Temperature dependence of the local structure of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$

The local structure of superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ is studied by atomic pair-distribution analysis of pulsed-neutron-scattering data. Collective displacements of chain oxygen atoms forming polarized microdomains are found at low temperatures, but the domains disappear above the spin-gap temperature. The size of the domains is smaller than 20 Å. These observations suggest an active role of the lattice in charge localization and formation of the spin gap.