Peer review of: Strange semimetal dynamics in SrIrO3 (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

This manuscript reports Raman measurements in polarized light on the SrIrO3 thin films that focus on 
the electronic Raman response. A careful and methodical analysis of the electronic Raman responses 
using the memory function formalism originally used by Opel et al in 2000 and developed here, leads 
the authors to identify two kinds of carriers, hole and electron pockets, depending on the polarizations 
of light used. 

They say that the Raman responses of these carriers cannot be interpreted as a Fermi Liquid but 
rather as a marginal Fermi liquid. From this point of view, they consider that the charge dynamic in 
SrIrO3 is similar to that of strongly correlated systems and they found that the static scattering rate 
versus T follows the Planckian limit observed from transport data in cuprates. By this study the 
authors highlight the power of Raman spectroscopy for studying the carrier dynamics and in particular, 
scattering rate, effective mass of charge carriers. 

The authors have carried out a very fine experimental study and an analysis of the Raman data using 
the formalism of the memory function which opens new perspectives for the exploitation of electron 
Raman spectroscopy. 

However, there are several points which concern me and which do not seem clear enough for me to be 
in favour of an immediate publication. 

The first point is the authors' assertion that Raman response cannot be interpreted in the context of a 
Fermi liquid model. 
The authors show curve fits in figures S4 and S5 of the supplementary information without giving the 
values of the parameters. what are the values of g, D, β, and λ? 

To what extent by varying g and /or λ in the FL model , it would not be possible to better fit the low 
energy part of the Raman spectra in figures S4 and S5 and so attenuate or even make the hump 
disappear around 100 cm-1? 

Importantly, if one of the main conclusion of the paper is that the dynamics of the carriers in SrIrO3 
thin films cannot be interpreted in terms of FL expression. I expect that at least, the S4 or S5 figure 
(with the parameters used) to be put in the main text and not in the supplementary information. 

In figure 3, which show the different stage of the process for extracting the Raman electronic 
background in X’Y’ configuration, is hard to understand why in panel ( c ) is plotted the temperature 
dependence of the electronic XY Raman response and not the one of X’Y’? Can the authors show the 
electronic XY Raman response and if they can't do it, they have to explain why? 

In view of the electronic Raman data, the authors propose that SrIrO3 is a strongly correlated material 
as it is the case of the cuprates. If this is actually the case, I would like the authors indicate or 
suggest which kind of strong electronic interactions occur in SrIrO3? SrIrO3 is a non-magnetic 
material, the correlations likely do not come from antiferromagnetic fluctuations as in cuprates, so, 
why make the analogy with cuprates? I would like the authors to be clearer on this point. 

Finally, I have a couple of questions: 

Editorial Note: Parts of this peer review file have been redacted as indicated to remove third-party material where no permission to publish could be obtained. -How the authors know the system they studied is free of constraints between the substrate and the 
thin film? 

-how the authors eliminate the Raman vibration modes of the air molecules at the second focus point 
in the confocal set-up? 

-Why the phonon peaks are smaller in intensity in the XY configuration with respect to the X’Y’ 
configuration? 

-The authors mentioned a double phonon around 600 cm-1 but I don’t see any single phonon at 300 
cm-1. Can the authors explain why? 


Reviewer #2 (Remarks to the Author): 

This paper presents the study of electronic dynamics in the strongly correlated Dirac semimetallic 
SrIrO3. Combined with the inelastic light scattering and ab-initio calculation, authors analyzed the 
electronic excitation in the scheme of the marginal Fermi liquid phenomenology. They show that the 
spectral shape can be modelled by using the marginal Fermi liquid theory, but the temperature 
dependence of scattering rate is not fully understood by the existing model. Although there are some 
issues to be addressed, the results are interesting. In my opinion, this manuscript may deserve the 
consideration in Nature Communications, after addressing the following issues. 

1) SrIrO3 is a Dirac semimetal which possesses electrons with Dirac-like dispersion and holes with 
conventional band dispersion. An important issue of this material is the dynamics of Dirac electrons. 
Authors clarified that the renormalization is slightly smaller for Dirac electrons than for holes. Is there 
a special mechanism that the renormalization of Dirac electron is smaller than that of holes? 

2) Authors present that the temperature dependence of scattering time appears to be proportional to 
T in all the temperature region and does not show the “low-T regime of marginal Fermi liquid theory”. 
This is an interesting result, but the reason is not clear. Why is “the low-T regime” missing in this 
material ? The materials nearby the quantum critical point often show such T-linear behavior. Is 
SrIrO3 is related to the quantum criticality ? 

3) The ARPES study reveals that there is a hole pockets nearby the Gamma-point in the momentum 
space, which substantially contributes the DC transport. In this work, authors do not mention the hole 
pocket nearby Gamma-point since the Raman scattering probes the pockets at the zone boundary. 
Nevertheless, in the final part of the main text and Figure 4, authors compare the results of Raman 
scattering with that of DC-transport. The missing information about hole pocket at Gamma-point may 
be critical for this comparison. It would be better to append the explanation about the comparison. 

4)Authors suggest that the electronic structure at the Fermi surface does not significantly change as a 
function of temperature. However, this statement may not be consistent with the previous research by 
optical conductivity which reveals a substantial change of electronic structure as a function of 
temperature. It would be better to append some explanation. 



Reviewer #3 (Remarks to the Author): 
The manuscript, entitled “Strange semimetal dynamics in SrIrO3”, brings a Raman study of SrIrO3, a 
member of iridate perovskites from the Ruddlesden-Popper series. Unlike other members of that 
series, which are layered, SrIrO3 is a three-dimensional system, exhibiting (semi)metallic behavior. 

The two most important achievements presented in this manuscript are: 

i) Independent measurements of the electron- and hole-like contributions to the Raman response, by 
probing different sections of the FS. 

ii) A fitting procedure that provides a relatively convincing case for a phenomenological description of 
the charge dynamics in terms of a scattering rate that is linear in frequency and temperature. 

I think that these results are interesting and eventually worth publishing in Nature Communications. 
However, I have some issues with the consistency of the new findings with the already published 
results. Furthermore, I have worries regarding the physical interpretation of the experimental data. In 
the light of these concerns, I would like to draw attention to few essential problems, mostly in relation 
to the nature of the scattering rate. Specifically: 

i) How important are the strain-relaxation effects for the present study? In the part of the manuscript 
that discusses experimental aspects it is said that fully strain-relaxed 50 nm thick films are measured, 
suggesting that the electron band structure and the Raman response are not affected by the film 
thickness. Are there any data to confirm this? 

ii) It appears that there is a good agreement in the literature that bulk SrIrO3 exhibits a metallic 
resistivity [PRB 89, 214106; PRB 95, 121102(R); J. Appl. Phys. 103, 103706]. In particular, in the 
recent study [PRB 89, 214106], for low temperatures (T<30K), it was found that the bulk resistivity 
follows the typical Fermi-liquid behavior quadratic in temperature. Above these temperatures the 
resistivity became linear in T, which is typical for metals and for the scattering on thermal phonons. 
The authors of [PRB 89, 214106] find no indication for an upturn in resistivity below 50 K reported in 
[J. Appl. Phys. 103, 103706], concluding that this upturn should be related to impurity or boundary 
scattering. The authors of the current manuscript should comment on these standard metallic 
behaviors observed in the resistivity curves, and appropriately cite the literature. 

iii) In the ARPES study, Ref. [14], it was found sharp quasiparticle peaks and a well-defined Fermi 
surface characterizes the hole pockets. Moreover, the study reported elliptical electron-like pockets. 
Given that the marginal Fermi liquid MFL predicts a vanishing qusiparticle weight at the Fermi level, 
what would be the interpretation of these sharp ARPES features in the context of the MFL 
phenomenology used in the current manuscript? The authors just say that “we have not yet reached 
this low-T regime”. When this limit should occur and why according to their expectations? 

iv) It seems that the coupling to lattice degrees of freedom cannot be so easily excluded and that 
electron-phonon scattering may provide a significant contribution to the total scattering rate. In 
general, the phonon subsystem that characterizes SrIrO3 should consist of many different optical and 
acoustic phonons. The authors observe many Raman active phonon modes, few of them involving 
rather small optical phonon energies. Furthermore, it is established in the literature that the bands in 
SrIrO3 are quite sensitive on in- and out-of-plane octahedral rotations, while the authors suggest a 
high sensitivity of flat bands associated with the hole-like Fermi surface to structural details. However, 
in the manuscript there is no mention of the coupling to phonons and no explanation of reasons for 
neglecting this coupling when considering the relaxation rates. 

v) Measurements of temperature-dependent resistivity would be quite helpful for the manuscript, as 
an independent check of the results obtained from the Raman response. This, or other transport 
measurements, would be particularly beneficial for the interpretation of results presented in Table 1 
and Fig. 5. 

A revised version of the manuscript should elaborate on these points with the aim of achieving a 
consistent physical picture of the studied system. This would secure a quality that is appropriate for 
manuscripts that aim at publication in Nature Communications.