Time: 09:30am, Mar. 25 (Wedn.), 2020 (UTC/GMT+08:00) 

Online Meeting Room (zoom.com): click here to join the meeting

Meeting ID: 405 160 862

Title: Localized Majorana Excitations in Fe-Based Superconductors: Experiment and Theory

Speaker:Ziqiang Wang (Boston College)

Abstract:

This talk focuses on recent developments in searching for Majorana zero modes (MZMs) in iron-based superconductors. We will mostly discuss bulk FeTeSe, where superconducting topological surface states – a new state of quantum matter – has been observed. Candidate MZMs have been detected by STM in magnetic field induced Abrikosov vortices. We argue that the conventional folklore requiring an external magnetic field to generate vortices changes in a fundamental way in superconductors with strong spin-orbit coupling. Topological defect excitations, dubbed quantum anomalous vortices (QAV), can nucleate around magnetic ions spontaneously in the absence of external magnetic fields. With increasing exchange coupling, the nature of the magnetic defect states changes from the Yu-Shiba states to such the QAV core states, including the MZM. We discuss several recent experiments that observed candidate MZMs at both growth-induced interstitial magnetic Fe impurities and Fe adatoms through surface atomic deposition, as well as a reversible transition between the Yu-Shiba states and the MZM bound to the QAV. In the last part, we discuss the recent discovery of partners of candidate MZMs at both ends of atomic line defects in monolayer FeTeSe superconductors. We argue that the missing Te/Se atoms along the defect line introduces strong Rashba spin-orbit coupling and turns it into a 1D time-reversal invariant topological superconductor with Kramers pairs of MZMs at both ends. We argue that Fe-based superconductors provide a promising and advantageous platform for studying the fundamental physics of localized Majorana excitation and topological quantum computing.