Spectral signatures of Holstein polarons

1 . 1. Self-trapping phenomenon. – Electrons or holes delocalised in a perfect rigid lattice can be “trapped” in a potential well produced by displacements of atoms if the particle-lattice interaction is sufficiently strong [1]. Such trapping is energetically favoured over wide-band Bloch states if the carrier’s binding energy exceeds the strain energy required to produce the trap. Since the potential itself depends on the carrier state, this highly non-linear process is called “self-trapping” (Fig. 1). A self-trapped state is referred to as “large” if it extends over multiple lattice sites. Alternatively, for a quasi-particle (QP) with extremely large effective mass m – practically confined to a single site – the state is designated as “small”. Self-trapping does not imply a breaking of translational invariance, i.e., in a crystal these eigenstates are still itinerant allowing, in principle, for coherent transport with an extremely small bandwidth. Introducing the concept of polarons into physics, the possibility of electron selftrapping was pointed out by Landau as early as 1933 [2]. Self-trapped polarons consisting of electrons accompanied by phonon clouds can be found, e.g., in alkali(ne) metal (earth) halides, II-IVand group-IV semiconductors, and organic molecular crystals [3]. With the