Photoinduced $sp^3$ Nanosize Domain with Frozen Shear Displacement in Graphite

A novel $sp^3$-bonded nanosize domain, known as a diaphite which is an intermediate state between a graphite and a diamond, is generated by the irradiation of visible laser pulse onto a graphite crystal. The $sp^3$ structure is well stabilized by shear displacement between neighboring graphite layers. We theoretically study the interlayer $sp^3$ bond formation with frozen shear displacement in a graphite crystal, using a classical molecular dynamics and a semi-empirical Brenner potential. We show that a pulse excitation under the fluctuation of shearing motion of carbons in an initial state can generate interlayer $sp^3$ bonds which freeze the shear, though no frozen shear appears if there is no fluctuation initially. Moreover, we investigate a pulse excitation under the coherent shearing motion and consequently obtain that the $sp^3$-bonded domain with frozen shear is efficiently formed. We conclude that the initial shear is important for the photoinduced $sp^3$ nanosize domain formation.