Superconductivity Magnetism and their Interplay in two-dimension

Abstract

The ability to understand, control, and manipulate quantum states of matter, such as superconductivity, magnetism, and topological order, is not only a fundamental scientific pursuit but also has significant technological implications. Further, the interface of two materials with distinct quantum states can lead to the emergence of novel phenomena that are not present in either of the individual materials. The thesis explores some of these aspects to study superconductivity, magnetism and investigate emergent Majorana zero modes that may arise at their interface. The first part of the thesis deals with investigating superconductivity with the first principles approach. Particularly we explore the aspects of tip-induced superconductivity and provide a microscopic understanding of its absence, emergence, and enhancement in materials like CuFeSb, hcp-Zr and ZrSiS. We further investigate and conclude that the doped ZrB2 is not a phonon-mediated superconductor. The second part of the thesis focuses on developing a microscopic Hamiltonian model that qualitatively and quantitatively explains the available experimental data. We develop a long-range anisotropic Heisenberg (XXZ) Hamiltonian and demonstrate that magnetic interactions beyond the first neighbor are crucial. Our findings show that by externally modulating exchange and anisotropic magnetic interactions through gate-induced charge carrier doping, magnetic ordering in CrI3 and CrBr3 monolayers can be substantially controlled up to room temperature. These results open up new possibilities for electrically controlled spintronic and magnetoelectric devices based on atomically thin crystals. In the final part of the thesis, we investigate the potential emergence of Majorana zero modes in superconductor/ferromagnetic van der Waals heterostructures. Specifically, we focus on the NbSe2/CrI3 heterostructure and employ first-principles calculations in conjunction with tight-binding and Bogoliubov-de Gennes Hamiltonians. Our analysis reveals the appearance of six pairs of nodal points within the hexagonal Brillouin zone under an applied in-plane magnetic field in the superconducting state. Additionally, we observe a Majorana flat band on the armchair edge of the NbSe2 nano-ribbon, which disappears on the zigzag edge. Based on these results, we conclude that vdW SC/FM heterostructures are a promising platform for investigating and realizing Majorana zero modes.