Physicists have long puzzled over a strange contradiction inside a family of minerals called rutile oxides. These materials all share the same crystal structure—but while some of them, like titanium dioxide, are firmly insulating, others, like ruthenium dioxide, conduct electricity like a metal. So far, physicists have had little idea of why this happens.

In a new study published in Physical Review B, researchers led by Kaushik Sen at the Indian Institute of Technology Delhi traced the answer back to phonons: the tiny vibrations that ripple through a material’s atomic lattice.

Their discovery reveals that metallic rutile oxides develop a fundamentally different relationship between electrons and phonons as they cool—settling a long-running scientific dispute along the way.