有机铁磁界面自旋极化接触构型效应

基于第一性原理,研究了三种不同的接触构型垂直吸附在镍表面的苯双硫分子的界面自旋极化.结果表明界面自旋极化强烈依赖于接触构型,接触构型的变化可使自旋极化由正值变为负值.通过分析投影态密度,发现界面处镍原子的3d轨道与硫原子的sp³杂化轨道发生了轨道杂化.模拟机械可控断裂结实验中的界面吸附构型,根据计算的界面自旋极化,利用Julliere模型得到磁电阻约为27%,与实验测量结果较为符合.     

Electrostatic coupling with interfacial free charge in ferroelectric-paraelectric bilayers and superlattices

Based on a thermodynamic model, we studied the interfacial free charge effect on the polarization, spatial inversion symmetry, and dielectric response in ferroelectric-paraelectric bilayer. The broken spatial inversion symmetry and imprint due to free charge are not found. Effect of free charge on the dielectric constant is not monotonic. Contradiction between the equilibrium polarization and non-linear field-induced dielectric polarization in paraelectric layer is found. Our study reveals that treating coupling strength as an “input” parameter taken dependent of the polarization is inappropriate to describe electrostatic coupling with interfacial free charge.     

铁电双层膜的反转动力学行为

基于Landau-Khalatnikov运动方程,本文研究了含有表面过渡层和铁电界面耦合的反转动力学行为(包括平均极化、反转时间、反转电流和矫顽场).研究结果表明:在铁电双层膜系统中存在一个竞争的机理,即表面过渡层与界面耦合的竞争作用.我们发现在双层膜反转过程中出现了反常行为,这些反常行为归因于表面过渡层与界面耦合之间的竞争.表面过渡层与界面耦合的共同行为对铁电双层膜的动力学特性起到了决定性的作用.     

Theoretical study on spin-polarized injection of electrical currents into polymer semiconductors

Different from electrons and holes in traditional inorganic semiconductors, the charge carriers in polymer semiconductors are spin polarons and spinless bipolarons. In this paper, a theoretical model is presented to describe the spin-polarized injection of electrical currents from a ferromagnetic contact into a nonmagnetic polymer semiconductor. In this model, a new relation of conductivity to concentration polarization for polymer semiconductors is introduced based on a three-channel model to describe the spin-polarized injection of electrical currents under large electrical current densities. The calculated results of the model reveal the effects of the polaron ratio, the carrier concentration polarization, the interfacial conductance, the bulk conductivity of materials, and the electrical current density, etc. on the spin polarization of electrical currents. As conclusions, the large and matched bulk conductivity of materials, the small spin-dependent interfacial conductance, the thin polymer thickness and the large enough electrical current are critical factors for upgrading the spin polarization of electrical currents in polymer semiconductors. Particularly, when the polaron ratio in polymer semiconductors approaches the concentration polarization of the ferromagnetic contact, a modest concentration polarization is sufficient for achieving a nearly complete spin-polarized injection of electrical currents.     

Engineering the Maxwell-Wagner polarization effect

Layered structures, when supporting the Maxwell-Wagner polarization mechanism, exhibit very large effective electric permittivity and thus can be used for miniaturizing purposes. However, the large interfacial dimensions evolved, limit the Maxwell-Wagner polarization at relatively low frequencies. Any element or mechanism that causes a spatial variation of charge density, contributes to the dielectric susceptibility of a medium. Thus, intentionally planted polarization states can be used for further exploiting the Maxwell-Wagner polarization mechanism.     

Two-dimensional intrinsic ferromagnetism at nitride-boride interfaces

We predict theoretically novel two-dimensional interface ferromagnetism at AlN/MgB(2)(0001) using first-principles calculations, where the interface is employed as an ordered structure of spin sites instead of point defects. Although N dangling bonds are apparently saturated, interfacial states exhibit spin polarization. Hund's coupling of the two N p(∥) orbitals as well as low density of states at the Fermi energy contribute to strong band ferromagnetism. Furthermore, first-principles electron transport calculations demonstrate that this interfacial spin polarization is responsible for quantum spin transport. The magnetization can be controlled by applied gate bias voltages.     

Atomic-scale compensation phenomena at polar interfaces

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density-functional theory to show how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by an interfacial charge generated, for example, by oxygen vacancies.     

Ferroelectric Ba0.75Sr0.25TiO3 tunable charge transfer in perovskite devices

Interfacial charge recombination is a main issue causing the efficiency loss of the perovskite solar cells (PSCs). Here, ferroelectric Ba_0.75Sr_0.25TiO₃ (BST) is introduced as a polarization tunable layer to promote the interfacial charge transfer of the PSCs. The coexistence of ferroelectric polarization and charge carriers in BST is confirmed by density functional theory (DFT) calculations. Experimental characterization demonstrates the polarization reversal and the existence of domain in BST film. The BST film conductivity is tested as 2.98×10^-4 S/cm, which is comparable to the TiO₂ being used as the electron transporting layer (ETL) in PSCs. The calculations results prove that BST can be introduced into the PSCs and the interfacial charge transfer can be tuned by ferroelectric polarization. Thus, we fabricated the BST-based PSCs with a champion power conversion efficiency (PCE) of 19.05% after poling.     

A theoretical exploration of the influencing factors for surface potential

Although it has been widely used to probe the interfacial property,dynamics,and reactivity,the surface potential remains intractable for directly being measured,especially for charged particles in aqueous solutions.This paper presents that the surface potential is strongly dependent on the Hofmeister effect,and the theory including ion polarization and ionic correlation shows significant improvement compared with the classical theory.Ion polarization causes a strong Hofmeister effect and further dramatic decrease to surface potential,especially at low concentration;in contrast,ionic correlation that is closely associated with potential decay distance overestimates surface potential and plays an increasing role at higher ionic concentrations.Contributions of ion polarization and ionic correlation are respectively assessed,and a critical point is detected where their contributions can be exactly counteracted.Ionic correlation can be almost neglected at low ionic concentrations,while ion polarization,albeit less important at high concentrations,should be considered across the entire concentration range.The results thus obtained are applicable to other interfacial processes.     

Dielectric Probing of Relaxation Behaviors in PMMA/Organoclay Nanocomposites: Effect of Organic Modification

Poly(methyl methacrylate) (PMMA)–clay nanocomposites (PCN) were prepared with PMMA/clay concentrates and POP-diamines modified clay via melt blending. Partially exfoliated clay layers were uniformly dispersed in the nanocomposites as evidenced from XRD study. The subglassβ relaxation and the segmental α relaxation were observed from the dynamic mechanical thermal analysis (DMTA). The PCNs at 5 wt% clay level from clay concentrate exhibited significantly higher glass-transition temperatures and dynamic storage moduli relative to the neat PMMA. The confinement effect from the tethering of polymer chain to the clay surface would contribute to such a dramatic difference. In addition to the normal-mode relaxations, the αβ-merging process above T _g and the subglass interfacial polarization were detected from the dielectric analysis. Significant increase in dielectric permittivities and dielectric losses due to interfacial polarization and ionic conduction at low frequency were observed for the PCNs. The intensity of the interfacial polarization process increases with the dispersion degree of clay layers and hence the process can be assigned to the space charge polarization of the ionic species in the clay intergalleries.     

自旋极化有机电致发光器件中单线态与三线态激子的形成及调控

由于有机半导体（OSC）材料自旋弛豫时间长、自旋扩散长度大,OSC自旋器件逐渐成为研究热点.对于有机电致发光器件（OLED）,通过自旋极化电极调控单线态和三线态激子比率是提高其效率的有效方法.本文从漂移扩散方程和载流子浓度连续性方程出发,结合朗之万定律建立了一个自旋注入、输运、复合的理论模型.计算了OSC中的极化电子、空穴浓度,得出了单线态和三线态激子的比率.分析了电场强度、自旋相关界面电导、电极和OSC电导率匹配和电极极化率等因素的影响.计算结果表明：两电极注入反向极化的载流子并提高载流子自旋极化率,有     

外场作用下铁电/铁磁双层膜的极化磁化性质

建立了铁电/铁磁双层膜模型,铁电层的电矩用连续标量描述,而铁磁层的自旋应用经典矢量描述.利用蒙特卡罗方法模拟了体系的热力学性质和极化、磁化行为.给出了零场下体系的内能、比热、极化和磁化随温度变化的关系,并分别研究了体系在外磁场和外电场下的极化和磁化行为.模拟结果表明,双层膜体系的内能、比热、极化和磁化性质因层间耦合系数的不同而明显不同,当界面耦合较弱时,双层膜表现出各自的热力学性质,当层间耦合增强到一定程度时,双层膜耦合为一个整体,表现出统一的热力学性质.该双层膜在外场中形成电滞回线和磁滞回线,并表现出偏置特性,界面耦合强度和温度影响滞后回线和偏置现象.     

Spin glass behaviour and extrinsic origin of magnetodielectric effect in non-multiferroic La(2)NiMnO(6) nanoparticles

We report magnetization, dielectric and dc transport properties of La(2)NiMnO(6) nanoparticles. Both dc and ac magnetization measurements indicated a metastable magnetic behaviour with random ferromagnetic and antiferromagnetic interactions below 110 K; critical slow-down, memory and rejuvenation properties signify the spin glass nature. The dc resistivity shows a semiconducting nature but the temperature dependent magnetoresistance (MR) shows a peak at the spin glass transition. The colossal dielectric property and its frequency dependence were interpreted using the Maxwell-Wagner (MW) interfacial polarization model. Impedance analysis along with magnetodielectric (MD) and magnetoresistance (MR) indicates that the observed MD originates from the combined effect of MR and MW interfacial polarization.     

Polarization of light by a polymer film containing elongated drops of liquid crystal with inhomogeneous interfacial anchoring

An optico-mechanical model describing the coherent (directed) transmittance and the degree of polarization of forward-transmitted light by a polymer film with elongated liquid-crystal (LC) drops has been developed. This model, based on the Foldy–Twersky and anomalous-diffraction approximations, makes it possible to analyze the optical response of a film under extension as a function of the film thickness, refractive index of the polymer, the sizes and anisometry parameters of liquid-crystal drops, their concentration, internal structure, polydispersity, and orientation of optical axes. The model is verified based on the comparison of numerical and experimental data for the inverse modification of interfacial anchoring by an ion-forming surfactant. The internal drop structure is determined by solving the problem of minimizing the volume free energy density. A comparative analysis of the calculated transmittance and degree of polarization for films with uniform homeotropic and modified inhomogeneous interfacial anchoring is performed. The spectral polarization characteristics of a film with elongated LC drops and single-domain internal structure, formed under mechanical extension with the aid of surfactants, are investigated.     

Evidence for electret polarization in polyoxymethylene

It has been found that under certain experimental conditions, polyoxymethylene films undergo a microscopic as well as macroscopic polarization resulting in electret formation. Thermally stimulated discharge (TSD) currents have been measured for polyoxymethylene film electrets of 100 μM prepared at various polarizing fields and polarization temperatures. Experimental evidence like the shift of the discharge current peak with polarization temperature, and a nonlinear dependence of the accumulated charge on field strength suggests the formation of a space charge and interfacial polarization. The observed TSD peak at 130°C coincides with the α -relaxation temperature in low-frequency dielectric measurements. Activation energies and relaxation times have been calculated on the basis of the experimental data.     

Interfacial Ferromagnetism in a Co/CdTe Ferromagnet/Semiconductor Quantum Well Hybrid Structure

The magnetization properties of a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure are investigated by several techniques. Exploiting the proximity effect between acceptor bound holes and magnetic ions we detect the magnetization curves by measuring the circular polarization of photoluminescence in an out-of-plane magnetic field. We show that magnetization originates from interfacial ferromagnet on Co-CdMgTe interface and the proximity effect is caused by magnetization of interfacial Co-CdMgTe ferromagnetic layer whose magnetic properties are very different from Co.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg--Landau--Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interfacial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectric materials.     

Ultrafast reorientation of dangling OH groups at the air-water interface using femtosecond vibrational spectroscopy

We report the real-time measurement of the ultrafast reorientational motion of water molecules at the water-air interface, using femtosecond time- and polarization-resolved vibrational sum-frequency spectroscopy. Vibrational excitation of dangling OH bonds along a specific polarization axis induces a transient anisotropy that decays due to the reorientation of vibrationally excited OH groups. The reorientation of interfacial water is shown to occur on subpicosecond time scales, several times faster than in the bulk, which can be attributed to the lower degree of hydrogen bond coordination at the interface. Molecular dynamics simulations of interfacial water dynamics are in quantitative agreement with experimental observations and show that, unlike in bulk, the interfacial reorientation occurs in a largely diffusive manner.     

Effect of Carrier Differences on Spin Polarized Injection into Organic and Inorganic Semiconductors

Spin polarized injection into organic and inorganic semiconductors are studied theoretically from the spin diffusion theory and Ohm＇s law, and the emphases are placed on the effect of the carrier differences on the current spin polarization. The mobility and the spin-flip time of carriers in organic and inorganic semiconductors are different. From the calculation, it is found that current spin polarization at a ferromagnetic/organic interface is higher than that at a ferromagnetic/inorganic interface because of different carriers in them. Effects of the conductivity matching, the spin dependent interracial resistances, and the bulk spin polarization of the ferromagnetic layer on the spin polarized injection are also discussed.