Quasiparticle and Nonquasiparticle Transport in Doped Quantum Paraelectrics
Charge transport in doped quantum paraelectrics (QPs) presents a number of puzzles, including a
pronounced T^2 regime in the resistivity. We analyze charge transport in a QP within a model of electrons
coupled to a soft transverse optical (TO) mode via a two-phonon mechanism. For T above the soft-mode
frequency but below some characteristic scale (E_0), the resistivity scales with the occupation number of
phonons squared, i.e., as T^2. The T^2 scattering rate does not depend on the carrier number density and is not
affected by a crossover between degenerate and nondegenerate regimes, in agreement with the experiment.
Temperatures higher than E_0 correspond to a nonquasiparticle regime, which we analyze by mapping the
Dyson equation onto a problem of supersymmetric quantum mechanics. The combination of scattering by
two TO phonons and by a longitudinal optical mode explains the data quite well.