Yao Kai-Lun

Magnetic Properties of a Mixed Spin-2 and Spin-5/2 Heisenberg Ferrimagnetic System on a Two-Dimensional Honeycomb Lattice: Green's Function Approach

The multisublattice Greens function technique is applied to study the magnetic properties of a mixed spin-2 and spin-5/2 Heisenberg ferrimagnetic system on a two-dimensional honeycomb lattice. The role of the different interactions in the Hamiltonian is explored. When only the nearest-neighbor interaction and the single-ion anisotropy are included, our results indicate that there are compensation points at finite temperatures. When the next-nearest-neighbor interaction exceeds a minimum value that depends on the other parameters in the Hamiltonian, the compensation point disappears. The next-nearest-neighbor interaction has the effect of changing the compensation temperature.

Block Entanglement in the Single-Hole Hubbard Model

We investigate the distribution of the entanglement of the one-dimensional single-hole Hubbard model (HM) and study the relationship between the entanglement and the quantum phase transition in the model. The von Neumann entropy of a block with neighbouring spins L for a single-hole HM is calculated using the density-matrix renormalization group. The distribution of the entanglement entropy in the ground state, as a function of block length, show a dramatic effect, i.e. effectively decoupling with centres, no matter how the Coulomb interaction u > 0 or u < 0. Contarily, for the Coulomb interaction u = 0 or close to zero, the entanglement entropy in the single-hole model reaches a saturation model for a certain block size. For a fixed size L = 40, the ground state entanglement entropy measure, as a function of u, shows a peak corresponding to the critical quantum phase transition.

Percolation model of the temperature dependence of exotic magnetic field in doped manganese perovskites

Magnetic transitions are studied for(La_(1-x)B_x)_(2/3)Ca_(1/3)MnO_3 system,based on a magnetic percolation theoretical model,where the system consists of paramagnetic and ferromagnetic domains.In the mean-field approximation,the resistance as a function of temperature and magnetic field are derived analytically and simulated numerically.It is found that the dependence of the critical temperature on magnetic field is linear when applied magnetic field is not too strong.Our theoretical predictions are in excellent agreement with recent experimental observations.

Nonlinear current－voltage characteristics of sintered tungsten－vanadium oxide

We have studied the densification behaviour, microstructure and electrical properties of WO3 ceramics with V2O5 as the additive ranging from 0.5 to 15mol%. Scanning electron microscopic photos indicated that the grain size of WO3-V2O5 specimens is smaller than that of pure WO3. The addition of V2O5 to WO3 showed a tendency to enhance the densification rate and to restrict the grain growth. Electrical properties of all specimens were measured for different electrodes at different temperatures. The formation of the grain boundary barrier layer was confirmed by the non-ohmic I-V behaviour. The nonlinear coefficient was obtained at the current density J=0.01, 0.1 and 1mA/cm(2) for a series of WO3-V2O5 samples. The V0.5mol% specimen showed an abnormal phenomenon that the nonlinear characteristics appeared at 350degreesC and disappeared at lower and higher temperatures. This implies that it could be applied as a high-temperature varistor. The double Schottky barrier model was adopted to explain the phenomena for the WO3-V(2)o(5) varistors.

Kinetic Monte Carlo Simulation of Deposition of Co Thin Film on Cu(001)

A kinetic Monte Carlo (kMC) simulation is conducted to study the growth of ultrathin film of Co on Cu(001) surface. The many-body, tight-binding potential model is used in the simulation to represent the interatomic potential. The film morphology of heteroepitaxial Co film on a Cu(001) substrate at the transient and final state conditions with various incident energies is simulated. The Co covered area and the thickness of the film growth of the first two layers are investigated. The simulation results show that the incident energy influences the film growth and structure. There exists a transition energy where the interfacial roughness is minimum. There are some void regions in the film in the final state, because of the influence of the island growth in the first few layers. In addition, there are deviations from ideal layer-by-layer growth at a coverage from 0 similar to 2 monolayers (ML).

Specific Heat and Charge Density of Quasi-One-Dimensional Interchain Coupling Organic Ferromagnets

On the basis of a generalized SSH model, an organic polymer ferromagnet theory is proposed at the finite temperature in the self-consistent mean field approximation, and the specific heat and charge density of the quasione-dimensional interchain coupling organic ferromagnets are presented. We find that an obvious feature is to present itself the round peak for the specific heat with the temperature. This indicates unambiguously the presence of the phase transition in the system. The transition temperature plays down with increasing of the interchain coupling t2 or decreasing of the electron repulsion u. The curves of charge density with the temperature debase monotonously. This result illustrates that the higher the temperature is, the more electrons are excited.

Ab initio Study of Electronic Structure and Magnetic Properties of Two Novel Neptunium (V) Sulfates: NaK3(NpO2)4(SO4)4(H2O)2 and NaNpO2SO4H2O

Ab initio within the full potential linearized augmented plane wave (FP-LAPW) method with the GGA+U approach is applied to study the electronic structures of two compounds NaK3(NpO2)4(SO4)4(H2O)2 and NaNpO2SO4H2O. The present calculations show that the major part of the spin magnetic moment in these two compounds is from Np(V) ions, and the origin of the cation-cation interactions between Np comes from the spin polarization effect within Np-ONp-Np bonds.

Strong ferromagnetic coupling of { Cu(Hpht)(N-3) center dot H2O}(n) with simultaneous end-on azido and carboxylato bridges: A first principle study

The compound {[Cu(Hpht)(N-3)]-H2O}(n) (Hpht = hydrogen phthalate) is formed by chains of copper atoms bridged simultaneously by syn-syn carboxylato and end-on azido bridges. Taking into account the large Cu-O(1)-C(7) bond angle of the single carboxylato bridge (131degrees), or the large Cu-N(11)-Cu bond angle of the azido bridge (111.9degrees), a moderately intrachain antiferromagnetic behavior should be expected for the compound. This paper is devoted to examining the apparently anomalous intrachain ferromagnetic behavior of {[Cu(Hpht)(N-3)].H2O}(n), using first principles within the full potential linearized augmented plane wave (FP-LAPW) method. The total energy, the density of states (DOS), and the spin distributions are obtained. The atomic spin distribution has been analyzed as resulting from the interplay of electron delocalization and spin polarization. The DOS reveals a surprisingly strong exchange interaction between the d type orbitals of the copper and the pi molecular orbitals of the two ligands.

Electronic and Magnetic Properties of the p-NPNN

In this paper, we study the electronic band structure and the ferromagnetic properties of the organic radical p-NPNN by employing density-functional theory with generalized gradient approximation (GGA) and local-spin density approximation (LSDA). The density of states, the total energy, and the spin magnetic moment are calculated. The calculations reveal that the delta-phase of p-NPNN has a stable ferromagnetic ground state. It is found that an unpaired electron in this compound is localized in a single occupied molecular orbital (SOMO) constituted primarily of pi(*) (NO) orbitals, and the main contribution of the spin magnetic moment comes from the pi(*) (NO) orbitals. By comparison, we find that the GGA is more suitable to describe free radical systems than LSDA.

Ab-Initio Study of Electronic Structure and Magnetic Properties of Pipz-H2[MnF4(HF2)]

The full-potential linearized augmented plane wave method was applied to study the electronic and the magnetic properties of the compound pipz-H-2[MnF4(HF2)](pipz=piperazine). The band structure, the total density of states, the partial density of states and the electron density were calculated to explain the electronic and the magnetic properties of pipz-H-2[MnF4(HF2)] in the ferromagnetic state. It is found that the magnetic moment of the molecule mainly comes from the Mn atoms with partial contribution from the F atoms. The symmetrical sigma/sigma, bonds via H atoms along Mn-F-H-F-Mn chains and the weak direct-exchange interaction between F(2), F(3) and Mn atoms have effect on the electronic structure and the magnetism of pipz-H-2[MnF4(HF2)].