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The development of simple, scalable, and cost-effective methods to prepare Van der Waals materials for thermoelectric applications is a timely research field, whose potential and possibilities are still largely unexplored. In this work, we present a systematic study of ink-jet printing and drop-casting deposition of 2H phase SnSe2 and WSe2 nanoflake assemblies, obtained by liquid phase exfoliation, and their characterization in terms of electronic and thermoelectric properties. The choice of optimal annealing temperature and time is crucial for preserving phase purity and stoichiometry and for removing dry residues of ink solvents at inter-flake boundaries, while maximizing the sintering of nanoflakes. An additional pressing is beneficial to improve nanoflake orientation and packing, thus enhancing electric conductivity. In nanoflake assemblies deposited by drop casting and pressed at 1 GPa, we obtained thermoelectric power factors at room temperature up to 2.2 × 10−4 mW m−1 K−2 for SnSe2 and up to 3.0 × 10−4 mW m−1 K−2 for WSe2.

In this paper the roles of texturing and thickness of different buffer layers employed in the fabrication of Fe(Se,Te) coated conductors are studied. Fe(Se,Te) thin films were deposited either on rolling assisted biaxially textured substrates substrates with highly textured CeO2 buffer layer and on HASTELLOY® C276 substrates with a randomly oriented native oxide grown through different oxidizing processes. It was found that the buffer layer has to be both thick enough to block Ni interdiffusion from the metallic tape to the Fe(Se,Te) overlayer, and textured enough to ensure good in-plane alignment of the superconducting film, and consequently good critical current densities (Jc). The best results in terms of critical temperature (Tc) and critical current were obtained using a highly textured 350 nm thick CeO2 buffer layer, but a fairly good Jc of 1.2 104 A cm−2 in self field at 5 K was also obtained using an HASTELLOY® C276 substrate with a 400 nm thick randomly oriented native oxide. The obtained results indicate that the superconducting properties of the Fe(Se,Te) coated conductors are determined by a trade-off between degree of texturing of the substrate and inhibition of Nickel interdiffusion.

In this work we investigate the effect of each different heat treatment stage in the fabrication of Bi2Sr2CaCu2O8+d superconducting wires on intra-grain and inter-grain superconducting properties. We measure magnetic critical temperature Tc values and transport critical current density Jc at temperatures from 4 K to 40 K and in fields up to 7 T. From an analysis of the temperature dependence of the self-field critical current density Jc(T) that takes into account weak link behavior and proximity effect, we study the grain boundaries (GB) transparency to supercurrents and we establish a relationship between GB oxygenation in the different steps of the fabrication process and the GB transparency to supercurrents. We find that grain boundary oxygenation starts in the first crystallization stage, but it becomes complete in the plateau at 836 {\deg}C and in slow cooling stages, and is further enhanced in the prolonged post annealing step. Such oxygenation makes GBs more conducting, thus improving the inter-grain Jc value and temperature dependence. On the other hand, from the inspection of the Tc values in the framework of the phase diagram dome, we find that grains are oxygenated already in the crystallization step up to the optimal doping, while successive slow cooling and post annealing treatments further enhance the degree of overdoping, especially if carried out in oxygen atmosphere rather than in air.

We fabricated field effect transistors which are transparent at visible wavelength, by realizing epitaxial ZnO/SrTiO3 heterostructures. High crystalline quality ZnO layers were grown by pulsed laser deposition on 110 oriented strontium titanate single crystals. By conventional photolithographic techniques we realized micrometric sized devices in both bottom-gate stacked layers geometry and planar side-gate configuration. Our transistors show 80% light transmittance, on/off ratios up to 10(6) and field effect mobilities up to 30 cm(2)/Vs at 77 K. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Femtosecond spectroscopy has been used to investigate the quasiparticle relaxation times in a nearly optimally doped chalcogenide superconducting FeSe0.5Te0.5 thin film. It is characterized by an enhancement of the critical temperature Tc = 19.1 K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time τR in the superconducting state where an energy gap Δ(T) is present. A fitting procedure based on a BCS-like assumption for τR gives Δ0 = 4.1 ± 0.4 meV for the superconducting energy gap extrapolated at T = 0, which corresponds to 2Δ0/kBTc = 4.9 ± 0.5. Accordingly, an estimation of the characteristic phonon energy and the electron–phonon coupling constant λ is reported and discussed. The obtained value λ ≈ 0.6 is in good agreement with other similar determinations obtained by different techniques and numerical approaches. The small value of λ confirms that a phonon-mediated coupling cannot be the only mechanism leading to the formation of the superconducting state in FeSeTe films.

Zinc oxide and strontium titanate are among the most interesting materials for oxide electronics due to their semiconducting and dielectric properties respectively. We obtain high electron mobility values in epitaxial ZnO thin films deposited on (110) oriented strontium titanate substrates. Values up to 400 cm 2 /V s are found below 50 K in samples grown by a two step method: firstly a thin ZnO relaxing layer is deposited on the SrTiO 3 (110) substrate at relatively low temperature (550 ∘ C) and then the deposition temperature is raised up to 750 ∘ C for the growth of a second ZnO layer. The realized epitaxial ZnO/SrTiO 3 heterostructures are used to fabricate field effect transistors transparent at visible wavelength. By conventional photolithographic techniques we realize micrometric sized devices in planar side-gate configuration. The transistors have an 80% transmittance, on–off ratios up to 10 6 , and field effect mobilities up to 30 cm 2 /V s.

We present resistivity and thermal conductivity measurements on bulk samples, prepared either by a standard method or by a one-step technique. The latter samples, due to their high density and purity, show residual resistivity values as low as 0.5 mW cm and thermal conductivity values as high as 215 W/mK, higher than the single crystal ones. Thermal and electrical data of all the samples are analysed in the framework of the Bloch-Gruneisen equation giving reliable parameter values. In particular the temperature resitivity coefficient, obtained both from resistivity and thermal conductivity, in the dirty sample comes out ten time larger than in the clean ones. This result supports the hypothesis of ref. [1] that p and s bands conduct in parallel, prevailing p conduction in clean samples and s conduction in dirty samples ., Comment: 8 pages, 5 figures, Presented at the BOROMAG workshop, June 17-19 2002, Genoa, Italy

We present a systematic characterization of Mg1−xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where π bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth λ by an inductance technique. Data fits based on the standard Bardeen–Cooper–Schrieffer two-band model yield the zero-temperature limit of both λ and superconducting gaps: we find that λ(0) increases weakly and quickly saturates with increasing x, whereas Δπ(0) and Δσ(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the π band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Δπ(0) that may be related to an inhomogeneous distribution of Li content.

Despite the long period of time elapsed since their discovery, intermetallic superconducting borocarbides in the form of thin films are rarely reported in the literature. In this paper we present our results on the growth of ErNi/sub 2/B/sub 2/C thin films by an ultra high vacuum laser ablation technique. The dependence of the film structure and properties on the deposition parameters has been studied and optimized. Strongly c-oriented superconducting films (rocking angle of 1/spl deg/) with a rather smooth surface have been obtained. Here we present the details of the deposition procedure as well as those of the structural, morphological, and electrical characterization. The feasibility of superconducting films with a tungsten buffer layer on MgO substrate has been studied successfully. Both buffer layers and bilayers have been characterized by X-ray reflectivity and atomic force microscopy.

We have shown that the superconducting properties of FeSe0.5 Te0.5 thin films are strongly dependent on the growth conditions and, in particular, the in-plane lattice constant of the substrate influences the crystallographic lattice parameters of the films, affecting the strain and is responsible for strong enhancements of the critical temperature Tc and for the introduction of different pinning mechanisms. Here we compare the structural, electrical and transport properties of superconducting Fe(Se0.5, Te0.5) epitaxial films deposited through pulsed laser ablation on three different substrates namely lanthanum aluminate (LaAlO3), strontium titanate (SrTiO3), and calcium fluoride (CaF2). We analyze in particular the critical current density Jc as a function of the temperature and magnetic field, and its anisotropy, which is related to the different pinning mechanisms in play. The film grown on SrTiO3 exhibits a higher critical current when the field is perpendicular to the film surface, opposite to what happens in the sample grown on LaAlO3 due to the presence of extrinsic pinning along the c-axis, while we observe almost no anisotropy on the thin film grown on CaF2.

Three years since the discovery by the Hosono’s group of Fe-based superconductors, an enormous number of compounds, belonging to several different families have been discovered and fundamental properties have been deeply investigated in order to clarify the interplay between magnetisms and superconductivity in these compounds. Indeed, the actual potential of these compounds for practical applications remains still unclear. Fe-based superconductors are midway between high temperature superconductors (HTSCs) and MgB 2 . In Fe-based superconductors the critical current is rather independent of the field, similarly to HTSCs, as a consequence of the exceptionally high upper critical field and strong pinning associated with nm-scale local modulations of the order parameter. They exhibit low anisotropy of the critical current with respect to the crystalline directions, as in the case of MgB 2 , which allows current flow along the c -axis. However, Fe-based superconductor polycrystalline materials currently available still exhibit electromagnetic granularity, like the HTSCs, which suppresses superconducting current flow over long length. Whether the nature of such granularity is extrinsic, as due to spurious phases or cracks between grains or intrinsic, as related to misalignment of adjacent grains, is under debate. These aspects will be reviewed in the light of the recent literature.

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

In this communication we present a transmission electron microscopy study on neutron-irradiated bulk samples. We give clear evidence that neutron irradiation creates nanometric amorphous regions within the crystal lattice. The density of these defects correctly scales with the neutron dose. These defects, whose size matches with the coherence length, act as pinning centers and are responsible for the strong increase of the critical current density at high field and for shifting the maxima of the pinning force curves toward higher fields.

Paraconductivity of MgB2 has been measured in homogeneous thin films grown by Hybrid physical chemical vapor deposition. In order to reduce the possible effects of spatial inhomogeneities, stripes of different widths were cut on the films. We measured several samples with different resistivity values; after subtracting the normal state resistivity, paraconductivity appears to be of the same order of magnitude in all the samples. The dependence on the reduced temperature epsilon = ln(T/Tc) is discussed and compared with the existing models. (c) 2007 Elsevier B.V. All rights reserved.

We measured the upper critical field H-c2 and the critical current density J(c) on neutron-irradiated MgB2 samples and studied the effects of damaging on pinning mechanisms. We observed a strong enhancement of the upper critical field and great improvement of the critical current magnetic field dependence in an intermediate range of fluence. We demonstrated that the critical current increase is not only due to the H-c2 improvement but the irradiation substantially changes the pinning force improving the critical current density at high magnetic field. (C) 2007 Elsevier B.V. All rights reserved.

The effect of disorder on normal state and super-conducting properties is studied in a series of neutron irradiated magnesium diboride thin films. Irradiation increases residual resistivity by more than two orders of magnitude and progressively decreases critical temperature down to the complete suppression of superconductivity. In this disorder-controlled set of samples, we analyze magnetoresistivity, extracting the scattering times in each band, and upper critical field. H-c2 is extremely high and only weakly depends on resistivity at low irradiation level. We propose a simple explanation for this puzzling topic, and we discuss the importance of reaching high magnetic field to correctly extrapolate H-c2 (0).

We studied the upper critical field in poly-crystalline MgB/sub 2/ samples in which disorder was introduced by different ways: Al substitution and neutron irradiation. We observed that by Al doping critical temperature T/sub c/ decreases, the resistivity increases but the upper critical field, H/sub c2/, decreases. This behavior can be explained considering that Al substitutions affect only /spl pi/ bands leaving /spl sigma/ bands in clean regime; the decrease of H/sub c2/ can be correlated with suppression of the larger gap in doped samples. On the other hand neutron irradiation produces an increase of H/sub c2/. Neutron irradiation was carried on up to fluences of 1.4/spl middot/10/sup 20/ n/cm/sup 2/. To obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the /sup 11/B isotope. Up to fluences of 10/sup 18/ n/cm/sup 2/ the critical temperature is slightly diminished (36 K) and the superconducting properties are improved; the upper critical field is significantly increased: from 13.5 T to 20.3 T at 12 K. For larger neutron fluences the critical temperature is suppressed down to 9.2 K and the superconducting properties come out strongly degraded.

The development of thin film deposition techniques allowed the growth of new artificial materials leading to the discovery of new high T/sub c/ superconducting compounds as infinite layer based superlattices [(BaCuO/sub 2/)/sub m//(CaCuO/sub 2/)/sub n/]. The order of the superlattice structure seems to be one of the crucial parameters to be improved so as to induce a superconductivity onset in these materials and to make them suitable for technological applications. In this paper we present a study on the correlation between structural disorder and transport properties of several artificial superlattices grown by pulsed laser ablation technique. Our goal was to simulate the layered structure of copper oxide superconductors, so as to create new artificial superconducting materials and study the charge transfer mechanism. We chose infinite layer phases (SrCuO/sub 2/, CaCuO/sub 2/, BaCuO/sub 3/) for the blocks containing CuO/sub 2/ planes and doped infinite layer phases (CaCu/sub 1-x/Sc/sub x/O/sub y/, SrCu/sub 1-x/Sc/sub x/O/sub y/, CaCu/sub 1-x/Ag/sub x/O/sub y/ and SrCu/sub 1-x/Ag/sub x/O/sub y/, with x ranging from 0.1 to 0.2) for the charge reservoir blocks. By means of reflection high energy electron diffraction (RHEED) and X-ray structural characterization as well as resistivity and Hall effect measurements, we perform a comparative analysis of structural and transport properties of these samples and we tried to explain them in terms of Anderson's theory of disorder.

Infinite layer based superlattices [(BaCuO/sub 2/)/sub m//(CaCuO/sub 2/)/sub n/] are a new kind of artificial superconductors with interesting properties both for fundamental comprehension purposes and for future possible applications. The artificial nature of these compounds offers the possibility of analyzing the influence of the structural properties on the superconducting behavior. For example we can study the correlation between the number of CuO/sub 2/ planes contained in the infinite layer block (CaCuO/sub 2/) and the transport properties. In this paper, we present transport properties characterization of some thin films of this family containing different numbers of CuO/sub 2/ planes. Such films have been grown by the pulsed laser deposition (PLD) technique. In order to investigate the influence of the number of CuO/sub 2/ planes and of the distance between them on critical temperature, charge concentration, and anisotropy, we perform measurements of resistivity, magnetoresistivity and Hall coefficient as functions of temperature and applied magnetic field.

The magnetic behaviour of type-II superconducting thin films subjected to a transverse external field can be explained taking demagnetising effects into account. We studied this problem following two different approaches: on the one hand, we computed flux diffusion and circulating currents, on the basis of a recent model presented in the literature; on the other hand, we carried out measurements of local magnetic field on the film surface by means of a self-made Hall probe magnetometer. We compared the measured and the computed hysteresis loops and magnetisation decays and we found good agreement. By the fit of the experimental hysteresis loops, we also obtained the dependence of the critical current density on temperature and local magnetic field in YBCO thin films.

The employment of superconducting thin films as electronic devices seems to be the most probable future application of these materials. For this application, thin films with extremely smooth surface and very good superconducting as well as structural properties are required. We deposited YBCO epitaxial thin films by pulsed laser ablation using a pulsed molecular oxygen flow synchronous with the arrival of the ablated species on the substrate. We studied the dependence of their surface morphology on the deposition pressure and we found an evident improvement in the surface quality by lowering the deposition pressure below mbar. Films grown in these conditions showed surface roughness lower than 1 ?, transition temperatures above 88 K and critical current densities of about at 4.2 K in self-field.

From recent literature, it became clear that the crystallographic lattice parameters and the superconducting properties of FeSe0.5Te0.5 thin films exhibit a non-trivial dependence on the in-plane lattice constant of the substrates on which they are grown. The strain, which depends on the type of growth, can play an important role both in enhancing the critical temperature Tc and in determining the pinning mechanisms. Here, we present the effects of the substrate on the superconducting properties of FeSe0.5Te0.5 thin films. After a comprehensive overview of the different substrates used, i.e. oxides and fluorides, we compare the superconducting properties of films grown on LaAlO3(001) and SrTiO3(001). We show that the pinning properties of the two types of film are completely different: the angular dependences of the critical current density are opposite due to the presence of extrinsic pinning along the c-axis on films grown on SrTiO3. The presence of strong correlated pinning for a field perpendicular to the surface in films grown on SrTiO3 is confirmed by the analysis of the activation energy for vortex motion U0 and supported by the observation through scanning tunnelling microscopy of nanoscale threading dislocations possibly induced by the lattice mismatch with the substrate, which are not seen on the films deposited on LaAlO3.

We report on measurements of the superconducting properties of FeSe0.5Te0.5 thin films grown on lanthanum aluminate. The films have high transition temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T. The temperature dependence of the upper critical field and the irreversibility lines are steep and anisotropic, as recently reported for single crystals. The critical current densities, assessed by magnetization measurements in a vector VSM, were found to be well above 109 A m-2 at low temperatures. In all samples, the critical current as a function of field orientation has a maximum, when the field is oriented parallel to the film surface. The maximum indicates the presence of correlated pinning centers. A minimum occurs in three films, when the field is applied perpendicular to the film plane. In the fourth film, instead, a local maximum caused by c-axis-correlated pinning centers was found at this orientation. The irradiation of two films with fast neutrons did not change the properties drastically, where a maximum enhancement of the critical current by a factor of two was found.

In this paper we carry out a direct comparison between transport and superconducting properties?namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field?of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO1 ? xFx with the largest Tc and the largest anisotropy and Fe1 + yTe1 ? xSex, with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAsO1 ? xFx series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe1 + y(Te1 ? x, Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1 + y(Te1 ? x, Sex) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the Tc and the upper critical field Hc2. The huge Hc2 values of the Fe1 + yTe1 ? xSex samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO1 ? xFx samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.

We study the critical current density Jc of a series of MgB2 epitaxial thin films with disorder introduced in a controlled way by neutron irradiation. We find that the in-field Jc is significantly increased for an intermediate neutron fluence of ~7.6 × 1017 cm−2, reaching ~1.8 × 105 A cm−2 at μ0H = 4 T, which is larger by more than one order of magnitude than the critical current density of the pristine sample. Moreover, in the same irradiated sample, Jc(μ0H = 9 T) is still as large as 1.1 × 104 A cm−2. By transmission electron microscopy, we find evidence of the presence of amorphous regions created by irradiation whose average size matches the coherence length ξab. Analysis of the shape of the critical current density and pinning force curves indicates that such regions with a suppressed superconducting order parameter behave as effective point-like pinning centers for magnetic flux lines.

We present a systematic study of the superconducting properties of high quality V3Si thin films, grown by high vacuum pulsed laser deposition from a stoichiometric target, a technique never reported in literature for this compound. By changing both the substrate (crystal structure and orientation) and the deposition conditions (substrate temperature, target-substrate distance, laser frequency and pulse energy) the critical temperature and the resistivity values of the samples have been finely tuned. Best results (TC = 16.1 K and Residual Resistivity Ratio RRR = 8.5) are obtained for deposition temperatures higher than 1200degC on both LaAlO3 and MgO substrates. All the samples are polycrystalline, that however does not affect the good quality of the transport properties. The possibility of multiband superconductivity in this compound is investigated, probing as a function of temperature: (i) the critical field up to 28 T using voltamperometric measurements, and (ii) the superfluid density via magnetic penetration depth measurements with a single coil inductive technique operating in the MHz region.