Fluctuation-induced non-equilibrium transition in a liquid-crystal metastable system

The research herein studies the Langevin dynamics that allows for an exchange of energy between liquid crystals and the thermal environment. This dynamics leads to fluctuation and dissipation behaviors in the motions of liquid crystals, and therefore drives the system toward non-equilibrium evolutional processes. In particular, for the operations of liquid-crystal metastable systems, the fluctuation could allow an excitation (non-equilibrium) transition against energy barriers to the globally-stable state. Implemented with an actual case of liquid crystal π configuration, this work statistically studies the non-equilibrium metastable transitions and shows the dependence of the transition-time on the correlations (of fluctuations).     

Point defect concentrations in metastable Fe-C alloys

Point defect species and concentrations in metastable Fe-C alloys are determined using density functional theory and a constrained free-energy functional. Carbon interstitials dominate unless iron vacancies are in significant excess, whereas excess carbon causes greatly enhanced vacancy concentration. Our predictions are amenable to experimental verification; they provide a baseline for rationalizing complex microstructures known in hardened and tempered steels, and by extension other technological materials created by or subjected to extreme environments.     

Exciton-plasma transition in GaAs

In this work, we study the stability of excitons at high density, i.e. we calculate the reduction of the exciton binding energy due to exciton-exciton interactions in a high-density exciton gas. We derive first the effective electron-hole interaction in the presence of free carriers and excitons. We use the static approximation. The exciton binding energy is calculated by the variational technique. The computations are specialized to GaAs. We investigate the critical density when the exciton binding disappears, which corresponds to the exciton plasma transition. We conclude that this transition occurs at higher density than the reverse plasma exciton transition, determined by the standard criteria a₀q_D =1.19 [Rogers F. J., Graboske H. C., Jr. and Harword D. J., Phys. Rev.A1, 1577 (1970)].     

Observation of metastable erbium trihydride

The γ phase of the erbium-hydrogen system is a hexagonal trihydride that is not predicted to be stable at room temperature without an overpressure of hydrogen gas. Herein, we report the creation of both, a thin film and powder of erbium trihydride that is metastable at ambient conditions. The presence of the hexagonal γ phase was determined by X-ray diffraction (XRD). The ratio of the total moles of hydrogen isotopes (hydrogen and deuterium) to moles of erbium, (H + D):Er, have been confirmed by elastic recoil detection (ERD)/Rutherford backscattering spectroscopy (RBS). Auger electron spectroscopy (AES) depth profiles and X-ray photoelectron spectroscopy (XPS) revealed the presence of an oxide layer that may account for the metastable behavior of the thin film.     

Different annealing behaviour of the metastable state in GaAs

We have studied the temperature dependence of the stable state (EL2) recovery after photoquenching in two different materials, grown by the LEC (material A) and the horizontal gradient freeze (B) techniques, by means of optical absorption measurements. The temperature of annealing of the metastable state is 110–120 K in material A and 40–50 K in material B, suggesting the existence of two different defects belonging to the EL2 family. Furthermore, one sample from the B ingot, which has been platically deformed, shows the two thresholds of temperature recovery (50 and 120 K) and also an increase of the EL2 concentration.     

Point defect mobility in irradiated GaAs

For the first time the process of primary radiation defect migration in GaAs at T = 300 K was detected using a direct experimental technique. The maximum value of the migration activation energy E_m ≈ 0.34 eV was determined.     

A dominant electrical defect in GaAs

Several as-grown Bridgman and Czochralski GaAs crystals, with dominant electrical levels from 0.13–0.20 eV below the conduction band, have been studied by the temperature-dependent Hall-effect, spark-source mass spectroscopy, and secondary-ion mass spectroscopy. It is shown that no impurity is of a sufficient concentration to account for these levels, and therefore they are composed of single or multiple defects. The detailed nature of the defects has not yet been established but may involve an As vacancy. It is believed that this is the first time that a dominant electrically active center has been shown to be a pure defect in any as-grown semiconductor.     

Electronic topological transition in metastable Al-Ge solid solutions

Al-rich Al-Ge solid solutions synthesized under high pressure demonstrate physical properties strikingly different from those of pure Al. In particular, enhanced superconductivity temperature (T_c), anomalies in the phonon spectra and decrease of the Debye temperature have been observed. We show from first-principles, based on calculations of the electronic spectra and Fermi surfaces of Al-Ge substitutional solid solutions, that an electronic topological transition (ETT) takes place in the system. We predict anomalies in transport properties to be revealed experimentally for Al-Ge solid solutions with the Ge concentration ≈10 at.%. The influence of the ETT on the thermodynamic properties of the system is discussed and, in particular, concentration dependence of the Debye temperature is reproduced in good agreement with experiment.     

Superconductivity in the metastable phases of transition metal hydrides

New inelastic neutron scattering data and the observation of an inverse isotope effect in metastable phases of titanium hydride give support for the realization of the recently proposed anharmonic (two-level systems mediated) mechanism of superconductivity in the hydrides of titanium, hafnium and zirconium. The model allows in combination with an optical phonon contribution to explain not only theT _c enhancement but also the change of the sign of the isotope effect experimentally observed for PdMH (M=Cu, Ag, Au) alloys.On leave from Institute of Physics & Technology Ulaanbaatar-51 Mongolia