Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals

Abstract

The London penetration depth, $\lambda (T)$, has been measured in several single crystals of Ba(Fe${}_{0.93}$Co${}_{0.07}$)${}_2$As${}_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $\lambda (T)$ follows a power-law, $\Delta \lambda (T)\sim T^n$ with $n=2.4\pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO${}_{1-x}$F${}_x$ and SmFeAsO${}_{1-x}$F${}_x$, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe${}_{0.93}$Co${}_{0.07}$)${}_2$As${}_2$ may lead to an anisotropic $s-$wave gap with point nodes that would explain the observed $\lambda (T)$.

arXiv comment4 pages

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