Poling effect on distribution of quenched random fields in a uniaxial relaxor ferroelectric

The frequency dependence of the dielectric permitivity's maximum has been studied for poled and unpoled doped relaxor strontium barium niobate $Sr_{0.61}Ba_{0.39}Nb_{2}O_{6}:Cr^{3+}$ (SBN-61:Cr). In both cases the maximum found is broad and the frequency dispersion is strong. The present view of random fields compensation in the unpoled sample is not suitable for explaining this experimental result. We propose a new mechanism where the dispersion of quenched random electric fields, affecting the nanodomains, is minimized after poling. We test our proposal by numerical simulations on a random field Ising model. Results obtained are in agreement with the polarization's measurements presented by Granzow et al. [Phys. Rev. Lett {\bf 92}, 065701 (2004)].Comment: 7 pages, 4 figure

Retention of improvement in gait stability over 14 weeks due to trip-perturbation training is dependent on perturbation dose

© 2018 Elsevier Ltd Perturbation training is an emerging approach to reduce fall risk in the elderly. This study examined potential differences in retention of improvements in reactive gait stability over 14 weeks resulting from unexpected trip-like gait perturbations. Twenty-four healthy middle-aged adults (41–62 years) were assigned randomly to either a single perturbation group (SINGLE, n = 9) or a group subjected to eight trip-like gait perturbations (MULTIPLE, n = 15). While participants walked on a treadmill a custom-built brake-and-release system was used to unexpectedly apply resistance during swing phase to the lower right limb via an ankle strap. The anteroposterior margin of stability (MoS) was calculated as the difference between the anterior boundary of the base of support and the extrapolated centre of mass at foot touchdown for the perturbed step and the first recovery step during the first and second (MULTIPLE group only) perturbation trials for the initial walking session and retention-test walking 14 weeks later. Group MULTIPLE retained the improvements in reactive gait stability to the perturbations (increased MoS at touchdown for perturbed and first recovery steps; p < 0.01). However, in group SINGLE no differences in MoS were detected after 14 weeks compared to the initial walking session. These findings provide evidence for the requirement of a threshold trip-perturbation dose if adaptive changes in the human neuromotor system over several months, aimed at the improvement in fall-resisting skills, are to occur

Validation of a tearing mode locking model using a database of disruptive plasmas at ASDEX Upgrade

An exploratory study is presented that aims at validating a model for mode locking on the basis of a large set of ASDEX Upgrade (AUG) discharges. Not discriminating between plasma configurations, the model allows to estimate the duration of the deceleration phase, as well as the critical mode width for locking. Both quantities are important for the design of disruption avoidance algorithms that aim to affect the MHD mode rotation. It was found that the model successfully described locking of large modes in those cases where the deceleration started in a quasi-stationary phase of the discharge (i.e. with low variability of the global plasma angular momentum prior to mode seeding) and where deceleration took place over temporal intervals comparable to the momentum confinement time. Theoretical braking curves and locking durations predicted with the model were in good quantitative agreement with the experiment. On the other hand, the model failed to reproduce the braking curves of modes appearing towards the end of a transient phase, e.g. during an impurity influx or when approaching the disruptive density limit. It can be concluded that the modes were not the primary cause of the plasma momentum losses within the scope of the model. A modified mode equation of motion is proposed, which accounts for transient variation of the plasma density, e.g. during the development of a MARFE, and its impact on braking predictions is discussed. Furthermore, it was observed that a substantial fraction of modes was rotating at the onset of a major disruptive event. Consequences of this observation on disruption prediction schemes in AUG are examined

XY models with disorder and symmetry-breaking fields in two dimensions

The combined effect of disorder and symmetry-breaking fields on the two-dimensional XY model is examined. The study includes disorder in the interaction among spins in the form of random phase shifts as well as disorder in the local orientation of the field. The phase diagrams are determined and the properties of the various phases and phase transitions are calculated. We use a renormalization group approach in the Coulomb gas representation of the model. Our results differ from those obtained for special cases in previous works. In particular, we find a changed topology of the phase diagram that is composed of phases with long-range order, quasi-long-range order, and short-range order. The discrepancies can be ascribed to a breakdown of the fugacity expansion in the Coulomb gas representation. Implications for physical systems such as planar Josephson junctions and the faceting of crystal surfaces are discussed.Comment: 17 pages Latex with 5 eps figures, change: acknowledgment extende

Competing orders in PZN-xPT and PMN-xPT relaxor ferroelectrics

Neutron and x-ray scattering studies on relaxor ferroelectric systems Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (PZN), Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ (PMN), and their solid solutions with PbTiO$_3$ (PT) have shown that inhomogeneities and disorder play important roles in the materials properties. Although a long-range polar order can be established at low temperature - sometimes with the help of an external electric field; short-range local structures called the ``polar nano-regions'' (PNR) still persist. Both the bulk structure and the PNR have been studied in details. The coexistence and competition of long- and short-range polar orders and how they affect the structural and dynamical properties of relaxor materials are discussed.Comment: Article submitted for JPSJ Special Topics (Novel States of Matter Induced by Frustration