Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S18: Ultracold Fermi Gases 
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Sponsoring Units: DAMOP Chair: Nir Navon, Yale University Room: 277 
Thursday, March 16, 2017 11:15AM  11:27AM 
S18.00001: A relation connecting thermodynamic quantities and transport coefficients in unitary Fermi gases ChihChun Chien, Hao Guo, Weimin Cai, Yan He From kinetic theory it is known that in a scaleinvariant system like an unitary Fermi gas, the sheer viscosity is proportional to the pressure at high temperatures, and their ratio is the relaxation time. This is an example of a relation connecting thermodynamic quantities (the pressure) and transport coefficients (the sheer viscosity). At low temperatures, however, the presence of superfluid calls for a revised relation. By implementing a gaugeinvariant linear response theory, we found that the sheer viscosity is related not only to the pressure and relaxation time, but also to the superfluid density and an additional response function involving a tensor structure of the fluctuations of the Cooper pairs. Incidentally, the additional response function is negligible as the system approaches the ground state. We have tested the relation with and without pairing fluctuations that are crucial in describing the BCSBEC crossover and reached qualitatively the same conclusion. A direct measurement of the relaxation time in quantum gases can be a challenge, and the new relation may be implemented experimentally for determining the relaxation time of unitary Fermi gases. [Preview Abstract] 
Thursday, March 16, 2017 11:27AM  11:39AM 
S18.00002: An Effective Series Expansion to the Equation of State of Unitary Fermi Gases Theja De Silva Using universal properties and a basic statistical mechanical approach, we propose an effective series expansion to the equation of state for unitary Fermi gases. The universal equation of state is written as a series solution to a selfconsistent integral equation where the general solution is a linear combination of Fermi functions. First, by truncating our series solution to four terms with already known exact theoretical inputs at limiting cases, namely the first \emph{three} virial coefficients and using the Bertsch parameter as a free parameter, we find a good agreement with experimental measurements in the entire temperature region in the normal state. This analytical equation of state agrees with experimental data up to the fugacity $z = 18$, which is a vast improvement over the other analytical equations of state available where the agreements is only up to $z \approx 7$. Second, by truncating our series solution to four terms using first \emph{four} virial coefficients, we find the Bertsch parameter $\xi= 0.35$, which is in good agreement with the direct experimental measurement of $\xi = 0.37$. This second form of equation of state shows a good agreement with selfconsistent Tmatrix calculations in the normal phase. [Preview Abstract] 
Thursday, March 16, 2017 11:39AM  11:51AM 
S18.00003: 1D3D Crossover In A SpinBalanced Fermi Gas Melissa C. Revelle, Ben A. Olsen, Jacob A. Fry, Randall G. Hulet We experimentally study the phases of an ultracold twospin component gas of atomic fermions ($^{6}$Li) confined to 1D tubes formed by a 2D optical lattice. Spinimbalanced trapped Fermi gases have been observed to phase separate in both 1D and 3D, but with qualitatively different features\footnote{Y.A. Liao et al., Nature 467, 567 (2010);G. B. Partridge et al., Science 311, 503 (2006); Y. Shin et al., Phys. Rev. Lett. 97, 030401 (2006).}. The difference between the phase separation in these regimes allows for the dimensionality of the system to be determined using phase diagrams. We observed the transition for a 1Dlike to 3Dlike Fermi gas by varying the atomic interactions and the tunneling rate between the 1D tubes. Using the inversion of the phase separation between 1D and 3D, we determined crossover point. By scaling the tunneling rate $t$ with respect to the pair binding energy $\epsilon_{B}$, we observe a collapse of the data and have identified a universal crossover point of $t/\epsilon_{B}=0.025(7)$\footnote{M.C. Revelle et al., arXiv:1605.06986v2 [physics.atomph] (2016)}. [Preview Abstract] 
Thursday, March 16, 2017 11:51AM  12:03PM 
S18.00004: FFLO superfluidity in a spin imbalanced Fermi gas$^{\mathbf{1}}$ Anna L. Marchant, Jacob A. Fry, Yi Jin, Melissa C. Revelle, Randall G. Hulet Ultracold atomic gases confined in optical lattices have proven to be highly versatile, tunable systems, capable of emulating condensed matter systems and realizing novel quantum states of matter. We use a pair of atomic states to create a pseudospin1/2 system and engineer a spin imbalance in the gas, analogous to applying a magnetic field to a superconductor. The atomic gas is confined in a 2D optical lattice, which produces an array of 1D tubes. Both the tunneling between tubes and interactions between atoms can be precisely controlled. We previously identified a universal crossover regime$^{2}$ from 1D to 3Dlike behavior in the phase separation of this spinimbalanced Fermi gas when varying the tunneling in the lattice. This crossover region is expected to be a promising regime in which to observe the elusive polarized superfluid known as FFLO where magnetism is accommodated by the formation of pairs with finite momentum. Here we present our current progress towards the observation of this exotic superfluid state. By compensating the optical potential along the weak axial direction of the lattice we can carry out 1D timeofflight expansion to study the momentum distribution of the spin imbalanced gas and thus search for experimental signatures of the FFLO phase. [1] Supported by the ONR, ARO MURI program, the Welch Foundation, and the NSF [2] M. C. Revelle et al., to be published in Phys. Rev. Lett., arXiv:1605.06986 (2016) [Preview Abstract] 
Thursday, March 16, 2017 12:03PM  12:15PM 
S18.00005: Phases and transport in spin and massimbalanced Fermi mixtures in one dimension Binbin Tian, Yuchi He, Michelle Tomczyk, Anthony TylanTyler, Patrick Irvin, Jeremy Levy, Roger Mong, David Pekker We study the interplay of both species (spin and transverse band index) and mass imbalance in a mixture of two or more species of fermions with attractive interactions in one dimension. Previous theoretical and experimental efforts have shown the existence of a FuldeFerrellLarkinOvchinnikov (FFLO) phase for the case of two species with equal mass, in addition to the fully paired and fully polarized phases. For the unequal mass case, there are signatures of trimer phases as well. We use DMRG to explore the rich possibilities of quantum phases and their transport signatures for the cases of two and more species of Fermions as we vary the interaction strengths and mass imbalances. With this we can gain insights into ongoing experiments with sketched nanowires in LAO/STO and ultracold atoms confined to onedimensional tubes. [Preview Abstract] 
Thursday, March 16, 2017 12:15PM  12:27PM 
S18.00006: pwave superfluid with imbalanced atomic population in harmonic trap Ammar Kirmani, Khandker Quader, Maxim Dzero We consider the problem of $p$wave superfluid pairing in an atomic Fermi gas across Feshbach resonance for imbalanced populations in presence of an optical trap. For our harmonic trapping potential, we employ Local Density Approximation {\bf (LDA)} through chemical potential. In twochannel mean field approximation pairing model, we show that depending on the distance from the trap's center, the pwave superfluid state will have the lowest energy. The ground state order parameter configuration we find is not invariant under parity and timereversal symmetry operations. We also present phase diagrams and density profiles in onechannel model for our singlet pwave pairing and show that the center of trap is occupied by pwave superfluid. The work of A. K. and M. D. was financially supported by the National Science Foundation Grant No. DMR1506547. [Preview Abstract] 
Thursday, March 16, 2017 12:27PM  12:39PM 
S18.00007: Eigenstate entanglement entropy in spinless fermion systems Lev Vidmar, Lucas Fabian Hackl, Eugenio Bianchi, Marcos Rigol The entanglement entropy of ground states of spinless fermion systems has been extensively studied in recent years. Here, we focus on its properties in the entire spectrum, which have remained largely unexplored. We discuss evidence of the fact that, for any subsystem that is a finite fraction of the entire system, the average eigenstate entanglement entropy is always smaller than that of the infinitetemperature thermal state. We also study the dependence of the entanglement entropy on the subsystem size. [Preview Abstract] 
Thursday, March 16, 2017 12:39PM  12:51PM 
S18.00008: Eigenstate entanglement entropy in spinless fermion systems II Lucas Hackl, Lev Vidmar, Eugenio Bianchi, Marcos Rigol The entanglement entropy of ground states of spinless fermion systems has been extensively studied in recent years. Here, we focus on its properties in the entire spectrum, which have remained largely unexplored. Using recently developed analytical techniques, we study the dependence of the entanglement entropy on the subsystem size for large classes of eigenstates. We also discuss the corresponding statistical ensemble that one uses to find the average eigenstate entanglement entropy. [Preview Abstract] 
Thursday, March 16, 2017 12:51PM  1:03PM 
S18.00009: Particle partition entanglement of onedimensional spinless fermions Hatem Barghathi, Emanuel CasianoDiaz, Adrian Del Maestro We investigate the scaling of the R\'{e}nyi entanglement entropies for a particle bipartition of interacting spinless fermions in one spatial dimension. In the TomonagaLuttinger liquid regime, we calculate the second R\'{e}nyi entanglement entropy and show that the leading order finitesize scaling is equal to a universal logarithm of the system size plus a nonuniversal constant. Higherorder corrections decay as a powerlaws in the system size with exponents that depend only on the Luttinger parameter. We confirm the universality of our results by investigating the one dimensional $tV$ model of interacting spinless fermions via exactdiagonalization techniques. The resulting sensitivity of the particle partition entanglement to boundary conditions and statistics points to its utility in future studies of novel quantum liquids. [Preview Abstract] 
Thursday, March 16, 2017 1:03PM  1:15PM 
S18.00010: Density Assisted Tunneling of Fermions in Optical Lattices Vito Scarola, Wenchao Xu, William Morong, Hoi Hui, Brian DeMarco The Hubbard model is a simple approximation to interacting particles tunneling in a lattice. Interactions beyond the ordinary Hubbard term can generate unconventional states of matter. We discuss a scheme that uses stimulated Raman transitions of fermions in optical lattices to introduce density assisted tunneling, an interaction that goes beyond the ordinary Hubbard interaction. We model stimulated Raman processes as a way to dynamically generate density assisted tunneling. We compute observables in the ensuing dynamics. We compare the observables with experiments that reveal evidence for density assisted tunneling. [Preview Abstract] 
Thursday, March 16, 2017 1:15PM  1:27PM 
S18.00011: Effect of impurity scattering on pairing and superfluidity in ultracold atomic Fermi gases on a 3D lattice Qijin Chen, Yanming Che We study the effect of nonmagnetic impurities in twocomponent atomic Fermi gases on pairing and superfluidity on a 3D optical lattice. For short range swave pairing, we find that while Anderson theorem holds for low density weak impurities in the BCS regime, it manefestly breaks as the density and impurity strength grow large. Meanwhile, this leads to a quantum critical phase transition between superconductor and insulator at zero $T$. As the pairing strength grows towards unitary regime, pairing is very robust and hard to destroy with nonmagnetic impurities. This result is close to the case in 3D continuum, but in sharp contrast to a dwave or pwave case, for which the superfluidity is much more sensitive to impurity densities. Preliminary result in the presence of a population imbalance will also be briefly mentioned. References: Q.J. Chen and J.R. Schrieffer, Phys. Rev. B 66, 014512 (2002). Yanming Che and Q.J. Chen, arXiv:1608.02110. [Preview Abstract] 
Thursday, March 16, 2017 1:27PM  1:39PM 
S18.00012: Unconventional Phases of Attractive Fermi Gases in Synthetic Hall Ribbons Sudeep Ghosh, Sebastian Greschner, Umesh Yadav, Tapan Mishra, Matteo Rizzi, Vijay B. Shenoy A novel way to produce quantum Hall ribbons in a cold atomic system is to use $M$ hyperfine states of atoms in a $1$D optical lattice to mimic an additional ``synthetic dimension''. A notable aspect here is that the SU($M$) symmetric interaction between atoms manifests as ``infinite ranged'' along the synthetic dimension. We study the many body physics of fermions with attractive interactions in this system. We use a combination of analytical field theoretic and numerical density matrix renormalization group (DMRG) methods to reveal its rich ground state phase diagram, including unconventional phases such as squished baryon fluids. Remarkably, changing the parameters entails novel crossovers and transitions, e. g., we show that increasing the magnetic field (that produces the Hall effect) may convert a ``ferrometallic'' state at low fields to a ``squished baryon superfluid''(with algebraic pairing correlations) at high fields. We also show that this system provides a unique o pportunity to study quantum phase separation in a multiflavor ultracold fermionic system. [Preview Abstract] 
Thursday, March 16, 2017 1:39PM  1:51PM 
S18.00013: Finitetemperature valencebondsolid transitions and thermodynamic properties of interacting SU(2$N)$ Dirac fermions Yu Wang We investigate the SU(2$N)$ symmetry effects with 2$N$ \textgreater 2 on the twodimensional interacting Dirac fermions at finite temperatures, including the valencebondsolid transition, the Pomeranchuk effect, the compressibility and the uniform spin susceptibility, by performing the determinant quantum Monte Carlo simulations of the halffilled SU(2$N)$ Hubbard model on a honeycomb lattice. The columnar valencebondsolid (cVBS) phase only breaks the threefold discrete symmetry, and thus can survive at finite temperatures. The disordered phase in the weak coupling regime is the thermal Dirac semimetal state, while in the strong coupling regime it is largely a Mott state in which the cVBS order is thermally melted. The calculated entropytemperature relations for various values of the Hubbard interaction $U$ show that, the Pomeranchuk effect occurs when the specific entropy is below a characteristic value of $S^{\mathrm{\ast }}$  the maximal entropy per particle from the spin channel of local moments. The SU(2$N)$ symmetry enhances the Pomeranchuk effect, which facilitates the interactioninduced adiabatic cooling. Our work sheds new light on future explorations of novel states of matter with ultracold largespin alkaline fermions. [Preview Abstract] 
(Author Not Attending)

S18.00014: Single shot imaging of trapped Fermi gas  Pauli crystals Mariusz Gajda, Jan Mostowski, Tomasz Sowinski, Magdalena ZaluskaKotur Atomic gas microscopes allow for direct monitoring of atoms with a resolution of the order of hundreds of nanometers. Inspired by this experimental achievements we study on a theoretical ground a singleshot picture of a twodimensional ideal Fermi gas in a harmonic trap. We show that identical fermions arrange themselves in spectacular geometric structures although no mutual interaction is present. This is because the indistinguishability of fermions prevents them from being at the same location. These unexplored geometric structures, Pauli crystals, emerge repeatedly in singleshot pictures of the manybody system. To observe the Pauli crystals one has to detect positions of all N particles. Such a detection, however, will never correspond to the pure geometry of the Pauli crystal because of quantum fluctuations. We show how to extract the pattern from a collection of the measured noisy structures. [Preview Abstract] 
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