A. Amato

Commensurate Spin Density Wave in LaFeAsO: A Local Probe Study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe Mössbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)muB at the iron site below T(N)=138 K, well separated from a structural phase transition at T(S)=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T(N) are reproduced.

Field and Temperature Dependence of the Superfluid Density in LaFeAsO1 xFx Superconductors: A Muon Spin Relaxation Study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.

Pressure induced static magnetic order in superconducting FeSe1-x.

We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure approximately 0.8 GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above T{c} and bulk superconductivity coexists and competes on short length scales with the magnetic order below T{c}. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.

Momentum-resolved superconducting gap in the bulk of Ba1−xKxFe2As2 from combined ARPES and μSR measurements

Here we present a calculation of the temperature-dependent London penetration depth, ?(T), in Ba1-xKxFe2As2 (BKFA) on the basis of the electronic band structure (Zabolotnyy et al 2009 Nature?457 569, Zabolotnyy et al 2009 Physica C 469?448) and momentum-dependent superconducting gap (Evtushinsky et al 2009 Phys. Rev. B 79 054517) extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of ?(T) by muon spin rotation (?SR) (Khasanov et al 2009 Phys. Rev. Lett.?102 187005). The value of ?(T=0), calculated with no adjustable parameters, equals 270?nm, while the directly measured one is 320?nm; the temperature dependence ?(T) is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new iron-based superconductors in general allows us to state that most of them bear two nearly isotropic gaps with coupling constants 2?/kBTc=2.5?1.5 and 7?2.

Evolution of two-gap behavior of the superconductor FeSe1-x.

The superfluid density, rho{s}, of the iron chalcogenide superconductor, FeSe1-x, was studied as a function of pressure by means of muon-spin rotation. The analysis of rho{s}(T) within the two-gap scheme reveals that the effect on both, the transition temperature T{c} and rho{s}(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

Microscopic coexistence of superconductivity and magnetism in Ba(1-x)K(x)Fe2As2.

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba(1-x)K(x)Fe(2)As(2) exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba(1-x)K(x)Fe(2)As(2) showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.

Superconducting properties of single-crystalline A x Fe 2 − y Se 2 ( A = Rb, K) studied using muon spin spectroscopy

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Effect of external pressure on the magnetic properties of LnFeAsO (Ln = La, Ce, Pr, Sm)

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Coexistence of low-moment magnetism and superconductivity in tetragonal FeS and suppression of T-c under pressure

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Structural and magnetic phase transitions in triclinic Ca10(FeAs)10(Pt3As8)

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