3,745 research outputs found
Finite temperature dynamics of the Anderson model
The recently introduced local moment approach (LMA) is extended to encompass
single-particle dynamics and transport properties of the Anderson impurity
model at finite-temperature, T. While applicable to arbitrary interaction
strengths, primary emphasis is given to the strongly correlated Kondo regime
(characterized by the T=0 Kondo scale ). In particular the
resultant universal scaling behaviour of the single-particle spectrum
D(\omega; T) \equiv F(\frac{\w}{\omega_{\rm K}}; \frac{T}{\omega_{\rm K}})
within the LMA is obtained in closed form; leading to an analytical description
of the thermal destruction of the Kondo resonance on all energy scales.
Transport properties follow directly from a knowledge of . The -dependence of the resulting resistivity , which is
found to agree rather well with numerical renormalization group calculations,
is shown to be asymptotically exact at high temperatures; to concur well with
the Hamann approximation for the s-d model down to ,
and to cross over smoothly to the Fermi liquid form in the low-temperature limit. The underlying
approach, while naturally approximate, is moreover applicable to a broad range
of quantum impurity and related models
Linear polarization sensitivity of SeGA detectors
Parity is a key observable in nuclear spectroscopy. Linear polarization
measurements of -rays are a probe to access the parities of energy
levels. Utilizing the segmentation of detectors in the Segmented Germanium
Array (SeGA) at the NSCL and analyzing the positions of interaction therein
allows the detectors to be used as Compton polarimeters. Unlike other segmented
detectors, SeGA detectors are irradiated from the side to utilize the
transversal segmentation for better Doppler corrections. Sensitivity in such an
orientation has previously been untested. A linear polarization sensitivity has been measured in the 350-keV energy range for SeGA detectors
using - correlations from a \nuc{249}{Cf} source.Comment: 7 pages, 9 figure
Charging axisymmetric space-times with cosmological constant
Ernst's solution generating technique for adding electromagnetic charge to
axisymmetric space-times in general relativity is generalised in presence of
the cosmological constant. Ernst equations for complex potentials are found and
they are traced back to an affective dual complex dynamical system, whose
symmetries are studied. In particular this method is able to generate charged,
asymptotically (A)dS black holes from their uncharged version: as an example,
it is shown explicitly how to pass from the Kerr-(A)dS to the Kerr-Newman-(A)dS
metric. A new solution describing a magnetic universe in presence of the
cosmological constant is also generated.Comment: 15 pages, v2: typos correcte
The Influence of Boron (B), Tin (Sn), Copper (Cu), and Manganese (Mn) on the Microstructure of Spheroidal Graphite Irons
Most spheroidal graphite irons (SGIs) have a matrix consisting of ferrite, pearlite, or a mix of the two. To achieve the desired matrix composition, pearlite promoters such as Mn, Cu, or Sn, are added to the molten metal. Among these elements, Sn is the most potent pearlite promoter. However, each has a different impact on the solidification, graphite precipitation, eutectoid transformation, and ultimately the final structure of the material. Research has shown that B promotes ferrite in fully pearlitic grades where Cu and Mn were used to promote pearlite. The present work investigates the effect of B in SGI with additions of Sn, Cu, and Mn, and the effects of varying amounts of the different pearlite promoters on the matrix composition. The results show that Mn alone at levels of approximately 0.9 wt% is not enough to promote a fully pearlitic matrix, while 0.5 wt% Cu combined with 0.67 wt% Mn is sufficient. Likewise, a fully pearlitic microstructure can be obtained by alloying with 0.06 wt% Sn and 0.67 wt% Mn. B was found to promote ferrite in fully pearlitic SGI alloyed with Sn or Cu. However, in the absence of those elements, B promoted pearlite when alloyed with just Mn. Graphite protrusions were observed on the graphite nodule surface only for B-added alloys with Sn and Cu. In these cases, it is believed B promotes ferrite by changing the growth mechanism of graphite after solidification from spherical to lamellar. However, a different graphite morphology is observed when B is added with just Mn. Thermal analysis data is in agreement with the microstructural observations regarding the ferrite promoting effect of B
Functional renormalization group approach to zero-dimensional interacting systems
We apply the functional renormalization group method to the calculation of
dynamical properties of zero-dimensional interacting quantum systems. As case
studies we discuss the anharmonic oscillator and the single impurity Anderson
model. We truncate the hierarchy of flow equations such that the results are at
least correct up to second order perturbation theory in the coupling. For the
anharmonic oscillator energies and spectra obtained within two different
functional renormalization group schemes are compared to numerically exact
results, perturbation theory, and the mean field approximation. Even at large
coupling the results obtained using the functional renormalization group agree
quite well with the numerical exact solution. The better of the two schemes is
used to calculate spectra of the single impurity Anderson model, which then are
compared to the results of perturbation theory and the numerical
renormalization group. For small to intermediate couplings the functional
renormalization group gives results which are close to the ones obtained using
the very accurate numerical renormalization group method. In particulare the
low-energy scale (Kondo temperature) extracted from the functional
renormalization group results shows the expected behavior.Comment: 22 pages, 8 figures include
Dynamic and spectral mixing in nanosystems
In the framework of simple spin-boson Hamiltonian we study an interplay
between dynamic and spectral roots to stochastic-like behavior. The Hamiltonian
describes an initial vibrational state coupled to discrete dense spectrum
reservoir. The reservoir states are formed by three sequences with rationally
independent periodicities typical for vibrational states in many nanosize
systems. We show that quantum evolution of the system is determined by a
dimensionless parameter which is characteristic number of the reservoir states
relevant for the initial vibrational level dynamics. Our semi-quantitative
analytic results are confirmed by numerical solution of the equation of motion.
We anticipate that predicted in the paper both kinds of stochastic-like
behavior (namely, due to spectral mixing and recurrence cycle dynamic mixing)
can be observed by femtosecond spectroscopy methods in nanosystems.Comment: 6 pages, 4 figure
Designing for emergence and innovation: Redesigning design
We reveal the surprising and counterintuitive truth that the design process, in and
of itself, is not always on the forefront of innovation. Design is a necessary but
not a sufficient condition for the success of new products and services. We
intuitively sense a connection between innovative design and emergence. The
nature of design, emergence and innovation to understand their interrelationships
and interdependencies is examined. We propose that design must harness the
process of emergence; for it is only through the bottom-up and massively
iterative unfolding of emergence that new and improved products and services
are successfully refined, introduced and diffused into the marketplace.
The relationships among design, emergence and innovation are developed.
What designers can learn from nature about emergence and evolution that will
impact the design process is explored. We examine the roles that design and
emergence play in innovation. How innovative organizations can incorporate
emergence into their design process is explored.
We demarcate the boundary between invention and innovation. We also
articulate the similarities and differences of design and emergence. We then
develop the following three hypotheses:
Hypothesis 1: “An innovative design is an emergent design.”
Hypothesis 2: “A homeostatic relationship between design and emergence is a
required condition for innovation.”Hypothesis 3: “Since design is a cultural activity and culture is an emergent
phenomenon, it follows that design leading to innovation is also an emergent
phenomenon”
We provide a number of examples of how design and emergence have worked
together and led to innovation. Examples include the tool making of early man;
the evolutionary chain of the six languages speech, writing, math, science,
computing and the Internet; the Gutenberg printing press and techniques of
collaborative filtering associated with the Internet.
We close by describing the relationship between human and naturally “designed”
systems and the notion a key element of a design is its purpose as is the case
with a living organism
Non-equilibrium Differential Conductance through a Quantum Dot in a Magnetic Field
We derive an exact expression for the differential conductance for a quantum
dot in an arbitrary magnetic field for small bias voltage. The derivation is
based on the symmetric Anderson model using renormalized perturbation theory
and is valid for all values of the on-site interaction including the Kondo
regime. We calculate the critical magnetic field for the splitting of the Kondo
resonance to be seen in the differential conductivity as function of bias
voltage. Our calculations for small field show that the peak position of the
component resonances in the differential conductance are reduced substantially
from estimates using the equilibrium Green's function. We conclude that it is
important to take the voltage dependence of the local retarded Green's function
into account in interpreting experimental resultsComment: 8 pages, 4 figures; Replaced by a fully revised version with minor
corrections in the tex
Expected length of the longest common subsequence for large alphabets
We consider the length L of the longest common subsequence of two randomly
uniformly and independently chosen n character words over a k-ary alphabet.
Subadditivity arguments yield that the expected value of L, when normalized by
n, converges to a constant C_k. We prove a conjecture of Sankoff and Mainville
from the early 80's claiming that C_k\sqrt{k} goes to 2 as k goes to infinity.Comment: 14 pages, 1 figure, LaTe
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