Unconventional London Penetration Depth in Single-Crystal $\mathrm{Ba}({\mathrm{Fe}}_{0.93}{\mathrm{Co}}_{0.07}{)}_2{\mathrm{As}}_2$ Superconductors

The London penetration depth $\lambda (T)$ has been measured in single crystals of $\mathrm{Ba}({\mathrm{Fe}}_{0.93}{\mathrm{Co}}_{0.07}{)}_2{\mathrm{As}}_2$. The observed low-temperature variation of $\lambda (T)$ follows a power law, $\Delta \lambda (T)\sim T^n$ with $n\approx 2.4\pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast with previous penetration depth measurements on single crystals of ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ and ${\mathrm{SmFeAsO}}_{1-x}{\mathrm{F}}_x$, which indicate an anisotropic but nodeless gap. We discuss possible explanations of the observed power law behavior.

Figure 1

Characterization of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ single crystals. (a) Longitudinal x-ray scans through the position of the (0 0 8) reflection for indicated temperatures. (b) Transverse ($\nu \nu 0$) scans through the position of the (1 1 8) reflection. Changes in the position of the peaks in (a) and (b) result from thermal expansion of the lattice. The asymmetric peak shape in the ($\nu \nu 0$) scan originates from a second grain with a slightly different orientation (sample mosaic) and is present at all temperatures. In (a) and (b) the horizontal axes depict the reciprocal lattice vectors relative to the alignment and lattice parameters at 300 K. The vertical arrows in (b) depict the positions of the split diffraction peaks at 10 K in the parent ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ compound (after Ref. 24). (c) In-plane resistivity (open circles), dc magnetization (filled circles), and in-plane penetration depth (squares).

Figure 2

Comparison of the low-temperature $\Delta \lambda (T)$ measured in $\mathrm{Ba}({\mathrm{Fe}}_{0.93}{\mathrm{Co}}_{0.07}{)}_2{\mathrm{As}}_2$ with the known $d$-wave (BSCCO) and $s$-wave (Nb) superconductors as well as ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$. Inset: $\Delta \lambda$ vs $(T/T_c{)}^2$ for three different samples of $\mathrm{Ba}({\mathrm{Fe}}_{0.93}{\mathrm{Co}}_{0.07}{)}_2{\mathrm{As}}_2$ from different batches. (Curves are offset for clarity.) Note the slight positive curvature indicating $n>2$.

Figure 3

Superfluid density, ${\rho }_s$, as a function of normalized temperature for $\mathrm{Ba}({\mathrm{Fe}}_{0.93}{\mathrm{Co}}_{0.07}{)}_2{\mathrm{As}}_2$ using $\lambda (0)=200\mathrm{nm}$ and 300 nm. Also shown for comparison are calculated curves for clean and dirty $s$-wave and $d$-wave pairings as well as the best fit of the ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ compound (circles) from Ref. 15.

Figure 4

(a) Fermi surface of nonsuperconducting (a) ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and (b) $\mathrm{Ba}({\mathrm{Fe}}_{0.93}{\mathrm{Co}}_{0.07}{)}_2{\mathrm{As}}_2$ with experimental As positions.