Ultrafast dynamics in unconventional superconductors.

Inna Vishik (MIT)

Ultrafast (sub-picosecond) spectrosopies have emerged as important tools for studying quantum materials, and one of the key capabilities of ultrafast techniques is identifying relaxation processes in the time domain which are relevant to the formation of exotic states such as high temperature superconductivity. I will present ultrafast pump-probe spectroscopy studies of the heavy fermion superconductor CeCoIn₅ and the electron-doped cuprate La_2-xCe_xCuO₄ (LCCO). In these experiments, a 70-femtosecond 800nm-wavelength pulse creates transient electronic excitations whose decay is probed by studying changes in reflectivity as a function of time. Although both the excitation and the probe energies far exceed the low-energy excitations in cuprates and heavy fermions, this technique is still sensitive to low energy excitations and low-energy spectral features because these ultimately determine the electrons decay to equilibrium. Both materials are considered model systems. CeCoIn5 is a clean d-wave superconductor at low temperature and a model Kondo-lattice system above T_c, and electron-doped cuprates have a simpler phase diagram than their hole-doped cousins. Both materials exhibit characteristic changes in pump-probe recovery dynamics across the key temperatures in the phase diagram the Kondo coherence temperature in CeCoIn₅ and T_c and T_N in LCCO. In both material systems, the low temperature state contains two components - heavy/light electrons in CeCoIn5 and superconductivity/antiferromagnetism in LCCO, which can be distinguished via their unique time-domain signatures.