Browsing by Author "Harju, A."
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- Ab-initio transport fingerprints for resonant scattering in graphene
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012-12-12) Saloriutta, K.; Uppstu, A.; Harju, A.; Puska, M.J.We 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. - Anderson localization in two-dimensional graphene with short-range disorder: One-parameter scaling and finite-size effects
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Fan, Z.; Uppstu, A.; Harju, A.We study Anderson localization in graphene with short-range disorder using the real-space Kubo-Greenwood method implemented on graphics processing units. Two models of short-range disorder, namely, the Anderson on-site disorder model and the vacancy defect model, are considered. For graphene with Anderson disorder, localization lengths of quasi-one-dimensional systems with various disorder strengths, edge symmetries, and boundary conditions are calculated using the real-space Kubo-Greenwood formalism, showing excellent agreement with independent transfer matrix calculations and superior computational efficiency. Using these data, we demonstrate the applicability of the one-parameter scaling theory of localization length and propose an analytical expression for the scaling function, which provides a reliable method of computing the two-dimensional localization length. This method is found to be consistent with another widely used method which relates the two-dimensional localization length to the elastic mean free path and the semiclassical conductivity. Abnormal behavior at the charge neutrality point is identified and interpreted to be caused by finite-size effects when the system width is comparable to or smaller than the elastic mean free path. We also demonstrate the finite-size effect when calculating the two-dimensional conductivity in the localized regime and show that a renormalization group β function consistent with the one-parameter scaling theory can be extracted numerically. For graphene with vacancy disorder, we show that the proposed scaling function of localization length also applies. Last, we discuss some ambiguities in calculating the semiclassical conductivity around the charge neutrality point due to the presence of resonant states. - Broken symmetry in density-functional theory: Analysis and cure
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2004) Harju, A.; Räsänen, E.; Saarikoski, H.; Puska, Martti J.; Nieminen, Risto M.; Niemelä, K.We present a detailed analysis of the broken-symmetry mean-field solutions using a four-electron rectangular quantum dot as a model system. Comparisons of the density-functional theory predictions with the exact ones show that the symmetry-breaking results from the single-configuration wave function used in the mean-field approach. As a general cure we present a scheme that systematically incorporates several configurations into the density-functional theory and restores the symmetry. This cure is easily applicable to any density-functional approach. - Broken symmetry in density-functional theory: Analysis and cure
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2004-04-01) Harju, A.; Räsänen, E.; Saarikoski, H.; Puska, M.J.; Nieminen, R.M.; Niemelä, K.We present a detailed analysis of the broken-symmetry mean-field solutions using a four-electron rectangular quantum dot as a model system. Comparisons of the density-functional theory predictions with the exact ones show that the symmetry-breaking results from the single-configuration wave function used in the mean-field approach. As a general cure we present a scheme that systematically incorporates several configurations into the density-functional theory and restores the symmetry. This cure is easily applicable to any density-functional approach. - Calculation of positron states and annihilation in solids: A density-gradient-correction scheme
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(1996) Barbiellini, B.; Puska, Martti J.; Korhonen, T.; Harju, A.; Torsti, T.; Nieminen, Risto M.The generalized gradient correction method for positron-electron correlation effects in solids [B. Barbiellini et al., Phys. Rev. B 51, 7341 (1995)] is applied in several test cases. The positron lifetime, energetics, and momentum distribution of the annihilating electron-positron pairs are considered. The comparison with experiments shows systematic improvement in the predictive power of the theory compared to the local-density approximation results for positron states and annihilation characteristics. - Calculation of positron states and annihilation in solids: A density-gradient-correction scheme
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(1996-06-15) Barbiellini, B.; Puska, M.J.; Korhonen, T.; Harju, A.; Torsti, T.; Nieminen, R.M.The generalized gradient correction method for positron-electron correlation effects in solids [B. Barbiellini et al., Phys. Rev. B 51, 7341 (1995)] is applied in several test cases. The positron lifetime, energetics, and momentum distribution of the annihilating electron-positron pairs are considered. The comparison with experiments shows systematic improvement in the predictive power of the theory compared to the local-density approximation results for positron states and annihilation characteristics. - Electronic states in finite graphene nanoribbons: Effect of charging and defects
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Ijäs, M.; Ervasti, M.; Uppstu, Christer; Liljeroth, P.; van der Lit, J.; Swart, I.; Harju, A.We study the electronic structure of finite armchair graphene nanoribbons using density-functional theory and the Hubbard model, concentrating on the states localized at the zigzag termini. We show that the energy gaps between end-localized states are sensitive to doping, and that in doped systems, the gap between the end-localized states decreases exponentially as a function of the ribbon length. Doping also quenches the antiferromagnetic coupling between the end-localized states leading to a spin-split gap in neutral ribbons. By comparing dI/dV maps calculated using the many-body Hubbard model, its mean-field approximation and density-functional theory, we show that the use of a single-particle description is justified for graphene π states in case spin properties are not the main interest. Furthermore, we study the effect of structural defects in the ribbons on their electronic structure. Defects at one ribbon terminus do not significantly modify the electronic states localized at the intact end. This provides further evidence for the interpretation of a multipeak structure in a recent scanning tunneling spectroscopy (STS) experiment resulting from inelastic tunneling processes [van der Lit et al., Nat. Commun. 4, 2023 (2013)]. Finally, we show that the hydrogen termination at the flake edges leaves identifiable fingerprints on the positive bias side of STS measurements, thus possibly aiding the experimental identification of graphene structures. - Electronic structure of rectangular quantum dots
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2003) Räsänen, E.; Saarikoski, H.; Stavrou, V. N.; Harju, A.; Puska, Martti J.; Nieminen, Risto M.We study the ground-state properties of rectangular quantum dots by using the spin-density-functional theory and quantum Monte Carlo methods. The dot geometry is determined by an infinite hard-wall potential to enable comparison to manufactured, rectangular-shaped quantum dots. We show that the electronic structure is very sensitive to the shape of the dot, and, at realistic sizes, the noninteracting picture determines the general behavior. However, close to the degenerate points where Hund’s rule applies, we find spin-density-wave-like solutions bracketing the partially polarized states. In the quasi-one-dimensional limit we find permanent charge-density waves, and at a sufficiently large deformation or low density, there are strongly localized stable states with a broken spin symmetry. - Electronic structure of rectangular quantum dots
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2003-06-10) Räsänen, Esa; Saarikoski, H.; Stavrou, V.N.; Harju, A.; Puska, M.J.; Nieminen, R.M.We study the ground-state properties of rectangular quantum dots by using the spin-density-functional theory and quantum Monte Carlo methods. The dot geometry is determined by an infinite hard-wall potential to enable comparison to manufactured, rectangular-shaped quantum dots. We show that the electronic structure is very sensitive to the shape of the dot, and, at realistic sizes, the noninteracting picture determines the general behavior. However, close to the degenerate points where Hund’s rule applies, we find spin-density-wave-like solutions bracketing the partially polarized states. In the quasi-one-dimensional limit we find permanent charge-density waves, and at a sufficiently large deformation or low density, there are strongly localized stable states with a broken spin symmetry. - Electronic transport in graphene-based structures: An effective cross section approach
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012-01-03) Uppstu, A.; Saloriutta, K.; Harju, A.; Puska, M.; Jauho, A.-P.We 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. - Enhancement models of momentum densities of annihilating electron-positron pairs: The many-body picture of natural geminals
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Makkonen, I.; Ervasti, M.M.; Siro, T.; Harju, A. - Exchange-correlation potentials for inhomogeneous electron systems in two dimensions from exact diagonalization: Comparison with the local-spin-density approximation
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Makkonen, I.; Ervasti, M.M.; Kauppila, V.J.; Harju, A. - Far-infrared spectra of lateral quantum dot molecules
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2006) Helle, M.; Harju, A.; Nieminen, Risto M.We study effects of electron–electron interactions and confinement potential on the magneto-optical absorption spectrum in the far-infrared (FIR) range of lateral quantum dot molecules (QDMs). We calculate FIR spectra for three different QDM confinement potentials. We use an accurate exact diagonalization technique for two interacting electrons and calculate dipole transitions between two-body levels with perturbation theory. We conclude that the two-electron FIR spectra directly reflect the symmetry of the confinement potential and interactions cause only small shifts in the spectra. These predictions could be tested in experiments with non-parabolic quantum dots (QDs) by changing the number of confined electrons. We also calculate FIR spectra for up to six non-interacting electrons and observe some additional features in the spectrum. - Generalized tight-binding transport model for graphene nanoribbon-based systems
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2010-06-01) Hancock, Y.; Uppstu, A.; Saloriutta, K.; Harju, A.; Puska, M.J.An extended tight-binding model that includes up to third-nearest-neighbor hopping and a Hubbard mean-field interaction term is tested against ab initio local spin-density approximation results of band structures for armchair- and zigzag-edged graphene nanoribbons. A single tight-binding parameter set is found to accurately reproduce the ab initio results for both the armchair and zigzag cases. Transport calculations based on the extended tight-binding model faithfully reproduce the results of ab initio transport calculations of graphene nanoribbon-based systems. - Generalized tight-binding transport model for graphene nanoribbon-based systems
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2010) Hancock, Y.; Uppstu, A.; Saloriutta, K.; Harju, A.; Puska, Martti J.An extended tight-binding model that includes up to third-nearest-neighbor hopping and a Hubbard mean-field interaction term is tested against ab initio local spin-density approximation results of band structures for armchair- and zigzag-edged graphene nanoribbons. A single tight-binding parameter set is found to accurately reproduce the ab initio results for both the armchair and zigzag cases. Transport calculations based on the extended tight-binding model faithfully reproduce the results of ab initio transport calculations of graphene nanoribbon-based systems. - Giant vortices in rotating electron droplets
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2006) Räsänen, E.; Saarikoski, H.; Yu, Y.; Harju, A.; Puska, Martti J.; Reimann, S. M.We predict the formation of giant vortices in quasi-two-dimensional quantum dots at high magnetic fields, i.e., in rapidly rotating electron droplets. Our numerical results for quantum dots confined by a flat, anharmonic potential show ground states where vortices are accumulated in the center of the dot, thereby leading to large cores in the electron and current densities. The phenomenon is analogous to what was recently found in rotating Bose-Einstein condensates. The giant-vortex states leave measurable signatures in the ground-state energetics. The conditions for the giant-vortex formation as well as the internal structure of the vortex cores are discussed. - Giant Vortices in Rotating Electron Droplets
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2006-06-14) Räsänen, E.; Saarikoski, H.; Harju, A.; Puska, M.J.; Yu, Y.; Reimann, S.M.We predict the formation of giant vortices in quasi-two-dimensional quantum dots at high magnetic fields, i.e., in rapidly rotating electron droplets. Our numerical results for quantum dots confined by a flat, anharmonic potential show ground states where vortices are accumulated in the center of the dot, thereby leading to large cores in the electron and current densities. The phenomenon is analogous to what was recently found in rotating Bose-Einstein condensates. The giant-vortex states leave measurable signatures in the ground-state energetics. The conditions for the giant-vortex formation as well as the internal structure of the vortex cores are discussed. - Lateral diatomic two-dimensional artificial molecules: Classical transitions and quantum-mechanical counterparts
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2002) Marlo, M.; Alatalo, M.; Harju, A.; Nieminen, Risto M.Structural properties of a finite number (N=2−20) of point charges (classical electrons) confined laterally in a two-dimensional two-minima potential are calculated as a function of the distance (d) between the minima. The particles are confined by identical parabolic potentials and repel each other through a Coulomb potential. Both ground-state and metastable electron configurations are discussed. Discontinuous transitions from one configuration to another as a function of d are observed for N=6, 8, 11, 16, 17, 18, and 19. We show that the structural transitions have quantum-mechanical counterparts also in the limit of noninteracting electrons. - Many-body wave function for a quantum dot in a weak magnetic field
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(1999) Harju, A.; Sverdlov, V. A.; Nieminen, Risto M.; Halonen, V.The ground states of parabolically confined electrons in a quantum dot are studied by both direct numerical diagonalization and quantum Monte Carlo (QMC) methods. We present a simple but accurate variational many-body wave function for the dot in the limit of a weak magnetic field. The wave function has the center-of-mass motion restricted to the lowest-energy state and the electron-electron interaction is taken into account by a Jastrow two-body correlation factor. The optimized wave function has an accuracy very close to the state-of-the-art numerical diagonalization calculations. The results and the computational efficiency indicate that the presented wave function combined with the QMC method suits ideally for studies of large quantum dots. - Modeling positronium beyond the single particle approximation
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-02-29) Zubiaga, A.; Ervasti, M. M.; Makkonen, I.; Harju, A.; Tuomisto, F.; Puska, M. J.Understanding the properties of the positronium atom in matter is of interest for the interpretation of positron annihilation experiments. This technique has a unique capability for the investigation of nanometer sized voids and pores in soft molecular materials (polymers, liquids or biostructures) and porous materials. However, detailed interpretations of the experimental data rely on modeling of the annihilation properties of positronium in the host material. New applications of the technique are being developed but the computational models still are based on single particle approaches and there is no way to address the influence of the electronic properties of the host material. In this work we discuss new directions of research.
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