Local Noise Measurements to Probe Correlated, Disordered and Doped Materials

Abstract

Emergent phenomena in quantum materials arise from the collective behaviour of interacting electrons and their coupling to the atomic lattice, leading to electronic phases that cannot be understood from the properties of individual electrons alone. A central example is superconductivity, where electrons form coherent pairs and move without resistance below a critical temperature. In strongly correlated materials such as the cuprate superconductors, superconductivity coexists with and competes against other electronic states. At the same time, disorder and doping strongly influence the local electronic environment and the way electronic correlations develop. Understanding these effects requires experimental methods that probe electronic behaviour locally and on very small length scales. This thesis uses scanning tunneling microscopy (STM) combined with local noise spectroscopy to study correlated and disordered quantum materials. In addition to measuring the average tunneling current, this approach makes use of current fluctuations, which contain information about electron dynamics and tunnelling statistics that is not accessible in standard STM measurements. A key achievement of this work is the development of a high-sensitivity cryogenic amplifier integrated into the STM, enabling reliable measurement of fast current fluctuations at the atomic scale with unprecedented resolution. This technique is first applied to the cuprate superconductor Bi2Sr2CaCu2O8+d, where noise measurements at energies associated with the pseudogap reveal signatures consistent with electron pairing above the onset of global superconductivity. This indicates that electron pairing extends beyond the superconducting state. We continue with the disordered superconductor NbN, where spectroscopy shows a robust superconducting state, while local noise measurements reveal deviations from the behaviour expected in cleaner superconductors, and show regions with in-gap states strongly modifying the local noise response, highlighting the sensitivity of charge transport to disorder. In sulfur-doped InAs, individual dopants are found to undergo dynamical ionization processes, leading to a random telegraph noise in the tunneling current. By accessing sufficiently high measurement frequencies, these fluctuations can be resolved directly, allowing the charge-state dynamics of single dopants to be quantified in a way that is not possible with conventional STM techniques. Together, these results show how local noise measurements can reveal electronic behaviour that remains hidden in conventional spectroscopic measurements of correlated, disordered, and doped quantum materials.