Free energy and torque for superconductors with different anisotropies of H_{c2} and lambda

TL;DR

This paper investigates how differing anisotropies in upper critical field and penetration depth affect the free energy and equilibrium orientation of uniaxial superconductors, with implications for materials like MgB₂.

Contribution

It introduces a theoretical analysis of free energy in superconductors with distinct anisotropies of H_{c2} and lambda, predicting orientation shifts.

Findings

Equilibrium orientation can shift from theta=pi/2 to theta=0 with increasing anisotropy difference.

The effects are relevant for understanding MgB₂ behavior.

Different anisotropies influence the superconductor's response to magnetic fields.

Abstract

The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma_H = H_{c2,ab}/H_{c2,c}, different from the anisotropy of the penetration depth gamma_{lambda} = lambda_c/lambda_{ab}. 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 MgB_2.