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Title:Scanning tunneling microscopy and spectroscopy study of phase transitions and topology in layered transition metal dichalcogenides
Author(s):Iaia, Davide
Director of Research:Madhavan, Vidya
Doctoral Committee Chair(s):Mason, Nadya
Doctoral Committee Member(s):Wagner, Lucas; Goldschmidt, Elizabeth
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Scanning tunneling microscopy
transition metal dichalcogenides
charge density waves
topological insulators
topological Weyl semimetal
metal to insulator transition
Abstract:Layered transition metal dichalcogenides (TMDs) are versatile materials which exhibit different crystal structures and different electronic behavior which ranges from metallic to semiconducting and insulating. TMDs host a large variety of physical phenomena, for instance, superconductivity, charge density wave (CDW), Mott physics, Weyl fermions and others. This class of materials has been known for decades but it received a renovated interest after the discovery of graphene and the development of tools to isolate single layers from layered materials. TMDs are suitable for electronics and optoelectronics applications thanks to the possibility of tuning their electronic properties, for instance by electrostatic gating, or by doping and pressure, and to the possibility of realizing tailored band structures by creating complex heterostructures, similarly to twisted graphene bilayer. This work discusses our Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) study of the phase transitions of TiSe$_2$ upon Cu intercalation and Pt doping and of the electronic properties of the type-II Weyl semimetal MoTe$_2$. Cu intercalated TiSe$_2$ is a superconductor below 4 K with an optimal doping concentration $x\,\sim\,0.08$. The CDW state, that emerges below 200 K, coexists with superconductivity, thus this material represents the perfect playground to investigate the coexistence of two competing phases, such as superconductivity and CDW. Our STM measurements provide the first direct observation of the existence of CDW domain walls and our STS data demonstrate that the domain walls host an extra population of fermions. The loss of the long-range CDW order together with the enhancement of the density of states at the domain walls might be the signature that the CDW domain walls play an important role in the emergence of superconductivity. This scenario is also supported by our STM data at 300 mK. The experiment below critical temperature shows that the density of domain walls affects the robustness of the superconducting state. In particular, superconductivity seems to be stronger in areas with higher density of domain walls. The fate of TiSe$_2$ upon Pt doping is different. Transport data demonstrate that the resistivity in Pt$_{1-x}$Ti$_x$Se$_2$ increases up to several orders of magnitude for $x\,\sim\,0.1$. We investigated the mechanism for the observed metal to insulator transition (MIT) by performing STM and STS measurements. The experiment reveals a new mechanism for MIT in Pt$_{1-x}$Ti$_x$Se$_2$, where CDW domain walls, induced by Pt doping, create a disordered network which subtracts electrons from the bulk and acts like the main channel of low energy transport. MoTe$_2$ is a TMD material with properties substantially different compared to TiSe$_2$. MoTe$_2$ is a type-II Weyl semimetal which hosts exotic physics like Weyl fermions, chiral anomaly, giant magnetoresistance, and topologically protected surface states known as Fermi arcs. To explore potential application for Weyl semimetals requires understanding their electronic properties. Here a detailed analysis of our Quasi-Particle Interference (QPI) study of MoTe$_2$ is provided. QPI allows one to probe the band structure of the materials below and above Fermi energy, therefore it is a powerful tool to investigate the existence of surface states such as Fermi arcs. Our experimental results combined to theoretical calculation indicate that topologically trivial surface states exist close to the Fermi arcs in momentum space and that the differences between the two types of surface states are difficult to resolve in QPI patterns. The experiment also demonstrates that the two non equivalent surfaces of MoTe$_2$ produce different QPI patterns, thus QPI can be used as a tool to investigate the effects induced by broken inversion symmetry. This thesis also includes a study on the topological crystalline insulator Pb$_{1-x}$Sn$_x$Se$_2$, which does not belong to the family of TMD materials. This study offers the possibility of completing the thesis work by introducing a description of topological crystalline insulators and 1D topological edge states. The system hosts two types of step edges: odd steps with height equal to a half-integer multiple of the lattice constant, and even steps with height equal to an integer multiple of the lattice constant. It is known that the odd steps host topological non trivial 1D edge modes. Here we investigate the origin of the 1D modes localized along the step edges. Our STM and STS measurements combined with theoretical calculation reveal that also the even steps host edge modes. The presence of these modes is due to the emergence of a particle-hole symmetry which allows for introducing a topological integer $\nu\,\in\,2Z$. The integer is non zero only at the odd steps. The results obtained can be extended to other topological crystalline insulators with rock-salt structure.
Issue Date:2020-12-03
Rights Information:c 2020 by Davide Iaia. All rights reserved.
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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