1,721,072 research outputs found
Topological transmission line networks
No full textThis thesis is mainly about the study of topological models and the corresponding proof-of-principle experimental demonstrations by transmission line networks. Due to the similarity between the network equation and the tight-binding model, the transmission line network is an ideal platform for mimicking the tight-binding model in both experiment and simulation. In the first half of the thesis, we propose a topological model with angular-momentum-orbital coupling, similar to the spin-orbital coupling in the quantum spin Hall effect. It exhibits nontrivial topological property in the non-zero angular momentum subspace. A transmission line network is built based on this model, and the one-way topological edge states are observed experimentally. Furthermore, we introduce the angular-momentum-preserved randomness to this tight-binding model to verify the topological states' robustness and study topological Anderson insulators theoretically. In the other half of the thesis, we study the non-Abelian band topology using transmission lines. The non-Abelian band topology is a new theory proposed to study the topological property of multiple bands (>2) PT (parity and time-reversal) symmetric systems, in which the fundamental group of the order-parameter space is a non-Abelian group. Several quasi-1D transmission line networks with three sublattices (3-band) are built in the experiment. By observing the eigenstate rotations on the eigenstate sphere in momentum space, we directly detect different non-Abelian quaternion charges experimentally. In addition, we propose a non-Abelian edge-bulk correspondence theory to predict the energy positions of interface states. This new theory can be inferred from the traditional Abelian view. Furthermore, we prove this non-Abelian edge-bulk correspondence by using the Jackiw-Rebbi argument. Finally, we design a circuit experiment to mimic the nodal points braiding in a 2D PT-symmetric system. It is expected to observe that two nodal points carrying the same non-Abelian charge fail to annihilate after collision with each other. The simulation results verify the feasibility of this design.</p
Topologically charged nodal points and surfaces in photonic systems
No full textThe realization of topological band degeneracies in the photonic system is reported in this thesis. The topological band degeneracies have recently attracted considerable attention because they usually carry topological charges in the momentum space, which induces several interesting phenomena, such as helicoid surface state, chiral anomaly, and Imbert– Fedorov shift. Therefore, the realization of these band degeneracies in a photonic system, especially through a sufficiently simple structure to facilitate attainable experiments, is an important topic. Weyl point, as a zero-dimensional point degeneracy in the momentum space, is the most common topological nodal point. A simple chiral woodpile photonic crystal is designed, and Weyl points with topological charges 1 and 2 are found. The topological charge distribution can be changed by modifying the material parameter, while the symmetry of the system is maintained. A tight-binding model is established to understand the physics and show these Weyl points in the real structure with full-wave simulation. The gapless surface states are shown in a ribbon of the woodpile photonic crystal. The unidirectional transport, which is immune to backscattering, is also demonstrated. Realizing the Weyl points in the simple fabricated woodpile structure provides a platform for exploring the Weyl physics toward infrared and optical frequency. Meanwhile, the topological band degeneracies can also exhibit as a two-dimensional plane on the surface of the Brillouin zone, forming a topological charge nodal surface. A metacrystal with a topologically nontrivial nodal surface operating at microwave frequency is designed, fabricated, and characterized. Compared with the nodal surface in acoustic wave designed by the tight-binding model, the nodal surface in the proposed system is protected by a combination of two-fold screw rotation and time-reversal symmetry. The surface-state arcs connecting the nodal surface and charge-2 Weyl point in the system are experimentally observed to demonstrate the existence of the nodal surface and its topological properties. These surface-state arcs are derived from the helicoid sheet surface states. The band structure of the surface state is also measured. This system can facilitate the realization of the new class of topological degeneracy in electromagnetic waves. We also show that a metacrystal with connected-spirals structure can exhibit high-order Weyl point as a gapless system that carries topological quadruple. We demonstrate the existence of topological quadruple index is protected by combining two-fold rotation symmetry and time-reversal symmetry. In this system, both gapless surface states induced by topological charge and gapped surface states induced by topological quadruple can co-exist. The hinge state as the distinct feature of the topological high-order mode can be found on the hinge of a finite sample. We believe this work can offer a proposal to realize high-order Weyl point experimentally in an EM wave system. The current work on Weyl point, topological nodal surface and high order Weyl point in photonic systems can help realize these topological degeneracies with a simple design and experimentally explore their novel physics properties.</p
APPLICATION OF MACHINE LEARNING TO METAMATERIALS FOR INVERSE DESIGN AND TOPOLOGICAL CLASSIFICATION
No full textIn this thesis, we report the application of machine learning in physics, including inverse design and clustering. With rapid advancements in machine learning algorithms, many researchers are leveraging these tools to solve various physical problems. These algorithms offer significant advantages, including speed, cost reduction, and the ability to detect subtle features that may be overlooked by humans, highlighting their vast potential. Here, we focus on two specific physics problems. The first problem is the inverse design of metamaterial microwave absorbers. While metamaterial microwave absorbers are widely used in electromagnetics, traditional design methods often require extensive time and effort to fine-tune parameters. We propose a tandem artificial neural network for the inverse design of metamaterial microwave absorbers, incorporating physical constraints into the cost function. This allows us to achieve effective end-to-end design of high-performance microwave absorbers. The second problem focuses on the clustering of non-Abelian topological phases, which have attracted attention from researchers in recent years for their unique properties. The complicated dataset of the non-Abelian system poses challenges for automated classification. We employ an unsupervised learning method based on the diffusion map algorithm for clustering non-Abelian phases without the need to calculate topological charges. Moreover, our algorithm can automatically infer the multiplication table of the non-Abelian charges carried by the samples. In addition, our approach can do the correct topological classification in the context of homotopy both with and without a fixed base point, highlighting the role of the fixed base point in non-Abelian topology. In summary, this thesis illustrates the immense potential of machine learning in addressing physical problems, underscoring its significance for both academic inquiry and practical applications. For practical applications, machine learning-based inverse design offers significant convenience for device design. Even in the absence of prior knowledge in topological physics, data-driven automatic clustering algorithms have successfully achieved challenging non-Abelian topological classifications, illustrating their use in exploring new physics. These two contributions pave the way for the further combination of machine learning algorithms and physics. It is believed that these works can provide additional perspectives for further exploration of machine learning applications in various fields of physics.</p
Three-dimensional nonreciprocal transport in photonic topological heterostructure of arbitrary shape
No full textElectromagnetic wave propagation in three-dimensional (3D) space typically suffers omnidirectional scattering when encountering obstacles. In this study, we used Chern vectors to construct a topological heterostructure, where large-volume nonreciprocal topological transport in 3D is achieved. The shape of the cross section in the heterostructure can be arbitrary designed, and we experimentally observed the distinctive cross-shaped field pattern transport, nonreciprocal energy harvesting, and the remarkable ability of electromagnetic wave to traverse obstacles and abrupt structure changes without encountering reflections in 3D space
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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