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Register a new userElectronic Structure of B-2$psigma$ and $ppi$ States in MgB$_2$, AlB$_2$ and ZrB$_2$ Single Crystals
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The effect of electron correlation (EC) on the electronic structure in MgB$_2$, AlB$_2$ and ZrB$_2$, is studied by examining the partial density of states (PDOS) of B-2$psigma$ and $ppi$ orbitals using the polarization dependence of x-ray emission and absorption spectra. The discrepancies between observed and calculated PDOSs cannot be attributed to EC effects. The present results suggest that the EC effect is less than the experimental error ($sim$ 0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.
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Based on first-principles calculations and effective model analysis, a Dirac nodal-net semimetal state is recognized in AlB$_2$-type TiB$_2$ and ZrB$_2$ when spin-orbit coupling (SOC) is ignored. Taking TiB$_2$ as an example, there are several topological excitations in this nodal-net structure including triple point, nexus, and nodal link, which are protected by coexistence of spatial-inversion symmetry and time reversal symmetry. This nodal-net state is remarkably different from that of IrF$_4$, which requires sublattice chiral symmetry. In addition, linearly and quadratically dispersed two-dimensional surface Dirac points are identified as having emerged on the B-terminated and Ti-terminated (001) surfaces of TiB$_2$ respectively, which are analogous to those of monolayer and bilayer graphene.
The electronic properties of the carbon substituted MgB$_2$ single crystals are reported. The carbon substitution drops T$_c$ below 2 K. In-plane resistivity shows a remarkable increase in residual resistivity by C-substitution, while the change of in-plane/out-of-plane Hall coefficients is rather small. Raman scattering spectra indicate that the E$_{2g}$-phonon frequency radically hardens with increasing the carbon-content, suggesting the weakening of electron-phonon coupling. Another striking C-effect is the increases of the second critical fields in both in-plane and out-of-plane directions, accompanied by a reduction in the anisotropy ratio. The possible changes in the electronic state and the origin of T$_c$-suppression by C-substitution are discussed.
We report the observation of superconducting vortices in pure and lightly Al doped MgB$_2$ single crystals. Low field experiments allow for the estimation of the London penetration depth, lambda ~ 1900 $AA$ for T$sim$6 K. Experiments in higher fields (e.g. 200 Oe) clearly show a triangular vortex lattice in both real space (13 $mu$m by 13 $mu$m Bitter decoration image of over 1000 vortices) and reciprocal space.
Unlike the widely studied $s$-type two-gap superconductor MgB$_2$, the chemically similar compounds ZrB$_2$ and HfB$_2$ do not superconduct above 1 K. Yet, it has been shown that small amounts of self- or extrinsic doping (in particular with vanadium), can induce superconductivity in these materials. Based on results of different macro- and microscopic measurements, including magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin rotation ($mu$SR), we present a comparative study of Zr$_{0.96}$V$_{0.04}$B$_2$ and Hf$_{0.97}$V$_{0.03}$B$_2$. Their key magnetic and superconducting features are determined and the results are considered within the theoretical framework of multiband superconductivity proposed for MgB$_2$. Detailed Fermi surface (FS) and electronic structure calculations reveal the difference between MgB$_2$ and transition-metal diborides.
We report a detailed study of the electronic and structural properties of the 39K superconductor mgbtwo and of several related systems of the same family, namely mgalbtwo, bebtwo, casitwo and cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional $sigma$-bonding bands contribute significantly at the Fermi level. Similarities and differences between mgbtwo and bebtwo (whose superconducting properties have not been yet investigated) are analyzed in detail. Our calculations for mgalbtwo show that metal substitution cannot be fully described in a rigid band model. casitwo is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.
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