Recent experiments have shown that junctions consisting of individual single-molecule magnets (SMMs) bridged between two electrodes can be fabricated in three-terminal devices, and that the characteristic magnetic anisotropy of the SMMs can be affected by electrons tunneling through the molecule. Vibrational modes of the SMM can couple to electronic charge and spin degrees of freedom, and this coupling also influences the magnetic and transport properties of the SMM. The effect of electron-vibron coupling on transport has been extensively studied in small molecules, but not yet for junctions of SMMs. The goals of this thesis will be two-fold: to present a novel approach for studying the effects of this electron-vibron coupling on transport through SMMs that utilizes both density functional theory calculations and model Hamiltonian construction and analysis, and to present a software framework based on this hybrid approach for the simulation of transport across user-defined SMMs. The results of these simulations will indicate a characteristic suppression of the current at low energies that is strongly dependent on the overall electron-vibron coupling strength and number of molecular vibrational modes considered. / Master of Science

Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties such as quantum tunneling of magnetization, quantum interference, spin filtering effects, strong spin-phonon coupling and strong hyperfine Stark effects. These effects allow applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Therefore, SMMs are of interest to physicists, chemists, and engineers. Recently, experimental fabrication of individual SMMs within transistor set-ups have been achieved, offering a new method to examine magnetic properties of individual SMMs. In this thesis, two types of SMMs, specifically Eu2(C8H8)3 and Ni9Te6(PEt3)8, are theoretically investigated by simulating their electron transport properties within three-terminal transistor set-ups. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). We explore the exchange coupling of iron atoms in two EMACs: [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an introduction to density functional theory (DFT). Chapter 2 presents a theoretical study of electron transport via Eu2(C8H8)3. This type of molecule is interesting since its magnetic anisotropy type changes with oxidation state. The unique magnetic properties lead to spin blockade effects at zero and low bias. In other words, the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. Chapter 3 discusses a theoretical study of electron transport via Ni9Te6(PEt3)8. The magnetic anisotropy of this magnetic cluster has cubic symmetry, which is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena such as low-bias blockade effects, negative conductance and discontinuous conductance lines, are observed. In Chapter 2 and 3 DFT-calculated magnetic anisotropy parameters are used and electron transport properties are calculated by solving master equations at low temperature. Chapter 4 examines the exchange coupling between iron ions in EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. The exchange coupling constants are calculated by using the least-squares fitting method, based on the DFT-calculated energies from different spin configurations. / Ph. D. / Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties. Its quantum effects are used to describe the behavior of SMMs at the smallest scales. These quantum properties could also be used to reveal possible applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Thus SMMs are of interest to physicists, chemists, and engineers. Recently, electron transport via individual SMMs was achieved in experiments. Electron transport is obviously affected by the magnetic properties of the SMM, thus one can examine magnetic properties of an SMM indirectly by measuring electron transport via the SMM. In this thesis, two types of SMMs, Eu2(C8H8)3 and Ni9Te6(PEt3)8, are investigated theoretically by simulating their electron transport properties. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). If a molecule has magnetic anisotropy, its magnetic properties change with the direction of its magnetic moment. We explore how iron atoms interact with each other in the EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an approximation method, density functional theory (DFT). DFT is a method to approximate electronic structure and magnetic properties of various many-body systems. Chapter 2 investigates theoretical electron transport via Eu2(C8H8)3. Eu2(C8H8)3 changes its type of magnetic anisotropy when it obtains an extra electron, which is different from most SMMs. If the Eu2(C8H8)3 is short of an extra electron, its magnetization direction is in-plane, that is, its magnetic energy is lowest when its magnetic moment is along any direction in a specific plane. If an extra electron is captured by Eu2(C8H8)3, its magnetization direction becomes out-of-plane, and its lowest energy is obtained when its magnetic moment is along the direction normal to the specific plane. The unique magnetic properties lead to blockade effects at low bias: the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. The bias voltage on a molecule equals the electrical potential difference between two ends of the molecule. Chapter 3 investigates theoretical electron transport via Ni9Te6(PEt3)8. Magnetic anisotropy of Ni9Te6(PEt3)8 is cubic symmetric, and its symmetry is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena are observed. These phenomena include (1) current is completely suppressed when bias is low; (2) current via SMM decreases while bias on SMM increases; (3) there are discontinuous lines in the figures that describe electrical conductance of current. Chapter 4 examines the iron atoms’ interaction strength in both [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Reasonable spin Hamiltonians are used to describe the energy of EMACs. Considering all possible directions of the spins of iron atoms in two EMACs, we calculate the energy of every possible spin configuration using DFT. The energy of each spin configuration can be expressed as an equation containing one or more coupling constants. We apply the least-squares fitting method to obtain the values of the coupling constants in the spin Hamiltonians.

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In this thesis we study the effect of electron-electron interactions on Chiral two-dimensional electron gas (C2DEGs). C2DEGs are a very good description of the low-energy electronic properties of single layer and multilayer graphene systems. The low-energy properties of single layer and multilayer graphene are described by Chiral Hamiltoninans whose band eigenstates have definite chirality. In this thesis we focus on the effect of electron-electron interactions on two of these systems: monolayer and bilayer graphene. In the first half of this thesis we use the massless Dirac Fermion model and random-phase-approximation to study the effect of interactions in graphene sheets. The interplay of graphene's single particle chiral eigenstates along with electron-electron interactions lead to a peculiar supression of spin susceptibility and compressibility, and also to an unusual velocity renormalization. We also report on a theoretical study of the influence of electron-electron interactions on ARPES spectra in graphene. We find that level repulsion between quasiparticle and plasmaron resonances gives rise to a gap-like feature near the Dirac point. In the second half we anticipate interaction driven integer quantum Hall effects in bilayer graphene because of the near-degeneracy of the eight Landau levels which appear near the neutral system Fermi level. We predict that an intra-Landau-level cyclotron resonance signal will appear at some odd-integer filling factors, accompanied by collective modes which are nearly gapless and have approximate q[superscrit 3/2] dispersion. We speculate on the possibility of unusual localization physics associated with these modes. / text

Commercial sterilization of medical devices may be performed using electron beam irradiators, which operate at various electron energies. The potential for activating components of the devices has been discussed, with current standards stating that an electron energy greater than 10 MeV requires assessment of potential induced radioactivity. There does not appear to be a literature citation for this energy limit, but it is the accepted default assumption within the industry. This research was directed at evaluating potential activation in medical products sterilized in electron beam as a function of the electron maximum energy. Monte Carlo simulation of a surrogate medical device was used to calculate photon and neutron fields resulting from electron irradiation, which were used to calculate concentrations for several radionuclides. The predominant mechanism for inducing radioactivity is photoneutron production in metal elements. Other mechanisms, including photoneutron production in deuterium with subsequent neutron capture, neutron capture of the photoneutrons produced in metal elements, and isomeric excitation, are all possible means of inducing radioactivity in similar conditions, but none made a perceptible contribution to activation in these experiments. The experiments confirmed that 10 MeV is a conservative assumption that any lower energy does not create significant activation. However, in the absence of a limited number of elements, the amount of induced radioactivity at 11 MeV and 12 MeV could also be considered insignificant. When based on an estimate of the amount of metal present in a medical device, the sum-of-fractions comparison to the US Nuclear Regulatory Commission exempt concentration limits is less than unity for all energies below 12.1 MeV, which suggests that there is minimal probability of significant induced activity at energies above the generally-accepted standard 10 MeV upper energy limit. / text

Electron crystallography is powerful for determination of complex structures. The newly-developed 3D electron diffraction (ED) methods make structure determination from nano- and micron-sized crystals much easier than using other methods, for example X-ray diffraction. Almost complete 3D ED data can be collected easily and fast from crystals at any arbitrary orientations. Dynamical effects are largely reduced compared to zonal ED patterns. 3D ED is powerful for phase identification and structure solution from individual nano- and micron-sized crystals, while powder X-ray diffraction (PXRD) provides information from all phases present in the samples. 3D ED methods and PXRD are complementary and their combinations are promising for studying multiphasic samples and complicated crystal structures. In this thesis, the feasibility and capability of 3D ED methods, specifically rotation electron diffraction (RED), in phase identification and structure determination of different kinds of crystalline materials with nano- or submicrometer-sized crystals are investigated. Experimental conditions for RED data collection and data processing in relation to data quality, as well as the challenges in the applications of RED are discussed. RED was combined with PXRD to identify phases from as-synthesized samples and to characterize atomic structures of eleven crystalline compounds. It was shown to be possible to identify as many as four distinct compounds within one sample containing submicron-sized crystals in a Ni-Se-O-Cl system. RED was also used to determine unit cell and symmetry of isoreticular metal-organic frameworks (SUMOF-7) and solve five zeolite structures with new frameworks, ITQ-51, ITQ-53, ITQ-54, EMM-23 and EMM-25 and that of a metal-organic framework (MOF), SUMOF-7I. The structure of an open-framework germanate SU-77 was solved by combining RED with PXRD. The structures of the zeolites and SU-77 were confirmed by Rietveld refinement against PXRD. High-resolution transmission electron microscopy was used to confirm the structure models of ITQ-51, EMM-25 and SUMOF-7I. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Accepted. Paper 6: Manuscript. Paper 7: Epub ahead of print. Paper 9: Manuscript. Paper 11: Manuscript.</p>