Some recent results on high-temperature superconductors: The Dutch scene

For the high-T _c superconductor La_1.85Sr_0.15CuO₄, we present results for the low-field magnetization and the lower and upper critical fieldsB _c1 (T) andB _c2(T). ForB _c1 (0) we find a value of about 20 mT. Extrapolating our resistive data for the transition midpoint taken in fields up to 12 T, we deduce a value of about 50 T forB _c2 (0). TheB _c2 values obtained by inductive measurements are significantly lower. We explain this by a largeB _c2 anisotropy due to the layered structure of these compounds. Losses in the Meissner state observed in magnetization and susceptibility measurements disappear after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample.     

Magnetic and transport properties of UBi2 and USb2 single crystals

The thermoelectric power, S(T), of USb₂ and UBi₂, which are tetragonal, uniaxial antiferromagnets below T _N?=?202?K and 180.8?K, respectively, have been examined between 0.4?K and 300?K. The values of S(T), up to now known above 70?K for USb₂ and unknown for UBi₂, are positive along the a-axis for both compounds in the whole examined temperature range. The S(T) data for the c-axis (the easy magnetization axis) are positive near room temperature for USb₂ and UBi₂ but becomes negative below 120?K and 170?K, respectively, with two very deep minima in S(T) dependence for USb₂. In the latter compound the Fermi surface, known from literature, is composed of the only cylindrical sheets that are slightly corrugated and parallel to the c-axis. UBi₂, the Fermi surface of which is composed of one spherical and two cylindrical sheets, shows corresponding minima although less pronounced than those in USb₂. Having at disposal the highest purity single crystals in comparison to those for which the resistivity, ρ(T), has been reported in literature, the ρ(T) anisotropy was re-examined for these two systems. Magnon and phonon contributions to their total electrical resistivity have been determined and the critical fluctuation behaviour of the resistivity near T _N for both dipnictides has been analysed. Although the magnetic susceptibilities of UBi₂ and USb₂ reveal a similarity, their transport properties are significantly different due to the difference in the Fermi surface topology.     

Influence of disorder on superconductivity in non-magnetic rare-earth nickel borocarbides

The effect of substitutional disorder on the superconducting properties of YNi₂B₂C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni₂B₂C and Y(Ni, Pt)₂B₂C compounds, the upper critical field H _c2(T) and the specific heat c _p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as T _c, the upper critical field H _c2(0) at T=0 and a characteristic positive curvature of H _c2(T) observed for these compounds near T _c. The H _c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni_1−x , Pt _x )₂B₂C samples for x=0.1.     

Microscopic theory of a strongly correlated two-dimensional electron gas

The rearrangement of the Fermi surface in a diluted two-dimensional electron gas beyond the topological quantum critical point has been examined within an approach based on the Landau theory of Fermi liquid and a nonperturbative functional method. The possibility of a transition of the first order in the coupling constant at zero temperature between the states with a three-sheet Fermi surface and a transition of the first order in temperature between these states at a fixed coupling constant has been shown. It has also been shown that a topological crossover, which is associated with the joining of two sheets of the Fermi surface and is characterized by the maxima of the density of states N(T) and ratio C(T)/T of the specific heat to the temperature, occurs at a very low temperature T _⋄ determined by the structure of a state with the three-sheet Fermi surface. A momentum region where the distribution n(p, T) depends slightly on the temperature, which is manifested in the maximum of the specific heat C(T) near T _*, appears through a crossover at temperatures T ∼ T _* > T _⋄. It has been shown that the flattening of the single-particle spectrum of the strongly correlated two-dimensional electron gas results in the crossover from the Fermi liquid behavior to a non-Fermi liquid one with the density of states N(T) ∝ T ^−α with the exponent α }~ 2/3.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

Viscosity and thermal conductivity from non-conformal potentials. I Theory and application to noble gases

The approximate non-conformal (ANC) theory provides effective spherical potentials for more than 40 pure real gases and predicts many of their binary mixtures in the case of the second volumetric virial coefficients B(T). Each substance is characterized by only three parameters; besides the usual minimum energy ε and its corresponding position r _m, a softness parameter s is proposed to take account of the form of a particular potential. In this work the ANC theory is extended to take account of viscosity η(T) and thermal conductivity λ(T) coefficients in dilute gases. Very simple expressions are derived to calculate the appropriate collision integral Ω as a function of temperature and in terms of the parameters ε, r _m and s, using a reference collision integral Ω₀(T). The theory is successfully applied to the noble gases Ne, Ar, Kr and Xe, with deviations from experiment for η(T) and λ(T) within experimental uncertainties. In the case of Ne, the ANC potential obtained here reproduces simultaneously η(T) and λ(T) as well as B(T) within experimental uncertainty, using a single set of parameters. A prediction for the radon gas is proposed.     

Superconductivity of SrTiO

Superconducting SrTiO _{3 - δ} was obtained by annealing single crystalline SrTiO₃ samples in ultra high vacuum. An analysis of the V ( I ) characteristics revealed very small critical currents I _c which can be traced back to an unavoidable doping inhomogeneity. R ( T ) curves were measured for a range of magnetic fields B at I ≪ I _c , thereby probing only the sample regions with the highest doping level. The resulting curves B _c2 ( T ) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO _{3 - δ} can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB₂. Received 4 December 2002 / Received in final form 10 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: jourdan@uni-mainz.de     

Magnetic flux distribution in the bulk of the pure type-II superconductor niobium measured with positive muons

The spin precession of positive muons in ultra-pure Nb single crystals of high perfection, cooled down in transverse magnetic fields 0.68B _c2≤B _appl≤0.84B _c2 (B _c2=upper critical field) from temperatures well above the superconducting transition temperatureT _c=9.25 K, has been investigated in the temperature range 2.6 K≤T≤8.0 K. The experiments confirm the periodic fieldB(r) of triangular flux-line lattices as calculated from numerical solutions of the microscopic BCS-Gor'kov theory. The observed broadening of the van Hove singularities in the field distribution is discussed in terms of the combined effects of muon diffusion and random perturbations of the flux-line lattice.     

Probing the field-induced variation of the chemical potential in Bi2Sr2CaCu2Oy via magneto-thermopower measurements

Approximating the shape of the magneto-thermoelectric power (TEP) ΔS(T,H) measured in Bi₂Sr₂CaCu₂O_y by an asymmetric linear triangle of the form ΔS(T,H)≃S _p (H)±B ^±(H)(T _c −T) with positive B ⁻(H) and B ⁺(H) defined below and above T _c , we observe that B ⁺(H) ≃2B ⁻(H). To account for this asymmetry, we explicitly introduce the field-dependent chemical potential μ(H) of holes into the Ginzburg-Landau theory and calculate both an average ΔS _av(T,H) and fluctuation contribution ΔS _fl(T,H) to the total magneto-TEP ΔS(T,H). As a result, we find a rather simple relationship between the field-induced variation of the chemical potential in this material and the above-mentioned magneto-TEP data around T _c , viz. Δ μ(H)∝S _p (H). Zh. éksp. Teor. Fiz. 116, 257–262 (July 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Percolation magnetoresistance in ferromagnetic superconductors

It is shown that the magnetoresistance (as a function of magnetic field H) in polycrystalline magnetic superconductors has the percolation character which is the consequence of the anisotropy of magnetic susceptibility. The magnetoresistance R_m(H), and the upper critical field H_c2(T) of ErRh₄B₄ are evaluated and compared with the experimental data.     

Thermal conductivity of USb2 and UBi2 single crystals

The thermal conductivity (κ) of single crystals of tetragonal uniaxial antiferromagnets USb₂ (T _N = 202 K) and UBi₂ (T _N = 180.8 K) has been measured along the a-axis (κ_a ) over the temperature range from 0.5 to 300 K and along the c-axis (κ_c ) from 0.5 to 70 K. The as-grown samples have residual resistivity ratio (RRR) values of about 500–600 and 100–150 for UBi₂ and USb₂, respectively. The anisotropy of the thermal conductivity (κ_a (T)/κ_c (T) ～ 5) and the low-T Lorenz ratios are discussed in relation to Fermi surface topology for both compounds.     

Anisotropy in the compton profile of copper

Directional Compton profiles of high statistical accuracy are measured by means of a 412 keV gamma-ray Compton spectrometer. The experimental anisotropies are in very satisfactory agreement with earlier measurements and there is good qualitative agreement between the experimental data and a recent band structure calculation. Quantitatively, however, the experimental anisotropy is significantly smaller than predicted by theory.A simple model calculation based on the Seitz approximation, in which higher order Fermi surface volumes all of spherical shape are taken into account in obtaining the momentum density, demonstrates that the major contribution to the Compton profile anisotropy in copper is due to the hybridization which has mixedd-electrons and nearly-free electrons in the top band. Any fine structure in the theoretical anisotropy due to the detailed shape of the Fermi surface cannot be resolved in the present experiment.The potential of analysing the electronic structure of transition metals in terms ofB(r), the Fourier transform of the momentum density, is discussed in detail. The major contribution to the anisotropy arises from localised peaks inB(r) centered on the sites of the translational lattice. Within the Seitz approximation it could be shown that these secondary maxima are caused by the presence of localisedd-electrons in the highest partly occupied band. In conclusion we anticipate that a proper treatment of electron correlation would produce a marked quantitative improvement in the agreement between the present experimental data and the Compton profiles obtained from current band structure calculations.     

Ullah-Dorsey Scaling of Fluctuation Conductivity Near the Superconducting Transition in (LaSe)1.14(NbSe2)

Magnetotransport measurements with the quasi two-dimensional misfit-layer compound (LaSe)_1.14(NbSe₂) with critical temperature T _c = 1.23 K are presented. The temperature dependence of the upper critical magnetic field H _c2 has been determined from an analysis of magnetoresistance data through the scaling procedure obtained theroretically by Ullah and Dorsey for the fluctuation conductivity. We show that in contrast to a direct determination of H _c2(T) with a positive curvature the temperature dependence of H _c2 obtained via the scaling procedure reveals a negative curvature in agreement with the Werthammer-Helfand-Hohenberg theory for conventional type-II superconductors.     

Comment on Vub from exclusive semileptonic B and D decays

The prospects for determining |V_ub| from exclusive B semileptonic decay are discussed. The double ratio of form factors (f^(B→ρ)/f^(B→K*))/(f^(D→ρ)/f^(D→K*)) is calculated using chiral perturbation theory. Its deviation from unity due to contributions that are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from , and can lead to a model independent determination of |V_ub| with an uncertainty from theory of about 10%.     

Quantum magnets with large single-ion easy-plane anisotropy in magnetic field

We propose a theory describing low-temperature properties of magnets with integer spin and large single-ion easy-plane anisotropy D in magnetic field H directed parallel to the hard axis. Considering the exchange interaction between spins as a perturbation and using the bosonic spin representation proposed in our recent paper [1] we find thermal corrections to the elementary excitation spectrum, magnetization and specific heat in the vicinity of the quantum critical point (QCP) H = H _c1(0) ∼ D in the first nonvanishing orders of the perturbation theory. An expression is found for the boundary of the paramagnetic phase H _c1(T) in the H-T plane. The effective interaction between bosons is derived near the QCP. The proposed theory describes well experimental data obtained in NiCl₂-4SC(NH₂)₂ (DTN).     

On the susceptibility exponent of random anisotropy magnets

The temperature dependence of the non-linear susceptibility ≈₂(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature T_f within the framework of the correlated molecular field theory. For the effective susceptibility exponent λ_s(T) = (T?T_f)≈₂d^-1_≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data.     

Lokale Messung von Driftgeschwindigkeit und Temperatur der Ionen einer Argon-Niederdruckentladung mit resonanter Laserstreuung

The radial ion drift velocity and temperature in an argon low-pressure discharge have been measured by a resonant laser scattering method with high spatial resolution. The detection limit for the drift velocities was 2 · 10⁴ cm · s^?1. In the vicinity of the discharge axis the drift velocity grows linear with the distance from the axis. The measurements have shown the existence of a radial dependence of the ion temperature T¹(r). Comparisons of axial ion temperature T_? with T_‖(r = 0) are an experimental demonstration of the anisotropy of the ion velocity distribution in this kind of discharges.     

Magnetic properties of CdSb doped with Ni

Magnetic properties of the group II–V semiconductor CdSb single crystals doped with Ni (2 at%) are investigated. Deviation of the zero-field-cooled susceptibility, χ_ZFC, from the field-cooled susceptibility is observed below 300 K, along with a broad maximum of χ_ZFC (T) at T_b in fields below the anisotropy field B_K∼4 kG. T_b(B) obeys the law [T_b(B)/T_b(0)]^1/2=1–B/B_K with T_b(0)∼100 K. The magnetization exhibits saturation above ∼20–30 kG, a weak temperature dependence and anisotropy of the saturation value M_s. The coercive field is much smaller then B_K and displays anisotropy inverted with respect to that of M_s. Such magnetic behavior is expected for spheroidal Ni-rich Ni_1−xSb_x nanoparticles with high aspect ratio, broad distribution of the sizes and with orientations of the major axis distributed around a preferred direction.     

Two-band nature of upper critical fields in MgB2 thin films investigated by 37 T pulsed magnet

We present experimental and calculated phase diagrams of the upper critical field H_c2 and the anisotropy parameter γ of various MgB₂ thin films prepared by the molecular beam epitaxy (MBE) and the multiple-targets sputtering system. Experimental data of the H_c2(T) and γ are analyzed by fitting the Gurevich theory. We obtained the result that for these MgB₂ films the H_c2(T) is explained as the case of D_π/D_σ>1, which means the films are classified to the cleaner π band than the σ band. We discuss temperature dependence of the transition width obtained from the magnetoresistance measurements.