Controllable orbital angular momentum monopoles in chiral topological semimetals

The study addresses whether chiral topological semimetals host orbital angular momentum (OAM) monopoles—monopole-like orbital textures where OAM aligns isotropically with crystal momentum—and whether their polarity can be controlled via structural chirality. Chiral cubic crystals with B20 structure are predicted to stabilize multifold fermions (Kramers–Weyl and double spin-1 nodes) at time-reversal invariant momenta that act as strong sources/sinks of OAM, enabling large orbital Hall effects and isotropic magnetoelectric responses valuable for orbitronics. Unlike achiral Weyl semimetals where mirror symmetries can pin opposite-polarity monopoles to the same energy (reducing net orbital polarization), chiral systems generically separate them in energy. Despite prior ARPES observations of multifold fermions in B20 crystals, direct experimental evidence of OAM monopoles and demonstration of polarity control via enantiomorphs have been lacking. The work uses circular dichroism ARPES (CD-ARPES) coupled with microscopic photoemission modeling to detect and characterize OAM monopoles in PtGa and PdGa and to test chirality-controlled polarity inversion.

Prior works predicted OAM textures and large orbital Hall effects associated with chiral crystals and multifold fermions (Kramers–Weyl). ARPES has observed multifold fermions in chiral B20 compounds, but without probing OAM texture. CD-ARPES has been used to infer OAM or Berry curvature in simpler systems (Au, Bi2Se3 surfaces; 2D materials), yet its general validity in bulk is complicated by experimental geometry, final-state effects, and multi-atom interference, with photon-energy-dependent sign reversals reported in other materials. Achiral Weyl semimetals can host OAM monopoles but often suffer cancellation due to mirror symmetry. This context motivates a microscopic, orbital-resolved approach to connect CD-ARPES to OAM textures in complex chiral semimetals and to test the role of structural chirality.

• Materials and crystal growth: Single crystals of PdGa and PtGa (B20, space group 198) were grown via self-flux. Surfaces along (001) and (111) were prepared by polishing and, prior to ARPES, by in situ sputtering (Ar, 1 keV, 20 min) and annealing (>870 K, ≥20 min).
• ARPES experiments: Soft X-ray CD-ARPES at the ADRESS beamline (Swiss Light Source) with a PHOIBOS-150 analyzer. Sample at ~20 K, base pressure <2×10^-10 mbar. Angular resolution ~0.1°, energy resolution 60–180 meV for photon energies 360 eV to 1.021 keV. Measurements with left- and right-handed circular polarization; dichroism CD = (I+ − I−)/(I+ + I−). For spectral cuts, analyzer transmission corrections and background subtraction were applied. Fermi-surface maps integrated within ±100 meV of EF. Experimental geometry varied to probe 3D textures around the R point; photon energies chosen so that kz planes pass through R.
• First-principles and modeling: DFT (Quantum ESPRESSO) with GGA-PBE (without SOC) provided bulk band structures. Wannier90 projective Wannier functions (Pd/Pt d and Ga p) were constructed; slab supercells (15-layer (001), 20-layer (111)) enabled surface-sensitive ARPES simulations.
• Wannier-ARPES simulations: Based on Fermi’s golden rule, dipole gauge matrix elements Mα(k,E) computed using an atomic-centered approximation, expanding orbitals and final states in spherical harmonics. Radial integrals Il(E) evaluated via KKR formalism and parameterized with Slater-type orbitals to interpolate across energy. Simulations incorporated orbital-resolved intrasite terms and intersite interference (phase factors e^{-ik·(rj−rl)}), enabling decomposition of circular dichroism into local OAM-related and interference contributions. Both bulk and slab calculations were performed to compare with experiment.
• Analysis strategy: Evaluate CD-ARPES maps and dispersions near the R-point double spin-1 multifold node, across multiple surfaces ((111), (001)), enantiomers, and photon energies (e.g., 360, 547, 767, 1,021 eV). Compare with simulations to interpret photon-energy-dependent rotation or inversion of polar textures and to link observed dichroism to OAM monopole textures.

• Direct imaging of OAM monopoles: CD-ARPES maps near the R point in PdGa and PtGa exhibit robust polar circular-dichroism textures with sign changes across positive/negative momenta relative to R, consistent with monopole-like OAM aligned with crystal momentum.
• Polarity control by structural chirality: The sign of the polar CD texture reverses between PdGa enantiomers, demonstrating that OAM monopole polarity is switched by crystal handedness (monopole vs antimonopole), validating the pseudovector transformation law.
• Photon-energy dependence and rotation of polar texture: In PtGa(111), increasing photon energy from 360 to 547 eV yields an overall sign inversion of CD; at higher energies (767, 1,021 eV) the apparent sign change along a fixed kx cut can vanish. Simulations reveal the polar texture persists but rotates over the Fermi surface with photon energy, so specific cuts may miss the sign change at high energies.
• Microscopic mechanism: Decomposition of CD into orbital-resolved contributions shows that local Pd d-orbital OAM carries a polar texture at each site; intrasite CD mirrors local OAM. Intersite interference terms, with photon-energy-dependent phases tied to atomic positions and kz, superpose to produce complex, energy-dependent CD patterns and rotate the effective polar axis.
• Robustness and universality: Despite final-state and interference complexities, the total CD-ARPES generally retains a polar texture across materials, surfaces, and energies, serving as a robust fingerprint of OAM monopoles. Simulations for a hypothetical centrosymmetric bulk (inversion restored) eliminate magnetic orbital moments and remove the polar texture (aside from surface-induced effects), underscoring the role of bulk chirality.
• Energy dependence across the node: The global OAM texture does not necessarily flip sign above vs below the node, whereas CD-ARPES can, due to differences between global (including delocalized Bloch contributions) and local atomic OAM; nonetheless, the polar momentum-space symmetry remains a hallmark of the monopole at R.

The observations address the core question of whether chiral topological semimetals host controllable OAM monopoles. Polar CD-ARPES textures near the R-point multifold fermions in PdGa and PtGa, and their reversal between enantiomers, provide direct evidence for monopole and antimonopole OAM textures and demonstrate chirality-based polarity control. The photon-energy-dependent evolution is reconciled by a microscopic model where local polar OAM at atomic sites, combined with intersite interference that depends on kz and photon energy, rotates the apparent polar axis in CD-ARPES without eliminating the underlying monopole texture. This establishes CD-ARPES, when interpreted with orbital-resolved photoemission theory, as a viable probe of OAM monopoles in bulk chiral crystals. The results are significant for orbitronics: isotropic, longitudinal OAM-momentum locking near multifold nodes offers direction-independent orbital polarization and magnetization responses, enabling robust device functionalities (e.g., domain switching with perpendicular anisotropy) even in polycrystalline films. Structural chirality emerges as a design parameter to set the sign of orbitronic responses. The work connects OAM monopoles to the quantum geometry of Bloch electrons, with implications for transport and optoelectronics.

The study provides direct experimental evidence of orbital-angular-momentum monopoles in chiral topological semimetals PdGa and PtGa via CD-ARPES and demonstrates polarity control through crystal handedness. A combined experimental–theoretical framework shows that robust polar dichroism textures originate from local magnetic d-orbital textures and photon-energy-dependent intersite interference, explaining apparent sign inversions and rotations with photon energy. These results establish a universal CD-ARPES fingerprint for OAM monopoles and underscore chiral crystals’ promise for orbitronic applications with isotropic responses. The methodology opens pathways to discover and characterize more intricate nodal OAM textures in quantum materials, which could be harnessed in future orbitronic devices.

• Interpretation of CD-ARPES in bulk materials is complicated by final-state effects, experimental geometry, and multi-atom interference, meaning CD is not a direct, proportional measure of total (global) OAM.
• Differences between local atomic OAM and global OAM (including delocalized Bloch contributions) can lead to differing signs across bands above/below the node in CD-ARPES, complicating straightforward extraction of global OAM.
• DFT (without SOC in the presented Wannierization) may not fully capture electronic structures and orbital textures in some topological materials; quantitative discrepancies increase at higher photon energies.
• Surface effects and slab artifacts (e.g., back-folded shadow bands) can affect simulated spectra; inversion-symmetric bulk simulations still show residual CD from surface symmetry breaking.
• The fingerprint relies on appropriate photon energies and momentum cuts; at certain energies, the polar sign change can be missed in specific cuts due to rotation of the polar texture.