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arXiv2019

Magnetoresistance in Metallic Ferroelectrics

Polar metals with ferroelectric-like displacements in metals have been achieved recently, half century later than Anderson and Blount's prediction. However, the genuine ferroelectricity with electrical dipolar switching has not yet been attained experimentally in the conducting materials, especially the ones possessing magnetic responses. Here we report the coexistence of ferroelectricity and magnetoresistance (MR) in the metallic PbNb0.12Ti0.88O3 (PNTO) thin films. We found that the conducting and magnetic responses of PNTO films are highly asymmetric. Negative MR up to 50% is observed under an in-plane magnetic field; the MR switches to positive with the magnetic field applied parallel to the surface normal. Such unique behavior is attributed to the moving electron caused effective magnetic field which couples with the spins of electrons, which form a dynamic multiferroic state in the metallic PNTO. These findings break a path to multiferroic metal and offer a great potential to the multi-functional devices.

cond-mat.mtrl-sci↗

arXiv2019

Calibration and electric characterization of p-MNOS RADFETs at different dose rates and temperatures

This paper describes the radiation response and I-V characteristics of the stacked p-MNOS based RADFETs measured at different dose rates and irradiation temperatures. It is shown that the enhanced charge trapping takes place at the interface of the thick gate dielectrics in the MNOS transistors at low dose rates (ELDRS). The sensitivity of the radiation effect to irradiation temperature has also experimentally revealed. We associate both effects with the temperature and dose rate dependence of the effective charge yield in the thick oxides described within the framework of the previously proposed model. We have also simulated the I-V characteristics of the transistors for different total doses and irradiation conditions. It has been found the used electric and radiation models consistently describe the observed dependencies of the RADFETs sensitivity on dose rates and irradiation temperatures for the devices with different thickness of insulators.

physics.app-ph↗

arXiv2019

Acoustical hooks: a new subwavelength self-bending beam

In this work, we report the observation of a new type of near-field curved acoustic beam different from the Airy-family beams both through simulations and experiment. This new self-bending acoustical beam is generate from a rectangular trapezoid of a polymer material immersed in water and has unique features. The radius of curvature of acoustical hook is less than the wavelength, it is represents the smallest radius of curvature ever recorded for any acoustical beams. The origin of this curved beam is in the vortices of intensity flow that appear inside the solid due to the conversion of the incident longitudinal wave mode to a shear wave in a solid. These vortices redirect the intensity flow, which causes the beam to bend. These results may be potentially useful when an object, that is in the path of the beam, must be avoid, such as in cancer treatments in which tumors are found behind bones such as ribs. It could also have potential applications in particle manipulations.

physics.app-ph↗

arXiv2019

Control of surface states of planar metamaterial based on moire effect

The possibility of continuous tuning of the spectral properties of two types of planar metamaterials based on the moire effect by changing their geometric parameters is demonstrated both experimentally and numerically. It is shown that for a one-dimensional moire metamaterial obtained by superposition of two microstrip photonic crystals with close periods, the position of the stop band in the spectrum can be controlled by changing these periods. For the two-dimensional moire metamaterial formed by two identical periodic crossed structures with hexagonal symmetry, the ability to control the frequency of the surface state mode by changing the crossing angle of these structures relative to each other has been experimentally and numerically shown. It is numerically demonstrated that if the moire metamaterial is irradiated by the horn antenna, a surface wave propagating in the metamaterial plane appears in all directions beginning from its intersection point with the axis of the incident wave beam. From the application point of view, moire metamaterials of this type can be considered as promising prototype of microwave filters, whose spectral properties can be continuously and smoothly mechanically rearranged.

physics.app-ph↗

arXiv2019

Si-based GeSn photodetectors towards mid-infrared imaging applications

This paper reports a comprehensive study of Si-based GeSn mid-infrared photodetectors, which includes: 1) the demonstration of a set of photoconductors with Sn compositions ranging from 10.5% to 22.3%, showing the cut-off wavelength has been extended to 3.65 um. The measured maximum D* of 1.1x10^10 cmHz^(1/2)W(-1) is comparable to that of commercial extended-InGaAs detectors; 2) the development of surface passivation technique on photodiode based on in-depth analysis of dark current mechanism, effectively reducing the dark current. Moreover, mid-infrared images were obtained using GeSn photodetectors, showing the comparable image quality with that acquired by using commercial PbSe detectors.

physics.app-ph↗

arXiv2019

Field-free switching of a Spin-orbit torque device aided by interlayer-coupling induced domain walls

The spin-orbit torque device is promising as a candidate for next generation magnetic memory, while the static in-plane field needed to induce deterministic switching is a main obstacle for its application in highly integrated circuits. Instead of introducing effective field into the device, in this work we present an alternative way to achieve the field-free current-driven magnetization switching. By adding Tb/Co multilayers at two ends of the current channel, assisting domain wall is created by interlayer exchange coupling. The field-free deterministic switching is achieved by the movement of the domain wall driven by current. By loop shift measurement we find the driven force exerted on the domain wall is determined by the direction of the in-plane moment in the domain wall. Finally, we modify the device into a synthetic antiferromagnetic structure to solve the problem of reading out signal. The present work broadens the choice of field-free spin-orbit torque device design and clearly depicts the difference between two different switching mechanism.

physics.app-ph↗

arXiv2019

Sensor properties of Pd doped SnO2 nanofiber enshrouded with functionalized MWCNT

In this work, tin oxide (SnO2)/multi-walled carbon nanotube (MWCNT) composite based ethanol sensor that exhibits fast response/recovery behaviour, large sensitivity, and good selectivity was demonstrated. First, Pd doped SnO2 (SnO2-Pd) nanofibers was prepared on the interdigitated electrodes (IDE) using electrospinning technique, followed by calcination at 600 0C for 2 h. Then, carboxylic acid modified multi-walled carbon nanotube (MWCNT-COOH) was dropped on the nanofibers and allowed to dry at RT. Next, the thin films were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Finally, chemical sensing behaviors of the sensors were analyzed against to ethanol (ETH), toluene (TOL), trichloroethylene (TCE), acetone (ACE) and binary mixing with water. It was demonstrated that increasing the temperature and gas concentration has led to increased sensor response for SnO2-Pd nanofiber system. However, there was no general rule found for MWCNT-COOH coated SnO2-Pd nanofiber system. A novel sensing mechanism was offered for the composite structure.

physics.app-ph↗

arXiv2019

Reconfigurable Electromagnetic Structures: X-Band and Beyond

Developing reconfigurable millimeter-wave (mmWave) antennas and devices is an outstanding challenge, with switch technologies being a primary impediment. Recently, it has been shown that vanadium dioxide (VO2), a thermochromic material whose resistance changes with temperature, could provide a path forward in developing reconfigurable mmWave devices. As an initial step towards this vision, we investigate the integration of VO2 switches in reconfigurable components at 15 GHz. In particular, a frequency reconfigurable antenna and a reconfigurable phase shifter are shown. The low loss and minimal parasitics of VO2 technology have the potential to enable devices at 15 GHz and beyond.

physics.app-ph↗

arXiv2019

Surface permeability and surface flow tortuosity of particulate porous media

The dispersion process in particulate porous media at low saturation levels takes place over the surface elements of constituent particles and, as we have found previously by comparison with experiments, can be accurately described by super-fast non-linear diffusion partial differential equations. To enhance the predictive power of the mathematical model in practical applications, one requires the knowledge of the effective surface permeability of the particle-in-contact ensemble, which can be directly related with the macroscopic permeability of the particulate media. We have shown previously that permeability of a single particulate element can be accurately determined through the solution of the Laplace-Beltrami Dirichlet boundary-value problem. Here, we demonstrate how that methodology can be applied to study permeability of a randomly packed ensemble of interconnected particles. Using surface finite element techniques we examine numerical solutions to the Laplace-Beltrami problem set in the multiply-connected domains of interconnected particles. We are able to rigorously estimate tortuosity effects of the surface flows in a particle ensemble setting.

cond-mat.soft↗

arXiv2019

Potential Dependent Ionic Sieving Through Functionalized Laminar MoS2 Membranes

Laminar MoS2 membranes show outstanding potential for practical applications in energy conversion/storage, sensing, and as nanofluidic devices. For water purification technologies, MoS2 membranes can form abundant nanocapillaries from layered stacks of exfoliated MoS2 nanosheets. These MoS2 membranes have previously demonstrated excellent ionic rejection with high water permeation rates, as well as long-term stability with no significant swelling when exposed to aqueous or organic solvents. Chemical modification of these MoS2 membranes has been shown to improve their ionic rejection properties, however the mechanism behind this improvement is not well understood. To elucidate this mechanism we report the potential dependant ion transport through functionalized MoS2 membranes. The ionic permeability of the MoS2 membrane was transformed by chemical functionalization with a simple naphthalene sulfonate dye (sunset yellow) and found to decrease by over a factor of ~10 compared to the pristine MoS2 membranes and those reported for graphene oxide and Ti3C2Tx (MXene) membranes. The effect of pH, solute concentration, and ionic size/charge on the ionic selectivity of the functionalized MoS2 membranes is also reported. The potential dependant study of these dye functionalized MoS2 membranes for ionic sieving with charge selectivity should enable future applications in electro-dialysis and ion exchange for water treatment technologies.

physics.app-ph↗

arXiv2019

Many-body effects in strongly-disordered III-nitride quantum wells: interplay between carrier localization and Coulomb interaction

The joint impact of Anderson localization and many-body interaction is observed in the optical properties of strongly-disordered III-nitride quantum wells, a system where the Coulomb interaction and the fluctuating potential are pronounced effects with similar magnitude. A numerical method is introduced to solve the 6-dimensional coupled Schrodinger equation in the presence of disorder and Coulomb interaction, a challenging numerical task. It accurately reproduces the measured absorption and luminescence dynamics of InGaN quantum wells at room-temperature: absorption spectra reveal the existence of a broadened excitonic peak, and carrier lifetime measurements show that luminescence departs from a conventional bimolecular behavior. These results reveal that luminescence is governed by the interplay between localization and Coulomb interaction, and provide practical insight in the physics of modern light-emitting diodes.

cond-mat.mtrl-sci↗

arXiv2019

Characterisation of Li in the surface film of a corrosion resistant Mg-Li(-Al-Y-Zr) alloy

The surface film formed upon Mg-Li(-Al-Y-Zr) following aqueous immersion and air-exposure was investigated. This alloy (which contains 30.3 at. % Li) possesses a bcc crystal structure and has been reported as being corrosion resistant. It was determined that the principal components of the surface film were Li2CO3 and Mg(OH)2 as characterised by grazing incidence X-ray diffraction (GIXRD). The detection of hcp grains near the alloy surface was observed by GIXRD and selected area electron diffraction (SAED). The spatial distribution of Li and Mg in the surface film was characterised by electron energy loss spectroscopy (EELS) and the distribution of other major elements in the alloy was characterised by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDXS). It was observed that Li was distributed throughout the alloy surface film and with an elevated concentration in the so-called outer layer.

physics.app-ph↗

arXiv2019

Geometric localization in supported elastic struts

Localized deformation patterns are a common motif in morphogenesis and are increasingly finding widespread applications in materials science, for instance as memory devices. Here we describe the emergence of spatially localized deformations in a minimal mechanical system by exploring the impact of growth and shear on the conformation of a semi-flexible filament connected to a pliable shearable substrate. We combine numerical simulations of a discrete rod model with theoretical analysis of the differential equations recovered in the continuum limit to quantify (in the form of scaling laws) how geometry, mechanics, and growth act together to give rise to such localized structures in this system. We find that spatially localized deformations along the filament emerge for intermediate shear modulus and increasing growth. Finally, we use experiments on a 3D printed multi-material model system to demonstrate that external control of the amount of shear and growth regulates the spatial extent of the localized strain texture.

cond-mat.soft↗

arXiv2019

Simulation and experimental studies of induction hardening behavior of a new medium-carbon, low-alloy wear resistance steel

Flux2D commercial software together with a Gleeble thermomechanical simulator has been employed to numerically and physically simulate the material properties profile of an induction hardened slurry transportation pipe made of a recently developed 0.4 wt.% C, Nb-microalloyed steel. After calculating the thermal history of a 400 mm diameter, 10 mm thick pipe at various positions through the thickness, different heating and cooling paths were physically simulated using the Gleeble machine to predict the through-thickness material microstructure and hardness profiles. The results showed that by coupling a phase transformation model considering the effect of heating rate on the austenite transformation temperatures which allows calculations for arbitrary cooling paths with calculated induction heating and quenching thermal cycles, it has been possible to design induction hardening parameters for a slurry transport pipe material.

physics.app-ph↗

arXiv2019

Transient Analysis during Maximum Power Point Tracking (TrAMPPT) to Assess Dynamic Response of Perovskite Solar Cells

Determination of the device performance parameters of perovskite solar cells is far from trivial as transient effects may cause large discrepancies in current-voltage measurements as a function of scan rate and pre-conditioning. Maximum power point tracking, MPPT, enables to determine the steady-state maximum power conversion efficiency. However, the MPPT does not provide any information on the device performance parameters, which are reliable only if extracted from current-voltage curves collected under steady-state conditions. We show that is possible to determine the shorter settling or delay time suitable to carry out J-V measurements under steady-state conditions by analysis of the transient device response around the MPP. This procedure proves to be more time-efficient than measurement J-V measurements at a variety of scan rates. Furthermore, the generic algorithm presented here can be implemented to assess changes in the dynamic response of devices during long-term device ageing.

physics.app-ph↗

arXiv2019

Performance Considerations of Thin Ferroelectrics (~10 nm HfO2, ~20 nm PZT) FDSOI NCFETs for Digital Circuits at Reduced Power Consumption

The paper presents simulation study of thin ferroelectrics (Si doped HfO2, PZT) PGP FDSOI NCFETs at circuit level for high performance, low VDD low-power digital circuits. The baseline PGP FDSOI MOSFET has 20 nm metal gate length with supply voltage varying from 0.5 V to 0.9 V. The circuits studied were 3-stage CMOS ring oscillator, NAND-2 and NOR-2 gates at a frequency of 20 GHz. The paper shows that HfO2 FDSOI NCFET based NAND-2 gates can provide significant reduction in average power consumption, which was ~66% that of baseline FDSOI MOSFET based NAND-2 gates for comparable performance. For the same performance, the average power consumption for PZT FDSOI NCFET based NAND-2 gate was ~86% that of baseline FDSOI MOSFET based NAND-2 gate. The power-delay product of HfO2 FDSOI NCFET based gates was found to be ~24% lower than baseline FDSOI MOSFET based gates and that of PZT FDSOI NCFET based gates was found to be ~21% less than that of baseline FDSOI MOSFET based gates. The performance of HfO2 FDSOI NCFET based gates with increased fan-in and fan-out was also found to be superior to PZT FDSOI NCFET based gates and baseline FDSOI MOSFET based gates.

physics.app-ph↗

arXiv2019

Extending the Near-infrared Band-edge Absorption Spectrum of Silicon by Proximity to a 2D Semiconductor

Because of its low-cost, silicon is the standard material for photovoltaic conversion. Yet, its band-edge absorption spectrum is narrower than the spectrum of the solar radiation, which reduces its conversion efficiency. In this paper, it is shown that the spectrum of absorbance of silicon can be extended to longer wavelengths by proximity to a two-dimensional (2D) semiconductor. Photo-induced Hall effect, together with standard absorption spectroscopy, was employed to estimate the increase of efficiency of absorbance of a 2D-platinum-diselenide/intrinsic-silicon bilayer. The bilayer shows a significantly higher absorption in the infrared as compared to the single films. Moreover, an overall increase of absorption efficiency by a factor twenty was measured in the entire spectrum of light of a halogen lamp. X-ray Photoelectron Spectroscopy (XPS) confirms that a reduction of the band-gap occurs in the silicon substrate at the interface between the two semiconductors. The results are interpreted in the framework of band-gap narrowing due to hole-plasma confinement in the Si, induced by electron-confinement in the 2D film. Possible application of the effect in photo-voltaic cells is discussed.

physics.app-ph↗

arXiv2019

Mn(II)-doped 2D perovskite for light emitting devices

Low dimensional perovskites are considered good candidates for light emitting applications given the high exciton binding energy which should in principle improve the radiative recombination efficiency. Yet, single-layered two-dimensional (2D) perovskite films are strongly limited by trap-assisted recombination and suffer from low luminescence yields, hampering their application in electroluminescence devices. Here, we use ad hoc synthetic and defect engineering strategies to overcome such issue. We employ metallic doping to controllably introduce luminescent impurities in a matrix made of 2D perovskite $NMA_{2}PbX_{4}$ based on the cation NMA = 1-naphtylmethylammonium. By means of temperature-dependent and time-resolved spectroscopy we demonstrate efficient energy transfer to $Mn^{2+}$ centres. Such process avoids the funnelling of the photo-excited species in inefficient recombination channels represented by intra-gap trap states and enhances photoluminescence, with quantum yield surpassing 20% in doped films. Eventually, we embody Mn-doped $NMA_{2}PbBr_{4}$ in a light emitting diode architecture and show, for the first time, electroluminescence from the $Mn^{2+}:^{4}T_{1}-^{6}A_{1}$ transition. This proof-of-concept demonstration shows the potential of doping in layered perovskites and prompt for the study of a wider range of host/guest structures.

physics.app-ph↗