Polaron mobility in the ``beyond quasiparticles" regime*

In a number of physical situations, from polarons, to non-Fermi liquids, to Dirac liquids,
one often faces a problem of computing particle mobility/conductivity when conditions for
applicability of the kinetic equation approach are violated. The corresponding
``beyond quasiparticles" (or ``overdamped") regime is formally identified as a state in which
an inelastic scattering time (computed formally within the standard perturbative approach)
is exceeding the typical thermal energy of quasiparticles. We employ the Diagrammatic Monte
Carlo method to study mobility of Frohlich polarons in the overdamped regime and find
two effects: a substantial delay in developing an exponential law at temperatures below
the optical mode frequency, and a mobility minimum at T close to optical ode frequency.
Both effects should be taken into account in interpreting mobility data in materials with strong
electron-phonon coupling.