Exchange field induced symmetry breaking in quantum hexaborides

TL;DR

The paper investigates spin symmetry breaking as a mechanism to induce site-dependent exchange fields in the quantum hexaborides EuB$_6$ and SmB$_6$ using spin-DFT with the $r^2SCAN$ functional. By comparing NM, FM, AFM, and PM configurations and modeling PM with Special Quasirandom Structures, it shows that polymorphous spin disorder creates inequivalent rare-earth sites with distinct exchange fields, driving electronic and magnetic symmetry breaking without changing valence. The results reconcile XAS/XANES observations of multiple local environments with a SB framework and predict stronger effects in SmB$_6$; they also suggest a magnetic-field test to verify SB. This SB perspective provides a robust, valence-preserving explanation for complex spectroscopic signatures in correlated materials and highlights the importance of local symmetry-breaking physics in interpreting quantum materials data.

Abstract

Symmetry breaking (SB) has proven to be a powerful approach for describing quantum materials: strong correlation, mass renormalization, and complex phase transitions are among the phenomena that SB can capture, even when coupled to a mean-field-like theory. Traditionally, corrective schemes were required to account for these effects; however, SB has emerged as an alternative that can also successfully describe the intricate physics of quantum materials. Here, we explore spin SB on EuB6 and SmB6 and how its relation to the exchange field can determine onsite properties, depending on the type of symmetry breaking. Using spin-polarized Density Functional Theory (DFT) calculations with the r2SCAN functional, we systematically compare four magnetic configurations, one totally symmetric - non-magnetic (NM) configuration - and three with different types of symmetry breaking: ferromagnetic (FM), antiferromagnetic (AFM) and a paramagnetic (PM) configuration - modeled through a Special Quasirandom Structure (SQS) method - to capture local symmetry-breaking effects. Our results show that the PM configuration produces distinct magnetic environments for the rare-earth atoms, leading to different exchange fields. These, in turn, induce symmetry breaking in the electronic and magnetic properties of Eu and Sm. Those results provide an alternative explanation for the experimental results on both materials, EuB6 and SmB6, where X-ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Structure (XANES) measurements suggest the presence of multiple atomic environments, previously attributed to a mixed-valence configuration.

Figures (9)

• Figure 1: Crystal structure of the hexaborides (SmB$_6$ or EuB$_6$) for the (a) NM, (b) FM, (c) AFM, and (d) PM configurations. Red are spin-up states, and blue arrows are spin-down states.
• Figure 2: Unfolded band structures for the NM monomorphous configuration. Panel (a) shows the results for EuB$_6$, while (b) illustrates SmB$_6$. The spin-up and spin-down bands are identical.
• Figure 3: Unfolded band structures for the FM configuration. Panel (a) shows the results for $EuB_6$, while (b) illustrates $SmB_6$.
• Figure 4: Unfolded band structures for the AFM configuration. Panel (a) shows the results for EuB$_6$, while (b) illustrates SmB$_6$.
• Figure 5: (a) Positions of inequivalent Eu/Sm atoms in the PM phase: red atoms denote asymmetric ($a$) sites, while blue atoms correspond to symmetric ($s$) sites. (b) Local environment around a symmetric atom. (c) Local environment around an asymmetric atom.