22,244 research outputs found
Two monotonic functions involving gamma function and volume of unit ball
In present paper, we prove the monotonicity of two functions involving the
gamma function and relating to the -dimensional volume of the
unit ball in .Comment: 7 page
Epitaxial graphene on SiC(0001): More than just honeycombs
The potential of graphene to impact the development of the next generation of
electronics has renewed interest in its growth and structure. The
graphitization of hexagonal SiC surfaces provides a viable alternative for the
synthesis of graphene, with wafer-size epitaxial graphene on SiC(0001) now
possible. Despite this recent progress, the exact nature of the graphene-SiC
interface and whether the graphene even has a semiconducting gap remain
controversial. Using scanning tunneling microscopy with functionalized tips and
density functional theory calculations, here we show that the interface is a
warped carbon sheet consisting of three-fold hexagon-pentagon-heptagon
complexes periodically inserted into the honeycomb lattice. These defects
relieve the strain between the graphene layer and the SiC substrate, while
still retaining the three-fold coordination for each carbon atom. Moreover,
these defects break the six-fold symmetry of the honeycomb, thereby naturally
inducing a gap: the calculated band structure of the interface is
semiconducting and there are two localized states near K below the Fermi level,
explaining the photoemission and carbon core-level data. Nonlinear dispersion
and a 33 meV gap are found at the Dirac point for the next layer of graphene,
providing insights into the debate over the origin of the gap in epitaxial
graphene on SiC(0001). These results indicate that the interface of the
epitaxial graphene on SiC(0001) is more than a dead buffer layer, but actively
impacts the physical and electronic properties of the subsequent graphene
layers
Transport properties and anisotropy in rare earth doped CaFe2As2 single crystals with Tc above 40 K
In this paper we report the superconductivity above 40 K in the electron
doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction
patterns indicate high crystalline quality and c-axis orientation. the
resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by
partial replacement of Ca by rare earth and a superconducting transition
reaches as high as 43 K, which is higher than the value in electron doping
FeAs-122 compounds by substituting Fe ions with transition metal, even
surpasses the highest values observed in hole doping systems with a transition
temperature up to 38 K. The upper critical field has been determined with the
magnetic field along ab-plane and c-axis, yielding the anisotropy of 2~3.
Hall-effect measurements indicate that the conduction in this material is
dominated by electron like charge carriers. Our results explicitly demonstrate
the feasibility of inducing superconductivity in Ca122 compounds via electron
doping using aliovalent rare earth substitution into the alkaline earth site,
which should add more ingredients to the underlying physics of the iron-based
superconductors.Comment: 21 pages, 7 figure
Valley dependent many-body effects in 2D semiconductors
We calculate the valley degeneracy () dependence of the many-body
renormalization of quasiparticle properties in multivalley 2D semiconductor
structures due to the Coulomb interaction between the carriers. Quite
unexpectedly, the dependence of many-body effects is nontrivial and
non-generic, and depends qualitatively on the specific Fermi liquid property
under consideration. While the interacting 2D compressibility manifests
monotonically increasing many-body renormalization with increasing , the
2D spin susceptibility exhibits an interesting non-monotonic dependence
with the susceptibility increasing (decreasing) with for smaller (larger)
values of with the renormalization effect peaking around .
Our theoretical results provide a clear conceptual understanding of recent
valley-dependent 2D susceptibility measurements in AlAs quantum wells.Comment: 5 pages, 3 figure
An external potential dynamic study on the formation of interface in polydisperse polymer blends
The formation of interface from an initial sharp interface in polydisperse
A/B blends is studied using the external potential dynamic method. The present
model is a nonlocal coupling model as we take into account the correlation
between segments in a single chain. The correlation is approximately expressed
by Debye function and the diffusion dynamics are based on the Rouse chain
model. The chain length distribution is described by the continuous Schulz
distribution. Our numerical calculation indicates that the broadening of
interface with respect to time obeys a power law at early times, and the power
law indexes are the same for both monodisperse and polydisperse blend. The
power law index is larger than that in the local coupling model. However there
is not a unified scaling form of the broadening of the interface width if only
the interfacial width at equilibrium is taken into account as the
characteristic length of the system, because the correlation makes an extra
characteristic length in the system, and the polydispersity is related to this
length.Comment: 15 pages, 5 figure
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