Geofysisch onderzoek binnen het \u27Nieuwerck\u27 van de kathedraal van Antwerpen (Antwerpen, provincie Antwerpen)

Van 11 tot en met 15 februari 2013 werd door ADEDE bvba in de tuin van het Nieuwerck te Antwerpen een non-destructieve geofysische prospectie uitgevoerd. Deze tuin bevindt zich ten oosten van de Onze-Lieve-Vrouwekathedraal, tussen de Groenplaats, de Melkmarkt en de Lijnwaadmarkt. De totale toegankelijke oppervlakte van het onderzoeksterrein (i. e. vrij van vegetatie) bedroeg 415m². Ondanks een verstoorde (puin)bodem en het versnipperde onderzoeksgebied (bomen, struiken en een vijver) was de inzet van geofysische detectiemethoden zinvol. De grondradar met 200MHz antenne leverde daarbij wel betere resultaten op dan de conductiviteits- en de magnetische susceptibiliteitmetingen met behulp van de EM31-MK2. Binnen het dossier van het Nieuwerck hebben de gekozen geofysische methodes aangetoond dat er een groot aantal structuren in de bodem kunnen geregistreerd worden, ook al is er een hoge graad van verstoring (door heterogene grond met puin). Na uitvoering van de detectie en terugkoppeling met bestaande boorgegevens konden met zekerheid drie en waarschijnlijk twaalf structuren herkend worden, alsook werd er een beeld gevormd van hoe de opbouw van de opvulling van het Nieuwerck in elkaar zit. Door de dikte van de opvulling konden dieperliggende structuren niet vastgesteld worden

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 E_LLL(H). Since the critical temperature versus applied magnetic field T_c(H) is, in fact, E_LLL(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 T_c(H) phase boundary is determined by the confinement topology in a unique way.Comment: 28 pages, 19 EPS figures, uses LaTeX's aipproc.sty, contribution to Euroschool on "Superconductivity in Networks and Mesoscopic Systems", held in Siena, Italy (8-20 september 1997

Solution-processable thienoisoindigo-based molecular donors for organic solar cells with high open-circuit voltage

Two acetylene-bridged Donor–Acceptor–Donor (D-A-D) type small pi-conjugated molecules involving triphenylamine or N-phenylcarbazole as donor blocks (D) and thienoisoindigo as the acceptor unit (A) were synthesized and characterized by UV–Vis absorption and cyclic voltammetry. These donor materials were mixed with [6,6]-phenyl-C61-butyric acid methyl ester to prepare bulk heterojunction solar cells by simple solution processing. Due to their low-lying highest occupied molecular orbital energy levels, high open-circuit voltages up to 0.99 V were measured. The triphenylamine end-capped derivative led to the best power conversion efficiency of ca 2.20%, which ranks among the highest reported value for thienoisoindigo-based materials

Commensurate and Incommensurate Vortex States in Superconductors with Periodic Pinning Arrays

As a function of applied field, we find a rich variety of ordered and partially-ordered vortex lattice configurations in systems with square or triangular arrays of pinning sites. We present formulas that predict the matching fields at which commensurate vortex configurations occur and the vortex lattice orientation with respect to the pinning lattice. Our results are in excellent agreement with recent imaging experiments on square pinning arrays [K. Harada et al., Science 274, 1167 (1996)].Comment: 9 pages, 3 figures. Accepted to Physical Review

Individual and Multi Vortex Pinning in Systems with Periodic Pinning Arrays

We examine multi and individual vortex pinning in thin superconductors with periodic pinning arrays. For multi-vortex pinning we observe peaks in the critical current of equal magnitude at every matching field, while for individual vortex pinning we observe a sharp drop in the critical current after the first matching field in agreement with experiments. We examine the scaling of the critical current at commensurate and incommensurate fields for varied pinning strength and show that the depinning force at incommensurate fields decreases faster than at the commensurate fields.Comment: 4 figuure

Directional vortex motion guided by artificially induced mesoscopic potentials

Rectangular pinning arrays of Ni dots define a potential landscape for vortex motion in Nb films. Magnetotransport experiments in which two in-plane orthogonal electrical currents are injected simultaneously allow selecting the direction and magnitude of the Lorentz force on the vortex-lattice, thus providing the angular dependence of the vortex motion. The background dissipation depends on angle at low magnetic fields, which is progressively smeared out with increasing field. The periodic potential locks in the vortex motion along channeling directions. Because of this, vortex-lattice direction of motion is up to 85o away from the applied Lorentz force direction.Comment: PDF file includes figure

Multiply quantized vortices in trapped Bose-Einstein condensates

Vortex configurations in rotating Bose-Einstein condensed gases trapped in power-law and anharmonic potentials are studied. When the confining potential is steeper than harmonic in the plane perpendicular to the axis of rotation, vortices with quantum numbers larger than one are energetically favorable if the interaction is weak enough. Features of the wave function for small and intermediate rotation frequencies are investigated numerically.Comment: 9 pages, 6 figures. Revised and extended article following referee repor

Vortex Pinning and the Non-Hermitian Mott Transition

The boson Hubbard model has been extensively studied as a model of the zero temperature superfluid/insulator transition in Helium-4 on periodic substrates. It can also serve as a model for vortex lines in superconductors with a magnetic field parallel to a periodic array of columnar pins, due to a formal analogy between the vortex lines and the statistical mechanics of quantum bosons. When the magnetic field has a component perpendicular to the pins, this analogy yields a non-Hermitian boson Hubbard model. At integer filling, we find that for small transverse fields, the insulating phase is preserved, and the transverse field is exponentially screened away from the boundaries of the superconductor. At larger transverse fields, a ``superfluid'' phase of tilted, entangled vortices appears. The universality class of the transition is found to be that of vortex lines entering the Meissner phase at H_{c1}, with the additional feature that the direction of the tilted vortices at the transition bears a non-trivial relationship to the direction of the applied magnetic field. The properties of the Mott Insulator and flux liquid phases with tilt are also discussed.Comment: 20 pages, 12 figures included in text; to appear in Physical Review

Shapiro steps in a superconducting film with an antidot lattice

Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in the voltage-current characteristics of a superconducting film with a square lattice of perforating microholes (antidots)in the presence of radiofrequent radiation. These equidistant steps appear at the second matching field H_2 when the flow of the interstitial vortex lattice in the periodic potential created by the antidots and the vortices trapped by them, is in phase with the applied rf frequency. Therefore, the observation of Shapiro steps clearly reveals the presence of mobile intersitial vortices in superconducting films with regular pinning arrays. The interstitial vortices, moved by the driving current, coexist with immobile vortices strongly pinned at the antidots.Comment: 6 pages text, 3 EPS figures, RevTeX, accepted for publication in PRB Rapid Communication

Critical Currents, Pinning Forces and Irreversibility Fields in (YxTml-x)Ba2Cu3O7 Single Crystals with Columnar Defects in Fields up to 50 T

We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6 10^11 Kr-ions/cm2, on the superconducting critical parameters of single crystalline (YxTm1-x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2 - 90 K revealed that: (i) in fields up to T the critical current Jc(H,T) is considerably enhanced and (ii) down to temperatures T ~ 40 K the irreversibility field Hirr(T) is strongly increased. The field range and magnitude of the Jc(H,T) and Hirr(T) enhancement increase with increasing irradiation dose. To interpret these observations, an effective matching field was defined. Moreover, introducing columnar defects also changes the pinning force fp qualitatively. Due to stronger pinning of flux lines by the amorphous defects, the superconducting critical parameters largely exceed those associated with the defect structures in the unirradiated as-grown material: Jc,irrad(77 K, 5 T) ^3 10* Jc,ref(77 K, 5 T).Comment: 11 pages, all PDF, contribution to Physica