Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H31: Weakly and Strongly Interacting Fermions 
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Sponsoring Units: DAMOP Chair: Marcos Rigol, Georegetown University Room: E141 
Tuesday, March 16, 2010 8:00AM  8:12AM 
H31.00001: Spectral function and RF spectroscopy of dilute Fermi gases William Schneider, Mohit Randeria The spectral function $A(k,\omega)$ is a quantity of fundamental interest in manybody physics and its occupied part can now be directly measured in angleresolved RF spectroscopy experiments. We describe a surprising universal feature [1] in {\it all} dilute Fermi gases: for $k \gg k_F$, there is incoherent spectral weight centered about {\it negative} $k^2/2m$ in a range of energies that scales linearly with k. The total weight of this feature is exactly $n(k) \sim C / k^4$, where $C$ is Tan's contact. This ``bending back'' in the dispersion, while natural for superfluid excitations, is quite unexpected for normal gases, such as the hardsphere Fermi gas and the Fermi liquid ground state of the highly spinimbalanced, attractive gas near unitarity. We also argue that in the superfluid, this feature is dominated by interaction effects which do not reflect the pairing gap; e.g., the spectral weight in this feature is not exponentially small in the BCS limit, but rather a power law in $k_F a$. This incoherent spectral feature has observable consequences for both angleresolved and usual RF spectroscopy. We will also describe observable properties of the lowenergy spectral function in both the normal and superfluid ground states. [1] W. Schneider and M. Randeria, arXiv:0910.2693 [Preview Abstract] 
Tuesday, March 16, 2010 8:12AM  8:24AM 
H31.00002: The phase diagram of ultracold fermionic atoms across the BCSBEC crossover in the presence of disorder Parag Ghosh, Predrag Nikolic We study the effect of disorder in a twocomponent attractive fermionic system with nearly resonant interactions placed in a periodic potential. Treating disorder as a weak perturbation we first obtain Born correction to thermodynamic potential and explore the conventional phenomena such as suppression of superfluidity and enhancement of insulating states. Then, going beyond Born approximation we investigate the possibility of obtaining unconventional paired states, namely a Bose glass of Cooper pairs. Such a state is expected to arise from efficient pairing of lowenergy fermions localized by the disorder potential, but lacks longrange order due to strong phase fluctuations. We discuss experimental signatures of states in our phase diagram. [Preview Abstract] 
Tuesday, March 16, 2010 8:24AM  8:36AM 
H31.00003: Viscosity in dilute Fermi gases: spectral functions and sum rules Edward Taylor, Mohit Randeria Recently there has been considerable interest in the viscosity of strongly interacting systems, especially in regimes where the mean free path is of the order of the interparticle spacing and a quasiparticle description breaks down. We derive exact results for the spectral functions and sum rules for the shear and bulk viscosities, ${\rm Re}\ \eta(\omega)$ and ${\rm Re}\ \zeta(\omega)$, of strongly interacting Fermi and Bose systems. The zerofrequency limits of these functions give the viscosities measured in hydrodynamic damping experiments. For a twocomponent Fermi gas, we find the exact sum rules $\int^{\infty}_{0}d\omega\; {\rm Re}\eta(\omega)/{\pi}={\varepsilon}/{3}{2\langle V\rangle}/{5}$ and $\int^{\infty}_{0}d\omega\; {\rm Re}\ \zeta(\omega)/{\pi} ={(\varepsilon +P)}/{3}{\rho c^2_{{s}}}/{2}$, where $\varepsilon$ is the internal energy density, $\langle V\rangle$ the potential energy density, $P$ the pressure, $\rho$ the mass density, and $c_{{s}}$ the speed of sound. These results are valid at all temperatures and for all values of $1/(k_F a)$ through the BCSBEC crossover. We will discuss the implications of these sum rules including universal highfrequency tails of the spectral functions. [Preview Abstract] 
Tuesday, March 16, 2010 8:36AM  8:48AM 
H31.00004: Thermodynamics of the unitary Fermi gas Kris Van Houcke, Felix Werner, Evgeny Kozik, Nikolay Prokof'ev, Boris Svistunov Current experiments with ultracold fermions open the possibility of exploring the phasediagram of models that are hard to tackle with present quantum manybody theory, such as the Hubbard model or the problem of BECBCS crossover. With such quantum emulators within reach, (numerical) tools for determining the unbiased equation of state are evermore important. Diagrammatic Monte Carlo (DiagMC) is a generic technique, capable of solving macroscopic quantum manybody systems. We will discuss DiagMC, and present results for the unitary Fermi gas. We obtain universal thermodynamic functions and compare with experimental results. [Preview Abstract] 
Tuesday, March 16, 2010 8:48AM  9:00AM 
H31.00005: Measuring the Contact in the BCSBEC Crossover Tara Drake, John Gaebler, Jayson Stewart, Deborah Jin We present results for the contact of a strongly interacting Fermi gas of $^{40}$K atoms. We measure the contact by fitting the tails of the momentum distribution and RF lineshape of the gas. We compare this result to our measurements of the generalized virial theorem. [Preview Abstract] 
Tuesday, March 16, 2010 9:00AM  9:12AM 
H31.00006: Probing the homogeneous spectral function of a trapped atomic Fermi gas using momentum resolved rf spectroscopy Qijin Chen Radio refequency spectroscopy is arguably the most direct probe for measuring the excitation gap in atomic Fermi gases. Recently, the Jin group developed momentum resolved rf spectroscopy, which has been demonstrated to be able to probe the averaged spectral function in a Fermi gas. However, the usefulness of this technique was limited by the inhomogeneity over the entire trap. On the other hand, the tomography technique developed by the Kettele group provides spatial resolution but not spectral information. Here we propose that using highly population imbalances, one can essentially measure the spectral function of a homogeneous Fermi gas using momentum resolved rf spectroscopy. We will present theoretically calculated spectral functions and spectral intensity maps for various cases over the entire BCSBEC crossover. References: Q.J. Chen and K. Levin, Phys. Rev. Lett. 102, 190402 (2009). Y. He, C.C. Chien, Q.J. Chen, and K. Levin, Phys. Rev. A 77, 011602(R) (2008). Y. He, Q.J. Chen, and K. Levin, Phys. Rev. A 72, 011602(R) (2005). [Preview Abstract] 
Tuesday, March 16, 2010 9:12AM  9:24AM 
H31.00007: Single impurity in cold Fermi superfluids Lei Jiang, Han Pu Although nonmagnetic impurities do not change macroscopic properties of the system in swave superfluids, the microscopic local properties will change such as local density of states. Spatially resolved radio frequency spectroscopy gives us a tool to detect local properties of ultracold Fermi superfluids and hence can be used to investigate the effects of impurities. Here we study these local properties of Fermi superfluids with a single impurity. We calculate the radio frequency spectrum using the Tmatrix formalism. This impurity can be either potential scattering or scattering with other atoms; either magnetic or nonmagnetic. There may exist bound or antibound states near the impurity site. We may use single impurity to detect Fermi pairing in radio frequency spectrum in population balanced system. We also compare magnetic impurity spectra with nonmagnetic ones. [Preview Abstract] 
Tuesday, March 16, 2010 9:24AM  9:36AM 
H31.00008: Observation of pseudogap phase in a strongly interacting Fermi gas John Gaebler, J.T. Stewart, T.E. Drake, D.S. Jin We use atom photoemission spectroscopy to study the singleparticle states of a Fermi gas in the BCSBEC crossover. Our measurements reveal a BCSlike dispersion with backbending that persists well above the transition temperature for pair condensation. This strongly supports the existence of incoherent, or uncondensed, manybody pairst at temperatures above the superfluid phase transition, which represents a significant departure from conventional BCS theory. This demonstration that pseudogap physics can emerge in a strongly interacting Fermi gas without the need for, or even the possibility of, explanations that rely on complex material properties should be considered in trying to understand the pseudgap phase observed in high Tc superconductors. [Preview Abstract] 
Tuesday, March 16, 2010 9:36AM  9:48AM 
H31.00009: Shorttime dynamics of an ultracold Fermi gas in radiofrequency spectroscopy Chun Kit Chung, C. K. Law We formulate the quantum dynamics of radiofrequency (rf) spectroscopy problem of an interacting ultracold Fermi gas by timedependent variational principle. We consider the initial state as a BardeenCooperSchrieffer (BCS) state describing pairing between two hyperfine spin levels. An rf field is used to transfer atoms to a third level. Dynamical equations obtained from the variational approach are shown to be derivable from the timedependent Bogoliubovde Gennes mean field theory. We show that the shorttime dynamics is governed by a set of linear equations, whose spectrum contains a discrete eigenvalue signifying a pair bound state formation in the final scattering channel. Such a bound state causes an oscillation pattern of order parameters in time. [Preview Abstract] 
Tuesday, March 16, 2010 9:48AM  10:00AM 
H31.00010: Theory of photoemission spectroscopy of Fermi gases in the BCSBEC crossover regime above Tc Shunji Tsuchiya, Ryota Watanabe, Yoji Ohashi We address recent photoemission experiments on ultracold Fermi gases by the JILA group. We investigate strongcoupling effects on singleparticle excitation spectra of Fermi gases in the BCSBEC crossover above Tc. We calculate the momentumresolved tunneling current into another hyperfine state, as well as the singleparticle spectral weight (SW) and density of states (DOS), within the Tmatrix approximation including effects of trapping potential. We clarify the spatial and temperature dependence of SW and DOS, and find that they exhibit the pseudogap behavior around the center of the trap. We compare the spatially averaged spectral weight with the measured excitation spectra, and discuss interpretations of experimental results. [Preview Abstract] 
Tuesday, March 16, 2010 10:00AM  10:12AM 
H31.00011: Temperature Dependence of Dynamic Structure Factors in Ultracold Fermi Gases: Bragg Scattering From BCS to BEC Hao Guo, ChihChun Chien, Kathryn Levin While the behavior of the static and dynamic structure factors in the ground state of a Fermi gas superfluid are well studied, little is known about the behavior at general temperatures $T$. This is an important issue: densitycorrelation function measurements of these structure factors through, for example, Bragg scattering, will indicate the transition temperature $T_c$, which is often difficult to identify. Equally importantly they can help to establish whether a pseudogap is present in the normal state. We show how to compute these structure factors for a homogeneous system using a natural extension of the BCSLeggett ground state to finite temperatures. We demonstrate how to include gauge invariance in a fully consistent fashion, along with pairing correlations. Our results should provide important predictions for future experiments. [Preview Abstract] 
Tuesday, March 16, 2010 10:12AM  10:24AM 
H31.00012: Pseudogap phase of ultracold Fermi gases at unitarity: a comparative study ChihChun Chien, Hao Guo, Yan He, Kathryn Levin The pseudogap phase in high temperature superconductors (HTSCs) has been an intensely studied subject but its origin remains a mystery. Similar pseudogap phase has been observed in ultracold fermions in BCSBoseEinstein condensation (BEC) crossover. We search for signatures of pseudogap in an unitary Fermi gas by studying spectral function, density of states, and radiofrequency (RF) spectroscopy from two widely used finitetemperature BCSBEC crossover theories. The two theories are based on Nozieres SchmittRink theory and BCSLeggett theory, respectively. To demonstrate the similarity between the pseudogap phases in HTSCs and a unitary Fermi gas, we test some criteria for pseudogap in HTSCs on the two crossover theories. Although evidence of a pseudogap in a unitary Fermi gas can be found in both theories, its behavior differs quantitatively. We compare theoretical and experimental RF spectra and show that the extended BCSLeggett theory has the advantage that it can be generalized to study RF spectrum across the superfluid transition temperature or polarized Fermi gases. [Preview Abstract] 
Tuesday, March 16, 2010 10:24AM  10:36AM 
H31.00013: Propagation of second sound in a superfluid Fermi gas in the unitary limit Emiko Arahata, Tetsuro Nikuni We study sound propagation in a uniform superfluid gas of Fermi atoms in the unitary limit. The existence of normal and superfluid components leads to appearance of two sound modes in the collisional regime, referred to as first and second sounds. The second sound is of particular interest as it is a clear signal of a superfluid component. Using Landau's twofluid hydrodynamic theory, we calculate hydrodynamic sound velocities and these weights in the density response function. The latter is used to calculate the response to a sudden modification of the external potential generating pulse propagation. The amplitude of a pulse which is proportional to the weight in the response function is calculated, on the basis of the approach of Noz\`eres and SchmittRink for the BCSBEC crossover. We show that, in a superfluid Fermi gas at unitarity, the secondsound pulse is excited with an appreciate amplitude by density perturbations. [Preview Abstract] 

H31.00014: ABSTRACT WITHDRAWN 
Tuesday, March 16, 2010 10:48AM  11:00AM 
H31.00015: Inhomogeneous superfluid phases of spatially separated trapped fermions Kuei Sun, Julia S. Meyer, Daniel E. Sheehy, Smitha Vishveshwara Cold atom experiments that separately trap two species of interacting fermionic atoms offer the possibility of controlling pairing by adjusting the separation between the two trapping potentials. A nonzero separation between the two traps is effectively equivalent to a spatiallydependent local chemical potential difference for the two species. We explore potential inhomogeneous FFLOtype superfluid phases within this setup as a function of interaction strength and trap separation. [Preview Abstract] 
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