Vortex dynamics has been investigated in the cubic $(\mathrm{K},\mathrm{Ba})\mathrm{BiO}{}_3$ superconductor using ac susceptibility measurements on a large frequency range $(0.03\mathrm{Hz}<\omega <60\mathrm{kHz})$. Power law diverging barriers have been obtained on both sides of the order-disorder transition line. The $\mu$ exponent remains close to $5/2$ (elastic creep value) in some part of the disordered phase and finally decreases at high temperature and/or high field, in good agreement with the recent plastic collective creep theory [J. Kierfeld, H. Nordborg, and V. M. Vinokur, Phys. Rev. Lett., 85, 4948 (2000)].

• Received 20 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.167002