Peer review of: Ultrafast coupled charge and spin dynamics in strongly correlated NiO (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 

Gillmeister et al. present ultrafast photoemission measurements of NiO finding a novel oscillatory 
signal associated with the antiferromagnetic order. This represents a novel type of ultrafast 
dynamics in a correlated material. Overall, I regard the work to be sufficiently important and well-
supported by the data and theory to justify inclusion in Nat. Comm. On the less positive side, 
there are many points in the manuscript, which I think require improvement in order to make the 
work publishable. 

1. The terms describing magnetic exchange seem to be defined in ways that are inconsistent. Is 
Jex the energy cost of breaking the strongest magnetic bond or the energy cost of flipping a spin? 
The text seems to imply the usual definition i.e. Jex is the cost of breaking a single bond, but the 
diagram in the right panel of Fig 5a implies that Jex is the cost of creating the state with a flipped 
spin spin. Fig 5 also implicitly defines J1 in a way that seems inconsistent with the text. I thought 
J1 was the rather weak nearest-neighbor exchange? Not the stronger 2nd neighbor exchange. 
2. Somewhat related there is an inconsistency between the theory and experiment, which see 
oscillations at ~Jex and 3xJex respectively. The suggestion that theory and experiment will merge 
for smaller values of Jex is not immediately plausible to me given how the theory scales with Jex in 
the higher limit of Jex in Fig. 4f. I think this issue would be better treated by either: (A) Explaining 
the reasoning behind this suggestion or (B) Presenting this issue as an unsovled problem that 
needs further work. 
3. I find that the very important comparison between the temperature-dependent decay timescale 
and TN is not treated adequately. The plot appears to be designed to test the hypothesis that the 
time-scale changes like ~sqrt(1-T/T_N). This would only really make sense if one assumes that 
the AFM order parameter also scales like ~sqrt(1-T/T_N), but there is no information that this is 
the case. I think the meaningful comparison is to directly plot the timescales and sqrt(I) against 
temperature where I is the intensity of the magnetic scattering the authors currently only present 
in the supplementary. It is crucial to consider errorbars in both temperature and time in order to 
convey the statistical significance of the results. 
4. The reasoning for the assignment of features V, C and S in Fig 1 should be explained. If it is 
based on a comparison to the literature, it would be better to cite specific experimental references 
rather than a wide range of different papers. 
5. The way in which Mott-related physics is introduced in the manuscript blurs the distinction 
between a charge-transfer and Mott-Hubbard type insulator in a way that is rather unhelpful for 
the readability of the manuscript. It first discusses 2PPE in NiO as involving LHB to UHB transitions 
before clarifying the fact that the relevant transition is O 2p to UHB. It would be better to be more 
careful and precise from the outset about when one is talking about a generic Mott insulator and 
when one is talking about NiO. The second sentence of the Fig. 1 caption is also ambiguous. Does 
the LHB refer to a composite O 2p + Ni d state or to just the lower energy Ni 3d states? I would 
not object to either notation, but “LHB” should have one clear meaning in the manuscript. 
6. What I think is the same state seems to be referred to using multiple inequivalent descriptions/ 
notations e.g. “hole in the ligand p state = holon =3d8L-1 = 3d8Z-1. This makes it harder to 
follow exact meanings, especially as “holon” is more commonly used to refer to a spineless 
quasiparticle in 1D physics 
7. The literature cited all seem to use L to denote a ligand hole. It’s not clear why L-1 is used here. 
Right now, the ligand and Ni states have different notation where the superscript is the change in 
electrons and total electrons, respectively. 
8. The meaning of the term “One color” in Figure 1 might not be immediately clear for all readers. 
9. The use of colored letters in Fig 1 is inconsistent. The color of the text label sometimes matches 
the color of the points, but not always. 
10. Information about the pump fluence is should be in the main manuscript rather than just in the 
supplementary. 

 
Reviewer #2: 
Remarks to the Author: 
Gillmeister et al perform ultrafast 2PPE experiment on NiO. They observe coherent THz 
oscillations, which they interpret in terms of Hund’s physics of spin interactions associated with 
electron injection into the eg symmetry orbitals. The experiment and theoretical analysis are well 
done, so I recommend publication. Such physics have not been observed in the time domain, as 
far as I am aware, and I believe it will be of broad interest to those investigating correlated 
materials in the time domain. It is quite sophisticated that the experiments are accompanied by 
high level theory. The authors may consider a few comments. 
 
1) T2g-eg transitions appear to be common in transition metal oxides, as suggested by 
observation of similar excitations in TiO2.1 Presumably, the electron lifetimes and the energy level 
structure has much influence on whether such oscillations can be expected. 
2) The authors see a temperature dependence of the oscillations that they interpret as the Neel 
temperature. They should also consider how electron-phonon interaction affects the eg orbitals in 
transition metals. 
3) The experimental observables occur at 3.8 eV and are interpreted in terms of the electronic 
structure of NiO. This is probably correct, and can be supported by the NiO film thickness 
dependence. The authors, however, do not emphasize how their results depend on the NiO film 
thickness. This is important, because 3.8 eV also corresponds to the bulk plasmon in Ag(100) 
substrate,2 so they could have a process, where a bulk plasmon is excited and transfers an 
electron to NiO.3 This certainly happens for Ag/TiO2 interfaces. Moreover Ag has interband 
transitions that start at this energy.4 For example, Ag d-sp band transitions occur at and above 
this energy, and could have an influence on the NiO dynamics. 
 
1 Argondizzo, A. et al. Ultrafast Multiphoton Pump-Probe Photoemission Excitation Pathways in 
Rutile TiO2. Phys. Rev. B 91, 155429 (2015). 
2 Reutzel, M., Li, A., Gumhalter, B. & Petek, H. Nonlinear Plasmonic Photoelectron Response of 
Ag(111). Phys. Rev. Lett. 123, 017404, (2019). 
3 Tan, S. et al. Plasmonic coupling at a metal/semiconductor interface. Nature Photon 11, 806-
812, (2017). 
4 Rosei, R. Temperature modulation of the optical transitions involving the Fermi surface in Ag: 
Theory. Phys. Rev. B 10, 474 (1974). 
 
 
 
Reviewer #3: 
Remarks to the Author: 

The manuscript by Gillmeister et al. describes a combined experiment and theoretical study of the 
photodoping and subsequent relaxation of NiO thin films. The data show long-lived excitations 
which live inside the gap. The description of this behavior is that of strongly correlated dynamics. 
The most compelling results in the data are how the decay time of the oscillations scales with the 
temperature. 
 
Overall, I like the paper and lean toward recommending it for publication. However, I found much 
of the theory discussion and the integration of the theory with the experiment to be quite 
confusing. I hope this can be clarified in the next round. 
 
To start, the first part of the manuscript discusses a clear charge-transfer insulator with clear 
upper and lower Hubbard bands and the charge-transfer band. The ensuing discussion explains 
how the p-holes become correlated (Zhang-Rice physics) but later we are told they are integrated 
out because they are noninteracting. 
 
I found it very difficult to reconcile Figure 4 with the experimental results. How exactly am I to do 
that? The equilibrium spectra seemed to have far more bands and gaps than the earlier discussion 
described. The nonequilibrium result had additional structure, but I was confused by this because I 
thought it was shown after the pump is off. By that point, the retarded Green's function should 
have returned to its equilibrium form because it rapidly does this (due to the fact that it represents 
the available quantum states and there is no dressing when the field is off). But this one has 
additional structure (I am guessing due to the occupancy in the UHB, but the explanation for this 
was not so clear). 
 
The oscillations shown in Fig. 4 were very rapid. Indeed, they would never be seen in a real 
experiment because the probes have short time width envelopes, but this physics is not included in 
the calculations if I followed them properly. 
 
So, for me, there seems to be a huge disconnect between the actual experiments and the theory 
that is supposed to illustrate the phenomena seen in the experiment. I hope this can be properly 
remedied, as I did like the paper on the whole. 
 
My recommendation is to reconsider after the manuscript is revised.