J. H. Cho

Synthesis, characterization, and superconducting and magnetic properties of electrochemically oxidized La2CuO4+δ and La2-xSrxCuO4+δ (0.01 ≤ x ≤ 0.33, 0.01 ≤ δ ≤ 0.36)

Abstract La2CuO4 can be electrochemically oxidized in aqueous base at 295 K to form a superconductor with Tc≈44 K [Wattiaux et al., C.R. Acad. Sci. (Paris) 310 (1990) 1047]. We have confirmed and extended this study. We conclude that the electrochemical reaction forms La2CuO4+δ. Two distinct superconducting phases are found at δ≈0.03 (Tc≈32 K) and δ 0 .08(T c ≈4 K) Bulk superconductivity is found in similarly prepared La2-xSrxCuO4+δ with Tc≈40 K for 0.01≤x≤0.15. Normal stat e magnetic susceptibility data for La2CuO4.11 and La1.92Sr0.08CuO4.07 are nearly identical with those of conventionally prepared La1.85Sr 0.15CuO4, indicating that the hole doping level (p) in the CuO2 planes of the three compounds is nearly the same. Iodometric titration experiments confirmed that the maximum doped-hole concentration from combined Sr-doping and electrochemical oxygen doping for 0≤x≤0.15 is p≈0.16 holes/formula unit. The electrochemical oxidation of the starting materials does not appear to be reversible.

Vortex fluctuations in polycrystalline Bi1.75Pb0.25Sr2Ca2Cu3O10

Abstract Reversible magnetization M(H, T) data are reported for polycrystalline Bi1.75Pb0.25Sr2Ca2Cu3O10. After subtraction of the normal state contribution, the data exhibit a characteristic fluctuations dominated behavior near the critical temperature, already observed experimentally and explained theoretically for single crystals of weakly coupled layered superconductors with the magnetic field along the c crystal direction. It is shown that the M H, T) behavior is compatible with the notion that only the field component along the c-axis of each crystalline grain results in an induced superconducting state magnetization.

The Phase Diagrams and Doped-Hole Segregation in La2CuO4+δ and La2−xSrxCuO4+δ (x ≤ 0.15, δ ≤ 0.12)

The magnetic and structural phase diagrams of the title systems are reviewed, with an emphasis on our recent results obtained from magnetic and structural neutron diffraction, thermogravimetric analysis, iodometric titration, magnetic susceptibility x(T), and 139La nuclear quadrupole resonance (NQR) measurements. From measurements on electrochemically oxidized polycrystalline samples, the known miscibility gap in the system La2CuO4+δ is found to lie between δ ≈ 0.01 and 0.06 at low temperatures, with a maximum phase separation temperature T ps ≈ 415 K. Within the miscibility gap, the superconducting transition temperature T c of the oxygen-rich phase and the Neel temperature T N of the oxygen- deficient phase are constant at ≈ 32 K and 250 K, respectively. Neutron diffraction measurements showed T ps = 260(5) K and T N = 245(3) K for a single crystal. Beyond the miscibility gap, two distinct superconducting phases are found in polycrystalline samples at δ ≈ 0.06–0.08 (T c ≈ 32–34 K) and 0.11–0.12 (T c ≈ 42–45 K), separated by another two-phase region. The doped-hole concentration in the CuO2 planes is found to be p ≈ 0.08 holes/Cu and ≈ 0.16 holes/Cu for the two phases, respectively. These data suggest that a large fraction of the excess oxygen atoms participate in oxygen-oxygen bonding in both phases. Measurements of T c versus pressure suggest pressure-induced changes in the doped-hole concentration in the CuO2 layers. Superstructure reflections observed in the neutron diffraction patterns of both phases suggest spatial ordering of the excess oxygen atoms. In the La2−xSrxCuO4+δ system, the phase separation disappears by x = 0.03 for δ ≈ 0.03. Bulk superconductivity is found below T c = 40 K for 0.01 ≤ x ≤ 0.15 for maximally oxidized samples. For δ = 0 and 0 < x < 0.08, our 139La NQR and X(T) data indicate that the doped holes condense into walls separating undoped nanoscopic domains, consistent with theory based on an electronic mechanism for phase separation in these systems. In the antiferromagnetic regime (x < 0.02) below TN, the doped-hole spins are found to freeze at a temperature T f = (815 K)x, whereas in the spin-glass regime 0.02 < x < 0.08, the spin-glass transition at T g ∝ 1/x is found to arise from cooperative freezing of the dynamically-ordered mesoscopic undoped domains.

Magnetic penetration depth of Ba0.625K0.375BiO3

Abstract The London penetration depth λ ( T ) of Ba 0.625 K 0.375 BiO 3 was determined for the first time from the spin-rotation-relaxation of muons implanted into a high purity polycrystalline sample, field-cooled in 0.1 and 0.3T external fields. The data showed that over 90% of the sample was superconductive. The temperature dependence of λ indicated weak-coupling BCS behaviour, with T c =26K and λ (0)=340nm.

Phase separation and doped-hole segregation in La2CuO4+δ and La2-xSrxCuO4+δ

Abstract Herein, we review the magnetic, superconducting and structural phase diagrams of the title systems, with an emphasis on our recent results form magnetic and structural neutron diffraction, magnetic susceptibility and system, and indicate the occurrence of frustrated phase separation on a nanoscopic length scale in the La 2- x Sr x CuO 4 system with O x ≲ 0.05. 139 La nuclear quadrupole resonance measurements. The results clarify the miscibility gap in the La 2 CuO 4+δ

Phase diagram and phase separation in La2−xSrxCuO4+δ

Abstract The phase diagram of the title system is remarkably rich, exhibiting regions of long-range antiferromagnetic order, short-range (spin-glass-like) order, high temperature superconductivity, or none of these. Here, the phase diagram is reviewed, with an emphasis on our recent studies using 139La NQR, powder and single crystal neutron diffraction, and magnetization measurements for 0≤χ≤0.15 and 0≤δ≤0.1.

Magnetic Properties of Single-Layer Cuprates

The evolution of the magnetic properties of the insulating parent cuprate compounds as they are doped into the metallic and superconducting state continues to be an important area of research. Understanding this evolution and its relationship to the microscopic distribution of the doped holes may offer insights into interactions important in the superconducting materials. In addition, the nature of this evolution in strongly correlated electron systems is interesting in its own right. In this paper, we will review some of the progress made in attempting to address this issue in the La2CuO4-type and related systems.

Phase separation kinetics in La2CuO4+δ and inhomogeneous hole doping in the antiferromagnetic regime (0 <x < 0.02) of La2−xSr x CuO4

The diffusion of the excess oxygen during phase separation in La2CuO4+δ was studied using thermal history-dependent normal state magnetic susceptibilityϰ(T, t) measurements versus temperatureT and timet as a probe. A large thermal hysteresis ofϰ(T) was observed for La2CuO4.044 between data obtained after quenching to 5 K and then warming, and data obtained while or after slowly cooling from 300 K. A model for the excess oxygen diffusion is presented, from which theϰ(T, t) data yield aT-independent activation energy of 0.24(3) eV for the diffusion coefficient of the excess oxygen from 150 to 220 K. In related work, we have used139La NQR andμSR measurements to probe the antiferromagnetic (AF) region (x<0.02) of the La2−xSrxCuO4 system below the Néel temperatureTN(x), from which we extract the Cu+2 staggered magnetizationM†(x, T). M†(x, T=0), extrapolated from above 30 K, was successfully modeled with spin-wave theory, assuming that the doped holes are mobile and are situated in walls in the CuO2 plane which uncouple undoped AF domains; these domains are coupled to those in adjacent CuO2 planes. This agreement supports the previous hypothesis that microsegregation of the (mobile) doped holes into domain walls occurs above ∼30 K, consistent with the phenomenology of Emery and Kivelson. Below ∼30 K, an anomalous increase inM†(x, T) is observed, such thatM†(x, T=0) is nearly independent ofx. We interpret this effect as arising from localization of the doped holes below ∼30 K.