LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996 Part $3 - Superconductivity 2: HTS - Weak links, etc. Tunneling Studies of Electronic State in High Tr Bi-System Morio Suzuki, Masanori Taira, and Xuguang Zheng Department of Physics, Faculty of Science and Engineering,Saga University, Saga 840, Japan The tunneling measurements were can'ied out for the junctions fabricated on a ceramic and a single crystal of Bi2Sr2CaCu2Os-z. It was found that the most tunnel junctions fabricated on a a-b plane of single crystal does not have a conductance peak (CP), while the most junctions on the ceramic have CP. Kashiwaya et al. theoretically predicts that the junction formed on a d-wave superconductor can give various tunnel conductance characteristics. In the present experiment, the tunneling characteristics in the high Tr Bi-system superconductor ware found to qualitatively agree with the theoretical prediction. 1. INTRODUCTION In spite of many experiments, the microscopic nmchanism, which brings a superconducting state in high Tc cupurate superconductors, has been unclear. The mechanism intimately relates to electronic states in a superconductor. From this view point, it is im- portant to make clear whether a high Tc cupurate is a s-wave superconductor or a d-wave superconductor. The tunneling measurement has given various tunnel conductance characteristics scattering from a s-wave to a d-wave. Recently, Kashiwaya et al. [1] theoret- ically predicted that the tunnel junction fabricated on a d-wave superconductor can give various tunnel conductance characteristics scattering from a s-wave to a d-wave. In order to test the prediction, in the present experiment the tmmeling measurement was carried out for two kinds of junctions fabricated on a high Tc Bi-system cermnic and a high T, Bi-system single crystal. 2. EXPERIMENTAL RESULTS AND DIS- CUSSIONS The high Tc Bi-systcm single crystal was pre- pared by a zone melting method, wtfile the high T, Bi-system ceramic by tlmrmal powder reaction. The tunnel junction was fabricated on the single crys- tal as following procedures. The single crystal was cleaved along an a-b plane. SiO is deposited on the a-b plane as a tunnel barrier, Ag was deposited on SiO film through a mask with a hole of 0.8 mm in a diameter, and then In was deposited on the whole back face as an ohrmic contact. These depositions were done in a vacuum of 5 x 10-s Tort. While the tunnel junction was fabricated on the ceramic as fol- lowing procedures. The ceramic surface was polished on a sandpaper, the depositions of SiO, Ag and In were performed by the above procedures. The tun- nel conductance characteristics, G(V) is obtained by a standard voltage modulation methods. The typical G(V) dlaracteristics are shown in Fig. l(a) for the Ag-Bi2Sr2CaCu2Os-x single- crystal junction and in Fig. l(b) for the Ag- Bi2Sr2CaCu2Os-x ceramic junction. As seen in Figs. l(a) and l(b), both G(V) characteristics in the two kinds of junctions at low temperature have a valley structure ranging from -30 mV to +30 mV, which is caused by a superconducting state. Of course, the structure weakens with increasing temperature, and disappear near a critical temperature of 90 K. The most tunnel junctions fabricated on the a-b plane have no CP at V=0 mV, while the most tmmel junc- tions fabricated oil the ceramic have CP. The height of CP also decreases with increasing temperature, mad disappear near the critical temperature of 90 K. Three models which bring CP have been proposed; 1) s-d exchange interaction [2], 2) Josephson effect [3], and 3) Andrcev reflection [1]. The C P caused by the s-d exchange interaction must tmdergo the Zee- man splitting under strong magnetic field. However, as seen in Fig. 2 the G(V) characteristics does not have any Zeeman splitting even at a magnetic field of 2.7 T. When the Josephson effect brings the CP, a tunnel current must flow across a junction just at V=O inV. However, in the present experiment there is no current at V=O mV, further the CP structure ranges from -10 mV to +10 inV. Therefore, the s- d exchange interaction and the Josephson effect are excluded from an origin of the CP. Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $3 1351