*PLEASE NOTE ROOM CHANGE*
Harvard Quantum Initiative Special Seminar
Friday, January 4
12:00 PM
Jefferson 250
Ilya Esterlis, Stanford
Title: Reconsidering the electron-phonon problem
Abstract: The electron-phonon interactions determine many of the salient features of
“conventional” metals including their transport properties (e.g. resistivity) at most
temperatures and, famously, their superconducting transition temperature, Tc. As the
electron-phonon coupling is not small, perturbation theory is not applicable. However, a
60 year old “theorem” by Migdal was generally thought to solve this problem, allowing
results to all orders in perturbation theory to be summed using the ratio of the electron
mass to the ion mass as a small parameter. We have revisited this problem by studying a
paradigmatic model problem (the “Holstein model”) using the Migdal approach, a strong
coupling expansion, and exact numerical methods. We show that Migdal’s theorem isn’t (a
theorem); results based on the Migdal-Eliashberg “approximation” are found to be amazingly
accurate for a range of couplings, but break down quantitatively and qualitatively (in a
regime where the Migdal theorem is still nominally applicable) at a point of crossover to
strong-coupling physics. These results give a new perspective (including an upper bound on
Tc) on the hunt for high temperature conventional superconductors.
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Clare Ploucha
Administrative Program Manager
Max Planck/Harvard Research Center for Quantum Optics
Department of Physics
17 Oxford Street, Jefferson 357
Cambridge, MA 02138
P: 617-495-3388
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