Spin-rotation symmetry breaking in the superconducting state of CuxBi2Se3

Abstract

Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break further symmetries. In particular, spin-rotational symmetry is expected to be broken in spin-triplet superconductors. However, experimental evidence for such symmetry breaking has not been conclusively obtained so far in any candidate compounds. Here, using 77Se nuclear magnetic resonance measurements, we show that spin-rotation symmetry is spontaneously broken in the hexagonal plane of the electron-doped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature Tc = 3.4 K. Our results not only establish spin-triplet superconductivity in this compound, but may also serve to lay a foundation for the research of topological superconductivity.

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Figure 1: The NMR spectra of Cu0.3Bi2Se3 with the magnetic field applied in the hexagonal plane.
Figure 2: Temperature dependence of the Knight shift with the magnetic field in the plane and along the c axis.
Figure 3: Angle dependence of the Knight shift reduction below Tc.

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Acknowledgements

We thank A. Yamakage, M. Sato, L. Fu, Y. Tanaka, K. Mizushima, Y. Yanase, K. Miyake, Y. Maeno, J. P. Hu, T. Xiang and J. A. Sauls for helpful discussion, and Y. S. Hor, F. Iwase, S. Maeda and K. Ueshima for participation in the initial stage of this work. This work was supported in part by MEXT research grants (Innovative area ‘Topological Quantum Phenomena’, No. 22103004 and ‘Topological Materials Science’, No. 15H05852), JSPS grants (nos. 24540320, 25220708, 25800197, 15K05140 and 16H04016), AFOSR (AOARD 124038) and by the CAS (grant No. XDB07020200).

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G.-q.Z. conceived the project. M.K. and K.S. synthesized and characterized the electrochemically intercalated single crystals under the supervision of Y.A. K.M. and G.-q.Z. performed NMR measurements. G.-q.Z. integrated different pieces of the work and wrote the manuscript with inputs from all co-authors. All authors discussed the results and interpretation.

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Correspondence to Guo-qing Zheng.

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The authors declare no competing financial interests.

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Matano, K., Kriener, M., Segawa, K. et al. Spin-rotation symmetry breaking in the superconducting state of CuxBi2Se3. Nature Phys 12, 852–854 (2016). https://doi.org/10.1038/nphys3781

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