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Atomic-Scale Spectroscopic Structure of Tunable Flat Bands, Magnetic Defects and Heterointerfaces in Two-Dimensional Systems.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Atomic-Scale Spectroscopic Structure of Tunable Flat Bands, Magnetic Defects and Heterointerfaces in Two-Dimensional Systems./
作者:	Kerelsky, Alexander .
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2020,
面頁冊數:	257 p.
附註:	Source: Dissertations Abstracts International, Volume: 81-07, Section: B.
Contained By:	Dissertations Abstracts International81-07B.
標題:	Condensed matter physics. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=27545232
ISBN:	9781392878873

Atomic-Scale Spectroscopic Structure of Tunable Flat Bands, Magnetic Defects and Heterointerfaces in Two-Dimensional Systems.
Kerelsky, Alexander .

Atomic-Scale Spectroscopic Structure of Tunable Flat Bands, Magnetic Defects and Heterointerfaces in Two-Dimensional Systems. - Ann Arbor : ProQuest Dissertations & Theses, 2020 - 257 p.

Source: Dissertations Abstracts International, Volume: 81-07, Section: B.

Thesis (Ph.D.)--Columbia University, 2020.

This item must not be sold to any third party vendors.

Graphene, a single atom thick hexagonally bonded sheet of carbon atoms, was first isolated in 2004 opening a whole new field in condensed matter research and material engineering. Graphene has hosted a whole array of novel physics phenomena as its carriers move at near the speed of light governed by the Dirac Hamiltonian, it has few scattering sites, it is easily gate-tunable, and hosts exciting 2D physics amongst many other properties. Graphene was only the tip of the iceberg in 2D research as researchers have since identified a whole family of materials with similar layered atomic structures allowing isolation into several atom thick monolayers. Monolayer material properties range from metals to semiconductors, superconductors, magnets and most other properties found in 3D materials. Naturally, this has led to making fully 2D heterostructures to study exciting physics and explore applications such as 2D transistors. It has recently been found that not only can you stack these materials at will but you can also tune their properties with an inter-layer twist between layers which at precise twist angles yields on-demand electronic correlations that can be easily tuned with experimental knobs leading to novel correlated phases. The pioneering techniques towards understanding each 2D material and heterostructures thereof have usually been with transport and optics. These techniques are inherently bulk macroscopic measurements which do not give insights into the nanoscale properties such as atomic-scale features or the nanoscale heterostructure properties that govern the systems. Atomic-scale structural and electronic insights are crucial towards understanding each system and providing proper guidelines for comprehensive theoretical understandings. In this thesis, we study the atomic-scale structural and electronic properties of various 2D systems using ultra-high vacuum (UHV) scanning tunneling microscopy and spectroscopy (STM/STS), a technique which utilizes electron tunneling with an atomically sharp tip to visualize atomic structure and low-energy spectroscopic properties. We focus on three major types of systems: twisted graphene heterostructures (magic angle twisted bilayer graphene and small angle double bilayer graphene), bulk and monolayer semiconducting transition metal dichalcogenides (TMDs), and 2D heterointerfaces (TMD - metal and graphene p-n junctions). We establish a number of state of the art methods to study these 2D systems in their cleanest, transport-experiment-like forms using surface probes like STM/STS including robust, clean, reliable contact methods and procedures towards studying micronscale exfoliated 2D samples atop hexagonal boron nitride (hBN) as well as photo-assisted STM towards studying semiconducting TMDs and other poorly conducting materials at low temperatures (13.3 Kelvin).We begin with one of the most currently mainstream topics of twisted bilayer graphene (tBG) where, near the magic angle of 1.1◦ the first correlated insulating and superconducting states in graphene were observed. A lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the moir´e pattern had precluded a coherent theoretical understanding of the correlated states up til our work. We establish novel, robust methods to measure these micron-scale samples with a surface scanning probe technique. We directly map the atomic-scale structural and electronic properties of tBG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). Contrary to previous understandings (which predicted two flat bands with a several meV separation in the system), we observe two distinct van Hove singularities (vHs) in the local density of states (LDOS) around the magic angle, with a doping-dependent separation of 40-57 meV. We find that the vHs separation decreases through the magic angle with a lowest measured value of 7-13 meV at 0.79◦ . When doped near half moir´e band filling where the correlated insulating state emerges, a correlation-induced gap splits the conduction vHs with a maximum size of 6.5 meV at 1.15◦ , dropping to 4 meV at 0.79◦ . We find that more crucial to the magic angle than the vHs separation is that the ratio of the Coulomb interaction (U) to the bandwidth (t) of each individual vHs is maximized (as opposed to the proximity of the individual vHs's), indicating that indeed electronic correlations are very important and suggesting a Cooper-like pairing mechanism based on electron-electron interactions. This establishes that magic angle tBG is to be understood in a single vHs picture where the band-width of the vHs is minimized. Spectroscopy maps show that three-fold (C3) rotational symmetry of the LDOS is broken in magic angle tBG, with an anisotropy that is strongest near the Fermi level, and is highly enhanced when the doping is in the vicinity of the correlated gap, indicating the presence of a strong electronic nematic susceptibility or even nematic order in tBG in regions of the phase diagram where superconductivity is observed. We. (Abstract shortened by ProQuest).

ISBN: 9781392878873Subjects--Topical Terms:

3173567
Condensed matter physics.
Subjects--Index Terms:

Graphene

Atomic-Scale Spectroscopic Structure of Tunable Flat Bands, Magnetic Defects and Heterointerfaces in Two-Dimensional Systems.
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506 $a This item must not be sold to any third party vendors.
520 $a Graphene, a single atom thick hexagonally bonded sheet of carbon atoms, was first isolated in 2004 opening a whole new field in condensed matter research and material engineering. Graphene has hosted a whole array of novel physics phenomena as its carriers move at near the speed of light governed by the Dirac Hamiltonian, it has few scattering sites, it is easily gate-tunable, and hosts exciting 2D physics amongst many other properties. Graphene was only the tip of the iceberg in 2D research as researchers have since identified a whole family of materials with similar layered atomic structures allowing isolation into several atom thick monolayers. Monolayer material properties range from metals to semiconductors, superconductors, magnets and most other properties found in 3D materials. Naturally, this has led to making fully 2D heterostructures to study exciting physics and explore applications such as 2D transistors. It has recently been found that not only can you stack these materials at will but you can also tune their properties with an inter-layer twist between layers which at precise twist angles yields on-demand electronic correlations that can be easily tuned with experimental knobs leading to novel correlated phases. The pioneering techniques towards understanding each 2D material and heterostructures thereof have usually been with transport and optics. These techniques are inherently bulk macroscopic measurements which do not give insights into the nanoscale properties such as atomic-scale features or the nanoscale heterostructure properties that govern the systems. Atomic-scale structural and electronic insights are crucial towards understanding each system and providing proper guidelines for comprehensive theoretical understandings. In this thesis, we study the atomic-scale structural and electronic properties of various 2D systems using ultra-high vacuum (UHV) scanning tunneling microscopy and spectroscopy (STM/STS), a technique which utilizes electron tunneling with an atomically sharp tip to visualize atomic structure and low-energy spectroscopic properties. We focus on three major types of systems: twisted graphene heterostructures (magic angle twisted bilayer graphene and small angle double bilayer graphene), bulk and monolayer semiconducting transition metal dichalcogenides (TMDs), and 2D heterointerfaces (TMD - metal and graphene p-n junctions). We establish a number of state of the art methods to study these 2D systems in their cleanest, transport-experiment-like forms using surface probes like STM/STS including robust, clean, reliable contact methods and procedures towards studying micronscale exfoliated 2D samples atop hexagonal boron nitride (hBN) as well as photo-assisted STM towards studying semiconducting TMDs and other poorly conducting materials at low temperatures (13.3 Kelvin).We begin with one of the most currently mainstream topics of twisted bilayer graphene (tBG) where, near the magic angle of 1.1◦ the first correlated insulating and superconducting states in graphene were observed. A lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the moir´e pattern had precluded a coherent theoretical understanding of the correlated states up til our work. We establish novel, robust methods to measure these micron-scale samples with a surface scanning probe technique. We directly map the atomic-scale structural and electronic properties of tBG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). Contrary to previous understandings (which predicted two flat bands with a several meV separation in the system), we observe two distinct van Hove singularities (vHs) in the local density of states (LDOS) around the magic angle, with a doping-dependent separation of 40-57 meV. We find that the vHs separation decreases through the magic angle with a lowest measured value of 7-13 meV at 0.79◦ . When doped near half moir´e band filling where the correlated insulating state emerges, a correlation-induced gap splits the conduction vHs with a maximum size of 6.5 meV at 1.15◦ , dropping to 4 meV at 0.79◦ . We find that more crucial to the magic angle than the vHs separation is that the ratio of the Coulomb interaction (U) to the bandwidth (t) of each individual vHs is maximized (as opposed to the proximity of the individual vHs's), indicating that indeed electronic correlations are very important and suggesting a Cooper-like pairing mechanism based on electron-electron interactions. This establishes that magic angle tBG is to be understood in a single vHs picture where the band-width of the vHs is minimized. Spectroscopy maps show that three-fold (C3) rotational symmetry of the LDOS is broken in magic angle tBG, with an anisotropy that is strongest near the Fermi level, and is highly enhanced when the doping is in the vicinity of the correlated gap, indicating the presence of a strong electronic nematic susceptibility or even nematic order in tBG in regions of the phase diagram where superconductivity is observed. We. (Abstract shortened by ProQuest).
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