BCS $d$-wave behavior in the THz electrodynamic response of electron-doped cuprate superconductors

Abstract: Although cuprate superconductors have been intensively studied for the past decades, there is no consensus regarding the microscopic origin of their superconductivity. In this work, we measure the low-energy electrodynamic response of slightly underdoped and overdoped La$_{2-x}$Ce$_x$CuO$_4$ thin films using time-domain terahertz (THz) spectroscopy to determine the temperature and field dependence of the superfluid spectral weight. We show that the temperature dependence obeys the relation \textit{n$_s$} $\propto$ $1-(T/T_c)^2$, typical for dirty limit BCS-like $d$-wave superconductors. Furthermore, the magnetic field dependence was found to follow a sublinear $\sqrt{B}$ form, which supports predictions based on a $d$-wave symmetry for the superconducting gap. These observations imply that the superconducting order in these electron-doped cuprates can be well described in terms of a disordered BCS $d$-wave formalism.