Strong pinning and elastic to plastic vortex crossover in Na-doped CaFe2As2 single crystals

N. HABERKORN; M. MIURA; B. MAIOROV; G. F. CHEN; W. YU; L. CIVALE

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

American Physical Society

Año: 2011 vol. 84 p. 94522 - 94529

0163-1829

We study the vortex dynamics of NaxCa1-xFe2As2 single crystals with x = 0.5 (underdoped) and 0.75 (optimally doped),having Tc ~ 19.4 K and 33.4 K respectively, by performing magnetization measurements of the critical current density (Jc) and flux creep rate (S) . We find that the Jc versus temperature (T) dependence is consistent with Tc pinning, indicating strong pinning associated with randomly distributed defects larger than the coherence length (). The temperature dependence of S shows a crossover between glassy (elastic) and plastic creep regimes. The boundary Tcr(H) between both creep regimes coincides with the upper limit of the regime of strong pinning by nanoparticles. The glassy exponent () in the optimally doped crystal is consistent with the thermal collective creep theory previously applied to cuprate superconductors, but in the under-doped sample the plateau in S(T) indicates that ~ 3 - 3.3, a value larger than the existing theoretical predictions. We discuss the quantum creep contributions in both samples.