K. Winzer

UPPER CRITICAL FIELD PECULIARITIES OF SUPERCONDUCTING YNI2B2C AND LUNI2B2C

We present new upper critical field Hc2(T) data in a broad temperature region from 0.3K to Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates. The absolute values for all T, in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors.

(BEDO-TTF)2 ReO4·(H2O): A new organic superconductor

The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The resistivity and thermopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H2O) but the onset for superconductivity observed in the ac-susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50 mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.

Superconducting rare earth transition metal borocarbides

Abstract We present an overview of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds, as well as of theoretical calculations with possible relevance to the mechanism of superconductivity. The interplay of superconductivity and magnetism for compounds with pure and mixed rare earth components is considered. We suggest that the incommensurate magnetic structure modulated along the a -axis is responsible for the pair breaking in Ho x R 1− x Ni 2 B 2 C; R=Y, Lu samples. The effect of doping (Cu, Co) at the transition metal site is considered experimentally and theoretically. The possible role of correlation effects due to the presence of the transition metal component in determining the electronic structure is discussed comparing the band structure calculation results with various electronic spectroscopies as well as de Haas–van Alphen data. Important thermodynamic properties of these systems are analyzed within multiband Eliashberg theory with special emphasis on the upper critical field H c2 ( T ) and the specific heat. In particular, the unusual positive curvature of H c2 ( T ) near T c observed for high-quality single crystals, polycrystalline samples of YNi 2 B 2 C, LuNi 2 B 2 C as well as to a somewhat reduced extent also for the mixed system Y 1− x Lu x Ni 2 B 2 C is explained microscopically. It is shown that in these well-defined samples the clean limit of type II superconductors is achieved. The values of H c2 ( T ) and of its positive curvature near T c (as determined both resistively and from magnetization as well as from specific heat measurements is an intrinsic quantity generic for such samples) decrease with growing impurity content. Both quantities thus provide a direct measure of the sample quality.

Observation of the Flux Line Lock-In Transition in Layered Superconductors

We have investigated the vortex structure in three different representatives of highly anisotropic layered superconductors: Tl2Ba2Ca2Cu3O10, Bi2Sr2CaCu2O8 and κ-(BEDT-TTF)2-Cu[N(CN)2]Br. Based on the equilibrium part of the magnetic torque we identify the vortex lock-in transition, i.e. the perfect alignment of the magnetic induction parallel to the superconducting planes. This Meissner state for pancake vortices is quantitatively confirmed by ideal screening of the perpendicular field component.

de Haas-van Alphen effect in the superconducting state of YNi2B2C

To examine the Fermi surfaces of the recently discovered quaternery compounds RENi2B2C measurements of the de Haas-van Alphen (dHvA) effect were made on YNi2B2C single crystals in magnetic fields up to 12 T. ForB‖c we observe two dHvA frequenciesF1=0.499 kT andF2=6.933 kT which corresponds to approximately 1,5% and 21% of the Brillouin zone cross section area. Both frequencies could be observed deep in the vortex state of the type-II superconductor, the lower dHvA frequency down to 2 T corresponding to roughly 1/5 of the upper critical fieldBc2 which was found to be 10.6 T in resistivity measurements. The field dependent quasiparticle damping in the superconducting state is very weak, since the amplitude of the dHvA oscillations seems to be unaffected by the phase transition atBc2.

Josephson effect and single-particle tunneling in YBa2Cu3O7−x and YbBa2Cu3O7−x single-crystal break junctions

Abstract We have studied the current-voltage characteristics of break junctions in single crystals of YBa 2 Cu 3 O 7− x and YbBa 2 Cu 3 O 7− x at helium temperature. For a YBa 2 Cu 3 O 7− x Josephson junction Fiske resonances in low magnetic fields have been observed in the I ( V ) characteristics, thus proving the tunneling nature of the contact. The reduced velocity of the electromagnetic wave in the contact area has been calculated to be c / c 0 ∼ 0.018 ( c is the Swihart velocity). The ratio of the barrier thickness d 1 to the relative dielectric constant of the barrier ϵ was found to be d I / ϵ ∼ 0.09 nm. In the quasiparticle tunneling regime, the YbBa 2 Cu 3 O 7− x break junctions showed in some cases I ( V ) characteristics with a relatively small excess current at “sub-gap” voltages and a steep rise of the current at the “gap” voltage V g = 2 Δ / e . The “BCS-like” form of the observed I ( V ) characteristics seems to be in favor of s wave superconductivity.

Pressure dependence of Tc for single crystal YBa2Cu3O7−x up to 10 GPa

Employing a novel pressure technique we investigate the pressure dependence of Tc of single crystals of YBa2Cu3O7−x under nearly hydrostatic pressure. We observe for three different crystals that Tc decreases under high pressure in marked contrast to what has been reported for ceramic samples so far.

Low-Temperature Specific Heat and Susceptibility of the Superheavy Fermion Compound CeCu4Ga

For the recently discovered heavy-fermion compound CeCu4Ga specific heat and susceptibility measurements are presented for temperatures from 100 mK to 2 K and from 30 mK to 4 K, respectively. The ratio γ = c/T of the specific heat shows a maximum at 0.9 K with the giant value of 3.15 J/mole K2. From γ(0) = 1.83 J/mole K2 we estimate an effective mass m* ≈ 380m0 which is by far the heaviest mass of all presently known highly correlated electron systems.

Two-T c and three-T c behavior of the Kondo superconductor (La, Y)Ce

The dependence of the superconducting transition temperatureTc on Ce impurity concentration is reported for the (La, Y)Ce system. Susceptibility and resistivity measurements down to 8 mK show three transition temperatures for nearly critical Ce concentrations in alloys with 15–22.5 at %Y, while for 25 at %Y only two transition temperatures are observed. The finite tail of the superconducting-normal phase boundary, which is responsible for the third transition, occurs at small values ofTK/Tc0 and vanishes for larger values, in contradiction to recent theoretical predictions of the pair-breaking in Kondo superconductors.

Fermi surface studies of the borocarbide superconductor YNi2B2C

The Fermi surface of the nonmagnetic borocarbide superconductor YNi2B2C was studied by the de Haas-van Alphen (dHvA) effect. Five branches of dH-vA frequencies are observed in the normal state for B ∥ c and the angular dependence was investigated with magnetic fields in the (110) plane. Together with the effective cyclotron masses, mc, the result is discussed in the context of electronic band-structure calculations.