Quantum spin liquid in the semiclassical regime

Quantum spin liquids have been at the forefront of correlated electron research ever since their original proposal in 1973, and the realization that they belong to the broader class of intrinsic topological orders, along with the fractional quantum Hall states. According to received wisdom, quantum spin liquids can arise in frustrated magnets with low spin $S$, where strong quantum fluctuations act to destabilize conventional, magnetically ordered states. Here we present a magnet that has a $Z_2$ quantum spin liquid ground state already in the semiclassical, large-$S$ limit. The state has both topological and symmetry related ground state degeneracy, and two types of gaps, a `magnetic flux' gap that scales linearly with $S$ and an `electric charge' gap that drops exponentially in $S$. The magnet is described by the spin-$S$ version of the spin-1/2 Kitaev honeycomb model, which has been the subject of intense studies in correlated electron systems with strong spin-orbit coupling, and in optical lattice realizations with ultracold atoms. The results apply to both integer and half-integer spins

4d and 5d compounds as the new frontier of the anisotropic spin physics

University of Minnesota Ph.D. dissertation. July 2017. Major: Physics. Advisor: Natalia Perkins. 1 computer file (PDF); x, 180 pages.I perform a series of studies of the magnetism of 4d and 5d transition metal compounds. In particular I concentrate on the realization of anisotropic magnetic Hamiltonians by use of the spin-orbit coupling to tie together the real space geometry and spin space magnetism. In the first part, I derive the magnetic Hamiltonians of Sr2IrO4 and Na2IrO3 from microscopic parameters. The difficulty of these calculation arises from the fact that many microscopic parameters, such as Hund's coupling, spin-orbit coupling, and crystal field distortions are all of the same order and thus have to be treated on an equal footing. The competition and cooperation of these interactions leads to a rich magnetic Hamiltonians with many different anisotropic interactions. My calculations provide a clear dependence of these interactions on the microscopic parameters. This in turn can be used experimentally to single out and enhance given anisotropies by changing the microscopic parameters. In the second part I propose experimental measurements for the anisotropic interactions. In particular I study how different anisotropic interactions contribute to the anisotropy in the Curie-Weiss temperatures of these compounds. I show that the difference of Curie-Weiss temperatures along particular axes gives a way to measure the strength of the anisotropic interactions in the compounds. In the last part, I study how the multitude of the magnetic anisotropies determine the magnetic ground state in 4d and 5d compounds. We have developed a new method to calculate the fluctuational contribution to the free energy in anisotropic Hamiltonians at any temperature within the magnetically ordered phase. The calculation can be done for both classical (which includes only thermal fluctuations) and quantum (quantum and thermal fluctuations) systems. I also study the effects of external magnetic field applied to the nearest neighbor Kitaev-Heisenberg model, a model of particular interest for alpha-RuCl3

Modeling of Ion/Target Interactions in Plasma Facing Components of Fusion Reactor

Nuclear fusion is a promising source of clean energy that can be one of the key future suppliers of the world’s increasing power demand. One of today’s main challenges faced by scientists and engineers regarding nuclear reactors is to design plasma-facing components (PFCs) that can withstand extreme conditions of temperature, pressure, and ions/particles irradiation. Material evolution and damage of PFCs are strongly related to the bombardment and diffusion processes of ions resulting from fusion fuel, i.e., deuterium and tritium and reaction products, i.e., helium. However, work is still needed in order to understand fuel diffusion in the presence of helium effects and damage produced in heterogeneous media of potential PFCs. This study simulates the diffusion of atoms in an alloy of changing solute concentration in an environment similar to that of a nuclear fusion reactor. The diffusion equation was solved numerically while taking into account the “potential diffusion” present in heterogeneous materials, as it was described analytically in recent studies. The solution was implemented in Fortran 90 code using SRIM software as an input generator and taking parameters found in literature. Our results show that heterogeneous membranes can greatly shift the deuterium concentration profile towards the vanadium back surface, increasing the material\u27s permeability. These outcomes suggest that vanadium alloys with heterogeneous solute concentration distribution should be empirically analyzed in order to understand how these concentration shifts affect material properties and fuel retention

Heights integrated model as instrument for simulation of hydHeights Integrated Model as Instrument for Simulation of Hydrodynamic, Radiation Transport, and Heat Conduction Phenomena of Laser-Produced Plasma in EUV Applications.

The HEIGHTS integrated model has been developed as an instrument for simulation and optimization of laser-produced plasma (LPP) sources relevant to extreme ultraviolet (EUV) lithography. The model combines three general parts: hydrodynamics, radiation transport, and heat conduction. The first part employs a total variation diminishing scheme in the Lax-Friedrich formulation (TVD-LF); the second part, a Monte Carlo model; and the third part, implicit schemes with sparse matrix technology. All model parts consider physical processes in three-dimensional geometry. The influence of a generated magnetic field on laser plasma behavior was estimated, and it was found that this effect could be neglected for laser intensities relevant to EUV (up to {approx}10{sup 12} W/cm{sup 2}). All applied schemes were tested on analytical problems separately. Benchmark modeling of the full EUV source problem with a planar tin target showed good correspondence with experimental and theoretical data. Preliminary results are presented for tin droplet- and planar-target LPP devices. The influence of three-dimensional effects on EUV properties of source is discussed

Learning crystal field parameters using convolutional neural networks

We present a deep machine learning algorithm to extract crystal field (CF) Stevens parameters from thermodynamic data of rare-earth magnetic materials. The algorithm employs a two-dimensional convolutional neural network (CNN) that is trained on magnetization, magnetic susceptibility and specific heat data that is calculated theoretically within the single-ion approximation and further processed using a standard wavelet transformation. We apply the method to crystal fields of cubic, hexagonal and tetragonal symmetry and for both integer and half-integer total angular momentum values $J$ of the ground state multiplet. We evaluate its performance on both theoretically generated synthetic and previously published experimental data on CeAgSb$_2$, PrAgSb$_2$ and PrMg$_2$Cu$_9$, and find that it can reliably and accurately extract the CF parameters for all site symmetries and values of $J$ considered. This demonstrates that CNNs provide an unbiased approach to extracting CF parameters that avoids tedious multi-parameter fitting procedures.Comment: 19 pages, 9 figure

Effect of Dual Ion Beam Irradiation (Helium and Deuterium) on Tungsten–Tantalum Alloys Under Fusion Relevant Conditions

The selection of tungsten (W) as a divertor material in ITER is based on its high melting point, low erosion, and strong mechanical properties. However, continued investigation has shown W to undergo severe morphology changes in fusion-like conditions. Recent literature suggests alloying W with other ductile refractory metals, viz. tantalum (Ta) may resolve some of these issues. These results provide further motivation for investigating W–Ta alloys as a plasma-facing component (PFC) for ITER and future DEMO reactors. Specifically, how these alloy materials respond to simultaneous He+ and D+ ion irradiation, and what is the effect on the surface morphology when exposed to fusion relevant conditions. In the present study, the surface morphology changes are investigated in several W–Ta targets (pure W, W-1%Ta, W-3%Ta, and W-5% Ta) due to simultaneous He+ and D+ ion irradiations. This comprehensive work allows for deeper understanding of the synergistic effects induced by dual ion irradiation on W and W–Ta alloy surface morphology. Pure W and W–Ta alloys were irradiated simultaneously by 100 eV He+ and/or D+ ions at various mixture ratios (100% He+, 60% D+ + 40% He+, 90% D+ + 10% He+ ions and 100% D+ ions), having a total constant He fluence of 6 × 1024 ion m−2, and at a target temperature of 1223 K. This work shows that slight changes in materials composition and He/D content have significant impact on surface morphology evolution and performance. While both the pure W and W–Ta alloys exhibit very damaged surfaces under the He+ only irradiations, there is a clear suppression of the surface morphology evolution as the ratio of D+/He+ ions is increased