Liang Jiu-Qing

Bulk Modulus and Electronic Band Structure of ZnGa2X4 (X = S,Se): a First-Principles Study

First-principles local density functional calculations are presented for the compounds ZnGa2X4 (X = S,Se). We investigate the bulk moduli and electronic band structures in a defect chalcopyrite structure. The lattice constants and internal parameters are optimized. The electronic structures are analysed with the help of total and partial density of states. The relation between the cohesive energy and the unit cell volume is obtained by fully relaxed structures. We derive the bulk modulus of ZnGa2X4 by fitting the Birch–Murnaghans equation of state. The extended Cohens empirical formula agrees well with our ab initio results.

Macroscopic Quantum States and Quantum Phase Transition in the Dicke Model

The energy spectrum of Dicke Hamiltonians with and without the rotating wave approximation for arbitrary atom-number is obtained analytically with the variational method, in which the effective pseudo-spin Hamiltonian resulted from the expectation value in the boson-field coherent state is diagonalized by the spin-coherent-state transformation. In addition to the ground-state energy an excited macroscopic quantum-state is found corresponding to the south-and-north-pole gauges of the spin-coherent states respectively. Our results of ground-state energies in exact agreement with various approaches show that these models exhibit a zero-temperature quantum phase transition of second-order for any number of atoms, which however was commonly considered as a phenomenon of the thermodynamic limit with the atom-number tending to infinite. The critical behavior of geometric phase is analyzed, which displays no singularity at the critical point.

Transmission spectra through a photonic double quantum well system

By means of the modal expansion method with an R-matrix propagation algorithm, effects of the coupling between resonance photonic states on the resonance tunneling through a double quantum well structure are investigated. We examine the effects on the transmission spectra due to variation of the second well width and middle barrier thickness. Drastic change of the tunneling spectra is found and analyzed when the wells are filled with left-hand media.

Solitons in a Heisenberg spin chain with an easy axis under an oscillating magnetic field

The Lax pair is given for the equations of motion of a continuum-Heisenberg-spin chain of an easy axis in an oscillating magnetic field. Using an inverse scattering transformation we derive soliton solutions for the case of the easy axis.

Electronic transport through a periodic array of quantum-dot rings

Using the tight-binding approximation and the transfer matrix method, this paper studies the electronic transport properties through a periodic array of quantum-dot (QD) rings threaded by a magnetic flux. It demonstrates that the even-odd parity of the QD number in a single ring and the number of the QD rings in the array play a crucial role in the electron transmission. For a single QD ring, the resonance and antiresonance transmission depend not only on the applied magnetic flux but also on the difference between the number of QDs on the two arms of the ring. For an array of QD rings, the transmission properties are related not only to the even–odd parity of the number N0 of QDs in the single ring but also to the even–odd parity of the ring number N in the array. When the incident electron energy is aligned with the site energy, for the array of N rings with N0 = odd the antiresonance transmission cannot occur but the resonance transmission may occur and the transmission spectrum has N resonance peaks (N − 1 resonance peaks) in a period for N = odd (for N = even). For the array of N rings with N0 = even the transmission properties depend on the flux threading the ring and the QD number on one arm of the ring. These results may be helpful in designing QD devices.

Atomistic simulation of fcc—bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential, we investigate the fcc-to-bcc phase transition in single crystal Al, caused by uniform compression. Results show that the fcc structure is unstable when the pressure is over 250 GPa, in reasonable agreement with the calculated value through the density functional theory. The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail. The bcc (011) planes are transited from the fcc (111) plane and the (111) plane. We suggest that the transition mechanism consists mainly of compression, shear, slid and rotation of the lattice. In addition, our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.

Spin-dependent tunnelling through an indirect double-barrier structure

We use the transfer matrix method to study the quantum tunnelling through an indirect-band-gap double-barrier like the GaAs/AlAs/GaAs/AlAs/GaAs heterostructures along the [001] axis, which is described by the tight-binding model. The X-valley quasi-bound state gives rise to the Fano resonance different from the direct double-barrier transition in a resonance-tunnelling diode. The quantitative calculations demonstrate that a relatively high spin-polarization of the transmission probability can be achieved as compared with the single-barrier tunnelling case. Moreover the extension to the multi-barrier device is provided and leads to an important observation that the spin polarization increases with the number of barriers.

Soliton Collision in a Ferromagnetic Spin Chain Driven by a Magnetic Field

By employing an inverse-scattering transformation, the exact solution of an N-soliton train in the spin chain with a time-dependent magnetic field is derived. As a special case the one-soliton solution (N = 1) exhibits explicitly the spin precession around the magnetic field and periodic shape-variation induced by the time varying field as well. In terms of the general solution, the inelastic two-soliton collision is analysed. The inelastic collision, by which we mean the soliton shape-change before and after collision, appears generally due to the time varying field.

Neutronics studies of solid targets for spallation neutron source using Monte Carlo simulation

The oxygen-induced migration of Ag clusters on Si (111)-7 x 7 surface was observed by in-situ STM. All experiments were carried out in UHV system. The Si (111)-7 x 7 surface was obtained by directly heating to 1200 degreesC for several times. At the beginning of Ag clusters growth on the Si (111)-7 x 7 surface, Ag clusters formed the uniform and well-ordered arrays on the surface. The monomers, dimers and clusters of Ag were observed. After exposure to oxygen, some Ag clusters vanished and conglomerated into bigger clusters. As known, there are two kinds of adsorbed oxygen species on the Si surface, ad-ins and ins-ins. The Ag clusters appear to move easily on the surface until being trapped by adsorbed atomic oxygen and pinned around the adsorbed oxygen. It was confirmed that the migration of Ag clusters is caused by oxygen decoration. The statistical results show that the Ag clusters diffuse much easily on partly oxidized Si (111)-7 x 7 surface than on Si (111)-7 x 7 surface. It was also found that the strong interaction between Ag cluster and adsorbed oxygen species causes a distinct change of the property of adsorbed oxygen, i.e. a change of surface ins-ins oxygen into ad-ins oxygen species, reflecting the role of substrate at room temperature.

Topological quantum-phase coherence in full counting statistics of transport electrons with two-body interaction

The full counting statistics of electron transport through two parallel quantum dots with antiparallel magnetic fluxes is investigated as a probe to detect the topological quantum-phase coherence (TQPC), which results in the characteristic oscillation of the zero-frequency cumulants including the shot noise and skewness. We show explicitly the phase transition of cumulant spectrum-patterns induced by the topology change of electron path-loops while the pattern period, which depends only on the topology (or Chern number), is robust against the variation of Coulomb interaction and interdot coupling strengths. Most importantly we report for the first time on a new type of TQPC, which is generated by the two-particle interaction and does not exist in the single-particle wave function interference. Moreover, the accurately quantized peaks of Fano-factor spectrum, which characterize the super- and sub-Poissonian shot noises, are of fundamental importance in technical applications similar to the superconducting quantum interference device.