Experimental Evidence for Quantum Interference and Vibrationally Induced Decoherence in Single-Molecule Junctions

We analyze quantum interference and decoherence effects in single-molecule junctions both experimentally and theoretically by means of the mechanically controlled break junction technique and density-functional theory. We consider the case where interference is provided by overlapping quasidegenerate states. Decoherence mechanisms arising from electronic-vibrational coupling strongly affect the electrical current flowing through a single-molecule contact and can be controlled by temperature variation. Our findings underline the universal relevance of vibrations for understanding charge transport through molecular junctions.

Figure 1

(a) Schematic representation of a single-molecule junction with two QDSs that are close in energy ($\Delta ϵ$) and different in their spatial symmetry. (b) In the resonant tunneling model including vibrationally induced decoherence [19], the current plateaus $I(T)$ are sensitive to vibrational excitation, which can be tuned by temperature. $I(T)$ is therefore a suitable observable to investigate vibrationally induced decoherence.

Figure 2

Overview of the molecular systems investigated.

Figure 3

$I\mathrm{\text{−}}V$ and $I\mathrm{\text{−}}T$ characteristics (a),(b) for gold electrodes (blind experiment), (c) for molecule $\mathbf{1}$, and (d) for molecules $\mathbf{1}–\mathbf{3}$, respectively. The temperature-dependent current $I(T)$ recorded at the first plateau is the key observation indicating vibrationally induced decoherence. (e) Molecular junction $\mathbf{3}$ provides a pair of QDSs located next to the Fermi level. The corresponding wave functions differ in symmetry. (f) Transmission functions for electron transport through molecular junction $\mathbf{3}$. The individual (broadened) peaks associated with the QDSs 1 and 2 at $ϵ-{ϵ}_{\mathrm{F}}\simeq -1.7\mathrm{eV}$ are relatively strong, but their phase-correct transmission (states $1+2$) shows only a very small peak at this energy due to destructive interference. When including all states, this peak is shifted to $-1.0\mathrm{eV}$ below ${ϵ}_{\mathrm{F}}$.

Figure 4

(a) Transmission function for electronic transport through molecular junction $\mathbf{4}$. The black curve is the superposition of the three individual contributions. In this molecular junction, the states close to the Fermi energy are not quasidegenerate and no destructive interference occurs. (b),(c) $I\mathrm{\text{−}}V$ and $I\mathrm{\text{−}}T$ characteristics upon stretching of molecular junctions $\mathbf{3}$ and $\mathbf{4}$, respectively.