Dai Xian-qi

Electronic properties of the SnSe–metal contacts: First-principles study*

The geometries and electronic properties of SnSe/metal contact have been investigated using first-principles calculation. It is found that the geometries of monolayer SnSe were affected slightly when SnSe adsorbs on M (M = Ag, Au, Ta) substrate. Compared with the corresponding free-standing monolayer SnSe, the adsorbed SnSe undergoes a semiconductor-to-metal transition. The potential difference ΔV indicates that SnSe/Ta contact is the best candidate for the Schottky contact of the three SnSe/M contacts. Two types of current-in-plane (CIP) structure, where a freestanding monolayer SnSe is connected to SnSe/M, are identified as the n-type CIP structure in SnSe/Ag contact and p-type CIP structure in SnSe/Au and SnSe/Ta contact. The results can stimulate further investigation for the multifunctional SnSe/metal contact.

First-Principles Calculations of Atomic and Electronic Properties of Tl and In on Si(111)

The atomic and electronic structures of Tl and In on Si(111) surfaces are investigated using the first-principles total energy calculations. Total energy optimizations show that the energetically favored structure is 1/3 ML Tl adsorbed at the T4 sites on Si(111) surfaces. The adsorption energy difference of one Tl adatom between (√3 × √3) and (1 × 1) is less than that of each In adatom. The DOS indicates that Tl 6p and Si 3p electrons play a very important role in the formation of the surface states. It is concluded that the bonding of Tl adatoms on Si(111) surfaces is mainly polar covalent, which is weaker than that of In on Si(111). So Tl atom is more easy to be migrated than In atom in the same external electric field and the structures of Tl on Si(111) is prone to switch between (√3 × √3) and (1 × 1).

Effect of electronic weak localization on the charge transfer for oxygen-stoichiometric Y1:2:3 systems

Abstract We report the result of the study on the oxygen stoichiometry in Y1:2:3 superconducting systems by the positron experiment. The correlations between local electron density n e and the oxygen stoichiometry and Tc have been directly given, which indicates that n e has a strong dependence on the oxygen stoichiometry and the anomalous changes were found near O T transition. The results proved that there generially exists a electronic weak localization in a tetragonal phase.

Study of photoirradiation for YBa2Cu3O6+x compounds and the electron structure by positron experiment

Abstract We present positron lifetime data of YBa 2 Cu 3 O 6+ x (x=0.92, 0.43) compounds for different photo-irradiation time. It is given that change of the local electron density and vacancy concentration with photoirradiation time. It is found that there is transform at the electronic structure of CuO chains. We also have discussed the effect of photoirradiations time on the electronic structure of YBa 2 Cu 3 O 6+ x systems and their charge reservoir layer and CuO 2 plane conduction.layer. The positron experimental results support the model of photoinduced oxygen-diffusion mechanism.