6,195 research outputs found
Q-Dependent Susceptibilities in Ferromagnetic Quasiperiodic Z-Invariant Ising Models
We study the q-dependent susceptibility chi(q) of a series of quasiperiodic
Ising models on the square lattice. Several different kinds of aperiodic
sequences of couplings are studied, including the Fibonacci and silver-mean
sequences. Some identities and theorems are generalized and simpler derivations
are presented. We find that the q-dependent susceptibilities are periodic, with
the commensurate peaks of chi(q) located at the same positions as for the
regular Ising models. Hence, incommensurate everywhere-dense peaks can only
occur in cases with mixed ferromagnetic-antiferromagnetic interactions or if
the underlying lattice is aperiodic. For mixed-interaction models the positions
of the peaks depend strongly on the aperiodic sequence chosen.Comment: LaTeX2e, 26 pages, 9 figures (27 eps files). v2: Misprints correcte
Generalized Supersymmetric Perturbation Theory
Using the basic ingredient of supersymmetry, we develop a simple alternative
approach to perturbation theory in one-dimensional non-relativistic quantum
mechanics. The formulae for the energy shifts and wave functions do not involve
tedious calculations which appear in the available perturbation theories. The
model applicable in the same form to both the ground state and excited bound
states, unlike the recently introduced supersymmetric perturbation technique
which, together with other approaches based on logarithmic perturbation theory,
are involved within the more general framework of the present formalism.Comment: 13 pages article in LaTEX (uses standard article.sty). No Figures.
Sent to Ann. Physics (2004
Roots of Unity: Representations of Quantum Groups
Representations of Quantum Groups U_q (g_n), g_n any semi simple Lie algebra
of rank n, are constructed from arbitrary representations of rank n-1 quantum
groups for q a root of unity. Representations which have the maximal dimension
and number of free parameters for irreducible representations arise as special
cases.Comment: 23 page
Nonequilibrium Forces Between Neutral Atoms Mediated by a Quantum Field
We study all known and as yet unknown forces between two neutral atoms,
modeled as three dimensional harmonic oscillators, arising from mutual
influences mediated by an electromagnetic field but not from their direct
interactions. We allow as dynamical variables the center of mass motion of the
atom, its internal degrees of freedom and the quantum field treated
relativistically. We adopt the method of nonequilibrium quantum field theory
which can provide a first principle, systematic and unified description
including the intrinsic field fluctuations and induced dipole fluctuations. The
inclusion of self-consistent back-actions makes possible a fully dynamical
description of these forces valid for general atom motion. In thermal
equilibrium we recover the known forces -- London, van der Waals and
Casimir-Polder forces -- between neutral atoms in the long-time limit but also
discover the existence of two new types of interatomic forces. The first, a
`nonequilibrium force', arises when the field and atoms are not in thermal
equilibrium, and the second, which we call an `entanglement force', originates
from the correlations of the internal degrees of freedom of entangled atoms.Comment: 16 pages, 2 figure
Logarithmic perturbation theory for quasinormal modes
Logarithmic perturbation theory (LPT) is developed and applied to quasinormal
modes (QNMs) in open systems. QNMs often do not form a complete set, so LPT is
especially convenient because summation over a complete set of unperturbed
states is not required. Attention is paid to potentials with exponential tails,
and the example of a Poschl-Teller potential is briefly discussed. A numerical
method is developed that handles the exponentially large wavefunctions which
appear in dealing with QNMs.Comment: 24 pages, 4 Postscript figures, uses ioplppt.sty and epsfig.st
Amino acids whose intracellular levels change most during aging alter chronological lifespan of fission yeast
Amino acid deprivation or supplementation can affect cellular and organismal lifespan, but we know little about the role of concentration changes in free, intracellular amino acids during aging. Here, we determine free amino-acid levels during chronological aging of non-dividing fission yeast cells. We compare wild-type with long-lived mutant cells that lack the Pka1 protein of the protein kinase A signalling pathway. In wild-type cells, total amino-acid levels decrease during aging, but much less so in pka1 mutants. Two amino acids strongly change as a function of age: glutamine decreases, especially in wild-type cells, while aspartate increases, especially in pka1 mutants. Supplementation of glutamine is sufficient to extend the chronological lifespan of wild-type but not of pka1Î cells. Supplementation of aspartate, on the other hand, shortens the lifespan of pka1Î but not of wild-type cells. Our results raise the possibility that certain amino acids are biomarkers of aging, and their concentrations during aging can promote or limit cellular lifespan
Symmetries of Large N Matrix Models for Closed Strings
We obtain the symmetry algebra of multi-matrix models in the planar large N
limit. We use this algebra to associate these matrix models with quantum spin
chains. In particular, certain multi-matrix models are exactly solved by using
known results of solvable spin chain systems.Comment: 12 pages, 1 eps figure, RevTex, some minor typos in the publised
version are correcte
Radiosurgery for brainstem metastases with and without whole brain radiotherapy: clinical series and literature review
Objective The objective of this study was to investigate outcomes for patients with brainstem metastases treated with stereotactic radiosurgery (SRS). Methods Patients with brainstem metastases treated with SRS between April 2006 and June 2012 were identified from a prospective database. Patient and treatment-related factors were recorded. Kaplan-Meier analysis was used to calculate survival and freedom from local and distant brain progression. Univariate and multivariate Cox regression was used to identify factors important for overall survival. Results In total, 44 patients received SRS for 48 brainstem metastases of whom 33 (75 %) also received whole brain radiotherapy (WBRT): 23 patients (52 %) WBRT prior to SRS, 6 (13.6 %) WBRT concurrently with SRS and 4 (9.0 %) WBRT after SRS. Eight patients received a second course ofWBRTat further progression. Median target volume was 1.33 cc (range 0.04â12.17) and median prescribed marginal dose was 15 Gy (range 10â22). There were four cases of local failure, and 6-month and 1-year freedom from local failure was 84.6 and 76.9 %, respectively. Median overall survival (OS) was 5.4 months. There were four cases of radionecrosis, 2 (4.8 %) of which were symptomatic. The absence of external beam brain radiotherapy (predominantly WBRT) showed a trend towards improved OS on univariate analysis. Neither local nor distant brain failure significantly impacted OS. Conclusion This retrospective series of patients treated with SRS for brainstem metastases, largely in combination with at least one course of WBRT, demonstrates that this approach is safe and results in good local control. In this cohort, no variables significantly impacted OS, including intracranial control
Duality and Symmetry in Chiral Potts Model
We discover an Ising-type duality in the general -state chiral Potts
model, which is the Kramers-Wannier duality of planar Ising model when N=2.
This duality relates the spectrum and eigenvectors of one chiral Potts model at
a low temperature (of small ) to those of another chiral Potts model at a
high temperature (of ). The -model and chiral Potts model
on the dual lattice are established alongside the dual chiral Potts models.
With the aid of this duality relation, we exact a precise relationship between
the Onsager-algebra symmetry of a homogeneous superintegrable chiral Potts
model and the -loop-algebra symmetry of its associated
spin- XXZ chain through the identification of their eigenstates.Comment: Latex 34 pages, 2 figures; Typos and misprints in Journal version are
corrected with minor changes in expression of some formula
- âŠ