UCLA

This work explores two key areas in the study of van der Waals heterostructures: their utilization in developing quantum tunneling devices and the observation of time-reversal symmetry breaking within superconductors.

In the first part, the research focuses on Quantum tunneling devices utilizing van der Waals heterostructures. These devices have garnered significant attention due to their ability to elucidate the nature of quasi-particle excitations and energy states through inelastic electron tunneling spectroscopy (IETS). Specifically, graphite-based tunnel junctions with insulating barriers like hexagonal boron nitride (h-BN) are pivotal in understanding quantum phenomena in non-magnetic tunnel junctions. Recent device-oriented studies on magnetic tunnel junctions indicate that Landau levels and magnon excitations play a role when electrons tunnel between graphite layers. However, to thoroughly investigate the tunneling mechanisms involving Landau levels and excitations, a comprehensive study involving magnetic field and temperature dependencies through IETS is essential. In this context, we fabricated tunnel junction devices with graphite electrodes and thin h-BN/MoS2 as insulating barriers, conducting extensive transport measurements. Our low-temperature findings revealed trivial phonon excitations without signs of magnon or Landau levels in these non-magnetic tunnel junctions. This study aims to understand quasiparticle behavior and spin-polarized Landau levels for enhancing the performance and reliability of magnetic tunnel junctions (MTJs), with applications in spintronics and quantum sensing.

In the second part, we move beyond the proximity effect to the direct study of time-reversal symmetry breaking in superconductors intrinsically, where van der Waals heterostructures have shown significant potential for applications involving stacking and twisting of layers. Concurrently, Iron-based d-wave superconductors have been realized to exhibit topological surface superconductivity. However, previous studies have predominantly been restricted to bulk characterizations and lack comprehensive device-oriented investigations. Here, for the first time, we demonstrate the evolution of symmetry breaking coexisting with superconductivity at the interface of twisted thin van der Waals Iron Selenide Josephson Junctions. A post-fabrication free approach for ultra-clean interface has been implemented. Twisting the bottom layer approximately 6 degrees in the vertically stacked Junction has displayed the emergence of ferromagnetism in addition to superconducting d-wave symmetry below the critical temperature. Further, the current trainable sequence by modulating the Josephson phase particle between the two superconducting degenerate ground states confirmed the spontaneous time-reversal symmetry breaking in the system.