Search

The photon beam brightness of synchrotron light facilities is increased by reducing the beam emittance. For the upgrade of the storage ring of the Swiss Light Source (SLS2), the lattice achieving a low emittance foresees longitudinal gradient bending (LGB) magnets producing high peak field values and quasi-hyperbolic field profiles to minimize emittance at locations of radiation. Two types of technologies are studied: a 1.4 T peak field magnet based on permanent magnet materials (PM-LGB) and a superconducting version (SC-LGB) working at almost 5 T peak, to be upgraded to 6 T in future. The baseline scenario consists in commissioning the SLS2 with 60 PM-LGB assemblies first and exchange at a later stage three PM-LGBs by superconducting ones providing a higher peak field value. Thus, the design must foresee interchangeability. In this paper, we focus on the magnetic and mechanical design aspects of the PM-LGB and on the main mechanical constraints of the SC-LGB.

At ambient pressure, lithium molybdenum purple bronze (${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$) is a quasi-one-dimensional solid in which the anisotropic crystal structure and the linear dispersion of the underlying bands produced by electronic correlations possibly bring about a rare experimental realization of Tomomaga-Luttinger liquid physics. It is also the sole member of the broader purple molybdenum bronzes family where a Peierls instability has not been identified at low temperatures. The present paper reports a pressure-induced series of phase transitions between 0 and 12 GPa. These transitions are strongly reflected in infrared spectroscopy, Raman spectroscopy, and x-ray diffraction. The most dramatic effect seen in optical conductivity is the metalization of the $c$ axis, concomitant to the decrease in conductivity along the $b$ axis. This indicates that high pressure drives the material away from its quasi-one-dimensional behavior at ambient pressure. Although the first pressure-induced structure of the series is resolved, the identification of the underlying mechanisms driving the dimensional change in the physics remains a challenge.

We report high resolution Fourier Transform Infrared (FTIR) spectra of nitrogen trifluoride (trifluoroamine, NF3) measured with the Zurich Bruker prototype spectrometer (ZP 2001) and the SLS THz/FTIR setup (2009 prototype) in the range 20–3000 cm - 1 at temperatures between 80 and 120 K using a collisional cooling cell designed with White cell multipath reflection optics and at room temperature (296 K). Except in the far-infrared with instrument limited resolution ( Δ ν FWHM = 0.0008 cm - 1 ) many of the spectra are nearly Doppler limited. Rovibrational transitions were assigned to 13 different vibrational bands and were used then in the fitting procedure. We re-investigated the bands 2 ν 4 , ν 1 , ν 2 + ν 4 , ν 1 + ν 4 , 2 ν 3 and ν 1 + ν 3 , which had been studied before, and we have analyzed the ν 2 + ν 3 , 2 ν 1 , ν 1 + ν 2 + ν 4 , ν 1 + ν 2 + ν 3 , ν 2 + 2 ν 3 , 3 ν 3 , ν 1 + 2 ν 3 bands for the first time. All our analyses refer to the isotopomer 14 NF3.

In the context of the ITER project, the winding geometry of the large-scale magnetic field coils has to be validated. A geometrical survey during manufacturing is combined with a magnetic survey of finished assemblies. We are developing a fluxmeter system, which is capable of sampling magnetic flux density values (or gradients thereof) at many locations distributed around the assembly. The measurement method uses low-frequency (300 mHz-1.2 Hz) ac excitation of the coil under study and acquires induced voltage values sensed by magnetic flux pickup coils. The sampling strategy leverages on the use of calibrated arrays of coils mounted on rigid supports. The supports, whose positions in space are measured with a laser tracker, carry 48 distinct coils and are placed at several locations around the perimeter of the magnet under study. Maps of induced voltage values, corresponding to time-varying magnetic flux, are recorded. In this contribution, we present experimental results obtained on the first available European double-pancake prototype (12 turns) for the toroidal field coils and compare the results with feedforward computation using magnetostatics. At 300 mHz, the global relative agreement between measured and computed signal amplitude is in the % range. It is necessary to take into account the knowledge of the actual shape of the 12-turn current filament.

We observed a strong difference of the pressure dependence of the infrared (IR) active molecular vibron of hydrogen in phase IV in the 200–310 GPa pressure range in comparison with the Raman vibrons. While the Raman vibron strongly splits (∼250 cm−1) at the transition from phases III to IV at 220 GPa, the IR vibron nearly does not change. This small spitting of IR vibron is not described by the graphene-like structure proposed for phase IV. The combined pressure dependence of Raman and IR vibrons provides a sensitive test for further theoretical models of phase IV.

Today, the vast majority of electron storage rings delivering synchrotron radiation for general user operation offer a dedicated infrared port. There is growing interest expressed by various scientific communities to exploit the mid-IR emission in microspectroscopy, as well as the far infrared (also called THz) range for spectroscopy. Compared with a thermal (laboratory-based source), IR synchrotron radiation sources offer enhanced brilliance of about two to three orders of magnitude in the mid-IR energy range, and enhanced flux and brilliance in the far-IR energy range. Synchrotron radiation also has a unique combination of a broad wavelength band together with a well defined time structure. Thermal sources (globar, mercury filament) have excellent stability. Because the sampling rate of a typical IR Fourier-transform spectroscopy experiment is in the kHz range (depending on the bandwidth of the detector), instabilities of various origins present in synchrotron radiation sources play a crucial role. Noise recordings at two different IR ports located at the Swiss Light Source and SOLEIL (France), under conditions relevant to real experiments, are discussed. The lowest electron beam fluctuations detectable in IR spectra have been quantified and are shown to be much smaller than what is routinely recorded by beam-position monitors., Journal of Synchrotron Radiation, 19 (1), ISSN:0909-0495, ISSN:1600-5775

The potential of superconducting tunneling junctions (STJ) as energy resolving photon counters was proved in the range of 1 eV to 6 keV. In our laboratory we are producing and studying Ta/Al1/AlOx/Al2/Ta STJs. The responsivity of a STJ depends on the energy of the photon, the geometry of the STJ, the temperature and the bias point. We measured the dependence of the responsivity on the bias and the temperature. We model our devices with four quasiparticle populations coupled by the usual Rothwarf-Taylor balance equations. This simple approach appears to explain some of the observed features and doesn't require the introduction of local traps in the quasiparticle dynamics.

Transition edge sensor devices made with Mo–Au proximity bilayer thermometers have been fabricated. The bilayers were DC-sputter deposited directly on 250 nm thick pre-etched Si 3 N 4 membranes. Mo was deposited at 700°C, whereas Au was deposited at room temperature in the same pumpdown in order to allow for a good proximity effect. The superconducting transition temperature of devices with 43 nm Mo and 200 nm Au was around 190 mK . The transition width of bilayers deposited on membranes was substantially larger than those measured on samples deposited on the bulk substrate in the same fabrication run (≈2 mK ) . We consider this difference to be the result of a large temperature gradient across the membrane during the deposition process. Devices were tested with a single stage SQUID array kept at 2.2 K . The resolution of a device that has a transition width of 30 mK is 30 eV at 6 keV . A detailed study shows that the detector noise roughly scales with αI 0 , confirming the importance of thermal fluctuation noise in the thermometer.

Superconducting tunneling junction devices are being developed as energy resolving photon counting imagers. We have fabricated a series of Ta/Al/AlOx/Al/Ta devices with variable Al layer thickness, dAl, and measured their response to a weak visible photon source. Single photon sensitivity is observed. We model our system with a set of modified Rothwarf–Taylor equations including quasiparticle losses, trapping, (back)tunneling and calculate the charge responsivity as a function of dAl with one fitting parameter. The quasiparticle loss time for Al and Ta is found to be τ l ( Al )=35 μs and τ l ( Ta )=0.14 μs in our devices, respectively.

Organic structures respond to pressure with a variety of mechanisms including degradation, intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organic structures are of particular interest because of the potential steric controls on the fate of pressurized molecules. Despite representing a range of opportunities, the simultaneous study of high pressures on different forms of chiral structures is poorly explored. We have combined synchrotron-source vibrational optical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvil cell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid, alanine.

We fabricated X-ray detection devices by instrumenting 2 Al/AlO x /Al/Nb tunneling junctions at each end of strips of epitaxial Ta. The Ta strip acts as photon absorber and is a few hundreds of microns long, the Al layers act as quasiparticle traps. The quality of the epitaxial Ta thin film as well as the absorber/trap interface was varied. Devices kept at 550 mK and biased around 300 μV were irradiated by 6 keV photons. The total charge collected by both junctions varies with the energy released by absorbed photons, depends on various quasiparticle loss processes, and is a function of the bias voltage of the tunneling devices. Our arrangement enables to distinguish between energy resolution degrading mechanisms occuring in the Ta absorber and those present in the vicinity of the tunneling junctions.

We are developing small arrays of microcalorimeters based on transition edge sensors made with Mo/Au bilayers deposited on silicon nitride membranes and Au absorbers. The superconducting transition of the bilayers is adjusted to be around 130 mK with a transition width better than a millikelvin by use of the proximity e!ect between the Au and Mo "lms. We built a dilution refrigerator and wired it for 2 channel operation in order to study thermal coupling issues between thermometers within the array. The device fabrication procedure as well as preliminary results are presented. ( 2000 Elsevier Science B.V. All rights reserved.

A specially designed device based on superconducting tunnel junctions was used to measure the diffusion of excess electronic quasiparticles generated in a superconducting film of tantalum by the absorption of x-ray photons. The device was made out of a thin film of epitaxial tantalum. At both ends, a microfabricated $\mathrm{Al}\ensuremath{-}{\mathrm{AlO}}_{x}\ensuremath{-}\mathrm{Al}\ensuremath{-}\mathrm{Nb}$ tunneling junction was placed onto the film. Both tunneling junctions were operated in a current-biased mode in a small magnetic field and were used to monitor the diffusion of excess quasiparticles generated in the Ta. For the data analysis, the Ta absorber has been modeled as a chain of cells, and the time evolution and diffusion of the distribution of energy carried by quasiparticles has been calculated using the Chang-Scalapino equations. In this way, we determined the values of the quasiparticle diffusion length $\ensuremath{\Lambda}=\sqrt{{D}_{\mathrm{abs}}{\ensuremath{\tau}}_{\mathrm{eff}}},$ the diffusion rate ${D}_{\mathrm{abs}},$ and the effective lifetime ${\ensuremath{\tau}}_{\mathrm{eff}}.$

Experimental investigations of the proximity effects in symmetric Nb/Al-AlOx-Al/Nb superconducting tunneling junctions with a fixed Nb thickiness of 200 nm, and Al layer thicknesses ranging from 4 to 120 nm, are reported. Both de Josephson-tunneling and quasiparticle-tunneling current voltage characteristics were measured, and analyzed using the proximity effect model proposed by Golubov and Kupriyanov. The numerical computations were performed without the approximation used by Golubov et nl. In particular, the order parameter was allowed to spatially vary in Nb and Al and the Usadel functions were not linearized. The overall shape of the experimental I-V curves, as well as the temperature dependence of the de Josephson current and of the gap energies are satisfactorily reproduced by the model calculations provided the parameter gamma(BN) describing the resistance at the Al-Nb, interface is taken into account. From ys, we extract a value for the quasiparticle transmission at the Nb/Al interface which is smaller than unity. [S0163-1829(99)01013-9].

The infrared beamline at the Swiss light source uses dipole radiation and is designed to transport light to four experimental stations, A, B, C, D. Branch A is dedicated to far IR work in vacuum; branch B is a micro-spectrometer; branch C is dedicated to high resolution spectroscopy in the gas phase; branch D is a pump and probe set-up. This contribution describes the optical layout and provides a brief survey of currently available experimental stations. The beamline is in regular user operation since 2009.

A new Nb/Al-AlOx-Al/Nb trilayer process using a movable mechanical slit for junction-detector applications is described. Best junctions have a quality factor of 42 mV with a Josephson current density of 1200 A/cm 2 at 4.2 K. The temperature dependence of the sub-gap current in the range of 0.45 to 4.2 K has been measured. Deviations from the thermally activated behavior due to imperfections in the tunneling barrier are observed. We compare our simple method with the usual whole wafer processes and we discuss the influence of proximity effects in these junctions in terms of the models proposed by Golubov et al. and by McMillan.

A new Nb/AlO x /Nb trilayer process for X-ray detector applications has been developed. The process employs a movable mechanical slit which, after the sputter deposition of the base Nb electrode followed by a thin layer of Al and its oxidation, is shifted to a second position where the Nb counter electrode is deposited. The overlap area of the two electrodes forms the junction are after final patterning by standard photolithography and reactive ion etching. The best junctions so far obtained have, at 4.2 K, typically a quality factor of 42 mV with a Josepshon current density of 1200 A/cm 2 , and a tunneling time τ tun in the range of 50 to 500 ns. We discuss several aspects of this technique in comparison with the selective niobium anodization process (SNAP) and the selective niobium etching process (SNEP).

The infrared response of coronene (C(24)H(12)) under pressure and temperature conditions up to 10 GPa and 300 °C is examined in situ using a diamond anvil cell and synchrotron-source Fourier transform infrared (FTIR) spectroscopy. Coronene is a polycyclic aromatic hydrocarbon that is present in the interstellar medium and meteorites which may have contributed to the Earth's primordial carbon budget. It appears to undergo a reversible phase transition between 2 and 3.2 GPa at ambient temperature; new intramolecular bonds in the region 840-880 cm(-1) result from compression. We document the shift of spectral features to higher wavenumbers with increasing pressure but find this change suppressed by increased temperature. By investigating the stability of coronene over a range of naturally occurring conditions found in a range of environments, we assess the survival of the molecule through various terrestrial and extraterrestrial processes. Coronene has previously been shown to survive atmospheric entry during Earth accretion; this can now be extended to include survival through geological processes such as subduction and silicate melting of the rock cycle, opening the possibility of extraterrestrial coronene predating terrestrial accretion existing on Earth.

We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(CoxFe1- x)2As2 compounds for Co-doping ranging between 0 and 18%. For the parent compound as well as for x=0.025, we observe the opening of a pseudogap, due to the spin-density-wave phase transition and inducing a reshuffling of spectral weight from low to high frequencies. For compounds with 0.051≤x≤0.11, we detect the superconducting gap, while at x=0.18 the material stays metallic at all temperatures. We describe the effective metallic contribution to the optical conductivity with two Drude terms, representing the combination of a coherent and an incoherent component, and extract the respective scattering rates. We establish that the dc transport properties in the normal phase are dominated by the coherent Drude term for 0≤x≤0.051 and by the incoherent one for 0.061≤x≤0.18, respectively. Finally, through spectral weight arguments, we give clear-cut evidence of moderate electronic correlations for 0≤x≤0.061, which then cross over to values appropriate for a regime of weak interacting and nearly-free electron metals for x≥0.11., New Journal of Physics, 12, ISSN:1367-2630

Epitaxial superconducting YBa2Cu3O7−x (YBCO) thin films have been grown on Si(100) using CaF2 as an intermediate buffer layer. The CaF2(100) layers were grown by molecular beam epitaxy while for the YBCO layers a laser ablation process was used. Electron channeling patterns of the YBCO surface show the characteristic four‐fold symmetry for c‐axis orientation. The epitaxial relationship is (001)YBCO//(001)Si and [010]YBCO//[110]Si. This corresponds to a lattice mismatch of only 1% in the a‐b‐YBCO interface plane. The 60‐nm‐thick layers are crack free. Magnetic shielding properties of the epitaxial YBCO layers have been studied by measuring the zero temperature penetration depth λab(0) and the power law dependence of the transition temperature versus magnetic field.

The superconductive transition of c-axis-oriented YBa2Cu3O7 thin films, as characterized by measurements of the temperature dependence of the magnetic penetration depth, is found to occur below the Ginzburg-Landau mean-field transition. The divergence of the penetration depth at the transition is in good quantitative agreement with the predictions of a percolation model.

Three thin film dc SQUIDs of varied construction were bombarded with energetic protons in the energy range of 50 to 280 MeV. Measurements of the voltage output of the dc SQUIDs were taken in open loop, as well as flux locked mode, in an environment of proton flux that was varied from 10/sup 4/ to 10/sup 7/ protons/cm/sup 2//s. Discrete voltage jumps corresponding to 0.01 to 0.001 flux quanta were observed in two of the three SQUIDs in the flux locked mode; discrete changes in the open loop SQUID output voltage were also observed. Some data appear to be consistent with proton-induced flux motion in the body of the SQUID loop. >

A method is described for fabricating superconducting weak links in thin films of YBa2Cu3O7−x grown in situ by laser deposition. Carefully controlled electrical pulses applied to patterned bridges at 77 K are shown to reduce the critical current from initial values of typically 10 mA to values as low as 20 μA. The current‐voltage characteristics of the Josephson weak links so formed exhibit constant‐voltage current steps that oscillate in amplitude as the applied microwave power is increased, implying that the current‐phase relation is periodic but not necessarily sinusoidal.

With a mutual-inductance method the effective kinetic inductance of square arrays of 10{sup 6} proximity-effect Pb-Cu-Pb Josephson junctions was measured. The magnetic field was tuned to achieve full frustration {ital f}=1/2, the frustration parameter {ital f}=={Phi}/{phi}{sub 0} being the ratio of the applied magnetic flux {Phi} per unit cell of the array and the superconducting flux quantum {phi}{sub 0}. A test of the temperature dependence of the free vortex correlation length as well as of the helicity modulus was performed and found to be consistent with a description of the phase transition at {ital f}=1/2 within the framework of the Kosterlitz-Thouless-Berezinski ideas. The results do not, however, provide information concerning the possible presence of an Ising-like transition, which may exist in the fully frustrated case.

A whole wafer fabrication procedure using selective etching of refractory materials to produce Nb- or Ta/Al/AlO x Al/Nb tunneling X-ray detectors is discussed. Fabrication aspects and preliminary results are presented.

Results from a comparison between current-voltage data measured on Nb/Al/AlO x /Al/Nb tunneling junctions to a proximity effect model proposed by Golubov et al. [J. Low Temp. Phys. 70 (1988) 83] are used to get an estimation of the quasiparticle collection as a function of the Al-film thickness (4 and 120 nm) in X-ray detectors.

Laser ablated thin YBa 2 Cu 3 O 7 (YBCO) films on SrTiO 3 substrates were used to fabricate step edge Josephson junction arrays. After forming the substrate into a checkerboard arrangement of steep mesas of lattice parameter 7.5μ m , a YBCO film was deposited and patterned into a square network with the same lattice parameter. The arrays thus consist of 10 3 ×10 3 wires, 1.5 μ m wide and some 10 6 junctions at the sites of the steps. In a perpendicular magnetic field the conductance of the arrays shows up to 40 periodic oscillations, the periodicity corresponding to one flux quantum per array unit cell.

Pulsed laser deposition (PLD) was used to grow superconducting Ta-films with critical temperatures close to bulk values (4.5 K) on sapphire substrates. Results are compared with films grown by e-beam evaporation. The PLD method allows the growth of superconducting Ta-films on substrates kept at ambient temperature but film surfaces are plagued by sub-micron particles. On the other hand, e-beam evaporation results in smooth surfaces but requires a substrate temperature of the order of 400 ° C for producing high-quality superconducting films. Critical temperatures, residual resistance ratios, and crystal structure are presented.

The feasibility of using of current-biased superconducting strips for radiation detection is investigated. Narrow Ta strips are exposed to 5.5 MeV α-particle radiation and the rise-time of the induced voltage pulses is measured as function of temperature and bias current. The rise-time of the voltage signal strongly depends on the site on the strip which is hit by the α-particle. In order to determine the spatial resolution of a superconducting strip detector, position-sensitive measurements were performed. The maximum lateral resolution estimated so far is 25μm in a 7μm wide, 340 nm thick and 0.6 mm long Ta-strip.

The ac sheet conductance of large two-dimensional arrays of proximity effect SNS Josephson junctions was measured as a function of temperature, frequency and applied perpendicular magnetic field. A quantitative analysis of the data shows that the normal to superconducting transition of unfrustrated arrays follows the predictions of the Kosterlitz-Thouless theory for phase transitions in two dimensions.

The ac response of large two-dimensional arrays of proximity-effect Josephson junctions to an oscillating driving field has been studied as a function of temperature, applied transverse magnetic field, and frequency. For an integer number of flux quanta per unit cell, a peak in dissipation and a drop in superfluid density are observed near the superconducting transition of the array. These features as well as their frequency dependence provide clear evidence for the vortex-unbinding transition predicted by the Kosterlitz-Thouless theory.

A fully optical version of the oblique evaporation technique through undercut photoresist stencils developed by Dolan is used to fabricate planar variable-thickness S-N-S microbridges. An experimental study of the static and dynamic behaviour of Pb-Cu-Pb devices is reported. I c R N -products are found in good agreement with the Likharev theory and its modifications. In the shortest microbridges we find I c R N - values of the order of the limiting value 2Δ pb (O)/e. The I-V curves show hysteresis only at the lowest temperatures of our experiments and exhibit sharp quantum steps when the samples are exposed to microwave radiation.

A design analysis is given for a bolometric infrared detector that uses the resistive transition of a high‐temperature superconductor as the temperature sensing element, and liquid nitrogen (LN) as the coolant. It is shown that for highly oriented c‐axis films, the measured low‐frequency noise causes little or no degradation of the performance. With the incoming radiation chopped at 10 Hz, noise equivalent powers (NEP) in the range (1–20)×10−12 W Hz−1/2 should be achievable. These values compare favorably with the NEP of other detectors operating at or above LN temperatures for wavelengths greater than 20 μm.

Low-frequency voltage fluctuations in thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ at and above the superconducting transition temperature have a spectral density proportional to the ratio of the average voltage across the film to the frequency. The ratio of the spectral density to the average voltage decreases markedly as the microstructure of the films is improved. In contrast to classic superconductors, the noise at the resistive transition does not arise from equilibrium temperature fluctuations. >

Ordering in a physical system strongly depends on its dimensionality d and on the number of degrees of freedom, n, of the quantity used to describe the order [1]. For instance, in a magnetic system n is the number of components of the spin (n = 1 for the Ising model, n = 2 for the XY model, n = 3 for the Heisenberg model), while the description of the ordered phase of a superfluid in terms of a complex order parameter implies n = 2. In this context, two-dimensional systems (d = 2) with n ≥ 2 hold, on account of the delicate problems they raise in connection with the possibility of a phase transition and with the nature of the ordered state that may occur at low temperatures [2–5], a very special position. Since the pioneering work of KOSTERLITZ and THOULESS [6] in the early 1970s, there has been considerable progress in the theoretical understanding of the critical behaviour of two-dimensional (2D) systems. It is now well established that topological excitations, such as vortices in 2D superfluids or dislocations in 2D solids, play a central role in critical phenomena occurring in these systems. For n = 2 there is a sharp transition, triggered by the unbinding of pairs of topological excitations, from a low-temperature phase showing quasi long-range (or topological) order to a disordered phase at high temperatures, while for n > 2 no phase transition at all occurs in two dimensions. Advances in material science, experimental techniques and numerical simulations have contributed to assess, in a variety of systems, the validity of the various theoretical ideas underlying the physics of phase transitions in two dimensions.

Artificial c -axis oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBACO/PBCO) supperlattices were grown on MgO and SrTiO 3 (100) substrates by an in-situ laser ablation process. Samples with stackings of two unit cells of YBCO, alternating with 3, 4, 5 and 15 unit cells of PBCO, respectively, were prepared. Information on the in-plane orientation was obtained from X-ray pole figure measurements. The effective inverse kinetic inductance L K - 1 (T), proportional to the effective density of Cooper pairs in the sample, was determined with complex-impedance measurements. The data reveal that the transition to the superconducting state, deduced from the temperature dependence of L K - 1 , depends both on the in-plane order of the crystal structure and on the quality of the interfaces between YBCO and PBCO.

The observation of quantum interference transitions in the dynamic resistance of a large two-dimensional array of proximity-effect junctions driven by superposed dc- and rf-currents is reported. It is shown that, under the synchronizing effect of the rf-field, the array behaves as a superposition of coherent quantum oscillators connected in series.

Planar arrays of Josephson junctions provide excellent model systems to study a variety of phase transitions in two dimensions. Experiments are reported in which the critical behaviour of periodic arrays is studied by measuring the dynamic response of the system to a small oscillating field. Tuning of frustration occurs by adjusting the magnetic flux threading a unit cell of the array, while by varying the driving frequency it is possible to probe the critical behaviour of the system at different length scales. The dynamic response near the phase transition of an un frustrated array is well described by a model incorporating the vortex unbinding mechanism predicted by the Kosterlitz-Thouless theory. Some aspects of the dynamic response of frustrated arrays are also discussed.