297 research outputs found
From Type-II Triply Degenerate Nodal Points and Three-Band Nodal Rings to Type-II Dirac Points in Centrosymmetric Zirconium Oxide
Using first-principles calculations, we report that ZrO is a topological
material with the coexistence of three pairs of type-II triply degenerate nodal
points (TNPs) and three nodal rings (NRs), when spin-orbit coupling (SOC) is
ignored. Noticeably, the TNPs reside around Fermi energy with large linear
energy range along tilt direction (> 1 eV) and the NRs are formed by three
strongly entangled bands. Under symmetry-preserving strain, each NR would
evolve into four droplet-shaped NRs before fading away, producing distinct
evolution compared with that in usual two-band NR. When SOC is included, TNPs
would transform into type-II Dirac points while all the NRs have gaped.
Remarkably, the type-II Dirac points inherit the advantages of TNPs: residing
around Fermi energy and exhibiting large linear energy range. Both features
facilitate the observation of interesting phenomena induced by type-II
dispersion. The symmetry protections and low-energy Hamiltonian for the
nontrivial band crossings are discussed.Comment: 7 pages, 5 figures, J. Phys. Chem. Lett. 201
Higher Derivative D-brane Couplings
This dissertation covers two different but related topics: the construction of consistent models in type IIB and heterotic string theories, and the higher derivative
couplings for D-brane action, which will enable us to relate some models of type IIB to the heterotic side through duality chain.
In the first part, we describe an alternative to the KKLT scenario, in which one can achieve de-Sitter space after fixing all moduli. We fix complex structure moduli
and the axio-dilaton by deriving the stability conditions for the critical points of the no-scale scalar potential that governs the dynamics of the complex structure moduli
and the axio-dilaton in compactifications of type IIB string theory on Calabi-Yau
three-folds.
In the second part, we show the existence of a class of flux backgrounds in heterotic string theory. The background metric we will consider is a T2 fibration over a K3 base times four-dimensional Minkowski space. Unbroken space-time supersymmetry determines all background fields except one scalar function which is related to the dilaton. The heterotic Bianchi identity gives the same differential equation for the dilaton, and we will discuss in detail the solvability of this equation for backgrounds preserving an N=2 supersymmetry.
In the third part, we obtain the higher derivative D-brane action by using both linearized T-duality and string disc amplitude computation. We evaluate disc amplitude of one R-R field C^(p-3) and two NS-NS fields in the presence of a single Dp-brane in type II string theory. We obtain the action for the higher derivative brane interactions among one R-R field C^(p-3) and two NS-NS B-fields after carefully comparing the supergravity amplitudes with the corresponding string amplitude up to alpha^r2 order. We also show that these higher derivative brane couplings are invariant under both R-R and NS-NS B-field gauge transformations, and compatible with linear T-duality
Higher derivative brane couplings from T-duality
The Wess-Zumino coupling on D-branes in string theory is known to receive
higher derivative corrections which couple the Ramond-Ramond potential to terms
involving the square of the spacetime curvature tensor. Consistency with
T-duality implies that the branes should also have four-derivative couplings
that involve the NS-NS B-field. We use T-duality to predict some of these
couplings. We then confirm these results with string worldsheet computations by
evaluating disc amplitudes with insertions of one R-R and two NS-NS vertex
operators.Comment: 30 pages, 1 figure, references adde
Four-derivative brane couplings from string amplitudes
We evaluate the string theory disc amplitude of one Ramond-Ramond field
C^(p-3) and two Neveu-Schwarz B-fields in the presence of a single Dp-brane in
type II string theory. From this amplitude we extract the four-derivative (or
equivalently order (alpha')^2) part of the Dp-brane action involving these
fields. We show that the new couplings are invariant under R-R and NS-NS gauge
transformations and compatible with linear T-duality.Comment: 31 pages, 7 figure
Quantum phases of SrCu2(BO3)2 from high-pressure thermodynamics
We report heat capacity measurements of SrCu(BO) under high
pressure along with simulations of relevant quantum spin models and map out the
phase diagram of the material. We find a first-order quantum phase
transition between the low-pressure quantum dimer paramagnet and a phase with
signatures of a plaquette-singlet state below T = K. At higher pressures,
we observe a transition into a previously unknown antiferromagnetic state below
K. Our findings can be explained within the two-dimensional
Shastry-Sutherland quantum spin model supplemented by weak inter-layer
couplings. The possibility to tune SrCu(BO) between the
plaquette-singlet and antiferromagnetic states opens opportunities for
experimental tests of quantum field theories and lattice models involving
fractionalized excitations, emergent symmetries, and gauge fluctuations.Comment: 6 pages + 8 pages supplemental informatio
Metastable Flux Configurations and de Sitter Spaces
We derive stability conditions for the critical points of the no-scale scalar
potential governing the dynamics of the complex structure moduli and the
axio-dilaton in compactifications of type IIB string theory on Calabi-Yau
three-folds. We discuss a concrete example of a T^6 orientifold. We then
consider the four-dimensional theory obtained from compactifications of type
IIB string theory on non-geometric backgrounds which are mirror to rigid
Calabi-Yau manifolds and show that the complex structure moduli fields can be
stabilized in terms of H_{RR} only, i.e. with no need of orientifold
projection. The stabilization of all the fields at weak coupling, including the
axio-dilaton, may require to break supersymmetry in the presence of H_{NS} flux
or corrections to the scalar potential.Comment: 24 page
Age-related sensitivity and pathological differences in infections by 2009 pandemic influenza A (H1N1) virus
<p>Abstract</p> <p>Background</p> <p>The highly pandemic 2009 influenza A H1N1 virus infection showed distinguished skewed age distribution with majority of infection and death in children and young adults. Although previous exposure to related antigen has been proposed as an explanation, the mechanism of age protection is still unknown.</p> <p>Methods</p> <p>In this study, murine model of different ages were inoculated intranasally with H1N1 (A/Beijing/501/09) virus and the susceptibility and pathological response to 2009 H1N1 infection were investigated.</p> <p>Results</p> <p>Our results showed that the younger mice had higher mortality rate when infected with the same dose of virus and the lethal dose increased with age. Immunohistochemical staining of H1N1 antigens in mice lung indicated infection was in the lower respiratory tract. Most bronchial and bronchiolar epithelial cells in 4-week mice were infected while only a minor percentage of those cells in 6-month and 1-year old mice did. The young mice developed much more severe lung lesions and had higher virus load in lung than the two older groups of mice while older mice formed more inducible bronchus-associated lymphoid tissue in their lungs and more severe damage in spleen.</p> <p>Conclusions</p> <p>These results suggest that young individuals are more sensitive to H1N1 infection and have less protective immune responses than older adults. The age factor should be considered when studying the pathogenesis and transmission of influenza virus and formulating strategies on vaccination and treatment.</p
Depth-Varying Friction on a Ramp-Flat Fault Illuminated by ∼3-Year InSAR Observations Following the 2017 Mw 7.3 Sarpol-e Zahab Earthquake
We use interferometric synthetic aperture radar observations to investigate the fault geometry and afterslip evolution within 3 years after a mainshock. The postseismic observations favor a ramp-flat structure in which the flat angle should be lower than 10°. The postseismic deformation is dominated by afterslip, while the viscoelastic response is negligible. A multisegment, stress-driven afterslip model (hereafter called the SA-2 model) with depth-varying frictional properties better explains the spatiotemporal evolution of the postseismic deformation than a two-segment, stress-driven afterslip model (hereafter called the SA-1 model). Although the SA-2 model does not improve the misfit significantly, this multisegment fault with depth-varying friction is more physically plausible given the depth-varying mechanical stratigraphy in the region. Compared to the kinematic afterslip model, the mechanical afterslip models with friction variation tend to underestimate early postseismic deformation to the west, which may indicate more complex fault friction than we expected. Both the kinematic and stress-driven models can resolve downdip afterslip, although it could be affected by data noise and model resolution. The transition depth of the sedimentary cover basement interface inferred by afterslip models is ∼12 km in the seismogenic zone, which coincides with the regional stratigraphic profile. Because the coseismic rupture propagated along a basement-involved fault while the postseismic slip may activate the frontal structures and/or shallower detachments in the sedimentary cover, the 2017 Sarpol-e Zahab earthquake may have acted as a typical event that contributed to both thick- and thin-skinned shortening of the Zagros in both seismic and aseismic ways
An analysis of the molecular evolution of Hepatitis B viral genotypes A/B/D using a Bayesian evolutionary method
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