John J. Neumeier

Two new electron cuprate superconductors, Pr1.85Th0.15CuO4−y and Eu1.85Ce0.15CuO4−y, and properties of Nd2−xCexO4−y

Abstract We report the first observation of superconductivity in the Pr 2− x Th x CuO 4− y and Eu 2− x Ce x CuO 4− y systems. Superconducting resistive onset temperatures of ∼ 23 K and ∼ 13 K were observed for Pr 1.85 Th 0.15 CuO 4− y and Eu 1.85 Ce 0.15 CuO 4− y , respectively; the corresponding transition midpoints are T 0.5 c = 19.5 and 8 K . Magnetic susceptibility data indicate that the superconductivity is a bulk property. Transition temperatures higher than previously reported for Pr 1.85 Ce 0.15 CuO 4− y and Sm 1.85 Ce 0.15 CuO 4− y were also observed. Oxygen content analysis reveals that the superconducting Ce-doped specimens have quite small oxygen deficiencies. Specific heat data for Nd 2 CuO 4− y ( y ≈0.07) and Nd 1.85 Ce 0.15 CuO 4− y ( y ≈0.02) display a large peak, presumably associated with magnetic ordering of Nd 3+ ions, Which decreases in magnitude with Ce doping. Pressure-dependent electrical resistivity measurements reveal an increase of T c with applied pressure for Nd 1.85 Ce 0.15 CuO 4− y ( y ≈0.02), at a rate of approximately d T c /d P ≈0.025 K/kbar.

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.

Neutron diffraction study of Pr valence and oxygen ordering in the Y1−xPrxBa2Cu3O7−δ system

Abstract The crystallographic changes that occur with increasing Pr concentration in the Y−xPrxBa2Cu3O7−δ system have been studied by neutron diffraction. As x is increased from 0 to 1, the orthorhombic a, b and c lattice parameters expand monotonically, while the orthorhombic distortion, espressed as b a , decreases, reflecting increased occupation of O(5) sites at the expense of O(4) chain site occupancy. The separation of the CuO2 planes in PrBa2Cu3O7−δ indicates that the Pr ions have an intermediate valence of ∼3.3 in this compound. Evidence is presented for increased interaction between the Pr ions and oxygen ions in adjacent CuO2 planes.

Pressure dependence of the superconducting transition temperature of (Y1−χPrχ)Ba2Cu3O7−δ compounds; Evidence for 4f electron hybridization

Abstract The effect of nearly hydrostatic pressure to 20 kbar on the superconducting transition temperature Tc of (Y1−χPrχ)Ba2Cu3 O7−δ compounds with 0⩽χ⩽0.5 was determined by means of low frequency ac electrical resistivity measurements. The application of pressure to this system reveals striking effects which appear to be associated with the hybridiazation between Pr 4f and valence band states. In this range of pressure P, Tc increases with P for 0⩽χ⩽0.2, exhibits a maximum at ∼6 kbar for χ=0.3, and decreases rapidly with P for χ⩾0.4, while the initial rate of increase of Tc with pressure (dTc/dP)P=0, increases with χ for 0⩽χ⩽0.3. These results suggest there is a pressure-induced electronic phase transition at ∼6 kbar foir χ=0.3 that shifts to lower values of P for higher values of χ.

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.

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.

Hybridization, hole localization and pair breaking in the high Tc superconducting system Y1-xPrxBa2Cu3O7-δ

Abstract The Y 1- x Pr x Ba 2 Cu 3 O 7- δ ( δ ≈ 0.05) system has a rich temperature—praseodymium concentration ( T - x ) phase diagram consisting of a metal—insulator transition at x cr ≈0.6, high T c superconductivity in the metallic phase (0⩽ x ≤ x cr ), and antiferromagnetic ordering of the Cu 2+ and praseodymium ions at Neel temperatures T N (Cu) > T N (Cu)> T N (Pr) in the insulating phase ( x cr ≤ x ⩽1). In the metallic phase, T c decreases monotonically with x and vanishes near x cr , the dependence of T c on pressure P and the upper critical field on temperature are both anomalous, and the low temperature specific heat can be resolved into a large γΓ contribution, reminiscent of heavy fermion behavior, and a spin 1 2 Kondo anomaly; in the insulating phase, the values of T N (Pr) are anomalously large. These phenomena are briefly reviewed within the framework of a picture that is based on appreciable hybridization of the praseodymium localized 4f states and the CuO 2 valence band states. A phenomenological model that incorporates the annihilation of mobile holes in the CuO 2 planes and superconducting electron pair breaking by the praseodymium ions can describe T c ( x , P ) for x ≤ 0.2.

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.

Upper critical magnetic field measurements in RBa2Cu3O7−δ (R=rareearth) compounds

Abstract 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.

Metastable A15 phase Nb3Si synthesized by high dynamic pressure

Abstract Nb3Si in the majority Ti3P-type tetragonal phase was subjected to high dynamic pressures of 0.82, 0.96, and 1.04 Mbar. Dynamic pressures were generated by the Lawrence Livermore National Laboratorys two-stage light-gas gun. This technique produces dynamic pressures of up to a Mbar and temperatures of up to a few thousand degrees Kelvin with high quench rates. High dynamic pressures induced a partial phase change to the cubic A15-type phase in our specimens. The synthesis of this phase is supported by X-ray diffraction, optical microscopy, electrical resistivity, specific heat, and ac magnetic susceptibility data. The midpoint of the resistive transition to the superconducting state occurs at 17.5 K in the specimen shocked to 0.96 Mbar. Upper critical magnetic field and the dependence of Tc on pressure have also been measured, yielding slopes of −1.20 tesla/K and — 3 × 10−5, respectively. The specific heat data indicate that about 67% of the specimen shocked to 1.04 Mbar was converted to the A15 ...