K2_SPH方法及其对二维非线性水波的模拟

K2_SPH方法通过泰勒级数展开和联立求解积分方程组的办法得到具有二阶精度的核近似方法.随着核近似精度的提高,K2_SPH需要对-些关键数值技术进行改进才能成功模拟非线性水波问题,例如自由表面边界和固壁边界.通过与传统SPH方法计算结果比较,K2_SPH方法在非线性自由表面计算精度和整个粒子系统中有关变量分布都有显著提高.     

液滴超声速气动破碎初期界面不稳定性

针对液滴破碎问题,获得并揭示两相界面演化特征机理.采用数值模拟方法,观察了超声速条件下的液滴气动破碎初期的界面不稳定性.基于数值模拟结果和线性稳定性理论,综合分析表明,Rayleigh-Taylor不稳定性和Kelvin-Helmholtz不稳定性均对源于驻点和外环之间中段附近处的主导扰动产生作用.保持其他流动特性不变,降低K-H不稳定性的影响,对数值模拟进行了专门改进,进一步验证了前述结论.      

CO_2/正己烷体系质扩散系数和黏度的分子动力学模拟研究

在CO_2驱油提高采收率技术(CO_2-EOR)中,质扩散系数和黏度参数决定了液相混合体系的扩散行为和流动特性。本文基于分子动力学模拟方法,对CO_2/正己烷体系的质扩散系数和黏度进行了研究。CO_2采用改进的EPM2模型,正己烷采用L-OPLS模型,模拟得到的纯质密度分布和自扩散系数与文献数据吻合良好。模拟计算了CO_2/正己烷体系在温度为303.15 K、383.15 K、压力为1～5 MPa范围条件下的质扩散系数和黏度。结果表明:在相同温度下,随压力增大,扩散系数模拟值与实验值相对偏差增大,偏差基本在20%以内。黏度模拟值与实验值相对偏差不超过15%.采用本文所用力场模型与数据处理方法可以用来预测CO_2/正己烷体系的质扩散系数和黏度。     

强激光间接驱动材料动态破碎过程的实验技术研究

强激光驱动加载已成为冲击波作用下材料动态破碎过程研究的一种有效手段.采用间接驱动方式,设计合适的腔型进行物理实验研究,可实现更大且更均匀的冲击加载一维区.采用数值模拟和物理实验方法,研究强激光间接驱动材料动态破碎过程的实验技术.首先,利用IRAD程序设计适用于开展动态破碎过程研究的半柱腔,其直径为2 mm、腔长为2 mm;进而通过物理实验获得此腔型下多个激光能量点、脉宽2 ns和3 ns条件下辐射峰值温度和波形;最后,利用流体模拟方法给出多种辐射波形下的冲击加载波形.利用高能X射线成像和光子多普勒干涉仪诊断给出间接驱动加载下层裂过程的物理图像和速度历史.经分析发现,间接驱动的加载一维区达到2 mm,平面性优于5%,能有效地开展相关物理实验研究.研究结果为新型柱腔设计、冲击加载技术及动态破碎过程研究提供了重要的研究基础.     

气溶胶抛撒过程中首次破碎液滴的尺寸分布

对气溶胶爆炸抛撒过程中,首次破碎液滴的尺寸分布进行理论和数值研究.基于热力学第二定律的最大熵增理论,建立首次破碎过程满足的方程组约束条件,给出破碎后液滴尺寸分布的确定方法,并对Air-blast喷管实验和Samirant相关实验进行数值模拟预测,与实验数据符合较好.     

深过冷液态Ni2 TiAl合金热物理性质的分子动力学模拟

采用嵌入原子法对深过冷条件下Ni2TiAl合金的焓和密度进行了分子动力学模拟,模拟获得了比热和密度在1200K至2000K范围内的温度依赖关系.结果表明过冷条件下Ni2TiAl合金的比热、密度和温度之间存在着线性关系,它们都随着温度的降低而降低.比热的大小与依据Neumann-Kopp法则估算的结果相近,同时模拟过程中的尺度效应经验证可以忽略.     

Ce_3Rh_3Si_2和CeRh_3Si_2的磁性模拟和晶场效应研究

根据晶场理论,通过对D.Kaczorowski等人测量的磁化率倒数与温度关系曲线的模拟,得到了稀土化合物CeRh3Si2和Ce3Rh3Si2的晶场分裂能和相应波函数.对于CeRh3Si2和Ce3Rh3Si2,它们的第一激发能和总的激发能分别为176K,855K和77.5K,428K.计算表明,Kramers离子Ce3+在晶场效应的作用下,基态简并部分消除得到了三个双重态,模拟得到的化合物的磁化率倒数与温度关系曲线与实验曲线吻合较好.     

第一性原理计算和Monte Carlo算法耦合对共掺杂稀磁半导体磁学性质的研究

将第一性原理计算和Monte Carlo算法耦合,对共掺杂稀磁半导体进行模拟,预测其磁学性质,并与同种过渡金属原子单掺杂稀磁半导体相比较进行分析.首先用第一性原理计算掺杂原子间的磁相互作用强度,作为后面Monte Carlo模拟的输入参数;然后利用Monte Carlo方法结合海森堡自旋模型和磁相互作用强度来模拟体系热力学磁化强度和磁化率,由此得到居里温度;用同样的方法模拟掺杂磁性体系的磁滞回线.计算结果显示,对应于掺Co、Al-Co、Fe、Al-Fe的ZnO基稀磁半导体的居里温度分别为346 K、450 K、743 K、467 K,与实验值很吻合;模拟这四种掺杂情况得到的磁滞回线也很符合理论分析和实验结果,且呈现和居里温度相同的变化规律.     

很热余核破碎瞬时的状态方程

用正则系综蒙特卡罗模拟方法研究余核破碎问题时,若对碎块位置的平均采取近似的办法,就可以得到余核(RT)系统破碎时的状态方程.本文用数值方法计算了该状态方程,得到了广义压强(P)-体积(V_RT)平面上的等温曲线;它与Van der Waals气体相应等温线有类似之处;相变温度就应该对应于只有一特点的等温曲线的温度.本文又指出,描写破碎问题的统计模型中的两个参数,即破碎温度(T)和破碎体积(V_RT),在一定的变化范围内都能符合碎块质量分布数据.也就是说,(T,V_RT)对不是唯一的.一定温度下与实验符合最好的一对参数,就是相应等温线的两相共存区内压强的极大点.     

非等温非牛顿黏性流体流动问题的修正光滑粒子动力学方法模拟

为准确、有效地模拟非等温非牛顿黏性流体的流动问题,本文基于一种不含核导数计算的核梯度修正格式和不可压缩条件给出了一种改进光滑粒子动力学(SPH)离散格式,它较传统SPH离散格式具有较高精度和较好稳定性.同时,为准确地描述温度场的演化过程,建立了非牛顿黏性的SPH温度离散模型.通过对等温Poiseuille流、喷射流和非等温Couette流、4:1收缩流进行模拟,并与其他数值结果作对比,分别验证了改进SPH方法模拟非牛顿黏性流动问题的可靠性和提出的SPH温度离散模型求解非等温流动问题的有效性和准确性.随后,运用改进SPH方法结合SPH温度离散模型对环形腔和C形腔内非等温非牛顿黏性流体的充模过程进行了试探性模拟研究,分析了数值模拟的收敛性,讨论了不同位置处热流参数对温度和流动的影响.     

Selective β-elimination in the reaction of 2-propanol to acetone on Rh(111)-p(2 × 1)-O

The formation of acetone from 2-propanol and Rh(111)-p(2 × 1)-O has been investigated by temperature programmed reaction and X-ray photoelectron spectroscopies and isotopic labeling experiments under ultrahigh vacuum conditions. Some 2-propanol forms 2-propoxide on Rh(111)-p(2× 1)-O below 250 K and selective β (with respect to the metal in 2-propoxide) C-H bond breaking at 270 K is the primary path for acetone evolution. A minor amount of reversible C-H bond activation is also observed. β-carbon-hydrogen bond breaking is proposed to be the rate-limiting step for the initial acetone evolution from 2-propanol on Rh(111)-p(2× 1)-O at high coverage based on kinetic isotope effects. The rate of acetone evolution is in part rate-limited by desorption, however, for low 2-propanol exposures. In addition, there is some oxygen exchange between the surface and the acetone at 320 K. Combustion to H₂O, CO and CO₂is a competing pathway. Irreversible γ-C-H bond breaking primarily leads to combustion. The reactivity of 2-propanol on the (2 × 1)-O surface is dramatically different from that on clean Rh(111), where nonselective decomposition to CO and H₂ is induced. The inhibition of extensive, nonselective C-H and C-C bond breaking is a crucial factor in determining the selectivity for β-dehydrogenation to produce acetone.     

Observation of a liquid-gas-type transition in the pyrochlore spin ice compound Dy2Ti2O7 in a magnetic field

Low temperature magnetization measurements on the pyrochlore spin ice compound Dy2Ti2O7 reveal that the ice-rule breaking spin flip, appearing at H approximately 0.9 T applied parallel to the [111] direction, turns into a novel first-order transition for T<0.36 K which is most probably of a liquid-gas type. T-linear variation of the critical field observed down to 0.03 K suggests the unusual situation that the entropy release across the transition remains finite [approximately 0.5 (J/K) x mol-Dy] as T-->0, in accordance with a breaking of the macroscopic degeneracy in the intermediate "kagomé ice" state.     

Symmetry Reduction and Exact Solutions of the （3＋1）-Dimensional Breaking Soliton Equation

By means of the generalized direct method, a relationship is constructed between the new solutions and the old ones of the （3＋1）-dimensional breaking soliton equation. Based on the relationship, a new solution is obtained by using a given solution of the equation. The symmetry is also obtained for the （3＋1）-dimensional breaking soliton equation. By using the equivalent vector of the symmetry, we construct a seven-dimensional symmetry algebra and get the optimal system of group-invariant solutions. To every case of the optimal system, the （3＋1）-dimensional breaking soliton equation is reduced and some solutions to the reduced equations are obtained. Furthermore, some new explicit solutions are found for the （3＋ 1）-dimensional breaking soliton equation.     

Enhanced critical currents of superconducting ErNi2B2C in the ferromagnetically ordered state

We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.     

Recent progress on isospin breaking corrections and their impact on the muon g－2 value

We describe some recent results on isospin breaking corrections which are of relevance for predictions of the leading order hadronic contribution to the muon anomalous magnetic moment aμ^had.LO when using τ lepton data. When these corrections are applied to the new combined data on the π^±π^0 spectral function, the prediction for aμ^had.LO based on τ lepton data gets closer to the one obtained using e^＋e^- data.     

A Note on ＂Doubly Periodic Propagating Wave for （2＋1）-Dimensional Breaking Soliton Equation＂

In a recent paper [Commun. Theor. Phys. （Beijing, China） 49 （2008） 268], Huang et al. gave a general variable separation solution to the （2＋1）-dimensional breaking soliton equation via a special Biicldund transformation and the variable separation approach. In terms of the derived variable separation solution and by introducing Jacobi elliptic functions, they claimed that nonelastic types of interaction between Jacobi elliptic function waves are investigated both analytically and graphically. We show that some inappropriateness or errors exist in their paper, and say nothing of the conclusion that some nonelastic types of interaction between Jacobi elliptic function waves in the （2＋1）-dimensional breaking soliton equation have been found.     

Scanning tunneling spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.     

Band renormalization and spin polarization of MoS2 in graphene/MoS2 heterostructures

Transition metal dichalcogenides exhibit spin–orbit split bands at the K‐point that become spin polarized for broken crystal inversion symmetry. This enables simultaneous manipulation of valley and spin degrees of freedom. While the inversion symmetry is broken for monolayers, we show here that spin polarization of the MoS₂ surface may also be obtained by interfacing it with graphene, which induces a space charge region in the surface of MoS₂. Polarization induced symmetry breaking in the potential gradient of the space charge is considered to be responsible for the observed spin polarization. In addition to spin polarization we also observe a renormalization of the valence band maximum (VBM) upon interfacing of MoS₂ with graphene. The energy difference between the VBM at the Γ‐point and K‐point shifts by ～150 meV between the clean and graphene covered surface. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Observation of potassium hyperfine interactions in x-irradiated KH2AsO4 through the method of electron spin echo envelope modulation

Hyperfine interaction frequencies of ¹H and ³⁹K nuclei near the AsO^4-₄ radical in X-ray irradiated KH₂AsO₄ (KDA) have been observed through the method of electron spin echo envelope modulation (ESEEM). This method enabled us to record nuclear hyperfine interaction (ENDOR-like) spectra around the ferroelectric phase transition of KDA for the first time. The ESEEM spectrum of ³⁹K exhibits a clear change when passing the ferroelectric phase transition temperature, but that of close protons does not. The result for close protons is in agreement with the symmetry breaking of the AsO^4-₄ site as observed via the EPR spectrum [5]. Finally, at 4.2 K the hyperfine interaction parameters of a ³⁹K nucleus near the AsO^4-₄ unit could be determined through the ESEEM method.