Energy landscapes, lowest gaps, and susceptibility of elastic manifolds at zero temperature

We study the effect of an external field on (1 + 1) and (2 + 1) dimensional elastic manifolds, at zero temperature and with random bond disorder. Due to the glassy energy landscape the configuration of a manifold changes often in abrupt, “first order”-type of large jumps when the field is applied. First the scaling behavior of the energy gap between the global energy minimum and the next lowest minimum of the manifold is considered, by employing exact ground state calculations and an extreme statistics argument. The scaling has a logarithmic prefactor originating from the number of the minima in the landscape, and reads ΔE ₁∼L ^θ[ln(L _z L ^{- ζ})]^-1/2, where ζ is the roughness exponent and θ is the energy fluctuation exponent of the manifold, L is the linear size of the manifold, and L_z is the system height. The gap scaling is extended to the case of a finite external field and yields for the susceptibility of the manifolds ∼L ^{2D + 1 - θ}[(1 - ζ)ln(L)]^1/2. We also present a mean field argument for the finite size scaling of the first jump field, h ₁∼L ^{d - θ}. The implications to wetting in random systems, to finite-temperature behavior and the relation to Kardar-Parisi-Zhang non-equilibrium surface growth are discussed. Received December 2000 and Received in final form April 2001     

Synchrotron radiation studies of mass-selected Fe nanoclusters deposited in situ

A portable UHV-compatible gas aggregation cluster source, capable of depositing clean mass-selected nanoclusters in situ, has been used at synchrotron radiation facilities to study the magnetic behaviour of exposed and Co-coated Fe clusters in the size range 250 to 540 atoms. X-ray magnetic circular dichroism (XMCD) studies of isolated and exposed 250-atom clusters show a 10% enhancement in the spin magnetic moment and a 75% enhancement in the orbital magnetic moment relative to bulk Fe. The spin moment monotonically approaches the bulk value with increasing cluster size but the orbital moment does not measurably decay till the cluster size is above ∼ 400 atoms. The total magnetic moments for the supported particles though higher than the bulk value are less than those measured in free clusters. Coating the deposited particles with Co in situ increases the spin moment by a further 10% producing a total moment per atom close to the free cluster value. At low coverages the deposited clusters are super-paramagnetic at temperatures above 10 K but a magnetic remanence at higher temperature emerges as the cluster density increases and for cluster films with a thickness greater than 50 ?(i.e. 2-3 layers of clusters) the remanence becomes greater than that of an Fe film of the same thickness produced by a conventional deposition source. Thick cluster-assembled film show a strong in-plane anisotropy. Received 14 December 2000     

Ground-state geometries and stability of NaMg clusters using ab initio molecular dynamics method

The ground-state geometries, energetics and the stability of ( n =1-12) clusters are studied using ab initio molecular dynamics method. Our results indicate that the ground-state geometries of large clusters () are different from those of clusters where a trivalent impurity Al is added to the same monovalent host Na. Other features observed are an early appearance of 3-dimensional structure and a pentagonal growth path from n =6 up to n =11. As expected, the ground-state geometry of is not an icosahedron but can be viewed as a distorted form of one of the low lying geometries of cluster. In the energetically favored structures impurity atom Mg is never located at the center of the cluster. The stability analysis based on the energetics shows (8 valence electrons) to be the most stable. In addition there is a remarkable even-odd pattern observed in the dissociation energy and the second difference in energy which is absent in earlier studies of and clusters. Received: 16 September 1998 / Received in final form: 15 February 1999     

Imaging and tunneling spectroscopy of gold nanocrystals and nanocrystal arrays

Scanning tunneling microscopy (STM) and spectroscopy (STS) have been used to determine the structural and electronic properties of thiol-passivated 29000 amu gold nanocrystals, both individually and in spontaneously formed quasi-two-dimensional arrays. Experiments were performed at temperatures of 300 K, 77 K, and 8 K. Even at room temperature, tunneling through these 1.7 nm nanocrystals is shown to give rise to a Coulomb blockade. At cryogenic temperatures, the spectroscopy of the nanocrystals in arrays and in isolation shows an incremental charging effect (the Coulomb staircase) and evidence is found for quantization of the electronic states. Received: 10 September 1998 / Received in final form: 16 February 1999     

Spatial and angular distributions of third harmonic generation from metal surfaces

Third harmonic generation from planar and spherical metal surfaces is studied theoretically through the standard Green function method, so that the results are not affected by the uncertainty associated with previous simplified models. In general the pattern of the non-linear scattering loosely resembles the pattern of Mie scattering. The strong backward scattering is uniquely related to the non-linear process. These results differ significantly from the predictions of the surface charge model. Received: 20 May 1998 / Received in final form: 25 August 1998 / Accepted: 1st September 1998     

Fe基非晶和纳米晶合金晶化及电化学腐蚀行为

应用单辊甩带法制备非晶态Fe78Si13B9和Fe73.5Si13.5B9Nb3Cu1薄带,并以非晶晶化退火法制备出纳米晶Fe73.5Si13.5B9Nb3Cu1薄带.利用X射线衍射(XRD)仪和示差扫描量热计(DSC)对该非晶薄带的非晶特性及其晶化过程进行了研究.并用电化学极化曲线的方法和电化学阻抗技术研究了非晶态Fe78Si13B9和纳米晶Fe73.5Si13.5B9Nb3Cu1合金在1mol/LNaOH溶液里的电化学腐蚀行为,用SEM对极化测试后的试样形貌进行了观察;同时还研究了不同的热处理温度对材料结构及在1mol/LNaOH溶液里耐腐蚀性能的影响.结果表明,该非晶薄带的晶化过程分为两步;纳米晶比非晶合金的耐腐蚀性要好;且随着热处理温度的升高,非晶和纳米晶的耐腐蚀性能都得到提高.     

燃烧法制备La_2O_3纳米晶的研究

以硝酸镧作为氧化剂,柠檬酸铵作为燃料,用少量尿素作引发剂,采用燃烧法制备了La_2O_3纳米晶.并研究了煅烧温度、原料计量比和释放气体等因素对样品的影响.室温固相反应法制得的粘稠膏状络合物LaCit,当加热至150 ℃时立即引起燃烧,得到灰色前驱物,前驱物经700 ℃煅烧2 h得到单晶纳米La_2O_3样品.利用XRD、TEM和SEM测试方法对样品表征,结果表明:样品形貌为透明矩形片状单晶,平均长度为100~150 nm,宽度为40~80 nm,厚度约为20 nm,产率为85;~87;,粒度分布均匀,具有良好的分散性.     

Grain size reduction of copper subjected to repetitive uniaxial compression combined with accumulative fold

This paper reports a novel method of repetitive uniaxial compression combined with accumulative fold for preparing bulk submicron- to nanocrystalline copper starting with a coarse grained counterpart. Grain size reduction and microstrain variations of the high purity copper samples after different passes of compression and fold are investigated by scanning electron microscope and x-ray diffraction (XRD), respectively. Our results show that the average grain size of samples decreases from about 830nm to 127nm as the number of compression passes increases to 30. Microstrain in the compressed sample is found to increase for the first 20 passes, but to decrease at the last 10 passes. The variations of compressive yield strength and the shift of XRD peaks to larger diffraction angles are observed in the squeezed sample. Our experimental results demonstrate that the repetitive uniaxial compression combined with accumulative fold is an effective method to prepare bulk nanocrystalline metallic materials, in particular for soft metals such as Cu, Al and Pb.     

Controllable Optical Bistability in a Crystal of Molecular Magnets System

We investigate the formation of optical bistability (OB) in a crystal of molecular magnets contained in a unidirectional ring cavity. The crystal is subjected to one dc magnetic field and two (probe and coupling) ac resonant magnetic field. The results show that OB can be controlled efficiently by adjusting the intensity of the control field, the detuning of probe magnetic field and the cooperation parameter. Furthermore, within certain parameter range, the optical multistablity (OM) can also be observed in the crystal medium. This investigation can be used for designing new types of nonelectronic devices for realizing switching process.