Classical and quantum two-dimensional anisotropic Heisenberg antiferromagnets

The classical and the quantum, spin $S=1/2, versions of the uniaxially anisotropic Heisenberg antiferromagnet on a square lattice in a field parallel to the easy axis are studied using Monte Carlo techniques. For the classical version, attention is drawn to biconical structures and fluctuations at low temperatures in the transition region between the antiferromagnetic and spin-flop phases. For the quantum version, the previously proposed scenario of a first-order transition between the antiferromagnetic and spin-flop phases with a critical endpoint and a tricritical point is scrutinized.Comment: 5 pages, 7 figures, accepted by Phys. Rev.

Quenched charge disorder in CuO2 spin chains: Experimental and numerical studies

We report on measurements of the magnetic response of the anisotropic CuO_2 spin chains in lightly hole-doped La_x (Ca,Sr)_14-x Cu_24 O_41, x>=5. The experimental data suggest that in magnetic fields B >~ 4T (applied along the easy axis) the system is characterized by short-range spin order and quasi-static (quenched) charge disorder. The magnetic susceptibility chi(B) shows a broad anomaly, which we interpret as the remnant of a spin-flop transition. To corroborate this idea, we present Monte Carlo simulations of a classical, anisotropic Heisenberg model with randomly distributed, static holes. Our numerical results clearly show that the spin-flop transition of the pure model (without holes) is destroyed and smeared out due to the disorder introduced by the quasi-static holes. Both the numerically calculated susceptibility curves chi(B) and the temperature dependence of the position of the anomaly are in qualitative agreement with the experimental data.Comment: 10 pages, REVTeX4. 11 figures; v2: Fig.2 replaced, small changes in Figs.1 and 11; minor revisons in Sec. III.C; accepted by Phys. Rev.

Dynamics of surface steps

In the framework of SOS models, the dynamics of isolated and pairs of surface steps of monoatomic height is studied, for step--edge diffusion and for evaporation kinetics, using Monte Carlo techniques. In particular, various interesting crossover phenomena are identified. Simulational results are compared, especially, to those of continuum theories and random walk descriptions.Comment: 13 pages in elsart style, 4 eps figures, submitted to Physica

Quantum phase transitions of the diluted O(3) rotor model

We study the phase diagram and the quantum phase transitions of a site-diluted two-dimensional O(3) quantum rotor model by means of large-scale Monte-Carlo simulations. This system has two quantum phase transitions, a generic one for small dilutions, and a percolation transition across the lattice percolation threshold. We determine the critical behavior for both transitions and for the multicritical point that separates them. In contrast to the exotic scaling scenarios found in other random quantum systems, all these transitions are characterized by finite-disorder fixed points with power-law scaling. We relate our findings to a recent classification of phase transitions with quenched disorder according to the rare region dimensionality, and we discuss experiments in disordered quantum magnets.Comment: 11 pages, 14 eps figures, final version as publishe

Classical and quantum anisotropic Heisenberg antiferromagnets

We study classical and quantum Heisenberg antiferromagnets with exchange anisotropy of XXZ-type and crystal field single-ion terms of quadratic and cubic form in a field. The magnets display a variety of phases, including the spin-flop (or, in the quantum case, spin-liquid) and biconical (corresponding, in the quantum lattice gas description, to supersolid) phases. Applying ground-state considerations, Monte Carlo and density matrix renormalization group methods, the impact of quantum effects and lattice dimension is analysed. Interesting critical and multicritical behaviour may occur at quantum and thermal phase transitions.Comment: 13 pages, 14 figures, conferenc

Interfacial adsorption phenomena of the three-dimensional three-state Potts model

We study the interfacial adsorption phenomena of the three-state ferromagnetic Potts model on the simple cubic lattice by the Monte Carlo method. Finite-size scaling analyses of the net-adsorption yield the evidence of the phase transition being of first-order and $k_{\rm B} T_{\rm C} / J = 1.8166 (2)$.Comment: 14 page

Boundary critical behaviour of two-dimensional random Ising models

Using Monte Carlo techniques and a star-triangle transformation, Ising models with random, 'strong' and 'weak', nearest-neighbour ferromagnetic couplings on a square lattice with a (1,1) surface are studied near the phase transition. Both surface and bulk critical properties are investigated. In particular, the critical exponents of the surface magnetization, 'beta_1', of the correlation length, 'nu', and of the critical surface correlations, 'eta_{\parallel}', are analysed.Comment: 16 pages in ioplppt style, 7 ps figures, submitted to J. Phys.

Virtual Compton scattering off nuclei in the Δ\Delta-resonance region

Virtual Compton scattering in the $\Delta$-resonance region is considered in the case of a target nucleus. The discussion involves generalized polarizabilities and is developed for zero-spin nuclei, focusing on the new information coming from virtual Compton scattering in comparison with real Compton scattering.Comment: 8 pages, LaTeX, 3 figures available from the author

Droplets in the coexistence region of the two-dimensional Ising model

The two-dimensional Ising model with fixed magnetization is studied using Monte Carlo techniques. At the coexistence line, the macroscopic, extensive droplet of minority spins becomes thermally unstable by breaking up into microscopic clusters. Intriguing finite--size effects as well as singularities of thermal and cluster properties associated with the transition are discussed.Comment: 7 pages, 3 figures included, submitted to J. Phys. A: Math. Ge

Invasive Obstetric Procedures and Cesarean Sections in Women With Known Herpes Simplex Virus Status During Pregnancy.

BackgroundNeonatal herpes is a potentially devastating infection that results from acquisition of herpes simplex virus (HSV) type 1 or 2 from the maternal genital tract at the time of vaginal delivery. Current guidelines recommend (1) cesarean delivery if maternal genital HSV lesions are present at the time of labor and (2) antiviral suppressive therapy for women with known genital herpes to decrease HSV shedding from the genital tract at the time of vaginal delivery. However, most neonatal infections occur in infants born to women without a history of genital HSV, making current prevention efforts ineffective for this group. Although routine serologic HSV testing of women during pregnancy could identify women at higher risk of intrapartum viral shedding, it is uncertain how this knowledge might impact intrapartum management, and a potential concern is a higher rate of cesarean sections among women known to be HSV-2 seropositive.MethodsTo assess the effects of prenatal HSV-2 antibody testing, history of genital herpes, and use of suppressive antiviral medication on the intrapartum management of women, we investigated the frequency of invasive obstetric procedures and cesarean deliveries. We conducted a retrospective cohort study of pregnant women delivering at the University of Washington Medical center in Seattle, Washington. We defined the exposure of interest as HSV-2 antibody positivity or known history of genital herpes noted in prenatal records. The primary outcome was intrapartum procedures including fetal scalp electrode, artificial rupture of membranes, intrauterine pressure catheter, or operative vaginal delivery (vacuum or forceps). The secondary outcome was incidence of cesarean birth. Univariate and multivariable logistic regressions were performed.ResultsFrom a total of 449 women included in the analysis, 97 (21.6%) were HSV-2 seropositive or had a history of genital herpes (HSV-2/GH). Herpes simplex virus-2/GH women not using suppressive antiviral therapy were less likely to undergo intrapartum procedures than women without HSV-2/GH (odds ratio [OR], 0.49; 95% confidence interval [CI], 0.25-0.95; P = .036), but this relationship was attenuated after adjustment for potential confounders (adjusted OR, 0.69; 95% CI, 0.34-1.41; P = .31). There was no difference in intrapartum procedures for women on suppressive therapy versus women without HSV-2/GH (OR, 1.17; 95% CI, 0.66-2.07; P = .60). Similar proportions of cesarean sections were performed within each group of women: 25% without history of HSV-2/GH, 30% on suppressive treatment, and 28.1% without suppressive treatment (global, P = .73).ConclusionsIn this single-site study, provider awareness of genital herpes infection either by HSV serotesting or history was associated with fewer invasive obstetric procedures shown to be associated with neonatal herpes, but it was not associated with an increased rate of cesarean birth