Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Z45: Topological Properties of Fe Based and Pnictide Superconducting and Magnetic MaterialsInvited Live Streamed
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Sponsoring Units: DCMP Chair: Yanna Chen, Northwestern University Room: McCormick Place W-375D |
Friday, March 18, 2022 11:30AM - 12:06PM |
Z45.00001: Topology, Superconductivity and Magnetism in the FeTe1−xSex superconductors Invited Speaker: Peter D Johnson There is mounting evidence from angle-resolved photoemission (ARPES),[1] NV optical detection of magnetic resonance (ODMR) [2] and magneto-optic Kerr effect (MOKE) measurements, that, with the onset of superconductivity in the FeTe1−xSex superconductors, a magnetic field develops in the surface region that breaks time-reversal symmetry and results in mass acquisition in a topological surface state. The combination of magnetism and topology offers the possibility of the Quantum Anomalous Hall Effect (QAHE). A competing ground state, topological superconductivity offers the platform for Majorana Fermions as has already been demonstrated on this surface.[ [3] Such fermions have the potential for application in qubit technology. However, superconductivity and the QAHE are competing ground states. Here we review these important observations and also examine the role of Ni substitution for Fe. The presence of Ni is known to quench the superconductivity, potentially providing a more favorable environment for the QAHE and thus the disappearance of the former might enhance the possibility of the latter. In fact, we show that with Ni substitution the size of the magnetic field is reduced and further that the magnitude of the field appears to scale with the local moment. Indeed, the surface magnetism disappears even faster than the superconductivity. |
Friday, March 18, 2022 12:06PM - 12:42PM |
Z45.00002: Observation of Majorana Zero Modes in Iron-based Superconductors* Invited Speaker: Hong-Jun Gao Majorana zero-modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics. They are believed to hold great promise for topological quantum computing. A breakthrough of Majorana zero mode has been achieved in a single material platform of high-T c iron- based superconductor, FeTe 0.55 Se 0.45 , by using scanning tunneling microscopy/spectroscopy (STM/STS) [1]. This material combines the advantages of a simple material, high- T c , and large ratio of Δ/E F . We also revealed the mechanism of two distinct classes of vortices presented in this system, which directly tied with the presence or absence of zero-bias peak [2]. We further found the Majorana conductance plateau in vortices on the iron-based superconductor FeTe 0.55 Se 0.45 [3]. Both the extrinsic instrumental convoluted broadening and the intrinsic quasiparticle poisoning can reduce the conductance plateau value, and when extrinsic instrumental broadening is removed by deconvolution, the plateau is found to nearly reach a 2e 2 /h quantized value. Moreover, we confirmed the existence of MZMs in the vortex cores of CaKFe 4 As 4 and LiFeAs [4,5], single-material superconductors belonging to the iron pnictide family. These results show the great potential of the single-material platforms for Majorana research in the future. |
Friday, March 18, 2022 12:42PM - 1:18PM |
Z45.00003: Quantum anomalous vortex and Majorana zero mode induced by magnetic impurities in Fe-based superconductors. Invited Speaker: Ziqiang Wang In conventional superconductors, an external magnetic field is needed to generate magnetic vortices. We argue that the presence of strong spin-orbit coupling changes this folklore in a fundamental way. Topological defect excitations, dubbed quantum anomalous vortex (QAV), can nucleate around magnetic ions spontaneously in the absence of external magnetic fields [1]. With increasing exchange coupling, magnetic impurity states transition from the vortex-free Yu-Shiba-Rusinov states to the QAV core states. Several Fe-based superconductors exhibit Z2 nontrivial band structures and superconducting topological surface states. We discuss recent experimental observations of QAV and candidate MZM at growth-induced interstitial magnetic Fe impurities [2], magnetic Fe adatoms deposited on the surface [3, 4], and the reversible transition between the Yu-Shiba-Rusinov states and integer quantized QAV core states including the MZM [4]. Other on-going experiments searching for the QAV and MZM will be discussed, as well as possible realizations of intrinsic chiral topological superconductivity in Fe-based superconductors. |
Friday, March 18, 2022 1:18PM - 1:54PM |
Z45.00004: Weyl points in iron pnictides and iron chalcogenides Invited Speaker: Rafael M Fernandes It has been recently realized that iron-based superconductors provide a flexible and novel platform to investigate the interplay between topology, unconventional superconductivity, and electronic correlations. Much of the collective effort has focused on topological phenomena related to the band inversion involving the Fe d-bands and an As/Se p-band. Here we show that, even in the absence of band inversion, nontrivial topological effects can also arise from the Fe d-orbitals alone, due to the existence of a glide-plane symmetry. In particular, we demonstrate that when the inversion symmetry with respect to the Fe plane is broken, a magnetic field can be used to create Weyl points. This symmetry can be broken explicitly, such as in the cases of CaKFe4As4 and monolayer FeSe (or FeTe), or spontaneously, such as in the case of a vestigial spin-vorticity density-wave order near an underlying spin-vortex crystal instability. Moreover, the Weyl points can have a topological charge of ±1 or ±2, depending on the relative scales of the spin-orbit coupling and of the Zeeman energy. We discuss experimental manifestations of these Weyl fermions, focusing on the case of CaKFe4As4, and their potential interplay with unconventional superconductivity. |
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