Abstract
We performed in situ transport measurements of ultrathin Bi(111) films grown on Si(111) surface, with a four-tip scanning tunneling microscope using metal-coated carbon nanotube (CNT) tips. When the distance between the current injection tip (nonmagnetic Pt-coated CNT tip) and voltage tip (magnetic CoFe-coated CNT tip) was smaller than 1 μm, we found a violation of Green's reciprocity theorem which should hold with no spin transport. This was interpreted as a signal of the current-induced spin polarization (CISP) that occurs due to the Rashba spin-splitting surface-state bands of the Bi(111). The result was reasonably in accord with quantitative analyses based on the CISP theory of Rashba systems.
Highlights
We performed in situ transport measurements of ultrathin Bi(111) films grown on Si(111) surface, with a four-tip scanning tunneling microscope using metal-coated carbon nanotube (CNT) tips
The main reason is that these surface states cannot survive in air and there have been no experimental apparatus that is capable of detecting spin-dependent transport in situ in ultrahigh vacuum (UHV)
The measurements were performed at room temperature (RT) in our four-tip scanning tunneling microscope (STM) system installed in an UHV scanning electron microscope (UHV SEM) [18]
Summary
The measurements were performed at room temperature (RT) in our four-tip STM system installed in an UHV scanning electron microscope (UHV SEM) [18]. The CNT-W tip was wholly coated with 10 nm thick Pt (nonmagnetic tips) or CoFe (magnetic tips) using the pulsed laser deposition technique [22] This procedure is effective for stabilizing the resistance of the CNT-W junction down to less than 10 k. We have applied a magnetic field as large as 150 mT along the CNT tip prior to installation, but the results did not seem to change whether the magnetic field was applied or not This has been demonstrated by magnetic force microscopy measurements for the CoFe-coated CNT tips with similar aspect ratio [25]. Bi was deposited on the 7 × 7 surface at RT, followed by annealing at ∼ 380 K Such procedure results in the formation of high-quality, single-crystalline, epitaxial Bi(111) films thicker than six BL (25 Å) as reported in [26, 27]. The thickness of the films in the present measurement was 10 BL, in which the surface-state contribution is over 60% of the total conductivity [11]
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