Electronic structure of MgB2 from angle-resolved photoemission spectroscopy

The first angle-resolved photoemission spectroscopy results from MgB2 single crystals are reported. Along the GammaK and GammaM directions, we observed three distinct dispersive features approaching the Fermi energy. These can be assigned to the theoretically predicted sigma (B 2p(x,y)) and pi (B 2p(z)) bands. In addition, a small parabolic-like band is detected around the Gamma point, which can be attributed to a surface-derived state. The overall agreement between our results and the band calculations suggests that the electronic structure of MgB2 is of a conventional nature, thus implying that electron correlations are weak and may be of little importance to superconductivity in this system.     

High-resolution angle-resolved photoemission study of large magnetoresistance topological semimetal CaAl_4

Extremely large magnetoresistance(XMR) has been explored in many nonmagnetic topologically nontrivial/trivial semimetals,while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures.In this paper,the magnetoresistance origin of single crystal CaAl_4 with C2/m structure at low temperature is investigated,exhibiting unsaturated magnetoresistance of ～3000% at 2.5 K and 14 T as the fingerprints of XMR materials.By the combination of ARPES and the first-principles calculations,we elaborate multiband features and anisotropic Fermi surfaces,which can explain the mismatch of isotropic two-band model.Although the structural phase transition from I4/mmm to C2/m has been recognized,the subtle impact on electronic structure is revealed by our ARPES measurements.Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl_4,our findings report CaAl_4 as a new reference material for exploring the XMR phenomena.     

High-resolution photoemission in low-dimensional conductors and superconductors

We review some recent spectroscopic results on low-dimensional systems, including high-T_c superconductors, and charge density wave compounds. We briefly discuss the reasons for the present interest in these materials, and the relevance of band mapping experiments by angle-resolved photoemission spectroscopy. The main focus of the paper is on high-energy resolution, which can be exploited to probe the quasiparticle excitations, and to study the characteristic instabilities of the Fermi surface. Gap spectroscopy in the ordered phases is a prominent part of this research, namely in the cuprates. The normal state properties, however, are often as interesting. This is particularly true in 1D, where the photoemission results indicate strong deviations from a Fermi liquid scenario.     

High-resolution angle-resolved photoemission study of oxygen adsorbed Fe/MgO(001)

We have investigated the electronic states of clean Fe(001) and oxygen adsorbed Fe(001)–p(1 × 1)-O films epitaxially grown on MgO(001) substrates by means of polarization-dependent angle-resolved photoemission spectroscopy(ARPES)and extensive density-functional theory(DFT) calculations. The observed Fermi surfaces and band dispersions of pure Fe near the Fermi level were modified upon oxygen adsorption. By the detailed comparison of ARPES and DFT results of the oxygen adsorbed Fe surface, we have clarified the orbital-dependent p–d hybridization in the topmost and second Fe layers.Furthermore, the observed energy levels and Fermi wave numbers for the oxygen adsorbed Fe surface were deviated from the DFT calculations depending on the orbital characters and momentum directions, indicating an anisotropic interplay of the electron correlation and p–d hybridization effects in the surface region.     

High-resolution angle-resolved photoemission from Cu(610)

Cu(610) has a mean terrace width of six atoms. Although the regular arrangement of steps was verified by SPALEED, no step-induced surface umklapp process could be identified in UPS spectra taken with HeI radiation. This fact allows an easy comparison with results from Cu(100). In particular, we demonstrate that the final state wave vector can be determined by triangulation using exclusively the Cu(610) surface, since equivalent k-space points contribute at different electron emission angles. However, spectra from equivalent points exhibit differences in the UPS linewidth and this gives additional information on the final state lifetime. Finally we discuss the modification of a Tamm state [its analogue is observed at on the (100) surface] due to confinement by the steps.     

High-resolution photoemission study of FeSr2YCu2O7+δ

We have performed high-resolution photoemission spectroscopy (PES) on FeSr₂YCu₂O_7+δ, of which superconductivity of T_c=49 K was recently reported. We clearly observed opening of a d-wave-like superconducting gap and estimated the maximum gap value (Δ_max) to be 10 meV at 15 K. This gap value gives 2Δ_max/k_BT_c∼5, suggesting a strong-coupling nature of superconductivity in FeSr₂YCu₂O_7+δ. Comparative PES study with superconducting and insulating samples shows that the valence band is rigidly shifted as a function of doping without evolution of additional states within the insulating gap.     

High-resolution photoemission study of adatom bonding effects: Cl (√2 × √2) R 45° on Cu (100)

We have investigated photoemission from an ordered $\text{(}\text{2}\text{×}\text{2}\text{) R 45°}$ Cl-overlayer on Cu (100) at good angular (Δθ ? 1°) and energy (ΔE = 100 meV) resolution. We observe two prominent adsorbate- induced features at 1.95 eV (FWHM ≈ 100 meV) and 5.4 eV (0.65 eV) below E_F. By determination of their angular momentum character, their spatial orientation and their energy dispersion, they are identified as “antibonding” and “bonding” resonances, respectively. Significant modification of the Cu d-band emission is observed and interpreted in terms of substrate orbitals which are involved in bonding.     

类MgB2硼化物晶体电子结构比较研究

应用准确的第一原理方法 ,对 8种类MgB2 超导体结构的二硼化物进行了电子结构比较研究 ,发现其中的超导体具有特殊的能带属性 .对其在高压下态密度的变化情况做了对照 .     

X-ray absorption spectroscopic study of a hot pressed MgB2

X-ray near edge absorption spectra (XANES) of boron and oxygen K edge of a hot-pressed MgB₂ (HP2) sample were measured by using synchrotron radiation. The HP2 was produced by annealing a commercial powder of MgB₂ in ambient air under a hydrostatic pressure of 2.0 GPa at 900 °C. No substantial change of the electronic structure in HP2 was observed as indicated in the B K-edge XANES. This sample maintains high superconductivity and enhances the critical current density. The XANES and the XRD combined indicate that both MgO and B₂O₃ in HP2 increase and more B₂O₃ than MgO was produced during the hot-press process. The better connection of the grain in HP2 can be the major cause to increase its critical current density. However, the impurity may also increase the flux pinning and therefore increases the current density.     

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

High-resolution resonant photoemission study of the 4f states in a typical Kondo system: CePd3

We exploited resonant photoemission at the Ce absorption edge to investigate the Ce 4f states in . High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds. The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations. Received: 13 October 1997 / Accepted: 21 January 1998     

Electronic structure of MgB2

Results of ab initio electronic structure calculations on the compound MgB₂ using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (E _F) and the experimental specific heat data. The T _c is found to be 24.7 K.     

Two-band superconductivity in MgB2

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]].     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

MgB2超导储能磁体可行性研究及结构优化

MgB2作为超导材料中的一颗新星,可以很好的应用于超导磁体。文中对比分析了作为超导磁体应用材料MgB2的超导特性,之后进行了MgB2磁体电磁设计。并通过与已有的Bi系超导磁体的对比,论证了MgB2在超导储能磁体应用中的可行性。针对MgB2超导特性进行磁体结构优化设计,得到两种新思路的螺管型磁体模型,都能够达到更好的经济性。