Successful PhD defense of Pranava Sivakumar

Non-reciprocal critical currents in superconductors labelled the superconducting diode effect has attracted a lot of attention in recent years for its potential to create energy-efficient superconducting devices for computing applications. In this thesis, the observation of similar non-reciprocal critical currents in Josephson junctions made of two van der Waals transition metal dichalcogenides 1T-NiTe₂ and 1T-PtTe₂, named the Josephson diode effect is reported. The hidden inversion-symmetry breaking in the structure of these materials naturally gives rise to a helical spin-momentum locking in the band structure. The origin of the effect is discussed in terms of a non-zero center of mass Cooper pair momentum created due to the shift of the spin-momentum locked states in an external Zeeman field. In 1T-NiTe₂, this hypothesis is verified by comparing the experimental results to a Ginzburg-Landau model of the Josephson junction expanded up to the second order. The presence of a finite momentum is further verified by measuring the evolution of the interference pattern in the presence of an additional in-plane magnetic field that creates finite Cooper pair momentum and investigating the modification to the pattern. The Cooper pair momentum estimated from these measurements are in good agreement with those estimated from the Josephson diode effect. In the case of 1T-PtTe₂, the presence of a helical spin-momentum locking is verified through measurement of the Josephson diode effect in lateral junctions. Extrinsic mechanisms that give rise to a Josephson diode effect are also identified and nullified to study the underlying physics. A generic current-phase relationship, analogous to that predicted in NiTe₂, is used to evaluate the evolution of the Josephson diode effect in the junction. Measurements of the Josephson diode effect in the presence of a magnetic flux at different in-plane magnetic fields reveal the presence of a large Josephson diode effect that is directly related to the second harmonic term in the current-phase relationship. The current-phase relationship (CPR) is shown to be unique and different from typical Josephson junctions. The existence of this CPR is further confirmed by looking at the periodicity of the diode effect with magnetic flux, which is half magnetic flux quantum periodic as opposed to the critical currents themselves, which have one magnetic flux quantum period. The evolution of the Josephson diode effect simulated with the second harmonic term obtained previously is also in good agreement with the measurements confirming the accuracy of the assumed current-phase relationship. The origin of the large second harmonic term, which corresponds to charge ‘4e’ supercurrents in the PtTe₂ junction, is discussed in terms of the helical spin-momentum locked states that suppresses normal reflections. The presence of a transparent interface that promotes coherent Andreev reflections is revealed by the presence of excess currents and fitting the critical currents as a function of temperature. The Josephson junctions of these materials with a distinctive CPR provide the unique possibility of tuning the relative phase between the two harmonics and hence rendering it useful for practical applications in separating the two harmonic supercurrents.