Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B14: Fe-based 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 Cu-based high temperature superconducting (HTS) materials, the undoped phase proximate to the superconductor is a robust Mott insulator. By contrast, the undoped phase proximate to Fe-based HTS is never an insulator. But this distinction may be deceptive because, while only a single Cu d-orbital is active in the former compounds, up to five Fe d-orbitals are active in the latter. Theory has long predicted that such orbital multiplicity allows an unusual new correlated state, sometimes referred to as an Orbital-selective 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 Fe-based Superconductors Andreas Kreisel, Brian Andersen, Peter Hirschfeld Recently, it has been observed that electronic correlations in iron pnictides and chalcogenides affect electrons in different d-orbitals 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 dxy 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 spin-fluctuation pairing theory, this makes the sign-changing s-wave 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 dyz and dxz 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 KFe2As2, 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 Hai-Hu Wen One of the on-going 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 in-gap quasi-particle states induced by the non-magnetic Cu impurities in Na(Fe0.97-xCo0.03Cux)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 (Li1-xFex)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 FeSe0.45Te0.55. Saheli Sarkar, John Van Dyke, Peter Sprau, Freek Massee, Ulrich Welp, Wai-Kwong 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 FeSe0.45Te0.55, 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 hole-like Fermi surfaces, and a strongly anisotropic gap on the electron-like Fermi surface. Moreover, I will show that the pinning of nematic fluctuations by defects can five rise to a dumbbell-like spatial structure of the induced impurity bound states, and explains the related C2 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 t2g electrons. This fully gapped state is spin-triplet (S = 1), orbital antisymmetric (L = 1), and has total angular momentum J = 0 (it is s-wave). The basic ingredients to stabilize this phase are strong Hund's interaction and sizable spin-orbit 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 electron-doped iron-chalcogenides continues to be a matter of wide debate. In these systems, the hole-like band has dropped below the Fermi energy, challenging the leading spin-fluctuation based theory of pairing that relies on the presence and nesting properties of both electron- and hole-Fermi 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 d-wave 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 low-energy band dispersions. We solved the pairing problem within the low-energy model of FeSe, which includes the orbital content of low-energy 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 intra-orbital and inter-orbital 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 Tc and breaks time-reversal symmetry below a certain T<Tc. We compare our results with recent experiments in stoichiometric FeSe and S-doped FeSe. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B14.00008: Nematicity and superconductivity in FeSe1-xSx 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 Fe-based superconductors. Here, we investigate the electronic state in FeSe1-xSx focusing on the relation between the nematicity and superconductivity based on a multi-orbital Hubbard model by including both the spin-orbit interaction and the higher-order many-body effects called the vertex correction. We find that, in the tetragonal phase, the strong ferro-orbital 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 yz-orbital component. The obtained abrupt change of the gap structure is consistent with the resent specific heat and thermal conductivity measurements on FeSe1-xSx. We also investigate the development of the orbital susceptibilities for FeSe1-xSx, and discuss the relation between the share modulus C66 and the Raman nematic susceptibility in detail. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B14.00009: Quantum Effects in the High-magnetic-field Superconductivity of a Quasi-two-dimensional Multiple-band Superconductor Kok Wee Song, Alexei Koshelev We investigate the superconducting transition of a two-band layered system with deep and shallow bands in the presence of the out-of-plane 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 out-of-plane 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 Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with sign change of the order parameter between the neighboring layers. This divergence may lead to the reentrant high-field superconductivity. In a general situation, when the Fermi level crosses the spin-up and spin-down bands, the FFLO states with a longer field-dependent modulating period are formed, which also may be reentrant. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B14.00010: Evolution of pairing interactions in Fe-based superconductors Daniel Jost, Thomas Boehm, Andreas Baum, Florian Kretzschmar, Bernhard Muschler, Rudolf Hackl The superconducting properties of the ferro-pnictides 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 sub-leading pairing interactions. We studied the energy gaps in various families including Ba(Fe1-xCox)2As2 and Ba1-xKxFe2As2 and found values similar to those obtained by other methods. In materials with clean gaps such as Ba1-xKxFe2As2 solely light scattering allows the observation of collective in-gap modes which, for their line shapes, temperature and doping dependences, and evolution of spectral weights, can be identified as Bardasis-Schrieffer excitons. The binding energies of these modes encode the relative coupling strengths in the sub-leading 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 Fe-based systems. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B14.00011: Discrete Superconducting Phases in FeSe-derived 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 Tc with tuning parameters such as doping and pressure. For the simplest iron-based superconductor FeSe (Tc > 8 K), its Tc can be greatly enhanced by doping electrons via many ways, even up to 65 K in monolayer FeSe/SrTiO3 with the help of phonons. However, it remains controversial whether there exists a superconducting dome in these electron-doped FeSe-derived 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 Fe-based superconductors Dmitry Chichinadze, Andrey Chubukov In this work, we analyze nodal points in multi-orbital Fe-based superconductors from the point of view of topology. We consider the case of $s^{+-}$ gap structure, with accidental nodes, and d-wave 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 inter-pocket pairing. Eventually, the nodes merge and annihilate and via Lifshitz-type transition. This happens in a multi-orbital model even if the gap symmetry is d-wave. We discuss Lifshitz transition in Fe-based superconductors from topological point of view. We show, both analytically and numerically, that in both s-wave and d-wave 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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