  We measured the activation energy gap of the N = 2 bubble states using
tilted-field technique in two GaAs/AlGaAs samples with Corbino geometry, and
observed monotonically decreasing energy gaps with the increasing total
magnetic field. The observed tilt angle dependence is discussed based on
magneto-orbital coupling and finite-size effect. We also found a bubble state
at an uncommon filling factor of 4.67, with an unanticipated energy gap of up
to 5.35 K and opposite in-plane field dependence.
