Andreev reflections and experimental current–voltage characteristics of break junctions of polycrystalline HTSC M.I. Petrov a, * , D.M. Gokhfeld a,b,c , D.A. Balaev a , K.A. Shaihutdinov a , R. K€ ummel c a Kirensky Intitute of Physics, Akademgorodok, 660036 Krasnoyarsk, Russia b Reshetnev Siberian State Aerospace University, 660014 Krasnoyarsk, Russia c Institut f€ ur Theoretische Physik und Astrophysik der Universit€ atW€ urzburg, D-97074 W€ urzburg, Germany Abstract The temperature evolution of current–voltage characteristics (CVCs) of break junctions made from polycrystalline Y 0:75 Lu 0:25 Ba 2 Cu 3 O 7 and La 1:85 Sr 0:15 CuO 4 is investigated. The experimental CVCs have hysteretic features that reflect a part of a curve with negative differential resistance. The temperature evolution of the CVCs is discussed within the framework of the K€ ummel–Gunsenheimer–Nicolsky theory for superconductor/normal-metal/superconductor junc- tions considering multiple Andreev reflections. It is shown that the shape of the CVCs of break junctions is determined by the ratio of the number of ‘‘short’’ and ‘‘long’’ intergrain normal regions in the polycrystalline HTSC under investigation. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Andreev reflections; Break junction; Current–voltage characteristics 1. Break junctions in HTSC The break junction technique allows to investigate quantum phenomena in bulk HTSCs. The formation of a microcrack in a bulk sample leads to a significant reduction of the effective cross section for charge transport, A. In the non-tunneling case two massive polycrystalline banks are connected by weak links which constitute narrow bottlenecks. The critical current and the current–voltage characteristics (CVCs) of the whole sample are determined by the cross section A. For this reason we can measure CVCs at bias currents that are much smaller than the critical current of the bulk sam- ples, and self-heating effects and vortex motion should not matter. La 1:85 Sr 0:15 CuO 4 (LSCO) and Y 0:75 Lu 0:25 Ba 2 Cu 3 O 7 (YBCO) were sintered by standard ceramic technology. The T c values of LSCO and YBCO are 38 and 92 K respectively. Details of the break junction technique are described in [1]. Comparison of sample resistances be- fore the creation of the microcracks and after (typical resistance after: <10 X) shows that the value of A de- creased by factors between approximately 30 and 100. The CVCs were measured by four probe technique un- der bias current. 2. Current–voltage characteristics A typical CVC of an YBCO break junction is shown in Fig. 1. The hysteretic peculiarity reflects a part of a current ðI Þ versus voltage ðU Þ curve with negative dif- ferential resistance which one would be able to observe under voltage bias. The temperature evolution of the CVC of a LSCO break junction is shown in Fig. 2a. * Corresponding author. Tel.: +7-3912-494600; fax: +7-3912- 438923. E-mail address: smp@iph.krasn.ru (M.I. Petrov). 0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.085 www.elsevier.com/locate/physc Physica C 408–410 (2004) 620–622