Observation of supercurrent in PbIn-graphene-PbIn Josephson junction

Superconductor-graphene-superconductor (SGS) junction provides a unique platform to study relativistic electrodynamics of Dirac fermions in graphene combined with proximity-induced superconductivity. We report the observation of the Josephson effect in proximity-coupled superconducting junctions of graphene in contact with ${\mathrm{Pb}}_{1-x}{\mathrm{In}}_x$ ($x=0.07$) electrodes for temperatures as high as $T$ $=$ 4.8 K, with a large value of $I_c{R}_N$ ($~$255 $\mu$ V). This demonstrates that ${\mathrm{Pb}}_{1-x}{\mathrm{In}}_x$ SGS junction would facilitate the development of the superconducting quantum information devices and superconductor-enhanced phase-coherent transport of graphene.

Figure 1

(a) Gate-voltage dependence of the junction resistance obtained from the Pb${}_{0.93}$In${}_{0.07}$-based SGS junction in the normal state in a perpendicular field of $H$ $=$ 4.2 kOe and at $T$ $=$ 6 mK. The charge neutrality point ($V_{\mathrm{CNP}}=-20$ V) is indicated by an arrow. Left inset: topographic AFM image of the Pb${}_{0.93}$In${}_{0.07}$ film showing granular morphology. Right inset: optical microscope image of the device. The dotted line denotes the boundary of the monolayer graphene. The narrow colored region indicates the 300-nm spacing of graphene layer between two Pb${}_{0.93}$In${}_{0.07}$ electrodes. The wide colored region of graphene layer used for the measurement of the quantum Hall plateaus, respectively. (b) Gate-voltage-dependent conductance of the narrow junction and the wide graphene sheet with $H$ $=$16 T at $T$ = 6 mK. The dotted line corresponds to $G$ $=$ $\nu e^2/h$ ($\nu =2,6,10,\dots$).

Figure 2

(a) Current-voltage ($I$-$V$) characteristic curve measured at $\Delta V_{\mathrm{BG}}=-40$ V with increasing and decreasing bias current. The critical ($I_c$) and the retrapping ($I_R$) currents are indicated. Inset: resistance vs temperature ($R$-$T$) curve of a Pb${}_{1-x}$In${}_x$ electrode, revealing $T_c$=7.0 K. (b) $I$-$V$ curves with five different temperatures at $\Delta V_{\mathrm{BG}}=-40$ V. (c) $T$ dependencies of $I_c$ and $I_R$ for $\Delta V_{\mathrm{BG}}=-40$ V. (d) $I_c-T$ curves for different back-gate voltages. The solid lines are the theoretical fits explained in the text.

Figure 3

(a) Gate-voltage dependence of $I_c$ (upper curve) and $I_R$ (lower curve) at $T=6$ mK with bias current swept from negative to positive polarity. (b) $I_c{R}_N$ product as a function of the back-gate voltage. (c) $\mathit{dI}/\mathit{dV}$ vs $V$ curves with varying $\Delta V_{\mathrm{BG}}=0$, $-20$, $-40$ V from bottom to top. The conductance with $\Delta V_{\mathrm{BG}}=0$ V is magnified by 2.5 times for the sake of clarity. The arrows indicate the subgap structures corresponding to the voltage of $V_n$ $=$ $2{\Delta }_{\mathrm{PbIn}}/\mathit{ne}$ ($n$ $=$ 1, 2, 3, 4). (d) The normalized critical current ($I_c$) as a function of magnetic flux ($\Phi$) measured at $\Delta V_{\mathrm{BG}}=-60$ V and $-20$ V for $T$ $=$ 48 mK and $-20$ V for $T$ $=$ 2.2 K. The maximum critical currents ($I_{c0}$) are $I_{c0}=5.8$, 2.8, 0.8 $\mu$A for $\Delta V_{\mathrm{BG}}$ ($T$)$=-60$ (0.048 K), $-20$ (0.048 K), and $-20$ (2.2 K) V, respectively. The black curve represents a calculation result of an ideal Fraunhofer pattern (see text).

Figure 4

(a) Gate-voltage dependent $I$-$V$ curves under the irradiation of a microwave of $f=6$ GHz at $T=48$ mK. Inset: color-coded plot of $\mathit{dI}/\mathit{dV}$ as a function of $I$ and square root of the microwave power ($P^{1/2}$) at $\Delta V_{\mathrm{BG}}=60$ V. (b) $I$-$V$ curves measured at various temperatures under the irradiation of a microwave with $f=6$ GHz at $\Delta V_{\mathrm{BG}}=60$ V. Inset: $\mathit{dI}/\mathit{dV}$-$V$ curve obtained at $T=4.2$, 4.8, and 5.4 K. The voltage was normalized by $\mathit{hf}/2e$ to clarify the existence of the Shapiro steps at $T=4.2$ and 4.8 K. (c) $I-V$ characteristic curves at 48 mK and $\Delta V_{\mathrm{BG}}=-60$ V with varying external microwave of frequency $f$ $=$ 6 GHz, 10 GHz, and 17 GHz from bottom to top. $\Delta V$ is the voltage interval between two different neighboring Shapiro steps. The curves are shifted for clarity. (d) The voltage interval $\Delta V$ as a function of microwave frequency $f$. The solid line shows a good agreement with the ac Josephson relation $\Delta V=\mathit{hf}/2e$.