Electrons leave the flatland: out-of-plane charge dynamics in layered materials

The goal of this thesis is to explore the "neglected" third dimension of layered materials, by studying the interlayer charge dynamics through dc conductivity, optical spectroscopy and ab-initio calculations. This project was designed to respond to the growing interest in layered materials and heterostructures. It is today evident that new properties originate by the interaction between neighbouring atomic planes, but the magnitude of the interlayer coupling has so far been poorly investigated. Reliable and reproducible experiments, in particular for dc conductivity, are today made possible thanks to the recent advancements in the field of materials microfabrication through focused ion beam. In the case of this thesis, it enabled me to review long-standing scientific paradigms and verify that previous experimental results differ by more than 2 orders of magnitude from the intrinsic material property. The subjects of study are metallic layered transition metal chalcogenides, as they offer a very flexible playground of crystalline structure and physical properties. The discussion will start in Chapter 3, where we will discuss the case of simple layered metals (2H-NbSe2, 2H-TaS2) or semimetal (ZrTe5), for which the anisotropic transport properties can be accurately predicted by their simple band structure calculation and Fermi surface topology. The anisotropic response will be treated in a more general term by considering the effective number of carriers which contributes to charge transport along a specific direction.

The discussion will continue by looking at the peculiar properties of 1T-TaS2, a prominent layered material for the rich set of charge density waves reconstruction, and the presence of a low temperature metal-insulator transition. Recent results from band structure calculations, photoemission spectroscopy and X-ray diffraction, on the contrary, suggests that strong interlayer interactions need to be accounted to justify the observed properties of this material. The experimental results in this thesis provide a clear evidence from transport properties in support of this new interpretation of a layered material where physical properties are strongly dictated by the way different layers interact with each other. This counterintuitive result is regarded as the consequence of the intertwined relation between the charge density wave and the orbital character of the conduction electrons bands, producing a c-axis oriented orbital texture.

The weak bond that holds together the crystalline planes in layered materials also creates the perfect condition for intercalation, or the growth of compounds with different crystalline layers stacked together, creating a natural heterostructure. We will explore the new physical properties that emerged in layered transition metal dichalcogenides when different layers are combined together to form a natural heterostructure, or when magnetic ions are intercalated. Surprising results are reported from the studies of interlayer conduction properties. The interaction between the different types of layers in the natural heterostructure of 4Hb-TaS2, influences the charge density wave and superconducting properties. In this situation, the conduction mechanism is highly anisotropic. In Chapter 6 we will see how conduction anisotropy is affected by the specific long-range magnetic order produced by the intercalated atoms.