4PO7-30 Origin of The T_c- suppression in (R_1-xPr_x)Ba₂Cu₃O₇

J.G. Lin 1, C.Y. Huang 1, and J.C. Ho 2. 1 Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan. 2 Department of Physics and National Institute for Aviation Research, Wichita State University, Wichita, KS 68260-0032 USA.

Presenting Author: J.G. Lin

We have systematically measured the normal state resistivity (r ), superconducting transition temperature (T_c) and the pressure effects of r and T_c (dT_c/dP and dr /dP, respectively) for the series of (R_1-xPr_x)Ba₂Cu₃O₇ samples with x = 0 to 0.3 and R = Yb, Dy and Nd. Our results have shown that the pressure effect on T_c is closely related to the pressure effect on the normal state resistivity, underlining the important role of the carrier concentration in determining T_c. The salient findings in this study include: (1) the negative dT_c/dP observed in the samples with high Pr-concentration were accompanied by a positive dr /dP, while the positive dT_c/dP in low Pr-concentration samples were with a negative dr /dP, and (2) the x-dependency of T_c and dT_c/dP varies with changing the R-ion in a systematic way with respect to the size of R-ion. In order to explain our data, we have used a phenomenological model by incorporating the concept of pressure-induced hole depletion into the model of pressure induced charge transfer. A good agreement between the experimental data and the theoretical fitting prompts us to believe that the T_c-suppression induced by Pr-doping originates from the heavy hybridization of Pr-4f and CuO₂ conduction bands.

*Supported by the National Science Council of R.O.C. under grant No. NSC 89-2112-M002-039.

4PO7-32 Strong dependence of superconducting transition temperature (T_c) on the rare earth ionic size in REBaSrCu₃O₇ (RE = Y, Dy, Nd and La) series

V.P.S. Awana 1, S.K. Malik 2, W.B. Yelon 3, Ram Prasad 4, and A.V. Narlikar 5. 1 Max Planck Institüt für festkörperforschung, Heisenbergstrasse-1, Stuttgart D-70569, Germany. 2 Tata Institute of Funadamental Research, Homi Bhabha Road, Mumbai 400005, India. 3 Research Reactor Facility, University of Missouri, Columbia, MO 65211, USA. 4 Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, India. 5 National Physical Laboratory, K.S. Krishnan Marg, New Delhi 110012, India.

Presenting Author: V.P.S. Awana

The REBaSrCu₃O₇ compounds with RE = Y, Dy, Nd and La have been synthesized by the standard solid state reaction route. Superconducting transition temperature (T_c), as measured by ac susceptibility technique, is 81K, 79K, 64K and 45K for samples with RE = Y, Dy, Nd and La, respectively. Rare earth dependence of T_c in REBaSrCu₃O₇ series is quite different than that observed in REBaSrCu₃O₇ (RE:123) series where T_c is nearly independent of the choice of the RE (except when RE=Ce, Pr and Tb). Neutron diffraction studies have been carried out on the REBaSrCu₃O₇ compounds and structural details have been obtained from the Rietveld analysis of the room temperature neutron diffraction data. The compounds with RE = Y, Dy are found to crystallize in the orthorhombic RE:123 structure (space group Pmmm) with orthorhombicity considerably smaller than that of the RE:123 compounds. In fact, the orthorhombicity reduces so much that the REBaSrCu₃O₇ compounds with light rare earths, RE = La and Nd, are tetragonal or almost tetragonal .

4PO7-33 Crystal structure of (Pb_1-xM_x)Sr₂(Y_1-yCa_y)Cu₂O_z and its Physical Properties

Y. Ichimaru, K. Satoh, S. Kambe, O. Ishii, and K. Yamaguchi, Graduate School of Science and Engineering, Yamagata Univ, 4-3-16, Jonan, Yonezawa, 992-8510, Japan

Presenting Author: Y. Ichimaru

It is well-known that (Pb_1-xM_x)Sr₂YCu₂O_y (M=Bi, Cu, V, Cd, Mg, Sr and Ca) compounds show superconductivity whose T_c ranges from 40K to 104K.

For exploring novel Pb-contained 1212 compounds, we examined the conditions for preparing the (Pb_2/3M_1/3)-1212 phase and the kind of elements substituted for the Pb site. First, quenching treatment was essential for forming a single phase of the (Pb_2/3M_1/3)-1212 compound (M = Cu, Fe, Ga, In and Ni).

It is revealed that when the M element is divalent or trivalent, a (Pb_2/3M_1/3)-1212 single phase was formed, indicating that a charge balance is an important factor for forming the (Pb_2/3M_1/3)-1212 phase. When M is silver, (Pb_2/3Ag_1/3)-1212 phase including some impurity. Particularly, preparation conditions such as M content and heating temperature were examined in detail.

We also investigated the substitution of Ca for Y for doping holes. It was found that Ca is substituted for Y for the (Pb_1-xM_x)Sr₂(Y_1-yCa_y)Cu₂O_z compounds when the M is divalent and trivalent.

4PO7-34 Influence of the Oxygen Content on the Superconductivity of Pr_1-xCa_xBa₂Cu₃O_7-d Synthesized Under High Pressure

Yufeng Xiong, Yushu Yao, Zhaojia Chen, Yongmin Ni, Duo Jin, and Zhongxian Zhao, Institute of Physics & Center for Condensed Matter Physics, CAS, P.O. Box 2711, Beijing 100080, China

Presenting Author: Y.F. Xiong

We have recently reported the 98 K superconductivity in bulk Pr_1-xCa_xBa₂Cu₃O_7-d , synthesized under high pressure. Our results demonstrated hole depletion is the primary cause of the suppression of superconductivity by Pr in PrBa₂Cu₃O_7-d and suggested that weakening the couple between the rare earth element and CuO₂ planes would be beneficial to raise the critical temperature in 123-structure.

In this study, we have investigated the influence of the oxygen content on the superconductivity of Pr_1-xCa_xBa₂Cu₃O_7-d . By changing the high pressure synthesis oxygen environment and the oxygen content of the precursors, we got a serials samples with different oxygen content. The full oxygen content x=0.5 sample only have 52 K T_c, but the suitable oxygen deficient sample has the T_c up to 106 K. In oxygen deficient high pressure synthesized Y_0.7Ca_0.3Ba₂Cu₃O_7-d sample we also got the 96 K T_c. These results imply Ca doping has introduced holes to the CuO₂ planes and compensated the hole depletion by oxygen deficient and Pr variant valence higher that 3+ . The highest T_c can be got when these three factors arrive a suitable condition.

4PO7-35 Oxygen States on the Cu-O Chain of YBa_2-xSr_xCu₃O_y

D.H. Ha, S. Byon, H.S. Min, and K.W. Lee, Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-600, Korea

Presenting Author: D.H. Ha

We have measured the mass changes of YBa_2-xSr_xCu₃O_y samples (0 £ x £ 1.0) with heating each up to 950° C at a rate of 2° C/min in Ar atmosphere. For pure YBa₂Cu₃O_y sample, there is only one maximum around 480° C in the curve of differential mass change with the temperature, whereas, another maximum was observed around 630° C for Sr-substituted ones. As-sintered samples show sharp superconducting transition. After annealing in Ar atmosphere, however, Sr-substituted samples show two-step superconducting transitions while the transition width of pure YBa₂Cu₃O_y sample remains relatively sharp. These results suggest that there are two states of oxygen on the Cu-O chain in YBa_2-xSr_xCu₃O_y superconductor. One is the same as that of pure YBa₂Cu₃O_y sample, and the other is more tightly bound state around the Sr ions due to the local contraction of the structure. The concentration of the latter increases as the Sr concentration increases.

*4PO7-36 Crystal growth and superconductivity of (Y,Ca)Ba₂Cu₃O_7-y with BaF2 additive

C. Chen, F. Wondre, A.J.S. Chowdhury, J.W. Hodby, and J.F. Ryan, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom

Presenting Author: C. Chen

The substitution of Ca for Y in single crystal YBCO provides an important system for investigating the effects of doping on superconductivity, as it covers a wide range from under- to over-doping. Based on our study on the improvement of superconductivity and crystal growth by additives, high quality crystals have been grown from the eutectic self-flux system with BaF2 additive. Single crystals of size up to 2x2 cm were obtained from non-contaminating yttria or BaZrO₃ crucibles. A cluster model was proposed to understand the function of F ion in the high temperature solution. The distribution coefficient of Ca between the solid and the liquid phases is <<1, which implies that crystals with Ca doping level > 0.2 are unfavourable thermodynamically. The preference of six-fold co-ordination for Ca doping in Y-site distorts the conductive layers of Cu-O planes, but has little effect on the lattice parameters for the low doping concentration. At high doping concentration, a small fraction of Ca also substitutes at the Ba-site close to Cu-O chains, and the crystals become increasingly tetragonal. Considering the disortion in planes and the substitution in Ba-site by Ca, modifications have been made for the calculation of the effective hole concentration, which leads to fit better into the universal parabolic relation between T_c and the hole concentration.

4PO7-37 Phase Formation and Superconductivity in [Y_1-xCa_x]SrBaCu_2.80(BO₃)_0.20O_y with 0.10 £ x £ 0.50

Angel Bustamante Dominguez 1, Víctor Hugo Barinotto C. 1, Jesús Flores S. 1, and Flavio Garcia 2. 1 Facultad de Ciencias Físicas, Universidad Nacional Mayor de San Marcos, Apartado Postal 14-0149, Lima - 14, Perú. 2 Centro Brasileiro de Pesquisas Fisicas, Departamento da Materia Condensada e Espectroscopia, Rua Xavier Sigaud 150, Urca, R.J. - Brasil, CEP 22290-180.

Presenting Author: A. Bustamante Dominguez

It was discovered that oxyanion groups - (CO₃)^2-, (NO₃)^-, (BO₃)^3-, (SO₄)^2-, (PO₄)^3- - can be also located on some cationic sites to provide further means for controlling the electronic properties of the superconducting regions. The coexistence of CuO⁴ and BO³ groups is common for many superconducting copper oxyborates. We report the phase formation and superconductivity in [Y_1-xCa_x]SrBaCu_2.80(BO₃)_0.20O_y with 0.10 £ x £ 0.50 prepared at normal pressure and temperature. The X-ray diffraction pattern shows that all samples have a tetragonal P4/mmm structure, indicating that the BO3 groups were incorporated in the structure. The ZFC magnetization versus temperature measurements under an applied field of 10 Oe using a SQUID magnetometer show that the materials are superconductors, with T_c » 75K, almost Ca independent, the observed differences are related to the volume fraction of Meissner effect. In temperature range 100-300K all samples show a typical Curie - Weiss behaviour.

4PO7-38 Superconductivity and magnetism in the system (Y_1-xCa_x)SrBaCu_2.80(PO₄)_0.20O_y with 0.10 £ x £ 0.40

Juan C. Gonzalez 1, Angel Bustamante 1, Albino Aguiar 2, and David Landinez 2. 1 Laboratorio de Superconductividad, Facultad de Ciencias Fisicas, Universidad Nacional Mayor de San Marcos, Apartado Postal 14-0149, Lima 14 - Peru. 2 Departamento de Fisica, Universidad Federal de Pernambuco, 50670-901 Recife PE, Brazil.

Presenting Author: J.C. Gonzalez

Oxy-anion (PO₄)^3- substituted phases of the composition [Y_1-xCa_x](SrBa)Cu_2.80(PO₄)_0.20O_y with 0.10 £ x £ 0.40 prepared at normal pressure and temperature conditions have been synthesized. X-ray diffraction pattern show that the samples containig Ca crystallize into 123 tetragonal structure with little impurities and Rietveld method was used to refine lattice constants. Tha ac susceptibility measurements using a SQUID show that the materials are superconductors with the variation of Ca concentration (for x = 0.10 T_c = 46° K and x = 0.40 T_c = 75° K). This demonstrates the importance of doping of Ca²⁺ into the site Y³⁺ in the generation of the holes in the CuO² layers. In the normal region (100 - 300° K) measurements of dc susceptibility show a typical Curie-Weiss behavior.

4PO7-39 Non conventional magnetic behavior of tetragonal YBa₂Cu₃O₆ doped with oxygen or lithium

M. Nicolas-Francillon 1, F. Maury 2, and P. Monod 3. 1 Laboratoire de Physique du Solide, ESPCI, 75231 Paris, Cédex 05, France. 2 Laboratoire des Solides Irradiés. Ecole Polytechnique, 91128 Palaiseau, Cédex, France. 3 LPMC-ENS, 75005 Paris, France.

Presenting Author: M. Nicolas-Francillon

Magnetization of pure tetragonal YBCO_6+e with various oxygen contents and of lithium doped YBCO_6+e samples was studied as a function of field (0<H<5T) and temperature (<300K). Below a given temperature, very particular magnetization curves as a function of magnetic field with no saturation effects are observed. Large deviations from simple paramagnetism appear and a Langevin model of superparamagnetism is not sufficient to understand all the data especially at very low temperature. The magnetization follows a very simple law provided the temperature is sufficiently low: M = M₀*Hⁿ with M₀ varying as » T^-3 and 0<n<1. All the data can be scaled with respect to either concentration or temperature. The magnetization of samples of different purity was studied as a function of temperature and field in order to detect specific impurity effects. However, the M(H) curves are always very similar to that of the spin glasses. Perhaps the studied systems undergo a transition from a paramagnetic state to a frozen spin state due to frustration. If so, the frustrated paramagnetic ions would be the oxidized Cu(1) ions of the chains in the A.F. phase or plane copper moments released by the Li substitution for Cu leading to a partially frozen disordered 2D phase.