Ultrafast coupled charge and spin dynamics in strongly correlated NiO

Abstract

Charge excitations across an electronic band gap play an important role in opto-electronics and light harvesting. In contrast to conventional semiconductors, studies of above-band-gap photoexcitations in strongly correlated materials are still in their infancy. Here we reveal the ultrafast dynamics controlled by Hund’s physics in strongly correlated photoexcited NiO. By combining time-resolved two-photon photoemission experiments with state-of-the-art numerical calculations, an ultrafast (≲10 fs) relaxation due to Hund excitations and related photo-induced in-gap states are identified. Remarkably, the weight of these in-gap states displays long-lived coherent THz oscillations up to 2 ps at low temperature. The frequency of these oscillations corresponds to the strength of the antiferromagnetic superexchange interaction in NiO and their lifetime vanishes slightly above the Néel temperature. Numerical simulations of a two-band t-J model reveal that the THz oscillations originate from the interplay between local many-body excitations and antiferromagnetic spin correlations.

Colloquium: Nonthermal pathways to ultrafast control in quantum materials

“…This technique has been used, for example, to study decoherence effects in the excitation process (Höfer et al, 1997;Ogawa et al, 1997;Reutzel et al, 2019), to shed light on the physics of high-temperature superconductors (Avigo et al, 2013;Parham et al, 2017;Smallwood et al, 2012;Yang et al, 2014Yang et al, , 2019, to track the melting and recovery of charge-density wave orders (Hellmann et al, 2010(Hellmann et al, , 2012Perfetti et al, 2006;Rettig et al, 2016;Rohwer et al, 2011;Schmitt et al, 2008;Zong et al, 2019b), to directly probe excitonic states (Cui et al, 2014;Madéo et al, 2020), to measure the relaxation dynamics of photocurrents (Güdde and Höfer, 2021;Reimann et al, 2018) and the coupling between electronic and lattice degrees of freedom (Gerber et al, 2017;Kemper et al, 2017;Na et al, 2019). tr-ARPES also permits the detection of transiently populated topological states (Belopolski et al, 2017;Sobota et al, 2012Sobota et al, , 2013Zhang et al, 2017), observation of Floquet-Bloch states (Mahmood et al, 2016;Wang et al, 2013), and identification of nonthermal electronic regimes (Gierz et al, 2013;Johannsen et al, 2013;Na et al, 2020). An exciting prospect is the implementation of new detection schemes extending time-and momentumresolved microscopy to FELs (Kutnyakhov et al, 2020).…”

Orbital-dependent photodynamics of strongly correlated nickel oxide clusters

“…3 NiO is also antiferromagnetic with a high Ne ´el temperature of 523 K, [4][5][6] making it a strong candidate for spintronic devices. Photoexcitation of NiO can invoke a ferromagnetic response, [5][6][7][8] and alter its insulator-like properties on ultrafast timescales. 9 Ultimately, the timescale that NiO redistributes photoexcitation and the related mechanism of local rearrangement of 3d-electrons dictate its activity in solar cell applications and its efficiency as opto-electronic and optomagnetic devices.…”

“…Ni has the lowest excitation energy of the first-row transition metal elements and an open 3d subshell, providing an extremely rich density of states. This enables electron-electron (e-e) scattering to be a prominent relaxation mechanism in bulk NiO, leading to excited state dynamics as short as 10s of fs 7,10 and polaron formation on the sub-ps timescale. 11 The d 8 ground state electron configuration of NiO formally consists of fully occupied t 2g and half-filled e g orbitals, with strong hybridization between the 2p and 3d bands.…”

“…11 The d 8 ground state electron configuration of NiO formally consists of fully occupied t 2g and half-filled e g orbitals, with strong hybridization between the 2p and 3d bands. 7 The d-shell of Ni is the most compact of all transition metals, 12 enabling NiO to contain signatures of both localized atomic-like states and band-like dispersive electronic states. NiO is classified as an intermediate charge-transfer insulator, suggesting that the charge transfer energy gap (D) between the O-2p band and the unoccupied Hubbard band, is smaller than the weakly screened Coulomb interaction (Hubbard U), 7 thereby leaving O-2p bands in the energy range of the occupied Hubbard band.…”

Low rank compression in the numerical solution of the nonequilibrium Dyson equation

“…The Keldysh formalism is a particularly versatile approach, as it is not limited by the dimension of the problem (like DMRG), and can be efficiently adjusted to realistic setups. Several recent studies have used these techniques in direct comparisons with experiments, including those involving transport properties [27] and periodic driving [11] in ultra-cold atomic systems, and pump-probe experiments in correlated solids [5,[28][29][30][31][32]. Many have already reached the level of first-principles description [33][34][35][36][37].…”