Scientific Highlights

Datum
12. Juli 2019
Weyl fermions

Weyl-Fermionen in einer weiteren Materialklasse entdeckt

Medienmitteilungen Materie und Material Materialforschung Forschung mit Synchrotronlicht

Eine besondere Art von Teilchen, sogenannte Weyl-Fermionen, waren bislang nur in bestimmten nicht-magnetischen Materialien gefunden worden. Jetzt aber haben PSI-Forschende sie erstmals auch in einem besonderen paramagnetischen Material experimentell nachgewiesen.

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3. Mai 2019
Type-II Weyl semimetal WP2

New study gives compelling evidence that tungsten diphosphide is a type-II Weyl semimetal

Researchers at NCCR MARVEL have combined first principles calculations with soft X-ray angle-resolved photoemission spectroscopy to examine tungsten diphosphide’s electronic structure, characterizing its Weyl nodes for the very first time. In agreement with density functional theory calculations, the results revealed two pairs of Weyl nodes lying at different binding energies. The observation of the Weyl nodes, as well as the tilted cone-like dispersions in the vicinity of the nodal points, provides compelling evidence that the material is a robust type-II Weyl semimetal with broken Lorentz invariance. This is as MARVEL researchers predicted two years ago. The research has been published in Physical Review Letters as an Editor's Suggestion.

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22. Oktober 2018
Electronic structure of overdoped La1.77Sr0.23CuO4. (left) dx2-y2 and dz2 band structure 
along the nodal direction. (middle) Light polarization analysis of the dx2-y2 and dz2 bands.
(right) Anti-nodal Fermi surface warping along the kz direction.

Cuprate Trilogy

In a trio of recent papers, a research group from the University of Zürich has made a number of new discoveries about the nature of cuprates' electronic structure and orbital composition. The results have important implications for superconductivity and pseudogaps in cuprates, and even the existence of type-II Dirac fermions in oxides.

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25. September 2018

Evidence of a Coulomb-Interaction-Induced Lifshitz Transition and Robust Hybrid Weyl Semimetal in Td-MoTe2

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of Td-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.

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1. September 2017
Dr. Nan Xu

Dr. Nan Xu awarded SPS 2017 Prize in Condensed Matter Physics

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).

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22. Mai 2017
ARPES recorded with right-handed circularly polarized (C+) 65 eV photons in the paramagnetic (150 K) insulating state of Ca2RuO4. Incident direction of the light is indicated by the blue arrow. Dark colours correspond to high intensities. (a) Constant energy map displaying the photoemission spectral weight at binding energy ε=E−EF=−5.2 eV. Solid and dashed lines mark the in-plane projected orthorhombic and tetragonal zone boundaries, respectively. Γi with i=1, 2, 3 label orthorhombic zone centres. S and X …

Realization of a combined band-Mott insulator

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.

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21. März 2017

Better graphene nanoribbons for electronics applications

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.

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20. Oktober 2016
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Novel insulating phase in iron-pnictide materials

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 NaFe1-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 NaFe0.5Cu0.5As resembles the situation in the parent compounds of the high-Tc cuprate superconductors.

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14. Juli 2016

Shedding light on the origins of high-Tc superconductivity in bismuth oxides

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.

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