Coexistence of magnetism and superconductivity in ultraclean underdoped YBa2Cu3O6.37

Muon spin rotation and magnetization measurements in ultraclean single crystals of superconducting YBa2Cu3O6+x (x=0.375) reveal transitions at T=19 K to a bulk superconducting state and below T=15 K to a disordered antiferromagnetic state. The superconducting state pins magnetic flux. At the lowest temperatures an internal magnetic field with a magnitude close to that seen in the antiferromagnetic parent compound is present throughout the bulk of the sample. The two states coexist on a nanometer length scale, and over a narrow region of oxygen doping.