Electronic structure of topological semimetals

Topology, an important topic in physics since several years, is handled as possible solution to many current-state problems in electronics and energy. It could allow to dramatically shrink computational devices or increase their speed without the current problem of heat dissipation, or topological principles can be used to introduce room temperature high-conduction paths within materials. Unfortunately, while many promising materials have been presented yet, the one breakthrough material is still missing. Current style materials are either consisting of toxical elements, obstructing possible use cases, or their electronic structure is too complex to investigate the interplay of all the facets of the electronic structure present in the mateirals. In this thesis, two very promising materials will be thoroughly introduced, namely TaIrTe4 and GaGeTe. Both materials have the potential, to lift one of the shortcomings mentioned. First, TaIrTe4 will be presented. TaIrTe4 is a simplistic Weyl semimetal in terms of its electronic and topological structure - the simplest yet known material. It hosts four Weyl points, the minimum amount of Weyl nodes possible in a non-centrosymmetric material. Predictions state, that these nodes are well separated throughout the Brillouin zone, and are connected by nearly parallel Fermi arcs. The existance of the topological states is proved in this thesis through angle-resolved photoemission spectroscopy (ARPES) and confirmed by spin polarization measurements on these states. GaGeTe is predicted to be a Bi2Se3-style topological insulator, but ARPES data presented shows, that no direct band gap could be observed. Yet, a topological state is still believed to be present. This makes this material interesting in many ways: its elemental composition is less toxic than bismuth and selenium, as well as it is the first realization of such a specific electronic structure. A full discussion of the electronic states close to the Fermi level including the possible existance of topological states is shown in this thesis

WSO/UV: World Space Observatory/Ultraviolet

We summarize the capabilities of the World Space Observatory (UV) Project (WSO/UV). An example of the importance of this project (with a planned launch date of 2007/8) for the study of Classical Novae is given.Comment: 4 pages, To appear in the proceeedings of the "Classical Nova Explosions" conference, eds. M. Hernanz and J. Jose, AI

Publisher Correction: Turning charge-density waves into Cooper pairs (npj Quantum Materials, (2020), 5, 1, (22), 10.1038/s41535-020-0225-5)

[ no abstract available] Correction to: npj Quantum Materials https://doi.org/10.1038/s41535-020-0225-5, published online 14 April 202

Bond-strength inversion in (In,Ga)As semiconductor alloys

The atomic-scale structure and vibrational properties of semiconductor alloys are determined by the energy required for stretching and bending the individual bonds. Using temperature-dependent extended x-ray absorption fine-structure spectroscopy, we have determined the element-specific In-As and Ga-As effective bond-stretching force constants in (In,Ga)As as a function of the alloy composition. The results reveal a striking inversion of the bond strength where the originally stiffer bond in the parent materials becomes the softer bond in the alloy and vice versa. Our findings clearly demonstrate that changes of both the individual bond length and the surrounding matrix affect the bond-stretching force constants. We thus show that the previously used common assumptions about the element-specific force constants in semiconductor alloys do not reproduce the composition dependence determined experimentally for (In,Ga)As.The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. n.◦312284 (CALIPSO), from the Friedrich-Schiller-Universität Jena under the ProChance Initiative (2.11.3-A1/2012-01), and from the German Research Foundation (DFG) under Grant No. SCHN 1283/2-1

Time-reversal symmetry breaking type-II Weyl state in YbMnBi2

Spectroscopic detection of Dirac and Weyl fermions in real materials is vital for both, promising applications and fundamental bridge between high-energy and condensed-matter physics. While the presence of Dirac and noncentrosymmetric Weyl fermions is well established in many materials, the magnetic Weyl semimetals still escape direct experimental detection. In order to find a time-reversal symmetry breaking Weyl state we design two materials and present here experimental and theoretical evidence of realization of such a state in one of them, YbMnBi2. We model the time-reversal symmetry breaking observed by magnetization and magneto-optical microscopy measurements by canted antiferromagnetism and find a number of Weyl points. Using angle-resolved photoemission, we directly observe two pairs of Weyl points connected by the Fermi arcs. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties

Time-reversal symmetry breaking type-II Weyl state in YbMnBi2YbMnBi_{2}

Spectroscopic detection of Dirac and Weyl fermions in real materials is vital for both, promising applications and fundamental bridge between high-energy and condensed-matter physics. While the presence of Dirac and noncentrosymmetric Weyl fermions is well established in many materials, the magnetic Weyl semimetals still escape direct experimental detection. In order to find a time-reversal symmetry breaking Weyl state we design two materials and present here experimental and theoretical evidence of realization of such a state in one of them, YbMnBi2. We model the time-reversal symmetry breaking observed by magnetization and magneto-optical microscopy measurements by canted antiferromagnetism and find a number of Weyl points. Using angle-resolved photoemission, we directly observe two pairs of Weyl points connected by the Fermi arcs. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties

Possible experimental realization of a basic Z(2) topological semimetal in GaGeTe

We report experimental and theoretical evidence that GaGeTe is a basic Z2 topological semimetal with three types of charge carriers: bulkoriginated electrons and holes as well as surface state electrons. This electronic situation is qualitatively similar to the classic 3D topological insulator Bi2Se3, but important differences account for an unprecedented transport scenario in GaGeTe. High-resolution angle-resolved photoemission spectroscopy combined with advanced band structure calculations show a small indirect energy gap caused by a peculiar band inversion at the T-point of the Brillouin zone in GaGeTe. An energy overlap of the valence and conduction bands brings both electron and holelike carriers to the Fermi level, while the momentum gap between the corresponding dispersions remains finite. We argue that peculiarities of the electronic spectrum of GaGeTe have a fundamental importance for the physics of topological matter and may boost the material’s application potentialThis work was supported by DFG under the Grant No. BO 1912/7-1 as well as Grant Nos. IS 250/2-1 and PF 324/4-1 of the SPP 1666 program, Grant No. RU 776/15-1 of the ERANET-Chemistry program, Tomsk State University Project No. 8.1.01.2018, and St. Petersburg University Project No. 15.61.202.2015. Calculations were performed at the SKIF-Cyberia supercomputer of Tomsk State University and supported by RSF, grant No. 18-12-00169. The authors acknowledge Diamond Light Source for the beamtime at I05 beamline under Proposal No. SI18586 as well as the BESSY II Berlin for the beamtime at 13 ARPES station under Proposal Nos. 171-05051CR and 172-05659CR/R. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement No. 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. S.B. is grateful to BMWi Germany for the support within the framework of WIPANOprogram (Grant No. N 03THW12H04). S.B. is also grateful to BMBF Germany for the support within the framework of Ukrainian-German Excellence Center UKRATOP project