Isao Tanaka

Chemical Oxidation on The Surface of La2CuO4 Single Crystals

Surface of La2CuO4 single crystals were oxidized by KMnO4 aqueous solutions. The oxidation conditions such as KMnO4 concentration, soaking time and solution temperature of KMnO4, were systematically controlled. The magnetic properties of the crystals were drastically dependent on the oxidation conditions. The superconducting transition properties under the optimum condition, 48 h, 150°C and 5 wt%, were T c = 28 K and ∆T c = 2K. The chemical oxidation was limited only to the surface region of the crystals. Therefore, chemical oxidation may be useful for the preparation of superconducting single-crystalline films.

TSFZ Growth of La2-xBaxCuO4 Single Crystals Under Varied Oxygen Partial Pressure

La2-xBaxCuO4 (LBCO) single crystals were grown under oxygen partial pressure (Po2) of 1~200 kPa by the traveling solvent floating zone (TSFZ) method. The Ba solubility in the LBCO crystals was maximum for growth under about 100 kPa, and was decreased with deviating Po2 from 100 kPa. The Ba solubility in the LBCO grown crystals was increased up to 0.19 under 100 kPa, but the grown crystals contained subgrains and inclusions of La4BaCu5O13 and CuO. The subgrain- and inclusion- free single crystals of LBCO with the Ba concentration x≤0.113, the typical size of 5 mm in diameter and 30 mm in length, were obtained under 100 kPa.

Thermal Expansion Anomaly in High Tc La1.85Sr0.15CuO4 Associated with the Superconductivity

Precision lattice parameter in La1.85Sr0.15CuO4 single crystal superconductor has been measured in a wide temperature range between 4.2 K and room temperature by the X-ray Bond’s method. An anomalous lattice contraction associated with the appearance of the superconductivity was found in c-plane. Along c-direction, on the contrary, an anomalous expansion of the lattice was observed. The relative spontaneous change in the lattice parameter in c-plane was 2.3xl0xad-5 and that of c-axis was 1.5×l0-5 at 4.2 K. The temperature dependence of the spontaneous lattice change was compared with currently existing theories for high Tc superconductivity.

Unusual Lattice Hardening below T c in La 2− x Sr x CuO 4

By measuring the ultrasound velocity for various modes, we found unusual temperature dependence of the elastic coefficients in single-crystalline La2−xSr x CuO4 with x = 0.09, 0.14 and 0.19. The longitudinal elastic coefficients show a rapid drop in La2−xSr x CuO4 at Tc, as is seen in conventional superconductors. As the temperature decreases, however, the lattice shows gradual hardening in La2−xSr x CuO4 both for the longitudinal and transverse modes of ultrasound in contrast to most of the conventional superconductors, in which the lattice continues to soften in the superconducting state. According to thermodynamic analysis, the temperature dependence of the elastic coefficients below Tc suggests that the second strain derivative of the superconducting condensation energy d2 O /d e 2 is negative in La2−xSr x CuO4, whereas the sign is positive in the conventional superconductors. An important possibility is that this unusual lattice hardening in the superconducting state is a common character of the high-Tc copper oxides.

Superconductivity of electron irradiated La1.85Sr0.15CuO4 single crystal

We studied the superconducting properties of an electron irradiated La1.85Sr0.15CuO4 single crystal. Point-like defects are introduced into the crystal with the irradiation. The irradiation decreases the correlation between CuO2 layers. However, both before and after the irradiation, the flux creep activation energy Ueff (J,T,H) can be expressed the same formula as Ueff (J,T,H) = const.*In(J)*(1-T/Tc)n/H, where n is a constant determined from the irreversibility line.

A New Method of Determining Tc(H) of Single-Crystal La2−xSrxCuO4 from the Field-Dependent Susceptibility Above Tc

Magnetic susceptibility of superconducting single-crystal La2−x Sr x:CuO4 (x = 0.12, 0.14 and 0.19) was measured with a SQUID magnetometer from 30 to 300 K under the magnetic fields (H) of 1 ~ 5 T. The field- dependent susceptibility is evident below 40 K under H perpendicular to the CuO2 planes and is assigned to the superconducting fluctuation-induced diamagnetism (SFD). The field-independent part was subtracted by taking the difference of the susceptibilities for the different H. The field-dependent part was fitted by using the two-dimensional form of the Lawrence-Doniach model for SFD. The magnetic-field dependence of the superconducting-transition temperature was obtained by this analysis. The values of the in-plane coherence length at 0 K are determined as 28.3, 29.2 and 32.5 A for the samples of x = 0.12, 0.14 and 0.19, respectively.