Free-electron Metal Research Articles

Intrinsic and extrinsic plasmons in the hard x-ray photoelectron spectra of nearly free electron metals

Intrinsic and extrinsic plasmons in the hard x-ray photoelectron spectra of nearly free electron metals

Transduction of UV-light energy into alternating-current electricity via a neglected internal photoelectric effect of metal foil-based nanogenerator

Massive free electrons in metals could escape from the metal surface to generate a convection current when colliding with high-energy photons (e.g. UV light). Namely, metals with specific work functions could transduce light energy into direct-current (DC) electricity via the famous external photoelectric effect. However, obtaining alternating-current (AC) electricity via a non-Faraday’s law of electromagnetic induction directly using metal foil is challenging. Inspired by our recent work, we herein for the first time report a subtle method to directly transduce UV light energy into AC electricity by metal foil nanogenerators and propose a new mechanism regarding the transduction of UV light into AC electricity entitled the neglected internal photoelectric effect (NIPE). According to the NIPE, a zinc foil-based NIPE nanogenerator (NIPEG) can output an optimal open-circuit voltage (Voc) and a short-circuit current (Isc) of 108 V and 320 μA, respectively. The maximum output power and the photoelectric conversion efficiency of the fabricated nanogenerator approach 23.9 W m−2 and 77.02%, respectively. Meantime, the electrical energy stored in the capacitor connected to the Zn foil-based nanogenerator after rectification is several orders of magnitude higher than that of the conventional Cu/PTFE/PET/Cu-based triboelectric nanogenerator and Cu/PTFE/Al-based water droplet nanogenerator. The herein approach potentially offers a novel energy harvesting and conversion avenue to develop self-powered and wearable electronic devices.

On the Electride Nature of Na-hP4.

Early quantum mechanical models suggested that pressure drives solids towards free-electron metal behavior where the ions are locked into simple close-packed structures. The prediction and subsequent discovery of high-pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na-hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na-pd hybrid orbitals point towards the center of an 11-membered penta-capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na-hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.

Nonlocal Soft Plasmonics in Planar Homogeneous Multilayers

Plasmonics is the study of resonant oscillations of free electrons in metals caused by incident electromagnetic radiation. Surface plasmons can focus and steer light on the subwavelength scale. Apart from metals, plasmonic phenomena can be observed in soft matter systems such as electrolytes which we study here. Resonant charge oscillations can be induced for ions in solution, however, due to their larger mass, they are plasmon-active in a lower frequency regime and on a larger wavelength scale. Our investigation focuses on spatial confinement which allows increasingly strong charge interactions and gives rise to nonlocality or spatial dispersion effects. We derive and discuss the nonlocal optical response of ionic plasmons using a hydrodynamic two-fluid model in a planar homogeneous three-layer system with electrolyte-dielectric interfaces. As in metals, we observe the emergence of additional longitudinal propagation modes in electrolytes which causes plasmonic broadening. Studying such systems enables us to identify and understand plasmonic phenomena in biological and chemical systems.

Size-dependent heat capacity in nano-oxide crystals

Recent developments suggest that heat capacity of nano-oxides is size-dependent, since the product of bulk modulus (B) and thermal expansion coefficient (α) scales directly with isometric heat capacity Cv. Dependence of heat capacity on crystal-size Cv(d) is explored by examining Bαao where ao is the lattice parameter. Here Bαao(d)'s show maxima at 33 nm for ceria and at 14 nm for MgO and decrease with decreasing crystal-size (d) implying similar size-dependence for Cv if the Grüneisen parameter is size insensitive. Hence, the Cv (d) contradicts with metal's heat capacities which are larger in nano-grains. Contributions from grain boundaries and surfaces were posited for the difference. Besides phonons, free electrons in metals contribute to heat capacity. The absence of electronic carriers in oxides likely explains the difference.

A Potential‐Based Boundary Element Implementation for Modeling Multiple Scattering from Local and Nonlocal Plasmonic Nanowires

AbstractA novel boundary element implementation that models multiple scattering of plasmonic nanowires is presented. The modeling is based on potentials and the materials constituting the wires can be local (described by the local response model) or nonlocal (described by the nonlocal hydrodynamic model). The nonlocal hydrodynamic model (HDM) provides an important approximation describing nonclassical effects associated with the collective motion of free electrons in metals. The modeling is challenging as different interface conditions are needed at a boundary which separates 1) a local medium from a local medium; 2) a local (nonlocal) medium from a nonlocal (local) medium; and 3) a nonlocal medium from a nonlocal medium. The algorithm can address constructs of arbitrary geometry and material composition within the HDM; thus, it becomes a complete numerical tool for exploration of nonlocal nanowires. Fictitious sources are imposed at the boundaries, linking the scattered fields to the imposed sources, and matching the interface conditions at the boundaries. This procedure yields a set of Boundary Integral Equations (BIEs). Then, the BIEs are numerically solved utilizing the Boundary Element Method (BEM). The results from the BEM solver are verified quantitatively and qualitatively showing good agreement in both the near‐ and the far‐field regimes.

OpenSANS: A Semi-Analytical solver for Nonlocal plasmonicS

Prompted by the renewed interest the past decade in the hydrodynamic model for the description of nanoplasmonic systems, we present OpenSANS, a MATLAB toolbox for the simulation of canonical (that is, standalone spherical and cylindrical) nanoparticles and planar stratified structures whose critical dimensions are of mere nanometres. The method comprises of a generalisation of Mie theory to properly address the hydrodynamic motion of free electrons in metals and a compact S matrix formulation for the efficient treatment of multiple layers. The toolbox is built in a modular manner; each part (routine) has its own important functionality and can be used independently from the others. Our primary goal is the presentation of a highly efficient and accessible solver for the nanoplasmonics community. This is achieved by the very semi-analyticity of the chosen method and by powerful vectorisation techniques optimised in a very popular environment, MATLAB. Program summaryProgram Title: OpenSANSCPC Library link to program files:https://doi.org/10.17632/hmk224xngb.1Code Ocean capsule:https://codeocean.com/capsule/2246190Licensing provisions: GPLv3Programming language: MATLAB R2018b and newer.Supplementary material: See Appendix A.Nature of problem: Solve the scattering problem from multilayered, dielectric, local or nonlocal metallic nanoparticles, excited by a plane wave and placed in a local (dielectric or metallic) background.Solution method: Mie theory, appropriately extended to incorporate nonlocal effects (longitudinal waves) as predicted by the Hard Wall-Hydrodynamic Model, combined with S matrix theory for efficient handling of the multilayered structure.

A Boundary Integral Equation Formalism for Modeling Multiple Scattering of Light from 3D Nanoparticles Incorporating Nonlocal Effects

AbstractA novel computational scheme that models the scattering of light at interfaces in a deep‐nanometric (deep‐nm) plasmonic system is presented. The nonclassical effects associated with the collective motion of free electrons in metals of the system are investigated by a nonlocal hydrodynamic (HD) model. Due to the HD model, three types of interfaces, that is, the dielectric–dielectric interface, the dielectric–metal interface, and the metal–metal interface, are distinguished. The scattering of light at these interfaces is described by Boundary Integral Equations (BIEs), which are constructed by using the surface equivalence principle, the concept of the Green' function and the interface conditions. The BIEs are numerically solved by the boundary element method (BEM) for 3D nanoparticles of arbitrary shapes. To validate, the computed results are first quantitatively contrasted with analytical solutions from the Mie theory for spherical core–shell structures, with good agreements being demonstrated in both the near field and the far field. Then, a qualitative study is performed for dimers consisting of spheres with various gap sizes for both the classical local response model and the nonlocal HD model. The observed trends agree well with the results reported in previous works.

Pressure-dependent thermal conductivity in Al, W, and Pt: Role of electrons and phonons

Understanding the pressure dependence on thermal transport of metals is crucial for high-pressure applications and fundamental research in earth science. As high-pressure thermal measurements are challenging, the theoretical approach and first-principles methods are widely adopted to investigate the pressure-dependent thermal transport in metals. However, these approaches are generally limited to the free-electron metals and the phonon contributions are neglected. The mechanisms behind the pressure effect on thermal transport of metals are not fully addressed. In this work, we implement rigorous mode-level first-principles calculations to reveal different mechanisms behind the pressure-dependent electronic and phonon thermal conductivity in aluminum (Al), tungsten (W), and platinum (Pt). While the overall thermal conductivity values of the three metals all increase with pressure, the mechanisms are different. For the electronic thermal conductivity part, the main contribution for the positive pressure effect on free-electron metal Al is the decrease of the electron-phonon scattering rate, while the dominant contribution in Pt and W, whose $d$ state electrons are abundant around the Fermi energy, is the increase of electron group velocity. Phonon thermal conductivity of Pt and Al is found to increase with pressure, but the rates of increase are different. In contrast, phonon thermal conductivity of W is nearly independent with pressure. Such pressure invariance is due to the competing effect between phonon lifetime decrease and phonon group velocity increase under pressure.

Lifetime of quasiparticles in the nearly free electron metal sodium

We report a high-resolution angle-resolved photoemission (ARPES) study of the prototypical nearly-free-electron metal sodium. The observed mass enhancement is slightly smaller than that derived in previous studies. The new results on the lifetime broadening increase the demand for theories beyond the random phase approximation. Our results do not support the proposed strong enhancement of the scattering rates of the charge carriers due to a coupling to spin fluctuations. Moreover, a comparison with earlier electron energy-loss data on sodium yields a strong reduction of the mass enhancement of dipolar electron-hole excitations compared to that of monopole hole excitations, measured by ARPES.

Electronic correlation in nearly free electron metals with beyond-DFT methods

For more than three decades, nearly free-electron elemental metals have been a topic of debate because the computed bandwidths are significantly wider in the local density approximation to density-functional theory (DFT) than indicated by angle-resolved photoemission (ARPES) experiments. Here, we systematically investigate this using first principles calculations for alkali and alkaline-earth metals using DFT and various beyond-DFT methods such as meta-GGA, G0W0, hybrid functionals (YS-PBE0, B3LYP), and LDA + eDMFT. We find that the static non-local exchange, as partly included in the hybrid functionals, significantly increase the bandwidths even compared to LDA, while the G0W0 bands are only slightly narrower than in LDA. The agreement with the ARPES is best when the local approximation to the self-energy is used in the LDA + eDMFT method. We infer that even moderately correlated systems with partially occupied s orbitals, which were assumed to approximate the uniform electron gas, are very well described in terms of short-range dynamical correlations that are only local to an atom.

Computational Analyses of Plasmonics of a Silver Nanoparticle in a Vacuum and in a Water Solution by Classical Electronic and Molecular Dynamics Simulations.

We present basic optical responses of a silver nanoparticle (Ag309) in a vacuum and a water solution obtained by classical electronic and molecular dynamics (CEMD) calculations, where the CEMD is our previously developed force-field based molecular dynamics simulation method that incorporates the classical equation of motion for free electrons in metal and an interaction with the applied oscillating electric field (Yamada, A. J. Chem. Phys. 2021, 155, 174118)). The present work is the follow-up of the previous study. Calculated absorption spectra in the visible region with various conditions are reported together with parameter determination for realistic analyses as well as the verification of the method. Time-domain optical responses of Ag309 in water solvent are as well analyzed in terms of the plasmon resonance excitation under a subpicosecond light pulse and its thermal relaxation.

Positron pair interactions in a nearly free-electron metal

The electric field surrounding a single positron in a metal is screened by an increase in the local electron density which, in the case of nearly free-electron metals (like Al, Na, etc.), has a radial distribution similar to that of the electron in positronium (Ps). In such metals, a singlet pair of positrons would experience an attractive interaction and at low enough electron densities could possibly form a bound state that is held together by exchange and correlation energies, thus forming structures analogous to that of the positronium molecule (Ps_2), with binding energies of a few tenths of an eV. Such di-positrons could be prevalent at positron densities of around 10^{18} cm^{-3} and, if so, would be evident from an apparent broadening of the sharp step at the Fermi surface in measurements of the electron momentum distribution by the angular correlation of the 2gamma annihilation radiation. Even if di-positrons are not directly formed in a metal, optical spectroscopy of Ps_2 formed in vacuum via pairs of positrons simultaneously being emitted from the surface could be applied to the direct measurement of the momentum distribution of Cooper pairs. If they exist, di-positrons in metals would yield interesting information about electron and positron interactions and at very high densities might allow the study of a di-positron Bose–Einstein condensate immersed in an electron gas.Graphic

Investigation of the Way of Phase Synchronization of a Self-Injected Bunch and an Accelerating Wakefield in Solid-State Plasma

The electron acceleration, in a laser wakefield accelerator, controlled through plasma density inhomogeneity is studied on a basis of 2.5-dimensional particle-in-cell simulation. The acceleration requires a concordance of the density scale length and shift of the accelerated electron bunch relative to wake bubble during electron acceleration. This paper considers the excitation of a wakefield in plasma with a density equal to the density of free electrons in metals, solid-state plasma (the original idea of Prof. T. Tajima), in the context of studying the wakefield process. As is known in the wake process, as the wake bubble moves through the plasma, the self-injected electron bunch shifts along the wake bubble. Then, the self-injected bunch falls into the phase of deceleration of the wake wave. In this paper, support of the acceleration process by maintaining the position of the self-injected electron bunch using an inhomogeneous plasma is proposed. It is confirmed that the method of maintaining phase synchronization proposed in the article by using a nonuniform plasma leads to an increase in the accelerating gradient and energy of the accelerated electron bunch in comparison with the case of self-injection and acceleration in a homogeneous plasma.

Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

Plasmonic vortex cavities multiply OAM

Nanooptics Orbital angular momentum (OAM) of light can be confined to metal surfaces in the form of plasmonic vortices, where nanoscale twisted light oscillates together with the free electrons in metal. Spektor et al. introduced the reflection from structural boundaries as a new degree of freedom to generate and control this surface-confined angular momentum. They designed vortex cavities with chiral boundaries, in which subsequent reflections increased the vortex OAM by multiples of the chiral cavity order. They then tracked the spatiotemporal dynamics of the plasmon pulse trains within the vortex cavities using time-resolved photoemission electron microscopy with sub-femtosecond resolution. These results may have applications in quantum initialization schemes and potentially achieve vortex lattice cavities with dynamically evolving topology. Sci.Adv. 10.1126/sciadv.abg5571 (2021).

Theory for heating of metals assisted by surface plasmon polaritons

We propose a model accounting for plasmons within a two temperature description, to investigate the role of surface plasmon polaritons (SPP) in the energy redistribution between laser excited electrons and the lattice, leading to surface restructuring of the material. This energy transfer can lead to the creation of laser induced surface structures in metals illuminated by an ultrashort laser pulse. The Two Temperature Model+Plasmon (TTM + P) equations are constructed by applying perturbation theory on the energy-, momentum- and density conservation equations of free electrons in metals. We consider three subsystems: the lattice, the thermalized electrons and the SPP, subject to an external laser field irradiation. The interference between the laser and SPP fields leads to spatially modulated energy absorption by the electronic system, and, through electron–phonon collisions, periodically shapes the lattice temperature. A numerical analysis is performed on a 1D model for gold. We show the emergence of a periodic modulation of the lattice temperature, which may contribute to laser-induced periodic surface structures (LIPSS).

Unveiling the anisotropic behavior of ultrafast electron transfer at the metal/organic interface

Ultrafast electron transfer between adsorbed organic molecules and metal substrates is studied. In particular, the dynamics of the active electron in the nitroethylene anion/metal-copper surface system has been followed in real time using a wave packet propagation approach, allowing a rigorous analysis of the decay of molecule-localized electronic resonances. We find that the strong coupling with the metal substrate leads to an extremely short lifetime (~1fs) of the π∗ molecular resonance. Comparison between the free-electron metal, Cu(100), and Cu(111) surfaces demonstrates that the electronic band structure of the substrate and the shape of the decaying molecular orbital lead to a highly marked anisotropy of the metal continuum states populated by resonant electron transfer from the adsorbate. This finding points at possible anisotropy effects in adsorbate–adsorbate interactions and it is of particular importance in molecular self assembly at metal surfaces, thus opening the way to a rational design of hybrid metal/organic interfaces with tailored electronic properties.

Tunable electron and hole injection channels at plasmonic Al-TiO2 interfaces.

Metallic nanostructures can strongly absorb light through their plasmon excitations, whose nonradiative decay generates hot electron-hole pairs. When the metallic nanostructure is interfaced with a semiconductor, the spatial separation of hot carriers plays the central and decisive roles in photovoltaic and photocatalytic applications. In recent years, free-electron metals like Al have attracted tremendous attentions due to the much higher plasmon frequencies that could extend to the ultraviolet regime. Here, the plasmon excitations and charge separations at the Al-TiO2 interfaces have been investigated using quantum-mechanical calculations, where the atomic structures and electronic dynamics are all treated from first-principles. It is found that the high-frequency plasmon of Al produces abundant and broad-band hot-carrier distributions, where the electron-hole symmetry is broken by the presence of the semiconductor band gap. Such an asymmetric hot-carrier distribution provides two competing channels, which can be controlled either by tuning the laser frequency, or by harnessing the plasmon frequency through the geometry and shape of the metallic nanostructure. Our study suggests that the Al plasmon offers a versatile and tunable pathway for the charge transfer and separation, and has general implications in plasmon-assisted photovoltaics and photocatalysis.

Ising-like models for stacking faults in a free electron metal.

We propose an extension of the axial next nearest neighbour Ising (ANNNI) model to a general number of interactions between spins. We apply this to the calculation of stacking fault energies in magnesium—particularly challenging due to the long-ranged screening of the pseudopotential by the free electron gas. We employ both density functional theory (DFT) using highest possible precision, and generalized pseudopotential theory (GPT) in the form of an analytic, long ranged, oscillating pair potential. At the level of first neighbours, the Ising model is reasonably accurate, but higher order terms are required. In fact, our ‘ ANNNI model’ is slow to converge—an inevitable feature of the free electron-like electronic structure. In consequence, the convergence and internal consistency of the ANNNI model is problematic within the most precise implementation of DFT. The GPT shows the convergence and internal consistency of the DFT bandstructure approach with electron temperature, but does not lead to loss of precision. The GPT is as accurate as a full implementation of DFT but carries the additional benefit that damping of the oscillations in the ANNNI model parameters are achieved without entailing error in stacking fault energies. We trace this to the logarithmic singularity of the Lindhard function.