341 research outputs found
New Global F-theory GUTs with U(1) symmetries
We construct global F-theory GUTs with SU(5) x U(1) gauge group defined by
specifying a fully resolved Calabi-Yau fourfold and consistent four-form
G-flux. Its specific U(1) charged matter spectrum allows the desired Yukawa
couplings, but forbids dangerous proton decay operators. The model we find: (1)
does not follow from an underlying higgsed E8 gauge group (2) leaves the class
of theories that can be analyzed with current split-spectral cover techniques.
This avoids recently proposed no-go theorems for models with hypercharge flux,
as required to break the GUT group. The appearance of additional fields is
related geometrically to considering a more general class of sections and 4-1
splits. We show explicitly that the four-dimensional chiral matter index can
still be computed using three-dimensional one-loop Chern-Simons terms.Comment: 24 pages, 2 figure
Complete Intersection Fibers in F-Theory
Global F-theory compactifications whose fibers are realized as complete
intersections form a richer set of models than just hypersurfaces. The detailed
study of the physics associated with such geometries depends crucially on being
able to put the elliptic fiber into Weierstrass form. While such a
transformation is always guaranteed to exist, its explicit form is only known
in a few special cases. We present a general algorithm for computing the
Weierstrass form of elliptic curves defined as complete intersections of
different codimensions and use it to solve all cases of complete intersections
of two equations in an ambient toric variety. Using this result, we determine
the toric Mordell-Weil groups of all 3134 nef partitions obtained from the 4319
three-dimensional reflexive polytopes and find new groups that do not exist for
toric hypersurfaces. As an application, we construct several models that cannot
be realized as toric hypersurfaces, such as the first toric SU(5) GUT model in
the literature with distinctly charged 10 representations and an F-theory model
with discrete gauge group Z_4 whose dual fiber has a Mordell-Weil group with
Z_4 torsion.Comment: 41 pages, 4 figures and 18 tables; added references in v
The zero-dimensional O(N) vector model as a benchmark for perturbation theory, the large-N expansion and the functional renormalization group
We consider the zero-dimensional O(N) vector model as a simple example to
calculate n-point correlation functions using perturbation theory, the large-N
expansion, and the functional renormalization group (FRG). Comparing our
findings with exact results, we show that perturbation theory breaks down for
moderate interactions for all N, as one should expect. While the
interaction-induced shift of the free energy and the self-energy are well
described by the large-N expansion even for small N, this is not the case for
higher-order correlation functions. However, using the FRG in its one-particle
irreducible formalism, we see that very few running couplings suffice to get
accurate results for arbitrary N in the strong coupling regime, outperforming
the large-N expansion for small N. We further remark on how the derivative
expansion, a well-known approximation strategy for the FRG, reduces to an exact
method for the zero-dimensional O(N) vector model.Comment: 13 pages, 13 figure
Dual-Wavelength Lasing in Quantum-Dot Plasmonic Lattice Lasers
Arrays of metallic particles patterned on a substrate have emerged as a
promising design for on-chip plasmonic lasers. In past examples of such
devices, the periodic particles provided feedback at a single resonance
wavelength, and organic dye molecules were used as the gain material. Here, we
introduce a flexible template-based fabrication method that allows a broader
design space for Ag particle-array lasers. Instead of dye molecules, we
integrate colloidal quantum dots (QDs), which offer better photostability and
wavelength tunability. Our fabrication approach also allows us to easily adjust
the refractive index of the substrate and the QD-film thickness. Exploiting
these capabilities, we demonstrate not only single-wavelength lasing but
dual-wavelength lasing via two distinct strategies. First, by using particle
arrays with rectangular lattice symmetries, we obtain feedback from two
orthogonal directions. The two output wavelengths from this laser can be
selected individually using a linear polarizer. Second, by adjusting the
QD-film thickness, we use higher-order transverse waveguide modes in the QD
film to obtain dual-wavelength lasing at normal and off-normal angles from a
symmetric square array. We thus show that our approach offers various design
possibilities to tune the laser output
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X-ray Ptychographic Imaging and Spectroscopic Studies of Plasma-Treated Plastic Films
Polyethylene terephthalate (PET) is a thermoplastic polyester with numerous applications in industry. However, it requires surface modification on an industrial scale for printing and coating processes and plasma treatment is one of the most commonly used techniques to increase the hydrophilicity of the PET films. Systematic improvement of the surface modification by adaption of the plasma process can be aided by a comprehensive understanding of the surface morphology and chemistry. However, imaging large surface areas (tens of microns) with a resolution that allows understanding the surface quality and modification is challenging. As a proof-of-principle, plasma-treated PET films were used to demonstrate the capabilities of X-ray ptychography, currently under development at the soft X-ray free-electron laser FLASH at DESY, for imaging macroscopic samples. In combination with scanning electron microscopy (SEM), this new technique was used to study the effects of different plasma treatment processes on PET plastic films. The studies on the surface morphology were complemented by investigations of the surface chemistry using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). While both imaging techniques consistently showed an increase in roughness and change in morphology of the PET films after plasma treatment, X-ray ptychography can provide additional information on the three-dimensional morphology of the surface. At the same time, the chemical analysis shows an increase in the oxygen content and polarity of the surface without significant damage to the polymer, which is important for printing and coating processes
Heterologous Overexpression and Mutagenesis of the Human Bile Salt Export Pump (ABCB11) Using DREAM (Directed REcombination-Assisted Mutagenesis)
Homologous recombination in Saccharomyces cerevisiae is a well-studied process. Here, we describe a yeast-recombination-based approach to construct and mutate plasmids containing the cDNA of the human bile salt export pump (BSEP) that has been shown to be unstable in E. coli. Using this approach, we constructed the necessary plasmids for a heterologous overexpression of BSEP in the yeast Pichia pastoris. We then applied a new site-directed mutagenesis method, DREAM (Directed REcombination-Assisted Mutagenesis) that completely bypasses E. coli by using S. cerevisiae as the plasmid host with high mutagenesis efficiency. Finally, we show how to apply this strategy to unstable non-yeast plasmids by rapidly turning an existing mammalian BSEP expression construct into a S. cerevisiae-compatible plasmid and analyzing the impact of a BSEP mutation in several mammalian cell lines
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