We report critical current density (Jc) in tetragonal FeS single crystals, similar to iron based superconductors with much higher superconducting critical temperatures (Tc's). The Jc is enhanced 3 times by 6% Se doping. We observe scaling of the normalized vortex pinning force as a function of reduced field at all temperatures. Vortex pinning in FeS and FeS0.94Se0.06 shows contribution of core-normal surface-like pinning. Reduced temperature dependence of Jc indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path (δl), in contrast to KxFe2−ySe2 where spatial variations in Tc (δTc) prevails.

We report the doping, temperature, and spatial evolutions of the electronic structure of NaFe(1-x)Co(x)As studied by scanning tunneling microscopy. In the parent state we directly observe the spin density wave gap, which exhibits unconventional features that are incompatible with simple Fermi surface nesting. The optimally doped sample has a single superconducting gap, but in the overdoped regime a novel pseudogaplike feature emerges. The pseudogaplike phase coexists with superconductivity in the ground state, persists well into the normal state, and shows strong spatial variations. The characteristics of the three distinct electronic states revealed here shed important new lights on the microscopic models for the iron-based superconductors.

We report quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase and small cyclotron mass indicate the presence of Dirac fermions. Angular-dependent magnetoresistance indicates possible quasi two dimensional (2D) Fermi surface whereas the deviation from the non-trivial Berry phase expected for Dirac states suggests contribution of parabolic bands at the Fermi level or spin-orbit coupling.

We report two-dimensional quantum transport and Dirac fermions in BaMnBi2 single crystals. BaMnBi2 is a layered bad metal with highly anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, small cyclotron mass and the first-principles band structure calculations indicate the presence of Dirac fermions in Bi square nets. Quantum oscillations in the Hall channel suggest the presence of both electron and hole pockets whereas Dirac and parabolic states coexist at the Fermi level.