Thermodynamic properties of Fermi gases in states with defined many-body spins

Ariel Sommer, Mark Ku, Lawrence W. Cheuk +1 more

Naoyuki Sakumichi, Norio Kawakami, Masahito Ueda

Vladimir Yurovsky

Second-order phase transitions have no latent heat and are characterized by a change in symmetry. In addition to the conventional symmetric and antisymmetric states under permutations of bosons and fermions, mathematical group-representation theory allows for non-Abelian permutation symmetry. Such symmetry can be hidden in states with defined total spins of spinor gases, which can be formed in optical cavities. The present work shows that the symmetry reveals itself in spin-independent or coordinate-independent properties of these gases, namely as non-Abelian entropy in thermodynamic properties. In weakly interacting Fermi gases, two phases appear associated with fermionic and non-Abelian symmetry under permutations of particle states, respectively. The second-order transitions between the phases are characterized by discontinuities in specific heat. Unlike other phase transitions, the present ones are not caused by interactions and can appear even in ideal gases. Similar effects in Bose gases and strong interactions are discussed.

Frédéric Chevy, Ch Salomon

Sylvain Nascimbène, Nir Navon, Frédéric Chevy +1 more

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.

P. D. Drummond, Hui Hu, Xia-Ji Liu

Ultra-cold atomic Fermi gases at nano-Kelvin temperatures provide a new paradigm for the foundations of quantum many-body theory. Here, we summarize our recent theoretical efforts to understand the intriguing properties of these strongly correlated Fermi gases. A number of powerful theoretical techniques have been developed, including (i) a quantitative diagrammatic theory taken to infinite order; (ii) a virial expansion at high temperature; (iii) exact solutions for one-dimensional many-body systems; and (iv) new quantum simulations using the Gaussian Fermi representation. We have employed these techniques to predict universal thermodynamics in the strongly interacting limit and exotic quantum superfluid phases with imbalanced spin populations. More insights into strongly interacting Fermi gases are anticipated, by extending the Gaussian Fermi simulations to new regimes.