Mapping the three-dimensional fermiology of the triangular lattice magnet EuAg$_4$Sb$_2$

TL;DR

This study maps the three-dimensional electronic structure and magnetic phase diagram of EuAg$_4$Sb$_2$, revealing complex magnetic transitions and detailed Fermi surface features through experimental measurements and theoretical calculations.

Contribution

It provides a comprehensive analysis combining quantum oscillations, ARPES, and first-principles calculations to elucidate the fermiology and magnetic states of EuAg$_4$Sb$_2$, highlighting the impact of magnetic exchange splitting.

Findings

Identification of multiple spin-split Fermi surface pockets.

Observation of temperature-dependent evolution of small Fermi pockets.

Good agreement between experimental data and theoretical calculations.

Abstract

In this paper, we report the temperature-field phase diagram as well as present a comprehensive study of the electronic structure and three-dimensional fermiology of the triangular-lattice magnet EuAg$_4$Sb$_2$, utilizing quantum oscillation measurements, angle-resolved photoemission spectroscopy and first-principles calculations. The complex magnetic phase diagram of EuAg$_4$Sb$_2$ highlights many transitions through nontrivial AFM states. Shubnikov-de Haas and de Haas-van Alphen oscillations were observed in the polarized ferromagnetic state of EuAg$_4$Sb$_2$, revealing three pairs of distinct spin-split frequency branches with small effective masses. A comparison of the angle-dependent oscillation data with first-principles calculations in the ferromagnetic state and angle-resolved photoemission spectra shows good agreement, identifying tubular hole pockets and hourglass-shaped hole pockets at the Brillouin zone center, as well as diamond-shaped electron pockets at the zone boundary. As the temperature increases, the frequency branches of the tiny hourglass pockets evolve into a more cylindrical shape, while the larger pockets remain unchanged. This highlights that variations in exchange splitting, driven by changes in the magnetic moment, primarily impact the small Fermi pockets without significantly altering the overall band structure. This is consistent with first-principles calculations, which show minimal changes near the Fermi level across ferromagnetic and simple antiferromagnetic states or under varying on-site Coulomb repulsion.