Fermi-Bose mixtures near broad interspecies Feshbach resonances

In this Letter we study dressed bound states in Fermi-Bose mixtures near broad interspecies resonances, and implications on many-body correlations. We present the evidence for a first order phase transition between a mixture of Fermi gas and condensate, and a fully paired mixture where extended fermionic molecules occupy a single pairing channel instead of forming a molecular Fermi surface. We further investigate the effect of Fermi surface dynamics and pair fluctuations and discuss the validity of our results.     

Trimers, molecules, and polarons in mass-imbalanced atomic Fermi gases

We consider the ground state of a single "spin-down" impurity atom interacting attractively with a "spin-up" atomic Fermi gas. By constructing variational wave functions for polarons, molecules, and trimers, we perform a detailed study of the transitions between these dressed bound states as a function of mass ratio r=m↑/m↓ and interaction strength. Crucially, we find that the presence of a Fermi sea enhances the stability of the p-wave trimer, which can be viewed as a Fulde-Ferrell-Larkin-Ovchinnikov molecule that has bound an additional majority atom. For sufficiently large r, we find that the transitions lie outside the region of phase separation of the imbalanced Fermi gas and should thus be observable in experiment, unlike the well-studied equal-mass case.     

Dressed Feshbach molecules in the BEC-BCS crossover

We present the random phase approximation (RPA) theory of the Bose-Einstein-condensation-Bardeen-Cooper-Schrieffer crossover in an atomic Fermi gas near a Feshbach resonance that includes the relevant two-body atomic physics exactly. This allows us to determine the probability for the dressed molecules in the Bose-Einstein condensate to be in the closed channel of the Feshbach resonance and to compare with the recent experiments of Partridge et al. [95, 020404 (2005)10.1103/PhysRevLett.95.020404] with , who have measured the same quantity.     

Feshbach-resonant interactions in 40K and 6Li degenerate Fermi gases

We theoretically examine a system of Fermi degenerate atoms coupled to bosonic molecules by a Feshbach resonance, focusing on the superfluid transition to a molecular Bose-Einstein condensate dressed by Cooper pairs of atoms. This problem raises interest because it is unclear at present whether bimodal density distributions observed recently in 40K and 6Li are due to a condensate of bosonic molecules or fermionic atom pairs. As opposed to 40K, we find that any measurable fraction of above-threshold bosonic molecules is necessarily absent for the 6Li system in question, which strongly implicates Cooper pairs as the culprit behind its bimodal distributions.     

Calculation of Partition Function of QCD at Finite Chemical Potential

In equilibrium statistical field theory, the partition function has fundamental importance. In this paper we propose a direct and general method for calculating the partition function and equation of state of QCD at finite chemical potential. It is found that the partition function is totally determined by the dressed quark propagator at finite chemical potential up to a multiplicative constant. From this a criterion for the phase transition between the Nambu and the Wigner phases is obtained. This general method is applied to two specific cases： the free quark theory and QCD with a model dressed quark propagator having confinement features. In the first case, the standard Fermi distribution at T = 0 is reproduced. In the second case, we apply the conclusion in previous works to obtain the dressed quark propagator at finite chemical potential and find the unphysical result that the baryon number density vanishes for all values of chemical potential. The reason for this result is discussed.     

Effective Hamiltonian for fermions in an optical lattice across a Feshbach resonance

We derive the Hamiltonian for cold fermionic atoms in an optical lattice across a broad Feshbach resonance, taking into account both multiband occupations and neighboring-site collisions. Under typical configurations, the resulting Hamiltonian can be dramatically simplified to an effective single-band model, which describes a new type of resonance between the local dressed molecules and the valence bond states of fermionic atoms at neighboring sites. On different sides of such a resonance, the effective Hamiltonian is reduced to either a t-J model for the fermionic atoms or an XXZ model for the dressed molecules. The parameters in these models are experimentally tunable in the full range, which allows for observation of various phase transitions.     

Tunable superfluidity and quantum magnetism with ultracold polar molecules

By selecting two dressed rotational states of ultracold polar molecules in an optical lattice, we obtain a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. In addition to XXZ spin exchange, the model features density-density interactions and density-spin interactions; all interactions are dipolar. We show that full control of all interaction parameters in both magnitude and sign can be achieved independently of each other and of the tunneling. As a first step towards demonstrating the potential of the system, we apply the density matrix renormalization group method to obtain the 1D phase diagram of the simplest experimentally realizable case. Specifically, we show that the tunability and the long-range nature of the interactions in the t-J-V-W model enable enhanced superfluidity. Finally, we show that Bloch oscillations in a tilted lattice can be used to probe the phase diagram experimentally.     

Phase behaviors of strongly correlated Fermi gases in one-dimensional confinements

We report on the ground state of models for strongly correlated one-dimensional Fermi systems by means of theoretical studies of two-component atomic Fermi gases in highly anisotropic harmonic traps. In this context, we consider (i) the Gaudin-Yang model for a Luttinger liquid with repulsive interactions, including an analysis of the emergence of Wigner molecules in the 2k _F → 4k _F crossover, and (ii) the lattice Hubbard model yielding Luttinger liquid and Mott insulator or band-insulator phases for repulsive interactions and the Luther-Emery phase for attractive interactions, including in the former case an analysis of the role of disorder. Our calculations use novel versions of density and spin-density functional theory and a density-matrix renormalization-group technique. We also discuss preliminary results and future perspectives in the study of nonsymmetric two-component Fermi gases.     

Magnetic properties of a one-dimensional integrable electron model with correlated hopping

We investigate ground-state properties of a one-dimensional correlated hopping electron model in the presence of an external magnetic field which is solvable by Bethe ansatz. We present a general method of calculating magnetization, susceptibility, and chemical potential in exactly solvable spin-1/2 fermion models by deriving a parametric representation of these functions and the magnetic field in terms of the charge and spin distributions, the dressed charge matrix, and the dressed energy at the Fermi points in parameter space. For the correlated hopping model, we numerically calculate the dressed properties-which are given by sets of coupled integral equations-for general values of the field, the band filling, and the interaction parameter. In the special limits of magnetic saturation or large interaction parameter the results are presented in analytic form.     

The chiral and deconfinement phase transitions

By introducing the dressed Polayakov loop or dual chiral condensate as a candidate order parameter to describe the deconfinement phase transition for light flavors, we discuss the interplay between the chiral and deconfinement phase transitions, and propose the possible QCD phase diagram at finite temperature and density. We also introduce a dynamical gluodynamic model with dimension-2 gluon condensate, which can describe the color electric deconfinement as well as the color magnetic confinement.     

Ultracold heteronuclear molecules and ferroelectric superfluids

We analyze the possibility of a ferroelectric transition in heteronuclear molecules consisting of Bose-Bose, Bose-Fermi, or Fermi-Fermi atom pairs. This transition is characterized by the appearance of a spontaneous electric polarization below a critical temperature. We discuss the existence of a ferroelectric Fermi liquid phase for Fermi molecules and the existence of a ferroelectric superfluid phase for Bose molecules characterized by the coexistence of ferroelectric and superfluid orders. Lastly, we propose an experiment to detect ferroelectric correlations through the observation of coherent dipole radiation pulses.     

Quarksonic matter at high isospin density

Analogous to the quarkyonic matter at high baryon density in which the quark Fermi seas and the baryonic excitations coexist,it is argued that a "quarksonic matter" phase appears at high isospin density where the quark(antiquark) Fermi seas and the mesonic excitations coexist.We explore this phase in detail in both large Nc and asymptotically free limits.In the large Nc limit,we sketch a phase diagram for the quarksonic matter.In the asymptotically free limit,we study the pion superfluidity and thermodynamics of the quarksonic matter by using both perturbative calculations and an effective model.     

Collective molecule formation in a degenerate Fermi gas via a Feshbach resonance

We model collisionless collective conversion of a degenerate Fermi gas of atoms into bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a classical field and seeding the pairing amplitudes with random phases. A dynamical instability of the Fermi sea against association with molecules drives the conversion. The model qualitatively reproduces several experimental observations [Regal et al., Nature (London), (2003)]. We predict that the initial temperature of the Fermi gas sets the limit for the efficiency of atom-molecule conversion.     

Breached pairing superfluidity: possible realization in QCD

We propose a wide universality class of gapless superfluids, and analyze a limit that might be realized in quark matter at intermediate densities. In the breached pairing color superconducting phase heavy s quarks, with a small Fermi surface, pair with light u or d quarks. The ground state has a superfluid and a normal Fermi component simultaneously. We expect a second-order phase transition, as a function of increasing density, from the breached pairing phase to the conventional color-flavor locked phase.     

Reconstruction of the Fermi surface in the pseudogap state of cuprates

Reconstruction of the Fermi surface of high-temperature superconducting cuprates in the pseudogap state is analyzed within a nearly exactly solvable model of the pseudogap state, induced by short-range order fluctuations of the antiferromagnetic (AFM), spin-density wave (SDW), or a similar charge-density wave (CDW) order parameter, competing with the superconductivity. We explicitly demonstrate the evolution from “Fermi arcs” (on the “large” Fermi surface) observed in the ARPES experiments at relatively high temperatures (when both the amplitude and phase of the density waves fluctuate randomly) towards the formation of typical “small” electron and hole “pockets,” which are apparently observed in the de Haas-van Alphen and Hall resistance oscillation experiments at low temperatures (when only the phase of the density waves fluctuate and the correlation length of the short-range order is large enough). A qualitative criterion for the quantum oscillations in high magnetic fields to be observable in the pseudogap state is formulated in terms of the cyclotron frequency, the correlation length of fluctuations, and the Fermi velocity. The text was submitted by the authors in English.     

Holographic metals and the fractionalized fermi liquid

We show that there is a close correspondence between the physical properties of holographic metals near charged black holes in anti-de Sitter (AdS) space, and the fractionalized Fermi liquid phase of the lattice Anderson model. The latter phase has a "small" Fermi surface of conduction electrons, along with a spin liquid of local moments. This correspondence implies that certain mean-field gapless spin liquids are states of matter at nonzero density realizing the near-horizon, AdS? × R2 physics of Reissner-Nordstr?m black holes.     

Ferromagnetic superconductivity driven by changing Fermi surface topology

We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase dia-gram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetization. The twin-peak DOS may be derived from a tight-binding, quasi-one-dimensional band structure, inspired by previous band-structure calculations.     

一维强相互作用极化费米子的相图

文章首先简要评述了目前强相互作用的极化冷费米原子体系的研究现状.在三维,人们对该体系基态存在着不同认识.为对这个问题有进一步了解,文章探讨了一维强相互作用极化费米气体.在均匀情况下,这是一个可积系统,可以得到该体系的一个严格相图.作者发现了一种非均匀的超流相在相空间占主导地位.在有外加束缚势的实验情况下,通过局域密度泛函近似,作者发现了两种新颖的相分离相.     

Cooper Molecules: Second Pairing of Cooper Pairs in Gapless Superconductor CeCoIn_5

We establish a quantum field theory of phase transitions in gapless superconductor CeCoIn5.It is found that uniform Cooper pair gases with pure gradient interactions with negative coefficient can undergo a BardeenCooper-Schrieffer (BCS) condensation below a critical temperature.In the BCS condensation state,bare Cooper pairs with opposite wave vectors are bound into Cooper molecules,and uncoupled bare Cooper pairs are transformed into a new kind of quasiparticle,i.e.,the dressed particles.The Cooper molecule s.ystem is a condensate or a superfluid,and the dressed particle s.ystem is a normal fluid.The critical temperature is derived anal.yticall.y.The critical temperature of the superconductor CeCoIn5 is obtained to be T_c = 2.289 K,which approaches the experimental data.The transition from the BCS condensation state to the normal state is a first-order phase transition.