| 11. |
- Karlsson, Krister, 1961-
(författare)
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Electronic Structure Calculations: Materials with Weak and Strong Correlations
- 1992
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is devoted to studies of various electronic properties which can be extracted from photoemission spectroscopy. Both weakly and strongly correlated systems are discussed, ranging from free electron metal surfaces to strongly correlated copper oxide compounds. In order to study such a variety of systems, different calculational schemes have to be employed. This work is mainly based on band structure calculations using the local density approximation. However, for strongly correlated systems the need for model Hamiltonians is crucial. Subjects discussed are: * inverse photoemission spectra from the Cd(0001) surface obtained by a multiple scattering formalism. The results are in good agreement with recent experimental data. Especially, the one-step model is used to demonstrate image states excitations by secondary beams. * artificial structures, such as semiconductor heterojunctions and short-period superlattices, which are of vital importance for modern electronic devices. A method for tuning the band offset at semiconductor interfaces is proposed; by varying (n,m) for the GaAs/(AlAs)n(GaAs)m interface, the magnitude of the valence band offset can be designed. The metallic superlattice Mo/V was also studied, with emphasis on the application of hydrogen storage. * Schottky barrier heights. Final state screening effects can influence the Schottky barrier height, as deduced from measured core level shifts. According to our calculations, such effects may be of the order 0.1-0.2 eV. * chemical shifts in Cu2O, CuO and NaCuO2, where the Cu atom is formally mono-, di- and trivalent. We find similar chemical shifts between Cu atoms in YBa2Cu3O6.5 as between the Cu atoms in the model compounds. This suggests that different Cu atoms in YBa2Cu3O6.5 are formally mono-, di- and trivalent. The Cu 2p core level spectra were calculated using the Anderson impurity model, with parameters obtained from ab initio calculations. The results are related to the formal valence of Cu, and can be explained by a charge counting argument.
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| 12. |
- Karlsson, Krister, 1961-, et al.
(författare)
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Final-state effects in photoemission from metal-semiconductor interfaces
- 1991
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 67:2, s. 236-239
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Tidskriftsartikel (refereegranskat)abstract
- In this Letter we stress the importance of final-state effects in photoelectron spectroscopy. In particular, we address the problem of Schottky-barrier formation, as studied via core-level shifts in photoemission. We have calculated the shift of the core-level distribution when a semiconductor surface is covered with a metal, using a wave-vector-dependent image-screening model. We conclude that final-state effects, which are generally neglected in this context, are in fact quite important. This conclusion is supported by experimental observations reported in the literature. © 1991 The American Physical Society.
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| 13. |
- Karlsson, Krister, 1961-, et al.
(författare)
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Shape of the Cu 2p core level photoemission spectrum for monovalent, divalent and trivalent Cu compounds
- 1992
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 4:11, s. 2801-2816
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Tidskriftsartikel (refereegranskat)abstract
- We calculate the shape of the 2p core level photoemission spectrum for Cu2O, CuO and NaCuO2, where Cu is formally mono-, di- and trivalent, respectively. Although the number of 3d electrons is similar in the three compounds, there is a large variation in the strength of the d9-like satellite and in the width of the main line. We relate the differences between Cu2O and CuO to the difference in the valence, while for NaCuO2 the detailed form of the hopping matrix elements is also found to be important. © 1992 IOP Publishing Ltd.
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| 14. |
- Karlsson, Krister, 1961-, et al.
(författare)
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Tailoring materials for quantum wells : band offsets at (001)-oriented GaAs/(AlAs)n(GaAs)minterfaces
- 1990
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 2:23, s. 5265-5269
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure at the interface between bulk GaAs(001) and short-period superlattices of (AlAs)n(GaAs)m has been calculated using ab initio pseudopotential techniques. The results show that the valence band offsets at such interfaces are very similar to those obtained experimentally for random alloy systems, but superior transport properties are anticipated for the ordered systems.
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| 15. |
- Lundell, Björn, et al.
(författare)
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Active Support for ER Modelling
- 1993
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A commonly used methodology in the design of a database system is to use the ER Model for the conceptual design together with the RDM during the implementation phase. This methodology requires that a mapping between the two data models be performed.This paper addresses the `lost semantics' problem caused by the impedance mismatch between the two models and reports on a prototype system which has been developed in order to bridge this gap. The aim of the system is to provide a tool for preserving the semantics of a conceptually rich model when automatically translating it into a working system.The graph oriented approach suggested by Dogac [Dog90] makes use of the implicit information from the ER Schema when identifying the correct update propagations. They suggest a preprocessor which must be run on all applications for this automatic generation of relational code. We take this a stage further and suggest how, using theemerging active technology of new DBMS, the techniques can be used to provide full, automatic and centralised support for an ER design. Details of an initial prototype (for non-active systems) and a refined prototype (for the latest release of a system, with triggers) are provided.There are a number of aspects to the architecture which has been developed: it is multi-level, for targeting different database systems; it preserves the ER Model in the working system; it can be used for both active and passive database systems. The early (passive) prototype has been designed to run on top of the commercial RDBMSINFORMIX. Work on both the passive and active designs targets the commercial RDBMS INFORMIX; the active design uses a general intermediate form of ECA rule as a target for translation.
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| 16. |
- Niklasson, Lars, et al.
(författare)
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Can Connectionist Models Exhibit Non-Classical Structure Sensitivity?
- 1994
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Several connectionist models have been supplying non-classical explanations to the challenge of explaining systematicity, i.e., structure sensitive processes, without merely being implementations of classical architectures. However, lately the challenge has been extended to include learning related issues. It has been claimed that when these issues are taken into account, only a restricted form of systematicity could be claimed by the connectionist models put forward so far. In this paper we investigate this issue further, and supply a model and results that satisfies even the revised challenge.
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| 17. |
- Niklasson, Lars, et al.
(författare)
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Connectionism : The Miracle Mind Model
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Abstract: Connectionism as a model of the mind has recently been challenging the Classical model, in which the mind is regarded as symbol manipulating system. The main arguments against Connectionism concern its inability to form mental representations for complex expressions, which can be used for structure sensitive operations. Some argue for hybrid models which combine some of the most attractive features of the Classical and Connectionist models. This paper starts off by examining the definitions of the different approaches and also their strengths and weaknesses. One section is devoted to the debate between the advocators of the different paradigms, including the arguments about the lack of compositionality and systematicity in Connectionist cognitive models. We then argue for the Connectionist approach as the most attractive model of the mind. This includes performing the "miracle" of defining structure sensitive operations on non-symbolic representations of concepts.
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| 18. |
- Niklasson, Lars, et al.
(författare)
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Connectionism and the Issues of Compositionality and Systematicity
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Connectionism as a model of the mind has been attacked by the advocators of the classical paradigm, who claim that Connectionism can only work if it is an implementation of Classical representations. This could be true for some of the models that claim to be Connectionist, but it will in this paper be shown that this is not true for Connectionist architectures that use non-symbolic representations. We will provide evidence in the form of simulation results that severely weaken of the arguments raised by Fodor and Pylyshyn and Fodor and McLaughlin, including their two main arguments, which are the lack of compositionality and systematicity.
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| 19. |
- Niklasson, Lars
(författare)
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Structure Sensitivity in Connectionist Models
- 1993
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- HS-IDA-TR-93-003. Annotation: Published in The Proceedings of the 1993 Connectionist Models Summer School, (Eds) Mozer et al., Lawrence Erlbaum, 1993.
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| 20. |
- Niklasson, Lars, et al.
(författare)
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Systematicity and Generalisation in Connectionist Compositional Representation
- 1993
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- It has been argued that models, that are claimed to be models of the mind, have to exhibit a behaviour closely related to human thought. This includes dealing with the issues of compositionality, systematicity and productivity. This paper starts by describing a non-concatenative mode of combination for connectionist patterns of neural activation. We then turn to the issue of systematicity, i.e. structure sensitive processes. We explore this issue to some level of detail, e.g. the importance of choosing the `right' type of representation and how the construction of the training set could result in different types of systematicity.
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