Behavior of the energy gap near a commensurate-incommensurate transition in double-layer quantum Hall systems at ν=1

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Abstract
The charged excitations in the system of the title are vortex-antivortex pairs in the spin texture described in the theory by Yang et al. which, in the commensurate phase, are bound together by a ‘‘string.’’ It is shown that their excitation energy drops as the string lengthens as 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 role of disorder in the tunneling and which appears not to produce a minimum in the excitation energy is also discussed. It is suggested that a similar transition could also occur in compressible Fermi-liquid-like states.
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