Superconductivity in Al-substituted Tantalum Germanites TaAl<i><sub>x</sub></i>Ge<sub>2－<i>x</i></sub> with a C40 Chiral Crystal Structure

We review superconductors with a chiral crystal structure （chiral superconductors）, which lack horizontal and vertical mirrors and an inversion center. Very few chiral superconductors have been reported so far; this is primarily because material design of chiral inorganic crystals still remains challenging. Therefore, we show that an element substitution of the existing chiral inorganic materials could be a useful method for the material discovery of chiral superconductors. We present a new family of chiral intermetallic superconductors TaAlxGe2－x. Magnetic susceptibility, magnetization curves, and electrical resistivity revealed type-II superconductivity, with a superconducting transition temperature Tc from 2.0 to 2.2 K in TaAlxGe2－x （x = 0.2－0.4）. An emergence of superconductivity is discussed in relation to changes in the lattice constants associated with the Al substitution. These results suggest that the C40-type chiral disilicides and digermanides TX2 （T = Nb, Ta and X = Si, Ge） are a promising family of materials for exploring chiral superconductors. Chiral superconductors could potentially have unique attractive properties, which have not been studied well in noncentrosymmetric superconductors, as chiral superconductors exhibit nontrivial physical phenomena, such as nonreciprocal electrical magnetochiral anisotropy and chirality-induced spin selectivity, in addition to the formation of unconventional Cooper pair states and enhancing the properties of superconductivity.