${\mathrm{NbSi}}_2$ crystallizes in the $P{6}_{2}22$ symmetry, featuring chiral chains of Si atoms. The absence of inversion symmetry, combined with its chiral structure, gives rise to unique physical properties. The breaking of inversion symmetry leads to the emergence of Weyl points, while the chiral structure enables the formation of chiral edge modes. As a result, ${\mathrm{NbSi}}_2$ serves as an ideal platform for exploring the interplay between phonon Weyl points and chiral phonon edge modes. For example, we identify the presence of a structure consisting of three Weyl points with a Chern number of $\mathcal{C}=+1$ around the $\overline{K}$ point. These nodes form unconventional Fermi arcs connecting the $\overline{\mathrm{\Gamma }}$ or $\overline{K}$ points, which mimic an effective Chern number of $\mathcal{C}=-2$.

Over the years, the in-plane and out-of-plane photonic spin Hall effects (PSHEs) have been treated as two independent phenomena. In this work, we establish a unified framework that reveals the interrelationship between the in-plane and out-of-plane PSHEs for arbitrarily polarized vortex beams undergoing reflection and transmission. We quantitatively demonstrate how the vortex-induced in-plane spin spatial shift relates to the out-of-plane spin angular shift of the fundamental Gaussian beam (FGB). Similarly, we show that the vortex-dependent out-of-plane spin spatial shift is quantitatively connected to the in-plane spin angular shift of the FGB. Moreover, we find that the vortex-related in-plane and out-of-plane spin angular shifts remain unaffected by the FGB's in-plane and out-of-plane spin spatial shifts. Additionally, we confirm that all of our theoretical results adhere to the principles of linear and angular momentum conservation.

Two-dimensional (2D) materials are categorized into van der Waals (vdW) and non-vdW types. However, no relevant descriptors have been proposed for identifying non-vdW 2D materials. Here, we identify the non-vdW 2D materials by calculating the thickness dependence of total energy of thin films truncated from surfaces. The non-vdW 2D materials exhibit a deviation from the law of finite-thickness excess energy (FTEE) inverse to the number of layers in the monolayer limit. This framework is applied to single-component systems, which successfully identifies non-vdW 2D materials including silicene and goldene that are overlooked in the dimensional analysis of the parent crystals. The anomalies of FTEE at specific thickness are found in Pb and Ga thin films, supporting the creation of plumbene and gallenene. The framework is also applied to multicomponent systems with cubic and hexagonal symmetry (zincblende- and fluorite-types and tungsten carbide- and PtSbBa-types, respectively). Among them, several non-vdW 2D materials are identified to be dynamically stable. Structural transformation from hexagonal to almost tetragonal symmetry is observed in metallic TiS and ZrTe thin films. Our framework is useful for identifying non-vdW 2D materials.

We investigated the influence of van der Waals interactions on the electronic structure at the interface between a pentacene monolayer and a graphite surface. Upon cooling below 130 K, the pentacene molecules form a densely packed monolayer characterized by newly developed dispersive bands. These bands, observed using low-energy angle-resolved photoelectron spectroscopy with photon energy varying from 7.2 to 8.5 eV, exhibit constant final-state characteristics that overlap with nondispersive molecular-orbital states. This overlap results in variations in photoemission intensity—both enhancement and suppressions—indicative of Fano resonance. Such resonance arises from the interaction between a discrete molecular state and a continuum state. The Fano profile analysis of spectral fine features reveals the wave-function connection by the broader spread in unoccupied states at the physisorbed interface.

In semiconductors, the electronic drift velocity can easily exceed the speed of sound in the material at moderate biases. The accelerated electrons lead to the generation and amplification of acoustic phonons that travel in the direction of the carriers via a process that is analogous to the generation of Cherenkov radiation by accelerated charged particles. Here, we demonstrate, using a high-Q acoustic cavity, orders of magnitude enhancement in subterahertz phonon amplification. The 0.44 THz acoustic cavity structure enables a long phonon lifetime (τ ≈ 0.56 ns), and we achieve a remarkable phonon gain efficiency of $0.3\times {10}^7\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{W}}^{-1}$. The study of high-frequency coherent acoustic phonons in semiconductor materials and heterostructures has led to significant advancements in various technological applications, including polaritonic devices, terahertz light modulation, phonon imaging, nondestructive testing, and ultrafast phononic circuits. These results further enhance our understanding of sound amplification in phononic structures, paving the way for future developments in phonon-based technologies.

The narrowband resonance characteristic of surface phonon polaritons limits the performance of many polar materials in near-field radiative heat transfer (NFRHT). To overcome this limitation, we propose a gradient in-plane coupled mode. Taking SiC as a typical narrowband phonon-polaritonic material, it is demonstrated that employing this strategy can achieve a 1.59 times enhancement in heat flux that surpasses those of single-groove and equivalent-depth metasurfaces. The near-field radiative heat flux, energy transmission coefficients, and magnetic field profiles are calculated using the rigorous coupled-wave analysis. The results reveal that the enhancement is attributed to the hybridization of multiband hyperbolic modes, which are induced by coupled cavity resonances in the grooves. Furthermore, the multiband hyperbolic modes lead to multiple resonance peaks in the spectral heat flux, which are not simply additive but result from the interference effects within the structure. Our work provides a paradigm for overcoming the limitations of single-phonon materials in NFRHT.

The optical absorbance of two-dimensional semiconductors is generalized to individual lattice sites through the topological marker formalism, yielding an absorbance marker. This marker allows us to investigate the atomic-scale variation of absorbance caused by impurities and thereby quantify the influence of disorder on the quantum geometry and the spread of Wannier functions of valence-band states. Applying this marker to transition-metal dichalcogenides reveals a very localized suppression of absorbance caused by potential impurities, rendering a reduction of absorbance in the macroscopic scale proportional to the impurity density, in good agreement with the experimental results of plasma-treated ${\mathrm{WS}}_2$.

Flat bands have become a pillar of modern condensed matter physics and photonics owing to the vanishing group velocity and diverging density of states. Here, we present a paradigmatic scheme to construct arbitrary flat bands on demand by introducing a new type of breathing superlattice, where both the number and spectral positions of isolated flat bands can be continuously tailored by simply controlling the breathing strength. Microscopically, the momentum-independent interband scatterings near the band edge protect them robustly against weak intracell disorder. By dimensional reduction, we establish a duality between the one-dimensional (1D) breathing superlattice and the 2D Harper-Hofstadter model, where cascade flat bands naturally emerge as the different orders of Landau levels in the weak magnetic flux limit. As a proof of concept, photonic flat bands at optical frequencies are experimentally demonstrated with all-dielectric photonic crystal slabs. Finally, we generalize our scheme to 2D systems to realize partial and omnidirectional flat bands, and discuss the achievement of high-quality factors. Our findings shed new light on the manipulation of flat bands with high band flatness and large usable bandwidth, paving the way for the development of advanced optical devices.

Domain-wall skyrmions are magnetic solitons embedded in a domain wall that are topologically equivalent to skyrmions. Here, we theoretically study antiferromagnetic domain-wall skyrmions and their current-driven motion within the Landau-Lifshitz-Gilbert phenomenology, and verify our findings with micromagnetic simulations. While the skyrmion Hall effect is expected to be suppressed in the current-induced motion of antiferromagnetic domain-wall skyrmions, we observe a finite Hall angle, which originates from the anisotropic spin configuration of domain-wall skyrmions. The skyrmion Hall effect is, however, conditionally suppressed and the motion aligns with the current applied in certain directions, which can be interpreted as principal axes of a domain-wall skyrmion that is easily identified from the symmetry of the spin configuration. Our work on antiferromagnetic domain-wall skyrmions shows that the dynamics of spin textures endowed with multiple soliton characteristics can be unconventional, which is envisaged to enrich the field of topological solitons.

We present terahertz spectroscopic measurements of quantum spin dynamics in the honeycomb magnet ${\mathrm{K}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ as a function of temperature, polarization and in an external magnetic field applied in the honeycomb plane. Magnetic excitations are resolved below the magnetic ordering temperature of $T_N=12$ K. In the applied magnetic field, we reveal characteristic field dependence not only for the magnetic excitations observed at zero field but also a rich set of modes emerging in finite fields. The observed magnetic excitations exhibit clear dependence on the terahertz polarization and characteristic features at field-induced phase transitions consistent with our high-field magnetization data. We cannot evidently resolve a continuumlike feature, even when the long-range magnetic order is presumably suppressed in the strong magnetic field, indicating that a Kitaev-type interaction, if existing, is subleading in this compound.

Silicon-based qubits are highly promising for scalable high compatibility with the existing semiconductor industry. However, a major challenge in utilizing 2-dimensional electron gas (2DEG) quantum well (QW) structures for qubits is maintaining high electron mobility ($\mu$) while achieving significant valley splitting ($E_v$). In this study, by employing a thorough chemical cleaning process for the chemical mechanical polish (CMP)-processed buffer layer, we minimized QW interface disorder in a $\mathrm{Si}/{\mathrm{Si}}_{0.77}{\mathrm{Ge}}_{0.23}$ 2DEG heterostructure, achieving record-high electron mobility of $750000{\mathrm{cm}}^2/\mathrm{Vs}$, a transport scattering time (${\tau }_t$) over 80 ps, and a valley splitting energy of $204\mu \mathrm{eV}$ simultaneously from magnetotransport measurements. These results highlight the heterostructure's low disorder and superior material properties, affirming the critical role of interface optimization in achieving high-mobility 2DEG systems. Our findings demonstrate that optimizing the quantum well interface in low-strain Si/SiGe structures can achieve both high mobility and a large in-plane $g$ factor conducive to quantum dot tuning, and a significant valley splitting calculated from the $g$ factor.

Two-dimensional massless Dirac fermions exhibit Dirac cones, which are classified into three types: type I, type II, and type III. In both type-I and type-II cones, the energy dispersion is linear in all momentum directions. Type-I cones are characterized by a nonovertilted structure, where the Dirac point serves as a local minimum (maximum) for the upper (lower) band. In contrast, type-II cones exhibit overtilted dispersions, leading to the coexistence of electron and hole pockets. At the critical tilt, the linear energy dispersion vanishes in one momentum direction, corresponding to a type-III Dirac cone. We further define a special case, termed the “narrow-sense” type-III cone, where not only the linear term but also quadratic and higher-order terms vanish, resulting in a completely flat dispersion along one direction. In this work, we numerically investigate the temperature ($T$) dependence of the electronic specific heat ($C$), as the Dirac cone is continuously tilted from type I to narrow-sense type III. A model with particle-hole symmetry is employed to ensure that the chemical potential ($\mu$) remains temperature independent. Our results reveal a notable crossover in $C$ near narrow-sense type III, where $C$ changes from $C\propto T^2$ below the crossover temperature ($T_{\mathrm{co}}$) to $C\propto T^{\frac{1}{2}}$ above $T_{\mathrm{co}}$. This crossover is attributed to the energy-dependent structure of the density of states. The present findings suggest a feasible approach for experimentally probing the degree of Dirac cone tilting near the narrow-sense type-III limit.

Pure as well as Ag- and Au-substituted ${\mathrm{YbInCu}}_4$ single crystals are structurally and chemically characterized and investigated by means of heat capacity, magnetization, resistivity, and ultrasonic measurements. We study the influence of different compositions of the initial melt as well as of Au and Ag substitutions on the valence change and investigate whether this change occurs via a first-order phase transition or via crossover. We construct a phase diagram of ${\mathrm{YbInCu}}_4$ as a function of various substitutions and show that the position of the critical end point of the valence transition depends on the substituent and on the conditions under which the samples are grown. Multiple thermal cycles through the first-order transition lead to a significant modification of the physical properties, which clearly demonstrates the influence of defects in substituted ${\mathrm{YbInCu}}_4$.

Majorana zero modes (MZMs) in condensed-matter systems have attracted great attention in the past two decades due to their interesting physics and potential application in topological quantum computing (TQC). However, the topologically protected nature of MZMs still need more experimental verifications. In this study, we have realized controllable creation of a topological boundary state at the corner of topological insulator (TI)-based Josephson corner junctions. This state demonstrates protected existence across a broad region in parametric space and exhibits a non-2π-period but 4π-period-compatible energy-phase relation. Our study suggests that TI-based Josephson junctions, as proposed in the Fu-Kane scheme of TQC, may provide a promising platform for hosting and braiding MZMs.

Recently, two-dimensional goldene as a single-atomic-layer gold has been experimentally fabricated [S. Kashiwaya et al., Nat. Synth. 3, 744 (2024)] and it is expected as a potential material for next-generation devices, instead of its three-dimensional bulk gold. In this study, two-dimensional Janus goldene $\mathrm{Au}\text{−}h$ ($h=\mathrm{Cl}/\mathrm{Br}/\mathrm{I}$) monolayers are constructed based on surface halogenation of the original single-layer goldene using first-principles simulations. The three Au-Cl, Au-Br, and Au-I structures exhibit high dynamical, thermodynamic, energetic, and mechanical stabilities for experimental fabrications. The electronic properties of $\mathrm{Au}\text{−}h$ monolayers are investigated using different exchange-correlation functionals. According to the obtained band structures, the $\mathrm{Au}\text{−}h$ are found to be metals that are expected to be suitable for electrocatalytic applications. Under surface halogenation, the crystal structure of the goldene part in Janus $\mathrm{Au}\text{−}h$ is found in the buckled form with a buckling distance of 0.46 to 0.59 Å. Janus $\mathrm{Au}\text{−}h$ monolayers exhibit strongly anisotropic mechanical characteristics. The findings in our work provide more information on the Janus $\mathrm{Au}\text{−}h$ monolayers which can stimulate further theoretical and experimental studies on these interesting materials.

Valley-contrasting Hall transport conventionally relies on the inversion symmetry breaking in two-dimensional systems, which greatly limits the selection range of valley materials. In particular, while monolayer transition metal dichalcogenides have been widely utilized as a well-known class of valley materials in valleytronics, the centrosymmetric nature hinders the realization of valley-contrasting properties in the bilayer counterparts. Here, taking ${\mathrm{MoS}}_2$ as an example, we discover valley-contrasting transport in bilayer transition metal dichalcogenides by exploring a nonlinear transport regime. Using effective models and first-principles calculations, our work demonstrates that nonvanishing nonlinear valley Hall conductivities emerge in a uniaxially strained ${\mathrm{MoS}}_2$ bilayer, owing to strain-induced band tilts of Dirac fermions. Compared with the valence bands, the conduction bands have smaller spin-orbit-coupling-induced band splittings and correspondingly they generate much more remarkable nonlinear valley Hall conductivity. Moreover, the nonlinear conductivities are highly tunable through modulating the strength and the direction of the strain, chemical potential, and interlayer gap. Our findings not only expand material choices for valleytronic applications, but also provides opportunities for designing advanced electronic devices that leverage nonlinear valley transports.