Electric-field-controlled memory effect in heterostructures for gas sensors

A capacitive gas sensor has been created on the basis of an n-SnO₂/SiO₂/p-Si heterostructure with two successive oxide layers. The presence of polar C₂H₅OH, NH₃, and H₂O gas molecules in air leads to a significant increase in the capacitance of the structure at room temperature. An important feature of the adsorption process is a memory effect, which is confined to the possibility of maintaining the capacitance value after removal of the active component from the gas mixture. The possibility of quenching the accumulated useful signal by electric-field pulses has been realized for the first time as applied to gas sensors. Pis’ma Zh. Tekh. Fiz. 25, 22–29 (June 26, 1999)     

Magnetoelectric effect in thick-film heterostructures of PZT and Ni-Zn ferrites

This paper presents an experimental study of the magnetoelectric (ME) properties of layered heterostructures consisting of PZT-46, PZT-19, and Ni-Zn ferrites (1000NN and 2000NN). Their magnetoelectric response is examined as a function of the configuration, shape, and thickness of the layers in the composites. The results demonstrate that increasing the thickness of the magnetostrictive layer from 0.35 to 3.5 mm has an adverse effect on the magnitude of the ME effect. A comparative analysis of the data for rectangular and disk-shaped samples shows that the ME response of the disks is a factor of 2.5 stronger.     

Acoustoelastic Effects in Elastic Media with Nonuniform Initial Stress

Abstract

A perturbative formalism for the treatment of the acoustoelastic effect in material plates with a stress profile in the thickness direction is proposed and compared with a numerical method. The latter consists of an application of the local interaction simulation approach (LISA). The reliability of both the analytical treatment and the numerical approach is tested by comparing the respective results for several cases of propagation of Rayleigh waves in specimens with various kinds of stress fields, including welded steel plates. Finally, the linearization obtained from the perturbation treatment is exploited for the solution of the “inverse problem,” i.e., the determination of the stress field from a set of RW times of flight with different wavelengths. A numerical example is provided to illustrate the applicability of the method, starting from a set of synthetic data obtained with LISA.     

Acoustoelastic Effects in Elastic Media with Nonuniform Initial Stress

A perturbative formalism for the treatment of the acoustoelastic effect in material plates with a stress profile in the thickness direction is proposed and compared with a numerical method. The latter consists of an application of the local interaction simulation approach (LISA). The reliability of both the analytical treatment and the numerical approach is tested by comparing the respective results for several cases of propagation of Rayleigh waves in specimens with various kinds of stress fields, including welded steel plates. Finally, the linearization obtained from the perturbation treatment is exploited for the solution of the ``inverse problem,' i.e., the determination of the stress field from a set of RW times of flight with different wavelengths. A numerical example is provided to illustrate the applicability of the method, starting from a set of synthetic data obtained with LISA.     

Acoustoelastic response of polycrystalline aggregates exhibiting transverse isotropy

Acoustoelasticity is an ultrasonic technique which has been used for the determination of active and residual stresses in common structural materials. This paper examines the effect of texture on the acoustoelastic response in polycrystalline bodies. In particular materials which are transversely isotropic aggregates of cubic crystals are studied. The second- and third-order elastic constants of the polycrystal are derived from the elastic properties of the constituent crystals, and the crystalline orientation relative to the body's symmetry axis. The acoustoelastic relations between velocity and deformation are then presented for the aggregate. Finally, evaluation of the acoustoelastic response for several ideal textures using data for aluminum single crystals shows that the response is highly dependent on the texture.     

Microwave based nanogenerator using the ratchet effect in Si/SiGe heterostructures

Ratchet based microwave current generators and detectors were developed in Si/SiGe heterostructures for wireless communication with the possibility of extending the detection limit to the terahertz range. A microwave induced ratchet current was generated in the two-dimensional electron gas by patterning an array of semicircular antidots in hexagonal geometry. The spatial asymmetry created by the semicircular antidots forces the electrons under the influence of the microwave electric field to move preferentially towards the direction of the semidisc axis. A photovoltage of the order of few millivolts was observed. Such a photovoltage was completely absent in a symmetric system consisting of circular antidots. The induced photovoltage increased monotonically with microwave power and was found to be independent of the microwave polarization. This device opens the possibility of employing silicon based heterostructures for nanogenerators and other wireless communication devices using microwaves.     

Semiconductor nanowire heterostructures

Recent progress on the synthesis and characterization of semiconductor nanowire heterostructures is reviewed. We describe a general method for heterostructure synthesis based on chemical vapour deposition and the vapour-liquid-solid growth of crystalline semiconducting nanowires. We then examine examples of nanowire heterostructures for which physical properties have been measured, considering the effects of synthetic conditions on the heterointerfaces as well as the electrical and optical characterization measurements that reveal heterointerface formation and quality. Finally, we identify areas of technical and conceptual progress that can contribute to the development of functional nanowire heterostructures.     

Acoustoelastic Effects on Borehole Flexural Waves in Anisotropic Formations under Horizontal Terrestrial Stress Field

Applying the Stroh theory and based on the works of Hwu and Ting (1989), the complex function solution of stress and displacement fields around an open borehole in intrinsic anisotropic formation under horizontal terrestrial stress field is obtained. For cross-dipole flexural wave propagation along borehole axis, using the perturbation method, the acoustoelastic equation describing the relation between the alteration in phase velocity and terrestrial stress as well as formation intrinsic anisotropy is derived. At last, the numerical examples are provided for both the cases of fast and slow formation where the symmetry axis of a transversely isotropic (TI) formation makes an angle with the borehole axis. The phase velocity dispersion curves of borehole flexural wave and the corresponding velocity-stress coefficient are investigated. Computational results indicate that different from the stressed intrinsic isotropic formation situation, the variation in the phase velocity of flexural wave in stressed intrinsic anisotropic formation is dominated by two factors, one is the intrinsic formation anisotropy itself and the other is the stress-induced anisotropy. The former factor merely causes the borehole flexural wave split while the latter factor induces the dispersion curves intersection for two flexural waves polarized orthogonally. The combined effect of the two factors could strengthen or weaken the phenomenon of crossover for flexural wave dispersion curves. Thus, the dispersion curves of flexural waves may not intersect even under the unequal horizontal terrestrial stress field, whereas it is still possible to observe the crossover of the flexural wave dispersion curves under the equal horizontal terrestrial stress field. The polarized direction of the low-frequency fast flexural wave is no longer consistent with the direction of the maximum horizontal terrestrial stress all the time. Therefore, the crossover of the borehole flexural wave dispersion curves means that the terrestrial stress must exist. On the other hand, we can't exclude the possibility of the existence of terrestrial stress even if the flexural wave dispersion curves do not intersect. Based on the above researches, the method for terrestrial stress inversion from borehole flexural wave dispersion curves obtained by cross-dipole sonic logging in stressed intrinsic anisotropic formation is simply discussed.     

Nitrogen ion irradiation effect on enhancing photocatalytic performance of CdTe/ZnO heterostructures

To deal with the increasingly deteriorating environment problems, more and more harsh requirements are put forward for photocatalysis application. Building semiconductor heterostructures has been proven to be an efficient way to enhance photocatalytic performance. A kind of CdTe/ZnO heterostructures were synthesized by a hydrothermal and successive ionic layer absorption and reaction (SILAR) method and achieved obviously efficient photocatalytic performance. Moreover, after the N ion irradiation treatment, the photocatalytic activity was further enhanced, which can be ascribed to the introduction of oxygen vacancy defects. The photocatalytic performance enhancement mechanism by coupling constructing heterostructures and ion irradiation are further studied to give us an overall understanding on ZnO nanowires.     

Carrier-mediated magnetoelectricity in complex oxide heterostructures

Increasing demands for high-density, stable nanoscale memory elements, as well as fundamental discoveries in the field of spintronics, have led to renewed interest in exploring the coupling between magnetism and electric fields. Although conventional magnetoelectric routes often result in weak responses, there is considerable current research activity focused on identifying new mechanisms for magnetoelectric coupling. Here we demonstrate a linear magnetoelectric effect that arises from a carrier-mediated mechanism, and is a universal feature of the interface between a dielectric and a spin-polarized metal. Using first-principles density functional calculations, we illustrate this effect at the SrRuO3/SrTiO3 interface and describe its origin. To formally quantify the magnetic response of such an interface to an applied electric field, we introduce and define the concept of spin capacitance. In addition to its magnetoelectric and spin capacitive behaviour, the interface displays a spatial coexistence of magnetism and dielectric polarization, suggesting a route to a new type of interfacial multiferroic.     

Shear effects of the stressed state in transversely isotropic plates. Part 1. Vortex effect

We present the results of an investigation of the Reissner vortex edge effect in transversely isotropic plates on the basis of an improved theory of the second approximation according to which transverse shears obey the fourth-order parabolic law, unlike the first approximation described by ordinary parabolas. We compared the degrees of decay of the edge effect in the first and second approximations and establish the ranges of parameters of the plate characterized by “strong” and “weak” edge effects in the second approximation. It is shown under what conditions one can neglect the vortex effect in the second approximation. Ukrainian Transport University, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 56 – 62, January – February, 1998.     

Shear effects of the stressed state in transversely isotropic plates. Part 2. Potential effect

We investigate the potential effect of the stressed state in transversely isotropic plates on the basis of an improved theory of the second approximation. The equations of this theory are deduced on the basis of the solution of the variational problem of finding the conditional extremum of the functional of total energy of the plate by the method of undetermined Lagrangian multipliers. The terms responsible for the vortex effect are excluded from these equations. We present the numerical results obtained for one-layer and three-layer hinged plates and show that the contribution of the potential effect to the stressed state is substantial and the results obtained by taking this effect into account are closer to the exact solution. For a plate rigidly fixed at the edges and subjected to cylindrical bending, the potential effect turns into an edge effect and this significantly increases the level of stresses in the restraint. Ukrainian Transport University, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 63 – 71, January – February, 1998.     

Overstressed interatomic bonds in stressed polymers

Infrared (IR) spectroscopy is used to find how the applied mechanical stress imposed upon a sample is distributed among the interatomic bonds. The distribution is highly heterogeneous: 80–95% of the bond population experience stresses close to the applied stress, the stress on the rest of the bond population varies over a wide range and reaches 1000–2000 kg/mm². The overstressed interatomic bonds lie in the amorphous regions of the polymer and are oriented in the direction of the mechanical force. The maximum stress on interatomic bonds is determined by the magnitude of the breaking stress on them. The breaking stress is shown to be a function of the applied stress, time, and temperature. This dependence is due to scission of stressed bonds induced by thermal fluctuations.     

Ferroelectric/superconductor heterostructures

This review covers the fabrication and characterization of ferroelectric/superconductor heterostructures such as Pb(Zr_xTi_1−x)O₃/YBa₂Cu₃O_7−δ (YBCO), BaTiO₃/YBCO and Ba_xSr_1−xTiO₃/YBCO etc. on various single crystal substrates. Pulsed laser deposition, laser molecular beam epitaxy, and magnetron-sputtering methods are compared. This report shows that pulsed laser deposition equipped with in situ reflection high-energy electron diffraction is a good method to control the growth mode of YBCO thin films. Furthermore, laser molecular beam epitaxy is a superb method for research of complex oxide films and their superlattices. Atomic force microscopy and transmission electron microscopy showed the ferroelectric films grown on the rough surface of the YBCO films produced high-density planar defects in the film and is detrimental to the ferroelectric/dielectric properties of the heterostructures. Therefore, for device usage, it is more advantageous to use SrRuO₃ than YBCO as the bottom electrode material. For growing atomically smooth surface films step-flow mode is highly recommended. Prospects of microwave device application of the ferroelectric/superconductor heterostructures are discussed, and proposed the BSTO films as the best candidate for passive microwave components.     

缺陷/应力交互对碳钢L_(cr)波声弹性系数的影响

基于L_(cr)波声弹性理论,探讨缺陷及其尺寸对L_(cr)波评价应力的影响机理。结合"当量法"预制不同直径盲孔,采用互相关系数函数计算L_(cr)波时间差,通过线性拟合得到L_(cr)波声弹性系数,基于弹塑性变形和圆孔应力集中理论澄清盲孔直径对L_(cr)波声弹性系数的影响机理。结果表明:各直径盲孔L_(cr)波时间差随应力增大基本呈线性增加,但其非线性特征亦逐渐明显,线性阶段的最大应力值小于试样屈服强度;L_(cr)波声弹性系数随盲孔直径增大逐渐减小,并趋于平稳。分析认为,盲孔应力集中是导致上述结果的主要原因,试样各向异性组织及盲孔深度也是其重要因素。     

Screening effect in contactless electroreflectance spectroscopy observed for AlGaN/GaN heterostructures with two dimensional electron gas

AlGaN/GaN heterostructures with a two dimensional electron gas (2DEG) at the interface have been investigated by contactless electroreflectance (CER) and photoreflectance (PR) spectroscopies. It has been shown that the 2DEG effectively screens the GaN layer and hence no signal related to a bandgap transition in the GaN layer is observed in CER spectra whereas the CER signal related to a bandgap transition in the AlGaN layer is very strong. The screening phenomenon is unimportant for PR spectroscopy due to different mechanism of the electromodulation. As a result both GaN and AlGaN related transitions are clearly observed in PR spectra. It has been proposed that the screening phenomena observed in CER can find application in contactless detection of the 2DEG in AlGaN/GaN heterostructures.     

Transients in the formation of nanowire heterostructures

We present results on the effect of seed particle reconfiguration on the growth of short InAs and InP nanowire segments. The reconfiguration originates in two different steady state alloy compositions of the Au/In seed particle during growth of InAs and InP. From compositional analysis of the seed particle, the In content in the seed particle is determined to be 34 and 44% during InAs and InP growth, respectively. When switching between growing InAs and InP, transient effects dominate during the time period of seed particle reconfiguration. We developed a model that quantitatively explains the effect and with the added understanding we are now able to grow short period (<10 nm) nanowire superlattices.     

Interface formation in monolayer graphene-boron nitride heterostructures

The ability to control the formation of interfaces between different materials has become one of the foundations of modern materials science. With the advent of two-dimensional (2D) crystals, low-dimensional equivalents of conventional interfaces can be envisioned: line boundaries separating different materials integrated in a single 2D sheet. Graphene and hexagonal boron nitride offer an attractive system from which to build such 2D heterostructures. They are isostructural, nearly lattice-matched, and isoelectronic, yet their different band structures promise interesting functional properties arising from their integration. Here, we use a combination of in situ microscopy techniques to study the growth and interface formation of monolayer graphene-boron nitride heterostructures on ruthenium. In a sequential chemical vapor deposition process, boron nitride grows preferentially at the edges of existing monolayer graphene domains, which can be exploited for synthesizing continuous 2D membranes of graphene embedded in boron nitride. High-temperature growth leads to intermixing near the interface, similar to interfacial alloying in conventional heterostructures. Using real-time microscopy, we identify processes that eliminate this intermixing and thus pave the way to graphene-boron nitride heterostructures with atomically sharp interfaces.