The topic of this thesis is vortex-physics in multi component Ginzburg- Landau models. These models describe a newly discovered class of supercon- ductors with multiple superconducting gaps, and posses many properties that set them apart from single component models. The work presented here relies on large scale computer simulations using various numerical techniques, but also some analytical methods. In Paper I, Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors, we show that in multiband supercon- ductors, even an extremely small interband proximity effect can lead to a qualitative change in the interaction potential between superconducting vor- tices by producing long-range intervortex attraction. This type of vortex interaction results in an unusual response to low magnetic fields, leading to phase separation into domains of two-component Meissner states and vortex droplets. In paper II, Type-1.5 superconductivity in multiband systems: Effects of interband couplings, we investigate the appearance of Type-1.5 superconduc- tivity in the case with two active bands and substantial inter-band couplings. such as intrinsic Josephson coupling, mixed gradient coupling, and density- density interactions. We show that in the presence of these interactions, the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields. In paper III, Semi-Meissner state and nonpairwise intervortex interactions in type-1.5 superconductors, we demonstrate the existence of nonpairwise in- teraction forces between vortices in multicomponent and layered supercon- ducting systems. Next, we consider the properties of vortex clusters in a semi-Meissner state of type-1.5 two-component superconductors. We show that under certain conditions nonpairwise forces can contribute to the forma- tion of very complex vortex states in type-1.5 regimes. In paper IV, Length scales, collective modes, and type-1.5 regimes in three- band superconductors, we consider systems where frustration in phase differ- ences occur due to competing Josephson inter-band coupling terms. We show that gradients of densities and phase differences can be inextricably inter- twined in vortex excitations in three-band models. This can lead to very long-range attractive intervortex interactions and the appearance of type-1.5 regimes even when the intercomponent Josephson coupling is large. We also show that field-induced vortices can lead to a change of broken symmetry from U (1) to U (1) × Z2 in the system. In the type-1.5 regime, it results in a semi-Meissner state where the system has a macroscopic phase separation in domainswithbrokenU(1)andU(1)×Z2 symmetries. In paper V, Topological Solitons in Three-Band Superconductors with Bro- ken Time Reversal Symmetry, we show that three-band superconductors with broken time reversal symmetry allow magnetic flux- carrying stable topolog- ical solitons. They can be induced by fluctuations or quenching the system through a phase transition. It can provide an experimental signature of the time reversal symmetry breakdown. / <p>QC 20130109</p>

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The properties and behaviors of charge transport mechanisms in highly insulating polymers are investigated by measuring conduction currents through thin film samples of low density polyethylene (LDPE). Measurements were obtained using a constant voltage method with copper electrodes inside a chamber adapted for measurements under vacuum and over a wide range of temperatures and applied fields. Field-dependent behaviors, including Poole-Frenkel conduction, space charge limited current (SCLC), and Schottky charge injection, were investigated at constant temperature. These field-dependent mechanisms were found to predict incorrect values of the dielectric constant and the field dependence of conductivity in LDPE was not found to be in agreement with SCLC predicted behavior. A model of thermally assisted hopping was a good fit at low applied fields and produced activation energies within the accepted range for LDPE. Low applied field measurements over the range of 213 K to 338 K were used to investigate two prominent hopping conduction mechanisms: thermally assisted hopping and variable range hopping. The observed temperature dependence of LDPE was found to be consistent with both thermally assisted hopping and variable range hopping. Activation energies determined for the range of temperatures were consistent with values reported in the literature for LDPE under similar conditions. A third aspect of charge transport behavior is a bulk response with time dependence. Conductivity behavior is examined in relation to transient current behavior, long time decay currents, and electrostatic discharge. Comparing charging and discharging cycles allowed qualitative separation of polarization and multiple trapping behaviors.

Presented here are electron-induced electron yield measurements from high-resistivity, high-yield materials to support a model for the yield of uncharged insulators. These measurements are made using a low-fluence, pulsed electron beam and charge neutralization to minimize charge accumulation. They show charging induced changes in the total yield, as much as 75%, even for incident electron fluences of <3 fC/mm2, when compared to an uncharged yield. The evolution of the yield as charge accumulates in the material is described in terms of electron recapture, based on the extended Chung and Everhart model of the electron emission spectrum and the dual dynamic layer model for internal charge distribution. This model is used to explain charge-induced total yield modification measured in high-yield ceramics, and to provide a method for determining electron yield of uncharged, highly insulating, high-yield materials. A sequence of materials with progressively greater charge susceptibility is presented. This series starts with low-yield Kapton derivative called CP1, then considers a moderate-yield material, Kapton HN, and ends with a high-yield ceramic, polycrystalline aluminum oxide. Applicability of conductivity (both radiation induced conductivity (RIC) and dark current conductivity) to the yield is addressed. Relevance of these results to spacecraft charging is also discussed.

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Water is one of the most important materials and has enormous impacts on life. Due to its delicate Hydrogen bond (H-bond) network, water shows various anomalous properties which has not been fully illuminated. Advanced experimental methods, such as scattering experiments and various spectroscopy techniques, have been developed and applied to study the nature of H-bond in liquid water. On the other hand, ab initio molecular dynamics (AIMD) have been widely adopted as an important theoretical tool to provide microscopic information of water on a sub-picosecond timescale. Recent AIMD studies based on the strongly constrained and appropriately normed (SCAN) exchange correlation functional yield an excellent description of the structural, electronic, and dynamic properties of liquid water. In this dissertation, we will focus on studying the structural, electronic and dynamic properties of liquid water as well as the modeling of the hydration structures of ions in aqueous solutions, using AIMD with potential energy surface provided by the novel SCAN functional. In the first work we represent an accurately predicted infrared spectrum of liquid water and show how the improvements are connected to the description of the underlying H-bond network. The second work mainly focusing on modeling the nuclear quantum effects (NQEs) and isotope effect of liquid water with a force field model based on artificial neural network, where qualitative agreements with experimental observations are achieved. In the third work, we study the isotope effect on the x-ray absorption spectra of liquid and attribute observed differences to the structural distinctions between light and heavy water as mentioned in the previous work. And in the last two projects, we systematically show the necessity of including NQEs of the hydrogen atom when modeling chloride ionic solution. Prominent changes in the hydration structure as well as electronic structure can be identified when NQEs are taken into consideration. / Physics

The efficiency of parallel tempering Monte Carlo is studied for a two-dimensional Ising system of length L with N=L^2 spins. An external field is used to introduce a difference in free energy between the two low temperature states. It is found that the number of replicas R_opt that optimizes the parallel tempering algorithm scales as the square root of the system size N. For two symmetric low temperature states, the time needed for equilibration is observed to grow as L^2.18. If a significant difference in free energy is present between the two states, this changes to L^1.02. It is therefore established that parallel tempering is sped up by a factor of roughly L if an asymmetry is introduced between the low temperature states. This confirms previously made predictions for the efficiency of parallel tempering. These findings should be especially relevant when using parallel tempering for systems like spin glasses, where no information about the degeneracy of low temperature states is available prior to the simulation.