• Polarons free from many-body self-interaction in density functional theory

    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.

  • Researchers identify, tune novel electronic states in kagome metal GdV6Sn6

    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.

  • cell2mol: a boon to the use of crystallographic repositories in molecular and materials design

    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. 

  • Simulations reveal non-volatile means of topological order control in 2D heterostructures

    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.

  • Novel quantum simulation method clarifies correlated properties of complex material 1T–TaS2

    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. 

  • New approach for a unified formalism for spectral and thermodynamic properties of interacting-electron systems 

    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.  

  • Electronic structure study of AV3Sb5 kagome metals bolsters understanding of correlated phenomena

    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.

  • Predicting the optical read-out of a qubit from first principles

    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.

  • Scientists propose open platform for managing data from chemical research

    EPFL scientists including NCCR MARVEL's deputy director Berend Smit have proposed an open platform for managing the vast amounts of diverse data produced in chemical research. They presented their vision in a paper recently published in Nature Chemistry. The article below was written by EPFL journalist Nik Papageorgiou. 

  • Researchers identify new paraelectric phase prototypes for use in computational engineering of functional materials

    A deeper investigation into the microscopic picture of the BaTiO3 perovskite revealed the persistence of intrinsic off-centerings in its cubic paraelectric phase. Interestingly, this feature is inconsistent with the space group often used to atomistically model this phase using density-functional theory (DFT) or similar methods. This prompted researchers, led by NCCR MARVEL’s Giovanni Pizzi and Michele Kotiuga at Professor Nicola Marzari’s Theory and simulation of materials laboratory at EPFL, to use a systematic symmetry analysis to construct representative structural models, supercells that satisfy a desired point symmetry but are built from a combination of lower-symmetry primitive cells. In this way, they identified two 40-atom structural prototypes—the smallest of the representative models that are both energetically and dynamically stable—for paraelectric BaTiO3. These prototypes, which are also common to many other ABO3 perovskites, offer structural models of paraelectric phases for use in the computational engineering of functional materials. The paper was recently published in Physical Review Research.

  • New paper describes on-surface synthesis and characterization of nitrogen-substituted undecacenes

    Heteroatom substitution—the replacement of carbon atoms with others such as sulfur and nitrogen—can be used to tailor the electronic, magnetic, and physico-chemical properties of acenes, a class of organic compounds and polycyclic aromatic hydrocarbons made up of linearly fused benzene rings. With potential applications in organic electronics, this fine-tuning could allow for precise engineering and property optimization. Synthesizing and characterizing larger acenes has, however, remained a challenge for solution chemistry because they feature low solubility and high reactivity. In the paper On-surface synthesis and characterization of nitrogen-substituted undecacenes, recently published in Nature Communications, NCCR MARVEL affiliated scientists and colleagues addressed this by demonstrating first on-surface synthesis of three undecacene analogs substituted with four nitrogen atoms on an Au(111) substrate and then characterizing the materials using various techniques. The work paves the way towards the precise fabrication of nitrogen-substituted acenes and their analogs, potential building-blocks of organic electronics and spintronics. 

  • ARPES gives first observation of dispersive excitons in a low-dimensional metallic system

    Already used in some solar cells, excitons, with their charge neutrality and expected mobility, have also been proposed as potential transmitters of quantum information. In either use, it’s essential to understand how and why these quasiparticles move. Investigations into exciton mobility have been inaccessible to traditional optical experiments though because they only create and detect excitons with negligible momentum. Now, using angle-resolved photoemission spectroscopy (ARPES), scientists led by NCCR MARVEL’s Professor Ming Shi at the Paul Scherrer Institute have detected several types of mobile excitons in the quasi-one-dimensional metallic trichalcogenide, TaSe3. They’ve also shown that certain exciton properties can be tuned by surface doping. The paper was recently published in Nature Materials.