Showing papers on "Electric field published in 2020"

Ultrahigh enhancement rate of the energy density of flexible polymer nanocomposites using core–shell BaTiO3@MgO structures as the filler

Abstract: Dielectric energy storage capacitors are critical components widely used in electronic equipment and power systems due to their advantages of ultrahigh power density and high voltage. Herein, a novel core–shell BaTiO3@MgO (BT@MO) nanostructure was fabricated, in which highly insulating MgO was directly coated on a continuous ferroelectric nanoparticle BaTiO3 shell through a chemical precipitation method to improve the breakdown strength and electric displacement under high electric field. A large electric displacement (D ≈ 9.8 μC cm−2 under 571.4 MV m−1) was observed along with a high discharge energy density (Ud ≈ 19.0 J cm−3) for BT@MO/P(VDF-HFP) composites, which was 187% higher than that for a P(VDF-HFP) film when the filler content was 3 wt%. The enhancement rate of Ud in this study achieved the highest level among the reported results. It was revealed that the highly insulating MgO shell can enhance the breakdown strength by preventing charge injection from electrodes and impeding the development of electrical stress during the breakdown process, as confirmed by the leakage current measurements and the finite element simulations. The core–shell BT@MO structured filler provided an effective way to improve the energy storage properties of the polymer-based dielectrics.

A highly scalable dielectric metamaterial with superior capacitor performance over a broad temperature

Abstract: Although many polymers exhibit excellent dielectric performance including high energy density with high efficiency at room temperature, their electric and dielectric performance deteriorates at high temperatures (~150°C). Here, we show that nanofillers at very low volume content in a high-temperature (high–glass transition temperature) semicrystalline dipolar polymer, poly(arylene ether urea), can generate local structural changes, leading to a marked increase in both dielectric constant and breakdown field, and substantially reduce conduction losses at high electric fields and over a broad temperature range. Consequently, the polymer with a low nanofiller loading (0.2 volume %) generates a high discharged energy density of ca. 5 J/cm3 with high efficiency at 150°C. The experimental data reveal microstructure changes in the nanocomposites, which, at 0.2 volume % nanofiller loading, reduce constraints on dipole motions locally in the glassy state of the polymer, reduce the mean free path for the mobile charges, and enhance the deep trap level.

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics.

Abstract: We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.

Electrical switching of magnetic order in an orbital Chern insulator

Abstract: Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favors ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields---a longstanding technological goal in spintronics and multiferroics---can be achieved only indirectly. Here, we experimentally demonstrate direct electric field control of magnetic states in an orbital Chern insulator, a magnetic system in which non-trivial band topology favors long range order of orbital angular momentum but the spins are thought to remain disordered. We use van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer to realize narrow and topologically nontrivial valley-projected moire minibands. At fillings of one and three electrons per moire unit cell within these bands, we observe quantized anomalous Hall effects with transverse resistance approximately equal to $h/2e^2$, which is indicative of spontaneous polarization of the system into a single-valley-projected band with a Chern number equal to two. At a filling of three electrons per moire unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate nonvolatile electric field induced reversal of the magnetic state. A theoretical analysis indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favored magnetic state. Voltage control of magnetic states can be used to electrically pattern nonvolatile magnetic domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultralow power magnetic memory.

Electret formation in transition metal oxides by electrochemical amorphization

Abstract: Transition metal oxides (TMOs) are an important class of materials that show a wide range of functionalities involving spin, charge, and lattice degrees of freedom. The strong correlation between electrons in d-orbitals and the multivalence nature give rise to a variety of exotic electronic states ranging from insulator to superconductor and cause intriguing phase competition phenomena. Despite a burst of research on the multifarious functionalities in TMOs, little attention has been paid to the formation and integration of an electret—a type of quasi-permanent electric field generator useful for nanoscale functional devices as an electric counterpart to permanent magnets. Here, we find that an electret can be created in LaMnO3 thin films by tip-induced electric fields, with a considerable surface height change, via solid-state electrochemical amorphization. The surface charge density of the formed electret area reaches ~400 nC cm−2 and persists without significant charge reduction for more than a year. The temporal evolution of the surface height, charge density, and electric potential are systematically examined by scanning probe microscopy. The underlying mechanism is theoretically analyzed based on a drift-diffusion-reaction model, suggesting that positively charged particles, which are likely protons produced by the dissociation of water, play crucial roles as trapped charges and a catalysis to trigger amorphization. Our finding opens a new horizon for multifunctional TMOs. A material that generates its own electric field has been developed by scientists in South Korea. An electret is the electrical equivalent of a magnet in that it is formed of two electric poles rather than two magnetic poles. Just as magnetic dipoles give rise to permanent magnets, electret materials create a quasi-permanent electric field. They are useful for microphones, photocopiers and many other electrical devices. Yong-Jin Kim and Chan-Ho Yang from the Korea Advanced Institute of Science and Technology, Daejeon, have created an electret using transition metal oxides. The researchers used a sharp tip of platinum-coated silicon to “write” a charged pattern with a density similar to that of commercially available electrets into a thin film of lanthanum manganite. These patterns persisted for more than a year. An electret can be created in a complex transition metal oxide LaMnO3 by tip-induced electric fields with a considerable surface height change via solid-state electrochemical amorphization. The surface charge density of the formed electret area reaches ~400 nC cm−2 and persists without significant charge reduction for more than a year. Our finding opens a new horizon for multifunctional transition metal oxides by providing an electric counterpart to permanent magnets.

A 1.86-kV double-layered NiO/β-Ga2O3 vertical p–n heterojunction diode

Abstract: In this Letter, high-performance vertical NiO/β-Ga2O3 p–n heterojunction diodes without any electric field managements were reported. The devices show a low leakage current density and a high rectification ratio over 1010 (at ±3 V) even operated at temperature of 400 K, indicating their excellent thermal stability and operation capability at high temperature. Given a type-II band alignment of NiO/β-Ga2O3, carrier transport is dominated by the interface recombination at forward bias, while the defect-mediated variable range hopping conduction is identified upon strong reverse electric field. By using the double-layer design of NiO with a reduced hole concentration of 5.1 × 1017 cm−3, the diode demonstrates an improved breakdown voltage (Vb) of 1.86 kV and a specific on-resistance (Ron,sp) of 10.6 mΩ cm2, whose power figure of merit (Vb2/Ron,sp) has reached 0.33 GW/cm2. The high breakdown voltage and low leakage current are outperforming other reported Ga2O3 based p–n heterojunctions and Schottky barrier diodes without field plate and edge termination structures. TCAD simulation indicates that the improved Vb is mainly attributed to the suppression of electric field crowding due to the decreased hole concentration in NiO. Such bipolar heterojunction is expected to be an alternative to increase the breakdown characteristics of β-Ga2O3 power devices.

Internal-Field-Enhanced Charge Separation in a Single-Domain Ferroelectric PbTiO3 Photocatalyst.

Abstract: Ferroelectric materials with spontaneous polarization-induced internal electric fields have drawn increasing attention in solar fuel production due to the intrinsic polarized structure. However, the origination of charge separation in these materials at the nano/microlevel is ambiguous owing to the complexity of the multielectric fields. Besides, the observed charge separation ability is far from theoretical expectation. Herein, by spatially resolved surface photovoltage spectroscopy, it is clearly demonstrated that the depolarization field in single-domain ferroelectric PbTiO3 (PTO) nanoplates is the main driving force for charge separation and it can effectively drive photogenerated electrons and holes to the positive and negative polarization facets, respectively. Moreover, the charge separation ability of PTO nanoplates increases with increasing particle size along the polarization direction, due to the increasing potential difference between the opposite polarization facets. Furthermore, this driving force for charge separation directly contributes to the enhancement of the photocatalytic hydrogen evolution reaction activity in ferroelectrics. Finally, it is proved that the screening field compensates part of the depolarization field and can be diminished by adding a dielectric layer on the ferroelectric surface. These findings demonstrate the importance of increasing the depolarization field and decreasing the screening field for efficient charge separation in ferroelectric semiconductor photocatalysts.

Strong Electric Field Observed at the Interface of Aqueous Microdroplets.

Abstract: Chemical reactions in aqueous microdroplets often exhibit unusual kinetic and thermodynamic properties not observed in bulk solution. While an electric field has been implicated at the water interface, there has been no direct measurement in aqueous microdroplets, largely due to the lack of proper measurement tools. Herein, we employ newly developed stimulated Raman excited fluorescence microscopy to measure the electric field at the water-oil interface of microdroplets. As determined by the vibrational Stark effect of a nitrile-bearing fluorescent probe, the strength of the electric field is found to be on the order of 107 V/cm. This strong electric field aligns probe dipoles with respect to the interface. The formation of the electric field likely arises from charge separation caused by the adsorption of negative ions at the water-oil interface of microdroplets. We suggest that this strong electric field might account in part for the unique properties of chemical reactions reported in microdroplets.

Fatigue resistant lead-free multilayer ceramic capacitors with ultrahigh energy density

Abstract: The critical role of electrical homogeneity in optimising electric-field breakdown strength (BDS) and energy storage in high energy density (0.7 − x)BiFeO3–0.3BaTiO3–xBi(Li0.5Nb0.5)O3 (BF–BT–xBLN) lead-free capacitors is demonstrated. The high BDS for bulk ceramics and multilayers (dielectric layer thickness ∼ 8 μm) of ∼260 and ∼950 kV cm−1, respectively, gives rise to record-performance of recoverable energy density, Wrec = 13.8 J cm−3 and efficiency, η = 81%. Under an electric field of 400 kV cm−1, multilayers are temperature stable up to 100 °C, frequency independent in the range 10−2 to 102 Hz, have low strain (<0.03%) and are fatigue-resistant up to 104 cycles (Wrec variation < 10%). These properties show promise for practical use in pulsed power systems.

Reversing Interfacial Catalysis of Ambipolar WSe2 Single Crystal.

Abstract: An improved understanding of the origin of the electrocatalytic activity is of importance to the rational design of highly efficient electrocatalysts for the hydrogen evolution reaction. Here, an ambipolar single-crystal tungsten diselenide (WSe2) semiconductor is employed as a model system where the conductance and carrier of WSe2 can be individually tuned by external electric fields. The field-tuned electrochemical microcell is fabricated based on the single-crystal WSe2 and the catalytic activity of the WSe2 microcell is measured versus the external electric field. Results show that WSe2 with electrons serving as the dominant carrier yields much higher activity than WSe2 with holes serving as the dominant carrier even both systems exhibit similar conductance. The catalytic activity enhancement can be characterized by the Tafel slope decrease from 138 to 104 mV per decade, while the electron area concentration increases from 0.64 × 1012 to 1.72 × 1012 cm-2. To further understand the underlying mechanism, the Gibbs free energy and charge distribution for adsorbed hydrogen on WSe2 versus the area charge concentration is systematically computed, which is in line with experiments. This comprehensive study not only sheds light on the mechanism underlying the electrocatalysis processes, but also offers a strategy to achieve higher electrocatalytic activity.

Large-Scale Electric-Field Confined Silicon with Optimized Charge-Transfer Kinetics and Structural Stability for High-Rate Lithium-Ion Batteries

Abstract: The stereospecific design of the interface effects can optimize the electron/Li-ion migration kinetics for energy-storage materials. In this study, an electric field was introduced to silicon-based materials (C-SiOx@Si/rGO) through the rational construction of multi-heterostructures. This was achieved by manipulating the physicochemical properties at the atomic level of advanced Li-ion batteries (LIBs). The experimental and density functional theory calculations showed that the unbalanced charge distribution generated a large potential difference, which in turn induced a large-scale electric-field response with a boosted interfacial charge transfer in the composite. The as-prepared C-SiOx@Si/rGO anode showed advanced rate capability (i.e., 1579.0 and 906.5 mAh g-1 at 1000 and 8000 mA g-1, respectively) when the migration paths of the Li-ion/electrons hierarchically optimized the large electric field. Furthermore, the C-SiOx@Si/rGO composite with a high SiOx@Si mass ratio (73.5 wt %) demonstrated a significantly enhanced structural stability with a 40% volume expansion. Additionally, when coupled with the LiNi0.8Co0.1Mn0.1O2 (NCM) cathode, the NCM//C-SiOx@Si/rGO full cell delivers superior Li-ion storage properties with high reversible capacities of 157.6 and 101.4 mAh g-1 at 500 and 4000 mA g-1, respectively. Therefore, the electric-field introduction using optimized electrochemical reaction kinetics can assist in the construction of other high-performance LIB materials.

Ultrafast vector imaging of plasmonic skyrmion dynamics with deep subwavelength resolution

Abstract: Plasmonic skyrmions are an optical manifestation of topological defects in a continuous vector field. Identifying them requires characterization of the vector structure of the electromagnetic near field on thin metal films. Here we introduce time-resolved vector microscopy that creates movies of the electric field vectors of surface plasmons with subfemtosecond time steps and a 10-nanometer spatial scale. We image complete time sequences of propagating surface plasmons as well as plasmonic skyrmions, resolving all vector components of the electric field and their time dynamics, thus demonstrating dynamic spin-momentum coupling as well as the time-varying skyrmion number. The ability to image linear optical effects in the spin and phase structures of light in the single-nanometer range will allow for entirely novel microscopy and metrology applications.

Decay of the coronal magnetic field can release sufficient energy to power a solar flare.

Abstract: Solar flares are powered by a rapid release of energy in the solar corona, thought to be produced by the decay of the coronal magnetic field strength. Direct quantitative measurements of the evolving magnetic field strength are required to test this. We report microwave observations of a solar flare, showing spatial and temporal changes in the coronal magnetic field. The field decays at a rate of ~5 Gauss per second for 2 minutes, as measured within a flare subvolume of ~1028 cubic centimeters. This fast rate of decay implies a sufficiently strong electric field to account for the particle acceleration that produces the microwave emission. The decrease in stored magnetic energy is enough to power the solar flare, including the associated eruption, particle acceleration, and plasma heating.

Temporal and spectral fingerprints of ultrafast all-coherent spin switching

Abstract: Future information technology demands ultimately fast, low-loss quantum control. Intense light fields have facilitated important milestones, such as inducing novel states of matter, accelerating electrons ballistically, or coherently flipping the valley pseudospin. These dynamics leave unique signatures, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute - the spin - between two states separated by a potential barrier is to trigger an all-coherent precession. Pioneering experiments and theory with picosecond electric and magnetic fields have suggested this possibility, yet observing the actual dynamics has remained out of reach. Here, we show that terahertz (1 THz = 10$^{12}$ Hz) electromagnetic pulses allow coherent navigation of spins over a potential barrier and we reveal the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO$_{3}$ with the locally enhanced THz electric field of custom-tailored antennas. Within their duration of 1 ps, the intense THz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the magnon resonance, and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with a numerical simulation. The switchable spin states can be selected by an external magnetic bias. The low dissipation and the antenna's sub-wavelength spatial definition could facilitate scalable spin devices operating at THz rates.

Measurement of magnetic field and relativistic electrons along a solar flare current sheet

Abstract: In the standard model of solar flares, a large-scale reconnection current sheet is postulated to be the central engine for powering the flare energy release1–3 and accelerating particles4–6. However, where and how the energy release and particle acceleration occur remain unclear owing to the lack of measurements of the magnetic properties of the current sheet. Here we report the measurement of the spatially resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a ‘magnetic bottle’. This spatial structure agrees with theoretical predictions1,7 and numerical modelling results. A strong reconnection electric field of about 4,000 V m−1 is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons instead concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is probably the primary site for accelerating and confining the relativistic electrons. Observations of the X8.2 solar flare, which happened on 2017 September 10, could spatially resolve the distribution of the energetic electrons along the reconnection current sheet. More than 99% of them are concentrated at the bottom of the current sheet, not at the reconnection X point.

Lattice distortion induced internal electric field in TiO2 photoelectrode for efficient charge separation and transfer.

Abstract: Providing sufficient driving force for charge separation and transfer (CST) is a critical issue in photoelectrochemical (PEC) energy conversion. Normally, the driving force is derived mainly from band bending at the photoelectrode/electrolyte interface but negligible in the bulk. To boost the bulky driving force, we report a rational strategy to create effective electric field via controllable lattice distortion in the bulk of a semiconductor film. This concept is verified by the lithiation of a classic TiO2 (Li-TiO2) photoelectrode, which leads to significant distortion of the TiO6 unit cells in the bulk with well-aligned dipole moment. A remarkable internal built-in electric field of ~2.1 × 102 V m−1 throughout the Li-TiO2 film is created to provide strong driving force for bulky CST. The photoelectrode demonstrates an over 750% improvement of photocurrent density and 100 mV negative shift of onset potential upon the lithiation compared to that of pristine TiO2 film. The driving force for charge transfer in photoelectrochemical systems is typically derived from band bending at a surface-electrolyte interface. In this work, battery-type lithiation of TiO2 generates a built-in electric field in the bulk material, giving a 750% enhancement in photocurrent density.

External electric field effects on chemical structure and reactivity

Abstract: In recent years, external electric fields (EEFs) have captured some spotlight as novel effectors of chemical change. EEFs directly impact the structure of molecular systems. For example, aligning an electric field along a specific bond‐axis leads to either shortening or elongation of the bond (and ultimately bond breaking). Furthermore, EEFs enable unprecedented control over chemical reactivity. Orienting an electric field along the so‐called “reaction‐axis,” the direction in which the electrons reorganize during the conversion from reactant to product, leads to catalysis or inhibition and can induce mechanistic crossover from concerted to stepwise reactions. Off‐reaction‐axis orientation enables control over the stereoselectivity of reactions and disables forbidden–orbital mixing. Two‐directional fields enable control over both reactivity and selectivity. In this advanced review, we offer an overview of this rapidly evolving research field with a focus on the valence bond modeling of EEF effects and the insight it offers. A wide variety of examples will be considered and a link to the experiment will be made throughout. We end by offering some perspectives in which we postulate that, as experimental techniques continue to mature, EEFs could potentially become a generally applicable “zapping” tool to facilitate elaborate chemical syntheses.

Pulsed electric field-assisted extraction of valuable compounds from microorganisms.

Abstract: Microorganisms (bacteria, yeast, and microalgae) are a promising resource for products of high value such as nutrients, pigments, and enzymes. The majority of these compounds of interest remain inside the cell, thus making it necessary to extract and purify them before use. This review presents the challenges and opportunities in the production of these compounds, the microbial structure and the location of target compounds in the cells, the different procedures proposed for improving extraction of these compounds, and pulsed electric field (PEF)-assisted extraction as alternative to these procedures. PEF is a nonthermal technology that produces a precise action on the cytoplasmic membrane improving the selective release of intracellular compounds while avoiding undesirable consequences of heating on the characteristics and purity of the extracts. PEF pretreatment with low energetic requirements allows for high extraction yields. However, PEF parameters should be tailored to each microbial cell, according to their structure, size, and other factors affecting efficiency. Furthermore, the recent discovery of the triggering effect of enzymatic activity during cell incubation after electroporation opens up the possibility of new implementations of PEF for the recovery of compounds that are bounded or assembled in structures. Similarly, PEF parameters and suspension storage conditions need to be optimized to reach the desired effect. PEF can be applied in continuous flow and is adaptable to industrial equipment, making it feasible for scale-up to large processing capacities.

Dripping, jetting and tip streaming

Abstract: Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Surface ionization wave propagation in the nanosecond pulsed surface dielectric barrier discharge: the influence of dielectric material and pulse repetition rate

Abstract: In this work, the propagation of the surface ionization wave (SIW) in the nanosecond pulsed surface dielectric barrier discharge with different dielectric materials and pulse repetition rates is investigated. The current waveforms at different locations along the route of the SIW propagation are obtained, based on a specially designed ground strip array geometry. The temporal evolution and spatial distribution of the electric field during the SIW propagation are measured by using the electric field induced second harmonic (EFISH) generation method. The distribution of the residual surface potential after the discharge is mapped with a Kelvin electrostatic probe, which verifies both the existence of the residual electric field and its opposite direction to that during the SIW propagation. It is found that with the dielectric material on which the surface charges decay faster, there are the well-pronounced primary and secondary SIWs with a higher velocity on the voltage rising edge and both the peak current and the peak electric field are also higher, with a less spatial attenuation along the SIW propagation route. It is demonstrated that the residual surface charges with the same polarity as the high-voltage pulse suppress the development of the surface ionization wave.

Electric-Field-Assisted Growth of Vertical Graphene Arrays and the Application in Thermal Interface Materials

Abstract: Owing to the development of electronic devices moving toward high power density, miniaturization, and multifunction, research on thermal interface materials (TIMs) is become increasingly significant. Graphene is regarded as the most promising thermal management material owing to its ultrahigh in-plane thermal conductivity. However, the fabrication of high-quality vertical graphene (VG) arrays and their utilization in TIMs still remains a big challenge. In this study, a rational approach is developed for growing VG arrays using an alcohol-based electric-field-assisted plasma enhanced chemical vapor deposition. Alcohol-based carbon sources are used to produce hydroxyl radicals to increase the growth rate and reduce the formation of defects. A vertical electric field is used to align the graphene sheets. Using this method, high-quality and vertically aligned graphene with a height of 18.7 μm is obtained under an electric field of 30 V cm−1. TIMs constructed with the VG arrays exhibit a high vertical thermal conductivity of 53.5 W m−1 K−1 and a low contact thermal resistance of 11.8 K mm2 W−1, indicating their significant potential for applications in heat dissipation technologies.

Electric-Field Switching of Magnetic Topological Charge in Type-I Multiferroics

Abstract: Applying electric field to control magnetic properties is a very efficient way for spintronics devices. However, the control of magnetic characteristics by electric fields is not straightforward, due to the time-reversal symmetry of magnetism versus spatial inversion symmetry of electricity. Such fundamental difficulty makes it challenging to modify the topology of magnetic skyrmionic states with electric field. Here, we propose a novel mechanism that realizes the electric-field (E) switching of magnetic topological charge (Q) in a controllable and reversible fashion, through the mediation of electric polarization (P) and Dzyaloshinskii-Moriya interaction (D). Such a mechanism is coined here EPDQ. Its validity is demonstrated in a multiferroic VOI_{2} monolayer, which is predicted to host magnetic bimerons. The change in magnetic anisotropy is found to play a crucial role in realizing the EPDQ process and its microscopic origin is discussed. Our study thus provides a new approach toward the highly desired electric-field control of magnetism.

Attosecond optoelectronic field measurement in solids

Abstract: The sub-cycle interaction of light and matter is one of the key frontiers of inquiry made accessible by attosecond science. Here, we show that when light excites a pair of charge carriers inside of a solid, the transition probability is strongly localized to instants slightly after the extrema of the electric field. The extreme temporal localization is utilized in a simple electronic circuit to record the waveforms of infrared to ultraviolet light fields. This form of petahertz-bandwidth field metrology gives access to both the modulated transition probability and its temporal offset from the laser field, providing sub-fs temporal precision in reconstructing the sub-cycle electronic response of a solid state structure. Characterization of light pulses is important in order to understand their interaction with matter. Here the authors demonstrate a nonlinear photoconductive sampling method to measure electric field wave-forms in the infrared, visible and ultraviolet spectral ranges.

Dual-controlled broadband terahertz absorber based on graphene and Dirac semimetal

Abstract: We proposed a dual-controlled broadband terahertz (THz) absorber based on graphene and Dirac semimetal. Calculated results show that the absorptance over 90% is achieved in the frequency range of 4.79-8.99 THz for both transverse electric (TE) and transverse magnetic (TM) polarizations. Benefiting from the advantage of the dielectric constant of these materials varying with chemical doping or gate voltage, the simulation results exhibit that the absorbance bandwidth can be controlled independently or jointly by varying the Fermi energy of the graphene or Dirac semimetal patterns instead of redesigning the absorbers. Impedance matching theory was introduced to analyze the absorption spectra changing with EF. The bandwidth and absorptivity of the proposed absorber are almost independent of changing the incident angle θ up to 35° and 40° for TE and TM modes, respectively. It works well even at a larger incident angle. Because of the symmetry of the structure, this designed absorber is polarization insensitive and almost the same absorptivity for both polarizations. Furthermore, the physical mechanisms were further disclosed by the electric field distributions. The proposed broadband and dual-controlled absorber may have potential applications in various fields of high-performance terahertz devices.