1,631 research outputs found
Resistivity phase diagram of cuprates revisited
The phase diagram of the cuprate superconductors has posed a formidable
scientific challenge for more than three decades. This challenge is perhaps
best exemplified by the need to understand the normal-state charge transport as
the system evolves from Mott insulator to Fermi-liquid metal with doping. Here
we report a detailed analysis of the temperature (T) and doping (p) dependence
of the planar resistivity of simple-tetragonal HgBaCuO
(Hg1201), the single-CuO-layer cuprate with the highest optimal . The
data allow us to test a recently proposed phenomenological model for the
cuprate phase diagram that combines a universal transport scattering rate with
spatially inhomogeneous (de)localization of the Mott-localized hole. We find
that the model provides an excellent description of the data. We then extend
this analysis to prior transport results for several other cuprates, including
the Hall number in the overdoped part of the phase diagram, and find little
compound-to-compound variation in (de)localization gap scale. The results point
to a robust, universal structural origin of the inherent gap inhomogeneity that
is unrelated to doping-related disorder. They are inconsistent with the notion
that much of the phase diagram is controlled by a quantum critical point, and
instead indicate that the unusual electronic properties exhibited by the
cuprates are fundamentally related to strong nonlinearities associated with
subtle nanoscale inhomogeneity.Comment: 22 pages, 5 figure
Dense packing crystal structures of physical tetrahedra
We present a method for discovering dense packings of general convex hard
particles and apply it to study the dense packing behavior of a one-parameter
family of particles with tetrahedral symmetry representing a deformation of the
ideal mathematical tetrahedron into a less ideal, physical, tetrahedron and all
the way to the sphere. Thus, we also connect the two well studied problems of
sphere packing and tetrahedron packing on a single axis. Our numerical results
uncover a rich optimal-packing behavior, compared to that of other continuous
families of particles previously studied. We present four structures as
candidates for the optimal packing at different values of the parameter,
providing an atlas of crystal structures which might be observed in systems of
nano-particles with tetrahedral symmetry
Random projections and the optimization of an algorithm for phase retrieval
Iterative phase retrieval algorithms typically employ projections onto
constraint subspaces to recover the unknown phases in the Fourier transform of
an image, or, in the case of x-ray crystallography, the electron density of a
molecule. For a general class of algorithms, where the basic iteration is
specified by the difference map, solutions are associated with fixed points of
the map, the attractive character of which determines the effectiveness of the
algorithm. The behavior of the difference map near fixed points is controlled
by the relative orientation of the tangent spaces of the two constraint
subspaces employed by the map. Since the dimensionalities involved are always
large in practical applications, it is appropriate to use random matrix theory
ideas to analyze the average-case convergence at fixed points. Optimal values
of the gamma parameters of the difference map are found which differ somewhat
from the values previously obtained on the assumption of orthogonal tangent
spaces.Comment: 15 page
A systemizing research framework for Web 2.0
Web 2.0 has recently been one of the most discussed topics in Information Systems science and practice. However, little consensus is found on what its components and characteristics actually are and what a comprehensive conceptualization might look like. This paper tries to shed light on these questions by systemizing the phenomenon’s characteristics in a hierarchical framework. In a first step, we apply content and cluster analysis on contributions of the field and inductively identify 103 raw categories which are then clustered into ten subcategories and two main categories. Namely these identified main categories of Web 2.0 are: ‘Technological Characteristics’ and ‘Socioeconomic Characteristics’. In a second step, we pretest and optimize the constructs for applicability and ambiguities and finally apply them to evaluate on the importance and weighting of the discovered subcategories. The resulting framework is found to comply with common quality measures for content analysis and classification schemes. It can be used to analyze and explore economic or social phenomena associated with Web 2.0 in a systematic manner
Building Taxonomies in IS and Management – A Systematic Approach Based on Content Analysis
Classification schemes such as taxonomies are important groundwork for research on many topics in Information Systems (IS) and Management. They make investigating topics manageable by allowing researchers to delimit their work to certain taxa or types and provide a basis for generalization. Opposed to theoretically grounded typologies, taxonomies are empirically derived from entities of a phenomenon under investigation and therefore have several advantages such as more detailed and exhaustive coverage. Nevertheless, research is still missing a clear set of procedures on how to empirically build taxonomies. We tackle this topic by suggesting an inductive approach based on the procedures of content and cluster analysis. Each of the proposed six steps is amended with comprehensive state of the art guidelines, suggestions, alternatives and formative measures of reliability and validity
Characterization of the self-palmitoylation activity of the transport protein particle component Bet3
Bet3, a transport protein particle component involved in vesicular trafficking, contains a hydrophobic tunnel occupied by a fatty acid linked to cysteine 68. We reported that Bet3 has a unique self-palmitoylating activity. Here we show that mutation of arginine 67 reduced self-palmitoylation of Bet3, but the effect was compensated by increasing the pH. Thus, arginine helps to deprotonate cysteine such that it could function as a nucleophile in the acylation reaction which is supported by the structural analysis of non-acylated Bet3. Using fluorescence spectroscopy we show that long-chain acyl-CoAs bind with micromolar affinity to Bet3, whereas shorter-chain acyl-CoAs do not interact. Mutants with a deleted acylation site or a blocked tunnel bind to Pal-CoA, only the latter with slightly reduced affinity. Bet3 contains three binding sites for Pal-CoA, but their number was reduced to two in the mutant with an obstructed tunnel, indicating that Bet3 contains binding sites on its surface
A method for dense packing discovery
The problem of packing a system of particles as densely as possible is
foundational in the field of discrete geometry and is a powerful model in the
material and biological sciences. As packing problems retreat from the reach of
solution by analytic constructions, the importance of an efficient numerical
method for conducting \textit{de novo} (from-scratch) searches for dense
packings becomes crucial. In this paper, we use the \textit{divide and concur}
framework to develop a general search method for the solution of periodic
constraint problems, and we apply it to the discovery of dense periodic
packings. An important feature of the method is the integration of the unit
cell parameters with the other packing variables in the definition of the
configuration space. The method we present led to improvements in the
densest-known tetrahedron packing which are reported in [arXiv:0910.5226].
Here, we use the method to reproduce the densest known lattice sphere packings
and the best known lattice kissing arrangements in up to 14 and 11 dimensions
respectively (the first such numerical evidence for their optimality in some of
these dimensions). For non-spherical particles, we report a new dense packing
of regular four-dimensional simplices with density
and with a similar structure to the densest known tetrahedron packing.Comment: 15 pages, 5 figure
Towards Understanding Photoperiodic Response in Grasses
In many plants, day length is the critical environmental parameter that controls flowering time. In long day plants, such as Arabidopsis and ryegrass (Lolium perenne), increasing day length in spring signals flowering, while in short day plants like rice, flowering is accelerated when days become shorter. Recently, significant progress has been made in understanding the molecular genetic mechanisms that govern this response. Most results have been obtained in the model plant Arabidopsis where CONSTANS (CO) is a critical candidate gene. Upstream of it is the GIGANTEA (GI) gene which is associated with the circadian clock mechanism (1). The FT gene is the immediate downstream genetic target of CO, and is a direct promoter of flowering (2). Characteristically, all three genes show circadian expression, albeit in different phases, and both the CO and FT genes are up-regulated under long-day (inductive) conditions. Work in ryegrass should help reveal both the conserved and divergent segments of the photoperiod response between different plant species
The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors
The condensation of an electron superfluid from a conventional metallic state
at a critical temperature is described well by the BCS theory. In the
underdoped copper-oxides, high-temperature superconductivity condenses instead
from a nonconventional metallic "pseudogap" phase that exhibits a variety of
non-Fermi liquid properties. Recently, it has become clear that a charge
density wave (CDW) phase exists within the pseudogap regime, appearing at a
temperature just above . The near coincidence of and
, as well the coexistence and competition of CDW and superconducting
order below , suggests that they are intimately related. Here we show that
the condensation of the superfluid from this unconventional precursor is
reflected in deviations from the predictions of BSC theory regarding the
recombination rate of quasiparticles. We report a detailed investigation of the
quasiparticle (QP) recombination lifetime, , as a function of
temperature and magnetic field in underdoped HgBaCuO
(Hg-1201) and YBaCuO (YBCO) single crystals by ultrafast
time-resolved reflectivity. We find that exhibits a local
maximum in a small temperature window near that is prominent in
underdoped samples with coexisting charge order and vanishes with application
of a small magnetic field. We explain this unusual, non-BCS behavior by
positing that marks a transition from phase-fluctuating SC/CDW composite
order above to a SC/CDW condensate below. Our results suggest that the
superfluid in underdoped cuprates is a condensate of coherently-mixed
particle-particle and particle-hole pairs
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