Hans F. Braun

Ruthenium-based layered cuprates RuSr2LnCu2O8 and RuSr2(Ln1+xCe1−x)Cu2O10 (LnSm, Eu and Gd)

Abstract We synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra. The (idealized) stoichiometries of these compounds are RuSr 2 LnCu 2 O 8 (Ru-1212) and RuSr 2 (Ln 1+ x Ce 1− x ) Cu 2 O 10 (Ru-1222), where Ln is one of the three neighboring lanthanides Sm, Eu and Gd. The analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds. Both 1212 and 1222 compounds are semiconducting when prepared in air and rapidly cooled to room temperature. Metallic behavior and superconductivity can be achieved for various compositions upon appropriate annealing.

Superconductors consisting of CuO2 and RuO2 layers

We have synthesized compounds which may be viewed as stacking of pairs of CuO2 layers alternating with RuO2 layers. There are two structural variants, RuSr2GdCu2O8-δ and RuSr2Gd1+xCe1−xCu2O10−δ, which are denoted shortly 1212 and 1222 structures. In their stacking sequence, they differ solely by the sheets separating the CuO2 bilayers. The synthesis of these phases is accompanied by the formation of small amounts of SrRuO3 impurities, which appear to have a much more devastating effect on the superconducting properties of the 1212 phase as compared to the 1222 phase. We present an investigation of the influence of various synthesis procedures on the appearance of impurity phases and on superconductivity in the 1212 system.

Effect of pressure on superconductivity in the quaternary compounds RENi2B2C (RE=Lu, Y, Tm, Er, Ho)

Abstract The effect of hydrostatic pressure up to 15 kbar on the superconducting transition temperature T c is reported for five compounds in the quaternary system RENi 2 B 2 C (RE=Y, Lu, Tm, Er, Ho). The compounds with RE=Y, Lu, Tm, Er display a modest linear variation of T c with pressure. The value of d T c /d p is ranging from −1.78×10 −5 K/bar up to +1.88×10 −5 K/bar. In contrast, for HoNi 2 B 2 C some kind of interplay between long-range magnetic order and superconductivity occurs, which is strongly modified under applied pressure.

The Depinning Line in Bi-Based High-Tc Superconductors

We have measured the depinning line of vortices in polycrystalline samples of Bi2Sr2CaCu2O8 by d.c.- and a.c.-susceptibility and vibrating reed methods in a wide temperature, magnetic field and frequency range. The main observed features are consistent with the diffusion character of the flux motion and can be understood with the recently updated theory for thermally assisted flux flow. We provide clear evidence that the depinning observed in the vibrating reed has the same origin as in susceptibility measurements.

Occurrence and composition dependence of superconductivity and magnetism in HoNi2B2C

Abstract The dependence of the superconducting and magnetic properties of the re-entrant superconductor HoNi 2 B 2 C on phase composition was investigated with X-ray powder diffractometry, AC susceptibility measurements at ambient and applied pressure, and resistivity measurements. The experimental results based on over 40 polycrystalline samples show that this phase occurs with three different types of behaviour: pure superconductivity. re-entrant behaviour and a non-superconducting state. Starting from a pure superconducting state, with changing phase composition anomalies develop around 5.5 K and 6 K in the AC susceptibility, followed by re-entrant superconductivity. The anomalies are attributed to the manifestation of modulated magnetic states, which are destructive to superconductivity. The application of hydrostatic pressure results in a modification of these anomalies. Further composition change leads to a complete loss of superconductivity. The influence of volume contraction and substitution effects will be discussed.

An insight into what superconducts in polycrystalline boron-doped diamonds based on investigations of microstructure

The discovery of superconductivity in polycrystalline boron-doped diamond (BDD) synthesized under high pressure and high temperatures [Ekimov, et al. (2004) Nature 428:542–545] has raised a number of questions on the origin of the superconducting state. It was suggested that the heavy boron doping of diamond eventually leads to superconductivity. To justify such statements more detailed information on the microstructure of the composite materials and on the exact boron content in the diamond grains is needed. For that we used high-resolution transmission electron microscopy and electron energy loss spectroscopy. For the studied superconducting BDD samples synthesized at high pressures and high temperatures the diamond grain sizes are ≈1–2 μm with a boron content between 0.2 (2) and 0.5 (1) at %. The grains are separated by 10- to 20-nm-thick layers and triangular-shaped pockets of predominantly (at least 95 at %) amorphous boron. These results render superconductivity caused by the heavy boron doping in diamond highly unlikely.

Superconductivity in polycrystalline boron-doped diamond synthesized at 20GPa and 2700K

Bulk sample (∼7.5mm3) of boron-doped diamond containing 2.6(0.6)at.%B was synthesized by means of direct reaction between boron carbide and graphite in multianvil apparatus at 20GPa and 2700K. Electrical resistance of the sample of B-doped polycrystalline diamond was measured in the temperature interval from 10mK to 300K and revealed a transition to superconducting state at 2.4–1.4K. Our results imply that increase of synthesis pressure from 8–9GPa [Ekimov et al., Nature 428, 542 (2004)] to 20GPa does not significantly affect boron content in diamond but decreases the temperature of the transition to superconducting state. We observed sharpening of the temperature interval of the transition to superconducting state in magnetic field that may suggest that superconductivity in our samples could arise from filaments of zero-resistant material.

Magnetization anomalies in the superconducting state of RuSr2GdCu2O8 and the magnetic study of Sr2GdRuO6

Abstract d.c. magnetization measurements performed on a polycrystalline sample of RuSr2GdCu2O8 (Ru-1212) showed distinct peaks of the magnetization upon entering the superconducting state. Since Sr2GdRuO6 (Sr-2116) is the precursor for the preparation of Ru-1212, a detailed investigation of the magnetic properties of Sr-2116 was carried out. Although similarities were observed in the magnetic behavior of Sr-2116 and Ru-1212 in the temperature range of the observed peaks, we can exclude, based on a quantitative comparison, that the anomalies observed for Ru-1212 are due to Sr-2116 impurities.

Optical investigation of the metal-insulator transition in NdBa2Cu3Ox A comparison with YBa2Cu3Ox

Abstract We have performed reflectance measurements on the (001) surfaces of twinned single crystals of NdBa 2 Cu 3 O x . The oxygen concentration x was varied between 6 ≤ x ≤ 7. The results were compared to former investigations on YBa 2 Cu 3 O x . The different dependence of the electronic transport properties on the oxygen content of both compounds can be explained by the different distributions of oxygen amongst the CuO chains.

What do we learn from vibrating high-temperature superconductors?

This work reviews the theory and experiments of vibrating high-temperature superconductors (period 1988-94). In particular we discuss results on the elastic coupling between the flux line lattice and pinning centres, magnetization measurements, flux diffusion, possible phase transitions and matching effects of the flux line lattice. Special emphasis has been devoted to the comparison of these results with results obtained by more conventional methods such as AC susceptibility, low-frequency torsional oscillators, I-V curves and surface impedance measurements.