V. V. Andrievskii

Magnetoresistance of nanocarbon materials based on carbon nanotubes

The results of experimental investigations of magnetoresistance in nanocarbon material (NCM) containing carbon nanotubes in magnetic field up to 5 T and at temperature up to 0.54 K are reported. The obtained experimental magnetoresistance curves of NCM are described satisfactorily within the framework of the shrinkage effect of wave function of localized state in a magnetic field along with the spin-polarization mechanism.

Weak localization and charge-carrier interaction effects in a two-dimensional hole gas in a germanium quantum well in a SiGe/Ge/SiGe heterostructure

The weak localization and interaction effects for charge carriers in a two-dimensional hole gas in a pure germanium quantum well in a SiGe∕Ge∕SiGe heterostructure with a hole density of 5.68×1011cm−2 and mobility of 4.68×104cm2V−1s−1 are investigated. The resistance measurements were made at temperatures from 46mKto10K and magnetic fields up to 15T. The magnetic-field dependence of the resistivity exhibits Shubnikov-de Haas oscillations and quantum Hall effect steps. At very low magnetic fields (B<0.1T) a weak localization effect for holes is revealed, which makes for a negative magnetoresistance and growth of the resistance with decreasing temperature (at T<2K). The manifestation of the interaction effect is observed and analyzed over a wide range of temperatures and magnetic fields. With increasing temperature the manifestation of the interaction-induced quantum correction passes from the diffusive regime to an intermediate and then to the ballistic regime. In all regions the behavior of the interaction...

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

The use of Shubnikov-de Haas oscillations for determining effective mass is illustrated by a study of the magnetotransport properties of the two-dimensional hole gas in Si1− x Ge x (x = 0.13, 0.36, 0.95, 0.98) quantum wells. For some samples the data cannot be fitted to standard theoretical curves in which the scattering of charge carriers is described by the conventional Dingle factor. The reasons for the discrepancies between the experiment the theory are: (i) the effect of spin splitting on the amplitude of the peak in the SdH oscillations; (ii) extra broadening of the Landau levels attributable to an inhomogeneous distribution of the carrier concentration; (iii) the coexistence of short and long-range scattering potentials; and, (iv) population of the second energy level in the quantum well. Ways of calculating the effective hole masses m* for all these cases are presented and values of m* are found for the heterostructures studied here.

Positive quasiclassical magnetoresistance and quantum effects in germanium quantum wells

Changes in the conductivity of p-type quantum-well heterostructures of Si(0.05)Ge(0.95) alloy are studied at temperatures ranging from 0.352-7.1 K and magnetic fields of up to 11 T. The distinctive feature of the sample was asymmetric doping, with layers of Si(0.4)Ge(0.6) with boron impurity concentrations of 2 . 10(18) and 8 . 10(18) cm(-3) positioned on opposite sides of the quantum well. Shubnikov-de Haas oscillations were observed clearly against the background of a high quasi-classical positive magnetoresistance. The field dependence of the magnetoresistance is well described by a function of the form rho(xx)(B)/rho(xx)(0)proportional to B(12/7), as predicted by a theory including the combined effect of both short-and long-range disorder. The contribution to the temperature and magnetic field dependences of the resistance owing to quantum corrections associated with weak localization and charge carrier interactions is determined. Strong spin-orbital scattering of holes on the quantum well is revealed by analyzing these corrections. A study of the variations in the amplitude of the Shubnikov-de Haas oscillations with temperature and magnetic field (including the monotonic behavior of the resistance with changing magnetic field) makes it possible to determine the effective mass of the charge carriers, m*=0.17m(0) The temperature dependence of the hole-phonon relaxation time was found by studying the overheating of charge carriers by an electric field

Overheating effect and hole-phonon interaction in SiGe heterostructures

The effect of charge-carrier overheating in a two-dimensional 2D hole gas is realized in a Si1�xGex quantum well, where x = 0.13, 0.36, 0.8, or 0.95. The Shubnikov–de Haas SdH oscillation amplitude is used as a “thermometer” to measure the temperature of overheated holes. The temperature dependence of the hole-phonon relaxation time is found from an analysis of the change of the dependence of the amplitude of the SdH oscillations on temperature and applied electrical field. Analysis of the temperature dependence of the hole-phonon relaxation time reveals a transition of the 2D system from the regime of “partial inelasticity” to conditions of small-angle scattering.

Spin precession observation in quantum corrections to resistance of Si0.7Ge0.3/Si0.2Ge0.8 heterostructure with 2DHG

The two-dimensional hole gas (2DHG) magnetoresistivity of a Si0.7Ge0.3/Si0.2Ge0.8 heterostructure in a wide range of temperatures T=0.335–20K and transport currents I=100–50μA is measured. In the vicinity of zero magnetic field, a sharp and positive in sign feature on smooth negative magnetoresistance is observed at the lowest temperatures. The amplitude of this feature quickly fades with increasing temperature and transport current. For the analysis of the experimental data the theory of weak localization for 2DHG is applied. The values of τso and τtr obtained are used for the first time to define zero magnetic field splitting in the hole energy spectrum for a Si0.7Ge0.3/Si0.2Ge0.8 heterostructure: Δ=2.97meV.

Manifestation of the spin-orbit interaction in bismuth films in a parallel magnetic field

The magnetic field dependence of the resistance of bismuth thin films (100–700A thick) at low temperatures (1.5–77K) are analyzed in the conceptual framework of quantum corrections to the conductivity due to weak localization and electron interaction effects. It is shown that the diversity and variability of the magnetoresistance curves in a parallel field upon variations of the thickness and temperature are due to the fact that the spin-orbit interaction time τso increases with increasing field, altering the relationship between τso and the phase relaxation time τφ. This result supports the hypothesis that the strong spin-orbit interaction manifested in the surface scattering of electrons is due to the existence of a potential gradient near the metal surface, and a parallel magnetic field alters the orientation of the spins, accompanied by a decrease of the rate of spin-orbit processes.

Quantum Interference and Spin-Splitting Effects in Si 1− X Ge X p -Type Quantum Well

The magnetoquantum and quantum-interference effects of two-dimensional hole gas in Si/SiGe-based heterostructures with Si 1−X Ge X quantum wells were studied in the temperature range 0.35 – 70K in a magnetic field up to 11T. In high magnetic fields (B > 1.5T) the magnetic field dependencies of the samples resistances exhibit the Shubnikov-de Haas oscillations. The positive growth and maximum of magnetoresistances are observed in weak magnetic fields limit (B < 0.1T), which can be attributed to the influence of weak localization of the charge carriers when the inelastic scattering time τ φ and spin orbit scattering time τSO have close values. It is shown that in the heterostructures studied splitting of the spin states occurs due to the influence of the perturbing potential (Rashba mechanism). The corresponding quantum times and spin splitting values are calculated.

Relaxation of highly nonequilibrium electrons in bismuth point contacts

Abstract The technique of electron focusing with a transverse magnetic field has been used to investigate inelastic relaxation of charge carriers in point contact at bismuth crystal. The observed features of the electron focusing line suggest a discrete nature of the electron—phonon relaxation of highly nonequilibrium electrons in the current-carrying point contact. It is found that the energy of electrons injected in a metal is determined not only by the voltage applied to the point contact, but also by the point contact temperature and the condition of heat removal from it. If the excess of electron energy becomes considerably higher than the Debye energy the phonon features of the EF line smoothly vanish, but a gigantic peak with average released energy of 40 meV appears. A more efficient channel of inelastic relaxation of highly nonequilibrium electrons opens.

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 τ h - ph sat weak magnetic fields demonstrated transition of the 2D system from regime of “partial inelasticity” characterized by dependence τ h - ph − 1 ∝ T2 to regime of small-angle scattering, described by dependence τ h - ph − 1 ∝ T5 with temperature increase. But in higher magnetic fields the dependence τ h - ph − 1 ∝ T3 manifests itself on dependences τh-ph(Th-ph). The possible explanations of such dependences are discussed.PACS: 72.15.Lh Relaxation times and mean free path; 72.20. My Galvanomagnetic and other magnetotransport effects; 72.20. −i Conductivity phenomena in semiconductors and insulators.