Comment on “Quantum Criticality and Nodal Superconductivity in the FeAs-Based SuperconductorKFe2As2”

Abstract

In a recent Letter [1], Dong et al. have observed a T 1.5 dependence of resistivity ρ in KFe 2 As 2 at the upper critical field B c2 = 5 T parallel to the c axis and have suggested the existence of a field-induced quantum critical point (QCP) at B c2 . The value of B c2 = 5 T was determined from the onset of a resistive transition. It is much higher than a perviously reported value of B c2 = 1.25 T [2], which was determined from a midpoint. The origin of the large difference in B c2 can be attributed to broad transitions (see the inset of Fig. 1 of Ref. 1, the ρ(T ) curve at zero field shows the onset at 4.8 K and zero resistivity at 2.5 K), which indicate distribution of the transition temperature and B c2 due to inhomogeneity in the sample. Since the observed exponent 1.5 is some kind of average over the inhomogeneous sample, it is questionable to relate it to a QCP. To clarify this, let us assume that some portion of a sample becomes quantum critical at B c2 = 5 T and that ρ in that portion varies as T 1.5 . Since B c2 for the rest of the sample is below 5 T, the rest is in the Fermi liquid regime and exhibits a T 2 dependence of ρ. Then, one can not expect ρ measured across the sample varies as T 1.5 .We substantiate below the argument against the proposed QCP, mentioning data obtained for high-quality single crystals with resistivity ratios of up to ∼600, which exhibit de Haas-van Alphen (dHvA) oscillations [3].

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

“…The compound KFe 2 As 2 corresponds to a large doping level of 0.5 hole/Fe. From ARPES and quantum oscillation studies of the Fermi surface and band structure calculations, 156,157 one finds the two previous hole pockets and an additional hole pocket about Γ, and a small 3D pocket about (00π). The previous electron pockets centered at X completely disappear and are replaced by two small quasi-cylindrical hole pockets with elliptical cross section nearby that are not centered at X.…”

“…de Haas-van Alphen (dHvA) magnetization oscillation measurements versus applied magnetic field were also carried out on superconducting KFe 2 As 2 crystals with T c = 3 K. 157 These measurements are important because, as for LaFePO above, there is no intrinsic crystallographic disorder in this superconducting compound.…”

NMR study of electronic correlations in Mn-doped Ba(Fe1−xCox)2AsLeBoeuf1, Texier2, Boselli3 et al. 2014Phys. Rev. B

“…However, high value of Sommerfeld coefficient and mass enhancement of certain bands derived from quantum oscillations in KFe 2 As 2 suggest electronic correlations may be stronger in the case of hole-doped 122 materials 13,26,27 . Recent theoritical work proposed that hole-doped KFe 2 As 2 is more correlated than electrondoped Co-122 because it stands closer, in terms of band filling, to an orbital-selective Mott transition driven by Hund's coupling between the different orbitals at play 28 .…”

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductorsKFe2As2,RbFe2As2, andTafti1, Ouellet2, Juneau-Fecteau3 et al. 2015Phys. Rev. B

“…The Fermi surface of KFe 2 As 2 is known in detail from quantum oscillations studies. 10,11 It consists of three quasi-2D hole-like cylinders centered on the Γ point, labelled α (small inner cylinder), ξ (middle cylinder), and β (large outer cylinder), as well as small cylindrical satellites at the corners of the Brillouin zone, labelled ε. There is also one small 3D hole pocket at the Z point of the Brillouin zone.…”