Theory of the optical conductivity of (TMTSF)2PF6in the midinfrared range

We propose an explanation of the midinfrared peak observed in the optical conductivity of the Bechgaard salt (TMTSF${)}_2$ ${\mathrm{PF}}_6$ in terms of electronic excitations. It is based on a numerical calculation of the conductivity of the quarter-filled, dimerized Hubbard model. The main result is that, even for intermediate values of U/t for which the charge gap is known to be very small, the first peak, and at the same time the main structure, of the optical conductivity is at an energy of the order of the dimerization gap, like in the infinite U case. This surprising effect is a consequence of the optical selection rules. © 1996 The American Physical Society.