Typically, chemical bonding is viewed in a static fashion, i.e. a specific bonding arrangement determines the properties of the system at fixed positions of the nuclei. I will introduce a system in which the chemical bonding undergoes a constant and substantial evolution through vibrations, even on the ground state of the material, and this dynamics defines unique material’s properties. 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 SmB6 centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm-B bonding modes within SmB6, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB6 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.

Dynamical bonding driving new type of mixed valency, phase transitions, Raman anomalies, and more, in samarium hexaboride

2019 Fall National Meeting

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