Ultrafast transport experiments on optically-driven K3C60

Abstract

Optical excitation has emerged as a powerful tool to investigate and control the properties of matter. Particular attention in this regard has been devoted to quantum materials, since they present a rich variety of entangled and topologically non-trivial phases that are extraordinarily sensitive to external parameters. Through coherent excitation with ultrashort laser pulses, it has been shown that complex transient phases can be induced on femtosecond timescales, demonstrating great potential for high-speed technology utilising non-equilibrium quantum phenomena.

Harnessing non-equilibrium phenomena in electronic devices is a natural next step for both scientific understanding and potential applications. Electronic devices allow properties to be measured under current and voltage biases, and provide a reliable platform for multiple components to interact, leading to more complex functionalities. However, conducting transport experiments on short-lived non-equilibrium phenomena necessitates a departure from conventional electronics. The first part of this thesis details the development of an ultrafast opto-electronic platform to carry out charge-transport and voltage measurements on timescales shorter than one picosecond.

Recent studies on the alkali-doped fulleride K3C60 have demonstrated signatures of a light-induced superconducting-like phase at temperatures far above its equilibrium critical temperature. To date, however, this phenomenon has predominantly been characterised by its optical features in the terahertz frequency range. In this work, the light-induced phase was investigated in MBE-grown K3C60 by means of sub-terahertz electronic transport. Two ultrafast transport experiments were conducted on granular thin films that revealed a finite enhancement in conductivity, characterised by nonlinear current-voltage behaviour in the excited state (Chapter 5) and inductive voltage dynamics upon in-current excitation (Chapter 6).

The results presented in this work complement a growing body of literature on ultrafast electronics. Furthermore, these experiments demonstrate the ultrafast electronic platform developed here as a valuable tool for investigating non-equilibrium phenomena, paving the way towards high-speed applications.