arXiv subjects

Vidar Gudmundsson

Publications and source records attributed to Vidar Gudmundsson.

At least 37 records · Page 2Linked to original sources

arXiv2023

Controlling the excitation spectrum of a quantum dot array with a photon cavity

We use a recently proposed quantum electrodynamical density functional theory (QEDFT) functional in a real-time excitation calculation for a two-dimensional electron gas in a square array of quantum dots in an external constant perpendicular magnetic field to model the influence of cavity photons on the excitation spectra of the system. The excitation is generated by a short elecrical pulse. The quantum dot array is defined in an AlGaAs-GaAs heterostructure, which is in turn embedded in a parallel plate far-infrared photon-microcavity. The required exchange and correlation energy functionals describing the electron-electron and electron-photon interactions have therefore been adapted for a two-dimensional electron gas in a homogeneous external magnetic field. We predict that the energies of the excitation modes activated by the pulse are generally red-shifted to lower values in the presence of a cavity. The red-shift can be understood in terms of the polarization of the electron charge by the cavity photons and depends on the magnetic flux, the number of electrons in a unit cell of the lattice, and the electron-photon interaction strength. We find an interesting interplay of the exchange forces in a spin polarized two-dimensional electron gas and the square lattice structure leading to a small but clear blue-shift of the excitation mode spectra when one electron resides in each dot.

cond-mat.mes-hall↗

arXiv2022

Role of planar buckling on the electronic, thermal, and optical properties of Germagraphene nanosheets

We report the electronic, the thermal, and the optical properties of a Germagraphene (GeC) monolayer taking into account buckling effects. The relatively wide direct band gap of a flat GeC nanosheet can be changed by tuning the planar buckling. A GeC monolayer has an sp$^2$ hybridization in which the contribution of an $s$-orbital is half of the contribution of a $p$-orbital leading to stronger $σ\text{-}σ$ bonds compared to the $σ\text{-}π$ bonds. Increasing the planar buckling, the contribution of an $s$-orbital is decreased while the contribution of a $p$-orbital is increased resulting in a sp$^3$-hybridization in which the $σ\text{-}π$ bond becomes stronger than the $σ\text{-}σ$ bond. As a result, the band gap of a buckled GeC is reduced and thus the thermal and the optical properties are significantly modified. We find that the heat capacity of the buckled GeC is decreased at low values of planar buckling, which is caused by the anticrossing of the optical and the acoustic phonon modes affecting phonon scattering processes. The resulting optical properties, such as the dielectric function, the refractive index, the electron energy loss spectra, the absorption, and the optical conductivity show that a buckled GeC nanosheet has increased optical activities in the visible light region compared to a flat GeC. The optical conductivity is red shifted from the near ultraviolet to the visible light region, when the planar buckling is increased. We can thus confirm that the buckling can be seen as another parameter to improve GeC monolayers for optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Single photon controlled steady state electron transport through a resonance DQD-Cavity system in a strong coupling regime

We perform theoretical calculations to study steady-state electron transport in a double quantum dot, DQD, coupled to a quantized cavity photon field both in resonance and off-resonance regimes considering weak and strong coupling. In the resonant strong coupling regime, photon exchanges between the energy states of the DQD and the cavity are found reflecting multiple Rabi-resonances. The electron occupation of the states and the transport current can be smoothly increased by tuning the cavity-environment coupling strength. Making the system off-resonant, but still in the strong coupling regime, the photon exchange is diminished and the electron occupation of the system and the transport current through it are prominent for high cavity-environment coupling strength. In the weak coupling regime between the DQD and the cavity, the system has almost the same response in the resonant and the off-resonant regimes at high values of the cavity-environment coupling strength.

cond-mat.mes-hall↗

arXiv2022

The effects of a far-infrared photon cavity field on the magnetization of a square quantum dot array

The orbital and spin magnetization of a cavity-embedded quantum dot array defined in a GaAs heterostructure are calculated within quantum-electrodynamical density-functional theory (QEDFT). To this end a gradient-based exchange-correlation functional recently employed for atomic systems is adapted to the hosting two-dimensional electron gas (2DEG) submitted to an external perpendicular homogeneous magnetic field. Numerical results reveal the polarizing effects of the cavity photon field on the electron charge distribution and nontrivial changes of the orbital magnetization. We discuss its intertwined dependence on the electron number in each dot, and on the electron-photon coupling strength. In particular, the calculated dispersion of the photon-dressed electron states around the Fermi energy as a function of the electron-photon coupling strength indicates the formation of magnetoplasmon-polaritons in the dots.

cond-mat.mes-hall↗

arXiv2022

Photon and magnetic field controlled electron transport of a multiply-resonant photon-cavity double quantum dot system

We study electron transport through double quantum dots (DQD) coupled to a cavity with a single photon mode. The DQD is connected to two electron reservoirs, and the total system is under an external perpendicular magnetic field. The DQD system exhibits a complex multi-level energy spectrum. By varying the photon energy, several anti-crossings between photon dressed electron states of the DQD-cavity system are found at low strength of the magnetic field. The anti-crossings are identified as multiple Rabi resonances arising from the photon exchange between these states. As the results, a dip in the current is seen caused by the multiple Rabi resonances. By increasing the strength of the external magnetic field, a dislocation of the current dip to a lower photon energy is found and the current dip can be diminished. The interplay of the strength of the magnetic field and the geometry of the states the DQD system can weaken the multiple Rabi resonances in which the exchange of photon between the anti-crossings is decreased. We can therefore confirm that the electron transport behavior in the DQD-cavity system can be controlled by manipulating the external magnetic field and the photon cavity parameters.

cond-mat.mes-hall↗

arXiv2022

Study of the buckling effects on the electrical and optical properties of the group III-Nitride monolayers

We consider electronic and optical properties of group III-Nitride monolayers using first-principle calculations. The group III-Nitride monolayers have flat hexagonal structures with almost zero planar buckling, $Δ$. By tuning the $Δ$, the strong $σ\text{-}σ$ bond through sp$^2$ hybridization of a flat form of these monolayers can be changed to a stronger $σ\text{-}π$ bond through sp$^3$ hybridization. Consequently, the band gaps of the monolayers are tuned due to a dislocation of the $s$- and $p$-orbitals towards the Fermi energy. The band gaps decrease with increasing $Δ$ for those flat monolayers, which have a band gap greater than $1.0$ eV, while no noticeable change or a flat dispersion of the band gap is seen for the flat monolayers, that have a band gap less than $1.0$ eV. The decreased band gap causes a decrease in the excitation energy, and thus the static dielectric function, refractive index, and the optical conductivity are increased. In contrast, the flat band gap dispersion of few monolayers in the group III-Nitride induces a reduction in the static dielectric function, the refractive index, and the optical conductivity. We therefore confirm that tuning of the planar buckling can be used to control the physical properties of these monolayers, both for an enhancement and a reduction of the optical properties. These results are of interest for the design of optoelectric devices in nanoscale systems.

cond-mat.mtrl-sci↗

arXiv2022

Effects of coupling strength of the electron-photon and the photon-environment interactions on the electron transport through multiple-resonances of a double quantum dot system in a photon cavity

We study electron transport properties through a double quantum dot (DQD) system coupled to a single mode photon cavity, DQD-cavity. The DQD system has a complex multilevel energy spectrum, in which by tuning the photon energy several anti-crossings between the electron states of the DQD system and photon dressed states are produced, which have not been seen in a simple two level DQD system. Three different regions of the photon energy are studied based on anti-crossings, where the photon energy ranges are classified as "low", "intermediate", and "high". The anti-crossings represent multiple Rabi-resonances, which lead to a current dip in the electron transport at the "intermediate" photon energy. Increasing the electron-photon coupling strength, $g_γ$, the photon exchanges between the anti-crossing states are changed leading to a dislocation of the multiple Rabi resonance states. Consequently, the current dip at the intermediate photon energy is further reduced. Additionally, we tune the cavity-environment coupling, $κ$, to see how the transport properties in the strong coupling regime, g$_γ>κ$, are changed for different directions of the photon polarization. Increasing $κ$ with a constant value of $g_γ$, a current enhancement in the intermediate photon energy is found, and a reduction in the current is seen for the "high" photon energy range. The current enhancement in the intermediate photon energy is caused by the weakening of the multiple Rabi-resonance in the system.

cond-mat.mes-hall↗

arXiv2022

Enhanced ultraviolet absorption in BN monolayers caused by tunable buckling

The optical properties of a hexagonal Boron Nitride (BN) monolayer across the UV spectrum are studied by tuning its planar buckling. The strong $σ\text{-}σ$ bond through sp$^2$ hybridization of a flat BN monolayer can be changed to a stronger $σ\text{-}π$ bond through sp$^3$ hybridization by increasing the planar buckling. This gives rise to the $s$- and $p$-orbital contributions to form a density of states around the Fermi energy, and these states dislocate to a lower energy in the presence of an increased planar buckling. Consequently, the wide band gap of a flat BN monolayer is reduced to a smaller band gap in a buckled BN monolayer enhancing its optical activity in the Deep-UV region. The optical properties such as the dielectric function, the reflectivity, the absorption, and the optical conductivity spectra are investigated. It is shown that the absorption rate can be enhanced by $(12\text{-}15)\%$ for intermediate values of planar buckling in the Deep-UV region, and $(15\text{-}20)\%$ at higher values of planar buckling in the near-UV region. Furthermore, the optical conductivity is enhanced by increased planar buckling in both the visible and the Deep-UV regions depending on the direction of the polarization of the incoming light. Our results may be useful for optoelectronic BN monolayer devices in the UV range including UV spectroscopy, deep-UV communications, and UV photodetectors.

cond-mat.mtrl-sci↗

arXiv2022

Thermal transport controlled by intra- and inter-dot Coulomb interactions in sequential and cotunneling serially-coupled double quantum dots

We study thermoelectric transport through a serial double quantum dot (DQD) coupled to two metallic leads with different thermal energies. We take into account the electron sequential and cotunneling effects via different master equation approaches. In the absence of intra- and inter-dot Coulomb interactions, a small peak in thermoelectric and heat currents is found for $E_{\rm L} \text{=} E_{\rm R}$ indicating the Coulomb blockade DQD regime, where $E_{\rm L}(E_{\rm R})$ is the energy of the state of the left(right) quantum dot. In the presence of intra- and inter-dot Coulomb interactions with strengths U$_{\rm intra}$, and U$_{\rm inter}$, respectively, avoided crossings or resonance energies between the intra- and the inter-dot two-electron states, 2ES, are found. These resonances induce extra transport channels through the DQD leading to strong side peaks in the thermoelectric and heat currents at $ E_{\rm L} \text{-} E_{\rm R} = \pm (U_{\rm intra} \text{-} U_{\rm inter})$ in addition to the main peak generated at $E_{\rm L} \text{=} E_{\rm R}$. The current side peaks are enhanced by increased strength of the Coulomb interactions. Interestingly, the current side peaks are enhanced when cotunneling terms are considered in which the resonances of the 2ESs assist the electron cotunneling process through the system. Furthermore, the issue of coherences is carefully checked in the DQD-leads system via different approaches to the master equation, which are the Pauli, the Redfield, a first order Lindblad, and the first- and second order von-Neumann methods. We realize that the Pauli method gives a wrong results for the thermoelectric transport when the role of the coherences is relevant.

cond-mat.mes-hall↗

arXiv2022

Electronic and Optical properties of Metallic Nitride: A comparative study between the MN (M=Al, Ga, In, Tl) monolayers

The electronic and the optical properties of metallic nitride (MN) monolayers are studied using a DFT formalism. In most of these monolayers, the electron density of the metallic atoms is much higher than that of the nitride atoms, and ionic, covalent, and metallic bonds are found in M-N bonds, resulting in fascinating electronic and optical properties. The optical band gap is varied from almost $0.0$ to $3.0$~eV for the MN monolayers depending on the bond type between the metallic and the nitride atoms, as well as the contribution of the type of orbitals around the Fermi energy. The optical properties such as the dielectric function, the excitation spectra, the refractive index, the reflectivity, and the optical conductivity of MN monolayers are calculated. The excitation energy and static dielectric constant are found to be inversely proportional to the band gap at low photon energy. The MN monolayers with a large band gap have good visible light functionality, while the MN monolayers with a lower band gap are found to be active in the infrared region. Furthermore, it is shown that the optical properties of MN monolayers show a strong anisotropy with respect to the polarization of the incoming light. Consequently, our results for the optical properties of MN monolayers show that they could be beneficial in optoelectronic device applications.

cond-mat.mtrl-sci↗

arXiv2021

Controlling thermoelectric, heat, and energy currents through a quantum dot in sequential and cotunneling Coulomb-blockade regimes

Thermal transport through a Coulomb-blockade quantum dot (QD) coupled to two metallic leads is studied using five different approaches to the master equation in which sequential and coutuneling terms are taken into account. In the presence of intradot Coulomb interactions, a plateau in the thermo-particle, the heat, and the energy currents is seen. The current plateau diminishes at a high thermal bias between the leads. It is shown that the Pauli, the Redfield, the Lindblad-type equation with first order tunneling rates, and first-order von-Neumann master equations give very similar thermal transport indicating the conservation of coherency in the electron transport in sequential tunneling between the QD and leads. In contrast, the thermal transport is suppressed when coutuneling processes are taken into account via a second-order von-Neumann master equation. The consideration of second order effects with respect to the QD-leads coupling brings in a wealth of virtual processes at the contact to each lead. These virtual processes directly weaken the effects of the contribution of the first order direct processes to the overall transport, and introduce important other aspects of the transport, as level broadening, energy shifts, and lifetimes in the time-domain. As a result the current plateau formed via the Coulomb interaction diminishes, when second order and cotunneling processes are considered.

cond-mat.mes-hall↗

arXiv2021

Unified approach to cyclotron and plasmon resonances in a periodic 2DEG hosting the Hofstadter butterfly

We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due to the lateral confinement and the magnetic field for a different number of electrons in each dot. In the limit of vanishing dot modulation of the 2DEG we find signs of the structure of the Hofstadter butterfly in the excitation spectra.

cond-mat.mes-hall↗

arXiv2021

DFT study of tunable electronic, magnetic, thermal, and optical properties of a Ga$_2$Si$_6$ monolayer

The electrical, magnetic, thermal and optical characteristics of Gallium (Ga) doped silicene are investigated using density functional theory (DFT). The effect of doping is studied by tuning dopant concentrations as well as examining varied doping distances, and atomic dopant interactions for the same substitutional doping concentration. The results indicate that the Ga atoms alter the band structure and the band gap in the silicene monolayer at various concentrations, which can be referred back to to the repulsive interaction of Ga-Ga atoms. The band gap is determined by the interaction strength of the Ga-Ga atoms, the Coulomb repulsive force, and it does not always widen as doping concentration increases. In addition, our spin-polarized DFT calculations show that these monolayers behave like nonmagnetic semiconductors, exhibiting symmetric spin-up and spin-down channels. The repulsive interaction between the Ga atoms causes a symmetry breaking of the monolayers. As a consequence, a Ga dopant can open the band gap, leading to better thermoelectric properties such as the Seebeck coefficient and the figure of merit, as well as an increase in the optical response. As a result of our estimates, Ga doped silicene monolayers could be advantageous in thermoelectric and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2021

High thermoelectric and optical conductivity driven by the interaction of Boron and Nitrogen dopant atoms with a 2D monolayer Beryllium Oxide

The electronic, thermal and optical properties of a monolayer BeO with Boron (B) and Nitrogen (N) co-dopant atoms are studied by means of a density functional theory computation. Our calculations reveal that BeO with BN-codopant atoms can give rise to more effective and outstanding performance for the thermal and optical responses. More significantly, the monolayer BeO with BN codopant atoms becomes a semiconductor with a direct band gap in comparison with the insulator behavior of pristine BeO. The particular attention of this work is paid to the influence of the atomic configuration and the interaction of the B and N dopant atoms with BeO. The interaction of the B and N atoms with the BeO monolayer diminishes degenerate energy states forming flat bands. It is also found that there is a strong attractive interaction between the O and N atoms forming a strong sigma bond breaking the symmetry of BeO structure. Consequently, the band gap is reduced leading to a semiconductor behavior with improved thermoelectric properties such as the Seebeck coefficient and the figure of merit. The reduced band gap and the flat bands induce a high optical responses such as the refractive index, the reflectivity and the optical conductivity in the visible light region. In addition, the anisotropy of a monolayer BeO with B and N atoms regarding different direction of electromagnetic polarization is presented. We anticipate that our results can be useful for design of both thermoelectric and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Enhanced electronic and optical responses of Nitrogen- or Boron-doped BeO monolayer: First principle computation

In this work, the electronic and optical properties of a Nitrogen (N) or a Boron (B) doped BeO monolayer are investigated in the framework of density functional theory. It is known that the band gap of a BeO monolayer is large leading to poor material for optoelectronic devices in a wide range of energy. Using substitutional N or B dopant atoms, we find that the band gap can be tuned and the optical properties can be improved. In the N(B)-doped BeO monolayer, the Fermi energy slightly crosses the valence(conduction) band forming a degenerate semiconductor structure. The N or B atoms thus generate new states around the Fermi energy increasing the optical conductivity in the visible light region. Furthermore, the influences of dopant atoms on the electronic structure, the stability, the dispersion energy, the density of states, and optical properties such as the plasmon frequency, the excitation spectra, the dielectric functions, the static dielectric constant, and the electron energy loss function are discussed for different directions of polarizations for the incoming electric field.

cond-mat.mtrl-sci↗

arXiv2021

Modulation of electronic and thermal proprieties of TaMoS$_2$ by controlling the repulsive interaction between Ta dopant atoms

We theoretically study the electronic and the thermal characteristics of Tantalum, Ta, doped Molybdenum disulfide, MoS$_2$, using density functional theory. It has been shown that the MoS$_2$ monolayer is not a good material for thermoelectric devices due to its relatively large band gap. We find that a Ta doped MoS$_2$ forming a TaMoS$_2$ monolayer can be useful for thermoelectric devices. The particular attention of this work is paid to the interaction effect between the Ta atoms in the MoS$_2$ structure. We find that the interaction type is repulsive. It introduces an asymmetry in the density of states, DOS, reducing the band gap. In the presence of a strong repulsive interaction of Ta-Ta atoms, new states in the DOS around the Fermi energy are found leading to a reduction of the band gap. Consequently, a high Seebeck coefficient and figure of merit are seen over a wide range of energy around the Fermi energy. In contrast, a small reduction of the band gap and a vanishing degeneracy of the valence and the conduction bands are observed for the case of a weak Ta-Ta repulsive interaction leading to less promising thermoelectric properties.

cond-mat.mes-hall↗

arXiv2021

Interaction effects in a two-dimensional AlSi$_6$P nanosheet: A first-principles study on the electronic, mechanical, thermal, and optical properties

Physical quilities such as electronic, mechanical, thermal, and optical properties of Al and P codoped silicene forming AlSi$_6$P nanosheets are investigated by first-principle calculations within density functional theory. A particular attention of this study is paid to the interaction effect between the Al and P atoms in the buckled silicene structure. We infer that the interaction type is attractive and it leads to an asymmetry in the density of states opening up a band gap. In contrast to BN-codoped silicene, the buckling in the Al-P codoped silicene is not much influenced by the dopants as the bond lengths of Si-P and Al-P are very similar to the Si-Si bond lengths. On the other hand, the longer bond length of Si-Al decreases the stiffness and thus induces fractures at smaller values of applied strain in AlSi$_6$P. The elastic and nonelastic regions of the stress-strain curve of AlSi$_6$P depend on the placement of the Al and P atoms. The finite band gap caused by the Al-P dopant leads to enhancement of the Seebeck coefficient and the figure of merit, and induces a redshift of peaks in the dielectric response, and the optical conductivity. Finally, properties of the real and imaginary parts of the dielectric function, the excitation spectra, the refractive index, and the optical response of AlSi$_6$P are reported.

cond-mat.mes-hall↗

arXiv2021

Electromagnetic field emitted by core-shell semiconductor nanowires driven by an alternating current

We consider tubular nanowires with a polygonal cross-section. In this geometry the lowest energy states are separated in two sets, one of corner and one of side-localized states, respectively. The presence of an external magnetic field transverse to the nanowire imposes an additional localization mechanism, the electrons being pushed sideways relatively to the direction of the field. This effect has important implications on the current density, as it creates current loops induced by the Lorentz force. We calculate numerically the electromagnetic field radiated by hexagonal, square, and triangular nanowires. We demonstrate that, because of the aforementioned localization properties, the radiated field can have a complex distribution determined by the internal geometry of the nanowire. We suggest that measuring the field in the neighborhood of the nanowire could be the basic idea of a tomography of the electron distribution inside it, if a smaller receiver antenna could be placed in that zone.

cond-mat.mes-hall↗