Appeal LP9512
April 20, 2005
April 20, 2005
Dr. Saad E. Hebboul,
Assistant Editor
Physical Review Letters
Re: LP9512
Evolution of excitons via biexcitons to an electron-hole plasma without level crossing between
band edge and exciton in a quantum wire
By: Masahiro Yoshita, Yuhei Hayamizu, Hidefumi Akiyama, et al.
Dear Dr. Saad E. Hebboul,
We APPEAL for review by a divisional associate editor of PRL. We learned that our
revised manuscript and cover letter of January 11, 2005, which we wrote to the assistant editor
and a divisional editor, went back to the previous referees A, B, and a new referee C by the
editorial decision, because the editor cannot skip the second-round review process before an
appealing process. Since the second-round review was concluded on February 28, 2005, we now
APPEAL for review by a divisional associate editor.
Our main reason for the APPEAL is because we believe that the referees have strong bias
between our present T-wire work and the previous V-groove-wire reports mentioned in their
comments. The referees A, B, and C claim that our data on our present T-wire are SIMILAR TO
or have NO ADVANTAGE OVER the previous reports on V-groove wires with very large disorder.
We want to ask neutral fair judgment on this case by a divisional associate editor.
We understand that the assistant editor selected the referees via a normal process. A
problem is, however, that there are only limited numbers of research groups in the field of
quantum-wire optics. In particular, sample sources of quantum wires are very limited in the
world, and most researchers cannot study quantum wires with high quality. Therefore, if
referees were selected from authors of the cited papers, or if some referee candidates declined to
review and recommended another reviewer who works close to this subject, our manuscript had
very high probability to go to our competitors.
Our T-shaped quantum wires have unprecedented high quality. In fact, our wires are
revealing new clear experimental data, which correct or deny some vague or wrong discussions
claimed by previous reports. This fact is widely accepted via our invited talks in open meetings
such as APS, CLEO/QELS, MRS, and JPS. Therein, we never had criticisms on the qualities of
our wires or data in comparison with previous V-groove-wire reports, which the referees made in
anonymous review process. We suspect that our competitors working on lower quality wires or
their close collaborators made biased comments as anonymous referees, because they felt that
Appeal LP9512
April 20, 2005
publication of higher quality data for higher quality samples is fatal for them. In any event, the
biased referee comments which intend to exclude new data on state-of-the-art high-quality
quantum wires from PRL journal are very unfair.
Furthermore, we are very unhappy about the style of the second-round reports by referees
A and B. In the previous cover letter of January 11, 2005, we wrote our complete answers to all
the first-round comments by referees A and B, stressing our 3 highlights. We made 5 major
revisions complying with some comments by the referees. However, the referees did not respond
directly to our 3 highlights and respective answers, but added or repeated their criticisms
neglecting our replies for their previous comments. Therefore, the arguments never converge. We
need assistance by a divisional editor to settle this situation.
We sincerely ask a divisional editor to fairly evaluate our manuscript and our points in
physics described in detail in the previous cover letter of January 11, 2005. Since divisional
editors such as Dr. Deveaud and Dr. Pinczuk are experimental experts in this field, they can
surely judge whether the referees are our competitors or neutral researchers by their names, and
can surely make a fair evaluation on our manuscript, our cover letters, and the referee
comments.
In our previous cover letter, we stressed our 3 highlights, 5 points of revisions, and our
complete answers to all the questions and concerns by referees A and B. As far as we examined
the second-round reports by referees A, B, and the report by referee C, our main 3 highlights
were not fairly recognized by the referees due to their biases. In the following, we point out
essential problems in the latest referee comments related to our main 3 highlights.
Highlight 1: on high quality of the single T wire
All the referees admit that our T-wire has unprecedented high quality, but seem to have a
biased belief that the V-groove wires studied in the previous reports (ref. [12] by Guillet et al. and
ref. [11] by Crottini et al.) had enough high quality to study universal 1D quantum-wire physics
in carrier-density dependent evolution of PL. This must be the reason why the referees made
unfair comments that our present results are similar to or simply confirming the results of the
previous V-groove-wire study.
Note, however, that the V-groove wires in ref. [12] by Guillet et al. and ref. [11] by Crottini
et al. show strongly localized PL. The PL is spatially chopped up into short pieces, and the PL
peak energy fluctuates more than 10 meV. Excitons are three-dimensionally confined by a
localization energy of more than 10 meV. Biexciton binding energy (1.5mV in ref. [12]) and
carrier-induced broadening of excitons and biexcitons are far smaller than the 10meV energy of
structural inhomogeneity and localization. Therefore, these systems are like 0D quantum dots
rather than 1D quantum wires. In fact, people often study physics of 0D quantum dots using
strongly disordered quantum wells. This major problem in the previously published papers
Appeal LP9512
April 20, 2005
[7-12] is clearly explained in the introduction of our manuscript and in the previous cover letter,
though the referees mentioned nothing on it in the second reports.
In our experiment, the PL peak of T-wire has narrow total width of 1.3 meV and a small
Stokes shift less than 0.3meV. We admit that structural inhomogeneity exists, which may
dominantly contribute to the broadening at the low-density limit. However, these energy values
of structural inhomogeneity (<1.3meV or 0.3meV) are smaller than relevant many-body
interaction energies such as a biexciton binding energy of 2.8meV and a carrier-induced width
increasing gradually to 12meV. Therefore, in the intermediate- and high-excitation regions of our
interest to investigate the 1D exciton-plasma crossover, the PL spectra are homogeneously
broadened and inhomogeneous broadening plays only a negligible role.
High wire quality that meets this criterion on small inhomogeneity is crucially important
to study 1D many-body physics of broad interest, and our present T-wire is the first sample with
such high quality. Therefore, the main comment by the referee B that "our T-wire is of high
structural interest but is not of broad interest" is wrong and very unfair.
We surely performed ensemble measurements with 1 micron spatial resolution on our
T-wire, as the referee C mentioned. We also admit that NSOM with 0.2 micron spatial resolution
used in ref. [11] by Crottini et al. is an advanced tool. However, the key difference between the
two experiments is not the 1/ 5 spatial resolution, but the values of localization energy or the
energy fluctuation along wires in comparison with the many-body interaction energies, which
separates the systems to 1D wires or 0D dots. The comments by the referee C that "the ref. [11]
measured a single quantum state in the quantum WIRE", and that "the referee sees no real
advantages in this highlight" are strongly biased and unfair.
Highlight 2: on crossover of biexcitons to an e-h plasma
The second highlight of our paper is the observed crossover of biexciton PL peak to an e-h
plasma peak as a result of continuous broadening. The highlight 2 is strongly linked to the
highlight 1. Note that the observed broadening in the crossover regime, or the intermediate- and
high-excitation-density regime, of our major interest is NOT inhomogeneous broadening, but is
intrinsic homogeneous broadening. Therefore, the claim by the referee C that ensemble
measurements do not detect intrinsic features of quantum wires is wrong. The argument by the
referees B and C that relevant spectral features were smeared out by broadening and were
invisible does not make sense, because the observed intrinsic homogeneous broadening is
nothing but the relevant quantum-wire physics. It is a pity that the referees did not change their
conclusions on the highlight 2 most probably due to biased judgment on the highlight 1.
In the second-round report, the referee A simply commented that the observed phenomena
are the same as those observed in V-groove wires in the previous reports. The referee A did not
respond to our respective answers and detailed explanation in the previous cover letter.
The referee B, in the second-round report, doubted our assignment of the high-density
peak to an e-h plasma, and claimed that the peak is due to broadened X and XX peaks. However,
Appeal LP9512
April 20, 2005
our direct evidence is given in the wire PL intensity plot of Fig. 2 (a) explained in the 8th and
10th paragraph of our manuscript. The wire PL intensity saturates above excitation intensity of
4x10^-4 W, because all the states in the wire are filled. In other words, degenerate e-h plasma is
formed. This PL intensity saturation is analogous to the saturation plateau observed in
time-resolved PL in ref. [12] by Guillet et al., which was assigned to a characteristic of e-h
plasma emission in ref. [12]. Thus, the doubt raised by the referee B must be completely
resolved.
The referee C claimed that the broader e-h plasma peak should coexist with the sharper
biexciton peak, as was observed in V-groove wires in the previous reports. This claim must stem
from the biased belief in the previous V-groove wire results: PL data of Guillet et al. and Crottini
et al. in the intermediate densities show that sharp biexciton PL coexists with broad background
plasma PL. The referee C proposed to add an additional fitting component of a broader
background curve in addition to the two Lorentzian curves of the exciton and the S-peak. It is
surly possible to increase fitting components or parameters, but our PL data in the crossover
regime is already very well fit by the two Lorentzian curves. Hence, the spectral weight of the
additional broader background curve is negligibly small.
Highlight 3: on the onset of the exciton band edge and band-gap renormalization (BGR)
The referee C approved that the onset of the exciton band edge and BGR is relevant, and
fully agreed to the comments of the authors. On the other hand, the referees A and B did not
respond directly to this point.
In the latest reports by the referees, they criticized about some of our terminologies used
in the manuscript and citation. If these are indeed not appropriate, we are willing to revise.
However, we feel that the referees' criticisms are extremes rather than fair comments. We want
to ask suggestions by a divisional editor.
On the term "free excitons"
We used a word "free exciton" in the manuscript, quantitatively noting that there is a
finite PL width of 1.3 meV and a Stokes shift less than 0.3 meV. We believe that the criticism by
the referee B, "there simply is no free delocalized exciton state in a disordered quantum wire
even if the disorder is arbitrarily small", is an extreme, and never makes a fair reason for
rejection.
We used the expression "the free exciton state is continuous over 10 micron" on the basis of
our spatially resolved PL. Obviously, "continuous" does not imply "coherent". We quantitatively
explained the finite PL width and the Stokes shift in the same paragraph. Though we
understand what the referee C meant, we do not think it is misleading in this context.
Appeal LP9512
April 20, 2005
On the term "low-energy edge of e-h plasma PL"
The referees B and C complained about the concept of low-energy edge of e-h plasma PL
and the definition of triangles in Fig. 3.
In general, e-h plasma PL denotes band-to-band transition between electrons and holes
forming a plasma, where low-energy edge (onset) and high-energy edge (cut-off) of PL correspond
to transitions at their band edges and Fermi edges, respectively. Because of many-body effects,
scatterings, and thermalization, the onset and the cut-off can have tail or broadening. It is true
that we did not give this explanation in the manuscript. However, such a general picture of e-h
plasma PL is well-known, and we believe that the concept of the low-energy edge of e-h plasma
PL is evident.
To interpret our experimental PL data on the basis of this picture and denote positions of
the edges, there is certain ambiguity due to tail or broadening. However, the discussion of Fig. 3
is not affected by such ambiguity. Therefore, the comment by the referee B that "any conclusions
drawn on this edge are obviously meaningless" is an unfair extreme statement. Indeed, in the
first-round reports, any complaints about our interpretation were NOT made by the referees A
and also B.
The definition of triangles in Fig. 3 is clearly given in the figure caption as "the positions
where the PL intensity decreases to one twentieth of the peak intensity". We may change "one
twentieth" to "one tenth", for example, but the text of the manuscript require no change. Since
Fig. 3 is now plotted in log-scale after the previous suggestion by the referee B, the PL tails are
emphasized. Thus, positions of low-energy edge may not be very obvious for those who are not
familiar with log-plots, but this is not the case with most physicists.
On the citation of Ref. 23 on a theory by Asano and Ogawa
It is not the main issue or purpose of our manuscript to justify a theory by Asano and
Ogawa on strong biexcitonic correlations in quantum wires. However, it is interesting and
related to the present experiment. We believe that citation of such a paper on the cond-mat
preprint server is allowed in PRL. We do not understand why the citation of a paper is not
acceptable.
Finally, we wish to remark that we improved our experiments and revised our manuscript
significantly in the previous resubmission to fully comply with the previous referee reports and
to emphasize our 3 highlights. However, the referees did not respond directly to these essential
points. This is the reason why we do not further revise our manuscript in this appeal. We are
willing to revise our manuscript, if a divisional editor recommends it.
We sincerely ask fair judgment of this case by a divisional editor.