Nanoscale devices for studying quantum fluids and electrostatic field-effects in superconducting nanoconstrictions

Guthrie, Andrew and Kafanov, Sergey and Tsepelin, Viktor (2020) Nanoscale devices for studying quantum fluids and electrostatic field-effects in superconducting nanoconstrictions. PhD thesis, Lancaster University.

Text (2020GuthriePhD)
2020_Andrew_Guthrie_PhD_Thesis.pdf - Published Version
Available under License Creative Commons Attribution.
Download (19MB)

Abstract

Nanofabricated devices, where the characteristic dimension is less than one micron (10^-6 m), have remarkable potential as highly sensitive tools to observe the quantum world to which we belong. By operating such devices at extremely low temperatures we can engineer physical scenarios never before seen in nature. Investigating the laws which govern at such small energy and length scales has the potential to upend our current understanding of the universe. This thesis introduces two main bodies of experimental work, both of which utilise nanoscale devices as precise detectors for various unique quantum phenomena. In Part I, nanomechanical devices with extremely high mass sensitivity were used as probes of quantum fluids, namely superfluid 4He. We present the first measurements of nanomechanical devices in superfluid 4He, measuring the spectra of thermal excitations, and use our devices to demonstrate a new quasiparticle driving mechanism, which we call the ‘phonon wind’. Here, a nanobeam can be moved coherently under the influence of a modulated flux of thermal excitations. We go on to use nanobeams as high-speed detectors for the phenomenon of quantized vortices in 4He, ultimately concluding with the systematic study of a single vortex trapped by a nanobeam. We also present the first measurements of micromechanical torsional tuning forks in superfluid 4He, demonstrating a unique multimode detection scheme where torsional oscillations are used to sense quantised vortices generated by flexural oscillations. In Part II, nanoelectronic devices with extremely high charge sensitivity were used as probes of the superconducting field-effect. The superconducting field-effect, as previously observed in nanoconstrictions, is a completely unexpected phenomenon and cannot theoretically coexist with the well-established Bardeen, Cooper and Schrieffer theory of superconductivity. Using a gated Dayem bridge coupled to a high-frequency circuit, we characterise this effect with far greater time resolution than previously possible, demonstrating that the observed effect is merely a result of electron tunnelling causing localised heating in the constriction.