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Register a new userFermi surface nesting, vacancy ordering and the emergence of superconductivity in IrSb compounds
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Fermi surface nesting, as a peculiar reciprocal space feature, is not only closely correlated with the real space superstructure, but also directly modulates the underlying electronic behavior. In this work, we elucidate the Fermi surface nesting feature of the IrSb compound with buckled-honeycomb-vacancy (BHV) ordering through Rh and Sn doping, and its correlation with structure and electronic state evolution. The advantageous substitution of atom sites (i.e., Rh on the Ir sites, Sn on the Sb sites, respectively), rather than the direct occupation of vacancies, induces the collapse of BHV order and the emergence of superconductivity. The distinct superconducting behavior of Rh and Sn incorporated systems are ascribed to the mismatch of Fermi surface nesting in the Sn case.
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We report the synthesis of single-crystal NbC, a transition metal carbide with various unusual properties. Transport, magnetic susceptibility, and specific heat measurements demonstrate that NbC is a conventional superconductor with a superconducting transition temperature ($T_c$) of 11.5 K. Our theoretical calculations show that NbC is a type-II Dirac semimetal with strong Fermi surface nesting, which is supported by our ARPES measurement results. We also observed the superconducting gaps of NbC using angle-resolved photoemission spectroscopy (ARPES) and found some unconventional behaviors. These intriguing superconducting and topological properties, combined with the high corrosion resistance, make NbC an ideal platform for both fundamental research and device applications.
Vacancies are prevalent and versatile in solid-state physics and materials science. The role of vacancies in strongly correlated materials, however, remains uncultivated until now. Here, we report the discovery of an unprecedented vacancy state forming an extended buckled-honeycomb-vacancy (BHV) ordering in Ir$_{16}$Sb$_{18}$. Superconductivity emerges by suppressing the BHV ordering through squeezing of extra Ir atoms into the vacancies or isovalent Rh substitution. The phase diagram on vacancy ordering reveals the superconductivity competes with the BHV ordering. Further theoretical calculations suggest that this ordering originates from a synergistic effect of the vacancy formation energy and Fermi surface nesting with a wave vector of (1/3, 1/3, 0). The buckled structure breaks the crystal inversion symmetry and can mostly suppress the density of states near the Fermi level. The peculiarities of BHV ordering highlight the importance of correlated vacancies and may serve as a paradigm for exploring other non-trivial excitations and quantum criticality.
Superconductivity and structural properties of Mg1-xAlxB2 materials have been systematically investigated. Evident modifications in both superconductivity and microstructure are identified to originate from the Al ordering along the c-axis direction. The resultant superstructure phase has an optimal composition of MgAlB4 with the superconducting transition at around 12K. Brief diagrams illustrating the superconductivity and structural features of Mg1-xAlxB2 materials along with the increase of Al concentration are presented.
The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface(FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, the pairing strength is enhanced simultaneously (2$Delta$/Tc$sim$7) on the nearly nested FS pockets, i.e. the inner holelike ($alpha$) FS and the two hybridized electronlike FSs, while the pairing remains weak (2$Delta$/Tc$sim$3.6) in the poorly-nested outer hole-like ($beta$) FS. Here we report that in the electron-doped BaFe$_{1.85}$Co$_{0.15}$As$_2$ the FS nesting condition switches from the $alpha$ to the $beta$ FS due to the opposite size changes for hole- and electron-like FSs upon electron doping. The strong pairing strength (2$Delta$/Tc$sim$6) is also found to switch to the nested $beta$ FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.
We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1-xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transi- tion temperature with hole doping coincides with the magnetic excitations splitting into two incom- mensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s+- symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight caused by the weakening of electron-electron correlations.
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