arXiv subjects

Xiaopeng Li

Publications and source records attributed to Xiaopeng Li.

227 records · Page 13Linked to original sources

arXiv2014

Chiral spin superfluidity and spontaneous spin Hall effect of interacting bosons

Recent experiments on ultracold atoms in optical lattices have synthesized a variety of tunable bands with degenerate double-well structures in momentum space. Such degeneracies in the single particle spectrum strongly enhance quantum fluctuations, and may lead to exotic many-body ground states. Here we consider weakly interacting spinor Bose gases in such bands, and discover a universal quantum "order by disorder" phenomenon which selects a novel chiral spin superfluid with remarkable properties such as spontaneous anomalous spin Hall effect and momentum space antiferromagnetism. For bosons in the excited Dirac band of a hexagonal lattice, such a state supports staggered spin loop currents in real space. We show that Bloch oscillations provide a powerful dynamical route to quantum state preparation of such a chiral spin superfluid. Our predictions can be readily tested in spin resolved time-of-flight experiments.

cond-mat.quant-gas↗

arXiv2014

$p$-wave chiral superfluidity from an $s$-wave interacting atomic Fermi gas

Chiral $p$-wave superfluids are fascinating topological quantum states of matter that have been found in the liquid $^3$He-A phase and arguably in the electronic Sr$_2$RuO$_4$ superconductor. They are shown fundamentally related to the fractional $5/2$ quantum Hall state which supports fractional exotic excitations. A common understanding is that such states require spin-triplet pairing of fermions due to $p$-wave interaction. Here we report by controlled theoretical approximation that a center-of-mass Wannier $p$-wave chiral superfluid state can arise from spin-singlet pairing for an $s$-wave interacting atomic Fermi gas in an optical lattice. Despite a conceptually different origin, it shows topological properties similar to the conventional chiral $p$-wave state. These include a non-zero Chern number and the appearance of chiral fermionic zero modes bounded to domain walls. Several signature quantities are calculated for the cold atom experimental condition.

cond-mat.quant-gas↗

arXiv2014

Spirals and skyrmions in two dimensional oxide heterostructures

We construct the general free energy governing long-wavelength magnetism in two-dimensional oxide heterostructures, which applies irrespective of the microscopic mechanism for magnetism. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations. The general conclusions are vetted by a microscopic derivation for a simple model with Rashba spin-orbit coupling.

cond-mat.str-el↗

arXiv2013

Formation and detection of a chiral orbital Bose liquid in an optical lattice

Recent experiments on $p$-orbital atomic bosons have suggested the emergence of a spectacular ultracold superfluid with staggered orbital currents in optical lattices. This raises fundamental questions like the effects of collective thermal fluctuations, and how to directly observe such chiral order. Here, we show via Monte Carlo simulations that thermal fluctuations destroy this superfluid in an unexpected two-step process, unveiling an intermediate normal phase with spontaneously broken time-reversal symmetry, dubbed "chiral Bose liquid". For integer fillings ($n\geq 2$) in the chiral Mott regime, thermal fluctuations are captured by an effective orbital Ising model, and Onsager's powerful exact solution is adopted to determine the transition from this intermediate liquid to the para-orbital normal phase at high temperature. A suitable lattice quench is designed to convert the staggered angular momentum, previously thought by experts difficult to directly probe, into coherent orbital oscillations, providing a smoking-gun signature of chiral order.

cond-mat.quant-gas↗

arXiv2013

Orbital phases of fermions in an asymmetric optical ladder

We study a quantum ladder of interacting fermions with coupled s and p orbitals. Such a model describes dipolar molecules or atoms loaded into a double-well optical lattice, dipole moments being aligned by an external field. The two orbital components have distinct hoppings. The tunneling between them is equivalent to a partial Rashba spin-orbital coupling when the orbital space (s, p) is identified as spanned by pseudo-spin 1/2 states. A rich phase diagram, including incommensurate orbital density wave, pair density wave and other exotic superconducting phases, is proposed with bosonization analysis. In particular, superconductivity is found in the repulsive regime.

cond-mat.quant-gas↗

arXiv2012

Stripe, checkerboard, and liquid-crystal ordering from anisotropic p-orbital Fermi surfaces in optical lattices

We study instabilities of single-species fermionic atoms in the p-orbital bands in two-dimensional optical lattices at noninteger filling against interactions. Charge density wave and orbital density wave orders with stripe or checkerboard patterns are found for attractive and repulsive interactions, respectively. The superfluid phase, usually expected of attractively interacting fermions, is strongly suppressed. We also use field theory to analyze the possible phase-transitions from orbital stripe order to liquid-crystal phases and obtain the phase diagram. The condition of nearly-perfect Fermisurface nesting, which is key to the above results, is shown robustly independent of fermion fillings in such p-orbital systems, and the $(2k_F,\pm2k_F)$ momentum of density wave oscillation is highly tunable. Such remarkable features show the promise of making those exotic orbital phases, which are of broad interest in condensed-matter physics, experimentally realizable with optical lattice gases.

cond-mat.quant-gas↗

arXiv2012

Topological orbital ladders

We unveil a topological phase of interacting fermions on a two-leg ladder of unequal parity orbitals, derived from the experimentally realized double-well lattices by dimension reduction. $Z_2$ topological invariant originates simply from the staggered phases of $sp$-orbital quantum tunneling, requiring none of the previously known mechanisms such as spin-orbit coupling or artificial gauge field. Another unique feature is that upon crossing over to two dimensions with coupled ladders, the edge modes from each ladder form a parity-protected flat band at zero energy, opening the route to strongly correlated states controlled by interactions. Experimental signatures are found in density correlations and phase transitions to trivial band and Mott insulators.

cond-mat.quant-gas↗

arXiv2012

Time reversal symmetry breaking of $p$-orbital bosons in a one-dimensional optical lattice

We study bosons loaded in a one-dimensional optical lattice of two-fold $p$-orbital degeneracy at each site. Our numerical simulations find an anti-ferro-orbital p$_x$+ip$_y$, a homogeneous p$_x$ Mott insulator phase and two kinds of superfluid phases distinguished by the orbital order (anti-ferro-orbital and para-orbital). The anti-ferro-orbital order breaks time reversal symmetry. Experimentally observable evidence is predicted for the phase transition between the two different superfluid phases. We also discover that the quantum noise measurement is able to provide a concrete evidence of time reversal symmetry breaking in the first Mott phase.

cond-mat.quant-gas↗

arXiv2011

Effective action approach to the p-band Mott insulator and superfluid transition

Motivated by the recent experiment on p-orbital band bosons in optical lattices, we study theoretically the quantum phases of Mott insulator and superfluidity in two-dimensions. 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 go beyond mean field theory and derive from a microscopic model an effective action that is capable of describing both the p-band Mott insulating and superfluid phases in strong coupling. We further calculate the excitation spectra near the quantum critical point and find two gapless modes away from the tip of the Mott lobe but four gapless modes at the tip. Our effective theory reveals how the phase coherence peak builds up in the Mott regime when approaching the critical point. We also discuss the finite temperature phase transition of p-band superfluidity.

cond-mat.quant-gas↗

arXiv2011

U(1) $\times$ U(1) / Z$_2$ Kosterlitz-Thouless transition of the Larkin-Ovchinnikov phase in an anisotropic two-dimensional system

We study Kosterlitz-Thouless (KT) transitions of the Larkin-Ovchinnikov (LO) phase for a two-dimensional system composed of coupled one-dimensional tubes of fermions. The LO phase here is characterized by a stripe structure (periodic in only one direction) in the order parameter. The low energy excitations involve the oscillation of the stripe and the fluctuation of the phase, which can be described by an effective theory composed of two anisotropic XY models. We compute from a microscopic model the coefficients of the XY models from which the KT transition temperatures are determined. We found the $T^{KT} \propto t_{\perp}$ for small intertube tunneling $t_{\perp}$. As $t_{\perp}$ increases the system undergoes a first-order transition to the normal phase at zero temperature. Our method can be used to determine the Goldstone excitations of any stripe order involving charge or spin degrees of freedom.

cond-mat.quant-gas↗

arXiv2010

Bose-Einstein supersolid phase for a novel type of momentum dependent interaction

A novel class of non-local interactions between bosons is found to favor a crystalline Bose-Einstein condensation ground state. By using both low energy effective field theory and variational wavefunction method, we compare this state not only with the homogeneous superfluid, as has been done previously, but also with the normal (non-superfluid) crystalline phase and obtain the phase diagram. The key characters are: the interaction potential displays a negative minimum at finite momentum which determines the wavevector of this supersolid phase; and the wavelength corresponding to the momentum minimum needs to be greater than the mean inter-boson distance.

cond-mat.quant-gas↗