Bosons with long range interactions on two-leg ladders in artificial magnetic fields

Motivated by experiments exploring the physics of neutral atoms in artificial magnetic fields, we study the ground state of bosons interacting with long range dipolar interactions on a two-leg ladder. Using two complimentary variational approaches, valid for weak interactions, we find rich physics driven by the long range forces. Generically, long range interactions tend to destroy the Meissner phase in favor of modulated density wave phases. Nearest neighbor interactions produce a novel interleg charge density wave phase, where the total density remains uniform, but the density on each leg of the ladder is modulating in space, out-of-phase with one another. At weak magnetic fields, next nearest neighbor interactions lead to a fully modulated biased ladder phase, where all the particles are on one leg of the ladder, and the density is modulating in space. This state simultaneously breaks $Z_{2}$ reflection symmetry and $U(1)$ symmetry associated with translation in real space. For values of the flux near $\phi = \pi$, we find that a switching effect occurs for arbitrarily weak interactions, where the density modulates in space, but the chiral current changes sign on every plaquette. Arbitrarily weak attractive interactions along the rungs destroy the Meissner phase completely, in favor of a modulated density wave phase. Varying magnetic field produces a cascade of first order transitions between modulated density wave states with different wave-vectors, which manifests itself as discrete jumps in the chiral current. Polarizing the dipoles along the ladder direction yields a region of phase space where a stable biased ladder phase occurs even at arbitrarily weak magnetic fields. We discuss the experimental consequences of our work, in particular, how the interleg charge density wave can manifest itself in recent experiments on bosons in synthetic dimensions.Comment: 12 pages, 4 figure

Spin dynamics in a spin-orbit coupled Fermi gas

We study the dynamics of a non-degenerate, harmonically trapped Fermi gas following a sudden ramp of the spin-orbit coupling strength. In the non-interacting limit, we solve the Boltzmann equation in the presence of spin orbit coupling analytically, and derive expressions for the dynamics of an arbitrary initial spin state. In particular we show that for a fully spin polarized initial state, the total magnetization exhibits collapse and revival dynamics in time with a period set by the trapping potential. In real space, this corresponds to oscillations between a fully polarized state and a spin helix. We numerically study the effect of interactions on the dynamics using a collisionless Boltzmann equation.Comment: 8 pages, 3 figures Expanded to include discussion of dynamics in the presence of a Zeeman field, rewritten section with interaction

Absence of damping of low energy excitations in a quasi-2D dipolar Bose gas

We develop a theory of damping of low energy, collective excitations in a quasi-2D, homogenous, dipolar Bose gas at zero temperature, via processes whereby an excitation decays into two excitations with lower energy. We find that owing to the nature of the low energy spectrum of a quasi-2D dipolar gas, such processes cannot occur unless the momentum of the incoming quasi-particle exceeds a critical value k_{crit}. We find that as the dipolar interaction strength is increased, this critical value shifts to larger momenta. Our predictions can be directly verified in current experiments on dipolar Bose condensates using Bragg spectroscopy, and provide valuable insight into the quantum many-body physics of dipolar gases.Comment: 4 pages, 2 figure

Spin waves in a spin-1 Bose gas

We present a theory of spin waves in a non-condensed gas of spin-1 bosons: providing both analytic calculations of the linear theory, and full numerical simulations of the nonlinear response. We highlight the role of spin-dependent contact interactions in the dynamics of a thermal gas. Although these interactions are small compared to the thermal energy, they set the scale for low energy long wavelength spin waves. In particular, we find that the polar state of Rb-87 is unstable to collisional mixing of magnetic sublevels even in the normal state. We augment our analytic calculations by providing full numerical simulations of a trapped gas, explicitly demonstrating this instability. Further we show that for strong enough anti-ferromagnetic interactions, the polar gas is unstable. Finally we explore coherent population dynamics in a collisionless transversely polarized gas.Comment: 10 pages, 7 figure

Landau damping in a collisionless dipolar Bose gas

We present a theory for the Landau damping of low energy quasi-particles in a collisionless, quasi-2D dipolar Bose gas and produce expressions for the damping rate in uniform and non-uniform systems. Using simple energy-momentum conservation arguments, we show that in the homogeneous system, the nature of the low energy dispersion in a dipolar Bose gas severely inhibits Landau damping of long wave-length excitations. For a gas with contact and dipolar interactions, the damping rate for phonons tends to decrease with increasing dipolar interactions; for strong dipole-dipole interactions, phonons are virtually undamped over a broad range of temperature. The damping rate for maxon-roton excitations is found to be significantly larger than the damping rate for phonons.Comment: 11 pages, 4 figure

Dynamics of correlations in a quasi-2D dipolar Bose gas following a quantum quench

We study the evolution of correlations in a quasi-2D dipolar gas driven out-of-equilibrium by a sudden ramp of the interaction strength. For sufficiently strong ramps, the momentum distribution, excited fraction and density-density correlation function all display pronounced features that are directly related to the appearance of a roton minimum in the underlying spectrum. Our study suggests that the evolution of correlations following a quench can be used as a probe of roton-like excitations in a dipolar gas. We also find that the build up of density-density correlations following a quench occurs much more slowly in the dipolar gas compared to a non-dipolar gas, owing to the long-range interactions.Comment: 8 pages, 4 figures. Expanded to include self-consistent Bogoliubov ansatz. Significance of results to experiments discusse

Coarsening dynamics of binary Bose condensates

We study the dynamics of domain formation and coarsening in a binary Bose-Einstein condensate that is quenched across a miscible-immiscible phase transition. The late-time evolution of the system is universal and governed by scaling laws for the correlation functions. We numerically determine the scaling forms and extract the critical exponents that describe the growth rate of domain size and autocorrelations. Our data is consistent with inviscid hydrodynamic domain growth, which is governed by a universal dynamical critical exponent of $1/z = 0.68(2)$. In addition, we analyze the effect of domain wall configurations which introduce a nonanalytic term in the short-distance structure of the pair correlation function, leading to a high-momentum "Porod"-tail in the static structure factor, which can be measured experimentally

Interaction-Tuned Dynamical Transitions in a Rashba Spin-Orbit Coupled Fermi Gas

We consider the time evolution of the magnetization in a Rashba spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We model the dynamics using a Boltzmann equation, which we solve in the Hartree-Fock approximation. The resulting non-linear system of equations gives rise to three distinct dynamical regimes with qualitatively different asymptotic behaviors of the magnetization at long times. The distinct regimes and the transitions between them are controlled by the interaction strength: for weakly interacting fermions, the magnetization decays to zero. For intermediate interactions, it displays undamped oscillations about zero and for strong interactions, a partially magnetized state is dynamically stabilized. The dynamics we find is a spin analog of interaction induced self-trapping in double-well Bose Einstein condensates. The predicted phenomena can be realized in trapped Fermi gases with synthetic spin-orbit interactions.Comment: 5 pages, 3 figure

Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas

We compute the finite-temperature phase diagram of a pseudospin-$1/2$ Bose gas with contact interactions, using two complementary methods: the random phase approximation (RPA) and self-consistent Hartree-Fock theory. We show that the inter-spin interactions, which break the (pseudo) spin-rotational symmetry of the Hamiltonian, generally lead to the appearance of a magnetically ordered phase at temperatures above the superfluid transition. In three dimensions, we predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong repulsive/attractive inter-species interactions respectively. The normal easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in electronic systems. For the case of inter-spin attraction, we also discuss the possibility of a \textit{bosonic} analog of the Cooper paired phase. This state is shown to significantly lose in energy to the transverse ferromagnet in three dimensions, but is more energetically competitive in lower dimensions. Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba and Dresselhaus-type couplings (as recently realized in experiment), we investigate the possibility of stripe ordering in the normal phase. Within our approximations however, we do not find an instability towards stripe formation, suggesting that the stripe order melts below the condensation temperature, which is consistent with the experimental observations of Ji \textit{et al.} [Ji \textit{et al.}, Nature Physics \textbf{10}, 314 (2014)].Comment: 5 pages, 3 figures. published versio

Strong correlation effects in a two-dimensional Bose gas with quartic dispersion

Motivated by the fundamental question of the fate of interacting bosons in flat bands, we consider a two-dimensional Bose gas at zero temperature with an underlying quartic single-particle dispersion in one spatial direction. This type of band structure can be realized using the NIST scheme of spin-orbit coupling [Y.-J. Lin, K. Jim\'{e}nez-Garcia, and I. B. Spielman, Nature $\textbf{471}$, 83 (2011)], in the regime where the lower band dispersion has the form $\varepsilon_{\textbf{k}} \sim k_{x}^{4}/4+k_{y}^{2}+\ldots$, or using the shaken lattice scheme of Parker $\textit{et al.}$ [C. V. Parker, L.-C. Ha and C. Chin, Nature Physics $\textbf{9}$, 769 (2013)]. We numerically compare the ground state energies of the mean-field Bose-Einstein condensate (BEC) and various trial wave-functions, where bosons avoid each other at short distances. We discover that, at low densities, several types of strongly correlated states have an energy per particle ($\epsilon$), which scales with density ($n$) as $\epsilon \sim n^{4/3}$, in contrast to $\epsilon \sim n$ for the weakly interacting Bose gas. These competing states include a Wigner crystal, quasi-condensates described in terms properly symmetrized fermionic states, and variational wave-functions of Jastrow type. We find that one of the latter has the lowest energy among the states we consider. This Jastrow-type state has a strongly reduced, but finite condensate fraction, and true off-diagonal long range order, which suggests that the ground state of interacting bosons with quartic dispersion is a strongly-correlated condensate reminiscent of superfluid Helium-4. Our results show that even for weakly-interacting bosons in higher dimensions, one can explore the crossover from a weakly-coupled BEC to a strongly-correlated condensate by simply tuning the single particle dispersion or density.Comment: 10 pages, 1 figur