Persistence of metastable vortex lattice domains in MgB2 in the presence of vortex motion.

Recently, extensive vortex lattice metastability was reported in MgB2 in connection with a second-order rotational phase transition. However, the mechanism responsible for these well-ordered metastable vortex lattice phases is not well understood. Using small-angle neutron scattering, we studied the vortex lattice in MgB2 as it was driven from a metastable to the ground state through a series of small changes in the applied magnetic field. Our results show that metastable vortex lattice domains persist in the presence of substantial vortex motion and directly demonstrate that the metastability is not due to vortex pinning. Instead, we propose that it is due to the jamming of counterrotated vortex lattice domains which prevents a rotation to the ground state orientation.

Observation of well-ordered metastable vortex lattice phases in superconducting MgB2 using small-angle neutron scattering.

The vortex lattice (VL) symmetry and orientation in clean type-II superconductors depends sensitively on the host material anisotropy, vortex density and temperature, frequently leading to rich phase diagrams. Typically, a well-ordered VL is taken to imply a ground-state configuration for the vortex-vortex interaction. Using neutron scattering we studied the VL in MgB(2) for a number of field-temperature histories, discovering an unprecedented degree of metastability in connection with a known, second-order rotation transition. This allows, for the first time, structural studies of a well-ordered, nonequilibrium VL. While the mechanism responsible for the longevity of the metastable states is not resolved, we speculate it is due to a jamming of VL domains, preventing a rotation to the ground-state orientation.

Production of ferritin by rat hepatoma cells in vitro. Demonstration of protein subunits and ferritin by immunofluorescence.

Using precipitating antibodies to ACI rat liver ferritin and to sodium-dodecyl-sulfate-dissociated protein subunits of ACI rat liver ferritin, we have demonstrated the presence of ferritin-positive sites and subunit-positive sites in situ in several rat hepatoma cell lines by immunofluorescence. Hepatoma cells from three transplantable rat hepatomas (Reuber H-139, Reuber H-35, and Morris 5123) were explanted and propagated. Rabbit antibodies specific for either protein subunits of ferritin or ferritin were prepared by affinity chromatography or by dissociation of antibody-antigen complexes with 0.1 M acetic acid followed by differential ultracentrifugation. Explants of Reuber H-139, Reuber H-35, and Morris 5123 hepatoma cells, grown either in ordinary McCoy's 5a medium or in such medium enriched with iron (0.002% Fe), gave positive immunofluorescence for subunits as well as ferritin. Exposure of a clonal strain of Morris 5123 hepatoma cells to iron-enriched culture medium for varying lengths of time of up to 24 hours resulted in progressive increase in the quantity of ferritin-specific immunofluorescent cytoplasmic material, which was at first present diffusely, and later in clumps. By contrast, during the initial 24-hour period, subunit-specific immunofluorescence remained at relatively low intensity, with diffuse distribution through the cytoplasma. Our findings indicate a) the presence, in the cytoplasm, of the three kinds of hepatoma cells, of unassembled or only partly assembled subunits of fragments of subunits as well as of ferritin, and b) rapid assembly of the protein subunits into apoferritin and ferritin after administration of iron, so that the concentration of subunits in the cytoplasm was not significantly increased.

Detection of protein subunits of ferritin in situ in cells by immunofluorescence.

Antisera raised in rabbits against protein subunits of rat liver ferritin contain antibodies (IgG) that precipitate subunits and other antibodies (IgG) that precipitate undissociated ferritin and apoferritin. Removal of the latter antibodies from IgG fractions of immune sera by repeated addition of ferritin leaves antibodies that specifically precipitate subunits. Using these subunit-specific antibodies, we have demonstrated the presence of subunit-positive sites in cells by immunofluorescence. Subunit-positive sites were best demonstrated in rats that had been loaded with iron. These sites were diffusely spread through the cytoplasm of hepatocytes and Kupffer cells in rat livers and of macrophages and fibroblasts in rat livers and hearts. With antibodies to ferritin, the presence of ferritin was demonstrated in the same kinds of cells. Although, by immunofluorescence, the intracellular localization of ferritin-positive and subunit-positive sites was similar, ferritin-positive immunofluorescence was most intense in Prussian blue-positive cytoplasmic granules in which high concentrations of ferritin were presumed to be present on the basis of electron microscopic studies. Our findings can be interpreted to indicate either the presence of unassociated subunits or of fragments of subunits in the cytoplasm of the several kinds of cells studied. It is also possible that partly assembled molecules of ferritin or apoferritin in these cells reacted with subunit-specific antibodies. Subunit-specific antibodies may be useful for localizing sites of assembly of ferritin molecules at the level of cell fine structure and for elucidation of the biosynthesis of ferritin.