Publications in 2021

  1. Nzar Rauf Abdullah, Botan Jawdat Abdullah, Hunar Omar Rshid, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
    Enhanced electronic and optical responses of nitrogen- or boron-doped BeO monolayer: First principle computation.
    Superlattices and Microstructures 162, 107102 (2022).
    Abstract In this work, the electronic and optical properties of a Nitrogen (N) or a Boron (B) doped BeO monolayer are investigated in the framework of density functional theory. It is known that the band gap of a BeO monolayer is large leading to poor material for optoelectronic devices in a wide range of energy. Using N or B dopant atoms, we find that the band gap can be tuned and the optical properties can be improved. In the N(B)-doped BeO monolayer, the Fermi energy slightly crosses the valence (conduction) band forming a degenerate semiconductor structure. The N or B atoms thus generate new states around the Fermi energy increasing the optical conductivity in the visible light region. Furthermore, the influences of dopant atoms on the electronic structure, the stability, the dispersion energy, the density of states, and optical properties such as the plasmon frequency, the excitation spectra, the dielectric functions, the static dielectric constant, and the electron energy loss function are discussed for different directions of polarizations for the incoming electric field.
    URL, DOI BibTeX

    @article{ABDULLAH2022107102,
    	title = "Enhanced electronic and optical responses of nitrogen- or boron-doped BeO monolayer: First principle computation",
    	journal = "Superlattices and Microstructures",
    	volume = 162,
    	pages = 107102,
    	year = 2022,
    	issn = "0749-6036",
    	doi = "https://doi.org/10.1016/j.spmi.2021.107102",
    	url = "https://www.sciencedirect.com/science/article/pii/S0749603621003037",
    	author = "Nzar Rauf Abdullah and Botan Jawdat Abdullah and Hunar Omar Rshid and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
    	keywords = "BeO monolayer, DFT, Electronic structure, Optical properties, Doping",
    	abstract = "In this work, the electronic and optical properties of a Nitrogen (N) or a Boron (B) doped BeO monolayer are investigated in the framework of density functional theory. It is known that the band gap of a BeO monolayer is large leading to poor material for optoelectronic devices in a wide range of energy. Using N or B dopant atoms, we find that the band gap can be tuned and the optical properties can be improved. In the N(B)-doped BeO monolayer, the Fermi energy slightly crosses the valence (conduction) band forming a degenerate semiconductor structure. The N or B atoms thus generate new states around the Fermi energy increasing the optical conductivity in the visible light region. Furthermore, the influences of dopant atoms on the electronic structure, the stability, the dispersion energy, the density of states, and optical properties such as the plasmon frequency, the excitation spectra, the dielectric functions, the static dielectric constant, and the electron energy loss function are discussed for different directions of polarizations for the incoming electric field."
    }
    
  2. Nicolae Filipoiu, Tudor Luca Mitran, Dragos Victor Anghel, Mihaela Florea, Ioana Pintilie, Andrei Manolescu and George Alexandru Nemnes.
    Investigation of Opto-Electronic Properties and Stability of Mixed-Cation Mixed-Halide Perovskite Materials with Machine-Learning Implementation.
    Energies 14 (2021).
    Abstract The feasibility of mixed-cation mixed-halogen perovskites of formula AxA’1−xPbXyX’zX”3−y−z is analyzed from the perspective of structural stability, opto-electronic properties and possible degradation mechanisms. Using density functional theory (DFT) calculations aided by machine-learning (ML) methods, the structurally stable compositions are further evaluated for the highest absorption and optimal stability. Here, the role of the halogen mixtures is demonstrated in tuning the contrasting trends of optical absorption and stability. Similarly, binary organic cation mixtures are found to significantly influence the degradation, while they have a lesser, but still visible effect on the opto-electronic properties. The combined framework of high-throughput calculations and ML techniques such as the linear regression methods, random forests and artificial neural networks offers the necessary grounds for an efficient exploration of multi-dimensional compositional spaces.
    URL, DOI BibTeX

    @article{en14175431,
    	author = "Filipoiu, Nicolae and Mitran, Tudor Luca and Anghel, Dragos Victor and Florea, Mihaela and Pintilie, Ioana and Manolescu, Andrei and Nemnes, George Alexandru",
    	title = "Investigation of Opto-Electronic Properties and Stability of Mixed-Cation Mixed-Halide Perovskite Materials with Machine-Learning Implementation",
    	journal = "Energies",
    	volume = 14,
    	year = 2021,
    	number = 17,
    	article-number = 5431,
    	url = "https://www.mdpi.com/1996-1073/14/17/5431",
    	issn = "1996-1073",
    	abstract = "The feasibility of mixed-cation mixed-halogen perovskites of formula AxA’1−xPbXyX’zX”3−y−z is analyzed from the perspective of structural stability, opto-electronic properties and possible degradation mechanisms. Using density functional theory (DFT) calculations aided by machine-learning (ML) methods, the structurally stable compositions are further evaluated for the highest absorption and optimal stability. Here, the role of the halogen mixtures is demonstrated in tuning the contrasting trends of optical absorption and stability. Similarly, binary organic cation mixtures are found to significantly influence the degradation, while they have a lesser, but still visible effect on the opto-electronic properties. The combined framework of high-throughput calculations and ML techniques such as the linear regression methods, random forests and artificial neural networks offers the necessary grounds for an efficient exploration of multi-dimensional compositional spaces.",
    	doi = "10.3390/en14175431"
    }
    
  3. Nzar Rauf Abdullah, Hunar Omar Rashid, Andrei Manolescu and Vidar Gudmundsson.
    Interaction effects in a two-dimensional AlSi$_6$P nanosheet: A first-principles study on the electronic, mechanical, thermal, and optical properties.
    (2021).
    BibTeX

    @misc{abdullah2021interaction,
    	title = "Interaction effects in a two-dimensional AlSi$_6$P nanosheet: A first-principles study on the electronic, mechanical, thermal, and optical properties",
    	author = "Nzar Rauf Abdullah and Hunar Omar Rashid and Andrei Manolescu and Vidar Gudmundsson",
    	year = 2021,
    	eprint = "2108.00387",
    	archiveprefix = "arXiv",
    	primaryclass = "cond-mat.mes-hall"
    }
    
  4. Anna Sitek, Kristinn Torfason, Andrei Manolescu and Ágúst Valfells.
    Edge Effect on the Current-Temperature Characteristic of Finite-Area Thermionic Cathodes.
    Phys. Rev. Applied 16, 034043 (September 2021).
    Abstract We perform a computational study, based on the molecular-dynamics method, of the shape of Miram curves obtained from microscale planar diodes. We discuss the smooth transition from the source-limited regime to the space-charge-limited regime due to the finite size of the emitter (i.e., the “knee” in the Miram curve). In our model we find that the smoothing occurs mostly due to the increased emission at the external edges of the emitting area, and that the knee becomes softer when the size of the emitting area decreases. We relate this to recent work that has described how a heterogeneous work function similarly affects the Miram curve.
    URL, DOI BibTeX

    @article{PhysRevApplied.16.034043,
    	title = "Edge Effect on the Current-Temperature Characteristic of Finite-Area Thermionic Cathodes",
    	author = "Sitek, Anna and Torfason, Kristinn and Manolescu, Andrei and Valfells, \'Ag\'ust",
    	journal = "Phys. Rev. Applied",
    	volume = 16,
    	issue = 3,
    	pages = 034043,
    	numpages = 6,
    	year = 2021,
    	month = "Sep",
    	publisher = "American Physical Society",
    	doi = "10.1103/PhysRevApplied.16.034043",
    	url = "https://link.aps.org/doi/10.1103/PhysRevApplied.16.034043",
    	abstract = "We perform a computational study, based on the molecular-dynamics method, of the shape of Miram curves obtained from microscale planar diodes. We discuss the smooth transition from the source-limited regime to the space-charge-limited regime due to the finite size of the emitter (i.e., the “knee” in the Miram curve). In our model we find that the smoothing occurs mostly due to the increased emission at the external edges of the emitting area, and that the knee becomes softer when the size of the emitting area decreases. We relate this to recent work that has described how a heterogeneous work function similarly affects the Miram curve."
    }
    
  5. Miguel Urbaneja Torres, Kristjan Ottar Klausen, Anna Sitek, Sigurdur I Erlingsson, Vidar Gudmundsson and Andrei Manolescu.
    Electromagnetic field emitted by core–shell semiconductor nanowires driven by an alternating current.
    Journal of Applied Physics 130, 034301 (2021).
    URL, DOI BibTeX

    @article{doi:10.1063/5.0055260,
    	author = "Torres,Miguel Urbaneja and Klausen,Kristjan Ottar and Sitek,Anna and Erlingsson,Sigurdur I. and Gudmundsson,Vidar and Manolescu,Andrei",
    	title = "Electromagnetic field emitted by core–shell semiconductor nanowires driven by an alternating current",
    	journal = "Journal of Applied Physics",
    	volume = 130,
    	number = 3,
    	pages = 034301,
    	year = 2021,
    	doi = "10.1063/5.0055260",
    	url = "https://doi.org/10.1063/5.0055260",
    	eprint = "https://doi.org/10.1063/5.0055260"
    }
    
  6. Muhammad Taha Sultan, Hákon Örn Árnason, Movaffaq Kateb, Andrei Manolescu, Halldór Gudfinnur Svavarsson and Ágúst Valfells.
    Enhanced photoemission from surface modulated GaAs:Ge.
    Nano Select 2, 2346-2357 (2021).
    Abstract Abstract The present work reports the evolution and growth of GeGaAs(O) polytype nanoislands over GaAs p-type substrate with photoemission application in mind. Several morphological transformations from NIs to simultaneously present nanopits/holes are observed as a function of annealing parameters that is, temperature (350-800°C) and time (5-90 minutes). Structural and elemental analyses are executed using atomic force microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Photoemission current of the nanostructured surfaces, measured upon exposure from 265 nm light emitting diode, is found to depend on the nanoislands size, which in turn depends on the annealing parameters. A maximum photoelectric emission is obtained for structure annealed at 650°C for 60 minutes, upon which an increment of roughly two orders of magnitude is observed.
    URL, DOI BibTeX

    @article{https://doi.org/10.1002/nano.202100012,
    	author = "Sultan, Muhammad Taha and Árnason, Hákon Örn and Kateb, Movaffaq and Manolescu, Andrei and Svavarsson, Halldór Gudfinnur and Valfells, Ágúst",
    	title = "Enhanced photoemission from surface modulated GaAs:Ge",
    	journal = "Nano Select",
    	volume = 2,
    	number = 12,
    	pages = "2346-2357",
    	keywords = "AFM, GaAsGe, nanostructure, photoemission, SEM",
    	doi = "https://doi.org/10.1002/nano.202100012",
    	url = "https://onlinelibrary.wiley.com/doi/abs/10.1002/nano.202100012",
    	eprint = "https://onlinelibrary.wiley.com/doi/pdf/10.1002/nano.202100012",
    	abstract = "Abstract The present work reports the evolution and growth of GeGaAs(O) polytype nanoislands over GaAs p-type substrate with photoemission application in mind. Several morphological transformations from NIs to simultaneously present nanopits/holes are observed as a function of annealing parameters that is, temperature (350-800°C) and time (5-90 minutes). Structural and elemental analyses are executed using atomic force microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Photoemission current of the nanostructured surfaces, measured upon exposure from 265 nm light emitting diode, is found to depend on the nanoislands size, which in turn depends on the annealing parameters. A maximum photoelectric emission is obtained for structure annealed at 650°C for 60 minutes, upon which an increment of roughly two orders of magnitude is observed.",
    	year = 2021
    }
    
  7. Nzar Rauf Abdullah, Mohammad T Kareem, Hunar Omar Rashid, Andrei Manolescu and Vidar Gudmundsson.
    Spin-polarised DFT modeling of electronic, magnetic, thermal and optical properties of silicene doped with transition metals.
    Physica E: Low-dimensional Systems and Nanostructures 129, 114644 (2021).
    Abstract The geometric, electronic, magnetic, thermal, and optical properties of transition metal (TM) doped silicene are systematically explored using spin-dependent density functional computation. We find that the TM atoms decrease the buckling degree of the silicene structure caused by the interaction between the dopant TM atoms and the Si atoms in the silicene layer plane which is quite strong. The phonon dispersion of TM-silicene structures are calculated to show the most stable structure. In some TM-silicenes, parallel bands and the corresponding van Hove singularities are observed in the electronic band structure without and with spin-polarization. These parallel bands are the origin of most of the transitions in the visible region. A high Seebeck coefficient is found in some TM-silicene without spin-polarization. In the presence of emergent spin-polarization, a reduction or a magnification of the Seebeck coefficient is seen due to a spin-dependent phase transition. We find that the preferred state is a ferromagnetic state with a very high Curie temperature. We observe a strong interaction and large orbital hybridization between the TM atoms and the silicene. As a result, a high magnetic moment emerges in TM-silicene. Our results are potentially beneficial for thermospin, and optoelectronic nanodevices.
    URL, DOI BibTeX

    @article{ABDULLAH2021114644,
    	title = "Spin-polarised DFT modeling of electronic, magnetic, thermal and optical properties of silicene doped with transition metals",
    	journal = "Physica E: Low-dimensional Systems and Nanostructures",
    	volume = 129,
    	pages = 114644,
    	year = 2021,
    	issn = "1386-9477",
    	doi = "https://doi.org/10.1016/j.physe.2021.114644",
    	url = "https://www.sciencedirect.com/science/article/pii/S1386947721000266",
    	author = "Nzar Rauf Abdullah and Mohammad T. Kareem and Hunar Omar Rashid and Andrei Manolescu and Vidar Gudmundsson",
    	keywords = "Magnetization, Thermal transport, Silicene, DFT, Electronic structure, Optical properties",
    	abstract = "The geometric, electronic, magnetic, thermal, and optical properties of transition metal (TM) doped silicene are systematically explored using spin-dependent density functional computation. We find that the TM atoms decrease the buckling degree of the silicene structure caused by the interaction between the dopant TM atoms and the Si atoms in the silicene layer plane which is quite strong. The phonon dispersion of TM-silicene structures are calculated to show the most stable structure. In some TM-silicenes, parallel bands and the corresponding van Hove singularities are observed in the electronic band structure without and with spin-polarization. These parallel bands are the origin of most of the transitions in the visible region. A high Seebeck coefficient is found in some TM-silicene without spin-polarization. In the presence of emergent spin-polarization, a reduction or a magnification of the Seebeck coefficient is seen due to a spin-dependent phase transition. We find that the preferred state is a ferromagnetic state with a very high Curie temperature. We observe a strong interaction and large orbital hybridization between the TM atoms and the silicene. As a result, a high magnetic moment emerges in TM-silicene. Our results are potentially beneficial for thermospin, and optoelectronic nanodevices."
    }
    
  8. Nzar Rauf Abdullah, Hunar Omar Rashid, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
    Role of interlayer spacing on electronic, thermal and optical properties of BN-codoped bilayer graphene: Influence of the interlayer and the induced dipole-dipole interactions.
    Journal of Physics and Chemistry of Solids 155, 110095 (2021).
    Abstract We demonstrate that the electronic, thermal, and optical properties of a graphene bilayer with boron and nitrogen dopant atoms can be controlled by the interlayer distance between the layers in which the interaction energy and the van der Waals interaction between the dopant atoms play an essential role. We find a conversion of an AA-to an AB-stacked bilayer graphene caused by the repulsive interaction between dopant atoms. At a short interlayer distance, a strong repulsive interaction inducing a strong electric dipole moment of the dopant atoms is found. This gives rise to a breaking of the high symmetry, opening up a bandgap. Consequently, a considerable change in thermoelectric properties such as the Seebeck coefficient and the figure of merit are seen. The repulsive interaction is reduced by increasing the interlayer distance, and at a large interlayer distance the conversion process of the stacking order vanishes. A small bandgap is found leading to a low Seebeck coefficient and a figure of merit. For both short and large interlayer distances, a prominent peak in the optical response is found in the visible range and the peak position is inversely proportional to the interlayer distance.
    URL, DOI BibTeX

    @article{ABDULLAH2021110095,
    	title = "Role of interlayer spacing on electronic, thermal and optical properties of BN-codoped bilayer graphene: Influence of the interlayer and the induced dipole-dipole interactions",
    	journal = "Journal of Physics and Chemistry of Solids",
    	volume = 155,
    	pages = 110095,
    	year = 2021,
    	issn = "0022-3697",
    	doi = "https://doi.org/10.1016/j.jpcs.2021.110095",
    	url = "https://www.sciencedirect.com/science/article/pii/S002236972100161X",
    	author = "Nzar Rauf Abdullah and Hunar Omar Rashid and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
    	keywords = "Thermoelectric, Bilayer graphene, DFT, Electronic structure, and optical properties",
    	abstract = "We demonstrate that the electronic, thermal, and optical properties of a graphene bilayer with boron and nitrogen dopant atoms can be controlled by the interlayer distance between the layers in which the interaction energy and the van der Waals interaction between the dopant atoms play an essential role. We find a conversion of an AA-to an AB-stacked bilayer graphene caused by the repulsive interaction between dopant atoms. At a short interlayer distance, a strong repulsive interaction inducing a strong electric dipole moment of the dopant atoms is found. This gives rise to a breaking of the high symmetry, opening up a bandgap. Consequently, a considerable change in thermoelectric properties such as the Seebeck coefficient and the figure of merit are seen. The repulsive interaction is reduced by increasing the interlayer distance, and at a large interlayer distance the conversion process of the stacking order vanishes. A small bandgap is found leading to a low Seebeck coefficient and a figure of merit. For both short and large interlayer distances, a prominent peak in the optical response is found in the visible range and the peak position is inversely proportional to the interlayer distance."
    }
    
  9. Movaffaq Kateb, Jon Tomas Gudmundsson, Pascal Brault, Andrei Manolescu and Snorri Ingvarsson.
    On the role of ion potential energy in low energy HiPIMS deposition: An atomistic simulation.
    Surface and Coatings Technology 426, 127726 (2021).
    Abstract We study the effect of the so-called ion potential or non-kinetic energies of bombarding ions during ionized physical vapor deposition of Cu using molecular dynamics simulations. In particular we focus on low energy high power impulse magnetron sputtering (HiPIMS) deposition, in which the potential energy of ions can be comparable to their kinetic energy. The ion potential, as a short-ranged repulsive force between the ions of the film-forming material and the surface atoms (substrate and later deposited film), is defined by the Ziegler-Biersack-Littmark potential. Analyzing the final structure indicates that, including the ion potential leads to a slightly lower interface mixing and fewer point defects (such as vacancies and interstitials), but resputtering and twinning have increased slightly. However, by including the ion potential the collision pattern changes. We also observed temporary formation of a ripple/pore with 5 nm height when the ion potential is included. The latter effect can explain the pores that have been observed experimentally in HiPIMS deposited Cu thin films by atomic force microscopy.
    URL, DOI BibTeX

    @article{KATEB2021127726,
    	title = "On the role of ion potential energy in low energy HiPIMS deposition: An atomistic simulation",
    	journal = "Surface and Coatings Technology",
    	volume = 426,
    	pages = 127726,
    	year = 2021,
    	issn = "0257-8972",
    	doi = "https://doi.org/10.1016/j.surfcoat.2021.127726",
    	url = "https://www.sciencedirect.com/science/article/pii/S0257897221009002",
    	author = "Movaffaq Kateb and Jon Tomas Gudmundsson and Pascal Brault and Andrei Manolescu and Snorri Ingvarsson",
    	keywords = "Ion potential, HiPIMS, Deposition, Molecular dynamics, ZBL potential",
    	abstract = "We study the effect of the so-called ion potential or non-kinetic energies of bombarding ions during ionized physical vapor deposition of Cu using molecular dynamics simulations. In particular we focus on low energy high power impulse magnetron sputtering (HiPIMS) deposition, in which the potential energy of ions can be comparable to their kinetic energy. The ion potential, as a short-ranged repulsive force between the ions of the film-forming material and the surface atoms (substrate and later deposited film), is defined by the Ziegler-Biersack-Littmark potential. Analyzing the final structure indicates that, including the ion potential leads to a slightly lower interface mixing and fewer point defects (such as vacancies and interstitials), but resputtering and twinning have increased slightly. However, by including the ion potential the collision pattern changes. We also observed temporary formation of a ripple/pore with 5 nm height when the ion potential is included. The latter effect can explain the pores that have been observed experimentally in HiPIMS deposited Cu thin films by atomic force microscopy."
    }
    
  10. Kristinn Torfason, Anna Sitek, Andrei Manolescu and Ágúst Valfells.
    Dynamics of a Field Emitted Beam From a Microscopic Inhomogeneous Cathode.
    IEEE Transactions on Electron Devices 68, 2461-2466 (2021).
    Abstract We investigate by molecular dynamics simulations (Torfason et al., 2015; 2016) a beam of electrons released via field emission from a planar cathode surface of 1 μ m 2 with an inhomogeneous two-level work function, Φ low and Φ high . A rectangular grid, where each cell can have one out of two values of the work function, is used as a model. The number of cells in the grid ranges from 6 × 6 to 96×96. We compare a periodic checkerboard arrangement with disordered distributions of patches. We perform multiple simulations and randomize the pattern each time. We study the beam behavior by calculating the position and velocity of each electron, r.m.s. emittance, and the brightness of the electron beam. The emittance increases while brightness decreases, with the mean distance between patches with Φ low when they are in minority, and they switch the behavior versus the mean distance between patches with Φ high when these patches are in minority, respectively. The Coulomb interaction between all particles is fully included in our simulations.
    DOI BibTeX

    @article{9380194,
    	author = "Torfason, Kristinn and Sitek, Anna and Manolescu, Andrei and Valfells, Ágúst",
    	journal = "IEEE Transactions on Electron Devices",
    	title = "Dynamics of a Field Emitted Beam From a Microscopic Inhomogeneous Cathode",
    	year = 2021,
    	volume = 68,
    	number = 5,
    	pages = "2461-2466",
    	doi = "10.1109/TED.2021.3063210",
    	abstract = "We investigate by molecular dynamics simulations (Torfason et al., 2015; 2016) a beam of electrons released via field emission from a planar cathode surface of 1 μ m 2 with an inhomogeneous two-level work function, Φ low and Φ high . A rectangular grid, where each cell can have one out of two values of the work function, is used as a model. The number of cells in the grid ranges from 6 × 6 to 96×96. We compare a periodic checkerboard arrangement with disordered distributions of patches. We perform multiple simulations and randomize the pattern each time. We study the beam behavior by calculating the position and velocity of each electron, r.m.s. emittance, and the brightness of the electron beam. The emittance increases while brightness decreases, with the mean distance between patches with Φ low when they are in minority, and they switch the behavior versus the mean distance between patches with Φ high when these patches are in minority, respectively. The Coulomb interaction between all particles is fully included in our simulations."
    }
    
  11. Anna Sitek, Kristinn Torfason, Andrei Manolescu and Ágúst Valfells.
    Space-Charge Effects in the Field-Assisted Thermionic Emission from Nonuniform Cathodes.
    Phys. Rev. Applied 15, 014040 (January 2021).
    Abstract We use computational simulations to study the electron emission and propagation in planar vacuum diodes. We show how space charge affects thermionic emission from cathodes with two different values of work function that form a checkerboard pattern of finite extent on the cathode surface. We confirm that, for intermediate cathode temperature, the local current density from low work function regions can exceed the space-charge limit for the entire cathode. As the cathode temperature rises, space-charge effects lead to homogeneous current density from the interior of the emitting area and a higher current density from its periphery. We show how beam emittance and brightness are affected, and show that the operating temperature for optimal brightness is such that it corresponds to the transition region between source-limited and space-charge-limited emissions. Finally, we show how beam current and beam quality depend on how fine grained the structure of the cathode is.
    URL, DOI BibTeX

    @article{PhysRevApplied.15.014040,
    	title = "Space-Charge Effects in the Field-Assisted Thermionic Emission from Nonuniform Cathodes",
    	author = "Sitek, Anna and Torfason, Kristinn and Manolescu, Andrei and Valfells, \'Ag\'ust",
    	journal = "Phys. Rev. Applied",
    	volume = 15,
    	issue = 1,
    	pages = 014040,
    	numpages = 8,
    	year = 2021,
    	month = "Jan",
    	publisher = "American Physical Society",
    	doi = "10.1103/PhysRevApplied.15.014040",
    	url = "https://link.aps.org/doi/10.1103/PhysRevApplied.15.014040",
    	abstract = "We use computational simulations to study the electron emission and propagation in planar vacuum diodes. We show how space charge affects thermionic emission from cathodes with two different values of work function that form a checkerboard pattern of finite extent on the cathode surface. We confirm that, for intermediate cathode temperature, the local current density from low work function regions can exceed the space-charge limit for the entire cathode. As the cathode temperature rises, space-charge effects lead to homogeneous current density from the interior of the emitting area and a higher current density from its periphery. We show how beam emittance and brightness are affected, and show that the operating temperature for optimal brightness is such that it corresponds to the transition region between source-limited and space-charge-limited emissions. Finally, we show how beam current and beam quality depend on how fine grained the structure of the cathode is."
    }
    
  12. Vidar Gudmundsson, Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Valeriu Moldoveanu.
    Self-induction and magnetic effects in electron transport through a photon cavity.
    Physica E: Low-dimensional Systems and Nanostructures 127, 114544 (2021).
    Abstract We explore higher order dynamical effects in the transport through a two-dimensional nanoscale electron system embedded in a three-dimensional far-infrared photon cavity. The nanoscale system is considered to be a short quantum wire with a single circular quantum dot defined in a GaAs heterostructure. The whole system, the external leads and the central system are placed in a constant perpendicular magnetic field. The Coulomb interaction of the electrons, the para- and diamagnetic electron-photon interactions are all treated by a numerically exact diagonalization using step-wise truncations of the appropriate many-body Fock spaces. We focus on the difference in transport properties between a description within an electric dipole approximation and a description including all higher order terms in a single photon mode model. We find small effects mostly caused by an electrical quadrupole and a magnetic dipole terms that depend strongly on the polarization of the cavity field with respect to the transport direction and the photon energy. When the polarization is aligned along the transport direction we find indications of a weak self-induction that we analyze and compare to the classical counterpart, and the self-energy contribution of high-order interaction terms to the states the electrons cascade through on their way through the system. Like expected the electron-photon interaction is well described in the dipole approximation when it is augmented by the lowest order diamagnetic part for a nanoscale system in a cavity in an external magnetic field.
    URL, DOI BibTeX

    @article{GUDMUNDSSON2021114544,
    	title = "Self-induction and magnetic effects in electron transport through a photon cavity",
    	journal = "Physica E: Low-dimensional Systems and Nanostructures",
    	volume = 127,
    	pages = 114544,
    	year = 2021,
    	issn = "1386-9477",
    	doi = "https://doi.org/10.1016/j.physe.2020.114544",
    	url = "https://www.sciencedirect.com/science/article/pii/S138694772031612X",
    	author = "Vidar Gudmundsson and Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Valeriu Moldoveanu",
    	keywords = "Self-induction, Electronic transport in nanoscale systems, Cavity quantum electrodynamics, Magneto-optical effects",
    	abstract = "We explore higher order dynamical effects in the transport through a two-dimensional nanoscale electron system embedded in a three-dimensional far-infrared photon cavity. The nanoscale system is considered to be a short quantum wire with a single circular quantum dot defined in a GaAs heterostructure. The whole system, the external leads and the central system are placed in a constant perpendicular magnetic field. The Coulomb interaction of the electrons, the para- and diamagnetic electron-photon interactions are all treated by a numerically exact diagonalization using step-wise truncations of the appropriate many-body Fock spaces. We focus on the difference in transport properties between a description within an electric dipole approximation and a description including all higher order terms in a single photon mode model. We find small effects mostly caused by an electrical quadrupole and a magnetic dipole terms that depend strongly on the polarization of the cavity field with respect to the transport direction and the photon energy. When the polarization is aligned along the transport direction we find indications of a weak self-induction that we analyze and compare to the classical counterpart, and the self-energy contribution of high-order interaction terms to the states the electrons cascade through on their way through the system. Like expected the electron-photon interaction is well described in the dipole approximation when it is augmented by the lowest order diamagnetic part for a nanoscale system in a cavity in an external magnetic field."
    }
    
  13. Nzar Rauf Abdullah, Hunar Omar Rashid, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
    Properties of BSi6N monolayers derived by first-principle computation.
    Physica E: Low-dimensional Systems and Nanostructures 127, 114556 (2021).
    Abstract The buckling effects due to BN-bonds in BN-codoped silicene, BSi6N, on structural stability, electronic band structure, and mechanical, thermal and optical properties are studied systematically by first-principle calculations within density functional theory. In the presence of BN-bonds, a high warping in BSi6N indicating a high buckling effect is found due to the presence of a repulsive interaction between B and N atoms. It thus breaks the sublattice symmetry of silicene and opens up a bandgap. The high buckling of BSi6N leads to a decrease in its stiffness and thus induces fractures at small values of applied strain. The finite bandgap caused by the BN-bonds leads to enhancement of the Seebeck coefficient and the figure of merit, and induces a redshift of a peak in the dielectric response. By increasing the distance between the B and N atoms i.e. for the BSi6N without BN-bonds, a flatter BSi6N is found compared to pristine silicene. The stiffness of the structure and the ultimate strain are increased. The breaking of the sublattice symmetry is very weak and a very small bandgap is revealed. As a result, the Seebeck coefficient and the figure of merit stay very small. A reduction in the optical response is seen due to an indirect bandgap.
    URL, DOI BibTeX

    @article{ABDULLAH2021114556,
    	title = "Properties of BSi6N monolayers derived by first-principle computation",
    	journal = "Physica E: Low-dimensional Systems and Nanostructures",
    	volume = 127,
    	pages = 114556,
    	year = 2021,
    	issn = "1386-9477",
    	doi = "https://doi.org/10.1016/j.physe.2020.114556",
    	url = "https://www.sciencedirect.com/science/article/pii/S1386947720316246",
    	author = "Nzar Rauf Abdullah and Hunar Omar Rashid and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
    	keywords = "Energy harvesting, Thermal transport, Silicene, Density functional theory, Electronic structure, Optical properties, Stress-strain curve",
    	abstract = "The buckling effects due to BN-bonds in BN-codoped silicene, BSi6N, on structural stability, electronic band structure, and mechanical, thermal and optical properties are studied systematically by first-principle calculations within density functional theory. In the presence of BN-bonds, a high warping in BSi6N indicating a high buckling effect is found due to the presence of a repulsive interaction between B and N atoms. It thus breaks the sublattice symmetry of silicene and opens up a bandgap. The high buckling of BSi6N leads to a decrease in its stiffness and thus induces fractures at small values of applied strain. The finite bandgap caused by the BN-bonds leads to enhancement of the Seebeck coefficient and the figure of merit, and induces a redshift of a peak in the dielectric response. By increasing the distance between the B and N atoms i.e. for the BSi6N without BN-bonds, a flatter BSi6N is found compared to pristine silicene. The stiffness of the structure and the ultimate strain are increased. The breaking of the sublattice symmetry is very weak and a very small bandgap is revealed. As a result, the Seebeck coefficient and the figure of merit stay very small. A reduction in the optical response is seen due to an indirect bandgap."
    }
    
  14. Jóhannes Bergur Gunnarsson, Kristinn Torfason, Andrei Manolescu and Ágúst Valfells.
    Space-Charge Limited Current From a Finite Emitter in Nano- and Microdiodes.
    IEEE Transactions on Electron Devices 68, 342-346 (2021).
    Abstract We simulate numerically the classical charge dynamics in a microscopic, planar, vacuum diode with a finite emitter area and a finite number of electrons in the gap. We assume that electrons are emitted under space-charge limited conditions with a fixed potential applied to the diode. The Coulomb interaction between all electrons is included using the method of molecular dynamics. We compare our results to the conventional 2-D Child-Langmuir and explain how it is limited in applicability for submicrometer diameter emitters. Finally, we offer some simple relations for understanding space-charge limited flow from very small emitters.
    DOI BibTeX

    @article{9266079,
    	author = "Gunnarsson, Jóhannes Bergur and Torfason, Kristinn and Manolescu, Andrei and Valfells, Ágúst",
    	journal = "IEEE Transactions on Electron Devices",
    	title = "Space-Charge Limited Current From a Finite Emitter in Nano- and Microdiodes",
    	year = 2021,
    	volume = 68,
    	number = 1,
    	pages = "342-346",
    	doi = "10.1109/TED.2020.3037280",
    	abstract = "We simulate numerically the classical charge dynamics in a microscopic, planar, vacuum diode with a finite emitter area and a finite number of electrons in the gap. We assume that electrons are emitted under space-charge limited conditions with a fixed potential applied to the diode. The Coulomb interaction between all electrons is included using the method of molecular dynamics. We compare our results to the conventional 2-D Child-Langmuir and explain how it is limited in applicability for submicrometer diameter emitters. Finally, we offer some simple relations for understanding space-charge limited flow from very small emitters."
    }
    
  15. Aleš Cahlík, Jack Hellerstedt, Jesús I Mendieta-Moreno, Martin Švec, Vijai M Santhini, Simon Pascal, Diego Soler-Polo, Sigurdur I Erlingsson, Karel Výborný, Pingo Mutombo, Ondrej Marsalek, Olivier Siri and Pavel Jelínek.
    Significance Of Nuclear Quantum Effects In Hydrogen Bonded Molecular Chains.
    ACS Nano 15, 10357-10365 (2021).
    URL, DOI BibTeX

    @article{doi:10.1021/acsnano.1c02572,
    	author = "Cahlík, Aleš and Hellerstedt, Jack and Mendieta-Moreno, Jesús I. and Švec, Martin and Santhini, Vijai M. and Pascal, Simon and Soler-Polo, Diego and Erlingsson, Sigurdur I. and Výborný, Karel and Mutombo, Pingo and Marsalek, Ondrej and Siri, Olivier and Jelínek, Pavel",
    	title = "Significance Of Nuclear Quantum Effects In Hydrogen Bonded Molecular Chains",
    	journal = "ACS Nano",
    	volume = 15,
    	number = 6,
    	pages = "10357-10365",
    	year = 2021,
    	doi = "10.1021/acsnano.1c02572",
    	note = "PMID: 34033457",
    	url = "https://doi.org/10.1021/acsnano.1c02572",
    	eprint = "https://doi.org/10.1021/acsnano.1c02572"
    }
    
  16. Simon Wozny, Martin Leijnse and Sigurdur I Erlingsson.
    Dynamic impurities in two-dimensional topological-insulator edge states.
    Phys. Rev. B 104, 205418 (November 2021).
    URL, DOI BibTeX

    @article{PhysRevB.104.205418,
    	title = "Dynamic impurities in two-dimensional topological-insulator edge states",
    	author = "Wozny, Simon and Leijnse, Martin and Erlingsson, Sigurdur I.",
    	journal = "Phys. Rev. B",
    	volume = 104,
    	issue = 20,
    	pages = 205418,
    	numpages = 9,
    	year = 2021,
    	month = "Nov",
    	publisher = "American Physical Society",
    	doi = "10.1103/PhysRevB.104.205418",
    	url = "https://link.aps.org/doi/10.1103/PhysRevB.104.205418"
    }