Magnetoresistance Experiments Research Articles

Boundary scattering and weak localization of electrons in a magnetic field.

The influence of boundary scattering on one- and two-dimensional weak localization is studied both analytically and by numerical simulation. Diffuse and specular boundary scattering are considered for two geometries, a metal film in a parallel magnetic field and a laterally restricted two-dimensional electron gas in a perpendicular field. The results, which extend the Al'tshuler-Aronov and Dugaev-Khmel'nitskii theories, are relevant to recent magnetoresistance experiments on high-mobility channels in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructures.

Low temperature measurements of UBe 13

Transport and specific heat measurements are reported on a new sample of UBe 13. Special focuses are given on the zero field specific heat : at low temperature (T< 150mK) where the impurity scattering dominates, at intermediate temperature where a quasi T 3 law characteristic of an axial state is obeyed and near the critical temperature where critical fluctuations are observed. Correlations are made with magnetoresistivity and thermal conductivity experiments.

High-field transitions in URu 2Si 2 Observed by magnetoresistivity and magnetization experiments

High-field magnetoresistivity and magnetization experiments have been performed on a monocrystalline sample of the antiferromagnetic superconductor URu 2Si 2, for a field direction along the tetragonal axis. At a temperature of 1.5K three sharp transitions are observed at fields of 35.8, 37.3 and 39.4T, suggesting a complex magnetization process. Large jumps in the magnetoresistance indicate that the magnetization process involves gap-like changes of the Fermi surface.

Metamagnetic-like transition in CeRu2Si2?

Extensive magnetization and magnetoresistivity experiments on CeRu2Si2 are described, with special emphasis on the metamagnetic-like transition that occurs atH*=80 kOe. By contrast to the low-field susceptibility, the differential susceptibility atH* almost diverges atT→0. It is suggested that high magnetic fields (H>H*) restore a situation where the interactions between particles have collapsed. Comparison is made with other well-known examples: TiBe2, UPt3, TmSe, and3He.

A.C. Susceptibility and Specific Heat in Low B.C. Fields of La2-xSrxCuO4 and YBa2Cu3O7 High Temperature Superconductors

We report the giant effect of small d.c. fields on a.c. diamagnetic transitions in La2-xSrxCuO4, and YBa2Cu3O7. An unexpected frequency effect is observed. In order to elucidate the apparent conflict Between the high critical field slope as measured by magnetoresistance experiments and the large depression of a.c. diamagnetic transitions, we locate the bulk transition by high resolution specific heat measurements in small fields. The results, together with d.c. magnetization measurements near Tc and SEM micrographs, suggest a model where superconducting regions with a high critical field are separated by weak links.

Magnetic structures and magnetoresistivity of MnRhAs

Measurements of the magnetisation, resistivity, magnetoresistivity and neutron diffraction experiments have been made on a powder sample of MnRhAs. The compound exhibits a metamagnetic behaviour below 160 K and shows both ferro- and antiferromagnetic couplings between 160 and 200 K. The magnetoresistivity reflects both transitions. Above T c = 200 K strong magnetic correlations are pointed out.

Multiband effects in weak localization.

Weak-localization effects, especially the dependence of the resistance on a magnetic field, have become a unique probe for determining characteristic times for conduction electrons in metals. Analysis of the magnetoresistance data is usually made with the theory of Hikami et al., which is derived in the model of free electrons and s-type impurity scattering. We show that the results of Hikami et al. are universally valid for electrons with an arbitrarily complex Fermi surface (anisotropic, several bands) and arbitrary impurity scattering (anisotropic, interband scattering). The complex band-structure and scattering properties enter only in the resistivity, the electronic density of states, and properly averaged lifetimes, which can be obtained from a magnetoresistance experiment.

Magneto Transport and EPR Measurements on (TSeT)2 Br

Abstract We report resistivity and magnetoresistance measurements under pressure down to 0.1 K and EPR experiment at 1 bar on (TSeT)2 Br prepared by an electrochemical method. The compound is known to undergo a phase transition at 293 K as a function of pressure leading to two conducting phases: a low and a high pressure phase. In the latter at low temperature the metallic state is stabilized under high pressure. In spite of large residual resistivity ratio (RRR), no sign of superconductivity has been detected down to 0.1 K. Transverse magnetoresistance experiments at 6 Kbar show very weak quadradic field dependence and no saturation up to 70 KG.

Electronic structure ofPbTePb1−xSnxTeTe superlattices

${\mathrm{P}\mathrm{b}\mathrm{T}\mathrm{e}/\mathrm{P}\mathrm{b}}_{0.88}$${\mathrm{Sn}}_{0.12}$Te Te superlattices were prepared with individual thicknesses of about 50 nm and periodicities of 100-250 nm on ${\mathrm{BaF}}_{2}$ substrates. The lattice constant mismatch of $\frac{\ensuremath{\Delta}a}{a}=2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ introduces a misfit strain in the constituent layers and there is an additional tensile strain caused by the substrate. These strains were determined by x-ray diffractometry. The electrical properties of this many-valley type-I superlattice were investigated by conductivity, Hall effect, and weak-field magnetoresistance (WFMR) experiments. For electrons or holes confined in the Pb-Sn-Te layers, the angular dependence of the WFMR data is consistent with quasi-two-dimensional transport. The band structure of these many-valley PbTe/Pb-Sn-Te superlattices is calculated by matching propagating or evanescent envelope functions at the boundaries of consecutive layers and considering the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ Hamiltonian for cubic IV-VI compounds at the $L$ point of the Brillouin zone. The evolution of the PbTb/Pb-Sn-Te band structure with increasing superlattice periodicity is calculated and energy-momentum dispersions are given for the samples of interest. The observed quasi-two-dimensional transport is explained by these $E(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$ relations, yielding, for appropriate superlattice periodicities, cylinders as surfaces of constant energy.

Negative and positive magnetoresistance in Cs adsorbed Si(111) n-inversion layers

In n-channel inversion layers on Cs adsorbed Si(111) surfaces, the range of electron-electron interaction constants g i 's is determined by magnetoresistance experiments (the positive magnetoresistance in the field parallel to the surface and the negative magnetoresistance in the field perpendicular to the surface) and the log T dependence of the conductivity. The results are 0.95 ⩽ g 4 ⩽ 1.52, 0.36 ⩽ g 1 ⩽ 0.55, 0 ⩽ g 2 ⩽ 1.14 and 0 ⩽ g 3 0.57 for n v = 6. For n v = 2 the value of g 1 is the same as the previous one and the values of the other g i 's are one third of the g i 's for n v = 6.

Reduction of the Shoenberg magnetic interaction by magnetic feedback

It is demonstrated that the oft-times severe waveform distortion arising from the Shoenberg magnetic interaction in the de Haas-van Alphen effect can be very effectively suppressed when measurements are made on spherical samples in the presence of a uniform magnetic feedback field. A digital spectrum analyzer is used in conjunction with a large ramping modulation field (∼1 kG peak-to-peak) to obtain a Fourier transform of a portion of the dHvA waveform roughly every second, and the dHvA effect itself is used to provide criteria for setting the correct gain in the magnetic feedback loop. Examples drawn from a study of the [110] orientation in Pb serve to illustrate the advantages and improvements in waveform studies which accrue from the use of magnetic feedback. The improvement is particularly impressive in the three-harmonic method for determining the spin-splitting factor g c for extremal orbits on the Fermi surface, and the value g c = 0.704 is found for the [110] ζ orbit in Pb (γ oscillations). A careful search has been made for low-frequency dHvA oscillations in Pb corresponding to those reported in ultrasonic attenuation and magnetoresistance experiments, but no such oscillations could be found in the magnetization.

An evidence of spin flopping in V 5S 8 by magnetoresistance experiments

An anomalous field dependence of the magnetoresistance of V 5S 8 is observed at 42 kOe at liquid helium temperature. This is attributed to the spin flopping transition predicted by our torque experiments.

Structure of the second valence band in PbTe

The orientation of the main axis of the constant-energy ellipsoids and the mass-anisotropy ratio $\frac{{m}_{l}}{{m}_{t}}$ of the secondary valence-band extrema of PbTe has been determined experimentally. These extrema are generally believed to be located at the $\ensuremath{\Sigma}$ point of the Brillouin zone. Investigation of the anisotropy behavior of the coefficient of the conductivity of warm holes and magnetoresistance measurements showed that the main axes of these ellipsoids are parallel to 100g and do not coincide with the symmetry direction of the $\ensuremath{\Sigma}$ point. The value of the mass-anisotropy ratio for the secondary valence band in PbTe is approximately 10. It has been derived from magnetoresistance experiments.

Bandstructure and Fermi surface of Cu2Sb by the LMTO method

The linear muffin-tin orbital (LMTO) method of bandstructure calculation has been applied to the simple tetragonal compound Cu2Sb. The d bands of Cu lie substantially below the Fermi level, and the Fermi surface is a recognizable distortion of the free-electron model. The Fermi surface has sheets in four bands. The first and second bands contain closed sheets, degenerate along a plane in the absence of spin-orbit splitting. The third band contains a multiply-connected sheet. The fourth band consists of undulating columns, degenerate along a plane with the third band in the absence of spin-orbit splitting, and of another closed sheet. Earlier de Haas-van Alphen results are explained semiquantitatively by the model, which also accounts for open orbits seen in high-field magnetoresistance experiments.

Transport in magnetic alloys: Scattering asymmetries (anisotropic scattering, skew scattering, side-jump)

The scattering of conduction electrons by magnetic impurities depends on the current direction with respect to the impurity moments (anisotropic scattering effect) and is different to the left and to the right (skew scattering and side-jump effects). The scattering asymmetries show up in magnetoresistance and Hall effect experiments and have been mainly studied for rare-earth impurities. They are of interest for determining the anisotropic terms of the conduction electron-impurity interaction, such as quadrupolar Coulomb terms or orbital exchange terms. In alloys with transition metal impurities they can reveal the unquenced orbital part of the magnetic moment.

A simple formula for the low-field longitudinal magnetoresistance: application to experiments performed on copper

For the low-field longitudinal magnetoresistance parameter P/sub /// a formula has been derived by solving iteratively the Boltzmann equation with an anisotropic relaxation time scattering term. This formula is applied to copper, the Fermi surface of which is approximated by two bands. A simple relation between P/sub /// and the ratio tau B/ tau N of the belly and neck relaxation times is established. Various magnetoresistance experiments reported in the literature are analysed within this model and in general a good agreement is found. Furthermore new experiments are reported where Frenkel defects at 4.6K were added to gold impurities in copper and both the magnetoresistance and the deviation from Matthiessen rule have been measured.

De Haas-van Alphen Effect inn-TypeBi2Te3

The de Haas-van Alphen (dHvA) effect has been measured in $n$-type ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$ with carrier concentrations ranging from 9\ifmmode\times\else\texttimes\fi{}${10}^{17}$ to 2.4\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Below 5\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, the conduction-band minima can be fitted to a six-ellipsoid model, the parameters of which differ considerably from those inferred from magnetoresistance experiments. The carrier density computed from the volume of the ellipsoids agrees well with that obtained from the limiting high-field Hall coefficient at 4.2\ifmmode^\circ\else\textdegree\fi{}K. At higher concentrations there is a large discrepancy between the two estimates of carrier density. The presence of a low-mobility high-mass band, lying 30 meV above the ellipsoidal minima, is postulated to explain the difference. The existence of this second band or of an anisotropic relaxation time are possible explanations of the discrepancy between the ellipsoidal parameters deduced from the dHvA and transport measurements.

Magnetoresistance and Fermi Surface Topology of Crystalline Mercury

The results of magnetoresistance experiments on a number of oriented single crystals of mercury are reported. Three distinct types of minima are observed in plots of magnetoresistance versus field direction, and these are attributed to open orbits normal to the planes {100}, ${0\overline{1}1}$, and {110}. A unique model of the Fermi surface, formulated in terms of the dimensions of the contact regions on the first zone, is derived to explain these data. An eight-plane-wave pseudopotential calculation including spin-orbit coupling is used to obtain a compensated Fermi surface in substantial agreement with this model and with the first-zone de Haas-van Alphen extremal areas.

Cyclotron resonance of electrons in silicon at temperatures up to 200 K

Cyclotron resonance signals from thermally generated electrons have been observed with ultra-high purity silicon samples over the temperature range 50-206 °K. A microwave spectrometer incorporating a confocal cavity and operating at a frequency of 139 Gc/s was used in these experiments. The transverse effective mass parameter m⊥ is found to increase with temperature by 6±3% at 102 °K. The longitudinal mass m|| appears to be unaffected by any temperature change. The collision times derived from the width of the resonance lines are in good agreement with mobility data in this temperature range. However, the collision time anisotropy ratio for intravalley acoustic phonon scattering ( / ) was found to be 0.90+0.05-0 11 in disagreement with magnetoresistance measurements which suggest a value of 0 67. The following values for the deformation potential constants are derived from the cyclotron resonance data: Ξu = 8.6 eV, Ξd = -3.4 eV. A comparison of the deformation potentials quoted above with those derived from the magnetoresistance experiments shows that the cyclotron resonance data give better agreement with the recent theoretical calculations of Goroff and Kleinman. The origin of the above discrepancy is obscure.

Effects of Electrical Current and Tensile Stress on the Domain Structure in Polycrystalline Grain-Oriented 3.25% Silicon Steel

A Kerr magneto-optic apparatus has been used to observe the effects of an electrical current and tensile stress on the domain structure in commercial, 3.25% silicon steel. It is noted that a current density of 0.27 A/mm2 produces almost complete saturation in a favorably oriented grain. This effect of current should be considered in elastoresistance and low-field magnetoresistance experiments. Upon application of a tensile stress of only 0.57 kg/mm2, a domain pattern resembling the `lozenge' pattern is shown to occur in certain grains. This indicates that the lozenge pattern is associated with stress and will probably not occur in a completely strain-free crystal. The corresponding strain-free pattern is identified here as a `chevron' pattern.