Showing papers by "David K. Christen published in 1981"

Critical fields H c and H c 2 of superconducting niobium

Abstract: New measurements of the thermodynamic critical field ${H}_{c}$ and the upper critical field ${H}_{c2}$ of a niobium crystal are interpreted by using realistic microscopic theory. The theory of ${H}_{c2}$ combined with band theory allows the ${H}_{c2}$ measurements to be compared with de Haas---van Alphen measurements. Eliashberg theory allows the ${H}_{c}$ measurements to be compared with other experiments that are sensitive to the electron-phonon interaction. Our measurements of ${H}_{c2}$ confirm the band theory of niobium as adjusted to match de Haas---van Alphen data. Our measurements of ${H}_{c}$ enable us to deduce the value of the electron-phonon-coupling parameter $\ensuremath{\lambda}$. Finally, we note a discrepancy between the value of the renormalization parameter $1+\ensuremath{\lambda}$ that is needed to match the ${H}_{c2}$ data with the band theory and the value of $1+\ensuremath{\lambda}$ that is required to give the observed ${H}_{c}$.

Observation of the flux-line lattice in superconducting V3Si

Abstract: A small-angle neutron-diffraction technique has been used to obtain the first direct observations of a well-defined flux-line lattice (FLL) in the mixed state of a single-crystal sample of the A-15 compound V 3 Si. From the measurements made at 4.85 K, the form factor of the FLL was deduced, and found to be nearly London-like. For a scattering geometry employing the neutron beam parallel to the flux lines, the sample rocking-curve widths reveal that there is no significant correlated bending of the flux lines greater than ± 0.3° from the mean direction of their axes. With the neutron beam orthogonal to the flux lines, the FLL morphology was determined, and found to be strongly correlated with the symmetry properties of the crystal axis parallel to the applied magnetic field.