Dynamics and hysteresis in square lattice artificial spin-ice

Dynamical effects under geometrical frustration are considered in a model for artificial spin ice on a square lattice in two dimensions. Each island of the spin ice has a three-component Heisenberg-like dipole moment subject to shape anisotropies that influence its direction. The model has real dynamics, including rotation of the magnetic degrees of freedom, going beyond the Ising-type models of spin ice. The dynamics is studied using a Langevin equation solved via a second order Heun algorithm. Thermodynamic properties such as the specific heat are presented for different couplings. A peak in specific heat is related to a type of melting-like phase transition present in the model. Hysteresis in an applied magnetic field is calculated for model parameters where the system is able to reach thermodynamic equilibrium.Comment: Revised versio

Exploration of Shared Genetic Architecture Between Subcortical Brain Volumes and Anorexia Nervosa

In MRI scans of patients with anorexia nervosa (AN), reductions in brain volume are often apparent. However, it is unknown whether such brain abnormalities are influenced by genetic determinants that partially overlap with those underlying AN. Here, we used a battery of methods (LD score regression, genetic risk scores, sign test, SNP effect concordance analysis, and Mendelian randomization) to investigate the genetic covariation between subcortical brain volumes and risk for AN based on summary measures retrieved from genome-wide association studies of regional brain volumes (ENIGMA consortium, n = 13,170) and genetic risk for AN (PGC-ED consortium, n = 14,477). Genetic correlations ranged from − 0.10 to 0.23 (all p > 0.05). There were some signs of an inverse concordance between greater thalamus volume and risk for AN (permuted p = 0.009, 95% CI: [0.005, 0.017]). A genetic variant in the vicinity of ZW10, a gene involved in cell division, and neurotransmitter and immune system relevant genes, in particular DRD2, was significantly associated with AN only after conditioning on its association with caudate volume (pFDR = 0.025). Another genetic variant linked to LRRC4C, important in axonal and synaptic development, reached significance after conditioning on hippocampal volume (pFDR = 0.021). In this comprehensive set of analyses and based on the largest available sample sizes to date, there was weak evidence for associations between risk for AN and risk for abnormal subcortical brain volumes at a global level (that is, common variant genetic architecture), but suggestive evidence for effects of single genetic markers. Highly powered multimodal brain- and disorder-related genome-wide studies are needed to further dissect the shared genetic influences on brain structure and risk for AN

A molecular-dynamics simulation of the orientational melting of potassium perchlorate

A constant-pressure molecular-dynamics simulation of the first-order phase transition of potassium perchlorate is presented. The simulation yields the correct structures in both the low-temperature ordered phase and the high-temperature orientationally disordered phase. While experiments show a rather ill-behaved reconstructive transition, the simulation, despite its excellent agreement with known crystallographic data, follows a very straightforward transition process. This suggests that the behaviour of the real sample, which is presumably connected with the large volume increase at the transition, is driven by effects on a non-microscopic scale

Raman light scattering experiments of orientationally disordered crystals, and the rigid molecule hypothesis

The widely used rigid molecule assumption is discussed in the light of Raman scattering experiments performed on orientationally disordered crystals. Environment effects inducing the deformation of molecules during their reorientations can be observed by choosing appropriate experimental conditions. Examples are given in the case of adamantane single crystals and neopentane

A Molecular Dynamics Simulation of the Orientationaly Disordered Phase of Potassium Perchlorate

The orientationaly disordered phase of a relatively large system of potassium perchlorate is simulated on a massively parallel computer with strict SIMD architecture. Crystallographic results are compared with known diffraction data. Correlation densities of orientational and translational variables are computed showing orientation-translation coupling of the perchlorate ions, and coupling between a perchlorate orientation and the translation of neighbouring potassium and perchlorate ions. The orientation-orientation correlation between neighbouring perchlorate ions shows antiferromagnetic pseudo-spin type coupling corresponding to ions in opposite $T_{\rm d}$ orientations. This correlation has a lifetime which is longer than the single molecule orientational lifetime, evidence of correlated orientational motion

Orientational disorder in plastic neopentane (paralipomena)

Neutron diffraction experiments performed on orientationally disordered neopentane (C(CD3)4) single crystals are interpreted using a TLS model adapted in order to account as well as possible for the orientational disorder. The molecules are found to be always localized around their most probable orientations at every temperature ; the orientational variables display a large, but practically temperature-independent, dispersion with no sign of reorientational dynamics. In contrast, the centre-of-mass displacements, always much larger than what one expects from the phonon contribution, strongly increase at high temperatures.Des expériences de diffraction de neutrons effectuées sur des monocristaux de néopentane (C(CD3)4) sont réinterprétées à l'aide d'un modèle TLS adapté afin de rendre compte autant que possible du désordre d'orientation. L'on obtient ainsi que les molécules sont localisées autour de leur orientation la plus probable à toutes les températures ; les variables orientationnelles montrent une dispersion importante mais pratiquement indépendante de la température, sans manifestation apparente de la dynamique réorientationnelle. Au contraire, les déplacements des centres de masse, toujours nettement plus grands que ce que l'on attendrait d'une contribution de phonons, s'accroissent fortement à haute température

Vortex lines in a cubic magnetic nanodot: structure and dynamics

Langevin simulations of cubic magnetic nanodots were performed using the Landau-Lifshitz equation with exchange and dipolar interactions. Vortices tend to organize as lines: we establish the structure and dynamics thereof for a large range of the dipolar versus exchange ratio d. These lines tend to be bent and twisted. For large values of the dipolar interaction, a complex network of vortex lines arises. Dynamics evidences low frequency collective gyrotropic motions of vortex lines which maintain their distance during motion

Temperature dependence of internal bandwidths in plastic adamantane by Raman light scattering

The ODIC phase of adamantane (208 K-541 K) was studied by Raman light scattering between 209 K and 500 K. The 1 220 cm-1, E symmetry, vibrational line was recorded with Eg and F2g symmetries. The Eg spectra, due to ordered molecules, are broadened by a residence time effect. The F2g spectra, which correspond to molecules as they rotate, could be attributed to the modification of the internal vibration by the environment during the molecular reorientations.La phase à désordre d'orientation de l'adamantane (208 K-541 K) a été étudiée par diffusion Raman de 209 K à 500 K. La bande de vibration à 1 220 cm -1 de symétrie E a été enregistrée avec les symétries Eg et F 2g. Les spectres Eg, dus aux molécules ordonnées, sont élargis par un effet de temps de résidence. Les spectres F2g, correspondant aux molécules en cours de réorientation, pourraient être attribués à la modification de la vibration interne par l'environnement pendant la rotation