Investigation of the photoplastic effect in vitreous semiconductors by cyclic nanoindentation

The dynamics of photoinduced changes in the internal friction and the Young’s modulus for films in the As-Se system is investigated by cyclic nanoindentation at infralow frequencies. It is revealed that the irradiation of the films leads to an increase in the Young’s modulus, a retardation of relaxation processes, and a decrease in the viscosity and nanohardness of the material. The results obtained are discussed in the framework of the concept of microheterogeneous glass structure.     

Nonlinear percolation near a metal-insulator transition in regular textures

Current percolation in weakly nonlinear two-dimensional periodic structures near a metal-insulator transition is studied. It is shown that the nonlinear conductivity exhibits critical behavior as a function of the density of the insulating and superconducting inclusions. The possibility of experimentally observing the effects predicted is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 521–524 (10 April 1997)     

Polaron percolation in diluted magnetic semiconductors

We theoretically study the development of spontaneous magnetization in diluted magnetic semiconductors as arising from a percolation of bound magnetic polarons. Within the framework of a generalized percolation theory we derive analytic expressions for the Curie temperature and the magnetization in the limit of low carrier density, obtaining excellent quantitative agreement with Monte Carlo simulation results and good qualitative agreement with experimental results.     

Magnetic percolation in diluted magnetic semiconductors

We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors. We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.     

Light-induced giant softening of network glasses observed near the mean-field rigidity transition

The longitudinal acoustic (LA) mode of bulk GexSe1-x glasses is examined in Brillouin scattering (BS) over the 0.15相似文献   

The Hall effect in granular metal films near the percolation threshold

We have measured the Hall effect in the granular metals Au-SiO₂ and W-Al₂O₃ near their percolation thresholds. Our results indicate that when conduction is by electronic tunneling between isolated grains, the Hall coefficient is negative for both cermets. In the bulk, Au has a negative coefficient and W a positive one.     

Softening of spin-wave stiffness near the ferromagnetic phase transition in diluted magnetic semiconductors

Employing the self-consistent Green's function approach, we studied the temperature dependence of the spin-wave stiffness in diluted magnetic semiconductors. Note that the Green's function approach includes the spatial and temperature fluctuations simultaneously which was not possible within conventional Weiss mean-field theory. It is rather interesting that we found the stiffness becomes dramatically softened as the critical temperature is approached, which seems to explain the mysterious sharp drop of magnetization curves in samples within diffusive regime.     

Changes in the gradient percolation transition caused by an Allee effect

The establishment and spreading of biological populations depends crucially on population growth at low densities. The Allee effect is a problem in those populations where the per capita growth rate at low densities is reduced. We examine stochastic spatial models in which the reproduction rate changes across a gradient g so that the population undergoes a 2D-percolation transition. Without the Allee effect, the transition is continuous and the width w of the hull scales as in conventional (i.e., uncorrelated) gradient percolation, w ∝ g(-0.57). However, with a strong Allee effect the transition is first order and w ∝ g(-0.26).     

Photoinduced defect formation in chalcogenide vitreous semiconductors

Journal of Applied Spectroscopy -     

Disordered magnet near the percolation threshold

A disordered Potts magnet containing a random mixture of ferromagnetic exchange constants J_a and J_b (J_a?J_b) near the percolation threshold is considered. The scaling form for the free energy contains two crossover exponents. Duality arguments in two dimensions show that these exponents are equal. They are also shown to be equal to unity in d = 6 ? ? dimensions to order ?.     

Role of rigidity in the fluid-solid transition

We examine the fluid-solid transition for a hard-disk system. By counting the near neighbors in the average configurations of a grand-canonical Monte Carlo simulation, this enables us to relate the thermodynamic transition with the rigidity theory, since we find that the coordination number in the fluid-solid transition is close to the coordination number predicted by a mean field rigidity theory, due to dynamical jamming of particles, where the contact region between disks is the radial ring outside a disk with a maximum allowed coordination number that is not bigger than six. Using these ideas, we were able to produce a continuous glass-like transition when nucleation of rigidity is suppressed.     

Amplitude of photostructural changes in chalcogenide vitreous semiconductors

It is shown that a position of an optical absorption edge (OAE) of amorphous AsSe and As₂S₃ films irradiated by light up to saturation is independent on T_exp the temperature at which the sample is exposed, and the amplitude of a reversible photoinduced shift of OAE ΔE is determined by its thermal shift ΔE as it is heated from T_exp up to the glass-transition temperature T_g. So, in order to obtain maximum photoinduced changes we need to use materials with maximum thermal variations of the forbidden zone width, and to tend to a greater difference between T_g and T_exp. The obtained results are well explained within the scope of the local heating model.