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In this work, the feasibility condition of Powder-In-Tube (PIT) processed wires of Fe(Se,Te) superconductor has been investigated. We faced several technical issues that are extensively described and discussed. In particular, we tested different metals and alloys as external sheaths (Cu, Ag, Nb, Ta, Ni, Fe, cupronickel, brass) concluding that the only sheath that does not affect substantially the Fe(Se,Te) phase is Fe. On the other hand, Fe sheath introduces excess iron in the Fe(Se,Te) phase, which affects the superconducting properties; we investigated the effects of the thermal treatments and of the powder composition in order to avoid it. The maximum Jc value obtained in our samples is 4*10^2 A/cm2, comparable to other published values of PIT conductors of the 11 family. We conclude that the fabrication of Fe(Se,Te) wires by PIT method is quite challenging and other approaches should be developed.

Promising results reported in our previous works led us to think that production of B powder plays a crucial role in MgB2 synthesis. A new method for boron preparation has been developed in our laboratory. This particular process is based on magnesiothermic reaction (Moissan process) with the addition of an initial step that gives boron powder with nano-metric grain size. In this paper we report our efforts regarding optimization of PIT method for these nanometric powders and the resolution of problems previously highlighted such as the difficulty in powder packaging and the high friction phenomena occurring during cold working. This increases cracking during the tape and wire manufacturing leading to its failure. Packaging problems are related to the amorphous nature of boron synthesized in our laboratory, so a crystallization treatment was applied to improve crystallinity of B powder. To prevent excessive friction phenomena we synthesized non-stoichiometric MgB2 and using magnesium as lubricant. Our goal is the Jc improvement, but a global physical-chemical characterization was also made to analyze the improvement given by our treatments: this characterization includes X-ray diffraction, resistivity vs. temperature measurement, SEM image, besides magnetic and transport Jc measurements., Comment: 15 pages, 11 figures

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

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out Density Functional Theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements., Comment: 13 pages, 5 figures accepted for publication on European Journal of Physics B

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.

In this paper we report a detailed $\mu^{+}$SR and {}$^{19}$F-NMR study of the La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ class of materials. Here, the diamagnetic La$_{1-y}$Y$_{y}$ substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic-superconducting phase transition by keeping the Y content constant ($y = 0.3$) and by varying the F content in the range $0.025 \leq x \leq 0.15$. Our results show how magnetism and superconductivity coexist for $x = 0.065$. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO$_{1-x}$F$_{x}$ materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce., Comment: Proceedings of QCNP12 Conference

The effect of Ru substitution on the local structure of layered SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ superconductor has been studied by As $K$- and Sm $L_3$ - edges x-ray-absorption spectroscopy. The extended x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As bondlengths in the Ru substituted samples with the latter being $\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 active iron-arsenide layer, its coupling to the rare-earth\textminus oxygen spacer layer needs to be considered for describing the electronic properties of these layered superconductors.

A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions, large magnetic hysteresis loops and high upper critical fields are observed. Interestingly, the global critical current density is much enhanced as compared to the values reported in literature for bulk samples of the same 11 family, reaching about 103 A/cm2 at zero field at 4.2 K as assessed by magnetic, transport and magneto-optical techniques. Even more importantly, its field dependence turns out to be very weak, such that at \mu_{0}H = 7 T it is suppressed only by a factor \sim2., Comment: 21 pages, 15 figures

We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce, Pr, La) by using muon-spin relaxation measurements and ab-initio calculations. Both magnetic transition temperature $T_m$ and Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our DFT calculations. The same calculations allow us to assign a value of 0.68(2) $\mu_B$ to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different $f$-electron overlap., Comment: 16 pages, 11 figures

The microstructural evolution throughout the first order tetragonal to orthorhombic structural transition is analyzed by powder diffraction analysis for two different systems belonging to the class of compounds referred to as 1111 oxy-pnictides: (La1-yYy)FeAsO and SmFeAs(O1-xFx). Both systems are characterized by a similar behaviour: on cooling microstrain along the tetragonal hh0 direction takes place and increases as the temperature is decreased. Just above the structural transition microstrain reaches its maximum value and then is abruptly suppressed by symmetry breaking. No volume discontinuity throughout the first order transition is observed and a groupsubgroup relationship holds between the tetragonal and the orthorhombic structures, thus suggesting that orbital ordering drives symmetry breaking. Microstrain reflects a distribution of lattice parameters in the tetragonal phase and explains the occurrence of anisotropic properties commonly attributed to nematic correlations; in this scenario the nematic behaviour is induced by the tendency towards ordering of Fe orbitals.

The effect of isoelectronic substitutions on the microscopic properties of LaFe1-xRuxAsO, for 0< x< 0.8, has been investigated by means of muSR and 139La 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 139La nuclear spin-lattice relaxation rate 1/T_1 can be suitably described within a two-band model. One band giving rise to the spin density wave ground-state, while the other one is characterized by weakly correlated electrons. Fe for Ru substitution yields to 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 doping 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 spin density wave ground-state., Comment: 6 pages, 8 figures

The evolution of the antiferromagnetic order parameter in CeFeAsO_{1-x}F_{x} as a function of the fluorine content x was investigated primarily via zero-field muon-spin spectroscopy. The long-range magnetic order observed in the undoped compound gradually turns into a short-range order at x=0.04, seemingly accompanied/induced by a drastic reduction of the magnetic moment of the iron ions. Superconductivity appears upon a further increase in doping (x>0.04) when, unlike in the cuprates, the Fe magnetic moments become even weaker. The resulting phase diagram evidences the presence of a crossover region, where the superconducting and the magnetic order parameters coexist on a nanoscopic range.

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 <= x <= 0.6) samples with the support of ab initio calculations and we find out that in the whole series: (i) the transport is dominated by electron bands only; (ii) the magnetoresistance exhibits distinctive features related to the presence of Dirac cones; indeed, ab initio calculations confirm the presence of anisotropic Dirac cones in the band structure; (iii) the low temperature mobility is exceptionally high and reaches 18.6 m2/(Vs) in the Ru-free sample at T=2K, in the extreme limit of a single Landau level occupied in the Dirac cones; (iv) the mobility drops abruptly above 10K-15K; (v) the disorder has a very weak effect on the band mobilities and on the transport properties; (vi) there exists a correlation between the temperature ranges of Dirac cones and SDW carrier condensation. These findings may be of crucial importance in the investigation of the pairing mechanism in the F-doped superconducting La(Fe,Ru)As(O,F) compounds related to this series of parent compounds., Comment: 18 pages, 12 figures, 1 table

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO$_{0.8}$F$_{0.2}$ superconductor. The field - temperature phase diagram of the superconducting state was drawn and, in particular, the features of the flux line lattice derived. The dependence of the intra-grain depinning energy on the magnetic field intensity was derived in the thermally-activated flux creep framework, enlightening a typical 1/H dependence in the high-field regime. The intra-grain critical current density was extrapolated in the zero temperature and zero magnetic field limit, showing a remarkably high value $J_{c0}(0) \sim 2 \cdot 10^{7}$ A/cm$^{2}$, which demonstrates that this material is rather interesting for the potential future technological applications., Comment: 6 pages, 6 figures

We prepared polycrystalline SmFeAsO1-xFx (Sm1111) bulk samples by sintering and hot isostatic pressing (HIP) in order to study the effects of phase purity and relative density on the intergranular current density. Sintered and HIPped Sm1111 samples are denser with fewer impurity phases, such as SmOF and the grain boundary wetting phase, FeAs. We found quite complex magnetization behavior due to variations of both the inter and intragranular current densities. Removing porosity and reducing second phase content enhanced the intergranular current density, but HIPping reduced Tc and the intragranular current density, due to loss of fluorine and reduction of Tc. We believe that the HIPped samples are amongst the purest polycrystalline 1111 samples yet made. However, their intergranular current densities are still small, providing further evidence that polycrystalline pnictides, like polycrystalline cuprates, are intrinsically granular., Comment: 14 pages, 6 figures

We report on the coexistence of magnetic and superconducting states in CeFeAsO1-xFx for x=0.06(2), characterized by transition temperatures T_m=30 K and T_c=18 K, respectively. Zero and transverse field muon-spin relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the CeFeAsO1-xFx phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies., Comment: 4 pages, 3 figs, accepted for publication as PRB Rapid comm

We report measurements of the phonon density of states as measured with inelastic x-ray scattering in SmFeAsO$_{1-x}$F$_y$ powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO$_{1-x}$F$_y$ single crystals allow us to identify the 21 meV A$_{1g}$ in-phase (Sm,As) and the 26 meV B$_{1g}$ (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors., Comment: 4 pages, 3 figures, submitted for SNS2010 conference proceedings

In this work we present a systematic experimental and theoretical study of the structural, transport and superconducting properties of Sm(Fe1-xRux)As(O0.85F0.15) 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 newly discovered multiband superconductors. Two different regions are identified: the Fe-rich phase (x<0.5) where superconducting and normal state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase (x>0.5) 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., Comment: Accepted on Physical Review B

$^{19}$F NMR measurements in SmFeAsO$_{1-x}$F$_x$, for $0.15\leq x\leq 0.2$, are presented. The nuclear spin-lattice relaxation rate $1/T_1$ increases upon cooling with a trend analogous to the one already observed in CeCu$_{5.2}$Au$_{0.8}$, a quasi two-dimensional heavy-fermion intermetallic compound with an antiferromagnetic ground-state. In particular, the behaviour of the relaxation rate either in SmFeAsO$_{1-x}$F$_x$ or in CeCu$_{5.2}$Au$_{0.8}$ can be described in the framework of the self-consistent renormalization theory for weakly itinerant electron systems. Remarkably, no effect of the superconducting transition on $^{19}$F $1/T_1$ is detected, a phenomenon which can hardly be explained within a single band model., Comment: 4 figures

The nuclear and magnetic structure of Fe1+y(Te1-x,Sex) (0 < x < 0.20) compounds was analyzed between 2 K and 300 K by means of Rietveld refinement of neutron powder diffraction data. Samples with x < 0.075 undergo a tetragonal to monoclinic phase transition at low temperature, whose critical temperature decreases with increasing Se content; this structural transition is strictly coupled to a long range antiferromagnetic ordering at the Fe site. Both the transition to a monoclinic phase and the long range antiferromagnetism are suppressed for 0.10 < x < 0.20. The onset of the structural and of the magnetic transition remains coincident with the increase of Se substitution. The low temperature monoclinic crystal structure has been revised. Superconductivity arises for x > 0.05, therefore a significant region where superconductivity and long range antiferromagnetism coexist is present in the pseudo-binary FeTe - FeSe phase diagram., Comment: 33 pages, 4 tables, 13 figures

High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that 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.

We present 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: SmFeAs(O1-xFx) with the largest Tc and the largest anisotropy and Fe1+y(Te1-x,Sex), with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAs(O1-xFx) series, we find that a single band description allows to extract an approximated estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data of 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: it dopes electrons in the system and it introduces localized magnetic moments, responsible for Kondo like scattering and likely pair-breaking of Cooper pairs. Hence, excess Fe plays a crucial role also in determining superconducting properties such as the Tc and the upper critical field Bc2. The huge Bc2 values of the Fe1+y(Te1-x,Sex) samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAs(O1-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.

The structural, magnetic and resistive properties of (La1-yYy)FeAsO compounds (y = 0.10, 0.20, 0.30) have been investigated by means of X-ray and neutron powder diffraction as well as by resistivity measurements. The temperatures at which the structural transition from tetragonal to orthorhombic and the magnetic ordering take place progressively reduce by similar amounts with increasing Y substitution on account of the progressive chemical pressure increase. We propose that the structural transition could be originated by a cooperative Jahn-Teller distortion involving the alignment of the fully occupied Fe 3d(z2) orbitals in the Fe plane along the y axis, leading to the branching of the cell parameters a and b. The magnetic structure develops after the occurrence of the structural transition, but before its completion., Comment: 27 pages, 9 figures

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, 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 behaviours 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, while 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, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

We have investigated the evolution of the low temperature specific heat anomaly (TN=5.4K in zero field) in polycrystalline SmFeAsO samples with magnetic fields up to 35T. The anomaly remains very sharp up to 16T and becomes rounded with little shift in temperature at higher fields. Doped (superconducting) SmFeAsO0.85F0.15 sample shows a similar behavior up to 16T. The initial slope of the critical field dBc/dT is 160T/K for undoped SmFeAsO and 70T/K for doped SmFeAsO0.85F0.15, with Bc(T) defined at the peak of the specific heat anomaly. The insensitivity to the application of an external magnetic field is unique to Sm and is not observed in CeFeAsO whose anomaly shifts with initial slope dBc/dT=5.7T/K. We argue that SmFeAsO(F) presents an unprecedented case of spin reorientation at the antiferromagnetic transition.

We report about muSR measurements on SmFeAsO1-xFx which helped us to identify the signature of diluted ferromagnetic inclusions, ubiquitous in the iron pnictides. These impurities are characterized by a Curie temperature close to room temperature and they seem responsible for a non negligible magnetic relaxation of the implanted muons, that should not be confused with intrinsic pnictide properties., Comment: The International Conference on FeAs High Tc Superconducting Multilayers and Related Phenomena (Superstripes2008), Rome, Italy, December 9-13, 2008

Superconducting epitaxial FeSe0.5Te0.5 thin films were prepared on SrTiO3 (001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy were studied with different experimental techniques: X-rays diffraction, reflection high energy electron diffraction, scanning tunnelling microscopy and atomic force microscopy. The substrate temperature during the deposition was found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained in the optimal conditions show an epitaxial growth with c axis perpendicular to the film surface and the a and b axis parallel to the substrates one, without the evidence of any other orientation. Moreover, such films show a metallic behavior over the whole measured temperature range and critical temperature above 17K, which is higher than the target one., Comment: 10 pages including 4 figures

In this paper the magnetic behaviour of undoped and 15% F doped SmFeAsO (Sm-1111) and LaFeAsO (La-1111) samples is presented and discussed. Magnetization measurements are not a simple tool to use for the characterisation of the new family of Fe-based superconductors because magnetic impurities can be easily formed during the preparation procedure and may affect the magnetic signal. In spite of this problem bulk magnetization measurements, properly treated, may give very useful information. In the undoped samples we gathered the main aspects of the physical behavior of the 1111 phase, i.e. the onset of the Spin Density Wave (SDW), the antiferromagnetic ordering at the Sm sublattice and the susceptibility increase with increasing temperature above the SDW temperature, and, in addition, we were able to estimate the Pauli contribution to susceptibility and therein the Wilson ratio both for LaFeAsO and SmFeAsO compounds, and the amplitude of the jump at the SDW temperature. In the doped samples, while the presence of magnetic signals due to impurities is dominating in the normal state, the superconducting behavior may be clearly observed and studied. In particular, in the Sm-1111 superconducting sample the coexistence-competition between superconductivity and antiferromagnetic ordering of the Sm ions was clearly observed., Comment: in press on Journal of Magnetism and Magnetic Materials

We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15 compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the a- and c-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase of the superconducting transition temperatures in the doped ones. The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution. Comparison with literature data indicates that chemical pressure cannot be the only mechanism which tunes drastically both TSDW and Tc in 1111 compounds. Our data suggest that structural disorder induced by the partial substitution in the La site or by doping could play an important role as well.

We present uSR investigations on SmFeAsO_1-xF_x showing coexistence of magnetic order and superconductivity only in a very narrow F-doping range. The sharp crossover between the two types of order is similar to that observed in LaFeAsO_1-xF_x, suggesting a common behavior for the 1111 pnictides. The analysis of the muon asymmetry demonstrates that the coexistence must be nanoscopic, i.e. the two phases must be finely interspersed over a typical length-scale of few nm. In this regime both the magnetic and the superconducting transition temperatures collapse to very low values. Our data suggest a competition between the two order parameters, Comment: new title; fig3 changed; new inset in fig4; little changes in abstract, introduction and conclusions. Accepted for publication in Phys Rev B

We measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criterion-dependence Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2/dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than mu_0H_LLL=8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc. In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound., Comment: 14 pages, 4 figures, 3 tables Submitted to Phys.Rev.B

We measured the Raman and the Infrared phonon spectrum of SmFeAsO polycrystalline samples. We also performed Density Functional Theory calculations within the pseudopotential approximation to obtain the structural and dynamical lattice properties of both the SmFeAsO and the prototype LaFeAsO compounds. The measured Raman and Infrared phonon frequencies are well predicted by the optical phonon frequencies computed at the Gamma point, showing the capability of the employed ab-initio methods to describe the dynamical properties of these materials. A comparison among the phonon frequencies of different oxypnictides suggests a possible role of the high frequency phonons in the pairing mechanism leading to superconductivity in these materials., Comment: text file (.tex) and 2 figures (.eps)

We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient, infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which below the magnetic transition carrier condensation occurs due to the opening of spin density wave (SDW) gap. This is accompanied by the mobility increase of not gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an increase of the metallic character on cooling consistent with electron doping, even though at room temperature values of all the properties nearly overlaps with those of SmFeAsO. However, with temperature decrease all anomalies related to the SDW instability are missed and the superconducting transition occurs. This suggests that doping breaks abruptly the symmetries of the Fermi surface inhibiting the SDW formation in favor of the superconducting transition, with no substantial changes in the density of states or in the effective mass., Comment: Submitted to SUST

The crystal structure of SmFeAsO has been investigated by means of Rietveld refinement of high resolution synchrotron powder diffraction data collected at 300 K and 100 K. The compound crystallizes in the tetragonal P4/nmm space group at 300 K and in the orthorhombic Cmma space group at 100 K; attempts to refine the low temperature data in the monoclinic P112/n space group diverged. On the basis of both resistive and magnetic analyses the tetragonal to orthorhombic phase transition can be located at T about 140 K., Comment: Submitted to: Superconductor Science and Technology PACS: 61.05.cp, 61.66.Fn, 74.10.+v, 74.62.Dh, 74.70.Dd

In this paper the magnetic behaviour of SmFeAsO and SmFeAs(O0.85F0.15) samples is presented and discussed. Molar susceptibility of SmFeAsO exhibits a local peak at T around 140K due to the establishment of a long range antiferromagnetic ordering of the Fe moments in Fe-As layers. This feature has already been observed with different techniques, and frequently ascribed to the onset of a Spin Density Wave (SDW). At TN around 6K another peak, which we attribute to the establishment of antiferromagnetic ordering of Sm ion sublattice, is observed. Furthermore, a temperature independent signal (Pauli paramagnetism, Landau and core diamagnetism...) is also present in the magnetic behaviour of this sample. In SmFeAs(O0.85F0.15) the antiferromagnetic ordering in Fe-As plane is suppressed and superconductivity occurs at T = 52 K, whereas the antiferromagnetic ordering of Sm ions at low temperature persists, leading to the coexistence and competition between superconducting and magnetic orderings. Above the transition temperature, after the subtraction of the Sm ion sublattice paramagnetic contribution and of the temperature independent contribution to the experimental susceptibility data, a Curie-Weiss behaviour for Fe is observed, with a magnetic moment of 1.4 Bohr magneton., Comment: submitted to Physical Review B

Recent investigations have shown that the first proposed explanations of the disagreement between experimental and theoretical value of isotope coefficient in MgB2 need to be reconsidered. Considering that in samples with residual resistivity of few mu-Ohm cm critical temperature variations produced by disorder effects can be comparable with variations due to the isotopic effect, we adopt a procedure in evaluating the B isotope coefficient which take account of these effects, obtaining a value which is in agreement with previous results and then confirming that there is something still unclear in the physics of MgB2., Comment: 8 pages, 3 figures Title has been changed A statement has been added in page 7 of the pdf file "Finally we would..." Reference 21 has been added Figure 1 anf Figure 2 have been changed

We have performed zero-field $^{75}$As nuclear magnetic resonance study of SmFeAsO$_{1-x}$F$_x$ (x=0, 0.07) polycrystals in a wide frequency range at various temperatures. $^{75}$As resonance line was found at around 265 MHz revealing the formation of the intermetallic FeAs clusters in the new layered superconductors. We have also demonstrated that NMR is a sensitive tool for probing the quality of these materials., Comment: Revised authorship

A comparative study of thermal properties of SmFeAsO, SmFeAs(O0.93F0.07) and SmFeAs(O0.85F0.15) samples is presented. Specific heat and thermal conductivity show clear evidences of the spin density wave (SDW) ordering below TSDW 135 K in undoped SmFeAsO. At low level of F-doping, SmFeAs(O0.93F0.07), SDW ordering is suppressed and superconducting features are not yet optimally developed in both specific heat and thermal conductivity. At optimal level of F-doping SmFeAs(O0.85F0.15) anomalies related to the superconducting transition are well noticeable. By a compared analysis of doped and undoped samples we conclude that, despite F-doping modifies definitely the electronic ground state, it does not substantially alter phonon and electron parameters, like phonon modes, Sommerfeld coefficient, electro-phonon coupling. The analysis of the thermal conductivity curves provides an evaluation of SDW and superconducting energy gap, showing that phonons can suitably probe features of electronic ground state.

SmFeAsO and the isostructural superconducting SmFeAs(O0.93F0.07) samples were prepared. Characterization by means of Rietveld refinement of X-ray powder diffraction data, scanning electron microscope observation, transmission electron microscope analysis, resistivity and magnetization measurements were carried out. Sintering treatment strongly improves the grain connectivity, but, on the other hand, induces a competition between the thermodynamic stability of the oxy-pnictide and Sm2O3, hence worsening the purity of the sample. In the pristine sample both magnetization and resistivity measurements clearly indicate that two different sources of magnetism are present: the former related to Fe ordering at 140 K and the latter due to the Sm ions that orders antiferromagnetically at low temperature. The feature at 140 K disappears in the F-substituted sample and, at low temperatures a superconducting transition appears. The magnetoresistivity curves of the F-substituted sample probably indicates very high critical field values., Comment: Submitted to: Superconductor Science and Technology

We carried out Raman measurements on neutron-irradiated and Al-doped MgB$_2$ samples. The irradiation-induced topological disorder causes an unexpected appearance of high frequency spectral structures, similar to those observed in lightly Al-doped samples. Our results show that disorder-induced violations of the selection rules are responsible for the modification of the Raman spectrum in both irradiated and Al-doped samples. Theoretical calculations of the phonon density of states support this hypothesis, and demonstrate that the high frequency structures arise mostly from contributions at ${\bf q}\not=0$ of the E$_{2g}$ phonon mode., Comment: 4 pages, 4 figures

We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B 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 10B 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 1-2x1018 cm-2. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behaviour 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 analysed., Comment: 22 pages, 12 figures. In press on Phys. Rev. B

We study the effect of neutron irradiation on the critical current density Jc of isotopically pure polycrystalline Mg11B2 samples. For fluences in the range 1017-1018 cm-2, Jc is enhanced and its dependence on magnetic field is significantly improved: we demonstrate that, in this regime, point-like pinning centers are effectively introduced in the system proportionally to the neutron fluence. Instead, for larger fluences, a strong suppression of the critical temperature accompanied by a decrease of both the upper critical field Bc2 and Jc is found., Comment: 13 pages, 3 igures

We prepared a series of Mg_{1-x}(AlLi)_{x}B_2 samples with 0≤x≤0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and X-ray diffraction confirms that Li enters the MgB2 structure even though in an amount less than nominal one. We performed susceptibility, resistivity and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg_{1-x}Al_{x}B_2 samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties., Comment: 15 pages, 13 figures; changes: "codoped" instead of "co-doped"; added comments in the Fig. 11 caption Comments 31/1/2006: 16 figures ; new revised version of the manuscript

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., Comment: 13 pages, 4 figures. Submitted to Appl.Phys.Lett

In this Letter, the study of the effect of Al substitution on the upper critical field, Bc2, in AlxMg1-xB2 samples is presented. We find a straightforward correlation between Bc2 and the sigma-band gap, Delta_sigma, evaluated by point-contact measurements. Up to x=0.2 Bc2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta_sigma. For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of Delta_sigma rather than by the increase of the sigma-band scattering rate, Comment: 5 pages, 4 figures