Metallicity without quasi-particles in room-temperature strontium titanate

Cooling oxygen-deficient strontium titanate to liquid-helium temperature leads to a decrease in its electrical resistivity by several orders of magnitude. The temperature dependence of resistivity follows a rough T3 behavior before becoming T2 in the low-temperature limit, as expected in a Fermi liquid. Here, we show that the roughly cubic resistivity above 100 K corresponds to a regime where the quasi-particle mean-free-path is shorter than the electron wave-length and the interatomic distance. These criteria define the Mott-Ioffe-Regel limit. Exceeding this limit is the hallmark of strange metallicity, which occurs in strontium titanate well below room temperature, in contrast to other perovskytes. We argue that the T3-resistivity cannot be accounted for by electron-phonon scattering à la Bloch–Gruneisen and consider an alternative scheme based on Landauer transmission between individual dopants hosting large polarons. We find a scaling relationship between carrier mobility, the electric permittivity and the frequency of transverse optical soft mode in this temperature range. Providing an account of this observation emerges as a challenge to theory. An expected metallic behavior that defies existing theory is observed by researchers in France. Xiao Lin from the Laboratoire Physique et Etude de Matériaux and co-workers show that room temperature strontium titanate behaves like a so-called ‘bad’ metal. Particles with an electric charge can flow through a metal with little resistance. But this resistivity increases with particle density. In conventional metals, this increase eventually stops at what is known as the Mott-Ioffe-Regel limit. Bad or strange metals, however, defy this rule and the resistivity continues to increase. This effect is thought to arise when the when the electrons work collectively, creating quasi-particles. Lin and colleagues show that this bad metal behavior can occur even in a material without quasi-particles. Instead, they argue the metallicity is caused by so-called Landauer transmission between individual dopants.