Orbital mixing as a design principle for tunable shift currents in quasi-one-dimensional ferroelectrics

Quasi-one-dimensional ferroelectrics provide a promising platform for exploring bulk photovoltaic effects with potential applications beyond conventional three-dimensional systems, where the relationship between characteristics of the electronic structures and shift-current responses remains largely unexplored. Using first-principles density functional theory, we investigate the shift-current responses of quasi-one-dimensional transition-metal oxytetrahalides MOX4 (M = Mo, W; X = Cl, Br), consisting of one-dimensional chains with ferroelectric distortions. Depending on the chemical composition and degree of polarization, there is a substantial change in the frequency-dependent shift current responses. We identify the orbital mixing between d-orbitals as the primary driving force, affecting both the matrix elements and shift vectors. Moreover, modulating the orbital mixing by strain results in a large increase in shift current in MoOBr4, demonstrating the applicability of the identified mechanism. Our findings provide an effective design principle for controlling shift currents in quasi-one-dimensional ferroelectrics, useful in enhancing efficiency in the photovoltaic, optoelectronic, and memory applications.