Dissertation Committee Chair: Vladimir Manucharyan
Committee:
Richard Greene
Steven Anlage
Jay Deep Sau
Ichiro Takeuchi
Abstract: A Josephson junction (JJ) is known as a weak link connecting two superconductors, in which the non-dissipative supercurrent flows. More than two superconductors also can form a single composite JJ, called a “multi-terminal JJ”, by being connected through a common weak link. The supercurrent in multi-terminal JJs may depend on multiple superconducting phase differences defined across the junction, which is attributed to the supercurrent-carrying sub-gap quasiparticle bound states called Andreev bound states (ABSs).
In this dissertation, we first investigate the supercurrent of three- and four-terminal JJs fabricated on hybrid two-dimensional Al/InAs (superconductor/semiconductor) heterostructures. We define the critical current of an N-terminal junction as a (N − 1)-dimensional hypersurface of the bias currents, which can be reduced to a set of critical current contours (CCCs) in the two-dimensional planes. The geometry of the CCCs are modified non-trivially in response to magnetic field, electrical gating and phase biasing, which is attributed to the multi-phase-dependent ABSs. We also show that our experimental observations are consistent with quantum transport simulations based on a tight-binding model of the hybrid superconductor/semiconductor system. Second, we demonstrate the multi-phase-dependence of ABSs in three-terminal JJs by tunneling spectroscopy measurements. The ABS energy spectrum mimics electronic band structure in solid, which makes multi-terminal JJs provide a new platform to study band topology in higher dimensional parameter space. Moreover, spin-splitting of ABS energies induced by the multi-phase and gapless energy spectrum facilitated by the presence of a discrete vortex, a nonzero winding of the superconducting phases, are investigated.