Growth dynamics of photoinduced domains in two-dimensional charge-ordered conductors depending on stabilization mechanisms

Photoinduced melting of horizontal-stripe charge orders in quasi-two-dimensional organic conductors θ-(BEDT-TTF)₂RbZn(SCN) ₄ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] and α-(BEDT-TTF)₂I₃ is investigated theoretically. By numerically solving the time-dependent Schrödinger equation, we study the photoinduced dynamics in extended Peierls-Hubbard models on anisotropic triangular lattices within the Hartree-Fock approximation. The melting of the charge order needs more energy for θ-(BEDT-TTF)₂RbZn(SCN) ₄ than for α-(BEDT-TTF)₂I₃, which is a consequence of the larger stabilization energy in θ-(BEDT-TTF) ₂RbZn(SCN)₄. After local photoexcitation in the charge ordered states, the growth of a photoinduced domain shows anisotropy. In θ-(BEDT-TTF)₂RbZn(SCN)₄, the domain hardly expands to the direction perpendicular to the horizontal-stripes. This is because all the molecules on the hole-rich stripe are rotated in one direction and those on the hole-poor stripe in the other direction. They modulate horizontally connected transfer integrals homogeneously, stabilizing the charge order stripe by stripe. In α-(BEDT-TTF)₂I₃, lattice distortions locally stabilize the charge order so that it is easily weakened by local photoexcitation. The photoinduced domain indeed expands in the plane. These results are consistent with recent observation by femtosecond reflection spectroscopy.