Your_manuscript LP9512 Yoshita

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1 1/6 2005/07/21 16:55 Subject: Your_manuscript LP9512 Yoshita From: Physical Review Letters Date: Mon, 28 Feb :32:45 UT To: Re: LP9512 Evolution of excitons to an electron-hole plasma via biexcitons without level crossing between band-edge annd excitons in a quantum wire by Masahiro Yoshita, Yuhei Hayamizu, et al. Dr. Masahiro Yoshita Inst. for Sol. St. Physics Univ. of Tokyo CREST, JST Kashiwanoha, Kashiwa Chiba , JAPAN Dear Dr. Yoshita, The above manuscript has been reviewed by our referees. A critique drawn from the reports is enclosed. On this basis, we judge that while the work probably warrants publication in some form, it does not meet the special criteria of importance and broad interest required for Physical Review Letters. In accordance with our standard practice (see enclosed memo), this concludes our review of your manuscript. The paper, with revision as appropriate, might be suitable for publication in Physical Review B. If you submit the paper to Physical Review, the editors of that journal will make the decision on publication of the paper, and may seek further review; however, our complete file will be available. If you submit this manuscript or a revision of it to Physical Review B, be sure to respond to all referee comments and cite the code number assigned to the paper to facilitate transfer of the records. Yours sincerely, Saad E. Hebboul Assistant Editor Physical Review Letters prl@ridge.aps.org Fax: Second Report of Referee A -- LP9512/Yoshita I have red the revised version of manuscript entitled Evolution of..., by Yoshita and coworkers. The authors made several changes in

2 2/6 2005/07/21 16:55 their manuscript: 1- the title has changed ; personnaly I found this new title less attractive than the previous one although I am not convinced by the concept of biexcitonic correlations. 2- all the figures have changed since the authors have performed again their experiments. Indeed the results are more clear now. From the scanning 5PL (fig 1a) and the imaging technique used, it is quite clear that the exciton and biexciton lines are spatially correlated. The localization is also very weak in this system as it nicely shown in the scanning 5PL. This allows also to observe weakly the onset of the band to band edge. Some disorder still exists probably due to the stem well which must be the origin of the inhomogeneous brodening of the 5PL line (1.5 mev). 3- However concerning the interpretation, I still believe that there are no new physical phenomena occuring. Indeed: a- the authors claim that neither the exciton peak nor the band edge show any shift at all pair densities studied. I think that this is not surprising since they work in a low intermediate pair density regime where nax, the density dimensionless parameter, is less than 0.1(n<105 cm-1). In this regime, excitons and biexcitons are very stable in a one dimensional wire, as reported before (Crottini et al, Oberli et al), and the shifts of the lines are small, as it is also evidenced in spectra on fig1b and on fig 2b. b- Then at higher pump powers, corresponding to 0.1< nax <1 (105<n<106 cm-1), the bound states (exciton, biexciton) interact with a continuum (free electron-hole pairs) leading to an important broadening of the lines, with only a single feature remaining visible and corresponding to the forming e-h plasma. The broadening becomes too important to resolve any shifts in the PL. The authors confirm in the same way as previous experimental and theoretical work (see for instance Piermarrocchi et al PRB63, , 2001), that BGR and screening are very small in quantum wires. Also as reported before (Guillet et al), the so called Mott transition is not a phase transition in this kind of 1D systems, but rather a continuous evolution of an exciton gas to a dense e-h plasma. c- Moreover, the concept of biexciton liquid, if it exists, can not be clearly demonstrated with only PL experiments because of the large broadenings occuring and the formation of the plasma. Therefore, I do not recommend publication in PRL. Second Report of Referee B -- LP9512/Yoshita This paper refers on cw micropl measurements under high excitation in T-shaped quantum wire. After a discussion of the previous referee's reports, the authors pointed out three main highlights to justify the publication in PRL. I cannot find them convincing. Let me comment them. 1) High quality of the single QWR The high quality of the QWR is based

3 3/6 2005/07/21 16:55 on the PL linewidth of 1.3 mev and on the Stoke shift less than 0.3 mev. These parameters are usually used as quality tests, in case of ensemble PL measurements, since they denote small inhomogeneous broadening. Indeed the authors attribute them to structural inhomogeneity, which is therefore quite small. Nevertheless, in the context of spatially resolved PL, the fact that the emission band are inhomogeously broadened simply means that the spatial resolution is not good enough to isolate the single quantum state. If a single quantum state were observed, the broadening will be homogeneous, by definition. However 1.3 mev of homogeneous broadening is not realistic: it is know that in QWs the homogeneous broadening is of the order of 0.1 mev and in QDs is even smaller. An other data considered by the authors as a test of high quality is the fact that the PL band is constant in energy position over 10 micrometers. Again this is a test for ensemble PL and denotes that the structural disorder is constant. It does not mean that the quantum state is delocalized over 10 micrometers. In this respect, the sentence: "... the free exciton state is continuous at least over 10 micrometers in the wire" is a bit misleading In conclusion, following the authors, the present paper refers on ensemble measurements. In other words the emission is collected from many different quantum states. Even if the sample quality is very high, the information obtained is averaged over different quantum states. On the contrary in previous cases the QWR inhomogeneous broadening was larger, BUT by using a SNOM (see for instance Crottini et al. [11]) they were able to detect a single quantum state in the QWR. Therefore I did not see real advantages in this highlight stressed by the authors, with respect to previous findings, as far as the intrinsic features of QWR are considered. 2) Crossover of biexciton to an e-h plasma. The authors wrote that the transition between biexciton and e-h plasma has been already observed. In previous reports the transition was gradual, with the coexistence of a broad background and a the sharp biexciton line. The novelty of this report is the continuous crossover, which is inferred by PL lineshape analysis. The authors claim that the PL peak changes continuously its character from biexcitons and e-h plasma. The problem I have with this claim is mainly related to point 1). In fact, in the Ref. [11] the biexciton FWHM is below 0.2 mev and the separation between e-h plasma and biexciton is trivial. In the present manuscript the larger broadening of the biexciton PL makes the separation more involved. Then the comparison with a lineshape fit will help the reader to get a conclusion. I am quite sure that a fit with a biexciton band plus an e-h band will agree with the data. Therefore the authors have to convince the reader (and the referee) that this is not the case. In addition the definition of low-energy edges of the plasma PL (triangles in Fig.3) is very handmade and I do not understand the meaning. 3) Onset of exciton band edge and band gap renormalization. This point is relevant and I agree with the comments of the authors.

4 4/6 2005/07/21 16:55 In conclusion I find the paper interesting but I cannot recommend publication in PRL. Report of Referee C -- LP9512/Yoshita The manuscript has been carefully reviewed before and both reviewers did not recommend to publish the paper in PRL. In their response, the referees "have completely overlooked or ignored all three essential highlights of the paper". Since this is a strong statement, I have carefully reread this manuscript. My thoughts are given below. Let us go through the results of this paper: The authors study a relatively weakly confined QWR of high optical quality (Fig. 1a, highlight 1 -- undisputed by the referees). At low powers, single-exition (X) emission dominates, then an additional -- most likely - biexcition (XX) peak sets in. With increasing power, the widths of the resonances increase while the resonance energies remain basically unchanged (up to 50 micro W). Then the broadening becomes so large that the individual lines can no longer be resolved and a single peak remains (Fig. 1b). In the same intensity range, also the emission from the arm QW at ev shows a similar behavior, though with a slightly less pronounced broadening. Parameters for resonance energies and widths are given in Fig. 2. In the text, the results are described on pages 3 and 4. In the top paragraph on page 5, the authors then write that "the spectral changes" from 50 micro W to 100 micro W indicate that "the overlap between excitons becomes strong and isolated X (and also XX) are rare. That is an e-h-plasma is formed simultaneously with X in the wire around 10 micro W and the e-h-system is completely changed to the 1D e-h plasma state at 50 micro W." To be honest, I do not see that the experimental data give evidence for the crossover of the system to an e-h-plasma. What I see are two peaks with linewidths that become so large that they can no longer be individually resolved. Why these broadened peaks should reflect a "new" e-h-plasma state for P > 50 micro W rather than broadened X and XX peaks seen at P < 50 micro W is not explained by the authors. There is also no additional experimental evidence for this claim -- time-resolved PL may potentially be helpful, see the comment by Ref. A on the paper by Guillet. In the next short paragraph, the authors claim that their results are consistent with an -- unpublished - theoretical results predicting that at high-density strong biexcitonic correlations, simlar to a biexciton liquid, exist. What is the purpose of this paragraph? Neither is the theoretical modelling described nor can the reader judge whether this

5 5/6 2005/07/21 16:55 claim is indeed valid. In Fig. 3, the authors then plot intensity-dependent PL spectra at 30 K on a logarithmic scale. Two additional small peaks are observed at low powers and are assigned to excited X states and to an onset of continuum states. This assignment is supported by previous work. These additional peaks can faintly be seen in the PL spectra at powers up to 22 micro W, a density where X and XX peaks are still well resolved. Also these peaks show essentially no power-induced spectral shift, indicating that the increase in carrier density mainly results in a line broadening rather than line shifts, as stated by the authors. In the final paragraph on p.6, the authors introduce a "low energy edge of the plasma PL", a concept which is NOT defined in the text. The PL peaks are defined by their resonance energies and widths and I do not see why any additional "edge" should be introduced, nor what that should mean. Any conclusions drawn based on this "edge" are obviously meaningless. A theoretical interpretation of the experimental results is not given in this manuscript. I trust that this gives a fair summary of the authors achievements, not overlooking or ignoring any of the highlights of this work. The summary shows that -- in my opinion -- the authors observe a power-induced broadening of the X and XX lines and that the claim that there is a gradual evolution from exciton over biexciton to e-h plasma is not substantiated. Certainly the summary shows that the results are so specialized that they are not of sufficient broad interest to merit publication in PRL. Therefore I recommend that this paper should not be published in Physical Review Letters. As the investigated quantum wires are of high structural interest, the experimental results may be of interest for some people working on quantum wire spectroscopy and thus may be considered as a brief report or regular article in Physical Review B if the following additional comments are satisfactorily addressed: 1. As acknowledged by the authors, the 1.3 mev width of the X peak at low powers is limited by inhomgeneous broadening. Still, on p.3, the authors assign this peak to a free exction. There simply is no "free", delocalized exciton state in a disordered quantum wire, even if the disorder is arbitrarily small. 2. Excitation-induced dephasing and the underlying physics is still not adequately addressed nor are appropriate references given (see my first report). 3. Either the theoretical model (ref. 23) should be explained and compared to the experimental results in detail or the paragraph should be omitted entirely.

6 6/6 2005/07/21 16:55 4. Either it should explained what "low-energy edges of the plasma PL" are and why this unclear concept needs to be introduced or the corresponding paragraph should be removed from the manuscript. Please see the following forms: Physical Review or Physical Review Letters? Resubmittal Policy (PRL)