Anisotropic phonon dynamics in Dirac semimetal PtTe2 thin films enabled by helicity-dependent ultrafast light excitation.

Coherent phonons have aroused considerable attention in condensed matter physics owing to their extraordinary capacity of reflecting and controlling the physical properties of matter. However, the investigation on the interaction between coherent phonons and other microscopic particles on the ultrafast timescale within topological systems continues to be an active and unresolved area. Here, we show the energy transfer of coherent optical phonons (COP) in Dirac semimetal PtTe2 thin films using ultrafast optical pump-probe spectroscopy. Specifically, the helicity-dependent light-driven anisotropic COP signals disclose their direct connection with the light-excited anisotropic spin-polarized electrons via an angular momentum transfer. Furthermore, we observe the notable decreases in the COP oscillation frequency and the decay rate with increasing temperatures due to the anharmonic phonon-phonon scattering and electron-phonon scattering in the COP dissipation process, respectively. Our work paves the way for uncovering the coherent phonons in Dirac semimetals for the potential applications in optoelectronics and opto-spintronics.

Light-induced giant enhancement of nonreciprocal transport at KTaO3-based interfaces.

Nonlinear transport is a unique functionality of noncentrosymmetric systems, which reflects profound physics, such as spin-orbit interaction, superconductivity and band geometry. However, it remains highly challenging to enhance the nonreciprocal transport for promising rectification devices. Here, we observe a light-induced giant enhancement of nonreciprocal transport at the superconducting and epitaxial CaZrO3/KTaO3 (111) interfaces. The nonreciprocal transport coefficient undergoes a giant increase with three orders of magnitude up to 105 A-1 T-1. Furthermore, a strong Rashba spin-orbit coupling effective field of 14.7 T is achieved with abundant high-mobility photocarriers under ultraviolet illumination, which accounts for the giant enhancement of nonreciprocal transport coefficient. Our first-principles calculations further disclose the stronger Rashba spin-orbit coupling strength and the longer relaxation time in the photocarrier excitation process, bridging the light-property quantitative relationship. Our work provides an alternative pathway to boost nonreciprocal transport in noncentrosymmetric systems and facilitates the promising applications in opto-rectification devices and spin-orbitronic devices.

Defect-induced helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films.

Nonlinear transport enabled by symmetry breaking in quantum materials has aroused considerable interest in condensed matter physics and interdisciplinary electronics. However, achieving a nonlinear optical response in centrosymmetric Dirac semimetals via defect engineering has remained a challenge. Here, we observe the helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films via the circular photogalvanic effect under normal incidence. This is activated by a controllable out-of-plane Te-vacancy defect gradient, which we unambiguously evidence with electron ptychography. The defect gradient lowers the symmetry, which not only induces the band spin splitting but also generates the giant Berry curvature dipole responsible for the circular photogalvanic effect. We demonstrate that the THz emission can be manipulated by the Te-vacancy defect concentration. Furthermore, the temperature evolution of the THz emission features a minimum in the THz amplitude due to carrier compensation. Our work provides a universal strategy for symmetry breaking in centrosymmetric Dirac materials for efficient nonlinear transport.

High-Throughput Electrosynthesis of Gradient Polypyrrole Film Using a Single-Electrode Electrochemical System.

As an effective approach for materials synthesis, bipolar electrochemistry has been earning a renewed interest nowadays thanks to its unique features compared to conventional electrochemistry. Indeed, the wireless mode of electrode reactions and the generation of a gradient potential distribution above the bipolar electrode are among the most appealing qualities of bipolar electrochemistry. In particular, the gradient potential distribution is a highly attractive characteristic for the fabrication of surfaces with gradients in their chemical properties or molecular functionalities. Herein, we report the high-throughput electrosynthesis of gradient polypyrrole films by means of a new electrochemical cell design named the single-electrode electrochemical system (SEES). SEESs are made by attaching an inert plastic board with holes onto an indium tin oxide electrode, constructing multiple microelectrochemical cells on the same electrode. This type of arrangement enables parallel electrochemical reactions to be carried out simultaneously and controlled in a contactless manner by a single electrode. Several experimental conditions for polypyrrole film growth were extensively investigated. Furthermore, the gradient property of the polymer films was evaluated by thickness determination, surface morphology analysis, and contact angle measurements. The use of SEES has been demonstrated as a convenient and cost-effective strategy for high-throughput electrosynthesis and electroanalytical applications and has opened up a new door for gradient film preparation via a rapid condition screening process.

Sizable spin-to-charge conversion in PLD-grown amorphous (Mo, W)Te2-xfilms.

We report on the spin-to-charge conversion (SCC) in Mo0.25W0.75Te2-x(MWT)/Y3Fe5O12(YIG) heterostructures at room temperature. The centimeter-scale amorphous MWT films are deposited on liquid-phase-epitaxial YIG by pulsed laser deposition technique. The significant SCC voltage is measured in the MWT layer with a sizable spin Hall angle of ∼0.021 by spin pumping experiments. The control experiments by inserting MgO or Ag layer between MWT and YIG show that the SCC is mainly attributed to the inverse spin Hall effect rather than the thermal or interfacial Rashba effect. Our work provides a novel spin-source material for energy-efficient topological spintronic devices.

Oxygen Vacancies Induced by Pd Doping in Ni-P2O5/MoO3 Hollow Polyhedral Heterostructures for Highly Efficient Diethylamine Gas Sensing.

Semiconductor metal-oxide materials have a high surface-to-volume ratio and many active sites, making them potentially useful for gas sensing. Dopants introduced into the lattice can improve the catalytic activity of oxides and promote the formation of oxygen vacancies, hence improving the sensing performance of the materials. However, the simple preparation of materials with high sensitivity, selectivity, and a low detection limit remains a challenge. Herein, we report on the synthesis of Ni-P2O5/MoO3 and Pd-doped Ni-P2O5/MoO3 hollow polyhedral heterostructures (HPHSs) and their application in diethylamine (DEA) sensing for the first time. The Pd-doped Ni-P2O5/MoO3 HPHS was synthesized by doping different proportions of palladium-containing precursors using hydrothermal and solid-state reaction techniques. The concentration of oxygen vacancies in the HPHS composite increased by increasing Pd doping from 2 to 6 weight percent (wt %) but later reduced, according to X-ray photoelectron spectroscopy (XPS) measurements. Pd6%Ni-P2O5/MoO3 has the highest sensitivity to DEA (Ra/Rg = 42.5) and is 5.0 times and 42.5 times more sensitive than the pure Ni-P2O5/MoO3 HPHS (Ra/Rg = 8.5) and commercial ammonium phosphomolybdate (Ra/Rg = 1) at 175 °C toward 10 ppm DEA. Moreover, the DEA sensor exhibits a low detection limit (Ra/Rg = 3.5@1 ppm) with a wide dynamic response (Ra/Rg = 145.5@50 ppm). The remarkable improvement in DEA sensitivity is attributed to the hollow polyhedral structure, heterostructures, and oxygen vacancies formed by Pd doping. This study confirms that developing Pd-doped Ni-P2O5/MoO3 HPHSs provides an innovative approach for DEA sensors.

Significant Reduction of the Dead Layers by the Strain Release in La0.7Sr0.3MnO3 Heterostructures.

Great efforts have been devoted to exploring the emergent phenomena occurring in heterostructures of correlated oxides. However, the presence of both magnetic and electrical dead layers in functional oxide films generally obstructs the device functionalization and miniaturization. Here, we demonstrate an effective strategy to significantly reduce the thickness of dead layers in a prototypical correlated oxide system, La0.7Sr0.3MnO3 (LSMO) grown on LaAlO3 (LAO) substrates, via strain engineering by inserting a Sr3Al2O6 buffer layer with a different thickness at heterointerfaces. In this way, the thicknesses of the magnetic and electrical dead layers of LSMO films on the LAO substrates notably decrease from 8 to 4 unit cells and from 13 to 9 unit cells, respectively. Our results provide a convenient method to minimize or even eliminate the dead layers of correlated oxides through the interfacial strain engineering, which has potential applications in nanoscale oxide spintronic devices.

Dithiothreitol-Lucigenin Chemiluminescent System for Ultrasensitive Dithiothreitol and Superoxide Dismutase Detection.

1,4-Dithiothreitol (DTT), a highly water-soluble and well-known reducing agent for preservation and regeneration of sulfhydryl groups in biomedical applications, has been developed as an efficient and stable coreactant of lucigenin for the first time. DTT efficiently reacts with lucigenin to generate intense chemiluminescence (CL), eliminating the need for external catalysts to facilitate the lucigenin CL. The DTT-lucigenin CL is approximately 15-fold more intense when compared with the lucigenin-H2O2 classical system. Superoxide dismutase (SOD) remarkably quenches the DTT-lucigenin CL. Based on this phenomenon, a newly developed CL approach for the determination of SOD was proposed with a linear range of 0.01-1.5 µg/mL and a limit of detection of 2.2 ng/mL. Various factors affecting the CL emission of the DTT-lucigenin probe were studied and optimized. Plausible mechanistic pathways for the CL coreaction of lucigenin with DTT were proposed and fully discussed. Our proposed method not only has the merit of being selective toward the target analytes but also eliminates the need for the complex synthesis of luminescent probes and facilitates the sensitive detection of SOD in human serum and cosmetics SOD raw material with satisfactory recoveries.

Lucigenin-pyrogallol chemiluminescence for the multiple detection of pyrogallol, cobalt ion, and tyrosinase.

Developing new, cheap, sensitive and selective chemiluminescence (CL) systems with multiple detection properties is still a big challenge for biological and environmental applications. Here, we report a new CL system having multiple detection applications (environmental and biological). The developed lucigenin-pyrogallol system gave an enhancement (190 times) over the conventional lucigenin-H2O2 CL system. Cobalt ion and tyrosinase can inhibit lucigenin-PG CL intensity. Based on these phenomena, we have developed new CL methods for the detection of pyrogallol (LOD = 0.94 µM), lucigenin (LOD = 0.42 µM), cobalt (LOD = 68 nM), and tyrosinase (LOD = 1.8 µg/mL). Furthermore, the developed system gave excellent recoveries in real samples within the range of 93.59-103.12% for the assay of pyrogallol, Co2+ and tyrosinase in coffee, lake water, and human plasma, respectively.

Development of luminol-fluorescamine-PVP chemiluminescence system and its application to sensitive tyrosinase determination.

Fluorescamine is a popular fluorescent probe. We report for the first time that luminol chemiluminescence (CL) can be enhanced by fluorescamine in the presence of PVP. The CL intensity of luminol-fluorescamine-PVP is about 26 times stronger than that of luminol. Both the removal of oxygen and the addition of superoxide dismutase (SOD) decrease CL intensity, thiourea and NaN3 have little effect on CL intensities, indicating that O2•- is critical for CL. Interestingly, o-quinone generated from phenol by tyrosinase obviously inhibited the CL intensity. Inspired by such quenching effect on the luminol-fluorescamine-PVP CL system, a sensitive CL sensing for the determination of tyrosinase activity was developed. The method can detect tyrosinase in the range of 0.07-1.5 µg mL-1 (0.19-4.02 U mL-1) with the detection limit of 0.035 µg mL-1 (0.094 U mL-1). Moreover, this method exhibits satisfied recoveries for the spiked human serum samples.

Silicotungstic acid as a highly efficient coreactant for luminol chemiluminescence for sensitive detection of uric acid.

Herein, we report luminol-silicotungstic acid (STA) chemiluminescence (CL) for the first time. The luminol-STA system resulted in remarkable CL enhancement (65 times) compared with the known classical luminol-H2O2 system because of the generation of the strong oxidizing agent tungsten trioxide from STA. Based on the quenching effect of uric acid, the new CL system is applied for the sensitive and selective assay of uric acid in its pure state (LOD 0.75 nM) and in real human urine with excellent recoveries in the range of 99.6-102.3%. Furthermore, this system permits the efficient detection of STA (LOD, 0.24 µM).

Direct observation of high spin polarization in Co2FeAl thin films.

We have studied the Co2FeAl thin films with different thicknesses epitaxially grown on GaAs (001) by molecular beam epitaxy. The magnetic properties and spin polarization of the films were investigated by in-situ magneto-optic Kerr effect (MOKE) measurement and spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES) at 300 K, respectively. High spin polarization of 58% (±7%) was observed for the film with thickness of 21 unit cells (uc), for the first time. However, when the thickness decreases to 2.5 uc, the spin polarization falls to 29% (±2%) only. This change is also accompanied by a magnetic transition at 4 uc characterized by the MOKE intensity. Above it, the film's magnetization reaches the bulk value of 1000 emu/cm3. Our findings set a lower limit on the thickness of Co2FeAl films, which possesses both high spin polarization and large magnetization.

Topological Phase Transition-Induced Triaxial Vector Magnetoresistance in (Bi1-xInx)2Se3 Nanodevices.

We report the study of a triaxial vector magnetoresistance (MR) in nonmagnetic (Bi1-xInx)2Se3 nanodevices at the composition of x = 0.08. We show a dumbbell-shaped in-plane negative MR up to room temperature as well as a large out-of-plane positive MR. MR at three directions is about in a -3%:-1%:225% ratio at 2 K. Through both the thickness and composition-dependent magnetotransport measurements, we show that the in-plane negative MR is due to the topological phase transition enhanced intersurface coupling near the topological critical point. Our devices suggest the great potential for room-temperature spintronic applications in, for example, vector magnetic sensors.