J.T. Markert

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 superconductivitity in Th-doped Nd2CuO4-y

Abstract We have discovered superconductivity with an onset at ≈20 K in the Nd2-xThxCuO4-y system at x = 0.15. The superconductivity appears to be associated with electrons in the CuO2 planes donated by tetravalent Th which substitutes for trivalent Nd. We have also verified the occurence of superconductivity in the Nd2-xCexCuO4-y system, recently reported by Tokura et al., where the superconductivity is apparently due to electrons donated by tetravalent, or intermediate valent (between +3 and +4), Ce which substitutes for the trivalent Nd ion. These results suggest that high Tc superconductivity in the layered copper oxide compounds involving electrons in the CuO2 planes is a general phenomenon.

Processing and superconducting properties of the electron-doped compounds Ln2−xMxCuO4−y (Ln=Pr,Nd,Sm,Eu,Gd;M=Ce,Th)

Abstract The variations in transport and magnetic properties as a function of annealing conditions are discussed for the Ln 2−x M x CuO 4−y compounds. A composition x∼0.15−0.16 and a small oxygen deficiency (y∼0.02) are required to optimize superconducting properties. For Nd 1.85 Ce 0.15 CuO 3.98 , critical current densities, inferred from magnetization measurements, are estimated to be as high as 10 6 A/cm 2 at 4.2 K in zero field. From resistive measurements of polycrystalline samples in fields up to 8 tesla, upper critical field H c2 slopes of −dH c2 /dT∼1.5T/K are obtained at low temperatures.

Sm1.85Th0.15CuO4−y: A new electron-doped copper oxide superconductor

Abstract We report here results from a study of superconductivity in the Sm 1.85 (Ce 0.15−x Th x )CuO 4−y system. We observe resistive superconducting transitions for all values of x, with transition midpoints ranging from 19.3 K for x =0 to 1.9 K for x =0.15. This is the first reported observation of superconductivity in the compound Sm 1.85 Th 0.15 CuO 4−y . Magnetic susceptibility measurements performed on this compound verify the presence of superconductivity. Additionally, electrical resistivity measurements done under pressure show a decrease in T c in Sm 1.85 Th 0.15 CuO 4−y at a rate dT c /dP=-0.043 K/kbar.

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 .

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.

Magnetism, specific heat, and pressure-dependent resistivity of the electron-doped compounds Ln2−xMxCuO4−y (Ln = Pr, Nd, Sm, Eu, Gd; M = Ce, Th)

Abstract Measurements of the magnetic susceptibility z the specific heat C, and the electrical resistivity ρ are presented for the Nd 2 CuO 4 -type structure insulating parent compounds Ln 2 CuO 4 (Ln = Pr, Nd, Sm, Eu, and Gd), the doped superconducting compounds Ln 2−x M x CuO 4−y (Ln = Pr, Nd, Sm, and Eu, M = Ce; Ln = Pr and Nd, M = Th), and the doped, semiconducting compound Gd 2−x Ce x CuO 4−y . The z and C data indicate that antiferromagnetic (AFM) order occurs in the undop compounds for Ln = Nd, Sm, and Gd at temperatures below 1.7 K, 5.95 K, and 6.6 K, respectively. The coexistence of superconductivity and AFM order is observed in Ln 1.85 M 0.15 CuO 4−y , with reduced Neel temperatures T N = 1.2 K for Ln = Nd, M = Ce, Th and T N = 4.6 K for Ln = Sm, M = Ce. Magnetic susceptibility data for Gd 2−x Ce x CuO 4−y display evidence for AFM ordering of Cu 2+ spins at ≈ 280 K for x = 0 and at ≈ 150 K for x = 0.15. For Nd 1.85 M 0.15 CuO 4−y , a very slight increase (decrease) in T c with applied pressure is observed for M = Ce (Th); this relative insensitivity and the depression of T c for M = Th is in contrast to the behavior observed for other high T c compounds.

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.

A comparative compositional study of Ln2-xMxCuO4-y electron-doped superconductors

Abstract We present the results of temperature-dependent magnetization and electrical resistivity measurements for a systematic series of compositions and concentrations of the electron-doped copper-oxide superconducting compounds Ln 2− x M x CuO 4− y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th). The concentration dependences of normal and superconducting properties for both the Ce- and Th- doped systems are nearly identical, indicating that the Ce dopant ions exhibit simple tetravalent behavior. A number of distinct measures of superconducting properties display similar sharply-peaked concentration dependences; such variation is discussed in terms of phase homogeneity.

Effect of pressure on critical parameters of electron-doped superconductors Ln2−xMxCuO4−y (Ln=Nd, Sm, Pr; M=Ce, Th)

Abstract The effects of pressure on the temperature of the superconductive transition and the critical field of irreversibility for n-type polycrystalline high- T c compounds Ln 2−x M x CuO 4−y (Ln=Nd, Sm, Pr; M=Ce, Th) was investigated. The pressure derivatives of critical parameters are less in magnitude than these for p-type high- T c compounds. This fact may be accounted for by structure peculiarities of electron-doped HTSC.