Nonclassical dynamics of Néel vector and magnetization accompanied by THz and high-harmonic radiation from ultrafast-light-driven NiO antiferromagnet insulator

Abstract: Ultrafast-light-driven strongly correlated antiferromagnetic insulators, such as prototypical NiO with large energy gap 4 eV, have recently attracted experimental attention using either above-gap [K. Gillmeister et al., Nat. Commun. 11, 4095 (2020)] or subgap [H. Qiu et al., Nat. Phys. 17, 388 (2021)] energy photons that are of fundamental interest in far-from-equilibrium quantum matter or spintronic applications, respectively. In the latter context, emission of THz radiation is also observed from NiO/Pt bilayers, where heavy metal (HM) Pt introduces strong spin-orbit coupling (SOC). However, microscopic mechanisms of such emission remain obscure because spintronic THz emitters have been amply studied using FM/HM (FM-ferromagnetic metal of conventional type) bilayers, where ultrafast demagnetization takes place and is directly related to THz emission. Conversely, in NiO total magnetization is zero prior to the fs laser pulse (fsLP) application. Here we employ the two-orbital Hubbard-Hund-Heisenberg model and study, via numerically exact nonequilibrium quantum many-body methods, the dynamics of its Neel vector and nonequilibrium magnetization. Additionally, we compute electromagnetic radiation by both time-dependent magnetization and local charge currents arising in either plain NiO or NiO with proximity SOC introduced by HM layer. Our analysis reveals nonclassical dynamics of Neel vector and nonequilibrium magnetization, changing only in length while not rotating, where the former is substantially reduced only in the case above-gap fsLP. In the plain NiO case, THz radiation of interest to applications is insignificant, but adding SOC enhances both current and magnetic dipole contributions to it. Above THz range, we find integer high-harmonic generation, as well as unusual noninteger harmonics for above-gap fsLP pump.