9,708 research outputs found
Noninvasive brain stimulation techniques can modulate cognitive processing
Recent methods that allow a noninvasive modulation of brain activity are able to modulate human cognitive behavior. Among these methods are transcranial electric stimulation and transcranial magnetic stimulation that both come in multiple variants. A property of both types of brain stimulation is that they modulate brain activity and in turn modulate cognitive behavior. Here, we describe the methods with their assumed neural mechanisms for readers from the economic and social sciences and little prior knowledge of these techniques. Our emphasis is on available protocols and experimental parameters to choose from when designing a study. We also review a selection of recent studies that have successfully applied them in the respective field. We provide short pointers to limitations that need to be considered and refer to the relevant papers where appropriate
Dipolar spin correlations in classical pyrochlore magnets
We study spin correlations for the highly frustrated classical pyrochlore
lattice antiferromagnets with O(N) symmetry in the limit T->0. We conjecture
that a local constraint obeyed by the extensively degenerate ground states
dictates a dipolar form for the asymptotic spin correlations, at all N 2
for which the system is paramagnetic down to T=0. We verify this conjecture in
the cases N=1 and N=3 by simulations and to all orders in the 1/N expansion
about the solvable N=infinity limit. Remarkably, the N=infinity formulae are an
excellent fit, at all distances, to the correlators at N=3 and even at N=1.
Thus we obtain a simple analytical expression also for the correlations of the
equivalent models of spin ice and cubic water ice, I_h.Comment: 4 pages revtex
Diagnosing Deconfinement and Topological Order
Topological or deconfined phases are characterized by emergent, weakly
fluctuating, gauge fields. In condensed matter settings they inevitably come
coupled to excitations that carry the corresponding gauge charges which
invalidate the standard diagnostic of deconfinement---the Wilson loop. Inspired
by a mapping between symmetric sponges and the deconfined phase of the
gauge theory, we construct a diagnostic for deconfinement that has the
interpretation of a line tension. One operator version of this diagnostic turns
out to be the Fredenhagen-Marcu order parameter known to lattice gauge
theorists and we show that a different version is best suited to condensed
matter systems. We discuss generalizations of the diagnostic, use it to
establish the existence of finite temperature topological phases in
dimensions and show that multiplets of the diagnostic are useful in settings
with multiple phases such as gauge theories with charge matter.
[Additionally we present an exact reduction of the partition function of the
toric code in general dimensions to a well studied problem.]Comment: 11 pages, several figure
Spin-nematic order in the frustrated pyrochlore-lattice quantum rotor model
As an example of ordering due to quantum fluctuations, we examine the
nearest-neighbor antiferromagnetic quantum O(n) rotor model on the pyrochlore
lattice. Classically, this system remains disordered even at zero temperature;
we find that adding quantum fluctuations induces an ordered phase that survives
to positive temperature, and we determine how its phase diagram scales with the
coupling constant and the number of spin components. We demonstrate, using
quantum Monte Carlo simulations, that this phase has long-range spin-nematic
order, and that the phase transition into it appears to be first order.Comment: 10 pages, 8 figure
Dynamic Kerr effect responses in the Terahertz-range
Dynamic Kerr effect measurements provide a simple realization of a nonlinear
experiment. We propose a field-off experiment where an electric field of one or
several sinusoidal cycles is applied to a sample in thermal equilibrium.
Afterwards, the evolution of the polarizability is measured. If such an
experiment is performed in the Terahertz-range it might provide valuable
information about the low-frequency dynamics in disordered systems. We treat
these dynamics in terms of a Brownian oscillator model and calculate the Kerr
effect response. It is shown that frequency-selective behaviour can be
expected. In the interesting case of underdamped vibrational motion we find
that the frequency-dependence of the phonon-damping can be determined from the
experiment. Also the behaviour of overdamped relaxational modes is discussed.
For typical glassy materials we estimate the magnitude of all relevant
quantities, which we believe to be helpful in experimental realizations.Comment: 26 pages incl. 5 figure
Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results
Aims: Recent observations have challenged our understanding of rotational
mixing in massive stars by revealing a population of fast-rotating objects with
apparently normal surface nitrogen abundances. However, several questions have
arisen because of a number of issues, which have rendered a reinvestigation
necessary; these issues include the presence of numerous upper limits for the
nitrogen abundance, unknown multiplicity status, and a mix of stars with
different physical properties, such as their mass and evolutionary state, which
are known to control the amount of rotational mixing. Methods: We have
carefully selected a large sample of bright, fast-rotating early-type stars of
our Galaxy (40 objects with spectral types between B0.5 and O4). Their
high-quality, high-resolution optical spectra were then analysed with the
stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the
temperature of the target. Several internal and external checks were performed
to validate our methods; notably, we compared our results with literature data
for some well-known objects, studied the effect of gravity darkening, or
confronted the results provided by the two codes for stars amenable to both
analyses. Furthermore, we studied the radial velocities of the stars to assess
their binarity. Results: This first part of our study presents our methods and
provides the derived stellar parameters, He, CNO abundances, and the
multiplicity status of every star of the sample. It is the first time that He
and CNO abundances of such a large number of Galactic massive fast rotators are
determined in a homogeneous way.Comment: accepted for publication by A&
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