1,816 research outputs found
An ensemble of random graphs with identical degree distribution
Degree distribution, or equivalently called degree sequence, has been
commonly used to be one of most significant measures for studying a large
number of complex networks with which some well-known results have been
obtained. By contrast, in this paper, we report a fact that two arbitrarily
chosen networks with identical degree distribution can have completely
different other topological structure, such as diameter, spanning trees number,
pearson correlation coefficient, and so forth. Besides that, for a given degree
distribution (as power-law distribution with exponent discussed
here), it is reasonable to ask how many network models with such a constraint
we can have. To this end, we generate an ensemble of this kind of random graphs
with (), denoted as graph space
where probability parameters and hold on ,
and indirectly show the cardinality of seems to be large
enough in the thermodynamics limit, i.e., , by varying
values of and . From the theoretical point of view, given an ultrasmall
constant , perhaps only graph model is small-world and other
are not in terms of diameter. And then, we study spanning trees number on two
deterministic graph models and obtain both upper bound and lower bound for
other members. Meanwhile, for arbitrary , we prove that graph model
does go through two phase transitions over time, i.e., starting by
non-assortative pattern and then suddenly going into disassortative region, and
gradually converging to initial place (non-assortative point). Among of them,
one "null" graph model is built
Spin excitations in KFeSe superconductor as studied by M\"ossbauer spectroscopy
M\"ossbauer spectroscopy was used to probe the site specific information of
the superconductor. Possibility of coexistence of
superconductivity and magnetism is discussed. A spin excitation gap, 5\,meV, is observed by analyzing the temperature dependence of the
hyperfine magnetic field (HMF) at the iron site within the spin wave theory.
Using a simple model suggested in the literature, the temperature dependence of
the HMF is well reproduced, suggesting that, below room temperature, the
iron-selenide superconductors can be regarded as ferromagnetically coupled spin
blocks that interact with each other antiferromagnetically to form the observed
checkerboard-like magnetic structure
Charge redistribution at the antiferromagnetic phase transition in SrFeAsF compound
The relationship between spin, electron, and crystal structure has been one
of the foremost issues in understanding the superconducting mechanism since the
discovery of iron-based high temperature superconductors. Here, we report
M\"ossbauer and first-principles calculations studies of the parent compound
SrFeAsF with the largest temperature gap (50\,K) between the structural
and antiferromagnetic (AFM) transitions. Our results reveal that the structural
transition has little effect on the electronic structure of the compound
SrFeAsF while the development of the AFM order induces a redistribution of the
charges near the Fermi level.Comment: 6 Pages, 7 Figure
Edge Modes and Asymmetric Wave Transport in Topological Lattices: Experimental Characterization at Finite Frequencies
Although topological mechanical metamaterials have been extensively studied
from a theoretical perspective, their experimental characterization has been
lagging. To address this shortcoming, we present a systematic laser-assisted
experimental characterization of topological kagome lattices, aimed at
elucidating their in-plane phononic and topological characteristics. We
specifically explore the continuum elasticity limit, which is established when
the ideal hinges that appear in the theoretical models are replaced by
ligaments capable of supporting bending deformation, as observed for instance
in realistic physical lattices. We reveal how the zero-energy floppy edge modes
predicted for ideal configurations morph into finite-frequency phonon modes
that localize at the edges. By probing the lattices with carefully designed
excitation signals, we are able to extract and characterize all the features of
a complex low-frequency acoustic regime in which bulk modes and topological
edge modes overlap and entangle in response. The experiments provide
unequivocal evidence of the existence of strong asymmetric wave transport
regimes at finite frequencies.Comment: 8 pages, 10 figure
Study of 57Fe Mossbauer effect in RFe2Zn20 ( R = Lu, Yb, Gd)
We report measurements of 57Fe Mossbauer spectra for RFe2Zn20 ( R = Lu, Yb,
Gd) from ~ 4.5 K to room temperature. The obtained isomer shift values are very
similar for all three compounds, their temperature dependence was analyzed
within the Debye model and resulted in an estimate of the Debye temperatures of
450-500 K. The values of quadrupole splitting at room temperature change with
the cubic lattice constant a in a linear fashion. For GdFe2Zn20, ferromagnetic
order is seen as an appearance of a sextet in the spectra. The 57Fe site
hyperfine field for T = 0 was evaluated to be ~ 2.4 T
Mechanically-induced disorder in CaFe2As2: a 57Fe Mossbauer study
57Fe Mossbauer spectroscopy was used to perform a microscopic study on the
extremely pressure and strain sensitive compound, CaFe2As2, with different
degrees of strain introduced by grinding and annealing. At the base
temperature, in the antiferromagnetic/orthorhombic phase, compared to a sharp
sextet Mossbauer spectrum of single crystal CaFe2As2, which is taken as an
un-strained sample, an obviously broadened sextet and an extra doublet were
observed for ground CaFe2As2 powders with different degrees of strain. The
Mossbauer results suggest that the magnetic phase transition of CaFe2As2 can be
inhomogeneously suppressed by the grinding induced strain to such an extent
that the antiferromagnetic order in parts of the grains forming the powdered
sample remain absent all the way down to 4.6 K. However, strain has almost no
effect on the temperature dependent hyperfine magnetic field in the grains with
magnetic order. Additional electronic and asymmetry information was obtained
from the isomer shift and quadrupole splitting. Similar isomer shift values in
the magnetic phase for samples with different degrees of strain, indicate that
the stain does not bring any significant variation of the electronic density at
57Fe nucleus position. The absolute values of quadrupole shift in the magnetic
phase decrease and approach zero with increasing degrees of strain, indicating
that the strain reduces the average lattice asymmetry at Fe atom position
Transition to collapsed tetragonal phase in CaFe2As2 single crystals as seen by 57Fe Mossbauer spectroscopy
Temperature dependent measurements of 57Fe Mossbauer spectra on CaFe2As2
single crystals in the tetragonal and collapsed tetragonal phases are reported.
Clear features in the temperature dependencies of the isomer shift, relative
spectra area and quadrupole splitting are observed at the transition from the
tetragonal to the collapsed tetragonal phase. From the temperature dependent
isomer shift and spectral area data, an average stiffening of the phonon modes
in the collapsed tetragonal phase is inferred. The quadrupole splitting
increases by ~25% on cooling from room temperature to ~100 K in the tetragonal
phase and is only weakly temperature dependent at low temperatures in the
collapsed tetragonal phase, in agreement with the anisotropic thermal expansion
in this material. In order to gain microscopic insight about these measurements
we perform ab initio density functional theory calculations of the electric
field gradient and the electron density of CaFe2As2 in both phases. By
comparing the experimental data with the calculations we are able to fully
characterize the crystal structure of the samples in the collapsed-tetragonal
phase through determination of the As z-coordinate. Based on the obtained
temperature dependent structural data we are able to propose charge saturation
of the Fe - As bond region as the mechanism behind the stabilization of the
collapsed-tetragonal phase at ambient pressure
A New Variation of Hat Guessing Games
Several variations of hat guessing games have been popularly discussed in
recreational mathematics. In a typical hat guessing game, after initially
coordinating a strategy, each of players is assigned a hat from a given
color set. Simultaneously, each player tries to guess the color of his/her own
hat by looking at colors of hats worn by other players. In this paper, we
consider a new variation of this game, in which we require at least correct
guesses and no wrong guess for the players to win the game, but they can choose
to "pass".
A strategy is called {\em perfect} if it can achieve the simple upper bound
of the winning probability. We present sufficient and necessary
condition on the parameters and for the existence of perfect strategy
in the hat guessing games. In fact for any fixed parameter , the existence
of perfect strategy can be determined for every sufficiently large .
In our construction we introduce a new notion: -regular partition
of the boolean hypercube, which is worth to study in its own right. For
example, it is related to the -dominating set of the hypercube. It also
might be interesting in coding theory. The existence of -regular
partition is explored in the paper and the existence of perfect -dominating
set follows as a corollary.Comment: 9 pages; The main theorem was improve
Strong anisotropy effect in iron-based superconductor CaFeCoAsF
The anisotropy of the Fe-based superconductors is much smaller than that of
the cuprates and the theoretical calculations. A credible understanding for
this experimental fact is still lacking up to now. Here we experimentally study
the magnetic-field-angle dependence of electronic resistivity in the
superconducting phase of iron-based superconductor
CaFeCoAsF, and find the strongest anisotropy effect of the
upper critical field among the iron-based superconductors based on the
framework of Ginzburg-Landau theory. The evidences of energy band structure and
charge density distribution from electronic structure calculations demonstrate
that the observed strong anisotropic effect mainly comes from the strong ionic
bonding in between the ions of Ca and F, which weakens the
interlayer coupling between the layers of FeAs and CaF. This finding provides a
significant insight into the nature of experimentally observed strong
anisotropic effect of electronic resistivity, and also paves an avenue to
design exotic two dimensional artificial unconventional superconductors in
future.Comment: 5 pages, 3 figures, accepted to Supercond. Sci. Techno
Growth and characterization of CaFeCoAsF single crystals by CaAs flux method
Millimeter sized single crystals of CaFeCoAsF were grown using a
self-flux method. It is found that high-quality single crystals can be grown
from three approaches with different initial raw materials. The chemical
compositions and crystal structure were characterized carefully. The c-axis
lattice constant is suppressed by the Co substitution. Superconductivity with
the critical transition as high as 21 K was confirmed by both the
resistivity and magnetic susceptibility measurements in the sample with =
0.12. Moreover, it is found that can be enhanced for about 1 K under the
very small hydrostatic pressure of 0.22 GPa, which is more quickly than that
reported in the polycrystalline samples. Our results is a promotion for the
physical investigations of 1111 phase iron-pnictide superconductors.Comment: 11 pages, 4 figures, submitted to Journal of Crystal Growt
- β¦