Observation of spin-mediated Mott-Hubbard exciton in the antiferromagnetic Mott insulator Sr₃Ir₂O₇

In a Mott insulator, doubly occupied and empty sites are predicted to form a bound state - known as Mott-Hubbard exciton - via exchange interactions. Although the Mott-Hubbard exciton is dipole-forbidden and thus invisble to many conventional probes, there have been efforts using terahertz light to unveil its existence and analogy with Cooper pairs in a doped Mott insulator. In this talk, we will discuss on our observation of a Mott-Hubbard exciton in Sr₃Ir₂O₇ using Raman scattering. Below the Neel temperature where spins align antiferromagnetically, an A_1g excitation appears at 105 meV. However, microscopic model for Raman scattering does not allow a zone-center single magnon. Indeed, applying a magnetic field up to 8 T along [001], the A_1g excitation remains unaffected with a width < 1 meV, which contrasts with an energy splitting of ~ 2 meV expected for a spin wave. Ruling out the magnetic origin, we assign this mode to an exciton across Mott gap ~ 130 meV. A_1g symmetry is consistent with numerical calculations that predict s-wave pairing is energetically favorable. Upon chemical doping, a small amount of Pt significantly broadens this mode, while the antiferromagnetic order is hardly affected. Our finding shows that a long-range antiferromagnetic order plays a role in the binding mechanism of Mott-Hubbard excitons.