arXiv subjects

Vidar Gudmundsson

Publications and source records attributed to Vidar Gudmundsson.

At least 19 recordsLinked to original sources

arXiv2026

Rashba-Cavity Engineering of Energy Gaps and Spin-Hall Conductivity in Semiconducting Artificial Graphene

We investigate the combined effect of a far - infrared cavity field and Rashba spin - orbit interaction on the band structure and transport properties of artificial graphene composed of quasi-2D InAs/GaAs quantum dots. The coupling to cavity photons is modeled by constructing a complete basis as the tensor product of the electronic Hilbert space and the Fock space. Our numerical calculations for the system embedded in a linear cavity predict the existence of both type-I and type-II Dirac points which can be distinguished by their response to Rashba interaction. Namely, Rashba coupling opens a gap at type-II Dirac points, while type-I Dirac points remain gapless. The possibility of gap-opening for type-II Dirac points is demonstrated analytically as well. For both cylindrical and linear cavities, we demonstrate the formation of electron - photon hybrid states and Rabi splittings between energy minibands. Multiple crossings, and anticrossings between Dirac-band replicas produce pronounced modifications of the spin-Hall conductivity, including strong anisotropy and oscillatory behavior controlled by cavity geometry and polarization of photons. Our results show that the interplay between Rashba and cavity couplings governs Dirac-point physics and provides a route toward tunable polaritonic spin-transport in engineered nanostructures.

cond-mat.mes-hall↗

arXiv2026

NIR and visible structured beam generation with a dual-force mechanical long-period fiber grating

This work presents tunable generation of vortex, vector, and flat-top 1060-nm NIR beams in a few-mode fiber with a dual-force mechanical long-period fiber grating. By varying the applied forces on the fiber grating, the core mode to higher-order mode excitation can be adjusted. Manipulating the beam transformation is achieved by controlling the polarization of the fiber eigenmodes and the mode-coupling efficiency. In addition, our dual-force scheme enables greater flexibility in the mode selection by introducing the force gradient as an additional degree of freedom. By precisely tuning the intensity ratio between fundamental and doughnut modes, we arrive at the generation of propagation-invariant flat-top beams, for which the transverse intensity distribution exhibits a near-uniform profile over a propagation distance of 5 m in free space. The transverse optical field of 532-nm green light from a frequency-doubled Nd-doped yttrium vanadate laser can also be manipulated and coupled into various intensity distributions in a few-mode fiber by using a mechanically induced long-period fiber grating. We show that a doughnut beam, Mexican-hat beam, and crater-lake beam can be generated from the input Gaussian beam via the coupling of the fundamental core mode to a series of co-propagating higher-order modes with properly applied forces and polarizations. Our experimental results establish dual-force mechanically induced LPGs as a flexible and reconfigurable fiber-based tool for structured beam engineering, offering a unified platform for in-situ and robust control of phase, polarization, and amplitude of optical fields across different spectral bands.

physics.optics↗

arXiv2026

Driven square lattice of quantum dots in a magnetic field coupled to a cylindrical FIR-photon cavity

We present a comprehensive computational study of driven quantum dot arrays in a square lattice configuration, subject to an external magnetic field and coupled to a cylindrical far-infrared photon cavity. The driving is introduced through a harmonic modulation of the full electron-photon interaction, therefore including both paramagnetic and diamagnetic contributions. The electron-electron Coulomb interactions are treated within density functional theory, while the electron-photon coupling is modeled using a many-body configuration interaction approach at each iteration of the density functional. By exploiting the unique properties of the cylindrical TE$_{011}$ cavity mode, we demonstrate selective enhancement of diamagnetic two-photon transitions. Our results reveal that the effectiveness of harmonic modulation of the electron-photon interaction is strongly dependent on both the driving frequency and the electron occupation number per dot. When the driving frequency approaches twice the cavity photon frequency, the system exhibits resonant behavior characterized by efficient photon pumping, occupation of higher-order photon replicas, and activation of collective radial Coulomb breathing modes. These findings establish a controllable mechanism for manipulating photon states in coupled quantum dot-cavity systems and provide insights into the interplay among harmonic modulation, photonic excitations, magnetic confinement, and many-body electron correlations in dimensionally reduced nanostructures.

cond-mat.mes-hall↗

arXiv2026

Suppression of Bloch Oscillations and Nonreciprocal Landau-Zener Tunneling in Bose-Einstein Quantum Droplets

We investigate the nonlinear Bloch dynamics and Landau-Zener (LZ) tunneling of quantum droplets in optical lattices. We show that the Lee-Huang-Yang (LHY) correction not only stabilizes the self-bound droplet, but also introduces nonlinear phase feedback that competes with the lattice-induced coherent motion. In the deep-lattice regime, applying a generalized super-Gaussian ansatz within the tight-binding model demonstrates that chirp accumulation modifies the internal phase profile and renormalizes mobility. The coherent Bloch oscillations (BO) are progressively arrested in the presence of the LHY interaction without dissipative damping. In the shallow-lattice regime, the system is mapped onto a nonlinear two-level Josephson-analog model in which the mean-field and LHY contributions enter through an effective nonlinear detuning, deforming the adiabatic spectrum and generating looped bands. Using the classical action-angle formulation, we demonstrate that the nonlinear LZ tunneling is governed by the underlying phase-space structure. In particular, the LHY correction suppresses the tunneling probability by modifying the separatrix action and renormalizing the exponential sweep-rate scaling through a nonlinear weighting factor. We further identify pronounced nonreciprocal LZ tunneling arising from branch-dependent population imbalance and the nonlinearly induced inertia. These results establish a unified mechanism in which the LHY interaction suppresses both coherent Bloch dynamics and interband tunneling by reorganizing the dynamical exchange among lattice motion, population imbalance, and internal phase modulation.

cond-mat.quant-gas↗

arXiv2026

First-Principles Study of Structural, Electronic, Thermal, and Optical Properties of Quasi-2D C2 N2 O Using GGA and HSE06

DFT and AIMD are used to investigate the structural, stability, electronic, thermal, and optical properties of the quasi-2D C2N2O structure. The structure exhibits thermal and energy stability, signifying robustness under ambient conditions, however less dynamical stability is observed. The electronic structure investigation reveals that C2N2O displays semiconducting properties with a moderate indirect band gap resulting from the hybridisation of p-orbitals of N, C, and O atoms, with band gap values of 2.3 eV (GGA) and 3.9 eV (HSE06). The optical properties, including the dielectric function, optical conductivity, and refractive index, are thoroughly analyzed to clarify the electronic transitions. The material exhibits considerable optical absorption in the visible and ultraviolet spectrum, with notable anisotropy between in-plane and out-of-plane polarizations. Furthermore, plasmon resonance occurs at around 3.8 eV, relating to the collective oscillations of charge carriers. The thermal properties indicate a heat capacity of around 382 J/mol.K at 300 K, which is close to and slightly above the standard Dulong-Petit limit for this structure, indicating near-complete excitation of lattice vibrational modes at room temperature. The lattice thermal conductivity is extremely low, reaching approximately 0.017 W/m.K at 300 K, primarily attributed to significant phonon scattering, evidenced by a scattering rate of roughly 3.2 1/ps in the phonon frequency ranges. The findings demonstrate that the C2N2O structure maintains structural stability while allowing for tunable electronic, optical, and thermal properties, making it a promising candidate for nanoscale optoelectronic and thermal control applications.

cond-mat.mtrl-sci↗

arXiv2025

Enhanced Efficiency of Intermediate-Band Semiconductor Solar Cells Embedded with Quantum Dot Superlattices

We present a multiscale approach for modeling an intermediate-band solar cell based on a GaAs-GaAlAs quantum dot superlattice of cubic symmetry. Our framework combines high-accuracy theoretical calculations of the superlattice band structure and miniband-related absorption coefficient with experimentally determined interband absorption data. The quantum-mechanically derived absorption spectrum is incorporated into a drift-diffusion transport model in COMSOL Multiphysics, where key processes, including thermal and radiative recombination, are taken into account. This integrated methodology enables realistic modeling of device performance. Our results identify an optimal superlattice constant of 14 nm, yielding a maximum solar cell efficiency of 13.3 percent. Further increase in the superlattice constant enhances the miniband-related absorption peak but reduces the generation rate in the n-type region, resulting in a net efficiency decrease. The proposed approach, integrating theoretical, experimental, and computational components, provides a reliable framework for assessing solar cells based on quantum dot superlattices.

cond-mat.mes-hall↗

arXiv2025

Signatures of broken symmetries in the excitations of a periodic 2DEG coupled to a cylindrical photon cavity

In a two-dimensional electron gas (2DEG) in a periodic lateral superlattice subjected to an external homogeneous magnetic field and in a cylindrical far-infrared photon cavity we search for effects of broken symmetries: Static ones, stemming from the unit cell of the system, and the external magnetic field together with the dynamic ones caused by the vector potential of the cavity promoting magnetic types of transitions, and the chirality of the excitation pulse. The Coulomb interaction of the electrons is described within density functional theory, but the electron-photon interactions are handled by a configuration interaction formalism within each step of the density functional approach, both for the static and the dynamic system. In the dynamical calculations we observe weak chiral effects that change character as the strength of the electron-photon interaction and the external magnetic field are increased. From the analysis of the chiral effects we identify an important connection of the para- and diamagnetic electron-photon interactions that promotes the diamagnetic interaction in the present system when the interaction strength is increased. Furthermore, the asymmetric potential in the unit cell of the square array activates collective oscillation modes that are not present in the system when the unit cell has a higher symmetry.

cond-mat.mes-hall↗

arXiv2024

Spin-phase transition in an array of quantum rings controlled by cavity photons

We model a spin-phase transition in a two-dimensional square array, or a lateral superlattice, of quantum rings in an external perpendicular homogeneous magnetic field. The electron system is placed in a circular cylindrical far-infrared photon cavity with a single circularly symmetric photon mode. Our numerical results reveal that the spin ordering of the two-dimensional electron gas in each quantum ring can be influenced or controlled by the electron-photon coupling strength and the energy of the photons. The Coulomb interaction between the electrons is described by a spin-density functional approach, but the para- and the diamagnetic electron-photon interactions are modeled via a configuration interaction formalism in a truncated many-body Fock-space, which is updated in each iteration step of the density functional approach. In the absence of external electromagnetic pulses this spin-phase transition is replicated in the orbital magnetization of the rings. The spin-phase transition can be suppressed by a strong electron-photon interaction. In addition, fluctuations in the spin configuration are found in dynamical calculations, where the system is excited by a time-dependent scheme specially fit for emphasizing the diamagnetic electron-photon interaction.

cond-mat.mes-hall↗

arXiv2024

Magneto-optical properties of electron gas in a chain of planar quantum rings: Effect of screening on the signature of quantum phase interference

The equilibrium properties and interminiband transitions for Hartree-interacting two-dimensional electron gas in a one-dimensional chain of planar quantum rings subjected to a transverse homogeneous magnetic field are examined theoretically and numerically. The proposed analytical models for the external modulation potential and the basis wave-function reflect the symmetry and the topology of the system allowing to get high accuracy results in comparatively short computational times. The calculated dependencies of the electronic band structure versus magnetic field show two detached minibands in the region of low energies, while higher minibands manifest multiple crossings and anticrossings. The existence of highly degenerate energy levels (referred to as miniband nodes) for certain values of magnetic field is revealed. The miniband nodes are preserved when taking into account the Coulomb interaction of the electrons. The interaction leads to an up-shift and a broadening of the minibands, as well as to a shift of the nodes toward the larger values of the magnetic field. The miniband nodes have their signature in the magnetization of the electron gas and in the interminiband transitions. The number of electrons per a unite cell of the system has a strong impact on the current density distribution, magnetization and the oscillator strength. The obtained results open new opportunities for a flexible manipulation of magneto-optical properties of future devices operating in far-infrared and THz regimes.

cond-mat.mes-hall↗

arXiv2024

The tuning of para- and diamagnetic cavity photon excitations in a square array of quantum dots in a magnetic field

We employ a ``real-time'' excitation scheme to calculate the excitation spectra of a two-dimensional electron system in a square array of quantum dots placed in a circular cylindrical far-infrared photon cavity subjected to a perpendicular homogeneous external magnetic field. The Coulomb interaction of the electrons is handled via spin density functional theory and the para- and the diamagnetic parts of the electron-photon coupling are updated according to a configuration interaction method in each iteration of the density functional calculation. The results show that an excitation scheme built on using the symmetry of the lateral square superlattice of the dots and the cylindrical cavity produces both para- and diamagnetic resonance peaks with oscillator strengths that can be steered by the excitation pulse parameters. The excitation method breaks the conditions for the generalized Kohn theorem and allows for insight into the subband structure of the electron system and can be used both in and outside the linear response regime.

cond-mat.mes-hall↗

arXiv2024

Magneto-optical properties of a quantum dot array interacting with a far-infrared photon mode of a cylindrical cavity

We model the equilibrium properties of a two-dimensional electron gas in a square lateral superlattice of quantum dots in a GaAs heterostructure subject to an external homogeneous perpendicular magnetic field and a far-infrared circular cylindrical photon cavity with one quantized mode, the TE011 mode. In a truncated linear basis constructed by a tensor product of the single-electron states of the noninteracting system and the eigenstates of the photon number operator, a local spin density approximation of density functional theory is used to compute the electron-photon states of the two-dimensional electron gas in the cavity. The common spatial symmetry of the vector fields for the external magnetic field and the cavity photon field in the long wavelength approximation enhances higher order magnetic single- and multi-photon processes for both the para- and the diamagnetic electron-photon interactions. The electron-photon coupling introduces explicit photon replicas into the bandstructure and all subbands gain a photon content, constant for each subband, that can deviate from an integer value as the coupling is increased or the photon energy is varied. The subbands show a complex Rabi anticrossing behavior when the photon energy and the coupling bring subbands into resonances. The complicated energy subband structure leads to photon density variations in reciprocal space when resonances occur in the spectrum. The electron-photon coupling polarizes the charge density and tends to reduce the Coulomb exchange effects as the coupling strength increases.

cond-mat.mes-hall↗

arXiv2023

Buckling effects in AlN monolayers: Shifting and enhancing optical characteristics from the UV to the near visible light range

The structural, electronic, and optical properties of flat and buckled AlN monolayers are investigated using first-principles approaches. The band gap of a flat AlN monolayer is changed from an indirect one to a direct one, when the planar buckling increases, primarily due to diminishing sp$^2$ overlapping and bond symmetry breaking in the conversion to sp$^3$ bonds. The sp$^3$ hybridization thus results in a stronger $σ\text{-}π$ bond rather than a $σ\text{-}σ$ covalent bond. The calculations of the phonon band structure indicates that the buckled AlN monolayers are structurally and dynamically stable. The optical properties, such as the dielectric function, the refractive index, and the optical conductivity of an AlN monolayer are evaluated for both flat systems and those impacted with planar buckling. The flat AlN monolayer has outstanding optical characteristics in the Deep-UV and absorbs more effectively in the UV spectrum due to its large band gap. The results reveal that optical aspects are enhanced along different directions of light polarization, with a considerable shift in the optical spectrum from Deep-UV into the visible range. Additionally, depending on the polarization direction of the incoming light, increased planar buckling enhances the optical conductivity in both the visible and the Deep-UV domains. The ability to modify the optical and electronic properties of these essential 2D materials using planar buckling technique opens up new technological possibilities, particularly for optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Controlling electronic, magnetic, thermal, and optical properties of boron-nitrogen codoped strontium oxide monolayer: Activation of optical transitions in the VL region

The electronic, thermal, magnetic and optical properties of BN-codoped strontium oxide (SrO) monolayers are studied taking into account the interaction effects between the B and the N dopant atoms. The indirect band gap of a pure two dimensional SrO is modified to a narrow direct band gap by tuning the B-N attractive interaction. The B or N separately doped SrO leads to a metallic behavior, while a BN-codoped SrO has a semiconductor character. The strong B-N attractive interaction changes a non-magnetic SrO to a magnetic system and reduces its heat capacity. An ab initio molecular dynamics, AIMD, calculations are also utilized to check the thermodynamic stability of the pure and BN-codoped SrO monolayers. The band gap reduction of SrO increases the optical conductivity shifting the most intense peak from the Deep-UV to the visible light region. The red shifted optical conductivity emerges due to the B-N attractive interaction. In addition, both iso- and anisotropic characters are seen in the optical properties depending on the strength of the B-N attractive interaction. It can thus be confirmed that the interaction effects of the BN-codopants can be used to control the properties of SrO monolayers for thermo- and opto-electronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Exploring electronic, optical, and phononic properties of MgX (X=C, N, and O) monolayers using first principle calculations

The electronic, the thermal, and the optical properties of hexagonal MgX monolayers (where X=C, N, and O) are investigated via first principles studies. Ab-initio molecular dynamic, AIMD, simulations using NVT ensembles are performed to check the thermodynamic stability of the monolayers. We find that an MgO monolayer has semiconductor properties with a good thermodynamic stability, while the MgC and the MgN monolayers have metallic characters. The calculated phonon band structures of all the three considered monolayers shows no imaginary nonphysical frequencies, thus indicating that they all have excellent dynamic stability. The MgO monolayer has a larger heat capacity then the MgC and the MgN monolayers. The metallic monolayers demonstrate optical response in the IR as a consequence of the metal properties, whereas the semiconducting MgO monolayer demonstrates an active optical response in the near-UV region. The optical response in the near-UV is beneficial for nanoelectronics and photoelectric applications. A semiconducting monolayer is a great choice for thermal management applications since its thermal properties are more attractive than those of the metallic monolayer in terms of heat capacity, which is related to the change in the internal energy of the system.

cond-mat.mtrl-sci↗

arXiv2023

Planar buckling controlled optical conductivity of SiC monolayer from Deep-UV to visible light region: A first-principles study

The electrical and optical properties of flat and planar buckled siligraphene (SiC) monolayer are examined using a first principles approach. Buckling between the Si and the C atoms in SiC structures influences and impacts the properties of the 2D nanomaterial, according to our results. The electron density of a planar SiC monolayer is calculated, as well as the effects of buckling on it. According to our findings, a siligraphene monolayer is a semiconductor nanomaterial with a direct electronic band gap that decreases as the planar buckling rises. The contributions to the density of states differ owing to changes in the system's structure. Another explanation is that planar buckling reduces the sp$^2$ overlapping, breaking the bond symmetry causing it to become a sp$^3$ bond. We show that increased planar buckling between the Si and the C atoms alters the monolayer's optical, mechanical, and thermal properties. A managed planar buckling increases the optical conductivity with a significant shift in the far visible range, as all optical spectra features are red shifted, still remaining visible. Instead of a $σ\text{-}σ$ covalent bond, the sp$^3$ hybridization produces a stronger $σ\text{-}π$ bond. Optical characteristics such as the dielectric function, the absorbance, and the optical conductivity of a SiC monolayer are investigated for both parallel and perpendicular polarization of the incoming electric field for both flat and planar buckled systems. The findings show that the optical properties are influenced for both of these two polarizations, with a significant change in the optical spectrum from the near visible to the far visible. The ability to manipulate the optical and electrical characteristics of this critical 2D material through planar buckling opens up new technological possibilities, especially for optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Optical conductivity enhancement and thermal reduction of BN-codoped MgO nanosheet: Significant effects of B-N atomic interaction

We investigate the electronic, the thermal, and the optical properties of BN-codoped MgO monolayers taking into account the interaction effects between the B and the N dopant atoms. The relatively wide indirect band gap of a pure MgO nanosheet can be changed to a narrow direct band gap by tuning the B-N attractive interaction. The band gap reduction does not only enhance the optical properties, including the absorption spectra and the optical conductivity, but also the most intense peak is shifted from the Deep-UV to the visible light region. The red shifting of the absorption spectra and the optical conductivity are caused by the attractive interaction. In addition, both isotropic and anisotropic characteristics are seen in the optical properties depending on the strength of the B-N attractive interaction. The heat capacity is reduced for the BN-doped MgO monolayer, which can be referred to changes in the bond dissociation energy. The bond dissociation energy decreases as the difference in the electronegativities of the bonded atoms decreases. The lower difference in the electronegativities leads to a weaker endothermic process resulting in reduction of the heat capacity. An ab initio molecular dynamics, AIMD, calculation is utilized to check the thermodynamic stability of the pure and the BN-codoped MgO monolayers. We thus confirm that the BN-codopant atoms can be used to gain control of the properties of MgO monolayers for thermo- and opto-electronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Hofstadter-like spectrum and Magnetization of Artificial Graphene constructed with cylindrical and elliptical quantum dots

In this paper a comparative study of the electronic and magnetic properties of quasi-two-dimensional electrons in an artificial graphene-like superlattice composed of circular and elliptical quantum dots is presented. A complete orthonormal set of basis wave functions, which has previously been constructed in the frame of the Coulomb gauge for the vector potential has been implemented for calculation of the energy dispersions, the Hofstadter spectra, the density of states and the orbital magnetization of the considered systems, taking into account both the translational symmetry of the superlattice and the wave function phase-shifts due to the presence of a transverse external magnetic field. Our calculations indicate a topological change in the miniband structure due to the ellipticity of the quantum dots, and non-trivial modifications of the electron energy dispersion surfaces in reciprocal space with the change of the number of magnetic flux quanta through the unit cell of the superlattice. The ellipticity of the QDs leads to an opening of a gap and considerable modifications of the Hofstadter spectrum. The orbital magnetization is shown to reveal significant oscillations with the change of the magnetic flux. The deviation from the circular geometry of quantum dots has a qualitative impact on the dependencies of the magnetization on both the magnetic flux and the temperature.

cond-mat.mes-hall↗

arXiv2023

Magnetic properties of a cavity-embedded square lattice of quantum dots or antidots

We apply quantum electrodynamical density functional theory to obtain the electronic density, the spin polarization, as well as the orbital and the spin magnetization of square periodic arrays of quantum dots or antidots subjected to the influence of a far-infrared cavity photon field. A gradient-based exchange-correlation functional adapted to a two-dimensional electron gas in a transverse homogeneous magnetic field is used in the theoretical framework and calculations. The obtained results predict a non-trivial effect of the cavity field on the electron distribution in the unit cell of the superlattice, as well as on the orbital and the spin magnetization. The number of electrons per unit cell of the superlattice is shown to play a crucial role in the modification of the magnetization via the electron-photon coupling. The calculations show that cavity photons strengthen the diamagnetic effect in the quantum dots structure, while they weaken the paramagnetic effect in an antidot structure. As the number of electrons per unit cell of the lattice increases the electron-photon interaction reduces the exchange forces that would otherwise promote strong spin splitting for both the dot and the antidot array.

cond-mat.mes-hall↗