Magnetic and transport properties of LaMnO3+δ, La1-xCaxMnO3, La1-xCaxMn1-yFeyO3 and La1-xSrxMn1-yFeyO3

Interest to hole-doped mixed-valence manganite perovskites is connected to the ‘colossal’ magnetoresistance. This effect or huge drop of the resistivity, ρ, in external magnetic field, B, attains usually the maximum value near the ferromagnetic Curie temperature, TC. In this thesis are investigated conductivity mechanisms and magnetic properties of the manganite perovskite compounds LaMnO3+, La1-xCaxMnO3, La1-xCaxMn1-yFeyO3 and La1- xSrxMn1-yFeyO3. When the present work was started the key role of the phase separation and its influence on the properties of the colossal magnetoresistive materials were not clear. Our main results are based on temperature dependencies of the magnetoresistance and magnetothermopower, investigated in the temperature interval of 4.2 - 300 K in magnetic fields up to 10 T. The magnetization was studied in the same temperature range in weak (up to 0.1 T) magnetic fields. LaMnO3+δ is the parent compound for preparation of the hole-doped CMR materials. The dependences of such parameters as the Curie temperature, TC, the Coulomb gap, Δ, the rigid gap, γ, and the localization radius, a, on pressure, p, are observed in LaMnO3+δ. It has been established that the dependences above can be interpreted by increase of the electron bandwidth and decrease of the polaron potential well when p is increased. Generally, pressure stimulates delocalization of the electrons in LaMnO3+δ. Doping of LaMnO3 with Ca, leading to La1-xCaxMnO3, changes the Mn3+/Mn4+ ratio significantly and brings an additional disorder to the crystal lattice. Phase separation in a form of mixture of the ferromagnetic and the spin glass phases was observed and investigated in La1- xCaxMnO3 at x between 0 and 0.4. Influence of the replacement of Mn by Fe is studied in La0.7Ca0.3Mn1−yFeyO3 and La0.7Sr0.3Mn1−yFeyO3. Asymmetry of the soft Coulomb gap and of the rigid gap in the density of localized states, small shift of the centre of the gaps with respect to the Fermi level and cubic asymmetry of the density of states are obtained in La0.7Ca0.3Mn1−yFeyO3. Damping of TC with y is connected to breaking of the double-exchange interaction by doping with Fe, whereas the irreversibility and the critical behavior of the magnetic susceptibility are determined by the phase separation and the frustrated magnetic state of La0.7Sr0.3Mn1−yFeyO3.

Resonant tunneling effects in semiconductor heterostructures

The thesis is devoted to a theoretical study of resonant tunneling phenomena in semiconductor heterostructures and nanostructures. It considers several problems relevant to modern solid state physics. Namely these are tunneling between 2D electron layers with spin-orbit interaction, tunnel injection into molecular solid material, resonant tunnel coupling of a bound state with continuum and resonant indirect exchange interaction mediated by a remote conducting channel. A manifestation of spin-orbit interaction in the tunneling between two 2D electron layers is considered. General expression is obtained for the tunneling current with account of Rashba and Dresselhaus types of spin-orbit interaction and elastic scattering. It is demonstrated that the tunneling conductance is very sensitive to relation between Rashba and Dresselhaus contributions and opens possibility to determine the spin-orbit interaction parameters and electron quantum lifetime in direct tunneling experiments with no external magnetic field applied. A microscopic mechanism of hole injection from metallic electrode into organic molecular solid (OMS) in high electric field is proposed for the case when the molecules ionization energy exceeds work function of the metal. It is shown that the main contribution to the injection current comes from direct isoenergetic transitions from localized states in OMS to empty states in the metal. Strong dependence of the injection current on applied voltage originates from variation of the number of empty states available in the metal rather than from distortion of the interface barrier. A theory of tunnel coupling between an impurity bound state and the 2D delocalized states in the quantum well (QW) is developed. The problem is formulated in terms of Anderson-Fano model as configuration interaction between the carrier bound state at the impurity and the continuum of delocalized states in the QW. An effect of this interaction on the interband optical transitions in the QW is analyzed. The results are discussed regarding the series of experiments on the GaAs structures with a -Mn layer. A new mechanism of ferromagnetism in diluted magnetic semiconductor heterosructures is considered, namely the resonant enhancement of indirect exchange interaction between paramagnetic centers via a spatially separated conducting channel. The underlying physical model is similar to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction; however, an important difference relevant to the low-dimensional structures is a resonant hybridization of a bound state at the paramagnetic ion with the continuum of delocalized states in the conducting channel. An approach is developed, which unlike RKKY is not based on the perturbation theory and demonstrates that the resonant hybridization leads to a strong enhancement of the indirect exchange. This finding is discussed in the context of the known experimental data supporting the phenomenon.

Tutors as mediators of exchange student experiences: evaluation of the international student tutor system at the University of Turku

As academic student mobility is increasing, improving the functionality of international operations is recognised as a competitive advantage at tertiary education institutions. Although many scholars have researched the experiences of exchange students, the role of student tutors and their contribution to exchange students’ experiences is still an unknown factor. This research examines international tutoring at the University of Turku, and aims to understand better the way tutoring contributes to exchange experiences and to explore the functionality of the tutor system and discover areas for improvements. To achieve these goals, the research seeks to answer the fundamental research question: What is the role of tutors in mediating exchange experiences? The theoretical framework combines literature on mediating exchange experiences, the phenomenon of studying abroad, the process of adaptation, the importance of cross-cultural communication, and the role of student tutors as mediators. Based on the literature review, a theoretical model for studying the mediation of exchange experiences is introduced. The model’s applicability and validity is examined through a case study. Three methods were used in the empirical research: surveys, participant observations, and interviews. These methods provided extensive data from three major parties of the tutor system: tutors, exchange students, and the international office. The findings of the research reveal that tutoring – instrumental leading and social and cultural mediating – generates both negative and positive experiences depending on the individuals’ expectations, motivations, relationships, and the nature of the tutoring. Although functional, there are a few weaknesses in the tutor system. Tutors tend to act as effective instrumental leaders, but often fail to create a friendship and contribute to the exchange students’ experience through social and cultural mediation, which is significantly more important in the exchange students’ overall experience in terms of building networks, adapting, gaining emotional experiences, and achieving the stage of personal development and mental change. Based on the weaknesses, three improvements are suggested: (1) increasing comprehensive sharing of information, effective communication, and collective cooperation, (2) emphasising the importance of social and cultural mediation and increasing the frequency of interaction between tutors and exchange students, and (3) improving the recruitment and training, revising the process of reporting and rewarding, and finally, enhancing services and coordination.

Expectations of elderly people on the nurse-client relationship and interaction in home care : "When I see the nurse's smiling face it feels likes sunshine on a rainy and gloomy day"

The purpose of the thesis was to explore expectations of elderly people on the nurse-client relationship and interaction in home care. The aim is to improve the quality of care to better meet the needs of the clients. A qualitative approach was adopted. Semi-structured theme interviews were used for data collection. The interviews were conducted during spring 2006. Six elderly clients of a private home care company in Southern Finland acted as informants. Content analysis was used as the method of data analysis. The findings suggest that clients expect nurses to provide professional care with loving-kindness. Trust and mutual, active interaction were expected from the nurse-client relationship. Clients considered it important that the nurse recognizes each client's individual needs. The nurse was expected to perform duties efficiently, but in a calm and unrushed manner. A mechanic performance of tasks was considered negative. Humanity was viewed as a crucial element in the nurse-client relationship. Clients expressed their need to be seen as human beings. Seeing beyond the illness was considered important. A smiling nurse was described to be able to alleviate pain and anxiety. Clients hoped to have a close relationship with the nurse. The development of a close relationship was considered to be more likely if the nurse is familiar and genuine. Clients wish the nurses to have a more attending presence. Clients suggested that the work areas of the nurses could be limited so that they would have more time to transfer from one place to another. Clients felt that they would benefit from this as well. The nurses were expected to be more considerate. Clients wished for more information regarding changes that affect their care. They wished to be informed about changes in schedules and plans. Clients hoped for continuity from the nurse-client relationship. Considering the expectations of clients promotes client satisfaction. Home care providers have an opportunity to reflect their own care behaviour on the findings. To better meet the needs of the clients, nurses could apply the concept of loving-kindness in their work, and strive for a more attending presence.

Worldly Saints : Social Interaction of Dominican Penitent Women in Italy, 1200-1500

Doctoral dissertation, University of Turku

Copresence video conference mediated interaction

Abstract

Interaction of prechilling, temperature, osmotic stress, and light in Picea abies seed germination

Summary

Sensitivity of temperate grassland species to elevated atmospheric CO2 and the interaction with temperature and water stress

Selostus: Kohonneen hiilidioksidipitoisuuden, lämpötilan ja kuivuuden vaikutus nurmikasveihin