G. J. Ye

Evolution of High-Temperature Superconductivity from a Low-T_{c} Phase Tuned by Carrier Concentration in FeSe Thin Flakes.

We report the evolution of superconductivity in an FeSe thin flake with systematically regulated carrier concentrations by the liquid-gating technique. With electron doping tuned by the gate voltage, high-temperature superconductivity with an onset at 48xa0K can be achieved in an FeSe thin flake with T_{c} less than 10xa0K. This is the first time such high temperature superconductivity in FeSe is achieved without either an epitaxial interface or external pressure, and it definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with T_{c}^{onset} as high as 48xa0K in bulk FeSe. Intriguingly, our data also indicate that the superconductivity is suddenly changed from a low-T_{c} phase to a high-T_{c} phase with a Lifshitz transition at a certain carrier concentration. These results help to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on the further pursuit of a higher T_{c} in these materials.

Coexistence of superconductivity and antiferromagnetism in single crystals A0.8Fe2?ySe2 (A=K, Rb, Cs, Tl/K and Tl/Rb): Evidence from magnetization and resistivity

We measure the resistivity and magnetic susceptibility in the temperature range from 5u2009K to 600u2009K for the single crystals AFe2−ySe2 (A=K0.8, Rb0.8, Cs0.8, Tl0.5K0.3 and Tl0.4Rb0.4). A sharp superconducting transition is observed in low-temperature resistivity and susceptibility, and the fully shielding fraction shows bulk susceptibility for all the crystals, while an antiferromagnetic transition is observed in susceptibility at Neel temperature (TN) as high as 500u2009K to 540u2009K depending on A. This indicates the coexistence of superconductivity and antiferromagnetism in these intercalated iron selenides. A sharp increase in resistivity arises from the structural transition due to Fe vacancy ordering at temperature slightly higher than TN. Occurrence of superconductivity in an antiferromagnetic ordered state with so high TN may suggest new physics in this type of unconventional superconductors.

Crystal structure, physical properties and superconductivity in AxFe2Se2 single crystals

We studied the correlation among structure, transport properties and superconductivity in different AxFe2Se2 single crystals (A=K, Rb and Cs). Two sets of (00l) reflections are observed in the x-ray single-crystal diffraction patterns, and they arise from the intrinsic inhomogeneous distribution of the intercalated alkali atoms. The occurrence of superconductivity is closely related to the c-axis lattice constant, and the A content is crucial to superconductivity. The hump observed in resistivity seems to be irrelevant to the superconductivity. There exist many deficiencies within the FeSe layers in AxFe2Se2, although their Tc does not change so much. In this sense, superconductivity is robust to the vacancies within the FeSe layers. Very high resistivity in the normal state should be ascribed to such defects in the conducting FeSe layers. AxFe2Se2 (A=K, Rb and Cs) single crystals show the same susceptibility behavior in the normal state, and no anomaly is observed in susceptibility at the hump temperature in resistivity. The clear jump in specific heat for RbxFe2Se2 and KxFe2Se2 single crystals indicates the good bulk superconductivity of these crystals.

Superconductivity and magnetic properties of single crystals of K 0 . 75 Fe 1 . 66 Se 2 and Cs 0 . 81 Fe 1 . 61 Se 2

We successfully grew the high-quality single crystals of

Enhanced superconductivity by rare-earth metal doping in phenanthrene

A_{x}

Pressure effect on superconductivity of AxFe2Se2 (A = K and Cs)

Fe

Phase diagram and physical properties of NaFe 1 − x Cu x As single crystals

_2

A crossover in the phase diagram of NaFe1−xCoxAs determined by electronic transport measurements

Se

Pressure effects on the superconducting properties of single-crystalline Co doped NaFeAs

_2

Phase diagram as a function of doping level and pressure in the Eu1-xLaxFe2As2 system

(