Observation of magnetic adatom-induced Majorana vortex and its hybridization with field-induced Majorana vortex in an iron-based superconductor

Abstract

Braiding Majorana zero modes is essential for fault-tolerant topological quantum computing. Iron-based superconductors with nontrivial band topology have recently emerged as a surprisingly promising platform for creating distinct Majorana zero modes in magnetic vortices in a single material and at relatively high temperatures. The magnetic field-induced Abrikosov vortex lattice makes it difficult to braid a set of Majorana zero modes or to study the coupling of a Majorana doublet due to overlapping wave functions. Here we report the observation of the proposed quantum anomalous vortex with integer quantized vortex core states and the Majorana zero mode induced by magnetic Fe adatoms deposited on the surface. We observe its hybridization with a nearby field-induced Majorana vortex in iron-based superconductor FeTe0.55Se0.45. We also observe vortex-free Yu-Shiba-Rusinov bound states at the Fe adatoms with a weaker coupling to the substrate, and discover a reversible transition between Yu-Shiba-Rusinov states and Majorana zero mode by manipulating the exchange coupling strength. The dual origin of the Majorana zero modes, from magnetic adatoms and external magnetic field, provides a new single-material platform for studying their interactions and braiding in superconductors bearing topological band structures.

Robust topological state against magnetic impurities observed in the superconductor PbTaSe2

“…The experiments demonstrated here suggest that the lowenergy topological surface state with a helical spin texture can be taken as an invariant in studying this superconductor with dilute magnetic impurities. This observation is consistent with the topological interpretation of the nonsplitting zeroenergy states observed at Fe adatoms in several topological superconductor candidates, including Fe(Te,Se), LiFeAs, and PbTaSe 2 [12,[29][30][31][32], and encourages further exploration of topological zero modes in similar platforms with a combination of topology, magnetism, and quantum ordering [33][34][35].…”

Yu-Shiba-Rusinov states in a superconductor with topological Z2 bands

“…It has been theoretically proposed that the arrangement of the YSR states on the superconductor surface can lead to topological superconductor platforms hosting Majorana zero modes [4][5][6][7][8][9][10][11]. The experimental progress serves hints of the Majorana existence [12][13][14][15]. Understanding the fundamentals of the YSR state is an important step toward understanding local excitations in superconductors and topological superconductivity.…”

“…1(a)) can manipulate its exchange coupling to the superconductor [16]. Recent experiments on Fe-based superconductor FeTe 0.55 Se 0.45 [12] show that the YSR states exhibit two zero-energy crossings with increasing tunnel barrier conductance G accompanied by a monotonic decrease in the the tip distance d to the magnetic Fe adatom. Since the exchange of the quasiparticle and the quasihole occur at zero energy, the crossing point is identified as a first-order quantum phase transition.…”

“…Since the exchange of the quasiparticle and the quasihole occur at zero energy, the crossing point is identified as a first-order quantum phase transition. With decreasing d, the exchange coupling was assumed to increase monotonically [12] and drive the two distinct energy crossings, in striking contrast to YSR states in conventional superconductors [1][2][3]. Evidence for the proposed transition from YSR states to quantum anomalous vortex core states spontaneously nucleated at magnetic Fe impurities [4] has also been observed [12].…”

“…With decreasing d, the exchange coupling was assumed to increase monotonically [12] and drive the two distinct energy crossings, in striking contrast to YSR states in conventional superconductors [1][2][3]. Evidence for the proposed transition from YSR states to quantum anomalous vortex core states spontaneously nucleated at magnetic Fe impurities [4] has also been observed [12]. In a different experiment where magnetic FeP molecules were adsorbed on the surface of conventional superconductors Pb(111), two zero-energy crossings have also been observed with increasing G [16].…”