29 research outputs found
Emergence of Topological and Strongly Correlated Ground States in trapped Rashba Spin-Orbit Coupled Bose Gases
We theoretically study an interacting few-body system of Rashba spin-orbit
coupled two-component Bose gases confined in a harmonic trapping potential. We
solve the interacting Hamiltonian at large Rashba coupling strengths using
Exact Diagonalization scheme, and obtain the ground state phase diagram for a
range of interatomic interactions and particle numbers. At small particle
numbers, we observe that the bosons condense to an array of topological states
with n+1/2 quantum angular momentum vortex configurations, where n = 0, 1, 2,
3... At large particle numbers, we observe two distinct regimes: at weaker
interaction strengths, we obtain ground states with topological and symmetry
properties that are consistent with mean-field theory computations; at stronger
interaction strengths, we report the emergence of strongly correlated ground
states.Comment: 14 pages, 9 figure
Cavity-Assisted Dynamical Spin-Orbit Coupling in Cold Atoms
We consider ultracold atoms subjected to a cavity-assisted two-photon Raman
transition. The Raman coupling gives rise to effective spin-orbit interaction
which couples atom's center-of-mass motion to its pseudospin degrees of
freedom. Meanwhile, the cavity photon is dynamically affected by the atom. This
feedback between atom and photon leads to a dramatic modification of the atomic
dispersion relation, and further leads to dynamical instability of the system.
We propose to detect the change of cavity photon number as a direct way to
demonstrate dynamical instability.Comment: 5 pages, 5 figure
Controlling Condensate Collapse and Expansion with an Optical Feshbach Resonance
We demonstrate control of the collapse and expansion of an 88Sr Bose-Einstein
condensate using an optical Feshbach resonance (OFR) near the 1S0-3P1
intercombination transition at 689 nm. Significant changes in dynamics are
caused by modifications of scattering length by up to +- ?10a_bg, where the
background scattering length of 88Sr is a_bg = -2a0 (1a0 = 0.053 nm). Changes
in scattering length are monitored through changes in the size of the
condensate after a time-of-flight measurement. Because the background
scattering length is close to zero, blue detuning of the OFR laser with respect
to a photoassociative resonance leads to increased interaction energy and a
faster condensate expansion, whereas red detuning triggers a collapse of the
condensate. The results are modeled with the time-dependent nonlinear
Gross-Pitaevskii equation.Comment: 5 pages, 3 figure
Half-quantum vortex state in a spin-orbit coupled Bose-Einstein condensate
We investigate theoretically the condensate state and collective excitations
of a two-component Bose gas in two-dimensional harmonic traps subject to
isotropic Rashba spin-orbit coupling. In the weakly interacting regime when the
inter-species interaction is larger than the intra-species interaction
(), we find that the condensate ground state has a
half-quantum-angular-momentum vortex configuration with spatial rotational
symmetry and skyrmion-type spin texture. Upon increasing the interatomic
interaction beyond a threshold , the ground state starts to involve
higher-order angular momentum components and thus breaks the rotational
symmetry. In the case of , the condensate becomes
unstable towards the superposition of two degenerate half-quantum vortex
states. Both instabilities (at and ) can be
determined by solving the Bogoliubov equations for collective density
oscillations of the half-quantum vortex state, and by analyzing the softening
of mode frequencies. We present the phase diagram as functions of the
interatomic interactions and the spin-orbit coupling. In addition, we directly
simulate the time-dependent Gross-Pitaevskii equation to examine the dynamical
properties of the system. Finally, we investigate the stability of the
half-quantum vortex state against both the trap anisotropy and anisotropy in
the spin-orbit coupling term.Comment: 13 pages, 18 figure
Finite-Temperature Study of Bose-Fermi Superfluid Mixtures
Ultra-cold atom experiments offer the unique opportunity to study mixing of
different types of superfluid states. Our interest is in superfluid mixtures
comprising particles with different statistics- Bose and Fermi. Such scenarios
occur naturally, for example, in dense QCD matter. Interestingly, cold atomic
experiments are performed in traps with finite spatial extent, thus critically
destabilizing the occurrence of various homogeneous phases. Critical to this
analysis is the understanding that the trapped system can undergo phase
separation, resulting in a unique situation where phase transition in either
species (bosons or fermions) can overlap with the phase separation between
possible phases. In the present work, we illustrate how this intriguing
interplay manifests in an interacting 2-species atomic mixture - one bosonic
and another fermionic with two spin components - within a realistic trap
configuration. We further show that such interplay of transitions can render
the nature of the ground state to be highly sensitive to the experimental
parameters and the dimensionality of the system.Comment: 9 pages, 7 figures; Accepted for publication in Phys. Rev.
Spin-orbit coupled weakly interacting Bose-Einstein condensates in harmonic traps
We investigate theoretically the phase diagram of a spin-orbit coupled Bose
gas in two-dimensional harmonic traps. We show that discrete Landau levels
develop at strong spin-orbit coupling. For a weakly interacting gas, quantum
states with skyrmion lattice patterns emerge spontaneously and preserve either
parity symmetry or combined parity-time-reversal symmetry. These phases can be
readily observed by experimentally engineering spin-orbit coupling and
interatomic interactions for a cloud of Rb atoms in a highly oblate
trap.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Modeling and analysis of the three-dimensional current density in sandwich-type single-carrier devices of disordered organic semiconductors
We present the results of a modeling study of the three-dimensional current density in single-carrier sandwich-type devices of disordered organic semiconductors. The calculations are based on a master-equation approach, assuming a Gaussian distribution of site energies without spatial correlations. The injection-barrier lowering due to the image potential is taken into account, so that the model provides a comprehensive treatment of the space-charge-limited current as well as the injection-limited current (ILC) regimes. We show that the current distribution can be highly filamentary for voltages, layer thicknesses, and disorder strengths that are realistic for organic light-emitting diodes and, that, as a result, the current density in both regimes can be significantly larger than as obtained from a one-dimensional continuum drift-diffusion device model. For devices with large injection barriers and strong disorder, in the ILC transport regime, good agreement is obtained with the average current density predicted from a model assuming injection and transport via one-dimensional filaments.
BCS and BEC p-wave pairing in Bose-Fermi gases
The pairing of fermionic atoms in a mixture of atomic fermion and boson gases
at zero temperature is investigated. The attractive interaction between
fermions, that can be induced by density fluctuations of the bosonic
background, can give rise to a superfluid phase in the Fermi component of the
mixture. The atoms of both species are assumed to be in only one internal
state, so that the pairing of fermions is effective only in odd-l channels. No
assumption about the value of the ratio between the Fermi velocity and the
sound velocity in the Bose gas is made in the derivation of the energy gap
equation. The gap equation is solved without any particular "ansatz" for the
pairing field or the effective interaction. The p-wave superfluidity is studied
in detail. By increasing the strength and/or decreasing the range of the
effective interaction a transition of the fermion pairing regime, from the
Bardeen-Cooper-Schrieffer state to a system of tightly bound couples can be
realized. These composite bosons behave as a weakly-interacting Bose-Einstein
condensate.Comment: 14 pages, 6 eps-figures. To be published in European Physical Journal
Solitary waves in the Nonlinear Dirac Equation
In the present work, we consider the existence, stability, and dynamics of
solitary waves in the nonlinear Dirac equation. We start by introducing the
Soler model of self-interacting spinors, and discuss its localized waveforms in
one, two, and three spatial dimensions and the equations they satisfy. We
present the associated explicit solutions in one dimension and numerically
obtain their analogues in higher dimensions. The stability is subsequently
discussed from a theoretical perspective and then complemented with numerical
computations. Finally, the dynamics of the solutions is explored and compared
to its non-relativistic analogue, which is the nonlinear Schr{\"o}dinger
equation. A few special topics are also explored, including the discrete
variant of the nonlinear Dirac equation and its solitary wave properties, as
well as the PT-symmetric variant of the model