The field of them1oelectrics started in early 19th century. Since the discovery of the Seebeck effect and the Peltier effect, thermoelectric modules have found their way into, mostly, niche applications such as radioisotope thermoelectric generators on space missions. Thermoelectric modules can also be used for cooling, utilizing the Peltier effect. Thermoelectrics are promising materials due to the operation nature of the modules. That is, they have no moving parts, no exhaust, long lifetime without maintenance, features that make them attractive for many applications. Despite these promising properties, thermoelectric modules are mostly used in niche applications. The main reason for this is conventional modules with the highest efficiency are commonly made of expensive and/or rare elements which prevents mass production. To tackle this problem, new materials are investigated to find a module that can be made widely available. Oxides are one possibility, where an added benefit is that they are chemically stable even at elevated temperature. The perovskite CaMnO3 is one of the more promising oxides, with elements that are abundant on earth and cheap. The material does suffer from low electrical conductivity which results in a low electrical conductivity and efficiency. A substantial effort has been put in to increase the efficiency of CaMnO3, hut it still needs improvement. In my thesis, I have investigated the CaMnO3 system. CaMnO3 was synthesized using co-reactive RF-magnetron sputtering and post annealing. The synthesis method is already known hut has not been used for deposition of perovskites. I have also demonstrated that this synthesis method can be used to dope CaMnO3 with niobium at appropriate levels for enhancing the efficiency.

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Barium Zirconate is a material valuable in many aspects. Among all the significant characteristics of (BZO), the possibility of hydrogen interstitials is of great importance. With the help of X-ray diffraction analysis, many kinds of structure-related information of a  crystal sample can be collected. In this thesis, the XRD pattern of hydrogen induced expansion and contraction of oxygen atoms in the BZO perovskite lattice is investigated both experimentally and theoretically.

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In this project, Density Functional Theory as implemented in Quantum Espressois used to calculate the electronic structures of monolayers and bulk of MoS2and WTe2. The calculations are carried out for four different types of pseudopotentialsgenerated using PSlibrary. It is found that the band structure ofmonolayer MoS2 is only properly described for the pseudopotentials includingspin-orbit coupling. In addition to this, a simple test was preformed to checkthe transferability of the pseudopotetnials by calculating the bulk modulus andlattice constants of molybdenum, tungsten and tellurium. The results obtainedwith the generated pseudopotentials were found to be in line with expectations.indicating that the pseudopotentials have a good transferability / I denna studie används täthetsfunktionalteori implementerad i Quantum Espressoför att beräkna elektronstrukturer för MoS2 och WTe2" i bulk och två dimensionellform.. Beräkningarna har utförts för fyra olika pseodopotentialer hämtadefrån PSlibrary. Det visar sig att bandstrukturen för två dimensionell MoS2enbart beskrivs korrekt för pseudopotentialer som inkluderar effekten av spinnbankoppling. Dessutom utfördes ett test av överförbarheten för pseudopotentialernagenom att beräkna trykmodul och gitterkonstanter för bulk molybden,bulk volfram och trigonal tellur. Framräknade värden för pseudopotentialernavar i enhelighet med experimentelavärden, vilket indikerar att pseudopotentialernahar en god överförbarhet.

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.