Ethan Fletcher (Senior Thesis, April 2021,
Advisor: Benjamin Frandsen
)

Abstract

V2O3 is an important material for the study of Mott insulators and other strongly correlated electron systems. Despite decades of research, a complete understanding of the metal-insulator transition in V2O3 has not been conclusively established. Here, we present comprehensive atomic and magnetic pair distribution function (PDF) analyses of V2O3 using both x-ray and neutron total scattering measurements, shedding new light on the mechanism of the transition from the point of view of short-range structural and magnetic correlations on both sides of the transition. We observe an abrupt structural transition with no hint of short-range monoclinic distortions above the transition temperature. This lack of structural fluctuations above the transition contrasts with the known presence of magnetic fluctuations in the high-temperature state, suggesting that the lattice degree of freedom plays a secondary role behind the spin degree of freedom in the transition mechanism.

Abstract

Quantum electrodynamics (QED) is a quantum field theory for electromagnetic interactions. By quantizing the electric field, we can analyze the properties of the electromagnetic interactions between quantum systems. Historically, QED has been able to predict the behavior of quantum particles with unprecedented success. However, the success of QED has relied upon perturbation theory to approximate results to a given order of precision. Physicists have largely abandoned analytical, closed form solutions to QED due to the problematic mathematical divergences in attempts to resolve systems non-perturbatively. By using the dressed vacuum as a basis, we may be able to renormalize systems which possess Hamiltonians of similar attributes. We use analytical tactics to avoid the divergence, and form a basis upon which the Hamiltonian can be normalized for the dressed vacuum, giving hope that we can obtain an analytical, closed-form solution for some systems in QED. By doing so, we accomplish an analytical, closed-form type of solution to problems within QED.

Abstract

Magnetic nanoparticles, such as Fe3O4, are responsive in a magnetic field. Three different sizes of Fe3O4 nanoparticles (5 nm, 12.5 nm, and 20 nm) were probed by vibrating sample magnetometry (VSM) and muon spin relaxation (μSR). VSM is a bulk magnetic probe, and μSR is a local magnetic probe. Particles of 5 nm, 12.5 nm, and 20 nm average sizes were found to exhibit strong superparamagnetic characteristics and distinct blocking temperatures. The blocked state transition occurred between 3 K and 45 K for the 5 nm particles, between 80 K and 160 K for 12.5 nm particles, and between 150 K and 300 K for the 20 nm particles. Both the VSM and μSR techniques showed spin-flip energy and magnetic anisotropy.

Jade Stevens (Capstone, August 2021,
Advisor: Benjamin Frandsen
)

Abstract

Magnetite nanoparticle samples in two sizes, 5 nm and 20 nm, undergo a 𝜇SR experiment and then asymmetry data and superparamagnetic fraction data are analyzed. Asymmetry spectra show characteristic behaviors for temperatures below, near, and above the blocking temperature, which is about 24 K for the 5 nm particles and 200 K for the 20 nm particles. Superparamagnetic fraction data and experimental data points for blocking temperature as a function of radius show that the gradual blocking transition is mostly explained by particle size distribution.

Abstract

Dilute magnetic semiconductors (DMSs) are of interest to physicists and materials scientists due to their potential applications in spintronics and quantum computing. The research I will present is not directly aimed at spintronic or quantum computing applications. Rather, it is aimed at understanding the detailed atomic and magnetic structure of DMSs, which will enable a more fundamental understanding of their properties and facilitate future applications. Two DMSs, Li(Zn,Mn)As and (Ba,K)(Zn,Mn)2As2 were investigated experimentally, with the data analyzed via pair distribution function (PDF) analysis of x-ray and neutron scattering data. Li(Zn,Mn)As was found to lack local structural deviations and possesses only weak magnetic order, while (Ba,K)(Zn,Mn)2As2 was found to have significant local structural deviations and possess more robust magnetic order. These findings suggest that useful magnetic correlations may be connected to local structural deviations. Keywords: dilute magnetic semiconductor; local structure; magnetic correlations; short-range magnetic order; spintronics

Jake Hughes (Capstone, August 2020,
Advisor: Benjamin Frandsen
)

Abstract

The properties of materials are strongly dependent on the crystal structure of the material. The atomic pair distribution function (PDF) method is a powerful method for investigating the structure of materials on length scales of several unit cells. Determining the positions of atoms within the unit cell is crucial to gaining an understanding of the overall structure of the material, but traditional methods of refining a model against experimental PDF data to determine atomic positions can be time intensive and possibly unreliable. We have taken a new approach to traditional PDF modeling using symmetry mode analysis enabled by the ISODISTORT program. Instead of using conventional Cartesian coordinates as fitting parameters, we fit symmetry mode amplitudes to try to reproduce simulated data and determine which distortion modes are active in each structure. This approach may lead to more effective modeling of experimental data and more accurate determinations of crystal structure.