Liling Sun

Superconductivity in the iron-based F-doped layered quaternary compound Nd[O1-xFx]FeAs

Here we report a new quaternary iron-arsenide superconductor Nd[O1?x Fx ]FeAs , with the onset resistivity transition at 51.9?K and Meissner transition at 51?K. This compound has the same crystal structure as LaOFeAs, and becomes the second superconductor after Pr[O1?x Fx]FeAs that superconducts above 50?K.

Superconductivity and phase diagram in iron-based arsenic-oxides ReFeAsO1−δ (Re = rare-earth metal) without fluorine doping

Here we report a new class of superconductors prepared by high-pressure synthesis in the quaternary family ReFeAsO1−δ (Re=Sm, Nd, Pr, Ce, La) without fluorine doping. The onset superconducting critical temperature (Tc) in these compounds increases with the reduction of the Re atom size, and the highest Tc obtained so far is 55 K in SmFeAsO1−δ. For the NdFeAsO1−δ compound with different oxygen concentration a dome-shaped phase diagram was found.

Superconductivity at 52 K in iron based F doped layered quaternary compound Pr[O1–xFx]FeAs

Abstract Here the authors report that by a high pressure synthesis method, superconductivity with an onset transition temperature at 52 K was discovered in a fluorine-doped quaternary iron-arsenide compound Pr[O1-xFx]FeAs, with the zero resistivity and Meissner transition appeared at 44 K and 50 K respectively. As the first non-cuprate compound that superconducts above 50 K, this discovery places these iron arsenide compounds to the second high temperature superconducting family explicitly.

Re-emerging superconductivity at 48 kelvin in iron chalcogenides

Pressure plays an essential role in the induction1 and control2,3 of superconductivity in iron-based superconductors. Substitution of a smaller rare-earth ion for the bigger one to simulate the pressure effects has surprisingly raised the superconducting transition temperature Tc to the record high 55 K in these materials4,5. However, Tc always goes down after passing through a maximum at some pressure and the superconductivity eventually tends to disappear at sufficiently high pressures1-3. Here we show that the superconductivity can reemerge with a much higher Tc after its destruction upon compression from the ambient-condition value of around 31 K in newly discovered iron chalcogenide superconductors. We find that in the second superconducting phase the maximum Tc is as high as 48.7 K for K0.8Fe1.70Se2 and 48 K for (Tl0.6Rb0.4)Fe1.67Se2, setting the new Tc record in chalcogenide superconductors. The presence of the second superconducting phase is proposed to be related to pressure-induced quantum criticality. Our findings point to the potential route to the further achievement of high-Tc superconductivity in iron-based and other superconductors.Pressure has an essential role in the production and control of superconductivity in iron-based superconductors. Substitution of a large cation by a smaller rare-earth ion to simulate the pressure effect has raised the superconducting transition temperature Tc to a record high of 55 K in these materials. In the same way as Tc exhibits a bell-shaped curve of dependence on chemical doping, pressure-tuned Tc typically drops monotonically after passing the optimal pressure. Here we report that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges above 11.5 GPa, after the Tc drops from the first maximum of 32 K at 1 GPa. The Tc of the re-emerging superconducting phase is considerably higher than the first maximum, reaching 48.0–48.7 K for Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2 and K0.8Fe1.78Se2.

Spin-singlet superconductivity with multiple gaps in PrFeAsO0.89F0.11

We report 75As and 19F NMR studies on the superconducting properties of the newly discovered compound PrFeAsO0.89F0.11 (Tc=45 K). The Knight shift in the direction of H∥ab-plane decreases below Tc to almost zero, which indicates spin-singlet pairing. The temperature dependencies of both the Knight shift and the spin lattice relaxation rate indicate that there are two gaps opening below Tc, with nodes in the gap function.

Superconductivity at 53.5 K in GdFeAsO1−δ

Superconducting Tl(2)Ba(2)CaCu(2)O(8) thin films (Tl-2212) have been reproducibly fabricated on both sides of 2 inch LaAlO(3)(001) substrates by using a so-called two-step process. A simple and flexible horizontal arrangement inside the crucible during thallination has been proposed, to ensure a good convection of thallous oxide vapor around the film surface, and thus a high film quality on both sides. A good uniformity over the whole area of the films has been realized. The films are strongly textured, with the c-axis perpendicular to the substrate surface, and they show the fourfold symmetry of the tetragonal crystal structure. The full width at half maximum of the (002) peak for both films on the best sample is 0.43 degrees and 0.55 degrees respectively. The films exhibit flat terraces with particles and shallow holes, but free of microcracks. The superconducting critical temperatures fall in the range 105-108.5 K. The critical current densities are between 1.0 x 10(6) and 4.8 x 10(6) A cm(-2) at 77 K. The microwave surface resistances at the central area of the films are around 500 mu Omega at 77 K and 10 GHz. These films are suitable for microwave applications.Here we report the fabrication and superconductivity of the iron-based arsenic-oxide GdFeAsO1-delta compound with oxygen-deficiency, which has an onset resistivity transition temperature at 53.5 K. This material has a same crystal structure as the newly discovered high-Tc ReFeAsO1-delta family (Re = rare earth metal) and a further reduced crystal lattice, while the Tc starts to decrease compared with the SmFeAsO1-delta system.

Superconductivity emerging from a suppressed large magnetoresistant state in tungsten ditelluride

The recent discovery of large magnetoresistance in tungsten ditelluride provides a unique playground to find new phenomena and significant perspective for potential applications. The large magnetoresistance effect originates from a perfect balance of hole and electron carriers, which is sensitive to external pressure. Here we report the suppression of the large magnetoresistance and emergence of superconductivity in pressurized tungsten ditelluride via high-pressure synchrotron X-ray diffraction, electrical resistance, magnetoresistance and alternating current magnetic susceptibility measurements. Upon increasing pressure, the positive large magnetoresistance effect is gradually suppressed and turned off at a critical pressure of 10.5 GPa, where superconductivity accordingly emerges. No structural phase transition is observed under the pressure investigated. In situ high-pressure Hall coefficient measurements at low temperatures demonstrate that elevating pressure decreases the population of hole carriers but increases that of the electron ones. Significantly, at the critical pressure, a sign change of the Hall coefficient is observed.

The role of F-doping and oxygen vacancies on the superconductivity in SmFeAsO compounds

By investigating the F-doping effect in the SmFeAsO1−xFx, SmFeAsO1−xF0.20 and SmFeAsO0.90Fx systems as well as the oxygen vacancy effect in the SmFeAsO1−y superconductors, we obtained the following results: (a) the substitution range of F for oxygen in the SmFeAsO1−xFx system prepared by the ambient pressure method is 0≤x≤0.125; (b) F cannot substitute for oxygen in samples without oxygen vacancies; (c) the oxygen-deficient SmFeAsO1−y superconductor cannot be prepared by the ambient pressure method; and (d) F-doping and oxygen vacancies both lead to lattice shrinkage. Oxygen-deficient SmFeAsO0.85 and F-doped SmFeAsO0.85F0.15 prepared by the high pressure method have higher superconducting transition temperature compared to SmFeAsO0.85F0.15 prepared by the ambient pressure method.

Phase diagram of a pressure-induced superconducting state and its relation to the Hall coefficient of Bi2Te3 single crystals

Pressure-induced superconductivity and its relation to the corresponding Hall coefficient (

Pressure effect on superconductivity of iron-based arsenic-oxide ReFeAsO0.85 (Re=Sm and Nd)

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