M. Baert

Flux phases and quantized pinning force in superconductor with a periodic lattice of pinning centres.

Flux phases have been studied in superconducting Pb/Ge multilayers with a square lattice of submicrometre holes. Sharp peaks in the magnetic-field dependence of the critical current have been observed at integer Hm and rational Hk/l matching fields. At integer fields an artificial flux line crystal is formed, consisting of a stable square lattice of single or multiple flux quanta trapped by the holes. The existence of such a crystal leads to the quantization of the pinning force. New rational flux lines patterns at H = Hk/l, qualitatively different from those previously seen in superconducting networks, have been identified.

Matching effects in Pb/Ge multilayers with the lattice of submicron holes

Abstract The matching effects and the enhancement of the critical current density j c have been studied in superconducting Pb/Ge multilayers with a square lattice of submicrometer holes. The sharp peaks in the j c vs. field curve at the integer and rational matching fields have been observed. The existence of multi-quanta vortices confined by the holes has been confirmed.

Stable vortex configurations in superconducting 2×2 antidot clusters

The vortex state of superconducting 2×2 antidot clusters has been studied by transport measurements. Characteristic oscillations have been observed in the magnetoresistance and superconducting-normal phase boundary at specific values of the magnetic flux Φ coming from the formation of stable vortex configurations. In analogy with semiconducting 2×2 quantum dot systems, a bistable diagonal state, perspective for single flux quantum logic applications, has been identified at half-flux filling, Φ/Φ0=0.5, where Φ0=h/2e is the flux quantum.

Upper critical field of Pb films with an antidot lattice

Abstract We have studied the effect of lateral microstructuring on the upper critical field of superconducting Pb films with a square antidot lattice of period d, formed by square antidots with size a × a. The critical field was determined resistively by keeping the resistance at a fixed value while varying field and temperature. In low fields, the critical field curve demonstrates a cusp-like structure with cusps at integer and rational multiples of φ0/d2. At low fields, the periodicity is thus governed by the size of the lattice cell and corresponds to a superconducting wire network regime. In higher fields, the crossover from a “collective” to a “single object” behaviour is observed, characterized by oscillations with a period φ0/a2 coming from the formation of superconducting edge states around the antidots.

Quantum interference and confinement phenomena in mesoscopic superconducting systems

The superconducting field (H)-temperature (T) phase boundary has been measured in mesoscopic AI samples of different topology: lines, open and filled squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j, has been achieved. Sharp integer and rational matching peaks in the j,(H) curve are observed. The possibility of the “quantum design” of the superconducting critical parameters (H,(T) and jc(T, H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated.

Commensurate vortex lattice in superconducting Nb/Ti multilayers

Abstract The field dependence of the critical current measured on [Nb/Ti] ∗ 10 multilayers up to 0.6 T is considered. The thickness of the Nb sublayers was varied between 5 and 8 nm. The titanium sublayers were 5 nm thick. When the multilayers are placed in a parallel field, a non-monotonic dependence of the critical current density with two local minima is found, which is explained in terms of the commensurate-incommensurate phase transition. The fields at which the minima are found correspond to the occurence of one- or two-row commensurate vortex lattice inside the multilayer. The influence of the Lorentz force and the sharpness of the interfaces on the shape of the J c ( B ) curves is discussed.

Enhanced pinning and matching effects in superconducting films with a lattice of submicrometer holes

Abstract Magnetization studies have been carried out on single Pb films with a square lattice of submicrometer holes. These submicrometer hole patterns create the possibility to trap an integer number, as well as a certain rational number of flux lines per hole corresponding to a regular organization of flux lines inside the structure. The stable flux line configurations lead to the appearance of sharp peaks in the magnetization M ( H ⊥ ).

Relation between structural and physical properties in magnetic and superconducting superlattices

The physical properties of superlattices are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. In this paper the relationship between structure and properties will be illustrated by the giant magnetoresistance (GMR) effect, as well as the dimensional transitions and pinning mechanisms in respectively magnetic and superconducting superlattices. In the first example we will analyse the effect of the sample structure and interface quality on the GMR of polycrystalline and epitaxial Fe/Cr superlattices grown by molecular beam epitaxy. Secondly, the effect of thermal annealing and ion irradiation on the electrical and magnetic properties of Ag/Fe multilayers is discussed. In a third example, the influence of a very thin Fe interlayer on the coupling phenomena in Fe/Nb multilayers is analysed. Finally, it will be shown that an additional lateral modulation (lattice of submicron holes) in Pb/Ge multilayers substantially changes the critical superconducting parameters.

Effect of dimensional crossover on the magnetoresistance of YBa2Cu3O7/PrBa2Cu3O7 superlattices

Abstract We report on magnetoresistance measurements of YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PrBCO) superlattices in fields up to 12 T at temperatures below, but not too far from, the superconducting transition temperature Tc. By varying the thickness of the PrBCO separator layer, the influence of the interlayer coupling on the superconducting properties is studied. An excellent fitting by the Larkin two-dimensional superconducting fluctuation theory with only two scaling parameters (A(T) and HΦ(T) for each R(T) curve shows that by taking thicker PrBCO layers it is possible to induce the dimensional crossover from the anisotropic 3D behavior in pure YBCO to the quasi-2D behavior in YBCO/PrBCO superlattices. The resistive transition broadening in a magnetic field can be related to giant conductivity fluctuations in the quasi-two-dimensional CuO2 superconducting double layers in YBCO. For Y:Pr=1:3 and 1:5 superlattices, the temperature dependence of the first scaling parameter A(T), determined by the fluctuation amplitude, follows quite well the Larkin β(T) function typical for 2D systems. For Y:Pr = 1:1 and 5:6 superlattices, the YBCO layers are not completely decoupled or too thick and A(T) deviates from β(T). The temperature dependence of the second scaling parameter HΦ(T), the phase coherence breaking field, is consistent with the recent theoretical prediction by Reizer for the quasi-two-dimensional electron-electron interactions. The resistive transition tail, where the characteristic resistance is much smaller than the normal state resistance, is not directly caused by these fluctuations but rather is related to thermally assisted flux motion.

Confinement and quantization effects in mesoscopic superconducting structures

We have studied quantization and confinement effects in nanostructured superconductors. Three different types of nanostructured samples were investigated: individual structures (line, loop, dot), 1-dimensional (1D) clusters of loops and 2D clusters of antidots, and finally large lattices of antidots. Hereby, a crossover from individual elementary “plaquettes”, via clusters, to huge arrays of these elements, is realized. The main idea of our study was to vary the boundary conditions for confinement of the superconducting condensate by taking samples of different topology and, through that, modifying the lowest Landau level ELLL(H). Since the critical temperature versus applied magnetic field Tc(H) is, in fact, ELLL(H) measured in temperature units, it is varied as well when the sample topology is changed through nanostructuring. We demonstrate that in all studied nanostructured superconductors the shape of the Tc(H) phase boundary is determined by the confinement topology in a unique way.