Publications in 2012

1. Olafur Jonasson, Chi-Shung Tang, Hsi-Sheng Goan, Andrei Manolescu and Vidar Gudmundsson.
Nonperturbative approach to circuit quantum electrodynamics.
Phys. Rev. E 86, 046701 (October 2012).
Abstract We outline a rigorous method which can be used to solve the many-body Schrödinger equation for a Coulomb interacting electronic system in an external classical magnetic field as well as a quantized electromagnetic field. Effects of the geometry of the electronic system as well as the polarization of the quantized electromagnetic field are explicitly taken into account. We accomplish this by performing repeated truncations of many-body spaces in order to keep the size of the many particle basis on a manageable level. The electron-electron and electron-photon interactions are treated in a nonperturbative manner using “exact numerical diagonalization.” Our results demonstrate that including the diamagnetic term in the photon-electron interaction Hamiltonian drastically improves numerical convergence. Additionally, convergence with respect to the number of photon states in the joint photon-electron Fock space basis is fast. However, the convergence with respect to the number of electronic states is slow and is the main bottleneck in calculations.
URL arXiv, DOI BibTeX

@article{PhysRevE.86.046701,
title = "Nonperturbative approach to circuit quantum electrodynamics",
author = "Jonasson, Olafur and Tang, Chi-Shung and Goan, Hsi-Sheng and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. E",
volume = 86,
issue = 4,
pages = 046701,
numpages = 8,
year = 2012,
month = "Oct",
doi = "10.1103/PhysRevE.86.046701",
publisher = "American Physical Society",
abstract = "We outline a rigorous method which can be used to solve the many-body Schrödinger equation for a Coulomb interacting electronic system in an external classical magnetic field as well as a quantized electromagnetic field. Effects of the geometry of the electronic system as well as the polarization of the quantized electromagnetic field are explicitly taken into account. We accomplish this by performing repeated truncations of many-body spaces in order to keep the size of the many particle basis on a manageable level. The electron-electron and electron-photon interactions are treated in a nonperturbative manner using “exact numerical diagonalization.” Our results demonstrate that including the diamagnetic term in the photon-electron interaction Hamiltonian drastically improves numerical convergence. Additionally, convergence with respect to the number of photon states in the joint photon-electron Fock space basis is fast. However, the convergence with respect to the number of electronic states is slow and is the main bottleneck in calculations.",
arxiv = "http://arxiv.org/abs/1203.5980"
}

2. Marian Niţă, D C Marinescu, Andrei Manolescu, Bogdan Ostahie and Vidar Gudmundsson.
Persistent oscillatory currents in a 1D ring with Rashba and Dresselhaus spin–orbit interactions excited by a terahertz pulse.
Physica E: Low-dimensional Systems and Nanostructures 46, 12 - 20 (2012).
Abstract Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahertz laser pulse in a one-dimensional mesoscopic ring with Rashba and Dresselhaus spin–orbit interactions (SOI) linear in the electron momentum. The characteristic interference effects result from the opposite precession directions imposed on the electron spin by the two SOI couplings. The time dependence of the currents is obtained by solving numerically the equation of motion for the density operator, which is later employed in calculating statistical averages of quantum operators on few electron eigenstates. The parameterization of the problem is done in terms of the SOI coupling constants and of the phase difference between the two laser components. Our results indicate that the amplitude of the oscillations is controlled by the relative strength of the two SOI's, while their frequency is determined by the difference between the excitation energies of the electron states. Furthermore, the oscillations of the spin current acquire a beating pattern of higher frequency that we associate with the nutation of the electron spin between the quantization axes of the two SOI couplings. This phenomenon disappears at equal SOI strengths, whereby the opposite precessions occur with the same probability.
URL arXiv, DOI BibTeX

@article{Niţă201212,
title = "Persistent oscillatory currents in a 1D ring with Rashba and Dresselhaus spin–orbit interactions excited by a terahertz pulse",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 46,
number = 0,
pages = "12 - 20",
year = 2012,
note = "",
issn = "1386-9477",
doi = "10.1016/j.physe.2012.08.017",
url = "http://www.sciencedirect.com/science/article/pii/S1386947712003232",
author = "Marian Niţă and D.C. Marinescu and Andrei Manolescu and Bogdan Ostahie and Vidar Gudmundsson",
abstract = "Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahertz laser pulse in a one-dimensional mesoscopic ring with Rashba and Dresselhaus spin–orbit interactions (SOI) linear in the electron momentum. The characteristic interference effects result from the opposite precession directions imposed on the electron spin by the two SOI couplings. The time dependence of the currents is obtained by solving numerically the equation of motion for the density operator, which is later employed in calculating statistical averages of quantum operators on few electron eigenstates. The parameterization of the problem is done in terms of the SOI coupling constants and of the phase difference between the two laser components. Our results indicate that the amplitude of the oscillations is controlled by the relative strength of the two SOI's, while their frequency is determined by the difference between the excitation energies of the electron states. Furthermore, the oscillations of the spin current acquire a beating pattern of higher frequency that we associate with the nutation of the electron spin between the quantization axes of the two SOI couplings. This phenomenon disappears at equal SOI strengths, whereby the opposite precessions occur with the same probability.",
arxiv = "http://arxiv.org/abs/1111.2949"
}

3. Kristinn Torfason, Andrei Manolescu, Valeriu Molodoveanu and Vidar Gudmundsson.
Excitation of collective modes in a quantum flute.
Phys. Rev. B 85, 245114 (June 2012).
Abstract We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum “flute” device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.
URL arXiv, DOI BibTeX

@article{PhysRevB.85.245114,
title = "Excitation of collective modes in a quantum flute",
author = "Torfason, Kristinn and Manolescu, Andrei and Molodoveanu, Valeriu and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 85,
issue = 24,
pages = 245114,
numpages = 9,
year = 2012,
month = "Jun",
doi = "10.1103/PhysRevB.85.245114",
publisher = "American Physical Society",
abstract = "We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum “flute” device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.",
arxiv = "http://arxiv.org/abs/1202.0566"
}

4. D C Marinescu and Andrei Manolescu.
Weak localization in a lateral superlattice with Rashba and Dresselhaus spin-orbit interaction.
Phys. Rev. B 85, 165302 (April 2012).
Abstract We calculate the weak localization (WL) correction to the conductivity of a lateral superlattice (LSL) with Rashba (R)-Dresselhaus (D) spin-orbit interaction (SOI). The superlattice is modeled as a sequence of parallel wires that support tunneling between adjacent sites, leading to the formation of extended Bloch states along its axis and a miniband in the energy spectrum. Our results, obtained by calculating the eigenvalues of the Cooperon operator in the diffusion approximation, indicate that the electron dephasing rate that determines the antilocalization correction is enhanced by a term proportional with the LSL potential and the bandwidth. Within the same formalism, the spin-relaxation rates associated with the localization corrections are found to exhibit a strong anisotropy dictated by the relative strength of the two SOI couplings, as well as by the orientation of the LSL axis.
URL, DOI BibTeX

@article{PhysRevB.85.165302,
title = "Weak localization in a lateral superlattice with Rashba and Dresselhaus spin-orbit interaction",
author = "Marinescu, D. C. and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 85,
issue = 16,
pages = 165302,
numpages = 8,
year = 2012,
month = "Apr",
doi = "10.1103/PhysRevB.85.165302",
abstract = "We calculate the weak localization (WL) correction to the conductivity of a lateral superlattice (LSL) with Rashba (R)-Dresselhaus (D) spin-orbit interaction (SOI). The superlattice is modeled as a sequence of parallel wires that support tunneling between adjacent sites, leading to the formation of extended Bloch states along its axis and a miniband in the energy spectrum. Our results, obtained by calculating the eigenvalues of the Cooperon operator in the diffusion approximation, indicate that the electron dephasing rate that determines the antilocalization correction is enhanced by a term proportional with the LSL potential and the bandwidth. Within the same formalism, the spin-relaxation rates associated with the localization corrections are found to exhibit a strong anisotropy dictated by the relative strength of the two SOI couplings, as well as by the orientation of the LSL axis.",
publisher = "American Physical Society"
}

5. Vidar Gudmundsson, Olafur Jonasson, Chi-Shung Tang, Hsi-Sheng Goan and Andrei Manolescu.
Time-dependent transport of electrons through a photon cavity.
Phys. Rev. B 85, 075306 (February 2012).
Abstract We use a non-Markovian master equation to describe the transport of Coulomb-interacting electrons through an electromagnetic cavity with one quantized photon mode. The central system is a finite-parabolic quantum wire that is coupled weakly to external parabolic quasi-one-dimensional leads at t=0. With a stepwise introduction of complexity to the description of the system and a corresponding stepwise truncation of the ensuing many-body spaces, we are able to describe the time-dependent transport of Coulomb-interacting electrons through a geometrically complex central system. We take the full electromagnetic interaction of electrons and cavity photons without resorting to the rotating-wave approximation or reduction of the electron states to two levels into account. We observe that the number of initial cavity photons and their polarizations can have important effects on the transport properties of the system. The quasiparticles formed in the central system have lifetimes limited by the coupling to the leads and radiation processes active on a much longer time scale.
URL arXiv, DOI BibTeX

@article{PhysRevB.85.075306,
title = "Time-dependent transport of electrons through a photon cavity",
author = "Gudmundsson, Vidar and Jonasson, Olafur and Tang, Chi-Shung and Goan, Hsi-Sheng and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 85,
issue = 7,
pages = 075306,
numpages = 13,
year = 2012,
month = "Feb",
doi = "10.1103/PhysRevB.85.075306",
publisher = "American Physical Society",
abstract = "We use a non-Markovian master equation to describe the transport of Coulomb-interacting electrons through an electromagnetic cavity with one quantized photon mode. The central system is a finite-parabolic quantum wire that is coupled weakly to external parabolic quasi-one-dimensional leads at t=0. With a stepwise introduction of complexity to the description of the system and a corresponding stepwise truncation of the ensuing many-body spaces, we are able to describe the time-dependent transport of Coulomb-interacting electrons through a geometrically complex central system. We take the full electromagnetic interaction of electrons and cavity photons without resorting to the rotating-wave approximation or reduction of the electron states to two levels into account. We observe that the number of initial cavity photons and their polarizations can have important effects on the transport properties of the system. The quasiparticles formed in the central system have lifetimes limited by the coupling to the leads and radiation processes active on a much longer time scale.",
arxiv = "http://arxiv.org/abs/1109.4728"
}

6. Olafur Jonasson, Chi-Shung Tang, Hsi-Sheng Goan, Andrei Manolescu and Vidar Gudmundsson.
Quantum magneto-electrodynamics of electrons embedded in a photon cavity.
New Journal of Physics 14, 013036 (2012).
Abstract We investigate the coupling between a quantized electromagnetic field in a cavity resonator and a Coulomb interacting electronic system in a nanostructure in an external magnetic field. The effects caused by the geometry of the electronic system and the polarization of the electromagnetic field are explicitly taken into account. Our numerical results demonstrate that the two-level system approximation and the Jaynes–Cummings model remain valid in the weak electron–photon coupling regime, while the quadratic vector potential in the diamagnetic part of the charge current leads to significant correction to the energy spectrum in the strong coupling regime. Furthermore, we find that coupling to a strong cavity photon mode polarizes the charge distribution of the system, requiring a large basis of single-electron eigenstates to be included in the model.
URL arXiv BibTeX

@article{1367-2630-14-1-013036,
author = "Olafur Jonasson and Chi-Shung Tang and Hsi-Sheng Goan and Andrei Manolescu and Vidar Gudmundsson",
title = "Quantum magneto-electrodynamics of electrons embedded in a photon cavity",
journal = "New Journal of Physics",
volume = 14,
number = 1,
pages = 013036,
url = "http://stacks.iop.org/1367-2630/14/i=1/a=013036",
year = 2012,
abstract = "We investigate the coupling between a quantized electromagnetic field in a cavity resonator and a Coulomb interacting electronic system in a nanostructure in an external magnetic field. The effects caused by the geometry of the electronic system and the polarization of the electromagnetic field are explicitly taken into account. Our numerical results demonstrate that the two-level system approximation and the Jaynes–Cummings model remain valid in the weak electron–photon coupling regime, while the quadratic vector potential in the diamagnetic part of the charge current leads to significant correction to the energy spectrum in the strong coupling regime. Furthermore, we find that coupling to a strong cavity photon mode polarizes the charge distribution of the system, requiring a large basis of single-electron eigenstates to be included in the model.",
arxiv = "http://arxiv.org/abs/1109.4594"
}

7. H G Svavarsson, J W Yoon, M Shokooh-Saremi, S H Song and R Magnusson.
Fabrication and Characterization of Large, Perfectly Periodic Arrays of Metallic Nanocups.
Plasmonics 7, 653-657 (2012).
Abstract Fabrication of plasmonic resonance devices composed of large arrays of highly ordered gold nanocups is presented. The nanostructures are generated from periodic photoresist templates created by interference lithography and subsequent reflow, deposition, and dislodging. The nanocups are hemispherical in shape and arranged in both rectangular and hexagonal arrays with periods of ~500 nm. Their ability to support surface plasmonic resonances is manifested experimentally by reflectance spectroscopy. Theoretical modeling to ascertain the plasmonic spectra of these nanostructures is performed. The computed spectra of the rectangular structure are in qualitative agreement with the measurements. A weaker correlation observed for the hexagonal structure is explained by its more intricate symmetry which complicates the spectral response.
URL PDF, DOI BibTeX

@article{Svavar2012,
year = 2012,
issn = "1557-1955",
journal = "Plasmonics",
volume = 7,
number = 4,
doi = "10.1007/s11468-012-9355-3",
title = "Fabrication and Characterization of Large, Perfectly Periodic Arrays of Metallic Nanocups",
url = "http://scitation.aip.org/content/aip/journal/apl/100/9/10.1063/1.3690951",
publisher = "Springer US",
keywords = "Nanocups; Periodic arrays; Nano-indented films; Plasmonics; Hexagonal arrays; Rectangular arrays",
author = "Svavarsson, H.G. and Yoon, J.W. and Shokooh-Saremi, M. and Song, S.H. and Magnusson, R.",
pages = "653-657",
abstract = "Fabrication of plasmonic resonance devices composed of large arrays of highly ordered gold nanocups is presented. The nanostructures are generated from periodic photoresist templates created by interference lithography and subsequent reflow, deposition, and dislodging. The nanocups are hemispherical in shape and arranged in both rectangular and hexagonal arrays with periods of ~500 nm. Their ability to support surface plasmonic resonances is manifested experimentally by reflectance spectroscopy. Theoretical modeling to ascertain the plasmonic spectra of these nanostructures is performed. The computed spectra of the rectangular structure are in qualitative agreement with the measurements. A weaker correlation observed for the hexagonal structure is explained by its more intricate symmetry which complicates the spectral response.",
language = "English",
pdf = "http://optics.hanyang.ac.kr/~shsong/Plasmonics%202012-03-Fabrication%20and%20Characterization%20of%20Large,%20Perfectly%20Periodic%20Arrays%20of%20Metallic%20Nanocups.pdf"
}

8. R Magnusson, H G Svavarsson, J Yoon, M Shokooh-Saremi and S H Song.
Experimental observation of leaky modes and plasmons in a hybrid resonance element.
Applied Physics Letters 100, 091106-091106-3 (2012).
Abstract We provide experimental evidence of a hybrid photonic device supporting simultaneously surface-plasmon polaritons and resonant leaky modes. A fabricated metallo-dielectric structure exhibits a pronounced plasmonic resonance at 799 nm wavelength and a modal resonance at 669 nm in transverse magnetic polarization. In transverse electric polarization, a weak modal resonance appears at 725 nm wavelength. We identify the corresponding modes by computing the attendant internal field distributions. Numerically computed spectra are in good agreement with our measurements. Since traditional modal and plasmonic devices find many uses, their hybrid versions may enable the extension of their applicability.
PDF, DOI BibTeX

@article{6163058,
author = "Magnusson, R. and Svavarsson, H. G. and Yoon, J. and Shokooh-Saremi, M. and Song, S. H.",
journal = "Applied Physics Letters",
title = "Experimental observation of leaky modes and plasmons in a hybrid resonance element",
year = 2012,
volume = 100,
number = 9,
pages = "091106-091106-3",
keywords = "light polarisation;polaritons;surface plasmon resonance;7320Mf",
doi = "10.1063/1.3690951",
issn = "0003-6951",
abstract = "We provide experimental evidence of a hybrid photonic device supporting simultaneously surface-plasmon polaritons and resonant leaky modes. A fabricated metallo-dielectric structure exhibits a pronounced plasmonic resonance at 799 nm wavelength and a modal resonance at 669 nm in transverse magnetic polarization. In transverse electric polarization, a weak modal resonance appears at 725 nm wavelength. We identify the corresponding modes by computing the attendant internal field distributions. Numerically computed spectra are in good agreement with our measurements. Since traditional modal and plasmonic devices find many uses, their hybrid versions may enable the extension of their applicability.",
pdf = "http://optics.hanyang.ac.kr/~shsong/APL%202012-02-Experimental%20observation%20of%20leaky%20modes%20and%20plasmons%20in%20a%20hybrid%20resonance%20element.pdf"
}

9. Gunnar Thorgilsson, Carlos J Egues, Daniel Loss and Sigurdur I Erlingsson.
Rashba spin orbit interaction in a quantum wire superlattice.
Phys. Rev. B 85, 045306 (January 2012).
Abstract In this work, we study the effects of a longitudinal periodic potential on a parabolic quantum wire defined in a two-dimensional electron gas with Rashba spin-orbit interaction. For an infinite wire superlattice we find, by direct diagonalization, that the energy gaps are shifted away from the usual Bragg planes due to the Rashba spin-orbit interaction. Interestingly, our results show that the location of the band gaps in energy can be controlled via the strength of the Rashba spin-orbit interaction. We have also calculated the charge conductance through a periodic potential of a finite length via the nonequilibrium Green's function method combined with the Landauer formalism. We find dips in the conductance that correspond well to the energy gaps of the infinite wire superlattice. From the infinite wire energy dispersion, we derive an equation relating the location of the conductance dips as a function of the (gate controllable) Fermi energy to the Rashba spin-orbit coupling strength. We propose that the strength of the Rashba spin-orbit interaction can be extracted via a charge conductance measurement.
URL arXiv PDF, DOI BibTeX

@article{PhysRevB.85.045306,
title = "Rashba spin orbit interaction in a quantum wire superlattice",
author = "Thorgilsson, Gunnar and Egues, J. Carlos and Loss, Daniel and Erlingsson, Sigurdur I.",
journal = "Phys. Rev. B",
volume = 85,
issue = 4,
pages = 045306,
numpages = 8,
year = 2012,
month = "Jan",
doi = "10.1103/PhysRevB.85.045306",