Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high-Tc) superconductors is one of the deepest problems in condensed matter physics. One candidate is the 'stripe' phase1-3, in which the charge carriers (holes) condense into rivers of charge that separate regions of antiferromagnetism. A related but lesser known system is the 'spin ladder', which consists of two coupled chains of magnetic ions forming an array of rungs. A doped ladder can be thought of as a high-Tc material with lower dimensionality, and has been predicted to exhibit both super-conductivity 4-6 and an insulating 'hole crystal'4,7,8 phase in which the carriers are localized through many-body interactions. The competition between the two resembles that believed to operate between stripes and superconductivity in high-Tc materials9. Here we report the existence of a hole crystal in the doped spin ladder of Sr 14Cu24O41 using a resonant X-ray scattering technique10. This phase exists without a detectable distortion in the structural lattice, indicating that it arises from many-body electronic effects. Our measurements confirm theoretical predictions4,7,8, and support the picture that proximity to charge ordered states is a general property of superconductivity in copper oxides.
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