Phase Diagram of the Two Dimensional Lattice Coulomb Gas

We use Monte Carlo simulations to map out the phase diagram of the two dimensional Coulomb gas on a square lattice, as a function of density and temperature. We find that the Kosterlitz-Thouless transition remains up to higher charge densities than has been suggested by recent theoretical estimates.Comment: 4 pages, including 6 in-line eps figure

Numerical Study of Spin and Chiral Order in a Two-Dimensional XY Spin Glass

The two dimensional XY spin glass is studied numerically by a finite size scaling method at T=0 in the vortex representation which allows us to compute the exact (in principle) spin and chiral domain wall energies. We confirm earlier predictions that there is no glass phase at any finite T. Our results strongly support the conjecture that both spin and chiral order have the same correlation length exponent $\nu \approx 2.70$. We obtain preliminary results in 3d.Comment: 4 pages, 2 figures, revte

Numerical Study of Order in a Gauge Glass Model

The XY model with quenched random phase shifts is studied by a T=0 finite size defect energy scaling method in 2d and 3d. The defect energy is defined by a change in the boundary conditions from those compatible with the true ground state configuration for a given realization of disorder. A numerical technique, which is exact in principle, is used to evaluate this energy and to estimate the stiffness exponent $\theta$. This method gives $\theta = -0.36\pm0.013$ in 2d and $\theta = +0.31\pm 0.015$ in 3d, which are considerably larger than previous estimates, strongly suggesting that the lower critical dimension is less than three. Some arguments in favor of these new estimates are given.Comment: 4 pages, 2 figures, revtex. Submitted to Phys. Rev. Let

Percolation properties of the 2D Heisenberg model

We analyze the percolation properties of certain clusters defined on configurations of the 2--dimensional Heisenberg model. We find that, given any direction \vec{n} in O(3) space, the spins almost perpendicular to \vec{n} form a percolating cluster. This result gives indications of how the model can avoid a previously conjectured Kosterlitz-Thouless phase transition at finite temperature T.Comment: 4 pages, 3 eps figures. Revised version (more clear abstract, some new references

Two phase transitions in the fully frustrated XYXY model

The fully frustrated $XY$ model on a square lattice is studied by means of Monte Carlo simulations. A Kosterlitz-Thouless transition is found at $T_{\rm KT} \approx 0.446$, followed by an ordinary Ising transition at a slightly higher temperature, $T_c \approx 0.452$. The non-Ising exponents reported by others, are explained as a failure of finite size scaling due to the screening length associated with the nearby Kosterlitz-Thouless transition.Comment: REVTEX file, 8 pages, 5 figures in uuencoded postscrip

Glassiness Vs. Order in Densely Frustrated Josephson Arrays

We carry out extensive Monte Carlo simulations on the Coulomb gas dual to the uniformly frustrated two dimensional XY model, for a sequence of frustrations f converging to the irraltional (3-sqrt 5)/2. We find in these systems a sharp first order equilibrium phase transition to an ordered vortex structure at a T_c which varies only slightly with f. This ordered vortex structure remains in general phase incoherent until a lower pinning transition T_p(f) that varies with f. We argue that the glassy behaviors reported for this model in earlier simulations are dynamic effects.Comment: 4 pages, 4 eps figure

Psychotic features, particularly mood incongruence, as a hallmark of severity of bipolar I disorder.

The occurrence of psychotic features within mood episodes in patients with bipolar I disorder (BD I) has been associated in some studies with a more severe clinical and socio-professional profile. In contrast, other studies establishing the associations of psychotic features in BD I, and in particular of mood-congruent (MC) and mood-incongruent (MI) features, with clinical characteristics have yielded contradictory results. However, many pre-existing studies have been affected by serious methodological limitations. Using a sample of thoroughly assessed patients with BD I our aims were to: (1) establish the proportion of those with MI and MC features, and (2) compare BD I patients with and without psychotic features as well as those with MI to those with MC features on a wide array of socio-demographic and clinical characteristics including course, psychiatric comorbidity and treatment. A sample of 162 treated patients with BD I (60.5% female, mean age = 41.4 (s.d: 10.2) years) was recruited within a large family study of mood disorders. Clinical, course and treatment characteristics relied on information elicited through direct diagnostic interviews, family history reports and medical records. (1) A total of 96 patients (59.3%) had experienced psychotic features over their lifetime. Among them, 44.8% revealed MI features at least once in their lives. (2) Patients with psychotic features were much less likely to be professionally active, revealed alcohol abuse more frequently and used health care, particularly inpatient treatment, more frequently than those without psychotic features. Within patients with psychotic symptoms, those with MI features showed more clinical severity in terms of a higher likelihood of reporting hallucinations, suicidal attempts and comorbid cannabis dependence. Our data provide additional support for both the distinction between BD-I with and without psychotic features as well as the distinction between MI and MC psychotic features. The more severe course of patients with psychotic features, and particularly those with MI psychotic features, highlights the need for thorough psychopathological evaluations to assess the presence of these symptoms to install appropriate treatment

Evidence of Two Distinct Dynamic Critical Exponents in Connection with Vortex Physics

The dynamic critical exponent $z$ is determined from numerical simulations for the three-dimensional (3D) lattice Coulomb gas (LCG) and the 3D XY models with relaxational dynamics. It is suggested that the dynamics is characterized by two distinct dynamic critical indices $z_0$ and $z$ related to the divergence of the relaxation time $\tau$ by $\tau\propto \xi^{z_0}$ and $\tau\propto k^{-z}$, where $\xi$ is the correlation length and $k$ the wavevector. The values determined are $z_0\approx 1.5$ and $z\approx 1$ for the 3D LCG and $z_0\approx 1.5$ and $z\approx 2$ for the 3D XY model. It is argued that the nonlinear $IV$ exponent relates to $z_0$, whereas the usual Hohenberg-Halperin classification relates to $z$. Possible implications for the interpretation of experiments are pointed out. Comparisons with other existing results are discussed.Comment: to appear in PR

Monte Carlo simulation of a two-dimensional continuum Coulomb gas

We study the classical two-dimensional Coulomb gas model for thermal vortex fluctuations in thin superconducting/superfluid films by Monte Carlo simulation of a grand canonical vortex ensemble defined on a continuum. The Kosterlitz-Thouless transition is well understood at low vortex density, but at high vortex density the nature of the phase diagram and of the vortex phase transition is less clear. From our Monte Carlo data we construct phase diagrams for the 2D Coulomb gas without any restrictions on the vortex density. For negative vortex chemical potential (positive vortex core energy) we always find a Kosterlitz-Thouless transition. Only if the Coulomb interaction is supplemented with a short-distance repulsion, a first order transition line is found, above some positive value of the vortex chemical potential.Comment: 10 pages RevTeX, 7 postscript figures included using eps