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


RR2
 Reviewers' Comments:
 Reviewer #1:
 Remarks to the Author:
 The authors have provided very good quality answers to all the points I raised. I consider the work
 to be an important contribution to ultrafast dynamics in correlated materials and support
 publication.
 I might even go so far as to say that the manuscript would be suitable for a higher impact journal.
 Reviewer #2:
 Remarks to the Author:
 I have read the revised manuscript by Gillmeister et al. carefully, and unfortunately I find it
 lacking. The authors preform a dynamical study of some presumably spin dependent phenomenon
 in NiO, but for the most part their experimental measurement and theoretical interpretation lacks
 an intelligible dynamical interpretation. Also, the author’s response to my earlier comments shows
 a lack of understanding of relatively well understood dynamical phenomena. For this reason, I do
 not believe that the article can be published in the present form.
 The authors excite the sample with photon energy >3.8 eV, and probe photoemission with a
 longer wavelength light(3.4 eV in fig. 2). They observe a 17 THz oscillation that appears to have a
 sine phase (Fig. 3), and persists for ~2 ps. The 2PPE spectroscopy of NiO has not been
 established, or at least the authors do not cite any relevant work. The excitation is said to occur
 into UHB, followed by relaxation into a photoinduced-ingap state (PIS) on 10 fs. The reader is
 never told whether this PIS is known from any other studies or theory. My guess is that doping the
 material should populate this state, and it should be known for this material. It presumably does
 not have to be invented to explain the results. Also, why should we believe that the relaxation
 from UHB to PIS occurs on 10 fs time scale? Have electron lifetimes been measured for this
 material? Is there related research that would support such fast relaxation?
 Following formation of PIS, the authors see an oscillation of unclear origin. They present an
 elaborate theory claiming electron correlation effects, but as far as I can tell, this is purely a
 ground state model that does not explain why this process occurs. For example, why is the probe
 pulse sensitive to the proposed dynamics? Is photoemission from on spin-configuration much more
 probable than another? Why? Does it depend on the detection energy? Why does it occur with
 approximately the sine phase, and can one learn something from this.
 The authors show a very nice photoelectron energy vs. delay time plot in Fig. 2a, and then they do
 not do anything with it other than to show that the high energy electron decay is fast. The phase
 of oscillations, actually, looks to be cos in this figure. Sin and cos matters, because it tells what
 kind of force induces the oscillations, but that does not seem to trouble the authors. Moreover, the
 oscillation appears not to be single frequency, though the data in Fig. 3, which are processed
 differently, do appear to follow a single damped sine behavior. I would expect that additional
 dynamics should appear in the two-dimensional plot in figure 2 because the carrier density and
 temperature are certainly varying over the 2 ps time period. But the authors do not make use of
 such information. To me this is a clear lack of thinking about what is responsible for the observed
 dynamics.
 Next, I will comment on the author’s response to my first comments, and some other aspects that
 I find troubling. My point in referring to the work of Shang1 is that photoexcitation of NiO is likely
 to change the occupation of t2g and eg orbitals, and thereby exert a force on the lattice. This will
 happen no matter what the laser fluence may be. Furthermore, the authors claim that the
 oscillation that they observe is linked to the Neel temperature of the lattice. That may be true for

 the initial state of the system, but once they excite the system, the electron and lattice
 temperature will be delay dependent. The authors do not discuss these temperatures, and their
 analysis is appropriate for the ground state. If the ground state Neel temperature is important for
 this experiment, it need not have been done in the time domain. The authors should at least
 present the pump power dependence of the oscillation to show that they are measuring a time-
 dependent phenomenon of the electronic system. Since the authors are studying a charge transfer
 insulator, one would expect that the charge, lattice, and spin degrees are strongly coupled.
 Next, concerning the possibility of the longitudinal plasmon in Ag being involved, the authors seem
 to have a concept of the plasmonic response that is not in the publication that I mentioned,2 and
 seems to be more in tune with popular literature. The authors should refer to Fig. 7 of the
 publication by Barman et al.3, which shows that the longitudinal plasmon excitation of Ag can
 inject electrons from the substrate into their 10-20 ML thick film. Assuming that these electrons
 have the Fermi velocity of ~1.5 nm/fs, it should not take very long time to be transported through
 their NiO film. The authors should show the pump photon energy dependence of their signal to
 convince this referee, that they are driving this process by exciting UHP internally rather than by
 excitation of the substrate.
 Finally, I should mention that I find the first paragraph of the discussion on page 10 particularly
 disappointing. The authors take several examples of coherent dynamics that happen in vacuum,
 and claim that theirs is particularly novel because they observe the dynamics in a solid. It seems
 that they are not aware of much work on coherent phenomena in solids that occurs on comparable
 time scales and involves spin, charge, and lattice degrees of freedom.4-6 They need to compare
 their results to relevant phenomena, if they want to make a statement about significance of their
 work.
 1 Shang, H. et al. Electron-phonon coupling in d-electron solids: A temperature-dependent study
 of rutile TiO2 by first-principles theory and two-photon photoemission. Physical Review Research
 1, 033153, doi:10.1103/PhysRevResearch.1.033153 (2019).
 2 Reutzel, M., Li, A., Gumhalter, B. & Petek, H. Nonlinear Plasmonic Photoelectron Response of
 Ag(111). Phys. Rev. Lett. 123, 017404, doi:10.1103/PhysRevLett.123.017404 (2019).
 3 Barman, S. R., Biswas, C. & Horn, K. Electronic excitations on silver surfaces. Phys. Rev. B 69,
 045413 (2004).
 4 Bovensiepen, U. Coherent and incoherent excitations of the Gd(0001) surface on ultrafast
 timescales. J. Phys.: Condens. Matter 19, 083201 (2007).
 5 Bao, J., Bragas, A. V., Furdyna, J. K. & Merlin, R. Optically induced multispin entanglement in a
 semiconductor quantum well. Nat Mater 2, 175-179 (2003).
 6 Winkelmann, A., Lin, W.-C., Bisio, F., Petek, H. & Kirschner, J. Interferometric Control of Spin-
 Polarized Electron Populations at a Metal Surface Observed by Multiphoton Photoemission. Phys.
 Rev. Lett. 100, 206601-206604 (2008).
 Reviewer #3:
 Remarks to the Author:
 I am happy with the changes and explanations made by the authors in this revised version of the
 manuscript. I now recommend it be accepted for publication.