Efros-Shklovskii law and localized states in weakly doped lanthanum manganites

The existence of clusters in weakly doped lanthanum manganites at temperatures about twice as high as their Curie temperature T _C has been shown. Electrical resistance in weakly doped lanthanum manganites obeys the Efros-Shklovskii law. The temperature and magnetic-field dependences of a cluster size determined from the magnetotransport properties have been described using the model of phase separation into small metallic droplets within the dielectric paramagnetic and antiferromagnetic matrices. The results agree with the existence of the Griffiths phase.     

Linear expansion, phase separation, and magnetic inhomogeneities in La0.92Ca0.08MnO3

A relation of the thermal expansion with magnetic and magnetotransport properties has been revealed in La_0.92Ca_0.08MnO₃ single crystals in the paramagnetic state. The magnetotransport and lattice properties and the specific features in the neutron scattering characteristics of the La_0.92Ca_0.08MnO₃ single crystals have been explained by the phase separation in the paramagnetic state into magnetic inhomogeneities (clusters) with short-range (～10 Å) and long-range (>10² Å) orders. The performed investigations have demonstrated that the clusters are closely related to the crystal lattice and that the magnetic inhomogeneities in the paramagnetic region are correlated to T ～ 250–300 K ? TC.     

Spin valve with a composite dysprosium-based pinned layer as a tool for determining Dy nanolayer helimagnetism

Spin valves with nanostructure CoFe/Dy/CoFe and three-layer structures metal/Dy/metal were prepared by magnetron sputtering. The measurements of field dependences of magnetoresistance and magnetization were held at different temperatures. The changes of magnetotransport properties of spin valve containing CoFe/Dy/CoFe structure were used for getting information on the magnetic ordering in the dysprosium layer. The characteristic changes of magnetotransport properties caused by the formation of helical ordering in dysprosium layer were detected. Special attention was paid to the estimation of Neel temperature and to the investigation of microstructure of dysprosium nanolayer.     

Energy and structure of copper clusters （n=70-150） studied by the Monte Carlo computer simulation

The structure and binding energy of copper clusters of the size range 70 to 150 were studied by using the embeddedatom method. The stability of the structure of the clusters was studied by calculating the average binding energy per atom, first difference energy and second difference energy of copper cluster. Most of the copper clusters of the size n=70-150 adopt an icosahedral structure. The results show that the trends are in agreement with theoretic prediction for copper clusters. The most stable structures for copper clusters are found at n=77, 90, 95, 131, 139.     

Band structure and density of States effects in co-based magnetic tunnel junctions

Utilizing Co/Al(2)O(3)/Co magnetic tunnel junctions with Co electrodes of different crystalline phases, a clear relationship between electrode crystal structure and junction transport properties is presented. For junctions with one fcc(111) textured and one polycrystalline (polyphase and polydirectional) Co electrode, a strong asymmetry is observed in the magnetotransport properties, while when both electrodes are polycrystalline the magnetotransport is essentially symmetric. These observations are successfully explained within a model based on ballistic tunneling between the calculated band structures (density of states) of fcc-Co and hcp-Co.     

Agn(n=2～10)团簇的几何结构和电子特性

应用密度泛函理论中B3LYP/LANL2DZ 方法优化计算并分析了Agn(n=2～10)团簇的基态几何结构及电子性质.同时计算和讨论了银团簇的原子化能、能级分布、能级间隙、电子亲和能和电离势,所得理论计算值与实验值符合较好.研究结果表明:银小团簇的结构不同于块体,且随团簇尺寸大小而相应变化,原子化能和电子亲和势随原子尺寸的增加而增加,团簇的费米能级、电子亲和势和电离势随团簇大小变化具有明显的奇偶振荡特性,并对此作了分析.团簇的电子性质和几何结构之间的密切关系及其随团簇尺寸大小变化的规律,可以从理论上确定团簇的最稳定结构,并可对实验观测结果做出解释.     

Transmission electron microscopy of thiol-capped Au clusters on C: Structure and electron irradiation effects

High-resolution transmission electron microscopy is used to study interactions between thiol-capped Au clusters and amorphous C support films. The morphologies of the clusters are found to depend both on their size and on the local structure of the underlying C. When the C is amorphous, larger Au clusters are crystalline, while smaller clusters are typically disordered. When the C is graphitic, the Au particles adopt either elongated shapes that maximize their contact with the edge of the C film or planar arrays when they contain few Au atoms. We demonstrate the influence of electron beam irradiation on the structure, shape and stability of the Au clusters, as well as on the formation of holes bounded by terraces of graphitic lamellae in the underlying C.     

Thickness and microstructure dependent transport and MR in La0.7Pb0.3MnO3 manganite films

Thickness dependent modifications in the structure and microstructure and their effect on the transport and magnetotransport in chemical solution deposition (CSD) grown La_0.7Pb_0.3MnO₃ (LPMO) manganite thin films grown on single crystalline LaAlO₃ (LAO) (1 0 0) substrates have been studied. X-ray diffraction (XRD) measurements show the reduction in the microstrain at the film-substrate interface with increasing thickness. Increase in grain size and island like grain growth with the reduction in surface roughness as a function of film thickness has been observed from the microstructural studies using atomic force microscopy (AFM) and lateral force microscopy (LFM) measurements. Improvement in the surface to volume ratio (D⁻¹) resulting in the modifications in transport and magnetotransport properties of the LPMO films has been discussed in detail in the light of thickness dependent microstructural effects on the resistivity and magnetoresistance (MR) behavior. The variations in intrinsic and extrinsic MR with D⁻¹ show an interesting interplay between them, which can be explained on the basis of thickness dependent grain size, grain boundary density and grain boundary nature.     

Formation of the structure of gold nanoclusters during crystallization

The structure formation in gold nanoparticles 1.6–5.0 nm in diameter is studied by molecular dynamics simulation using a tight-binding potential. The simulation shows that the initial fcc phase in small Au clusters transforms into other structural modifications as temperature changes. As the cluster size increases, the transition temperature shifts toward the melting temperature of the cluster. The effect of various crystallization conditions on the formation of the internal structure of gold nanoclusters is studied in terms of microcanonical and canonical ensembles. The stability boundaries of various crystalline isomers are analyzed. The obtained dependences are compared with the corresponding data obtained for copper and nickel nanoparticles. The structure formation during crystallization is found to be characterized by a clear effect of the particle size on the stability of a certain isomer modification. Nickel and copper clusters are shown to exhibit common features in the formation of their structural properties, whereas gold clusters demonstrate much more complex behavior.     

Tuning magnetotransport through a magnetic kink crystal in a chiral helimagnet

We consider magnetotransport properties in a conducting chiral helimagnet, where the magnetic kink crystal (MKC) is formed under weak magnetic field applied perpendicular to the helical axis. The MKC behaves as a magnetic superlattice potential and results in Bragg scattering of conduction electrons. Tuning of the weak magnetic field enables us to control the size of the superlattice Brillouin zone and gives rise to a series of divergent resistivity anomalies originating from resonant Bragg scatterings. We discuss as well a nontrivial magnetic structure in the resonant states realized in the subsystem of the itinerant electrons.     

Magnetism of free and supported clusters: a comparative study

The presented work investigates the differences in magnetic properties of free and supported clusters via ab-initio calculations. The electronic structure of the clusters was calculated using a spin polarized relativistic multiple-scattering Green’s function formalism. We focus on Fe clusters of 2–9 atoms, either free or supported by Ni(001), and on Co clusters of 2–7 atoms, either free or supported by Au(111). For the supported clusters, the spin and orbital magnetic moments depend on the cluster size nearly monotonically, while for the free clusters large quasi-oscillations of magnetic moments with the cluster size were observed. Similarly, for supported clusters, the local spin magnetic moments decrease nearly linearly with increasing coordination number, while for free clusters of the same size range the trend is much more complicated. These findings are consistent with the fact that the spectral distribution function contains much sharper features for free clusters than for supported clusters.      

The non-ideal solvation of NaCl in solvent: a simulation study

The non-ideality of the solvation of NaCl in a family of modified hybrid water solvent environments with varying local structural order is investigated using intensive molecular dynamics simulations. The results show that there are ion clusters of different size in all environments studied. The size distribution of the ion clusters is sensitive to the local structure of the solvent, which is determined by the hydrogen bonding strength. The ions are inclined to be in contact in solvents with high translational order, for competition between structure breaking among solvent molecules and structure formation around hydrated ions.     

密度泛函理论研究ScnO(n=1—9)团簇的结构、稳定性与电子性质

采用基于密度泛函理论的BP86/CEP-121G （O原子采用6-311G^**基组）方法,对Sc_nO （n=1—9）团簇的几何结构、能量与稳定性、电子结构性质及其随团簇尺寸的变化趋势进行了研究.随着团簇原子个数的增加,O原子从位于Sc_n团簇结构的边缘转变为占据团簇的内部位置.O原子的掺入增加了Sc_n团簇的稳定性,使其能隙升高,并改变了其稳定性及电子结构性质随团簇尺寸变化的规律;含有偶数个Sc原子的氧化物团簇比其周围邻近的含有奇数个Sc原子的氧化物团簇具有相对较高的稳定性.Sc_nO团簇电离势的理论计算值与实验值符合得较好,而其电子亲和势呈现振荡交替上升的变化趋势;用最大化学硬度规律等方法表征了Sc_nO氧化物团簇的稳定性和电子结构性质. 关键词： nO团簇')" href="#">Sc_nO团簇 几何结构 电子性质 密度泛函理论     

硅团簇熔化行为的紧束缚分子动力学研究

利用紧束缚分子动力学方法研究了硅团簇Si_n(n=5—10)的熔化行为.给出了团簇 熔化潜热 和熔点随团簇尺寸的变化关系，表明团簇熔化潜热和熔点强烈依赖于团簇的原子数.计算结 果表明硅团簇熔化机理与金属团簇熔化有很大不同，金属小团簇的熔化是一个从低温类固态 向高温类固态转变的过程，在转变温区，类固态和类液态处于动力学共存，而硅团簇在转变 温区则是处于一种中间态，这种中间态既不是类固态又不是类液态.比较了用不同计算方法 和定义方法所得硅团簇熔点. 关键词： 紧束缚 硅团簇 熔化潜热     

Size and composition dependence of the frozen structures in Co-based bimetallic clusters

This Letter studies the size-dependent freezing of Co, Co-Ni, and Co-Cu clusters by using molecular dynamics with embedded atom method. Size effect occurs in these three types of clusters. The clusters with large sizes always freeze to form their bulk-like structures. However, the frozen structures for small sizes are generally related to their compositions. The icosahedral clusters are formed for Co clusters (for ?3.2 nm diameter) and also for Co-Ni clusters but at a larger size range (for ?4.08 nm). Upon the Co-Cu clusters, decahedral structure is obtained for small size (for 2.47 nm). The released energy induced the structural transformation plays a key role in the frozen structures. These results indicate that the preformed clusters with special structures can be tuned by controlling their compositions and sizes.     

Experiments on clusters

In this contribution we discuss four different types of experiments that have been conducted at molecular beams of neutral clusters. The size of particularly stable sodium chloride clusters and their corresponding geometrical structure is inferred from intensity anomalies in mass spectra. This information is obtained either for charged or for neutral clusters depending on whether the clusters are ionized by electron impact or by multiphoton absorption. The important role of fragmentation in mass spectrometry of xenon clusters is revealed by multiphoton ionization; dissociative reactions occurring on the time scale of 10⁻⁷ s with respect to the ionizing event can be analyzed. The solvation energy of negatively charged carbon dioxide clusters as a function of cluster size is obtained from electron attachment spectra. A resonance in the ion yield close to zero eV electron energy signifies that all clusters except for the monomer feature a positive electron affinity. An analysis of the kinetic energy of fragment ions, originating from delayed dissociation of triply charged carbon dioxide clusters, reveals that the size distribution of their fission fragments is extremely symmetric.     

Entropic effects on the size dependence of cluster structure

We show that the vibrational entropy can play a crucial role in determining the equilibrium structure of clusters by constructing structural phase diagrams showing how the structure depends upon both size and temperature. These phase diagrams are obtained for example rare gas and metal clusters.     

Anomalous interlayer magnetoresistance in bilayer crystals

The interlayer magnetotransport of a model layered metal is calculated semiclassically. Each layer contains parallel quasi-1D wires but the orientation of wires within each layer is perpendicular to the orientation of wires in adjacent layers. The model has a highly anisotropic amplitude for interlayer electron transfer and is used to illustrate simply the effects that this anisotropy has on the magnetotransport. Strong positive magnetoresistance is calculated for magnetic fields parallel to the current, with the size of magnetoresistance varying inversely with the interlayer hopping amplitude. For fields perpendicular to the current, the magnetoresistance depends qualitatively on the orientation of the field: it scales linearly with the field strength B when the field points toward intersections of 1D Fermi surfaces belonging to individual layers, and scales as √B when the field points between intersections. In a weak field, the resistance is maximum when the field is orientated parallel to the current and minimum when it is perpendicular to the current. Magnetoresistance oscillations are also studied. The implications for more general models of multilayer metals are discussed.     

Structure determination of isolated metal clusters via far-infrared spectroscopy

We present a new method for the size selective structure determination of small isolated metal clusters in the gas phase. The technique is applied to cationic vanadium clusters containing 6 to 23 atoms, whose far infrared absorption spectra are measured in the 140-450 cm(-1) spectral range. The spectra are unique for each cluster size and are true fingerprints of the cluster's structure. By comparing the experimental spectra to spectra obtained from density-functional theory, the geometric cluster structure can be identified.     

铟团簇结构和电子性质的密度泛函研究

用密度泛函方法研究了Inn(n=2-7)团簇的稳定结构和电子性质。结果表明：自旋多重度对结构的影响不大;对于基态结构,n≤5时为平面结构,n≥6时为立体结构,n=6为结构转变点;平均结合能曲线随团簇尺寸增大逐渐平缓;能隙、结合能的二阶差分和电离势随团簇尺寸的变化趋势完全一致,均反映出In4团簇的基态结构较为稳定,具有较强的非金属性。