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Semiconductor nanodevices as a probe of strong electron correlations


Type

Thesis

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Abstract

An electron is usually considered to have only one type of kinetic energy, but could it have more, for both its spin and charge, or by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the linear Tomonaga- Luttinger Liquid (TLL) model, with hallmark predictions, such as spin-charge separation, having already been observed. Recent theoretical work has focused on extending the theory to deal with more realistic curved dispersions. However, experimental realisations have remained elusive until recently. Here, we report on measurements of many-body modes using a momentum-resolved tunnelling spectroscopy technique in gated 1D wires connected via air-bridges. We map the 1D dispersion in a variety of devices, both in and out of equilibrium, and observe the formation of two separate Fermi seas, associated with spin and charge excitations, at energies up to five times that of the Fermi energy, which cannot be accounted for by the noninteracting model. By reducing the length of the system, we also observe the emergence of higher-order ‘replica’ parabolic dispersions with higher momenta or negative effective mass, which is consistent with one of the leading nonlinear Luttinger theories. Determining the bare electron mass m_0 in crystals is also another problem often hindered by many-body effects. Here, Fermi-liquid physics renormalises the band mass, making the observed values density-dependent. In 1D geometries, however, the effect of interactions is strongly amplified, and they naturally decouple from the mass. By changing the level of confinement in the wires, we are able to tune the electron density down to about 18 electrons per micron or, equivalently, an interaction parameter of rs~4. This allows us to extract a constant bare mass value of m_0 = 0.0545m_e, about 20% lighter than observed in GaAs in geometries of higher dimensionality. Finally, by progressively occupying more 1D subbands, our system also allows us to change the amount of inter-subband screening by over 50%, consequently varying the effective interaction strength, in situ, all the way from the weakly to the strongly interacting regime. This ability is the first of its kind for 1D solid-state system. Our spectroscopy technique therefore offers itself as an important and powerful tool for probing strongly correlated systems where new emerging phenomena are occurring. This knowledge is now being used to develop systems for improved energy efficiency and could prove useful in designing the next generation of sustainable materials.

Description

Date

2021-12-03

Advisors

Ford, Christopher

Keywords

Luttinger Liquid, One-dimensional systems, Strong Electron Correlations, Emerging Phenomena, Tunnelling Spectroscopy, Semiconductor Physics

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
Engineering and Physical Sciences Research Council (1948695)
Clare-Yale Travel Grant

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