21,147 research outputs found
Hybrid quantum-classical modeling of quantum dot devices
The design of electrically driven quantum dot devices for quantum optical
applications asks for modeling approaches combining classical device physics
with quantum mechanics. We connect the well-established fields of
semi-classical semiconductor transport theory and the theory of open quantum
systems to meet this requirement. By coupling the van Roosbroeck system with a
quantum master equation in Lindblad form, we introduce a new hybrid
quantum-classical modeling approach, which provides a comprehensive description
of quantum dot devices on multiple scales: It enables the calculation of
quantum optical figures of merit and the spatially resolved simulation of the
current flow in realistic semiconductor device geometries in a unified way. We
construct the interface between both theories in such a way, that the resulting
hybrid system obeys the fundamental axioms of (non-)equilibrium thermodynamics.
We show that our approach guarantees the conservation of charge, consistency
with the thermodynamic equilibrium and the second law of thermodynamics. The
feasibility of the approach is demonstrated by numerical simulations of an
electrically driven single-photon source based on a single quantum dot in the
stationary and transient operation regime
First Class Constrained Systems and Twisting of Courant Algebroids by a Closed 4-form
We show that in analogy to the introduction of Poisson structures twisted by
a closed 3-form by Park and Klimcik-Strobl, the study of three dimensional
sigma models with Wess-Zumino term leads in a likewise way to twisting of
Courant algebroid structures by closed 4-forms H.
The presentation is kept pedagogical and accessible to physicists as well as
to mathematicians, explaining in detail in particular the interplay of field
transformations in a sigma model with the type of geometrical structures
induced on a target. In fact, as we also show, even if one does not know the
mathematical concept of a Courant algebroid, the study of a rather general
class of 3-dimensional sigma models leads one to that notion by itself.
Courant algebroids became of relevance for mathematical physics lately from
several perspectives - like for example by means of using generalized complex
structures in String Theory. One may expect that their twisting by the
curvature H of some 3-form Ramond-Ramond gauge field will become of relevance
as well.Comment: 25 pages, invited contribution to the Wolfgang Kummer memorial volum
Inverse Quantum Chemistry: Concepts and Strategies for Rational Compound Design
The rational design of molecules and materials is becoming more and more
important. With the advent of powerful computer systems and sophisticated
algorithms, quantum chemistry plays an important role in rational design. While
traditional quantum chemical approaches predict the properties of a predefined
molecular structure, the goal of inverse quantum chemistry is to find a
structure featuring one or more desired properties. Herein, we review inverse
quantum chemical approaches proposed so far and discuss their advantages as
well as their weaknesses.Comment: 43 pages, 5 figure
Distribution functions and current-correlations in normal-metal--superconductor hetero-structures
We introduce electron-like and hole-like distribution functions, which
determine the currents and the fluctuation spectra of the currents measured at
a normal-conductor--superconductor hetero-structure. These distribution
functions are expressed with the help of newly defined partial densities of
states for hetero-structures. Voltage measurements using a weakly coupled
contact on such a structure show the absence of a contact resistance to the
superconducting reservoir and illustrate how the interface to the
superconductor acts as an Andreev mirror. We also discuss the current-current
correlations measured at two normal contacts and argue that the appearance of
positive correlations is a purely mesoscopic effect, which vanishes in the
limit of a large number of channels and in the average over an ensemble.Comment: 8 pages REVTeX, 1 figur
Applicability of Effective Pair Potentials for Active Brownian Particles
We have performed a case study investigating a recently proposed scheme to
obtain an effective pair potential for active Brownian particles [Farage et
al., Phys. Rev. E 91, 042310 (2015)]. Applying this scheme to the Lennard-Jones
potential, numerical simulations of active Brownian particles are compared to
simulations of passive Brownian particles interacting by the effective pair
potential. Analyzing the static pair correlations, our results indicate a
limited range of activity parameters (speed and orientational correlation time)
for which we obtain quantitative, or even qualitative, agreement. Moreover, we
find a qualitatively different behavior for the virial pressure even for small
propulsion speeds. Combining these findings we conclude that beyond linear
response active particles exhibit genuine non-equilibrium properties that
cannot be captured by effective pair interaction alone
Gradient-Driven Molecule Construction: An Inverse Approach Applied to the Design of Small-Molecule Fixating Catalysts
Rational design of molecules and materials usually requires extensive
screening of molecular structures for the desired property. The inverse
approach to deduce a structure for a predefined property would be highly
desirable, but is, unfortunately, not well-defined. However, feasible
strategies for such an inverse design process may be successfully developed for
specific purposes. We discuss options for calculating 'jacket' potentials that
fulfill a predefined target requirement - a concept that we recently introduced
[T. Weymuth, M. Reiher, MRS Proceediungs, 2013, 1524,
DOI:10.1557/opl.2012.1764]. We consider the case of small-molecule activating
transition metal catalysts. As a target requirement we choose the vanishing
geometry gradients on all atoms of a subsystem consisting of a metal center
binding the small molecule to be activated. The jacket potential can be
represented within a full quantum model or by a sequence of approximations of
which a field of electrostatic point charges is the simplest. In a second step,
the jacket potential needs to be replaced by a chemically viable chelate-ligand
structure for which the geometry gradients on all of its atoms are also
required to vanish. In order to analyze the feasibility of this approach, we
dissect a known dinitrogen-fixating catalyst to study possible design
strategies that must eventually produce the known catalyst.Comment: 40 pages, 6 tables, 5 figure
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