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Catalogue of phonon modes in several cuprate hightemperature superconductors from density functional theory
by N. J. Jabusch, P. Dayal, A. F. Kemper
This Submission thread is now published as
Submission summary
Authors (as registered SciPost users):  Noah Jabusch 
Submission information  

Preprint Link:  scipost_202210_00077v1 (pdf) 
Code repository:  https://github.com/kemperlab/axsfcellconversion.git 
Date accepted:  20221222 
Date submitted:  20221019 21:59 
Submitted by:  Jabusch, Noah 
Submitted to:  SciPost Physics Core 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Computational 
Abstract
Cuprates are promising candidates for study in developing higher temperature superconductors. A thorough understanding of a material's phonon modes enables further investigation of its emergent properties, however, no complete reference of the phonon modes exists. Here, using density functional theory, we evaluate the phonon frequencies and atomic displacements for $\text{La}_2\text{CuO}_4$, $\text{Bi}_2\text{Sr}_2\text{CuO}_6$, and $\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_8$ in their tetragonal structures. The phonon modes for all materials agree with those expected from space group symmetry and display instabilities corresponding to known lowtemperature structural phase transitions.
Author comments upon resubmission
We appreciate the feedback received and the opportunity to resolve the indicated weaknesses. We have clarified the details of our determination of the crystal structure, adding a direct comparison between the values of lattice constants and atomic positions used with those reported in the literature.
In regards to the reviewer’s second point, due to time constraints we are unable to complete magnetic calculations or those required for generating plots for the density of states F(\omega). We indeed found negative/imaginary frequency modes, though these are correlated to the materials’ known structural phase transitions at low temperature (we did our calculations in the tetragonal phase). We have added references to the Bohnen paper to situate our own work in relation to the suggested one.
Finally, we have added substantial discussion and several background sources about the role of electronphonon interactions in cuprates generally and their impact on superconductivity specifically, as requested by the third review concern.
Below, we provide a pointbypoint response to the referee.
Sincerely,
N. Jabusch (for the authors)
List of changes
Received Report:
Strengths
1) the paper presents a catalogue of the phonon modes in three representative highTc cuprate superconductor based on the DFT.
2) the calculations are strightforward
3) the presentation is clear
Weaknesses
1) it is not clear how close the relaxed structure used in these calculations is to the actual experimentally measured (especially for the oxygen position). The problem is that even a few percent differences might become crucail for the phonons. Given that 214 is an antiferromagnetic insulator whereas it is a nonmagnetic metal in the DFT I am a bit concerned about the accuracy of the calculations. In any case the authors should discuss this.
When performing density functional theory calculations, it is indeed often the case that both the lattice constants and internal degrees of freedom do not precisely match the experimental results. In essence, this is due to the inexact description of DFT. The typical approach is to ensure that at least the DFT result is stable – this is achieved by minimizing both the lattice parameters and atomic positions. In our work, we performed one or both of these steps (depending on computational time availability), and find results that are in reasonable agreement with the initial (experimental) inputs.
We have added further details that address these points in the calculation details in Sec. 2A.
2) could the authors also show not only the tabular frequencies of various phonons but also the spectrum plot F(\omega) to see if the phonons are overall stable (i.e. there are no negative dispersions) and discuss this in the manuscript. I am a bit afraid that non
magnetic DFT calculations can show some negative dispersions, signalling some problems in not accounting for the effect of the magnetism in the calculations. For the reference the authors could orient themselves to the classical work
Europhys. Lett., 64 (1), pp. 104110 (2003)
The referee is correct that negative frequencies can appear due to either a neglect of magnetism in the calculations, or due to structural instabilities. In fact, we do find negative frequencies at the high symmetry points in the Brillouin zone that correspond, which we term “soft modes.” They are discussed in detail in Sec. 3B.
Due to time constraints, we are limited to the calculation of the phonon modes at the highsymmetry points, and cannot investigate the spectrum/dispersion.
3) Overall I think the Introduction will benefit from having
a bit broader discussion on the role of phonons in the field of HighTc and the ambiguity of the alpha^2F(omega) extracted from DFT and claimed in the recent experiments. In the context of YBaCuO superconductors some works on the phonons have been made
We appreciate the referee’s suggestion. We have incorporated a discussion of these issues in the introduction (section 1), and added additional references concerning the study of YBCO.
Report
Overall I believe the paper might be interesting provided the authors make an iteration to improve the presentation reflecting the weak points (critical remarks) outlined above. Once the authors provide a satisfactory explanation the paper can be recommended for publication
We thank the referee for the positive assessment.
Published as SciPost Phys. Core 6, 018 (2023)