De-Cheng Tian

The electronic structure and chemical stability of the AlB2-type transition-metal diborides

The electronic structures of the A1B(2)-type transition-metal diborides TMB(2) (TM=Sc, Ti, V, Cr, Mn, Y, ZT, Nb, Mo, Hf, Ta) have been calculated by using the self-consistent LMTO-ASA method. The binding mechanism is discussed and the rigid-band model is shown to provide a fairly good description of their electronic structures. The existence of a pseudogap in the total density of states is found to be a common feature of these compounds. The variation of the chemical stabilities of these diborides is analysed and we find that the trends can be understood in terms of the band-filling concept of the bonding states. The results are compared with other theoretical and experimental work.

Fabrication and enhanced magnetoresistance of SiO2-coated Fe3O4 nanosphere compact

Magnetoresistance (MR) of a nanostructured material, monodisperse Fe3O4 nanospheres of about 200nm coated with thin SiO2 and compactly cold pressed and sintered, has been investigated. A high MR, up to 10.8% at 1T and 17% at 8T, has been observed at 100K. This enhanced MR is likely arising from the spin-polarized tunneling of conducting electrons through the insulating SiO2 boundaries. The decrease of the MR with the temperature increasing was attributed to the existence of the local spins in the grain boundaries. In addition, the MR ratio decreases with the SiO2 thickness increasing and sintering temperature decreasing. It suggests that the magnetic property of the insulating barrier is crucial for enhancing the low-field MR at a high temperature.

ELECTRONIC-STRUCTURE AND EQUATION OF STATE OF TIB2

The electronic structure of TiB2 has been calculated using the self-consistent LMTO-ASA method. Our results have been compared with experimental and other theoretical work. We find that the pseudo-gap at the Fermi level is the competing effect of Ti 3d resonance and strong hybridization between Ti 3d and B 2p states. The bonding nature of TiB2 is discussed. We have carried out calculations of the equation of state (EOS) Of TiB2; it is found that our first-principles calculated results support the universal model of EOS proposed by Vinet et al. The room-temperature isotherm is also plotted.

Application of a probability model for relaxation to the dielectric and -relaxation of amorphous polyethylene terephthalate

A probability model (PM) developed recently, which is an alternative to the conventional Havriliak-Negami (HN) dispersion function for polymer systems, is applied for the first time to the study of the dielectric alpha- and beta-relaxation of amorphous polyethylene terephthalate (PET). It has been found that the temperature dependence of the maxima of the imaginary part of the dielectric function and of the half-width of the distribution in the PM of the alpha-relaxation exhibit Vogel-Fulcher-type temperature behaviour; the temperature dependence of the maxima of the imaginary part of the dielectric function of the beta ation is proportional to the ratio of the gauche content to the trans content of the ethylene glycol linkages in PET. Finally, the mechanisms of the alpha- and beta-relaxation are elucidated concisely.

Effect of impurities on thermoelectric power in thallium blue bronze Tl0.3MoO3

Abstract Temperature dependence of thermoelectric power of thallium blue bronze Tl 0.3 MoO 3 doped with K and W atoms impurities are presented. The experimental results display that thermoelectric power data for all the doped samples can be fitted to an expression of the type, S = At + B / T , below and above T p . With the addition of impurities, the absolute values of the thermoelectric power decrease and the temperature of the sign change of thermoelectric power T s increases. The mechanisms of drift diffusion and phonon drag of the thermoelectric power are involved. The donor effect of the impurities and the influence of weak inter-chain coupling are proposed.

Metallic Phase in Quantum Hall Systems due to Inter-Landau-Band Mixing

The electronic eigenstates of a quantum Hall (QH) system are chiral states. Strong inter-Landau-band mixings among these states can occur when the bandwidth is comparable to the spacing of two adjacent Landau bands. We show that mixing of localized states with opposite chirality can delocalize electronic states. Based on numerical results, we propose the existence of a metallic phase between two adjacent QH phases and between a QH phase and the insulating phase. This result is consistent with non-scaling behaviors observed in recent experiments on quantum-Hall-liquid-to-insulator transition.

Crystal growth and electrical properties of thallium molybdenum purple bronze TlMo6O17 by molten salt electrolysis

Abstract Large size and high quality single crystals of quasi-two-dimensional thallium molybdenum purple bronze TlMo 6 O 17 have been grown by electrolytic reduction of molten salt of Tl 2 CO 3 MoO 3 . The crystal structure is trigonal with space group P 3 m1 determined by X-ray diffraction and four-circle single crystal diffraction. The lattice parameters of the unit cell are: a = b =5.5282 A, and c =13.6991 A. The temperature dependence of resistivity and magnetic susceptibility confirmed that a metal-to-metal transition occurs near 110 K.

Short-time dynamics behavior at the gas–liquid critical point

Abstract The dynamic relaxation process for the two dimension fluid at the critical point was studied, by using the constant pressure and constant temperature (NPT) molecular dynamics (MD). The system was initially at the state of very high temperature and of the density little higher than the critical density, and then it was quenched to the state of the critical temperature and critical pressure. The initial increase of the density was observed. The new dynamic exponent θ has been obtained and it manifests the universal short-time dynamic behavior also exists in the fluid phase.

Peierls phase transition and nonlinear transport study of doped blue bronzes K0.3-xNaxMoO3

Temperature dependence of Ohmic resistance, and nonlinear current-voltage (I-V) characteristics at 77 K were systematically studied in pure potassium blue bronze K0.3MoO3 and Na-doped alloy bronzes K0.3-xNaxMoO3 (x = 0.02, 0.05, 0.10). No considerable influence of Na ions doping in K0.3MoO3 was found on the Peierls transition temperature, while obvious enchancement on semiconductor gap and remarkable influence on the first threshold field were observed. We attribute the first phenomenon to two opposite effects of Na ions, and the depinning model is proposed to account for the latter. The analytical form of nonlinear charge-density wave current I-CDW in a moderate field range fits the power-law relation, i.e. I-CDW proportional to (V/V-T - 1)(alpha), where the exponent alpha is about 3/2 and slightly enhanced by Na ions doping.

Are Guggenheim formulae and Landau theory still applicable to expanded liquid metals

In the present work, we have found that Guggenheim formulae are still applicable to the liquid-vapor coexistence curves of expanded fluid metals only by adjusting their coefficients rather than exponents. In addition, the equation of state of expanded liquid cesium has been derived based on Landau theory of continuous phase transitions including the sixth-order term of the order parameter. Both the equation of state and the coexistence curves obtained are in good agreement with experimental data