Surface superconductivity on SrFe₂As₂ single crystals induced by ion implantation

¹ Industrial Research Limited - P.O. Box 31310, Lower Hutt 5040, New Zealand
² National Isotope Centre, GNS Science - P.O. Box 31312, Lower Hutt 5040, New Zealand
³ Institute of Materials Science and Graduate School of Pure & Applied Sciences, University of Tsukuba 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan

^a s.chong@irl.cri.nz

Received: 10 January 2011
Accepted: 26 March 2011

Abstract

This study presents an unprecedented route in inducing superconductivity in SrFe₂As₂ single crystals by ion implantation. Na⁺ implantation into SrFe₂As₂ single crystals did not induce superconductivity nor did it shift the spin-density wave (SDW) anomaly at 205 K. In contrast, above a threshold of 2 at.% dosage, all the K⁺- and Ca²⁺-implanted crystals show a superconducting transition below 25 K in magnetisation and resistivity measurements. Due to the shallow implantation depth of the ions, the observed superconductivity is in principle confined to the surface or sub-surface level, and the normal state can be recovered by thermal annealing. This strongly suggests superconductivity in these samples is induced by surface or sub-surface crystallographic strain. The deduced anisotropy ratio of the upper critical field is above 3 in all the superconducting samples, and more important is the obtained c-axis coherence lengths are longer than the interlayer spacing in SrFe₂As₂ implying strong interlayer coupling in these superconductors.