4PO7-60 Effect of V-Substitution on Superconductivity of Bi-2223 System

D.R. Mishra, Dipten Bhattacharya, and R.G. Sharma, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India

Presenting Author: D.R. Mishra

The effect of substitution of Vanadium (V) in Bi-Pb-Sr-Ca-Cu-O ceramic at Bi and Cu sites has been investigated in detail by carrying out X-ray diffraction (XRD), dc resistivity, ac susceptibility, and low field (1-10 Oe) ac magnetisation studies. Enhancement in T_c for smaller concentrations (up to 0.15 at%) of V at either of the two sites was observed which was followed by a drop in T_c for the higher V concentrations (up to 0.5 at%). V plays a role of substituting element more than just a sintering agent as reflected in the rapid decrease in T_c when substituted at the Cu-site. Further, the magnetisation values are found to be higher in the substituted samples. Substitution of Cu-site as opposed to Bi-site yields even higher magnetisation. The drop in T_c and rise in magnetisation can be understood from the point of view of magnetic pair breaking effect and flux-pinning by the defects. The difference in the rate of drop in T_c and rise in irreversible magnetisation depending on whether V is substituted at Bi or Cu site points out that V, indeed, is incorporated in the Cu-O plane and the reservoir block and substitution in Cu-O plane is far more sensitive in modulating the T_c and the irreversible magnetisation.

4PO7-61 Possible phase separation in Pb-doped Bi-2223 superconductors

Liuwan Zhang 1, Tingguo Chen 2, Bisong Cao 1, Meihong Zhu 1, and H.S. Huang 1. 1 Physics Department, Tsinghua University, Beijing 100084, China. 2 Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050.

Presenting Author: H.S. Huang

Upon annealing in nitrogen at 730° C, the resistivity of the Pb-doped Bi-2223 superconductors was initially increased,then slowly decreased.There exists a resistivity peak during the subsequent heat cycling. Furthermore the relaxation process at the temperature below and above the peak temperature is much different. In addition many transition steps were observed in the ac susceptibility curve. All above phenomena were not observed for both Pb-free and Pb-doped Bi-2212 superconductors.Phase separation and phase transition can explain our results.

4PO7-62 The Effect of Cr-doping on the Development of (Bi, Pb)-2223 Phase

Minghua Pu 1,2, Wenhai Song 1, Bing Zhao 1, Weidong Huang 1, Yuping Sun 1, and Jiaju Du 1. 1 Institute of Solid State Physics, Academia Sinica, P.O. Box 1129, Hefei 230031, China. 2 Department of Material Science, University of Science and Technology of Wuhan, Wuhan 430081, China.

Presenting Author: Y. Pu

The phase development of (Bi,Pb)-2223 superconducting materials doped with and without Cr-doping has been studied. CrO3 with amounts no more than 2wt% is added prior to sintering. XRD analysis shows that the formation rate of (Bi,Pb)-2223 phase increases with the increase of Cr content, and that the valence of Cr in the doped samples is changed from +6 to +3 during the development of (Bi,Pb)-2223 phase. Results of ac-susceptibility indicate that T_c of (Bi,Pb)-2223 is not sensitive to Cr content. The effect of Cr-doping on the flux pinning of (Bi,Pb)-2223 is also investigated, which suggests that chromic oxides may be a suitable candidate to improve the flux pinning of (Bi,Pb)-2223 superconductors.

4PO7-63 Epitaxial strain-induced insulator-superconductor transition in undoped and lightly-doped La₂CuO₄

X.X. Xi and Weidong Si, Department of Physics, the Pennsylvania State University, University Park, PA 16802

Presenting Author: X.X. Xi

In high temperature superconducting (HTS) cuprates, the antiferromagnetic insulating parent materials are transformed to superconductors when charge carriers are doped into the system. For example in La₂CuO₄ (the ``214'' compound), superconductivity is obtained either by substituting La<sup>3+</sup> with Sr<sup>2+</sup> (La<sub>2-x</sub>Sr<sub>x</sub>CuO<sub>4</sub> with x>0.05) or by inserting interstitial O<sup>2-</sup> (La<sub>2</sub>CuO<sub>4-d</sub> with d >0.01), both doping holes into the CuO<sub>2</sub> planes. Here we report the result of pulsed-laser-deposited undoped (x=0) and lightly-doped (x=0.02) ``214'' thin films which demonstrates that the insulator-superconductor (I-S) transition in HTS can also be induced by epitaxial strain. When deposited in pure molecular oxygen atmospheres, the ``214'' films are insulating under tensile strain, but as the strain is gradually changed to compressive the films eventually become superconducting. The result shows that the effect of epitaxial strain, in this work as well as in a previous work by Locquet et al., is to influence the insertion of interstitial oxygen into the ``214'' films, which contributes to both hole doping and interlayer coupling.

4PO7-64 Cu²⁺ spin localization induced by Oxygen-deficiency in CuO plane

Gaojie Xu 1,2, Changjin Zhang 2, Zhiqiang Mao 2, Zejun Din 1, Qirong Pu 1, and Yuheng Zhang 2. 1 Department of Astronomy and Applied Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China. 2 Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China.

Presenting Author: G. Xu

Single-phase La_2-xSr_xCuO_4-d samples with various oxygen content were synthesized. The transport properties and magnetism of La_2-xSr_xCuO_4-d system were studied by means of resistivity increases rapidly with the increase of oxygen deficiency of CuO₂ plane, and metal-insulator transition appears in this course. The ESR signals coming from Cu²⁺ spins also strengthen with the increase of oxygen deficiency. Oxygen-deficiency in CuO₂ loch plane destroys the ideal CuO octahedra and causes the Cu²⁺h spins localization. The relation between the localized Cu²⁺bch spins scattering and the charge transport in CuO₂ layer is discussed based on the experimental results.

4PO7-65 High Temperature X-ray Diffraction Investigation of La₂CuO_4+d

C. Dong, L. Chen, F. Wu, H. Chen, G.C. Che, and Z.X. Zhao, National Lab. for Superconductivity, Institute of Physics, Chinese Academy of Siences, P.O. Box 603, Beijing 100080, P.R. China

Presenting Author: C. Dong

Oxygen-rich La₂CuO_4+d (d =0.08) sample prepared by electrochemical oxidation was studied using high temperature X-ray diffraction and thermal gravimetric analysis (TGA). The results give an interesting picture about the phase transitions during the heating process. When temperature is higher than 80°C, the phase with T_c of 37K decomposes into two new phases. One is an oxygen-rich (with d near 0.09) phase with T_c around 32K and another is a comparatively oxygen-poor phase (d around 0.06) with T_c about 17K. The O/T phase transition temperature of the 17K-phase take places around 150°C. The oxygen loss is not obvious until 180°C according to the TGA experiment. Abrupt oxygen loss occurs around 200°C, and the oxygen-rich 32K-phase and the 17K-phase will gradually discompose. Sample will exist in AF and SC phase separation region when heated to 450°C and cooled to room temperature. The reason for the two-step oxygen loss will also be presented.

4PO7-66 Spin and charge ordering in Cu-rich phase separated La₂CuO_4+d system

B.R. Zhao 1, X.L. Dong 1, P.S. Luo 1, Z.X. Zhao 1, L.-M. Peng 2, Y.M. Ni 1, B. Xu 1, and F. Wu 1. 1 National Laboratory for Superconductivity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 Beijing Laboratory of Electron Microscopy, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China.

Presenting Author: B.R. Zhao

Phase separation and magnetic correlation have been investigated for Cu-rich La₂CuO_4+d system. The evidence of the possibility of substitution of Cu²⁺ for La³⁺ was observed directly for the designed Cu-rich La₂CuO_4+d samples. It is shown that the antiferromagnetic exchange interaction indicated by T_N enhances within the suitable region of Cu-excess content. TEM and magnetization analyses show the charge and spin ordering directly by incommensurate modulation structure and ferromagnetic hysteresis loops, which closely associate with the degree of the development of phase separation. It is suggested that the detected ferromagnetic correlation is the display of polarized spin ordering state caused by the accumulated holes (in one kind of ordering state) due to phase separation.

*4PO7-67 Doping-induced enhancement of the transition rate of tilted O octahedra tunneling within multiwell potentials in La_2-xSr_xCuO₄

Francesco Cordero 1, Rosario Cantelli 2, Annalisa Paolone 2, and Maurizio Ferretti 3. 1 CNR, Area di Ricerca di Roma-Tor Vergata, Ist. Acustica "O.M. Corbino", I-00133 Roma, and INFM, Italy, 2 Univ. di Roma "La Sapienza", Dip. Fisica, P.le A. Moro 2, I-00185 Roma, and INFM, Italy. 3 Univ. di Genova, Dip. Chimica e Chimica Fisica, Via Dodecanneso 31, I-16146 Genova, and INFM, Italy.

Presenting Author: F. Cordero

Recent anelastic relaxation experiments (acoustic absorption versus temperature) demonstrated that the O octahedra in La₂CuO₄ can switch between the minima of a multiwell potential both in a slow collective fashion (also observed with NQR relaxation) and locally with a tunneling driven dynamics. New results are presented for low Sr doping (x £ 0.03) on the absorption peak (5 K for x = 0, f = 6 kHz), which indicates local tilt motion with a temperature dependent characteristic frequency t ^-1(T). The peak is drastically shifted to lower temperature for x > 0, demonstrating that doping enhances the local tilt rate t ^-1. These observations can be qualitatively explained supposing that a considerable fraction of the octahedra, presumably in correspondence to the frozen collective tilt waves, are able to tunnel between different tilt orientations forming tunneling systems (TS, like the two-level systems in glasses). The transitions of the TS between their eigenstates occur with a rate t ^-1 determined by the interaction between the octahedra with phonons and other excitations. The large enhancement of the rate even by small doping demonstrates that the local tilt rate is governed by charge and spin excitations.

4PO7-68 Evidence for Two type Charge Carriers in La2-xSrxCuO4: An Electron Paramagnetic Resonance Study

Alexander Shengelaia 1, Boris I. Kochelaev 2, Kazimierz Conder 3, Hugo Keller 1, and K.A. Muller 1. 1 Institute of Physics, University of Zurich, CH-8057 Zurich, Switzerland. 2 Department of Physics, Kazan State University, Kazan, 420008, Russia. 3 Laboratorium fur Festkorperphysik, ETH Zurich, CH-8093 Zurich, Switzerland.

Presenting Author: A. Shengelaya

A number of experiments on high-T_c superconductors suggest the possible existence of two quasiparticles: a heavy Jahn-Teller (JT) type polaron and a light fermion. In the present study we performed Electron Paramagnetic Resonance (EPR) experiments on La_2-xSr_xCuO₄ with different oxygen isotopes ¹⁶O and ¹⁸O. In order to observe the EPR signal, La_2-xSr_xCuO₄ was doped with 2% of Mn²⁺ ions which replace the Cu ions in copper-oxygen layer and serve as an EPR probe.In the very underdoped regime x<0.06 two distinct EPR lines are observed: a narrow line and a broad line. We found that the narrow line is not affected by oxygen isotope substitution, while the linewidth of broad signal exhibits a huge isotope effect. The linewidth increases by about factor of two upon substitution of ¹⁶O by ¹⁸O. We assign the narrow line to the light carriers and the broad one to the heavy polarons. With increasing doping the two EPR lines merge into a single EPR line, and the isotope effect on the linewidth gradually decreases towards optimum doping. Our results strongly suggest the presence of two quasiparticles in underdoped La_2-xSr_xCuO₄. Moreover, the isotope effect observed on the EPR signal provides the first microscopic evidence for polaronic charge carriers in cuprate superconductors.