matter and an asymmetric normal phase is favored. With increasing coupling strength, we show that the stress on the superfluid phase to accommodate a number asymmetry increases. Near the infinite-scattering length limit, we calculate the single-particle excitation spectrum and the ground-state energy at various polarizations. A picture of weakly-interacting quasi-particles emerges for modest polarizations. In this regime near infinite scattering length, and for modest polarizations, a homogeneous phase with a finite population of excited quasi-particle states characterized by a gapless spectrum should be favored over the phase separated state. These states may be realized in cold atom experiments.

Fermion-mediated long-range interactions of bosons in the one-dimensional Bose-Fermi-Hubbard model

Physical Review A, 2010

The ground-state phase diagram of mixtures of spin polarized fermions and bosons in a 1D periodic lattice is discussed in the limit of large fermion hopping and half filling of the fermions. Numerical simulations performed with the density matrix renormalization group (DMRG) show besides bosonic Mott insulating (MI), superfluid (SF), and charge density-wave phases (CDW) a novel phase with spatial separation of MI and CDW regions. We derive an effective bosonic theory which allows for a complete understanding and quantitative prediction of the bosonic phase diagram. In particular the origin of CDW phase and the MI-CDW phase separation is revealed as the interplay between fermion-induced mean-field potential and long range interaction with alternating sign.

Molecular superfluid phase in systems of one-dimensional multicomponent fermionic cold atoms

Physical Review A, 2008

We study a simple model of N -component fermions with contact interactions which describes fermionic atoms with N = 2F + 1 hyperfine states loaded into a one-dimensional optical lattice. We show by means of analytical and numerical approaches that, for attractive interaction, a quasi-long-range molecular superfluid phase emerges at low density. In such a phase, the pairing instability is strongly suppressed and the leading instability is formed from bound-states made of N fermions. At small density, the molecular superfluid phase is generic and exists for a wide range of attractive contact interactions without an SU(N ) symmetry between the hyperfine states.

Fermion mediated long-range interactions of bosons in the 1D Bose-Fermi-Hubbard model

2009

Critical Temperature and Thermodynamics of Attractive Fermions at Unitarity

Physical Review Letters, 2006

The unitarity regime of the BCS-BEC crossover can be realized by diluting a system of twocomponent lattice fermions with an on-site attractive interaction. We perform a systematic-errorfree finite-temperature simulations of this system by diagrammatic determinant Monte Carlo. The critical temperature in units of Fermi energy is found to be Tc/εF = 0.152(7). We also report the behavior of the thermodynamic functions, and discuss the issues of thermometry of ultracold Fermi gases.

Few-Body States in Fermi-Systems and Condensation Phenomena

1999

Residual interactions in many particle systems lead to strong correlations. A multitude of spectacular phenomenae in many particle systems are connected to correlation effects in such systems, e.g. pairing, superconductivity, superfluidity, Bose-Einstein condensation etc. Here we focus on few-body bound states in a many-body surrounding.

Pair Correlations of an Expanding Superfluid Fermi Gas

Physical Review Letters, 2006

The pair correlation function of an expanding gas is investigated with an emphasis on the BEC-BCS crossover of a superfluid Fermi gas at zero temperature. At unitarity quantum Monte Carlo simulations reveal the occurrence of a sizable bunching effect due to interactions in the spin updown channel which, at short distances, is larger than that exhibited by thermal bosons in the Hanbury-Brown and Twiss effect. We propose a local equilibrium ansatz for the pair correlation function which we predict will remain isotropic during the expansion even if the trapping potential is anisotropic, contrary to what happens for non-interacting gases. This behavior is understood to be a consequence of the violation of scaling of the pair correlation function due to interactions.

Superfluidity in two-component fermionic systems

Eprint Arxiv 0705 1201, 2007

Different types of superfluid ground states have been investigated in systems of two species of fermions with Fermi surfaces that do not match. This study is relevant for cold atomic systems, condensed matter physics and quark matter. In this paper we consider this problem in the case the fermionic quasi-particles can transmute into one another and only their total number is conserved. We use a BCS approximation to study superconductivity in two-band metallic systems with inter and intra-band interactions. Tuning the hybridization between the bands varies the mismatch of the Fermi surfaces and produces different instabilities. For inter-band attractive interactions we find a first order normal-superconductor and a homogeneous metastable phase with gapless excitations. In the case of intra-band interactions, the transition from the superconductor to the normal state as hybridization increases is continuous and associated with a quantum critical point. The case when both interactions are present is also considered.

Spin 1/2 Fermions in the Unitary Regime: A Superfluid of a New Type (original) (raw)

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