Free energy and torque for superconductors with different anisotropies of H(c2) and lambda.
The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma(H)=H(c2,a)/H(c2,c), different from the anisotropy of the penetration depth gamma(lambda)=lambda(c)/lambda(a). With increasing difference between gamma(H) and gamma(lambda), the equilibrium orientation of the crystal relative to the applied field may shift from theta=pi/2 (theta is the angle between the field and the c axis) to lower angles and reach theta=0 for large enough gamma(H). These effects are expected to take place in MgB2.
Kogan
V G
VG
Ames Laboratory DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.