Bistability of quantum magnetotransport in a multilayer Ge/p-Ge1−x Six heterostructure with wide potential wells

Two metastable states of a multilayer Ge/p-Ge_1−x Si_x heterosystem with wide (∼ 35 nm) potential wells (Ge) are observed in strong magnetic fields B at low temperatures. In the first state, the Hall resistivity exhibits an inflection near the value ρ_xy=h/e ² scaled to one Ge layer. The longitudinal magnetoresistivity ρ_xx(B) possesses a minimum in the range of fields where this inflection occurs. The temperature evolution of the inflection in ρ_xy(B), the minimum of ρ _xx(B), and the value of ρ_xy at the inflection indicates a weakly expressed state of the quantum Hall effect with a uniform current distribution over the layers. In the second metastable state, an unusually wide plateau near h/2e ² with a very weak field dependence is observed in ρ_xy(B). Estimates show that in these samples the Fermi level lies below but close to the top of the inflection in the bottom of the well. For this reason, the second state can be explained by separation of a hole gas in the Ge layers into two sublayers, and the saturation of ρ_xy(B) near h/2e ² can be explained by the formation of a quantum Hall insulator state. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 290–297 (25 August 1999)     

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρ _xx and transverse ρ _xy resistivities and magnetic susceptibility χ _ac of La_0.8Sr_0.2MnO₃ single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρ _xy ∼ρ _xx , which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χ _ac. As a result, the curve of ρ _xy versus ρ _xx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρ _xx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽T _C . The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials. Zh. éksp. Teor. Fiz. 116, 671–683 (August 1999)     

Weak-field Hall resistance and effective carrier density measurements across the metal-insulator transition in Si-MOS structures

The weak-field Hall voltage in Si-MOS structures with different mobility is studied on both sides of the metal-insulator transition. In the vicinity of the critical density on the metallic side of the transition, the Hall voltage is found to deviate by 6–20 % from its classical value. The deviation does not correlate with the strong temperature dependence of the diagonal resistivity ρ _xx (T). In particular, the smallest deviation in R _xy is found in the highest-mobility sample, which exhibits the largest variation in the diagonal resistivity ρ _xx with temperature. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 1, 48–52 (10 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Fractional quantum Hall effect at a suppressed energy gap

In the fractional quantum Hall effect regime, the diagonal (ρ_xx) and Hall (ρ_xy) magnetoresistivity tensor components of the two-dimensional electron system (2DES) in gated GaAs/Al_xGa_1−x As heterojunctions are measured together with the capacitance between 2DES and the gate. The 1/3-and 2/3-fractional quantum Hall effects are observed at rather low magnetic fields where the corresponding fractional minima in the thermodynamic density of the states have already disappeared, thus, implying the suppression of the quasiparticle energy gaps. The text was submitted by the authors in English.     

Quantized Hall effect in disordered GaAs layers with 3D spectrum in tilted magnetic fields

The quantum Hall effect structure in the transverse magnetoresistance R _xx and the Hall resistance R _xy of heavily doped GaAs layers with a three-dimensional spectrum of the charge carriers is investigated for different field orientations. The characteristic structures (minima in R _xx and plateaus in R _xy ) shift much more slowly to higher fields and are suppressed much more rapidly in comparison with the expected angular dependence for a two-dimensional system. The results are discussed in terms of the anisotropic change of the three-dimensional conductivity tensor with magnetic field rotation. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 305–308 (25 August 1998)     

On the phase boundaries of the integer quantum Hall effect. Part II

It has been shown that the observation of the transitions between the dielectric phase and the integer-quantum-Hall-effect phases with the quantized Hall conductivity σ _xy ^q ≥ 3e ²/h announced in a number of works is unjustified. In these works, the crossing points of the magnetic-field dependence of the diagonal resistivity ρ _xx at different temperatures T and ω_cτ = 1 have been misidentified as the critical points of the phase transitions. In fact, these crossing points are due to the sign change of the derivative dρ _xx /dT owing to the quantum corrections to the conductivity. Here, ω_c = eB/m is the cyclotron frequency, τ is the transport relaxation time, and m is the effective electron mass.     

Carrier mobility in nonstoichiometric n-Bi2Te3−x Sex solid solutions

A study has been made of the thermoelectric and galvanomagnetic properties of n-Bi₂Te_3−x Se_x solid solutions (x=0.3 and 0.36) in the temperature range 80–300 K. The lowest carrier concentrations, (0.8–1)×10¹⁸ cm⁻³, were obtained by displacing the solid solution from the stoichiometric to a Te-rich composition. At such carrier concentrations, the second subband in the conduction band of n-Bi₂Te_3−x Se_x is not filled, which results in a growth of mobility because of the absence of interband scattering, and brings about an increase of thermoelectric efficiency in the 80–120-K range. Fiz. Tverd. Tela (St. Petersburg) 39, 483–487 (March 1997)     

Quantum corrections to the conductance of n-GaAs films in a strong magnetic field

The conductance of doped n-GaAs films is studied experimentally as a function of magnetic field and temperature in strong magnetic fields right up to the quantum limit (ħω_c = E _F). The Hall conductance G _xy is virtually independent of temperature T until the transverse conductance G _xx is quite large compared with e ²/h. In strong fields, when G _xx becomes comparable to e ²/h, G _xy starts to depend on T. The difference between the conductances G _xx at the two temperatures 4.2 and 0.35 K depends only weakly on the magnetic field H over a wide range of magnetic fields, while the conductances G _xx themselves vary strongly. The results can be explained by quantum corrections to the conductance as a result of the electron-electron interaction in the diffusion channel. The possibility of quantization of the Hall conductance as a result of the electron-electron interaction is discussed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 201–206 (10 February 1998)     

Magnetoresistivity and acoustoelectronic effects in a tilted magnetic field in <Emphasis Type="Italic">p</Emphasis>-Si/SiGe/Si structures with an anisotropic <Emphasis Type="Italic">g</Emphasis> factor

Magnetoresistivity ρ _xx and ρ _xy and the acoustoelectronic effects are measured in p-Si/SiGe/Si with an impurity concentration p = 1.99 × 10¹¹ cm⁻² in the temperature range 0.3–2.0 K and an tilted magnetic field up to 18 T. The dependence of the effective g factor on the angle of magnetic field tilt θ to the normal to the plane of a two-dimensional p-Si/SiGe/Si channel is determined. A first-order ferromagnet-paramagnet phase transition is observed in the magnetic fields corresponding to a filling factor ν = 2 at θ ≈ 59°–60°.     

A size effect in the optical properties of powdered TiO2

The spectral reflectivity ρ in the region 0.36–2.1 μm and its change Δρ with irradiation by 30 keV electrons versus the average grain sizer _av in a TiO₂ (rutile) powder has been studied in the range 1–7.5 μm. It has been established that the dependence of ρ onr _av differs for different regions of the spectrum, but there is a common increase of ρ in the size range 2.5–4 μm. The value of Δρ after irradiation is also less in powders with this range of sizes. It has been shown that the dependence Δρ=f(r _av) with increasing electron fluence from 2·10¹⁵ to 4·10¹⁶ is strengthened for one band and weakened for another. Tomsk Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 41, No. 12, pp. 52–58, December, 1998.     

Topological Berry phase and semiclassical quantization of cyclotron orbits for two dimensional electrons in coupled band models

The semiclassical quantization of cyclotron orbits for two-dimensional Bloch electrons in a coupled two band model with a particle-hole symmetric spectrum is considered. As concrete examples, we study graphene (both mono and bilayer) and boron nitride. The main focus is on wave effects – such as Berry phase and Maslov index – occurring at order (^h/_2p)\hbar in the semiclassical quantization and producing non-trivial shifts in the resulting Landau levels. Specifically, we show that the index shift appearing in the Landau levels is related to a topological part of the Berry phase – which is basically a winding number of the direction of the pseudo-spin 1/2 associated to the coupled bands – acquired by an electron during a cyclotron orbit and not to the complete Berry phase, as commonly stated. As a consequence, the Landau levels of a coupled band insulator are shifted as compared to a usual band insulator. We also study in detail the Berry curvature in the whole Brillouin zone on a specific example (boron nitride) and show that its computation requires care in defining the “k-dependent Hamiltonian” H(k), where k is the Bloch wavevector.     

Electron density redistribution in Sn-doped Bi2Te3

X-ray photoelectron spectroscopy is used to investigate the redistribution of the density of electronic states in the valence band, and of the binding energies and chemical shifts of core levels in bismuth telluride caused by introduction of impurity tin atoms. A substantial increase in the density of electronic states below the valence-band top at energies μ≈15–30 meV has been revealed. This feature in the energy spectrum accounts for the unusual behavior of the kinetic coefficients in p-Bi₂Te₃:Sn crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1969–1972 (November 1999)     

Magnetoresistance of the semiconducting spinelide Cu0.625Ga0.375Cr2Se4 having a low-temperature long-range magnetic order-spin glass transition

A study is reported of the dependence of magnetoresistance Δρ/ρ on the square of magnetization σ ² of the semiconducting spinelide Cu_0.625Ga_0.375Cr₂Se₄, which exhibits a low-temperature transition from long-range magnetic order (LRMO) to the spin glass (SG) state in strong magnetic fields. It is shown that at the freezing temperature T _f the Δρ/ρ(σ ²) relations change their slope, and that below T _f this slope is about one half that for T>T f. This finding, together with the earlier observation that the freezing temperature does not depend on the frequency of the ac magnetic field in which it was measured, suggests that the spin-glass phase consists of spins of individual Cr³⁺ ions, and that the SG-LRMO crossover is a phase transition. Fiz. Tverd. Tela (St. Petersburg) 40, 315–317 (February 1998)     

Low-temperature resistivity of YBa2Cu3O6+x single crystals in the normal state

A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa₂Cu₃O_6+x (x≈0.37) is done in two alternative ways, namely, by applying a magnetic field and by reducing the hole concentration through oxygen rearrangement. The in-plane normal-state resistivity ρ_ab obtained in the two cases is quite similar; its temperature dependence can be fitted by a logarithmic law in a temperature range of almost two decades. However, an alternative representation of the temperature dependence of σ_ab=1/ ρ _ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σ_c=1/ρ_c followed a power law, and neither σ_c(T), nor ρ_c(T) could be fitted by log T. It follows from the ρ_c measurements that the transformation at T=0 is split into two transitions: superconductor-normal-metal and normal-metal-insulator. In our samples, they are separated in oxygen content by Δx≈0.025. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 834–839 (10 June 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

On the theory of the integral quantized Hall effect

Summary In the present paper the integral quantum Hall effect is studied using the Schrauben functions which are suitable eigenfunctions to describe the quantum transport in uniform electric and magnetic fields. The effect of Landau band structure on the Hall quantization is investigated. A model calculation of the conductivities σ_xy and σ_yy is presented and the onset of a Hall current dissipation is discussed. Also, the quantum oscillations of a free-electron gas into the quantum Hall regime are studied, including the electric-field effect.     

Anomalous Hall effect in granular Fe/SiO2 films in the tunneling-conduction regime

It is established that the Hall effect in Fe/SiO₂ nanocomposite films in the activational tunneling conduction range is anomalous, i.e., the Hall resistivity ρ_h is proportional to the magnetization and is due to the spin-orbit interaction. The parametric coupling of the Hall and longitudinal (ρ_xx) resistances ρ_h ∝ ρ _xx ^m (with temperature as the parameter) is characterized by a much lower value of the exponent m than in a uniform ferromagnetic metal. This circumstance is attributed to the characteristic features of the Hall effect mechanism in the hopping regime — in our case, the interference of the amplitudes of tunneling transitions in a set of three granules. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 87–92 (25 July 1999)     

Anisotropy of the superconducting properties of YBa2Cu3O7−x single crystals with reduced oxygen content

In an investigation of the resistivity anisotropy of YBa₂Cu₃O_7−x single crystals with suboptimal oxygen content it is observed that the superconducting transition for the component ρc of the resistivity tensor is shifted to lower temperatures with respect to the transition for the component ρab. A similar shift is also observed for the transition in the temperature dependence of the dynamic magnetic susceptibility. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 309–313 (25 August 1998)     

Realizability of Point Processes

There are various situations in which it is natural to ask whether a given collection of k functions, ρ _j (r ₁,…,r _j ), j=1,…,k, defined on a set X, are the first k correlation functions of a point process on X. Here we describe some necessary and sufficient conditions on the ρ _j ’s for this to be true. Our primary examples are X=ℝ ^d , X=ℤ ^d , and X an arbitrary finite set. In particular, we extend a result by Ambartzumian and Sukiasian showing realizability at sufficiently small densities ρ ₁(r). Typically if any realizing process exists there will be many (even an uncountable number); in this case we prove, when X is a finite set, the existence of a realizing Gibbs measure with k body potentials which maximizes the entropy among all realizing measures. We also investigate in detail a simple example in which a uniform density ρ and translation invariant ρ ₂ are specified on ℤ; there is a gap between our best upper bound on possible values of ρ and the largest ρ for which realizability can be established.     

Specific features of Bi2Te3 doping with Sn

The effect of doping bismuth telluride with tin, on its electrophysical properties, has been studied. It is shown that the main features in the transport coefficients of Bi₂Te₃:Sn can be explained by the existence of resonant Sn states within the valence band. The existence of resonant Sn states was confirmed by codoping Bi₂Te₃:Sn with the electroactive impurity I. Fiz. Tverd. Tela (St. Petersburg) 40, 1428–1432 (August 1998)