MARVEL Junior Seminar — November 2021

Zoom meeting link:

https://epfl.zoom.us/j/93881551248

The MARVEL Junior Seminars aim to intensify interactions between the MARVEL Junior scientists belonging to different research groups.

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.

MARVEL Junior Seminar Organizing Committee — Michele Kotiuga, Sara Fiore, Stefano Falletta, Kristians Cernevics, Max Veit and Patrick Mayor

Talk 1 — Smart local orbitals for efficient calculations within density functional theory and beyond

Guido Gandus^1,2, Angelo Valli³, Daniele Passerone², Robert Stadler³

¹ Integrated Systems Laboratory, ETH Zurich
² nanotech@surfaces, Empa, Dübendorf
³ Institute for Theoretical Physics, TU Wien

Localized basis sets in the projector augmented wave formalism allow for computationally efficient calculations within density functional theory (DFT). However, achieving high numerical accuracy requires an extensive basis set, which also poses a fundamental problem for the interpretation of the results. We present a way to obtain a reduced basis set of atomic orbitals through the subdiagonalization of each atomic block of the Hamiltonian. The resulting local orbitals (LOs) inherit the information of the local crystal field. In the LO basis, it becomes apparent that the Hamiltonian is nearly block-diagonal, and we demonstrate that it is possible to keep only a subset of relevant LOs which provide an accurate description of the physics around the Fermi level. This reduces to some extent the redundancy of the original basis set, and at the same time it allows one to perform post-processing of DFT calculations, ranging from the interpretation of electron transport to extracting effective tight-binding Hamiltonians, very efficiently and without sacrificing the accuracy of the results.

Talk 2 — The search for magnetic order in the hexagonal manganites: a first-principles investigation of magneto-structural coupling

Tara Tošić¹, Nicola Spaldin¹, Quintin Meier²

¹ Materials Theory Laboratory, D-MATL, ETH Zurich
² LITEN, CEA Grenoble, France

The hexagonal manganites, h-RMnO₃, are multiferroic materials that combine improper ferroelectricity with frustrated antiferromagnetic order. We use a combination of symmetry analysis and density functional theory to rationalise the magnetic ground states, which vary with no obvious trend across the series of R chemistries (R=In, Sc, Y and Dy to Lu).

Within a Landau Free energy formalism, we highlight the couplings between structural and magnetic order parameters responsible for the magnetic ground state. Our density functional calculations show that the magnetic ordering is particularly sensitive to one structural order parameter, associated with a K1 distortion mode, which generates a triangular breathing of the Mn and O ions in the ab plane. Using our first-principles results, we then extract the parameters of a magnetic model Hamiltonian as a function of K1 for three representative compounds: LuMnO₃, ErMnO₃ and YMnO₃. We summarise the magnetic ground state dependence on K1 in a phase diagram and show that the spanned parameters within our model follow the trend of the A-site radii.

Check the list of the next MARVEL Junior Seminars here.