Melting of vortex lattice in the magnetic superconductor RbEuFe4As4

Abstract

The iron-based superconductors are characterized by strong fluctuations due to high transition temperatures and small coherence lengths. We investigate fluctuation behavior in the magnetic ironpnictide superconductor RbEuFe4As4 by calorimetry and transport. We find that the broadening of the specific-heat transition in magnetic fields is very well described by the lowest-Landau-level scaling. We report calorimetric and transport observations for vortex-lattice melting, which is seen as a sharp drop of the resistivity and a step of the specific heat at the magnetic-field-dependent temperature. The melting line in the temperature/magnetic-field plane lies noticeably below the upper-critical-field line and its location is in quantitative agreement with theoretical predictions without fitting parameters. Finally, we compare the melting behavior of RbEuFe4As4 with other superconducting materials showing that thermal fluctuations of vortices are not as prevalent as in the high-temperature superconducting cuprates, yet they still noticeably influence the properties of the vortex matter. :1906.10236v2 [cond-mat.supr-con] arXiv

Vortex-lattice melting and paramagnetic depairing in the nematic superconductor FeSe

“…IV B 1), respectively.˜ ≈ 4.5 is inferred from Fig. 6 where we find that the heat-capacity curve for H ⊥ = 1-2 T matches that of H = 7 T. We obtain Gi = 5 × 10 −4 for FeSe, which is several orders of magnitude larger than in classical superconductors, e.g., Nb (Gi ≈ 10 −11 ), but 033319-3 slightly lower than in cuprate superconductors (10 −1 < Gi < 10 −3 ) [31]. This large value of Gi strongly suggests that thermal fluctuations cannot be neglected in FeSe.…”

“…IV A. As expected for thermally induced fluctuations, this broadening finally reduces progressively with decreasing temperatures for T < 3 K [see [30] and RbEuFe 4 As 4 [31]. It was interpreted as a second-order melting transition between a vortex glass and a vortex liquid [76].…”

“…The recently discovered Fe-based superconductors offer another interesting platform for the study of vortex matter. As anticipated theoretically [46], thermal fluctuations of intermediate magnitude between cuprates and conventional materials, accompanied by a clear vortex-melting anomaly, were highlighted in the 122 and 1144 families via high-resolution thermodynamic measurements [30,31,47]. In parallel, a first-order superconducting transition detected in the magnetostriction of KFe 2 As 2 stressed the relevance of Zeeman depairing in these materials and raised the possibility of observing a FFLO phase [48][49][50].…”

“…This first-order transition represents the only genuine phase transition for superconductivity in a magnetic field, since H c2 (T ) becomes a broad crossover. Since then calorimetric features related to melting were also reported for conventional low-T c superconductors, e.g., Nb 3 Sn, SnMo 6 S 8 , and for Fe-based superconductors [28][29][30][31]. However, the fate of the melting transition for T → 0 remains unclear since the low-temperature/high-field region is usually inaccessible, as in cuprates due to the high values of H c2 (0), or because residual disorder disrupts the melting transition [26].…”

“…It measures the temperature shift with respect to T c (H ) normalized by the fluctuation broadening [31] and…”