Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B14: Febased Superconductors  Multiorbital SuperconductivityFocus

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Sponsoring Units: DMP Chair: Rudolf Hackl, Walther Meissner Inst Room: LACC 304B 
Monday, March 5, 2018 11:15AM  11:51AM 
B14.00001: Visualizing Orbital Selective Mottness/Hundness and Cooper Pairing in FeSe Invited Speaker: J.C. DAVIS In Cubased high temperature superconducting (HTS) materials, the undoped phase proximate to the superconductor is a robust Mott insulator. By contrast, the undoped phase proximate to Febased HTS is never an insulator. But this distinction may be deceptive because, while only a single Cu dorbital is active in the former compounds, up to five Fe dorbitals are active in the latter. Theory has long predicted that such orbital multiplicity allows an unusual new correlated state, sometimes referred to as an Orbitalselective Mott phase or as a Hund's Metal phase, to appear. 
Monday, March 5, 2018 11:51AM  12:03PM 
B14.00002: Consequences of Orbital Selectivity for Magnetism and Superconductivity in Febased Superconductors Andreas Kreisel, Brian Andersen, Peter Hirschfeld Recently, it has been observed that electronic correlations in iron pnictides and chalcogenides affect electrons in different dorbitals quite differently. The resulting reduced coherence of the quasiparticle states has consequences for the normal state properties and affects the superconducting state. The renormalization of the d_{xy} orbital is known to be largest, thus its quasiparticle weight smallest. In the Fe based systems, this leads to a reduction of Néel type (π,π) magnetic fluctuations and makes stripe fluctuations relatively stronger. Within a modified spinfluctuation pairing theory, this makes the signchanging swave state more competitive. In this work, we investigate these effects of orbital selectivity with a focus on the FeSe system, which allows us to study the effect of nematicity due to the breaking of tetragonal symmetry without magnetic order at low temperatures. Consequences include different renormalization of the d_{yz} and d_{xz} orbital states, leading to an anisotropic superconducting order parameter and enhancements of the (π,0) magnetic fluctuations, an effect seen in neutron scattering experiments. The strongest effects of the reduced coherence are observed in the KFe_{2}As_{2}, system where we discuss implications for the superconducting order parameter. 
Monday, March 5, 2018 12:03PM  12:15PM 
B14.00003: To investigate the superconducting order parameters in iron based superconductors by quantum impurities HaiHu Wen One of the ongoing frontier studies on iron based superconductors is about the pairing mechanism. We have conducted extensive STM/STS studies on many different iron based superconductors and try to give a general understanding on the order parameters among different systems by using quantum impurities as the detector. We found clear evidence of the ingap quasiparticle states induced by the nonmagnetic Cu impurities in Na(Fe0.97xCo0.03Cux)As possessing both the electron and hole pockets, giving the finger prints evidence of the S+ pairing[1]. Concerning the iron chalcogenide superconductors without hole Fermi pockets, we have conducted systematic investigations. Our STS spectrum clearly indicates the presence of double superconducting gaps in (Li1xFex)OHFeSe phase. Further analysis based on QPI allows assign the gaps to different Fermi surfaces[2]. Finally use a phase related technique we show the sign reversal of order parameters on the Fermi surfaces without hole pocket[3]. 
Monday, March 5, 2018 12:15PM  12:27PM 
B14.00004: Orbital superconductivity, defects and pinned nematic fluctuations in the doped iron chalcogenide FeSe_{0.45}Te_{0.55.} Saheli Sarkar, John Van Dyke, Peter Sprau, Freek Massee, Ulrich Welp, WaiKwong Kwok, James Davis, Dirk Morr In this talk I will demonstrate that the differential conductance dI/dV, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide FeSe_{0.45}Te_{0.55}, posseses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two holelike Fermi surfaces, and a strongly anisotropic gap on the electronlike Fermi surface. Moreover, I will show that the pinning of nematic fluctuations by defects can five rise to a dumbbelllike spatial structure of the induced impurity bound states, and explains the related C_{2} symmetry in the Fourier transformed differential conductance. 
Monday, March 5, 2018 12:27PM  12:39PM 
B14.00005: L^{.}S pairing for the iron based superconductors Elio Koenig, Piers Coleman In this talk we propose L^{.}S pairing as a candidate superconducting state for the iron based superconductors within a three orbital model of t_{2g} electrons. This fully gapped state is spintriplet (S = 1), orbital antisymmetric (L = 1), and has total angular momentum J = 0 (it is swave). The basic ingredients to stabilize this phase are strong Hund's interaction and sizable spinorbit coupling. Crucially, details of the Fermi surface geometry are not important, by consequence the L^{.}S mechanism is universally applicable to all known pnictide and chalcogenide compounds. While early proposals for spin triplet pairing were dismissed due to the observation of clear Knight shift data, we show that the L^{.}S state does explain the Knight shift. Furthermore, the L^{.}S mechanism may be explored both within the weak coupling, itinerant and the strong coupling, atomistic pictures. It therefore has the potential to unify the success of both theoretical scenarios and is suitable for different compounds with dissimilar degree of correlations. 
Monday, March 5, 2018 12:39PM  12:51PM 
B14.00006: Quantum Monte Carlo study of pairing in systems with incipient bands Thomas Maier, Vivek Mishra, Douglas Scalapino The nature and mechanism of superconductivity in the extremely electrondoped ironchalcogenides continues to be a matter of wide debate. In these systems, the holelike band has dropped below the Fermi energy, challenging the leading spinfluctuation based theory of pairing that relies on the presence and nesting properties of both electron and holeFermi surface pockets. Here, we use dynamic cluster quantum Monte Carlo calculations of a bilayer Hubbard model to gain insight into this problem. These calculations show that s^{+} pairing remains robust and dominant over dwave pairing, even when the hole band does not cross the Fermi energy. The gap on the submerged band has the opposite sign and larger magnitude than the gap on the electron band. Consistent with previous phenomenological calculations, this demonstrates that an incipient band can have a decisive effect on the pairing symmetry of the Fermi surface states. 
Monday, March 5, 2018 12:51PM  1:03PM 
B14.00007: Anisotropic pairing gap in FeSe Jian Kang, Debanjan Chowdhury, Andrey Chubukov, Rafael Fernandes Superconductivity in FeSe occurs deep inside the nematic phase, but in the absence of any form of magnetic order. As a result, FeSe provides a unique opportunity to investigate pairing in an electronic nematic state, which reconstructs the lowenergy band dispersions. We solved the pairing problem within the lowenergy model of FeSe, which includes the orbital content of lowenergy excitations. We found deep minima of a superconducting gap on the distorted hole pocket. The location of the minima depends on two features: the relative magnitude of the intraorbital and interorbital pairing interactions, and the relative sign of the nematic order parameters on hole and electron pockets. We also discuss a possibility that superconducting order evolves below T_{c} and breaks timereversal symmetry below a certain T<T_{c}. We compare our results with recent experiments in stoichiometric FeSe and Sdoped FeSe. 
Monday, March 5, 2018 1:03PM  1:15PM 
B14.00008: Nematicity and superconductivity in FeSe_{1x}S_{x} Youichi Yamakawa, Hiroshi Kontani The rich variety of the phase diagrams in FeSe systems demonstrates the strong interplay between the nematicity, magnetism, and superconductivity in Febased superconductors. Here, we investigate the electronic state in FeSe_{1x}S_{x} focusing on the relation between the nematicity and superconductivity based on a multiorbital Hubbard model by including both the spinorbit interaction and the higherorder manybody effects called the vertex correction. We find that, in the tetragonal phase, the strong ferroorbital fluctuations on xz and yz orbitals develop equally, and its divergence corresponds to the nematic transition. In the orthorhombic phase with nematicity, however, the orbital fluctuation on the yz orbital becomes dominant even if the induced orbital polarization is small, and the superconducting gap becomes large on the FSs with large yzorbital component. The obtained abrupt change of the gap structure is consistent with the resent specific heat and thermal conductivity measurements on FeSe_{1x}S_{x}. We also investigate the development of the orbital susceptibilities for FeSe_{1x}S_{x}, and discuss the relation between the share modulus C_{66} and the Raman nematic susceptibility in detail. 
Monday, March 5, 2018 1:15PM  1:27PM 
B14.00009: Quantum Effects in the Highmagneticfield Superconductivity of a Quasitwodimensional Multipleband Superconductor Kok Wee Song, Alexei Koshelev We investigate the superconducting transition of a twoband layered system with deep and shallow bands in the presence of the outofplane magnetic field. As the shallow band’s Fermi energy and cyclotron frequency may be comparable, the Landau quantization has to be accounted for exactly. The interlayer tunneling lifts the Landau levels to dispersive bands along the outofplane momentum with two van Hove singularities at the zone center and the boundary. The pairing susceptibility has strong infrared divergence when either the same or opposite van Hove points simultaneously cross the Fermi level. The latter case yields the alternating FuldeFerrellLarkinOvchinnikov (FFLO) state with sign change of the order parameter between the neighboring layers. This divergence may lead to the reentrant highfield superconductivity. In a general situation, when the Fermi level crosses the spinup and spindown bands, the FFLO states with a longer fielddependent modulating period are formed, which also may be reentrant. 
Monday, March 5, 2018 1:27PM  1:39PM 
B14.00010: Evolution of pairing interactions in Febased superconductors Daniel Jost, Thomas Boehm, Andreas Baum, Florian Kretzschmar, Bernhard Muschler, Rudolf Hackl The superconducting properties of the ferropnictides and chalcogenides vary substantially across the families. In particular one finds almost clean as well as strongly anisotropic energy gaps suggesting that small changes in the electronic structure entail strong variations in the pairing. Raman spectroscopy offers a window into the structure of the energy gaps and, in addition, unveils fingerprints of subleading pairing interactions. We studied the energy gaps in various families including Ba(Fe_{1x}Co_{x})_{2}As_{2} and Ba_{1x}K_{x}Fe_{2}As_{2} and found values similar to those obtained by other methods. In materials with clean gaps such as Ba_{1x}K_{x}Fe_{2}As_{2} solely light scattering allows the observation of collective ingap modes which, for their line shapes, temperature and doping dependences, and evolution of spectral weights, can be identified as BardasisSchrieffer excitons. The binding energies of these modes encode the relative coupling strengths in the subleading pairing channels and show along with theoretical RPA and fRG calculations that spin fluctuations contribute substantially to Cooper pairing. So far it is open whether this conclusion can be generalized to all Febased systems. 
Monday, March 5, 2018 1:39PM  1:51PM 
B14.00011: Discrete Superconducting Phases in FeSederived Superconductors Tianping Ying, Minxiang Wang, Ziyi Zhao, ZhiZhen Zhang, Xiaoyu Jia, Yingcheng Li, Bin Lei, Qing Li, Yunjie Yu, Erjian Cheng, zhenghua An, Yuanbo Zhang, Wei Yang, Xianhui Chen, Shiyan Li A general feature of unconventional superconductors is the existence of a superconducting dome in the phase diagram, i.e., the continuous evolution of the transition temperature T_{c} with tuning parameters such as doping and pressure. For the simplest ironbased superconductor FeSe (T_{c} > 8 K), its T_{c} can be greatly enhanced by doping electrons via many ways, even up to 65 K in monolayer FeSe/SrTiO_{3} with the help of phonons. However, it remains controversial whether there exists a superconducting dome in these electrondoped FeSederived superconductors. Here, we report the observation of a series of discrete superconducting phases in FeSe thin flakes by finely tuning the intercalation of Li and Na ions with a solid ion gating technique. Such discrete superconducting phases contrast sharply to the superconducting dome in most unconventional superconductors. Our results demonstrate that the existence of discrete superconducting phases is intrinsic and universal in FeSe intercalated by alkali/alkaline earth/rear earth metals. It likely comes from the spontaneous phase separation of these ions between FeSe layers. 
Monday, March 5, 2018 1:51PM  2:03PM 
B14.00012: Winding numbers of nodal points in Febased superconductors Dmitry Chichinadze, Andrey Chubukov In this work, we analyze nodal points in multiorbital Febased superconductors from the point of view of topology. We consider the case of $s^{+}$ gap structure, with accidental nodes, and dwave gap with nodes along symmetry directions. In both cases, the nodes are not monolithic and can be moved by varying an external parameter, e.g., a degree of interpocket pairing. Eventually, the nodes merge and annihilate and via Lifshitztype transition. This happens in a multiorbital model even if the gap symmetry is dwave. We discuss Lifshitz transition in Febased superconductors from topological point of view. We show, both analytically and numerically, that in both swave and dwave cases, the merging nodal points have winding numbers of opposite signs. This is consistent with a general reasoning that the total winding number is a conserved quantity in the Lifshitz transition. 
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