ELSEVIER Physica C 281 (1997) 211-217 PHYmA© Metal-insulator transition in YBaz_xCaxCu 3 07-3 system M.K. Chattopadhyay a, S.K. Ghatak b, T.K. Dey a,* ~ High Temperature Superconductivity Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur 721302, India b Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Received 6 March 1997; revised 29 April 1997; accepted 12 May 1997 Abstract Temperature dependence of the electrical resistivity of Ca-substituted YBa2_xCaxCu307_ ~ pellets (with x = 0-2) is presented. A metal-insulator transition accompanied by six orders change in resistivity has been observed as Ca concentration changes from x = 0 to x = 2. Samples with low Ca-content display superconducting transitions but with much depressed Too. For samples with intermediate Ca concentrations, the resistivity (p) follows a In T dependence approximately between 90 and 200 K. The temperature range over which the In T dependence is observed shrinks with increasing Ca-content in the sample. Results have been discussed in the light of strain induced localization effect and marginal Fermi liquid model (MFL). © 1997 Elsevier Science B.V. 1. Introduction It has now been established that the superconduc- tivity in high-To oxides is intimately connected with unusual normal state properties. The unusual charac- teristics are most evident in thermal behaviour of transport properties viz., resistivity and thermo-elec- tric power. Most widely used procedures for unravel- ling the electronic conduction processes in these systems are the substitution of ions (either rare earth, alkaline earth or 3d element/similar ion) [1]. The replacement of copper in YBa2Cu307_ 8 (YBCO) with small amount of divalent (Zn, Ni etc.) and trivalent (Ga, A1 etc.) reduces the superconducting transition temperature (T~) and increases the resistiv- ity. The largest reduction of T c was found for Zn * Corresponding author. Tel.: +91 3222 55221 ext. 77390; fax: + 91 3222 55303. e-mail: tapasdey@hijli.iitkgp.ernet.in. substitution on the copper site in the Cu-O plane and is of the order of 13 K per atomic percent of Zn atom [2]. The substitution of Y by other trivalent lanthanides is found to produce less dramatic changes in superconducting properties [1,3]. It is generally observed that the c-axis increases monotonically with increase in the ionic radius of rare earth ions. The transition temperature T c decreases approximately linearly with increasing radius of rare earth ions [1,3,4]. This suppression of T c is qualitatively inter- preted in terms of hybridization between valence band states of CuO 2 layer and local 4f state of rare earth ion. The decrease in density of states at the Fermi level due to compression effect of rare earth ion can also be a contributing factor for the suppres- sion of To. The resistivity in the normal state around room temperature increases by an order of magni- tude with increase in ionic size of rare earth ion. The resistivity of RBa2Cu3_xZn/OT_ 8 at 295 K in- 0921-4534/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PIIS0921-4534(97)01439-1