准周期层状铁磁超晶格的实空间重整化群方法

本文用实空间重整化群方法讨论了准周期层状铁磁超晶格的磁自旋波,用Reduce语言推导了decimation变换公式,从而求得了局域格林函数、局域态密度和约化磁矩。发现局域态密度的带宽和约化磁矩与最近邻相互作用J₁、J₂及格点自旋s_a、s_b密切相关。     

Magnetic properties and percolation threshold in diluted B-spinel ZnCr2xAl2−2xS4: a study through high-temperature expansions

By making use of high-temperature series expansions (HTSE) of the correlation functions, we study the thermal and disorder variation of the short-range order (SRO) in the particular B-spinel ZnCr_2xAl_2−2xS₄. We developed the HTSE for the q-dependent static structure factor S(q) to the order 6 in reciprocal temperature including both the nearest- and next-nearest-neighbour interactions J₁ and J₂, respectively. Respecting the experimental fact that the broad diffuse peak of the neutron is situated at the particular wave vector q₀=[0 0 0.79] and is insensitive to the temperature for a given ratio of dilution x, we have estimated the thermal variation of J₁ and J₂ in the case of the pure compound.

The bond percolation threshold x_p of the ZnCr_2xAl_2−2xS₄ is determined by studying the disorder variation of the correlation length ξ. The x_p is considered as the concentration at which ξ vanishes. The obtained values are x_p=0.27 when only J₁ is considered and 0.23 when both J₁ and J₂ are taken into account.     

Surface roughness dependence of the first-order probability density function for integrated speckle

The surface roughness dependence of the first-order probability density function (PDF) of an integrated speckle pattern, produced in the condition of circular detection aperture and gaussian scattering spot, was theoretically investigated. The mutual intensity J_A(x₁, y₁; x₂, y₂) containing two roughness parameters, dispersion of surface height ø² lateral correlation length x_c, was calculated. The exact first-order probability density function was analytically derived and numerically calculated by means of the Karhunen-Loeve expansion and fast Fourier transform (FFT). As a diffuse object became smooth, the first order probability density function was changed from negative exponential to sharp peak gaussian centered around mean intensity.     

Very strong interlayer exchange coupling in epitaxial Fe/Fe1−xSix/Fe trilayers (x=0.4–1.0)

Fe/Fe_1−xSi_x/Fe (x=0.4–1.0) wedge-type epitaxial trilayers with improved homogeneity are grown by co-evaporation from two electron-beam sources. The coupling strengths of the bilinear (J₁) and biquadratic (J₂) coupling terms are derived from Brillouin light scattering (BLS) spectra and longitudinal MOKE hysteresis loops. The total coupling strength J=J₁+J₂ increases dramatically with increasing x and reaches values in excess of 6 mJ/m².     

自旋为1的双层平方晶格阻挫模型的相变

采用双时格林函数方法研究了自旋为1的双层平方晶格阻挫模型的相变行为.详细探讨了层间耦合相互作用J_c和单离子各向异性参数D对奈尔态(AF1)和共线态(AF2)之间相转换的影响.结果显示:只要参数J_c和D不同时为零,奈尔态和共线态在J₂=J₁/2(这里J₁和J₂分别描述的是系统自旋间最近邻和次近邻交换作用)时的相变温度相等,两个态共存.在低于相变点的温度范围内,AF1-AF2态之间可以发生相转换,其相变类型为一阶相变.当J₂≠J₁/2时,尽管AF1-AF2态有不同相变温度,但它们也可以共存.如果AF1(AF2)态的相变温度大,在低温,AF1(AF2)态更稳定;在高温,AF2(AF1)态更稳定;在中间温度范围内,AF1-AF2态之间也可以发生一阶相转换.     

Resource reduction for simultaneous generation of two types of continuous variable nonclassical states

We demonstrate experimentally the simultaneous generation and detection of two types of continuous variable nonclassical states from one type-0 phase-matching optical parametric amplification (OPA) and subsequent two ring filter cavities (RFCs). The output field of the OPA includes the baseband ω₀ and sideband modes ω₀±nω_fsubjects to the cavity resonance condition, which are separated by two cascaded RFCs. The first RFC resonates with half the pump wavelength ω₀ and the transmitted baseband component is a squeezed state. The reflected fields of the first RFC, including the sideband modes ω₀±ω_f, are separated by the second RFC, construct Einstein–Podolsky–Rosen entangled state. All freedoms, including the filter cavities for sideband separation and relative phases for the measurements of these sidebands, are actively stabilized. The noise variance of squeezed states is 10.2 dB below the shot noise limit (SNL), the correlation variances of both quadrature amplitude-sum and quadrature phase-difference for the entanglement state are 10.0 dB below the corresponding SNL.     

Electronic and magnetic properties of single-layer and double-layer VX_2(X=Cl,Br) under biaxial stress

First-principles calculations and Monte Carlo simulations reveal that single-layer and double-layer VX_2(X = Cl, Br)can be tuned from antiferromagnetic(AFM) semiconductors to ferromagnetic(FM) state when biaxial tensile stress is applied. Their ground states are all T phase. The biaxial tensile stress at the phase transition point of the double-layer VX_2 is larger than that of the single-layer VX_2. The direct band gaps can be also manipulated by biaxial tensile stress as they increases with increasing tensile stress to a critical point and then decreases. The Néel temperature(TN) of double-layer VX_2 are higher than that of single-layer. As the stress increases, the TNof all materials tend to increase. The magnetic moment increases with the increase of biaxial tensile stress, and which become insensitive to stress after the phase transition points.Our research provides a method to control the electronic and magnetic properties of VX_2 by stress, and the single-layer and double-layer VX_2 may have potential applications in nano spintronic devices.     

On meson resonances and chiral symmetry

We study meson resonances with quantum numbers J^P=1⁺ in terms of the chiral SU(3) Lagrangian. At leading order a parameter-free prediction is obtained for the scattering of Goldstone bosons off vector mesons with J^P=1⁻ once we insist on approximate crossing symmetry of the unitarized scattering amplitude. A resonance spectrum arises that is remarkably close to the empirical pattern. In particular, we find that the strangeness-zero resonances h₁(1380), f₁(1285) and b₁(1235) are formed due to strong and channels. This leads to large coupling constants of those resonances to the latter states.     

Role of the interfacial state: a comparison of Co/Al2O3/NiFe and La0.7Ca0.3MnO3/Al2O3/La0.7Ca0.3MnO3 tunnel junctions

Two junctions of Co/Al₂O₃/NiFe (J₁) and La_0.7Ca_0.3MnO₃/Al₂O₃/La_0.7Ca_0.3MnO₃ (J₂) were prepared to compare their tunneling magnetoresistance (TMR) in consideration of interfacial state effects. The structural and transport properties of the layered samples were characterized by X-ray and magnetic measurements, showing indeed an interfacial state dependence. The influences such as from a CoO sublayer in J₁ and from interfacial coherence in J₂ were discussed. The largest TMR observed amounts to 16% (290 K) for J₁ and 65% (40 K) for J₂.     

Robustness of the unidirectional stripe order in the kagome superconductor CsV3Sb5

V-based kagome materials AV₃Sb₅ (A=K, Rb, Cs) have attracted much attention due to their novel properties such as unconventional superconductivity, giant anomalous Hall effect, charge density wave (CDW) and pair density wave. Except for the 2a₀×2a₀ CDW (charge density wave with in-plane 2×2 superlattice modulation) in AV₃Sb₅, an additional 1×4 (4a₀) unidirectional stripe order has been observed at the Sb surface of RbV₃Sb₅ and CsV₃Sb₅. However, the stability and electronic nature of the 4a₀ stripe order remain controversial and unclear. Here, by using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically study the 4a₀ stripe order on the Sb-terminated surface of CsV₃Sb₅. We find that the 4a₀ stripe order is visible in a large energy range. The STM images with positive and negative bias show contrast inversion, which is the hallmark for the Peierls-type CDW. In addition, below the critical temperature about 60 K, the 4a₀ stripe order keeps unaffected against the topmost Cs atoms, point defects, step edges and magnetic field up to 8 T. Our results provide experimental evidences on the existence of unidirectional CDW in CsV₃Sb₅.     

Pressure-induced phase transition of B-type Y_2O_3

The synthesized monoclinic(B-type) phase of Y_2O_3 has been investigated by in situ angle-dispersive x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. A phase transition occurs from monoclinic(B-type) to hexagonal(A-type) phase at 23.5 GPa and these two phases coexist even at the highest pressure. Parameters of isothermal equation of state are V_0= 69.0(1) ~3, K_0= 159(3) GPa, K_0= 4(fixed) for the B-type phase and V_0= 67.8(2) ~3, K_0= 156(3) GPa,K'_0= 4(fixed) for the A-type phase. The structural anisotropy increases with increasing pressure for both phases.     

Fabrication, microstructure and critical current density of screen-printed Ag-(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes

The influence of the sintering conditions on the microstructure and critical current density J_c has been studied on screen-printed Ag-(Bi, Pb)₂Sr₂Ca₂Cu₃O_x tapes with a ceramics mono-layer core. Three kinds of fabrication processes, which consist of a combination of cold working (rolling and/or pressing) and sintering, are applied. Four times repetition of pressing and sintering after the pre-sintering produces the highest c-axis alignment and achieves J_c= 1.5 × 10⁴ A/cm² (77 K, 0 T). The J_c versus θ data with an angle θ between B and the c-axis elucidate the relation between the anisotropy ratio γ=J_c(Bc)/J_c(B|c and the half-height angular width Δθ of a peak for Bc. This is related to both grain alignment and the J_c value. An increase in J_c, which comes from an improvement for grain alignment, enhances γ and narrows Δθ. The J_c versus θ data are fitted to the expression J_c(B, θ)=J _c(B, 90°)/[(γ−1)|cos θ|ⁿ+1] by regarding both γ and n as adjustable parameters. Fabrication of screen-printed tapes with multilayers (1≤N≤5) is presented, where the critical current increases from 8.0 A to 30.2 A at 77 K and 0 T as N increases.     

An optimized cluster density matrix embedding theory

We propose an optimized cluster density matrix embedding theory(CDMET).It reduces the computational cost of CDMET with simpler bath states.And the result is as accurate as the original one.As a demonstration,we study the distant correlations of the Heisenberg J_1-J_2 model on the square lattice.We find that the intermediate phase(0.43■J_2■0.62) is divided into two parts.One part is a near-critical region(0.43■ J_2■ 0.50).The other part is the plaquette valence bond solid(PVB) state(0.51■J_2■ 0.62).The spin correlations decay exponentially as a function of distance in the PVB.     

三角对称晶场下d8电子组态的EPR参量的普遍公式及其应用

本文考虑到d⁸电子组态中所有自旋三重态和自旋单态对基态的影响,用高阶微扰法导出了d⁸电子组态在三角对称晶场下光谱和基态EPR普遍公式。发现在三角对称晶场下,d⁸电子组态离子的g_⊥ > g_‖或g_⊥ < g_‖和D > 0或D < 0是以立方对称晶场为界,晶体中顺磁离子的配体沿C₃轴伸长或压缩所致。利用所得公式计算了NiCl₂和NiSiF₆·6H₂O晶体的EPR参量,其结果与实验吻合较好。     

Magnetic and transport properties of YBa2Cu3Oy superconductor foams

The recently reported superconducting YBa₂Cu₃O_y (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a T_c of 92 K and have a magnetization J_c of 40,000 A/cm² at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to J_c^ab/J_c^c7.     

虚阴极产生微波的一些基本特性

用Vlasov-Poisson方程对相对论电子束在单板、双板间的传播过程进行了数值模拟,给出了单板模型空间电荷积累最大的位置,不同位置上的电流J、电子数密度n、电场E的振荡频率随入射电子数密度n₀、入射速度v₀的变化关系,双板模型空间电荷积累最大的位置,J、n、E的振荡频率随入射流J₀及两板间距离的变化关系。虚阴极位置的数值结果与稳态理论给出的结果相近,它的振荡频率符合经验公式(1~√2π)ω_peb。单板时入射电子数密度按速度服从高斯分布,能散△E_n/E_n < 10%时的数值结果给出与单能情况基本相同的结论。     

Ising spin glass on a D = 3 hierarchical lattice: Effects of tailed coupling distributions

Within a real-space renormalization-group framework, we approach the cubic lattice through a D = 3 diamond-like hierarchical lattice. The model is a standard, nearest-neighbor, Ising spin glass with coupling constants {J_ij} distributed according to the family of continuous probability distributions P_q(J_ij) ∝ 1/[1 + (q − 1)J_ij²/2J²]^{1/(q − 1)} (if 1 + (q − 1) J_ij²/2J² > 0, and zero otherwise; q ). Such distributions, which arise naturally in the treatment, within the recently proposed nonextensive thermostatistics, of anomalous diffusion, reproduce the usual, Gaussian case, for q → 1. Moreover, they present a second moment J_ij² proportional to (5 − 3q)⁻¹ for q < 5/3, diverging for q ≥ 5/3, but keeping a finite width at midheight. In the limit q → 3, P_q(J_ij) collapses with the abscissa, and so the width at midheight diverges. We compute the q-dependence of the spin-glass critical temperature T_c. We show numerically that T_c does not scale with J_ij^21/2 (contrary to the usual belief), but rather with the width at midheight of P_q(J_ij). Our results suggest that T_c vanishes as −1/q when q → −∞; furthermore, we verified that T_c diverges exponentially when q approaches 3 from below.     

部分道路关闭引起的交通激波特性研究

本文根据实际交通中经常遇到的交通事故或部分道路施工等情况, 建立了部分道路关闭的交通流模型. 采用平均场理论分析和确定性NS元胞自动机规则分别对模型进行解析和数值模拟, 结果表明, 系统存在三种稳定的物理状态:低密度相、激波相和高密度相, 并找到了系统发生相变的临界密度. 理论分析和数值模拟能很好地符合.     

Conductivity peaks broadening in the quantum Hall regime

We argue that it is the hopping transport that is responsible for broadening of the σ_xx peaks. Explicit expressions for the width Δν of a peak as a function of the temperature T, current J and frequency ω are found. It is shown that Δν grows with T as (T/T₁)^κ, where κ is the inverse localization-length exponent. The current J is shown to act like the effective temperature T_eff(J) ∝ J^1/2 if . Broadening of the ohmic ac-conductivity peaks with frequency ω is found to be determined by the effective temperature