tailieunhanh - Superconductor_2

This book contains a collection of works intended to study theoretical and experimental aspects of superconductivity. Here you will find interesting reports on low-Tc superconductors (materials with Tc 30 K). Certainly this book will be useful to encourage further experimental and theoretical researches in superconducting materials | 8 Charged Particle Irradiation Studies on Bismuth Based High Temperature Superconductors MgB2 A Comparative Survey Variable Energy Cyclotron Centre 1 AF Bidhan Nagar Kolkata-700 064 India 1. Introduction In the field of superconductivity the discovery of Lanthanum Cuprate La2-xSrxCuC 4 ushered in a new era- the so called High Tc superconductors HTSC . High Tc Cuprate Superconductors are quite intriguing and unique in their behaviour in contrast to their low Tc counterparts. Defects and disorder play a crucial role in controlling various physical properties like Tc resistivity Critical Current Density Jc etc. in these hole doped superconductors. The nonstoichiometries in these compounds in particular with respect to oxygen bring out fascinating properties oxygen playing the role of hole carrier. These compounds are based on layered perovskite structure. Superconductivity essentially resides in CuC2 plane with other layers containing multivalent metal ions functioning as charge reservoir layers pumping holes or electrons to the superconducting CuC2 layer and thereby controlling the Cu-C-Cu coupling and Tc. The cuprates are essentially quasi 2-dimensional systems with a weak interlayer coupling along c-direction between two CuC2 layers residing in ab-plane. This also gives rise to anisotropy in various physical properties like conductivity Jc etc. It is seen that Tc increases in general with more number of CuC2 layers and with more anisotropy. This millennium saw a non cuprate system MgB2 which is quite simple compared to cuprates yet with a fairly high Tc of 40K. This has got some similarity with the conventional superconductors in that it is BCS type superconductor with holes in the antibonding band of Boron coupling with phonons of E2g vibrational mode. MgB2 possesses hexagonal AlB2 type structure with Mg ions sandwitched between boron hexagons. Boron is sp2 hybridised with in plane ơ-band primarily participating in superconductivity and the out .