| 1. |
- Karlsson, Fredrik, 1975-
(författare)
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The acquisition of contrast : a longitudinal investigation of initial s+plosive cluster development in Swedish children
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This Thesis explores the development of word-initial s+plosive consonant clusters in the speech of Swedish children between the ages of 1;6 and 4;6. Development in the word-initial consonant clusters is viewed as being determined by 1) the children’s ability to articulate the target sequence of consonants, 2) the level of understanding of which acoustic features in the adult model production are significant for the signalling of the intended distinction, and 3) the children’s ability to apply established production patterns only to productions where the acquired feature agrees with the adult target, to achieve a contrast between rival output forms. This Thesis employs a method where output forms are contrasted with attempted productions of potential homonym target words. Thus, development is quantified as an increase in the manifestations of phonetic features where it agrees with the adult norm, coupled by a decrease in the same feature in output forms where it is inappropriate according to the specifications of the phonological system of the ambient language. Acoustic investigations of cues of voicing, aspiration, place of articulation and syllable onset complexity, and auditory investigations of place, manner and syllable onset complexity were conducted. The Thesis has four outcomes. One, a description of the perceptual quality of the productions in terms of place, manner, voicing and syllable onset complexity is presented. Two, a developmental sequence of stable acquisition of these features is proposed; manner is shown to be acquired first, followed by syllable onset complexity and place of articulation. Evidence is provided that the voiced/aspirated distinction is still being acquired at the end of the investigated age period. Three, the developmental use of acoustic cues of place and voicing are described. Voice Onset Time and Spectral Skewness are shown to be used by children in order to increase the likeness to the adult target in terms of voicing and place of articulation. Aspiration Amplitude is shown to be used as an auxiliary cue to Voice Onset Time. The place cues Spectral Tilt Change, F2, Spectral Mean and Spectral Variance were shown to be used in order to refine already produced consonants rather than approach the adult target model. Four, the Thesis provides evidence of periods of confusions in the output of children. With the reductions of these patterns of confusion, evidence is provided of children’s re-organisation of their internal representation of the consonant to be produced.
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| 2. |
- Karlsson, Max, 1990-
(författare)
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Dynamics in Blue Emitting Metal Halide Perovskites for Light Emitting Diodes
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lighting comprises a large part of the global electricity consumption as of today, and the use of lighting in illumination and displays is only projected to grow. It is therefore imperative to meet this energy demand, not only by means of greener energy production, but also with materials that are both more efficient to fabricate as well as to use. Low cost and energy efficient light sources therefore play an important role in minimizing further greenhouse emissions from the way we choose to live.Metal halide perovskites are a group of semiconductors that have received a great amount of attention during the past years due to impressive - and continuously increasing - performance as active materials implemented in solar cells and light emitting diodes. This is due to highly desirable optoelectronic properties combined with low-cost, solution-processable fabrication methods. Simple bandgap-tunability is easily achieved by compositional and dimensional engineering, allowing perovskite emission to span a broad wavelength region from ultraviolet to near infrared. As with previous technologies, attaining stable, bright, and pure blue light has proven difficult also in metal halide perovskites. This thesis investigates some of the challenges in achieving blue emission in mixed-halide and mixed-dimensional perovskites for light-emitting-diode applications.Mixed-halide alloying provides the most straightforward way of tuning the bandgap of perovskites. Unfortunately, mixed bromide/chloride-perovskites (used to achieve blue light) suffer from both spectral and temporal instabilities, as well as severe luminescence quenching at the large chloride contents necessary for blue emission. The spectral instability arises from a segregation of halides into regions of differing halide content, and hence different bandgap, resulting in a shift in emission color during operation. Although the origins of the poor temporal stability of perovskite light emitting diodes are manifold, one of the main problems are the low barriers for halide migration under the applied electric field during operation, rapidly degrading the device properties.We first find that compositional heterogeneities, stemming from rapid uncontrolled film growth, both lowers the threshold for further halide segregation as well as serves as centers for non-radiative recombination, resulting in reduced luminescence yield. We show that by carefully moderating the crystallization dynamics it is possible to achieve films with a homogeneous composition, thereby mitigating the negative effects arising from material inhomogeneities. We identify means of how growth environment, stoichiometric tuning and chelating additives can be used to favorably control film formation and provide guidelines that can be more widely applied in the fabrication of perovskite films and devices. We continue by investigating the role of Br/Cl-alloying on device efficiency and stability in green to blue emitting perovskite LEDs. We find that chloride incorporation, while having only a minor impact on efficiency at moderate levels, detrimentally affects device stability even in small amounts. We ascribe this phenomenon to an increased mobility of halogen ions in the mixed-halide lattice resulting from an increased chemically and structurally disordered landscape with reduced migration barriers. We assign this as the major obstacle towards stable blue-emitting mixed-halide perovskite light emitting diodes.In the last work we investigate blue emitting mixed-dimensional Ruddlesden-Popper perovskites (RPPs) comprising of multiple-quantum-well-structures of varying bandgap. Successful implementation in LEDs has been attributed to efficient carrier funneling from large bandgap (donor) regions to low bandgap regions (acceptors) resulting in improved luminescence yields due to trap state filling from the locally increased carrier density. However, due to the enhanced carrier concentrations in acceptor domains, Auger recombination quickly outcompetes radiative recombination mechanisms already at moderate pump fluences or carrier injection densities in RPPs. We show that by moderating the inter-well carrier transfer, while at the same time providing adequate defect passivation, high quantum yields can be maintained even at large carrier densities. We thereby show that RPPs can support a large density of carriers without compromising luminescence efficiency, paving the way for their use in high brightness applications by engineering the funneling and recombination processes in these materials.The work in this thesis provides new insights on various dynamical processes in metal halide perovskites aimed at light emitting applications. The hope is that it will contribute toward the understanding of these systems and help in bringing these materials closer to practical use.
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| 3. |
- Koudouridis, Georgios
(författare)
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Radio Resource Allocation and Utilization in Multiple Radio Access Networks
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Meeting in a sustainable way the demands of an unprecedented explosive growth in the number of connected devices and mobile data traffic, next-generation mobile networks are expected to make flexible use of spectrum and provide higher capacity by a denser deployment of radio access network infrastructure. In addition, the allocation and the utilisation of radio access resources should improve spectrum usage efficiency and keep energy consumption low, implying the capability to effectively exploit the denser deployment of radio accesses and the co-existence of radio accesses belonging to one or multiple radio access technologies. The term radio access (RA) is used to denote the radio resources associated with a frequency carrier that can be allocated to one or multiple users for their data transmissions.In a network consisting of multiple radio accesses, improvements in spectral and energy efficiency can be achieved when either channel gains are increased or interference is eliminated or both. There are at least three possible approaches to improve performance, namely, (i) by opportunistically utilizing channel conditions of multiple radio accesses, (ii) by mitigating interference across and within radio accesses, and (iii) by redistributing traffic load among radio accesses. In this work, four different technical solutions for downlink transmissions have been studied and evaluated with respect to throughput, spectral efficiency and energy efficiency performance: (a) multi-access transmit diversity, which refers to the dynamic selection of multiple radio accesses for the transmission of a user’s data, (b) inter-cell interference coordination, which mitigates interference by dividing radio access bandwidth among neighbouring nodes, (c) power on/off of access nodes, which mitigates interference by switching off radio accesses of interfering neighbouring nodes, and (d) radio access load balancing, which effectively distributes load by associating users to radio accesses where the expected rate is higher. For the implementation of the technical solutions different algorithms have been devised and their performance have been evaluated for different user distribution scenarios and different heterogeneous multi-radio access networks deployments consisting of at least a tier of macro-cellular and/or a tier of pico-cells of different densities.The technical solutions are combined into a framework for the allocation and utilisation of radio access resources in heterogeneous multi-radio access dense networks. The framework consists of two subsequent steps: (1) a step solving the multi-radio allocation problem which associates users with a single or multiple radio accesses in the network on the basis of the expected data rates, and (2) a step solving the multi-radio utilisation problem that determines which of the associated radio access(s) should be used at any time for the user data transmissions in the downlink on the basis of the expected instantaneous data rate. To solve the first problem, we employ the flexible spectrum access solution that performs load balancing by associating users to multiple radio accesses while keeping radio accesses without users switched off. For the second problem, we utilise different multi-radio transmit diversity schemes while taking into account different forms of static inter-cell interference coordination. The evaluation of our framework, which is performed by means of simulations, demonstrates significant performance improvements in terms of user throughput, cell-edge throughput, spectral efficiency and energy efficiency.
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| 4. |
- Niva, Mikael, 1964-
(författare)
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Life History Strategies in Linnaea borealis
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- About 70% of the plant species in the temperate zone are characterised by clonal growth, clonal species are also in majority in the Arctic and Subarctic where they affect the structure and composition of the vegetation. It is therefore of great importance to increase our knowledge about clonal plants and their growth and life histories. I have investigated how ramets of the stoloniferous plant Linnaea borealis are affected by the naturally occurring variation in environmental factors, such as: light, nutrient and water availability. Moreover, I examined the seed set and how supplemental hand pollination affects seed set in L. borealis, and also investigated the significance of the apical meristem for shoot population fitness. All studies were performed under field conditions in northern Sweden in a Subarctic environment and most are experimental.The results show that nutrient resorption from senescing leaves is not significantly affecting the growth and nutrient pools of the ramet. This implies that the growth of L. borealis ramets is not governed by micro-site resource availability. However, removal of light competition resulted in increased branching and number of lateral meristems produced, reduced growth, and decreased root:shoot ratio on a per ramet basis. Thus, ramets of L. borealis can efficiently exploit favourable light patches through plastic growth. Apical dominance exerts a significant effect on shoot population fitness and can be lost through rodent grazing. However, loss of apical dominance is dependent on the timing of grazing, if the apical meristem is removed early in the autumn the ramet can repair the loss until the next summer. If grazing occur during spring the dry weight and leaf area production is affected negatively. Seed production in L. borealis in the Abisko area varies between years and sites, and was unaffected by supplemental hand pollination treatment, implying that there is no lack of pollinator activity.
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| 5. |
- Åström, Maria, 1965-
(författare)
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Defining Integrated Science Education and Putting It to Test
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis is made up by four studies, on the comprehensive theme of integrated and subjectspecific science education in Swedish compulsory school. A literature study on the matter is followed by an expert survey, then a case study and ending with two analyses of students’ science results from PISA 2003 and PISA 2006. The first two studies explore similarities and differences between integrated and subject-specific science education, i. e. Science education and science taught as Biology, Chemistry and Physics respectively. The two following analyses of PISA 2003 and PISA 2006 data put forward the question whether there are differences in results of students’ science literacy scores due to different types of science education.The expert survey compares theories of integration to the Swedish science education context. Also some difference in intention, in the school case study, some slight differences in the way teachers plan the science education are shown, mainly with respect to how teachers involve students in their planning.The statistical analysis of integrated and subject-specific science education comparing students’ science results from PISA 2003 shows no difference between students or between schools. The analysis of PISA 2006, however, shows small differences between girls’ results with integrated and subject-specific science education both in total scores and in the three scientific literacy competencies. No differences in boys’ results are shown on different science educations.
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