Fröhlich interaction dominated by a single phonon mode in CsPbBr3

The excellent optoelectronic performance of lead halide perovskites has generated great interest in their fundamental properties. The polar nature of the perovskite lattice means that electron-lattice coupling is governed by the Fröhlich interaction. Still, considerable ambiguity exists regarding the phonon modes that participate in this crucial mechanism. Here, we use multiphonon Raman scattering and THz time-domain spectroscopy to investigate Fröhlich coupling in CsPbBr3. We identify a longitudinal optical phonon mode that dominates the interaction, and surmise that this mode effectively defines exciton-phonon scattering in CsPbBr3, and possibly similar materials. It is additionally revealed that the observed strength of the Fröhlich interaction is significantly higher than the expected intrinsic value for CsPbBr3, and is likely enhanced by carrier localization in the colloidal perovskite nanocrystals. Our experiments also unearthed a dipole-related dielectric relaxation mechanism which may impact transport properties.

Green Synthesis ZnO/TiO2 for High Recyclability Rapid Sunlight Photodegradation Textile Dyes Applications

Composite ZnO/TiO2 have been successfully synthesized by green synthesis method with various calcination temperature 500oC, 600oC, 700oC, and 800oC (TiO2 concentrations: 2.5 g and 5 g) for photocatalyst application. In this study, Calopogonium mucunoides leaf extract was used as reducing and stabilizing agent. The synthesized composites were characterized by using Fourier Transform Infra-Red (FTIR), X-Ray Diffraction (XRD), and UV-Visible spectroscopy. The XRD spectra shows the hexagonal phase with wurtzite structure of ZnO and anatase for TiO2. The best degradation performance is 98.26% (only 10 min) for ZnO/TiO2 (5 g) with calcination temperature is 800oC. This is due to the highest distance between two optical phonon mode Δ(LO-TO) and lowest attenuating and propagating constant. The composite ZnO/TiO2 shows high potentials photodegradation of organic dyes with the high stable recyclability up to 5 cycles (> 95%) only for every 15 minutes. High potentials for applicability with the concept environmentally friendly principles and stability for circular chemistry, and efficiency of use the energy and chemicals.

Effects of Density and Diameter on Surface Optical Phonon Modes in GaAs Nanowire Bundles

We report the systematic investigation of the surface optical phonon modes in Au-catalyzed GaAs nanowires grown on an Au pre-patterned GaAs(111)B substrate using μ-Raman spectroscopy. We employed electron-beam dose rate as a control parameter during the substrate patterning step for adjusting the nanowire base diameter and coverage, which are independent from the nanowire growth conditions. We have experimentally studied the effect of the fill factor and average diameter on the surface optical phonon modes and explained the red-shift and broadening of the surface optical phonon frequencies by employing the dielectric continuum model. The surface optical phonon mode shift is exhibited to be sensitive to fill factor, rather than base diameter. The decrease in the average diameter from 280 nm to 180 nm results in the asymmetric broadening and red-shift of the surface optical phonon frequency (~1.83 cm−1) but the theoretical calculation from the isolated single nanowire-based dielectric continuum model cannot solely explain the behaviors of the surface optical phonon mode. In contrast, the change in the fill factor from 0.01 to 0.83 results in a shift of the surface optical phonon frequency (~6.5 cm−1) from the GaAs bulk value. The red-shift and asymmetric broadening of the surface optical phonons, in an agreement with the Maxwell-Garnett approximation, are consequences of dipolar interaction of randomly aligned neighboring nanowires and the polar nature of GaAs nanowire bundles. This work suggests the pre-patterning parameter dependent surface optical phonon characteristics of GaAs nanowire bundles which are of great importance in the nondestructive characterization of low-dimensional opto-electronic materials and devices.

Extremely low-frequency phonon material and its temperature- and photo-induced switching effects

Extremely low-frequency optical phonon mode in Rb-intercalated two-dimensional cyanide-bridged Co–W bimetal assembly and its temperature- and photo-induced switching effect.

Charge transport in a polar metal

The fate of electric dipoles inside a Fermi sea is an old issue, yet poorly explored. Sr$${}_{1-x}$$1−xCa$${}_{x}$$xTiO$${}_{3}$$3 hosts a robust but dilute ferroelectricity in a narrow ($$0.0018\ <\ x\ <\ 0.02$$0.0018<x<0.02) window of substitution. This insulator becomes metallic by removal of a tiny fraction of its oxygen atoms. Here, we present a detailed study of low-temperature charge transport in Sr$${}_{1-x}$$1−xCa$${}_{x}$$xTiO$${}_{3-\delta }$$3−δ, documenting the evolution of resistivity with increasing carrier concentration ($$n$$n). Below a threshold carrier concentration, $${n}^{* }(x)$$n*(x), the polar structural-phase transition has a clear signature in resistivity and Ca substitution significantly reduces the 2 K mobility at a given carrier density. For three different Ca concentrations, we find that the phase transition fades away when one mobile electron is introduced for about $$7.9\pm 0.6$$7.9±0.6 dipoles. This threshold corresponds to the expected peak in anti-ferroelectric coupling mediated by a diplolar counterpart of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. Our results imply that the transition is driven by dipole–dipole interaction, even in presence of a dilute Fermi sea. Charge transport for $$n\ <\ {n}^{* }(x)$$n<n*(x) shows a non-monotonic temperature dependence, most probably caused by scattering off the transverse optical phonon mode. A quantitative explanation of charge transport in this polar metal remains a challenge to theory. For $$n\ge {n}^{* }(x)$$n≥n*(x), resistivity follows a T-square behavior together with slight upturns (in both Ca-free and Ca-substituted samples). The latter are reminiscent of Kondo effect and most probably due to oxygen vacancies.

Role of RF power on physical properties of RF magnetron sputtered GaN/p-Si(1 0 0) thin film

GaN thin films were deposited on p-Si(1 0 0) substrates using RF magnetron sputtering at various RF powers. Influence of RF power on morphological, optical and structural properties of GaN thin films were investigated and presented in detail. XRD results proved that the films were polycrystalline in structure with (1 0 0) and (1 1 0) planes of hexagonal GaN. It was found that increasing RF power led to deterioration of crystal structure of the films due to increased decomposition of GaN. Stress in GaN thin films was calculated from XRD measurements and the reasons for this stress were discussed. Furthermore, it was analyzed and interpreted whether the experimental measurement results support each other. E2 (high) optical phonon mode of hexagonal GaN was obtained from the analysis of Raman results. UV-Vis spectroscopy results showed that optical band gap of the films varied by changing RF power. The reasons of this variation were discussed. AFM study of the surfaces of the GaN thin films showed that some of them were grown in Stranski-Krastanov mode and others were grown in Frank-Van der Merwe mode. AFM measurements revealed almost homogeneous, nanostructured, low-roughness surface of the GaN thin films. SEM analysis evidenced agglomerations in some regions of surface of the films and their possible causes have been discussed. It has been inferred that morphological, optical, structural properties of GaN thin film can be changed by controlling RF power, making them a potential candidate for LED, solar cell, diode applications.

Uniaxial pressure control of competing orders in a high-temperature superconductor

Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa2Cu3O6.67under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along theaaxis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.