Macroscopic Superconducting Current through a Silicon Surface Reconstruction with Indium Adatoms: $\mathrm{Si}(111)\mathrm{\text{−}}(\sqrt{7}\times \sqrt{3})\mathrm{\text{−}}\mathrm{In}$

Macroscopic and robust supercurrents are observed by direct electron transport measurements on a silicon surface reconstruction with In adatoms [$\mathrm{Si}(111)\mathrm{\text{−}}(\sqrt{7}\times \sqrt{3})\mathrm{\text{−}}\mathrm{In}$]. The superconducting transition manifests itself as an emergence of the zero resistance state below 2.8 K. $I\mathrm{\text{−}}V$ characteristics exhibit sharp and hysteretic switching between superconducting and normal states with well-defined critical and retrapping currents. The two-dimensional (2D) critical current density $J_{2\mathrm{D},c}$ is estimated to be as high as $1.8\mathrm{A}/\mathrm{m}$ at 1.8 K. The temperature dependence of $J_{2\mathrm{D},c}$ indicates that the surface atomic steps play the role of strongly coupled Josephson junctions.

Figure 1

(a) Atomic structural model of the $(\sqrt{7}\times \sqrt{3})\mathrm{\text{−}}\mathrm{In}$ surface proposed by Kraft et al. [15]. The red and blue spheres represent In and surface Si atoms, respectively. The dashed lines indicate the $(\sqrt{7}\times \sqrt{3})$ unit cell. The In atoms are assumed to be on the hollow sites of the $\mathrm{Si}(111)\mathrm{\text{−}}1\times 1$ for simplicity. (b) LEED pattern taken at beam energy $E=86\mathrm{eV}$. The dashed lines indicate the reciprocal unit cell. (c) High-resolution STM image taken at a sample voltage $V_s=-0.015\mathrm{V}$ and a tunneling current $I_t=270\mathrm{pA}$. (d) Large-scale STM image taken at $V_s=2\mathrm{V}$ and $I_t=160\mathrm{pA}$. (e) Height profile taken along the dashed line in (d).

Figure 2

(a) Drawing of the sample patterning. The central and the four outer squares (purple areas) are made of $(\sqrt{7}\times \sqrt{3})\mathrm{\text{−}}\mathrm{In}$, and the surrounding regions (gray areas) consist of sputtered Si surfaces. (b) Calculated current density distribution on the sample. The bright green (dark purple) represents a high (low) current density. The red dotted line indicates the flow of a bias current. (c) Temperature dependence of zero bias resistances. $R_{0,\mathrm{I}}$ (red lines) and $R_{0,\mathrm{II}}$ (blue lines) are zero bias resistances measured using the probe configurations I and II, respectively, with dc bias currents of $1\mu \mathrm{A}$. Left inset: $R_{0,\mathrm{I}}$ and $R_{0,\mathrm{II}}$ for a larger temperature range. Right insets: schematic drawings of the probe configurations I and II.

Figure 3

(a) Temperature dependence of $I\mathrm{\text{−}}V$ characteristics obtained with configuration I. The inset shows $I\mathrm{\text{−}}V$ characteristics taken by inverting sweeping directions at 1.80 K. The arrows indicate the sweeping directions. (b) Temperature dependences of critical current $I_c$ (green squares) and retrapping current $I_r$ (pink squares). The data taken with configurations I and II are shown by closed and open squares, respectively. The blue solid and red dotted lines show theoretical fits. For details, see the text. The inset sketches the relation between current flow and atomic steps.