1,723,266 research outputs found
Monte Carlo simulation of ultrafast processes in photoexcited semiconductors: Coherent and incoherent dynamics
Full text linkThe ultrafast dynamics of photoexcited carriers in a semiconductor is investigated by using a Monte Carlo simulation. In addition to a ‘‘conventional'' Monte Carlo simulation, the coherence of the external light field and the resulting coherence in the carrier system are fully taken into account. This allows us to treat the correct time dependence of the generation process showing a time-dependent linewidth associated with a recombination from states off resonance due to stimulated emission. The subsequent dephasing of the carriers due to scattering processes is analyzed. In addition, the simulation contains the carrier-carrier interaction in Hartree-Fock approximation giving rise to a band-gap renormalization and excitonic effects which cannot be treated in a conventional Monte Carlo simulation where polarization effects are neglected. Thus the approach presents a unified numerical method for the investigation of phenomena occurring close to the band gap and those typical for the energy relaxation of hot carriers
Analysis of Coherent and Incoherent Phenomena in Photoexcited Semiconductors: A Monte Carlo Approach
Full text linkA generalized Monte Carlo procedure for the ultrafast dynamics of photoexcited carriers in a semiconductor is presented, where the coherence in the carrier system, as well as band renormalization and excitonic effects in the Hartree-Fock approximation are fully taken into account. The details of the coherent generation process, the energy relaxation, and dephasing of the carriers are analyzed. The approach presents a numerical method for the investigation of phenomena occurring close to the band gap and those typical for the relaxation of hot carriers
Ultrafast phenomena in photoexcited semiconductors
Full text linkThe authors review the physics of ultrafast dynamics in semiconductors and their heterostructures, including both the observed experimental phenomena and the theoretical description of the processes.
These are probed by ultrafast optical excitation, generating nonequilibrium states that can be monitored by time-resolved spectroscopy. Light pulses create coherent superpositions of states, and the dynamics of the associated phase relationships can be directly investigated by means of many-pulse experiments. The commonly used experimental techniques are briefly reviewed. A variety
of different phenomena can be described within a common theoretical framework based on the density-matrix formalism. The important interactions of the carriers included in the theoretical description are the phonon interactions, the interactions with classical and quantum light fields, and the Coulomb interaction among the carriers themselves. These interactions give rise to a strong interplay between phase coherence and relaxation, which strongly affects the non equilibrium dynamics. Based on the general theory, the authors review the physical phenomena in various semiconductor structures including superlattices, quantum wells, quantum wires, and bulk media.
Particular results which have played a central role in understanding the microscopic origins of the relaxation processes are discussed in detail
Collective Identities and the Integration of Core State Powers: Introduction to the Special Issue
Full text linkThis special issue explores the relationship between collective identities and the integration of core state powers, that is, the delegation of powers to a centralized institution in policy areas that are essential for the functioning of the modern state. In this introductory article we present the main conceptualizations of the contributions to the special issue that define our understanding of collective identities and core state powers. We discuss the multi-level nature of collective identities and we discuss theoretical expectations over the link between collective identities and core state powers. Finally, we briefly present each of the contributions to the issue and discuss how they relate to the broad research goals, the special issue and to each other
Ultrafast Relaxation of Photoexcited Carriers: The Role of Coherence in the Generation Process
Full text linkA self-consistent description of the ultrafast dynamics of photoexcited carriers in semiconductors based on a generalized Monte Carlo solution of the semiconductor Bloch equations is presented. The problem of photogeneration and its theoretical description are discussed. We show that some of the approaches commonly used fail in describing correctly the effect of carrier-carrier interaction in the low-density limit. By including terms which have the structure of ‘‘in-scattering'' terms (vertex corrections) for the interband polarization, the experimentally observed features in the carrier dynamics are well described in the whole density range
Analysis of coherent and incoherent ultrafast dynamics in photoexcited semiconductors: a Monte Carlo approach
No full textIn the present paper we present a generalized Monte Carlo method recently developed by the authors for the solution of the coupled set of quantum kinetic equations for the distribution functions and the interband polarization. The aim of this method is to combine the advantages of the description within a fully quantum mechanical picture with the power of the Monte Carlo technique for the treatment of stochastic processes. It is based on a decomposition of the kinetic equations into a coherent and an incoherent part. The former is integrated directly while the latter is solved by means of a Monte Carlo simulation. Band renormalization and excitonic effects are included in a self-consistent way. The calculations allow us to treat carrier thermalization and relaxation, as well as the dephasing process, on the same level
The role of coherence for carrier relaxation in photo-excited semiconductors
No full textThe role played by the dynamics of the interband polarization for the correct description of the generation of carriers by a short laser pulse is investigated. The calculations are based on the semiconductor Bloch equations which are solved by means of a generalized Monte Carlo simulation. It turns out that some of the approaches commonly used for a microscopic modelling of the dephasing process fail in describing correctly the effect of carrier-carrier interaction in the low-density limit. By including terms which have the structure of "in-scattering" terms (vertex corrections) for the interband polarization, the experimentally observed features in the carrier dynamics are well reproduced in the whole density range
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
Common currency, common identity? The impact of the Euro introduction on European identity
Full text linkDoes European state building go hand in hand with European nation building? This article engages with the scholarly debate on the dynamic relationship between the construction of supranational political institutions that exert key functions of sovereignty and collective identities by investigating the extent to which the adoption of the Euro as a currency is associated with a decrease in the share of Europeans who identify exclusively with their nation and not with the European Union. In detail, by using a dynamic panel-data model on 26 European Union countries in the post-Maastricht period (1996–2017), our results show that the Euro has fostered European identity, leading to a small but significant decrease (-3%) in the share of Europeans with exclusive national identity
- …
