arXiv subjects

Nzar Rauf Abdullah

Publications and source records attributed to Nzar Rauf Abdullah.

At least 37 records · Page 2Linked to original sources

arXiv2022

Thermal transport driven by Coulomb interactions in quantum dots: Enhancement of thermoelectric and heat currents

We investigate thermal transport in a serial asymmetric double quantum dot (DQD) coupled to two electron reservoirs with different temperatures. The inter- and intra-Coulomb interactions are taken into account in a Coulomb blockade DQD where the electron sequential tunneling via four different master equation approaches is considered. In the absence of Coulomb interactions, a neglectable thermoelectric and heat currents is found identifying as the Coulomb blockade DQD regime. In the presence of Coulomb interactions, intra- and inter-Coulomb interactions, crossings energies between the intra- and the inter-dot many-body electron states are observed. The crossings induce extra channels in the energy spectrum of the DQD that enhance thermoelectric and heat currents. The extra channels form several peaks in the thermoelectric and heat currents in which intensity and position of the peaks depend on strength of the inter- and intra-dot Coulomb interactions. In addition, the problem of coherences and incoherences are studied using different approaches to the master equation, which are the first order von-Neumann, the Redfield, a first order Lindblad, and the Pauli methods. We find that all methods give almost similar thermal transport when the role of the coherences is irrelevant in the DQD.

cond-mat.mes-hall↗

arXiv2022

Temperature dependence of the single photon source efficiency based on QD-cQED

We study a photonic circuit consisting of a quantum dot, QD, coupled to a photon cavity over a wide range of temperature up to room temperature. A key component of such a system is presented here in the form of a Purcell-enhanced single-photon source based on Cavity Quantum Electrodynamics, cQED. We use a real set of pure dephasing data extracted from experimental measurements of InGaAs QD to calculate the effective QD-cavity coupling strength, the Purcell factor, and the single photon efficiency emerged from the QD-cavity system in the cases without and with detuning. In the non-detuned system, the effective coupling strength between the QD and the resonator decreases with increasing temperature, results in a decrease in efficiency. However, when the temperature of the QD-cavity system increases under Purcell effect conditions, the detuned QD-cavity system induces spontaneous emission rate enhancement. As a result, we found that the increase in efficiency can be obtained under a certain condition, when the maximum effective coupling strength and the Purcell factor are related to the spontaneous emission and the pure dephasing rates. Additionally, the influences of the pumping mechanism on the efficiency of the QD-system were examined and showed that the pumping process can be used to further increase in efficiency. Our results can be advantageous for advanced quantum optics applications once temperature is taken into account.

quant-ph↗

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

Properties of BC$_6$N monolayer derived by first-principle computation: Influences of interactions between dopant atoms

The properties of graphene-like BC$_6$N semiconductor are studied using density functional theory taking into account the attractive interaction between B and N atoms. In the presence of a strong attractive interaction between B and N dopant atoms, the electron charge distribution is highly localized along the B-N bonds, while for a weaker attractive interaction the electrons are delocalized along the entire hexagonal ring of BC$_6$N. Furthermore, when both B and N atoms are doped at the same site of the hexagon, the breaking of the sub-lattice symmetry is low producing a small bandgap. In contrast, if the dopant atoms are at different sites, a high sub-lattice symmetry breaking is found leading to a large bandgap. The influences of electron localization/delocalization and the tunable bandgap on thermal behaviors such as the electronic thermal conductivity, the Seebeck coefficient, and the figure of merit, and optical properties such as the dielectric function, the excitation spectra, the refractive index, the electron energy loss spectra, the reflectivity, and the optical conductivity are presented. An enhancement with a red shift of the optical conductivity at low energy range is seen while a reduction at the high energy range is found indicating that the BC$_6$N structure may be useful for optoelectronic devices in the low energy, visible range.

cond-mat.mes-hall↗

arXiv2021

Study of BC$_{14}$N-bilayer graphene: Effects of atomic spacing and interatomic interaction between B and N atoms

We study the effects of an attractive interaction between the boron (B) and the nitrogen (N) atoms doped in a bilayer graphene (BLG), BC$_{14}$N, on the electronic, the thermal and the optical properties for two different types of a doping process: First, both the B and the N atoms are doped in the same layer while the other layer is undoped. Second, the B and N atoms are doped in both layers. An attractive interaction between the B and N atoms does not influence the interlayer interaction in the first case, while it does in the second case. We find that the strong B-N attractive interaction in one layer induces metallic behavior due to the crossing of the valence band and the Fermi energy, while the strong attractive interaction between both layers induces a semiconductor property arising from the emergence a bandgap. We therefore confirm that the metallic-like BLG is not a good material for thermal devices because it has a low figure of merit, while we notice that the semiconductor-like BLG has a high Seebeck coefficient and figure of merit as well as a low thermal conductivity. The strong attractive interaction of the B-N atoms between the layers gives rise to a prominent peak to appear in dielectric function, the excitation and the absorption spectra in the low energy, visible range, while a very weak peak is seen in the case of a strong attractive interaction between the B and N doped in one layer. Controlling the B and N atomic configurations in the BLG may help to improve the material for use in both thermoelectric and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2021

Role of interlayer spacing on electronic, thermal and optical properties of BN-codoped bilayer graphene:\break Influence of the interlayer and the induced dipole-dipole interactions

We demonstrate that the electronic, thermal, and optical properties of a graphene bilayer with boron and nitrogen dopant atoms can be controlled by the interlayer distance between the layers in which the interaction energy and the van der Waals interaction between the dopant atoms play an essential role. We find a conversion of an AA- to an AB-stacked bilayer graphene caused by the repulsive interaction between dopant atoms. At a short interlayer distance, a strong repulsive interaction inducing a strong electric dipole moment of the dopant atoms is found. This gives rise to a breaking of the high symmetry, opening up a bandgap. Consequently, a considerable change in thermoelectric properties such as the Seebeck coefficient and the figure of merit are seen. The repulsive interaction is reduced by increasing the interlayer distance, and at a large interlayer distance the conversion process of the stacking order vanishes. A small bandgap is found leading to a low Seebeck coefficient and a figure of merit. For both short and large interlayer distances, a prominent peak in the optical response is found in the visible range and the peak position is inversely proportional to the interlayer distance.

cond-mat.mes-hall↗

arXiv2021

Conversion of the stacking orientation of bilayer graphene due to \break the interaction of BN-dopants

A conversion of AA- to AB-stacking bilayer graphene (BLG) due to interlayer interaction is demonstrated. Two types of interlayer interactions, an attractive and a repulsive, between the Boron and Nitrogen dopant atoms in BLG are found. In the presence of the attractive interaction, an AA-stacking of BN-codoped BLG is formed with a less stable structure leading to weak mechanical properties of the system. Low values of the Young modulus, the ultimate strength and stress, and the fracture strength are observed comparing to a pure BLG. In addition, the attractive interaction induces a small bandgap that deteriorates the thermal and optical properties of the system. In contrast, in the presence of a repulsive interaction between the B and N atoms, the AA-stacking is converted to a AB-stacking with a more stable structure. Improved mechanical properties such as higher Young modulus, the ultimate strength and stress, fracture strength are obtained comparing to the AA-stacked BN-codoped BLG. Furthermore, a larger bandgap of the AB-stacked bilayer enhances the thermal and the optical characteristics of the system.

cond-mat.mes-hall↗

arXiv2020

Self-induction and magnetic effects in electron transport through a photon cavity

We explore higher order dynamical effects in the transport through a two-dimensional nanoscale electron system embedded in a three-dimensional far-infrared photon cavity. The nanoscale system is considered to be a short quantum wire with a single circular quantum dot defined in a GaAs heterostructure. The whole system, the external leads and the central system are placed in a constant perpendicular magnetic field. The Coulomb interaction of the electrons, the para- and diamagnetic electron-photon interactions are all treated by a numerically exact diagonalization using step-wise truncations of the appropriate many-body Fock spaces. We focus on the difference in transport properties between a description within an electric dipole approximation and a description including all higher order terms in a single photon mode model. We find small effects mostly caused by an electrical quadrupole and a magnetic dipole terms that depend strongly on the polarization of the cavity field with respect to the transport direction and the photon energy. When the polarization is aligned along the transport direction we find indications of a weak self-induction that we analyze and compare to the classical counterpart, and the self-energy contribution of high-order interaction terms to the states the electrons cascade through on their way through the system. Like expected the electron-photon interaction is well described in the dipole approximation when it is augmented by the lowest order diamagnetic part for a nanoscale system in a cavity in an external magnetic field.

cond-mat.mes-hall↗

arXiv2020

Properties of bilayer graphene-like Si$_{2}$C$_{14}$ semiconductor using first-principle calculations

We theoretically investigate silicon doped bilayer graphene, Si-BLG, with different stoichiometry of Si-BLG structures. The dangling bonds of C-Si atoms are found at low concentration ratio of Si atoms inducing sp$^3$-hybridization of buckled pattern in the structure. The Si-BLG with dangling bonds revel strong mechanical response at low strain of a uniaxial load, and the fracture strain is seen at low strain ratio. The sp$^3$-hybridization forms a small bandgap which induces an intermediate thermal and optical response of the system. In contrast, at higher Si concentration ratio, the Young modulus and fracture strain are increased comparing to the low Si concentration ratio. This is due to presence of high number of C-Si bonds which have a high tolerant under uniaxial load. In addition, a relatively larger bandgap or an overlap between valence and conduction bands are found depending on the Si configurations. In the presence of gaped Si-BLG, the thermal and optical response are high. We thus obtain a high Seebeck coefficient and a figure of merit with low electronic thermal conductivity which are useful for thermoelectric nanodevices, and an enhancement of optical response is acquired with a redshift in the visible range.

cond-mat.mtrl-sci↗