A. Juneau-Fecteau

Direct measurement of the upper critical field in cuprate superconductors

In the quest to increase the critical temperature Tc of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field Hc2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect Hc2 in the cuprates YBa2Cu3Oy, YBa2Cu4O8 and Tl2Ba2CuO6+δ, allowing us to map out Hc2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa2Cu3Oy is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from Hc2, suffers a sudden 20-fold collapse. This reveals that phase competition—associated with Fermi-surface reconstruction and charge-density-wave order—is a key limiting factor in the superconductivity of cuprates.

Universal Heat Conduction in the Iron Arsenide Superconductor KFe2As2: Evidence of a d-Wave State

The thermal conductivity κ of the iron arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c axis. A residual linear term at T→0, κ(0)/T is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of κ(0)/T in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity ρ(0) was 10 times larger. This independence of κ(0)/T on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate Γ(c) for suppressing T(c) to zero; the magnitude of κ(0)/T, and its dependence on current direction and on magnetic field; the temperature dependence of κ(T).

Superconductivity in the noncentrosymmetric half-Heusler compound LuPtBi: A candidate for topological superconductivity

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1. 0Ka ndHc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. The combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.

From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and the pairing mechanism. We also compare KFe2As2 with the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

Sudden reversal in the pressure dependence ofTcin the iron-based superconductor CsFe2As2: A possible link between inelastic scattering and pairing symmetry

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.

Pressure-induced Fermi-surface reconstruction in the iron-arsenide superconductor Ba 1 − x K x Fe 2 As 2 : Evidence of a phase transition inside the antiferromagnetic phase

E. Hassinger, ∗ G. Gredat, F. Valade, S. René de Cotret, A. Juneau-Fecteau, J.-Ph. Reid, H. Kim, M. A. Tanatar, R. Prozorov, B. Shen, H.-H. Wen, 5 N. Doiron-Leyraud, and Louis Taillefer 5, † Département de physique & RQMP, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 Ames Laboratory, Ames, Iowa 50011, USA Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Center for Superconducting Physics and Materials, National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China Canadian Institute for Advanced Research, Toronto, Ontario, Canada M5G 1Z8 (Dated: May 1, 2014)

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductors KFe2As2 ,R bFe2As2 ,a nd CsFe2As2

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a d-wave state favored by high-Q scattering at low pressure changes to a state with s+- symmetry favored by low-Q scattering at high pressure.

Doping evolution of the superconducting gap structure in the underdoped iron arsenide Ba 1 − x K x Fe 2 As 2 revealed by thermal conductivity

The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K). In zero field, there is no residual linear term in kappa(T) as T goes to 0 at any doping, whether for in-plane or inter-plane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order.

Change of pairing symmetry in the iron-based superconductor KFe2As2

Different probes have found different superconducting pairing states in different iron-based high-temperature superconductors. Now transport measurements suggest that pressure drives the superconducting state in KFe2As2 from d-wave to s-wave.The pairing mechanism in iron-based superconductors is the subject of ongoing debate. Proximity to an antiferromagnetic phase suggests that pairing is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+-), while the latter yield a standard s-wave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a change of pairing state. The critical temperature Tc decreases with pressure initially, and then suddenly increases, above a critical pressure Pc. The constancy of the Hall coefficient through Pc rules out a change in the Fermi surface. There is compelling evidence that the pairing state below Pc is d-wave, from bulk measurements at ambient pressure. Above Pc, the high sensitivity to disorder argues for a particular kind of s+- state. The change from d-wave to s-wave is likely to proceed via an unusual s + id state that breaks time-reversal symmetry. The proximity of two distinct pairing states found here experimentally is natural given the near degeneracy of d-wave and s+- states found theoretically. These findings make a compelling case for spin-fluctuation-mediated superconductivity in this key iron-arsenide material.