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Group IVA of 2D Xenes materials (Silicene, Germanene, Stanene, Plumbene)
Published in Zongyu Huang, Xiang Qi, Jianxin Zhong, 2D Monoelemental Materials (Xenes) and Related Technologies, 2022
Yundan Liu, Dan Mu, Jincheng Zhuang
As shown in Figure 3.12b, the calculated band gap induced by spin-orbit coupling in plumbene is 0.467 eV. Such large value of gaps can be attributed to the strong SOC effect caused by the heavy Pb atoms as well as the px/py orbital character of the low-energy states. The gap is opened at Dirac points and Γ point, which means that plumbene is a two-dimensional topological insulator with Z2 invariant.93 Functionalization is an effective way to tune the band structure of plumbene. For example, the band gap of plumbene has been successfully tuned by modifying with F, Cl, Br, I, H, and SiH3, which can achieve an incasement as high as 1 eV on the band gap size.93 Wang et al. reported that free-standing plumbene is a topological insulator with a large gap through electron doping, and the nontrivial state is very robust with respect to external strain. Therefore, plumbene was regarded as an ideal candidate for realizing the quantum spin Hall effect at room temperature (RT), promising a great potential for the application of new quantum device.97
History and Mental State Examination
Published in Richard Kerslake, Elizabeths Templeton, Lisanne Stock, Revision Guide for MRCPsych Paper A, 2018
Since the discovery of TIs, especially 3D TI materials including bismuth chalcogenides (i.e., Bi2Se3, Bi2Te3, Bi2Te2Se, …), many new opportunities have emerged in physics and device structures [48]. First, the topologically robust surface states in TIs protected by time-reversal-symmetry enable the coherent transport for the massless Dirac fermion with an ultrahigh mobility on the surfaces. In addition, the quantum spin Hall effect occurs in TI systems without the need of applied magnetic field that induces spatial separation between the traffic lanes of electrons. Importantly, it prohibits any elastic backscattering from nonmagnetic impurities due to the topological phase as a result of a large intrinsic spin-orbit coupling. Thus, TIs have potential for applications in low-power-dissipation electronic devices. Second, the spin-momentum-lock mechanism promises potential applications using an effective spin-polarized current, which can be generated by simply applying a lateral electric-field across a TI film. Hence, it is believed that TIs are a suitable platform for spin-tronic applications.
Hexagonal honeycomb silicon: Silicene
Published in Klaus D. Sattler, Silicon Nanomaterials Sourcebook, 2017
Xin Tan, Sean C. Smith, Zhongfang Chen
In recent years, tremendous theoretical efforts have been devoted to assessing the fundamental properties of freestanding silicene, and various exciting, rich, physical and chemical properties have been predicted (Cahangirov et al. 2009; Chen et al. 2012; Feng et al. 2012). For example, the electronic structures of silicene and graphene are similar: both have a Dirac cone and linear electronic dispersion around K points (Cahangirov et al. 2009). Experimentally observable quantum spin Hall effect (QSHE) and quantum anomalous Hall effect (QAHE) were also predicted in silicene (Liu et al. 2011).
Broadband valley-locked waveguide states of elastic wave in topological phononic crystal plates with asymmetric double-sided pillars
Published in Mechanics of Advanced Materials and Structures, 2022
Shao-yong Huo, Guan-hong Xie, Shi-jia Qiu, Xiao-chao Gong, Shao-zhang Fan, Chun-ming Fu, Zhen-ye Li
TIs, with novel topological interface/edge modes, have renewed our understanding for condensed matter physics, which have been observed in a variety of quantum Hall families of both fermionic and bosonic systems such as quantum Hall effect (QHE), quantum spin Hall effect (QSHE), and quantum valley Hall effect (QVHE), and provide building blocks of various topological devices [13]. For the classical acoustic wave system, the realization of topologically protected acoustic propagation mainly relies on two wide categories of mechanisms. The first one is to break the time-reversal symmetry of system to simulate the QHE by introducing circulating fluid flow [14,15] and rotating gyroscopes [16]. The other one is to break the spatial inversion symmetry of system to simulate the QSHE and QVHE by reducing the symmetry of lattice including the translational, specular, or rotational symmetries [17–23]. Due to the fact that designing the QSH or QVH acoustic TI is the convenient and flexible just by reasonably disposing of structural parameters and symmetries without adding any external field, various schemes have been proposed to realize them and some novel applications have emerged, such as topological acoustic antenna [24], topological acoustic splitter and concentrator [25,26].
Subwavelength thermally controlled acoustic topological interface states in split hollow spheres
Published in Mechanics of Advanced Materials and Structures, 2023
Guifeng Wang, Yanhong Guan, Zhenyu Chen, Xinsheng Xu, Zhenhuan Zhou, C.W. Lim
The research on topologically protected phenomena in acoustics [22, 23] and mechanics [24, 25] is extended from analogous research in condensed matter physics [26]. Generally, the topological protection in two dimensional (2D) structures results from some common effects: quantum Hall effect (QHE) [22, 27], quantum valley Hall effect (QVHE) [28, 29], and quantum spin Hall effect (QSHE) [30, 31], etc. In QHE, an external field is necessary to interact with the wave propagation medium and break the time-reversal symmetry (TRS). Spinning rotors [32, 33], fluids [34], or other rotating inclusions are generally employed to break TRS in acoustic systems. By contrast, breaking spatial inversion symmetry (SIS) is preferred to obtain topological protections in acoustics and elastic systems because classical phonons are not very sensitive to the changes in magnetic fields [35]. As early as 1979, Su et al. [36] created the Su-Schrieffer-Heeger (SSH) model, which is a one-dimensional (1D) long-chain polyenes and topological non-trivial phases were captured. Recently, Chen et al. [37] designed a 1D topological metamaterial beam by periodically and alternatively attaching elastic foundations on a homogenous beam. In two-dimensional (2D) systems, Al Ba’ba’a et al. [38] reported a topological insulator by introducing QVHE to a 2D spring-mass system. Chen et al. [39] utilized QSHE to design a 2D membrane-type metamaterial and observed topologically protected interface modes (TPIMs). Although there are more advance and complicated topology concepts such as the quantum anomalous Hall effect (QAHE), the related research is not inside the scope of the current study and it is subject for future studies.
Tailoring geometric phases of two-dimensional functional materials under light: a brief review
Published in International Journal of Smart and Nano Materials, 2020
For 2D materials, such light driven phase transformation is also seen experimentally. One of the most famous examples that attracts great attention recently is monolayer transition metal dichalcogenide (TMD). The phase transition of TMD monolayers is very promising, as they usually show two phases, namely, 2 H and 1 T’. Both of them are of significant importance with promising physical properties. The 2 H phase has an intermediate bandgap, with low energy bands lie at the K and K’ points of the first Brillouin zone [32]. Owing to the inversion symmetry breaking, the electron states at these two points (electronic valleys) can be selectively excited by different handed circularly polarized light [33]. Thus, 2 H TMD can serve as a good valleytronic platform in the future, for very low energy consumption information storage devices. On the other hand, the 1 T’ TMD monolayer is typically a semimetal, with inversion symmetry. It has two Dirac points in the first Brillouin zone, which can open a finite bandgap once spin orbit coupling is included. It thus is classified as a 2D Z2 topological insulator, which hosts fault-tolerant quantum spin Hall effect and is applicable in future quantum computing devices [34]. From geometric point of view, the phase transformation between 2 H and 1 T’ in TMD only involves a short distance sliding of one layer chalcogen atoms and a subsequent spontaneous M-M dimerization (Peierls distortion). Thus, it is also a martensitic phase transition with diffusionless atomic motions, and may occur ultrafast in a short timescale [35]. Several strategies have been proposed to induce phase transitions from the ground state 2 H to metastable 1 T’ phase (except for monolayer WTe2 whose 1 T’ is energetically lower than 2 H), especially in the MoTe2, as the two phases in this system differ least than other analogues.