In this talk I’ll first describe our studies of a variety of cuprate and iron-based superconductors by means of cryogenic scanning tunneling spectroscopy (STS) as a function of magnetic field, energy, wave-vector and doping level. Our investigation together with further comparison to angle-resolved photoemission spectroscopy (ARPES) and inelastic neutron scattering (INS) experiments suggests that the commonalities among the cuprate and iron-based superconductors include the proximity to competing orders, antiferromagnetic spin fluctuations and magnetic resonances in the superconducting state , and the unconventional pairing symmetries with sign-changing order parameters on different parts of the Fermi surface. These findings suggest that strong electronic correlations with repulsive pairing interactions of varying signs in the momentum space may be essential to the occurrence of high-Tc superconductivity, which is in contrast to the attractive pairing mediated by electron-phonon coupling in conventional superconductivity. Finally, interesting ideas for possible realization, detection and applications of Majorana modes associated with the edge states of topological superconductors and the proximity effects of topological insulators will be discussed.
Nai-Chang Yeh is currently the Fletcher Jones Foundation Co-Director of the Kavli Nanoscience Institute and Professor of Physics at the California Institute of Technology (Caltech). She received her B.Sc. in Physics from the National Taiwan University in 1983 and Ph.D. in Physics from the Massachusetts Institute of Technology in January 1988. From January 1988 to August 1989, she was a visiting scientist at IBM, Thomas J. Watson Research Center in New York. She joined the Caltech faculty in August 1989, and became the first woman tenured in physics at Caltech in 1995. Her professional honors include: Wu Chien-Shiung Distinguished Lectureship, National Central University, Taiwan (2012); CNMM Chair Professorship, Tsinghua University, Beijing, China (2012-2014); Fellow, American Association for the Advancement of Science (AAAS) (2007); Fellow, American Physical Society (APS) (2004); Distinguished Alumni Award, Department of Physics, National Taiwan University (2003); Fellow, The Institute of Physics, UK (2001); Achievement Awards, Southern California Chinese-American Faculty Association (2001); Outstanding Young Researcher Award, Overseas Chinese Physicists Association (1998); Packard Fellowship for Science and Engineering (1992-1997); and Sloan Research Fellowship (1990-1992).