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Superconductivity induced by structural reorganization in the electron-doped cuprate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mm
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The superposition of strongly anisotropic magnetic and superconducting properties in GdSr2RuCu2O8 (Gd1212) stimulates interest in the fabrication of oriented samples of macroscopic size by Melt-Texturing Growth (MTG). Up to now, the... more
The superposition of strongly anisotropic magnetic and superconducting properties in GdSr2RuCu2O8 (Gd1212) stimulates interest in the fabrication of oriented samples of macroscopic size by Melt-Texturing Growth (MTG). Up to now, the results obtained are unsatisfactory due to the small size and the random distribution of the oriented domains. In order to achieve MTG samples with higher oriented volume fractions, we
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Biaxially aligned YBa2Cu3O7−y thick films were obtained by Pulsed Laser Deposition on cube textured non-magnetic Ni-V metallic substrates, with NiO as a first buffer layer. The critical temperature and critical current density at 77K and... more
Biaxially aligned YBa2Cu3O7−y thick films were obtained by Pulsed Laser Deposition on cube textured non-magnetic Ni-V metallic substrates, with NiO as a first buffer layer. The critical temperature and critical current density at 77K and self field of the YBCO thick film grown on an epitaxial CeO2/NiO buffer layer architecture are 90.5K and 0.6 MA/cm2, respectively.
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To draw a complete vortex phase diagram for a CaKFe4As4 polycrystalline iron-based superconductor, different kinds of magnetic measurements have been performed focusing on the critical parameters of the sample. Firstly, magnetic moment... more
To draw a complete vortex phase diagram for a CaKFe4As4 polycrystalline iron-based superconductor, different kinds of magnetic measurements have been performed focusing on the critical parameters of the sample. Firstly, magnetic moment versus field measurements m(H) were performed at low fields in order to evaluate the lower critical field Hc1. After that, by performing relaxation measurements m(t), a field crossover Hcross was detected in the framework of a strong pinning regime. The irreversibility field Hirr as a function of the temperature curve was then drawn by plotting the critical current densities Jc versus the field for temperatures near Tc. Jc(H) has demonstrated a second magnetization peak effect phenomenon, and the second peak field Hsp has been identified and plotted as a function of temperature, providing information about an elastic to plastic transition in the vortex lattice. Finally, the upper critical field Hc2 as a function of the temperature has been obtained. H...
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Several members of the RPt<inf>4</inf>Ge<inf>12</inf> skutterudites class (R = Sr, Ba, Th, La, Pr) show at low temperature the phenomenon of superconductivity, with R = Pr exhibiting the highest transition... more
Several members of the RPt<inf>4</inf>Ge<inf>12</inf> skutterudites class (R = Sr, Ba, Th, La, Pr) show at low temperature the phenomenon of superconductivity, with R = Pr exhibiting the highest transition temperature of about 8 K. PrPt<inf>4</inf>Ge<inf>12</inf> shows as well a wide variety of strongly correlated electron behavior, including metal-insulator transitions, spin fluctuations, and various magnetically ordered states. In this work, we have applied the point contact spectroscopy to investigate the superconducting order parameter of the PrPt<inf>4</inf>Ge<inf>12</inf> compound. Superconductivity in PrPt<inf>4</inf>Ge<inf>12</inf> is progressively suppressed by partially substituting the Pr ions with Ce ions. For this reason, electrical resistance measurements as a function of the temperature (R-T), in different applied magnetic fields have also been performed and analysed on Pr<inf>1-x</inf>Ce<inf>x</inf>Pt<inf>4</inf>Ge<inf>12</inf> samples for x = 0, 0.07, 0.10.
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We analyze the magnetic behavior of a CaKFe4As4 polycrystalline sample fabricated by a mechanochemically assisted synthesis route. By means of DC magnetization (M) measurements as a function of the temperature (T) and DC magnetic field... more
We analyze the magnetic behavior of a CaKFe4As4 polycrystalline sample fabricated by a mechanochemically assisted synthesis route. By means of DC magnetization (M) measurements as a function of the temperature (T) and DC magnetic field (H) we study its critical parameters and pinning features. The critical temperature Tc has been evaluated by M(T) curves performed in Zero Field Cooling-Field Cooling conditions. These curves show the presence of a little magnetic background for temperatures above Tc, as also confirmed by the hysteresis loops M(H). Starting from the M(H) curves, the critical current density Jc of the sample has been calculated as a function of the field at different temperatures in the framework of the Bean critical state model. The Jc(H) values are in line with the ones reported in the literature for this typology of samples. By analyzing the temperature dependence of the critical current density Jc(T) at different magnetic fields, it has been found that the sample i...
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ABSTRACT In order to build high-temperature superconductor (HTS) multichip modules (MCMs), it is necessary to grow several epitaxial layers of YBCO that are separated by thick dielectric layers without seriously affecting the quality of... more
ABSTRACT In order to build high-temperature superconductor (HTS) multichip modules (MCMs), it is necessary to grow several epitaxial layers of YBCO that are separated by thick dielectric layers without seriously affecting the quality of the YBCO layers. In this work, we have successfully fabricated YBCO/YSZ/SiO2/YSZ/YBCO structures on single-crystal LaAlO3 substrates using a combination of pulsed laser deposition for the YBCO layers and ion-beam-assisted rf sputtering to obtain biaxially aligned YSZ intermediate layers. The bottom YBCO layer had a Tc∼89 K, Jc∼7.2×105 A/cm2 at 77 K, whereas the top YBCO layer had a Tc∼86 K, Jc∼6×105 A/cm2 at 77 K. The magnetic field and temperature dependence of Jc for the YBCO films in the multilayer have been obtained. The results for each of the YBCO layers within the YBCO/YSZ/SiO2/YSZ/YBCO structure are quite similar to those for a good quality single-layer YBCO film.
Research Interests: Engineering, Materials Engineering, Condensed Matter Physics, Materials Science, Superconductivity, and 14 moreApplied Physics, Nanotechnology, HIGH TEMPERATURE SUPERCONDUCTIVITY, Magnetic field, Pulsed Laser Deposition, Mathematical Sciences, Physical sciences, Sputtering, Temperature Dependence, Silicon Oxide, Single Crystal, Epitaxy, Copper Oxide, and Electric Conductivity
The correlation in type-II superconductors between the creep rate S and the Second Magnetization Peak (SMP) phenomenon which produces an increase in Jc, as a function of the field (H), has been investigated at different temperatures by... more
The correlation in type-II superconductors between the creep rate S and the Second Magnetization Peak (SMP) phenomenon which produces an increase in Jc, as a function of the field (H), has been investigated at different temperatures by starting from the minimum in S(H) and the onset of the SMP phenomenon detected on a FeSe0.5Te0.5 sample. Then the analysis has been extended by considering the entire S(H) curves and comparing our results with those of many other superconducting materials reported in literature. In this way, we find evidence that the flux dynamic mechanisms behind the appearance of the SMP phenomenon in Jc(H) are activated at fields well below those where the critical current starts effectively to increase. Moreover, the found universal relation between the minimum in the S(H) and the SMP phenomenon in Jc(H) shows that both can be attributed to a sequential crossover between a less effective pinning (losing its effectiveness at low fields) to a more effective pinning ...
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The fundamental harmonic of the AC magnetic susceptibility of a Fe(Se,Te) single crystal iron based superconductor has been studied at different AC field amplitudes and frequencies with and without a superimposed DC magnetic field with... more
The fundamental harmonic of the AC magnetic susceptibility of a Fe(Se,Te) single crystal iron based superconductor has been studied at different AC field amplitudes and frequencies with and without a superimposed DC magnetic field with the aim of analyzing its superconducting properties that can be also suitable for power applications such as the critical current density and the pinning energy in AC regime. From the analysis of the fundamental Cole-Cole plots a finite influence of flux creep phenomena has been detected. Its study, by means of the Arrhenius plots, evidences thermal activation energy U values as high as 103 K, whose dependence on the DC and AC fields, and of the temperature, has been also obtained. The magnetic field dependence shows the existence of the strong pinning regime characterized by plastic deformations of the vortex lattice, whereas the temperature dependence of the critical current density Jc(T) is consistent with the δl pinning model. Finally, the irreversibility field values Hirr(T) have been extracted from the Jc(T) at the different AC frequencies showing very high values if compared with the literature
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We have performed DC magnetic moment measurements as a function of magnetic field m(H) and time m(t) on a FeSeo.94 and a silver doped FeSeo.94 + 6 wt% Ag. In particular, from the m(H) measurements, the field dependence of the critical... more
We have performed DC magnetic moment measurements as a function of magnetic field m(H) and time m(t) on a FeSeo.94 and a silver doped FeSeo.94 + 6 wt% Ag. In particular, from the m(H) measurements, the field dependence of the critical current density Jc(H) of the samples has been extracted at different temperatures in the framework of the Bean critical state model. Starting from the Jc(H) of the samples, we have studied the influence of the silver doping on the temperature dependence of the irreversibility field Hirr(T) above which the pinning energy is so reduced that the vortices are free to move. The obtained Hirr(T) values have been fitted with the equation Hirr(T) = Hirr(0) (1-T/T*)n whose results have been compared with the literature. After that, we have analyzed the silver doping effects on the field dependence of the pinning energy U(H), extracted from relaxation measurements at different temperatures and magnetic fields, obtaining that both samples undergo to a pinning reg...
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The correlation between the appearance of a peak effect in the critical current of a superconducting material and the presence of twin boundaries, involved in a crossover between different pinning regimes, is investigated by means of dc... more
The correlation between the appearance of a peak effect in the critical current of a superconducting material and the presence of twin boundaries, involved in a crossover between different pinning regimes, is investigated by means of dc magnetic measurements on a FeSe0.5Te0.5 crystal. In particular, by analyzing the temperature dependence of the critical current density J c(T) for different magnetic fields H, a crossover from a weak pinning regime to a strong pinning regime has been revealed. The analysis shows that this crossover can be ascribed to the presence of twin boundary defects inside the sample, and can be associated to the onset of the peak effect and interpreted as the start of the vortex dynamic processes responsible for the increase of J c with the field. On the basis of the information extracted by our analysis, a plausible dynamic scenario involving the contribution of the different pinning regimes depending on the applied field has been described, and the relative H(T) vortex phase diagram has been determined. Moreover, in our description, the peak in the J c(H) curve corresponds to the end of the processes leading to the peak effect and it is confirmed to be related to the transition from an elastic to a plastic deformation regime in the vortex lattice
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Ion irradiation of superconductors allows both the establishment of their radiation hardness and the modification and optimization of their properties useful for applications. In this work, we investigate the effects of proton irradiation... more
Ion irradiation of superconductors allows both the establishment of their radiation hardness and the modification and optimization of their properties useful for applications. In this work, we investigate the effects of proton irradiation with different energies on Fe(Se,Te) thin films grown on CaF$_2$ and on buffered YSZ substrates. These systems allowed us to perform preliminary studies for the development of Fe(Se,Te) coated conductors. Critical temperature and critical current were measured for different levels of displacement damage, and with ion implantation happening at various depths inside the substrate, as evaluated by simulations with Monte Carlo codes. All measurements evidenced that these Fe(Se,Te) films are robust against proton induced damage, and that an increase of critical current can be achieved introducing pointlike defects in the superconductor. However, we find that damage induced in the substrate also plays a crucial role in modifying superconducting film properties through a variation of the strain exerted on the film. This strain degrades superconducting parameters and should be minimized.
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The development of novel superconducting photon detectors requires the investigation of non-traditional materials to exploit their specific properties and realize novel/enhanced devices. Nd2−xCexCuO4±5 is a moderate high transition... more
The development of novel superconducting photon detectors requires the investigation of non-traditional materials to exploit their specific properties and realize novel/enhanced devices. Nd2−xCexCuO4±5 is a moderate high transition temperature cuprate superconductor exhibiting n-type conduction which properties can be changed by modifying its cerium and oxygen content. Ultra-thin films of this compound have been deposited by dc sputtering technique, and systematically characterized by using X-ray diffraction and electrical measurements. Design and patterning of sample geometries have been performed by optical and electron beam lithography in order to obtain sub-micron wide strips for measurements of photon detection.
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We report theoretical and experimental results on the transition metal pnictide WP. The theoretical outcomes based on tight-binding calculations and density functional theory indicate that WP is a three-dimensional superconductor with an... more
We report theoretical and experimental results on the transition metal pnictide WP. The theoretical outcomes based on tight-binding calculations and density functional theory indicate that WP is a three-dimensional superconductor with an anisotropic electronic structure and nonsymmorphic symmetries. On the other hand, magnetoresistance experimental data and the analysis of superconducting fluctuations of the conductivity in external magnetic field indicate a weakly anisotropic three-dimensional superconducting phase.
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Irradiation effects are often used to improve the effective pinning in superconductors, but sometimes they can reveal detrimental for superconducting properties. Fe(Se,Te) has been proven to be a very robust material against irradiation,... more
Irradiation effects are often used to improve the effective pinning in superconductors, but sometimes they can reveal detrimental for superconducting properties. Fe(Se,Te) has been proven to be a very robust material against irradiation, in particular proton irradiation, thus configuring as an ideal material to work in harsh environments such as particle accelerators or fusion reactors. Anyway, the study of the pinning activation energy in Fe(Se,Te) thin film irradiated by 3.5 MeV protons suggests that this treatment can modify the anisotropy of the films pinning. Thus here we present the result of further investigation analyzing the effect of proton irradiation on the critical current and the pinning force both for the magnetic field applied parallel and perpendicular to the sample surface. We find that, although a slight effect on the critical current anisotropy is observed, the pinning landscape is not affected by the irradiation process. This confirms that Fe(Se,Te) can be consi...
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In a quench event determined by instability of the flux-flow regime, the quenching current <italic>I</italic><sup> *</sup> can be much higher than the critical current... more
In a quench event determined by instability of the flux-flow regime, the quenching current <italic>I</italic><sup> *</sup> can be much higher than the critical current <italic>I<sub>c</sub></italic>; thus, it sets the upper limit to the superconducting operational regime. In iron-based Fe(Se,Te) superconducting thin films, which can be relevant to fabricate conductors for high magnetic field applications, substantial differences between the <italic>I<sub>c</sub> </italic> and <italic>I</italic><sup>*</sup> behavior as a function of the applied magnetic field have recently been observed. Both current values are strongly influenced by flux pinning; thus, anisotropy effects are expected to play a role. Here, we report the first study of the angular dependence on the magnetic field orientation of the quenching current in Fe(Se,Te) thin films grown on a CaF<sub>2</sub> substrate, which have already shown a weak anisotropic critical current dependence on the applied magnetic field orientation. The quenching current is found even less sensitive than the critical current with respect to angular variations.
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Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific... more
Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content only by a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here, we report the occurrence of superconductivity in oxygen-deficient Nd_2-xCe_xCuO_4 thin films grown in oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between CuO_2 planes in the crystal structure. Since this distance is correlated to the concentration of oxygens in apical positions, and since oxygen content cannot substantially increase during annealing, our experiments indicate that the superconducting phase transition has to be ascribed to a migration of oxygen ions to apical positions during...
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Cooling efficiency and thermal stability is strictly demanding for practical applications of superconductors operating at current values close to the critical current, such as superconducting detectors. Indeed, a thermally unstable device... more
Cooling efficiency and thermal stability is strictly demanding for practical applications of superconductors operating at current values close to the critical current, such as superconducting detectors. Indeed, a thermally unstable device can show premature quench, i.e. it can suddenly switch from the superconducting state to the normal one at a current value lower than the expected one, which can result in false counts. Cooling by direct contact with a liquid He bath is considered the best way to obtain thermal stability in a superconducting device. Other, cheaper cooling techniques can be suitable to achieve satisfactory working conditions. In this work, we evaluate the impact of three different cooling environments, namely liquid He in a standard cryostat and both dynamic and static He gas in a cryogen-free cryostat, on current voltage characteristics (CVCs) acquired in ultra-thin superconducting microbridges suitable for detectors. In particular, we use the Flux-Flow Instability phenomenon as a tool to analyze voltage stability in CVCs in the three different environments and we find that cryogen-free techniques have performance comparable to liquid He cooling.
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Epitaxial ultra-thin films of the electron-doped compound Nd2-xCexCuO4±δ (NCCO) have been fabricated by dc sputtering technique. The NCCO parent compound, namely Nd2CuO4, has been used as buffer layer to improve the crystalline properties... more
Epitaxial ultra-thin films of the electron-doped compound Nd2-xCexCuO4±δ (NCCO) have been fabricated by dc sputtering technique. The NCCO parent compound, namely Nd2CuO4, has been used as buffer layer to improve the crystalline properties because of the very small lattice parameter mismatch, as well as compatible depositions conditions, compared to NCCO. A deep morphological and structural characterization has been carried out on several samples, by means of surface analysis techniques and X-ray diffraction, in order to optimize the growth procedure.
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The magnetization M of an Fe(Se, Te) single crystal has been measured as a function of temperature T and dc magnetic field H. The sample properties have been analyzed in the case of a magnetic field parallel to its largest face H||ab.... more
The magnetization M of an Fe(Se, Te) single crystal has been measured as a function of temperature T and dc magnetic field H. The sample properties have been analyzed in the case of a magnetic field parallel to its largest face H||ab. From the M(T) measurement, the Tc of the sample and a magnetic background have been revealed. The superconducting hysteresis loops M(H) were between 2.5 K and 15 K showing a tilt due to the presence of a magnetic signal measured at T > Tc. From the M(H) curves, the critical current density Jc(H) has been extracted at different temperatures showing the presence of a second magnetization peak phenomenon. By extracting and fitting the Jc(T) curves at different fields, a pinning regime crossover has been identified and shown to be responsible for the origin of the second magnetization peak phenomenon. Then, the different kinds of pinning centers of the sample were investigated by means of Dew-Hughes analysis, showing that the pinning mechanism in the sa...
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In a quench event determined by instability of the flux-flow regime, the quenching current <italic>I</italic><sup> *</sup> can be much higher than the critical current... more
In a quench event determined by instability of the flux-flow regime, the quenching current <italic>I</italic><sup> *</sup> can be much higher than the critical current <italic>I<sub>c</sub></italic>; thus, it sets the upper limit to the superconducting operational regime. In iron-based Fe(Se,Te) superconducting thin films, which can be relevant to fabricate conductors for high magnetic field applications, substantial differences between the <italic>I<sub>c</sub> </italic> and <italic>I</italic><sup>*</sup> behavior as a function of the applied magnetic field have recently been observed. Both current values are strongly influenced by flux pinning; thus, anisotropy effects are expected to play a role. Here, we report the first study of the angular dependence on the magnetic field orientation of the quenching current in Fe(Se,Te) thin films grown on a CaF<sub>2</sub> substrate, which have already shown a weak anisotropic critical current dependence on the applied magnetic field orientation. The quenching current is found even less sensitive than the critical current with respect to angular variations.
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We study the nonlinear effects in the current-voltage characteristics of Nb superconducting thin films, induced both by the current dependence of the pinning force and by the electric-field dependence of the flux-flow resistance of... more
We study the nonlinear effects in the current-voltage characteristics of Nb superconducting thin films, induced both by the current dependence of the pinning force and by the electric-field dependence of the flux-flow resistance of Abrikosov vortices driven by high electric currents. Despite of the quite strong pinning in our samples, by increasing the bias current, in a temperature dependent magnetic-field range, the moving vortex system undergoes a dynamical transition from a disordered to an ordered vortex lattice. Such transition leads to a quite sharp reduction of the dynamic pinning force corresponding to a peak in the current dependence of the differential flux-flow resistance. On the same samples, in a higher voltage-velocity regime, for a different temperature dependent magnetic-field range, an instability of the moving vortex lattice also occurs, with a sudden jump in the $I\text{\ensuremath{-}}V$ curve from the non linear flux-flow branch to the linear normal resistive state. Within the Larkin-Ovchinnikov velocity dependence of the flux-flow resistance, this flux-flow instability has been studied as function of the magnetic field and temperature in order to get out its nonequilibrium electronic nature. Finally, we propose a dynamic phase diagram to describe the vortex lattice motion driven by high electric currents in the presence of such a disordered pinning landscape.
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The process of developing superconducting materials for large scale applications is mainly oriented to optimize flux pinning and the current carrying capability. A powerful approach to investigate pinning properties is to combine high... more
The process of developing superconducting materials for large scale applications is mainly oriented to optimize flux pinning and the current carrying capability. A powerful approach to investigate pinning properties is to combine high resolution imaging with transport measurements as a function of the magnetic field orientation, supported by a pinning modelling. We carry out Transmission Electron Microscopy, Electron Energy Loss Spectroscopy and critical current measurements in fields up to 16 T varying the angle between the field and c-axis of Fe(Se,Te) epitaxial thin films deposited on CaF2 substrates. We find evidence of nanoscale domains with different Te:Se stoichiometry and/or rotated and tilted axes, as well as of lattice distortions and two-dimensional defects at the grain boundaries. These elongated domains are tens of nm in size along the in-plane axes. We establish a correlation between these observed microstructural features and the pinning properties, specifically stron...
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Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ (NCCO) epitaxial thin films have been deposited on (100) SrTiO$_3$ substrates by DC sputtering technique in different atmosphere. The as-grown samples show different dependence of the in-plane... more
Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ (NCCO) epitaxial thin films have been deposited on (100) SrTiO$_3$ substrates by DC sputtering technique in different atmosphere. The as-grown samples show different dependence of the in-plane resistivity at low temperature, when they are grown in pure argon atmosphere or in oxygen. Moreover, an unusual behaviour is also found when transport takes place in the presence of an external magnetic field. It is commonly accepted that the higher anisotropic properties of NCCO crystalline cell with respect to the hole doped YBCO and LSCO and the electric conduction mainly confined in the CuO$_2$ plane, strongly support the two-dimensional (2D) character of the current transport in this system. Results on the temperature dependence of the resistance, as well as on the magnetoresistance and the Hall coefficient, obtained on epitaxial NCCO thin films in the over-doped region ($x\ge0.15$) of the phase diagram are presented and discussed.
In superconducting films, the dissipative flux flow state plays a crucial role in the stability of those devices that operate under bias current whose value is very close to the critical current value, such as superconducting electronics... more
In superconducting films, the dissipative flux flow state plays a crucial role in the stability of those devices that operate under bias current whose value is very close to the critical current value, such as superconducting electronics and superconducting single particle/photon detectors. Therefore, we study the voltage stability versus electrical current transport by proper current-voltage measurement modes in superconducting films, investigating the effects of ion irradiation on the relevant current parameters.
We analyze Fiske resonances of one-dimensional parallel arrays of underdamped Josephson tunnel junctions. They appear in the current voltage (I–V) characteristics as resonant current singularities (steps) at finite voltages Vm when a... more
We analyze Fiske resonances of one-dimensional parallel arrays of underdamped Josephson tunnel junctions. They appear in the current voltage (I–V) characteristics as resonant current singularities (steps) at finite voltages Vm when a magnetic field H is applied perpendicular to the array cells. We present measurements of current step amplitudes Icm, and of the maximum Josephson current Ic0 as a function of H, for arrays made of four, six, and ten small Josephson junctions. The I–V characteristics of the arrays exhibit three, five, and eight resonant current steps, respectively, at increasing voltages. In all devices we find that the current amplitude of the highest order step has just one maximum occurring at H≈1/2H*, being H* the first field value where Ic0(H*)≈Ic0(0). Numerical simulations of the phase dynamics in small parallel arrays as a function of the applied magnetic flux are performed. The results of the simulation reproduce the experimentally observed features.
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Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the... more
Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(SeTe x ) and FeSe samples by current–voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, whereas the activation energy as a function of temperature, is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.
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The role of a layered structure in superconducting pinning properties is still at a debate. The effects of the vortex shape, which can assume for example a staircase form, could influence the interplay with extrinsic pinning coming from... more
The role of a layered structure in superconducting pinning properties is still at a debate. The effects of the vortex shape, which can assume for example a staircase form, could influence the interplay with extrinsic pinning coming from the specific defects of the material, thus inducing an effective magnetic field dependence. To enlighten this role, we analysed the angular dependence of flux pinning energy U(H,θ) as a function of magnetic field in FeSe0.5Te0.5 thin film by considering the field components along the ab-plane of the crystal structure and the c-axis direction. U(H,θ) has been evaluated from magneto-resistivity measurements acquired at different orientations between the applied field up to 16 T and FeSe0.5Te0.5 thin films grown on a CaF2 substrate. We observed that the U(H,θ) shows an anisotropic trend as a function of both the intensity and the direction of the applied field. Such a behaviour can be correlated to the presence of extended defects elongated in the ab-pl...
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We performed a detailed investigation of the superconducting properties of polycrystalline Pr1−xCexPt4Ge12 pellets. We report the effect of Ce substitution, for x = 0.07, on magnetic field phase diagram H-T. We demonstrate that the upper... more
We performed a detailed investigation of the superconducting properties of polycrystalline Pr1−xCexPt4Ge12 pellets. We report the effect of Ce substitution, for x = 0.07, on magnetic field phase diagram H-T. We demonstrate that the upper critical field is well described by the Ginzburg–Landau model and that the irreversibility field line has a scaling behaviour similar to cuprates. We also show that for magnetic fields lower than 0.4 T, the activation energy follows a power law of the type ?−1/2, suggesting a collective pinning regime with a quasi-2D character for the Ce-doped compound with x = 0.07. Furthermore, by means of a point contact Andreev reflection spectroscopy setup, we formed metal/superconductor nano-junctions as small as tens of nanometers on the PrPt4Ge12 parent compound (x = 0). Experimental results showed a wide variety of conductance features appearing in the dI/dV vs. V spectra, all explained in terms of a modified Blonder–Tinkham–Klapwijk model considering a sup...
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We report the experimental results of the field H and temperature T dependent magnetization M measurements of MgB 2 superconducting thin films deposited by pulsed laser deposition. MgB 2 thin films were obtained from a precursor layer... more
We report the experimental results of the field H and temperature T dependent magnetization M measurements of MgB 2 superconducting thin films deposited by pulsed laser deposition. MgB 2 thin films were obtained from a precursor layer deposited in UHV at room temperaure from stoichiometric target, then an annealing treatment in Mg atmosphere allowed to crystallize the superconducting phase. From the hysteresis loops at each temperature of measurement the critical current density Jc(H) has been extracted according to Brandt's model for a transverse geometry. At low temperatures an anomalous behavior has been observed in the hysteresis loops: as the exernal magnetic field approaches the zero field region on the upper and lower branches, a double peak of the magnetization occurs. This magnetic flux jumping can be explained within an adiabatic critical state model, and can be ascribed to thermomagnetic instabilities as in bulk samples.