Dr. Masahiro Yoshita Inst. for Sol. St. Physics Univ. of Tokyo CREST, JST Kashiwanoha, Kashiwa Chiba , JAPAN. Dear Dr.

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1 Subject: Your_manuscript LP9512 Yoshita From: Physical Review Letters Date: Tue, 13 Jul :56:22 UT To: Re: LP9512 Crossover of excitons to an electron-hole plasma with biexcitonic correlations in a single 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. 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@aps.org Fax: Report of Referee A -- LP9512/Yoshita This paper deals with micro-photoluminescence experiments under high excitation power in a high quality single T-shaped quantum wire. This investigation is made in order to evidence many-body effects in a one-dimensional system and eventually observe a Mott transition from a gas of excitons to an e-h plasma, like an insulator-metal transition. The authors try to give a new interpretation of the spectral modifications occuring as the pump power is increased in terms of "bi-excitonic correlations". This last concept is to my opinion not well argued and not convincing. My comments are the following: 1) The experimental work is of very good quality, however numerous similar investigations have been reported in the literature, which are not always quoted in the manuscript (the references should be properly cited), concerning:

2 - the observation of biexcitons in a single quantum wire by SNOM [Crottini et al. Phys. Stat. Sol. (b) 221, 277 (2000)] - the gradual transition from a diluted gas of excitons to a dense electron-hole plasma in a single V-shaped QWR by means of micropl has already been observed by Guillet and coworkers [Phys. Rev. B 67, (2003)] (biexcitons are also observed, and the e-h plasma is unambiguously identified through time-resolved PL); see also Vouilloz et al, Sol. State Comm. 108, 945 (1998) in a lower density regime. - the authors themselves have published in Phys. Rev. B 67, (R) 2003, their work on "Coulomb correlated electron -hole plasma and gain in a quantum wire laser of high uniformity", which deals with the same topic as in the present manuscript. 2) Concerning the essential points of the paper : a) the existence of biexcitons has been already established in QWRs. However in the present work the presence of biexcitons at high power density is not convincing at all and not well justified. Indeed some crucial questions arise from the spectra of fig 1: - First, why the lines at very low pump power are gaussians? This means that the lines are not homogeneously broadened and this might create difficulties for the assignement of the lines. For instance the second peak at low energy of the micropl could be due, at all power densities, to an exciton localized in a one monolayer thickness fluctuation. The energy difference is exactly 3 mev as reported in ref 13 by the authors. - The authors claim that 3 mev is the binding energy of the biexciton. This value seems to be quite large for a quantum wire where the lateral confinement is 20 mev and the binding energy of the exciton is only 10 mev (ref 13). In V-shaped quantum wires where the binding energy of the exciton is about 20 mev, the biexciton binding is only 1-2 mev [Crottini et al; Guillet et al; Banyai et al PRB 36, 6099 (1987) predict EX2/EX ~ 0.1 in such wires]. Could the authors give some comment on this point? - It is well known that due to thickness fluctuations on the hetero-interfaces during the growth, there is strong localization of excitons on a submicrometer scale [see for instance Oberli et al, Physica E 2, 862, (1998); Crottini et al, PRB, 63, (R) (2001); Guillet et al, PRB 68, (2003)]. In order to assign properly the lines one has to be certain that the observed biexciton is spatially correlated to the observed exciton, i.e. that the emission comes from the same position under the laser spot. Since the authors have perfomed some scanning micropl in previous studies of the same wire, did they do similar scans for these measurements versus the excitation power? - This brings me to the next point: all PL experiments have been performed at an excitation energy of ev, that means that absorption occurs not only in the wire but also in the arm well. This would yield in a rapid diffusion in the arm well before recombination in the wire, which would mean that the emission does not come from the same position as the exciting spot. Therefore the origin of the biexcitonic line is open to question. Could the authors comment on this point?

3 - The authors should present all spectra as a function of the power density in Wcm-2 or give the number nax where ax should be the Bohr radius in the wire and not that of the bulk material. nax is the only parameter that allows comparison between experiments in different systems and gives the order of magnitude of the density regime where the experiments are perfomed. Moreover if there is some diffusion then the authors should correct the estimated pair densities by this diffusion factor. - b) I agree with the authors that as the power density is increased a dense electron-hole plasma is formed progressively in the system. This is a dynamical process where excitons should coexist with free carriers at some power densities for nax <1. Biexcitons should also be formed in the appropriate density regime. This is shown in fig 2 c where we observe that the biexciton remains stable over only one order of magnitude (the slope is 2 between 0.01 and 0.1 mw). Then it is quite difficult to claim that the transition to an e-h plasma is made via a "biexcitonic liquid" (this concept needs to my opinion an exact definition). I believe that PL results are not enough experimental facts to confirm this interpretation. As for the theoretical calculations in 1D high density e-h system the authors don't present the results and reference 22 is unpublished. Experimental findings and physical interpretations cannot be based on unfounded statements and I suggest that this paragraph should be skipped in the discussion unless the authors can provide a reliable reference or communicate their results. - The fitting of the spectra in fig 1 gives loretzians curves. This is not surprising since the homogeneous broadening becomes larger than the inhomogeneous one. But the line at 1 mw shows that the fitting is not so correct which means that at high pump power there are different contributions, essentially an e-h plasma is formed and may be excitons or biexcitons still exist. In this sense the authors confirm the observations made previously by other groups that a broad line attributed to the e-h plasma appears almost at the same energy position as the exciton. c) About the band gap renormalization, it is clear enough from the spectra presented in fig 3 that there is a red shift of the plasma PL edge line (shown on fig 3 by an inverted triangle) as the excitation power is increased. Moreover it is the dominant feature on the spectra, while the "onset" of the "exciton band edge" is two orders of magnitude less important, so it is difficult to say what happens to this "onset". How can the authors finally conclude that BGR does not exist in their QWR? In conclusion I believe that the presence of biexcitons at high pump power is not clearly demonstrated and the interpretation in terms of "bi-excitonic correlations" needs to be defined and justified. Therefore I cannot recommend publication in Physical Review Letters. Report of Referee B -- LP9512/Yoshita The authors report photoluminescence studies on high quality T-shaped quantum wires. Density dependent PL spectra are analyzed to investigate the effects of many-body interactions on the optical properties of quasi-one-dimensional semiconductor nanostructures, specifically the transition from a dilute exciton gas to a dense electron-hole plasma.

4 This topic is not new, it has been the subject of a number of similar experimental studies (Ambigapathy, PRL 78, 3579,1997; Guillet, PRB 67, , 2003, old work by Wegscheider...) and a considerable amount of theoretical work. Compared to previous experimental studies, disorder effects are less pronounced in the wires studied by Yoshita et al. I do not recommend to publish this paper for the following reasons: (i) The density-dependent experimental PL data shown in this paper are very similar to those reported before by the same group of authors (PRB 67, , 2003). (ii) The authors present an analysis of these data based on a phenomenological line shape model. Based on this analysis, they conclude that there is no direct transition between exciton and plasma emission, and that biexciton emission is observed in an intermediate density regime. While biexcitons are generally not included in the prevailing theoretical models for bandgap renormalization in quantum wires, it is well known from numerous studies of single quantum dots and wires that biexciton emission contributes significantly to the PL spectra at intermediate densities (see e.g. A. Crottini et al, Solid state comm. 121, 401, 2002, Brunner et al, PRL 73, 1138 (1994),Wu et al. PRB 62, (2000)). Thus I do not believe that this finiding is of sufficient general interest to merit publication in PRL. (iii) The fact that the exciton peak in the QWR PL spectra shows no appreciable shift has been reported before. As such there are, in my opinion, not enough new conclusions that can be drawn from this study and I do not see that it meets the special criteria of broad interest that should be fulfilled for publication in Physical Review Letters. More specifically, I have several problems with the qualitative lineshape model introduced by the authors. (i) Disorder effects are completely neglected and I think that inhomogenous line broadening is not even mentioned. Yet, the low density PL lines show a width of about mev, much broader than the radiative linewidth of the order of 0.1 mev. How do the authors explain this finite linewidth if it is not due to disorder effects. Disorder may be present even if the PL spectra seem spatially uniform along the wire axis. (ii) The authors assign the two PL peaks for powers of < mw to "free" and "localized" exitons in the wire. What exactly do they mean with these words. It is well understood from the disorder models introduced by Zimmermann and coworkers and from studies of exciton localization in nanostructures and observation of effects such as level repulsion, that there is a rather broad distribution of exciton "sizes" (more precisely extents of center-of-mass wavefunctions). It is also clear from the theoretical work that all exciton states in a disordered quantum wire are localized. Thus the notion of a "free" and a "localized" exciton seems oversimplified and needs to be explained more precisely. Why can't the two peaks be attributed to monolayer fluctuations of the QWR thickness? (iii) Simply looking at the data in Fig. 1 it seems that the peak ev shows almost no spectral shift, whereas the low energy

5 peak shifts quite appreciably and monotonously with increasing intensity. The analysis if Fig. 2(b), upper image seems not really to reflect this impression. Why is this so? (iv) The low energy peak shows a substantial line broadening with increasing power. This may either be due to many-body effects arising from interactions with carriers inside the QWR or from excitation-induced dephasing to the interactions with carriers in QW states which are also populated. Such excitation-induced dephasing is known to be important for optical nonlinearities of low-dimensional nanostructures (see e.g. Guenther et al. PRL 89, (2002)) but not discussed in this ms. How can the authors rule out such effects? (v) What is plotted in Fig. 2c, the area of the PL spectra of the two peaks or just the maximum intensity? As the PL linewidth is changing, I guess that one should compare areas. As the assignment of the PL to biexciton PL seems based only on the density dependence, this point needs to be worked out more clearly. (vi) How do the authors qualitatively interpret the strong difference in density-dependent broadening of the two peaks? (vii) The authors mention that at a certain density, the line shape changes from Gaussian to Lorentzian. This is not apparent from the figures. It may be helpful for the reader to plot the PL spectra also on a logarithmic scale to see this transition more clearly. In summary, I think that the qualitative model introduced by the authors needs to be explained in much more detail. The claimed transition from single exciton -> biexciton -> e-h-plasma is not readily visible in the PL spectra and only emerges out of the modeling of the data. Thus it is mandatory that this modeling is precisely described and transparent to the reader. With a thorough and in-depth description of the model introduced by the authors and of the data analysis, the paper may be considered for publication as a regular article in PRB if the points raised in this report are satisfactorily addressed. Please see the following forms: Physical Review or Physical Review Letters?