52 research outputs found
How chromium doping affects the correlated electronic structure of V2O3
The archetypical strongly correlated Mott-phenomena compound V2O3 is known to
show a paramagnetic metal-insulator transition driven by doping with chromium
atoms and/or (negative) pressure. Via charge self-consistent density-functional
theory+dynamical mean-field theory calculations we demonstrate that these two
routes cannot be understood as equivalent. To this end, the explicit
description of Cr-doped V2O3 by means of supercell calculations and the virtual
crystal approximation is performed. Already the sole introduction of chromium's
additional electron to the system is shown to modify the overall correlated
electronic structure substantially. Correlation-induced charge transfers
between Cr and the remaining V ions occur and the transition-metal orbital
polarization is increased by the electron doping, in close agreement with
experimental findings.Comment: 6 pages, 8 figure
Electron correlation and magnetism at the LaAlO/SrTiO interface: A DFT+DMFT investigation
We shed light on the interplay between structure and many-body effects
relevant for itinerant ferromagnetism in LaAlO/SrTiO heterostructures.
The realistic correlated electronic structure is studied by means of the
(spin-polarized) charge self-consistent combination of density functional
theory (DFT) with dynamical mean-field theory (DMFT) beyond the realm of static
correlation effects. Though many-body behavior is also active in the
defect-free interface, a ferromagnetic instability occurs only with oxygen
vacancies. A minimal Ti two-orbital - description for the
correlated subspace is derived. Magnetic order affected by quantum fluctuations
builds up from effective double exchange between nearly-localized and
mobile electrons.Comment: refinements, final versio
Formation of orbital-selective electron states in LaTiO/SrTiO superlattices
The interface electronic structure of correlated LaTiO/SrTiO
superlattices is investigated by means of the charge self-consistent
combination of the local density approximation (LDA) to density functional
theory (DFT) with dynamical mean-field theory (DMFT). Utilizing a
pseudopotential technique together with a continuous-time quantum Monte-Carlo
approach, the resulting complex multiorbital electronic states are addressed in
a coherent fashion beyond static mean-field. General structural relaxations are
taken into account on the LDA level and cooperate with the driving forces from
strong electronic correlations. This alliance leads to an Ti()
dominated low-energy quasiparticle peak and a lower Hubbard band in line with
photoemission studies. Furthermore correlation effects close to the
band-insulating bulk SrTiO limit as well as the Mott-insulating bulk
LaTiO limit are studied via realistic single-layer embeddings.Comment: minor refinements, added referenc
Approaching finite-temperature phase diagrams of strongly correlated materials: a case study for V2O3
Examining phase stabilities and phase equilibria in strongly correlated
materials asks for a next level in the many-body extensions to the
local-density approximation (LDA) beyond mainly spectroscopic assessments. Here
we put the charge-self-consistent LDA+dynamical mean-field theory (DMFT)
methodology based on projected local orbitals for the LDA+DMFT interface and a
tailored pseudopotential framework into action in order to address such
thermodynamics of realistic strongly correlated systems. Namely a case study
for the electronic phase diagram of the well-known prototype Mott-phenomena
system VO at higher temperatures is presented. We are able to describe
the first-order metal-to-insulator transitions with negative pressure and
temperature from the self-consistent computation of the correlated total energy
in line with experimental findings.Comment: 12 pages, 15 figures, new data adde
Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3
Albeit structurally and electronically very similar, at low temperature the
quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator
transition via the appearance of a charge-density-wave state, while BaVSe3
apparently remains metallic down to zero temperature. This different behavior
upon cooling is studied by means of density functional theory and its
combination with the dynamical mean-field theory and the rotationally-invariant
slave-boson method. We reveal several subtle differences between these
chalcogenides that provide indications for the deviant behavior of BaVSe3 at
low temperature. In this regard, a smaller Hubbard U in line with an increased
relevance of the Hund's exchange J plays a vital role.Comment: 16 pages, 11 figures, published versio
Review article: A European perspective on wind and storm damage – from the meteorological background to index-based approaches to assess impacts
Wind and windstorms cause severe damage to natural and human-made environments. Thus, wind-related risk assessment is vital for the preparation and mitigation of calamities. However, the cascade of events leading to damage depends on many factors that are environment-specific and the available methods to address wind-related damage often require sophisticated analysis and specialization. Fortunately, simple indices and thresholds are as effective as complex mechanistic models for many applications. Nonetheless, the multitude of indices and thresholds available requires a careful selection process according to the target sector. Here, we first provide a basic background on wind and storm formation and characteristics, followed by a comprehensive collection of both indices and thresholds that can be used to predict the occurrence and magnitude of wind and storm damage. We focused on five key sectors: forests, urban areas, transport, agriculture and wind-based energy production. For each sector we described indices and thresholds relating to physical properties such as topography and land cover but also to economic aspects (e.g. disruptions in transportation or energy production). In the face of increased climatic variability, the promotion of more effective analysis of wind and storm damage could reduce the impact on society and the environment
The new Felsenkeller 5 MV underground accelerator
The field of nuclear astrophysics is devoted to the study of the creation of
the chemical elements. By nature, it is deeply intertwined with the physics of
the Sun. The nuclear reactions of the proton-proton cycle of hydrogen burning,
including the 3He({\alpha},{\gamma})7Be reaction, provide the necessary nuclear
energy to prevent the gravitational collapse of the Sun and give rise to the by
now well-studied pp, 7Be, and 8B solar neutrinos. The not yet measured flux of
13N, 15O, and 17F neutrinos from the carbon-nitrogen-oxygen cycle is affected
in rate by the 14N(p,{\gamma})15O reaction and in emission profile by the
12C(p,{\gamma})13N reaction. The nucleosynthetic output of the subsequent phase
in stellar evolution, helium burning, is controlled by the
12C({\alpha},{\gamma})16O reaction.
In order to properly interpret the existing and upcoming solar neutrino data,
precise nuclear physics information is needed. For nuclear reactions between
light, stable nuclei, the best available technique are experiments with small
ion accelerators in underground, low-background settings. The pioneering work
in this regard has been done by the LUNA collaboration at Gran Sasso/Italy,
using a 0.4 MV accelerator.
The present contribution reports on a higher-energy, 5.0 MV, underground
accelerator in the Felsenkeller underground site in Dresden/Germany. Results
from {\gamma}-ray, neutron, and muon background measurements in the
Felsenkeller underground site in Dresden, Germany, show that the background
conditions are satisfactory for nuclear astrophysics purposes. The accelerator
is in the commissioning phase and will provide intense, up to 50{\mu}A, beams
of 1H+, 4He+ , and 12C+ ions, enabling research on astrophysically relevant
nuclear reactions with unprecedented sensitivity.Comment: Submitted to the Proceedings of the 5th International Solar Neutrino
Conference, Dresden/Germany, 11-14 June 2018, to appear on World Scientific
-- updated version (Figure 2 and relevant discussion updated, co-author A.
Domula added
5G transport network requirements for the next generation fronthaul interface
To meet the requirements of 5G mobile networks, several radio access technologies, such as millimeter wave communications and massive MIMO, are being proposed. In addition, cloud radio access network (C-RAN) architectures are considered instrumental to fully exploit the capabilities of future 5G RANs. However, RAN centralization imposes stringent requirements on the transport network, which today are addressed with purpose-specific and expensive fronthaul links. As the demands on future access networks rise, so will the challenges in the fronthaul and backhaul segments. It is hence of fundamental importance to consider the design of transport networks alongside the definition of future access technologies to avoid the transport becoming a bottleneck. Therefore, we analyze in this work the impact that future RAN technologies will have on the transport network and on the design of the next generation fronthaul interface. To understand the especially important impact of varying user traffic, we utilize measurements from a real-world 4G network and, taking target 5G performance figures into account, extrapolate its statistics to a 5G scenario. With this, we derive both per-cell and aggregated data rate requirements for 5G transport networks. In addition, we show that the effect of statistical multiplexing is an important factor to reduce transport network capacity requirements and costs. Based on our investigations, we provide guidelines for the development of the 5G transport network architecture.Peer ReviewedPostprint (published version
Review article: A European perspective on wind and storm damage – from the meteorological background to index-based approaches to assess impacts
Wind and windstorms cause severe damage to natural and human-made environments. Thus, wind-related risk assessment is vital for the preparation and mitigation of calamities. However, the cascade of events leading to damage depends on many factors that are environment-specific and the available methods to address wind-related damage often require sophisticated analysis and specialization. Fortunately, simple indices and thresholds are as effective as complex mechanistic models for many applications. Nonetheless, the multitude of indices and thresholds available requires a careful selection process according to the target sector. Here, we first provide a basic background on wind and storm formation and characteristics, followed by a comprehensive collection of both indices and thresholds that can be used to predict the occurrence and magnitude of wind and storm damage. We focused on five key sectors: forests, urban areas, transport, agriculture and wind-based energy production. For each sector we described indices and thresholds relating to physical properties such as topography and land cover but also to economic aspects (e.g. disruptions in transportation or energy production). In the face of increased climatic variability, the promotion of more effective analysis of wind and storm damage could reduce the impact on society and the environment
5G infrastructures supporting end-user and operational services:The 5G-XHaul architectural perspective
We propose an optical-wireless 5G infrastructure offering converged fronthauling/backhauling functions to support both operational and end-user cloud services. A layered architectural structure required to efficiently support these services is shown. The data plane performance of the proposed infrastructure is evaluated in terms of energy consumption and service delay through a novel modelling framework. Our modelling results show that the proposed architecture can offer significant energy savings but there is a clear trade-off between overall energy consumption and service delay.Peer ReviewedPostprint (author's final draft
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