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<jats:title>Abstract</jats:title> <jats:p>In stacks of transition metal dichalcogenide monolayers with arbitrary twisting angles, we explore a new class of bright excitons arising from the pronounced Förster coupling, whose dimensionality is tuned by its in-plane momentum. The low energy sector at small momenta is two-dimensional, featuring a Mexican Hat dispersion, while the high energy sector at larger momenta becomes three-dimensional (3D) with sizable group velocity both in-plane and out-of-plane. By choices of the spacer thickness, versatile surface or interface exciton modes localized at designated layers can emerge out of the cross-dimensional bulk dispersion for a topological origin, which can be mapped to the Su-Schrieffer-Heeger soliton. Moreover, step-edges in spacers can be exploited for engineering lateral interfaces to enable interlayer communication of the topological interface exciton. Combined with the polarization selection rule inherited from the monolayer building block, these exotic exciton properties open up new opportunities for multilayer design towards 3D integration of valley exciton optoelectronics.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Photon-energy dependence of photoemission from seven-atoms-wide armchair graphene nanoribbons is studied experimentally and theoretically up to hν = 95 eV. A strong hv dependence of the normal emission from the valence band maximum (VB<jats:sub>1</jats:sub>) is observed, sharply peaked at hv = 12 eV. The detailed analysis of the light-polarization dependence of the photoemission from VB<jats:sub>1</jats:sub> unambiguously&amp;#xD;characterizes the symmetry of the state. The experimental observations are analyzed based on ab initio one-step theory of photoemission. Off-normal emission is studied in detail and its relation to the standing-wave character of the valence band states is discussed. Excellent agreement with the earlier experiment [Senkovskiy et al. 2018 2D Materials 5 035007] is obtained. Rapid variations of the intensity with the ribbon transverse photoelectron momentum are predicted from the ab initio theory, which are at variance with the prediction of the popular tight-binding rigid-wall model. These findings are instrumental for the study of the electronic structure of nanoribbons with angle-resolved photoemission. Moreover, the strong enhancement of the photoyield could trigger the GNR application as narrow-band photodetectors and contribute to the design of novel photocathodes for vacuum ultraviolet photodetection.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The growth of monolayer hexagonal boron nitride (h-BN) on Ir(110) through low-pressure chemical vapor deposition is investigated using low energy electron diffraction and scanning tunneling microscopy. We find that the growth of aligned hexagonal boron nitride on Ir(110) requires a growth temperature of 1500\,K, whereas lower growth temperatures result in coexistence of aligned h-BN with twisted h-BN. The presence of the h-BN overlayer suppresses the formation of the nano-faceted ridge pattern known from clean Ir(110). Instead, we observe the formation of a $(1 \times n)$ reconstruction, with n such that the missing rows are in registry with the h-BN/Ir(110) moir\'{e} pattern. Our moir\'{e} analysis showcases a precise methodology for determining both the moir\'{e} periodicity and the h-BN lattice parameter on an fcc(110) surface. Aligned h-BN on Ir(110) is found to be slightly compressed compared to bulk h-BN, with a monolayer lattice parameter of $a_{\rm{h-BN}} = (0.2489 \pm 0.0006)$\,nm. The lattice mismatch with the substrate along $\left[ 1 \bar{1} 0 \right]$ gives rise to a moir\'{e} periodicity of $a_{\rm{m}} = 2.99 \pm 0.08$\,nm.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The true character of physical phenomena is thought to be reinforced as the system becomes disorder-free. In contrast, the two-dimensional (2D) superconductor is predicted to turn fragile and resistive away from the limit I → 0, B → 0, in the pinning-free regime. It is intriguing to note that the very vortices responsible for achieving superconductivity by pairing, condensation, and, thereby reducing the classical dissipation, render the state resistive driven by quantum fluctuations in the T → 0. While cleaner systems are being explored for technological improvements, the 2D superconductor turning resistive when influenced by weak electric and magnetic fields has profound consequences for quantum technologies. A metallic ground state in 2D is beyond the consensus of both Bosonic and Fermionic systems, and its origin and nature warrant a comprehensive theoretical understanding supplemented by in-depth experiments. A real-time observation of the influence of vortex dynamics on transport properties so far has been elusive. We explore the nature and fate of a low-viscous, clean, 2D superconducting state formed on an ionic-liquid gated few-layered MoS<jats:sub>2</jats:sub> sample. The vortex-core being dissipative, the elastic depinning, intervortex interaction, and the subsequent dynamics of the vortex-lattice leave transient signatures in the transport characteristics. The temperature and magnetic field dependence of the transient nature and the noise characteristics of the magnetoresistance confirm that quantum fluctuations are solely responsible for the Bose metal state and the fragility of the superconducting state.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Layered ferromagnetic materials are significant for nano-spintronic devices, however, low transition temperature and air instability remain major challenges for layered ferromagnetic compounds. Herein, we have synthesized layered crystals CrIr2Sn10 with ferromagnetic transition below 315 K. The ratio of the magnetization between in-plane and out-of-plane is 41. Moreover, the magnetism of CrIr2Sn10 is derived from the highly spin-polarized Cr atoms. CrIr2Sn10 will be a promising platform for 2D magnetism and spintronic devices.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate a pentagonal monolayer of palladium diselenide, a stable two-dimensional system, as a material realization of a crystalline phase with nontrivial topological electronic properties. We find that its electronic structure involves an atomic obstructed insulator related to higher-order topology, which is a consequence of the selenium-selenium bond dimerization along with inversion and time-reversal symmetry. By means of first-principles calculations and the analysis of symmetry indicators and topological invariants, we also characterize the electronic corner states associated with the atomic obstruction and compute the corresponding corner charge for a finite geometry, which is found to be not quantized but still inversion-protected. Applying tensile strain to the finite geometry we verify the robustness of the corner states and also achieve a strain-controlled variation of the corner charge magnitude.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Photocatalytic reduction of carbon dioxide (CO2) has been expected to be an effective way to reduce carbon emissions. Designing photocatalytic materials with long-term effectiveness is the key of photocatalytic technology. In this work, CoO nanoparticles loaded on the surface of reduced graphene oxide (rGO) membranes on silicon substrate were in-situ fabricated by one-step method. The resulting materials can convert CO2 into carbon monoxide (CO) up to 70 h at a steady rate of ~185 ± 30 µmol g-1 h-1 with a selectivity of nearly 100%. This material system contained rich oxygen vacancies and generated new oxygen vacancies during the photocatalytic process. Oxygen vacancies mediate the interactions with excitons: (i) promoting the dissociation of free excitons; (ii) leading to form bound excitons under the coupling effect with phonons, inhibiting the recombination of photogenerated electrons and holes as well as enhancing the long-term effectiveness of photocatalytic CO2 reduction. We hope this work can provide valuable insights for the design and optimization of photocatalytic materials.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recent growth of two-dimensional (2D) layered crystals of MoSi2N4 and WSi2N4 has sparked significant interest due to their outstanding properties and potential applications. This development has paved the way for a new and large family of 2D materials with a general formula of MA2Z 4. In this regard, motivated by this exciting family, we propose two structural phases (1T-and 1H -) of M Si2N4 (M = Ge, Sn, and Pb) monolayers and investigate their structural, vibrational, mechanical, electronic and optical properties by using first-principles methods. The two phases have similar cohesive energies, while the 1T structures are found to be more energetically favorable than their 1H counterparts. The analysis of phonon spectra and ab initio molecular dynamics simulations indicate that all the suggested monolayers, except for 1H -GeSi2N4, are dynamically and thermally stable even at elevated temperatures. The elastic stability and mechanical properties of the proposed crystals are examined by calculating their elastic constants (Cij), in-plane stiffness (Y2D), Poisson’s ratio (ν), and ultimate tensile strain (UTS). Remarkably, the considered systems exhibit prominent mechanical features such as substantial in-plane stiffness and high UTS. The calculated electronic band structures reveal that both the 1T- and 1H -M Si2N4 nanosheets are wide-band-gap semiconductors and their energy band gaps span from visible to ultraviolet region of the optical spectrum, suitable for high-performance nanoelectronic device applications. Lastly, the analysis of optical properties shows that the designed systems have isotropic optical spectra, and depending on the type of the system, robust absorption of ultraviolet and visible light (particularly in 1H -PbSi2N4 monolayer) is predicted. Our study not only introduces new members to the family of 2D MA2Z 4 crystals but also unveils their intriguing physical properties and suggests them as promising candidates for diverse nanomechanical and optoelectronic applications.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The investigation of exotic properties in two-dimensional (2D) topological superconductors has garnered increasing attention in condensed matter physics, particularly for applications in topological qubits. Despite this interest, a reliable way of fabricating topological Josephson junctions (JJs) utilizing topological superconductors has yet to be demonstrated. Controllable structural phase transition presents a unique approach to achieving topological JJs in atomically thin 2D topological superconductors. In this work, we report the pioneering demonstration of a structural phase transition from the superconducting to the semiconducting phase in the 2D topological superconductor 2M-WS2. We reveal that the metastable 2M phase of WS2 remains stable in ambient conditions but transitions to the 2H phase when subjected to temperatures above 150°C. We further locally induced the 2H phase within 2M-WS2 nanolayers using laser irradiation. Notably, the 2H phase region exhibits a hexagonal shape, and scanning tunneling microscopy (STM) uncovers an atomically sharp crystal structural transition between the 2H and 2M phase regions. Moreover, the 2M to 2H phase transition can be induced at the nanometer scale by a 200 keV electron beam. The electrical transport measurements further confirmed the superconductivity of the pristine 2M-WS2 and the semiconducting behavior of the laser-irradiated 2M-WS2. Our results establish a novel approach for controllable topological phase change in 2D topological superconductors, significantly impacting the development of atomically scaled planar topological JJs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Silicon's potential for having magnetic properties is fascinating, as combining its electronic capabilities with magnetic response seems promising for spintronics. In this work, the mechanisms that drive the change from diamagnetic behavior in pure silicates like SiO2 to paramagnetic behavior in transition metal-doped silicates like Rhodonite silicate (CaMn3Mn(Si5O15)) are explored. This naturally occurring Rhodonite (R) -silicate was thinned down while retaining its magnetic properties by liquid-phase scalable exfoliation. Exfoliating R-silicate into the two-dimensional (2D) structure by LPE increases magnetic coercivity, and the internal resistance to demagnetization (ΔHc) up to ~23.95 Oe compared to 7.08 Oe for its bulk phase. DFT spin-polarized calculations corroborate those findings and explain that the origin of the magnetic moment comes mainly from the Mn in the doped 2D silicate due to the asymmetrical components of the Mn d and Si p states in the valence band. This result is further illustrated by the spin component differential charge densities showing that Mn and Si atoms display a residual up spin charge. Rhodonite's unusual magnetic behavior has considerable potential for spintronics, data storage, and sensing technologies. Unraveling the complicated interplay of silicates' structural, magnetic, and electronic properties provides critical insights for future research and the development of novel materials and composites.</jats:p>