Electron correlation effects and scattering rates in Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductor

TL;DR

This study uses ARPES to explore the electronic structure and scattering rates in Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors, revealing unique Fermi surface features and non-Fermi liquid behavior, differing from other iron-based superconductors.

Contribution

It provides detailed ARPES measurements showing distinct Fermi surface topology and non-Fermi liquid self-energy effects in Fe$_{1+y}$Te$_{1-x}$Se$_x$, highlighting differences from other iron pnictides.

Findings

Fermi surface topology differs from other iron pnictides.

Observation of non-Fermi liquid behavior in self-energy.

Weak to moderate $k_z$ dispersion of hole pockets.

Abstract

Using angle-resolved photoemission spectroscopy we have studied the low-energy electronic structure and the Fermi surface topology of Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors. Similar to the known iron pnictides we observe hole pockets at the center and electron pockets at the corner of the Brillouin zone (BZ). However, on a finer level, the electronic structure around the $\Gamma$- and $Z$-points in $k$-space is substantially different from other iron pnictides, in that we observe two hole pockets at the $\Gamma$-point, and more interestingly only one hole pocket is seen at the $Z$-point, whereas in $1111$-, $111$-, and $122$-type compounds, three hole pockets could be readily found at the zone center. Another major difference noted in the Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors is that the top of innermost hole-like band moves away from the Fermi level to higher binding energy on going from $\Gamma$ to $Z$, quite opposite to the iron pnictides. The polarization dependence of the observed features was used to aid the attribution of the orbital character of the observed bands. Photon energy dependent measurements suggest a weak $k_z$ dispersion for the outer hole pocket and a moderate $k_z$ dispersion for the inner hole pocket. By evaluating the momentum and energy dependent spectral widths, the single-particle self-energy was extracted and interestingly this shows a pronounced non-Fermi liquid behaviour for these compounds. The experimental observations are discussed in context of electronic band structure calculations and models for the self-energy such as the spin-fermion model and the marginal-Fermi liquid.