Kohn anomalies and enhanced superconductivity in simple systems under pressure: Insights from the nearly free electron model

Rousseau, Bruno; Errea, Ion; Bergara, Aitor

doi:10.1016/j.jpcs.2010.03.026

Cited by 5 publications

(6 citation statements)

References 23 publications

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“…The intensity of the Kohn anomalies is much stronger and the transverse acoustic modes become unstable with imaginary frequencies. Therefore, even though the electronic band structure could be understood within the free-electron-like approximation, phonons seem to be far from this picture, contrary to the case of sodium 40,45 . This fact also questions the stability of the I4 1 /amd tetragonal phase in the ultimate high-pressure limit where the electrons are expected to be free.…”

Section: B Vibrational Propertiesmentioning

confidence: 95%

“…Besides DFPT, we have also estimated D e making use of the free-electron Lindhard response function within the Random Phase Approximation (RPA) as in Ref. 40. In the latter approach the dynamical matrix is analytical at any q so that we have not restricted the calculations to the 6 × 6 × 6 grid.…”

Section: Computational Detailsmentioning

confidence: 99%

“…Considering the validity of the free-electron-like approximation to describe the electronic band structure, we have calculated the phonon dispersion within this approximation, assuming that the D e contribution to the dynamical matrix can be calculated with the Lindhard response function at the RPA level as in Ref. 40, therefore, neglecting correlation and exchange in the electronic response. In this free-electron limit we can obtain the spectra along high-symmetry lines without any Fourier interpolation, evidencing the presence of the Kohn anomalies at the |q + G| = 2k f points, and confirming that the kinks present in the DFPT result are Kohn anomalies.…”

Section: B Vibrational Propertiesmentioning

confidence: 99%

“…Combining different approximations in V scf to calculate χ 0 and δV scf to calculate χ, the relevant contributions to the dynamical matrices can be unveiled. For instance, in the Lindhard RPA limit χ 0 is built from free-electron bands (V scf = 0) and exchange-correlation is neglected to build χ (δV scf = δV e,p + δV H ), limit in which D e is reduced to a simple analytical expression 40 .…”

Section: B Vibrational Propertiesmentioning

confidence: 99%

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Anharmonic effects in atomic hydrogen: Superconductivity and lattice dynamical stability

et al. 2016

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We present first-principles calculations of metallic atomic hydrogen in the 400-600 GPa pressure range in a tetragonal structure with space group I41/amd, which is predicted to be its first atomic phase. Our calculations show a band structure close to the free-electron-like limit due to the high electronic kinetic energy induced by pressure. Bands are properly described even in the independent electron approximation fully neglecting the electron-electron interaction. Linear-response harmonic calculations show a dynamically stable phonon spectrum with marked Kohn anomalies. Even if the electron-electron interaction has a minor role in the electronic bands, the inclusion of electronic exchange and correlation in the density response is essential to obtain a dynamically stable structure. Anharmonic effects, which are calculated within the stochastic self-consistent harmonic approximation, harden high-energy optical modes and soften transverse acoustic modes up to a 20% in energy. Despite a large impact of anharmonicity has been predicted in several high-pressure hydrides, here the superconducting critical temperature is barely affected by anharmonicity, as it is lowered from its harmonic 318 K value only to 300 K at 500 GPa. We attribute the small impact of anharmonicity on superconductivity to the absence of softened optical modes and the fairly uniform distribution of the electron-phonon coupling among the vibrational modes.

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Section: B Vibrational Propertiesmentioning

confidence: 95%

Section: Computational Detailsmentioning

confidence: 99%

Section: B Vibrational Propertiesmentioning

confidence: 99%

Section: B Vibrational Propertiesmentioning

confidence: 99%

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Anharmonic effects in atomic hydrogen: Superconductivity and lattice dynamical stability

et al. 2016

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“…In contrast to Li, the bcc-to-fcc phase transition in Na is not due to phonon softening but, rather, the Kohn anomalies in the phonon spectrum, which result in dynamic instability. 6,42 On the other hand, acoustic phonon softening along ΓK induces the fcc-to-cI16 transition in Na, which is similar to the fcc-to-cI16 transition in Li. 40 The cI16-Na and tI19-Na structures have 9 and 29 atoms in their primitive cells, respectively, therefore both of their phonon spectra are dominated by numerous optical phonon branches.…”

Section: B Electronic and Phonon Band Structuresmentioning

confidence: 91%

Is sodium a superconductor under high pressure?

Tutchton

Chen

2017

The Journal of Chemical Physics

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Superconductivity has been predicted or measured for most alkali metals under high pressure, but the computed critical temperature (T) of sodium (Na) at the face-centered cubic (fcc) phase is vanishingly low. Here we report a thorough, first-principles investigation of superconductivity in Na under pressures up to 260 GPa, where the metal-to-insulator transition occurs. Linear-response calculations and density functional perturbation theory were employed to evaluate phonon distributions and the electron-phonon coupling for bcc, fcc, cI16, and tI19 Na. Our results indicate that the maximum electron-phonon coupling parameter, λ, is 0.5 for the cI16 phase, corresponding to a theoretical peak in the critical temperature at T≈1.2 K. When pressure decreases or increases from 130 GPa, T drops quickly. This is mainly due to the lack of p-d hybridization in Na even at 260 GPa. Since current methods based on the Eliashberg and McMillian formalisms tend to overestimate the T (especially the peak values) of alkali metals, we conclude that under high pressure-before the metal-to-insulator transition at 260 GPa-superconductivity in Na is very weak, if it is measurable at all.

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Possible enhancement of the superconducting Tc due to sharp Kohn-like soft phonon anomalies

Cunyuan

Enrico

Baggioli

et al. 2023

J. Phys.: Condens. Matter

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Phonon softening is a ubiquitous phenomenon in condensed matter systems which is usually associated with charge density wave (CDW) instabilities and anharmonicity. The interplay between phonon softening, CDW and superconductivity is a topic of intense debate. In this work, the effects of anomalous soft phonon instabilities on superconductivity are studied based on a recently developed theoretical framework that accounts for phonon damping and softening within the Migdal-Eliashberg theory. Model calculations show that the phonon softening in the form of a sharp dip in the phonon dispersion relation, either acoustic or optical (including the case of Kohn-type anomalies typically associated with CDW), can cause a manifold increase of the electron-phonon coupling constant λ. This, under certain conditions, which are consistent with the concept of optimal frequency introduced by Bergmann and Rainer, can produce a large increase of the superconducting transition temperature Tc. In summary, our results suggest the possibility of reaching high-temperature superconductivity by exploiting soft phonon anomalies restricted in momentum space.

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Kohn anomalies and enhanced superconductivity in simple systems under pressure: Insights from the nearly free electron model

Cited by 5 publications

References 23 publications

Anharmonic effects in atomic hydrogen: Superconductivity and lattice dynamical stability

Anharmonic effects in atomic hydrogen: Superconductivity and lattice dynamical stability

Is sodium a superconductor under high pressure?

Possible enhancement of the superconducting Tc due to sharp Kohn-like soft phonon anomalies

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