High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

Spin relaxation of conduction electrons in GaAs

We have used optical spin orientation techniques to measure T₁ of conduction electrons in GaAs (NinA ≈ 10¹⁷ cm^-3) for 4.7 K ? T ? 200 K. From Hall effect measurements we estimated the electron momentum relaxation time τ_p. For 50 K ? T ? 200 K, the product T₁τ_p agrees with our earlier order of magnitude estimate of the D'yakonov-Perel' mechanism, in which band structure induced precession is strongly narrowed by momentum relaxation. The Elliott mechanism is one to two orders of magnitude weaker.     

Mobility of electrons in SnS2−xSex

The electrical resistivities and Hall constants of the semiconducting compounds SnS_2?xSe_x have been measured at temperatures ranging from 100 to 450 K and three donor ionization energies (0.013, 0.086 and 0.25 eV) have been identified. The Hall mobilities exhibit for T> 200K a temperature dependence of the form μ～ (T/T₀)^?n separating the SnS_2?xSe_x compounds into two groups one behaving like SnS₂ and the other like SnSe₂.     

Hall and transverse even effects in the vicinity of a quantum critical point in Tm1 ? x Yb x B12

The angular, temperature, and magnetic field dependences of the resistance recorded in the Hall effect geometry are studied for the rare-earth dodecaboride Tm_{1 ? x} Yb _x B₁₂ solid solutions where the metal-insulator and antiferromagnetic-paramagnetic phase transitions are observed in the vicinity of the quantum critical point x _c ?? 0.3. The measurements performed on high-quality single crystals in the temperature range 1.9?C300 K for the first time have revealed the appearance of the second harmonic contribution, a transverse even effect in these fcc compounds near the quantum critical point. This contribution a is found to increase drastically both under the Tm-to-ytterbium substitution in the range x > x _c and with an increase in the external magnetic field. Moreover, as the Yb concentration x increases, a negative peak of a significant amplitude appears on the temperature dependences of the Hall coefficient R _H(T) for the Tm^{1 ? x} Yb _x B₁₂ compounds, in contrast to the invariable behavior R _H(T) ?? const found for TmB₁₂. The complicated activation-type behavior of the Hall coefficient is observed at intermediate temperatures for x ?? 0.5 with activation energies E _g /k _B ?? 200 K and E _a/k _B = 55?C75 K, and the sign inversion of R _H(T) is detected at liquid-helium temperatures in the coherent regime. Renormalization effects in the electron density of states induced by variation of the Yb concentration are analyzed. The anomalies of the charge transport in Tm_{1 ? x} Yb _x B₁₂ solid solutions in various regimes (charge gap formation, intra-gap many-body resonance, and coherent regime) are discussed in detail and the results are interpreted in terms of the electron phase separation effects in combination with the formation of nanosize clusters of rare earth ions in the cage-glass state of the studied dodecaborides. The data obtained allow concluding that the emergence of Yb-Yb dimers in the Tm_{1 ? x} Yb _x B₁₂ cage-glass matrix is the origin of the metal-insulator transition observed in the achetypal strongly correlated electron system of YbB₁₂.     

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni₅₀Mn₃₅In_{15 ? x} Si _x (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220?C280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150?C200 ??? cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.     

Halleffekt und anisotropie der beweglichkeit der elektronen in ZnO

Hallconstant, conductivity and Hall mobility of ZnO crystals were measured as function of temperature (4 °K < T < 370 °K) and orientation. Value and anisotropy of mobility can be explained (50 °K < T < 370 °K) by polar optical scattering, deformation potential sc., piezoelectric sc. and sc. by ionized impurities. The anisotropy of mobility is caused only by piezoelectric sc. Maximum values of μ_H are reached for μ_H ⊥ c, with 2400 cm²/V sec at 40 °K and for μ_H ¦ c with 1350cm²/Vsec at 60 °. Below 50 °K Hallconstant, conductivity and Hall mobility are influenced by impurity band conduction processes. The crystals have impurity concentration in the 10¹⁶ cm^?3 range, but they show different donor activation energies depending on growth conditions: Type I: 38,4 meV (50 °K < T < 100 °K) and Type II: 20,3 meV (50 °K < T < 100 °K) and 6 meV (25 °K < T < 50 °K).     

Spin-polaron transport and magnetic phase diagram of iron monosilicide

The study of galvanomagnetic, magnetic, and magnetooptical characteristics of iron monosilicide in a wide range of temperatures (1.8–40 K) and magnetic fields (up to 120 kOe) has revealed the origin of the low-temperature sign reversal of the Hall coefficient in FeSi. It is shown that this effect is associated with an increase in the amplitude of the anomalous component of the Hall resistance ρ_H (the amplitude increases by more than five orders of magnitude with decreasing temperature in the range 1.8–20 K). The emergence of the anomalous contribution to ρ_H is attributed to the transition from the spin-polaron to coherent regime of electron density fluctuations in the vicinity of Fe centers and to the formation of nanosize ferromagnetic regions, i.e., ferrons (about 10 Å in diameter), in the FeSi matrix at T<T_C=15 K. An additional contribution to the Hall effect, which is observed near the temperature of sign reversal of ρ_H and is manifested as the second harmonic in the angular dependences ρ_H(?), cannot be explained in the framework of traditional phenomenological models. Analysis of magnetoresistance of FeSi in the spin-polaron and coherent spin fluctuation modes shows that the sign reversal of the ratio Δρ(H)/ρ accompanied by a transition from a positive (Δρ /ρ>0, T>T_m) to a negative (Δρ/ρ<0, T<T_m) magnetoresistance is observed in the immediate vicinity of the mictomagnetic phase boundary at T_m=7 K. The linear asymptotic form of the negative magnetoresistance Δρ/ρ ∝?H in weak magnetic fields up to 10 kOe is explained by the formation of magnetic nanoclusters from interacting ferrons in the mictomagnetic phase of FeSi at T<T_m. The results are used for constructing for the first time the low-temperature magnetic phase diagram of FeSi. The effects of exchange enhancement are estimated quantitatively and the effective parameters characterizing the electron subsystem in the paramagnetic (T>T_C), ferromagnetic (T_m<T< T_C), and mictomagnetic (T<T_m) phases are determined. Analysis of anomalies in the aggregate of transport, magnetic, and magnetooptical characteristics observed in the vicinity of H_m≈35 kOe at T<T_m leads to the conclusion that a new collinear magnetic phase with M∥H exists on the low-temperature phase diagram of iron monosilicide.     

Electron-phonon scattering in potassium at high magnetic fields

We have measured the temperature dependence of both the zero-field resistivity and the transverse magnetoresistance of polycrystalline potassium wires (?(300 K)/?(4.2 K)=140 to 6000) in fieldsH?35 kG and at temperaturesT?4.2 K. Our principal findings are: 1) The presence of a large magnetic fieldH=35 kG does not alter the temperature dependence of ? from that observed atH=0; below 4.2 K theT-dependent part of the resistivities,_?T (H=0) and_?T (H=35 kG), fit well to the function exp (?Θ*/T) with the same Θ*=23K. 2) Deviations from Matthiessen's rule are significantly reduced in a strong field so that the magnitude of_?T (H=0) approaches that of_?T (H=35 kG) as sample purity decreases. 3) The slope of the high-field linear magnetoresistance increases slightly (?8%) from 1.5 K to 4.2 K. We attribute the exponential temperature dependence of_?T (H) to the freezing out of electron-phonon umklapp processes as has been shown for the zero-field resistivity. The reduction in deviations from Matthiessen's rule at high fields can be understood within semiclassical theory, but the latter cannot explain the failure of_?T (H) to saturate at high fields. A proposal by Young that electron-phonon umklapp scattering may contribute aT-dependent high-field linear magnetoresistance in potassium is considered.     

Neutron scattering investigation of the tricritical point in DyPO4

Neutron scattering experiments on DyPO₄ were performed without and with external field at temperatures between 1.65 and 3.5 K in order to determine the phase diagram and critical properties. In contrast to previous results the M(H)-curve shows typical first order behaviour fot T ? 2 K and H ? 4.2. kG. The H_c(T)-curve agrees quite well with the previous results. Critical scattering was observed at 1.65 K around the (110) magnetic Gragg-peak. A structure refinement at room temperature was also done mainly in order to check for the importance of extinction.     

Scaling behavior of the Hall coefficient of Si:P at the metal-insulator transition

Measurements of the Hall coefficient R _H (T, B) of Si:P with P concentration N between 3.54 and 7.0·10¹⁸ cm^?3 are reported for the temperature range 0.04 K ≤ T ≤ 4K and in magnetic fields up to 7 T. Even far above the metal-insulator transition (MIT), a sign change of the temperature coefficient similar to the behavior of the conductivity σ(T) in moderate fields is not observed in R _H (T). Field and temperature dependence of R _H both increase as the MIT (at the critical concentration N _c = 3.52 · 10¹⁸ cm^?3) is approached. A careful extrapolation to T → 0 and B → 0 indicates that R _H ^?1 scales to zero as R _H ^?1 ～| N ? N _c ^μH with μ _H = (0.44 ± 0.04) in agreement with previous results.     

Metal-nonmetal transition and superconductivity in frozen aluminium-hydrogen films

A study of the influence of the hydrogen concentration c_H on the superconducting transition temperature T_c and the d.c.-conductivity σ of Al-films has been performed. The samples were prepared by co-deposition of Al and H (H₂) onto a sapphire substrate held at a temperature of about 5 K. Within the experimental errors, there is no difference between the behaviour of Al-H and Al-H₂ respectively. σ shows a decrease with increasing c_H and a metal-insulator transition at c_H?0.68. The T_c values exhibit a maximum T_c ? 5.3 K for 0.22 ? c_H ? 0.5. Annealing experiments result in a decrease of T_c and an increase of σ. An analysis of the annealing experiments leads us to the conclusion that the as-prepared Al-H (Al-H₂) films have an amorphous structure, which transforms during annealing in a granular system of Al grains embedded in insulating AlH₃. No H-specific effect has been detected.     

High temperature EPR linewidths in MnO and MnS

EPR linewidths ΔH in MnO and MnS were studied at 300–1100°K. The remarkable constancy of _χT(ΔH) for T ? 3T_N follows from an extension of the Mori-Huber theories and delineates the region where critical-like behavior first becomes apparent as T approaches T_N.     

Normal state transport properties in single crystals of Ba1−xKxFe2As2 andNdFeAsO1−xFx

Hall effect and magnetoresistance Δρ/ρ(0) (MR) in the normal state have been measured on single crystals of Ba_1?xK_xFe₂As₂ and NdFeAsO_1?xF_x. Detailed analysis reveal the following conclusions: (1) For the parent phases of Ba_1?xK_x Fe₂As₂ and NdFeAsO_1?xF_x, large Hall effect and MR with strong temperature dependence were observed below a characteristic temperature corresponding to the antiferromagnetic/ structural transition. The field dependence of the Hall resistivity ρ_xy exhibits a non-linear behavior, which is accompanied by the violation of the B-square feature of the longitudinal magnetoresistivity Δρ_xx(B)/ρ_xx(0). A closer inspection further indicates that they are well related to each other and could be attributed to the multi-band effect or spin-related scattering. (2) The superconducting samples show much smaller Hall coefficient and MR in the normal state. The Hall coefficient shows a weaker temperature dependence compared to the parent phase, while the mean scattering rate 1/τ_H has a power-law like temperature dependence as 1/τ_H ∝ Tⁿ (n = 2–3). (3) For a Ba_1?xK_xFe₂ As₂ sample with T_c = 36 K, the field dependence of MR is complicated and the feature varies in different temperature regions. A drastic change of Δρ/ρ(0) was found between 80 K and 100 K, which corresponds very well to the maximum of the temperature derivative of the resistivity. This may be attributed to the spin-related scattering of electrons. (4) A comparison between the parent phase and the superconducting sample with T_c = 50 K in NdFeAsO_1?xF_x suggests that the electronic transport properties in the normal state cannot be easily understood with the simple multi-band model, while a picture concerning a suppression to the quasiparticle density of states at the Fermi energy is more reasonable.     

Universal explicit expression for quantum Hall conductance at any temperatures and chemical potentials

This paper generalises the theorem already obtained [Solid State Commun. 127 (2003) 505] for the high mobility, dissipationless, integer quantum Hall systems at T=0 K to the T>0 K situations. The results obtained are again suitable at both microscopic and macroscopic scales. In comparison [Solid State Commun. 127 (2003) 505], this generalised form gives a universal explicit expression for the Hall conductance σ_xy(μ,T) between any two points selected in such a system as a function of chemical potential and temperature. Further, thermal deviation Δσ_xy(μ,T) from the exact quantised values of σ_xy(μ,T) and the minimum slopes of Hall plateaux in the T>0 cases, observed already in experiments, are also derived in theory. Similar to those in the T=0 K case [Solid State Commun. 127 (2003) 505], the overall quantum Hall behaviour of the system can again be obtained from this theory by simply selecting two points on the two Hall contacts.     

Temperature dependence of effective mobility edge and inelastic scattering time in silicon MOS inversion layers in strong magnetic fields

Temperature (T) dependence on Hall conductivity (σ_xy) in Si MOS inversion layers measured in 15 T at T=1.4?10 K is investigated by comparing dσ_xy/dN_s, N_s electron concentration, to the calculation based on an effective mobility edge (E_c) model. Temperature dependence of inelastic scattering time is discussed in connection to the T-dependence of E_c.     

Low-temperature transport and ferromagnetism in GaAs-based structures with Mn

GaAs structures with implanted Mn and, additionally, with Mg for increasing the hole concentration in the implanted Mn layer are synthesized and investigated. SQUID magnetometer measurements revealed the existence of ferromagnetism in the temperature range 4.2 K ≤ T < 400 K, which is associated with the formation of the Ga_1?x Mn _x As solid solution and MnAs and Ga_1?y Mn _y clusters in the sample as a result of rapid high-temperature annealing. At temperatures from 4.2 to approximately 200 K, the anomalous Hall effect associated with additional magnetization of the sample is observed. As the temperature increases from 4.2 K, the colossal negative magnetoresistance is transformed into a positive magnetoresistance at T ≈ 35 K.     

Metal-nonmetal transition and superconductivity in frozen tin-hydrogen films

We have studied the d.c.-conductivity σ and the superconducting transition temperature T_c in frozen mixtures of Sn and molecular H₂ (Sn-H₂) or atomic H(Sn-H). The samples were prepared by co-deposition of Sn and H₂ (H) onto a sapphire substrate held at a temperature of about 5 K. Both systems show a quite different behaviour. While the Sn-H₂ system exhibits a metal-insulator transition at c_H2 = 0.6 which agrees well with percolation theory, the Sn-H samples show a metal-semiconductor transition already at c_H?0.32. Both systems have an increased T_c in the metallic region. H₂ seems to be mobile in Sn-H₂ films even down to 5 K after annealing of the samples at 10 K. Atomic H, on the other hand, is bound up to T?200 K.     

Crystal fields and the Kondo properties of CeAl3

Crystal fields are included in a model calculation of the magnetization of the Kondo compound CeAl₃. Using parameters taken from experiment, the predicted magnetization at T=1.4 and 4.2K for applied magnetic fields H less than 10T is in good agreement with experiment. Using this magnetization, the model also correctly predicts the decrease in the specific heat C as a function of H for 0.35 ?T?0.9K. In particular, it correctly predicts that this decrease is approximately linear in H and that $\text{(}\text{?H}\text{?H}\text{)}{}_{\mathrm{T}}$ is approximately temperature independent.     

Hall coefficients for A15 compounds: Nb-Sn and V-Si

The Hall coefficients, R_H, for A15 structure single crystal V₃Si and polycrystal V25.25 at.% Si and Nb 26 at.% Sn have been measured as functions of temperature. The data between the superconducting transition temperature, T_c, and 80 K for the Nb-Sn show a small dip centred arounf 30 K which accompanies a cubic-to-tetragonal lattice distortion commencing at about 45 K. R_H for the VSi samples, which are believed to be non-transforming, is temperature independent in the range 17–40 K and equal to 2.0 ± 0.2×10^?10m³C^?1. These result are discussed in relation to the effect of the tetragonal distortion on the band structures of these compounds.     

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe_1?x S_x (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe_1?x S_x with x≥0.27 and HgSe are similar in properties.