Insulator-metal transition and quasi-flat-band of Shastry-Sutherland lattice.

Insulator-metal transition is investigated self-consistently on the frustrated Shastry-Sutherland lattice in the framework of Slave-Boson mean-field theory. Due to the presence of quasi-flat band structure characteristic, the system displays a spin-density-wave (SDW) insulating phase at the weak doping levels, which is robust against frustration, and it will be transited into an SDW metallic phase at high doping levels. As further increasing the doping, the temperature or the frustration on the diagonal linking bonds, the magnetic order m will be monotonically suppressed, resulting in the appearance of a paramagnetic metallic phase. Although the Fermi surface of the SDW metallic phase may be immersed by temperature, the number of mobile charges is robust against temperature at weak doping levels.

Possible Pairing Symmetry in the FeSe-Based Superconductors Determined by Quasiparticle Interference.

We study the momentum-integrated quasiparticle interference (QPI) in the FeSe-based superconductors. This method was recently proposed theoretically and has been applied to determine the pairing symmetry in these materials experimentally. Our findings suggest that, if the incipient bands and the superconducting (SC) pairing on them are taken into consideration, then the experimentally measured bound states and momentum-integrated QPI can be well fitted, even if the SC order parameter does not change sign on the Fermi surfaces. Therefore, we offer an alternative explanation to the experimental data, calling for more careful identification of the pairing symmetry that is important for the pairing mechanism.

Majorana zero modes in the hopping-modulated one-dimensional p-wave superconducting model.

We investigate the one-dimensional p-wave superconducting model with periodically modulated hopping and show that under time-reversal symmetry, the number of the Majorana zero modes (MZMs) strongly depends on the modulation period. If the modulation period is odd, there can be at most one MZM. However if the period is even, the number of the MZMs can be zero, one and two. In addition, the MZMs will disappear as the chemical potential varies. We derive the condition for the existence of the MZMs and show that the topological properties in this model are dramatically different from the one with periodically modulated potential.

Possible superconductivity in Sr2IrO4 probed by quasiparticle interference.

Based on the possible superconducting (SC) pairing symmetries recently proposed, the quasiparticle interference (QPI) patterns in electron- and hole-doped Sr2IrO4 are theoretically investigated. In the electron-doped case, the QPI spectra can be explained based on a model similar to the octet model of the cuprates while in the hole-doped case, both the Fermi surface topology and the sign of the SC order parameter resemble those of the iron pnictides and there exists a QPI vector resulting from the interpocket scattering between the electron and hole pockets. In both cases, the evolution of the QPI vectors with energy and their behaviors in the nonmagnetic and magnetic impurity scattering cases can well be explained based on the evolution of the constant-energy contours and the sign structure of the SC order parameter. The QPI spectra presented in this paper can be compared with future scanning tunneling microscopy experiments to test whether there are SC phases in electron- and hole-doped Sr2IrO4 and what the pairing symmetry is.

Superfluid density in the s±-wave state of clean iron-based superconductors.

Based on a phenomenological model and the Kubo formula, we investigate the superfluid density ρ(s)(T) and then the penetration depth λ(T) of the iron-based superconductors in the coexistence region of the spin-density wave and superconductivity, and also in the overdoped region. Our calculations show a dramatic increase of λ(0) with the decrease of the doping concentration x below x = 0.1. This result is consistent with the experimental observations. At low temperatures, ρ(s)(T) shows an exponential-law behavior, while at higher temperatures, the linear-in-T behavior is dominant before it trends to vanish. It is in qualitative agreement with the direct measurement of superfluid density in films of Fe-pnictide superconductor at x = 0.08. The evolution of Δλ(T) can be roughly fitted by a power-law function with the exponent depending on the doping concentration. We show that the Uemura relation holds for the iron-based superconductors only at very low doping levels.

Model of vortex states in hole-doped iron-pnictide superconductors.

Based on a phenomenological model with competing spin-density-wave (SDW) and extended s-wave superconductivity, the vortex states in Ba(1-x)K(x)Fe2As2 are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.