17,451 research outputs found
Epitaxial graphene on SiC(0001): More than just honeycombs
The potential of graphene to impact the development of the next generation of
electronics has renewed interest in its growth and structure. The
graphitization of hexagonal SiC surfaces provides a viable alternative for the
synthesis of graphene, with wafer-size epitaxial graphene on SiC(0001) now
possible. Despite this recent progress, the exact nature of the graphene-SiC
interface and whether the graphene even has a semiconducting gap remain
controversial. Using scanning tunneling microscopy with functionalized tips and
density functional theory calculations, here we show that the interface is a
warped carbon sheet consisting of three-fold hexagon-pentagon-heptagon
complexes periodically inserted into the honeycomb lattice. These defects
relieve the strain between the graphene layer and the SiC substrate, while
still retaining the three-fold coordination for each carbon atom. Moreover,
these defects break the six-fold symmetry of the honeycomb, thereby naturally
inducing a gap: the calculated band structure of the interface is
semiconducting and there are two localized states near K below the Fermi level,
explaining the photoemission and carbon core-level data. Nonlinear dispersion
and a 33 meV gap are found at the Dirac point for the next layer of graphene,
providing insights into the debate over the origin of the gap in epitaxial
graphene on SiC(0001). These results indicate that the interface of the
epitaxial graphene on SiC(0001) is more than a dead buffer layer, but actively
impacts the physical and electronic properties of the subsequent graphene
layers
A comparison of robust Mendelian randomization methods using summary data.
The number of Mendelian randomization (MR) analyses including large numbers of genetic variants is rapidly increasing. This is due to the proliferation of genome-wide association studies, and the desire to obtain more precise estimates of causal effects. Since it is unlikely that all genetic variants will be valid instrumental variables, several robust methods have been proposed. We compare nine robust methods for MR based on summary data that can be implemented using standard statistical software. Methods were compared in three ways: by reviewing their theoretical properties, in an extensive simulation study, and in an empirical example. In the simulation study, the best method, judged by mean squared error was the contamination mixture method. This method had well-controlled Type 1 error rates with up to 50% invalid instruments across a range of scenarios. Other methods performed well according to different metrics. Outlier-robust methods had the narrowest confidence intervals in the empirical example. With isolated exceptions, all methods performed badly when over 50% of the variants were invalid instruments. Our recommendation for investigators is to perform a variety of robust methods that operate in different ways and rely on different assumptions for valid inferences to assess the reliability of MR analyses
Concise sharpening and generalizations of Shafer's inequality for the arc sine function
In this paper, by a concise and elementary approach, we sharpen and
generalize Shafer's inequality for the arc sine function, and some known
results are extended and generalized.Comment: 5 page
Experimental demonstration of phase-remapping attack in a practical quantum key distribution system
Unconditional security proofs of various quantum key distribution (QKD)
protocols are built on idealized assumptions. One key assumption is: the sender
(Alice) can prepare the required quantum states without errors. However, such
an assumption may be violated in a practical QKD system. In this paper, we
experimentally demonstrate a technically feasible "intercept-and-resend" attack
that exploits such a security loophole in a commercial "plug & play" QKD
system. The resulting quantum bit error rate is 19.7%, which is below the
proven secure bound of 20.0% for the BB84 protocol. The attack we utilize is
the phase-remapping attack (C.-H. F. Fung, et al., Phys. Rev. A, 75, 32314,
2007) proposed by our group.Comment: 16 pages, 6 figure
Some integral inequalities on time scales
In this paper, some new integral inequalities on time scales are presented by
using elementarily analytic methods in calculus of time scales.Comment: 8 page
Regulation of differentiation- and proliferation-inducers on Lewis antigens, α-fucosyltransferase and metastaticotential in hepatocarcinoma cells
The expressions of Lewis (Le) antigens, α-1,3/1,4 fucosyltransferases (α-1,3/1,4 FuTs), and metastatic potential after the treatment of 2 differentiation inducers, all- trans retinoic acid (ATRA), 8-bromo-cyclic 3′,5′adenosine monophosphate (8-Br-cAMP); and 2 proliferation inducers, epidermal growth factor (EGF) and phobol-12-myristate-13-acetate (PMA), on 7721 human hepatocarcinoma cell line were studied. Cell adhesion to human umbilical vein endothelial cells (HUVEC), cell migration through transwell and invasion through matrigel were selected as the indexes of metastatic potential-related phenotypes. Using fluorescence-labelled antibodies and flow-cytometric analysis, it was found that 7721 cells mainly expressed sialyl Lewis X (SLex) and a less amount of sialyl dimeric Lewis X (SDLex) antigens on the cell surface. Their expressions were down-regulated by ATRA, and up-regulated by EGF. SLexantigen was also decreased and increased by the treatment of 8-Br-cAMP and PMA respectively. With Northern blot to detect the mRNAs of α-1,3/1,4 FuTs, the main enzymatic basis for the change in SLexexpression was found to be the alteration of the expression of α-1,3 FuT-VII. It was evidenced by the observations that α-1,3 FuT-VII was the main α-1,3/1,4 FuT in 7721 cells, while α-1,3/1,4 FuT-III and α-1,3 FuT-VI were expressed rather low. The changes in the expressions of SLexantigen and α-1,3 FuT-VII resulted in the altered cell adhesion to tumour necrosis factor-α stimulated HUVEC, since only the monoclonal antibody of the SLex, but not other monoclonal antibodies blocked the adhesion of 7721 cells to HUVEC. The migration and invasion of 7721 cells were also reduced by the treatment of ATRA or 8-Br-cAMP, and elevated by EGF or PMA. The above findings indicate that the metastatic potential of 7721 cells is suppressed by differentiation-inducers and promoted by proliferation-inducers. © 2001 Cancer Research Campaign http://www.bjcancer.co
Dynamical Axion Field in Topological Magnetic Insulators
Axions are very light, very weakly interacting particles postulated more than
30 years ago in the context of the Standard Model of particle physics. Their
existence could explain the missing dark matter of the universe. However,
despite intensive searches, they have yet to be detected. In this work, we show
that magnetic fluctuations of topological insulators couple to the
electromagnetic fields exactly like the axions, and propose several experiments
to detect this dynamical axion field. In particular, we show that the axion
coupling enables a nonlinear modulation of the electromagnetic field, leading
to attenuated total reflection. We propose a novel optical modulators device
based on this principle.Comment: 5 pages, 3 figure
Tuning Jeff = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridate Sr3Ir2O7
Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting
between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a
metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+
for Sr2+) or application of high pressure up to 35 GPa. Our study of
single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high
hydrostatic pressure P leads to a drastic reduction in the electrical
resistivity by as much as six orders of magnitude at a critical pressure, PC =
13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35
GPa produces no fully metallic state at low temperatures, possibly as a
consequence of localization due to a narrow distribution of bonding angles
{\theta}. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7
readily induces a robust metallic state in the resistivity at low temperatures;
the magnetic ordering temperature is significantly suppressed but remains
finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results
are discussed along with comparisons drawn with Sr2IrO4, a prototype of the
Jeff = 1/2 insulator.Comment: five figure
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