Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs

Abstract

Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids.

Link between Weyl-fermion chirality and spin texture

“…In summary, using different polarization states of the exciting light, as well as advanced first-principles calculations, we clarified the spin-and orbital-textures of the topological states. This revealed fingerprints of the topological character of the quasiparticle states in reminiscence of previous results for type-I Weyl semimetals [36]. We captured a pair of WPs exhibiting a strong CD with reversed sign, giving evidence for the location of the WPs in energy-momentum space near the Fermi level and their respective chirality.…”

“…4d shows a rich texture with sign changes at the positions of the Weyl points. This shows the correlation between the chirality of the Weyl points and the orbital texture of the electronic structure around them [36]. Considering possible matrix element effects we find a good agreement of the calculated values with the measurements, that show a crossing of the changing chirality around k y = 0.2 Å−1 (highlighted by dotted lines).…”

“…The CDAD can be shown to be approximately proportional to the projection of the orbital angular momentum L on the light propagation direction [35,36]. A recent study further demonstrated that momentum mapping of the orbital angular momentum reflects the chirality of the WPs in a Weyl semimetal [36]. In our experimental geometry, where the light incidence is aligned in the y-z plane, the observed CDAD is sensitive to the L y component of the orbital angular momentum.…”

“…A fingerprint of the Weyl points and the associated chiral Dirac states can be obtained through the excitation by circularly polarized light in a photoemission experiment [35,36].…”

“…It has been shown that the CDAD probes the orbital angular momentum of the wave function [38]. The CDAD can be shown to be approximately proportional to the projection of the orbital angular momentum L on the light propagation direction [35,36]. A recent study further demonstrated that momentum mapping of the orbital angular momentum reflects the chirality of the WPs in a Weyl semimetal [36].…”