L. F. Cohen

Vortex dynamics in superconducting MgB2 and prospects for applications.

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are ‘pinned’ in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the ‘persistent’ supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.

High critical current density and improved irreversibility field in bulk MgB2 made by a scaleable, nanoparticle addition route

Bulk samples of MgB2 were prepared with 5, 10, and 15 wt % Y2O3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ∼3–5 nm YB4 nanoparticles embedded within MgB2 grains of ∼400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density JC was observed, most notably for 10% doping. At 4.2 K, the lower bound JC value was ∼2×105 A cm−2 at 2 T. At 20 K, the corresponding value was ∼8×104 A cm−2. Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K.

Magnetic relaxation phenomena and cluster glass properties of La 0.7 − x Y x Ca 0.3 MnO 3 manganites

The dynamic magnetic properties of the distorted perovskite system

Spray-Deposited Li-Doped ZnO Transistors with Electron Mobility Exceeding 50 cm(2)/Vs

{\mathrm{La}}_{0.7\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}

Specific heat and magnetic order in LaMnO/sub 3+[delta]/

(0l~xl~0.15)

SERS platforms for high density DNA arrays

have been investigated by ac susceptibility and dc magnetization measurements, including relaxation and aging studies. All investigated samples display a metal-insulator transition. As yttrium is added in the compounds the overall results show evidence for the gradual appearance of a cluster glass behavior. For the

Thin-film alternating current nanocalorimeter for low temperatures and high magnetic fields

x=0.15

Influence of strain and microstructure on magnetotransport in La0.7Ca0.3MnO3 thin films

sample, magnetization measurements as a function of time at various temperatures show that the magnetic relaxation is maximum at a given temperature, well below the ferromagnetic transition. This maximum coincides in temperature with a frequency-dependent feature in the imaginary part of the ac susceptibility, associated with a freezing process. This is interpreted as due to ferromagnetic clusters, which grow with decreasing temperature down to a temperature at which they freeze due to severe intercluster frustration.

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability.

Ambient spray pyrolysis is used for the deposition of high quality polycrystalline ZnO films utilizing blends of precursor solutions based on Zinc and Lithium acetates and the demonstration of n-channel thin-film transistors with electron mobility exceeding 50 cm(2)/Vs (see figure).