Calorimetric determination of the magnetic phase diagram of underdoped ortho II YBa2Cu3O6.54 single crystals

The recent discovery of a charge order in underdoped YBa(2)Cu(3)Oy raised the question of the interplay between superconductivity and this competing phase. Understanding the normal state of high-temperature superconductors is now an essential step towards the description of the pairing mechanism in those materials and determining the upper critical field is therefore of fundamental importance. We present here a calorimetric determination of the field- temperature phase diagram in underdoped YBa(2)Cu(3)Oy single crystals. We show that the specific heat saturates in high magnetic fields. This saturation is consistent with a normal state without any significant superconducting contribution and a total Sommerfeld coefficient gamma(N)similar to 6.5 +/- 1.5 mJ mol(-1) K-2 putting strong constraints on the theoretical models for the Fermi surface reconstruction.