A cluster algorithm for resistively shunted Josephson junctions

We present a cluster algorithm for resistively shunted Josephson junctions and similar physical systems, which dramatically improves sampling efficiency. The algorithm combines local updates in Fourier space with rejection-free cluster updates which exploit the symmetries of the Josephson coupling energy. As an application, we consider the localization transition of a single junction at intermediate Josephson coupling and determine the temperature dependence of the zero bias resistance as a function of dissipation strength.Comment: 4 page

Thermodynamics of the 3D Hubbard model on approach to the Neel transition

We study the thermodynamic properties of the 3D Hubbard model for temperatures down to the Neel temperature using cluster dynamical mean-field theory. In particular we calculate the energy, entropy, density, double occupancy and nearest-neighbor spin correlations as a function of chemical potential, temperature and repulsion strength. To make contact with cold-gas experiments, we also compute properties of the system subject to an external trap in the local density approximation. We find that an entropy per particle $S/N \approx 0.65(6)$ at $U/t=8$ is sufficient to achieve a Neel state in the center of the trap, substantially higher than the entropy required in a homogeneous system. Precursors to antiferromagnetism can clearly be observed in nearest-neighbor spin correlators.Comment: 4 pages, 6 figure

Diagrammatic Quantum Monte Carlo solution of the two-dimensional Cooperon-Fermion model

We investigate the two-dimensional cooperon-fermion model in the correlated regime with a new continuous-time diagrammatic determinant quantum Monte Carlo (DDQMC) algorithm. We estimate the transition temperature $T_{c}$, examine the effectively reduced band gap and cooperon mass, and find that delocalization of the cooperons enhances the diamagnetism. When applied to diamagnetism of the pseudogap phase in high-$T_{c}$ cuprates, we obtain results in a qualitative agreement with recent torque magnetization measurements.Comment: 8 pages, 11 figure

Simulation results for an interacting pair of resistively shunted Josephson junctions

Using a new cluster Monte Carlo algorithm, we study the phase diagram and critical properties of an interacting pair of resistively shunted Josephson junctions. This system models tunneling between two electrodes through a small superconducting grain, and is described by a double sine-Gordon model. In accordance with theoretical predictions, we observe three different phases and crossover effects arising from an intermediate coupling fixed point. On the superconductor-to-metal phase boundary, the observed critical behavior is within error-bars the same as in a single junction, with identical values of the critical resistance and a correlation function exponent which depends only on the strength of the Josephson coupling. We explain these critical properties on the basis of a renormalization group (RG) calculation. In addition, we propose an alternative new mean-field theory for this transition, which correctly predicts the location of the phase boundary at intermediate Josephson coupling strength.Comment: 21 pages, some figures best viewed in colo

Swift/UVOT grism monitoring of NGC 5548 in 2013: an attempt at MgII reverberation mapping

Reverberation-mapping-based scaling relations are often used to estimate the masses of black holes from single-epoch spectra of AGN. While the radius-luminosity relation that is the basis of these scaling relations is determined using reverberation mapping of the H$\beta$ line in nearby AGN, the scaling relations are often extended to use other broad emission lines, such as MgII, in order to get black hole masses at higher redshifts when H$\beta$ is redshifted out of the optical waveband. However, there is no radius-luminosity relation determined directly from MgII. Here, we present an attempt to perform reverberation mapping using MgII in the well-studied nearby Seyfert 1, NGC 5548. We used Swift to obtain UV grism spectra of NGC 5548 once every two days from April to September 2013. Concurrent photometric UV monitoring with Swift provides a well determined continuum lightcurve that shows strong variability. The MgII emission line, however, is not strongly correlated with the continuum variability, and there is no significant lag between the two. We discuss these results in the context of using MgII scaling relations to estimate high-redshift black hole masses.Comment: 8 pages, 7 figures, accepted for publication in Ap

Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCu2_2O3_3 and CaV2_2O5_5

We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder system, both by perturbation around the dimer limit and by quantum Monte Carlo simulations. We discuss the influence of the inter-ladder coupling on the spin gap and the dispersion, and present results for the temperature dependence of the uniform susceptibility. The latter was found to be parameterized well by a mean-field type scaling ansatz. Finally we discuss fits of experimental measurements on SrCu$_2$O$_3$ and CaV$_2$O$_5$ to our results.Comment: 7 pages, 8 figure

Quantum spin correlations in an organometallic alternating sign chain

High resolution inelastic neutron scattering is used to study excitations in the organometallic magnet DMACuCl$_3$. The correct magnetic Hamiltonian describing this material has been debated for many years. Combined with high field bulk magnetization and susceptibility studies, the new results imply that DMACuCl$_3$ is a realization of the $S=1/2$ alternating antiferromagnetic-ferromagnetic (AFM-FM) chain. Coupled-cluster calculations are used to derive exchange parameters, showing that the AFM and FM interactions have nearly the same strength. Analysis of the scattering intensities shows clear evidence for inter-dimer spin correlations, in contrast to existing results for conventional alternating chains. The results are discussed in the context of recent ideas concerning quantum entanglement.Comment: 5 pages, 4 figures included in text. Submitted to APS Journal

Neel Temperature of Quasi-Low-Dimensional Heisenberg Antiferromagnets

The N\'eel temperature, $T_{\rm N}$, of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of inter-chain (inter-layer) to intra-chain (intra-layer) coupling $J'/J$ down to $J'/J\simeq 10^{-3}$. We find that $T_{\rm N}$ obeys a modified random-phase approximation-like relation for small $J'/J$ with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulae describing $T_{\rm N}$ for a wide range of $J'$ and useful for the analysis of experimental measurements are presented.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let