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Strong pinning theory of thermal vortex creep in type II superconductors | | Martin Buchacek
; Vadim B. Geshkenbein
; Roland Willa
; Gianni Blatter
; | | Date: |
21 Mar 2019 | | Abstract: | We study thermal effects on pinning and creep in type-II superconductors
where vortices interact with a low density $n_p$ of strong point-like defects
with pinning energy $e_p$ and extension $xi$, the vortex core size. Defects
are classified as strong if the interaction between a single pin and an
individual vortex leads to the appearance of bistable solutions describing
pinned and free vortex configurations. Extending the strong pinning theory to
account for thermal fluctuations, we provide a quantitative analysis of vortex
depinning and creep. We determine the thermally activated transitions between
bistable states using Kramer’s rate theory and find the non-equilibrium
steady-state occupation of vortex states. The latter depends on the temperature
$T$ and vortex velocity $v$ and determines the current--voltage (or
force--velocity) characteristic of the superconductor at finite temperatures.
We find that the $T=0$ linear excess-current characteristic $v propto (j-j_c)
, Theta(j-j_c)$ with its sharp transition at the critical current density
$j_c$, keeps its overall shape but is modified in three ways due to thermal
creep: a downward renormalization of $j_c$ to the thermal depinning current
density $j_mathrm{dp}(T) < j_c$, a smooth rounding of the characteristic
around $j_mathrm{dp}(T)$, and the appearance of thermally assisted flux flow
(TAFF) ${v propto j exp(-U_0/k_{
m scriptscriptstyle B} T)}$ at small drive
$j ll j_c$, with the activation barrier $U_0$ defined through the energy
landscape at the intersection of free and pinned branches. This characteristic
emphasizes the persistence of pinning of creep at current densities beyond
critical. | | Source: | arXiv, 1903.9083 | | Services: | Forum | Review | PDF | Favorites | |
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