We analyze the phase diagram of single and bi-layer cuprates using the gauge-field description of the t-J model. For $T>T_{ \text{ BE}}$ the in-plane fermion-pairing order parameter $\Delta_{ \scriptscriptstyle\parallel}$ is eliminated by gauge field fluctuations, leading us to predict the absence of a spin-gap phase in single-layer cuprates. For bi-layer cuprates the inter-layer order parameter $\Delta_\perp$ is enhanced by spin correlations, and is less affected by gauge fluctuations. We believe that the spin gap in bi-layer materials is due to inter-layer fermion pairing, and that for these underdoped samples the superconducting gap may be nodeless.
Categories: cond-mat