Charge Fluctuations in YBa₂Cu₃O₇₋ₓ High-Temperature Superconductors

Understanding the behaviour of the electrons in the high-temperature copper oxide superconductors remains a challenging problem. An important class of models argues that the distribution of electronic charge and spin is not homogeneous: rather, spin and charge adopt a dynamic arrangement in which the spins on the copper form antiferromagnetic stripes, separated by domain walls containing the charge. The dynamic behaviour of the spins has been extensively studied by neutron scattering, and recent results have shown that the low-frequency fluctuations for different classes of materials display a universal spatial behaviour that is consistent with the stripe picture. But arguments for the existence of the stripe phases are difficult to sustain without a demonstration that charge is distributed in the domain walls. Here we report phonon measurements for the YBa ₂ Cu ₃ O _7-x high-temperature superconductors, which reveal the presence of charge fluctuations. The inferred periodicity is that expected if the charge is located in the domain walls separating the spin stripes. Our results therefore provide strong support for the existence of a dynamic stripe phase in the high-temperature superconductors.