Quantum transport and field-induced superconductivity in carbon nanotubes.

摘要

For my thesis, I conducted experiments to investigate superconductivity and superconducting proximity effect in carbon nanotubes. The measurements are carried out on carbon nanotube field-effect transistors (CNTFETs) made of individual carbon nanotubes. Carbon nanotubes are synthesized by chemical vapor deposition (CVD) on heavily doped silicon substrate covered by a layer of thermally grown silicon oxide, so that the substrate serves as a back gate. Different metals (superconducting and normal metal) have been deposited to make source and drain contacts with the carbon nanotube. For the investigation of intrinsic superconductivity in carbon nanotubes, I fabricated low-resistance CNTFETs with contacts made of palladium, a normal metal. In certain special gate voltage ranges, the conductance of carbon nanotube increases with decreasing temperature below a critical value. I suggest that this is due to intrinsic superconductivity in the carbon nanotube when the Fermi level of the carbon nanotube is shifted into van Hove singularities of its density of states by a gate voltage. The increase of conductance at low temperature is then attributed to Andreev reflection occurring at the CNT/Pd interfaces when Cooper pairs form in the carbon nanotube In our devices, we measured critical temperatures up to 30 K, which is higher than the critical temperatures previously reported for intrinsic superconductivity in carbon nanotubes (ranging from 0.5 K to 15 K). Moreover, I have measured the general low temperature transport phenomena of these devices, including quantized conductance, coherent interference, and single electron tunneling. I will also discuss carbon nanotubes used as nanoscale probes for superconducting proximity effect for CNTFETs with superconduting electrodes.