Two-Species Fermion Mixtures with Population Imbalance

Iskin, M.; Melo, C. A. R. Sá de

doi:10.1103/physrevlett.97.100404

Cited by 136 publications

(200 citation statements)

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“…Recently, density polarized gases in optical lattices were considered in Ref. [20] but that work concentrates on the insulating phases and does not resolve the competition between FFLO, BCS-BP, and phase separation, moreover the discussion there is limited to zero temperature. Finite temperature phase diagrams for trapped imbalanced gases have been considered in [11,14,21,22].…”

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

et al. 2007

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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 (BP), 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.PACS numbers: 03.75. Ss, 03.75.Hh,74.25.Dw Recent advances in the experiments of ultracold Fermi gases have shown great potential for elucidating longstanding problems in many different fields of physics related to strongly correlated Fermions. For instance, in recent experiments [1,2,3,4,5] spin-density imbalanced, or polarized, Fermi gases were considered. Such systems make it possible to study pairing with mismatched Fermi surfaces, potentially leading to non-standard phases such as that appearing in FFLO-states [6,7] or BP-states [8] (Sarma-states). These possibilities have been considered extensively in condensed-matter, nuclear, and highenergy physics [9]. The experiments in trapped gases have shown clear evidence of the separation of the gas into a BCS core region and a normal state shell around it, i.e. phase separation. Although an FFLO-type state has been predicted to appear in these systems as well [10,11], it is likely to be difficult to observe since it appears in the edges of the trap except for large polarizations.Recent experiments [12,13] on Fermi gases confined in optical lattices have already demonstrated the potential of these systems for a multitude of studies of new phases, dimensionality effects, and dynamics. In this letter, we calculate the phase diagram for an attractively interacting Fermi gas in an optical lattice at zero and finite temperatures. In particular, we consider the possibility of the single mode FFLO-phase, where the order parameter is space-dependent, and of the BCS-BP phase, where compared to the standard BCS phase the excess polarization is carried by additional Bogoliubov excitations. We investigate their competition with the phase separation (PS) of the gas into normal and BCS superfluid regions. Our results reveal that a typical phase diagram as a function of polarization and temperature is as shown in Fig. 1: phase separation is expected for small polarizations and temperatures, whereas at zero temperature the FFLO state appears after some critical polarization. At a finite temperature, the FFLO-phase competes with the BCS-BP phase. As the temperature increases the BCS-BP phase becomes energetically favorable, and as the temperature is increased even further the BCS-BP phase gives way to a normal polarized Fermi gas.Furthermore, the phase diagrams reveal a Lifshitz point which is surrounded by the normal, FFLO, and BCS-BP phases. The transitions around this point are of second order, but the FFLO ph...

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

et al. 2007

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“…Some recent works have investigated the relevant problem of imbalanced Fermi gases at finite temperature [16,17,18]. However, there are only few recent theoretical studies exploring, concomitantly, the temperature effects and the stability of population and mass imbalanced Fermi gases [14,19,20,21].…”

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Temperature effects in a Fermi gas with population imbalance

Caldas¹,

Mota²

2008

J. Stat. Mech.

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We investigate temperature effects in a Fermi gas with imbalanced spin populations. From the general expression of the thermal gap equation we find, in weak coupling limit, an analytical expression for the transition temperature Tc as a function of various possibilities of chemical potential and mass asymmetries between the two particle species. For a range of asymmetry between certain specific values, this equation always has two solutions for Tc which has been interpreted as a reentrant phenomena or a pairing induced by temperature effect. We show that the lower Tc is never related to a stable solution. The same results are obtained in strong coupling limit. The thermodynamical potential is carefully analyzed to avoid the consideration of the unstable solutions. We also obtain the tricritical points for the chemical potential and mass imbalanced cases, and beyond these points we properly minimize the thermodynamic potential to find the stable and metastable first order transition lines. The recent advances in experiments with ultracold fermionic atoms have provided the possibility for the understanding of superfluidity in several physical situations, from high temperature superconductivity to the pairing of quarks in the cores of neutron stars.When a two fermion species system have the same number of spin-up and spin-down particles, its ground state is described by the well known Bardeen-CooperSchrieffer (BCS) theory of superconductivity [1]. In this case, if the temperature is below a certain critical temperature (T c ), fermions with opposite spins interact near their common Fermi surface resulting in pair formation, even at arbitrarily weak coupling. For temperatures above T c the system is found to be in the normal state i.e., the fermions are unpaired. [6,7], and also the possibility of new coexisting phases in the BEC regime [8]. Other pairing mechanisms beyond BCS, such as P-wave superfluidity, have also been investigated [9]. Observation of phase separation between a fully paired superfluid core surrounded by the unpaired excess atoms, have been reported independently by the Rice [10,11] and MIT [12,13] groups. This phase separation can be viewed in terms of three phase transitions of different nature. Differently from the standard BCS thermodynamical phase transition we mentioned above, in an imbalanced system the phase * hcaldas@ufsj.edu.br † motaal@ufsj.edu.br transitions that may happen are: I. At zero temperature (T ), increasing the chemical potential or number asymmetry the system undergoes a (first-order) quantum phase transition to the normal state [6]; II. Still at T = 0, first order phase transitions occur from the normal to a phase separation (PS) phase, and from PS to a spatially homogeneous (magnetized) superfluid as the interaction parameter 1/k F a is varied [8,14], with a tricritical point sitting in the fully polarized line [14], and III. Lowering the temperature, a system with different and fixed number particles phase separates into a unpolarized superfluid core surrounded by a pol...

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“…In [95,96]. In FermiBose systems, a different and complementary approach to fermionic superfluidity has been suggested in which the interaction between fermionic atoms is provided by bosonic atoms taking over the role of phonons in the solid state superconductor [97,98,99,29].…”

Section: Tuning Of Heteronuclear Interactionsmentioning

confidence: 99%

Heteronuclear quantum gas mixtures

Ospelkaus¹,

Ospelkaus²

2008

J. Phys. B: At. Mol. Opt. Phys.

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Abstract. In this PhD tutorial article, we present experiments with quantum degenerate mixtures of fermionic and bosonic atoms in 3-dimensional optical lattices. This heteronuclear quantum gas mixture offers a wide range of possibilities for quantum simulation, implementation of condensed matter Hamiltonians, quantum chemistry and ultimately dense and quantum degenerate dipolar molecular samples.We When loaded into a 3D optical lattice, a whole zoo of novel quantum phases has been predicted for Fermi-Bose mixtures. We present the first realization of Fermi-Bose mixtures in 3D optical lattices as a novel quantum many body system [3]. We study the phase coherence of the bosonic cloud in the 3D optical lattice as a function of the amount of fermionic atoms simultaneously trapped in the lattice. We observe a loss of phase coherence at much lower lattice depth than for a pure bosonic cloud and discuss possible theoretical scenarios including adiabatic processes, mean field FermiBose Hubbard scenarios, and disorder-enhanced localization scenarios.After considering this many-body limit of mixtures in lattices, we show how fermionic heteronuclear Feshbach molecules can be created in the optical lattice [4] as a crucial step towards all ground state dense dipolar molecular samples. We develop rf association as a novel molecule association technique, measure the binding energy, lifetime, and association efficiency of the molecules. We develop a simple theoretical single channel model of the molecules trapped in the lattice [5] which gives excellent quantitative agreement with the experimental data.

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Two-Species Fermion Mixtures with Population Imbalance

Cited by 136 publications

References 30 publications

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

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

Temperature effects in a Fermi gas with population imbalance

Heteronuclear quantum gas mixtures

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