Role of electron correlations in some Weyl systems

We are discussing a model to understand previously obtained results on Weyl semimetals as realized in MoTe2 using DFT and DFT+U calculations. The model is motivated from general principles and we use it to investigate the effects of Coulomb correlations originating from the localized nature of the Mo-d orbitals. We find that such correlations can eliminate or create pairs of Weyl points as the strength of the Coulomb interaction is varied. The effect of the spin-orbit coupling (SOC) is to split each Weyl point, which is assumed present in the absence of SOC, into pairs of spin-chiral partners.

Rydberg series of dark excitons and the conduction band spin-orbit splitting in monolayer WSe2

Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction are at the heart of studies of the rich physics of excitons in monolayers of transition metal dichalcogenides (TMD). Those archetypes of two-dimensional systems promise a design of new optoelectronic devices. In intrinsic TMD monolayers the basic, intravalley excitons, are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is dark, although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in tungsten diselenide monolayer, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band.

Introduction

The fundamental and technological importance of transition metal oxides, and the relationship of the present work to previous monographs dealing with transition metal oxides are reviewed. The relatively abundant 3d-transition metal oxides are contrasted with the rarer 4d- and 5d-transition metal oxides that exhibit a unique interplay between spin-orbit, exchange, crystalline electric field and Coulomb correlations. The combined effect of these fundamental interactions yields peculiar quantum states and empirical trends that markedly differ from those of their 3d counterparts. General trends in the electronic structure are related to generalized phase diagrams of the magnetic and insulating ground states. The intriguing absence of experimental evidence for predicted topological states and superconductivity in these materials are discussed.

Proximity control of interlayer exciton-phonon hybridization in van der Waals heterostructures

Van der Waals stacking has provided unprecedented flexibility in shaping many-body interactions by controlling electronic quantum confinement and orbital overlap. Theory has predicted that also electron-phonon coupling critically influences the quantum ground state of low-dimensional systems. Here we introduce proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials, creating new electrically neutral hybrid eigenmodes. Specifically, we explore how the internal orbital 1s-2p transition of Coulomb-bound electron-hole pairs in monolayer tungsten diselenide resonantly hybridizes with lattice vibrations of a polar capping layer of gypsum, giving rise to exciton-phonon mixed eigenmodes, called excitonic Lyman polarons. Tuning orbital exciton resonances across the vibrational resonances, we observe distinct anticrossing and polarons with adjustable exciton and phonon compositions. Such proximity-induced hybridization can be further controlled by quantum designing the spatial wavefunction overlap of excitons and phonons, providing a promising new strategy to engineer novel ground states of two-dimensional systems.