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The electronic ground state of iron-based materials is unusually sensitive to electronic correlations. Among others, its delicate balance is profoundly affected by the insertion of magnetic impurities in the FeAs layers. Here, we address the effects of Fe-to-Mn substitution in the non-superconducting Sm-1111 pnictide parent compound via a comparative study of SmFe1-xMnxAsO samples with x(Mn) = 0.05 and 0.10. Magnetization, Hall effect, and muon-spin spectroscopy data provide a coherent picture, indicating a weakening of the commensurate Fe spin-density-wave (SDW) order, as shown by the lowering of the SDW transition temperature TSDW with increasing Mn content, and the unexpected appearance of another magnetic order, occurring at T∗≈ 10 and 20 K for x= 0.05 and 0.10, respectively. We attribute the new magnetic transition at T∗, occurring well inside the SDW phase, to a reorganization of the Fermi surface due to Fe-to-Mn substitutions. These give rise to enhanced magnetic fluctuations along the incommensurate wavevector Q2= (π± δ, π± δ) , further increased by the RKKY interactions among Mn impurities., Scientific Reports, 11, ISSN:2045-2322

The local structure of La(Fe1-xMnx)AsO, showing suppression of long-range magnetic order by Mn-substitution, has been studied. Extended X-ray absorption fine structure (EXAFS) measurements at As K-edge and Fe K-edge are used to probe the atomic correlations in the FeAs-layer. We have found that both Fe-As and Fe-Fe bondlengths tend to increase with increasing Mn concentration. The associated mean square relative displacements (MSRD) reveal a gradual increase indicating increased local disorder in the FeAs-layer with Mn content. The results reveal that the FeAs-layer thickness, measured by the As-height (hAs) from the Fe-Fe plane, is correlated with the Neel temperature. Furthermore, Fe Kβ X-ray emission spectroscopy (XES) shows a sharp increase in the Fe local magnetic moment with Mn substitution. The importance of the FeAs layer thickness in describing magnetic correlations in La(Fe1−xMnx)AsO is discussed.

The GDG fabrication process developed at CNR-SPIN is a promising method for producing Bi-2212 wires without overpressure processing. In this paper, critical current measurements performed at ENEA on both short and long samples in magnetic fields up to 14 T are compared. The results show no significant difference in Je between short and long (∼1 m) samples, and suggest very good longitudinal uniformity, validating the effectiveness and scalability of the process. In addition, the suitability of Inconel 625 for reaction and test barrels is demonstrated, and the reasons for obtaining lower than expected absolute Je values are discussed.

The recently discovered ${\mathrm{LaFe}}_{2}{\mathrm{As}}_{2}$ superconducting compound, member of the 122 family of iron pnictide superconductors, becomes superconducting below ${T}_{c}\ensuremath{\approx}13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, yet its nominal doping apparently places it in the extreme overdoped limit, where superconductivity should be suppressed. In this work, we investigate the normal state of magneto- and thermoelectric transport and specific heat of this compound. The experimental data are consistent with the presence of highly compensated electron and hole bands, with \ensuremath{\sim}0.42 electrons per unit cell just above ${T}_{\mathrm{c}}$, and high effective masses $\ensuremath{\sim}3{m}_{0}$. The temperature dependence of transport properties strongly resembles that of conventional superconductors, pointing to a key role of electron-phonon coupling. From this evidence, ${\mathrm{LaFe}}_{2}{\mathrm{As}}_{2}$ can be regarded as the connecting compound between unconventional and conventional superconductors.

High temperature superconductors are strongly coupled systems which present a complicated phase diagram with many coexisting phases. This makes it difficult to understand the mechanism which generates their singular transport properties. Hydrodynamics, which mostly relies on the symmetries of the system without referring to any specific microscopic mechanism, constitutes a promising framework to analyze these materials. In this paper we show that in the strange metal phase of the cuprates, a whole set of transport coefficients are described by a universal hydrodynamic framework once one accounts for the effects of quantum critical charge density waves. We corroborate our theoretical prediction by measuring the DC transport properties of Bi-2201 close to optimal doping, proving the validity of our approach. Our argument can be used as a consistency check to understand the universality class governing the behavior of high temperature cuprate superconductors., Version to appear on Physical Review Research

The epitaxial growth of Zr-doped CeO2 (CZO) films by chemical solution deposition (CSD) on single crystal substrates such as Al2O3, SrTiO3 and Y2O3-stabilized ZrO2 (YSZ) and their use as buffer layers for Fe(Se,Te) film growth are reported. The growth of 30 nm thin CZO films is mostly dominated by dewetting issues, leading to highly incomplete substrate coverage due to the strong tendency of CZO grains to agglomerate. This effect is less severe on YSZ substrate and, in addition, can be fully controlled by setting the deposition temperature at 950 °C and operating under Ar-H2 reducing atmosphere. These conditions promote the growth of epitaxial and compact CZO films on YSZ, showing a flat surface morphology with root-mean-square roughness around 2.5 nm. Preliminary results on Fe(Se,Te) film deposited by pulsed laser deposition on CZO-buffered YSZ are reported. Although good epitaxial growth is achieved on CZO buffer layer, Fe(Se,Te) films exhibit poor superconducting properties. On the other hand, Fe(Se,Te) films show excellent properties when a seed layer is used, exhibiting a sharp superconducting transition at about 17 K and a critical current density exceeding 1 MA cm-2 at 4.2 K in self field. The main role of the seed layer, consisting of a non-superconducting Fe(Se,Te) film, is to favour the crystal and chemical matching between buffer layer and film, allowing for the growth of high-quality superconducting Fe(Se,Te) at deposition temperature as low as 200 °C. In perspective of applications, this result is a significant step towards process simplification and cost reduction of Fe(Se,Te)-based coated conductor technology development, being the first demonstration of superconducting Fe(Se,Te) films grown on CSD buffer layer.

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.

At the present day, iron-based superconductors (IBS) can be 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.

In this paper, the feasibility of producing Fe(Se,Te)-coated conductors on cost-effective, unbuffered Invar 36 substrates is studied. A suitable substrate with a sharp {001} texture and a misorientation angle lower than 10° was developed, and the possibility of growing epitaxial thin films without any buffer layer was demonstrated. It was also shown that the presence of a Fe(Se,Te) seed layer can improve the in- and out-of-plane orientation of the film. A high-resolution scanning electron microscopy study was performed, and the absence of a superconducting transition has been attributed to Ni poisoning based on X-ray microanalysis.

The local structure of La(Fe1−x Mn x )AsO has been investigated using temperature dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements. The EXAFS data reveal distinct behavior of Fe–As and Fe–Fe atomic displacements with a clear boundary between x ⩽ 0.02 and x > 0.02. The Fe–As bondlength shows a gradual thermal expansion while the Fe–Fe bond manifests a temperature dependent anomaly at ∼180 K for x > 0.02. It is interesting to find characteristically different nature of Fe–As and Fe–Fe bondlengths shown by the temperature dependent mean square relative displacements. Indeed, the Fe–As bond, stiffer than that of the Fe–Fe, gets softer for x ⩽ 0.02 and hardly shows any change for x > 0.02. On the other hand, Fe–Fe bond tends to be stiffer for x ⩽ 0.02 followed by a substantial softening for x > 0.02. Such a distinction has been seen also in the As K-edge x-ray absorption near edge structure, probing local geometry around As atom together with the valence electronic structure. The results suggest that local atomic displacements by Mn substitution inducing increased iron local magnetic moment that should be the main reason for its dramatic effect in iron-based superconductors.

Here we report a new versatile technique to manufacture MgB2 massive samples, called reverse advance internal magnesium diffusion (r-AIMI). The idea focuses on the goal of obtaining dense bulk or wire samples depending on synthesis conditions. In respect to the traditional AIMI procedure, in which a central Mg rod is covered with a B corona, here a Mg tube is filled with B powder and clad in a Ti external sheath, which is quite similar to the traditional (powder in tube) technique. After cold deformation, during which several intermediate low temperature heat treatments are necessary in order to relax the Ti sheath and Mg tube, samples are reacted at high temperature and ambient pressure to form a dense MgB2 core. The MgB2 phase results are totally disconnected from metallic sheath, and can be easily extracted and characterized. Critical current density measurements show values exceeding 106 A cm−2 below 1.5 T at 20 K. In the last part of the paper, we show the effect of final heat-treatment performed under high pressure to eliminate the present void and connect the external sheath to the internal MgB2 core and so permitting the electric transfer necessary for power applications of wires.

Electric and thermoelectric transport properties are mutually intertangled in diffusive transport equations. In particular, in high mobility multiband systems an anomalous behavior may occur, which can be tracked down to the properties of the individual bands. Here, we present magnetoelectric and magnetothermoelectric transport properties of a ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ high-quality single crystal, for different magnetic field directions up to 30 T. We detect a giant Nernst effect and an anomalous field dependence of the Seebeck coefficient. The extraction of the Peltier tensor coefficients ${\ensuremath{\alpha}}_{xx}, {\ensuremath{\alpha}}_{xy}$, and ${\ensuremath{\alpha}}_{xz}$ allows us to disentangle the main transport mechanisms in play. The large ${\ensuremath{\alpha}}_{xy}$ and ${\ensuremath{\alpha}}_{xz}$ values and their field dependence provide evidence of the presence of a high mobility band, compatible with a Dirac dispersion band, crossing the Fermi level, and suggest a possible three-dimensional nature of the Dirac fermions.

In this paper we explore the effects of 3.5 MeV proton irradiation on Fe(Se,Te) thin films grown on CaF2. In particular, we carry out a systematic experimental investigation with different irradiation fluences up to 7.30x10^16 cm^-2 and different proton implantation depths, in order to clarify whether and to what extent the critical current is enhanced or suppressed, what are the effects of irradiation on the critical temperature, the resistivity and the critical magnetic fields, and finally what is the role played by the substrate in this context. We find that the effect of irradiation on superconducting properties is generally small as compared to the case of other iron-based superconductors. Such effect is more evident on the critical current density Jc, while it is minor on the transition temperature Tc, on the normal state resistivity and on the upper critical field Hc2 up to the highest fluences explored in this work. In addition, our analysis shows that when protons implant in the substrate far from the superconducting film, the critical current can be enhanced up to 50% of the pristine value at 7 T and 12 K, while there is no appreciable effect on critical temperature and critical fields together with a slight decrease in resistivity. On the contrary, when the implantation layer is closer to the film-substrate interface, both critical current and temperature show a decrease accompanied by an enhancement of the resistivity and the lattice strain. This result evidences that possible modifications induced by irradiation in the substrate may affect the superconducting properties of the film via lattice strain. The robustness of the Fe(Se,Te) system to irradiation induced damage makes it a promising compound for the fabrication of magnets in high-energy accelerators., Comment: 14 pages, 11 figures, submitted to Superconductor Science and Technology

We study the electronic structure of the SmFeAsO(1-x)F(x) alloy by means of first-principle calculations. We find that, contrary to common believe, F-doping does not change the charge balance between electrons and holes free-carriers in SmFeAsO(1-x)F(x). For energies within a narrow energy range across E_F, the effect of F-doping on the band structure dispersion is tiny in both the paramagnetic and stripe antiferromagnetic phase. The charge balance between the conducting FeAs-layer and the SmO(1-x)F(x) charge reservoir layer is not influenced by the compositional change. The additional charge carried by fluorine, with respect to the oxygen, is compensated by a change in the oxidation state of the Sm ion from 3+ to 2+. A comparison with the SmFe(1-x)Co(x)AsO system shows that such charge compensation by the Sm ion is not shared by donors substituting at the Fe site., Comment: J. Phys.: Condens. Matter (2019)

The interplay between magnetism and superconductivity is tuned as a function of the external pressure in LnFeAsO[Formula: see text]Fx at [Formula: see text] values close to the appearance of superconductivity for lanthanide ion Ln=La[Formula: see text]Y[Formula: see text] and Ce. The results show that in these compounds, both superconductivity and magnetism are reinforced by pressure. Furthermore, muon spin rotation and relaxation measurements reveal qualitative remarkable differences among the samples: Ln=Ce displays nanoscopic coexistence of magnetism and superconductivity in the same volume fraction while Ln=La[Formula: see text]Y[Formula: see text] shows macroscopic phase separation. The Ln-dependent results can be due to a complex combination of the external and chemical pressure, the latter being driven by the different average ionic Ln radius.

In this Letter we report high-resolution synchrotron X-ray powder diffraction and transmission electron microscope analysis of Mn-substituted LaFeAsO samples, demonstrating that a static incommensurate modulated structure develops across the low-temperature orthorhombic phase, whose modulation wave-vector depends on the Mn content. The incommensurate structural distortion is likely originating from a charge-density-wave instability, a periodic modulation of the density of conduction electrons associated with a modulation of the atomic positions. Our results add a new component in the physics of Fe-based superconductors, indicating that the density wave ordering is charge-driven., Comment: To be published in the Physical Review Letters

Among the recently discovered Fe-based superconducting compounds, the Ba-122 phase has proved to be the more attracting for the development of powder in tube (PIT) processed conductors. In fact, after some years of development, critical current densities ( Jc ) of about 105 A/cm2 at 4.2 K and magnetic fields up to 10 T have been obtained in (Ba0.6K0.4)Fe2As 2, PIT wires and tapes. To develop a safe upscaling method, the synthesis of the powders is a crucial point. In order to avoid the use of highly reactive K we have developed BaFe2(P1-xAsx)2 PIT tapes. This compound has proved to be very stable and in form of crystals and thin films exhibits excellent critical current: We succeeded in manufacturing PIT tapes with this phase with Jc values of about 103 A/cm2 at 4.2 K in self-field. Detailed microstructural and chemical investigations of the samples have been performed by high-resolution TEM, scanning TEM, and electron energy loss spectrometry analysis. A deformation network inside the grains that can induce strong pinning has been observed. However, chemical inhomogeneities at the grain boundaries are also present, which limit the transport current.

We have investigated the effects of the isovalent ruthenium substitution in LaFeRuAsO, by extensive 75As NQR (nuclear quadrupole resonance) measurements, supported by DFT (density functional theory) calculations, in order to characterize both the lattice and electronic structure details. The evidence for five different local configurations around the arsenic site emerges upon increasing ruthenium for iron substitution. DFT calculations confirm the attribution of the measured electric field gradients (EFGs) to ruthenium atom occupancies (0, 1, 2, 3, and 4) on the nearest-neighbour sites of arsenic. It is found that the low-frequency (Ru-free) NQR peak remains almost unaffected upon ruthenium substitution, providing an experimental confirmation that ruthenium does not introduce delocalized carriers in the iron plane.

Herewith, we review the available experimental data of thermoelectric transport properties of iron-based superconductors and parent compounds. We discuss possible physical mechanisms into play in determining the Seebeck effect, from whence one can extract information about Fermi surface reconstruction and Lifshitz transitions, multiband character, coupling of charge carriers with spin excitations and its relevance in the unconventional superconducting pairing mechanism, nematicity, quantum critical fluctuations close to the optimal doping for superconductivity, correlation. Additional information is obtained from the analysis of the Nernst effect, whose enhancement in parent compounds must be related partially to multiband transport and low Fermi level, but mainly to the presence of Dirac cone bands at the Fermi level. In the superconducting compounds, large Nernst effect in the normal state is explained in terms of fluctuating precursors of the spin density wave state, while in the superconducting state it mirrors the usual vortex liquid dissipative regime. A comparison between the phenomenology of thermoelectric behavior of different families of iron-based superconductors and parent compounds allows to evidence the key differences and analogies, thus providing clues on the rich and complex physics of these fascinating unconventional superconductors.

The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally doped ${\mathrm{SmFeAsO}}_{0.88}{\mathrm{F}}_{0.12}$ iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents ($x$) by means of muon-spin spectroscopy ($\ensuremath{\mu}\mathrm{SR}$), both in zero and in transverse fields, to probe the magnetic and the superconducting order, respectively. The pure superconducting phase (at $xl0.03$) is replaced by a crossover region at intermediate Mn values ($0.03\ensuremath{\le}xl0.08$), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for $x=0.08$, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of ${T}_{c}$ and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors (M. N. Gastiasoro et al., Phys. Rev. Lett. 117, 257002 (2016)), which includes correlation-enhanced RKKY couplings between the impurity moments.

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.

In dc and ac practical applications of MgB2 superconducting wires, an important role is represented by the material sheath that has to provide, among other things, a suitable electrical and thermal stabilization. One way to obtain a large-enough amount of low-resistivity material into the conductor architecture is to use it as the external sheath. In this paper, we study ex situ multifilamentary MgB2 wires fabricated with oxide-dispersion-strengthened copper (GlidCop) as the external sheath in order to reach a good compromise between critical current density and thermal properties. We prepared three GlidCop samples with different contents of dispersed submicroscopic Al2O3 particles. We characterized the superconducting and thermal properties, and we showed that the good thermal conductivity, together with the good mechanical properties and a reasonable critical current density, makes the GlidCop composite wire a useful conductor for applications where high thermal conductivity is required at temperatures above 30 K, such as superconducting fault-current limiters.

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.

MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means of the ex-situ powder-in-tube method using either high-energy ball milled and low temperature synthesized powders. All sample were sintered at 920 C in Ar flow. The milling time and the revolution speed were tuned in order to maximize the critical current density in field (Jc): the maximum Jc value of 6 x 10e4 A/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with powders milled for 144h at 180rpm. Vorious synthesis temperature were also investigated (730-900 C) finding a best Jc value for the wire prepared with powders synthesized at 745 C. We speculate that this optimal temperature is due to the fluidifying effect of unreacted magnesium content before the sintering process which could better connect the grains.

Among the recently discovered Fe-based superconducting compounds, the (K,Ba)Fe2As2 phase is attracting great attention within the scientific community interested in conductor developments. In fact, after some years of development, critical current densities Jc of about 105 A cm−2 at fields up to more than 10 T have been obtained in powder-in-tube (PIT) processed wires and tapes. Here we explore the crucial points in the wire/tape fabrication by means of the ex situ PIT method. We focus on scaling-up processes, which are crucial for industrial fabrication. We analyzed the effects on the microstructure of the different heat and mechanical treatments. By an extensive microstructural analysis correlated with the transport properties, we addressed the issues concerning the phase purity, internal porosity, and crack formation in the superconducting core region. Our best conductors, with a filling factor of about 30% heat-treated at 800 °C, exhibited Tc = 38 K, the highest value measured in this kind of superconducting tape. The microstructure analysis shows clean and well-connected grain boundaries but rather poor density: the measured Jc of about 3 104 A cm−2 in the self field is suppressed by less than a factor of seven at 7 T. Such not-yet-optimized Jc values can be accounted for by the reduced density, while the moderate in-field suppression and a rather high n-factor confirm the high homogeneity and uniformity of these tapes.

FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density Jc({\theta}) at {\theta} = 180{\deg} (H || c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no Jc({\theta}) peaks for H || c were observed in films on CaF2 and LaAlO3. Jc({\theta}) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg-Landau (AGL) approach with appropriate anisotropy ratio {\gamma}J. The scaling parameter {\gamma}J is decreasing with decreasing temperature, which is different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO substrates., Comment: accepted for publication in SUST

We have measured the thermal and electrical conductivity of silver and silver alloy sheaths for Bi-2223 tapes. The thermal conductivity at 10 K varied from 1 to 104 W m−1 K−1 (the upper values are for Ag sheaths and the lower, 1–102 W m−1 K−1, for the alloy sheaths). The large spread of data for the alloy sheaths is due to the difference in composition and microstructure, which in case of the AgMg system depends on the thermal and mechanical history of alloy.

The electronic anisotropy in MgB2, which arises from its layered crystal structure is not completely clear until now. High quality c-oriented films offer the opportunity of studying such property. MgB2 thin films were deposited by using two methods both based on room temperature precursor deposition (by Pulsed Laser Ablation) and ex-situ annealing in Mg atmosphere. The two methods differ for the starting targets: stoichiometric MgB2 in one case and Boron in the other. The two films presented in this paper are grown by means of the two techniques on MgO substrates and are both c-oriented; they present TC values of 31.5 and 37.4 K respectively. Upper critical field measurements, up to 9T, with the magnetic field in perpendicular and parallel directions in respect to the film surface evidenced anisotropy ratios of 1.8 and 1.4 respectively. In this paper we will discuss this remarkable and surprising difference also in comparison with the literature data., Presented at EUCAS 2001. submitted to Physica C

We report on the growth of epitaxial Fe1+δ Se0.5Te0.5 thin films on 0°, 5°, 10°, 15° and 20° vicinal cut CaF2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe1+δ Se0.5Te0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab- and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for Hab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength.

We study normal state electrical, thermoelectrical, and thermal transport in polycrystalline ${\mathrm{BiS}}_{2}$-based compounds, which become superconducting by F doping on the O site. In particular, we explore undoped ${\mathrm{LaOBiS}}_{2}$ and doped ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ samples, prepared either with or without high-pressure annealing, in order to evidence the roles of doping and preparation conditions. The high-pressure annealed sample exhibits room temperature values of resistivity \ensuremath{\rho} around 5 m\ensuremath{\Omega}cm, Seebeck coefficient $S$ around $\ensuremath{-}20\ensuremath{\mu}V/K$, and thermal conductivity \ensuremath{\kappa} around 1.5 W/Km, while the Hall resistance ${R}_{H}$ is negative at all temperatures and its value is $\ensuremath{-}10{}^{\ensuremath{-}8} m{}^{3}/\mathrm{C}$ at low temperature. The sample prepared at ambient pressure exhibits ${R}_{H}$ positive in sign and five times larger in magnitude, and $S$ negative in sign and slightly smaller in magnitude. These results reveal a complex multiband evolution brought about by high-pressure annealing. In particular, the sign inversion and magnitude suppression of ${R}_{H}$, indicating increased electron-type carrier density in the high-pressure sample, may be closely related to previous findings about change in lattice parameters and enhancement of superconducting ${T}_{c}$ by high-pressure annealing. As for the undoped sample, it exhibits 10 times larger resistivity, 10 times larger $|S|$, and 10 times larger $|{R}_{H}|$ than its doped counterpart, consistent with its insulating nature. Our results point out the dramatic effect of preparation conditions in affecting charge carrier density as well as structural, band, and electronic parameters in these systems.

Bi2Sr2CaCu2O8+x (Bi-2212) superconducting long-length wires are mainly limited in obtaining high critical currents densities (JC) by the internal gas pressure generated during the heat treatment, which expands the wire diameter and dedensifies the superconducting filaments. Several ways have been developed to increase the density of the superconducting filaments and therefore decreasing the bubble density: much higher critical currents have been reached always acting on the final as-drawn wires. We here try to pursue the same goal of having a denser wire by acting on the deformation technique, through a partial use of the groove-rolling at different wire processing stages. Such technique has a larger powders compaction power, is straightforwardly adaptable to long length samples, and allows the fabrication of samples with round, square or rectangular shape depending on the application requirements. In this paper we demonstrate the capability of this technique to increase the density in Bi-2212 wires which leads to a three-fold increase in Jc with respect to drawn wires, making this approach very promising for fabricating Bi-2212 wires for high magnetic field magnets, i.e. above 25 T.

We report an infrared spectroscopy study of a 200 nm thick FeSe$_{0.5}$Te$_{0.5}$ film grown on LaAlO$_3$ with T$_c$=13.7 K. We analyze the 20 K normal state absolute reflectance R$_N$ measured over a broad infrared range and the reflectance ratio R$_S$/R$_N$, R$_S$ being the superconducting state reflectance, measured at 6 K in the terahertz range down to 12 cm$^{-1}$. We show that the normal state model conductivity is given by two Drude components, one of which much broader and intense than the other. In the superconducting state, we find that a gap $\Delta$=37$\pm$3 cm$^{-1}$ opens up in the narrow Drude band only, while the broad Drude band results to be ungapped, at least in the explored spectral range. Our results show that only a two-band model can coherently describe both normal and superconducting state data., Comment: 11 pages, 3 figures, accepted for publication in Supercond. Sci. Technol

We have studied the electronic and magnetic properties of SmFe(1-x)RuxAsO0.85F0.15 (x = 0, 0.05, 0.25, 0.33, 0.5) by high-resolution x-ray absorption and x-ray emission spectroscopy. The local Fe magnetic moment (μ) tends to decrease for a small Ru substitution, but it shows a clear increase with further substitution. It appears that impurity scattering prevails in reducing the μ with small Ru substitution due to an extended Ru d-band. A nanoscale phase separation, that decouples the FeAs layers from the spacer layers, drives the increase of μ at higher Ru substitution. The results provide important information on nanoscale phase separation due to isoelectronic substitution in the active layers of iron-based 1111-superconductors and its effect on the local magnetic properties.

The fluctuation-induced magnetoconductivity of the Bi2Sr2Ca2Cu3O10+x phase is studied above zero-field critical temperature Tc(0) and for moderate magnetic fields. It is found that the Gaussian approximation for superconducting fluctuations underestimates the negative fluctuation magnetoconductance drastically in the Tc(0) < T < Tc(0) + 20 K temperature range. Taking into account the critical fluctuation contribution on the base of self-consistent Hartree approximation makes it possible to explain the data quantitatively in terms of the only Aslamazov-Larkin contribution for different magnetic fields and temperatures, consistently with the zero field data.

We present the results of collaboration between the groups of Naples and Genoa Universities/INFM, focused on thin films superconducting borocarbides. In particular a careful study of the deposition of non-magnetic LuNi 2 B 2 C thin films by Pulsed Laser Deposition (PLD) on MgO and other substrates will be presented. While samples with good c-orientation and superconducting properties are obtained in wide range of deposition parameters, in-plane micrometric oriented structures are obtained only in a narrow range of conditions, as evidenced by X-rays measurements and AFM (atomic force microscopy) analyses. A special set-up for the in situ interchanging of shadow masks has been developed in order to deposit the diferent patterned layers and different buffer and/or barrier layers. New results on the temperature dependence of the LuNi 2 B 2 C gap using S/N contact junctions are presented and discussed. The obtained value of the superconducting gap and its temperature dependence agrees very well with the BCS prediction. We present also microwave measurements of the surface impedance performed on large area films obtained by sputtering technique on low loss sapphire substrates. The data confirm the overall BCS-like behavior of these compounds.

In this paper we present measurements of thermal and electrical conductivity performed on several alloys and composite alloys tipically used as sheaths of BSCCO tapes: AgAu, AgMg, AgAu-AgMg. The thermal conductivity of the alloys is analysed in terms of electron and phonon contribution and we emphasize that in not dilute alloys the phonons contribute largely to the heat transport. In order to reduce the overall thermal conductivity, this latter contribution has to be minimized.

Thin films and artificial materials deposition, applied to the study of high temperature superconductors, can be a powerful method to achieve better comprehension of the conduction mechanisms in these materials and, in perspective, to obtain new superconducting compounds. We simulate the layered structure of the superconducting oxides by alternating infinite layer compounds ( CaCuO 2, SrCuO 2, BaCuO 2) both pure and doped either with chemical substitutions or stoichiometry variations. By the first way, we obtained superlattices with a semiconducting electrical behaviour, probably because of the disorder induced by such high chemical substitutions. The second way led to superconducting films with transition temperatures around 80 K and low anisotropy. Furthermore, their artificial nature offers an unique possibility of analysing the influence of the structural properties on the superconducting behaviour, such as the correlation between the number of CuO 2 planes contained in the infinite layer block ( CaCuO 2) and the transport properties.

In this paper we report results on the growth of borocarbides ErNi 2 B 2C and DyNi 2 B 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. Moreover the possibility to obtain superconducting films with a tungsten buffer layer on MgO substrates has been successfully checked. Here we present details on the deposition procedure as well as on the structural, morphological, and physical characterization.

We discuss thermoelectric measurements of high-temperature superconductors around the superconducting transition. The Seebeck effect coefficient has been widely used to study the dissipative phenomena of the vortex lattice and its interaction with quasiparticles, in conjunction with the resistivity to which it is correlated in the framework of all existing theories. As is well known, Seebeck effect measurements are far from the accuracy of resistivity measurements because the presence of a thermal gradient across the sample may alter the shape of the curve. We make a quantitative analysis of the errors involved in ac measurement techniques; the error percentage grows considerably in the low-dissipation region where the Seebeck effect vanishes. We propose a method that drastically reduces the experimental artifacts; moreover, we present the Seebeck effect measurements of a Y-Ba-Cu-O thin film, performed with the standard steady-flux ac technique and with the one proposed here, and we discuss the results in light of our calculations. @S0163-1829~98!06842-8#

We report on the magnetic and superconducting properties of LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ by means of zero- and transverse-field (ZF/TF) muon-spin spectroscopy measurements ($\ensuremath{\mu}$SR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-$\ensuremath{\mu}$SR data give access to the superfluid density, which shows a marked two-dimensional character with a dominant $s$-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.

We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2(001) substrate. High critical current density values – larger than 1 MA/cm2 in self field and liquid helium – are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through transmission electron microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what was observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction.

Local structure of SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ ($x$ = 0.0, 0.05, 0.25 and 0.5) superconductors has been investigated by temperature dependent As $K$-edge extended x-ray absorption fine structure. The effect of Ru substitution remains confined to the iron-arsenide layer but neither the static disorder nor the Fe-As bond strength suffers any change for $x \le$ 0.25. With further Ru substitution the static disorder increases while the Fe-As bond strength remains unchanged. Also, the Ru-As distance ($\sim$2.42 \AA), different from the Fe-As distance ($\sim$2.39 \AA), does not show any change in its force constant with the Ru substitution. These observations suggest that the SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ system breaks down to coexisting local electronic phases on isoelectric substitution in the active FeAs layer.

The effect of Ru substitution on the local structure of layered SmFe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO${}_{0.85}$F${}_{0.15}$ superconductor has been studied by As $K$- and Sm ${L}_{3}$-edge x-ray-absorption spectroscopy. The extended x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As bond lengths in the Ru substituted samples with the latter being $\ensuremath{\sim}$0.03 \AA{} longer. Local disorder induced by the Ru substitution is mainly confined to the FeAs layer while the SmO spacer layer sustains a relative order, consistent with the x-ray-absorption near-edge structure spectra. The results suggest that, in addition to the order/disorder in the active FeAs layer, its coupling to the rare-earth--oxygen spacer layer needs to be considered for describing the electronic properties of these layered superconductors.

The effect of isoelectronic substitutions on the microscopic properties of LaFe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO, for $0\ensuremath{\le}x\ensuremath{\le}0.8$, has been investigated by means of $\ensuremath{\mu}$SR and ${}^{139}$La NMR. It was found that Ru substitution causes a progressive reduction of the N\'eel temperature (${T}_{N}$) and of the magnetic order parameter without leading to the onset of superconductivity. The temperature dependence of ${}^{139}$La nuclear spin-lattice relaxation rate $1/{T}_{1}$ can be suitably described within a two-band model. One band giving rise to the metallic antiferromagnetic ground state, while the other one is characterized by weakly correlated electrons. Ru for Fe substitution yields a progressive decrease of the density of states at the Fermi level close to the one derived from band structure calculations. The reduction of ${T}_{N}$ with increasing Ru content follows the predictions of the ${J}_{1}$-${J}_{2}$ model on a square lattice, which appears to be an effective framework to describe the magnetic properties of the magnetic ground state.

We explore the electronic, transport and thermoelectric properties of Fe1+ySexTe1−x compounds to clarify the mechanisms of superconductivity in Fe-based compounds. We carry out first-principles density functional theory (DFT) calculations of structural, electronic, magnetic and transport properties and measure resistivity, Hall resistance and Seebeck effect curves. All the transport properties exhibit signatures of the structural/magnetic transitions, such as discontinuities and sign changes of the Seebeck coefficient and of the Hall resistance. These features are reproduced by calculations provided that antiferromagnetic correlations are taken into account and experimental values of lattice constants are considered in DFT calculations. On the other hand, the temperature dependences of the transport properties can not be fully reproduced, and to improve the agreement between experiment and DFT calculations it is necessary to go beyond the constant relaxation time approximation and take into account correlation effects.

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.

The past four years of incredibly intense research into Fe-based superconductors have brought about many unexpected surprises. Our understanding of their behavior and physical properties is constantly evolving. Unlike any other superconductors, those containing iron span diverse groups of materials: pnictides, chalcogenides, intermetallics and oxides. Some major properties of the materials are quite similar, yet each group has its own distinct features. Significant effort has been put into identifying new superconducting compositions, modifying the existing ones with new dopants and treatments, and producing single crystals, thin films, wires and polycrystalline bulk material. A wide array of experimental techniques was applied to study Fe-based superconductors and the result is a tremendous amount of data collected over a period of less than four years. Theoretical debates are still lively, and there is an ongoing search for possible universalities and commonalities with other unconventional superconductors, like high-Tc cuprates or heavy fermion materials. The three-dimensional electronic structures of Fe-based superconductors, as well as their extreme sensitivity to disorder, present serious challenges for both theoretical analysis and the interpretation of experiments. However, some key properties emerge from multiple studies. Unconventional, multiband superconductivity originating from an electronic mechanism has found both experimental and theoretical support. There has been great progress in the understanding of various anisotropies of superconducting gap structures, including the possibility of gap nodes even if the gap symmetry is s-wave. Similar to high-Tc cuprates, the superconducting phase has a dome-like shape on T-doping or T-pressure phase diagrams. The anisotropy of the superconducting gap evolves with doping and is likely to become stronger at the dome's edge. In many Fe-based superconductors there is a range where superconductivity coexists and competes with long-range magnetic order, and magnetic fluctuations are considered by some to be of the utmost importance for the pairing mechanism. Others argue that orbital fluctuations, possibly in combination with phonons, are crucial for the pairing. Fe-based superconductors show extremely large upper critical fields and relatively low electronic anisotropy, which are crucial aspects for power applications. The expectations are high, though it remains unclear what maximal current densities can be supported by a properly designed bulk material with optimal pinning centers. This focus issue of Superconductor Science and Technology is a snapshot of some of the recent progress in materials preparation, experiments and theory. It includes articles on the search for new Fe-based superconductors and on the search for superconductivity at extreme conditions. Particular attention is devoted to: the effects of chemical substitutions; the development of thin films; the introduction of artificial defects to increase critical current density; and a general analysis of vortex physics. The articles on fundamental aspects of superconductivity include: the discussion of various experimental problems; an in-depth analysis of the nodal and nodeless pairing states; the discussion of the pairing mechanism; and the effects of pair-breaking due to disorder. Also discussed are nematic correlations and the coexistence of magnetism and superconductivity. The papers collected in this issue present a detailed review of the accomplishments of the last four years of research into Fe-based superconductors, up to and including last-minute developments. We hope that this combination will make this special section of Superconductor Science and Technology both interesting and useful to a broad spectrum of physicists and materials scientists.

We discuss the resistivity, the Hall, the Seebeck and the Nerst effect in the mixed state taking the motion of Abrikosov and Josephson vortices into account. Measurements on a Bi(2223) tape point out how the contribution of Josephson vortices to the resistivity prevails at low temperatures while the thermomagnetic effects are, in a first approximation, due to the motion of Abrikosov vortices.

We report the synthesis procedure and the chemical-physical characterization of the YNi2B2C superconductor. From magnetic measurements we show that this material is a type-II superconductor. We evaluate the penetration depth, the coherence length and, thus, the Ginzburg-Landau parameter. Despite the low value of κ(12), a very large region of reversibility in the H-T plane is observed.