Monte Carlo Simulations of Ultrathin Magnetic Dots

In this work we study the thermodynamic properties of ultrathin ferromagnetic dots using Monte Carlo simulations. We investigate the vortex density as a function of the temperature and the vortex structure in monolayer dots with perpendicular anisotropy and long-range dipole interaction. The interplay between these two terms in the hamiltonian leads to an interesting behavior of the thermodynamic quantities as well as the vortex density.Comment: 10 figure

Phase transition in ultrathin magnetic films with long-range interactions: Monte Carlo simulation of the anisotropic Heisenberg model

Ultrathin magnetic films can be modeled as an anisotropic Heisenberg model with long-range dipolar interactions. It is believed that the phase diagram presents three phases: An ordered ferromagnetic phase I, a phase characterized by a change from out-of-plane to in-plane in the magnetization II, and a high-temperature paramagnetic phase III. It is claimed that the border lines from phase I to III and II to III are of second order and from I to II is first order. In the present work we have performed a very careful Monte Carlo simulation of the model. Our results strongly support that the line separating phases II and III is of the BKT type.Comment: 7 page

Predictions of molecular chirality and helical twisting powers: A theoretical study

A theoretical study of a number of chiral molecules has been undertaken using a molecular Monte Carlo simulation approach coupled with calculations of molecular chirality based on a chirality order parameter. Results for a variety of TADDOL (alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxolan-4,5-dimethanol) derivatives show good agreement with experimental findings for the sign, magnitude, and the temperature dependence of the helical twisting power (HTP). For a photochromic chiral dopant with variable HTP we are able to model the reduction in the HTP when photoisomerization occurs. Our studies on a liquid crystalline material with a single chiral center have reproduced a temperature dependent twist inversion in the material. We discuss the temperature and solvent dependence of the helical twisting power and argue that in all the systems studied here, preferential selection of certain molecular conformations at different temperatures and in different solvents are able to explain the observed experimental behavior of the HTP

Escape configuration lattice near the nematic-isotropic transition: Tilt analogue of blue phases

We predict the possible existence of a new phase of liquid crystals near the nematic-isotropic ($NI$) transition. This phase is an achiral, tilt-analogue of the blue phase and is composed of a lattice of {\em double-tilt}, escape-configuration cylinders. We discuss the structure and the stability of this phase and provide an estimate of the lattice parameter.Comment: 5 pages, 6 figures (major revision, typos corrected, references added

Orientational transition in a nematic liquid crystal at a patterned surface

T. J. Atherton and J. Roy Sambles, Physical Review E, Vol. 74, article 022701 (2006) "Copyright © 2006 by the American Physical Society."We consider a semi-infinite nematic in contact with a periodic patterned surface with alternate planar and homeotropic stripes. Extending the work of Barbero et al., we find the free energy (assuming K1=K3) for the situations where the easy direction on the planar stripe is either perpendicular or parallel to the length of the stripes. We find the bulk free energy difference between the structures to be proportional to √ K2/K1 and so we consider the possibility of a spontaneous transition between the two states if the azimuthal anchoring energy is sufficiently weak and K1≠K2. We compute the critical azimuthal anchoring energy for such a transition in terms of the relative width of the stripes and the period of the pattern and find it to be ~10−6 J m−2, comparable to experimental values

Magnetic friction due to vortex fluctuation

We use Monte Carlo and molecular dynamics simulation to study a magnetic tip-sample interaction. Our interest is to understand the mechanism of heat dissipation when the forces involved in the system are magnetic in essence. We consider a magnetic crystalline substrate composed of several layers interacting magnetically with a tip. The set is put thermally in equilibrium at temperature T by using a numerical Monte Carlo technique. By using that configuration we study its dynamical evolution by integrating numerically the equations of motion. Our results suggests that the heat dissipation in this system is closed related to the appearing of vortices in the sample.Comment: 6 pages, 41 figure

The FDA “black box” warning on antidepressant suicide risk in young adults: More harm than benefits?

The decision made in the year 2004 by the U.S. Food and Drug Administration (FDA) to require a boxed warning on antidepressants regarding the risk of suicidality in young adults still represents a matter of controversy. The FDA warning was grounded on industry-sponsored trials carried one decade ago or earlier. However, within the past decade, an increasing number of reports have questioned the actual validity of the FDA warning, especially considering a decline in the prescription of the antidepressant drugs associated with an increase in the rate of suicidal events among people with severe depression. The present report provides an overview of the FDA black box warning, also documenting two Major Depressive Disorder patients whose refusal to undergo a pharmacological antidepressant treatment possibly led to an increased risk for suicidal behaviors. The concerns raised by the FDA black box warning need to be considered in real-world clinical practice, stating the associated clinical and public health implications

Post-traumatic stress disorder (PTSD) in parents of children with type 1 diabetes during Covid-19 pandemic

Introduction The Post-traumatic Stress Disorder (PTSD) is a group of persistent psychological and physiological symptoms due to a traumatic, severe, event. Only few studies focused on the effects of Covid-19 on psychosocial outcomes in children with Type 1 Diabetes (T1D) and their parents. Aim of the study The aim of this study was to evaluate the presence PTSD in parents of children with T1D during COVID-19 pandemic lockdown. Patients and methods In the period between March and May 2020 we submitted the "Impact of Event Scale - Revised" (IES-R) questionnaire to the parents of 34 children with Type 1 Diabetes, asking them to express their emotions about the ongoing Covid-19 pandemic. Results A total of thirty mothers (mean age 43.0 +/- 4.2 years) and 25 fathers (mean age 45.6 +/- 5.9 years) participated in the survey and completed the questionnaires. 29.1% of parents had a score that allows to define a clinically relevant level of PTSD; ten mothers and 6 fathers had a PTSD clinically relevant score, corresponding, respectively, to 28.4 and 24% of total mothers and fathers. Finally, mothers and fathers, both express PTSD symptoms mainly in the form of intrusion and hyperarousal. Conclusions The present study confirms a high prevalence symptoms related to PTSD in mothers and fathers of children with Type 1 Diabetes. We believe that psychosocial outcomes of the COVID-19 pandemic should be taken into account in the planning of the next future assistance for children with T1D

Layer dynamics of a freely standing smectic-A film

We study the hydrodynamics of a freely-standing smectic-A film in the isothermal, incompressible limit theoretically by analyzing the linearized hydrodynamic equations of motion with proper boundary conditions. The dynamic properties for the system can be obtained from the response functions for the free surfaces. Permeation is included and its importance near the free surfaces is discussed. The hydrodynamic mode structure for the dynamics of the system is compared with that of bulk systems. We show that to describe the dynamic correlation functions for the system, in general, it is necessary to consider the smectic layer displacement $u$ and the velocity normal to the layers, $v_z$, together. Finally, our analysis also provides a basis for the theoretical study of the off-equilibrium dynamics of freely-standing smectic-A films.Comment: 22 pages, 4 figure