Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B8: Spectroscopy of Strongly Interacting Fermi Gases |
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Sponsoring Units: DAMOP Chair: Erich Mueller, Cornell University Room: 414/415 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B8.00001: Observation of Spin-Polarons in a strongly interacting Fermi liquid Invited Speaker: We have observed spin-polarons in a highly imbalanced mixture of fermionic atoms using 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-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 narrow width signals a long lifetime of the spin-polaron, much longer than the collision rate with spin up atoms, as it must be for a proper quasi-particle. The peak position allows to directly measure the polaron energy. The broad pedestal at high energies reveals physics at short distances and is thus ``molecule-like'': It is exactly matched by the spin up spectra. The comparison with the area under the polaron peak allows to directly obtain the quasi-particle weight $Z$. We observe a smooth transition from polarons to molecules. At a critical interaction strength of $1/k_F a = 0.7$, the polaron peak vanishes and spin up and spin down spectra exactly match, signalling the formation of molecules. This is the same critical interaction strength found earlier to separate a normal Fermi mixture from a superfluid molecular Bose-Einstein condensate. The spin-polarons determine the low-temperature phase diagram of imbalanced Fermi mixtures. In principle, polarons can interact with each other and should, at low enough temperatures, form a superfluid of p-wave pairs. We will present a first indication for interactions between polarons. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B8.00002: Photoemission Spectroscopy for Ultracold Atoms Invited Speaker: We perform momentum-resolved rf spectroscopy on a Fermi gas of potassium-40 atoms in the region of the BCS-BEC crossover. This measurement is analogous to photoemission spectroscopy, which has proven to be a powerful probe of excitation gaps in superconductors. We measure the single-particle spectral function, which is a fundamental property of a strongly interacting system and is directly predicted by many-body theories. For a strongly interacting Fermi gas near the transition temperature for the superfluid state, we find evidence for a large pairing gap. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B8.00003: Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms Invited Speaker: Ultracold Fermi atoms allow the realization of the crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC), by varying with continuity the attraction between fermions of different species. In this context, radio-frequency spectroscopy provides a microscopic probe to infer the nature of fermionic pairing. In the strongly-interacting regime, this pairing affects a wide temperature range comprising the critical temperature Tc, in analogy to the pseudo-gap physics for high-temperature superconductors. By including final-state interactions affecting the excited level of the transition, calculations are here reported of radio-frequency spectra of ultracold Fermi atoms with balanced populations, both below and above Tc, and compared with available experimental data. In the superfluid phase below Tc our calculation rests on the use of the BCS-RPA approximation, while in the normal phase above Tc it includes the Azlamazov-Larkin type contribution which is familiar in the theory of ``paraconductivity'' fluctuations in superconductors, besides the density-of-states contribution. In both cases, the limit of a molecular spectrum is correctly recovered in the BEC regime of the crossover. A competition is revealed between pairing-gap effects which tend to push the oscillator strength toward high frequencies away from threshold and final-state effects which tend instead to pull the oscillator strength toward threshold. In addition, an energy scale associated with pairing is extracted from the spectra and related to a universal quantity recently introduced for Fermi gases. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B8.00004: Theory of RF Spectroscopy in the Normal And Superfluid Phases of Ultracold Fermi Gases Invited Speaker: In this talk we present an overview of radio frequency (RF) spectroscopy in the atomic Fermi superfluids, addressing both momentum integrated and momentum resolved experiments. A general purpose of these RF experiments is to extract the pairing gap size and we present several methodologies for accomplishing this. In addition, we discuss the effects of traps, population imbalance, and final state interactions over the entire range of temperatures. By comparing theory and experiment, we show how a broad range of experimental phenomena can be accomodated within the BCS-Leggett description of BCS-BEC crossover. We also briefly touch on commonalities between photoemission in the cuprate superconductors and RF spectroscopy in the ultracold gases. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B8.00005: Spin polarons, Molecules and Twin Peaks in rf spectra of Fermi gases at unitarity Invited Speaker: We examine pairing, molecule, and spin-polaron formation in strongly-interacting Fermi gases and discuss how radio-frequency (RF) spectroscopy can reveal these phenomena. For an unpolarized gas at unitarity, we show how the double-peak structures observed in recent experiments arise due to the inhomogeneity of the trapped gas. The emergence of stable molecules in the BEC regime results in a two-peak structure in the RF spectrum with clearly visible medium effects on the low-energy part of the molecular wavefunction. For the highly-imbalanced case, we show the existence of a well- defined quasiparticle (a spin polaron) on both sides of the Feshbach resonance, we evaluate its lifetime, and we illustrate how its energy may be measured by RF spectroscopy. The main experimental features observed above the critical temperature in the recent experiments are recovered with no fitting parameters. [Preview Abstract] |
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