2,094 research outputs found
Spin-chain model of a many-body quantum battery
Recently, it has been shown that energy can be deposited on a collection of
quantum systems at a rate that scales super-extensively. Some of these schemes
for `quantum batteries' rely on the use of global many-body interactions that
take the batteries through a correlated short cut in state space. Here, we
extend the notion of a quantum battery from a collection of a priori isolated
systems to a many-body quantum system with intrinsic interactions.
Specifically, we consider a one-dimensional spin chain with physically
realistic two-body interactions. We find that the spin-spin interactions can
yield an advantage in charging power over the non-interacting case, and we
demonstrate that this advantage can grow super-extensively when the
interactions are long ranged. However, we show that, unlike in previous work,
this advantage is a mean-field interaction effect that does not involve
correlations and that relies on the interactions being intrinsic to the
battery.Comment: 9 pages, 6 figure
Simulation Studies of Nanomagnet-Based Architecture
We report a simulation study on interacting ensembles of Co nanomagnets that
can perform basic logic operations and propagate logic signals, where the state
variable is the magnetization direction. Dipole field coupling between
individual nanomagnets drives the logic functionality of the ensemble and
coordinated arrangements of the nanomagnets allow for the logic signal to
propagate in a predictable way. Problems with the integrity of the logic signal
arising from instabilities in the constituent magnetizations are solved by
introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each
nanomagnet. The enhanced stability allows for more complex components of a
logic architecture capable of random combinatorial logic, including horizontal
wires, vertical wires, junctions, fanout nodes, and a novel universal logic
gate. Our simulations define the focus of scaling trends in nanomagnet-based
logic and provide estimates of the energy dissipation and time per nanomagnet
reversal
Barley staining
GRAIN discolouration or staining is a quality defect in W.A. barley which substantially reduces the marketable value oi the crop. It also causes heavy losses to individual growers whose grain is docked or rejected
Experimental consequences of the s-wave cos(k(x))cos(k(y)) superconductivity in the iron pnictides
The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with emphasis on the cos(k(x))cos(k(y)) nodeless order parameter recently derived by Seo [arXiv:0805.2958, Phys. Rev. Lett. (to be published)]. We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin-relaxation time. This extended s-wave symmetry has line zeros in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the cos(k(x))cos(k(y)) superconducting gap is largest on the smallest (hole) Fermi surface. For the 1/T-1 NMR spin-relaxation rate, the interband contribution is consistent with the current experimental results, including a (nonuniversal) T-3 behavior and the absence of a coherence peak. However, the intraband contribution is considerably larger than the interband contributions and still exhibits a small enhancement in the NMR spin-relaxation rate right below T-c in the clean limit
Supersolidity in electron-hole bilayers with a large density imbalance
We consider an electron-hole bilayer in the limit of extreme density
imbalance, where we have a single particle in one layer interacting
attractively with a Fermi liquid in the other parallel layer. Using an
appropriate variational wave function for the dressed exciton, we provide
strong evidence for the existence of the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) phase in electron-hole bilayers with a large density imbalance.
Furthermore, within this unusual limit of FFLO, we find that a dilute gas of
minority particles forms excitons that condense into a two-dimensional
"supersolid".Comment: 6 pages, 2 figure
Shuttle-launch triangular space station
A triangular space station deployable in orbit is described. The framework is comprized of three trusses, formed of a pair of generally planar faces consistine of foldable struts. The struts expand and lock into rigid structural engagement forming a repetition of equilater triangles and nonfolding diagonal struts interconnecting the two faces. The struts are joined together by node fittings. The framework can be packaged into a size and configuration transportable by a space shuttle. When deployed, the framework provides a large work/construction area and ample planar surface area for solar panels and thermal radiators. A plurity of modules are secured to the framework and then joined by tunnels to make an interconnected modular display. Thruster units for the space station orientation and altitude maintenance are provided
Non-saturating magnetoresistance of inhomogeneous conductors: comparison of experiment and simulation
The silver chalcogenides provide a striking example of the benefits of
imperfection. Nanothreads of excess silver cause distortions in the current
flow that yield a linear and non-saturating transverse magnetoresistance (MR).
Associated with the large and positive MR is a negative longitudinal MR. The
longitudinal MR only occurs in the three-dimensional limit and thereby permits
the determination of a characteristic length scale set by the spatial
inhomogeneity. We find that this fundamental inhomogeneity length can be as
large as ten microns. Systematic measurements of the diagonal and off-diagonal
components of the resistivity tensor in various sample geometries show clear
evidence of the distorted current paths posited in theoretical simulations. We
use a random resistor network model to fit the linear MR, and expand it from
two to three dimensions to depict current distortions in the third (thickness)
dimension. When compared directly to experiments on AgSe and
AgTe, in magnetic fields up to 55 T, the model identifies
conductivity fluctuations due to macroscopic inhomogeneities as the underlying
physical mechanism. It also accounts reasonably quantitatively for the various
components of the resistivity tensor observed in the experiments.Comment: 10 pages, 7 figure
Fulde-Ferrell-Larkin-Ovchinnikov states in one-dimensional spin-polarized ultracold atomic Fermi gases
We present a systematic study of quantum phases in a one-dimensional
spin-polarized Fermi gas. Three comparative theoretical methods are used to
explore the phase diagram at zero temperature: the mean-field theory with
either an order parameter in a single-plane-wave form or a self-consistently
determined order parameter using the Bogoliubov-de Gennes equations, as well as
the exact soluble Bethe ansatz method. We find that a spatially inhomogeneous
Fulde-Ferrell-Larkin-Ovchinnikov phase, which lies between the fully paired BCS
state and the fully polarized normal state, dominates most of the phase diagram
of a uniform gas. The phase transition from the BCS state to the
Fulde-Ferrell-Larkin-Ovchinnikov phase is of second order, and therefore there
are no phase separation states in one-dimensional homogeneous polarized gases.
This is in sharp contrast to the three-dimensional situation, where a phase
separation regime is predicted to occupy a very large space in the phase
diagram. We conjecture that the prediction of the dominance of the phase
separation phases in three dimension could be an artifact of the
non-self-consistent mean-field approximation, which is heavily used in the
study of three-dimensional polarized Fermi gases. We consider also the effect
of a harmonic trapping potential on the phase diagram, and find that in this
case the trap generally leads to phase separation, in accord with the
experimental observations for a trapped gas in three dimension. We finally
investigate the local fermionic density of states of the
Fulde-Ferrell-Larkin-Ovchinnikov ansatz. A two-energy-gap structure is shown
up, which could be used as an experimental probe of the
Fulde-Ferrell-Larkin-Ovchinnikov states.Comment: 22 papes, 19 figure
The BCS Functional for General Pair Interactions
The Bardeen-Cooper-Schrieffer (BCS) functional has recently received renewed
attention as a description of fermionic gases interacting with local pairwise
interactions. We present here a rigorous analysis of the BCS functional for
general pair interaction potentials. For both zero and positive temperature, we
show that the existence of a non-trivial solution of the nonlinear BCS gap
equation is equivalent to the existence of a negative eigenvalue of a certain
linear operator. From this we conclude the existence of a critical temperature
below which the BCS pairing wave function does not vanish identically. For
attractive potentials, we prove that the critical temperature is non-zero and
exponentially small in the strength of the potential.Comment: Revised Version. To appear in Commun. Math. Phys
Classical magnetotransport of inhomogeneous conductors
We present a model of magnetotransport of inhomogeneous conductors based on
an array of coupled four-terminal elements. We show that this model generically
yields non-saturating magnetoresistance at large fields. We also discuss how
this approach simplifies finite-element analysis of bulk inhomogeneous
semiconductors in complex geometries. We argue that this is an explanation of
the observed non-saturating magnetoresistance in silver chalcogenides and
potentially in other disordered conductors. Our method may be used to design
the magnetoresistive response of a microfabricated array.Comment: 12 pages, 13 figures. Minor typos correcte
- …