Modern Physics Letters B

In this letter we use a Semi-Classical Impurity Band Model and obtain by Monte Carlo simulation the density of states (DOS) and the impurity charge distribution inside a quantum well (QW) of Ga_1−xAl_xAs/GaAs. We show the existence of a Coulomb gap as has been observed in bulk semiconductors. The DOS is not very sensitive to the QW width close to the Coulomb gap, at least in the range from 1 to 4 times the Bohr radius, and shows a behavior D(E)∝|E−E_F| which indicates a two-dimensional signature. We show that the neutral donors concentrate in the center of the well according to a distribution whose width and decay rate depend on the compensation and impurity concentration respectively. Those effects are expected to be observed by infra-red absorption experiments and useful in device diagnosis.

Hydrogen vibration spectra for both polycrystalline Ti₂CuH_1.03 and amorphous Ti₂CuH_1.26 have been measured in the energy range of 50–210 meV using a beryllium filter detector spectrometer. From the experimental curves it can be seen that although there is some similarity in vibrational density of states of the hydrogen atoms in these two samples, large differences between them are evident. The optical vibration peak of crystalline sample reveals a narrow frequency distribution with a FWHM of 54 meV and consists apparently of two peaks at 92 and 147 meV. In the amorphous sample, the optical peak occurs at nearly the same frequency as the main peak of the former but with strongly increased width (~104 meV FWHM). The result shows that on the average, the H-sites occupied in amorphous and crystalline structures are mainly tetrahedral type holes, but the distribution of local environment for the hydrogen atoms in the alloy glass is much broader because the polyhedra in it may be heavily distorted and there is local fluctuation in its chemical composition.

Lead sulphide thin films have been deposited by conventional chemical bath technique. The effect of Cu doping is studied on the resistivity and the band gap of these films. An anamolous behavior is obtained in the case of variation of band gap with impurity (mol%) which shows that certain values of impurity have prominent features. While the resistivity decreases first with the increase in impurity, after a particular value the resistivity adopts a rising trend, and on still higher doping the decreased resistivity again starts to take place.

X-ray, electrical conductivity and infrared absorption studies for the system Zn_1−xCu_xMn_xFe_2−xO₄ have been carried out. All the compounds in the range 0≤x≤1 show cubic symmetry. It is found that all the compounds of the system possess low mobility values in the order of 10⁻⁹cm²/Vsec and conduction is due to hopping process. The electrical conductivity measurements show that the activation energy decreases with increasing concentration of Cu²⁺ and Mn³⁺ in the crystal lattice. X-ray intensity calculations and IR studies indicated the presence of Zn²⁺ at tetrahedral sites, Cu²⁺ and Mn³⁺ at octahedral sites, and Fe³⁺ distributed both tetrahedral and octahedral sites. The probable ionic configuration for the system is suggested as

Some dielectric parameters e.g. dielectric constant, resistivity, dielectric strength, loss tangent, coercive field and saturation polarization of barium titanate-sodium niobate composites have been calculated as a function of composition and frequency. assuming a very simplified model and a 3–0 connectivity pattern. Most of the parameters have been found to vary linearly with composition at a given frequency.

The critical temperature of the 124-class of high temperature superconductors is investigated by employing the model of charge carriers interacting with the oxygen-displacive modes of the lattice. The role played by the fixed stoichiometry in entailing a relatively narrow range of critical temperature is emphasized.

Hybridization of Nd(fπ) and Ce(fπ) orbitals with composite O(pπ)-O(pπ) bonding/ Cu(dπ*)-O(pπ) antibonding orbitals at the Fermi energy (E_F) is shown to promote high-T_c superconductivity in Nd_2−xCe_xCuO₄. Dynamic Jahn-Teller coupling of these hybrid molecular orbitals to the lattice leads to Cooper pairing as it does in other high-T_c superconductors, such as La_2−xSr_xCuO₄, where O(pπ) character at E_F is dominant.

Scanning electron microscopy, energy dispersive X-ray spectroscopy, and polarized light microscopy have shown that the addition of AgO to the EuBa₂Cu₃O_y and GdBa₂Cu₃O_y systems results in the growth of very large grains. Distribution of silver particle appears to influence the grain formation and growth in the superconducting composites.

We study the Nielsen-Olesen vortices in the three dimensional noncompact Abelian Higgs model with a Chern-Simons term. The vortices are fractionally charged anyons (i.e., particles with fractional spin and statistics). In a certain limit we can express the theory in terms of these topological excitations by performing a duality transformation on the lattice. The result is a topologically conserved current describing the vortices, minimally coupled to a dynamical gauge field with a Chern-Simons term. Spin, statistics and size of the excitations are all preserved under this transformation.