DSpace Collection:
https://dspace.lboro.ac.uk/2134/8270
Fri, 18 Apr 2014 18:20:40 GMT2014-04-18T18:20:40ZMobile electron pairs on lattices in the UV Model
https://dspace.lboro.ac.uk/2134/8563
Title: Mobile electron pairs on lattices in the UV Model
Authors: Jackson, Steven
Abstract: We investigate the conditions under which the ground state of a low-density quasi-twodimensional
electron (or hole) system is a Bose-Einstein condensate of mobile dimers.
Such a ground state would require an effective attraction between electrons but an
effective repulsion between dimers to prevent clustering. A UV model is assumed;
this is not specific to the pairing mechanism but can be obtained from a Fr¨ohlich-
Coulomb model by the Lang-Firsov transformation. We survey the parameter space for
each lattice restricting the dimer Hilbert spaces to the low-energy sector since we are
interested in low-lying states and low densities. Singlet dimers are mobile on a triangular
lattice; in the simplest case the effective Hamiltonian for dimer hopping is that of a
kagome lattice. However, a dimer condensate is never the ground state in the triangular
lattice, as dimers will either cluster or dissociate. For a square lattice with nearest- and
next-nearest-neighbour hopping we find a substantial region in which dimers form a
ground state. These dimers turn out to be very light since they can propagate by a
“crab-like” motion without requiring virtual transitions. For a perovskite layer we find
a substantial region in which dimers, which are also light and mobile due to crab-like
motion, form a ground state. Our findings indicate that the existence of stable small
mobile bipolarons is very sensitive to the lattice structure.
We secondly identify circumstances under which triplet dimers are strictly localised
by interference in certain one- and two-dimensional lattices. We find that strict localisation
is possible for the square ladder and some two-dimensional bilayers. We thirdly
investigate the electronic properties of Graphene. We identify the origin of Graphene’s
Dirac points and subsequently identify Dirac points in other two- and three-dimensional
lattices. We finally investigate the dynamics of electrons and dimers on various oneand
two-dimensional lattices by the use of Green’s functions.
Description: This thesis is confidential until 31/12/2021. A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.Sat, 01 Jan 2011 00:00:00 GMThttps://dspace.lboro.ac.uk/2134/85632011-01-01T00:00:00ZOn the formation of stationary spatial patterns of luminescence in two-dimensional exciton systems
https://dspace.lboro.ac.uk/2134/7107
Title: On the formation of stationary spatial patterns of luminescence in two-dimensional exciton systems
Authors: Paraskevov, Alexander V.
Abstract: The thesis is organized as follows:
In Chapter 1 the introduction and the description of motivating
experiments are given. It also contains the formulation of the
problems for the research.
Chapter 2 describes two kinetic mechanisms suggested to explain the ring-shaped luminescence pattern formation and, to investigate the influence of one of them, the results of molecular dynamics simulation of a spatiotemporal evolution of the locally photoexcited electrons and holes localized in two separate layers. In particular,
it is shown that the ring-shaped spatial pattern of luminescence can be formed due to the strong in-layer Coulomb interactions in the excitation spot at high photoexcitation power. The strong
interactions become possible because of the bilayer geometry.
In Chapter 3 a simple microscopic mechanism which explains the
linear dependence of the radiative lifetime of free-moving
two-dimensional excitons on their effective temperature is
suggested. It is shown that there exists a characteristic effective
temperature (of about few Kelvin) defined by exciton-acoustic phonon
interaction at which the radiative lifetime is minimal. Below this temperature the lifetime starts to increase with decreasing temperature. The correspondence with previous theoretical and experimental results is discussed.
In Chapter 4 the external ring fragmentation is considered under the assumption that excitons at the ring become statistically degenerate at low temperatures. In particular, the exciton condensate density at the ring has been found as a function of the polar angle at zero
temperature with the involvement of exciton formation and
recombination processes. Starting from the quasi one-dimensional
Gross-Pitaevskii equation with a spatially uniform pumping-and-decay term, an exact analytical solution is derived yielding the spatial fragmentation of the exciton ring in a certain range of parameters.
Chapter 5 is Appendix. It contains some results which are essential for the whole thesis.
Description: A Doctoral Thesis. Restricted access with a Moratorium period of 20 years ending 5th October 2030. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.Fri, 01 Jan 2010 00:00:00 GMThttps://dspace.lboro.ac.uk/2134/71072010-01-01T00:00:00Z