Jahn-Teller Solitons, Structural Phase Transitions, and Phase Separation

It is demonstrated that under common conditions a molecular solid subject to Jahn-Teller interactions supports stable $Q$-ball-like nontopological solitons. Such solitons represent a localized lump of excess electric charge in periodic motion accompanied by a time-dependent shape distortion of a set of adjacent molecules. The motion of the distortion can correspond to a true rotation or to a pseudorotation about the symmetric shape configuration. These solitons are stable for Jahn-Teller coupling strengths below a critical value; however, as the Jahn-Teller coupling approaches this critical value, the size of the soliton diverges signaling an incipient structural phase transition. The soliton phase mimics features commonly attributed to phase separation in complex solids.

Figure 1

Electron charge density profile for a Jahn-Teller soliton. Local distortions create an attractive potential well that enhances the electron charge density. Friedel oscillations in the charge density set in beyond the distorted region $r>r_0$.

Figure 2

(a) Variational energy of Jahn-Teller soliton $E$ versus coupling constant $g$. The soliton energy behaves as $\sqrt{g_c^2-g^2}/g$, thus vanishing as $g\to g_c$. The vanishing soliton energy signals the onset of a structural phase transition. (b) Sketch of $U(\xi )$ for several values of $g$. At $g_c$, $U$ vanishes at the local minimum at ${\xi }_0=|{\kappa }_3|/2{\kappa }_4$.