458 research outputs found
Superfluidity and spin superfluidity in spinor Bose gases
We show that spinor Bose gases subject to a quadratic Zeeman effect exhibit
coexisting superfluidity and spin superfluidity, and study the interplay
between these two distinct types of superfluidity. To illustrate that the basic
principles governing these two types of superfluidity are the same, we describe
the magnetization and particle-density dynamics in a single hydrodynamic
framework. In this description spin and mass supercurrents are driven by their
respective chemical potential gradients. As an application, we propose an
experimentally accessible stationary state, where the two types of
supercurrents counterflow and cancel each other, thus resulting in no mass
transport. Furthermore, we propose a straightforward setup to probe spin
superfluidity by measuring the in-plane magnetization angle of the whole cloud
of atoms. We verify the robustness of these findings by evaluating the
four-magnon collision time, and find that the time scale for coherent
(superfluid) dynamics is separated from that of the slower incoherent dynamics
by one order of magnitude. Comparing the atom and magnon kinetics reveals that
while the former can be hydrodynamic, the latter is typically collisionless
under most experimental conditions. This implies that, while our
zero-temperature hydrodynamic equations are a valid description of spin
transport in Bose gases, a hydrodynamic description that treats both mass and
spin transport at finite temperatures may not be readily feasible
Hydrodynamic modes of partially condensed Bose mixtures
We generalize the Landau-Khalatnikov hydrodynamic theory for superfluid
helium to two-component (binary) Bose mixtures at arbitrary temperatures. In
particular, we include the spin-drag terms that correspond to viscous coupling
between the clouds. Therefore, our theory not only describes the usual
collective modes of the individual components, e.g., first and second sound,
but also results in new collective modes, where both constituents participate.
We study these modes in detail and present their dispersions using
thermodynamic quantities obtained within the Popov approximation
Spin Hall mode in a trapped thermal Rashba gas
We theoretically investigate a two-dimensional harmonically-trapped gas of
identical atoms with Rashba spin-orbit coupling and no interatomic
interactions. In analogy with the spin Hall effect in uniform space, the gas
exhibits a spin Hall mode. In particular, in response to a displacement of the
center-of-mass of the system, spin-dipole moment oscillations occur. We
determine the properties of these oscillations exactly, and find that their
amplitude strongly depends on the spin-orbit coupling strength and the quantum
statistics of the particles
Quantum rotor model for a Bose-Einstein condensate of dipolar molecules
We show that a Bose-Einstein condensate of heteronuclear molecules in the
regime of small and static electric fields is described by a quantum rotor
model for the macroscopic electric dipole moment of the molecular gas cloud. We
solve this model exactly and find the symmetric, i.e., rotationally invariant,
and dipolar phases expected from the single-molecule problem, but also an axial
and planar nematic phase due to many-body effects. Investigation of the
wavefunction of the macroscopic dipole moment also reveals squeezing of the
probability distribution for the angular momentum of the molecules
Spin drag Hall effect in a rotating Bose mixture
We show that in a rotating two-component Bose mixture, the spin drag between
the two different spin species shows a Hall effect. This spin drag Hall effect
can be observed experimentally by studying the out-of-phase dipole mode of the
mixture. We determine the damping of this mode due to spin drag as a function
of temperature. We find that due to Bose stimulation there is a strong
enhancement of the damping for temperatures close to the critical temperature
for Bose-Einstein condensation.Comment: 1 figur
Quantitative Probe of Pairing Correlations in a Cold Fermionic Atom Gas
A quantitative measure of the pairing correlations present in a cold gas of
fermionic atoms can be obtained by studying the dependence of RF spectra on
hyperfine state populations. This proposal follows from a sum rule that relates
the total interaction energy of the gas to RF spectrum line positions. We argue
that this indicator of pairing correlations provides information comparable to
that available from the spin-susceptibility and NMR measurements common in
condensed-matter systems.Comment: 5 pages, 1 figur
Spin-wave amplification and lasing driven by inhomogeneous spin transfer torques
We show that an inhomogeneity in the spin-transfer torques in a metallic
ferromagnet under suitable conditions strongly amplifies incoming spin waves.
Moreover, at nonzero temperatures the incoming thermally occupied spin waves
will be amplified such that the region with inhomogeneous spin transfer torques
emits spin waves spontaneously, thus constituting a spin-wave laser. We
determine the spin-wave scattering amplitudes for a simplified model and
set-up, and show under which conditions the amplification and lasing occurs.
Our results are interpreted in terms of a so-called black-hole laser, and could
facilitate the field of magnonics, that aims to utilize spin waves in logic and
data-processing devices.Comment: 5 pages, 4 figure
Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids
Magnons in ferromagnets behave as a viscous fluid over a length scale, the
momentum-relaxation length, below which momentum-conserving scattering
processes dominate. We show theoretically that in this hydrodynamic regime
viscous effects lead to a sign change in the magnon chemical potential, which
can be detected as a sign change in the nonlocal resistance measured in spin
transport experiments. This sign change is observable when the
injector-detector distance becomes comparable to the momentum-relaxation
length. Taking into account momentum- and spin-relaxation processes, we
consider the quasiconservation laws for momentum and spin in a magnon fluid.
The resulting equations are solved for nonlocal spin transport devices in which
spin is injected and detected via metallic leads. Because of the finite
viscosity we also find a backflow of magnons close to the injector lead. Our
work shows that nonlocal magnon spin transport devices are an attractive
platform to develop and study magnon-fluid dynamics
Current-driven and field-driven domain walls at nonzero temperature
We present a model for the dynamics of current- and field-driven domain-wall
lines at nonzero temperature. We compute thermally-averaged drift velocities
from the Fokker-Planck equation that describes the nonzero-temperature dynamics
of the domain wall. As special limits of this general description, we describe
rigid domain walls as well as vortex domain walls. In these limits, we
determine also depinning times of the domain wall from an extrinsic pinning
potential. We compare our theory with previous theoretical and experimental
work
Quantum theory of a vortex line in an optical lattice
We investigate the quantum theory of a vortex line in a stack of
weakly-coupled two-dimensional Bose-Einstein condensates, that is created by a
one-dimensional optical lattice. We derive the dispersion relation of the
Kelvin modes of the vortex line and also study the coupling between the Kelvin
modes and the quadrupole modes. We solve the coupled dynamics of the vortex
line and the quadrupole modes, both classically as well as quantum
mechanically. The quantum mechanical solution reveals the possibility of
generating nonequilibrium squeezed vortex states by strongly driving the
quadrupole modes.Comment: Minor changes in response to a referee repor
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