9,746 research outputs found
The One-Dimensional ANNNI model in a Transverse Field: Analytic and numerical study of Effective Hamiltonians
We consider a spin- chain with competing nearest and
next-nearest neighbor interactions within a transverse magnetic field, which is
known to be an equiavelent to the ANNNI model. When studing thermodynamics of
the 2D ANNNI model Villain and Bak arrived to a free fermion approximation that
neglects heavy excitations from the ferromagnetic ground state, which is an
appropriate description close to the paramagnetic-ferromagnetic transition. In
the vicinity of the floating-phase/anti-phase transition another sort of
quasiparticles, but free fermions too, appears to be convenient. Although free
fermions are a suitable tool for investigation of the phase diagram and the
critical properties, they are defined on the fictitious lattice which makes the
analysis non-rigorous. Here we deal with a proper fermion scheme which is
especially effective %devised to describe the floating-phase/anti-phase
transition. for performing exact diagonalization calculations for cyclic
chains. Systems up to size has been analysed and the predictions of the
effective fermion Hamiltonian has been confirmed. Various predictions for the
infinite system and the critical properties are derived.Comment: 30 RevTeX pages, 10 postscript figure
Domain walls and chaos in the disordered SOS model
Domain walls, optimal droplets and disorder chaos at zero temperature are
studied numerically for the solid-on-solid model on a random substrate. It is
shown that the ensemble of random curves represented by the domain walls obeys
Schramm's left passage formula with kappa=4 whereas their fractal dimension is
d_s=1.25, and therefore is NOT described by "Stochastic-Loewner-Evolution"
(SLE). Optimal droplets with a lateral size between L and 2L have the same
fractal dimension as domain walls but an energy that saturates at a value of
order O(1) for L->infinity such that arbitrarily large excitations exist which
cost only a small amount of energy. Finally it is demonstrated that the
sensitivity of the ground state to small changes of order delta in the disorder
is subtle: beyond a cross-over length scale L_delta ~ 1/delta the correlations
of the perturbed ground state with the unperturbed ground state, rescaled by
the roughness, are suppressed and approach zero logarithmically.Comment: 23 pages, 11 figure
Average persistence in random walks
We study the first passage time properties of an integrated Brownian curve
both in homogeneous and disordered environments. In a disordered medium we
relate the scaling properties of this center of mass persistence of a random
walker to the average persistence, the latter being the probability P_pr(t)
that the expectation value of the walker's position after time t has not
returned to the initial value. The average persistence is then connected to the
statistics of extreme events of homogeneous random walks which can be computed
exactly for moderate system sizes. As a result we obtain a logarithmic
dependence P_pr(t)~{ln(t)}^theta' with a new exponent theta'=0.191+/-0.002. We
note on a complete correspondence between the average persistence of random
walks and the magnetization autocorrelation function of the transverse-field
Ising chain, in the homogeneous and disordered case.Comment: 6 pages LaTeX, 3 postscript figures include
Particle acceleration close to the supermassive black hole horizon: the case of M87
The radio galaxy M87 has recently been found to be a rapidly variable TeV
emitting source. We analyze the implications of the observed TeV
characteristics and show that it proves challenging to account for them within
conventional acceleration and emission models. We discuss a new pulsar-type
scenario for the origin of variable, very high energy (VHE) emission close to
the central supermassive black hole and show that magneto-centrifugally
accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk
photons to the TeV regime, leading to VHE characteristics close to the observed
ones. This suggests, conversely, that VHE observations of highly under-luminous
AGNs could provide an important diagnostic tool for probing the conditions
prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High
Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for
publication in International Journal of Modern Physics
Nonequilibrium Dynamics and Aging in the Three--Dimensional Ising Spin Glass Model
The low temperature dynamics of the three dimensional Ising spin glass in
zero field with a discrete bond distribution is investigated via MC
simulations. The thermoremanent magnetization is found to decay algebraically
and the temperature dependent exponents agree very well with the experimentally
determined values. The nonequilibrium autocorrelation function shows
a crossover at the waiting (or {\em aging}) time from algebraic {\em
quasi-equilibrium} decay for times to another, faster algebraic
decay for with an exponent similar to one for the remanent
magnetization.Comment: Revtex, 11 pages + 4 figures (included as Latex-files
Observation of Galactic Gamma-ray Sources with VERITAS
We report on VERITAS observations at energies above 200 GeV of known or
potential galactic gamma-ray sources. The observed objects comprise pulsars,
pulsar wind nebulae, high-mass X-ray binaries and gamma-ray sources with
unknown counterparts in other wavelengths. Among the highlights are the
observation of variable gamma-ray emission from the X-ray binary LS I +61 303
and the detection of MGRO J1906+06/HESS J1906+063, an extended gamma-ray source
which could not be associated with any obvious counterpart at lower energies.Comment: Fixed typos in source name
Finite temperature behavior of strongly disordered quantum magnets coupled to a dissipative bath
We study the effect of dissipation on the infinite randomness fixed point and
the Griffiths-McCoy singularities of random transverse Ising systems in chains,
ladders and in two-dimensions. A strong disorder renormalization group scheme
is presented that allows the computation of the finite temperature behavior of
the magnetic susceptibility and the spin specific heat. In the case of Ohmic
dissipation the susceptibility displays a crossover from Griffiths-McCoy
behavior (with a continuously varying dynamical exponent) to classical Curie
behavior at some temperature . The specific heat displays Griffiths-McCoy
singularities over the whole temperature range. For super-Ohmic dissipation we
find an infinite randomness fixed point within the same universality class as
the transverse Ising system without dissipation. In this case the phase diagram
and the parameter dependence of the dynamical exponent in the Griffiths-McCoy
phase can be determined analytically.Comment: 23 pages, 12 figure
Continuous loading of an electrostatic trap for polar molecules
A continuously operated electrostatic trap for polar molecules is
demonstrated. The trap has a volume of ~0.6 cm^3 and holds molecules with a
positive Stark shift. With deuterated ammonia from a quadrupole velocity
filter, a trap density of ~10^8/cm^3 is achieved with an average lifetime of
130 ms and a motional temperature of ~300 mK. The trap offers good starting
conditions for high-precision measurements, and can be used as a first stage in
cooling schemes for molecules and as a "reaction vessel" in cold chemistry.Comment: 4 pages, 3 figures v2: several small improvements, new intr
Ground states versus low-temperature equilibria in random field Ising chains
We discuss with the aid of random walk arguments and exact numerical
computations the magnetization properties of one-dimensional random field
chains. The ground state structure is explained in terms of absorbing and
non-absorbing random walk excursions. At low temperatures, the magnetization
profiles follow those of the ground states except at regions where a local
random field fluctuation makes thermal excitations feasible. This follows also
from the non-absorbing random walks, and implies that the magnetization length
scale is a product of these two scales. It is not simply given by the
Imry-Ma-like ground state domain size nor by the scale of the thermal
excitations.Comment: 7 pages LaTeX, 8 eps-figures include
Ground state properties of fluxlines in a disordered environment
A new numerical method to calculate exact ground states of multi-fluxline
systems with quenched disorder is presented, which is based on the minimum cost
flow algorithm from combinatorial optimization. We discuss several models that
can be studied with this method including their specific implementations,
physically relevant observables and results: 1) the N-line model with N
fluxlines (or directed polymers) in a d-dimensional environment with point
and/or columnar disorder and hard or soft core repulsion; 2) the vortex glass
model for a disordered superconductor in the strong screening limit and 3) the
Sine-Gordon model with random pase shifts in the strong coupling limit.Comment: 4 pages RevTeX, 3 eps-figures include
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