MARVEL Junior Seminar — December 2018

The MARVEL Junior Seminars aim at intensifying interactions between the MARVEL Junior scientists belonging to different research groups located at EPFL. The EPFL community interested in MARVEL research topics is very welcome to attend. We believe that these events will be central for establishing a vibrant community.

Each seminar consists of two presentations of 25 minutes each, allowing to present on a scientific question in depth, followed by time for discussion. The discussion is facilitated and timed by the chair.

Pizza is served at 11:45 in front of the auditorium (note that it will take place in ELA2 this time!), and after the seminar at 13:30 you are cordially invited for coffee and dessert to continue discussion with the speakers.

MARVEL Junior Seminar Organizing Committee — Francesco Ambrosio, Davide Campi, Edgar Engel, Gloria Capano, Michele Pizzochero, Kun-Han Lin, Francesco Maresca and Patrick Mayor

Check the list of the next MARVEL Junior Seminars here.

Abstract — Free Energies from Density Functional Theory Calculations

Patrick Gono, Francesco Ambrosio, Julia Wiktor, Assil Bouzid, Alfredo Pasquarello 

Chair of atomic scale simulation (CSEA), Institute of Physics, EPFL 

The oxygen evolution reaction (OER) is often limiting the efficiency of photocatalytic water splitting, a promising source of clean hydrogen fuel. Catalytic efficiency depends exponentially on the free energy steps and barriers associated with the reaction. Therefore, theoretical studies aim for extreme precision when describing these. The OER can be formulated as a series of four proton-coupled electron transfer steps. The free energies associated with these hydrogen removals are strongly dependent on the structure of the catalyst interface, as well as that of the solvent. In this talk, I will address three methods that enable us to calculate free energies from density functional theory simulations. The application of these methods will be demonstrated on the solvated rutile TiO2 interface, a benchmark material often studied in relation with photocatalysis. In the collaborative spirit of the MARVEL project, the talk will aim to provide a tutorial, a presentation of the methods themselves, rather than just the results of our research.

Abstract — First-principle prediction of a Dirac semimetal and its topological phase transition

QuanSheng Wu^{1, 2}, Christophe Piveteau², Zhida Song³ and Oleg V. Yazyev¹ 

¹ Chair of computational condensed matter physics (C3MP), Institute of Physics, EPFL

² Institut für Theoretische Physik, ETHZ

³ Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Science, Beijing, China

Three-dimensional topological Dirac semimetals [1] are materials realizing a novel state of quantum matter described by the massless Dirac equation. The four-component Dirac spinor is composed of two two-component Weyl fermions of opposite chirality. Although the degeneracy of the Dirac point (DP) is not protected in topological sense due to its zero net Chern number, it can still be protected by the space group symmetries, e.g. C4v, C6v and non-symmorphic symmetries,  In this talk, I will show a new predicted Dirac semimetal MgTa2N3 [2] based on first-principles calculations and symmetry analysis. In this material, the Fermi level is located exactly at the Dirac point without additional Fermi surface pockets. The band inversion associated with the Dirac cone involves the d orbitals of two structurally inequivalent Ta atoms with octahedral and trigonal prismatic coordination spheres. We further show that the lattice symmetry breaking can realize topological phase transitions from the Dirac semimetal phase to a triple nodal point semimetal, Weyl semimetal or topological insulator. The topologically protected surface states and the non-protected Fermi arc surface states are also studied. In the end, I will briefly introduced an open-source software WannierTools [3] which is very useful for the study of topological materials. 

[1] Wang, Z. J et al., Three-dimensional Dirac semimetal and quantum transport in Cd3As2, PhysRevB.88.125427 (2013)
[2] Wu Q. S et al., MgTa2N3: A new reference Dirac semimetal, PhysRevB.98.081115 (2018)
[3] Wu Q. S et al., WannierTools : An open-source software package for novel topological materials, j.cpc.2017.09.033 (2018)