Transport and Optical Study on Topological Semimetals and 2D Materials

This thesis is mainly focus on the 3D topological semimetal and 2D materials. If you ask a physicist to name the most significant discovery in solid-state physics over the recent 50 years, and highly likely, if not high-temperature superconductivity, will be topological materials or 2D materials. The study of them has profoundly impacted our understanding of the solid-state system, as recognized by four Nobel prizes. Chapter 1 is a brief introduction of topological semimetal and 2D materials. Chapter 2 focuses on a quantum oscillatory study on the Dirac type-II semimetallic candidates PdTe2 and PtTe2. In high-quality single crystals of both compounds, i.e. displaying carrier mobilities between 10^3 and 10^4 cm2/Vs, we observed a large non-saturating magnetoresistivity (MR) which in PtTe2 at a temperature T = 1.3 K, leads to an increase in the resistivity up to 5*10^4% under a magnetic field μ0H = 62 T. These high mobilities correlate with their light effective masses in the range of 0.04 to 1 bare electron mass according to our measurements. For PdTe2 the experimentally determined Fermi surface cross-sectional areas show an excellent agreement with those resulting from band-structure calculations. Surprisingly, this is not the case for PtTe2 whose agreement between calculations and experiments is relatively poor even when electronic correlations are included in the calculations. Therefore, our study provides a strong support for the existence of a Dirac type-II node in PdTe2 and probably also for PtTe2. Band structure calculations indicate that the topologically non-trivial bands of PtTe2 do not cross the Fermi-level (εF). In contrast, for PdTe2 the Dirac type-II cone does intersect εF , although our calculations also indicate that the associated cyclotron orbit on the Fermi surface is located in a distinct kz plane with respect to the one of the Dirac type-II node. Therefore it should yield a trivial Berry-phase. Chapter 3 presents a study on the Fermi-surface of the Dirac type-II semi-metallic candidate NiTe2. In contrast to its isostructural compounds like PtSe2, band structure calculations predict NiTe2 to display a tilted Dirac node very close to its Fermi level that is located along the Γ to A high symmetry direction within its first Brillouin zone (FBZ). The angular dependence of the dHvA frequencies is found to be in agreement with the first-principle calculations when the electronic bands are slightly shifted with respect to the Fermi level (εF ), and therefore provide support for the existence of a Dirac type-II node in NiTe2. Nevertheless, we observed mild disagreements between experimental observations and density Functional theory calculations as, for example, nearly isotropic and light experimental effective masses. This indicates that the dispersion of the bands is not well captured by DFT. Despite the coexistence of Dirac-like fermions with topologically trivial carriers, samples of the highest quality display an anomalous and large, either linear or sublinear magnetoresistivity. This suggests that Lorentz invariance breaking Dirac-like quasiparticles dominate the carrier transport in this compound. Chapter 4 describes a study on a 2D superconductor. The superconducting α-phase thin molybdenum carbide flakes were first synthesized, and a subsequent sulfurization treatment induced the formation of vertical heterolayer systems consisting of different phases of molybdenum carbide— ranging from α to γ′ and γ phases—in conjunction with molybdenum sulfide layers. These transition-metal carbide/disulfide heterostructures exhibited critical superconducting temperatures as high as 6 K, higher than that of the starting single-phased α-Mo2C (4 K). We analyzed possible interface configurations to explain the observed moire patterns resulting from the vertical heterostacks. Our density-functional theory (DFT) calculations indicate that epitaxial strain and moire patterns lead to a higher interfacial density of states, which favors superconductivity. Such engineered heterostructures might allow the coupling of superconductivity to the topologically nontrivial surface states featured by transition-metal carbide phases composing these heterostructures potentially leading to unconventional superconductivity. Moreover, we envisage that our approach could also be generalized to other metal carbide and nitride systems that could exhibit high-temperature superconductivity. In chapter 5, We introduce a new moire system: InSe/GaSe hetero-structure. To date, the moire physics has been constrained mainly by two factors i) the dimensionality, defined by the stacking of monolayers, and ii) the twist angle ϕ which unveils novel phases only at quite precise values (e.g. superconductivity, orbital magnetism, correlated insulator states). Here, we overcome these practical limitations through a new class of heterostructures composed of strongly coupled layers of γ-InSe on ϵ-GaSe revealing compelling evidence for the moire potential even in thick stacked layers and at arbitrary values of ϕ. We detect a pronounced interlayer exciton (Xi) of very small radius according to the Stark effect, that is composed of several superimposed emissions uniformly spaced in energy ΔE displaying a pronounced ϕ-dependence. In the interfacial area, similar behavior is also shown by the intralayer exciton of GaSe (X0(Ga)). The strong correlation between ϕ and δEimplies localization of excitations at the moire potential minima. In contrast to transition metal dichalcogenides (TMDs), the moire potential modulates the multi-component Xi over the entire range of ϕ due to their direct band-gap at the center of the Brillouin zone. Therefore, γ-InSe/ϵ-GaSe interfaces offer an unprecedented level of moire exciton tunability not yet achieved in other vans der Waals heterostructures. Our results unveil clear pathways for quantum optoelectronics while offering opportunities to study electronic correlations over a broad range of moire periodicities and layer thicknesses.