2,618 research outputs found
Carrier Multiplication in Graphene
Graphene as a zero-bandgap semiconductor is an ideal model structure to study
the carrier relaxation channels, which are inefficient in conventional
semiconductors. In particular, it is of fundamental interest to address the
question whether Auger-type processes significantly influence the carrier
dynamics in graphene. These scattering channels bridge the valence and
conduction band allowing carrier multiplication - a process that generates
multiple charge carriers from the absorption of a single photon. This has been
suggested in literature for improving the efficiency of solar cells. Here we
show, based on microscopic calculations within the density matrix formalism,
that Auger processes do play an unusually strong role for the relaxation
dynamics of photo-excited charge carriers in graphene. We predict that a
considerable carrier multiplication takes place, suggesting graphene as a new
material for high-efficiency solar cells and for high-sensitivity
photodetectors
Graphene as Gain Medium for Broadband Lasers
In contrast to conventional structures, efficient non-radiative carrier
recombination counteracts the appearance of optical gain in graphene. Based on
a microscopic and fully quantum-mechanical study of the coupled carrier,
phonon, and photon dynamics in graphene, we present a strategy to obtain a
long-lived gain: Integrating graphene into a photonic crystal nanocavity and
applying a high-dielectric substrate gives rise to pronounced coherent light
emission suggesting the design of graphene-based laser devices covering a broad
spectral range
Impact of doping on the carrier dynamics in graphene
We present a microscopic study on the impact of doping on the carrier
dynamics in graphene, in particular focusing on its influence on the
technologically relevant carrier multiplication in realistic, doped graphene
samples. Treating the time- and momentum-resolved carrier-light,
carrier-carrier, and carrier-phonon interactions on the same microscopic
footing, the appearance of Auger-induced carrier multiplication up to a Fermi
level of 300 meV is revealed. Furthermore, we show that doping favors the
so-called hot carrier multiplication occurring within one band. Our results are
directly compared to recent time-resolved ARPES measurements and exhibit an
excellent agreement on the temporal evolution of the hot carrier multiplication
for n- and p-doped graphene. The gained insights shed light on the ultrafast
carrier dynamics in realistic, doped graphene sample
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Conceptualizing Energy Security
Energy security is one of the main targets of energy policy. However, the term has not been clearly defined, which makes it hard to measure and difficult to balance against other policy objectives. We review the multitude of definitions of energy security. They can be characterized according to the sources of risk, the scope of the impacts, and the severity filters in the form of the speed, size, sustention, spread, singularity and sureness of impacts. Using a stylized case study for three European countries, we illustrate how the selection of conceptual boundaries along these dimensions determines the outcome. This can be avoided by more clearly separating between security of supply and other policy objectives. This leads us to the definition of energy security as the continuity of energy supplies relative to demand. If security is defined from the perspective of private utilities, end consumers or public servants, the concept could further be reduced to the continuity of specific commodity or service supplies, or the impact of supply discontinuities on the continuity of the economy.Literature Revie
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Measuring Energy Security
Continuity of energy supplies is a central aspect of concerns about energy security. Although the continuity of supplies can be influenced by a large number of risks, most models only analyse a small subset of risk sources and often neglect interdependencies between them. In this paper we introduce a probabilistic time-series model that quantifies the impact of inter-dependent natural, technical and human risk sources on energy supply continuity. Based on a case study of Italian gas and electricity markets we conclude that typical simplifications in time-series models and alternative approaches lead to a bias, which justifies the usage of detailed time-series models of interdependent risks such as the framework suggested in this paper, even though more detailed versions of this and other frameworks may quickly become very resource intensive
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