London penetration depth of electron-irradiated Ba1-xKxFe2As2 single-crystals

TL;DR

This study investigates how electron irradiation and subsequent annealing affect the London penetration depth in Ba1-xKxFe2As2 single crystals, revealing non-monotonic behavior and recovery of critical temperature.

Contribution

It provides a detailed experimental analysis of the effects of electron irradiation and annealing on penetration depths and critical fields in Ba1-xKxFe2As2 superconductors.

Findings

Penetration depth changes are non-monotonic with annealing temperature.

Superconducting critical temperature Tc is fully recovered after irradiation.

Magneto-optic measurements confirm uniformity and determine lower critical field.

Abstract

We have characterized an electron-irradiated Ba1-xKxFe2As2 (x = 0.53) single-crystal using two different experimental techniques: magneto-optic measurements and microwave measurements. The crystal has been measured before as well as after the 2.5 MeV electron irradiation process. After irradiation it was annealed in a number of steps, between 90 deg C and 180 deg C, and measured after each annealing step. Most microwave measurements were performed by means of a copper cavity, taking advantage of the TE011 and TM110 modes, allowing for the determination of the London penetration depths changes {\delta}{\lambda}ab(T) and {\delta}{\lambda}c(T), i.e. perpendicular and parallel to the sample c-axis. Appropriate equations, based on perturbation theory, were derived to calculate the penetration depths changes {\delta}{\lambda}ab and {\delta}{\lambda}c for a rectangular prism geometry. The sample showed a full recovery of its Tc, however the observed behavior of {\delta}{\lambda}c and {\delta}{\lambda}ab was not monotonic vs annealing temperature, displaying a minimum of {\delta}{\lambda}c and {\delta}{\lambda}ab at 120 deg C. This finding was confirmed by magneto-optic measurements, where besides verifying the sample uniformity and the absence of visible defects, the lower critical field Hc1 of the Ba1-xKxFe2As2 single-crystal was obtained and the London penetration depth {\lambda}ab(0) was calculated.