Single pair of Weyl nodes in the spin-canted structure of EuCd$_2$As$_2$

TL;DR

This study demonstrates that EuCd$_2$As$_2$ hosts a single pair of Weyl points due to a specific magnetic structure with Eu moments canted out-of-plane, confirmed by neutron diffraction, DFT, and transport measurements, making it a minimal Weyl semimetal.

Contribution

The paper identifies EuCd$_2$As$_2$ as a Weyl semimetal with a single pair of Weyl points caused by magnetic canting, a novel magnetic configuration affecting electronic topology.

Findings

EuCd$_2$As$_2$ has a magnetic structure with Eu moments canted ~30° out-of-plane.

Density functional theory shows canting creates a single set of Weyl points.

Transport measurements indicate chiral anomaly consistent with Weyl physics.

Abstract

Time reversal symmetry breaking Weyl semimetals are unique among Weyl materials in allowing the minimal number of Weyl points thus offering the clearest signatures of the associated physics. Here we present neutron diffraction, density functional theory and transport measurement results which indicate that EuCd$_2$As$_2$ , under ambient field, strain and pressure, is such a material with a single pair of Weyl points. Our work reveals a magnetic structure (magnetic space group $C2'/m'$) with Eu moments pointing along the [210] direction in-plane and canted $\sim$ 30$^{\circ}$ out-of-plane. Density functional theory calculations using this structure show that the observed canting drastically alters the relevant electronic bands, relative to the in-plane order, leading to a single set of well defined Weyl points. Furthermore, we find the canting angle can tune the distance of the Weyl points above the Fermi level, with the smallest distance at low canting angles. Finally, transport measurements of the anomalous Hall Effect and longitudinal magnetoresistance exhibit properties indicative of a chiral anomaly, thus supporting the neutron scattering and DFT results suggesting EuCd$_2$As$_2$ is close to the ideal situation of the Weyl "Hydrogen atom".