Double-Exchange Enhanced Magnetic Blue-Shift of Mott Gaps

TL;DR

This paper investigates how magnetic order influences Mott gaps, revealing a magnetic blue-shift effect enhanced by exchange and double-exchange mechanisms, with implications for spintronics and magnonics.

Contribution

It demonstrates that magnetic order increases Mott gaps via a magnetic blue-shift, incorporating exchange and double-exchange effects, and interprets optical data in $ ext{α}$-MnTe accordingly.

Findings

Magnetic order causes a blue-shift in Mott gaps.

Double-exchange contributes additional blue-shift.

Optical conductivity in $ ext{α}$-MnTe shows this effect.

Abstract

A substantial energy gap of charge excitations induced by strong correlations is the characteristic feature of Mott insulators. We study how the Mott gap is affected by long-range antiferromagnetic order. Our key finding is that the Mott gap is increased by the magnetic ordering: a magnetic blue-shift (MBS) occurs. Thus, the effect is proportional to the exchange coupling in the leading order in the Hubbard model. In systems with additional localized spins the double-exchange mechanism induces an additional contribution to the MBS which is proportional to the hopping in the leading order. The coupling between spin and charge degrees of freedom bears the potential to enable spin-to-charge conversion in Mott systems on extreme time scales determined by hopping and exchange only, since a spin-orbit mediated transfer of angular momentum is not involved in the process. In view of spintronic and magnonic applications, it is highly promising to observe that several entire classes of compounds show exchange and double-exchange effects. Exemplarily, we show that the magnetic contribution to the band-gap blue-shift observed in the optical conductivity of $\alpha$-MnTe is correctly interpreted as the MBS of a Mott gap.