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The magnetocaloric effect (MCE) is the temperature change in a magnetic material due to a change in an applied magnetic field. How the MCE behaves in different magnetic materials and at different phase transitions is fundamental to understand. The driver of the MCE is a change in entropy which has multiple contributions: magnetic, lattice, and electron. In this thesis the MCE is studied in a simple antiferromagnetic (AFM) model andin a realistic noncollinear spin glass Neodymium model using Monte Carlo and Atomistic Spin Dynamics simulations. For the simple AFM system, no clear results were achieved, indicating that MCE in AFM materials is not due to a change solely in the magnetic entropy. For the complex magnetic material Nd, a more clear result is seen, indicating that frustration in the system might be important to the MCE in noncollinear materials. Nd results also signify more phase transitions than previously reported.

This thesis focuses on the development of nonlinear optical techniques and the measurement of topological properties of the Weyl semimetals. The first portion of this thesis describes technical developments of the nonlinear optical spectroscopic probes rotational anisotropy second harmonic generation (RA-SHG) and transient grating. In our work on SHG, we describe a fast-reflective optic-based rotational anisotropy nonlinear harmonic generation spectrometer built upon synchronization of stepper motors and a voice-coil fast turning motor with data recorded by a data acquisition card. This device enables fast accumulation of significantly more data points than traditional SHG spectrometers and further allows spectral measurement over a broad wavelength range to be performed without optical realignment. We then describe the Fourier domain RA-SHG, allows direct measurements of the RA-SHG signal components of Cn symmetry. This method is based on the fast scanning RA-SHG device described above and operates by recording the nth harmonics of the fast scanning signal using a lock-in amplifier. Finally, we describe a novel method of performing transient grating measurements based on low power laser diodes, a laser diode pulser, a digital delay generator, and a data acquisition card. The RA-SHG technique was applied to the chiral Weyl semimetal RhSi, where a spectrum of the sole SHG tensor element χ(2) i jk was measured over the unprecedented 0.275-1.5 eV incoming photon energy range. Our data shows evidence of a strong surface state response and are detailed enough to reveal the second order corrections to the linear band structure as well as the Pauli blocking condition which was observed to occur at ∼630 meV. We also describe measurements of the linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE) in RhSi deriving from topological Fermi arc states. While the magnitude of the CPGE response broadly matched theoretical predictions, the data also exhibit an inexplicably high degree of symmetry in the response as a function of incoming polarization in both CPGE and LPGE channels. Collaborative work on the SHG spectrum from TaAs is also described, from which we attribute the origin of the SHG response peak to the third cumulant of the Bloch wavefunction. Further collaborative studies of the CPGE in RhSi (111) revealed a response that was likely due to the topological band structure, but that also shows that the theoretically predicted quantized CPGE was not observed due to impurities and from contributions from sources other than the Weyl nodes. Finally, we briefly summarized how the crystal structure of PrAlGe1-xSix was revealed to be non-centrosymmetric using the RA-SHG technique. Transition from intrinsic to extrinsic anomalous Hall effect by tuning the dopant concentration x was studied in this ferromagnetic Weyl semimetal. / Physics

Semilocal density functionals up to the generalized gradient approximation (GGA) level cannot accurately describe band gaps of bulk solids. Meta-GGA density functionals with a dependence on the kinetic energy density ingredient (τ) can potentially give wider band gaps compared with GGAs. The recently developed TASK meta-GGA functional yields excellent band gaps of bulk solids. The accuracy of the TASK functional for band gaps of bulk solids cannot be straightforwardly transferred to low-dimensional materials due to reduced screening in low-dimensional materials. We have developed mTASK from TASK by changing (a) the tight upper-bound for one or two-electron systems (h0X) from 1.174 to 1.29 and (b) the limit of the interpolation function fX(α → ∞) of the TASK functional that interpolates the exchange enhancement factor FX(s,α) from α = 0 to 1, so that mTASK has the screening appropriate for low-dimensional materials. These two conditions guarantee the increased nonlocality within the generalized Kohn-Sham scheme in the mTASK functional and yield a better description of band gaps of low-dimensional materials. We computed the band gaps of bulk solids from mTASK having a wide range of gaps such as Ge, CdO, ZnS, MgO, NiF, Ar. The improvement in the band gaps from mTASK is more consistent than TASK for the large-gaps crystals. We have studied the band structures in two forms of transition metal dichalcogenide (TMD) monolayers, i.e., monolayer hexagonal (1H) and monolayer trigonal (1T) and their nanoribbons. The mTASK functional systematically improves the band gaps and is in close agreement with the experiments or the hybrid level HSE06 density functional for 2D single-layer and nanoribbon systems. In the second part of this assessment, we explore the large tunability of band gaps and optical absorption of phosphorene nanoribbons under mechanical bending from first-principles. Bending can induce an unoccupied edge state in armchair phosphorene nanoribbons. The electronic and optical properties of nanoribbons drastically change because of this edge state. GW-Bethe–Salpeter equation calculations for armchair phosphorene nanoribbons at different bending curvatures show that the absorption peaks generally shift toward the high energy direction with increasing curvature. Our study suggests that bright excitons can also be formed from the transition from the valence bands to the edge state when the edge state completely separates out from the continuum conduction bands. We systematically study the role of the edge state to form bound excitons at large curvatures. Our analysis suggests that the optical absorption peaks of zigzag phosphorene nanoribbons shift toward the low-energy region, and the height of the absorption peaks increases while increasingthe bending curvature. In the third part of this assessment, we extend our study of phosphorene nanoribbons to MoS2 nanoribbons under bending from GW and Bethe-Salpeter equation approaches. We find three critical bending curvatures for armchair MoS2 nanoribbons, and the edge and non-edge band gaps show a non-monotonic trend with bending. The edge band gap shows an oscillating feature with ribbon width n, with a period of ∆n=3. The binding energy and the lowest exciton energy decrease with the curvature. The large tunability of optical properties of bent MoS2 nanoribbon is applicable in tunable optoelectronic nanodevices. / Physics

<p>In this dissertation, we present work on developing superconducting circuits intended to advance the implementation of Asynchronous Ballistic Reversible Computation using Fluxon Logic. In the first Chapter we introduce the need for developing reversible computing, and discuss implementing asynchronous reversible computing using fluxons in superconducting circuits. In Chapter 2, we introduce basic superconductivity physics, including the Josephson effects, which is necessary to know for understanding the behavior of Josephson junction transmission lines. In Chapter 3, we introduce tools to physically understand the behavior of topologically protected solitons, 'fluxons', in Josephson junction transmission lines. Finally, in Chapter 4, we briefly discuss the history of fluxon-based computation devices and present current state of the art design of such reversible computation devices, including the fluxon Rotary gate that we have developed. Taken together, these represent advances in the direction of implementing asynchronous reversible computing in practice.</p>

<p>  </p> <p>Research that seeks to understand and develop the growth of III-nitride materials by molecular beam epitaxy (MBE) is beneficial to a broad range of the device community. MBE and the III-nitrides have been used to develop transistors, diodes, electroacoustic devices, solar cells, LEDs, LDs, intersubband devices, and quantum-cascade lasers. In this work we focus on the growth of III-nitride materials specifically for applications in near-infrared intersubband (NIR ISB) optical devices, however all this work is broadly applicable. </p> <p><br></p> <p>We begin by investigating the reduced indium incorporation in non-polar m-plane InGaN films. We find that InGaN grown on m-plane GaN has an effective activation energy for thermal decomposition of 1 eV, nearly half that reported for similar c-plane films. We produce high quality m-plane In0.16Ga0.84N and utilize it in AlGaN/InGaN devices designed for near-infrared ISB absorption measurements. We continue this work by exploring the growth of low-temperature AlGaN, necessary for these devices. We find that the utilization of an indium surfactant during low-temperature AlGaN growth enhances adatom diffusion, resulting in smoother surface morphologies, sharper interfaces, and reduced defects within the material. This growth method also prevents the anomalous suppression of the AlGaN growth rate, which we link to a reduction in the formation of high-aluminum containing defects. These investigations result in the demonstration of an Al0.24Ga0.76N/In0.16Ga0.84N heterostructure with a conduction band offset large enough to enable NIR ISB transitions.</p> <p><br></p> <p>Lastly, we explore the novel material ScAlN. This material’s large bandgap, large spontaneous polarization, ferroelectricity, and ability to be lattice matched to GaN at ~18% scandium composition make it an ideal candidate for a variety of devices, including NIR ISB devices. We investigate the reported temperature dependence of ScAlN’s <em>c</em>-lattice constant and confirm this dependence is present for high growth-temperature ScxAl1-xN with 0.11 < x < 0.23. We find that this temperature dependence is no longer present below a certain composition-dependent growth temperature. This finding, coupled with observations that samples grown at lower temperatures exhibit lower defect densities, smoother surfaces, and homogeneous chemical compositions suggest that high growth temperatures lead to defect generation that may cause the observed change in lattice parameters. We demonstrate lattice-matched, 50 repeat Sc0.18Al1-xN/GaN heterostructures with ISB absorption in excess of 500 meV with FWHM as little as 45 meV. </p>

Detta projekts mål har varit att undersöka tre olika 2D-materials triboelektriska egenskaper och därmed placera dem i en triboelektrisk serie. Detta utfördes genom att använda en triboelektrisk nanogenerator (TENG) och mäta den resulterande spänningen. Tio stycken motmaterial applicerades mot varje 2D-material på nanogeneratorn. Utifrån resultatet var det möjligt uppmärka typiska vågformer för en TENG, alltså kunde resultatet från mätningen antas vara från den triboelektriska effekten. 2D-materialen placerades tillsammans med dess motmaterial i en triboelektrisk serie och sorterades sedan för att bestämma dess elektronaffinitet. För de tre 2D-materialen hade de gemensamt att ETFE och FEP tillhör den positiva sidan av den triboelektriska serien relativt de 2D-materialen. Resten, alltså: cellofan, kapton, LDPE, nylon, PEEK, PEI, polypropylene och PTFE, placerades negativt i deras respektives 2D-materials serie. Dock blev resultatet ej som förväntat, då ordningen på motmaterialen i serien kunde antas vara samma för alla 2D-material, men detta var inte vad som hittades. Anledningen till detta kan möjligtvis vara ytladdningar som kan ha överförts till materialen medans de hanterades, eller på grund av ytstrukturen av 2D-materialen. Därför föreslås att detta arbete kan förbättras genom mer varsam hantering och spridning av materialen över dess plattform.

Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the easurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical diversity and ambient temperature low cost manufacturing. In this thesis we investigate the switching and conducting properties of photochromic dithienylethene derivatives. Such molecules change their conformation in solution when acted upon by light. Photochromic molecules are attractive candidates for use in molecular electronics because of the switching between different states with different conducting properties. The possibility of optically controlling the conductance of the molecule attached to leads may lead to new device implementations. The switching reaction is investigated with potential energy calculations for different values of the reaction coordinate between the closed and the open isomer. The electronic and atomic structure calculations are performed with density functional theory (DFT). It is concluded that there is a large potential energy barrier separating the open and closed isomer and that switching between open and closed forms must involve excited states. The conducting properties of the molecule inserted between gold leads is calculated within the Non Equilibrium Green Function theory. The transmission function is calculated for the two isomers with different basis sizes for the gold contacts, as well as the electrostatic potential, for finite applied bias voltages. We conclude that a Au 6s basis give qualitatively the same result as a Au spd basis close to the Fermi level. The transmission coefficient at the Fermi energy is around 10 times larger in the closed molecule compared to the open. This will result in a large difference in conductivity. It is also found that the large difference in conductivity will remain for small applied bias voltages. The results are consistent with earlier work. / QC 20101119