New type of conductivity?

Experiments yielded rapid rise in the Hall gr_xy and magnetoresistance ρ_xx, ρ_zz in Hg_0.76Cd_0.24Te to almost linear dependence in strong magnetic fields. This paper relates it to the states, which are extended at the Hall edge for ρ_xy and between the Hall edges for ρ_xx, ρ_zz, while bulk states are localized. Theory agrees with experiments, and suggests that thin enough samples may have zero magnetoresistance in strong enough magnetic fields.     

Experimental observation of a striking similarity between quantum hall transport coefficients

The derivative of the Quantum Hall resistance, ρ_xy, with respect to the carrier density, n, has been measured for a two-dimensional electron gas in a GaAs-Al_xGa_1?xAs heterostructure, as a function of magnetic field. dρ_xy/dn exhibits a remarkable similarity to the diagonal resistivity, ρ_xy, to the extent that one is almost directly proportional to the other. Our result suggests the possibility of a fundamental connection between the two quantities.     

Scaling in the quantum Hall effect regime in n-InGaAs/GaAs nanostructures

The longitudinal ρ _xx (B) and Hall ρ _xy (B) magnetoresistances are investigated experimentally in the integer quantum Hall effect (QHE) regime in n-InGaAs/GaAs double quantum well nanostructures in the range of magnetic fields B = (0–16) T and temperatures T = (0.05–70) K before and after IR illumination. The results are evaluated within the scaling hypothesis with regard to electron-electron interaction.     

Magnetoresistance in dilute p‐Si/SiGe in parallel and tilted magnetic fields

We report the results of an experimental study of the magnetoresistance ρ_xx and ρ_xy in two samples of p‐Si/SiGe with low carrier concentrations p = 8.2 × 10¹⁰ cm^‐2 and p = 2 × 10¹¹ cm^‐2. The research was performed in the temperature range of 0.3–2 K and in the magnetic fields of up to 18 T, parallel or tilted with respect to the two‐dimensional (2D) channel plane. The large in‐plane magnetoresistance can be explained by the influence of the in‐plane magnetic field on the orbital motion of the charge carriers in the quasi‐2D system. The measurements of ρ_xx and ρ_xy in the tilted magnetic field showed that the anomaly in ρ_xx, observed at filling factor ν = 3/2 is practically nonexistent in the conductivity σ_xx. The anomaly in σ_xx at ν = 2 might be explained by overlapping of the levels with different spins 0 ↑ and 1 ↓ when the tilt angle of the applied magnetic field is changed. The dependence of g‐factor g^*(Θ)/g^*(0⁰) on the tilt angle Θ was determined.     

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)     

Spin-dependent resistivity and quantum Hall ferromagnetism in two-dimensional electrons confined to AlAs quantum wells

We observe a strong dependence of the amplitude and field position of longitudinal resistivity (ρ_xx) peaks in the spin-resolved integer quantum Hall regime on the spin orientation of the Landau level (LL) in which the Fermi energy resides. The amplitude of a given peak is maximal when the partially filled LL has the same spin as the lowest LL, and amplitude changes as large as an order of magnitude are observed as the sample is tilted in field. In addition, the field position of both the ρ_xx peaks and plateau–plateau transitions in the Hall resistance shift depending on the spin orientation of the LLs. The spin dependence of the resistivity points to a new explanation for resistivity spikes, associated with first-order quantum Hall ferromagnetic transitions, that occur at the edges of quantum Hall states.     

高温超导体霍尔电阻和霍尔角在涡旋玻璃相变附近的普适标度律及统一霍尔电阻方程

发现了一个钉扎效应影响霍尔电阻ρ_xy和霍尔角θ_H的普适标度律. 同时,根据纵向电阻ρ_xx的扩展幂律形式和对霍尔电导σ_xy的微观分析,给出了一个对有一次或多次霍尔反号的高温超导体都适用的霍尔电阻方程. 关键词： 高温超导体 涡旋玻璃相变 普适标度律 霍尔电阻方程     

Weak localization effects in a quasi-one-dimensional electron system over liquid helium

Magnetoresistance ρ_xx measurements are performed for a quasi-one-dimensional electron system over liquid helium in the gas-scattering region (the temperature range 1.3–2.0 K). The measurements show that, as the magnetic field increases, the magnetoresistance ρ_xx first decreases and then passes through a minimum and increases according to the law ρ_xx～B ². It is suggested that the negative magnetoresistance observed in the experiment is caused by the weak localization effects. The results of the experiment are in qualitative agreement with the theoretical model describing the weak localization effects in a one-dimensional nondegenerate electron system.     

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)     

Self-assembled nanocrystalline CdSe thin films

The topic of this contribution is the investigation of quantum states and quantum Hall effect in electron gas subjected to a periodic potential of the lateral lattice. The potential is formed by triangular quantum antidots located on the sites of the square lattice. In such a system the inversion center and the four-fold rotation symmetry are absent. The topological invariants which characterize different magnetic subbands and their Hall conductances are calculated. It is shown that the details of the antidot geometry are crucial for the Hall conductance quantization rule. The critical values of lattice parameters defining the shape of triangular antidots at which the Hall conductance is changed drastically are determined. We demonstrate that the quantum states and Hall conductance quantization law for the triangular antidot lattice differ from the case of the square lattice with cylindrical antidots. As an example, the Hall conductances of magnetic subbands for different antidot geometries are calculated for the case when the number of magnetic flux quanta per unit cell is equal to three.     

Quantum Hall effect in the bulk semiconductors bismuth and antimony tellurides: Proof of the existence of a current-carrier reservoir

The quantization of the Hall resistivity ρ_xy in the form of plateaus in the dependence of ρ_xy on the magnetic field B is observed in the semiconductors Bi₂Te₃ and Sb₂Te₃; the minima of the transverse magnetoresistivity ρ_xx correspond to the start of the plateaus. The quantization of ρ_xy is due to the presence of a current-carrier reservoir. An impurity band with a high density of states or a different band with a much higher current-carrier effective mass serves as the reservoir. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 754–758 (10 December 1999)     

Temperature dependence of the hall effect in amorphous Ni-Si-B alloys between 1.5 and 290 K: Evidence for magnetic ordering near a resistance minimum

The Hall effect in the metallic glasses Ni₇₆Si₁₂B₁₂, Ni₇₈Si₈B₁₄ and Ni₈₀Si₁₀B₁₀ has been measured as a function of temperature (1.5 ≤ T ≤ 290 K) and magnetic field (B ^<_～ 1.2 Tesla). At room temperature the Hall resistivity ρ_H is negative and depends linearly on the magnetic field for all three compositions. With decreasing temperature Ni₈₀Si₁₀B₁₀ exhibits a negative spontaneous Hall coefficient and a transition to ferromagnetism at about 110 K. In case of Ni₇₆Si₁₂B₁₂ and Ni₇₈Si₈B₁₄ an anomalous part contributes below ～ 40 K and ρ_H changes from negative to positive sign at about 9 K. For these two alloys we find that the initial slope of the ρ_H(B) curves follows a Curie-Weiss law with Tθ ≈ 0 K and the anomalous contribution of the Hall resistivity can be described by a Brillouin function for superparamagnetism.     

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)     

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)     

Realization of dual,tunable, ordinary- and quantized-Hall resistances in doubly connected GaAs/AlGaAs heterostructures

An experimental configuration based on gated, double boundary GaAs/AlGaAs devices provides the first demonstration of dual distinct, tunable Hall resistances in the same sample over the weak- and strongmagnetic field limits, including the simultaneous observability of dual quantized Hall effects of dissimilar filling factors. These experiments suggest that the ordinary Hall resistance examined in experiment is essentially a topological resistance determined by connectivity, which need not reflect all “local” values of the off-diagonal resistivity, θ _M ρ _xy .     

Thermal conductivity and thermal Hall effect in Bi- and Y-based high-T c superconductors

Measurements of the thermal conductivity (k_xx) and the thermal Hall effect (k_xy) in high magnetic fields in Y- and Bi-based high-T _c superconductors are presented. We describe the experimental technique and test measurements on a simple metal (niobium). In the high-T _c superconductors k_xx and k_xy increase below T _c and show a maximum in their temperature dependence. k_xx has contributions from phonons and quasiparticle (QP) excitations, whereas k_xy is purely electronic. The strong increase of k_xy below T _c gives direct evidence for a strong enhancement of the QP contribution to the heat current and thus for a strong increase of the QP mean free path. Using k_xy and the magnetic field dependence of k_xx we separate the electronic thermal conductivity ( k _xx ^el ) of the CuO ₂ -planes from the phononic thermal conductivity ( k _xx ^ph ). In YBa₂Cu₃O _{7 - δ} k _xx ^el shows a pronounced maximum in the superconducting state. This maximum is much weaker in Bi₂Sr₂CaCu₂O _{8 + δ} , due to stronger impurity scattering. The maximum of k _xx ^el is strongly suppressed by a magnetic field, which we attribute to the scattering of QPs on vortices. An additional magnetic field independent contribution to the maximum of k_xx occurs in YBa₂Cu₃O _{7 - δ} , reminiscent of the contribution of the CuO-chains, as determined from the anisotropy in untwined single crystals. Our data analysis reveals that below T _c as in the normal state a transport (τ) and a Hall ( ) relaxation time must be distinguished: The inelastic (i.e. temperature dependent) contribution to τ is strongly enhanced in the superconducting state, whereas displays the same temperature dependence as above T _c . We determine also the electronic thermal conductivity in the normal state from k_xy and the electrical Hall angle. It shows an unusual linear increase with temperature. Received 23 August 2000     

Frequency-dependent Hall effect in spintronic systems under zero magnetic field

It is shown that in an electronic system with finite Rashba coupling and in the absence of external magnetic field, the Hall resistivity (ρ_xy) is finite at both zero and finite frequencies. This Hall resistivity is determined by the reactive part (real part) of the inverse dielectric functions. This allows us to probe the real part of the dielectric function in a spintronic system by using a transport measurement.     

Exchange interaction effects in the crossing of spin-polarized Landau levels in a silicon-germanium heterostructure: transition into a ferromagnetic state

The crossing of spin-split Landau levels in a Si/SiGe heterostructure is investigated by means of magneto-transport experiments in tilted magnetic fields. We observe a transition from a paramagnetic into a fully spin polarized state. During the transition strongly enhanced maxima in the transverse resistivity ρ_xx appear when the parallel field component is oriented along the Hall bar. We assign this effect to an energy level structure strongly modified by exchange interaction effects between different Landau levels. Surprisingly the maximum in ρ_xx totally disappears when the parallel field component is perpendicular to the Hall bar.     

Quantized transport in two-dimensional spin-ordered structures

We study in detail the transport properties of a model of conducting electrons in the presence of double exchange between localized spins arranged on a 2D Kagome lattice, as introduced by Ohgushi, Murakami and Nagaosa. The relationship between the canting angle of the spin texture θ and the Berry phase field flux per triangular plaquette φ is derived explicitly and we emphasize the similarities between this model and Haldane's honeycomb lattice version of the quantum Hall effect. The quantization of the transverse (Hall) conductivity σ _xy is derived explicitly from the Kubo formula and a direct calculation of the longitudinal conductivity σ _xx shows the existence of a metal–insulator transition as a function of the canting angle θ (or flux density φ). This transition might be linked to that observable in the manganite compounds or in the pyrochlore ones, as the spin ordering changes from ferromagnetic to canted.     

Topological invariant and the quantization of the Hall conductance

The topological aspects of wavefunctions for electrons in a two dimensional periodic potential with a magnetic field are discussed. Special attention is paid to the linear response formula for the Hall conductance σ_xy. It is shown that the quantized value of σ_xy is related to the number of zeros of wavefunctions in the magnetic Brillouin zone. A phase of wavefunctions cannot be determined in a unique and smooth way over the entire magnetic Brillouin zone unless the magnetic subband carries no Hall current.