Effective theory and emergent SU(2) symmetry in the flat bands of attractive Hubbard models

Tutkimustuotos: Lehtiartikkeli

Tutkijat

Organisaatiot

  • Swiss Federal Institute of Technology Zurich

Kuvaus

In a partially filled flat Bloch band electrons do not have a well defined Fermi surface and hence the low-energy theory is not a Fermi liquid. Neverethless, under the influence of an attractive interaction, a superconductor well described by the Bardeen-Cooper-Schrieffer (BCS) wave function can arise. Here we study the low-energy effective Hamiltonian of a generic Hubbard model with a flat band. We obtain an effective Hamiltonian for the flat band physics by eliminating higher lying bands via perturbative Schrieffer-Wolff transformation. At first order in the interaction energy we recover the usual procedure of projecting the interaction term onto the flat band Wannier functions. We show that the BCS wave function is the exact ground state of the projected interaction Hamiltonian and that the compressibility is diverging as a consequence of an emergent SU(2) symmetry. This symmetry is broken by second order interband transitions resulting in a finite compressibility, which we illustrate for a one-dimensional ladder with two perfectly flat bands. These results motivate a further approximation leading to an effective ferromagnetic Heisenberg model. The gauge-invariant result for the superfluid weight of a flat band can be obtained from the ferromagnetic Heisenberg model only if the maximally localized Wannier functions in the Marzari-Vanderbilt sense are used. Finally, we prove an important inequality D≥W2 between the Drude weight D and the winding number W, which guarantees ballistic transport for topologically nontrivial flat bands in one dimension.

Yksityiskohdat

AlkuperäiskieliEnglanti
Artikkeli 245149
Sivut1-17
JulkaisuPhysical Review B (Condensed Matter and Materials Physics)
Vuosikerta 94
Numero24
TilaJulkaistu - 30 joulukuuta 2016
OKM-julkaisutyyppiA1 Julkaistu artikkeli, soviteltu

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