Weak Screening Research Articles

Electrical conductivity of the thermal dusty plasma under the conditions of a hybrid plasma environment simulation facility

We discuss the inductively heated plasma generator (IPG) facility in application to the generation of the thermal dusty plasma formed by the positively charged dust particles and the electrons emitted by them. We develop a theoretical model for the calculation of plasma electrical conductivity under typical conditions of the IPG. We show that the electrical conductivity of dusty plasma is defined by collisions with the neutral gas molecules and by the electron number density. The latter is calculated in the approximations of an ideal and strongly coupled particle system and in the regime of weak and strong screening of the particle charge. The maximum attainable electron number density and corresponding maximum plasma electrical conductivity prove to be independent of the particle emissivity. Analysis of available experiments is performed, in particular, of our recent experiment with plasma formed by the combustion products of a propane–air mixture and the CeO2 particles injected into it. A good correlation between the theory and experimental data points to the adequacy of our approach. Our main conclusion is that a level of the electrical conductivity due to the thermal ionization of the dust particles is sufficiently high to compete with that of the potassium-doped plasmas.

Structure-Based Design of Potent and Selective Inhibitors of the Metabolic Kinase PFKFB3.

A weak screening hit with suboptimal physicochemical properties was optimized against PFKFB3 kinase using critical structure-guided insights. The resulting compounds demonstrated high selectivity over related PFKFB isoforms and modulation of the target in a cellular context. A selected example demonstrated exposure in animals following oral dosing. Examples from this series may serve as useful probes to understand the emerging biology of this metabolic target.

Electrostatically transparent graphene quantum-dot trap layers for efficient nonvolatile memory

In this study, we have demonstrated nonvolatile memory devices using graphene quantum-dots (GQDs) trap layers with indium zinc oxide (IZO) semiconductor channel. The Fermi-level of GQD was effectively modulated by tunneling electrons near the Dirac point because of limited density of states and weak electrostatic screening in monolayer graphene. As a result, large gate modulation was driven in IZO channel to achieve a subthreshold swing of 5.21 V/dec (300 nm SiO2 gate insulator), while Au quantum-dots memory shows 15.52 V/dec because of strong electrostatic screening in metal quantum-dots. Together, discrete charge traps of GQDs enable stable performance in the endurance test beyond 800 cycles of programming and erasing. Our study suggests the exciting potential of GQD trap layers to be used for a highly promising material in non-volatile memory devices.

Dual-gated MoS2/WSe2 van der Waals tunnel diodes and transistors.

Two-dimensional layered semiconductors present a promising material platform for band-to-band-tunneling devices given their homogeneous band edge steepness due to their atomically flat thickness. Here, we experimentally demonstrate interlayer band-to-band tunneling in vertical MoS2/WSe2 van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS2 and WSe2 layers are independently controlled by the two symmetric gates. The same device can be gate modulated to behave as either an Esaki diode with negative differential resistance, a backward diode with large reverse bias tunneling current, or a forward rectifying diode with low reverse bias current. Notably, a high gate coupling efficiency of ∼80% is obtained for tuning the interlayer band alignments, arising from weak electrostatic screening by the atomically thin layers. This work presents an advance in the fundamental understanding of the interlayer coupling and electron tunneling in semiconductor vdW heterostructures with important implications toward the design of atomically thin tunnel transistors.

Synthesis and physical properties of a new caged compound Ce3Pd20As6 of the C6Cr23-type structure

We have found that Ce3Pd20As6 crystallizes into a cubic C6Cr23-type structure. Combination of electron probe microanalysis of the chemical composition and Rietveld analysis of the powder X-ray diffraction pattern has revealed an inhomogeneous atomic composition of variable stoichiometry. The physical properties of Ce3Pd20As6 were investigated by measuring the magnetization, electrical resistivity and specific heat. The 4f electrons of Ce3+ ions are well localized but do not show phase transition down to 0.5K. The metallic electrical resistivity shows a weak Kondo screening. The residual resistivity ratio is rather low probably due to the variable stoichiometry. The magnetization curve and magnetic entropy suggest the Γ8 quartet crystalline-electric-field ground state at least one of two Ce sites.

Dynamics of the fully stripped ion-hydrogen atom charge exchange process in dense quantum plasmas

The plasma screening effects of dense quantum plasmas on charge exchange processes of a fully stripped ion colliding with a hydrogen atom are studied by the classical trajectory Monte Carlo method. The inter-particle interactions are described by the exponential cosine-screened Coulomb potentials. It is found that in weak screening conditions, cross sections increase with the increase of the ionic charge Z. However, in strong screening conditions, the dependence of cross sections on the ionic charge is related to the incident particle energy. At high energies, cross sections show a linear increase with the increase of Z, whereas at low energies, cross sections for Z≥4 become approximately the same. The He2+ and C6+ impacting charge exchange cross sections in dense quantum plasmas are also compared with those in weakly coupled plasmas. The interactions are described by the static screened Coulomb potential. It is found that for both He2+ and C6+, the oscillatory screening effects of dense quantum plasmas are almost negligible in weak screening conditions. However, in strong screening conditions, the oscillatory screening effects enhance the screening effects of dense quantum plasmas, and the enhancement becomes more and more significant with the increase of the screening parameter and the ionic charge.

Tunable electronic and optical properties of monolayer silicane under tensile strain: a many-body study.

The electronic structure and optical response of silicane to strain are investigated by employing first-principles calculations based on many-body perturbation theory. The bandgap can be efficiently engineered in a broad range and an indirect to direct bandgap transition is observed under a strain of 2.74%; the semiconducting silicane can even be turned into a metal under a very large strain. The transitions derive from the persistent downward shift of the lowest conduction band at the Γ-point upon an increasing strain. The quasi-particle bandgaps of silicane are sizable due to the weak dielectric screening and the low dimension; they are rapidly reduced as strain increases while the exciton bound energy is not that sensitive. Moreover, the optical absorption edge of the strained silicane significantly shifts towards a low photon energy region and falls into the visible light range, which might serve as a promising candidate for optoelectronic devices.

Collision time measurements in a sonoluminescing microplasma with a large plasma parameter.

The plasma which forms inside of a micron-sized sonoluminescing bubble in water for under a nanosecond has been probed with 3 ns long laser pulses. A comparison of the response to 532 and 1064 nm light indicates that the plasma number density is about 2×10(21) cm(-3) and that transport properties are dominated by strong screening and correlation effects. The spherical shape, well-defined atomic density, and blackbody temperature make the sonoluminescing plasma a test bed for theories of strongly coupled plasmas. The plasma in this experiment distinguishes between competing theories of strong, intermediate, and weak effective screening.

Debye Screening Effect on Scaling Behavior of Longest Relaxation Time of Biological Polyelectrolyte Chain

The scaling relationship of the longest relaxation time of a single chain of semiflexible biological polyelectrolyte has been investigated by performing well-established coarse-grained Brownian dynamics simulations. Two kinds of longest relaxation times were estimated from time-sequences of chain trajectories, and their behaviors were interpreted by applying the scaling law for different molecular weights of polyelectrolyte and Debye lengths. The scaling exponents for longest stress relaxation and rotational relaxation are found in the ranges of 1.67-1.79 and 1.65-1.81, respectively, depending on the physicochemical interaction of electrostatic Debye screening. The scaling exponent increases with decreasing screening effect, which is a special feature of polyelectrolytes differing from neutral polymers. It revealed that the weak screening allows a polyelectrolyte chain to follow the behavior in good solvent due to the strong electrostatic repulsion between beads.

Two Approximate Analytic Eigensolutions of the Hellmann Potential with any Arbitrary Angular Momentum

The parametric Nikiforov-Uvarov (pNU) and asymptotic iteration method (AIM) are applied to study the approximate analytic bound state eigensolutions (energy levels and wave functions) of the radial Schr¨odinger equation (SE) for the Hellmann potential which represents the superposition of the attractive Coulomb potential (-a/r) and the Yukawa potential bexp(-δ/r)/r of arbitrary strength b and screening parameter d in closed form. The analytical expressions to the energy eigenvalues Enl yield quite accurate results for a wide range of n; l in the limit of very weak screening but the results become gradually worse as the strength b and the screening coefficient d increase. The calculated bound state energies have been compared with available numerical data. Special cases of our solution like pure Coulomb and Yukawa potentials are also investigated.

Charge exchange and ionization cross sections of H+ + H collision in dense quantum plasmas

The plasma screening effects of dense quantum plasmas on H+ + H charge exchange and ionization cross sections are calculated by the classical trajectory Monte Carlo method. For charge exchange cross sections, it is found that the screening effects reduce cross sections slightly in weak screening conditions. However, cross sections are reduced substantially in strong screening conditions. For ionization cross sections, with the increase of screening effects, cross sections for low energies increase more rapidly than those for high energies. When the screening effects are strong enough, it is found that ionization cross sections decrease with the increase of incident H+ energy. In addition, the cross sections have been compared with those in weakly coupled plasmas. It is found that in weak screening conditions, plasma screening effects in the two plasmas are approximately the same, while in strong screening conditions, screening effects of dense quantum plasmas are stronger than those of weakly coupled plasmas.

Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials

Layered materials of graphene and MoS₂, for example, have recently emerged as an exciting material system for future electronics and optoelectronics. Vertical integration of layered materials can enable the design of novel electronic and photonic devices. Here, we report highly efficient photocurrent generation from vertical heterostructures of layered materials. We show that vertically stacked graphene-MoS₂-graphene and graphene-MoS₂-metal junctions can be created with a broad junction area for efficient photon harvesting. The weak electrostatic screening effect of graphene allows the integration of single or dual gates under and/or above the vertical heterostructure to tune the band slope and photocurrent generation. We demonstrate that the amplitude and polarity of the photocurrent in the gated vertical heterostructures can be readily modulated by the electric field of an external gate to achieve a maximum external quantum efficiency of 55% and internal quantum efficiency up to 85%. Our study establishes a method to control photocarrier generation, separation and transport processes using an external electric field.

Inverse bremsstrahlung heating rate for dense plasmas in laser fields

We report a theoretical analysis of inverse bremsstrahlung heating rate in the eikonal approximation. The present analysis is performed for a dense plasma using the screened electron-ion interaction potential for the ion charge state Zi = 1 and for both the weak and strong plasma screening cases. We have also compared the eikonal results with the first Born approximation (FBA) [M. Moll et al., New J. Phys. 14, 065010 (2012)] calculation. We find that the magnitudes of inverse bremsstrahlung heating rate within the eikonal approximation (EA) are larger than the FBA values in the weak screening case (κ = 0.03 a.u.) in a wide range of field strength for three different initial electron momenta (2, 3, and 4 a.u.). But for strong screening case (κ = 0.3 a.u.), the heating rates predicted by the two approximations do not differ much after reaching their maximum values. Furthermore, the individual contribution of photoemission and photoabsorption processes to heating rate is analysed for both the weak and strong screening cases. We find that the single photoemission and photoabsorption rates are the same throughout the field strength while the multiphoton absorption process dominates over the multiphoton emission process beyond the field strength ≈ 4×108 V/cm. The present study of the dependence of heating rate on the screening parameter ranging from 0.01 to 20 shows that whereas the heating rate predicted by the EA is greater than the FBA up to the screening parameter κ = 0.3 a.u., the two approximation methods yield results which are nearly identical beyond the above value.

Salt-Responsive Liquid Crystal Hydrogels: Neurofilament Network Structure and Mechanical Modulation

Neurofilaments (NFs), the class of intermediate filaments found in neurons, are one of three structural protein groups that collectively form the cytoskeletal network. The active structure that NFs assume in the cell, and that permits them to fulfill their adaptable physiological role, is a space-filling expanded hydrogel held together by tunable interfilament associations. These interfilament associations are predominantly electrostatic and established by the polyampholytic nature of the sidearms, the unstructured C-termini of the three constituent subunits: NF-Low (NF-L), NF-Medium (NF-M), and NF-High (NF-H) [1]. Using synchrotron x-ray scattering and polarized optical microscopy, we examined the strength of these regulatory electrostatic associations and their effect on filament re-orientation organization. As a function of decreasing ionic strength, for binary and ternary NF systems, three distinct salt-induced hydrogel phases are found: nematic liquid-crystal (NG), isotropic (IG), and anisotropic NF blue phase (BG). At low ionic strength (< 5mM) with weak screening, sidearm overlap is maximal producing BG hydrogels with very high elastic moduli comparable to cross-linked gels with a phenomenal ability to retain shape and water over long-times. Upon melting into the IG phase, with reduced sidearm overlap, filaments re-organize crossing at large angles to minimize electrostatic repulsion. At still higher salt concentrations, NF gels exhibit an abrupt transition from the IG with mesh size ≈ 1000A to an Onsager-type oriented nematic liquid-crystalline gel with interfilament spacing d ≈ 500A (NF-LM) and 700A (NF-LH)[1]. Remarkable to the NF system is the fully reversible interchange of NF hydrogel properties, most notably the elastic modulus transition from chemical-gel mimicking BG to physical-gel-like (IG and NG), prompted merely by variation in ionic strength. Supported by DOE-BES DOE-DE-FG02-06ER46314 and NSF DMR-1101900.[1] R. Beck, J. Deek, J.B.Jones, C.R. Safinya. Nature Materials. 9, 40 (2010)

Non-equilibrium ionic assemblies of oppositely charged nanoparticles

Structure and evolution kinetics of non-equilibrium clusters formed in a solution of oppositely charged nanoparticles are studied using a recently developed kinetic Monte Carlo simulation scheme (Jha et al., Soft Matter, 2012, 8, 227–234). A diverse range of dynamic cluster configurations are obtained by varying the interaction strength between nanoparticles, screening length, and packing density of nanoparticles. Structural details of the resulting clusters are obtained using the correlations of local bond orientational order parameters. At low-salt concentrations (weak screening), clusters with structures ranging from NaCl-type cubic aggregates to fibril-like chains are observed, while at high-salt concentrations (strong screening), disordered compact clusters are observed. A chain-folding barrier model is proposed to explain the kinetically trapped fibril-like assemblies. In higher-density solutions, large ionic clusters or percolated gel structures are observed. Our work demonstrates the structural richness of non-equilibrium ionic assemblies of oppositely charged nanoparticles and elucidates the effect of ion correlations on the determination of the structure of assemblies of oppositely charged nanoparticles. These “nanoionic composites” hold great promise in a variety of emerging applications such as templated polymerization of charged molecules and assembly of charged nano-objects.

Specific features of electric charge screening in few-layer graphene films

We present a nonlinear Thomas–Fermi theory which describes the electric charge screening in a system including two charged substrate layers separated by a few-layered graphene film. We show that by increasing the charge at the interfaces, the system can be turned from the weak screening regime where the whole film responds to the external charge to the strong screening regime where the external charge is screened by a surface charge distribution confined to the bounding graphene layers. The transition from weak to strong screening is shown to turn on relatively quickly, and it occurs when the applied external charge/external field reaches a certain crossover magnitude. The possibilities for experimental observation of the predicted crossover are discussed.

Electron-phonon mediated heat flow in disordered graphene

We calculate the heat flux and electron-phonon thermal conductance in a disordered graphene sheet, going beyond a Fermi's golden rule approach to fully account for the modification of the electron-phonon interaction by disorder. Using the Keldysh technique combined with standard impurity averaging methods in the regime ${k}_{F}l\ensuremath{\gg}1$ (where ${k}_{F}$ is the Fermi wave vector and $l$ is the mean free path), we consider both scalar potential (i.e., deformation potential) and vector-potential couplings between electrons and phonons. We also consider the effects of electronic screening at the Thomas-Fermi level. We find that the temperature dependence of the heat flux and thermal conductance is sensitive to the presence of disorder and screening, and reflects the underlying chiral nature of electrons in graphene and the corresponding modification of their diffusive behavior. In the case of weak screening, disorder enhances the low-temperature heat flux over the clean system (changing the associated power law from ${T}^{4}$ to ${T}^{3}$), and the deformation potential dominates. For strong screening, both the deformation potential and vector-potential couplings make comparable contributions, and the low-temperature heat flux obeys a ${T}^{5}$ power law.

Electrostatic Interactions and Aqueous Two-Phase Separation Modes of Aqueous Mixed Oppositely Charged Surfactants System

Electrostatic interactions play an important role in setting the aqueous two-phase separation behaviors of mixtures of oppositely charged surfactants. The aqueous mixture of cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfonate (AS) is actually a five-component system, comprised of CTAB, AS, complex salt (cetyltrimethylammonium dodecylsulfonate, abbreviated as CTA(+)AS(-)), NaBr, and water. In the three-dimensional pyramid phase diagram, the aqueous two-phase region with excess AS or with excess CTAB extends successively from the region very near to the NaBr-H2O line through the CTAB-AS-H2O conventional mixing plane to the CTA(+)AS(-)-AS-H2O side plane or to the CTA(+)AS(-)-CTAB-H2O side plane, respectively. Large or small molar ratios between the counterions and their corresponding surfactant ions for oppositely charged surfactants located in the NaBr side or the CTA(+)AS(-) side of the pyramid imply strong or weak electrostatic screening. Electrostatic screening of counterions alters the electrostatic attractions between the oppositely charged head groups or the electrostatic repulsions between the like-charged head groups in excess, and the electrostatic free energy of aggregation thus affects the aqueous two-phase separation modes. Composition analysis, rheological property investigation, and TEM images suggest that there are two kinds of aqueous two-phase systems (ATPSs). On the basis of these experimental results and Kaler's cell model, two kinds of phase separation modes were proposed. Experimental results also indicate that all of the top phases are surfactant-rich, and all of the bottom phases are surfactant-poor; the density difference between the top phase and the bottom phase in one ATPS is very small; the interfacial tension (σ) of the ATPS is ultralow.

Condensation of electron-hole pairs in a two-layer graphene system: Correlation effects

Cooper pairing of spatially separated electrons and holes in graphene bilayer is studied beyond the mean-field approximation. Suppression of the screening at large distances, caused by appearance of the gap, is considered self-consistently. A mutual positive feedback between appearance of the gap and enlargement of the interaction leads to a sharp transition to correlated state with greatly increased gap above some critical value of the coupling strength. At coupling strength below the critical, this correlation effect increases the gap approximately by a factor of two. The maximal coupling strength achievable in experiments is close to the critical value. This indicated importance of correlation effects in closely-spaced graphene bilayers at weak substrate dielectric screening. Another effect beyond mean-field approximation considered is an influence of vertex corrections on the pairing, which is shown to be very weak.

Tu1224 A Systematic Review and Meta-Analysis of Pancreatic Autoantibody's (Pab) Diagnostic Accuracy vs Standard Diagnosis in Patients With Inflammatory Bowel Disease

Introduction: Serological tests are becoming an important aspect of the diagnosis of inflammatory bowel diseases (IBD). Pancreatic autoantibody (PAB) has been described as an immunological marker of IBD. The purpose of the present study is to synthesize available data from the literature on the diagnostic accuracy of PAB against standard diagnosis of IBD (with clinical, radiological and histological criteria). Methods: We searched articles and abstracts indexed in Medline/PubMed, Scopus, EMBASE and Google Scholar using the following search terms: pancreatic autoantibody and inflammatory bowel disease. There was no language or time limit in our literature search. Papers included had to mention diagnostic accuracy measures (e.g. sensitivity, specificity) of PAB against standard diagnosis of IBD. The Standards for Reporting of Diagnostic Accuracy Initiative and the Review of Methodological Standards were used to assess study quality. Pooled sensitivities and specificities were calculated for PAB with use of the DerSimonian-Laird random effects model; an assessment of heterogeneity was performed with use of Cochran's Q statistic. A pooled diagnostic OR was calculated for each of the diagnostic tests evaluated. All statistical analyses were performed with use of MetaDisc software. Results: Seventeen studies were included in the meta-analysis. The pooled sensitivity was 27.6% (95% CI 26.1-29.1%; range 11-38.2%), specificity 97.3% (95% CI 96.6-97.9%; range 92-100%), and the diagnostic odds ratio was 13,33 (95% CI 7,93-22,41; range 4.10-137.54), revealing moderately good discriminating characteristics, supported also by the area under the curve (AUC) (Symmetric Curve AUC: 0.4050) in the Summary Receiver Operating Characteristic (SROC) curve (Figure 1). The results were heterogeneous, as all I-square values were high and statistically significantly different from zero (p<0.001). Conclusion: This is the first meta-analysis of PAB's diagnostic accuracy for the detection of IBD. Due to its low sensitivity, PAB appears to be a weak screening test for IBD; however, its high specificity indicates moderate power to diagnose IBD but it would be particularly valuable in difficult IBD cases, in conjunction with standard diagnostic criteria.