Dipole Evolution Research Articles

Interface dipole evolution from the hybrid coupling between nitrogen-doped carbon quantum dots and polyethylenimine featuring the electron transport thin layer at Al/Si interfaces

The assessment of electron transport layer (ETL) for rear-contact engineering of silicon (Si) based optoelectronics has been considered as one of the critical challenges that leverage the performance improvement and device reliability. In this work, the hybrid design of ETL, obtained from the solution-processed nitrogen-doped carbon quantum dots (NCQDs) incorporated with organic polyethylenimine (PEI) demonstrates the feasible contact characteristics for the modification of Si/Al contacts, which greatly facilitates the transport and collection of photoexcited electrons in the Si-based optoelectronics. The aspects of microstructures, functional groups, chemical features, interfacial characteristics and band structures of NCQD/PEI are explicated, visualizing that the evolution of interface dipoles mediated by the overall outcome of physisorption and chemisorption effects, modifies the surface potential difference and results in the explicit reduction of the Al work function from 4.3 eV for pristine Al to 3.23 eV based on the optimized constitutional design (0.10 % NCQD in PEI). These findings are practically employed on the Si-based hybrid solar cells at Si/Al interfaces, fulfilling the conversion-efficiency improvement by 30.9 % compared with reference cells without ETL employment, which are experimentally interpreted by the efficient electron transport across the Si/Al heterojunction and charge collection.

Spatiotemporal Evolution Disparities of Vegetation Trends over the Tibetan Plateau under Climate Change

The Tibetan Plateau has experienced profound climate change with significant implication for spatial vegetation greenness. However, the spatiotemporal disparities of long-term vegetation trends in response to observed climate change remain unclear. Based on remote-sensing vegetation images indicated by the normalized difference vegetation index (NDVI) from two long-term combined datasets, GIMMS and MODIS, we identified two spatiotemporal evolution patterns (SEPs) in long-term vegetation anomalies across the Tibetan Plateau. This new perspective integrates spatial and temporal NDVI changes during the growing seasons over the past four decades. Notably, the dipole evolution pattern that rotates counterclockwise from May to September accounted for 62.8% of the spatial mean amplitude of vegetation trends, dominating the spatiotemporal disparities. This dominant pattern trend is attributed to simultaneous effects of spatial warming and rising CO2, which accounted for 75% and 15%, respectively, along with a lagged effect of dipole precipitation, accounting for 6%. Overall, wetting and warming promote greening evolution in the northern Tibetan Plateau, while slight drying and warming favor browning evolution in the southern Tibetan Plateau. These findings provide insights into the combined effects of climate change on spatiotemporal vegetation trends and inform future adaptive strategies in fragile regions.

Deformation and destruction of north-eastward drifting dipoles

We systematically study the evolution of Larichev–Reznik dipoles in an equivalent-barotropic quasi-geostrophic beta-plane model in high-resolution numerical simulations. Our results shed light on the self-organization and rich dynamics of dipolar vortices, which are ubiquitous in geophysical flows. By varying both dipole strength and initial angle α0 of dipole tilt to the zonal direction, we discover new breakdown mechanisms of the dipole evolution. The dipoles are quickly destroyed by Rossby wave radiation, if initial tilt is too large or the dipole is too weak; otherwise, via damped oscillations, the dipoles tend to adjust themselves to different states drifting eastward. Two competing physical mechanisms that govern dipole transformations are found: (1) spontaneous dipole instability due to a growing critical linear mode and (2) meridional separation of dipole partners that accumulates over the adjustment period and prevents the above instability. Which mechanism prevails depends on the initial tilt and dipole strength, and the details of this are discussed.

Dihadron production in DIS at small x at next-to-leading order: Transverse photons

We calculate the next-to-leading order corrections to dihadron production in deep inelastic scattering (DIS) at small $x$ using the color glass condensate formalism for the case when the virtual photon is transverse polarized. Similar to the case of longitudinal photon exchange all UV and soft singularities cancel while the collinear divergences are absorbed into quark and antiquark-hadron fragmentation functions. Rapidity divergences lead to Jalilian-Marian, Iancu, McLerran, Wiegert, Leonidov, Kovner (JIMWLK) evolution of dipoles and quadrupoles which describe multiple scatterings of the quark antiquark dipole on the target proton/nucleus and contain all the QCD dynamics of the target leading to a finite final result for the dihadron production cross section.

Competition between oxygen and water molecules on SiO2/P-doped Si surface: The electrical dipole evolution on water/oxygen-adsorbed oxide surface

The performance of gas sensors is greatly influenced under humidity environment due to the dynamic correlation between water and oxygen molecules. In this work, we utilize the charge trapping characteristic to study surface gas adsorption and demonstrate the competitive adsorption behaviors between oxygen and water vapor from the perspective of electrical dipole by observing the evolution of second harmonic generation (SHG). By analyzing the trapping time constant of time-dependent SHG spectrum, we propose a dynamic model of the way water molecules weaken oxygen adsorption and develop water adsorption layer. Results show that the electric field created by charge trapping would further align the water molecules and enhance the SHG intensity. Here, a viewpoint of analyzing the net surface dipole change due to the adsorption–desorption on SiO2 surface to realize the energy preferable state is presented and provides a pathway for solving the water–sensor interaction problems in gas sensor.

One-loop corrections to dihadron production in DIS at small x

We calculate the one-loop corrections to dihadron production in Deep Inelastic Scattering (DIS) at small x using the Color Glass Condensate formalism. We show that all UV and soft singularities cancel while the collinear divergences are absorbed into quark and anti quark-hadron fragmentation functions. Rapidity divergences lead to JIMWLK evolution of dipoles and quadrupoles describing multiple-scatterings of the quark anti-quark dipole on the target proton/nucleus. The resulting cross section is finite and can be used for phenomenological studies of dihadron angular correlations at small x in a future Electron-Ion Collider (EIC).

Geometric integration of classical spin dynamics via a mean-field Schrödinger equation

The Landau-Lifshitz equation describes the time evolution of magnetic dipoles and can be derived by taking the classical limit of a quantum mechanical spin Hamiltonian. To take this limit, one constrains the many-body quantum state to a tensor product of coherent states, thereby neglecting entanglement between sites. Expectation values of the quantum spin operators produce the usual classical spin dipoles. One may also consider expectation values of polynomials of the spin operators, leading to quadrupole and higher-order spin moments, which satisfy a dynamical equation of motion that generalizes the Landau-Lifshitz dynamics [Zhang and Batista, Phys. Rev. B 104, 104409 (2021)]. Here we reformulate the dynamics of these ${N}^{2}\ensuremath{-}1$ generalized spin components as a mean-field Schr\"odinger equation on the $N$-dimensional coherent state. This viewpoint suggests efficient integration methods that respect the local symplectic structure of the classical spin dynamics.

Recent Changes in Drought Events over South Asia and Their Possible Linkages with Climatic and Dynamic Factors

South Asia is home to one of the fastest-growing populations in Asia, and human activities are leaving indelible marks on the land surface. Yet the likelihood of successive observed droughts in South Asia (SA) and its four subregions (R-1: semi-arid, R-2: arid, R-3: subtropical wet, and R-4: tropical wet and dry) remains poorly understood. Using the state-of-the-art self-calibrated Palmer Drought Severity Index (scPDSI), we examined the impact of different natural ocean variability modes on the evolution, severity, and magnitude of observed droughts across the four subregions that have distinct precipitation seasonality and cover key breadbaskets and highly vulnerable populations. The study revealed that dryness had significantly increased in R-1, R-2, and R-4 during 1981–2020. Temporal analysis revealed an increase in drought intensity for R-1 and R-4 since the 2000s, while a mixed behavior was observed in R-2 and R-3. Moreover, most of the sub-regions witnessed a substantial upsurge in annual precipitation, but a significant decrease in vapor pressure deficit (VPD) during 1981–2020. The increase in precipitation and the decline in VPD partially contributed to a significant rise in Normalized Difference Vegetation Index (NDVI) and a decrease in dryness. In contrast, a strong positive correlation was found between drought index and precipitation, and NDVI across R-1, R-2, and R-4, whereas temperature and VPD exhibited a negative correlation over these regions. No obvious link was detected with El-Niño Southern Oscillation (ENSO) events, or Indian Ocean Dipole (IOD) and drought evolution, as explored for certain regions of SA. The findings showed the possibility that the precipitation changes over these regions had an insignificant relationship with ENSO, IOD, and drought onset. Thus, the study results highlight the need for considering interactions within the longer climate system in describing observed drought risks rather than aiming at drivers from an individual perspective.

Eccentric Dipole Evolution during the Last Reversal, Last Excursions, and Holocene Anomalies. Interpretation Using a 360-Dipole Ring Model

The eccentric dipole (ED) is the next approach of the geomagnetic field after the generally used geocentric dipole. Here, we analyzed the evolution of the ED during extreme events, such as the Matuyama-Brunhes polarity transition (~780 ka), the Laschamp (~41 ka) and Mono Lake (~34 ka) excursions, and during the time of two anomalous features of the geomagnetic field observed during the Holocene: the Levantine Iron Age Anomaly (LIAA, ~1000 BC) and the South Atlantic Anomaly (SAA, analyzed from ~700 AD to present day). The analysis was carried out using the paleoreconstructions that cover the time of the mentioned events (IMMAB4, IMOLEe, LSMOD.2, SHAWQ-Iron Age, and SHAWQ2k). We found that the ED moves around the meridian plane of 0–180° during the reversal and the excursions; it moves towards the region of the LIAA; and it moves away from the SAA. To investigate what information can be extracted from its evolution, we designed a simple model based on 360-point dipoles evenly distributed in a ring close to the inner core boundary that can be reversed and their magnitude changed. We tried to reproduce with our simple model the observed evolution of the ED, and the total field energy at the Earth’s surface. We observed that the modeled ED moves away from the region where we set the dipoles to reverse. If we consider that the ring dipoles could be related to convective columns in the outer core of the Earth, our simple model would indicate the potential of the displacement of the ED to give information about the regions in the outer core where changes start for polarity transitions and for the generation of important anomalies of the geomagnetic field. According to our simple model, the regions in which the most important events of the Holocene occur, or in which the last polarity reversal or excursion begin, are related to the regions of the Core Mantle Boundary (CMB), where the heat flux is low.

Dipole and Nondipole Evolution of the Historical Geomagnetic Field From Instrumental, Archeomagnetic, and Volcanic Data

AbstractAlthough a large number of magnetic declination and inclination measurements are available for the historical period from 1400 to 1900 CE, even the evolution of the axial dipole moment, the most prominent quantity of the geomagnetic field, is highly disputed for this time interval. Here, a new iterative Bayesian spherical harmonic model is constructed that combines historical (direct) and archeomagnetic or volcanic (indirect) records to better constrain the geomagnetic field evolution over this historical epoch. The resulting geomagnetic field reconstruction (BIGMUDIh.1) benefits from recently published and revised data for critical periods and regions. The highly variable data uncertainties and qualities are tackled with a weighting and selection scheme. Model uncertainties are mainly controlled by selection criteria applied to indirect intensity records and are estimated using a bootstrapping approach. A major improvement with respect to the established historical model gufm1 consists in the incorporation of archeointensities, which enables a direct estimation of the axial dipole evolution prior to 1840 CE. BIGMUDIh.1 yields a 2 µT (6%) decrease of the axial dipole component between 1500 and 1600 CE, followed by a relatively stable period until 1900 CE. Modeled declinations agree well with historical records and the gufm1 model derived from such records. Furthermore, the evolution of the South Atlantic Anomaly, driven by reverse flux patches (RFPs) at the core‐mantle boundary, is outlined. Periods of accelerated movement of the northern magnetic dip pole appear to be connected to the formation and changes of RFPs in the Northern Hemisphere.

Dynamics of free electron wave packets with coherent extreme ultraviolet four-wave mixing fields

We discuss the generation of cascaded extreme ultraviolet four-wave mixing fields with a single-gas-cell configuration and use them to study the dynamics of free electron wave packets of the noble gases. The temporal evolution of atomic dipoles in the vicinity of one-electron excitation Rydberg states 3s3p6np of argon and 4s4p6n′p of krypton is reflected in the intensity profiles of the four-wave mixing spectra. Different interference pathways for these dipole-allowed transitions are revealed in the time-delay-dependent phase shift of the four-wave mixing fields. The results of this study may be promising for studies of more complex systems.

COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations

We present the geomagnetic field model COV-OBS.x2 that covers the period 1840–2020. It is primarily constrained by observatory series, satellite data, plus older surveys. Over the past two decades, we consider annual differences of 4-monthly means at ground-based stations (since 1996), and virtual observatory series derived from magnetic data of the satellite missions CHAMP (over 2001–2010) and Swarm (since 2013). A priori information is needed to complement the constraints carried by geomagnetic records and solve the ill-posed geomagnetic inverse problem. We use for this purpose temporal cross-covariances associated with auto-regressive stochastic processes of order 2, whose parameters are chosen so as to mimic the temporal power spectral density observed in paleomagnetic and observatory series. We aim this way to obtain as far as possible realistic posterior model uncertainties. These can be used to infer for instance the core dynamics through data assimilation algorithms, or an envelope for short-term magnetic field forecasts. We show that because of the projection onto splines, one needs to inflate the formal model error variances at the most recent epochs, in order to account for unmodeled high frequency core field changes. As a by-product of the core field model, we co-estimate the external magnetospheric dipole evolution on periods longer than 2 years. It is efficiently summarized as the sum of a damped oscillator (of period 10.5 years and decay rate 55 years), plus a short-memory (6 years) damped random walk.

Dislocation self-interaction in TiAl: Evolution of super-dislocation dipoles revealed by atomistic simulations

As one of the fundamental outcomes of dislocation self-interaction, dislocation dipoles have an important influence on the plastic deformation of materials, especially on fatigue and creep. In this work, super-dislocation dipoles in γ-TiAl and α2-Ti3Al were systematically investigated by atomistic simulations, with a variety of dipole heights, orientations and annealing temperatures. The results indicate that non-screw super-dipoles transform into locally stable dipolar or reconstructed cores at low temperature, while into isolated or interconnected point defect clusters and stacking fault tetrahedra at high temperature via short-range diffusion. Non-screw super-dipoles in γ-TiAl and α2-Ti3Al exhibit similar features as fcc and hcp metals, respectively. Generally, over long-term annealing where diffusion is significant, 60° super-dipoles in γ-TiAl are stable, whereas the stability of super-dipoles in α2-Ti3Al increases with dipole height and orientation angle. The influence on mechanical properties can be well evaluated by integrating these results into mesoscale or constitutive models.

Dipole evolution: perspectives for collectivity and \u03b3*A collisions

The transverse, spatial structure of protons is an area revealing fundamental properties of matter, and provides key input for deeper understanding of emerging collective phenomena in high energy collisions of protons, as well as collisions of heavy ions. In this paper eccentricities and eccentricity fluctuations are predicted using the dipole formulation of BFKL evolution. Furthermore, first steps are taken towards generation of fully exclusive final states of γ*A collisions, by assessing the importance of colour fluctuations in the initial state. Such steps are crucial for the preparation of event generators for a future electron-ion collider. Due to the connection between an impact parameter picture of the proton structure, and cross sections of ep and pp collisions, the model parameters can be fully determined by fits to such quantities, leaving results as real predictions of the model.

Modeling dynamic rotation of defect dipoles and poling time dependence of piezoelectric effect in ferroelectrics

A phase field model combined with a kinetic Monte Carlo method has been used to describe the formation of domain structures in a ferroelectric system with rotatable defect dipoles. Our simulation shows that domain configurations are strongly dependent on the evolution of defect dipoles at different poling times. A smaller domain size with a larger piezoelectric coefficient can be obtained by optimizing the poling time, while long time poling will produce larger domains that could degrade piezoelectric effects. Our modeling results demonstrated the fundamental principle for enhancing the piezoelectric response by combining domain engineering with defect engineering through the control of the poling strategy.

Defect dipole evolution and its impact on the ferroelectric properties of Fe‐doped KTN single crystals

AbstractThe ferroelectric and piezoelectric properties of doped potassium tantalum niobate crystals with different Fe doping amounts and the adjustability of the properties are investigated. The hysteresis loops and current density curves show that the defect dipoles have an obvious effect on domain reorientation, and the effect decreases with increasing doping amount. The ferroelectric and piezoelectric properties can be adjusted via the defect dipoles, and the adjustability is reduced with increasing doping amount. A change of the doping amount leads to defect dipole structure evolution in the crystals, in which the defect dipoles transform from a polar structure to a nonpolar structure, which is the reason for the transition of the domain reorientation determined by the defect dipoles. This result has proved that introducing defects is an effective way to improve and regulate perovskite properties, and the doping amount is one of the important factors controlling the defect dipoles.

Leading-color fully differential two-loop soft corrections to QCD dipole showers

We compute the next-to-leading order corrections to soft-gluon radiation differentially in the one-emission phase space. We show that their contribution to the evolution of color dipoles can be obtained in a modified subtraction scheme, such that both one- and two-emission terms are amenable to Monte-Carlo integration. The two-loop cusp anomalous dimension is recovered naturally upon integration over the full phase space. We present two independent implementations of the new algorithm in the two event generators Pythia and Sherpa, and we compare the resulting fully differential simulation to the CMW scheme.

The frequency dependence of microstructure evolution in a ferroelectric nano-film during AC dynamic polarization switching

The frequency dependence of the electromechanical response of a barium titanate nano-thin film was studied through phase-field simulation. A two-dimensional phase-field model based on Landau–Devonshire energy density function was established in this work. The time-dependent Ginzburg–Landau equation was utilized to calculate the dynamics of the microstructure upon the application of an AC electric field. A segment of barium titanate thin film was modeled with 20 nm in thickness and 80 nm in width. Periodic boundary conditions were applied to both ends of the nano-thin film to represent an infinite length-to-thickness ratio. It was observed from the phase-field results that the loading frequency of the electric field can noticeably affect the hysteresis and butterfly loops of the nano-thin film through competition with the electric dipole evolution. A high-frequency electric field tends to yield a close-to-linear response of the thin film. Meanwhile, it was discovered that the existence of $$180^{\circ }$$ domain walls and their dynamics (oscillation) within the thin film have remarkable influence on the overall response.

Defect structure‐electrical property relationship in Mn‐doped calcium strontium titanate dielectric ceramics

AbstractCa0.6Sr0.4TiO3 (CST) ceramics with different amounts of Mn dopant (0‐2.0 mol%) were prepared by solid‐state reaction method. The electric field and temperature stability of energy storage performance was found to be greatly enhanced with moderate doped level of 0.5 mol%. The dielectric loss‐frequency spectra revealed the existence and evolution of defect dipoles at elevated temperature, which was confirmed directly by electron paramagnetic resonance (EPR) spectra. The response of defect dipoles was characterized by thermally stimulated depolarization current (TSDC), where the activation energy and the concentration evolution of defect dipoles were calculated, with the highest values observed for 0.5% doped samples. The dissociation of defect dipoles and the movement of free were analyzed by high‐temperature impedance spectra analysis, with the activation energy of 1.04‐1.60 eV, and 0.5% doped samples also demonstrated the highest Ea. The relationship between microscopic defect structure and macroscopic electrical behavior was established in this work.

"Watching" Polaron Pair Formation from First-Principles Electron-Nuclear Dynamics.

The formation of polaron pairs is one of the important photophysical processes that take place after the excitation in semiconducting organic polymers. First-principles Ehrenfest excited-state dynamics is a unique tool to investigate ultrafast photoinduced charge carrier dynamics and related nonequilibrium processes involving correlated electron-nuclear dynamics. In this work the formation of polaron pairs and their dynamical evolution in an oligomer of seven thiophene units is investigated with a combined approach of first-principles exciton-nuclear dynamics and wavelet analysis. The real-time formation of a polaron pair can be observed in the dipole evolution during the excited-state dynamics. The possible driving force of the polaron pair formation is investigated through qualitative correlation between the structural dynamics and the dipole evolution. The time-dependent characteristics and spectroscopic consequences of the polaron pair formation are probed using the wavelet analysis.