Optical conductivity in the CuO double chains of PrBa₂Cu₄O₈: Consequences of charge fluctuation

We calculate the optical conductivity of the CuO double chains of PrBa$_2$Cu$_4$O$_8$ by the mean-field approximation for the coupled two-chain Hubbard model around quarter filling. We show that the $\sim$40 meV peak structure, spectral shape, and small Drude weight observed in experiment are reproduced well by the present calculation provided that the stripe-type charge ordering presents. We argue that the observed anomalous optical response may be due to the presence of stripe-type fluctuations of charge carriers in the CuO double chains; the fast time scale of the optical measurement should enable one to detect slowly fluctuating order parameters as virtually a long-range order.