Direct observations of vortices in a high-Tc superconductor (La1-xSrx)2CuO4 by scanning SQUID microscopy

In order to investigate the behavior of vortices in high-Tc superconductors under low magnetic fields, we have performed scanning SQUID microscopy (SSM) on the ab-surfaces of (La1-xSrx)2CuO4 (LSCO) single crystals at low temperatures. The observed magnetic images clearly demonstrated vortices in units of Φ0 (=hc/2e) below Tc. Vortices trapped inside the overdoped LSCO tended to be arranged in a one-dimensional manner, while those in optimally doped crystals were randomly distributed. From the variable-temperature SSM measurements, it was directly visualized that the normal state with regular magnetic-field distribution gradually changed into the inhomogeneous Meissner state, and finally vortices evolved, with decreasing temperature.