T. E. Kuzmicheva

Single crystal growth and characterization of tetragonal FeSe1−x superconductors

The plate-like single crystals of tetragonal (P4/nmm) FeSe1−x superconductors were grown using the KCl–AlCl3 flux technique which produced single crystalline tetragonal samples of about 4 × 4 × 0.1 mm3 dimensions. The energy dispersive X-ray spectroscopy established a ratio of Feu2006:u2006Se = 1u2006:u20060.96 ± 0.02. The resistivity and magnetization measurements revealed a sharp superconducting transition at Tc = 9.4 K. Multiple Andreev reflections spectroscopy pointed to the existence of two-gap superconductivity with the gap values ΔL = 2.4 ± 0.2 meV and ΔS = 0.75 ± 0.1 meV at 4.2 K.

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature Tc ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, ΔΓ = 5.1–6.5 meV, ΔL = 3.8–4.8 meV, and ΔS = 0.9–1.9 meV (at T ≪ Tc), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ∼12, and ∼20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming ΔΓ and ΔL are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λΓL ≈ λLΓ ≪ λSΓ, λSL.

“Break-junction” technique in application to layered superconductors (Review Article)

A comprehensive study of superconductor-constriction-superconductor contacts, obtained using the “break-junction” technique in layered superconductors. Depending on the constriction transparency, tunneling and SnS Andreev spectroscopies could be used to directly determine the values of the superconducting gaps, characteristic BCS ratios and temperature dependences of the gaps in cuprates, magnesium diboride and iron pnictides and chalcogenides. Based on these results we can estimate the gap anisotropy and the electron-boson coupling constants. The advantages and drawbacks of “break-junction” technique are discussed, and we demonstrate that this method is powerful enough for the study of optical phonon modes in high-temperature superconducting cuprates and for creating contacts with selective transparency in Mg1-xAlxB2 compounds.

Determination of the electron-phonon coupling constants from the experimental temperature dependences of superconducting gaps in MgB2

Experimental temperature dependences Δσ, π(T) of the energy of superconducting gaps for MgB2 samples with the critical temperatures 22 K < Tc < 41 K have been fitted by selecting the renormalized electron-phonon coupling constants λij with the use of the Moskalenko-Shul system of equations, the expression for the frequency of collective plasma oscillations obtained by Leggett for two-gap superconductors, and two fitting parameters. We previously obtained the dependences Δσ, π(T) by the multiple Andreev reflection spectroscopy of superconductor-constriction-superconductor junctions based on MgB2 with various degree of disorder of the crystal structure. It has been shown that the intraband pairing constants are decisive for the superconductivity mechanism in MgB2; in this case, √VσσVππ/Vσπ = 8–22 and the ratio of the interband constants α can range from 3 to 11. The set of the Eliashberg coupling constants λij0 has been qualitatively determined for relatively pure MgB2 with maximum values Tc ≈ 40 K. The leading constant is 0.7 < λσσ0 ≈ λeff0 < 0.9 and depends on the choice of the upper integration limit in the Bardeen-Cooper-Schrieffer (BCS) model and the effective Coulomb repulsion μiff*. The characteristic ratio for the gap in the σ band is 2Δσ/kBTc = 5.0–6.5.

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

1 P.N. Lebedev Physical Institute RAS, 119991 Moscow, Russia 2 M.V. Lomonosov Moscow State University, 119991 Moscow, Russia 3 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409 Moscow, Russia 4 International Laboratory of High Magnetic Fields and Low Temperatures, 53-412 Wroclaw, Poland 5 National University “Higher School of Economics”, 101000 Moscow, Russia

Observation of bosonic resonances in GdO 1− x F x FeAs by intrinsic multiple Andreev reflection effect spectroscopy

The current–voltage characteristics and dynamic conductance spectra of Andreev arrays in nearly optimal GdO1 − xFxFeAs superconductors with the critical temperature Tc = 46–50 K have been studied. A reproducible fine structure that accompanies large gap features and caused by boson emission during the process of multiple Andreev reflections has been observed. At T = 4.2 K, the bosonic mode energy ε0 = (12 ± 2) meV has been obtained.

Superconducting gap symmetry in BaFe

T.E. Kuzmicheva, S.A. Kuzmichev, 1 A.V. Sadakov, S.Yu. Gavrilkin, A.Yu. Tsvetkov, X. Lu, H. Luo, A. N. Vasiliev, 4, 5 V. M. Pudalov, Xiao-Jia Chen, and Mahmoud Abdel-Hafiez 7, ∗ P.N. Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia M.V. Lomonosov Moscow State University, Faculty of Physics, 119991 Moscow, Russia Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China National Research South Ural State University, Chelyabinsk 454080, Russia National University of Science and Technology (MISiS), Moscow 119049, Russia Center for High Pressure Science and Technology Advanced Research, Shanghai, 201203, China Center for High Pressure Science and Technology Advanced Research, Beijing, 100094, China (Dated: June 8, 2018)

_{1.9}

The superconducting order parameters in optimally doped Ba0.65K0.35Fe2As2 single crystals have been directly measured using multiple Andreev reflection effect spectroscopy of superconductor–normal metal–superconductor break-junctions. We determine two superconducting gaps, which are nodeless in the kxky-plane of the momentum space, and resolve a substantial in-plane anisotropy of the large gap. The temperature dependences of the gaps indicate a strong coupling within the bands where ΔL develops, a weak coupling in the condensate with the small gap ΔS, and a moderate interband interaction between the two condensates. The own critical temperatures of both condensates have been estimated (under the hypotherical assumption of zero interband interaction).

Ni

We present direct measurements of the superconducting order parameter in nearly optimal FeSe Te single crystals with the critical temperature TC ≈ 14 K. Using the intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of the lower critical field, we directly determined two superconducting gaps, ΔL ≈ 3.3−3.4 meV and ΔS ≈ 1 meV, and their temperature dependences. We show that a twoband model fits well the experimental data. The estimated electron–boson coupling constants indicate a strong intraband and a moderate interband interaction.