*Markus Kriener1, M. Kamitani1, T. Koretsune1,2, S. Bahramy1,3, M. Sakano3, R. Yukawa4, K. Horiba4, H. Kumigashira4, K. Ishizaka1,3, R. Arita1,3, Y. Tokura1,3, Y. Taguchi1,3
(1. RIKEN Center for Emergent Matter Science (CEMS), Wako(Japan), 2. Department of Physics, Tohoku University, Sendai(Japan), 3. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo(Japan), 4. Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki(Japan))
Keywords:polar superconductivity, negative-U mechanism, valence-skipping elements, valence fluctuations, enhancement of superconducting Tc