Low-Temperature Penetration Depth of κ−(ET)2Cu[N(CN)2]Br and κ−(ET)2Cu(NCS)2

Abstract

We present high-precision measurements of the penetration depth of single crystals of $\kappa -\left(\mathrm{ET}{)}_2\mathrm{Cu}\right[\mathrm{N}\left(\mathrm{CN}{)}_2\right]\mathrm{Br}$ and $\kappa -\left(\mathrm{ET}{)}_2\mathrm{Cu}\right(\mathrm{NCS}{)}_2$ at temperatures down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth $\left({\lambda }_{\parallel }\right)$ varies as a fractional power law, ${\lambda }_{\parallel }\sim T^{\frac{3}{2}}$. While this may be taken as evidence for novel pair excitation processes, we show that the data are also consistent with a quasilinear variation of the superfluid density, as is expected for a $d$-wave superconductor with impurities or a small residual gap. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, ${\lambda }_{\perp }\left(0\right)=100\mathrm{}\pm 20\mu \mathrm{m}$.