Self-Assembled Nanowires with Giant Rashba Split Bands

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission. High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. Angle-resolved photoemission reveals fully occupied one-dimensional (1D) bands with a Rashba-type split dispersion. Local $dI/dV$ spectra further indicate well-confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana fermions.

Figure 1

(a) STM image of Pt-Si nanowires ($120\mathrm{nm}\times 50\mathrm{nm}$ at 1.0 V). Inset is the line profile along the line A indicating the width (1.6 nm) and separation ($5a_0$) of nanowires. (b) Magnified STM image of the region B in (a) ($13.7\mathrm{nm}\times 13.7\mathrm{nm}$ at 500 mV). (c) Line profiles on the nanowire and valley along the red and blue lines in (b), which show $3b_0$ and $2b_0$ periodicities, respectively. (d) Fourier transformed STM image and (e) low-energy-electron diffraction pattern display a mixture of $3b_0$ and $2b_0$ periodicities along the $b$ axis.

Figure 2

(a) ARPES intensity plot of Pt-Si nanowires along the nanowire direction, the $b$ axis ($h\nu =46\mathrm{eV}$). Red and blue parabolic lines around the $\Gamma$ point are interpreted as Rashba-type spin-split bands. The spectral peak positions from the momentum (filled circles) and energy (open circles) distribution curves were used in the parabolic fitting of band dispersions. These Rashba-type bands are repeated along nanowires with a periodicity of $2\pi /3b_0$. $\Gamma$ and $X$ points indicate the $\mathrm{Si}(110)-1\times 1$ Brillouin zone center and boundary, respectively, and ${\Gamma }_3$ is the $\times 3$ surface Brillouin zone center. $E_R$ and $\Delta k_R$ are 81 meV and $0.12{\AA }^{-1}$, respectively. The black and gray parabolic lines around the $\Gamma$ point are the heavy hole band and the valence band edge of Si(110) bulk, respectively. (b) Similar ARPES intensity plot around the $\Gamma$ point taken at a different photon energy ($h\nu =60\mathrm{eV}$).

Figure 3

(a) Detailed filled-state STM image of the Pt-Si nanowires ($8.3\mathrm{nm}\times 8.3\mathrm{nm}$ at $-500\mathrm{mV}$). The boxed curve at the bottom is the averaged height across nanowires. (b) Line-averaged (along nanowires) $dI/dV$ spectra across nanowires for the same area as (a). $dI/dV$ conductance curves are obtained on $64\times 64$ grid points over the area of (a) and the intensity of the $dI/dV$ curves are converted to the color scale. Dotted lines with arrows indicate the positions of subwires (red), a central trench (green), and a valley (blue). (c) $dI/dV$ curves averaged along a subwire (red), a central trench (green), and a valley (blue). The small arrows in (b) and (c) indicate the characteristic peak-and-dip feature in LDOS. During the $dI/dV$ conductance measurement the bias voltage was set at $+500\mathrm{mV}$.

Figure 4

Simultaneously acquired (a) topography ($8.3\mathrm{nm}\times 8.3\mathrm{nm}$ at 500 mV) and (b)–(e) bias-dependent $dI/dV$ conductance maps. Bias voltages are denoted on the maps. Red, green, and blue arrows and the dashed lines indicate subwires, central trenches, and valleys, respectively, as in Fig 3. (f) Subtraction of the spectroscopy map at $-300\mathrm{mV}$ from that at $-200\mathrm{mV}$. This corresponds to the difference of the $dI/dV$ conductance of the peak and the dip.