1. Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, UK
2. Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, UK
3. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK

Correspondence to: J. P. Attfield² Correspondence and requests for materials should be addressed to J.P.A. (Email: j.p.attfield@ed.ac.uk).

The mechanism of high-transition-temperature (high-T _c) superconductivity in doped copper oxides is an enduring problem. Antiferromagnetism is established as the competing order^{1, 2}, but the relationship between the two states in the intervening 'pseudogap' regime has become a central puzzle³. The role of the crystal lattice, which is important in conventional superconductors, also remains unclear. Here we report an anomalous increase of the distance between copper oxide planes on cooling, which results in negative thermal volume expansion, for layered ruthenium copper oxides^{4, 5} that have been doped to the boundary of antiferromagnetism and superconductivity. We propose that a crossover between these states is driven by spin ordering in the ruthenium oxide layers, revealing a novel mechanism for negative lattice expansion in solids. The differences in volume and lattice strain between the distinct superconducting and antiferromagnetic states can account for the phase segregation phenomena found extensively in low-doped copper oxides, and show that Cooper pair formation is coupled to the lattice. Unusually large variations of resistivity with magnetic field are found in these ruthenium copper oxides at low temperatures through coupling between the ordered Ru and Cu spins.

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