I. B. Berkutov

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.

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

Asymmetric magnetoresistance in the graphite intercalation compounds with cobalt

ABSTRACT The results of measurements of the magnetoresistance for graphite intercalation compounds with cobalt based on highly oriented pyrolytic graphite and fine crystalline pyrolytic graphite in the temperature range from 1.6 K to 293 K and in a magnetic field up to 16 T are presented. For the investigated intercalation compounds, the effect of asymmetry of the magnetoresistance relative to the magnetic field direction is revealed. It is shown that this effect can be satisfactorily explained within the Segal model of asymmetric magnetoresistance in thin films with large magnetic anisotropy.

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-orbit interaction in thin bismuth films

The magnetic-field dependences of the resistance of thin (100–700 A thick) bismuth films at low temperatures are analyzed using quantum corrections to the conductivity with weak electron localization. It is shown that the spin-orbit scattering time τso is much shorter than the phase relaxation time τφ of the electrons (the case of a strong spin-orbit interaction). It is found that τso tends to increase with the film thickness. This shows that the surface scattering of the electrons plays a dominant role in spin-orbit processes. Apparently, strong spin relaxation in the presence of surface scattering is due to the gradient of the internal crystal-field potential near the surface of the metal, resulting in lifting of the spin degeneracy and in the appearance of a spin gap (Rashba mechanism).The magnetic-field dependences of the resistance of thin (100–700 A thick) bismuth films at low temperatures are analyzed using quantum corrections to the conductivity with weak electron localization. It is shown that the spin-orbit scattering time τso is much shorter than the phase relaxation time τφ of the electrons (the case of a strong spin-orbit interaction). It is found that τso tends to increase with the film thickness. This shows that the surface scattering of the electrons plays a dominant role in spin-orbit processes. Apparently, strong spin relaxation in the presence of surface scattering is due to the gradient of the internal crystal-field potential near the surface of the metal, resulting in lifting of the spin degeneracy and in the appearance of a spin gap (Rashba mechanism).

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.

Weak localization and interaction effects in acceptor graphite intercalation compounds

The presented work is devoted to investigations of manifestation of quantum effects of weak localization and interaction of charge carriers in electrical conductivity of acceptor graphite intercalation compounds (CICs). As shown by studies intercalation leads to a decrease in the resistivity and to change the resistivity temperature coefficient from negative sign in the source graphite on a positive sign in intercalated graphite. At the low temperature for all GICs specimens the minimum in the temperature dependence of resistivity is observed. In terms of the model of charge carriers weak localization and interaction for two-dimensional systems temperature dependence of phase relaxation time, localization radius and charge carriers screening constant for all GICs are estimated.

Germanium quantum well with two subbands occupied: Kinetic properties

Multisubband transport of the p-type Si0.4Ge0.6/Ge/Si0.4Ge0.6 heterostructure has been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Two frequency Shubnikov–de Haas oscillations indicate occupation of two subbands. This allows us to determine the densities and mobilities of the charge carriers on each subband. Shubnikov–de Haas oscillations reveal two 2D conduction subbands with carrier effective masses of 0.112m0 and 0.131m0. The quantum Hall ferromagnetic states which results from the crossing of two Landau levels with opposite spin and different subband was observed in SiGe systems for the first time.

Interference effects in silicon-germanium heterostructures with quantum wells of different widths

Weak localization effects and the interactions of charge carriers are studied in two Si0.7Ge0.3/Si0.2Ge0.8/Si0.7Ge0.3 p-type heterostructures, where one or two quantum levels are filled, respectively. A weak localization effect for two-dimensional charge carriers is found to occur in weak magnetic fields when the spin-orbital and inelastic scattering times are close, which is indicative of splitting of the spin states under the influence of a perturbing potential related to the formation of a two-dimensional potential well (Rashba mechanism). In higher magnetic fields when one quantum level is occupied, interaction effects appear that are caused by Coulomb interactions with a scatterer. When the two quantum levels are occupied, the dominant mechanism is scattering on Friedel oscillations of the charge carrier density induced by an impurity electric field. In all regions, the quantum corrections are in good agreement with modern theoretical predictions.