K. Enomoto

Sample size dependence of excess resistance near critical field in mesoscopic superconducting Al disk

We report the size dependence of superconducting transition in mesoscopic Al disks with 1.0, 0.7, and 0.4? m in diameter performed by resistance measurements under magnetic field. All the samples show excess resistance peaks near Hc2, which are 3 ~ 6 times larger than normal state resistance Rn, whereas there are no resistance peaks at H = 0. We find that the resistance peak rapidly increases and becomes the largest at the magnetic field just below the transition of vorticity L : 0 ? 1 with no relation to the sample size. The resistance peak, which shows the presence of large energy dissipation, would be caused by characteristic vortex dynamics confined in the small geometries.

UGe2: Low-temperature resistivity measurements in a wide range of magnetic field and pressure

The ground state of the ring-shape magnetic nanoparticle is studied. Depending on the geometrical and magnetic parameters of the nanoring, there exist different magnetisation configurations (magnetic phases): two phases with homogeneous magnetisation (easy-axis and easy-plane phases) and two inhomogeneous (planar vortex phase and out-of-plane one). The existence of a new intermediate out-ofplane vortex phase, where the inner magnetisation is not strongly parallel to the easy axis, is predicted. Possible transitions between different phases are analysed using the combination of analytical calculations and micromagnetic simulations. r 2006 Elsevier B.V. All rights reserved. PACS: 75.75.+a; 75.40.Mg; 75.30.Kz

I‐V Characteristics in the Superconducting State of a Mesoscopic Al Square

We have measured the I–V characteristics in a mesoscopic Al square at low temperatures. The resistance at sufficiently low current is zero in magnetic field H < 250 G for T = 900 mK. At H < 250 G, the current Ic above which a finite voltage appears periodically has minima with increasing magnetic field. These minima are ascribed to the transitions between different vortex states in the sample. The resistance V/I exceeds the normal state value RN at |I|< Ic for H < 250 G. The experimental results suggest that the field dependence of Ic as well as the resistance larger than RN is attributed to the dynamics of the vortices in the superconducting Al square.

Fermi Surface and Electronic Properties of κ‐(BETS)2FeCl4

Shubnikov‐de Haas (SdH) and angular dependent magnetoresistance oscillations are measured in the organic conductor κ‐(BETS)2FeCl4. This salt has a quasi‐two dimensional Fermi surface, which is almost consistent with the band structure calculation. The SdH oscillations suggest the presence of a large internal field of 10.6 T. The ratio of the interlayer to intralayer transfer integral is estimated as 1/146 from the width of the coherence peak.

Possibility of FFLO State in Organic Superconductor λ‐(BETS)2FeCl4

A magnetic two dimensional organic conductor λ‐(BETS)2FeCl4 has a superconducting phase only under high magnetic fields between 18T and 42 T. This field induced superconductivity is qualitatively understood by Jaccarino‐Peter effect, and an inhomogeneous superconducting phase, the so‐called FFLO phase, whose order parameter oscillates in space, is expected to be stabilized. Under high magnetic fields parallel to the conducting layers, characteristic features in the interlayer resistance are observed, suggesting that the Josephson vortices are strongly pinned at some fields. The results are explained in terms of a commensurability effect between the vortex lattice and the oscillating order parameter.