Browse Articles

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 > 90% and >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.

Negative ion formation from water molecule following absorption of vacuum ultraviolet light

Negative ion formations through ion-pair photodissociation or dissociative photoionization of H2O and D2O have been investigated by using an imaging technique in conjunction with the pulsed synchrotron light in the photon energy range between 17 and 41 eV. Cross sections for the formations of H−/D−, O−, and OH−/OD− were measured as a function of the photon energy. The large isotope effect observed in the previous mass spectroscopic study [Hunniford et al., J. Phys. B: At., Mol. Opt. Phys. 40, 1225 (2007)] were only partially reproduced in the present study. The kinetic energies of O− and H−/D− vary with increasing photon energy, which have been analyzed based on the kinematics. Through the analysis, the mechanisms of the two-body or three-body ion-pair photodissociations have been revealed. It has also been found that the axial recoil approximation breaks in the three-body ion-pair dissociation, forming O−.

Electron scattering from prebiotic ethylene glycol: An R-matrix study

Ethylene glycol is a prebiotically relevant complex organic molecule detected in interstellar and cometary environments, yet quantitative low-energy electron–ethylene glycol scattering data remain limited for astrochemical modeling. This work presents an R-matrix study of low-energy electron collisions with ethylene glycol over a 0–12 eV energy range, using static exchange (SE), static exchange plus polarization (SEP), and configuration interaction (CI) models with 6-311G* and cc-pVTZ basis sets. We compute elastic, excitation, and differential cross-sections within a close-coupling framework. The dataset offers benchmark inputs for astrochemical models, supporting interpretation of ethylene glycol abundances in space and refining constraints on electron-induced prebiotic pathways.

Reduced dynamical maps in finite temperature vibronic coupling models via Choi matrices: Numerical methods and applications

We present a streamlined implementation of a computational framework for constructing and analyzing reduced dynamical maps for complex system–bath models at finite temperature. The methodology is based on three established ingredients of quantum dynamics: the Choi–Jamiołkowski isomorphism for the representation of quantum channels, thermofield (TFD) purification of thermal environments, and tensor-train (TT) propagation of the resulting enlarged pure state. The reduced map is obtained from a single unitary propagation in a thermofield-doubled Hilbert space and represented in matrix form through the Choi–Jamiołkowski isomorphism. The TFD evolution is implemented in the TT representation, enabling efficient propagation of high-dimensional purified thermal states. We illustrate the methodology for exciton transfer in the Fenna–Matthews–Olson complex with site-dependent structured spectral densities represented by discretized bosonic environments. The resulting maps are used to analyze decoherence, relaxation, and finite-memory effects, and to assess the crossover to an effectively time-local description. The proposed approach provides a route to compute reduced propagators and to post-process them into memory kernels, transfer tensors, and effective kinetic rate descriptions for complex molecular systems.

Investigation of Coulomb scattering centers using a substrate bias effect in <i>n</i> -channel metal–oxide–semiconductor field-effect transistors with AlSiO/AlN/ <i>p</i> -type GaN gate structures

Electron scattering factors are analyzed for GaN metal–oxide–semiconductor field-effect transistors (MOSFETs) using gate-biased Hall-effect measurements. The fabricated MOSFETs and Hall devices have AlSiO/AlN/p-type GaN gate structures formed on bottom p+/n+ GaN junctions. Because the capacitance of the bottom p+/n+ junction is much greater than the gate capacitance, the p-type body layer and the substrate electrode are short-circuited. Thus, the body potential is controlled by the substrate bias (Vsub). To investigate Coulomb scattering due to Mg dopants, MOSFETs with Mg-doped and undoped body layers are prepared. Comparing the sheet carrier densities Ns estimated from a capacitance–voltage curve and the Hall effect reveals that almost all electrons contribute to conduction in both channels. In the plot of Hall-effect mobility as a function of Ns, Coulomb scattering components are proportional to Nsγ. If Coulomb scattering is mainly caused by bulk impurities, γ should be unity. However, γ &amp;lt; 1 is obtained, indicating the existence of Coulomb scattering centers other than Mg atoms. The effective electric field perpendicular to the channels is essentially different between both channels, resulting in differences in the scattering magnitudes. Vsub allows control of the effective channel field independent of the gate bias. The effective channel field in the Mg-doped channel at Vsub = 0 V is almost equivalent to that in the undoped channel at −40 V. Then, the Hall-effect mobilities are almost equal. We conclude that Coulomb scattering centers are mainly located near the interface between the gate insulator and the channel.

Cation/anion and monovalent/divalent selectivity in negatively charged nanopores: The effect of modeling the charged surface groups

The anomalous mole fraction effect (AMFE) is widely regarded as a hallmark of calcium vs monovalent ion selectivity in negatively charged pores. While AMFE is well understood in highly cation-selective narrow ion channels, its microscopic origin in wide synthetic nanopores, where anions may also contribute to transport, remains less clear. Here, we use a reduced Nernst–Planck + Local Equilibrium Monte Carlo framework to study ionic transport in a negatively charged polyethylene terephthalate nanopore, with particular emphasis on how the modeling of surface carboxyl (COO−) groups influences charge inversion, ionic currents, and AMFE. We systematically compare fixed point-charge models and explicit-particle representations of surface oxygens and identify two controlling parameters: the distance of closest approach (DCA) between ionic charges and pore charges and grid spacing that modulates localization (while keeping average surface charge constant). By fitting pore diffusion coefficients to three experimental conductance points, we reproduce the entire experimental AMFE curve as well as anion leakage in CaCl2 seen in experiments and molecular dynamics simulations. Remarkably, vastly different microscopic models of the surface groups yield indistinguishable device-level conductance curves when the DCA and grid spacing are matched, despite substantial differences in local Ca2+ concentration profiles. Our results demonstrate that AMFE in wide nanopores is governed by the strong adsorption of divalent cations to pore charges compared to monovalent cations, the resulting changes in ionic mobilities, and increased anion leakage with increasing calcium mole fraction. In wide pores, therefore, selectivity between monovalent and divalent cations is modulated by cation vs anion selectivity.

Halide-dependent photoelectrical response and charge transport in EVA-embedded Cs2TiClxBr6−x perovskite thin films

Lead-free titanium halide perovskites with the formula Cs2TiClxBr6−x were integrated into an ethylene–vinyl acetate (EVA) matrix to form hybrid perovskite thin films with tunable optoelectronic properties. Importantly, this work goes beyond compositional tuning by demonstrating that the polymer–perovskite hybrid architecture actively governs interfacial charge transport and polarization phenomena. Structural and optical characterization confirmed the formation of vacancy-ordered double perovskites with visible-light absorption and photoluminescence features associated with defect-mediated electronic relaxation. The electrical and photoinduced responses of the composites were investigated using electrochemical impedance spectroscopy with symmetric fluorine-doped tin oxide (FTO)│EVA│FTO configurations. Upon illumination, the films exhibited a marked decrease in impedance, revealing the halide-dependent modulation of charge transport, interfacial polarization, and pseudocapacitive behavior. Mixed-halide compositions displayed improved electrical uniformity and reduced transport resistance compared to pristine EVA, indicating enhanced photoinduced charge dynamics. Hirshfeld surface analysis was employed as a structural descriptor to quantify halide surface exposure, thereby linking composition-dependent electronic localization to the observed photoelectrical response. As a functional demonstration of these charge dynamics, the hybrid films were also evaluated under photocatalytic conditions, where light-driven formate and hydrogen generation corroborated the effective charge separation inferred from impedance measurements.

Effect of ensemble averaging on Green–Kubo estimation of short-time stress relaxation modulus in all-atom molecular dynamics simulations of an unentangled polymer melt

Response to “Comment on ‘Determining angle of arrival of radio frequency fields using subwavelength, amplitude-only measurements of standing waves in a Rydberg atom sensor”’ [J. Appl. Phys. 138, 114402 (2025)]

Analysis of factors influencing self-Q-switching of erbium-doped fiber

Self-Q-switched erbium-doped fiber (EDF) lasers combine a high damage threshold, a compact, miniaturizable structure, and the ability to generate stable Q-switched pulses. However, in communication systems, the self-Q-switching behavior of gain fibers can cause optical damage. To investigate these issues, we investigated high-gain erbium-doped fiber used as a saturable absorber (EDF-SA). Experiments show that, within a specific length range, increasing the EDF-SA length significantly enhances Q-switching performance: It increases the maximum single-pulse energy and reduces pulse width—improvements that are important for high-performance all-fiber Q-switched lasers. We attribute this improvement to the greater pump-light absorption efficiency of longer absorbers. Further experiments show that, for a fixed EDF length, peak power and single-pulse energy increase with pump power up to a point and then decline. This reversal results from amplified spontaneous emission at high pump powers, which depletes stored energy before Q-switching occurs. Moreover, inserting a non-Q-switched EDF segment effectively suppresses self-Q-switching in high-gain EDFs, providing a practical means to protect communication devices from optical damage. We attribute the ability of EDFs to produce Q-switched pulses primarily to ion-pair interactions and fast carrier recovery dynamics rather than to the saturable-absorption curve alone. Finally, based on measured data, we hypothesize that the observed self-pulsing originates from relaxation oscillations, and it was observed that stronger reabsorption leads to a lower relaxation oscillation frequency, while higher pump power results in a higher relaxation oscillation frequency. These results deepen the understanding of the physical mechanisms underlying self-pulsing in gain fibers.

Numerical and experimental study on forming characteristics of jet from nylon–water composite liner and its penetration performance on steel targets

Compared to traditional metal jets, shaped charge water jets offer the advantage of low-collateral-damage when destroying explosives. To investigate the forming characteristics and penetration performance of Nylon–Water Composite Jet (NWCJ), this study designed a 3D-printed Nylon–Water Composite Liner. Combining numerical simulation with experimental validation, the formation and penetration characteristics of polymer–liquid composite jets were studied. First, numerical simulations analyzed the forming characteristics and penetration performance of the nylon–water jet. Subsequently, the shaped charge was fabricated using selective laser sintering powder printing technology, overcoming the key technical challenge of solid–liquid coupling. Finally, static armor penetration tests validated the reliability of numerical simulations. Results indicate that compared to the Single Nylon Liner, the NWCJ exhibits significantly greater head expansion and higher velocity during the formation process. By regulating the water layer thickness, the NWCJ can effectively control penetration depth while maintaining hole-enlarging capability. The differing kinetic energy decay sequences of nylon and water reveal the penetration mechanism of polymer–liquid composites. This study provides guidance for designing polymer–liquid composite liner structures and enhances understanding of the formation and penetration of polymer–liquid composite jets under explosive loading.