Superconducting and ferromagnetic phases in SrTiO3/LaAlO3 oxide interface structures: possibility of finite momentum pairing

We introduce a model to explain the observed ferromagnetism and superconductivity in LAO/STO oxide interface structures. Because of the polar catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface layer. We argue that this charge localizes and orders ferromagnetically via exchange with the conduction electrons. Ordinarily, this ferromagnetism would destroy superconductivity, but, due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming a Fulde-Ferrell-Larkin-Ovchinikov-type condensate of Cooper pairs at finite momentum, which is surprisingly robust in the presence of strong disorder.     

退火氛围对掺杂ZnO薄膜磁性的影响

采用直流磁控反应共溅法制备了非磁性元素Al和磁性元素Co掺杂的ZnO薄膜, 样品原位真空退火后再空气退火处理. 利用X射线衍射仪(XRD) 和物理性能测量仪(PPMS) 对薄膜的结构和磁性进行了表征. XRD和PPMS结果表明, 不同的退火氛围对掺杂薄膜的结构和磁性有着很大的影响. 真空退火的Al掺杂ZnO薄膜没有观察到铁磁性, 而空气退火的样品却显示出明显的室温铁磁性, 铁磁性的来源与空气退火后导致Al和ZnO基体间电荷转移增强有关. 而对于Co掺杂ZnO薄膜, 真空退火后再空气退火, 室温铁磁性明显减弱. 其磁性变化与Co离子和ZnO基体间电荷转移导致磁性增强和间隙Co原子被氧化导致磁性减弱有关.     

Spin-resolved inverse photoemission of ferromagnetic surfaces

Inverse photoemission (IPE) with spin-polarized electrons provides a way to measure separately the exchange-split majority and minority bands in ferromagnets above the Fermi level. Consequently spin-resolved IPE turns out to be an outstanding technique for obtaining information on surface magnetism: the magnetization of the topmost atomic layer may be investigated by measuring the exchange splitting of electronic states that are localized within the surface layer. Theoretical models of ferromagnetism may be tested by observing the temperature behavior of bands which contribute to the ferromagnetism of the material. The magnetic coupling of an adsorbate to the ferromagnetic substrate may be studied by probing adsorbate-induced bands. Results for the Ni(110) surface serve as an illustration to discuss the status of spin-resolved IPE and its application to the field of surface magnetism.     

Effect of Ar ion irradiation on the room temperature ferromagnetism of undoped and Cu-doped rutile TiO_2 single crystals

Remarkable room-temperature ferromagnetism was observed both in undoped and Cu-doped rutile TiO_2 single crystals(SCs).To tune their magnetism,Ar ion irradiation was quantitatively performed on the two crystals in which the saturation magnetizations for the samples were enhanced distinctively.The post-irradiation led to a spongelike layer in the near surface of the Cu-doped TiO_2.Meanwhile,a new CuO-like species present in the sample was found to be dissolved after the post-irradiation.Analyzing the magnetization data unambiguously reveals that the experimentally observed ferromagnetism is related to the intrinsic defects rather than the exotic Cu ions,while these ions are directly involved in boosting the absorption in the visible region.     

Angle-resolved UV photoelectron spectroscopy of ethylene and benzene on nickel

A review of results obtained by Angle-Resolved UV-Photoelectron Spectroscopy (ARUPS) using linearly polarized synchrotron radiation is presented for two model systems, ethylene/Ni and benzene/Ni. It is shown that for these systems detailed conclusions concerning adsorbate/substrate and adsorbate/adsorbate interactions can be derived from ARUPS spectra using symmetry selection rules, and in combination with model calculations. In particular, electronic structure, bonding, orientation and symmetry of the adsorbates in dilute and saturated layers will be discussed. It is shown that at high adsorbate coverages lateral interactions in the adsorbate layer play a dominant role. Steric effects in densely packed layers can lead to a reorientation of the molecules as compared to the orientation of the single molecules. The ARUPS spectra of well ordered, densely packed layers exhibit significant (up to 2 eV) dispersion of the various adsorbate bands and allow detailed conclusions on two-dimensional adsorbate band structures.     

Ionic monolayers on metals: I. layers of like ions

We estimate the energy of a layer of like ions adsorbed on an atomically smooth face of a metal crystal. We consider only adions of large radius, so that the method of electrostatic images can be used in evaluating the energy. For periodic layers of high density, we find large deviations from the dipole approximation of Topping. Also, we estimate the increase in electrostatic energy accompanying disordering of a layer, and use the result to discuss thermal disordering of Na and Cs layers on W(110). We emphasize that the charge transfer to a layer of like ions must be small. For example, O and S anions of integral charge cannot form densely packed layers on metals.     

Exciton–dopant and exciton–charge interactions in electronically doped OLEDs

The electronic dopants, like tetrafluorocyanoquinodimethane (F₄–TCNQ) molecules, used for p-doping of hole transport layers in organic light-emitting diodes (OLEDs) are found to quench the electroluminescence (EL) if they diffuse into the emissive layer. We observed EL quenching in OLED with F₄-TCNQ doped N,N′-diphenyl-N′N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine hole transport layer at large dopant concentrations, >5%. To separate the effects of exciton–dopant quenching, from exciton–polaron quenching we have intentionally doped the emissive layer of (8-tris-hydroxyquinoline) with three acceptors (A) of different electron affinities: F₄-TCNQ, TCNQ, and C₆₀, and found that C₆₀ is the strongest EL-quencher, while F₄-TCNQ is the weakest, contrary to intuitive expectations. The new effects of charge transfer and usually considered energy transfer from exciton to neutral (A) and charged acceptors (A⁻) are compared as channels for non-radiative Ex–A decay. At high current loads the EL quenching is observed, which is due to decay of Ex on free charge carriers, hole polarons P⁺. We consider contributions to Ex–P⁺ interaction by short-range charge transfer and describe the structure of microscopic charge transfer (CT)-processes responsible for it. The formation of metastable states of ‘charged excitons’ (predicted and studied by Agranovich et al. Chem. Phys. 272 (2001) 159) by electron transfer from a P to an Ex is pointed out, and ways to suppress non-radiative Ex–P decay are suggested.     

两种新型电荷转移盐的合成和光谱研究

合成了两种新的有机盐[NO₂BzPy]Cl(1)和[NO₂BzPyNH₂]Cl(2),(1)[或(2)]与K[TCNQ]₂进一步反应生成[NO₂BzPy] [TCNQ]₂ (3)和[NO₂BzPyNH₂] [TCNQ]₂(4)。光谱数据表明：TCNQ盐中存在TCNQ0和TCNQ-,并形成了一维TCNQ分子柱,且TCNQ₀和TCNQ-之间存在相互作用,部分电荷从[TCNQ]-₂向阳离子([NO₂BzPy]+和[NO₂BzPyNH₂]+)转移。     

控制纳米结构以调控氧化锌的发光、磁性和细胞毒性

氧化锌(ZnO) 纳米材料因其在UV 激光器、发光二极管、太阳能电池、稀磁半导体、生物荧光标示、靶向药物等领域中的广泛应用而成为最热门的研究课题之一. 调节和优化ZnO 纳米结构的性质是ZnO 的实际应用迫切所需. 在此, 通过发展聚乙烯吡咯烷酮导向结晶法、微波加热强制水解法、表面活性剂后处理法, 成功地制备出了尺寸、表面电荷或成分可调的球、半球、棒、管、T 型管、三脚架、片、齿轮、两层、多层、带盖罐子、碗等一系列ZnO 纳米结构. 通过简单地改变ZnO 纳米粒子的尺寸、形貌和表面电荷或成分, 有效地调控ZnO 本身的发光强度和位置, 并近90 倍地增强了荧光素染料的荧光强度; 诱使了强度可调的室温铁磁性; 实现了对ZnO纳米颗粒的细胞毒性的系统性调控.     

V,Cr,Mn掺杂MoS2磁性的第一性原理研究

基于第一性原理的自旋极化密度泛函理论分别研究了过渡金属V, Cr, Mn掺杂单层MoS₂的电子结构、 磁性和稳定性. 结果表明: V和Mn单掺杂均能产生一定的磁矩, 而磁矩主要集中在掺杂的过渡金属原子上, Cr单掺杂时体系不显示磁性. 进一步讨论双原子掺杂MoS₂ 体系中掺杂原子之间的磁耦合作用发现, Mn掺杂的体系在室温下显示出稳定的铁磁性, 而V掺杂则表现出非自旋极化基态. 形成能的计算表明Mn掺杂的MoS₂体系相对V和Cr 掺杂结构更稳定. 由于Mn掺杂的MoS₂ 不仅在室温下可以获得比较好的铁磁性而且其稳定性很高, 有望在自旋电子器件方面发挥重要的作用. 关键词： 2')" href="#">单层MoS₂ 掺杂 铁磁态 第一性原理     

Ferromagnetic behaviour in semiconductors: a new magnetism in search of spintronic materials

The future of the spintronic technology requires the development of magnetic semiconductor materials. Most research groups have focused on diluted magnetic semiconductors because of the promising theoretical predictions and initial results. In this work, the current experimental situation of ZnO based diluted magnetic semiconductors is presented. Recent results on unexpected ferromagnetic-like behaviour in different nanostructures are also revised, focusing on the magnetic properties of Au and ZnO nanoparticles capped with organic molecules. These experimental observations of magnetism in nanostructures without the typical magnetic atoms are discussed. The doubts around the intrinsic origin of ferromagnetism in diluted magnetic semiconductors along with the surprising magnetic properties in absence of the typical magnetic atoms of certain nanostructures should make us consider new approaches in the quest for room temperature magnetic semiconductors.     

Two-dimensional XSe_2(X = Mn,V) based magnetic tunneling junctions with high Curie temperature

Two-dimensional(2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research,magnetic tunneling junctions(MTJs) based on X Se_2(X = Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance(TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe_2. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias.The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer X Se_2(X = Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.     

Gate-induced band ferromagnetism in an organic polymer

We propose that a chain of five-membered rings (polyaminotriazole) should be ferromagnetic with an appropriate doping that is envisaged to be feasible with a field-effect transistor structure. The ferromagnetism is confirmed by a spin density functional calculation, which also shows that ferromagnetism survives the Peierls instability. We explain the magnetism in terms of the Mielke and Tasaki flatband ferromagnetism with the Hubbard model. This opens a new possibility of band ferromagnetism in purely organic polymers.     

Interaction of the ground state of quarter-filled one-dimensional strongly correlated electronic system with localized spins

We study numerically the ground-state properties of the one-dimensional quarter-filled strongly correlated electronic system interacting antiferromagnetically with localized S = 1/2 spins. It is shown that the charge-ordered state is significantly stabilized by the introduction of relatively small coupling with the localized spins. When the coupling becomes large the spin and charge degrees of freedom behave quite independently and the ferromagnetism is realized. Moreover, the coexistence of ferromagnetism with charge order is seen in the strongly interacting region. The present theoretical results are to be compared with the experiments on phthalocyanine compounds.     

Theoretical study of enhanced ferromagnetism and tunable magnetic anisotropy of monolayer CrI3 by surface adsorption

Magnetic anisotropy energy (MAE) plays a key role for 2D magnetic materials, which have attracted significant attention for their promising applications in spintronic devices. Based on first-principles calculations, we have investigated the influence of surface adsorption on the ferromagnetism and MAE of monolayer CrI₃. We find that Li adsorption can dramatically enhance its ferromagnetism, and tune its easy magnetization axis to the in-plane direction from original out-of-plane at certain coverage of Li. The monotonic enhancement of in-plane magnetism in CrI₃ as the coverage of Li increases are attributed to electrostatic doping induced by charge transfer between Li atoms and I atoms, as supported by the charge doping simulation. The tunable robust magnetic anisotropy may open new promising applications of CrI₃–based materials in spintronic devices.     

Characterization of organic material micro-structures transferred by laser in nanosecond and picosecond regimes

The laser-induced forward transfer technique has been performed on thin layers of conducting organic materials for applications in plastic micro-electronics.This process is a promising alternative for fabrication of organic electronic components on flexible supports when usual techniques, such like ink-jet printing, cannot be considered. For example, when the organic material has no solubility properties or when complex architectures are needed.Experiments on the influence of pulse duration (nanosecond and picosecond) and wavelength on a large range of fluences have been proceeded using different lasers. An optimization of the process has been carried out by inserting a thin layer of absorbing metallic material between the substrate and the organic film. The advantage of this technique is to preserve organic layers from being damaged by thermal and photochemical effects during the interaction.The morphology and thickness of the deposit have been investigated by optical and scanning electronic microscopy. This experimental study is supplemented by electrical characterization of the deposits.     

Stepping Stone Mechanism: Carrier-Free Long-Range Magnetism Mediated by Magnetized Cation States in Quintuple Layer

The long-range magnetism observed in group-V tellurides quintuple layers is the only working example of carrierfree dilute magnetic semiconductors(DMS), whereas the physical mechanism is unclear, except the speculation on the band topology enhanced van Vleck paramagnetism. Based on DFT calculations, we find a stable longrange ferromagnetic order in a single quintuple layer of Cr-doped Bi_2Te_3 or Sb_2Te_3, with the dopant separation more than 9 ?. This configuration is the global energy minimum among all configurations. Different from the conventional super exchange theory, the magnetism is facilitated by the lone pair derived anti-bonding states near the cations. Such anti-bonding states work as stepping stones merged in the electron sea and conduct magnetism.Further, spin orbit coupling induced band inversion is found to be insignificant in the magnetism. Therefore, our findings directly dismiss the common misbelief that band topology is the only factor that enhances the magnetism.We further demonstrate that removal of the lone pair derived states destroys the long-range magnetism. This novel mechanism sheds light on the fundamental understanding of long-range magnetism and may lead to discoveries of new classes of DMS.     

Optical properties and component composition of layers of cyanine dyes on dielectric supports: influence of asymmetry of the molecular electron density distribution

The component composition of molecular layers of dicarbocyanine dyes of cationic type on glass is determined by the absorption spectroscopy methods. For symmetric and asymmetric molecules the component composition depends on the chemical structure of a molecule and its interaction with the substrate, which are responsible for the electron density distribution in a molecule. The interaction with the substrate depends on the thickness of a layer and the orientation of a layer molecule on the substrate. The increase in the separation between the molecule and the substrate in thicker layers decreases the interaction with the substrate and the asymmetry of a molecule. In symmetric and slightly asymmetric dyes the value of the charge difference on the end groups decreases with increasing layer thickness, whereas in dyes with a large asymmetry, both the value and the sign of the charge difference change. The above results are confirmed by data on the computer simulation of the charge distribution in a dicarbocyanine cation upon its interaction with the oxygen anion on the substrate.     

Electrically Controlled Dimensionality of Magnetic Systems in Organic Materials

Electrically controlling charge density in materials using electronic device structures shows various interesting phenomena, such as electrically induced ferromagnetism and superconductivity, owing to strong charge interactions. However, electrically controlled dimensionality of magnetic systems has not yet been fully investigated. Here we report electrically controlled magnetic interactions and dimensionality of magnetic systems in organic materials from a microscopic viewpoint, which were revealed by electron spin resonance (ESR) spectroscopy using ionic liquid-gated devices. The ESR investigation demonstrated that the magnetic systems’ dimensionality of electrically accumulated charges varied from zero dimensional to two dimensional in a regioregular polymer, regioregular poly(3-hexylthiophene) (RR-P3HT), by increasing charge densities. This phenomenon is in contrast to those in a small molecule pentacene and a regiorandom polymer, regiorandom poly(3-hexylthiophene) (RRa-P3HT), where it varied from zero to three dimensional when their charge densities increased. Moreover, the formation of the complete spinless states of electrically induced charges was observed. Our investigation demonstrates the novel magnetic systems based on electrically induced charges.     

Electrical control of magnetism in oxides

Recent progress in the electrical control of magnetism in oxides,with profound physics and enormous potential applications,is reviewed and illustrated.In the first part,we provide a comprehensive summary of the electrical control of magnetism in the classic multiferroic heterostructures and clarify the various mechanisms lying behind them.The second part focuses on the novel technique of electric double layer gating for driving a significant electronic phase transition in magnetic oxides by a small voltage.In the third part,electric field applied on ordinary dielectric oxide is used to control the magnetic phenomenon originating from charge transfer and orbital reconstruction at the interface between dissimilar correlated oxides.At the end,we analyze the challenges in electrical control of magnetism in oxides,both the mechanisms and practical applications,which will inspire more in-depth research and advance the development in this field.