http://theses.ncl.ac.uk/jspui/handle/10443/5374 Sat, 08 Aug 2026 06:24:11 GMT 2026-08-08T06:24:11Z http://theses.ncl.ac.uk/jspui/handle/10443/6792 Title: Self Interacting Fuzzy Dark Matter and its Observational Implications Authors: Indjin, Milos Abstract: We investigate aspects of Fuzzy Dark Matter (FDM) in both the non-interacting and repul sively self-interacting regimes, critically discussing the role of interactions on virialised profiles, using our findings to fit to galactic rotation curves of a range of dark matter dominated galax ies, thus proposing a way to numerically reconstruct a galaxy from analysing observational curves. Specifically, we firstly analyse the ground state solution of the Gross-Pitaevskii-Poisson equations- the soliton core. A generalised ansatz is formed along with a soliton-specific di mensionless interaction strength. The virial theorem is utilised to explain core dependencies on boson (m) and total mass, and self-interaction (g), and a degeneracy in the pairing of m and g is also uncovered. Moreover, we introduce a Super-Gaussian profile from empirically fitting to numerically generated self-interacting soliton ground states. This profile, joined with a Navarro-Frenk-White (NFW) halo profile, is used to fit self-interacting FDM to observational data of dark matter dominated galaxies, finding the crucial result that a single m and g pair fits all selected galaxies. Thirdly, we present three dimensional (3D) simulation results of self interacting FDM, and we demonstrate a scheme which allows us to reconstruct density profiles whose inferred rotation curves replicate observational data. In particular, we present a sim ulated self-interacting FDM halo corresponding to a galaxy for which we have observational data, specifically galaxy UGCA444. We find an elegant form of the core-halo mass relation, depending on the total energy of the system and qualitatively comment on the variation in granule number and size depending on total energy. Next, we analyse the dynamical aspects of the halo. In particular we (i) investigate the effects of m and g on the oscillation frequency of soliton cores; (ii) reveal evidence for a cooling (condensation) process, where the core density grows on time scales longer than the age of the universe; (iii) finally, we undertake a preliminary investigation of the orbital dynamics of test particles in our recreated galaxy, commenting on the velocity dispersion and the implications for the age gradient of stars in disc galaxies. Description: Ph. D. Thesis. Wed, 01 Jan 2025 00:00:00 GMT http://theses.ncl.ac.uk/jspui/handle/10443/6792 2025-01-01T00:00:00Z http://theses.ncl.ac.uk/jspui/handle/10443/6783 Title: A study of imbalanced quantum droplets Authors: Flynn, Thomas A Abstract: Gases of ultracold atoms have proved to be a highly versatile platform for studying quantum physics due in part to their high controllability. One example of this control is the manipulation of interactions. A single-component Bose gas in free space is unstable to collapse under attractive contact interactions, whereas, in an attractive two-component Bose gas the collapse is arrested by the repulsion from quantum fluctuations, forming self-bound liquid droplets: quantum droplets. Quantum droplets are therefore a quantum fluid in which quantum fluctuations are not only present but integral. During formation quantum droplets preserve a constant density ratio such that the droplet can be approxi mately described by its total density rather than in terms of two independent component densities. This approximation, known as density locking, is powerful but limiting. Droplet experiments typically begin with a population imbalance during formation, meaning that imbalanced droplets may be the most prevalent. Motivated by a link with an experi mental 174YbCs mixture, which deviates from many of the assumptions built into the density-locked model, the primary focus of this thesis is: how are droplets modified by imbalances that break the assumptions of density locking? This thesis begins by showing that, up to a saturation limit, free-space droplets can lower their energy through population imbalancing but correspondingly become less stably bound. By perturbing these imbalanced droplets, breathing modes are observed to have a complex parameter space of multiple, superimposed, decaying modes with time-dependent frequencies and decay rates. All droplets are experimentally formed in harmonic traps, and so the free-space imbal anced results are extended to study how harmonic traps affect the structure and breathing modes of imbalanced droplets. Beyond the droplet formation, there is a crucial question around the stability of the imbalance during a release into free space. A key result of this thesis is that imbalances can be retained during either an instantaneous or linearly-ramped trap release. Finally, the imbalanced droplet results above are applied to explore the 174YbCs pa rameter space, by measuring droplet size, peak densities and lifetimes. A variety of dynamics are studied from droplet formation and excitations, to proposing potential ex periments serving as a platform to study the non-equilibrium dynamics of droplets. Description: PhD Thesis Wed, 01 Jan 2025 00:00:00 GMT http://theses.ncl.ac.uk/jspui/handle/10443/6783 2025-01-01T00:00:00Z http://theses.ncl.ac.uk/jspui/handle/10443/6773 Title: Charge control of defect colour centres in hexagonal boron nitride Authors: Prasad, Madhava Krishna Abstract: Colour centres in hexagonal boron nitride (hBN) have gained significant interest as they are active at room temperature and embedded in a two-dimensional material. The latter leads to minimal total internal reflection and coupling to photonic devices with nanometre proximity. Colour centres in hBN are particularly interesting as they have applications in quantum information, communications, metrology and LED technology. Current research has largely focused on photoluminescence from these defects. However, the integration of solid-state emitters into electronics necessitates the defects to be electrically driven. It is therefore essential to understand charge control of these defects. Recently, it was discovered that the charge state of defects in hBN can be controlled by interfacing it with graphene. It was observed that charge transfer between optically active defects in hBN and graphene led to the quenching of photoluminescence from these emitters. Therefore, interfacing hBN with graphene offers a pathway to inject carriers into hBN for the electrical control of defects. This is especially useful as graphene, also being a two-dimensional material, is a suitable gate to apply vertical electric fields across emitters in hBN. In this thesis, the study of charge transfer between hBN and graphene has been ex plored both from a theoretical and experimental standpoint. The theoretical approach involves using density functional theory to perform electronic structure calculations of de fects in hBN and subsequently deriving defect properties, such as band structure, defect geometry, and formation energies. The results of these calculations allowed the determina tion of the degree of charge transfer between defects in hBN and graphene. Furthermore, periodic boundary conditions and the choice of methodology for the assignment of charge to different layers had a significant impact on the derived value of charge transfer. A methodology to determine charge transfer that was robust against these effects was de veloped. The impact of the thickness of encapsulating hBN and graphene layers on charge transfer and the thermodynamics of the defect was also explored. The energy path for the Charge control of defect colour centres in hexagonal boron nitride reorientation of the defect between two equilibrium structures was studied to determine metastable intermediate states. It was found that the energy ordering of different phases of a defect was sensitive to the encapsulation of the host layer by graphene and hBN. As such, encapsulating layers can impact the spectroscopic properties of the defect. Experimentally, site control of emitters in hBN was studied. This involved the growth of an aluminium oxide (Al2O3) spacer between graphene and hBN to mitigate the quench ing of emitters. The Al2O3 was then patterned using electron beam lithography and etched to create an array of pillars. hBN was then deposited on top hosting emitters, with the aim of emitters being quenched in regions where there was direct contact with graphene and active in regions where an intermediate Al2O3 layer was present. Emitters were found predominantly on the edges of the pillars and quenching occurred in the regions where the pillars were absent. Hence spatial contol over colour centres in hBN was achieved. Description: Ph. D. Thesis. Wed, 01 Jan 2025 00:00:00 GMT http://theses.ncl.ac.uk/jspui/handle/10443/6773 2025-01-01T00:00:00Z http://theses.ncl.ac.uk/jspui/handle/10443/6765 Title: Phase transition and conductivity modification of porous silicon via CO2 laser processing Authors: Qadi, Haifa Ali Ibrahim Abstract: This study investigates how laser irradiation modifies the optical, structural and electrical properties of porous silicon (PS). PS samples were fabricated by electrochemically etching p-type crystalline silicon wafers at a current density of 63 mA/cm2 for 20 minutes. A continuous wave CO2 laser (10.6 µm, 5–40 W) was then applied. A comprehensive suite of characterisation techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Kelvin probe force microscopy (KPFM), Raman spectroscopy and current-voltage (I-V) measurements was used to analyse the samples before and after laser irradiation. The unique interaction between the 10.6 µm CO2 laser and PS is attributed to the significant IR absorption of PS at this wavelength, unlike bulk silicon. Upon laser irradiation, vibrational excitation triggers localised heating and the formation of heat zones, which induce two distinct stages. During the pre-melting consolidation stage, localised heating leads to partial melting of pore walls, causing adjacent pores to merge. As pores coalesce, the number of individual pores decreases, and the material between them consolidates, resulting in thicker pore walls, wider remaining pores, and an overall reduction in porosity. SEM and AFM analyses reveal a decrease in PS layer thickness and a smoother surface, with a roughness decreasing from 1.48 to 0.82 nm, approaching that of the underlying crystalline silicon substrate. In the melting stage, where the temperature exceeds the melting point of the silicon, the AFM images show a complete pore collapse, indicating that the silicon has fully melted. Raman spectroscopy transitions from a broadened, redshifted spectrum with an additional lower-frequency peak to a sharp crystalline peak at 520 cm-1 (with a 2.8 cm-1 full width at half maximum) in the laser-modified regions. Increasing laser intensity and exposure further broadens the peak and induces a redshift, accompanied by the emergence of a secondary peak, suggesting stress effects. XRD confirms the reformation of a single-crystalline structure, while XPS analysis reveals significant oxidation and the formation of a silicon oxide layer in the laser-irradiated areas, a layer that when removed by HF etching restores the Raman spectral profile. Electrical characterisation shows that the laser-processed areas exhibit metallic-like behaviour, as evidenced by I-V measurements and KPFM data indicating a high-work function network. This behaviour is attributed to the uneven distribution of boron during electrochemical etching, leading to the formation of heavily doped silicon nanowires. Upon laser irradiation, these regions melt and recrystallise, resulting in localised metallic-like conduction because of the high density of charge carriers and a reduced bandgap. Overall, the controlled laser irradiation process offers a precise means of tailoring PS properties, with significant implications for advanced applications in optical waveguides, biosensors, photovoltaics, microelectronics, energy storage, and plasmonic devices Description: PhD Thesis Wed, 01 Jan 2025 00:00:00 GMT http://theses.ncl.ac.uk/jspui/handle/10443/6765 2025-01-01T00:00:00Z