Photoinduced melting of horizontal-stripe charge orders in quasi-two-dimensional organic conductors theta-(BEDT-TTF)2RbZn(SCN)4[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] and alpha-(BEDT-TTF)2I3 is investigated theoretically. By numerically solving the time-dependent Schrodinger 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 theta-(BEDT-TTF)2RbZn(SCN)4 than for alpha-(BEDT-TTF)2I3, which is a consequence of the larger stabilization energy in theta-(BEDT-TTF)2RbZn(SCN)4. After local photoexcitation in the charge ordered states, the growth of a photoinduced domain shows anisotropy. In theta-(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 alpha-(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.
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