Search

The pressure dependence of elastic properties of ZnS in zinc-blende (ZB) and wurtzite (WZ) structures are investigated by the generalized gradient approximation (GGA) within the plane-wave pseudopotential density functional theory (DFT). Our results are in good agreement with the available experimental data and other theoretical results. From the high-pressure elastic constants obtained, we find that the ZB and WZ structures of ZnS are unstable when the applied pressures are larger than 15.8 GPa and 21.3 GPa, respectively. The sound velocities along different directions for the two structures are also obtained. It is shown that as pressure increases, the sound velocities of the shear wave decrease, and those of all the longitudinal waves increase. An analysis has been made to reveal the anisotropy and highly noncentral forces in ZnS.

The pressure induced phase transition of ZnS from the wurtzite (WZ) and the zincblende (ZB) structures to the rocksalt (RS) structure and the temperature induced phase transition from the ZB structure to the WZ structure axe investigated by ab initio plane-wave pseudopotential density-functional theory (I)FT), together with the quasi-harmonic Debye model. It is found that the zero-temperature transition pressures from the WZ-ZnS and the ZB-ZnS to the RS-ZnS are 17.20 and 17.37 GPa, respectively. The zero-pressure transition temperature from the ZB-ZnS to the WZ-ZnS is 1199 K. All these results are consistent with the available experimental data. Moreover, the dependences of the normalized primitive cell volume V/V-0 on pressure and thermal expansion coefficient alpha on temperature are also obtained successfully.

The phase transition and melting curves of CaF2 are investigated by using the general utility lattice programme (GULP) via the shell model with molecular dynamics method. By calculating the entropy H (at 0K) and Gibbs free energy G ⁄ (at 300K), we flnd that the phase transition pressure from the face-centred cubic (fcc) structure to the orthorhombic structure is 11.40GPa and 9.33GPa at 0K and 300K, respectively. The modifled melting point of the fcc CaF2 is in the range of 1650{1733K at 0GPa. All these results are well consistent with the available experimental data and other theoretical results. We also obtain that the melting temperature of high pressure phase is 990{1073K at 10GPa. Moreover, the temperature dependences of the elastic constants Cij, bulk module B and shear module G are also predicted.

The controlled growth of Zn-polar ZnO films on Al-terminated α-Al2O3 (0001) substrates is investigated by the radio-frequency plasma-assisted molecular beam epitaxy method. Prior to the growth, α-Al2O3 (0001) surface is modified by an ultrathin MgO layer, which serves as a uniform template for epitaxy of Zn-polar ZnO films. The microstructures of ZnO/MgO/Al2O3 interface are investigated by in-situ reflection high-energy electron diffraction observations and ex-situ high-resolution transmission electron microscopy characterization. It is found that under Mg-rich condition, the achievement of the wurtzite MgO ultrathin layer plays a key role in the subsequent growth of Zn-polar ZnO. An interfacial atomic model is proposed to explain the mechanism of polarity selection of both MgO and ZnO films

We report on the growth of rock salt MgO films on sapphire (0001) substrates by rf plasma-assisted molecular beam epitaxy. A two-step method, i.e. high temperature epilayer growth after low-temperature buffer layer growth, was adopted to obtain the single crystal MgO film. The epitaxial orientation between the MgO epilayer and the sapphire (0001) substrate was studied by using in situ reflection high energy electron diffraction and ex situ x-ray diffraction, and it is found that the MgO film grows with [111] orientation. The role of the low temperature buffer layer in the improvement of crystal quality of the MgO epilayer is discussed based on the cross-sectional scanning electron microscopy.

Susceptibility measurements on Bi2Sr2CaCu2O8 single crystals do reveal field-induced-suppression of sharp phase transition. No apparent anisotropy of the susceptibility was observed when magnetic field was applied along the three main directions a, b and c. The three dimensional isotropic fluctuation diamagnetism theory is proposed to explain our experimental finding of isotropic susceptibility. In a wide range of magnetic field, the temperature dependence of susceptibility does not show any obvious change in its character when the sample enters into expected vortex-lattice state from vortex-liquid state.

We investigated the influences of a uniform and a nanoscaled periodic magnetic field on electron's resonant tunneling through quantum-wire arrays (QWA). It is shown that, due to the magnetic quantum-size effect and additional destructive interference induced by the nanoscaled periodic magnetic field, ballistic conductance spectrum for electron tunneling through the QWA is strongly modified; while for uniform magnetic modulation, it cannot modify the resonant structure of ballistic conductance.

Pb-doped HgBa2CaCu2Oy films were prepared on the (100) SrTiO3 substrate by using laser ablation followed by post-Hg-vapor annealing. These films are dominated by c-axis-oriented Hg-1212 phase as indicated by x-ray diffraction and electromagnetic measurements. The transition temperature Tc0 is 110 K. The critical current density Jc achieved 106 A/cm2 at 77 K and zero applied field.

The phase tuning characteristics of a double-longitudinal-mode He-Ne laser with optical feedback are demonstrated. The phase behaviors of the two adjacent longitudinal modes are studied with different laser cavity lengths at the weak feedback level, and it is found that the phase difference of the two modes can be 0 to 180° along with the changing of the length of external cavity. The high-density intensity fringes are obtained with asymmetric feedback cavity at a strong feedback level. In particular, the phase difference is also observed between the two high-density fringes of adjacent modes, which have the potential to develop a direction sensitive laser feedback interferometer with high resolution. A simple theoretical analysis is presented, which fits well with the experimental results.