Bilayer counterflow transport at filling factor 1 in the strong interacting regime

We review magneto-transport properties of interacting GaAs bilayer hole systems, with very small inter-layer tunneling, in a geometry where equal currents are passed in opposite directions in the two, independently contacted layers (counterflow). In the quantum Hall state at total bilayer filling ν=1 both the longitudinal and Hall counterflow resistances tend to vanish in the limit of zero temperature, suggesting the existence of a superfluid transport mode in the counterflow geometry. As the density of the two layers is reduced, making the bilayer more interacting, the counterflow Hall resistivity (ρ_xy) decreases at a given temperature while the counterflow longitudinal resistivity (ρ_xx), which is much larger than ρ_xy, hardly depends on density. Our data suggest that the counterflow dissipation present at any finite temperature is a result of mobile vortices in the superfluid created by the ubiquitous disorder in this system.     

Activation energy and hall-coefficient measurements in lightly dopedn-InP in the thermal freeze-out region

Summary The direct-current resistivity, β, and Hall coefficient,R _H, of lightly dopedn-type InP samples were measured at temperatures (T) down to 12K and magnetic fields up to 4.8 kG. A sharp exponential increase in β, asT was decreased, was observed for temperatures below 80 K. The Hall coefficient showed a similar trend,i.e. R _H increased sharply asT was reduced below 80 K. This is attributed to the freeze-out of conduction electrons onto their donor sites. The donor activation energy,E _d, calculated from the temperature dependence of the resistivity, was less than the theoretical prediction. An enhanced dielectric constant would be a possible candidate for such behaviour. The initial decrease in β (asT is reduced) recorded in the higher-temperature region is due to impurity scattering probably combined with lattice scattering. The authors of this paper have agreed to not receive the proofs for correction.     

Concentration dependence of the anomalous Hall effect in Fe/SiO2 granular films below the percolation threshold

The anomalous Hall effect is studied on Fe_x(SiO₂)_1?x nanocomposite films with x<0.7 in the vicinity of the percolation transition (x _c ≈0.6). It is found that, as the transition is approached from the side of metallic conduction, the Hall angle nonmonotonically varies, passing through a minimum. A qualitative model for describing the concentration dependence of the anomalous Hall effect is proposed. The model is based on that of the conductivity of a two-phase system near the percolation threshold [9, 10]. The anomalous Hall effect is governed by two conduction channels: one of them (a conducting network) is formed by large metal clusters that are separated by narrow dielectric interlayers below the percolation threshold, and the other is represented by the dielectric part of the medium containing Fe grains; in this part of the medium, the anomalous Hall effect occurs through the interference of amplitudes from the tunneling junctions in a set of three grains. It is shown that, at x<x _c , the network may give rise to a “shunting” effect, which makes the effective Hall voltage even less than the Hall voltage of the dielectric component.     

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.     

Finite Larmor radius and Hall effects on thermal instability of a compressible plasma

The effects of compressibility, finite Larmor radius (FLR) and Hall currents are considered on the thermal instability of a plasma in the presence of a uniform horizontal magnetic field. For stationary convection, the compressibility has a stabilizing effect whereas FLR and Hall currents have stabilizing as well as destabilizing effects. For (C _pβ/g)<1, the system is stable. The magnetic field, FLR and Hall currents introduce oscillatory modes in the system for (C _pβ/g)>1.     

Temperature- and magnetic field dependence of the Hall effect in the heavy fermion system CeCu6

The temperature dependence of the Hall coefficient and the magnetic field dependence of the Hall resistivity of CeCu₆ have been determined in the temperature range 80 mK<T<10 K and in magnetic fields up to 10T. The Hall coefficientR _H shows a very strong temperature dependence with two extrema and a change of sign, and the Hall resistivity _xy has a strong field dependence with up to two changes of sign. The observed behavior can partially be explained by the field- and temperature dependence of the skew scattering contribution to the Hall coefficient.     

Hall effect of tetragonal and orthorhombic SrRuO3 films

High quality orthorhombic and tetragonal SrRuO₃ thin films were grown by pulsed laser deposition on SrTiO₃(001) and Ba_0.75Sr_0.25TiO₃ buffered LaAlO₃(001) substrates. Resistivity vs. temperature curves showed a slope change at a Curie temperature of 147.5 ± 2 K for 40 nm thick films irrespective of crystalline symmetry. The Hall resistivity of both films contained an anomalous Hall contribution. The anomalous Hall coefficient was positive throughout the whole temperature range for the tetragonal film, whereas it showed a sign change at 143 K for the orthorhombic film. This is a strong indication that the Berry‐phase mechanism is the dominant anomalous Hall effect mechanism in SrRuO₃. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Space-charge relaxation and electrical conduction in?K0.5Na0.5NbO3 at high temperatures

Sodium potassium niobate K_0.5Na_0.5NbO₃(KNN) ceramic was synthesized by a solid-state technique. The X-ray diffraction of the sample at room temperature showed a monoclinic phase. The real part (ε′) and imaginary part (ε″) of dielectric permittivity of the sample were measured in a frequency range from 40 Hz to 1 MHz and in a temperature range from 350 to 850 K. The ε′ deviated from Curie–Weiss law above 702 K, due to additional dielectric contributions resulting from universal dielectric response and thermally activated space charges at high temperatures. This anomaly arose from a Debye dielectric dispersion that slowed down following an Arrhenius law. We have established a link between the dielectric relaxation and the conductivity.     

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)     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

Piezo-Optical Effect of Excitons in CuCl

We calculated the stress-induced variations in real part and imaginary part of the dielectric functions in copper chloride (CuCl) by Kramers-Kronig analysis from the piezoreflectance measurements of the 1s Z₃ and 1s Z₁₂ excitons in two configurations of the applied stress X // [001] and X // [110]. From the data of the deduced stress-induced variations in real part and imaginary part of the dielectric functions, the three independent complex components W ₁₁ , W ₁₂ , and W ₄₄ of the fourth-rank linear piezo-optical tensor were determined.     

Solvation energy of multiply charged anions and dielectric constant for finite system: a microscopic theory based bottom-up and top-down approach

This is the first time a microscopic theory-based bottom-up approach has been implemented to derive an analytical expression for the solvation energy for a finite (N) system, including the bulk. This bottom-up approach provides the information on solvation energies of anionic solutes in finite-size clusters, including the bulk (N = ∞), from the knowledge of the detachment energies for the system containing a few numbers of solvent molecules. However, in the case of dielectric constant, a microscopic theory-based top-down approach has been prescribed to derive an analytical expression for the static dielectric constant for the finite system. In this approach, the knowledge of the dielectric constant for the bulk provides a scheme to obtain the same quantity for a wide numbers of solvent molecules. As an illustrative example, the hydrated doubly charged anions, SO^?2 ₄.NH₂O and C₂O^?2 ₄.NH₂O, have been considered, and the calculated bulk solvation energy for the SO^?2 ₄.NH₂O system is found to be in very good agreement (within 5%) with the available experimental result. However, the same quantity calculated based on the Born model is found to be largely deviating (32%) from the experimental result. The calculated results of the dielectric constant for these two systems support the linear theory of dielectric constant.     

Superconductivity in the BaPb1−x Bi x O3 system

Superconductivity properties of the BaPb_1−x Bi _x O₃ system have been investigated by precise measurements of electrical resistivity and Hall coefficient in specimens prepared by the hot press technique. Sharp superconducting transitions have been observed in those specimens, indicating the homogeneities of the specimens thus prepared. superconductivity has been observed for 0.05<x≦0.35 and a maximum critical temperature of 11.7 K has been observed atx=0.25. The carrier concentration deduced from the Hall coefficient has been found to show nearly the samex dependence as does the critical temperature. The upper critical field has been determined from the magnetic field dependence of the lowcurrent-density resistivity. Discussions are given about the superconductivity in the BaPb_1−x Bi _x O₃ system on the basis of the experiment results.     

Investigations of the exciton condensate

Recent experiments on quantum Hall bilayers in the vicinity of total filling factor 1 (ν_T=1) have revealed many exciting observations characteristic of a superfluidic exciton condensate. We report on our experimental work involving the ν_T=1 exciton condensate in independently contacted bilayer two-dimensional electron systems. We observe previously reported phenomena as a zero-bias resonant tunneling peak, a quantized Hall drag resistivity, and in counter-flow configuration, the near vanishing of both ρ_xx and ρ_xy resistivity components. At balanced electron densities in the layers, we find for both drag and counter-flow current configurations, thermally activated transport with a monotonic increase of the activation energy for d/ℓ_B<1.65 with activation energies up to 0.4 K. In the imbalanced system the activation energies show a striking asymmetry around the balance point, implying that the gap to charge excitations is considerably different in the separate layers that form the bilayer condensate. This indicates that the measured activation energy is neither the binding energy of the excitons, nor their condensation energy.     

Gap at the Fermi level in the intermetallic vacancy system RBiSn(R=Ti,Zr,Hf)

The new class of intermetallic compounds RNiSn(R=Ti,Zr,Hf) may be characterised by the presence of an ordered sublattice of Ni atom vacancies in comparison with normal metals RNi₂Sn with no Ni vacancies. We report unusual transport and optical properties of the RNiSn system. The electrical resistivity of RNiSn is very high (3<p<100) mOhm*cm; the temperature coefficient of resistivity (TCR) is negative and strongly dependent on the annealing conditions. For some samples ZrNiSn and for a single crystal of TiNiSn the resistivity can be described by the Mott's law at temperatures 0.1<T<20 K. A phase transition nearT=100 K without change of crystal structure was deduced from Hall effect data and the temperature dependence of the lattice constant. Preliminary data on transport phenomena in RPtSn and RPdSn(R=Ti,Zr,Hf) compounds are also reported. The unusual properties of RNiSn system might be related to a gap of the electron spectrum near the Fermi energy.     

Anisotropic transport properties of U4Cu4P7

The electrical resistivity of U₄Cu₄P₇ for the current flowing in the basal plane of this tetragonal antiferromagnetic compound has been measured from 2 to 1000 K. Contrary to the case where the current flows along thec axis, we find an increasing resistivity with increasing temperature over the whole temperature range. This ferromagnetic-like behavior in the basal plane and the antiferromagnetic-like behavior perpendicular to it, is explained with the complex magnetic structure of the compound. The Hall effect forB parallel to thec axis shows an antiferromagnetic-like temperature dependence. The ordinary part of the Hall effect in the paramagnetic phase gives in an one-band model a carrier concentration of 0.7 holes per uranium atom, which is quite unique in uranium compounds by its sign and which results from the presence of two kinds of uranium atoms with different bonding, valence, magnetic moment and exchange. Quantitative differences between various samples of U₄Cu₄P₇ are related to a mixed-layer stacking polytype of U₄Cu₄P₇ and UCuP₂ units.     

On the electrical resistivity in intermediate valence compounds arising from electron-phonon scattering

The static and frequency dependent electrical resistivity in intermediate valence compounds arising from electron-phonon scattering is calculated by applying the memory function method to the periodic Anderson model. It is shown that in the static limit the resistivity may be considerably enhanced due to a finite fraction of 4f electrons being present at the Fermi level. This partial filling of the 4f shell is also responsible for the fact that the real part of the conductivity, which by Drude's formula is a Lorentzian of width 1/τ₀, is drastically reduced to an effective width 1/τ.     

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.     

The Hall effect in amorphous (Zr0.64Ni0.36)1−x M x alloys (M=Al,Ga)

Values of the room temperature Hall coefficients and electrical resistivity of amorphous melt spun (Zr_0.64Ni_0.36)_1–x Al _x and (Zr_0.64Ni_0.36)_1–x Ga _x alloys forx=0–0.25 are reported. Addition of Al or Ga to Zr_0.64Ni_0.36 dramatically increases the already positive Hall coefficient of this alloy and also increases the electrical resistivity and crystallization temperature.     

Quantum Hall effect in restricted geometries

The confinement of electrons in narrow quasi-two-dimensional conducting channels, modelled with a parabolic well, leads to asymmetric Hall plateaus about complete Landau-level fillings and to saw-toothed oscillations of the dc resistivity _xx as a function of the magnetic field B. The peaks in _xx are displaced to lower B and drastically reduced from their wide-channel values. The peak values of _xx increase with increasing channel width. The corrections to σ_yx for finite channel widths and the response to oscillating electric fields are evaluated.