8,602 research outputs found
Sublattice Asymmetric Reductions of Spin Values on Stacked Triangular Lattice Antiferromagnet CsCoBr
We study the reductions of spin values of the ground state on a stacked
triangular antiferromagnet using the spin-wave approach. We find that the spin
reductions have sublattice asymmetry due to the cancellation of the molecular
field. The sublattice asymmetry qualitatively analyzes the NMR results of
CsCoBr.Comment: 5pages, 5figure
Photon Statistics for Single Molecule Non-Linear Spectroscopy
We consider the theory of the non-linear spectroscopy for a single molecule
undergoing stochastic dynamics and interacting with a sequence of two laser
pulses. General expressions for photon counting statistics are obtained, and an
exact solution to the problem of the Kubo-Anderson process is found. In the
limit of impulsive pulses the information on the photon statistics is contained
in the molecule's dipole correlation function. The selective limit where
temporal resolution is maintained, the semi-classical approximation and the
fast modulation limit exhibit general behaviors of this new type of
spectroscopy. We show how the design of the external field leads to rich
insights on dynamics of individual molecules which are different than those
found for an ensemble
Fluctuation-dissipation relations and critical quenches in the transverse field Ising chain
Dynamic correlation and response functions of classical and quantum systems
in thermal equilibrium are connected by fluctuation-dissipation theorems, which
allow an alternative definition of their (unique) temperature. Motivated by
this fundamental property, we revisit the issue of thermalization of closed
many-body quantum systems long after a sudden quench, focussing on the
non-equilibrium dynamics of the Ising chain in a critical transverse field. We
show the emergence of distinct observable-dependent effective temperatures,
which rule out Gibbs thermalization in a strict sense but might still have a
thermodynamic meaning.Comment: 5 pages, 3 figure
Magnetic Flux Loss and Flux Transport in a Decaying Active Region
We estimate the temporal change of magnetic flux perpendicular to the solar
surface in a decaying active region by using a time series of the spatial
distribution of vector magnetic fields in the photosphere. The vector magnetic
fields are derived from full spectropolarimetric measurements with the Solar
Optical Telescope aboard Hinode. We compare a magnetic flux loss rate to a flux
transport rate in a decaying sunspot and its surrounding moat region. The
amount of magnetic flux that decreases in the sunspot and moat region is very
similar to magnetic flux transported to the outer boundary of the moat region.
The flux loss rates [] of magnetic elements with positive and
negative polarities are balanced each other around the outer boundary of the
moat region. These results suggest that most of the magnetic flux in the
sunspot is transported to the outer boundary of the moat region as moving
magnetic features, and then removed from the photosphere by flux cancellation
around the outer boundary of the moat region.Comment: 16 pages, 7 figures, Accepted for publication in Ap
Intraband Optical Spectral Weight in the presence of a van Hove singularity: application to BiSrCaCuO
The Kubo single band sum rule is used to determine the optical spectral
weight of a tight binding band with further than nearest neighbour hopping. We
find for a wide range of parameters and doping concentrations that the change
due to superconductivity at low temperature can be either negative or positive.
In contrast, the kinetic energy change is always negative. We use an ARPES
determined tight binding parametrization of BiSrCaCuO
to investigate whether this can account for recent observations of a positive
change in the spectral weight due to the onset of superconductivity. With this
band structure we find that in the relevant doping regime a straightforward BCS
calculation of the optical spectral weight cannot account for the experimental
observations.Comment: 10 page 9 figure
Mode-Dependent nonequilibrium temperature in aging systems
We introduce an exactly solvable model for glassy dynamics with many
relaxational modes, each one characterized by a different relaxational
time-scale. Analytical solution of the aging dynamics at low temperatures shows
that a nonequilibrium or effective temperature can be associated to each
time-scale or mode. The spectrum of effective temperatures shows two regions
that are separated by an age dependent boundary threshold. Region I is
characterized by partially equilibrated modes that relax faster than the modes
at the threshold boundary. Thermal fluctuations and time-correlations for modes
in region I show that those modes are in mutual thermal equilibrium at a unique
age-dependent effective temperature . In contrast, modes with
relaxational timescales longer than that of modes at the threshold (region II)
show diffusive properties and do not share the common temperature .
The shift of the threshold toward lower energy modes as the system ages, and
the progressive shrinking of region II, determines how the full spectrum of
modes equilibrates. As is usually done in experiments, we have defined a
frequency-dependent effective temperature and we have found that all modes in
region I are mutually equilibrated at the temperature
independently of the probing frequency. The present model aims to explain
transport anomalies observed in supercooled liquids in terms of a collection of
structurally disordered and cooperative rearranging mesoscopic regions.Comment: 26 pages, 11 figure
Zero-th law in structural glasses: an example
We investigate the validity of a zeroth thermodynamic law for non-equilibrium
systems. In order to describe the thermodynamics of the glassy systems, it has
been introduced an extra parameter, the effective temperature which generalizes
the fluctuation-dissipation theorem (FDT) to off-equilibrium systems and
supposedly describes thermal fluctuations around the aging state. In particular
we analyze two coupled systems of harmonic oscillators with Monte Carlo
dynamics. We study in detail two types of dynamics: sequential dynamics, where
the coupling between the subsystems comes only from the Hamiltonian; and
parallel dynamics where there is another source of coupling: the dynamics. We
show how in the first case the effective temperatures of the two interacting
subsystems are different asymptotically due to the smallness of the thermal
conductivity in the aging regime. This explains why, in structural glasses,
different interacting degrees of freedom can stay at different effective
temperatures, and never thermalize.Comment: 10 pages. Contribution to the Proceedings of the ESF SPHINX meeting
`Glassy behaviour of kinetically constrained models' (Barcelona, March 22-25,
2001). To appear in a special issue of J. Phys. Cond. Mat
Exact Analysis of ESR Shift in the Spin-1/2 Heisenberg Antiferromagnetic Chain
A systematic perturbation theory is developed for the ESR shift and is
applied to the spin-1/2 Heisenberg chain. Using the Bethe ansatz technique, we
exactly analyze the resonance shift in the first order of perturbative
expansion with respect to an anisotropic exchange interaction. Exact result for
the whole range of temperature and magnetic field, as well as asymptotic
behavior in the low-temperature limit are presented. The obtained g-shift
strongly depends on magnetic fields at low temperature, showing a significant
deviation from the previous classical result.Comment: 4 pages, 3 figures,to be published in Phys. Rev. Let
Universality of One-Dimensional Heat Conductivity
We show analytically that the heat conductivity of oscillator chains diverges
with system size N as N^{1/3}, which is the same as for one-dimensional fluids.
For long cylinders, we use the hydrodynamic equations for a crystal in one
dimension. This is appropriate for stiff systems such as nanotubes, where the
eventual crossover to a fluid only sets in at unrealistically large N. Despite
the extra equation compared to a fluid, the scaling of the heat conductivity is
unchanged. For strictly one-dimensional chains, we show that the dynamic
equations are those of a fluid at all length scales even if the static order
extends to very large N. The discrepancy between our results and numerical
simulations on Fermi-Pasta-Ulam chains is discussed.Comment: 7 pages, 2 figure
New possibility of the ground state of quarter-filled one-dimensional strongly correlated electronic system interacting with localized spins
We study numerically the ground state properties of the one-dimensional
quarter-filled strongly correlated electronic system interacting
antiferromagnetically with localized spins. It is shown that the
charge-ordered state is significantly stabilized by the introduction of
relatively small coupling with the localized spins. When the coupling becomes
large the spin and charge degrees of freedom behave quite independently and the
ferromagnetism is realized. Moreover, the coexistence of ferromagnetism with
charge order is seen under strong electronic interaction. Our results suggest
that such charge order can be easily controlled by the magnetic field, which
possibly give rise to the giant negative magnetoresistance, and its relation to
phthalocyanine compounds is discussed.Comment: 5pages, 4figure
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