Screening Length Research Articles

Dynamics of strongly coupled two-component plasma via ultrafast spectroscopy.

A combination of ultrafast emission and transmission spectroscopy is presented that provides a model-independent temperature measurement and tracking of the expansion dynamics for a dense, strongly coupled plasma. For femtosecond laser breakdown of hydrogen gas at 10 bar, we observe a 30,000 K two-component plasma for hundreds of picoseconds where both electrons and protons have a strong coupling parameter value of $\Gamma \sim{0.5}$Γ∼0.5. Furthermore, the plasma's degree of ionization (45%) results in a condition where the Debye screening length (6 Å) is less than the interatomic spacing (13 Å). Plasma formation occurs under an isochoric initial condition, which simplifies hydrodynamic modeling of the plasma channel expansion. The channel radius is found to accelerate at a constant rate until the front is moving with the speed of sound. Comparing hydrogen and deuterium for the same breakdown conditions grants unique insight into the hydrodynamics of strongly coupled plasma due to their nearly identical electronic structure yet large mass difference. The ultimate goal of these experiments is to access a plasma regime where continuum mechanics become nonlocal, as compared with the hydrodynamic motion described by the Navier-Stokes equations.

Scaling Behavior of Bipolar Nanopore Rectification with Multivalent Ions

We present a scaling behavior of a rectifying bipolar nanopore as a function of the parameter ξ = RP/(λzif), where RP is the radius of the pore, λ is the characteristic screening length of the elec...

Probing energy landscapes in multilayer heterostructures: Challenges and opportunities

Quantitative characterization of energy landscapes at buried interfaces is essential for assessing their functionality. This perspective highlights recent developments in reconstructing internal potential profiles based on hard x-ray photoemission experiments that have proved to be a rich source of information. We show that band-edge profiles can be reconstructed from core-level photoelectron spectra by performing a comprehensive search for the best-fit set of associated layer-resolved spectra. The use of hard x-rays allows heterostructures to be probed over length scales comparable to relevant electronic screening lengths. Significantly, this information takes our understanding of such systems to a new level that is not currently achievable using any other experimental technique.

Calculation of self-consistent electrophysical parameters of semiconductor gas sensors based on samarium sulfide for semiconductor Fourier spectroscopy of volatile hydrocarbons contained in atmospheric air

The developed method of semiconductor gas Fourier spectroscopy is based on the relationship of the obtained calibration characteristics of the concentration sensors for volatile hydrocarbons contained in atmospheric air (in our case, it is methane and propane), with electrophysical parameters of the gas sensitive layers. The study used gas sensors based on samarium sulfide with reproducible and stable calibration characteristics. The working layers of hydrocarbon concentration sensors were obtained by two different methods: the explosive spraying method and the sol-gel coating technology. In this paper we present a system of equations that includes the main electrophysical characteristics of a gas-sensitive film which is solved by numerical methods. These equations reflect a unique relationship between the Debye screening length (Ld), film thickness (d), Fermi level (EF), concentration of impurity levels (Nd), dielectric constant of the medium (ε), crystal lattice constant (a), and mobility of the main charge carriers (μ) with the observed data on the change in the conductivity of thin films of samarium sulfide in the processes of adsorption of methane and propane molecules on their surface. In the present work, we obtained the first calculated data of such parameters for the current prototype images of gas sensors of methane and propane.

Energy dispersive spectroscopic measurement of charge traps in MoTe2

Spectroscopic techniques are vital to determine the energy distribution of trapped states in semiconducting materials to assess the quality and efficiency of electronic devices. However, there is a need for a sensitive spectroscopic technique that can be used with atomically thin materials which are thinner than the Debye screening length as the current methods, such as deep level trap spectroscopy and admittance spectroscopy, are incompatible with the long emission times and temperature sensitivities of these materials. In this work we expand the threshold voltage transient phenomenon into an energy dispersive spectroscopic technique, dubbed the threshold voltage transient spectroscopy technique. This is applied to few-layer ${\mathrm{MoTe}}_{2}$ with the trap concentration and subsequently the density of trap states found in a region between the valence band edge and the midgap, which clearly shows the density of states in the tail end of the valence band.

A statistical field theory of salt solutions of ‘hairy’ dielectric particles

In this paper, we formulate a field-theoretical model of dilute salt solutions of electrically neutral spherical colloid particles. Each colloid particle consists of a ‘central’ charge that is situated at the center and compensating peripheral charges (grafted to it) that are fixed or fluctuating relative to the central charge. In the framework of the random phase approximation, we obtain a general expression for electrostatic free energy of solution and analyze it for different limiting cases. In the limit of infinite number of peripheral charges, when they can be modelled as a continual charged cloud, we obtain an asymptotic behavior of the electrostatic potential of a point-like test charge in a salt colloid solution at long distances, demonstrating the crossover from its monotonic decrease to damped oscillations with a certain wavelength. We show that the obtained crossover is determined by certain Fisher–Widom line. For the same limiting case, we obtain an analytical expression for the electrostatic free energy of a salt-free solution. In the case of nonzero salt concentration, we obtain analytical relations for the electrostatic free energy in two limiting regimes. Namely, when the ionic concentration is much higher than the colloid concentration and the effective size of charge cloud is much bigger than the screening lengths that are attributed to the salt ions and the central charges of colloid particles. The proposed theory could be useful for theoretical description of the phase behavior of salt solutions of metal-organic complexes and polymeric stars.

Conformational Properties of Comb-shaped Polyelectrolytes with Negatively Charged Backbone and Neutral Side Chains Studied by a Generic Coarse-grained Bead-and-Spring Model

A generic coarse-grained bead-and-spring model, mapped onto comb-shaped polycarboxylate-based (PCE) superplasticizers, is developed and studied by Langevin molecular dynamics simulations with implicit solvent and explicit counterions. The agreement on the radius of gyration of the PCEs with experiments shows that our model can be useful in studying the equilibrium sizes of PCEs in solution. The effects of ionic strength, side-chain number, and side-chain length on the conformational behavior of PCEs in solution are explored. Single-chain equilibrium properties, including the radius of gyration, end-to-end distance and persistence length of the polymer backbone, shape-asphericity parameter, and the mean span dimension, are determined. It is found that with the increase of ionic strength, the equilibrium sizes of the polymers decrease only slightly, and a linear dependence of the persistence length of backbone on the Debye screening length is found, in good agreement with the theory developed by Dobrynin. Increasing side-chain numbers and/or side-chain lengths increases not only the equilibrium sizes (radius of gyration and mean span) of the polymer as a whole, but also the persistence length of the backbone due to excluded volume interactions.

Screening length for finite-size ions in concentrated electrolytes.

The classical Debye-Hückel (DH) theory clearly accounts for the origin of screening in electrolyte solutions and works rather well for dilute electrolyte solutions. While the Debye screening length decreases with the ion concentration and is independent of ion size, recent surface-force measurements imply that for concentrated solutions, the screening length exhibits an opposite trend; it increases with ion concentration and depends on the ionic size. The screening length is usually defined by the response of the electrolyte solution to a test charge but can equivalently be derived from the charge-charge correlation function. By going beyond DH theory, we predict the effects of ion size on the charge-charge correlation function. A simple modification of the Coulomb interaction kernel to account for the excluded volume of neighboring ions yields a nonmonotonic dependence of the screening length (correlation length) on the ionic concentration, as well as damped charge oscillations for high concentrations.

Photosensing System Using Photosystem I and Gold Nanoparticle on Graphene Field-Effect Transistor.

In this study, a light sensor is fabricated based on photosystem I (PSI) and a graphene field-effect transistor (FET) that detects light at a high quantum yield under ambient conditions. We immobilized PSI on a micrometer-sized graphene FET using Au nanoparticles (AuNPs) and measured the I-V characteristics of the modified graphene FET before and after light irradiation. The source-drain current (Isd) increased upon illumination, exhibiting a photoresponsivity of 4.8 × 102 A W-1, and the charge neutrality point of graphene shifted by -12 mV. This system represents the first successful photosensing system based on proteins and a solution-gated graphene FET. The probable mechanism of this negative shift can be explained by the increase in negative charge carriers in graphene induced by a hole trap in the AuNP resulting from electron transfer from the AuNP to PSI. Photoresponses were only observed in the presence of two surface-active agents, n-hexyltrimethylammonium bromide and sodium dodecylbenzenesulfonate, because they caused the formation of a hydrophobic environment on the surface of the graphene. The lipid layer of these agents caused dissociation of ascorbate ions from the graphene sheet, thereby expanding the Debye screening length of the electrolyte solution. The hydrophobic environment above graphene also enhanced hole storage in the AuNP through electron transfer from the AuNP to PSI.

Ground-state atlas of a three-dimensional semimetal in the quantum limit

An interplay between electron correlation and reduced dimensionality due to the Landau quantization gives rise to exotic electronic phases in three-dimensional semimetals under high magnetic field. Using an unbiased theoretical method, we clarify for the first time comprehensive ground-state phase diagrams of a three-dimensional semimetal with a pair of electron and hole pockets in the quantum limit. For the electron interaction, we consider either screened Coulomb repulsive interaction or an attractive electron-electron interaction mediated by a screened electron-phonon coupling, where a screening length is generally given by a dimensionless constant times magnetic length $l$. By solving the parquet RG equation numerically and employing a mean-field argument, we construct comprehensive ground-state phase diagrams of the semimetal in the quantum limit for these two cases, as a function of the Fermi wave length and the screening length (both normalized by $l$). In the repulsive interaction case, the ground state is either excitonic insulator (EI) in strong screening regime or Ising-type spin density wave in weak screening regime. In the attractive interaction case, the ground state is either EI that breaks the translational symmetries (strong screening regime), topological EI, charge Wigner crystal (intermediate screening regime), plain charge density wave or possible non-Fermi liquid (weak screening regime). We show that the topological EI supports a single copy of massless Dirac fermion at its side surface, and thereby exhibit a $\sqrt{H_{\perp}}$-type surface Shubnikov-de Haas (SdH) oscillation in in-plane surface transports as a function of a canted magnetic field $H_{\perp}$. Armed with these theoretical knowledge, we discuss implications of recent transport experiments on graphite under the high field.

Where Do the Ions Reside in a Highly Charged Droplet?

Aqueous droplets in atmospheric and electrosprayed aerosols are charged due to presence of multiple ionic species. We examine the ion spatial distribution and the surface electric field in aqueous charged nanodrops by using atomistic modeling and analytical theory. We find that in nanoscopic liquid drops the concentration of simple ions is higher in the outer droplet shells, reduces gradually toward the drop center, and dies-off toward the vapor-droplet interface. The behavior of the ion spatial distribution is supported by a general analytical theory that takes into account a fluctuating droplet interface, an effective screening length of the charges and the finite size of a solvated ion. We compute the electric potential and the electric field near the droplet surface using a multipole expansion. We emphasize the significance of the fluctuations of the normal component of the electric field in ion evaporation via the Born model. In the presence of a highly charged peptide, we find that the peptide is situated mainly in the droplet interior and occasionally near the droplet surface. The simple ions are mainly near the droplet surface. The study provides insight into droplet chemistry and electrospray ionization mass spectrometry findings.

Direct measurement of a beta function and an indirect check of the Schwinger effect near the boundary in Dirac semimetals

The electric field inside typical conductors drops down exponentially with the screening length determined by an intrinsic length scale of the system such as the density of mobile carriers. We show that in a classically conformal system with boundaries, where the intrinsic length scale is absent, the screening of an external electric field is governed by the quantum conformal anomaly associated with the renormalization of the electric charge. The electric field decays algebraically with a fractional power determined by the beta function of the system. We argue that this `anomalous conformal screening effect' is an indirect manifestation of the Schwinger pair production in relativistic field theory. We discuss the experimental feasibility of the proposed phenomenon in Dirac and Weyl semimetals what would allow direct experimental access to the beta function.

Effect of Roughness and Elasticity on Interactions between Charged Colloidal Spheres.

The effects of realistic roughness and elasticity on the interactions between charged silica spheres are studied as a function of surface potential, screening length, interfacial energy, and roughness. The repulsive force Frep that must be overcome to bring charged spheres into contact is relatively insensitive to elasticity unless spheres are hundreds of times softer than silica. Frep is also insensitive to roughness and interfacial energy. In contrast, roughness has a large effect on the binding energy of spheres and the force Fsep to separate them. Both are lowered by 1 to 2 orders of magnitude by the measured surface roughness of less than 1 nm on 1 μm silica spheres. The reason is that interactions between rigid spheres are dominated by the highest surface peaks rather than the entire spherical surface. Elasticity can increase the pull-off force of rough spheres by a factor of 2 or more because additional surface area can be brought into contact. The implications of these results for shear-thickening transitions are discussed.

Unified description of sound velocities in strongly coupled Yukawa systems of different spatial dimensionality

Sound velocities in classical single-component fluids with Yukawa (screened Coulomb) interactions are systematically evaluated and analyzed in one-, two-, and three spatial dimensions (D=1,2,3). In the strongly coupled regime, the convenient sound velocity scale is given by Q2/Δm, where Q is the particle charge, m is the particle mass, n is the particle density, and Δ=n−1/D is the unified interparticle distance. The sound velocity can be expressed as a product of this scaling factor and a dimension-dependent function of the screening parameter, κ=Δ/λ, where λ is the screening length. A unified approach is used to derive explicit expressions for these dimension-dependent functions in the weakly screened regime (κ≲3). It is also demonstrated that for stronger screening (κ≳3), the effect of spatial dimensionality virtually disappears, the longitudinal sound velocities approach a common asymptote, and a one-dimensional nearest-neighbor approximation provides a relatively good estimate for this asymptote. This result is not specific to the Yukawa potential, but equally applies to other classical systems with steep repulsive interactions. An emerging relation to a popular simple freezing indicator is briefly discussed. Overall, the results can be useful when Yukawa interactions are relevant, in particular, in the context of complex (dusty) plasmas and colloidal suspensions.

Multicomponent mutual diffusion in the warm, dense matter regime.

We present the formulation, simulations, and results for multicomponent mutual diffusion coefficients in the warm, dense matter regime. While binary mixtures have received considerable attention for mass transport, far fewer studies have addressed ternary and more complex systems. We therefore explicitly examine ternary systems utilizing the Maxwell-Stefan formulation that relates diffusion to gradients in the chemical potential. Onsager coefficients then connect the macroscopic diffusion to microscopic particle motions, evinced in trajectories characterized by positions and velocities, through various autocorrelation functions (ACFs). These trajectories are generated by molecular dynamics (MD) simulations either through the Born-Oppenheimer approximation, which treats the ions classically and the electrons quantum-mechanically by an orbital-free density-functional theory, or through a classical MD approach with Yukawa pair-potentials, whose effective ionizations and electron screening length derive from quantal considerations. We employ the reference-mean form of the ACFs and determine the center-of-mass coefficients through a simple reference-frame-dependent similarity transformation. The Onsager terms in turn determine the mutual diffusion coefficients. We examine a representative sample of ternary mixtures as a function of density and temperature from those with only light elements (D-Li-C, D-Li-Al) to those with highly asymmetric mass components (D-Li-Cu, D-Li-Ag, H-C-Ag). We also follow trends in the diffusion as a function of number concentration and evaluated the efficacy of various approximations such as the Darken approximation.

Growth interactions between icosahedral quasicrystals

We investigate the solidification pathways of a population of icosahedral quasicrystals in a liquid through in situ synchrotron x-ray tomography. The wealth of three-dimensional space- and time-resolved data enables us to test the predictions of various models and theories of crystallization on a quasi-crystal-forming alloy. Remarkably, we find the general evolution equation---that accounts for the competing effects of growth, coalescence, and coarsening---fits well our experimental data on surface area concentration. Furthermore, we quantify the orientation selection, screening length, and coarsening rate of the dodecahedra, and compare these results to that of periodic crystals. The latter is a full order of magnitude smaller than that of elemental metal dendrites in the limit of zero volume fraction, a reflection of the low solid-liquid interfacial energy of the icosahedral phase. Our paper provides the critical input data for microstructural models used for integrated computational materials engineering of complex intermetallics, including quasicrystals.

Finite dust clusters under strong magnetic fields

Experiments on dust clusters trapped in the sheath of a radio frequency discharge have been performed for different magnetic field strengths ranging from a few milliteslas to 5.8 T. The dynamics of the dust clusters are analyzed in terms of their normal modes. From that, various dust properties such as the kinetic temperature, the dust charge, and the screening length are derived. It is found that the kinetic temperature of the cluster rises with the magnetic field, whereas the dust charge nearly remains constant. The screening length increases slightly at intermediate magnetic field strengths. Generally, the dust properties seem to correlate with magnetization parameters of the plasma electrons and ions, however only to a small degree.

Properties of Yukawa Crystals and Liquid under Phase Equilibrium Conditions

The equilibrium properties of a system of particles whose interaction is described by the Yukawa potential are studied. Liquids and crystals with bcc and fcc lattices near the melting line are considered. The latent heat of melting for crystals and the density change during the phase transition are calculated. We analyze the temperature dependences of the thermodynamic quantities near the melting line and calculate the thermal expansion coefficient and the heat capacity at constant pressure. We show that the state of a Yukawa system cannot be uniquely defined by a pair of dimensionless quantities, the temperature and the pressure, which use the interaction potential parameters (the product of the charges and the screening length) as scales. In particular, the position of the melting line is ambiguously determined on a two-parameter phase diagram constructed in dimensionless quantities.

Controlling Magnetic Properties in Bulk Ferromagnetic Metals By Electrochemical Charging and Discharging of Hydrogen Atoms

Controlling magnetic properties of materials with external voltages has attracted much interest owing to its promising applications in various areas like actuation. However, the voltage effect in metals is usually restricted to few atomic layers as well as too small for real use due to strong electric field screening. Here, for the first time, we show that bulk magnetic properties of ferromagnetic metal can be tuned significantly through charging and discharging of hydrogen atoms. We manipulated the coercivity of micrometer-sized SmCo5 grains by a fraction of tesla with a voltage of only around 1 V. Employing this approach, voltage-assisted magnetization reversal is realized at room temperature. Through both experiments and simulation, we found that the huge voltage effect originates from the significant change of magnetocrystaline anisotropy in surface regions of SmCo5 grains due to hydrogen insertion and removal, which modifies the nucleation field during magnetization reversal. This work opens up a route for overcoming the screening length and has implication, for example, for using SmCo5 in sensors and actuators.

Short-path-length corrections to Djordjevic-Gyulassy-Levai-Vitev energy loss

We compute the correction to the energy loss of a hard parton due to short separation distances between the creation of the particle and the in-medium scattering center that stimulates bremsstrahlung radiation, to first order in opacity. In deriving the result we make full use of the large formation time assumption, which results in a significant reduction of the number of diagrams contributing to the small separation distance correction. An asymptotic analysis of our small separation distance correction term finds that the correction dominates at large $\sim 100$ GeV parent parton energies; scales like $L$ with the size of the system for small $L$, but like $L^0$ at larger $L$; and breaks color triviality. An extensive numerical investigation of the correction term confirms the aforementioned analytic findings, reveals that the correction term does not go to zero for large $L$, finds that the correction is sensitive to the mass of the parent parton, and shows a crucial dependence of the energy loss on a proper treatment of the physics of separation distances on the order of the Debye screening length. However, an examination of the large formation time approximation shows that it is violated for much of the phase space of the emitted radiation, implying a need to investigate the sensitivity of jet quenching results from relaxing this approximation. Our result constitutes an important step toward understanding partonic energy loss in small colliding systems.