分子束外延制备的垂直易磁化MnAl薄膜结构和磁性

系统地研究了利用分子束外延方法在GaAs(001) 衬底上外延生长的MnAl_x薄膜的结构和垂直易磁化特性随组分及生长温度的依赖关系. 磁性测试表明, 可在较大组分范围内 (0.4≤x≤1.2) 获得大矫顽力的垂直易磁化MnAl_x薄膜, 然而同步辐射X射线衍射和磁性测试发现当x≤0.6时MnAl薄膜出现较多的软磁相, 当x >0.9时, MnAl薄膜晶体质量和化学有序度逐渐降低, 组分为MnAl_0.9时制备的薄膜有最好的[001]取向. 随着生长温度的增加, MnAl_0.9薄膜的有序度、垂直磁各向异性常数、矫顽力和剩磁比均增加, 350℃时制备的MnAl_0.9薄膜化学有序度高达0.9, 其磁化强度、剩磁比、矫顽力和垂直磁各向异性常数分别为265emu/cm³、93.3%、8.3kOe (1 Oe=79.5775A/m)和7.74Merg/cm³ (1 erg=10^-7J). 不含贵金属及稀土元素、良好的垂直易磁化性质、 与半导体材料结构良好的兼容性以及磁性能随不同生长条件的可调控 性使得MnAl薄膜有潜力应用于多种自旋电子学器件. 关键词： 分子束外延 大矫顽力材料 磁各向异性     

垂直磁各向异性L10-Mn1.67Ga超薄膜分子束外延生长与磁性研究

具有超强垂直磁各向异性的L1₀-Mn_xGa薄膜由于其与半导体材料结构及工艺的高度兼容性而受到广泛关注, 其超高垂直磁各向异性能和极低的磁阻尼因子预示着L1₀-Mn_xGa薄膜在高热稳定性自旋电子学器件中将发挥重要作用. 而L1₀-Mn_xGa超薄膜对于降低L1₀-Mn_xGa基垂直磁各向异性隧道结中的磁矩翻转临界电流密度有着重要的意义. 本文采用分子束外延的方法, 在半导体GaAs衬底上成功制备出了一系列不同厚度的L1₀-Mn_1.67Ga薄膜, 厚度范围为1-5 nm. 生长过程中反射式高能电子衍射原位检测以及X射线衍射结果均表明了其良好的单晶相. 磁性测量结果表明, 厚度在1 nm以上的L1₀-Mn_1.67Ga薄膜均可以保持垂直磁各向异性特征, 厚度为5 nm的L1₀-Mn_1.67Ga薄膜的垂直磁各向异性能可达到14.7 Merg/cm³. 这些结果为基于L1₀-Mn_1.67Ga的垂直磁各向异性隧道结在自旋转移扭矩驱动的磁随机存储器等低功耗器件的集成及应用提供了重要的实验支持.     

Surface kinetics of GaN evaporation and growth by molecular-beam epitaxy

The kinetics of surface processes during the growth of GaN by molecular-beam epitaxy (MBE) with ammonia as the source of reactive nitrogen is studied theoretically and experimentally. A model of surface processes is developed taking into account specific effects of the blocking of NH₃ adsorption sites by Group III and Group V surface species. Parameters of the model (respective kinetic rate constants) are determined from comparison with experimental data. It is shown that the evaporation rate of GaN in ammonia atmosphere is much lower than that in vacuum. Kinetics of GaN growth under gallium-rich and nitrogen-rich conditions are compared. Under nitrogen-rich conditions the GaN surface is predicted to be enriched by NH_x surface radicals, in contrast to the case of growth under gallium-rich conditions or of free evaporation in vacuum. It is shown that use of the nitrogen-rich conditions allows one to increase the growth temperature by 80–90°C compared with the case of gallium-rich conditions or plasma-activated MBE. The increased growth temperature is favorable in improving the optical and electrical properties of the material grown.     

Dynamic scaling of migration-driven aggregate growth

We study the kinetic behaviour of the growth of aggregates driven by reversible migration between any two aggregates. For a simple model with the migration rate K(i;j)=K′(i;j)∝i^uj^v at which the monomers migrate from the aggregates of size i to those of size j, we find that the aggregate size distribution in the system with u+v≤3 and u<2 approaches a conventional scaling form, which reduces to the Smoluchovski form in the u=1 case. On the other hand, for the system with u<2, the average aggregate size S(t) grows exponentially in the u+v=3 case and as (tlnt)^{1/(5-2u)} in another special case of v=u-2. Moreover, this typical size S(t) grows as t^{1/(3-u-v)} in the general u-2相似文献   

Influence of velocity spatiotemporal correlations on the anomalous scaling exponents of passive scalars

In this paper, we consider spatial-temporal correlation functions of the turbulent velocities. With numerical simulations on the Gledzer--Ohkitani--Yamada (GOY) shell model, we show that the correlation function decays exponentially. The advecting velocity field is regarded as a colored noise field, which is spatially and temporally correlative. For comparison, we are also given the scaling exponents of passive scalars obtained by the Gaussian random velocity field, the multi-dimensional normal velocity field and the She--Leveque velocity field, introduced by She, et al. We observe that extended self-similarity scaling exponents H(p)/ H(2) of passive scalar obtained by the colored noise field are more anomalous than those obtained by the other three velocity fields.     

含有广义守恒律的生长方程标度奇异性的直接标度分析

利用直接标度分析方法研究一个含有广义守恒律生长方程的标度奇异性,得到强弱耦合区域的奇异标度指数.作为其特殊情况,这个方程包含Kardar-Parisi-Zhang（KPZ）方程、 Sun-Guo-Grant（SGG）方程以及分子束外延（MBE）生长方程,并能对其进行统一的研究.研究发现, KPZ方程和SGG方程,无论在弱耦合还是在强耦合区域内都遵从自仿射Family -Vicsek正常标度规律;而MBE 方程在弱耦合区域内服从正常标度,在强耦合区域内能呈现内禀奇异标度行为.这里所得到生长方程的奇异标度性质与利用重正化群理论、数值模拟以及实验相符很好. 关键词： 标度奇异性 强耦合 弱耦合     

Numerical study of anomalous dynamic scaling behaviour of (1+1)-dimensional Das Sarma-Tamborenea model

In order to discuss the finite-size effect and the anomalous dynamic scaling behaviour of Das Sarma-Tamborenea growth model,the (1+1)-dimensional Das Sarma-Tamborenea model is simulated on a large length scale by using the kinetic Monte-Carlo method.In the simulation,noise reduction technique is used in order to eliminate the crossover effect.Our results show that due to the existence of the finite-size effect,the effective global roughness exponent of the (1+1)-dimensional Das Sarma-Tamborenea model systematically decreases with system size L increasing when L > 256.This finding proves the conjecture by Aarao Reis[Aarao Reis F D A 2004 Phys.Rev.E 70 031607].In addition,our simulation results also show that the Das Sarma-Tamborenea model in 1+1 dimensions indeed exhibits intrinsic anomalous scaling behaviour.     

Evidence of self-affine multiplicity scaling of charged-particle multiplicity distribution in hadron-nucleus interaction

A self-affine analysis of charged-particle multiplicity distribution (protons + pions) in π ⁻-AgBr interaction at 350 GeV/c is performed according to the two-dimensional factorial moment methodology using the concept of Hurst exponent in X _cosθ -X _ϕ phase space. Comparing with the results obtained from self-similar analysis, the self-affine analysis shows a better power-law behaviour. Corresponding results are compared with shower multiplicity distribution (pions). Multifractal behaviour is observed for both types of distributions.      

Scaling behavior of the time-fractional equations for molecular-beam epitaxy growth: scaling analysis versus numerical stimulations

The scaling behavior of the time-fractional molecular-beam epitaxy (TFMBE) equations in 1+1 dimensions is investigated by numerical simulations and scaling analysis, respectively. The growth equations studied include the time-fractional linear molecular-beam epitaxy (TFLMBE) and the time-fractional Lai-Das Sarma-Villain (TFLDV). Growth exponents are obtained using the two methods. The analytical results are consistent with the corresponding numerical solutions based on Caputo-type fractional derivative.     

含时空关联噪声的非局域及各向异性Kardar-Parisi -Zhang方程的直接标度分析

将直接标度分析方法推广应用到含时间空间关联噪声的非局域及各向异性KardarParisiZhang方程的动力学标度分析中,分别得到了方程在强耦合区和弱耦合区的标度指数值.在弱耦合区得到的标度指数能与使用动力学重整化方法得到的结果相吻合 关键词： 表面生长 标度分析 KPZ方程     

Effects of memory on scaling behaviour of Kardar--Parisi--Zhang equation

In order to describe the time delay in the surface roughing process the Kardar-Parisis-Zhang (KPZ) equation with memory effects is constructed and analysed using the dynamic renormalization group and the power counting mode coupling approach by Chattopadhyay [2009 Phys. Rev. E 80 011144]. In this paper, the scaling analysis and the classical self-consistent mode-coupling approximation are utilized to investigate the dynamic scaling behaviour of the KPZ equation with memory effects. The values of the scaling exponents depending on the memory parameter are calculated for the substrate dimensions being 1 and 2, respectively. The more detailed relationship between the scaling exponent and memory parameter reveals the significant influence of memory effects on the scaling properties of the KPZ equation.     

Anomalous Scaling of Surface Growth Equations with Spatially and Temporally Correlated Noise

Based on the scaling idea of local slopes by Lopez et al. [Phys. Rev. Lett. 94 （2005） 166103], we investigate anomalous dynamic scaling of （d ＋ 1）-dimensional surface growth equations with spatially and temporally correlated noise. The growth equations studied include the Kardar-Parisi-Zhang （KPZ）, Sun-Guo-Grant （SGG）, and Lai-Das Sarma-Villain （LDV） equations. The anomalous scaling exponents in both the weak- and strong-coupling regions are obtained, respectively.     

含关联噪声的空间分数阶随机生长方程的动力学标度行为研究

为探讨含关联噪声的空间分数阶随机生长方程的动力学标度行为,本文利用Riesz分数阶导数和Grümwald-Letnikov分数阶导数定义方法研究了关联噪声驱动下的空间分数阶Edwards-Wilkinson (SFEW)方程在1+1维情况下的数值解,得到了不同噪声关联因子和分数阶数时的生长指数、粗糙度指数、动力学指数等,所求出的临界指数均与标度分析方法的结果相符合.研究表明噪声关联因子和分数阶数均影响到SFEW方程的动力学标度行为,且表现为连续变化的普适类.     

Anomalous Scaling of Surface Growth Equations with Spatially and Temporally Correlated Noise

Based on the scaling idea of local slopes by López et al. [Phys. Rev. Lett. 94 (2005) 166103], we investigate anomalous dynamic scaling of (d+1)-dimensional surface growth equations with spatially and temporally correlated noise. The growth equations studied include the Kardar-Parisi-Zhang (KPZ), Sun-Guo-Grant (SGG), and Lai-Das Sarma-Villain (LDV) equations. The anomalous scaling exponents in both the weak- and strong-coupling regions are obtained, respectively.     

含时空关联噪声生长方程的标度奇异性分析

基于动力学重整化群理论研究表面界面生长动力学标度奇异性问题, 得到含时空关联噪声的表面生长方程标度奇异指数的一般结果,并将此方法应用于几种典型的局域生长方程——Kardar-Parisi-Zhang（KPZ）方程、线性生长方程、Lai-Das Sarma-Villain（LDV）方程.结果表明,在长波长极限下局域生长方程的动力学标度奇异性与最相关项、基底维数以及噪声有关,并且若出现标度奇异性,只会是超粗化（super rough）奇异标度行为,而不是内禀（intrinsically）奇异标度行为. 关键词： 标度奇异性 动力学重整化群理论 时空关联噪声     

单体迁移驱使的聚集体生长

本文提出了一个可解的聚集体生长模型。在系统中，在质量为l的B聚集体的作用下，一个单体从质量为i的A聚集体迁移到质量为j的A聚集体，这个迁移反应速率核为K(i;j;l) i^μ j^ν l^varpi（ω≥0）。利用平均场速率方程方法，本文得到了几种不同情况下的聚集体质量分布的解析解。对μ=ν的系统，在μ<3/2的情况下系统的聚集体质量分布a_k(t)具有一种普适的标度形式。对于μ≠ν的系统，只有在μ<ν和μ+ν<2的情况下a_k(t)才满足标度形式。同时，在μ+ν>2（μ≠ν）或者μ>3/2（μ=ν）的情况下，系统将发生类凝胶相变。此外，本文也研究了反应核为K(i;j;l)μ(i^μ j^ν+ i^νj^μ )lω的系统的动力学标度行为。结果表明，聚集体质量分布只有在μ+ν<3的情况下才遵循标度律，而在其他情况下系统将在足够长的时间后发生类凝胶相变。     

Renormalization group study of Mullins' equation for molecular beam epitaxy with conserved noise

The dynamics of driven interfaces under conserved noise in a continuum model of growth by a molecular beam has been studied by means of the Noziéres-Gallet dynamic renormalization group technique, using the results of Sun and Plischke for the case of non-conserved noise. Relaxation of the growing film is due to both surface tension and surface diffusion. In (1 + 1) dimensions, four growth regimes have been found. None of these are purely diffusive. One of these fixed points has negative surface tension and is stable with respect to renormalization group flow. This is an unstable growth state in which the creation of large slopes in the interface configuration is expected. In (2 + 1) dimensions, seven growth regimes have been found, in which three are purely diffusive. There is also one fixed point with a negative surface tension. However, this fixed point is unstable with respect to renormalization group flow, and is therefore expected to crossover into the other growth regimes at large system size and long times.     

Extensive numerical study of the anomalous dynamic scaling of the Wolf-Villain model

In order to study the microscopic physical mechanisms of roughness surfaces exhibiting the anomalous scaling behavior, the Wolf-Villain model in 1+1 and 2+1 dimensions is investigated by the kinetic Monte-Carlo simulation on long time and large length scale (the growth time and the system size are respectively extended to t=2²⁹, for 1+1 dimensions, and t=2²¹, L×L=512×512 for 2+1 dimensions). In the 2+1-dimensional simulations, the noise reduction technique is employed so as to eliminate the crossover effects in the growth process. Our calculations show that the Wolf-Villain model in 1+1 dimensions very probably exhibits intrinsic anomalous scaling behavior in the time and length simulation range of this paper, and the 2+1-dimensional Wolf-Villain model leads to a pyramidal mounded morphology. Some properties of the mounded pattern in the 2+1-dimensional Wolf-Villain model are discussed in the final part of this presentation.     

表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.