Caging-Pnictogen-Induced Superconductivity in Skutterudites IrX₃ (X = As, P)

Here, we report on a new kind of compound, X_δIr₄X_12-δ (X = P, As), the first hole-doped skutterudites superconductor. We provide atomic-resolution images of the caging As atoms using scanning transmission electron microscopy (STEM). By inserting As atoms into the caged structure under a high pressure, superconductivity emerges with a maximum transition temperature (T_c) of 4.4 K (4.8 K) in IrAs₃ (IrP₃). In contrast to all of the electron-doped skutterudites, the electronic states around the Fermi level in X_δIr₄X_12-δ are dominated by the caged X atom, which can be described by a simple body-centered tight-binding model, implying a distinct pairing mechanism. Our density functional theory (DFT) calculations reveal an intimate relationship between the pressure-dependent local-phonon mode and the enhancement of T_c. The discovery of X_δIr₄X_12-δ provides an arena to investigate the uncharted territory of hole-doped skutterudites, and the method proposed here represents a new strategy of carrier doping in caged structures, without introducing extra elements.