Enhancement of superconductivity in FeNbxSe0.95 by hole carrier doping

Abstract

FeSe has attracted considerable attention over the past few years due to its unique superconductivity and electronic properties. Until now, electron-doping approaches, including interlayer intercalation, K-coating, the interface-induced effect and liquid-gating technology, have been effective methods for enhancing the superconducting transition temperature (T_c) of FeSe bulks or thin films. Due to the multiband nature of FeSe, hole carriers also participate in electrical transport as electron counterparts. However, whether hole doping is able to enhance T_c or not, similarly, heavy electron-doping still remains an open issue. In this work, hole-doping was confirmed in bulk FeSe by adding small amounts of Nb, based on Hall measurements and first-principle theoretical calculations. Subsequently, an enhanced T_c was obtained. Morphology and structural investigations indicated that Nb enters the lattice of β-FeSe by substituting the Fe sites when the amount of Nb added is relatively low. With increased Nb substitution, the T_c value of FeNb_xSe_0.95 distinctly increases, with the highest value achieved in FeNb_0.04Se_0.95. Excess Nb addition resulted in the formation of Nb_xSe_y impurities and T_c reduction. Hall measurements indicate the dominant nature of hole-type carriers in FeNb_0.04Se_0.95 below 95 K, and this is in sharp contrast to the un-doped and heavily electron-doped FeSe systems. This result is further verified by first-principle theoretical calculations, which show that Fe_0.95Nb_0.05Se possesses a larger hole-pocket at the Γ point and a smaller electron-pocket at the M point, compared with FeSe. This work provides a new approach for enhancing the superconductivity of the FeSe system based on hole-doping, and proposes complementary understanding of the intrinsic mechanism of iron-based superconductivity.