  Electronic phases with symmetry properties matching those of conventional
liquid crystals have recently been discovered in transport experiments on
semiconductor heterostructures and metal oxides at milli-Kelvin temperatures.
We report the spontaneous onset of a onedimensional, incommensurate modulation
of the spin system in the high-temperature superconductor YBa2Cu3O6.45 upon
cooling below ~150 K, while static magnetic order is absent above 2 K. The
evolution of this modulation with temperature and doping parallels that of the
in-plane anisotropy of the resistivity, indicating an electronic nematic phase
that is stable over a wide temperature range. The results suggest that soft
spin fluctuations are a microscopic route towards electronic liquid crystals,
and nematic order can coexist with high-temperature superconductivity in
underdoped cuprates.
