359 research outputs found
Strong Ramsey games: Drawing on an infinite board
Consider the following strong Ramsey game. Two players take turns in claiming a previously
unclaimed edge of the complete graph on n vertices until all edges have been claimed. The first
player to build a copy of K5 is declared the winner of the game. If none of the players win,
then the game ends in a draw. A simple strategy stealing argument shows that the second player
cannot expect to ever win this game. Moreover, for sufficiently large n, it follows from Ramsey’s
Theorem that the game cannot end in a draw and is thus a first player win. A famous question
of Beck asks whether the minimum number of moves needed for the first player to win this game
on Kn grows with n. This seems unlikely but is still wide open. A striking equivalent formulation
of this question is whether the same game played on the infinite complete graph is a first player
win or a draw.
The target graph of the strong Ramsey game does not have to be K5, it can be any predetermined fixed graph. In fact, it can even be a k-uniform hypergraph (and then the game is played
on the infinite k-uniform hypergraph). Since strategy stealing and Ramsey’s Theorem still apply,
one can ask the same question: is this game a first player win or a draw? The same intuition
which lead most people (including the authors) to believe that the K5 strong Ramsey game on
the infinite board is a first player win, would also lead one to believe that the H strong Ramsey
game on the infinite board is a first player win for any uniform hypergraph H. However, in this
paper we construct a 5-uniform hypergraph for which the corresponding game is a draw
Controlling the quality factor of a tuning-fork resonance between 9 K and 300 K for scanning-probe microscopy
We study the dynamic response of a mechanical quartz tuning fork in the
temperature range from 9 K to 300 K. Since the quality factor Q of the
resonance strongly depends on temperature, we implement a procedure to control
the quality factor of the resonance. We show that we are able to dynamically
change the quality factor and keep it constant over the whole temperature
range. This procedure is suitable for applications in scanning probe
microscopy.Comment: 5 pages, 6 figure
Nanoscale piezoelectric response across a single antiparallel ferroelectric domain wall
Surprising asymmetry in the local electromechanical response across a single
antiparallel ferroelectric domain wall is reported. Piezoelectric force
microscopy is used to investigate both the in-plane and out-of- plane
electromechanical signals around domain walls in congruent and
near-stoichiometric lithium niobate. The observed asymmetry is shown to have a
strong correlation to crystal stoichiometry, suggesting defect-domain wall
interactions. A defect-dipole model is proposed. Finite element method is used
to simulate the electromechanical processes at the wall and reconstruct the
images. For the near-stoichiometric composition, good agreement is found in
both form and magnitude. Some discrepancy remains between the experimental and
modeling widths of the imaged effects across a wall. This is analyzed from the
perspective of possible electrostatic contributions to the imaging process, as
well as local changes in the material properties in the vicinity of the wall
Damping of optomechanical disks resonators vibrating in air
We report on miniature GaAs disk optomechanical resonators vibrating in air
in the radiofrequency range. The flexural modes of the disks are studied by
scanning electron microscopy and optical interferometry, and correctly modeled
with the elasticity theory for annular plates. The mechanical damping is
systematically measured, and confronted with original analytical models for air
damping. Formulas are derived that correctly reproduce both the mechanical
modes and the damping behavior, and can serve as design tools for
optomechanical applications in fluidic environment
Local probing of ionic diffusion by electrochemical strain microscopy: spatial resolution and signal formation mechanisms
Electrochemical insertion-deintercalation reactions are typically associated
with significant change of molar volume of the host compound. This strong
coupling between ionic currents and strains underpins image formation
mechanisms in electrochemical strain microscopy (ESM), and allows exploring the
tip-induced electrochemical processes locally. Here we analyze the signal
formation mechanism in ESM, and develop the analytical description of operation
in frequency and time domains. The ESM spectroscopic modes are compared to
classical electrochemical methods including potentiostatic and galvanostatic
intermittent titration (PITT and GITT), and electrochemical impedance
spectroscopy (EIS). This analysis illustrates the feasibility of spatially
resolved studies of Li-ion dynamics on the sub-10 nanometer level using
electromechanical detection.Comment: 49 pages, 17 figures, 4 tables, 3 appendices, to be submitted to J.
Appl. Phys
Strong Ramsey games: drawing on an infinite board
We consider the strong Ramsey-type game R(k)(H,ℵ0), played on the edge set of the infinite complete k-uniform hypergraph KkN. Two players, called FP (the first player) and SP (the second player), take turns claiming edges of K^k_N with the goal of building a copy of some finite predetermined k-uniform hypergraph H. The first player to build a copy of H wins. If no player has a strategy to ensure his win in finitely many moves, then the game is declared a draw.
In this paper, we construct a 5-uniform hypergraph H such that R(5)(H,ℵ0) is a draw. This is in stark contrast to the corresponding finite game R(5)(H,n), played on the edge set of K5n. Indeed, using a classical game-theoretic argument known as \emph{strategy stealing} and a Ramsey-type argument, one can show that for every k-uniform hypergraph G, there exists an integer n0 such that FP has a winning strategy for R(k)(G,n) for every n≥n0
Production of antihydrogen at reduced magnetic field for anti-atom trapping
We have demonstrated production of antihydrogen in a 1T solenoidal
magnetic field. This field strength is significantly smaller than that used in
the first generation experiments ATHENA (3T) and ATRAP (5T). The
motivation for using a smaller magnetic field is to facilitate trapping of
antihydrogen atoms in a neutral atom trap surrounding the production region. We
report the results of measurements with the ALPHA (Antihydrogen Laser PHysics
Apparatus) device, which can capture and cool antiprotons at 3T, and then
mix the antiprotons with positrons at 1T. We infer antihydrogen production
from the time structure of antiproton annihilations during mixing, using mixing
with heated positrons as the null experiment, as demonstrated in ATHENA.
Implications for antihydrogen trapping are discussed
A novel antiproton radial diagnostic based on octupole induced ballistic loss
We report results from a novel diagnostic that probes the outer radial
profile of trapped antiproton clouds. The diagnostic allows us to determine the
profile by monitoring the time-history of antiproton losses that occur as an
octupole field in the antiproton confinement region is increased. We show
several examples of how this diagnostic helps us to understand the radial
dynamics of antiprotons in normal and nested Penning-Malmberg traps. Better
understanding of these dynamics may aid current attempts to trap antihydrogen
atoms
Alpha Antihydrogen Experiment
ALPHA is an experiment at CERN, whose ultimate goal is to perform a precise
test of CPT symmetry with trapped antihydrogen atoms. After reviewing the
motivations, we discuss our recent progress toward the initial goal of stable
trapping of antihydrogen, with some emphasis on particle detection techniques.Comment: Invited talk presented at the Fifth Meeting on CPT and Lorentz
Symmetry, Bloomington, Indiana, June 28-July 2, 201
Abnormal phenomena in a one-dimensional periodic structure containing left-handed materials
The explicit dispersion equation for a one-dimensional periodic structure
with alternative layers of left-handed material (LHM) and right-handed material
(RHM) is given and analyzed. Some abnormal phenomena such as spurious modes
with complex frequencies, discrete modes and photon tunnelling modes are
observed in the band structure. The existence of spurious modes with complex
frequencies is a common problem in the calculation of the band structure for
such a photonic crystal. Physical explanation and significance are given for
the discrete modes (with real values of wave number) and photon tunnelling
propagation modes (with imaginary wave numbers in a limited region).Comment: 10 pages, 4 figure
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