1,721,066 research outputs found
Microscopic modeling of energy dissipation and decoherence in nanoscale materials and devices
Full text linkPrimary goal of this thesis work is to develop and implement microscopic modeling strategies able to describe semiconductor-based nanomaterials and nanodevices, overcoming both the intrinsic limits of the semiclassical transport theory and the huge computational costs of non Markovian approaches. The progressive reduction of modern optoelectronic devices space-scales, triggered by the evolution on semiconductor heterostructures at the nanoscale, together with the decrease of the typical time-scales involved, pushes device miniaturization toward limits where the application of the traditional Boltzmann transport theory becomes questionable, and a comparison with more rigorous quantum transport approaches is imperative. In spite of the quantum-mechanical nature of electron and photon dynamics in the core region of typical solid-state nanodevices, the overall behavior of such quantum systems is often governed by a highly non-trivial interplay between phase coherence and dissipation/dephasing. To this aim, the crucial step is to adopt a quantum mechanical description of the carrier subsystem; this can be performed at different levels, ranging from phenomenological dissipation/decoherence models to quantum-kinetic treatments. However, due to their high computational cost, non-Markovian Green' s-function as well as density-matrix approaches like quantum Monte Carlo techniques or quantum-kinetics are currently unsuitable for the design and optimization of new-generation nanodevices. On the other end, the Wigner-function technique is a widely used approach which, in principle, is well suited to describe an interplay between coherence and dissipation: in fact it can be regarded both as a phase space formulation of the electronic density matrix and a quantum equivalent of the classical distribution function. The evolution of this quasi-distribution function is governed by the Wigner-equation, which is usually solved by applying local spatial boundary conditions. However, such a scheme has recently shown some intrinsic limits. In this thesis work we analyze both the reasons for these unphysical features -pointing out the needing of different and purely quantum approaches- and the limits in which they should not appear, thus justifying why these problems had not been encountered in numerous quantum-transport simulations based on this procedure. For these reasons here we present a novel single-particle simulation strategy able to describe the interplay between coherence and dissipation/dephasing. In the presence of one- as well as two-body scattering mechanisms, we apply the mean-field approximation to the many-body Lindblad-type (hence, positive-definite) scattering superoperators provided by a recently proposed Markov approach, and we derive a closed equation of motion for the electronic single-particle density matrix. Although the resulting scattering superoperator turns out to be, at finite or high carrier densities, nonlinear and non-Lindblad, we prove that it is able to guarantee the positivity of the evolution (in striking contrast with conventional Markov approaches) independently of the scattering mechanisms, an essential prerequisite of any reliable kinetic treatment of semiconductor quantum devices; furthermore, it may be extended to the cases of quantum systems with open spatial boundaries (in this regard, it provides a formal derivation of a recently proposed Lindblad-like device-reservoir scattering superoperator). The proposed theoretical scheme is able, one the one hand, to recover the space-dependent Boltzmann equation and, on the other, to point out the regimes where a relevant role may be played by scattering-nonlocality effects, e.g. scattering-induced variations of the spatial charge-density which may not be provided by semiclassical treatments. Supplementing our analytical investigation with a number of simulated experiments in homogeneous as well as inhomogeneous GaN-based systems, we provide a rigorous treatment of scattering nonlocality in semiconductor nanostructures: in particular, we show how the scattering-nonlocality effects (i) are particularly significant in the presence of a carrier localization on the nanometric space scale, (ii) cause a speedup of the diffusion and (iii) in superlattice structures induce, with respect to scattering-free evolutions, a suppression of coherent oscillations between adjacent wells. These genuine quantum effects may be predicted also by other simplified treatments of the dissipation/decoherence like, e.g., the Relaxation Time Approximation: the latter however turns out to be, contrary to the proposed microscopic theoretical scheme, totally nonlocal, e.g. it is unable to recover the local character of the Boltzmann collision term in the semiclassical limit and it leads, especially for the case of quasielastic dissipation processes, to a significant overestimation of the diffusion speedu
Scattering nonlocality in quantum charge transport: Application to semiconductor nanostructures
No full textOur primary goal is to provide a rigorous treatment of scattering nonlocality in semiconductor nanostructures. On the one hand, starting from the conventional density-matrix formulation and employing as ideal instrument for the study of the semiclassical limit the well-known Wigner-function picture, we shall perform a fully quantum-mechanical derivation of the space-dependent Boltzmann equation. On the other hand, we shall examine the validity limits of such semiclassical framework, pointing out, in particular, regimes where scattering-nonlocality effects may play a relevant role; to this end we shall supplement our analytical investigation with a number of simulated experiments, discussing and further expanding preliminary studies of scattering-induced quantum diffusion in GaN-based nanomaterials. As for the case of carrier-carrier relaxation in photoexcited semiconductors, our analysis will show the failure of simplified dephasing models in describing phonon-induced scattering nonlocality, pointing out that such limitation is particularly severe for the case of quasielastic dissipation processe
Electron-phonon coupling in metallic carbon nanotubes: Dispersionless electron propagation despite dissipation
Full text linkA recent study [Rosati, Dolcini, and Rossi, Appl. Phys. Lett. 106, 243101 (2015)] has predicted that, while in semiconducting single-walled carbon nanotubes (SWNTs) an electronic wave packet experiences the typical spatial diffusion of conventional materials, in metallic SWNTs, its shape remains essentially unaltered up to micrometer distances at room temperature, even in the presence of the electron-phonon coupling. Here, by utilizing a Lindblad-based density-matrix approach enabling us to account for both dissipation and decoherence effects, we test such a prediction by analyzing various aspects that were so far unexplored. In particular, accounting for initial nonequilibrium excitations, characterized by an excess energy E0, and including both intra- and interband phonon scattering, we show that for realistically high values of E0 the electronic diffusion is extremely small and nearly independent of its energetic distribution, in spite of a significant energy-dissipation and decoherence dynamics. Furthermore, we demonstrate that the effect is robust with respect to the variation of the chemical potential. Our results thus suggest that metallic SWNTs are a promising platform to realize quantum channels for the nondispersive transmission of electronic wave packet
Phonon-induced quantum diffusion in Carbon-based materials
Full text linkEmploying as prototypical systems the metallic single-walled carbon nanotubes (SWNTs), we investigate the electronic propagation in Carbon-based materials, showing that their linear electronic spectra protect the spatial shape of electronic wavepackets from phononinduced diffusion, up to micrometric scale already at room temperature. To this end, we employ a recently proposed nonlinear Lindblad-based density-matrix approach, which allows us to account for the interplay between phase coherence and dissipation/decoherence, avoiding both huge computational costs of non-Markovian approaches or the limitations of oversimplified dephasing model
Quantum diffusion due to scattering non-locality in nanoscale semiconductors
No full textIn view of its local character, the semiclassical or Boltzmann theory is intrinsically unable to describe transport phenomena on ultrashort space and time scales, and to this purpose genuine quantum-transport approaches are imperative. By employing a density-matrix simulation strategy recently proposed, we shall demonstrate its power and flexibility in describing quantum diffusion phenomena in nanoscale semiconductors. In particular, as for the case of carrier-carrier relaxation in photoexcited semiconductors, our analysis will show the failure of simplified dephasing models in describing phonon-induced scattering non-locality, pointing out that such limitation is particularly severe for the case of quasielastic dissipation processe
Microscopic modeling of scattering quantum non-locality in semiconductor nanostructures
Full text linkIn spite of their intrinsic validity limits, a number of Boltzmann-like simulation schemes are extensively employed in the investigation of semiconductor nanomaterials and nanodevices. Such modeling strategies, based on the neglect of carrier phase coherence, are definitely unable to describe space-dependent ultrafast phenomena. In this letter, we shall propose a quantum-mechanical modeling strategy able to properly account for scattering-induced spatial non-locality. Its power and flexibility will be demonstrated via a few simulated experiment
Phonon-induced quantum diffusion in semiconductors2014 International Workshop on Computational Electronics (IWCE)
No full textStarting from a density-matrix treatment of carrierphonon interaction based on a recent reformulation of the Markov limit, we provide a detailed investigation of phononinduced quantum diffusion in semiconductor nanostructures. In particular, as for the case of carrier-carrier relaxation in photoexcited semiconductors, our analysis shows the failure of simplified dephasing models in describing phonon-induced scattering nonlocality, pointing out that such limitation is particularly severe for the case of quasielastic dissipation processes
Wigner-function formalism applied to semiconductor quantum devices: Failure of the conventional boundary condition scheme
Full text linkThe Wigner-function formalism is a well-known approach to model charge transport in semiconductor nanodevices. The primary goal of the present article is to point out and explain the intrinsic limitations of the conventional quantum-device modeling based on such a Wigner-function paradigm, providing a definite answer to open questions related to the application of the conventional spatial boundary condition scheme to the Wigner transport equation. Our analysis shows that (i) in the absence of energy dissipation (coherent limit) the solution of the Wigner equation equipped with given boundary conditions is not unique, and (ii) when dissipation and decoherence phenomena are taken into account via a relaxation-time approximation, the solution, although unique, is not necessarily a physical Wigner function
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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