2,960 research outputs found
Time-dependent spin-wave theory
We generalize the spin-wave expansion in powers of the inverse spin to
time-dependent quantum spin models describing rotating magnets or magnets in
time-dependent external fields. We show that in these cases, the spin operators
should be projected onto properly defined rotating reference frames before the
spin components are bosonized using the Holstein-Primakoff transformation. As a
first application of our approach, we calculate the reorganization of the
magnetic state due to Bose-Einstein condensation of magnons in the magnetic
insulator yttrium-iron garnet; we predict a characteristic dip in the
magnetization which should be measurable in experiments.Comment: 6 pages, 5 figures, final version published in PR
On the Geometry and Entropy of Non-Hamiltonian Phase Space
We analyze the equilibrium statistical mechanics of canonical, non-canonical
and non-Hamiltonian equations of motion by throwing light into the peculiar
geometric structure of phase space. Some fundamental issues regarding time
translation and phase space measure are clarified. In particular, we emphasize
that a phase space measure should be defined by means of the Jacobian of the
transformation between different types of coordinates since such a determinant
is different from zero in the non-canonical case even if the phase space
compressibility is null. Instead, the Jacobian determinant associated with
phase space flows is unity whenever non-canonical coordinates lead to a
vanishing compressibility, so that its use in order to define a measure may not
be always correct. To better illustrate this point, we derive a mathematical
condition for defining non-Hamiltonian phase space flows with zero
compressibility. The Jacobian determinant associated with time evolution in
phase space is altogether useful for analyzing time translation invariance. The
proper definition of a phase space measure is particularly important when
defining the entropy functional in the canonical, non-canonical, and
non-Hamiltonian cases. We show how the use of relative entropies can circumvent
some subtle problems that are encountered when dealing with continuous
probability distributions and phase space measures. Finally, a maximum
(relative) entropy principle is formulated for non-canonical and
non-Hamiltonian phase space flows.Comment: revised introductio
Searching for the Slater Transition in the Pyrochlore CdOsO with Infrared Spectroscopy
Infrared reflectance measurements were made on the single crystal pyrochlore
CdOsO in order to examine the transformations of the
electronic structure and crystal lattice across the boundary of the metal
insulator transition at . All predicted IR active phonons are
observed in the conductivity over all temperatures and the oscillator strength
is found to be temperature independent. These results indicate that charge
ordering plays only a minor role in the MIT and that the transition is strictly
electronic in nature. The conductivity shows the clear opening of a gap with
. The gap opens continuously, with a temperature
dependence similar to that of BCS superconductors, and the gap edge having a
distinct dependence. All of these
observables support the suggestion of a Slater transition in CdOsO.Comment: 4 pages, 4 figure
Evening choruses in the Perth Canyon and their potential link with Myctophidae fishes
An evening chorus centered at near 2.2 kHz was detected across the years 2000 to 2014 from seabed receivers in 430-490 m depth overlooking the Perth Canyon, Western Australia. The chorus reached a maximum level typically 2.1 h post-sunset and normally ran for 2.1 h (between 3 dB down points). It was present at lower levels across most of the hours of darkness. Maximum chorus spectrum levels were 74-76 dB re 1 µPa2/Hz in the 2 kHz 1/3 octave band, averaging 6-12 dB and up to 30 dB greater than pre-sunset levels. The chorus displayed highest levels over April to August each year with up to 10 dB differences between seasons. The spatial extent of the chorus was not determined but exceeded the sampling range of 13-15 km offshore from the 300 m depth contour and 33 km along the 300 m depth contour. The chorus comprised short damped pulses. The most likely chorus source is considered to be fishes of the family Myctophidae foraging in the water column. The large chorus spatial extent and its apparent correlation with regions of high productivity suggest it may act as an acoustic beacon to marine fauna indicating regions of high biomass
Time of arrival in the presence of interactions
We introduce a formalism for the calculation of the time of arrival t at a
space point for particles traveling through interacting media. We develop a
general formulation that employs quantum canonical transformations from the
free to the interacting cases to construct t in the context of the Positive
Operator Valued Measures. We then compute the probability distribution in the
times of arrival at a point for particles that have undergone reflection,
transmission or tunneling off finite potential barriers. For narrow Gaussian
initial wave packets we obtain multimodal time distributions of the reflected
packets and a combination of the Hartman effect with unexpected retardation in
tunneling. We also employ explicitly our formalism to deal with arrivals in the
interaction region for the step and linear potentials.Comment: 20 pages including 5 eps figure
Prevalence of Small-scale Jets from the Networks of the Solar Transition Region and Chromosphere
As the interface between the Sun's photosphere and corona, the chromosphere
and transition region play a key role in the formation and acceleration of the
solar wind. Observations from the Interface Region Imaging Spectrograph reveal
the prevalence of intermittent small-scale jets with speeds of 80-250 km/s from
the narrow bright network lanes of this interface region. These jets have
lifetimes of 20-80 seconds and widths of 300 km or less. They originate from
small-scale bright regions, often preceded by footpoint brightenings and
accompanied by transverse waves with ~20 km/s amplitudes. Many jets reach
temperatures of at least ~100000 K and constitute an important element of the
transition region structures. They are likely an intermittent but persistent
source of mass and energy for the solar wind.Comment: Figs 1-4 & S1-S5; Movies S1-S8; published in Science, including the
main text and supplementary materials. Reference: H. Tian, E. E. DeLuca, S.
R. Cranmer, et al., Science 346, 1255711 (2014
Minivoids in the Local Volume
We consider a sphere of 7.5 Mpc radius, which contains 355 galaxies with
accurately measured distances, to detect the nearest empty volumes. Using a
simple void detection algorithm, we found six large (mini)voids in Aquila,
Eridanus, Leo, Vela, Cepheus and Octans, each of more than 30 Mpc^3. Besides
them, 24 middle-size "bubbles" of more than 5 Mpc^3 volume are detected, as
well as 52 small "pores". The six largest minivoids occupy 58% of the
considered volume. Addition of the bubbles and pores to them increases the
total empty volume up to 75% and 81%, respectively. The detected local voids
look like oblong potatoes with typical axial ratios b/a = 0.75 and c/a = 0.62
(in the triaxial ellipsoide approximation). Being arranged by the size of their
volume, local voids follow power law of volumes-rankes dependence. A
correlation Gamma-function of the Local Volume galaxies follows a power low
with a formally calculated fractal dimension D = 1.5. We found that galaxies
surrounding the local minivoids do not differ significantly from other nearby
galaxies on their luminosity, but have appreciably higher hydrogen
mass-to-luminosity ratio and also higher star formation rate. We recognize an
effect of local expansion of typical minivoid to be \Delta H/H_0~(25+-15)%.Comment: 23 pages, 18 figures. Astrophysical Journal, accepte
Diffusion controlled initial recombination
This work addresses nucleation rates in systems with strong initial
recombination. Initial (or `geminate') recombination is a process where a
dissociated structure (anion, vortex, kink etc.) recombines with its twin
brother (cation, anti-vortex, anti-kink) generated in the same nucleation
event. Initial recombination is important if there is an asymptotically
vanishing interaction force instead of a generic saddle-type activation
barrier. At low temperatures, initial recombination strongly dominates
homogeneous recombination. In a first part, we discuss the effect in one-,
two-, and three-dimensional diffusion controlled systems with spherical
symmetry. Since there is no well-defined saddle, we introduce a threshold which
is to some extent arbitrary but which is restricted by physically reasonable
conditions. We show that the dependence of the nucleation rate on the specific
choice of this threshold is strongest for one-dimensional systems and decreases
in higher dimensions. We discuss also the influence of a weak driving force and
show that the transport current is directly determined by the imbalance of the
activation rate in the direction of the field and the rate against this
direction. In a second part, we apply the results to the overdamped sine-Gordon
system at equilibrium. It turns out that diffusive initial recombination is the
essential mechanism which governs the equilibrium kink nucleation rate. We
emphasize analogies between the single particle problem with initial
recombination and the multi-dimensional kink-antikink nucleation problem.Comment: LaTeX, 11 pages, 1 ps-figures Extended versio
Brownian markets
Financial market dynamics is rigorously studied via the exact generalized
Langevin equation. Assuming market Brownian self-similarity, the market return
rate memory and autocorrelation functions are derived, which exhibit an
oscillatory-decaying behavior with a long-time tail, similar to empirical
observations. Individual stocks are also described via the generalized Langevin
equation. They are classified by their relation to the market memory as heavy,
neutral and light stocks, possessing different kinds of autocorrelation
functions
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