4,300 research outputs found
Variable - temperature scanning optical and force microscope
The implementation of a scanning microscope capable of working in confocal,
atomic force and apertureless near field configurations is presented. The
microscope is designed to operate in the temperature range 4 - 300 K, using
conventional helium flow cryostats. In AFM mode, the distance between the
sample and an etched tungsten tip is controlled by a self - sensing
piezoelectric tuning fork. The vertical position of both the AFM head and
microscope objective can be accurately controlled using piezoelectric coarse
approach motors. The scanning is performed using a compact XYZ stage, while the
AFM and optical head are kept fixed, allowing scanning probe and optical
measurements to be acquired simultaneously and in concert. The free optical
axis of the microscope enables both reflection and transmission experiments to
be performed.Comment: 24 pages, 9 figures, submitted to the journal "Review of Scientific
Instruments
Phase Fluctuations near the Chiral Critical Point
The Helmholtz free energy density is parametrized as a function of
temperature and baryon density near the chiral critical point of QCD. The
parametrization incorporates the expected critical exponents and amplitudes. An
expansion away from equilibrium states is achieved with Landau theory. This is
used to calculate the probability that the system is found at a density other
than the equilibrium one. Such fluctuations are predicted to be very large in
heavy ion collisions.Comment: 7 pages, 8 figures, Winter Workshop on Nuclear Dynamics 201
Cognitive Computation sans Representation
The Computational Theory of Mind (CTM) holds that cognitive processes are essentially computational, and hence computation provides the scientific key to explaining mentality. The Representational Theory of Mind (RTM) holds that representational content is the key feature in distinguishing mental from non-mental systems. I argue that there is a deep incompatibility between these two theoretical frameworks, and that the acceptance of CTM provides strong grounds for rejecting RTM. The focal point of the incompatibility is the fact that representational content is extrinsic to formal procedures as such, and the intended interpretation of syntax makes no difference to the execution of an algorithm. So the unique 'content' postulated by RTM is superfluous to the formal procedures of CTM. And once these procedures are implemented in a physical mechanism, it is exclusively the causal properties of the physical mechanism that are responsible for all aspects of the system's behaviour. So once again, postulated content is rendered superfluous. To the extent that semantic content may appear to play a role in behaviour, it must be syntactically encoded within the system, and just as in a standard computational artefact, so too with the human mind/brain - it's pure syntax all the way down to the level of physical implementation. Hence 'content' is at most a convenient meta-level gloss, projected from the outside by human theorists, which itself can play no role in cognitive processing
A new approach to the complex-action problem and its application to a nonperturbative study of superstring theory
Monte Carlo simulations of a system whose action has an imaginary part are
considered to be extremely difficult. We propose a new approach to this
`complex-action problem', which utilizes a factorization property of
distribution functions. The basic idea is quite general, and it removes the
so-called overlap problem completely. Here we apply the method to a
nonperturbative study of superstring theory using its matrix formulation. In
this particular example, the distribution function turns out to be positive
definite, which allows us to reduce the problem even further. Our numerical
results suggest an intuitive explanation for the dynamical generation of 4d
space-time.Comment: 7 pages, 4 figures, PRD version somewhat extended from the original
versio
320-Channel Dual Phase Lock-in Optical Spectrometer
The development of a multiple-channel lock-in optical spectrometer (LIOS) is
presented, which enables parallel phase-sensitive detection at the output of an
optical spectrometer. The light intensity from a spectrally broad source is
modulated at the reference frequency, and focused into a high-resolution
imaging spectrometer. The height at which the light enters the spectrometer is
controlled by an acousto-optic deflector, and the height information is
preserved at the output focal plane. A two-dimensional InGaAs focal plane array
collects light that has been dispersed in wavelength along the horizontal
direction, and in time along the vertical direction. The data is demodulated
using a high performance computer-based digital signal processor. This parallel
approach greatly enhances (by more than 100x) the speed at which spectrally
resolved lock-in data can be acquired. The noise performance of a working
system optimized for the 1300 nm wavelength range is analyzed using a laser
diode light source. Time-resolved absorption traces are obtained for InAs
quantum dots embedded in a GaAs matrix, and for dispersed films of PbSe
nanocrystals.Comment: 27 pages, 11 figures, submitted to the journal "Review of Scientific
Instruments
A proper understanding of Millikan
Ruth Millikan’s teleological theory of mental content is complex and often misunderstood. This paper motivates and clarifies some of the complexities of the theory, and shows that paying careful attention to its details yields answers to a number of common objections to teleological theories, in particular, the problem of novel mental states, the problem of functionally false beliefs, and problems about indeterminacy or multiplicity of function
The QCD phase diagram at nonzero quark density
We determine the phase diagram of QCD on the \mu-T plane for small to
moderate chemical potentials. Two transition lines are defined with two
quantities, the chiral condensate and the strange quark number susceptibility.
The calculations are carried out on N_t =6,8 and 10 lattices generated with a
Symanzik improved gauge and stout-link improved 2+1 flavor staggered fermion
action using physical quark masses. After carrying out the continuum
extrapolation we find that both quantities result in a similar curvature of the
transition line. Furthermore, our results indicate that in leading order the
width of the transition region remains essentially the same as the chemical
potential is increased.Comment: 12 pages, 6 figure
An effective chiral Hadron-Quark Equation of State
We construct an effective model for the QCD equation of state, taking into
account chiral symmetry restoration as well as the deconfinement phase
transition. The correct asymptotic degrees of freedom at the high and low
temperature limits are included (quarks hadrons). The model
shows a rapid crossover for both order parameters, as is expected from lattice
calculations. We then compare the thermodynamic properties of the model at
which turn out to be in qualitative agreement with lattice data,
while apparent quantitative differences can be attributed to hadronic
contributions and excluded volume corrections. Furthermore we discuss the
effects of a repulsive vector type quark interaction at finite baryon number
densities on the resulting phase diagram of the model. Our current model is
able to reproduce a first-order liquid gas phase transition as expected, but
does not show any signs of a first order deconfinement or chiral phase
transition. Both transitions rather appear as a very wide crossover in which
heavily medium modified hadron coexist with free quarks.Comment: 19 pages, 13 figures Version accepted by J. Phys.
Collider Bounds on Lee-Wick Higgs Bosons
We study the constraints on the Lee-Wick Higgs sector arising from direct
collider searches. We work in an effective-field theory framework, where all of
the Lee-Wick partners are integrated out, with the sole exception of the
Lee-Wick Higgs bosons. The resulting theory is a two-Higgs doublet model where
the second doublet has wrong-sign kinetic and mass terms. We include the bounds
coming from direct Higgs searches at both LEP and Tevatron using the code
HiggsBounds, and show the currently excluded parameter space. We also analyze
the prospects of LHC Run-I, finding that with a total integrated luminosity of
5 fb and a center-of-mass energy of 7 TeV, most of the parameter space
for the SM-like CP-even Higgs will be probed.Comment: 26 pages, 5 figures, matches version published by PR
EoS of finite density QCD with Wilson fermions by Multi-Parameter Reweighting and Taylor expansion
The equation of state (EoS), quark number density and susceptibility at
nonzero quark chemical potential are studied in lattice QCD simulations
with a clover-improved Wilson fermion of 2-flavors and RG-improved gauge
action. To access nonzero , we employ two methods : a multi-parameter
reweighting (MPR) in and and Taylor expansion in . The use
of a reduction formula for the Wilson fermion determinant enables to study the
reweighting factor in MPR explicitly and heigher-order coefficients in Taylor
expansion free from errors of noise method, although calculations are limited
to small lattice size. As a consequence, we can study the reliability of the
thermodynamical quantities through the consistency of the two methods, each of
which has different origin of the application limit.
The thermodynamical quantities are obtained from simulations on a lattice with an intermediate quark mass(. The MPR
and Taylor expansion are consistent for the EoS and number density up to
and for the number susceptibility up to . This
implies within a given statistics that the overlap problem for the MPR and
truncation error for the Taylor expansion method are negligible in these
regions.
In order to make MPR methods work, the fluctuation of the reweighting factor
should be small. We derive the equation of the reweighting line where the
fluctuation is small, and show that the equation of the reweighting line is
consistent with the fluctuation minimum condition.Comment: 20 pages, 11 figures. Accepted to JHEP. Discussions are added.
Figures for Taylor coefficients (Fig. 7) are modifie
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