Giant orbital diamagnetism of three-dimensional Dirac electrons in Sr$ _3$PbO antiperovskite

TL;DR

This study demonstrates that the giant diamagnetism observed in the three-dimensional Dirac electron system Sr$_3$PbO originates from the orbital contribution of Dirac electrons, confirming theoretical predictions with microscopic NMR evidence.

Contribution

The paper provides experimental proof that the giant diamagnetism in Sr$_3$PbO is due to orbital effects of Dirac electrons, using $^{207}$Pb NMR to separate spin and orbital contributions.

Findings

Orbital diamagnetism in Sr$_3$PbO is confirmed as the main origin.

NMR measurements successfully separate spin and orbital contributions.

Results align with theoretical predictions of giant orbital diamagnetism in Dirac systems.

Abstract

In Dirac semimetals, inter-band mixing has been known theoretically to give rise to a giant orbital diamagnetism when the Fermi level is close to the Dirac point. In Bi$ _{1-x}$Sb$ _x$ and other Dirac semimetals, an enhanced diamagnetism in the magnetic susceptibility $\chi$ has been observed and interpreted as a manifestation of such giant orbital diamagnetism. Experimentally proving their orbital origin, however, has remained challenging. Cubic antiperovskite Sr$ _3$PbO is a three-dimensional Dirac electron system and shows the giant diamagnetism in $\chi$ as in the other Dirac semimetals. $ ^{207}$Pb NMR measurements are conducted in this study to explore the microscopic origin of diamagnetism. From the analysis of the Knight shift $K$ as a function of $\chi$ and the relaxation rate $T_1^{-1}$ for samples with different hole densities, the spin and the orbital components in $K$ are successfully separated. The results establish that the enhanced diamagnetism in Sr$ _3$PbO originates from the orbital contribution of Dirac electrons, which is fully consistent with the theory of giant orbital diamagnetism.