Hall Resistivity Ρxy Research Articles

Observation of Quantum Hall effect in an ultra-thin (Bi0.53Sb0.47)2Te3 film

We report the observation of the Quantum Hall effect from the topological surface states in both the Dirac electron and Dirac hole regions in a 4 quintuple layer (Bi0.53Sb0.47)2Te3 film grown on GaAs (111)B substrates. The Fermi level is sitting within the enlarged bulk band gap due to the quantum confinement of the ultra-thin film and can be tuned through the Dirac point by gate biases. Furthermore, the Hall resistance Rxy shows even denominator plateaus, which could be fractional Quantum Hall states. This may be due to the hybridization between the top and bottom surface states and suggests the possible way to manipulate the interaction of two surfaces for potential spintronic devices.

Quantum oscillations in strong magnetic fields, berry phase, and superconductivity in three-dimensional topological Bi2–x Cu x Se3 insulators

Two-dimensional (2D) Shubnikov–de Haas oscillations and 2D Hall oscillations are observed in 3D copper-doped Bi2Se3 single crystals in magnetic fields up to 19.5 T at temperatures down to 0.3 K. Three samples with a high bulk carrier concentration (n ≈ 1019–1020 cm–3) are studied. The rotation of the samples in a magnetic field shows that these oscillations are related to numerous parallel 2D conducting channels 1–5 nm thick. Their basic kinetic parameters are found. Quantized Hall resistance Rxy is detected in 1-nm-thick 2D conducting channels at high fields. The distance Δ(1/Rxy) between the steps in the field dependence of 1/Rxy is found to be constant for different Landau levels, 1.3e2/h per 1-nm-thick layer. The constructed fan diagrams of 2D Landau levels for various angles of sample inclination with respect to the magnetic field direction allowed us to conclude that the Berry phase in the 2D conducting channels is γ ≈ π and independent of the magnetic field direction. When studying the angular dependence of upper resistive critical magnetic field Hc2 in one of the superconducting samples, we showed that it can be considered as a bulk superconductor consisting of superconducting layers with an effective thickness of about 50 nm.

Fractional quantum Hall effect at Landau level fillingν=4/11

We report low temperature electronic transport results on the fractional quantum Hall effect of composite fermions at Landau level filling nu = 4/11 in a very high mobility and low density sample. Measurements were carried out at temperatures down to 15mK, where an activated magnetoresistance Rxx and a quantized Hall resistance Rxy, within 1% of the expected value of h/(4/11)e^2, were observed. The temperature dependence of the Rxx minimum at 4/11 yields an activation energy gap of ~ 7 mK. Developing Hall plateaus were also observed at the neighboring states at nu = 3/8 and 5/13.

Fermi level tuning and weak localization/weak antilocalization competition of bulk single crystalline Bi2−xSbxSe2Te compounds

From the investigation of the electrical transport properties of single crystalline Bi2−xSbxSe2Te (x = 0.0, 0.6, 0.8, 1.0, 1.2 and 1.4) compounds, we observed a systematic change of the Fermi level from n-type metallic (x = 0.0 and 0.6) or small-gap semiconducting (x = 0.8) to p-type semiconducting (x = 1.0) and metallic (x = 1.2 and 1.4), respectively, with increasing Sb-substitution concentration based on temperature-dependent electrical resistivity ρ(T) and Hall resistivity ρxy(H) measurements, respectively. The parent compound Bi2Se2Te exhibits linear negative magnetoresistance at low magnetic fields (H ⩽ 1 T) by weak localization. The intermediate doped compounds of x = 0.8 and 1.0 showed weak antilocalization (WAL) and weak localization (WL) crossover behavior from the field-dependent magnetoresistance measurements at low temperatures. From the Hikami–Larkin–Nagaoka analysis of the compounds (x = 0.8 and 1.0), we found that there is a competing behavior between WL and WAL in terms of Sb-doping and magnetic field strength.

Effect of MgO/Co interface and Co/MgO interface on the spin dependent transport in perpendicular Co/Pt multilayers

Effect of the metal/oxide interface on spin-dependent transport properties in perpendicular [Co/Pt]3 multilayers was investigated. The saturation Hall resistivity (ρxy) is significantly increased by 45% with 1.4 nm thick CoO layer inserted at the top Co/MgO interface; whereas it is increased only 25% with 1 nm thick CoO layer at the bottom MgO/Co interface. The interfacial structures characterized by X-ray photoelectron spectroscopy show that the MgO/Co interface and Co/MgO interface including chemical states play a dominant role on spin-dependent transport, leading to different anomalous Hall behavior.

Giant planar Hall effect in reactive sputtered epitaxial CrxFe3–xO4 films

The epitaxial CrxFe3-xO4 films were fabricated by dc reactive sputtering on MgO (001) substrates. The longitudinal resistivity ρxx was enhanced 3 orders of magnitude with the increase of Cr content x from 0 to 0.87. The tunneling magnetoresistance like planar Hall effect in the CrxFe3-xO4 films was observed while the angle between ⟨100⟩ in (001) oriented films and the direction of magnetic field is 45° and 135°, respectively. Both the phase relation of angular dependent planar Hall resistivity ρxy and the numeral relation of ρxy with longitudinal resistivity difference ρ//−ρ⊥ cannot be understood by the planar Hall effect expression in isotropic magnetic medium. The largest planar Hall resistivity was ∼105 μΩ cm for x = 0.71, which is one, two, and six orders of magnitude larger than that in Fe3O4, GaMnAs, and ferromagnetic metals, respectively. The giant planar Hall resistivity ρxy is weak saturated at high fields and increases with the magnetic field. This giant planar Hall effect in the highly resistive CrxFe3-xO4 films is closely correlated to the longitudinal resistivity and antiphase boundaries.

Thickness dependence of the charge-density-wave transition temperature in VSe2

A set of three-dimensional charge-density-wave (3D CDW) VSe2 nano-flakes with different thicknesses were obtained by the scotch tape-based micro-mechanical exfoliation method. Resistivity measurements showed that the 3D CDW transition temperature Tp decreases systematically from 105 K in bulk to 81.8 K in the 11.6 nm thick flake. The Hall resistivity ρxy of all the flakes showed a linear dependent behavior against the magnetic field with a residual electron concentration of the order of ∼1021 cm−3 at 5 K. The electron concentration n increases slightly as the thickness d decreases, possibly due to the CDW gap is reduced with the decrease of the thickness.

Magnetotransport along a boundary: from coherent electron focusing to edge channel transport

We study theoretically how electrons, coherently injected at one point on the boundary of a two-dimensional electron system, are focused by a perpendicular magnetic field B onto another point on the boundary. Using the non-equilibrium Green's function approach, we calculate the generalized four-point Hall resistance Rxy as a function of B. In weak fields, Rxy shows the characteristic equidistant peaks observed in the experiment and explained by classical cyclotron motion along the boundary. In strong fields, Rxy shows a single extended plateau reflecting the quantum Hall effect. In intermediate fields, we find superimposed upon the lower Hall plateaus anomalous oscillations, which are neither periodic in 1/B (quantum Hall effect) nor in B (classical cyclotron motion). The oscillations are explained by the interference between the occupied edge channels, which causes beatings in Rxy. In the case of two occupied edge channels, these beatings constitute a new commensurability between the magnetic flux enclosed within the edge channels and the flux quantum. Introducing decoherence and a partially specular boundary shows that this new effect is quite robust.

Reservoir model for two-dimensional electron gases in quantizing magnetic fields: A review

We collect and review works which treat two-dimensional electron gases (2DEGs) in quantum wells (QWs, mostly GaAs/GaAlAs heterostructures) in the presence of quantizing magnetic fields as open systems in contact with outside reservoirs. If a reservoir is sufficiently large, it pins the Fermi level to a certain energy. As a result, in a varying external magnetic field, the thermodynamic equilibrium will force oscillations of the electron density in and out of the QW. This leads to a number of physical phenomena in magneto-transport, interband and intraband magneto-optics, magnetization, magneto-plasma dispersion, etc. In particular, as first proposed by Baraff and Tsui, the density oscillations in and out of QW lead to plateaus in the integer quantum Hall effect (IQHE) at values observed in experiments. The gathered evidence, especially from magneto-optical investigations, allows us to conclude that, indeed, in most GaAs/GaAlAs hetrostructures one deals with open systems in which the electron density in QWs oscillates as the magnetic field varies. Relation of the density oscillations to other factors, such as electron localization, and their combined influence on the quantum transport in 2DEGs, is discussed. In particular, a validity of the classical formula for the Hall resistivity ρxy = B/Nec is considered. It is concluded that the density oscillations are not sufficient to be regarded as the only source of plateaus in IQHE, although such claims have been sometimes made in the past and present. Still, our general conclusion is that the reservoir approach should be included in various descriptions of 2DEGs in the presence of a magnetic field. An attempt has been made to quote all the relevant literature on the subject.

Structural ordering driven anisotropic magnetoresistance, anomalous Hall resistance, and its topological overtones in full-Heusler Co2MnSi thin films

We report the evolution of crystallographic structure, magnetic ordering, and electronic transport in thin films of full-Heusler alloy Co2MnSi deposited on (001) MgO with annealing temperatures (TA). By increasing the TA from 300 °C to 600 °C, the film goes from a disordered nanocrystalline phase to B2 ordered and finally to the L21 ordered alloy. The saturation magnetic moment improves with structural ordering and approaches the Slater-Pauling value of ≈5.0μB per formula unit for TA=600 °C. At this stage, the films are soft magnets with coercive and saturation fields as low as ≈7 mT and 350 mT, respectively. Remarkable effects of improved structural order are also seen in longitudinal resistivity (ρxx) and residual resistivity ratio. A model based upon electronic transparency of grain boundaries illucidates the transition from a state of negative dρ/dT to positive dρ/dT with improved structural order. The Hall resistivity (ρxy) derives contribution from the normal scattering of charge carriers in external magnetic field, the anomalous effect originating from built-in magnetization, and a small but distinct topological Hall effect in the disordered phase. The carrier concentration (n) and mobility (μ) have been extracted from the high field ρxy data. The highly ordered films are characterized by n and μ of 1.19 × 1029 m−3 and 0.4 cm2V−1s−1 at room temperature. The dependence of ρxy on ρxx indicates the dominance of skew scattering in our films, which shows a monotonic drop on raising the TA. The topological Hall effect is analyzed for the films annealed at 300 °C. We find maximum topological contribution to Hall resistivity around 0.5 T while it approach to zero with increasing magnetic field. The anisotropic magnetoresistance changes its sign from positive to negative as we go from as deposited to 600 °C annealed film, suggesting a gradual increase in the half-metallic character.

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity ρxx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μq of our device. It is found that at the direct I-QH transition, μqBc ≈ 0.37 which is considerably smaller than 1. In contrast, at Bc, ρxx is close to the Hall resistivity ρxy, i.e., the classical mobility μBc is ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.

Effect of interfacial structures on anomalous Hall behavior in perpendicular Co/Pt multilayers

A large enhancement of anomalous Hall resistivity was obtained in the perpendicular [Co/Pt]3 multilayers sandwiched by MgO/CoO hybrid bilayers. For example, the saturation Hall resistivity (ρxy) is greatly increased, which is 250% and 67% larger than that in pure [Co/Pt]3 multilayers and that in [Co/Pt]3 multilayers sandwiched by pure MgO layers, respectively. Meanwhile, the perpendicular magnetic anisotropy in the multilayers with MgO/CoO hybrid bilayers was enhanced. The large enhancement of ρxy originates from the modified metal/oxide interfacial structures, together with improved crystallization of core [Co/Pt]3 multilayers, due to the insertion of hybrid bilayers.

Anomalous Hall effect in perpendicularly magnetized [formula omitted] thin films

Mn3−xGa (x=0.1, 0.4, 0.7) thin films on MgO and SrTiO3 substrates were investigated with magnetic anisotropy perpendicular to the film plane. An anomalous Hall effect was observed for the tetragonal distorted lattice in the crystallographic D022 phase. The Hall resistivity ϱxy was measured in a temperature range from 20 to 330K. The determined skew scattering and side jump coefficients are discussed with regard to the film composition and used substrate and compared to the crystallographic and magnetic properties.

The magnetoresistance and Hall effect in CeFeAsO: a high magnetic field study

The longitudinal electrical resistance and the transverse Hall resistance of CeFeAsO are simultaneously measured up to a magnetic field of 45T using the facilities of pulsed magnetic field at Los Alamos. Distinct behaviour is observed in both the magnetoresistance Rxx(μ0H) and the Hall resistance Rxy(μ0H) while crossing the structural phase transition at Ts≈150K. At temperatures above Ts, little magnetoresistance is observed and the Hall resistivity follows linear field dependence. Upon cooling down the system below Ts, large magnetoresistance develops and the Hall resistivity deviates from the linear field dependence. Furthermore, we found that the transition at Ts is extremely robust against the external magnetic field. We argue that the magnetic state in CeFeAsO is unlikely a conventional type of spin-density-wave (SDW).

Large enhancement of the anomalous Hall effect in Co/Pt multilayers sandwiched by MgO layers

We report a large enhanced anomalous Hall effect (AHE) in the Co/Pt multilayers sandwiched by two MgO layers. The Hall resistivity (ρxy) was over an order of magnitude larger than that in pure Co/Pt multilayers. By optimizing the thickness of MgO layers, a high field sensitivity value of 2445 V/A T for Hall sensors was achieved. The enhancement of AHE is mainly attributed to the MgO–Pt interfacial effect.

Sign reversal of the Hall resistivity in the electron-doped [formula omitted] thin films

Transport properties including both the longitudinal resistivity ρxx(H) and the Hall resistivity ρxy(H) are systematically studied in the optimally doped La2-xCexCuO4 (x=0.105) thin films. We have found sign reversals of the Hall resistivity ρxy(H) in two different temperature regions, i.e., 15.5–20K and 36.5–38K. In addition, ρxx(H) and ρxy(H) show H2 dependence in the temperature region of 40–50K, which is similar to the behavior of the compensated metal. The abnormal sign evolution in ρxy(H,T) can be attributed to the competition between the two different carriers, i.e., electrons and holes.

Hall effect and magnetoresistance in the heavy fermion superconductor CeCo(In1–xCdx)5

We present measurements of the low temperature Hall effect and magnetoresistance in the heavy fermion superconductor CeCo(In0.925Cd0.075)5 as a function of applied magnetic field. The Hall resistivity ρxy is observed to be negative indicating electron dominated transport. Features corresponding to the possible destabilization of the antiferromagnetic ground state are observed. Magnetic field sweeps show that the resistivity ρxx(H) is hysteretic in this regime, indicating that these transitions could be first order in nature.

Extended Power Law and Hall Anomaly of High-Temperature Superconductors

Starting from the free energy of a moving vortex, we obtain the extended power law form of longitudinal resistivity ρxx and the analytical form of Hall resistivity ρxy. Based on them, we obtain a scaling relation which agrees well with the experimental data of different kinds of high-temperature superconductors. Furthermore, the theoretical results well fit the scaling results.

Universal scaling relations of the Hall resistivity and Hall angle near the vortex glass transition and the unified Hall resistivity equation

Universal scaling relations of the pinning effect on the Hall resistivity ρxy and Hall angle θH were shown. Considering the extended power law form of ρxx and the microscopic analysis of σxy，we derive unified ρxy equations for superconductors with and without double sign reversal.

Enhanced Hall effect in FexGe1−x nanocomposite films

Enhanced Hall effect has been observed in the FexGe1−x nanocomposite films composed of nanoscale Fe grains embedded in amorphous Ge matrix. The Fe grain size, the saturation magnetization, and the interparticle interaction increase with increasing Fe atomic fraction x. The transport mechanism of the films changes from semiconducting to metallic character as x increases, and the percolation phenomenon ocurrs at x∼0.5. The Hall resistivity (ρxy) reaches its maximum of ∼126μΩcm at x=0.5, which is ∼140 times larger than that of Fe films. The ordinary and extraordinary Hall coefficients are enhanced by two orders in magnitude compared to pure Fe films and four orders compared to the bulk Fe. Upon annealing at 400°C, the enhanced Hall effect disappears with the disappearance of the Fe grains. This enhanced Hall effect can be attributed to the local quantum interference effect due to the presence of ∼1–2nm Fe grains.