We consider the two-dimensional Fermi gas at finite temperature with attractive short-range interactions. Using the virial expansion, which provides a controlled approach at high temperatures, we determine the spectral function and contact for the normal state. Our calculated spectra are in qualitative agreement with recent photoemission measurements [M. Feld et al., Nature (London) 480, 75 (2011).], thus suggesting that the observed pairing gap is a feature of the high-temperature gas rather than being evidence of a pseudogap regime just above the superfluid transition temperature. We further argue that the strong pair correlations result from the fact that the crossover to bosonic dimers occurs at weaker interactions than previously assumed.

We use the virial expansion to investigate the behavior of the two-component, attractive Fermi gas in the high-temperature limit, where the system smoothly evolves from weakly attractive fermions to weakly repulsive bosonic dimers as the short-range attraction is increased. We present a new formalism for computing the virial coefficients that employs a diagrammatic approach to the grand potential and allows one to easily include an effective range R * in the interaction. In the limit where the thermal wavelength λ R * , the calculation of the virial coefficients is perturbative even at unitarity and the system resembles a weakly interacting Bose-Fermi mixture for all scattering lengths a. By interpolating from the perturbative limits λ/|a| 1 and R * /λ 1, we estimate the value of the fourth virial coefficient at unitarity for R * = 0 and we find that it is close to the value obtained in recent experiments. We also derive the equations of state for the pressure, density and entropy, as well as the spectral function at high temperatures.

Path-integral analyses originally pioneered in the study of the complex-phase problem afflicting lattice calculations of finite-density quantum chromodynamics are generalized to non-relativistic Fermi gases with repulsive interactions. Using arguments similar to those previously applied to relativistic theories, we show that the analogous problem in nonrelativistic systems manifests itself naturally in Tan's contact as a nontrivial cancellation between terms with varied dependence on extensive thermodynamic quantities. We analyze that case under the assumption of gaussian phase distribution, which is supported by our Monte Carlo calculations and perturbative considerations. We further generalize these results to observables other than the contact, as well as to polarized systems and systems with fixed particle number. Our results are quite general in that they apply to repulsive multi-component fermions, are independent of dimensionality or trapping potential, and hold in the ground state as well as at finite temperature. Introduction.-Cold-atom experimentalists continue to engineer astonishing techniques to probe fundamental properties of quantum mechanics by means of multi-component gases [1-9]. Access to the properties of these remarkable systems has expanded from the simplest ther-modynamic quantities to observables characterizing nu-anced dynamical and information-theoretic properties (see e.g. [10, 11]). Bridging a broad range of interactions, compositions, and dimensions, this ever-expanding repertoire of techniques is both celebrated and envied by the-orists, as attempting to answer similar questions about low-temperature, strongly correlated fermions is accompanied by a long list of nontrivial complications [12, 13]. Of these impediments, one which is not only particularly formidable but is also shared with lattice studies of quantum chromodynamics (QCD) is the complex phase problem associated on the one hand with non-relativistic, repulsive or imbalanced Fermi systems and on the other with QCD at finite quark density [14, 15]. In the QCD case, the problem can be traced back to the breaking of time-reversal invariance at finite chemical potential (which also appears in quasi-relativistic systems like low-energy graphene away from the Dirac point) [16, 17], which bears a strong resemblance to the (mass-or spin-) imbalanced non-relativistic Fermi gas [18]. Repulsive interactions, on the other hand, do not break time-reversal invariance per se, however that symmetry is lost upon decoupling via a Hubbard-Stratonovich transformation [19, 20]. As explained below, the partition function for N f identical fermion species then takes the path integral form

PACS 34.50.-s -Atomic and molecular scattering PACS 31.15.ac -Few-body systems PACS 05.30.Fk -Fermion systems (quantum statistical mechanics)Abstract -We investigate the three-body properties of two identical ↑ fermions and one distinguishable ↓ atom interacting in a strongly confined two-dimensional geometry. We compute exactly the atom-dimer scattering properties and the three-body recombination rate as a function of collision energy and mass ratio m ↑ /m ↓ . We find that the recombination rate for fermions is strongly energy dependent, with significant contributions from higher partial waves at low energies. For m ↑ m ↓ , the s-wave atom-dimer scattering below threshold is completely described by the scattering length. Furthermore, we examine the ↑↑↓ bound states (trimers) appearing at large m ↑ /m ↓ and find that the energy spectrum for the deepest bound trimers resembles that of a hydrogen atom confined to two dimensions.Introduction. -Ultracold atomic gases have proven to be an extremely versatile system, providing elegant realizations of a range of quantum many-body phenomena such as the BCS-BEC crossover and the Mott transition [1,2]. In particular, the cold-atom system involves short-range interactions that can be tuned to have large scattering lengths, thus rendering the low-energy physics insensitive to the details of the interaction potentials and essentially "universal". This has enabled the study of universal few-body physics, which in turn has had major consequences for the many-body system. For instance, the scattering length of diatomic molecules (dimers) was necessary for a complete description of the BCS-BEC crossover [3][4][5].Few-body inelastic processes in the cold-atom system are also important since they limit the lifetime of the gas and constrain the densities that can be achieved in experiment. Furthermore, they can act as an indirect probe of the quantum system, e.g., the first experimental evidence for the Efimov effect [6] was deduced from threebody losses [7]. There is even the prospect of generating strongly correlated phases using few-body loss processes: dimer-dimer dissipation has already been shown experimentally to induce correlations [8], while it has recently been proposed that three-body dissipation can be used to engineer the Pfaffian state in two dimensions (2D) [9].In this paper, we investigate the universal three-body problem of two identical ↑ fermions and one ↓ particle in

Recent advances in surface treatments of Niobium superconducting radio frequency (SRF) cavities have led to substantially increased Q-factors and maximum surface field. This poses theoretical challenges to identify the mechanisms responsible for such performance enhancements. We report theoretical results for the effects of inhomogeneous surface disorder on the superheating fieldthe surface magnetic field above which the Meissner state is globally unstable. We find that inhomogeneous disorder, such as that introduced by infusion of Nitrogen into the surface layers of Niobium SRF cavities, can increase the superheating field above the maximum for superconductors in the clean limit or with homogeneously distributed disorder. Homogeneous disorder increases the penetration of screening current, but also suppresses the maximum supercurrent. Inhomogeneous disorder in the form of an impurity diffusion layer biases this trade-off by increasing the penetration of the screening currents into cleaner regions with larger critical currents, thus limiting the suppression of the screening current to a thin dirty region close to the surface. Our results suggest that the impurity diffusion layers play a role in enhancing the maximum accelerating gradient of Nitrogen treated Niobium SRF cavities. *

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We report theoretical results for the electronic contribution to thermal and electrical transport for chiral superconductors belonging to even or odd-parity E 1 and E 2 representations of the tetragonal and hexagonal point groups. Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk Fermionic excitations, and -as we highlight -the structure of the impurity potential. The anomalous thermal Hall effect is shown to be sensitive to the structure of the electron-impurity t-matrix, as well as the winding number, ν, of the chiral order parameter, ∆(p) = |∆(p)| e iνφ p . For heat transport in a chiral superconductor with isotropic impurity scattering, i.e., pointlike impurities, a transverse heat current is obtained for ν = ±1, but vanishes for |ν| > 1. This is not a universal result. For finite-size impurities with radii of order or greater than the Fermi wavelength, R ≥h/p f , the thermal Hall conductivity is finite for chiral order with |ν| ≥ 2, and determined by a specific Fermi-surface average of the differential cross-section for electron-impurity scattering. Our results also provide quantitative formulae for interpreting heat transport experiments for superconductors predicted to exhibit broken time-reversal and mirror symmetries.arXiv:1911.06299v1 [cond-mat.supr-con]

The reliable detection of environmental molecules in the presence of noise is an important cellular function, yet the underlying computational mechanisms are not well understood. We introduce a model of two interacting sensors which allows for the principled exploration of signal statistics, cooperation strategies and the role of energy consumption in optimal sensing, quantified through the mutual information between the signal and the sensors. Here we report that in general the optimal sensing strategy depends both on the noise level and the statistics of the signals. For joint, correlated signals, energy consuming (nonequilibrium), asymmetric couplings result in maximum information gain in the low-noise, high-signalcorrelation limit. Surprisingly we also find that energy consumption is not always required for optimal sensing. We generalise our model to incorporate time integration of the sensor state by a population of readout molecules, and demonstrate that sensor interaction and energy consumption remain important for optimal sensing.