# Dynamical Bonding Driving Mixed Valency in a Metal Boride

## TL;DR

This paper introduces a new model for SmB$_6$ that explains its mixed valency and exotic properties through dynamical bonding effects involving thermal population of different Sm-B bonding modes, driven by B motion.

## Contribution

It proposes a unified understanding of SmB$_6$'s properties based on unrecognized dynamical bonding, linking valence fluctuations, Fermi surface, and phase transitions.

## Key findings

- Explains thermal valence fluctuations and magnetic Fermi surface.
- Accounts for pressure-induced phase transitions.
- Describes isotope and temperature dependence of spectral features.

## Abstract

Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed-valent semiconductor, and later - a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB$_6$ centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm-B bonding modes within SmB$_6$, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB$_6$ from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure-induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope.