Tracking local magnetic dynamics via high-energy charge excitations in a relativistic Mott insulator

Abstract

We use time-and energy-resolved optical spectroscopy to investigate the coupling of electron-hole excitations to the magnetic environment in the relativistic Mott insulator Na2IrO3. We show that, on the picosecond timescale, the photoinjected electron-hole pairs delocalize on the hexagons of the Ir lattice via the formation of quasi-molecular orbital (QMO) excitations and the exchange of energy with the short-range-ordered zig-zag magnetic background. The possibility of mapping the magnetic dynamics, which is characterized by typical frequencies in the THz range, onto high-energy (1-2 eV) charge excitations provides a new platform to investigate, and possibly control, the dynamics of magnetic interactions in correlated materials with strong spin-orbit coupling, even in the presence of complex magnetic phases.Addressing the way local charge excitations delocalize and interact with the magnetic environment is a urgent task in the study of correlated materials. The possible impacts range from the clarification of the microscopic origin of the spin correlations that give rise to exotic magnetic states in transition-metal oxides [1][2][3][4][5] to the possibility of photo-stimulating novel functionalities in materials in which the charge and magnetic excitations are strongly intertwined [6][7][8]. The family of 5d transition-metal oxides offers a particularly interesting playground to achieve these goals. In these materials, the on-site Coulomb repulsion U is reduced to ≈ 2 eV by the increased average radius of the orbitals and the spin-orbit coupling (SOC) interaction is pushed up to λ SOC ≈ 0.7 eV by the large mass of the metal atoms [9][10][11]. As a consequence of the interplay of U , λ SOC and the delocalization driven by the hopping, neither a local Mott-like picture nor a delocalized orbital scenario is fully appropriate to capture the rich phenomenology of these materials: from Mott insulating states in which the local moments have also orbital character [9, 12] to exotic spin-liquid phases [1] driven by bond-directional magnetic interactions [4].Here, we focus on the honeycomb relativistic Mott insulator Na 2 IrO 3 and adopt a non-equilibrium approach [8]. We investigate how electron-hole excitations, photoinjected across the Mott gap [13,14], delocalize on the honeycomb lattice and release energy to the local magnetic background. Our experiment shows that the delocalization of local photostimulated charges is mediated by the excitations of specific QMOs. The perfect symmetry matching between the relevant QMOs and the zig-zag magnetic order allows the energy transfer to the magnetic background on the picosecond timescale. Exploiting the SOC-driven intertwining of magnetism and high-bindingenergy electronic states, we show that the melting of the short-range zig-zag order is mapped onto the modification of the optical properties in the near infrared region. This finding provides a new platform to track the magnetic dynamics in 5d transition metal oxides, as well as in many other relativistic correlated materi...

Nonequilibrium quasistationary spin disordered state in the Kitaev-Heisenberg magnet $α$-RuCl$_3$

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