K. Zhang

Solubility and superconductivity in RE(Ba2-xREx)Cu3O7+δ systems (RE=Nd, Sm, Eu, Gd, Dy)

Solid solutions of RE(Ba2-xREx)Cu3O7+ delta (RE=Nd, Sm, Eu, Gd, Dy) for x=0 to x=0.5 have been investigated. X-ray and resistivity measurements show that there exists a solid solution region, through which the structure changes from orthorhombic to tetragonal and the superconducting properties are depressed. The solubility limits depend strongly on the size of the rare-earth ion, with the smallest (Dy) showing no appreciable solubility. The superconducting transition temperature versus x for all of the rare-earth-ion substitutions falls on a universal curve, indicating that the Ba sites are extremely ionic and magnetically isolated.

Upper critical fields and high superconducting transition temperatures of La1.85Sr0.15CuO4 and La1.85Ba0.15CuO4

The superconducting transition temperature Tc of La1.85Sr0.15CuO4 is measured to be 34.7 K (midpoint) with a transition width (10–90%) of 2.6 K. The critical field slope at Tc is 2.13 T/K. Using the Werthamer, Helfand, and Hohenberg expression in the dirty limit we estimate the upper critical field Hc2(0) to be at least 50 T for this system. The measured transition width broadens initially with increasing field to 4 K, then saturates at this value for fields above 2 T. The critical field slope for the lower Tc material La1.85Ba0.15CuO4 is measured to be 1.78 T/K, which yields an Hc2(0) of at least 36 T.

Nucleation and growth kinetics of the tetragonal to orthorhombic transition in YBa2Cu3O7

The nucleation and growth geometry of the tetragonal to orthorhombic transformation in YBa2Cu3Ox is determined to be grain‐boundary nucleation after saturation. In this geometry the orthorhombic phase rapidly nucleates along the grain boundaries, then propagates into the grains as a planar front governed by the much slower volume diffusion of oxygen. This conclusion is reached by fitting the volume fraction of the superconducting, and hence orthorhombic, phase as a function of annealing time at 400 °C to the Avrami relation. The volume fraction of the superconducting phase is obtained using magnetization versus field data.

Superconducting critical field and normal state susceptibility of rare-earth-doped YBa2Cu3O7 − δ

Abstract We have measured resistivity, critical field, and normal state susceptibility for samples of (RE)Ba2Cu3O7 − δ where RE ≡ Sm, Gd, Eu, Dy, Ho, Er. We see little effect on Tc for the RE substitutions compared with the pure compound. The normal state susceptibility follows a Curie law for all samples (except samarium and europium) yielding the approximate free ion moment. The critical field slopes near Tc lie between approximately 2 T K−1 and approximately 3 T K−1 and are strongly dependent on the width of the superconducting transition in zero field, indicating that sample quality has a dramatic effect on the initial critical field slope near Tc.

Magnetic ordering and crystal field effects in REBa2Cu3O7−x (RE=Gd, Dy, Ho, Er) (abstract)

Heat capacity, resistivity and magnetic susceptibility data have been used to study electronic and magnetic properties of REBa2Cu3O7−x compounds. Neutron diffraction on the Ho compound shows identical crystal structure to YBa2Cu3O6.8. All the compounds are superconducting at Tc=(93±2) K. Magnetic transitions are observed in the heat capacity data at Tm=2.2, 0.95, 0.17, and 0.59 K for RE=Gd, Dy, Ho, and Er, respectively, and the magnetically ordered state is found to coexist with superconductivity. Estimates have been obtained for the magnetic moment in the crystal field ground state and the energy splitting of the first excited state crystal field level.