SiC MESFETs and GaN HEMTs posses an enormous potential in power amplifiers at microwave frequencies due to their wide bandgap features of high electric field strength, high electron saturation velocity and high operating temperature. The high power density combined with the comparably high impedance attainable by these devices also offers new possibilities for wideband power microwave systems. Similarly Si-LDMOS being low cost and lonely silicon based RF power transistor has great contributions especially in the communication sector. The focus of this thesis work is both device study and their application in different classes of power amplifiers. In the first part of our research work, we studied the performance of transistors in device simulation using physical transistor structure in Technology Computer Aided Design (TCAD). A comparison between the physical simulations and measured device characteristics has been carried out.  We optimized GaN HEMT, Si-LDMOS and enhanced version of our previously fabricated and tested SiC MESFET transistor for enhanced RF and DC characteristics. For large signal AC performance we further extended the computational load pull (CLP) simulation technique to study the switching response of the power transistors. The beauty of our techniques is that, we need no lumped or distributive matching networks to study active device behavior in almost all major classes of power amplifiers. Using these techniques, we studied class A, AB, pulse input class-C and class-F switching response of SiC MESFET. We obtained maximum PAE of 78.3 % with power density of 2.5 W/mm for class C and 84 % for class F power amplifier at 500 MHz. The Si-LDMOS has a vital role and is a strong competitor to wideband gap semiconductor technology in communication sector. We also studied Si-LDMOS (transistor structure provided by Infineon Technologies at Kista, Stockholm) for improved DC and RF performance. The interface charges between the oxide and RESURF region are used not only to improve DC drain current and RF power, gain & efficiency but also enhance its operating frequency up to 4 GHz. In the second part of our research work, six single stage (using single transistor) power amplifiers have been designed, fabricated and characterized in three phases for applications in communications, Phased Array Radars and EW systems. In the first phase, two class AB power amplifiers are designed and fabricated. The first PA (26 W) is designed and fabricated at 200-500 MHz using SiC MESFET. Typical results for this PA at 60 V drain bias at 500 MHz are, 24.9 dB of power gain, 44.15 dBm output power (26 W) and 66 % PAE. The second PA is designed at 30-100 MHz using SiC MESFET. At 60 V drain bias Pmax is 46.7 dBm (~47 W) with a power gain of 21 dB. In the second phase, for performance comparison, three broadband class AB power amplifiers are designed and fabricated at 0.7-1.8 GHz using SiC MESFET and two different GaN HEMT technologies (GaN HEMT on SiC and GaN HEMT on Silicon substrate). The measured maximum output power for the SiC MESFET amplifier at a drain bias of Vd= 66 V at 700 MHz the Pmax was 42.2 dBm (~16.6 W) with a PAE of 34.4 %. The results for GaN HEMT on SiC amplifier are; maximum output power at Vd = 48 V is 40 dBm (~10 W), with a PAE of 34 % and a power gain above 10 dB. The maximum output power for GaN HEMT on Si amplifier is 42.5 dBm (~18 W) with a maximum PAE of 39 % and a gain of 19.5 dB. In the third phase, a high power single stage class E power amplifier is implemented with lumped elements at 0.89-1.02 GHz using Silicon GaN HEMT as an active device. The maximum drain efficiency (DE) and PAE of 67 and 65 % respectively is obtained with a maximum output power of 42.2 dBm (~ 17 W) and a maximum power gain of 15 dB.

KTiOPO4 (KTP) and Rb-doped KTP (RKTP) are two of the most attractive nonlinear opticalmaterials for engineering of periodically poled domain structures, commonly used as frequencyconversiondevices for laser radiation via the quasi-phase matching (QPM) technique. Thesematerials have excellent non-linearity, wide transparency windows and high resistance to opticaldamage. Furthermore their large domain-velocity anisotropy allows the fabrication of highaspect-ratio domain structures, needed for many QPM applications. To create highly efficientdevices, precise control over the structure uniformity and duty-cycle is required. Constantimprovement of the domain engineering techniques has allowed pushing the limits of theachievable domain aspect-ratio. For this development to continue, a deeper understanding of theformation dynamics and stability of the domain gratings is of utmost importance. As the domainsizein nanostructured devices decreases, the density of the domains walls (DWs) increases andtheir properties are ever more important for device performance. Indeed, more knowledge on thedomain wall properties, and the means to engineer them, could enable new applicationsexploiting these properties.This thesis presents studies on domain wall properties and dynamics in KTP and RKTP. Thesub-millisecond dynamics of grating formation in RKTP under an applied electric field has beenstudied in the high-field regime using online second harmonic generation. The effects ofdifferent pulse shapes were compared and single triangular pulses were found to be superior interms of the resulting grating quality.The high-temperature stability of domain gratings was investigated. The domain wall motioninduced by annealing was shown to be highly anisotropic along the a- and b-crystal axes, anddependent on the period of the grating period.The local charge transportation at the domains and domain walls in KTP was characterized usingatomic force microscopy, demonstrating a fourfold increase of conductivity at the walls.Voltage-cycling measurements revealed memristive-like characteristics, attributed to the effectof ionic motion and local charge accumulation. The enhanced conductivity of charged domainwalls was used as an imaging tool, to study domain wall dynamics while inducing motionthrough the application of an external field.Finally, the interplay between ionic motion, spontaneous polarization and polarization reversalwas investigated, showing direct evidence of elastic modulus modification during localpolarization switching. / <p>QC 20171113</p>

Stresses and impurities may influence elastic properties, phase stability and magnetic behavior of metals and their alloys. A physical understanding of this influence is of great importance to both fundamental science and technological applications. The diverse methods used in this work allowed us to shed light on the various aspects of the problem. In particular, in this work the thermodynamic, magnetic and elastic properties of Fe and Fe-Ni alloys at Earth’s inner core conditions were investigated by means of the ab initio theory. The main features of these calculations are on one side the extreme pressure-temperature conditions; on the other side the strong-correlation effects, which at these conditions may play an unexpected role. That is why I used different approaches, ranging from molecular dynamics to the dynamical mean field theory. Interesting possibility for the effect of non-hydrostatic stresses on the stability of the body-centered cubic (bcc) phase of iron was observed. If detected, it could allow for an explanation of striking contradictions in high-pressure experiments. The influence of the alloying with Ni on the stability of Fe was studied. It was shown that the observed reverse of the stability trend under pressure is associated with the suppression of ferromagnetism at conditions of Earth’s inner core. The strong correlation effects were observed in Fe3Ni by means of the dynamical mean field theory, revealing that the local environment of iron atoms is crucial for the strength of the on-site electronic correlations. There is also an exciting experimental finding of our colleagues indicating that magnetism in pure nickel survives at very high pressures up to 260 GPa, i.e. up to the highest pressure at which magnetism in any material has ever been observed. Our calculations of the pressure dependence of the effective exchange interaction parameter and the hyperfine field support the picture of the ordered ferromagnetic state in Ni at multimegabar pressures. Further, hydrogen is believed to be an important light impurity in Earth’s core. Thereupon the hydrogen containing FeOOH was also investigated. The prediction of the effect of symmetrization of the hydrogen bond under pressure was made. The universality of applied methods allowed us to study the elastic constants of TiN, which is of high relevance to the industry of cutting tools. The importance of taking into account the finite temperature effects in the calculations of the elastic properties was demonstrated. Another case of practical interest is the Fe-Cr system, a prototype of many industrial steels. For instance, it is used in cooling pipes of pressure vessel reactors. We studied the effect of hydrostatic pressure on the phase stability of Fe-Cr alloys and revealed intriguing differences in the ordering tendencies depending on the Cr concentration and magnetic state of the alloy. We showed how variation of the ordering tendency between the Fe and Cr atoms emerges due to suppression of the local magnetic moment on the Cr atoms. Noteworthy, hydrogen is not only the basic material playing fundamental role on and in the Earth, it is also a very promising source of fuel, which does not pollute the environment. In this sense the problem of hydrogen storage in Pd is of separate but related interest and it was theoretically investigated in the present work. The effect of vacancies on the energetically preferable position of hydrogen in the Pd cell was addressed. My theoretical results supported the experimental suggestion of multiple occupation of Pd vacancies by hydrogen.

Condensed matter physics is the study of the macroscopic and microscopic properties of condensed phases of matter. For quite some time, Landau’s symmetry breaking theory was believed to describe and explain the nature of any phase transition. However, since the late 1980s, it has become apparent that it is necessary to introduce some new kind of order, named topological order, that transcends the traditional symmetry description. In this thesis we will study the Kitaev model, which is a Hamiltonian lattice model that allows one to incorporate the concept of topological order, as well as the corresponding operators and algebras. First, we consider the model on an infinite lattice, and show how to relate local and global degrees of freedom of the anyons/quasi-particles living on sites to the ribbon operators. Afterwards, we introduce holes and an external boundary to the lattice, and examine the ramifications of this generalization in terms of the ground state degeneracy. Lastly, we verify that the algebra formed by boundary site operators has the structure of a quasi-Hopf algebra.

First-principles methods based on Density functional theory (DFT) are now adopted routinely to calculate the properties of materials. However, one of the biggest challenges of DFT is to describe the electronic behaviors of random alloys. One of the aims of this thesis is to study binary alloys, e.g. Ti-Al, Cu-Au, and multi-component alloys by using two models for chemically random structures: the special quasi-random structure (SQS) and coherent potential approximation (CPA). I investigate these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. Within the SQS approach, the LLD effect can be modeled in a straightforward manner by relaxing the positions of atoms in the supercell. However, within this approach, it is difficult to model the random multi-components alloys due to the large size of the supercells. On the other hand, the CPA approach uses single-site approximation and thus it is not limited by the number of alloy components. But CPA suffers from the neglect of the local lattice relaxation effect, which in some systems and for some properties could be of significant importance. In my studies, the SQS and CPA approaches are combined with the pseudopotential method as implemented in the Vienna Ab-initio Simulation Package (VASP) and the Exact Muffin-Tin Orbitals (EMTO) methods, respectively. The mixing energies or formation enthalpies and elastic parameters of fcc Ti1-xAlx and Cu1-xAux (0 =&lt; x =&lt; 1) random solid solutions and high-entropy multicomponent TiZrVNb, TiZrNbMo and TiZrVNbMo alloys are calculated as a function of concentration. By comparing the results with and without local lattice relaxations, we find that the LLD effect is negligible for the elastic constants C11, C12, and C44. In general, the uncertainties in the elastic parameters associated with the symmetry lowering in supercell studies turn out to be superior to the differences between the two alloy techniques including the effect of LLD. However, the LLD effect on the mixing energies or formation enthalpies is significant and depends on the degree of size mismatch between alloy constituents. In the cases of random Cu-Au and high-entropy alloys, the formation enthalpies and mixing energies are significantly decreased when the LLD effect is considered. This finding sets the limitations of CPA for the mixing energies or formation enthalpies of alloys with large atomic size differences. The other goal of the thesis is to study the effect of exchange-correlation functionals on the formation energies of ordered alloys. For this investigation, we select the Cu-Au binary system which has for many years been in the focus of DFT and beyond DFT schemes. The Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation term in DFT is a mature approach and have been adopted routinely to investigate the properties of metallic alloys. In most cases, PBE provides theoretical results in good agreement with experiments. However, the ordered Cu-Au system turned out to be a special case where large deviations between the PBE predictions and observations occur. In this work, we make use of a recently developed exchange-correlation functional, the so-called quasi-nonuniform exchange-correlation approximation (QNA), to calculate the lattice constants and formation energies for ordered Cu-Au alloys as a function of composition. The calculations are performed using the EMTO method and verified by a full-potential method. We find that the QNA functional leads to an excellent agreement between theory and experiment. The PBE strongly overestimates the lattice constants for ordered Cu3Au, CuAu, CuAu3 compounds and also for the pure metals which are nicely corrected by the QNA approach. The errors in the formation energies of Cu3Au, CuAu, CuAu3 relative to the experimental data decrease from 38-45% obtained with PBE to 5-9% calculated for QNA. This excellent result demonstrates that one can reach superior accuracy within DFT for the formation energies and there is no need to go beyond DFT. Furthermore, it shows that error cancellation can be very effective for the formation energies as well and that the main DFT errors obtained at PBE or LDA levels originate from the core-valence overlap region, which is correctly captured by QNA due to its particular construction. Our findings are now extended to disordered alloys, which is briefly discussed already in one of my published papers. / <p>Qc 20170630</p>

With the discovery of the quantum Hall effect more than thirty years ago, a whole new field emerged—that of topological quantum matter. This field is now a very mature one, and many different aspects are covered in the literature. The main text of this thesis introduces the field and gives a background to topological quantum matter, as well as topological aspects of superconductivity and the Abelian fractional quantum Hall (FQH) states.  Together with the main text there are five articles that address five different questions, all connected to topological quantum matter. In the first article, representative wave functions for the Abelian FQH states are calculated using conformal field theory methods. Before this paper was published, similar constructions had been restricted to flat geometries, but in this paper we generalize the analysis to the simplest curved geometry, namely the sphere. On top of being of interest for numerical studies (which usually are performed on a sphere), the response of the FQH liquids to curvature can be used to detect a topological quantity, the shift, which is the average orbital spin of the constituent electrons. In the second article, we construct an effective field theory for the two-dimensional spinless, chiral p-wave superconductor that faithfully describes the topological properties of the bulk state, and also provides a model for the subgap states at vortex cores and edges. In particular, it captures the topologically protected zero-modes and has the correct ground state degeneracy on the torus. In the third paper, tools for a hydrodynamic theory for insulators in three dimensions are derived. Specifically, we use functional bosonization to write insulators as a condensation phase of the U(1) gauge theory obtained in the functional bosonization language. In the fourth paper, we investigate the edge Majorana modes in the two-dimensional chiral p-wave superconductor. We define the model on surfaces with different geometries—the annulus, the cylinder, the Möbius band, and a cone—and with different configurations of magnetic flux threading holes in these surfaces. In particular, we address the following question: Given that, in the absence of magnetic flux, the ground state on the annulus does not support Majorana modes, while the one on the cylinder does, how is it possible that the conical geometry can interpolate smoothly between the two? In the fifth and last article, we demonstrate that two-dimensional chiral superconductors on curved surfaces spontaneously develop magnetic flux. We propose this geo-Meissner effect as an unequivocal signature of chiral superconductivity that could be observed in layered materials under stress. We also employ the effect to explain some puzzling questions related to the location of Majorana modes. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript.</p>

To investigate the existence  of  molecular heterogeneties in mixtures of DMSO-water, the dynamics were measured with the method of dynamic light scattering (DLS). Three different compositions (20 mol %, 33 mol% and 60 mol%) were included in the study and measured at room temperature (295 K) and for one composition (33 mol %) also a temperature dependence (from 295 K down to 263 K) was measured. Measurements were done on samples both with and without nanoparticles acting as tracers for the DLS. The diffusion coefficients of DMSO in water was extracted from the analysis and the results from samples without nanoparticles are consistent with diffusion of DMSO molecules reported previously, except for the highest concentration,  and showed a clear Arrhenius behaviour with an activation energy of 26±1kJ/mol. The viscosity was extracted from the diffusioncoefficient of the nanoparticles in the solutions and followed an expected trend regarding the concentration as well as for the higher temperatures, but deviated for lower temperatures due to an unexpected drastic change in the diffusion coefficient around the temperature T= 273 K. The reason for the drastic change could be connected to a possible liquid-liquid phase separation in the DMSO-water  mixture. The hydrodynamical radii was estimated using Stokes-Einsteins equation and had a small but unsure concentration dependence so the size could only be confirmed to be around 0.43 nm at T= 295 K and increased with temperature up to 1.6 nm atT= 263 K, indicating clustering effects and supporting the theory of molecular heterogeneity in DMSO-water mixtures. This was a pilot study to aproposed x-ray experiment at NanoMax at MaxIV to capture the nanoscale fluctuations present in binary solutions.

This thesis presents results from large scale Monte Carlo simulations of systems subject to a superfluid phase transition in the presence of disorder. The simulations are performed by state-of-the-art, collective Monte Carlo algorithms treating phase degrees of freedom in effective models with amplitude fluctuations integrated out. In Paper I a model system for the possible solid to supersolid transition in 4He is presented.The Wolff cluster algorithm is used to study how the presence of linearly correlated random defects is able to alter the universality class of the 3-dimensional XY-model. In the pure case the superfluid density and heat capacity have singular onsets, which are not seen in the supersolid experiments where instead a smooth onset is obtained. Using finite size scaling of Monte Carlo data, we find a similar smooth onset in our simulations, governed by exponents  ν=1 for the superfluid density and α=-1 for the heat capacity. These results are in qualitative agreement with experiments for the observed transition in solid 4He. In Paper II a systematic investigation of the scaling result z=d for the dynamic critical exponentat the Bose glass to superfluid quantum phase transition is performed. The result z=d has been believed to be exact for about 20 years, but although it has been questioned lately no accurate estimate of z has been available. An effective link current model of quantum bosons at T=0 with disorder in 2D is simulated using highly effective worm Monte Carlo simulations.The data analysis is based on a finite size scaling approach todetermine the quantum correlation time from simulationdata for boson world lines without any a priori assumption on the critical parameters. The resulting critical exponents are z=1.8 \pm 0.05, ν=1.15 \pm 0.03, and η=-0.3 \pm 0.1. This suggests that z=d is not satisfied. / <p>QC 20111206</p>

QC 20110309

Special quasi-random structure (SQS) and coherent potential approximation (CPA) are techniques widely employed in the first-principles calculations of random alloys. The aim of the thesis is to study these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. We compare the elastic parameters obtained from SQS and CPA calculations. For the CPA and SQS calculations, we employ the Exact Muffin-Tin Orbitals (EMTO) method and the pseudopotential method as implemented in the Vienna Ab initio Simulation Package (VASP), respectively. We compare the predicted trends of the VASP-SQS and EMTO-CPA parameters against composition. As a first case study, we investigate the elastic parameters of face centered cubic (fcc) Ti1−xAlx(0≤x≤100at.%) random solid solutions as a function of Al content (x). The EMTO-CPA and VASP-SQS results are in good agreement with each other. Comparing the lattice constants from SQS calculations with and without local lattice relaxations, we find that in Ti-rich (Al-rich) side the lattice constants remain almost unchanged (slightly increase) upon atomic relaxations. Taking local lattice distortions into consideration decreases the C11 and C44 elastic parameters, but their trends are not significantly affected. The C12 elastic constant, on the other hand, is almost unchanged when atomic relaxations are included. In general, the uncertainties in the elastic parameters associated with the symmetry lowering in supercell studies turn out to be superior to the differences between the two alloy techniques including the effect of LLD. We also investigate the elastic properties of random fcc Cu1−xAux(0≤x≤100 at.%) alloys as a function of Au content employing the CPA and SQS approaches. It is found that the CPA and SQS values forC11andC12 are consistent with each other no matter whether the atomic relaxations are taken into account or not. On the other hand, the EMTO-CPA values for C44 are slightly larger than those from SQS calculations especially for Cu-rich alloys which we ascribe to the differences in the DFT solvers rather than the differences between CPA and SQS. The Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation term in density functional theory (DFT) is a mature approach and have been adopted routinely to investigate the properties of metallic alloys. In most of the cases, PBE provides theoretical results in good agreement with experiments. However, the ordered Cu-Au system turned out to be a special case where large deviations between the PBE predictions and observations occur. In this work, we make use of a recently developed exchange-correlation functional, the so-called quasi-non-uniform exchange-correlation approximation (QNA), to calculate the lattice constants and formation energies for ordered Cu-Au alloys as a function of composition. The calculations are performed using the EMTO method. We find that the QNA functional leads to excellent agreement betweent heory and experiment. The PBE strongly overestimates the lattice constants for ordered Cu3Au, CuAu, CuAu3 compounds and also for the pure metals which is nicely corrected by the QNA approach. The errors in the formation energies of Cu3Au, CuAu, CuAu3relative to the experimental data decrease from 38-45% obtained with PBE to 5-9% calculated for QNA. / <p>QC 20151216</p>