Condensed Spin Excitation of Quantized Dirac Fermions in the Quasi-Two-Dimensional semimetal BaMnBi$_2$

Abstract

Dirac semimetals provide a new platform for the quantum Hall effect at low magnetic fields. In the presence of strong spin-orbit coupling, a spin-split Landau level is expected to enhance the bulk quasiparticle excitation. Here we report NMR spectroscopy that site-selectively probes dynamic spin susceptibility on the magnetic semimetal BaMnBi$_2$. We find that spontaneous staggered fields from antiferromagnetic Mn moments are completely canceled at the Bi layer hosting Dirac fermions. The nuclear spin-lattice relaxation rate $1/T_1$ follows the cubic temperature dependence down to low temperatures under the in-plane field, manifesting the chemical potential close to the Dirac point. $1/T_1$ becomes constant below 20 K under the out-of-plane field, where the well-separated Laudau level appears. The strong anisotropy of $1/T_1$ exceeding 100 suggests spin-split Landau levels in the quantum Hall regime.