### Browsing by Author "Harju, Ari, Adjunct Prof., Aalto University, Department of Applied Physics, Finland"

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Item Electronic properties of graphene from tight-binding simulations(Aalto University, 2014) Uppstu, Andreas; Harju, Ari, Adjunct Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Aalto Distinguished Prof., Aalto University, Department of Applied Physics, FinlandGraphene is an effectively two-dimensional semimetallic material consisting of a sheet of carbon atoms arranged in a hexagonal lattice. Due to its high electron mobility and special electronic properties, it is considered to be a promising candidate for various future electronic applications. Freestanding graphene was discovered as late as in 2004, but since then it has become the focus of numerous studies, sparking not only scientific but also significant commercial interest. Realizing the potential of the material requires both theoretical, numerical, and experimental studies. An important computational model for studying the electronic properties of graphene is the so-called tight-binding (TB) model. In the TB model, the charge carriers of a material are described using effective parameters, which can be either derived from more complex models or fitted to experimental or computational results. In this Thesis, the TB model was used to study both the local density of states (LDOS) of graphene as well as electronic transport in graphene. Simulating the LDOS of graphene is important, since it may be more or less directly measured using scanning tunneling microscopy (STM), and is thus extremely helpful for characterizing the properties of nanometer-sized graphene samples. The results presented in this Thesis, which show good agreement between simulations and STM measurements, help to determine the electronic structure of graphene quantum dots on various substrates. Simulations of electronic transport aid in making graphene useful for semiconductor applications. Graphene may be turned semiconducting by various means, such as by cutting it into ribbons or by adding disorder. In this Thesis, it was showed how scaling theory can be utilized to obtain the conductance of a mesoscopically sized disordered graphene device using first-principles-based results and how the localization length of the charge carriers can be obtained effectively using the so-called Kubo-Greenwood method. The results aid in interpreting experimental conductance measurements and in estimating how strong disorder is required to turn graphene into an effective semiconductor.Item Quantum computation with two-electron spins in semi-conductor quantum dots(Aalto University, 2015) Hiltunen, Tuukka; Harju, Ari, Adjunct Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Quantum Many-body Physics Group; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Aalto Distinguished Prof., Aalto University, Department of Applied Physics, FinlandA quantum computer would exploit the phenomena of quantum superposition and entanglement in its functioning and with them offer pathways to solving problems that are too hard or complex to even the best classical computers built today. The implementation of a large-scale working quantum computer could bring about a change in our society rivaling the one started by the digital computer. However, the field is still in its infancy and there are many theoretical and practical issues needing to be solved before large-scale quantum computing can become reality. In digital computers, data is stored in bits. The quantum equivalent is called a qubit (quantum bit) and it is basically a quantum mechanical two-level system that can be in a superposition of its two basis states. There are many different proposals for implementing qubits, but one of the most promising ones is to encode the qubit using electron spins trapped in semiconductor quantum dots. Singlet-triplet qubits are spin qubits where the two-electron spin eigenstates are used as the qubit's basis. The required one and two-qubit operations have already been demonstrated experimentally by several research groups around the world in this qubit architecture. The most severe factor limiting the implementation of larger systems of qubits is decoherence. The qubits are not isolated systems, they interact with their environment, which can lead to the loss of quantum information. Few-electron systems can be simulated accurately using first principle methods that become too taxing when the particle number increases. The topic of this thesis is the simulation of quantum dot singlet-triplet qubit systems using accurate exact diagonalization based methods. The emphasis is on the realistic description of qubit operations, both single-qubit ones and those involving the interaction between neighboring qubits. The decoherence effects are also discussed alongside with certain proposals to alleviate their effects.Item Simulating impurities and edges in graphene(Aalto University, 2016) Ervasti, Mikko; Harju, Ari, Adjunct Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Distinguished Prof., Aalto University, Department of Applied Physics, FinlandGraphene is a two-dimensional allotrope of carbon with incredible mechanical strength, high charge carrier mobility and excellent thermal conductivity. These remarkable properties present numerous potential applications in nanoelectronics and related fields. However, using graphene in a field-effect transistor requires opening a band gap, which can be achieved by cutting graphene into ribbons. Furthermore, the electronic structure and transport properties of graphene are modified by various kinds of defects, such as vacancies, impurities and grain boundaries. Both the defects and edges can host magnetic states that are useful in spintronics applications. In this Thesis, impurities and edges in graphene are simulated using computational techniques. Part of the research has been done in collaboration with experimental groups. The computational simulations provide the necessary link between theory and experiment, aiding in the interpretation of the measurements. The main computational methods used are tight-binding, exact diagonalization and density functional theory, of which the tight-binding and exact diagonalization methods were implemented by the author. Exact diagonalization was used to evaluate correlation energies and reference data to exchange-correlation functionals in two-dimensional quantum dots, electron-positron annihilation in three-dimensional quantum dots, and many-body properties of finite graphene nanoribbons. The research sheds light on the electronic and magnetic properties of graphene. By using the first-principles density functional theory, the formation energies of silicon and silicon-nitrogen impurities were evaluated to identify the relevant low-energy configurations. By fitting to tight-binding models, the transport properties of systems containing randomly distributed impurities were determined. Moreover, hydrogen adatoms with noncollinear spins were shown to scatter the electron spin strongly close to the charge neutrality point. The narrow finite graphene nanoribbons were found to have only small band gaps, and the simulated scanning tunneling microscopy maps and spectra of the ribbons agreed with the experiments. The precise atomic structure at the graphene-hexagonal boron nitride interfaces was determined with the help of simulations, and the interfaces were shown to host electronic states similar to those on the graphene edges. Overall, the theoretical and computational results build up the knowledge and understanding of graphene-related systems.Item Solving topological lattice models on coprocessors(Aalto University, 2016) Siro, Topi; Harju, Ari, Adjunct Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Quantum Many-Body Physics; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Prof. Emeritus, Aalto University, Department of Applied Physics, FinlandUnderstanding how the various electronic properties of matter emerge from the motion and interaction of electrons has been an important goal of physics since the early 1900s. One important tool has been the study of quantum lattice models, which can be considered as simplified depictions of solids. Provided that the system is small enough, it is possible to solve the low energy spectrum accurately with numerical methods. Like most problems in modern physics, studying lattice models requires extensive numerical computation. Traditionally, computer programs have been written to run on the central processing unit, but in recent years, various new parallel computing coprocessors have been introduced. Graphics processing units, which were originally added to render images on the computer screen, can now also be used for general purpose computation. Another new platform is the Xeon Phi coprocessor, specifically designed to accelerate parallel programs. Both of these coprocessors are parallel systems, where there are hundreds or thousands of computational threads running concurrently. This poses challenges in designing and implementing algorithms that benefit from the parallelism. In this Thesis, we implement the exact diagonalization method on graphics processors and the Xeon Phi. We apply it on topological lattice models, which have been under intense study recently. They feature topological phases that cannot be explained with Landau's symmetry-breaking theory, but instead require studying the topological properties of the ground state. One key quantity in identifying the phases is the Chern number that is related to the transverse conductance in quantum Hall phases. In the so called checkerboard model, we show that the topological ground state can withstand strong local impurities. With increasing impurity density, we observe transitions to a metallic state and an insulating state. In another model, the Haldane-Hubbard model, we study the phase diagram with changing on-site interaction and sublattice potential. We find an interesting intermediate topological phase, where the symmetry of the up and down spins breaks spontaneously.