Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H45: Exotic Quantum Phases in Optical Lattices: FFLO, P-band Physics, and Beyond |
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Sponsoring Units: DAMOP Chair: Marcos Rigol, Georgetown University Room: A310 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H45.00001: Stable Fulde-Ferrell-Larkin-Ovchinnikov pairing states in 2D and 3D optical lattices Zi Cai, Yupeng Wang, Congjun Wu We present the study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states in the $p$-orbital bands in both two and three-dimensional optical lattices. Due to the quasi one-dimensional band structure which arises from the unidirectional hopping of the orthogonal $p$-orbitals, the pairing phase space is not affected by spin imbalance. Furthermore, interactions build up high dimensional phase coherence which stabilizes the FFLO states in 2D and 3D optical lattices in a large parameter regime in phase diagram. These FFLO phases are stable with imposing the inhomogeneous trapping potential. Their entropies are comparable to those of the normal states at finite temperatures. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H45.00002: Robust Larkin-Ovchinnikov-Fulde-Ferrell phases in a wide class of lattice models Meng Cheng, Christopher Varney, Kai Sun, Victor Galitski We consider BCS pairing of fermions on lattice whose normal state breaks both time-reversal and spatial inversion symmetries. Due to the asymmetric band structure, unusual pairing states exist: Cooper pairs condense at finite momentum, which is known as the Fulde- Ferrel-Larkin-Ovchinnikov (FFLO) state. A one-dimensional lattice model of spinless fermions is studied in detail and two types of FFLO states are found: (1) a FF state with spontaneous supercurrent and (2) a nodeless LO state where the amplitude of order parameter oscillates. This conclusion is obtained via mean-field theory, bosonization, and exact diagonalization. The transition between the two phases can be tuned by the filling. We also find that the FF state is a topological superconductor. We further consider a generalization to two dimensions, where similar physics is realized. [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H45.00003: Spectral Functions of FFLO states in coupled chains Nandini Trivedi, Karim Bouadim, Yen-Lee Loh, Valery Rousseau Polarized Fermi gases hold the possibility of an exotic and fragile modulated superfluid known as a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Quasi-one-dimensional systems of ultracold fermions are the ideal place to look for FFLO physics. Using various methods [1] (including determinant quantum Monte Carlo, stochastic Green function, and Bogoliubov-de Gennes methods), we study the correlation functions and quantum dynamics of polarized Fermi gases in single chains and coupled chains. Our results indicate that fluctuating domain walls lead to spectral weight near the Fermi energy in the spin-resolved density of states, that are a signature of Andreev reflections and fluctuating bound states. We derive bounds for the optimal interchain coupling to maximize the critical temperature of the FFLO state, in order to aid detection of these FFLO states in cold atom experiments [2].\\[4pt] [1] Y.-L. Loh and N. Trivedi, Phys. Rev. Lett. {\bf 104}, 165302 (2010).\\[0pt] [2] Y-an. Liao et. al Nature {\bf 467}, 567-569 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H45.00004: Understanding the Mass-Imbalanced Highly-Polarized Fermi Gases Michael Kolodrubetz, Bryan Clark The phase diagram of spin-polarized single atomic species Fermi gases has been well-studied theoretically and experimentally. However, cold gases containing multiple atomic species open up the possibility of seeing more exotic states. Recent variational calculations (arXiv:1002.0101v2 [cond-mat.quant-gas]) suggest a complicated phase diagram for a light impurity interacting via a short-range potential with a sea of heavier fermions. In particular, at large mass ratio the polaron is expected to give way to more complicated many-body bound states, such as the trimer or the FFLO molecule. We extend these results beyond this variational ansatz, sampling over many-body states with an arbitrary number of particle-hole pairs. We will discuss the phase diagram resulting from these simulations, including implications for the stability of the trimer and FFLO phases. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H45.00005: Spectroscopy of the soliton lattice formation in quasi-one-dimensional fermionic superfluids with population imbalance Roman Lutchyn, Maxim Dzero, Victor Yakovenko Motivated by recent experiments in low-dimensional trapped fermionic superfluids we study quasi-1D superfluid with imbalanced populations between two hyperfine states and analyze its properties using the exact mean field solution for the order parameter. When population imbalance exceeds some critical value the superfluid order parameter develops spatial inhomogeneities and can be described by a soliton lattice formation. Emergence of the soliton lattice is accompanied by the formation of the spin density wave with the majority fermions residing at the points in space where Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) order parameter vanishes. We show that the presence of the spin density wave leads to the formation of the band of the ``subgap states,'' which serves as a hallmark of the quasi-1D FFLO state. We employ the soliton lattice description to discuss the possibilities for the experimental detection of the quasi-1D FFLO phase: elastic and inelastic optical Bragg scattering experiments and radio-frequency spectroscopy. We demonstrate that these measurements allow one to extract necessary information about the inhomogeneous superfluid phase to unambiguously identify quasi-1D FFLO state. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H45.00006: $(2k_F, 2k_F)$ density-wave orders of interacting p-orbital fermions in square optical lattice Zixu Zhang, W. Vincent Liu We study instabilities of spinless fermionic atoms in the p- orbital bands in two dimensional optical lattices at non- integer filling against interactions. Stripe charge-density- wave or orbital-density-wave orders are found for attractive and repulsive interactions, respectively. A surprising result is that the superfluid phase, usually expected of attractively interacting fermions, is less energetically favored. Nesting quasi-one-dimensional Fermi surfaces in such systems are independent of filling, which ensures that the stripe density- wave orders occur in a large parameter regime. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H45.00007: $U(1) \times Z_2$ transition from the Mott insulator to $p_x+ip_y$ Bose-Einstein superfluid phase Xiaopeng Li, Erhai Zhao, W. Vincent Liu Motivated by the recent experiment on p-band bosons in optical lattices [arXiv:1006.0509 (2010)], we study theoretically the quantum phases and phase transition of a two-dimensional extended Bose-Hubbard model with p-orbital degrees of freedom. The system features a novel superfluid phase with transversely staggered orbital current at weak interaction, and a Mott insulator phase with antiferro-orbital order at strong coupling and commensurate filling. We derive an effective theory from a microscopic model to describe the quantum phase transition from Mott to superfluid phase. We also calculate the excitation spectra near quantum critical point and find two gapless modes away from the Mott tip but four gapless modes at the tip point. We describe how the phase coherence builds up in the Mott regime when approaching the critical point. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H45.00008: Induced $p$-wave superfluidity at unitarity in strongly imbalanced Fermi gases Kelly Patton, Daniel Sheehy We compute the induced interaction among the majority spin-up fermions, due to the presence of the minority spin-down fermions, in a population imbalanced Fermi gas. This interaction leads to an instability of the spin-polaron Fermi liquid, favoring a $p$-wave superfluid. For the majority component, near unitarity, the transition temperature is found to be within experimental reach, of order a few percent of the Fermi energy. As a probe of this phase, the radio-frequency spectroscopic line-shape is calculated for the $p_{x}+ip_{y}$ ground state. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H45.00009: Polaron Metastability Kayvan Sadegzadeh We investigate the metastability associated with the first order transition from normal to superfluid phases along the BEC-BCS crossover in partially polarised Fermi gases. The momentum thresholds and rates of key decay processes involved are presented in the context of the system's phase diagram, together with metastability regions. In the limit of a single polaron, this region extends from the interaction strength at which a polarised phase of molecules becomes the groundstate ($\frac{1}{k_{F\uparrow}a} 0.73$), to the value of the crossing point from a single polaron to molecule groundstate ($\frac{1}{k_{F\uparrow}a} 0.9$). Finally, we propose experiments to explore the metastability of this Fermi liquid and the various decay processes, and to observe the $\frac{1}{k_{F\uparrow}a} 0.9$ value. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H45.00010: Possibility of $\pi$-Josephson junction and spontaneous current in a spin-polarized Fermi gas Takashi Kashimura, Shunji Tsuchiya, Yoji Ohashi We theoretically propose an idea to realize a $\pi$-phase in a superfluid Fermi gas, where the phase of the superfluid order parameter differs by $\pi$ across a Josephson junction. When a weak nonmagnetic potential barrier is embedded in a superfluid Fermi gas with population imbalance ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of atoms with pseudospin $\sigma=\uparrow, \downarrow$), this barrier may be {\it magnetized} in the sense that some of excess atoms $N_\uparrow-N_\downarrow>0$ are localized around it. This magnetic barrier behaves like a {\it ferromagnetic junction} discussed in superconductivity literature, which twists the phase of superfluid order parameter by $\pi$. We confirm this idea by solving an attractive Hubbard model within the mean-field theory at $T=0$. We also show that, when this ferromagnetic barrier is realized in a ring-shaped (or torus) trap, the system becomes the so-called $\pi$-Josephson junction, where spontaneous circulating current flows due to the phase twist at the juntion. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H45.00011: Many-body spectral moment sum rules for the Bose Hubbard model James Freericks, Volodomyr Turkowski, Hulikal Krishnamurthy Exact results for many-body interacting systems are rare. Here we derive a series of exact results for the single-band Bose-Hubbard model. In particular, we derive spectral moment sum rules for the Green's functions of the Bose-Hubbard model. Unlike the fermionic sum rules, the bosonic ones depend on complicated expectation values of the bosons that go beyond just needing to know the local particle density. Nevertheless, they can be used to benchmark the quality of different numerical calculations of spectral functions. These sum rules hold with arbitrary values of the interaction strength and even into nonequilibrium situations, similar to what is seen for the fermionic case. We present some case studies comparing the exact moments to those found with other numerical techniques like the VCA approximation. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H45.00012: Bosonic Hubbard-Holstein model and its realization in optical lattices Man-Hong Yung, Kuei Sun, Alan Aspuru-Guzik The Hubbard-Holstein (HH) model describes the interplay between the Coulomb interaction and the electron-phonon coupling for fermionic systems. Motivated by the recent experimental progresses in optical lattices, we investigate a bosonic version of the HH model, where the two competing many-body interactions of the HH model become a bosonic two-body interaction and a boson-phonon coupling. In the regime of weak boson-phonon coupling, the mean-field phase diagram shows that overall effects of the phonons is to expand the domain of superfluidity. This bosonic Hubbard-Holstein (BHH) model can be realized in a pair of overlapping optical lattices, where bosonic particles trapped in one optical lattice are perturbed by more massive particles trapped in the other lattice. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H45.00013: Number Density Distributions of Ultracold Bosons in 3D Optical Lattices Joe Garrett, Eric Duchon, Nandini Trivedi We calculate the probability, $P(n)$, of finding $n$ bosons at a site and the probability of hopping in a uniform optical lattice as a function of the temperature, $T$, and the repulsive interaction between bosons, $U/t$, as a function of hopping energy. We examine the characteristic $P(n)$ distribution for the Mott Insulator, quantum critical region and superfluid and determine its behavior across thermal and quantum phase transitions using quantum Monte Carlo. The behavior of the local kinetic energy is estimated using the probability of hopping. These results illuminate number squeezing in the Mott Insulator and the quantum critical region described in [1].\\[4pt] [1] Y. Kato, et al., {\em Nature Physics} {\bf 4}, 617 (2008). [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H45.00014: Surface Majorana modes in ultra-cold fermion systems with unconventional Cooper pairings Yi Li, Congjun Wu The rapid progress of dipolar fermions provides a new opportunity to investigate unconventional Cooper pairings and exotic topological properties. We study the zero energy modes for the single and multiple-component dipolar gases along the surface perpendicular to the z-direction, which are a flat band of Majorana fermions. Under time-reversal symmetry breaking perturbations, such as vortices, the degeneracy of the surface Majorana modes is lifted. We also investigated the spontaneous time-reversal symmetry breaking effect in such systems. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H45.00015: Attractive Bose-Hubbard model with three-body constraint Kwai-Kong Ng, Ming-Fong Yang We numerically study the quantum and thermal phase transitions of the Bose-Hubbard model with particle numbers per site restricted to less than three. The bosons experience on-site attractions while the nearest-neighbor interactions are repulsive. Using particular two-loops algorithm in the QMC simulations, we study the exotic dimer superfluid at small hopping and low density regime. The nature of the phase transitions between the dimer superfluid and the atomic superfluid will be discussed. [Preview Abstract] |
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