1. National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, USA
2. Department of Physics and Astronomy, SUNY Stony Brook, Stony Brook, New York 11794, USA
3. Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA
4. University of Groningen, 9747 AG Groningen, The Netherlands
5. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
6. Department of Materials Science & Engineering, University of Wisconsin, Madison, Wisconsin 53706, USA
7. Nanoelectronics Research Institute, AIST, 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki, 305-8568, Japan
8. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA
9. Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T-1Z1, Canada

Correspondence to: P. Abbamonte^1,2 Email: abbamonte@bnl.gov

Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high-T _c) superconductors is one of the deepest problems in condensed matter physics. One candidate is the 'stripe' phase^{1, 2, 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-T _c material with lower dimensionality, and has been predicted to exhibit both superconductivity^{4, 5, 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-T _c materials⁹. Here we report the existence of a hole crystal in the doped spin ladder of Sr₁₄Cu₂₄O₄₁ 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 predictions^{4, 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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