Thermodynamic evidence for nematic superconductivity in   Cu$_x$Bi$_2$Se$_3$
  Unconventional superconductivity is characterized by the spontaneous symmetry breaking of the macroscopic superconducting wavefunction in addition to the gauge symmetry breaking, such as rotational-symmetry breaking with respect to the underlying crystal-lattice symmetry. Particularly, superconductivity with spontaneous rotational-symmetry breaking in the wavefunction amplitude and thus in bulk properties, not yet reported previously, is intriguing and can be termed "nematic" superconductivity in analogy to nematic liquid-crystal phases. Here, based on specific-heat measurements of the single-crystalline Cu$_x$Bi$_2$Se$_3$ under accurate magnetic-field-direction control, we report thermodynamic evidence for nematic superconductivity, namely, clear two-fold-symmetric behavior in a trigonal lattice. The results indicate realization of an "odd-parity nematic" state, feasible only by macroscopic quantum condensates and distinct from nematic states in liquid crystals. The results also confirm topologically non-trivial superconductivity in Cu$_x$Bi$_2$Se$_3$. 
Shingo Yonezawa, Kengo Tajiri, Suguru Nakata, Yuki Nagai, Zhiwei Wang,   Kouji Segawa, Yoichi Ando, Yoshiteru Maeno
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.soft cond-mat.str-el