All-Optically Controlled Topological Transistor Based on $X\mathrm{enes}$

We theoretically propose an $X\mathrm{ene}$ ($X=\mathrm{Si}$, $\mathrm{Ge}$, or $\mathrm{Sn}$) transistor that can be operated with high and low threshold light parameters. The results reveal that a spin-dependent nonconductive path in the $X\mathrm{ene}$ superlattice can be formed by utilizing an off-resonant light-induced topological phase transition and the band mismatch between illuminated and unilluminated regions. This topological transistor can be switched between an on state with a $100\mathrm{\%}$ spin-polarized weak current, an on state with a nonpolarized strong current, and an off state with a controllable breakdown voltage, just by adjusting the polarization state of circularly polarized light. With the assistance of an electric field, the $X\mathrm{ene}$ transistor can be operated at low light parameters, the threshold parameter of the transistor can be reduced to much lower than the spin-orbit coupling strength, and the breakdown voltage can be larger than the bulk band gap of the unilluminated $X\mathrm{ene}$. All the results indicate that the proposed $X\mathrm{ene}$ nanosystems are promising candidates for topological electronic devices.

Figure 1

(a) Schematic representation of $X\mathrm{ene}$ transistor decomposed into source electrode, central device, and drain electrode. Four areas of the central nanoribbon are covered periodically by areas of a nontransparent material with length $N_x=5$ and width $N_y=20$; the central device region is divided into two parts, and a pair of circularly polarized light beams irradiate it. LCP, left-circularly-polarized; RCP, right-circularly-polarized.

Figure 2

Illustration of (a) LCP light (${\lambda }_{\mathrm{\Omega }}>0$), (b) RCP light (${\lambda }_{\mathrm{\Omega }}<0$), and (c) no light (${\lambda }_{\mathrm{\Omega }}=0$) irradiating the entire device region. (d) Illustration of LCP and RCP light irradiating parts I and II, respectively, of the central device region. (e) Drain current of a stanene transistor as a function of the bias voltage $V_{\mathrm{bias}}$ with a light parameter $|{\lambda }_{\mathrm{\Omega }}|=2{\lambda }_{\mathrm{SO}}^{\mathrm{Sn}}$. (f) Drain current of silicene and germanene transistors in off state versus bias voltage $V_{\mathrm{bias}}$ with parameters $|{\lambda }_{\mathrm{\Omega }}|=2{\lambda }_{\mathrm{SO}}^{\mathrm{Si}}$ and $|{\lambda }_{\mathrm{\Omega }}|=2{\lambda }_{\mathrm{SO}}^{\mathrm{Ge}}$. (g) On:off ratio of stanene transistor versus number of opaque regions $N_{\mathrm{Dark}}$ at temperature $T=4.2$ K and bias voltage $V_{\mathrm{bias}}=20$ meV. (h) On:off ratio of $X\mathrm{ene}$ transistor as a function of the bias voltage $V_{\mathrm{bias}}$ at 4.2 K. (i) On:off ratio versus $V_{\mathrm{bias}}$ at 300 K.

Figure 3

(a)–(d) Energy-band diagrams of stanene with light parameters (a) ${\lambda }_{\mathrm{\Omega }}=0$, (b) ${\lambda }_{\mathrm{\Omega }}={\lambda }_{\mathrm{SO}}^{\mathrm{Sn}}/2$, (c) ${\lambda }_{\mathrm{\Omega }}=2{\lambda }_{\mathrm{SO}}^{\mathrm{Sn}}$, and (d) ${\lambda }_{\mathrm{\Omega }}=-2{\lambda }_{\mathrm{SO}}^{\mathrm{Sn}}$. (e),(f) Schematic illustration of edge states in the real-space picture with (e) RCP $({\lambda }_{\mathrm{\Omega }}>{\lambda }_{\mathrm{SO}}^{\mathrm{Sn}})$ and (f) LCP $({\lambda }_{\mathrm{\Omega }}<-{\lambda }_{\mathrm{SO}}^{\mathrm{Sn}})$ light. The solid and dashed (red and blue) lines correspond to the upper and lower (spin-up and spin-down) edge states, respectively.

Figure 4

(a) Schematic illustration of an optically and electrically controlled $X\mathrm{ene}$ transistor in which back gates are added to the illuminated regions. (b) Drain current of $X\mathrm{ene}$ transistor in the off state as a function of the bias voltage $V_{\mathrm{bias}}$ with $|{\lambda }_{\mathrm{\Omega }}|=|{\lambda }_E|=3{\lambda }_{\mathrm{SO}}^X/4$. (c) On and off states of the transistor for different values of the light parameter ${\lambda }_{\mathrm{\Omega }}$ and the staggered potential ${\lambda }_E$. (d) On:off ratio of the $X\mathrm{ene}$ transistor as a function of the bias voltage $V_{\mathrm{bias}}$ at 4.2 K. (e) On:off ratio versus $V_{\mathrm{bias}}$ with $|{\lambda }_{\mathrm{\Omega }}|=|{\lambda }_E|={\lambda }_{\mathrm{SO}}^X$ at 300 K.