Search

Experiments on high TcRBa2Cu3O7 − δ (R = rare earth; δ ≈ 0.1) compounds have revealed a variety of interesting effects that involve the 4f electrons of the R ions. Schottky anomalies due to the partial lifting of the degeneracy of the Hund's rules ground state multiplets of the R ions by the crystalline electric field (CEF) are found in the low temperature specific heat. With some exceptions, the anisotropy of the room temperature paramagnetic susceptibility correlates with the sign of the second order Stevens factor of the CEF hamiltonian. Specific heat anomalies due to the antiferromagnetic ordering of the R3+ ions in RBa2Cu3O7 − σ compounds with R  Nd, Sm, Dy and Er can be described well by the anisotropic twodimensional Ising model with an exchange interaction parameter ratio that ranges from approximately 50 for neodymium to approximately 4 for dysprosium. The magnetic ordering temperatures and shapes of the magnetic specific heat anomalies change markedly when the superconductivity of the compounds is quenched by increasing the oxygen vacancy concentration to δ ≳ 0.5, indicating that RKKY and/or superexchange interactions are involved in the magnetic ordering of the R3+ ions in these materials. The anomalous pressure dependence of Tc and the normal state electrical resistivity in the (Y1 − xPrxx)Ba2Cu3O7 − δ system suggest that the praseodymium 4f states are hybridized with valence band states, which may be responsible for the lack of metallic behavior and superconductivity in PrBa2Cu3O7 − δ. The compound PrBa2Cu3O7 − δ appears to exhibit some form of complex antiferromagnetic order at approximately 16 K and has a γT contribution to the specific heat at low temperatures with a large γ value of approximately 169 mJ (mol praseodymium)−1 K−2, reminiscent of heavy fermion behavior.

Electrical resistivity p, magnetization M, and specific heat C measurements have been made on polycrystalline RBa2Cu3O7–δ compounds (R = Y or a rare earth element) which, except for R = Ce, Pr and Tb, exhibit superconductivity with TC ≈ 90K. Measurements of C(T) below Tc reveal (1) a contribution linear in T and Debye and Einstein lattice contributions for R = Y, and (2) electronic Schottky anomalies due to crystalline electric field effects and magnetic ordering for many R ions with partially-filled 4f electron shells. Measurements of ρ(T, H) and M(T, H) yield upper critical field slopes near Tc of ≥ 3T/K and critical current densities ≥3.5 × 104 A/cm2 at 4.2 K in zero field.

X-ray measurements on glassy alloys (TE)1−x(TL)x formed between transition metals occurring “early” (TE) and “late” (TL) in the Periodic Table show that (a) the first-broad-peak scattering vector Qp increases approximately linearly with atomic concentration x, (b) the very approximate average nearest-neighbor separation d1 as simply d 1 ≾4π Q p −1 increases approximately linearly with 2

Magnetic properties have been investigated for polycrystalline samples of RBa 2 Cu 2 O 7−δ with R = Y and all the rare earth elements except Pm. The normal-state magnetic susceptibilities of these compounds can be described as χ ( T )= χ 0 + Nμ 2 eff /[3 k b ( T − ϑ )], with μ eff comparable to the values of free R 3+ ions. The superconducting-state magnetizations for all measured superconducting compounds are very different for different R while the field and temperature dependence of critical current densities J c ( H , T ) inferred from the hysteresis of M ( H ) isotherms at different temperatures are similar. J c decreases with H , rapidly in low fields and slowly in high fields, and decreases by two orders of magnitude as the temperatures increase from 4.2 to 65 K. At 4.2 K and 180 kOe, the critical current densities of Gd, Yb, and Tm compounds range from 1.6 to 17 kA/cm 2 .

Upper critical magnetic field initial slopes -d H c2 /d T , have been measured for RBa 2 Cu 3 O 7−δ compounds where R=Y , Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu. If H at the resistive midpoint is defined as H c2 , then -d H c2 /d T falls within the range of 0.81 (R=Lu) to 2.86 (R=Eu) T/K. Extrapolations to T =0 K using the standard, three-dimensional, dirty-limit theory of theory of Werthamer, Helfand, Hohenberg and Maki yield (for R=Eu) (a) zero-temperature upper critical fields H c 2 ( T =0 K)=102–183 T, where the range is determined by the degree of paramagnetic limitation; (b) H c 2 ( T =77 K) ≃ 43 T, the upper critical field at the boiling point of liquid nitrogen. Excessive broadening of the resistive transition to the superconducting state which lowers | -d H c2 / dT | is discussed in terms of intergranular conduction and crystal anisotropy with comparison drawn to single crystal data.

The magnetization M and electrical resistivity rho have been measured at 0 T/sub 0/, high H >> k/sub B/T/g..mu../sub B/, Larkin-Khmel'nitskii (LK) prediction ..delta..M (LK) = ..delta..M/sub s/(1more » - H/sub 0//H) (..delta..M/sub s/ is the impurity saturation magnetization, H/sub 0/ proportional cV/sub 0/, V/sub 0/ is the RKKY coefficient) does not account for the (i) T-dependence of ..delta..M, (ii) relatively low fields 40 9 or approx. = H/sub s/ 9 or approx. = 130 kG (at 520 < or = c < or = 3100-ppm Mn) required for saturation of the impurity magnetization at 1.2 K; (f)« less

The effect of hydrostatic pressures up to 300 atmospheres on the critical magnetic field of superconducting lead has been measured at a temperature slightly below the critical temperature. The pressures were applied using helium gas as the pressure transmitting fluid. The critical fields of the pressurized specimen and a zero-pressure comparison specimen were measured alternately using a sensitive d.c. ballistic method. The pressure-induced critical field difference ΛHc was found to vary linearly with pressure with a slope (δHc/δp)T = −9.60 ± 0.11 × 10−9 gauss/ dyne cm−2 at about 7.09° K. The factors influencing the magnitude of the pressure shift are discussed on the basis of the Bardeen theory. The pressure shift for lead, unlike that for other super-conductors which have been measured, is about the same as that which is estimated from a consideration of zero-point vibration-amplitude changes only.

Measurements of the magnetic-field dependence of the electrical resistivity in bulk specimens of very-short-coherence-distance superconducting alloys suggest the presence of fluctuation superconductivity up to at least 3 TC, and up to 40 kG in the vicinity of 2.5 TC.

Superconductivity with critical transition temperature Tc near 90 K has been found in RBa2Cu3O7-δ compounds where R = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and with Tc=56 K for R = La by electrical resistivity and low field magnetic susceptibility measurements. All compounds (except La) form in the orthorhombic, layered perovskite structure which is isomorphic to YBa2Cu3O7-δ. The lattice parameters of the compounds decrease in accordance with the metallic radii of the R trivalent ions. The electrical resistivity of the compounds studied (except for R = Nd) decreases with decreasing temperature and exhibit a sharp superconducting transition. Upper critical magnetic field Hc2 of the heavy R compounds (R from Dy to Yb) were measured and the highest initial slope |dHc2/dTc|Tc yet found for above 90 K superconductors was exhibited in TmBa2Cu3O7-δ with a value of 2.8 tesla/K.