Hole Injection Induces Local Motion of Surface Atoms through Electron-lattice Interaction
Komori Group
Strong excitation of local vibration can induce the motions of atoms and molecules in various materials. The most effective way to realize the highly excited vibration is to convert electronic excitation to vibrational one through the coupling between them. Moreover, the electronic excitation at extended states can realize a nonlocal manipulation of the atoms and molecules. At the clean Ge(001) surface, the carriers injected into the surface states reversibly and nonlocally alter the tilting orientation of the Ge dimers [1]. The change rate linearly depends on the tunneling current for both the electron and hole injections, indicating the single electron excitation process. Using scanning tunneling microscopy (STM) and density functional theory, we have studied the microscopic processes of this nonlocal atom manipulation through the surface electronic excitation induced by hole injections [2] .
Figure 1 shows STM images and models illustrating the change of the dimer tilting orientation. We can continuously monitor the local structure change as the tunneling current increase or decrease when the STM tip is fixed. Actually, the creation of the kink pair shown in Fig. 1(b) was detected at the positions above the points A-D as different values of the tunneling current. With increasing the excitation energy of the injected hole, the kink creation rate per electron first increases, once saturates, and then increases again as shown in Fig. 2(a). The electronic states (Fig. 2(b)) and their coupling to two rocking modes of the adjacent dimer pair, out-of phase and in phase as illustrated in Fig. 2(c), were calculated for understanding the observed rate of the kink creation. The both modes strongly couple the unoccupied π*, occupied π and σ states of the dimers, and the occupied dimer back-bond states. The dimer orientation change is mainly induced by the carriers injected to the π* and π states. The observed two-step increase of the creation rate by the hole injection is attributed to the crossover of the electronic states from the π to σ. The hole injected to the B1 state induces the rocking mode vibration, but its energy is not enough to induce the change of the tilting orientation.
References
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