Composition dependence of bulk superconductivity in YFe2Ge2

Abstract

In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ∼ 100 mJ/mol K 2 and a T 3/2 temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2kBTc/ . Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimised by precipitating from a slightly iron-rich melt, following by annealing.The iron-based superconductor YFe 2 Ge 2 [1] exhibits strong electronic correlations: its Sommerfeld ratio is enhanced by an order of magnitude over density functional theory (DFT) estimates [2][3][4], the normal state resistivity ρ follows a non-Fermi liquid temperature dependence, photoemission spectroscopy has revealed renormalised energy bands [5], and it displays enhanced magnetic fluctuations [6,7]. Further interest in this material derives from theoretical proposals for the superconducting state, which include s ± [3] or triplet pair wavefunctions [4] and from its striking similarities to some of the iron pnictide superconductors [8,9]. Moreover, several structurally and electronically related materials have recently been examined, some of which were found to superconduct at low temperatures [10][11][12][13], including a new iron-based superconductor, LaFeSiH [14].

Unconventional Bulk Superconductivity in YFe2Ge2 Single Crystals

“…The diamagnetic screening and the sharp heat capacity jump at the transition both suggest 100% bulk superconductivity in the highest quality samples. This is consistent with the mean free path l and the scattering rate τ −1 extracted from resistivity measurements, using the approach described in [18]: the residual resistivity ρ 0 ≃ 0.45 μΩ cm of LT-grown crystals translates to l ≃ 2400 Å, far larger than the coherence length ξ ≃ 180 Å [17], and to τ −1 ≃ 0.09 meV=ℏ, well below the threshold of about 0.2 meV expected within the Abrikosov-Gorkov approach [18,30] from the bulk T c . The suppressed jump height and residual Sommerfeld coefficient in heat capacity measurements could alternatively result from strong inhomogeneity of composition or disorder level, producing a large nonsuperconducting fraction [2].…”

“…The complete diamagnetic screening in the magnetisation measurement and the sharp transition in heat capacity both suggest 100% bulk superconductivity in the high-quality samples. This is consistent with the mean free path and the scattering rate τ −1 extracted from resistivity measurements, using the approach described in [16]: the residual resistivity ρ 0 0.45 µΩ cm of LT-grown crystals translates to 2400 Å, far larger than the coherence length ξ 180 Å [15], and to τ −1 0.09 meV/ , well below the threshold of about 0.2 meV expected within the Abrikosov-Gorkov approach [16,22] from the bulk T c . The three growth methods used in this study produce YFe 2 Ge 2 crystals with different morphology, as illustrated in Fig.…”

“…The modified flux growth from polycrystal precursors (MF) leads to a marked improvement, reaching ρ 0 1 µΩ cm and RRR 200 in the best batches. These samples show full resistive transitions, which often split into two steps, and they show broad heat capacity anomalies similar to those observed in the best polycrystals with similar RRR values [15,16]. Crystals grown in the liquid transport setup (LT) with a horizontal temperature gradient display the lowest residual resistivity ρ 0 0.45 µΩ cm and reach resistance ratios of up to RRR 430.…”

“…Initially, no heat capacity anomalies were observed at the superconducting transition in polycrystals [13] or single crystals [14], despite considerable efforts on optimizing the crystal growth. Further, systematic improvements of polycrystal quality produced the first thermodynamic evidence of bulk superconductivity in YFe 2 Ge 2 [15], clarified the primary origin of crystalline imperfections in YFe 2 Ge 2 and demonstrated that disorder rapidly suppresses the transition at a rate commensurate with expectations for a sign-changing order parameter [16]. This motivated a renewed effort to produce bulk superconducting single crystals that are required for specialised experiments probing the strongly correlated normal state and the superconducting gap symmetry in YFe 2 Ge 2 .…”

“…Methods -Our crystal growth experiments are designed to enhance Fe occupancy on the Fe site, which in polycrystals proved to be the determining factor for achieving low disorder [16]. A comprehensive crystal growth study using the conventional Sn-flux method demonstrated that lowering the growth temperatures leads to higher quality [14,17].…”