Competition of Superconductivity and Charge Density Wave in Selective Oxidized ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ Thin Flakes

Competition of Superconductivity and Charge Density Wave in Selective Oxidized ${CsV}_{3} {Sb}_{5}$ Thin Flakes

Yanpeng Song, Tianping Ying, Xu Chen, Xu Han, Xianxin Wu, Andreas P. Schnyder, Yuan Huang, Jian-gang Guo, and Xiaolong Chen

Phys. Rev. Lett. 127, 237001 – Published 1 December 2021

Abstract

The recently discovered layered kagome metals $A V_{3} {Sb}_{5}$ ( $A = K$ , Rb, and Cs) with vanadium kagome networks provide a novel platform to explore correlated quantum states intertwined with topological band structures. Here we report the prominent effect of hole doping on both superconductivity and charge density wave (CDW) order, achieved by selective oxidation of exfoliated thin flakes. A superconducting dome is revealed as a function of the effective doping content. The superconducting transition temperature ( $T_{c}$ ) and upper critical field in thin flakes are significantly enhanced compared with the bulk, which are accompanied by the suppression of CDW. Our detailed analyses establish the pivotal role of van Hove singularities in promoting correlated quantum orders in these kagome metals. Our experiments not only demonstrate the intriguing nature of superconducting and CDW orders, but also provide a novel route to tune the carrier concentration through both selective oxidation and electric gating. This establishes ${CsV}_{3} {Sb}_{5}$ as a tunable 2D platform for the further exploration of topology and correlation among $3 d$ electrons in kagome lattices.

Received 22 June 2021
Revised 29 October 2021
Accepted 2 November 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.237001

Physics Subject Headings (PhySH)

Charge density waves Superconducting phase transition Superconductivity

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yanpeng Song^1,*, Tianping Ying ^1,2,*, Xu Chen¹, Xu Han¹, Xianxin Wu^3,4,†, Andreas P. Schnyder³, Yuan Huang ¹, Jian-gang Guo^1,5,‡, and Xiaolong Chen ^1,5,6,§

¹Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan
³Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
⁴CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China
⁵Songshan Lake Materials Laboratory, Dongguan 523808, China
⁶School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

^*Y. S. and T. Y. contributed equally to this work.
^†xianxin.wu@fkf.mpg.de
^‡jgguo@iphy.ac.cn
^§chenx29@iphy.ac.cn

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Vol. 127, Iss. 23 — 3 December 2021

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Figure 1
Selective oxidation of ${CsV}_{3} {Sb}_{5}$ thin flakes. (a) Side view of the crystal structure of ${CsV}_{3} {Sb}_{5}$ . (b) Room-temperature x-ray diffraction pattern of the ${CsV}_{3} {Sb}_{5}$ single crystal with a preferred orientation along the [00l] direction. The acquired bulk crystal is shown in the inset. (c) Optical image of a typical fabricated device. (d) Illustration of the dry-transfer and selective-oxidation processes.
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Figure 2
XPS measurements for bulk and thin flakes. (a) The optical images of bulk (left) and the exfoliated ${CsV}_{3} {Sb}_{5}$ thin flakes on a ${SiO}_{2} / Si$ substrate (right). (b)–(d) The high-resolution XPS spectra of Cs $3 d$ , V $2 p$ , and Sb $3 d$ for bulk crystal and thin flakes. The additional peak at 532.9 eV shown in (d) comes from the substrate.
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Figure 3
Low-temperature transport property of ${CsV}_{3} {Sb}_{5}$ thin flakes. (a) Temperature-dependent resistance of a 82 nm ${CsV}_{3} {Sb}_{5}$ thin fake. Upper inset shows the field-dependent $R (T)$ curves at low temperatures. Lower inset is the upper critical field $H_{c 2}$ along the $c$ axis and $a b$ plane (raw data from which $H_{c 2}^{a b}$ extracted can be found in Fig. S5 [41]). The solid lines are the Ginzburg-Landau fitting curves. (b) Temperature-dependent resistance for ${CsV}_{3} {Sb}_{5}$ thin fakes with various thicknesses. Each curve is vertically shifted to show the $T_{c}^{onset}$ and $T_{c}^{zero}$ more clearly. The $T_{CDW}$ is determined by $d ρ / d T$ and the raw data can be found in Figs. S6 and S7 [41]. (c) Temperature-thickness phase diagram of ${CsV}_{3} {Sb}_{5}$ . Rhombus and circle symbols denote the $T_{CDW}$ and $T_{c}$ , respectively. We use $T_{c}^{mid}$ , where the resistance drops 50% of its normal state, as $T_{c}$ , and the transition widths $T_{c}^{onset} - T_{c}^{zero}$ are taken as the error bar.
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Figure 4
Gate-tuning investigation of the selectively oxidized ${CsV}_{3} {Sb}_{5}$ thin flake. (a) Optical image of the fabricated device. The thickness of the sample and the BN gate insulator are 160 and 120 nm, respectively. The ${CsV}_{3} {Sb}_{5}$ thin flake was intentionally oxidized before the device fabrication. Positive gate voltage was applied on the top gate. (b) Temperature-dependent resistance under various gate voltages, with the results summarized in (c). (d) The nonlinear relationship of flake thickness versus effective doping in our toy model estimation. The surface contribution will dramatically increase approaching the 2D limit.
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Figure 5
Band structure, density of states (DOS), and phase diagram of hole doped ${CsV}_{3} {Sb}_{5}$ . (a) Electronic band structure and (b) V $d$ -orbital DOS for ${Cs}_{x} V_{3} {Sb}_{5}$ ( $x = 1$ , 0.97, 0.95). (c) Phase diagram of ${CsV}_{3} {Sb}_{5}$ with the variation of hole-doping content, where only the top layer is assumed to be oxidized. Phase diagram in the low doping region $(0 % \sim 1 %)$ can be found in Fig. S11 [41], where 15 data with the flake thickness above 100 nm are included.
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Physical Review Letters

Competition of Superconductivity and Charge Density Wave in Selective Oxidized CsV3Sb5 Thin Flakes

Yanpeng Song, Tianping Ying, Xu Chen, Xu Han, Xianxin Wu, Andreas P. Schnyder, Yuan Huang, Jian-gang Guo, and Xiaolong Chen

Phys. Rev. Lett. 127, 237001 – Published 1 December 2021

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Competition of Superconductivity and Charge Density Wave in Selective Oxidized ${CsV}_{3} {Sb}_{5}$ Thin Flakes