Research at the Center
Research at the Center involves the development of theoretical methods, algorithms and software for the calculations of properties of matter with the goal of being able to predict materials properties from basic equations of physics.
Calculations of the properites of electronic systems
Improved energy functionals, including orbital density dependence (ODD), to describe electronic stystems are being developed, as well as numerical algorithms and software for carrying out calculations using this extended functional form. In calculations of silicon and diamond crystal the computational effort with our new method was only three times longer than with the commonly used GGA functionals and the scaling was confirmed to be the same. Since ODD functionals are potentially much more accurate than GGA - in some cases correcting for qualitative errors in GGA - a factor of three is a small Ôprice to payÕ. Furthermore, the calculations are easy to parallelize so the calculations take roughly the same time if three times more compute sockets are used. The method is working now both in our own code QuantIce (Gaussian based code for molecules) and in GPAW (a grid based code with PAW treatment of inner electrons and periodic boundary conditions for simulations of solids). Several other groups are starting to make use of this methodology in collaborative projects. This extended functional form and the efficient algorithm that has been developed opens the possibility to study band gaps in solids and to study electronic defect states in, for example, oxides used for solar cells. It opens up a whole new range of applications for simulation studies of solids and surfaces of solids.
Transport of electrons in nano-structures
Studies of transport of electrons through two-dimensional semiconductor
structures on the nanoscale in the presence of perpendicular magnetic field
and the interplay between the geometry of the system, the leads, and the
magnetic length. A non-Markovian equation of motion for the
reduced density operator is being used. The transient and the steady state charge and current distributions have been shown to depend on the geometry of the system.
A time-dependent Lippmann-Schwinger scattering theory is used to study the
transport spectroscopy in a time-modulated double quantum point contact
system in the presence of a perpendicular magnetic field. The magnetotransport properties involving inter-subband and inter-sideband transitions were found to be tunable by adjusting the time-modulated split-gates and the applied magnetic field. The observed magnetic field induced Fano resonance feature may be useful for the application of quantum switching [9].
A double quantum wire system containing a coupling element
in the middle barrier between the two parallel quantum wires is being studied.
Explicit account is taken for the finite length of the double quantum wire
with a time-dependent switching-on potential coupling the double-wire
system and the leads. Tuning of the magnetic field and the coupling
window between the wires, the time-dependent current and
the charge distribution of the many-electron states are calculated in order to explore
interwire transfer effects for developing efficient quantum interference
nanoelectronics.
The dynamics of a space-charge limited, photo-injected,
electron beam in a microscopic vacuum diode are being studied. In a series of simulations
of molecular dynamics type, where electrons are treated as point charges,
the space-charge effects in a micrometer-scale vacuum
diode have been analyzed. The breakup of a single pulse injected
with a current density beyond the Child-Langmuir limit has been simulated, and it was found
that continuous injection of current into the diode gap results in a
well-defined train of electron bunches corresponding to THz frequency.
Spintronics and spin transitions
Codes were developed to simulate transport in ballistic nanostructures in the presence of both magnetic field and Rashba spin-orbit interaction. The formalism involves non-equilibrium lattice Green's function for multi-terminal setups. A fast, recursive algorithm for calculating the Green's function in multiterminal system is being developed. So-called spin-valve systems are also studied and an explanation has been proposed of novel spin transport behaviour recently observed experimentally. Using a combination of microscopic theory describing spin-dependent scattering on magnetic impurities and a network model describing the experimental setup, a model was devised that seems to fit the observed data quite well.
Rate theory for spin transitions is being formulated and applied to the rate of decay of a metastable magnetic state in nano-clusters of metal atoms on solid surfaces. Also, the effect of hydrogen on the spin states of adsorbed metallic nano-clusters are being studied.
Long time scale simulations of atomic systems
Sofware has been developed for long time scale simulations in solids, EON, and it has been interfaced with the NorduGrid using the ARC middleware. This is part of IcelandÕs contribution to this Nordic collaboration and both makes EON available to other Nordic groups. Simulations of hydrogen in metals . The software can use (1) MEAM potentials by linking in the LAMMPS software, and (2) DFT codes through ASE2 (including VASP and GPAW). This opens up the possibility to do simulations on a much wider range of solids and with more accuracy than possible before. The simulations are also being carried out on the Amazon Cloud.
Education
The Center participates in the Nordic Master Program in Computational Chemistry and Physics and its members teach an introductory course on atomic scale simulations, as well as
advanced, special topics courses on Rates of transitions and long time scale evolution and Transport through molecules and nanostructures.