Incommensurate Phonon Anomaly and the Nature of Charge Density Waves in Cuprates*

While charge density wave (CDW) instabilities are ubiquitous to superconducting cuprates, the different ordering wavevectors in various cuprate families have hampered a unified description of the CDW formation mechanism. Here we investigate the temperature dependence of the low energy phonons in the canonical CDW ordered cuprate La_1.875Ba_0.125CuO₄. We discover that the phonon softening wavevector associated with CDW correlations becomes temperature dependent in the high-temperature precursor phase and changes from a wavevector of 0.238 reciprocal space units (r.l.u.) below the ordering transition temperature up to 0.3 r.l.u. at 300 K. This high-temperature behavior shows that ``214''-type cuprates can host CDW correlations at a similar wavevector to previously reported CDW correlations in non-``214'' type cuprates such as YBa₂Cu₃O_6+x. This indicates that cuprate CDWs may arise from the same underlying instability despite their apparently different low temperature ordering wavevectors.