Migdal-Eliashberg theory breaks down to polaron/bipolaron states before phonon softening at extreme densities, with variational upper bounds on coupling λ showing this occurs well before softening in 2D/3D systems.
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In the Holstein model, polaronic and bipolaronic states emerge before phonon softening across wide fillings in 2D and 3D, via an intermediate pseudogap mixed state where Luttinger theorem is broken.
In the 2D attractive Hubbard model, Tc is enhanced near Van Hove singularities only for weak interactions, while the global maximum Tc occurs at intermediate coupling away from the singularity.
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Breakdown of the Migdal-Eliashberg theory for electron-phonon systems. Role of polarons/bi-polarons
Migdal-Eliashberg theory breaks down to polaron/bipolaron states before phonon softening at extreme densities, with variational upper bounds on coupling λ showing this occurs well before softening in 2D/3D systems.
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Limits of validity for Migdal-Eliashberg theory: role of polarons/bi-polarons
In the Holstein model, polaronic and bipolaronic states emerge before phonon softening across wide fillings in 2D and 3D, via an intermediate pseudogap mixed state where Luttinger theorem is broken.
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Superconductivity near two-dimensional Van Hove singularities: a determinant quantum Monte Carlo study
In the 2D attractive Hubbard model, Tc is enhanced near Van Hove singularities only for weak interactions, while the global maximum Tc occurs at intermediate coupling away from the singularity.