Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Graphene: Emerging matter in two dimensions

We briefly review recent advances in graphene science and technology in the context of the contributions to this special issue.     

The Performance of an Electrorheological Fluid in Dynamic Squeeze Flow: The Influence of Solid Phase Size

This paper is concerned with an experimental investigation of a comparison of the rheological performance of an ER fluid, consisting of a carrier with a range of solid phase sizes, in oscillatory squeeze-flow under DC conditions. The fluid is sandwiched between two parallel plane electrodes, the upper one stationary and the lower one oscillating normal to its plane. It is seen that the dynamic performance of the fluid, in terms of the capacity for the transmission of imposed forces across the fluid, is highly dependent on the size of the dispersed solid phase and has an optimum value which is dependent on the mean value of the interelectrode gap. In addition the paper includes some measurements of the current through the fluid which may help to shed some further light on the mechanism of the ER effect. Finally the implications of the results to vibration control are discussed. Copyright 1999 Academic Press.     

Fine structure in magnetization of individual fluxoid states

Each time a vortex enters or exits a small superconductor, a different fluxoid state develops which can be characterized by its vorticity, i.e., the number of fluxoids inside. We have studied magnetization response of such individual states and found clear signatures of first and second order transitions within the states, which reveal the existence of distinct vortex phases for a fixed number of fluxoids. We attribute the transitions to the merger of individual vortices into a single giant vortex and switching between different arrays of vortices.     

Giant intrinsic carrier mobilities in graphene and its bilayer

We have studied temperature dependences of electron transport in graphene and its bilayer and found extremely low electron-phonon scattering rates that set the fundamental limit on possible charge carrier mobilities at room temperature. Our measurements show that mobilities higher than 200 000 cm2/V s are achievable, if extrinsic disorder is eliminated. A sharp (thresholdlike) increase in resistivity observed above approximately 200 K is unexpected but can qualitatively be understood within a model of a rippled graphene sheet in which scattering occurs on intraripple flexural phonons.     

Magnetisation of isolated single crystalline Fe-nanoparticles measured by a ballistic Hall micro-magnetometer

We present here the first magnetisation measurements on isolated single crystalline Fe-nanoparticles performed with a ballistic Hall micro-magnetometer. The measurements have a sensitivity of and thus provide us the possibility to study the mechanisms of magnetisation reversal in a single nanoparticle. The magnetic properties of the nanoparticles are influenced by their crystal structure and shape, and the presence of an oxide surface layer. They exhibit curling of the magnetic moments, but also a novel hysteresis behaviour. The spin configurations found for the system agree well with numerical calculations based on a Heisenberg Hamiltonian including the exchange and dipole interaction and surface anisotropy. Received: 1st September 1998 and Received in final form 21 June 1999     

Ultrasonic study of molecular interaction in binary liquid mixtures at 30°C

Densities ρ and ultrasonic speeds u of the binary mixtures of tetrahydrofuran (THF) with 1-butanol and tert-butanol, at 30°C, over the entire composition range were measured. From these data isentropic compressibility, K _s, intermolecular free length L _f, relative association R _A, acoustic impedance Z, molar sound speed R _m, deviations in isentropic compressibility ΔK _s, and excess volume V ^E were calculated. The variation of these parameters with composition of the mixture helps us in understanding the nature and extent of interaction between unlike molecules in the mixtures. Further, theoretical values of ultrasonic speed were evaluated using theories and empirical relations. The relative merits of these theories and relations were discussed.