Massless or heavy due to two-fold symmetry: Surface-state electrons at W(110)

The C₂ᵥ symmetry of the W(110) surface influences strongly the spin-polarized Dirac-cone-like surface state within a spin-orbit-induced symmetry gap. We present a detailed angle-resolved photoemission study with s- and p-polarized light along three different symmetry lines. The Dirac-cone-like feature appears along ΓH and ΓS, while it is strongly deformed along ΓN. A two-fold Σ₃ symmetry of the d-type surface state is identified from photoemission experiments using linearly polarized light. Our results are well described by model calculations based on an effective Hamiltonian with C₂ᵥ symmetry. The flattened Dirac cone of the surface state is caused by hybridization with bulk continuum states of Σ₁ and Σ₂ symmetry. The spin texture of this state obtained from the model calculations shows a quasi-one-dimensional behavior. This finding opens a new avenue in the study of d-electron-based persistent spin helix systems and/or weak topological insulators.