Department of Physics. Colloquium Series

Speaker: Guo-qing Zheng;

Department of Physics,

Okayama University, Japan

Wednesday, September 6, 2017, 4:00PM

Higgins 310

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Topological Superconducting States Revealed by NMR

 

Topological Insulators (TIs) are materials in which the bulk is insulating but the surface hosts gapless metallic states due to non-zero topological invariants of the bulk band structure. A topological superconductor is analogous to a TI in that the superconducting gap function has a nontrivial topological invariant. The gapless quasiparticle states on the surface (called Majorana fermions) of a topological superconductor have potential applications in topological quantum computing.

Although a great success has been achieved in the study of topological insulators, progress in establishing bulk topological superconductivity has been slow until recently [1]. In this colloquium, I will present some of our results in the quest of topological superconductivity in doped topological insulators Cu_xBi₂Se₃ [1] and Sn_1-xIn_xTe [2,3], as well as in strongly-correlated superconductors (K,Rb)₂Cr₃As₃ [4].

In Cu_0.3Bi₂Se₃,  our ⁷⁷Se nuclear magnetic resonance (NMR) measurements indicate that spin rotation symmetry is spontaneously broken in the hexagonal plane below the superconducting transition temperature Tc=3.4 K [1]. This is the first evidence for such symmetry breaking found in any superconductors. Our results not only establish spin-triplet (odd parity) superconductivity in this compound, but also show that it is a topological superconductor belonging to DIII class. I will also discuss our most recent effort to obtain single crystals of Cu_xBi₂Se₃ with various doping contents.

 In Rb₂Cr₃As₃  (T_c=4.8 K ), we found strong ferromagnetic spin fluctuations in the normal state and point nodes in the superconducting gap [4], which suggest that this compound may be a solid-state analogue of superfluid ³He.

 

[1] K. Matano, M. Kriener, K. Segawa, Y. Ando and G.-q. Zheng, Nature Physics 12, 852 (2016).

[2] S. Maeda, S. Katsube, and G.-q. Zheng, J. Phys. Soc. Jpn. 86, 024702 (2017)

[3] S. Maeda, R. Hirose, K. Matano, M. Novak, Y. Ando, G.-q. Zheng, arXiv:1705.08636

[4] J. Yang, Z. T. Tang, G. H. Cao and G.-q. Zheng, Phys. Rev. Lett. 115, 147002 (2015).