The influence of the cooling method on thermal stability is a non-trivial aspect for superconducting material to be used in several applications, for example, in photon detection. Indeed, a wasteful cooling can induce quenches in the device that can lead to false counts. So far, the efficient cooling has been realized by immersion of the superconducting device in a liquid He bath. However, cheaper cryogen-free (CF) cooling techniques are now commercially available, as well as cryocoolers are becoming the only way to obtain a cooling environment in the liquid He shortage. Here we consider three different cooling methods: one is by liquid He in a standard cryostat and the other two are a dynamic or a static He gas cooling in a CF cryostat. Then, we are able to evaluate the performance of the cooling method by the impact on current-voltage curves at very high bias currents. In particular, we acquire current-voltage characteristics on ultra-thin microbridges made by two different superconducting materials commonly used in detectors fabrication, which are NbN and NbTiN. Here, the flux-flow instability (FFI) phenomenon is used as a tool to determine the influence of the three different cooling techniques on the voltage stability of the devices under current biasing. It results that the CF cooling method has performance comparable to liquid He bath. This finding supports the spread of the CF technique for applications, and it validates the FFI as a tool to test superconducting materials.

A Fe(Se,Te) iron based crystal has been studied using dc magnetic measurements as a function of temperature (T) and magnetic field (H). Firstly, the field dependence of the critical current density Jc has been extracted from the superconducting hysteresis loops m(H) at different temperatures within the Bean critical state model in order to obtain information about the transport properties of the sample. The knowledge of the critical current density values as a function of the field for temperatures near Tc allow us to determine the values and the behavior of the irreversibility field Hirr as a function of the temperature. On the other hand, by means of zero field cooling m(T) measurements at different magnetic fields, the upper critical field Hc2 has been evaluated as a function of the temperature. Both irreversibility field and upper critical field have been fitted by the equation H(T) = H(0)(1 − T/T*)n allowing us to obtain the values of the related parameters which are in good agreement with those found in literature for the IBS systems.

Morphological, compositional and magnetic properties of single crystals of Ba2CuGe2O7 grown in oxygen and in dry air have been investigated. It is shown that the use of different atmospheres influences the morphological and compositional characteristics, probably because of some secondary reactions, that occur on the surface of the sample when oxygen is used, but it does not change the structural and magnetic properties. In the case of samples grown in oxygen, a thin, dark superficial layer forms. In this layer impurity particles of BaCu2Ge2O7 are present, while the core is formed by pure Ba2CuGe2O7.

In superconducting materials a dynamical rearrangement of the vortex lattice occurs by forcing vortices at high velocities, until the system can become unstable. This phenomenon is known as vortex lattice instability, in which a sudden transition drives the superconducting system abruptly to the normal state. We present an experimental study on submicron bridges of NbN and NbTiN ultra-thin films with a thickness of few nanometers. The nanoscale effect on vortex lattice instability is investigated not only by the ultra-thin thickness in wide bridges, but also by changing the direction of the external magnetic field applied parallel and perpendicular to the c-axis epitaxial films Indeed, measurements are performed for both orientations and show the vortex lattice instability, regardless of the superconducting material. Critical currents I-c as well as instability currents I* have been compared. However, only in the parallel configuration an unusual flying birds' feature appears in the magnetic field dependence of current switching, as a consequence of the ratio I*/I-c that is approaching 1. This amazing tendency becomes relevant for practical applications involving nanostructures, since by scaling down sample thickness and rotating the external field towards the in-plane orientation, the ultra-thin film geometry can mimic the bridge narrowing down to the nanoscale.

We study the temperature dependence of the upper critical magnetic field H-c2 in a layered iron-based superconducting (IBS) material of the 11-family, namely the Fe(Se,Te) thin film grown on CaF2 substrate. On the basis of intrinsic anisotropy as well as system dimensionality, it turns useful to make a comparison with an ultrathin film conventional low temperature superconductors (LTS) such as NbN, mostly used for device applications. We compare the anisotropy factors as a function of temperature and magnetic field. Both materials present a peculiar behavior: The LTS behaves as a strong anisotropic system, whereas the IBS shows very weak anisotropic features. The strong NbN character can be directly ascribed to the dimensionality of the ultrathin film employed, thus revealing a geometry effect. The weak Fe(Se,Te) trend should be related to its layered crystallographic structure, thus probing an intrinsic origin of its anisotropy. These characteristics become relevant for the potential application of this material in coated conductor technology.

At the present day, iron-based superconductors (IBS) can he considered as competitors of cuprates for the fabrication of superconducting cables for high field application in the presence of particle irradiation. In particular, it has been demonstrated that 11-IBS are very robust against proton irradiation. In this paper, we analyze the effect of this kind of irradiation on the anisotropy of superconducting and pinning properties of Fe(Se,Te) thin films grown on CaF2. By electrical transport measurements, we evaluate the upper critical field as a function of the temperature and the pinning activation energy as function of the applied magnetic field, for different orientation between the material crystal structure and the field itself, in pristine and irradiated samples. The aim is to recognize if and how proton irradiation modifies the anisotropy of the material properties.

Anisotropy effects on flux pinning and flux flow are strongly effective in cuprate as well as iron-based superconductors due to their intrinsically layered crystallographic structure. However Fe(Se,Te) thin films grown on CaF2 substrate result less anisotropic with respect to all the other iron based superconductors. We present the first study on the angular dependence of the flux flow instability, which occurs in the flux flow regime as a current driven transition to the normal state at the instability point (I*, V*) in the current-voltage characteristics. The voltage jumps are systematically investigated as a function of the temperature, the external magnetic field, and the angle between the field and the Fe(Se,Te) film. The scaling procedure based on the anisotropic Ginzburg-Landau approach is successfully applied to the observed angular dependence of the critical voltage V*. Anyway, we find out that Fe(Se,Te) represents the case study of a layered material characterized by a weak anisotropy of its static superconducting properties, but with an increased anisotropy in its vortex dynamics due to the predominant perpendicular component of the external applied magnetic field. Indeed, I* shows less sensitivity to angle variations, thus being promising for high field applications.

In a superconducting photon detector, the effective transition to the normal state can be induced by the instability of the flux flow regime. Indeed, in the presence of self-magnetic field, the local suppression of superconductivity induced by photon absorption determines vortex nucleation and flux flow regime, which can make the superconducting state unstable. Understanding such instability can boost the performances of those superconducting devices based on this resistive switching. Here, we present the study of the geometry influence on such instability in NbN and NbTiN ultrathin films. Despite the same patterned microbridge geometry, the two superconductors show different behaviors at very low applied magnetic fields. A comparison with other superconductors outlines the possibility to tune the resistive switching by geometry effects in interesting materials for devices applications. Finally, we also report the influence of the cooling environment on the electric critical power in superconducting thin films.

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.

In a quench event determined by instability of the flux-flowregime, the quenching current I* can be much higher than the critical current Ic; 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 Ic and I* 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 themagnetic field orientation of the quenching current in Fe(Se, Te) thin films grown on a CaF2 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.

Thin films of the electron-doped high-temperature superconductor Nd2−xCexCuO4±δ have been deposited by dc sputtering technique on (100) SrTiO3 substrates. A tuning of the oxygen content in the as-grown non-superconducting samples has been achieved by changing the oxygen partial pressure during the growth in the Argon sputtering atmosphere. All samples show the superconducting transition after a suitable two-step thermal treatment in an oxygen-reducing environment. Structural and electrical transport properties on the as-grown as well as on the superconducting samples have been investigated. We find that the structural properties are consistent with a deficiency of the oxygen content with respect to optimally annealed samples, and that the transition to the superconducting phase is always accompanied by an increase of the c-axis lattice parameter. Measurements of the Hall coefficient RH as a function of temperature and in the normal state of our epitaxial films are presented and discussed. RH results negative for all the films regardless of the oxygen content and it decreases with the temperature. In particular, the Hall coefficient is only about 10% lower than the value measured in the as-grown oxygen-deficient phase, in contrast to the results reported in literature. The removal of the excess oxygen in as-grown samples seems not to be the only requirement for triggering the superconducting transition in electron-doped compounds. The microstructural change associated with the increase of the c-axis parameter in our deoxygenated samples could help in understanding the microscopic mechanism underlying the reduction process of n-type superconductors, which is still under debate.

Iron-based superconductors are an interesting class of materials that have attracted significant interest during the last years, and which could be employed in a variety of potential applications due to their high upper critical field H c2 . In order to exploit the properties of these materials for practical applications, in this paper we explore two different routes for the fabrication of polycrystalline samples belonging to 11-family. In particular, the influence of dedicated heat treatments has been investigated to optimize their overall superconducting properties. Then, we compared the pinning properties of these two types of samples to correlate the fabrication process with the pinning landscape. The final aim is to obtain an increased T c and H c2 , as well as an enhanced pinning efficacy, which could lead to higher critical current density J c relevant for practical uses.

Superconducting vortices are a well known class of vortices, each of them carrying a single magnetic flux quantum. In this chapter the authors present the results of low temperature Magnetic Force Microscopy experiments to investigate the nucleation and dynamics of superconducting vortices in magnetically coupled Superconductor/Ferromagnet (S/F) heterostructures made by Nb/Py. It is here shown that by controlling the thicknesses of both S and F layer, the formation of spontaneous vortex-antivortex pairs (V-AV) can be favored and their confinement and mobility can be tuned. The experimental results are compared with two theoretical models dealing with the spontaneous nucleation of V/AV pairs in the limits of S thickness respectively greater and smaller than the London penetration depth. It is shown that vortex nucleation and confinement is regulated by the intensity of the out-of-plane component of the magnetization with respect to a critical magnetization set by the thickness of both S and F layers. Additionally, external field cooling processes were used to probe in-field vortex nucleation and V-AV unbalancing, whereas the sweeping of an external magnetic field when below the superconducting critical temperature was used to force the vortex into motion, probing the vortex mobility/rigidity and the vortex avalanche events.

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 report the first experimental observation of the quenching of the superconducting state in current-voltage characteristics of an iron-based superconductor, namely, in Fe(Se,Te) thin films. Based on available theoretical models, our analysis suggests the presence of an intrinsic flux-flow electronic instability along with non-negligible extrinsic thermal effects. The coexistence and competition of these two mechanisms classify the observed instability as halfway between those of low-temperature and of high-temperature superconductors, where thermal effects are, respectively, largely negligible or predominant.

Electrical transport measurements and transient optical pump-probe experiments have been performed on epitaxial films of the electron doped Nd1.83Ce0.17CuO4-delta compound for studying the non-equilibrium carrier dynamics in this material. Samples have been grown on (001)-oriented SrTiO3 substrates by dc sputtering in a mixed atmosphere of both Ar and O-2. X-ray diffraction analysis and scanning electron microscope equipped with a wavelength dispersive spectroscopy detector have been used to characterize the structure and the composition of the thin films. Time-resolved femtosecond pump-and-probe spectroscopy has been also carried out on our samples in the temperature range 4.2K-300 K.

Electrical transport measurements and transient optical pump-probe experiments have been performed on epitaxial films of the electron doped Nd1.83Ce0.17Cu4−δ compound for studying the non-equilibrium carrier dynamics in this material. Samples have been grown on (001)-oriented SrTiO3 substrates by dc sputtering in a mixed atmosphere of both Ar and O2. X-ray diffraction analysis and scanning electron microscope equipped with a wavelength dispersive spectroscopy detector have been used to characterize the structure and the composition of the thin films. Time-resolved femtosecond pump-and-probe spectroscopy has been also carried out on our samples in the temperature range 4.2K-300 K.

In several superconducting applications, low energy losses are strictly demanding. In superconducting films, the dissipative flux flow state plays a role in the stability of the superconducting state in those devices that can operate just above the critical current (I-c). Therefore, details of the current-voltage (I-V) characteristics and in particular of current instabilities from the flux flow state to the normal one can become significant. We study current stability in the flux flow dissipative state by a proper current-voltage measurement mode. Low-temperature superconducting films have been investigated to demonstrate that the current stability range above I-c can be increased by light ion irradiation.

The magnetic properties of the triple-layered Sr(4)Ru(3)O(10) have been investigated by means of neutron scattering diffraction. At zero field we find that the magnetic moments are ferromagnetically coupled and oriented along the c-axis with no signatures of either long-range antiferromagnetic order or ferromagnetic components in the ab-plane. The field dependence of the reflection intensity points to a metamagnetic response involving only the planar magnetic moments. The structural refinement indicates a distinct rearrangement of the unit cell as a function of both temperature and in-plane applied field. We show that at the temperature T* ~/= 50 K, below which the metamagnetic behavior is observed, the c-axis lattice parameter exhibits a rapid increase while the in-plane amplitude saturates. A similar upturn of the in-plane lattice parameter after the quench of the c-axis amplitude occurs above a critical magnetic field.