Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E14: Febased Superconductors  Electron Correlation and Orbital SelectivityFocus

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Sponsoring Units: DMP Chair: Adriana Moreo, Univ of Tennessee, Knoxville Room: LACC 304B 
Tuesday, March 6, 2018 8:00AM  8:36AM 
E14.00001: Theory of electron correlation and orbital selectivity in Febased superconductors Invited Speaker: Rong Yu It has been recognized that the electron correlation plays a crucial role in understanding the properties of the ironbased superconductors [1]. In this talk I will review recent theoretical progresses on the study of the electron correlations in ironbased superconductors [25]. I show that not only are the ironbased superconductors close to a Mott localization, but the multiorbital nature of these systems can accommodate a novel orbitalselective Mott phase, in which some orbital is Mott localized while the others are still itinerant. The orbitalselective Mott phase anchors the understanding of the strong orbital selectivity in the ironbased superconductors, which has been extensively studied both experimentally and theoretically [5,6]. Using a U(1) slavespin theory [2,7], I discuss the materialsspecific aspects of the orbitalselective electron correlation in several iron selenides and iron pnictides and, furthermore, address its connection to the nematicity in these systems. 
Tuesday, March 6, 2018 8:36AM  8:48AM 
E14.00002: Hund’s Induced FermiLiquid Instabilities and Enhanced Quasiparticle Interactions Luca De Medici Hund’s coupling is shown to generally favor, in a doped halffilled Mott insulator, an increase in the compressibility culminating in a Fermiliquid instability towards phase separation. The largest effect is found near the frontier between an ordinary and an orbitally decoupled (“Hund’s”) metal. The increased compressibility implies an enhancement of quasiparticle scattering, thus favoring other possible symmetry breakings. This physics is shown to happen in simulations of the 122 Febased superconductors, possibly implying the relevance of this mechanism in the enhancement of the critical temperature for superconductivity. 
Tuesday, March 6, 2018 8:48AM  9:00AM 
E14.00003: Hunddriven enhancement of the electronic compressibility in FeSe Pablo Villar Arribi, Luca De Medici We compute the compressibility of the conduction electrons in both bulk orthorhombic FeSe and monolayer FeSe on SrTiO_{3} substrate, including dynamical electronic correlations within slavespin meanfield + density functional theory. Results show a zone of enhancement of the electronic compressibility crossing the interactiondoping phase diagram of these compounds in accord with previous simulations on iron pnictides and in general with the phenomenology of Hund's metals. 
Tuesday, March 6, 2018 9:00AM  9:12AM 
E14.00004: Orbital Selectivity from high and lowenergy scales: the key feature of Ironbased supercondutors physics Laura Fanfarillo, Lara Benfatto, Elena Bascones, Belen Valenzuela, Massimo Capone Unconventional superconductivity is found in correlated materials as a low temperature bridge between phases dominated by high and lowenergy scale of electronic interactions (e.g. Mott physics vs Fermi Liquid regime). The understanding of the nature and strength of correlations is key to unveil the nature of the pairing itself and its role as competitive/cooperative order with other phases. 
Tuesday, March 6, 2018 9:12AM  9:24AM 
E14.00005: SpinOrbit Coupling and Magnetic Anisotropy in IronBased Superconductors Brian Andersen, DANIEL SCHERER We determine theoretically the effect of spinorbit coupling on the magnetic excitation spectrum of itinerant multiorbital systems, with specific application to ironbased superconductors. Our microscopic model includes a realistic tenband kinetic Hamiltonian, atomic spinorbit coupling, and multiorbital Hubbard interactions. Our results are in excellent agreement with a large body of experiemnts, and highlight the remarkable variability of the resulting magnetic anisotropy despite constant spinorbit coupling. At the same time, the magnetic anisotropic exhibits robust universal behavior upon changes in the bandstructure corresponding to different materials of ironbased superconductors. A natural explanation of the observed universality emerges when considering optimal nesting as a resonance phenomenon. Our study should be of relevance to other itinerant system with spinorbit coupling and nesting tendencies in the bandstructure. 
Tuesday, March 6, 2018 9:24AM  9:36AM 
E14.00006: Orbitally Resolved Quasiparticle Weight Renormalization Factors in Febased Superconductors. Shinibali Bhattacharyya, KRISTOFER BJORNSON, Andreas Kreisel, MARIA Chatzieleftheriou, DANIEL SCHERER, BRIAN ANDERSEN, Peter Hirschfeld We study the quasiparticle weight renormalization via spin and charge fluctuations in Febased pnictides within a multiorbital tightbinding model with onsite interactions treated in weak coupling theory. The leading contribution to the quasiparticle scattering is calculated from the secondorder selfenergy diagram with the polarization operator calculated in the randomphase approximation. We find oneparticle renormalization factors for orbital weights on each Fermi sheet, from the first order frequency derivative of the real part of the dynamic selfenergy at the Fermi level. The orbitally resolved renormalization factor Z modifies the spinfluctuation pairing through suppression of pair scattering processes in the corresponding orbital channel, resulting in orbitalselective Cooper pairing of electrons. We use the modified spinfluctuation pairing interaction to compare our results with experimentally observed anisotropic gap structures of certain Fepnictides, to test the validity of current phenomenological theories of orbitally selective spin fluctuation pairing. 
Tuesday, March 6, 2018 9:36AM  9:48AM 
E14.00007: Electronic correlations in the quasione dimensional iron superconductor BaFe_{2}S_{3} Elena Bascones, José María Pizarro In 2015 superconductivity with Tc~24 K was discovered in BaFe_{2}S_{3} [1]. Superconductivity appears under pressure, when a stripe antiferromagnetic phase is suppressed. Contrary to what happens in other 2D materials BaFe_{2}S_{3} is not a layered material but it contains quasione dimensional Fe_{2}S_{3} ladders and the antiferromagnetic state is insulating. BaFe_{2}S_{3} has been claimed to be a Mott insulator. By using slavespin methods we have analyzed the correlation strength of BaFe_{2}S_{3. }We find that at zero pressure BaFe_{2}S_{3 }is very strongly correlated. Some of the orbitals can be considered to be localized while others remain metallic. At pressures at which the superconductivity appears the correlations have been reduced to values similar to those in iron superconductors, close to the Hund metal crossover, but still shows important orbital differentiation. Our work emphasizes the role of intermediate correlations in the appearance of highTc superconductivity. 
Tuesday, March 6, 2018 9:48AM  10:00AM 
E14.00008: Theoretical study on the ironbased ladder: metalinsulator and antiferromagnetic transitions Yang Zhang, LingFang lin, JunJie Zhang, Elbio Dagotto, Shuai Dong The recent discovery of superconductivity in BaFe_{2}X_{3}_{ }(X=S/Se) under high pressure has stimulated researchers’ enthusiasm for the study of 123type iron chalcogenides. These materials own quasionedimensional twoleg ladders, which is structurally and thus physically different from previously studied ironbased superconductors with twodimensional iron sheets. For the Sbased case, our firstprinciples calculations show that the lattice constants as well as local magnetic moments are gradually suppressed with increasing pressure, followed by a firstorder magnetic transition at a critical pressure^{[}^{1}^{]}. The selfdoping effect, namely the electrons transfer from S to Fe, may play a key role in this transition^{[}^{1}^{]}. Although the superconducting dome has also been reported in the Sebased case, our calculations on BaFe_{2}Se_{3} have unveiled several qualitative differences from BaFe_{2}S_{3}. Sequential transitions, including structural, electronic, and magnetic transitions, are found with increasing pressure^{[}^{2}^{]}. 
Tuesday, March 6, 2018 10:00AM  10:12AM 
E14.00009: Spin dynamics within the block orbitalselective Mott phase in one dimensional iron chains and ladders Jacek Herbrych, Nitin Kaushal, Alberto Nocera, Gonzalo Alvarez, Adriana Moreo, Elbio Dagotto Ironbased superconductors display a variety of magnetic phases originating in the competition between electronic, orbital, and spin degrees of freedom. Theoretical investigations [1] of the 1D multiorbital Hubbard model revealed the existence of an orbitalselective Mott phase (OSMP) with block spin order, i.e., antiferromagnetically coupled ferromagnetic (FM) spin islands. Recent inelastic neutron scattering experiments on quasi1D BaFe_{2}Se_{3} and doped RbFe_{2}Se_{3} compounds confirm the relevance of the spinblock phases [24]. Moreover, the spectrum unveiled exotic features in the dynamical spin structure factor S(q,ω) including a lowenergy acoustic mode and a highenergy optical mode. In our work [5] we present the first theoretical study of the S(q,ω) within the blockOSMP using the DMRG method. In agreement with experimental results we find: a dispersive (acoustic) mode for momentum q<π/2, which arises from the dynamics of FM islands, and a dispersionless (optical) mode for q>π/2 attributed to local block Hund excitations. 
Tuesday, March 6, 2018 10:12AM  10:24AM 
E14.00010: Proliferation of Competing Magnetic Orders in Iron Pnictides from the Interplay of Quantum Fluctuations and SpinOrbit Coupling Morten Holm Christensen, Peter Orth, Brian Andersen, Rafael Fernandes The magnetic phase diagram of the iron pnictides has been the subject of extensive studies in recent years. Experiments on a number of different compounds have revealed the emergence of several distinct magnetic orders as the putative quantum critical point is approached. Here we demonstrate that such a proliferation of magnetic orders can be naturally explained as a consequence of the interplay between strong quantum fluctuations and spinorbit coupling (SOC), observed to be sizable in the pnictides. A finite SOC results in spin anisotropy which, at the meanfield level, leads to the appearance of new phases by allowing admixtures of single and doubleQ phases. Beyond meanfield we employ a renormalization group (RG) approach for the quantum phase transition and show that the RG flow of the spinanisotropic system is fundamentally different than the isotropic one. While the isotropic system only displays fixed trajectories resulting in firstorder transitions, the anisotropic case features an additional stable Gaussian fixed point. This indicates an enhanced magnetic degeneracy near the quantum phase transition. Such a scenario can naturally account for the fact that several types of magnetic order appear in close proximity near optimal doping in the experimental phase diagram. 
Tuesday, March 6, 2018 10:24AM  10:36AM 
E14.00011: Resilient nodeless dwave superconductivity in monolayer FeSe Daniel Agterberg, Tatsuya Shishidou, Joseph O'Halloran, Philip Brydon, Michael Weinert Monolayer FeSe exhibits the highest transition temperature among the iron based superconductors and appears to be fully gapped, seemingly consistent with s wave superconductivity. Here, we develop a theory for the superconductivity based on coupling to fluctuations of checkerboard magnetic order (which has the same translation symmetry as the lattice) [1]. The electronic states are described by a symmetry based kplike theory and naturally account for the states observed by angle resolved photoemission spectroscopy. We show that a prediction of this theory is that the resultant superconducting state is a fully gapped, nodeless, d wave state. This state, which would usually have nodes, stays nodeless because, as seen experimentally, the relevant spinorbit coupling term has an energy scale smaller than the superconducting gap [2]. 
Tuesday, March 6, 2018 10:36AM  10:48AM 
E14.00012: Role of the chargetransfer induced electric field in potassiumdosed FeSe layers Young Woo Choi, Hyoung Joon Choi Potassiumdosed FeSe layers are investigated using the density functional theory combined with the dynamical meanfield theory. We show that K dosing induces the charge transfer from K atoms to the topmost FeSe layer, and subsequently, ionized K atoms generate a strong local electric field. Role of this chargetransfer induced electric field is discussed with emphasis on its impacts on the electronic structure and electron correlation among Fe 3d orbitals. By controlling the concentration of K atoms, we systematically investigate the evolution of the electronic structure of both FeSe mono and bilayers. Notably, K dosing reduces bandwidths of the Fe 3d bands near the Fermi level and significantly enhances electron correlation. We also discuss the structural changes of FeSe layers due to K dosing. Our results illustrate that charge transfer from external agents in electrondoped FeSe systems can have nontrivial effects other than electron doping and account for their enhanced electron correlation. 
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