Session Q3: Invited Session: Recent Advances in Pnictide Superconductors

Study of the Multiorbital Hubbard Model for the Fe-Superconductors Beyond Weak Coupling

A variety of experimental and theoretical investigations indicate that the pnictides and chalcogenides are materials with on-site Hubbard repulsion intermediate between weak coupling, where simple nesting ideas apply, and strong coupling where the spins are localized. For this reason, it is desirable to broaden the range of couplings theoretically studied as well as the many-body models and techniques employed. In this talk, an extensive analysis of model Hamiltonians for the Fe-based superconductors is presented. The multiorbital Hubbard models with two, three, and five orbitals are studied, via the Hartree-Fock approximation and exact diagonalization techniques. The main topics to be discussed are: magnetic ordering tendencies [1], range of realistic Hubbard repulsion and Hund couplings [2], orbital-weight redistribution at the Fermi surface and comparison with photoemission data [3], low-temperature transport properties [4], and competing pairing channels [5]. The possible magnetic states of the $\sqrt{5}\times \sqrt{5}$ Fe-vacancy arrangement will also be presented [6]. The experimental reports of local moments at room temperature leads to our most recent efforts employing a three-orbital spin-fermion model, analyzed via Monte Carlo simulations, to study the temperature dependence of the (anisotropic) conductance [7]. It is concluded that considerable progress has been made in the understanding of these materials in spite of their difficult range of intermediate couplings. However, the existence of several open problems will also be discussed.\\[4pt] [1] R. Yu {\it et al.}, Phys. Rev. B {\bf 79}, 104510 (2009); A. Moreo {\it et al.}, Phys. Rev. B {\bf 79}, 134502 (2009).\\[0pt] [2] Q. Luo {\it et al.}, Phys. Rev. B {\bf 82}, 104508 (2010); A. Nicholson {\it et al.}, Phys. Rev. B {\bf 84}, 094519 (2011).\\[0pt] [3] M. Daghofer {\it et al.}, Phys. Rev. B {\bf 81}, 180514(R) (2010).\\[0pt] [4] X. Zhang and E. Dagotto, Phys. Rev. B {\bf 84}, 132505 (2011). See also Q. Luo {\it et al.}, Phys. Rev. B {\bf 83}, 174513 (2011).\\[0pt] [5] A. Nicholson {\it et al.}, Phys. Rev. Lett. {\bf 106}, 217002 (2011); M. Daghofer {\it et al.}, Phys. Rev. Lett. {\bf 101}, 237004 (2008).\\[0pt] [6] Q. Luo {\it et al.}, Phys. Rev. B {\bf 84}, 140506(R) (2011).\\[0pt] [7] S. Liang {\it et al.}, submitted.   

Doping - dependent anisotropy of the superconducting gap in underdoped pnictide superconductors

The in-plane London penetration depth, $\Delta\lambda(T)$, was studied in single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (``Ba122") and Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_{x}$)$_2$As$_2$]$_5$ (``10-3-8"). Whereas in Ba122 magnetism and superconductivity coexist in the underdoped regime, the 10-3-8 compound exhibits a clear separation of two order parameters. By comparing the results obtained in these two systems, we could study general features of the superconducting gap structure as function of doping in the underdoped regime. Similar to all other pnictides, the low-temperature variation of London penetration depth exhibits a power-law behavior, $\Delta\lambda(T)= AT^n$, in both systems. Moving towards the underdoped edge of the superconducting dome, the exponent $n$ decreases well below scattering - limited value of $n=2$ and, at the same time, the pre-factor $A$ increases. Both trends indicate an increasing anisotropy of the superconducting gap in more underdoped compounds. These and previous results suggest that the development of the superconducting gap anisotropy towards the underdoped edge of the superconducting dome is an intrinsic property of iron pnictides, similar to the known tendency on the overdoped side where magnetism and superconductivity do not interfere.\\[4pt] In collboration with M.A. Tanatar, H. Kim, The Ames Laboratory; Bing Shen, Hai-Hu Wen, Nanjing University; and N. Ni, R.J. Cava, Princeton University.   

Nematic susceptibility and quantum criticality in Fe-pnictide superconductors

The concept of broken symmetry is ubiquitous in condensed matter physics because it allows us to write the proper Hamiltonian description of an electron in a solid. Determining which symmetry is broken and how it is broken is therefore crucial. I will present results from transport and magnetization experiments on both underdoped and overdoped pnictide superconductors of the ``122'' structural motif that help reveal the intrinsic symmetry of the electronic ground state. We reveal the nature of the nematic susceptibility as optimal doping is approached on the underdoped side, and the breakdown of Fermi liquid like quasiparticles from the overdoped side. We discuss the universality of these properties to other pnictides.   

Controlling vortex pinning and vortex phase diagrams of FeAs-based superconductors through particle irradiation and substitution

The prominent vortex pinning features of the Ba-122 and Sm-1111 family of pnictide superconductors are presented. For isovalently doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$ we observe the systematic evolution of vortex pinning with increasing P-doping from fishtail behavior to a distinct peak effect near the irreversibility field to a reversible magnetization and Bean Livingston surface barriers. The enhancement of vortex pinning resulting from heavy ion and proton irradiation is shown to arise from delta-Tc-type pinning. These results will be compared to those on optimal doped BaKFe$_2$As$_2$ and SmFeAs(O$_{1-x}$F$_x$). High-energy heavy-ion irradiation induced defects lead to a decrease in the superconducting anisotropy, an increase in the slope of the temperature dependence of the irreversibility line and only small suppression of Tc. In all cases, we see a large enhancement of the critical current following particle irradiation. In particular, on BaKFe$_2$As$_2$ irradiated to a dose matching field of 21 T with 1.3-GeV Pb-ions, Jc $\sim$ 4 MA/cm$^2$ at 5 K and in 7 T $||$ c is achieved, comparable to results for YBCO coated conductors at the same temperature and field.\\[4pt] In collaboration with L. Fang, Y. Jia, J. A. Schlueter, H. Claus, C. Chaparro, G. Sheet, A. E. Koshelev, G. W. Crabtree, W. K. Kwok, Materials Science Division, Argonne National Laboratory, Argonne, Illinois, USA; S. F. Xu, Physics Division, Argonne National Laboratory, Argonne, Illinois, USA; H. F. Hu, J. M. Zuo, Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois, USA; A. Kayani, Physics Department, Western Michigan University, Kalamazoo, Michigan, USA; H.-H. Wen, University of Nanjing, Nanjing, China; and N. D. Zhigadlo, J. Karpinski, Solid State Physics Laboratory, ETH Zurich, Switzerland.\\[4pt] This work was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the DoE, Office of Basic Energy Sciences (LF, YJ, HC, AEK, CC, GS, GWC, HFH, JMZ), by the DoE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 (UW, JAS, WKK) and by the ATLAS accelerator at Argonne (SFZ).   

Unusual electronic structure and pairing in the K$_{x}$Fe$_{2-y}$Se$_{2}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ superconductors

In this talk, we present the angle resolved photoemission study of the unusual electronic structure and pairing behavior in two rather unique iron based superconductors: K$_{x}$Fe$_{2-y}$Se$_{2}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ For K$_{x}$Fe$_{2-y}$Se$_{2}$, large electron Fermi surfaces are observed around the zone corners with an almost isotropic superconducting gap of 10.3 meV, while there is no hole Fermi surface near the zone center, which demonstrate the inter-band scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Moreover, two insulating and one semiconducting parental phases of K$_{x}$Fe$_{2-y}$Se$_{2}$ were identified. The two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is mesoscopically phase-separated from the superconducting/semiconducting phase in the superconductor/semiconductor [2]. For BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$, which is a prototypical iron-based superconductor with nodal gap behaviors, we have determined the systematic change of its low energy electronic structures as a function of the Phosphor concentration. We found the so-called iso-valent doping actually introduce significant amount of holes into the system. The chemical pressure effect is largely a doping effect in addition to the non-rigid band behavior [3]. Moreover, we report the direct observation of a circular line node on the largest hole Fermi surface around the Z point at the Brillouin zone boundary. We found that the nodes are due to the strong three dimensional character of this Fermi surface (large kz dispersion, strong mixing of d$_{3z2-r2}$ orbitals), instead of d-wave pairing or other scenarios involving the electron pockets [4]. \\[4pt] [1] Y. Zhang et al. Nature. Materials 10, 273 (2011).\\[0pt] [2] F. Chen et al. arXiv:1106.3026v1[cond-mat.supr-con] (2011).\\[0pt] [3] Z. R. Ye et al. arXiv:1105.5242v1[cond-mat.supr-con] (2011).\\[0pt] [4] Y. Zhang et al. arXiv:1109.0229v1[cond-mat.supr-con] (2011).