  Due to competing long range ferromagnetic order, the transition metals Fe, Co
and Ni are not superconductors at ambient pressure. While superconductivity was
observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to
be discovered in Co and Ni under any experimental conditions. Here, we report
emergence of superconductivity in the recently discovered high-density
nonmagnetic face centered cubic phase in Co thin films below a transition
temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact
spectroscopy and resistance, and four-probe measurements of resistance at
ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of
Co within first-principles density functional theory, and show that its
superconductivity below 5 K originates from anomalous softening of
zone-boundary phonons and their enhanced coupling with electrons upon biaxial
strain.
