M. Aprili

Spontaneous Supercurrent Induced by Ferromagnetic π 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.

High-quality planar high-Tc Josephson junctions

Reproducible high-Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge (1 to 5μm wide) is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7−δ film through a gold mask such as the nonprotected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower fluence through a narrow slit (20 nm) opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. This process can be used to produce complex Josephson circuits.

Spin imbalance and spin-charge separation in a mesoscopic superconductor

Injection of spin-polarized electrons into a superconductor leads to both spin and charge imbalance. If charge relaxation occurs faster than spin relaxation, it is possible to observe excess spin at almost no extra charge.

Using ion irradiation to make high-Tc Josephson junctions

In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.

High Tc superconducting quantum interference devices made by ion irradiation

The authors describe a method to make superconducting quantum interference devices (SQUIDs) formed in a single layer of high Tc superconducting materials. The superconducting loop is patterned using ion beam irradiation through an in situ suitable gold mask. Josephson junctions are defined by a lower fluence irradiation through narrow slits opened in a polymethyl methacrylate resist. The critical current and the resistance of the SQUIDs at a given temperature can be adjusted precisely by choosing the fluence of irradiation to make the device operational at temperatures between 4.2K and the Tc of the superconducting material.

Ferromagnetic 0-pi junctions as classical spins.

The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson junctions shows a spontaneous half quantum vortex, sustained by a supercurrent of undetermined sign. This supercurrent flows in the electrode of a Josephson junction used as a detector and produces a phi(0)/4 shift in its magnetic diffraction pattern. We have measured the statistics of the positive or the negative sign shift occurring at the superconducting transition of such a junction. The randomness of the shift sign, the reproducibility of its magnitude, and the possibility of achieving exact flux compensation upon field cooling are the features which show that 0-pi junctions behave as classical spins, just as magnetic nanoparticles with uniaxial anisotropy.

Direct dynamical coupling of spin modes and singlet Josephson supercurrent in ferromagnetic Josephson junctions

I. Petković, M. Aprili, S. E. Barnes, F. Beuneu, and S. Maekawa 1 Laboratoire de Physique des Solides, Université Paris-Sud, CNRS, UMR 8502, 91405 Orsay, France. 2 Theory of Condensed Matter Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, United Kingdom. Physics Department, University of Miami, Coral Gables, FL 33124, USA. Laboratoire des Solides Irradiés, CNRS, École Polytechnique, F-91128 Palaiseau, France. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. CREST, Japan Science and Technology Agency, Sanbancho, Tokyo 102-0075, Japan.∗ (Dated: April 11, 2009)

Scanning thermal imaging of an electrically excited aluminum microstripe

We study the Joule heating of a 1.25 μm wide aluminum microstripe excited by an electrical current. The temperature changes are measured with a scanning thermal microscope that uses a small fluorescent particle as a sensor. The lateral resolution observed for this sample is better than 300 nm. We have compared the temperature distribution in the stripe with a simple analytical model of heat propagation in the wire and the substrate. A good qualitative agreement is observed, although the measured temperature is much smaller than the estimated one, showing that the heat transfer between the hot wire and the fluorescent probe is not fully efficient.

Subharmonic gap structures and Josephson effect in MgB2 /Nb microconstrictions

Superconducting micro-constrictions between Nb tips and high quality MgB

Properties of thin and ultra-thin YBCO films grown by a Co-evaporation technique

_{2}