Spontaneous supercurrent induced by ferromagnetic pi-junctions

We present magnetization measurements of mesoscopic superconducting niobium loops containing a ferromagnetic (PdNi) pi-junction. The loops are prepared on top of the active area of a micro Hall-sensor based on high mobility GaAs/AlGaAs heterostructures. We observe asymmetric switching of the loop between different magnetization states when reversing the sweep direction of the magnetic field. This provides evidence for a spontaneous current induced by the intrinsic phase shift of the pi-junction. In addition, the presence of the spontaneous current near zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in half integer flux quantization in the loop at low temperatures.Comment: 4 pages, 4 figure

Superconductivity on the localization threshold and magnetic-field-tuned superconductor-insulator transition in TiN films

Temperature- and magnetic-field dependent measurements of the resistance of ultrathin superconducting TiN films are presented. The analysis of the temperature dependence of the zero field resistance indicates an underlying insulating behavior, when the contribution of Aslamasov-Larkin fluctuations is taken into account. This demonstrates the possibility of coexistence of the superconducting and insulating phases and of a direct transition from the one to the other. The scaling behavior of magnetic field data is in accordance with a superconductor-insulator transition (SIT) driven by quantum phase fluctuations in two-dimensional superconductor. The temperature dependence of the isomagnetic resistance data on the high-field side of the SIT has been analyzed and the presence of an insulating phase was confirmed. A transition from the insulating to a metallic phase is found at high magnetic fields, where the zero-temperature asymptotic value of the resistance being equal to h/e^2.Comment: 5 pages, 4 eps figures, RevTeX4, Published versio

1/f flux flow noise due to a coexistence of qualitatively different vortex states

We investigate the vortex-motion voltage noise in a hybrid structure consisting of a weak-pinning amorphous Nb0.7Ge0.3 microbridge on top of which a strong-pinning, longitudinal Nb line with a narrow interruption in the middle is added. The Nb part enforces a branching of the applied current, causing a modulation of the current density within the Nb0.7Ge0.3, where vortex motion induces a voltage. When the Nb0.7Ge0.3 is sufficiently dissipative, the modulation is strong and the vortex dynamics is spatially dependent. Under these circumstances, the distribution function for normal excitations in vortex cores varies considerably over the sample, which results in a coexistence of distinct vortex states spreading from nearly equilibrium to strongly nonequilibrium ones. This leads to a range of characteristic times for the voltage fluctuations and, consequently, to the frequency (f) dependence of the noise being of 1/f type. The noise originates in the fluctuations of the vortex-core size around the average set by the nonequilibrium effects in vortex motion

Flux-flow instabilities in amorphous Nb0.7Ge0.3 microbridges

We report measurements of the electric field vs current density [E(J)] characteristics in the mixed state of amorphous Nb0.7Ge0.3 microbridges. Close to the transition temperature Tc the Larkin-Ovchinnikov theory of nonlinear flux flow and the related instability describes the data quantitatively up to ${\approx}$70% of the upper critical magnetic field Bc2 and over a wide electric-field range. At lower temperatures the nonlinearities of E(J) can be described by electron heating which reduces Bc2 and leads to a second type of flux flow instability, as shown by a scaling analysis of the high-dissipation data

Effect of Microwaves on the Current-Phase Relation of Superconductor-Normal-Metal-Superconductor Josephson Junctions

We investigate the current-phase relation (CPR) of long diffusive superconductor–normal-metal–superconductor (Nb/Ag/Nb) Josephson junctions in thermodynamic equilibrium and under microwave irradiation. While in equilibrium good agreement with the predictions of quasiclassical theory is found, we observe that the shape of the CPR can be strongly affected by microwave irradiation. Close to a Josephson-phase difference phi~=pi, the supercurrent can be strongly suppressed when increasing the rf power. Our results can be understood in terms of microwave excitation of low-lying Andreev bound states across the minigap in the junction. In the frequency interval studied, this mechanism becomes important, when the minigap closes at phi~=pi

Hall-magnetometry on ferromagnetic micro-rhombs

Micropatterned Hall-sensors are used to measure the magnetic switching of nanopatterned rhombic Ni-elements at temperatures below 1.2 K. The length of the short axis (�waist�) was systematically varied between 700 and 100 nm while the length of 2 ìm of the long axis was kept fixed. The thickness of the Ni-rhombs was 70 nm. Whereas the hysteresis loops of the broader rhombs show many small steps, which seem to be connected to domain wall movement and pinning, the narrowest two only reveal one single step during magnetization reversal. The process of magnetization reversal is additionally studied by Lorentz transmission electron microscopy. Placing Hall-sensors on both ends of acicular Ni elements allows one to investigate the switching process on both sides simultaneousl