Time-Resolved Optical Reflectivity of the Electron-Doped ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_{4+\delta }$ Cuprate Superconductor: Evidence for an Interplay between Competing Orders

We use pump-probe spectroscopy to measure the photoinduced reflectivity $\Delta R$ of the electron-doped cuprate superconductor ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_{4+\delta }$ at a value of $x$ near optimal doping, as a function of time, temperature, and laser fluence. We observe the onset of a negative $\Delta R$ signal at $T^{*}\approx 75\mathrm{K}$, above the superconducting transition temperature, $T_c$, of 23 K. The relatively slow decay of $\Delta R$, compared to the analogous signal in hole doped compounds, allows us to resolve time-temperature scaling consistent with critical fluctuations. A positive $\Delta R$ signal onsets at $T_c$ that we associate with superconducting order. We find that the two signals are strongly coupled below $T_c$, in a manner that suggests a repulsive interaction between superconductivity and another fluctuating order.