J. Aarts

Coupling of two superconductors through a ferromagnet: evidence for a pi junction

We report measurements of the temperature dependence of the critical current, I(c), in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature I(c) generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which I(c) rises again. Such a sharp cusp can be explained only by assuming that the junction changes from a 0-phase state at high temperatures to a pi phase state at low temperatures.

Thermally assisted flux flow at small driving forces

The theory for thermally assisted flux flow (TAFF) in the limit of small driving forces is used to derive exact expressions for the time-dependent behaviour of the magnetisation and permeability. The problem is especially relevant for the high-temperature superconductors where large variations of the transition temperature in small DC fields are observed as a function of the frequency of the probing AC field. The parameters of the theory are extensively discussed in relation to the present understanding of flux pinning.

Collective and plastic vortex motion in superconductors at high flux densities

The ‘mixed state’ of type II superconductors occurs when magnetic flux penetrates the material (in the form of vortices) without destroying the superconducting ground state. Zero resistivity is retained if the vortices are pinned by crystalline defects, but is destroyed by vortex motion. This provides the practical motivation for studying vortices in random pinning potentials. But theinsights so obtained also bear on the more general class of problems involving the dynamics of elastic media in the presence of quenched disorder (for example, mechanical friction). Moreover, the magnetic vortex system is highly tunable and permits questions concerning frictional, plastic and elastic flow to be investigated on the scale of single vortices. Remarkable results have been obtained on the dynamics of this system, but have been largely restricted to well separated vortices at very low flux densities. Scanning tunnelling microscopy has the potential to resolve individual vortices at much higher flux densities, and here we show that the imaging rates can be sufficiently high toresolve the dynamics in this flux regime. We find that, in thepresence of strongly pinning line defects, the vortex lattice remains pinned until the number of vortices is about twice that ofthe defects, at which point plastic creep commences. But in thepresence of weak intrinsic point disorder, the vortices creep coherently along one of the principle axes of the vortex lattice, where they exhibit striking and unanticipated velocity modulations that appear to be related to the lattice periodicity.

Disorder effects in epitaxial thin films of (La,Ca)MnO3

We have investigated as-grown sputtered films of La0.7Ca0.3MnO3 in a thickness range between 5 and 200 nm on SrTiO3 substrates. The films are epitaxial, strained, and smooth. All films order magnetically around 175 K. Very thin films show full magnetization at low temperatures, but the temperature of the metal–insulator transition is appreciably lower than the magnetic ordering temperature. In thick films, the magnetization is much lower than expected. Both effects are probably related to structural disorder as found by transmission electron microscopy.

Superconductivity in CeCu2Si2 (invited)

The specific heat, dc magnetization, and resistivity have been measured on a number of samples of CeCu2Si2, which was previously found to assume a novel superconducting state below 0.5 K [F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979)]. By comparing the transition temperatures of samples which have undergone different heat treatments, we demonstrate that small fractions of spurious phases hinder the onset of superconductivity but are gradually removed with annealing. With a very pure sample obtained in this way, we have fully confirmed all the characteristics of the superconducting phase transition reported before on less pure samples. However, even after this heat treatment, the samples show clear signs of striking inhomogeneities, manifested by a static Meissner effect of bulk samples representing typically only a few percent of the volume. An increase of this Meissner signal up to 60 vol% is observed after powdering the samples. In addition to measurements in the superconducting state, the intrin...

Observations on intensity oscillations in reflection high-energy electron diffraction during epitaxial growth of Si(001) and Ge(001)

Intensity oscillations have been found in the specular beam of reflection high‐energy electron diffraction patterns during growth of Si(001) and Ge(001) by molecular beam epitaxy. The reported results demonstrate the dependence of the amplitude and damping of the oscillations on different parameters such as substrate temperature, electron beam angle of incidence, and azimuth.

Enhancement of the superconducting transition temperature in Nb/permalloy bilayers by controlling the domain state of the ferromagnet.

In (S/F) hybrids the suppression of superconductivity by the exchange field h(ex) of the ferromagnet can be partially lifted when different directions of h(ex) are sampled simultaneously by the Cooper pair. In F/S/F trilayers where the magnetization directions of the F layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F layer yield a similar effect. We study the transport properties of Ni(0.80)Fe(0.20)/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples.

Inverse spin switch effects in ferromagnet-superconductor-ferromagnet trilayers with strong ferromagnets

In ferromagnet-superconductor-ferromagnet

Gapless superconductivity and variation of Tc in the heavy-fermion system CeCu2Si2☆

(F∕S∕F)

Interface-Induced Room-Temperature Ferromagnetism in Hydrogenated Epitaxial Graphene

trilayers where the magnetization directions of the