Publications in 2020

1. Kristján Óttar Klausen, Anna Sitek, Sigurdur Ingi Erlingsson and Andrei Manolescu.
Majorana Zero Modes in Nanowires with Combined Triangular and Hexagonal Geometry.
Nanotechnology (2020).
Abstract The effects of geometry on the hosting of Majorana zero modes are explored in core-shell nanowires with a hexagonal core and a triangular shell, and vice versa. The energy interval separating electronic states localized in the corners from states localized on the sides of the shell is shown to be larger for a triangular nanowire with a hexagonal core, than a triangular one. We build the topological phase diagram for both cases and compare them to earlier work on prismatic nanowires with the same core and shell geometry. We suggest that a dual core nanowire is needed to allow for braiding operation of Majorana zero modes at the nanowire end plane.
URL, DOI BibTeX

@article{10.1088/1361-6528/ab932e ,
abstract = "The effects of geometry on the hosting of Majorana zero modes are explored in core-shell nanowires with a hexagonal core and a triangular shell, and vice versa. The energy interval separating electronic states localized in the corners from states localized on the sides of the shell is shown to be larger for a triangular nanowire with a hexagonal core, than a triangular one. We build the topological phase diagram for both cases and compare them to earlier work on prismatic nanowires with the same core and shell geometry. We suggest that a dual core nanowire is needed to allow for braiding operation of Majorana zero modes at the nanowire end plane.",
author = "Kristján Óttar Klausen and Anna Sitek and Sigurdur Ingi Erlingsson and Andrei Manolescu",
doi = "10.1088/1361-6528",
journal = "Nanotechnology",
title = "Majorana Zero Modes in Nanowires with Combined Triangular and Hexagonal Geometry",
url = "http://iopscience.iop.org/10.1088/1361-6528/ab932e",
year = 2020
}

2. Robert Magnusson, Kyu J Lee, Hafez Hemmati, Pawarat Bootpakdeetam, Jonathan Vasilyev, Fairooz A Simlan, Nasrin Razmjooei, Yeong Hwan Ko, Shanwen Zhang, Sun-Goo Lee and Halldor G Svavarsson.
Properties of resonant photonic lattices: Bloch mode dynamics, band flips, and applications.
In Connie J Chang-Hasnain, Andrei Faraon and Weimin Zhou (eds.). High Contrast Metastructures IX 11290. (2020), 1 – 10.
Abstract We review guided-mode resonant photonic lattices by addressing their functionalities and potential device applications. The 1D canonical model is rich in properties and conceptually transparent, with all the main conclusions being applicable to 2D metasurfaces and periodic photonic slabs. We explain the operative physical mechanisms grounded in lateral leaky Bloch modes. We summarize the band dynamics of the leaky stopband. With several examples, we demonstrate that Mie scattering is not causative in resonant reflection. Illustrated applications include a wideband reflector at infrared bands as well as resonant reflectors with triangular profiles. We quantify the improved efficiency of a silicon reflector operating in the visible region relative to loss reduction as realizable with sample hydrogenation. A resonant polarizer with record performance is presented.
URL, DOI BibTeX

@inproceedings{10.1117/12.2547322 ,
abstract = "We review guided-mode resonant photonic lattices by addressing their functionalities and potential device applications. The 1D canonical model is rich in properties and conceptually transparent, with all the main conclusions being applicable to 2D metasurfaces and periodic photonic slabs. We explain the operative physical mechanisms grounded in lateral leaky Bloch modes. We summarize the band dynamics of the leaky stopband. With several examples, we demonstrate that Mie scattering is not causative in resonant reflection. Illustrated applications include a wideband reflector at infrared bands as well as resonant reflectors with triangular profiles. We quantify the improved efficiency of a silicon reflector operating in the visible region relative to loss reduction as realizable with sample hydrogenation. A resonant polarizer with record performance is presented.",
author = "Robert Magnusson and Kyu J. Lee and Hafez Hemmati and Pawarat Bootpakdeetam and Jonathan Vasilyev and Fairooz A. Simlan and Nasrin Razmjooei and Yeong Hwan Ko and Shanwen Zhang and Sun-Goo Lee and Halldor G. Svavarsson",
booktitle = "High Contrast Metastructures IX",
doi = "10.1117/12.2547322",
editor = "Connie J. Chang-Hasnain and Andrei Faraon and Weimin Zhou",
keywords = "guided-mode resonance, leaky-mode resonance, resonant waveguide gratings, periodic photonic films, metasurfaces, leaky-band dynamics, subwavelength nanophotonics, metamaterials",
organization = "International Society for Optics and Photonics",
pages = "1 -- 10",
publisher = "SPIE",
title = "{Properties of resonant photonic lattices: Bloch mode dynamics, band flips, and applications}",
url = "https://doi.org/10.1117/12.2547322",
volume = 11290,
year = 2020
}

3. H V Haraldsson, K Torfason, A Manolescu and Á Valfells.
Molecular Dynamics Simulations of Mutual Space-Charge Effect Between Planar Field Emitters.
IEEE Transactions on Plasma Science , 1-7 (2020).
Abstract Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space-charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space-charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given.
DOI BibTeX

@article{9093900 ,
abstract = "Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space-charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space-charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given.",
author = "H. V. {Haraldsson} and K. {Torfason} and A. {Manolescu} and Á. {Valfells}",
doi = "10.1109/TPS.2020.2991582",
journal = "IEEE Transactions on Plasma Science",
number = "",
pages = "1-7",
title = "Molecular Dynamics Simulations of Mutual Space-Charge Effect Between Planar Field Emitters",
volume = "",
year = 2020
}

4. Nzar Rauf Abdullah, Hunar Omar Rashid, Mohammad T Kareem, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Effects of bonded and non-bonded B/N codoping of graphene on its stability, interaction energy, electronic structure, and power factor.
Physics Letters A 384, 126350 (2020).
Abstract We model boron and nitrogen doped/codoped monolayer graphene to study its stability, interaction energy, electronic and thermal properties using density functional theory. It is found that a doped graphene sheet with non-bonded B or N atoms induces an attractive interaction and thus opens up the bandgap. Consequently, the power factor is enhanced. Additionally, bonded B or N atoms in doped graphene generate a repulsive interaction leading to a diminished bandgap, and thus a decreased power factor. We emphasis that enhancement of the power factor is not very sensitive to the concentration of the boron and nitrogen atoms, but it is more sensitive to the positions of the B or N atoms in ortho, meta, and para positions of the hexagonal structure of graphene. In the B and N codoped graphene, the non-bonded dopant atoms have a weak attractive interaction and interaction leading to a small bandgap, while bonded doping atoms cause a strong attractive interaction and a large bandgap. As a result, the power factor of the graphene with non-bonded doping atoms is reduced while it is enhanced for graphene with bonded doping atoms.
URL, DOI BibTeX

@article{ABDULLAH2020126350 ,
abstract = "We model boron and nitrogen doped/codoped monolayer graphene to study its stability, interaction energy, electronic and thermal properties using density functional theory. It is found that a doped graphene sheet with non-bonded B or N atoms induces an attractive interaction and thus opens up the bandgap. Consequently, the power factor is enhanced. Additionally, bonded B or N atoms in doped graphene generate a repulsive interaction leading to a diminished bandgap, and thus a decreased power factor. We emphasis that enhancement of the power factor is not very sensitive to the concentration of the boron and nitrogen atoms, but it is more sensitive to the positions of the B or N atoms in ortho, meta, and para positions of the hexagonal structure of graphene. In the B and N codoped graphene, the non-bonded dopant atoms have a weak attractive interaction and interaction leading to a small bandgap, while bonded doping atoms cause a strong attractive interaction and a large bandgap. As a result, the power factor of the graphene with non-bonded doping atoms is reduced while it is enhanced for graphene with bonded doping atoms.",
author = "Nzar Rauf Abdullah and Hunar Omar Rashid and Mohammad T. Kareem and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
doi = "https://doi.org/10.1016/j.physleta.2020.126350",
issn = "0375-9601",
journal = "Physics Letters A",
keywords = "Energy harvesting, Thermal transport, Graphene, Density functional theory, Electronic structure",
number = 12,
pages = 126350,
title = "Effects of bonded and non-bonded B/N codoping of graphene on its stability, interaction energy, electronic structure, and power factor",
url = "http://www.sciencedirect.com/science/article/pii/S0375960120301602",
volume = 384,
year = 2020
}

5. Nzar Rauf Abdullah, Hunar Omar Rashid, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Modeling electronic, mechanical, optical and thermal properties of graphene-like BC $_6$ N materials: Role of prominent BN-bonds.
arXiv preprint arXiv:2003.08467 (2020).
Abstract We model monolayer graphene-like materials with BC6N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC6N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC6N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC6N without BN-bonds has more dynamical stability. Furthermore, all the BC6N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC6N systems caused by hot carrier–assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices.
arXiv BibTeX

@article{abdullah2020modeling ,
abstract = "We model monolayer graphene-like materials with BC6N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC6N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC6N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC6N without BN-bonds has more dynamical stability. Furthermore, all the BC6N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC6N systems caused by hot carrier--assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices.",
arxiv = "https://arxiv.org/abs/2003.08467",
author = "Abdullah, Nzar Rauf and Rashid, Hunar Omar and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "arXiv preprint arXiv:2003.08467",
title = "Modeling electronic, mechanical, optical and thermal properties of graphene-like BC $\_6$ N materials: Role of prominent BN-bonds",
year = 2020
}

6. 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.
arXiv preprint arXiv:2005.10914 (2020).
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.
arXiv BibTeX

@article{gudmundsson2020self ,
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.",
arxiv = "https://arxiv.org/abs/2005.10914",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Moldoveanu, Valeriu",
journal = "arXiv preprint arXiv:2005.10914",
title = "Self-induction and magnetic effects in electron transport through a photon cavity",
year = 2020
}

7. Ionel Stavarache, Constantin Logofatu, Muhammad Taha Sultan, Andrei Manolescu, Halldor Gudfinnur Svavarsson, Valentin Serban Teodorescu and Magdalena Lidia Ciurea.
SiGe nanocrystals in SiO 2 with high photosensitivity from visible to short-wave infrared.
Scientific Reports 10, 1–9 (2020).
Abstract Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.
DOI BibTeX

@article{stavarache2020sige ,
abstract = "Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.",
author = "Stavarache, Ionel and Logofatu, Constantin and Sultan, Muhammad Taha and Manolescu, Andrei and Svavarsson, Halldor Gudfinnur and Teodorescu, Valentin Serban and Ciurea, Magdalena Lidia",
doi = "10.1038/s41598-020-60000-x",
journal = "Scientific Reports",
number = 1,
pages = "1--9",
publisher = "Nature Publishing Group",
title = "SiGe nanocrystals in SiO 2 with high photosensitivity from visible to short-wave infrared",
volume = 10,
year = 2020
}

8. M T Sultan, J T Gudmundsson, A Manolescu, V S Teodorescu, M L Ciurea and H G Svavarsson.
Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing.
Applied Surface Science 511, 145552 (2020).
Abstract Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.
URL, DOI BibTeX

@article{SULTAN2020145552 ,
abstract = "Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.",
author = "M.T. Sultan and J.T. Gudmundsson and A. Manolescu and V.S. Teodorescu and M.L. Ciurea and H.G. Svavarsson",
doi = "10.1016/j.apsusc.2020.145552",
issn = "0169-4332",
journal = "Applied Surface Science",
keywords = "TiO, SiGe, Nanoparticles, HiPIMS, GiXRD, Interface, Photo-spectra",
pages = 145552,
title = "Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing",
url = "http://www.sciencedirect.com/science/article/pii/S0169433220303081",
volume = 511,
year = 2020
}