Observation of interface superconductivity in a ${\mathrm{SnSe}}_{2}$/epitaxial graphene van der Waals heterostructure

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Observation of interface superconductivity in a ${SnSe}_{2}$ /epitaxial graphene van der Waals heterostructure

Yi-Min Zhang, Jia-Qi Fan, Wen-Lin Wang, Ding Zhang, Lili Wang, Wei Li, Ke He, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Phys. Rev. B 98, 220508(R) – Published 19 December 2018

Abstract

We report on direct observation of interface superconductivity in single-unit-cell ${SnSe}_{2}$ films grown on graphitized SiC(0001) substrate. The tunneling spectrum in the superconducting state reveals a rather conventional character with a fully gapped order parameter. The occurrence of superconductivity is further confirmed by the observation of vortices under external magnetic field. Through interface engineering, we unravel the mechanism of superconductivity that originates from a two-dimensional electron gas formed at the interface of ${SnSe}_{2}$ and graphene. Our finding opens up novel strategies to hunt for and understand interface superconductivity based on van der Waals heterostructures.

Received 28 February 2018
Revised 30 November 2018

DOI:https://doi.org/10.1103/PhysRevB.98.220508

Physics Subject Headings (PhySH)

Superconducting phase transition

Dichalcogenides Interfaces Low-temperature superconductors Multilayer thin films Van der Waals systems

Molecular beam epitaxy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yi-Min Zhang¹, Jia-Qi Fan¹, Wen-Lin Wang¹, Ding Zhang^1,2, Lili Wang^1,2, Wei Li^1,2, Ke He^1,2, Can-Li Song^1,2,*, Xu-Cun Ma^1,2,†, and Qi-Kun Xue^1,2,‡

¹State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
²Collaborative Innovation Center of Quantum Matter, Beijing 100084, China

^*clsong07@mail.tsinghua.edu.cn
^†xucunma@mail.tsinghua.edu.cn
^‡qkxue@mail.tsinghua.edu.cn

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Vol. 98, Iss. 22 — 1 December 2018

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Figure 1
(a) Sketch of a ${SnSe}_{2}$ /graphene heterostructure. (b) Topographic image ( $300 nm \times 300 nm$ , $V = 3.5$ V, $I = 20$ pA) of in situ grown ${SnSe}_{2}$ films on graphitized SiC(0001). The dashed square marks the region where ZBC maps are acquired. (c) Zoom-in STM topography ( $18 nm \times 18 nm$ , $V = 50$ mV, $I = 100$ pA) on 1-UC ${SnSe}_{2}$ film. The bright spots denote the Se atoms at the top layer. (d) Thickness-dependent $d I / d V$ spectra of ${SnSe}_{2}$ films. The black and red triangles mark VBM and CBM near the $Γ$ point of ${SnSe}_{2}$ , respectively, with their middles marked by short vertical lines. Tunneling gap is set at $V = 1.5$ V and $I = 150$ pA. The lock-in bias modulation has a magnitude of 10 meV.
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Figure 2
(a) Tunneling $d I / d V$ spectra ( $V = 6$ mV, $I = 100$ pA) of 1-UC ${SnSe}_{2}$ film on bilayer graphene as a function of temperature as indicated. For clarity the spectra have been vertically offset, with their zero conductance positions marked by correspondingly colored horizontal lines. The same convention is used throughout, unless otherwise noted. The blue line at the bottom shows the best fit of experimental data (black curve) to the BCS Dynes formula with a single isotropic $s$ -wave gap. (b) A series of $d I / d V$ spectra ( $V = 10$ mV, $I = 100$ pA) acquired at 0.4 K and along a 40 nm trajectory in the inserted STM image ( $30 nm \times 30 nm$ , $V = 10$ mV, $I = 100$ pA). Instead of colored lines, the black lines mark the zero conductance positions for clarity.
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Figure 3
(a) ZBC map ( $120 nm \times 120 nm$ ) showing three individual isolated vortices (emphasized by white circles) of a 1-UC ${SnSe}_{2}$ /graphene heterostructure at 0.5 T. The colored dots along the arrowed line mark the positions where the $d I / d V$ spectra of the same color code are acquired in (c). The tunneling junction [also applies to (b) and (c)] is set at $V = 8$ mV and $I = 100$ pA. (b) ZBC map at a higher magnetic field of 1.0 T. (c) Linecut of $d I / d V$ spectra across a vortex core in (a) and at 0.4 K.
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Figure 4
(a) Tunneling spectra at 0.4 K vs sketched ${SnSe}_{2}$ /graphene heterostructures on every upper-left corner of the curves. For simplicity the atoms of SiC are not displayed. The tunneling junction is set at $V = 10$ mV and $I = 100$ pA except for the bottom one ( $V = 8$ mV, $I = 100$ pA). (b) A $98 nm \times 98 nm$ STM topographic image ( $V = 10$ mV, $I = 100$ pA) of 1-UC ${SnSe}_{2}$ films prepared on monolayer, bilayer, and trilayer grpaphene coexisting substrate. The red dashes encircle the region of ${SnSe}_{2}$ films situated on monolayer graphene. (c) Simultaneous ZBC map ( $64 pixels \times 64 pixels$ ) revealing graphene layer-dependent conductance contrast at zero field. (d) Dependence of $d I / d V$ spectra ( $V = 0.5$ V, $I = 150$ pA) on the ${SnSe}_{2}$ film thickness, measured in an intermediate energy range of $\pm 0.5$ eV. The lock-in has a bias modulation of 5 meV. The magenta triangles mark the CBM at the M point of ${SnSe}_{2}$ , located far above $E_{F}$ (vertical dashes). Inset shows the energy band scheme for a ${SnSe}_{2}$ /graphene heterostructure. Electron transfer from graphene to the ${SnSe}_{2}$ films is indicated by the orange arrow. The band gap opening near the Dirac point ( $E_{D}$ ) of graphene due to the SiC substrate is also shown.
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Physical Review B

covering condensed matter and materials physics

Observation of interface superconductivity in a ${SnSe}_{2}$ /epitaxial graphene van der Waals heterostructure

Yi-Min Zhang, Jia-Qi Fan, Wen-Lin Wang, Ding Zhang, Lili Wang, Wei Li, Ke He, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Phys. Rev. B 98, 220508(R) – Published 19 December 2018

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Physical Review B

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Observation of interface superconductivity in a SnSe2/epitaxial graphene van der Waals heterostructure

Yi-Min Zhang, Jia-Qi Fan, Wen-Lin Wang, Ding Zhang, Lili Wang, Wei Li, Ke He, Can-Li Song, Xu-Cun Ma, and Qi-Kun Xue

Phys. Rev. B 98, 220508(R) – Published 19 December 2018

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Observation of interface superconductivity in a ${SnSe}_{2}$ /epitaxial graphene van der Waals heterostructure