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PUBLISHED ONLINE: 30 MAY 2016 | DOI: http://dx.doi.org/10.1038/nphys3781
Web End =10.1038/NPHYS3781
Spin-rotation symmetry breaking in the superconducting state of CuxBi2Se3
K. Matano1, M. Kriener2, K. Segawa2, Y. Ando2,3 and Guo-qing Zheng1,4*
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.
In most superconductors including copper oxide and iron pnictide high-temperature superconductors, the electron pairs (Cooper pairs) are in the spin-singlet state with total spin S=0 (refs 1,2).
Spin-triplet superconductivity or fluidity with S=1 possesses
internal structure, which gives rise to rich physics such as further symmetry breaking, order parameter collective modes and multiple phases of the condensate. This is best illustrated by superfluid 3He, where chargeless 3He atoms form spin-triplet pairs3,4. One of the important properties associated with spin-triplet condensates is spin-rotational symmetry breaking. In superfluid 3He, however, as there is no interaction to dictate that the paired spins point to a particular direction, the system preserves spin-rotation symmetry.In superconductors such as UPt3 (ref. 5) or Sr2RuO4 (ref. 6), it has been believed that the Cooper pairs are in the spin-triplet state.Therefore, evidence for possible spin-rotation symmetry breaking had been actively searched for, as in solids the spin pairs can favour a particular direction of the underlying lattice. Unfortunately, all such eorts have been unsuccessful7,8, which has been interpreted as due to a weak spinorbit coupling acting on the Cooper pairs9.
This greatly hindered the firm establishment of the physics of spin-triplet superconductivity in these compounds5,10.
Topological insulators (TIs) are a new class of quantum materials with strong spinorbit coupling (SOC) and are characterized by topological invariants of the valence...