C.L. Seaman

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.

Non fermi liquid ground states in strongly correlated f-electron materials

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M1−xUxPd3 (M = Sc, Y), U1−xThxPd2Al3, and UCu5−xPdx. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility χ for T ≪ T0, where To is a characteristic temperature: P(T) ∼ 1 –aT/T0, where a < 0 or > 0, C(T)/T ∼ (-1/To) In (T/bT0), and χ(T) ∼ 1 −c(T/To)1/2. In several of the f-electron systems, the characteristic temperature To can be identified with the Kondo temperature Tk.

Non fermi liquid behavior in strongly correlated f-electron materials

Evidence for non Fermi liquid (NFL) behavior in y1−x UxPd3 and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Low temperature electrical resistivity, specific heat, and magnetic susceptibility measurements on the Th1−xUxPd2Al3 system reveal unconventional Kondo behavior with NFL low temperature characteristics. Magnetic susceptibility measurements on UCu3.5Pd1.5, which has previously been shown to exhibit NFL behavior, are presented. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity ϱ ∼ 1−aT with either positive or negative coefficient a, a logarithmically diverging specific heat C/T ∼−lnT, and T1/2 asymptotic behavior of the magnetic susceptibility χ ∼ 1 − T1/2.

Low temperature specific heat of Ln2CuO4 (Ln=Pr, Nd, Sm, Eu and Gd) and Nd1.85M0.15CuO4−y(M=Ce and Th)

Abstract Specific heat measurements in the temperature T range 0.5 K ⩽ T ⩽ 30 K were performed on insulating Ln 2 CuO 4 compounds (Ln = Pr, Nd, Sm, Eu and Gd) and on superconducting Nd 1.85 M 0.15 CuO 4 − y compounds (M = Ce and Th). The specific heat of Eu 2 CuO 4 and Pr 2 CuO 4 were analyzed to obtain electronic contributions and Debye and Einstein lattice contributions. The data for the compounds Nd 2 CuO 4 , Sm 2 CuO 4 and Gd 2 CuO 4 show magnetic ordering of the trivalent Ln ions at temperatures T M of 1.7 K, 5.94 K and 6.66 K, respectively. Magnetic ordering is also observed in Nd 1.85 M 0.15 CuO 4 − y at T M = 1.2 for both M = Ce and M = Th. The data for doped and undoped Nd 2 CuO 4 were compared to derive an electronic contribution to the specific heat of Nd 1.85 Ce 0.15 CuO 4 − y , with γ ≈ 53 mJ/mol K 2 .

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.

Increase in the flux‐pinning energy of YBa2Cu3O7−δ by shock compaction

Magnetic flux relaxation data on a composite specimen of YBa2Cu3O7−δ +Ag shock compacted at 167 kbar show that the flux‐pinning energy at 70 K and 10 kOe is enhanced by a factor of 2–3. This enhancement persists after annealing at 890 °C in oxygen for 24 h. The increased pinning energy is apparently caused by shock‐induced 〈100〉 and 〈110〉 line defects having a density up to 1012 cm−2. Annealing replaces line defects with (001) extrinsic stacking faults having displacement vectors of 1/6[031]. The average separation of the stacking faults in the [001] direction is 0.04 μm, which is comparable to the separation between fluxoids at 10 kOe. These results indicate that intragranular critical current densities can be increased significantly in bulk materials by shock processing. High densities of flux‐pinning sites could be induced efficiently in industrial‐scale quantities of bulk high Tc oxides by explosives.

Observation of the two-channel quadrupolar Kondo effect in Y1-xUxPd3

Abstract We present measurements of electrical resistivity p ( T ), magnetic susceptibility x ( T ), and specific heat C ( T ) for the system Y 1-x U x Pd 3 . The data are consistent with nearly localized tetravalent uranium ions leading to “Fermi-level tuning” and competition between single ion Kondo (0 x ≤0.2) and cooperative spin glass-like (0.3≤ x ≤0.5) behavior. The Kondo behavior is unusual and well described by the two-channel quadrupolar Kondo effect. At low temperatures T ⪡ T K , where T K is the Kondo temperature, the electrical resistivity varies nearly linearly with temperature p ( T )/ p (0)≈ 1 - T /( aT K ), and the electronic specific heat diverges logarithmically ΔC / T ≈ -(1/ T K)1 nT with a finite residual T 0 entropy S (0)≈( R /2) ln 2. This appears to be the first example of the quadrupolar Kondo effect and two-channel behavior in a dilute alloy and the first example of “marginal Fermi liquid” phenomenology in a fully three-dimensional system.

On narrowing of the resistive superconducting transition in applied magnetic fields and positive curvature in Hc2(T) in the (Gd1−xPrx)Ba2Cu3O7−δ system

Abstract The superconducting transition temperature T c and the temperature dependence of the upper critical field H c2 were measured resistively on polycrystalline samples in the (Gd 1− x Pr x )Ba 2 Cu 3 O 7−δ system. The experimental results reveal that T c decreases monotonically with increasing x and vanishes at x ≈0.49. Whereas H c2 of a sample with x =0.10 has an almost linear temperature dependence and the superconducting transition broadens with increasing magnetic field, a sample with x =0.45 shows strong positive curvature in the H c2 versus temperature curve and narrowing of the superconducting transition width with increasing magnetic field. Both phenomena observed in the sample with x =0.45 can be explained qualitatively in terms of a negative exchange interaction between the superconducting charge carriers and the Pr 4f electrons.

Superconductivity in La2−xNaxCuO4−y

Abstract Observation of bulk superconductivity in the La 2−x Na x CuO 4−y system is reported for the first time. Like other La 2−x M x CuO 4−y (M  Ba, Sr, and Ca) systems, the M  Na form has the K 2 NiF 4 -type structure. This Na-doped compound can be obtained for only a limited range of sample preparation conditions. Measurements of the electrical resistivity and the magnetic susceptibility are presented. The midpoint of the resistive superconducting transition increases only slightly with concentration x from T c = 16 K for x = 0.11 to T c = 18 K for x = 0.41. The 10%–90% transition widths are ΔT c ≈ 3 K . Semiconducting behavior is observed for x ≤ 0.08. Susceptibility measurements exhibit a large fractional Meissner effect that increases with x. Diamagnetic onset temperatures also increase with x from T onset = 22 K for x = 0.11 to at least T onset = 36 K for x = 0.41. The x = 0.41 specimen also exhibits unusual χ vs T behavior, possibly indicating that the superconductivity is two-component in nature.