Disorder-dependent superconducting phase diagram at high magnetic fields in Fe1+ySexTe

Abstract

We compare the superconducting phase-diagram under high magnetic fields (up to H = 45 T) of Fe1+ySe0.4Te0.6 single crystals originally grown by the Bridgman-Stockbarger (BRST) technique, which were annealed to display narrow superconducting transitions and the optimal transition temperature Tc 14 K, with the diagram for samples of similar stoichiometry grown by the travelingsolvent floating-zone technique as well as with the phase-diagram reported for crystals grown by a self-flux method. We find that the so-annealed samples tend to display higher ratios Hc2/Tc, particularly for fields applied along the inter-planar direction, where the upper critical field Hc2(T ) exhibits a pronounced downward curvature followed by saturation at lower temperatures T . This last observation is consistent with previous studies indicating that this system is Pauli limited. An analysis of our Hc2(T ) data using a multiband theory suggests the emergence of the Fulde-Ferrel-Larkin-Ovchnikov state at low temperatures. A detailed structural x-ray analysis, reveals no impurity phases but an appreciable degree of mosaicity in as-grown BRST single-crystals which remains unaffected by the annealing process. Energy-dispersive x-ray analysis showed that the annealed samples have a more homogeneous stoichiometric distribution of both Fe and Se with virtually the same content of interstitial Fe as the non-annealed ones. Thus, we conclude that the excess of Fe, in contrast to structural disorder, contributes to decrease the superconducting upper-critical fields of this series. Finally, a scaling analysis of the fluctuation conductivity in the superconducting critical regime, suggests that the superconducting fluctuations have a two-dimensional character in this system.

Anomalous metallic state and anisotropic multiband superconductivity in Nb3Pd0.7Se7

“…Remarkably, H c2 for fields along the b−axis saturates at a value of H c2 (T → 0 K) ∼ 14.1 T which is 4.26 × H p [= 1.84T c (≃ 1.8 K)], where H p is the Pauli limiting field in the weak coupling regime. To put this value in perspective, compare the ratio H c2 (T → 0 K)/T c = 14.1 T/1.8 K = 7.83 with the corresponding ratios for Fe 1+y Se 0.45 Te 0.55 , (∼ 50 T/14 K ≃ 3.57) [21], CeCoIn 5 (∼ 12 T/2.3 K = 5.2) [22], URu 2 Si 2 (∼ 12.5 T/1.5 K = 8.33) [23] which, according to all evidence, are unconventional and strongly-correlated superconductors. Notice that this ratio for Nb 3 Pd 0.7 Se 5 also surpasses the respective one for Nb 2 Pd 0.81 S 5 (∼ 37 T/6.6 K ≃ 5.6) [7], or for the Chevrel-phase PbMo 6 S 8 (∼ 60 T/13.3 K ≃ 4.51) [24], and obviously the ratio for the widely used Nb 3 Sn compound (∼ 30 T/18 K ≃ 1.67) [25].…”

“…Notice, that in quasi-one-dimensional or quasi-two-dimensional systems such as these, both charge-density waves [34] coupling to lattice distortions, and spin-density waves [35] resulting from electronic correlations lead to sharp, first-order like anomalies in their physical properties such as the resistivity [36], in contrast to what is seen here. In fact, it would seem that these compounds are more akin to the Fe chalcogenide superconductors: for example in the Fe 1+y Se x Te 1−x series the resistivity in the metallic state is known to display a − log T dependence above the superconducting transition which can be suppressed upon careful annealing [21].…”

Iron chalcogenide superconductors at high magnetic fields

“…It indicates that the spin-paramagnetic effect is the dominant pair-breaking mechanism for both H ab and H c [110,111,114]. For the Fe(Te,Se) in the clean limit (grown by Bridgman-Stockbarger technique) the µ 0 H c2 (T ) is Pauli limited.…”

“…For the Fe(Te,Se) in the clean limit (grown by Bridgman-Stockbarger technique) the µ 0 H c2 (T ) is Pauli limited. This may suggest the emergence of the FFLO state at low temperatures [114]. The dominance of spin-paramagnetic effect in Fe(Te,Se) may be due to the disorder induced by Te(Se) substitution/vacancies and excess Fe in Fe(2) site [92,115].…”

In-plane Fulde-Ferrel-Larkin-Ovchinnikov instability in a superconductor–normal metal bilayer system under nonequilibrium quasiparticle distribution

“…Now there is a growing body of experimental evidence for the FFLO phase, generated by the applied magnetic field, reported from various measurements [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] . However, any unambiguous experimental results, which can be interpreted only as a fingerprint of the FFLO-state, are not reported by now.…”