| 1. |
|
|
| 2. |
- Agåker, Marcus, et al.
(författare)
-
A five-axis parallel kinematic mirror unit for soft X-ray beamlines at MAX IV
- 2020
-
Ingår i: Journal of Synchrotron Radiation. - : INT UNION CRYSTALLOGRAPHY. - 0909-0495 .- 1600-5775. ; 27, s. 262-271
-
Tidskriftsartikel (refereegranskat)abstract
- With the introduction of the multi-bend achromats in the new fourth-generation storage rings the emittance has decreased by an order of magnitude resulting in increased brightness. However, the higher brightness comes with smaller beam sizes and narrower radiation cones. As a consequence, the requirements on mechanical stability regarding the beamline components increases. Here an innovative five-axis parallel kinematic mirror unit for use with soft X-ray beamlines using off-axis grazing-incidence optics is presented. Using simulations and measurements from the HIPPIE beamline at the MAX IV Laboratory it is shown that it has no Eigen frequencies below 90 Hz. Its positioning accuracy is better than 25 nm linearly and 17-35 mu rad angularly depending on the mirror chamber dimensions.
|
|
| 3. |
- Andersen, Jesper N, et al.
(författare)
-
Strong Phonon Replicas in Be 1 s Photoemission Spectra
- 2001
-
Ingår i: Physical Review Letters. - 1079-7114. ; 86:19, s. 4398-4401
-
Tidskriftsartikel (refereegranskat)abstract
- The Be 1s core level photoemission line from metallic Be is shown to contain unexpected internal fine structure. We argue that this fine structure is caused by intrinsic excitation of a narrow band of optical phonons in the 1s photoemission process. The general importance of the present results for high resolution core level photoemission investigations of metals is pointed out.
|
|
| 4. |
- Anderson, Jonathan, 1975, et al.
(författare)
-
Thin graphite overlayers: Graphene and alkali metal intercalation
- 2007
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 601:4, s. 1167-1175
-
Tidskriftsartikel (refereegranskat)abstract
- Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the pi states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage I compounds C8K or CgRb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 x 2) ordered alkali metal layer adjacent to the uppermost carbon layer. (c) Elsevier B.V. All rights reserved.
|
|
| 5. |
- 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.
|
|
| 6. |
- Bentmann, H., et al.
(författare)
-
Strong Linear Dichroism in Spin-Polarized Photoemission from Spin-Orbit-Coupled Surface States
- 2017
-
Ingår i: Physical Review Letters. - 0031-9007. ; 119:10
-
Tidskriftsartikel (refereegranskat)abstract
- A comprehensive understanding of spin-polarized photoemission is crucial for accessing the electronic structure of spin-orbit coupled materials. Yet, the impact of the final state in the photoemission process on the photoelectron spin has been difficult to assess in these systems. We present experiments for the spin-orbit split states in a Bi-Ag surface alloy showing that the alteration of the final state with energy may cause a complete reversal of the photoelectron spin polarization. We explain the effect on the basis of ab initio one-step photoemission theory and describe how it originates from linear dichroism in the angular distribution of photoelectrons. Our analysis shows that the modulated photoelectron spin polarization reflects the intrinsic spin density of the surface state being sampled differently depending on the final state, and it indicates linear dichroism as a natural probe of spin-orbit coupling at surfaces.
|
|
| 7. |
- Bianchi, M., et al.
(författare)
-
One-dimensional spin texture of Bi(441): Quantum spin Hall properties without a topological insulator
- 2015
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:16
-
Tidskriftsartikel (refereegranskat)abstract
- The high index (441) surface of bismuth has been studied using scanning tunneling microscopy (STM), angle resolved photoemission spectroscopy (APRES), and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localized states are observed, both of which are linearly dispersing in one in-plane direction (k(x)), and dispersionless in the orthogonal in-plane direction (k(y)), and both of which have a Dirac-like crossing at k(x) = 0. Spin ARPES reveals a strong in-plane polarization, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its possible origin as an edge state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one-dimensional quantum spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk.
|
|
| 8. |
- Curcio, Davide, et al.
(författare)
-
Current-driven insulator-to-metal transition without Mott breakdown in Ca2RuO4
- 2023
-
Ingår i: Physical Review B. - 2469-9950. ; 108:16
-
Tidskriftsartikel (refereegranskat)abstract
- The electrical control of a material's conductivity is at the heart of modern electronics. Conventionally, this control is achieved by tuning the density of mobile charge carriers. A completely different approach is possible in Mott insulators such as Ca2RuO4, where an insulator-to-metal transition (IMT) can be induced by a weak electric field or current. While the driving force of the IMT is poorly understood, it has been thought to be a breakdown of the Mott state. Using in operando angle-resolved photoemission spectroscopy, we show that this is not the case: The current-induced conductivity is caused by the formation of in-gap states with only a minor reorganization of the Mott state. Electronic structure calculations show that these in-gap states form at the boundaries of structural domains that emerge during the IMT. At such boundaries, the overall gap is drastically reduced, even if the structural difference between the domains is small and the individual domains retain their Mott character. The inhomogeneity of the sample is thus key to understanding the IMT, as it leads to a nonequilibrium semimetallic state that forms at the interface of Mott domains.
|
|
| 9. |
- Dahl, J., et al.
(författare)
-
Formation of polar InN with surface Fermi level near the valence band maximum by means of ammonia nitridation
- 2012
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 86:24
-
Tidskriftsartikel (refereegranskat)abstract
- Development of InN films for devices is hindered due to metallic In clusters, formed readily during growth, and unintentional n-type conductivity of the nominally undoped films, including surface electron-accumulation layers via the Fermi level pinning into the conduction band. Plasma nitridation eliminates even large In clusters from the surface by changing them to two-dimensional InN [Yamaguchi and Nanishi, Appl. Phys. Expr. 2, 051001 (2009)]. Here we utilized a similar approach, that is, nitridation of In-covered surfaces with ammonia (NH3) to grow thin, up to 25 nm thick polar InN films on Si(111) and GaN(0001) substrates. By means of scanning tunneling microscopy and spectroscopy, as well as photoelectron spectroscopy, we show that this simple NH3 nitridation provides the hitherto not reported formation of polar InN(000-1) films with the surface Fermi level close to the valence band maximum, as recent calculations [Belabbes et al., Phys. Rev. B 84, 205304 (2011)] predict. DOI: 10.1103/PhysRevB.86.245304
|
|
| 10. |
- Dziawa, P., et al.
(författare)
-
Topological crystalline insulator states in Pb1-xSnxSe
- 2012
-
Ingår i: Nature Materials. - 1476-1122 .- 1476-4660. ; 11:12, s. 1023-1027
-
Tidskriftsartikel (refereegranskat)abstract
- Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection(1,2). In this study we show that the narrow-gap semiconductor Pb1-xSnxSe is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.
|
|
