  Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method,
and the crystals were quenched in ice-water rapidly after high temperature
growth. The quenched crystal undergoes a collapsed tetragonal structural phase
transition around 80 K revealed by the temperature dependent X-ray diffraction
measurements. Superconductivity below 25 K was observed in the collapsed phase
by resistivity and magnetization measurements. The isothermal magnetization
curve measured at 2 K indicates that this is a typical type-II superconductor.
For comparison, we systematically characterized the properties of the furnace
cooled, quenched, and post-annealed single crystals, and found strong internal
crystallographic strain existing in the quenched samples, which is the key for
the occurrence of superconductivity in the undoped CaFe2As2 single crystals.
