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[ Annulé] – Quantum Materials : Jeudi 27 janvier à 14h

Kota Katsumi (University of Tokyo & MPQ-Paris)


ZOOM link : https://univ-grenoble-alpes-fr.zoom.us/j/92554547821?pwd=ZVdPdWVMNGovUHZlK2c3UjBWQ3Nidz09

Title : Absence of the superconducting collective excitations in the optically excited nonequilibrium state of underdoped YBa2Cu3Oy


Observation of superconducting (SC) like transient response in the c-axis optical conductivity far above the SC transition temperature Tc in underdoped YBa2Cu3Oy (YBCO) under intense laser pulse irradiation has attracted significant attention among the broad field of condensed matter physics [W. Hu et al., Nat. Mater. 13, 705 (2014) and S. Kaiser et al., Phys. Rev. B 89, 184516 (2014)]. Afterward, various theoretical and experimental studies have been devoted to elucidating its microscopic origin. However, one remaining problem is that one cannot distinguish the 1/ω-like SC response and the Drude response of the quasiparticle excitation with an extremely low scattering rate from the measured optical conductivity in the terahertz (THz) frequency range. To circumvent this problem, we investigated the photoinduced nonequilibrium state in underdoped YBCO samples using the THz nonlinear optical responses arising from the SC collective excitations: the Higgs mode and the ac-driven Josephson current. We observed the near-infrared (NIR) pump-induced 1/ω-like increase in the imaginary part of the c-axis optical conductivity above Tc, consistent with the previous studies. Besides, we found that the NIR pump-induced change in the reflected THz pulse emerged below the pseudogap opening temperature, indicating its relevance to the pseudogap. On the other hand, neither the THz nonlinear optical response of the Higgs mode nor the ac-driven Josephson current was observed in such a photoexcited state,revealing that the photoinduced state exhibiting the 1/ω-like response is distinct from the superconductivity in equilibrium. Our results suggest that the photoexcited nonequilibrium state provides an essential clue to understanding the pseudogap.