We report on the electronic structure of the elemental topological semimetal α-Sn on InSb(001). High-resolution angle-resolved photoemission data allow to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional p-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.

Two-dimensional (2D) atom lattices provide model setups with Coulomb correlations that induce competing ground states. Here, SiC emerges as a wide-gap substrate with reduced screening. We report the first artificial high-Z atom lattice on SiC(0001) by Sn adatoms, based on experimental realization and theoretical modeling. Density-functional theory of our triangular structure model closely reproduces the scanning tunneling microscopy. Photoemission data show a deeply gapped state (∼2  eV gap), and, based on our calculations including dynamic mean-field theory, we argue that this reflects a pronounced Mott-insulating scenario. We also find indications that the system is susceptible to antiferromagnetic superstructures. Such artificial lattices on SiC(0001) thus offer a novel platform for coexisting Coulomb correlations and spin-orbit coupling, with bearing for unusual magnetic phases and proposed topological quantum states of matter.

We thoroughly examine the ground state of the triangular lattice of Pb on Si(111) using scanning tunneling microscopy and spectroscopy. We detect electronic charge order, and disentangle this contribution from the atomic configuration which we find to be 1-down-2-up, contrary to previous predictions from density functional theory. Applying an extended variational cluster approach we map out the phase diagram as a function of local and nonlocal Coulomb interactions. Comparing the experimental data with the theoretical modeling leads us to conclude that electron correlations are the driving force of the charge-ordered state in Pb/Si(111). These results resolve the discussion about the origin of the well-known 3 × 3 reconstruction. By exploiting the tunability of correlation strength, hopping parameters, and bandfilling, this material class represents a promising platform to search for exotic states of matter, in particular, for chiral topological superconductivity.In a frustrated lattice of uncompensated spins the exchange interactions cannot be saturated completely on every site. This leads to competing ground states where either a specific magnetic order or a spin liquid phase can emerge [1][2][3][4][5]. When nonlocal Coulomb interactions are involved or the system is doped away from half filling, the formation of charge-order (CO) is another possibility. These scenarios are often accompanied by superconductivity arising in the vicinity of such ordered phases. Yet, candidate materials are limited to very few bulk solids, such as cobaltates [6,7] and organic compounds [8,9]. However, due to the complexity of these materials, the occurrence of particular phases is not fully understood. In contrast, atomic two-dimensional (2D) lattices with a triangular net, experimentally generated by epitaxial submonolayer deposition on an insulating substrate, are intriguingly simple in structure. Thus they provide versatile model systems for the study of strong electron correlations. The generically rich phase diagram of correlated triangular systems has been pointed out in theoretical studies of lattice models [10,11] and surface systems [12,13], including CO and the possibility of topological superconductivity [14][15][16][17][18][19][20]. In this respect, the atomic architecture allows us to tune the interactions by variation of the adatom species as well as the substrate which provides screening and mediates the electron hopping [21]. In addition, dopants such as alkali atoms have been demonstrated to change the band filling [22].The case in point are group-IV adsorbates (Sn, Pb) on semiconductor surfaces such as Si, Ge, or SiC [12,21,23].The key concept here is that unsaturated adsorbate orbitals exist with half filling which are subject to significant local and nonlocal Coulomb interactions. These surface systems thus represent a rich playground for the investigation of correlation physics in a frustrated lattice, including the formation of unusual symmetrybroken ground states. The experimental system is a triangular array...

We report on the electronic structure of α-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energies angle-resolved photoemission (ARPES) allows for the direct observation of the linearly dispersing 2D topological surface states (TSSs) that exist between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α-Sn films where the hybridization gap in TSS coexists with the topologically non-trivial electronic structure and one can expect the presence of a 1D helical edge states.

Preparation of SiC(0001) substrates is of high relevance to graphene growth. Yet, if only a smooth surface could be achieved, heteroepitaxy of many other two-dimensional materials comes into reach. Here we report a novel approach to hydrogen etching of SiC, based on stepwise ultrapure H exposure with slow substrate cooling rates. For the first time, the atomic evolution of the surface structure is witnessed by scanning tunneling microscopy. A detailed picture of the gas phase chemistry emerges, such as a zipper-like material desorption at step edges. The Si−C sheets are removed in layer-by-layer fashion, leading to large terraces with straight rims. The process ultimately results in an atomically smooth surface with complete H-passivation, with no detectable defect states in photoemission. The degree of perfection achieved suggests the use of this substrate as a versatile nanostructure template.