Negative magnetoresistance in the nearest-neighbour hopping conduction in granular gold film

The low temperature (0.5-55 K) conduction of semicontinuous gold film vacuum deposited at T \approx 50 K is studied. The film is near the percolation threshold (thickness 3.25 nm). Its resistance is extremely sensitive to the applied voltage U. At low enough U the film behaves as an insulator (two-dimensional granular metal). In this state the dependences R(T) \propto \exp (1/T) (for T \leq 20 K) and R(U) \propto \exp (1/U)) (for T \leq 1 K and U > 0.1 V) are observed. Magnetoresistance (MR) is negative and can be described by \Delta R(H)/R(0) \propto -H^2/T. This negative MR which manifests itself for nearest-neighbour hopping is rather uncommon and, up to now, has not been clarified. The possible mechanisms of such case of negative MR are discussed.Comment: 9 pages, LATEX, 6 figures. To be published in Physica B. Fig.4 is JPG file, in case of troubles with it, appeal for help and advice to: [email protected]

Magnetotransport studies of SiGe-based p-type heterostructures: problems of the effective mass determination

The Shubnikov–de Haas oscillations method of the effective mass extraction was illustrated by the magnetotransport properties investigation of two-dimensional hole gas in Si₁₋xGex (x = 0.13, 0.36, 0.95, 0.98) QWs. We have found that for certain samples our data cannot be fitted to standard theoretical curves in which the scattering of charge carriers is described by conventional Dingle factor. It is demonstrated that reasons of deviations of the experiment from the theory are as follows; (i) influence of the spin splitting on amplitude of SdH oscillations maxima; (ii) extra broadening of the Landau levels attributed to existence of inhomogeneous distribution of the carrier concentration; (iii) the influence of the concurrent existence of short and long-range scattering potentials; (iv) the population of second energy level in the quantum well. The ways to calculate the effective masses m* of holes in all cases are presented and values of m* are found for studied heterostructures

Effect of microstructures on the electron-phonon interaction in the disordered metals Pd60_{60}Ag40_{40}

Using the weak-localization method, we have measured the electron-phonon scattering times $\tau_{ep}$ in Pd$_{60}$Ag$_{40}$ thick films prepared by DC- and RF-sputtering deposition techniques. In both series of samples, we find an anomalous $1/\tau_{ep} \propto T^2\ell$ temperature and disorder dependence, where $\ell$ is the electron elastic mean free path. This anomalous behavior cannot be explained in terms of the current concepts for the electron-phonon interaction in impure conductors. Our result also reveals that the strength of the electron-phonon coupling is much stronger in the DC than RF sputtered films, suggesting that the electron-phonon interaction not only is sensitive to the total level of disorder but also is sensitive to the microscopic quality of the disorder.Comment: accepted for publication in Phys. Rev.

TEMPERATURE DEPENDENCE OF THE ELECTRIC RESISTIVITY OF AMORPHOUS BISMUTH FILMS

The reversible temperature-induced variation of the electric resistivity is studied on amorphous bismuth films condensed onto helium temperature substrates in the region of existence of the amorphous phase. The films studied (10 to 130 Å thick) had negative temperature resistance coefficient. In the range 10-30 K the resistance difference (in reference to the initial value) is proportional to T4 ; however, superconducting fluctuations allowed for, it becomes proportional to T2. For thin films (< 30 Å) it varies as T2 in both the cases ; at temperature above 30 K it is proportional to T. The results obtained are discussed in terms of up-to-date theoretical models

The overheating effects in germanium quantum well with two subbands occupied

The charge carrier overheating effect was studied in the p-type Si₀.₄Ge₀.₆/Ge/Si₀.₄Ge₀.₆ heterostructure with two subband occupy. The temperature dependences of hole-phonon relaxation time τh-ph at weak magnetic fields demonstrated transition of the 2D system from regime of “partial inelasticity” characterized by dependence τ⁻¹h-ph ∝ T² to regime of small-angle scattering, described by dependence τ-1h-ph ∝ T⁵ with temperature increase. But in higher magnetic fields the dependence τ⁻¹h-ph ∝ T³ manifests itself on dependences τh-ph(Th-ph). The possible explanations of such dependences are discussed.Ефект перегріву носіїв заряду вивчався в гетероструктурі Si₀.₄Ge₀.₆/Ge/Si₀.₄Ge₀.₆ p-типу з двома зайнятими підзонами. В слабких магнітних полях температурні залежності часу дірково-фононної релаксації h-ph τ демонструють перехід двовимірної системи з режиму «часткової непружності», яка характеризується залежністю τ⁻¹h-ph ∝ T² до режиму малокутового розсіювання, що описується залежністю τ-1h-ph ∝ T⁵ з підвищенням температури. В більш високих магнітних полях залежність τ⁻¹h-ph ∝ T³ змінюється на -ph -ph ( ) τh hT . Обговорюються можливі варіанти пояснення таких спостережень