F. C. Chou

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.

Coexistence of stripes and superconductivity: Tc amplification in a superlattice of superconducting stripes

Abstract Here we report direct evidence for charge ordering in the oxygen doped La 2 CuO 4.1 superconductor by x-ray diffraction using a third generation synchrotron source. We observe 1D charge ordering and oxygen ordering in an optimally doped superconductor (T c =40K) well beyond the phase separation regime. The order parameter of the anharmonic 1D CDW shows a critical behavior (T co -T) 0.5 T co =190K. The oxygen ordering and the polaron 1D CDW with its associated orbital wave, realize a superlattice of quantum stripes with a wavevector that at the density of charge carriers, r s ∼6, in the superconducting stripes satifies the shape resonance condition for T c amplification.

Preparation, magnetization and electrical resistivity of electrochemically oxidized La2CuO4+δ single crystals

Abstract We report that oxygen can be intercalated at room temperature into single crystal La 2 CuO 4 through slow electrochemical oxidation in aqueous base. Magnetic susceptibility χ( T ) measurements indicate that homogeneous La 2 CuO 4+δ crystals can be obtained with T c ≈40 K. With increasing δ, two distinct superconducting phases are observed with T c ≈32 and 40 K, respectively; the latter is lower than the maximum T c ≈45 K of the corresponding polycrystalline samples, whereas the former is very similar. The resistivity anisotropy ϱ c /ϱ ab increases from ∼ 100 at 290 K to ∼ 300 at 50 K. The susceptibility anisotropy, χ c −χ ab , is significantly larger than in the La 2 CuO 4 parent crystals. The magnetically determined critical current density J c ( H ) of a La 2 CuO 4+δ crystal with T c = 40 K decreases exponentially with magnetic field H , with J c (0) ∼ 5 × 10 4 and 9 × 10 4 A/cm 2 for H ‖ c and H ‖ ab , respectively.

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.

Study of Cu–O and Cu–La Pair Correlations in Superconducting La 2CuO 4.1 System by Cu K-Edge EXAFS

We report Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra up to high momentum transfer measured on a superconducting La 2 CuO 4.1 single crystal using in-plane polarization of synchrotron light. The EXAFS data with a high signal to noise ratio has enabled the precise determination of the Debye-Waller factors and the pair correlations of Cu–O (planar) and Cu–La pairs of the structurally complex superconducting system. We have observed an anomalous behaviour of the pair correlations at ∼170 K and attributed it to an interplay between temperature-dependent structural modulation of the perovskite and rock-salt layers in the title compound.

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.