Shijie Shen

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.

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.

Ca2O3Fe2.6S2: an antiferromagnetic Mott insulator at proximity to bad metal

We report here the first layered iron oxychalcogenide Ca2O3Fe2.6S2 that contains both planar [Ca2FeO2](2+) and [Fe2OS2](2-) layers with the shortest Fe-Fe bond length. This compound is a narrow band gap (~0.073 eV) Mott insulator. The observed antiferromagnetic (AFM) transition at 77 K is due to the ordered Fe vacancies, which can be suppressed by partial substitution of Se for S. We show that the vacancy-free phase Ca2O3Fe3S2 may become a metal with moderate electron correlation comparable to the parent compound LaOFeAs of corresponding superconductors. Our results imply that iron oxychalcogenide can be converted from an AFM Mott insulator into a bad metal like iron pnictides through Fe-Fe bond length shrinking.

Bandgap narrowing in the layered oxysulfide semiconductor Ba3Fe2O5Cu2S2: Role of FeO2 layer*

A new layered Cu-based oxychalcogenide Ba3Fe2O5Cu2S2 has been synthesized and its magnetic and electronic properties were revealed. Ba3Fe2O5Cu2S2 is built up by alternatively stacking [Cu2S2]2− layers and iron perovskite oxide [(FeO2)(BaO)(FeO2)]2− layers along the c axis that are separated by barium ions with Fe3+ fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuCh-based (Ch = S, Se, Te) compounds the +3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba3Fe2O5Cu2S2 is an antiferromagnetic semiconductor with a Neel temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3d states of Fe ions that antiferromagnetically arranged in FeO2 layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr3Sc2O5Cu2S2.

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.

A comparative study of doping effects of V and Cr on a SrAlSi superconductor

It is well known that the superconducting transition temperature (Tc) is very sensitive to the electronic density of states at the Fermi energy (N(EF)) and the Debye frequency (ΘD) for a BCS superconductor. However, which one plays a leading role in the ternary silicides AeMSi (Ae = Ca/Sr/Ba and M = Al/Ga) system is still unclear. Here, we report a comparative study of doping effects by V and Cr at Al sites in SrAlSi, which has a relatively high Tc among AeMSi systems. It is found that V and Cr atoms can be successfully introduced into SrAlSi with the actual solid solution limit up to 16 at% and 13 at%, respectively. In this case, the incorporation of V and Cr will have nearly identical effects in decreasing ΘD. Hall effect measurements demonstrate that V and Cr have dramatically different effects on the carrier concentration upon doping. In SrAl1−xVxSi, the carrier concentration is decreased by about three orders of magnitude and V dopants lead to the quenching of superconductivity at the highest doping level. In contrast, Cr almost does not change the carrier concentration, leading to a minor change in Tonsetc of 0.6 K. Our results provide solid evidence that the N(EF) should be responsible for the Tc in the transition metal doped AeMSi system.