  In this letter we report on the generation of cold electron beams using a
Cs3Sb photocathode grown by co-deposition of Sb and Cs. By cooling the
photocathode to 90 K we demonstrate a significant reduction in the mean
transverse energy validating the long standing speculation that the lattice
temperature contribution limits the mean transverse energy or thermal emittance
near the photoemission threshold, opening new frontiers in generating
ultra-bright beams. At 90 K, we achieve a record low thermal emittance of 0.2
$\mu$m (rms) per mm of laser spot diameter from an ultrafast (sub-picosecond)
photocathode with quantum efficiency greater than $7\times 10^{-5}$ using a
visible laser wavelength of 690 nm.
