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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.

Anisotropy of transport and magnetic properties of parent compounds of iron based superconductors is a key ingredient of superconductivity. In this work, we investigate in-plane and out-of-plane properties, namely thermal, electric, thermoelectric transport and magnetic susceptibility in a high quality BaFe2As2 single crystal of the 122 parent compound, using a combined experimental and theoretical approach. Combining the ab initio calculation of the band structure and the measured in-plane and out-of-plane resistivity, we evaluate the scattering rates which turn out to be strongly anisotropic and determined by spin excitations in the antiferromagnetic state. The observed anisotropy of thermal conductivity is discussed in terms of anisotropy of sound velocities which we estimate to be [Formula: see text]. Remarkably, we find that thermal conductivity is characterized by a sizeable electronic contribution at low temperature, which is ascribed to the high purity of our crystal.

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.

By carrying out magnetotransport measurements up to 30 T, we succeeded in detecting and analyzing quantitatively Shubnikov--de Haas oscillations in two crystals of the $RE\mathrm{FeAsO}$ ($RE=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth}$) family of iron pnictides, namely, the undoped SmFeAsO and the superconducting fluorine underdoped SmFeAs(O,F). We combine experimental outcomes and ab initio calculations of electronic structure to get the following direct evidences: (i) the Shubnikov--de Haas oscillations are determined by a two-dimensional hole band, whose effective mass is ${m}^{*}=(0.50\ifmmode\pm\else\textpm\fi{}0.1)\phantom{\rule{0.16em}{0ex}}{m}_{e}$ in the undoped sample and ${m}^{*}=(0.89\ifmmode\pm\else\textpm\fi{}0.1)\phantom{\rule{0.16em}{0ex}}{m}_{e}$ in the F-doped sample, while classical magnetotransport is dominated by an electron band; (ii) in the superconducting compound we give a quantitative account of the low effectiveness of electron doping associated with fluorine substitution.

Structural and transport properties in the normal and superconducting states are investigated in a Ca0.8La0.2FeAs2 single crystal with Tc = 27 K, belonging to the newly discovered 112 family of iron based superconductors. The transport critical current density Jc for both field directions measured in a focused ion beam patterned microbridge reveals a weakly field dependent and low anisotropic behaviour with a low temperature value as high as Jc(B = 0) ∼ 105 A/cm2. This demonstrates not only bulk superconductivity but also the potential of 112 superconductors towards applications. Interestingly, this superconducting compound undergoes a structural transition below 100 K which is evidenced by temperature-dependent X-ray diffraction measurements. Data analysis of Hall resistance and magnetoresistivity indicate that magnetotransport properties are largely dominated by an electron band, with a change of regime observed in correspondence of the onset of a structural transition. In the low temperature regime, the contribution of a hole band to transport is suggested, possibly playing a role in determining the superconducting state.

In this paper we analyze transport data of different families of Fe-based superconductors, within a pseudogap framework. The Fe(Te, Se) samples exhibit s-shaped resistivity ρ(T) curves, while SmFeAs(O, F) ones present a departure from their high-temperature linear behavior. In either case, a characteristic temperature can be identified. These temperatures correspond to those at which abrupt changes in the temperature behavior of Hall resistance and of Seebeck coefficient occur, suggesting that they are signatures of pseudogaps opening in the density of states. The direct correlation between these characteristic temperatures and the superconducting transition temperatures suggests that the pseudogap and the superconducting state originate from the same mechanism. Scaling procedures of resistivity curves confirm such proportionality. On the other hand, excess Fe content in Fe(Te, Se) samples affects the pseudogap temperature much more strongly than the superconducting Tc. Finally, we find out that the pseudogap in the 1111 family is almost insensitive to disorder, as in high-Tc cuprates.

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.

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.

Investigations of $\mathrm{Mg}{\mathrm{B}}_{2}$ and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity, but manifestations of anisotropic multiband effects in the critical current density ${J}_{c}$ have not been addressed experimentally, mostly because of the difficulties in measuring ${J}_{c}$ along the $c$ axis. To investigate the effect of very different intrinsic anisotropies of \ensuremath{\sigma} and \ensuremath{\pi} electron bands in $\mathrm{Mg}{\mathrm{B}}_{2}$ on current transport, we grew epitaxial films with a tilted $c$ axis (\ensuremath{\theta} \ensuremath{\sim} 19.5\ifmmode^\circ\else\textdegree\fi{}), which enabled us to measure the components of ${J}_{c}$ along both the $ab$ plane and the $c$ axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the tilted-$c$-axis films. The measured field and temperature dependencies of the anisotropic ${J}_{c}(H)$ show that ${J}_{c,\mathrm{L}}$ parallel to the terraced steps is higher than ${J}_{c,\mathrm{T}}$ perpendicular to the terraced steps, and ${J}_{c}$ of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches \ensuremath{\sim}10% of the depairing current density ${J}_{d}$ in the $ab$ plane, while magneto-optical imaging revealed much higher ${J}_{c}$ at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of ${J}_{c}$ in the tilted-$c$-axis films and suggests that the apparent anisotropy of ${J}_{c}$ is affected by current pairbreaking effects in the weaker \ensuremath{\pi} band. Our results indicate that the out-of-plane current transport mediated by the \ensuremath{\pi} band could set the ultimate limit of ${J}_{c}$ in $\mathrm{Mg}{\mathrm{B}}_{2}$ polycrystals.

We investigate Seebeck effect in REFeAsO (RE=rare earth)compounds as a function of temperature and magnetic field up to 30T. The Seebeck curves are characterized by a broad negative bump around 50K, which is sample dependent and strongly enhanced by the application of a magnetic field. A model for the temperature and field dependence of the magnon drag contribution to the Seebeck effect by antiferromagnetic (AFM) spin fluctuation is developed. It accounts for the magnitude and scaling properties of such bump feature in our experimental data. This analysis allows to extract precious information on the coupling between electrons and AFM spin fluctuations in these parent compound systems, with implications on the pairing mechanism of the related superconducting compounds.

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.

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained.

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.

In this work we investigate the Ru substituted LaFeAsO compound, by studying the magnetotransport behaviour and its relationship with the band structure, in different regimes of temperature, magnetic field and Ru content. In particular we analyse the magnetoresistance of LaFe1-xRuxAsO (0, 18 pages, 12 figures, 1 table

We report on measurements of electric transport and voltage noise in FeTe${}_{0.5}$Se${}_{0.5}$ epitaxial thin films. Detailed structural investigations demonstrate the high quality of the samples, also testified by a superconducting transition temperature of $18$ K, larger than the value observed for bulk compounds. An unusual dependence of the 1/$f$ noise is found by varying the applied voltage and the temperature. Above a threshold voltage the 1/$f$ noise shows a nonquadratic dependence on applied voltage with a temperature-dependent exponent. Conversely, at low voltages the 1/$f$ noise amplitude follows always a quadratic law. In the superconductive transition region, a simple percolative model allows one to estimate the dimensionality of the system, which is found to be two-dimensional, with a critical exponent similar to values reported for other high-${T}_{c}$ superconductors. The experimental findings on noise properties give evidence of unusual transport processes, occurring in this new superconducting material, and can provide useful information on possible conduction mechanisms.

We report measurements of Hc2(T) up to 85 Tesla on Ba1-xKxAs2Fe2 single crystals and FeSe1-xTex films tuned by doping and strain. We observed an Hc2 enhancement by nearly 25 T at 30 K for the optimally-doped Ba1-xKxAs2Fe2 as compared to the previous results and extraordinarily high slopes dHc2/dT = 250-500 T/K near Tc in FeSe1-xTex indicating an almost complete suppression of the orbital pair-breaking. Theoretical analysis of Hc2(T) in FeSe1-xTex and the optimally doped Ba1-xKxAs2Fe2 predicts an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state for H//ab and T < 3-10 K, and shows that Hc2 in multiband Fe based superconductor can be enhanced by doping and strain much more effectively than by the conventional way of increasing disorder., Accepted for publication in Physical Review B

In this work we present a systematic experimental and theoretical study of the structural, transport, and superconducting properties of $\text{Sm}({\text{Fe}}_{1\ensuremath{-}x}{\text{Ru}}_{x})\text{As}({\text{O}}_{0.85}{\text{F}}_{0.15})$ polycrystalline samples as a function of Ru content $(x)$ ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of F-doped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these recently discovered multiband superconductors. Two different regions are identified: the Fe-rich phase $(xl0.5)$ where superconducting and normal-state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase $(xg0.5)$ where the system is metallic and strongly compensated and the presence of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic features and related spin fluctuations may be the cause for the suppression of superconductivity.

We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $\Delta_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $\Delta_A$ gap results to be well determined ($\Delta_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $\Delta_B$ is more uncertain ($\Delta_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$\Delta/kT_c$ values close to 2 and 7., Comment: Published Version

We present a simple method allowing the determination of the degree of texture of superconducting grains inside Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3O10+x tapes from a set of magnetic measurements. This procedure is based on the considerable anisotropy existing between the in-plane and out-of-plane critical current densities of individual crystallites in Bi-based superconducting compounds. By this method, we have monitored the evolution of the precursor Bi2Sr2CaCu2O8+y grain texturing in unreacted multifilamentary tapes in the course of the cold rolling deformation process. Evidence for anisotropic texturing, i.e., different longitudinal and transversal angle with respect to the rolling direction, has been observed. The accuracy of the method is discussed and the results compared with the current literature.

Epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by pulsed laser ablation deposition on different substrates. High purity phase and fully epitaxial growth were obtained. By varying the film thickness, superconducting transition temperatures up to 21 K were observed, significantly larger than the bulk value 16.2 K. Structural analyses indicated that the c-axis is smaller than the bulk value but it is almost independent of the film thickness and the a-axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

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.

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 influence of the disorder introduced in polycrystalline $\mathrm{Mg}{\mathrm{B}}_{2}$ samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and $^{10}\mathrm{B}$ we employed isotopically enriched $^{11}\mathrm{B}$ which contains 40 times less $^{10}\mathrm{B}$ than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low $^{10}\mathrm{B}$ content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range $1--2\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.

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.

We have investigated the charge transfer mechanism in artificial superlattices by Seebeck effect measurements. Such a technique allows a precise determination of the amount of charge transferred on each ${\mathrm{CuO}}_{2}$ plane. A systematic characterization of thermoelectric power in $({\mathrm{BaCuO}}_{2+\ensuremath{\delta}}{)}_{2}/({\mathrm{CaCuO}}_{2}{)}_{n}$ and $({\mathrm{Ba}}_{0.9}{\mathrm{Nd}}_{0.1}{\mathrm{CuO}}_{2+\ensuremath{\delta}}{)}_{2}/({\mathrm{CaCuO}}_{2}{)}_{n}$ superlattices demonstrates that electrical charge distributes uniformly among the ${\mathrm{CuO}}_{2}$ planes in the Ca block. The differences observed in the Seebeck effect behavior between the Nd-doped and undoped superlattices are ascribed to the different metallic character of the Ba block in the two cases. Finally, the special role of structural disorder in superlattices with $n=1$ is pointed out by such analysis.

We present thermal conductivity measurements on very pure and dense bulk samples, as indicated by residual resistivity values as low as $0.5\ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}$ and thermal conductivity values higher than 200 W/mK. In the normal state we found that the Wiedemann-Franz law, in its generalized form, works well suggesting that phonons do not contribute to the heat transport. The thermal conductivity in the superconducting state has been analyzed by using a two-gap model. Thanks to the large gap anisotropy we were able to evaluate quantitatively intraband scattering relaxation times of $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ bands, which depend on the disorder in different way; namely, as the disorder increases, it reduces more effectively the relaxation times of $\ensuremath{\pi}$ than that of $\ensuremath{\sigma}$ bands, as suggested by a recent calculation [Mazin et al., Phys. Rev. Lett. 89, 107002 (2002)].

We show that the growth of the heterostructure LaGaO3 /SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3 /SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.

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 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.

Structural refinement, lattice microstrain and spontaneous strain analyses have been carried out and a phase diagram has been drawn., to be published on Journal of Physics: Condensed Matter

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.

In this letter we present the effect of neutron irradiation up to fluences of 3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the critical temperature is slightly diminished (36 K) and the superconducting properties are significantly improved; the upper critical field is increased from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K. For larger neutron fluences the critical temperature is suppressed down to 12 K and the superconducting properties come out strongly degraded.

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.