Pseudogap phenomenon and effects of population imbalance in the normal state of a unitary Fermi gas

Abstract

We investigate strong-coupling corrections to single-particle excitations in the normal state of a spin-polarized unitary Fermi gas. Within the framework of an extended T -matrix approximation, we calculate the single-particle density of states, as well as the single-particle spectral weight, to show that the so-called pseudogap phenomenon gradually disappears with increasing magnitude of an effective magnetic field. In the highly spin-polarized regime, the calculated spin polarization as a function of the effective magnetic field agrees well with the recent experiment on a 6 Li Fermi gas. Although this experiment has been considered to be incompatible with the existence of the pseudogap in an unpolarized Fermi gas, our result clarifies that the observed spin polarization in the highly spin-polarized regime and the pseudogap in the unpolarized limit can be explained in a consistent manner when one correctly includes effects of population imbalance on single-particle excitations. Since it is a crucial issue to clarify whether the pseudogap exists or not in the BCS-BEC crossover regime of an ultracold Fermi gas, our results would be useful for the understanding of this strongly interacting fermion system.

Strong-coupling corrections to spin susceptibility in the BCS-BEC-crossover regime of a superfluid Fermi gas

“…In a previous paper [63], including strong pairing fluctuations within an extended T -matrix approximation (ETMA) [64][65][66], we examined strong-coupling corrections to the spin susceptibility χ, which is experimentally accessible [67][68][69][70], in the normal state near the superfluid phase transition temperature T c , to clarify that this magnetic quantity is useful for the study of pseudogap physics in an ultracold Fermi gas with an s-wave pairing interaction. (Here, spin σ =↑, ↓ is actually pseudospin, describing two atomic hyperfine states contributing to the pair formation.)…”

Superfluid Phase Transition and Strong-Coupling Effects in an Ultracold Fermi Gas with Mass Imbalance

“…To understand the origin of this unphysical result, it is helpful to compare the present system with a polarized Fermi gas, where the breakdown of the NSR theory and TMA have also been pointed out 15,16,14 . For this purpose, we rewrite the kinetic energies ξ p,σ =↑,↓ as,…”

“…In this sense, effects of mass imbalance is similar to those of population imbalance (where the mismatch of the Fermi surfaces between the two components is induced by setting N ↑ = N ↓ .) In the latter spin-polarized system, the reason for the breakdown of the NSR theory and TMA is known to be the incomplete treatments of magnetic fluctuations and pseudogap effects associated with pairing fluctuations 15,16,14 . It has also been pointed out 14 in the case of spin-polarized Fermi gas that one can eliminate this serious problem when the self-energy in Fig.…”

“…In the latter spin-polarized system, the reason for the breakdown of the NSR theory and TMA is known to be the incomplete treatments of magnetic fluctuations and pseudogap effects associated with pairing fluctuations 15,16,14 . It has also been pointed out 14 in the case of spin-polarized Fermi gas that one can eliminate this serious problem when the self-energy in Fig. 1(b) is used.…”

Zero-Temperature Properties of a Strongly Interacting Superfluid Fermi Gas in the BCS–BEC Crossover Region

“…(Note that the specific heat, as well as the spin susceptibility vanish at T = 0.) In this paper, we numerically calculate χ n within the framework of an extended T -matrix approximation (ETMA) [21][22][23][24][25] which has been developed to evaluate the spin susceptibility χ s in the BCS-BEC crossover region. An advantage of ETMA is that one may quantitatively describe the self-energy shift, which is also referred as "Hartree shift" in the previous work [26][27][28].…”