Zheng-Cai Li

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.

Competing orders and spin-density-wave instability in La(O1−xFx)FeAs

The interplay between different ordered phases, such as superconducting, charge or spin ordered phases, is of central interest in condensed-matter physics. The very recent discovery of superconductivity with a remarkable Tc=26 K in Fe-based oxypnictide La(O1−xFx)FeAs (see Kamihara Y. et al., J. Am. Chem. Soc., 130 (2008) 3296) is a surprise to the scientific community and has generated tremendous interest. The pure LaOFeAs itself is not superconducting but shows an anomaly near 150 K in both resistivity and dc magnetic susceptibility. Here we provide combined experimental and theoretical evidences showing that a spin-density-wave (SDW) state develops at low temperature, in association with electron Nematic order. The electron-doping by F suppresses the SDW instability and induces the superconductivity. Therefore, the La(O1−xFx)FeAs offers an exciting new system showing competing orders in layered compounds.

Structural and magnetic phase diagram of CeFeAsO 1− x F x and its relation to high-temperature superconductivity

Recently, high-transition-temperature (high-Tc) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-Tc superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x=0.10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x>0.06, resulting in an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-Tc superconductivity in these Fe pnictides.

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.

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.

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

Here we report pressure effects on the superconducting transition temperature (Tc) of the ReFeAsO0.85 (Re=Sm and Nd) system without fluorine doping. In situ measurements under high pressure showed that Tc of the two compounds decreases monotonously over the pressure range investigated. The pressure coefficients dTc/dP in SmFeAsO0.85 and NdFeAsO0.85 were different, revealing the important influence of the deformation in layers on Tc. Theoretical calculations suggested that the electron density of states decreases with increasing pressure, following the same trend of experimental data.

Superconductivity in some heavy rare-earth iron arsenide REFeAsO1-δ (RE = Ho, Y, Dy and Tb) compounds

Here, we report some new iron arsenide superconductors of REFeAsO1?? (RE = Ho, Y, Dy and Tb) that were successfully synthesized by a high-pressure synthesis method with a special rapid quenching process, with the onset superconducting critical temperatures at 50.3, 46.5, 52.2 and 48.5?K for RE = Ho, Y, Dy and Tb, respectively, as determined from the resistivity measurements.

High-pressure study on LaFeAs(O1−xFx) and LaFeAsOδ with different Tc

We report studies on pressure dependence of superconducting transition temperature (Tc) of LaFeAsOδ (δ< 0.3), LaFeAs(O0.5F0.5) and LaFeAs(O0.89F0.11) samples. In situ resistance measurements under high pressure showed that the Tc of these three compounds increases with pressure initially, reaches a maximum value and then decreases with further increasing pressure, although Tcs at ambient pressure are different. The onset Tc of LaFeAsOδ is ~50 K at 1.5 GPa, which is the highest record in La-based oxypnictide system. The significant change in Tc induced by pressure is attributed to the orbital degeneracy and the electron density of state at the Fermi level.

High pressure study on LaFeAsO with different Tc

We report studies on pressure dependence of superconducting transition temperature (Tc) of LaFeAsOδ (δ< 0.3), LaFeAs(O0.5F0.5) and LaFeAs(O0.89F0.11) samples. In situ resistance measurements under high pressure showed that the Tc of these three compounds increases with pressure initially, reaches a maximum value and then decreases with further increasing pressure, although Tcs at ambient pressure are different. The onset Tc of LaFeAsOδ is ~50 K at 1.5 GPa, which is the highest record in La-based oxypnictide system. The significant change in Tc induced by pressure is attributed to the orbital degeneracy and the electron density of state at the Fermi level.