Phase transitions in planar magnetic nanostructures

Using numerical micromagnetics we have studied the ground state magnetization distribution of square planar ferromagnetic elements ("nanostructures"). As the element size is reduced from 250 to 2 nm at constant thickness (2-35 nm), we find that the magnetization distribution undergoes up to three phase transitions involving as many as three different near single domain states. One of these phase transitions is analogous to the reorientation phase transition observed in continuous ultrathin magnetic films. © 1998 American Institute of Physics.

A model for diffusive and displacive phase transitions: Thermo-chemo-mechanical coupling effects

A diffuse interface phase field model is proposed for the unified analysis of diffusive and displacive phase transitions under nonisothermal conditions. Two order parameters are used for the description of the phenomena: one is related to the solute mass fraction and the other to the strain. The model governing equations come from the balance of linear momentum, the solute mass balance (which will lead to the Cahn-Hilliard equation) and the balance of internal energy. Thermodynamic restrictions allow to define constitutive relations for the thermodynamic forces and for the mechanical and chemical dissipations. Numerical tests carried out at different values of the initial temperature show that the model is able to describe the main features of both the displacive and the diffusive phase transitions, as well as their effect on the temperature. © 2010, Advanced Engineering Solutions.

Stress-induced domain dynamics and phase transitions in epitaxially grown VO₂ nanowires.

We demonstrate that surface stresses in epitaxially grown VO₂ nanowires (NWs) have a strong effect on the appearance and stability of intermediate insulating M₂ phases, as well as the spatial distribution of insulating and metallic domains during structural phase transitions. During the transition from an insulating M1 phase to a metallic R phase, the coexistence of insulating M₁ and M₂ phases with the absence of a metallic R phase was observed at atmospheric pressure. In addition, we show that, for a VO₂ NW without the presence of an epitaxial interface, surface stresses dominantly lead to spatially inhomogeneous phase transitions between insulating and metallic phases. In contrast, for a VO₂ NW with the presence of an epitaxial interface, the strong epitaxial interface interaction leads to additional stresses resulting in uniformly alternating insulating and metallic domains along the NW length.

Photomechanically Induced Phase Transitions in Ferroelectric Liquid Crystals

We report optically induced phase transtions occurring in two different host ferroelectric liquid crystals; SCE13 a multicomponentmixture optimised for room temperature performance, and CE8 a single component liquid crystal. These act as host liquid crystals for a novel guest azo dye, which can be made to photoisomerise using low power density U.V. illumination, resulting in dramatic changes in sample properties. We have shown that the magnitude of spontaneous polarisation of systems can be isothermally and reversibly induced or reduced, with the consequent appearance or disappearance of optical switching hysteresis. We discuss the parameters controlling the behaviour of the systems under U.V. illumination and suggest mechansims by which the transitions may occur. © 1993, Taylor & Francis Group, LLC. All rights reserved.

Superradiance: exploiting quantum phase transition in the real world

Superradiance (SR), or cooperative spontaneous emission, has been predicted by R. Dicke before the invention of the laser. During the last few years one can see a renaissance of both experimental and theoretical studies of the superradiant phase transition in a variety of media, ranging from quantum dots and Bose condensates through to black holes. Until recently, despite of many years of research, SR has been considered as a phenomenon of pure scientific interest without obvious potential applications. However, recent investigations of the femtosecond SR emission generation from semiconductors have opened up some practical opportunities for the exploitation of this quantum optics phenomenon. Here we present a brief review of some features, advantages and potential applications of the SR generation from semiconductor laser structures

Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams.

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.

Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H 25NH3)2PbI4(1-y)Br4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices. © 2013 AIP Publishing LLC.

Efficient InGaAsP/InP multiple quantum well optical phase modulator for the 1.5-1.55μm fiber band

We report an InGaAsP/InP MQW phase modulator operating over the entire 1.55μm fiber band with high phase modulation efficiency and low loss modulation. The spectral dependence of the electro-refraction in a MQW structure is measured for the first time.