Jelena Stajic

BCS–BEC crossover: From high temperature superconductors to ultracold superfluids

Abstract We review the BCS to Bose–Einstein condensation (BEC) crossover scenario which is based on the well known crossover generalization of the BCS ground state wavefunction Ψ 0 . While this ground state has been summarized extensively in the literature, this Review is devoted to less widely discussed issues: understanding the effects of finite temperature, primarily below T c , in a manner consistent with Ψ 0 . Our emphasis is on the intersection of two important problems: high T c superconductivity and superfluidity in ultracold fermionic atomic gases. We address the “pseudogap state” in the copper oxide superconductors from the vantage point of a BCS–BEC crossover scenario, although there is no consensus on the applicability of this scheme to high T c . We argue that it also provides a useful basis for studying atomic gases near the unitary scattering regime; they are most likely in the counterpart pseudogap phase. That is, superconductivity takes place out of a non-Fermi liquid state where preformed, metastable fermion pairs are present at the onset of their Bose condensation. As a microscopic basis for this work, we summarize a variety of T-matrix approaches, and assess their theoretical consistency. A close connection with conventional superconducting fluctuation theories is emphasized and exploited.

Heat Capacity of a Strongly Interacting Fermi Gas

We have measured the heat capacity of an optically trapped, strongly interacting Fermi gas of atoms. A precise addition of energy to the gas is followed by single-parameter thermometry, which determines the empirical temperature parameter of the gas cloud. Our measurements reveal a clear transition in the heat capacity. The energy and the spatial profile of the gas are computed using a theory of the crossover from Fermi to Bose superfluids at finite temperatures. The theory calibrates the empirical temperature parameter, yields excellent agreement with the data, and predicts the onset of superfluidity at the observed transition point.

Rise of the Machines

![Figure][1] Illustration of how a virtual interviewer sees a human. The interviewer assesses the psychological state of the human by tracking and analyzing their facial expressions, body posture, and speech. CREDITS: (PHOTO) NICK DOLDING/GETTY IMAGES; DATA: BALTRUSAITIS, TADAS, PETER

Thermodynamics of interacting fermions in atomic traps

We calculate the entropy in a trapped, resonantly interacting Fermi gas as a function of temperature for a wide range of magnetic fields between the BCS and Bose-Einstein condensation end points. This provides a basis for the important technique of adiabatic sweep thermometry and serves to characterize quantitatively the evolution and nature of the excitations of the gas. The results are then used to calibrate the temperature in several ground breaking experiments on (6)Li and (40)K.

Nature of superfluidity in ultracold Fermi gases near Feshbach resonances

We study the superfluid state of atomic Fermi gases using a BCS-Bose-Einstein-condensation crossover theory. Our approach emphasizes noncondensed fermion pairs which strongly hybridize with their (Feshbach-induced) molecular boson counterparts. These pairs lead to pseudogap effects above

Density Profiles of Strongly Interacting Trapped Fermi Gases

{T}_{c}

Particle density distributions in Fermi gas superfluids: Differences between one- and two-channel models in the Bose-Einstein-condensation limit

and non-BCS characteristics below. We discuss how these effects influence the experimental signatures of superfluidity.

Heat-loving quantum oscillations

We study density profiles in trapped fermionic gases, near Feshbach resonances, at all T< or =Tc and in the near Bose-Einstein condensation and unitary regimes. For the latter, we characterize and quantify the generally neglected contribution from noncondensed Cooper pairs. As a consequence of these pairs, our profiles are rather well fit to a Thomas-Fermi (TF) functional form, and equally well fit to experimental data. Our work lends support to the notion that TF fits can be used in an experimental context to obtain information about the temperature.

Hunting the elusive (quasi)particles

We show how to describe the

A topological superconductor

T \neq 0