Direct measurement of the upper critical field in cuprate superconductors

In the quest to increase the critical temperature T(c) of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field H(c2) is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect H(c2) in the cuprates YBa(2)Cu(3)O(y), YBa(2)Cu(4)O(8) and Tl(2)Ba(2)CuO(6+δ), allowing us to map out H(c2) across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa(2)Cu(3)O(y) is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from H(c2), suffers a sudden 20-fold collapse. This reveals that phase competition—associated with Fermi-surface reconstruction and charge-density-wave order—is a key limiting factor in the superconductivity of cuprates.