Expansion of vortex cores by strong electronic correlation in La 2 − x Sr x CuO 4 at low magnetic induction

The vortex core radius ${\ensuremath{\rho}}_{v},$ defined as the peak position of the supercurrent around the vortex, has been determined by muon spin rotation measurements in the mixed state of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ for $x=0.13,$ 0.15, and 0.19. At lower doping $(x=0.13$ and 0.15), ${\ensuremath{\rho}}_{v}(T)$ increases with decreasing temperature T, which is opposite to the behavior predicted by the conventional theory. Moreover, ${\ensuremath{\rho}}_{v}(\stackrel{\ensuremath{\rightarrow}}{T}0)$ is significantly larger than the Ginzburg-Landau coherence length determined by the upper critical field, and shows a clear tendency to decrease with increasing the doping x. These features can be qualitatively reproduced in a microscopic model involving antiferromagnetic electronic correlations.