Unusual Interplay between Superconductivity and Field-Induced Charge Order in YBa2Cu3 Oy

We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field independent above a characteristic field HDOS, and that the HDOS(T) line displays an unusual inflection near the onset of the long-range 3D charge-density wave order. The unusual S shape of HDOS(T) is suggestive of two mutually exclusive orders that eventually establish a form of cooperation in order to coexist at low T. On theoretical grounds, such a collaboration could result from the stabilization of a pair-density wave state, which calls for further investigation in this region of the phase diagram.