Won Beom Choi

Manipulating electron quantum 1D channels is an important element in the field of quantum information due to their ballistic and phase coherence properties. In GaAs and graphene based two dimensional gas systems, these edge channels have been investigated with both integer and fractional quantum Hall (QH) effects, contributing to the realization of electron interferometer and anyon braiding. Often, at the p–n junction in the QH regime, the presence of a depletion region due to a band gap or the formation of gaps between the zeroth Landau levels (zLLs) suppresses interaction between the co-propagating edge channels of opposing doping regimes and helps to preserve the phase coherence of the channels. Here, we observe a new type of p–n junction in hexagonal boron nitride (hBN) aligned graphene that lacks both the zLL and band gap. In this system, a van Hove singularity (vHS) emerges at the p–n junctions under magnetic fields of several Tesla, owing to the doping inversion near the secondary Dirac point. By fabricating devices with independently tunable global bottom and local top gates, we enable the study of interactions between p-type and n-type QH edge channels through magnetic breakdown associated with the vHS. These findings provide valuable insights into the interactions of superlattice-induced QH edge channels in hBN-aligned graphene.

Imaging local magnetic fields produced by nano- and micrometer-scale superconductors has become a vital tool that can not only reveal crucial information on the vortex dynamics and order parameters of the superconducting materials but also visualize the working mechanism of superconducting devices made for quantum information. Here, we performed measurements of the magnetic field distributions of mesoscopic superconducting structures with various geometries by combining a thin ferromagnetic metal layer as a magneto-optical sensing element that responds to the environmental magnetic fields and a custom-made sensitive Sagnac interferometer. The sensitivity of the technique enables the observation of magnetic flux jumps due to individual vortex entries into nanostructured superconductors. In addition, with the control of incident power at a focused laser spot, we induce thermally driven movement of vortices that leaves a trace of a microscopic optical heating pattern.