Colloquium

Title: Pumping and Probing Novel Quantum Materials by Time- and Angle-Resolved Photoemission

Speaker: Zhang Wentao, UC Berkeley

Location: Room 111, Physics Building

Time: 14:00-15:00, Tue, July 28, 2015

Abstract:

After giving a brief introduction to the application of ultrashort laser pulses in novel quantum material and the technique of time- and angle-resolved photoemission (trARPES), I will present two examples of my recent studies on cuprate high temperature superconductor by trARPES. 1) Ultrafast studies of high Tc superconductor reveal a softening of the electron-boson interaction in concomitance with a softening/closure of the superconducting gap. The effect is negligible in the normal state and for a metallic compound, suggesting that the electron-boson interaction might be a booster for pairing in cuprates. 2) We study the initial response of a high-temperature superconductor for both coherent pairing and non-equilibrium quasiparticle recombination following perturbation with an ultrashort infrared pulse. At temperature far below the superconducting transition temperature, the build-up of maximum response in electronic states takes as long as 0.9 ps, while at high excitation density, in the normal states, and outside of the Fermi arc in the Brillouin zone, the electronic states respond the photon pulse immediately. These observations demonstrate a much slower build-up of the maximum response in perturbing a coherent pairing system than would occur if pumping a metal or non-coherently gapped state, indicating that fast stimulated emission of Cooper pairs plays an important role in governing the quasiparticles recombination in superconductors.