  Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism
of superconductivity for underdoped cuprates. The gluing force of the
superconducting mechanism is an attraction between spin vortices on two
different N\'eel sublattices, centered around the empty sites described in
terms of fermionic holons. The spin fluctuations are described by bosonic
spinons with a gap generated by the spin vortices. Due to the no-double
occupation constraint, there is a gauge attraction between holon and spinon
binding them into a physical hole. Through gauge interaction the spin vortex
attraction induces the formation of spin-singlet (RVB) spin pairs with a
owering of the spinon gap. Lowering the temperature the approach exhibits two
crossover temperatures: at the higher crossover a finite density of incoherent
holon pairs are formed leading to a reduction of the hole spectral weight, at
the lower crossover also a finite density of incoherent spinon RVB pairs are
formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic
vortices appear in the plasma phase. Finally, at a even lower temperature the
hole pairs become coherent, the magnetic vortices become dilute and
superconductivity appears. The superconducting mechanism is not of BCS-type
since it involves a gain in kinetic energy (for spinons) coming from the spin
interactions.
