镁基合金自由枝晶生长的相场模拟研究

利用定量相场模型, 以Mg-0.5 wt.%Al合金为例模拟了基面((0001)面)内镁基合金的等温自由枝晶生长过程. 通过研究该合金体系数值模拟的收敛性, 获得了最优化值耦合参数λ = 5.5及网格宽度Δx/W₀ = 0.4, 并在该参数下系统研究了各向异性强度和过饱和度对枝晶尖端生长速度、尖端曲率半径、Péclet数及稳定性常数σ^* 的影响. 结果表明, 由微观可解性理论得到的稳定性系数σ^* 与ε₆ 拟合值σ^*≅ ε₆ ^1.81905, 更接近理想值σ ^* (ε₆) ≅ε₆ ^1.75. 此外, 当过饱和度Ω < 0.6时, 稳定性系数σ ^* 不随ε₆ 的变化而变化, 而当Ω > 0.6时, 稳定性系数σ ^* 随着ε₆ 的增加而减小. 这反映了枝晶的生长由扩散控制向动力学控制的转变. 随着过饱和度的增加, 枝晶形貌由雪花状枝晶向圆状枝晶转变.     

Quantum superreplication of states and gates

Although the no-cloning theorem forbids perfect replication of quantum information, it is sometimes possible to produce large numbers of replicas with vanishingly small error. This phenomenon, known as quantum superreplication, can occur for both quantum states and quantum gates. The aim of this paper is to review the central features of quantum superreplication and provide a unified view of existing results. The paper also includes new results. In particular, we show that when quantum superreplication can be achieved, it can be achieved through estimation up to an error of size O(M/N²), where N and M are the number of input and output copies, respectively. Quantum strategies still offer an advantage for superreplication in that they allow for exponentially faster reduction of the error. Using the relation with estimation, we provide i) an alternative proof of the optimality of Heisenberg scaling in quantum metrology, ii) a strategy for estimating arbitrary unitary gates with a mean square error scaling as log N/N², and iii) a protocol that generates O(N²) nearly perfect copies of a generic pure state U|0>while using the corresponding gate U only N times. Finally, we point out that superreplication can be achieved using interactions among k systems, provided that k is large compared to M²/N².     

Spectral density analysis of the chiral transition in nf=4 finite temperature QCD

We apply the spectral density reweighting technique to the analysis of the chiral phase transition in finite-temperature lattice quantum chromodynamics (QCD) with four flavors of dynamical staggered fermions and m_qa=0.025. Our simulations were performed using the hybrid Monte Carlo method for L_tL³ lattices with L_t=4 and L=6, 8 and 10. We calculate partition function zeros, as well as the maxima of the specific heat and of the susceptibilities for the Polyakov loop and for the chiral condensate. A finite size scaling analysis of these data leads to preliminary results for the critical coupling β_c, for the critical exponent ν, for the latent heat, and for the jumps in the Polyakov loop and in the chiral condensate.     

一端附壁高分子链的Monte Carlo研究

采用简立方格点上的Monte Carlo模拟,研究一端被无限大不可穿透平面壁吸附的高分子链的均方末端距<R²>,以及高分子链的质量中心到平面吸附壁的平均距离<Z>,与链长N、参数u(u=e^-ε/kT,ε是链骨架原子间的相互作用能量,k是玻耳兹曼常数,T是热力学温度)的关系。结果表明:<R²>和<Z>都服从标度律,<R²>=αN^γ,<Z>=βN^η,其中,γ、η、α、β都是u的函数;u从1减小到0.5,则γ从1.01增大到1.19,η从0.51增大到0.60.     

Perturbative QCD correction to the B → π transition form factor

We report on the perturbative O(_s) correction to the light-cone QCD sum rule for the B → π transition form factor f⁺. The correction to the product f_Bf⁺ in leading twist approximation is found to be about 30%, that is similar in magnitude to the corresponding O(_s) correction in the two-point sum rule for f_B. The resulting cancellation of large QCD corrections in f⁺ eliminates one important uncertainty in the sum-rule prediction for this form factor.     

二维圆盘颗粒体系声学行为的数值研究

数值测量了卸载过程中二维单分散圆盘颗粒系统的横波、纵波声速、声衰减系数、非线性系数随压强的变化以及声衰减系数随频率的变化.结果表明,二维(2D)圆盘颗粒体系的横波、纵波声速均随压强呈分段幂律标度:当压强P10~(-4)时,横波、纵波声速随压强的增大而减小;当P10~(-4)时,有v_t~P~(0.202),v_l~P~(0.338).进一步得到其剪切模量和体积模量的比值G/B也随压强呈幂律标度,G/B~P~(-0.502),暗示在低压强下,与三维(3D)球形颗粒体系类似,2D圆盘颗粒体系也处于L玻璃态.水平激励和垂直激励下2D圆盘颗粒系统的衰减系数随频率变化也呈现分段行为:当频率f0.05时,衰减系数不随f变化;当f0.05时,横波纵波的衰减系数α~f;当f0.35时,横波衰减系数α_T~f~2,纵波衰减系数α_L~f~(1.5).此外,竖直水平激励下的2D圆盘颗粒系统的非线性系数和衰减系数随压强也呈现与声速类似的分段规律:当P10~(-4)时,横波非线性系数β_T~P~(-0.230),其余都不随压强变化.当P10~(-4)时,两者均随压强增大呈幂律减小:β_T~P~(-0.703),β_L~P~(-0.684),α_T~P~(-0.099),α_L~P~(-0.105).进而得到2D圆盘颗粒系统中散射相关的特征长度?~*随压强呈幂律标度,当P10~(-4)时,?~*~P~(-0.595);当P10~(-4)时,?~*~P~(0.236).     

N=2 6-dimensional supersymmetric E6 breaking

We study the N=2 supersymmetric E₆ models on the 6-dimensional space–time where the supersymmetry and gauge symmetry can be broken by the discrete symmetry. On the space–time M⁴×S¹/(Z₂×Z₂′)×S¹/(Z₂×Z₂′), for the zero modes, we obtain the 4-dimensional N=1 supersymmetric models with gauge groups SU(3)×SU(2)×SU(2)×U(1)², SU(4)×SU(2)×SU(2)×U(1), and SU(3)×SU(2)×U(1)³ with one extra pair of Higgs doublets from the vector multiplet. In addition, considering that the extra space manifold is the annulus A² and disc D², we list all the constraints on constructing the 4-dimensional N=1 supersymmetric SU(3)×SU(2)×U(1)³ models for the zero modes, and give the simplest model with Z₉ symmetry. We also comment on the extra gauge symmetry breaking and its generalization.     

Flux creep and flux flow for thallium ceramics in a pulsed magnetic field

The resistive properties of Tl-ceramics with T_c|=0=114 K were investigated in a pulsed magnetic field (B) up to 30 T and in the temperature range of 4.2 K<T<140 K. It was shown that the character of the field dependence of the resistance differs qualitatively in high-and low-temperature regions. At low (T80K) temperatures the dynamic magnetoresistance arising in the sample is analogous to that observed earlier in LaSrCuO [1] and YBaCuO [2] ceramics. This magnetoresistance is defined by the magnetic field variation rate and leads to the appearance of a minimum at the maximum of the magnetic field pulse, i.e. at . In the region of high temperatures (80 K T<T_c) or magnetic fields (at T60K) the sample resistance rises monotonically with B increase, and dynamic resistance is not observed. In this temperature range the existence of a scaling relation is shown (here B^* and T^* meet the condition k=(B^*, T^*)/ _n(T^*)=const) for the ceramics resistance (B,T), which can be represented as . An estimate for the upper critical field B_c2(0)B_o=1030±40 T is obtained.     

随机双指数记忆耗散系统的非马尔可夫扩散

针对具有双指数耗散记忆核函数的两自由度耦合系统, 本文利用Laplace变换导出了热宽带噪声激励下该系统响应二阶矩的解析表达式. 并观察到位移二阶矩不同于单自由度情形下的反常扩散:<x²(t)> ∝ t^α (0<α<2, α≠1), 而是随时间及噪声等参数变化呈现普遍的振荡扩散现象.分析可得, 阻尼耦合因子B使粒子远离简谐势场的束缚, <x²(t)>随B的增大扩散加剧而摩擦系数增大却使其趋于平稳状态.进一步, 若两热噪声互关联时, 较小的互关联时间对二阶矩的影响较大, 反之作用较小. 伴随互关联强度递增, 位移二阶矩的扩散加剧, 位移间的相关性加强, 与物理直观相符. 关键词： 热噪声 非马尔可夫扩散 广义朗之万方程 关联性     

Control schemes for synchronizing two subnetworks with weak couplings

In this paper, we propose a well-designed network model with a parameter and study full and partial synchronization of the network model based on the stability analysis. The network model is composed of a star-coupled subnetwork and a globally coupled subnetwork. By analyzing the special coupling configuration, three control schemes are obtained for synchronizing the network model. Further analysis indicates that even if the inner couplings in each subnetwork are very weak, two of the control schemes are still valid. In particular, if the outer coupling weight parameter θ is larger than (n2 2n)/4, or the subnetwork size n is larger than θ2, the two subnetworks with weak inner couplings can achieve synchronization. In addition, the synchronizability is independent of the network size in case of 0 < θ < n/(n + 1). Finally, we carry out some numerical simulations to confirm the validity of the obtained control schemes. It is worth noting that the main idea of this paper also applies to any network consisting of a dense subnetwork and a sparse network.     

Electron spectroscopy observation of the 1s2s2p(S) state in atomic fluorine by means of electron impact excitation of the hf molecule

The energy of the 1s2s²2p⁶²S) core excited state in atomic fluorine is determined by studying a satellite in the electron beam excited KVV Auger electron spectrum of HF. The satellite is assigned to an atomic autoionization transition 1s2(²2p⁶(²S) → 1ss²2p⁴ (¹D). The energy of the 1s2s²2P⁶(²s) initial state has been determined to be 676.5 eV. Transitions to the 1s²2s²2p⁴³p(³P)and 1s²2s²2P⁴(¹S) states are also observed. The intensity for these transitions is very low.     

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa₂Cu₃O_7−δ and Nb have been fabricated. This procedure allows connections between high-T_c and low-T_c superconductors at different crystal sides of the high-T_c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa₂Cu₃O_7−δ to the Nb. Critical current densities up to 600 A/cm² are obtained at T=4.2 K, with typical R_nA values of 0.8 μΩ cm². The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa₂Cu₃O_7−δ crystal axes is explained by considering a predominant d_x²−y² order parameter symmetry of the YBa₂Cu₃O_7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-T_c superconductor.     

Universal behaviors of magnon-mediated spin transport in disordered nonmagnetic metal-ferromagnetic insulator heterostructures

We numerically investigate magnon-mediated spin transport through nonmagnetic metal/ferromagnetic insulator (NM/FI) heterostructures in the presence of Anderson disorder, and discover universal behaviors of the spin conductance in both one-dimensional (1D) and 2D systems. In the localized regime, the variance of logarithmic spin conductance σ²(lnG_T) shows a universal linear scaling with its average ⟨lnG_T⟩, independent of Fermi energy, temperature, and system size in both 1D and 2D cases. In 2D, the competition between disorder-enhanced density of states at the NM/FI interface and disorder-suppressed spin transport leads to a non-monotonic dependence of average spin conductance on the disorder strength. As a result, in the metallic regime, average spin conductance is enhanced by disorder, and a new linear scaling between spin conductance fluctuation rms(G_T) and average spin conductance G_T is revealed which is universal at large system width. These universal scaling behaviors suggest that spin transport mediated by magnon in disordered 2D NM/FI systems belongs to a new universality class, different from that of charge conductance in 2D normal metal systems.     

AlC分子 X4∑-和B4∑-电子态的光谱性质

采用Davidson修正的内收缩多参考组态相互作用方法(MRCI+Q) 结合Dunning等的相关一致基aug-cc-pVnZ (n=D,T,Q,5,6) 计算了AlC分子X⁴∑^-和B⁴∑^-态的势能曲线, 并利用总能量外推公式将这两个态的总能量分别外推至完全基组极限. 对势能曲线进行核价相关修正及相对论修正, 并详细讨论了基组、核价相关和相对论修正 等对X⁴∑^-和B⁴∑^-电子态的能量和光谱常数的影响. 拟合核价相关及相对论效应修正的外推势能曲线, 得到了AlC分子X⁴∑^- 和B⁴∑^-电子态的主要光谱常数T_e, R_e, ω_e, ω_ex_e, ω_ey_e, B_e和α_e. 它们与实验结果符合较好. 求解双原子分子核运动的径向Schrödinger方程, 找到了无转动的AlC分子两个电子态的全部振动态. 针对每一振动态, 还分别计算了其相应的振动能级和惯性转动常数等分子常数. 它们与已有的实验结果一致. 关键词： 光谱常数 分子常数 核价相关修正 相对论修正     

πN scattering and electromagnetic corrections in the perturbative chiral quark model

We apply the perturbative chiral quark model to give predictions for the electromagnetic O(p²) low-energy couplings of the ChPT effective Lagrangian that define the electromagnetic mass shifts of nucleons and first-order (e²) radiative corrections to the πN scattering amplitude. We estimate the leading isospin-breaking correction to the strong energy shift of the π⁻p atom in the 1s state, which is relevant for the experiment “Pionic Hydrogen” at PSI.     

Does the quenched reduced U(∞) chiral model break spontaneously in weak coupling?

The U(N) chiral model, when quenched using Parisi's rule, has a [U(1) × U(1)]^N/U(1) global invariance. To determine whether this symmetry breaks spontaneously in weak coupling for N=∞, a one-loop calculation of the distribution of eigenvalues of the single U(N) matrix of the model is performed. This distribution is shown to be uniform on the unit circle and hence, no symmetry breaking occurs. Further, the order parameter | tr U|²/N², which should be zero at N=∞ in the absence of spontaneous symmetry breaking, is evaluated in the weak coupling phase for one, two and three dimensions for N varying from 2 to 50 by Monte Carlo simulation of the quenched model. The data indicate that this parameter indeed goes to zero as N→∞ implying that the symmetry does not break.     

若干高精度恒稳的半显式差分格式

本文构造了解色散方程u₁=au_xxx的若干三层恒稳的半显式差分格式。第Ⅰ、Ⅱ类格式的局部截断误差的阶为O(τ²+h²+(τ²)/(h³));而第Ⅲ、Ⅳ类格式的局部截断误差的阶为O(τ²+h⁴+((τ)/(h))²+τh)。用判别稳定性的Von Neumann准则可证明:第Ⅰ、Ⅱ类格式及当参数α≤1时的第Ⅲ、Ⅳ类格式都是无条件稳定的,并且当必须的边界条件给定时它们可以显式地进行计算。     

Sneutrino–Higgs mixing in WW and ZZ production in supersymmetry with R-parity violation

We consider new s-channel scalar exchanges in e⁺e⁻→ZZ, W⁺W⁻ in supersymmetry with a small lepton number violation. We show that a small bilinear R-parity violating term which leads to sneutrino–Higgs mixing can give rise to a significant scalar resonance enhancement in e⁺e⁻→ZZ, W⁺W⁻. We use the LEP2 measurements of the WW and ZZ cross-sections to place useful constraints on this scenario. We also find, under conservative assumptions on the relevant parameter space involved, that such an exchange of the sneutrino-like admixture in e⁺e⁻→ZZ, W⁺W⁻ may be accessible to a 500 GeV e⁺e⁻ collider.