Surface magnetism in 3He: Thermodynamics

A phenomenological formulation of the enhancement of the susceptibility of liquid ³He in contact with a dielectric surface is presented. The effective Hamiltonian includes a surface term consisting of both the quadratic and the quartic dependence on the surface order parameter. The partition function for this system is calculated using a novel application of the transfer matrix methods. We find a susceptibility that contains the Curie-Weiss-like surface contribution as well as the appropriate field dependence in agreement with experiments. In addition, we find a weak temperature-dependent anomaly in specific heat as well.Nordita Visiting Professor.     

Photoluminescence of Ca4Ga2S7:Ce3+ and Ca4Ga2S7:Pr3+

The photoluminescence of Ca₄Ga₂S₇:Ce³⁺ and Ca₄Ga₂S₇:Pr³⁺ was studied. Ca₄Ga₂S₇ was found to have an fcc structure witha = 5.67 Å. The photoluminescence of the rare-earth-doped materials is due to Ce³⁺ and Pr³⁺ intracenter transitions. The lifetimes of Ce³⁺ and Pr³⁺ excited states fall in the range 10^-5 to 10^-7 s, and the frequency factor is 10¹¹-10^-12 s^-1     

Vortex motion in coupled DyBa2Cu3O7/(Y0.6Pr0.4)Ba2Cu3O7 heterostructures

Multilayer structures containing 24 Å thick DyBa₂Cu₃O₇ layers, separated by 96 Å of an (Y_0.6Pr_0.4)Ba₂Cu₃O₇ alloy, are studied to investigate the effect of coupling on vortex dynamics. With the magnetic field perpendicular to theab plane, and as a function of the number of superconducting layers in the structure, we find that the activation energy for flux motion increases, first linearly, and then saturates. This linear increase is taken as evidence that pancake vortices belonging to different DyBa₂Cu₃O₇ layers are stacked and have a coupled motion. Above a characteristic number of superconducting layers,N _c , shear of the vortex structures becomes important and the thermally activated process only displaces a stack ofN _c pancake vortices, meaning that the vortex lattice is turning three dimensional. In these structures we findN _c to be 2 to 3.     

Solid3He magnetism in the classical approximation

The classical analog of the Hamiltonian proposed for³He spins by Stipdonk and Hetherington is studied. First- and second-order phase transitions similar to those found experimentally are obtained. This paper introduces the use of a Gaussian ensemble rather than a canonical ensemble in the Monte Carlo method. This ensemble is equivalent to the canonical ensemble in the limit of large systems except in the energy range inside of a first-order transition. This new method has general applicability, but is especially useful for studying first-order transitions, since it can eliminate the problem of hysteresis associated with them.     

Light scattering mechanism in(Y/Pr)Ba 2 Cu 3 O 7 superlattices

An extra peak was observed in the region of 290–320 cm^–1 of Raman spectra of a high quality (001) (Y₂Pr₁)(Ba₂Cu₃O₇) superlattice in Z(X,Y)Z geometryat low temperature. It was identified as an even combination of B_1u vibrations in Y-subcells. We propose an asymmetric field mechanism for the Raman intensity of this extra phonon.This work is supported by Deutsche Forschungsgemeinschaft (SFB 252). E.S gratefully acknowledges the support by Alexander von Humboldt Foundation.     

Theoretical Consideration on Triplet Superconductivity in UPt3 and Sr2RuO4

Two representative superconductors, UPt₃ and Sr₂RuO₄, are comparatively studied on the basis of a general symmetry argument. The existing experimental data set for both systems strongly and commonly points to a triplet pairing. For the former, heavy fermion material, UPt₃, a non-unitary bipolar state is the most possible candidate, whereas for the latter system, Sr₂RuO₄, determination of a precise pairing symmetry class, either unitary or non-unitary, is debated.     

Low-temperature unit-cell parameters of La1−x Pr x CaBaCu3O7 compounds

The suppression of the superconductivity by praseodymium in La_1−x Pr _x CaBaCu₃O₇ may be explained in terms of the difference in the electronic or crystal structure between these compounds. Three compositions with x=0.0, 0.5, 1.0 were, therefore, investigated by X-ray diffraction in order to determine the unit-cell dimensions at room temperature as well as at lower temperatures down to 5 K. The results showed no considerable changes in the orthorhombic unit-cell dimensions on decreasing the temperature to 5 K. Unit-cell volumes at low temperatures were 0.99 of the room temperature values. The intensity sequence of reflections in the diffraction pattern was the same for all samples throughout the range of temperature studied.     

Pr3+摩尔浓度对CaTiO3:Pr3+红色长余辉材料的影响

制备了不同Pr^3 摩尔浓度下的CaTiOs：Pr^3 红色长余辉材料，测量了磷光体的的初始亮度、余辉曲线、激发光谱和发射光谱、热释光曲线。研究发现CaTiO3：Pr^3 合适的Pr^3 摩尔浓度为0．1％—0．2％，在该摩尔浓度下，材料具有较好初始亮度和余辉时间。     

层状CoPS3中磁性的压力与载流子掺杂可控调节

最近发现的层状铁磁材料吸引了研究者的广泛兴趣,但是其有限的数量严重阻碍了进一步的发展.作为一个更大的家族,层状反铁磁材料为加深对基本磁性物理的理解和推动高性能自旋电子学的应用提供了一个出色的平台.本文中,我们选取了过渡金属磷三硫族化合物中代表性的反铁磁CoPS3材料,并通过系统的第一性原理计算证实压力或载流子掺杂可有效...     

Spectroscopic and laser properties of SrLaGa3O7:Pr crystals

The spectroscopic properties of trivalent praseodymium (Pr³⁺) doped SrLaGa₃O₇ (SLG) crystals have been investigated at various temperatures. Absorption, emission, excitation and lifetime measurements have been performed and discussed in the framework of the Judd-Ofelt approach. Stimulated emission cross sections for the strongest transitions of Pr³⁺ ion have been calculated. Stimulated emission has been observed for the first time in Pr³⁺ doped SLG crystal, we have obtained laser operation at 488 n, at cryogenic temperatures and at 645 nm at room temperature.     

溶胶-凝胶法制备Fe掺杂ZnO基稀磁半导体的结构与磁性

采用了添加抗坏血酸作为还原剂的改良溶胶-凝胶法制备了单相的稀磁半导体Zn1-xFexO(x=0.01、0.05、0.07、0.10) 粉末.X射线衍射谱(XRD)显示所有Zn1-xFexO样品在室温下都呈现出P63mc的六角晶格结构,同时单位晶胞体积呈现出随着Fe离子掺杂量的提高而增大的趋势.X射线光电子能谱(XPS)证明在Zn1-xFexO晶格中的掺杂元素Fe主要以Fe2 的方式存在.比饱和磁化强度(σs)随Fe掺杂量的增加而提高,并在样品Zn0.90Fe0.10O中获得最大值0.43Am2/kg,定性地解释了铁磁性的来源.     

Photoluminescence and energy storage traps in CaTiO3:Pr3+

Red-emitting CaTiO₃:Pr³⁺ phosphors are fabricated using a solid state method and structurally characterized by X-ray diffraction and field emission scanning electron microscopy. The optical properties are investigated using photoluminescence emission, excitation, photoluminescence decay curves, diffused reflectance and thermoluminescence spectra, and persistent phosphorescence decay curves. The optimal fabrication temperatures for photoluminescence and persistent phosphorescence are found at 1200–1300 °C for photoluminescence and 1400 °C for persistent phosphorescence. The energy storage traps for persistent phosphorescence in the system are analyzed based on the dependence of photoluminescence and persistent phosphorescence on sintering temperature. The distribution of energy storage traps is further characterized by thermoluminescence spectra and the parameters of the traps are calculated. Oxygen vacancies as the main trapping centers play the key role for persistent phosphorescence in CaTiO₃:Pr³⁺.