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1.	Fuhrer, Andreas, et al. (författare) Few electron double quantum dots in InAs/InP nanowire heterostructures 2007 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 7:2, s. 243-246 Tidskriftsartikel (refereegranskat)abstract We report on fabrication of double quantum dots in catalytically grown InAs/InP nanowire heterostructures. In the few-electron regime, starting with both dots empty, our low-temperature transport measurements reveal a clear shell structure for sequential charging of the larger of the two dots with up to 12 electrons. The resonant current through the double dot is found to depend on the orbital coupling between states of different radial symmetry. The charging energies are well described by a capacitance model if next-neighbor capacitances are taken into account.
2.	Lindwall, Greta, et al. (författare) Zero-phonon linewidth and phonon satellites in the optical absorption of nanowire-based quantum dots 2007 Ingår i: Physical Review Letters. - 1079-7114. ; 99:8, s. 1-087401 Tidskriftsartikel (refereegranskat)abstract The optical properties of quantum dots embedded in a catalytically grown semiconductor nanowire are studied theoretically. In comparison to dots in a bulk environment, the excitonic absorption is strongly modified by the one-dimensional character of the nanowire phonon spectrum. In addition to pronounced satellite peaks due to phonon-assisted absorption, we find a finite width of the zero-phonon line already in the lowest-order calculation.
3.	Weber, Carsten, et al. (författare) Density-matrix theory of quantum cascade lasers: Localization effects 2009 Ingår i: Workshop proceedings terahertz and mid infrared radiation: Basic research and practical applications. - 9781424438488 ; , s. 5-6 Konferensbidrag (refereegranskat)abstract A density-matrix theory is employed to study dynamical effects in a THz quantum cascade laser. Varying the tunnel coupling across the injection barrier by changing the barrier width, different regimes are distinguished where stationary and dynamics-induced localization effects are found and discussed.
4.	Weber, Carsten, et al. (författare) Probing Confined Phonon Modes by Transport through a Nanowire Double Quantum Dot 2010 Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 104:3, s. 1-036801 Tidskriftsartikel (refereegranskat)abstract Strong radial confinement in semiconductor nanowires leads to modified electronic and phononic energy spectra. We analyze the current response to the interplay between quantum confinement effects of the electron and phonon systems in a gate-defined double quantum dot in a semiconductor nanowire. We show that current spectroscopy of inelastic transitions between the two quantum dots can be used as an experimental probe of the confined phonon environment. The resulting discrete peak structure in the measurements is explained by theoretical modeling of the confined phonon mode spectrum, where the piezoelectric coupling is of crucial importance.
5.	Almqvist, Tim, et al. (författare) Domain formation and self-sustained oscillations in quantum cascade lasers 2019 Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 92:4 Tidskriftsartikel (refereegranskat)abstract Abstract: We study oscillations in quantum cascade lasers due to traveling electric field domains, which are observed both in simulations and experiments. These oscillations occur in a range of negative differential resistance and we clarify the condition determining whether the boundary between domains of different electric field can become stationary. Graphical abstract: [Figure not available: see fulltext.].
6.	Ayedh, Hussein, et al. (författare) Acoustic phonons in nanowires with embedded heterostructures 2011 Ingår i: Journal of Nanomaterials. - : Hindawi Limited. - 1687-4129 .- 1687-4110. ; 2011 Tidskriftsartikel (refereegranskat)abstract Abstract in Undetermined Acoustic phonons are studied in finite nanowires based on the elastic continuum model. The phonon dispersion relations are compared for different types of approximations both for pure nanowires and nanowire heterostructures including cubic and wurtzite materials. We find that the isotropic approximation highly underestimates the frequency of torsional modes. The impact of heterostructures on the lowest modes is rather weak and can be described by an average of the different structure parameters.
7.	Banit, F, et al. (författare) Self-consistent theory of the gain linewidth for quantum-cascade lasers 2005 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 86:4, s. 1-41108 Tidskriftsartikel (refereegranskat)abstract The linewidth in intersubband transitions can be significantly reduced below the sum of the lifetime broadening for the involved states, if the scattering environment is similar for both states. This is studied within a nonequilibrium Green function approach here. We find that the effect is of particular relevance for a recent, relatively low doped, Terahertz quantum-cascade laser.
8.	Carosella, F., et al. (författare) Free-carrier absorption in quantum cascade structures 2012 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 85:8 Tidskriftsartikel (refereegranskat)abstract We show that the free-carrier absorption in quantum cascade lasers (QCLs) is very small and radically different from the classical Drude result due to the orthogonality between the direction of the carrier free motion and the electric field of the laser emission. A quantum mechanical calculation of the free-carrier absorption and intersubband oblique absorption induced by interface defects, Coulombic impurities, and optical phonon absorption/emission is presented for QCLs with a double-quantum-well design. The interaction between the electrons and the optical phonons dominates at room temperature.
9.	Carosella, F., et al. (författare) One-dimensional massless Dirac bands in semiconductor superlattices 2014 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 89:23 Tidskriftsartikel (refereegranskat)abstract Semiconductor superlattices may display dispersions that are degenerate either at the zone center or zone boundary [G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures, Monographies de Physique (Les Editions de Physique, Les Ulis, 1988); C. Sirtori, F. Capasso, D. L. Sivco, and A. Y. Cho, Appl. Phys. Lett. 64, 2982 (1994)]. We show that they are linear upon the wave vector in the vicinity of the crossing point. This establishes a realisation of massless Dirac bands within semiconductor materials. We show that the eigenstates and the corresponding Wannier functions of these superlattices have peculiar symmetry properties. We discuss the stability of the properties of such superlattices versus the electron in-plane motion. As a distinct fingerprint, the inter-sub-band magnetoabsorption spectrum for such superlattices is discussed.
10.	Damtie, Fikeraddis A., et al. (författare) Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots 2016 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:6 Tidskriftsartikel (refereegranskat)abstract Multiple exciton generation (MEG) is a process in which more than one electron hole pair is generated per absorbed photon. It allows us to increase the efficiency of solar energy harvesting. Experimental studies have shown the multiple exciton generation yield of 1.2 in isolated colloidal quantum dots. However real photoelectric devices require the extraction of electron hole pairs to electric contacts. We provide a systematic study of the corresponding quantum coherent processes including extraction and injection and show that a proper design of extraction and injection rates enhances the yield significantly up to values around 1.6.
11.	Damtie, Fikeraddis A., et al. (författare) Time dependent study of multiple exciton generation in nanocrystal quantum dots 2016 Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596 .- 1742-6588. ; 696:1 Tidskriftsartikel (refereegranskat)abstract We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton ESX in resonance to a double exciton EDX. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a 4 nm PbS quantum dot.
12.	Damtie, Fikeraddis A., et al. (författare) Two-dimensional action spectroscopy of excitonic systems : Explicit simulation using a phase-modulation technique 2017 Ingår i: Physical Review A. - 2469-9926. ; 96:5 Tidskriftsartikel (refereegranskat)abstract Two-dimensional (2D) spectroscopy has been intensively used to study electronic and vibronic coherences in biological systems and semiconductors. This technique studies coherent as well as incoherent signals that arise from the nonlinear interaction of a sequence of laser pulses. In this paper we present a direct evaluation of the 2D signal based on elementary quantum kinetics in order to compare with the common approximate diagrammatic approaches. Here we consider incoherent action signals such as fluorescence or photocurrent as the observable, which is easily accessible in a measurement. These observables are calculated by solving the time evolution of the density matrix in the Lindblad form, which can take into account all possible decoherence processes. The phase modulation technique is used to separate the relevant nonlinear signals from the other possible interaction pathways. The approach can be used to calculate 2D spectra of any quantum system. For our model system we find a good agreement for the quantum beating between the coupled states.
13.	Dorsch, Sven, et al. (författare) Heat Driven Transport in Serial Double Quantum Dot Devices 2021 Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:2, s. 988-994 Tidskriftsartikel (refereegranskat)abstract Studies of thermally induced transport in nanostructures provide access to an exciting regime where fluctuations are relevant, enabling the investigation of fundamental thermodynamic concepts and the realization of thermal energy harvesters. We study a serial double quantum dot formed in an InAs/InP nanowire coupled to two electron reservoirs. By means of a specially designed local metallic joule-heater, the temperature of the phonon bath in the vicinity of the double quantum dot can be enhanced. This results in phonon-assisted transport, enabling the conversion of local heat into electrical power in a nanosized heat engine. Simultaneously, the electron temperatures of the reservoirs are affected, resulting in conventional thermoelectric transport. By detailed modeling and experimentally tuning the interdot coupling, we disentangle both effects. Furthermore, we show that phonon-assisted transport is sensitive to excited states. Our findings demonstrate the versatility of our design to study fluctuations and fundamental nanothermodynamics.
14.	Dupont, E., et al. (författare) A phonon scattering assisted injection and extraction based terahertz quantum cascade laser 2012 Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 111:7 Tidskriftsartikel (refereegranskat)abstract A lasing scheme for terahertz quantum cascade lasers, based on consecutive phonon-photon-phonon emissions per module, is proposed and experimentally demonstrated. The charge transport of the proposed structure is modeled using a rate equation formalism. An optimization code based on a genetic algorithm was developed to find a four-well design in the GaAs/Al0.25Ga0.75As material system that maximizes the product of population inversion and oscillator strength at 150 K. The fabricated devices using Au double-metal waveguides show lasing at 3.2 THz up to 138 K. The electrical characteristics display no sign of differential resistance drop at lasing threshold, which, in conjunction with the low optical power of the device, suggest-thanks to the rate equation model-a slow depopulation rate of the lower lasing state, a hypothesis confirmed by non-equilibrium Green's function calculations. [http://dx.doi.org/10.1063/1.3702571]
15.	Franckie, Martin, et al. (författare) Comparative analysis of quantum cascade laser modeling based on density matrices and non-equilibrium Green's functions 2014 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:10 Tidskriftsartikel (refereegranskat)abstract We study the operation of an 8.5 mu m quantum cascade laser based on GaInAs/AlInAs lattice matched to InP using three different simulation models based on density matrix (DM) and non-equilibrium Green's function (NEGF) formulations. The latter advanced scheme serves as a validation for the simpler DM schemes and, at the same time, provides additional insight, such as the temperatures of the sub-band carrier distributions. We find that for the particular quantum cascade laser studied here, the behavior is well described by simple quantum mechanical estimates based on Fermi's golden rule. As a consequence, the DM model, which includes second order currents, agrees well with the NEGF results. Both these simulations are in accordance with previously reported data and a second regrown device. (C) 2014 AIP Publishing LLC.
16.	Franckie, Martin, et al. (författare) Impact of interface roughness distributions on the operation of quantum cascade lasers 2015 Ingår i: Optics Express. - 1094-4087. ; 23:4, s. 5201-5212 Tidskriftsartikel (refereegranskat)abstract We study the impact of interface roughness on the operation of mid-IR and THz quantum cascade lasers. Particular emphasis is given towards the differences between the Gaussian and exponential roughness distribution functions, for which we present results from simulation packages based on nonequilibrium Green's functions and density matrices. The Gaussian distribution suppresses scattering at high momentum transfer which enhances the lifetime of the upper laser level in mid-IR lasers. For THz lasers, a broader range of scattering transitions is of relevance, which is sensitive to the entire profile of the interface fluctuations. Furthermore we discuss the implementation of interface roughness within a two band model. (C) 2015 Optical Society of America
17.	Franckie, Martin, et al. (författare) Injection schemes in THz quantum cascade lasers under operation 2013 Ingår i: Proceedings of SPIE. - : SPIE. - 9780819496966 ; 8846 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract The two main design schemes for Terahertz quantum cascade lasers, based on tunnelling and scattering injection, respectively, are theoretically compared. We apply our simulation package based on the non-equilibrium Green’s function technique. Our results provide a good description of the gain degradation with temperature. Thermal backfilling contributes to decrease of population inversion in both cases. However, the dropping inversion cannot account for the total reduction of gain.
18.	Gnodtke, C, et al. (författare) Phonon-assisted tunneling through quantum dot stacks 2006 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:11, s. 1-115338 Tidskriftsartikel (refereegranskat)abstract The impact of electron-phonon interaction on the coherent transport through a quantum dot stack is investigated using the nonequilibrium Green's function technique. While the main resonance is due to the alignment of the single dot levels, satellite peaks occur due to resonances with phonon replica of the bare levels. This effect is analyzed in detail and we show that the inclusion of nondiagonal self-energies leads to a significant reduction of the induced current, which is interpreted as an interference effect.
19.	Goldozian, Bahareh, et al. (författare) Quantifying the impact of phonon scattering on electrical and thermal transport in quantum dots 2019 Ingår i: European Physical Journal: Special Topics. - : Springer Science and Business Media LLC. - 1951-6355 .- 1951-6401. ; 227:15-16, s. 1959-1967 Tidskriftsartikel (refereegranskat)abstract We report the inclusion of phonon scattering to our recently established numerical package QmeQ for transport in quantum dot systems. This enables straightforward calculations for a large variety of devices. As examples we show (i) transport in a double-dot structure, where energy relaxation is crucial to match the energy difference between the levels, and (ii) the generation of electrical power by contacting cold electric contacts with quantum dot states, which are subjected to heated phonons.
20.	Goldozian, Bahareh, et al. (författare) Transport in serial spinful multiple-dot systems: The role of electron-electron interactions and coherences. 2016 Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6 Tidskriftsartikel (refereegranskat)abstract Quantum dots are nanoscopic systems, where carriers are confined in all three spatial directions. Such nanoscopic systems are suitable for fundamental studies of quantum mechanics and are candidates for applications such as quantum information processing. It was also proposed that linear arrangements of quantum dots could be used as quantum cascade laser. In this work we study the impact of electron-electron interactions on transport in a spinful serial triple quantum dot system weakly coupled to two leads. We find that due to electron-electron scattering processes the transport is enabled beyond the common single-particle transmission channels. This shows that the scenario in the serial quantum dots intrinsically deviates from layered structures such as quantum cascade lasers, where the presence of well-defined single-particle resonances between neighboring levels are crucial for device operation. Additionally, we check the validity of the Pauli master equation by comparing it with the first-order von Neumann approach. Here we demonstrate that coherences are of relevance if the energy spacing of the eigenstates is smaller than the lead transition rate multiplied by ħ.
21.	Gustafsson, Fredrik, et al. (författare) Fler randomiserade tester hade lärt oss mer om pandemin 2021 Ingår i: Ny teknik. - 0550-8754. Tidskriftsartikel (populärvet., debatt m.m.)
22.	Hedse, Albin, et al. (författare) Pulse overlap artifacts and double quantum coherence spectroscopy 2023 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 158:14 Tidskriftsartikel (refereegranskat)abstract The double quantum coherence (DQC) signal in nonlinear spectroscopy gives information about the many-body correlation effects not easily available by other methods. The signal is short-lived, consequently, a significant part of it is generated during the pulse overlap. Since the signal is at two times the laser frequency, one may intuitively expect that the pulse overlap-related artifacts are filtered out by the Fourier transform. Here, we show that this is not the case. We perform explicit calculations of phase-modulated two-pulse experiments of a two-level system where the DQC is impossible. Still, we obtain a significant signal at the modulation frequency, which corresponds to the DQC, while the Fourier transform over the pulse delay shows a double frequency. We repeat the calculations with a three-level system where the true DQC signal occurs. We conclude that with realistic dephasing times, the pulse-overlap artifact can be significantly stronger than the DQC signal. Our results call for great care when analyzing such experiments. As a rule of thumb, we recommend that only delays larger than 1.5 times the pulse length should be used.
23.	Kalaee, Alex Arash Sand, et al. (författare) Electron extraction from excited quantum dots with higher order coulomb scattering 2020 Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 4:3 Tidskriftsartikel (refereegranskat)abstract The electron kinetics in nanowire-based hot-carrier solar cells is studied, where both relaxation and extraction are considered concurrently. Our kinetics is formulated in the many-particle basis of the interacting system. Detailed comparison with simplified calculations based on product states shows that this includes the Coulomb interaction both in lowest and higher orders. While relaxation rates of 1 ps are obtained, if lowest order processes are available, timescales of tens of ps arise if these are not allowed for particular designs and initial conditions. Based on these calculations we quantify the second order effects and discuss the extraction efficiency, which remains low unless an energy filter by resonant tunnelling is applied.
24.	Kalaee, Alex Arash Sand, et al. (författare) Positivity of entropy production for the three-level maser 2021 Ingår i: Physical Review A. - 2469-9926. ; 103:1 Tidskriftsartikel (refereegranskat)abstract Entropy production is a key concept of thermodynamics and allows one to analyze the operation of engines. For the Scovil-Schulz-DuBois heat engine, the archetypal three-level thermal maser coupled to thermal baths, it was argued that the common definition of heat flow may provide negative entropy production for certain parameters [Boukobza and Tannor, Phys. Rev. Lett. 98, 240601 (2007)PRLTAO0031-900710.1103/PhysRevLett.98.240601]. Here, we show that this can be cured if corrections for detuning are properly applied to the energies used for the bath transitions. This method can be used more generally for the thermodynamical analysis of optical transitions where the modes of the light field are detuned from the transition energy.
25.	Karbaschi, Hossein, et al. (författare) Nonlinear thermoelectric efficiency of superlattice-structured nanowires 2016 Ingår i: Physical Review B. - 1098-0121. ; 94:11 Tidskriftsartikel (refereegranskat)abstract We theoretically investigate nonlinear ballistic thermoelectric transport in a superlattice-structured nanowire. By a special choice of nonuniform widths of the superlattice barriers - analogous to antireflection coating in optical systems - it is possible to achieve a transmission which comes close to a square profile as a function of energy. We calculate the low-temperature output power and power-conversion efficiency of a thermoelectric generator based on such a structure and show that the efficiency remains high also when operating at a significant power. To provide guidelines for experiments, we study how the results depend on the nanowire radius, the number of barriers, and on random imperfections in barrier width and separation. Our results indicate that high efficiencies can indeed be achieved with today's capabilities in epitaxial nanowire growth.
26.	Karlström, Olov, et al. (författare) A diagrammatic description of the equations of motion, current and noise within the second-order von Neumann approach 2013 Ingår i: Journal of Physics A: Mathematical and Theoretical. - : IOP Publishing. - 1751-8113 .- 1751-8121. ; 46:6 Tidskriftsartikel (refereegranskat)abstract We investigate the second-order von Neumann approach from a diagrammatic point of view and demonstrate its equivalence with the resonant tunneling approximation. The investigation of higher order diagrams shows that the method correctly reproduces the equation of motion for the single-particle reduced density matrix of an arbitrary non-interacting many-body system. This explains why the method reproduces the current exactly for such systems. We go on to show, however, that diagrams not included in the method are needed to calculate exactly higher cumulants of the charge transport. This thorough comparison sheds light on the validity of all these self-consistent second-order approaches. We analyze the discrepancy between the noise calculated by our method and the exact Levitov formula for a simple non-interacting quantum dot model. Furthermore, we study the noise of the canyon of current suppression in a two-level dot, a phenomenon that requires the inclusion of electron-electron interaction as well as higher order tunneling processes.
27.	Karlström, Olov, et al. (författare) Analysing the capacitance–voltage measurements of vertical wrapped-gated nanowires 2008 Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 19:43 Tidskriftsartikel (refereegranskat)abstract The capacitance of arrays of vertical wrapped-gate InAs nanowires is analysed. With the help of a Poisson–Schrödinger solver, information about the doping density can be obtained directly. Further features in the measured capacitance–voltage characteristics can be attributed to the presence of surface states as well as the coexistence of electrons and holes in the wire. For both scenarios, quantitative estimates are provided. It is furthermore shown that the difference between the actual capacitance and the geometrical limit is quite large, and depends strongly on the nanowire material.
28.	Karlström, Olov, et al. (författare) Canyon of current suppression in an interacting two-level quantum dot 2011 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 83:20 Tidskriftsartikel (refereegranskat)abstract Motivated by the recent discovery of a canyon of conductance suppression in a two-level equal-spin quantum dot system [Phys. Rev. Lett. 104, 186804 (2010)], the transport through this system is studied in detail. At low bias and low temperature a strong current suppression is found around the electron-hole symmetry point independent of the couplings, in agreement with previous results. By means of a Schrieffer-Wolff transformation we are able to give an intuitive explanation to this suppression in the low-energy regime. In the general situation, numerical simulations are carried out using quantum rate equations. The simulations allow for the prediction of how the suppression is affected by the couplings, the charging energy, the position of the energy levels, the applied bias, and the temperature. We find that, away from electron-hole symmetry, the parity of the couplings is essential for the current suppression. It is also shown how broadening, interference, and a finite interaction energy cause a shift of the current minimum away from degeneracy. Finally we see how an increased population of the upper level leads to current peaks on each side of the suppression line. At sufficiently high bias we discover a coherence-induced population inversion.
29.	Karlström, Olov, et al. (författare) Increasing thermoelectric performance using coherent transport 2011 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 84:11 Tidskriftsartikel (refereegranskat)abstract We show that coherent electron transport through zero-dimensional systems can be used to tailor the shape of the system's transmission function. This quantum-engineering approach can be used to enhance the performance of quantum dots or molecules in thermal-to-electric power conversion. Specifically, we show that electron interference in a two-level system can substantially improve the maximum thermoelectric power and the efficiency at maximum power by suppressing parasitic charge flow near the Fermi energy and by reducing electronic heat conduction. We discuss possible realizations of this approach in molecular junctions or quantum dots.
30.	Kiesslich, G, et al. (författare) Counting statistics and decoherence in coupled quantum dots 2006 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:3, s. 1-033312 Tidskriftsartikel (refereegranskat)abstract We theoretically consider charge transport through two quantum dots coupled in series. The corresponding full counting statistics for noninteracting electrons is investigated in the limits of sequential and coherent tunneling by means of a master equation approach and a density matrix formalism, respectively. We clearly demonstrate the effect of quantum coherence on the zero-frequency cumulants of the transport process, focusing on noise and skewness. Moreover, we establish the continuous transition from the coherent to the incoherent tunneling limit in all cumulants of the transport process and compare this with decoherence described by a dephasing voltage probe model.
31.	Kiesslich, G, et al. (författare) Decoherence and current fluctuations in tunneling through coupled quantum dots 2005 Ingår i: [Host publication title missing]. - : AIP. - 1551-7616 .- 0094-243X. ; 780, s. 439-441 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract The electronic transport through two coupled quantum dots in series can be described either in a fully coherent approach (e.g. density matrix description or nonequilibrium Green's functions) or in a simple sequential tunneling treatment (Pauli master equation with Fermi's Golden rule for coupling between the quantum dots). It turns out that both descriptions provide the same average current for noninteracting electrons. In contrast, the zero-frequency spectral power density and the skewness are different for intermediate coupling strengths between the quantum dots reflecting their sensitivity on coherence in the tunneling process.
32.	Kiesslich, G, et al. (författare) Super-Poissonian current fluctuations in tunneling through coupled quantum dots 2006 Ingår i: Springer Proceedings in Physics: Proc. of the 14th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors. - 9783540365877 ; 110, s. 23-27 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract The shot noise behavior of sequential tunneling through two vertically coupled quantum dots is studied by means of a master equation approach. In particular, we propose a mechanism to observe super-Poissonian shot noise in such a system. The crucial ingredient is the Coulomb interaction between electrons in individual quantum dots.
33.	Kiesslich, Gerold, et al. (författare) Theory of Nonlinear Transport for Ensembles of Quantum Dots 2008 Ingår i: Semiconductor Nanostructures. - 1434-4904. ; , s. 211-220 Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract This article reviews our work on the description of electronic transport through self-assembled quantum dots. Our main interest is in the effect of Coulomb interaction on quantum dot charging (capacitance-voltage characteristics), on the average current (current-voltage characteristics), and on current fluctuations (quantum shot noise) in quantum dot layers embedded in pn- or resonant tunneling devices. Our studies show the particular importance of understanding those interaction mechanisms for future device applications.
34.	Kiršanskas, Gediminas, et al. (författare) Phenomenological position and energy resolving Lindblad approach to quantum kinetics 2018 Ingår i: Physical Review B. - 2469-9950. ; 97:3 Tidskriftsartikel (refereegranskat)abstract A general theoretical approach to study the quantum kinetics in a system coupled to a bath is proposed. Starting with the microscopic interaction, a Lindblad master equation is established, which goes beyond the common secular approximation. This allows for the treatment of systems, where coherences are generated by the bath couplings while avoiding the negative occupations occurring in the Bloch-Wangsness-Redfield kinetic equations. The versatility and accuracy of the approach is verified by its application to three entirely different physical systems: (i) electric transport through a double-dot system coupled to electronic reservoirs, (ii) exciton kinetics in coupled chromophores in the presence of a heat bath, and (iii) the simulation of quantum cascade lasers, where the coherent electron transport is established by scattering with phonons and impurities.
35.	Kirsanskas, Gediminas, et al. (författare) QmeQ 1.0 : An open-source Python package for calculations of transport through quantum dot devices 2017 Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 221, s. 317-342 Tidskriftsartikel (refereegranskat)abstract QmeQ is an open-source Python package for numerical modeling of transport through quantum dot devices with strong electron-electron interactions using various approximate master equation approaches. The package provides a framework for calculating stationary particle or energy currents driven by differences in chemical potentials or temperatures between the leads which are tunnel coupled to the quantum dots. The electronic structures of the quantum dots are described by their single-particle states and the Coulomb matrix elements between the states. When transport is treated perturbatively to lowest order in the tunneling couplings, the possible approaches are Pauli (classical), first-order Redfield, and first-order von Neumann master equations, and a particular form of the Lindblad equation. When all processes involving two-particle excitations in the leads are of interest, the second-order von Neumann approach can be applied. All these approaches are implemented in QmeQ. We here give an overview of the basic structure of the package, give examples of transport calculations, and outline the range of applicability of the different approximate approaches. Program summary: Program Title: QmeQ Program Files doi: http://dx.doi.org/10.17632/8687mrhgg9.1 Licensing provisions: BSD 2-Clause. Programming language: Python External libraries: NumPy, SciPy, Cython Nature of problem: Calculation of stationary state currents through quantum dots tunnel coupled to leads. Solution method: Exact diagonalization of the quantum dot Hamiltonian for a given set of single particle states and Coulomb matrix elements. Numerical solution of the stationary-state master equation for a given approximate approach. Restrictions: Depending on the approximate approach the temperature needs to be sufficiently large compared to the coupling strength for the approach to be valid.
36.	Kristinsdottir, Liney Halla, et al. (författare) Signatures of Wigner localization in epitaxially grown nanowires 2011 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 83:4 Tidskriftsartikel (refereegranskat)abstract It was predicted by Wigner in 1934 that an electron gas will undergo a transition to a crystallized state when its density is very low. Whereas significant progress has been made toward the detection of electronic Wigner states, their clear and direct experimental verification still remains a challenge. Here we address signatures of Wigner molecule formation in the transport properties of InSb nanowire quantum-dot systems, where a few electrons may form localized states depending on the size of the dot (i.e., the electron density). Using a configuration interaction approach combined with an appropriate transport formalism, we are able to predict the transport properties of these systems, in excellent agreement with experimental data. We identify specific signatures of Wigner state formation, such as the strong suppression of the antiferromagnetic coupling, and are able to detect the onset of Wigner localization, both experimentally and theoretically, by studying different dot sizes.
37.	Kristinsdottir, Liney Halla, et al. (författare) Thermopower as a tool to investigate many-body effects in quantum systems 2014 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:8 Tidskriftsartikel (refereegranskat)abstract Measuring the thermopower of a confined quantum system reveals important information about its excitation spectrum. Our simulations show how this kind of transport spectroscopy is able to extract a clear signal for the onset of Wigner localization in a nanowire segment. This demonstrates that thermopower measurements provide a tool for investigating complex many-body quantum effects, which is less intrusive than the usual charge-stability diagram as no high source-drain bias is required. While the effect is most pronounced for weak tunnel coupling and low temperatures, the excited states also significantly affect the thermopower spectrum at moderate temperature, adding distinct features to the characteristic thermopower lineshape. (C) 2014 AIP Publishing LLC.
38.	Kristinsdottir, Liney Halla, et al. (författare) Total current blockade in an ultracold dipolar quantum wire. 2013 Ingår i: Physical Review Letters. - 1079-7114. ; 110:8 Tidskriftsartikel (refereegranskat)abstract Cold-atom systems offer a great potential for the future design of new mesoscopic quantum systems with properties that are fundamentally different from semiconductor nanostructures. Here, we investigate the quantum-gas analogue of a quantum wire and find a new scenario for the quantum transport: Attractive interactions may lead to a complete suppression of current in the low-bias range, a total current blockade. We demonstrate this effect for the example of ultracold quantum gases with dipolar interactions.
39.	Krivas, K. A., et al. (författare) Influence of interface roughness in quantum cascade lasers 2015 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 118:11 Tidskriftsartikel (refereegranskat)abstract We use a numerical model based on non-equilibrium Green's functions to investigate the influence of interface roughness (IFR) scattering in terahertz quantum cascade lasers. We confirm that IFR is an important phenomenon that affects both current and gain. The simulations indicate that IFR causes a leakage current that transfers electrons from the upper to the lower laser state. In certain cases, this current can greatly reduce gain. In addition, individual interfaces and their impact on the renormalized single particle energies are studied and shown to give both blue-and red-shifts of the gain spectrum. (C) 2015 AIP Publishing LLC.
40.	Lassen, Benny, et al. (författare) Electron transport through nanosystems driven by Coulomb scattering 2007 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 76:7, s. 1-075316 Tidskriftsartikel (refereegranskat)abstract Electron transmission through nanosystems is blocked if there are no states connecting the left and the right reservoir. Electron-electron scattering can lift this blockade, and we show that this feature can be conveniently implemented by considering a transport model based on many-particle states. We discuss typical signatures of this phenomenon, such as the presence of a current signal for a finite bias window.
41.	Lee, S.-C, et al. (författare) Quantum mechanical wavepacket transport in quantum cascade laser structures 2006 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:24, s. 1-245320 Tidskriftsartikel (refereegranskat)abstract We present a viewpoint of the transport process in quantum cascade laser structures in which spatial transport of charge through the structure is a property of coherent quantum mechanical wave functions. In contrast, scattering processes redistribute particles in energy and momentum but do not directly cause spatial motion of charge.
42.	Matthews, J., et al. (författare) Heat flow in InAs/InP heterostructure nanowires 2012 Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 86:17 Tidskriftsartikel (refereegranskat)abstract The transfer of heat between electrons and phonons plays a key role for thermalmanagement in future nanowire-based devices, but only a few experimental measurements of electron-phonon (e-ph) coupling in nanowires are available. Here, we combine experimental temperature measurements on an InAs/InP heterostructure nanowire system with finite element modeling to extract information on heat flow mediated by e-ph coupling. We find that the electron and phonon temperatures in our system are highly coupled even at temperatures as low as 2 K. Additionally, we find evidence that the usual power-law temperature dependence of electron-phonon coupling may not correctly describe the coupling in nanowires and show that this result is consistent with previous research on similar one-dimensional electron systems. We also compare the strength of the observed e-ph coupling to a theoretical analysis of e-ph interaction in InAs nanowires, which predicts a significantly weaker coupling strength than observed experimentally.
43.	Ndebeka-Bandou, Camille, et al. (författare) Dopant Engineering of Inter-Subband Linewidth and Lineshape in Multiwell Heterostructures 2013 Ingår i: Applied Physics Express. - 1882-0778. ; 6:9 Tidskriftsartikel (refereegranskat)abstract We show by numerical diagonalization of the electronic Hamiltonian including screened coulombic impurities that the inter-subband absorption lineshape and linewidth in heterostructures can be controlled by a suitable location of the dopants. We also point out that the usual optical conductivity calculations that employ the self-consistent Born approximation often lead to incorrect lineshapes, although the trends for the linewidths versus the dopant location are the same as found in the present numerical approach. (c) 2013 The Japan Society of Applied Physics
44.	Ndebeka-Bandou, C., et al. (författare) Free carrier absorption and inter-subband transitions in imperfect heterostructures 2014 Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 29:2 Forskningsöversikt (refereegranskat)abstract We present the results of a quantum mechanical modelling of the free carrier absorption (FCA) in semiconductor heterolayers. Elastic and inelastic scatterers are considered with emphasis on the interface defects (optical phonons) contributions to the induced photon absorption for elastic (inelastic) scatterers. Various approaches to FCA are also presented (perturbation, Green's function technique). The connection between inter-subband absorption and FCA is thoroughly discussed. The absorption lineshape and its modification by suitable doping is presented.
45.	Ndebeka-Bandou, Camille, et al. (författare) Relevance of intra- and inter-subband scattering on the absorption in heterostructures 2012 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 101:19 Tidskriftsartikel (refereegranskat)abstract We analyze the absorption lineshape for inter-subband transitions in disordered quasi two-dimensional heterostructures by an exact calculation. The intra-subband scatterings control the central peak, while the tails of the absorption line are dominated by the inter-subband scattering terms. Our numerical study quantitatively assesses the magnitude of the free carrier absorption. The accuracy of different models currently used for gain/absorption is discussed.
46.	Nelander, Rikard, et al. (författare) Fingerprints of spatial charge transfer in Quantum Cascade Lasers 2007 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102, s. 1-113104 Tidskriftsartikel (refereegranskat)abstract We show that mid-infrared transmission spectroscopy of a quantum cascade laser provides clear-cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for a lambda~7.4 µm InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes hand in hand with a decrease of discrete intersubband absorption peaks and an increase of broad, high-energy absorption toward the continuum delocalized states above the barriers.
47.	Nelander, Rikard, et al. (författare) Temperature degradation of the gain transition in terahertz quantum cascade lasers - the role of acoustic phonon scattering 2009 Ingår i: physica status solidi (c). - : Wiley. - 1610-1634 .- 1862-6351 .- 1610-1642. ; 6:2, s. 579-582 Konferensbidrag (refereegranskat)abstract The temperature degradation in terahertz quantum cascade lasers is investigated with the focus on the role acoustic phonon scattering. A self-energy describing the electron-acoustic phonon interaction is derived and used to study the importance in transport and optical properties. We observe a strong degradation of the gain peak with temperature. Despite the very small coupling to electrons, the exclusion of acoustic phonon scattering alters the current and peak gain in the order of 5%. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
48.	Nelander, Rikard, et al. (författare) Temperature dependence and screening models in quantum cascade structures 2009 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 106:6 Tidskriftsartikel (refereegranskat)abstract Different screening models in quantum cascade lasers are compared by calculating the contribution of intrasubband impurity scattering to the optical linewidth as a function of temperature. We find a strong impact of impurity scattering, which increases substantially with temperature. A simple isotropic bulk screening model works well if the screening length is of the order of or longer than the period length of the cascade structure. (C) 2009 American Institute of Physics. [doi:10.1063/1.3226072]
49.	Nelander, Rikard, et al. (författare) Temperature dependence of the gain profile for THz quantum cascade lasers 2008 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92:8, s. 1-081102 Tidskriftsartikel (refereegranskat)abstract We study the rapid decrease of peak gain in resonant-phonon THz Quantum Cascade Lasers with increasing temperature. The effect of various microscopic scattering processes on the gain profile as a function of temperature is discussed. We argue that increased broadening, prim arily due to increased impurity scattering, and not diminishing population inversion, is the main reason for the reduction of peak gain.
50.	Nilsson, Henrik, et al. (författare) Correlation-induced conductance suppression at level degeneracy in a quantum dot. 2010 Ingår i: Physical Review Letters. - 1079-7114. ; 104:18 Tidskriftsartikel (refereegranskat)abstract The large, level-dependent g factors in an InSb nanowire quantum dot allow for the occurrence of a variety of level crossings in the dot. While we observe the standard conductance enhancement in the Coulomb blockade region for aligned levels with different spins due to the Kondo effect, a vanishing of the conductance is found at the alignment of levels with equal spins. This conductance suppression appears as a canyon cutting through the web of direct tunneling lines and an enclosed Coulomb blockade region. In the center of the Coulomb blockade region, we observe the predicted correlation-induced resonance. Our findings are supported by numerical and analytical calculations.

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