Professor Stephen Blundell

Abstract:

Muon-spin rotation (mu SR) measurements have been used to study the superconducting vortex properties of layered organic superconductors based on molecular donors such as BEDT-TTF. The mu SR is particularly sensitive to the degree of local ordering of pancake vortices and can detect when the pancake layers become decoupled by intrinsic or defect-driven decoupling mechanisms, or by thermally driven motion. Further novel features of the vortex system occur when the field is tilted away from a crystal axis. Knowledge of the vortex phase behaviour allows appropriate parameter regions to be selected for reliable determination of the superconducting penetration depth lambda and studies of the temperature dependence of lambda have shown a T-linear term at low fields that is suppressed with increasing field. Systematic studies of lambda across the range of organic superconductors have revealed a strong correlation between lambda and T-c. In contrast to the linear scaling T-c proportional to lambda(-2) seen in high T-c cuprates, the organics show an overall correlation better described as T-c proportional to lambda(-3). One interpretation is that the superconducting carriers are only a small fraction of the total carrier concentration in these low-T-c superconductors. Understanding this result may give us some important clues about the nature of the superconductivity in the organics.

Abstract:

Compositions in the solid solution Fe2-xRhxMo 3N which adopt the filled β-manganese structure have been prepared by the reductive nitridation of mixed binary metal oxides; single-phase samples can be prepared for 0.3 ≤ x ≤ 0.75. Those compositions with x ≤ 1 have been shown, by a combination of magnetometry, neutron diffraction and μSR, to be ferromagnetic at low temperatures. The Curie temperature is highest (Tc = 110 K) for x = 0.5, with a saturation magnetisation of 2.26(1) μB per formula unit. These compositions display metallic behaviour, with a change in the temperature gradient of the resistivity being observed at the Curie temperature. Fe0.5Rh1.5Mo 3N is also metallic but does not show long-range magnetic order. © The Royal Society of Chemistry 2005.

Abstract:

We report muon-spin rotation, heat capacity, magnetization, and ac magnetic susceptibility measurements of the magnetic properties of the layered spin-12 antiferromagnet NaNiO2. These show the onset of long-range magnetic order below TN=19.5K. Rapid muon depolarization, persisting from TN to about 5 K above TN, is consistent with the presence of short-range magnetic order. The temperature and frequency dependence of the ac susceptibility suggests that magnetic clusters persist above 25 K and that their volume fraction decreases with increasing temperature. A frequency dependent peak in the ac magnetic susceptibility at Tsf=3K is observed, consistent with a slowing of spin fluctuations at this temperature. A partial magnetic phase diagram is deduced. © 2005 The American Physical Society.