Spin texture of time-reversal symmetry invariant surface states on W(110)

Abstract

We find in the case of W(110) previously overlooked anomalous surface states having their spin locked at right angle to their momentum using spin-resolved momentum microscopy. In addition to the well known Dirac-like surface state with Rashba spin texture near the Γ-point, we observe a tilted Dirac cone with circularly shaped cross section and a Dirac crossing at 0.28 × Γ N within the projected bulk band gap of tungsten. This state has eye-catching similarities to the spin-locked surface state of a topological insulator. The experiments are fortified by a one-step photoemission calculation in its density-matrix formulation.In the past decade topological insulators have attracted large scientific interest because of their unusual electronic properties [1][2][3] . Topologically protected Dirac-type surface states appearing in the bulk band gap give rise to metallic behavior at their surfaces 2,3 . The rigid spin-locking of these surface states perpendicular to the crystal momentum bears high potential for the development of novel spintronic devices and improvement of existing electronic devices, suitable for spin injection and manipulation without applying external magnetic fields 3 . It was a surprise that recently strong spin-polarized surface states with linear dispersion resembling a Dirac-cone were found on metallic surfaces [4][5][6] . This was unexpected as, e.g. W(110) has no similarities to known topological insulators except the strong spin-orbit interaction due to a large atomic number. Instead of the fundamental band gap of an insulator, tungsten exhibits a spin-orbit induced local band gap 7 . The energy range of the observed surface state is populated by d electrons while the fundamental band gap in known topological insulators (e.g. Bi 2 Se 3 ) is caused by p electrons.Miyamoto et al. 4,5 found a "massless" (i.e., Dirac-like) surface state with linear dispersion and Rashba-type spin signature in a large energy range of 220 meV, which is an anomalous behavior in metals. Further experimental 8-12 as well as theoretical work 4,5,7,9,13 was performed in order to clarify the origin of this anomalous surface resonances on W(110). Additionally, the "direct neighbors" of W(110) in the periodic table, Mo(110) and Ta (110), were investigated. The analogous surface resonance was confirmed in our previous work for Mo (110) 14 , whereas Ta(110) does not show a "Dirac-like" surface state 15 . For Mo(110) we found a second state with anomalous dispersion behavior in the middle between the Γ and N points 14 . Also, recent work of K. Miyamoto et al. 16 experimentally confirmed the prediction of ref. 9, i.e. the change of spin polarization comparing p-and s-polarized light excitation. This effect is explained by an orbital-symmetry-selective excitation of states. This new work also comes to the conclusion that p-polarized light reflects at least the sign of the ground state spin polarization.Here, we give for the first time evidence for a time-reversal symmetry invariant surface state with high spin polarization i...

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