M. Tropeano

Università degli Studi di Genova, Genova, Liguria, Italy

Are you M. Tropeano?

Claim your profile

Publications (74)114.66 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: 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 recently 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.
    Physical Review B 04/2013; 81(18):184504. · 3.77 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: 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.
    physica status solidi (b) 03/2013; 250(3):599-602. · 1.49 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev reflection spectroscopy. The results show that the SmFe_{1-x}Ru_{x}AsO_{0.85}F_{0.15} system keeps a multigap character at least up to x=0.50, and that the gap amplitudes Delta_1 and Delta_2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Delta_i/(kB Tc) remain approximately constant only as long as Tc > 30 K, but increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based superconductors, irrespective of their family. Based on first-principle calculations of the bandstructure and of the density of states projected on the different bands, we show that this trend, as well as the Tc dependence of the gaps and the reduction of Tc upon Ru doping, can be explained within an effective three-band Eliashberg model as being due to a suppression of the superfluid density at finite temperature that, in turns, modifies the temperature dependence of the characteristic spin-fluctuation energy.
    Superconductor Science and Technology 01/2013; 25(8). · 2.76 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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.
    Physica Status Solidi (B). 01/2013; 250:599.
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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 MgB2. 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 MgB2, 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.
    Physica C Superconductivity 11/2012; 482:68–73. · 0.72 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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.
    Superconductor Science and Technology 05/2012; 25(11). · 2.76 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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.
    Superconductor Science and Technology 05/2012; 25. · 2.76 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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.
    Physical Review B 02/2012; 85:054518. · 3.77 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: In this paper, we investigate the Ru-substituted LaFeAsO compound by studying the magnetotransport behavior and its relationship with the band structure in different regimes of temperature, magnetic field, and Ru content. In particular, we analyze the magnetoresistance of LaFe1-xRuxAsO (0
    Physical Review B 01/2012; 85:019902-1. · 3.77 Impact Factor
  • Physical Review B - Condensed Matter and Materials Physics. 01/2012; 85(5).
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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 or 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.
    Physical Review B 11/2011; 84:195123. · 3.77 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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.
    Physical Review B 10/2011; 84(13):134524. · 3.77 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Superconducting fluctuations (SF) in SmFeAsO$_{0.8}$F$_{0.2}$ (characterized by superconducting transition temperature $T_{c} \simeq 52.3$ K) are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response to magnetic fields up to 1 T above $T_{c}$ is similar to what previously reported for underdoped cuprate superconductors and it can be justified in terms of metastable superconducting islands at non-zero order parameter lacking of long-range coherence because of strong phase fluctuations. In the high-field regime ($H \gtrsim 1.5$ T) scaling arguments predicted on the basis of the Ginzburg-Landau theory of conventional SF are found to be applicable, at variance with what observed in the low-field regime. This fact enlightens that two different phenomena are simultaneously present in the fluctuating diamagnetism, namely the phase SF of novel character and the conventional SF. High magnetic fields (1.5 T $\lesssim H \ll H_{c2}$) are found to suppress the former while leaving unaltered the latter one.
    Physical Review B 08/2011; 84:064507. · 3.77 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: In this Letter we propose a new phase diagram for the SmFeAs(O(1-x)F(x)) system, based on careful analysis of synchrotron powder diffraction data, SQUID, and muon spin rotation measurements. The tetragonal to orthorhombic structural transition is slightly affected by F content and is retained for the superconducting samples, even at optimal doping. These findings relate the AFM transition on a different ground with respect to the structural one and suggests that orbital ordering could be the driving force for symmetry breaking.
    Physical Review Letters 06/2011; 106(22):227001. · 7.94 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: 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 lines were derived. The dependence of the intragrain 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 intragrain critical current density was extrapolated in the zero-temperature and zero-magnetic-field limit, showing a remarkably high value J(c0)(0) similar to 2 x 10(7) A/cm(2), which demonstrates that this material is rather interesting for potential future technological applications.
    Physical Review B 05/2011; 83:174514. · 3.77 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The bulk intrinsic magnetism of Co changes dramatically when nonmagnetic La Sm and Nd in RECoAsO (RE¼La, Nd, Sm). Although the itinerant ferromagnetism occurs below 80-100 K with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 57 K and 45 K for Sm and at 69 K and 14 K for Nd. This is evidenced in isothermal magnetization plots as well. It is clear that Sm=Nd magnetic moments interact with the ordered Co spins in adjacent layer and thus transforms the ferromagnetic (FM) ordering to AFM.
    Journal of Applied Physics 03/2011; 109:07E121. · 2.21 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We have prepared polycrystalline Sm1111 bulk samples by sintering and hot isostatic pressing (HIP) and studied the influence of processing on the microstructure and intergranular current. After sintering and HIPping, samples are denser with much less impurity SmOF and grain boundary wetting FeAs. They then show significantly larger hysteresis loops than as- prepared samples. But Tc of HIPed samples is lower than samples with optimal doping, indicating loss of fluorine during later processing. Nevertheless, even after the fluorine loss and its associated carrier density reduction produced by the HIP treatment, both Magneto-Optical and remanent magnetization analyses showed that the intergranular current was enhanced. We conclude that the denser microstructure and smaller impurity phase content of HIPped samples enhances their connected fraction, even as the doping state of the sample is degraded by F loss. Our study emphasizes that current transport in polycrystalline pnictides is a complex balance involving the need to control carrier density, especially at grain boundaries, while densifying the microstructure and avoiding normal-state grain boundary wetting phases like FeAs.
    03/2011;
  • [show abstract] [hide abstract]
    ABSTRACT: We have microscopically investigated the interplay between magnetism (M) and superconductivity (SC) of the RFeAsO (1111) oxy-pnictide for R=La, Sm and Ce as a function of F doping, isoelectronic Fe/Ru substitution and external pressure. In contrast to earlier data, our results suggest a unique behaviour in different 1111 families at the M-SC crossover, showing a sharp crossover between the two types of order as a function of F doping [1-3]. In the optimally e^--doped SmFe1-xRuxAsO0.85F0.15 compound, magnetic order appears in the FeAs layers for 0.1
    03/2011;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: We study the effect of synthesis temperature on the phase formation in nano(n)-SiC added bulk MgB2 superconductor. In particular we study: lattice parameters, amount of carbon (C) substitution, microstructure, critical temperature (Tc), irreversibility field (Hirr), critical current density (Jc), upper critical field (Hc2) and flux pinning. Samples of MgB2+(n-SiC)x with x=0.0, 0.05 & 0.10 were prepared at four different synthesis temperatures i.e. 850, 800, 750, and 700oC with the same heating rate as 10oC/min. We found 750oC as the optimal synthesis temperature for n-SiC doping in bulk MgB2 in order to get the best superconducting performance in terms of Jc, Hc2 and Hirr. Carbon (C) substitution enhances the Hc2 while the low temperature synthesis is responsible for the improvement in Jc due to the smaller grain size, defects and nano-inclusion induced by C incorporation into MgB2 matrix, which is corroborated by elaborative HRTEM (high-resolution transmission electron microscopy) results. We optimized the the Tc(R=0) of above 15K for the studied n-SiC doped and 750 0C synthesized MgB2 under 140 KOe field, which is one of the highest values yet obtained for variously processed and nano-particle added MgB2 in literature to our knowledge.
    Superconductor Science and Technology 01/2011; 24:045013. · 2.76 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: High-purity epitaxial FeSe0.5Te 0.5 thin films with different thicknesses were grown by pulsed laser ablation on different substrates. By varying the film thickness, T c values of up to 21 K 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 T c. The latter result indicates the important role of the compressive strain in enhancing T c: the compressive strain derives from the Volmer–Weber growth of the films. The critical temperature is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.
    Journal of Superconductivity and Novel Magnetism 01/2011; 24(1):35-41. · 0.70 Impact Factor