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The flexoelectric effect, which manifests itself as a strain-gradient-induced electrical polarization, has triggered great interest due to its ubiquitous existence in crystalline materials without the limitation of lattice symmetry. Here, we propose a flexoelectric photodetector based on a thin-film heterostructure. This prototypical device is demonstrated by epitaxial LaFeO3 thin films grown on LaAlO3 substrates. A giant strain gradient of the order of 106/m is achieved in LaFeO3 thin films, giving rise to an obvious flexoelectric polarization and generating a significant photovoltaic effect in the LaFeO3-based heterostructures with nanosecond response under light illumination. This work not only demonstrates a novel self-powered photodetector different from the traditional interface-type structures, such as the p–n and Schottky junctions but also opens an avenue to design practical flexoelectric devices for nanoelectronics applications.

Flexoelectricity has been proven to be an effective way in tailoring a material’s properties. Here, we report the flexoelectricity-induced transport modulation to the two-dimensional electron gas a...

Log-periodic quantum oscillations discovered in transition-metal pentatelluride give a clear demonstration of discrete scale invariance (DSI) in solid-state materials. The peculiar phenomenon is convincingly interpreted as the presence of two-body quasi-bound states in a Coulomb potential. However, the modifications of the Coulomb interactions in many-body systems having a Dirac-like spectrum are not fully understood. Here, we report the observation of tunable log-periodic oscillations and DSI in ZrTe5 and HfTe5 flakes. By reducing the flakes thickness, the characteristic scale factor is tuned to a much smaller value due to the reduction of the vacuum polarization effect. The decreasing of the scale factor demonstrates the many-body effect on the DSI, which has rarely been discussed hitherto. Furthermore, the cut-offs of oscillations are quantitatively explained by considering the Thomas-Fermi screening effect. Our work clarifies the many-body effect on DSI and paves a way to tune the DSI in quantum materials., Accepted by npj Quantum Materials

Integration of transition metal dichalcogenides (TMDs) on ferromagnetic materials (FM) may yield fascinating physics and promise for electronics and spintronic applications. In this work, high-temperature anomalous Hall effect (AHE) in the TMD ZrTe

Two-dimensional electron gas (2DEG) in SrTiO3 (STO)-based heterostructures has been a subject of intense scientific interest in recent years. In this work, the metallic transition metal dichalcogenides ZrTe2 was grown on STO by pulsed laser deposition and AlN was subsequently deposited as a protection layer. The high-resolution transmission electron microscopy and electron energy loss spectroscopy results demonstrated the system as a multilayer structure of AlN/ZrTe2/ZrO2/STO due to interface redox reactions and implied a conductive STO surface. The remarkable Shubnikov–de Haas oscillations detected by angular dependent magnetotransport measurements reveal clear evidence of a high mobility quasi-2DEG in the STO-based interface. Moreover, evidence for extra carriers with three-dimensional features is observed implying the multiband contributions, which provide an explanation for some anomalous behavior in STO-based heterostructures. In addition, the thickness dependence study suggests the charge transfer effect between the capping metallic topological material ZrTe2 and the interfacial 2DEG. This work provides insight into the intrinsic electronic structure of STO-based heterostructures, and the integrated systems can serve as a platform for studying the interplay of the 2DEG with attractive materials as well as developing practical device applications.

We investigated the tunability of the transport and magnetic properties in 7.5 nm La 0.7 Sr 0.3 MnO 3 (LSMO) epitaxial films in a field effect geometry with the ferroelectric copolymer P(VDF-TrFE) as the gate insulator. Two different switching behaviors were observed upon application of gate voltages with either high or low magnitudes. The application of single voltage pulses of alternating polarity with an amplitude high enough to switch the remanent polarization of the ferroelectric copolymer led to a 15% change of the resistance of the LSMO channel at temperature 300 K (but less than 1% change at 20 K). A minimal shift of the peak in the resistance-temperature plot was observed, implying that the Curie temperature T C of the manganite layer is not changed. Alternatively, the application of a chain of low voltage pulses was found to shift T C by more than 16 K, and a change of the channel resistance by a 45% was obtained. We attribute this effect to the field-assisted injection and removal of oxygen vacancies in the LSMO layer, which can occur across the thickness of the oxide film. By controlling the oxygen migration, the low-field switching route offers a simple method for modulating the electric and magnetic properties of manganite films.

Binary ferroelectric materials such as hafnium oxide have been intensively studied, which are expected to exhibit robust ferroelectricity comparable to the perovskite-based ferroelectrics at nanoscale. Here, using the combination of X-ray diffraction (XRD) associate with Transmission Electron Microscope (TEM), we reveal the epitaxial growth of Al-doped HfO2 possessing various phase structures on different oriented SrTiO3 substrates. The well-oriented films show different ferroelectricity indicated by piezoresponse force microscopy (PFM). The film grown on the (111) SrTiO3 substrate shows the largest electromechanical effect. These results, with the analyses of structural characterizations, demonstrate the orientation control of phase transitions and ferroelectricity in Al-doped HfO2 thin films.

The roles of oxygen gas in crystal orientation, surface morphology and electrical resistivity of Pt thin films grown on MgO(001) substrate by magnetron sputtering are studied. With a well-controlled oxygen ratio (15% oxygen) during sputtering deposition with Ar-O2 mixture ambient, (001) epitaxial growth of Pt film on MgO substrate is achieved with an epitaxial orientation relationship of (001)Pt//(001)MgO and [100]Pt//[100]MgO. Microstructural and electrical characterizations reveal that the (001) Pt thin films possess very smooth surface and good conductivity. The formation and subsequent decomposition of platinum oxides in the Pt films grown with more than 30% oxygen result in an increase of surface roughness and electrical resistivity. The high-quality Pt(001) film has large potential for integrated electronic device applications.

CuCrSe2 compounds were synthesized by mechanical alloying followed by annealing and spark plasma sintering at temperatures of 873 K, 923 K, 973 K and 1023 K. Our investigation revealed that high SPS temperature (above 973 K) is the key to obtaining CuCrSe2 single phase completely free of a metallic secondary phase CuCr2Se4. The ultralow thermal conductivity caused by the Cu ions disorder, complex layered structure and dislocation leads to the favorable thermoelectric performance in CuCrSe2 compound. A maximum ZT value over 0.75@873 K is achieved in the absolutely pure sample sintered at 1023 K.

In this study, we demonstrate the crystal structure-controlled growth of superconductive Mo2C thin films on a sapphire(0001) substrate in the pulsed-laser deposition. Detailed characterizations indicate that the thin film grown at 700 °C adopts the face centered cubic structure with the preferred orientation of the 〈111〉 direction. It was also demonstrated that the sapphire substrate and face centered cubic structured Mo2C thin film have the orientation relationship of {0006}sapphire//{111}Mo2C and [010]sapphire//[10] Mo2C due to a small in-plane lattice mismatch between their corresponding lattice spacing perpendicular to the {111} atomic plane of the Mo2C thin film. Electrical transport measurements show that the Mo2C thin films exhibit superconducting characteristics. This study provides a practical approach to controlling the crystal structure of superconductive Mo2C thin films through the optimization of substrate orientation.

Thin film flexoelectricity is attracting more attention because of its enhanced effect and potential application in electronic devices. Here we find that a mechanical bending induced flexoelectricity significantly modulates the electrical transport properties of the interfacial two-dimensional electron gas (2DEG) at the LaAlO_{3}/SrTiO_{3} (LAO/STO) heterostructure. Under variant bending states, both the carrier density and mobility of the 2DEG are changed according to the flexoelectric polarization direction, showing an electric field effect modulation. By measuring the flexoelectric response of LAO, it is found that the effective flexoelectricity in the LAO thin film is enhanced by 3 orders compared to its bulk. These results broaden the horizon of study on the flexoelectricity effect in the hetero-oxide interface and more research on the oxide interfacial flexoelectricity may be stimulated.

Photo-enhanced flexoelectricity or flexoelectricity-enhanced photovoltaic effect, named photo-flexoelectric, is an interesting topic and has application potential in photo-electro-mechanical devices. However, this effect is far from being well understood. In this work, we demonstrate the photoflexoelectric effect in perovskite-structured SrTiO3 (STO) single crystal and reveal the coupling mechanism between its photovoltaic and flexoelectric effect. Driven by the flexoelectric field, light-induced electrons can tunnel through the Schottky junction at the Au/STO interface, giving rise to enhanced flexoelectricity, i.e., photoflexoelectric effect. Thermal annealing in vacuum induces oxygen vacancies in STO and results in stronger light absorption and enlarged photoflexoelectric effect.

The coming Big Data Era requires progress in storage and computing technologies. As an emerging memory technology, Resistive RAM (RRAM) has shown its potential in the next generation high-density storage and neuromorphic computing applications, which extremely demand low switching voltage and power consumption. In this work, a 10 nm-thick amorphous GeS2 thin film was utilized as the functional layer of RRAM in a combination with Ag and Pt electrodes. The structure and memory performance of the GeS2-based RRAM device was characterized — it presents high on/off ratio, fast switching time, ultralow switching voltage (0.15 V) and power consumption (1.0 pJ and 0.56 pJ for PROGRAM and ERASE operations, respectively). We attribute these competitive memory characteristics to Ag doping phenomena and subsequent formation of Ag nano-islands in the functional layer that occurs due to diffusion of Ag from electrode into the GeS2 thin film. These properties enable applications of GeS2 for low energy RRAM device.

This work aims at improving the quality of the highly (1 0 0)-orientated SnSe thin films for thermoelectric applications. The as-deposited films were obtained by controlling the basic parameters including target-to-substrate distance, deposition time and growth temperature through pulsed-laser deposition. The films quality was further improved by vacuum thermal annealing. The microstructure and crystalline structure of the films were studied by X-ray photoelectron spectroscopy, X-ray diffraction, electron probe micro-analyzer, electron back-scatter diffraction, atomic force microscope and Raman spectroscopy. The SnSe thin films grown on SiO2/Si substrate at 673 K followed by thermal annealing at 673 K for 30 min show the best crystal quality and uniform orientation with mirror-like surface, and the corresponding Seebeck coefficient and power factor are about 383 μV/K and 15.4 μW/m⋅K2, respectively. Angle resolved polarized Raman spectroscopy proved that the surface of the SnSe films is the b-c plane with preferred (1 0 0) orientation crystalline over a large area, providing an important way to prepare thermoelectric thin film devices by pulse laser deposition.

In this work, we successfully synthesized p -type Mg 2(1-x) Li 2x Si 0.4 Sn 0.6 (x = 0.00, 0.01, 0.03, 0.07, 0.09) samples using a home-made melt spinning system combined with spark plasma sintering. Compared with the samples prepared by a traditional method, the processing time is reduced from typically several days to less than one hour. It is found that the electrical conductivity rises rapidly with the increase of Li content owing to the enhanced carrier density, while the Seebeck coefficient decreases concomitantly to some extent. Meanwhile, Li doping dramatically reduces the thermal conductivity, leading to an enhanced figure of merit ZT ~ 0.58 at 760 K.

In this study, via a home-made induction melting process combined with spark plasma sintering technique, we successfully synthesize the nanostructured skutterudites NdxYbyFe3CoSb12 (x + y = 1.0, x = 0.3, 0.4, 0.5, 0.6, 0.7) in less than 3 h, which is significantly faster than that of the solid state reaction (168 h) and the traditional melt spinning method (40 h). Systematical microstructures analysis of the sintered bulk materials reveals the largely refined matrix grains (100–300 nm) and ubiquitous YbSb precipitates (~ 30 nm), resulting in a very low lattice thermal conductivity. A peak ZT up to 1.02 at 760 K is obtained for Nd0.6Yb0.4Fe3CoSb12 compound.

Oxygen-rich polycrystalline NiOx films were prepared by means of magnetron sputtering. Thickness-dependent bipolar resistive switching behaviors revealed that the 20 nm-thick NiOx film presented a clockwise current-voltage loop, while the 60 nm-thick NiOx film achieved an anti-clockwise current-voltage loop. Redox reactions between penetrated Ag ions and drifted oxygen ions in the whole 20 nm-thick NiOx films resulted in the clockwise current-voltage loops. Filamentary conducting paths composed by oxygen vacancies were responsible for the anti-clockwise resistive switching loops of the 60 nm-thick NiOx film.

Excellent thermoelectric performance is obtained over a broad temperature range from 300 K to 800 K by doping single crystals of SnSe. The average value of the figure of merit ZT, of more than 1.17, is measured from 300 K to 800 K along the crystallographic b-axis of 3 at% Na-doped SnSe, with the maximum ZT reaching a value of 2 at 800 K. The room temperature value of the power factor for the same sample and in the same direction is 2.8 mW mK−2, which is an order of magnitude higher than that of the undoped crystal. Calculations show that Na doping lowers the Fermi level and increases the number of carrier pockets in SnSe, leading to a collaborative optimization of the Seebeck coefficient and the electrical conductivity. The resultant optimized carrier concentration and the increased number of carrier pockets near the Fermi level in Na-doped samples are believed to be the key factors behind the spectacular enhancement of the average ZT.

Immobilization of the suspended nanoparticles is essential for many microfluidic applications. This work reports a novel biomimetic method to immobilize nanoparticles by using a common adhesive tape as the substrate of microfluidic chip. It mimics the clams’ feeding system that utilizes the mucus (i.e., sticky fluid) to capture small phytoplankton particles in water. This work proves experimentally that this method has a better immobilization effect and a stronger shear stress resistance than the traditional methods using hard glass substrates. Moreover, we have applied this method to immobilize Au nanorods for the detection of R6G of various concentrations using the surface-enhanced Raman scattering (SERS) effect. This method enjoys several major merits: the sticky adhesive tape can seal the microfluidic structure easily, avoiding the bonding process; the immobilization is easy and environmental friendly, without the need for expensive reagents or complex processes; the adhesive tape substrate allows the flexibility of microfluidic chips; and the adhesive tape substrate can be stripped off for off-chip detection and can be replaced easily for the reuse of microfluidic structures. With these, the biomimetic method may find potential applications in environmental sensing, biocatalysis and biosynthesis using microchips.

Two-dimensional (2D) metallic transition metal dichalcogenides (TMDs) exhibit fascinating quantum effects, such as charge-density-wave (CDW) and weak antilocalization (WAL) effect. Herein, low temperature synthesis of 1T phase VSe2 single crystals with thickness ranging from 3 to 41 nm by chemical vapor deposition (CVD) is reported. The VSe2 shows a decreasing phase transition temperature of the CDW when the thickness is decreased. Moreover, low-temperature magnetotransport measurements demonstrate a linear positive and non-saturating magnetoresistance (MR) of 35% from a 35 nm thick VSe2 at 15 T and 2 K due to CDW induce mobility fluctuations. Surprisingly, Kohler's rule analysis of the MR reveals the non-applicability of Kohler's rule for temperature above 50 K indicating that the MR behavior cannot be described in terms of semiclassical transport on a single Fermi surface with a single scattering time. Furthermore, WAL effect is observed in the 4.2 nm thick VSe2 at low magnetic fields at 2 K, revealing the contribution of the quantum interference effect at the 2D limit. The phase coherence length [Formula: see text] and spin-orbit scattering length [Formula: see text] were determined to be 73 nm and 18 nm at 2 K, respectively. Our work opens new avenues to study the fundamental quantum phenomena in CVD-deposited TMDs.

Strain engineering plays a critical role in ferroelectric memories. In this work, we demonstrate dynamic strain modulation on tunneling electroresistance in a four-unit-cell ultrathin $\mathrm{BaTi}{\mathrm{O}}_{3}$ metal/ferroelectric/semiconductor tunnel junction by applying mechanical stress to the device. With an extra compressive strain induced by mechanical stress, which is dynamically applied beyond the lattice mismatch between the $\mathrm{BaTi}{\mathrm{O}}_{3}$ layer and the $\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate, the ON/OFF current ratio increases significantly up to a record high value of ${10}^{7}$, whereas a mechanical erasing effect can be observed when a tensile stress is applied. This dynamic strain engineering gives rise to an efficient modulation of ON/OFF ratio due to the variation of $\mathrm{BaTi}{\mathrm{O}}_{3}$ polarization. This result sheds light on the mechanism of electroresistance in the ferroelectric tunnel junctions by providing direct evidence for polarization-induced resistive switching, and also provides another stimulus for memory state operation.

Discovery of log-periodic quantum oscillations may open a new chapter in the nearly 90-year history of quantum oscillations., Quantum oscillations are usually the manifestation of the underlying physical nature in condensed matter systems. Here, we report a new type of log-periodic quantum oscillations in ultraquantum three-dimensional topological materials. Beyond the quantum limit (QL), we observe the log-periodic oscillations involving up to five oscillating cycles (five peaks and five dips) on the magnetoresistance of high-quality single-crystal ZrTe5, virtually showing the clearest feature of discrete scale invariance (DSI). Further, theoretical analyses show that the two-body quasi-bound states can be responsible for the DSI feature. Our work provides a new perspective on the ground state of topological materials beyond the QL.

(Ba1−x,Srx)(Zr0.1,Ti0.9)O3 (BSZT) ceramics with x = 0, 0.05, 0.15, 0.25, 0.35 and 0.45 were prepared by conventional solid state reaction method. The structural characterization with X-ray diffraction and scanning electron microscopy indicate a monotonical drop in lattice constants and grain size with the increase of Sr concentration. Consequently, the Curie temperature and remnant polarization of the ceramics exhibit a strong compositional dependence. A linear relationship between the Curie temperature and Sr concentration is revealed. At x = 0.45, the BSZT ceramics show substantially high tunability of over 55 % under 20 kV/cm dc electric field with very low dielectric loss value of 0.0025 at room temperature, suggesting the BSZT ceramics could be a promising alternative to traditional (Ba,Sr) TiO3 ferroelectrics for developing high frequency tunable dielectric devices.

An anomalous transverse voltage near the superconducting transition is observed at the LaAlO3/SrTiO3 heterointerface. In contrast to the normal Hall effects, the observed anomalous transverse voltage persists even at zero magnetic field and is an even function of the magnetic field. It also responds anisotropically to out-of-plane and in-plane magnetic fields. Due to the two-dimensional nature of this superconducting electron system, this anomalous transverse signal is highly tunable via electrostatic gating. Strikingly, the temperature dependence of this transverse voltage exhibits a gate-tunable sign reversal behavior and can even undergo multiple sign reversals. Thorough analyses indicate that the anomalous transverse signal can be largely attributed to the guided vortex motion in the two-dimensional superconducting system. Our findings not only reveal important aspects of vortex dynamics at the strongly correlated oxide interface but also may promote the development of electrically tunable vortex dynamics.

We report on room-temperature synthesis of Ag nanoparticles (NPs) decorated silver molybdate nanowires (SMNs) using a solution-based chemical reaction method. We show that AgxMoyOz (Ag2Mo2O7, Ag2MoO4 and Ag6Mo10O33) nanomaterials can be selectively synthesized by altering the pH value of the reaction media in the presence of silicotungstic acid at room temperature. This reaction process uses isopropyl alcohol as a free radical scavenger and occurs over a short reaction time compared to the traditional hydrothermal method. Moreover, we demonstrate that under UV-light illumination of different periods, Ag NPs of controllable distribution densities are formed in situ on the surface of SMNs in tungstosilicate acid environment. We show that the SERS sensitivity of such Ag NP–SMN complex depends on the distribution density of Ag NPs, with the best structure capable of detecting the p-aminothiophenol at a concentration as low as 1.0 × 10−11 M. The same substrate is further used to enhance the Raman signal from 2-mercaptopyridine from which a quantitative relationship between the analyte concentration and its corresponding Raman intensity can be established.

Endoscopic ultrasound radial array transducers were fabricated using PZT ceramic tubes by a rotate-and-dice method. 50-element 14 MHz and 100-element 3 MHz radial array transducers have been fabricated and characterized with a pulse-echo response measurement. It was found that both transducers exhibit reasonably good performance. The −6 dB bandwidth of these two array transducers was found to be 46% and 56%, respectively. Besides, the insertion loss of the transducers was found to be 26 dB and 32 dB, respectively. The results show that the rotate-and-dice method is an effective way to fabricate radial array transducers for various applications.

A systematic study on laser micro-machining of PMN-PT single crystals has been performed. The PMN-PT single crystal has been micromachined using a 355 nm Nd:YAG laser. The kerf profile and cutting quality of the laser micro-machined PMN-PT single crystal were studied as functions of the laser process parameters. It was found that the direct proportional relationship between the depth of the kerf and the laser power provides an effective reference to determine the depth required to be ablated. It is significant for fabricating micro-devices using the single crystal by laser micro-machining. Besides the physical behavior, the piezoresponse of the PMN-PT single crystal has also been investigated by PFM. The results reveal that the laser micro-machining would not affect the macroscopic piezoelectric performance of the PMN-PT single crystal. The present work shows the feasibility of micro-patterning the PMN-PT single crystal into any geometry using a Nd:YAG laser.

Tuning transport properties of two-dimensional electron gas (2DEG) at the LaAlO 3 /SrTiO 3 interface by surface modification is of great interest due to its rich physics and potential application in polar liquid or gas sensors. From first-principle calculations, we found that the adsorption of a thin layer of polar water molecules on the surface of LaAlO 3 can remarkably enhance the carrier density of the interfacial 2DEG by over at least 50% which is qualitatively consistent with reported experimental result. This result sheds light on the charge coupling between surface polar molecules and interfacial 2DEG and suggests that sensor devices may be built using the hetero-oxide interfacial 2DEG.

Silver nanoparticles embedded in homoepitaxial SrTiO3 thin films are prepared by laser molecular beam epitaxy technique combined with nanocluster beam source. Microstructural analysis reveals that Ag nanoparticles are well isolated within the film with an average size of 16 nm. The existence of Ag nanoparticles alters the epitaxial growth orientation of SrTiO3 and makes the film deviate from the epitaxial growth orientation. The surface plasmon resonance of Ag nanoparticles is studied by optical absorption spectrum and photoluminescence spectrum. The position of surface plasmon resonance peak in the absorption spectrum is explained using modified Mie's theory.

The electrical properties and Raman spectroscopy of [001]-oriented pure lead-free Na0.5Bi0.5TiO3 (NBT) and manganese-doped NBT single crystals have been investigated. With the effect of Mn-doping, the remnant polarization P r has a great enhancement from 15.2 μC/cm2 to 30.0 μC/cm2, and the pyroelectric coefficient is improved significantly from 178 μC/m2K to 241 μC/m2K. The Mn-doping has further altered the domain structure of the crystals, as revealed by piezoresponse force microscopy study. The Raman studies suggest that the substitution effect of Mn ions on B-site cations of the perovskite structure results in enhancement of the performance of the Mn-doped NBT single crystals.

Lead magnesium niobate–lead titanate (PMN–PT) single crystal has ultra-high piezoelectric properties compared with conventional piezoelectric materials and is suitable for high-performance ultrasonic transducer application. In this paper, an improved process has been used for fabricating an annular array ultrasonic transducer using PMN-PT single crystal. A 35 MHz annular array with 8 elements has been built. The bandwidth of the array elements were around 45%. At the center frequency, the insertion loss ranged from −21dB to −28dB. The electrical cross-talk between the adjacent elements was in a range of −24dB to −31dB.

Ferroelectric vortex is one of unique domain structures in the hexagonal RMnO3 (R=Sc, Y, Ho-Lu) systems. This vortex pattern is quite sensitive to crystal imperfections, such as lattice defects and oxygen vacancies, which has been previously observed and studied in a single-crystal structure. Here we report epitaxial growth of hexagonal YMnO3 thin films on platinum-coated Al2O3 (0001) substrates. High-quality epitaxial YMnO3(0001)/Pt(111)/Al2O3(0001) heterostructures with sharp interfaces have been achieved and characterised by using X-ray diffractometry and transmission electron microscopy. Reversible ferroelectric domain structures have been achieved and observed with well-established piezoresponse hysteresis. Furthermore, the ferroelectric vortex domain patterns with a typical size of ~20 nm have been observed, representing a significant progress in the fabrication and exploration of topological vortices in hexagonal RMnO3 thin films. Ferroelectric polarization vortices, although commonly observed in bulk, they rarely form in low dimensions due to the lack of perfect crystal structures in thin films. A team led by Min Zeng at South China Normal University showed how to epitaxially grow high-quality hexagonal YMnO3 films on platinum-coated aluminum oxide substrates. Facilitated by the lattice strains generated at the flat interfaces between YMnO3 and the substrates, local domains with size of approximately 20 nm were observed, which were characterized by vortex-like structures with opposite ferroelectric polarizations. The work represents a significant progress in the fabrication and exploration of topological vortices in hexagonal RMnO3 thin films. It could open the possibility for new designs of memory devices using ferroelectric oxides.

Among the interfacial transport modulations to the $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LAO/STO) heterostructure, mechanical strain has been proven to be an effective approach by growing the LAO/STO films on different substrates with varying lattice mismatches to STO. However, this lattice-mismatch-induced strain effect is static and biaxial, hindering the study of the strain effect in a dynamic way. In this work we realize dynamic and uniaxial strain to the LAO/STO oxide heterostructure at low temperature, through mechanical coupling from a magnetostrictive template. This anisotropic strain results in symmetry breaking at the interface and induces further splitting of the electronic band structure and therefore produces different conductivities along the $x$ and $y$ in-plane directions. In particular, we observe that along the strained direction the interface conductivity decreases by up to $70%$ under a tensile strain, while it increases by $6.8%$ under a compressive strain at 2 K. Also, it is revealed that the modulation on the interfacial transport property can be anisotropic, i.e., the resistance changes differently when an excitation current is parallel or perpendicular to the strain direction. This approach of strain engineering provides another degree of freedom for control of transport properties of oxide heterostructures and opens an additional way to investigate strain effects in materials science.

In this work, lead-free 1–3 composites based on piezoelectric 0.947Na0.5Bi0.5TiO3–0.053BaTiO3 (NBT–0.053BT) single crystal and epoxy are fabricated for ultrasonic transducer applications by a modified dice-and-fill method. Excellent properties for ultrasonic transducer applications have been achieved, such as high electromechanical coupling coefficient (kt = 73%), lower acoustic impedance (Z = 16 MRayl) and moderate dielectric constant. Based on this lead-free piezoelectric single crystal composite, single-element ultrasonic transducer and linear array have been fabricated and characterized. Both types of transducers exhibit similar performance with broad bandwidth of exceeding 100%. The promising results show that these lead-free composites have the potential to be used for high-performance ultrasonic transducers.

We use a multi-fluid model to describe the dynamic behavior of electrons, ions, and neutrals in a weakly ionized three-component plasma and investigate the differentially rotational instability. Charged species and neutrals are supposed to have the same angular velocity to eliminate the equilibrium current and collisions in the unperturbed state. By taking into account collisions between electrons/ions and neutrals, we derive the analytical expression of the general local dispersion relation and examine the collision effect on the rotational instability in different limiting cases. Collisions on neutrals are neglected in the first case to obtain a simplified dispersion, in which collisions play an important role in the instability criteria. In the low-frequency region, the previous result in the magnetohydrodynamic model is reproduced. The collision effect in the high-frequency case for non-magnetized electrons is found to be similar to the low-frequency region, but can induce instability for both the negative and positive radial derivative of rotational frequency.

The ground-state structural, electronic, magnetic, optical and dielectric properties of MnTiO3 are calculated using density functional theory within the generalized gradient approximation. The structure parameters obtained agree well with experimental results. The electronic structure results show that the G-type antiferromagnetic phase of LN-type MnTiO3 has an indirect band gap of 0.85 eV. The calculated local magnetic moment of Mn ion is 4.19 μB. The calculated Born effective charges (BECs, denoted by tensor Z*) show that the Z* of Ti and O atoms are significantly and anomalously large. Interestingly, ferroelectric spontaneous polarization of large magnitude is predicted to be along [111] direction with a magnitude of 87.95–105.22 μC/cm2. B-site Ti ions in 3 d0 state dominate ferroelectric polarization of multiferroic MnTiO3, whereas A-site Mn ions having partially filled 3 d5 orbitals are considered to contribute to its antiferromagnetic properties. Furthermore, it is predicted that multiferroic MnTiO3 shows good dielectric and optical properties.

A geometric design for array elements of a piezoelectric linear array transducer is proposed and evaluated. The design concept is based on the half-concave geometry, in which the radiating surface is concave while the other surface remains plane. A ∼1.8 MHz piezoelectric linear array transducer with half-concave elements has been designed, fabricated and evaluated. A dicing method was developed to shape the concave surfaces of the piezo elements and matching layers. By comparing the transducer performance, a traditional linear array transducer with similar dimension has been fabricated. It was found that the half-concave array transducer has significantly broader −6 dB bandwidth (96%), higher effective electromechanical coupling coefficient (0.62), and lower insertion loss (−21 dB) compared to those (76%, 0.55, and −25 dB, respectively) of the plane array transducer. The enhanced coupling coefficient and bandwidth are caused by the broaden resonance of the elements, which is induced by the continuously varying thickness in the designed geometry. The increased sensitivity is mainly attributed to the focused radiating surface.

Bi0.85La0.15FeO3 (BLFO) nanotubes with an average diameter of about 200 nm and wall thickness of about 20 nm are fabricated by sol–gel alumina template technique, and their room-temperature multiferroic properties are studied. Piezoelectricity and ferroelectricity in the BLFO nanotubes are revealed by piezoresponse force microscopy study of individual nanotube. Doping the BiFeO3 nanotubes with La reduces the crystallization temperature and results in a reduced lose of Bi and therefore improved multiferroic properties of the nanotubes. Enhanced weak ferromagnetism is also observed, and it is attributed to the nano-crystalline structure of the nanotubes.

A novel spinel (Zn,Co)Fe 2 O 4 film is deposited on (0 0 1) SrTiO 3 (STO) substrate by pulsed laser deposition. Microstructure analysis reveals that the film is composed of pillar phase and matrix phase, and both of them have the same chemical composition. The pillar phase has the cube-on-cube orientation relationship with STO substrate, which is consistent with the reported orientation relationship between spinel CoFe 2 O 4 films and STO substrate. However, the matrix phase forms to 90° domains with a unique orientation relationship with STO substrate of spinel(1 1¯ 1)||STO(0 0 1) and spinel[1 1 0]||spinel[1 1 2] (90° domains)||STO[1 1 0]. Magnetic property study reveals that the film exhibits relatively larger saturation magnetization and lower coercive field, compared to the reported magnetic properties of the CoFe 2 O 4 films on STO substrate.

Interfacial microstructure and electrical properties of HfAlO"x films deposited by RF magnetron sputtering on compressively strained Si"8"3Ge"1"7/Si substrates were investigated. HfSiO"x-dominated amorphous interfacial layer (IL) embedded with crystalline HfSi"x nano-particles were revealed by high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy depth profile study. About 280mV-wide clockwise capacitance-voltage(C-V) hysteresis for the HfAlO"x film deposited in Ar+N"2 mixed ambient was observed. Oxygen vacancies and interfacial defects in the HfSiO"x IL, as well as trapped charges in the boundaries between the HfSi"x nano-particles and surrounded amorphous HfSiO"x may be responsible for the large C-V hysteresis.

Multiferroic BaTiO3/CoFe2O4 superlattice films are deposited by laser molecular beam epitaxy. The film growth modes are studied by in situ reflection high energy electron diffraction and the film structures are revealed by high resolution transmission electron microscopy study. Ferroelectric switching behavior was studied by piezoresponse force microscopy, and it shows that good ferroelectricity was retained in the superlattice. Such a multiferroic superlattice also shows a magnetic exchange coupling under room temperature. Detailed analysis reveals that different growth modes and the substrate strain effect may be responsible for the magnetic exchange coupling.

Ultrasonic-assisted wet chemical etching process was developed for thinning and pattering of single crystalline 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 (0.70PMN–0.30PT) thin layers for micro-electromechanical systems (MEMS) applications. 50 kHz ultrasonic bath with appropriate intensity turned out to be an effective way to speed up the etching process while maintain a constant etching rate. The relationships of etching rates versus chemical concentrations, temperatures and crystalline orientations were investigated. It was found that the chemical etching is thermally activated with an activation energy Ea of 67.7 kcal/mol, and the highest etching rate (6.2 µm/min) was achieved by ultrasonic-assisted etching along the cub direction at room temperature. The feasibility of micro-scale patterning of PMN–PT single crystal layers was also demonstrated by using patterned Au/Cr layer as the etching mask.

Nanoclusters (NC) as charge storage nodes have been applied in nonvolatile, high-speed, high-density and low-power memory devices. Compared with conventional floating gate memory, where a layer of poly-Si is used for charge storage, a memory device composed of nanoclusters isolated by dielectrics benefits from a relatively low operating voltage, high endurance, fast write-erase speeds and better immunity to soft errors due to the quantum confinement and Coulomb blockade effects. Recent patents in this field have proposed several innovated structures and fabrication methods for nanocluster based floating gate flash memory and single-electron memory devices.

Dependence of oxygen partial pressures on structural and electrical characteristics of HfAlO (Hf:Al=1:1) high- k gate dielectric ultra-thin films grown on the compressively strained Si 83 Ge 17 by pulsed-laser deposition were investigated. The microstructure and the interfacial structure of the HfAlO thin films grown under different oxygen partial pressures were studied by transmission electron microscopy, and the their electrical properties were characterized by capacitance–voltage ( C − V ) and conductance–voltage measurements. Dependence of interfacial layer thickness and C – V characteristics of the HfAlO films on the growth of oxygen pressure was revealed. With an optimized oxygen partial pressure, an HfAlO film with an effective dielectric constant of 16 and a low interface state density of ∼2.1×10 10 cm −2 eV −1 was obtained.

Barium strontium zirconate titanate [(Ba1-xSrx)(ZrxTi1-x)O3, BSZT-100x], thin films with x = 0.025 and 0.05 (BSZT-2.5 and BSZT-5) were grown on MgO (100) substrates by pulsed laser deposition. X-ray diffraction measurements revealed that the BSZT films had grown epitaxially on the MgO (100) substrates. The refractive index and thickness of the BSZT-2.5 and BSZT-5 films were determined using a prism coupler. The electro-optic (E-O) properties were measured at 632.8 nm wavelength using a phase modulation detection method. The BSZT films exhibited a predominantly quadratic E-O behavior and the E-O coefficient of BSZT-2.5 and BSZT-5 were found to be 1.52 × 10−18 m2/V2 and 2.04 × 10−18 m2/V2, respectively.

A metal-insulator-semiconductor (MIS) structure containing an HfO 2 /SiO 2 stack tunnel layer, isolated Germanium (Ge) nanocrystals, and an HfO 2 capping layer, was obtained by an electron-beam evaporation method. A high-resolution transmission electron microscopy (HRTEM) study revealed that uniform and pronounced Ge nanocrystals had formed after annealing. Raman spectroscopy provided evidence for the formation of Ge-Ge bonds and the optimal annealing temperature for the crystallization ratio of the Ge. The electric properties of the MIS structure were characterized by capacitance-voltage (C-V) and current-voltage (I-V) measurements at room temperature. Negative photoconductivity was observed when the structure was under a forward bias, which screened the bias voltage, resulting in a decrease in the current at a given voltage and a negative shift in flat band voltage. A relatively high stored charge density of 3.27×10 12 cm -2 was also achieved.