Two-dimensional magnetism in correlated systems

Project Information

2DMAGICS

Grant agreement ID: 839551

DOI

10.3030/839551

Project closed

EC signature date 9 April 2019

Start date 1 December 2020

End date 30 November 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 168 477,04

EU contribution

€ 168 477,04

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - 2DMAGICS (Two-dimensional magnetism in correlated systems)

Reporting period: 2020-12-01 to 2022-11-30

Summary of the context and overall objectives of the project

The aim of the 2DMAGICS project consisted in elaborating a microscopic description and theoretical predictions of magnetism and magnetic interactions of advanced two-dimensional materials and layered three-dimensional heterostructures. The main objects of investigation were two-dimensional group V semiconductors, transition metal dichalcogenides, layered nodal-line semimetals, and artificial surface systems built up by regular atomic structures deposited on substrates. Interest in these systems has not only been triggered by their potential applications in nanoscale and flexible electronics, but also by a number of fascinating physical features, such as competing charge- and spin-ordered, as well as Mott insulating states, superconductivity, anomalous optical properties, and promising magnetic characteristics for applications in spintronics.

Using materials from this class for technological applications requires an accurate theoretical understanding and, ideally, prediction of their properties. These materials are characterized by a complex interplay between spin-orbit coupling and strong non-local Coulomb correlations, which represents an outstanding challenge for a consistent theoretical description. Within 2DMAGICS the proposed low-dimensional systems have been studied using a combination of already existing theoretical approaches and novel methods. The latter were developed in the framework of the project in order to provide a realistic multiscale description of many-body effects beyond the state-of-the-art. This allowed 2DMAGICS to resolve a number of fundamental issues in the general field of magnetism and, based on these accomplishments, to achieve an accurate description and prediction of various properties of realistic materials.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A method for a complete description of spin dynamics in itinerant electronic systems including exchange interactions and the equation of motion for the local magnetic moment has been developed. The results have been published in Phys. Rev. B 105, 155151 (2022).

A multi-mode generalization of the fluctuating field method that allows one to describe dynamical symmetry breaking towards a magnetically ordered phase has been introduced. The free energy of a prototypical one-dimensional molecule with strong magnetic fluctuations has been calculated. The results have been published in Phys. Rev. B 105, 035118 (2022). Further, a multi-channel extension of this method was developed. The improved method allows one to describe the interplay between the leading collective electronic fluctuations in the system numerically exactly. The multi-channel fluctuating field approach has been applied to an electronic model with long-range Coulomb interactions in order to investigate the competition between the charge- and spin-density wave instabilities that emerge in the system. The results of this study have been submitted to a peer reviewed journal for publication.

A novel method, dubbed D-TRILEX, for describing collective electronic effects in realistic multi-orbital systems has been developed and implemented as a program package. The method was benchmarked against existing exact solutions in the single- and multi-orbital cases. The results and description of the D-TRILEX program package are published in Phys. Rev. B 103, 245123 (2021) and in SciPost Phys. 13, 036 (2022).

The D-TRILEX method has been applied to study many-body effects in two-dimensional indium selenide (InSe), a material with unique characteristics of the electronic spectrum. We have demonstrated that the monolayer phase of InSe is a rare example of a system, where collective electronic fluctuations lead to the formation of exotic states of matter, such as coexisting charge density wave and ferromagnetic orderings. The results of this study have been published in npj Comput. Mater. 8, 118 (2022).

The magnetic susceptibilities for a system of lead adatoms on a silicon surface have been calculated using the D-TRILEX method. This allowed us to investigate the temperature vs doping level phase diagram of the material. As a result, we have found several charge- and spin-density wave phases, a signature of which has recently been observed experimentally. The results of this study have been submitted to a peer reviewed journal for publication.

While working on the project the PI obtained a permanent researcher position at CNRS, France. The PI regularly gives scientific seminars and also participates in several collaborations at the host institution (Ecole Polytechnique, France). To disseminate the results of the research, the PI gave invited talks in scientific groups at University Paris-Saclay, University of Manchester, University of Hamburg, and at the TRIQS meeting at College de France (Paris, France). The PI also attended the GDR 2426 Quantum Mesoscopic Physics meeting (Aussois, France) and the Psi-k conference (Lausanne, Switzerland) with poster presentations. Results of the research are regularly prepared for scientific publication. In addition, the PI contributed to a review paper entitled “Quantitative theory of magnetic interactions in solids”.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

D-TRILEX allowed the PI to challenge the state-of-the-art solution of several interacting electronic problems. Recently, D-TRILEX was used to investigate spatial magnetic fluctuations in a highly-anisotropic three-orbital system modelling transition metal oxide compounds [Phys. Rev. Lett. 127, 207205 (2021)]. The results allowed us to reconsider a commonly believed mean-field-based statement that correlations usually tend to increase the anisotropy of a system. On the contrary, it has been found that strong non-local magnetic fluctuations enhanced by the large Hund's coupling J drastically reduce the orbital anisotropy of the perovskite structure and consequently become isotropic in orbital space (see attached figure). Further, the D-TRILEX method was used to address the problem of the metal-to-insulator phase transitions in the two-orbital Hubbard-Kanamori model on a cubic lattice. In the half-filled model with two different bandwidths of the orbitals state-of-the-art methods predict an orbital-selective Mott transition (OSMT). This means that the transition occurs for the narrower band at the critical value of the interaction, while the wider band remains metallic. However, applying the D-TRILEX approach shows that strong magnetic fluctuations prevent the OSMT and favor the Néel transition to the antiferromagnetic state [Phys. Rev. Lett. 129, 096404 (2022)]. Remarkably, the Néel transition occurs for both orbitals at the same critical temperature and does not display an orbital-selective character. In the quarter-filled model, where two identical orbitals split by the crystal filed, a systematic consideration of the non-local collective electronic fluctuations provided by the D-TRILEX approach allowed us to reveal an enormously broad coexistence region of metallic and Mott insulating phases [arXiv:2204.02116 (2022)]. Importantly, this coexistence region is completely missing in the state-of-the-art picture of the phase transition. The obtained results suggest, that the developed method significantly improves the state-o-the-art description of strongly-interacting electronic systems and has great potential for further application to realistic calculations for magnetic materials.

The momentum-resolved static spin susceptibility for the doped t_2g model.