T.J. Goodwin

Superconducting, magnetic, electronic transport, and structural properties of (R1.5Ce0.5)Sr2Cu2NbO10, R=Pr, Nd, Sm, Eu

Abstract The (R 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 system has been characterized for R=Pr, Nd, Sm, and Eu using X-ray diffraction, resistivity, and magnetization measurements. (Nd 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 has been previously reported and is confirmed to be a superconductor with T c near 28 K. (Sm 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 and (Eu 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 are also superconductors with T c s near 28 K. (Pr 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 is not superconducting and is an insulator. In addition, magnetic susceptibility data reveal two magnetic ordering transitions at 17 K and 53 K for (Pr 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 . In contrast, no such transitions are observed for R=Nd, Sm, or Eu down to 2 K. This behavior is analogous to that found for R=Pr or Cm in the RBa 2 Cu 3 O 7− x system and for R=Pr in the RBa 2 Cu 2 NbO 8 system. A proposal is made for the generalization of a correlation between an anomalously high Neel temperature and the lack of superconductivity in RBa 2 Cu 3 O 7- x related structures.

Vortex pinning in polycrystalline Eu1.5Ce0.5Sr2Cu2NbO10 from rotational magnetic measurements

Abstract Rotational magnetization-vector measurements were carried out on a superconducting polycrystalline disk of EuCSCNO. The results at 4.2 K and low fields resemble recent results for YBa 2 Cu 3 O 7−δ (YBCO) and Ba 0.57 K 0.43 BiO 3 (BKBO) in revealing a broad distribution in the strength of the vortex pinning torques. This resemblance extends to higher fields, where it was found that the average pinning torque per vortex ( τ p ), which is directly related to the critical current density, decreases steadily with increasing field in a manner consistent with vortex bundling. Quantitatively, however, the values of τ p for EuCSCNO are smaller by an order of magnitude than those for YBCO but are only moderately smaller than those for BKBO. This comparison supports the notion that the vortex pinning may derive in part from deviations from oxygen stoichiometry, which are normally present in YBCO but are essentially absent in EuCSCNO and BKBO. Moreover, as in YBCO and BKBO, the τ p in EuCSCNO at fixed field is found to diminish rapidly as the temperature rises towards T c .

Magnetic and transport properties of HgBa2CuO4+δ

Abstract The upper critical field and the critical current density of a ceramic sample of superconducting HgBa 2 CuO 4+δ have been estimated from resistivity measurements and magnetic hysteresis loop data. The resistivity measurements were done over a temperature range from 10 to 350 K and in a magnetic field up to 5 T. The results showed that the resistive transition has a strong field dependence. A small coherence length of 2.7 nm was estimated in this HgBa 2 CuO 4+δ compound. Furthermore the inductive critical current density, deduced from magnetic hysteresis loop d3ta taken in fields up to 1 kOe, is estimated as 16 × 10 5 A/cm 2 at T = 5 K. This value is relatively low in comparison with the well-studied oxide superconductor YBa 2 Cu 3 O 7−δ .

Superconductivity and magnetism in niobium doped YBa 2 Cu 3 O 7 related high T C ceramics

Magnetic characterization has been performed on the members of the cuprateniobate RBa2Cu2NbO8 (R = Pr, Nd, and La) series and R1.5Ce0.5Sr2Cu2NbO10 (R = Pr, Eu, Nd, and Sm) series. The PrBCNO samples show a signature in the magnetization of a magnetic ordering at 12K. The PrCSCNO sample is nonsuperconducting and shows two distinct orderings at 17K and 53K. No such magnetic phase transition is observed down to 2K in the Nd and La based RBCNO materials or the Nd, Sm, and Eu based RCSCNO materials. Measurements of the lower critical field curve, dc irreversibility line, and critical curent densities are reported for each of the superconducting NdCSCNO, SmCSCNO, and EuCSCNO compounds.

The specific heat of (Y1−xPrx)Ba2Cu3O7: Effects of Pr3+ singlet-ground state ordering and PrO hybridization

Abstract Specific-heat measurements suggest that the Pr valence in (Y 1−x Pr x )Ba 2 Cu 3 O 7 is +3, and that Pr hybridization with the CuO 2 planes results in an entropy excess over that expected from the Pr 3+ crystal-field states.

Relating structural properties and oxygen content to the electronic and magnetic states of (Eu1.5−xPrxCe0.5)Sr2Cu2NbO10-δ

Abstract Structural parameters, phase purity, and oxygen stoichiometry for the (Eu 1.5−x Pr x Ce 0.5 )Sr 2 Cu 2 NbO 10-δ system are studied using X-ray diffraction, Rietveld refinement, and thermogravimetric analysis. Relationships between these properties and the suppression of T c and unusual magnetism in these materials are examined.

Structural characterization and magnetic ordering in (La1−xPrx)Ba2Cu2NbO8

Abstract The perovskite-based system (La 1−x Pr x )Ba 2 Cu 2 NbO 8 has been synthesized for 0≤x≤1 and characterized using x-ray diffraction and magnetization measurements. Lattice parameters determined by Rietveld refinement are given as a function of composition. Antiferromagnetic ordering of the Pr sublattice is observed for x≥0.5, the Neel temperature (T N ) is observed to increase linearly with Pr content, and the previously reported T N of 12 K for PrBa 2 Cu 2 NbO 8 is confirmed. These results are interpreted in terms of established Pr magnetism in PrBa 2 Cu 2 NbO 8 and similar magnetism in other well-documented high-T c cuprate structures.

Absence of superconductivity in (Pr1.5Ce0.5)Sr2Cu2NbO10—A general correlation with magnetic ordering☆

Abstract (Pr 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 is found to be an insulating analog to (Nd 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 , (Sm 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 and (Eu 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 which have superconducting transition temperatures near 28 K. (Pr 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 is also observed to display two magnetic ordering transitions, near 15 K and 50 K, which are not observed for the Nd, Sm or Eu based materials. This compound is a new example of the absence of superconductivity in a PrBa 2 Cu 3 O 7 related structure which exhibits an unusually high magnetic ordering temperature relative to the other members of the rare earth series. An argument is made for a general correlation between the absence of superconductivity and the presence of magnetic ordering in these structures. The observation of this correlation in the (R 1.5 Ce 0.5 )Sr 2 Cu 2 NbO 10 system is compared to similar observations in RBa 2 Cu 3 O 7 and RBa 2 Cu 2 NbO 8 . The physical basis for the general nature of this correlation is discussed in terms of f electron hybridization and the unusual crystal field splitting observed for the Pr ion in the YBCO structure.