The magneto-quantum and quantum interference effects in a two-dimensional gas of p-type charge carriers are studied for three quantum wells made of practically pure germanium in a Si0.6Ge0.4/Si0.2Ge0.8/Si0.6Ge0.4 heterostructure. The quantum well widths were 8 nm for sample I, 19.5 nm for sample II, and 25.6 nm for sample III. The dependences of resistance on the magnetic field for all samples exhibit Shubnikov–de Haas oscillations. Their analysis made it possible to calculate the kinetic characteristics of charge carriers for the cases of one (sample I) and two occupied subbands (samples II and III). In the region of weak magnetic fields (B < 0.1 T), the effect of weak localization of holes was revealed, which determines the negative magnetoresistance and the increase in resistance with decreasing temperature. The manifestation of the charge carriers interaction effect at various temperatures and magnetic fields is discovered and analyzed. A transition from the diffusion mode of manifestation of the quantum correction to the intermediate, and then to the ballistic mode is observed. In all regions, the behavior of the quantum correction due to the charge carriers interaction effect is in good agreement with modern theoretical predictions. The temperature dependences of the hole-phonon relaxation time are calculated. In weak magnetic fields, with an increase in the temperature of the 2D system, a transition from the “partial inelasticity” mode, characterized by the dependence τhph−1∝T2, to the small-angle scattering mode, described by the relation τhph−1∝T5, takes place. In stronger magnetic fields for samples with two occupied subbands, the dependence τhph−1∝T3was observed. Possible explanations for this dependence are presented.

Using the example of a pressed sample consisting of chromium dioxide nanoparticles coated with insulating shells, we study the relationship between the electron transport system and magnetic subsystem in granular spin-polarized metals. It is shown that the spin-polarized tunneling transport current can affect the coercivity fields of the percolation cluster formed in the sample with decreasing temperature.

The magnetoresistance of bulk specimens of graphite nanoplatelets obtained by different methods is studied in magnetic fields up to 2.2 T. It has been established that magnetoresistance is negative for graphite nanoplatelets prepared by chemical treatment of source graphite with a solution of potassium permanganate in sulfuric acid. This negative magnetoresistance can be explained in terms of the model of charge carrier's weak localization in a system with imperfect structure. It has been established that the magnetoresistance is positive and independent of temperature for graphite nanoplatelets produced by sonication method. Moreover, magnetoresistance is linear relative to a magnetic field in fields above ∼0.7 T. It is shown that linear magnetoresistance can be explained in the terms of the Abrikosov's model of quantum linear magnetoresistance.

Quantum effects in p-type Si0.2Ge0.8/Ge/Si0.2Ge0.8 heterostructure with an extremely high mobility of charge carriers μH = 1367000 cm2/(V ⋅ s) have been comprehensively studied. An analysis of Shubnikov–de Haas oscillations yielded effective mass of charge carriers, which proved to be very low, m* = 0.062m0, and the value of fluctuations of hole density along the channel δp = 3.5 ⋅ 109 cm–2. The fractional Hall effect (filling numbers 8/3, 7/3, 5/3, 4/3) observed at temperatures up to 5 K has been discovered in strong magnetic fields. The studies of quantum interference effects related to weak localization and electron-electron interaction between charge carriers, which have been conducted in such a high-mobility system for the first time, enabled calculation of spin splitting Δ = 1.07 meV and the Fermi-liquid coupling constant F0σ=−0.12, which agree with results obtained earlier.

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 sat weak magnetic fields demonstrated transition of the 2D system from regime of “partial inelasticity” characterized by dependence τh-ph−1 ∝ T2 to regime of small-angle scattering, described by dependence τh-ph−1 ∝ T5 with temperature increase. But in higher magnetic fields the dependence τh-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.

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.

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.

The magnetoresistance and electrical resistivity of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) were studied in the temperature range of 1.6-100.3 and 1.6-286K, respectively, using a standard four-probe technique. The possible mechanisms of the observed effects are discussed in detail. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The behaviors of resistance, magnetoresistance (up to 5 T), and Hall electromotive force (EMF) with varying temperature (10–300 K) and measuring current (A–10 mA) are studied for the Si sample with CrSi2 nanocrystallites (NC) in the plane (111). The conduction in such heterostructure proceeds in the plane with the NC and is the conduction of a two‐dimensional system of charge carriers that shows some unusual effects. The temperature variation of resistivity may be treated as the result of the effect of thermal activation but in this case it is characterized by a low activation energy different in value in different temperature ranges. This suggests that the mechanism of conduction is more complex. It is found that the conduction is determined by the effect of temperature variation not only on carrier concentration but also on its mobility. Magnetoresistivity is also of different shape in different temperature ranges. All the above features are treated in terms of the proposed model of electron hopping through the conduction band (or hole hopping through the valence band). A peculiar effect of giant reduction in resistivity with increasing the measuring current has been revealed. Discussed are some possible factors responsible for this effect.

A conductivity quasi-two-dimensional system formed by nanocrystallites CrSi2 located in the crystallographic (111) plane of silicon has been considered. At low temperatures the system exhibits several unique properties: (i) the activation energy in the temperature dependence of resistance is appreciably lower than in the case of impurity condition; (ii) the carrier mobility is very high but it decreases rapidly with the growing temperature; (iii) the magnetoresistance is linear and decreases rapidly as the temperature rises. To explain these features, a model has been proposed which assigns special importance to the charges at the nanocrystallites which appear due to the escape of the electrons to the conduction band (or holes to the valence band) of silicon.

We present magneto-transport properties of the two dimensional hole gas (2DHG) in fully strained Ge quantum wells grown on Si0.2Ge0.8/Si (100) substrates. Comparison is made between heterostructures that are modulation doped in both normal and inverted configurations. Using Shubnikov de Haas oscillations at temperatures down to 90 mK (inverted structure) and to 1.5K (normal structure), an extremely high hole mobility (0.51-1.34) × 106 cm2/Vs has been observed, along with the lowest value of effective mass (0.063-0.070) m0 to date. The 2DHG is confirmed to be in a pure Ge channel, with low background impurity scattering that improves the 2DHG transport.

In this paper we present structural characterization and magneto-transport properties of the two dimensional hole gas in strained germanium heterostructures. An extremely high hole mobility has observed, along with the lowest value of effective mass to date, using Schubnikov de Haas oscillations for both normal and inverted structures. The channel is confirmed to be pure germanium, with low background impurity scattering that improves the hole transport.

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−xGex (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.

The magnetoresistance (MR) of a two-dimensional hole gas in a quantum well of compressively strained Si0.05Ge0.95 has been investigated as a function of temperature. The MR shows a maximum at intermediate magnetic fields between the regions of weak localization and the Shubnikov-de Haas oscillations, which is discussed in terms of a recent theoretical study of the electron-electron interaction effect by Sedrakyan and Raikh (SR). The magnetic field MR dependence is clearly observed to cross over from quadratic to linear at T=7.8 K and B≈0.3 T. It is shown that the SR theory provides a good description of both the measured quadratic and positive linear MR, but over estimates the field position of the MR maximum and does not account for the shift in position with temperature that is observed. Earlier theories of electron-electron interaction (by Altshuler and Aronov, Gornyi and Mirlin) show a better agreement with the experimentally observed behavior of the MR maximum, but fit the low field MR less accurately.

The effect of charge carrier overheating is studied in the p-type Si/Si(0.87)Ge(0.13)/Si heterostructure. The hole temperature T(h) can be calculated using three different methods: From a comparison of the changes in the amplitude of the Shubnikov-de Haas oscillations under the influences of temperature and current, from a comparison of the change of phase relaxation time in the weak localization effect obtained at different temperatures and minimum current and at a given temperature but at different values of current, and from a comparison of the temperature and current dependence of the sample resistance. The temperature dependences values of T(h) obtained using three different methods were identical and exhibit transition of the two-dimensional system under study from the regime of "partial inelasticity" to that of small angle scattering at temperature lowering.

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.

Quantum interference effects, such as weak localization and electron-electron interaction (EEI), have been investigated in magnetic fields up to 11 T for hole gases in a set of Si 1− x Ge x quantum wells with 0.13< x <0.95. The temperature dependence of the hole phase relaxation time has been extracted from the magneto-resistance between 35 mK and 10 K. The spin-orbit effects that can be described within the Rashba model were observed in low magnetic fields. A quadratic negative magneto-resistance was observed in strong magnetic fields, due to the EEI effect. The hole-phonon scattering time was determined from hole overheating in a strong magnetic field.

Changes in the conductivity of p-type quantum-well heterostructures of Si0.05Ge0.95 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 Si0.4Ge0.6 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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3536348]

The Shubnikov-de Haas oscillations of the conductivity of a two-dimensional gas of holes in quantum wells consisting of pure germanium and silicon with low germanium content (13%) are analyzed to determine the effective masses and the g factor in these regions. The magnetic-field dependences of the resistivity rho(xx) obtained at temperatures from 33 mK to 4 K in magnetic fields up to I I T are used for the analysis. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3075945]