Observation of a Strongly Interacting Fermi gas

We are the first group to produce and study a strongly-interacting, degenerate Fermi gas of atoms, near a Feshbach resonance, in the so-called BEC-BCS crossover regime. A cigar-shaped cloud of fermionic ⁶Li atoms is confined and rapidly cooled to degeneracy in a cigar-shaped CO₂ laser trap, using a magnetic field to induce strong interactions. Upon abruptly turning off the trap, the gas exhibits a spectacular anisotropic expansion, "elliptic flow," rapidly expanding in the transverse direction while remaining nearly stationary along the axial direction (OHaraScience2002). Similar behavior is observed in a quark-gluon plasma, a state of matter that existed microseconds after the Big Bang, which has been recreated in experiments with heavy ion colliders. [Read more]

First Evidence for Superfliud Hydrodynamics in a Strongly Interacting Fermi Gas

Studies of collective modes in optically trapped gases are useful because they provide information about microscopic interactions by monitoring macroscopic observables. We studied the frequency and damping rate of the radial breathing mode in an optically trapped gas of fermions in the strongly-interacting regime. The strength and sign of the interactions between particles is controlled via application of an external magnetic field.  After cooling a gas of atoms well into the degenerate regime, we found that the damping rate decreased, while the frequency remained precisely at the hydrodynamic value, signalling superfluid hydrodynamics (Kinast2004). [Read more]

Heat Capacity of a Strongly Interacting Fermi Gas

We studied the thermodynamics of a strongly interacting Fermi gas by adding a known energy and measuring the change in an empirical temperature determined from the cloud profile. Calibrating the temperature scale by comparing to theoretical predictions enabled a study of the heat capacity, albeit in a model-dependent way. This was the first attempt at thermometry and thermodynamics measurements in the BEC-BCS crossover regime of a Fermi gas near a Feshbach resonance. We observed a transition in the heat capacity, which we interpreted as the onset of a high temperature superfluid state (KinastScience2005). [Read more]

Model-Independent Thermodynamic Measurements in a Fermi Gas

Our group made the first model-independent measurement of the thermodynamics of a strongly interacting Fermi gas, by measuring the energy and entropy of the cloud. We showed the the virial theorem holds when the gas is tuned to a Feshbach resonance, to that the total energy is equal to twice the potential energy in a harmonic trap. By measuring the mean square cloud size along one axis and the corresponding harmonic oscillation frequency, we precisely determined the potential energy, hence the total energy, of this strongly interacting many-body system. By adiabatically sweeping the bias magnetic field to tune the gas to a weakly interacting regime, a second measurement of the cloud size determined the entropy of the weakly interacting gas. A round trip sweep verifed that the sweep was adiabatic, thereby determining the entropy of the strongly interacting gas. The measured curve of energy verus entropy enabled a new temperature calibration for this strongly-interacting many-body system, independent of any theoretical model (Luo2007).

Measurement of Sound Velocity

We measure sound propagation in an optically-trapped degenerate Fermi gas of spin-up and spin-down ⁶Li atoms with magnetically tunable interactions. A sound wave is excited by using a green 532 nm beam, which passes through a cylindrical telescope to create a knife-like repulsive optical potential near the center of the trapped cigar-shaped cloud. Pulsing the knife beam creates two outward propagating sound waves. Measurements are made throughout the BEC-BCS crossover region, from a Bose condensate of dimer pairs below the Feshbach resonance, to the resonant Fermi superfluid regime, and finally to a weakly interacting Fermi gas above the Feshbach resonance. At resonance, the sound velocity exhibits universal scaling with the Fermi velocity (Joseph2007).

Observation of Nearly Perfect Rotational Flow

In our initial attempts to measure the shear viscosity, we discovered that an expanding, strongly interacting Fermi gas exhibits nearly perfect, irrotational flow in the normal fluid regime. We found that the effective moment of inertia was highly suppressed in the normal fluid regime due to the extremely small shear viscosity, as there is no torque to produce a rigid rotation component. We created a rotating cigar-shaped cloud and measured the effective moment of inertia by determining the initial angular momentum of a rotating cloud and its angular velocity as a function of time. The results are in very good agreement with hydrodynamic predictions (Clancy2007).

Progress in the Search for Perfect Universal Fluids

Perfect fluids are currently of great interest, connecting diverse fields of physics from nuclear matter and quark-gluon plasmas to string theory and cold atoms. As defined by a recent conjecture from the string theory community, perfect fluids exhibit a minimum ratio of the shear viscosity h to the entropy density s in strongly interacting scale invariant systems. Ultracold Fermi gases, tuned to a collisional (Feshbach) resonance, are strongly interacting and scale invariant, with thermodynamic and transport properties that are universal functions of the density n and temperature T. Measurements of the equilibrium properties, like the energy and entropy, currently test predictions employing state-of-the-art non-perturbative many-body methods. However, measurement and prediction of universal transport coefficients presents new challenges. In recent papers (CaoScience2011) and (CaoNJP2011), we report the measurement of the universal quantum viscosity in a unitary Fermi gas, and compare the ratio h/s to that of a perfect fluid.

Nonlinear Hydrodynamics and Shock Waves in Quantum Matter

Fermi gases provide a new paradigm for studying nonlinear wave propagation in quantum matter. In contrast to a weakly interacting Bose-Einstein condensate, which is not hydrodynamic in the normal fluid regime above the superfluid transition temperature, strongly interacting Fermi gases are hydrodynamic in both the superfluid and normal fluid regimes. We collide two strongly interacting atomic Fermi gas clouds and observe traveling shock waves, enabling studies in both the dispersive regime, where the quantum kinetic energy is important and in the dissipative regime, where dissipation stablizes the shock wave front. Studies of vortices and measurements the shock wave speed will reveal new properties of shock waves in the strongly interacting gas (Joseph2011).

Radio Frequency Spectroscopy of Quasi-Two-Dimensional Fermi Gases

A reflected CO₂ laser beam forms creates an optical potential comprising periodic pancake-shaped traps, each containing several hundred atoms. As the individual sites are spaced by 5.3 m, site-resolved measurements are possible. [Standing wave FIGURE]. We use radio-frequency spectroscopy to measure pairing interactions in this unique mesoscopic system. In contrast to previous studies in nearly two-dimensional systems, we the transverse Fermi energy in our experiments is comparable to the harmonic oscillator level spacing in the tightly confining direction. In this case, the Fermi energy exceeds the confinement-induced dimer pairing energy even in the strongly interacting regime. Near the Feshbach resonance, we observe unexpected behavior. The spectra are not fit by predictions based on confinement-induced dimer pairs. Instead better fit by assuming transitions between polaronic states in two dimensions (Zhang2012).