Transport Properties of Mgb 2

1. Introduction The discovery of the superconductivity in MgB 2 [1] at a temperature as high as 39 K has stimulated a enormous interest about the comprehension of its mechanism [2][3]. The study of the electron transport properties may give an insight into the normal state conduction process and into the electronic structure of MgB 2. From this point of view the results are contradictory. In fact, some measurements, such as magnetoresistivity [4] and the Seebeck effect [5][6][7][8], indicate that MgB 2 behaves like a simple metal, while the Hall effect measurements [9] are very similar to those of the high temperature superconductors and the thermal conductivity [8][10] does not show any kind of anomaly at the superconducting transition. However, it should be stressed that most measurements have been performed on sintered samples in which grain boundaries can affect transport properties complicating a clear understanding of MgB 2 basic properties. Actually, since the coherence length, ξ≈ 50 Å, is larger than the junction length between the grains, the transport properties that involve the motion of Cooper pairs are less affected than in cuprates superconductors. Thus, the resistivity transitions are sharp and the absence of weak links has been emphasized by magnetization