[Paper Review] Heavy Weyl fermion state in CeRu$_4$Sn$_6$

Yuanfeng Xu, Changming Yue+2 more|arXiv (Cornell University)|Aug 16, 2016

Rare-earth and actinide compounds4 citations

TL;DR

This paper proposes CeRu₄Sn₆ as the first heavy Weyl fermion state in a strongly correlated heavy fermion material, where strong f-electron correlations from Ce 4f electrons renormalize the quasiparticle bands to host both type-I and type-II Weyl points near the Fermi level. Surface calculations reveal topologically protected Fermi arcs only on the (010) surface, distinguishable from the (001) surface due to separated bulk Fermi surface projections, offering a detectable signature via ARPES or quantum oscillations.

ABSTRACT

A new type of topological state in strongly corrected condensed matter systems, heavy Weyl fermion state, has been found in a heavy fermion material CeRu$_4$Sn$_6$, which has no inversion symmetry. Both two different types of Weyl points, type I and II, can be found in the quasi-particle band structure obtained by the LDA+Guztwiller calculations, which can treat the strong correlation effects among the f-electrons from Cerium atoms. The surface calculations indicate that the topologically protected Fermi arc states exist on the (010) but not on the (001) surfaces.

Motivation & Objective

• To investigate the existence of topological Weyl fermion states in strongly correlated heavy fermion materials like CeRu₄Sn₆.
• To determine how strong electron correlations from Ce 4f electrons influence the formation of Weyl points and topological surface states.
• To identify the surface-specific emergence of Fermi arcs and their detectability in angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation experiments.
• To assess the robustness of the heavy Weyl fermion state under variations in electronic structure parameters such as f-orbital occupation and double counting potential.

Proposed method

• Employed LDA+Gutzwiller method to self-consistently treat strong electron correlations in CeRu₄Sn₆, accounting for f-electron hybridization and renormalization.
• Used WannierTools and Green's function techniques to compute surface states from the renormalized tight-binding Hamiltonian derived from LDA+Gutzwiller results.
• Projected bulk Fermi surfaces onto (001) and (010) surface Brillouin zones to analyze topological surface state connectivity and separation.
• Varied the double counting potential in LDA+Gutzwiller to simulate changes in Ce valence and assess stability of Weyl points and Fermi arcs.
• Generated a phase diagram based on f-orbital occupation (n_f) to map the stability region of the heavy Weyl fermion state.
• Analyzed the role of spin-orbit coupling and lack of inversion symmetry in enabling Weyl node formation in the quasiparticle band structure.

Experimental results

Research questions

• RQ1 Does CeRu₄Sn₆ host Weyl points in its quasiparticle band structure due to strong electron correlations, despite lacking inversion symmetry?
• RQ2 Are both type-I and type-II Weyl points stabilized in the correlated electronic structure of CeRu₄Sn₆?
• RQ3 Why do topologically protected Fermi arcs appear only on the (010) surface and not on the (001) surface, despite both surfaces hosting Weyl points?
• RQ4 How robust is the heavy Weyl fermion state against changes in the f-orbital occupation and double counting potential?
• RQ5 Can external tuning via pressure, strain, or doping modify the position and number of Weyl points in this system?

Key findings

• CeRu₄Sn₆ hosts both type-I and type-II Weyl points in its quasiparticle band structure, confirmed by LDA+Gutzwiller calculations that include strong f-electron correlations.
• The heavy Weyl fermion state is robust for f-orbital occupation n_f > 0.87, with the state persisting across a wide range of correlation parameters.
• Fermi arcs are topologically protected and clearly visible only on the (010) surface due to well-separated projections of bulk Fermi surfaces, while they are masked on the (001) surface by overlapping projections.
• For n_f > 0.92, the number of Weyl point pairs increases to 12, leading to more complex Fermi arc patterns on the (010) surface.
• The phase diagram indicates high tunability of the Weyl state via external perturbations such as pressure, strain, or chemical doping, which can shift the effective Ce valence and thus control the Weyl node configuration.
• The surface state pattern on (010) is detectable via ARPES or quantum oscillation experiments, offering a direct experimental signature of the heavy Weyl fermion state.