International Journal of Modern Physics B

There is a class of quantum Hamiltonians known as Rokhsar–Kivelson (RK)–Hamiltonians for which static ground state properties can be obtained by evaluating thermal expectation values for classical models. The ground state of an RK–Hamiltonian is known explicitly, and its dynamical properties can be obtained by performing a classical Monte Carlo simulation. We discuss the details of a Diffusion Monte Carlo method that is a good tool for studying statics and dynamics of perturbed RK–Hamiltonians without time discretization errors. As a general result we point out that the relation between the quantum dynamics and classical Monte Carlo simulations for RK–Hamiltonians follows from the known fact that the imaginary-time evolution operator describing optimal importance sampling, where the exact ground state is used as guiding function, is Markovian. Thus quantum dynamics can be studied by classical Monte Carlo for any Hamiltonian provided its ground state is known explicitly and that there is no sign problem.

The magnetic field B dependence of the critical current I_c for the vortex phase of a disordered superconductor is studied numerically at zero temperature. The I_c(B) increases rapidly near the upper critical field B_c2 similar to the peak effect (PE) phenomenon observed in many superconductors. The real space configuration across the PE changes continuously from a partially ordered domain (polycrystalline) state into an amorphous state. For B≥0.4B_c2, the topological defect density n_d(b) increases as e^αBk with k>1. There is no evidence of a phase transition in the vicinity of the PE suggesting that an order-disorder transition is not essential for the occurrence of the PE phenomenon. An alternative view is presented wherein the vortex system with high dislocation density undergoes jamming at the onset of the PE.

By applying a special Bäcklund transformation, a quite general variable separation solution for new (2+1)-dimensional soliton equation is derived. In addition to some types of the usual localized excitations such as dromion, lumps, ring soliton, oscillated dromion and breathers soliton structures can be easily constructed by selecting the arbitrary functions appropriately, a new novel class of localized structures like fractal-dromion, fractal-lump, peakon, compacton and folded excitation of this system are found by selecting appropriate functions. Some interesting novel features of these structures are revealed.

Hydrostatic pressure effect on superconducting transition temperature and the structural transition temperatures were measured upto 1.0 GPa in the La_1.25Nd_0.6Sr_0.15CuO₄ single crystals. We found that the low temperature tetragonal phase was suppressed with drastic increase of T_c at a low pressure of ~0.1 GPa. By comparing the pressure effect on La_1.48Nd_0.4Sr_0.12CuO₄ single crystals, we confirm that pressure is an effective two-way control parameter for controlling stripes and superconductivity in La-Nd-Sr-Cu-O single crystals.

Nearly monodisperse silica spheres with a diameter of ~90 nm containing NiFe₂O₄ nanoparticles were successfully prepared by hydrolysis of TEOS in the presence of NiFe₂O₄ nanoparticles, synthesized through a hydrothermal method. The colloid particles were characterized by XRD, TEM and VSM. The results indicate that the particles possess better magnetic properties at room temperature, compared with paramagnetic colloids with magnetic core by co-precipitation method. The colloid particles assembled under additional magnetic fields could have potential application in biomedical systems.

Within the framework of the dielectric-continuum model, the polarization eigenvector, the dispersion relations, and the electron-phonon interaction Fröhlich-like Hamiltonian of the interface optical (IO) phonons in multilayer spherical heterostructures are derived by using transfer-matrix method. The dispersion relations and electron-phonon coupling function strengths of the IO phonons are investigated for 5-layer CdS/HgS/CdS/HgS/H₂O spherical heterostructures. We find that there are seven IO phonon branches in 5-layer CdS/HgS/CdS/HgS/H₂O spherical heterostructures, but only one has a more important contribution to the coupling function .

Aluminium nitride (AlN) thin films were fabricated on a glass substrate by reactive magnetron sputtering. Raman microscopy was then employed to follow the characteristics of their optical and acoustic phonon modes. At the optimal sputtering time of 30 minutes, the defect-induced first and second order Raman spectra were observed in 400–800 cm^-1 band which were mostly related to the coating compositions. However, at the 30-, 60- and 90-minute sputtering, crystals of submicron size order of AlN were achieved. This could be clearly identified by the presence of Raman peak at 658–662 cm^-1. Powder X-ray diffraction (PXRD) patterns revealed the development of (002) and (101) planes of hexagonal wurtzite AlN phase. The optimal average grain size measured by atomic force microscopy (AFM) is at 330 nm. It was found that the hardness was strongly dependent on roughness of the film, the maximum of which was achieved at 20.00 GPa. The presence of F-type defects in AlN films was investigated by X-band (~9.44 GHz) ESR spectrometer at 295 K. The ESR experiments were carried out by applying magnetic field perpendicular to AlN film, which showed the ESR six-peak multiplet signal at ~290 mT arising from superhyperfine interactions between nuclear spin I = 5/2 of ²⁷Al and electron spins trapped in nitrogen vacancies. The ESR signals are simulated and the ESR parameters are calculated. The vacancies are clearly randomly distributed as the ESR signals are independent of rotation angle (φ) about the normal of the film. All these results were analyzed and presented as a function of the deposition parameters and composition, and crystalline phases existed in the films.