Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B61: FeBased Superconductors  Mayorana / TopologicalFocus

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Sponsoring Units: DMP DCMP DCOMP Chair: Yanwei Ma Room: Mile High Ballroom 4B 
Monday, March 2, 2020 11:15AM  11:51AM 
B61.00001: Signature of Dispersing 1D Majorana Channels in an Ironbased Superconductor Invited Speaker: Vidya Madhavan Majorana fermions can be realized as quasiparticle excitations in a topological superconductor, whose 
Monday, March 2, 2020 11:51AM  12:03PM 
B61.00002: Longerrange exchange interaction in ironbased superconductors: Firstprinciples studies Weiguo Yin The nature of strong electronic correlations in ironbased superconductors (FeSCs) was shown to be kin to manganites where localized spins and itinerant electrons coexist and are coupled by Hund’s rule interaction, leading to fierce competition between superexchange antiferromagnetism and orbitaldegenerate doubleexchange ferromagnetism [1,2]. This implies that in a lowenergy spinonly description of FeSCs where the itinerantelectron degree of freedom were integrated out, the effective exchange interactions would have been long ranged. The spin dynamics in FeSCs has been interpreted by using such spinonly Heisenberg models with exchange couplings extended to the third nearest neighbors, i.e., the J1J2J3 model. Here, we use firstprinciples calculations on the K2Fe4Se5 and KFeAgTe2 insulators to demonstrate the importance of J4 the effective exchange between the fourth nearest neighbors. We further reveal that the band gap of K2Fe4Se5 or KFeAgTe2 is not of Mott type but the gap of Hund's metal. Our findings shed a new light on the understanding of FeSCs and the puzzling strong magnetic fluctuations in FeSe in particular. [1] PRL 105, 107004 (2010); [2] PRB 86, 081106(R) (2012). 
Monday, March 2, 2020 12:03PM  12:15PM 
B61.00003: Quasiparticle selfconsistent GW analysis in ironbased superconductors Katsuhiro Suzuki, Taishi Ogura, Hirofumi Sakakibara, Takao Kotani, Hiroaki Ikeda Since the early theoretical study of ironbased superconductors, the density functional theory (DFT) has given great contributions to understand material properties, e.g. magnetic/orbital structure, pairing mechanism of superconductivity, etc. However, the LDA/GGA exchangecorrelation potential in conventional DFT calculations is insufficient to describe the electronic state observed experimentally, such as in the angular resolved photoemission spectroscopy (ARPES) and de Hass van Alphen (dHvA) measurement [13]. In this study, we calculated the electronic band structure of ironbased superconductors with the quasiparticle selfconsistent GW (QSGW) method. We found that the QSGW can improve the LDA/GGA band structure and Fermi surface in most cases. We will summarize the result and discuss the superconducting gap in the obtained QSGW band structure. [1] A. A. Kordyuk et al., Phys. Rev. B 83 134513 (2011). [2] K. Okazaki et al., Science 337 1314 (2012). [3] T. Terashima et al., J. Phys. Soc. Jpn. 79 053702 (2010). 
Monday, March 2, 2020 12:15PM  12:27PM 
B61.00004: Search for evidence of quantum anomalous vortices in Ironbased Topological Superconductor Fe_{1+y}Te_{1−x}Se_{x }_{ } Lu Li, Lu Chen, Ziji Xiang, Colin B Tinsman, Genda Gu Topological superconductivity makes it possible for faulttolerant quantum computations. Febased superconductors are a rich material family of intermetallic compounds for hightemperature superconductivity. The interplay between these two exciting fields creates an emergent new field, marked by recent discoveries of the superconducting Dirac surface state and Majorana bound states at the vortex cores in ironbased superconductors Fe_{1+y}Te_{1−x}Se_{x}. However, the direct electronic transport consequence is not clear out of these topological superconducting states. We are left wondering how to operate these novel excitations. We solve the problem by thermally drive ordinary superconducting vortices around these Majorana zero modes and search for a transverse electrical response. This scheme is the Nernst effect in the superconducting state. A finite Nernst signal is observed at zero fields in the superconducting states of Fe_{1+y}Te_{1−x}Se_{x}. This anomalous Nernst signal shows fieldsymmetric dependence on the external magnetic field. Our experiments provide the first evidence of broken timereversal symmetry in topological superconductors and point to a new method to engage Majorana zero mode excitations in transport properties. 
Monday, March 2, 2020 12:27PM  12:39PM 
B61.00005: The Triplet Resonating Valence Bond State and Superconductivity in Hund's Metals: I Concept and Symmetries Piers Coleman, Yashar Komijani, Elio Koenig A central idea in strongly correlated systems is that doping a Mott insulator leads to a superconductor by transforming the resonating valence bonds (RVBs) into spinsinglet Cooper pairs. Here, we argue that a spintriplet RVB (tRVB) state, driven by spatially, or orbitally anisotropic ferromagnetic interactions can provide the parent state for triplet superconductivity. One of the new features of the concept, is that triplet RVB bonds can resonate into the interior of an atom, where they are supported as Hund's coupled triplet states between orbitals at the same site. Remarkably though, simple symmetry arguments due to P. W. Anderson, mean that such a process can only develop coherence with triplet pairs on the Fermi surfacce, if the there are two symmetry related atoms per unit cell, leading to a state that has staggered structure of onsite pair correlations that can be detected using Josephson tunneling. A natural candidate for this physics are the iron based superconductors, as will be described in the two following talks [1]. 
Monday, March 2, 2020 12:39PM  12:51PM 
B61.00006: Title: The Triplet Resonating Valence Bond State and Superconductivity in Hund's Metals: II Meanfield theory Piers Coleman, Yashar Komijani, Elio Koenig Triplet resonating valence bond (tRVB) states are entangled states that are produced in anisotropic magnetic environments [1], just like how singlet RVB states arise in frustated antiferromagnetic systems. A single Iron atom in a cubic environment contains miniature triplet states resonating between various t2g orbitals of the dshell due to the large Hund's coupling. I will discuss how this can be used to derive a local interorbital triplet pairing applicable to Ironbased superconductors [2]. While the order parameter contains significant interband pairing, the projection on the Fermi surface appears as an oddparity intraband triplet pairing which enjoys a weakcoupling instability and gaps out the entire Fermi surface. In analogy with the RVB theory, a Gutzwiller projected variational meanfield is suggested for the ground state. Furthermore, we augment our meanfield theory using a KotliarLiu slaveboson approach to include the effect of intra and interorbital interactions. Experimental signatures of the proposed triplet superconductor are discussed in the next talk. 
Monday, March 2, 2020 12:51PM  1:03PM 
B61.00007: The Triplet Resonating Valence Bond State and Superconductivity in
Hund's Metals: III Phenomenology in iron based superconductors. Piers Coleman, Yashar Komijani, Elio Koenig Based on the triplet Resonating Valence Bond concept introduced in the previous two talks, I will discuss the prospects and signatures of this theory in the context of iron based superconductors. I will explicitly concentrate on the spintriplet, orbital antisymmetric pairing state as derived in part II and on four experimental techniques of fundamental interest: (1) nuclear magnetic resonance, (2) neutron scattering, (3) tunneling spectroscopy, (4) scanning Josephson tunneling microscopy. First, I will demonstrate that this pairing state develops an anisotropic, partial suppression of the Knight shift and discuss existing experimental data on single crystals. Second and third, I will demonstrate that the relative sign between triplet dvectors of electron and hole pockets crucially affects finite frequency spinsusceptibility and local density of states. I will discuss the consequences thereof for key experimental observables, i.e. a sharp subgap spinresonance and characteristic quasiparticle interference patterns, that previously were interpreted as evidence for s_{+} pairing. Finally, I describe experimental signatures of the staggered order parameter in scanning Josephson microscopy. 
Monday, March 2, 2020 1:03PM  1:15PM 
B61.00008: Unconventional superconductivity and Hund's induced electron correlations: a Cooperative mechanism Laura Fanfarillo, Angelo Valli, Massimo Capone The phenomenology of ironbased superconductors shows that the normal state is a bad metal characterized by orbitalselective correlations induced by the Hund's exchange coupling, suggesting an important role of the electronelectron interactions. On the other hand theories based on the exchange of bosons (e.g. spin fluctuations) describe correctly a variety of phenomena pointing towardsa more standard pairing mechanism. 
Monday, March 2, 2020 1:15PM  1:27PM 
B61.00009: Hundenhanced electronic compressibility in LiFeP, LiFeAs and NaFeAs tuned by chemical substitution Pablo Villar Arribi, Luca De Medici We study the electronic structure of LiFeP, LiFeAs and NaFeAs in their paramagnetic metallic phase including dynamical electronic correlations within a density functional theory + slavespin meanfield framework. Our results show that the three compounds are next to the crossover between a normal and a Hund’s metal, where a region of enhanced electronic compressibility that may be relevant for superconductivity is present in this type of systems. While LiFeP and LiFeAs lie in the normal metallic regime, with LiFeAs in the vicinity of the crossover, NaFeAs is in the Hund’s metal regime, which explains the different behavior observed in these materials. Our findings correlate positively with the experimental trends for doping and pressuredependence of superconductivity in these compounds, as well as the predicted mass renormalizations and Sommerfeld coefficients. 
Monday, March 2, 2020 1:27PM  1:39PM 
B61.00010: Modified pairing structure due to momentumdependent correlations in ironbased superconductors Shinibali Bhattacharyya, Peter Hirschfeld, Thomas Maier, Douglas J Scalapino We discuss the influence of momentumdependent correlation effects arising from particlehole interactions on the superconducting gap structure of ironbased superconductors. Within the Eliashberg formalism, we obtain a modified linearized gap equation arising from the spinfluctuation interaction in the weakcoupling regime which includes selfconsistent renormalizations of quasiparticle weights. The modification of the particleparticle interaction vertex by particlehole processes modifies the superconducting gap structure, i.e. momentumdependent enhancement or suppression of gap amplitude compared to traditional spinfluctuation pairing calculations. We obtain good agreement with experimentally observed anisotropic gap structures in LiFeAs, indicating that inclusion of nonlocal correlation effects in the existing weakcoupling theories can account for observed signatures of the correlated ironbased superconductors. 
Monday, March 2, 2020 1:39PM  1:51PM 
B61.00011: Systematic derivation of ab initio effective Hamiltonians for ironbased superconductors Takahiro Misawa, Kota Ido, Yuichi Motoyama, Kazuyoshi Yoshimi Ironbased superconductors have attracted much attentions since its discovery at 2008. Owing to huge amount of studies, more than a dozen ironbased superconductors including related compounds have been found. In this study, combining the firstprinciples calculations and accurate numerical methods for treating quantum lattice models [1], we systematically obtain and analyze firstprinciple effective Hamiltonians of more than 20 ironbased superconductor and the related compounds. We perform the band calculation using Quantum ESPRESSO [2], derive the ab initio effective Hamiltonians using RESPACK [3], and analyze the Hamiltonians using mVMC [4]. As a result, we show the materials dependence of the microscopic parameters/physical quantities, and their relations with the superconducting transition temperatures. 
Monday, March 2, 2020 1:51PM  2:03PM 
B61.00012: Topological bands in Fe(Se,Te) Tamaghna Hazra, Himanshu Lohani, Amit Ribak, Yuval Nitzav, Huixia Fu, Binghai Yan, Mohit Randeria, Amit Kanigel Bulk FeSe_{0.45}Te_{0.55} has recently emerged as a promising candidate to host topological superconductivity on its surface, with experimental signatures for a Dirac surface state and Majorana bound states in vortex cores. However, ARPES measurements of the bulk band structure show essentially no k_{z} dispersion, in apparent contradiction with DFT predictions for the significant k_{z} dispersion that drives the band inversion. 
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