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Register a new userSuperfluid Expansion of a Strongly Interacting Fermi Gas
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Added by
Christian Schunck H.
Publication date
2006
fields
Physics
and research's language is
English
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We study the expansion of a rotating, superfluid Fermi gas. The presence and absence of vortices in the rotating gas is used to distinguish superfluid and normal parts of the expanding cloud. We find that the superfluid pairs survive during the expansion until the density decreases below a critical value. Our observation of superfluid flow at this point extends the range where fermionic superfluidity has been studied to densities of 1.2 10^{11} cm^{-3}, about an order of magnitude lower than any previous study.
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We study a rotating atomic Fermi gas near a narrow s-wave Feshbach resonance in a uniaxial harmonic trap with frequencies $Omega_perp$, $Omega_z$. Our primary prediction is the upper-critical angular velocity, $omega_{c2} (delta,T)$, as a function of temperature $T$ and resonance detuning $delta$, ranging across the BEC-BCS crossover. The rotation-driven suppression of superfluidity at $omega_{c2}$ is quite distinct in the BCS and BEC regimes, with the former controlled by Cooper-pair depairing and the latter by the dilution of bosonic molecules. At low $T$ and $Omega_zllOmega_perp$, in the BCS and crossover regimes of $0 lesssim delta lesssim delta_c$, $omega_{c2}$ is implicitly given by $hbar sqrt{omega_{c2}^2 +Omega_perp^2}approx 2Delta sqrt{hbar Omega_perp/epsilon_F}$, vanishing as $omega_{c2} simOmega_perp(1-delta/delta_c)^{1/2}$ near $delta_capprox 2epsilon_{F} + fracgamma 2epsilon_{F} ln(epsilon_F/hbarOmega_perp)$ (with $Delta$ the BCS gap and $gamma$ resonance width), and extending bulk result $hbaromega_{c2} approx 2Delta^2/epsilon_{F}$ to a finite number of atoms in a trap. In the BEC regime of $delta < 0$ we find $omega_{c2} toOmega^-_perp$, where molecular superfluidity can only be destroyed by large quantum fluctuations associated with comparable boson and vortex densities.
Many-body fermion systems are important in many branches of physics, including condensed matter, nuclear, and now cold atom physics. In many cases, the interactions between fermions can be approximated by a contact interaction. A recent theoretical advance in the study of these systems is the derivation of a number of exact universal relations that are predicted to be valid for all interaction strengths, temperatures, and spin compositions. These equations, referred to as the Tan relations, relate a microscopic quantity, namely, the amplitude of the high-momentum tail of the fermion momentum distribution, to the thermodynamics of the many-body system. In this work, we provide experimental verification of the Tan relations in a strongly interacting gas of fermionic atoms. Specifically, we measure the fermion momentum distribution using two different techniques, as well as the rf excitation spectrum and determine the effect of interactions on these microscopic probes. We then measure the potential energy and release energy of the trapped gas and test the predicted universal relations.
Quantum-degenerate Fermi gases provide a remarkable opportunity to study strongly interacting fermions. In contrast to other Fermi systems, such as superconductors, neutron stars or the quark-gluon plasma, these gases have low densities and their interactions can be precisely controlled over an enormous range. Here we report observations of vortices in such a gas that provide definitive evidence for superfluidity. By varying the pairing strength between two fermions near a Feshbach resonance, one can explore the crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of loosely bound pairs whose size is comparable to, or even larger than, the interparticle spacing. The crossover realizes a novel form of high-T_C superfluidity and it may provide new insight for high-T_C superconductors. Previous experiments with Fermi gases have revealed condensation of fermion pairs. While these and other studies were consistent with predictions assuming superfluidity, the smoking gun for superfluid behavior has been elusive. Our observation of vortex lattices directly displays superfluid flow in a strongly interacting, rotating Fermi gas.
We report on the expansion of a Fermi-Fermi mixture of Li-6 and K-40 atoms under conditions of strong interactions realized near the center of an interspecies Feshbach resonance. We observe two different phenomena of hydrodynamic behavior. The first one is the well-known inversion of the aspect ratio. The second one is a collective expansion, where both species stick together and despite of their different masses expand jointly. Our work constitutes a first step to explore the intriguing many-body physics of this novel system.
We report on the observation of a quenched moment of inertia as resulting from superfluidity in a strongly interacting Fermi gas. Our method is based on setting the hydrodynamic gas in slow rotation and determining its angular momentum by detecting the precession of a radial quadrupole excitation. The measurements distinguish between the superfluid or collisional origin of hydrodynamic behavior, and show the phase transition.
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