Munekazu Horikoshi

Measurement of Universal Thermodynamic Functions for a Unitary Fermi Gas

Dissecting Fermion Interactions Electrons, protons, and other building blocks of our universe belong to a class of particles we call fermions. Different interfermion interactions give rise to different forms of matter. In the strongly interacting resonant regime, however, fermionic systems have thermodynamic properties that depend only on the interparticle spacing and scaled temperature. Horikoshi et al. (p. 442; see the Perspective by Marwan) precisely characterize the thermodynamics in this universal regime for a system of ultracold fermionic lithium atoms. Analysis of a large number of trapped-gas density profiles confirms that the results depend neither on trap geometry nor the absolute temperature of the gas. The results are relevant to studies of all strongly interacting fermionic systems, including neutron stars and nuclear matter. Cold Fermi gases are used to study resonant fermion-fermion interactions. Thermodynamic properties of matter generally depend on the details of interactions between its constituent parts. However, in a unitary Fermi gas where the scattering length diverges, thermodynamics is determined through universal functions that depend only on the particle density and temperature. By using only the general form of the equation of state and the equation of force balance, we measured the local internal energy of the trapped gas as a function of these parameters. Other universal functions, such as those corresponding to the Helmholtz free energy, chemical potential, and entropy, were calculated through general thermodynamic relations. The critical parameters were also determined at the superfluid transition temperature. These results apply to all strongly interacting fermionic systems, including neutron stars and nuclear matter.

Nonuniversal Efimov atom-dimer resonances in a three-component mixture of 6Li.

We observed an enhanced atom-dimer loss due to the existence of Efimov states in a three-component mixture of 6Li atoms. We measured the magnetic-field dependence of the atom-dimer loss in the mixture of atoms in state |1> and dimers formed in states |2> and |3>, and found two peaks corresponding to the degeneracy points of the energy levels of |23> dimers and the ground and first excited Efimov trimers. We found that the locations of these peaks disagree with universal theory predictions, in a way that cannot be explained by nonuniversal two-body properties. We constructed theoretical models that characterize the nonuniversal three-body physics of three-component 6Li atoms in the low-energy domain.

Collisional Properties of p-Wave Feshbach Molecules

We have observed p-wave Feshbach molecules for all three combinations of the two lowest hyperfine spin states of 6Li. By creating a pure molecular sample in an optical trap, we measured the inelastic collision rates of p-wave molecules. We have also measured the elastic collision rate from the thermalization rate of a breathing mode which was excited spontaneously upon molecular formation.

Rectified momentum transport for a kicked Bose-Einstein condensate.

We report the experimental observation of rectified momentum transport for a Bose-Einstein condensate kicked at the Talbot time (quantum resonance) by an optical standing wave. Atoms are initially prepared in a superposition of the 0 and -2hkl momentum states using an optical pi/2 pulse. By changing the relative phase of the superposed states, a momentum current in either direction along the standing wave may be produced. We offer an interpretation based on matter-wave interference, showing that the observed effect is uniquely quantum.

Critical temperature and condensate fraction of a fermion pair condensate.

We report on measurements of the critical temperature and the temperature dependence of the condensate fraction for a fermion pair condensate of 6Li atoms. Bragg spectroscopy is employed to determine the critical temperature and the condensate fraction after a fast magnetic field ramp to the molecular side of the Feshbach resonance. Our measurements reveal evidence of level off of the critical temperature and limiting behavior of condensate fraction near the unitarity limit.

Ground-state thermodynamic quantities of homogeneous spin-1/2 fermions from the BCS region to the unitarity limit

We experimentally determined various thermodynamic quantities of interacting two-component fermions at the zero-temperature limit from the Bardeen-Cooper-Schrieffer (BCS) region to the unitarity limit. The obtained results are very accurate in the sense that the systematic error is within 4% around the unitarity limit. Using this advantage, we can compare our data with various many-body theories. We found that an extended

Strong-coupling corrections to ground-state properties of a superfluid Fermi gas

{\it T}

All-optical production of dual Bose–Einstein condensates of paired fermions and bosons with 6Li and 7Li

-matrix approximation, which is a strong-coupling theory involving fluctuations in the Cooper channel, well reproduces our experimental results. We also found that the superfluid order parameter

Appropriate Probe Condition for Absorption Imaging of Ultracold 6Li Atoms

{\it \Delta}

Fast and efficient production of Bose-Einstein condensate atoms based on an atom chip

calculated by solving the ordinary BCS gap equation with the chemical potential of interacting fermions is close to the binding energy of the paired fermions directly observed in a spectroscopic experiment and that obtained using a quantum Monte Carlo method. Since understanding the strong-coupling properties of a superfluid Fermi gas in the BCS-BEC (Bose-Einstein condensation) crossover region is a crucial issue in condensed matter physics and nuclear physics, the results of the present study are expected to be useful in the further development of these fields.