NanoSQUID Magnetometers and High Resolution Scanning SQUID Microscopy

Recent interest in the development of small sized superconducting quantum interference devices (SQUIDs) has been motivated by the applicability of these sensors for the investigation of small, local, magnetic signals, such as the magnetization reversal of small magnetic clusters and the observation of local magnetic structures using a scanning SQUID microscope (SSM). Further miniaturization of the sensors offers a possibility to enhance the sensitivity and spatial resolution in these experiments. In the first part of this thesis the development of miniature SQUIDs based on Niobium nanobridges was described. From a fabrication point of view the realization of such devices offers less practical limitations compared to the development of miniature sensors based on “classical” Josephson tunnel junctions. The nanobridge based SQUIDs were patterned by means of focused ion beam milling. Since the ion beam profile is inhomogeneous, structures with widths that are smaller than the beam diameter can be created by letting two beam profiles overlap. Simulations and experiments aimed towards the determination of the effects of the ion induced damage on patterned devices have proven that, in Niobium structures patterned with a 25 keV Ga focused ion beam, superconductivity is suppressed as far as 35 nm inwards from the surface at T = 4.2 K. This result was taken into account when modeling the dimensions of realized nanobridges

Temperature dependence measurements of the supercurrent-phase relationship in niobium nanobridges

The current-phase relationship has been measured as a function of temperature for niobium nanobridges with different widths. A deformation from Josephson-like sinusoidal characteristics at high temperatures to sawtooth shaped curves at intermediate and multivalued relationships at low temperatures was observed. Based on this, possible hysteresis in the current-voltage characteristics of niobium nanobridge superconducting quantum interference devices can be attributed to phase slippage

Electrically Excited, Localized Infrared Emission from Single Carbon Nanotubes

Carbon nanotube field-effect transistors (CNTFETs) produce band gap derived infrared emission under both ambipolar and unipolar transport conditions. We demonstrate here that heterogeneities/defects in the local environment of a CNTFET perturb the local potentials and, as a result, the characteristic bias dependent motion of the ambipolar light emission. Such defects can also introduce localized infrared emission due to impact excitation by carriers accelerated by a voltage drop at the defect. The correlation of the change in the motion of the ambipolar light emission and of the stationary electroluminescence with the electrical characteristics of the CNTFETs shows that stationary electroluminescence can identify "environmental defects" in carbon nanotubes and help evaluate their influence on electrical transport and device operation. A number of different defects are studied involving local dielectric environment changes (partially polymer-covered nanotubes), nanotube-nanotube contacts in looped nanotubes, and nanotube segments close to the electronic contacts. Random defects due to local charging are also observed