Breakdown of the Migdal-Eliashberg theory and a theory of lattice-fermionic superfluidity | |
Article | |
关键词: STRONG-COUPLING LIMIT; SUPERCONDUCTORS; TRANSITION; VIBRATIONS; ELECTRONS; MOTION; MODEL; | |
DOI : 10.1103/PhysRevB.106.054518 | |
来源: SCIE |
【 摘 要 】
We show that the Migdal-Eliashberg theory loses validity at a finite value lambda c of the electron-phonon coupling lambda regardless of the underlying model Hamiltonian. The value of lambda c is approximately between 3.0 and 3.7. The new phase that emerges at lambda > lambda(c) breaks the lattice translational symmetry. Depending on the filling fraction and crystal symmetry, it is an insulator or a Fermi liquid. Its characteristic feature is a gap or a pronounced depression of the fermionic density of states near the Fermi level. We establish the breakdown from within the Migdal-Eliashberg theory by demonstrating that the normal state specific heat is negative for lambda 3.7 and the quasiparticle lifetime vanishes in the strong coupling limit. At fixed lambda > lambda(c), the transition to the new phase occurs at a critical temperature higher than the superconducting transition temperature. In addition, there is a first-order phase transition between the new phase and the superconducting state as we vary lambda across lambda(c) at fixed temperature. We put forward a new theory-lattice-fermionic theory of superfluidity-that bridges the gap between the Migdal-Eliashberg approach and the physics at stronger coupling. At small lambda, our theory reduces to the Migdal-Eliashberg theory, and past lambda(c), it describes the new phase and a range of other phenomena.
【 授权许可】
Free