Locating the pseudogap closing point in cuprate superconductors: Absence of entrant or reentrant behavior

Many current descriptions of the pseudogap in underdoped cuprates envision a doping-dependent transition line T*(p) which descends monotonically toward zero just beyond optimal doping. There is much debate as to the location of the terminal point p* where T*(p) vanishes, whether or not there is a phase transition at T* and exactly how T*(p) behaves below T-c within the superconducting dome. One perspective sees T*(p) cutting the dome and continuing to descend monotonically to zero at p* approximate to 0.19 holes/Cu-referred to here as entrant behavior. Another perspective derived from photoemission studies is that T*(p) intersects the dome near p 0.23 holes/Cu then turns back below T-c, falling to zero again around p* approximate to 0.19-referred to here as reentrant behavior. By examining field-dependent thermodynamic data for Bi2Sr2CaCu2O8+delta we show that neither entrant nor reentrant behavior is supported. Rather, p* sharply delimits the pseudogap regime: For p < 0.19 the pseudogap is always present, independent of T. Similar results are found for Y0.8Ca0.2Ba2Cu3O7-delta. For both materials, T* (p) is not a temperature but a crossover scale, approximate to E*(p)/2k(B), reflecting instead the underlying pseudogap energy E*(p) which vanishes as p -> p* approximate to 0.19.