# Quasiclassical Elienberger Equations

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This means, in this scenario the electron motion is diffusive and green functions are nearly isotropic. Usadel’s approach not only deals to ‘strongly dirty’ samples but also ‘moderately dirty’ samples. The theory of upper critical field for MgB2 was developed separately by Alex Gurevich [133], and Golubov and Koshelov [134]. Their theoretical models are based on multiband generalization of the quasiclassical Usadel equations. The main difference between these two approaches is that Gurevich model considers the small band anisotropy where as Golubov and Koshelov model addresses the case of very high anisotropy in quasi-2D band [134]. In this chapter, first I provide an over view of Gurevich theory [135] and I apply this theory to extract interband scattering rates and intraband diffusivities of MgB2 ultrathin

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To address the physics behind observed anomalous behavior of Hc2 (T), Gurevich, and Gloubov and Koshelov theory was developed. The unique benefit of this approach is that the details of complex Fermi surface of MgB2 is not necessary to calculate Hc2 [132]. The impurity scattering is accounted by the nature of normal state electronic diffusivities and which are controlled by their respective intraband scattering rates in the and π bands, and interband scattering rates , , and