Publications in 2013

1. T Khaleque, H G Svavarsson and R Magnusson.
Fabrication of nano-imprinted resonant structures for thin-film solar cell applications.
In Photonics Conference (IPC), 2013 IEEE. (2013), 580-581.
Abstract A simple fabrication approach of replicating nanoscale patterns with 300-nm periods on thermoplastic polymethyl methacrylate substrates using thermal nano-imprint lithography is reported. The resonant structures hold potential application in light trapping for thin-film solar cells.
DOI BibTeX

@inproceedings{6656696,
author = "Khaleque, T. and Svavarsson, H.G. and Magnusson, R.",
booktitle = "Photonics Conference (IPC), 2013 IEEE",
title = "Fabrication of nano-imprinted resonant structures for thin-film solar cell applications",
year = 2013,
month = "Sept",
pages = "580-581",
keywords = "nanofabrication;nanolithography;optical fabrication;radiation pressure;replica techniques;soft lithography;solar cells;light trapping;nanoimprinted resonant structure fabrication;thermal nanoimprint lithography;thermoplastic polymethyl methacrylate substrates;thin-film solar cells;Gratings;Heating;Optical device fabrication;Photovoltaic cells;Substrates;Temperature measurement",
doi = "10.1109/IPCon.2013.6656696",
abstract = "A simple fabrication approach of replicating nanoscale patterns with 300-nm periods on thermoplastic polymethyl methacrylate substrates using thermal nano-imprint lithography is reported. The resonant structures hold potential application in light trapping for thin-film solar cells."
}

2. Y B Zhu, P Zhang, A Valfells, L K Ang and Y Y Lau.
Novel Scaling Laws for the Langmuir-Blodgett Solutions in Cylindrical and Spherical Diodes.
Phys. Rev. Lett. 110, 265007 (June 2013).
Abstract It is found that the Langmuir-Blodgett solutions for the space charge limited current density, for both cylindrical and spherical diodes, may be approximated by J = 4/9?_0\sqrt(2e/m)(E_c^(3/2)/D) over a wide range of parameters, where $E_c$ is the surface electric field on the cathode of the vacuum diode and D is the anode-cathode spacing. This dependence is valid whether $R_a/R_c$ is greater than or less than unity, where $R_a$ and $R_c$ are, respectively, the anode and cathode radius. Minor empirical corrections to the above scaling yield fitting formulas that are accurate to within 5% for 310E-5<$R_c/R_a$<500. An explanation of this scaling is given. An accurate transit time model yields the Langmuir-Blodgett solutions even in the Coulomb blockade regime for a nanogap, where the electron number may be in the single digits, and the transit time frequency is in the THz range.
URL, DOI BibTeX

@article{PhysRevLett.110.265007,
title = "Novel Scaling Laws for the Langmuir-Blodgett Solutions in Cylindrical and Spherical Diodes",
author = "Zhu, Y. B. and Zhang, P. and Valfells, A. and Ang, L. K. and Lau, Y. Y.",
journal = "Phys. Rev. Lett.",
volume = 110,
issue = 26,
pages = 265007,
numpages = 5,
year = 2013,
month = "Jun",
doi = "10.1103/PhysRevLett.110.265007",
abstract = "It is found that the Langmuir-Blodgett solutions for the space charge limited current density, for both cylindrical and spherical diodes, may be approximated by J = 4/9?_0\sqrt(2e/m)(E_c^(3/2)/D) over a wide range of parameters, where $E_c$ is the surface electric field on the cathode of the vacuum diode and D is the anode-cathode spacing. This dependence is valid whether $R_a/R_c$ is greater than or less than unity, where $R_a$ and $R_c$ are, respectively, the anode and cathode radius. Minor empirical corrections to the above scaling yield fitting formulas that are accurate to within 5% for 310E-5<$R_c/R_a$<500. An explanation of this scaling is given. An accurate transit time model yields the Langmuir-Blodgett solutions even in the Coulomb blockade regime for a nanogap, where the electron number may be in the single digits, and the transit time frequency is in the THz range.",
publisher = "American Physical Society"
}

3. Kristinn Torfason, Andrei Manolescu, Sigurdur I Erlingsson and Vidar Gudmundsson.
Thermoelectric current and Coulomb-blockade plateaus in a quantum dot.
Physica E: Low-dimensional Systems and Nanostructures 53, 178 - 185 (2013).
Abstract A Generalized Master Equation (GME) is used to study the thermoelectric currents through a quantum dot in both the transient and steady-state regime. The two semi-infinite leads are kept at the same chemical potential but at different temperatures to produce a thermoelectric current which has a varying sign depending on the chemical potential. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. We observe a saw-teeth like profile of the current alternating with plateaus of almost zero current. Our calculations go beyond the linear response with respect to the temperature gradient, but are compatible with known results for the thermopower in the linear response regime.
URL arXiv, DOI BibTeX

@article{Torfason2013178,
title = "Thermoelectric current and Coulomb-blockade plateaus in a quantum dot",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 53,
number = 0,
pages = "178 - 185",
year = 2013,
note = "",
issn = "1386-9477",
doi = "10.1016/j.physe.2013.05.005",
url = "http://www.sciencedirect.com/science/article/pii/S1386947713001689",
author = "Kristinn Torfason and Andrei Manolescu and Sigurdur I. Erlingsson and Vidar Gudmundsson",
abstract = "A Generalized Master Equation (GME) is used to study the thermoelectric currents through a quantum dot in both the transient and steady-state regime. The two semi-infinite leads are kept at the same chemical potential but at different temperatures to produce a thermoelectric current which has a varying sign depending on the chemical potential. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. We observe a saw-teeth like profile of the current alternating with plateaus of almost zero current. Our calculations go beyond the linear response with respect to the temperature gradient, but are compatible with known results for the thermopower in the linear response regime.",
arxiv = "http://arxiv.org/abs/1303.3160"
}

4. Olafur Jonasson, Vidar Gudmundsson, Andrei Manolescu, Chi-Shung Tang and Hsi-Sheng Goan.
Symmetric excitation and de-excitation of a cavity QED system.
The European Physical Journal B 86, 1-7 (2013).
Abstract We calculate the time evolution of a cavity-QED system subject to a time dependent sinusoidal drive. The drive is modulated by an envelope function with the shape of a pulse. The system consists of electrons embedded in a semiconductor nanostructure which is coupled to a single mode quantized electromagnetic field. The electron-electron as well as photon-electron interaction is treated exactly using exact numerical diagonalization and the time evolution is calculated by numerically solving the equation of motion for the systems density matrix. We find that the drive causes symmetric excitation and de-excitation where the system climbs up the Jaynes-Cummings ladder and descends back down symmetrically into its original state. This effect is known at low electron-photon coupling strengths but our main finding is how robust the effect is even at ultra-strong coupling strength where the JC-model does not give qualitatively correct results. We investigate the robustness of this symmetric behavior with respect to the drive de-tuning and pulse duration.
URL arXiv, DOI BibTeX

@article{,
year = 2013,
issn = "1434-6028",
journal = "The European Physical Journal B",
volume = 86,
number = 6,
doi = "10.1140/epjb/e2013-40330-x",
title = "Symmetric excitation and de-excitation of a cavity QED system",
url = "http://dx.doi.org/10.1140/epjb/e2013-40330-x",
publisher = "Springer-Verlag",
keywords = "Mesoscopic and Nanoscale Systems",
author = "Jonasson, Olafur and Gudmundsson, Vidar and Manolescu, Andrei and Tang, Chi-Shung and Goan, Hsi-Sheng",
pages = "1-7",
language = "English",
abstract = "We calculate the time evolution of a cavity-QED system subject to a time dependent sinusoidal drive. The drive is modulated by an envelope function with the shape of a pulse. The system consists of electrons embedded in a semiconductor nanostructure which is coupled to a single mode quantized electromagnetic field. The electron-electron as well as photon-electron interaction is treated exactly using exact numerical diagonalization and the time evolution is calculated by numerically solving the equation of motion for the systems density matrix. We find that the drive causes symmetric excitation and de-excitation where the system climbs up the Jaynes-Cummings ladder and descends back down symmetrically into its original state. This effect is known at low electron-photon coupling strengths but our main finding is how robust the effect is even at ultra-strong coupling strength where the JC-model does not give qualitatively correct results. We investigate the robustness of this symmetric behavior with respect to the drive de-tuning and pulse duration.",
arxiv = "http://arxiv.org/abs/1207.6797"
}

5. Kristian Spilling, Ása Brynjólfsdóttir, Dagmar Enss, Heiko Rischer and Halldór Guðfinnur Svavarsson.
The effect of high pH on structural lipids in diatoms.
Journal of Applied Phycology 25, 1435-1439 (2013).
Abstract We tested the hypothesis that increased pH reduces the amount of structural lipids. To do this, we used three different diatoms (Phaeodactylum tricornutum CCAP strain, P. tricornutum TV strain and Amphiprora sp). We tested the effect of rapid increase from pH?7.5 to 10 by adding NaOH. The total lipid content was reduced by 13, 36 and 47 % in the P. tricornutum CCAP strain, TV strain and Amphiprora sp., respectively, 1 h after increasing the pH. The P. tricornutum CCAP strain was used for further testing the effect of pH on the lipid content during active growth. This strain was cultivated at pH?7.5 and 10, and the pH was regulated by the CO2 inflow. The growth rate was similar (0.3 day?1) in both pH treatments, but the lipid content in the pH?10 treatment was on average 28 % lower than in the pH?7.5 treatment. Our data support the hypothesis that structural lipids are reduced when pH increases to high levels. The results suggest that regulating the pH during algae cultivation could be used to refine the lipid composition in the harvested algal biomass.
URL PDF, DOI BibTeX

@article{spilling2013effect,
year = 2013,
issn = "0921-8971",
journal = "Journal of Applied Phycology",
volume = 25,
number = 5,
doi = "10.1007/s10811-012-9971-5",
title = "The effect of high pH on structural lipids in diatoms",
url = "http://dx.doi.org/10.1007/s10811-012-9971-5",
publisher = "Springer Netherlands",
keywords = "Membrane lipids; Glycolipids; Phospholipids; Polyunsaturated fatty acids",
author = "Spilling, Kristian and Brynjólfsdóttir, Ása and Enss, Dagmar and Rischer, Heiko and Svavarsson, Halldór Guðfinnur",
pages = "1435-1439",
language = "English",
abstract = "We tested the hypothesis that increased pH reduces the amount of structural lipids. To do this, we used three different diatoms (Phaeodactylum tricornutum CCAP strain, P. tricornutum TV strain and Amphiprora sp). We tested the effect of rapid increase from pH?7.5 to 10 by adding NaOH. The total lipid content was reduced by 13, 36 and 47 % in the P. tricornutum CCAP strain, TV strain and Amphiprora sp., respectively, 1 h after increasing the pH. The P. tricornutum CCAP strain was used for further testing the effect of pH on the lipid content during active growth. This strain was cultivated at pH?7.5 and 10, and the pH was regulated by the CO2 inflow. The growth rate was similar (0.3 day?1) in both pH treatments, but the lipid content in the pH?10 treatment was on average 28 % lower than in the pH?7.5 treatment. Our data support the hypothesis that structural lipids are reduced when pH increases to high levels. The results suggest that regulating the pH during algae cultivation could be used to refine the lipid composition in the harvested algal biomass."
}

6. P Jonsson, Marjan Ilkov, A Manolescu, A Pedersen and A Valfells.
Tunability of the terahertz space-charge modulation in a vacuum microdiode.
Physics of Plasmas 20, 023107 (2013).
Abstract Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the terahertz range. In this computational study, it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage, and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependant on the vacuum electric field in the diode, described by a simple power law. It is also found that, for a fixed value of the electric field, the frequency increases with diminishing size of the emitting spot on the cathode. Some observations are made on the spectral quality, and how it is affected by the gap spacing in the diode and the initial velocity of the electrons.
URL arXiv, DOI BibTeX

@article{jonsson:023107,
author = "P. Jonsson and Marjan Ilkov and A. Manolescu and A. Pedersen and A. Valfells",
collaboration = "",
title = "Tunability of the terahertz space-charge modulation in a vacuum microdiode",
publisher = "AIP",
year = 2013,
journal = "Physics of Plasmas",
volume = 20,
number = 2,
eid = 023107,
numpages = 7,
pages = 023107,
keywords = "molecular dynamics method; plasma diodes; plasma oscillations; plasma simulation; plasma transport processes; space charge",
doi = "10.1063/1.4793451",
abstract = "Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the terahertz range. In this computational study, it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage, and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependant on the vacuum electric field in the diode, described by a simple power law. It is also found that, for a fixed value of the electric field, the frequency increases with diminishing size of the emitting spot on the cathode. Some observations are made on the spectral quality, and how it is affected by the gap spacing in the diode and the initial velocity of the electrons.",
arxiv = "http://arxiv.org/abs/1301.6533"
}

7. V Gudmundsson, O Jonasson, Th. Arnold, C-S Tang, H -S Goan and A Manolescu.
Stepwise introduction of model complexity in a generalized master equation approach to time-dependent transport.
Fortschritte der Physik 61, 305–316 (2013).
Abstract We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using exact numerical diagonalization.
URL arXiv, DOI BibTeX

@article{PROP:PROP201200053,
author = "Gudmundsson, V. and Jonasson, O. and Arnold, Th. and Tang, C-S. and Goan, H.-S. and Manolescu, A.",
title = "Stepwise introduction of model complexity in a generalized master equation approach to time-dependent transport",
journal = "Fortschritte der Physik",
volume = 61,
number = "2-3",
publisher = "WILEY-VCH Verlag",
issn = "1521-3978",
url = "http://dx.doi.org/10.1002/prop.201200053",
doi = "10.1002/prop.201200053",
pages = "305--316",
keywords = "Open system, Coulomb interaction, photon cavity, time-dependent transport.",
year = 2013,
abstract = "We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using exact numerical diagonalization.",
arxiv = "http://arxiv.org/abs/1203.3048"
}

8. Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Magnetic-field-influenced nonequilibrium transport through a quantum ring with correlated electrons in a photon cavity.
Phys. Rev. B 87, 035314 (January 2013).
Abstract We investigate magnetic-field-influenced time-dependent transport of Coulomb interacting electrons through a two-dimensional quantum ring in an electromagnetic cavity under nonequilibrium conditions described by a time-convolutionless non-Markovian master equation formalism. We take into account the full electromagnetic interaction of electrons and cavity photons. A bias voltage is applied to semi-infinite leads along the x axis, which are connected to the quantum ring. The magnetic field is tunable to manipulate the time-dependent electron transport coupled to a photon field with either x or y polarization. We find that the lead-system-lead current is strongly suppressed by the y-polarized photon field at magnetic field with two flux quanta due to a degeneracy of the many-body energy spectrum of the mostly occupied states. On the other hand, the lead-system-lead current can be significantly enhanced by the y-polarized field at magnetic field with half-integer flux quanta. Furthermore, the y- polarized photon field perturbs the periodicity of the persistent current with the magnetic field and suppresses the magnitude of the persistent current. The spatial and temporal density distributions reflect the characteristics of the many-body spectrum. The vortex formation in the contact areas to the leads influences the charge circulation in the ring.
URL arXiv, DOI BibTeX

@article{PhysRevB.87.035314,
title = "Magnetic-field-influenced nonequilibrium transport through a quantum ring with correlated electrons in a photon cavity",
author = "Arnold, Thorsten and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 87,
issue = 3,
pages = 035314,
numpages = 13,
year = 2013,
month = "Jan",
doi = "10.1103/PhysRevB.87.035314",
publisher = "American Physical Society",
abstract = "We investigate magnetic-field-influenced time-dependent transport of Coulomb interacting electrons through a two-dimensional quantum ring in an electromagnetic cavity under nonequilibrium conditions described by a time-convolutionless non-Markovian master equation formalism. We take into account the full electromagnetic interaction of electrons and cavity photons. A bias voltage is applied to semi-infinite leads along the x axis, which are connected to the quantum ring. The magnetic field is tunable to manipulate the time-dependent electron transport coupled to a photon field with either x or y polarization. We find that the lead-system-lead current is strongly suppressed by the y-polarized photon field at magnetic field with two flux quanta due to a degeneracy of the many-body energy spectrum of the mostly occupied states. On the other hand, the lead-system-lead current can be significantly enhanced by the y-polarized field at magnetic field with half-integer flux quanta. Furthermore, the y- polarized photon field perturbs the periodicity of the persistent current with the magnetic field and suppresses the magnitude of the persistent current. The spatial and temporal density distributions reflect the characteristics of the many-body spectrum. The vortex formation in the contact areas to the leads influences the charge circulation in the ring.",
arxiv = "http://arxiv.org/abs/1209.2888"
}

9. Tanzina Khaleque, Halldor Gudfinnur Svavarsson and Robert Magnusson.
Fabrication of resonant patterns using thermal nano-imprint lithography for thin-film photovoltaic applications.
Opt. Express 21, A631–A641 (July 2013).
Abstract A single-step, low-cost fabrication method to generate resonant nano-grating patterns on poly-methyl-methacrylate (PMMA; plexiglas) substrates using thermal nano-imprint lithography is reported. A guided-mode resonant structure is obtained by subsequent deposition of thin films of transparent conductive oxide and amorphous silicon on the imprinted area. Referenced to equivalent planar structures, around 25% and 45% integrated optical absorbance enhancement is observed over the 450-nm to 900-nm wavelength range in one- and two-dimensional patterned samples, respectively. The fabricated elements provided have 300-nm periods. Thermally imprinted thermoplastic substrates hold potential for low-cost fabrication of nano-patterned thin-film solar cells for efficient light management.
URL, DOI BibTeX

@article{Khaleque:13,
author = "Tanzina Khaleque and Halldor Gudfinnur Svavarsson and Robert Magnusson",
journal = "Opt. Express",
keywords = "Waveguides; Deposition and fabrication; Guided waves; Solar energy; Nanolithography; Subwavelength structures",
number = "S4",
pages = "A631--A641",
publisher = "OSA",
title = "Fabrication of resonant patterns using thermal nano-imprint lithography for thin-film photovoltaic applications",
volume = 21,
month = "Jul",
year = 2013,
url = "http://www.opticsexpress.org/abstract.cfm?URI=oe-21-104-A631",
doi = "10.1364/OE.21.00A631",
abstract = "A single-step, low-cost fabrication method to generate resonant nano-grating patterns on poly-methyl-methacrylate (PMMA; plexiglas) substrates using thermal nano-imprint lithography is reported. A guided-mode resonant structure is obtained by subsequent deposition of thin films of transparent conductive oxide and amorphous silicon on the imprinted area. Referenced to equivalent planar structures, around 25\% and 45\% integrated optical absorbance enhancement is observed over the 450-nm to 900-nm wavelength range in one- and two-dimensional patterned samples, respectively. The fabricated elements provided have 300-nm periods. Thermally imprinted thermoplastic substrates hold potential for low-cost fabrication of nano-patterned thin-film solar cells for efficient light management."
}

10. Karel Vborn, Goran Mihajlović, Axel Hoffmann and Sigurdur I Erlingsson.
Magnetic field dependence of non-local lateral spin-valve signals beyond the Hanle effect.
Journal of Physics: Condensed Matter 25, 216007 (2013).
Abstract We present a theoretical model of spin transport in metallic lateral valves that takes into account spin scattering on magnetic impurities. We show that the model agrees with recent experimental findings of increasing non-local spin signals by in-plane magnetic field, which is parallel to the injected spins. The increase arises due to reduction of conduction electron spin flips on magnetic impurities present at the metalferromagnet interfaces as they freeze out under application of the magnetic field.
URL, DOI BibTeX

@article{0953-8984-25-21-216007,
author = "Karel Vborn and Goran Mihajlović and Axel Hoffmann and Sigurdur I Erlingsson",
title = "Magnetic field dependence of non-local lateral spin-valve signals beyond the Hanle effect",
journal = "Journal of Physics: Condensed Matter",
volume = 25,
number = 21,
pages = 216007,
url = "http://stacks.iop.org/0953-8984/25/i=21/a=216007",
doi = "10.1088/0953-8984/25/21/216007",
year = 2013,
abstract = "We present a theoretical model of spin transport in metallic lateral valves that takes into account spin scattering on magnetic impurities. We show that the model agrees with recent experimental findings of increasing non-local spin signals by in-plane magnetic field, which is parallel to the injected spins. The increase arises due to reduction of conduction electron spin flips on magnetic impurities present at the metalferromagnet interfaces as they freeze out under application of the magnetic field."
}