Vortex creep down to 0.3 K in superconducting Fe(Te,Se) single crystals

We report on a study of the vortex creep in Fe1+δ(TexSe1-x) single crystals (x=0.5 and 0.4) down to 0.28 K (∼Tc/50) and up to μ0Ha=2T. The relaxation of the current density [J(t)] has been measured during 20 hours and the decay of J(t) can be well described by a J(t)â̂[ln(t/t0)]-1/μ law. We show that the relaxation exponent μ tends towards 0 for T<2K and μ0Ha<0.1T [i.e. J(t)→(t0/t)α] and increases for increasing T and/or Ha. Our measurements strongly suggest that the logarithmic creep rate R=-dln(J)/dln(t) remains finite at zero temperature (R|T→0→2%) and hence that quantum creep plays a dominant role in the relaxation process at low temperature. A maximum is observed in both the temperature and field dependence of R(t=100s,T,Ha), which can be associated to a crossover from a single vortex (one-dimensional) to a bundle (three-dimensional) creep regime.