48,090 research outputs found
Quantum sine-Gordon dynamics on analogue curved spacetime in a weakly imperfect scalar Bose gas
Using the coherent state functional integral expression of the partition
function, we show that the sine-Gordon model on an analogue curved spacetime
arises as the effective quantum field theory for phase fluctuations of a weakly
imperfect Bose gas on an incompressible background superfluid flow when these
fluctuations are restricted to a subspace of the single-particle Hilbert space.
We consider bipartitions of the single-particle Hilbert space relevant to
experiments on ultracold bosonic atomic or molecular gases, including, e.g.,
restriction to high- or low-energy sectors of the dynamics and spatial
bipartition corresponding to tunnel-coupled planar Bose gases. By assuming full
unitary quantum control in the low-energy subspace of a trapped gas, we show
that (1) appropriately tuning the particle number statistics of the
lowest-energy mode partially decouples the low- and high-energy sectors,
allowing any low-energy single-particle wave function to define a background
for sine-Gordon dynamics on curved spacetime and (2) macroscopic occupation of
a quantum superposition of two states of the lowest two modes produces an
analogue curved spacetime depending on two background flows, with respective
weights continuously dependent on the corresponding weights of the superposed
quantum states.Comment: 12 pages, 1 figur
Amplification of the quantum superposition macroscopicity of a flux qubit by a magnetized Bose gas
We calculate a measure of superposition macroscopicity for a
superposition of screening current states in a superconducting flux qubit
(SFQ), by relating to the action of an instanton trajectory
connecting the potential wells of the flux qubit. When a magnetized
Bose-Einstein condensed (BEC) gas containing
atoms is brought into a proximity of the flux
qubit in an experimentally realistic geometry, we demonstrate the appearance of
a two- to five-fold amplification of over the bare value without
the BEC, by calculating the instantion trajectory action from the
microscopically derived effective flux Lagrangian of a hybrid quantum system
composed of the flux qubit and a spin- atomic Bose gas. Exploiting the
connection between and the maximal metrological usefulness of a
multimode superposition state, we show that amplification of in
the ground state of the hybrid system is equivalent to a decrease in the
quantum Cram\'{e}r-Rao bound for estimation of an externally applied flux. Our
result therefore demonstrates the increased usefulness of the BEC--SFQ hybrid
system as a sensor of ultraweak magnetic fields below the standard quantum
limit.Comment: 10 pages, 2 figure
Domain formation in membranes with quenched protein obstacles: Lateral heterogeneity and the connection to universality classes
We show that lateral fluidity in membranes containing quenched protein
obstacles belongs to the universality class of the two-dimensional random-field
Ising model. The main feature of this class is the absence of a phase
transition: there is no critical point, and macroscopic domain formation does
not occur. Instead, there is only one phase. This phase is highly
heterogeneous, with a structure consisting of micro-domains. The presence of
quenched protein obstacles thus provides a mechanism to stabilize lipid rafts
in equilibrium. Crucial for two-dimensional random-field Ising universality is
that the obstacles are randomly distributed, and have a preferred affinity to
one of the lipid species. When these conditions are not met, standard Ising or
diluted Ising universality apply. In these cases, a critical point does exist,
marking the onset toward macroscopic demixing.Comment: 10 pages, 10 figure
Red blood cells and other non-spherical capsules in shear flow: oscillatory dynamics and the tank-treading-to-tumbling transition
We consider the motion of red blood cells and other non-spherical
microcapsules dilutely suspended in a simple shear flow. Our analysis indicates
that depending on the viscosity, membrane elasticity, geometry and shear rate,
the particle exhibits either tumbling, tank-treading of the membrane about the
viscous interior with periodic oscillations of the orientation angle, or
intermittent behavior in which the two modes occur alternately. For red blood
cells, we compute the complete phase diagram and identify a novel
tank-treading-to-tumbling transition at low shear rates. Observations of such
motions coupled with our theoretical framework may provide a sensitive means of
assessing capsule properties.Comment: 11 pages, 4 figure
Metastability and uniqueness of vortex states at depinning
We present results from numerical simulations of transport of vortices in the
zero-field cooled (ZFC) and the field-cooled (FC) state of a type-II
superconductor. In the absence of an applied current , we find that the FC
state has a lower defect density than the ZFC state, and is stable against
thermal cycling. On the other hand, by cycling , surprisingly we find that
the ZFC state is the stable state. The FC state is metastable as manifested by
increasing to the depinning current , in which case the FC state
evolves into the ZFC state. We also find that all configurations acquire a
unique defect density at the depinning transition independent of the history of
the initial states.Comment: 4 pages, 4 figures. Problem of page size correcte
Fluids with quenched disorder: Scaling of the free energy barrier near critical points
In the context of Monte Carlo simulations, the analysis of the probability
distribution of the order parameter , as obtained in simulation
boxes of finite linear extension , allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of at
the peaks () and the value at the minimum in-between ()
becomes -independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead should be observed, where is the
"violation of hyperscaling" exponent. Since is substantially non-zero,
the scaling of with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, versus
was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.Comment: sent to J. Phys. Cond. Mat
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