7,136 research outputs found
Spin-spin Correlation in Some Excited States of Transverse Ising Model
We consider the transverse Ising model in one dimension with
nearest-neighbour interaction and calculate exactly the longitudinal spin-spin
correlation for a class of excited states. These states are known to play an
important role in the perturbative treatment of one-dimensional transverse
Ising model with frustrated second-neighbour interaction. To calculate the
correlation, we follow the earlier procedure of Wu, use Szego's theorem and
also use Fisher-Hartwig conjecture. The result is that the correlation decays
algebraically with distance () as and is oscillatory or
non-oscillatory depending on the magnitude of the transverse field.Comment: 5 pages, 1 figur
Exact solution of a 2d random Ising model
The model considered is a d=2 layered random Ising system on a square lattice
with nearest neighbours interaction. It is assumed that all the vertical
couplings are equal and take the positive value J while the horizontal
couplings are quenched random variables which are equal in the same row but can
take the two possible values J and J-K in different rows. The exact solution is
obtained in the limit case of infinite K for any distribution of the horizontal
couplings. The model which corresponds to this limit can be seen as an ordinary
Ising system where the spins of some rows, chosen at random, are frozen in an
antiferromagnetic order. No phase transition is found if the horizontal
couplings are independent random variables while for correlated disorder one
finds a low temperature phase with some glassy properties.Comment: 10 pages, Plain TeX, 3 ps figures, submitted to Europhys. Let
Boundary correlation function of fixed-to-free bcc operators in square-lattice Ising model
We calculate the boundary correlation function of fixed-to-free boundary
condition changing operators in the square-lattice Ising model. The correlation
function is expressed in four different ways using block Toeplitz
determinants. We show that these can be transformed into a scalar Toeplitz
determinant when the size of the matrix is even. To know the asymptotic
behavior of the correlation function at large distance we calculate the
asymptotic behavior of this scalar Toeplitz determinant using the Szeg\"o's
theorem and the Fisher-Hartwig theorem. At the critical temperature we confirm
the power-law behavior of the correlation function predicted by conformal field
theory
Finite Temperature and Dynamical Properties of the Random Transverse-Field Ising Spin Chain
We study numerically the paramagnetic phase of the spin-1/2 random
transverse-field Ising chain, using a mapping to non-interacting fermions. We
extend our earlier work, Phys. Rev. 53, 8486 (1996), to finite temperatures and
to dynamical properties. Our results are consistent with the idea that there
are ``Griffiths-McCoy'' singularities in the paramagnetic phase described by a
continuously varying exponent , where measures the
deviation from criticality. There are some discrepancies between the values of
obtained from different quantities, but this may be due to
corrections to scaling. The average on-site time dependent correlation function
decays with a power law in the paramagnetic phase, namely
, where is imaginary time. However, the typical
value decays with a stretched exponential behavior, ,
where may be related to . We also obtain results for the full
probability distribution of time dependent correlation functions at different
points in the paramagnetic phase.Comment: 10 pages, 14 postscript files included. The discussion of the typical
time dependent correlation function has been greatly expanded. Other papers
of APY are available on-line at http://schubert.ucsc.edu/pete
Exact renormalization of the random transverse-field Ising spin chain in the strongly ordered and strongly disordered Griffiths phases
The real-space renormalization group (RG) treatment of random
transverse-field Ising spin chains by Fisher ({\it Phys. Rev. B{\bf 51}, 6411
(1995)}) has been extended into the strongly ordered and strongly disordered
Griffiths phases and asymptotically exact results are obtained. In the
non-critical region the asymmetry of the renormalization of the couplings and
the transverse fields is related to a non-linear quantum control parameter,
, which is a natural measure of the distance from the quantum critical
point. , which is found to stay invariant along the RG trajectories and
has been expressed by the initial disorder distributions, stands in the
singularity exponents of different physical quantities (magnetization,
susceptibility, specific heat, etc), which are exactly calculated. In this way
we have observed a weak-universality scenario: the Griffiths-McCoy
singularities does not depend on the form of the disorder, provided the
non-linear quantum control parameter has the same value. The exact scaling
function of the magnetization with a small applied magnetic field is calculated
and the critical point magnetization singularity is determined in a simple,
direct way.Comment: 11 page
Critical Behavior and Griffiths-McCoy Singularities in the Two-Dimensional Random Quantum Ising Ferromagnet
We study the quantum phase transition in the two-dimensional random Ising
model in a transverse field by Monte Carlo simulations. We find results similar
to those known analytically in one-dimension. At the critical point, the
dynamical exponent is infinite and the typical correlation function decays with
a stretched exponential dependence on distance. Away from the critical point
there are Griffiths-McCoy singularities, characterized by a single,
continuously varying exponent, z', which diverges at the critical point, as in
one-dimension. Consequently, the zero temperature susceptibility diverges for a
RANGE of parameters about the transition.Comment: 4 pages RevTeX, 3 eps-figures include
Health system constraints to optimal coverage of the prevention of mother-to-child HIV transmission programme in South Africa: lessons from the implementation of the national pilot programme
Background: It is three years since the government of South Africa began implementing a PMTCT programme. Over this period of time attempts have been made to scale up this programme across all provinces under routine health service conditions.
Objectives: To report on the uptake and performance of South Africa\'s national pilot programme for preventing mother to child HIV transmission (PMTCT) and to identify health system constraints to optimal coverage.
Methods: Routine programme data were collected from antenatal records and delivery registers at the pilot sites and interviews were conducted with health workers on site and with provincial programme managers.
Results: Routine PMTCT programme data were collected from all 18 pilot sites for the period January to December 2002. During this period, of 84406 women attending the sites for first antenatal visits, 47267 (56%) agreed to an HIV test. 14340 (30%) of the women tested were HIV positive and of these 7853 (55%) were dispensed nevirapine. 7950 (99%) of infants born to women identified as being HIV positive received nevirapine syrup. 58% (4196/7237) of HIV positive women expressed an intention to exclusively formula feed, and 42% (3041/7237) intended to exclusively breastfeed. 1907 infants were due for 12 month HIV testing between January and December 2002, of these 949 (50%) infants were tested.
Conclusions: Programme effectiveness was limited by the low rate of HIV test acceptance, poor delivery of nevirapine to mothers and inability to track mother-infant pairs postnatally for 12-month HIV testing of infants. Infant feeding intentions of mothers suggest inadequate counselling and possible negative effects of the provision of free formula milk. The poor perfor- mance of the main components of this programme will seriously reduce its operational effectiveness. There is a need for greater integration of VCT within antenatal care, a review of the current policy of providing free formula milk and an alternative model for mother-infant follow up. African Health Sciences Vol. 5 (3) 2005: pp. 213-21
Localization transitions in non-Hermitian quantum mechanics
We study the localization transitions which arise in both one and two
dimensions when quantum mechanical particles described by a random
Schr\"odinger equation are subjected to a constant imaginary vector potential.
A path-integral formulation relates the transition to flux lines depinned from
columnar defects by a transverse magnetic field in superconductors. The theory
predicts that the transverse Meissner effect is accompanied by stretched
exponential relaxation of the field into the bulk and a diverging penetration
depth at the transition.Comment: 4 pages (latex) with 3 figures (epsf) embedded in the text using the
style file epsf.st
Zero--Temperature Quantum Phase Transition of a Two--Dimensional Ising Spin--Glass
We study the quantum transition at in the spin- Ising
spin--glass in a transverse field in two dimensions. The world line path
integral representation of this model corresponds to an effective classical
system in (2+1) dimensions, which we study by Monte Carlo simulations. Values
of the critical exponents are estimated by a finite-size scaling analysis. We
find that the dynamical exponent, , and the correlation length exponent,
, are given by and . Both the linear
and non-linear susceptibility are found to diverge at the critical point.Comment: RevTeX 10 pages + 4 figures (appended as uuencoded, compressed
tar-file), THP21-9
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