Research on partial electronic density of states in superoonducting diborides by x-ray absorption and emission spectroscopies

测量了超导体MgB2的同结构二硼化物AlB2单晶和MgB2单晶在B - k边附近的x射线发射（XES）和吸收（XAS）光谱。根据x -光谱和XAS光谱的偏振依赖性，得到了B-2p-σ和p-π轨道的偏态密度（PDOS）。PDOS的总体特征与AlB2和MgB2波段计算结果吻合较好。与MgB2类似，AlB2在费米能级附近也有相当数量的PDOS，但AlB2在费米能级附近的p-σ轨道上几乎没有PDOS，即p-σ态存在赝隙，p-π态存在宽金属态。相反，MgB2在费米能级附近的p-σ轨道上存在相当数量的PDOS，即：在p-π态和p-σ态中观察到宽金属态，这一结果间接支持了p-σ空穴在MgB2中超导性发生中起重要作用的假设。利用MgB2、AlB2单晶x射线发射光谱和吸收光谱的偏振依赖性，研究了电子相关（EC）对偏态密度（PDOS）的影响。虽然与电子相关效应相反，但观测到的pdos与理论值存在一些差异。目前的结果表明，EC效应小于实验误差（0.2 eV），这间接支持了电子-声子相互作用在超导发生中起重要作用的假设。

X-ray emission (XES) and obsorption (XAS) spectra near the B K-edge were measured on single-crystalline AlB2 which is an isostructural diboride of superconducting MgB2, and on single-crystalline MgB2. The partial density of states (PDOS) of B-2p-σ and p-π orbitals were derived from the polarization dependence of XES and XAS spectra.The overall features of PDOS were found to be in good agreement with the result of band calculation of AlB2 and MgB2. There are considerable amounts of PDOS near the Fermi energy in AlB2 similarly to that in MgB2, but there are almost no PDOS in p-σ orbitals of AlB2 near the Fermi energy, i.e., a pseudo-gap in p-σ state and a broad metallic state in p-π state are observed, On the contrary, there are considerable amount of PDOS in p-σ orbitals of MgB2 near the Fermi energy, i.e., broad metallic states in p-π state and p-σ state are observedThe present result indirectly supports scenarios that the p-σ holes play an important role in the occurrence of superconductivity in MgB2.The effect of electron correlation (EC) on the partial density of state (PDOS) were studied by the polarization dependence of x-ray emission and absorption spectra of MgB2, AlB2 single crystals.There are some discrepancies between the observed PDOSs and theoretical ones, although these are contrary to the electron-correlation effects. The present results suggest that the EC effect is less than the experimental error (0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.