The magnetic properties of axially confined, hydrogenated single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24 are systematically explored by density functional theory. Emphasis is placed on the relation between the ground-state magnetic moments of SWCNTs and zigzag graphene nanoribbons (ZGNRs). Comparison between the SWCNTs considered here and ZGNRs of equal length gives rise to two basic questions: 1) how does the nanotube curvature affect the antiferromagnetic order known to prevail for ZGNRs, and 2) to what extent do the magnetic moments localized at the SWCNT edges deviate from the zero-curvature limit of n/3 μB? In response to these questions, it is found that systems with n≥7 display preference for antiferromagnetic order at any length investigated, whereas for n=5, 6 the magnetic phase varies with tube length. Furthermore, elementary patterns are identified that describe the progression of the magnitude of the magnetic moment with n for the longest tubes explored in this work. The spin densities of the considered SWCNTs are analyzed as a function of the tube length L, with L ranging from 3 to 11 transpolyene rings for n≥7 and from 3 to 30 rings for n=5 and 6. Magnetic carbon nanostructures are explored by density functional theory calculations on axially confined, single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24. For SWCNTs with n≥7, antiferromagnetic (AFM) order is favored energetically over ferromagnetic (FM) order for all lengths L investigated, whereas for n=5, 6 the magnetic phase varies with tube length (see picture).

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<p>Scientific progress often relies on unexpected discoveries and unique observations. In</p> <p>fact, many of the most groundbreaking scientific advances throughout history have been the</p> <p>result of serendipitous events. For instance, the discovery of penicillin by Alexander Fleming</p> <p>was a result of him noticing a mold growing on a petri dish that was contaminating his</p> <p>bacterial culture. Similarly, the discovery of the cosmic microwave background radiation,</p> <p>which is considered one of the strongest pieces of evidence for the Big Bang theory, was</p> <p>the result of two scientists accidentally stumbling upon it while conducting a completely</p> <p>different experiment. These types of unexpected discoveries can lead to new avenues of</p> <p>research and open up entirely new fields of study. During my PhD, I experienced a similar</p> <p>phenomenon when I stumbled upon an anomaly in my experimental data that led me down a</p> <p>completely new path of investigation. This unexpected discovery not only provided me with</p> <p>new insights into the underlying mechanisms of my research, but also opened new avenues for</p> <p>future research directions. It was a reminder that sometimes the greatest scientific progress</p> <p>can come from the most unexpected places.</p> <p>My primary focus was initially directed towards topological superconductivity. However,</p> <p>this research direction was modified by unexpected findings while characterizing a SQUID.</p> <p>Specifically, a unique response by a Josephson junction was observed when exposed to an inplane</p> <p>magnetic field. Chapter 1 details our experimental results on the SQUID. We observed</p> <p>intriguing effects resulting from the in-plane magnetic field in the asymmetric evolution of</p> <p>the Fraunhofer pattern suggesting the existence of additional underlying physics in the heterostructure,</p> <p>which may have been previously overlooked. This serendipitous finding served</p> <p>as the impetus to explore simpler superconducting devices such as nanowires and rings.</p> <p>Remarkably, subsequent investigations into the critical current of a superconducting ring revealed</p> <p>a bi-modal histogram arising from the application of an in-plane magnetic field, which</p> <p>was an unforeseen outcome. This adds to our observations made in chapter 1. Chapter 2 details</p> <p>the unique properties of Al-InAs superconducting rings. Further experiments involving</p> <p>a superconducting nanowire resulted in the observation of non-reciprocal critical current under</p> <p>an in-plane magnetic field perpendicular to the current direction, subsequently referred to as the superconducting diode effect. Chapter 3 delves into the non-reciprocal properties</p> <p>of an Al-InAs superconducting nanowire. Our findings revealed the diamagnetic source of</p> <p>non-reciprocity generic to multi-layer superconductors. Finally, chapter 4 provides a detailed</p> <p>account of the fabrication processes for the superconducting devices, along with a discussion</p> <p>of the measurement techniques employed to unveil the underlying physics.</p>

In this thesis, Density Functional Theory and Time-Dependent Density-Functional Theory approaches are applied to study the optical and magnetic properties of several types of nanostructures. In studies of the optical properties we mainly focused on the plasmonic and excitonic effects in pure and transition metal-doped noble metal nanochains and their conglomerates. In the case of pure noble metal chains, it was found that the (collective) plasmon mode is pronounceable when the number of atoms in the chain is larger than 5. The plasmon energy decreases with further with increasing number of atoms (N) and is almost N-independent when N is larger than 20. In the case of coupled pure chains it was found that the plasmon energy grows as square root of the number of chains, and reaches the visible light energy 1.8eV for the case of three parallel chains. Doping of pure Au chains with transition-metal atoms leads in many cases to formation of additional plasmon peaks close in energy to the undoped chain peak. This peak comes from the local charge oscillations around the potential minima created by the impurity atom. The effect is especially pronounced for Ni-doped chains. In the multiple-chain case, we find an unusual hybridization of the two different (local and collective) plasmon modes. Changing the chain size and chemical composition in the array can be used to tune the absorption properties of nanochains. The case of coupled finite (plasmonic) and infinite (semiconductor, excitonic) chains was also analyzed. We find that one can get significant exciton-plasmon coupling, including hybridized modes and energy transfer between these excitations, in the case of doped chains. The impurity atoms are found to work as attraction centers for excitons. This can be used to transform the exciton energy into local plasmon oscillations with consequent emission at desired point (at which the impurity is located). In a related study the optical properties of single layer MoS2 was analyzed with a focus on the possibility of ultrafast emission, In particular, it was found that the system can emit in femto-second regime under ultrafast laser pulse excitations. Finally, we have studied the magnetic properties of FeRh nanostructures to probe whether there is an antiferromagnetic to ferromagnetic transition as a function of the ratio of Fe and Rh atoms, as in the bulk alloy.. Surprisingly, the ferromagnetic phase is found to be much more stable for these nanostructures as compared to the bulk, which suggests that band-type effects may be responsible for this transition in the bulk, i.e. the transition cannot be described in terms of modification of the Heisenberg model parameters.

Applications, converter topologies, and control schemes are examined for superconductive magnetic energy storage (SMES) systems. Diurnal load leveling for electric utilities and compensation for fluctuations in photovoltaic (PV) power generation are the primary applications discussed. It is demonstrated that a SMES system implemented with standard AC/DC converters offers energy storage capacity large enough, and dynamic response fast enough, to compensate for PV fluctuations due to changes in weather conditions. The method of control is developed so that the charging and discharging of the SMES system are changed in response to PV fluctuations, and the combined SMESIPV power output is smooth and controllable. An innovative control scheme is introduced for SMES that can simultaneously regulate real power and voltage independently without hardware modifications to the standard ACIDC bridge arrangement normally used for coordinated control of real and reactive power. The combination of SMES and PV systems could benefit from DCIDC converters that take advantage of the DC nature of both. It is established that DClDC converters can respond with sufficient speed to handle variations in PV power. A converter topology is devised where two DC/DC converters in cascade effectively maintain a PV array at its maximum power point and simultaneously control a SMES system to compensate for PV fluctuations. An alternative cascade configuration of an AC/DC converter with a DCIDC converter is proposed that could significantly reduce the reactive power requirements and improve the operational characteristics of a large scale SMES system connected to the utility grid. / Master of Science

<p> The magnetic structures of antiferromagnetic manganese pyrophosphate and copper pyrophosphate have been determined by single crystal neutron diffraction techniques. More detailed features of the magnetic structure have been determined by nuclear magnetic resonance (NMR). A discrepancy between previous NMR measurements on Mn2P2o7 and the single crystal neutron measurements was resolved by postulating a low temperature crystallographic phase transition. Information about the dependence of the transferred hyperfine interaction upon the separation of the relevant ions is obtained for Cu2P2o7 from the NMR data and from previously determined deviations from a higher symmetry phase. The origin of the magnetic anisotropy energy in Cu2P2o7 is discussed. </p> / Thesis / Doctor of Philosophy (PhD)

Compounds in the series La1-xTiO3 have been prepared in the powder and single crystalline forms. The structures of these compounds were studied with x-ray, neutron, and electron diffraction techniques. In all cases, a distortion from the ideal perovskite structure was found. Throughout the series, transitions occur in structural, magnetic, and electrical properties. The structure of La.67TiO3 was found to be of a distorted perovskite type arising from distortions in the titanium octahedra as a result of an ordering of the lanthanum positions and A-site cation vacancies. Although this phase has been reported many times, the crystal structure has not been determined. The La.67TiO3 and La.70TiO3 phases have the same structure and were assigned to the Pmmn space group. At La75.TiO3, a Ti3+ concentration of 2 8%, the symmetry of the structure changes from Pmmn to Imma, and distortion of the perovskite structure is caused by octahedral tilting within the lattice. A further structural transition occurs at a Ti3+ concentration of 64%, La8gTiO3, where the symmetry changes from Imma to Pnma. Again distortions from the ideal perovskite structure are caused by octahedral tilts within the system. Transitions were also observed in electrical and magnetic properties. These transitions occur at the same Ti3+ concentrations as the transitions observed in the structural properties, which suggests a correlation between properties based on Ti3+ concentration. The occurrence of A-site cation vacancies within the lattice is directly related to the Ti3+ concentrations within the individual materials. Changes in the Ti3+ concentration affect the size of the unit cell as well as the electrical and magnetic properties which range from insulators at low Ti3+ concentrations to metals at high Ti3+ concentrations. / Thesis / Master of Science (MSc)

<p> In this thesis, we investigated a recent and interesting issue in magnetism; spin ice, which is a term used for systems where there is a analogy between their magnetic structure and the proton structure of water ice. Until now, only three spin ices, Dy2Ti2O7, Ho2Ti2O7, and Ho2Sn2O7, have been discovered. In 2002, Ho2Ru2O7 was proposed as a candidate spin ice by Bansal et al. Given the similar structure and experimental behaviors of Ho2Ru2O7 and known spin ice systems, it has been an issue whether Ho2Ru2O7 is the fourth spin ice.</p> <p> In order to determine whether the new candidate is spin ice or not, the magnetic characteristics of Ho2Ru2O7 have been investigated. The frustrated system Ho2Ru2O7 has a pyrochlore structure with magnetic spins located on lattice of corner sharing tetrahedra. The crystal field originated <1 1 1> anisotropy of this sites and ferromagnetic interaction of spins give the preference of a two spin in and two spin out to a center of the tetrahedra.</p> <p> High quality polycrystalline samples were prepared and crystal growth attempts were made, then various measurements have been conducted. DC susceptibility data were used to determine the effective magnetic moment and value of Weiss temperature(θ). Zero field cooled (ZFC) and field cooled (FC) susceptibility data show a small irreversibility below 95K, which indicate ruthenium antiferromagnetic ordering. AC susceptibility measurements show a strong frequency dependence of the susceptibility which is a feature characteristic of spin glass or superparamagnetic materials.</p> <p> Specific heat experiments were conducted to also confirm the existence of Ru magnetic ordering at 95 K. The Debye temperature is estimated to be ~441 K.</p> <p> μSR measurements were conducted at TRIUMF. The measurements of Ho2Ru2O7 show signatures of ruthenium ordering near 95K and holmium near 1.4K. In agreement with previous neutron scattering experiment we conclude that the ground state of Ho2Ru2O7 is antiferromagnetic rather than spin ice. Presumably the ruthenium ordering acts to preclude the holmium moments entering the spin ice state.</p> / Thesis / Master of Science (MSc)