Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C41: Magnetism, Unconventional Superconductivity and Pressure Effects in CaKFe4As4Invited
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Sponsoring Units: DCMP GMAG Chair: Ilya Eremin, Ruhr Univ Bochum Room: LACC 502A |
Monday, March 5, 2018 2:30PM - 3:06PM |
C41.00001: NMR studies of magnetism and superconducting properties of CaK(Fe1-xNix)4As4 Invited Speaker: Yuji Furukawa We have carried out nuclear magnetic resonance measurements on the recent discovered new iron-based superconductor CaKFe4As4 (Tc = 35 K). Based on NMR data, we will discuss the details of superconducting properties and magnetic fluctuations of CaKFe4As4 [1], which will be compared with those in other Fe-based SC such as 122 systems. In addition, we will report the first observation of the hedgehog spin vortex crystal order in the antiferromagnetic state in Ni-doped CaK(Fe1-xNix)4As4 (Tn = 52 K and Tc= 9 K for x=0.05) [2,3]. Furthermore, the Ni doping x dependence of magnetic fluctuations will be presented and we will discuss a possible quantum critical behavior in in Ni-doped CaK(Fe1-xNix)4As4. |
Monday, March 5, 2018 3:06PM - 3:42PM |
C41.00002: Magnetic degeneracy and intertwined orders in iron-based superconductors Invited Speaker: Rafael Fernandes Recent experiments in hole-doped iron-based materials have unveiled a surprising competition between different magnetic ground states near optimal doping, where the superconducting transition temperature acquires its largest value. In particular, besides the usual stripe-type magnetic phase, so-called C4 phases have been observed in a variety of different materials, challenging our understanding of the magnetic phase diagram of these unconventional superconductors. Here, we present a model that captures all these distinct magnetic phases and attributes the experimental observations to an emergent magnetic degeneracy arising from the interplay between spin-orbit coupling and quantum magnetic fluctuations. This results in a rich magnetic landscape with stripe spin density waves, charge-spin density waves, and spin-vortex crystals intertwined with composite vestigial orders that break different symmetries of the system and transform as B2g, B2u, and A2u irreducible representations of the point group of the lattice. We argue that, while a charge-spin density wave is realized in hole-doped 122 materials, a spin-vortex crystal emerges in the newly discovered 1144 system due to its peculiar crystalline structure, which provides a conjugate field to the composite order that is intertwined with the spin-vortex crystal phase. Finally, we also discuss the potential implications of this magnetic degeneracy to the superconducting properties of these systems. |
Monday, March 5, 2018 3:42PM - 4:18PM |
C41.00003: Inelastic neutron scattering study on spin resonance in CaKFe4As4 Invited Speaker: Motoyuki Ishikado The iron-based superconductors are extensively studied since the discovery in 2008, because of much material variations and the unconventional mechanisms of their high transition temperature superconductivity. Recently, new-structure-type iron-base superconductors AeAFe4As4 (where Ae = Ca, Sr, or Eu and A = K, Rb, or Cs) were found. The AeAFe4As4 family exhibits Tc=32 ~ 38 K, whose Tc is similar to that of so-called 122 system, and Ae and A layers stack alternatively between Fe2As2 layers owing to the large difference between their ionic radii. This unique crystal structure gives us opportunity to theoretically and experimentally investigate iron based superconductivity. In the session, we report the dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe4As4 by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations [1]. Powder INS measurements show that the spin resonance at Qres = 1.17(1) Å-1, corresponding to the (π, π) nesting wave vector in tetragonal notation, evolves below Tc. The characteristic energy of the spin resonance Eres = 12.5 meV is smaller than twice the size of the superconducting gap (2Δ). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe4As4 is the s± symmetry. |
Monday, March 5, 2018 4:18PM - 4:54PM |
C41.00004: Quasiparticle interference imaging in pure and Ni-doped CaKFe4As4 and in related systems Invited Speaker: Isabel Guillamon Quasiparticle interference has been shown to be a powerful tool to investigate bandstructure and superconducting gap opening in the pnictide superconductors. Here I will review efforts in the recently discovered family of 1144 materials, particularly in pure and Ni-doped CaKFe4As4. Pure CaKFe4As4 shows the highest Tc among stoichiometric pnictide materials (35 K). It is a two-gap, sign-changing superconductor [1, 2] located at optimal doping. Quasiparticle interference in the pure compound shows the opening of a superconducting gap in the hole bands around the zone center. The top of some bands is very close to the Fermi level, probably influencing the structure of Caroli de Gennes Matricon localized states within vortex cores. Ni doping reduces Tc and induces a magnetic transition where a unique hedgehog magnetic order has been proposed [3]. I will show atomic scale imaging and quasiparticle interference measurements and discuss those properties of the bandstructure and the vortex lattice that could be related to the decrease in the critical temperature. Finally, I will compare this with very recent results in the P-doped BaFe2As2 system. |
Monday, March 5, 2018 4:54PM - 5:30PM |
C41.00005: Electronic properties, low-energy Hamiltonian, and superconducting instabilities in CaKFe4As4. Invited Speaker: Tilmann Hickel We analyze the electronic properties of the recently discovered stoichiometric superconductor CaKFe4As4 by combining an ab initio approach and a projection of the band structure to a low-energy tight-binding Hamiltonian, based on the maximally localized Wannier orbitals of the 3d Fe states. We identify the key symmetries as well as differences and similarities in the electronic structure between CaKFe4As4 and the parent systems CaFe4As4 and KFe2As2. In particular, we find CaKFe4As4 to have a significantly more quasi-two-dimensional electronic structure than the latter systems. Finally, we study the superconducting instabilities in CaKFe4As4 by employing the leading angular harmonics approximation and find two potential A1g-symmetry representations of the superconducting gap to be the dominant instabilities in this system. |
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