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SCIENCE CHINA Physics, Mechanics & Astronomy, Volume 60, Issue 3: 037411(2017) https://doi.org/10.1007/s11433-016-0499-x

Superconductivity with two-fold symmetry in topological superconductor Sr$_{\textbf{x}}$Bi$_{\textbf{2}}$Se$_{\textbf{3}}$

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  • ReceivedDec 22, 2016
  • AcceptedDec 30, 2016
  • PublishedJan 11, 2017
PACS numbers

Abstract

Topological superconductivity is the quantum condensate of paired electrons with an odd parity of the pairing function. By using a Corbino-shape like electrode configuration, we measure the c-axis resistivity of the recently discovered superconductor Sr$_{x}$Bi$_{2}$Se$_{3}$ with the magnetic field rotating within the basal planes, and find clear evidence of two-fold superconductivity. The Laue diffraction measurements on these samples show that the maximum gap direction is either parallel or perpendicular to the main crystallographic axis. This observation is consistent with the theoretical prediction and strongly suggests that Sr$_{x}$Bi$_{2}$Se$_{3}$ is a topological superconductor.


Acknowledgment

We thank Profs. GuoQing Zheng and Liang Fu for helpful discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 0402/11534005, and 11190023), the Ministry of Science and Technology of China (Grant No. 2016YFA0300401). The work in Brookhaven was supported by the Office of Science, U.S. Department of Energy (Grant No. DE-SC0012704). J. Schneeloch and R. D. Zhong are supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.


Supplement

The supporting information is available online at phys.scichina.com and http://link.springer.com/journal/11433 The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.


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  • Figure 1

    (Color online) Superconducting properties of S1. (a) Temperature dependence of magnetic susceptibility of S1 measured with ZFC and FC processes at 3 Oe. (b) Temperature dependence of resistivity of S1 at zero field. The insets show the photograph of S1 with the electrodes (above) and the Corbino-shape like electrode configuration (below). The field is applied within the basal plane of the sample. (c) The experimental Laue diffraction patterns of the cleaved top surface of S1 (left), theoretically simulated Laue diffraction patterns (right), and the derived lattice structure of the terminated Se surface (above). (d) Angular dependence of c-axis resistivity of S1 measured at 0.5 T and 1.9 K. The inset shows the representation of the same data by polar coordinates.

  • Figure 2

    (Color online) The angular dependence of resistivity of S1-S3 measured at various fields and temperatures. (a), (c), (e) The angular dependence of c-axis resistivity measured at 1.9 K and different magnetic fields for S1 (a), S2 (c), and S3 (e). (b), (d), (f) The angular dependence of resistivity measured at 0.5 T and different temperatures for S1 (b), S2 (d), and S3 (f).

  • Figure 3

    (Color online) Superconducting transition at different magnetic fields and $H_{c2}$ anisotropy. (a), (b) The magnetic field dependence of resistivity data of S1 measured at $\theta^{\mathrm{max}}$ and $\theta^{\mathrm{min}}$. (c) Extracted upper critical fields $H_{c2}$($T$) for the two field orientations. The dots represent the experimental data and solid lines are the fitting results. (d)-(i) The similar results for S2 ((d)-(f)) and S3 ((g)-(i)).

  • Figure 4

    (Color online) Analysis of orientation of the two-fold resistivity. The blue curves in (a)-(c) are the angular dependence of the resistivity data for S1-S3 respectively, measured at 1.9 K and 0.5 T. The red lines indicate $\theta^{\min}$ direction for each sample. $\theta^{\min}$ = $176.3^{\circ}$ or –$3.7^{\circ}$ (a), $176.7^{\circ}$ or –$3.3^{\circ}$ (b), and $93.9^{\circ}$ or –$86.1^{\circ}$ (c). The illustrations of the corresponding crystal structures are also overlaid below the data in (a)-(c). The green balls here represent the Se atoms on the terminated layer.

  • Figure 5

    (Color online) Angular dependence of resistivity of Ba$_{0.65}$K$_{0.35}$Fe$_{2}$As$_{2}$ single crystal. (a) Temperature dependence of resistivity of Ba$_{0.65}$K$_{0.35}$Fe$_{2}$As$_{2}$ single crystal at zero field. The superconducting transition is very sharp and the superconducting transition temperature is about 38 K. (b) The angular dependence of resistivity measured at 15 T and different temperatures for the Ba$_{0.65}$K$_{0.35}$Fe$_{2}$As$_{2}$ single crystal.

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