Search

The correlated semiconductor vanadium dioxide (VO2) exhibits an insulator–metal transition (IMT) near room temperature, which is of interest in various device applications. Precise IMT temperature control is crucial to determine the use cases across technologies such as thermochromic windows, actuators for robots or neuronal oscillators. Doping the cation or anion sites can modulate the IMT by several tens of degrees and control hysteresis. However, modeling the effects of control parameters (e.g., doping concentration, type of dopants) is challenging due to complex experimental procedures and limited data, hindering the use of traditional data-driven machine learning approaches. Symbolic regression (SR) can bridge this gap by identifying nonlinear expressions connecting key input parameters to target properties, even with small data sets. In this work, we develop SR models to capture the IMT trends in VO2 influenced by different dopant parameters. Using experimental data from the literature, our study reveals a dual nature of the IMT temperature with varying tungsten (W) doping concentrations. The symbolic model captures data trends and accounts for experimental variability, providing a complementary approach to first-principles calculations. Our feature-driven analysis across a broader class of dopants informs selectivity and provides qualitative insights into tuning phase transition properties valuable for neuromorphic computing and thermochromic windows. [ABSTRACT FROM AUTHOR]

Grain boundaries (GBs) in two-dimensional (2D) materials often have a profound impact on various material properties from mechanical to optical to electronic, yet predicting all possible GB formations is a challenge. Here, we introduce a workflow based on an evolutionary algorithm for exploring possible GBs formed at a lateral 2D interface. In a departure from conventional genetic algorithm based structure optimization methods, we perform genetic operations in the near interface region that allow us to be computationally efficient. We benchmark our method using graphene, which is a well-studied 2D material with a wide range of point defects. An empirical potential was used as the surrogate of the evolutionary search. More than 11.5 × 106 structures in total were evaluated for 128 GB orientations, and for each orientation, the ten best structures are recorded. A subset of low energy GBs predicted by empirical potential based search was relaxed by first-principles calculations and used to validate the energetic rank order. With the validated formation energy, we rank-ordered the best 128 GB structures and performed a detailed statistical analysis of primitive rings to find the correlation between the ring distribution and the formation energy. We found that for low energy GBs (below 0.5 eV / Å), Stone–Wales defects will dominate, while structures with a higher energy (0.5 – 1.1 eV / Å) show an increasing population of heptagons and nine-membered rings to form seven-nine defect pairs. For structures with energy higher than 1.1 eV / Å , the percentage of octagons and nine-membered rings increases, which indicates that these two types of rings are not energetically favorable. Our proposed methodology is broadly applicable to explore defective low dimensional materials and represents a powerful tool that enables a systematic search of GBs of lateral interfaces for 2D materials. [ABSTRACT FROM AUTHOR]

Surface enhanced Raman scattering (SERS) study is an interesting active area of research, where periodically patterned plasmonic substrates play a key role in SERS enhancement. Surface plasmon resonance excitation generates tremendous electromagnetic near-fields (E) in the form of localized or propagating near-fields contributing to the Raman signal (E4 process) to a great extent and overall enhancement is reported to be as high as 1010 or even more. Independently, graphene alone can enhance the Raman signal due to chemical enhancement. In the present study, we have attempted to achieve high SERS from the R6G Raman active probe using plasmonic patterned substrates in the presence and absence of graphene oxide. Plasmonic ladder-like patterned substrates are fabricated using laser interference lithography, which is cost effective, simple to operate, and has potential for large scale nanofabrication. By combining graphene oxide with R6G, we have found additional two time enhancement compared to that obtain from R6G alone on the plasmonic patterned substrate. Further, we have also attempted to understand the underlying mechanism to correlate the uniform and reproducible SERS through Raman mapping and Finite difference time domain computation. Our finding can potentially be applied for SERS investigation at a low molecular concentration. [ABSTRACT FROM AUTHOR]

We report on the impact of zinc doping upon the magneto-electric properties of CaBaCo4O7. We show that the presence of less than 1% Zn at the Co sites decreases TC from 64K to 48K and generates two magnetic transitions, concomitant with changes of slope of dielectric permittivity, at 54K and 77 K, respectively. Powder neutron diffraction and magnetic data show the existence of an incommensurate magnetic phase and a spin-flop transition in this temperature range. The crossover of the magnetodielectric effect of this oxide, from positive below 50K to negative above this temperature emphasizes the primordial role of the incommensurate magnetic phase in the evolution of these properties. [ABSTRACT FROM AUTHOR]

We study the transformations between amorphous phases of Si through molecular simulations using the environment dependent interatomic potential (EDIP) for Si. Our results show that upon pressure, the material undergoes a transformation from the low density amorphous (LDA) Si to the high density amorphous (HDA) Si. This transformation can be reversed by decompressing the material. This process, however, exhibits clear hysteresis, suggesting that the transformation LDA ↔ HDA is first-order like. The HDA phase is predominantly five-fold coordinated, whereas the LDA phase is the normal tetrahedrally bonded amorphous Si. The HDA phase at 400K and 20GPa was submitted to an isobaric annealing up to 800 K, resulting in a denser amorphous phase, which is structurally distinct from the HDA phase. Our results also show that the atomic volume and structure of this new amorphous phase are identical to those of the glass obtained by an isobaric quenching of the liquid in equilibrium at 2000K and 20 GPa down to 400 K. The similarities between our results and those for amorphous ices suggest that this new phase is the very high density amorphous Si. [ABSTRACT FROM AUTHOR]

We demonstrate a complete polarization analysis of soft x-rays with an energy of 1.1 keV using a free-standing W/B4C multilayer phase retarders and a beryl crystal analyzer. The W/B4C multilayer exhibits five times increase in transmission over that previously reported. The beryl crystal proves to be a suitable analyzer for the polarization analysis with a 10% s-component of reflectivity resulting in an extinction ratio close to 0.002 at 1.1 keV. The combination of multilayer phase retarders and crystal analyzers should open up the field of the soft x-ray polarization analysis in the energy range between 1 keV and 2 keV. [ABSTRACT FROM AUTHOR]

Diamond has unique properties which make it the ideal material for use in synchrotron instrumentation. X-ray optics made of diamond are almost transparent, they possess strength, and are subject to very low thermal expansion; therefore they will be able to withstand the powerful beams generated by fourth-generation light sources without compromising brilliance. For this reason, several groups are attempting fabrication of refractive lenses and zone plates made of diamond. Lithography and, in general, microfabrication technology, are the ultimate tools for the innovation of synchrotron focusing optics. We propose to combine modern silicon microtechnology with advanced deposition methods to fabricate nanocrystalline-diamond lenses for third- and fourth-generation synchrotron sources. The fabrication method is described here and microfocusing synchrotron tests are illustrated. [ABSTRACT FROM AUTHOR]

Biosensors typically operate in liquid media for detection of biomarkers and suffer from fouling resulting from nonspecific binding of protein molecules to the device surface. In the current work, using a coupled field finite element fluid-structure interaction simulation, we have identified that fluid motion induced by high intensity sound waves, such as those propagating in these sensors, can lead to the efficient removal of the nonspecifically bound proteins thereby eliminating sensor fouling. We present a computational analysis of the acoustic-streaming phenomenon induced biofouling elimination by surface acoustic-waves (SAWs) propagating on a lithium niobate piezoelectric crystal. The transient solutions generated from the developed coupled field fluid solid interaction model are utilized to predict trends in acoustic-streaming induced forces for varying design parameters such as voltage intensity, device frequency, fluid viscosity, and density. We utilize these model predictions to compute the various interaction forces involved and thereby identify the possible mechanisms for removal of nonspecifically-bound proteins. For the range of sensor operating conditions simulated, our study indicates that the SAW motion acts as a body force to overcome the adhesive forces of the fouling proteins to the device surface whereas the acoustic-streaming induced hydrodynamic forces prevent their reattachment. The streaming velocity fields computed using the finite element models in conjunction with the proposed particle removal mechanism were used to identify the optimum conditions that lead to improved removal efficiency. We show that it is possible to tune operational parameters such as device frequency and input voltage to achieve effective elimination of biofouling proteins in typical biosensing media. Our simulation results agree well with previously reported experimental observations. The findings of this work have significant implications in designing reusable, selective, and highly sensitive biosensors. [ABSTRACT FROM AUTHOR]

Sr2TiMnO6, a double perovskite associated with high degree of B-site cation disorder was investigated in detail for its structural, magnetic, and dielectric properties. Though x-ray powder diffraction analysis confirms its cubic structure, first order Raman scattering and infrared reflectivity spectra indicate a breaking of the local cubic symmetry. The magnetization study reveals an anomaly at 14 K owing to a ferrimagnetic/canted antiferromagneticlike ordering arising from local Mn-O-Mn clusters. Saturated M-H hysteresis loops obtained at 5 K also reflect the weak ferromagnetic exchange interactions present in the system and an approximate estimation of Mn3+/Mn4+ was done using the magnetization data for the samples sintered at different temperatures. The conductivity and dielectric behavior of this system has been investigated in a broad temperature range of 10 to 300 K. Intrinsic permittivity was obtained only below 100 K whereas giant permittivity due to conductivity and Maxwell-Wagner polarization was observed at higher temperatures. X-ray photoemission studies further confirmed the presence of mixed oxidation states of Mn and the valence band spectra analysis was carried out in detail. [ABSTRACT FROM AUTHOR]

Fluid motion induced on the surface of 100 MHz focused surface acoustic wave (F-SAW) devices with concentric interdigital transducers (IDTs) based on Y-cut Z-propagating LiNbO3 substrate was investigated using three-dimensional bidirectionally coupled finite element fluid-structure interaction models. Acoustic streaming velocity fields and induced forces for the F-SAW device are compared with those for a SAW device with uniform IDTs (conventional SAW). Both, qualitative and quantitative differences in the simulation derived functional parameters, such as device displacements amplitudes, fluid velocity, and streaming forces, are observed between the F-SAW and conventional SAW device. While the conventional SAW shows maximum fluid recirculation near input IDTs, the region of maximum recirculation is concentrated near the focal point of the F-SAW device. Our simulation results also indicate acoustic energy focusing by the F-SAW device leading to maximized device surface displacements, fluid velocity, and streaming forces near the focal point located in the center of the delay path, in contrast to the conventional SAW exhibiting maximized values of these parameters near the input IDTs. Significant enhancement in acoustic streaming is observed in the F-SAW device when compared to the conventional ones; the increase in streaming velocities was computed to be 352% and 216% for tangential velocities in propagation and transverse directions, respectively, and 353% for the normal velocity. Consequently, the induced streaming force for F-SAW is 480% larger than that for conventional SAW. In biosensing applications, this allows for the removal of smaller submicron sized particles by F-SAW which are otherwise difficult to remove using the conventional SAW. The F-SAW presents an order of magnitude reduction in the smallest removable particle size compared to the conventional device. Our results indicate that the acoustic energy focusing and streaming enhancement brought about by the F-SAW device manifests itself as enhanced biofouling removal efficiency of F-SAW throughout the device delay path compared to the conventional device, thereby providing enhanced device sensitivity, selectivity, and reusability. Furthermore, contrary to the conventional SAW in which the smallest particle is removable near the input IDTs, the F-SAW device removes the smallest particle near the device focal point. The results of this work are shown to have significant implications in typical biosensing and microfluidic applications. In a broader context, the results of the present study demonstrate a technique of enhancing streaming induced flows, which is of great importance to contemporary problems involving microfluidic and sensing applications of piezoelectric devices. [ABSTRACT FROM AUTHOR]

Aurivillus intergrowth Bi4Ti3O12–5BiFeO3 was demonstrated to be ferroelectric that evoked the possibility of achieving high temperature magnetoelectric property in this family of compounds. X-ray diffraction studies confirmed its structure to be orthorhombic [Fmm2; a=5.5061(11) Å, b=5.4857(7) Å, c=65.742(12) Å]. However, transmission electron microscopy established the random incidence of intergrowth at nanoscale corresponding to n=6 and n=7 members of the Aurivillius family. Diffuse ferroelectric orthorhombic to paraelectric tetragonal phase transition around 857 K was confirmed by dielectric and high temperature x-ray diffraction studies. Polarization versus electric field hysteresis loops associated with 2Pr of 5.2 μC/cm2 and coercive field of 42 kV/cm were obtained at 300 K. [ABSTRACT FROM AUTHOR]

Manganitelike double perovskite Sr2TiMnO6 (STMO) ceramics fabricated from the powders synthesized via the solid-state reaction route, exhibited dielectric constants as high as ∼105 in the low frequency range (100 Hz–10 kHz) at room temperature. The Maxwell–Wagner type of relaxation mechanism was found to be more appropriate to rationalize such high dielectric constant values akin to that observed in materials such as KxTiyNi(1-x-y)O and CaCu3Ti4O12. The dielectric measurements carried out on the samples with different thicknesses and electrode materials reflected the influence of extrinsic effects. The impedance studies (100 Hz–10 MHz) in the 180–300 K temperature range revealed the presence of two dielectric relaxations corresponding to the grain boundary and the electrode. The dielectric response of the grain boundary was found to be weakly dependent on the dc bias field (up to 11 V/cm). However, owing to the electrode polarization, the applied ac/dc field had significant effect on the low frequency dielectric response. At low temperatures (100–180 K), the dc conductivity of STMO followed a variable range hopping behavior. Above 180 K, it followed the Arrhenius behavior because of the thermally activated conduction process. The bulk conductivity relaxation owing to the localized hopping of charge carriers obeyed the typical universal dielectric response. [ABSTRACT FROM AUTHOR]

Focused interdigital transducers (F-IDTs) patterned on surfaces of piezoelectric substrates can be used to generate surface acoustic waves (SAW) with high intensity and high beam-width compression ratio. A three dimensional coupled field finite element model of a focused SAW (F-SAW) device with interdigital transducers shaped as concentric circular arcs based on a YZ LiNbO3 substrate is developed in this study. This model was utilized to investigate the effect of geometric shape of transducers on the focusing properties of F-IDTs to identify the optimal design for potential microfluidic applications. The transducer design parameters investigated in the current study include number of finger pairs, degree of arc, geometric focal length, and wavelength of F-SAW. The transient response of the device on application of impulse and ac electrical inputs at the transmitting FIDT fingers were utilized to deduce the device frequency response and propagation characteristics of F-SAWs, respectively. The influence of applied input voltage on the propagation characteristics is also investigated. The insertion loss calculated for the various F-IDT designs was used to identify the optimal transducer configuration for sensing and microfluidic applications. The focusing properties as well as the wave propagation characteristics for the various F-IDT designs were evaluated in terms of the amplitude field and displacement contours generated in regions close to and at the focal point. Comparison with a conventional SAW device operating at megahertz frequency range and uniform IDT design is also made. Our study indicates that the focusing property of the device is significantly influenced by the geometric shape of the F-IDTs. The streaming phenomenon induced by F-SAW propagation, when in contact with a fluid medium, is discussed in detail. The simulated amplitude fields generated using ac analysis for the various designs in conjunction with wave propagation parameters derived using perturbational techniques such as Campbell–Jones are utilized to calculate the streaming forces and velocities based on successive approximation technique applied to Navier–Stokes equation (Nyborg’s theory). The maximum streaming force and velocity are obtained at the focal point of the F-SAW device. The magnitude of the generated streaming force and induced streaming velocity are strongly influenced by the transducer configurations. Based on the simulation results of this study, we provide guidelines for designing various F-IDTs to suite desired applications. F-SAW devices operating with higher applied input voltages and at higher frequencies, with optimal geometric length and larger degree of arc, are best suited for actuation and fluid microtransport. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of SmTbFe17-xAlx (x=0–8) and SmTbFe17-xSix (x=0–3.5) were investigated by x-ray-diffraction and magnetization studies. All the compounds stabilize in rhombohedral structure. The lattice parameters are found to increase with Al substitution whereas they are found to decrease with Si substitution. The rates of the decrease of saturation magnetization (Ms) value with the substitution of Al and Si are almost the same. The easy direction of magnetization (EMD) is in the ab plane up to an Al concentration of x=7 and has a tendency to shift towards the c axis at x=8. All the compounds with Si are seen to have the EMD in the ab plane. Electron-magnon scattering is seen to contribute to the electrical resistivity in both Al- and Si-substituted compounds; in the temperature range of 30–60 K and at higher temperatures, scattering due to phonons is observed. [ABSTRACT FROM AUTHOR]

The electrical characteristics of a hydrogen sensor utilizing a metal/AlN/Si(111) structure, which behaves similarly to a metal-insulator-semiconductor-type capacitor, have been investigated. The AlN layer, which acts as the insulator, was deposited by plasma source molecular-beam epitaxy. The device employed alternating gates of Pd and Al, and an Al back contact, all grown by magnetron sputtering. The Pd gate structures were found to be sensitive to hydrogen, down to parts per million concentrations in the surrounding flow, while the Al gate structures showed no response to hydrogen. Both structures show similar capacitance versus voltage as well as conductance versus voltage curves. Therefore the combination of Al and Pd gates can be utilized in a balanced hydrogen sensor structure. Furthermore, the frequency dependence of the capacitance and conductance curves clearly indicated the presence of series resistance effects, but with multiple time constants, requiring a complex equivalent circuit representation. Evidence was found for the presence of carrier traps at the AlN/Si interface, with energy levels distributed throughout the silicon band gap, and with an estimated surface density ranging between 8×10[sup 14] and 1.5×10[sup 15] m[sup -2] eV[sup -1]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This study describes the relative effect on the permanent magnet characteristics viz. remanence (B,.), intrinsic coercivity (H[SUBci]), Curie temperature (T[SUBc]), and rectangularity of the intrinsic demagnetization curve, when A1, Cu, Ga, and Nb are added individually to NdFeB. Each elemental addition causes significant improvement in H[SUBci] but the level of improvement differs from one additive element to the other. The addition of Nb is favored over other elements for realizing maximum enhancement in H[SUBci] and rectangularity of the demagnetization curve. The microstructural features of the sintered samples of NdFeB with elemental addition show the formation of a new phase, in addition to the phases (φ η, and Nd-rich) generally found in the ternary sample. The factors influencing the permanent magnet characteristics of sintered samples are the distribution of the Nd-rich phase in the intergranular region, the size and distribution of the minor phases at the grain junctions, the formation and distribution of new phases due to alloying addition, and the solubility of the dopant element in various phases coexisting in the sample. [ABSTRACT FROM AUTHOR]

A study on the impact of TiN deposition on thin HfO2 films formed on Si substrates is conducted using x-ray photoelectron spectroscopy and a monoenergetic positron beam. For the predeposition sample, the positrons implanted into Si are found to diffuse toward the HfO2/Si interface under the influence of the electric field, suggesting the presence of negative charges in HfO2.

Deals with a study which investigated the magnetic, structural and electrical properties of iron-rich rare-earth intermetallic compounds. Discussion on x-ray diffraction; Methodology of the study; Results and discussion.

Deals with the electrical resistivity studies that have been carried out on the C 15-type Laves phase intermetallic compounds and their hydrides in the temperature range 16-300 K. Experimental details; Results and discussion; Conclusion.

Presents a study that performed an extensive series of measurements on symmetrical barrier bound-to-continuum and asymmetrical barrier bound-to-bound quantum well infrared photodetectors consisting of only a single well. Behavior of the optical and transport properties; Behavior of the usual multiquantum well detectors; Factor that allowed for an accurate calculation of the potential drops across each barrier, the photoinduced carrier depletion in the quantum well.

Presents a series of measurements and theoretical analysis on an undoped single-quantum-well infrared photodetector. Description of the structure; Description of the responsivity spectrum; Presentation of a theoretical calculation of the voltage drops.

Presents information on a study which investigated the magnetic, magnetomechanical and electrical properties of the terbium dysprosium iron cobalt composition. Experimental procedure; Results; Discussion.

Focuses on a study which investigated the magnetic and electrical properties of pseudobinary alloys using x-ray diffraction studies. Information on pseudobinary alloys; Methodology of the study; Results and discussion.

Deals with a study which analyzed iron Mossbauer measurements on Ho[sub0.85]Tb[sub0.15]Fe[sub2]H[subx] and Dy[sub0.73]Tb[sub0.27]Fe[sub2]H[subx]. Temperature variation; Quadrupole coupling constant; Isomer shift.

Presents a study which analyzed from a scattering theory viewpoint the time delays, dwell times, resonant state lifetimes and electron sheet densities associated with tunneling through quantum well structures. Relation between the time delays and the dwell time matrix; Relation between the dwell time matrix and sheet density; Resonant state effects.

Presents a study which examined the theoretical basis of the Tsu-Esaki tunneling current formula. Description of the tunneling current formula; Details on antisymmetrization and matrix elements; Expressions for the left-incoming and right-incoming wave functions with energy E in the tunneling direction.

Investigates the lateral degrees of freedom in resonant tunneling heterostructure devices. Relaxation of spatially non-uniform configurations; Discussion on the possibility of non-uniform configurations with spatially localized oscillation; Formulation of the model.

Presents a study which examined the effect of hydrogen on the magnetic properties of holmium[sub0.85]terbium[sub0.15]iron[sub2] and dysprosium[0.73]terbium[0.27]iron[sub2]. Magnetization measurements; Experimental details; Results and discussion.

Focuses on a study which obtained the lattice parameters of alloys of dysprosium, terbium, iron and nickel. Methods; Results and discussion; Conclusions.

Deals with ac-susceptibility study on the intermetallic compounds Zr[sub1-x]Ho[subx]Co[sub2]. Description of the magnetic transition; Spin reorientation; Effect of hydrogen absorption on the exchange interactions.

Presents a study that examined Cubic Laves phase alloys of holmium-terbium-iron-nickel. Importance of rare earth and transition metals; Information on powder-x-ray diffractograms.

Presents a study that measured the magnetostriction and magnetochemical coupling coefficient on Ho[sub0.85]Tb[sub0.15]Fe[sub2-x]Co[subx] alloys. Impact of the increase of cobalt concentration on saturation magnetostriction and magnetomechanical coupling coefficient; Determination of magnetomechanical coupling coefficient; Change in the easy direction of magnetization.

We determine the electronic properties and dielectric response of zirconia (ZrO2) with oxygen vacancies (O vacancies) and Ti doping using first-principles density functional theory calculations based on pseudopotentials and a plane wave basis. We find significantly enhanced static dielectric response in zirconia with Ti doping and introduction of oxygen vacancies. Softening of phonon modes are responsible for the enhanced dielectric response of doped samples compared to pure zirconia. [ABSTRACT FROM AUTHOR]

The effect of Al on the structural and magnetic properties of Pr3(Fe1-xAlx)27.5Ti1.5 (x=0.1, 0.2, and 0.3) compounds has been investigated by x-ray-diffraction, magnetization, and Curie temperature measurements. All these compounds have formed in monoclinic symmetry with A2/m space group with Pr(Fe,Ti)12 as secondary phase and traces of α-Fe. The monotonic decreases in MS both at 300 and 80 K are discussed on the basis of electron transfer from the 3p band of Al to the 3d band of Fe near EF. The variations in TC are explained using the Friedel approach. In all the compounds, the easy magnetization direction is away from the b axis. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of (Sm0.9Pr0.1)3(Fe1-xAlx)27.5Ti1.5 [x=0.1, 0.2, 0.3] compounds have been investigated by x-ray diffraction, magnetization, and Curie temperature measurements. The lattice parameters are found to increase with increasing Al concentration. The saturation magnetization is seen to decrease with the substitution of Al and is attributed to the modification in the DOS by Al. The Curie temperature increased for low concentrations of Al and then decreased. The easy direction of magnetization (EMD) in all the compounds is normal to the (204) plane. The anisotropy fields decrease with increasing Al concentration. [ABSTRACT FROM AUTHOR]