Significantly enhanced critical current densities in MgB2 tapes made by a scaleable, nano-carbon addition route

Nanocarbon-doped Fe-sheathed MgB2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, Jc for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport Jc for the 5 at% doped tapes reached 1.85x104 A/cm2 at 10 T and 2.8x103 A/cm2 at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intra-granular dislocations and the dispersed nanoparticles embedded within MgB2 grains induced by the C doping. The mechanism for the enhancement of flux pinning is also discussed. These results indicate that powder-in-tube-processed MgB2 tape is very promising for high-field applications.Comment: 13 pages, 5 figures. to be published soo

Systematic Determination of Absolute Absorption Cross-section of Individual Carbon Nanotubes

Determination of optical absorption cross-section is always among the central importance of understanding a material. However its realization on individual nanostructures, such as carbon nanotubes, is experimentally challenging due to the small extinction signal using conventional transmission measurements. Here we develop a technique based on polarization manipulation to enhance the sensitivity of single-nanotube absorption spectroscopy by two-orders of magnitude. We systematically determine absorption cross-section over broad spectral range at single-tube level for more than 50 chirality-defined single-walled nanotubes. Our data reveals chirality-dependent one-dimensional photo-physics through the behaviours of exciton oscillator strength and lifetime. We also establish an empirical formula to predict absorption spectrum of any nanotube, which provides the foundation to determine quantum efficiencies in important photoluminescence and photovoltaic processes