Characterizing the pseudogap in the high-Tc 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 63 Cu NMR measurements. In the slightly overdoped TlSr 2 CaCu 2 O 6.8 , the PG is strongly field dependent and shown to be due to the superconducting fluctuations. By contrast, the PG in the underdoped YBa 2 Cu 4 O 8 does not depend on magnetic fields up to 28.5 T. These results imply that there exists a field-insensitive PG up to a certain doping level beyond which it is taken over by the superconducting fluctuations-induced one.