Ab initio nonadiabatic molecular dynamics study on spin–orbit coupling induced spin dynamics in ferromagnetic metals

Understanding the photoexcitation induced spin dynamics in ferromagnetic metals is important for the design of photo-controlled ultrafast spintronic device.In this work,by the ab initio nonadiabatic molecular dynamics simulation,we have studied the spin dynamics induced by spin–orbit coupling(SOC)in Co and Fe using both spin-diabatic and spin-adiabatic representations.In Co system,it is found that the Fermi surface(E_(F))is predominantly contributed by the spin-minority states.The SOC induced spin flip will occur for the photo-excited spin-majority electrons as they relax to the E_(F),and the spin-minority electrons tend to relax to the EFwith the same spin through the electron–phonon coupling(EPC).The reduction of spin-majority electrons and the increase of spin-minority electrons lead to demagnetization of Co within100 fs.By contrast,in Fe system,the E_(F) is dominated by the spin-majority states.In this case,the SOC induced spin flip occurs for the photo-excited spin-minority electrons,which leads to a magnetization enhancement.If we move the E_(F) of Fe to higher energy by 0.6eV,the E_(F) will be contributed by the spin-minority states and the demagnetization will be observed again.This work provides a new perspective for understanding the SOC induced spin dynamics mechanism in magnetic metal systems.

Quantitative determination of the critical points of Mott metal–insulator transition in strongly correlated systems

Mottness is at the heart of the essential physics in a strongly correlated system as many novel quantum phenomena occur in the metallic phase near the Mott metal–insulator transition. We investigate the Mott transition in a Hubbard model by using the dynamical mean-field theory and introduce the local quantum state fidelity to depict the Mott metal–insulator transition. The local quantum state fidelity provides a convenient approach to determining the critical point of the Mott transition. Additionally, it presents a consistent description of the two distinct forms of the Mott transition points.

Metal-to-insulator transition in two-dimensional ferromagnetic monolayer induced by substrate

Two-dimensional （2D） ferromagnetic （FM） materials have great potential for applications in next-generation spin- tronic devices. Since most 2D FM materials come from van der Waals crystals, stabilizing them on a certain substrate without killing the ferromagnetism is still a challenge. Through systematic first-principles calculations, we proposed a new family of 2D FM materials which combines TaX （X= S, Se or Te） monolayer and A1203（0001） substrate. The TaX monolayers provide magnetic states and the A1203（0001） substrate stabilizes the former. Interestingly, the A1203（0001） substrate leads to a metal-to-insulator transition in the TaX monolayers and induces a band gap up to 303 meV. Our study paves the way to explore promising 2D FM materials for practical applications in spintronics devices.

Possible Origins for Paramagnetic-ferromagnetic and Insulator-metal Transitions as well as Colossal Magnetoresistance in Manganites

Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.

Pressure induced insulator to metal transition in quantum spin liquid candidate NaYbS_(2)

Pressure induced insulator to metal transition followed by the appearance of superconductivity has been observed recently in inorganic quantum spin liquid candidate NaYbSe_(2).In this paper,we study the properties of isostructural compound NaYbS_(2)under pressure.It is found that the resistance of Na YbS_(2)single crystal exhibits an insulating state below 82.9 GPa,but with a drop of more than six orders of magnitude at room temperature.Then a minimum of resistance is observed at about 100.1 GPa and it moves to lower temperature with further compression.Finally,a metallic state in the whole temperature range is observed at about 130.3 GPa accompanied by a non-Fermi liquid behavior below 100 K.The insulator to metal transition,non-monotonic resistance feature and non-Fermi liquid behavior of NaYbS_(2)under pressure are similar to those of NaYbSe_(2),suggesting that these phenomena might be the universal properties in NaLnCh_(2)(Ln=rare earth,Ch=O,S,Se)system.

Dynamical anisotropic magnetoelectric effects at ferroelectric/ferromagnetic insulator interfaces

The interfacial magnetoelectric interaction originating from multi-orbital hopping processes with ferroelectricassociated vector potential is theoretically investigated for complex-oxide composite structures.Large mismatch in the electrical permittivity of the ferroelectric and ferromagnetic materials gives rise to giant anisotropic magnetoelectric effects at their interface.Our study reveals a strong linear dynamic magnetoelectric coupling which genuinely results in electric control of magnetic susceptibility.The constitutive conditions for negative refractive index of multiferroic composites are determined by the analysis of light propagation.

Ferromagnetic-insulators-modulated transport properties on the surface of a topological insulator

Transport properties on the surface of a topological insulator （TI） under the modulation of a two-dimensional （2D） ferromagnet/ferromagnet junction are investigated by the method of wave function matching. The single ferromagnetic barrier modulated transmission probability is expected to be a periodic function of the polarization angle and the planar rotation angle, that decreases with the strength of the magnetic proximity exchange increasing. However, the transmission probability for the double ferromagnetic insulators modulated n-n junction and n-p junction is not a periodic function of polarization angle nor planar rotation angle, owing to the combined effects of the double ferromagnetic insulators and the barrier potential. Since the energy gap between the conduction band and the valence band is narrowed and widened respectively in ranges of 0 ≤ 0 〈π/2 and r/2 〈 0 ≤ π, the transmission probability of the n-n junction first increases rapidly and then decreases slowly with the increase of the magnetic proximity exchange strength. While the transmission probability for the n-p junction demonstrates an opposite trend on the strength of the magnetic proximity exchange because the band gaps contrarily vary. The obtained results may lead to the possible realization of a magnetic/electric switch based on TIs and be useful in further understanding the surface states of TIs.

Growth and transport properties of topological insulator Bi2Se3 thin film on a ferromagnetic insulating substrate

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi2Se3 thin film on a single-crystalline LaCoO3 thin film on SrTiO3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi2Se3 films with highest quality on LaCoO3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi2Se3/LaCoO3 heterostructures, which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.

Ferromagnetic barrier-induced negative differential conductance on the surface of a topological insulator

The effect of the negative differential conductance of a ferromagnetic barrier on the surface of a topological insulat（ is theoretically investigated. Due to the changes of the shape and position of the Fermi surfaces in the ferromagnetic barrie the transport processes can be divided into three kinds： the total, partial, and blockade transmission mechanisms. The bias voltage can give rise to the transition of the transport processes from partial to blockade transmission mechanisms, which results in a considerable effect of negative differential conductance. With appropriate structural parameters, the currenl voltage characteristics show that the minimum value of the current can reach to zero in a wide range of the bias voltag and then a large peak-to-valley current ratio can be obtained.

Erratum:Magnetic Proximity Effect in an Antiferromagnetic Insulator/Topological Insulator Heterostructure with Sharp Interface[Chin.Phys.Lett.38(2021)057303]

We should add the following acknowledge:Jing Teng thanks the support from the Youth Innovation Promotion Association Project,Chinese Academy of Sciences.

Tunneling Conductance in Quantum-Wire/Ferromagnetic-Insulator/d-Wave Superconductor Junction

我们在更金色的 Tinkham Klapwijk 模型的框架在量电线 /ferromagnetic-insulator/d 波浪超导体连接(q/FI/d ) 学习了伪粒子运输。我们基于 Bogoliubov-de Gennes 方程在零温度和有限温度作为偏爱电压的功能在 q/FI/d 计算通道传导力。与在 normal-metal/insulator/d 波浪超导体连接的盒子不同，零偏爱的传导力山峰为单个模式的盒子消失。通道传导力系列取决于交换相互作用的大小在铁磁性绝缘体。

Coupling ferromagnetic ordering electron transfer channels and surface reconstructed active species for spintronic electrocatalysis of water oxidation

Sluggish reaction kinetics of oxygen evolution reaction(OER), resulting from multistep proton-coupled electron transfer and spin constriction, limits overall efficiency for most reported catalysts. Herein, using modeled ZnFe_(2-x)Ni_xO_(4)(0 ≤ x ≤ 0.4) spinel oxides, we aim to develop better OER electrocatalyst through combining the construction of ferromagnetic(FM) ordering channels and generation of highly active reconstructed species. The number of symmetry-breaking Fe–O–Ni structure links to the formation of FM ordering electron transfer channels. Meanwhile, as the number of Ni^(3+)increases, more ligand holes are formed, beneficial for redirecting surface reconstruction. The electro-activated ZnFe_(1.6)Ni_(0.4)O_(4) shows the highest specific activity, which is 13 and 2.5 times higher than that of ZnFe_(2)O_(4) and unactivated ZnFe_(1.6)Ni_(0.4)O_(4), and even superior to the benchmark IrO_(2) under the overpotential of 350 mV. Applying external magnetic field can make electron spin more aligned, and the activity can be further improved to 39 times of ZnFe_(2)O_(4). We propose that intriguing FM exchange-field interaction at FM/paramagnetic interfaces can penetrate FM ordering channels into reconstructed oxyhydroxide layers, thereby activating oxyhydroxide layers as spin-filter to accelerate spin-selective electron transfer. This work provides a new guideline to develop highly efficient spintronic catalysts for water oxidation and other spin-forbidden reactions.

Metal magnetic memory testing for early damage assessment in ferromagnetic materials

In order to investigate the physical mechanism of metal magnetic memory testing, both the influences of earth magnetic field and applied stress on magnetic domain structure were discussed. Static tension and fatigue tests for low carbon steel plate specimens were carried out on hydraulic servo testing machine of MTS810 type and magnetic signals were measured during the processes by the type of EMS-2003 instrument. The results indicate that the initial magnetic signals of specimens are different before loading. The magnetic signals curves are transformed from initial random to regular pattern due to the effect of two types of loads. However, the shape and distribution of magnetic signal curves in the elastic region are different from that of plastic region in tension test. While in fatigue test those magnetic signals curves corresponding to different cycles are similar. The H_p(y) value of magnetic signals on the fracture zone increases dramatically at the breaking transient time and positive-negative magnetic poles occur on the two parts of fracture zone.

Impact of the Insulator on the Electric Field and Generation Characteristics of Vacuum Arc Metal Plasmas

The field electron emission plays a vital role in the process of vacuum discharge breakdown. The electric field strength at the cathode tip is significant to the generation char- acteristics of vacuum arc metal plasmas. To increase the field strength at the cathode tip, a coaxial electrode plasma source was employed with an insulator settled between the electrodes. The math expression of the field strength is derived based on the Gauss theory. The impact of the insulator on the electric field and parameters of plasmas were investigated by MAXWELL 3D simulation software and the Langmuir probe. In addition, a composite insulator was adopted to further strengthen the field strength. A series of experiments were performed to focus on the role of the composite insulator in detail. The experimental and simulation results indicate that, a reasonable layout of the insulator, especially the composite insulator, can effectively increase the field strength at the cathode tip and the plasma density.

Rubidium-induced phase transitions among metallic,band-insulating,Mott-insulating phases in 1T-TaS_(2)

Realizing phase transitions via non-thermal sample manipulations is important not only for applications,but also for uncovering the underlying physics.Here,we report on the discovery of two distinct metal–insulator transitions in 1T-TaS_(2) via angle-resolved photoemission spectroscopy and in-situ rubidium deposition.At 205 K,the rubidium deposition drives a normal metal–insulator transition via filling electrons into the conduction band.While at 225 K,however,the rubidium deposition drives a bandwidth-controlled Mott transition as characterized by a rapid collapsing of Mott gap and a loss of spectral weight of the lower Hubbard band.Our result,from a doping-controlled perspective,succeeds in distinguishing the metallic,band-insulating,and Mott-insulating phases of 1T-TaS_(2),manifesting a delicate balance among the electronitineracy,interlayer-coupling and Coulomb repulsion.We also establish an effective method to tune the balance between these interactions,which is useful in seeking exotic electronic phases and designing functional phase-changing devices.

Numerical and experimental evaluation for density-related thermal insulation capability of entangled porous metallic wire material

Entangled porous metallic wire material(EPMWM)has the potential as a thermal insulation material in defence and engineering.In order to optimize its thermophysical properties at the design stage,it is of great significance to reveal the thermal response mechanism of EPMWM based on its complex structural effects.In the present work,virtual manufacturing technology(VMT)was developed to restore the physics-based 3D model of EPMWM.On this basis,the transient thermal analysis is carried out to explore the contact-relevant thermal behavior of EPMWM,and then the spiral unit containing unique structural information are further extracted and counted.In particular,the thermal resistance network is numerically constructed based on the spiral unit through the thermoelectric analogy method to accurately predict the effective thermal conductivity(ETC)of EPMWM.Finally,the thermal diffusivity and specific heat of the samples were obtained by the laser thermal analyzer to calculate the ETC and thermal insulation factor of interest.The results show that the ETC of EPMWM increases with increasing temperature or reducing density under the experimental conditions.The numerical prediction is consistent with the experimental result and the average error is less than 4%.

Hybrid metal-insulator-metal structures on Si nanowires array for surface enhanced Raman scattering

Surface enhanced Raman scattering(SERS)is an efficient technique to detect low concentration molecules.In this work,periodical silicon nanowires(Si NWs)integrated with metal-insulator-metal(MIM)layers are employed as SERS substrates.Laser interference lithography(LIL)combined with reactive ion etching(RIE)is used to fabricate large-area periodic nanostructures,followed by decorating the MIM layers.Compared to MIM disks array on Si surface,the SERS enhancement factor(EF)of the MIM structures on the Si NWs array can be increased up to 5 times,which is attributed to the enhanced electric field at the boundary of the MIM disks.Furthermore,high density of nanoparticles and nanogaps serving as hot spots on sidewall surfaces also contribute to the enhanced SERS signals.Via changing the thickness of the insulator layer,the plasmonic resonance can be tuned,which provides a new localized surface plasmon resonance(LSPR)characteristic for SERS applications.

High density Al2O3/TaN-based metal-insulatormetal capacitors in application to radio equency integrated circuits

Metal-insulator-metal （MIM） capacitors with atomic-layer-deposited Al2O3 dielectric and reactively sputtered TaN electrodes in application to radio frequency integrated circuits have been characterized electrically. The capacitors exhibit a high density of about 6.05 fF/μm^2, a small leakage current of 4.8 × 10^-8 A/cm^2 at 3 V, a high breakdown electric field of 8.61 MV/cm as well as acceptable voltage coefficients of capacitance （VCCs） of 795 ppm/V2 and 268ppm/V at 1 MHz. The observed properties should be attributed to high-quality Al2O3 film and chemically stable TaN electrodes. Further, a logarithmically linear relationship between quadratic VCC and frequency is observed due to the change of relaxation time with carrier mobility in the dielectric. The conduction mechanism in the high field ranges is dominated by the Poole-Frenkel emission, and the leakage current in the low field ranges is likely to be associated with trap-assisted tunnelling. Meanwhile, the Al2O3 dielectric presents charge trapping under low voltage stresses, and defect generation under high voltage stresses, and it has a hard-breakdown performance.

Plasmonically induced reflection in metal-insulator-metal waveguides with two silver baffles coupled square ring resonator

A plasmonic waveguide coupled system that is composed of a square ring cavity and a metal-insulator-metal （MIM） waveguide with two silver baffles is proposed. The transmission and reflection properties of the proposed plasmonic system are investigated numerically using the finite element method. The normalized Hz field distributions are calculated to analyze the transmission mode in the plasmonic system. The extreme destructive interference between light mode and dark mode causes plasmonically induced reflection （PIR） window in the transmission spectrum. The PIR window is fitted using the coupled mode theory. The analytical result agrees with the simulation result approximately. In addition, the PIR window can be controlled by adjusting structural parameters and filling different dielectric into the MIM waveguide and the square ring cavity. The results provide a new approach to designing plasmonic devices.

Preparation of ferromagnetic metal fine fibers by organic gel-thermal reduction process

The organic gel-thermal reduction process was used for the preparation of ferromagnetic metal Ni, Co and Fe fine fibers from the raw materials of citric acid, lactic acid and metal salts. The structure, thermal decomposition process and morphologies of the gel precursors and fibers derived from thermal reduction of these gel precursors were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermo-gravimetric/differential scanning calorimetry and scanning electron microscopy. The results show that spinnability of gel largely depends on molecular structure of metal-carboxylate complex that is a linear-type structure formed in the gel. As a result, the gels exhibit a good spinnability. Metal Ni, Co and Fe fine fibers are featured with diameters of around 1 μm and a high aspect ratio up to 1×106.