  We investigate a long-range interaction between $64D_{5/2}$ Rydberg-atom
pairs and antiblockade effect employing a two-color excitation scheme. The
first color (pulse A) is set to resonantly excite the Rydberg transition and
prepare a few seed atoms, which establish a blockade region due to strong
long-range interaction between Rydberg-atom pairs. The second color (pulse B)
is blue detuned relative to Rydberg transition and enables further Rydberg
excitation of atoms by counteracting the blockade effect. It is found that a
few seed atoms lead to a huge difference of the Rydberg excitation with pulse
B. The dynamic evolution of antiblockade excitation by varying the pulse-B
duration at 30-MHz blue detuning is also investigated. The evolution result
reveals that a small amount of seed atoms can trigger an avalanche Rydberg
excitation. A modified superatom model is used to simulate the antiblockade
effect and relevant dynamic evolution. The simulations are consistent with the
experimental measurements.
