In-Plane Anisotropy of the Upper Critical Field of La2−xSrxCuO4 in the Underdoped and Overdoped Regimes

Abstract

For La 2-x Sr x CuO 4 single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, ρ c , under constant magnetic fields, changing the field direction in the ab-plane. At x = 0.08, 0.12 and 0.16 in the underdoped and almost optimally-doped regimes, clear anisotropy of ρ c with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H c2 , and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d x 2 -y 2 pairing. The magnitude of the fourfold component of H c2 , ΔH c2 /H c2 , is largest in x = 0.08 among the three crystals. At x = 0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.