Author: Isabelle Y. Phinney Publisher: ISBN: Category : Languages : en Pages : 86
Book Description
Two-dimensional systems, and, most recently, moire systems, have risen to the forefront of condensed matter physics with the advent of experimental techniques that allow for controlled stacking of van der Waals heterostructures [17, 54]. For example, it was recently discovered that when two pieces of atomically thin carbon (graphene) are twisted at 1.1° with respect to one another, they display a variety of effects, including superconducting behavior [10]. Experimental investigation of the behavior of small-angle twisted bilayer graphene (SA-TBG) as a function of twist angle is imperative to understanding the mechanisms that play into the many interesting, strongly-interacting phenomena that the moire system displays. In this thesis, I present three experiments which explore electron-electron and electron-phonon interactions in SA-TBG. I first consider SA-TBG as a host for a viscous electron fluid and look for the onset of fluid behavior via electron transport. Then I investigate the temperature dependence of resistivity in SA-TBG devices at a number of angles. The final experiment examines magnetophonons in three devices above the magic angle and compares the findings to theoretical results.
Author: Younghee Kim Publisher: ISBN: Category : Physics Languages : en Pages :
Book Description
ABSTRACT: Single layer graphene (SLG) is a novel 2D system consisting of a single sheet of carbon atoms arranged in a honeycomb lattice, and exhibits a unique, linear low-energy dispersion. Bilayer graphene (BLG), two sheets stacked together, is an equally interesting system displaying a second unique, but hyperbolic, dispersion. Graphite consists of Bernal stacked graphene layers. Graphite band structure at K-point mimics the band structure of BLG, while at H- point it is similar to SLG. Hence, depending on their momentum along the c axis, K-point electrons in bulk graphite behave as massive Dirac fermions in BLG. The carriers at the H-point have a character of massless particles due to the eectively vanishing inter-layer coupling. In this work, we study the inter-Landau level transitions of graphene and grpahite to probe the electron structure and electron-phonon interactions by using magneto-Raman spectroscopy. Raman spectroscopy is used extensively to characterize graphene, as the material is composed almost entirely of symmetric sp2 bonded carbon. After the invention of SLG significant efforts have been made to investigate phonons, electron-phonon, and electron-electron interactions in graphene using Raman sepctroscopy. Among these categories, we studied E2g phonon corrected by Landau quantizations. The phonon is predicted to have a resonant character when the phonon energy matches with the energy of asymmetric inter-LL transition energy, so-called anticrossing behavior. In graphite, we study the E2g phonon shift and broadening coupled to the inter-LL excitations at H- and K-points. Moreover, we probe a series of Raman peaks due to the symmetric inter-LL excitations over a broad magnetic eld range including including the low energy transitions involving the electron-hole mixed LL-1 and LL0 LLs, enables an accurate determination of the SWM parameters. Also, in the highest magnetic eld range (>35 T) the E35 T) the E2g peak narrows due to suppression of electron-phonon interaction. It allow us to determine the phonon life time through energy time conservation. More deeply, we study the temperature dependence of inter-LL excitations. Surprisingly, Raman peaks shift to the higher energy and broad with increasing the temperature. In SLG, we study magneto-phonon resonance (MPR) of E2g phonon. The interesting aspect is that the electron-phonon coupling strength depends on the lling factor and polarization of incident and scattered photons. We probe the MPR dependence on various doping level and polarizations. Moreover, we observe the surprising Raman scattering intensity in the middle of the MPR anticrossing gap. Such an unusual MPR ne structure is shown to be a result of eective mixing and splitting electron-phonon coupled modes caused by random fluctuations of strain-induced pseudo-magnetic fields. Finally, we present a Raman spectroscopy study of Bi2Se3 and Sb2Te3 crystals in the temperature range between 5 K and 300 K. We uncover a characteristic temperature dependence of the phonon peak position and linewidth, and interpret it in the context of thermal expansion and three-phonon anharmonic decay. We present experimental procedures and describe the magneto-Raman probes we built. in order to conduct these experiments. Summaries of collaborative work on magneto-elastic eects of the molecular-based magnets by using magneto-Raman spectroscopy and Aharov- Bohm oscillations of Type-II Quantum dots by using magneto-photoluminescence are given.
Author: Astrid Weston Publisher: Springer Nature ISBN: 3031120930 Category : Technology & Engineering Languages : en Pages : 148
Book Description
This thesis provides the first atomic length-scale observation of the structural transformation (referred to as lattice reconstruction) that occurs in moiré superlattices of twisted bilayer transition metal dichalcogenides (TMDs) at low (θ 2 ̊) twist angles. Such information is essential for the fundamental understanding of how manipulating the rotational twist-angle between two adjacent 2-dimensional crystals subsequently affects their optical and electrical properties./ppStudies using Scanning transmission electron microscopy (STEM), a powerful tool for atomic-scale imaging, were limited due to the complexity of the (atomically-thin) sample fabrication requirements. This work developed a unique way to selectively cut and re-stack monolayers of TMDs with a controlled rotational twist angle which could then be easily suspended on a TEM grid to meet the needs of the atomically thin sample requirements. The fabrication technique enabled the study of the two common stacking-polytypes including 3R and 2H (using MoS2 and WS2 as the example) as well as their structural evolution with decreasing twist-angle./ppAtomic-scale studies were followed by a comprehensive investigation of their electronic properties using scanning probe microscopy and electrical transport measurements of the artificially-engineered structures. The electronic structure of two common stacking-polytypes (3R and 2H) were strikingly different, as revealed by conductive atomic force microscopy. Further studies focused on the 3R-stacking polytype to reveal room-temperature out-of-plane ferroelectricity using tools such as kelvin probe force microscopy, scanning electron microscopy and electrical transport measurements. This work highlights that the unique intrinsic properties of TMDs (i.e. semiconductors with strongly light-matter interaction) combined with the additional twisted degree-of-freedom has great potential to create atomically thin transistors/LEDs with built-in memory storage functions and will further aid in the development of the next generation of optoelectronics.
Author: Minhao He Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Moiré quantum materials, stacked 2D atomic layers with a small twist angle or a slight lattice mismatch, have become emerging platforms to study the physics of correlation, topology, and the interplay between the two. A plethora of novel states, including unconventional superconductivity, generalized Wigner crystal, orbital magnetism and its associated quantum anomalous Hall effect (QAHE), have been observed within their characteristic flat bands. Moiré quantum materials provide unprecedented opportunities in elucidating the nature of these quantum states of matter because of their high degree of tunability, such as doping, displacement field, pressure, and Coulomb screening.This dissertation presents transport and optical studies on the emergent correlated and topological states in representative twisted multilayer graphene systems. First, we identify correlated metallic states with isospin symmetry breaking in the phase diagram of twisted double bilayer graphene (tDBG). We employ pressure as a dynamic tuning knob to control the electronic correlation in tDBG. Magneto transport further reveals symmetry-broken Chern insulators at high magnetic field and anomalous Hall effect near zero field. In the second part, we study the correlated phase diagram of twisted monolayer-bilayer graphene (tMBG). We reveal abundant orbital magnetic states across a wide range of twist angle, some with exotic isospin polarized ground states. The high field Landau fan exhibits a variety of closely competing Chern insulators and hysterestic first-order phase transitions in between. We discuss these results and their implication regarding the physics of isospin symmetry, orbital magnetism and Hofstadter butterfly spectrum in graphene Moiré superlattice. Finally, we propose a new optical probe for twisted graphene by using the Rydberg excitons in an adjacent monolayer WSe2. We demonstrate a new set of Moiré Rydberg excitons in the heterostructure of WSe2/twisted bilayer graphene, whose doping tunable Moiré potential unravels the underlying flat bands in twisted bilayer graphene.
Author: Gregory William Burg Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
In recent years, the twist angle between vertically stacked, two-dimensional van der Waals materials has come to the forefront as a key parameter for modifying the characteristics of van der Waals heterostructures and tuning the strength of electron-electron interactions. This dissertation discusses three heterostructures in which the twist angles between layers are controlled to enable interactions and probe collective states. The first heterostructure is rotationally aligned double bilayer graphene separated by WSe2, which exhibits energy and momentum conserving tunneling that is characterized by large peaks in the interlayer current when the bands of the graphene bilayers energetically align as a function of interlayer voltage, followed by negative differential resistance. Simulations of the tunneling current, which closely match the experimental data, and in-plane magnetotunneling measurements confirm that tunneling electrons conserve energy and momentum at all interlayer and gate biases. At the condition of equal and opposite densities in the graphene bilayers, however, the data show a large tunneling conductance that is not captured by calculations. The tunneling conductance exhibits a strong temperature dependence, much greater than expected for single particle tunneling. In addition, the conductance is suppressed at high and low layer densities, and with the application of an in-plane magnetic field, which suggests the tunneling enhancement originates from the condensation of spatially separated electron-hole pairs. The second heterostructure consists of two bilayer graphene stacked and twisted to a small angle near 1°. At this angle, non-dispersive bands appear in the low energy band structure, which support strong electron-electron interactions. Correlated insulators are observed at 1/2 filling of the flat conduction band at finite transverse electric fields, when the band is maximally isolated in energy from neighboring bands. Furthermore, in an in-plane magnetic field, additional correlated insulators appear at 1/4 and 3/4 filling, suggesting an interaction induced lifting of spin and valley degeneracy. Finally, alternating twist quadrilayer graphene is discussed, which consists of four graphene monolayers stacked and twisted with equal but alternating twist angles between layers. Two twist angles are considered that are slightly removed from the optimal flat band angle. At the larger angle, correlated insulators appear at half filling of the flat bands with no signatures of superconductivity. At the smaller angle, the correlated insulators weaken, and signs of superconductivity appear near half filling of the bands. The results suggest an asymmetry in the angle dependence of the two different correlated states
Author: Marta Pita Vidal Publisher: ISBN: Category : Languages : en Pages :
Book Description
Under mesoscopic conditions, strong electron-electron interactions and weak electron-phonon coupling in graphene lead to an hydrodynamic behaviour of electron dynamics, resulting in unusual transport phenomena. During the last years, there have been several theoretical studies of macroscopic signatures of this collective cooperation of electrons. Here, we want to distinguish this hydrodynamic regime studying the conductance through a narrow geometrical constriction in graphene. To do so, we have fabricated high-quality, low-disorder graphene nano-constriction devices encapsulated by hexagonal boron nitride, where electron-electron scattering dominates impurity scattering. We carried out systematic four-probe conductance measurements on devices with different constriction widths as a function of carrier density and temperature. The observation of quantum transport phenomena that are inconsistent with the non-interacting ballistic free-fermion model suggests a macroscopic transport signature of electron viscosity.
Author: Guorui Wang Publisher: Springer Nature ISBN: 9811580294 Category : Technology & Engineering Languages : en Pages : 139
Book Description
This thesis shares new findings on the interfacial mechanics of graphene-based materials interacting with rigid/soft substrate and with one another. It presents an experimental platform including various loading modes that allow nanoscale deformation of atomically thin films, and a combination of atomic force microscopy (AFM) and Raman spectroscopy that allows both displacement and strain to be precisely measured at microscale. The thesis argues that the rich interfacial behaviors of graphene are dominated by weak van der Waals force, which can be effectively modulated using chemical strategies. The continuum theories are demonstrated to be applicable to nano-mechanics and can be used to predict key parameters such as shear/friction and adhesion. Addressing key interfacial mechanics issues, the findings in thesis not only offer quantitative insights in the novel features of friction and adhesion to be found only at nanoscale, but will also facilitate the deterministic design of high-performance graphene-based nanodevices and nanocomposites.
Author: Isabelle Y. Phinney
Author: Younghee Kim
Author: Nicolas Ray
Author: Astrid Weston
Author: Shanglong Ning
Author: Bartholomew Mears Ludbrook
Author: Minhao He
Author: Gregory William Burg
Author: Marta Pita Vidal
Author: Guorui Wang