A. V. Sadakov

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

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO0.88F0.12FeAs samples with bulk critical temperature Tc = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are ΔL = 10.5 ± 2 meV and Δs = 2.3 ± 0.4 meV, respectively.

Magnetic and superconducting properties of FeAs-based high-Tc superconductors with Gd

We report on successful synthesis under high pressure of a series of polycrystalline GdFeAsO1 − xFx high-Tc superconductors with different oxygen deficiency x = 0.12−0.16 and also with no fluorine. We have found that the high-pressure synthesis technique is crucial for obtaining the single-phase superconducting materials: by sythesizing the same compounds with no pressure in ampoules, we obtained non-superconducting materials with an admixture of incidental phases. Critical temperature for all the materials was in the range 40 to 53 K. The temperature derivative of the critical field dHc2/dT is remarkably high, indicating potentially high value of the second critical field Hc2 ∼ 130 T.

Quantum oscillations in heavily doped bismuth chalcogenides

The observation of Shubnikov-de Haas and Hall oscillations in high-quality Bi2 − xCuxSe3 single crystals is reported. Measurements carried out upon rotating the samples with respect to the magnetic field demonstrate that the oscillations originate from two-dimensional surface states in three-dimensional single crystals and are determined only by the perpendicular component of the magnetic field.

Quantum oscillations in strong magnetic fields, berry phase, and superconductivity in three-dimensional topological Bi2–xCuxSe3 insulators

Two-dimensional (2D) Shubnikov–de Haas oscillations and 2D Hall oscillations are observed in 3D copper-doped Bi2Se3 single crystals in magnetic fields up to 19.5 T at temperatures down to 0.3 K. Three samples with a high bulk carrier concentration (n ≈ 1019–1020 cm–3) are studied. The rotation of the samples in a magnetic field shows that these oscillations are related to numerous parallel 2D conducting channels 1–5 nm thick. Their basic kinetic parameters are found. Quantized Hall resistance Rxy is detected in 1-nm-thick 2D conducting channels at high fields. The distance Δ(1/Rxy) between the steps in the field dependence of 1/Rxy is found to be constant for different Landau levels, 1.3e2/h per 1-nm-thick layer. The constructed fan diagrams of 2D Landau levels for various angles of sample inclination with respect to the magnetic field direction allowed us to conclude that the Berry phase in the 2D conducting channels is γ ≈ π and independent of the magnetic field direction. When studying the angular dependence of upper resistive critical magnetic field Hc2 in one of the superconducting samples, we showed that it can be considered as a bulk superconductor consisting of superconducting layers with an effective thickness of about 50 nm.

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

Manganese(III) complexes with tetradentate (N2O2) Schiff bases and dicyanamide

New Mn(III) complexes with Schiff bases and dicyanamide are synthesized: [Mn(Salpn)N(CN)2]n (two polymorphous modifications, Ia and Ib), {[Mn(5-BrSalen)N(CN)2] · CH3OH}n (II), and [Mn(3-MeOSalen)N(CN)2(H2O)] (III), where SalpnH2 = N,N′-bis(salicylidene)-1,3-diaminopropane, 5-BrSalenH2 = N,N′-bis(5-bromosalicylidene)-1,2-diaminoethane, and 3-MeOSalenH2 = N,N′-bis(3-methoxysalicylidene)-1,2-diaminoethane. Complexes Ia, Ib, and II have the polymer structure in which the dicyanamide anion binds the paramagnetic Mn(III) complexes with the Schiff bases into one-dimensional chains. Unlike them, in complex III the monomer units containing water and the dicyanamide anion as terminal ligands form dimers due to hydrogen bonds. The study of the magnetic properties of complexes Ia and II shows a weak antiferromagnetic interaction between the Mn3+ ions through the dicyanamide bridges in these complexes.

Growth and properties of layered oxycarbonate Bi2Sr4Cu2CO3O8 single crystals

Layered oxycarbonate Bi2Sr4Cu2CO3O8 single crystals have been obtained for the first time by free growth in closed vapor-phase cavities. The morphology, structure, composition, and superconducting properties of these crystals have been investigated.

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 vortex pinning and liquid-glass transition have been studied in BaFe2–xNixAs2 single crystals with different doping levels (x = 0.065, 0.093, 0.1, 0.14, 0.18). We found that Ni-doped Ba-122 has rather narrow vortex-liquid state region. Our results show that the temperature dependence of the resistivity as well as I−V characteristics of Ni-doped Ba-122 is consistent with 3D vortex-glass model. It was found that -pinning gives the main contribution to overall pinning in 122 Ni-doped system. The vortex phase diagrams for different doping levels were built based on the obtained data of temperature of the vortex-glass transition Tg and the upper critical magnetic field Hc2.

Ni

The crystals of a perylene complex with the polymeric cluster containing [Mn6O2(O2CCHCl2)10(H2O)2] units assembled by 3,10-perylenequinone bridges, {[Mn6O2(O2CCHCl2)10(H2O)2(C20H10O2)]2n·(C20H12)n} (I), were obtained in the reaction of [Mn12O12(O2CCHCl2)16(H2O)4] with perylene. Unlike the Mn12 oxodichloroacetate cluster, the reaction of the Mn12 oxobenzoate cluster, [Mn12O12(O2CPh)16(H2O)4], with perylene does not occur. However, it turned out that the crystals isolated from the reaction are a new Mn12 oxobenzoate cluster containing three molecules of coordinated H2O and solvate molecules of hexane, [Mn12O12(O2CC6H5)16(H2O)3]·2C6H12 (II). Crystal structures of complexes I and II as well as the magnetic properties of the former were studied.