Anisotropy of flux-flow resistivity in UPt3

The ac prenetration depth, \lambda_ac(T,H,f), has been measured in superconducting UPt3 single crystals for H\parallel c and H\perp c in the range f=0.01-1MHz and T=0.1-0.4 K. The contributions from bulk pinning and surface pinning have been separated out to yield the flux-flow resistivity \rho_f(H). With H\perp c, rho_f displays magneto-resistance at low field which agrees with previous dc measurements and the characteristic scaling law of clean crystals with anisotropic gap. When H\parallel c, the low-field \rho_f is three times larger. We interpret this property as evidence for flux line with unconventional core structure.