2PO9-80 In-situ thermoelectric power measurement during oxygen intercalation of La₂CuO_4+d in an electrochemical cell

Y.S. Song, Z.G. Li, and P.H. Hor, Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77204

Presenting Author: Y.S. Song

We have performed in-situ thermoelectric power measurement of La₂CuO_4+d during oxidation and reduction process in an electrochemical cell. The galvanostatic technique is used for the oxidation and reduction with a constant current of 10 m A. Thermoelectric power decreases monotonically from 580 m V/K to 25 m V/K upon charging up to Q=0.13, where d =Q/2. We will present thermoelectric power data as a function of charge carrier concentration.

2PO9-81 Superconductivity and charge redistribution under high pressure in the underdoped La_2-xSr_xCuO₄

Tatsuya Honma 1,2, Bernd Lorenz 1, Zugang Li 1, and Pei-Herng Hor 1. 1 Dept. of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77004, USA. 2 Department of Physics, Asahikawa Medical College, Asahikawa 078-8510, Japan.

Presenting Author: T. Honma

Superconducting transition under high pressure in La_1.916Sr_0.084CuO₄ was investigated by ac resistance measurement with a four probe method and by the ac mutual inductance technique to 17 kbar. The sample was prepared by solid state reaction. The superconducting transition curve obtained from both measurements showed a single transition at 1 bar. With increasing pressure, the resistive superconducting transition curve shifted to the higher temperature below ~9 kbar with single transition. Above ~9 kbar, the superconducting transition curve became broadening with increasing pressure. This broadening is reversible under pressure cycling. Similar results were also observed from the inductance measurement. The broadening of the superconducting transition curve under high pressure suggests that the single transition curve at 1 bar has transformed into double transition: a higher-T_c phase and a lower-T_c phase. In this system, it is well known that Hall coefficient does not change under high pressure. Therefore, the total hole concentration does not change under pressure. We attribute the double transition to the charge redistribution under high pressure.

2PO9-82 Different tunneling regimes in La_1.85Sr_0.15CuO_4-d break junction

F. Bobba 1, Y. Bugoslavsky 2, Anna Maria Cucolo 1, Andrea Jossa 1, H. Kojima 3, and I. Tanaka 3. 1 Physics Department and INFM Unit, University of Salerno, Via S. Allende, 84081 Baronissi, Italy. 2 General Physics Institute, Vavilov St. 38, 117942, Moscow, Russia. 3 Institute of Inorganic Synthesis, Yamanashi University, Miyamae 7, Kofu, Yamanashi 400, Japan.

Presenting Author: F. Bobba

We have performed tunneling spectroscopy on optimally doped La_1.85Sr_0.15CuO_4-d single crystals by break junctions tecnique. The optimally doped La_1.85Sr_0.15CuO_4-d single crystals were grown by the travelling solvent floating zone method, showing resistive transitions T_c(R = 0) = 30-34 K. By carefully adjusting the spacing between the two broken electrodes, we were able to investigate different conductance characteristics along the a-b planes in different tunneling regimes.

In the weak link regime, with junctions resistances less than few tens of ohm, point contact-like conductance characteristics were observed, due to weak contacts between the two electrodes. In this regime we observed the Josephson effect of the order of mA at low temperature, that decreases monotonically in temperature and vanishes at T » 30 K.

In the quasi-particle regime, in which the junction resistances ranged between few hundreds and few KW , we have found a well developed zero bias conductance peak and an increasing background for high bias. This observation is consistent with the Andreev bound states theoretically predicted for a d-wave symmetry of the order parameter for high T_c superconductors.

2PO9-83 Transport properties of Nd_2-yCe_yCuO_4-d with y ~ 0.14

T. Fujita, N. Kikugawa, M. Ito, K. Yamane, and T. Matsumoto, Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

Presenting Author: T. Fujita

Electron-doped Nd_2-yCe_yCuO_4-d (NCCO) shows an antiferromagnetic (AF) order for a wide range up to y ~ 0.14 due to the superexchange interaction between Cu²⁺ ions, as compared with typical hole-doped cuprates La_2-xSr_xCuO₄ in which AF order is suppressed rapidly with x. In order to study the transport properties in the carrier-doped NCCO, we have measured the in-plane and out-of-plane resistivities r _ab and r _c of NCCO around the border between the superconducting (SC) and AF regions.

After reduction by Ar annealing at 770 ° C for ~30 hours, the temperature dependence of r _ab(r _c) is metallic (nonmetallic) for y=0.13 above ~200K. In addition, we obtained the very large resistivity ratio typically r _c/r _ab 15000 in this temperature range. This suggests that electronic state of NCCO also has a two-dimensional nature common to the cuprates. In the AF phase, both r _ab and r _c increase logarithmically with decreasing temperature below ~40K, while anisotropy parameter decreases monotonously to r _c/r _ab 7000 for y=0.13 at ~4.2K.

The absolute value is as low as r _ab ~2.3m s cm for the AF sample at ~4.2K despite of the nonmetallic conduction. In the samples which shows SC transition, however, r _ab remains metallic down to T_c.

2PO9-84 Superconductivity in the tetragonal lattice of underdoped La_2-xSr_xCuO₄

T. Fujita, J. Hori, T. Goko, N. Kikugawa, and S. Iwata, Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

Presenting Author: T. Fujita

Experimentally superconductivity of La_2-xSr_xCuO₄ (LSCO) is observed mainly in the orthorhombic phase. However, a tetragonal lattice is assumed in most theoretical models, in which the tilting of CuO₆ octahedrons or buckling of CuO₂ planes is not taken into full consideration. Controlling the lattice by anisotropic pressure we recently achieved a substantial enhancement of the superconducting transition temperature T_c in optimally doped LSCO. In the underdoped region T_c is more sensitive to the orthorhombic distortion. Our new results are as follows. 1) Even at low pressures we succeeded in estimating the potential value T_c^T for the tetragonal lattice by extrapolating T_c in the high-pressure tetragonal phase. The T_c^T is always higher than the observed value T_c^O in the orthorhombic phase. 2) The T_c suppression is closely related to the degree of orthorhombic distortion or buckling and can be expressed as D T_c = T_c^T - T_c^O % T_d², where Td is the transition temperature from the tetragonal to orthorhombic structure. 3) The estimated T_c^T of under-doped LSCO at the ambient pressure decreases linearly with decreasing the carrier concentration x below 0.15, as is the case with the Bose-Einstein condensation. 4) It should be specially noted that T_c^T appears to drop abruptly from a finite value (10 K) to zero around at x = 0.05.

2PO9-86 Saturation of the phase coherence length at low temperatures in Pr_2-xCe_xCuO₄*

P. Fournier, C.J. Lobb, and R.L. Greene, Center for Superconductivity Research and Department of Physics, University of Maryland, College Park, MD 20742

Presenting Author: P. Fournier

We present a detailed analysis of the properties of Pr_2-xCe_xCuO₄ thin films in the low temperature, non-metallic, regime. We show that saturation trends previously observed in the resistivity at high fields (H > H_c2) and low temperatures (down to 30mK) in superconducting (SC) films [1], can also be observed for non-superconducting (NSC) cerium contents (x < 0.12). Using longitudinal and Hall conductivities, we show that the non-metallic regimes for x < 0.12 (NSC) and 0.12 < x < 0.15 (SC) are equivalent. Using 2D weak-localization theory, we evaluate the phase coherence length from fits to the magnetoresistance of NSC compositions. A puzzling saturation is shown to correlate with the saturations observed in the resistivity. We speculate on the possible origin of this saturation of the phase coherence length.

[1] P. Fournier et al., Phys. Rev. Lett. 81, 4720 (1998).

*Supported by NSF, grant DMR-9732736.

2PO9-87 Hall effect in Nd_1.85Ce_0.15CuO_y with controlled oxygen content

J.I. Martin 1, A. Serquius 2, F. Prado 2, D. Jaque 1, A. Caneiro 2, and J.L. Vicent 1. 1 Depto. Fisica de Materiales, Universidad Complutense, Madrid, Spain. 2 Instituto Balseiro y Centro Atomico Bariloche, CNEA, 8400, Argentina.

Presenting Author: J.L. Vicent

The superconducting behavior of the electron doped Nd_2-xCe_xCuO_y cuprates is extremely sensitive to small variations in the "x" and "y" contents. In this work, we have prepared Nd_1.85Ce_0.15CuO_y ceramic samples with accurate control of the oxygen content to study their electronic properties. In particular, the oxygen content "y" has been varied in the range 3.995< y <4.007 by annealing under controlled oxygen partial pressure at 900C and subsequent quenching at liquid nitrogen temperature. In this range the samples change from superconducting for the lowest oxygen content sample (T_c = 19K) to non-superconducting for the highest "y" value. Normal state Hall effect measurements have been performed up to 9 T and in the range 20-200K. The superconducting samples present a negative Hall effect with R_H almost temperature independent. The increase in the absolute value of R_H with oxygen content concentration will be discussed.

2PO9-88 Strong influence of magnetic field on the thermal expansion anomaly in La_2-xSr_xCuO₄ and Ba_1-xK_xBiO₃ systems

A.I. Golovashkin 1, N.V. Anshukova 1, L.I. Ivanova 2, I.B. Krinetskii 3, and A.P. Rusakov 3. 1 P.N.Lebedev Physical Institute RAS, Moscow, Russia. 2 Moscow Steel and Alloys Institute, Moscow, Russia. 3 Moscow State University, Moscow, Russia.

Presenting Author: A.I. Golovashkin

The strong influence of magnetic field on the thermal expansion anomaly has been observed in La_1.9Sr_0.1CuO₄ single crystals and Ba_0.6K_0.4BiO₃ polycrystals with pseudocubic lattice symmetry. The measured samples were well tested, pure without traces of magnetic impurities (<10^-17 cm^-3). The observed magnetic field influence was the most prominent in the area of negative thermal expansion at low temperatures. With magnetic field growing (up to 40 kOe) the temperature interval with negative a was decreasing. The strong anisotropy of a was measured for La_1.9Sr_0.1CuO₄ single crystals. The mechanism of magnetic field suppression of thermal expansion anomaly is discussed.

*2PO9-89 Thermal Conductivity of Sr₂RuO₄ in Oriented Magnetic Field

Makariy A. Tanatar, Shuichi Nagai, Yoshiteru Maeno, Zhiqiang Mao, and Takehiko Ishiguro, Dept. of Physics, Kyoto Univ., Kyoto 606-8502, Japan and Japan Science and Technology Corp., Kawaguchi 332-0012, Japan

Presenting Author: M.A. Tanatar

Thermal conductivity k was used for studying unconventional superconducting state of Sr₂RuO₄. High quality single crystals with T_c 1.37 K, close to the value for impurity free material (1.5 K), were used. Studies were performed for heat flow within conducting plane as a function of temperature (in 0.3 to 3 K range) and magnetic field, oriented both normal and parallel to the conducting layer. Linear decrease of k/T towards zero value at zero temperature is found. Field dependence of thermal conductivity in field normal to the layer is consistent with the behavior of type-II superconductor. This dependence gives lower and upper critical fields of 0.01 T and 0.075 T, respectively. In parallel magnetic field, however, k/T vs. H dependence is principally different from ordinary type-II behavior. It shows rapid restoration of conductivity towards normal state in narrow vicinity of the upper critical field. T-H phase diagram of this behavior is established. Field-induced formation of a superconducting state with nodes in the gap is discussed as a possible reason for the observed behavior.