Magnetoresistance in semiconductor structures with hopping conductivity: effects of random potential and generalization for the case of acceptor states

We reconsider the theory of magnetoresistance in hopping semiconductors. First, we have shown that the random potential of the background impurities affects significantly preexponential factor of the tunneling amplitude which becomes to be a short-range one in contrast to the long-range one for purely Coulomb hopping centers. This factor to some extent suppresses the negative interference magnetoresistance and can lead to its decrease with temperature decrease which is in agreement with earlier experimental observations. We have also extended the theoretical models of positive spin magnetoresistance, in particular, related to a presence of doubly occupied states (corresponding to the upper Hubbard band) to the case of acceptor states in 2D structures. We have shown that this mechanism can dominate over classical wave-shrinkage magnetoresistance at low temperatures. Our results are in semi-quantitative agreement with experimental data.Comment: 19 pages, 3 figure

Structural, magnetic, electrical and supraconducting properties of the high temperature superconductor Nd(SrBa)Cu3O6+z Effect of argon annealing

We have studied the structural and superconducting properties of two samples superconductors Nd(SrBa)Cu3O6+z. One was a conventionally annealed sample at 450°C in oxygen (0) and the other was heated in argon at 850°C followed by oxygen annealing (A0). The Tc of the sample (O) was 68 K and that of (AO) sample was 78 K. This was confirmed by our AC susceptibility, resistivity and Seebeck effect measurements. Further, there was an enhancement in the shielding and the intergranular critical current. The X-ray and neutron data showed a tetragonal structure, and that the (O) sample had a higher impurity level arising from Nd entering the Sr/Ba sites that may result in a decrease in the carrier density (p) which also was confirmed by our thermopower measurements. A combination of several factors such as changes in the Cu (1) apical oxygen distance, chain oxygen ordering, increase in p and in-phase purity for the (AO) sample may qualitatively account for the observed data.We have studied the structural and superconducting properties of two samples superconductors Nd(SrBa)Cu3O6+z. One was a conventionally annealed sample at 450°C in oxygen (0) and the other was heated in argon at 850°C followed by oxygen annealing (A0). The Tc of the sample (O) was 68 K and that of (AO) sample was 78 K. This was confirmed by our AC susceptibility, resistivity and Seebeck effect measurements. Further, there was an enhancement in the shielding and the intergranular critical current. The X-ray and neutron data showed a tetragonal structure, and that the (O) sample had a higher impurity level arising from Nd entering the Sr/Ba sites that may result in a decrease in the carrier density (p) which also was confirmed by our thermopower measurements. A combination of several factors such as changes in the Cu (1) apical oxygen distance, chain oxygen ordering, increase in p and in-phase purity for the (AO) sample may qualitatively account for the observed data

Weak Localization and Electron-Electron Interactions theories applied to the positive magnetoresistance in metallic Si1-yNiy:H alloys

Electrical conductivity of hydrogenated amorphous silicon nickel alloys (Si1-yNiy:H) is discussed on the metallic side of the metal insulator transition (MIT), at very low temperature, and in the presence of magnetic field up to 4.5T. The experimental results are analyzed in the light of weak localization and electron-electron interaction theories in a strong spin-orbit case. The zero magnetic field conductivity is almost found to vary according to the formula σ(B=0,T)=σ0+m.T1/2. Physical interpretations of our results are given to understand the diffusion mechanism