Lattice Polarons and Switching in Molecular Nanowires and Quantum Dots

Abstract

Conducting electrons in inorganic and organic matter interact with vibrating ions. If phonon frequencies are sufficiently low, the local deformation of ions, caused by the electron itself, creates a potential well, which traps the electron even in a perfect crystal lattice. This self-trapping phenomenon was predicted by Landau [1]. It was studied in greater detail by Pekar [2], Frohlich [3], Feynman [4], Rashba [5], Devreese [6], and other authors in the effective mass approximation for the electron placed in a continuous polarizible medium, which leads to a so-called large or continuous polaron. Large-polaron wave functions and corresponding lattice distortions spread over many lattice constants (see Fig. 8.1). The trapping is never complete in the perfect lattice. Due to finite phonon frequencies, ion polarizations follow polaron motion if the motion is sufficiently slow. Hence, large polarons with a low kinetic energy propagate through the lattice as free electrons but with an enhanced effective mass. Open image in new window FIGURE 8.1. An electron shifts the equilibrium position of Na+ and Cl− ions in the ionic lattice of and forms a large (or intermediatej-radius polaron.