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We investigate by molecular dynamics simulations [arXiv:1412.4537 [physics.plasm-ph], arXiv:1608.06789 [physics.plasm-ph]] a beam of electrons released via field emission from a planar cathode surface of 1 ${\rm \mu m}^2$ with an inhomogeneous two-level work function, $\phi_{\rm low}$ and $\phi_{\rm high}$. A rectangular grid, where each cell can have one out of two values of the work function, is used as a model. The number of cells in the grid ranges from 6x6 to 96x96. We compare a periodic checkerboard arrangement with disordered distributions of patches. We perform multiple simulations and randomize the pattern each time. We study the beam behavior by calculating the position and velocity of each electron, r.m.s. emittance, and the brightness of the electron beam. The emittance increases while brightness decreases, with the mean distance between patches with $\phi_{\rm low}$ when they are in minority, and they switch the behavior vs. the mean distance between patches with $\phi_{\rm high}$ when these patches are in minority, respectively. The Coulomb interaction between all particles is fully included in our simulations., Comment: 8 pages, 8 figures

Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space-charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space-charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given., 8 pages, 9 figures

Publisher's version (útgefin grein), Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1., This work was supported by TE Contract no.30/2018 (PN-III-P1-1.1-TE-2016-2050, within PNCDI III), M-ERA.NET PhotoNanoP Contract No. 33/2016, PCE Contract No. 122/2017, PCCDI Contract No. 75/2018, and financed by CNCS-UEFISCDI, and by Romanian Ministry of Research and Innovation through NIMP Core Program PN19-03 (Contract No. 21N/08.02.2019)., "Peer Reviewed"

We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent.

Publisher's version (útgefin grein), In this work, we theoretically model the time-dependent transport through an asymmetric double quantum dot etched in a two-dimensional wire embedded in a far-infrared (FIR) photon cavity. For the transient and the intermediate time regimes, the current and the average photon number are calculated by solving a Markovian master equation in the dressed-states picture, with the Coulomb interaction also taken into account. We predict that in the presence of a transverse magnetic field the interdot Rabi oscillations appearing in the intermediate and transient regime coexist with slower non-equilibrium fluctuations in the occupation of states for opposite spin orientation. The interdot Rabi oscillation induces charge oscillations across the system and a phase difference between the transient source and drain currents. We point out a difference between the steady-state correlation functions in the Coulomb blocking and the photon-assisted transport regimes., The Research Fund of the University of Iceland, the Icelandic Research Fund, grant No. 163082-051, and the Icelandic Infrastructure Fund. CST acknowledges support from Ministry of Science and Technology of Taiwan under grant No. 106-2112-M-239-001-MY3. V. M. acknowledges financial support by the CNCS-UEFISCDI Grant PN-III-P4-ID-PCE-2016-0084. The computations were performed on resources provided by the Icelandic High Performance Computing Centre at the University of Iceland. The initial version of the manuscript was placed on the preprint server arXiv as: (arXiv: 1809.06930v1 [cond-mat.mes-hall])., "Peer Reviewed"

Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization ...

Unlike single layer graphene, in the case of $AB$-stacked bilayer graphene (BLG) one can induce a non-zero energy gap by breaking the inversion symmetry between the two layers using a perpendicular electric field. This is an essential requirement in field-effect applications, particularly since the induced gap in BLG systems can be further tuned by the magnitude of the external electric field. Doping is another way to modify the electronic properties of graphene based systems. We investigate here BLG systems doped with boron and nitrogen in the presence of external electric field, in the framework of density functional theory (DFT) calculations. Highly doped BLG systems are known to behave as degenerate semiconductors, where the Fermi energy depends on the doping concentration but, in addition, we show that the electronic properties drastically depend also on the applied electric field. By changing the magnitude and the orientation of the electric field, the gap size and position relative to the Fermi level may be tuned, essentially controlling the effect of the extrinsic doping. In this context, we discuss in how far the external electric field may suitably adjust the effective doping and, implicitly, the conduction properties of doped BLG systems., 10 pages, 13 figures

We study the effect of the so-called ion potential or non-kinetic energies of bombarding ions during ionized physical vapor deposition of Cu using molecular dynamics simulations. In particular we focus on low energy HiPIMS deposition, in which the potential energy of ions can be comparable to their kinetic energy. The ion potential, as a short-ranged repulsive force between the ions of the film-forming material and the surface atoms (substrate and later deposited film), is defined by the Ziegler-Biersack-Littmark potential. Analyzing the final structure indicates that, including the ion potential leads to a slightly lower interface mixing and fewer point defects (such as vacancies and interstitials), but resputtering and twinning have increased slightly. However, by including the ion potential the collision pattern changes. We also observed temporary formation of a ripple/pore with 5~nm height when the ion potential is included. The latter effect can explain the pores in HiPIMS deposited Cu thin films observed experimentally by atomic force microscopy., 8 pages, 5 figures

We calculate the charge and heat current associate with electrons, generated by a temperature gradient and chemical potential difference between two ends of a tubular nanowire of 30 nm radius in the presence of an external magnetic field perpendicular to its axis. We consider a nanowire based on a semiconductor material, and use the Landauer-Bttiker approach to calculate the transport quantities. We obtain the variation of the Seebeck coefficient ($S$), thermal conductivity kappa, and the figure of merit ($ZT$), with respect to the temperature up to 20\,K, and with the magnetic field up to 3 T. In particular we show that the Seebeck coefficient can change sign in this domain of parameters. In addition $\kappa$ and $ZT$ have oscillations when the magnetic field increases. These oscillations are determined by the energy spectrum of the electrons., Comment: 6 pages, 6 figures

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., RevTeX - pdfLaTeX, 15 pages with 15 included pdf-figures

We model monolayer graphene-like materials with BC$_6$N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC$_6$N are found based on the BN bonds: In the presence of BN bonds, an even number of $\pi$-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of $\pi$-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC$_6$N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC$_6$N without BN-bonds has more dynamical stability. Furthermore, all the BC$_6$N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC$_6$N systems caused by hot carrier--assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices., Comment: RevTeX, 9 pages with 6 included jpg figures

We model boron and nitrogen doped/codoped monolayer graphene to study its stability, interaction energy, electronic and thermal properties using density functional theory. It is found that a doped graphene sheet with non-bonded B or N atoms induces an attractive interaction and thus opens up the bandgap. Consequently, the power factor is enhanced. Additionally, bonded B or N atoms in doped graphene generate a repulsive interaction leading to a diminished bandgap, and thus a decreased power factor. We emphasis that enhancement of the power factor is not very sensitive to the concentration of the boron and nitrogen atoms, but it is more sensitive to the positions of the B or N atoms in ortho, meta, and para positions of the hexagonal structure of graphene. In the B and N codoped graphene, the non-bonded dopant atoms have a weak attractive interaction and interaction leading to a small bandgap, while bonded doping atoms cause a strong attractive interaction and a large bandgap. As a result, the power factor of the graphene with non-bonded doping atoms is reduced while it is enhanced for graphene with bonded doping atoms., RevTeX, 10 pages with 9 included pdf figures

The geometric, electronic, magnetic, thermal, and optical properties of transition metal (TM) doped silicene are systematically explored using spin-dependent density functional computation. We find that the TM atoms decrease the buckling degree of the silicene structure caused by the interaction between the dopant TM atoms and the Si atoms in the silicene layer plane which is quite strong. In some TM-silicenes, parallel bands and the corresponding van Hove singularities are observed in the electronic band structure without and with spin-polarization. These parallel bands are the origin of most of the transitions in the visible and the UV regions. A high Seebeck coefficient is found in some TM-silicene without spin-polarization. In the presence of emergent spin-polarization, a reduction or a magnification of the Seebeck coefficient is seen due to a spin-dependent phase transition. We find that the preferred state is a ferromagnetic state with a very high Curie temperature. We observe a strong interaction and large orbital hybridization between the TM atoms and the silicene. As a result, a high magnetic moment emerges in TM-silicene. Our results are potentially beneficial for thermospin, and optoelectronic nanodevices., Comment: RevTeX, 8 pages with 11 included eps figures

The buckling effects due to BN-bonds in BN-codoped silicene, BSi$_6$N, on structural stability, electronic band structure, and mechanical, thermal and optical properties are studied systematically by first-principle calculations within density functional theory. In the presence of BN-bonds, a high warping in BSi$_6$N indicating a high buckling effect is found due to the presence of a repulsive interaction between B and N atoms. It thus breaks the sublattice symmetry of silicene and opens up a bandgap. The high buckling of BSi$_6$N leads to a decrease in its stiffness and thus induces fractures at small values of applied strain. The finite bandgap caused by the BN-bonds leads to enhancement of the Seebeck coefficient and the figure of merit, and induces a redshift of a peak in the dielectric response. By increasing the distance between the B and N atoms i.e. for the BSi$_6$N without BN-bonds, a flatter BSi$_6$N is found compared to pristine silicene. The stiffness of the structure and the ultimate strain are increased. The breaking of the sublattice symmetry is very weak and a very small bandgap is revealed. As a result, the Seebeck coefficient and the figure of merit stay very small. A reduction in the optical response is seen due to an indirect bandgap., Comment: RevTeX, 9 pages with 6 included figures

The effects of geometry on the hosting of Majorana zero modes are explored in core-shell nanowires with a hexagonal core and a triangular shell, and vice versa. The energy interval separating electronic states localized in the corners from states localized on the sides of the shell is shown to be larger for a triangular nanowire with a hexagonal core, than a triangular one. We build the topological phase diagram for both cases and compare them to earlier work on prismatic nanowires with the same core and shell geometry. We suggest that a dual core nanowire is needed to allow for a braiding operation of Majorana zero modes at the nanowire end plane., Comment: 8 pages, 10 figures

We simulate numerically the classical charge dynamics in a microscopic, planar, vacuum diode with a finite emitter area and a finite number of electrons in the gap. We assume electrons are emitted under space-charge limited conditions with a fixed potential applied to the diode. The Coulomb interaction between all electrons is included using the method of molecular dynamics. We compare our results to the conventional two-dimensional Child-Langmuir and explain how it is limited in applicability for sub-micron diameter emitters. Finally, we offer some simple relations for understanding space-charge limited flow from very small emitters., Comment: 5 pages, 5 figures

We consider core–shell nanowires with prismatic geometry contacted with two or more superconductors in the presence of a magnetic field applied parallel to the wire. In this geometry, the lowest energy states are localized on the outer edges of the shell, which strongly inhibits the orbital effects of the longitudinal magnetic field that are detrimental to Majorana physics. Using a tight-binding model of coupled parallel chains, we calculate the topological phase diagram of the hybrid system in the presence of non-vanishing transverse potentials and finite relative phases between the parent superconductors. We show that having finite relative phases strongly enhances the stability of the induced topological superconductivity over a significant range of chemical potentials and reduces the value of the critical field associated with the topological quantum phase transition., This work was partially financed by the research funds of Reykjavik University and by the Icelandic Research Fund. TDS was supported in part by NSF DMR-1414683.

The distinctive prismatic geometry of semiconductor core-shell nanowires leads to complex localization patterns of carriers. Here, we describe the formation of optically active in-gap excitonic states induced by the interplay between localization of carriers in the corners and their mutual Coulomb interaction. To compute the energy spectra and configurations of excitons created in the conductive shell, we use a multi-electron numerical approach based on the exact solution of the multi-particle Hamiltonian for electrons in the valence and conduction bands, which includes the Coulomb interaction in a nonperturbative manner. We expose the formation of well separated quasidegenerate levels, and focus on the implications of the electron localization in the corners or on the sides of triangular, square and hexagonal cross sections. We obtain excitonic in-gap states associated with symmetrically distributed electrons in the spin singlet configuration. They acquire large contributions due to Coulomb interaction, and thus are shifted to much higher energies than other states corresponding to the conduction electron and the vacancy localized in the same corner. We compare the results of the multi-electron method with those of an electron-hole model and we show that the latter does not reproduce the singlet excitonic states. We also obtain the exciton lifetime and explain selection rules which govern the recombination process., This work was supported by the Icelandic Research Fund

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., RevTeX, 7 pages with 7 included figures

Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3–xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J–V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J–V characteristics. The measured J–V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and pos...

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic-inorganic materials and related devices, particularly based on CH3NH3PbI3-xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3-xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)+ dipoles. (Graph Presented).

We monitored the evolution in time of pinhole-free structures based on FTO/TiO2/CH3NH3PbI2.6Cl0.4 layers, with and without spiro-OMeTAD and counter electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen atmosphere. In the absence of electrodes, no degradation occurs. While devices with Au show only a 10% drop in power conversion efficiency, remaining stable after a further overheating at 70 °C, >90% is lost when using Ag, with the process being slower for Mo/Ag. We demonstrate that iodine is dislocated by the electric field between the electrodes, and this is an intrinsic cause for electromigration of I– from the perovskite until it reaches the anode. The iodine exhaustion in the perovskite layer is produced when using Ag electrodes, and AgI is formed. We hypothesize that in the presence of Au the iodine migration is limited due to the buildup of I– negative space charge accumulated at the perovskite–OMeTAD interface.

Publisher's version (útgefin grein)., Transport properties of a quantum dot coupled to a photon cavity are investigated using a quantum master equation in the steady-state regime. In the off-resonance regime, when the photon energy is smaller than the energy spacing between the lowest electron states of the quantum dot, we calculate the current that is generated by photon replica states as the electronic system is pumped with photons. Tuning the electron-photon coupling strength, the photocurrent can be enhanced by the influences of the photon polarization, and the cavity-photon coupling strength of the environment. We show that the current generated through the photon replicas is very sensitive to the photon polarization, but it is not strongly dependent on the average number of photons in the environment., This work was financially supported by the Research Fund of the University of Iceland, the Icelandic Research Fund, grant no. 163082-051, and the Icelandic Infrastructure Fund. The computations were performed on resources provided by the Icelandic High Performance Computing Center at the University of Iceland. NRA acknowledges support from University of Sulaimani and Komar University of Science and Technology. CST acknowledges support from Ministry of Science and Technology of Taiwan under grant No. 106-2112-M-239-001-MY3.

Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing.

We study the electronic transport through a pair of distant nanosystems ($S_a$ and $S_b$) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double-system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem $S_b$ induces a photon-assisted current in the other subsystem $S_a$ which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of back-action, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system., Comment: 9 pages, 5 figures, to appear in Phys. Rev. B

We theoretically investigate the characteristics of the electron transport through a two-dimensionala quantum dot system in the $xy$-plane coupled to a photon cavity and a photon reservoir, the environment. The electron-photon coupling, $g_{\gamma}$, and the cavity-reservoir coupling, $\kappa$, are tuned to study the system in the weak, $g_{\gamma} \leq \kappa$, and the strong coupling regime, $g_{\gamma} > \kappa$. An enhancement of current is both seen with increasing $g_{\gamma}$ and $\kappa$ in the weak coupling regime for both $x$- and $y$-polarization of the photon field. This is a direct consequence of the Purcell effect. The current enhancement is due to the contribution of the photon replica states to the electron transport in which intraband transitions play an important role. The properties of the electron transport are drastically changed in the strong coupling regime with an $x$-polarized photon field in which the current is suppressed with increasing $g_{\gamma}$, but it is still increasing with $\kappa$. This behavior of the current is related to the population of purely electronic states and depopulation of photon replica states., Comment: RevTeX, 7 pages with 6 included eps figures

We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. The system is considered to be microstructured from a two-dimensional electron gas in a GaAs heterostructure. The 3D photon cavity is active in the far-infrared or the terahertz regime. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity&ndash, photon reservoir coupling, the mean photon number in the reservoir, the electron&ndash, photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron&ndash, photon coupling strength. In the selected range of the parameters, the electron&ndash, photon coupling and the cavity-environment coupling strengths, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity&ndash, reservoir coupling, while they decrease with increasing electron&ndash, photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.

We show that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockage regime when coupled to external leads and placed in a three-dimensional rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads we identify a slow high order transition between the ground states of the two quantum dots. The Fourier power spectrum for photon-photon correlations in the steady state shows a Fano-type of a resonance for the frequency of the slow transition. Full account is taken of the geometry of the multi-level electronic system, and the electron-electron Coulomb interactions together with the para- and diamagnetic electron-photon interactions are treated with step wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly., RevTeX - pdflatex, 10 pages with 11 included pdf-figures

We carry out numerical calculations of the scattering cross section of tubular semiconductor nanocylinders in the optical range. The scattering is investigated for the transversal incidence of light, i.e., along the diameter of the cylinder, with both transverse electric and transverse magnetic polarization. These subwavelength nanostructures support Mie resonances and, when the length of the cylinder is comparable to the wavelength, guided modes that can overlap with the Mie modes giving rise to sharp Fano resonances. We show that a varying internal radius affects each mode differently, allowing for an extra degree of freedom for tuning the spectral position of the resonant peaks., Comment: Conference ICTON 2019

Publisher's version (útgefin grein), We theoretically investigate thermoelectric effects in a quantum dot system under the influence of a linearly polarized photon field confined to a 3D cavity. A temperature gradient is applied to the system via two electron reservoirs that are connected to each end of the quantum dot system. The thermoelectric current in the steady state is explored using a quantum master equation. In the presence of the quantized photons, extra channels, the photon replica states, are formed generating a photon-induced thermoelectric current. We observe that the photon replica states contribute to the transport irrespective of the direction of the thermal gradient. In the off-resonance regime, when the energy difference between the lowest states of the quantum dot system is smaller than the photon energy, the thermoelectric current is almost blocked and a plateau is seen in the thermoelectric current for strong electron–photon coupling strength. In the resonant regime, an inversion of thermoelectric current emerges due to the Rabi-splitting. Therefore, the photon field can change both the magnitude and the sign of the thermoelectric current induced by the temperature gradient in the absence of a voltage bias between the leads., This work was financially supported by the Research Fund of the University of Iceland, the Icelandic Research Fund, grant no. 163082-051, and the Icelandic Infrastructure Fund. The computations were performed on resources provided by the Icelandic High Performance Computing Center at the University of Iceland. N.R.A. acknowledges support from the University of Sulaimani and Komar University of Science and Technology. C.-S.T. acknowledges support from the Ministry of Science and Technology of Taiwan under grant No. 106-2112-M-239-001-MY3.

We model bilayer graphene-like materials with Si$_2$C$_{14}$ and BC$_{14}$N stoichiometry, where the interlayer interactions play important roles shaping the physical properties of the systems. We find the interlayer interaction in Si$_2$C$_{14}$ to be repulsive due to the interaction of Si-Si atoms, and in BC$_{14}$N it is attractive due to B and N atoms for both the AA- and the AB-stacking. The repulsive interlayer interaction opens up a bandgap in Si$_2$C$_{14}$ while the attractive interlayer interaction in BC$_{14}$N induces a small indirect bandgap or overlaping of the valence conduction bands. Furthermore, the repulsive interaction decreases the Young modulus while the attractive interaction does not influence the Young modulus much. The stress-strain curves of both the AA- and the AB-stackings are suppressed compared to pure graphine bilayers. The optical response of Si$_2$C$_{14}$ is very sensitive to an applied electric field and an enrichment in the optical spectra is found at low energy. The enrichment is attributed to the bandgap opening and increased energy spacing between the $\pi{\text -}\pi^*$ bands. In BC$_{14}$N, the optical spectra are reduced due to the indirect bandgap or the overlapping of the $\pi{\text -}\pi^* $ bands. Last, a high Seebeck coefficient is observed due to the presence of a direct bandgap in Si$_2$C$_{14}$, while it is not much enhanced in BC$_{14}$N., Comment: RevTeX, 11 pages with 5 included figures

Transparent boundary conditions for the time-dependent Schrodinger equation are implemented using the R -matrix method. The employed scattering formalism is suitable for describing open quantum systems and provides the framework for the time-dependent coherent transport. Transmission and reflection of wave functions at the edges of a finite quantum system are essential for an accurate and efficient description of the time-dependent processes on large time scales. We detail the computational method and point out the numerical advantages stemming from the open system approach based on the R -matrix formalism. The approach is used here to describe time-dependent transport across nanostructured interfaces relevant for photovoltaic applications.

Electron switching in waveguides coupled to a photon cavity is found to be strongly influenced by the photon energy and polarization. Therefore, the charge dynamics in the system is investigated in two different regimes, for off- and on-resonant photon fields. In the off-resonant photon field, the photon energy is smaller than the energy spacing between the first two lowest subbands of the waveguide system, the charge splits between the waveguides implementing a $\sqrt{\rm NOT}$-quantum logic gate action. In the on-resonant photon field, the charge is totally switched from one waveguide to the other due to the appearance of photon replica states of the first subband in the second subband region instigating a quantum-NOT transition.In addition, the importance of the photon polarization to control the charge motion in the waveguide system is demonstrated.The idea of charge switching in electronic circuits may serve to built quantum bits., Comment: 6 pages with 6 included eps figures

A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field.

We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime., RevTeX, 8 pages with 9 included eps figures

We study the surface morphology and growth process of SiGe nanoislands on TiO2 films, deposited over Si (001) substrates by high power impulse magnetron sputtering, followed by varying annealing parameters (i.e., 500–750 °C for 30 min to 20 h). Structural analysis was performed by atomic force microscopy, scanning electron microscopy, and grazing incidence x-ray diffraction. Two structural schemes were taken into consideration, i.e., SiGe grown over pre-annealed TiO2 (scheme I) and as-grown TiO2 (scheme II). Photoluminescence (PL) study of the structures revealed spectral features comprised of multiple peak features related to localized and surface states within the oxide layer, along with a peak due to the SiGe nano-islands. It was observed that the spectral feature and intensity depend on the surface morphology and the crystallinity of the underlying TiO2 layer. The structures were subjected to low temperature PL measurements, and the spectra were de-convoluted in order to validate the origin of the obtained spectra. Structural analysis revealed that pre-annealing the underlying polycrystalline TiO2 film, prior to deposition of SiGe layers (scheme I), facilitates the formation of SiGe nanoislands, preferably along the grain boundaries (due to their higher interfacial energy). In comparison, for the case of SiGe deposited over as-grown TiO2 (scheme II), annealing of the structure resulted in random distribution of nanoislands across the entire film. The size of the nanoislands grew with increased annealing time up until the point they started to coalesce, forming discontinuous SiGe layers and eventually leading to melting of the layer.

Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.

We studied the effect of short term furnace annealing over the photoconductive properties of tristacked layer i.e. TiO2/(SiGe/TiO2)3. The structure was prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A transmission electron microscopy and grazing incidence spectroscopy was used to analyze the morphology of the structure. Photoconductive properties were studied by measuring photocurrent spectra at different applied voltages and temperatures. Tristack layers were obtained with 5–10 nm SiGe nanocrystals (NCs) by annealing at 600 °C for 5 min. No sign of SiO2 formation was found inside stacked layers. A maximum in the photocurrent spectra was observed at 994 nm at 300 K but it red-shifted gradually to 1045 nm with decrease in temperature to 100 K. This transition in peak maxima is attributed to SiGe NCs, due to lattice vibration and to contribution of non-radiative recombination at low temperatures.

A molecular dynamics based code has been developed for high-fidelity simulations of nano- and microscale vacuum electronics. The force acting upon any electron in the system is calculated directly from the applied fields and Coulomb interaction with every other electron in the system and all of the image charge. This code captures scattering and other important discrete particle effects in proximity to the cathode better than Particle-in-Cell and fluid codes that are commonly used for simulation of electron beams. Electron emission is treated stochastically in a self-consistent manner with regard to spacecharge effects.

We describe a molecular dynamics based simulation code for high-fidelity models of electron emission and propagation in micro-diodes and near cathode regions. Some practical uses of the code are described.

We calculate the electromagnetic field radiated by tubular nanowires with prismatic geometry and infinite length. The polygonal geometry has implications on the electronic localization; the lowest energy states are localized at the edges of the prism and are separated by a considerable energy gap from the states localized on the facets. This localization can be controlled with external electric or magnetic fields. In particular, by applying a magnetic field transverse to the wire the states may become localized on the lateral regions of the shell, relatively to the direction of the field, leading to channels of opposite currents. Because of the prismatic geometry of the nanowire the current distribution, and hence the radiated electromagnetic field, have an anisotropic structure, which can be modified by the external fields. In this work we study hexagonal, square and triangular nanowires., 4 pages, 3 figures

We describe the code used for high resolution molecular dynamics (MD) simulations of vacuum nanodiodes. The code includes full Coulomb interaction of electrons and offers several types of emission mechanisms with different geometries. It has been successfully used to study field emission and Child-Langmuir limited emissions.

This article is part of the Thematic Series "Topological materials"., In this paper we consider charge current generated by maintaining a temperature difference over a nanowire at zero voltage bias. For topological insulator nanowires in a perpendicular magnetic field the current can change sign as the temperature of one end is increased. Here we study how this thermoelectric current sign reversal depends on the magnetic field and how impurities affect the size of the thermoelectric current. We consider both scalar and magnetic impurities and show that their influence on the current are quite similar, although the magnetic impurities seem to be more effective in reducing the effect. For moderate impurity concentration the sign reversal persists., This work was supported by: RU Fund 815051 TVD and ERC Starting Grant 679722

We consider electrons in tubular nanowires with prismatic geometry and infinite length. Such a model corresponds to a core-shell nanowire with an insulating core and a conductive shell. In a prismatic shell the lowest energy states are localized along the edges (corners) of the prism and are separated by a considerable energy gap from the states localized on the prism facets. The corner localization is robust in the presence of a magnetic field longitudinal to the wire. If the magnetic field is transversal to the wire the lowest states can be shifted to the lateral regions of the shell, relatively to the direction of the field. These localization effects should be observable in transport experiments on semiconductor core-shell nanowires, typically with hexagonal geometry. We show that the conductance of the prismatic structures considerably differs from the one of circular nanowires. The effects are observed for sufficiently thin hexagonal wires and become much more pronounced for square and triangular shells. To the best of our knowledge the internal geometry of such nanowires is not revealed in experimental studies. We show that with properly designed nanowires these localization effects may become an important resource of interesting phenomenology., Comment: 10 pages, 12 figures

The dynamic effects observed in the J-V measurements represent one important hallmark in the behavior of the perovskite solar cells. Proper measurement protocols (MPs) should be employed for the experimental data reproducibility, in particular for a reliable evaluation of the power conversion efficiency (PCE), as well as for a meaningful characterization of the type and magnitude of the hysteresis. We discuss here several MPs by comparing the experimental J-V characteristics with simulated ones using the dynamic electrical model (DEM). Pre-poling conditions and bias scan rate can have a dramatic influence not only on the apparent solar cell performance, but also on the hysteretic phenomena. Under certain measurement conditions, a hysteresis-free behavior with relatively high PCEs may be observed, although the J-V characteristics may be far away from the stationary case. Furthermore, forward-reverse and reverse-forward bias scans show qualitatively different behaviors regarding the type of the hysteresis, normal and inverted, depending on the bias pre-poling. We emphasize here that correlated forward-reverse or reverse-forward bias scans are essential for a correct assessment of the dynamic hysteresis. In this context, we define a hysteresis index which consistently assigns the hysteresis type and magnitude. Our DEM simulations, supported by experimental data, provide further guidance for an efficient and accurate determination of the stationary J-V characteristics, showing that the type and magnitude of the dynamic hysteresis may be affected by unintentional pre-conditioning in typical experiments., Comment: 11 pages 7, figures

We investigate in how far the hysteresis-free behavior of perovskite solar cells can be reproduced using particular pre-conditioning and measurement conditions. As there are currently more and more reports of perovskite solar cells without J-V hysteresis it is crucial to distinguish between genuine performance improvements and measurement artifacts. We focus on two of the parameters that influence the dynamic J-V scans, namely the bias scan rate and and the bias poling voltage, and point out measurement conditions for achieving a hysteresis-free behavior. In this context we discuss the suitability of defining a hysteresis index (HI) for the characterization of dynamic J-V scans. Using HI, aging effects are also investigated, establishing a potential connection between the sample degradation and the variation of the maximal hysteresis on one hand, and relaxation time scale of the slow process, on the other hand. Pre-poling induced recombinations effects are identified. In addition, our analysis based on sample pre-biasing reveals potential indication regarding two types of slow processes, with two different relaxation time scales, which provides further insight regarding ionic migration., Comment: 11 pages, 11 figures

In contrast to monolayer graphene, in bilayer graphene (BLG) one can induce a tunable bandgap by applying an external electric field, which makes it suitable for field effect applications. Extrinsic doping of BLGs enriches the electronic properties of the graphene-based family, as their behavior can be switched from an intrinsic small-gap semiconductor to a degenerate semiconductor. In the framework of density functional theory (DFT) calculations, we investigate the electronic and thermoelectric properties of BLGs doped with extrinsic impurities from groups III (B, Al, Ga), IV (Si, Ge) and V (N, P, As), in the context of applied external electric fields. Doping one monolayer of the BLG with p- or n-type dopants results in a degenerate semiconductor, where the Fermi energy depends on the type of the impurity, but also on the magnitude and orientation of the electric field, which modifies the effective doping concentration. Doping one layer with isoelectronic species like Si and Ge opens a gap, which may be closed upon applying an electric field, in contrast to the pristine BLG. Furthermore, dual doping by III-V elements, in a way that the BLG system is formed by one n-type and one p-type graphene monolayer, leads to intrinsic semiconductor properties with relatively large energy gaps. Si-Si and Ge-Ge substitutions render a metallic like behavior at zero field similar to the standard BLG, however with an asymmetric density of states in the vicinity of the Fermi energy. We analyze the suitability of the highly doped BLG materials for thermoelectric applications, exploiting the large asymmetries of the density of states. In addition, a sign change in the Seebeck coefficient is observed by tuning the electric field as a signature of narrow bands near the Fermi level., Comment: 10 pages, 10 figures

We study the validity of Hund's first rule for the spin multiplicity in circular molecules - made of real or artificial atoms such as quantum dots - by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule {\it always} defines the ground state in a molecule with $4N$ atoms at half-filling. In all other cases (i.e. number of atoms {\it not} multiple of four, or a $4N$ molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined {primarily} by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized {\it mathematically} for any values of the on-site and long range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction., 8 pages, 2 figures, 2 tables

The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO 2 cap/(GeSi/TiO 2 ) 2 multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO 2 cap/GeSi/TiO 2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO 2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO 2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5–10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20–30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.

We analyse theoretically the possibilities of contactless control of in-gap states formed by a pair of electrons confined in a triangular quantum ring. The in-gap states are corner-localized states associated with two electrons occupying the same corner area, and thus shifted to much higher energies than other corner states, but still they are below the energies of corner-side-localized states. We show how the energies, degeneracy and splittings between consecutive levels change with the orientation of an external electric field relatively to the polygonal cross section. We also show how absorption changes in the presence of external electric and magnetic fields., 4 pages, 2 figures