Heat and charge transport properties of MgB2

Abstract

A polycrystalline sample of the MgB 2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8K and 300K in zero magnetic field. The electrical resistivity shows a superconducting transition at Tc = 38.7K and, similarly to borocarbides, a T 2.4 behaviour up to 200K. The electron diffusion thermopower and its bandstructure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electronphonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below Tc is observed resulting in a T 3 behaviour below 7K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.

Thermal conductivity of single-crystallineMgB2

“…The κ(T ) values are about an order of magnitude higher than previously reported for polycrystalline samples. 23,24,25,26 Also the overall temperature dependence of κ is quite different from those earlier data. Instead of a monotonous increase with temperature we note a distinct maximum of κ(T ) at T ∼ 65 K. The cause of these differences is obviously the strong influence of intergrain boundaries on the heat transport in polycrystals, which masks the intrinsic mechanisms of quasiparticle scattering.…”

“…23,24,25,26 This observation is quite unusual for superconductors with nonnegligible phonon heat transport, because the opening of the superconducting gap rapidly reduces the rate of phonon scattering on electrons and should lead to a fast increase of κ ph below T c . The assumptions that either the phonon-electron scattering is much weaker than phonondefect scattering, or that κ ph is negligibly small in the vicinity of T c , which, in principle, might explain the absence of a κ(T ) feature at T c , are all incompatible with the observation that applying a relatively weak external magnetic field of 0.63 kOe, introducing some additional quasiparticles in the cores of vortices, considerably reduces the thermal conductivity at intermediate temperatures (see Fig.…”

Influence of carbon substitution on the heat transport in single crystallineMgB2

“…14, 15,16,17,19 Selected κ(H) data at several fixed temperatures for x=0, 0.03, and 0.06 are shown in Fig. 2.…”

Magnetotransport study of MgB2superconductor

“…The second term corresponds to the electron-phonon scattering. Fitting the electron thermal conductivity to equation (10) supplies α = 7.66 x 10 -7 with 1.5% accuracy. Then we arrive at the Debye temperature T D = 844 K, which agrees with values usually reported for MgB 2 and is remarkably lower than T D estimated from thermoelectric data.…”