20,587 research outputs found
Phonons in potassium doped graphene: the effects of electron-phonon interactions, dimensionality and ad-atom ordering
Graphene phonons are measured as a function of electron doping via the
addition of potassium adatoms. In the low doping regime, the in-plane carbon
G-peak hardens and narrows with increasing doping, analogous to the trend seen
in graphene doped via the field-effect. At high dopings, beyond those
accessible by the field-effect, the G-peak strongly softens and broadens. This
is interpreted as a dynamic, non-adiabatic renormalization of the phonon
self-energy. At dopings between the light and heavily doped regimes, we find a
robust inhomogeneous phase where the potassium coverage is segregated into
regions of high and low density. The phonon energies, linewidths and tunability
are remarkably similar for 1-4 layer graphene, but significantly different to
doped bulk graphite.Comment: Accepted in Phys. Rev. B as a Rapid Communication. 5 pages, 3
figures, revised text with additional dat
Motor regulation results in distal forces that bend partially disintegrated Chlamydomonas axonemes into circular arcs
The bending of cilia and flagella is driven by forces generated by dynein
motor proteins. These forces slide adjacent microtubule doublets within the
axoneme, the motile cytoskeletal structure. To create regular, oscilla- tory
beating patterns, the activities of the axonemal dyneins must be coordinated
both spatially and temporally. It is thought that coordination is mediated by
stresses or strains, which build up within the moving axoneme, and somehow
regulate dynein activity. While experimenting with axonemes subjected to mild
proteolysis, we observed pairs of doublets associate with each other and form
bends with almost constant curvature. By model- ing the statics of a pair of
filaments, we show that the activity of the motors concentrates at the distal
tips of the doublets. Furthermore, we show that this distribution of motor
activity accords with models in which curvature, or curvature-induced normal
forces, regulates the activity of the motors. These observations, together with
our theoretical analysis, provide evidence that dynein activity can be
regulated by curvature or normal forces, which may, therefore, play a role in
coordinating the beating of cilia and flagella
Evaluation of the effect of tyrothricin on beta-hemolytic streptococci in salva. Part I: The effect of salvia upon bacteria. Part II: Effect of tyrothricin on the New York 5 strain of Streptococcus pyogenes in saliva
Part II of thesis by Brancato, Noyes, and Swift. Part I of thesis by Swift. Thesis (M.A.)--Boston UniversityThe antibacterial effect of saliva has been known for many years. Still the exact nature of the antagonistic action of saliva upon bacteria is as yet unsettled. Most workers agree, however, that the salivary bacterial inhibitory action is brought about in at least six ways:
The first antibacterial effect is changes in pH, which affect the growth of oral organisms. Furthermore, this change in pH is dependent on diet and on the type of organisms in the oral cavity.
The second is the mechanical factors involved, for saliva not only flushes bacteria from the mouth, but dilutes the number of organisms as well.
The third is the antibacterial action of the cellular components in saliva. The leukocytes in saliva have a phagocytic action, and the non-phagocytic epithelial cells slough off in sheets, carrying with them thousands of organisms which have lodged in the partially turned edges of the necrotic cells .
The fourth antibacterial action is ascribed to the presence of immune bodies in the saliva which lyse or agglutinate the oral bacteria.
The fifth is the presence of oral bacteria which are antagonistic to new invaders.
And the sixth is the presence of enzymes that lyse some oral bacteria or alter their cell membranes thereby inhibiting further growth.
In recent years a great deal of investigation has been made to ascribe the enzymatic effect as the chief antibacterial agent in saliva; however, contradictory work has been done to try to attribute the chief antibacterial action of salivary cocci.
Indeed the antibacterial effect of saliva is not always present, for the bacteriostatic effect of saliva is variable from day to day and from individual to individual.
The only way of reducing the number of oral bacteria is to add to the saliva an antibiotic. Tyrothricin was used.
In an attempt to delineate the range of concentration of tyrothricin per ml. effective against the New York 5 strain of Streptococcus pryogenes in saliva, this experiment was carried out. It was molded after the unpublished work of Belding concerning the effect of tyrothricin on the Oxford Strain of Staphylococcus aureus in saliva.
The required inoculum of approximately one million organisms per ml was obtained by growing cultures of the streptococci under uniform conditions and setting up a table of the absorbances and viable cell counts, from which dilution factors for further cultures could be estimated. Controls were set up for determining possible inhibition of tyrothricin and/or test organisms by the various diluting fluids including saliva.
Final concentrations per ml of 10, 25, 50, 75, and 100 µg of tyrothricin integrated with saliva and an approximated number of streptococci were plated out after 30 and 60 minutes exposure periods and were counted after 24 and 48 hours of incubation at 37°C.
Whereas 1 µg per ml of tyrothricin reduced markedly the number of streptococci suspended in water during a 30 minute exposure period and 10 µg per ml, under similar conditions, caused complete inhibition, 10 µg per ml of the antibiotic was ineffective against this test organism suspended in saliva during a 30 minute exposure period but caused about an 80 per cent reduction in viable organisms during 60 minutes exposure. The length of the exposure period necessary for effective inhibition varied inversely with the concentration of tyrothricin per ml, 100 µg per ml causing a 98 per cent reduction of viable organisms during an exposure period of 1 minute. For the 30 minute exposure period, the quantity of tyrothricin effective against this strain of streptococci mixed in saliva would fall in the 10 µg - 25 µg per ml range and for shorter exposure periods, the concentration per ml would have to be greater.
Cultures completely negative during 24 hours incubation at 37°C, showed a typical growth during 48 hours. This is considered indicative of the bacteriostatic action of tyrothricin which, prolonged, resulted in the death of large numbers of the streptococci.
The results which were obtained in these experiments serve chiefly to point out the way for further work and to form a basis for the general conclusions listed below:
1. The action of tyrothricin on bacteria is inhibited by saliva to a large degree.
2. The minimal amounts of tyrothricin necessary to produce complete inhibition of growth of Streptococcus pyogenes in saliva is between 25 and 50 µg per ml acting for 30 minutes.
3. There is an effective reduction of Streptococcus pyogenes in saliva by concentrations of tyrothricin between 10 and 25 µg per ml acting for 30 minutes.
4. Tyrothricin acts immediately upon contact with Streptococcus pyogenes.
5. The action of tyrothricin on Streptococcus pyogenes in saliva is apparently bacteriostatic and not of a permanent nature as manifested by growth of atypical colonies during 48 hours incubation.
6. Tyrothricin above a concentration of 50 µg per ml had a definite reducing effect on the bacterial population of this saliva.
7. Saliva also has a bactericidal or bacteriostatic (or both) action against Streptococcus pyogenes
Probing wrong-sign Yukawa couplings at the LHC and a future linear collider
We consider the two-Higgs-doublet model as a framework in which to evaluate
the viability of scenarios in which the sign of the coupling of the observed
Higgs boson to down-type fermions (in particular, -quark pairs) is opposite
to that of the Standard Model (SM), while at the same time all other tree-level
couplings are close to the SM values. We show that, whereas such a scenario is
consistent with current LHC observations, both future running at the LHC and a
future linear collider could determine the sign of the Higgs coupling
to -quark pairs. Discrimination is possible for two reasons. First, the
interference between the -quark and the -quark loop contributions to the
coupling changes sign. Second, the charged-Higgs loop contribution to the
coupling is large and fairly constant up to the largest
charged-Higgs mass allowed by tree-level unitarity bounds when the -quark
Yukawa coupling has the opposite sign from that of the SM (the change in sign
of the interference terms between the -quark loop and the and loops
having negligible impact).Comment: 28 pages, 21 figure
Better bound on the exponent of the radius of the multipartite separable ball
We show that for an m-qubit quantum system, there is a ball of radius
asymptotically approaching kappa 2^{-gamma m} in Frobenius norm, centered at
the identity matrix, of separable (unentangled) positive semidefinite matrices,
for an exponent gamma = (1/2)((ln 3/ln 2) - 1), roughly .29248125. This is much
smaller in magnitude than the best previously known exponent, from our earlier
work, of 1/2. For normalized m-qubit states, we get a separable ball of radius
sqrt(3^(m+1)/(3^m+3)) * 2^{-(1 + \gamma)m}, i.e. sqrt{3^{m+1}/(3^m+3)}\times
6^{-m/2} (note that \kappa = \sqrt{3}), compared to the previous 2 * 2^{-3m/2}.
This implies that with parameters realistic for current experiments, NMR with
standard pseudopure-state preparation techniques can access only unentangled
states if 36 qubits or fewer are used (compared to 23 qubits via our earlier
results). We also obtain an improved exponent for m-partite systems of fixed
local dimension d_0, although approaching our earlier exponent as d_0
approaches infinity.Comment: 30 pp doublespaced, latex/revtex, v2 added discussion of Szarek's
upper bound, and reference to work of Vidal, v3 fixed some errors (no effect
on results), v4 involves major changes leading to an improved constant, same
exponent, and adds references to and discussion of Szarek's work showing that
exponent is essentially optimal for qubit case, and Hildebrand's alternative
derivation for qubit case. To appear in PR
Actively stressed marginal networks
We study the effects of motor-generated stresses in disordered three
dimensional fiber networks using a combination of a mean-field, effective
medium theory, scaling analysis and a computational model. We find that motor
activity controls the elasticity in an anomalous fashion close to the point of
marginal stability by coupling to critical network fluctuations. We also show
that motor stresses can stabilize initially floppy networks, extending the
range of critical behavior to a broad regime of network connectivities below
the marginal point. Away from this regime, or at high stress, motors give rise
to a linear increase in stiffness with stress. Finally, we demonstrate that our
results are captured by a simple, constitutive scaling relation highlighting
the important role of non-affine strain fluctuations as a susceptibility to
motor stress.Comment: 8 pages, 4 figure
The CP-conserving 2HDM after the 8 TeV run
We confront the most common CP-conserving 2HDM with the LHC data analysed so
far while taking into account all previously available experimental data. A
special allowed corner of the parameter space is analysed - the so-called
wrong-sign scenario where the Higgs coupling to down-type quarks changes sign
relative to the Standard Model while the coupling to the massive vector bosons
does not.Comment: 6 pages, 2 figures, to appear in the proceedings of the 22nd
International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS
2014), 28 April - 2 May 2014 Warsaw (Poland
The Wrong Sign limit in the 2HDM
A sign change in the Higgs couplings to fermions and massive gauge bosons is
still allowed in the framework of two-Higgs doublet models (2HDM). In this work
we discuss the possible sign changes in the Higgs couplings to fermions and
gauge bosons, while reviewing the status of the 8-parameter CP-conserving 2HDM
after the Large Hadron Collider 8 TeV run.Comment: 6 pages, 3 figures. Proceedings of the Second Annual Conference on
Large Hadron Collider Physics, Columbia University, New York, U.S.A, June
2-7, 2014. arXiv admin note: text overlap with arXiv:1407.439
Mass-degenerate Higgs bosons at 125 GeV in the Two-Higgs-Doublet Model
The analysis of the Higgs boson data by the ATLAS and CMS Collaborations
appears to exhibit an excess of h --> gamma\gamma events above the Standard
Model (SM) expectations; whereas no significant excess is observed in h --> ZZ*
--> {four lepton} events, albeit with large statistical uncertainty due to the
small data sample. These results (assuming they persist with further data)
could be explained by a pair of nearly mass-degenerate scalars, one of which is
a SM-like Higgs boson and the other is a scalar with suppressed couplings to
W+W- and ZZ. In the two Higgs doublet model, the observed \gamma\gamma and ZZ*
--> {four lepton} data can be reproduced by an approximately degenerate CP-even
(h) and CP-odd (A) Higgs boson for values of \sin(\beta-\alpha) near unity and
0.7 < \tan\beta < 1. An enhanced \gamma\gamma signal can also arise in cases
where m_h ~ m_H, m_H ~ m_A, or m_h ~ m_H ~ m_A. Since the ZZ* --> {four lepton}
signal derives primarily from a SM-like Higgs boson whereas the \gamma\gamma
signal receives contributions from two (or more) nearly mass-degenerate states,
one would expect a slightly different invariant mass peak in the ZZ* --> {four
lepton} and \gamma\gamma channels. The phenomenological consequences of such
models can be tested with additional Higgs data that will be collected at the
LHC in the near future.Comment: 18 pages, 19 pdf figures, v2: references added, v3&v4: added refs and
explanation
Persistence in the zero-temperature dynamics of the -states Potts model on undirected-directed Barab\'asi-Albert networks and Erd\"os-R\'enyi random graphs
The zero-temperature Glauber dynamics is used to investigate the persistence
probability in the Potts model with , ,..., states on {\it directed} and {\it
undirected} Barab\'asi-Albert networks and Erd\"os-R\'enyi random graphs. In
this model it is found that decays exponentially to zero in short times
for {\it directed} and {\it undirected} Erd\"os-R\'enyi random graphs. For {\it
directed} and {\it undirected} Barab\'asi-Albert networks, in contrast it
decays exponentially to a constant value for long times, i.e, is
different from zero for all values (here studied) from ; this shows "blocking" for all these values. Except that for
in the {\it undirected} case tends exponentially to zero;
this could be just a finite-size effect since in the other "blocking" cases you
may have only a few unchanged spins.Comment: 14 pages, 8 figures for IJM
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