arXiv subjects

A. Harju

Publications and source records attributed to A. Harju.

At least 19 recordsLinked to original sources

arXiv2015

Scaling behavior of spin transport in hydrogenated graphene

We calculate the spin transport of hydrogenated graphene using the Landauer-Büttiker formalism with a spin-dependent tight-binding Hamiltonian. The advantages of using this method is that it simultaneously gives information on sheet resistance and localization length as well as spin relaxation length. Furthermore, the Landauer-Büttiker formula can be computed very efficiently using the recursive Green's function technique. Previous theoretical results on spin relaxation time in hydrogenated graphene have not been in agreement with experiments. Here, we study magnetic defects in graphene with randomly aligned magnetic moments, where interference between spin-channels is explicitly included. We show that the spin relaxation length and sheet resistance scale nearly linearly with the impurity concentration. Moreover, the spin relaxation mechanism in hydrogenated graphene is Markovian only near the charge neutrality point or in the highly dilute impurity limit.

cond-mat.mes-hall↗

arXiv2014

Orbital-Free Density Functional Theory Implementation with the Projector Augmented-Wave Method

We present a computational scheme for orbital-free density functional theory (OFDFT) that simultaneously provides access to all-electron values and preserves the OFDFT linear scaling as a function of the system size. Using the projector augmented-wave method (PAW) in combination with real-space methods we overcome some obstacles faced by other available implementation schemes. Specifically, the advantages of using the PAW method are two fold. First, PAW reproduces all-electron values offering freedom in adjusting the convergence parameters and the atomic setups allow tuning the numerical accuracy per element. Second, PAW can provide a solution to some of the convergence problems exhibited in other OFDFT implementations based on Kohn-Sham codes. Using PAW and real-space methods, our orbital-free results agree with the reference all-electron values with a mean absolute error of 10~meV and the number of iterations required by the self-consistent cycle is comparable to the KS method. The comparison of all-electron and pseudopotential bulk modulus and lattice constant reveal an enormous difference, demonstrating that in order to assess the performance of OFDFT functionals it is necessary to use implementations that obtain all-electron values. The proposed combination of methods is the most promising route currently available. We finally show that a parametrized kinetic energy functional can give lattice constants and bulk moduli comparable in accuracy to those obtained by the KS PBE method, exemplified with the case of diamond.

physics.comp-ph↗

arXiv2013

Self-consistent energy approximation for orbital-free density-functional theory

Employing a local formula for the electron-electron interaction energy, we derive a self-consistent approximation for the total energy of a general $N$-electron system. Our scheme works as a local variant of the Thomas-Fermi approximation and yields the total energy and density as a function of the external potential, the number of electrons, and the chemical potential determined upon normalization. Our tests for Hooke's atoms, jellium, and model atoms up to $\sim 1000$ electrons show that reasonable total energies can be obtained with almost a negligible computational cost. The results are also consistent in the important large-$N$ limit.

physics.chem-ph↗

arXiv2013

Electronic states in finite graphene nanoribbons: Effect of charging and defects

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. Furthermore, we study the effect of structural defects in the ribbons on their electronic structure. Defects at one ribbon termini do not significantly modify the electronic states localized at the intact end. This provides further evidence for the interpretation of a multi-peaked structure in a recent scanning tunneling spectroscopy (STS) experiment resulting from inelastic tunneling processes [J. van der Lit et al., Nature Commun., in press (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.

cond-mat.mes-hall↗

arXiv2013

Interaction of chlorine with Stone-Wales de- fects in graphene and carbon nanotubes, and thermodynamical prospects of chlorine-induced nanotube unzipping

We study the binding of chlorine atoms to carbon nanotubes and graphene at a Stone-Wales defect and to the sidewalls of pristine nanotubes. We show using ab initio thermodynamics that if the environment is chlorine-rich enough, the unzipping of carbon nanotubes into graphene nanoribbons with chlorinated edges may be thermodynamically feasible. By controlling the Cl chemical potential through temperature and pressure, opening selectively tubes below a threshold diameter might be possible. Additionally, we find increased binding energies for chlorine atoms bound to Stone-Wales defects as compared to defect-free graphene and nanotubes, indicating that defects might act as nucleation sites for tube unzipping. On graphene, no more than a single Cl atom may be bound to the defect in ambient conditions, limiting possible Cl-induced changes in the resulting nanoribbons.

cond-mat.mtrl-sci↗

arXiv2012

High-temperature surface superconductivity in rhombohedral graphite

Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature.

cond-mat.supr-con↗

arXiv2012

Fractional periodicity of persistent current in coupled quantum rings

We study the transmission properties of a few-site Hubbard rings with up to second-nearest neighbor coupling embedded to a ring-shaped lead using exact diagonalization. The approach captures all the correlation effects and enables us to include interactions both in the ring and in the ring-shaped lead, and study on an equal footing weak and strong coupling between the ring and the lead as well as asymmetry. In the weakly coupled case, we find fractional periodicity at all electron fillings at sufficiently high Hubbard U, similar to isolated rings. For strongly coupled rings, on the contrary, fractional periodicity is only observed at sufficiently large negative gate voltages and high interaction strengths. This is explained by the formation of a bound correlated state in the ring that is effectively weakly coupled to the lead.

cond-mat.str-el↗

arXiv2012

Fracturing graphene by chlorination: a theoretical viewpoint

Motivated by the recent photochlorination experiment [B. Li et al., ACS Nano 5, 5957 (2011)], we study theoretically the interaction of chlorine with graphene. In previous theoretical studies, covalent binding between chlorine and carbon atoms has been elusive upon adsorption to the graphene basal plane. Interestingly, in their recent experiment, Li et al. interpreted their data in terms of chemical bonding of chlorine on top of the graphene plane, associated with a change from sp2 to sp3 in carbon hybridization and formation of graphene nanodomains. We study the hypothesis that these domains are actually fractured graphene with chlorinated edges, and compare the energetics of chlorine-containing graphene edge terminations, both in zigzag and armchair directions, to chlorine adsorption onto infinite graphene. Our results indicate that edge chlorination is favored over adsorption in the experimental conditions with radical atomic chlorine and that edge chlorination with sp3-hybridized edge carbons is stable also in ambient conditions. An ab initio thermodynamical analysis shows that the presence of chlorine is able to break the pristine graphene layer. Finally, we discuss the possible effects of the silicon dioxide substrate on the chlorination of graphene.

cond-mat.mes-hall↗

arXiv2011

Hydrogenated graphene on the silicon dioxide surface

Hydrogenation of graphene on the α-quartz (0001) SiO_2 substrate is studied, considering different surface terminations in order to take into account the amorphic nature of the material. Our ab initio calculations show that the formation of graphane by hydrogen adsorption on graphene is energetically favored on hydroxyl and oxygen terminated surfaces, whereas silicon termination and reconstruction of the oxygen termination hinder adsorption. Our results indicate that in order to fabricate graphane on SiO_2, it is beneficial to oxygenize the surface and saturate it with hydrogen. For the pristine graphene on the substrate, we find only marginal changes in the low-energy band structure for all surface terminations.

cond-mat.mes-hall↗

arXiv2011

Spin-asymmetric graphene nanoribbons in graphane on silicon dioxide

Hydrogenated graphene, graphane, is studied on oxygen-terminated silicon dioxide substrate using ab initio calculations. A structure with hydrogenation only on one side of the graphene layer is found stable and its hydrogen configurations are presented. Additionally, we form zigzag graphene nanoribbons by selectively removing hydrogens from the epitaxial graphane layer. In these ribbons, the spin degeneracy of the freestanding antiferromagnetic zigzag ribbons is broken, and band gaps of different magnitude emerge for the opposite spin species. This degeneracy breaking is due to a charge imbalance in the substrate below the ribbon, introduced through the asymmetric alignment of the substrate atoms with respect to the edges of the graphene ribbon.

cond-mat.mes-hall↗

arXiv2010

Lattice density-functional theory on graphene

A density-functional approach on the hexagonal graphene lattice is developed using an exact numerical solution to the Hubbard model as the reference system. Both nearest-neighbour and up to third nearest-neighbour hoppings are considered and exchange-correlation potentials within the local density approximation are parameterized for both variants. The method is used to calculate the ground-state energy and density of graphene flakes and infinite graphene sheet. The results are found to agree with exact diagonalization for small systems, also if local impurities are present. In addition, correct ground-state spin is found in the case of large triangular and bowtie flakes out of the scope of exact diagonalization methods.

cond-mat.mes-hall↗

arXiv2010

Vortices in quantum droplets: Analogies between boson and fermion systems

The main theme of this review is the many-body physics of vortices in quantum droplets of bosons or fermions, in the limit of small particle numbers. Systems of interest include cold atoms in traps as well as electrons confined in quantum dots. When set to rotate, these in principle very different quantum systems show remarkable analogies. The topics reviewed include the structure of the finite rotating many-body state, universality of vortex formation and localization of vortices in both bosonic and fermionic systems, and the emergence of particle-vortex composites in the quantum Hall regime. An overview of the computational many-body techniques sets focus on the configuration interaction and density-functional methods. Studies of quantum droplets with one or several particle components, where vortices as well as coreless vortices may occur, are reviewed, and theoretical as well as experimental challenges are discussed.

cond-mat.quant-gas↗

arXiv2010

Reduced density-matrix functional theory in quantum Hall systems

We apply reduced density-matrix functional theory to the parabolically confined quantum Hall droplet in the spin-frozen strong magnetic field regime. One-body reduced density matrix functional method performs remarkably well in obtaining ground states, energies, and observables derivable from the one-body reduced density matrix for a wide range of system sizes. At the strongly correlated regime, the results go well beyond what can be obtained with the density functional theory. However, some of the detailed properties of the system, such as the edge Green's function, are not produced correctly unless we use the much heavier two-body reduced density matrix method.

cond-mat.mes-hall↗

arXiv2009

Magnetism in tunable quantum rings

We have studied the spin structure of circular four-electron quantum rings using tunable confinement potentials. The calculations were done using the exact diagonalization method. Our results indicate that ringlike systems can have oscillatory flips between ferromagnetic and antiferromagnetic behaviour as a function of the magnetic field. Furthermore, at constant external magnetic fields there were seen similar oscillatory changes between ferromagnetism and antiferromagnetism when the system parameters were changed. Ackording to our results, the magnetism of quantum rings could be tuned by system parameters.We have studied the spin structure of circular four-electron quantum rings using tunable confinement potentials. The calculations were done using the exact diagonalization method. Our results indicate that ringlike systems can have oscillatory flips between ferromagnetic and antiferromagnetic behaviour as a function of the magnetic field. Furthermore, at constant external magnetic fields there were seen similar oscillatory changes between ferromagnetism and antiferromagnetism when the system parameters were changed. According to our results, the magnetism of quantum rings could be tuned by system parameters.

cond-mat.mes-hall↗

arXiv2009

Optimal confinement potential in quantum Hall droplets

We find that the confinement potential of a few electron quantum dot can be tuned to significantly increase the overlap with certain quantum Hall trial wave functions. Besides manipulating inter-electron interaction, this approach may prove useful in quantum point contact experiments, which involve narrow constrictions.

cond-mat.mes-hall↗

arXiv2009

Quantum Hall droplet laterally coupled to a quantum ring

We study a two-dimensional cylindrically-symmetric electron droplet separated from a surrounding electron ring by a tunable barrier using the exact diagonalization method. The magnetic field is assumed strong so that the electrons become spin-polarized and reside on the lowest Fock-Darwin band. We calculate the ground state phase diagram for 6 electrons. At weak coupling, the phase diagram exhibits a clear diamond structure due to the blockade caused by the angular momentum difference between the two systems. We find separate excitations of the droplet and the ring as well as the transfer of charge between the two parts of the system. At strong coupling, interactions destroy the coherent structure of the phase diagram, while individual phases are still heavily affected by the potential barrier.

cond-mat.mes-hall↗

arXiv2008

Coreless Vortices in Rotating Two-Component Quantum Droplets

The rotation of a quantum liquid induces vortices to carry angular momentum. When the system is composed of multiple components that are distinguishable from each other, vortex cores in one component may be filled by particles of the other component, and coreless vortices form. Based on evidence from computational methods, here we show that the formation of coreless vortices occurs very similarly for repulsively interacting bosons and fermions, largely independent of the form of the particle interactions. We further address the connection to the Halperin wave functions of non-polarized quantum Hall states.

cond-mat.mes-hall↗

arXiv2008

Effects of thickness in quantum dots at strong magnetic fields

We study the effects of thickness on the ground states of two-dimensional quantum dots in high magnetic fields. To be specific, we assume the thickness to be small so that only the lowest state in the corresponding direction is occupied, but which however leads to a modification of the effective interaction between the electrons. We find the ground state phase diagram and demonstrate the emergence of new phases as the thickness is accounted for. Finally, the wave functional form and vortex structure of different phases is analyzed.

cond-mat.mes-hall↗