arXiv subjects

Ari Harju

Publications and source records attributed to Ari Harju.

50 records · Page 3Linked to original sources

arXiv2013

Lagrange mesh and exact diagonalization for numerical study of semiconductor quantum dot systems with application in singlet-triplet qubits

We present a highly flexible computational scheme for studying correlated electrons confined by an arbitrary external potential in two-dimensional semiconductor quantum dots. The method starts by a Lagrange mesh calculation for the single-particle states, followed by the calculation of the Coulomb interaction matrix elements between these, and combining both in the exact diagonalization of the many-body Hamiltonian. We apply the method in simulation of double quantum dot singlet-triplet qubits. We simulate the full quantum control and dynamics of one singlet-triplet qubit. We also use our method to provide an exact diagonalization based first-principles model for studying two singlet-triplet qubits and their capacitative coupling via the long-distance Coulomb interaction.

cond-mat.mes-hall↗

arXiv2012

Ab-initio transport fingerprints for resonant scattering in graphene

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.

cond-mat.mes-hall↗

arXiv2012

High-field magnetoresistance revealing scattering mechanisms in graphene

We show that the type of charge carrier scattering significantly affects the high-field magnetoresistance of graphene nanoribbons. This effect has potential to be used in identifying the scattering mechanisms in graphene. The results also provide an explanation for the experimentally found, intriguing differences in the behavior of the magnetoresistance of graphene Hall bars placed on different substrates. Additionally, our simulations indicate that the peaks in the longitudinal resistance tend to become pinned to fractionally quantized values, as different transport modes have very different scattering properties.

cond-mat.mes-hall↗

arXiv2012

Y-junction splitting spin states of moving quantum dot

The development of a working quantum computer utilizing electron spin states as qubits is a major goal for many theorists and experimentalists. The future applications of quantum information technology would also require a reliable method for the transportation of quantum information. A promising such method is the propagation of electrons by a surface acoustic wave (SAW). In this paper, we simulate the SAW transfer of two interacting electrons through a Y-shaped junction. Our results show that the singlet and triplet states can be differentiated in the Y-junction by an electrostatic detuning potential, an effect that could be used in for example measuring the state of a two-spin qubit.

cond-mat.mes-hall↗

arXiv2012

Exact diagonalization of the Hubbard model on graphics processing units

We solve the Hubbard model with the exact diagonalization method on a graphics processing unit (GPU). We benchmark our GPU program against a sequential CPU code by using the Lanczos algorithm to solve the ground state energy in two cases: a one-dimensional ring and a two-dimensional square lattice. In the one-dimensional case, we obtain speedups of over 100 and 60 in single and double precision arithmetic, respectively. In the two-dimensional case, the corresponding speedups are over 110 and 70.

physics.comp-ph↗

arXiv2012

Exchange-correlation potentials for inhomogeneous electron systems in two dimensions from exact diagonalization: comparison with the local-spin-density approximation

We consider electronic exchange and correlation effects in density-functional calculations of two-dimensional systems. Starting from wave function calculations of total energies and electron densities of inhomogeneous model systems, we derive corresponding exchange-correlation potentials and energies. We compare these with predictions of the local-spin-density approximation and discuss its accuracy. Our data will be useful as reference data in testing, comparing and parametrizing exchange and correlation functionals for two-dimensional electronic systems.

cond-mat.mes-hall↗

arXiv2011

Electronic transport in graphene-based structures: an effective cross section approach

We show that transport in low-dimensional carbon structures with finite concentrations of scatterers can be modeled by utilising scaling theory and effective cross sections. Our reults are based on large scale numerical simulations of carbon nanotubes and graphene nanoribbons, using a tightbinding 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-1D 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 towards using first principles methods to evaluate the conductance of mesoscopic systems, consisting of millions of atoms.

cond-mat.mes-hall↗

arXiv2011

Quantum confined electronic states in atomically well-defined graphene nanostructures

Despite the enormous interest in the properties of graphene and the potential of graphene nanostructures in electronic applications, the study of quantum confined states in atomically well-defined graphene nanostructures remains an experimental challenge. Here, we study graphene quantum dots (GQDs) with well-defined edges in the zigzag direction, grown by chemical vapor deposition (CVD) on an iridium(111) substrate, by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). We measure the atomic structure and local density of states (LDOS) of individual GQDs as a function of their size and shape in the range from a couple of nanometers up to ca. 20 nm. The results can be quantitatively modeled by a relativistic wave equation and atomistic tight-binding calculations. The observed states are analogous to the solutions of the text book "particle-in-a-box" problem applied to relativistic massless fermions.

cond-mat.mes-hall↗

arXiv2011

Spin dynamics at the singlet-triplet crossings in double quantum dot

We simulate the control of the spin states in a two-electron double quantum dot when an external detuning potential is used for passing the system through an anticrossing. The hyperfine coupling of the electron spins with the surrounding nuclei causes the anticrossing of the spin states but also the decoherence of the spin states. We calculate numerically the singlet-triplet decoherence for different detuning values and find a good agreement with experimental measurement results of the same setup. We predict an interference effect due to the coupling of T0 and T+ states.

cond-mat.mes-hall↗

arXiv2010

Charge dynamics in two-electron quantum dots

We investigate charge dynamics in a two-electron double quantum dot. The quantum dot is manipulated by using a time-dependent external voltage that induces charge oscillations between the dots. We study the dependence of the charge dynamics on the external magnetic field and on the periodicity of the external potential. We find that for suitable parameter values, it is possible to induce both one-electron and two-electron oscillations between the dots.

cond-mat.mes-hall↗

arXiv2010

Control of a two-electron quantum ring with an external magnetic field

We investigate the use of external time-dependent magnetic field for the control of the quantum states in a two-electron quantum ring. The hyperfine interaction of the confined electrons with surrounding nuclei couples the singlet state with the three triplet states. When the external magnetic field is changed, the singlet ground state becomes degenerate with the triplet states allowing singlet-triplet transitions. By choosing different speeds for the magnetic field switching the final quantum state of the system can be manipulated. We evaluate suitable magnetic field values and time scales for efficient quantum ring control.

cond-mat.mes-hall↗

arXiv2009

Gate-controlled current switch in graphene

We study numerically cross conductances in a four-terminal all-graphene setup. We show that far away from the Dirac point current flows along zigzag directions, giving the possibility to guide the current between terminals using a tunable pn-junction. The device operates as a gate-controlled current switch, and the electronic properties of graphene are crucial for efficient performance.

cond-mat.mes-hall↗

arXiv2005

Ground-state of two-dimensional finite electron systems in the Quantum Hall regime

We study electronic structures of quasi-two-dimensional finite electron systems in high magnetic fields. The solutions in the fractional quantum Hall regime are interpreted as quantum liquids of electrons and off-electron vortices. The ground states are classified according to the number of vortices inside the electron droplet. The theory predicts observable effects due to vortex formation in the chemical potentials and magnetization of electron droplets. We compare the transitions in the theory to those found in electron transport experiments on a quantum dot device and find significant correspondence.

cond-mat.mes-hall↗

arXiv2005

Variational Monte Carlo for Interacting Electrons in Quantum Dots

We use a variational Monte Carlo algorithm to solve the electronic structure of two-dimensional semiconductor quantum dots in external magnetic field. We present accurate many-body wave functions for the system in various magnetic field regimes. We show the importance of symmetry, and demonstrate how it can be used to simplify the variational wave functions. We present in detail the algorithm for efficient wave function optimization. We also present a Monte Carlo -based diagonalization technique to solve the quantum dot problem in the strong magnetic field limit where the system is of a multiconfiguration nature.

cond-mat.mes-hall↗