  We report a two-color magneto-optical trap (MOT) for ytterbium atoms
operating at a low magnetic field gradient down to 2 G/cm where a conventional
MOT using the singlet transition (6s^2 1S0 -> 6s6p 1P1) is unable to trap
atoms. By simultaneously applying laser light on both the broad-linewidth
singlet transition and the narrow-linewidth triplet transition (6s^2 1S0 ->
6s6p 3P1), we load and trap 4.0 x 10^5 atoms directly from an atomic beam at
700 K. In the two-color MOT, the slowing and trapping functions are separately
performed by the singlet transition light and the triplet transition light,
respectively. The two-color MOT is highly robust against laser power imbalance
even at very low magnetic field gradients.
