Session S2: Focus Session: Spectroscopy of Strongly Interacting Fermi Gases

TBA

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Universal Relations for the Strongly-interacting Fermi Gas

A set of universal relations between various properties of any few-body or many-body system consisting of fermions with two spin states and a large but finite scattering length have been derived by Shina Tan. They involve an extensive quantity called the contact that measures the density of pairs of fermions with different spins and very small separations. We present simple derivations of these universal relations using quantum field theory methods, namely renormalization and the operator product expansion. We identify the contact density as the expectation value of a local operator that annihilates and creates a pair at a point.   

Measurements of the Paired Fraction in the BEC-BCS Crossover

Pairing in fermionic systems is the essential ingredient of superfluidity and superconductivity. I will report quantitative measurements of the paired fraction of a two-spin Fermi gas of $^6$Li atoms as a function of interaction and temperature. The paired fraction is determined by tuning a laser probe to resonance between the paired state and an excited molecular triplet level. A transition to the molecular state leads to a detectable loss of atoms, as in a previous experiment where the closed-channel fraction was measured by driving transitions to a molecular singlet level\footnote {G.B. Partridge, K.E. Strecker, R.I. Kamar, M.W. Jack, and R.G. Hulet PRL \textbf{95} 020404 (2005).}. Depletion of correlated pairs occurs rapidly, and is easily distinguished from photoassociation of unpaired atoms. By driving the dominant triplet transition, the rate of excitation can be much faster than the time scale for pair reformation. This method can be used to quantitatively explore ``preformed'' pairing that occurs above T$_c$, a phenomenon reminiscent of high-temperature superconductors.   

Observation of Spin Polarons in a Tunable Fermi Liquid of Ultracold Atoms

We have observed spin polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms via tomographic RF spectroscopy. Feshbach resonances allow to freely tune the interactions between the two spin states involved. A single spin down atom immersed in a Fermi sea of spin up atoms can do one of two things: For strong attraction, it can form a molecule with exactly one spin up partner, but for weaker interaction it will spread its attraction and surround itself with a collection of majority atoms. This spin down atom dressed with a spin up cloud constitutes the spin- or Fermi polaron. We have observed a striking spectroscopic signature of this quasi-particle for various interaction strengths, a narrow peak in the spin down spectrum that emerges above a broad background. The spectra allow us to directly measure the polaron energy and the quasi-particle residue $Z$. The polarons are found to be only weakly interacting with each other, and can thus be identified with the quasi-particles of Landau's Fermi liquid theory. At a critical interaction strength, we observe a transition from spin one-half polarons to spin zero molecules. At this point the Fermi liquid undergoes a phase transition into a superfluid Bose liquid.   

Photoemission Spectroscopy of an Ultra Cold Fermi Gas

We report on our latest results using photoemission spectroscopy to directly probe the elementary excitations and energy dispersion of a strongly interacting Fermi gas of atoms. In these photoemission experiments, an rf photon ejects an atom from our strongly interacting system via a spin-flip transition to a weakly interacting state. This new measurement technique for ultracold atom gases, like photoemission spectroscopy for electronic materials, directly probes low energy single-particle excitations and thus can reveal excitation gaps and/or pseudogaps.   

Three-body Recombination of Fermionic Atoms with Large Scattering Lengths

The 3-body recombination rate at threshold for fermions with three spin states and large scattering lengths is calculated in the zero-range approximation. The only parameters in this limit are the three scattering lengths and the Efimov parameter, which can be complex valued. Semi-analytic results are obtained for the cases of negative scattering lengths, two of which are equal. The general result is applied to the three lowest hyperfine states of Lithium-6 atoms in regions of the magnetic field in which the three scattering lengths are all large and negative. Comparisons with recent experiments provide indications of loss features associated with Efimov trimers near the 3-atom threshold.   

Detection of Fermi Pairing by Electromagnetically Induced Transparency

Fermi pairing in the strongly interacting degenerate Fermi gas of two hyperfine spin states lies at the heart of many exotic phases that may shed light on long-standing problems in many branches of physics.~ The probe spectroscopy in an electromagnetically induced transparency, where one of the spin states is coupled to the ground state via an excited state by a pair of Raman laser fields, is proposed as quite a genetic tool in providing valuable insights into the nature of Fermi paring.~ This technique has the capability of allowing the local spectroscopic response to be determined in a nondestructive manner.~ EIT spectra in the presence of both BCS and pseudo gaps are presented. A dressed state picture is constructed to facilitate a simple physical explanation, based on quantum interference, to the Fermi pairing signature in these EIT spectra.