Pressure dependence of shear elastic mode near structural transition in the band Jahn-teller model

The temperature dependence of the pressure derivative of the shear elastic constant $\text{dC}{}_{\mathrm{<mtext>S</mtext>}}\text{dp}$ for degenerate electronic system is worked out in the band Jahn Teller model and is shown to follow C_S linearly as the temperature decreases and approaches the transition temperature T_M. The constant of proportionality gives useful informations on the characteristics of the band Jahn-Teller transition. This analysis can explain well the recently observed experimental results on Nb₃Sn.     

Random exchange interactions and the “frustration effect”

A random distribution of positive (ferromagnetic) and negative (antiferromagnetic) exchange interactions on a lattice is considered. Depending on the relative abundance of positive bonds, conventional magnetically ordered structures, as well as the spin glass phase, are obtained.     

Accuracy of scalar approximation for single-mode fibers

Total dispersion in single-mode fibers has been computed from the exact numerical solution of scalar and vector wave equations and the accuracy of the scalar approximation has been discussed. It has been shown that for larger values of relative index difference between core and cladding, although the values of total dispersion obtained by the two approaches differ appreciably in the vicinity of zero dispersion, the optimum core radii and the zero dispersion wavelengths obtained by the two methods differ negligibly for all practical purposes.     

Influence of alloy composition on the dependence of the diffusivity-mobility ratio in ternary semiconductors on a quantizing magnetic field

An expression is derived for the diffusivity-mobility ratio in degenerate narrow-gap ternary semiconductors in the presence of a quantizing magnetic field. With the help of this expression, the diffusivity-mobility ratio is shown, taking n-Hg_1–xCd_xTe as an example, to have an oscillatory dependence on a quantizing magnetic field. The influence of alloy composition on such dependence is also investigated.     

Simple theory of the density-of-states function in heavily doped non-linear optical and optoelectronic materials

We study theoretically the dispersion relation of the conduction electrons and the corresponding density-of-state (DOS) function in heavily doped non-linear optical, tetragonal, III–V, ternary and quaternary materials forming band tails. It has been found, taking CdGeAs₂, Cd₃As₂, InAs, In_1−xGa_xAs_yP_1−y lattice matched to InP and Hg_1−xCd_xTe as examples of the aforementioned materials that the complex nature of the energy spectrum, the oscillatory DOS for the negative values of the energy and the creation of a new forbidden zone is due to the presence of the crystal field splitting constant, the anisotropic effective electron mass and the anisotropic spin–orbit splitting for the tetragonal and non-linear optical materials and because of the interaction of the impurity atoms in the tails with the spin–orbit splitting of the valence bands for the other compounds. Analytically, the presence of non-removable poles in the dispersion relation of the undoped material creates the complex energy spectrum for the corresponding heavily doped sample and the solution of the Boltzmann transport equation will introduce new physical ideas and new experimental findings under different external conditions. The results of the perturbed III–V semiconductors whose unperturbed energy band structures are defined by the two band model of Kane and that of parabolic energy bands are special cases of our generalized analysis and no oscillations in the DOS are found in the aforementioned cases. The well-known results of the DOS for all materials in the absence of doping have also been obtained from the theoretical analysis under certain limiting conditions. This compatibility is the indirect test of our generalized formalism.     

Critical coupling and coherent perfect absorption for ranges of energies due to a complex gain and loss symmetric system

We consider a non-Hermitian medium with a gain and loss symmetric, exponentially damped potential distribution to demonstrate different scattering features analytically. The condition for critical coupling (CC) for unidirectional wave and coherent perfect absorption (CPA) for bidirectional waves are obtained analytically for this system. The energy points at which total absorption occurs are shown to be the spectral singular points for the time reversed system. The possible energies at which CC occurs for left and right incidence are different. We further obtain periodic intervals with increasing periodicity of energy for CC and CPA to occur in this system.     

Control of adhesion via internally pressurized subsurface microchannels

While pressure sensitive adhesives in general consist of a layer of viscoelastic glue sandwiched between two adherents, we explore here the design of an adhesive embedded with microchannels which remain either open to atmosphere or pressurized to different positive and negative pressures. We subject these layers to indentation by a rigid cylinder such that in addition to adhesion between the indenter and the adhesive surface, the inner walls of the channels too self-adhere; during retraction of the indenter, these surfaces debond, but at a different load, thus resulting in hysteresis. When these channels are pressurized to different extents, the contact areas of various interfaces vary, so also the resultant hysteresis. For experiments with constant depth of indentation, the hysteresis increases and attains maxima at an intermediate value of the internal pressure inside the channels. The hysteresis increases also with the skin thickness of the adhesive over the channels. These results show that subsurface channels in an adhesive allow active manipulation of adhesion over a large range via coupled effect of geometry of channels, their surface characteristics, and the pressure inside.