Irreversibility and melting in YBCO through AC susceptibility measurements

Abstract

We report on the response of an untwinned YBCO single crystal to an AC magnetic field over a wide region of the (B,T) diagram (up to 11T) by means of high resolution micro-Hall probe measurements. Our results suggest the existence of two distinct behaviors of the crystal, depending on the amplitude Of the DC field B. For B less than a critical value B0 (≈8T), shielding currents appear in a discontinuous way in the sample. At higher fields shielding currents appear in a continuous way. Our experimental method allows to detect a magnetic irreversibility in the sample too weak to be resolved by recent dc SQUID magnetometry experiments. This leads to suggest that a recently observed jump in dc magnetization [1,2] at the melting transition occurs simultaneously with irreversibility. AC susceptibility becomes so for the first time a very precise mean to track the melting (or irreversibility) line.