Strain and high temperature superconductivity: unexpected results from direct electronic structure measurements in thin films

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.     

小尺寸单轴应变Si PMOS沟道晶面/晶向选择实验新发现

小尺寸单轴应变Si p型金属氧化物半导体(PMOS)沟道反型层迁移率与晶面/晶向密切相关,应变PMOS优化设计时应合理选择沟道的晶面/晶向.目前,文献已有1.5 GPa应力强度下单轴应变Si PMOS沟道反型层迁移率按晶面/晶向排序的理论模型.然而,在器件实际制造过程中,覆盖SiN应力膜工艺是固定的,由于沟道弹性劲度系数具有各向异性,这样,不同晶面/晶向应变PMOS沟道所受应力强度不同,进而导致在实际工艺下沟道反型层迁移率晶面/晶向排序理论模型"失效".针对该问题,本文采用中国科学院微电子研究所40 nm工艺流程制备了不同晶面/晶向40 nm沟道小尺寸单轴应变Si PMOS与未应变Si PMOS,并通过器件转移特性测试,获得了小尺寸单轴应变Si PMOS反型层迁移率晶面/晶向排序结论.此有关小尺寸单轴应变Si PMOS沟道反型层迁移率晶面/晶向排序的相关结论,由于考虑了工艺实现因素,与文献理论预测排序结果相比,更适于指导实际器件制造;相关分析方法也可为其他应变材料沟道MOS相关问题的解决提供重要技术参考.     

Measurements of elastic constants in thin films of colossal magnetoresistance material

Measurements of elastic constants of strained 200 and 400 nm thin films, as well as unstrained samples, of the colossal magnetoresistance (CMR) material La0.67Ca0.33MnO3 are presented. Since the peak resistance temperature of a strained CMR film decreases as the film thickness decreases, it is of interest to see if features in the elastic constants, reflecting structural or magnetic changes, follow the peak resistance temperature. It is observed that features in the elastic constants appear not only at the peak resistance temperatures of the CMR samples, but also at a temperature about 17 K higher. A new technique, thin-film resonant ultrasound spectroscopy, was used to make the measurements.     

Low-temperature intracenter relaxation times of shallow donors in germanium

The relaxation times of localized states of antimony donors in unstrained and strained germanium uniaxially compressed along the [111] crystallographic direction are measured at cryogenic temperatures. The measurements are carried out in a single-wavelength pump–probe setup using radiation from the Novosibirsk free electron laser (NovoFEL). The relaxation times in unstrained crystals depend on the temperature and excitation photon energy. Measurements in strained crystals are carried out under stress bar S > 300, in which case the ground-state wavefunction is formed by states belonging to a single valley in the germanium conduction band. It is shown that the application of uniaxial strain leads to an increase in the relaxation time, which is explained by a decrease in the number of relaxation channels.     

Tight-binding parameterization of α-Sn quasiparticle band structure

The diamond structure of tin (α-Sn) can be stabilized in nanocrystals embedded in a suitable host. We developed highly accurate parameterizations for tight-binding simulation of such structures incorporating strain and spin-orbit interaction. Parameters are obtained by fitting to ab initio GW quasiparticle band structures of unstrained α-Sn as well as geometries under uniform compressive or tensile strain. The optical response calculated from this fit is in excellent agreement with experiments. As an application, confinement induced band gaps in strained and unstrained 3 nm nanocrystals are computed. It is found that compressive and tensile strain raises and lowers the gap, respectively.     

像散光束的光束参数与光强二阶矩的关系

采用光束的二阶强度矩的方法讨论三维像散光束的光束特性。通过计算像散光束在自由空间、球面光学系统中的传输特性以及坐标系旋转时的变换特性 ,给出了光束的束腰位置、束腰半径、瑞利长度、有效波面曲率半径、近场和远场的方位角、光束的光束参数积等参数与光强二阶矩之间的关系。     

Independent variables in foam clusters

We find, by counting the degrees of freedom of two-dimensional bubble clusters (finite or periodic) of given topology and bubble areas, that the Plateau laws determine a unique configuration of a finite free cluster, but allow an infinite number of configurations of a periodic cluster. Each of these configurations is associated with a particular strain (stress) state of the cluster; there is in general one unstrained configuration, which corresponds to the minimum of the (surface) energy. Configurations of given topology that satisfy Plateau's laws may only exist in certain ranges of bubble area ratios and/or strains. Received 31 May 2001 and Received in final form 12 September 2001     

Retarded thermal oxidation of strained Si substrate

Strained Si is recognized as a necessary technology booster for modern integrated circuit technology. However, the thermal oxidation behaviors of strained Si substrates are not well understood yet despite their importance. In this study,we for the first time experimentally find that all types of strained Si substrates(uniaxial tensile, uniaxial compressive,biaxial tensile, and biaxial compressive) show smaller thermal oxidation rates than an unstrained Si substrate. The possible mechanisms for these retarded thermal oxidation rates in strained Si substrates are also discussed.     

Unstrained and strained flamelets for LES of premixed combustion

The unstrained and strained flamelet closures for filtered reaction rate in large eddy simulation (LES) of premixed flames are studied. The required sub-grid scale (SGS) PDF in these closures is presumed using the Beta function. The relative performances of these closures are assessed by comparing numerical results from large eddy simulations of piloted Bunsen flames of stoichiometric methane–air mixture with experimental measurements. The strained flamelets closure is observed to underestimate the burn rate and thus the reactive scalars mass fractions are under-predicted with an over-prediction of fuel mass fraction compared with the unstrained flamelet closure. The physical reasons for this relative behaviour are discussed. The results of unstrained flamelet closure compare well with experimental data. The SGS variance of the progress variable required for the presumed PDF is obtained by solving its transport equation. An order of magnitude analysis of this equation suggests that the commonly used algebraic model obtained by balancing source and sink in this transport equation does not hold. This algebraic model is shown to underestimate the SGS variance substantially and the implications of this variance model for the filtered reaction rate closures are highlighted.     

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.      

Oscillation of surface in-plane lattice spacing during epitaxial growth of BaTiO3 and SrTiO3 on SrTiO3(1 0 0)

In-plane lattice spacing oscillation was observed as functions of surface coverage and growing film thickness by reflection high-energy electron diffraction during the growth of strained BaTiO₃ and unstrained SrTiO₃ epitaxial layers on SrTiO₃(1 0 0). For BaTiO₃/SrTiO₃, the oscillation continued until the critical thickness is reached, whereas for SrTiO₃/SrTiO₃, oscillation was observed only at the initial few monolayer growth. The origin of oscillations is discussed in relation to the non-tetragonal elastic distortion occurring at the free edges of 2D single monolayer islands. The amplitude of the oscillation strongly depended on the island size and density.     

Influence of kinematical constraints on multiplicity distributions and neutral charged correlations in the independent cluster emission model

Energy-momentum conservation in the cluster production process is introduced in the independent cluster emission model using the generating functional formalism. In a simple version of the model clusters are produced which decay into a fixed number of pions. The ?? model and a model with isospin conservation in the cluster decay are used to calculate the charge distribution among the secondaries of cluster decay. Multiplicity characteristics like average multiplicity, second moments and associated average neutral multiplicities and second moments are calculated. The results are compared with experimental data.     

Comparison of two kinds of active layers for high-power narrow-stripe distributed feedback laser diodes

Usually GaAs/AlGaAs is utilized as an active layer material in laser diodes operating in the spectral range of 800--850 nm. In this work, in addition to a traditional unstrained GaAs/AlGaAs distributed feedback (DFB) laser diode, a compressively strained InGaAlAs/AlGaAs DFB laser diode is numerically investigated in characteristic. The simulation results show that the compressively strained DFB laser diode has a lower transparency carrier density, higher gain, lower Auger recombination rate, and higher stimulated recombination rate, which lead to better a device performance, than the traditional unstrained GaAs/AlGaAs DFB laser diode.     

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     

Direct photoemission spectroscopy and electronic properties of in situ grown,strained high-Tc and related oxide films

We have optimized laser ablation thin film deposition and transfer procedures within synchrotron vault, specifically to perform angle-integrated and angle-resolved photoemission spectroscopy (ARPES) on high-T_c and related films without cleaving the samples. However, the chain-containing phases like YBCO-123 easily loose surface oxygen and do not exhibit stable Fermi edge, hence are not suitable for ARPES studies. Direct in situ ARPES studies on strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO evolves with doping, yet changes even more significantly with strain. The strain changes the FS topology from hole-like to electron-like, and causes band dispersion along k_x and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the ‘usual’ flat band remaining ≈30 meV below E_F measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; T_c is enhanced in all our strained samples. Implications for the evolving high-T_c theory and studies of nano-engineered film heterostructures are briefly discussed.     

potential theory and analytic properties of self-similar fractal and multifractal distributions

By the use of recursion relations and analytic techniques we deduce general analytic results pertaining to the electrostatic potential, moments, and Fourier transform of exactly self-similar fractal and multifractal charge distributions. Three specific examples are given: the binomial distribution on the middle-third Cantor set, which is a multifractal distribution, the uniform distribution on the Menger sponge, which illustrates the added complication of higher dimensionality, and the uniform distribution on the von Koch snowflake, which illustrates the effect of rotations in the defining transformations.     

不同晶系应变Si状态密度研究

应变Si技术是当前微电子领域研究发展重点,态密度是其材料的重要物理参量.本文基于应力相关KP理论,建立了(001),(101)和(111)晶面施加双轴应力形成的四方、单斜及三角晶系应变Si导带、价带态密度模型.结果表明,除单斜和三角晶系导带底态密度外,应力对其余各态密度均有显著影响.本文所得模型数据量化,可为应变Si材料物理的理解及其他物理参数模型的建立奠定重要理论基础. 关键词： 应变Si KP 态密度     

The effects of strain and surface roughness scattering on the quasi-ballistic characteristics of a Ge nanowire p-channel field-effect transistor

The effects of strain and surface roughness scattering on the quasi-ballistic hole transport in a strained gate-all-around germanium nanowire p-channel field-effect transistor （pFET） are investigated in this work. The valence subbands are shifted up and warped more parabolically by the influence of HfO2 due to the lattice mismatch. However, the boundary force only shifts the subbands downwards and has little effect on the reshaping of bands. Strain induced by HfO2 increases both the hole mobility and ON-current （/ON）, but has little effect on the hole mobility. The/ON is degraded by the surface roughness scattering in both strained and unstrained devices.     

Dipole capture of a slow electron by a water cluster

It is shown that the anomalously large cross section for attachment of a slow electron to a water cluster (H₂O) _n≥50 that is observed in molecular-beam experiments may be explained by the capture of the electron by a long-range field of the permanent electric dipole moment of the cluster. The cross section values are used to estimate the dipole moment of the cluster as a function of its diameter n. The values obtained significantly exceed the random dipole moments in the case of the proton-disordered cluster structure and indicate the ferroelectric ordering of the orientations of dipole moments of the molecules included in the cluster.     

Multipolar corrections to the Clausius-Mossotti formula for the effective dielectric constant of a polydisperse suspension of spheres

We study multipolar corrections to the Clausius-Mossotti formula for the effective dielectric constant of a polydisperse suspension of spheres with identical uniform dielectric constant embedded in a uniform background. The first few moments of the spectral density occurring in the Bergman representation are evaluated exactly to second order in the volume fraction. We present numerical results for a log-normal distribution of sphere radii.