Role of CuO chains in vortex core structure in YBa2Cu3O{7-delta}

The Bogoliubov-deGennes equations are solved for a proximity model for YBa_2Cu_3O_{7-\delta} in a magnetic field. The model explicitly includes the effects of the one-dimensional CuO chains, whose influence on the vortex core structure is studied. The rapid vortex core contraction as a function of field which is seen experimentally at low magnetic fields is naturally explained by the presence of the chains.Comment: 9 pages, 5 figure

DIANE JORGE

The Classical Association conference held at the University of the Orange Free State in January devoted a brief period of a plenary session to an obituary for Diane Jorge, who was to have presented a paper on Catullus but was killed in a tragic motor car accident in Namibia in December 1994. It was a tribute to her achievement and a mark of our loss that her memory was honoured in this unprecendented way, as she was one of the youngest of our colleagues and one who still had to gain a permanent post at a university

ST PAUL’S ENCOUNTER WITH ATHENIAN STOICS AND EPICUREANS

The account in Acts 17 is approached from an historical point ofview in the context of Athens’ situation as an ‘autonomous’ city in aprovince of the Roman Empire. Despite the allusions to the trial ofSocrates, the circumstantial evidence suggests that Paul was notformally put on trial, and if the hearing was more of a public debatethen one might have expected more of a three-cornered exchange.Commentaries on Acts 17 generally focus on Christological issuesreflected in Luke’s account of Paul’s encounter with Stoics andEpicureans in Athens, and naturally treat the episode as a chapter inthe history of Christianity, but the aim here is to approach theepisode more from an historical point of view in the context ofAthens’ situation in a province of the Roman Empire.1 It is argued, ifonly in summary form, that Luke’s text is not meant to be taken asreferring to a formal trial, especially when one allows for literaryinfluences and Luke’s structuring of Paul’s challenges in this periodin Greece

EXTENSION PROGRAMS ON THE SOIL BANK

Teaching/Communication/Extension/Profession,

Soliton Solutions for ABS Lattice Equations II: Casoratians and Bilinearization

In Part I [arXiv:0902.4873 [nlin.SI]] soliton solutions to the ABS list of multi-dimensionally consistent difference equations (except Q4) were derived using connection between the Q3 equation and the NQC equations, and then by reductions. In that work central role was played by a Cauchy matrix. In this work we use a different approach, we derive the $N$-soliton solutions following Hirota's direct and constructive method. This leads to Casoratians and bilinear difference equations. We give here details for the H-series of equations and for Q1; the results for Q3 have been given earlier.Comment: 36 page

The c axis optical conductivity of layered systems in the superconducting state

In this paper, we discuss the c axis optical conductivity Re [sigma_c(omega)] in the high T_c superconductors, in the superconducting state. The basic premise of this work is that electrons travelling along the c axis between adjacent CuO_2 layers must pass through several intervening layers. In earlier work we found that, for weak inter-layer coupling, it is preferable for electrons to travel along the c axis by making a series of interband transitions rather than to stay within a single (and very narrow) band. Moreover, we found that many of the properties of the normal state optical conductivity, including the pseudogap could be explained by interband transitions. In this work we examine the effect of superconductivity on the interband conductivity. We find that, while the onset of superconductivity is clearly evident in the spectrum, there is no clear signature of the symmetry of the superconducting order parameter.Comment: 6 pages, 4 figure

Low temperature magnetization and the excitation spectrum of antiferromagnetic Heisenberg spin rings

Accurate results are obtained for the low temperature magnetization versus magnetic field of Heisenberg spin rings consisting of an even number N of intrinsic spins s = 1/2, 1, 3/2, 2, 5/2, 3, 7/2 with nearest-neighbor antiferromagnetic (AF) exchange by employing a numerically exact quantum Monte Carlo method. A straightforward analysis of this data, in particular the values of the level-crossing fields, provides accurate results for the lowest energy eigenvalue E(N,S,s) for each value of the total spin quantum number S. In particular, the results are substantially more accurate than those provided by the rotational band approximation. For s <= 5/2, data are presented for all even N <= 20, which are particularly relevant for experiments on finite magnetic rings. Furthermore, we find that for s > 1 the dependence of E(N,S,s) on s can be described by a scaling relation, and this relation is shown to hold well for ring sizes up to N = 80 for all intrinsic spins in the range 3/2 <= s <= 7/2. Considering ring sizes in the interval 8 <= N <= 50, we find that the energy gap between the ground state and the first excited state approaches zero proportional to 1/N^a, where a = 0.76 for s = 3/2 and a = 0.84 for s = 5/2. Finally, we demonstrate the usefulness of our present results for E(N,S,s) by examining the Fe12 ring-type magnetic molecule, leading to a new, more accurate estimate of the exchange constant for this system than has been obtained heretofore.Comment: Submitted to Physical Review B, 10 pages, 10 figure

Magnetic properties and critical behavior of disordered Fe_{1-x}Ru_x alloys: a Monte Carlo approach

We study the critical behavior of a quenched random-exchange Ising model with competing interactions on a bcc lattice. This model was introduced in the study of the magnetic behavior of Fe_{1-x}Ru_x alloys for ruthenium concentrations x=0%, x=4%, x=6%, and x=8%. Our study is carried out within a Monte Carlo approach, with the aid of a re-weighting multiple histogram technique. By means of a finite-size scaling analysis of several thermodynamic quantities, taking into account up to the leading irrelevant scaling field term, we find estimates of the critical exponents \alpha, \beta, \gamma, and \nu, and of the critical temperatures of the model. Our results for x=0% are in excellent agreement with those for the three-dimensional pure Ising model in the literature. We also show that our critical exponent estimates for the disordered cases are consistent with those reported for the transition line between paramagnetic and ferromagnetic phases of both randomly dilute and $\pm J$ Ising models. We compare the behavior of the magnetization as a function of temperature with that obtained by Paduani and Branco (2008), qualitatively confirming the mean-field result. However, the comparison of the critical temperatures obtained in this work with experimental measurements suggest that the model (initially obtained in a mean-field approach) needs to be modified