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Kohlenstoff-Nanoröhrchen als elektronische und nanoelektromechanische Hybridsysteme im Quantenlimes

Fachliche Zuordnung Experimentelle Physik der kondensierten Materie
Förderung Förderung von 2010 bis 2017
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 163841188
 
Erstellungsjahr 2018

Zusammenfassung der Projektergebnisse

Both regarding their electronic and their mechanical properties, carbon nanotubes provide an outstanding material system. The excitation and addition spectrum of few to single trapped charges in the unperturbed nanotube band structure can be observed and manipulated with high precision. At the same time, the transversal vibration mode of a suspended nanotube provides a nanomechanical system close to the quantum limit in cryogenic experiments, strongly coupling motion and charge transport. With the objective of integrating a carbon nanotube quantum dot into a microwave optomechanical system, we have successfully developed a multitude of experimental and scientific building blocks. As a first step we have established the so-called overgrowth technique, where clean carbon nanotubes are grown as last fabrication step into predefined electrode geometries. This has enabled us to perform transport spectroscopy of unperturbed electronic systems in the nanotube band structure. We have been able to demonstrate novel transport selection rules specific to the Kondo effect, by identifying the participating quantum states in the case of broken SU(4) symmetry. In the case of a nanotube strongly coupled to its leads, we have demonstrated how the chiral angle can be estimated from secondary patterns in the electronic Fabry-Pérot interference. Conversely, for a single, strongly confined electron, we have been able to show how an axial magnetic field provides extraordinary tuning of the longitudinal single particle wave function envelope. At the same time, the overgrown, suspended nanotubes act as transversal nano-electromechanical resonators. Here, we were able to demonstrate the electromechanical damping by an externally applied magnetic field, as well as damping by a viscous liquid helium environment. Using the vibration mode as a charge sensor, we have precisely traced the charging of the quantum dot embedded in the nanotube in the Kondo regime. We have shown that the large Kondo conductance is carried via virtual occupation of the quantum dot alone, while a sequential tunneling model is sufficient to describe charge accumulation and mechanical resonance frequency tuning. The second major component of the planned optomechanical system is a microwave resonator. With this in mind we have built up a millikelvin / gigahertz transmission measurement setup in a dilution refrigerator, and established the fabrication of superconducting coplanar waveguide resonators. Using a molybdenumrhenium alloy, we have been able to build and characterize at millikelvin temperatures coplanar resonators that survive the carbon nanotube chemical vapor deposition growth environment. The temperature-dependent behaviour of the molybdenum-rhenium resonators can be described well by the characteristic combination of Mattis-Bardeen theory and two level system dissipation. Even so, the overgrowth technique turned out to be risky, easily damaging predefined complex chip structures as e.g. the coplanar resonators or gate oxides, and leading to insufficient fabrication yield. For this reason we have recently developed a nanotube transfer technique, growing the nanotubes on readily available and reusable commercial quartz tuning forks. The nanotubes can be deposited onto electrodes and electrically cut. This technique now forms the base of device fabrication and experimental work. Ongoing efforts target the demonstration of Coulomb-blockade enhanced optomechanical coupling.

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