Magdalena Huefner

Imaging localized states in graphene nanostructures

Probing techniques with spatial resolution have the potential to lead to a better understanding of the microscopic physical processes and to novel routes for manipulating nanostructures. We present scanning-gate images of a graphene quantum dot which is coupled to source and drain via two constrictions. We image and locate conductance resonances of the quantum dot in the Coulomb-blockade regime as well as resonances of localized states in the constrictions in real space.

Spatial mapping and manipulation of two tunnel-coupled quantum dots

The metallic tip of a scanning force microscope operated at

Microcantilever Q control via capacitive coupling

300\text{}\mathrm{m}\mathrm{K}

Single-vortex pinning and penetration depth in superconducting NdFeAsO1−xFx

is used to locally induce a potential in a fully controllable double quantum dot defined via local anodic oxidation in a GaAs/AlGaAs heterostructure. Using scanning gate techniques we record spatial images of the current through the sample for different numbers of electrons on the quantum dots (i.e., for different quantum states). Owing to the spatial resolution of current maps, we are able to determine the spatial position of the individual quantum dots, and investigate their apparent relative shifts due to the voltage applied to a single gate.

Mapping leakage currents in a nanostructure fabricated via local anodic oxidation

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ∼10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO2 thin film in high vacuum.

The Aharonov?Bohm effect in a side-gated graphene ring

We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO1-xFx, one of the highest-Tc iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, Fdepin ≃ 4.5 pN, corresponding to a critical current up to Jc ≃ 7×105 A/cm2. As a result, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO1-xFx, λab = 320 ± 60 nm, which is larger than previous bulk measurements.

Magnetoresistance of atomic-size contacts realized with mechanically controllable break junctions

The functionality of nanostructures fabricated via local anodic oxidation is limited by undesired leakage currents. We use low-temperature scanning gate microscopy to pin down the spatial position where leakage currents are most likely to occur. We show that leakage currents do not flow homogeneously along the complete barrier but at distinct weak points such as crossings of two oxide lines. These findings can be used to improve the design of such nanostructures.