Charge-transfer optical absorption in linear magnetic insulators in a strong electric field

The effect of a strong electric field on the charge-transfer optical absorption is studied in linear narrow-band Mott-Hubbard insulators (or semiconductors) such as alkali-TCNQ's (tetracyanoquinodimethanes) and antiferromagnetic Heisenberg chains, using the half-filled-band Hubbard model with strong on-site Coulomb repulsion. It is shown that, for an antiferromagnetic ground state, the main effect of the field is to severely (i.e., nonlinearly with the applied field) squeeze the unperturbed continuum absorption band into a narrower band of unevenly spaced discrete rungs of a Stark ladder. The intensity of the absorption into the Stark-ladder states has exponential tails both above and below the main band, showing a discrete version of the Franz-Keldysh effect. For random and ferromagnetic spin configurations, the absorption profile narrows and the discrete absorption resonances smear into sharp continuum structures