### Browsing by Author "Harju, Ari"

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Item Ab initio transport fingerprints for resonant scattering in graphene(American Physical Society (APS), 2012) Saloriutta, Karri; Uppstu, Andreas; Harju, Ari; Puska, Martti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have recently shown that by using a scaling approach for randomly distributed topological defects in graphene, reliable estimates for transmission properties of macroscopic samples can be calculated based even on single-defect calculations [A. Uppstu et al., Phys. Rev. B 85, 041401 (2012)]. We now extend this approach of energy-dependent scattering cross sections to the case of adsorbates on graphene by studying hydrogen and carbon adatoms as well as epoxide and hydroxyl groups. We show that a qualitative understanding of resonant scattering can be gained through density functional theory results for a single-defect system, providing a transmission “fingerprint” characterizing each adsorbate type. This information can be used to reliably predict the elastic mean free path for moderate defect densities directly using ab initio methods. We present tight-binding parameters for carbon and epoxide adsorbates, obtained to match the density-functional theory based scattering cross sections.Item Band geometry, Berry curvature and superfluid weight(2017-01-27) Liang, Long; Vanhala, Tuomas I.; Peotta, Sebastiano; Siro, Topi; Harju, Ari; Törmä, Päivi; Department of Applied Physics; Quantum Dynamics; Quantum Many-Body PhysicsWe present a theory of the superfluid weight in multiband attractive Hubbard models within the Bardeen-Cooper-Schrieffer (BCS) mean field framework. We show how to separate the geometric contribution to the superfluid weight from the conventional one, and that the geometric contribution is associated with the interband matrix elements of the current operator. Our theory can be applied to systems with or without time reversal symmetry. In both cases the geometric superfluid weight can be related to the quantum metric of the corresponding noninteracting systems. This leads to a lower bound on the superfluid weight given by the absolute value of the Berry curvature. We apply our theory to the attractive Kane-Mele-Hubbard and Haldane-Hubbard models, which can berealized in ultracold atom gases. Quantitative comparisons are made to state of the art dynamical mean-field theory and exact diagonalization results.Item Computational study of quantum dot qubits using Lagrange mesh method and exact diagonalization(2013) Ritala, Juha; Harju, Ari; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; Nieminen, RistoQuantum computation using quantum circuit model is based on quantum bits and gates, which are quantum analogues to the bits and logical gates in classical computing. The computations are carried out by performing single- and two-qubit quantum gate operations on the input qubits (quantum bits). In this thesis, a set of computational methods to study these gate operations in the case of semiconductor quantum dot based spin qubits is presented. This set consists of three parts. Lagrange mesh method is used to calculate the single-electron states in a quantum dot system. These states are then used in an exact diagonalization calculation to obtain the many-electron ground state, which is then evolved using exact diagonalization based dynamics. The presented set of methods is used to simulate single-qubit gates, and it is found to be successful for this purpose. The Lagrange mesh method is extremely versatile as it can handle an arbitrary quantum dot confinement potential without the need to calculate any integrals. This feature is achieved by approximating the potential matrix element integrals using a Gauss quadrature. The high accuracy of the Lagrange mesh method despite the seemingly crude approximation is investigated, and a reasonable cause for it in the case of low degree polynomial potentials is found. A hypothesis that the Gauss quadrature approximation is extremely accurate for an arbitrary polynomial potential is made. The convergence of the states calculated with the Lagrange mesh method is tested and compared to an alternative method based on localized Gaussian basis functions.Item Conformal Field theory and Numerics for Fractional Quantum Hall Systems(2008) Nissinen, Jaakko; Harju, Ari; Teknillisen fysiikan laitos; Teknillinen korkeakoulu; Helsinki University of Technology; Nieminen, RistoThe subject of this work is the application of conformal field theory in the fractional quantum Hall effect. The fractional quantum Hall effect is observed in a two-dimensional electron gas in a strong perpendicular magnetic field and at low temperature. The electron system forms a strongly correlated quantum liquid and exhibits the exact quantization of the Hall conductivity in to a fractional value vet/h, where v is the fractional filling fraction of Landau-levels in the system. In addition, the elementary excitations of the electron fluid have fractional charge and fractional statistics of anyons. First, we review the basic of two-dimensional conformal field theory and show how certain conformal quantum field theories can be solved non-perturbatively by using the representation theory of conformal symmetries. The low-energy effective field theory in the bulk of a fractional quantum Hall state is known to be a 2+1 dimensional topological field theory. The states of the topological quantum field theory are directly connected to the holomorphic correlation functions of an equivalent two-dimensional conformal field theory. It is known that the gapless excitations at the boundary of the quantum Hall state are described by the same conformal field theory. By using this connection, the so-called Laughlin and Moore-Read states are derived in detail using conformal field theory. The fractional charge and statistics of the quasiparticle excitations in these states are obtained from the conformal field theory description. Furthermore, the non-Abelian statistics of the Moore-Read states are verified explicitly from the quasiparticle wave functions. We argue how the different fractional quantum Hall phases of matter are described by the quantum numbers obtained using conformal field theory. Lastly, we study model interactions that have the Laughlin v = 1/3 and the Moore-Read v = 1/2 states as exact ground states. We compute the matrix elements of these interactions and diagonalize the Hamiltonian matrix in the disk geometry for both states. The low-energy edge excitations of these states are studied in the finite geometry and we verify that they are in accordance with the conformal field theory on the boundary. By creating a localized density disturbance in the system, we create the e/4 quasiparticle of the Moore-Read state, and by studying the properties of the edge theory it is seen that quasiparticle has the same properties as the non-Abelian quasihole in the conformal field theory. The obtained results verify the correspondence between the bulk state and the boundary conformal field theory at the edge in the fractional quantum Hall states in question.Item Density Functional Theory and Variational Monte Carlo Calculations for 2-Dimensional Electron Systems(2011) Kauppila, Ville; Makkonen, Ilja; Perustieteiden korkeakoulu; Harju, AriItem Edge tunnelling in quantum Hall systems(2008) Webb, Christian; Harju, Ari; Teknillisen fysiikan laitos; Teknillinen korkeakoulu; Helsinki University of Technology; Puska, MarttiThe quantum Hall effect is one of the curious macroscopic quantum effects that are encountered in condensed matter physics when strong magnetic fields, low temperatures and systems with low disorders are probed. The hallmark of the effect is the extremely precise quantization of the Hall conductance for certain values of the magnetic field. A lot of curious physics is related to the quantum Hall effect. For example, exotic quasiparticles are believed to emerge in these systems and there is hope for these quasiparticles to be of great importance in future technology. Also these systems are of fundamental theoretical importance. It has been found that there is a need for a new kind of notion of order in these systems. This new kind of order is called topological order and different phases are distinguished through topological quantum numbers. One of these topological quantum numbers can be extracted from the Green's function of the electrons on the edge of a quantum Hall system. The theoretical aim of this thesis is to review some of the physics related to the quantum Hall effect and the notion of topological order in quantum Hall systems. From a computational point of view, the goal of the thesis is to numerically investigate the microscopic counterparts of quantum Hall systems - quantum dots. In particular, computations are performed to see how well the notion of topological order works in microscopic systems.Item Electric structure and transmission properties of graphene antidot lattices(2014-08-26) Simula, Kristoffer; Harju, Ari; Perustieteiden korkeakoulu; Alava, MikkoItem Electronic and transport properties in geometrically disordered graphene antidot lattices(2015) Fan, Z.; Uppstu, A.; Harju, Ari; Department of Applied Physics; Quantum Many-Body PhysicsA graphene antidot lattice, created by a regular perforation of a graphene sheet, can exhibit a considerable band gap required by many electronics devices. However, deviations from perfect periodicity are always present in real experimental setups and can destroy the band gap. Our numerical simulations, using an efficient linear-scaling quantum transport simulation method implemented on graphics processing units, show that disorder that destroys the band gap can give rise to a transport gap caused by Anderson localization. The size of the defect-induced transport gap is found to be proportional to the radius of the antidots and inversely proportional to the square of the lattice periodicity. Furthermore, randomness in the positions of the antidots is found to be more detrimental than randomness in the antidot radius. The charge carrier mobilities are found to be very small compared to values found in pristine graphene, in accordance with recent experiments.Item Electronic and transport properties of graphene nanoribbons with applications for device design(2010) Uppstu, Andreas; Harju, Ari; Informaatio- ja luonnontieteiden tiedekunta; Perustieteiden korkeakoulu; School of Science; Nieminen, RistoGraphene is an allotrope of carbon consisting of a single sheet of atoms arranged in a hexagonal lattice, and graphene nanoribbons are quasi-one dimensional graphene strips of nanometer width. In this thesis, generalized tight-binding methods for modelling the electronic structure and the transport properties of graphene nanoribbons are studied and compared to computationally heavier ab initio methods. The tight-binding model is a commonly used method to compute the electronic properties of carbon-based materials. The method relies on the assumption that the electrons are tightly bound to the lattice sites, and the energy cost for an electron to jump to a neighboring lattice site is taken as a free parameter. The model is extended by introducing further than nearest neighbor hopping, and a mean field Hubbard term that models the on-site Coulombic repulsion. The extended model presented in this thesis is then used to compute the electronic transport properties of notched graphene nanoribbons with zigzag edges, which may possibly be used as spin-injection devices or spin filters.Item Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene(AMERICAN CHEMICAL SOCIETY, 2021-06-22) Kumar, Avijit; Banerjee, Kaustuv; Ervasti, Mikko M.; Kezilebieke, Shawulienu; Dvorak, Marc; Rinke, Patrick; Harju, Ari; Liljeroth, Peter; Department of Applied Physics; Atomic Scale Physics; Computational Electronic Structure TheoryOrganic charge-transfer complexes (CTCs) formed by strong electron acceptor and strong electron donor molecules are known to exhibit exotic effects such as superconductivity and charge density waves. We present a low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) study of a two-dimensional (2D) monolayer CTC of tetrathiafulvalene (TTF) and fluorinated tetracyanoquinodimethane (F4TCNQ), self-assembled on the surface of oxygen-intercalated epitaxial graphene on Ir(111) (G/O/Ir(111)). We confirm the formation of the charge-transfer complex by dI/dV spectroscopy and direct imaging of the singly occupied molecular orbitals. High-resolution spectroscopy reveals a gap at zero bias, suggesting the formation of a correlated ground state at low temperatures. These results point to the possibility to realize and study correlated ground states in charge-transfer complex monolayers on weakly interacting surfaces.Item Electronic transport in graphene-based structures: An effective cross-section approach(American Physical Society (APS), 2012) Uppstu, Andreas; Saloriutta, Karri; Harju, Ari; Puska, Martti J.; Jauho, Antti-Pekka; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe show that transport in low-dimensional carbon structures with finite concentrations of scatterers can be modeled by utilizing scaling theory and effective cross sections. Our results are based on large-scale numerical simulations of carbon nanotubes and graphene nanoribbons, using a tight-binding model with parameters obtained from first-principles electronic structure calculations. As shown by a comprehensive statistical analysis, the scattering cross sections can be used to estimate the conductance of a quasi-one-dimensional system both in the Ohmic and localized regimes. They can be computed with good accuracy from the transmission functions of single defects, greatly reducing the computational cost and paving the way toward using first-principles methods to evaluate the conductance of mesoscopic systems, consisting of millions of atoms.Item Energetics and structure of grain boundary triple junctions in graphene(2017-07-06) Hirvonen, Petri; Fan, Zheyong; Ervasti, Mikko M.; Harju, Ari; Elder, Ken R.; Ala-Nissilä, Tapio; Department of Applied Physics; Oakland UniversityGrain boundary triple junctions are a key structural element in polycrystalline materials. They are involved in the formation of microstructures and can influence the mechanical and electronic properties of materials. In this work we study the structure and energetics of triple junctions in graphene using a multiscale modelling approach based on combining the phase field crystal approach with classical molecular dynamics simulations and quantum-mechanical density functional theory calculations. We focus on the atomic structure and formation energy of the triple junctions as a function of the misorientation between the adjacent grains. We find that the triple junctions in graphene consist mostly of five-fold and seven-fold carbon rings. Most importantly, in addition to positive triple junction formation energies we also find a significant number of orientations for which the formation energy is negative.Item Equivalence of the equilibrium and the nonequilibrium molecular dynamics methods for thermal conductivity calculations: From bulk to nanowire silicon(2018-03-26) Dong, Haikuan; Fan, Zheyong; Shi, Libin; Harju, Ari; Ala-Nissila, Tapio; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Multiscale Statistical and Quantum Physics; Bohai UniversityMolecular dynamics (MD) simulations play an important role in studying heat transport in complex materials. The lattice thermal conductivity can be computed either using the Green-Kubo formula in equilibrium MD (EMD) simulations or using Fourier's law in nonequilibrium MD (NEMD) simulations. These two methods have not been systematically compared for materials with different dimensions and inconsistencies between them have been occasionally reported in the literature. Here we give an in-depth comparison of them in terms of heat transport in three allotropes of Si: three-dimensional bulk silicon, two-dimensional silicene, and quasi-one-dimensional silicon nanowire. By multiplying the correlation time in the Green-Kubo formula with an appropriate effective group velocity, we can express the running thermal conductivity in the EMD method as a function of an effective length and directly compare it to the length-dependent thermal conductivity in the NEMD method. We find that the two methods quantitatively agree with each other for all the systems studied, firmly establishing their equivalence in computing thermal conductivity.Item Force and heat current formulas for many-body potentials in molecular dynamics simulation with applications to thermal conductivity calculations(2015) Fan, Z.; Pereira, L.F.C.; Wang, H.-Q.; Zheng, J.-C.; Donadio, D.; Harju, Ari; Department of Applied Physics; Quantum Many-Body PhysicsWe derive expressions of interatomic force and heat current for many-body potentials such as the Tersoff, the Brenner, and the Stillinger-Weber potential used extensively in molecular dynamics simulations of covalently bonded materials. Although these potentials have a many-body nature, a pairwise force expression that follows Newton's third law can be found without referring to any partition of the potential. Based on this force formula, a stress applicable for periodic systems can be unambiguously defined. The force formula can then be used to derive the heat current formulas using a natural potential partitioning. Our heat current formulation is found to be equivalent to most of the seemingly different heat current formulas used in the literature, but to deviate from the stress-based formula derived from two-body potential. We validate our formulation numerically on various systems described by the Tersoff potential, namely three-dimensional silicon and diamond, two-dimensional graphene, and quasi-one-dimensional carbon nanotube. The effects of cell size and production time used in the simulation are examined.Item Fundamental Properties of Quantum Rings(2007) Gylfadóttir, Sigridur Sif; Harju, Ari; Teknillisen fysiikan ja matematiikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Nieminen, RistoItem Grafeenin ja boorinitridin kiraalinen rajapinta(2014-09-29) Vierimaa, Ville; Harju, Ari; Perustieteiden korkeakoulu; Puska, MarttiItem Graphene photodetection and the Seebeck effect(2012) Hiltunen, Tuukka; Harju, Ari; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science; Nieminen, RistoGraphene is a very rapidly rising star among nanomaterials. It has great potential in both terms of commercial applications, and understanding of fundamental physics. Graphene's unique properties make it a promising new material in nano-electronics. It has been proposed that it could one day replace silicon in semiconductor technology. Among the numerous future applications are graphene based photodetectors. In this field, graphene could offer fundamentally different applications compared to the traditional photodetectors based on the IV and III-V semiconductors. The main subject of this Master's thesis is the photothermoelectric effect (or the Seebeck effect) in graphene. It is considered as one of the main mechanisms in the generation of photocurrents in graphene. The photoelectic effect is also briefly discussed in terms of optical transition rates. This Master's thesis is a computational study. The modeling of graphene's electronic states is done with the tight-binding approximation. The photocurrents are simulated using the Landauer-Büttiker transport formalism and the Green's function method for the computation of the transmission probabilities. The conductances and Seebeck coefficients of various graphene based systems are computed. The obtained computational results are compared to existing computational and experimental studies.Item Homogeneous nonequilibrium molecular dynamics method for heat transport and spectral decomposition with many-body potentials(American Physical Society, 2019-02-28) Fan, Zheyong; Dong, Haikuan; Harju, Ari; Ala-Nissilä, Tapio; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Multiscale Statistical and Quantum Physics; Bohai UniversityThe standard equilibrium Green-Kubo and nonequilibrium molecular dynamics (MD) methods for computing thermal transport coefficients in solids typically require relatively long simulation times and large system sizes. To this end, we revisit here the homogeneous nonequilibrium MD method by Evans [Phys. Lett. A 91, 457 (1982)PYLAAG0375-960110.1016/0375-9601(82)90748-4] and generalize it to many-body potentials that are required for more realistic materials modeling. We also propose a method for obtaining spectral conductivity and phonon mean-free path from the simulation data. This spectral decomposition method does not require lattice dynamics calculations and can find important applications in spatially complex structures. We benchmark the method by calculating thermal conductivities of three-dimensional silicon, two-dimensional graphene, and a quasi-one-dimensional carbon nanotube and show that the method is about one to two orders of magnitude more efficient than the Green-Kubo method. We apply the spectral decomposition method to examine the long-standing dispute over thermal conductivity convergence vs divergence in carbon nanotubes.Item Many body effects in graphene quantum dots(2010) Perkkiö, Lauri; Harju, Ari; Informaatio- ja luonnontieteiden tiedekunta; Nieminen, RistoItem Modeling the electronic and transport properties of graphene nanostrutures(2010) Ijäs, Mari; Harju, Ari; Kemian laitos; Kemian tekniikan korkeakoulu; School of Chemical Engineering; Nieminen, RistoGraphene is a versatile material suggested for nanoelectronics applications. The purpose of this thesis is to provide deeper understanding in the properties of the possible circuit elements and the effect of interactions. The energetics of graphene nanoflakes were studied within the Hubbard model using both exact diagonalization and approximate methods for solving the ground state. To improve the treatment of the interactions, maintaining the computational cost low at the same time, a lattice density functional theory method was developed for the flakes. The densities of states and charge densities were calculated for arbitrarily-shaped large graphene structures within the tight-binding picture. Localized single-electron states due to structural confinement were found. The conductance of one-dimensional nanostructures, consisting of a few-site gated nanodot weakly coupled to the leads, was studied using within linear response using the Kubo formula. Analytical expressions for the conductance due to external bias voltage were derived and using them resonance structure in the conductance were located. The results were confirmed using another method, based on magnetic flux-induced persistent currents in a ring-shaped structure. Also quasi-one-dimensional structures resembling narrow graphene nanoribbons were studied using the second method. The effect of interactions was also studied and splitting of the resonances was observed. This effect was explained in terms of an energetic competition between the Hubbard repulsion and the gate voltage applied on the dot.