Probing the interaction between coupled Weyl points via magneto-infrared spectroscopy.

Weyl semimetals (WSMs) are a newly discovered quantum state of matter with topological protection characterized by a pair of Weyl points (WPs) with linear dispersion in all directions. When the bands of two WPs overlap, their interaction can significantly change the energy dispersion, resulting in unique responses in WSM. Here, we present a combined theoretical and experimental study of Niobium Phosphide (NbP), an established WSM. Our magneto-infrared reflectance measurements show that the Landau levels in NbP are strongly modified from an isolated WP picture. By considering the interaction between two WPs, our model can reach a semi-quantitative agreement with the experiment. Our result signifies the crucial role of the interaction effect in understanding the rich physical phenomena in WSM. We also discuss other unique implications of this interaction picture, which could be used to distinguish a WSM from a Dirac semimetal.