Doping dependence of the gap anisotropy in ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ studied by millimeter-wave spectroscopy

We measure the penetration depth of optimally doped and underdoped ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ in the millimeter frequency domain (4–7 ${\mathrm{cm}}^{\mathrm{-}1}$) and for temperatures $2\mathrm{K}<~T<~300\mathrm{K}.$ The penetration depth as a function of temperature reveals significant changes on electron doping. It shows quadratic temperature dependence in underdoped samples, but increases almost exponentially at optimal doping. Significant changes in the gap anisotropy (or even in the gap symmetry) may account for this transition.