# Metallicity without quasi-particles in room-temperature strontium titanate

## TL;DR

This paper reveals that strontium titanate exhibits strange metallic behavior at room temperature, with resistivity exceeding the Mott-Ioffe-Regel limit, and proposes a novel interpretation involving Landauer transmission and polarons.

## Contribution

It introduces the concept of metallicity without quasi-particles in strontium titanate and links resistivity behavior to Landauer transmission and large polarons, challenging existing theories.

## Key findings

- Resistivity follows T^3 behavior above 100K and T^2 at low temperatures.
- Resistivity exceeds the Mott-Ioffe-Regel limit, indicating strange metallicity.
- A scaling relationship connects mobility, permittivity, and soft mode frequency.

## Abstract

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 T$^{3}$ behavior before becoming T$^{2}$ in the low-temperature limit, as expected in a Fermi liquid. Here, we show that the roughly cubic resistivity above 100K 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 T$^{3}$-resistivity cannot be accounted for by electron-phonon scattering \`{a} la Bloch-Gruneisen and consider an alternative scheme based on Landauer transmission between individual dopants hosting large polarons. We find a scaling relationship between the 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.