Dependence of the specific heat of a La1.85Sr0.15CuO4 superconducting single crystal on the magnetic field direction in the a-b plane

The low-temperature specific heat of a La_1.85Sr_0.15CuO₄ superconducting single crystal was investigated in magnetic fields up to 8 T and with four orientations — in the a-b plane (along the (100) and (110) directions) and at angles of 45° and 90° with respect to the a-b plane (along the (103) and (001) directions). Anisotropy was observed in the field dependence of the specific heat in the a-b plane. The specific heat was found to be minimum with the field oriented in the direction of the a axis and maximum with the field oriented in a direction making an angle of 45° with the a axis. This can be explained by the anisotropy of the energy gap, whose minimum lies along the (110) direction. For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field strength. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 683–687 (10 December 1997)