Unconventional sequence of correlated Chern insulators in magic-angle twisted bilayer graphene

The interplay between strong electron-electron interactions and band topology can lead to novel electronic states that spontaneously break symmetries. The discovery of flat bands in magic-angle twisted bilayer graphene (MATBG) with nontrivial topology has provided a unique platform in which to search for new symmetry-broken phases. Recent scanning tunneling microscopy and transport experiments have revealed a sequence of topological insulating phases in MATBG with Chern numbers $C=\pm 3, \, \pm 2, \, \pm 1$ near moir\'e band filling factors $\nu = \pm 1, \, \pm 2, \, \pm 3$, corresponding to a simple pattern of flavor-symmetry-breaking Chern insulators. Here, we report high-resolution local compressibility measurements of MATBG with a scanning single electron transistor that reveal a new sequence of incompressible states with unexpected Chern numbers observed down to zero magnetic field. We find that the Chern numbers for eight of the observed incompressible states are incompatible with the simple picture in which the $C= \pm 1$ bands are sequentially filled. We show that the emergence of these unusual incompressible phases can be understood as a consequence of broken translation symmetry that doubles the moir\'e unit cell and splits each $C=\pm 1$ band into a $C=\pm 1$ band and a $C=0$ band. Our findings significantly expand the known phase diagram of MATBG, and shed light onto the origin of the close competition between different correlated phases in the system

Grating based plasmonic band gap cavities

Cataloged from PDF version of article.We report on a comparative study of grating based plasmonic band gap cavities. Numerically, we calculate the quality factors of the cavities based on three types of grating surfaces; uniform, biharmonic and Moiré surfaces. We show that for biharmonic band gap cavities, the radiation loss can be suppressed by removing the additional grating component in the cavity region. Due to the gradual change of the surface profile in the cavity region, Moiré type surfaces support cavity modes with higher quality factors. Experimentally, we demonstrate the existence of plasmonic cavities based on uniform gratings. Effective index perturbation and cavity geometries are obtained by additional dielectric loading. Quality factor of 85 is obtained from the measured band structure of the cavity. © 2009 Optical Society of America

Doctor of Philosophy

dissertationPlasmonics, the study of light metal interactions, has shown great potential in the fields of spectroscopy, catalysis, medicine, and photovoltaics. There is significant interest in the design of metal nanoparticles that interact with specific wavelengths of light. By changing the size, shape, and metal composition, nanoparticles can be tuned to interact with different regions of the electromagnetic spectrum. Gold and silver are the two most common metals used in plasmonics, but are inefficient in the ultraviolet (UV) wavelength range and expensive. This has driven the research of plasmonics with non-noble metals that support plasmons in the UV and are cost-effective. Aluminum has been widely considered to be the ideal metal to fill this application has favorable plasmonic properties in the UV wavelength range and is cheap. However, Al is difficult to structure at the nanoscale due to its rapidly forming and chemically stable native oxide. Here we report the simple, large scale fabrication of Aluminum nanoparticle antennas. These nanoparticles have plasmon resonances in the UV, visible, near infrared, and infrared wavelengths. We demonstrate the utility of these nanoparticles as substrates for surface-enhanced infrared absorption spectroscopy, a wavelength range that is usually not associated with Aluminum. We also demonstrate that this fabrication technique allows for the fabrication of more complex nanoparticle geometries. These complex nanoparticles geometries have utility in both fundamental studies, and in surface-enhanced spectroscopies. We also demonstrate the potential of magnesium as a plasmonic metal. Magnesium is shown to support plasmon resonances from the UV to near infrared wavelengths. We investigate the plasmonic properties of nanostructured magnesium films and demonstrate that pure magnesium does not support sharp nanoscale features. By alloying magnesium with aluminum, its plasmonic and structural properties are improved

Plasmonic band gap cavities

Ankara : The Department of Physics and the Institute of Engineering and Science of Bilkent University, 2008.Thesis (Ph.D.) -- Bilkent University, 2008.Includes bibliographical references leaves 46-51.Surface plasmon polaritons (SPP’s) are trapped electromagnetic waves coupled to free electrons in metals that propagate at the metal-dielectric interfaces. Due to their surface confinement and potential in sub-wavelength optics, SPP’s have been extensively studied for sensing and nanophotonic applications. Dielectric structures and metallic surfaces, both periodically modulated, can form photonic band gaps. Creating a defect cavity region in the periodicity of dielectrics allows specific optical modes to localize inside a cavity region. However, despite the demonstration of numerous plasmonic surfaces and unlike its dielectric counterparts, low index modulation in metallic surfaces limits the formation of plasmonic defect cavity structures. This thesis describes new approaches for plasmonic confinement in a cavity through the use of selective loading of grating structures as well as through the use of Moiré surfaces. In our first approach, we demonstrate that a high dielectric superstructure can perturb the optical properties of propagating SPPs dramatically and enable the formation of a plasmonic band gap cavity. Formation of the cavity is confirmed by the observation of a cavity mode in the band gap both in the infrared and the visible wavelengths. In addition to the confinement of SPP’s in the vertical direction, such a cavity localizes the SPP’s in their propagation direction. Additionally, we have demonstrated that such biharmonic grating structures can be used to enhance Raman scattering and photoluminescence (PL). Using biharmonic grating structure 105 times enhancement in Raman signal and 30 times enhancement in PL were measured. Furthermore, we show that metallic Moiré surfaces can also serve as a basis for plasmonic cavities with relatively high quality factors. We have demonstrated localization and slow propagation of surface plasmons on metallic Moiré surfaces. Phase shift at the node of the Moiré surface localizes the propagating surface plasmons in a cavity and adjacent nodes form weakly coupled plasmonic cavities. We demonstrate group velocities around v = 0.44c at the center of the coupled cavity band and almost zero group velocity at the band edges can be achieved. Furthermore, sinusoidally modified amplitude about the node suppresses the radiation losses and reveals a relatively high quality factor for plasmonic cavities.Kocabaş, AşkınPh.D

Observation of Rydberg moir\'e excitons

Rydberg excitons, the solid-state counterparts of Rydberg atoms, have sparked considerable interest in harnessing their quantum application potentials, whereas a major challenge is realizing their spatial confinement and manipulation. Lately, the rise of two-dimensional moir\'e superlattices with highly tunable periodic potentials provides a possible pathway. Here, we experimentally demonstrate this capability through the observation of Rydberg moir\'e excitons (XRM), which are moir\'e trapped Rydberg excitons in monolayer semiconductor WSe2 adjacent to twisted bilayer graphene. In the strong coupling regime, the XRM manifest as multiple energy splittings, pronounced redshift, and narrowed linewidth in the reflectance spectra, highlighting their charge-transfer character where electron-hole separation is enforced by the strongly asymmetric interlayer Coulomb interactions. Our findings pave the way for pursuing novel physics and quantum technology exploitation based on the excitonic Rydberg states.Comment: 24 pages, including 4 figures and 6 supplementary figure

Atomic and Electronic Structure of Graphene and Graphene Intercalation Compounds. X-Ray Standing Wave and Scanning Tunnelling Microscopy Studies

The morphology of graphene/Iridium(111) was studied by x-ray standing wave (XSW) measurements. A dependence of the moire corrugation on the graphene coverage is observed. A comparison with density functional theory (DFT) reveals a discrepancy on the corrugation caused by stress appearing from the cool down from the preparation temperature. The model of rehybridised graphene due to cluster adsorption is supported by a structure analysis. Graphene intercalation compounds were investigated by scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), and XSW. It is shown that intercalation takes place via cracks and holes at wrinkles and wrinkle crossings. The superstructures of caesium intercalated graphene are resolved. For intercalants interacting mainly via van der Waals forces it could be shown that the graphene-intercalant distance is dependent on the charge transfer. Moreover, the structure analysis supports that oxygen intercalation leads to quasi freestanding graphene. A rigid-band model is introduced and applied to graphene intercalation compounds. Scanning tunnelling microscopy measurements reveal clear indications for Dirac electron scattering at defects. In these processes the pseudo-spin is not conserved leading to both inter- and intravalley scattering

Optical Properties of Al@Al2O3 Nanorod as a UV and Visible Wavelengths Plasmonic Nanostructure

AbstractAluminum as a new plasmonic material shows deep ultraviolet plasmon resonances which are broadly tunable. The use of aluminum plasmonic nanostructures offers new approaches, such as access high energy regions of the spectrum, low-cost and sustainable material. Therefore, aluminum is capable of being alternative plasmonic material compared to conventional gold and silver nanostructures. In this research, surface plasmon resonance properties of Al@Al2O3 core@shell nanorods in different dielectric environments were investigated. Using boundary element method and MNPBEM simulation package the sensitivity of aluminum plasmon resonance to the presence of Al2O3 layer, different aspect ratios and different dielectric mediums were studied. Results show that in Al nanorods with diameter of 3nm increasing length from 3 to 7nm plasmon longitudinal peak wavelength monotonously increase from 138nm to 213nm. In Al@Al2O3 nanorods with the same size and presence of 0.5nm Al2O3 oxide layer the peak wavelengths dramatically shift to higher values from 307nm to 514nm in the middle of visible region. Plasmon resonance sensitivity to medium refractive index was also investigated. Both aluminum and Al@Al2O3 nanorods exhibit red shift of longitudinal plasmon resonance wavelength by increasing refractive index from 1 to 2. Furthermore, red shift of plasmon peak wavelength by refractive index is linear in both cases. Finally results show that plasmonic response of Al@Al2O3 nanorods depend sensitively on presence of oxide layer, size and dielectric medium. As a new plasmonic material, Al@Al2O3 nanorods are capable for tremendous application due to wide ranges of plasmon resonances from deep UV to the middle of visible region

Infrared photoresistance as a sensitive probe of electronic transport in twisted bilayer graphene

We report on observation of the infrared photoresistance of twisted bilayer graphene (tBLG) under continuous quantum cascade laser illumination at a frequency of 57.1 THz. The photoresistance shows an intricate sign alternating behavior under variations of temperature and back gate voltage, and exhibits giant resonance-like enhancements at certain gate voltages. The structure of the photoresponse correlates with weaker features in the dark dc resistance reflecting the complex band structure of tBLG. It is shown that the observed photoresistance is well captured by a bolometric model describing the electron and hole gas heating, which implies an ultrafast thermalization of the photoexcited electron–hole pairs in the whole range of studied temperatures and back gate voltages. We establish that photoresistance can serve a highly sensitive probe of the temperature variations of electronic transport in tBLG