Physics of Josephson Diodes Formed from 1T-Transition Metal Dichalcogenides

This book provides a clear and lucid introduction to the field of non-reciprocal supercurrent transport in Josephson junctions, particularly the Josephson diode effect in junctions fabricated from mechanically exfoliated transition metal dichalcogenides and its microscopic mechanism. Superconducting materials that display a non-reciprocity in their critical current, namely a supercurrent diode effect (SDE), and Josephson junctions (JJs) that display a Josephson diode effect (JDE) have recently been discovered just a few years ago. These phenomena have attracted much attention for their potential in creating energy-efficient superconducting electronics. The SDE was discovered for the first time only in 2020 and the JDE shortly afterwards. JJs are a critical element of many superconducting devices and, in particular, superconducting qubits that are under intense study for the development of quantum computers. In order to make use of devices that display a JDE, a detailed and comprehensive understanding of the physical origin or origins of this effect is essential, which is the main topic of this dissertation. In addition to the published results, the dissertation contains detailed information on the basic theoretical aspects of superconductivity, Josephson junctions, and the experimental methods that are necessary to achieve these results, which is suitable for undergraduate and graduate students or any reader with knowledge on basic condensed matter physics.

Chapter 1.Introduction and scope of the thesis.- Chapter 2.Theoretical Foundations.- Chapter 3.Experimental methods.- Chapter 4.Josephson diode effect induced by finite momentum Cooper pairing in a topological Rashba system 1T-NiTe2.- Chapter 5.Helical spin-momentum locking and tunable second-order junctions in the Dirac semimetal 1T-PtTe2 probed by the Josephson diode effect.- Chapter 6.Conclusions and outlook.

Pranava Keerthi Sivakumar has done his BS-MS (Chemistry) at the Indian Institute of Science, Bengaluru, India. He completed his Ph.D. in Physics at the Max Planck Institute of Microstructure Physics and Martin Luther University in Halle, Germany under the supervision of Prof. Stuart S. P. Parkin and graduated with the highest honors (summa cum laude) for his dissertation. During his Ph.D., he worked on a variety of topics including electrical detection of topological spin textures, current-induced antiskyrmion motion, and anomalous Hall effects in non-collinear antiferromagnetic systems. Most notably, as described in his dissertation, he set up and carried out low-noise electrical transport measurements on Josephson junctions of two-dimensional van der Waals transition metal dichalcogenides sandwiched by superconducting niobium electrodes at ultra-low temperatures in a dilution refrigerator setup assembled by himself during his Ph.D. He has identified large non-reciprocal critical currents in the presence of a magnetic field, and experimentally verified a mechanism based on the formation of finite-momentum Cooper pairs, that can reliably explain the existence of this effect in systems with spin-momentum locking due to broken inversion symmetry. The discovery itself and the in-depth understanding of its underlying physics provided in his work is both technologically and fundamentally significant; as such effects have recently attracted a lot of attention in the condensed matter physics community for both their potential in creating dissipation less superconducting devices and as a tool in identifying unconventional superconductivity.