BCS and BEC p-wave pairing in Bose-Fermi gases

The pairing of fermionic atoms in a mixture of atomic fermion and boson gases at zero temperature is investigated. The attractive interaction between fermions, that can be induced by density fluctuations of the bosonic background, can give rise to a superfluid phase in the Fermi component of the mixture. The atoms of both species are assumed to be in only one internal state, so that the pairing of fermions is effective only in odd-l channels. No assumption about the value of the ratio between the Fermi velocity and the sound velocity in the Bose gas is made in the derivation of the energy gap equation. The gap equation is solved without any particular ansatz for the pairing field or the effective interaction. The p-wave superfluidity is studied in detail. By increasing the strength and/or decreasing the range of the effective interaction a transition of the fermion pairing regime, from the Bardeen-Cooper-Schrieffer state to a system of tightly bound couples can be realized. These composite bosons behave as a weakly-interacting Bose-Einstein condensate.     

All-optical production of a degenerate Fermi gas

We achieve degeneracy in a mixture of the two lowest hyperfine states of 6Li by direct evaporation in a CO2 laser trap, yielding the first all optically produced degenerate Fermi gas. More than 10(5) atoms are confined at temperatures below 4 microK at full trap depth, where the Fermi temperature for each state is 8 microK. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose-Einstein condensation of strongly bound pairs.     

Non-fermi liquid and pairing in electron-doped cuprates

We study the normal state and pairing instability in electron-doped cuprates in a model with long-ranged antiferromagnetic spin fluctuations close to an antiferromagnetic quantum-critical point. We show that the fermionic self-energy has a non-Fermi-liquid form leading to peculiar frequency dependencies of the conductivity and the Raman response. We solve the pairing problem and demonstrate that T(c) is determined by the curvature of the Fermi surface, and the pairing gap delta (kappa, omega) is strongly nonmonotonic along the Fermi surface. The normal state frequency dependencies, the value of T(c) is approximately 10 K, and the kappa dependence of the gap agree with the experiment.     

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.     

Evolution of the normal state of a strongly interacting Fermi gas from a pseudogap phase to a molecular Bose gas

Wave-vector resolved radio frequency spectroscopy data for an ultracold trapped Fermi gas are reported for several couplings at T(c), and extensively analyzed in terms of a pairing-fluctuation theory. We map the evolution of a strongly interacting Fermi gas from the pseudogap phase into a fully gapped molecular Bose gas as a function of the interaction strength, which is marked by a rapid disappearance of a remnant Fermi surface in the single-particle dispersion. We also show that our theory of a pseudogap phase is consistent with a recent experimental observation as well as with quantum Monte Carlo data of thermodynamic quantities of a unitary Fermi gas above T(c).     

On the temperature dependence of ballistic Coulomb drag in nanowires

We have investigated within Fermi liquid theory the dependence of Coulomb drag current in a passive quantum wire on the applied voltage V across an active wire and on the temperature T for any values of eV/k(B)T. We assume that the bottoms of the 1D minibands in both wires almost coincide with the Fermi level. We conclude that: (1) within a certain temperature interval the drag current can be a descending function of the temperature T; (2) the experimentally observed temperature dependence T(-0.77) of the drag current can be interpreted within the framework of Fermi liquid theory; (3) at relatively high applied voltages the drag current saturates as a function of the applied voltage; and (4) the screening of the electron potential by metallic gate electrodes can be of importance.     

Pairing in spin polarized two-species fermionic mixtures with mass asymmetry

We discuss on the pairing mechanism of fermions with mismatch in their Fermi momenta due to a mass asymmetry. Using a variational ansatz for the ground state we also discuss the BCS-BEC crossover of this system. It is shown that the breached pairing solution with a single Fermi surface is stable in the BEC regime. We also include the temperatures effect on the fermion pairing within an approximation that is valid for temperatures much below the critical temperature.     

Nodal d + id pairing and topological phases on the triangular lattice of Na(x)CoO(2).yH(2)O: evidence for an unconventional superconducting state

We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density x(c) where the normal state Fermi surface crosses the isolated zeros. For spin-singlet pairing, we show that the second-nearest-neighbor (d+id)-wave pairing can be the dominant pairing channel. The gapless critical state at x (c) approximately 0.25 has six Dirac points and is topologically nontrivial with a T3 spin relaxation rate below T(c). This picture provides a possible explanation for the unconventional superconducting state of Na(x)Co O(2). yH(2)O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.     

Protected nodes and the collapse of Fermi arcs in high-T{c} cuprate superconductors

Angle resolved photoemission on underdoped Bi2Sr2CaCu2O8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to T{c}. This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At T{c} the point nodes of the d-wave gap abruptly expand into finite length "Fermi arcs." This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase.     

Competing types of order in two-dimensional bose-fermi mixtures

Using a functional renormalization group approach we study the zero temperature phase diagram of two-dimensional Bose-Fermi mixtures of ultracold atoms in optical lattices, in the limit when the velocity of bosonic condensate fluctuations is much larger than the Fermi velocity. For spin-1/2 fermions we obtain a phase diagram, which shows a competition of pairing phases of various orbital symmetry (s, p, and d) and antiferromagnetic order. We determine the value of the gaps of various phases close to half filling, and identify subdominant orders as well as short-range fluctuations from the renormalization group flow. For spinless fermions we find that p-wave pairing dominates the phase diagram.     

Resonance superfluidity in a quantum degenerate Fermi gas

We consider the superfluid phase transition that arises when a Feshbach resonance pairing occurs in a dilute Fermi gas. We apply our theory to consider a specific resonance in potassium ((40)K), and find that for achievable experimental conditions, the transition to a superfluid phase is possible at the high critical temperature of about 0.5T(F). Observation of superfluidity in this regime would provide the opportunity to experimentally study the crossover from the superfluid phase of weakly coupled fermions to the Bose-Einstein condensation of strongly bound composite bosons.     

Nonadiabatic channels in the superconducting pairing of fullerides

We show the intrinsic inconsistency of the conventional phonon mediated theory of superconductivity in relation to the observed properties of Rb3C60. The recent, highly accurate measurement of the carbon isotope coefficient alpha(C) = 0.21, together with the high value of T(c) (30 K) and the very small Fermi energy E(F) (0.25 eV), unavoidably implies the opening of nonadiabatic channels in the superconducting pairing. We estimate these effects and show that they are actually the key elements for the high value of T(c) in these materials compared to the very low values of graphite intercalation compounds.     

Thermodynamics of a Fermi liquid beyond the low-energy limit

We consider the nonanalytic temperature dependences of the specific heat coefficient, C(T)/T, and spin susceptibility, chi(s)(T), of 2D interacting fermions beyond the weak-coupling limit. We demonstrate within the Luttinger-Ward formalism that the leading temperature dependences of C(T)/T and chi(s)(T) are linear in T, and are described by the Fermi liquid theory. We show that these temperature dependences are universally determined by the states near the Fermi level and, for a generic interaction, are expressed via the spin and charge components of the exact backscattering amplitude of quasiparticles. We compare our theory to recent experiments on monolayers of He3.     

Dynamical mean-field theory for pairing and spin gap in the attractive hubbard model

We solve the attractive Hubbard model for arbitrary interaction strengths within dynamical mean-field theory. We compute the transition temperature for superconductivity and analyze electron pairing in the normal phase. The normal state is a Fermi liquid at weak coupling and a non-Fermi-liquid state with a spin gap at strong coupling. Away from half filling, the quasiparticle weight vanishes discontinuously at the transition between the two normal states.     

Unconventional anisotropic s-wave superconducting gaps of the LiFeAs iron-pnictide superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy on Fe-based superconductor LiFeAs (T(c)=18 K). We reveal multiple nodeless superconducting (SC) gaps with 2Δ/k(B)T(c) ratios varying from 2.8 to 6.4, depending on the Fermi surface (FS). We also succeeded in directly observing a gap anisotropy along the FS with magnitude up to ~30%. The anisotropy is fourfold symmetric with an antiphase between the hole and electron FSs, suggesting complex anisotropic interactions for the SC pairing. The observed momentum dependence of the SC gap offers an excellent opportunity to investigate the underlying pairing mechanism.     

Population of closed-channel molecules in trapped Fermi gases with broad Feshbach resonances

We compute the fraction of closed-channel molecules in trapped atomic Fermi gases, over the entire range of accessible fields and temperatures. We use a two-channel model of Bardeen-Cooper-Schrieffer-Bose-Einstein-condensation crossover theory at general temperature , and show that this fraction provides a measure of the T-dependent pairing gap. Our calculations, containing no free parameters, are in good quantitative agreement with recent low- measurements in (6)Li. We present readily testable predictions for the dependencies of the closed-channel fraction on temperature and Fermi momentum.     

Spin-orbit coupled Fermi gases across a Feshbach resonance

In this Letter we study both ground state properties and the superfluid transition temperature of a spin-1/2 Fermi gas across a Feshbach resonance with a synthetic spin-orbit coupling, using the mean-field theory and the exact solution of two-body problem. We show that a strong spin-orbit coupling can significantly enhance the pairing gap for negative scattering length a(s), due to increased density of state at Fermi surface. Strong spin-orbit coupling can also significantly enhance the superfluid transition temperature Tc to a sizable fraction of Fermi temperature when a(s) ≤ 0, while it suppresses Tc slightly for positive a(s). The interaction energy and pair size at resonance are also discussed.     

First-order pairing transition and single-particle spectral function in the attractive hubbard model

A dynamical mean-field theory analysis of the attractive Hubbard model in the normal phase is carried out upon restricting to solutions where superconducting order is not allowed. A clear first-order pairing transition as a function of the coupling takes place at all the electron densities out of half filling between a Fermi liquid, stable for UU(c), and it is accompanied by phase separation. The spectral function in the metallic phase is constituted by a low-energy structure around the Fermi level, which disappears discontinuously at U = U(c), and two high-energy features (Hubbard bands), which persist in the insulating phase.     

Achieving a BCS transition in an atomic Fermi gas

We consider a gas of cold fermionic atoms having two spin components with interactions characterized by their s-wave scattering length a. At positive scattering length the atoms form weakly bound bosonic molecules which can be evaporatively cooled to undergo Bose-Einstein condensation, whereas at negative scattering length BCS pairing can take place. It is shown that, by adiabatically tuning the scattering length a from positive to negative values, one may transform the molecular Bose-Einstein condensate into a highly degenerate atomic Fermi gas, with the ratio of temperature to Fermi temperature T/T(F) approximately 10(-2). The corresponding critical final value of k(F)/a/, which leads to the BCS transition, is found to be about one-half, where k(F) is the Fermi momentum.