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Determination of the time scale of photoemission from the measurement of spin polarization
by Mauro Fanciulli, J. Hugo Dil
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Submission summary
| Authors (as registered SciPost users): | Hugo Dil · Mauro Fanciulli |
| Submission information | |
|---|---|
| Preprint Link: | https://arxiv.org/abs/1806.05895v1 (pdf) |
| Date submitted: | 2018-06-19 02:00 |
| Submitted by: | Fanciulli, Mauro |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
Abstract
The Eisenbud-Wigner-Smith (EWS) time delay of photoemission depends on the phase term of the matrix element describing the transition. Because of an interference process between partial channels, the photoelectrons acquire a spin polarization which is also related to the phase term. The analytical model for estimating the time delay by measuring the spin polarization, as introduced in M. Fanciulli et al., PRL 118, 067402 (2017), will be expanded here. In particular, a distinction between scattering EWS and interfering EWS time delay will be made, providing an insight in the chronoscopy of photoemission.
Current status:
Reports on this Submission
Report #1 by Anonymous (Referee 2) on 2018-8-1 (Invited Report)
- Cite as: Anonymous, Report on arXiv:1806.05895v1, delivered 2018-08-01, doi: 10.21468/SciPost.Report.545
Strengths
adds important information to the previously published experimental data [39] (up to page 11 and App. A)
Weaknesses
1 - contains too many unnecessary considerations which overshadow central result and its application to actual experimental data
2 - after the derivation of the time delays I would have expected suggestions how to measure or estimate the unknowns (in part done in [39])
Report
I have a split opinion on the manuscript "Determination of the time scale of photoemission from
the measurement of spin polarization" by Fanciulli and Dil. On the one hand, I am thankful for
more elaborate follow-up papers of PRLs and other letter-style papers, on the other hand, there
is not enough additional information in the manuscript (apart from an exceedingly long and in
part confusing text) to warrant publication.
It starts with the concept of "interference". Since the electrons are distinguishable (spin)
there is no two-path interference in this particular setup. Instead, as explained in Refs.
[69,76,80], different initial and final states which may also include the direction of the
outgoing electron will lead to different matrix elements and the observed polarization as shown
in [72] for photoionization from Xe p-states. For this particular case the relative delay of the
two contributing partial waves (s- and d-waves) can be evaluated and even classically estimated
individually [6]. At this point (bottom of page 6: "The effects seen in photoionization are
found also in photoemission from crystals"), a discussion on the matrix elements as in
[69,76,80] and their influence on the result of Eq. (7) would have helped which is done
implicitly in Sec. 2.1 by geometrical considerations influencing the polarization. Now, Eq. (14)
could be inserted in Eq. 7 (side remark: the function $c(r,t)$ has two maxima/minima with $c=\pm 1$)
and evaluated for the data at hand (appendix A and top of page 19). If the authors had stopped
here this would have been a nice paper with a very general result applied to a specific
experimental situation, add a short discussion about error estimates and that's it.
Instead, the second half of the manuscript contains a very general, lengthy discussion on
possible time delays under various assumptions without clear indication why the reader should
bother. And why should one vary the binding energy instead of the photon energy to change the
kinetic energy of the outgoing electron? The argument of the Brillouin zone is not valid as the
crystal structure should enter both the initial and final states of the electron (that is one of
the reasons for the observed angular dependence of polarization) and why should a procedure
become simpler by changing both initial and final states instead of the final state alone? Why
should this have a smaller effect on the matrix elements than the "conventional" change of
photon energy? This is not clear to me and adds unnecessary confusion in the manuscript as, of
course, the photon energy has been changed in experiment (at least that is how I interpret Fig.
2a of [39]).
Finally, EWS(s) and EWS time delays are evaluated which we have learned are complicated
functions of various parameters. Fig. 4 shows the delays as functions of r (are these results of
the evaluation of Eqs. (26) and (27) from appendix A? Or is it silently assumed that $\dot r=0$?
This is not indicated in the text. Where does the energy dependence of the polarization come
from? Why the choice of other parameters? We find a reference to the experiment [39] only for
the parameter choice in Fig. 5.). We see in Figs. 4 and 5 that any value from 0 to infinity
appears to be possible (we have long dropped any signs) with uncertainties ("spurious effects")
in all directions. It would probably have helped to analyze matrix elements as in [69,76,80] to
assess the influences of different effects.
All in all, half of the paper would have been nice to read, maybe including an extended
discussion on the derivation of the equations from [69,76,80]. Application to the data from
[39], estimate for the error bars and then stop. All the rest of the paper is questionable,
confusing, (in my view) unnecessary.
Requested changes
I recommend to revise the paper by making it more concise and strongly reducing its second half. Instead, a discussion of ways to experimentally determine the unknowns (e.g., discuss the experimental setup of [39]) and estimate remaining uncertainties would be interesting.