title: Non-thermal hot electrons ultrafastly generating hot optical phonons in graphite.
abstract: Investigation of the non-equilibrium dynamics after an impulsive impact provides insights into couplings among various excitations. A two-temperature model (TTM) is often a starting point to understand the coupled dynamics of electrons and lattice vibrations: the optical pulse primarily raises the electronic temperature T(el) while leaving the lattice temperature T(l) low; subsequently the hot electrons heat up the lattice until T(el) = T(l) is reached. This temporal hierarchy owes to the assumption that the electron-electron scattering rate is much larger than the electron-phonon scattering rate. We report herein that the TTM scheme is seriously invalidated in semimetal graphite. Time-resolved photoemission spectroscopy (TrPES) of graphite reveals that fingerprints of coupled optical phonons (COPs) occur from the initial moments where T(el) is still not definable. Our study shows that ultrafast-and-efficient phonon generations occur beyond the TTM scheme, presumably associated to the long duration of the non-thermal electrons in graphite.
authors.affiliation: ISSP, University of Tokyo, Kashiwa-no-ha, Kashiwa, Chiba 277-8581, Japan. ishiday@issp.utokyo.ac.jp
authors.forename: Y
authors.initials: Y
authors.lastname: Ishida
authors.forename: T
authors.initials: T
authors.lastname: Togashi
authors.forename: K
authors.initials: K
authors.lastname: Yamamoto
authors.forename: M
authors.initials: M
authors.lastname: Tanaka
authors.forename: T
authors.initials: T
authors.lastname: Taniuchi
authors.forename: T
authors.initials: T
authors.lastname: Kiss
authors.forename: M
authors.initials: M
authors.lastname: Nakajima
authors.forename: T
authors.initials: T
authors.lastname: Suemoto
authors.forename: S
authors.initials: S
authors.lastname: Shin