Eﬀect of the magnetic ﬁeld on the branching point of the resistive transition in the Y 1x Pr x Ba 2 Cu 3 O 7d system M.S. da Luz * , C.A.M. dos Santos * , B. Ferreira, A.J.S. Machado Grupo de Supercondutividade, Departamento de Engenharia de Materiais, FAENQUIL, 12.600-000, Lorena-SP AI-6, Brazil Abstract This work presents results about the inﬂuence of the applied magnetic ﬁeld on the branching point of the resistive transition in granular samples of the Y 1x Pr x Ba 2 Cu 3 O 7d (Y123 + Pr) system. Single phase samples of x ¼ 0:0 and 0.35 composition were prepared by solid state reaction method. Using several resistance curves as a function of the tem- perature measured at diﬀerent applied currents we deﬁned branching points at each magnetic ﬁeld which was ranged from 0 up to 9 T. We have observed that a line in the H –T phase diagram separates the weak coupling state from decoupled regime of the superconducting clusters. Measurements of current–voltage curves and magneto-resistance hysteresis loops agree with such discussion. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Branching point; Josephson coupling; Magneto-transport properties After the discovery of the high temperature super- conductors, eﬀects of weak coupling between isolated superconducting clusters or grains have received great attention [1–3]. The most common observable eﬀect is related to the broadening of the superconducting tran- sition which is marked by the existence of two distinct temperatures labeled as T Ci and T Cj . The T Ci deﬁnes the temperature in which Cooper pairs appear inside clus- ters and T Cj is the temperature below which supercon- ducting clusters are connected by Josephson eﬀect [1–3]. The ﬁrst regime is ohmic and the second is extremely current dependent [1]. Recently, our group observed an ohmic saturation of the magneto-resistance at high current limit in the Y 1x Pr x Ba 2 Cu 3 O 7d (Y123 + Pr) polycrystalline samples [4]. Based on this result we have suggested that a normal current can ﬂow in parallel with the supercurrent. This idea has been supported by the resistively shunted junction model [4,5]. In order to obtain more information about the cou- pling eﬀects in granular or weakly coupled supercon- ductors, we have studied the eﬀects of the magnetic ﬁeld on the branching point of the resistive transition in the Y123 + Pr system. Two polycrystalline samples of the Y123 + Pr system with x ¼ 0:0 and 0.35 compositions were prepared by solid state reaction method using high purity Y 2 O 3 , BaCO 3 , Pr 6 O 11 and CuO powders. These powders were mixed in appropriated amounts, calcined at 800 °C in air during 24 h, reground, compacted into pellets and sintered at 900 °C during 24 h. Crystalline phases were identiﬁed by X-ray powder diﬀractometry. Magneto- transport properties were measured by standard four probe method in an Oxford system using low-resistance sputtered Au contacts. Fig. 1 presents the resistance as a function of the temperature for the samples with x ¼ 0:0 and 0.35 measured at diﬀerent currents in applied magnetic ﬁeld of 4 and 1 T, respectively. Two distinct behaviors can be seen below the critical temperature T Ci . The ﬁrst regime, between T Ci and T Cj , is ohmic and the drop in the elec- trical resistance can be related to the formation of iso- lated superconducting clusters which reduce the equivalent resistance of the samples with decreasing the temperature. Below T Cj the resistance curves broad gradually with increasing the applied current. This * Corresponding authors. E-mail addresses: mariodaluz@bol.com.br (M.S. da Luz), cams@demar.faenquil.br (C.A.M. dos Santos). 0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.176 www.elsevier.com/locate/physc Physica C 408–410 (2004) 460–461