Field-free platform for Majorana-like zero mode in superconductors with a topological surface state

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Field-free platform for Majorana-like zero mode in superconductors with a topological surface state

Songtian S. Zhang, Jia-Xin Yin, Guangyang Dai, Lingxiao Zhao, Tay-Rong Chang, Nana Shumiya, Kun Jiang, Hao Zheng, Guang Bian, Daniel Multer, Maksim Litskevich, Guoqing Chang, Ilya Belopolski, Tyler A. Cochran, Xianxin Wu, Desheng Wu, Jianlin Luo, Genfu Chen, Hsin Lin, Fang-Cheng Chou, Xiancheng Wang, Changqing Jin, Raman Sankar, Ziqiang Wang, and M. Zahid Hasan

Phys. Rev. B 101, 100507(R) – Published 31 March 2020

Abstract

Superconducting materials exhibiting topological properties are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. In this letter, we explore the possibility of a field-free platform for generating Majorana zero energy excitations by depositing magnetic Fe impurities on the surface of candidate topological superconductors, LiFeAs and $PbTaS e_{2}$ . We use scanning tunneling microscopy to probe localized states induced at the Fe adatoms on the atomic scale and at sub-Kelvin temperatures. We find that each Fe adatom generates a striking zero-energy bound state inside the superconducting gap, which do not split in magnetic fields up to 8 T, underlining a nontrivial topological origin. Our findings point to magnetic Fe adatoms evaporated on bulk superconductors with topological surface states for exploring Majorana zero modes and quantum information science under field-free conditions.

Received 21 May 2019
Revised 13 March 2020
Accepted 16 March 2020

DOI:https://doi.org/10.1103/PhysRevB.101.100507

Physics Subject Headings (PhySH)

Impurities in superconductors Majorana bound states Superconductivity Topological superconductors

Superconductors

Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Songtian S. Zhang^1,*, Jia-Xin Yin ^1,*,†, Guangyang Dai², Lingxiao Zhao², Tay-Rong Chang^3,4,5, Nana Shumiya¹, Kun Jiang⁶, Hao Zheng⁷, Guang Bian⁸, Daniel Multer¹, Maksim Litskevich¹, Guoqing Chang¹, Ilya Belopolski¹, Tyler A. Cochran¹, Xianxin Wu², Desheng Wu², Jianlin Luo², Genfu Chen², Hsin Lin⁹, Fang-Cheng Chou¹⁰, Xiancheng Wang², Changqing Jin², Raman Sankar^9,10, Ziqiang Wang⁶, and M. Zahid Hasan^1,11,‡

¹Laboratory for Topological Quantum Matter and Advanced Spectroscopy, Department of Physics, Princeton University, Princeton, New Jersey 08544, USA
²Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
⁴Center for Quantum Frontiers of Research & Technology (QFort), Tainan 701, Taiwan
⁵Physics Division, National Center for Theoretical Sciences, Hsinchu, 30013 Taiwan
⁶Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
⁷Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240, China
⁸Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, USA
⁹Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
¹⁰Center for Condensed Matter Science, National Taiwan University, Taipei 10617, Taiwan
¹¹Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*These authors contributed equally to this work.
^†Corresponding author: jiaxiny@princeton.edu
^‡mzhasan@princeton.edu

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Vol. 101, Iss. 10 — 1 March 2020

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Images

Figure 1
(a), (b) Transport measurement of LiFeAs and $PbTaS e_{2}$ , respectively, showing a large residual-resistance ratio of 48 and 130, respectively, indicating the high quality of the samples. Inset left: enlarged resistivity curve near the superconducting transition temperature. Inset right: calculated band structure along a high symmetry direction, with the topological surface state featuring spin-momentum locking. (c), (d) Topographic 3D image of randomly scattered Fe adatoms on the LiFeAs and $PbTaS e_{2}$ surface, respectively, demonstrating the atomic nature of the Fe deposition at cryogenic conditions. Insets: 2D zoom-in view of adatoms.
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Figure 2
(a) Impurity state of a Fe adatom, featuring a zero-energy state. Inset: topographic image of the impurity (upper) and its $d I / d V$ map at zero-energy (lower). (b) $d I / d V$ spectra on eight different Fe adatoms. (c) Intensity map of $d I / d V$ spectrum across the Fe adatom, showing decay of the zero-energy state without detectable splitting. (d) Selected spectrums from (c). Inset: spatial evolution of the zero-energy state. (e) High energy resolution $d I / d V$ spectrum of the zero-energy state. (f) Vector magnetic field perturbation to the zero-energy state, showing suppression of the coherence of the spectrum.
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Figure 3
(a) $d I / d V$ spectra taken far from any vortices on a pristine sample as a function of magnetic field. Inset: gap evolution with field. (b) $d I / d V$ spectra on and far from a vortex core (imaged inset) at $B = 0.04 T$ . Inset: $d I / d V$ map at $Δ_{T} = 0.7 mV$ . (c) $d I / d V$ spectra at a Fe adatom (red) and away (blue). Inset: topographic image of Fe adatom. (d) Subtraction of the two spectra in (c), characterizing the emergence of a pair of in-gap states (red) at $\pm Δ_{T}$ ( $ɛ = 0$ ). (e) Intensity map of $d I / d V$ spectrum taken across the Fe adatom. (f) Spatial evolution of the coherence peak and in-gap state intensity. (g) $d I / d V$ spectra on seven Fe adatoms (red), showing the robust in-gap state at $\pm Δ_{T}$ ( $ɛ = 0$ ); spectrum on vortex core (orange) exhibits a peak at the same energy. Inset: deviation from zero-energy for ten measured IFIs (offset for clarity). Circles (triangles) denote bound states at the negative (positive) energy. (h) Deconvoluted tunneling spectra of pristine $PbTaS e_{2}$ (blue) and at the Fe adatom (red) with both superconducting tip gap and thermal broadening effect at 0.4 K subtracted. Inset: spectra convoluted with a superconducting tip gap and Fermi distribution function at 0.4 K, plotted as open blue and red circles, respectively.
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