Vacuum Interface Research Articles

Toward understanding the dimensional crossover of canonical spin-glass thin films

Spin-glass thin films exhibit many features different from the bulk. The freezing temperatures of spin-glass films are suppressed for reduced thickness and follow the Kenning relation. The dynamics are altered near the vacuum interface. These phenomena are closely related to the lower critical dimension of spin glasses, the spin-glass correlation length, and the dimensional crossover from d = 3 to d = 2. In this article, we review the experimental facts and theoretical perspectives for spin-glass thin films. We focus on canonical spin-glass systems with the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between magnetic impurities in a nonmagnetic host. Open questions to be addressed are emphasized.

Spin Spiral State at a Ferromagnetic Gd Vacuum Interface.

Centrosymmetric bulk magnets made of layered Gd intermetallics had been discovered recently to exhibit helical spin spirals with a wavelength of ≈2 nm that transform into skyrmion lattices at certain magnetic fields. Here we report on the observation of a spin spiral state at the Gd(0001) surface. Spin-polarized scanning tunneling microscopy images show striped regions with a periodicity of about 2nm. These stripes rearrange upon application of an external magnetic field, thereby unambiguously confirming their magnetic origin. Density functional theory calculations explain that competing exchange interactions in the surface layer of Gd(0001) together with a magnetovolume fine-tuning of the exchange interaction to the next Gd layer favor a chiral 2nm conical spin spiral at the surface, arising as a general behavior of the Gd monolayer.

Propagation Characteristics of Electrostatic Surface Plasma Waves at the Spin‐Polarized Quantum Plasma–Vacuum Interface

ABSTRACTThis investigation explores the characteristics of electrostatic surface plasma waves within the framework of a spin‐polarized quantum plasma. Utilizing the spin‐polarized quantum hydrodynamic model and incorporating essential elements like Fermi pressure and Bohm potential, we derive the dispersion relation governing surface plasma waves at a plasma–vacuum interface. Through Fourier decomposition of the hydrodynamic model, we establish the dispersion relation that outlines the behavior of surface plasmons under conditions of small amplitude. Quantum effects, encompassing degenerate pressure, and Bohm potential are considered with specific attention given to the spin polarization effect, treating spin up, and spin down electrons as distinct species. The resulting dispersion relation demonstrates that, regardless of the degree of spin matching, Bohm potential significantly alters the phase speed in the limit of a large wave vector. Increasing spin mismatch in the quantum plasma leads to a decrease in the phase speed of the surface mode for a fixed value of the plasmonic coupling parameter . Our findings bear relevance to graphene‐based plasmonic systems, aligning with some of the observations reported in Gao et al. (2013) and Guo et al. (2019).

Cooperation mechanism between the vacuum levels and interface dipole energy of triboelectric materials for real-time vibration recognition

Triboelectric Nanogenerator (TENG) device, acting as sensors of energy harvesting and material identification, are attracting intensive attention. Nevertheless, vibration wave recognition cannot be achieved by the developed devices owing to device structure and working mechanism limitations. Here, a cooperation mechanism between the vacuum level and interface dipole energy of triboelectric materials is proposed to explore triboelectric charge generation and migration. Hence, metallized cottons of high specific surface area encapsulated by polydimethylsiloxane (PDMS) of low elastic modulus are developed as TENG-based sensors to realize vibration recognition. The results indicate electrons flow from PDMS to metallized cotton with the generation of an extremely thin interfacial dipole barrier layer because the vacuum level of metallized cotton is higher than that of PDMS, resulting in the accumulation of triboelectric charges on the PDMS surface under vibration, further leading to generating distinct triboelectric signals that can be simply differentiated using signal amplitude or peak shape with 98.3 % recognition accuracy for vibration waveform. Besides, combined with the deep machine learning and triboelectric effect, a vibration perception system integrated with a TENG-based sensor, data processing and display modules is also developed for real-time vibration monitoring. This work contributes to further clarifying the theoretical mechanisms of triboelectric charge excitation under vibration.

Exploring Antiviral Drugs on Monolayer Black Phosphorene: Atomistic Theory and Explainable Machine Learning-Assisted Platform.

Favipiravir (FP) and ebselen (EB) belong to a diverse class of antiviral drugs known for their significant efficacy in treating various viral infections. Utilizing molecular dynamics (MD) simulations, machine learning, and van der Waals density functional theory, we accurately elucidate the binding properties of these antiviral drugs on a phosphorene single-layer. To further investigate these characteristics, this study employs four distinct machine learning models-Random Forest, Gradient Boosting, XGBoost, and CatBoost. The Hamiltonian of antiviral molecules within a monolayer of phosphorene is appropriately trained. The key aspect of utilizing machine learning (ML) in drug design revolves around training models that are efficient and precise in approximating density functional theory (DFT). Furthermore, the study employs SHAP (SHapley Additive exPlanations) to elucidate model predictions, providing insights into the contribution of each feature. To explore the interaction characteristics and thermodynamic properties of the hybrid drug, we employ molecular dynamics and DFT calculations in a vacuum interface. Our findings suggest that this functionalized 2D complex exhibits robust thermostability, indicating its potential as an effective and enabled entity. The observed variations in free energy at different surface charges and temperatures suggest the adsorption potential of FP and EB molecules from the surrounding environment.

Investigation of pyruvic acid photolysis at the air-liquid interface as a source of aqueous secondary organic aerosols

Pyruvic acid (PA) is a ubiquitous 2-oxocarboxylic acid in the atmosphere. Its photochemical process at the air-liquid (a-l) interface has been suggested as an important source of aqueous secondary organic aerosols. We investigated the photochemical reaction pathways of PA at the a-l interface using synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) microreactor. Results from mass spectral analysis and the determination of appearance energies (AEs) indicate that photolysis of PA can generate radicals, then they recombine with carboxylic acids and simple molecular oligomers. Furthermore, the preliminary products could form larger oligomers via radical reaction or esterification in the presence of hydroxyl and carboxyl functional groups. Mass spectral comparison shows that most photochemical reactions would complete within 4 h. The expanded photochemistry-driven reaction flowchart of PA is proposed based on the newly discovered products. Our results reveal that the interfacial PA photochemical reactions have different mechanisms from the bulk liquid due to the interfacial properties, such as molecular density, composition, and ion concentration. Our findings show that in situ mass spectral analysis with bright photon ionization is useful to elucidate the contribution of a-l interfacial reactions leading to aqSOA formation.

A High Vacuum, High Electric Field Pulsed Power Interface Based on a Ceramic Insulator

Improving the flashover voltage in a vacuum interface between a pulsed-power system and a vacuum region has been a goal for many years. The interface problem is difficult because of the electrical, mechanical, and vacuum issues that must be satisfied simultaneously. In this paper, according to the theories of vacuum flashover and the design rules for high electric field insulators, a ceramic insulated vacuum interface is presented. The electrostatic field along the ceramic surface was obtained by ANSYS simulations. By optimizing the field shaper and shielding rings, the electric fields were well distributed and the fields around the cathode and anode triple junctions were effectively controlled. The high voltage test was carried out on a ~40 GW, ~100-ns pulser, and results show that the vacuum interface can work stably with the hold-off voltage of 600 kV. The experimental results agree with the theoretical and simulated results. Finally, the influence of the surface treatment on the ceramic flashover characteristics was also discussed.

Probing fragment ion reactivity towards functional groups on coordination polymer surfaces.

Functionalization of surface-grown coordination polymer layers by ion soft-landing of highly reactive molecular fragment ions is demonstrated. The ions form covalent bonds to terminal functional groups of the polymer at the vacuum interface, opening new perspectives for controlled bond formation using reactive ions.

On effective radiational gravity acceleration at the interface of dense plasmas and vacuum

On effective radiational gravity acceleration at the interface of dense plasmas and vacuum

Electric-field-induced ion evaporation from the ionic liquid–vacuum interface

We studied ion evaporation from a planar interface between the room-temperature ionic liquid (RTIL) and vacuum under external electric fields by using molecular dynamics (MD) simulations. We calculated the ion evaporation rate (je) as a function of the electric field normal to the RTIL surface (En), and results under Langevin, dissipative particle dynamics (DPD), and Berendsen thermostats were compared. It was found that DPD thermostat is more suitable for simulating the ion evaporation phenomenon. We also compared results under different methods for applying the electric field (constant potential, constant charge, and constant field methods) and found that je-En curves from the three methods agree with each other. Temporal evolution of electric fields in vacuum was further analyzed, and variations of electric fields over time were traced to the induced electric field between the evaporated ion and RTIL film. This work could guide the selection of proper methods for MD simulations of electrospray in the pure ion regime and lays the foundation to study more complex electrospray phenomena using MD simulations.

Post Deposition Interfacial Néel Temperature Tuning in Magnetoelectric B:Cr2O3

AbstractBoron (B) alloying transforms the magnetoelectric antiferromagnet Cr2O3 into a multifunctional single‐phase material which enables electric field driven π/2 rotation of the Néel vector. Nonvolatile, voltage‐controlled Néel vector rotation is a much‐desired material property in the context of antiferromagnetic spintronics enabling ultralow power, ultrafast, nonvolatile memory, and logic device applications. Néel vector rotation is detected with the help of heavy metal (Pt) Hall‐bars in proximity of pulsed laser deposited B:Cr2O3 films. To facilitate operation of B:Cr2O3‐based devices in CMOS (compementary metal‐oxide semiconductor) environments, the Néel temperature, TN, of the functional film must be tunable to values significantly above room temperature. Cold neutron depth profiling and X‐ray photoemission spectroscopy depth profiling reveal thermally activated B‐accumulation at the B:Cr2O3/ vacuum interface in thin films deposited on Al2O3 substrates. The B‐enrichment is attributed to surface segregation. Magnetotransport data confirm B‐accumulation at the interface within a layer of ≈50 nm thick where the device properties reside. Here TN enhances from 334 K prior to annealing, to 477 K after annealing for several hours. Scaling analysis determines TN as a function of the annealing temperature. Stability of post‐annealing device properties is evident from reproducible Néel vector rotation at 370 K performed over the course of weeks.

Dissipative Transport and Field Emission of Electrons in 2D Carbon Heterostructures with a Quantum Barrier

The influence of the thickness of a tunnel-thin carbon layer depleted in charge carriers in an enriched noncrystalline carbon matrix on the non-dissipative transport and field emission of electrons is studied. It has been established that the “geometric” increase in the electron self-energy increases the transparency of potential barriers for non-dissipative transport in the heterostructure and at the solid–vacuum interface. It is shown that the introduction of a quantum barrier into the carbon matrix increases the transverse current and rectifying properties of heterostructures, increases the field current density and the steepness of the current–voltage characteristics.

A versatile 16-pole ion trap setup for investigating photophysics of biomolecular ions.

A linear 16-pole ion trap-based experimental setup has been designed, implemented, and characterized to investigate the photophysics of biomolecules in the gas phase. Electrospray ionization is employed to generate the ions in the gas phase at atmospheric pressure. The voltage configuration on the ion funnel, the ion optic device in the first vacuum interface, is used to control the energy of the ions. A home-built quadrupole mass-filter is utilized for the mass-selection of the ions of interest. A 16-pole ion trap designed and built in-house is implemented for ion trapping. The instrument's versatility and capability are showcased by demonstrating the fragmentation patterns of protonated and deprotonated tryptophan, as well as describing the photodetachment decay of deprotonated indole.

On vacuum free boundary problems in ideal compressible magnetohydrodynamics

AbstractWe survey some recent results on vacuum free boundary problems in three‐dimensional ideal compressible magnetohydrodynamics, restate the main theorems in our works (Secchi and Trakhinin, Nonlinearity 27(1) (2014) 105–169; Trakhinin and Wang, Arch. Ration. Mech. Anal. 239(2) (2021) 1131–1176; Trakhinin and Wang, Math. Ann. 383(1–2) (2022) 761–808; Trakhinin and Wang, SIAM J. Math. Anal. 54(6) (2022) 5888–5921), and provide an alternative proof for the linear well‐posedness of the full plasma–vacuum interface problem under the noncollinearity condition.

Photoelectron spectroscopy from a liquid flatjet.

We demonstrate liquid-jet photoelectron spectroscopy from a flatjet formed by the impingement of two micron-sized cylindrical jets of different aqueous solutions. Flatjets provide flexible experimental templates enabling unique liquid-phase experiments that would not be possible using single cylindrical liquid jets. One such possibility is to generate two co-flowing liquid-jet sheets with a common interface in vacuum, with each surface facing the vacuum being representative of one of the solutions, allowing face-sensitive detection by photoelectron spectroscopy. The impingement of two cylindrical jets also enables the application of different bias potentials to each jet with the principal possibility to generate a potential gradient between the two solution phases. This is shown for the case of a flatjet composed of a sodium iodide aqueous solution and neat liquid water. The implications of asymmetric biasing for flatjet photoelectron spectroscopy are discussed. The first photoemission spectra for a sandwich-type flatjet comprised of a water layer encapsulated by two outer layers of an organic solvent (toluene) are also shown.

The plasmon–polariton scattering by random-impedance surface defects: The interplay between localization and outflow

We study the scattering of TM-polarized surface plasmon-polariton (SPP) by the finite section of flat metal–vacuum interface with a random impedance. We analyze the solution to the integral equation that connect the scattered field and the incident plasmon–polariton, and is valid for any strength of the scattering and dissipative characteristics of the conducting half-space. We show that the norm of the intermode scattering operator as a measure of scattering strength is not only determined by the parameters of the random impedance (the variance, correlation radius, the length of the heterogeneous section of the interface), but also crucially depends on the metal conductivity. For a small norm of the integral operator, the incident surface plasmon polariton radiates effectively into vacuum, resulting in excitation of quasi-isotropic Norton-type waves above the conducting surface. The intensity of the leaking field is expressed in terms of the pair correlation function of the impedance, whose dependence on wave numbers of incident and scattered waves demonstrates the possibility to observe a phenomenon similar to Wood’s anomalies of wave scattering by periodic gratings. Under strong scattering regime, the radiation into the upper half-space is highly suppressed and the SPP wave is mainly backscattered from the heterogeneous surface segment. For the lossless conducting half-space, the surface plasmon–polariton becomes unstable for arbitrarily small fluctuations of the conductor polarizability. The mirroring should also take place at small norm of the scattering operator, yet in this case it is related to Anderson’s localization of the SPP within the disordered section.

Generation and detection of sound at the effect of femtosecond pulses on a metal film on a dielectric substrate

The excitation of sound by a femtosecond laser pulse in a metal layer on a dielectric substrate has been studied. The modulations of the reflection coefficient of the metal, which arise due to the propagation of sound in it, are described in detail. It is shown that, in addition to oscillations corresponding to odd harmonics of sound waves, oscillations corresponding to even harmonics can be present on the profile of the Fourier image of the reflection coefficient change. The efficiency of even harmonic generation depends on the substrate material and the metal film thickness. The dependence of the reflection coefficient change on time has been studied. It has been established that if the electron heat flux reaches the metal–dielectric interface before the energy of the electrons is transferred to the lattice, then modulations are present both due to sound that occurs at the metal–vacuum interface and at the metal–dielectric interface. The wavelength of probe radiation also affects the reflection coefficient change. If real and imaginary parts of the permittivity at the wavelength of probing radiation are comparable in magnitude, then the Fourier image of the reflection coefficient change contains a smaller number of maxima, and the change in the reflection coefficient over time is accompanied by splitting of peaks.

Charged Ice Particle Beams with Selected Narrow Mass and Kinetic Energy Distributions.

Small ice particles play an important role in atmospheric and extraterrestrial chemistry. Circumplanetary ice particles that are encountered by space probes at hypervelocities play a critical role in the determination of surface and subsurface properties of their source bodies. Here we present an apparatus for the generation of low-intensity beams of single mass-selected charged ice particles under vacuum. They are produced via electrospray ionization of water at atmospheric pressure and undergo evaporative cooling when transferred to vacuum through an atmospheric vacuum interface. m/z selection is achieved through two subsequent quadrupole mass filters operated in the variable-frequency mode within a range of m/z values between 8 × 104 and 3 × 107. Velocity and charge of the selected particles are measured using a nondestructive single-pass image charge detector. From the known electrostatic acceleration potentials and settings of the quadrupoles the particle masses could be obtained and be accurately controlled. It has been shown that the droplets are frozen within the transit time of the apparatus such that ice particles are present after the quadrupole stages and finally detected. The demonstrated correspondence between particle mass and specific quadrupole potentials in this device allows preparation of beams of single particles with a repetition rate between 0.1 and 1 Hz with various diameter distributions from 50 to 1000 nm at 30-250 eV of kinetic energy per charge. This corresponds to velocities and particle masses quickly available between 600 m/s (80 nm) and 50 m/s (900 nm) and particle charge numbers (positive) between 103 and 104[e], depending upon size.

A direct liquid sampling interface for photoelectron photoion coincidence spectroscopy.

We introduce an effective and flexible high vacuum interface to probe the liquid phase with photoelectron photoion coincidence (liq-PEPICO) spectroscopy at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source. The interface comprises a high-temperature sheath gas-driven vaporizer, which initially produces aerosols. The particles evaporate and form a molecular beam, which is skimmed and ionized by VUV radiation. The molecular beam is characterized using ion velocity map imaging, and the vaporization parameters of the liq-PEPICO source have been optimized to improve the detection sensitivity. Time-of-flight mass spectra and photoion mass-selected threshold photoelectron spectra (ms-TPES) were recorded for an ethanolic solution of 4-propylguaiacol, vanillin, and 4-hydroxybenzaldehyde (1 g/l of each). The ground state ms-TPES band of vanillin reproduces the reference, room-temperature spectrum well. The ms-TPES for 4-propylguaiacol and 4-hydroxybenzaldehyde are reported for the first time. Vertical ionization energies obtained by equation-of-motion calculations reproduce the photoelectron spectral features. We also investigated the aldol condensation dynamics of benzaldehyde with acetone using liq-PEPICO. Our direct sampling approach, thus, enables probing reactions at ambient pressure during classical synthesis procedures and microfluidic chip devices.

Photo-plasmonic effect as the hot electron generation mechanism

Based on the effective Schrödinger–Poisson model a new physical mechanism for resonant hot-electron generation at irradiated half-space metal–vacuum interface of electron gas with arbitrary degree of degeneracy is proposed. The energy dispersion of undamped plasmons in the coupled Hermitian Schrödinger–Poisson system reveals an exceptional point coinciding the minimum energy of plasmon conduction band. Existence of such exceptional behavior is a well-know character of damped oscillation which in this case refers to resonant wave–particle interactions analogous to the collisionless Landau damping effect. The damped Schrödinger–Poisson system is used to model the collective electron tunneling into the vacuum. The damped plasmon energy dispersion is shown to have a full-featured exceptional point structure with variety of interesting technological applications. In the band gap of the damped collective excitation,depending on the tunneling parameter value, there is a resonant energy orbital for which the wave-like growing of collective excitations cancels the damping of the single electron tunneling wavefunction. This important feature is solely due to dual-tone wave-particle oscillations, characteristics of the collective excitations in the quantum electron system leading to a resonant photo-plasmonic effect, as a collective analog of the well-known photo-electric effect. The few nanometer wavelengths high-energy collective photo-electrons emanating from the metallic surfaces can lead to a much higher efficiency of plasmonic solar cell devices, as compared to their semiconductor counterpart of electron–hole excitations at the Fermi energy level. The photo-plasmonic effect may also be used to study the quantum electron tunneling and electron spill-out at metallic surfaces. Current findings may help to design more efficient spasers by using the feature-rich plasmonic exceptional point structure.