Gap evolution in v = 1/2 bilayer quantum hall systems

Fractional quantum Hall states in bilayer systems at the total filling fraction v = 1/2 are examined numerically within some ranges of layer separation and interlayer tunneling. It is shown that the ground state changes continuously from a two-component state to a one-component state as the interlayer tunneling rate is increased, while the lowest excited state changes discontinuously. This fact explains the unusual behavior of the observed activation energies which reveals an upward cusp as a function of interlayer tunneling. Some trial wave functions for the ground state and quasihole excited states are inspected.