Fabrication and Transport Study of Superconductor-Based Quantum Devices


Student thesis: Doctoral Thesis

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Award date15 Nov 2023


Superconductor-based devices have shown superiorities in sensors, deep space exploration, quantum communication and quantum computing. It is essential to fully capture the basic properties of each unit. Graphene-based superconductor-normal metal-superconductor Josephson junctions (JJs), for its high stability and tuneability, have been a popular medium of choice for studying the fundamentals as well as applications of superconducting devices for more than a decade. However, the full spectrum and consequences of the interactions between the graphene Josephson junction and the environment have not been fully mapped. Here we studied the overdamped phase diffusion behavior of Graphene-based JJs. We investigate the zero-bias behavior of Josephson junctions made of encapsulated graphene boron nitride heterostructures in the long ballistic junction regime. For temperatures down to 2.7 K, the junctions appear non-hysteretic with respect to the switching and retrapping currents IC and IR. A small nonzero resistance is observed even around zero-bias current and scales with temperature as dictated by the phase diffusion mechanism. By varying the graphene carrier concentration, we are able to confirm that the observed phase diffusion mechanism follows the trend for an overdamped Josephson junction. This is in contrast with the majority of graphene-based junctions which are underdamped and shorted by the environment at high frequencies.

Moreover, superconductor-based devices are good candidates for studying the properties of new materials. Infinite-layer (IL) nickelates as a new family of unconventional superconductors have captured a lot of enthusiasm for the similarities and distinctions to cuprates. In which, one of the most important properties of superconductivity is the pairing symmetry of the Cooper pairs. A Number of works have been done on IL nickelates trying to show the superconducting pairing mechanism, including STM, ARPES, and mutual inductance works. However, these works cannot directly illustrate the phase information of the pairing. Here we designed three types of phase-sensitive transport devices: the Andreev reflection spectrum, DC superconducting quantum interference devices (SQUID), and single junction SQUID. All the devices can provide strong evidence of the type of the pairing symmetry of IL nickelate. By implementing the technique of using overexposed e-beam resist, we will perform nanofabrication on the IL thin film to make the edge contacts. For the Andreev reflection spectrum, a contact from a normal metal to IL nickelates will be fabricated to see whether we can observe the zero-bias conductance peak which is one of the key signatures of d-wave superconductivity. For the device with DC SQUID geometry, the interference spectrum can stretch out the phase contribution of d-wave pairing with the corner SQUID layout. Furthermore, single junction SQUID measurements can provide the phase contribution of the phase difference of d-wave nodes without being affected by the trapping flux in the device. By performing these phase-sensitive transport measurements, strong evidence of the pairing symmetry of nickelates will be unveiled. This will be a pivotal approach to the understanding of high-temperature superconductivity and the pairing mechanism of nickelates.