Title:
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The geometry and topology of change-ordered quantum fields in low-dimensional systems
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Charge ordering is a phenomenon in which the electron field in a
material spontaneously breaks the symmetry of the underlying
crystal lattice, selecting out a new period and, in dimensions higher
than one, a particular direction in space.
In the first half of this thesis I study the consequences of charge
ordering in ID, applying a self-consistent mean-field approach. A
system with rational filling p/q forms a period q charge density
wave, which I demonstrate exhibits a quantized adiabatic particle
transport upon being dragged through a full period. I show that an
irrationally-filled system is quasiperiodic, and use the equivalence
to show that ID quasicrystals fit into the topological classification of
free fermion systems known as the Tenfold Way. Using a free energy
analysis I demonstrate that incommensurate charge order provides
a new non-local growth mechanism for ID quasicrystals, potentially
greatly increasing the number of known, naturally-occurring,
examples.
In the second half of this thesis I address the question of whether
the ID charge ordering mechanism, the Peierls instability, applies in
dimensions higher than one, focussing on the prototypical 2D
charge-ordered material niobium diselenide, NbSe2' In this case I
definitively rule out such 'weak-coupling' theories, and show that it
is necessary to consider a model of a strong electron-phonon
coupling dependent on both the ingoing and outgoing electron
momenta and the electronic bands scattered between. The model
provides the first consistent theoretical account of the full range of
experimental results on the system, including a particle/hole
asymmetric gap centred above the Fermi level which opens in one
band only, the softening of phonon frequencies over a wide range of
momenta, and the existence of a pseudogap regime over a range of
temperatures, with the latter explained as suppression of charge
order through fluctuations of the phonon field.
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