Nanoscale physics

Thermally assisted and quantum vortex creep in type-II superconductors with columnar defects

Kasatkin A.L. , Tsvitkovskyi V.P.
G.V. Kurdyumov Institute for Metal Physics, 36 Vernadsky Str., 03142 Kiev
E-mail: tsvad@i.ua

The critical current density jc value in superconductors depends on the material defect nanostructure ability to pin Abrikosov vortices and prevent their motion which causes electrical resistivity emergence. It is commonly believed that extended linear (columnar) defects with radii comparable to the coherence length ξ of the superconducting state are the most effective pinning sites for vortices. 
       In the present work we consider mechanisms of vortex escape from extended linear defects due to thermally activated and quantum vortex depinning processes. At high enough temperatures and low current densities vortex depinning from extended linear defect in a type-II superconductor proceeds due to fluctuation formation of partially depinned vortex loops in the bulk of superconductor (or vortex “tongues” near its surface) which have a size that exceeds a certain critical value lc(j), dependent on transport current density j, and, correspondingly, with the excitation energy greater than the critical value Uc(j) : lc(j) ~ Uc(j)~ j-1 [1]. On the other hand at low temperatures still there is a possibility of vortex escape from columnar defect due to quantum creep of pinned vortices which can proceed  at j < jc (jc – is the depinning critical current density) due to macroscopic quantum tunneling of vortex segments from the pinning potential well of linear defect. This process leads to a finite probability (T,j) of vortex escape and corresponding decay rate for magnetic moment  and resistivity  even at T0:  = 0 exp{-(1-j/jc )}, where parameters   ( > 0), 0,  depend on the model of vortex dynamics, namely on vortex mass, dissipation, Magnus force and the way for their  account [2]. Experimentally the quantum creep phenomenon was observed in some experimental works.

1.	Brandt E.H. Large range of validity of linear elasticity of the vortex lattice 
in high-Tc superconductors // Phys. Rev. Lett.-1992.- 69, N7.- P.1105-1108.
2.   Morais Smith C.,  Caldeira A.O., Blatter G. Relaxation rates of flux-lines and flux-pancakes in the presence of correlated disorder // Physica C.- 1997.- 276. – P. 42-56.