Enhanced current flow through meandering and tilted grain boundaries in YBCO films

Grain boundaries (GBs) have been shown to limit critical current density, Jc, in YBa2Cu3O7 (YBCO) coated conductors. Here we use transport measurements and scanning Hall probe microscopy coupled with current reconstruction to demonstrate that GB geometry, such as the in-plane meandering observed in films grown by metalorganic deposition (MOD) on rolling assisted biaxially textured substrate (RABiTS), can lead to higher GB Jc. We observe current-induced flux entry into such a coated conductor, then model its behavior by imaging films with single, straight GBs tilted at various angles to the applied current.Comment: 3 pages, 3 figures. For submission to Applied Physics Letters. Movies and higher resolution figures at http://www.stanford.edu/group/moler/rdinner

Evolution of Directed Triangle Motifs in the Google+ OSN

Motifs are a fundamental building block and distinguishing feature of networks. While characteristic motif distribution have been found in many networks, very little is known today about the evolution of network motifs. This paper studies the most important motifs in social networks, triangles, and how directed triangle motifs change over time. Our chosen subject is one of the largest Online Social Networks, Google+. Google+ has two distinguishing features that make it particularly interesting: (1) it is a directed network, which yields a rich set of triangle motifs, and (2) it is a young and fast evolving network, whose role in the OSN space is still not fully understood. For the purpose of this study, we crawled the network over a time period of six weeks, collecting several snapshots. We find that some triangle types display significant dynamics, e.g., for some specific initial types, up to 20% of the instances evolve to other types. Due to the fast growth of the OSN in the observed time period, many new triangles emerge. We also observe that many triangles evolve into less-connected motifs (with less edges), suggesting that growth also comes with pruning. We complement the topological study by also considering publicly available user profile data (mostly geographic locations). The corresponding results shed some light on the semantics of the triangle motifs. Indeed, we find that users in more symmetric triangle motifs live closer together, indicating more personal relationships. In contrast, asymmetric links in motifs often point to faraway users with a high in-degree (celebrities)

η\eta and η′\eta' mesons in the Dyson-Schwinger approach at finite temperature

We study the temperature dependence of the pseudoscalar meson properties in a relativistic bound-state approach exhibiting the chiral behavior mandated by QCD. Concretely, we adopt the Dyson-Schwinger approach with a rank-2 separable model interaction. After extending the model to the strange sector and fixing its parameters at zero temperature, T=0, we study the T-dependence of the masses and decay constants of all ground-state mesons in the pseudoscalar nonet. Of chief interest are $\eta$ and $\eta^\prime$. The influence of the QCD axial anomaly on them is successfully obtained through the Witten-Veneziano relation at T=0. The same approach is then extended to T>0, using lattice QCD results for the topological susceptibility. The most conspicuous finding is an increase of the $\eta^\prime$ mass around the chiral restoration temperature $T_{\rm Ch}$, which would suggest a suppression of $\eta^\prime$ production in relativistic heavy-ion collisions. The increase of the $\eta^\prime$ mass may also indicate that the extension of the Witten-Veneziano relation to finite temperatures becomes unreliable around and above $T_{\rm Ch}$. Possibilities of an improved treatment are discussed.Comment: 13 pages, 15 figure

Exclusive channels in semi-inclusive production of pions and kaons

We investigate the role of exclusive channels in semi-inclusive electroproduction of pions and kaons. Using the QCD factorization theorem for hard exclusive processes we evaluate the cross sections for exclusive pseudoscalar and vector meson production in terms of generalized parton distributions and meson distribution amplitudes. We investigate the uncertainties arising from the modeling of the nonperturbative input quantities. Combining these results with available experimental data, we compare the cross sections for exclusive channels to that obtained from quark fragmentation in semi-inclusive deep inelastic scattering. We find that rho^0 production is the only exclusive channel with significant contributions to semi-inclusive pion production at large z and moderate Q^2. The corresponding contribution to kaon production from the decay of exclusively produced phi and K^* is rather small.Comment: 33 pages, 18 figure

Dimensional structural constants from chiral and conformal bosonization of QCD

We derive the dimensional non-perturbative part of the QCD effective action for scalar and pseudoscalar meson fields by means of chiral and conformal bosonization. The related structural coupling constants L_5 and L_8 of the chiral lagrangian are estimated using general relations which are valid in a variety of chiral bosonization models without explicit reference to model parameters. The asymptotics for large scalar fields in QCD is elaborated, and model-independent constraints on dimensional coupling constants of the effective meson lagrangian are evaluated. We determine also the interaction between scalar quarkonium and the gluon density and obtain the scalar glueball-quarkonium potential.Comment: 21 pages, LaTe

Time transfer through optical fibers over a distance of 73 km with an uncertainty below 100 ps

We demonstrate the capability of accurate time transfer using optical fibers over long distances utilizing a dark fiber and hardware which is usually employed in two-way satellite time and frequency transfer (TWSTFT). Our time transfer through optical fiber (TTTOF) system is a variant of the standard TWSTFT by employing an optical fiber in the transmission path instead of free-space transmission of signals between two ground stations through geostationary satellites. As we use a dark fiber there are practically no limitations to the bandwidth of the transmitted signals so that we can use the highest chip-rate of the binary phase-shift modulation available from the commercial equipment. This leads to an enhanced precision compared to satellite time transfer where the occupied bandwidth is limited for cost reasons. The TTTOF system has been characterized and calibrated in a common clock experiment at PTB, and the combined calibration uncertainty is estimated as 74 ps. In a second step the remote part of the system was operated at Leibniz Universitaet Hannover, Institut fuer Quantenoptik (IQ) separated by 73 km from PTB in Braunschweig. In parallel, a GPS time transfer link between Braunschweig and Hannover was established, and both links connected a passive hydrogen maser at IQ with the reference time scale UTC(PTB) maintained in PTB. The results obtained with both links agree within the 1-sigma uncertainty of the GPS link results, which is estimated as 0.72 ns. The fiber link exhibits a nearly 10-fold improved stability compared to the GPS link, and assessment of its performance has been limited by the properties of the passive maser

eta and eta' mesons and dimension 2 gluon condensate <A^2>

The study of light pseudoscalar quark-antiquark bound states in the Dyson-Schwinger approach with the effective QCD coupling enhanced by the interplay of the dimension 2 gluon condensate and dimension 4 gluon condensate , is extended to the eta-eta' complex. We include the effects of the gluon axial anomaly into the Dyson-Schwinger approach to mesons. The calculated masses, mixing and two-photon decay widths of eta and eta' mesons are in agreement with experiment. Also, in a model-independent way, we give the modification of the Gell-Mann--Okubo and Schwinger nonet relations due to the interplay of the gluon anomaly and SU(3) flavor symmetry breaking.Comment: 15 pages, 2 tables, 1 eps figure, revtex

Sequential Flavour Symmetry Breaking

The gauge sector of the Standard Model (SM) exhibits a flavour symmetry which allows for independent unitary transformations of the fermion multiplets. In the SM the flavour symmetry is broken by the Yukawa couplings to the Higgs boson, and the resulting fermion masses and mixing angles show a pronounced hierarchy. In this work we connect the observed hierarchy to a sequence of intermediate effective theories, where the flavour symmetries are broken in a step-wise fashion by vacuum expectation values of suitably constructed spurion fields. We identify the possible scenarios in the quark sector and discuss some implications of this approach.Comment: 22 pages latex, no figure

Pseudoscalar Meson Mixing in Effective Field Theory

We show that for any effective field theory of colorless meson fields, the mixing schemes of particle states and decay constants are not only related but also determined exclusively by the kinetic and mass Lagrangian densities. In the general case, these are bilinear in terms of the intrinsic fields and involve non-diagonal kinetic and mass matrices. By applying three consecutive steps this Lagrangian can be reduced into the standard quadratic form in terms of the physical fields. These steps are : (i) the diagonalization of the kinetic matrix, (ii) rescaling of the fields, and (iii) the diagonalization of the mass matrix. In case, where the dimensions of the non-diagonal kinetic and mass sub-matrices are respectively, $k\times k$ and $n\times n$, this procedure leads to mixing schemes which involve $[k(k-1)/2] + [n(n-1)/2]$ angles and $k$ field rescaling parameters. This observation holds true irrespective with the type of particle interactions presumed. The commonly used mixing schemes, correspond to a proper choice of the kinetic and mass matrices, and are derived as special cases. In particular, $\eta$-$\eta '$ mixing, requires one angle, if and only if, the kinetic term with the intrinsic fields has a quadratic form.Comment: REVTeX, 6 page

Non-leptonic two-body decays of the Bc meson in light-front quark model and QCD factorization approach

We study exclusive non-leptonic two-body $B_c\to(D_{(s)},\eta_c,B_{(s)})+F$ decays with $F$(pseudoscalar or vector meson) being factored out in QCD factorization approach. The non-leptonic decay amplitudes are related to the product of meson decay constants and the form factors for semileptonic $B_c$ decays. As inputs in obtaining the branching ratios for a large set of non-leptonic $B_c$ decays, we use the weak form factors for the semileptonic $B_c\to(D_{(s)},\eta_c,B_{(s)})$ decays in the whole kinematical region and the unmeasured meson decay constants obtained from our previous light-front quark model. We compare our results of the branching ratios with those of other theoretical studies.Comment: 11 pages, 3 figures, minor corrections, version to appear in PR