MoTe2: A Type-II Weyl Topological Metal

Based on the ab initio calculations, we show that MoTe2, in its low-temperature orthorhombic structure characterized by an X-ray diffraction study at 100 K, realizes 4 type-II Weyl points between the N-th and N+1-th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2 - the smallest number of arcs consistent with time-reversal symmetry.Comment: Published versio

Automated construction of symmetrized Wannier-like tight-binding models from ab initio calculations

Wannier tight-binding models are effective models constructed from first-principles calculations. As such, they bridge a gap between the accuracy of first-principles calculations and the computational simplicity of effective models. In this work, we extend the existing methodology of creating Wannier tight-binding models from first-principles calculations by introducing the symmetrization post-processing step, which enables the production of Wannier-like models that respect the symmetries of the considered crystal. Furthermore, we implement automatic workflows, which allow for producing a large number of tight-binding models for large classes of chemically and structurally similar compounds, or materials subject to external influence such as strain. As a particular illustration, these workflows are applied to strained III-V semiconductor materials. These results can be used for further study of topological phase transitions in III-V quantum wells

Automated construction of symmetrized Wannier-like tight-binding models from ab initio calculations

Wannier tight-binding models are effective models constructed from first-principles calculations. As such, they bridge a gap between the accuracy of first-principles calculations and the computational simplicity of effective models. In this work, we extend the existing methodology of creating Wannier tight-binding models from first-principles calculations by introducing the symmetrization post-processing step, which enables the production of Wannier-like models that respect the symmetries of the considered crystal. Furthermore, we implement automatic workflows, which allow for producing a large number of tight-binding models for large classes of chemically and structurally similar compounds or materials subject to external influence such as strain. As a particular illustration, these workflows are applied to strained III-V semiconductor materials. These results can be used for further study of topological phase transitions in III-V quantum wells

Wannier90 as a community code: new features and applications

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.The WDG acknowledges financial support from the NCCR MARVEL of the Swiss National Science Foundation, the European Union’s Centre of Excellence E-CAM (Grant No. 676531), and the Thomas Young Centre for Theory and Simulation of Materials (Grant No. TYC-101).Peer reviewe

Common workflows for computing material properties using different quantum engines

The prediction of material properties based on density-functional theory has become routinely common, thanks, in part, to the steady increase in the number and robustness of available simulation packages. This plurality of codes and methods is both a boon and a burden. While providing great opportunities for cross-verification, these packages adopt different methods, algorithms, and paradigms, making it challenging to choose, master, and efficiently use them. We demonstrate how developing common interfaces for workflows that automatically compute material properties greatly simplifies interoperability and cross-verification. We introduce design rules for reusable, code-agnostic, workflow interfaces to compute well-defined material properties, which we implement for eleven quantum engines and use to compute various material properties. Each implementation encodes carefully selected simulation parameters and workflow logic, making the implementer’s expertise of the quantum engine directly available to non-experts. All workflows are made available as open-source and full reproducibility of the workflows is guaranteed through the use of the AiiDA infrastructure.This work is supported by the MARVEL National Centre of Competence in Research (NCCR) funded by the Swiss National Science Foundation (grant agreement ID 51NF40-182892) and by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 824143 (European MaX Centre of Excellence “Materials design at the Exascale”) and Grant Agreement No. 814487 (INTERSECT project). We thank M. Giantomassi and J.-M. Beuken for their contributions in adding support for PseudoDojo tables to the aiida-pseudo (https://github.com/aiidateam/aiida-pseudo) plugin. We also thank X. Gonze, M. Giantomassi, M. Probert, C. Pickard, P. Hasnip, J. Hutter, M. Iannuzzi, D. Wortmann, S. Blügel, J. Hess, F. Neese, and P. Delugas for providing useful feedback on the various quantum engine implementations. S.P. acknowledges support from the European Unions Horizon 2020 Research and Innovation Programme, under the Marie Skłodowska-Curie Grant Agreement SELPH2D No. 839217 and computer time provided by the PRACE-21 resources MareNostrum at BSC-CNS. E.F.-L. acknowledges the support of the Norwegian Research Council (project number 262339) and computational resources provided by Sigma2. P.Z.-P. thanks to the Faraday Institution CATMAT project (EP/S003053/1, FIRG016) for financial support. KE acknowledges the Swiss National Science Foundation (grant number 200020-182015). G.Pi. and K.E. acknowledge the swissuniversities “Materials Cloud” (project number 201-003). Work at ICMAB is supported by the Severo Ochoa Centers of Excellence Program (MICINN CEX2019-000917-S), by PGC2018-096955-B-C44 (MCIU/AEI/FEDER, UE), and by GenCat 2017SGR1506. B.Z. thanks to the Faraday Institution FutureCat project (EP/S003053/1, FIRG017) for financial support. J.B. and V.T. acknowledge support by the Joint Lab Virtual Materials Design (JLVMD) of the Forschungszentrum Jülich.Peer reviewe

Identifying topological states in matter

The rise of topological insulators, semimetals and superconductors established the topology of the electronic band structure as a fundamental material property. Topological materials can realize exotic novel quantum states such as an integer quantum Hall state in the absence of an external magnetic field, quasiparticle states needed for topological quantum computing, and many more. The topological nature of these states makes them insensitive to small perturbations, which has profound practical consequences. Consequently, the ability to reliably identify topological states is crucial in understanding and predicting many physical effects. In this work, we propose a general approach for calculating any topological invariant, based on the charge centers of hybrid Wannier functions. The method is illustrated in the context of Chern insulators, Z 2 topological insulators and Weyl semi- metals. Most importantly, we present Z2Pack, an easy-to-use software implementing this technique. It can be used as a post-processing tool for first-principles calculations or as a standalone package for tight-binding or k.p models. The fully automated calculation of topological invariants makes Z2Pack ideally suited for both the search for topological states of matter in existing materials and the design of materials or heterostructures with desirable topology

Identifying Topological Semimetals

Geometric properties of electron states in crystalline solids lead to a topological classification of materials. A remarkable consequence of this topological viewpoint is that it reveals a deep link between the bulk properties of a material and electronic states which form on its surface. This leads to unique transport properties, the most well-known example being the integer quantum Hall effect. In topological semimetals, the bulk features of interest are nodes in the band structure, where occupied and unoccupied states are not separated by an energy gap. This leads to interesting low-energy excitations, some of which are the condensed matter equivalent of fundamental particles. The Weyl Fermion for example is realized in topological semimetals, which is theoretically postulated but eludes experimental verification in high-energy physics. Crystals however do not have a continuous translational symmetry, and thus do not need to fulfill the so-called Lorentz invariance present in high-energy physics. This allows for Fermions to exist in materials which do not have a fundamental counterpart. The main topic of this thesis is the study and identification of topological semimetals. We propose a mechanism for Weyl Fermions to form under the influence of an external magnetic field. This effect could help explain the anisotropic negative magnetoresistance in transition metal dipnictides. We also study several novel topological material candidates, hosting a plethora of Weyl Fermions and topological nodal lines. In addition to studying specific material examples, we also present several tools and algorithms which enhance the process of identifying topological materials. First, we present an algorithm for evaluating the phase diagram of a system with discrete phases. This is useful in identifying topological phases, but also applicable to other fields of computational physics. Furthermore, we develop tools that simplify the creation of k·p and tight-binding models to study crystalline systems. A particular focus lies on the construction of models which preserve the crystal symmetries, since these play a crucial role in determining the topology of a material. And finally, we develop an algorithm that reliably finds and classifies topological nodal features