Timescales of excited state relaxation in α−RuCl3 observed by time-resolved two-photon photoemission spectroscopy

Nonequilibrium dynamics of $α$-RuCl$_{3}$ -- a time-resolved magneto-optical spectroscopy study

“…The mechanism within the LER is conceivable, as already observed in similar Mott-Hubbard systems. At low excitation densities, the holons and doublons form bound pairs, a Mott-Hubbard exciton, (also reported for α-RuCl 3 in [31]) which then recombine within 10s of ps, while their excess energy is dispersed through magnons and potentially other bosonic exciations into the whole system. The clearly different non-linear fluence dependence of the initial demagnetization dynamics within the IFR thus can be understood in terms of an increasing unbound holon-doublon density perturbing the zigzag order.…”

“…Previous magneto-optical studies on magnetically ordered systems reported similar time scales for an initial photo-induced demagnetization process [49,50], although the relevance of the involved microscopic degrees of freedom in different systems lead to a wide spreading of time scales in the literature [51]. Recent 2PPES [31] directly revealed a sub-ps lifetime of doublons in the UHB after photo-excitation across the Mott gap, which is initially unexpected for such a large Mott-gapped insulator. In the same study some signatures of sub ps creation of Mott-Hubbard excitons have been reported, which has been interpreted as a reason of the fast decay of the doublons in the UHB in line with [29].…”

“…The holon-doublon density is indeed crucial in understanding the photo-induced quench of the zigzag antiferromagnetic state. Up to a threshold fluence τ D is constant, which points towards the initial formation of Mott-Hubbard excitons by the photo-excited holons and doublons referring to [31]. Above this threshold fluence the amount of excess free holons and doublons increases and causes a screening effect, such that the formed excitons break up into free holons and doublons.…”

“…Different to the hopping of single holons and doublons the movement of holons and doublons paired up to a Mott-Hubbard exciton does not lead to a further disordering of the zigzag ordered groundstate, hence leaving zero net disorder behind, and they only contribute to the magnetization through magnon emission in the recombination process. At the time, the presence of Mott-Hubbard excitons can be regarded as a bottleneck for the reestablishment of long-range antiferromagnetic zigzag order, as in α-RuCl 3 the holon-doublon bound together to an exciton are long-lived with lifetimes exceeding 100s of ps [31]. Thinking of an antiferromagnetically disordered zigzag groundstate by the presence of Mott-Hubbard excitons with long lifetime and breaking to single holons and doublons is in good agreement with the start of the recovery process at long times for low pump fluences.…”

“…The pump photons induce intersite d-d charge transfer transitions resulting in doubly occupied and non-occupied sites creating multiple spinless quasiparticles, the doublons and holons. A recent time-resolved two-photon photoemission spectroscopy (2PPES) study reported the condensation of these quasiparticles forming Mott-Hubbard excitons within a few picoseconds after photoexcitation [31], which are expected to interfere with the zigzag ordered antiferromagnetic background. In this report, we systematically investigate the magnetization dynamics and the subsequent recovery processes from fs to ns time scales by transient magnetic linear dichroism measurements, which have been performed for different applied pump fluences varying by more than one order of magnitude, at different temperatures below and above the Néel temperature T N ≈ 7 K and in external magnetic fields strengths from 0 − ±7 T applied along the ab honeycomb-planes.…”