Peer review of: Tuning the hysteresis of a metal-insulator transition via lattice compatibility (2020)


RR2
Reviewers' comments:
Reviewer #1 (Remarks to the Author):
The authors have addressed all my concerns and I recommend publication.
Reviewer #2 (Remarks to the Author):
    The authors have undertaken more extensive characterization of the prepared films to rule out the co-
 existence of M2 and triclinic polymorphs. I remain somewhat perplexed by some of the rationale that
 has been put forth, specifically the idea that surface stresses in free-standing nanobeams beams are
 more likely than residual stresses in vapor-deposited thin films –surely facile deformation of the
 former provides a means of relaxing such stresses. Nevertheless, under the specific conditions
 examined here, the authors make a compelling case for there not being a significant concentration of
 the M2 phase. Since hysteretic phenomena have been studied extensively in nanobeams, I suggest
 that the authors provide an expanded discussion clearly delineating the differences between the
 observations here and previous studies of the origin of hysteresis in VO2 nanobeams. Such a
 discussion would allow this work to better connect to the considerable literature on nanobeams and
 would initiate a much needed discussion as the role not just of stresses but other parameters such as
 oxygen stoichiometry and point defect concentration.
 In a similar vein to the above point, by contrasting the lattice parameters extracted from Rietveld
 refinements for undoped VO2 films, can the authors delineate how much strain there might be in the
 films on Si and sapphire?
 The scan rate dependence of hysteresis widths is quite remarkable (hysteresis increases to 24C at
 faster scan rates) and points to the role in correlated oxides of not just thermodynamics but also
 activation barriers, which likely have some electronic structure contributions. Figure R4 is quite useful
 and it would be nice if the authors could more explicitly comment on the kinetic aspects of the
 transition. I am not asking for more experiments but just a more detailed discussion. There are
 nuances here that are clearly not well understood that ought to be highlighted and not entirely
 relegated to the SI.
 The authors make a compelling point about the role of disorder in suppressing the magnitude of the
 MIT. An alternative view comes from the idea of the orbital-selective Mott transition in VO2 (e.g., J.
 Appl. Phys. 125, 082539, (2019)) in epitaxial VO2 films that seem rather similar to the ones analyzed
 here wherein the authors find that even before the onset of the Peirl’s instability, the residual stresses
 engender breaking of the degeneracy of the electronic states, thereby inducing an orbital-selective
 Mott transition (OSMT). Such an OSMT is then inevitably lower in magnitude than a full coupled Mott-
 Peirls transition and provides a more physical rationale for the suppressed transition. The work I am
 citing further provides notable caveats about the generalizability of the OSMT to other correlated
 oxides. This again gets to the crux of my critique that co-factor analysis and single-element
 substitution appears to work under a specific set of circumstances (that enable relaxation of interfacial
 stresses in a particular manner and under certain density of defects where the transitions are not
 nucleation limited) but electronic structure matters and substantially confounds hysteretic phenomena
 in correlated systems. As such, some tempering of the claims regarding broad applicability of
 crystallographic analysis-that ultimately alters only phonon density of states- to systems with strongly
 coupled lattice, electronic, and spin degrees of freedom is necessary.
 I have a further question about the lattice compatibility analysis with seeking to get to a co-factor of
 two. Unlike in previous systems subject to lattice compatibility conditions, tungsten is an aliovalent
 dopant that must inevitably be charge compensated by localized vanadium reduction or an oxygen
 interstitial (it is likely polaron distortion on vanadium centers that dominates). Nevertheless, an
 important idea for why co-factor analysis works here –amplifying an argument that Reviewer 3 seems

 to make-is that the local distortion/polaron formation likely yields an abundance of nucleation sites
 that at a minimum ensure that at low scan rates the phase transitions are not nucleation limited. This
 might be a second happy accident for tungsten that facilitates observation of this phenomenon since a
 homovalent dopant that meets lattice compatibility conditions might still have remained nucleation
 limited across the entire range of scan rates.
 Reviewer #3 (Remarks to the Author):
 Now I recommend this manuscript for being published on Nature Communications. Comment: I was
 not stating that the disorder aspect is dominant compared to the lattice-compatibility aspect, but my
 opinion is that adding some discussion about the disorder effect on the hysteresis may make the
 manuscript more complete.