Penetration depth and core radius μSR measurements in the vortex state near the lower critical field

muSR measurements of the magnetic field distribution in the high-T-c, superconductor YBa2Cu3O6+x, and conventional superconductor NbSe2, near the lower critical field are presented. The muon spin polarization signal is analyzed in terms of a Ginzburg-Landau (GL) field distribution. Our analysis reveals that the increase in the vortex core radius levels off below mu(0)H = 0.05 T in underdoped YBa2Cu3O6+x and the penetration depth stops decreasing. We find that lambda = 2000 Angstrom in YBa2Cu3O6.50, lambda = 1850 Angstrom in YBa2Cu3O6.67, and lambda = 1600 Angstrom in Nbse(2). At still lower external magnetic fields, the GL field distribution. no longer fits the muSR signal, likely due to a transition to the Meissner state. (C) 2002 Elsevier Science B.V. All rights reserved.