QUEST2D

Materials whose electronic properties are governed by emergent quantum electronic phenomena, such as topological insulators, Mott insulators, unconventional superconductors, and 2D magnets fall into the broad label of Quantum Materials. In this project, we will study collective excitations, such as magnons, paramagnons, excitons, and plasmons in two-dimensional (2D) quantum materials of two types: 2D crystals with either magnetic order, such as CrI3,  oxychlorides, and Fe2GeTe3, or superconducting order, such as NbSe2, and Van der Waals heterostructures combining them both with each other and with other 2D crystals such as graphene and transition metal dichalcogenides. The study of collective excitations is of paramount importance for two reasons. First, collective excitations govern the response of these materials to experimental probes and are thus essential to turn experimental data into physical insight about the materials. Second, collective excitations behave as quasiparticles that can be used to store and carry information, both classical and quantum, providing a platform for advanced technologies, and can also be the glue for superconducting pairing. In this project we address, from the theory standpoint, several open questions in the field of 2D crystals that feature either magnetic or superconducting order, as well their Van der Waals heterostructures. In all cases, hybridization between different classes the collective modes plays a crucial role and requires going beyond the state-of-the-art methods. We consider two types of hybridization. In 2D crystals with strong spin-orbit coupling or strong magneto-dichroism, the spin response function is coupled to the charge and optical response functions. The second type of hybridization studied here occurs in Van der Waals heterostructures that bring different 2D crystals. In both cases, the standard approximation of treating the spin, charge and optical response functions as decoupled objects fails. The main scientific questions that we shall address in this project are: 1) To understand the role of spin-orbit coupling, and the concomitant spin splitting, in the spin fluctuations of the nearly ferromagnetic phase of two dimensional 2H-NbSe2, and the relevance of the spin-fluctuations as pairing mechanism for superconductivity in this material.
2) To quantify the magnitude of the optically induced magnetization and second harmonic generation in ferromagnetic 2D crystals (such as CrI3) and alloys (such as WSe2:V), as well as antiferromagnetic crystals such as FeOCl. 3) To model hybrid collective modes, such as plasmon-magnon, Higgs mode-plasmon, and magnon-magnon, that arise either in single 2D crystals, because of strong spin-orbit coupling, or in Van der Waals heterojunctions, including twisted bilayers, because of interlayer interactions, and to explore new magnetic resonance methods made possible by them. Our methods combine the use of density functional theory (DFT) calculations with model Hamiltonians both for  electrons and spins. In both cases, the calculation of response functions is essential, and counts with the track record of the proponents computing excitons, plasmons and magnons, including the development of our own methods for this matter [COS20a,COS20b, MOU20]. In this project we shall develop two new computational tools. On one side, we shall complete a generalized Random Phase Approximation (g-RPA) that permits to compute the spin and charge response function of systems with strong spin orbit coupling. On the other hand we shall develop a numerical method to compute non-linear response functions, based on DFT, addressing the band disentangling problem by means of machine learning techniques. This project has an enormous potential to be disruptive in several fronts: We will try to solve the long-standing mystery of the origin of superconductivity in 2H- NbSe2, and its connection with the charge density wave and spin fluctuations. Our g-RPA formalism will permit us to understand the response of coupled spin-charge dynamics of 2D crystals with large spin-orbit coupling, going beyond a six-decade-old paradigm that treats them separately. This will unveil the existence of novel hybrid collective modes, such as magnonplasmon polaritons in 2D conducting ferromagnets. Our understanding of hybrid collective modes will permit us to design new ways to excite and probe magnetic resonance in 2D materials, opening thereby a new window to explore them. In addition, the new hybrid modes may have topological gaps at energies accessible to conventional probes. The proponent team brings together experts in 2D materials and in the computation of excitons, magnons and plasmons, with a wide-ranging toolbox of calculation methods that combines models, DFT, machine learning and an outstanding track publication, citation record, and several relevant recent papers in high profile journals.