4PO7-70 Transport mechanisms in Zn, Ni and Mg doped La_1.85Sr_0.15CuO_y systems

Hongjie Yan 1, Qingming Zhang 1, Bin Wang 2, Zhiqiang Mao 2, and Yuheng Zhang 2. 1 National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing, 210093, P.R. China. 2 Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui, 230026, P.R. China.

Presenting Author: Q.M. Zhang

Different transport mechanisms have been comparatively studied for La_1.85Sr_0.15Cu_1-xM_xO_y (M=Zn, Ni, Mg)(0 x 0.3) systems. Structure analysis shows that among the Zn, Ni and Mg doped systems, Zn has the strongest influence on structural distortion. Phase transition from tetragonal to orthorhombic can be observed in Zn doped samples as x³ 0.15, and no similar transition is observed in Ni and Mg dopant systems. The resistivity measurements show that Zn and Mg dopings suppress the superconductivity more extensively than Ni doping, and Ni doping influences normal-state transport properties much strongly. For three doped systems the r (T) can be fitted using linear function r =r ₀+(dr /dT)T at temperature range of 150~300K. The fitting results reveal that r ₀ and dr /dT exhibit different dependence on x in different doping level, indicating that in lower doped level impurity scattering dominates the normal-state transport properties of three doped systems. And in higher doped level the different mechanisms are needed for the normal-state transport properties of three doped systems. Compared with magnetic measurements, the analysis suggests that different suppressions of the superconductivity may be closely related to magnetic correlation effects induced by Zn, Ni and Mg dopings.

4PO7-71 Different effects of Zn, Ni and Mg dopings on thermoelectric power in La-based cuprate systems

Hongjie Yan 1, Qingming Zhang 1, Bin Wang 2, Zhiqiang Mao 2, and Yuheng Zhang 2. 1 National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing, 210093, P.R. China. 2 Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui, 230026, P.R. China.

Presenting Author: Q.M. Zhang

Different effects of Zn, Ni and Mg dopings on thermoelectric power (TEP) for La_1.85Sr_0.15Cu_1-xM_xO_y (0 x 0.3) systems have been comparatively studied. Experimental results show that the temperature dependence of TEP exhibits quite different behaviors in three doped systems: in Zn doped samples the TEP values decrease as the Zn content increases at whole doped range. Yet, the TEP values increase with increasing x for both Mg (0£ x£ 0.3) and Ni (0£ x£ 0.2) doped series. An anomalous change can be observed in the Ni doped sample with x=0.3, its value is smaller than that of the sample with x=0.2. Furthermore, the TEP behavior of Ni doped samples is rather different from those of the Zn and Mg doped samples in higher doping level (x³ 0.1): Ni doping shows stronger influence on the TEP than Zn and Mg dopings. The analysis suggests that the unusual behavior of TEP for Zn doped samples should attribute to crystal distortion induced by Zn doping. The TEP behaviors of Ni and Mg doped samples in lower doped level (x<0.1) should be explained by variation of carrier concentration, and in higher doped level (x 0.1) their TEP behaviors can be interpreted by the magnetic correlation effects.

4PO7-72 Effects of Diamagnetic Dilution and Frustrating Second Nearest Neighbour Exchange on Neel Temperature in La₂Cu_(1-y)Zn_yO₄

Rebekah E. Martin, Byron C. Den Hertog, and Michel J.P. Gingras, Department of Physics, University of Waterloo, Waterloo ON, N2L 3G1, Canada

Presenting Author: R.E. Martin

Recent experimental work done on Zn diluted La₂Cu_(1-y)Zn_yO₄ has shown that the transition to long range Neel order disappears at a Zn concentration, y, which is well below the percolation threshold for that system. While the depression of T_N for increased concentrations of non-magnetic atoms is to be expected, the fact that the transition to an ordered state completely disappears around y=0.25 has not yet been satisfactorily explained. Also, nuclear quadrupole resonance (NQR) studies of this material have revealed a peak of the spin lattice relacation at a temperature T_f(y) < T_N(y) that possibly suggests a transverse freezing. We show that both the rapid depression of T_N and the existence of a transverse freezing temperature in this system can be explained by random frustration in the system. This random frustration arises from the combined effect of random dilution and the frustrating second nearest neighbour anti-ferromagnetic exchange interaction. This introduces a canting into the system which destroys long range order at larger dilutions. We will present numerical results obtained from quasi-2D XY and Heisenberg model Monte Carlo simulations which we compare to experiment.

4PO7-73 Effect of substitution way on the structure and superconductivity in La_1.85Sr_0.15Cu_1-xSn_xO_4-y and La_2-2xSr_2xCu_1-xSn_xO₄

G.D. Liu, G.C. Che, and Z.X. Zhao, National Laboratory for Superconductivity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China

Presenting Author: G.D. Liu

In this paper, the effect of substitution way on the structure and superconductivity in La_1.85Sr_0.15Cu_1-xSn_xO_4-y and La_2-2xSr_xCu_1-xSn_xO₄ systems were investigated. La_1.85Sr_0.15Cu_1-xSn_xO_4-y is a superconductor with the traditional substitution way. In the superconductor, Sn⁴⁺ ions replace for Cu²⁺ on the CuO₂ plane to induce extra electron in the conducting layer, which leads to decreasing T_c significantly. La_2-2xSr_xCu_1-xSn_xO₄ is a novel kind of (Sr,Sn)-doping La₂CuO₄. In this superconductor, Sr²⁺ ions substitute for La³⁺ to provide carries of hole, Sn⁴⁺ ions replace for Cu²⁺ on the CuO₂ plane to induce extra electron, and the number of Sr²⁺ and Sn⁴⁺ ions always keeps a strict ratio relation of 2:1 when changing x. It is significant that such double substitution effect of Sr and Sn would result in an offset of electrons and holes. At least, the offset effect would result in a rather low carrier concentration in La_2-2xSr_xCu_1-xSn_xO₄ compounds and has a significant effect on T_c. However, the superconducting critical temperature T_c keeps higher values, even T_c can reach about 33K for the sample with x=0.11. The reason why La_2-2xSr_xCu_1-xSn_xO₄ can keep higher T_c at heavy Sn-doping was explained by the model of amphoteric effect of Sn-doping on the carrier concentration.

4PO7-74 Low-temperature specific heat as a probe of quasiparticles in La_1.78Sr_0.22Cu_1-yNi_yO₄

J.-Y. Lin 1, C.F. Chang 2, and H.D. Yang 2. 1 Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan ROC. 2 Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan ROC.

Presenting Author: J.-Y. Lin

Recently, experiments of low-temperature specific heat (LTSH) have revealed valuable information about the electronic states of cuprate superconductors. These data also provide a critical test of theory about the magnetic field (H) and impurity effects on quasiparticles in cuprates. To make a comprehensive study on these issues, we have measured LTSH of La_2-2xSr_xCu_1-xSn_xO₄. For clean samples, a T² electronic contribution in LTSH has been identified, while it disappears in Ni-doped samples. Zero field g increases with Ni doping. In magnetic fields, the increase in g follows Hln(1/H) as predicted by theory. The scaling of LTSH breaks down with the presence of impurities.

4PO7-75 Superconductivity in Polycrystalline Sm_1.85(Ce_1-xTh_x)_0.15CuO_4-y Compounds

R.F. Jardim 1, E.A. Early 2, and M.B. Maple 3. 1 Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970, São Paulo, Brazil. 2 NIST, Gaithersburg, MD, 20899-8442, USA. 3 Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA, 92093, USA.

Presenting Author: R.F. Jardim

We report a systematic study on structural and superconducting properties of polycrystalline Sm_1.85(Ce_1-xTh_x)_0.15CuO_4-y; 0 £ x £ 1; samples sintered at temperatures as high as 1250 °C. Electrical resistivity and magnetic susceptibility measurements reveal that the onset of the superconducting critical temperature T_ci decreases systematically with the increasing Th content in this series. We have also found that T_ci in the compound Sm_1.85Th_0.15CuO_4-y assumes values as high as 13 K which is significantly higher than the one first reported of ~ 9 K. Measurements performed in samples subjected to different sintering temperatures T_s show that T_ci increases with increasing sintering temperature and attains its maximum value in samples sintered at 1200 °C. This feature is more pronounced samples with high Th content and sintered at lower temperatures T_s < 1100 °C, suggesting a very low Th diffusion into Sm₂CuO_4-y matrix. This latter assumption is confirmed by X-ray diffraction analysis which shows a systematic decrease in the volume fraction of Th-rich additional phase with increasing sintering temperatures. The results are qualitatively explained utilizing a simple model of inhomogeneous anionic and cationic distribution inside the grains.

4PO7-76 Different effect of Fe doping in La214 system and Bi2201 system

Gaojie Xu 1,2, Zejun Din 1, Zhiqiang Mao 2, Qirong Pu 1, and Yuheng Zhang 1. 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 samples of La_1.85-xSr_0.15+xCu_1-xO_4-d system and Bi₂Sr_1.6+xLa_0.4-xCu_1-xFe_xO_y system were synthesized. The crystal structure, transport properties and the state of Fe 3d electrons of these two systems were investigated by means of XRD, resistivity, thermoelectric power (TEP) measurement, and electron spin resonance (ESR) spectrum. Resistivity results show that Fe doping in the both systems leads to the remarkable increase of resistivity, and causes the charge transport behavior to change from a metal to an insulator. TEP measurements indicate that Fe doping results in the absolute value of dS/dT decreasing. An obvious upshift of the temperature dependent TEP peak with Fe content increasing is observed for the La214 system, which is contrast with the Bi2201 system. ESR results show that a strong ESR signal coming from Fe⁶³⁺ spins was detected in Fe doped La214 system. However no ESR signal was observed for Bi₂Sr_1.6+xLa_0.4-xCu_1-xFe_xO_y. It is suggested that there exists different state of Fe 3d electrons between Fe doped La214 system and Bi2201 system. The transport properties of the two systems are discussed based on these experimental results.

4PO7-77 A comparative study between chemical and hydrostatic pressure on the Hg,Re-1223 system

Marcos T.D. Orlando 1, Evandro V.L. Mello 1, A.G. Cunha 1, H. Belich 2, T. Burghardrt 3, A. Eichler 3, A. Sin 4, X. Obradors 4, and E.M. Baggio-Saitovitch 2. 1 Depto. Física, Universidade Federal do Espírito Santo, Vitoria ES 29060-900, Brazil. 2 Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro RJ 22290-180, Brazil. 3 Institut für Technische Physik, TU-Braunschweig, Braunschweig 38106, Germany. 4 Instituto de Ciencias de Materiales de Barcelona (CISC) Barcelona E-08193, Spain.

Presenting Author: M.T.D. Orlando

A comparative and quantitative study of chemical pressure and low external hydrostatic pressure (< 1 GPa) on the Hg_1-xRe_xBa₂Ca₂Cu₃O_8+y, (Hg,Re)-1223, (0.0 < x < 0.30) pellets has shown that the samples with x = 0.18 nominal content has the larger measured dT_c/dP=6.8(2)K/GPa. The samples quality was verified by means of Rietveld analysis of the X-ray diffraction data, AC susceptibility, and by AC resistance. There was a c-axis reduction between x=0.00 up to x=0.30, without any a-axis modification (chemical pressure), however the Re content variation do not change T_c. On the other hand, the resistance as function of temperature measurements have revealed an increase of T_c under applied external hydrostatic pressure, which depends strongly on the Re doping value. Taken into account that all different Re content samples have optimum oxygen dopping, we argue that the Re concentration does not increase T_c, as the external hydrostatic pressure does, but provides a strong influence on the intrinsic term (dT_c'/dP).

4PO7-78 Substitution Effects in Mercury Superconductors

Karel Knizek 1, Miroslav Veverka 1, Emil Pollert 1, and Ingrid Bryntse 2. 1 Institute of Physics ASCR, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic. 2 Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-10691, Stockholm, Sweden.

Presenting Author: K. Knizek

The effect of Sr substitution for Ba in Hg-superconductors was investigated. The samples were prepared by the encapsulation method under an oxygen partial pressure controlled by the redox couple Mn₃O₄/Mn₂O₃. The oxygen pressure during preparation influences the resulting phase composition and the oxygen stoichiometry of the superconductor. The critical temperature is decreasing with Sr content. However, for an individual oxygen pressure the increasing dependence of the oxygen stoichiometry on the Sr content was observed. This behavior is explained by different equilibrium oxygen pressures over phases with different Sr content. In order to discern the actual influence of Sr/Ba substitution, samples of equal oxygen stoichiometry, i.e. samples annealed under different oxygen pressures, should be compared. An explanation for these observations is proposed, based on structural changes introduced by replacing Ba by smaller Sr. The appropriate coordination of Ba/Sr site is formed by a cooperation of oxygen O₂ (in BaO plane) shift in c-direction and increased occupation of O₃ site (in HgO plane).

4PO7-79 Enhancement of superconductivity in the double mercury layer compound Hg₂Ba₂(Y_1-xCa_x)Cu₂O_8-d by Pb-doping

P. Toulemonde 1, A. Sin 1, P. Bordet 1, J.L. Tholence 2, and P. Odier 1. 1 Laboratoire de Cristallographie, CNRS, 25 avenue des martyrs, F-38042 Grenoble cedex 09, France. 2 LEPES, CNRS, 25 avenue des martyrs, F-38042 Grenoble cedex 09, France.

Presenting Author: P. Odier

The mercury compound Hg-2212 is a promising superconductor. Under pressure it presents the highest T_c enhancement ever measured: 50 K under 20 GPa. Nevertheless the compounds, synthesized under high pressure - high temperature, remain underdoped because of the Ca solubility limit (^xCa < 0.40).

To reach the optimal T_c we have used Pb doping (on Hg site) to fill up the oxygen vacancies of the bilayer Hg₂O_2-d and thus stabilize the structure. By this HP-HT way we achieve a maximum T_c onset of 84 K and the full substitution of Y by Ca. However the diamagnetic transitions are broad.

To improve the superconducting transitions we have tried a "sol-gel technique" for the " Ba₂(Y_1-xCa_x)Cu₂O_y " precursor synthesis. This process permits to have very fine homogeneous and reactive powders. Then we have done the final synthesis of (Hg_1-yPb_y)₂Ba₂(Y_1-xCa_x)Cu₂O_8-d series by reacting mercury (gas) on the precursor (solid) in a vacuum sealed quartz tube. The properties of sol-gel processed compounds are compared with those formed by HP-HT technique.