arXiv:0801.1015 · Physical Review A

Noise correlations of the ultracold Fermi gas in an optical lattice

Tomi Paananen, T. K. Koponen, Päivi Törmä, J.-P. Martikainen

Published May 7, 2008• 17 citations

Abstract

In this paper we study the density noise correlations of the two component Fermi gas in optical lattices. Three different types of phases, the BCS state (Bardeen, Cooper, and Schieffer), the FFLO state (Fulde, Ferrel, Larkin, and Ovchinnikov), and the BP (breach pair) state are considered. We show how these states differ in their noise correlations. The noise correlations are calculated not only at zero temperature, but also at nonzero temperatures paying particular attention to how much the finite temperature effects might complicate the detection of different phases. Since one-dimensional systems have been shown to be very promising candidates to observe FFLO states, we apply our results also to the computation of correlation signals in a one-dimensional lattice. We find that the density noise correlations reveal important information about the structure of the underlying order parameter as well as about the quasiparticle dispersions.

DOI: 10.1103/physreva.77.053602

Related papers

Superfluid density of the ultra-cold Fermi gas in optical lattices

Tomi Paananen

In this paper we study the superfluid density of the two component Fermi gas in optical lattices with population imbalance. Three different type of phases, the BCS-state (Bardeen, Cooper, and Schrieffer), the FFLO-state (Fulde, Ferrel, Larkin, and Ovchinnikov), and the Sarma state, are considered. We show that the FFLO superfluid density differs from the BCS/Sarma superfluid density in an important way. Although there are dynamical instabilities in the FFLO phase, when the interaction is strong or densities are high, on the weak coupling limit the FFLO phase is found to be stable.

arXiv:0811.3623Journal of Physics B Atomic Molecular and Optical PhysicsJul 27, 200918 citations

Finite-Temperature Phase Diagram of a Polarized Fermi Gas in an Optical Lattice

T. K. Koponen, Tomi Paananen, J.-P. Martikainen +1 more

We present phase diagrams for a polarized Fermi gas in an optical lattice as a function of temperature, polarization, and lattice filling factor. We consider the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma or breached pair, and BCS phases, and the normal state and phase separation. We show that the FFLO phase appears in a considerable portion of the phase diagram. The diagrams have two critical points of different nature. We show how various phases leave clear signatures to momentum distributions of the atoms which can be observed after time of flight expansion.

arXiv:cond-mat/0701484Physical Review LettersSep 19, 200794 citations

Collective modes and the speed of sound in the Fulde-Ferrell-Larkin-Ovchinnikov state

Miikka O. J. Heikkinen, Päivi Törmä

We consider the density response of a spin-imbalanced ultracold Fermi gas in an optical lattice in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We calculate the collective mode spectrum of the system in the generalized random-phase approximation and find that, although the collective modes are damped even at zero temperature, the damping is weak enough to have well-defined collective modes. We calculate the speed of sound in the gas and show that it is anisotropic due to the anisotropy of the FFLO pairing, which implies an experimental signature for the FFLO state.

arXiv:1103.0696Physical Review AMay 25, 201119 citations

FFLO state in 1-, 2- and 3-dimensional optical lattices combined with a non-uniform background potential

T. K. Koponen, Tomi Paananen, J.-P. Martikainen +2 more

We study the phase diagram of an imbalanced two-component Fermi gas in optical lattices of 1–3 dimensions (1D–3D), considering the possibilities of the Fulde–Ferrel–Larkin–Ovchinnikov (FFLO), Sarma/breached pair, BCS and normal states as well as phase separation, at finite and zero temperatures. In particular, phase diagrams with respect to average chemical potential and the chemical potential difference of the two components are considered, because this gives the essential information about the shell structures of phases that will occur in the presence of an additional (harmonic) confinement. These phase diagrams in 1D, 2D and 3D show in a striking way the effect of Van Hove singularities on the FFLO state. Although we focus on population imbalanced gases, the results are relevant also for the (effective) mass imbalanced case. We demonstrate by LDA calculations that various shell structures such as normal–FFLO–BCS–FFLO–normal, or FFLO–normal, are possible in presence of a background harmonic trap. The phases are reflected in noise correlations: especially in 1D the unpaired atoms leave a clear signature of the FFLO state as a zero-correlation area ('breach') within the Fermi sea. This strong signature occurs both for a 1D lattice as well as for a 1D continuum. We also discuss the effect of Hartree energies and the Gorkov correction on the phase diagrams.

arXiv:0711.4065New Journal of PhysicsApr 30, 200863 citations

Polarized Fermi gases in asymmetric optical lattices

Xiaoling Cui, Yupeng Wang

The zero-temperature phase diagrams of imbalanced two-species Fermi gases are investigated in asymmetric optical lattices with arbitrary potential depths, based on the exact spectrum instead of the Fermi-Hubbard model. We study the effect of lattice potentials and atomic densities to the fully paired Bardeen-Cooper-Schrieffer (BCS) state and particularly the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. It is found that the increasing lattice potential favors BCS at low densities because of the enhanced effective coupling, whereas FFLO is favored at intermediate densities when the system undergoes a dimensional crossover. Finally, using the local density approximation, we study the evolution of the phase profile in the presence of external harmonic traps by merely tuning the lattice potentials.

arXiv:0906.1937Physical Review AFeb 22, 20106 citations

Fermionic superfluidity in optical lattices

T. K. Koponen

This is a study of the theory of ultracold atomic Fermi gases, especially in optical lattices, ie. periodic potentials created with laser light. Superfluidity is studied especially in the case of spin population imbalanced Fermi gases and phase diagrams are calculated from mean field theory. The Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase is shown to occupy a larger region of the phase diagram than previously predicted for purely harmonically trapped systems. Methods for detecting the FFLO phase are suggested. As another topic, excitations in a superfluid Fermi gas, their signatures and use in sensor applications are studied. The thesis is divided in chapters in the following way: Chapter 1 is an introduction to the research field of ultracold quantum gases, including references to some of the most important experiments done with the fermionic isotopes. Chapter 2 is an introduction to optical lattices and their major experimental characteristics. Chapter 3 deals with extensions of BCS-theory, mainly the FFLO and the detection of the phases predicted by the extensions. In chapter 4 I discuss excitations in BCS-like states and their applications in sensing electro-magnetic fields. Chapter 5 contains the conclusions.

Jan 1, 20081 citation