S. A. Kuzmichev

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

Ya. G. Ponomarev, S. A. Kuzmichev, M. G. Mikheev, M. V. Sudakova, S. N. Tchesnokov, N. Z. Timergaleev, A. V. Yarigin, M. V. Lomonosov Moscow State University, Department of Physics, 119899 Moscow, Russia, E. G. Maksimov, S. I. Krasnosvobodtsev and A. V. Varlashkin, P. N. Lebedev Physics Institute, RAS, Moscow, Russia, M. A. Hein, G. Müller, H. Piel, Bergische Universität Wuppertal, Fachbereich Physik, D-42097 Wuppertal, Germany, L. G. Sevastyanova, O. V. Kravchenko, K. P. Burdina, B. M. Bulychev, M. V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow, Russia.

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 Fe : Se = 1 : 0.96 ± 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.

Lower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, T-c. We investigated the temperature dependence of the lower critical field H-c1(T) of a Ca0.32Na0.68Fe2As2 (T-c approximate to 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same T-c. We estimated the range of the large gap value Delta(L) = 6-8 meV (depending on small variation of T-c) and its a k space anisotropy of about 30%, and the small gap Delta(S) approximate to 1.7 +/- 0.3 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps.

Multiple Andreev Reflections Spectroscopy of Two-Gap 1111- and 11 Fe-Based Superconductors

Using the “break-junction” technique, we prepared and studied superconductor–constriction–superconductor (ScS) nanocontacts in polycrystalline samples of Fe-based superconductors CeO0.88F0.12FeAs (Ce-1111;

Investigation of a superconducting Mg1−xAlxB2 system by tunneling and microjunction (Andreev) spectroscopies

T_{C}^{\mathrm{bulk}} = 41 \pm1~\mathrm{K}

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

), LaO0.9F0.1FeAs (La-1111;

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

T_{C}^{\mathrm{bulk}} = 28 \pm1~\mathrm {K}

Temperature dependence of superconducting gaps in Mg1−xAlxB2 system investigated by SnS-Andreev spectroscopy

), and FeSe (

Leggett’s mode in Mg1−x AlxB2

T_{C}^{\mathrm{bulk}} = 12 \pm1~\mathrm{K}

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

). We detected two subharmonic gap structures related with multiple Andreev reflections, indicating the presence of two superconducting gaps with the BCS-ratios 2ΔL/kBTC=4.2÷5.9 and 2ΔS/kBTC∼1≪3.52, respectively. Temperature dependences of the two gaps ΔL,S(T) in FeSe indicate a k-space proximity effect between two superconducting condensates. For the studied iron-based superconductors, we found a linear relation between the gap ΔL and magnetic resonance energy, Eres≈2ΔL.