News & Scientific Highlights

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of T_d-MoTe₂. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations.

The SPS 2017 Prize in Condensed Matter Physics, sponsored by IBM, has been awarded to Dr. Nan Xu for his excellent work on topological quantum states. Dr. Nan Xu is a joint postdoc of Paul Scherrer Institute (PSI) and the École Polytechnique Fédérale de Lausanne (EPFL).

For decades, the mechanism of Mott phase in Ca₂RuO₄ has puzzled researchers. This material is a paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling. Progress has been impeded by the lack of knowledge about the low-energy electronic structure. With our recent contribution, we provided-- using angle-resolved photoemission electron spectroscopy -- the band structure of the paramagnetic insulating phase of Ca₂RuO₄.

Turning the semimetal graphene into a technologically useful semiconductor is challenging. One way of opening a band gap is to cut graphene into nanometre-wide ribbons, but even atomic-level roughness at the ribbon edges can seriously degrade the mobility of charge carriers. Recent advances in on-surface chemistry have made it possible to obtain graphene nanoribbons with atomically precise edges through direct synthesis from molecular building blocks. Here, we report the synthesis, full structural and electronic characterization of 9-atom wide graphene nanoribbons with significantly improved electronic properties.

The first example of an insulating phase which is close to the superconducting phase in an iron-pnictide system has been recently observed in heavy Cu-doped NaFe_1-xCuxAs (x > 0.3). A combined study by angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations revealed that on-site Coulomb repulsion and enhanced Hund’s rule coupling are responsible for the insulating behavior. The results show that the insulating phase in NaFe_0.5Cu_0.5As resembles the situation in the parent compounds of the high-T_c cuprate superconductors.

Researchers have overcome a number of challenges in order to employ an advanced probe in the study of an unusual material, barium bismuth oxide (BaBiO3) – an insulating parent compound of a family of high-temperature superconductors known since the late 80s. In order to finally realize the experiments, the researchers grew and studied thin films of the material completely in situ under ultrahigh vacuum conditions. The results show that superconductivity in bismuth oxides emerges out of a novel insulating phase, where hole pairs located on combinations of the oxygen orbitals are coupled with distortions of the crystal lattice.

The study of nontrivial topological semimetals (TSM) is an emerging subject, providing a new frontier in topological aspects beyond insulators. Here, we have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin polarized. The measured spin texture fulfills the requirement of mirror and time-reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first principles calculations.