The pairing symmetry in the iron-based superconductor Ba1-xKxFe2As2 may change from nodeless s-wave near x~0.4 and Tc>30 K, to nodal (either d-wave or s-wave) at x=1 and Tc<4 K. Theoretical interest has been focused on this possibility, where in the
transition region both order parameters would be present and time reversal symmetry would be broken. We report specific heat in magnetic fields down to 0.4 K of three single crystals, free of low temperature magnetic anomalies, of heavily overdoped Ba1-xKxFe2As2, x= 0.91, 0.88, and 0.81, Tc(mid) ~ 5.6, 7.2 and 13 K and Hc2 ~ 4.5, 6, and 20 T respectively. The data can be analyzed in a two gap scenario, Delta2/Delta1 ~ 4, with the field dependence of gamma (=C/T as T->0) showing an S-shape vs H, with the suppression of the lower gap by 1 T and gamma ~ H**1/2 overall. Although such a non-linear gamma vs H is consistent with deep minima or nodes in the gap, it is not clear evidence for one, or both, of the gaps being nodal. Following the established analysis of the specific heat of d-wave cuprate superconductors containing line nodes, we present the specific heat/H**1/2 vs T/H**1/2 of these Ba1-xKxFe2As2 samples which all, due to the absence of magnetic impurities, convincingly show the scaling for line node behavior for the larger gap. There is however no clear observation of the nodal behavior C ~ alpha*T**2 in zero field at low temperatures, with alpha ~ 2 mJ/molK**3 being consistent with the data. This, with the scaling, leaves the possibility of extreme anisotropy in a nodeless larger gap, Delta2, such that the scaling works for fields above 0.25 to 0.5 T (0.2 to 0.4 K in temperature units), where this an estimate for the size of the deep minima in the Delta2 ~ 20-25 K gap. Thus, the change from nodeless to nodal gaps in Ba1-xKxFe2As2 may be closer to the KFe2As2 endpoint than x=0.91.
The editors have dedicated this special issue on superconducting materials to Ted Geballe in honor of his numerous seminal contributions to the field of superconducting materials over more than 60 years, on the year of his 95th birthday. Here, as an
executive summary, are just a few highlights of his research in superconductivity, leavened with some anecdotes, and ending with some of Teds general insights and words of wisdom.
The cubic A15 structure metals, with over 60 distinct member compounds, held the crown of highest Tc superconductor starting in 1954 with the discovery of Tc=18 K in Nb3Sn. Tc increased over the next 20 years until the discovery in 1973 of Tc = 22.3
K (optimized to approximately 23 K a year later) in sputtered films of Nb3Ge. Attempts were made to produce - via explosive compression - higher (theorized to be 31-35 K) transition temperatures in not stable at ambient conditions A15 Nb3Si. However, the effort to continue the march to higher Tc values in A15 Nb3Si only resulted in a defect-suppressed Tc of 19 K by 1981. Focus in superconductivity research partially shifted with the advent of heavy Fermion superconductors (CeCu2Si2, UBe13, and UPt3 discovered in 1979, 1983 and 1984 respectively) and further shifted away from A15 superconductors with the discovery of the perovskite structure cuprate superconductors in 1986 with Tc=35 K. However, the A15 superconductors, and specifically doped Nb3Sn, are still the material of choice today for most applications where high critical currents (e. g. magnets with dc persistent fields up to 21 T) are required. Thus, this article discusses superconductivity, and the important physical properties and theories for the understanding thereof, in the A15 superconductors which held the record Tc for the longest time (32 years) of any known class of superconductor since the discovery of Tc=4.2 K in Hg in 1911. The discovery in 2008 of Tc=38 K at 7 kbar in A15 Cs3C60 (properly a member of the fullerene superconductor class), which is an insulator at 1 atm pressure and otherwise also atypical of the A15 class of superconductors, will be briefly discussed.
The strong power law behavior of the specific heat jump $Delta C$ vs. $T_c$ ($Delta C/T_c sim T_c ^{alpha}, alphaapprox 2$), first observed by Budko, Ni, and Canfield (BNC)[1], has been confirmed with several families of the Fe-based superconducting
compounds with a series of doping. We show here that this anomalous non-BCS behavior is an intrinsic property of the multiband superconducting state paired by a dominant interband interaction ($V_{inter} > V_{intra}$) reflecting the relation $frac{Delta_h}{Delta_e} sim sqrt{frac{N_e}{N_h}}$ near $T_c$, as in the $pm$S-wave pairing state. Then this $Delta C$ vs. $T_c$ relation can continuously change from the perfect BNC scaling to a considerable deviation at lower $T_c$ region with a moderate variation of the impurity scattering rate.
We report specific heat measurements on the Fe-based superconductor BaFe2(As0.7P0.3)2, a material on which previous penetration depth, NMR, and thermal conductivity measurements have observed a high density of low-energy excitations, which have been
interpreted in terms of order parameter nodes. Within the resolution of our measurements, the low temperature limiting C/T is found to be linear in field, i.e. we find no evidence for a Volovik effect associated with nodal quasiparticles in either the clean or dirty limit. We discuss possible reasons for this apparent contradiction.
