The excited state dynamics of correlated electron and electron-phonon systems triggered by an oscillating electric-field pulse of large amplitude are theoretically investigated. A "negative-temperature" state and inversion of electron-electron and electron-phonon interactions are induced even by a symmetric monocycle pulse. This fact is numerically demonstrated, using the exact diagonalization method, in a band-insulator phase of one-dimensional three-quarter-filled strongly dimerized extended Peierls-Hubbard and Holstein models. When the total-energy increment is maximized as a function of the electric field amplitude, the occupancy of the bonding and antibonding orbitals is inverted to produce a negative-temperature state. Around this state, the dependences of time-averaged electron-electron and electronphonon correlation functions on interaction parameters are opposite to those in the ground state.
Using the classical molecular dynamics and the semiempirical Brenner's potential, we theoretically study the interlayer σ bond formation, as cooperative and nonlinear phenomena induced by visible light excitations of a graphite crystal. We have found several cases, wherein the excitations of certain lattice sites result in new interlayer bonds even at non-excited sites. We have also found that, a new interlayer bond is easier to be formed around a bond, if it is already existing. As many more sites are going to be excited, the number of interlayer bonds increases nonlinearly with the number of excited sites. This nonlinearity shows 1.7 power of the total number of excited sites, corresponding to about three-or four-photon process.
We theoretically study the early-stage real-time dynamics of the interlayer -bond formation by visible-light irradiation of graphite crystal. An electron-hole pair, generated as an interlayer charge-transfer excitation in the visible region, mostly dissipates away into the two-dimensional semimetallic electronic continuum as plus and minus free carriers. However, by a small but finite probability, this electron-hole pair self-localizes during the lattice relaxation, resulting in a local interlayer contraction to form a bond. Our theory for this dynamics is composed of two parts. The first part describes the quantum and spontaneous breakage of the translational symmetry, or the self-localization of this electron-hole pair, in a simplified way. While the subsequent second one, by using a Brenner's potential, describes the classical dynamics of a further local lattice distortion which occurs after this self-localization and also describes the final interlayer -bond formation. We thus estimate this probability of self-localization and show the conditions that the interlayer bond can be formed. Consequently, we find that the self-localization occurs by the probability of about 2% and the subsequent bond formation is achieved when the excitation energy is more than 4.5 eV corresponding to about three visible photons.
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