Pengcheng Dai The University of Tennessee (UT) Institute of Physics, Chinese Academy of Sciences (IOP) Evolution of spin excitations in high- temperature FeAs-based superconductors

Chenglin Zhang, Miaoyin Wang, L. W. Harriger, O. Lipscombe UT/ORNL Meng Wang, Huiqian Luo, Shiliang Li IOP/Beijing Jeff Lynn, Songxue Chi NIST center for neutron research M. D. Lumsden, D. L. Abernathy HFIR and SNS, ORNL G. F. Chen, Nanlin Wang IOP, Beijing D. T. Adroja, T. G. Perring ISIS Tao Xiang (IOP, Beijing), Jiangping Hu (Purdue, IOP, Beijing)

Phase diagrams of copper oxide and iron arsenide superconductors. Mazin, Nature 464, 183 (2010).

Statement of the problem How spin-waves in the parent compounds are modified as holes and electrons are doped into the parent compounds of high-T c superconductors? How superconductivity interacts with spin excitations? What spin excitations can tell us about gap symmetry?

Lattice structures of iron-based superconductors LaFeAsO 1111 Tn=150 K BaFe2As2 122 Tn= K LiFeAs 111 Tn ? FeTe 11 Tn=60 K

Localized magnets What are the effective exchange couplings in FeAs? Exchange couplings between local moments Fermi surface nesting in metals La 2 CuO 4 Coldea et al. PRL (2001) J = 112 meV

Spin structures of Fe-based parent compounds CaFe2As2 122 FeTe 11

Spin structures of Fe-based parent compounds (Rb,K,Cs)Fe1.6Se2 Tn=550 K, and parent compound is an insulator!

The Heisenberg Model

Low Temperature Ca(122) Ca(122) Jun Zhao et. al., Nature Physics 5, 555 (2009)

SJ1a = 49SJ1b = -5.7SJ2 = 19SJc = 5.3 meV Magnetic exchange couplings in CaFe 2 As 2 Jun Zhao et al., Nature Physics 5, 555 (2009).

Wave vector dependence of spin-waves in BaFe 2 As 2

Model calculation of spin-waves in BaFe 2 As 2 SJ1a = 59 meVSJ1b = -9 meVSJ2= 13 meVSJ3 = 2 meV, Similar to Jun Zhao et al., Nature Physics 5, 555 (2009).

Comparison of Low T Exchange Couplings J 1a J 1b J2J2 JcJc BaFe 2 As 2 (7K) CaFe 2 As 2 (10K)

Spin waves in FeTe

SJ1a = -17 meV SJ1b = -51 meV SJ2a=SJ2b = 22 meV SJ3 = 6.8 meV Lispcombe et al., PRL (2011).

Spin structures of insulating parent compounds Spin structures of Rb 0.8 Fe 1.6 Se 2 insulating parent compounds

Spin waves of insulating along c-axis Spin waves of insulating RbFe 1.6 Se 2 along c-axis

Spin waves of in the ab-plane Spin waves of RbFe 1.6 Se 2 in the ab-plane

Model spin waves of Model spin waves of RbFe 1.6 Se 2

J 1a J 1b J2J2 JcJc BaFe 2 As 2 (7K) CaFe 2 As 2 (10K) J 1a J 1b J2J2 JcJc FeTe (7K) RbFe 1.6 As 2 (5 K) to Bottom line, similarities between different Fe- based parent compounds

Why does this have anything to do with superconductivity?

Electron-doping hardly affects spin excitations in Fe-based superconductors

The effective of electron-doping on spin excitations

Low-energy spin excitations knows superconductivity, and can mediate pairing.

Summary Spin waves in parent compounds have a common feature that is associated with J2 of the effective exchange coupling constant. Electron-doping hardly affects the spin excitations in Fe-based superconductors.