Han Ru-Shan

Electronic structures of doped fullerenes with boron and nitrogen atoms

Abstract The electronic structures of substitutional doped fullerenes C59B, C59N, C58B2, C58N2, C58BN have been calculated using a tight-binding approximation method. The results indicate that a boron and a nitrogen atom induce a acceptor state and a donor state respectively. Two boron and two nitrogen atoms make the impurity states deep.

Electronic structural properties and superconductivity of diborides in the MgB2 structure

We calculate the critical temperature Tc of a wide range of diborides which have the same crystal structure as MgB2. Their electronic structure is also calculated in the framework of the local density approximation method of density functional theory by using the pseudopotential plane wave approach. The Hopfield factors η of these materials are calculated by the frozen phonon method. Our results show that most of these diborides have low η, and hence low or no Tc; this is consistent with experimental observations. The most important result of our calculation is that AgB2 and AuB2 have higher Tc than MgB2. The high Tc of these two materials comes from the combination of the high density of states and high deformation potential of the σ bands.

Electronic structures of Ag-adsorbed Si(111) surface

Abstract The electronic structure of the Si(111) 3 × 3 -Ag surface is investigated by the first principles DV-Xα SCC and SCM methods with the use of a model suggested by Yang et al. and an embedded model. The calculated total energy and the binding energy of the Ag 4 Si 13 H 16 cluster for Yangs model are lower than that for the embedded model. The variation in binding energy of the AgSi 7 H 13 cluster with the Ag position has an energy minimum at the Ag position of 0.05 A above the first Si layer which implies that the atomic geometry of Yangs model is more reasonable. The contour map of charge density distribution reveals that the binding between Ag and Si atoms is weak.

Effects of Conduction Electron Band Structure on Transport of Quantum Dot Systems

We study the effects of the energy band structure of conduction-electron on the transport properties of an interacting quantum dot system. By applying the nonequilibrium Keldysh Green function technique, we show that the finite width of electron band in leads causes the negative differential conductance in some regions of the applied voltage. We also show that the van Hove singularities in the density of states of conduction-electron do not qualitatively change the differential conductance of the system, and hence can be safely ignored. Therefore, the wide band approximation used in the previous investigations is partially justified.

Inter-layer coupling and its contribution to superconductivity: Application to high-Tc oxides

Abstract We investigate the role of inter-layer coupling and its contribution to superconductivity. It is concluded that this coupling will reduce Coulomb pseudopotential and hence have considerable effect on the enhancement of Tc. Recently discovered high-Tc superconductivity in some exotic oxides is discussed in this model.

Josephson-Like Effects in the Mesoscopic Electric Circuit*

The quantum theory for mesoscopic electric circuit with charge discreteness is briefly described. The Schrodinger equation of the mesoscopic electric circuit with external source which is a time function has been proposed. The Josephson-like effects in the mesoscopic electric circuit have been addressed.

First-principles calculation of transport property in nano-devices under an external magnetic field

The mesoscopic quantum interference phenomenon (QIP) can be observed and behaves as the oscillation of conductance in nano-devices when the external magnetic field changes. Excluding the factor of impurities or defects, specific QIP is determined by the sample geometry. We have improved a first-principles method based on the matrix Greens function and the density functional theory to simulate the transport behaviour of such systems under a magnetic field. We have studied two kinds of QIP: universal conductance fluctuation (UCF) and Aharonov–Bohm effect (A–B effect). We find that the amplitude of UCF is much smaller than the previous theoretical prediction. We have discussed the origin of difference and concluded that due to the failure of ergodic hypothesis, the ensemble statistics is not applicable, and the conductance fluctuation is determined by the flux-dependent density of states (DOSs). We have also studied the relation between the UCF and the structure of sample. For a specific structure, an atomic circle, the A–B effect is observed and the origin of the oscillation is also discussed.

Time-Reversal Symmetry Breaking: Evidence for Spin Pairing in High-{Tc} Superconductors

Effective spin coupling leads to local triplet pairing in the antiferromagnetically ordered CuO2 plane as shown by the K-J model [Guo et al. Chin. Phys. Lett. 18 (2001) 103]. The precession of the spin triplet (S = 1, Sz = 0) in the CuO2 plane does not hold time-reversal invariance due to the dimpled CuO structure, which not only modifies the gap function but also contributes to the asymmetry of the high-Tc tunnelling conductance. In principle, the effect of time-reversal symmetry breaking can be manifested by the variation of the conductance asymmetry by applying a magnetic field at superconductor-insulator-normal metal junction, which should provide direct evidence for triplet pairing in the high-Tc superconductors.

Electron Spin Pairing and Excitations in High-Tc Superconductors

Effective spin coupling between conduction electrons mediated by local Cu moments in the doped CuO2 plane may induce the instability of the normal states of conduction electrons and pairing at Cu sites, where the superexchange interaction between local moments influenced by hole doping has been found to be Ising-like in recent experiments. By a mean field approach, we show that spin pairing and strong pair-pair interaction in the doped antiferromagnetic ordered Ising-like CuO2 plane gives rise to high-temperature superfluid condensation and the exceptional features of quasi-particle excitation in cuprates such as spin gap, pseudogap, and the linear temperature dependence of resistivity ρab in the normal states.

Electron Spin Pairing and the Phase Diagram of High-Tc Superconductors

The origin of the instability of the normal state of electrons in the superconducting copper oxides is shown by the K-J model, in which the superexchange (K) between local moments and the Kondo exchange (J) between electron and local moment are considered. The suppression of superexchange via impurity doping may induce effective spin coupling between electrons and triplet pairing (S = 1, Sz = 0). The spin pairing theory explains the phase diagram of high-Tc superconductors, especially the superconducting transition temperature Tc, the pseudogap temperature T* and the magnetic crossover temperature Tn as a function of the doped hole concentration. The universal expression for the empirical law of the superconducting transition temperature is derived from the theory.