Elemental topological insulator with tunable Fermi level: strained α-Sn on InSb(001)

We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.

Au-induced quantum chains on Ge(001)-symmetries, long-range order and the conduction path

Atomic nanowires on the Au/Ge(001) surface are investigated for their structural and electronic properties using scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). STM reveals two distinct symmetries: a c(8 × 2) describing the basic repeating distances, while the fine structure on top of the wires causes an additional superstructure of p(4 × 1). Both symmetries are long-range ordered as judged from low-energy electron diffraction. The Fermi surface is composed of almost perfectly straight sheets. Thus, the electronic states are one-dimensionally confined. Spatial dI/dV maps, where both topography and density of states (DOS) are probed simultaneously, reveal that the DOS at low energies, i.e. the conduction path, is oriented along the chain direction. This is fully consistent with the recently reported Tomonaga-Luttinger liquid phase of Au/Ge(001), with the density of states being suppressed by a power-law towards the Fermi energy.

Three-dimensional spin rotations at the Fermi surface of a strongly spin-orbit coupled surface system

The spin texture of the metallic two-dimensional electron system (sqrt[3]×sqrt[3])-Au/Ge(111) is revealed by fully three-dimensional spin-resolved photoemission, as well as by density functional calculations. The large hexagonal Fermi surface, generated by the Au atoms, shows a significant splitting due to spin-orbit interactions. The planar components of the spin exhibit a helical character, accompanied by a strong out-of-plane spin component with alternating signs along the six Fermi surface sections. Moreover, in-plane spin rotations toward a radial direction are observed close to the hexagon corners. Such a threefold-symmetric spin pattern is not described by the conventional Rashba model. Instead, it reveals an interplay with Dresselhaus-like spin-orbit effects as a result of the crystalline anisotropies.

Symmetry-breaking phase transition without a Peierls instability in conducting monoatomic chains

The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm. By scanning tunneling microscopy and electron diffraction we reveal a second-order phase transition at 585 K. It leads to charge ordering with transversal and vertical displacements and complex interchain correlations. However, the structural phase transition is not accompanied by the electronic signatures of a charge density wave, thus precluding a Peierls instability as origin. Instead, this symmetry-breaking transition exhibits three-dimensional critical behavior. This reflects a dichotomy between the decoupled 1D electron system and the structural elements that interact via the substrate. Such substrate-mediated coupling between the wires thus appears to have been underestimated also in related chain systems.

Renormalization of bulk magnetic electron states at high binding energies

The quasiparticle dynamics of electrons in a magnetically ordered state is investigated by high-resolution angle-resolved photoemission of Ni(110) at 10 K. The self-energy is extracted for high binding energies reaching up to 500 meV, using a Gutzwiller calculation as a reference frame for correlated quasiparticles. Significant deviations exist in the 300 meV range, as identified on magnetic bulk bands for the first time. The discrepancy is strikingly well described by a self-energy model assuming interactions with spin excitations. Implications relating to different electron-electron correlation regimes are discussed.

New model system for a one-dimensional electron liquid: self-organized atomic gold chains on Ge(001)

Unique electronic properties of self-organized Au atom chains on Ge(001) in novel c(8 x 2) long-range order are revealed by scanning tunneling microscopy. Along the nanowires an exceptionally narrow conduction path exists which is virtually decoupled from the substrate. It is laterally confined to the ultimate limit of single atom dimension, and is strictly separated from its neighbors, as not previously reported. The resulting tunneling conductivity shows a dramatic inhomogeneity of 2 orders of magnitude. The atom chains thus represent an outstandingly close approach to a one-dimensional electron liquid.