Highlights

Understanding at the atomistic level how the properties of transition-metal elements lead to efficient electrochemical processes is critically important in the development of new cathode materials for lithium-ion batteries. While density-functional theory (DFT) calculations with local and semilocal exchange-correlation functionals can play an important role in producing first-principles predictions for these materials, they can also produce unsatisfactory results because of self-interaction errors. In the paper “Accurate Electronic Properties and Intercalation Voltages of Olivine-Type Li-Ion Cathode Materials from Extended Hubbard Functionals,” recently published in Physical Review X Energy, NCCR MARVEL researchers and colleagues carried out a comparative study of four electronic-structure methods for selected olivine-type cathode materials. They found that the DFT+U+V method clearly outperforms the others and is able to describe the interactions accurately, opening the door for the study of more complex cathode materials as well as for a reliable exploration of the chemical space of compounds for Li-ion batteries.

Simone Gallarati and a team led by Professor Clemence Corminboeuf, head of the Laboratory for Computational Molecular Design at EPFL, have created a dataset containing thousands of organic molecules that have been mined from the literature and from the Cambridge Crystallographic Database, and enriched with species generated in a combinatorial fashion by recombining molecular building blocks.  The result is a map that could help researchers navigate organocatalyst space and enable informed catalyst design. It’s also the starting point for a multitude of fragment-based reaction optimization methods. 

Zeolites are nanoporous frameworks that can be applied in a number of industrial processes, particularly in separation and catalysis. While much effort has been put into identifying and synthesizing new zeolite structures, success has been largely theoretical. While massive databases of hypothetical zeolites have been generated, containing millions of new framework structures, none has been made in the lab. In the paper “Ranking the synthesizability of hypothetical zeolites with the sorting hat,” NCCR MARVEL’s Michele Ceriotti, EPFL professor and head of the School of Engineering’s Laboratory of Computational Science and Modelling, and colleagues present a data-driven procedure for distinguishing known structures from hypothetical ones and categorizing them into compositional classes. The approach helps identify promising synthetic candidates and suggests likely chemical compositions, providing a sort of recipe for materials chemists.

A paper describing the Open Software Services for Classrooms and Research (OSSCAR) platform has recently been published in the journal Computer Physics Communications. Developed through a collaboration between NCCR MARVEL and the Centre Européen de Calcul Atomique et Moléculaire (CECAM), OSSCAR provides an open collaborative environment for developing and accessing educational resources through web applications. Associated tools are easy to use, and create a uniform and open environment that can be used by a large academic community and are meant to facilitate learning and help people avoid duplicating efforts in the creation of teaching material. Contributions to expand the educational content of the OSSCAR project are welcome.

Polarons can affect numerous phenomena in a material but have been difficult to model correctly. In the paper “Many-Body Self-Interaction and Polarons,” recently published jointly in Physical Review Letters and in Physical Review B as an Editor’s Suggestion, researchers Stefano Falletta and Professor Alfredo Pasquarello of the Chair of Atomic Scale Simulation at EPFL advance the conceptual understanding of the self-interaction problem in density functional theory, paving the way to efficient calculations of polarons in large systems, in systematic studies involving large sets of materials, and in molecular dynamics evolving over long time periods.

Transition-metal-based “kagome” materials, referred to as such because of the resemblance of their structure to a Japanese woven bamboo pattern, are rich platforms for investigating novel topological electronic states and correlated phenomena. Until now, topologically nontrivial Dirac surface states (TDSSs) have not been definitively observed in the idealized 2D kagome lattice, nor has the question of whether such states or van Hove singularities (VHSs) can be tuned been explored extensively. In the paper “Tunable Topological Dirac Surface States and Van Hove Singularities in Kagome Metal GdV6Sn6,” recently published in Science Advances, researchers led by Professor Ming Shi at the Paul Scherrer Institute address both topics, identifying and manipulating these features in the magnetic kagome material GdV6Sn6. The results establish the material as an excellent platform for studying the nontrivial topology, magnetism and correlation effects and open a new avenue of potential development for use in spintronic devices.

Applying quantum chemistry (QC) approaches to the high-throughput screening of crystallographic data repositories could enable huge advances in molecular and materials design. The challenge, however, particularly in transition metal complexes, is to retrieve all the information needed to perform QC computations. In the paper "cell2mol: encoding chemistry to interpret crystallographic data," recently published in Nature Publishing's journal Computational Materials, Sergi Vela and the team led by Professor Clémence Corminboeuf, head of the Computational Molecular Design Laboratory at EPFL, present a novel solution. A fully automatic pipeline for characterizing molecular crystals, cell2mol greatly simplifies the construction of QC-ready datasets. The authors show that the code can interpret the large chemical diversity and structural complexity contained in crystallographic repositories by simply encoding the chemist's view when visualizing a crystal structure. Under this perspective, the unit cell is understood by the hierarchy of molecular fragments, and their Lewis structures and formal charges. The code, as well as reliable QC-ready databases of transition metals with incomparable chemical diversity, are now available. 

Quantum spin Hall insulators (QSHI) could be used to power several applications, but many would rely on the possibility of switching between a topological and a trivial insulating phase. This control is typically achieved through strain or electric fields, which require energy consumption to be maintained. Developing an approach that allows the material to stay in the topological or trivial state even after the field is removed would be a huge boon to the development of such devices. In the paper “Twist-resilient and robust ferroelectric quantum spin Hall insulators driven by van der Waals interactions,” recently published in npj 2D Materials and Applications, researchers Dr. Antimo Marrazzo and Dr. Marco Gibertini, both formerly of NCCR MARVEL, and now in Italy at the Universities of Trieste and Modena, respectively, propose combining a ferroelectric layer with another two-dimensional (2D) trivial insulator to achieve this end. They give the example of a heterostructure made of a well-known ferroelectric material, In2Se3, and the easily exfoliable trivial insulator, CuI. They show that the approach is indeed a legitimate   strategy for engineering non-volatile ferroelectric control of topological order in 2D heterostructures, and also that the effect is robust and can survive up to room temperature.

A team led by Philipp Werner, professor of physics at the University of Fribourg and leader of NCCR MARVEL’s Phase 3 project Continued Support, Advanced Simulation Methods, has applied their advanced quantum simulation method to the investigation of the complex material 1T–TaS2. The research, recently published in Physical Review Letters, helped resolve a conflict between earlier experimental and theoretical results, showing that the surface region of 1T–TaS2 exhibits a nontrivial interplay between band insulating and Mott insulating behavior when the material is cooled to below 180 K. 

Addressing increased need for more accurate predictions of materials spectra and thermodynamics, NCCR MARVEL researchers Tommaso Chiarotti and Nicola Marzari, and colleague Andrea Ferretti, introduce a treatment of frequency dependence allowing for the algorithmic inversion method on sum over poles (AIM-SOP) for solving Dyson-like equations and handling frequency-dependent quantities in dynamical theories. Specializing to the case of many-body perturbation theory applied to the homogeneous electron gas, they show that the AIM-SOP approach provides a unified formalism for generating the spectral and thermodynamic properties of an interacting-electron system. The authors are now working on generalizing the AIM-SOP method to non-homogeneous crystalline systems.  

The recently discovered layered kagome metals AV3Sb5 (A=K, Rb, Cs) exhibit diverse correlated phenomena, thought to be linked to so-called Van Hove singularities (VHSs) in the material. Using a combination of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory, researchers led by NCCR MARVEL’s Professor Ming Shi at the Paul Scherrer Institute directly revealed the sublattice properties of 3d-orbital VHSs in CsV3Sb5. The research reveals important insights into the material’s electronic structure and provides a basis for understanding correlation phenomena in the metals.

Phonon-assisted luminescence is a key property of defect centers in semiconductors. It can be measured to perform the readout of the information stored in a quantum bit or used to detect temperature variations. The investigation of phonon-assisted luminescence is now generally carried out through models that incorporate restrictive assumptions and so fail to be predictive. The paper “Phonon-assisted luminescence in defect centers from many-body perturbation theory,” recently published in Physical Review Letters by researchers led by NCCR MARVEL’s Prof. Nicola Marzari and PhD student Francesco Libbi of EPFL’s Theory and Simulation of Materials laboratory, outlines a novel approach to predicting luminescence and studying exciton-phonon couplings with a many-body perturbation theory framework, an analysis that has never been performed for defect centers.