Characteristics of ferroelectric-ferroelastic domains in N{\'e}el-type skyrmion host GaV4_4S8_8

GaV$_4$S$_8$ is a multiferroic semiconductor hosting N{\'e}el-type magnetic skyrmions dressed with electric polarization. At T$_s$ = 42K, the compound undergoes a structural phase transition of weakly first-order, from a non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral structure at low temperatures. Below T$_s$, ferroelectric domains are formed with the electric polarization pointing along any of the four $\left< 111 \right>$ axes. Although in this material the size and the shape of the ferroelectric-ferroelastic domains may act as important limiting factors in the formation of the N{\'e}el-type skyrmion lattice emerging below T$_C$=13\:K, the characteristics of polar domains in GaV$_4$S$_8$ have not been studied yet. Here, we report on the inspection of the local-scale ferroelectric domain distribution in rhombohedral GaV$_4$S$_8$ using low-temperature piezoresponse force microscopy. We observed mechanically and electrically compatible lamellar domain patterns, where the lamellae are aligned parallel to the (100)-type planes with a typical spacing between 100 nm-1.2 $\mu$m. We expect that the control of ferroelectric domain size in polar skyrmion hosts can be exploited for the spatial confinement and manupulation of N{\'e}el-type skyrmions

Scaling of the conductance distribution near the Anderson transition

The single parameter scaling hypothesis is the foundation of our understanding of the Anderson transition. However, the conductance of a disordered system is a fluctuating quantity which does not obey a one parameter scaling law. It is essential to investigate the scaling of the full conductance distribution to establish the scaling hypothesis. We present a clear cut numerical demonstration that the conductance distribution indeed obeys one parameter scaling near the Anderson transition

Numerical verification of universality for the Anderson transition

We analyze the scaling behavior of the higher Lyapunov exponents at the Anderson transition. We estimate the critical exponent and verify its universality and that of the critical conductance distribution for box, Gaussian and Lorentzian distributions of the random potential

The three-dimensional Anderson model of localization with binary random potential

We study the three-dimensional two-band Anderson model of localization and compare our results to experimental results for amorphous metallic alloys (AMA). Using the transfer-matrix method, we identify and characterize the metal-insulator transitions as functions of Fermi level position, band broadening due to disorder and concentration of alloy composition. The appropriate phase diagrams of regions of extended and localized electronic states are studied and qualitative agreement with AMA such as Ti-Ni and Ti-Cu metallic glasses is found. We estimate the critical exponents nu_W, nu_E and nu_x when either disorder W, energy E or concentration x is varied, respectively. All our results are compatible with the universal value nu ~ 1.6 obtained in the single-band Anderson model.Comment: 9 RevTeX4 pages with 11 .eps figures included, submitted to PR

Remote optical fiber sensing based on reflectance due to the swelling of polymer particles produced by dispersion polymerization

Poly (4-vinyl pyridine) particles produced by dispersion polymerization were suspended in a hydrogel membrane and used for remote measurements employing fiber optic sensors. The refractive index of poly(4-vinyl pyridine) particles is higher than the refractive index of the hydrogel. At low pH, protonation of the poly(vinyl pyridine) causes the particles to swell reducing their refractive index and lowering membrane turbidity. The membrane response to pH was monitored using instrumentation originally that was constructed by Michael Civiello and was further developed in this work. A new hydrogel, HYPAN, was evaluated as the medium in which the particles are suspended. Methods for attaching a polymer layer to the distal end of an optical fiber were also examined. A 1310 nm wavelength source and an InGaAs detector were installed in the remote sensing instrument to improve signal quality by lowering the noise associated with the signal. The noise associated with the instrument response results from two sources, one being Rayleigh scattering of the laser pulse as it travels down the optical fiber. The other contribution to the background is the reflection from the optical fiber connector/polymer interface at the distal end of the optical fiber. The use of longer wavelength source compared to 834 nm reduced the background signal. The noise associated with the signal at the longer wavelength, however, is increased compared to the original, 834 nm. The use of angled connectors in the instrument reduced the background of the signal and enhanced the turbidity signal by a factor of three. Using and improving the instrumentation designed for remote optical sensing, it was possible to detect pH changes through 500 meters of optical fiber. The particles produced from 4-vinyl pyridine showed quick response times and relatively large changes in response when placed in a HYPAN membrane. The pKa of these particles was determined to be approximately 4.6 through 500 meters of fiber. Sensors were constructed on the top of an optical fiber connector, because we could not develop a convenient method for attaching a polymer layer on the distal end of an unconnectorized optical fiber

Néel-type skyrmion lattice with confined orientation in the polar magnetic semiconductor GaV4S8

Following the early prediction of the skyrmion lattice (SkL)—a periodic array of spin vortices—it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices