SciPost Submission Page
Possible superconductivity from incoherent carriers in overdoped cuprates
by M. Culo, C. Duffy, J. Ayres, M. Berben, Y.-T. Hsu, R. D. H. Hinlopen, B. Bernath and N. E. Hussey
This is not the current version.
|As Contributors:||Nigel Hussey|
|Date submitted:||2021-02-23 18:11|
|Submitted by:||Hussey, Nigel|
|Submitted to:||SciPost Physics|
The non-superconducting state of overdoped cuprates is conjectured to be a strange metal comprising two distinct charge sectors, one governed by coherent quasiparticle excitations, the other seemingly incoherent and characterized by non-quasiparticle (Planckian) dissipation. The zero-temperature superfluid density n_s(0) of overdoped cuprates exhibits an anomalous depletion with increased hole doping p, falling to zero at the edge of the superconducting dome. Over the same doping range, the effective zero-temperature Hall number n_H(0) transitions from p to 1 + p. By taking into account the presence of these two charge sectors, we demonstrate that in the overdoped cuprates Tl2Ba2CuO6+\delta and La2-xSrxCuO4, the growth in n_s(0) as p is decreased from the overdoped side may be compensated by the loss of carriers in the coherent sector. Such a correspondence is contrary to expectations from conventional BCS theory and implies that superconductivity in overdoped cuprates emerges uniquely from the sector that exhibits incoherent transport in the normal state.
Submission & Refereeing History
You are currently on this page
Reports on this Submission
Anonymous Report 2 on 2021-5-28 (Invited Report)
1 )Comprehensive overview of multiple probes of electronic density in a broad doping range spanning critical doping and across multiple families of cuprates.
2) The experimental review is clearly focused around a single physical insight.
The physical insight is not rooted in the data presented.
The manuscript presents a review of several probes of electronic density in a broad doping range. These measurements are synthesized to support the argument that the superconductivity in the cuprates emerges directly from the incoherent part (non-Fermi surface) part of electronic spectral weight.
The main argument is that neither density n_H, as inferred from Hall measurements using the Fermi liquid phenomenology, or the density n_s, as inferred from superfluid density using BCS phenomenology, correspond to the total electronic density 1+p (holes) expected for cuprates well into the overdoped regime. The Authors observe that although n_H(p) and n_s(p) differ from 1+p in the magnitude and their stronger doping dependence, the sum total of n_H(p) and n_s(p) does check with 1+p within the accuracy of the data. They then argue that the neat partition of 1+p into n_H and n _s in a broad doping range above critical doping might suggest that n_H and n_s correspond to spectral weights of two distinct but coexisting electronic excitations, n_H to quasiparticles on the Fermi surface and n_s to the excitations outside of the Fermi surface ("incoherent"), and that because n_s is more directly associated with the superconductivity -- both in it physical interpretation and in its doping dependence -- the superconductivity must emerge directly from the incoherent part of the electronic excitations.
The argument n_H + n _s = 1+p in Tl2201 is strongly dependent on the validity of the analysis of Hall resistivity in Ref. 14 where the Hall coefficient has been found to change by about a factor of two between p=0.27 and p=0.23, much larger than the relative change in 1+p in the same doping range. The magnitude of the Hall coefficient n_H in Ref 14 has been inferred form the high field measurement which show a strong field and temperature dependence of R_H, with the variance comparable with with the factor of 2 required to distinguish reliably the value of n_H(p) and 1+p at p=0.23. Although the inferred value of n_H(p=0.27 ) in Ref 14 is consistent with the quantum oscillations measurements (Ref 29) in Tl2201, no quantum oscillation measurements exist at p=0.23 and the the uncertainty of the value of n_H in Ref. 14 cannot be reliably established.
The main point argued in this manuscript does present several interesting possibilities for the understanding of the physics of cuprates, and will be met with interest by its readers. However, the argument in the Manuscripts relies on several weak interpretational steps (BCS-like interpretation of n_s, Fermi-liquid-like interpretation of Hall resistivity) to make a strong leap in their interpretation o the nature of the superconducting state. In particular, it is not clear what is the basis for interpretation of penetration depth measurements in terms of electronic density in the absence of any quantitative description of the superconductivity emerging from incoherent excitations. It is also not clear how big is the interpretational error bar on n_H in Ref. 14.
That said, the range of experimental studies collected together in this manuscript will be of broad interest and will stimulate further discussion of the physics of cuprates.
Anonymous Report 1 on 2021-4-8 (Invited Report)
An original idea in a pretty mature field.
i) Ambiguity of concepts
ii) Incomplete presentation of data
ii) Restricted to the cuprate "bubble"
The paper argues that cuprate superconductivity is an instability of incoherent carriers. The origin of high-temperature superconductivity in cuprates is a mystery several decades old. I think that the authors make several relevant observations and find a very intriguing link between the evolution of superfluid density and a subset of carriers. However, I cannot recommend the acceptance of the paper in its current form. My objections belong to three categories: i) Imprecision of language; ii) absence of connection to other strange metals or superconductors other than cuprates; and Iii) the logical flow.
i) What is “an incoherent carrier”? The authors do not define what they mean by this expression but seem to suggest that it refers to entities displaying Planckian dissipation. What about electrons in copper, which copper display a T-linear resistivity with a Planckian prefactor (see ref. 68)?
ii) Berg et al. in PNAS 117, 2852 (2020) invoke incoherent carriers in a well-defined fashion. These are “hot” carriers, which are almost classical due to their lower degeneracy temperature or shorter lifetime. Interestingly, the strange metal Sr3Ru2O7, like overdoped cuprates displays T-linear resistivity with a Planckian prefactor (ref. 68) despite its well-defined Fermi surface.
iii) Strontium titanate has also a superconducting dome. Like cuprates, its superfluid density as a function of doping decreases as soon as the peak Tc is attained (Collignon et al., PRB, 96, 224506 (2017)). This appears to be a generic feature of any superconducting dome. If for whatever reason, adding an electron, instead of enhancing Tc (because of enhanced density of states) pulls it down, then you expect the superfluid density to do the same, because adding an electron reduces foremost the ratio of the superconducting gap to the carrier lifetime. Why should one exempt cuprates from this general rule?
iv) The very first sentence of the abstract qualifies the non-superconducting state of overdoped cuprates as a strange metal. Doesn’t this contradict previous works by Hussey and collaborators, such as PRB 68, 100502 (2003)? I am also surprised by the logical flow. The paper begins with a conjecture and presents arguments in favor of this conjecture, instead of starting with observations and ending with a conclusion.
v) Fig. 1A compares two normalized quantities and finds a striking correlation. However, it raises numerous questions. What is the amplitude of superfluid density in physical units (cm-3 for example)? This information cannot be found in ref. 7. So can one compare it with the Hall density of the normal state? What is the amplitude of the residual zero-temperature specific heat and how does it evolve with doping? Was it measured on single crystals? At what temperature? Inside the superconducting state? How can rule out that this zero-temperature residual term in not caused by uncontrolled disorder? Given that the source of this information is unpublished (ref. 31) and the importance of this figure in the authors’ scenario, I recommend a more detailed presentation of the data condensed in this figure.