The Effects of Rare EARTH(3+) Ions and Oxygen Deficiency in High Critical Temperature Rare Earth BARIUM(2) COPPER(3) OXYGEN(7-DELTA) Superconductors

The effects of R^{3+} ions and oxygen deficiency in RBa_2 Cu_3O_{7-delta} compounds (R = Y or a rare earth) were investigated by means of X-ray diffraction, electrical resistivity, magnetic susceptibility, and low temperature specific heat measurements. The series of RBa_2 Cu _3O_{7-delta} compounds exhibited superconductivity with superconducting transition temperatures T_{c} ~ 90 K, except for the compounds with R = Ce, Pr, Pm, and Tb. Lattice parameters of RBa_2 Cu_3O_{7-delta} decrease monotonically with the ionic size of R ^{3+} ions. There are no substantial changes due to the magnetic moments of R ^{3+} ions but detectable and systematic changes due to ionic size of R^ {3+} ions in the T_{ c} of the superconducting compounds. X-ray diffraction analysis of magnetically aligned specimens of RBa_2 Cu_3O _{7-delta} indicated that the c-axis of the crystallites aligns parallel to the aligning field H_{A}, for R = Y, Nd, Sm, Dy, and Ho and perpendicular to H _{A} for R = Eu, Er, Tm, and Yb. Measurements of the magnetic hysteresis DeltaM at 4.3 K are consistent with the X -ray evidence, except for R = Ho. The compounds with R = Y, Eu, Ho, Tm, and Yb exhibit no specific heat feature associated with magnetic ordering for temperature above 0.5 K. However, electronic Schottky specific heat anomalies due to the CEF were observed in RBa_2 Cu_3O_{7 -delta} compounds with R = Ho, Tm, and Yb. Features in the specific heat associated with antiferromagnetic ordering of the R^{3+} ions coexisting with the superconductivity were observed for superconducting RBa_2 Cu_3O _{7-delta} compounds with R = Nd, Sm, Gd, Dy, and Er. Specific heat anomalies due to antiferromagnetic ordering of R^{3+} ions in the T_{c}~ 90 K superconducting RBa_2 Cu_3O_{7-delta} (delta ~ 0.1) compounds with R = Nd, Sm, Dy, and Er can be well described by an anisotropic two-dimensional Ising model. The magnetic specific heat anomalies and magnetic ordering temperatures of these nonsuperconducting RBa_2 Cu_3O_{7 -delta}(delta _sp{~ }{>} 0.5) compounds are markedly different than those of the superconducting compounds. The rate of decrease of T_{ c} with increasing oxygen vacancy concentration delta for NdBa _2 Cu_3O_{7-delta } is more rapid than that of YBa_2 Cu_3O_{7-delta} , and the critical oxygen concentration delta_{c} for NdBa _2 Cu_3O_{7- delta}, at which superconductivity is suppressed, moves to a lower value with a less prominent lower plateau than that of YBa_2 Cu_3O_{7-delta}. The low-temperature specific heat measurements reveal that the magnetic ordering of the Nd^{3+ } ions in NdBa_2 Cu _3O_{7-delta}, which occurs at ~0.5 K for 0 _sp{~}{<} delta _sp{~}{<} 0.1, is suppressed in the range 0.2 _sp {~}{<} delta _sp{~}{<} 0.4 , and reappears as delta is increased for delta _sp{ ~}{>} 0.5 where NdBa _2 Cu_3O_{7- delta} loses its superconductivity. The specific heat anomaly due to the antiferromagnetic ordering of Dy^{3+} ions in DyBa _2 Cu_3O_{7- delta} is also changed by the oxygen deficiency.