3,130 research outputs found
Line-Graph Lattices: Euclidean and Non-Euclidean Flat Bands, and Implementations in Circuit Quantum Electrodynamics
Materials science and the study of the electronic properties of solids are a
major field of interest in both physics and engineering. The starting point for
all such calculations is single-electron, or non-interacting, band structure
calculations, and in the limit of strong on-site confinement this can be
reduced to graph-like tight-binding models. In this context, both
mathematicians and physicists have developed largely independent methods for
solving these models. In this paper we will combine and present results from
both fields. In particular, we will discuss a class of lattices which can be
realized as line graphs of other lattices, both in Euclidean and hyperbolic
space. These lattices display highly unusual features including flat bands and
localized eigenstates of compact support. We will use the methods of both
fields to show how these properties arise and systems for classifying the
phenomenology of these lattices, as well as criteria for maximizing the gaps.
Furthermore, we will present a particular hardware implementation using
superconducting coplanar waveguide resonators that can realize a wide variety
of these lattices in both non-interacting and interacting form
Quantum Graphs II: Some spectral properties of quantum and combinatorial graphs
The paper deals with some spectral properties of (mostly infinite) quantum
and combinatorial graphs. Quantum graphs have been intensively studied lately
due to their numerous applications to mesoscopic physics, nanotechnology,
optics, and other areas.
A Schnol type theorem is proven that allows one to detect that a point
belongs to the spectrum when a generalized eigenfunction with an subexponential
growth integral estimate is available. A theorem on spectral gap opening for
``decorated'' quantum graphs is established (its analog is known for the
combinatorial case). It is also shown that if a periodic combinatorial or
quantum graph has a point spectrum, it is generated by compactly supported
eigenfunctions (``scars'').Comment: 4 eps figures, LATEX file, 21 pages Revised form: a cut-and-paste
blooper fixe
Pseudo-random graphs
Random graphs have proven to be one of the most important and fruitful
concepts in modern Combinatorics and Theoretical Computer Science. Besides
being a fascinating study subject for their own sake, they serve as essential
instruments in proving an enormous number of combinatorial statements, making
their role quite hard to overestimate. Their tremendous success serves as a
natural motivation for the following very general and deep informal questions:
what are the essential properties of random graphs? How can one tell when a
given graph behaves like a random graph? How to create deterministically graphs
that look random-like? This leads us to a concept of pseudo-random graphs and
the aim of this survey is to provide a systematic treatment of this concept.Comment: 50 page
Effect of polycrystallinity on the optical properties of highly oriented ZnO grown by pulsed laser deposition
We report the results of photoluminescence and reflectance measurements on highly c-axis oriented polycrystalline ZnO grown by pulsed laser deposition. The samples measured were grown under identical conditions and were annealed in-situ at various temperatures for 10-15 min. The band-edge photoluminescence spectra of the material altered considerably with an increase in grain size, with increased free exciton emission and observable excitonic structure in the reflectance spectra. The green band emission also increased with increasing grain size. A deformation potential analysis of the effect of strain on the exciton energy positions of the A- and B-excitons demonstrated that the experimental exciton energies could not be explained solely in terms of sample strain. We propose that electric fields in the samples due to charge trapping at grain boundaries are responsible for the additional perturbation of the excitons. This interpretation is supported by theoretical estimates of the exciton energy perturbation due to electric fields. The behaviour of the green band in the samples provides additional evidence in favour of our model
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