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Breakdown of the Migdal approximation at Lifshitz transitions with giant zero-point motion in the H3S superconductor.

Jarlborg T, Bianconi A - Sci Rep (2016)

Bottom Line: The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Γ point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations.The maximum Tc = 203 K occurs at 160 GPa where EF/ω0 = 1 in the small Fermi surface pocket at Γ.A Feshbach-like resonance between a possible BEC-BCS condensate at Γ and the BCS condensate in different k-space spots is proposed.

View Article: PubMed Central - PubMed

Affiliation: DPMC, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland.

ABSTRACT
While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the Migdal-approximation becomes questionable. We show that the neglected hydrogen zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Γ point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations. The maximum Tc = 203 K occurs at 160 GPa where EF/ω0 = 1 in the small Fermi surface pocket at Γ. A Feshbach-like resonance between a possible BEC-BCS condensate at Γ and the BCS condensate in different k-space spots is proposed.

No MeSH data available.


Related in: MedlinePlus

The band structure for H3S along X–M–Γ–R for a for the simple cubic double cell (sc BZ) with the lattice parameter changing between 5.4 and 6.2 a.u.
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f9: The band structure for H3S along X–M–Γ–R for a for the simple cubic double cell (sc BZ) with the lattice parameter changing between 5.4 and 6.2 a.u.

Mentions: In order to associate the crossing of the chemical potential of the narrow peak in the DOS with the Lifshitz transitions on the topology of the Fermi surfaces we have plotted the electronic bands in a narrow energy range near the chemical potential for both the sc BZ and bcc BZ in Figs 9 and 10. These band plots show that several Lifshitz transitions appear for increasing pressure. From Fig. 9 it is seen that a local band maximum crosses EF in the sc BZ at about 2/3 of the Γ − M distance when a is ~5.8. a.u. (see Figs 1 and 2) and the same band crosses the chemical potential between N and H in Fig. 10 in the bands for the bcc BZ. This Lifshitz transition occurs at P = 210 GPa. The energy difference E2 between the chemical potential and this local band maximum which is associated with the vHs goes from −200 to +100 meV when a decreases from 6.2 to 5.6 a.u. This gives a neck disrupting Lifshitz transition in the Fermi surface at 210 GPa where the neck disappears at low pressure in the N-H direction20. The Fermi surface neck appears at the point where the narrow peaks in the DOS crosses the chemical potential. Another band is seen to be approaching the chemical potential in Fig. 9 between X and M when the pressure goes up. However, this potential band crossing (which is not seen on a symmetry line in the band plots for the small bcc cell) will not reach EF to make a FS pocket unless P is increased even more.


Breakdown of the Migdal approximation at Lifshitz transitions with giant zero-point motion in the H3S superconductor.

Jarlborg T, Bianconi A - Sci Rep (2016)

The band structure for H3S along X–M–Γ–R for a for the simple cubic double cell (sc BZ) with the lattice parameter changing between 5.4 and 6.2 a.u.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4837402&req=5

f9: The band structure for H3S along X–M–Γ–R for a for the simple cubic double cell (sc BZ) with the lattice parameter changing between 5.4 and 6.2 a.u.
Mentions: In order to associate the crossing of the chemical potential of the narrow peak in the DOS with the Lifshitz transitions on the topology of the Fermi surfaces we have plotted the electronic bands in a narrow energy range near the chemical potential for both the sc BZ and bcc BZ in Figs 9 and 10. These band plots show that several Lifshitz transitions appear for increasing pressure. From Fig. 9 it is seen that a local band maximum crosses EF in the sc BZ at about 2/3 of the Γ − M distance when a is ~5.8. a.u. (see Figs 1 and 2) and the same band crosses the chemical potential between N and H in Fig. 10 in the bands for the bcc BZ. This Lifshitz transition occurs at P = 210 GPa. The energy difference E2 between the chemical potential and this local band maximum which is associated with the vHs goes from −200 to +100 meV when a decreases from 6.2 to 5.6 a.u. This gives a neck disrupting Lifshitz transition in the Fermi surface at 210 GPa where the neck disappears at low pressure in the N-H direction20. The Fermi surface neck appears at the point where the narrow peaks in the DOS crosses the chemical potential. Another band is seen to be approaching the chemical potential in Fig. 9 between X and M when the pressure goes up. However, this potential band crossing (which is not seen on a symmetry line in the band plots for the small bcc cell) will not reach EF to make a FS pocket unless P is increased even more.

Bottom Line: The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Γ point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations.The maximum Tc = 203 K occurs at 160 GPa where EF/ω0 = 1 in the small Fermi surface pocket at Γ.A Feshbach-like resonance between a possible BEC-BCS condensate at Γ and the BCS condensate in different k-space spots is proposed.

View Article: PubMed Central - PubMed

Affiliation: DPMC, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland.

ABSTRACT
While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the Migdal-approximation becomes questionable. We show that the neglected hydrogen zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Γ point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations. The maximum Tc = 203 K occurs at 160 GPa where EF/ω0 = 1 in the small Fermi surface pocket at Γ. A Feshbach-like resonance between a possible BEC-BCS condensate at Γ and the BCS condensate in different k-space spots is proposed.

No MeSH data available.


Related in: MedlinePlus