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We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe 2 . On both surfaces, we identify a surface state that forms a large Fermi arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. This implies that the observation of surface Fermi arcs alone does not allow the identification of WTe 2 as a topological Weyl semimetal. We further use the identification of the two different surfaces to clarify the number of Fermi surface sheets in WTe 2 . DOI: 10.1103/PhysRevB.94.121112 Transition-metal dichalcogenides (TMDs) have long been studied in many-body physics as model systems for metalinsulator transitions, multiband superconductivity, and charge density waves [1][2][3]. In recent years, the interest in TMDs intensified because of the promising optoelectronic properties of monolayer or few-layer devices based on hexagonal semiconducting MX 2 compounds with M = W,Mo and X = Se,S [4,5]. Unlike these materials, WTe 2 crystallizes in the orthorhombic, noncentrosymmetric 1T structure (P mn2 1 space group) and is semimetallic due to a small overlap of valence and conduction bands at the Fermi level [6,7]. Recent theoretical work [8] predicts that WTe 2 is an example of a new type of Weyl semimetal with strongly tilted Weyl cones that arise from topologically protected crossings of valence and conduction bands causing touching points between electron and hole pockets near the Fermi level. In type-I Weyl semimetals, realized for example in TaAs [9][10][11][12][13], the projections of opposite chirality Weyl points onto a surface are isolated from the bulk continuum and must be connected by well-defined Fermi arcs. This is not generally the case for type-II Weyl points, which are necessarily accompanied by bulk carrier pockets. The surface Fermi arcs corresponding to the bulk Weyl points in these materials can emerge within the projection of the bulk carrier pockets, rendering the robust identification of their topological nature challenging. Indeed, very recent angle-resolved photoemission spectroscopy (ARPES) experiments on the related Mo x W 1-x Te 2 and MoTe 2 systems report conflicting interpretations of the topological character of potential surface states [14][15][16]. ARPES studies on pure WTe 2 have to date not reported any surface states [17][18][19].In addition, WTe 2 is attracting interest because of its nonsaturating magnetoresistance [7] and the recent discovery of pressure induced superconductivity [20,21]. A possible relation between these phenomena and the topological nature of the low-energy surface excitations in WTe 2 is an intriguing prospect but has not yet been established. To date, even the basic bulk electronic structure underlying these phenomena remains controversial. The complex magnetotransport proper...

The adsorption of Ni, Co, Mn, Ti, and Zr at the (√ 2 × √ 2)R45 •-reconstructed Fe 3 O 4 (001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the (√ 2 × √ 2)R45 • reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (√ 2 × √ 2)R45 • reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1 × 1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe 2 O 4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe 3 O 4 .

Keywords: strontium titanate, angle resolved photoemission spectroscopy, two-dimensional electron gas, oxygen vacancies Combining the tunability of semiconductor heterostructures with the rich properties of correlated electron systems is a central goal in materials science. The two-dimensional electron gas (2DEG) observed at the LaAlO 3 /SrTiO 3 (LAO/STO) interface [1] emerged as a particularly promising model system in this regard since it combines high mobility with properties such as gate-tunable superconductivity that cannot be realized in conventional semiconductor heterostructures or bulk correlated electron systems. [2][3][4][5] The interface 2DEG resides solely in STO and can be stabilized in different ways. These include interfacing STO with other crystalline insulating materials such as spinel Al 2 O 3 or perovkite GdTiO 3 , [6,7] deposition of amorphous overlayers, [Y. Chen et al, Nano Lett. 2011, 11, 3774. 8,9] and electrolyte gating. [10] A similar 2DEG has also been observed on the bare surface of STO following irradiation with