756 research outputs found
A two-channel Kondo impurity in the spin-1/2 chain: Consequences for Knight shift experiments
A magnetic impurity in the spin-1/2 chain is a simple realization of the
two-channel Kondo problem since the field theoretical descriptions in the
spin-sector are identical. The correlation functions near the impurity can be
calculated. Using a modified version of the numerical transfer matrix DMRG, we
are able to accurately determine local properties close to the impurity in the
thermodynamic limit. The local susceptibilities (Knight-shifts) show an
interesting behavior in a large range around the impurities. We are able to
make quantitative experimental predictions which would allow to observe
two-channel Kondo physics for the first time directly by doping of spin-1/2
chain compounds.Comment: 2 pages in revtex format including 2 embedded figures (using epsf
Wigner crystal vs. Friedel oscillations in the 1D Hubbard model
We analyze the fermion density of the one-dimensional Hubbard model using
bosonization and numerical DMRG calculations. For finite systems we find a
relatively sharp crossover even for moderate short range interactions into a
region with density waves as a function of density. The results show
that the unstable fixed point of a spin-incoherent state can dominate the
physical behavior in a large region of parameter space in finite systems. The
crossover may be observable in ultra cold fermionic gases in optical lattices
and in finite quantum wires.Comment: 6 pages, 6 figures. Published version. The most recent file can be
found at http://www.physik.uni-kl.de/eggert/papers/index.htm
Universal cross-over behavior of a magnetic impurity and consequences for doping in spin-1/2 chains
We consider a magnetic impurity in the antiferromagnetic spin-1/2 chain which
is equivalent to the two-channel Kondo problem in terms of the field
theoretical description. Using a modification of the transfer-matrix density
matrix renormalization group (DMRG) we are able to determine local and global
properties in the thermodynamic limit. The cross-over function for the impurity
susceptibility is calculated over a large temperature range, which exhibits
universal data-collapse. We are also able to determine the local
susceptibilities near the impurity, which show an interesting competition of
boundary effects. This results in quantitative predictions for experiments on
doped spin-1/2 chains, which could observe two-channel Kondo physics directly.Comment: 5 pages in revtex format including 3 embedded figures (using epsf).
The latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/crossover.pdf . Accepted by PR
Impurity corrections to the thermodynamics in spin chains using a transfer-matrix DMRG method
We use the density matrix renormalization group (DMRG) for transfer matrices
to numerically calculate impurity corrections to thermodynamic properties. The
method is applied to two impurity models in the spin-1/2 chain, namely a weak
link in the chain and an external impurity spin. The numerical analysis
confirms the field theory calculations and gives new results for the crossover
behavior.Comment: 9 pages in revtex format including 5 embedded figures (using epsf).
To appear in PRB. The latest version in PDF format can be found at
http://fy.chalmers.se/~eggert/papers/DMRGimp.pd
Spin- and charge-density oscillations in spin chains and quantum wires
We analyze the spin- and charge-density oscillations near impurities in spin
chains and quantum wires. These so-called Friedel oscillations give detailed
information about the impurity and also about the interactions in the system.
The temperature dependence of these oscillations explicitly shows the
renormalization of backscattering and conductivity, which we analyze for a
number of different impurity models. We are also able to analyze screening
effects in one dimension. The relation to the Kondo effect and experimental
consequences are discussed.Comment: Final published version. 15 pages in revtex format including 22
epsf-embedded figures. The latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/density-osc.pd
Data warehouse design and legal visualization – the applicability of H2 for reporting
The steady increase of regulations and its acceleration due to the financial crisis heavily affect the management of regulatory compliance. Regulations, such as Basel III and Solvency II particularly impact data warehouses and lead to many organizational and technical changes. From an IS perspective modeling techniques for data warehouse requirement elicitation help to manage conceptual requirements. From a legal perspective attempts to visualize regulatory requirements – so called legal visualization approaches – have been developed. This paper investigates whether a conceptual modeling technique for regulatory-driven data warehouse requirements is applicable for representing data warehouse requirements in a legal environment. Applying the modeling technique H2 for Reporting in three extensive modeling projects provides three contributions. First, evidence for the applicability of a modeling technique for regulatory-driven data warehouse requirements is given. Second, lessons learned for further modeling projects are provided. Third, a discussion towards a combined perspective of information modeling and legal visualization is presented.<br /
Phase diagram of an impurity in the spin-1/2 chain: two channel Kondo effect versus Curie law
We consider a magnetic s=1/2 impurity in the antiferromagnetic spin chain as
a function of two coupling parameters: the symmetric coupling of the impurity
to two sites in the chain and the coupling between the two sites .
By using field theory arguments and numerical calculations we can identify all
possible fixed points and classify the renormalization flow between them, which
leads to a non-trivial phase diagram. Depending on the detailed choice of the
two (frustrating) coupling strengths, the stable phases correspond either to a
decoupled spin with Curie law behavior or to a non-Fermi liquid fixed point
with a logarithmically diverging impurity susceptibility as in the two channel
Kondo effect. Our results resolve a controversy about the renormalization flow.Comment: 5 pages in revtex format including 4 embedded figures (using epsf).
The latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/phase-diagram.pd
Low-energy local density of states of the 1D Hubbard model
We examine the local density of states (DOS) at low energies numerically and
analytically for the Hubbard model in one dimension. The eigenstates represent
separate spin and charge excitations with a remarkably rich structure of the
local DOS in space and energy. The results predict signatures of strongly
correlated excitations in the tunneling probability along finite quantum wires,
such as carbon nanotubes, atomic chains or semiconductor wires in scanning
tunneling spectroscopy (STS) experiments. However, the detailed signatures can
only be partly explained by standard Luttinger liquid theory. In particular, we
find that the effective boundary exponent can be negative in finite wires,
which leads to an increase of the local DOS near the edges in contrast to the
established behavior in the thermodynamic limit.Comment: 6 pages, 4 figures, more information can be found at
http://www.physik.uni-kl.de/eggert/papers/index.htm
The Foresighted Driver: Future ADAS Based on Generalized Predictive Risk Estimation
Separably developed functionality as well as increasing situation complexity poses
problems for building, testing, and validating future Advanced Driving Assistance Systems
(ADAS). These will have to deal with situations in which several current ADAS domains
interplay. We argue that a generalized estimation of the future ADAS functions’ benefit is
required for efficient testing and evaluations, and propose a quantification based on an estimation of the predicted risk. The approach can be applied to several different types of risks and to such
diverse scenarios as longitudinal driving, intersection crossing and lane changes with several
traffic participants. Resulting trajectories exhibit a proactive, ”foresighted” driver behavior
which smoothly avoids potential future risks
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