In-plane fourfold symmetry of the upper critical field observed in La(Sr)214 single crystals

In order to establish the existence of in-plane anisotropy of the upper critical field Hc2(θ), the out-of-plane resistivity ρc measurements were performed on La(Sr)214 single crystals with rectangular and cylindrical shape under rotating magnetic field applied within the ab-plane. Although observed ρc shows non-sinusoidal twofold symmetry, clear fourfold symmetry was obtained after subtracting twofold sinusoidal component in ρc which is due to the unavoidable misalignment of the magnetic field with respect to ab-plant. Hc2(θ) is estimated from the extracted fourfold component of ρc with the flux flow theory. Angular dependence of Hc2(θ) was well fitted by cos(4θ). Since the fourfold component of Hc2(θ) was largest at θ = nπ/2, which corresponds to the a-axis direction, the present results strongly suggest the dx2-y2type symmetry in La(Sr)214. In addition, the difference in fourfold component of Hc2 at θ = nπ/2 and nπ/2 + π/4 was found to increase with decreasing temperature