M. Roslova

Evidence of d-wave superconductivity in K1-xNaxFe2As2 (x=0,0.1) single crystals from low-temperature specific-heat measurements

From the measurement and analysis of the specific heat of high-quality K_(1-x)Na_xFe_2As_2 single crystals we establish the presence of large T^2 contributions with coefficients alpha_sc ~ 30 mJ/mol K^3 at low-T for both x=0 and 0.1. Together with the observed square root field behavior of the specific heat in the superconducting state both findings evidence d-wave superconductivity on almost all Fermi surface sheets with an average gap amplitude of Delta_0 in the range of 0.4 - 0.8 meV. The derived Delta_0 and the observed T_c agree well with the values calculated within the Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime.

Investigation of LiFeAs by means of “break-junction” technique

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 ΔL = (2.5–3.4) meV and the small gap ΔS = (0.9–1) meV at T = 4.2 K for the TClocal ≈ (10.5–14) K (the contact area critical temperature which deviation causes the variation of ΔL). The BCS-ratio is found to be 2ΔL/kBTC = 4.6–5.6, whereas 2ΔS/kBTC ≪ 3.52 results from induced superconductivity in the bands with the small gap.

Uniform Patterns of Fe-Vacancy Ordering in the Kx(Fe,Co)(2-y)Se2 Superconductors

The Fe-vacancy ordering patterns in the superconducting KxFe2–ySe2 and nonsuperconducting Kx(Fe,Co)2–ySe2 samples have been investigated by electron diffraction and high angle annular dark field sc...

Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides

Angle-resolved photoemission spectroscopy (ARPES) is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictides systems. Our ARPES results show linear-in-energy dependent scattering rates which are constant in a wide range of control parameter and which depend on the orbital character of the bands. We demonstrate that the linear energy dependence gives rise to weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. In the superconducting phase the related small effective Fermi energy favors a Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein (BE)\,\cite{Bose1924} crossover state.

Crystal growth, transport phenomena and two-gap superconductivity in the mixed alkali metal (K1−zNaz)xFe2−ySe2 iron selenide

Using the self-flux technique we grew superconducting (K1−zNaz)xFe2−ySe2 (z = 0.3) single crystals. EDX mapping revealed the uniform elements distribution on the crystal surface while XRD measurements indicate that the crystals are compositionally inhomogeneous on the nanoscale. The physical properties of the as-prepared sample are characterized by electrical resistivity, magnetization and specific heat measurements. Resistivity measurements show the onset of the superconducting transition at 33 K and zero resistivity at 31.7 K. The large upper critical field Hc2(0) was estimated as high as about 140 T for the in-plane field and 38 T for the out-of-plane field. The anisotropy of Habc2(0)/Hcc2(0) and coherence lengths ξab(0)/ξc(0) was found to be around 3.7. The pioneering studies by multiple Andreev reflections effect spectroscopy (“break-junction” technique) revealed the presence of two anisotropic superconducting gaps ΔL = (9.3 ± 1.5) meV, ΔS = (1.9 ± 0.4) meV, and provided a measurement of the ΔL(T) temperature dependence. The Bardeen–Cooper–Schrieffer (BCS) theory ratio ratio for the large gap 2ΔL/kBTbulkc ≈ 6.3 points to a strong electron–boson coupling in the “driving” condensate characterized by the ΔL order parameter.

Local structure and hyperfine interactions of 57Fe in NaFeAs studied by Mössbauer spectroscopy.

Detailed 57Fe Mössbauer spectroscopy measurements on superconducting NaFeAs powder samples have been performed in the temperature range 13 K ≤ T < 300 K. The 57Fe spectra recorded in the paramagnetic range (T > TN ≈ 46 K) are discussed supposing that most of the Fe2+ ions are located in distorted (FeAs4) tetrahedra of NaFeAs phase, while an additional minor (<10%) component of the spectra corresponds to impurity or intergrowth NaFe2As2 phase with a nominal composition near NaFe2As2. Our results reveal that the structural transition (TS ≈ 55 K) has a weak effect on the electronic structure of iron ions, while at T ≤ TN the spectra show a continuous distribution of hyperfine fields HFe. The shape of these spectra is analyzed in terms of two models: (i) an incommensurate spin density wave modulation of iron magnetic structure, (ii) formation of a microdomain structure or phase separation. It is shown that the hyperfine parameters obtained using these two methods have very similar values over the whole temperature range. Analysis of the temperature dependence HFe(T) with the Bean–Rodbell model leads to ζ = 1.16 ± 0.05, suggesting that the magnetic phase transition is first order in nature. A sharp evolution of the VZZ(T) and η(T) parameters of the full Hamiltonian of hyperfine interactions near T ≈ (TN,TS) is interpreted as a manifestation of the anisotropic electron redistribution between the dxz-, dyz- and dxy-orbitals of the iron ions.

High-energy electronic interaction in the 3d band of high-temperature iron-based superconductors

D. V. Evtushinsky, A. N. Yaresko, V. B. Zabolotnyy, J. Maletz, T. K. Kim, A. A. Kordyuk, 4 M. S. Viazovska, M. Roslova, 6 I. Morozov, 6 R. Beck, S. Wurmehl, 7 H. Berger, B. Büchner, 7 and S. V. Borisenko Institute for Solid State Research, IFW Dresden, P.O.Box 270116, D-01171 Dresden, Germany Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Diamond Light Source Ltd., Didcot, Oxfordshire, OX11 0DE, United Kingdom Institute of Metal Physics of National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine Humboldt University of Berlin, Rudower Chaussee 25, 12489 Berlin Moscow State University, 119991 Moscow, Russia Institut für Festkörperphysik, Technische Universität Dresden, D-01171 Dresden, Germany Institut de Physique Applique, Ecole Politechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland

Weak-coupling superconductivity in a strongly correlated iron pnictide

Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations.

Combined resistivity and Hall effect study on NaFe1−xRhxAs single crystals

Electrical transport measurements are used to study the Rh-doped NaFeAs superconductor series with a focus on the tetragonal phase. The resistivity curvature has an anomalous temperature dependence evidencing in the phase diagram two crossover regions of changes in the scattering rate, the effective mass as well as of the charge carrier density. The first crossover region is directly connected to the structural transition and resembles the onset of resistivity anisotropy. The second crossover region can as well be deduced from the temperature dependent Hall coefficient. A comparison to literature NMR data suggests this region to be connected with nematic fluctuations far above the tetragonal to orthorhombic phase transition.

Downscaling Effect on the Superconductivity of Pd3Bi2X2 (X = S or Se) Nanoparticles Prepared by Microwave-Assisted Polyol Synthesis

Pd3Bi2S2 and Pd3Bi2Se2 have been successfully prepared in the form of nanoparticles with diameters of ∼50 nm by microwave-assisted modified polyol synthesis at low temperatures. The composition and morphology of the samples have been studied by means of powder X-ray diffraction as well as electron microscopy methods, including X-ray intensity mapping on the nanoscale. Superconducting properties of the as-prepared samples have been characterized by electrical resistivity measurements down to low temperatures (∼0.2 K). Deviations from the bulk metallic behavior originating from the submicrometer nature of the samples were registered for both phases. A significant critical-field enhancement up to 1.4 T, i.e., 4 times higher than the value of the bulk material, has been revealed for Pd3Bi2Se2. At the same time, the critical temperature is suppressed to 0.7 K from the bulk value of ∼1 K. A superconducting transition at 0.4 K has been observed in nanocrystalline Pd3Bi2S2. Here, a zero-temperature upper critical field of ∼0.5 T has been estimated. Further, spark plasma-sintered Pd3Bi2S2 and Pd3Bi2Se2 samples have been investigated. Their superconducting properties are found to lie between those of the bulk and nanosized samples.