REVIEW
NMR study of charge density waves under hydrostatic pressure in YBa2Cu3Oy
Superconductivity · Submitted 1 Sep 2019 · 44 references
Pith's verdict: New NMR pressure data that genuinely sharpen the CDW-vs-x-ray contradiction in YBCO, wrapped in a speculative Tc model that should not carry the paper's weight.
T0 · AI review
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Referee report
The paper
The paper claims
17O NMR shows charge-density waves in YBa2Cu3Oy are barely weakened at 1.9 GPa, contradicting x-ray reports of complete suppression, and supports gradual CDW fade as the driver of pressure-enhanced Tc.
The reading
What carries the argument
The load-bearing probe is the dimensionless 17O quadrupole broadening $\delta\nu_{\rm quad}/\nu_{\rm quad}$ (short-range CDW amplitude) and the field-induced quadrupole splitting $\Delta\nu_{\rm quad}$ (long-range CDW), measured in untwinned single crystals with two oxygen-ordered superstructures. The quantitative argument rests on three small relations: the pressure-induced doping $\Delta p = b\,p_0\,P$ with $b \approx 0.014$ GPa$^{-1}$; the decomposition $\mathrm{d}T_0/\mathrm{d}P_{\rm tot} = \mathrm{d}T_0/\mathrm{d}P_{\rm dop} + \mathrm{d}T_0/\mathrm{d}P_{\rm intr}$, which turns the measured total slope of $-1.1$ K/GPa for the Hall-sign-change temperature $T_0$ into an intrinsic suppression rate of $-3.6$ K/GPa once the $+2.5$ K/GPa doping contribution is removed; and the linear extrapolation $T_0(P) = T_0(p_0+\Delta p) + (\mathrm{d}T_0/\mathrm{d}P_{\rm intr})\,P$, which places the vanishing of $T_0$—and hence of charge order—near $P_c \approx 17$ GPa. A Curie-Weiss fit to the O-VIII sample's broadening gives a $25\pm12\%$ amplitude decrease at 1.9 GPa, consistent with a quadratic $A(P) \propto (P_c-P)^2$ vanishing at 15 GPa.
Extended reading notes
Core claim
The paper's central claim is that a pressure of 1.9 GPa does not suppress charge-density-wave order in YBa2Cu3Oy. The NMR quadrupole broadening that tracks the short-range CDW amplitude is essentially unchanged at both hole dopings measured (p0=0.109 and 0.125), and the quadrupole splitting that marks the high-field long-range CDW shows no more than a possible ten-percent change in amplitude while its onset field rises from about 9.9 T to 16.8 T (3.6 T/GPa), following the pressure-induced increase of the upper critical field. This directly contradicts x-ray studies reporting complete CDW disappearance by about 1 GPa. The paper argues that the contradiction can be resolved by recognizing an induced doping effect: pressure both intrinsically weakens CDW order and slightly increases hole doping, which masks the weakening near p0=0.109. With these two effects separated, the data are consistent with CDW order vanishing on a pressure scale of roughly 10-20 GPa—matching the scale on which Tc saturates near 107 K—so that most of the Tc enhancement under pressure may result from the weakening of CDW competition.
Load-bearing premise
The quantitative story hinges on the assumption that the CDW's intrinsic suppression rate stays constant all the way to 15 GPa, and that pressure raises hole doping by a fixed percentage per gigapascal; if either assumption fails, the predicted disappearance near 17 GPa and the claimed link to rising Tc do not follow.
Editorial extensions
If this is right
- Short-range CDW order persists at 1.9 GPa in both studied dopings, so the earlier x-ray conclusion of complete suppression by about 1 GPa must be either wrong, probing a different quantity, or affected by pressure-cell or temperature-calibration artifacts.
- The onset field for the high-field long-range CDW rises by about 3.6 T/GPa, tracking the upper critical field, so superconductivity and field-induced CDW order remain tightly coupled under pressure.
- Separating pressure-induced doping from the intrinsic effect yields an intrinsic CDW suppression rate near -3.6 K/GPa, comparable in magnitude to $\mathrm{d}T_c/\mathrm{d}P \approx +3.8$ K/GPa, which is what a strong competition scenario needs.
- Extrapolating with the model puts CDW disappearance at roughly 10-20 GPa, the same pressure scale on which Tc in YBa2Cu3O6.6 saturates near 107 K.
- By the paper's estimate, about 70% of the Tc increase under pressure is attributable to weakening CDW competition, with only about 30% coming from the doping increase.
Reading between the lines
- If the paper is right, then pressure is a continuous, tunable knob for the CDW-superconductivity competition across the cuprate phase diagram, and the apparent robustness of CDW near p0≈0.11 is a compensation effect rather than intrinsic resilience; this predicts that CDW should weaken visibly once measurements are pushed past roughly 5 GPa.
- A directly testable consequence is that the high-field CDW onset field should continue to track the upper critical field at higher pressures; a divergence above a few GPa would break the proposed link between the two orders.
- The same doping-correction scheme could be applied to other underdoped cuprates with pressure data, and compounds without competing CDW order should show much weaker Tc pressure derivatives than YBa2Cu3Oy does.
Original abstract
The effect of hydrostatic pressure (P) on charge density waves (CDW) in YBa2Cu3Oy has recently been controversial. Using NMR, we find that both the short-range CDW in the normal state and the long-range CDW in high fields are, at most, slightly weakened at P=1.9 GPa. This result is in contradiction with x-ray scattering results finding complete suppression of the CDW at ~1 GPa and we discuss possible explanations of this discrepancy. Quantitative analysis, however, shows that the NMR data is not inconsistent with a disappearance of the CDW on a larger pressure scale, typically ~10-20 GPa. We also propose a simple model reconciling transport data with such a hypothesis, provided the pressure-induced change in doping is taken into account. We conclude that it is therefore possible that most of the spectacular increase in Tc upon increasing pressure up to ~15~GPa arises from a concomitant decrease of CDW strength.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The effect of hydrostatic pressure (P) on charge density waves (CDW) in YBa2Cu3Oy has recently been controversial. Using NMR, we find that both the short-range CDW in the normal state and the long-range CDW in high fields are, at most, slightly weakened at P=1.9 GPa. This result is in contradiction with x-ray scattering results finding complete suppression of the CDW at ~1 GPa and we discuss possible explanations of this discrepancy. Quantitative analysis, however, shows that the NMR data is not inconsistent with a disappearance of the CDW on a larger pressure scale, typically ~10-20 GPa. We also propose a simple model reconciling transport data with such a hypothesis, provided the pressure-induced change in doping is taken into account. We conclude that it is therefore possible that most of the spectacular increase in Tc upon increasing pressure up to ~15~GPa arises from a concomitant decrease of CDW strength.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The effect of hydrostatic pressure (P) on charge density waves (CDW) in YBa2Cu3Oy has recently been controversial. | background 0.97 | 0.03 | 0.02 | asserted 0.98 |
| s1 | Using NMR, we find that both the short-range CDW in the normal state and the long-range CDW in high fields are, at most, slightly weakened at P=1.9 GPa. | result 1.00 | 0.98 | 0.20 | asserted 0.99 |
| s2 | This result is in contradiction with x-ray scattering results finding complete suppression of the CDW at ~1 GPa and we discuss possible explanations of this discrepancy. | interpretation 0.67 | 0.43 | 0.05 | asserted 1.00 |
| s3 | Quantitative analysis, however, shows that the NMR data is not inconsistent with a disappearance of the CDW on a larger pressure scale, typically ~10-20 GPa. | interpretation 0.66 | 0.90 | 0.10 | hedged possibility 0.87 |
| s4 | We also propose a simple model reconciling transport data with such a hypothesis, provided the pressure-induced change in doping is taken into account. | interpretation 0.39 | 0.83 | 0.05 | hedged possibility 0.60 |
| s5 | We conclude that it is therefore possible that most of the spectacular increase in Tc upon increasing pressure up to ~15~GPa arises from a concomitant decrease of CDW strength. | interpretation 1.00 | 0.34 | 0.83 | hedged possibility 0.96 |
What was examined, and what was not
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Figures
Showing 4 of the paper's 8 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:1909.00389 by the authors.
References
Discussion
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