Dec 26 - Dec 30 Xiaopeng Li
Dec 30
1. arXiv:1112.6422 [pdf, other]
Nonequilibrium dynamic critical scaling of the quantum Ising chain
Michael Kolodrubetz, Bryan K. Clark, David A. HuseThe one-dimensional transverse-field Ising chain is a prototypical example of a quantum phase transition. While its equilibrium scaling has been understood for more than half a century, here we investigate the nonequilibrium quantum critical dynamics as the system is swept slowly through the critical point (a Kibble-Zurek ramp). Kibble-Zurek scaling is well understood for ramps that end at the quantum critical point or deep in the ordered phase. In this Letter, we solve for the full finite-size scaling forms of excess heat and spin-spin correlation function for an arbitrary time during the ramp. On the ferromagnetic side of the transition, we see negative spin correlations at a distance set by the ramp rate, demonstrating qualitatively the athermal nature of the ramp. We then confirm the universality of the scaling forms by numerically simulating Mott-insulating bosons in a tilted potential, an experimentally-realizable system in the same universality class. Our results indicate that the time scales necessary to see this dynamic scaling should be within the reach of present-day cold atom experiments.
2. arXiv:1112.6393 [pdf, ps, other]
Berry phase and Anomalous Hall Effect in a Three-orbital Tight-binding Hamiltonian
Yan He, Joel Moore, C.M. Varma
We consider the Anomalous Hall (AH) state induced by interactions in a three-orbital per unit-cell model. To be specific we consider a model appropriate for the Copper-Oxide lattice to highlight the necessary conditions for time-reversal breaking states which are AH states and which are not. We compare the singularities of the wave-functions of the three-orbital model, which are related to the nonzero Berry curvature, and their variation with a change of gauge to those in the two-orbital model introduced in a seminal paper by Haldane. Explicit derivation using wave-functions rather than the more powerful abstract methods may provide additional physical understanding of the phenomena.
3. arXiv:1112.6334 [pdf, ps, other]
Interplay between sublattice and spin symmetry breaking in graphene
D. Soriano, J. Fernández-Rossier
We study the effect of sublattice symmetry breaking on the electronic, magnetic and transport properties of two dimensional graphene as well as zigzag terminated one and zero dimensional graphene nanostructures. The systems are described with the Hubbard model within the collinear mean field approximation. We prove that for the non-interacting bipartite lattice with unequal number of atoms in each sublattice midgap states still exist in the presence of a staggered on-site potential $\pm \Delta/2$. We compute the phase diagram of both 2D and 1D graphene with zigzag edges, at half-filling, defined by the normalized interaction strength $U/t$ and $\Delta/t$, where $t$ is the first neighbor hopping. In the case of 2D we find that the system is always insulating and, we find the $U_c(\Delta)$ curve above which the system goes antiferromagnetic.
In 1D we find that the system undergoes a phase transition from non-magnetic insulator for $U<U_C(\Delta)$ to a phase with ferromagnetic edge order and antiferromagnetic inter-edge coupling. The conduction properties of the magnetic phase depend on $\Delta$ and can be insulating, conducting and even half-metallic, yet the total magnetic moment in the system is zero. We compute the transport properties of a heterojunction with two non-magnetic graphene ribbon electrodes connected to a finite length armchair ribbon and we find a strong spin filter effect.
4. arXiv:1112.6051 [pdf, ps, other]
1/(N-1) expansion for a finite U Anderson model away from half-filling
Akira OguriWe apply recently developed 1/(N-1) expansion to a particle-hole asymmetric SU(N) Anderson model with finite Coulomb interaction U. To leading order in 1/(N-1) it describes the Hartree-Fock random phase approximation (HF-RPA), and the higher-order corrections describe systematically the fluctuations beyond the HF-RPA. We show that the next-leading order results of the renormalized parameters for the local Fermi-liquid state agree closely with the numerical renormalization group results at N=4. It ensures the reliability of the next-leading order results for N>4. Our expansion scheme uses the standard Feynman diagrams, and has wide potential applications.
5. arXiv:1112.6022 [pdf, other]
Synthetic 3D Spin-Orbit Coupling
Brandon M. Anderson, Gediminas Juzeliūnas, Ian B. Spielman, Victor M. Galitski
We describe a method for creating a three-dimensional analogue to Rashba spin-orbit coupling in systems of ultracold atoms. This laser induced coupling uses Raman transitions to link four internal atomic states with a tetrahedral geometry, and gives rise to a Dirac point that is robust against environmental perturbations. We present an exact result showing that such a spin-orbit coupling in a fermionic system always rise to a molecular bound state.
6. arXiv:1112.5893 [pdf]
Spectroscopic Imaging STM Studies of Electronic Structure in the Superconducting and Pseudogap Phases of Cuprate High-Tc Superconductors
Kazuhiro Fujita, Andrew R. Schmidt, Eun-Ah Kim, Michael J. Lawler, Dung Hai Lee, J. C. Davis, Hiroshi Eisaki, Shin-ichi Uchida
One of the key motivations for the development of atomically resolved spectroscopic imaging STM (SI-STM) has been to probe the electronic structure of cuprate high temperature superconductors. In both the d-wave superconducting (dSC) and the pseudogap (PG) phases of underdoped cuprates, two distinct classes of electronic states are observed using SI-STM. The first class consists of the dispersive Bogoliubov quasiparticles of a homogeneous d-wave superconductor. These are detected below a lower energy scale |E|={\Delta}0 and only upon a momentum space (k-space) arc which terminates near the lines connecting k=\pm({\pi}/a0,0) to k=\pm(0, {\pi}/a0). In both the dSC and PG phases, the only broken symmetries detected in the |E|\leq {\Delta}0 states are those of a d-wave superconductor. The second class of states occurs at energies near the pseudogap energy scale |E| {\Delta}1 which is associated conventionally with the 'antinodal' states near k=\pm({\pi}/a0,0) and k=\pm(0, {\pi}/a0). We find that these states break the 90o-rotational (C4) symmetry of electronic structure within CuO2 unit cells, at least down to 180o rotational (C2) symmetry (nematic) but in a spatially disordered fashion. This intra-unit-cell C4 symmetry breaking coexists at |E| {\Delta}1 with incommensurate conductance modulations locally breaking both rotational and translational symmetries (smectic). The properties of these two classes of |E| {\Delta}1 states are indistinguishable in the dSC and PG phases. To explain this segregation of k-space into the two regimes distinguished by the symmetries of their electronic states and their energy scales |E| {\Delta}1 and |E|\leq{\Delta}0, and to understand how this impacts the electronic phase diagram and the mechanism of high-Tc superconductivity, represents one of a key challenges for cuprate studies.
7. arXiv:1112.5852 [pdf, ps, other]
Different types of the dimensional crossover in the quasi-one-dimensional spinless fermion systems
A.V. RozhkovIt is known that many-body correlations qualitatively modify the properties of a one-dimensional metal. However, for a quasi-one-dimensional metal these correlations are suppressed, at least partially. We study conditions under which the one-dimensional effects significantly influence the dimensional crossover of a quasi-one-dimensional metal. It is proved (i) that even a system with very high anisotropy of the single-particle hopping might behave on both sides of the crossover as an ordinary weakly non-ideal Fermi gas. Further, (ii) to demonstrate well-developed signatures of one-dimensional correlations the system must have extremely (exponentially) high anisotropy. Between cases (i) and (ii) an intermediate regime lies: (iii) the one-dimensional phenomena affect the two-particle susceptibilities, but do not reveal themselves in single-particle quantities. Unlike the normal state properties, (iv) the ordering transition is always very sensitive to the anisotropy: the mean field theory quickly becomes invalid as the anisotropy increases. An expression for the transition temperature is derived. The attributes (i-iv) are used to classify the weakly interacting quasi-one-dimensional fermion systems.
8. arXiv:1112.5720 [pdf, ps, other]
Frustrated spin chains in strong magnetic field: dilute two-component Bose gas regime
A. K. Kolezhuk, F. Heidrich-Meisner, S. Greschner, T. VekuaWe study the ground state of frustrated spin-S chains in a strong magnetic field in the immediate vicinity of saturation. In strongly frustrated chains, the magnon dispersion has two degenerate minima at inequivalent momenta $\pm Q$, and just below the saturation field the system can be effectively represented as a dilute one-dimensional lattice gas of two species of bosons that correspond to magnons with momenta around $\pm Q$. We present a theory of effective interactions in such a dilute magnon gas that allows us to make quantitative predictions for arbitrary values of the spin. With the help of this method, we are able to establish the magnetic phase diagram of frustrated chains close to saturation and study phase transitions between several nontrivial states, including a two-component Luttinger liquid, a vector chiral phase, and phases with bound magnons. We study those phase transitions numerically and find a good agreement with our analytical predictions.
9. arXiv:1112.5662 [pdf, other]
Clustered Wigner crystal phases of cold polar molecules in arrays of one-dimensional tubes
Michael Knap, Erez Berg, Martin Ganahl, Eugene Demler
We analyze theoretically polar molecules confined in planar arrays of one dimensional tubes. In the classical limit, if the number of tubes is finite, new types of "clustered Wigner crystals" with increasingly many molecules per unit cell can be stabilized by tuning the in-plane angle between the dipolar moments and the tube direction. Quantum mechanically, these phases melt into distinct "clustered Luttinger liquids." We map the phase diagram of the system and find that the requirements for cluster formation are reachable in current experiments. We discuss possible experimental signatures of clustered phases.
Dec 26
1. arXiv:1112.5616 [pdf, ps, other]
Mapping the Berry Curvature from Semiclassical Dynamics in Optical Lattices
H. M. Price, N. R. Cooper
We propose a general method by which experiments on ultracold gases can be used to determine the topological properties of the energy bands of optical lattices, as represented by the map of the Berry curvature across the Brillouin zone. The Berry curvature modifies the semiclassical dynamics and hence the trajectory of a wavepacket undergoing Bloch oscillations. However, in two dimensions these trajectories may be complicated Lissajous-like figures, making it difficult to extract the effects of Berry curvature in general. We propose how this can instead be done using a "time-reversal" protocol. This compares the mean velocity of a wavepacket under positive and negative external force, and allows a clean measurement of the Berry curvature over the Brillouin zone. We discuss how this protocol may be implemented and explore the semiclassical dynamics for three specific systems: the asymmetric hexagonal lattice; and two "optical flux" lattices in which the Chern number is nonzero. Finally, we discuss general experimental considerations for observing Berry curvature effects in ultracold gases.
2. arXiv:1112.5586 [pdf, other]
Ground-state properties of hard-core bosons in two dimensions: the semi-classical approach revisited
Tommaso Coletta, Nicolas Laflorencie, Frédéric MilaMotivated by some inconsistencies in the way quantum fluctuations are included beyond the classical treatment of hard-core bosons on a lattice in the recent literature, we revisit the large-S semi-classical approach to hard-core bosons on the square lattice at T=0. First of all, we show that, if one stays at the purely harmonic level, the only correct way to get the 1/S correction to the density is to extract it from the derivative of the ground state energy with respect to the chemical potential, and that to extract it from a calculation of the ground state expectation value of the particle number operator, it is necessary to include 1/\sqrt{S} corrections to the harmonic ground state. Building on this alternative approach to get 1/S corrections, we provide the first semiclassical derivation of the momentum distribution, and we revisit the calculation of the condensate density. The results of these as well as other physically relevant quantities such as the superfluid density are systematically compared to quantum Monte Carlo simulations. This comparison shows that the logarithmic corrections in the dilute Bose gas limit are only captured by the semi-classical approach if the 1/S corrections are properly calculated, and that the semi-classical approach is able to reproduce the 1/k divergence of the momentum distribution at k=0. Finally, the effect of 1/S^2 corrections is briefly discussed.
3. arXiv:1112.5609 (cross-list from quant-ph) [pdf, ps, other]
Quantum Magnetomechanics with Levitating Superconducting Microspheres
O. Romero-Isart, L. Clemente, C. Navau, A. Sanchez, J. I. Cirac
We show that by magnetically trapping a superconducting microsphere close to a quantum circuit, it is experimentally feasible to perform ground state cooling and to prepare quantum superpositions of the center-of-mass motion of the microsphere. Due to the absence of clamping losses and time dependent electromagnetic fields, the mechanical motion of micrometer-sized metallic spheres in the Meissner state is predicted to be extremely well isolated from the environment. Hence, we propose to combine the technology of magnetic mictrotraps and superconducting qubits to bring relatively large objects to the quantum regime.
Dec 19 - Dec 23 Zixu Zhang
Dec 23
None
Dec 22
1. arXiv:1112.5082 [pdf, ps, other]
Exploring spin-orbital models with dipolar fermions in zig-zag optical lattices
G. Sun, G. Jackeli, L. Santos, T. Vekua
Ultra-cold dipolar spinor fermions in zig-zag type optical lattices can mimic spin-orbital models relevant in solid-state systems, as transition-metal oxides with partially filled d-levels, with the interesting advantage of reviving the quantum nature of orbital fluctuations. We discuss two different physical systems in which these models may be simulated, showing that the interplay between lattice geometry and spin-orbital quantum dynamics produces a wealth of novel quantum phases.
Dec 21
None
Dec 20
1. arXiv:1112.3973 [pdf, ps, other]
Competition between superconductivity and nematic order : anisotropy of superconducting coherence length
Eun Gook Moon, Subir Sachdev
We study the interplay between nematic order and superconductivity, motivated by a recent experiment on FeSe observing strongly distorted vortex shapes (Song et al., Science 332, 1410 (2011)). We show that the nematic order strongly enhances the anisotropy in the superconducting coherence length, beyond that expected from considerations of the Ginzburg-Landau theory. We obtain universal functions describing the coupling between the nematic order and superconductivity, and discuss connections of our results to the experiments.
2. arXiv:1112.4199 [pdf, other]
Direct observation of quantum phonon fluctuations in a one dimensional Bose gas
Julien Armijo
At temperature T=0, classical thermodynamics predicts the complete absence of excitations, however, due to the Heisenberg uncertainty principle, a quantum observable never fully comes to rest. Quantum fluctuations have intriguing consequences like Casimir-Polder forces or the Hawking radiation near black holes, and drive all quantum phase transitions. In matter fields, they rule the correlation properties at low temperature, and are dramatically enhanced in reduced dimensions. In 1D, they forbid long range order and Bose-Einstein condensation even at T=0. However, their microscopic observation in continuous systems has remained elusive. Here, we report direct observation of quantum density fluctuations in the \textit{collective} modes of a continuous matter field, using ultracold atomic 1D clouds and statistical analysis of \textit{in situ} density profiles. When $k_B T<\mu$, where $k_B$ is the Boltzman constant and $\mu$ the chemical potential, fluctuations are dominated by the quantum rather than thermal term, and negative density correlations, i.e. antibunching, is observed. In our cloud centers, $\mu/k_B T \simeq 4$ and up to 20% of the microscopically observed fluctuations are quantum phonons. Moreover, using a novel analysis at variable observation length, we monitor non-locally the emergence of quantum fluctuations at short distances as a clear deviation from a classical field prediction. Our work opens new roads for studies of beyond-thermodynamical physics in quantum many-body systems.
3. arXiv:1112.4244 [pdf, ps, other]
Exact relaxation dynamics of a localized many-body state in the 1D bose gas
Jun Sato, Rina Kanamoto, Eriko Kaminishi, Tetsuo Deguchi
Through an exact method we numerically solve the time evolution of the density profile for an initially localized state in the one-dimensional bosons with repulsive short-range interactions. We show that a localized state with a density notch is constructed by superposing one-hole excitations. The initial density profile overlaps the plot of the squared amplitude of a dark soliton in the weak coupling regime. We observe the localized state collapsing into a flat profile in equilibrium for a large number of particles such as N=1000. The relaxation time increases as the coupling constant decreases, which suggests the existence of off-diagonal long-range order. We show a recurrence phenomenon for a small number of particles such as N=20.
4. arXiv:1112.4257 [pdf, other]
d-wave Superfluid with Gapless Edges in a Cold Atom Trap
Anne-Louise Gadsbolle, H. Francis Song, Karyn Le Hur
We consider a strongly repulsive fermionic gas in a two-dimensional optical lattice confined by a harmonic trapping potential. To address the strongly repulsive regime, we consider the $t-J$ Hamiltonian. The presence of the harmonic trapping potential enables the stabilization of coexisting and competing phases. In particular, at low temperatures, this allows the realization of a d-wave superfluid region surrounded by purely (gapless) normal edges. Solving the Bogoliubov-de Gennes equations and comparing with the local density approximation, we show that the proximity to the Mott insulator is revealed by a downturn of the Fermi liquid order parameter at the center of the trap where the d-wave gap has a maximum. The density profile evolves linearly with distance.
5. arXiv:1112.4468 [pdf, ps, other]
Counterflow of spontaneous mass currents in trapped spin-orbit coupled Fermi gases
E. Doko, A. L. Subasi, M. Iskin
We use the Bogoliubov-de Gennes formalism and study the ground-state phases of trapped spin-orbit coupled Fermi gases in two dimensions. Our main finding is that the presence of a symmetric (Rashba type) spin-orbit coupling spontaneously induces counterflowing mass currents in the vicinity of the trap edge, i.e. $\uparrow$ and $\downarrow$ particles circulate in opposite directions with equal speed. These currents flow even in noninteracting systems, but their strength decreases toward the molecular BEC limit, which can be achieved either by increasing the spin-orbit coupling or the interaction strength. These currents are also quite robust against the effects of asymmetric spin-orbit couplings in $x$ and $y$ directions, gradually reducing to zero as the spin-orbit coupling becomes one dimensional. We compare our results with those of chiral p-wave superfluids/superconductors.
Dec 19
1. arXiv:1112.3657 [pdf, other]
Strongly interacting one-dimensional bosons in arbitrary-strength optical lattices: from Bose-Hubbard to sine-Gordon and beyond
Achilleas Lazarides, Masudul Haque
We analyze interacting one-dimensional bosons in the continuum, subject to a periodic sinusoidal potential of arbitrary depth. Variation of the lattice depth tunes the system from the Bose-Hubbard limit for deep lattices, through the sine-Gordon regime of weak lattices, to the complete absence of a lattice. Using the Bose-Fermi mapping between strongly interacting bosons and weakly interacting fermions, we derive the phase diagram in the parameter space of lattice depth and chemical potential. The same formalism is used to derive the equation of state and energy gaps at arbitrary lattice depth. The latter are compared to recent experiments. In the presence of a harmonic trap, we derive density profiles, which range from the familiar "wedding-cake" structures for deep lattices to more peculiar profiles for shallow lattices. Generically, both incompressible (gapped) and compressible phases coexist in the trap.
Dec 12 - Dec 16 Bin Wang
Dec 16
1. arXiv:1112.3419 [pdf, ps, other]
Title: Wigner crystal induced by dipole-dipole interaction in one-dimensional optical lattices
Authors: Zhihao Xu, Shu Chen
We demonstrate that the static structure factor, momentum distribution and density distribution provide clear signatures of the emergence of Wigner crystal for the fermionic dipolar gas with strongly repulsive dipole-dipole interactions trapped in one-dimensional optical lattices. Our numerical evidences are based on the exact diagonalization of the microscopic effective lattice Hamiltonian of few particles interacting with long-range interactions. As a comparison, we also study the system with only nearest-neighbor interactions, which displays quite different behaviors from the dipolar system in the regime of strong repulsion.
2. arXiv:1112.3355 [pdf, other]
Title: Observing complex bound states in the spin-1/2 Heisenberg XXZ chain using local quantum quenches
Authors: Martin Ganahl, Elias Rabel, Fabian H. L. Essler, Hans Gerd Evertz
We consider the non-equilibrium evolution in the spin-1/2 XXZ Heisenberg chain for fixed magnetization after a local quantum quench. This model is equivalent to interacting spinless fermions. Initially an infinite magnetic field is applied to n consecutive sites and the ground state is calculated. At time t=0 the field is switched off and the time evolution of observables such as the z-component of spin is computed using the Time Evolving Block Decimation (TEBD) algorithm. We find that the observables exhibit strong signatures of linearly propagating spinon and bound state excitations. These persist even when integrability-breaking perturbations are included. Since bound states ("strings") are notoriously difficult to observe using conventional probes such as inelastic neutron scattering, we conclude that local quantum quenches are an ideal setting for studying their properties. We comment on implications of our results for cold atom experiments.
Dec 15
1. arXiv:1112.3157 [pdf, ps, other]
Title: Simulating Z_2 topological insulators with cold atoms in a one-dimensional optical lattice
Authors: Feng Mei, Shi-Liang Zhu, Zhi-Ming Zhang, C. H. Oh, N. Goldman
We propose an experimental scheme to simulate and detect the properties of time-reversal invariant topological insulators, using cold atoms trapped in one-dimensional bichromatic optical lattices. This system is described by a one-dimensional Aubry-Andre model with an additional SU(2) gauge structure, which captures the essential properties of a two-dimensional Z2 topological insulator. We demonstrate that topologically protected edge states, with opposite spin orientations, can be pumped across the lattice by sweeping a laser phase adiabatically. This process constitutes an elegant way to transfer topologically protected quantum states in a highly controllable environment. We discuss how density measurements could provide clear signatures of the topological phases emanating from our one-dimensional system.
2. arXiv:1112.3331 [pdf, other]
Title: Possible spin liquid state in the spin 1/2 J1-J2 antiferromagnetic Heisenberg model on square lattice: A tensor product state approach
Authors: Ling Wang, Zheng-Cheng Gu, Xiao-Gang Wen, Frank Verstraete
We study the spin 1/2 J1-J2 antiferromagnetic Heisenberg model on square lattice by using a recently proposed cluster update method for tensor product states (TPSs). The ground state energies in the thermodynamic limit are in good agreement with the state of art exact diagonalization study, and the energy differences between these two studies are of the order of 10^-3J1 per site. For small bond dimension D(D<5), we find a paramagnetic ground state with mixed columnar and staggered dimer orders of a magnitude 10^-2 in the range of 0.47<J2=J1<0.6, which agrees with previous results. For large bond dimension D(D>=5), we observe all these orders vanish, implying the emergence of a spin liquid phase.
3. arXiv:1112.3311 [pdf, ps, other]
Title: Topological Nematic States and Non-Abelian Lattice Dislocations
Authors: Maissam Barkeshli, Xiao-Liang Qi
An exciting new prospect in condensed matter physics is the possibility of realizing fractional quantum Hall (FQH) states in simple lattice models without a large external magnetic field. A fundamental question is whether qualitatively new states can be realized on the lattice as compared with ordinary fractional quantum Hall states. Here we propose new symmetry-enriched topological states, topological nematic states, which are a dramatic consequence of the interplay between the lattice translation symmetry and topological properties of these fractional Chern insulators. When a partially filled flat band has a Chern number N, it can be mapped to an N-layer quantum Hall system. We find that lattice dislocations can act as wormholes connecting the different layers and effectively change the topology of the space. Lattice dislocations become defects with non-trivial quantum dimension, even when the FQH state being realized is by itself Abelian. Our proposal leads to the possibility of realizing the physics of topologically ordered states on high genus surfaces in the lab even though the sample has only the disk geometry.
Dec 14
1. arXiv:1112.2846 [pdf, other]
Title: Dipolar fermions in a two-dimensional square lattice
Authors: Anne-Louise Gadsbolle, G. M. Bruun
We consider dipolar fermions in a two-dimensional square lattice and a harmonic trapping potential. The anisotropy of the dipolar interaction combined with the lattice leads to transitions between phases with density order of different symmetries. We show that the attractive part of the dipolar interaction results in a superfluid phase which is suppressed by density order. The trapping potential is demonstrated to make the different phases co-exist, forming ring and island structures. The phases with density and superfluid order can overlap forming regions with supersolid order.
2. arXiv:1112.2799 [pdf, ps, other]
Title: Critical temperature of a Rashba spin-orbit coupled Bose gas in harmonic traps
Authors: Hui Hu, Xia-Ji Liu
We investigate theoretically Bose-Einstein condensation of an ideal, trapped Bose gas in the presence of Rashba spin-orbit coupling. Analytic results for the critical temperature and condensate fraction are derived, based on a semi-classical approximation to the single-particle energy spectrum and density of states, and are compared with exact results obtained by explicitly summing discrete energy levels for small number of particles. We find a significant decrease of the critical temperature and of the condensate fraction due to a finite spin-orbit coupling. For large coupling strength and finite number of particles $N$, the critical temperature scales as $N^{2/5}$ and $N^{2/3}$ in three and two dimensions, respectively, contrasted to the predictions of $N^{1/3}$ and $N^{1/2}$ in the absence of spin-orbit coupling. Finite size corrections in three dimensions are also discussed.
Dec 13
1. arXiv:1112.2596 [pdf, ps, other]
Title: Spontaneous generation of spin-orbit coupling in magnetic dipolar Fermi gases
Authors: T. Sogo, M. Urban, P. Schuck, T. Miyakawa
The stability of an unpolarized two-component dipolar Fermi gas is studied within mean-field theory. Besides the known instability towards spontaneous magnetization with Fermi sphere deformation, another instability towards spontaneous formation of a spin-orbit coupled phase with a Rashba-like spin texture is found. A phase diagram is presented and consequences are briefly discussed.
2. arXiv:1112.2395 [pdf, other]
Title: Shear viscosity and spin diffusion coefficient of a two-dimensional Fermi gas
Author: G. M. Bruun
Using kinetic theory, we calculate the shear viscosity and the spin diffusion coefficient as well as the associated relaxation times for a two-component Fermi gas in two dimensions, as a function of temperature, coupling strength, polarization, and mass ratio of the two components. It is demonstrated that the minimum value of the viscosity decreases with the mass ratio, since Fermi blocking becomes less efficient. We furthermore analyze recent experimental results for the quadrupole mode of a 2D gas in terms of viscous damping obtaining a qualitative agreement using no fitting parameters.
3. arXiv:1112.2368 (cross-list from cond-mat.str-el) [pdf, ps, other]
Title: Fractional charge and statistics in the fractional quantum spin Hall effect
Authors: Yuanpei Lan, Shaolong Wan
In this paper, we consider there exist two types of fundamental quasihole excitation in the fractional quantum spin Hall state and investigate their topological properties by both Chern-Simons field theory and Berry phase technique. By the two different ways, we obtain the identical charge and statistical angle for each type of quasihole, as well as the identical mutual statistics between two different types of quasihole excitation.
4. arXiv:1112.2241 [pdf, other]
Title: Spin Liquid Ground State of the Spin-1/2 Square $J_1$-$J_2$ Heisenberg Model
Authors: Hong-Chen Jiang, Hong Yao, Leon Balents
We perform highly accurate density matrix renormalization group (DMRG) simulations to investigate the ground state properties of the spin-1/2 antiferromagnetic square lattice Heisenberg $J_1$-$J_2$ model. Based on studies of numerous long cylinders with circumferences of up to 10 lattice spacings, we obtain strong evidence for a topological quantum spin liquid state in the region $0.41\leq J_2/J_1\leq 0.62$, separating conventional N\'eel and striped antiferromagnetic states for smaller and larger $J_2/J_1$, respectively. The quantum spin liquid is characterized numerically by the absence of magnetic or valence bond solid order, and non-zero singlet and triplet energy gaps. Furthermore, we positively identify its topological nature by measuring a non-zero topological entanglement entropy and a non-trivial finite size dimerization effect depending upon the parity of the circumference of the cylinder.
5. arXiv:1112.2551 [pdf, ps, other]
Title: Transport in a one-dimensional isotropic Heisenberg model at high temperature
Author: Marko Znidaric
Magnetization transport in a one-dimensional isotropic spin 1/2 Heisenberg model is studied. It is shown that in a nonequilibrium steady state at high temperature and constant small driving the magnetization current depends on the system length L as 1/L^{0.5}, meaning that the diffusion constant diverges as L^{0.5}. Spectral properties of a superoperator governing the relaxation towards a nonequilibrium steady state are also discussed.
Dec 12
1. arXiv:1112.2035 [pdf, ps, other]
Title: Long-lived super-molecules and thier implication to many-body physics
Authors: S.-J. Huang, Y.-T. Hsu, H. Lee, Y.-C. Chen, A. G. Volosniev, N. T. Zinner, D.-W. Wang
We demonstrate that, in the presence of a microwave field, the effective interaction between polar molecules can be tuned to be attractive and so shallow that none or only a few bound states (super- molecules) can exist in the dilute limit. Binding energies, average sizes, and phase diagrams for both s-orbital (bosons) and p-orbital (fermions) dimers are systematically studied, together with bosonic trimer states. We explicitly show that the non-adiabatic transition rate can be easily tuned small for such ground state super-molecules, so that the system can be stable from collapse even near the associated potential resonance. Our results therefore suggest a feasible cold molecule system to investigate both novel few-body and many-body physics (for example, the p-wave BCS-BEC crossover for fermions and the paired condensate for bosons) that can not be easily accessed in single species atomic gases.
Dec 5 - Dec 9 Johannes Schachenmayer
Dec 9
1. arXiv:1112.1852 [pdf, ps, other]
Topological Quantum Critical Points and Collective Modes of Non-Abelian Honeycomb Optical Lattice
Fa-Di Sun, Xiao-Lu Yu, Shao-Jian Jiang, Heng Fan, W. M. Li
Motivated by recent experiments carried out by Esslinger's group at ETH [arXiv:1111.5020], we developed the local effective field theories to describe the universal properties of the topological quantum critical points in non-Abelian honeycomb optical lattice. We find that the ETH experiments, which manipulate Dirac points by changing the lattice anisotropy, can be realized in adjusting non-Abelian gauge potentials. From the effective metric tensor, which is induced by non-Abelian gauge potentials, we have identified two different types of local effective field theories. In one effective field theory the Fermi surface is a Dirac point, the short range Hubbard-like interactions are irrelevant and the collective modes are all damped. In the other effective field theory the Fermi surface is a nodal line, where exits a dimension crossover which leads to the short range Hubbard-like interactions being marginal and the collective modes being undamped.
Dec 8
1. arXiv:1112.1622 [pdf, other]
Oscillatory pairing amplitude and magnetic compressible-incompressible transitions in imbalanced fermionic superfluids in optical lattices of elongated tubes
Kuei Sun, C. J. Bolech
We study two-species fermion gases with attractive interaction in two-dimensional optical lattices producing an array of elongated tube confinements. Focusing on the interplay of Cooper pairing, spin imbalance (or magnetization) and intertube tunneling, we find the pairing gap can exhibit oscillatory behavior both along the tubes axis and across the tubes, reminiscent of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. We obtain a Bose-Hubbard-like phase diagram that shows the magnetization of the system undergoes an incompressible-compressible transition as a function of magnetic field and intertube tunneling strength. We find the parity of tube-filling imbalance in incompressible states is protected by that of the oscillatory pairing gap. Finally, we discuss signatures of this transition and thus (indirectly) of the FFLO pairing in cold atom experiments.
Dec 7
1. arXiv:1112.1204 [pdf, ps, other]
Dynamic Kosterlitz-Thouless transition in 2D Bose mixtures of ultra-cold atoms
L. Mathey, Kenneth J. Günter, Jean Dalibard, A. Polkovnikov
We propose a realistic experiment to demonstrate a dynamic Kosterlitz-Thouless transition in ultra-cold atomic gases in two dimensions. With a numerical implementation of the Truncated Wigner Approximation we simulate the time evolution of several correlation functions, which can be measured via matter wave interference. We demonstrate that the relaxational dynamics is well-described by a real-time renormalization group approach, and argue that these experiments can guide the development of a theoretical framework for the understanding of critical dynamics.
2 arXiv:1112.1154 [pdf, other]
Pairing properties of cold fermions in the honeycomb lattice
Benoît Grémaud
The pairing properties of ultracold fermions, with an attractive interaction, loaded in a honeycomb (graphene-like) optical lattice are studied in a mean-field approach. We emphasize, in the presence of a harmonic trap, the unambiguous signatures of the linear dispersion relation of the band structure around half-filling (i.e. the massless Dirac fermions) in the local order parameter, in particular in the situations of either imbalance hoping parameters or imbalance populations. It can also be observed in the system response to external perturbation, for instance by measuring the pair destruction rate when modulating the optical lattice depth. Going beyond the mean-field level, we estimate the critical temperature for the "condensation" of the preformed pairs.
3. arXiv:1112.1100 [pdf, other]
Three-sublattice order in the SU(3) Heisenberg model on the square and triangular lattice
Bela Bauer, Philippe Corboz, Andreas M. Läuchli, Laura Messio, Karlo Penc, Matthias Troyer, Frédéric Mila
We present a numerical study of the SU(3) Heisenberg model of three-flavor fermions on the triangular and square lattice by means of the density-matrix renormalization group (DMRG) and infinite projected entangled-pair states (iPEPS). For the triangular lattice we confirm that the ground state has a three-sublattice order with a finite ordered moment which is compatible with the result from linear flavor wave theory (LFWT). The same type of order has recently been predicted also for the square lattice [PRL 105, 265301 (2010)] from LFWT and exact diagonalization. However, for this case the ordered moment cannot be computed based on LFWT due to divergent fluctuations. Our numerical study clearly supports this three-sublattice order, with an ordered moment of m=0.2-0.4 in the thermodynamic limit.
Dec 6
1. arXiv:1112.0985 [pdf, ps, other]
Effect of disorder close to the superfluid transition in a two-dimensional Bose gas
B. Allard, T. Plisson, Markus Holzmann, G. Salomon, Alain Aspect, Philippe Bouyer, Thomas Bourdel
We experimentally study the effect of disorder on trapped quasi two-dimensional (2D) $^{87}$Rb clouds in the vicinity of the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. The disorder correlation length is of the order of the Bose gas characteristic length scales (thermal de Broglie wavelength, healing length) and disorder thus modifies the physics at a microscopic level. We analyze the coherence properties of the cloud through measurements of the momentum distributions, for two disorder strengths, as a function of its degeneracy. For moderate disorder the emergence of coherence remains steep but is shifted to a lower entropy. In contrast, for strong disorder, the growth of coherence is hindered. Such studies of dirty atomic Bose gases are relevant to the understanding of superfluid-insulator transitions occurring in several condensed matter systems.
2. arXiv:1112.0972 [pdf, ps, other]
Frustration and time reversal symmetry breaking for a Bose-Fermi mixture
Krzysztof Sacha, Katarzyna Targonska, Jakub Zakrzewski
The modulation of an optical lattice potential that breaks time-reversal symmetry enables realization of complex tunneling rates in the corresponding tight-binding model. A superfluid Fermi gas in a triangular lattice potential with complex tunnelings is considered. The superfluid pairing function does not reveal any frustration despite the fact that ground state of bosons in such a lattice corresponds to a non-trivial Bloch wave. On the other hand, a Bose-Fermi mixture of bosonic molecules and unbound fermions in the lattice due to boson-fermion coupling shows an interesting behavior. Fermions become infested by a frustration of bosons as shown by the corresponding pairing function.
3. arXiv:1112.0899 [pdf, ps, other]
Creating atom-number states around tapered optical fibres by loading from an optical lattice
T. Hennessy, Th. Busch
We describe theoretically a setup in which a tapered optical nanofibre is introduced into an optical lattice potential for cold atoms. Firstly, we consider the disturbance to the geometry of the lattice potential due to scattering of the lattice lasers from the dielectric fibre surface and show that the resulting distortion to the lattice can be minimized by placing the fibre at an appropriate position in the lattice. We then calculate the modifications of the local potentials that are achievable by transmitting off-resonant light through the fibre. The availability of such a technique holds the potential to deterministically create and address small well-defined samples of atoms in the evanescent field of the tapered nanofibre.
4. arXiv:1112.0843 [pdf, ps, other]
Optomechanically-Based Probing of Spin-Charge Separation in Ultracold Gases
Qing Sun, W. M. Liu, An-Chun Ji
We propose a new approach to investigate the spin-charge separation in 1D quantum liquids via the optomechanical coupled atom-cavity system. We show that, one can realize an effective two-modes optomechanical model with the spin/charge modes playing the role of mechanical resonators. By tuning the weak probe laser under a pump field, the signal of spin-charge separation could be probed explicitly in the sideband regime via cavity transmissions. Moreover, the spin/charge modes can be addressed separately by designing the probe field configurations, which may be beneficial for future studies of the atom-cavity systems and quantum many-body physics.
5. arXiv:1112.0719 [pdf, other]
The nature of self-localization in open optical lattices
Holger Hennig, Ragnar Fleischmann
We analytically describe a novel self-trapping transition (called self-localization) of Bose-Einstein condensates (BECs) in one-dimensional optical lattices in the presence of weak dissipation. Self-localization has recently been observed in several studies based upon the discrete nonlinear Schr\"odinger equation (DNLS) including boundary dissipation, however, its origin is hitherto an open question. We show that self-localization is based upon a self-trapping bifurcation in the system paired with a logarithmic dependence on the system size. Furthermore, we establish that the origin of the bifurcation is the Peierls-Nabarro barrier, an energy threshold describing the stability of self-trapped states. Beyond the mean-field approximation (including higher-order correlation functions) the bifurcation becomes even sharper which is also reflected by a sudden change of the coherence of the condensate. While the bifurcation can be readily studied in ongoing experiments in optical lattices, our results allow for the preparation of robust and long-time coherent quantum states.
Dec 5
1. arXiv:1112.0554 [pdf, ps, other]
Dipole Interaction Mediated Laser Cooling of Polar Molecules to Ultra-cold Temperatures
Sebastian D. Huber, Hans Peter Büchler
We present a method to design a finite decay rate for excited rotational states in polar molecules. The setup is based on a hybrid system of polar molecules with atoms driven into a Rydberg state. The atoms and molecules are coupled via the strong dipolar exchange interaction between two rotation levels of the polar molecule and two Rydberg states. Such a controllable decay rate opens the way to optically pump the hyperfine levels of polar molecules and it enables the application of conventional laser cooling techniques for cooling polar molecules into quantum degeneracy.
2. arXiv:1112.0362 [pdf, ps, other]
Contact Measurements on Atomic BEC
R. J. Wild, P. Makotyn, J. M. Pino, E. A. Cornell, D. S. Jin
A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with delta-function interactions. We report on measurements of the contact, using RF spectroscopy, for an $^{85}$Rb atomic Bose-Einstein condensate (BEC). For bosons, the fact that contact spectroscopy can be used to probe the gas on short timescales is useful given the decreasing stability of BECs with increasing interactions. A complication is the added possibility, for bosons, of three-body interactions. In investigating this issue, we have located an Efimov resonance for $^{85}$Rb atoms with loss measurements and thus determined the three-body interaction parameter. In our contact spectroscopy, in a region of observable beyond-mean-field effects, we find no measurable contribution from three-body physics.
3. arXiv:1112.0334 [pdf, other]
Quantised superflow glitches in an annular Bose-Einstein condensate
Stuart Moulder, Scott Beattie, Robert P. Smith, Naaman Tammuz, Zoran Hadzibabic
We study metastability and decay of multiply-charged superflow in a ring-shaped atomic Bose-Einstein condensate. Supercurrent corresponding to a giant vortex with topological charge up to q=10 is phase-imprinted optically and detected both interferometrically and kinematically. We observe q=3 superflow persisting for up to a minute and show that the eventual decay of the supercurrent occurs in a cascade of clearly quantised steps, corresponding to collective jumps of atoms between discrete q values. Finally, we show that these glitches in the superflow occur stochastically, and measure their time-resolved counting statistics.
