ORAL SESSION 4C5: Substitution Effect on HTS

Thursday, Feb. 23, 10:00 a.m. – 12:15 p.m., Room 302A (GRB)

Chairs: J.D. Jorgensen (Argonne Nat'l. Lab), R.J. Cava (Princeton)

4C5.1 Nanodomains, Dopant Siting, Ideal Phase Diagram, and Filamentary Model of HTS

J.C. Phillips, Lucent Bell Labs. (Retired), Murray Hill, NJ 07974-0636

Presenting Author: J.C. Phillips

With the discovery of ferromagnetic HTS there is growing interest in filamentary models, which readily explain this phenomenon, which is incompatible with continuum (effective medium) theories of the macroscopic London or microscopic BCS type. For the last twelve years the author has developed such a theory, arguing that it is necessary to explain high T_c's. In the last three years the theory has been greatly strengthened through successful analysis of the metal-insulator transition in the simplest known cases (Si:P or Ge:Ga). These have been shown to exhibit all the main features of HTS, namely, an intermediate (non-Fermi liquid) phase; pseudogaps; giant electron-phonon interactions at dopants in the intermediate phase, and two distinct normal-state transitions, that at lower dopant density being smooth, that at higher density being first-order [1].

[1] J.C. Phillips, Phil. Mag. B 79, 527, 1477 (1999).

4C5.2 Mechanical against Chemical Pressure in High T_c Cuprates

M. Marezio, F. Licci, A. Gauzzi, and E. Gilioli, Institute MASPEC-CNR, Parco Area delle Scienze 37/A, 43010 Fontanini Parma, Italy

Presenting Author: M. Marezio

The effect of the Sr substitution for Ba on the critical temperature of all superconducting cuprates, with the exception of (La_1-xBa_x)₂CuO₄, results in negative dT_c/dx coefficients. On the contrary, the effect of mechanical pressure on T_c is usually positive. In the case of the Hg-based cuprates record-posting T_c's are obtained if the materials are subjected to mechanical pressure, provided that they are either underdoped or optimally doped.

By studying the system Y(Ba_1-xSr_x)₂Cu₃O_7-d in detail we show that the structural parameter which scales with T_c is the strain of the Ba layers. In this system the mechanical pressure and the Sr substitution for Ba, that is the chemical pressure, do not have the same effect on T_c, because the former increases the strain of the Ba layers whereas the latter decreases it. If the strain on the BaO layers is needed to optimize the charge transfer and consequently the critical temperature, the substitution of Ba with a smaller cation is not what is needed to increase T_c. Since in the periodic table among the divalent cations there is nothing larger than Ba except Ra, the only way to obtain a T_c increase in the system YBa₂Cu₃O₇, would be to synthesize YRa₂Cu₃O₇. More recent results seem to show, though, that the curve T_c=f(x) for Y(Ba_1-xSr_x)₂Cu₃O_7-d might go through a minimum, at least at d » 0.4.

4C5.3 Charge Neutral Dopings in 123 Compounds

Ö. Rapp and P. Lundqvist, Department of Solid State Physics, Kungliga Tekniska Högskolan, SE 100 44 Stockholm, Sweden

Presenting Author: Ö. Rapp

Charge neutral dopings on RE site in 123 compounds represent in a sense a minimal distorsion of the 123 superconductor with preserved orthorhombic structure, and small changes in lattice parameters and charge density. Yet the effect on the superconducting T_c can be dramatic.

Samples of the type (RE)_1-2xCa_xM_xBa₂Cu₃O_7-? with RE =Y, Sm, or Nd, and M= Th, or Pr, and with x up to 0.1-0.2, have been studied by EDS analysis in an electron microscope, X-ray and neutron diffraction, muon spin relaxation, and resistive measurements. T_c is linearly depressed with dopant concentration in all cases, with -dT_x/dx in the range 150- 230 K for varying combinations of RE and M.

The depression of T_c in CaTh doped systems was found to be correlated with an increase in the normal state resistivity. Conventional disorder theories can qualitatively describe this relation. For the CaPr doping, differences were found compared to CaTh doping both from the calculations of bond valence sums based on the interatomic distances, and from the normal state resistivity measurements.

4C5.4 CLBLCO: A Tetragonal Family of 1-2-3 Superconductors

Y. Eckstein, Department of Physics and the Crown Center for Superconductivity, Technion, 32000 Haifa, Israel

Presenting Author: Y. Eckstein

(Ca_xLa_(1-x))(Ba_(1.75-x)La_(0.25+x))Cu₃O_y ("CLBLCO") is a family of high-T_c superconductors having essentially the same structure as YBCO. However (a), the materials are tetragonal for all levels of oxygen doping and at all temperatures. Thus there are no ordered chains in the CuI layer; it is their presence in the usual 1-2-3 materials, which may complicate the interpretation of data. Moreover, (b) the doping level can be controlled over a wide range of y values, from complete underdoping to complete overdoping. These two features make this an attractive system to study. The system is charge-compensated, i.e. the average oxidation state (valence) of the copper is independent of x. However, T_c does vary with x, showing that there is internal charge transfer between CuII and CuI layers.

This review talk will cover the following topics: resistivity, thermoelectric power, µsr, oxygen relaxation (from measurements at high pressure), crystal structure (from neutron diffraction) and its relation to T_c, and the effect of doping with nickel (which substitutes for copper in both the CuI and CuII layers). The materials will be compared with YBCO, the flagship of the 1-2-3 family.

4C5.5 Superconducting and Magnetic Properties of Hg-1234, Cu-1234 and BC-1223 Superconductors with T_c above 115K

Sung-Ik Lee, National Creative Research Initiative Center for Superconductivity, Department of Physics, Pohang University of Science and Technology, Korea

Presenting Author: S.-I. Lee

We report the experimental results on the reversible magnetization of the four-layer compounds HgBa₂Ca₃Cu₄O₁₀ (T_c > 5 K) and CuBa₂Ca₃Cu₄O₁₀ (T_c > 7 K), and the three-layer compound B_0.6C_0.4(Sr_0.25Ba_0.75)₂Ca₂Cu₃O₉ (T_c > 9 K).

The magnetizations were analyzed using the Hao-Clem model, the modified Lawrence-Doniach model, the vortex fluctuation model, and the high-field scaling law. From these analyses, we obtained various thermodynamic parameters such as the penetration depth and the critical fields, and clarified the dimensional nature of the superconductors. Even though the structures look similar, the physical properties of these compounds are quite different. We will discuss the correlation between the transition temperature and the interlayer coupling or the carrier concentration within the CuO₂ plane. The role of the apical oxygen in superconductivity will be also discussed. If this measurement is properly analyzed, we can understand the secrets why the transition temperatures of these compounds are not higher than 135 K.

4C5.6 Substitution Effect on Bi₂Sr₂(Ca_1-xY_x)Cu₂O_8+d Studied by X-ray Absorption Spectroscopy

R.S. Liu 1, I.J. Hsu 1, J.M. Chen 2, R.G. Liu 2, L.Y. Jang 2, J.F. Lee 2, and K.D.M. Harris 3. 1 Department of Chemistry, National Taiwan University, Taipei, Taiwan, R.O.C. 2 Synchrotron Radiation Research Center (SRRC), Hsinchu, Taiwan, R.O.C. 3 School of Chemistry, University of Birmingham, Birmingham, United Kingdom.

Presenting Author: R.S. Liu

The Bi₂Sr₂(Ca_1-xY_x)Cu₂O_8+d system over the homogeneity range x = 0 ~ 1.0 has a maximal superconducting transition temperature (T_c) of around 92 K at x = 0.2. Across the homogeneity range x = 0.5 ~ 1.0, the materials undergo a Metal-Insulator Transition. The hole distribution of overdoped, optimum doped and underdoped states in Bi₂Sr₂(Ca_1-xY_x)Cu₂O_8+d has been investigated by high-resolution O K-edge and Cu L-edge X-ray-absorption near-edge-structure (XANES) spectra. Near the O 1s edge, a well-pronounced pre-edge peak with maximum at ~528.3 eV is found, which is ascribed to the excitations of O 1s electron to O 2p hole states located in the CuO₂ planes. The intensity of this pre-edge peak decreases as the Y doping increases, demonstrating that the chemical substitution of Y³⁺ for Ca²⁺ in Bi₂Sr₂(Ca_1-xY_x)Cu₂O_8+d gives rise to a decrease in hole concentrations within the CuO₂ planes. The results from the Cu L-edge absorption spectra are consistent with those from O 1s X-ray absorption. The local structure of the pyramidal CuO₅ in Bi₂Sr₂(Ca_1-xY_x)Cu₂O_8+d has been determined from extended x-ray absorption fine structure (EXAFS) spectra. It was found that the axial Cu-O bond distances contract and the equatorial Cu-O bond distances expand as the Y content increases. Such results are strongly correlated to a decrease in the hole concentration within the CuO₂ plane, which controls the superconductivity. Moreover, the correlation between T_c and critical current density (J_c) as a function of doping concentration will be reported. It is important to point out that the highest J_c across the system is appeared at the overdoping side with x = 0 in Bi₂Sr₂(Ca_1-xY_x)Cu₂O_8+d .

4C5.7 ¹³⁹La and ⁶³Cu NQR in Zn-doped Lanthanum Cuprate

B.J. Suh 1, F. Borsa 2, Z.H. Jang 2, Y. Zhuo 3, and S.-I. Lee 3. 1 Department of Physics, The Catholic University of Korea, Korea. 2 Ames Laboratory and Department of Physics, Iowa State University, Ames, IA. 3 Pohang Superconductivity Center, Postech, Korea

Presenting Author: F. Borsa

It is well known that the superconducting transition temperature T_c is dramatically suppressed by very small substitution of Zn for Cu in La_2-xSr_xCuO₄ high temperature superconducting cuprate. ¹³⁹La and ⁶³Cu NQR relaxation measurements have been performed on a series of La_1.87Sr_0.13Cu_1-yZn_yO₄ (y * 0.03) to investigate the effects of Zn-doping on the spin and/or hole dynamics in CuO₂ plane and hence on superconductivity. At low temperature, ¹³⁹La NQR spin-lattice relaxation rate T1-1 of Zn-doped sample exhibits a strong enhancement which is similar to the spin glass behavior observed in lightly hole doped La_2-xSr_xCuO₄. From the ⁶³Cu NQR T1-1, we found that the temperature (T*) of spin pseudogap [i.e., the maximum of (T1T)-1] decreases as T_c decreases by Zn doping. This is in contrast with the results in a low-temperature -tetragonal (LTT) phase rare-earth doped La_1.67Eu_0.2Sr_0.13CuO₄, where T_c is totally suppressed but T* is unchanged or even enhanced. These anomalous magnetic properties are discussed in the context of microscopic segregation of doped holes into hole-rich domain walls separating undoped spin domains.