Evaluation of some basic positron-related characteristics of SiC

Ultra-high Photovoltage (2.45 V) Forming in Graphene Heterojunction via Quasi-Fermi Level Splitting Enhanced Effect

Owing to the fast response speed and low energy consumption, photovoltaic vacuum-ultraviolet (VUV) photodetectors show prominent advantages in the field of space science, high-energy physics, and electronics industry. For photovoltaic devices, it is imperative to boost their open-circuit voltage, which is the most direct indicator to measure the photoelectric conversion capability. In this report, a quasi-Fermi level splitting enhanced effect under illumination, benefiting from the variable Fermi level of graphene, is proposed to significantly increase the potential difference up to 2.45 V between the two ends of p-Gr/i-AlN/n-SiC heterojunction photovoltaic device. In addition, the highest external quantum efficiency of 56.1% (under the VUV irradiation of 172 nm) at 0 V bias and the ultra-fast photoresponse of 45 ns further demonstrate the superiority of high-open-circuit-voltage devices. The proposed device design strategy and the adopted effect provide a referential way for the construction of various photovoltaic devices.

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An open-circuit voltage up to 2.45 V is achieved by the graphene heterojunction device

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An efficient band assembly is designed to induce the larger quasi-Fermi level splitting

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An EQE as high as 56.1% (172 nm) and a rise time as short as 45 ns are achieved

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The adopted effect provides a referential way for various photovoltaic devices

First-principles calculations of momentum distributions of annihilating electron-positron pairs in defects in UO2

We performed first-principles calculations of the momentum distributions of annihilating electron-positron pairs in vacancies in uranium dioxide. Full atomic relaxation effects (due to both electronic and positronic forces) were taken into account and self-consistent two-component density functional theory schemes were used. We present one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) along with line-shape parameters S and W. We studied the effect of the charge state of the defect on the Doppler spectra. The effect of krypton incorporation in the vacancy was also considered and it was shown that it should be possible to observe the fission gas incorporation in defects in UO₂ using positron annihilation spectroscopy. We suggest that the Doppler broadening measurements can be especially useful for studying impurities and dopants in UO₂ and of mixed actinide oxides.

Near-ultraviolet lateral photovoltaic effect in Fe3O4/3C-SiC Schottky junctions

In this paper, we report a sensitive lateral photovoltaic effect (LPE) in Fe₃O₄/3C-SiC Schottky junctions with a fast relaxation time at near-ultraviolet wavelengths. The rectifying behavior suggests that the large build-in electric field was formed in the Schottky junctions. This device has excellent position sensitivity as high as 67.8 mV mm^-1 illuminated by a 405 nm laser. The optical relaxation time of the LPE is about 30 μs. The fast relaxation and high positional sensitivity of the LPE make the Fe₃O₄/3C-SiC junction a promising candidate for a wide range of ultraviolet/near-ultraviolet optoelectronic applications.

Defect-induced magnetism in neutron irradiated 6H-SiC single crystals

Defect-induced magnetism is firstly observed in neutron irradiated SiC single crystals. We demonstrated that the intentionally created defects dominated by divacancies (V(Si)V(C)) are responsible for the observed magnetism. First-principles calculations revealed that defect states favor the formation of local moments and the extended tails of defect wave functions make long-range spin couplings possible. Our results confirm the existence of defect-induced magnetism, implying the possibility of tuning the magnetism of wide band-gap semiconductors by defect engineering.

Identification of lattice vacancies on the two sublattices of SiC

The identification of atomic defects in solids is of pivotal interest for understanding atomistic processes and solid state properties. Here we report on the exemplary identification of vacancies on each of the two sublattices of SiC by making use of (i) electron irradiation, (ii) measurements of the positron lifetimes, (iii) coincident Doppler broadening studies of the positron-electron annihilation radiation, and (iv) a comparison of the experimental data with theoretical studies. After 0.3 MeV electron irradiation, carbon vacancies V(C) are identified, where, after 0.5 MeV electron irradiation, predomi-nantly silicon vacancies V(Si) are observed. After 2.5 MeV irradiation, divacancies V(Si)-V(Si) are detected. The present results are expected to be of general importance for reliable identification of defects and atomic processes in complex solids.

A theory of thermal fluctuations in DNA miniplasmids

A recent analysis of the normal modes of vibration of a ring formed by bringing together and sealing, with or without the addition of twist, the ends of rods that are straight when stress free is taken as the basis for a theory of the statistical thermodynamics of a canonical ensemble of DNA minicircles with specified linking number difference deltaLk and number N of base pairs. It is assumed that N corresponds to a circumference in the range of one or two persistence lengths. For such an ensemble, the theory yields an expression for the average writhe (Wr), which can be employed to calculate the free energy, entropy, and enthalpy of supercoiling, deltaGsc, deltaSsc, and deltaHsc. The results obtained for the dependence of deltaGsc on deltaLk and N are in accord with experimental observations of equilibrium distributions of topoisomers of plasmids with N approximately 200 bp.

Configurational transitions in Fourier series-represented DNA supercoils

A new Fourier series representation of supercoiled DNA is employed in Langevin dynamics simulations to study large-scale configurational motions of intermediate-length chains. The polymer is modeled as an ideal elastic rod subject to long-range van der Waals' interactions. The van der Waals' term prevents the self-contact of distant chain segments and also mimics attractive forces thought to stabilize the association of closely spaced charged rods. The finite Fourier series-derived polymer formulation is an alternative to the piecewise B-spline curves used in past work to describe the motion of smoothly deformed supercoiled DNA in terms of a limited number of independent variables. This study focuses on two large-scale configurational events: the interconversion between circular and figure-8 forms at a relatively low level of supercoiling, and the transformation between branched and interwound structures at a higher superhelical density.