194 research outputs found
Exact spin-orbital separation in a solvable model in one dimension
A one-dimensional model of coupled spin-1/2 spins and pseudospin-1/2 orbitals
with nearest-neighbor interaction is rigorously shown to exhibit spin-orbital
separation by means of a non-local unitary transformation. On an open chain,
this transformation completely decouples the spins from the orbitals in such a
way that the spins become paramagnetic while the orbitals form the soluble XXZ
Heisenberg model. The nature of various correlations is discussed. The more
general cases, which allow spin-orbital separation by the same method, are
pointed out. A generalization for the orbital pseudospin greater than 1/2 is
also discussed. Some qualitative connections are drawn with the recently
observed spin-orbital separation in Sr2CuO3.Comment: 5 page
Semiclassical dynamics of domain walls in the one-dimensional Ising ferromagnet in a transverse field
We investigate analytically and numerically the dynamics of domain walls in a
spin chain with ferromagnetic Ising interaction and subject to an external
magnetic field perpendicular to the easy magnetization axis (transverse field
Ising model). The analytical results obtained within the continuum
approximation and numerical simulations performed for discrete classical model
are used to analyze the quantum properties of domain walls using the
semiclassical approximation. We show that the domain wall spectrum shows a band
structure consisting of 2 non-intersecting zones.Comment: 15 pages, 9 figure
Quantum internal modes of solitons in 1d easy-plane antiferromagnet in strong magnetic field
In presence of a strong external magnetic field the dynamics of solitons in a
one-dimensional easy-plane Heisenberg antiferromagnet exhibits a number of
peculiarities. Dynamics of internal soliton degrees of freedom is essentially
quantum, and they are strongly coupled to the "translational" mode of soliton
movement. These peculiarities lead to considerable changes in the response
functions of the system which can be detected experimentally.Comment: 8 pages, RevTeX, 6 figures, uses psfig.sty, submitted to PR
First- and second-order transitions of the escape rate in ferrimagnetic or antiferromagnetic particles
Quantum-classical escape-rate transition has been studied for two general
forms of magnetic anisotropy in ferrimagnetic or antiferromagnetic particles.
It is found that the range of the first-order transition is greatly reduced as
the system becomes ferrimagnetic and there is no first-order transition in
almost compensated antiferromagnetic particles. These features can be tested
experimentally in nanomagnets like molecular magnets.Comment: 11 pages, 3 figures, to appear in Europhys. Let
A multiplex endpoint RT-PCR assay for quality assessment of RNA extracted from formalin-fixed paraffin-embedded tissues
<p>Abstract</p> <p>Background</p> <p>RNA extracted from formalin-fixed paraffin-embedded (FFPE) samples is chemically modified and degraded, which compromises its use in gene expression studies. Most of the current approaches for RNA quality assessment are not suitable for FFPE derived RNA.</p> <p>Results</p> <p>We have developed a single-tube multiplex endpoint RT-PCR assay specifically designed to evaluate RNA extracted from FFPE tissues for mRNA integrity and performance in reverse transcription - quantitative real-time PCR (RT-qPCR) assays. This single-tube quality control (QC) assay minimises the amount of RNA used in quality control. mRNA integrity and the suitability of RNA for RT-PCR is evaluated by the multiplex endpoint RT-PCR assay using the <it>TBP </it>gene mRNA as the target sequence. The RT-PCR amplicon sizes, 92, 161, 252 and 300 bp, cover a range of amplicon sizes suitable for a wide range of RT-qPCR assays. The QC assay was used to evaluate RNA prepared by two different protocols for extracting total RNA from needle microdissected FFPE breast tumour samples. The amplification products were analysed by gel electrophoresis where the spectrum of amplicon sizes indicated the level of RNA degradation and thus the suitability of the RNA for PCR. The ability of the multiplex endpoint RT-PCR QC assay to identify FFPE samples with an adequate RNA quality was validated by examining the C<sub>q </sub>values of an RT-qPCR assay with an 87 bp amplicon.</p> <p>Conclusions</p> <p>The multiplex endpoint RT-PCR assay is well suited for the determination of the quality of FFPE derived RNAs, to identify which RT-PCR assays they are suitable for, and is also applicable to assess non-FFPE RNA for gene expression studies. Furthermore, the assay can also be used for the evaluation of RNA extraction protocols from FFPE samples.</p
Emergent Ising degrees of freedom in frustrated two-leg ladder and bilayer Heisenberg antiferromagnets
Based on exact diagonalization data for finite quantum Heisenberg
antiferromagnets on two frustrated lattices (two-leg ladder and bilayer) and
analytical arguments we map low-energy degrees of freedom of the spin models in
a magnetic field on classical lattice-gas models. Further we use
transfer-matrix calculations and classical Monte Carlo simulations to give a
quantitative description of low-temperature thermodynamics of the quantum spin
models. The classical lattice-gas model yields an excellent description of the
quantum spin models up to quite large temperatures. The main peculiarity of the
considered frustrated bilayer is a phase transition which occurs at low
temperatures for a wide range of magnetic fields below the saturation magnetic
field and belongs to the two-dimensional Ising model universality class.Comment: 17 pages, 8 figure
The 2D XY model on a finite lattice with structural disorder: quasi-long-range ordering under realistic conditions
We present an analytic approach to study concurrent influence of quenched
non-magnetic site-dilution and finiteness of the lattice on the 2D XY model.
Two significant deeply connected features of this spin model are: a special
type of ordering (quasi-long-range order) below a certain temperature and a
size-dependent mean value of magnetisation in the low-temperature phase that
goes to zero (according to the Mermin-Wagner-Hohenberg theorem) in the
thermodynamic limit. We focus our attention on the asymptotic behaviour of the
spin-spin correlation function and the probability distribution of
magnetisation. The analytic approach is based on the spin-wave approximation
valid for the low-temperature regime and an expansion in the parameters which
characterise the deviation from completely homogeneous configuration of
impurities. We further support the analytic considerations by Monte Carlo
simulations performed for different concentrations of impurities and compare
analytic and MC results. We present as the main quantitative result of the work
the exponent of the spin-spin correlation function power law decay. It is non
universal depending not only on temperature as in the pure model but also on
concentration of magnetic sites. This exponent characterises also the vanishing
of magnetisation with increasing lattice size.Comment: 13 pages, 7 eps figures, style files include
Expansion Around the Mean-Field Solution of the Bak-Sneppen Model
We study a recently proposed equation for the avalanche distribution in the
Bak-Sneppen model. We demonstrate that this equation indirectly relates
,the exponent for the power law distribution of avalanche sizes, to ,
the fractal dimension of an avalanche cluster.We compute this relation
numerically and approximate it analytically up to the second order of expansion
around the mean field exponents. Our results are consistent with Monte Carlo
simulations of Bak-Sneppen model in one and two dimensions.Comment: 5 pages, 2 ps-figures iclude
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