Spin valve effect in a magnetic nanoelectromechanical shuttle

We study spin-dependent shuttle phenomena in a nanoelectromechanical single electron transistor (NEM-SET) with magnetic leads by considering the coupling between the transport of spin-polarized electrons and mechanical oscillations of the nanometer quantum dot. It is shown that there are two different bias-voltage thresholds for the shuttle instability in electronic transport through the NEM-SET, respectively, corresponding to parallel (P) and antiparallel (AP) magnetization alignments. In between the two thresholds, the electronic transport is in the shuttling regime for the P alignment but in the tunneling regime for the AP one, resulting in a very large spin valve effect.     

Quantum shuttle phenomena in a nanoelectromechanical single-electron transistor

An analytical analysis of quantum shuttle phenomena in a nanoelectromechanical single-electron transistor has been performed in the realistic case, when the electron tunneling length is much greater than the amplitude of the zero point oscillations of the central island. It is shown that when the dissipation is below a certain threshold value, the vibrational ground state of the central island is unstable. The steady state into which this instability develops is studied. It is found that if the electric field E between the leads is much greater than a characteristic value E(q), the quasiclassical shuttle picture is recovered, while if E0) shuttle vibrations.     

Quantum-limited sensitivity of single-electron-transistor-based displacement detectors

We consider a model of a quantum-mechanical resonator capacitively coupled to a single electron transistor (SET). The tunnel current in the SET is modulated by the vibrations of the resonator, and thus the system operates as a displacement detector. We analyze the effect of the backaction noise of charge fluctuations in the SET onto the dynamics of the resonator and evaluate the displacement sensitivity of the system. The relation between the "classical" and "quantum" parts of the SET charge noise and their effect on the measured system are also discussed.     

Coulomb promotion of spin-dependent tunneling

We study transport of spin-polarized electrons through a magnetic single-electron transistor (SET) in the presence of an external magnetic field. Assuming the SET to have a nanometer size central island with a single-electron level we find that the interplay on the island between coherent spin-flip dynamics and Coulomb interactions can make the Coulomb correlations promote rather than suppress the current through the device. We find the criteria for this new phenomenon--Coulomb promotion of spin-dependent tunneling--to occur.     

Effect of a constant magnetic field on echo signals in high-temperature superconductor powders

The generation of acoustic and vortex oscillations in high-temperature superconductor (HTSC) powders excited by radiofrequency (rf) pulses was analyzed in detail in our earlier publications. The rf magnetic field stimulates oscillations of magnetic vortices on the surface of an HTSC grain, which are transformed into lattice vibrations via the pinning centers at the surface, thus inducing a propagating acoustic wave. The allowance for second-order nonlinearity in the gradient of deviation of the crystal lattice from its equilibrium position in the equation for the acoustic wave leads to a dependence of the natural frequency of crystal lattice vibrations on the amplitude and duration of pulses exciting these vibrations. Such a dependence is responsible for echo signals that can be detected experimentally. The proposed model makes it possible to interpret most experimental results for BiPbSrCaCuO superconducting samples. We consider the effect of a constant magnetic field on the amplitude and the echo signal decay time. We observed a clearly manifested peak that was not described by other authors. The model proposed here provides an obvious explanation for this peak.     

Spin-controlled nanomechanics induced by single-electron tunneling

We consider dc-electronic transport through a nanowire suspended between normal- and spin-polarized metal leads in the presence of an external magnetic field. We show that magnetomotive coupling between the electrical current through the nanowire and vibrations of the wire may result in self-excitation of mechanical vibrations. The self-excitation mechanism is based on correlations between the occupancy of the quantized electronic energy levels inside the nanowire and the velocity of the nanowire. We derive conditions for the occurrence of the instability and find stable regimes of mechanical oscillations.     

Cluster of solar active regions and onset of coronal mass ejections

round-the-clock solar observations with full-disk coverage of vector magnetograms and multi-wavelength images demonstrate that solar active regions(ARs) are ultimately connected with magnetic field. Often two or more ARs are clustered, creating a favorable magnetic environment for the onset of coronal mass ejections(CMEs). In this work, we describe a new type of magnetic complex: cluster of solar ARs. An AR cluster is referred to as the close connection of two or more ARs which are located in nearly the same latitude and a narrow span of longitude. We illustrate three examples of AR clusters, each of which has two ARs connected and formed a common dome of magnetic flux system. They are clusters of NOAA(i.e., National Oceanic and Atmospheric Administration) ARs 11226 11227, 11429 11430, and 11525 11524. In these AR clusters, CME initiations were often tied to the instability of the magnetic structures connecting two partner ARs, in the form of inter-connecting loops and/or channeling filaments between the two ARs. We show the evidence that, at least, some of the flare/CMEs in an AR cluster are not a phenomenon of a single AR, but the result of magnetic interaction in the whole AR cluster. The observations shed new light on understanding the mechanism(s) of solar activity. Instead of the simple bipolar topology as suggested by the so-called standard flare model, a multi-bipolar magnetic topology is more common to host the violent solar activity in solar atmosphere.     

Role of thermal vibrations in magnetic phase transitions

This article addresses the contradiction that exists in studies on magnetic phase transitions between theoretical models that ignore the role of thermal vibrations and analysis of neutron diffraction data that always incorporates them. Ignoring thermal vibrations in both theoretical models and analysis of diffraction data leads to the latter giving different magnetic-order parameters for different reciprocal lattice lines. This leads to a unique consequence, the assumption to neglect a physical phenomenon turns a single-valued experimental observable into a multiple-valued one where all values are equally valid. This assumption is clearly unacceptable and must be rejected. Diffraction data constrain all theoretical models to incorporate thermal vibrations and represent the exchange interaction as temperature dependent, J_ij (T), instead of the current practice, J_ij (0).     

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.     

Structural phase transition in a large cluster

The effect of a phase transition between structures in a large cluster with a pair interatomic interaction on the thermodynamic parameters of the cluster is analyzed. The statistical parameters of a cluster consisting of 923 atoms are determined for an icosahedron and a face-centered cubic (fcc) structure. The specific heat and entropy of this cluster are calculated in the case when the transition between the icosahedron and fcc structures has the greatest effect on these parameters, so that at zero temperature this cluster has the structure of an icosahedron, and as the temperature increases to the melting point it assumes an fcc structure. Even with this, the contribution of the excitations of the atomic configurations to the thermodynamic parameters of a cluster is small compared with the excitation of vibrations in the cluster. The contribution of a configurational excitation in the thermodynamic parameters of a cluster becomes substantial for the liquid state of clusters.     

Local vibrations in fcc crystals with two-parametric substitutional impurities

The threshold parameters of defects (the mass defect and the relative change in the force constants) are determined at which local vibrations start to occur in an fcc crystal with substitutional impurities. The characteristics of local vibrations are investigated, and the influence of the defect parameters on the frequency of local vibrations and their decay rate with distance from the impurity atom is analyzed. The frequencies and the intensities of local vibrations are calculated for the nearest neighboring atoms of an impurity, which, combined with the impurity atom, form a defect cluster.     

Analysis of Co-Tunneling Current in Fullerene Single-Electron Transistor

Single-electron transistors (SETs) are nano devices which can be used in low-power electronic systems. They operate based on coulomb blockade effect. This phenomenon controls single-electron tunneling and it switches the current in SET. On the other hand, co-tunneling process increases leakage current, so it reduces main current and reliability of SET. Due to co-tunneling phenomenon, main characteristics of fullerene SET with multiple islands are modelled in this research. Its performance is compared with silicon SET and consequently, research result reports that fullerene SET has lower leakage current and higher reliability than silicon counterpart. Based on the presented model, lower co-tunneling current is achieved by selection of fullerene as SET island material which leads to smaller value of the leakage current. Moreover, island length and the number of islands can affect on co-tunneling and then they tune the current flow in SET.     

基于片段的核磁共振筛选方法识别NSD1 SET结构域的全新苗头化合物

包含SET结构域的核受体结合蛋白1（NSD1）是一种组蛋白甲基转移酶,它能够特异性的甲基化组蛋白H3赖氨酸第36位（H3K36）.异常表达的NSD1主要发现于Sotos综合症患者体内,但它同样也能导致其他多种人类疾病的发生.目前已有靶向组蛋白甲基转移酶DOT1L和EZH2的小分子抑制剂报道,然而,靶向NSD1的化学探针分子尚未被发现.本文使用基于片段的核磁共振（NMR）筛选方法寻找到3个以NSD1蛋白作为靶点的苗头化合物,利用化学位移扰动分析技术测定了这些化合物与NSD1的结合亲和力.另外,利用分子对接方法选择获得苗头化合物与NSD1蛋白的最可能的结合模型.结果显示苗头化合物1结合于NSD1天然底物S-腺苷酸甲硫氨酸（SAM）的结合口袋中.我们的研究成果为进一步以结构为指导的从苗头化合物到先导化合物的衍化奠定了基础.     

Magnetism of Co overlayers and nanostructures on W(0 0 1): A first-principles study

The magnetic properties of Co nanostructures and a Co monolayer on W(0 0 1) have been studied in the framework of density functional theory. Different geometries such as planar and three-dimensional clusters have been considered, with cluster sizes varying between 2 and 13 atoms. The calculations were performed using the real-space linear muffin-tin orbital method (RS-LMTO-ASA). With respect to the stability of the magnetic state, we predict an antiferromagnetic (AFM) structure for the ground state of the planar Co clusters and a ferromagnetic (FM) state for the three-dimensional clusters. For the three-dimensional clusters, one of the AFM arrangements leads to frustration due to the competing FM and AFM exchange interactions between different atoms in the cluster, and gives rise to a non-collinear state with energy close to that of the FM ground state. The relative role of the Co–Co and Co–W exchange interactions is also investigated.     

Magnetic properties of small Yttrium clusters

The magnetic properties of small Y_N clusters are studied by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Several types of cluster geometries are considered in order to see the effects of the size and symmetry of the structures on the magnetic properties. The average magnetic moments are found to be constant over large domains of variations in the interatomic distance, a fact that can be explained by the existing closed shell electronic configurations at least for one spin direction in all our magnetic solutions. Small energy gains upon the onset of magnetization are obtained, which reveals the low stability of the magnetic solutions. Our results contradict the prediction of a magnetic-nonmagnetic transition at a large cluster size (about 90 atoms) for these kinds of systems. Received: 27 April 1998 / Received in final form: 23 June 1998 / Accepted: 17 July 1998     

Steady-state and transient Raman gain in magnetoactive narrow band-gap semiconductors

Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, a detailed analytical investigation is undertaken to study both steady-state and the transient Raman gain in transversely magnetized narrow band-gap semiconductors arising from electron density perturbations and molecular vibrations of the medium. Using the fact that the origin of stimulated Raman scattering (SRS) lies in the third-order susceptibility of the medium, we obtain an expression for the gain coefficient of the backward Stokes mode in steady-state and transient regimes and study the dependence of the magnetic field and pump pulse duration on its growth rate. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. An externally applied magnetic field substantially enhances the transient SRS gain coefficient in narrow band-gap semiconductors, which can be of great use in the compression of scattered pulses.     

Magnetoelastic interactions in the Belorizky model

It is shown that interactions of spins with lattice vibrations in the Belorizky model leads Within the framework of Zubareev's Greens functions technique the magnetic properties of S=1 spin structures are studied. Expressions for the magnetization and collective excitation spectra are found.     

First-Principles Study of Magnetic Properties of TM_(13) and TM_(13)@Au_(32) Clusters(TM = Mn,Co)

The structural and magnetic properties of TM_(13 )and TM_(13)@Au_(32 )clusters(TM=Mn,Co)are studied by firstprinciples calculations.We find that the Au_(32 )cluster can tune not only the magnetic moment but also the magnetic coupling properties between the TM atoms of the TM cluster.The Au_(32 )cluster can increase the net magnetic moment of Mn_(13 )clusters while reducing that of Co_(13 )clusters.The interaction between Au and Mn atoms induces more Mn atoms to form spin parallel coupling,resulting in an increase of the total magnetic moment of Mn_(13 )clusters,while for the Co_(13 )clusters,the interaction between Au and Co atoms does not change the magnetic coupling states between the Co atoms,but reduces the magnetic moment of the Co atoms,leading to a decrease of the total magnetic moment of this system.Our findings indicate that the encapsulation of Au_(32 )clusters can not only raise the chemical stability of TM clusters,but also can tune their magnetic coupling properties and magnetic moment,which enables such systems to be widely applied in fields of spintronics and medical science.     

Thermodynamics of carrier-mediated magnetism in semiconductors

We propose a model of carrier-mediated ferromagnetism in semiconductors that accounts for the temperature dependence of the carriers. The model permits analysis of the thermodynamic stability of competing magnetic states, opening the door to the construction of magnetic phase diagrams. As an example, we analyze the stability of a possible reentrant ferromagnetic semiconductor, in which increasing temperature leads to an increased carrier density such that the enhanced exchange coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised.     

Single-electron transport in electrically tunable nanomagnets

We study a single-electron transistor (SET) based upon a II-VI semiconductor quantum dot doped with a single-Mn ion. We present evidence that this system behaves like a quantum nanomagnet whose total spin and magnetic anisotropy depend dramatically both on the number of carriers and their orbital nature. Thereby, the magnetic properties of the nanomagnet can be controlled electrically. Conversely, the electrical properties of this SET depend on the quantum state of the Mn spin, giving rise to spin-dependent charging energies and hysteresis in the Coulomb blockade oscillations of the linear conductance.