Eu₂Cd₂As₂ as a candidate Weyl Hydrogen Atom

It is an unexpected turn in condensed matter physics that we find ourselves hunting for solutions to the relativistic wave equation. Exotic particles such as the massless chiral Weyl Fermion which have been extensively and unsuccessfully sought in high energy physics are finding new life as quasiparticle excitations in carefully constructed solid state electronic band structures. In condensed matter, such particles arise from non-trivial topological invariants and thus, not only do they yield exotic transport properties, but the associated states are also topologically protected and therefore potentially useful. Yet finding them in ideal configurations has proven quite challenging.

In its simplest realization, a Weyl Fermion can be created in a material with only a single fourfold degenerate linear band crossing (i.e. a Dirac point) at the Fermi energy that then undergoes time reversal symmetry breaking. This splits the Dirac point into two opposite chirality Weyl points which are intrinsically protected by their chirality and separation in momentum space. Such a configuration, dubbed the Weyl ‘Hydrogen Atom’, gives not only the most direct comparison with toy models but should also exhibit the clearest signatures of the associated topological physics. Here we present the results of neutron scattering, transport, magnetization, and first principles studies which strongly suggest that ferromagnetic EuCd₂As₂ may be just such a Weyl ‘Hydrogen Atom’. To solve the magnetic structure, we performed neutron scattering using isotopic Eu and Cd which revealed a previously unexpected, canted structure. Using this magnetic symmetry for band structure calculations we found only a single pair of Weyl points near the Fermi energy. Angle and field dependent transport measurements evidenced the existance of the Chiral anomaly supporting the predictions of the first principles calculations. Together these results suggest EuCd₂As₂ as a rare example of a Weyl Hydrogen Atom.