Linear-in temperature resistivity from an isotropic Planckian scattering rate.

A variety of 'strange metals' exhibit resistivity that decreases linearly with temperature as the temperature decreases to zero, in contrast to conventional metals where resistivity decreases quadratically with temperature. This linear-in-temperature resistivity has been attributed to charge carriers scattering at a rate given by ħ/τ = αkT, where α is a constant of order unity, ħ is the Planck constant and k is the Boltzmann constant. This simple relationship between the scattering rate and temperature is observed across a wide variety of materials, suggesting a fundamental upper limit on scattering-the 'Planckian limit'-but little is known about the underlying origins of this limit.