Publications in 2017

1. 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,
title = "Controlled Coulomb effects in core-shell quantum rings",
author = "Sitek, Anna and Torres, Miguel Urbaneja and Torfason, Kristinn and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1704.06136",
arxiv = "https://arxiv.org/abs/1704.06136",
year = 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."
}

2. 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.
ChemArxiv (2017).
Abstract Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI(3-x)Cl(x)/spiro OMeTAD/Au configuration. We report normal(NH)and inverted hysteresis(IH) in the J-V characteristics occurring for the same device structure, the behavior strictly depending on the pre-poling bias. NH typically appears at pre-poling biases larger than the open circuit bias, while pronounced IH occurs for negative bias pre-poling. The transition from NH to IH is marked by a 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 (DEM). Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the non-linear polarization. Introducing a three step measurement protocol, which includes stabilization, pre-poling and measurement, we put forward the difficulties and possible solutions for a correct PCE evaluation.
URL arXiv BibTeX

@article{nemnes2016electronic,
title = "Normal and inverted hysteresis in perovskite solar cells",
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",
journal = "ChemArxiv",
url = "http://chemarxiv.org/id/eprint/243",
arxiv = "https://arxiv.org/abs/1704.03300",
year = 2017,
abstract = "Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI(3-x)Cl(x)/spiro OMeTAD/Au configuration. We report normal(NH)and inverted hysteresis(IH) in the J-V characteristics occurring for the same device structure, the behavior strictly depending on the pre-poling bias. NH typically appears at pre-poling biases larger than the open circuit bias, while pronounced IH occurs for negative bias pre-poling. The transition from NH to IH is marked by a 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 (DEM). Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the non-linear polarization. Introducing a three step measurement protocol, which includes stabilization, pre-poling and measurement, we put forward the difficulties and possible solutions for a correct PCE evaluation."
}

3. George Alexandru Nemnes, Camelia Visan and Andrei Manolescu.
Electronic and thermal conduction properties of halogenated porous graphene nanoribbons.
J. Mater. Chem. C, pages - (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 zig-zag 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, DOI BibTeX

@article{C7TC00029D,
author = "Nemnes, George Alexandru and Visan, Camelia and Manolescu, Andrei",
title = "Electronic and thermal conduction properties of halogenated porous graphene nanoribbons",
journal = "J. Mater. Chem. C",
year = 2017,
pages = "-",
publisher = "The Royal Society of Chemistry",
doi = "10.1039/C7TC00029D",
url = "http://dx.doi.org/10.1039/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 zig-zag 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."
}

4. Peng Zhang, Ágúst Valfells, L K Ang, J W Luginsland and Y Y Lau.
100 years of the physics of diodes.
Applied Physics Reviews 4, 011304 (2017).
Abstract The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the quantum regime, and to ultrafast processes. We review the influence of magnetic fields, multiple species in bipolar flow, electromagnetic and time dependent effects in both short pulse and high frequency THz limits, and single electron regimes. Transitions from various emission mechanisms (thermionic-, field-, and photoemission) to the space charge limited state (CL) will be addressed, especially highlighting the important simulation and experimental developments in selected contemporary areas of study. We stress the fundamental physical links between the physics of beams to limiting currents in other areas, such as low temperature plasmas, laser plasmas, and space propulsion.
URL, DOI BibTeX

@article{doi:10.1063/1.4978231,
author = "Peng Zhang and Ágúst Valfells and L. K. Ang and J. W. Luginsland and Y. Y. Lau",
title = "100 years of the physics of diodes",
journal = "Applied Physics Reviews",
volume = 4,
number = 1,
pages = 011304,
year = 2017,
doi = "10.1063/1.4978231",
url = "http://dx.doi.org/10.1063/1.4978231",
eprint = "http://dx.doi.org/10.1063/1.4978231",
abstract = "The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the quantum regime, and to ultrafast processes. We review the influence of magnetic fields, multiple species in bipolar flow, electromagnetic and time dependent effects in both short pulse and high frequency THz limits, and single electron regimes. Transitions from various emission mechanisms (thermionic-, field-, and photoemission) to the space charge limited state (CL) will be addressed, especially highlighting the important simulation and experimental developments in selected contemporary areas of study. We stress the fundamental physical links between the physics of beams to limiting currents in other areas, such as low temperature plasmas, laser plasmas, and space propulsion."
}

5. 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,
author = "Razvan Chirla and Cristian-Dorin Horea and Traian-Octavian Costea and Radu Dragomir and Andrei Manolescu and Cătălin Paşcu Moca",
title = "Shiba states coupled to a resonant cavity",
journal = "AIP Conference Proceedings",
volume = 1796,
number = 1,
pages = 030002,
year = 2017,
doi = "10.1063/1.4972367",
url = "http://aip.scitation.org/doi/abs/10.1063/1.4972367",
eprint = "http://aip.scitation.org/doi/pdf/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."
}

6. 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 (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,
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",
title = "Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state",
journal = "Annalen der Physik",
volume = 529,
number = "1-2",
issn = "1521-3889",
url = "http://dx.doi.org/10.1002/andp.201600177",
doi = "10.1002/andp.201600177",
pages = 1600177,
arxiv = "http://arxiv.org/abs/1605.08248",
year = 2017,
note = 1600177,
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."
}

7. 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,
title = "In-gap corner states in core-shell polygonal quantum rings",
author = "Sitek, Anna and Tolea, Mugurel and Nita, Marian and Serra, Lloren{$\c{c}$} and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "Scientific Reports",
volume = 7,
pages = 40197,
arxiv = "http://arxiv.org/abs/1607.02107",
year = 2017,
month = "Jan",
doi = "10.1038/srep40197",
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."
}