Three-Dimensional Charge Density Wave and Surface-Dependent Vortex-Core States in a Kagome Superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$

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Three-Dimensional Charge Density Wave and Surface-Dependent Vortex-Core States in a Kagome Superconductor ${CsV}_{3} {Sb}_{5}$

Zuowei Liang, Xingyuan Hou, Fan Zhang, Wanru Ma, Ping Wu, Zongyuan Zhang, Fanghang Yu, J.-J. Ying, Kun Jiang, Lei Shan, Zhenyu Wang, and X.-H. Chen

Phys. Rev. X 11, 031026 – Published 2 August 2021

Abstract

The transition-metal-based kagome metals provide a versatile platform for correlated topological phases hosting various electronic instabilities. While superconductivity is rare in layered kagome compounds, its interplay with nontrivial topology could offer an engaging space to realize exotic excitations of quasiparticles. Here, we use scanning tunneling microscopy to study a newly discovered $Z_{2}$ topological kagome metal ${CsV}_{3} {Sb}_{5}$ with a superconducting ground state. We observe charge modulation associated with the opening of an energy gap near the Fermi level. When across single-unit-cell surface step edges, the intensity of this charge modulation exhibits a $π$ -phase shift, suggesting a three-dimensional $2 \times 2 \times 2$ charge density wave ordering. Interestingly, while conventional Caroli–de Gennes–Matricon bound states are observed inside the superconducting vortex on the Sb surfaces, a robust zero-bias conductance peak emerges that does not split in a large distance when moving away from the vortex center on the Cs $2 \times 2$ surfaces, resembling the Majorana bound states arising from the superconducting Dirac surface states in ${Bi}_{2} {Te}_{3} / {NbSe}_{2}$ heterostructures. Our findings establish ${CsV}_{3} {Sb}_{5}$ as a promising candidate for realizing exotic excitations at the confluence of nontrivial lattice geometry, topology and multiple electronic orders.

Received 8 March 2021
Revised 12 May 2021
Accepted 7 June 2021

DOI:https://doi.org/10.1103/PhysRevX.11.031026

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Charge density waves Local density of states Majorana bound states Topological materials Topological superconductors

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zuowei Liang^1,§, Xingyuan Hou ^2,§, Fan Zhang^3,2,§, Wanru Ma¹, Ping Wu¹, Zongyuan Zhang², Fanghang Yu¹, J.-J. Ying¹, Kun Jiang³, Lei Shan ^2,4,*, Zhenyu Wang ^1,†, and X.-H. Chen^1,5,6,‡

¹Department of Physics and Chinese Academy of Sciences Key laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
²Information Materials and Intelligent Sensing Laboratory of Anhui Province, Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China
³Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601, China
⁵CAS Center for Excellence in Quantum Information and Quantum Physics, Hefei, Anhui 230026, China
⁶Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

^*Corresponding author. lshan@ahu.edu.cn
^†Corresponding author. zywang2@ustc.edu.cn
^‡Corresponding author. chenxh@ustc.edu.cn
^§These authors contributed equally to this work

Popular Summary

In recent years, “kagome metals” have leapt to the forefront of investigations into new and exotic electronic states. In these metals, atoms are arranged into layered sets of overlapping triangles—a lattice that resembles the Japanese kagome weaving pattern. Recently, researchers discovered a new family of kagome metals that exhibit a superconducting ground state. Investigations into these materials have also dropped hints at several other unusual electronic behaviors. One example is Majorana bound states, peculiar quasiparticles hosted in a particular class of materials called topological superconductors, which are proposed as a key ingredient for fault-tolerant quantum computation. Here, we present a scanning tunneling microscopy study of one of these new kagome superconductors and find clues for such exotic states.

In our scans of the kagome metal ${CsV}_{3} {Sb}_{5}$ , we find a 3D charge density wave state, a repeating pattern in which electrons crowd together and spread apart. Inside the superconducting vortices, we observe a robust zero-energy quasiparticle state. The spatial characteristic of this zero-energy state is in sharp contrast to the normal bound states found in conventional superconductors but reminiscent of the Majorana bound states observed in topologically nontrivial superconductors. Detailed analysis further supports the presence of Majorana bound states in this compound.

Our findings are likely to stimulate interest in kagome superconductors as hosts of Majorana states.

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Vol. 11, Iss. 3 — July - September 2021

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Condensed Matter Physics

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Figure 1
(a) Crystal structure of ${CsV}_{3} {Sb}_{5}$ and the illustration of a top view of the lattice. The size of Cs atoms represents the CDW modulation. (b) Temperature-dependent resistivity of ${CsV}_{3} {Sb}_{5}$ . A clear anomaly occurs around 94 K, indicating the formation of charge order. The superconducting transition (marked as SC) that occurred at low temperature is shown in the inset. (c)–(e) Topographies of the Cs, Sb, and half-Cs surfaces, respectively. All of them show clear $3 Q$ - $2 \times 2$ CDW patterns. (f) Topography obtained on Sb surfaces, showing additional $1 Q$ modulation. This $1 Q$ modulation is observed on large Sb surfaces with very few Cs atoms on the top, and it becomes weaker with more surrounding Cs atoms (SM 4 [31]). The insets display the Fourier transforms of these topographic images. The lattice peak is marked by green circles, the $3 Q$ -CDW vector in red circles, while the $1 Q$ -CDW vector is shown in blue. (g)–(j) Typical $d I / d V$ spectra for these surfaces. The purple and pink dots in (e) denote the locations where spectra are obtained for half-Cs surface. STM setup condition: (c) $V_{S} = - 70 mV$ , $I_{t} = 80 pA$ , (d) $V_{S} = - 70 mV$ , $I_{t} = 120 pA$ , (e) $V_{S} = - 50 mV$ , $I_{t} = 700 pA$ , (f) $V_{S} = 100 mV$ , $I_{t} = 150 pA$ , (g) $V_{S} = - 60 mV$ , $I_{t} = 150 pA$ , $V_{m} = 3 mV$ , (h) $V_{S} = - 50 mV$ , $I_{t} = 450 pA$ , $V_{m} = 4 mV$ , (i) $V_{S} = - 60 mV$ , $I_{t} = 250 pA$ , $V_{m} = 3 mV$ , (j) $V_{S} = - 50 mV$ , $I_{t} = 130 pA$ , $V_{m} = 2 mV$ .
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Figure 2
(a) Topographic image of the half-Cs surface showing a single-unit-cell step. (b) Height profile obtained along the pink line shown in (a). (c) Close-up of the step edge. The dashed pink lines track the chains with CDW modulation on the upper side. A $π$ -phase jump can be observed between the upper and lower sides. (d) Illustration of the CDW patterns near a single-unit-cell step. (e),(f) Normalized $d I / d V$ maps ( $d I / d V (r, eV) / [I (r, eV) / V]$ ) and their FTs at $- 80$ and $- 300 mV$ , respectively. The white circle denotes one of the $3 Q$ -CDW wave vectors. The clearly dispersing vector is marked as $q_{1}$ . (g) The energy dispersion of $q_{1}$ extracted from line cuts of the FTs and a parabolic fit, $E = 9.77 k^{2} - 0.6$ . STM setup condition: (a) $V_{S} = - 100 mV$ , $I_{t} = 80 pA$ , (e) $V_{S} = - 80 mV$ , (f) $V_{S} = - 300 mV$ , $I_{t} = 450 pA$ , $V_{m} = 4 mV$ .
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Figure 3
(a) Temperature evolution of the superconducting gap. (b) Superconducting spectra obtained on different types of surfaces. The spectra of half-Cs, Sb $3 Q - 2 \times 2$ , and Sb $1 Q$ regions are obtained in one field of view with the same tip. STM setup condition: (a),(b) $V_{S} = - 2 mV$ , $I_{t} = 200 pA$ , $V_{m} = 0.1 mV$ .
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Figure 4
(a) $d I / d V$ map showing a superconducting vortex on the Cs surface with $3 Q - 2 \times 2$ CDW. (b) Tunneling spectra obtained in the vortex core (red) and outside the vortex (dark blue). (c),(d) Color map of the spatially resolved $d I / d V$ spectra across the vortex. The traces are marked by the dark and pink arrows as shown in (a). (e) The waterfall plot of (c), vertically offset for clarity. (f) Spatial dependence of FWHM of the ZBCP when moving away from the vortex core for Cs surface (blue) and Sb surface (orange). The error bar denotes the energy step used in the measurements. (g) $d I / d V$ map showing a vortex on the Sb surface with additional $1 Q$ -CDW. (h) Spatial evolution of the $d I / d V$ spectra across the vortex. Dashed lines in (c)–(e) and (h) track the peak splitting. We note that the splitting of the peak is not clear near the starting point due to the limit of energy resolution; therefore, we track the visible splitting peaks near the vortex edge and use linear extrapolation to obtain the dashed lines (see Figs. S10 and S13 in Supplemental Material [31]). STM setup condition: (a) $V_{S} = - 5 mV$ , $I_{t} = 200 pA$ , $V_{m} = 0.5 mV$ , (g) $V_{S} = - 6 mV$ , $I_{t} = 200 pA$ , $V_{m} = 0.5 mV$ , (c),(d),(h) $V_{S} = - 5 mV$ , $I_{t} = 200 pA$ , $V_{m} = 0.1 mV$ .
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Physical Review X

Three-Dimensional Charge Density Wave and Surface-Dependent Vortex-Core States in a Kagome Superconductor CsV3Sb5

Zuowei Liang, Xingyuan Hou, Fan Zhang, Wanru Ma, Ping Wu, Zongyuan Zhang, Fanghang Yu, J.-J. Ying, Kun Jiang, Lei Shan, Zhenyu Wang, and X.-H. Chen

Phys. Rev. X 11, 031026 – Published 2 August 2021

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Three-Dimensional Charge Density Wave and Surface-Dependent Vortex-Core States in a Kagome Superconductor ${CsV}_{3} {Sb}_{5}$