Superconductors via Interface Mechanism and High Pressure

Since the discovery of superconductivity in 1911, researchers have continued the search for superconductors with higher transition temperatures (T_c). Many strides have been made with the increase of the T_c, as can been seen from the evolution of record T_c over time. We believe a holistic multidisciplinary enlightened empirical approach can be most fruitful. Two paths have been adopted to raise T_c: to realize novel mechanism inspired by the theoretical models and to discover new compounds guided by experience and insight empirically. Over the last few years, we discovered nonbulk superconductivity in rare-earth doped CaFe₂As₂ (Ca122) with an onset T_c of 49 K, which is higher than the T_c of any known compounds formed of the constituent elements at ambient or under high pressures. We successfully induced superconductivity with a T_c of 25 K in the mixed phase region of two non-superconducting phases in undoped Ca122. We also observed a universal increase of T_c under high pressure in BSCCO. Our systematical studies in R-doped Ca122 and undoped Ca122 both suggest that the observations may be attributed to an interface-induced T_c and our investigation on BSCCO reveals that ultra-high pressure may help to break away from the predictions by the rigid band model and reach even higher T_cs.