8,615 research outputs found
The Fourth Gravity Test and Quintessence Matter Field
After the previous work on gravitational frequency shift, light deflection
(arXiv:1003.5296) and perihelion advance (arXiv:0812.2332), we calculate
carefully the fourth gravity test, i.e. radar echo delay in a central gravity
field surrounded by static free quintessence matter, in this paper. Through the
Lagrangian method, we find the influence of the quintessence matter on the time
delay of null particle is presence by means of an additional integral term.
When the quintessence field vanishes, it reduces to the usual Schwarzschild
case naturally. Meanwhile, we also use the data of the Viking lander from the
Mars and Cassini spacecraft to Saturn to constrain the quintessence field. For
the Viking case, the field parameter is under the order of .
However, is under for the Cassini case.Comment: 9 pages, 1 figure, Eur. Phys. J. C in pres
Bounds on the spectral radius of nonnegative matrices and applications in graph spectra
In this paper, we give upper and lower bounds for the spectral radius of a
nonnegative irreducible matrix and characterize the equality cases. These
bounds theoretically improve and generalize some known results of Duan et
al.[X. Duan, B. Zhou, Sharp bounds on the spectral radius of a nonnegative
matrix, Linear Algebra Appl. (2013),
http://dx.doi.org/10.1016/j.laa.2013.08.026]. Finally, applying these bounds to
various matrices associated with a graph, we obtain some new upper and lower
bounds on various spectral radiuses of graphs, which generalize and improve
some known results.Comment: 10 pages, 1 figures, 15 conferenc
The normalized Laplacian spectra of the double corona based on -graph
For simple graphs , and , we denote their double corona based
on -graph by . This paper determines the
normalized Laplacian spectrum of in terms of
these of , and whenever , and are regular. The
obtained result reduces to the normalized Laplacian spectra of the -vertex
corona and -edge corona by
choosing or as a null-graph, respectively. Finally, applying the
results of the paper, we construct infinitely many pairs of normalized
Laplacian cospectral graphs.Comment: 9 pages, 19 conferenc
Some improved bounds on two energy-like invariants of some derived graphs
Given a simple graph , its Laplacian-energy-like invariant and
incidence energy are the sum of square root of its all Laplacian
eigenvalues and signless Laplacian eigenvalues, respectively. Applying the
Cauchy-Schwarz inequality and the Ozeki inequality, along with its refined
version, we obtain some improved bounds on and of the -graph and -graph for a regular graph. Theoretical analysis
indicates that these results improve some known results. In addition, some new
lower bounds on and of the line graph of a semiregular graph are
also given.Comment: 18 pages, 32 conferenc
The Q-generating function for graphs with application
For a simple connected graph , the -generating function of the numbers
of semi-edge walks of length in is defined by
. This paper reveals that the
-generating function may be expressed in terms of the
-polynomials of the graph and its complement . Using this
result, we study some -spectral properties of graphs and compute the
-polynomials for some graphs obtained by the use of some operation on
graphs, such as the complement graph of a regular graph, the join of two
graphs, the (edge)corona of two graphs and so forth. As another application of
the -generating function , we also give a combinatorial
interpretation of the -coronal of , which is defined to be the sum of the
entries of the matrix . This result may be used to
obtain the many alternative calculations of the -polynomials of the
(edge)corona of two graphs. Further, we also compute the -coronals of the
join of two graphs and the complete multipartite graphs
Photon-mediated electronic correlation effects in irradiated two-dimensional Dirac systems
Periodically driven systems can host many interesting and intriguing
phenomena. The irradiated two-dimensional Dirac systems, driven by circularly
polarized light, are the most attractive thanks to intuitive physical view of
the absorption and emission of photon near Dirac cones. Here, we assume that
the light is incident in the two-dimensional plane, and choose to treat the
light-driven Dirac systems by making a unitary transformation to capture the
photon-mediated electronic correlation effects, instead of using usual Floquet
theory. In this approach, the electron-photon interaction terms can be
cancelled out and the resultant effective electron-electron interactions can
produce important effects. These effective interactions will produce a
topological band structure in the case of 2D Fermion system with one Dirac
cone, and can lift the energy degeneracy of the Dirac cones for graphene. This
method can be applicable to similar light-driven Dirac systems to investigate
photon-mediated electronic effects in them.Comment: 5 pages, 4 figure
On the normalized Laplacian spectra of some subdivision joins of two graphs
For two simple graphs and , we denote the subdivision-vertex join
and subdivision-edge join of and by and
, respectively. This paper determines the normalized Laplacian
spectra of and in terms of these of
and whenever and are regular. As applications, we construct
some non-regular normalized Laplacian cospectral graphs. Besides we also
compute the number of spanning trees and the degree-Kirchhoff index of
and for regular graphs and .Comment: 15 pages,19 conferece
Hidden chiral symmetry protected topological insulators in a ladder dimer model
We construct a two-leg ladder dimer model by using two orbitals instead of
one in Su-Schrieffer-Heeger (SSH) dimer chain and find out a chiral symmetry in
it. In this model, the otherwise-hidden additional chiral symmetry allows us to
define two chiral massless fermions and show that there exist interesting
topological states characterized by and
corresponding zero mode edge states. Our complete phase diagram reveals that
there is an anomalous topologically nontrivial region in addition to the normal
one similar to that of SSH model. The anomalous region features that the
inter-cell hopping constants can be much smaller than the intra-cell ones,
being opposite to the normal region, and the zero mode edge states do not have
well-defined parity each. Finally, we suggest that this ladder dimer model can
be realized in double-well optical lattices, ladder polymer systems, and adatom
double chains on semiconductor surfaces.Comment: 6 pages, 5 figure
Floquet Weyl fermions in circularly-polarised-light-irradiated three-dimensional stacked graphene systems
Using Floquet theory, we illustrate that Floquet Weyl fermions can be created
in circularly-polarised-light-irradiated three-dimensional stacked graphene
systems. One or two semi-Dirac points can be formed due to overlapping of
Floquet sub-bands. Each pair of Weyl points have a two-component semi-Dirac
point parent, instead of a four-component Dirac point parent. Decreasing the
light frequency will make the Weyl points move in the momentum space, and the
Weyl points can approach to the Dirac points when the frequency becomes very
small. The frequency-amplitude phase diagram is worked out. It is shown that
there exist Fermi arcs in the surface Brillouin zones in
circularly-polarised-light-irradiated semi-infinitely-stacked and
finitely-multilayered graphene systems. The Floquet Weyl points emerging due to
the overlap of Floquet sub-bands provide a new platform to study Weyl fermions.Comment: 7 pages, 4 figures (minor improvement
Standardization, Distance, Host Galaxy Extinction of Type Ia Supernova and Hubble Diagram from the Flux Ratio Method
In this paper we generalize the flux ratio method Bailey et al. (2009) to the
case of two luminosity indicators and search the optimal luminosity-flux ratio
relations on a set of spectra whose phases are around not only the date of
bright light but also other time. With these relations, a new method is
proposed to constrain the host galaxy extinction of SN Ia and its distance. It
is first applied to the low redshift supernovas and then to the high redshift
ones. The results of the low redshift supernovas indicate that the flux ratio
method can indeed give well constraint on the host galaxy extinction parameter
E(B-V), but weaker constraints on R_{V}. The high redshift supernova spectra
are processed by the same method as the low redshift ones besides some
differences due to their high redshift. Among 16 high redshift supernovas, 15
are fitted very well except 03D1gt. Based on these distances, Hubble diagram is
drew and the contents of the Universe are analyzed. It supports an acceleration
behavior in the late Universe. Therefore, the flux ratio method can give
constraints on the host galaxy extinction and supernova distance independently.
We believe, through further studies, it may provide a precise tool to probe the
acceleration of the Universe than before.Comment: 33 pages, 9 figures and 6 table
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