  A pivotal ongoing debate about cuprate superconductors (HTS) is the location
of the transition temperatures for the superconducting wave function phase
coherence and condensation, Tphase and Tcond. This shall elucidate which of two
very different interactions dictate the macroscopic superconducting phase
diagram of HTS: either those between normal-state carriers or those between
preformed vortices and antivortices. Here, we present unambiguous experimental
determinations of Tphase and Tcond in the prototypical HTS La2-xSrxCuO4 as a
function of the doping level x. Tphase is measured as a sharp change in the
exponent alpha of the voltage-current characteristics (V proportional to
I^alpha). Tcond is determined from the critical rounding of the ohmic
resistivity above Tphase. Our measurements indicate that the transition to
macroscopic superconductivity is accompanied by phase coherence due to
vortex-antivortex binding and also that, for all x, Tcond lies only a few
Kelvin above Tphase, limiting then the shift of the transition due to
vortex-antivortex correlations.
