The effects of charge doping and dimensionality on the electronic structure of van der Waals magnet CrGeTe₃

Abstract

The recent discovery of the persistence of long-range magnetic order when van der Waals layered magnets are thinned towards the monolayer limit has provided a tunable platform for the engineering of novel magnetic structures and devices. In 2017, van der Waals semiconducting material CrGeTe₃ was first demonstrated to have long-range magnetic order persisting down to the bilayer, marking the beginning of the 2D magnets revolution, making it an ideal system to study the effects of charge doping and dimensionality on the electronic and magnetic properties. In this thesis, the evolution of the electronic structure of CrGeTe₃ is studied as a function of electron doping in the surface layer. From angle-resolved photoemission spectroscopy, spectroscopic fingerprints are observed that show this electron doping drives a marked increase in Tc, reaching values more than double that of the undoped material, in agreement with recent studies using electrostatic gating. Together with density functional theory calculations and Monte Carlo simulations, it is shown that, surprisingly, the increased Tc is mediated by the population of spin-minority Cr t2g states, forming a half-metallic 2D electron gas at the surface. This promotes a novel variant of double exchange, and unlocks a significant influence of the Ge - which was previously thought to be electronically inert in this system – in mediating Cr-Cr exchange. Additionally, the progress towards the growth and characterisation of monolayer CrGeTe₃, using molecular beam epitaxy, is detailed. The effects of varying Ge concentration, during growth, on the structural, electronic, and magnetic properties are investigated, however, the true composition and magnetic properties of the grown materials remain an open question.