Edge states and local electronic structure around an adsorbed impurity in a topological superconductor

Recently topological superconducting states has attracted a lot of interest. In this work, we consider a topo- logical superconductor with $Z_2$ topological mirror order [1] and s$pm$-wave superconducting pairing symmetry, within a two-orbital model originally designed for iron-based superconductivity [2]. We predict the existence of gapless edge states. We also study the local electronic structure around an adsorbed interstitial magnetic impurity in the system, and find the existence of low-energy in-gap bound states even with a weak spin polar- ization on the impurity. We also discuss the relevance of our results to the recent STM experiment on Fe(Te,Se) compound with adsorbed Fe impurity [3], for which our density functional calculations show the Fe impurity is spin polarized.

A unified description of superconducting pairing symmetry in electron-doped Fe-based-122 compounds

The pairing symmetry is examined in highly electron-doped Ba(Fe$_{1-x}$Co$_x$As)$_2$ and A$_y$Fe$_2$Se$_2$ (with A=K, Cs) compounds, with similar crystallographic and electronic band structures. Starting from a phenomenological two-orbital model, we consider nearest-neighbor and next-nearest-neighbor intraorbital pairing interactions on the Fe square lattice. In this model, we find a unified description of the evolution from $s_pm$-wave pairing ($2.0 < n lesssim 2.4$) to $d$-wave pairing ($2.4 lesssim n lesssim 2.5$) as a function of electron filling. In the crossover region a novel time-reversal symmetry breaking state with $s_pm+id$ pairing symmetry emerges. This minimal model offers an overall picture of the evolution of superconductivity with electron doping for both $s_pm$-wave [Ba(Fe$_{1-x}$Co$_x$As)$_2$] and $d$-wave [A$_y$Fe$_2$Se$_2$] pairing, as long as the dopants only play the role of a charge reservoir. However, the situation is more complicated for Ba(Fe$_{1-x}$Co$_x$As)$_2$. A real-space study further shows that when the impurity scattering effects of Co dopants are taken into account, the superconductivity is completely suppressed for $n > 2.4$. This preempts any observation of $d$-wave pairing in this compound, in contrast to A$_y$Fe$_2$Se$_2$.

Emergent topological mirror insulator in t2g-orbital systems

The electronic band structure of iron pnictides exhibits four Dirac cones, which are due to crystal symmetry and orbital bonding orientation. This hallmark signature presents the pnictide family as an ideal candidate in the search for quasi-two-dimensional topological crystalline insulators. In this report, we explore interaction-induced topological phases which cannot be described by conventional local order parameters. Based on a model Hamiltonian our symmetry analysis shows that sponta- neous novel topological phases may be realized in compounds with tetragonal crystal field symmetry, where the electrons occupy the two degenerate t2g energy levels at low temperature. We identify two stable topological phases in the ground state, which emerge from spontaneous orbital current order. These currents are driven by electronic correlations caused by inter-orbital Coulomb interactions. The first topological phase is an anomalous orbital Hall phase, characterized by a nonzero Chern number, while the second topological phase has a vanishing Chern number, though with an extra Z2-like invariant that preserves parity. More specifically, the interaction-induced novel phase of the quasi-two-dimensional topological crystalline insulator is protected by mirror reflection symmetries and therefore may be realized in pnictides.

Phase diagram of doped BaFe$_2$As$_2$ superconductor under broken $C_4$ symmetry

We develop a minimal multiorbital tight-binding model with realistic hopping parameters. The model breaks the symmetry of the tetragonal point group by lowering it from $C_4$ to $D_{2d}$, which accurately describes the Fermi surface evolution of the electron-doped BaFe$_{2-x}$Co$_x$As$_2$ and hole-doped Ba$_{1-y}$K$_y$Fe$_2$As$_2$ compounds. An investigation of the phase diagram with a mean-field $t$-$U$-$V$ Bogoliubov-de Gennes Hamiltonian results in agreement with the experimentally observed electron- and hole-doped phase diagram with only one set of $t$, $U$ and $V$ parameters. Additionally, the self-consistently calculated superconducting order parameter exhibits $s^pm$-wave pairing symmetry with a small d-wave pairing admixture in the entire doping range, % The superconducting $s^pm + d$-wave order parameter which is the subtle result of the weakly broken symmetry and competing interactions in the multiorbital mean-field Hamiltonian.

Fermi surface evolution and checker-board block-spin antiferromagnetism in $A_x$Fe$_{2-y}$Se$_{2}$

We develop an effective multiorbital mean-field t-J Hamiltonian with realistic tight-binding and exchange parameters to describe the electronic and magnetic structures of iron-selenide based superconductors $A_x$Fe$_{2-y}$Se$_2$ for iron vacancy doping in the range $0 leq y leq 0.4$. The Fermi surface topology extracted from the spectral function of angle-resolved photoemission spectroscopy (ARPES) experiments is adequately accounted for by a tight-binding lattice model with random vacancy disorder. Since introducing iron vacancies breaks the lattice periodicity of the stochiometric compound, it greatly affects the electronic band structure. With changing vacancy concentration the electronic band structure evolves, leading to a reconstruction of the Fermi surface topology. For intermediate doping levels, the realized stable electronic structure is a compromise between the solutions for the perfect lattice with $y=0$ and the vacancy stripe-ordered lattice with $y=0.4$, which results in a competition between vacancy random disorder and vacancy stripe order. A multiorbital hopping model is parameterized by fitting Fermi surface topologies to ARPES experiments, from which we construct a mean-field t-J lattice model to study the paramagnetic and antiferromagnetic (AFM) phases of K$_{0.8}$Fe$_{1.6}$Se$_2$. In the AFM phase the calculated spin magnetization of the t-J model leads to a checker-board block-spin structure in good agreement with neutron scattering experiments and {it ab}-{it initio} calculations.