6,145 research outputs found
MatsubaraFunctions.jl: An equilibrium Green's function library in the Julia programming language
The Matsubara Green's function formalism stands as a powerful technique for
computing the thermodynamic characteristics of interacting quantum
many-particle systems at finite temperatures. In this manuscript, our focus
centers on introducing MatsubaraFunctions.jl, a Julia library that implements
data structures for generalized n-point Green's functions on Matsubara
frequency grids. The package's architecture prioritizes user-friendliness
without compromising the development of efficient solvers for quantum field
theories in equilibrium. Following a comprehensive introduction of the
fundamental types, we delve into a thorough examination of key facets of the
interface. This encompasses avenues for accessing Green's functions, techniques
for extrapolation and interpolation, as well as the incorporation of symmetries
and a variety of parallelization strategies. Examples of increasing complexity
serve to demonstrate the practical utility of the library, supplemented by
discussions on strategies for sidestepping impediments to optimal performance.Comment: 37 pages, 10 figure
Grain Refinement in Iron-Based Materials
A process for manufacturing an iron-based alloy comprising forming targeted fine oxide and/or carbide dispersoids in a melt, and sequentially precipitating transition-metal nitrides on the dispersoids for heterogeneous nucleation of equiaxed grains. An iron-based cast alloy having a highly equiaxed fine grain structure
Physical Constraints and Functional Characteristics of Transcription Factor-DNA Interaction
We study theoretical ``design principles'' for transcription factor-DNA
interaction in bacteria, focusing particularly on the statistical interaction
of the transcription factors (TF's) with the genomic background (i.e., the
genome without the target sites). We introduce and motivate the concept of
`programmability', i.e. the ability to set the threshold concentration for TF
binding over a wide range merely by mutating the binding sequence of a target
site. This functional demand, together with physical constraints arising from
the thermodynamics and kinetics of TF-DNA interaction, leads us to a narrow
range of ``optimal'' interaction parameters. We find that this parameter set
agrees well with experimental data for the interaction parameters of a few
exemplary prokaryotic TF's. This indicates that TF-DNA interaction is indeed
programmable. We suggest further experiments to test whether this is a general
feature for a large class of TF's.Comment: 9 pages, 4 figures; revised version as published in PNA
On cardinal invariants and generators for von Neumann algebras
We demonstrate how virtually all common cardinal invariants associated to a
von Neumann algebra M can be computed from the decomposability number, dec(M),
and the minimal cardinality of a generating set, gen(M). Applications include
the equivalence of the well-known generator problem, "Is every separably-acting
von Neumann algebra singly-generated?", with the formally stronger questions,
"Is every countably-generated von Neumann algebra singly-generated?" and "Is
the gen invariant monotone?" Modulo the generator problem, we determine the
range of the invariant (gen(M), dec(M)), which is mostly governed by the
inequality dec(M) leq c^{gen(M)}.Comment: 22 pages; the main additions are Theorem 3.8 and Section
Analytic continuation of multipoint correlation functions
Conceptually, the Matsubara formalism (MF), using imaginary frequencies, and
the Keldysh formalism (KF), formulated in real frequencies, give equivalent
results for systems in thermal equilibrium. The MF has less complexity and is
thus more convenient than the KF. However, computing dynamical observables in
the MF requires the analytic continuation from imaginary to real frequencies.
The analytic continuation is well-known for two-point correlation functions
(having one frequency argument), but, for multipoint correlators, a
straightforward recipe for deducing all Keldysh components from the MF
correlator had not been formulated yet. Recently, a representation of MF and KF
correlators in terms of formalism-independent partial spectral functions and
formalism-specific kernels was introduced by Kugler, Lee, and von Delft [Phys.
Rev. X 11, 041006 (2021)]. We use this representation to formally elucidate the
connection between both formalisms. We show how a multipoint MF correlator can
be analytically continued to recover all partial spectral functions and yield
all Keldysh components of its KF counterpart. The procedure is illustrated for
various correlators of the Hubbard atom.Comment: 56 pages, 8 figure
Real-frequency quantum field theory applied to the single-impurity Anderson model
A major challenge in the field of correlated electrons is the computation of
dynamical correlation functions. For comparisons with experiment, one is
interested in their real-frequency dependence. This is difficult to compute, as
imaginary-frequency data from the Matsubara formalism require analytic
continuation, a numerically ill-posed problem. Here, we apply quantum field
theory to the single-impurity Anderson model (AM), using the Keldysh instead of
the Matsubara formalism with direct access to the self-energy and dynamical
susceptibilities on the real-frequency axis. We present results from the
functional renormalization group (fRG) at one-loop level and from solving the
self-consistent parquet equations in the parquet approximation. In contrast to
previous Keldysh fRG works, we employ a parametrization of the four-point
vertex which captures its full dependence on three real-frequency arguments. We
compare our results to benchmark data obtained with the numerical
renormalization group and to second-order perturbation theory. We find that
capturing the full frequency dependence of the four-point vertex significantly
improves the fRG results compared to previous implementations, and that solving
the parquet equations yields the best agreement with the NRG benchmark data,
but is only feasible up to moderate interaction strengths. Our methodical
advances pave the way for treating more complicated models in the future.Comment: 25 pages, 20 figure
Stochastic Physics, Complex Systems and Biology
In complex systems, the interplay between nonlinear and stochastic dynamics,
e.g., J. Monod's necessity and chance, gives rise to an evolutionary process in
Darwinian sense, in terms of discrete jumps among attractors, with punctuated
equilibrium, spontaneous random "mutations" and "adaptations". On an
evlutionary time scale it produces sustainable diversity among individuals in a
homogeneous population rather than convergence as usually predicted by a
deterministic dynamics. The emergent discrete states in such a system, i.e.,
attractors, have natural robustness against both internal and external
perturbations. Phenotypic states of a biological cell, a mesoscopic nonlinear
stochastic open biochemical system, could be understood through such a
perspective.Comment: 10 page
Towards an Ontological Modelling of Preference Relations
Preference relations are intensively studied in Economics,
but they are also approached in AI, Knowledge Representation, and
Conceptual Modelling, as they provide a key concept in a variety of
domains of application. In this paper, we propose an ontological foundation
of preference relations to formalise their essential aspects across
domains. Firstly, we shall discuss what is the ontological status of the
relata of a preference relation. Secondly, we investigate the place of preference
relations within a rich taxonomy of relations (e.g. we ask whether
they are internal or external, essential or contingent, descriptive or nondescriptive
relations). Finally, we provide an ontological modelling of
preference relation as a module of a foundational (or upper) ontology
(viz. OntoUML).
The aim of this paper is to provide a sharable foundational theory of
preference relation that foster interoperability across the heterogeneous
domains of application of preference relations
Naive Pluripotent Stem Cells Derived Directly from Isolated Cells of the Human Inner Cell Mass.
Conventional generation of stem cells from human blastocysts produces a developmentally advanced, or primed, stage of pluripotency. In vitro resetting to a more naive phenotype has been reported. However, whether the reset culture conditions of selective kinase inhibition can enable capture of naive epiblast cells directly from the embryo has not been determined. Here, we show that in these specific conditions individual inner cell mass cells grow into colonies that may then be expanded over multiple passages while retaining a diploid karyotype and naive properties. The cells express hallmark naive pluripotency factors and additionally display features of mitochondrial respiration, global gene expression, and genome-wide hypomethylation distinct from primed cells. They transition through primed pluripotency into somatic lineage differentiation. Collectively these attributes suggest classification as human naive embryonic stem cells. Human counterparts of canonical mouse embryonic stem cells would argue for conservation in the phased progression of pluripotency in mammals.This work was supported by the Medical Research Council, Biotechnology and Biological Sciences Research Council, Swiss National Science Foundation (SNF)/Novartis SNF (F.v.M.) and core funding to the Cambridge Stem Cell Institute from the Wellcome Trust and Medical Research Council. AS is a Medical Research Council Professor.This is the final version of the article. It first appeared from Cell Press via http://dx.doi.org/10.1016/j.stemcr.2016.02.00
Integrated impedance bridge for absolute capacitance measurements at cryogenic temperatures and finite magnetic fields
We developed an impedance bridge that operates at cryogenic temperatures
(down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This
is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed
circuit board containing the device under test and thereby parallel to the
magnetic field. The measured amplitude and phase of the output signal allows
for the separation of the total impedance into an absolute capacitance and a
resistance. Through a detailed noise characterization, we find that the best
resolution is obtained when operating the HEMT amplifier at the highest gain.
We obtained a resolution in the absolute capacitance of
6.4~aF at 77 K on a comb-drive actuator, while maintaining
a small excitation amplitude of 15~. We show the magnetic field
functionality of our impedance bridge by measuring the quantum Hall plateaus of
a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of
12.8~.Comment: 7 pages, 5 figure
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