Basal-Plane Magnetic Anisotropies of High-κ d-Wave Superconductors in a Mixed State: A Quasiclassical Approach

We study the basal-plane anisotropies of reversible magnetization and torque in a mixed state of layered d-wave superconductors based on the quasiclassical version of the BCS-Gor'kov theory. Both the longitudinal magnetization (ML) and torque (τ) show fourfold oscillations as a function of the field angle χ. The relationship between the node position and the oscillatory patterns shown by ML and τ is clarified. It is also shown that the sign of the τ (χ)-oscillation does not change between Hc1 and Hc2, while the sign of the ML (χ)-oscillation changes. The newly obtained result for τ indicates that the torque experiment can allow us to detect the in-plane anisotropies of Hc2 even in a material with strong fluctuations such as cuprate or organic superconductors, where the Hc2 itself cannot be determined experimentally.