  We analyse the dynamics of quantum correlations between two qubits coupled to
a linear chain of oscillators. The chain mediates interactions between the
qubits and acts as a non-Markovian reservoir. The the model is amenable to an
analytical solution when the initial state of the chain is Gaussian}. We study
the dynamics of the qubits concurrence starting from a separable state and
assuming that the chain spectrum is gapped {and the chain is initially in a
thermal state. We identify three relevant regimes that depend on the strength
of the qubit-chain coupling in relation to the spectral gap. These are (i) the
weak coupling regime, where the qubits are entangled at the asymptotics; (ii)
the strong coupling regime, where the concurrence can exhibit collapses
followed by revivals with exponentially attenuated amplitude; and (iii) the
thermal damping regime, where the concurrence rapidly vanishes due to the
chain's thermal excitations. In all cases, if entanglement is generated, this
occurs after a finite time has elapsed. This time scale depends exponentially
on the qubits distance and is determined by the spectral properties of the
chain. Entanglement irreversible decay, on the other hand, is due to the
dissipative effect induced by the coupling with the chain and is controlled by
the coupling strength between the chain and qubits. This study unravels the
basic mechanisms leading to entanglement in a non-Markovian bath and allows to
identify the key resources for realising quantum coherent dynamics of open
systems.
