  The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular
emphasis on the sample-quality issue. The compound has drawn attention as the
only superconductor (Tc = 1.0 K) that has been found in the family of
{\alpha}-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits
two characteristic structural transitions from the cubic pyrochlore structure,
with the inversion symmetry broken at the first one at 200 K. Recently, it has
attracted increasing attention as a candidate spin-orbit coupled metal (SOCM),
in which specific Fermi liquid instability is expected to lead to an odd-parity
order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett.
115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu:
Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016)
134512]. We review our previous experimental results in comparison with those
of other groups in the light of the theoretical prediction of the SOCM, which
we consider meaningful and helpful for future progress in understanding this
unique compound.
