Grafeenin spinriippuva johtavuus

Graphene is a 2-dimensional allotrope of carbon which has attracted a lot of interest since its discovery in 2004. It has high electron mobility and small intrinsic spin-orbit-coupling, leading to long spin persistence lengths. This makes it an interesting material for spintronics as it can be used as a platform for spin-dependent defects. Its spin behavior is almost completely determined by the defects, which can be used to tune its properties. The electronic transport properties of graphene are most commonly studied by Landauer-Büttiker and Kubo-Greenwood methods. They both work well with the tight-binding model, which recovers the electronic band structure of graphene correctly, and they give results consistent with each other. The difference between the two is that Landauer-Büttiker is more suitable for narrow ribbons, while Kubo-Greenwood performs better in wider systems. In this thesis, a spinful version of the Kubo-Greenwood method is derived and then implemented on top of an existing spinless version. The implementation is done with Nvidia CUDA and it runs on graphics processing units. Different spin-dependent defects in graphene are used to test the implementation and validate the method. While the spinful version of the method is easy to implement, it turns out that it lacks the ability to completely describe spin-flipping current. Spin-conserving conductivity is given correctly by the method and spin polarization is proposed to describe the spin behavior better. Long-ranged scatterers are found to scatter spin more efficiently than they do charge and they are suggested to be partly responsible for the experimentally found short spin relaxation times.Grafeeni on hiilen kaksiulotteinen allotrooppi, joka löydettiin vuonna 2004. Sen elektroneilla on suuri liikkuvuus ja pieni spin–rata-kytkentä, jonka ansiosta spinin relaksaatiopituus kasvaa huomattavan suureksi. Grafeeni on erityisen kiinnostava materiaali spintroniikassa, jossa sitä voidaan käyttää alustana spin-riippuville epäpuhtauksille. Sen spinin ominaisuudet ovat lähes täysin epäpuhtauksien määrittelemät, joten spin käytöstä voidaan säätää niiden avulla. Grafeenin johtavuusominaisuuksia tutkitaan yleensä joko Landauer-Büttiker- tai Kubo-Greenwood- menetelmillä. Molemmat toimivat hyvin tiukan sidoksen mallin kanssa ja antavat keskenään vertailukelpoisia tuloksia. Menetelmien erona ovat systeemit, joihin ne soveltuvat. Landauer-Büttiker toimii paremmin kapeammissa nauhoissa, kun taas Kubo-Greenwood soveltuu hyvin leveämpiin, täysin kaksiulotteisiin systeemeihin. Tässä työssä johdetaan spinillinen versio Kubo-Greenwood-menetelmästä ja toteutetaan se aikaisemman spinittömän version pohjalta. Toteutus tehdään Nvidian CUDA-kielellä ja sitä voidaan ajaa näytönohjaimilla. Toteutusta testataan erilaisilla spin-riippuvilla epäpuhtausmalleilla. Spinillinen versio on helposti johdettavissa spinittömästä versiosta, mutta käy ilmi, ettei se kuvaa täysin spinien välistä virtaa. Menetelmä antaa kuitenkin oikein spinin säilyttävän konduktiivisuuden ja spinin polarisaatio osoittautuu paremmaksi ominaisuudeksi kuvaamaan spinin käytöstä. Tutkituista epäpuhtauksista pitkän matkan sirottajat sirottivat spiniä suhteessa varaukseen eniten ja niitä ehdotetaan osatekijäksi kokeissa havaittuihin lyhyisiin relaksaatiopituuksiin

Attracting international degree students to Finnish higher education institutions: a study on decision making processes of Chinese and South Korean students

OBJECTIVE OF THE STUDY The research on international student mobility in Finland has been limited and often focused on exchange students. Furthermore studies in the field are often descriptive in nature and concentrated on satisfaction and the adjustment of foreign students to Finnish society and education system. The objective of this research is to create understanding of the decision making processes of potential foreign degree students from China and South Korea. This understanding is meant to create knowledge than can be utilized in recognizing ways for Finnish higher education institutions, including universities and universities of applied sciences, to attract international degree students. METHODOLOGY The research belongs to the discipline of services marketing and consumer research. The research was conducted as a qualitative research through personal semi-structured interviews. A total of 13 potential students were interviewed for the research. The interview data was examined by utilizing the theoretical framework on international students' decision-making process by Cubillo et al. The framework uses purchase intention of foreign higher education as an independent variable dependent on five factors. FINDINGS The study illustrated that reasons related to individual socioeconomic advancement were the main motivation behind the decision to study abroad, while country image effect, institution image and program evaluation proved to be the most important facilitators to achieve this desired advancement. Total cost of international education, including both tuition and general cost of living, was also a significant deciding factor. Advice from family was deemed less important than advice from friends and professors. Personal experience of the destination country was more important to students who were considering Finland. Compared to the rest of the sample, students considering Finland also considered program evaluation more important than institution image or city effect

Efficient molecular dynamics simulations with many-body potentials on graphics processing units

Graphics processing units have been extensively used to accelerate classical molecular dynamics simulations. However, there is much less progress on the acceleration of force evaluations for many-body potentials compared to pairwise ones. In the conventional force evaluation algorithm for many-body potentials, the force, virial stress, and heat current for a given atom are accumulated within different loops, which could result in write conflict between different threads in a CUDA kernel. In this work, we provide a new force evaluation algorithm, which is based on an explicit pairwise force expression for many-body potentials derived recently (Fan et al., 2015). In our algorithm, the force, virial stress, and heat current for a given atom can be accumulated within a single thread and is free of write conflicts. We discuss the formulations and algorithms and evaluate their performance. A new open-source code, GPUMD, is developed based on the proposed formulations. For the Tersoff many-body potential, the double precision performance of GPUMD using a Tesla K40 card is equivalent to that of the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) molecular dynamics code running with about 100 CPU cores (Intel Xeon CPU X5670 @ 2.93 GHz). (C) 2017 Elsevier B.V. All rights reserved.Peer reviewe

Olkkarin kävijöiden kokemukset : Matalan kynnyksen kohtaamispaikan merkitys sekä vaikutukset kävijöiden elämään ja hyvinvointiin

Opinnäytetyöni aiheena on Seinäjoella sijaitsevan matalan kynnyksen kohtaamispaikan, Olkkarin, kävijöiden kokemukset Olkkarista sekä merkitykset, joita he paikalle antavat. Tämän lisäksi tutkin sitä miten Olkkarissa käyminen on vaikuttanut kävijöiden elämään ja hyvinvointiin. Teoriapohjana opinnäytetyössä on kolme käsitettä: vertaistuki, hyvinvointi ja osallisuus. Tutkimukseni on laadullinen tutkimus, johon aineisto on kerätty teemahaastatteluin. Haastattelin seitsemää Olkkarin kävijää, jotka ilmoittautuivat vapaaehtoisiksi tutkimukseen. Haastattelut toteutettiin Olkkarin tiloissa keväällä 2015. Haastatteluiden perusteella Olkkari on kävijöilleen tärkeä paikka. Kävijät osallistuvat monipuolisesti erilaiseen Olkkarin toimintaan oman mielenkiintonsa mukaan ja löytävät toiminnasta itselleen mielekkäitä asioita. Keskeistä on ihmisen tapaaminen. Olkkarissa kävijät saavat vertaistukea sekä kokemuksia osallisuudesta. Olkkari on kävijöille merkittävä paikkana, jossa saa olla oma itsensä ja kokea olevansa hyväksytty. Olkkarissa käymisen hyvät kokemukset leviävät myös muuhun elämään ihmissuhteiden ja parantuneen itsetunnon kautta.The objective of my thesis is to investigate visitors’ experiences about Olkkari, a low-threshold meeting place located in Seinäjoki. In addition, I study the meanings visitors give to Olkkari and the effects it has on visitors’ life and well-being. As a theoretical background I use three concepts: peer support, well-being and inclusion. The study is a qualitative research and I used theme interviews to gather the material. I interviewed seven visitors of Olkkari, who volunteered for the research. The interviews were conducted at Olkkari premises in spring 2015. Based on the interviews, Olkkari is an important place to its visitors. Visitors participate in Olkkari’s activities in various ways based on their personal interests and get meaningful things from it. Meeting people is essential. In Olkkari visitors receive peer support and get experiences of inclusion. Olkkari has meaning to its visitors as a place where you can be yourself and feel being accepted. Good experiences gained in Olkkari spread to rest of visitors’ lives via human relationships and improved self-esteem

Scattering from spin-polarized charged impurities in graphene

We study the spin relaxation of charge carriers in graphene in the presence of spin-polarized charged impurities by calculating the time evolution of initially polarized state. The spin relaxation time shows completely different energy behaviour for short-ranged and long-ranged spin scatterers and can be used to identify the dominant source of spin scattering. Our results agree well with recent experimental findings and indicate that their spin relaxation is likely caused by long-ranged scatterers.Peer reviewe

Phosphorene under electron beam

Phosphorene, a single sheet of black phosphorus, is an elemental two-dimensional material with unique properties and potential applications in semiconductor technology. While few-layer flakes of the material have been characterized using transmission electron microscopy, very little is known about its response to electron irradiation, which may be particularly important in the context of top-down engineering of phosphorus nanostructures using a focused electron beam. Here, using first-principles simulations, we study the production of defects in a single phosphorene sheet under impacts of energetic electrons. By employing the McKinley-Feshbach formalism and accounting for the thermal motion of atoms, we assess the cross section for atom displacement as a function of electron energy. We further investigate the energetics and dynamics of point defects and the stability of ribbons and edges under an electron beam. Finally, we show that P atomic chains should be surprisingly stable, and their atomic structure is not linear giving rise to the absence of a gap in the electronic spectrum.Peer reviewe