Longitudinal Ρxx Research Articles

Localization and interference induced quantum effects at low magnetic fields in InGaAs/GaAs structures

The longitudinal ρxx(B, T) and Hall ρxy(B, T) resistances are experimentally investigated in n-InGaAs/GaAs nanostructures with a single and double quantum wells in the magnetic field range B = 0–2.5 T and temperatures T = 1.8–20 K. It is shown that the origin of the temperature-independent point located at ωcτ≅1 on the ρxx(B, T) curves is due to the combined action of the classical cyclotron motion and the quantum interference effects of weak localization and electron-electron interaction. The results obtained indicate that the transition from the dielectric phase to the phase of the quantum Hall effect is a crossover from weak localization (quantum interference effects in a weak magnetic field) to strong localization in quantizing magnetic fields in the quantum Hall effect regime.

Quantum Hall transitions in the presence of Landau levels mixing in n-InGaAs/InAlAs structures

We provide a systematic measurement of the longitudinal ρxx and Hall ρxy resistivities of quantum Hall transition in a two-dimensional electron system In0.9Ga0.1As/In0.81Al0.19As with strong spin-orbit coupling. For half-integer filling factors the linear temperature dependence of the effective quantum Hall effect plateau-to-plateau transition width ΔB(T) is observed in contrast to scaling behavior for systems with short-range disorder. The recent prediction that the width of transition region remains finite when extrapolated to zero temperature, resulting from Landau level mixing, is more preferable. The shift of the transition point in magnetic field with the temperature is found to originate from the mixing between Landau levels due to the inelastic scattering.

Insulator-quantum Hall transition in n-InGaAs/GaAs heterostructures

The longitudinal ρxx(B,T) and Hall ρxy(B,T) resistances in magnetic fields B up to 12 T at temperatures T = 1.8–80 K are studied experimentally in n-In0.2Ga0.8As/GaAs nanostructures with single and double strongly-coupled quantum wells separated by different barrier widths. It is shown that for ω c τ ≅ 1 there is a critical magnetic field near which the scaling relation ρ x x ∝ | B − B C | T − κ, which is indicative of a phase transition from a dielectric state to a quantum hall state, is satisfied. It is found that the critical index κ depends on the width of the barrier between the double quantum wells. The nature of this behavior is discussed.

Quantum magnetotransport inn-InGaAs/GaAs structures with electron density changes caused by infrared radiation

An experimental study of the longitudinal ρxx(B, T) and Hall ρxy (B, T) magnetoresistance as a function of the n-InGaAs/GaAs nanostructure transverse magnetic field, with single or double tightly-coupled quantum wells, depending on the width of the well, for magnetic field B = 0–12 T and temperature T = 0.05–100 K ranges, before and after low-temperature illumination by infrared radiation. Before illumination, a change in the samples' temperature dependence of zero-field resistivity ρ(T) was detected, from “dielectric” (dρ/dT < 0) to “metallic” (dρ/dT > 0). It is shown that the temperature dependence of resistivity is set by the mobility temperature dependence μ(T), the “dielectric” portion of which is related to the quantum corrections to conductivity in the diffusion and ballistic regimes, whereas the “metallic” portion is associated to the scattering of the carriers by acoustic and optic phonons. A slight change in the magnetic-field dependence of the longitudinal magnetoresistance ρxx(B, T) was observed with temperature, near the induction value corresponding to μB = 1. We also found unusual temperature dependence for conductivity components σxx(B, T) and σxy (B, T), at μB = 1. The σxx(B, T) curve has an observable temperature-independent point, whereas σxy (B, T) strongly depends on T at μB = 1. It is established that this regularity is caused by the nature of the temperature dependence of the charge carrier mobility μ(T), in both the diffusion and ballistic regimes. After IR illumination, positive persistent photoconductivity is observed in all samples, associated with a twofold increase in the charge carrier concentration. Resistivity in a zero-magnetic field ρ(T) for such samples also transitions from “dielectric” to “metallic” conductivity, at temperatures lower than before illumination. It is shown that the particularities of the transport after illumination are related to the manifestation of charge carrier concentration temperature dependence.

Quantum Hall plateau-plateau transitions in n-InGaAs/GaAs heterostructures before and after IR illumination

The longitudinal ρxx(B,T) and Hall ρxy(B,T) magnetoresistances in n-InGaAs/GaAs heterostructures with a single quantum well are studied experimentally before and after IR illumination in the quantum Hall regime in magnetic fields B = 0–12 T and at temperatures T = 0.4–4.2 K. The temperature dependence of the width of quantum Hall plateau-plateau transitions is analyzed in terms of a two-parameter scaling theory.

Scaling in the Quantum Hall Regime for a Double Quantum Well Nanostructure in High Magnetic Field

The longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are investigated in the integer quantum Hall effect regime in n-InGaAs/GaAs double quantum well nanostructures in the magnetic fields B up to 16 T at temperatures T = (0.05-4.2) K before and after IR illumination. The analysis of the quantum Hall effect plateau-plateau transitions based on the scaling hypothesis with regard to electron-electron interaction was carried out.

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

The longitudinal ρxx(B) and Hall ρxy(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.

The effect of infrared radiation on quantum magnetotransport in n-InGaAs/GaAs with two strongly coupled quantum wells

Longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are measured as a function of transverse magnetic field in n-InGaAs/GaAs nanostructures with strongly coupled double quantum wells in the temperature range T = 0.05–60 K and magnetic fields B = 0–16 T before and after low-temperature infrared irradiation. The appearance of persistent photoconductivity causes the type of the temperature dependence of resistance to be changed: from insulator-like (dρ/dT &amp;lt; 0) for T ≤ 8 K to metallic-like (dρ/dT &amp;gt; 0) at higher temperatures. It is shown that this is related to the temperature dependence of electron density. The strong temperature dependence of electron mobility observed before the illumination practically disappears after infrared irradiation.

Temperature dependence of quantum lifetime in n-InGaAs/GaAs structures with strongly coupled double quantum wells

Longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are experimentally investigated as a function of in-plane and transverse magnetic fields in n-InGaAs/GaAs nanostructures with strongly-coupled double quantum wells in the temperature range T = 1.8–70 K and magnetic fields B = 0–9.0 T. Experimental data on the temperature dependence of quantum lifetime in diffusive (kBT/τtr ≪ 1) and ballistic (kBT/τtr ≫ 1) regimes are reported. It has been found that in the ballistic regime in the temperature range where kBT/EF &amp;lt; 0.1, the observed quadratic temperature dependence of quantum lifetime is determined by inelastic electron–electron scattering. However, the temperature dependence of quantum lifetime cannot be quantitatively described by the existing theories in the whole temperature range.

Temperature dependence of the bandwidth of delocalized states for n-InGaAs/GaAs in the quantum Hall effect regime

Temperature and magnetic-field dependences of longitudinal ρxx(B,T) and Hall ρxy(B,T) resistivities of n-InxGa1−xAs/GaAs nanostructures with single and double quantum wells are investigated in the quantum Hall regime at B = 0–16 Т and T = 0.05–70 K, before and after IR illumination. The temperature dependence of the QHE plateau-plateau transition width is analyzed and information about temperature dependences of the bandwidth of delocalized states in the center of Landau subbands is obtained.

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

A comparative study of the longitudinal ρxx and transverse ρxy resistivities and magnetic susceptibility χac of La0.8Sr0.2MnO3 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 ρxy∼ρxx, 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 χac. As a result, the curve of ρxy versus ρxx 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/ρxx 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⩽TC. 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.

Longitudinal and transverse Hall resistivity of epitaxial SrRuO3 films

We report the longitudinal ρxx and transverse Hall ρxy resistivities of epitaxial SrRuO3 (SRO) under varied applied fields. The Hall resistivity, ρxy, is proportional to 1/(T−Tc) in the paramagnetic state of SRO films, where Tc is the asymptotic Curie temperature. The Lorentz resistivity Δρ(H)Lorentz in a fixed magnetic field in the ferromagnetic regime of SRO films is proportional to A(1−T/Tc)p with p∼0.5. RH is positive for T&amp;gt;125 K and changes sign at T∼125 K for SRO films. The RH remains negative down to low temperatures.