2 research outputs found
Evidence for two-gap superconductivity in (Ba,K)Fe_2As_2 by directional point contact Andreev reflection spectroscopy
Directional point-contact Andreev-reflection spectroscopy measurements on the
BaKFeAs single crystals are presented. The spectra
show significant differences when measured in the plane in comparison with
those measured in the direction of the crystal. In the latter case only a
reduced point-contact conductance around zero bias has been revealed persisting
well above and probably related to the structural and magnetic
transitions in the system. Within the plane two superconducting energy
gaps are detected below . Here a reduced conductance above could
also be found. The fits of the -plane data to the superconducting s-wave
two-gap model indicate that the smaller gap has a size below the BCS value
while the large gap reveals much higher coupling strength.Comment: published versio
Interplay between magnetism and superconductivity and appearance of a second superconducting transition in alpha-FeSe at high pressure
We synthesized tetragonal alpha-FeSe by melting a powder mixture of iron and
selenium at high pressure. Subsequent annealing at normal pressure results in
removing traces of hexagonal beta- FeSe, formation of a rather sharp transition
to superconducting state at Tc ~ 7 K, and the appearance of a magnetic
transition near Tm = 120 K. Resistivity and ac-susceptibility were measured on
the annealed sample at hydrostatic pressure up to 4.5 GPa. A magnetic
transition visible in ac-susceptibility shifts down under pressure and the
resistive anomaly typical for a spin density wave (SDW) antiferromagnetic
transition develops near the susceptibility anomaly. Tc determined by the
appearance of a diamagnetic response in susceptibility, increases linearly
under pressure at a rate dTc/dP = 3.5 K/GPa. Below 1.5 GPa, the resistive
superconducting transition is sharp; the width of transition does not change
with pressure; and, Tc determined by a peak in drho/dT increases at a rate ~
3.5 K/GPa. At higher pressure, a giant broadening of the resistive transition
develops. This effect cannot be explained by possible pressure gradients in the
sample and is inherent to alpha-FeSe. The dependences drho(T)/dT show a
signature for a second peak above 3 GPa which is indicative of the appearance
of another superconducting state in alpha-FeSe at high pressure. We argue that
this second superconducting phase coexists with SDW antiferromagnetism in a
partial volume fraction and originates from pairing of charge carriers from
other sheets of the Fermi surface