K. N. Yang

Superconducting and normal state properties of Y1−xMxBa2Cu3O7−δ (M=Pr, Na)

Superconducting and normal state properties of compounds in the series Y1−xMxBa2Cu3O7−δ for 0 ≤ × ≤ 1 and Y1−xMxBa2Cu3O7−δ in the limited region of solubility 0 ≤ × ≤ 0.5 have been investigated by means of X-ray diffraction, electrical resistivity ϱ, and magnetic susceptibility χ measurements. For both systems, the superconducting transition temperature Tc decreases monotonically with the Pr or Na concentration x. The temperature T coefficient of the normal state electrical resistivity α ≡ (1/ϱ)∂ϱ/∂T changes from positive to negative at x ≈ 0.4 for Pr and x ≈ 0.2 for Na. Magnetic susceptibility χ vs T data indicate a mixed valent state for the Pr substitutions with an effective magnetic moment μeff≈ 2.7 μB per Pr ion and the gradual development of magnetic moment for Na substitutions that attains a value μeff ≈ 3 μB per Na ion in the range 0.2 ≤ × ≤ 0.5, although the Na+ ions are presumably nonmagnetic. The depression of Tc in both systems is probably due to the fact that the Pr and Na ions are not trivalent in the orthorhombic YBa2Cu3O7− δ structure (the valence of Pr apparently lies between 3+ and 4+, while the valence of Na is 1+).

UFe4P12 and CeFe4P12: Nonmetallic isotypes of superconducting LaFe4P12

The new compound UFe4P12, which was found to be isostructural to superconducting LaFe4P12 and have a lattice constant of 7.7729 A, is a semiconductor and shows ferromagnetic order below 3.15 K. CeFe4P12 is also a semiconductor, and its magnetic susceptibility is unusually small in comparison to LaFe4P12. The semiconducting behaviors of both UFe4P12 and CeFe4P12 seem anomalous and may arise from strong f‐electron hybridization.

High temperature superconductivity in rare-earth (R)-barium copper oxides (RBa2)Cu3O9−σ

Abstract The series of rare-earth (R)-barium copper oxides with nominal composition (RBa2)Cu3O9−σ and the prototype compound (YBa2)Cu3O9−σ have been prepared from the R (except Pm), Y, and Cu oxides and Ba carbonate by sintering, followed by arc-melting and annealing in pure oxygen. X-ray diffraction measurements show that, except for R = Ce, Tb, and Lu, the compounds have the perovskite-type structure reported by Cava et al. for high superconducting transition temperature Tc (YBa2)Cu3O9−σ. Electrical resistivity and magnetic susceptibility vs temperature measurements reveal super-conductive onsets above 90 K for all R elements, except La, Ce, Pr, and Tb, for which no superconductivity was observed down to 4.2 K. Light (heavy) rare-earth compounds tend to have relatively little (large) orthorhombic splitting in their X-ray patterns, relatively high (low) normal-state resistivities with negative (positive) temperature coefficients, and broad (sharp) superconductive transitions. The negligible destructive effect of the R magnetic moments on superconductivity indicates that the superconducting electrons are primarily associated with the Cu-O polyhedral clusters and interact only weakly with the R ions, suggesting that the (RBa2)Cu3O9−σ compounds may constitute an interesting new class of “high Tc” magnetic superconductors.

RBa2Cu3O7−δ (R = rare earth) high-Tc magnetic superconductors

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.

Compounds in mixed phase CeBa2Cu3Oy and TbBa2Cu3Oy systems

An analysis of the composition of Ce and Tb barium copper oxide samples with the nominal composition CeBa2Cu3Oy and TbBa2Cu3Oy, prepared with the same solid state reaction method used for the other superconducting rare earth (R) barium copper oxide RBa2Cu3O7-δ compounds, is described. The analysis indicates that these samples are multiphase and consist mostly of 1) the perovskite phases BaCeO3 or BaTbO3, 2) CuO, and 3) BaCuO2. No trace of the oxygen deficient orthorhombic perovskite type structure of YBa2Cu3O7-δ could be found in the X-ray diffraction patterns. Consequently, the compounds do not display superconductivity. The magnetization measurements indicate that BaCeO3 is weakly paramagnetic which is consistent with tetravalent Ce. The BaTbO3 compound exhibits an antiferromagnetic transition at a Néel temperature TN-35 K, and its magnetic susceptibility above TN can be described by a Curie-Weiss law with an effective moment that corresponds to tetravalent Tb.

4f Electron effects in high Tc RBa2Cu3O7 − δ (R = rare earth) superconductors☆

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 Hunds 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.

Intermediate valence behavior of ternary cerium and uranium transition metal borides

Low-temperature specific heat, electrical resistivity, ac magnetic susceptibility, and dc magnetization measurements were made on ternary cerium and uranium transition metal borides with the general formula CeT3B2 (T=Co, Ru, Rh, and Ir) and UT3B2 (T=Co, Ru, and Ir). The compound CeRu3B2 was found to exhibit superconductivity below 0.68 K. The values of the electronic specific heat coefficient range from 9.7 mJ/mole-K2 for CeCo3B2 to 64 mJ/mole-K2 for UIr3B2. The electrical resistivity versus temperature curves of all of the compounds exhibit negative curvature and are reminiscent of valence fluctuation behavior. In the case of CeIr3B2, the electrical resistivity attains a maximum value at 180 K, while the dc magnetic susceptibility has a temperature dependence that is typical of intermediate valence Ce compounds, approaching a finite value at zero temperature. The electrical resistivity of the ferromagnetic compound CeRh3B2 reveals a rapid decrease in spin disorder resistivity below 120 K. The dc magnetic susceptibility of this material can be described as the sum of a constant term and a Curie-Weiss contribution with an effective magnetic moment of 1.01 µB per formula unit and a Curie-Weiss temperature of 119 K. Magnetization measurements on CeRh3B2 yield a saturation magnetic moment of 0.37 µB per formula unit and a Curie temperature of 113 K.

Normal and superconducting state magnetic properties of RBa2Cu3O7−δ compounds

Abstract 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 .

High temperature superconductivity in (M1−xM′x)2CuO4−δ and related compounds (M = Y, La, Eu, Sm; M′ = Ba, Sr)

Abstract Compounds of the form (M 1−x M′ x ) 2 CuO 4−δ and related compounds where M and M′ are Y, various rare earths from La to Lu, and the alkaline earths Sr and Ba, have been investigated in connection with high temperature superconductivity. High temperature superconductivity is confirmed for the system (La 1−x Ba x ) 2 CuO 4−δ , (La 1−x Sr x ) 2 CuO 4−δ and (Y 1−x Ba x ) 2 CuO 4−δ with superconducting transition temperature T c onsets of 30 K, 38 K and 90 K, respectively. We have found that the related systems (Eu 1−x Ba x ) 2 CuO 4−δ and (Sm 1−x Ba x ) 2 CuO 4−δ also exhibit high temperature superconductivity with T c onsets of 95 K and 65 K, respectively. The highest T c onset observed in this investigation was 97 K for a sample with the nominal composition of the spinel structure Y 0.33 Ba 0.67 Cu 2 O 4−δ . Measurements of the specific heat C as a function of temperature T on a La 0.8 Sr 0.2 CuO 4−δ sample reveal a break in slope in the C/T vs T curve at the T c midpoint, but no clearly discernable jump in C at T c . A linear term ≈ λ′T in C was observed at low temperature in the superconducting state.

Onset of superconductivity at 107K in YBa2Cu3O7 − δ at high pressure

Electrical resistivityρ measurements under pressure have been carried out on the high-temperature superconductor YBa2Cu3O7 − δ as a function of temperature T between 1 and 300 K at various pressures between 8 kbar and 149 kbar. The superconducting transition temperature Tc increases almost linearly with pressure at the rate dTc/dP ∼- 0.13 K/kbar. The onset of Tc, defined as the temperature at whichρ(T) drops to 90% of its extrapolated normal state value, increases from ∼95 K at 8 kbar to 107 K at 149 kbar. These results suggest that higher pressures will yield yet higher values of Tc.