We study the relationship between precipitate morphology and superconductivity in K x Fe 1.6+y Se 2 single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature T s , whereas it aggregates into micrometer-sized rectangular bars and aligns as disconnected chains in the furnace-cooled samples. Accompanying this change in morphology the superconducting shielding fraction is strongly reduced. By post-annealing above T s followed by quenching in room temperature water, the network recovers with superconducting shielding fraction approaching 80% for the furnace-cooled samples. A reversible change from network to bar chains was realized by a secondary heat treatment in annealed samples showing large shielding fraction, i.e., heating above T s followed by slow cooling across T s . The large shielding fraction observed in K x Fe 1.6+y Se 2 single crystals actually results from a uniform and contiguous distribution of superconducting phase. Through the measurements of temperature dependent x-ray diffraction, it is found that superconducting phase precipitates while iron vacancy ordered * Corresponding author: yliu@ameslab.gov 2 phase forms together by cooling across T s in K x Fe 1.6+y Se 2 single crystals. It is a solid solution above T s , where iron atoms randomly occupy the both Fe1 and Fe2 sites in iron vacancy disordering status; and phase separation is driven by the iron vacancy order-disorder transition upon cooling. However, neither additional iron in the starting mixtures nor as-quenching at high temperatures can extend the miscibility gap to KFe 2 Se 2 side.
Non-invasive magnetic field sensing using optically -detected magnetic resonance of nitrogenvacancy (NV) centers in diamond was used to study spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. Vector magnetic fields were measured on the surface of conventional, Pb and Nb, and unconventional, LuNi2B2C, Ba0.6K0.4Fe2As2, Ba(Fe0.93Co0.07)2As2, and CaKFe4As4, superconductors, with diffraction -limited spatial resolution using variable -temperature confocal system. Magnetic induction profiles across the crystal edges were measured in zero-field-cooled (ZFC) and field-cooled (FC) conditions. While all superconductors show nearly perfect screening of magnetic fields applied after cooling to temperatures well below the superconducting transition, Tc, a range of very different behaviors was observed for Meissner expulsion upon cooling in static magnetic field from above Tc. Substantial conventional Meissner expulsion is found in LuNi2B2C, paramagnetic Meissner effect (PME) is found in Nb, and virtually no expulsion is observed in iron-based superconductors. In all cases, good correlation with macroscopic measurements of total magnetic moment is found. Our measurements of the spatial distribution of magnetic induction provide insight into microscopic physics of the Meissner effect.
Neutron diffraction and magnetic susceptibility studies show that orthorhombic single-crystals of topological semimetals Sr(Mn 0.9 Cu 0.1 )Sb 2 and Sr(Mn 0.9 Zn 0.1 )Sb 2 undergo three-dimensional C-type antiferromagnetic (AFM) ordering of the Mn 2+ moments at T N = 200 ± 10 and 210 ± 12 K, respectively, significantly lower than that of the parent SrMnSb 2 with T N = 297 ± 3 K. Magnetization versus applied magnetic field (perpendicular to MnSb planes) below T N exhibits slightly modified de Haas van Alphen oscillations for the Zn-doped crystal as compared to that of the parent compound. By contrast, the Cu-doped system does not show de Haas van Alphen magnetic oscillations, suggesting that either Cu substitution for Mn changes the electronic structure of the parent compound substantially, or that the Cu sites are strong scatterers of carriers that significantly shorten their mean free path thus diminishing the oscillations. Density functional theory (DFT) calculations including spin-orbit coupling predict the C-type AFM state for the parent, Cu-, and Zn-doped systems and identify the a-axis (i.e., perpendicular to the Mn layer) as the easy magnetization direction in the parent and 12.5% of Cu or Zn substitutions. In contrast, 25% of Cu content changes the easy magnetization to the b-axis (i.e., within the Mn layer). We find that the incorporation of Cu and Zn in SrMnSb 2 tunes electronic bands near the Fermi level resulting in different band topology and semimetallicity. The parent and Zn-doped systems have coexistence of electron and hole pockets with opened Dirac cone around the Y-point whereas the Cu-doped system has dominant hole pockets around the Fermi level with a distorted Dirac cone. The tunable electronic structure may point out possibilities of rationalizing the experimentally observed de Haas van Alphen magnetic oscillations.
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