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 ΓH̅ and ΓS̅ while it is strongly deformed along ΓN̅ . 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