G.C. Che

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.

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.

Scanning tunneling spectroscopy of SmFeAsO 0.85 : Possible evidence for d -wave order-parameter symmetry

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) have been examined for an Al nanocluster periodic array grown on p-type silicon substrate. Local density of states at Al and Si sites within the nanocluster has been extracted from site-resolved STS spectra by taking into account tip-induced band bending (TIBB) effect. Besides, it has been clarified that the surface-potential- energy shift caused by TIBB effect directly influences the tunneling current spectra. Consequently, a good correspondence has been found between the experimental spectra and the theoretical local density of states except for the energy gap. The energy gap has been experimentally determined as 1.7 eV, which is much larger than the predicted value. This discrepancy could be ascribed to the local-density approximation rather than the TIBB effect.

Magnetic relaxation and critical current density of the new superconductor MgB2

Magnetic relaxation rate, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to Tc at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. A large separation between the bulk irreversibility field Hirr(T) and the upper critical field Hc2(T) has been found. It is thus proposed that there is a quantum vortex liquid due to strong quantum fluctuation of vortices at 0 K. It is also found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore, the phase line Hirr(T) is attributed to quantum vortex melting in the rather clean system at a finite temperature.

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.

Crystal structures and superconductivity of superconducting phases in the Tl-Ba-Ca-Cu-O system

Abstract We found a series of superconducting phases in the Tl-Ba-Ca-Cu-O system and determined the crystal structures of these superconducting phases by means of the X-ray powder diffraction method. All of these superconducting phases belong to the tetragonal symmetry. They have the same lattice constant a and different lattice constant c . These superconducting phases can be divided into two types. The space group of type I is P4/mmm. The space group of type II is 14/mmm. The chemical formulae of type I and type II can be generalized as TlBa 2 Ca n −1 Cu n O 2 n +2.5 ( n =2, 3, 4) and Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1, 2, 3), respectively. T c increases as n increases. On the condition that n is the same, type II has a higher T c than type I. The relations among the crystal structures of the superconducting phases are discussed. We proposed a method which can deduce the crystal structure of the superconducting phase from the d 1 -value of the first diffraction line (00 l ) or lattice parameter c and which is confirmed by experimental data.

Influence of carbon concentration on the superconductivity in MgCxNi3

The influence of carbon concentration on the superconductivity (SC) in MgCxNi3 has been investigated by measuring the low temperature specific heat combined with first principles electronic structure calculation. It is found that the specific heat coefficient gamma(n) = C-en/T of the superconducting sample (xapproximate to1) in normal state is twice that of the non-superconducting one (xapproximate to0.85). The comparison of measured gamma(n) and the calculated electronic density of states (DOS) shows that the effective mass renormalization changes remarkably as the carbon concentration changes. The large mass renormalization for the superconducting sample and the low transition temperature T-c(7K) indicate that more than one kind of boson mediated electron-electron interaction exists in MgCxNi3.