Abstract
Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr3Ir2O7. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr3Ir2O7 indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase.
Highlights
Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs
The condensation of the relevant triplet exciton at the quantum critical point (QCP) gives rise to an antiferromagnetic ground state hosting a well-defined excitonic longitudinal mode[4], which coexists with transverse modes that are a generic feature of ordered antiferromagnets
We have isolated and characterized a longitudinal magnetic mode in Sr3Ir2O7, which merges with the electron-hole continuum at certain points in the Brillouin zone, and which softens upon heating concurrent with a decrease in the material’s resistivity
Summary
Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr3Ir2O7 indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase. Effective electron-hole attraction can arise from on-site electron-electron Coulomb repulsion U via magnetic exchange interactions between the electron and hole[10] In this case, the soft exciton is expected to be a spin-triplet, which passes through a quantum critical point (QCP) with increasing effective U. The condensation of the relevant triplet exciton at the QCP gives rise to an antiferromagnetic ground state hosting a well-defined excitonic longitudinal mode[4], which coexists with transverse modes that are a generic feature of ordered antiferromagnets This longitudinal mode features excitonic character, in the sense that it modifies the local spin amplitude by creating electron-hole pairs[4]. We identify and study a longitudinal mode in Sr3Ir2O7, the presence of which is the key experimental signature of an antiferromagnetic excitonic insulator
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