Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L14: Febased Superconductors  TheoryFocus

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Sponsoring Units: DMP Chair: Wei Ku, Shanghai Jiao Tong Univ Room: LACC 304B 
Wednesday, March 7, 2018 11:15AM  11:51AM 
L14.00001: New Directions in Theoretical Studies of Ironbased Superconductors Invited Speaker: Adriana Moreo The discovery of high T_{c} superconductivity in ironbased pnictides and chalcogenides brought to the forefront the need to develop efficient theoretical procedures to treat multiorbital models of interacting electrons. Among the many challenges, we need to clarify the role that the orbital degree of freedom plays in pairing and how its interaction with magnetic and lattice degrees of freedom leads to the stabilization of exotic phases such as the nematic state. Theoretical studies in the strong and weak coupling limits cannot address the physically relevant intermediate regime, with a mixture of itinerant and localized degrees of freedom. Traditional numerical methods, such as Lanczos or quantum Monte Carlo, have either a too rapidly growing Hilbert space with increasing size or sign problems. For this reason, it is necessary to develop new models and techniques, and also better focus on systems where both experiments and accurate theory can be used in combination to reach a real understanding of iron pairing tendencies. Examples of recent advances along these directions that will be discussed in this talk include: i) The development of spinfermion models [1] that allow studies in the difficult nematic regime with a finite shortrange antiferromagnetic correlation length above the ordering critical temperatures. This type of studies also allow the inclussion of doping, quenched disorder, and the study of transport and realfrequency responses; ii) The application of the Density Matrix Renormalization Group approach to multiorbital Hubbard models in chain and ladder structures [2] triggered by the discovery of superconductivity at high pressure in ladder ironbased compounds such as BaFe_{2}S_{3} and BaFe_{2}Se_{3}. In this context, the recently reported [2] pairing tendencies unveiled at intermediate Hubbard U will be discussed. 
Wednesday, March 7, 2018 11:51AM  12:03PM 
L14.00002: Origin of Enhancement of Nematicity Due to Superconductivity in FeSe_{1x}S_{x} Xiao Chen, Saurabh Maiti, Rafael Fernandes, Peter Hirschfeld The origin of both superconductivity and nematicity in bulk FeSe is currently under debate. Important clues can be obtained by investigating the interplay between these two orders. Recent thermal expansion measurements on pure and Sdoped bulk FeSe have found that in contrast to other ironbased systems, the nematic order parameter is enhanced due to the onset of superconductivity, rather than suppressed as in the canonical Ba122 system. Here we study a model where electronic nematicity is described by a dwave Pomeranchuk instability, and superconductivity in the nematic phase is assumed to arise from an anisotropic pairing interaction in the orthorhombic phase. Within meanfield theory, we discuss the question of under which circumstances enhancement of nematic order below the superconducting transition is possible. 
Wednesday, March 7, 2018 12:03PM  12:15PM 
L14.00003: Effect of Nematicity In FeSe On Fermionic Excitations RuiQi Xing, Laura Classen, Andrey Chubukov In this work we study the effect of nematicity in FeSe on the temperature variation of the energies of fermions on $d_xz/d_yz$ and $d_{xy}$ orbitals at the M point ($=(\pi,\pi)$ in 2Fe zone). In the absence of nematicity, the energies of $d_{xz}$ and $d_{yz}$ orbitals are degenerate at $M$ even in the presence of spinorbit coupling, and $d_{xy}$ excitation is also doubly degenerate. A nematic order at $T < T_s$ should split these degeneracies. Recent ARPES experiments demonstrated that, besides splitting, nematic order causes a prominent change of the temperature dependence of excitations on $d_{xz}/d_{yz}$ orbitals, whose energy was increasing by magnitude with decreasing $T$ at $T > T_s$, but changes trend and decreases below $T_s$, while d_xy orbital does not exhibit such a behavior (A. Fedorov et al., Scientific Reports 6, 36834(2016); A. Coldea et al., arXiv:1706.00338(2017)). We attribute this phenomenon to the influence of $B_{1g}$ orbital order at the hole pockets on the dispersion at $M$ via the selfenergy effect. We argue that the selfenergy predominantly affects d_xz/d_yz orbitals at M because hole Fermi pockets are made of the same orbitals. 
Wednesday, March 7, 2018 12:15PM  12:27PM 
L14.00004: Theory for Hidden Spin Resonance versus S^{+− } Superconductivity in Heavily ElectronDoped FeSe Jose Rodriguez We describe heavily electrondoped FeSe by a Hubbard model over a square lattice of iron atoms with only d+ = d_{xz} + i d_{yz} and d− = d_{xz} − i d_{yz} degenerate orbitals. Nearest neighbor and nextnearest neighbor hopping parameters are chosen so that perfect nesting exists between an electrontype Fermi surface and a holetype Fermi surface at the center and at the corner of the oneiron Brillouin zone, respectively. The latter results in an instability to a hidden spindensity wave state that exhibits two spin1 Goldstone modes at wavenumber Q_{AF}=(π/a,π/a). The spectrum of magnetic excitations is obtained by a calculation of the dynamic spin susceptibility within the randomphase approximation (RPA). These results are compared to the ring of lowenergy magnetic excitations observed in heavily electrondoped FeSe around Q_{AF} by inelastic neutron scattering [1]. The nature of Cooper pairs that result from the virtual exchange of magnetic excitations predicted by RPA is also determined. It will be compared to exact calculations that find evidence for S^{+−} Cooper pairing in the limit of strong onsite Coulomb repulsion[2]. 
Wednesday, March 7, 2018 12:27PM  12:39PM 
L14.00005: Fermi Surface Pockets in Heavily ElectronDoped FeSe by Hidden Magnetic Order Ronald Melendrez, Jose Rodriguez We study the Hubbard model over a square lattice of iron atoms, each containing only d+ = d_{xz} + i d_{yz} and d− = d_{xz} − i d_{yz} degenerate orbitals. Superexchange interactions via the chalcogenide atoms are also added. Nearest neighbor (1) and nextnearest neighbor (2) hopping parameters are chosen so that perfect nesting exists between an electrontype and a holetype Fermi surface at the center and at the corner of the oneiron Brillouin zone, respectively. A hidden spindensity wave (hSDW) instability exists for superexchange coupling constants J_{2} > 0.5 J_{1}. Quasiparticle excitation energies disperse along Dirac cones at the intersections of the Fermi surfaces with the principal axes. The hSDW state also exhibits two spin1 Goldstone modes at wavenumber Q_{AF}=(π/a,π/a). Virtual emission/absorption of the latter by hSDW quasiparticles results in a migration of the Dirac cones towards the corner of the twoiron Brillouin zone. This agrees with Schwingerbosonslavefermion mean field theory and exact calculations at the limit of strong onsite Coulomb repulsion, which find electrontype Fermi surface pockets at the corner of the twoiron Brillouin zone[1]. 
Wednesday, March 7, 2018 12:39PM  12:51PM 
L14.00006: Dynamical susceptibility near a longwavelength critical point with a nonconserved order parameter Avraham Klein, Samuel Lederer, Debanjan Chowdhury, Erez Berg, Andrey Chubukov Many strongly correlated electronic systems evince longwavelength phase transitions, such as nematic transitions. The dynamical behavior near this transition strongly depends on conservation laws. If the system's order parameter is conjugate to a conserved quantity, such as charge, then fluctuations must vanish at the long wavelength limit, i.e. the polarization Π(q=0,Ω ≠0)=0 identically. This is because a conserved quantity such as total charge cannot fluctuate in time. In contrast, when the order parameter is not conserved, there can by strong fluctuations even at long wavelengths, so Π(q=0,Ω) is nonzero. 
Wednesday, March 7, 2018 12:51PM  1:03PM 
L14.00007: Symmetry classification and phenomenology of paired states in iron based superconductors Oskar Vafek, Paul Eugenio We use the symmetry adapted low energy theory to classify the superconducting states in iron based materials with only hole, or only electron, pockets. In addition, the low energy effective theory is used to study pairing mechanisms which would stabilize the various states, as well as their phenomenology. The combined role of Hund's coupling and spinorbit interactions will be highlighted as a possible way to overcome the detrimental effects of intraorbital repulsion in achieving Cooper pairing. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L14.00008: OrbitalSelective Pairing and Nematic Order in the IronBased Superconductors Haoyu Hu, Emilian Nica, Rong Yu, Qimiao Si The considerations of the orbitalselective Mott phase in the normal state [1,2] motivated the theoretical proposal for orbitalselective pairing [3]. The experimental evidence for the latter has come from both the iron pnictides [4] and iron selenides [5]. Motivated by the recent experiments, we theoretically study the role of the nematic order in the orbitalselective pairing within multiorbital models for superconductivity driven by shortrange magnetic exchange interactions. We consider several ways that characterize the nematicity in the Hamiltonian, and present systematic results on how the pairingstate competition [6], the orbital dependency of the intraorbital pairing amplitudes, the pairing symmetry and the gap anisotropy depend on the nematicity. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L14.00009: Intrinsic interfacial monolayers and their effect on the hightemperature superconductor FeSe / SrTiO_{3} Hunter Sims, Donovan Leonard, Axiel Birenbaum, Zhuozhi Ge, Lian Li, Valentino Cooper, Matthew Chisholm, Sokrates Pantelides It was recently demonstrated that monolayer FeSe on SrTiO_{3 }is a superconductor with T_{c} between 60 and 100 K, compared to 8 K in bulk FeSe. In contrast, T_{c} has been measured to be only 3.7 K in bilayer FeSe deposited on graphene (extrapolated to about 2 K in a monolayer), pointing to the major role of the interface in enhancing superconductivity. Here we determine the atomic structure of an interfacial layer and identify its role in driving the increase in T_{c} using a combination of quantum mechanical calculations and scanning transmission electron microscopy. Within our DFT calculations, this interfacial layer hosts magnetic and orbital order not found in the typical TiO_{2} surface termination nor in other surface reconstructions. Interactions between this interfacial monolayer and FeSe generate symmetrybreaking distortions in the film that are favorable for increasing T_{c} and are not present in other possible FeSe / STO interface structures. We propose that this may provide a path forward toward the design and enhancement of other twodimensional superconductors. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L14.00010: Magnetic Fluctuations in SingleLayer FeSe Tatsuya Shishidou, Michael Weinert, Daniel Agterberg The highT_{C} superconductivity in monolayer FeSe films is revisited from the viewpoint of magnetic fluctuations. Using densityfunctional calculations, we derive the spinspiral energy dispersion E(q) – including the energies of the checkerboard (CB), collinear stripe (CL), and staggereddimer antiferromagnetic (AFM) configurations – and map it onto the S=1 squarelattice spin model. We find that (i) there is a plateau in E(q) extending around the CB state, (ii) the q states in the plateau share very similar electronic band structure, (iii) Heisenberg exchange interactions are in the frustrated regime J_{2}/J_{1}~0.5, and (iv) both biquadratic (K) and fourspin (F) exchange interactions are important: K~F~0.2 J_{1}. These results reveal that both quantum and entropic (thermal) effects favor the CBAFM type fluctuations and naturally account for the CBlike features observed in the normalstate electronic structure[1] and the fullygapped yet anisotropic superconducting order parameter[2]. 
Wednesday, March 7, 2018 1:39PM  1:51PM 
L14.00011: Phonon linewidth due to electronphonon interactions with strong forward scattering in FeSe thin films on oxide substrates Yan Wang, Louk Rademaker, Elbio Dagotto, Steven Johnston The discovery of an enhanced superconducting transition temperature T_{c} in monolayers of FeSe grown on several oxide substrates has opened a different route to highT_{c} superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electronphonon interaction across the interface that is peaked at small momentum transfers. Here, we examine the implications of such a coupling on the phononic properties of the system. We find that while a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge instabilities at longwavelengths, the inclusion of Coulombic screening significantly reduces the phonon broadening, thus explaining the lack of anomalously broad phonon linewidths in the FeSe interface systems as shown by experiments. We also show in what parameter range and conditions, the T_{c} enhancement and the replica band feature in the electron spectral function can persist with the screened electronphonon coupling in the forward scattering direction. 
Wednesday, March 7, 2018 1:51PM  2:03PM 
L14.00012: Collective modes and shorttime dynamics of the timereversal symmety
broken s+is superconductor Marvin Mueller, Pengtao Shen, Maxim Dzero, Ilya Eremin Motivated by the recent observation of the timereversal symmetry broken 
Wednesday, March 7, 2018 2:03PM  2:15PM 
L14.00013: Theory of fullygapped s++ wave states in BaFe2(As,P)2 and LiFeAs: Analysis beyond the MigdalEliashberg formalism Hironori Nakaoka, Youichi Yamakawa, Hiroshi Kontani Both the spin fluctuation mediated S+ wave state and the orbital fluctuation mediated S++ wave state have been studied in various Febased superconductors. In BaFe2(As,P)2, the spin fluctuation theory predicts that the horizontal node appears on the hole like Fermi surface composed of z2 orbital [1]. In contrast, fullygapped s++ wave state is obtained based on the orbital fluctuation theory [2], by introducing the phenomenological quadrupole interaction. To confirm the validity of this phenomenological approach, we analyze the twodimensional Hubbard model for BaFe2(As,P)2, by focusing on the vertex corrections dropped in conventional MigdalEliashberg gap equation. We find that strong interorbital attractive pairing interaction among z2, xz, yz, xy orbitals is caused by the orbital fluctuations, due to the quadrupole interaction given by the AslamazovLarkin vertex correction. Therefore, the fullygapped s++ wave state is naturally obtained, consistently with Ref. [2]. In addition, we apply the same method to LiFeAs, and succeed in reproducing the experimental gap structure in terms of the orbital fluctuation mechanism. [1] K. Suzuki et al, JPSJ 80,013710 (2011). [2] T. Saito et al, PRB 88, 045115 (2013) 
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