Kinetics of a Phonon-Mediated Laser-Driven Structural Phase Transition in Sn₂P₂Se₆

Round  1

Reviewer 1 Report

The contribution by Kubli et al is very exciting, although the absence of SACLA data that is mentioned is a shame. I am assuming that these results are written up separately to follow this report, which would make good sense
The analysis of X-ray data is convincing, and interesting temperature dependence is reported. The dynamic range of the data is impressive, I wonder if this is also achieved at SACLA (but this will need to wait for follow up)
The main results analyse the fluence dependence of the superlattice reflections kinetics in figure 2 and the probing depth dependence in 3. These raise some questions. On page 3 the 1/e penetration of the pump is given as 20nm, based on presumable a ground state cross section. However the fluence is relatively high; can the authors please discuss the analysis based on ground state cross section? E.g. what is the ratio of photons for absorbing centers in the interaction region? Only if this is significantly below unity can a linear analysis be assumed.
On page 7 it is stated that “Saturation at high fluences (Fig. 2b) is
likely an effect of the spatial inhomogeneity in the pumped region; since the pump penetration depth is limited to 20 nm and the field penetration depth of the x-rays is in all cases larger than 70 nm, the pump does not initially excite the entire probed volume of the sample.” It would be useful here to point out that penetration of the pump beyond that expected for the linear response is not possible if the number of photons is far smaller than absorbing centers. If this was not the case, the non-linear cross section would determine the penetration. Have Z-scan experiments been reported for this material? If so, please include citation. The fluence is given however not the power density and a discussion of possible non-linear reactions is not included. Furthermore, the monoclinic spacegroup is biaxial and birefringence analysis of cross section and photolysis is possible. It would be useful if the mentioned ellipsometry data is included in a SI document, with conclusions included in the main document.
Author Response

Thank you for your valuable review. Please find my response in the attached PDF.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments on

M. Kubli et al.:”Dynamics of a phonon-mediated laser-driven structural phase transition in Sn₂P₂Se₆”

In this manuscript, the authors present interesting new results of the behavior of the modulated system Sn₂P₂Se₆ under the influence of a rapid laser pulse. The combination of the fs-laser and the real-time data-acquisition at the Swiss Light Source provides information about the structural variation after the external perturbation. The manuscript is in well written and deserves publication in Appl. Sci.. There are, however, some points that I would like the authors to consider in detail in an amended version:

1.       Interpretation related aspects:

a.       The appearance of higher order satellites may have two origins: A true higher order effect of the modulation as described by higher order expansion of the free energy, on the one hand, and a deviation of the shape of the modulation wave (squaring) sometimes being called “modulation harmonics”, on the other hand. While in section 4.1 the first alternative seems to be favored, in the abstract provide a different aspect. I would like the authors to clarify their interpretation.

b.      The fast laser-heating process obviously produces non-equilibrium states. If the total deposited energy were used to heat the sample volume affected by the laser, a very large temperature increase of several 100 K would result. Hence, the efficiency of  the energy transfer from the electron system to the lattice seems to be crucial. Moreover, it is well-known that in incommensurate systems that are driven by soft phonon modes, the competing order parameter may be a component of the internal strain. In fact, kinetic experiments of A₂BX₄-systems and also of SrTiO₃ as well as of multiferroics have shown that relaxation processes are slowed down which may be attributed to the creation and annihilation of strain fields. In their interpretation, the authors should at least consider these possible correlations as a reason of the delayed relaxation.

c.       The data have been obtained on single crystals with a specific orientation with respect to the laser beam. In view of the anisotropy of this monoclinic system, heat transport is expected to be direction-dependent. The mentioned DFT-calculations should provide information about this effect. The authors should comment on this instead of just stating a “weighted average”.
Moreover, it should also be considered that new types of excitations appear within the INC-phase, namely phasons and amplitudons, and topological defects like discommensurations may affect the heat transport.

d.      Finally, I would ask the authors to consider a modification of their title: One could replace “dynamics” by “kinetics” in order to point out that this is a non-equilibrium study. And the term “structural dynamics” may also be misleading, since it is not the phonon dispersion which is studied here.

 2.        Formal aspects:

a.       The authors report on experimental results that have exclusively been obtained at the Swiss Light Source. Hence, the remark on the SACLA free electron laser  (line 74f) looks like an artificial self-citation and should be dropped.

b.      The satellite position (-0.81 1 4.07) (lines 107 f) is probably incorrect.

c.       The satellite wave vectors observed in the Brillouin zones (-1 1 0) and (-1 1 4) differ significantly ((0.11 0 0.08) vs. (0.09 0 0.07)). The authors should comment on this finding and discuss the possible influence of the asymmetric line shape and/or resolution effects.

d.      The labels (-140) in Fig. 4b/c need to be checked.

e.      Typos:
line 87: “measurements”
line 156: “parameters are summarized”

 Author Response

Thank you for your valuable review. Please find my response in the attached PDF.

Author Response File: Author Response.pdf