Glassy transition in the vortex lattice of Ba(Fe _0.93 Rh _0.07 ) ₂ As ₂ superconductor probed by NMR and ac-susceptibility

Abstract

-By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.Introduction. -The discovery of iron-based superconductors has brought to a renewed interest in the study of the mixed phase of type-II superconductors and of the nature of the frozen vortex state. In this regard, the effect of quenched disorder on the flux lines lattice (FLL) properties has to be considered, together with the onset of a vortex glassy state [1][2][3][4]. In the case of weakly anisotropic superconductors, such as the 122 family of iron-pnictides, the high temperature (T ) region of the phase diagram is dominated, just below the transition temperature T c , by a high mobility state of vortex lines [5]. On the other hand, the nature of the low temperature vortex lattice (VL) dynamics is still debated, since it involves the complex interplay among the pinning forces, the intervortex repulsion/attraction [6] and the thermal excitations. In a recent theoretical work [7] the free energy of the vortex matter was explicitly calculated for a three dimensional (3D) superconductor, in presence of quenched disorder. Two transitions were found: a first-order melting, at which the quasi-long-range order is destroyed, and a second-order glassy transition. Such double transition has been recently experimentally observed in cuprates [8]. By taking advantage of the work carried out in cuprates,

Strain relaxation behaviour of vortices in a multiferroic superconductor

“…Values of ~200-300 K (~17-26 meV) for the activation energy estimated from Arrhenius treatment of acoustic loss peak 2 (table A1) compare with ~10-70 K (~1-6 meV) from Vogel-Fulcher analysis of the AC magnetic loss peaks (section A.7). These are comparable with a previous determination of 120 ± 20 K for vortex freezing in Ba(Fe 0.93 Rh 0.07 ) 2 As 2 [54].…”

A view from inside iron-based superconductors

“…Still, the IBS are characterized by an anisotropy which is not so pronounced and, accordingly, by a lower FLL mobility. [17,18,19,20,21] At the same time they show upper critical fields often exceeding 60 Tesla [22,23], making the IBS rather promising for applications. The high upper critical fields imply rather short coherence lengths, reaching few nanometers along the FeAs planes, so that these materials are weakly sensitive to disorder and, in particular, to non-magnetic impurities.…”

Evidence of unconventional low-frequency dynamics in the normal phase of Ba(Fe1−xRhx)Bossoni1, Carretta2, Halperin3 et al. 2013Phys. Rev. BSelf Cite214220View full textAdd to dashboardCite

“…2), as expected when the solid/glassy vortex phase sets in. 8,9,19 Since a LF dynamic is present one should expect an effect also on 1/T 1 , which probes the spectral density at the nuclear Larmor frequency. In the inset of Fig.…”