Temperature-Independent Cuprate Pseudogap from Planar Oxygen NMR

Round 1

Reviewer 1 Report

I have just one major recommendation and a comment:
Most of the figures, in particular the first 3 (B and C panels), would profit from larger and clearer symbols and labels.

There are at least two families of cuprates - the 4, 5. and 6 layer Tl-materials from Osaka and the Y2Ba4Cu7O15-x compounds from Zurich, which are not mentioned in the analysis. Still, since to my best knowledge neither have planar oxygen NMR results published, so there are not changing the current, planar oxygen analysis, but should be discussed with planar copper data.

Overall, the paper is well written and scientifically sound, so I suggest to publish it in Cond. Mat.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Nachtigal and coworkers report an extensive overview on the behaviour of 17O NMR 1/T1 and paramagnetic shift Ks in high Tc superconductors, concentrating on the normal state behaviour and on the doping dependence of the pseudogap. The impressive amount of data reported in this manuscript are useful for anyone working in the field of high temperature superconductivity and, even if the main discussion is rather qualitative, the manuscript certainly deserves to be published. Nevertheless, I think that it still needs some improvements before it can be finally accepted for publication.

• The fact that the normal state data can be fitted reasonably well by a temperature independent gap and that it decreases upon increasing doping is not new. The authors analyse the data in terms of a U and V-shaped gap and ascribe the discrepancies between the pseudogap values derived from Ks and 1/T1 to inhomogeneities. Couldn’t it be simply that the U and V-shaped models are not the most suitable ones to describe the pseudogap?
• A reader would expect, from the whole of the data reported in the manuscript and from their fits, that at the end a summarizing plot of the pseudogap behaviour vs doping or vs Tc is reported. Otherwise reporting such a huge amount of data without giving a conclusive analysis of the pseudogap behaviour vs these physical parameters weakens a lot the manuscript.
• In many figures the data are hardly readable: the symbols are too small and there are too many data overlapping. The authors should make the plots more clear and introduce guides to the eye to allow the reader to follow their trend. It’s a pity to report the data in this way in such a nice overview.
• The authors make an interesting point on 63Cu 1/T1 for H//ab and point out that its value at Tc does not depend very much on doping and on the family of high Tc superconductors. On the other hand for H//c it does depend. Since the relaxation for in-plane magnetic field is affected also by in-plane fluctuations, the same that contribute to 1/T1 for H//c, one would expect that there is an extra relaxation mechanism that compensates the doping dependence of the in-plane fluctuations. What is it?

Author Response

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Reviewer 3 Report

The manuscript “Temperature independent cuprate pseudogap from planar oxygen NMR”, by J. Nachtigal, et al., seems to contain results that are interesting and may trigger further research on the long-standing problem of the nature of the pseudogap in cuprates.

I would like to propose to the authors the following three issues, which they may wish to address in their manuscript.

1. The authors adopt a single-particle language when describing the data, which corresponds to the idea that there is “something” affecting the charge carriers in the system but, at least as far as NMR is concerned, the only effect of this “something” is to modify the density of states of the carriers. It is very likely that besides self-energy corrections, affecting the single-particle properties, also vertex corrections modify the response of the system to an external probe, so that interpretations based on the density of states only should be taken with a word of caution.

2. Possibly related to the above issue, there are two (in principle different, maybe unrelated) “pseudogap” phenomena. The first is usually understood as a weak pseudogap behavior, namely an overall suppression of the density of states (likely seen in shift measurements), the other is a strong pseudogap behavior (seen in relaxation time experiments), which entails a (e.g., U- or V-shaped) suppression of the density of states around the Fermi energy. In principle, one could have weak pseudogap without strong pseudogap, strong pseudogap with full recovery of the density of states outside the pseudogapped region, or strong pesudogap with reduced density of states even outside the U- or V-shaped region. I think it would be very useful for the reader if the authors comment on this, based on their analysis.

3. Do the authors think that the pseudogap is really open at all temperatures, or that at a given temperature (e.g., of the order of the exchange energy) it may eventually close? In this latter case, how would the nuclear relaxation look like, if one was able to explore that temperature range? Would one find a straight line extrapolating to zero at high temperature?

Once the authors have considered addressing the issues listed above, I suggest that their manuscript may be accepted for publication.

Author Response

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Author Response File: Author Response.pdf