282 research outputs found
The "Square Kagome" Quantum Antiferromagnet and the Eight Vertex Model
We introduce a two dimensional network of corner-sharing triangles with
square lattice symmetry. Properties of magnetic systems here should be similar
to those on the kagome lattice. Focusing on the spin half Heisenberg quantum
antiferromagnet, we generalise the spin symmetry group from SU(2) to SU(N). In
the large N limit, we map the model exactly to the eight vertex model, solved
by Baxter. We predict an exponential number of low-lying singlet states, a
triplet gap, and a two-peak specific heat. In addition, the large N limit
suggests a finite temperature phase transition into a phase with ordered
``resonance loops'' and broken translational symmetry.Comment: 5 pages, revtex, 5 eps figures include
Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)
We have studied the low temperature a.c. magnetic susceptibility of the
diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the
frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y
or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that
the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges
for x > 0.4 and persists to x = 1.98. This behavior can be understood as a
non-monotonic dependence of the quantum spin relaxation time with dilution. The
results suggest that the observed spin freezing is fundamentally a single spin
process which is affected by the local environment, rather than the development
of spin-spin correlations as earlier data suggested.Comment: 26 pages, 9 figure
Ordering of the pyrochlore Ising model with the long-range RKKY interaction
The ordering of the Ising model on a pyrochlore lattice interacting via the
long-range RKKY interaction, which models a metallic pyrochlore magnet such as
Pr_2Ir_2O_7, is studied by Monte Carlo simulations. Depending on the parameter
k_F representing the Fermi wavevector, the model exhibits rich ordering
behaviors
Ordered Phase of the Dipolar Spin Ice under [110]-Magnetic Fields
We find that the true ground state of the dipolar spin ice system under
[110]-magnetic fields is the ``Q=X'' structure, which is consistent with both
experiments and Monte Carlo simulations. We then perform a Monte Carlo
simulation to confirm that there exists a first order phase transition under
the [110]-field. In particular this result indicates the existence of the first
order phase transition to the ``Q=X'' phase in the field above 0.35 T for
Dy2Ti2O7. We also show the magnetic field-temperature phase diagram to
summarize the ordered states of this system.Comment: 4 pages, 5 figures, in RevTex4, submitted to J. Phys. Soc. Jp
Dipolar Interactions and Origin of Spin Ice in Ising Pyrochlore Magnets
Recent experiments suggest that the Ising pyrochlore magnets and display qualitative
properties of the spin ice model proposed by Harris {\it et al.} \prl {\bf 79},
2554 (1997). We discuss the dipolar energy scale present in both these
materials and consider how they can display spin ice behavior {\it despite} the
presence of long range interactions. Specifically, we present numerical
simulations and a mean field analysis of pyrochlore Ising systems in the
presence of nearest neighbor exchange and long range dipolar interactions. We
find that two possible phases can occur, a long range ordered antiferromagnetic
one and the other dominated by spin ice features. Our quantitative theory is in
very good agreement with experimental data on both
and . We suggest that the nearest neighbor exchange in
is {\it antiferromagnetic} and that spin ice behavior
is induced by long range dipolar interactions.Comment: 4 postscript figures included. Submitted to Physical Review Letters
Contact: [email protected]
Sigma-2: Multiple sequence alignment of non-coding DNA via an evolutionary model
<p>Abstract</p> <p>Background</p> <p>While most multiple sequence alignment programs expect that all or most of their input is known to be homologous, and penalise insertions and deletions, this is not a reasonable assumption for non-coding DNA, which is much less strongly conserved than protein-coding genes. Arguing that the goal of sequence alignment should be the detection of <it>homology </it>and not <it>similarity</it>, we incorporate an evolutionary model into a previously published multiple sequence alignment program for non-coding DNA, Sigma, as a sensitive likelihood-based way to assess the significance of alignments. Version 1 of Sigma was successful in eliminating spurious alignments but exhibited relatively poor sensitivity on synthetic data. Sigma 1 used a <it>p</it>-value (the probability under the "null hypothesis" of non-homology) to assess the significance of alignments, and, optionally, a background model that captured short-range genomic correlations. Sigma version 2, described here, retains these features, but calculates the <it>p</it>-value using a sophisticated evolutionary model that we describe here, and also allows for a transition matrix for different substitution rates from and to different nucleotides. Our evolutionary model takes separate account of mutation and fixation, and can be extended to allow for locally differing functional constraints on sequence.</p> <p>Results</p> <p>We demonstrate that, on real and synthetic data, Sigma-2 significantly outperforms other programs in specificity to genuine homology (that is, it minimises alignment of spuriously similar regions that do not have a common ancestry) while it is now as sensitive as the best current programs.</p> <p>Conclusions</p> <p>Comparing these results with an extrapolation of the best results from other available programs, we suggest that conservation rates in intergenic DNA are often significantly over-estimated. It is increasingly important to align non-coding DNA correctly, in regulatory genomics and in the context of whole-genome alignment, and Sigma-2 is an important step in that direction.</p
Low Temperature Spin Freezing in Dy2Ti2O7 Spin Ice
We report a study of the low temperature bulk magnetic properties of the spin
ice compound Dy2Ti2O7 with particular attention to the (T < 4 K) spin freezing
transition. While this transition is superficially similar to that in a spin
glass, there are important qualitative differences from spin glass behavior:
the freezing temperature increases slightly with applied magnetic field, and
the distribution of spin relaxation times remains extremely narrow down to the
lowest temperatures. Furthermore, the characteristic spin relaxation time
increases faster than exponentially down to the lowest temperatures studied.
These results indicate that spin-freezing in spin ice materials represents a
novel form of magnetic glassiness associated with the unusual nature of
geometrical frustration in these materials.Comment: 24 pages, 8 figure
Non-regular eigenstate of the XXX model as some limit of the Bethe state
For the one-dimensional XXX model under the periodic boundary conditions, we
discuss two types of eigenvectors, regular eigenvectors which have
finite-valued rapidities satisfying the Bethe ansatz equations, and non-regular
eigenvectors which are descendants of some regular eigenvectors under the
action of the SU(2) spin-lowering operator. It was pointed out by many authors
that the non-regular eigenvectors should correspond to the Bethe ansatz
wavefunctions which have multiple infinite rapidities. However, it has not been
explicitly shown whether such a delicate limiting procedure should be possible.
In this paper, we discuss it explicitly in the level of wavefunctions: we prove
that any non-regular eigenvector of the XXX model is derived from the Bethe
ansatz wavefunctions through some limit of infinite rapidities. We formulate
the regularization also in terms of the algebraic Bethe ansatz method. As an
application of infinite rapidity, we discuss the period of the spectral flow
under the twisted periodic boundary conditions.Comment: 53 pages, no figur
Deformed strings in the Heisenberg model
We investigate solutions to the Bethe equations for the isotropic S = 1/2
Heisenberg chain involving complex, string-like rapidity configurations of
arbitrary length. Going beyond the traditional string hypothesis of undeformed
strings, we describe a general procedure to construct eigenstates including
strings with generic deformations, discuss general features of these solutions,
and provide a number of explicit examples including complete solutions for all
wavefunctions of short chains. We finally investigate some singular cases and
show from simple symmetry arguments that their contribution to zero-temperature
correlation functions vanishes.Comment: 34 pages, 13 figure
Mutations in FUS lead to synaptic dysregulation in ALS-iPSC derived neurons
Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset neurodegenerative disorder characterized by progressive muscular weakness due to the selective loss of motor neurons. Mutations in the gene Fused in Sarcoma (FUS) were identified as one cause of ALS. Here, we report that mutations in FUS lead to upregulation of synaptic proteins, increasing synaptic activity and abnormal release of vesicles at the synaptic cleft. Consequently, FUS-ALS neurons showed greater vulnerability to glutamate excitotoxicity, which raised neuronal swellings (varicose neurites) and led to neuronal death. Fragile X mental retardation protein (FMRP) is an RNA-binding protein known to regulate synaptic protein translation, and its expression is reduced in the FUS-ALS lines. Collectively, our data suggest that a reduction of FMRP levels alters the synaptic protein dynamics, leading to synaptic dysfunction and glutamate excitotoxicity. Here, we present a mechanistic hypothesis linking dysregulation of peripheral translation with synaptic vulnerability in the pathogenesis of FUS-ALS.</p
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