Observation of Resonance Condensation of Fermionic Atom Pairs

Abstract

We have observed condensation of fermionic atom pairs in the BCS-BEC crossover regime. A trapped gas of fermionic 40K atoms is evaporatively cooled to quantum degeneracy and then a magnetic-field Feshbach resonance is used to control the atom-atom interactions. The location of this resonance is precisely determined from low-density measurements of molecule dissociation. In order to search for condensation on either side of the resonance, we introduce a technique that pairwise projects fermionic atoms onto molecules; this enables us to measure the momentum distribution of fermionic atom pairs. The transition to condensation of fermionic atom pairs is mapped out as a function of the initial atom gas temperature T compared to the Fermi temperature T(F) for magnetic-field detunings on both the BCS and BEC sides of the resonance.

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

“…2, we include a point (green star) having T F of La 1.9 Sr 0.1 CuO 4 in the horizontal and the Nernst onset T on of the same system [13,14] on the vertical axes, respectively. Interestingly, this point for "hypothetical underdoped LSCO" lies very close to the T BE line, similar to those of bulk 4 He and ultracold 40 K. This suggests that the effect of pair overlapping in the very underdoped region may be modest, and/or the coherence length could be comparable to the intercarrier distance. Indeed, the new estimate for ξ from the Nernst study [14] supports the latter picture.…”

“…Even for such a system regarded as a prototype for BE condensation, we see a 30 % reduction from its ideal-gas value of T BE = 3.2 K, presumably caused by the finite size and interactions of He atoms. In addition to these factors, 2-dimensional aspects of the cuprates could also cause a further re-duction of T c from T B as discussed in Section V. Recently, condensation of fermionic ultracold 40 K gas has been achieved in the BE-BCS crossover region [40]. We also plot a point for 40 K (red star) by multiplying 10 8 to both T c and T F .…”

“…10 (a) and (d)) and the "binding" process ( Fig. 10 (b) and (c)) is reminiscent of the Feshbach resonance [40,84] appearing in the BE-BCS crossover of ultracold atoms. Note that some of these processes in Fig.…”

Towards far-from-equilibrium quantum field dynamics: A functional renormalisation-group approach

“…In field theory, they also provide a way to study strongly correlated fermions beyond mean-field and Boltzmann approximations [8]. Like these, the results presented here are expected to be of high relevance, e.g., for the description of ultracold degenerate Fermi gases close to the BEC-BCS crossover [12].In this paper we derive dynamic equations for quantum fields far from equilibrium. As a central result, we derive a new time evolution equation for the non-equilibrium effective action by use of functional renormalisation group (RG) techniques, cf.…”

FFLO order in ultra-cold atoms in three-dimensional optical lattices

“…These experiments represent an exciting opportunity to simulate the fundamental mechanisms and models of condensed matter physics, for instance Cooper pairing of fermions and the Hubbard model, without the additional complexities presented by real materials. A number of experiments have already demonstrated the possibilities for ultra-cold atomic gases, including inducing superfluidity in fermionic systems and probing the BEC-BCS crossover [2][3][4][5].…”