Observation of Distinct Spatial Distributions of the Zero and Nonzero Energy Vortex Modes in (Li_{0.84}Fe_{0.16})OHFeSe.

The energy and spatial distributions of vortex bound state in superconductors carry important information about superconducting pairing and the electronic structure. Although discrete vortex states, and sometimes a zero energy mode, had been observed in several iron-based superconductors, their spatial properties are rarely explored. In this study, we used low-temperature scanning tunneling microscopy to measure the vortex state of (Li,Fe)OHFeSe with high spatial resolution. We found that the nonzero energy states display clear spatial oscillations with a period corresponding to bulk Fermi wavelength; while in contrast, the zero energy mode does not show such oscillation, which suggests its distinct electronic origin. Furthermore, the oscillations of positive and negative energy states near E_{F} are found to be clearly out of phase. Based on a two-band model calculation, we show that our observation is more consistent with an s_{++} wave pairing in the bulk of (Li, Fe)OHFeSe, and superconducting topological states on the surface.

Observation of Discrete Conventional Caroli-de Gennes-Matricon States in the Vortex Core of Single-Layer FeSe/SrTiO_{3}.

Using low-temperature scanning tunneling microscopy (STM), we studied the vortex states of single-layer FeSe film on a SrTiO_{3} (100) substrate, and the local behaviors of superconductivity at sample boundaries. We clearly observed multiple discrete Caroli-de Gennes-Matricon states in the vortex core, and quantitative analysis shows their energies well follow the formula: E=µΔ^{2}/E_{F}, where µ is a half integer (±1/2,±3/2,±5/2…) and Δ is the mean superconducting gap over the Fermi surface. Meanwhile, a fully gapped spectrum without states near zero bias is observed at the [110]_{Fe} oriented boundary of 1 and 2 ML FeSe films, and atomic step edge of 1 ML FeSe. Accompanied with theoretical calculations, our results indicate an s-wave pairing without sign change in the high-T_{C} FeSe/SrTiO_{3} superconductor.

Two-dimensional stable transition metal carbides (MnC and NbC) with prediction and novel functionalizations.

In this paper, manganese carbide (MnC) and niobium carbide (NbC) are predicted as stable monolayer metallic materials, whose Young's moduli are 50.06 N m-1 and 44.07 N m-1, respectively. The ab initio molecular dynamics (AIMD) results show that both MnC and NbC could hold their structure up to 1000 K, showing favorable thermal properties. These monolayers also show good properties for promising application in Li ion batteries because of their high specific capacities and low diffusion barriers. The MnC monolayer is ferromagnetic and the Curie temperature simulated by the Monte-Carlo method is about 205 K. The electronic band of MnC shows a metal to half-metal transition by passivation of Cl or Br atoms, and the functionalization methods also cause the metallic NbC monolayer to exhibit the quantum spin Hall effect (QSHE). These novel transition metal carbide monolayers hold great promise for 2D spintronic and electronic device applications.