Negative thermal expansion of two-dimensional magnets

Abstract

Thermal expansion phenomena are of both engineering and fundamental interest. The conventional theory of thermal expansion of solids considers anharmonic phonon effects but largely neglects other kind of elementary excitations. A recent experiment reported a negative thermal expansion (NTE) of a two-dimensional magnet CrBr 3 at low-temperatures. Our first-principles calculations, if only the effects of atomic vibrations are considered, predict a low-temperature NTE but lower by several orders of magnitude than experimental data. We propose magnetic excitations (magnons) as another important contributor to the NTE. We extend the theory of thermal expansion by including the contribution from magnons to Helmholtz free energy. First-principles calculations based on this theory confirm the dominate role of magnons in the negative thermal expansion of CrBr 3 as well as in other two-dimensional magnets.