Electronic state of PuCoGa5 and NpCoGa5 as probed by polarized neutrons

By using single crystals and polarized neutrons, we have measured the orbital and spin components of the microscopic magnetization in the paramagnetic state of NpCoGa(5) and PuCoGa(5). The microscopic magnetization of NpCoGa(5) agrees with that observed in bulk susceptibility measurements and the magnetic moment has spin and orbital contributions as expected for intermediate coupling. In contrast, for PuCoGa(5), which is a superconductor with a high transition temperature, the microscopic magnetization in the paramagnetic state is small, temperature-independent, and significantly below the value found with bulk techniques at low temperatures. The orbital moment dominates the magnetization.     

Photoemission and the electronic structure of PuCoGa5

The electronic structure of the first Pu-based superconductor PuCoGa5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Exceptional agreement between calculation and experiment defines a path forward for understanding the electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. The results for PuCoGa5 clearly indicate 5f electron behavior on the threshold between localized and itinerant. Comparisons to delta phase Pu metal show a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant.     

非传统超导PuCoGa5晶体结构和电子性质的密度泛函理论研究

基于密度泛函理论的全势能线性缀加平面波方法（FLAPW）,采用局域自旋密度近似（LSDA）及LSDA+U方法报道了Pu基超导体系PuCoGa₅的晶格参数,原子占位和电子性质.LSDA+U方法不仅考虑Pu-5f而且考虑了Co-3d的库伦排斥作用U和Hund交换相关作用J.结果表明LSDA+U在同时考虑Pu和Co的强关联作用时晶格参数和原子占位与相关理论和实验数据吻合较好,特别是PuCoGa₅中Pu-5f的强定域特征尤其是自旋向下的电子.此外,Pu-5f与Co-3d电子的轨道杂化明显强于Ga-4p电子.     

Electronic band structure and Fermi surface of CaB6 studied by angle-resolved photoemission spectroscopy

We report high-resolution angle-resolved photoemission spectroscopy (ARPES) on CaB6. The band structure determined by ARPES shows a 1 eV energy gap at the X point between the valence and the conduction bands. We found a small electron pocket at the X point, whose carrier number is estimated to be (4-5) x 10(19) cm(-3), in good agreement with the Hall resistivity measurement with the same crystal. The experimental results are discussed in comparison with band structure calculations and theoretical models for the high-temperature ferromagnetism.     

LaNi5晶体表面态的计算研究

用"团簇埋入自洽计算法"对LaNi5晶体表面进行了全电子、全势场、自旋极化的从头计算.在原子纵向坐标充分弛豫的条件下,得到处于最低基态总能量下LaNi5晶体的非平整表面空间结构及其电子结构.LaNi5晶体最表面La原子向外凸出,Ni原子向里收缩,凹凸不平的表面层增加了表面原子与氢原子的接触面积;而表面层的有效体积增大了约9%,有利于氢原子的进入.LaNi5晶体表面态的费米能量大大高于体材料的费米能量.在费密面上主要是Ni的3d电子,价带未填满,显示金属性.LaNi5晶体表面第一、第二层有1.15个电子从La原子向Ni原子转移,这两层有反向的微小自旋磁矩,从而使表面显示顺磁性.得到了LaNi5晶体表面的价带电子态密度.用过渡态方法计算了LaNi5晶体表面的电离能和电子亲和势.所有计算结果显示:LaNi5晶体表面的性质与体性质显著不同,而与氢化物LaNi5H7的性质非常相近.这说明LaNi5晶体的表面结构有利于氢原子的吸收.     

Electronic structure and Fermi surface of Bi2Sr2CaCu2O8

Applying angle-resolved photoemission spectroscopy of high angular and energy resolution we have measured the electronic structure of single crystals of Bi₂Sr₂CaCu₂O₈ near the Fermi energy. Along the high symmetry direction X a band is observed to disperse upwards and to cross the Fermi level, whose unoccupied part constitutes the hole-like carriers responsible for the superconductivity. From spectra along the M direction we find evidence for an electron pocket around the M point. The measured band widths appear to be drastically reduced compared with band structure calculations indicating strong electronic correlation effects. From the observation of Fermi-Dirac-like cut-offs in the spectra due to band crossings through the Fermi level we can confirm the existence and, in particular, the shape of the Fermi surface as calculated by band structure calculations.     

Effect of Coulomb correlations on the electronic structure of PuCoGa(5)

We investigate the effect of strong Coulomb correlations on the electronic structure of the Pu-based superconductor PuCoGa5 by employing the relativistic local spin density approximation+ Hubbard U (LSDA+U) method. The inclusion of intra-atomic Coulomb U and exchange J parameters leads to a significant reconstruction of the f states electronic structure over that given by the LSDA approach. At variance with the LSDA, the LSDA+U suggests "jj"-like coupling for the Pu 5f manifold.     

Electronic structure of carbon nanohorns near the Fermi level

The effect of pentagonal defects on the electronic structure at the tip of carbon nanohorns is investigated within the continuum gauge field-theory model. It is found that the existence of a localized electron state at the Fermi level (a true zero-mode state) results in enhanced charge density near the tip. Using a self-consistent perturbation scheme, the eigenfunctions and the local density of states near the pentagonal defects are numerically calculated.     

Fermi surface and magnetic structure of TmGa3

We carry out measurements of the two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) to reconstruct the complex multisheet Fermi surface (FS) of the cubic rare-earth (RE) compound TmGa3. We discover a correlation between the antiferromagnetic structures and the nesting of the FS along the [110] directions. Moreover, we propose methods to estimate the density of states at the Fermi energy ( EF) and the electronic contribution to the specific heat [we obtain N(EF) = 13.6 states/Ryd cell and gamma = 2.4 mJ/mole K2].     

LaNi5晶体表面态的计算研究

用“团簇埋入自洽计算法”对LaNi₅晶体表面进行了全电子、全势场、自旋极化的从头计算. 在原子纵向坐标充分弛豫的条件下，得到处于最低基态总能量下LaNi₅晶体的非平整表面空间结构及其电子结构. LaNi₅晶体最表面La原子向外凸出，Ni原子向里收缩，凹凸不平的表面层增加了表面原子与氢原子的接触面积；而表面层的有效体积增大了约9%，有利于氢原子的进入. LaNi₅晶体表面态的费米能量大大高于体材料的费米能量. 在费密面上主要是Ni的3d电子，价带未填满，显示金属性. LaNi₅晶体表面第一、第二层有1.15个电子从La原子向Ni原子转移，这两层有反向的微小自旋磁矩，从而使表面显示顺磁性. 得到了LaNi₅晶体表面的价带电子态密度. 用过渡态方法计算了LaNi₅晶体表面的电离能和电子亲和势. 所有计算结果显示：LaNi₅晶体表面的性质与体性质显著不同，而与氢化物LaNi₅H₇的性质非常相近. 这说明LaNi₅晶体的表面结构有利于氢原子的吸收. 关键词： 5')" href="#">LaNi₅ 表面 电子结构 团簇埋入自洽计算法     

Electronic surface resonances and crystal surface structure

A new method of determination of the lateral structure of crystal surfaces is presented. The method is based on earlier work showing the existence of resonances in the elastic scattering of low-energy electrons at crystal surfaces. The method consists of: (a) Measurement of the surface resonance band structure Ek_∥E, k_∥ respectively denote the electron energy and surface-parallel momentum for which resonances occur) and (b) Interpretation of E(k_∥) to determine the lateral variation of the effective potential acting on electrons at the surface.The surface resonance band structure is measured by a net-current electron reflection method. The measurement method is basically the same as used previously but here its precision is greatly enhanced by the use of digital methods of data handling including a digital filter to remove background due to inelastic and non-resonance elastic scattering. The surface resonance band structure is interpreted by a two-dimensional nearly-free electron scheme. In this scheme the interaction elements are Fourier coefficients of an effective potential which is an average of a pseudopotential with respect to the depth distribution of electron density in a surface resonance — the surface-weighted pseudopotential. Experimental surface resonance band structure for Ni(001), Ni(001) p(2 × 2)O and two different Ni(001) c(2 × 2)O surfaces (one of them with an oxygen-saturated Ni substrate) are presented for E = 1–30 eV and k_∥ running halfway from $\text{}\text{?}$gG towards H? in the surface Brillouin zone for Ni(001). The experimental results are fitted, using the nearly-free electron scheme, to determine the Fourier coefficients of the surface-weighted pseudopotential. Surface potential variations synthesized from the above data are discussed in comparison with the atomic arrangements known from LEED. It is demonstrated that the new method can give a correct picture of the lateral structure of surfaces. It is emphasized that these results are obtained without costly equipment or computations called for by other methods.     

de Haas-van Alphen effect and the Fermi surface of PrNi5

The Fermi surface of PrNi₅ has been studied by the measurements of the de Haas-van Alphen (dHvA) effect at temperatures between 0.3 and in magnetic fields up to 12 T. Two dHvA frequencies have been obtained. The electronic structure of PrNi₅ was calculated using the full potential linearized augmented plane wave method. Five sheets of the Fermi surface and the multiple extremal cross sections were determined. First and second sheet have a hole-like structure. The agreement between theory and experiment is obtained by a small downward shift ( 0.1 eV) of the Fermi energy which is probably due to an underestimation of the role of 4 f electrons. Received 9 May 2000 and Received in final form 20 September 2000     

稳定的GaAs (2 5 1 1)高密勒指数表面的电子结构

采用格林函数方法对具有稳定结构的GaAs(2 5 11)(1×1)表面的电子结构特性进行了计算. 结果表明：对于理想的GaAs(2 5 11)表面，基本带隙内的表面态主要处在三个能量区域，即 -0.1—0.1eV，085—10eV和1.4—1.6eV之间；吸附两个As原子形成(1×1)再构后，表面态的变化主要表现在0.85—1.0eV之间的表面态完全消失.结合电子数目规则，可以确定处在 - 0.1—0.1eV之间的表面态为全部填满的阴离子悬挂键态或再构引起的As As二聚体键的表 面态，而处在1.4— 关键词： 高密勒指数表面 电子结构 电子数目规则     

Superconductivity caused by the pairing of plutonium 5f Eelectrons in PuCoGa5

On the basis of electronic structure calculations we identify the superconductivity in the novel, high-temperature superconductor PuCoGa5 to be caused by the pairing of Pu 5f electrons. Assuming delocalized Pu 5f states, we compute theoretical crystallographic constants very near to the experimental ones, and the calculated specific heat coefficient compares reasonably to the measured coefficient. The theoretical Fermi surface is quasi-two-dimensional and the material appears to be close to a magnetic phase instability.     

Magnetic Susceptibility and Spin Fluctuations in the Nonsuperconducting Phase of PuCoGa5

Physics of the Solid State - Within the Hubbard model with allowance for Hund’s and spin–orbit interactions, the concepts of thermal spin fluctuations in a strongly correlated system of...     

Electronic band structure and Fermi surface of new 3.7 K superconductor LiCu2P2 from first-principles calculations

By means of the first-principles calculations, we have studied the structural, electronic properties and the Fermi surface (FS) of the newly discovered ThCr₂Si₂-like iron- and arsenic-free 3.7 K superconductor LiCu₂P₂. Our studies showed that LiCu₂P₂ is an unusual “intermediate” system, which may be classified as a (p + d) metal, unlike the majority of 122 phases, which are d or sp metals. Unlike FeAs 122 phases having a typical cylinder-like (2D) FS topology, the Fermi surface of LiCu₂P₂ adopts a 3D-like two-sheet topology formed by disconnected electronic-like and hole-like pockets. The picture of inter-atomic bonding for LiCu₂P₂ is highly anisotropic and includes mixed covalent-ionic bonds CuP (inside [Cu₂P₂] blocks), direct PP bonds (between the adjacent [Cu₂P₂] blocks) together with ionic bonds owing to Li → [Cu₂P₂] charge transfer.