Scientific Highlights

20. September 2021
PtGa highlight

Unpaired Weyl Point observed for the first time in crystalline solid

Flows need sources and sinks. That’s why, in a new class of exotic materials called Weyl semimetals, the sources and sinks of Berry curvature – dubbed Weyl points – were believed to exist only in pairs. Now researchers at PSI have observed unpaired Weyl points for the first time in a crystalline solid. This discovery, which upends conventional thinking and the so-called Nielson-Niomiya no-go theorem, demonstrates the unique properties of "nodal wall" Weyl semimetals in comparison to conventional Weyl systems having only zero-dimensional Weyl nodes.

26. August 2021
toc fig nno

Creating novel quantum phases via the heterostructure engineering

Within this synergetic collaboration, PSI scientists have investigated the correlation between magnetic and electronic ordering in NdNiO3 by tuning its properties through proximity to a ferromagnetic manganite layer. The main outcome is that the stray magnetic field from the manganite layer causes a novel ferromagnetic-metallic (FM-M) phase in NNO. This work demonstrates the utilization of heterostructure engineering for creating novel quantum phases.

18. August 2021
Comparison between Pr4Fe2As2Te0.88O4 and Ba(Fe1−xCox)2As2

Novel structural orthorhombicity in an iron-pnictide superconductor

Researchers from University of Zurich describe the experimental observation of a new orthorhombic structural phase in the superconducting iron-pnictide compound Pr4Fe2As2Te0.88O4. In contrast to nematicity found in underdoped iron pnictides this phase transition is not electronically driven. 

12. Juli 2019
Weyl fermions

Weyl fermions discovered in another class of materials

Media Releases Matter and Material Materials Research Research Using Synchrotron Light

A particular variety of particles, the so-called Weyl fermions, had previously only been detected in certain non-magnetic materials. But now researchers at PSI have experimentally proved their existence for the first time in a specific paramagnetic material.

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.

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.

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.

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).

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.