Quasiparticle dynamics and competing order in cuprate superconductors

We report time-resolved optical measurements that reveal quasiparticle and collective mode dynamics in the presence of competing order in cuprate superconductors. In these measurements, we use low-intensity short pulses of light to perturb the equilibrium state and time-resolve the ensuing change in optical reflectivity at a photon energy of 1.5 eV. The perturbing pulse generates a nonequilibrium population of quasiparticles near the Fermi energy by allowed dipole transitions as well as collective excitations through a Raman process. Tracking the relaxation of the single particle and collective modes through the phase space of temperature, carrier concentration, and magnetic field allows us to observe the interaction between the competing phases. In this talk I will describe measurements in \begin{itemize} \item YBCO ortho III and VIII in which photoexcitation is observed to generated collective oscillations of CDW order whose phase begins to rotate by 180 at the superconducting transition temperature (T$_{\mathrm{c}})$. \item Nd $_{\mathrm{2-x}}$Ce$_{\mathrm{x}}$CuO$_{\mathrm{4+\delta }}$ that indicate excitation of a collective mode that displays quantum critical dynamics above T$_{\mathrm{c}}$ and competition with superconductivity below. \item HgBa$_{\mathrm{2}}$CuO$_{\mathrm{4+\delta }}$ that indicate a cusp in the quasiparticle recombination lifetime at T$_{\mathrm{c}}$ that we associate with quasiparticle coherence effects. The size of the cusp is maximal at 8{\%} hole concentration, possibly coinciding with the peak of a competing CDW phase, and decreases rapidly with applied magnetic field. Lastly, we observe a complex, non-monotonic temperature dependence in the dynamics near hole concentration of 18{\%}, providing evidence for competing phases within the superconducting dome. \end{itemize}