Interfacial-Strain-Induced Structural and Polarization Evolutions in Epitaxial Multiferroic BiFeO_3(001) Thin Films

Varying the film thickness is a precise route to tune the interfacial strain to manipulate the properties of the multiferroic materials.Here,to explore the effects of the interfacial strain on the properties of the multiferroic BiFeO_3films,we investigated thickness-dependent structural and polarization evolutions of the BiFeO_3 films.The epitaxial growth with an atomic stacking sequence of BiO/TiO_2 at the interface was confirmed by scanning transmission electron microscopy.Combining X-ray diffraction experiments and first-principles calculations,a thickness-dependent structural evolution was observed from a fully strained tetragonality to a partially relaxed one without any structural phase transition or rotated twins.The tetragonality(c/a) of the BiFeO_3 films increases as the film thickness decreases,while the polarization is in contrast with this trend,and the size effect including the depolarization field plays a crucial role in this contradiction in thinner films.These findings offer an alternative strategy to manipulate structural and polarization properties by tuning the interfacial strain in epitaxial multiferroic thin films.

Electrolyte-gated transistors for neuromorphic applications

Von Neumann computers are currently failing to follow Moore’s law and are limited by the von Neumann bottleneck.To enhance computing performance,neuromorphic computing systems that can simulate the function of the human brain are being developed.Artificial synapses are essential electronic devices for neuromorphic architectures,which have the ability to perform signal processing and storage between neighboring artificial neurons.In recent years,electrolyte-gated transistors(EGTs)have been seen as promising devices in imitating synaptic dynamic plasticity and neuromorphic applications.Among the various electronic devices,EGT-based artificial synapses offer the benefits of good stability,ultra-high linearity and repeated cyclic symmetry,and can be constructed from a variety of materials.They also spatially separate“read”and“write”operations.In this article,we provide a review of the recent progress and major trends in the field of electrolyte-gated transistors for neuromorphic applications.We introduce the operation mechanisms of electric-double-layer and the structure of EGT-based artificial synapses.Then,we review different types of channels and electrolyte materials for EGT-based artificial synapses.Finally,we review the potential applications in biological functions.

Photon-interactions with perovskite oxides

Photons with variable energy, high coherency, and switchable polarization provide an ideal tool-kits for exploring the cutting-edge scientific questions in the condensed matter physics and material sciences. Over decades, extensive researches in the sample fabrication and excitation have employed the photon as one of the important means to synthesize and explore the low-dimensional quantum materials. In this review, we firstly summarize the recent progresses of the state-of-the-art thin-film deposition methods using excimer pulsed laser, by which syntactic oxides with atomic-unit-cell-thick layers and extremely high crystalline quality can be programmatically fabricated. We demonstrate that the artificially engineered oxide quantum heterostructures exhibit the unexpected physical properties which are absent in their parent forms. Secondly, we highlight the recent work on probing the symmetry breaking at the surface/interface/interior and weak couplings among nanoscale ferroelectric domains using optical second harmonic generation. We clarify the current challenges in the insitu characterizations under the external fields and large-scale imaging using optical second harmonic generation. The improvements in the sample quality and the non-contact detection technique further promote the understanding of the mechanism of the novel properties emerged at the interface and inspire the potential applications, such as the ferroelectric resistive memory and ultrahigh energy storage capacitors.

High-performance terahertz modulators induced by substrate field in Te-based all-2D heterojunctions

High-performance active terahertz modulators as the indispensable core components are of great importance for the next generation communication technology.However,they currently suffer from the tradeoff between modulation depth and speed.Here,we introduce two-dimensional(2D)tellurium(Te)nanofilms with the unique structure as a new class of optically controlled terahertz modulators and demonstrate their integrated heterojunctions can successfully improve the device performances to the optimal and applicable levels among the existing all-2D broadband modulators.Further photoresponse measurements confirm the significant impact of the stacking order.We first clarify the direction of the substrate-induced electric field through first-principles calculations and uncover the unusual interaction mechanism in the photoexcited carrier dynamics associated with the charge transfer and interlayer exciton recombination.This advances the fundamental and applicative research of Te nanomaterials in high-performance terahertz optoelectronics.

Two hydroxypyruvate reductases encoded by OsHPR1 and OsHPR2 are involved in photorespiratory metabolism in rice

Mutations in the photorespiration pathway dis- play a lethal phenotype in atmospheric air, which can be fully recovered by elevated C02. An exception is that mutants of peroxisomal hydroxypyruvate reductase （HPR1） do not have this phenotype, indicating the presence of cytosolic bypass in the photorespiration pathway. In this study, we constructed overexpression of the OsHPR1 gene and RNA interference plants of OsHPR1 and OsHPR2 genes in rice （Oryza sativo L. cv. Zhonghua 11）. Results from reverse transcription-polymerase chain reaction （RT-PCR）, Western blot, and enzyme assays showed that HPR1 activity changed significantly in corresponding transgenic lines without any effect on HPR2 activity, which is the same for HPR2. However, metabolite analysis and the serine glyoxylate aminotransferase （SGAT） activity assay showed that the metabolite flux of photorespiration was disturbed in RNAi lines of both HPR genes. Furthermore, HPR1 and HPR2 proteins were located to the peroxisome and cytosol, respectively, by transient expression experiment. Double mutant hprl x hpr2 was generated by crossing individual mutant of hprl and hpr2. The phenotypes of all transgenic lines were determined in ambient air and C02-elevated air. The phenotype typical of photorespiration mutants was observed only where activity of both HPRI and HPR2 were downregulated in the same line. These findings demonstrate that two hydroxypyruvate reductases encoded by OsHPRI and OsHPR2 are involved in photorespiratory metabolism in rice.

Effects of spectral linewidth of ultrashort pulses on the spa-tiotemporal distribution of diffraction fields

The spatiotemporal characteristics of electromagnetic pulses with ultrabroad spectral bandwidth in the far field are analyzed by using classical scalar diffraction theory. The effects of the ratio of the frequency width to the central frequency on the diffraction spatial distribution are discussed. It is concluded that the diffraction spatial distribution of the pulsed radiation gets narrower than a monochromatic wave when the frequency width of the pulse is comparable to or larger than its central frequency.

Evaluation of simulated reservoirs by using the oblique-incidence reflectivity difference technique

Dear Editors,The evaluation of the oil-gas resources’potential depends on the characterization of reservoirs.The geometry of the pore-crack structures and the compositions of the pore walls have influence on hydrocarbon production about capillarity and permeability[1].Besides,it has been known that the adsorption phenomenon in micro-structure plays a significant role in oil-gas reservoirs.Some techniques have been applied to characterize conventional and unconventional reservoirs,

Effects of BaTiO3 and SrTiO3 as the buffer layers of epitaxial BiFeO3 thin films

BiFeO_3 (BFO) thin films with BaTiO_3 (BTO) or SrTiO_3 (STO) as buffer layer were epitaxially grown on SrRuO_3-covered SrTiO_3 substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.

Oxygen vacancies effects on phase diagram of epitaxial La1-xSrxMnO3 thin films

We investigated the effects of oxygen vacancies on the structural,magnetic,and transport properties of La_(1-x)Sr_xMnO_3(x=0.1,0.2,0.33,0.4,and 0.5)grown around a critical point(without/with oxygen vacancies)under low oxygen pressure(10 Pa)and high oxygen pressure(40 Pa).We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature,and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments.These results show that in epitaxial La_(1-x)Sr_xMnO_3 thin films,the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies.Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.

Optical spectroscopy study of charge density wave orderOptical spectroscopy study of charge density wave order in Sr_3Rh_4Sn_(13) and (Sr_(0.5)Ca_(0.5))3_Rh_4Sn_(13)

We perform optical spectroscopy measurement on single-crystal samples of Sr_3Rh_4Sn_(13) and(Sr_(0.5)Ca_(0.5))_3Rh_4Sn_(13).Formation of CDW energy gap was clearly observed for both single-crystal samples when they undergo the phase transitions.The existence of residual Drude components in σ_1(ω) below T_(Cdw) indicates that the Fermi surface is only partially gapped in the CDW state.The obtained value of 2△/k_BT_(CDW) is roughly 13 for both Sr_3Rh_4Sn_(13) and(Sr_(0.5)Ca_(0.5))_3Rh_4Sn_(13) compounds,which is considerably larger than the mean-field value based on the weak-coupling BCS theory.The measurements provide optical evidence for the strong coupling characteristics of the CDW phase transition.

Effect of mechanical force on domain switching in BiFeO3 ultrathin films

Ferroelectric polarization can be switched by an external applied electric field and may also be reversed by a mechanical force via flexoelectricity from the strain gradient.In this study,we report the mechanical writing of an epitaxial BiFeO3(BFO)thin film and the combined action of an applied mechanical force and electric field on domain switching,where the mechanical force and electric field are applied using the tip of atomic force microscopy.When the applied force exceeds the threshold value,the upward polarization of the BFO thin film can be reversed by pure mechanical force via flexoelectricity;when an electric field is simultaneously applied,the mechanical force can reduce the coercive electric field because both the piezoelectricity from the homogeneous strain and the flexoelectricity from strain gradient contribute to the internal electric field in the film.The mechanically switched domains exhibit a slightly lower surface potential when compared with that exhibited by the electrically switched domains due to no charge injection in the mechanical method.Furthermore,both the mechanically and electrically switched domains exhibit a tunneling electroresistance in the BFO ferroelectric tunnel junction.