Data centers are becoming a big consumer of the world's electricity because of the explosive growth of their scale, which has become a serious concern. In recent years, many advanced data center network architectures have been proposed, which employ richly-connected topologies and multi-path routing to achieve high bisection bandwidth. Unfortunately, they undergo inefficient network energy usage. To address this issue, many network-wide energy conservation techniques have been proposed in the community. In the paper, we explore a new dimension in saving data center network energy, i.e., letting flows exclusively occupy link resources when transmission and using a flow preemption algorithm to schedule them. The key insight behind this design choice is that exclusively occupying link resources usually results in higher link utilization because there is no collision among the traffic flows. By combining both flow preemption and energy-aware routing, we can save much more network power than with traditional ways.