Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Gases Do not adsorb on the interstitial channels of closed-ended single-walled carbon nanotube bundles

We have experimentally determined the binding energies of Xe, CH4, and Ne on samples of closed-ended single-wall nanotube (SWNT) bundles. We find values for these quantities which are larger by approximately 75% on the SWNT samples than the values found for the same adsorbates on planar graphite. We have also determined the monolayer capacity of a SWNT sample using Xe and Ne adsorption. A comparison of all of our results leads us to conclude that none of the gases studied adsorb on the interstitial channels in the SWNT bundles.     

Experimental evidence of dipolar interaction in bilayer nanocomposite magnets

We use magnetic thin film hard/non/soft-magnetic trilayer systems to probe the nature of the hard–soft phase interaction and the role played by dipolar fields in one-dimensional (d) magnetic systems. We have systematically investigated six wedge samples where the thickness of a Cu spacer layer (t _Cu) was gradually changed to create a varying interfacial effect on the interaction between a CoPt hard layer and a Fe soft layer. Magneto-optical Kerr effect was used to obtain the magnetization loops at 28 points on each sample, and the nucleation field (H _N ) as a function of t _Cu was employed to characterize the layer interaction as a function of t _Cu. H _N (t _Cu) show a RKKY oscillatory behavior in addition to a non-negligible dipolar contribution, which had an exponential dependence. The dipolar term, which cannot be always neglected, is affected by the interface roughness and also by the CoPt crystallinity. Therefore, we cannot always consider exchange coupling to be the dominant interaction in one-d hard–soft magnetic bilayer systems, particularly, during magnetic reversal.