Session P3: Invited Session: Alkaline Iron Selenides vs Iron Pnictides: Properties and Their Implications

Neutron Scattering Study on the New 245 Iron Selenide Superconductors

We determine using neutron and x-ray diffraction method the sample composition, crystalline structure and magnetic order of the recently discovered $A_2$Fe$_4$Se$_5$ superconductors ($A$=K, Rb, Cs, Tl/K or Tl/Rb). Contrary to initial belief that these materials are heavily electron-doped variety of the BaFe2As2 family of Fe-based superconductors, they are almost charge balanced with the Fe valence close to 2+ as in the 11 iron selenide superconductors, and crystalize in an Fe vacancy-ordered lattice structure [1,2]. Coexisting with superconductivity is a novel block antiferromagnetic order which conforms to the tetragonal crystalline symmetry and possesses a very large ordered magnetic moment 3.3$\mu_B$ per Fe and a very high ordering temperature above 500 K [1]. Such Fe vacancy ordered crystal structure and coexisting antiferromagnetism and superconductivity occur in all 5 types of new iron selenide superconductors discovered so far. With Fe vacancy number departs from the chemical formulas $A_2$Fe$_4$Se$_5$, an imperfect version of the Fe vacancy order results at base temperature while phase separation into two vacancy-ordered phases exists at the intermediate temperature range [4]. The Fe site disorder renders the materials insulating and destroys the superconductivity as spin-glass disorder does in previous 11 iron selenide superconductors [5]. \\[4pt] [1] W. Bao et al., Chin. Phys. Lett. {\bf 28}, 086104 (2011).\\[0pt] [2] P. Zavalij etal., Phys. Rev. B {\bf 83}, 132509 (2011).\\[0pt] [3] F. Ye et al., Phys. Rev. Lett. {\bf 107} 137003 (2011).\\[0pt] [4] W. Bao et al., arXiv: 1102.3674 (2011).\\[0pt] [5] T.J. Liu et al., Nat. Materials {\bf 9}, 716 (2010).   

Distinct Fermi Surface Topology and nearly Isotropic Superconducting Gap in A$_{x}$Fe$_{2-y}$ Se$_{2}$ (A=K, Tl, Rb) Superconductors

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the newly discovered A$_{x}$Fe$_{2-y}$Se$_{2}$ [A=K, (Tl,K) and (Tl,Rb)] superconductors[1,2,3] 1. Distinct Fermi surface topology, consisting of two electron-like Fermi surface sheets around the $\Gamma $(0,0) point and an electron-like Fermi surface sheet near the M($\pi $,$\pi )$ point, was revealed in all these samples. This is in strong contrast to the Fermi surface topology of other Fe-based superconductors where hole-like Fermi surface sheets are present near the $\Gamma $(0,0) point. 2. Both the electron-like Fermi surface sheet near M point and the large electron-like Fermi surface sheet near $\Gamma $ point show nearly isotropic superconducting gap without nodes 3. The doping evolution of the electronic structure from insulating samples to the superconducting samples is consistent with a phase separation picture. The information on the Fermi surface topology and superconducting gap of this new A$_{x}$Fe$_{2-y}$Se$_{2}$ superconductor will provide key insights and constraints to understand the superconductivity mechanism in iron-based superconductors. \\[4pt] [1]. D. X. Mou, X. J. Zhou et. al, Phys. Rev. Lett. \textbf{106}, 107001 (2011). \\[0pt] [2]. L. Zhao, X. J. Zhou et. al, Phys. Rev. B \textbf{83}, 140508(R) (2011). \\[0pt] [3]. L. Yu, X. J. Zhou et al., unpublished.   

Pairing strength and symmetries of 122 iron selenides in comparison with iron pnictides

High temperature superconductivity with comparable transition temperatures has been observed in the vicinity of an antiferromagnetic phase, in both 122-alkaline iron selenides and 122-iron pnictides. In contrast to iron pnictides, where the parent state is an antiferromagnetic semimetal, the parent state of 122-iron selenides is a large moment, antiferromagnetic insulator. This provides a clear indication of strong electronic correlations. The 122-selenides possess only electron pockets, while the pnictides have both hole and electron pockets. In addition, the observed block spin magnetic order in 122-selenides can not be explained by Fermi surface nesting. At the same time, the comparable $T_c$ suggests a commonality in the underlying mechanism for superconductivity in the two classes of materials. Motivated by these observations and considerations, we present a comparative strong coupling analysis of the pairing strength and symmetries in these two classes of materials [1,2]. The analysis of appropriate five orbital $t-J_1-J_2$ models, reveals a similar pairing phase diagram for both materials, with $A_{1g}$ $s(x^2y^2)$ and $B_{1g}$ $d(x^2-y^2)$ as two dominant pairing channels. The pairing amplitudes in both materials are of comparable strength, making it natural for a comparable maximum $T_c$ . In contrast to the pnictides case, an $A_{1g}$ $s(x^2+y^2)$ state is not competitive, making the dominant pairing channels fully gapped. We also discuss the magnetism of the vacancy-ordered insulating 122 iron selenides [3], showing that the observed block-spin state occurs over a wide parameter range. The predicted magnetic excitation spectrum has been verified by inelastic neutron scattering experiments. Our study also reveals some commonality with the magnetism of the parent iron pnictides [4].\\[4pt] Work was done in collaboration with Rong Yu, Predrag Nikolic, Jian-Xin Zhu, Qimiao Si and Elihu Abrahams. \\[4pt] [1] Rong Yu, Pallab Goswami, Qimiao Si, Predrag Nikolic, and Jian-Xin Zhu, ``Pairing strength and symmetries of 122 iron selenides in comparison with iron pnictides,'' to be published; arXiv:1103.3259. \\[0pt] [2] Pallab Goswami et al, ``Superconductivity in Multi-orbital $t-J_1-J_2$ Model and its Implications for Iron Pnictides,'' Europhys. Lett. {\bf 91}, 37006 (2010).\\[0pt] [3] Rong Yu, Pallab Goswami, and Qimiao Si, ``The magnetic phase diagram of an extended $J_1-J_2$ model on a modulated square lattice and its implications for the antiferromagnetic phase of $\mathrm{K}_y\mathrm{Fe}_x\mathrm{Se}_2$,'' Phys. Rev. B {\bf 84}, 094451 (2011). \\[0pt] [4] Pallab Goswami et al, ``Spin Dynamics of a $J_1-J_2$ Antiferromagnet and its Implications for Iron Pnictides,'' Phys. Rev. B {\bf 84}, 155108 (2011).   

NMR investigation of iron-selenide and iron-arsenide high $T_{c}$ superconductors

We have investigated the electronic, magnetic, and superconducting properties of the iron-selenide high $T_{c}$ superconductor K$_{x}$Fe$_{2-y}$Se$_{z}$ ($T_{c}=33$\ K) with $^{77}$Se NMR [1]. We will compare the results with those observed for FeSe in ambient and applied pressures ($T_{c}>9$\ K) [2], and with iron-arsenides [3]. Similarities and dissimilarities will be pointed out, with primary focus on the anomalous normal state properties. Our latest work on K$_{x}$Fe$_{2-y}$Se$_{z}$ was carried out in collaboration with D. Torchetti, M. Fu, D. Christensen, K. Nelson (McMaster), H. Lei, and C. Petrovic (Brookhaven National Lab).\\[4pt] [1] D. Torchetti et al., PR {\bf B83}, 104508 (2011).\\[0pt] [2] T. Imai et al. PRL {\bf 102}, 177005 (2009).\\[0pt] [3] F.L. Ning et al., PRL {\bf 104}, 037001 (2010); JPSJ {\bf 78}, 103711 (2009).