Probing Ising Superconductivity and Light-Induced Phase Transitions in Two-Dimensional Transition Metal Dichalcogenides

Abstract

2H-NbSe2 is a superconducting transition metal dichalcogenide that retains its superconductivity in its two-dimensional (2D) form. Owing to strong spin orbit coupling, 2D 2H-NbSe2 demonstrates unconventional superconductivity that allows both spin-singlet and spin-triplet Cooper pairing. There is experimental and theoretical evidence that 2D 2H-NbSe2 is an Ising superconductor. However, a direct measurement of its Ising nature has not yet been proven. In this thesis, I present the results of measurements on a 2H-NbSe2 superconducting spin filter tunnel junction fabricated with mechanically exfoliated 2D flakes. Using this device geometry, it may be possible to elucidate the pairing nature in 2H-NbSe2. Intense ultrashort laser pulses can photo-excite non-equilibrium states in materials leading to transient new phases and exotic states. In bulk materials these photo-excited states are short-lived. 1T′-MoTe2 in bulk form undergoes a reversible transition at ~250 K from the monoclinic phase (1T′-MoTe2) to the inversion-symmetry breaking, orthorhombic phase (Td-MoTe2). Td-MoTe2 is a candidate type II Weyl semimetal that is predicted to demonstrate exotic quantum phenomena. These phases in 2D MoTe2 flakes are characterized using ultrafast optical-pump probe techniques. The results indicate a permanent photo-induced structural change occurs in thin Td-MoTe2 at high fluences. Given the durability of this structural change, further characterization of the sample is achieved using magnetotransport measurements.