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10 pages, 280 KB

Open AccessReview

Pseudo-Quantum Electrodynamics: 30 Years of Reduced QED

by Eduardo C. Marino, Leandro O. Nascimento, Van Sérgio Alves and Danilo T. Alves

Entropy 2024, 26(11), 925; https://doi.org/10.3390/e26110925 - 30 Oct 2024

Cited by 2 | Viewed by 1361

Charged quasiparticles, which are constrained to move on a plane, interact by means of electromagnetic (EM) fields which are not subject to this constraint, living, thus, in three-dimensional space. We have, consequently, a hybrid situation where the particles of a given system and the EM fields (through which they interact) live in different dimensions. Pseudo-Quantum Electrodynamics (PQED) is a U(1) gauge field theory that, despite being strictly formulated in two-dimensional space, precisely describes the real EM interaction of charged particles confined to a plane. PQED is completely different from QED(2 + 1), namely, Quantum Electrodynamics of a planar gauge field. It produces, for instance, the correct

1 / r

Coulomb potential between static charges, whereas QED(2 + 1) produces

ln r

potential. In spite of possessing a nonlocal Lagrangian, it has been shown that PQED preserves both causality and unitarity, as well as the Huygens principle. PQED has been applied successfully to describe the EM interaction of numerous systems containing charged particles constrained to move on a plane. Among these are p-electrons in graphene, silicene, and transition-metal dichalcogenides; systems exhibiting the Valley Quantum Hall Effect; systems inside cavities; and bosonization in (2 + 1)D. Here, we present a review article on PQED (also known as Reduced Quantum Electrodynamics). Full article

(This article belongs to the Special Issue PQED: 30 Years of Reduced Quantum Electrodynamics)

11 pages, 9639 KB

Open AccessEditor’s ChoiceArticle

Ferrovalley and Quantum Anomalous Hall Effect in Janus TiTeCl Monolayer

by Yufang Chang, Zhijun Zhang, Li Deng, Yanzhao Wu and Xianmin Zhang

Materials 2024, 17(13), 3331; https://doi.org/10.3390/ma17133331 - 5 Jul 2024

Cited by 4 | Viewed by 1428

Abstract

Ferrovalley materials are garnering significant interest for their potential roles in advancing information processing and enhancing data storage capabilities. This study utilizes first-principles calculations to determine that the Janus monolayer TiTeCl exhibits the properties of a ferrovalley semiconductor. This material demonstrates valley polarization with a notable valley splitting of 80 meV. Additionally, the Berry curvature has been computed across the first Brillouin zone of the monolayer TiTeCl. The research also highlights that topological phase transitions ranging from ferrovalley and half-valley metals to quantum anomalous Hall effect states can occur in monolayer TiTeCl under compressive strains ranging from −1% to 0%. Throughout these strain changes, monolayer TiTeCl maintains its ferromagnetic coupling. These characteristics make monolayer TiTeCl a promising candidate for the development of new valleytronic and topological devices. Full article

(This article belongs to the Special Issue Research on Interfaces and Transportation Phenomena in Materials Under Extreme Conditions)

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17 pages, 3174 KB

Open AccessEditor’s ChoiceArticle

Bond-Orbital-Resolved Piezoelectricity in Sp²-Hybridized Monolayer Semiconductors

by Zongtan Wang, Yulan Liu and Biao Wang

Materials 2022, 15(21), 7788; https://doi.org/10.3390/ma15217788 - 4 Nov 2022

Cited by 1 | Viewed by 2060

Abstract

Sp²-hybridized monolayer semiconductors (e.g., planar group III-V and IV-IV binary compounds) with inversion symmetry breaking (ISB) display piezoelectricity governed by their σ- and π-bond electrons. Here, we studied their bond-orbital-resolved electronic piezoelectricity (i.e., the σ- and π-piezoelectricity). We formulated a tight-binding [...] Read more.

Δ

). As

Δ

varied from positive to negative, the former decreased continuously, but the latter increased piecewise and jumped at

Δ = 0

due to the criticality of the π-electrons’ ground-state geometry near this quantum phase-transition point. This led to a piezoelectricity predominated by the π-electrons for a small

| Δ |

. By constructing an analytical model, we clarified the microscopic mechanisms underlying the anomalous π-piezoelectricity and its subtle relations with the valley Hall effect. The validation of our models was justified by applying them to the typical sp² monolayers including hexagonal silicon carbide, Boron-X (X = N, P, As, Ab), and a BN-doped graphene superlattice. Full article

(This article belongs to the Special Issue Metasurfaces Meet Two-Dimensional Materials)

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12 pages, 7885 KB

Open AccessArticle

Rashba Splitting and Electronic Valley Characteristics of Janus Sb and Bi Topological Monolayers

by Qi Gong and Guiling Zhang

Int. J. Mol. Sci. 2022, 23(14), 7629; https://doi.org/10.3390/ijms23147629 - 10 Jul 2022

Cited by 3 | Viewed by 2305

Abstract

Janus Sb and Bi monolayers as a new class of 2D topological insulator materials, which could be fulfilled by asymmetrical functionalizations with methyl or hydroxyl, are demonstrated by first-principles spin–orbit coupling (SOC) electronic structure calculations to conflate nontrivial topology, Rashba splitting and valley-contrast circular dichroism. Cohesive energies and phonon frequency dispersion spectra indicate that all Janus Sb and Bi monolayers possess a structural stability in energetic statics but represent virtual acoustic phonon vibrations of the hydrogen atoms passivating on monolayer surfaces. Band structures of Janus Sb and Bi monolayers and their nanoribbons demonstrate they are nontrivial topological insulators. Rashba spin splitting at G point in Brillouin zone of Janus Bi monolayers arises from the strong SOC p_x and p_y orbitals of Bi bonding atoms together with the internal out-of-plane electric field caused by asymmetrical functionalization. Janus Sb and Bi monolayers render direct and indirect giant bandgaps, respectively, which are derived from the strong SOC p_x and p_y orbitals at band-valley Brillouin points K and K′ where valley-selective circular dichroism of spin valley Hall insulators is also exhibited. Full article

(This article belongs to the Special Issue Molecular Structure, Electronic and Vibrational Spectra Theoretical Calculations in Materials Sciences)

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11 pages, 3498 KB

Open AccessArticle

Valley Hall Elastic Edge States in Locally Resonant Metamaterials

by Wenbo Fang, Chunyu Han, Yuyang Chen and Yijie Liu

Materials 2022, 15(4), 1491; https://doi.org/10.3390/ma15041491 - 17 Feb 2022

Cited by 11 | Viewed by 3293

Abstract

This paper presents a locally resonant metamaterial periodically rearranged as a local resonator, that is hexagonal holes arranged in a thin plate replace the elastic local resonator to achieve the quantum valley Hall effect. Due to the

C_{3 v}

symmetry in the [...] Read more.

C_{3 v}

symmetry in the primitive hexagonal lattice, one Dirac point emerges at high symmetry points in the Brillouin zone in the sub-wavelength area. Rotating the beam element of the resonator can break the spatial inversion symmetry to lift the Dirac degeneracy and form a new bandgap. Thus, the band inversion is discovered by computing the relationship between the associated bandgap and the rotational parameter. We also confirmed this result by analyzing the vortex chirality and calculating the Chern number. We can discover two kinds of edge states in the projected band obtained by computing the supercell composed of different topological microstructures. Finally, the propagation behavior in various heterostructures at low frequencies was analyzed. It is shown that these valley Hall elastic insulators can guide elastic waves along sharp interfaces and are immune to backscattering from defects or disorder. By utilizing elastic resonators, a simple reconfigurable topological elastic metamaterial is realized in the sub-wavelength area. Full article

(This article belongs to the Special Issue Mechanical Metamaterials: Optimization and New Design Ideas)

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22 pages, 5061 KB

Open AccessReview

Progress in Topological Mechanics

by Shengjie Zheng, Guiju Duan and Baizhan Xia

Appl. Sci. 2022, 12(4), 1987; https://doi.org/10.3390/app12041987 - 14 Feb 2022

Cited by 24 | Viewed by 5057

Abstract

Topological mechanics is rapidly emerging as an attractive field of research where mechanical waveguides can be designed and controlled via topological methods. With the development of topological phases of matter, recent advances have shown that topological states have been realized in the elastic media exploiting analogue quantum Hall effect, analogue quantum spin Hall effect, analogue quantum valley Hall effect, higher-order topological physics, topological pump, topological lattice defects and so on. This review aims to introduce the experimental and theoretical achievements with defect-immune protected elastic waves in mechanical systems based on the abovementioned methods, respectively. From these discussions, we predict the possible perspective of topological mechanics. Full article

(This article belongs to the Special Issue Advances in Metamaterials for Sound and Vibration Control)

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