We have investigated the evolution of the atomic and electronic structure of Sn deposited on the Ag(001) surface using low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. We find that, for all Sn coverages $x$ up to 1 ML, the thermodynamically favoured configurations correspond to the formation of a bimetallic Sn-Ag substitutional surface alloy in the topmost layer. The LEED data reveal distinct superstructures for varying Sn coverages. We find that the effective sizes of Sn atoms on the Ag(001) surface are significantly larger than those of surface or bulk Ag atoms. As a result, the formation of the substitutional surface alloy relieves tensile stress at the surface. The theoretically computed “unfolded” band structure agrees excellently with ARPES data. Surprisingly, this is true even in cases where experiments and calculations suggest different atomic structures. Our findings suggest that the significant features in the band structure are predominantly influenced by the surface potential, which is less sensitive to the precise details of the atomic structure, but rather to the average surface composition.

Fractionalization remains one of the most fascinating manifestations of strong interactions in quantum many-body systems. In quantum magnetism, the existence of spinons—collective magnetic excitations that behave as quasiparticles with fractional quantum numbers—is proven in spin chains, but the criteria for their appearance in higher dimensions remain disputed. Motivated by experiments reporting the observation of spinons at high energies in the square-lattice Heisenberg antiferromagnet, we adopt the approach of extrapolating from where spinons are well defined. We study the dynamical properties of a Gutzwiller-projected wave function, the staggered-flux state, on a rectangular spin-1/2 Heisenberg lattice as a function of the spatial coupling ratio, $\gamma =J_y/J_x$. By studying the spectrum and the spinon separation distribution we show how, as the system evolves from one-dimensional (1D) towards 2D, the spinons become progressively more confined over most of reciprocal space, but remain deconfined at specific wave vectors.

We study the transverse-field Ising model on a square lattice with bond and site dilution at zero temperature by stochastic series expansion quantum Monte Carlo simulations. Tuning the transverse field $h$ and the dilution $p$, the quantum phase diagram of both models is explored. Both quantum phase diagrams show long-range order for small $h$ and small $p$. The ordered phase of each is separated from the disordered (quantum) Griffiths phase by second-order phase transitions on two critical lines touching at a multicritical point. Using Binder ratios we locate quantum critical points with high accuracy. The order-parameter critical exponent $\beta$ and the average correlation-length exponent ${\nu }_{\mathrm{av}}$ are determined along the critical lines and at the multicritical points via finite-size scaling. We find three internally consistent sets of critical exponents and compare them with potentially connected universality classes. The quantum Griffiths phase in the vicinity of the phase transition lines is analyzed through the local susceptibility. Our results indicate that activated scaling occurs not only at the percolation transition, but also at the phase transition line for $p$ smaller than the percolation threshold $p_c$.

The flow of time moves in one direction in any spatial position and orientation in this universe. Chiral objects, which lack mirror symmetry, retain their chirality regardless of their position or orientation. Despite being seemingly independent, time and chirality share common features such as universality—applying to any position and orientation—and a binary nature, such as forward/backward time flow versus left/right chirality. We introduce the concept of time chirality and discuss the conjugate relationship between time chirality and traditional chirality. We explore how time chirality can manifest in certain magnetic states, and examine the physical phenomena associated with time-chiral magnetic states. This discussion offers a fresh perspective on true time-reversal symmetry breaking and temporal nonreciprocity.

We investigate a variant of spin ice whose degenerate ground states are densely packed monopole configurations. An applied field drives this model through a ${\mathbb{Z}}_2$ confinement transition. This phase change is a variant of the U(1) Kasteleyn transition, but instead of saturated order the system has fluctuations in the confined phase, and shows critical scaling on both sides of the transition. Remarkably, the magnetic response scales with the critical exponent of the specific heat in the 3D Ising universality class. We prove this universality using a Kramers–Wannier duality to map to an Ising model. The dual order parameter maps back to a nonlocal string order parameter in the original model, invisible to any local probe. Further, we describe the transition in terms of a bosonic field theory including a pairing term.

Using the valley degree of freedom as a carrier of information for storage and processing, valley polarization plays a crucial role. A variety of mechanisms for valley polarization have been proposed, among which the valley-layer coupling mechanism involves the induction of valley polarization by an out-of-plane electric field. Here, through first-principles calculations, it is found that the weak valley-layer coupling can exist in centrosymmetric $1T\text{−}{\mathrm{FeCl}}_2$ monolayer. It is crucial to note that valley-layer coupling only occurs with out-of-plane magnetization and vanishes with in-plane magnetization. Compared to monolayers with strong valley-layer coupling, ${\mathrm{FeCl}}_2$ requires an extremely strong electric field to achieve the same magnitude of valley splitting. Valley polarization switching can be achieved by manipulating the directions of magnetization and electric field. Reversing only one of these directions switches the valley polarization, whereas reversing both simultaneously leaves it unchanged. Moreover, the simply stacked bilayer ${\mathrm{FeCl}}_2$, as a $PT$-antiferromagnet, can spontaneously achieve valley polarization without an external electric field, highlighting its potential for miniaturization, ultradensity, and ultrafast performance. Our work provides guidelines for identifying materials with weak valley-layer coupling, and further enables the regulation of valley polarization through electric field and stacking engineering.

Scattering of electron in the field of a narrow solenoid with alternating current is considered. Using the eikonal approximation, the wave function of electron, the differential and total scattering cross sections are found. Unlike the case of direct current, the total cross section in the case of alternating current turns out to be finite. An oscillating asymmetry in the differential scattering cross section is discovered. The possibility of experimental observation of the effect is discussed.

We investigate superconductivity in a two-dimensional material described by a two-band heavy-fermion model, where hybridization between a dispersive band and a flat band introduces a quasiflat dispersion to the otherwise localized flat-band electrons. The enhanced density of states in the quasiflat band raises the crossover temperature for an inhomogeneous preformed Cooper pair state. The superconducting phase stiffness and the Berezinskii-Kosterlitz-Thouless (BKT) temperature are governed by the Fermi surface contribution induced by hybridization. We compute the crossover and BKT temperatures, revealing a dome-like dependence on doping. When the pairing amplitude exceeds the energy width of the quasiflat band, superconductivity is suppressed, and the inhomogeneous pairing regime expands linearly with increasing interaction strength. However, in the opposite case, the BKT temperature reaches a maximum value that is only numerically less than the energy width of the quasiflat band. We also discuss our results in the context of superconductivity in graphene-based systems.

We investigate the interplay between non-Hermitian skin effect (NHSE) and topological properties in two-dimensional topological superconductor. Two kinds of non-Hermiticity are considered. The first is the spin-independent nonreciprocal hopping, which respects particle-hole symmetry (PHS) and dictates that the bulk as well as topological edge modes localize at opposite corners, manifested as the NHSE protected by PHS. The direction of the localization can be conveniently characterized by the sign of the winding numbers. The other is the spin-dependent nonreciprocal hopping, where the NHSE is protected by time-reversal symmetry (TRS). The Kramers doublets are localized at opposite corners, namely the TRS protected $Z_2$ NHSE. When an external Zeeman field is applied, its internal symmetry changes from the symplectic class to the orthogonal class, eliminating the NHSE for the states in the spectrum continuum. For the zero-energy states that are isolated, the NHSE still has its effects despite the orthogonality. For the spin-independent case, edge states can be effectively tuned by the direction of the Zeeman field, where at certain directions the zero-energy edge state can be free of NHSE and uniformly distributed. Our work paves the way for the study of the interplay between topology and non-Hermiticity in superconducting systems.

In organic semiconductor devices, the deposition of organic layers may result in intermixed regions or rough interfaces between layers. To examine how roughness at organic-organic interfaces influences device performance, we conducted mesoscopic device simulations using a three-dimensional kinetic Monte Carlo algorithm. We simulated devices containing interfaces with periodic corrugation of either triangular or rectangular cross section. Our results show how the shape and size of interfacial roughness impacts on both charge and exciton dynamics of unipolar and bipolar devices. We first analyzed bilayer devices where the two layers are energetically offset. We find interfaces with triangular cross section display strong carrier funneling to the tips. This funneling translates to pronounced inhomogeneity in the spatial distribution of charge carriers, excitons, and excitonic losses. The tips act as injection hot spots, increasing the current density by up to two orders of magnitude, depending on the energy offset, compared to a flat-interface device. In contrast, the internal quantum efficiency of bipolar devices is surprisingly unaffected by interfacial morphology. In bipolar three-layer devices, we used this enhancement in current density to improve charge injection toward the central emissive layer. The recombination zone within the emissive layer can also be tuned through the configuration and size of the morphology.

The collective amplitude mode of the order parameter in displacive ferroelectrics, termed the ferron, represents the amplitude fluctuations of long-range ordered polarization. At temperatures well below phase-transition temperature $T_c$, the energy of ferron excitation is significantly gapped in the long-wavelength limit. As $T_c$ is approached, this gap softens dramatically to minimal or gapless values, and thereby should lead to a substantial contribution to thermal properties. In this context, we explore the role of ferrons in heat capacity and thermal transport by incorporating a microscopic self-consistent phase-transition theory for displacive ferroelectricity, in contrast to the conventional treatment of attributing thermal properties solely to acoustic phonons. Using ferroelectric ${\mathrm{PbTiO}}_3$ as a case study, we show that the softening of ferrons near the phase transition is essential to accurately capturing the experimental temperature and electric-field dependencies of thermal properties.

Classification of the non-Hermitian phases in a high-dimensional lattice becomes challenging due to interplay of the band topology and non-Hermiticity. The significant increase in data dimensions and the number of categories has rendered traditional supervised learning and unsupervised manifold learning unsuccessful. Here, we propose self-attention assistant machine learning for clustering non-Hermitian phases in a two-dimensional lattice. By incorporating the self-attention mechanism, the model can effectively capture long-range dependencies and important patterns, resulting in a more compact and information-rich latent space. It can achieve Altland-Zirnbauer classification with the Bloch vector dataset and distinguish the phases of eigenstates’ localized behavior with the competition between non-Hermitian skin effect and topological localization. Our results provide a general method for characterizing non-Hermitian phases in a two-dimensional lattice via machine learning.

The interaction of energetic ions with materials is a fundamental process that occurs in fusion reactors, impacting radiation damage, heat deposition, and overall material performance. While previous work has significantly advanced our understanding of electronic stopping in metals, the impact of alloying and irradiation-induced defects remains underexplored, despite their importance for materials in extreme environments. In this work, we combine ab initio simulations with measurements to study electronic stopping in both pristine and defect-containing Fe and EUROFER97. By systematically assessing the influence of alloying and defects, we examine the extent to which structural modifications alter energy dissipation. Our findings contribute to a more rigorous understanding of electronic stopping in fusion-relevant materials and the reliability of simplified approaches in radiation damage simulations.

The perpendicular shape anisotropy spin transfer torque magnetic random-access memory (PSA-STT-MRAM) is a recent concept proposed to maintain the thermal stability of standard MRAM at small diameters, considering thick vertical pillars as the free layer. To explore the specific physics of such systems with a three-dimensional nature, we have performed simulations combining a micromagnetic model coupled self-consistently with spin-dependent transport equations. The 3D shape induces flower states at the upper and lower surfaces. In addition, the fieldlike component of STT is found to be larger than in thin elements such as in standard MRAMs, suggesting that it needs to be considered. The combination of both effects leads to the efficient excitation of edge ferromagnetic resonance modes, playing a key role in magnetization reversal. These results highlight features of 3D nanomagnetic systems, largely disregarded so far. These must be considered to reliably describe PSA-STT-MRAM, but more broadly in the rising field of 3D nanomagnetism.

Using ab initio molecular dynamics combined with machine learning, we simulate bulk amorphous ${\mathrm{MoS}}_2$, where a layered structure forms, consisting of 1T-phase, 1H-phase, and defect regions. After extracting isolated monolayers from the evolving bulk structure, we investigate their structural and magnetic properties. Our simulations reveal that the 1T-phase region gradually diminishes over time, while the thermodynamically more stable 1H-phase region expands. The defect regions, interspersed between these phases, shift position as the system evolves. The inherent disorder in the amorphous structure leads to electron localization at Mo sites in some of the 1T-phase or defect regions, which induces spin polarization and results in the formation of localized magnetic moments. These magnetic moments exhibit both short-range antiferromagnetic and ferromagnetic couplings via a superexchange mechanism mediated by sulfur ligands, depending on the disordered local structures at different time points. In particular, the spin directions are noncollinear, with slight tilting from the out-of-plane axis that varies between Mo atoms. These findings highlight that the magnetic properties of the amorphous ${\mathrm{MoS}}_2$ monolayer can be tuned by modulating structural disorder and evolution, offering exciting prospects for applications in spintronics and magnetic technologies.

Monoclinic $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, a promising wide-band-gap semiconducting material, exhibits complex, anisotropic diffusional characteristics and mass transport behavior as a results of its low-symmetry crystal structure. From first-principles calculations combined with master diffusion equations, we determine three-dimensional diffusion tensors for neutral (${\text{O}}_{\text{i}}^0$) and $2-$ charged oxygen interstitials (${\text{O}}_{\text{i}}^{2-}$). Systematic exploration of the configurational space identifies stable configurations in these two dominant charge states and their corresponding formation energies. By connecting every pair of low-energy configurations considering both interstitial or interstitialcy hops, we construct three-dimensional diffusion networks and evaluate hopping barriers of all transition pathways in networks. Combining the collection of (i) defect configurations and their formation energies and (ii) the hopping barriers that link them, we construct and solve the master diffusion equations for ${\text{O}}_{\text{i}}^0$ and ${\text{O}}_{\text{i}}^{2-}$ separately through the Onsager approach, resulting in the respective three-dimensional diffusion tensors $D_{{\text{O}}_{\text{i}}}^0$ and $D_{{\text{O}}_{\text{i}}}^{2-}$. Both ${\text{O}}_{\text{i}}^0$ and ${\text{O}}_{\text{i}}^{2-}$ present the fastest diffusion along the $b$ axis, demonstrating significant anisotropy. The predicted self-diffusivities along [100] and [$\overline{2}01$] fall within the range of previously reported values from isotopically labeled oxygen tracer experiments, though some quantitative differences remain.

Trigonal ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ is a well-known ferromagnetic material that has recently drawn much attention due to the discovery of a zero-field skyrmion state, an unusual anomalous Hall effect, a topological Hall effect, and a topological Nernst effect. This quasi-van der Waals layered material with intercalated Cr atoms possesses many peculiar features that depend on the amount of Cr intercalation, although the microscopic magnetic ground state is still elusive. We reveal the structural and magnetic properties of trigonal ${\mathrm{Cr}}_{1.22}{\mathrm{Te}}_2$ by low-temperature x-ray diffraction, magnetization, temperature-dependent Raman spectroscopy, and single-crystal neutron-diffraction studies. Magnetization measurements under small applied magnetic field indicate two successive magnetic transitions, one from a ferromagnetic (FM) state to an antiferromagnetic (AFM) state ($T_{\mathrm{C}}=197\mathrm{K}$) and second from an AFM state to a paramagnetic state ($T_{\mathrm{N}}=211\mathrm{K}$). The FM transition is sharp with a strong presence of magnetoelastic coupling, but is not accompanied by any structural phase transition. The magnetic structure obtained from zero-field single-crystal neutron diffraction reveals that the Cr1 and Cr2 moments are ferromagnetically aligned along the $c$ axis, while the Cr3 and intercalated Cr4 atoms induce an AFM component in the $ab$ plane leading to an umbrellalike spin structure which possesses a finite spin chirality. The presence of a finite spin chirality is responsible for the observation of the topological Hall effect.