Tianping Ying

Superconducting Phases in Potassium-Intercalated Iron Selenides

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in A(x)Fe(2-y)Se(2) (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, K(x)Fe(2)Se(2)(NH(3))(y) (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K(0.3)Fe(2)Se(2)(NH(3))(0.47) corresponds to the 44 K phase with lattice constant c = 15.56(1) Å and K(0.6)Fe(2)Se(2)(NH(3))(0.37) to the 30 K phase with c = 14.84(1) Å. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH(3) has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K(0.3)Fe(2)Se(2) and K(0.6)Fe(2)Se(2) SC phases. K(0.3)Fe(2)Se(2)(NH(3))(0.47) and K(0.6)Fe(2)Se(2)(NH(3))(0.37) stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of T(c) with doping is not dome-like.

Exploring FeSe-based superconductors by liquid ammonia method

Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K~46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2(NH3)y (x ≈ 0.3 and 0.6), determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.

Anderson localization of electrons in single crystals: LixFe7Se8

The Anderson localization, which was predicted more than half a century ago, is realized for electron system in a 3D single crystal. Anderson (disorder-induced) localization, proposed more than half a century ago, has inspired numerous efforts to explore the absence of wave diffusions in disordered media. However, the proposed disorder-induced metal-insulator transition (MIT), associated with the nonpropagative electron waves, has hardly been observed in three-dimensional (3D) crystalline materials, let alone single crystals. We report the observation of an MIT in centimeter-size single crystals of LixFe7Se8 induced by lattice disorder. Both specific heat and infrared reflectance measurements reveal the presence of considerable electronic states in the vicinity of the Fermi level when the MIT occurs, suggesting that the transition is not due to Coulomb repulsion mechanism. The 3D variable range hopping regime evidenced by electrical transport measurements at low temperatures indicates the localized nature of the electronic states on the Fermi level. Quantitative analyses of carrier concentration, carrier mobility, and simulated density of states (DOS) fully support that LixFe7Se8 is an Anderson insulator. On the basis of these results, we provide a unified DOS picture to explain all the experimental results, and a schematic diagram for finding other potential Anderson insulators. This material will thus serve as a rich playground for both theoretical and experimental investigations on MITs and disorder-induced phenomena.

Understanding Doping, Vacancy, Lattice Stability, and Superconductivity in KxFe2−ySe2

Metal‐intercalated iron selenides are a class of superconductors that have received much attention but are less understood in comparison with their FeAs‐based counterparts. Here, the controversial issues such as Fe vacancy, the real phase responsible for superconductivity, and lattice stability have been addressed based on first‐principles calculations. New insights into the distinct features in terms of carrier doping have been revealed.

New Layered Iron Sulfide NaFe1.6S2: Synthesis and Characterization

Na was intercalated between [Fe2S2] layers for the first time, giving a novel compound NaFe(1.6)S2. This material adopts a CaAl2Si2-type structure with ~20% iron vacancies and represents the first layered compound in a ternary Na-M-X (M = Fe, Co, Ni; X = S, Se) system. First-principles calculations reveal that phonon dynamics is an important factor for it to prefer the CaAl2Si2-type rather than the ThCr2Si2-type structure. It features a magnetic transition at 205 K and is a narrow-band-gap semiconductor.

Structures and Physical Properties of Layered Oxyselenides Ba2MO2Ag2Se2 (M = Co, Mn)

Two new layered oxyselenides, Ba2MO2Ag2Se2 (M = Co, Mn), have been successfully synthesized via solid-state reaction. It is found that these two compounds, consisting of the infinite MO2 square planes and antifluorite-type Ag2Se2 layers separated by barium, possess new structural features while keeping I4/mmm symmetry. A detailed calculation on the discrete coordination of transition metals by oxygen in the two compounds and Ba2ZnO2Ag2Se2 revealed quite different energy landscapes. The calculated results indicate that the manganese compound favors adoption of the I4/mmm space group, while the cobalt compound could be at the boundary of the transition between the I4/mmm and Cmca phases. In Ba2CoO2Ag2Se2, the coexistence of a large barium ion and a Ag2Se2 layer expands the oxide layer significantly and results in the largest Co-O bond length in the square-planar sheet ever reported. Ba2CoO2Ag2Se2 is near-stoichiometric, whereas Ba2MnO2Ag2Se2 contains 7% silver vacancies, which is explained by the mixed valence of the manganese ion between 2+ and 3+. In Ba2CoO2Ag2Se2, the zero-field-cooled and field-cooled susceptibilities bifurcate at 159 K, located between two antiferromagnetic (AFM) transitions. Meanwhile, Ba2MnO2Ag2Se2 shows high-temperature Curie-Weiss behavior, followed by a low-temperature AFM transition with TN = 32 K. They both exhibit semiconducting behavior with resisitivities of about 10(5)Ω cm at room temperature. The optical band gaps are determined to be 1.49 and 1.18 eV for Ba2CoO2Ag2Se2 and Ba2MnO2Ag2Se2, respectively. Band structure calculations reveal that Ba2CoO2Ag2Se2 is a direct-gap semiconductor, with a calculated band gap of 1.147 eV; however, Ba2MnO2Ag2Se2 failed to reproduce the semiconducting behavior within an A-type AFM model.