Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C41: Magnetism, Unconventional Superconductivity and Pressure Effects in CaKFe_{4}As_{4}Invited

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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(Fe_{1x}Ni_{x})_{4}As_{4} Invited Speaker: Yuji Furukawa We have carried out nuclear magnetic resonance measurements on the recent discovered new ironbased superconductor CaKFe_{4}As_{4} (T_{c} = 35 K). Based on NMR data, we will discuss the details of superconducting properties and magnetic fluctuations of CaKFe_{4}As_{4} [1], which will be compared with those in other Febased 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 Nidoped CaK(Fe_{1x}Ni_{x})_{4}As_{4} (T_{n }= 52 K and T_{c}= 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 Nidoped CaK(Fe_{1x}Ni_{x})_{4}As_{4}. 
Monday, March 5, 2018 3:06PM  3:42PM 
C41.00002: Magnetic degeneracy and intertwined orders in ironbased superconductors Invited Speaker: Rafael Fernandes Recent experiments in holedoped ironbased 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 stripetype magnetic phase, socalled C_{4} 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 spinorbit coupling and quantum magnetic fluctuations. This results in a rich magnetic landscape with stripe spin density waves, chargespin density waves, and spinvortex crystals intertwined with composite vestigial orders that break different symmetries of the system and transform as B_{2g}, B_{2u}, and A_{2u} irreducible representations of the point group of the lattice. We argue that, while a chargespin density wave is realized in holedoped 122 materials, a spinvortex 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 spinvortex 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 ironbased 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, newstructuretype ironbase superconductors AeAFe_{4}As_{4} (where Ae = Ca, Sr, or Eu and A = K, Rb, or Cs) were found. The AeAFe_{4}As_{4} family exhibits T_{c}=32 ～ 38 K, whose T_{c} is similar to that of socalled 122 system, and Ae and A layers stack alternatively between Fe_{2}As_{2} 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 newstructuretype ironbased superconductor CaKFe_{4}As_{4} 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 Q_{res} = 1.17(1) Å^{1}, corresponding to the (π, π) nesting wave vector in tetragonal notation, evolves below T_{c}. The characteristic energy of the spin resonance E_{res} = 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 CaKFe_{4}As_{4} is the s_{±} symmetry. 
Monday, March 5, 2018 4:18PM  4:54PM 
C41.00004: Quasiparticle interference imaging in pure and Nidoped CaKFe_{4}As_{4} 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 Nidoped CaKFe_{4}As_{4}. Pure CaKFe_{4}As_{4} shows the highest T_{c} among stoichiometric pnictide materials (35 K). It is a twogap, signchanging 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 T_{c} 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 Pdoped BaFe_{2}As_{2} system. 
Monday, March 5, 2018 4:54PM  5:30PM 
C41.00005: Electronic properties, lowenergy Hamiltonian, and superconducting instabilities in CaKFe_{4}As_{4}. Invited Speaker: Tilmann Hickel , Felix Ahn , Felix Lochner , Ilya Eremin We analyze the electronic properties of the recently discovered stoichiometric superconductor CaKFe_{4}As_{4} by combining an ab initio approach and a projection of the band structure to a lowenergy tightbinding 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 CaKFe_{4}As_{4} and the parent systems CaFe_{4}As_{4} and KFe_{2}As_{2}. In particular, we find CaKFe_{4}As_{4} to have a significantly more quasitwodimensional electronic structure than the latter systems. Finally, we study the superconducting instabilities in CaKFe_{4}As_{4} by employing the leading angular harmonics approximation and find two potential A_{1g}symmetry representations of the superconducting gap to be the dominant instabilities in this system. 
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