Basal-Plane Magnetic Anisotropies of High-kappa 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 ($M_L$) and torque ($\tau$) show fourfold oscillations as a function of the field angle $\chi$. The relationship between the node position and the oscillatory patterns shown by $M_L$ and $\tau$ is clarified. It is also shown that the sign of the $\tau (\chi)$-oscillation does not change between $H_{c1}$ and $H_{c2}$, while the sign of the $M_L (\chi)$-oscillation changes. The newly obtained result for $\tau$ indicates that the torque experiment can allow us to detect the in-plane anisotropies of $H_{c2}$ even in a material with strong fluctuations such as cuprate or organic superconductors, where the $H_{c2}$ itself cannot be determined experimentally.