Giant anomalous Hall effect in quasi-two-dimensional layered antiferromagnet Co1/3NbS2

The discovery of the anomalous Hall effect (AHE) in bulk metallic antiferromagnets (AFMs) motivates the search of the same phenomenon in two-dimensional (2D) systems, where a quantized anomalous Hall conductance can, in principle, be observed. Here we present experiments on microfabricated devices based on Co1/3NbS2, a layered AFM that was recently found to exhibit AHE in bulk crystals below the Neel temperature T-N = 29 K. Transport measurements reveal a pronounced resistivity anisotropy, indicating that upon lowering temperature the electronic coupling between individual atomic layers is increasingly suppressed. The experiments also show an extremely large anomalous Hall conductivity of approximately 400 S/cm, more than one order of magnitude larger than in the bulk, which demonstrates the importance of studying the AHE in small exfoliated crystals, less affected by crystalline defects. Interestingly, the corresponding anomalous Hall conductance, when normalized to the number of contributing atomic planes, is similar to 0.6 e(2)/h per layer, approaching the value expected for the quantized anomalous Hall effect. The observed strong anisotropy of transport and the very large anomalous Hall conductance per layer make the properties of Co1/3NbS2 compatible with the presence of partially filled topologically nontrivial 2D bands originating from the magnetic superstructure of the antiferromagnetic state. Isolating atomically thin layers of this material and controlling their charge density may therefore provide a viable route to reveal the occurrence of the quantized AHE in a 2D AFM.

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

Edoardo Martino

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.

EPFL2020

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

Edoardo Martino

EPFL2020