Anisotropy of the superconducting transition in magnetic fields in a Nd$_{1.85}$Ce$_{0.15}$CuO$_4$ single crystal

The anisotropy of superconducting properties of a Nd$_{1.85}$Ce$_{0.15}$CuO$_4$ single crystal is studied by resistance measurements over the temperature range 2-30 K in magnetic fields of 0, 1, 2, 4, and 6 $T$ parallel to the $a-b$ plane. A strong anisotropy of $T_c(H)$ and $H_{c2}(T)$ is observed for different orientations of magnetic field in the $a-b$ plane. This anisotropy leads to a twofold symmetry of $T_c(H)$ and $H_{c2}(T)$, and the gap node direction is determined. An analysis of experimental data shows that this result can be attributed to a change in the local symmetry of the copper atom environment, which manifests itself as a reduction from tetragonal to orthorhombic symmetry in the low-temperature region. The comparison with La$_{1.85}$Sr$_{0.15}$CuO$_4$ suggests that the mechanisms of superconductivity in electron and hole doped superconductors are similar, and the difference observed in the experiment is related to the structural features of these materials.