Surfaces and interfaces o er new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides 1-4 . Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO 3 (001) surface 5-7 to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO 3 /SrTiO 3 interface 8-13 , our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO 3 -based 2DELs.Carrier concentration is a key parameter defining the ground state of correlated electron systems. At the LaAlO 3 /SrTiO 3 interface, the 2DEL density can be tailored by field-effect gating. As the system is depleted of carriers, its ground state evolves from a high-mobility 2DEL 4 into a two-dimensional superconductor 8-10 with pseudogap behaviour 11 and possible pairing above T c (ref. 12). An analogous 2DEL can be induced by doping the (001) surface of SrTiO 3 . As for the interface, the surface 2DEL is confined by a band-bending potential in SrTiO 3 and consists of an orbitally polarized ladder of quantum confined Ti t 2g electrons that are highly mobile in the surface plane [5][6][7]14 . Thus far, the surface 2DEL has been studied only at carrier densities around 2 × 10 14 cm −2 , approximately a factor of five higher than typically observed at the LaAlO 3 /SrTiO 3 interface [5][6][7] . In the following, we present ARPES data extending to lower carrier densities that are directly comparable to the LaAlO 3 /SrTiO 3 interface. We achieve this by preparing SrTiO 3 (001) wafers in situ, which results in well-ordered clean surfaces that can be studied by ARPES over extended timescales, as they are less susceptible to the ultraviolet-induced formation of charged oxygen vacancies reported for cleaved SrTiO 3 5,7,15,16 . Details of the sample preparation are given in Methods. Figure 1a shows an energy-momentum intensity map for a 2DEL with a carrier density of n 2D ≈ 2.9 × 10 13 cm −2 estimated from the Luttinger volume of the first light subband and the two equivalent heavy subbands (see Supplementary Section 2). The most striking features of this data are replica bands at higher binding energy following the dispersion of the primary quasiparticle (QP) bands. The replica bands are all separated by approximately 100 meV and progressively lose intensity, but can be visualized up to the third replica in the curvature plot shown in Fi...

We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe 2 . Using laser-based angle-resolved photoemission, we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topological Weyl semimetal state with eight type-II Weyl points. We further use systematic electronic structure calculations simulating different Weyl point arrangements to discuss the robustness of the identified Weyl semimetal state and the topological character of Fermi arcs in MoTe 2 .

No abstract

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...

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies for the β and γ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies, and provides strong evidence for an electronic origin of the observed non-linear frequency dependence of the self-energies, leading to 'kinks' in the quasiparticle dispersion of Sr2RuO4. * present address: ISIS Facility, Rutherford