Absence of a Spin Gap in the Superconducting Ladder Compound Sr 2 Ca 12 Cu 24 O 41

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Abstract
Transport and copper-63 nuclear magnetic resonance measurements of the Knight shift and relaxation time T 1 performed on the two-leg spin ladders of Sr 2 Ca 12 Cu 24 O 41 single crystals as a function of pressure show a collapse of the gap in ladder spin excitations when superconductivity is stabilized at 31 kilobars. This result suggests that the superconducting phase in these materials may be connected to this transition and the collapse of the spin gap, and support the prediction made with exact diagonalization techniques in two-leg isotropic t − J ladder models of a transition between a low-doping spin gap phase and a gapless regime.
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