Wydarzenia

Seminarium Międzynarodowego Laboratorium Silnych Pól Magnetycznych i Niskich Temperatur PAN

1. Improvement of the superconducting properties of poly-crystalline FeSe by silver addition
2. Vortex dynamics studies of FeSe crystals

Elena Nazarova¹, Krastyo Buchkov²

Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria

1. Improvement of the superconducting properties of polycrystalline FeSe by silver addition 

We studied the influence of different Ag additions (up to 10 wt %) on the superconducting properties of FeSe_0.94. The structural investigations (XRD and SEM) indicated that Ag is present in three different forms. Ag at grain boundaries supports the excellent intergrain connections and reduces _ΔT to values smaller than 1K at B=0 and _ΔT ≤ 2.74 K at B=14 T. Ag insertion in the crystal lattice unit cell provides additional carriers and changes the electron hole imbalance in FeSe_0.94. This results in an increase in the magnetoresistive effect (MR) and critical temperature (T_c). Reacted Ag forms a small amount (~1%) of Ag₂Se impurity phase, which may increase the pinning energy in comparison with that of the undoped sample. The enhanced upper critical field (B_c2) is also a result of the increased impurity scattering. Thus, unlike cuprates Ag addition enhances the T_c, B_c2, pinning energy and MR making the properties of polycrystalline FeSe_0.94 similar to those of single crystals.

2. Vortex dynamics studies of FeSe crystals

The superconducting properties of mm-sized Fe_1.02Se crystals grown by a flux method are investigated. The vortex states and dynamics are analyzed using a combined method of the fundamental and the 3rd harmonic ac magnetic susceptibility as a function of the temperature at different ac magnetic field amplitudes and frequencies and with various superimposed dc fields. The DC magnetization experiments – relaxation and M(H) hysteresis show a non-zero critical current at least as high at 12 K. It is observed a symmetric pseudo peak effect in the superconducting

M(H) loops, which has been interpreted in terms of the composition of two different contributions of our multidomain crystal FeSe sample. Additionally, the presence of dendritic phenomena of vortices (Flux Jumps) in the region of interdomain magnetic response was detected.