The coexistence of magnetic ordering and superconductivity in YBa2(Cu0.94Fe0.06)3O9−σ

We have made superconducting samples of the Y-Ba-Cu-O system with 6% Fe substituted for the Cu. The sample has been verified by X-ray diffraction to be a single-phased 1-2-3 compound. The 6% Fe substitution reduces T_c from 94 to 80 K. The room temperature Mössbauer spectrum shows two pairs of doublets, indicating there are two distinct Cu sites in the sample. The most surprising result is that magnetic ordering of the Fe coexists with superconductivity of the material at 4.2 K.     

共掺杂痕量Tb3+的Y2O2S:RE3+(Eu3+、Sm3+、Dy3+等)磷光体中Tb3+对RE3+能量传递的机理

共掺杂痕量的Tb3+,能使Y2O2S:RE3+ (RE3+=Eu3+、Sm3+、Dy3+等)磷光体的阴极射线和254nm紫外光激发下发光效率明显增强,此类增强的特点是:Tb3+浓度低(几十ppm),增强倍数高(1—6倍)和被敏化离子种类多。对一定浓度的痕量Tb3+,Sm3+和Dy3+浓度在10-5—10-2范围内,增强倍数为常数,而Eu3+随着浓度的增加、增强倍数逐渐减小,这表明Tb3+→RE3+不是共振能量传递。从激发光谱和Tb3+各条谱线强度变化的比例可判断能量传递不是光的再吸收。根据阴极射线激发后的热释光曲线以及增强倍数与温度(室温到液氮温度)关系实验,提出了能量传递模型。认为是Tb3+加入后形成等电子陷阱。它对空穴有大的俘获截面,并可以束缚一个激子。Tb3+→RE3+不是空穴类型的能量传递,而是在热扰动下束缚激子变为自由激子后引起的激子能量传递。     

Charm changing weak decays 1/2+ → 3/2+ + 0−/γ in SU(4) and SU(8) w

Weak decay modes (1/2⁺ → 3/2⁺ + 0⁻/γ) of charmed baryons are studied. Relations among the various decay amplitudes are derived in isospin, SU(3), SU(4) and SU(8) _w symmetries. Sextet dominance in SU(3) forbidsB(3) →D(10) +P(3*) decays. 20″ dominance in SU(4) specifies all the decays in terms of Θ⁻ decays. Weak decays of Θ* ₃ ⁺⁺ and Θ⁻ are also discussed. SU(8) _w symmetry predictsα , which is consistent with the experimental value.     

Optical spectroscopy of Cr3+ and Rh2+ in sodium-beta-̋alumina

Ion exchange techniques were used to substitute Cr³⁺ and Rh²⁺ ions for the sodium ions present in sodium-beta″-alumina single crystals. The optical absorption and fluorescence spectra were obtained and the fluorescence lifetimes measured for both types of samples. In each case the emission consisted of two bands having significantly different decay times. The temperature dependences of the fluorescence intensities and lifetimes were measured for these bands. The results are interpreted in terms of the energy level structures of these ions in different crystal field sites in this host lattice and the diffusion of ions in the conduction plane which changes these sites.     

The 2ν1 + ν3 and 3ν3 bands of 14N16O2

The spectra of the 2ν₁ + ν₃ and 3ν₃ bands of ¹⁴N¹⁶O₂ have been recorded by means of high-resolution Fourier transform spectroscopy and have been extensively analyzed. The (2 0 1) and (0 0 3) rotational levels deduced from the analysis have been reproduced within the experimental uncertainty using a Hamiltonian which takes into account the Coriolis interaction coupling the vibrational states of the diads {(2 2 0), (2 0 1)} and {(0 2 2), (0 0 3)}. Finally, precise sets of vibrational energies, and spin-rotation, rotational, and coupling constants have been derived for these vibrational states.     

Local tetragonal distortions of the (CrF6)3− and (FeF6)3− octahedral clusters in Cr3+- and Fe3+-doped tetragonal Rb2KGaF6 crystals

The local tetragonal distortions (α???α₀) (where α is the angle defined as tgα?=?R_⊥/R_//, R_⊥ and R_// are the metal–ligand distances parallel with and normal to the C₄ axis, α₀?=?45° is the same angle in cubic symmetry) of (CrF₆)^3? and (FeF₆)^3? octahedral clusters in the tetragonal Rb₂KGaF₆ crystals are estimated by analyzing their electron paramagnetic resonance (EPR) zero-field splittings D. The results indicate that the two impurity octahedra and hence the host (GaF₆)^3? octahedra are tetragonally elongated. The distortion (α???α₀) in magnitude differs from impurity to impurity because of the different sizes and natures of these impurities. These results are analogous to those in ABX₃ and doped ABX₃ perovskite crystals where the cubic-to-tetragonal phase transition is due to the rotation of BX₆ octahedra associated with the release or elongation of B–X bond along the C₄ rotational axis.     

Determination of charge-compensated C3v (Ⅱ) centers for Er3+ ions in CdF2 and CaF2 crystals

A unified theoretical method is established to determine the charge-compensated C3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 crystals by simulating the electron paramagnetic resonance(EPR)parameters and Stark energy levels.The potential(Er³⁺–F^?–O₄^2?)and(Er³⁺-F₇^?-O₄^2?)structures for theC3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 crystals are checked by diagonalizing 364×364 complete energy matrices in the scheme of superposition model.Our studies indicate that the C3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 may be ascribed to the local(Er³⁺-F^?-O₄^2?)structure,where the upper ligand ion F?undergoes an off-center displacement by?Z≈0.3?A for CdF2 and?Z≈0.29?A for the CaF2 along the C3 axis.Meanwhile,a local compressed distortion of the(ErFO4)6?cluster is expected to be?R≈0.07?A for CdF2:Er3+and?R≈0.079?A for CaF2:Er3+.The considerable g-factor anisotropy for Er3+ions in each of both crystals is explained reasonably by the obtained local parameters.Furthermore,our studies show that a stronger covalent effect exists in the C3v(Ⅱ)center for Er3+in CaF2 or CaF2,which may be due to the stronger electrostatic interaction and closer distance between the central Er3+ion and ligand O2?with the(Er³⁺-F^?-O₄^2?)structure.     

Time-resolved spectroscopy and energy transfer in Pr3+-doped Gd2(SO4)3·8H2O

The luminescence of Pr³⁺ in gadolinium sulfate hydrate is reported for 195 nm laser excitation. the Pr³⁺ ion acts as a sensitizer of the host lattice emission. The Pr³⁺Gd³⁺ energy transfer occurs in two different ways. In the octahydrate the transfer occurs from the lowest component of the 4f5d configuration of Pr³⁺, but in the samples with less water there is energy transfer from the¹ S _o level of Pr³⁺ to several Gd³⁺ levels. The Pr³⁺ emission in the two modifications is, of course, also strikingly different.Deceased 24 December 1994     

Investigation of magnetic ordered state in YBa2Cu3O6+x (0.06<X<0.32) bY μSR

The magnetic state properties have been studied in YBa₂Cu₃O_6+x with (0.06SR-studies two precession signals with the frequencies close to 4MHz was observed at T=20÷80K. Possible reasons of the phenomena at T<100K are discussed.     

Optimization of synthesis conditions and composition of blue phosphors [(B2O3)0.5(Al2O3)0.5] · 2SiO2: Eu2+

It is found that [(B₂O₃)_0.5(Al₂O₃)_0.5] · 2SiO₂: Eu²⁺ systems with different concentrations of europium exhibit intense photoluminescence in the blue-violet range. When the system is excited by a 325-nm helium-cadmium laser, the spectra of blue phosphors exhibit a maximum in the interval 419–439 nm, while excitation by a pulsed nitrogen laser with a wavelength of 337 nm causes luminescence between 429 and 461 nm. Phosphor with the composition [(B₂O₃)_0.5(Al₂O₃)_0.5] · 2SiO₂: Eu (3 mol %) (x = 0.151, y = 0.062) excited by the radiation of a helium-cadmium laser and phosphor of the same composition having been subjected to additional vacuum annealing and excited by the radiation of a nitrogen laser (x = 0.145, y = 0.084) are found to be optimal to provide primary blue color in the EBU and NTSC TV standards. Phosphors of the composition [(B₂O₃)_0.5(Al₂O₃)_0.5] · 2SiO₂: Eu²⁺ (7 mol %) offer the highest luminescence intensity both before and after vacuum annealing.     

Relationship between (2 + 1) and (3 + 1)-Friedmann–Robertson–Walker cosmologies; linear,non-linear,and polytropic state equations

It is shown that Friedmann–Robertson–Walker (FRW) cosmological models coupled to a single scalar field and to a perfect fluid fitting a wide class of matter perfect fluid state equations, determined in (3+1) dimensional gravity can be related to their (2+1) cosmological counterparts, and vice-versa, by using simple algebraic rules relating gravitational constants, state parameters, perfect fluid and scalar field characteristics. It should be pointed out that the demonstration of these relations for the scalar fields and potentials does not require the fulfilment of any state equation for the scalar field energy density and pressure. As far as to the perfect fluid is concerned, one has to demand the fulfilment of state equations of the form p+ = f(). If the considered cosmologies contain the inflation field alone, then any (3+1) scalar field cosmology possesses a (2+1) counterpart, and vice-versa. Various families of solutions are derived, and we exhibited their correspondence; for instance, solutions for pure matter perfect fluids and single scalar field fulfilling linear state equations, solutions for scalar fields coupled to matter perfect fluids, a general class of solutions for scalar fields subjected to a state equation of the form p + = are reported, in particular Barrow–Saich, and Barrow–Burd–Lancaster–Madsen solutions are exhibited explicitly, and finally perfect fluid solutions for polytropic state equations are given.     

掺Ce3+、Tb3+的M3Y2(BO3)4(M＝Ca,Sr)

采用固相反应的方法;经二次灼烧合成了掺Ce3+、Tb3+的Ca3Y2(BO3)4和Sr3Y2(BO3)4磷光体;分析了合成过程中铈的还原情况.用X-射线衍射分析确定了它们的结构均为正交晶系,空间群P21cn测定了Ce3+和Tb3+在两种基质中的光谱,得到Ce3+波长位移的某些规律,观察到Ce3+对Tb3+的敏化作用.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

Electrical and magnetic properties of Gd(Ba2-xLax)Cu3O7+δ

We have studied the structural, electrical, and magnetic properties of the normal and superconducting states Gd(Ba2-xLax)Cu3O7+ [Gd(BaLa)123] samples with 0.0 x 0.8 prepared by the standard solid-state reaction. XRD characterization shows an orthorhombic-tetragonal structural transition at x=0.2. Iodometric titration analysis shows the oxygen content of the samples increase with the increase of La doping. The resistivity curves show that for x0.15, there is metallic behavior, and for x0.2, there is a gradual insulating transition behavior in the normal state. The metal-insulator and superconductor-insulator transitions occur between x=0.35 and x=0.4. The superconducting transition temperature decreases with the increase of La content as two-step curve. The normal-state resistivity is fitted for two and three dimensional variable range hopping (2D&3D-VRH) and Coulomb gap (CG) regimes, separately. The results show that the dominant mechanism is CG for x0.35, and VRH for x0.4. The pinning energy U, derived from the thermally activated flux creep (TAFC) model and Ambegaokar-Halperin (AH) theory, shows a power-law relation as UH -. The critical current density decreases with the increase of La doping and magnetic field. The E-J curves show that the induced electric field increases with the increase of magnetic field and La concentration. The magnetization measurements indicate that the critical penetration fields and magnetic current density decrease with La doping.     

Luminescence and absorption of Tb3+in mo·Al2O3·B2O3·Tb2O3 glasses

Quantum yields of the green Tb luminescence for 254 nm excitation of glass compositions in the system MO·Al₂O₃· B₂O₃·Tb₂O₃ (M = Mg, Ca, Sr, Ba and Zn) were studied in relation to absorption and excitation spectra. Yields as high as 80% were observed. The Tb 4f-5d absorption maximum ranges from 218 to 232 nm, always at a longer wavelength than the glass matrix absorption. The yield strongly depends on the spectral position of the 4f-5d absorption, due to competing impurity absorption at 254 nm.     

Energy transfer and frequency upconversion in Yb3+–Er3+-doped PbO-GeO2 glass containing silver nanoparticles

We report the infrared-to-visible frequency upconversion in Er³⁺–Yb³⁺-codoped PbO-GeO₂ glass containing silver nanoparticles (NPs). The optical excitation is made with a laser at 980 nm in resonance with the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. Intense emission bands centered at 525, 550, and 662 nm were observed corresponding to Er³⁺ transitions. The simultaneous influence of the Yb³⁺→Er³⁺ energy transfer and the contribution of the intensified local field effect due to the silver NPs give origin to the enhancement of the whole frequency upconversion spectra.     

CH3CH2+O(3P)反应机理的理论研究

利用abinitio方法对CH3CH2+O(3P)反应进行了理论研究,在MP2/6311+G(d,p)水平上优化得到了反应途径上的反应物、中间体、过渡态和产物的几何构型和谐振频率,并在QCISD(T)/6311+G(d,p)水平上进行单点能计算.计算结果表明:CH2O+CH3、CH3CHO+H和CH2CH2+OH是主要反应产物,其中CH2O+CH3主要来自反应通道A1:(R)→IM1→TS3→(A),CH3CHO+H主要来自反应通道B1:(R)→IM1→TS4→(B),CH2CH2+OH主要来自直接抽提反应通道C1和C2:(R)→TS1(TS2)→(C).计算结果同时表明该反应生成CO的通道能垒是非常高的,CO应该不是主要产物.     

Sr2Mg(BO3)2:Tb3+的真空紫外发光性质

采用高温固相反应法合成了Tb3+激活的Sr2Mg(BO3)2荧光粉.利用XRD表征荧光粉的相纯度.研究了材料在VUV-UV范围的激发光谱和在VUV-UV光激发下的发射光谱及荧光衰减曲线.结果显示:Sr2Mg(BO3)2:Tb3+荧光粉的基质吸收带主峰位置大约位于178nm,Tb3+的最低自旋允许和最低自旋禁阻f-d跃迁吸收带分别位于235和278 nm,172nm激发下荧光粉的最强发射光谱主峰在543 nm,色坐标为(0.30,0.45),Tb3+的荧光寿命值约为2.8 ms.