Flat Bands at the Fermi Level in Unconventional Superconductor YFe2Ge2

TL;DR

This paper investigates the electronic structure of the unconventional superconductor YFe2Ge2, revealing very heavy flat bands near the Fermi level through ARPES and theoretical calculations, shedding light on its many-body physics.

Contribution

It provides the first direct observation of heavy flat bands at the Fermi level in YFe2Ge2 and demonstrates the effectiveness of GW and DMFT calculations in describing its electronic structure.

Findings

Heavy bands with effective mass ~25 m_e near E_F

Flat bands located at X points of the Brillouin zone

Good agreement between ARPES data and GW/DMFT calculations

Abstract

We report heavy electron behavior in unconventional superconductor YFe$_2$Ge$_2$ ($T_C \,{=}\, 1.2$ K). We directly observe very heavy bands ($m_\mathrm{eff}\sim 25 m_e$) within $\sim$10 meV of the Fermi level $E_{F}$ using angle-resolved photoelectron spectroscopy (ARPES). The flat bands reside at the X points of the Brillouin zone and are composed principally of $d_{xz}$ and $d_{yz}$ orbitals. We utilize many-body perturbative theory, GW, to calculate the electronic structure of this material, obtaining excellent agreement with the ARPES data with relatively minor band renormalizations and band shifting required. We obtain further agreement at the Dynamical Mean Field Theory (DMFT) level, highlighting the emergence of the many-body physics at low energies (near $E_F$) and temperatures.