Quasiclassical theory of superconducting states under magnetic fields: Thermodynamic properties

We present a simple calculational scheme for superconducting properties under magnetic fields. A combination of an approximate analytic solution with a free energy functional in the quasiclassical theory provides a wide use formalism for spatial-averaged thermodynamic properties, and requires a little numerical computation. The theory covers multiband superconductors with various set of singlet and unitary triplet pairings in the presence of an impurity scattering. It is also applicable to analyze experimental results in a rotating magnetic field with help of band structure calculations. We demonstrate the application to s-wave, d_{x^2-y^2}-wave and two-band s-wave pairings, and discuss the validity of the theory comparing with previous numerical studies.