Stripes in quantum Hall double-layer systems

We present results of a study of double-layer quantum Hall systems in which each layer has a high-index Landau level that is half filled. Hartree-Fock calculations indicate that, above a critical layer separation, the system becomes unstable to the formation of a unidirectional coherent charge densi ty wave (UCCDW), which is related to stripe states in single-layer systems. The UCCDW state supports a quantized Hall effect when there is tunneling b etween layers, and is always stable against formation of an isotropic Wigne r crystal for Landau indices N greater than or equal to1. The state does be come unstable to the formation of modulations within the stripes at large e nough layer separation. The UCCDW state supports low-energy modes associate d with interlayer coherence. The coherence allows the formation of charged soliton excitations, which become gapless in the limit of vanishing tunneli ng. We argue that this may result in a critical Hall state, characterized b y a power law I-V characteristic in tunneling experiments.