Ordering Phenomena in Cuprates

Abstract

Results of light-scattering experiments in Bi₂Sr₂CaCu₂O_8+δ (Bi-2212), La_2–xSr_xCuO₄ (LSCO), and YBa₂Cu₃O_6+x (Y-123) single crystals are presented. The emphasis is placed on the low doping range where the evolution of the metallic state from the antiferromagnetic one is studied. Along the diagonals of the CuO₂ planes the response is metallic even at small doping levels well below p = 0.05 where superconductivity disappears. In contrast, along the principal axes the carriers exhibit insulator-like characteristics up to approximately p=0.21. In the underdoped range charge-ordering fluctuations are observed manifesting themselves as quasielastic peaks at low temperatures. The symmetries at which the new response is found are compatible with the neutron results on the spin system. Possible connections between the localization phenomena, the pseudogap and the charge and spin-ordering instabilities as well as the implications for superconductivity are discussed.