Ultrathin MgB2 films fabricated on Al2O3 substrate by hybrid physical-chemical vapor deposition with high Tc and Jc

Ultrathin MgB2 superconducting films with a thickness down to 7.5 nm are epitaxially grown on (0001) Al2O3 substrate by hybrid physical-chemical vapor deposition method. The films are phase-pure, oxidation-free and continuous. The 7.5 nm thin film shows a Tc(0) of 34 K, which is so far the highest Tc(0) reported in MgB2 with the same thickness. The critical current density of ultrathin MgB2 films below 10 nm is demonstrated for the first time as Jc ~ 10^6 A cm^{-2} for the above 7.5 nm sample at 16 K. Our results reveal the excellent superconducting properties of ultrathin MgB2 films with thicknesses between 7.5 and 40 nm on Al2O3 substrate.Comment: 7 pages, 4 figures, 2 table

Nonlinear Near-Field Microwave Microscope For RF Defect Localization in Superconductors

Niobium-based Superconducting Radio Frequency (SRF) cavity performance is sensitive to localized defects that give rise to quenches at high accelerating gradients. In order to identify these material defects on bulk Nb surfaces at their operating frequency and temperature, it is important to develop a new kind of wide bandwidth microwave microscopy with localized and strong RF magnetic fields. By taking advantage of write head technology widely used in the magnetic recording industry, one can obtain ~200 mT RF magnetic fields, which is on the order of the thermodynamic critical field of Nb, on submicron length scales on the surface of the superconductor. We have successfully induced the nonlinear Meissner effect via this magnetic write head probe on a variety of superconductors. This design should have a high spatial resolution and is a promising candidate to find localized defects on bulk Nb surfaces and thin film coatings of interest for accelerator applications.Comment: 4 pages, 6 figures Journal-ref: 2010 Applied Superconductivity Conferenc

Mgb2 Nonlinear Properties Investigated under Localized High RF Magnetic Field Excitation

In order to increase the accelerating gradient of Superconducting Radio Frequency (SRF) cavities, Magnesium Diboride (MgB2) opens up hope because of its high transition temperature and potential for low surface resistance in the high RF field regime. However, due to the presence of the small superconducting gap in the {\pi} band, the nonlinear response of MgB2 is potentially quite large compared to a single gap s-wave superconductor (SC) such as Nb. Understanding the mechanisms of nonlinearity coming from the two-band structure of MgB2, as well as extrinsic sources, is an urgent requirement. A localized and strong RF magnetic field, created by a magnetic write head, is integrated into our nonlinear-Meissner-effect scanning microwave microscope [1]. MgB2 films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor deposition technique on dielectric substrates, are measured at a fixed location and show a strongly temperature-dependent third harmonic response. We propose that at least two mechanisms are responsible for this nonlinear response, one of which involves vortex nucleation and penetration into the film. [1] T. M. Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, S. M. Anlage, "Nonlinear Near-Field Microwave Microscope for RF Defect Localization in Superconductors", IEEE Trans. Appl. Supercond. 21, 2615 (2011).Comment: 6 pages, 6 figure

The Ultrasensitivity of Living Polymers

Synthetic and biological living polymers are self-assembling chains whose chain length distributions (CLDs) are dynamic. We show these dynamics are ultrasensitive: even a small perturbation (e.g. temperature jump) non-linearly distorts the CLD, eliminating or massively augmenting short chains. The origin is fast relaxation of mass variables (mean chain length, monomer concentration) which perturbs CLD shape variables before these can relax via slow chain growth rate fluctuations. Viscosity relaxation predictions agree with experiments on the best-studied synthetic system, alpha-methylstyrene.Comment: 4 pages, submitted to Phys. Rev. Let

Pion condensation in quark matter with finite baryon density

The phase structure of the Nambu -- Jona-Lasinio model at zero temperature and in the presence of baryon- and isospin chemical potentials is investigated. It is shown that in the chiral limit and for a wide range of model parameters there exist two different phases with pion condensation. In the first, ordinary phase, quarks are gapped particles. In the second, gapless pion condensation phase, there is no energy cost for creating only $u$- or both $u$ and $d$ quarks, and the density of baryons is nonzero.Comment: 7 pages, 6 figures; two references adde

Source apportionment of atmospheric ammonia before, during, and after the 2014 APEC summit in Beijing using stable nitrogen isotope signatures

Stable nitrogen isotope composition (δ15N) offers new opportunities to address the long-standing and ongoing controversy regarding the origins of ambient ammonia (NH3), a vital precursor of PM2.5 (particulate matters with aerodynamic diameter equal or less than 2.5 µm) inorganic components, in the urban atmosphere. In this study, the δ15N values of NH3 samples collected from various sources were constrained using a novel and robust chemical method coupled with standard elemental analysis procedures. Independent of the wide variation in mass concentrations (ranging from 33 (vehicle) to over 6000 (human excreta) µg m−3), different NH3 sources have generally different δ15N values (ranging from −52.0 to −9.6 ‰). Significantly high δ15N values are seen as a characteristic feature of all vehicle-derived NH3 samples (−14.2 ± 2.8 ‰), which can be distinguished from other sources emitted at environmental temperature (−29.1 ± 1.7, −37.8 ± 3.6, and −50.0 ± 1.8 ‰ for livestock, waste, and fertilizer, respectively). The isotope δ15N signatures for a range of NH3 emission sources were used to evaluate the contributions of the different sources within measured ambient NH3 in Beijing, using an isotope mixing model (IsoSource). The method was used to quantify the sources of ambient NH3 before, during and after the 2014 Asia-Pacific Economic Cooperation (APEC) summit, when a set of stringent air quality control measures were implemented. Results show that the average NH3 concentrations (the overall contributions of traffic, waste, livestock, and fertilizer) during the three periods were 9.1 (20.3, 28.3, 23.6, and 27.7 %), 7.3 (8.8, 24.9, 14.3, and 52.0 %), and 12.7 (29.4, 23.6, 31.7, and 15.4 %) µg m−3, respectively, representing a 20.0 % decrease first and then a 74.5 % increase in overall NH3 mass concentrations. During (after) the summit, the contributions of traffic, waste, livestock, and fertilizer decreased (increased) by 56.7 (234.2), 12.0 (−5.0), 39.4 (120.8), and −87.7 % (−70.5 %) when compared with periods before (during) the summit, respectively, signifying that future NH3 control efforts in megacities like Beijing should prioritize traffic sector as well as livestock breeding. The results show that isotope ratio measurements of NH3 to be a valuable tool to quantify the atmospheric sources of NH3 in urban atmospheres

Kondo effect of an adatom in graphene and its scanning tunneling spectroscopy

We study the Kondo effect of a single magnetic adatom on the surface of graphene. It was shown that the unique linear dispersion relation near the Dirac points in graphene makes it more easy to form the local magnetic moment, which simply means that the Kondo resonance can be observed in a more wider parameter region than in the metallic host. The result indicates that the Kondo resonance indeed can form ranged from the Kondo regime, to the mixed valence, even to the empty orbital regime. While the Kondo resonance displays as a sharp peak in the first regime, it has a peak-dip structure and/or an anti-resonance in the remaining two regimes, which result from the Fano resonance due to the significant background leaded by dramatically broadening of the impurity level in graphene. We also study the scanning tunneling microscopy (STM) spectra of the adatom and they show obvious particle-hole asymmetry when the chemical potential is tuned by the gate voltages applied to the graphene. Finally, we explore the influence of the direct tunneling channel between the STM tip and the graphene on the Kondo resonance and find that the lineshape of the Kondo resonance is unaffected, which can be attributed to unusual large asymmetry factor in graphene. Our study indicates that the graphene is an ideal platform to study systematically the Kondo physics and these results are useful to further stimulate the relevant experimental studies on the system.Comment: 8 pages, 5 figure

Effects of the sintering atmosphere on the superconductivity of SmFeAsO1-xFx compounds

A series of SmFeAsO1-xFx samples were sintered in quartz tubes filled with air of different pressures. The effects of the sintering atmosphere on the superconductivity were systematically investigated. The SmFeAsO1-xFx system maintains a transition temperature (Tc) near 50 K until the concentration of oxygen in quartz tubes increases to a certain threshold, after which Tc decreases dramatically. Fluorine losses, whether due to vaporization, reactions with starting materials, and reactions with oxygen, proved to be detrimental to the superconductivity of this material. The deleterious effects of the oxygen in the sintering atmosphere were also discussed in detail.Comment: 9 pages, 5 figure