MARGINALLY STABLE DIAMAGNETIC SCREENING BELOW THE VORTEX-GLASS TRANSITION OF YBA2CU3O7

By high-resolution magnetometry, we measured the decay of the diamagne tic moments M(t, T, H) of a single crystal in response to fields betwe en 0.1 and 10 kOe applied parallel to the c-axis. The logarithmic deri vatives, S = -dln M/dln t, determined for 0.5 less than or equal to t less than or equal to 10(4) s, reveal the vortex-glass (VG) type relax ation characterized by the exponent mu = 1.20(5) for temperatures belo w well-defined values T(H). Above T*(H), the screening currents displ ay an algebraic decay, M similar to t(-T/Uo), the characteristic energ y of which, U-0, vanishes at the VG transition T-g(H). We argue that t his marginally stable screening results from flux entry to the VG phas e proceeding via dislocation loops. (C) 1998 Elsevier Science B.V. All rights reserved.