Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L5: Focus Session: Orbital Fluctuations and Nematicity in FeSC |
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Sponsoring Units: DMP DCOMP Chair: Kazuhiko Kuroki, Osaka University Room: Juan Gorman Room 005 |
Wednesday, March 4, 2015 8:00AM - 8:36AM |
L5.00001: Experimental Signatures of Orbital Fluctuations in Iron Based Superconductors Invited Speaker: Wei-Cheng Lee Understanding the high temperature superconductivity has been one of main subjects in the condensed matter physics. The discovery of new classes of high-temperature superconductors, iron pnictides in 2008, launched an international wave of research in the past few years. While the magnetic interactions are certainly important in these materials, there have been significant evidences suggesting that the orbital degrees of freedom could play an important role as well. In this talk, I will demonstrate that the orbital degrees of freedom do play a significant role in physical properties of iron-based superconductors. At the level of single particle properties, while the orbital order in the quasi-1D dxz and dyz bands has been proposed to be a possible driving mechanism for the structural phase transition, our study shows that the fluctuations associated with the orbital order could further drive a non-Fermi liquid behavior in the critical region of the orbital ordering phase transition. I will show that this non-Fermi liquid behavior could induce a zero-bias anomaly in the point contact spectroscopy, which has been observed in a variety of iron based superconductors. As for the magnetic properties, we also find that the orbital order and fluctuations can qualitatively change the nature of the spin excitation spectrum, giving rise to the novel incommensurate-to-commensurate transformation observed in a recent neutron scattering measurement. In the superconducting state, we predict that a new collective excitation, termed as orbital resonance mode, could exist generally in the iron-based superconductors, which in principle can be measured by Raman spectroscopy. Our findings offer a new perspective on the pairing mechanism of iron based superconductors, and suggest that orbital degrees of freedom could provide a new route to high temperature superconductivity. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L5.00002: Orbital-selective Correlation Effects in Alkali Metal Iron Pnictides Weicheng Lv, Rong Yu, Jian-Xin Zhu, Qimiao Si There is growing evidence for the substantial electronic correlations in the iron based superconductors. In particular, recent experiments have revealed strong orbital-selective correlation effects in the series of alkali metal iron pnictides AFe$_2$As$_2$ (A = K, Rb, Cs). Among the important questions is how these systems, with a 3d-electron filling $n=5.5$ per site, differs from the parent iron pnictides, which has $n=6$. Here, we address these issues in a five-orbital Hubbard model with filling $n=5.5$, using the $U(1)$ slave-spin method. As the lattice parameters increase from K to Rb, then to Cs, we are able to identify the systematics in the orbital-selective Mott behavior as the correlation effects are enhanced due to the reduced bandwidth. We discuss the implications of our results for the quasiparticle mass as well as for the spin spectral weight. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L5.00003: Spin and Orbital Nematic Susceptibility of Iron Based Superconductors Christopher Bishop, Shuhua Liang, Anamitra Mukherjee, Niravkumar Patel, Elbio Dagotto, Adriana Moreo The spin and orbital nematic susceptibility of the iron-based superconductors are calculated in the undoped limit using a three-orbital (xz, yz, xy) spin-fermion model that includes spin, orbital, and lattice degrees of freedom [1]. The results are in very good agreement with experiments by J-H. Chu et al., Science 337, 710 (2012). Recently, Raman scattering experiments[2] indicate a Curie-Weiss behavior of the orbital nematic susceptibility in BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. This behavior is observed in our numerical simulations after considering the coupling between the spin and orbital nematic order parameters. A Landau-Ginzburg formalism is used to analytically derive equations for both the spin and orbital susceptibility that fit well the numerical data. [1] S. Liang et al., arXiv:1405.6395 (2014), to appear in PRB. [2] Y.-X. Yang, et al., ``Raman scattering as a probe of charge nematic fluctuations in iron based superconductors,'' {\em JPS Conf. Proc. 3 , 015001}, 2014. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L5.00004: Diverging Nematic Susceptibility, Physical Meaning of $T^*$ scale, and Pseudogap in the Spin Fermion Model for Pnictides Shuhua Liang, Anamitra Mukherjee, Niravkumar Patel, Chris Bishop, Elbio Dagotto, Adriana Moreo Using Monte Carlo simulations including the lattice degree of freedom [1], for the first time the nematic susceptibility of the spin fermion model for the pnictides is calculated [2]. The results are in very good agreement with experiments by J-H. Chu et al., Science 337, 710 (2012). Our study suggests a nematicity in the spin fermion model primarily originating on magnetism, but with the lattice/orbital also playing an important role by boosting up critical temperatures and separating the structural $T_S$ and N\'eel $T_N$ transitions. At $T>T_S$, Curie-Weiss behavior is observed with a characteristic temperature $T^*$ being the $T_N$ of the purely electronic system. In this regime, a pseudogap in the density of states and short-range magnetic order is observed. \\[4pt] [1] S. Liang et al., PRL 111, 047004 (2013)\\[0pt] [2] S. Liang et al., arXiv:1405.6395 (2014), to appear in PRB. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L5.00005: Symmetry of reentrant tetragonal phase in Ba$_{\mathrm{1-x}}$Na$_{\mathrm{x}}$Fe$_{2}$As$_{2}$: Magnetic versus orbital ordering mechanism Dmitry Khalyavin, Stephen Lovesey, Pascal Manuel, Frank Kruger, Stephan Rosenkranz, Jared Allred, Omar Chmaissem, Ray Osborn Magneto-structural phase transitions in Ba$_{\mathrm{1-x}}$A$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ (A $=$ K, Na) materials have been analyzed for both magnetically and orbitally driven mechanisms, using symmetry methods formulated within the Landau theory of phase transitions. Both mechanisms predict identical orthorhombic space group symmetries for the nematic and magnetic phases observed over much of the phase diagram, but they predict different tetragonal space-group symmetries for the newly discovered reentrant tetragonal phase in Ba$_{\mathrm{1-x}}$Na$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ (x $\sim $ 0.25). In a magnetic scenario, magnetic order with moments along the $c$-axis, as found experimentally, does not allow any type of orbital order, but in an orbital scenario, we have determined two possible orbital patterns, specified by \textit{P4/mnc1'}and \textit{I4221'} space groups, which do not require atomic displacements relative to the parent \textit{I4/mmm1'} symmetry and, in consequence, are indistinguishable in conventional diffraction experiments. We demonstrate that the three possible space groups are however, distinct in resonant X-ray Bragg diffraction patterns created by Templeton {\&} Templeton scattering. This provides an experimental method of distinguishing between magnetic and orbital models. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L5.00006: The phase diagram of Sr$_{\mathrm{1-x}}$Na$_{\mathrm{x}}$Fe$_{2}$As$_{2}$: evidence of magnetic C4 phase universality K.M. Taddei, J.M. Allred, D.E. Bugaris, M. Krogstad, S. Rosenkranz, R. Osborn, H. Claus, D.Y. Chung, S.H. Lapidus, M.G. Kanatzidis, O. Chmaissem Determination of the nature of superconductivity in the high Tc iron based superconductors requires understanding the material's magnetic behavior out of which superconductivity arises. The apparent competition between superconductivity and magnetism in these materials and the appearance of superconductivity upon suppression of magnetism suggests magnetic fluctuations as a possible superconducting pairing mechanism. A recent study of the sodium doped barium 122 system which established the existence of a new magnetic phase formed within the AFM dome and its coexistence with superconductivity, has generated intense interest in the nature of this novel magnetic phase and given new insights to the driving force behind the magnetic transitions and preceding nematic fluctuations in this system. To search for evidence of a universality to this new magnetic and tetragonal C4 phase in the hole doped iron pnictides, polycrystalline Sr$_{\mathrm{1-x}}$Na$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ samples were synthesized for measurements with x-ray and neutron diffraction experiments. In this talk, I will present results that show a more robust magnetic C4 phase, with a higher ordering temperature and stability over a larger range of compositions than has been seen in the Ba$_{\mathrm{1-x}}$Na$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ counterparts. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L5.00007: Enhancement of superconductivity near a nematic quantum critical point Samuel Lederer, Yonatan Schattner, Erez Berg, Steven Kivelson In both the hole-doped cuprate and iron-based high temperature superconductors, there is evidence of a nematic quantum critical point at a critical doping near the ``optimal doping'' at which the superconducting Tc is maximal. Thus motivated by experiments, but without pretense that the theory is directly applicable to these materials, we consider a low $T_c$ metallic superconductor weakly coupled to the soft fluctuations associated with proximity to a nematic quantum critical point. We show that: 1) a BCS-Eliashberg treatment remains valid outside of a parametrically narrow interval about the nematic quantum critical point; 2) the symmetry of the superconducting state (d-wave, s-wave, p-wave) is typically determined by the non-critical interactions, but Tc is enhanced by the nematic fluctuations in all channels; 3) in 2D, this enhancement grows rapidly upon approach to criticality up to the point at which the weak coupling approach breaks down, but in 3D the enhancement is much weaker. Finally, we note some consequences of the nematic-fluctuation-mediated pairing interaction, such as highly anisotropic gap functions and new collective modes. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L5.00008: Manifestation of nematic degrees of freedom in the Raman response function of iron pnictides Una Karahasanovic, Joerg Schmalian The electronic nematic phase in pnictides, characterized by the broken C4 symmetry, is believed to be generated by the presence of magnetic fluctuations associated with the striped phase, and occurs as a thin sliver in the phase diagram, above the magnetic transition temperature. Detecting the presence of nematic degrees of freedom in iron-based superconductors is a difficult task, since it involves measuring four spin correlation functions. We show that the nematic degrees of freedom manifest themselves in the experimentally measurable Raman response function, which is a density-density correlation weighted by an appropriate form factor. We calculate the Raman response function in the large N limit by considering Aslamazov-Larkin type of diagrams that contain series of inserted boxed-like diagrams that resemble the nematic coupling constant of the theory. These diagrams effectively account for collisions between spin fluctuations. We demonstrate that the Raman response function diverges at the structural phase transition. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L5.00009: Magnetic and orbital ordering in the iron-based superconductors: role of spin-orbit coupling Felix Ahn, Johannes Knolle, Rafael Fernandes, Ilya Eremin We analyze the magnetic ordering in the iron-based superconductors in presence of spin-orbit coupling. Based on several tight-binding parametrizations of the 3$d$ electron states we show how the spin-orbit coupling introduces the anisotropy of the magnetization of the striped antiferromagnetic state by lifting the degeneracy of all three components of the magnetization $m_x$, $m_y$ and $m_z$. The orientation of the magnetic moment is determined by the contribution of the $xy$, $xz$, and $yz$ orbitals to the electronic states near the Fermi level of the electron and hole bands and is determined by the electron filling. We find that within an itinerant approach the magnetic ordering is most favorable along the wavevector of the striped AF state. This appears to be a natural consequence of the spin-orbit coupling in the striped AF state where the ferro-orbital order of the $xz$ and $yz$ orbitals is only a consequence of the striped AF order. We further analyze the role of spin-orbit coupling for the C$_4$ magnetic structure where SDW order parameters with both wavevectors, $Q_x = (\pi,0)$ and $Q_y=(0,\pi)$, coexist. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L5.00010: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L5.00011: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L5.00012: Topological properties in Iron-Based Superconductors Jiangping Hu, Ningning Hao, X.X. Wu We show the existence of non-trivial topological properties in Iron-based superconductors. Several examples are provided, including (1) the single layer FeSe grown on SrTiO3 substrate, in which an topological insulator phase exists due to the band inversion at M point; (2) CaFeAs2, a staggered intercalation compound that integrates both quantum spin hall and superconductivity in which the nontrivial topology stems from the chain-like As layers away from FeAs layers; (3) the Fe(Te,Se) thin films in which the nontrivial Z2 topological invariance originates from the parity exchange at ? point that is controlled by the Te(Se) height. These results lay ground for integrating high Tc superconductivity with topological properties to realize new emergent phenomena, such as majorana particles, in iron-based high temperature superconductors. Reference: (1) NingNing Hao and Jiangping Hu, Topological phases in the Single Layer FeSe"; Phys. Rev. X 4, 031053 (2014). (2) X Wu, C Le, Y Liang, S Qin, H Fan and J. P. Hu Effect of As-chain layers in CaFeAs 2" Phys. Rev. B 89 205102 (2014) (3) X. Wu, S. Qin, Y. Liang, C. Le, H. Fan, and J. Hu, CaFeAs2: a Staggered Intercalation of Quantum Spin Hall and High Temperature Superconductivity," arXiv:1405.6401 (2014). [Preview Abstract] |
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