Slush vortex mater phase in YBa2Cu3O7 superconductor at tilted magnetic field
The vortex matter in high critical temperature superconductors displays novel phases, mainly due to the increasing role of the thermal fluctuations. The aim of our research effort is to explore unresolved vortex matter issues using high quality single crystals. One of the issues not yet well understood is the slush vortex matter phase. It has been proposed, that this phase exists, above the intersection point between the Bragg glass line and the line of the onset of the peak effect. Figure 1 (left panel) shows the temperature variation of the ac susceptibility, measured under a dc magnetic field H=90 kOe, tilted 30o in respect of the c-axis of the crystal. Our experimental data offer significant evidence that the sharp drop of the χ’ (indicated with arrow if Fig. 1) corresponds to the transmission from the liquid to slush vortex state.
Frequency dependence of the magnetic ac susceptibility in the peak effect region of Ba1-xkxFe2As2 superconductor (superconducting materials)The vortex matter (Abrikosov state) in type II superconductors under the influence of weak pinning centers constitutes an interesting physical system having many similarities with conventional matter. Despite the intensive research effort having been devoted, in studying the vortex matter, the discovery of new superconductors still keeps many surprises and not well understood novel physical effects. One of the most intriguing effect observed in all type II superconductors with weak pinning centers, interesting both from basic physics and applications’ point of view, is the non-monotonous variation of the critical current. In the related literature, this phenomenon is called peak effect (mainly in low-Tc superconductors), second magnetization peak (in high Tc superconductors) or sometimes fishtail effect. Experimentally, the peak effect appears as a diamagnetic negative peak both in dc and ac-susceptibility measurements as a function of temperature, or as non-monotonous variations of the dc-magnetization as a function of dc-magnetic field.
The onset of the peak effect (local minimum of the screening current), relates to the Bragg-glass to vortex glass transition. Mikitik and Brandt using Lindemann type criteria, and the results of the collective pinning theory, estimated the vortex matter phase diagram considering both thermal fluctuations and random point disorder. Usually, the weak frequency dependence of the ac-susceptibility in type II superconductors is related to flux creep of vortices, being a slow process, contrary to the strong frequency dependence predicted for a linear conductor. For Ohmic behavior E=ρJ, the ac-susceptibility is linear and frequency dependent (e.g. max χ''~1/√ω). On the other hand, for a "Bean" superconductor (Ε= E0(J/Jc)n, n>>1) the ac-susceptibility is non-linear and frequency independent. As we see below we have observed a weak frequency dependence of the ac-susceptibility, a fact implying that the departure from the critical state model is small. According to Faraday's law, a frequency change of the external magnetic induction is equivalent with the change of the electric field level developed in the superconductor.Following Brandt's scaling arguments when the superconductor can be described by a power-law current-voltage E~Jn a change of frequency by a factor k and the amplitude of the ac-field by factor k1/n-1 then the susceptibility does not change e.g. χ(f/B01/n-1}). Based on this, one expects that a frequency increasing for constant amplitude of the ac-field leads also, to an increasing of the susceptibility.
Fig. 3 shows the temperature variation of the fundamental ac-susceptibility χ1, measured under a dc-magnetic field μ0Hdc=7 Tesla and an ac-amplitude of H0=15 Oe for several frequencies.
Vortex matter of properties pf K0.8Fe2-ySe2 superconductor
Superconductivity in intercalated iron selenides is an interesting topic, because the physical mechanism being responsible for the superconducting state may be related to the quantum matter issues, topological superconductivity and Majorana zero modes.
Figure 6 (left panel) shows preliminary results on the vortex matter properties deduced from ac-susceptibility measurements in a K0.8Fe2-ySe2 single crystal for dc magnetic fields (0-9 Tesla), applied parallel to the c-axis and ab-plane. The experimental data were collected using frequencies from 10 Hz up to 10 kHz. From the diamagnetic onset, the irreversibility lines, both for H||c-axis and H||ab-plane, were estimated as a function of temperature and frequency.