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<jats:title>Abstract</jats:title> <jats:p>The structure evolution and origin of ultrahigh dielectric properties have been investigated in the low temperature range from 300 K to 5 K for [001]-oriented 0.68Pb(Mg<jats:sub>1/3</jats:sub>Nb<jats:sub>2/3</jats:sub>)O<jats:sub>3</jats:sub>–0.33PbTiO<jats:sub>3</jats:sub> (PMN–33PT) crystal. The experimental results reveal that a short-range ordered monoclinic M<jats:sub>A</jats:sub> is the dominant phase at ambient temperature. As the temperature drops below 270 K, the M<jats:sub>A</jats:sub> transforms into monoclinic M<jats:sub>C</jats:sub>, and the M<jats:sub>C</jats:sub> remains stable until 5 K. Although no phase transition occurs from 5 K to 245 K, polar nanoregions (PNRs) display visible changes. The instability of PNRs is suggested as responsible for the low temperature relaxation. The ultrahigh dielectric constant at room temperature is associated with the instability of local structure and phase transition. Our research provides an insight into the design of high-performance ferroelectric materials.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Complex permittivity and electrical impedance have been measured along the <jats:italic>c</jats:italic>-axis in single crystals BaFe<jats:sub>2</jats:sub>As<jats:sub>2</jats:sub>, which are the conductors known as the parent compound of 122-type iron superconductor. The resultant relative errors defined in the study indicate the existence of the transformation between complex permittivity and electrical impedance in the conductors, and these two physics quantities possibly reveal different aspects of the consistent superconductivity-relevant physics picture.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Speckle patterns are a fundamental tool in a variety of physical and optical applications. Here, we investigate a method of precisely tuning the intensity statistics of random speckle patterns into a desirable pattern that possesses the same spatial correlation length and similar statistics distribution. This tuning mechanism relies on the derivation of the transform function and transmission matrix, which achieves different contrasts while maintaining the same average value or energy level. The statistics properties of the generated speckle patterns are further investigated by analyzing the standard deviation under different fitting parameters. Precisely tuning the intensity statistics of random speckle patterns could be useful for both fundamental research and practical applications, such as microscopy, imaging, and optical manipulation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Temperature and excitation dependent photoluminescence (PL) of InGaN epilayer grown on <jats:italic>c</jats:italic>-plane GaN/sapphire template by molecular beam epitaxy (MBE) has been systematically investigated. The emission spectra of the sample consisted of strong multiple peaks associated with one stimulated emission (SE) located at 430 nm and two spontaneous emissions (SPE) centered at about 450 nm and 480 nm, indicating the co-existence of shallow and deep localized states. The peak energy of SE exhibiting weak s-shaped variation with increasing temperature revealed the localization effect of excitons. Moreover, an abnormal increase of the SPE intensity with increasing temperature was also observed, which indicated that the carrier transfer between the shallow and deeper localized states exists. Temperature dependent time-resolved PL (TRPL) demonstrated the carrier transfer processes among the localized states. In addition, a slow thermalization of hot carriers was observed in InGaN film by using TRPL and transient differential reflectivity, which is attributed to the phonon bottleneck effect induced by indium aggregation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigated the optical properties of hybrid exciton–plasmon coupling ensembles composed of ZnSe/ZnS quantum dots and Ag nanoparticles in aqueous solution. We modulated their average interval by changing the ratio of quantum dots and Ag nanoparticles. The transition from dramatic PL enhancement to PL quenching state was experimentally observed, according to the continuous decrease of the PL lifetime. The PL enhancement rate exceeded 10, with the Purcell factor of 3.5. Meanwhile, the proportion of fast decay increased from 0.3 to 0.6, corresponding to the proportion of slow decay decreased from 0.7 to 0.4. Our experiment is important for the hybrid exciton–plasmon coupling system to be practicable in optoelectronic application.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We put forward a two-step route to synthesize vanadium diselenide (VSe<jats:sub>2</jats:sub>), a typical transition metal dichalcogenide (TMD). To obtain the VSe<jats:sub>2</jats:sub> film, we first prepare a vanadium film by electron beam evaporation and we then perform selenization in a vacuum chamber. This method has the advantages of low temperature, is less time-consuming, has a large area, and has a stable performance. At 400 °C selenization temperature, we successfully prepare VSe<jats:sub>2</jats:sub> films on both glass and Mo substrates. The prepared VSe<jats:sub>2</jats:sub> has the characteristic of preferential growth along the <jats:italic>c</jats:italic>-axis, with low transmittance. It is found that the contact between Al and VSe<jats:sub>2</jats:sub>/Mo is ohmic contact. Compared to Mo substrate, lower square resistance and higher carrier concentration of the VSe<jats:sub>2</jats:sub>/Mo sample reveal that the VSe<jats:sub>2</jats:sub> film may be a potential material for thin film solar cells or other semiconductor devices. The new synthetic strategy that is developed here paves a sustainable way to the application of VSe<jats:sub>2</jats:sub> in photovoltaic devices.</jats:p>

<jats:p>Transmission line theory uses the complex nature of permeability and permittivity of a conventional magnetic absorber to evaluate its absorption properties and mechanism. However, because there is no method to obtain the electromagnetic parameters of a metamaterial-absorber integrated layer (composed of a medium layer and a periodic metal array), this theory is seldom used to study the absorption properties of the metamaterial absorber. We propose a symmetry model to achieve an equivalent complex permittivity and permeability model for the integrated layer, which can be combined with the transmission line theory to calculate metamaterial absorption properties. The calculation results derived from both the transmission line theory and the high-frequency structure simulator are in good agreement. This method will be beneficial in practical investigations of the absorption mechanism of a metamaterial absorber.</jats:p>

<jats:p>The responsivity and the noise of a detector determine the sensitivity. Thermal energy usually affects both the responsivity and the noise spectral density. In this work, the noise characteristics and responsivity of an antenna-coupled AlGaN/GaN high-electron-mobility-transistor (HEMT) terahertz detector are evaluated at temperatures elevated from 300 K to 473 K. Noise spectrum measurement and a simultaneous measurement of the source–drain conductance and the terahertz photocurrent allow for detailed analysis of the electrical characteristics, the photoresponse, and the noise behavior. The responsivity is reduced from 59 mA/W to 11 mA/W by increasing the detector temperature from 300 K to 473 K. However, the noise spectral density maintains rather constantly around 1–2 pA/Hz<jats:sup>1/2</jats:sup> at temperatures below 448 K, above which the noise spectrum abruptly shifts from Johnson-noise type into flicker-noise type and the noise density is increased up to one order of magnitude. The noise-equivalent power (NEP) is increased from 22 pW/Hz<jats:sup>1/2</jats:sup> at 300 K to 60 pW/Hz<jats:sup>1/2</jats:sup> at 448 K mainly due to the reduction in mobility. Above 448 K, the NEP is increased up to 1000 pW/Hz<jats:sup>1/2</jats:sup> due to the strongly enhanced noise. The sensitivity can be recovered by cooling the detector back to room temperature.</jats:p>

<jats:p>The conventional stationary Al content AlGaN electron blocking layer (EBL) in ultraviolet light-emitting diode (UV LED) is optimized by employing a linearly graded AlGaN inserting layer which is 2.0 nm Al<jats:sub>0.3</jats:sub>Ga<jats:sub>0.7</jats:sub>N/5.0 nm Al<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Ga<jats:sub>1−<jats:italic>x</jats:italic> </jats:sub>N/8.0 nm Al<jats:sub>0.3</jats:sub>Ga<jats:sub>0.7</jats:sub>N with decreasing value of <jats:italic>x</jats:italic>. The results indicate that the internal quantum efficiency is significantly improved and the efficiency droop is mitigated by using the proposed structure. These improvements are attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes, which result in an increased hole injection efficiency and a decreased electron leakage into the p-type region. In addition, the linearly graded AlGaN inserting layer can generate more holes in EBL due to the polarization-induced hole doping and a tunneling effect probably occurs to enhance the hole transportation to the active regions, which will be beneficial to the radiative recombination.</jats:p>

<jats:p>A power metal-oxide-semiconductor field-effect transistor (MOSFET) with dielectric trench is investigated to enhance the reversed blocking capability. The dielectric trench with a low permittivity to reduce the electric field at reversed blocking state has been studied. To analyze the electric field, the drift region is segmented into four regions, where the conformal mapping method based on Schwarz–Christoffel transformation has been applied. According to the analysis, the improvement in the electric field for using the low permittivity trench is mainly due to the two electric field peaks generated in the drift region around this dielectric trench. The analytical results of the electric field and the potential models are in good agreement with the simulation results.</jats:p>