Theory of the optical conductivity of (TMTSF)$_2$PF$_6$ in the mid-infrared range

TL;DR

This paper explains the mid-infrared peak in the optical conductivity of (TMTSF)$_2$PF$_6$ using numerical calculations of a dimerized Hubbard model, revealing that the main peak aligns with the dimerization gap due to optical selection rules.

Contribution

It provides a novel explanation for the mid-infrared peak in (TMTSF)$_2$PF$_6$ based on numerical analysis of the dimerized Hubbard model, highlighting the role of optical selection rules.

Findings

Main peak in optical conductivity is at the dimerization gap energy.

The peak position is consistent across different intermediate $U/t$ values.

Optical selection rules explain the peak's energy despite small charge gaps.

Abstract

We propose an explanation of the mid-infrared peak observed in the optical conductivity of the Bechgaard salt (TMTSF)$_2$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.