arXiv subjects

S. Pace

Publications and source records attributed to S. Pace.

At least 19 recordsLinked to original sources

arXiv2017

Geometry Effects on Switching Currents in Superconducting Ultra Thin Films

Vortex dynamics is strongly connected with the mechanisms responsible for the photon detection of superconducting devices. Indeed, the local suppression of superconductivity by photon absorption may trigger vortex nucleation and motion effects, which can make the superconducting state unstable. In addition, scaling down the thickness of the superconducting films and/or the width of the bridge geometry can strongly influence the transport properties of superconducting films, e.g. affecting its critical current as well as its switching current into the normal state. Understanding such instability can boost the performances of those superconducting devices based on nanowire geometries. We present an experimental study on the resistive switching in NbN and NbTiN ultra-thin films with a thickness of few nanometers. Despite both films were patterned with the same microbridge geometry, the two superconducting materials show different behaviors at very low applied magnetic fields. A comparison with other low temperature superconducting materials outlines the influence of geometry effects on the superconducting transport properties of these materials particularly useful for devices applications.

cond-mat.supr-con↗

arXiv2016

Speed limit to the Abrikosov lattice in mesoscopic superconductors

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material Mo$_3$Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current driven switching to the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a limiting maximum velocity as a function of the applied magnetic field. Based on time dependent Ginzburg-Landau simulations we argue that the observed behavior is due to the high velocity vortex dynamics confined on a mesoscopic scale. We build up a general phase diagram which includes all possible dynamic configurations of Abrikosov lattice in a mesoscopic superconductor.

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arXiv2010

Quasiparticle scattering time in superconducting films: from dirty to clean limit

We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean free paths in a state far from equilibrium, that is the highly dissipative flux-flow state driven up to the instability point. From the measured current-voltage curves we derive the vortex critical velocity $v^{*}$ for several temperatures. From the $v^{*}(T)$ values, the quasiparticle energy relaxation time $\tau_{\epsilon}$ is evaluated within the Larkin-Ovchinnikov model and numerical calculations of the quasiparticle energy relaxation rates are carried out to support the experimental findings. Besides the expected constant behavior of $\tau_{\epsilon}(T)$ for the dirty samples, we observe a strong temperature dependence of the quasiparticle energy relaxation time in the clean samples. This feature is associated with the increasing contribution from the electron-phonon scattering process as the dirty limit is approached from the clean regime.

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arXiv2010

Evidence of a new low field cross-over in the vortex critical velocity of type-II superconducting thin films

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux flow state related to the vortex critical velocity v* is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v* ~ B^-1/2, in the low field range a new cross-over field, Bcr1, is observed below which v* decreases by further lowering the external magnetic field B. We ascribe this unexpected cross-over to vortex channeling due to a fan-like penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields, that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.

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arXiv2007

Superconductivity in Sr2RuO4-Sr3Ru2O7 eutectic crystals

Superconducting behavior has been observed in the Sr2RuO4-Sr3Ru2O7 eutectic system as grown by the flux-feeding floating zone technique. A supercurrent flows across a single interface between Sr2RuO4 and Sr3Ru2O7 areas at distances that are far beyond those expected in a conventional proximity scenario. The current-voltage characteristics within the Sr3Ru2O7 macrodomain, as extracted from the eutectic, exhibit signatures of superconductivity in the bilayered ruthenate. Detailed microstructural, morphological and compositional analyses address issues on the concentration and the size of Sr2RuO4 inclusions within the Sr3Ru2O7 matrix. We speculate on the possibility of inhomogeneous superconductivity in the eutectic Sr3Ru2O7 and exotic pairing induced by the Sr2RuO4 inclusions.

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arXiv2007

New aspects in the Bragg Glass-Disordered phase transition: an analysis based on the 3rd harmonics of the AC magnetic susceptibility

We analyse the phase transition between the Bragg Glass and the Disordered phase in the vortex lattice in type-II superconductors, both by analytical computations and experimental investigations. It is known that if the Peak Effect can be detected, a Bragg Glass/Disordered phase transition takes place. We show that, in some conditions, this transition can occur without the observation of the Peak Effect Phenomenon. We introduce a method based on the 3rd harmonics of the AC magnetic susceptibility to detect the transition also in these cases. Using this method, we obtain an experimental confirmation of the theoretical predictions on sphere shaped V3Si single crystals, in the high fields/low temperatures range too, where previous experimental studies failed to detect the Bragg/Disordered phase transition.

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arXiv2005

H(T) Phase Diagram in Nb3Sn: A Different Behavior in Single Crystals, Polycristalline Samples and Multifilamentary Wires

A comparative study of magnetic behavior in a DC magnetic field up to 19 Tesla has been undertaken on different Nb3Sn samples, i.e. single crystals, polycrystal samples and multifilamentary wires. From the 1st and 3rd harmonics of the AC magnetic susceptibility a peak effect was experimentally observed in the single crystal and in the polycrystal, whereas this phenomenon is absent in the measured wires. The corresponding H vs T phase diagram reveals a different behavior between wires on one hand and polycrystalline and single crystal samples on the other. In particular, from the 3rd harmonics it has been observed that vortex thermally activated flux creep phenomena are relevant in the wires, whereas the static critical state models are more appropriate to describe single crystals.

cond-mat.supr-con↗

arXiv2005

Identification of the vortex glass phase by harmonics of the AC magnetic susceptibility

We compared the AC magnetic susceptibility behaviour for the vortex glass phase and for the creep phenomena with an inhomogeneous pinning potential. The temperature dependence of the harmonics of the susceptibility have been numerically simulated with these two models, and we studied them as a function of the frequency, in terms of Cole-Cole plots. From our analysis we show that it is possible to distinguish between the two different phases, because of their clear differences in the Cole-Cole plots behaviour with the frequency.

cond-mat.supr-con↗

arXiv2004

Analysis of coupling losses in multifilamentary untwisted BSCCO/Ag tapes through a.c. susceptibility measurements

Losses as function of the a.c. magnetic field amplitude ($B_0$) were measured at 77 K in untwisted BSCCO(2223)/Ag tapes, at different frequencies, by measuring the imaginary part of the a.c. susceptibility. In particular, loss measurements were performed in the portions of the same tape, obtained by cutting it in pieces with different length, starting from around 12 cm down to 1 cm. The results show that the measured losses depend on the sample length but this observed behaviour is not always due to the coupling mechanism among the filaments. In this work we discuss the observed experimental behaviour of different typology of tapes by analysing data comparing them with analytical models in order to fully characterize the tapes with regard to the coupling mechanism.

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arXiv2004

Magnetic relaxation of type II superconductors in a mixed state of entrapped and shielded flux

The magnetic relaxation has been investigated in type II superconductors when the initial magnetic state is realized with entrapped and shielded flux (ESF) contemporarily. This flux state is produced by an inversion in the magnetic field ramp rate due to for example a magnetic field overshoot. The investigation has been faced both numerically and by measuring the magnetic relaxation in BSCCO tapes. Numerical computations have been performed in the case of an infinite thick strip and of an infinite slab, showing a quickly relaxing magnetization in the first seconds. As verified experimentally, the effects of the overshoot cannot be neglected simply by cutting the first 10-100 seconds in the magnetic relaxation. On the other hand, at very long times, the magnetic states relax toward those corresponding to field profiles with only shielded flux or only entrapped flux, depending on the amplitude of the field change with respect to the full penetration field of the considered superconducting samples. In addition, we have performed numerical simulations in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes; this allowed us to interpret correctly also the first seconds of the $M(t)$ curves.

cond-mat.supr-con↗

arXiv2004

Irreversible Dynamics of Abrikosov Vortices in Type-two Superconductors

The voltage shift RfIc with respect to the flux flow ohmic behavior of the current-voltage characteristic in type II superconductors is ascribed to the irreversible processes occurring when a vortex crosses defects. We include irreversible effects of the vortices-defects interaction into an effective law of motion. The obtained current-voltage characteristic at finite temperature is in agreement with experimental data.

cond-mat.supr-con↗

arXiv2003

Study of coupling loss on bi-columnar BSCCO/Ag tapes by a.c. susceptibility measurements

Coupling losses were studied in composite tapes containing superconducting material in the form of two separate stacks of densely packed filaments embedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometry is quite favorable for studying the coupling currents and in particular the role of superconducting bridges between filaments. By using a.c. susceptibility technique, the electromagnetic losses as function of a.c. magnetic field amplitude and frequency were measured at the temperature T = 77 K for two tapes with different matrix composition. The length of samples was varied by subsequent cutting in order to investigate its influence on the dynamics of magnetic flux penetration. The geometrical factor $χ_0$ which takes into account the demagnetizing effects was established from a.c. susceptibility data at low amplitudes. Losses vs frequency dependencies have been found to agree nicely with the theoretical model developed for round multifilamentary wires. Applying this model, the effective resistivity of the matrix was determined for each tape, by using only measured quantities. For the tape with pure silver matrix its value was found to be larger than what predicted by the theory for given metal resistivity and filamentary architecture. On the contrary, in the sample with a Ag/Mg alloy matrix, an effective resistivity much lower than expected was determined. We explain these discrepancies by taking into account the properties of the electrical contact of the interface between the superconducting filaments and the normal matrix. In the case of soft matrix of pure Ag, this is of poor quality, while the properties of alloy matrix seem to provoke an extensive creation of intergrowths which can be actually observed in this kind of samples.

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arXiv2003

Third Harmonics of the AC Magnetic Susceptibility: a method for the study of flux dynamics in High Temperature Superconductors

The temperature dependence of the 1st and 3rd harmonics of the AC magnetic susceptibility has been measured on melt grown YBCO samples for different frequencies and amplitudes of the AC magnetic field and intensity of a contemporaneously applied DC field. With the help of critical state models and of numerical simulations, we have devised a novel method, based on the combined analysis of the 1st and the 3rd harmonics that allows to distinguish different temperature ranges dominated by the different dissipative magnetic flux regimes. In particular, we identified three principal zones in the temperature dependence of the real part of the 3rd harmonic: the zone 1, in the temperature range below the peak of the imaginary part of the 1st harmonic, Tp, and the zone 2, characterized by negative values in a temperature region of the real part of the 3rd harmonics, just above Tp, both dominated by the creep regime; the zone 3, just below Tc, in which we revealed the presence of Thermally Assisted Flux Flow (TAFF). By the identification of these zones, an estimation of the value of the pinning potential can be obtained.

cond-mat.supr-con↗

arXiv2003

Harmonics of the AC susceptibility as probes to differentiate the various creep models

We measured the temperature dependence of the 1st and the 3rd harmonics of the AC magnetic susceptibility on some type II superconducting samples at different AC field amplitudes, hAC. In order to interpret the measurements, we computed the harmonics of the AC susceptibility as function of the temperature T, by integrating the non-linear diffusion equation for the magnetic field with different creep models, namely the vortex glass-collective creep (single-vortex, small bundle and large bundle) and Kim-Anderson model. We also computed them by using a non-linear phenomenological I-V characteristics, including a power law dependence of the pinning potential on hAC. Our experimental results were compared with the numerically computed ones, by the analysis of the Cole-Cole plots. This method results more sensitive than the separate component analysis, giving the possibility to obtain detailed information about the contribution of the flux dynamic regimes in the magnetic response of the analysed samples.

cond-mat.supr-con↗

arXiv2003

Third Harmonic Susceptibility and the Irreversibility Line of Fe/MgB2 Tapes

We report a study of the pinning properties of Fe/MgB2 tapes performed by means of ac magnetic susceptibility measurements. In particular, the third harmonic response has been measured as a function of the dc magnetic field up to 9 T, at different temperatures, for various frequencies and amplitudes of the ac field. The irreversibility line has been determined using the third harmonic onset criterion for samples made with different ball-milled MgB2 powders. The study of the harmonic response allows us to probe the nature of the vortex pinning and to analyse its influence on the enhancement of the irreversibility field in strongly ball-milled samples. We have also investigated the dynamical regimes governing the vortex motion in the tapes by comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field.

cond-mat.supr-con↗

arXiv2003

Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements

In a type-II superconductor the generation of higher harmonics in the magnetic response to an alternating magnetic field is a consequence of the non-linearity in the I-V relationship. The shape of the current-voltage (I-V) curve is determined by the current dependence of the thermal activation energy U(J) and is thus related to the dynamical regimes governing the vortex motion. In order to investigate the vortex dynamics in MgB2 bulk superconductors we have studied the fundamental (chi1) and third (chi3) harmonics of the ac magnetic susceptibility. Measurements have been performed as a function of the temperature and the dc magnetic field, up to 9 T, for various frequencies and amplitudes of the ac field. We show that the analysis of the behaviour in frequency of chi3(T) and chi3(B) curves can provide clear information about the non-linearity in different regions of the I-V characteristic. By comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field we are able to resolve the crossover between a dissipative regime governed by flux creep and one dominated by flux flow phenomena.

cond-mat.supr-con↗

arXiv2002

Vortex dynamics and pinning properties analysis of MgB2 bulk samples by ac susceptibility measurements

The flux lines dynamics have been investigated on MgB2 bulk superconductors obtained by reactive liquid infiltration by measuring the ac magnetic susceptibility. The fundamental and third harmonics have been studied as a function of temperature, dc magnetic field, ac field amplitude and frequency. In order to determine the dynamical regimes governing the vortex motion, the experimental results have been compared with susceptibility curves obtained by numerical calculations of the non-linear diffusion equation for the magnetic field. The frequency behaviour of the third harmonic response, that cannot be explained by frequency dependent critical state models, has been related to the current dependence of the flux creep activation energy U(J) in the diffusion coefficient. In this way we have shown that the measured curves are correctly interpreted within the framework of a vortex glass description.

cond-mat.supr-con↗

arXiv2001

Low frequency transport measurements in GdSr2RuCu2O8

Low frequency transport measurements are performed on GdSr$_2$RuCu$_2$O$_8$ pellets. The observed current-voltage curves are qualitatively explained in the framework of a simple phenomenological model accounting for coexistence of ferromagnetism and superconductivity in the sample. A Curie temperature $T_{cM}$=133 K and a superconducting critical temperature $T_{cS}$=18 K, with an onset temperature $T_{cO}$=33 K, are extracted from the analysis of the current-voltage curves.

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