Characterizing the pseudogap in the high-T-c superconductors using very high magnetic fields: implications on the phase diagram

We find contrastive response of the pseudogap (PG) to high magnetic fields up to 28.5 T based on Cu-63 NMR measurements. In the slightly overdoped TlS r2CaCu2O6.8, the PG is strongly field dependent and shown to be due to the superconducting fluctuations. By contrast, the PG in the underdoped YBa2CU4 O8 does not depend on magnetic fields up to 28.5 T. These results imply tha t there exists a field-insensitive PG up to a certain doping level beyond w hich it is taken over by the superconducting fluctuations-induced one. (C) 2001 Elsevier Science B.V. All rights reserved.