BEHAVIOR OF THE ENERGY-GAP NEAR A COMMENSURATE-INCOMMENSURATE TRANSITION IN DOUBLE-LAYER QUANTUM HALL SYSTEMS AT NU=1

The charged excitations in the system of the title are vortex-antivort ex pairs in the spin texture described in the theory by Yang ct al. wh ich, in the commensurate phase, are bound together by a ''string.'' It is shown that their excitation energy drops as the string lengthens a s the parallel magnetic field approaches the critical value, then goes up again in the incommensurate phase. This produces a sharp downward cusp at the critical point. An alternative description based on the ro le of disorder in the tunneling and which appears not to produce a min imum in the excitation energy is also discussed. It is suggested that a similar transition could also occur in compressible Fermi-liquid-lik e states.