Development of Green's Function STM and Application to Study of Nano Electronic Transport Dynamics

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1. Designing and Construction of a low-temperature four-tip STM (Green's function STM): We have constructed the microscope, having the following features which make the machine world-wide unique. (a) It works in ultra-high vacuum, with aid of scanning electron microscope, enabling positioning the four tips with nanometer precision. (b) It cools the sample and tips down to 7 K with keeping them for 20 hours. (c) It attains atomic-resolution in STM by each tip. (d) It drives the four tips in systematic ways by a single PC controller (e) Its multi-functional preamplifier enables three modes of measurements, usual STM/S, four-point-probe conductivity measurements, and double-tip STS (Green's function) modes.2. Development of metal-covered carbon nanotube tips: We have succeeded in fabricating conductive carbon nanotube tips of 10 nm in diameter, by coating them with PtIr thin layers. This makes the minimum tip spacing in four-point-probe measurement down to about 20 nm, which is far smalle … More r than by usual metal tips. When the carbon nanotube tips are covered by a ferromagnetic (NiFe) thin layer, the tip becomes a ferromagnetic tip which is useful for a magnetic force microscope and spin-polarized STM.3. Applications: We have started various measurements by using the low-temperature four-tip STM. We have measured the electrical resistance of a thin CoSi2 nanowire of about 40 nm in diameter at room temperature. The results show that the electrical conduction is still diffusive at 20 nm probe spacing. The temperature dependence of conductivity of a quasi-one-dimensional metallic surface, Si(111)-4x1-In, was measured. The results show that the carrier conduction mechanism is different between the chain-direction and inter-chain direction. The transport along the chains shows a metal-insulator transition by Peierls transition while that across the chains is due to the Si substrate. We are now trying to obtain a signal of transconductance between two tips, which is aiming the measurement of Green's function. Less