Thomas P. Papageorgiou

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.

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.

Large carbon-isotope shift of TC in boron-doped diamond

We report the carbon-isotope effect for boron-doped diamond (BDD). Resistive as well as specific-heat measurements reveal a 0.2K shift in the superconducting transition temperature Tc between BDDs containing C13 and C12. This is more than two times larger than could be expected from the mass difference by the use of the simple BCS formula in case the phenomenon is related to electron-phonon mediated superconductivity in BDD.

Superconducting RuSr2GdCu2O8 studied by SQUID magnetometry

For polycrystalline RuSr 2 GdCu 2 O 8 (Ru-1212), distinct peaks have been reported in dc magnetization in the superconducting state of the sample. Sr 2 GdRuO 6 (Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in the same temperature regime. Based on measurements performed on both bulk and powdered samples of Ru-1212 and Sr-2116, we exclude the possibility that the observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities. The effect is related to the superconductivity of Ru-1212, but it is not an intrinsic property of this compound. We provide evidence that the observation of magnetization peaks in the superconducting state of Ru-1212 is due to flux motion generated by the movement of the sample in an inhomogeneous field, during the measurement in the superconducting quantum interference device (SQUID) magnetometer. We propose several tests that help to decide whether the features observed in a SQUID magnetization measurement of Ru-1212 represent a property of the compound or not.

Magnetization of superconducting RuSr2GdCu2O8 and of concurrent phases

Abstract The appearance of impurity phases belonging to the quaternary oxide system SrO–GdO 1.5 –CuO–RuO 2 in Ru-1212 samples is difficult to avoid. While the formation of the ferromagnetic compound SrRuO 3 may be suppressed by a precursor route involving Sr 2 GdRuO 6 , this latter compound shows magnetization anomalies below 30 K. We present evidence that magnetization anomalies observed for some Ru-1212 samples in this temperature range are not due to trace amounts of Sr 2 GdRuO 6 . Rather, the effect appears to be related to the establishment of inter-granular superconductivity and to the vortex dynamics in Ru-1212.

Magnetization, vortex state and specific heat in the superconducting state of RuSr2GdCu2O8

Magnetic-field changes

Critical state models for a granular ferromagnetic superconductor

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Possible pitfalls in SQUID magnetometry of superconducting samples: The case of RuSr2GdCu2O8

0.2 Oe over the scan length in magnetometers that necessitate sample movement are enough to create artifacts in the dc magnetization measurements of the weakly ferromagnetic superconductor RuSr

Dissolution of boron in diamond under high pressures: experimental evidences

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A Phase Diagram Approach to Superconductivity and Magnetism in Rutheno-Cuprates

GdCu