Bilayer graphene encapsulated in tungsten diselenide can host a weak topological phase with pairs of helical edge states. The electrical tunability of this phase makes it an ideal platform to investigate unique topological effects at zero magnetic field, such as topological superconductivity. Here we couple the helical edges of such a heterostructure to a superconductor. The inversion of the bulk gap accompanied by helical states near zero displacement field leads to the suppression of the critical current in a Josephson geometry. Using superconducting quantum interferometry we observe an even-odd effect in the Fraunhofer interference pattern within the inverted gap phase. We show theoretically that this effect is a direct consequence of the emergence of helical modes that connect the two edges of the sample. The absence of such an effect at high displacement field, as well as in bare bilayer graphene junctions, supports this interpretation and demonstrates the topological nature of the inverted gap.

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Bilayer graphene (BLG) was recently shown to host a band-inverted phase with unconventional topology emerging from the Ising-type spin-orbit interaction (SOI) induced by the proximity of transition metal dichalcogenides with large intrinsic SOI. Here, we report the stabilization of this band-inverted phase in BLG symmetrically encapsulated in tungsten diselenide (WSe2) via hydrostatic pressure. Our observations from low temperature transport measurements are consistent with a single particle model with induced Ising SOI of opposite sign on the two graphene layers. To confirm the strengthening of the inverted phase, we present thermal activation measurements and show that the SOI-induced band gap increases by more than 100% due to the applied pressure. Finally, the investigation of Landau level spectra reveals the dependence of the level-crossings on the applied magnetic field, which further confirms the enhancement of SOI with pressure.

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Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS2-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS2-like and PbS-like lattices incommensurability.

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In two-dimensional materials research, oxidation is usually considered as a common source for the degradation of electronic and optoelectronic devices or even device failure. However, in some cases a controlled oxidation can open the possibility to widely tune the band structure of 2D materials. In particular, we demonstrate the controlled oxidation of titanium trisulfide (TiS₃), a layered semicon-ductor that has attracted much attention recently thanks to its quasi-1D electronic and optoelectron-ic properties and its direct bandgap of 1.1 eV. Heating TiS₃ in air above 300 °C gradually converts it into TiO₂, a semiconductor with a wide bandgap of 3.2 eV with applications in photo-electrochemistry and catalysis. In this work, we investigate the controlled thermal oxidation of indi-vidual TiS₃ nanoribbons and its influence on the optoelectronic properties of TiS₃-based photodetec-tors. We observe a step-wise change in the cut-off wavelength from its pristine value ~1000 nm to 450 nm after subjecting the TiS₃ devices to subsequent thermal treatment cycles. Ab-initio and many-body calculations confirm an increase in the bandgap of titanium oxysulfide (TiO₂-xSx) when in-creasing the amount of oxygen and reducing the amount of sulfur.

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In transition metal dichalcogenides, defects have been found to play an important role, affecting doping, spin-valley relaxation dynamics, and assisting in proximity effects of spin-orbit coupling. Here we study localized states in WS2 and how they affect tunneling through van der Waals heterostructures of h-BN/graphene/ WS2/metal. The obtained conductance maps as a function of bias and gate voltage reveal single-electron transistor behavior (Coulomb blockade) with a rich set of transport features including excited states and negative differential resistance regimes. Applying a perpendicular magnetic field, we observe a shift in the energies of the quantum levels and information about the orbital magnetic moment of the localized states is extracted.

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We present measurements of weak localization on hexagonal boron nitride encapsulated bilayer MoS2. From the analysis we obtain information regarding the phase coherence and the spin diffusion of the electrons. We find that the encapsulation with boron nitride provides higher mobilities in the samples, and the phase coherence shows improvement, while the spin relaxation does not exhibit any significant enhancement compared to nonencapsulated MoS2. The spin relaxation time is in the order of a few picoseconds, indicating a fast intravalley spin-flip rate. Lastly, the spin-flip rate is found to be independent from electron density in the current range, which can be explained through counteracting spin-flip scattering processes based on electron-electron Coulomb scattering and extrinsic Bychkov-Rashba spin-orbit coupling.

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This thesis presents an experimental work on electronic transport in two-dimensional (2D) van der Waals systems. The variety of the physics of the chapters reflects the exploratory character of the Ph.D. study and demonstrates the versatility and the unique properties of some of the members of the vast group of van der Waals materials. In the experiments we explore the fundamental properties of carriers and states in 2D materials via electrical transport. The first three chapters include experimental work on planar transport in multiterminal devices from transition metal dichalcogenides (2H-MoS2) and trichalogenides (TiS3). In the fist chapter, we study how intravalley spin relaxation and the phase coherence affects weak localization in boron nitride encapsulatedMoS2. In TiS3 we explore its electronic properties and in order to avoid disorder induced localization, we protect the devices with hexagonal boron nitride. An improvement in the quality of the transport and signatures of charge-density-wave transition are observed. Lastly, multi-terminal transport in 1T/1T0-MoS2 and its carrier transport mechanism are investigated, with special emphasis on how to establish low-temperature electrical contacts to 2H-MoS2. The next part of the thesis shifts to vertical transport in van der Waals heterostructures. Firstly, we use WS2 as tunneling barrier between monolayer graphene and metal contact. We observe sequential tunneling through localized states. By studying the ground and excited states, we gain information about their spatial sizes and the magnetic moments of these states. Lastly, we explore heterostructures of graphene on WSe2 and potential effects on the band structure due to the dielectric environment and the proximity induced spin-orbit coupling. @en

We have studied electrical transport as a function of carrier density, temperature and bias in multi-terminal devices consisting of hexagonal boron nitride (h-BN) encapsulated titanium trisulfide (TiS₃) sheets. Through the encapsulation with h-BN, we observe metallic behavior and high electron mobilities. Below ∼60 K an increase in the resistance, and non-linear transport with plateau-like features in the differential resistance are present, in line with the expected charge density wave (CDW) formation. Importantly, the critical temperature and the threshold field of the CDW phase can be controlled through the back-gate.

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Phase engineering of MoS₂ transistors has recently been demonstrated and has led to record low contact resistances. The phase patterning of MoS₂ flakes with laser radiation has also been realized via spectroscopic methods, which invites the potential of controlling the metallic and semiconducting phases of MoS₂ transistors by simple light exposure. Nevertheless, the fabrication and demonstration of laser-patterned MoS₂ devices starting from the metallic polymorph have not been demonstrated yet. Here, we study the effects of laser radiation on 1T/1T′$-MoS₂ transistors with the prospect of driving an in situ phase transition to the 2H-polymorph through light exposure. We find that although the Raman peaks of 2H-MoS₂ become more prominent and the ones from the 1T/1T$′ phase fade after the laser exposure, the semiconducting properties of the laser-patterned devices are not fully restored, and the laser treatment ultimately leads to the degradation of the transport channel.

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TiS₃ nanosheets have proven to be promising candidates for ultrathin optoelectronic devices due to their direct narrow band-gap and the strong light-matter interaction. In addition, the marked in-plane anisotropy of TiS₃ is appealing for the fabrication of polarization sensitive optoelectronic devices. Herein, we study the optical contrast of TiS₃ nanosheets of variable thickness on SiO₂/Si substrates, from which we obtain the complex refractive index in the visible spectrum. We find that TiS₃ exhibits very large birefringence, larger than that of well-known strong birefringent materials like TiO₂ or calcite, and linear dichroism. These findings are in qualitative agreement with ab initio calculations that suggest an excitonic origin for the birefringence and linear dichroism of the material.

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We have studied temperature- and electric-field-dependent carrier transport in single flakes of MoS2 treated with n-butyllithium. The temperature dependence of the four-terminal resistance follows the Efros-Shklovskii variable range hopping conduction mechanism. From measurements in the Ohmic and non-Ohmic regime, we estimate the localization length and the average hopping length of the carriers, as well as the effective dielectric constant. Furthermore, a comparison between two- and four-probe measurements yields a contact resistance that increases significantly with decreasing temperature.

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