Multiband-Moments Model for the Conductivities of Chromium

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Klemens' s moment integral formulation of the transport coefficients has been expressed in multiband form. A two-band moments model consisting of a standard and a nonstandard band has been adduced to explain the anomalies in the electrical and thermal conductivities of chromium $\ensuremath{\sigma}$ and $k$, respectively, in the vicinity of the N\'eel temperature as if they were mostly the result of the changes in the band structure that occur during the antiferromagnetic transformation. It can be seen that the behavior of the Lorenz number $L=\frac{k}{\ensuremath{\sigma}T}$ clearly indicates that a BCS-type gap has opened over a portion of the Fermi surface during this transformation. The zero-temperature-gap ratio is $2{\mathit{\ensuremath{\Delta}}}_{0}=5.2$, a value that is in excellent agreement with the value ($2{\mathit{\ensuremath{\Delta}}}_{0}=5.1$) found experimentally by Barker et al. using optical reflectivity techniques. The ratio $R$ of the antiferromagnetic and paramagnetic Fermi surfaces is 0.51, a value in very good agreement with the theoretical prediction of Asano and Yamashita. It is possible to show that a recently observed minimum in the thermal resistivity near the N\'eel temperature can occur even in the case of elastic scattering.