Closed-channel contribution in the BCS-BEC crossover regime of an ultracold Fermi gas with an orbital Feshbach resonance

Abstract

We theoretically investigate strong-coupling properties of an ultracold Fermi gas with an orbital Feshbach resonance (OFR). Including tunable pairing interaction associated with an OFR within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink (NSR), we examine the occupation of the closed channel. We show that, although the importance of the closed channel is characteristic of the system with an OFR, the occupation number of the closed channel is found to actually be very small at the superfluid phase transition temperature T c , in the whole BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, when we use the scattering parameters for an ultracold 173 Yb Fermi gas. The occupation of the closed channel increases with increasing the temperature above T c , which is more remarkable for a stronger pairing interaction. We also present a prescription to remove effects of an experimentally inaccessible deep bound state from the NSR formalism, which we meet when we theoretically deal with a 173 Yb Fermi gas with an OFR.

Controlling the interaction of ultracold alkaline-earth atoms

“…This forces us to take into account the scattering state in the closed channel, that is to say, the so-called closed channel is not really "closed". The multi-orbital nature brings the physics into a new parameter regime that has not been explored before [39,[58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]. Here we simply give couple examples.…”

“…First of all, the strong influence of the closed channel leads to a strong momentum dependence of the scattering amplitude, which can lead to an even higher transition temperature for forming Fermi superfluid, comparing to the single-channel wide resonance in alkaline-metal atom [58]. In the superfluid phase, since both two channels are occupied by the scattering states, it requires two order pairing order parameters to describes such a Fermi superfluid, which adds a new twist to the BEC-BCS crossover physics [39,[58][59][60][61][62][63][64][65]. This is reminiscent of multi-band superconductor in solid-state materials.…”

Frustrated orbital Feshbach resonances in a Fermi gas

“…This has permitted the experimental study of strongly interacting multiorbital Fermi systems, including polarons [30] and ultracold molecules [31]. Concurrently, the OFR has attracted much attention as a promising candidate for realizing a Fermi superfluid in nonalkali atomic gases [22,[32][33][34][35][36][37][38][39][40][41][42]. The orbital degree of freedom has furthermore led to progress towards the implementation of the Kondo model in cold atoms [43].…”

Single-particle Excitations and Strong Coupling Effects in the BCS–BEC Crossover Regime of a Rare-Earth Fermi Gas with an Orbital Feshbach Resonance

“…In cold Fermi gas physics, an orbital Feshbach resonance (OFR) has recently attracted much attention as a promising pairing mechanism of a superfluid gas of group 2 (rare earth) Fermi atoms. [1][2][3][4][5][6][7][8][9][10][11] The ordinary broad magnetic Feshbach resonance (MFR), 13) which is the pairing mechanism of superfluid 40 K 14) and 6 Li Fermi gases, [15][16][17] strongly relies on the character of the group 1 (alkali metal) elements that one electron occupies the outermost s-orbital, giving the total electron spin S = 1/2. Thus, MFR does not exist in the group 2 elements, because their ground state always has two electrons in the outermost s-orbital, giving the total electron spin S = 0.…”

“…We briefly note that we have recently explained this in the case of a Gaussian fluctuation theory. 10) This paper is organized as follows: In Sec. 2, we present our formulation.…”