Search

The behaviors of resistance, magnetoresistance (up to 5 T), and Hall electromotive force (EMF) with varying temperature (10-300 K) and measuring current ( [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This is a survey of some recent results concerning Bernstein-type inequalities for entire functions of the exponential type, harmonic majorants for classes of subharmonic functions, Phragmen-Lindelöf function, the Krein description of the Cartwright classes of entire functions with the finite logarithmic integral, structure of the Martin boundary of the Denjoy domains, smoothness properties of the Green function, etc. We generalize the classical Bernstein theorem concerning the constructive description of classes of functions uniformly continuous on the real line. Approximation of continuous bounded functions by entire functions of exponential type on an unbounded closed proper subset of the real line or on an unbounded quasismooth (in the sense of Lavrentiev) curve in the complex plane is studied. We discuss Totik's extension of the classical Bernstein theorem on polynomial approximation of piecewise analytic functions on a closed interval. The results are achieved by the application of methods and techniques of modern geometric function theory and potential theory. [ABSTRACT FROM AUTHOR]

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 Si1− xGe 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. [ABSTRACT FROM AUTHOR]

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 ρxx obtained at temperatures from 33 mK to 4 K in magnetic fields up to 11 T are used for the analysis. [ABSTRACT FROM AUTHOR]

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 5.68×1011 cm-2 and mobility of 4.68×104 cm2 V-1 s-1 are investigated. The resistance measurements were made at temperatures from 46 mK to 10 K and magnetic fields up to 15 T. The magnetic-field dependence of the resistivity exhibits Shubnikov-de Haas oscillations and quantum Hall effect steps. At very low magnetic fields (B<0.1 T) a weak localization effect for holes is revealed, which makes for a negative magnetoresistance and growth of the resistance with decreasing temperature (at T<2 K). 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 quantum correction is in good agreement with the modern theoretical predictions. [ABSTRACT FROM AUTHOR]

The Shubnikov–de Haas oscillations in a two-dimensional hole gas in a quantum well of pure germanium in a SiGe/Ge/SiGe heterostructure with a hole concentration pH=5.68×1011 cm2 and mobility μ=4.68×104 cm2V-1s-1 are investigated in magnetic fields up to 15 T at temperatures from 40 mK to 4 K. The observed deviation from the known relation describing the conductivity oscillations in the Shubnikov–de Haas effect are explained by additional broadening of the Landau levels due to the existence of a nonuniform distribution of the concentration of charge carriers, and, accordingly, of their energy, in the plane of the two-dimensional gas. It is assumed that the latter is due to natural atomic-step variations of the well width. The effective hole mass (m*=0.112m0) is determined from the temperature dependence of the oscillation amplitude, and its dependence on magnetic field is used to determine the quantum scattering time and the value of the carrier concentration fluctuations. [ABSTRACT FROM AUTHOR]