Limits...
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 DOS for H3S for cubic 64-site supercells. The (blue) thin line is the DOS for the perfectly ordered supercell. The (red) heavy line shows the DOS for the disordered supercell with u(S) = 0.01a and u(H) = 0.033a Zero Point Motion.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4837402&req=5

f4: The DOS for H3S for cubic 64-site supercells. The (blue) thin line is the DOS for the perfectly ordered supercell. The (red) heavy line shows the DOS for the disordered supercell with u(S) = 0.01a and u(H) = 0.033a Zero Point Motion.

Mentions: The electronic calculations for the ZPM in H3S at P = 210 GPa, a = 5.6 a.u., have been carried out using a large supercell in which the lattice is disordered. Each atom is assigned randomized displacements, ux, uy, uz in such a way that the distribution of all displacement amplitudes (/u/) has a bell shaped (Gaussian) distribution with the FWHM width equal to the averaged displacement amplitude (<u>) at was described before and in ref. 42. In Fig. 4 we show the energy renormalization of the DOS due to the calculated zero point motion (ZPM). Here we consider a 2 × 2 × 2 extension of the cubic unit cell with 64 atoms totally which permits to calculate u from 192 displacement vectors, which is a reasonably good statistics for calculating <u>. As was discussed above, it is sufficient to do the calculation for one disordered configuration when the supercells are large with at least 48 atoms. Because of the large mass difference between S and H we here allow larger <u> for H (uH) than for S (uS). For small <u> it can be shown that correlation of vibrational movements and anharmonic terms are small56. But uH is large, and in the generation of a disordered configuration for uH/a = 0.05, as for the expected ZPM, several pairs of H come too close to each other. Therefore, in order to avoid large anharmonic effects at this stage, we calculate the electronic structure for a supercell with uS/a = 0.01 (which is close to the expected ZPM for S) and uH/a = 0.033 (which is 2/3 of the expected ZPM for H).


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 DOS for H3S for cubic 64-site supercells. The (blue) thin line is the DOS for the perfectly ordered supercell. The (red) heavy line shows the DOS for the disordered supercell with u(S) = 0.01a and u(H) = 0.033a Zero Point Motion.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: The DOS for H3S for cubic 64-site supercells. The (blue) thin line is the DOS for the perfectly ordered supercell. The (red) heavy line shows the DOS for the disordered supercell with u(S) = 0.01a and u(H) = 0.033a Zero Point Motion.
Mentions: The electronic calculations for the ZPM in H3S at P = 210 GPa, a = 5.6 a.u., have been carried out using a large supercell in which the lattice is disordered. Each atom is assigned randomized displacements, ux, uy, uz in such a way that the distribution of all displacement amplitudes (/u/) has a bell shaped (Gaussian) distribution with the FWHM width equal to the averaged displacement amplitude (<u>) at was described before and in ref. 42. In Fig. 4 we show the energy renormalization of the DOS due to the calculated zero point motion (ZPM). Here we consider a 2 × 2 × 2 extension of the cubic unit cell with 64 atoms totally which permits to calculate u from 192 displacement vectors, which is a reasonably good statistics for calculating <u>. As was discussed above, it is sufficient to do the calculation for one disordered configuration when the supercells are large with at least 48 atoms. Because of the large mass difference between S and H we here allow larger <u> for H (uH) than for S (uS). For small <u> it can be shown that correlation of vibrational movements and anharmonic terms are small56. But uH is large, and in the generation of a disordered configuration for uH/a = 0.05, as for the expected ZPM, several pairs of H come too close to each other. Therefore, in order to avoid large anharmonic effects at this stage, we calculate the electronic structure for a supercell with uS/a = 0.01 (which is close to the expected ZPM for S) and uH/a = 0.033 (which is 2/3 of the expected ZPM for H).

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