Speaker
Description
Ab-initio codes are usually very good at giving values for the electron-phonon Macmillan coupling, and, with a bit of extra work, for its momentum resolved counterpart. However, any further information, such as whether there are hidden structures within the electron phonon coupling, and how to control them, is completely lost in the ab-initio process. We here provide an analytic understanding of the constituent parts of the electron phonon coupling for systems with Fermi surfaces, and show that it can be split in two main contributions. Once is “energetic”, and vanished if the band is flat, the other is quantum geometric, and vanishes if the band has no wavefunction variation. We show, using a “Gaussian” approximation that analytic formulas can be obtained for both of these. We also show that in Kagome ScV$_6$Sn$_6$ materials, when coupled with a quantum loop calculation, this correctly predicts the phonon softening wavevector, while in MgB$_2$, a famous superconductor, 92% of the electron phonon coupling is contained in the quantum geometric part!
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