Over the past decade, ultracold gases have been studied extensively, both theoretically and experimentally. This is due to the discovery that the relevant measure for the interatomic interactions in such systems can be controlled experimentally by simply applying a magnetic field. Consequently, these gases can be conveniently studied in both the weakly and the strongly coupled regime. In particular, it is possible to tune the magnetic field such that the interaction strength between the atoms diverges. This is called the unitarity limit. Interestingly, at unitarity the system becomes almost scale invariant, which means that the only scales left in the problem are set by the temperature and the density of the gas. The unitary gas then exhibits universal behavior. It therefore constitutes a perfect candidate for the application of the AdS/CFT duality, also known as holography, to a nonrelativistic condensed-matter system. According to this duality, a strongly coupled, scale-invariant system can be described using a dual gravitational model with one additional spatial dimension. In this thesis we apply the AdS/CFT duality to construct holographic models for ultracold gases at unitarity. To this end, we start by studying properties of a well-known holographic model of a condensed-matter system, namely the holographic superconductor. We then show how we can use a very similar model to describe elementary fermions with a nonzero mass in holography. Subsequently, we study these holographic models in the nonrelativistic limit. We find that the resulting models exhibit thermodynamic properties that are qualitatively similar to those of a unitary Fermi gas. Lastly, we take some first steps towards holographically modelling Bose gases at unitarity. In particular, we show that we can describe massive elementary bosons using AdS/CFT.
We calculate nonperturbative self-consistent fermionic and bosonic spectral functions of ultracold Fermi gases with the spectral functional approach. This approach allows for a direct real-time computation of nonperturbative correlation functions, and in the present work we use spectral Dyson-Schwinger equations. We focus on the normal phase of the spin-balanced Fermi gas and provide numerical results for the full fermionic and bosonic spectral functions. The spectral functions are then used for the determination of the equation of state, the Tan contact, and ejection rf spectra at unitarity. These results are compared to experimental data, the self-consistent
T-matrix approach, and lattice results. Our approach offers a wide range of applications, including the ab initio calculation of transport and spectral properties of the superfluid phase in the BCS-BEC crossover.
Computing dynamical properties of strongly interacting quantum many-body systems poses a major challenge to theoretical approaches. Usually, one has to resort to numerical analytic continuation of results on imaginary frequencies, which is a mathematically ill-defined procedure. Here we present an efficient method to compute the spectral functions of the two-component Fermi gas near the strongly interacting unitary limit directly in real frequencies. To this end, we combine the Keldysh path integral that is defined in real time with the self-consistent T -matrix approximation. The latter is known to predict thermodynamic and transport properties in good agreement with experimental observations in ultracold atoms. We validate our method by comparison with thermodynamic quantities obtained from imaginary-time calculations and by transforming our real-time propagators to imaginary time. By comparison with state-of-the-art numerical analytic continuation of the imaginary-time results, we show that our real-time results give qualitative improvements for dynamical quantities. Moreover, we show that no significant pseudogap regime exists in the self-consistent T -matrix approximation above the critical temperature Tc , an issue that has been under significant debate. We close by pointing out the versatile nature of our method as it can be extended to other systems, like the spin- or mass-imbalanced Fermi gas, other Bose-Fermi models, two-dimensional systems, and systems out of equilibrium. Published by the American Physical Society 2024
We construct systematic expansions around four and two spatial dimensions for a Fermi gas near the unitarity limit. Near four spatial dimensions such a Fermi gas can be understood as a weakly-interacting system of fermionic and bosonic degrees of freedom. To the leading and next-to-leading orders in the expansion over ε=4-d, with d being the dimensionality of space, we determine the thermodynamic functions, the fermion quasiparticle spectrum, the critical polarizations, and the critical temperature as functions of the binding energy of the two-body state. We also show that the unitary Fermi gas near two spatial dimensions reduces to a weakly-interacting Fermi gas and calculate the thermodynamic functions and the fermion quasiparticle spectrum in the expansion over \barε=d-2. We discuss the matching of the two systematic expansions around four and two spatial dimensions in order to extract physical observables at d=3. We find good agreement of the results with those from recent Monte Carlo simulations.
We explore a generalization of nonrelativistic fermionic statistics that interpolates between bosons and fermions, in which up to
K particles may occupy a single-particle state. We show that it can be mapped exactly to
K flavors of fermions with imaginary polarization. In particular, for
K=2, we use such a mapping to derive the virial coefficients and relate them to those of conventional spin-1/2 fermions in an exact fashion. We also use the mapping to derive next-to-leading-order perturbative results for the pressure equation of state. Our results indicate that the
K=2 particles are more strongly coupled than conventional spin-
1/2 fermions, as measured by the interaction effects on the virial expansion and on the pressure equation of state. In the regime set by the unitary limit, the proposed
K=2 deformation represents a universal many-body system whose properties remain largely unknown. In particular, the system can be expected to become superfluid at a critical temperature
Tc higher than that of the unitary limit. We suggest it may be possible to realize this system experimentally by engineering a polarized coupling to an electrostatic potential. Finally, we show that the
K=2 system does not display a sign problem for determinantal Monte Carlo calculations, which indicates that
Tc can at least in principle be calculated with conventional methods.
We will present recent experimental work on the thermodynamics of strongly interacting Fermi gases. We have developed a general method to probe with high precision the Equation of State (EoS) of locally homogeneous ultracold gases. This allows stringent tests of recent many-body theories. First, we focus on the finite-temperature EoS of the unpolarized unitary gas. Precise thermometry is provided by adding to the Fermi gas of 6Li a trace of bosonic 7Li. We show that the low-temperature properties of the strongly interacting normal phase are well described by Fermi liquid theory and we localize the superfluid transition. Second, we address the zero-temperature EoS of the spin-polarized system in the BEC-BCS crossover. Surprisingly, despite strong interactions, the polarized phase behaves as a mixture of two ideal gases: a Fermi gas of majority atoms and a non-interacting gas of dressed quasi-particles, the Fermi polarons. We also obtain the equation of state of the superfluid state as a function of interaction strength, that we compare to Monte Carlo simulations and to the Lee-Huang-Yang corrections for low-density bosonic and fermionic superfluids.