7Li NMR investigation of Li-Li pair ordering in the paraelectric phase of weakly substitutionally disordered K(1-x)Li(x)TaO3

Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed 7Li NMR at temperatures (150-400?K) far above the nominal glass transition temperature (≈50 K for Li concentration x=0.03). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.     

EPR, optical absorption and superposition model study of Fe³(+) doped strontium nitrate single crystals

Electron paramagnetic resonance (EPR) study of Fe3(+) ions doped strontium nitrate (SN) single crystals is performed at liquid nitrogen temperature and at X band frequency. The spin Hamiltonian (SH) parameters are determined from the resonance lines observed at different angular rotations. The crystal field parameters (CFPs) are evaluated using superposition model of Newman. The Zeeman g-factor and zero-field splitting parameters (ZFSPs) of Fe3(+) ion in SN (truncated SH considered) are: g=1.9989 ± 0.002 and ∣D∣=(338 ± 5) × 10?? cm?1, ∣E∣=(10 ± 5)× 10?? cm?1, a=(458 ± 5)× 10?? cm?1, respectively. The Fe3(+) ion enters the lattice substitutionally replacing the Sr2(+) sites of cubic symmetry. The local site symmetry of Fe3(+) ion in the crystal is orthorhombic (lower than that of the host). The optical absorption study of the crystal is also done at room temperature in the wavelength range 195-925 nm. The energy values of different orbital levels are determined. The observed bands are assigned as transitions from the (6)A?(g)(S) ground state to various excited states of Fe3(+) ion in a cubic crystal field approximation. The observed band positions are fitted with four parameters, the Racah interelectronic repulsion parameters (B and C), the cubic crystal field splitting parameter (Dq) and the Trees correction (α) yielding: B=934, C=2059, Dq=1450, and α=90 (in cm?1). On the basis of EPR and optical data, the nature of metal-ligand bonding in this crystal is discussed. The ZFSPs are also determined theoretically using microscopic SH theory based on perturbation theory and CFPs, B(kq) obtained from superposition model. The values of ZFSPs thus obtained are ∣D∣=(340 ± 5) × 10?? cm?1 and ∣E∣=(15 ± 5) × 10?? cm?1.     

CaF2:Co2+光谱和顺磁g因子的统一解释

基于半自洽场d轨道理论和完全对角化方法,从杂质局部结构统一解释了CaF₂:Co²⁺的光谱和顺磁 g 因子. 杂质局部结构存在塌缩效应,但仍保持八配位立方对称. 在此基础上,用微扰法和完全对角化方法讨论了g因子对晶场参量D_q的依赖关系. Macfarlane的 g 因子三阶微扰公式是好的近似公式,但随着D_q的减小,其有效性变差. 最后,文章讨论了基于完全对角化和自旋哈密顿理论的 g 因子公式的适用性,及其在轨道简并情况下的困难.     

Magnetic and structural studies of the double perovskites Ba2REMoO6

A magnetic, electronic and structural study of the double perovskites Ba₂REMoO₆ (RE=Sm, Eu, Gd, Dy) has been performed. All materials crystallise in the cubic symmetry space group and the cell volume decreases as RE varies from Sm to Dy in accordance with Vegard's law. An antiferromagnetic transition is observed below T_N=130 and 112 K for RE=Sm and Eu, respectively. The Néel temperatures of these ordered rare earth molybdenum double perovskites are much higher than previously observed in double perovskites containing Eu or Sm and a 4d or 5d transition metal arranged in an ordered rock salt configuration. The high Néel temperatures arise due to a strong superexchange magnetic interaction via the Mo-O-RE-O-Mo pathway. All of the phases are electronically insulating and there is no evidence of magnetoresistance at any temperature.     

固体ASm2I5的d-f跃迁荧光和反射光谱(A=K,Rb,Cs,T1)

本文系统地研究了化合物ASm2I5(A=K,Rb,Cs,T1)固体粉末的荧光光谱和反射光谱.讨论了Sm2+在立方晶体场中的分裂能随着碱金属离子半径的增大而减小和f-d激发能随着A-I(A=Rb,T1)键的共价性增加而明显降低的现象.并从晶场效应和化学键性质两个方面解释了ASm2I5(A=K,Rb,Cs)和ASm2I5(A=Rb,T1)中的Sm2+荧光光谱分别发生蓝移和红移的现象.     

超高压下CsBr的结构与相变

 采用金刚石对顶压砧高压装置（DAC）、同步辐射X光源和能散法，对CsBr粉末样品进行了原位高压X光衍射实验，最高压力达115 GPa。观测到在53 GPa左右压力下，CsBr的最强衍射峰(110)劈裂成两个峰，标志了简单立方结构向四方结构的转变；在0至最高压力范围内（相应于V/V₀为1至0.463）测量了晶轴比c/a；在115 GPa内未观测到样品的金属化现象。     

天然萤石的阴极射线发光谱研究

本文报导产自我国几个主要萤石矿床中若干萤石样品的阴极射线发光谱.除样品Mo-25以外,所研究样品发光谱的主要谱线均来源于取代Ca2+的Dy3+和Sm3+离子的f-f跃迁.此外,还观察到几条Er3+、Pr3+和Tb3+离子的发光谱线.谱线的相对强度与样品中各种稀土离子的相对含量有关.群论分析表明,萤石中Dy3+离子的局部对称性低于立方,或者存在着一种以上的Dy3+环境.在稀土含量最低的样品(Mo-25)的谱中,可见区只观察到一条中心在490nm处的宽带,它可能是Mn2+离子的4T1g(G)→4A1g(G)跃迁产生的.由于萤石中Mn2+的晶场分裂参量Dq反常地低,故能产生八配位Mn2+离子少见的绿色发光.     

Mössbauer spectra of the Γ8 quadruplet in the presence of spin-lattice relaxation

The influence of crystal field fluctuations on Mössbauer hyperfine spectra is analyzed. These fluctuations lead to violation of cubic symmetry of the environment, to a time-dependent splitting of a quadruplet and, as a consequence, to a temporal modulation of the hyperfine interaction. It is found that fluctuations of E-type do not lead to a complete averaging of the hyperfine interaction in the fast relaxation limit, unlike the case of e.g. magnetic relaxation and any other type of crystal field fluctuations. Moreover, for ions with J=7/2 in the fast relaxation limit the relaxation spectra of ₈ are identical to the spectra of the ₈ doublet.     

Crystallographic and magnetic structure of HAVAR under high-pressure using diamond anvil cell (DAC)

Annealed (H1) and cold-rolled (H2) HAVAR has been studied using high-pressure synchrotron X-ray diffraction. A structural phase transformation was discovered at ～13 GPa at ambient temperature, transforming from m ??3 m (S.G. 225) to P 63/m m c (S.G. 194) symmetry. The transition was not reversible on pressure release. The low-pressure cubic phase was found to be more compressible than the high-pressure hexagonal phase. Conventional Mössbauer and NFS shows that the HAVAR is not magnetic at room temperature and no splitting is observed. The SQUID indicates a huge difference in the temperature dependence of the magnetic susceptibility between the cold Rolled HAVAR compared to the annealed HAVAR.     

抽运激光聚焦特性对预脉冲诱导的类氖J=0→1 X射线激光近场像的影响

研究了在最佳除焦、预脉冲光束单独散焦及主预脉 时散焦三种不同抽运激光聚焦条件下,预脉冲诱导的类氖铁和锌Ｊ＝０→１Ｘ射线激光的近场像。本实验是在Ａｓｔｅｒｉ ｉｖ磺激光装置上进行的,实验中主、预脉冲间的时间间隔为５ｎｓ。实验发现在抽运激光主、预脉冲最佳聚焦条件下,Ｘ射线激光束在２．５ｃｍ长的线状等离子体输出端沿靶面（垂直）方向分裂成两束;在抽运预脉冲单独散焦条件下,仍然观察到这一分裂现象;但是当抽运     

Atomistic Simulation of the Equation of State of SrF 2 Using Electron Gas Interionic Potentials

We have determined the equation of state (EOS) of SrF 2 in the cubic (C1, Fm3m ) and orthorhombic (C23, Pbnm ) phases using Electron Gas Interionic Potentials (EGIP) that incorporate many-body energy components. Thermal effects are included by means of a quasi-harmonic Debye model. The zero pressure unit cell length ( a 0 ), lattice energy ( E latt ), bulk modulus ( B 0 ), and EOS of the C1 phase are predicted in good agreement with the observed data. Moreover, the computed EOS satisfy very well the empirical Vinet EOS. Fittings of m / m 0 m p data ( m = a, b, c ) to linear forms of the Vinet EOS reveal that SrF 2 (C23) is more compressible along the b and c axes than along the a direction. Finally, the {\rm C1}\rightleftharpoons {\rm C23} transition is predicted to occur at p t =3.92 GPa, which is between the observed direct ( p t =5.0 GPa) and reverse C1 ( p t =1.7 GPa) phase transitions.     

Low-temperature broken-symmetry phases of spiral antiferromagnets

We study Heisenberg antiferromagnets with nearest- (J1) and third- (J3) neighbor exchange on the square lattice. In the limit of spin S-->infinity, there is a zero temperature (T) Lifshitz point at J(3)=1/4J(1), with long-range spiral spin order at T=0 for J3>1/4J(1). We present classical Monte Carlo simulations and a theory for T>0 crossovers near the Lifshitz point: spin rotation symmetry is restored at any T>0, but there is a broken lattice reflection symmetry for 0< or =T相似文献   

Magnetic and magnetocaloric properties of R6Mn23 (R=Y, Nd, Sm, Gd-Tm, Lu) compounds

The magnetic and magnetocaloric properties of the R₆Mn₂₃ compounds (R=Y, Nd, Sm, Gd-Tm, Lu) are investigated from DC magnetization measurements. The results are analyzed and discussed in connection with previously published data. These binaries crystallize in the cubic Th₆Mn₂₃ type of structure (Fm-3m). The Mn sublattice orders at high temperature (398 K≤T_C≤505 K) with a collinear ferrimagnetic structure. The R sublattice orders at lower temperature (<100 K) with a non-collinear arrangement. By opposition with the usual behaviour in intermetallics, light rare-earth compounds (R=Nd and Sm) have a lower ground state magnetization than the heavy rare-earth compounds (R=Gd-Tm). This manifests in their magnetocaloric response near the R ordering temperature: the compounds with R=Gd-Tm display a normal magnetocaloric effect of moderate magnitude (<50 mJ cm⁻³ K⁻¹ for a field variation of 5 T) while those with R=Nd and Sm present an inverse magnetocaloric effect of weaker magnitude. The potential interest of these phases for cooling applications is briefly discussed.     

Hysteresis in the random-field Ising model and bootstrap percolation

We study hysteresis in the random-field Ising model with an asymmetric distribution of quenched fields, in the limit of low disorder in two and three dimensions. We relate the spin flip process to bootstrap percolation, and show that the characteristic length for self-averaging L small star, filled increases as exp[exp(J/Delta)] in 2D, and as exp(exp[exp(J/Delta)]) in 3D, for disorder strength Delta much less than the exchange coupling J. For system size 1infinity for both square and cubic lattices. For lattices with coordination number 3, the limiting magnetization shows no jump, and h(coer) tends to J.     

Elastic stability of DNA configurations. II. Supercoiled plasmids with self-contact

Configurations of protein-free DNA miniplasmids are calculated with the effects of impenetrability and self-contact forces taken into account by using exact solutions of Kirchhoff's equations of equilibrium for elastic rods of circular cross section. Bifurcation diagrams are presented as graphs of excess link, DeltaL, versus writhe, W, and the stability criteria derived in paper I of this series are employed in a search for regions of such diagrams that correspond to configurations that are stable, in the sense that they give local minima to elastic energy. Primary bifurcation branches that originate at circular configurations are composed of configurations with D(m) symmetry (m=2,3,...). Among the results obtained are the following. (i) There are configurations with C2 symmetry forming secondary bifurcation branches which emerge from the primary branch with m=3, and bifurcation of such secondary branches gives rise to tertiary branches of configurations without symmetry. (ii) Whether or not self-contact occurs, a noncircular configuration in the primary branch with m=2, called branch alpha, is stable when for it the derivative dDeltaL/dW, computed along that branch, is strictly positive. (iii) For configurations not in alpha, the condition dDeltaL/dW>0 is not sufficient for stability; in fact, each nonplanar contact-free configuration that is in a branch other than alpha is unstable. A rule relating the number of points of self-contact and the occurrence of intervals of such contact to the magnitude of DeltaL, which in paper I was found to hold for segments of DNA subject to strong anchoring end conditions, is here observed to hold for computed configurations of protein-free miniplasmids.     

Instantons and Asymmetric Vacua

It is shown that with asymmetric classical vacua the quantum mechanical instanton approach to the energy splitting of degenerate states applies even though the degenerate state in one well is not the quantum mechanical ground state of that well. The instanton approach leads to the correct leading exponential behavior of the energy splitting E, but the prefactor is much more difficult to compute due to the asymmetric nature of the fluctuation potential V(_c(t)), which is in turn a direct consequence of the asymmetry of the two classical minima between which the instanton interpolates.     

The calculation of the two-loop spin splitting functions $P_{ij}^{(1)}(x)$

We present the calculation of the two-loop spin splitting functions $P_{ij}^{(1)}(x)(i,j=q,g)$ contributing to the next-to-leading order corrected spin structure function g ₁ (x, Q²). These splitting functions, which are presented in the ${?erline {? MS}}$ scheme, are derived from the order $←pha_{s}^{2}$ contribution to the anomalous dimensions $αmma_{ij}^{m}(i,j=q,g)$. The latter correspond to the local operators which appear in the operator product expansion of two electromagnetic currents. Some of the properties of the anomalous dimensions will be discussed. In particular our findings are in agreement with the supersymmetric relation $αmma_{qq}^{m}+αmma_{gq}^{m}-αmma_{qg}^{m}-αmma_{gg}^{m}=0$ up to order $←pha_{s}^{2}$.     

Symmetry analysis of surface band structures in magnetic fields

The method of invariants is used to study the structure of the surface electronic energy bands in the vicinity of symmetry points in the Brillouin zone (BZ). Each of the 17 surface groups has been examined both in the presence and in the absence of spin-orbit coupling. The symmetry points for spinless particles follow a parabolic law while conic “neutrino-like” shape turns out to be the usual shape for spin −1/2 particles. Compulsory Herring degeneracy is found to occur at the sides of the BZ normal to the glide planes. The possibility of cubic rather then linear splitting in the dispersion law in the center of the BZ for groups containing 3- and 6-fold axes is demonstrated. The electron Hamiltonian for all 17 surface groups in the presence of external magnetic fields has been derived and the splitting of the degenerate energy levels in this situation has been discussed.     

Spin fluctuations and Curie temperature in the compounds R2M17 with nonmagnetic elements

The magnetic properties of materials for permanent magnets based on binary compounds R₂M₁₇ (R=Y, Sm; M=Fe, Co), also including additions of the nonmagnetic elements N, Al, and Si, are investigated a the theory of dynamical fluctuations of the electronic spin density. It is shown that the Curie temperature is determined by the ratio of the exchange splitting energy (proportional to the magnetization at T=0) and the rms value of the fluctuations (proportional to the local spin susceptibility). The fluctuations are much larger in iron compounds than in cobalt alloys. This results not only in quantitative differences in their characteristics but also in a qualitatively different change in the properties of these materials on nitriding. Fiz. Tverd. Tela (St. Petersburg) 41, 77–83 (January 1999)     

Orbital angular momentum of 3d impurities in metals

It is shown that 3d impurities often have a significant orbital moment due to the high symmetry of the crystalline electric field in typical metallic hosts. The EPR behavior which has been observed for such impurities is clarified and further predictions are made. The configurations of 3d impurities in noble metals are established: for example, Fe has 6 3d electrons andS=2. A spin-orbit splitting of about 0.02 eV is expected for Fe impurities, which may be observable by inelastic neutron scattering. The influence of orbital angular momentum on the Kondo effect is briefly discussed.