We present resistivity and thermal conductivity measurements on bulk samples, prepared either by a standard method or by a one-step technique. The latter samples, due to their high density and purity, show residual resistivity values as low as 0.5 µΩ cm and thermal conductivity values as high as 215 W/mK, higher than the single crystal ones. Thermal and electrical data of all the samples are analysed in the framework of the Bloch-Gruneisen equation giving reliable parameter values. In particular the temperature resitivity coefficient, obtained both from resistivity and thermal conductivity, in the dirty sample comes out ten time larger than in the clean ones. This result supports the hypothesis of ref.[1] that π and σ bands conduct in parallel, prevailing π conduction in clean samples and σ conduction in dirty samples .
IntroductionSince the discovery of superconductivity in Magnesium diboride, this compound appeared to be a "simple" metal where most of the electronic properties follow in a first approximation the basic transport laws of a metal where electron-phonon interaction is dominant. We remind that magnetoresistance follows the Kohler law [2,3], resistivity follows the Bloch-Grüneisen relationship [4,5], Seebeck effect follows the Mott law [4] [6]. A further analysis is evidencing the important role in the transport properties of the peculiar band structure of this compound. Two kinds of bands having quite different character contribute to the transport [7,8]: two σ bands, which are hole-type and two-dimensional (2D), and two π bands, which are electron-type and threedimensional (3D). Calculations showed that σ bands are more strongly coupled with phonon modes and in particular with the E 2g mode. A recent paper [1] suggests that just due to the different parity of the two bands, interband impurity scattering turns out to be negligible and the σ and π bands behave as two separate conduction channels in parallel. In this paper we discuss some implications that this fact does have on transport properties, namely electrical and thermal conductivity.