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

TL;DR

This study uses NMR spectroscopy to explore spin excitations in the Dirac fermion system BaMnBi$_2$, revealing spin-split Landau levels and the influence of magnetic fields on quasiparticle excitations.

Contribution

It provides the first site-selective NMR evidence of spin-split Landau levels and spin dynamics in a Dirac semimetal with strong spin-orbit coupling.

Findings

Spontaneous antiferromagnetic fields are canceled at the Bi layer.

The relaxation rate follows a cubic temperature dependence near the Dirac point.

Strong anisotropy in relaxation rate indicates spin-split Landau levels.

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.