Ultracold atomic Fermi gases present an opportunity to study
strongly interacting fermionic systems in a controlled and uncomplicated setting. The ability to tune attractive interactions has led to the discovery of superfluidity in these systems with an extremely high transition temperature with respect to the Fermi temperature near T/TF=0.2. This superfluidity is the electrically neutral analogue of superconductivity; however, superfluidity in atomic Fermi gases occurs in the limit ofstrong interactions and defies a conventional Bardeen–Cooper– Schrieffer (BCS) description. For these strong interactions, it is predicted that the onset of pairing and superfluidity can occur at different temperatures. Thus, for a range of temperatures, a pseudogap region may exist, in which the system retains some of the characteristics of the superfluid phase—such as a BCS-like dispersion and a partially gapped density of states—but does not exhibit superfluidity. By making two independent measurements—the direct observation of pair condensation in momentum space and a measurement of the single-particle spectral function using an analogue to photoemission spectroscopy6—we directly probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with back-bending near the Fermiwavevector kF, which persists well above the transition temperature for pair condensation
The crossover from weak to strong coupling for a three dimensional continuum model of fermions interacting via an attractive contact potential is studied above the superconducting critical temperature Tc. The pair-fluctuation propagator, the one-loop self-energy, and the spectral function are investigated in a systematic way from the superconducting fluctuation regime (weak coupling) to the bosonic regime (strong coupling). Analytic and numerical results are reported. In the strongcoupling regime, where the pair fluctuation propagator has bosonic character, two quite different peaks appear in the spectral function at a given wave vector, a broad one at negative frequencies and a narrow one at positive frequencies. The broad peak of the spectral function at negative frequencies is asymmetric about its maximum, with its spectral weight decreasing by increasing coupling and temperature. In this regime, two crossover temperatures T * 1 (at which the two peaks in the spectral function merge in one peak) and T * 0 (at which the maximum of the lower peak crosses zero frequency) can be identified, with Tc ≪ T * 0 < T * 1 . By decreasing coupling, the two-peak structure evolves smoothly. In the weak-coupling regime, where the fluctuation propagator has diffusive Ginzburg-Landau character, the overall line-shape of the spectral function is more symmetric and the two crossover temperatures approach Tc. The systematic analysis of the spectral function identifies specific features which allow one to distinguish by ARPES whether a system is in the weak-or strong-coupling regime. Connection of the results of our analysis with the phenomenology of cuprate superconductors is also attempted and rests on the recently introduced two-gap model , according to which a crossover from weak to strong coupling is realized when moving in the Brillouin zone away from the nodal points toward the M points where the d-wave gap acquires its maximum value.PACS numbers: 74.25.Jb
Exciton bound states in solids between electrons and holes are predicted to form a superfluid at high temperatures. We show that by employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superfluidity of electron-hole pairs should be achievable for the first time. The transition temperatures are well above liquid helium temperatures. Because the sample parameters needed for the device have already been attained in similar graphene devices, our work suggests a new route toward realizing high-temperature superfluidity in existing quality graphene samples.
We consider the BCS-BEC (Bose-Einstein-condensate) crossover for a system of trapped Fermi atoms at finite temperature, both below and above the superfluid critical temperature, by including fluctuations beyond mean field. We determine the superfluid critical temperature and the pair-breaking temperature as functions of the attractive interaction between Fermi atoms, from the weak- to the strong-coupling limit (where bosonic molecules form as bound-fermion pairs). Density profiles in the trap are also obtained for all temperatures and couplings.
Resonances in the superconducting properties, in a regime of crossover from BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band superconductor where the chemical potential is tuned near the band edge of the second miniband generated by quantum confinement effects. The shape resonances at T=0 in the superconducting gaps (belonging to the class of Feshbach-like resonances) is manifested by interference effects in the superconducting gap at the first large Fermi surface when the chemical potential is in the proximity of the band edge of the second miniband. The case of a superlattice of quantum wells is considered and the amplification of the superconducting gaps at the Lifshitz transition of the type neck-collapsing of Fermi surface topology is clearly shown. The results are found to be in good agreement with available experimental data on a superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers
Wave-vector resolved radio frequency (rf) spectroscopy data for an ultracold trapped Fermi gas are reported for several couplings at Tc, 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 Tc.
The Cu-O pair-distribution function in a Bi 2 Sr 2 CaCu 2 O 8ϩy superconductor has been measured by polarized Cu K-edge extended x-ray-absorption fine structure at TϽT c . The results show an anomalous long Cu-O ͑planar͒ distance, 1.96 Å, assigned to distorted CuO 2 stripes of width W intercalated with undistorted stripes of width L. From the measurement of Lϭ15Ϯ0.5 Å we have calculated the energies E n of the bottom of the one-dimensional subbands of the superlattice and found that the Fermi level E F is tuned to a ''shape resonance'' E F ϪE n Ͻប D , where D is the Debye frequency, giving the T c amplification.
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