Phenomenological theory of the underdoped phase of a high-T$_c$   superconductor
  We model the Fermi surface of the cuprates by one-dimensional nested parts near $(0,\pi)$ and $(\pi,0)$ and unnested parts near the zone diagonals. Fermions in the nested regions form 1D spin liquids, and develop spectral gaps below some $\sim T^*$, but superconducting order is prevented by 1D phase fluctuations.   We show that the Josephson coupling between order parameters at $(0,\pi)$ and $(\pi,0)$ locks their relative phase at a crossover scale $T^{**}< T^*$. Below $T^{**}$, the system response becomes two-dimensional, and the system displays Nernst effect. The remaining total phase gets locked at $T_c < T^{**}$, at which the system develops a (quasi-) long-range superconducting order. 
A. M. Tsvelik and A. V. Chubukov