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Nitrogen-driven ferromagnetism and perpendicular magnetic anisotropy in two-dimensional transition-metal nitrides
Data-centric technologies demand energy-efficient, densely integrable spintronic building blocks, motivating the search for two-dimensional (2D) ferromagnets combining room-temperature Curie temperatures (TC) and perpendicular magnetic anisotropy (PMA). Compared with halides and chalcogenides, transition-metal nitrides (TMN2) feature short metal–nitrogen bonds and strong p-d hybridization, which can substantially reshape crystal-field splitting and magnetic exchange pathways. Through systematic first-principles screening of hexagonal h-TMN2 (TM = 3,4,5d) monolayers, we identify h-VN2 and h-CrN2 as the only stable candidates exhibiting intrinsic PMA and half-metallicity with sizable spin-flip gaps (Δsf = 0.27 eV for h-VN2 and 0.25 eV for h-CrN2). Phonon spectra and ab initio molecular dynamics simulations confirm their dynamical and thermal stability, with h-VN2 preserving crystalline integrity up to 800 K. Monte Carlo simulations confirm out-of-plane easy axes with TC values of ≈305 K for h-VN2 and ≈134 K for h-CrN2. Notably, h-VN2 exhibits pronounced magnetoelectric tunability: its half-metallicity and PMA are robustly preserved within a broad biaxial strain window (−2%–+3%), maintaining near-room-temperature TC. In bilayer configurations, vertical stacking further strengthens exchange interactions, enhancing TC to ∼477 K. Furthermore, h-VN2 sustains its ferromagnetism and PMA at both graphene and MoS2 interfaces, underscoring its compatibility with existing 2D platforms. These results highlight h-VN2 as a versatile platform where nitrogen-mediated exchange facilitates robust, high-temperature ferromagnetism for next-generation van der Waals spintronics.
Electronic and optical bandgaps of WO3 and WO3−x: Adsorbed water-induced Burstein–Moss shift
The reported bandgap of stoichiometric WO3 spans a range of 2.6–3.4 eV. The present study evaluated the underlying reason for this wide variation using multiple spectroscopic techniques that preferentially probe the surface, near-surface, or bulk region of WO3 and WO3−x. The results show that the bulk electronic bandgap of WO3 is ∼2.2 eV, which is much lower than the value of 2.7–2.8 eV measured through optical, photoelectrochemical, and photoemission probes that are more surface sensitive. The difference between the electronic and optical bandgap is likely due to the presence of a surface e− accumulation layer on the stoichiometric WO3 due to doping by adsorbed water. In contrast, the presence of oxygen vacancy defects in the bulk lattice of WO3−x causes degenerate bulk doping, a Burstein–Moss shift of Fermi energy to higher electron energies within the CB. However, exposure of WO3−x to ambient humid O2 causes surface oxidation and the formation of an e− depletion layer on the surface. This study highlights the important role of ambient O2 and water in modulating the carrier concentration and electron affinity of semiconductors.
Virial stress in systems of active Brownian particles in the presence of translational and rotational inertia
We elucidate the stress in a system of active Brownian particles augmented with translational and rotational inertia (ABP+TRI). Stress tensors are derived for periodic systems as well as systems confined between walls by employing Lagrange’s equations of motion of the first kind for the rotational motion. Using Langevin simulations of an ideal active gas in two dimensions, we confirm the existence of an equation of state for periodic systems that depends on translational and rotational inertia in general. Confinement implies a strong polarization of the propulsion direction near a wall and an enhanced density, both of which increase with increasing rotational inertia. This affects the local stress tensor normal to the confining walls, leading to a breakdown of the equation of state. Yet the local stress in the bulk part of the confined systems is identical with that of the periodic system. Importantly, for both kinds of boundary conditions, the so-called swim stress is not included in the local stress tensor; therefore, in general, the swim stress is not representative of the stress in systems of ABP+TRIs.
Ferromagnetic resonances in thin films—Electrodynamic analysis and experiments employing multimode rectangular cavity and broadband coplanar waveguide techniques
Rigorous electrodynamic analysis of resonances in rectangular TMn,m,0z mode resonators containing a thin in-plane magnetized ferromagnetic film is presented. Apart from the resonance frequencies and Q-factors, the electromagnetic field distribution is analyzed showing the appearance of a giant microwave magnetic field component perpendicular to the sample at the resonance frequencies of both the dominant and higher order modes. This confirms the plasmonic-like behavior of resonances in thin ferromagnetic films, where the term is used in the electrodynamic sense of subwavelength resonances. The conductivity of metallic ferromagnetic films is accounted for in computations. Resonance curve broadening due to conductivity is predicted for larger conductivities and/or for thicker samples. Results of rigorous computations are compared with the perturbation theory. A rectangular TMn,m,0z mode cavity is used for measurements of the saturation magnetization and Gilbert damping factor of thin CoFeB films at several frequencies in the range 4–12 GHz showing agreement with the broadband coplanar waveguide technique. The Gilbert damping factor of a single-crystal yttrium iron garnet film grown on a GGG substrate and an amorphous CoFeB thin film were determined from measurements of their Q-factors, employing a vector network analyzer, and considering the sample as a magnetic plasmon-like resonator coupled to the coplanar line.
Molecular dynamics study of ice melting on a silver plate
Ice fall incidents, such as detached ice chunks from bridge stay cables, pose not only serious safety hazards to pedestrians and vehicles below but also significant serviceability issues, as bridge closures required for inspection or ice removal can lead to costly disruptions. The current trend of climate change exacerbates this kind of hazard. The availability of a reliable engineering tool, such as an accurate numerical model grounded in nanoscale melt-front physics, is imperative to provide a clear insight into the ice detachment mechanism and develop effective de-icing solutions. Classical molecular dynamics simulations are conducted in the current study to investigate the melting of an ice cube in an atomically flat silver slab. The TIP4P/ice water model is adopted and the simulation is conducted in canonical ensemble with a layer-resolved Langevin thermostat. The phase evolution is tracked via the averaged tetrahedral order parameter, while systematically varying five controls: the depth of heated layers, the silver substrate thickness, the ice thickness, the lateral confinement, and the ice crystal contact orientation (basal vs prism). Results show that melting is controlled primarily by the substrate temperature; variations in heat-conducting-layer count had a minor influence and converged to similar end states. Doubling the ice thickness increases the melt time approximately by three times, whereas relaxing periodic boundaries reshapes the melt into domes or spreading films. Presenting the basal plane instead of a prism plane accelerates loss of crystalline order. Collectively, the simulations yield a numerically consistent set of parameters that not only advances the existing knowledge of nanoscale ice melting simulation but can also be transferred to continuum-scale de-icing simulations, enabling accurate modeling of melt-induced ice detachment from structural components, such as bridge stay cables.
Current sheet formation in planar inductive pulsed plasma thrusters
Formation of a well-defined, magnetically impermeable current sheet is central to the operation of pulsed plasma thrusters. Existing scaling laws for inductive pulsed plasma thrusters, however, are primarily derived from models that either overlook or greatly simplify the current sheet formation process. Data obtained from a compact, low discharge energy planar inductive pulsed plasma thruster were used to gain insight into the fundamental physics governing current sheet formation in these devices. It was found that the maximum inductive coupling and current density in the sheet scaled with the logarithm of the ratio of the resistive diffusion to ionization timescales. The ratio of these timescales was shown to describe the combined role of diffusion and ionization in influencing the electron population within the current sheet. When the ionization timescale was fast relative to the diffusive timescale, the thruster discharge was found to be much more effective at concentrating current in the sheet. A connection between the strength of the current sheet and the thruster design and operational parameters is derived and used to identify the parameter space over which efficient thruster operation is possible.
Interplay between electronic and phononic energy dissipation channels in the adsorption of CO on Cu(110)
In this work, we investigate the relative importance of electronic and phononic energy dissipation during the molecular adsorption of CO on Cu(110). Initial sticking probabilities as a function of impact energy for CO impinging at normal incidence at a surface temperature of 90 K were computed using classical trajectory simulations. To this aim, we use a full-dimensional potential energy surface constructed using an atomistic neural network trained on density functional theory data obtained with the nonlocal vdW-DF2 exchange–correlation functional. Two models are compared: one allowing only energy transfer and dissipation from the molecule to lattice vibrations, and the other also incorporating the effect of molecular energy loss due to the excitation of electron–hole pairs, modeled within the local-density friction approximation. Our results reveal, first, that the molecule mainly transfers energy to lattice vibrations, and this channel determines the adsorption probabilities, with electronic friction playing a minor role. Second, once the molecule is trapped near the surface (where electronic density is higher), electron–hole pair excitations accelerate energy dissipation, significantly promoting CO thermalization. Still, the faster energy dissipation when electron–hole pair excitations are accounted for accelerates the accommodation of the adsorbed molecules in the chemisorption well but does not significantly alter their lateral displacements over the surface.
Dynamic charges effect on infrared dielectric response of polar materials
Predictive modeling of the infrared dielectric function in polar materials is crucial for thermal management and infrared devices design. While the Green–Kubo molecular dynamics (MD) framework provides a nonperturbative route to compute dielectric responses from dipole fluctuations, yet it commonly relies on the fixed-charge approximation that neglects dynamic charge redistribution during atomic motion. Here, we employ a machine-learning neuroevolution potential with dynamic charges combined with Green–Kubo MD to investigate the dynamic charge effect on the infrared dielectric response of rutile TiO2, a material with large Born effective charges. Our results show that dynamic charge effects become increasingly important at elevated temperatures and are essential for accurately predicting longitudinal optical phonon features and infrared reflectance. This work establishes that accurate prediction of infrared optical properties in polar materials under thermal excitation requires explicit treatment of dynamic charge evolution.
Population transfer in quantum <i>β</i> -Fermi–Pasta–Ulam–Tsingou chains with fixed ends
We perform a series of numerically accurate tensor-train (TT) simulations of population dynamics and energy transfer in quantum β-Fermi–Pasta–Ulam–Tsingou (FPUT) chains comprising 10 to 30 nonlinear oscillators. The dynamics are propagated over timescales ranging from 18 to 60 fundamental vibrational periods. We demonstrate that the chain anharmonicity opens new energy-transfer channels, accelerates initial ballistic energy propagation, reduces (partial) recurrence periods relative to the harmonic limit, and regularizes oscillatory wavepacket motion. This work serves as the first proof-of-principle demonstration of the high efficiency of TT methods for simulating quantum FPUT dynamics.
Controlling the spin-wave nonreciprocity of a crescent-shaped nanowire via curvature and magnetic field
Recent studies on spin-wave propagation in ferromagnetic waveguides have highlighted the role of nonreciprocity resulting from the chiral nature of dipolar interactions in curved elements. However, the impact of spin-wave mode type on nonreciprocity remains unexplored. Using micromagnetic simulations supported by analytical modeling, we systematically analyzed the propagation of edge, fundamental, and width-quantized spin-wave modes in a ferromagnetic nanowire with a crescent-shaped cross section. Our results show that the strength and sign of nonreciprocity depend on the mode type, as well as on the curvature magnitude of the nanowire’s top and bottom surfaces and the strength of the external magnetic field. Interestingly, changing the mode type, for instance, induced by altering the curvature or magnetic field, results in a significant change in the dispersion relation asymmetry. This effect underscores the important role of spin-wave profiles in nonreciprocity, deepens our fundamental understanding of spin-wave dynamics in curved geometries, and paves the way for designing magnonic waveguides with tailored properties.
Simulating closed- and open-quantum photoinduced electron dynamics for time-resolved NEXAFS
We present a real-time method based on the propagation of the time-dependent Schrödinger equation in the space of electronic states to compute near edge x-ray absorption fine structure (NEXAFS) spectra of molecules from the ground or a valence excited state. Transition dipole moments between a core and a valence state are computed from linear-response time-dependent density functional theory implemented in the Amsterdam Modeling Suite package by using Slater–Condon rules following two distinct core and valence excited-state calculations. The implementation is compatible with any singly excited ansatz and generalizable to correlated wavefunction methods. The method has been applied to the ultrafast internal conversion observed in the gas-phase thymine, when excited to bright ππ* (S2) state. We have computed the NEXAFS O K-edge from the electronic ground state, S2, and the dark nπ* (S1) state. We have reproduced the experimental spectrum [Wolf et al., Nat. Commun. 8, 29 (2017)] after the pump, showing the peak at 526.5 eV associated with S1. The stochastic Schrödinger equation has been used to get a time-resolved NEXAFS signal, introducing the experimental S2 → S1 decay time of 60 fs. An implicit pump initializes the thymine in the S2 state, and an x-ray pulse probes the system at various delay times (and distinct thymine structures), leading to a time-resolved spectral profile that captures the S2 → S1 population transfer. Slower relaxation from S1 to the ground state has been also considered in a multiple-channel modeling of the dynamics. Ground- and excited-state NEXAFS spectra of cis and trans isomers of azobenzene have been also computed.
Leveraging mechanical frustration in the flatland: A pathway to ambient-pressure strain-engineered superconductivity
Pressure-enabled superconductivity has so far been associated mainly with bulk materials that require extreme external pressures, which sharply limits device relevance despite spectacular transition temperatures. This Perspective argues that mechanically frustrated van der Waals heterostructures could act as an internal-pressure platform for the more restricted but technologically more accessible class of two-dimensional superconductors whose transition temperatures are sensitive to modest biaxial in-plane compression. The strategy is not proposed as a route to reproducing megabar-pressure hydride superconductivity, but as a way to translate the pressure-tunable behavior already observed in layered superconductors into a stable, substrate-free architecture. Rather than claiming that superconductivity has already been achieved in such systems, this article assesses why the concept is timely, what the recent literature now makes possible, what a prototype CrS2|PtTe2 stack does and does not establish, and which scientific questions must be answered next. In my view, the importance of this direction lies in its potential to translate pressure-enabled superconductivity from a high-pressure discovery platform into an applied-physics design strategy.
Dissipative quantum geometric phase in the spin-boson system
We explore how the dissipative geometric phase evolves within the spin-boson model, focusing specifically on coupling to an Ohmic bath in the weakly coherent regime. To determine the non-unitary time evolution of the system’s reduced density matrix, we employ the non-interacting blip approximation (NIBA). For the localized pure initial state used throughout, we derive a compact Bloch-sphere expression showing that the mixed-state geometric phase is a weighted azimuthal winding of the dissipative trajectory. We then map geometric-phase accumulation across various system–bath coupling strengths, temperatures, static biases, and bath cutoff frequencies. Benchmarking representative results against the numerically exact time-evolving matrix product operator (TEMPO) technique shows that NIBA reproduces the population dynamics almost indistinguishably. It also captures the geometric phase quantitatively and qualitatively, although TEMPO reveals a clearer trend in the stationary coherence. Our results show that quantum dissipation suppresses the geometric phase through two complementary mechanisms: thermal noise reduces the state’s purity, while static bias localizes the dynamics and reduces the accessible geometric area.
Modeling cyclic voltammetry and electrochemical impedance spectroscopy measurements of PEDOT:PSS layers with finite density of states
This paper examines the impact of a finite density of states on hole transport in conjugated polymers, with a focus on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). To this end, a two-phase drift-diffusion model of PEDOT:PSS is extended to include a Fermi–Dirac, near-equilibrium occupation of the available hole states and a generalized Einstein relation between hole diffusivity and mobility in the PEDOT phase. A dedicated procedure is developed to impose the boundary conditions and to strengthen the convergence of the model equations. Numerical solutions of the extended model are compared to the results that neglect the above-mentioned aspects. We identify those conditions where the limited DoS affects the cyclic voltammetry and the electrochemical impedance spectroscopy response of a circular PEDOT:PSS-coated electrode in contact with an electrolyte. The results provide new insights into the electronic and ionic behavior of these electrodes and can be useful for preventing the inaccurate extraction of PEDOT:PSS properties or even the misinterpretation of cyclic voltammetry and electrochemical impedance spectroscopy experiments.
Elucidating Au–C bonding via laser spectroscopy of gold monocarbide
Gold monocarbide (AuC) has been produced and characterized using laser spectroscopy, representing the first reported observation of AuC. We recorded the optical spectrum of gas-phase AuC between 400 and 700 nm, assigning excitations from the X2Π1/2((2σ)2(2π)1) ground state to states arising from the (2σ)2(3σ*)1 and (2σ)1(2π)2 configurations. Dispersed-fluorescence spectra are used to study the vibrational and spin–orbit structure of the ground state, branching ratios and radiative lifetimes of the excited states, and the Au–C bond dissociation energy. A molecular orbital diagram is used to rationalize the nature of AuC’s low-lying electronic states. The data serve as valuable benchmarks of relativistic theory and are relevant to quantum science and precision measurements with cold molecules.
Laser-driven shock compression of pyrimidine: Insights from Raman scattering
Pyrimidine (C4H4N2), a nitrogen-rich heterocyclic molecule, is central to advanced green energetic materials, where ring nitrogen enhances both energy yield and stability. Understanding its phase evolution, structural response, and susceptibility to shock initiation under dynamic loading is, therefore, critical for assessing its behavior in extreme environments. Here, we investigate the dynamic response of pyrimidine under laser-driven shocks up to 4.2 GPa and determine its Hugoniot and phase diagram using in situ time-resolved Raman spectroscopy as the sole diagnostic, without any velocimetry methods. From the temporal evolution of Raman modes at 300 and 500 mJ pump energies, we directly extract shock velocities of 2.85 ± 0.05 and 3.09 ± 0.11 km/s, in excellent agreement with one-dimensional radiation-hydrodynamics simulations (2.86 and 3.10 km/s). These measurements, combined with impedance-mismatch analysis using aluminum, enable construction of the Hugoniot equation of state. The shock—particle velocity relationship is found to be linear, us = 1.736 + 1.561 up. Line-shape changes and detailed analysis of time-resolved Raman spectra further reveal a liquid → solid-I (orthorhombic) transition near ∼1 GPa and a subsequent solid-I → solid-II (low symmetry) transition at around 2.1 GPa. This work offers new insights into the dynamic stability and structural transformations of nitrogen-rich molecular systems relevant to advanced energetic formulations and determines the Hugoniot equation of state using time-resolved Raman spectroscopy.
Laser-induced, blackbody-radiation-assisted rovibrational cooling of symmetric-top molecular ions: NH3+ and ND3+
Quantum-state preparation of molecular ions is a prerequisite for precision spectroscopy and controlled studies of cold ion–molecule dynamics. While such control has been extensively developed for diatomic ions and proposed for linear polyatomic ions, corresponding strategies for symmetric-top molecular ions remain largely unexplored. We present a theoretical investigation of blackbody-radiation (BBR)–assisted rovibrational dynamics and laser cooling in the symmetric-top ions NH3+ and ND3+, prepared in specific rovibrational states by resonance enhanced multiphoton ionization of the neutral precursor. State-resolved radiative lifetimes and equilibration times are computed, revealing that vibrationally excited states decay rapidly, while the ground-state redistribution is dominated by slow BBR-driven rovibrational transitions, as pure rotational transitions are forbidden in the nonpolar NH3+ and ND3+ ions. BBR-assisted laser pumping via the ν2 umbrella-bending mode efficiently cools rotational levels within fixed K manifolds; however, ΔK = 0 selection rules induce a bottleneck, limiting access to the absolute rovibrational ground state for some initially prepared states. Isotopic substitution to ND3+ slows the redistribution dynamics due to the lower transition dipole moment. At room temperature, these cooling schemes yield &gt; 90% and &gt;85% of the population in selected rovibrational states of the NH3+ and ND3+ ions, respectively. In contrast, at temperatures below 100 K, BBR-induced redistribution is strongly suppressed for ions initially produced in the rovibrational ground state, effectively freezing the population for extended storage times. The schemes presented here provide a practical pathway for preparing state-selected molecular ions for precision spectroscopy and controlled ion–molecule reaction studies.
Self-powered photodetection covering UV–visible regime realized in BiSb alloy/silicon heterostructure
This study successfully fabricated a quasi-single-oriented bismuth–antimony (BiSb) (0112) alloy via a dual-target magnetron sputtering method and formed a heterojunction with silicon to construct a self-powered photodetector. Experimental results indicate that the device exhibits a significant on/off ratio under both 365 and 632 nm illumination. In particular, under zero-bias conditions at 632 nm, it achieves a photoresponsivity of 0.163 A/W, a time constant of 0.8 ms, and a detectivity (D*) of 2.4 × 1010 Jones. Based on the energy band structure and carrier transport mechanism, the vital role of topological surface states in low-Sb BiSb alloys in photogenerated carrier separation is revealed, and the positive impact of high film quality on device performance is confirmed. The novelty of this work lies in the first realization of a dual-target magnetron-sputtered BiSb/Si heterojunction self-powered photodetector, clearly elucidating the optoelectronic mechanism of tunable BiSb alloys with topological or semi-metallic characteristics. This research not only broadens the application prospect of BiSb topological alloys in optoelectronic devices but also offers a feasible strategy for high-efficiency, low-power photodetectors toward 6G communication.
Electronic structure and spectroscopy of ClSO+
Recent synchrotron photoionization mass spectrometry studies have identified ClSO+ as a prominent product in the VUV photochemistry of Cl2SO and revealed the vibronically structured near-threshold ionization of the ClSO radical. Motivated by these advances, we provide a focused, high-accuracy theoretical characterization of ClSO+. Coupled-cluster calculations including all-electron scalar-relativistic and explicitly correlated protocols yield equilibrium structures and benchmark adiabatic (9.47 eV) and vertical (9.67 eV) ionization energies for ClSO. The ClSO+ ground-state potential is predicted to be strongly bound, with a Cl–S bond dissociation energy of 3.21 eV at the CCSD(T)/aug-cc-pV(5+d)Z level. MRCI+Q and EOM-CCSD methods define the low-lying singlet manifold of ClSO+. We report rotational constants and IR intensities for key isotopologues, quantify geometry changes upon ionization, and predict vertical electronic excitations. This reveals a dominant allowed transition in the near-UV, alongside low-lying dark states that relax strongly along the Cl–S stretching coordinate. Our results provide a molecular framework for interpreting the threshold ionization spectra of ClSO and for assigning or anticipating ClSO+ features in VUV/UV experiments.
Modeling photocathode stoichiometric effects on quantum efficiency
Useful models of quantum efficiency (QE) for growth characterization but also simulation require material and optical parameters that exhibit complex dependencies on frequency and composition. We combine a Moments model of QE with a Lorentz–Drude–Resonant (LDR) model for metals and its extension to an Adachi–Drude–Resonant (ADR) model for semiconductors for the optical parameters based on density functional theory (DFT) simulations. A rapid numerical implementation is developed for characterization studies that will be advantageous in simulating particle accelerators and Free Electron Lasers using particle-in-cell codes. Changes in stoichiometry are then related to variations in QE, reflectivity, and laser penetration depth as governed by the ADR parameterization for alkali antimonide materials compared to a baseline DFT simulation of Cs3Sb. The correlation between LDR/ADR parameters and stoichiometry allows for an modifiable parameter library enabling user alterations for simulations to explore the effects of changes in composition or to compare/contrast different photocathode bulk materials in beam optics codes.