Weak Screening Research Articles

Securitization, bank behaviour and financial stability: A systematic review of the recent empirical literature

We systematically review the recent empirical literature to investigate whether and how securitization influences bank behaviour and its implication on financial stability. We find that, in the years preceding the 2007–2009 financial crisis, banks with higher credit and market risk were more likely to securitize assets. Banks became riskier and increased systemic risk as they took advantage of securitization in order to obtain capital relief. There is robust evidence indicating that mortgage securitization led to a deterioration in bank lending standards via weaker screening, lower denial rates, and misreporting of credit quality. For corporate loan securitization, the empirical findings on lax bank lending are inconclusive. However, it is evident that securitization resulted in poorer ex-post bank monitoring of corporate borrowers. Even though Europe is the second largest securitization market, there is a dearth of evidence on the impact of securitization on European banks' lending behaviour. Research is also very limited on the post-crisis regulatory incentive aligning mechanisms such as the risk retention requirements and the credit ratings reform. Finally, evidence on securitization activity in emerging markets, such as Latin America and China, where securitization volumes have been increasing recently, is relatively non-existent.

Enhancement factors of parity- and time-reversal-violating effects for monofluorides

Heavy polar diatomic molecules are currently one of the leading candidates for probing physics beyond the Standard Model via studies of time-reversal (T) and parity (P) violations. In this work, we analyze the effective electric field (Eeff) that is required for determining the electron electric dipole moment (eEDM), and the scalar-pseudoscalar (S-PS) interaction constant (Ws), in group 12 and group 2 systems. We use a relativistic coupled cluster method for our calculations, and find that group 12 monofluorides have large Eeff and Ws (for example, the values of Eeff and Ws of CnF, the heaviest group 12 fluoride, are 662 GV/cm and 3360 kHz, respectively). The reason for this is the contraction of the valence s and p orbitals due to the weak screening effect of the outermost core's d electron. The calculations of Eeff and Ws show that their ratio, (Ws/Eeff), increases with Z. Based on these results, as well as experimental suitability, we propose SrF and CdF as new candidate molecules for experiment.

Excitation-induced transition to indirect band gaps in atomically thin transition-metal dichalcogenide semiconductors

Monolayers of transition metal dichalcogenides (TMDCs) exhibit an exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. High densities of excited charge carriers in the various band-structure valleys cause strong many-body renormalizations that influence both the electronic properties and the optical response of the material. We investigate electronic and optical properties of the typical monolayer TMDCs MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ in the presence of excited carriers by solving semiconductor Bloch equations on the full Brillouin zone. With increasing carrier density, we systematically find a reduction of the exciton binding energies due to Coulomb screening and Pauli blocking. Together with excitation-induced band-gap shrinkage this leads to redshifts of excitonic resonances up to the dissociation of excitons. As a central result, we predict for all investigated monolayer TMDCs that the $\Sigma$-valley shifts stronger than the K-valley. Two of the materials undergo a transition from direct to indirect band gaps under carrier excitation similar to well-known strain-induced effects. Our findings have strong implications for the filling of conduction-band valleys with excited carriers and are relevant to transport and optical applications as well as the emergence of phonon-driven superconductivity.

Frequency-dependent substrate screening of excitons in atomically thin transition metal dichalcogenide semiconductors

Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. The van der Waals nature of interlayer coupling makes it easy to integrate TMDC layers into heterostructures with different dielectric or metallic substrates. This allows to tailor electronic and optical properties of these materials, as Coulomb interaction inside atomically thin layers is very susceptible to screening by the environment. Here we theoretically investigate dynamical screening effects in TMDCs due to bulk substrates doped with carriers over a large density range, thereby offering three-dimensional plasmons as tunable degree of freedom. We report a wide compensation of renormalization effects leading to a spectrally more stable exciton than predicted for static substrate screening, even if plasmons and excitons are in resonance. We also find a nontrivial dependence of the single-particle band gap on substrate doping density due to dynamical screening. Our investigation provides microscopic insight into the mechanisms that allow for manipulations of TMDC excitons by means of arbitrary plasmonic environments on the nanoscale.

Ion Correlation and Collective Dynamics in BMIM/BF4-Based Organic Electrolytes: From Dilute Solutions to the Ionic Liquid Limit.

Quantifying ion association and collective dynamical processes in organic electrolytes is essential for fundamental property interpretation and optimization for electrochemical applications. The extent of ion correlation depends on both the ion concentration and dielectric strength of the solvent; ions may be largely uncorrelated in sufficiently high-dielectric solvents at low concentration, but properties of concentrated electrolytes are dictated by correlated and collective ion processes. In this work, we utilize molecular dynamics simulations to characterize ion association and collective ion dynamics in organic electrolytes composed of binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM+][BF4-] and 1,2-dichloroethane, acetone, acetonitrile, and water solvents. We illustrate different physical regimes of characteristically distinct ion correlations for the systematic range of electrolyte concentrations and solvent dielectric strengths. Dilute electrolytes composed of low-dielectric solvents exhibit significant counterion correlation in the form of ion pairing and clustering driven by both weak screening and relatively low solvation energies. This regime is characterized by enhanced ion coordination numbers and near equality of cation and anion diffusion coefficients, despite the significantly different ion sizes. In contrast, ion correlation in highly concentrated electrolytes is dominated by the anti-correlated motion of both like-charge and opposite-charge ions, approaching neat ionic liquid behavior. We show that the cross-over of these correlation regimes is clearly illuminated by quantifying the fractional self and distinct contributions to the net ionic conductivity. For organic electrolytes composed of low-dielectric solvents, we conclude that significant ion correlation exists at all concentrations but the nature of the correlation changes markedly from the dilute electrolyte to the pure ionic liquid limit.

N-aryl-piperidine-4-carboxamides as a novel class of potent inhibitors of MALT1 proteolytic activity

Starting from a weak screening hit, potent and selective inhibitors of the MALT1 protease function were elaborated. Advanced compounds displayed high potency in biochemical and cellular assays. Compounds showed activity in a mechanistic Jurkat T cell activation assay as well as in the B-cell lymphoma line OCI-Ly3, which suggests potential use of MALT1 inhibitors in the treatment of autoimmune diseases as well as B-cell lymphomas with a dysregulated NF-κB pathway. Initially, rat pharmacokinetic properties of this compound series were dominated by very high clearance which could be linked to amide cleavage. Using a rat hepatocyte assay a good in vitro-in vivo correlation could be established which led to the identification of compounds with improved PK properties.

Self-medication practice and associated factors among pregnant women in Addis Ababa, Ethiopia

BackgroundSelf-medication which is the act of obtaining and using one or more medicines without medical supervision is a common practice among pregnant women. Unless proper caution is taken, it may result in maternal and fetal adverse outcomes. In Ethiopia, information on self-medication practice during pregnancy is scanty. Hence, this study aimed to assess self-medication practice and associated factors among pregnant women in government health centers in Addis Ababa.MethodsAn institution-based mixed study design using a sequential explanatory approach was employed among 617 pregnant women and nine key informants in Addis Ababa from May 8, 2017, to June 30, 2017. Multi-stage sampling technique was used to select study participants, and purposive sampling technique was used to select the key informants. The quantitative data were collected using a structured interview questionnaire and analyzed using Statistical Product and Service Solutions (SPSS) version 23.0 whereas semi-structured questionnaire was used for in-depth interviews. Binary logistic regression was used for quantitative data analysis, and thematic analysis method was used for qualitative data.ResultsThe prevalence of self-medication practice was 26.6%. Previous medication use (Adjusted odds ratio (AOR) = 4.20, 95% CI 2.70–6.53), gestational period (AOR = 0.63, 95% CI 0.41–0.98), education on self-medication (AOR = 0.36, 95% CI 0.21–0.62), previous pregnancy and delivery related problems (AOR = 1.71, 95% CI 1.06–2.76), and knowledge about risk of self-medication (AOR = 0.64, 95% CI 0.42–0.97) were significantly associated with self-medication practice. Lack of attention and priority of program designers, absence of strategies and guidelines; weak screening mechanisms, and regulatory enforcement were cited by the key informants as contributing factors for self-medication practices.ConclusionsConsiderable proportion of pregnant women practiced self-medication, including medicines categorized to have high risks. Gestational period, previous medication use, education on self-medication, previous pregnancy- and delivery-related problems, and knowledge were significantly associated with self-medication practice. In addition, there are correctable gaps in program designing, screening of pregnant women, regulatory enforcement, and strategies and guidelines. Hence, necessary measures at all levels must be taken to reduce risks of self-medication during pregnancy.

Colloquium : Excitons in atomically thin transition metal dichalcogenides

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here we review recent progress in our understanding of the excitonic properties in monolayer TMDs and lay out future challenges. We focus on the consequences of the strong direct and exchange Coulomb interaction, discuss exciton-light interaction and effects of other carriers and excitons on electron-hole pairs in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Full superconducting dome of strong Ising protection in gated monolayer WS2

Many recent studies show that superconductivity not only exists in atomically thin monolayers but can exhibit enhanced properties such as a higher transition temperature and a stronger critical field. Nevertheless, besides being unstable in air, the weak tunability in these intrinsically metallic monolayers has limited the exploration of monolayer superconductivity, hindering their potential in electronic applications (e.g., superconductor-semiconductor hybrid devices). Here we show that using field effect gating, we can induce superconductivity in monolayer WS2 grown by chemical vapor deposition, a typical ambient-stable semiconducting transition metal dichalcogenide (TMD), and we are able to access a complete set of competing electronic phases over an unprecedented doping range from band insulator, superconductor, to a reentrant insulator at high doping. Throughout the superconducting dome, the Cooper pair spin is pinned by a strong internal spin-orbit interaction, making this material arguably the most resilient superconductor in the external magnetic field. The reentrant insulating state at positive high gating voltages is attributed to localization induced by the characteristically weak screening of the monolayer, providing insight into many dome-like superconducting phases observed in field-induced quasi-2D superconductors.

Measurement of the muon antineutrino double-differential cross section for quasielastic-like scattering on hydrocarbon atEν∼3.5 GeV

We present double-differential measurements of anti-neutrino quasi-elastic scattering in the MINERvA detector. This study improves on a previous single differential measurement by using updated reconstruction algorithms and interaction models, and provides a complete description of observed muon kinematics in the form of a double-differential cross section with respect to muon transverse and longitudinal momentum. We include in our signal definition zero-meson final states arising from multi-nucleon interactions and from resonant pion production followed by pion absorption in the primary nucleus. We find that model agreement is considerably improved by a model tuned to MINERvA inclusive neutrino scattering data that incorporates nuclear effects such as weak nuclear screening and two-particle, two-hole enhancements.

Ab initio study of the Coulomb interaction in NbxCo clusters: Strong on-site versus weak nonlocal screening

By means of ab-initio calculations in conjunction with the random-phase approximation (RPA) within the full-potential linearized augmented plane wave method we study the screening of the Coulomb interaction in NbxCo (1<=x<=9) clusters. In addition, these results are compared with pure bcc Nb bulk. We find that for all clusters the onsite Coulomb interaction in RPA is strongly screened whereas the inter-site non-local Coulomb interaction is weakly screened and for some clusters it is unscreened or even anti-screened. This is in strong contrast with pure Nb bulk, where the inter-site Coulomb interaction is almost completely screened. Further, constrained RPA calculations reveal that the contribution of the Co 3d ! 3d channel to the total screening of the Co 3d electrons is small. Moreover, we find that both the onsite and inter-site Coulomb interaction parameters decrease in a reasonable approximation linearly with the cluster size and for clusters having more than 20 Nb atoms a transition from 0D to 3D screening is expected to take place.

Legal Status of the Strategic Environmental Assessment (SEA), Practice of Application and Challenges in Pakistan

This study addresses legal status of strategic environmental assessment (SEA), practice of application and challenges in Pakistan. The source of data was based upon a review of the literature, as well as structured and semi structured interviews with selected experts and consultants of the various backgrounds of Pakistan. This study shows that Pakistan transposed SEA in their national and provincial Environmental Protection Acts except for Punjab province. However Pakistan conducted a few SEA studies including SEA studies of the Azad Jammu and Kashmir Hydropower Plans, Gilgit city master plan, Poverty and Social Assessment of Trade and Transport Sector Reforms in Pakistan and the Cumulative and Induced Impact Assessment study of the Upper Indus River Basin (UIRB) of Pakistan. The main challenges for SEA in Pakistan include: devolution environment from federal to provinces, less technical staff for SEA studies, weak knowledge, and training about SEA in the Environmental Protection Agencies, weak political priority for environmental, weak environmental screening and scoping process, lack of financial resources for primary data collection and SEA studies. Despite the fact that the SEA is implemented in rudimentary stages in Pakistan, few SEA studies conducted in Pakistan only for donor-funded policies, plans, and programmes.

Electrostatic analogy for symmetron gravity

The symmetron model is a scalar-tensor theory of gravity with a screening mechanism that suppresses the effect of the symmetron field at high densities characteristic of the solar system and laboratory scales but allows it to act with gravitational strength at low density on the cosmological scale. We elucidate the screening mechanism by showing that in the quasi-static Newtonian limit there are precise analogies between symmetron gravity and electrostatics for both strong and weak screening. For strong screening we find that large dense bodies behave in a manner analogous to perfect conductors in electrostatics. Based on this analogy we find that the symmetron field exhibits a lightning rod effect wherein the field gradients are enhanced near the ends of pointed or elongated objects. An ellipsoid placed in a uniform symmetron gradient is shown to experience a torque. By symmetry there is no gravitational torque in this case. Hence this effect unmasks the symmetron and might serve as the basis for future laboratory experiments. The symmetron force between a point mass and a large dense body includes a component corresponding to the interaction of the point mass with its image in the larger body. None of these effects have counterparts in the Newtonian limit of Einstein gravity. We discuss the similarities between symmetron gravity and the chameleon model as well as the differences between the two.

Assessing the performance of self-consistent hybrid functional for band gap calculation in oxide semiconductors

In this paper we assess the predictive power of the self-consistent hybrid functional scPBE0 in calculating the band gap of oxide semiconductors. The computational procedure is based on the self-consistent evaluation of the mixing parameter α by means of an iterative calculation of the static dielectric constant using the perturbation expansion after discretization method and making use of the relation . Our materials dataset is formed by 30 compounds covering a wide range of band gaps and dielectric properties, and includes materials with a wide spectrum of applications such as thermoelectrics, photocatalysis, photovoltaics, transparent conducting oxides, and refractory materials. Our results show that the scPBE0 functional provides better band gaps than the non self-consistent hybrids PBE0 and HSE06, but scPBE0 does not show significant improvement on the description of the static dielectric constants. Overall, the scPBE0 data exhibit a mean absolute percentage error of 14% (band gaps) and 10% (). For materials with weak dielectric screening and large excitonic biding energies scPBE0, unlike PBE0 and HSE06, overestimates the band gaps, but the value of the gap becomes very close to the experimental value when excitonic effects are included (e.g. for SiO2). However, special caution must be given to the compounds with small band gaps due to the tendency of scPBE0 to overestimate the dielectric constant in the proximity of the metallic limit.

Electrostatic interaction of macroparticles in a plasma in the strong screening regime

We have studied the electrostatic interaction of spherical particles in an equilibrium plasma or an electrolyte in the moderate and strong screening regimes when the macroparticle size is comparable with or much larger than the Debye screening radius. We have developed an approximate theory of the electrostatic interaction of macroparticles in the case of constant potentials of their surfaces in the weak or moderate screening regimes. In this theory, the charges of macroparticles with a fixed spacing between them are determined using vacuum capacitive coefficients, which are corrected taking into account the plasma screening effects. The force of interaction with the resultant charges is calculated based on the solution of the problem of interaction in a homogeneous dielectric (vacuum) and is multiplied by the plasma factor. We have also obtained an approximate solution to the problem in the strong screening regime. Comparison with the exact solution has demonstrated high accuracy of the proposed methods of calculation.

True Bilayer Exciton Condensate of One-Dimensional Electrons.

We theoretically predict that a true bilayer exciton condensate, characterized by off-diagonal long-range order and global phase coherence, can be created in one-dimensional solid state electron systems. The mechanism by which this happens is to introduce a single particle hybridization of electron and hole populations, which locks the phase of the relevant mode and hence invalidates the Mermin-Wagner theorem. Electron-hole interactions then amplify this tendency towards off-diagonal long-range order, enhancing the condensate properties by more than an order of magnitude over the noninteracting limit. We show that the temperatures below which a substantial condensate fraction would form could reach hundreds of Kelvin, a benefit of the weak screening in one-dimensional systems.

Room temperature observation of biexcitons in exfoliated WS2 monolayers

Single layers of WS2 are direct gap semiconductors with high photoluminescence (PL) yield holding great promise for emerging applications in optoelectronics. The spatial confinement in a two-dimensional monolayer together with the weak dielectric screening leads to huge binding energies for the neutral excitons as well as other excitonic complexes, such as trions and biexcitons whose binding energies scale accordingly. Here, we report on the existence of biexcitons in mechanically exfoliated WS2 flakes from 78 K up to room temperature. Performing temperature and power dependent PL measurements, we identify the biexciton emission channel through the superlinear behavior of the integrated PL intensity as a function of the excitation power density. On the contrary, neutral and charged excitons show a linear to sublinear dependence in the whole temperature range. From the energy difference between the emission channels of the biexciton and neutral exciton, a biexciton binding energy of 65–70 meV is determined.

Electrical Double Layers near Charged Nanorods in Mixture Electrolytes

The electrical double layers (EDLs) in the mixtures of room-temperature ionic liquids and water play an important role in many applications but have only begun to receive widespread attention recently. Here, we report the molecular dynamics simulations of EDLs near rigid polyanion nanorods immersed in electrolytes containing ionic liquids [C2mim][TfO], water, and Na+ ions. When the water content in bulk electrolyte is high, the EDLs near the rods are similar to those in aqueous electrolytes except that TfO– ions accumulate notably near the cation layer adsorbed on the rods. When the water content in the bulk electrolyte becomes very low, even though water is greatly enriched in the interfacial region, the microenvironment in the region offers weak dielectric screening and the EDL exhibits features common to those in neat ionic liquids, e.g., charge overscreening. Na+ ions are readily adsorbed on the sulfonate groups of the polyanion rod in the range of water contents explored here but can experience an energy barrier when moving from the bulk electrolyte to the rod surface if the water content is very low. Introducing Na+ ions into the system displaces C2mim+ ions from the rod’s surface. However, the number of displaced C2mim+ ions is far less than the Na+ newly adsorbed on the rod, and thus, charge overscreening is enhanced. We highlight the important role of ion–ion correlations in determining the EDL characteristics and their response to the variation of mixture electrolytes’ water/Na+ ion contents revealed here and discuss their technical implications.

How to Design Rating Schemes of Risk Matrices: A Sequential Updating Approach.

Risk matrices have been widely used as a risk evaluation tool in many fields due to their simplicity and intuitive nature. Designing a rating scheme, i.e., determining the number of ratings used in a risk matrix and assigning different ratings to different cells, is an essential part of risk matrix construction. However, most of the related literature has focused on applying a risk matrix to various fields, instead of researching how to design risk matrices. Based on the analysis of several current rules, we propose a new approach, namely, the sequential updating approach (SUA), to design the rating scheme of a risk matrix in a reliable way. In this article, we propose three principles and a rating algorithm based on these principles. The three principles, namely, adjusted weak consistency, consistent internality, and continuous screening, characterize a good rating scheme. The resulting rating scheme has been proven to be unique. A global rating algorithm is then proposed to create the design that satisfies the three principles. We then explore the performance of the SUA. An illustrative application is first given to explain the feasibility of our approach. The sensitivity analysis shows that our method captures a resolution-reliability tradeoff for decisionmakers in choosing an appropriate rating scheme for a risk matrix. Finally, we compare the designs based on the SUA and Cox's axioms, highlighting the advantages of the SUA.

Femtosecond-timescale buildup of electron mobility in GaAs observed via ultrabroadband transient terahertz spectroscopy

The buildup of photoconductivity in an archetypal polar semiconductor GaAs was monitored on the timescale of carrier-carrier and carrier-lattice correlations, with sub-50 fs temporal resolution provided by ultra-broadband transient terahertz spectroscopy. Two relevant timescales are identified: the transition in photoconductivity from a regime of extremely weak screening to a screened Drude plasma was observed on the ∼280 fs timescale, followed by the gradual increase and saturation of electron mobility on a timescale of ∼160 fs. The Drude photoconductivity picture following carrier photoinjection was found to stabilize only on the timescale corresponding to several lattice and plasma oscillations.