Diss. (sammanfattning) Uppsala : Uppsala universitet, 2006. / Härtill 6 uppsatser.

Diss. (sammanfattning) Umeå : Univ., 2003. / Härtill 5 uppsatser.

<p>This thesis presents results from numerical simulations of the Nernst effect dueto phase fluctuations in models of two-dimensional granular superconductors. Inaddition other transport properties, such as thermal conductivity and electrical re-sistivity are calculated. The models are based on a phase only description withLangevin or resistively and capacitively shunted Josephson junction (RCSJ) dy-namics, generalized to be valid for any type of two-dimensional lattice structure.All transport coefficients are evaluated from equilibrium correlation functions usingKubo formulas.</p><p>In Paper I, anomalous sign reversals of the Nernst signal eN , corresponding tovortex motion from colder to hotter regions, are observed. These are attributedto geometric frustration effects close to magnetic fields commensurate with theunderlying lattice structure. The effect is seen also in systems with moderategeometric disorder, and should thus be possible to observe in real two-dimensionalgranular superconductors or Josephson junction arrays.</p><p>Paper II presents two different derivations of an expression for the heat current inLangevin and RCSJ dynamics. The resulting expression is through our simulationsseen to obey the required Onsager relation, as well as giving consistent resultswhen calculating κ and eN via Kubo formulas and through the responses to anapplied temperature gradient. In zero magnetic field and at low-temperatures, thecontribution to the thermal conductivity κ in RCSJ dynamics is calculated usinga spin-wave approximation, and is shown to be independent of temperature anddiverge logarithmically with system size. At higher temperatures, κ shows a non-monotonic temperature dependence. In zero magnetic field κ has a anomalouslogarithmic size dependence also in this regime. The off-diagonal component ofthe thermoelectric tensor αxy is calculated and displays the very same ∼1/T dependence at low temperatures predicted from calculations based on Gaussiansuperconducting fluctuations.</p>

<p>The objective of this thesis is to evaluate a generic process for fabrication and characterization of the Superconductive coplanar waveguide (CPW) resonators. Superconductive CPW resonators with various lengths and shapes are designed to investigate their electrical and magnetic properties as well as resonance properties and sensitivities. In the first part of thesis, two different models are introduced in order to estimate the nonlinear kinetic inductance of a superconducting CPW resonator. The first model is based on Bean critical-state model and the second one is based on current dependence of London penetration depth. The existence of a shift in resonant frequency  of Superconductive CPW resonator caused by a non-linear kinetic inductance is also shown experimentally. Simulations were carried out to estimate the nonlinear kinetic inductance due to the self- induced magnetic field penetration.</p><p>The rest of the thesis is concerned with development of very smooth Aluminum (Al) thin films with RMS (Root Mean Square) roughness 1~nm and CAD (Computer Aid Design) of superconductive CPW resonators. Experimental investigation of a generic fabrication technique for superconductive CPW resonator is carried out. Many resonators are fabricated with different design parameters, such as centerline or gap width, film thickness and gap capacitors length. The fabrication process is described in detail. Electron Beam Lithography is used to fabricate Nb and Al CPW resonators which are coupled to outer conductors via gap capacitors. We have fabricated GHz frequency CPW resonators with quality factors, Q up 5X10^5.</p>

<p>We have studied various aspects of the <i>critical</i> properties of the Abelian Higgs model. The initial motivation to study this model is its relation to superconductivity, but the results extend beyond the realms of superconductivity. This thesis contains an introductory part and three research papers, all related to different aspects of the Abelian Higgs model.</p><p>Paper 1: We have investigated the properties of the model using a dual vortex representation. By focusing on the propagators of the gauge field <b>A</b> and the dual gauge field <b>h</b> we find a nice demonstration of the fact that <i>the dual</i> of a neutral condensate is isomorphic to a charged condensate. Finally this also provides firm support for the existence of a stable charged fixed point in the theory, distinct from the 3DXY fixed point.</p><p>Paper 2: The critical fluctuations in the Abelian Higgs model are vortex loops. We have studied the geometrical properties of these loops, and by using duality we have obtained scaling relations between the fractal dimension D_H of the loops and the anomalous dimension η_φ of the dual field theory.</p><p>Paper 3: We have calculated the GL parameter κ_tri separating a first order metal to superconductor transition from a second order one, κ_tri =(0.76±0.04))/√2. We also argue qualitatively that this κ_tri is the value separating type-I and type-II behavior, in contrast to the conventional value 1=√2. The calculations have been done including fluctuations in the amplitude and the phase of the matter-field, as well as fluctuations in the gauge field.</p><p>Paper 4: We have determined the effective interaction between vortices in the Ginzburg-Landau model from large-scale Monte-Carlo simulations. We find a change, in the form of a crossover, from attractive to repulsive effective vortex interactions in an intermediate range of Ginzburg-Landau parameters κε[0.76; 1]=√2, depending on temperature. We present a simple physical picture of the crossover, and relate it to observations in Ta and Nb elemental superconductors which have low-temperature values of κ in the relevant range.</p> / Papers I - 4 reprinted with kind permission of the American Physical Society. Copyright the American Physical Society.

We have studied various aspects of the critical properties of the Abelian Higgs model. The initial motivation to study this model is its relation to superconductivity, but the results extend beyond the realms of superconductivity. This thesis contains an introductory part and three research papers, all related to different aspects of the Abelian Higgs model. Paper 1: We have investigated the properties of the model using a dual vortex representation. By focusing on the propagators of the gauge field <b>A</b> and the dual gauge field <b>h</b> we find a nice demonstration of the fact that the dual of a neutral condensate is isomorphic to a charged condensate. Finally this also provides firm support for the existence of a stable charged fixed point in the theory, distinct from the 3DXY fixed point. Paper 2: The critical fluctuations in the Abelian Higgs model are vortex loops. We have studied the geometrical properties of these loops, and by using duality we have obtained scaling relations between the fractal dimension DH of the loops and the anomalous dimension ηφ of the dual field theory. Paper 3: We have calculated the GL parameter κtri separating a first order metal to superconductor transition from a second order one, κtri =(0.76±0.04))/√2. We also argue qualitatively that this κtri is the value separating type-I and type-II behavior, in contrast to the conventional value 1=√2. The calculations have been done including fluctuations in the amplitude and the phase of the matter-field, as well as fluctuations in the gauge field. Paper 4: We have determined the effective interaction between vortices in the Ginzburg-Landau model from large-scale Monte-Carlo simulations. We find a change, in the form of a crossover, from attractive to repulsive effective vortex interactions in an intermediate range of Ginzburg-Landau parameters κε[0.76; 1]=√2, depending on temperature. We present a simple physical picture of the crossover, and relate it to observations in Ta and Nb elemental superconductors which have low-temperature values of κ in the relevant range. / Papers I - 4 reprinted with kind permission of the American Physical Society. Copyright the American Physical Society.

In this thesis, we study an isolated vortex in an s-wave superconductor by solving the Bogoliubov-de Gennes equations self-consistently on a disc. We calculate the order parameter and supercurrent profiles, as well as the distribution of quasiparticle states. In contrast to quasi-classical treatments, the ratio Δ∞/EF between the order parameter and the Fermi energy is not assumed negligible. We study a regime where this ratio is on the order of 10-1, relevant to high-temperature superconductors. In this regime, we find a Friedel-like oscillation in the order parameter profile at low temperatures. This oscillation is attributed to an increased level spacing of the quasiparticle states, causing a decrease of the number of states present inside the superconducting energy gap. The results are in good agreement with previously published works. In future studies, the method used in this thesis will be generalized to d-wave superconductors. / I detta examensarbete studeras en ensam virvel i en s-vågssupraledare genom att självkonsistent lösa Bogoliubov och de Gennes' ekvationer på en cylinderskiva. Vi beräknar ordningsparameter- och superströmsprofiler, samt fördelningen av kvasipartikeltillstånd. Till skillnad från i kvasiklassiska metoder så antas inte kvoten Δ∞/EF mellan ordningsparametern och Fermi-energin vara negligerbar. Vi studerar en regim där denna kvot är av storleksordningen 10-1, vilket är fallet i högtemperatur-supraledare. Vid låga temperaturer finner vi i denna regim en Friedelliknande oscillation i ordningsparameterprofilen. Denna oscillations förklaras genom att separationen mellan kvasipartikeltillstånd ökar, vilket får som effekt att färre tillstånd ryms innanför det supraledande energigapet. Våra resultat överensstämmer väl med tidigare publicerade artikler. I framtida studier kommer metoden vi använder i detta examensarbete att generaliseras till d-vågssupraledare.

This thesis presents a brief synopsis of the derivations of the BCS and Eliashberg equations. An analytic formula for the critical temperature $T_c$ in Eliashberg theory is derived, which contains a sum of iterative integral corrections. These iterative integral corrections are the result of an iterative expression for the gap quotient $\Delta(\iw, T)/\Delta(0,T)$, which is derived. At the critical temperature this expression contains no reference to the critical temperature itself due to the gap approaching zero in this limit, $\lim_{T \rightarrow T_c} \Delta(\iw, T) = 0$. This enables explicit calculation of the critical temperature through the aforementioned iterative expression.\\ \\The behaviour of the iterative expression and its corrections are explored numerically with a toy spectral function $\sF$. Through these numerical experiments, this formula is found to be consistent with, though not equal to the successful McMillan formula for the coupling parameter $\lambda$ in the range $0.3 \leq \lambda \leq 1.5$. Below this value, the McMillan formula is found to approach zero critical temperature $T_c$ more rapidly, raising the future question of which of the two expressions is most successful in predicting the critical temperature $T_c$ in this range. \\ \\ For a toy spectral function with a single mode, the zeroth order correction of the iterative expression for the critical temperature $T_c$ is found to be adequate for most practical purposes due to the magnitude of measurement errors in real life measurements of model parameters. / Detta examensarbete går igenom en kort derivation av BCS och Eliashberg ekvationerna. En analytisk formel för den kritiska temperaturen $T_c$ i Eliashbergteori ges. Denna formel innehåller en summa av iterativa integraler som resulterar från ett uttryckt för energigapets kvot. Vid den kritiska temperaturen så kan man explicit lösa ut denna och på så sätt få ett analytiskt uttryck. Den uttrycket för den kritiska temperaturen utforskas numeriskt med en leksaks-spektralfunktion. Genom dessa numeriska experiment visas det hur det iterativa uttrycket sammanstämmer med McMillans formel för kopplingsparametern $0.3 &lt; \lambda &lt; 1.5$, även om dem ej är lika. Under detta intervall så närmar sig McMillans uttryck noll snabbare, vilket höjer frågan vilken utav dem två uttrycken som fungerar bäst i denna gräns.  För en leksaks-spektralfunktion med ett läge så räcker den nollte korrektionen i det iterativa uttrycket för att få godtagbara resultat, med bakgrund av dem relativt stora mätfelen för riktiga parametrar.

Electronic structure calculations constitute a valuable tool to predict the properties of materials. In this study we propose an efficient scheme to study correlated electron systems with essentially only one free parameter, the screening length of the Coulomb potential. A general reformulation of the exchange energy of the correlated electron shell is combined with this method in order to analyze the calculations. The results are interpreted in terms of different polarization channels, due to different multipoles. The method is applied to various actinide compounds, in order to increase the understanding of the complicate behaviour of 5f electrons in these systems. We studied the non-magnetic phase of δ-Pu, where the spin polarization is taken over by a spin-orbit-like term that does not break the time reversal symmetry. We also find that a non-trivial high multipole of the magnetization density, the triakontadipole, constitutes the ordering parameter in the mysterious hidden order phase of the heavy-fermion superconductor URu2Si2. This type of multipolar ordering is also found to play an essential role in the hexagonal-based superconductors UPd2Al3,  UNi2Al3 and UPt3 and in the dioxide insulators UO2, NpO2 and PuO2. The triakontadipole moments are also present in all magnetic actinides we considered, except for Cm. These results led us to formulate a new set of rules for the ground state of a system, that are valid in presence of strong spin-orbit coupling interaction instead of those of Hund; the Katt's rules. Finally, we applied our method to a new class of high-Tc superconductors, the Fe-pnictides, where the Fe 3d electrons are moderately correlated. In these materials we obtain the stabilization of a low spin moment solution, in agreement with experiment, over a large moment solution, due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 705

A superconducting nanowire single-photon detector (SNSPD) is an emerging, and today commercially available technology, for photon-counting and quantum cryptography. Yet, the photon detection event is not fully understood and current modeling efforts require substantial computational resources which motivates studies of simpler models.  This thesis introduces a model for vortex dynamics in thin-layered superconductors, such as SNSPDs, using a simplified approach, which leads to a 2D Coulomb gas model where the vortices are modeled as electrostatic charges. The model is carefully constructed from the method of images to describe a wire with open boundary conditions and an applied supercurrent. Subsequently, equilibrium and non-equilibrium properties are sampled with the Metropolis-Hastings algorithm and further analyzed and discussed. The suggested model is shown to be effective and successfully reproduces expected SNSPD behavior; most importantly critical behavior and voltage pulses which are directly measured during detection events. In conclusion, a 2D Coulomb gas model can be a preferred alternative for modeling vortex dynamics in SNSPDs at a small computational cost, motivating further development and studies. / Supraledande nanotråd-enfotondetektorer (SNSPD:er) är en framväxande och idag kommersiellt tillgänglig teknologi som används för räknande av fotoner samt inom kvantkryptografi. Ändå är fotondetektionshändelsen inte helt förstådd och de nuvarande modelleringar kräver substantiell datorkraft vilket motiverar studier av enklare modeller. Det här examensarbetet introducerar en model för vortexdynamik i tunnskiktade supraledare såsom SNSPD:er genom ett förenklat tillvägagångssätt som leder till en 2D Coulombgas-modell där ett vortex modelleras som en elektrisk laddning. Modellen är noggrant konstruerad med öppna randvillkor och den så kallade frysta spegelbildsmetoden samt en pålagd superström. Då samlas mätvärden in på jämvikts- samt icke-jämviktsegenskaper hos systemet som vidare analyseras, jämförs och diskuteras. Den föreslagna modellen visas vara effektiv och reproducerar framgångsrikt förväntat beteende hos SNSPD:er; framför allt kritiskt beteende och spänningstoppar som direkt uppmäts i en fysisk detektionshändelse. Sammanfattningsvis, kan en 2D Coulombgas-modell vara ett föredraget alternativ för att modellera vortexdynamik hos SNSPD:er för en liten beräkningskostnad, vilket motiverar fler studier av detta.