Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd$_2$As$_2$

TL;DR

This study reveals that EuCd$_2$As$_2$ exhibits Weyl semimetal states induced by spin fluctuations in its paramagnetic phase, challenging the notion that magnetic order is necessary for such states.

Contribution

It demonstrates that Weyl semimetal states can arise from spin fluctuations without long-range magnetic order in centrosymmetric materials.

Findings

Degeneracy of bands lifted in paramagnetic phase

Presence of Weyl nodes near the Fermi level

Weyl states can exist without magnetic order

Abstract

Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd$_2$As$_2$. We attribute this effect to the itinerant electrons experiencing quasistatic and quasi-long-range ferromagnetic fluctuations. Moreover, the spin nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not an essential requirement for WSM states in centrosymmetric systems, and that WSM states can emerge in a wider range of condensed-matter systems than previously thought.