Observation of diamagnetic precursor to the Meissner state above Tc in high- Tc La 2− xSr xCuO 4 cuprates by scanning SQUID microscopy

Abstract

We have studied the magnetic imaging of the superconducting and anomalous normal states for underdoped La 2− x Sr x CuO 4 (LSCO) thin films deposited by a PLD method and nearly optimal-doped LSCO single crystals grown by a traveling solvent floating zone method by scanning SQUID microscopy. The La 2− x Sr x CuO 4 (LSCO) films and their T c's are 18–19 K. The SSM used here has a pick-up coil of 10 μm diameter and the magnetic flux sensitivity is less than 5 μΦ 0 Hz −1/2. Below T c, clear quantized vortices are observable. Above T c, however, the magnetic image never becomes uniform, and inhomogeneous diamagnetic domains appear. The nucleation of diamagnetic domains starts above 80 K, then they develop with expanding their area as temperature is reduced and connected with the Meissner state at T c. The diamagnetic amplitude of these domains is significantly large in the first stage and then decreases with reducing temperature.