层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶，主要研究了其磁电特性.结果表明，样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低，其磁性经历了一个很复杂的转变过程.当x=0时，在T^*=231K出现二维短程铁磁有序，在<     

掺Ba的Bi-Sr-Cu-O单晶Raman散射研究

利用Ｒａｍａｎ散射研究了Bi₂Sr_2-xBa_xCuO_y（ｘ＝０，０．２，０．４）单晶样品的声子振动性质。实验结果表明，在Bi₂Sr_2-xBa_xCuO_y体系的Ｒａｍａｎ谱中主要出现以下几个频率的特征振动模，即１９６，３００，４６０，６２５和６６０ｃｍ^－１；而频率为１９６，３００，６２５，６６０ｃｍ^－１<     

Magnetic and electrical properties of Mg1−x CuxO solid solutions and magnetic shielding in Cu-Mg1−x CuxO structures

Mg_1−x Cu_xO solid solutions having an NaCl structure with 0⩽x⩽0.20 are synthesized and Cu-Mg_1−x Cu_xO structures are prepared for superconductivity studies. The magnetic susceptibility χ, electron paramagnetic resonance (EPR), and electrical conductivity of the solid solutions are studied at temperatures of 5–550 K. It is shown that χ ⁻¹(T) obeys the Curie-Weiss law with a paramagnetic Curie temperature Θ close to zero and an effective magnetic moment μ _eff=1.9 μ _B, close to the 1.73 μ _B of a Cu²⁺ ion with spin S=1/2. The width ΔH of the EPR line depends weakly on temperature and increases as x is raised. The volume narrowing of the EPR linewidth ΔH is used to estimate the exchange interaction parameter, 3×10⁻⁴ eV. The g-factor is close to 2 and is temperature independent. The electrical conductivity of Mg_1−x Cu_xO at T=300 K is ≈10⁻¹¹–10⁻¹² Ω⁻¹ cm⁻¹ for x=0 and increases to 10⁻⁵–10⁻⁶ Ω⁻¹ cm⁻¹ for x=0.15–0.20. The conductivity is p-type. Magnetic shielding is observed in Cu-Mg_1−x Cu_xO structures with x=0.15 and 0.20. The possible connection of this phenomenon with interference superconductivity in the contact layer of the structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 293–296 (February 1999)     

Sn-based type-VIII single-crystal clathrates with a large figure of merit

Single-crystal samples of type-VIII Ba₈Ga_{16 - x}Cu_xSn₃₀ (x=0, 0.03, 0.06, 0.15) clathrates were prepared using the Sn-flux method. At room temperature the carrier density, n, is 3.5-5×10¹⁹ cm^-3 for all the samples, the carrier mobility, μ_H, increases to more than twice that of Ba₈Ga₁₆Sn₃₀ for all the Cu doping samples, and consequently the electrical conductivity is enhanced distinctly from 1.90×10⁴ S/m to 4.40×10⁴ S/m, with the Cu composition increasing from x=0 to x=0.15. The Seebeck coefficient, α , decreases slightly with the increases in Cu composition. The κ values are about 0.72 W/mK at 300 K and are almost invariant with temperature up to 500 K for the samples with x=0 and x=0.03. The lattice thermal conductivity, κ_L, decreases from 0.59 W/mK for x=0 to 0.50 W/mK for x=0.03 at 300 K. The figure of merit for x=0.03 reaches 1.35 at 540 K.     

Tb1－xDyxFe2－y-Fe掺杂BaTiO3多层膜中的磁电耦合

用溶胶-凝胶法制备了Fe掺杂BaTiO₃粉体，在1350℃下烧结成圆片状多晶样品，并与Tb_1-xDy_xFe_2-y胶合成磁电(ME)双层膜或三层膜.实验分析表明Fe：BaTiO₃依然是四方相钙钛矿结构，但是居里温度及相变潜热均略低于纯净BaTiO₃.研究了Tb_1-xDy_xFe_2-y-Fe∶BaTiO₃双层膜和Tb_1-xDy_xFe_2-y-Fe∶BaTiO₃-Tb_1-xDy_xFe_2-y三层膜的ME效应.在2.8×10⁴A/m的磁场下，两者的横向ME电压系数均达其峰值，分别为6.225和26.25mV·(A·m^-1)^-1·cm^-1.并且，用掺杂BaTiO₃制备的双层膜和三层膜的横向ME电压系数均为相同条件下用纯净BaTiO₃制备的双层膜和三层膜的横向ME电压系数的1.5倍.另外由于不含铅，锆等有害物质，符合环保要求，因此采用掺杂BaTiO₃制备的磁电效应器件具有深入研究和应用价值.     

矩形截面多量子阱脊形波导模式特性

本文根据多量子阱双折射率的性质,运用有效折射率法分析了矩形截面多量子阱脊形波导E_mn^x模和模E_mn^y的模式特性。     

Co掺杂对铁磁金属La0.8Sr0.2MnO3磁电阻影响机理

通过对La_0.8Sr_0.2Mn_1-yCo_yO₃(y≤02)饱和磁矩和输运的测量,研究了Co对La_0.8Sr_0.2MnO₃的磁电阻影响机制.结果表明,在La_0.8Sr_0.2Mn_1-yCo_yO₃（y≤02）中Co³⁺离子是低自旋态.由于Mn³⁺—O—Co³⁺—O—Mn³⁺类型的磁交换与Mn³⁺-Mn⁴⁺离子间双交换作用相比较弱,Curie温度T_C附近的磁电阻随着Co掺杂量的增加而降低.与此相反,由于Co²⁺离子与e_g巡游电子的反铁磁交换耦合作用,低温区间的磁电阻随着Co掺杂量的增加而升高.     

BeCl分子电子激发态的多参考组态相互作用计算

运用含Davidson修正的多参考组态相互作用方法,在aug-cc-pVTZ基组水平上,对BeCl分子基态和相同多重度的几个低电子激发态进行了势能扫描计算.通过群论原理确定各电子态对称性及离解极限.将其中基态(X²Σ⁺)和第一激发态(A²Π})对应的势能曲线拟合到Murrell-Sorbie解析势能函数形式,得到基态(X²Σ⁺)的离解能及主要光谱常数(括号中为文献[6]提供的实验值)为D_e=3.74eV,R_e=0.18173nm(0.17970),w_e=857.4cm¹(847.2),w_ex_e=5.03cm^-1(5.14),B_e=0.7103cm^-1(0.7285),α_e=0.0059cm^-1(0.0069),第一激发态(A²Π)的D_e=3.02eV,R_e=0.18369nm(0.18211),w_e=832.7cm^-1(822.1),w_ex_e=5.93cm^-1(5.24),B_e=0.6953cm^-1(0.7094),α_e=0.0065cm^-1(0.0068),计算结果与实验值符合得较好.另外,通过Level程序求解双原子径向核运动的Schrödinger方程得到J=0时BeCl分子这两个电子态的全部振动能级.     

弹性薄壁桿件的平衡与稳定的一般理论

In this paper a theory of equilibrium and stability of elastic thin-walled cylinders is proposed. The theory is based on the following assumptions: 1) The cross section of the cylinder is uncleformable. 2) The cylinder is under a system of initial stresses σ_z⁰=- P⁰/F-M_y⁰/I_xx x + M_x⁰/I_yyy. This theory may be regarded as a generalization of V. Z. Vlasoff's theory of stability of thin-walled rods, and includes the theory of Karman-Chien and Adaduroff as a special case. For cases of simply supported cylinders and cantilever cylinders, a method of solution using trigonometric series is proposed which is much simpler than the methods used by Karman-Chien and Adaduroff.     

梯形截面多量子阱脊形波导模式特性

本文根据多量子阱双折射率的性质,运用有效折射率法和WKB法对具有梯形截面多量子阱脊形波导的模式特性作了分析,导出了E_mn^x模和E_mn^y模的本征值方程.     

Super-Diffusivity in a Shear Flow Model¶from Perpetual Homogenization

This paper is concerned with the asymptotic behavior solutions of stochastic differential equations dy _t =dω _t −∇Γ(y _t ) dt, y ₀=0 and d=2. Γ is a 2 &\times; 2 skew-symmetric matrix associated to a shear flow characterized by an infinite number of spatial scales Γ₁₂=−Γ₂₁=h(x ₁), with h(x ₁)=∑ _{n =0} ^∞γ _n h ⁿ (x ₁/R _n ), where h ⁿ are smooth functions of period 1, h ⁿ (0)=0, γ _n and R _n grow exponentially fast with n. We can show that y ^t has an anomalous fast behavior (?[|y ^t |²]∼t ^1+ν with ν > 0) and obtain quantitative estimates on the anomaly using and developing the tools of homogenization. Received: 1 June 2001 / Accepted: 11 January 2002     

Phase separation, ferromagnetism and magnetic irreversibility in La1−xSrxMn1−yFeyO3

Magnetic susceptibility, χ(T), is investigated in ceramic La_1−xSr_xMn_1−yFe_yO₃ (LSMFO) samples with x=0.3 and y=0.15−0.25. A ferromagnetic (FM) transition observed in LSMFO is accompanied with an appreciable decrease of the transition temperature with increasing y, which is connected to breaking of the FM double-exchange interaction by doping with Fe. Strong magnetic irreversibility, observed in low (B=10 G) field, gives evidence for frustration of the magnetic state of LSMFO. The FM transition, which is expanded with increasing B, is more pronounced in the samples with y=0.15-0.20 and broadens considerably at y=0.25, where the irreversibility is increased. Well above the transition, χ(T) exhibits a Curie-Weiss asymptotic behavior, yielding very large values of the effective Bohr magneton number per magnetic ion, incompatible with those of Mn or Fe single ions. At y=0.15 and 0.20 a critical behavior of χ⁻¹(T)∼(T/T_C−1)^γ in the region of the FM transition is characterized by influence of two different magnetic systems, a 3D percolative one with γ₌γ_p≈1.8 and T_C=T_C^(p), and a non-percolative 3D Heisenberg spin system, with γ=γ_H≈1.4 and T_C=T_C^(H), where T_C^(p)<T_C^(H). At y=0.25 the percolative contribution to the critical behavior of χ(T) is not observed. The dependence of χ on T and y gives evidence for phase separation, with onset already near the room temperature, leading to generation of nanosize FM particles in the paramagnetic host matrix of LSMFO. The ferromagnetism of LSMFO is attributable to percolation over the system of such particles and generation of large FM clusters, whereas the frustration is governed presumably by a system of smaller weakly-correlated magnetic units, which do not enter the percolative FM clusters.     

Surface segregation of ZrC from a carbide solid solution

The surface segregation of zirconium carbide from carbide solid solutions is investigated. The spontaneous surface segregation of ZrC grains from solid solutions in the pseudobinary ZrC-NbC system is revealed for the first time. It is demonstrated that the ZrC precipitation is associated with the decomposition of the car-bide homogeneous solid solutions Zr_1?x Nb_xC ≡ (ZrC)_1?x (NbC)_x. The boundaries of the latent solid-phase decomposition region formed at T<1200 K are determined for the solid solutions formed by ZrC_y and NbC _y′ carbides with different nonstoichiometry. The experimental and theoretical estimates obtained for the segregation energy of ZrC are equal to ?50 and ?31 kJ mol^?1, respectively.     

Influence of a magnetic field on the hydrogen equilibrium pressure of RCo5Hx

The hydrogen equilibrium abessure above a two-phase mixture RCo₅H_x + RCo₅H_y can be changed slightly by applying a magnetic field. For H=10 kOe a relative change Δp/p ≈ 6×10⁻³ has been measured for R=La, x=3.35, y=4.3, T=295 K.     

Self-gravitating general-relativistic cosmic strings

We examine the coupled Einstem-Euler-Lagrange equations for nonstationary cosmic strings. Self-consistent solutions to all the equations are found under the assumption that the energy-momentum tensor is of the formT _t ^t =T _z ^z while all other components vanish. It is shown that the strings are necessarily static in this case and that the scalar field potential must be of the usual quartic form with the coupling constants satisfying e²=8.     

Coexistence of superconductivity and magnetic order in YBa2Cu4O8

⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility and powder x-ray-diffraction measurements were used to study superconductivity and magnetic order in YBa₂(Cu_1?xFe_x)₄O_8+δ. T_c is decreasing with x, disappearing for x>x_c≈0.04. For xc iron substitutes Cu, predominantly in the Cu(1) site exhibiting a single quadrupole Mössbauer spectrum at 90 K. For x>x_c magnetic order is observed in the Cu(2) site, T_N=380 (5) K for x=0.1 and H_eff (Cu(2), 4.2 K)=510(2) kOe. However, the most surprising discovery is that for x=0.025, for which T_c=27(2) K, the Fe in the Cu(1) site orders magnetically at T_N=30(2) K and H_eff (Cu(1), 4.2 K)=461(2) kOe. The coexistence and competition between superconductivity and magnetic order in the Cu(1) and Cu(2) sites in YBa₂Cu₄O₈ are discussed in terms of the previously observed phase diagrams for Y_1?xPr_xBa₂(Cu_1?yFe_y)₃O_z.     

Study of data on the associated momentum on the trigger side in highP T hadron production

The British-French-Scandinavian collaboration has recently studied the non trigger charged mean momentum in different rapidity regions on the trigger hemisphere, 〈p _x〉, in the collision of two hadrons at the CERN Intersecting Storing Rings (ISR). In particular, they give for the rapidity regions |y|<0.5 and |y|<1 the values of the slope, α, of 〈p _x〉 with the trigger momentum,p _T ^t . Several authors have analysed those values of α in the framework of hard scattering models which predict values independent ofp _T ^t for 〈z _c〉, the longitudinal momentum fraction of the outgoing hard scattered system taken by the trigger. From this analysis they give estimates of 〈z _c〉 of very difficult reconcilliation with those calculated in the Feynman, Field and Fox hard scattering model or in the QCD treatment of highp _T hadron production. The authors of the present paper have looked for, and found, other data whose model independent analysis is more feasible than that of the data mentioned above. More specifically, we analyse in the framework of the hard scattering models, but otherwise model independently, data on 〈p _x〉 in two other rapidity regions (|y|<3, 2<|y|<3) and find that consistence of the average slopes, α, in these two regions is only achieved with mean values of 〈z _c〉 significantly, increasing withp _T ^t and close in value to those obtained by Feynman et al. [6, 7].     

EPR study of the symmetry breaking effect in ferroelectric cesium dihydrogen phosphate doped with Cr5+ ions

The (PO₄)^3? units in a CsH₂PO₄ (CDP) crystal were replaced in a small fraction of sites by (CrO₄)^3? groups and the EPR of the Cr⁵⁺ center was investigated. Splitting of the EPR line appears at T¹_c=245 K, 91 K higher that the ferroelectric transition temperature T_c=154 K. The electronic wave function of Cr⁵⁺ (3d¹) is identified as d_x2_?y2. The d_x2_?y2 function couples with the near protons and the reorientation of this unit in the two possible configurations occurs in the paraelectric phase and breaks the symmetry far above T_c. The observed correlation time 10^?9 sec and associated activation energy ΔU=0.215 eV are discussed.     

Improving magnetocaloric properties of Fe68–xCrxTb5B23Nb4 (x = 0, 2, 4, 6 and 8) metallic glasses having high glass-forming ability with tunable Curie temperature

Abstract

The impacts of adding Cr on the Curie temperature (T_C), glass-forming ability (GFA), and magnetocaloric effect were studied in Fe_68?xCr_xTb₅B₂₃Nb₄ (x = 0, 2, 4, 6 and 8) metallic glasses prepared by suction casting. GFA depends on Cr content in the composition. For Fe_68?xCr_xTb₅B₂₃Nb₄ bulk metallic glasses (BMGs), with critical diameters up to ~3 mm can be produced by suction casting and maximum value of GFA was found for x = 6. By exchanging Cr with Fe partially, T_C could effectively be adjusted in a quite broad temperature interval from 487 K for x = 0 to 267 K for x = 8, whereas maximum magnetic entropy change decreased from 1.16 to 0.53 Jkg^?1 K^?1 and refrigeration capacity (RC) changed from 116 to 45.05 J/kg under a low field change of 2 T. Though T_C is shifted to room temperature, maximum magnetic entropy change and RC decreased almost half of the base alloy. To enhance these properties (Fe_0.62Cr_0.06Tb_0.05B_0.23Nb_0.04)_100?yCu_y (y = 0.75, 1), metallic glasses are prepared. By the help of small addition of Cu, magnetocaloric properties can be effectively increased without changing the T_C. These findings show that the successful synthesis of the Fe-based Fe₆₂Cr₆Tb₅B₂₃Nb₄ and (Fe_0.62Cr_0.06Tb_0.05B_0.23Nb_0.04)_100?yCu_y (y = 0.75, 1) BMGs near room temperature, could be considered as promising candidates as magnetic refrigerant materials.     

Effect of hydrogen doping on the magnetic properties of Gd2CuO4

We report the results of ac-susceptibility and dc-magnetization measurements for H_yGd₂CuO₄ (0y0.54). It is shown thatH doping lowers the weak ferromagnetic component in the material. The distinct hysteresis loops observed atT=77 K for both non- and hydrogenated samples change its shape withy. The magnetic ordering temperatures T _N ^Cu and T _N ^Gd , as determined from the temperature dependencies of ac-susceptibility, remain unchanged with sample's hydrogenation. This result seems to indicate that extra electrons are not doped onto the Cu-O planes of Gd₂CuO₄. The frequency dependencies ofx(, T) andx(, T) for bothy=0 andy=0.15 samples are analysed., The maximums ofx andx found at about 200K are considered in terms of susceptibility dependence on the spin-lattice relaxation time (). The anomalies in ac-susceptibility found recently in Gd₂CuO₄ atT _a=8 K andT _b=9.5 K decrease significantly withy. Results are discussed in the context of available data on 214T-type compounds.