${\mathrm{KFe}}_2{\mathrm{Se}}_2$ is the Parent Compound of K-Doped Iron Selenide Superconductors

We elucidate the existing controversies in the newly discovered K-doped iron selenide (${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}$) superconductors. The stoichiometric ${\mathrm{KFe}}_2{\mathrm{Se}}_2$ with $\sqrt{2}\times \sqrt{2}$ charge ordering was identified as the parent compound of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}$ superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in ${\mathrm{KFe}}_2{\mathrm{Se}}_2$ by either Se vacancies or interacting with the antiferromagnetic ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$ compound. In total, four phases were found to exist in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}$: parent compound ${\mathrm{KFe}}_2{\mathrm{Se}}_2$, superconducting ${\mathrm{KFe}}_2{\mathrm{Se}}_2$ with $\sqrt{2}\times \sqrt{5}$ charge ordering, superconducting ${\mathrm{KFe}}_2{\mathrm{Se}}_{2-z}$ with Se vacancies, and insulating ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$ with $\sqrt{5}\times \sqrt{5}$ Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy conditions.

Figure 1

STM characterization of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ films grown by MBE. (a) The crystal structure of ${\mathrm{KFe}}_2{\mathrm{Se}}_2$. (b) Topographic image (3.9 V, 0.02 nA, $90\times 90\mathrm{nm}$) of a ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ film. (c) and (d) Atomic-resolution STM topography ($10\times 10\mathrm{nm}$) of ${\mathrm{KFe}}_2{\mathrm{Se}}_2$. The two images belong to the same area, but with different bias voltages: $-90\mathrm{mV}$ for (c) and 50 mV for (d). The tunneling current is 0.02 nA for both. Panel (c) shows the uniform $1\times 1$-Se square lattice. Inhomogeneity in electronic structure is revealed in (d) with two distinct regions labeled by I and II. The area ratio between I and II is about 1:5. (e) The $\sqrt{2}\times \sqrt{2}$ charge ordering in region I. The imaging condition: 40 mV, 0.02 nA. (f) $dI/dV$ spectrum (set point, 25 mV, 0.1 nA) of region I, which reveals that region I is nonsuperconducting. (g) The $\sqrt{2}\times \sqrt{5}$ charge ordering (see also inset) in region II. Imaging condition: 30 mV, 0.02 nA. (h) $dI/dV$ spectrum (0.4 K; set point, 20 mV, 0.1 nA) showing that there is a superconducting gap opened in region II. Arrows mark the smaller gap.

Figure 2

Vacancy-induced bound states in superconducting gap. (a) and (b) STM topography (40 mV, 0.02 nA) and $dI/dV$ spectrum (0.4 K; set point, 25 mV, 0.1 nA) of a single Fe vacancy. The defect is attributed to Fe vacancy by examining the symmetry of the STM image. (c) and (d) STM topography ($-95\mathrm{mV}$, 0.02 nA) and $dI/dV$ spectrum (0.4 K; set point, 25 mV, 0.1 nA) of a single Se vacancy. The magnetic moment of a Se vacancy comes from the four adjacent Fe atoms. More bound states are induced by the Se vacancy because of the lower spatial symmetry.

Figure 3

The origin of superconductivity in ${\mathrm{KFe}}_2{\mathrm{Se}}_2$. (a) Schematic showing the relationship between insulating ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$ and superconducting ${\mathrm{KFe}}_2{\mathrm{Se}}_2$. Across the interface between ${\mathrm{KFe}}_2{\mathrm{Se}}_2$ and ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$, the lattice structure is the same except for a $\sqrt{5}\times \sqrt{5}$ Fe vacancy order in ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$. (b) $\sqrt{5}\times \sqrt{5}$ superstructure in region II. The image belongs to the same area as Fig. 1g, but with a different bias voltage (70 mV). The Moiré pattern is marked by arrows.

Figure 4

Superconductivity in ${\mathrm{KFe}}_2{\mathrm{Se}}_{2-z}$. (a) STM topography (75 mV, 0.02 nA) of an area with Se vacancies. (b) Superconducting gap (0.4 K; set point, 25 mV, 0.1 nA) at a location away from Se vacancies.