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Despite the simple crystal structure, LiFeAs having nontrivial band structure and anisotropy of superconducting properties seems the most exotic compound among iron-based superconducting pnictides. Nonetheless, the available experimental studies are not so numerous due to LiFeAs chemical instability even in presence of a trace amount of water vapors or oxygen. Here, using the Andreev spectroscopy of break junctions in LiFeAs single crystals, we unambiguously show a coexistence of three superconducting condensates below critical Tc = 17 K. The temperature dependences of the order parameters and the anisotropy of the middle and the large gaps, as well as the excess Andreev current, and zero-bias conductance are directly measured. We estimate the partial band contributions to the total conductance. Additionally, considered is an influence of the band structure topology near the Fermi level on the dynamic conductance in the Andreev regime. [ABSTRACT FROM AUTHOR]

Magnetic field dependent modulated microwave absorption (MMWA) measurements have been carried out to investigate vortex pinning effects in single crystals of the iron-based high-Tc superconductor Ba(Fe1-xCox)2As2 with three different cobalt doping levels of x = 0.07, 0.09 and 0.11. The dependence of the MMWA hysteresis loops on temperature, magnetic field and Co concentration have been measured and analyzed using a theoretical model of microwave absorption in superconductors. The analysis reveals that in the underdoped crystal (x = 0.07) the so called δTc-pinning due to magnetically ordered regions defines the temperature dependence of the critical current density, while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the pinning is governed by structural imperfections due to the inhomogeneous distribution of the cobalt dopant and has the so called δl character. [ABSTRACT FROM AUTHOR]

High-quality KFe2As2 (K122) single crystals synthesized by different techniques have been studied by magnetization and specific heat (SH) measurements. The adopted phenomenological analysis of the normal state properties shows that there are two types of samples both affected by disordered magnetic phases: (i) cluster-glass (CG) like or (ii) Griffiths phase (G) like. For (i) at low applied magnetic fields the T-dependence of the zero-field-cooled (ZFC) linear susceptibility χl exhibits an anomaly with an irreversible behavior in ZFC and field-cooled (FC) data. This anomaly is related to the freezing temperature Tf of a CG. For the investigated samples the extrapolated Tf to B = 0 varies between 56 and 90 K. Below Tf we observed a magnetic hysteresis in the field dependence of the isothermal magnetization M( B). The frequency shift of the freezing temperature $\delta T_{{\rm f}} = {\rm \Delta }T_{{\rm f}} /[T_{{\rm f}} {\rm \Delta }({\rm ln}\,\nu )]\sim 0.05$ has an intermediate value, which provides evidence for the formation of a CG-like state in the K122 samples of type (i). The frequency dependence of their Tf follows a conventional power-law divergence of critical slowing down $\tau = \tau _{0} [T_{{\rm f}} (\nu )/T_{{\rm f}} (0)- 1]^{- z\nu '} $ with the critical exponent zν′ ≈ 10 and a relatively long characteristic time constant $\tau _{0} \approx 6.9 \times 10^{- 11} \,{\rm s}$ also supporting a CG behavior. The large value of the Sommerfeld coefficient obtained from SH measurements of these samples was related to the magnetic contribution from a CG. Samples from (ii) did not show a hysteresis behavior for χl( T) and M( B). Below some crossover temperature T* ≈ 50-80 K a power-law dependence in the $\chi _{{\rm l}} \,{\propto} \,T^{\lambda _{{\rm G}} - 1} $, with a non-universal λG was observed, suggesting a quantum G-like behavior. In this case χl and M( B) can be scaled using the scaling function $M_{{\rm s}} (T,B) = B^{1- \lambda _{{\rm G}} } Y(\mu B/k_{{\rm b}} T)$ with the scaling moment µ ∼ 3.5 µB. The same non-universal exponent was found also in SH measurements, where the magnetic contribution $C/T\,{\propto} \,T^{\lambda _{{\rm G}} - 1} $. [ABSTRACT FROM AUTHOR]

In our tunneling investigation using Andreev superconductor-normal metal-superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap Δ = (2.5-3.4) meV and the small gap Δ = (0.9-1) meV at T = 4.2 K for the T ≈ (10.5-14) K (the contact area critical temperature which deviation causes the variation of Δ). The BCS-ratio is found to be 2Δ/ k T = 4.6-5.6, whereas 2Δ/ k T ≪ 3.52 results from induced superconductivity in the bands with the small gap. [ABSTRACT FROM AUTHOR]

We present thermal expansion studies on single-crystalline LaFe 1 - x Co x AsO with doping levels of x = 0, 0.026, and 0.034, and on BaFe 2 As 2 . Anomalies in the thermal expansion coefficient α enable reading off the structural and magnetic transition temperatures T S and T N . The evolution of the orthorhombic distortion at T S is illustrated by length changes of the a - and b -axes and the experimentally determined orthorhombicity δ. The results are compared to BaFe 2 As 2. Finally, the electronic phase diagram is discussed. [ABSTRACT FROM AUTHOR]

Highlights • Confirmation of twin structure in the orthorhombic BaFe 2 As 2 bulk single crystals. • Absence of orthorhombic domains in a fully strained 20 nm BaFe 2 As 2 thin film. • Absence of orthorhombic domains in a partially strained 60 nm BaFe 2 As 2 thin film. Abstract The ternary iron arsenide compound BaFe 2 As 2 exhibits a structural phase transition from tetragonal to orthorhombic at a temperature of about 140 K. The twin lamellae arising below this transition temperature were studied in undoped single crystalline bulk and epitaxial thin film samples using electron backscatter diffraction in a scanning electron microscope equipped with a helium cryostat. Applying this technique on bulk single crystals a characteristic twin lamella size in the range of 0.1 μm up to a few μm was observed. In contrast, in epitaxially strained thin films the phase transition is not observed at temperatures above 19 K. [ABSTRACT FROM AUTHOR]

The impact of synthesis conditions, post-preparation heating procedure, aging and influence of pressure on the superconducting properties of FeSe 0.4 Te 0.6 crystals is reported. Two FeSe 0.4 Te 0.6 single crystals were used in the study, prepared from stoichiometric melt but cooled down with very different cooling rates, and investigated using magnetic bulk and electrical-resistivity methods. The fast-cooled crystal contains large inclusions of F e 3 Se 2.1 Te 1.8 and exhibits bulk superconductivity in its as-prepared state, while the other is homogeneous and shows only traces of superconductivity. AC susceptibility measurements under hydrostatic pressure show that the superconducting transition temperature of the inhomogeneous crystal increases from 12.3 K at ambient pressure to T sc = 17.9 K at 9 kbar. On the other hand, neither pressure nor mechanically-induced stress is sufficient to induce superconductivity in the homogeneous crystal. However, an additional heat treatment at 673 K followed by fast cooling down and/or long-term aging at ambient conditions leads to the appearance of bulk superconductivity also in the latter sample. This sample remains homogeneous on a scale down to few μm but shows an additional magnetic phase transition around 130 K suggesting that it must be inhomogeneous. For comparison also F e 3 Se 2.1 Te 1.8 polycrystals have been prepared and their magnetic properties have been studied. It appears that this phase is not superconducting by itself. It is concluded that nano-scale inhomogeneities that appear in the F e S e x Te 1 − x system due to a spinodal decomposition in the solid state are necessary for bulk superconductivity, possibly due to minor changes in the crystal structure and microstructure. Macroscopic inclusions quenched by fast cooling from high temperatures lead obviously to strain and hence variations in the lattice constants, an effect that is further supported by application of pressure/stress. [ABSTRACT FROM AUTHOR]

Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe 0.6 Se 0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8 K. In contrast, fast-cooled samples are superconducting below about 14 K but are composed of several chemical phases: they consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 20–30 vol%). These have different crystal structures and chemical compositions and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe 0.6 Se 0.4 is non-superconducting. [ABSTRACT FROM AUTHOR]

The superconducting properties of LaFeAsO1-xFx under conditions of optimal electron doping are investigated upon the application of external pressure up to ∼23 kbar. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature Tc and the low-temperature saturation value for the ratio ns/m* (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of ∼30% is reported for ns/m* at the maximum pressure value while Tc is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of nonmagnetic impurities on multiband superconductivity into account. In particular, the unique possibility to modify the ratio between intraband and interband scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model. [ABSTRACT FROM AUTHOR]

We have studied the magnetic ordering in Ba1-xNaxFe2As2 with 0.25 ⩽ x ⩽ 0.4 by unpolarized and polarized neutron diffraction using single crystals. Unlike most FeAs-based compounds that magnetically order, Na-doped BaFe2As2 exhibits two successive magnetic transitions: For x = 0.35, upon cooling, magnetic order occurs at ∼ 70 K with in-plane magnetic moments being arranged as in pure or Ni-, Co-, or K-doped BaFe2As2 samples. At a temperature of ∼ 46 K a second phase transition occurs, which the single-crystal neutron-diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba065Na0.35Fe2As2 point along the c direction. The two nearly degenerate magnetic states document orbital degeneracy to persist in the superconducting phase. [ABSTRACT FROM AUTHOR]

We present 75As nuclear magnetic resonance measurements on single crystals of BaFe2As2, BaFe1.8Co0.2As2, and BaFe1.82Cu0.18As2. While only Co doping induces bulk superconductivity on a broad doping range, the spin fluctuations probed by the nuclear spin-lattice relaxation rate (T1T)-1 are identical for both dopings down to Tc. Below this temperature, (T1T)-1 of the Cu-doped sample continues to rise, proving that (a) there is a quantum critical point below the superconducting dome, and (b) adding electrons does not affect the spin fluctuations. Consequently, we analyze the Knight shift data in terms of a two-component scenario, with one hyperfine coupling to an itinerant degree of freedom and the other to Fe moments. [ABSTRACT FROM AUTHOR]

We report a low-temperature specific heat study of high-quality single crystals of the heavily hole-doped superconductor Ca0.32Na0.68Fe2As2. This compound exhibits bulk superconductivity with a transition temperature Tc ≈ 34 K, which is evident from the magnetization, transport, and specific heat measurements. The zero-field data manifest a significant electronic specific heat in the normal state with a Sommerfeld coefficient γ Ri 53 mJ/mol K2. Using a multiband Eliashberg analysis, we demonstrate that the dependence of the zero-field specific heat in the superconducting state is well described by a three-band model with an unconventional s± pairing symmetry and gap magnitudes Δ1 of approximately 2.35, 7.48, and -7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling, which contributes significantly to the relatively high value of Tc. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-Tc- iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions. [ABSTRACT FROM AUTHOR]

We present a systematic study of the electronic specific-heat jump (ΔCel) at the superconducting transition temperature Tc of K1-xNaxFe2As2. Both Tc and ΔCel monotonously decrease with increasing x. The specific heat jump scales approximately with a power law, ΔCel∝Tβc, with β≈2 determined by the impurity scattering rate, in contrast to most iron-pnictide superconductors, where the remarkable Bud'ko-Ni-Canfield (BNC) scaling ΔCel∝T3 has been found. Both the T dependence of Cel(T) in the superconducting state and the nearly quadratic scaling of ΔCel at Tc are well described by the Eliashberg theory for a two-band d-wave superconductor with weak pair breaking due to nonmagnetic impurities. The disorder induced by the Na substitution significantly suppresses the small gaps, leading to gapless states in the slightly disordered superconductor, which results in a large observed residual Sommerfeld coefficient in the superconducting state for x > 0. [ABSTRACT FROM AUTHOR]

We report the angular dependence of three distinct de Haas-van Aiphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB2 at temperatures down to 0.3 K and magnetic fields up to 14 T. Comparison with the Fermi surface calculations considering an incommensurate cycloidal magnetic order suggests that two of the observed dHvA oscillations arise from electronlike Fermi surface sheets formed by bands with strong B-px,y character. The third orbit could correspond to a Cr-d derived Fermi surface sheet. The measured effective masses of these Fermi surface sheets display strong enhancements of a factor of about 2 over the calculated band masses which can be attributed to electron-phonon coupling and electronic correlations. Signatures of further heavy d-electron bands that are predicted by the calculations are not observed in the temperature and field range studied. In the view that the B-p bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB2, we consider possible implications of our findings for nonmagnetic CrB2 and an interplay of itinerant antiferromagnetism with superconductivity. [ABSTRACT FROM AUTHOR]

From the measurement and analysis of the specific heat of high-quality K1-xNaxFe2As2 single crystals we establish the presence of large T2 contributions with coefficients asc ≈30 mJ/mol K3 at low T for both x = 0 and x =0.1. Together with the observed √B behavior of the specific heat in the superconducting state both findings give evidence of d-wave superconductivity on almost all Fermi-surface sheets with an average gap amplitude of Δ0 in the range of 0.4-0.8 meV. The derived Δ0 and observed Tc agree well with the values calculated within Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime. [ABSTRACT FROM AUTHOR]

We provide direct experimental evidence for the identical effect of the in-plane Fe1-xCox and of the out-of-plane O1-xFx chemical dilution on the itinerant spin-density-wave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still, it is clearly shown that the sizable in-plane disorder induced by the Fe1-xCox, substitution is highly effective in suppressing Tc. Differently from what is observed in CeFeAsO1-xFx, the ordered magnetic phase of the Ce sublattice is preserved throughout the whole phase diagram in CeFe1-xCoxAsO (x ⩽ 0.2). An intriguing effect is encountered, whereby the magnetic coupling among Ce3+ ions is enhanced by the superconducting phase. [ABSTRACT FROM AUTHOR]

We report a systematic study on the electronic structure and superconducting (SC) gaps in electron-doped NaFe0.95Co0.05 As superconductor using angle-resolved photoemission spectroscopy. Holelike Fermi sheets are at the zone center and electronlike Fermi sheets are at the zone corner, and are mainly contributed by xz and yz orbital characters. Our results reveala Δ/KgTc in the range of 1.8-2.1, suggesting a weak-coupling superconductivity in these compounds. Gap closing above the transition temperature (Tc) shows the absence of pseudogaps. Gap evolution with temperature follows the BCS gap equation near the Γ, Z, and M high symmetry points. Furthermore, an almost isotropic superconductivity along the kz direction in the momentum space is observed by varying the excitation energies. [ABSTRACT FROM AUTHOR]

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector (0.5 ± &dgr;, 0.5 ∓ &dgr;, 0) with &dgr; ∼ 0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight. [ABSTRACT FROM AUTHOR]

We investigate the electronic properties and the superconducting gap characteristics of a single crystal of hole- doped 122 Fe-pnictide Ba0.65Na0.35Fe2As2 by means of specific-heat measurements. The specific heat exhibits a pronounced anomaly around the superconducting transition temperature Tc = 29.4 K and a small residual part at low temperature. In a magnetic field of 90 kOe, the transition is broadened and Tc is lowered insignificantly by an amount ~1.5 K. We estimate a high electronic coefficient in the normal state with a value 57.5 mJ mol-1 K-2, being consistent with hole-doped 122 compounds. The temperature-dependent superconducting electronic specific heat cannot be described with single-gap BCS theory under the weak-coupling approach. Instead, our analysis implies a presence of two s-wave-like gaps with magnitudes Δ1(0)/kBTc = 1.06 and Δ2(0)/kB Tc = 2.08 with respective weights of 48% and 52%. While our results have qualitative similarities with other hole-doped 122 materials, the gap's magnitude and their ratio are quite different. [ABSTRACT FROM AUTHOR]

Single crystals of the new unconventional superconductor LiFe1-xCoxAs with x= 0, 0.025, 0.05 were grown by a new approach using the self-flux technique. The superconducting transition temperature was found to decrease upon Co doping at the Fe site. Apparently, in LiFeAs this doping scheme suppresses superconductivity, in contrast to the effects of Co doping in other Fe-As compounds, where it suppresses the spin-density wave and establishes superconductivity. Angle-resolved photoemission spectroscopy shows that the bottom of electron-like bands sinks by about 17 meV upon 5% Co doping, which indicates that the chemical substitution of Co for Fe in LiFeAs results in charge doping. [ABSTRACT FROM AUTHOR]

We study angular-dependent magnetoresistance in iron-based superconductors Ba1-xNaxFe2As2 and FeTe1-xSex. Both superconductors have relatively small anisotropies γ~2 and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field Hc2(T) for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field. [ABSTRACT FROM AUTHOR]

19F NMR measurements in optimally electron-doped La1-yYyFe1-xMnxAsO0.89F0.11 superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds (y=0 and y=0.2) where the chemical pressure is varied by substituting La with Y. In the y=0.2 series superconductivity is suppressed for Mn contents an order of magnitude larger than for y=0. For both series a peak in the 19F NMR nuclear spin-lattice relaxation rate 1/T1 emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. 19F NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the y=0 series, at variance with what one would expect for Q =(π/a,0) spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at Q =(π/a,0) and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn. [ABSTRACT FROM AUTHOR]