Tris(ethane-1,2-diamine)copper(II) bis­(trifluoro­acetate)

In the title complex, [Cu(H2NCH2CH2NH2)3](CF3COO)2, the environment of the Cu atom is distorted octa­hedral, formed by six N atoms from three chelating ethane-1,2-diamine ligands. The Cu—N distances range from 2.050 (2) to 2.300 (2) Å. This complex cation and the two trifluoro­acetate anions are connected by weak N—H⋯O and N—H⋯F hydrogen bonds, forming a three-dimensional framework. In both anions, the F atoms are disordered over two positions; in one the site-occupancy factors are 0.55 and 0.45, in the other the values are 0.69 and 0.31

Mass enhancements and band shifts in strongly hole overdoped Fe-based pnictide superconductors: KFe2_2As2_2 and CsFe2_2As2_2

The interplay of high and low-energy mass renormalizations with band-shifts reflected by the positions of van Hove singularities (VHS) in the normal state spectra of the highest hole-overdoped and strongly correlated AFe$_2$As$_2$ (A122) with A = K, Cs is discussed phenomenologically based on ARPES data and GGA band-structure calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient $\gamma$ from K122 to Cs122 is ascribed to an enhanced coupling to low-energy bosons in the vicinity of a quantum critical point to an unknown, yet incommensurate phase different from the commensurate Mott one. We find no sizeable increase in correlations for Cs122 in contrast to F. Eilers et al., PRL v. 116, 237003 (2016) [3]. The empirical (ARPES) VHS positions as compared with GGA-predictions point even to slightly weaker correlations in Cs122 in accord with low-$T$ magnetic susceptibility $\chi(T)$ data and a decreasing Wilson ratio $\propto \chi(0)/\gamma$.Comment: 8 pages, 7 figures, updated references, and a Note for arXiv-reader

3D visualizations of nano-scale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)0.95Fe1.75Se2

Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides (AxFe2-ySe2) continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of AxFe2-ySe2 superconductors. Here we report that the 3D visualizations of nano-scale phase separation in (Na0.32K0.68)0.95Fe1.75Se2 single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in (Na0.32K0.68)0.95Fe1.75Se2 is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change ({\Delta}R/R) identify two characteristic temperatures T*~100 K (the onset temperature of coupling between nano-metallic and AFM phases) and TH~230 K (the onset temperature of metallic-interface-phase). An energy-transfer channel between the nano-metallic and AFM phases is inferred. This proposed channel provides a new insight into the pairing mechanism of alkaline iron selenide superconductors.Comment: 17 pages, 5 figures; Physical Review Materials, in pres

Crystal growth, transport phenomena and two-gap superconductivity in the mixed alkali metal (K1-zNaz)xFe 2-ySe2 iron selenide

Using the self-flux technique we grew superconducting (K 1-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. This journal is © the Partner Organisations 2014

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 $\Delta_L=(2.5 \div 3.4)$ meV and the small gap $\Delta_L=(0.9 \div 1)$ meV at $T = 4.2$ K for the $T_C^{local} \approx (10.5 \div 14)$ K (the contact area critical temperature which deviation causes the variation of $\Delta_L$). The BCS-ratio is found to be $2\Delta_L/k_BT_C = (4.6 \div 5.6)$, whereas $2\Delta_S/k_BT_C \ll 3.52$ results from induced superconductivity in the bands with the small gap.Comment: 7 pages, 5 figures. Published in Pis'ma v ZhETF 95, 604-610 (2012

Effect of Sodium Substitution on Structural and Magnetic Properties of KFe2-y Se-2

WOS: 000382398300026We have synthesized the superconducting K1-x Na (x) Fe2-y Se-2 (x = 0.07 and 0.25) crystal by using the flux technique. SEM images demonstrate that two samples consist of successive plate-like surfaces oriented into ab plane. EDS analysis yields the average compositions as (Na0.20K0.80)(0.96)Fe(1.78)Se(2)for x = 0.07 and (Na0.33K0.67)(0.96)Fe1.73Se2 for x = 0.25. XRD pattern gives the (00 a"") peaks. The main peaks are indexed to the I4/m space group corresponding to reflected intensities from (002), (006), (008), and (0010) planes related to the peaks of the phase K2Fe4Se5. The lattice parameters are calculated as a = 8.701(4) and c = 14.185(2) for x = 0.07, and a = 8.689(3) and c = 14.054(4) for x = 0.25. ZFC magnetization measurement exhibits a very sharp transition which indicates the presence of superconductivity in the crystal. The superconducting transition temperatures, T (c) , obtained from magnetization measurements are estimated to be 31.0 and 28.4 K for x = 0.07 and 0.25, respectively. The critical current value, J (c) (0), deduced from the M-H loops are approximately 2.8 x10(4) A/cm (2) for x = 0.07 and 2.87 x10(2) A/cm (2) for x = 0.25 at 5 K. The volume pinning force, F (p) , of x = 0.07 sample is found larger than that of the x = 0.25 one.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [112T767]; Research Fund of Cukurova University, Adana, TurkeyCukurova University [FDK-2015-4036]; Volkswagen FoundationVolkswagen; Russian Foundation for Basic ResearchRussian Foundation for Basic Research (RFBR) [15-03-99628]This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under Grant No. 112T767 and Research Fund of Cukurova University, Adana, Turkey, under grant contract no.: FDK-2015-4036. LM, IM, and AB would like to acknowledge the financial support by the Volkswagen Foundation and the Russian Foundation for Basic Research (Grant No. 15-03-99628). The authors wish to thank Dr. Sabine Wurmehl, Michael Schulze and Prof. B. Buchner, from IFW Dresden (Germany) for their hospitality and support during the preparation of the sample