Voltage-induced conformational changes and current control in charge transfer through molecules

Transport through molecular contacts with a sluggish intramolecular vibrational mode strongly coupled to excess charges is studied far from equilibrium. A Born-Oppenheimer approximation in steady state reveals voltage-dependent energy surfaces, which cause abrupt conformational changes of the molecular backbone. These are directly related to transitions between current plateaus, which are relatively robust against thermal fluctuations. In a regime accessible in experiments this allows the operation of a molecular junction as a current switch or as a molecular machine in form of a valve controlled by time-dependent bias and gate voltages.

Figure 1

Sketch of the biphenyl molecule showing the rotational angle $\phi$. Connection to the electrodes is established via the thiol groups.

Figure 2

Frank-Condon-factor ${\cos }^2\theta \left(\phi \right)$ as a function of $\phi$ and $V_s$. As described in the text, a finite electric field breaks the symmetry of the one-electron eigenstates ($\theta \left(\phi \right)=\pi /4$) and induces a preference for angles $\phi \approx \pi /2$ in the coupled system.

Figure 3

Energy surfaces of the bare molecular states $|0\rangle ,|\pm \rangle$ and the BOS $V_{\mathrm{eff}}$ vs the torsional angle $\phi$ for $V_b=0.5V$, $V_g=0.5V$. In the upper part bare molecular eigenstates are used, in the lower one a Stark shift between the left and right side of $V_S=0.3V_b$ is assumed.

Figure 4

Angle-dependent current and probability density for the $T=80\text{K}$ rotational state without (top) and with (bottom) Stark shift. Parameters are the same as in Fig. 3.

Figure 5

Net current in units of $e\Gamma$ (top) and mean torsional angle (degrees, bottom) in steady state vs gate voltage $V_g$ and bias voltage $V_b$ (in units of $V$) with $V_S=0$.

Figure 6

Current and equilibrium angle vs $V_g$ at fixed $V_b=3V$, for $\kappa =0$. Arrows denote the opening (closing) of transition channels.

Figure 7

Same as Fig. 5 but with $V_S=0.3V_b$. The vertical line at $V_b=3V$ marks the cut displayed in Fig. 8.

Figure 8

Current $\langle I\rangle$ in units of $e\Gamma$ (solid) and average twist angle $\langle \phi \rangle /\pi$ (dashed) at $V_b=3V$ vs. $V_g$ (in $V$). To the left (right) of the maximal current arrows indicate the opening (closing) of transition channels between molecular states $|0\rangle ,|\pm \rangle ,|D\rangle$.