788 research outputs found
A solvable quantum antiferromagnet model
We introduce a quantum antiferromagnet model, having exactly soluble
thermodynamic properties. It is an infinite range antiferromagnetic Ising model
put in a transverse field. The free energy gives the ground state energy in the
zero temperature limit and it also gives the low temperature behaviour of the
specific heat, the exponential variation of which gives the precise gap
magnitude in the excitation spectrum of the system. The detailed behaviour of
the (random sublattice) staggard magnetisation and susceptibilities are
obtained and studied near the N\'eel temperature and the zero temperature
quantum critical point.Comment: 4 pages, 2 eps figure
Quantum phase transition in a disordered long-range transverse Ising antiferromagnet
We consider a long-range Ising antiferromagnet put in a transverse field
(LRTIAF) with disorder. We have obtained the phase diagrams for both the
classical and quantum case. For the pure case applying quantum Monte Carlo
method, we study the variation of order parameter (spin correlation in the
Trotter direction), susceptibility and average energy of the system for various
values of the transverse field at different temperatures. The antiferromagnetic
order is seen to get immediately broken as soon as the thermal or quantum
fluctuations are added. We discuss generally the phase diagram for the same
LRTIAF model with perturbative Sherrington-Kirkpatrick (SK) type disorder. We
find that while the antiferromagnetic order is immediately broken as one adds
an infinitesimal transverse field or thermal fluctuation to the pure LRTIAF
system, an infinitesimal SK spin glass disorder is enough to induce a stable
glass order in the LRTIAF. This glass order eventually gets destroyed as the
thermal or quantum fluctuations are increased beyond their threshold values and
the transition to para phase occurs. Analytical studies for the phase
transitions are discussed in detail in each case. These transitions have been
confirmed by applying classical and quantum Monte Carlo methods. We show here
that the disordered LRTIAF has a surrogate incubation property of the SK spin
glass phase.Comment: 16 pages, 12 figures. accepted in PR
Do Water Fountain Jets Really Indicate the Onset of the Morphological Metamorphosis of Circumstellar Envelopes?
The small-scale bipolar jets having short dynamical ages from "water fountain
(WF)" sources are regarded as an indication of the onset of circumstellar
envelope morphological metamorphosis of intermediate-mass stars. Such process
usually happens at the end of the asymptotic giant branch (AGB) phase. However,
recent studies found that WFs could be AGB stars or even early planetary
nebulae. This fact prompted the idea that WFs may not necessarily be objects at
the beginning of the morphological transition process. In the present work, we
show that WFs could have different envelope morphologies by studying their
spectral energy distribution profiles. Some WFs have spherical envelopes that
resembles usual AGB stars, while others have aspherical envelopes which are
more common to post-AGB stars. The results imply that WFs may not represent the
earliest stage of the morphological metamorphosis. We further argue that the
dynamical age of a WF jet, which can be calculated from maser proper motions,
may not be the real age of the jet. The dynamical age cannot be used to justify
the moment when the envelope begins to become aspherical, nor to tell the
concrete evolutionary status of the object. A WF jet could be the innermost
part of a larger well-developed jet, which is not necessarily a young jet.Comment: 21 pages, 4 figures, accepted for publication in MNRA
4p states and X-Ray Spectroscopy
The 4p states in transition metals and their compounds usually play minor
roles on their physical quantities. Recent development of resonant x-ray
scattering (RXS) at the K-edge of transition metals, however, casts light on
the 4p states, because the signals on orbital and magnetic superlattice spots
are brought about by the modulation in the 4p states. The 4p states are
extending in solids and thereby sensitive to electronic states at neighboring
sites. This characteristic determines the mechanism of RXS that the intensity
on the orbital superlattice spots are mainly generated by the lattice
distortion and those on magnetic superlattice spots by the coupling of the 4p
states with the orbital polarization in the 3d states at neighboring sites.
Taking up typical examples for orbital and magnetic RXS, we demonstrate these
mechanisms on the basis of the band structure calculation. Finally, we study
the MCD spectra at the K-edge, demonstrating that the same mechanism as the
magnetic RXS is working.Comment: 9 pages, 9 figures, submitted to Physica Scripta (comment
Log-aesthetic Curves as Similarity Geometric Analogue of Euler's Elasticae
In this paper we consider the log-aesthetic curves and their generalization
which are used in CAGD. We consider those curves under similarity geometry and
characterize them as stationary integrable flow on plane curves which is
governed by the Burgers equation. We propose a variational formulation of those
curves whose Euler-Lagrange equation yields the stationary Burgers equation.
Our result suggests that the log-aesthetic curves and their generalization can
be regarded as the similarity geometric analogue of Euler's elasticae
- …