1,181 research outputs found
Raman frequency shift in oxygen functionalized carbon nanotubes
In terms of lattice dynamics theory, we study the vibrational properties of
the oxygen-functionalized single wall carbon nanotubes (O-SWCNs). Due to the
C-O and O-O interactions, many degenerate phonon modes are split and even some
new phonon modes are obtained, different from the bare SWCNs. A distinct Raman
shift is found in both the radial breathing mode and G modes, depending not
only on the tube diameter and chirality but also on oxygen coverage and
adsorption configurations. With the oxygen coverage increasing, interesting, a
nonmonotonic up- and down-shift is observed in G modes, which is contributed to
the competition between the bond expansion and contraction, there coexisting in
the functionalized carbon nanotube.Comment: 4 pages, 3 figures, 1 tabl
Electronic structure of superconducting graphite intercalate compounds: The role of the interlayer state
Although not an intrinsic superconductor, it has been long--known that, when
intercalated with certain dopants, graphite is capable of exhibiting
superconductivity. Of the family of graphite--based materials which are known
to superconduct, perhaps the most well--studied are the alkali metal--graphite
intercalation compounds (GIC) and, of these, the most easily fabricated is the
CK system which exhibits a transition temperature K. By increasing the alkali metal concentration (through high pressure
fabrication techniques), the transition temperature has been shown to increase
to as much as K in CNa. Lately, in an important recent
development, Weller \emph{et al.} have shown that, at ambient conditions, the
intercalated compounds \cyb and \cca exhibit superconductivity with transition
temperatures K and K respectively, in excess
of that presently reported for other graphite--based compounds. We explore the
architecture of the states near the Fermi level and identify characteristics of
the electronic band structure generic to GICs. As expected, we find that charge
transfer from the intercalant atoms to the graphene sheets results in the
occupation of the --bands. Yet, remarkably, in all those -- and only
those -- compounds that superconduct, we find that an interlayer state, which
is well separated from the carbon sheets, also becomes occupied. We show that
the energy of the interlayer band is controlled by a combination of its
occupancy and the separation between the carbon layers.Comment: 4 Figures. Please see accompanying experimental manuscript
"Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca" by
Weller et a
Optimal transport on wireless networks
We present a study of the application of a variant of a recently introduced
heuristic algorithm for the optimization of transport routes on complex
networks to the problem of finding the optimal routes of communication between
nodes on wireless networks. Our algorithm iteratively balances network traffic
by minimizing the maximum node betweenness on the network. The variant we
consider specifically accounts for the broadcast restrictions imposed by
wireless communication by using a different betweenness measure. We compare the
performance of our algorithm to two other known algorithms and find that our
algorithm achieves the highest transport capacity both for minimum node degree
geometric networks, which are directed geometric networks that model wireless
communication networks, and for configuration model networks that are
uncorrelated scale-free networks.Comment: 5 pages, 4 figure
Electronic Structure and Optical Properties of the Co-doped Anatase TiO Studied from First Principles
The Co-doped anatase TiO, a recently discovered room-temperature
ferromagnetic insulator, has been studied by the first-principles calculations
in the pseudo-potential plane-wave formalism within the local-spin-density
approximation (LSDA), supplemented by the full-potential linear augmented plane
wave (FP-LAPW) method. Emphasis is placed on the dependence of its electronic
structures and linear optical properties on the Co-doping concentration and
oxygen vacancy in the system in order to pursue the origin of its
ferromagnetism. In the case of substitutional doping of Co for Ti, our
calculated results are well consistent with the experimental data, showing that
Co is in its low spin state. Also, it is shown that the oxygen vacancy enhances
the ferromagnetism and has larger effect on both the electronic structure and
optical properties than the Co-doping concentration only.Comment: 12 pages, 4 figure
Magnetism in Cr-doped ZnS: Density-functional theory studies
We investigated the magnetism and aggregation trends in cubic Zn1-xCrxS using
the density-functional theory calculations.We demonstrate that all studied
configurations show ground state half-metallic ferromagnetism (HMF); and Cr
impurities are energetically favorable to planar cluster into delta-doping
structures. The single-layer delta-doping structures of Zn0.75Cr0.25S and
Zn0.875Cr0.125S show ferromagnetic stabilization energies (\Delta E_AF) of
0.551 and 0.561 eV/Cr-Cr pair, respectively. The half-layer delta-doping
structure of Zn0.875Cr0.125S and double-layer delta-doping structure of
Zn0.75Cr0.25S show \Delta E_AF of 0.394 and 0.166 eV/Cr-Cr pair, respectively.
Furthermore, our studies indicate that the cubic ZnS/CrS heterostructure, one
extreme situation of the delta-doping structure, also shows ground state HMF.
The origin of HMF is discussed using a simple crystal field model. Finally, we
anticipate the potential spintronics application of Zn1-xCrxS.Comment: 13 pages, 4 figure
Chemotaxis When Bacteria Remember: Drift versus Diffusion
{\sl Escherichia coli} ({\sl E. coli}) bacteria govern their trajectories by
switching between running and tumbling modes as a function of the nutrient
concentration they experienced in the past. At short time one observes a drift
of the bacterial population, while at long time one observes accumulation in
high-nutrient regions. Recent work has viewed chemotaxis as a compromise
between drift toward favorable regions and accumulation in favorable regions. A
number of earlier studies assume that a bacterium resets its memory at tumbles
-- a fact not borne out by experiment -- and make use of approximate
coarse-grained descriptions. Here, we revisit the problem of chemotaxis without
resorting to any memory resets. We find that when bacteria respond to the
environment in a non-adaptive manner, chemotaxis is generally dominated by
diffusion, whereas when bacteria respond in an adaptive manner, chemotaxis is
dominated by a bias in the motion. In the adaptive case, favorable drift occurs
together with favorable accumulation. We derive our results from detailed
simulations and a variety of analytical arguments. In particular, we introduce
a new coarse-grained description of chemotaxis as biased diffusion, and we
discuss the way it departs from older coarse-grained descriptions.Comment: Revised version, journal reference adde
Effects of thermal fluctuation and the receptor-receptor interaction in bacterial chemotactic signalling and adaptation
Bacterial chemotaxis is controlled by the conformational changes of the
receptors, in response to the change of the ambient chemical concentration. In
a statistical mechanical approach, the signalling due to the conformational
changes is a thermodynamic average quantity, dependent on the temperature and
the total energy of the system, including both ligand-receptor interaction and
receptor-receptor interaction. This physical theory suggests to biology a new
understanding of cooperation in ligand binding and receptor signalling
problems. How much experimental support of this approach can be obtained from
the currently available data? What are the parameter values? What is the
practical information for experiments? Here we make comparisons between the
theory and recent experimental results. Although currently comparisons can only
be semi-quantitative or qualitative, consistency is clearly shown. The theory
also helps to sort a variety of data.Comment: 26 pages, revtex. Journal version. Analysis on another set of data on
adaptation time is adde
Multi-component Transparent Conducting Oxides: Progress in Materials Modelling
Transparent conducting oxides (TCOs) play an essential role in modern
optoelectronic devices through their combination of electrical conductivity and
optical transparency. We review recent progress in our understanding of
multi-component TCOs formed from solid-solutions of ZnO, In2O3, Ga2O3 and
Al2O3, with a particular emphasis on the contributions of materials modelling,
primarily based on Density Functional Theory. In particular, we highlight three
major results from our work: (i) the fundamental principles governing the
crystal structures of multi-component oxide structures including (In2O3)(ZnO)n,
named IZO, and (In2O3)m(Ga2O3)l(ZnO)n, named IGZO; (ii) the relationship
between elemental composition and optical and electrical behaviour, including
valence band alignments; (iii) the high-performance of amorphous oxide
semiconductors. From these advances, the challenge of the rational design of
novel electroceramic materials is discussed.Comment: Part of a themed issue of Journal of Physics: Condensed Matter on
"Semiconducting Oxides". In Press (2011
Triggering of the 2014 M_w7.3 Papanoa earthquake by a slow slip event in Guerrero, Mexico
Since their discovery two decades ago, slow slip events have been shown to play an important role in accommodating strain in subduction zones. However, the physical mechanisms that generate slow slip and the relationships with earthquakes are unclear. Slow slip events have been recorded in the Guerrero segment of the CocosâNorth America subduction zone. Here we use inversion of position time series recorded by a continuous GPS network to reconstruct the evolution of aseismic slip on the subduction interface of the Guerrero segment. We find that a slow slip event began in February 2014, two months before the magnitude (M_w) 7.3 Papanoa earthquake on 18 April. The slow slip event initiated in a region adjacent to the earthquake hypocentre and extended into the vicinity of the seismogenic zone. This spatio-temporal proximity strongly suggests that the Papanoa earthquake was triggered by the ongoing slow slip event. We demonstrate that the triggering mechanism could be either static stress increases in the hypocentral region, as revealed by Coulomb stress modelling, or enhanced weakening of the earthquake hypocentral area by the slow slip. We also show that the plate interface in the Guerrero area is highly coupled between slow slip events, and that most of the accumulated strain is released aseismically during the slow slip episodes
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