Showing papers on "Variable-range hopping published in 1982"

DC conduvtivity of V2O5TeO2 glasses

Abstract: The dc conductivity of semiconducting vanadium tellurite glasses of compositions in the range 50 to 80 mol% V2O5 has been measured in the temperature region 77 to 400 K. Measurements have been made on annealed samples at different annealing temperatures. Annealing the samples at temperature of about 250°C causes the appearance of a complex crystalline phase resulting in an increase of conductivity. Results are reported for amorphous samples of different compositions. The conductivity of tellurite glasses is slightly higher than the corresponding composition of phosphate glasses, but the general trend of the increase of conductivity and decrease of high temperature activation energy with increasing V2O5 content is similar in the two systems. The data have been analysed in the light of existing models of polaronic hopping conduction. A definite conclusion about the mechanics of conduction (adiabatic or nonadiabatic) is difficult in the absence of a precise knowledge of the characteristic phonon frequency v0. Adiabatic hopping is indicated for v0∼1011 Hz, however this value leads to unreasonably low value for the Debye temperature θD, and higher values for v0∼1013 hz satifiies the conditions for nonadiabatic hopping which appears to be the likely mechanism of conduction in V2O5TeO2 glasses. The low temperature data (< 100 K) can be fitted to Mott's variable range hopping, which when combined with ac conductivity data gives reasonable values of α, but a high value for the disorder energy.

Correlation Effects on Variable Range Hopping Conduction and the Magnetoresistance

Abstract: Effects of intra-state correlation on the variable range hopping are investigated in the absence and presence of a magnetic field. In the absence of a magnetic field, Mott's expression for the resistivity, ρ∝exp [ T 0 / T ] 1/4 , still holds in its temperature dependence. However, the prefactor T 0 now depends on two types of localization lengths brought about by the intra-state correlation. Magnetoresistance is found to be positive due to the intra-state correlation. It shows a linear dependence on a magnetic field in lower magnetic fields and saturates above a certain magnetic field.

Diffusion in a one-dimensional disordered system

Abstract: The low-frequency properties of a disordered one-dimensional diffusion model are determined. Of particular interest is the case where the distribution of hopping elements diverges for small values of the hopping elements. The density of states and the leading correction terms are calculated at low frequencies and the diffusion constant and correlation length are determined. The scaling assumptions of Alexander et al. are verified. The model considered is also one for phonons with random force constants and the localization length for low-frequency phonons is determined. The case where there is both off-diagonal and diagonal disorder is considered.

Exciton-plasma Mott transition in Si

Abstract: The transition between free excitons and an electron-hole plasma is described for the first time by a theory based on full random-phase-approximation (RPA) screening in the plasma. The onset of exciton binding predicted is in excellent agreement with experimental data attributed to exciton dissociation in Si. It is suggested that the many-body electron-hole interaction may be well described by static RPA screening near the Mott transition.

Transport and optical properties of the hydrazine intercalation complexes of 1T-TaS2

Abstract: The reversible interconversion of 1T-TaS2 with the hydrazine complexes 3R1.1 TaS2(N2H4)4/3 and 3RI TaS2(N2H4)2/3 has been studied by thermogravimetric analysis, optical transmission, and electrical resistivity measurements. The low-temperature resistivity of both hydrazine complexes is high, exceeding 103 Omega cm when below 20K, and for the 3RI.I complex in rho varies as T-1/3. This can be interpreted as two-dimensional variable range hopping at the Fermi level, EF, in a system in which the density of states at EF is reduced from the expected metallic value by the presence of a strong Fermi-surface driven structural distortion.

Interplay of Disorder and Coulomb Interactions in Anderson-Localized States

Abstract: This paper discusses a general theoretical treatment to investigate the rival effects of disorder and electron-electron interactions in the Anderson-localized regime. First it is shown that the intrastate interaction gives rise to the occurrence of singly occupied states as well as doubly occupied states near and below the Fermi level and that this results in a Curie-type spin susceptibility at low temperatures. Secondly it is shown that electron-electron interactions between Anderson-localized states lead to both ferromagnetic (direct exchange) and antiferromagnetic (kinetic-type exchange) interactions among spins of singly occupied states. Specific heat anomaly beyond the T-linear behavior, in particular its magnetic field dependence, and magnetic susceptibility at very low temperatures are calculated by taking into account the coexistence of ferromagnetic and antiferromagnetic interactions within a spin pair model. It is shown that the experimental results on Si:P can be well explained by the present treatment. Finally the effects of electron-electron interactions on the magnetoresistance of the variable range hopping conduction are investigated. It is shown that magnetoresistance is positive as far as the change of localization length due to a magnetic field is neglected. Taking into account the latter effect which is responsible for negative magnetoresistance, the total magnetoresistance including positive and negative parts is calculated for the system of lT-TaS2 which is considered to be two dimensional. The peculiar features of magnetoresistance observed in that system can be explained well by the present theory.

Variable range hopping in Al implanted sapphire

Abstract: Variable range hopping conductivity (i.e. resistivity proportional to exp T−1/4) has been observed in Al implanted sapphire. The conductivity of these implanted layers was studied as a function of implanted doses.

Variable range hopping conductivity in copper-indium-gallium-selenium-tellurium solid solutions

Abstract: Electrical conductivity of CuIn0.3Ga0.7SeTe solid solution (pentenary alloy) have been studied. Variable range hopping conductivity of Mott's T-1/4 type was observed in the temperature region 100 to 370K for both pentenary alloy and the related ternary compound CuGaTe2. The values obtained of the effective Bohr's radius are of the order of 10 AA.

Magnetoresistance in Sputtered Hydrogenated Amorphous Silicon

Abstract: Magnetoresistance (MR) measurements have been made on sputtered hydrogenated amorphous silicon films for magnetic fields up to 1 T in the temperature range 100-300 K. The effect of a systematic increase of the hydrogen content in the films has been studied. In α-Si films with no hydrogen MR was always negative and it increased with decreasing temperature. In hydrogenated films MR was positive and its magnitude about a decade larger than the negative MR. The positive MR showed a maximum at low fields ~ 0.2 T. Most of the observed features in the temperature range 200 K < T < 300 K can be explained by a hopping model in which MR is a result of the modification of the spin-flip relaxation time by the external magnetic field. It is concluded that the change of sign in MR with increasing hydrogen content is most probably due merely to the decrease in the density of states in the gap which causes a narrowing in the distribution of the hopping relaxation times. The obtained results support the idea that the variable range hopping mechanism dominates the electrical conduction in sputtered α-Si below room temperature. Further, the results indicate that spin dependent effects have a strong influence on the electrical transport in amorphous semiconductors.