Intra-unit-cell magnetic correlations near optimal doping in YBa$_2$Cu$_3$O$_{6.85}$
Résumé
The pseudo-gap phenomenon in copper oxide superconductors is central to any description
of these materials as it prefigures the superconducting state itself. A magnetic intra-unit-cell
order was found to occur just at the pseudo-gap temperature in four cuprate high-Tc
superconducting families. Here we present polarized neutron-scattering measurements of
nearly optimally doped YBa2Cu3O6.85, carried out on two different spectrometers, that reveal
several features. The intra-unit-cell order consists of finite-sized planar domains that are very
weakly correlated along the c axis. At high temperature, only the out-of-plane magnetic
components correlate, indicating a strong Ising anisotropy. An aditional in-plane response
develops at low temperature, giving rise to an apparent tilt of the magnetic moment. The
discovery of these two regimes puts stringent constraints, which are tightly bound to the
pseudo-gap physics, on the intrinsic nature of intra-unit-cell order.
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