Observation of Weyl Nodes in Robust Type-II Weyl Semimetal WP2

Distinct to type-I Weyl semimetals (WSMs) that host quasiparticles described by the Weyl equation, the energy dispersion of quasiparticles in type-II WSMs violates Lorentz invariance and the Weyl cones in the momentum space are tilted. Since it was proposed that type-II Weyl fermions could emerge from (W, Mo)Te-2, and (W, Mo)P-2 families of materials, a large number of experiments have been dedicated to unveiling the possible manifestation of type-II WSMs, e.g., surface-state Fermi arcs. However, the interpretations of the experimental results are very controversial. Here, using angle-resolved photoemission spectroscopy supported by the first-principles calculations, we probe the tilted Weyl cone bands in the bulk electronic structure of WP2 directly, which are at the origin of Fermi arcs at the surfaces and transport properties related to the chiral anomaly in type-II WSMs. Our results ascertain that, due to the spin-orbit coupling, the Weyl nodes originate from the splitting of fourfold degenerate band-crossing points with Chem numbers C = +/- 2 induced by the crystal symmetries of WP2, which is unique among all the discovered WSMs. Our finding also provides a guiding line to observe the chiral anomaly that could manifest in novel transport properties.

Spin-orbit coupling effects in the band structure of surface alloys

Luca Moreschini

At the surface of a solid, quantum mechanics allows the existence of two-dimensional Bloch waves as solutions of the Schrödinger equation. These "surface" states are interesting both for fundamental and practical reasons, and have been extensively explored, e.g., on pure metals. Conversely, relatively few studies exist of the electronic states of artificially-grown two-dimensional structures. This work is an investigation, mainly by angle-resolved photoemission, of different heterogeneous surfaces of defined stoichiometry, formed by a large-Z p metal on a noble metal substrate, which constitute what is called a "surface alloy". A breakthrough has been obtained on the Bi/Ag(111) (√3 × √3)R30° structure, which presents a band dispersion split into two spin polarized replicas in momentum space by the spin-orbit coupling. The effect is known as Rashba-Bychkov splitting in semiconductors, and it is a genuine effect of the symmetry breaking occurring at the surface, but in Bi/Ag(111) it is much larger than reported in any other compound. This sets this class of materials among the suitable candidates for spintronics applications, which have been object of rising interest in the last years. We have put our attention on other surface alloys, varying substrates (Ag(111) and Cu(111)) and adsorbates (Bi, Pb, Sb) in order to set the basis for a unifying description of the quantities involved in the determination of the size of the splitting, and in particular to discriminate between atomic and structural contributions. An analogous band structure is found in all the systems, consisting namely of two "sets" of bands with negative effective mass, originating from the hybridization between the surface states of the substrate and the p electrons of the adsorbate. We have identified a significant in-plane component of the electric potential gradient, which is not present in pure metal surfaces, as the new element most likely responsible for the large size of the splitting. At the same time, the results show that a necessary condition for it to be effective in enhancing the splitting is a strong atomic spin-orbit parameter. Our findings are confirmed by first-principle calculations, which reproduce quantitatively the band structure, and by a nearly free electron model, which qualitatively supports our interpretation of the role of the in-plane gradient. A parallel project of this thesis has concerned Yb-based Kondo systems, as studied by hard x-ray photoemission and inelastic x-ray scattering. At these high energies, photoemission probes a thicker region below the sample surface, where heavy-fermion compounds stabilize in a different electronic configuration with respect to that of the bulk. We have shown that the temperature dependence of the 4f spectral function is in line with the predictions of the Anderson impurity model, a point which has often been questioned on the basis of previous results of low-energy photoemission. On the other hand, by a comparison with inelastic scattering, we have noticed that the latter leads to a better agreement with the data from thermodynamic and transport measurements. We conclude that, despite the unique capability of photoemission of accessing directly the electron spectral function, photon-in–photon-out spectroscopies are a more trustworthy probe of the bulk electronic structure.

EPFL2009

Robust Type-II Weyl Semimetal Phase in Transition Metal Diphosphides XP2 (X = Mo, W)

Gabriel Albert Autes, Oleg Yazyev

The recently discovered type-II Weyl points appear at the boundary between electron and hole pockets. Type-II Weyl semimetals that host such points are predicted to exhibit a new type of chiral anomaly and possess thermodynamic properties very different from their type-I counterparts. In this Letter, we describe the prediction of a type-II Weyl semimetal phase in the transition metal diphosphides MoP2 and WP2. These materials are characterized by relatively simple band structures with four pairs of type-II Weyl points. Neighboring Weyl points have the same chirality, which makes the predicted topological phase robust with respect to small perturbations of the crystalline lattice. In addition, this peculiar arrangement of the Weyl points results in long topological Fermi arcs, thus making them readily accessible in angle-resolved photoemission spectroscopy.

Amer Physical Soc2016

Spin-orbit coupling effects in the band structure of surface alloys

Luca Moreschini

EPFL2009