Dipolar Effects Research Articles

Berry curvature dipole and nonlinear Hall effect in two-dimensional Nb2n+1SinTe4n+2

Recent experiments have demonstrated interesting physics in a family of two-dimensional composition-tunable materials ${\mathrm{Nb}}_{2n+1}{\mathrm{Si}}_{n}{\mathrm{Te}}_{4n+2}$. Here we show that, owing to their intrinsic low symmetry, metallic nature, tunable composition, and ambient stability, these materials offer a good platform for studying the Berry curvature dipole (BCD) and nonlinear Hall effect. Using first-principles calculations, we find that the BCD exhibits pronounced peaks in monolayer ${\mathrm{Nb}}_{3}{\mathrm{SiTe}}_{6}$ (the $n=1$ case). Its magnitude decreases monotonically with $n$ and completely vanishes in the $n\ensuremath{\rightarrow}\ensuremath{\infty}$ limit. This variation manifests a special hidden dimensional crossover of the low-energy electronic states in this system. The resulting nonlinear Hall response from the BCD in these materials is discussed. Our work reveals pronounced geometric quantities and nonlinear transport physics in ${\mathrm{Nb}}_{2n+1}{\mathrm{Si}}_{n}{\mathrm{Te}}_{4n+2}$ family materials, which should be readily detected in experiment.

Detecting nuclear spins in an organosilane monolayer using nitrogen-vacancy centers for analysis of precursor self-assembly on diamond surface

We demonstrated the correlation spectroscopy of organosilane monolayers using an ensemble of shallow nitrogen-vacancy centers as the quantum sensor. Several types of organosilane monolayers were grown directly on the diamond surface by exposing the surface to a silane precursor vapor. The feasibility of detecting 1H and 19F in the monolayer was examined by correlation spectroscopy measurements. The effect of the magnetic dipole–dipole interaction on the peak width was also discussed by comparing the spectrum of the monolayer with that of surface-attached 1H and that of immersion oil. The results highlight the feasibility of nitrogen-vacancy centers as the spin probe for physicochemical analyses of monolayers grown on the diamond surface.

Quantitative magnetization transfer MRI unbiased by on-resonance saturation and dipolar order contributions.

To demonstrate the bias in quantitative MT (qMT) measures introduced by the presence of dipolar order and on-resonance saturation (ONRS) effects using magnetization transfer (MT) spoiled gradient-recalled (SPGR) acquisitions, and propose changes to the acquisition and analysis strategies to remove these biases. The proposed framework consists of SPGR sequences prepared with simultaneous dual-offset frequency-saturation pulses to cancel out dipolar order and associated relaxation (T1D ) effects in Z-spectrum acquisitions, and a matched quantitative MT (qMT) mathematical model that includes ONRS effects of readout pulses. Variable flip angle and MT data were fitted jointly to simultaneously estimate qMT parameters (macromolecular proton fraction [MPF], T2,f , T2,b , R, and free pool T1 ). This framework is compared with standard qMT and investigated in terms of reproducibility, and then further developed to follow a joint single-point qMT methodology for combined estimation of MPF and T1 . Bland-Altman analyses demonstrated a systematic underestimation of MPF (-2.5% and -1.3%, on average, in white and gray matter, respectively) and overestimation of T1 (47.1 ms and 38.6 ms, on average, in white and gray matter, respectively) if both ONRS and dipolar order effects are ignored. Reproducibility of the proposed framework is excellent (ΔMPF = -0.03% and ΔT1 = -19.0 ms). The single-point methodology yielded consistent MPF and T1 values with respective maximum relative average bias of -0.15% and -3.5 ms found in white matter. The influence of acquisition strategy and matched mathematical model with regard to ONRS and dipolar order effects in qMT-SPGR frameworks has been investigated. The proposed framework holds promise for improved accuracy with reproducibility.

Dipole-lattice nanoparticle resonances in finite arrays.

We investigate how the periodic lattices define the collective optical characteristics of the silicon and titanium nanoparticle arrays. We examine the effects of dipole lattice on the resonances of optical nanostructures, including those made of lossy materials, such as titanium. Our approach involves employing coupled-electric-magnetic-dipole calculations for finite-size arrays, as well as lattice sums for effectively infinite arrays. Our model shows that the convergence to the infinite-lattice limit is faster when the resonance is broad, requiring fewer array particles. Our approach differs from previous works by altering the lattice resonance through modifications in the array period. We observed that a higher number of nanoparticles is necessary to achieve convergence to the infinite-array limit. Additionally, we observe that the lattice resonances excited next to higher diffraction orders (such as second order) converge more quickly toward the ideal case of an infinite array than the lattice resonances related to the first diffraction order. This work reports on the significant advantages of using a periodic arrangement of lossy nanoparticles and the role of collective excitation in enhancing response from transition metals, such as titanium, nickel, tungsten, and so on. The periodic arrangement of nanoscatterers allows for the excitation of strong dipoles, boosting the performance of nanophotonic devices and sensors by improving the strength of localized resonances.

Multiferroic properties induced by defect dipoles in thin Ca3Mn2O7 films at room temperature

Improper ferroelectrics are a class of materials with the potential to design multiferroic properties. In this paper, we report on the effect of defect dipoles on the ferromagnetic and ferroelectric properties of Ca3Mn2O7 improper ferroelectric thin films. Two kinds of defects dipoles, Mn3+-oxygen vacancy (Mn3+-VO∙∙) and Li+-Al3+ doping, were studied. The experimental results show that both Mn3+-VO∙∙ and Li+-Al3+ defect dipoles can introduce ferromagnetism in the films, and Li+-Al3+ defect dipole can also improve ferroelectricity with self-polarization. The first-principle calculations based on local density approximation demonstrate the local lattice distortion, including title MnO6 octahedron and off-center displacement of Mn ion, plays crucial role in multiferroic properties of the film. This work gives an effective method to design the ferroelectricity and ferromagnetic properties in improper ferroelectric oxides with layered structure through constructing defect dipoles.

Multi-Technique Approach for Work Function Exploration of Sc2O3 Thin Films.

Thin films based on scandium oxide (Sc2O3) were deposited on silicon substrates to investigate the thickness effect on the reduction of work function. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), energy dispersive X-ray reflectivity (EDXR), atomic force microscopy (AFM), and ultraviolet photoelectron spectroscopy (UPS) measurements were performed on the films deposited by electron-beam evaporation with different nominal thicknesses (in the range of 2-50 nm) and in multi-layered mixed structures with barium fluoride (BaF2) films. The obtained results indicate that non-continuous films are required to minimize the work function (down to 2.7 eV at room temperature), thanks to the formation of surface dipole effects between crystalline islands and substrates, even if the stoichiometry is far from the ideal one (Sc/O = 0.38). Finally, the presence of BaF2 in multi-layered films is not beneficial for a further reduction in the work function.

Long-range electrostatic contribution to electron-phonon couplings and mobilities of two-dimensional and bulk materials

Charge transport plays a crucial role in manifold potential applications of two-dimensional materials, including field-effect transistors, solar cells, and transparent conductors. At most operating temperatures, charge transport is hindered by scattering of carriers by lattice vibrations. Assessing the intrinsic phonon-limited carrier mobility is thus of paramount importance to identify promising candidates for next-generation devices. Here we provide a framework to efficiently compute the drift and Hall carrier mobility of two-dimensional materials through the Boltzmann transport equation by relying on a Fourier-Wannier interpolation. Building on a recent formulation of long-range contributions to dynamical matrices and phonon dispersions [Phys. Rev. X 11, 041027 (2021)], we extend the approach to electron-phonon coupling including the effect of dynamical dipoles and quadrupoles. We identify an unprecedented contribution associated with the Berry connection that is crucial to preserve the Wannier-gauge covariance of the theory. This contribution is not specific to two-dimensional crystals, but also concerns the three-dimensional case, as we demonstrate via an application to bulk SrO. We showcase our method on a wide selection of relevant monolayers ranging from ${\mathrm{SnS}}_{2}$ to ${\mathrm{MoS}}_{2}$, graphene, BN, InSe, and phosphorene. We also discover a nontrivial temperature evolution of the Hall hole mobility in InSe whereby the mobility increases with temperature above 150 K due to the Mexican-hat electronic structure of the InSe valence bands. Overall, we find that dynamical quadrupoles are essential and can impact the carrier mobility in excess of 75%.

How Do Surface Polar Molecules Contribute to High Open-Circuit Voltage in Perovskite Solar Cells?

To date, the improvement of open-circuit voltage (VOC ) offers a breakthrough for the performance of perovskite solar cells (PSCs) toward their theoretical limit. Surface modification through organic ammonium halide salts (e.g., phenethylammonium ions PEA+ and phenmethylammonium ions PMA+ ) is one of the most straightforward strategies to suppress defect density, thereby leading to improved VOC . However, the mechanism underlying the high voltage remains unclear. Here, polar molecular PMA+ is applied at the interface between perovskite and hole transporting layer and a remarkably high VOC of 1.175V is obtained which corresponds to an increase of over 100mV in comparison to the control device. It is revealed that the equivalent passivation effect of surface dipole effectively improves the splitting of the hole quasi-Fermi level. Ultimately the combined effect of defect suppression and surface dipole equivalent passivation effect leads to an overall increase in significantly enhanced VOC . The resulted PSCs device reaches an efficiency of up to 24.10%. Contributions are identified here by the surface polar molecules to the high VOC in PSCs. A fundamental mechanism is suggested by use of polar molecules which enables further high voltage, leading ways to highly efficient perovskite-based solar cells.

Simulated FMR to study in plane magnetic anisotropy of 3x3 array constituted by square base nickel nanopillars

A study about the magnetic anisotropy in plane of 3x3 arrays constituted by square base nickel nanopillars (NPs), with D = 30 nm width and L = 120 nm long, is presented. Variations in the effective anisotropy, due to the hard dipolar interactions, were provoked by modification in the interpillar distance, a-D = 5, 10, 20 and 50 nm. The data to study the anisotropy field, HA, was collected from micromagnetic simulation performed by means of a ferromagnetic resonance (FMR) standard problem using the OOMMF package. The interpillar distance (face to face) was varied from the lower value, a-D = 5 nm where the dipolar interactions are very strong, to the maximum value, a-D = 50 nm were no longer significant are the dipole effects (to rescue the behavior of a single NP). Based on the isolated nanopillar symmetry, the initial proposed model for the collected data was the tetragonal anisotropy. The data showed a breaking of this symmetry for a-D ≤ 20 nm due to strong dipole interpillars interactions. Taking into account the array characteristics and the effects of local magnetization configuration at the corners of NPs, it was possible to propose a new model. The results are well described by the proposed model for all interpillar distance and, for a-D = 50 nm, the model coincides with the tetragonal symmetry compatible with the single pillar.

Take Advantage of the N‐Nucleophilicity of Imine in Catalytic Cyclization Reactions

AbstractImines, or Schiff bases are basic and important synthons in modern chemical synthesis, which are widely used in methodological studies, synthesis of natural products or pharmaceutical agents, construction of organic porous materials and relative late‐stage modifications. In most cases, the imines often act as “electrophilic reagents” in reactions on the basis of their C‐electrophilicity owing to the dipole effect of the C=N bond. However, on the other hand, reactions initiated by the N‐nucleophilicity of imine, draw relatively less attentions. In this concept article, we try to give a concise summary of the reactions, especially the cyclization reaction that are initiated by the N‐nucleophilicity of imine. The basic measurement data and discussions of the N‐nucleophilicity of different imines, relative possible mechanisms, catalytic strategies, and reaction types in this context are briefly discussed. We hope the imines, as one of the basic chemical synthons in organic synthesis, will be utilized in broader scope in the future by taking advantage of their N‐nucleophilicity.

Modelling the effect of dipole ordering on charge-carrier mobility in organic semiconductors

The hopping mobility in organic semiconductors is strongly influenced by the correlated on-site noise — i.e. the dipole–dipole interaction between neighbouring molecules. In this paper, we use Kinetic Monte Carlo (KMC) to study the effect of dipole moment and structural order on the hole mobility of organic molecular semiconductors. While the effect on the charge mobility of the dipole–dipole interactions can be approximately reproduced by a global Gaussian disorder model, our present results show that there are non-trivial local effects that are not incorporated into such an approach. The dipole–dipole interactions give rise to large energy barriers that restrict charged particles to smaller and smaller regions as the dipole magnitude increases. While this effect can only reduce the mobility in any given system, we note that if the dipoles self-organize, potentially driven by the dipole moment itself or by an increase in packing density, the negative effect on the mobility can be reduced.

Thermal performance analysis of magnetized convectively heated ferromagnetic nanoparticles for radiative cross nanofluid flow

The main purpose of this work is to investigate the innovation of flow of two-dimensional (2D) ferrofluid flow passing upon stretched surface having the effect of a magnetic dipole. The said problem is selected since its use is very common in biomedical as well as engineering applications together with systems of drug targeting under magnetic effects. The impacts of magnetic dipole along with radiation parameter in flow of ferromagnetic cross fluid for a stretched region are investigated in detail by use of suitable similarity variables the nonlinear, ODEs are obtained. The desired ODEs are resolved numerically by using RK fifth-order method parallel to shooting scheme. Impacts of ferromagnetic interaction, viscous dissipation, Curie temperature, and Buongiorno parameters with convective boundary conditions are perceived for velocity, temperature and concentration fields. Furthermore, the terms like velocity, mass transfer and thermal gradients are under consideration and pictorial scrutinizing is done. Moreover, temperature of ferromagnetic fluid upsurges for enlargement in the values of ferrohydrodynamics interaction and Curie temperature parameters. Concentration of ferrofluid reveals a contrary tendency against the parameters of thermophoresis and Brownian motion. Owing to extensive application prospects of magnetized nanofluids, this area has received interest from engineers and researchers; nanofluids have a noteworthy preference over ordinary fluids.

Pt-based Rare Earth Alloy as Efficient and Robust Electrocatalyst for High-temperature Proton Exchange Membrane Fuel Cells.

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are crucial in future energy system. However, the activity and stability of the electrocatalysts are severely restricted by high temperature and phosphoric acid poisoning. Herein, PtCe alloy as oxygen reduction reaction (ORR) electrocatalyst for HT-PEMFCs exhibits fantastic performance. Ce can increase the electronic density of Pt weakening phosphoric acid poisoning and improving ORR activity. The optimized electronic structure can also reduce the dipole effect between Pt and O, which suppresses the irreversible oxidation of Pt. Additionally, the dramatically negative heat of formation in PtCe catalyst brings high kinetic barrier of metal diffusion and dissolution. With this electrocatalyst, the HT-PEMFCs show a preeminent peak power of 605 mW cm-2 with 0.3 mgPt cm-2. After 30000 cycles of accelerated stability test, the peak power density only decreases by 31.6 %, achieving the goal of Department of Energy in 2020 (＜40 % loss).

Magic angle HAXPES

The use of higher energy X-ray sources for photoelectron spectroscopy is receiving considerable attention due to the increased availability of laboratory-based instrumentation and an improved insight into the structures and interfacial properties of technological materials. In traditional X-ray photoelectron spectroscopy the design of the instrument often compensates for anisotropy in photoelectron emission through consideration of the angles between the X-ray source and the electron analyser. X-ray polarisation and non-dipole effects in photoemission are usually assumed to be negligible. However, for high energy XPS (HAXPES) both may be significant. Polarisation at synchrotron sources is an important consideration and non-dipole effects are generally more significant at higher photon energies. In this article we demonstrate that, for certain polarisations, ‘magic angle’ geometries exist that minimise the effects of both dipole and non-dipole contributions in photoemission. However, it is not possible to find such geometries for unpolarised X-rays; achieving a ‘magic angle’ geometry in HAXPES requires the X-rays to have a degree of linear polarisation of 1/3 or greater.

Erratum: Dipolar tidal effects in scalar-tensor theories [Phys. Rev. D 101 , 021501(R) (2020)

Received 1 March 2023DOI:https://doi.org/10.1103/PhysRevD.107.069901© 2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlternative gravity theoriesGravitational wavesPhysical SystemsBinary starsGravitation, Cosmology & Astrophysics

Improvement of the magnon-magnon coupling strength in Y3Fe5O12/Py heterostructures

Magnon-quasiparticle couplings perform magnon information transferences and transports, thus controlling the coupling strengths is significant for novel information function devices. Here, we report the modulation of the magnon-magnon (M−M) coupling strength in Y3Fe5O12/permalloy (YIG/Py) heterostructures via ferromagnetic resonance (FMR) technique. The anti-crossing gap gc enhances 80 Hz by changing the radio frequency (rf) microwave power, which could be ascribed to the enhancement of the dipolar interaction and spin pumping effect. Furthermore, the gc decreases with the increasing angle θ between external magnetic field and rf current due to the increase of magnon relaxation. Moreover, the FMR linewidths change with increasing angle θ due to the magnetic anisotropy parameters and the two-magnon relaxation mechanism induced by the more oblique external field.

Improving photocatalytic performance of defective titania for carbon dioxide photoreduction by Cu cocatalyst with SCN- ion modification

Titanium dioxide (TiO2) with defects is a promising semiconductor photocatalyst for photoreduction CO2, due to its unique electronic properties. However, defective TiO2 is difficult to achieve a high yield of CO2 photoreduction, especially the high-value-added C2+ hydrocarbons, due to the slow transfer of multielectron/proton and sluggish C-C coupling kinetics. Here, we combine the photoinduced deposition of copper (Cu) nanoparticle on defective titania (TiO2-SBO) with thiocyanate anion (SCN-, KSCN) surface modification via impregnation route to prepare a series of high performance photocatalysts (SCN-Cu/TiO2-SBO). It was found that the production rate and selectivity of C2H4 of CO2 over sample SCN-Cu/TiO2-SBO-3 as a representative are 4.7 μmol·g−1·h−1 and 40%, while the CO and CH4 yields rise by 2 times and 5 times as compared with those over bare TiO2-SBO. This is because the Cu cocatalyst can serve as the active site for C-C coupling (via intermediate *CO-COH) and the SCN-induced surface dipole effect can enhance the carrier’s dynamic behavior and accelerate the multielectron transfer, while the SCN- ions with nucleophilicity are conducive to CO2 adsorption as well as CO hydrogenation and the multi-proton coupling process of C2+ products. Furthermore, first principle calculations illustrate that the Cu species can effectively reduce the reaction energy of the key intermediate *CO-COH of ethylene, and SCN- ions benefit to the adsorption of CO2 molecules, and thereby being favor for the generation of ethylene.

Applications of triadic hybridized-cross nanomaterials suspended in engine oil using quadratic and linear convection with magnetic dipole

This work comprises the contribution of buoyancy forces, linear and quadratic convection on cross Nano fluid under the effects of dipole effects with chemical reaction. The flow of a cross model (CM) with mass and thermal transport has been modeled in Cartesian coordinates for the two-dimensional fluid flow problem. Temperature gradient effects have been incorporated in mass transport and concentration gradient impacts in thermal transport phenomenon with chemical reaction. The governing laws are derived under boundary layer principle (BLP) in the conformation of coupled nonlinear partial differential equations. Afterwards, coupled partial differential equations are converted into a scheme of ordinary differential equations (ODEs) by incorporating the similarity transformation. The nonlinear modeled equations are solved numerically with the aid of finite element scheme as well as analysis of grid independent is reported based on the reliability of the utilized approach. Several graphs and tables have been prepared to observe the bearing of multiple emerging parameters regarding velocity, temperature as well as concentration fields. Comparison has been made to confirm the efficiency of a finite element scheme. It is perceived that higher values of a Schmidt number control the fluid concentration. Moreover, velocity is diminished against higher values of Weissenberg parameter.

Dielectric constant, dielectric loss, conductivity, capacitance and model analysis of electronic electroactive polymers

In this study, the dielectric constant, dielectric loss, conductivity and capacitance of the electronic electroactive polymer (EEAP) were systematically tested experimentally, and the effects of different frequencies, effective voltages, electrode types, and specimen thicknesses on the dielectric relaxation properties of polymers were analyzed, and finally the Debye, Cole-Cole and Havriliak-Negami functions were used to model the dielectric constants. The experimental results show that the dielectric constant of the polymer depends strongly on the frequency, especially in the low frequency stage from 10−1 to 100 Hz. The effect of different effective voltages on the dielectric properties is basically identical, and the accuracy of the test can be improved at this AC voltage avoiding the effect of electrostatic stress. At the same effective voltage and thickness, the polymer with G-C electrodes shows the best electrical parameters, the composite electrode form of G-C electrodes contributes to the conductive properties of the polymer and can be applied to the later design of actuators. Compared with the Debye model, the errors of the experimental and calculated values of the Cole-Cole and Havriliak-Negami models are smaller, 1.1102 and 0.7130, respectively. The dielectric behavior of EEAPs differs from that of epoxy materials, probably due to the influence of electric dipole moments, bound charges, and dipole effects. The paper has some reference value for further revealing the dielectric properties and relaxation time distribution of the polymer, and provides guidance for the later application of EEAP materials in flexible robots.

Accessing the thermodynamics of Walter‐B fluid with magnetic dipole effect past a curved stretching surface

AbstractThe effects of magnetic dipole, porosity and heat transfer on the flow of ferromagnetic Walter‐B fluid past a curved stretching sheet is investigated with the gyrotactic microorganisms motion and cubic autocatalysis chemical reactions. The momentum, energy, homogeneous–heterogeneous chemical reactions and gyrotactic microorganisms motion equations, form the problem's governing equations, which are transformed into non‐linear ordinary differential equations via similarity transformations and are solved using an efficient and strong technique namely homotopy analysis method. The effects of the parameters under consideration on dimensionless velocity, temperature, homogeneous–heterogeneous chemical reaction, and motile microorganisms are graphically depicted and discussed in detail. Velocity decreases with the increasing values of curvature parameter while the magnetic dipole effect and Curie temperature parameter increase the heat transfer. The homogeneous chemical reaction and the motile density of microorganisms are enhanced with the rising values of elasticity and ferromagnetic interaction parameters.