Imaging the effects of magnetic proximity coupling on the electronic structure of transition metal dichalcogenides

Abstract

The aim of this thesis is to exploit proximity coupling effects to induce and manipulate magnetic states in transition metal dichalcogenides (TMDs), and to investigate the resulting effects on the electronic structure using predominantly photoemission-based techniques. The results presented here focus on three candidate materials that have been suggested to be on the brink of ferromagnetic instabilities, namely NbS₂, VSe₂ and VTe₂.

It has been demonstrated that intercalating magnetic ions between NbS₂ layers leads to the emergence of novel bulk magnetic textures. However, the magnetic phenomena at the termination-dependent surfaces of such intercalated TMDs, where proximity coupling effects with the underlying bulk material will play a crucial role, remain relatively unexplored. Motivated by this, I present a termination-dependent characterisation of the electronic structure of three intercalated TMDs, namely V₁/₃NbS₂, Cr₁/₃NbS₂ and Fe₁/₃NbS₂, obtained by utilising spatially-resolved angle-resolved photoemission spectroscopy (ARPES). Through this, methods to reliably identify unique surface terminations in this class of compounds are demonstrated. Further measurements focusing on the NbS₂-terminated surface of V₁/₃NbS₂ reveal that an RKKY-like interaction between the monolayer-like NbS₂ surface layer and underlying magnetic V intercalates generates a valley-dependent Zeeman spin splitting of the itinerant Nb-derived surface states, exceeding 50 meV. This energy scale is of comparable size to the intrinsic spin–orbit splitting and demonstrates a new route to control valley-spin splittings that provides the largest proximity coupling of a TMD monolayer realised to date.

The thesis concludes by investigating monolayer vanadium dichalcogenides, compounds for which charge density wave (CDW) states may suppress ferromagnetism. Here, I present our progress towards destabilising the CDW order in these compounds by inducing magnetism via proximity coupling to a ferromagnetic substrate. The results include ARPES characterisations of VSe₂ and VTe₂ monolayers, where it is shown that both compounds exhibit metal-insulator transitions in their CDW states. Finally, X-ray magnetic circular dichroism measurements of van der Waals heterostructures consisting of monolayer VTe₂ grown on ferromagnetic Cr₂Te₃ thin films are presented which provide evidence of a magnetic proximity coupling between the substrate and overlayer.