IQIM Virtual Seminar, November 20, 2020 – Mo Chen

Speaker: Mo Chen, Postdoctoral Scholar, Painter Group
Title: A synthetic monopole source of Kalb-Ramond field in diamond

Abstract: Monopoles are key to characterizing gauge fields and topological phases and they serve as important resources to study fundamental physics. While their observation is elusive in high-energy physics, monopole sources of artificial gauge fields have been recently identified in synthetic matter. String theory, a potentially unifying framework that encompasses quantum mechanics, promotes the conventional \emph{vector} gauge fields of electrodynamics to \emph{tensor} gauge fields, and predicts the existence of more exotic \emph{tensor monopoles} in 4D space. Here we report the observation of a tensor monopole in a 4D parameter space synthesized by the spin degrees of freedom of a single solid-state defect in diamond. Using two complementary methods, we reveal the existence of the tensor monopole through measurements of its quantized topological invariant. By introducing a fictitious external field that breaks chiral symmetry, we further observe a novel phase transition to a topological semimetal phase characterized by nodal rings protected by mirror symmetries. Our work represents the first detection of tensor monopoles in a solid-state system and opens up the possibility of emulating exotic topological phases inspired by string theory.