| 1. |
- Thiagarajan, Balasubramanian, et al.
(författare)
-
Spin-valley locking in the normal state of a transition-metal dichacogenide superconductor
- 2016
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
-
Tidskriftsartikel (refereegranskat)abstract
- Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.
|
|
| 2. |
- Belopolski, Ilya, et al.
(författare)
-
Observation of a linked-loop quantum state in a topological magnet
- 2022
-
Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 604:7907, s. 647-652
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state1–13. Over the past decades, these invariants have come to play a central role in describing matter, providing the foundation for understanding superfluids5, magnets6,7, the quantum Hall effect3,8, topological insulators9,10, Weyl semimetals11–13 and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet14–20. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material’s three-torus, T3, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk–boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.
|
|
| 3. |
- Krieger, Jonas A., et al.
(författare)
-
Weyl spin-momentum locking in a chiral topological semimetal
- 2024
-
Ingår i: Nature Communications. - 2041-1723. ; 15:1
-
Tidskriftsartikel (refereegranskat)abstract
- Spin-orbit coupling in noncentrosymmetric crystals leads to spin-momentum locking – a directional relationship between an electron’s spin angular momentum and its linear momentum. Isotropic orthogonal Rashba spin-momentum locking has been studied for decades, while its counterpart, isotropic parallel Weyl spin-momentum locking has remained elusive in experiments. Theory predicts that Weyl spin-momentum locking can only be realized in structurally chiral cubic crystals in the vicinity of Kramers-Weyl or multifold fermions. Here, we use spin- and angle-resolved photoemission spectroscopy to evidence Weyl spin-momentum locking of multifold fermions in the chiral topological semimetal PtGa. We find that the electron spin of the Fermi arc surface states is orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion at the Γ point, which is consistent with Weyl spin-momentum locking of the latter. The direct measurement of the bulk spin texture of the multifold fermion at the R point also displays Weyl spin-momentum locking. The discovery of Weyl spin-momentum locking may lead to energy-efficient memory devices and Josephson diodes based on chiral topological semimetals.
|
|
| 4. |
- Xu, Su-Yang, et al.
(författare)
-
Lifshitz transition and Van Hove singularity in a three-dimensional topological Dirac semimetal
- 2015
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 92:7
-
Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional (3D) Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report results on the electronic structure of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial ground state. Our studies reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene. Our results identify an example of a band saddle-point singularity in 3D Dirac materials. This is in contrast to its two-dimensional analogs such as graphene and the Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological bulk ground state.
|
|
| 5. |
- Inagaki, Shunsuke, et al.
(författare)
-
Effects of adsorbed molecular ordering to the superconductivity of a two-dimensional atomic layer crystal
- 2023
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 7:2
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) adsorption on the physical properties of the two-dimensional (2D) atomic layer superconductor (ALSC) In/Si(111)-(7×3) has been studied by angle-resolved photoelectron spectroscopy, transport measurements, and scanning tunneling microscopy. Hole doping from the adsorbed molecules has been reported to increase the superconducting transition temperature Tc of this ALSC, and the molecular spin tends to decrease it. Owing to its large electron affinity and its nonexistent spin state, the adsorption of PTCDA was expected to increase Tc. However, the PTCDA adsorption dopes only a small number of holes in the In layers and causes a suppression of Tc with a sharp increase in the normal-state sheet resistance followed by an insulating transition. Taking the disordering of the arrangement of PTCDA into account, we conclude that the increase in resistance is due to the localization effect originating from the random potential that is induced by the disordered PTCDA molecules. The present result also indicates the importance of the crystallinity of a 2D molecular film adsorbed on ALSCs.
|
|
| 6. |
- Karakachian, Hrag, et al.
(författare)
-
One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons
- 2020
-
Ingår i: Nature Communications. - : NATURE RESEARCH. - 2041-1723. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics. Here, the authors investigate armchair graphene nanoribbons by angle-resolved photoelectron spectroscopy, and show the development of a width-dependent semiconducting gap driven by quantum confinement effects, and an ideal one-dimensional electronic behaviour.
|
|
| 7. |
- Karakachian, Hrag, et al.
(författare)
-
Periodic Nanoarray of Graphene pn-Junctions on Silicon Carbide Obtained by Hydrogen Intercalation
- 2022
-
Ingår i: Advanced Functional Materials. - : Wiley-V C H Verlag GMBH. - 1616-301X .- 1616-3028. ; 32:18
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene pn-junctions offer a rich portfolio of intriguing physical phenomena. They stand as the potential building blocks for a broad spectrum of future technologies, ranging from electronic lenses analogous to metamaterials in optics, to high-performance photodetectors important for a variety of optoelectronic applications. The production of graphene pn-junctions and their precise structuring at the nanoscale remains to be a challenge. In this work, a scalable method for fabricating periodic nanoarrays of graphene pn-junctions on a technologically viable semiconducting SiC substrate is introduced. Via H-intercalation, 1D confined armchair graphene nanoribbons are transformed into a single 2D graphene sheet rolling over 6H-SiC mesa structures. Due to the different surface terminations of the basal and vicinal SiC planes constituting the mesa structures, different types of charge carriers are locally induced into the graphene layer. Using angle-resolved photoelectron spectroscopy, the electronic band structure of the two graphene regions are selectively measured, finding two symmetrically doped phases with p-type being located on the basal planes and n-type on the facets. The results demonstrate that through a careful structuring of the substrate, combined with H-intercalation, integrated networks of graphene pn-junctions could be engineered at the nanoscale, paving the way for the realization of novel optoelectronic device concepts.
|
|
| 8. |
- Marković, Igor, et al.
(författare)
-
Electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7
- 2020
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 117:27, s. 15524-15529
-
Tidskriftsartikel (refereegranskat)abstract
- The interplay between spin-orbit coupling and structural inversion symmetry breaking in solids has generated much interest due to the nontrivial spin and magnetic textures which can result. Such studies are typically focused on systems where large atomic number elements lead to strong spin-orbit coupling, in turn rendering electronic correlations weak. In contrast, here we investigate the temperature-dependent electronic structure of Ca3Ru2O7, a 4d oxide metal for which both correlations and spin-orbit coupling are pronounced and in which octahedral tilts and rotations combine to mediate both global and local inversion symmetry-breaking polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spinreorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridization mediated by a hidden Rashba-type spin- orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridization is actually the key driver for the phase transition, reflecting a delicate interplay between spin-orbit coupling and strong electronic correlations and revealing a route to control magnetic ordering in solids.
|
|
| 9. |
- Mazzola, Federico, et al.
(författare)
-
The sub-band structure of atomically sharp dopant profiles in silicon
- 2020
-
Ingår i: npj Quantum Materials. - : Springer Science and Business Media LLC. - 2397-4648. ; 5:1
-
Tidskriftsartikel (refereegranskat)abstract
- The downscaling of silicon-based structures and proto-devices has now reached the single-atom scale, representing an important milestone for the development of a silicon-based quantum computer. One especially notable platform for atomic-scale device fabrication is the so-called Si:P δ-layer, consisting of an ultra-dense and sharp layer of dopants within a semiconductor host. Whilst several alternatives exist, it is on the Si:P platform that many quantum proto-devices have been successfully demonstrated. Motivated by this, both calculations and experiments have been dedicated to understanding the electronic structure of the Si:P δ-layer platform. In this work, we use high-resolution angle-resolved photoemission spectroscopy to reveal the structure of the electronic states which exist because of the high dopant density of the Si:P δ-layer. In contrast to published theoretical work, we resolve three distinct bands, the most occupied of which shows a large anisotropy and significant deviation from simple parabolic behaviour. We investigate the possible origins of this fine structure, and conclude that it is primarily a consequence of the dielectric constant being large (ca. double that of bulk Si). Incorporating this factor into tight-binding calculations leads to a major revision of band structure; specifically, the existence of a third band, the separation of the bands, and the departure from purely parabolic behaviour. This new understanding of the band structure has important implications for quantum proto-devices which are built on the Si:P δ-layer platform.
|
|
| 10. |
- Mende, Max, et al.
(författare)
-
Strong Rashba Effect and Different f−d Hybridization Phenomena at the Surface of the Heavy-Fermion Superconductor CeIrIn5
- 2022
-
Ingår i: Advanced Electronic Materials. - : Wiley. - 2199-160X. ; 8:3
-
Tidskriftsartikel (refereegranskat)abstract
- New temperature scales and remarkable differences from bulk properties have increasingly placed the surfaces of strongly correlated f materials into the focus of research activities. Applying first-principles calculations and angle-resolved photoelectron spectroscopy measurements, a strong Rashba effect and spin-split surface states at the CeIn surface of the heavy-fermion superconductor CeIrIn5 are revealed. The unveiled 4f-derived electron landscape is remarkably distinct for surface and bulk Ce implying the existence of novel temperature scales near the surface region in this material. These results show that ab initio calculations can reliably predict the unusual electronic and spin structure of surfaces of strongly correlated 4f systems where Rashba spin-orbit-coupling phenomena emerge. It is suggested that the structural blocks of such materials can be combined with magnetically active layers for engineering of novel nanostructural objects with appropriate substrates where the diversity of f-driven properties can be applied for the development of novel functionalities.
|
|
