The Physics of High-Temperature Superconducting Cuprates in van der Waals Heterostructures

Abstract

Advances in crystal handling techniques specialized to van der Waals (vdW) materials - crystals with only weak vdW bonds between strongly bonded crystal planes - have brought about a new generation of devices with emergent properties in atomically thin layers and interfaces, which do not appear in their bulk counterpart. Many vdW materials, such as the cuprate high temperature superconductor Bi2Sr2CaCu2O8+x (BSCCO), spontaneously react with air and thermally decompose even at room temperature, rendering the powerful arsenal of vdW crystal handling techniques inapplicable. In this Dissertation, we develop a versatile suite of novel vdW fabrication techniques so that we can handle chemically sensitive vdW crystals like any other. We first demonstrate that the vdW heterointerface between graphene and hexagonal boron nitride can be electrochemically intercalated with lithium in a controlled way, like the interface between graphene crystals. Next, we use a scanning nano X-ray diffraction probe to visualize the crystal structure of an exfoliated BSCCO crystal 2 unit cells thick with 100 nm spatial resolution. We find that while the exfoliated and bulk crystals have the same incommensurate lattice modulations (ILM), the ILM spatial distribution is different in the exfoliated crystal, where it is correlated with the local strain. Next, we create BSCCO Hall bar devices down to 2 unit cells, where we observe Hall sign reversal both above and below the superconducting transition temperature. As the samples become thinner, the region over which the Hall resistance reverses sign enlarges due to enhanced superconducting fluctuations in the atomically thin samples as well as a decrease of carrier mobility. We obtain quantitative agreement between theory and experiment, which establishes excess charges at the core of superconducting vortices as the origin of Hall sign reversal below Tc. Finally, we engineer twist Josephson junctions between BSCCO crystals, with quality approaching that of intrinsic Josephson junctions in single crystals. Our angle-dependent critical current clearly reflects the expected d-wave symmetry of the order parameter. At 45 degree twist angle, we observe fractional Shapiro steps which indicate the presence of a second harmonic in the Josephson junction current-phase relation, originating from co-tunneling of Cooper pairs at the twist junction. Such a process is expected to support an emergent interfacial topological superconducting phase, persisting to the superconducting transition temperature of the cuprate crystal.