Abstract

One of the leading issues in the understanding of high-T_c superconductivity is the origin of the pseudo-gap phase. This phenomenon occurs typically in the low hole doping side (underdoped region) of the copper oxide superconductoring phase diagram. This phase is remarkable as all physical properties display, below a characteristic temperature T^*, anomalies from the behavior expected for a standard metal, although no symmetry breaking has been evidenced so far. The pseudo-gap state is usually described in two ways: either it represents a precursor of the superconducting (SC) state or it is related to an order parameter competing with the SC one. Using polarized elastic neutron diffraction, we report signature of a spontaneous magnetic order in YBa_2Cu_3O_{6+x} only in its underdoped phase. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudo-gap phase in high-T_c cuprates. Magnetic intensity occurs only on top of nuclear Bragg peaks with no additionnal Bragg peaks indicating no translational breaking symmetry of the lattice. The magnetic order basically exhibits the pattern of the orbital moments expected from the circulating currents (CC) theory of the pseudo-gap state, being more specifically only compatible with the CC phase with two currents per CuO_2 plaquette.