ZnO:V3+晶体光谱和电子顺磁共振谱的理论研究

采用强场耦合图像,建立了3d2电子组态在三角对称中的45×45能量矩阵,通过对角化完全能量矩阵的方法,计算出了ZnO:V3 晶体的45条光谱能级和5个电子顺磁共振谱参量(零场分裂D,g因子g//,g⊥和超精细结构常数A//,A⊥),计算结果与实验数值能很好地符合.计算中还发现V3 杂质中心的局部结构与基质晶体的结构不一致,即V3 在ZnO中并不占据准确的Zn2 位置,而是沿c3轴方向位移一段距离△Z≈0.003 nm.作者对上述这些结果的合理性进行了讨论.     

Exchange Interactions at Surfaces of Fe, Co, and Gd

Magnetic exchange interactions at low-index surfaces of bcc iron, hcp cobalt, and hcp gadolinium are studied using ab initio electronic structure calculations. Interlayer exchange couplings derived from total-energy differences are enhanced at the surfaces over their bulk counterparts. This trend is in contrast to a surface reduction of on-site exchange parameters formulated within a classical Heisenberg model. A particular attention is paid to the sensitivity of exchange interactions at a Gd(0001) surface to relaxation of interlayer distances. The calculated results do not provide support for recently observed surface enhancement of the Curie temperature of the Gd metal.     

Difference in magnetic properties between Co-doped ZnO powder and thin film

This paper reports that the Zn掺钴;氧化锌粉末;氧化锌薄膜;磁性差异;晶体取向ZnO, Co-doped, crystalline orientation, magnetismProject supported by the Shanghai Nanotechnology Promotion Center (Grant No~0452nm071).2006-09-152006-11-29This paper reports that the Zn0.95Co0.05O polycrystalline powder and thin film were prepared by sol-gel technique under the similar preparation conditions. The former does not show typical ferromagnetic behaviour, while the latter exhibits obvious ferromagnetic properties at 5 K and room temperature. The UV-vis spectra and x-ray absorption spectra show that Co2＋ ions are homogeneously incorporated into ZnO lattice without forming secondary phases.The distinct difference between film and powder sample is the c-axis （002） preferential orientation indicated by the x-ray diffraction pattern and field emission scanning electron microscopy measurement, which may be the reason why Zn0.95Co0.05O film shows ferromagnetic behaviour.     

EPR and optical absorption spectra of VO2+ ions in calcium oxalate monohydrate single crystals

Abstract

Electron paramagnetic resonance (EPR) and Optical absorption spectra of vanadyl ions in calcium oxalate monohydrate single crystals have been studied. EPR spectrum at room temperature exhibits an isotropic octet spectrum. The detailed EPR analysis indicates that vanadyl ions exhibit liquid like behaviour. EPR spectrum has been studied at different temperatures. It is found that at 255 K, the EPR spectrum completely disappears. The optical absorption spectrum exhibits three absorption bands characteristic of vanadyl ions in tetragonal symmetry. From the optical data, the crystal field and tetragonal parameters have been evaluated.     

EPR measurements of weak exchange interactions coupling unpaired spins in model compounds

I review electron paramagnetic resonance (EPR) measurements performed to evaluate very weak exchange interactions (defined as ?_ex(i, j) = ?J _ij S _i S _j , with 10^?3 cm^?1<|J _ij|<1 cm^?1) between unpaired spins, transmitted through long and weak chemical pathways typical of protein structures. They are performed in appropriate model compounds, mainly copper derivatives of amino acids and peptides, making use of the phenomenon of exchange narrowing and collapse of the resonances. I describe the theoretical basis and the implementations of the method to different situations, including selected experimental values of the exchange couplingsJ between metal centers, and briefly discuss correlations betweenJ and the structure of the paths. Results obtained in relatively simple EPR experiments performed at room temperature in single-crystal samples are compared with those obtained from thermodynamic magnetic measurements having higher experimental difficulties. The experimental information allows describing the role of molecular segments typical of biomolecules (H bonds, aromatic ring stacking, cation-π contacts, etc.) in the transmission of the exchange interaction. The values ofJ obtained in some model compounds are compared with those obtained in proteins to conclude that the magnitudes of the exchange interactions are useful to characterize long and weak biologically relevant chemical pathways. One observes that these exchange couplings are weakly dependent on the nature of the unpaired spins and strongly dependent on the chemical pathway. Thus, measurements of exchange couplings in model compounds may provide useful information about biological function, particularly about electron transfer in proteins.     

LiH晶体中离子间多体相互作用与高压下状态方程研究

在计算LiH晶体结合能时,将离子间多体关联效应和离子压缩效应当作两个独立的物理机理同时引入晶体能量表达式,未引入可调参数而直接计算电子交换能.结果表明多体交换关联效应对体系的常压和高压特性都具有显著影响.当考虑最邻近和次邻近离子贡献时,等温状态方程的计算结果与实验观测数据在0—90 GPa压力范围相一致. 关键词： LiH晶体 状态方程 多体交换作用     

Finite size effects in ZnO nanoparticles: An electron paramagnetic resonance (EPR) analysis

ZnO nanoparticles were synthesized by solid coprecipitation method with consecutive high energy ball milling procedure. By reducing the particle size of ZnO to nano dimensions strong nano‐size effects were observed. In order to characterize the ZnO defect structure, EPR has been applied. It was observed that below 50 nm the surface defects play a dominant role in the electronic properties of ZnO. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multifrequency EPR spectroscopy of Ho3+ ions in synthetic forsterite

Electron paramagnetic resonance (EPR) of Ho³⁺ single ions and Ho³⁺?Mg²⁺-vacancy-Ho³⁺ associates in holmium-doped forsterite single crystals are studied at 9.4, 37.3 and 65–250 GHz. Crystals were grown from melt by the Czochralski technique in slightly oxidizing atmosphere. For both centers, directions of the principal magnetic axes and parameters of the effective spin Hamiltonians describing dependences of electron-nuclear levels on applied magnetic field are obtained. For Ho³⁺ substituting Mg²⁺ in the M2 site as the single ion and for Ho³⁺ ions in dimer centers, values of crystal field parameters related to a real crystal lattice structure are estimated in the framework of the exchange charge model. The calculated crystal field energies, values of theg-factors of the ground Ho³⁺ quasi-doublet and the directions of the corresponding magnetic moments agree satisfactorily with the data obtained from measurements of EPR and optical absorption and site-selective luminescence spectra.     

High-frequency tunable EPR spectroscopy of Tm3+ ions in synthetic forsterite

Paramagnetic centers formed by impurity Tm³⁺ ions in synthetic forsterite Mg₂SiO₄ were studied by high-frequency tunable electron paramagnetic resonance spectroscopy in the frequency range of 150–315 GHz. Crystals were grown from the melt by the Czochralski technique in slightly oxidizing atmosphere. Several centers distinguished by different zero-field splitting between the ground and first excited singlets were found and investigated. Parameters of the effective spin Hamiltonian for these centers describing the dependence of electron-nuclear sublevels on magnetic field were determined.     

Structural and electrical properties of VO2/ZnO nanostructures

We examined the structural and electrical properties of uniformly-oriented VO₂/ZnO nanostructures. VO₂ was deposited on ZnO templates by using a direct current-sputtering deposition. Scanning electron microscope and transmission electron microscope measurements indicated that b-oriented VO₂ were uniformly crystallized on ZnO templates with different lengths. VO₂/ZnO formed nanorods on ZnO nanorods with length longer than 250 nm. X-ray absorption fine structure at the V K edge of VO₂/ZnO showed M₁ and R phases of VO₂ at 30 and 100 °C, respectively, suggesting structural-phase transition occurring between the two temperatures. Temperature-dependent resistance measurements of VO₂/ZnO nanostructures revealed metal-to-insulator transition at 65 °C and 55 °C during a heating and a cooling, respectively, regardless of ZnO length. Asymmetry behavior of resistance curves from VO₂/ZnO nanostructure during a heating and a cooling was attributed from a strong bond of VO₂ and ZnO.     

Influence of magnetic dipole and magnetoelastic interactions on the phase states of 2D non-Heisenberg ferromagnetic with complex exchange interactions

The phase states of the 2D non-Heisenberg ferromagnetic with anisotropic bilinear and biquadratic exchange interactions are investigated. The limiting cases of the system under consideration are the two-dimensional XY-model with biquadratic exchange interaction and the isotropic Heisenberg ferromagnetic. The account of the magnetic dipole interaction leads to the realization of spatially inhomogeneous quadrupolar phase. The stability regions of various phase transitions for different values of the material parameters are studied. The phase diagram is built. Besides, the temperature phase transitions are investigated. The influence of the magnetoelastic interaction on the formation of the long-range quadrupolar order is determined.     

EPR and optical spectroscopy of Yb3+ ions in single crystal CsCaF3

CsCaF₃ crystals doped with Yb³⁺ were studied using EPR and optical spectroscopy methods. Several types of paramagnetic centers of Yb³⁺ were found including a paramagnetic center in an uncommon 12-coordinated position. The schemes of the energy levels of the observed centers are determined and the potentials of the respective crystalline fields are calculated.     

Photoluminescence spectroscopy of defects in ZnO nano/microwires

Photoluminescence spectroscopy is used to study defects found in single ZnO nano/microwires at 90 K. The defect, acting as binding site for bound exciton (BX) transition, is represented by BF, the fractional intensity of the BX peak in the whole near-band edge ultraviolet (UV) luminescence. The concentration of defects as origins of the visible emissions is proportional to the intensity fraction DF, i.e., the intensity fraction of visible emissions in the sum total of all UV and visible luminescences. By comparing BF and DF, it is concluded that the two defects are not correlated to each other. The former kind of defect is considered to be related to the blueshift of the near-band edge peak as the radius of the nano/microwires decreases at room temperature.     

Optical and EPR spectroscopy of manganese ions in Sr3Ga2Ge4O14 crystals

Results of investigations of the spectroscopic properties of manganese-activated single crystals of Sr₃Ga₂Ge₄O₁₄ by the methods of optical and EPR spectroscopy are reported. It is shown that magnagese activator ions form substitutional centers Mn³⁺ and Mn²⁺ in 1a-octahedral positions of the Sr₃Ga₂Ge₄O₁₄ lattice. Changes in the opticla properties of Sr₃Ga₂Ge₄O₁₄: Mn after vacuum thermal annealing are attributed to charge transfer of some of the manganese ions (Mn³⁺→Mn²⁺). The relationship between the spectroscopical properties of Mn²⁺ ions and the crystallochemical structure of the system are discussed. I. Franko L’vov State University, 50, Dragomanov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 779–783, November–December, 1997.     

EPR study of exchange interactions of manganese ions in a CdGeAs<Subscript>2</Subscript> matrix

The results of an EPR study of the CdGeAs₂ compound doped with 6 at % Mn are reported. The experimental data are analyzed under the assumption that magnetic centers of the following two types are formed in the system: Mn_Cd, Mn ions that replace Cd²⁺ and have spin S = 5/2, and Mn_Ge, Mn ions that replace Ge⁴⁺ and form the Mn²⁺ + 2p complex with two “heavy” holes with spin S = 1/2. The absence of signals from isolated centers and the Lorentzian shape of an absorption curve suggest a strong exchange narrowing of the spectrum and the extension of an isotropic exchange interaction involving Mn_Ge to distances much longer than the lattice parameter. It is found that the exchange interaction between Mn²⁺ + 2p complexes is ferromagnetic, and it is stronger than the characteristic superexchange interaction involving Mn_Cd centers by three to four orders of magnitude. The form of the temperature dependence of susceptibility obtained by the double integration of spectra is indicative of the formation of nanoscale regions, which weakly interact with each other, with ferromagnetically ordered Mn²⁺ + 2p complexes at ～250 K.     

Phase transition in Cu2+-doped RbH2 PO4 as observed by EPR

The electron paramagnetic resonance (EPR) spectra of Cu²⁺-doped RbH₂ PO₄ at elevated temperatures indicate a phase transition at 358 K. The EPR-silent state at this temperature is attributed to a so-called polymeric phase transition. After the transition when the temperature is lowered to 293 K, the EPR signal does not appear; therefore, the transition is irreversible. This result seems to be in agreement with the other observations. The EPR spectra for the sample indicate the presence of two sites for Cu²⁺, and the values of EPR parameters are in accord with the literature on Cu²⁺-doped single crystals. Any other phase transitions could not to be observed at low temperatures down to 113 K.     

Absorption and EPR spectra of Cr3+ ions in a LaBeAl11O19 crystal

Electron spectra of optical absorption and EPR of Cr³⁺ ions in a LaBeAl₁₁O₁₉ crystal are investigated. It is shown that the Cr³⁺ ions occupy, three different octahedral positions of Al³⁺ in the LaBeAl₁₁O₁₉ structure, namely, 12k, 2a, and 4f₂; the ratio of their intensitites is 1∶2∶30, respectively. Parameters of the Cr³⁺ centers are determined and its is shown that the optical absorption spectra in the visible region are practically determined by the Cr³⁺ (III) occupying the 4f₂-positions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 275–277, March–April, 1999.     

Charge exchange in collisions of H+2 with H-

The process of mutual neutralization in slow H+2+H- collisions is considered within the multi-channel Landau-Zener model. The calculated total mutual neutralization cross section is in satisfactory agreement with the experimental data available in the CM energy range 20～2000 eV.     

Covalent bonding and J–J mixing effects on the EPR parameters of Er~(3+) ions in GaN crystal

The EPR parameters of trivalent Er~(3+) ions doped in hexagonal Ga N crystal have been studied by diagonalizing the 364×364 complete energy matrices. The results indicate that the resonance ground states may be derived from the Kramers doublet Γ_6. The EPR g-factors may be ascribed to the stronger covalent bonding and nephelauxetic effects compared with other rare-earth doped complexes, as a result of the mismatch of ionic radii of the impurity Er~(3+)ion and the replaced Ga~(3+) ion apart from the intrinsic covalency of host Ga N. Furthermore, the J–J mixing effects on the EPR parameters from the high-lying manifolds have been evaluated. It is found that the dominant J–J mixing contribution is from the manifold ~2K_(15/2), which accounts for about 2.5%. The next important J–J contribution arises from the crystal–field mixture between the ground state ~4I_(15/2) and the first excited state~4I_(13/2), and is usually less than 0.2%. The contributions from the rest states may be ignored.