ac transport in graphene-based Fabry-Pérot devices

We report on a theoretical study of the effects of time-dependent fields on electronic transport through graphene nanoribbon devices. The Fabry-Pérot interference pattern is modified by an ac gating in a way that depends strongly on the shape of the graphene edges. While for armchair edges the patterns are found to be regular and can be controlled very efficiently by tuning the ac field, samples with zigzag edges exhibit a much more complex interference pattern due to their peculiar electronic structure. These studies highlight the main role played by geometric details of graphene nanoribbons within the coherent transport regime. We also extend our analysis to noise power response identifying under which conditions it is possible to minimize the current fluctuations as well as exploring scaling properties of noise with the length and width of the systems.

Figure 1

(Color online) Top: scheme of the device considered in the text. Below, we show the low-energy dispersion and the atomic structure of (a) an armchair-edge graphene nanoribbon (AGNR) and (b) a zigzag-edge graphene nanoribbon (ZGNR). In all calculations we use $\mathcal{N}=14$ atoms along the width and lengths of $L_1=20.6\text{nm}$ or $L_2=440\text{nm}$ for the AGNR and $\mathcal{N}=10$ and $L=24.4\text{nm}$ for the ZGNR case.

Figure 2

(Color online) Fabry-Pérot patterns for an (a) AGNR $\left(L=20.6\text{nm}\right)$ and a (b) ZGNR $\left(L=24.4\text{nm}\right)$. White and dark blue colors correspond to maximum $\left(G^{\text{max}}=4e^2/h=G_0\right)$ and minimum conductances ($G^{\text{min}}\simeq 0.7G_0$ for AGNR and $G^{\text{min}}\simeq 0.2G_0$ for ZGNR), respectively.

Figure 3

(Color online) Conductance of AGNR computed as a function of the ac field intensity. The solid line is for $\hslash \Omega =5.0\times {10}^{-4}\gamma$, the dashed red line for $\hslash \Omega =1.0\times {10}^{-3}\gamma$, and the dotted (blue) line for $\hslash \Omega =\Delta =1.5\times {10}^{-3}\gamma$. Panel (a) corresponds to $V_g=0$ and in (b) the gate voltage is tuned in such a way that $G_{\text{dc}}=1G_0$.

Figure 4

(Color online) The panels marked with (a), (b), (c), and (d) are Fabry-Pérot conductance interference patterns for an AGNR (as a function of bias and gate voltages) calculated for different driving frequencies and amplitudes selected in Fig. 3b. White and dark blue colors correspond to maximum and minimum conductances, respectively.

Figure 5

(Color online) Contour plots showing the conductance (top) and the noise power $\overline{S}$ (bottom) as a function of the driving amplitude and frequency. White and blue colors correspond to minimum and maximum amplitudes, respectively.

Figure 6

(Color online) Contour plot showing the conductance for a ZGNR as a function of the driving amplitude and frequency. White and blue colors correspond to maximum and minimum amplitudes, respectively. The dots (a), (b), and (c) are the set ac parameters predetermined to obtain the full FP interference patterns shown in Fig. 7.

Figure 7

(Color online) FP patterns calculated for a ZGNR at (a) dc and two ac conditions [(b) $V_{\text{ac}}=0.02\gamma$, $\hslash \Omega =0.008\gamma$ and (c) $V_{\text{ac}}=0.11\gamma$, $\hslash \Omega =0.085\gamma$] marked on Fig. 6.