Evidence for magnetic Weyl fermions in a correlated metal

Abstract

1Recent discovery of both gapped and gapless topological phases in weakly correlated electron systems has introduced various relativistic particles and a number of exotic phenomena in condensed matter physics [1][2][3][4][5] . The Weyl fermion 6-8 is a prominent example of three dimensional (3D), gapless topological excitation, which has been experimentally identified in inversion symmetry breaking semimetals 4,5 . However, their realization in spontaneously time reversal symmetry (TRS) breaking magnetically ordered states of correlated materials has so far remained hypothetical 7, 9, 10 . Here, we report a set of experimental evidence for elusive magnetic Weyl fermions in Mn 3 Sn, a non-collinear antiferromagnet that exhibits a large anomalous Hall effect even at room temperature 11 . Detailed comparison between our angle resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations reveals significant bandwidth renormalization and damping effects due to the strong correlation among Mn 3d electrons. Moreover, our transport measurements have unveiled strong evidence for the chiral anomaly of Weyl fermions, namely, the emergence of positive magnetoconductance only in the presence of parallel electric and magnetic fields. The magnetic Weyl fermions of Mn 3 Sn have a significant technological potential, since a weak field (∼ 10 mT) is adequate for controlling the distribution of Weyl points and the large fictitious field (∼ a few 100 T) in the momentum space. Our discovery thus lays the foundation for a new field of science and technology involving the magnetic Weyl excitations of strongly correlated electron systems.Traditionally, topological properties have been considered for the systems supporting gapped bulk excitations 1 . However, over the past few years three dimensional gapless systems such asWeyl and Dirac semimetals have been discovered, which combine two seemingly disjoint notions 2 of gapless bulk excitations and band topology [2][3][4][5] . In 3D inversion or TRS breaking systems, two nondegenerate energy bands can linearly touch at pairs of isolated points in the momentum (k) space, giving rise to the Weyl quasiparticles. The touching points or Weyl nodes act as the unit strength (anti) monopoles of underlying Berry curvature [4][5][6][7] , leading to the protected zero energy surface states also known as the Fermi-arcs 4,5,7 , and many exotic bulk properties such as large anomalous Hall effect (AHE) 12 , optical gyrotropy 13 , and chiral anomaly 6,[14][15][16][17][18][19] . Interestingly, the Weyl fermions can describe low energy excitations of both weakly and strongly correlated electron systems. In weakly correlated, inversion symmetry breaking materials, where the symmetry breaking is entirely caused by the crystal structure rather than the collective properties of electrons, the ARPES has provided evidence for long-lived bulk Weyl fermions and the surface Fermi arcs 4, 5 .On the other hand, the magnetic Weyl fermions have been predicted for several...

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

“…It is a future subject to investigate the possible current-induced AF domain wall motion 37 . Finally, the topological aspect of the magnetic state (magnetic Weyl state) of Mn 3 Sn, which has been recently pointed out by theoretical 38 and experimental 39 studies, would make it interesting to investigate the Berry curvature effects in the MOKE at low frequency, and further make the topological defects even more attractive as the bulk-edge correspondence for such a magnetic Weyl semimetallic state may lead to a Fermi arc in a magnetic domain wall, and contribute to the MOKE signal in an unconventional fashion 37, 40 .…”

Large anomalous Hall effect in the chiral-lattice antiferromagnet CoNb3S6

“…Reformulation of the SOC-induced intrinsic mechanism of AHE in ferromagnets to the Berry phase curvature in momentum space has been fruitful in predicting and describing the AHE in several other systems, including Weyl (semi)metals 3 , non-collinear antiferromagnets 4 , non-coplanar magnets 5–7 , and other nontrivial spin textures 8–11 . Recent observations of the large anomalous Hall effect in metals with possible Weyl 12–14 and massive Dirac fermions 15,16 and/or complex spin textures, e.g., skyrmion bubbles 17 , have generated interest in such materials, especially for the role of correlated topological states in the emergent electronic properties. Here we present a large AHE in CoNb 3 S 6 that cannot be understood in terms of conventional mechanisms of the AHE.…”

Electric‐Field Control of Magnetic Order: From FeRh to Topological Antiferromagnetic Spintronics

“…For example, experimental evidence for time-reversalsymmetry-breaking Weyl fermions has been reported by Kuroda et al in AFM Mn 3 Sn, [128] where both Weyl points near the Fermi level (Figure 10c,d) and the chiral anomaly have been demonstrated via the angle-resolved photoemission spectroscopy and magnetotransport measurements, respectively. For example, experimental evidence for time-reversalsymmetry-breaking Weyl fermions has been reported by Kuroda et al in AFM Mn 3 Sn, [128] where both Weyl points near the Fermi level (Figure 10c,d) and the chiral anomaly have been demonstrated via the angle-resolved photoemission spectroscopy and magnetotransport measurements, respectively.…”

“…[112] These results are rather exciting because they counter the longstanding wisdom that the AHE is an intrinsic feature of FM materials. [104,116,117] Basically, it is an emerging research field that especially focuses on the links between AFM spintronics and topological structures in real and momentum space, such as Majorana fermions in AFM topological superconductors, [104] topologically protected AFM skyrmions, [118][119][120][121][122][123][124] exotic AHE, [125][126][127] and magnetic Weyl fermions [128][129][130][131] in AFM systems. [113][114][115] Recently, the concept of topological AFM spintronics has been emphasized.…”