Skip to main content

Topological Materials: New Fermions, Realization of Single Crystals and their Physical Properties

Periodic Reporting for period 3 - TOPMAT (Topological Materials: New Fermions, Realization of Single Crystals and their Physical Properties)

Reporting period: 2020-07-01 to 2021-12-31

Topology, a mathematical concept, recently became a hot and truly transdisciplinary topic in condensed matter physics, solid state chemistry and materials science. Since there is a direct connection between real space: atoms, valence electrons, bonds and orbitals, and reciprocal space: bands, Fermi surfaces and Berry curvature, a simple classification of topological materials in a single particle picture should be possible. Moreover, the properties of single crystals with particular topological electronic structures can mimic phenomena found in high energy physics and cosmology. New classes of quantum materials are found in insulators and semimetals that exhibit non-trivial topologies: they display a plethora of novel phenomena including: topological surface states; new Fermions such as Weyl, Dirac or Majorana; and non-collinear spin textures such as Skyrmions. A hallmark of many of these new quantum properties that are derived from fundamental symmetries of the bulk, is that they are topologically protected. A general scheme to identify all inorganic topological materials including novel Fermions was proposed that is based on the symmetries and the Wyckhoff positions of relevant space groups. The translation of these theoretical concepts into realizable materials is one focus of this project. The next steps are to apply this approach to magnetic space groups so as to identify new chiral or magnetic Fermions. A systematic investigation with the related methods was applied to all known antiferromagnetic materials by Bernevig et al. and the PI. For ferromagnetic materials the determination of the topological properties is straight forward, based on the Berry curvature. A series of chiral crystals were synthesized, in case of PdGa even with crystals of both chiralities with the goal to see the relation between chirality of different surfaces to the handedness of the crystal. The PI and her team have already synthesized more than 100 different topological materials as high-quality single crystals, see list on the TOPMAT webpage. The properties were characterized by applying high magnetic fields and high pressure, to tune topological phase transitions, electrical transport properties and surface states. Particularly high-pressure Hall measurements have been developed. Hall measurements allow the investigation of the fundamental electrical transport properties of topological materials such as their carrier densities, oscillation frequencies, mobilities, and anomalous and topological Hall effects. Despite of the high impact for basic science and for condensed matter science, reflected by the high impact publications, the relativistic topological materials can serve as model systems for high energy physics and astrophysics via quasiparticles as models for axions, gravitational anomaly etc.. Another new direction is the use of the topological materials for redox catalysis and for high efficient energy conversion. A series of patents is filed in this context.
• All existing inorganic compounds (based on the symmetries and the Wyckhoff positions of relevant space groups) are theoretically classified into topological or trivial – prediction of new topological compounds including novel Fermions, Nature 2019 (Objective 1: Concepts to materials)
• All centrosymmetric ferromagnets with crossing bands around the Fermi energy are Weyl semimetals or Weyl metals, all ferro- and ferrimagnetic cubic Heusler compounds were investigated and the compounds with large Nernst and Hall effect will be synthesized in next year
• A giant Anomalous Hall Effect was identified in Co3Sn2S2 (Nature Physics 2018) and Co2MnGa (NPG Materials Asia 2019, Phys. Rev. B 2019),(Objective 2c: Hall measurements under ambient conditions).
• Magnetic Weyl materials for high temperature QAH were identified including Co3Sn2S2 and MnAlGe.
• All these quasi 2D magnets are showing an Anomalous Hall effect per magnetic layer which is close to the Quantum Hall effect up to reasonable high temperatures. Thin films of MnAlGe are planned.
• All antiferromagnet compounds with a known antiferromagnetic structure are theoretically classified into topological or trivial – based on general symmetry consideration – new single crystals will be synthesized in the next year.
• Recently a 3D Quantum Hall Effect was realized by others in ZrTe5, we reproduced the results in ultraclean HfTe5 and have observed even a fractional QHE. Also in the Heusler compound YPtBi, we have strongly reduced the defect density to realize a zero Landau level already at low magnetic fields (Objective 2.c: Growth of single crystals with low defect density and tuned Fermi energy, Science under review).
• Chiral new Fermions CoSi, PdGa and PtAl were synthesized and the predicted large chiral Fermi arcs were observed in ARPES and STM (Objectives 1 and 2b, Nature, Nature Physics 2019).
• Compounds with new Fermions were synthesized and characterized with giant chiral Fermi arcs and tested for catalysis.
• Axion as a quasiparticle was identified for the first time in Ta2Se8I– Nature 2019.
• New Skyrmion materials were identified in Mn2RhSn with a much lower moment than the anti-Skyrmion Mn2PtSn and in MnPtGa with a different crystal structure and composition.
• A high-pressure Hall measurement set up has been developed. Hall measurements under pressure were used to measure the phase diagram of the axion material Ta2Se8I (Objective 3: Electronic properties including high pressure Hall measurements).
• Further investigation of topological materials under pressure are on the way. A single crystal platform with more than 250 compounds has been established within the project (see TOPMAT webpage, Objective 4: Platform for high quality single crystals of topological materials) and more than 100 samples were sent to collaborators in Germany, Europe and worldwide.
Surprisingly all Objectives up to the month 24 are fulfilled.
The next step is the investigation of the interface between different chiralities. PdGa has the same space group as the Skyrmion crystals as for example FeGe, which are magnetic. Skyrmions in Heusler compounds and in materials with B20 structure are also available and under investigation.
Two crystals grown by the Laser Otical Floating Zone method within the TOPMAT project