Publications in 2017

1. Gunnar Thorgilsson, Sigurdur I Erlingsson and Andrei Manolescu.
Thermoelectric current in tubular nanowires in transverse electric and magnetic fields.
Journal of Physics: Conference Series 906, 012021 (2017).
Abstract In the presence of a transverse magnetic field, the charge current in nanowires can flow from the hot to the cold reservoir, but also backwards. The sign change can be obtained by increasing the temperature bias or the magnetic field. This behavior occurs when the magnetic field is sufficiently strong. Here, we will investigate how the size of the anomalous backward-flowing current is affected by an electric field perpendicular to the nanowire. The interplay of the electric and magnetic field modifies the dispersion curves, which will show up in the transport properties. We will also investigate how the presence of impurities affects the anomalous current. The electric field affects backscattering due to impurities, and thus the thermoelectric current reversal. Preliminary results show that the current reversal can survive in the presence of impurities.
URL BibTeX

@article{1742-6596-906-1-012021 ,
abstract = "In the presence of a transverse magnetic field, the charge current in nanowires can flow from the hot to the cold reservoir, but also backwards. The sign change can be obtained by increasing the temperature bias or the magnetic field. This behavior occurs when the magnetic field is sufficiently strong. Here, we will investigate how the size of the anomalous backward-flowing current is affected by an electric field perpendicular to the nanowire. The interplay of the electric and magnetic field modifies the dispersion curves, which will show up in the transport properties. We will also investigate how the presence of impurities affects the anomalous current. The electric field affects backscattering due to impurities, and thus the thermoelectric current reversal. Preliminary results show that the current reversal can survive in the presence of impurities.",
author = "Gunnar Thorgilsson and Sigurdur I. Erlingsson and Andrei Manolescu",
journal = "Journal of Physics: Conference Series",
number = 1,
pages = 012021,
title = "Thermoelectric current in tubular nanowires in transverse electric and magnetic fields",
url = "http://stacks.iop.org/1742-6596/906/i=1/a=012021",
volume = 906,
year = 2017
}

2. A Slav, C Palade, I Stavarache, V S Teodorescu, M L Ciurea, R Müller, A Dinescu, M T Sultan, A Manolescu, J T Gudmundsson and H G Svavarsson.
Influence of preparation conditions on structure and photosensing properties of GeSi/TiO2 multilayers.
In 2017 International Semiconductor Conference (CAS) (). (October 2017), 63-66.
Abstract The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO2 cap/(GeSi/TiO2)2 multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO2 cap/GeSi/TiO2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5-10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20-30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.
DOI BibTeX

@inproceedings{8101154 ,
abstract = "The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO2 cap/(GeSi/TiO2)2 multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO2 cap/GeSi/TiO2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5-10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20-30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.",
author = "A. Slav and C. Palade and I. Stavarache and V. S. Teodorescu and M. L. Ciurea and R. Müller and A. Dinescu and M. T. Sultan and A. Manolescu and J. T. Gudmundsson and H. G. Svavarsson",
booktitle = "2017 International Semiconductor Conference (CAS)",
doi = "10.1109/SMICND.2017.8101154",
issn = "",
keywords = "Ge-Si alloys;X-ray diffraction;multilayers;nanofabrication;nanoparticles;nanostructured materials;photoconductivity;photodetectors;rapid thermal annealing;scanning electron microscopy;semiconductor materials;semiconductor superlattices;semiconductor thin films;sputter deposition;titanium compounds;transmission electron microscopy;GeSi-TiO2;SEM;TEM;XRD;high power impulse magnetron sputtering;multilayers;photocurrent measurement;photosensing properties;rapid thermal annealing;two-step annealing;visible-NIR spectroscopy;Annealing;Films;Nanocrystals;Nonhomogeneous media;Photoconductivity;Scanning electron microscopy;Silicon;GeSi nanocrystals;TiO2;photosensing",
month = "Oct",
number = "",
pages = "63-66",
title = "Influence of preparation conditions on structure and photosensing properties of GeSi/TiO2 multilayers",
volume = "",
year = 2017
}

3. Nzar Rauf Abdullah, Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Photon-induced tunability of the thermospin current in a Rashba ring.
arXiv preprint arXiv:1712.03386 (2017).
Abstract The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin-orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov-Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field.
arXiv BibTeX

@article{abdullah2017photon ,
abstract = "The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin-orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov-Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field.",
arxiv = "https://arxiv.org/abs/1712.03386",
author = "Abdullah, Nzar Rauf and Arnold, Thorsten and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "arXiv preprint arXiv:1712.03386",
title = "Photon-induced tunability of the thermospin current in a Rashba ring",
year = 2017
}

4. Vidar Gudmundsson, Thorsteinn H Jonsson, Maria Laura Bernodusson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state.
Annalen der Physik 529, 1600177–n/a (2017).
Abstract We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions.
URL arXiv, DOI BibTeX

@article{ANDP:ANDP201600177 ,
abstract = "We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions.",
arxiv = "https://arxiv.org/abs/1605.08248",
author = "Gudmundsson, Vidar and Jonsson, Thorsteinn H. and Bernodusson, Maria Laura and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
doi = "10.1002/andp.201600177",
issn = "1521-3889",
journal = "Annalen der Physik",
note = 1600177,
number = "1-2",
pages = "1600177--n/a",
title = "Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state",
url = "http://dx.doi.org/10.1002/andp.201600177",
volume = 529,
year = 2017
}

5. G A Nemnes, Camelia Visan and A Manolescu.
Electronic and thermal conduction properties of halogenated porous graphene nanoribbons.
J. Mater. Chem. C 5, 4435-4441 (2017).
Abstract We investigate the electronic and thermal properties of porous graphene (PG) structures passivated with halogen atoms as possible candidates for efficient thermoelectric devices in the framework of density functional theory (DFT) calculations. Armchair and zigzag halogenated PG nanoribbons are analyzed comparatively. The electronic properties are consistent with the expected behavior for the two types of terminations, however with marked influences introduced by the different halogen atoms. Depending on the pore sizes and halogen type pseudo-gaps in the phononic band structure are visible in the low frequency range, which are particularly important for the thermal conduction at low temperatures. The gaps are systematically displaced towards lower energies as the atomic number of the halogen increases. At the same time, the electronic gap decreases, which is also essential for attaining a large figure of merit in a thermoelectric device. This opens the possibility of tuning both electronic and thermal properties of PG structures by halogen passivation.
URL arXiv, DOI BibTeX

@article{C7TC00029D ,
abstract = "We investigate the electronic and thermal properties of porous graphene (PG) structures passivated with halogen atoms as possible candidates for efficient thermoelectric devices in the framework of density functional theory (DFT) calculations. Armchair and zigzag halogenated PG nanoribbons are analyzed comparatively. The electronic properties are consistent with the expected behavior for the two types of terminations{,} however with marked influences introduced by the different halogen atoms. Depending on the pore sizes and halogen type pseudo-gaps in the phononic band structure are visible in the low frequency range{,} which are particularly important for the thermal conduction at low temperatures. The gaps are systematically displaced towards lower energies as the atomic number of the halogen increases. At the same time{,} the electronic gap decreases{,} which is also essential for attaining a large figure of merit in a thermoelectric device. This opens the possibility of tuning both electronic and thermal properties of PG structures by halogen passivation.",
arxiv = "https://arxiv.org/abs/1612.07949",
author = "Nemnes, G. A. and Visan, Camelia and Manolescu, A.",
doi = "10.1039/C7TC00029D",
issue = 18,
journal = "J. Mater. Chem. C",
pages = "4435-4441",
publisher = "The Royal Society of Chemistry",
title = "Electronic and thermal conduction properties of halogenated porous graphene nanoribbons",
url = "http://dx.doi.org/10.1039/C7TC00029D",
volume = 5,
year = 2017
}

6. Gudmundsson Vidar, Abdulla Nzar Rauf, Sitek Anna, Goan Hsi-Sheng, Tang Chi-Shung and Manolescu Andrei.
Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity.
Annalen der Physik 530, 1700334 (2017).
Abstract Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.
URL arXiv, DOI BibTeX

@article{doi10.1002andp201700334 ,
abstract = "Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.",
arxiv = "https://arxiv.org/abs/1706.03483",
author = "Gudmundsson Vidar and Abdulla Nzar Rauf and Sitek Anna and Goan Hsi-Sheng and Tang Chi-Shung and Manolescu Andrei",
doi = "10.1002/andp.201700334",
eprint = "https://onlinelibrary.wiley.com/doi/pdf/10.1002/andp.201700334",
journal = "Annalen der Physik",
keywords = "configuration interactions, electroluminescence, electron transport, photon cavity, photon correlations",
number = 2,
pages = 1700334,
title = "Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity",
url = "https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.201700334",
volume = 530,
year = 2017
}

7. Neculai Plugaru, George Alexandru Nemnes, Lucian Filip, Ioana Pintilie, Lucian Pintilie, Keith Tobias Butler and Andrei Manolescu.
Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO3 (001) Surfaces.
The Journal of Physical Chemistry C 121, 9096-9109 (2017).
Abstract The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic–inorganic materials and related devices, particularly based on CH3NH3PbI3–xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3–xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)+ dipoles.
URL, DOI BibTeX

@article{doi:10.1021/acs.jpcc.7b00399 ,
abstract = "The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20\% in the laboratory, has strongly motivated research on this class of organic–inorganic materials and related devices, particularly based on CH3NH3PbI3–xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3–xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)+ dipoles.",
author = "Plugaru, Neculai and Nemnes, George Alexandru and Filip, Lucian and Pintilie, Ioana and Pintilie, Lucian and Butler, Keith Tobias and Manolescu, Andrei",
doi = "10.1021/acs.jpcc.7b00399",
eprint = "http://dx.doi.org/10.1021/acs.jpcc.7b00399",
journal = "The Journal of Physical Chemistry C",
number = 17,
pages = "9096-9109",
title = "Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO3 (001) Surfaces",
url = "http://dx.doi.org/10.1021/acs.jpcc.7b00399",
volume = 121,
year = 2017
}

8. George Alexandru Nemnes, Cristina Besleaga, Viorica Stancu, Daniela Emilia Dogaru, Lucia Nicoleta Leonat, Lucian Pintilie, Kristinn Torfason, Marjan Ilkov, Andrei Manolescu and Ioana Pintilie.
Normal and Inverted Hysteresis in Perovskite Solar Cells.
The Journal of Physical Chemistry C 121, 11207-11214 (2017).
Abstract Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3–xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J–V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J–V characteristics. The measured J–V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and possible solutions for a correct photoconversion efficiency evaluation.
URL, DOI BibTeX

@article{doi:10.1021/acs.jpcc.7b04248 ,
abstract = "Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3–xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J–V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J–V characteristics. The measured J–V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and possible solutions for a correct photoconversion efficiency evaluation.",
author = "Nemnes, George Alexandru and Besleaga, Cristina and Stancu, Viorica and Dogaru, Daniela Emilia and Leonat, Lucia Nicoleta and Pintilie, Lucian and Torfason, Kristinn and Ilkov, Marjan and Manolescu, Andrei and Pintilie, Ioana",
doi = "10.1021/acs.jpcc.7b04248",
eprint = "http://dx.doi.org/10.1021/acs.jpcc.7b04248",
journal = "The Journal of Physical Chemistry C",
number = 21,
pages = "11207-11214",
title = "Normal and Inverted Hysteresis in Perovskite Solar Cells",
url = "http://dx.doi.org/10.1021/acs.jpcc.7b04248",
volume = 121,
year = 2017
}

9. Razvan Chirla, Cristian-Dorin Horea, Traian-Octavian Costea, Radu Dragomir, Andrei Manolescu and Cătălin Paşcu Moca.
Shiba states coupled to a resonant cavity.
AIP Conference Proceedings 1796, 030002 (2017).
Abstract When a quantum dot is embedded into a superconducting environment, it leads to the formations of localized Shiba states inside the gap. If the Coulomb interaction is sufficiently small, the Shiba states consist of a pair of singlet states and a doublet, that compete for the ground state and induce a quantum phase transition. In the presence of an external microwave field, the Shiba energy spectrum is significantly modified. Moreover, the transmission of the cavity inherits features that can pinpoint the exact location of the quantum critical point. In terms of methods used, our analytical calculations are supplemented by state of the art numerical renormalization group calculations.
URL, DOI BibTeX

@article{doi:10.1063/1.4972367 ,
abstract = "When a quantum dot is embedded into a superconducting environment, it leads to the formations of localized Shiba states inside the gap. If the Coulomb interaction is sufficiently small, the Shiba states consist of a pair of singlet states and a doublet, that compete for the ground state and induce a quantum phase transition. In the presence of an external microwave field, the Shiba energy spectrum is significantly modified. Moreover, the transmission of the cavity inherits features that can pinpoint the exact location of the quantum critical point. In terms of methods used, our analytical calculations are supplemented by state of the art numerical renormalization group calculations.",
author = "Razvan Chirla and Cristian-Dorin Horea and Traian-Octavian Costea and Radu Dragomir and Andrei Manolescu and Cătălin Paşcu Moca",
doi = "10.1063/1.4972367",
eprint = "http://aip.scitation.org/doi/pdf/10.1063/1.4972367",
journal = "AIP Conference Proceedings",
number = 1,
pages = 030002,
title = "Shiba states coupled to a resonant cavity",
url = "http://aip.scitation.org/doi/abs/10.1063/1.4972367",
volume = 1796,
year = 2017
}

10. Halldor G Svavarsson, Johannes E Valberg, Hronn Arnardottir and Asa Brynjolfsdottir.
Carbon dioxide from geothermal gas converted to biomass by cultivating coccoid cyanobacteria.
Environmental Technology 0, 1-8 (2017).
Abstract ABSTRACTThe Blue Lagoon is a geothermal aquifer with a diverse ecosystem located within the Reykjanes UNESCO Global Geopark on Iceland’s Reykjanes Peninsula. Blue Lagoon Ltd., which exploits the aquifer, isolated a strain of coccoid cyanobacteria Cyanobacterium aponinum (C. aponinum) from the geothermal fluid of the Blue Lagoon more than two decades ago. Since then Blue Lagoon Ltd. has cultivated it in a photobioreactor, for use as an active ingredient in its skin care products. Until recently, the cultivation of C. aponinum was achieved by feeding it on 99.99% (4N) bottled carbon dioxide (CO2). In this investigation, C. aponinum was cultivated using unmodified, non-condensable geothermal gas (geogas) emitted from a nearby geothermal powerplant as the feed-gas instead of the 4N-gas. The geogas contains roughly 90% vol CO2 and 2% vol hydrogen sulfide (H2S). A comparison of both CO2 sources was made. It was observed that the use of geogas did enhance the conversion efficiency. A 13 weeks’ average CO2 conversion efficiency of C. aponinum was 43% and 31% when fed on geogas and 4N-gas, respectively. Despite the high H2S concentration in the geogas, sulfur accumulation in the cultivated biomass was similar for both gas sources. Our results provide a model of a CO2 sequestration by photosynthetic conversion of otherwise unused geothermal emission gas into biomass.
URL, DOI BibTeX

@article{doi:10.1080/09593330.2017.1349840 ,
abstract = "ABSTRACTThe Blue Lagoon is a geothermal aquifer with a diverse ecosystem located within the Reykjanes UNESCO Global Geopark on Iceland’s Reykjanes Peninsula. Blue Lagoon Ltd., which exploits the aquifer, isolated a strain of coccoid cyanobacteria Cyanobacterium aponinum (C. aponinum) from the geothermal fluid of the Blue Lagoon more than two decades ago. Since then Blue Lagoon Ltd. has cultivated it in a photobioreactor, for use as an active ingredient in its skin care products. Until recently, the cultivation of C. aponinum was achieved by feeding it on 99.99\% (4N) bottled carbon dioxide (CO2). In this investigation, C. aponinum was cultivated using unmodified, non-condensable geothermal gas (geogas) emitted from a nearby geothermal powerplant as the feed-gas instead of the 4N-gas. The geogas contains roughly 90\% vol CO2 and 2\% vol hydrogen sulfide (H2S). A comparison of both CO2 sources was made. It was observed that the use of geogas did enhance the conversion efficiency. A 13 weeks’ average CO2 conversion efficiency of C. aponinum was 43\% and 31\% when fed on geogas and 4N-gas, respectively. Despite the high H2S concentration in the geogas, sulfur accumulation in the cultivated biomass was similar for both gas sources. Our results provide a model of a CO2 sequestration by photosynthetic conversion of otherwise unused geothermal emission gas into biomass.",
author = "Halldor G. Svavarsson and Johannes E. Valberg and Hronn Arnardottir and Asa Brynjolfsdottir",
doi = "10.1080/09593330.2017.1349840",
eprint = "https://doi.org/10.1080/09593330.2017.1349840",
journal = "Environmental Technology",
note = "PMID: 28662603",
number = 0,
pages = "1-8",
publisher = "Taylor & Francis",
title = "Carbon dioxide from geothermal gas converted to biomass by cultivating coccoid cyanobacteria",
url = "https://doi.org/10.1080/09593330.2017.1349840",
volume = 0,
year = 2017
}

11. Vidar Gudmundsson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Electroluminescence caused by the transport of interacting electrons through parallel quantum dots in a photon cavity.
arXiv preprint arXiv:1706.03483 (2017).
Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.
arXiv BibTeX

@article{gudmundsson2017electroluminescence ,
abstract = "We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.",
arxiv = "https://arxiv.org/abs/1706.03483",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1706.03483",
title = "Electroluminescence caused by the transport of interacting electrons through parallel quantum dots in a photon cavity",
year = 2017
}

12. Y Li, E Júlíusson, H Pálsson, H Stefánsson and Á Valfells.
Machine learning for creation of generalized lumped parameter tank models of low temperature geothermal reservoir systems.
Geothermics 70, 62 - 84 (2017).
Abstract Abstract Lumped parameter tank models have gained renewed interest in recent years as an alternative tool for geothermal reservoir analysis and production planning. The models can be structured in various ways regarding the number of tanks, connections between the tanks and the parameters representing the physical properties of the geothermal system. It usually requires a time consuming and difficult process of trials and errors to manually decide the optimal configuration of a tank model. Inspired by recent development in the use of machine learning methods, we propose a method for automatically generating accurate and computationally feasible generalized tank models for isothermal, single phase, reservoirs. This is an extension of earlier work on complexity reduction of generalized tank models (Li et al., 2016). Here, a recursive “switch-back” method is constructed to maximize prediction accuracy of the model. It is also shown how the K-means clustering algorithm can be used to aggregate production wells in generalized tank models. One synthetic example and one field application from t Reykir geothermal fields in Iceland are used to illustrate the effectiveness of these methods.
URL, DOI BibTeX

@article{LI201762 ,
abstract = "Abstract Lumped parameter tank models have gained renewed interest in recent years as an alternative tool for geothermal reservoir analysis and production planning. The models can be structured in various ways regarding the number of tanks, connections between the tanks and the parameters representing the physical properties of the geothermal system. It usually requires a time consuming and difficult process of trials and errors to manually decide the optimal configuration of a tank model. Inspired by recent development in the use of machine learning methods, we propose a method for automatically generating accurate and computationally feasible generalized tank models for isothermal, single phase, reservoirs. This is an extension of earlier work on complexity reduction of generalized tank models (Li et al., 2016). Here, a recursive “switch-back” method is constructed to maximize prediction accuracy of the model. It is also shown how the K-means clustering algorithm can be used to aggregate production wells in generalized tank models. One synthetic example and one field application from t Reykir geothermal fields in Iceland are used to illustrate the effectiveness of these methods.",
author = "Y. Li and E. Júlíusson and H. Pálsson and H. Stefánsson and Á. Valfells",
doi = "doi.org/10.1016/j.geothermics.2017.05.009",
issn = "0375-6505",
journal = "Geothermics",
keywords = "Geothermal reservoir modeling, Lumped-parameter models, Machine learning, Complexity analysis, Information criterion, Complexity reduction algorithm, Switch-back method, Model selection, -means clustering algorithm",
pages = "62 - 84",
title = "Machine learning for creation of generalized lumped parameter tank models of low temperature geothermal reservoir systems",
url = "http://www.sciencedirect.com/science/article/pii/S0375650517300639",
volume = 70,
year = 2017
}

13. G A Nemnes, Cristina Besleaga, A G Tomulescu, Ioana Pintilie, L Pintilie, K Torfason and A Manolescu.
Dynamic electrical behavior of halide perovskite based solar cells.
Solar Energy Materials and Solar Cells 159, 197 - 203 (2017).
Abstract Abstract A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions.
URL arXiv, DOI BibTeX

@article{NEMNES2017197 ,
abstract = "Abstract A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions.",
arxiv = "http://arxiv.org/abs/1606.00335",
author = "G.A. Nemnes and Cristina Besleaga and A.G. Tomulescu and Ioana Pintilie and L. Pintilie and K. Torfason and A. Manolescu",
doi = "https://doi.org/10.1016/j.solmat.2016.09.012",
issn = "0927-0248",
journal = "Solar Energy Materials and Solar Cells",
keywords = "Perovskite solar cell, Hysteresis, Dynamic electrical model",
pages = "197 - 203",
title = "Dynamic electrical behavior of halide perovskite based solar cells",
url = "http://www.sciencedirect.com/science/article/pii/S0927024816303531",
volume = 159,
year = 2017
}

14. Vidar Gudmundsson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Time-dependent current into and through multilevel parallel quantum dots in a photon cavity.
Phys. Rev. B 95, 195307 (May 2017).
Abstract We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots embedded in a short wire placed in a photon cavity. A plunger gate is used to place specific many-body states of the interacting system in the bias window defined by the external leads. We show how the transport phenomena active in the many-level complex central system strongly depend on the gate voltage. We identify a resonant transport through the central system as the two spin components of the one-electron ground state are in the bias window. This resonant transport through the lowest energy electron states seems to a large extent independent of the detuned photon field when judged from the transport current. This could be expected in the small bias regime, but an observation of the occupancy of the states of the system reveals that this picture is not entirely true. The current does not reflect slower photon-active internal transitions bringing the system into the steady state. The number of initially present photons determines when the system reaches the real steady state. With two-electron states in the bias window we observe a more complex situation with intermediate radiative and nonradiative relaxation channels leading to a steady state with a weak nonresonant current caused by inelastic tunneling through the two-electron ground state of the system. The presence of the radiative channels makes this phenomena dependent on the number of photons initially in the cavity.
URL arXiv, DOI BibTeX

@article{PhysRevB.95.195307 ,
abstract = "We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots embedded in a short wire placed in a photon cavity. A plunger gate is used to place specific many-body states of the interacting system in the bias window defined by the external leads. We show how the transport phenomena active in the many-level complex central system strongly depend on the gate voltage. We identify a resonant transport through the central system as the two spin components of the one-electron ground state are in the bias window. This resonant transport through the lowest energy electron states seems to a large extent independent of the detuned photon field when judged from the transport current. This could be expected in the small bias regime, but an observation of the occupancy of the states of the system reveals that this picture is not entirely true. The current does not reflect slower photon-active internal transitions bringing the system into the steady state. The number of initially present photons determines when the system reaches the real steady state. With two-electron states in the bias window we observe a more complex situation with intermediate radiative and nonradiative relaxation channels leading to a steady state with a weak nonresonant current caused by inelastic tunneling through the two-electron ground state of the system. The presence of the radiative channels makes this phenomena dependent on the number of photons initially in the cavity.",
arxiv = "https://arxiv.org/abs/1611.09453",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
doi = "10.1103/PhysRevB.95.195307",
issue = 19,
journal = "Phys. Rev. B",
month = "May",
numpages = 12,
pages = 195307,
publisher = "American Physical Society",
title = "Time-dependent current into and through multilevel parallel quantum dots in a photon cavity",
volume = 95,
year = 2017
}

15. Andrei Manolescu, Anna Sitek, Javier Osca, Lloren$ç$ Serra, Vidar Gudmundsson and Tudor Dan Stanescu.
Majorana states in prismatic core-shell nanowires.
Phys. Rev. B 96, 125435 (September 2017).
Abstract We consider core-shell nanowires with conductive shell and insulating core and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e., along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e., thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell.
URL arXiv, DOI BibTeX

@article{PhysRevB.96.125435 ,
abstract = "We consider core-shell nanowires with conductive shell and insulating core and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e., along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e., thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell.",
arxiv = "https://arxiv.org/abs/1705.04950",
author = "Manolescu, Andrei and Sitek, Anna and Osca, Javier and Serra, Lloren{$\c{c}$} and Gudmundsson, Vidar and Stanescu, Tudor Dan",
doi = "10.1103/PhysRevB.96.125435",
issue = 12,
journal = "Phys. Rev. B",
month = "Sep",
numpages = 13,
pages = 125435,
publisher = "American Physical Society",
title = "Majorana states in prismatic core-shell nanowires",
volume = 96,
year = 2017
}

16. M Ni{ţ\ua}, M Ţolea, D C Marinescu and A Manolescu.
Hund and anti-Hund rules in circular molecules.
Phys. Rev. B 96, 235101 (December 2017).
Abstract We study the validity of Hund's first rule for the spin multiplicity in circular molecules—made of real or artificial atoms such as quantum dots—by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction.
URL, DOI BibTeX

@article{PhysRevB.96.235101 ,
abstract = "We study the validity of Hund's first rule for the spin multiplicity in circular molecules—made of real or artificial atoms such as quantum dots—by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction.",
author = "Ni{\c{t}\u{a}}, M. and {\c{T}}olea, M. and Marinescu, D. C. and Manolescu, A.",
doi = "10.1103/PhysRevB.96.235101",
issue = 23,
journal = "Phys. Rev. B",
month = "Dec",
numpages = 7,
pages = 235101,
publisher = "American Physical Society",
title = "Hund and anti-Hund rules in circular molecules",
volume = 96,
year = 2017
}

17. Sigurdur I Erlingsson, Andrei Manolescu, George Alexandru Nemnes, Jens H Bardarson and David Sanchez.
Reversal of Thermoelectric Current in Tubular Nanowires.
Phys. Rev. Lett. 119, 036804 (July 2017).
Abstract We calculate the charge current generated by a temperature bias between the two ends of a tubular nanowire. We show that in the presence of a transversal magnetic field the current can change sign; i.e., electrons can either flow from the hot to the cold reservoir, or in the opposite direction, when the temperature bias increases. This behavior occurs when the magnetic field is sufficiently strong, such that Landau and snaking states are created, and the energy dispersion is nonmonotonic with respect to the longitudinal wave vector. The sign reversal can survive in the presence of impurities. We predict this result for core-shell nanowires, for uniform nanowires with surface states due to the Fermi level pinning, and for topological insulator nanowires.
URL arXiv, DOI BibTeX

@article{PhysRevLett.119.036804 ,
abstract = "We calculate the charge current generated by a temperature bias between the two ends of a tubular nanowire. We show that in the presence of a transversal magnetic field the current can change sign; i.e., electrons can either flow from the hot to the cold reservoir, or in the opposite direction, when the temperature bias increases. This behavior occurs when the magnetic field is sufficiently strong, such that Landau and snaking states are created, and the energy dispersion is nonmonotonic with respect to the longitudinal wave vector. The sign reversal can survive in the presence of impurities. We predict this result for core-shell nanowires, for uniform nanowires with surface states due to the Fermi level pinning, and for topological insulator nanowires.",
arxiv = "https://arxiv.org/abs/1705.01569",
author = "Erlingsson, Sigurdur I. and Manolescu, Andrei and Nemnes, George Alexandru and Bardarson, Jens H. and Sanchez, David",
doi = "10.1103/PhysRevLett.119.036804",
issue = 3,
journal = "Phys. Rev. Lett.",
month = "Jul",
numpages = 6,
pages = 036804,
publisher = "American Physical Society",
title = "Reversal of Thermoelectric Current in Tubular Nanowires",
volume = 119,
year = 2017
}

18. Anna Sitek, Miguel Urbaneja Torres, Kristinn Torfason, Vidar Gudmundsson and Andrei Manolescu.
Controlled Coulomb effects in core-shell quantum rings.
arXiv preprint arXiv:1704.06136 (2017).
Abstract We analyse theoretically the possibilities of contactless control of in-gap states formed by a pair of electrons confined in a triangular quantum ring. The in-gap states are corner-localized states associated with two electrons occupying the same corner area, and thus shifted to much higher energies than other corner states, but still they are below the energies of corner-side-localized states. We show how the energies, degeneracy and splittings between consecutive levels change with the orientation of an external electric field relatively to the polygonal cross section. We also show how absorption changes in the presence of external electric and magnetic fields.
arXiv BibTeX

@article{sitek2017controlled ,
abstract = "We analyse theoretically the possibilities of contactless control of in-gap states formed by a pair of electrons confined in a triangular quantum ring. The in-gap states are corner-localized states associated with two electrons occupying the same corner area, and thus shifted to much higher energies than other corner states, but still they are below the energies of corner-side-localized states. We show how the energies, degeneracy and splittings between consecutive levels change with the orientation of an external electric field relatively to the polygonal cross section. We also show how absorption changes in the presence of external electric and magnetic fields.",
arxiv = "https://arxiv.org/abs/1704.06136",
author = "Sitek, Anna and Torres, Miguel Urbaneja and Torfason, Kristinn and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1704.06136",
title = "Controlled Coulomb effects in core-shell quantum rings",
year = 2017
}

19. Anna Sitek, Mugurel Tolea, Marian Nita, Lloren$ç$ Serra, Vidar Gudmundsson and Andrei Manolescu.
In-gap corner states in core-shell polygonal quantum rings.
Scientific Reports 7, 40197 (January 2017).
Abstract We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states.
arXiv, DOI BibTeX

@article{sitek2017gap ,
abstract = "We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states.",
arxiv = "http://arxiv.org/abs/1607.02107",
author = "Sitek, Anna and Tolea, Mugurel and Nita, Marian and Serra, Lloren{$\c{c}$} and Gudmundsson, Vidar and Manolescu, Andrei",
doi = "10.1038/srep40197",
journal = "Scientific Reports",
month = "Jan",
pages = 40197,
title = "In-gap corner states in core-shell polygonal quantum rings",
volume = 7,
year = 2017
}