4,921 research outputs found
On a conjecture about tricyclic graphs with maximal energy
For a given simple graph , the energy of , denoted by , is defined as the sum of the absolute values of all eigenvalues of its
adjacency matrix, which was defined by I. Gutman. The problem on determining
the maximal energy tends to be complicated for a given class of graphs. There
are many approaches on the maximal energy of trees, unicyclic graphs and
bicyclic graphs, respectively. Let denote the graph with vertices obtained from three copies of and a path by
adding a single edge between each of two copies of to one endpoint of the
path and a single edge from the third to the other endpoint of the
. Very recently, Aouchiche et al. [M. Aouchiche, G. Caporossi, P.
Hansen, Open problems on graph eigenvalues studied with AutoGraphiX, {\it
Europ. J. Comput. Optim.} {\bf 1}(2013), 181--199] put forward the following
conjecture: Let be a tricyclic graphs on vertices with or
, then with equality
if and only if . Let denote the set of all
connected bipartite tricyclic graphs on vertices with three vertex-disjoint
cycles , and , where . In this paper, we try to
prove that the conjecture is true for graphs in the class ,
but as a consequence we can only show that this is true for most of the graphs
in the class except for 9 families of such graphs.Comment: 32 pages, 12 figure
Anomalous thermoelectric transport of Dirac particles in graphene
We report a thermoelectric study of graphene in both zero and applied
magnetic fields. As a direct consequence of the linear dispersion of massless
particles, we find that the Seebeck coefficient Sxx diverges with 1 /, where
n2D is the carrier density. We observe a very large Nernst signal Sxy (~ 50
uV/K at 8 T) at the Dirac point, and an oscillatory dependence of both Sxx and
Sxy on n2D at low temperatures. Our results underscore the anomalous
thermoelectric transport in graphene, which may be used as a highly sensitive
probe for impurity bands near the Dirac point
Spin Seebeck effect from antiferromagnetic magnons and critical spin fluctuations in epitaxial FeF2 films
We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown
FeF2(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy
over the temperature range of 3.8 - 250 K. Both the magnetic field- and
temperature-dependent SSE signals below the N\'eel temperature (TN=70 K) of the
FeF2 films are consistent with a theoretical model based on the excitations of
AFM magnons without any net induced static magnetic moment. In addition to the
characteristic low-temperature SSE peak associated with the AFM magnons, there
is another SSE peak at TN which extends well into the paramagnetic phase. All
the SSE data taken at different magnetic fields up to 12 T near and above the
critical point TN follow the critical scaling law very well with the critical
exponents for magnetic susceptibility of 3D Ising systems, which suggests that
the AFM spin correlation is responsible for the observed SSE near TN
Field-effect mobility enhanced by tuning the Fermi level into the band gap of Bi2Se3
By eliminating normal fabrication processes, we preserve the bulk insulating
state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as
indicated by the activated temperature dependence of the resistivity at low
temperatures. We perform low-energy electron beam irradiation (<16 keV) and
electrostatic gating to control the carrier density and therefore the Fermi
level position in the nanodevices. In slightly p-doped Bi2-xCaxSe3 devices,
continuous tuning of the Fermi level from the bulk valence band to the band-gap
reveals dramatic enhancement (> a factor of 10) in the field-effect mobility,
which suggests suppressed backscattering expected for the Dirac fermion surface
states in the gap of topological insulators
A thermodynamically consistent quasi-particle model without density-dependent infinity of the vacuum zero point energy
In this paper, we generalize the improved quasi-particle model proposed in J.
Cao et al., [ Phys. Lett. B {\bf711}, 65 (2012)] from finite temperature and
zero chemical potential to the case of finite chemical potential and zero
temperature, and calculate the equation of state (EOS) for (2+1) flavor Quantum
Chromodynamics (QCD) at zero temperature and high density. We first calculate
the partition function at finite temperature and chemical potential, then go to
the limit and obtain the equation of state (EOS) for cold and dense QCD,
which is important for the study of neutron stars. Furthermore, we use this EOS
to calculate the quark-number density, the energy density, the quark-number
susceptibility and the speed of sound at zero temperature and finite chemical
potential and compare our results with the corresponding ones in the existing
literature
Systems Biology of Gastric Cancer: Perspectives on the Omics-Based Diagnosis and Treatment
Gastric cancer is the fifth most diagnosed cancer in the world, affecting more than a million people and causing nearly 783,000 deaths each year. The prognosis of advanced gastric cancer remains extremely poor despite the use of surgery and adjuvant therapy. Therefore, understanding the mechanism of gastric cancer development, and the discovery of novel diagnostic biomarkers and therapeutics are major goals in gastric cancer research. Here, we review recent progress in application of omics technologies in gastric cancer research, with special focus on the utilization of systems biology approaches to integrate multi-omics data. In addition, the association between gastrointestinal microbiota and gastric cancer are discussed, which may offer insights in exploring the novel microbiota-targeted therapeutics. Finally, the application of data-driven systems biology and machine learning approaches could provide a predictive understanding of gastric cancer, and pave the way to the development of novel biomarkers and rational design of cancer therapeutics
Independent tuning of electronic properties and induced ferromagnetism in topological insulators with heterostructure approach
The quantum anomalous Hall effect (QAHE) has been recently demonstrated in
Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures
below 100 mK. In those materials, the spins of unfilled d-electrons in the
transition metal dopants are exchange coupled to develop a long-range
ferromagnetic order, which is essential for realizing QAHE. However, the
addition of random dopants does not only introduce excess charge carriers that
require readjusting the Bi/Sb ratio, but also unavoidably introduces
paramagnetic spins that can adversely affect the chiral edge transport in QAHE.
In this work, we show a heterostructure approach to independently tune the
electronic and magnetic properties of the topological surface states in
(BixSb1-x)2Te3 without resorting to random doping of transition metal elements.
In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium
iron garnet (YIG), a high Curie temperature (~ 550 K) magnetic insulator, we
find that the TI surface in contact with YIG becomes ferromagnetic via
proximity coupling which is revealed by the anomalous Hall effect (AHE). The
Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is
uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is
varied, the AHE resistivity scales with the longitudinal resistivity. In this
approach, we decouple the electronic properties from the induced ferromagnetism
in TI. The independent optimization provides a pathway for realizing QAHE at
higher temperatures, which is important for novel spintronic device
applications.Comment: Accepted by Nano Letter
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