Superconductivity and magnetism in RbxFe2−ySe2: Imp

Abstract

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed RbxFe2−ySe2 single crystals is presented. Magnetization experiments evidence that annealing of RbxFe2−ySe2 at 413 K, well below the onset of phase separation Tp 489 K, neither changes the magnetic nor the superconducting properties of the crystals. In addition, annealing at 563 K, well above Tp, suppresses the superconducting transition temperature Tc and leads to an increase of the antiferromagnetic susceptibility accompanied by the creation of ferromagnetic impurity phases, which are developing with annealing time. However, annealing at T = 488 K Tp increases Tc up to 33.3 K, sharpens the superconducting transition, increases the lower critical field, and strengthens the screening efficiency of the applied magnetic field. Resistivity measurements of the as-grown and optimally annealed samples reveal an increase of the upper critical field along both crystallographic directions as well as its anisotropy. Muon spin rotation and scanning transmission electron microscopy experiments suggest the coexistence of two phases below Tp: a magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens just at Tp does not affect the volume fraction of the two phases, although the magnetic field distribution in the samples changes substantially. This suggests that the microstructure of the sample, caused by mesoscopic phase separation, is modified by annealing just at Tp, leading to an improvement of the superconducting properties of RbxFe2−ySe2 and an enhancement of Tc.

Influence of microstructure on superconductivity in KxFe2−ySe2 and evidence for a new parent phase K2Fe7Se8

“…It is interesting to see that, even up to 28 K (close to T c ), the MHL is still open showing a capability of carrying superconducting current up to about 2 T. This suggests that the superconducting area, although being small in volume fraction, has robust superconductivity as in the FeAs-based 122 samples. The behaviour of magnetization in K x Fe 2− y Se 2 samples after different thermal treatments was also observed by other groups2225. Here, we use the Bean critical state model, although it may be inappropriate for an inhomogeneous superconductor, to calculate the superconducting critical current density ( J c ) and present the data in Fig.…”

Isoelectronic substitutions and aluminium alloying in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor

“…If the superconductivity is indeed fully suppressed at the structural phase boundary, as is known for many other materials (see, e.g. references [28][29][30][31][32][33]), then the superconducting HEA system studied here offers a good opportunity to investigate this suppression on the fundamental BCC crystal lattice. [19] (blue solid line), the transition metals and their alloys in crystalline form [34,35] (tangerine dashed line) and amorphous vapor-deposited films [36,37] (azzurro dotted line) are also shown.…”

Effect of external pressure on Tc of as‐grown and thermally treated superconducting Rbx Fe2–ySe2 single crystals

“…1 the magnetization as a function of temperature of the as‐grown and annealed Rb 0.85 Fe 1.9 Se 2 samples is shown. The discrepancy between the absolute values of magnetization among the studied samples at 10 K is caused by extremely low H c1 , being very sensitive to the annealing conditions 17. A clear onset of diamagnetic transition was found at 27.1 K for the as‐grown sample.…”

“…16. All annealing treatments were performed according to the procedure described by Weyeneth et al 17; initially the samples of Rb x Fe 2– y Se 2 were sealed in evacuated quartz ampoules and annealed in a single zone furnace at temperatures 215 °C, 240 °C, 265 °C and 290 °C. For all samples the annealing time was kept constant (3 h).…”