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M. Tropeano,
M. R. Cimberle,
C. Ferdeghini,
G. Lamura,
A. Martinelli,
A. Palenzona,
I. Pallecchi,
A. Sala,
I. Sheikin, F. Bernardini,
M. Monni,
S. Massidda,
M. Putti
[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. · 3.69 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The mechanism of superconductivity and magnetism and their possible interplay have recently been under debate in pnictides. A likely pairing mechanism includes an important role of spin fluctuations and can be expressed in terms of the magnetic susceptibility χ. The latter is therefore a key quantity in the determination of both the magnetic properties of the system in the normal state and of the contribution of spin fluctuations to the pairing potential. A basic ingredient to obtain χ is the independent-electron susceptibility χ0. Using LaO1−xFxFeAs as a prototype material, in this report we present a detailed ab initio study of χ0(q,ω) as a function of doping and of the internal atomic positions. The resulting static χ0(q,0) is consistent with both the observed M-point-related magnetic stripe phase in the parent compound and with the existence of incommensurate magnetic structures predicted by ab initio calculations upon doping.
Physical Review B 04/2013; 81(10):104503. · 3.69 Impact Factor
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G Lamura,
T Shiroka,
P Bonfà,
S Sanna, F Bernardini,
R De Renzi,
R Viennois,
E Giannini,
A Piriou,
N Emery,
M R Cimberle,
M Putti
[show abstract]
[hide abstract]
ABSTRACT: The evolution of magnetic order in Fe1+ySexTe1-x crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent density functional theory calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase becomes progressively disordered with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment 'freezes' at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.
Journal of Physics Condensed Matter 03/2013; 25(15):156004. · 2.55 Impact Factor
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G. Lamura,
T. Shiroka,
P. Bonfà,
S. Sanna, F. Bernardini,
R. De Renzi,
R. Viennois,
E. Giannini,
A. Piriou,
N. Emery,
M. R. Cimberle,
M. Putti
[show abstract]
[hide abstract]
ABSTRACT: The evolution of the magnetic order in FeSeTe crystals as a function of Se
content was investigated by means of ac/dc magnetometry and muon-spin
spectroscopy. Experimental results and self-consistent DFT calculations both
indicate that muons are implanted in vacant iron-excess sites, where they probe
a local field mainly of dipolar origin, resulting from an antiferromagnetic
(AFM) bicollinear arrangement of iron spins. This long-range AFM phase
disorders progressively with increasing Se content. At the same time all the
tested samples manifest a marked glassy character that vanishes for high Se
contents. The presence of local electronic/compositional inhomogeneities most
likely favours the growth of clusters whose magnetic moment "freezes" at low
temperature. This glassy magnetic phase justifies both the coherent muon
precession seen at short times in the asymmetry data, as well as the glassy
behaviour evidenced by both dc and ac magnetometry.
03/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We investigate the pressure phase diagram of FeTe, predicting structural and
magnetic properties in the normal state at zero temperature within density
functional theory (DFT). We carefully examined several possible different
crystal structures over a pressure range up to $\approx 30 $ GPa: simple
tetragonal (PbO type), simple monoclinic, orthorhombic (MnP type), hexagonal
(NiAs and wurzite type) and cubic (CsCl and NaCl type). We predict pressure to
drive the system through different magnetic ordering (notably also some
ferromagnetic phases) eventually suppressing magnetism at around 17GPa. We
speculate the ferromagnetic order to be the reason for the absence of a
superconducting phase in FeTe at variance with the case of FeSe.
Physical Review B 01/2013; 87:094516. · 3.69 Impact Factor
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[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.
01/2013;
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Physical Review B 01/2012; 85:019902-1. · 3.69 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We address the in-plane pressure-dependent electrodynamics of graphite
through synchrotron based infrared spectroscopy and ab initio Density
Functional Theory calculations. The Drude term remarkably increases upon
pressure application, as a consequence of an enhancement of both electron and
hole charge densities. This is due to the growth of the band dispersion along
the k_z direction between the K and H points of the Brillouin zone. On the
other hand, the mid-infrared optical conductivity between 800 and 5000 cm-1 is
almost flat, and very weakly pressure dependent, at least up to 7 GPa. This
demonstrates a surprising robustness of the graphene-like universal quantum
conductance of graphite, even when the interlayer distance is significantly
reduced.
11/2011;
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[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. · 3.69 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report on theoretical calculations of the optical conductivity of Ba(Fe1-xCox)2As2, as obtained from density functional theory within the full-potential linear augmented plane-wave method. A thorough comparison with experiment shows that we are able to reproduce most of the observed experimental features, in particular, a magnetic peak located at about 0.2 eV, which we ascribe to antiferromagnetic ordered magnetic stripes. We also predict a large in-plane anisotropy of this feature, which agrees very well with measurements on detwinned crystals. The effect of Co doping, as well as the dependence of plasma frequency on the magnetic order, is also investigated.
Phys. Rev. B. 10/2010; 83(5).
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M. Tropeano,
M. R. Cimberle,
C. Ferdeghini,
G. Lamura,
A. Martinelli,
A. Palenzona,
I. Pallecchi,
A. Sala,
M. Putti, F. Bernardini,
M. Monni,
S. Massidda,
I. Sheikin
[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 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
Physical Review B 04/2010; 81:184504. · 3.69 Impact Factor
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A. Perucchi,
D. Nicoletti,
M. Ortolani,
C. Marini,
R. Sopracase,
S. Lupi,
U. Schade,
M. Putti,
I. Pallecchi,
C. Tarantini,
M. Ferretti,
C. Ferdeghini,
M. Monni, F. Bernardini,
S. Massidda,
P. Dore
[show abstract]
[hide abstract]
ABSTRACT: The possibility of multiband conductivity and multigap superconductivity is explored in oriented V3Si thin films by means of reflectance and transmittance measurements at terahertz frequencies. The temperature dependence of the transmittance spectra in the normal state gives evidence of two bands contributing to the film conductivity. This outcome is consistent with electronic structure calculations performed within density functional theory. On this basis, we performed a detailed data analysis and found that all optical data can be consistently accounted for within a two-band framework, with the presence of two optical gaps in the superconducting state corresponding to 2Δ/kTc values close to 1.8 and 3.8.
Physical Review B 03/2010; 81(9):092509. · 3.69 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The effects of high-temperature vacuum-annealing induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 C, the average Mg content in the MgB2 crystals systematically decreased, while Tc remains essentially unchanged and the superconducting transition slightly broadens from 0.55 K to 1.3 K. The reduction of the superconducting volume fraction was noticeable already after annealing at 875 C. Samples annealed at 975 C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(VMg)xB2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence Tc, in agreement with observations. The significant reduction of the superconducting volume fraction without noticeable Tc reduction indicates the coexistence, within the same crystal, of superconductive and non-superconductive electronic phases, associated with regions rich and poor in Mg vacancies. Comment: 21 pages, 1 table, 6 figures, PRB accepted
02/2010;
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[show abstract]
[hide abstract]
ABSTRACT: The mechanism of superconductivity and magnetism and their possible interplay have recently been under debate in pnictides. A likely pairing mechanism includes an important role of spin fluctuations and can be expressed in terms of the magnetic susceptibility chi. The latter is therefore a key quantity in the determination of both the magnetic properties of the system in the normal state, and of the contribution of spin fluctuations to the pairing potential. A basic ingredient to obtain chi is the independent-electron susceptibility chi0. Using LaO1-xFxFeAs as a prototype material, in this report we present a detailed ab-initio study of chi0(q,omega), as a function of doping and of the internal atomic positions. The resulting static chi0(q,0) is consistent with both the observed M-point related magnetic stripe phase in the parent compound, and with the existence of incommensurate magnetic structures predicted by ab-initio calculations upon doping. Comment: 15 pages, 8 figures
Physical Review B 01/2010; 81:104503. · 3.69 Impact Factor
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A. Perucchi,
D. Nicoletti,
M. Ortolani,
C. Marini,
R. Sopracase,
S. Lupi,
U. Schade,
M. Putti,
I. Pallecchi,
C. Tarantini,
M. Ferretti,
C. Ferdeghini,
M. Monni, F. Bernardini,
S. Massidda,
P. Dore
[show abstract]
[hide abstract]
ABSTRACT: The possibility of multi-band conductivity and multi-gap superconductivity is
explored in oriented V3Si thin films by means of reflectance and transmittance
measurements at terahertz frequencies. The temperature dependence of the
transmittance spectra in the normal state gives evidence of two bands
contributing to the film conductivity. This outcome is consistent with
electronic structure calculations performed within density functional theory.
On this basis, we performed a detailed data analysis and found that all optical
data can be consistently accounted for within a two-band framework, with the
presence of two optical gaps in the superconducting state corresponding to
2D=kTc values close to 1.8 and 3.8.
Physical Review B 07/2009; 81:092509. · 3.69 Impact Factor
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C Bersier,
A Floris,
P Cudazzo,
G Profeta,
A Sanna, F Bernardini,
M Monni,
S Pittalis,
S Sharma,
H Glawe,
A Continenza,
S Massidda,
E K U Gross
[show abstract]
[hide abstract]
ABSTRACT: Superconductivity in Pb, H under extreme pressure and CaBeSi, in the framework of the density functional theory for superconductors, is discussed. A detailed analysis on how the electron–phonon and electron–electron interactions combine together to determine the superconducting gap and critical temperature of these systems is presented. Pb, H under pressure and CaBeSi are multigap superconductors. We will address the question under which conditions does a system exhibits this phenomenon. The presented results contribute to the understanding of multiband and anisotropic superconductivity, which has received a lot of attention since the discovery of MgB2, and show how it is possible to describe the superconducting properties of real materials on a fully ab initio basis.
Journal of Physics Condensed Matter 03/2009; 21(16):164209. · 2.55 Impact Factor
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S Massidda, F Bernardini,
C Bersier,
A Continenza,
P Cudazzo,
A Floris,
H Glawe,
M Monni,
S Pittalis,
G Profeta,
A Sanna,
S Sharma,
E K U Gross
[show abstract]
[hide abstract]
ABSTRACT: Superconductivity in intercalated graphite CaC6 and H under extreme pressure is discussed in the framework of superconducting density functional theory. A detailed analysis of how the electron–phonon and electron–electron interactions combine together to determine the superconducting gap and critical temperature (Tc) of these systems is presented. In particular, we discuss the effect on the calculated Tc of the anisotropy of the electron–phonon interaction and of the different approximations for screening the Coulomb repulsion. These results contribute to the understanding of multigap and anisotropic superconductivity, which has received a lot of attention since the discovery of MgB2, and show how it is possible to describe the superconducting properties of real materials on a fully ab initio basis.
Superconductor Science and Technology 01/2009; 22(3):034006. · 2.66 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We use first-principles techniques to investigate the properties of native defects and impurities in MgB2. We focus our investigation on those defects that could have a relevant role in Al and Li co-doped MgB2. This system shows an anomalous dependence of superconductivity on doping concentration, not yet understood. Structural effects such as segregation and clustering have been put forward to justify experimental evidence. Here we show that native defects have very high formation energies, that Al and Li do not form aggregates and incorporate as substitutional defects randomly distributed on the Mg sublattice sites. There exists a competition between segregation and incorporation of Li, the latter being slightly favored energetically. Our results suggest that anomaly in the superconducting behavior must be accounted for by electronic structure effects.
EPL (Europhysics Letters) 01/2007; 76(3):491. · 2.17 Impact Factor
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R S Gonnelli,
D Daghero,
G A Ummarino,
A Calzolari,
M Tortello,
V A Stepanov,
N D Zhigadlo,
K Rogacki,
J Karpinski, F Bernardini,
S Massidda
[show abstract]
[hide abstract]
ABSTRACT: We present the first results of directional point-contact measurements in Mg1-xMnxB2 single crystals, with x up to 0.015 and bulk Tc down to 13.3 K. The order parameters Deltasigma and Deltapi were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. BothDeltapi and Deltasigma decrease with the critical temperature of the junctions TAC, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principles band-structure calculations.
Physical Review Letters 08/2006; 97(3):037001. · 7.37 Impact Factor
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R. S. Gonnelli,
D. Daghero,
G. A. Ummarino,
A. Calzolari,
M. Tortello,
V. A. Stepanov,
N. D. Zhigadlo,
K. Rogacki,
J. Karpinski, F. Bernardini,
S. Massidda
[show abstract]
[hide abstract]
ABSTRACT: We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model. Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principle band structure calculations. Comment: 4 pages, 5 eps figures. New theoretical results added, text and some figures changed. References added
10/2005;
