M. Vybornov

Superconductivity in Y-Ni-B base compounds

Carbon introduced in Y-Ni-B ternary alloy material was shown to stabilise aquaternary compound YNi2B2C which is also responsible for the appearance of superconductivity in Y-Ni-B alloys YNi4−xB1+x. Rietveld refinement of a nearly single-phase material of YNi2B2C confirmed isotypism with the filled ThCr2Si2-type (zB=0.353(1)). Magnetic and specific heat measurements indicate type-II superconductivity in YNi2B2C with a Tc-onset of 15.2 K and a normalised specific heat discontinuity ΔC/γTc=1.57. We present a first estimate of the upper critical field μ0Hc2(0)=3.7 T, the coherence length ξGL=9.4 nm and the Ginzburg-Landau parameter κ=10.5 of this new compound.

Superconductivity and magnetism in quaternary borocarbides

Abstract We present a status report of the new rare earth transition metal borocarbides and discuss in particular the magnetic properties, results of heat capacity measurements in connection with pressure experiments together with band structure calculations and properties of the superconducting state.

Structural analysis of superconducting NdBa1.5Sr0.5Cu3o7−δ

Two samples of NdBa1.5Sr0.5Cu3O7−δ with δ<0.1, prepared by the solid state reaction, were subjected to two different annealing conditions. Based on Rietveld refinement of powder neutron diffraction (PND) patterns, the occurrence of a single phase with tetragonal symmetry was found for the sample, which was annealed at high temperature in oxygen atmosphere (T-sample). The other sample, which was heated at high temperature in argon atmosphere followed by low temperature annealing in oxygen atmosphere, however, showed coexistence of both orthorhombic and tetragonal symmetries (O/T-sample). Refinement of powder X-ray diffraction data of the T-sample indicated cation disorder between Sr and Nd ions at the (12, 12, z) and the rare earth sites, respectively. The relation to oxygen disorder is discussed. Superconducting transition temperatures, obtained by electrical resistance measurements, were Tc (R = 0) = 83 K and Tc(R = 0) = 71 K for the O/T-and T-sample, respectively. The lower Tc for the T-sample is correlated with an elongated bond distance between the Cu(2) ion and apical oxygen.

Magnetoelastic interactions in RE2CuO4 system

Abstract We present thermal expansion data for R 2 CuO 4 (R = Pr, Nd, Sm, Gd, Eu) compounds and specific heat measurements of Sm and Eu compounds in the temperature range from liquid helium to room temperature. In the case of Pr, Nd, Gd, and Eu compounds the coefficient of thermal expansion (α) increases monotonously with rising temperature whereas α of the Sm compound shows a pronounced minimum at about 60 K. These findings are interpreted in terms of the crystal field induced electric quadrupole moment of the 4 f electrons.

The effect of chemical pressure upon the magnetism of (Ba1−xSrx)2PrNbCu2O8

Abstract We present specific-heat and susceptibility measurements and a structural analysis of (Ba1−xSrx)2PrNbCu2O8 in order to explore the effect of the chemical pressure on the magnetic properties of the Pr sublattice. These systems are similar to PrBa2Cu3O7−δ for which the Cu-O chain layers have been fully replaced by layers of Nb-O octahedra. The overall volume compression of - 5.5% across the series is found to be rather inhomogeneous yielding an anomalous expansion of the average Pr-O bond lenght of 3.8% which is accompanied by a reduction of TN from 11.6 K to 2.3 K. The variation of the magnetic properties across the solid solution is discussed in terms of hybridization of the Pr4f states with the Cu-O valence bands.

Magnetic order in Sm2−xGdxCuO4

Abstract We performed specific heat and susceptibility measurements of the pseudoternary system Sm2−xGdxCuO4 in order to establish the magnetic phase diagram of the Rare Earth sublattice.

Specific heat of RNi2B2C(R = Y, Er) and YNi4B

Abstract Comparative heat capacity measurements of YNi 4 B and YNi 2 B 2 C show that the incorporation of carbon in the latter causes a significant lattice stiffening. From the magnetic and specific heat data of ErNi 2 B 2 C we deduce that superconductivity and antiferromagnetism of the Er sublattice coexists below T N .

Magnetism and superconductivity in Sr2RyCe0.5MCu2O10−δ (R = Pr, Nd, Sm, Eu, Gd; M = Nb, Ta)

Abstract We present magnetic and specific heat measurements of the series Sr 2 R y Ce 0.5 M Cu 2 O 10−δ ( R = Pr, Nd, Sm, Eu, Gd; M = Nb, Ta). Superconductivity occurs for y > 0.8 ( R = Nd, Sm, Eu), however, for Gd only in combination with M = Nb, while the corresponding Pr compounds are found to be nonsuperconducting.

Magnetism, specific heat and structural chemistry of (Ba1−xSrx)2PrNbCu2O8

Abstract From magnetic and specific heat measurements together with a structural analysis of this series we deduce that the chemical pressure due to Ba/Sr substitution is rather inhomogeneous and is accompanied with the reduction of the Pr antiferromagnetism.