59,422 research outputs found
Dimensional crossover in SrRuO within slave-boson mean-field theory
Motivated by the anomalous temperature dependence of the c-axis resistivity
of SrRuO, the dimensional crossover from a network of perpendicular
one-dimensional chains to a two-dimensional system due to a weak hybridization
between the perpendicular chains is studied. The corresponding two-orbital
Hubbard model is treated within a slave-boson mean-field theory (SBMFT) to take
correlation effects into account such as the spin-charge separation on the
one-dimensional chains. Using an RPA-like formulation for the Green's function
of collective spinon-holon excitations the emergence of quasiparticles at
low-temperatures is examined. The results are used to discuss the evolution of
the spectral density and the c-axis transport within a tunneling approach. For
the latter a regime change between low- and high-temperature regime is found in
qualitative accordance with experimental data
Efficient Learning of a One-dimensional Density Functional Theory
Density functional theory underlies the most successful and widely used
numerical methods for electronic structure prediction of solids. However, it
has the fundamental shortcoming that the universal density functional is
unknown. In addition, the computational result---energy and charge density
distribution of the ground state---is useful for electronic properties of
solids mostly when reduced to a band structure interpretation based on the
Kohn-Sham approach. Here, we demonstrate how machine learning algorithms can
help to free density functional theory from these limitations. We study a
theory of spinless fermions on a one-dimensional lattice. The density
functional is implicitly represented by a neural network, which predicts,
besides the ground-state energy and density distribution, density-density
correlation functions. At no point do we require a band structure
interpretation. The training data, obtained via exact diagonalization, feeds
into a learning scheme inspired by active learning, which minimizes the
computational costs for data generation. We show that the network results are
of high quantitative accuracy and, despite learning on random potentials,
capture both symmetry-breaking and topological phase transitions correctly.Comment: 5 pages, 3 figures; 4+ pages appendi
Space/time noncommutative field theories and causality
As argued previously, amplitudes of quantum field theories on noncommutative
space and time cannot be computed using naive path integral Feynman rules. One
of the proposals is to use the Gell-Mann--Low formula with time-ordering
applied before performing the integrations. We point out that the previously
given prescription should rather be regarded as an interaction point
time-ordering. Causality is explicitly violated inside the region of
interaction. It is nevertheless a consistent procedure, which seems to be
related to the interaction picture of quantum mechanics. In this framework we
compute the one-loop self-energy for a space/time noncommutative \phi^4 theory.
Although in all intermediate steps only three-momenta play a role, the final
result is manifestly Lorentz covariant and agrees with the naive calculation.
Deriving the Feynman rules for general graphs, we show, however, that such a
picture holds for tadpole lines only.Comment: 16 pages, LaTeX, uses feynmf macros, one reference added; ooops,
version 2 was an older one
Comments concerning the paper "Measurement of negatively charged pion spectra in inelastic p+p interactions at 20, 31, 40, 80 and 158 GeV/c" by the NA61 collaboration
New data from the NA61 collaboration on the production of negative pions in
p+p interactions at beam momenta between 20 and 158 GeV/c are critically
compared to available results in the same energy range. It is concluded that
the NA61 data show some discrepancies with the previous results. This concerns
in particular the total yields, the integrated rapidity distributions and
the double differential cross sections.Comment: 16 pages, 13 figure
Electronic structure of an electron on the gyroid surface, a helical labyrinth
Previously reported formulation for electrons on curved periodic surfaces is
used to analyze the band structure of an electron bound on the gyroid surface
(the only triply-periodic minimal surface that has screw axes). We find that an
effect of the helical structure appears as the bands multiply sticking together
on the Brillouin zone boundaries. We elaborate how the band sticking is lifted
when the helical and inversion symmetries of the structure are degraded. We
find from this that the symmetries give rise to prominent peaks in the density
of states.Comment: RevTeX, 4 pages, 6 figure
Fast and rewritable colloidal assembly via field synchronized particle swapping
We report a technique to realize reconfigurable colloidal crystals by using
the controlled motion of particle defects above an externally modulated
magnetic substrate. The transport of particles is induced by applying a uniform
rotating magnetic field to a ferrite garnet film characterized by a periodic
lattice of magnetic bubbles. For filling factor larger than one colloid per
bubble domain, the particle current arises from propagating defects where
particles synchronously exchange their position when passing from one occupied
domain to the next. The amplitude of an applied alternating magnetic field can
be used to displace the excess particles via a swapping mechanism, or to
mobilize the entire colloidal system at a predefined speed
When is an alternative possibility robust?
According to some, free will requires alternative possibilities. But not any old alternative possibility will do. Sometimes, being able to bring about an alternative does not bestow any control on an agent. In order to bestow control, and so be directly relevant qua alternative to grounding the agent's moral responsibility, alternatives need to be robust. Here, I investigate the nature of robust alternatives. I argue that Derk Pereboom's latest robustness criterion is too strong, and I suggest a different criterion based on the idea that what agents need to be able to do is keep open the possibility of securing their blamelessness, rather than needing to directly ensure their own blamelessness at the time of decision
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