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A novel material is obtained by modifying pristine YBCO superconductor via simultaneous partial substitution of Sr for Ba and Se for O. The intended stoichiometry was YBa 2 − x SrxCu3 O 7 − x Sex with x = 0. 5 , 0.75 and 1; and x ≈ 1 yielded the most interesting results considered in detail. We confirmed that Se atoms indeed enter the superconductor lattice cell. The obtained polycrystalline material contains five phases, with the major phase (> 4 4 wt.%) being YBa 1. 4 Sr 0. 6 Cu3O6 Se 0. 5 . The synthesized superconductor demonstrates unique properties, including (i) two superconducting transitions with T c 1 ≈ 3 6 K and T c 2 ≈ 1 3 K yielding the Wohlleben effect; (ii) strong-coupling d-wave superconductivity (with 2 Δ / (k B T c) ≈ 5); (iii) Schottky anomaly in the heat capacity; and, most importantly, (iv) a very unusual anomaly in the heat capacity which can be associated with the quantum criticality typically observable at much higher fields. The fact that the quantum criticality is visible in weaker fields (∼ 7 T) may open opportunities for exploration of the interplay between superconductivity and pair density waves by wide research community particularly when its pure phase is available. To facilitate this, the stability of single-phase YBa 1. 4 Sr 0. 6 Cu3O6 Se 0. 5 is analyzed using machine learning methods. [ABSTRACT FROM AUTHOR]

Nematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high-Tc superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La2O2Fe2OM2M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C2 fluctuations while neutron diffraction indicates that global C4 symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity. [ABSTRACT FROM AUTHOR]

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 spectrum of the most strongly hole-overdoped correlated AFe2As2 (A122) with A=K, Cs is discussed phenomenologically from ARPES data and band-structure (GGA) calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient γ 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 Eilers et al. (Phys. Rev. Lett. 116, 237003 2016). 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 χ(T) data and a decreasing Wilson ratio ∝ χ(0)/γ. [ABSTRACT FROM AUTHOR]

We present a systematic study of the electronic specific-heat jump (ΔCel) at the superconducting transition temperature Tc of K1-xNaxFe2As2. Both Tc and ΔCel monotonously decrease with increasing x. The specific heat jump scales approximately with a power law, ΔCel∝Tβc, with β≈2 determined by the impurity scattering rate, in contrast to most iron-pnictide superconductors, where the remarkable Bud'ko-Ni-Canfield (BNC) scaling ΔCel∝T3 has been found. Both the T dependence of Cel(T) in the superconducting state and the nearly quadratic scaling of ΔCel at Tc are well described by the Eliashberg theory for a two-band d-wave superconductor with weak pair breaking due to nonmagnetic impurities. The disorder induced by the Na substitution significantly suppresses the small gaps, leading to gapless states in the slightly disordered superconductor, which results in a large observed residual Sommerfeld coefficient in the superconducting state for x > 0. [ABSTRACT FROM AUTHOR]

We report AC loss data of superconducting pancake coils (PC) wound from several non-transposed YBCO tapes connected in parallel. Due to coupling losses, their magnetization AC losses in AC fields applied parallel to the tape surface are significantly higher than for single-tape PCs. It was experimentally demonstrated that these coupling losses can be effectively reduced by using double-PCs in which the parallel tapes between the two PCs are connected in reversed order. This special connection between PCs imitates a twisting of parallel tapes and results in their decoupling. An AC current applied additionally with an external AC magnetic field to the double-PCs does not impede the decoupling of the tapes. We propose a simple model to calculate the magnetization AC losses of PCs by approximating the coils by face-to-face stacks of YBCO tapes. [ABSTRACT FROM AUTHOR]

A cellular superconducting material consisting of thin (1–20 μm) MgB2− x layers and magnesium granules of about 100 μm has been produced. The critical temperature T c of this superconductor decreases with the thickness of the MgB2− x layers. In unalloyed magnesium diboride, the curvature of the temperature dependence of the upper critical field H c2( T) changes gradually from downward to pronounced upward as the temperature T c decreases from 38 to 36 K. [ABSTRACT FROM AUTHOR]

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 spectrum of the most strongly hole-overdoped correlated AFe2As2 (A122) with A=K, Cs is discussed phenomenologically from ARPES data and band-structure (GGA) calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient γ 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 Eilers et al. (Phys. Rev. Lett. 116, 237003 2016). 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 χ(T) data and a decreasing Wilson ratio ∝ χ(0)/γ. [ABSTRACT FROM AUTHOR]