Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L39: Fe-based Superconductors: Orbital Effects and NematicityFocus
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Sponsoring Units: DMP DCOMP Chair: Lilia Boeri, TU Gratz Room: 386 |
Wednesday, March 15, 2017 11:15AM - 11:51AM |
L39.00001: Interplay and competition between the magnetism, superconductivity and orbital order in iron-based superconductors Invited Speaker: Maxim Khodas In this talk I will present the theory explaining the interrelations between different macroscopic orderings universally found in iron based superconductors. The nematic order sets in at the tetragonal to orthorhombic transition. In most of iron pnictides such order is a byproduct of stripe magnetism. In FeSe, the nematic transition is not followed by magnetism at ambient conditions. I will present a unifying description general enough to describe both of these two different scenarios. We employ the parquet Renormalization Group (RG) to study the interplay between different ordering tendencies. It describes renormalizations by the processes at energies in the interval from the band width down to the temperature or Fermi energy whichever is larger. The RG flow of susceptibilities favors the nematic instability even though the magnetism is the only interaction channel with attraction at the bare level. In most of iron pnictides the Fermi energy is large and RG is not effective. Therefore, the leading instability is magnetic or superconducting. In FeSe the small Fermi energy allows for RG flow to run long enough to promote the nematic instability, and at the same time to suppress magnetism. The remaining second leading instability is superconducting. In result, the presented theory allows us to describe both FeSe and iron pnictides. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L39.00002: Interplay between magnetism, superconductivity, and orbital order in 5-pocket model for iron-based superconductors -- parquet renormalization group study Maxim Khodas, Laura Classen, Ruiqi Xing, Andrey Chubukov We report the results of the parquet renormalization group (pRG) analysis of the 5-pocket model for Fe-based superconductors. We use as an input the fact that excitations near all five pockets are made out of $d_{xz}$, $d_{yz}$, and $d_{xy}$ orbitals. We argue, based on symmetry, that there are 40 different coupling constants, which describe the interactions between low-energy fermions in the orbital basis. All couplings flow under pRG. We find that there are four stable fixed trajectories of the pRG flow, separated by several unstable ones. Along the stable trajectories, the 5-pocket model effectively reduces either to a 3-pocket or a 4-pocket one.In both cases superconductivity wins over SDW, if the Fermi energies are small enough such that pRG runs over a wide energy range. The superconducting state has $s^{+-}$ gap structure for both effective models, but the magnitude of the gap on hole pockets is different for 3-pocket and 4-pocket cases. Furthermore, the flow to effective 3- or 4-pocket models distinguishes between the origin of the nematic order. For the former care it is a vestigial magnetic order, while for the latter case it comes from spontaneous orbital order. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L39.00003: Orbital selective Mott physics in the 111 iron pnictides Rong Yu, Jianxin Zhu, Qimiao Si Motivated by recent ARPES measurements on the iron pnictides compounds $A$FeAs ($A$=Li,Na), we study the effects of electron correlations on the bandstructures in these compounds via microscopic multiorbital Hubbard models using the U(1) slave-spin theory. We find that the calculated phase diagrams of both compounds at the commensurate filling $n=6$ contain a common orbital selective Mott phase (OSMP) besides a metallic one and a Mott insulating one. The OSMP is stabilized in a much wider parameter range in LiFeAs than in NaFeAs, as a consequence of a larger energy splitting between the Fe $d_{xy}$ orbital and the $d_{xz/yz}$ orbitals, as well as suppressed hoppings between the $d_{xy}$ and $d_{xz/yz}$ orbitals in LiFeAs. Meanwhile, the onset Coulomb coupling for the orbital selective Mott transition (OSMT) in LiFeAs shows a strong temperature dependence. This pushes the LiFeAs system close to an OSMT with a strongly suppressed quasiparticle spectral weight in the $d_{xy}$ orbital at high temperatures, similar to the iron chalcogenides. Our finding indicates that the orbital selective Mott physics is a common feature for both iron pnictides and iron chalcogenides. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L39.00004: Orbital selective pairing and gap structures of iron-based superconductors Brian M. Andersen, Andreas Kreisel, P. O. Sprau, A. Kostin, J. C. Seamus Davis, P. J. Hirschfeld Recent experiments in the superconducting phase of iron-based superconductors have mapped out the detailed momentum dependence of the superconducting gap structure. We discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. We propose that a key ingredient for an improved itinerant pairing theory is orbital selectivity, i.e. incorporating less coherent quasiparticles occupying specific orbital states into the pairing theory. This modifies the usual spin-fluctuation pairing via suppression of pair scattering processes involving those incoherent states and results in orbital selective Cooper pairing of electrons in the remaining states. We show that this paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both bulk and monolayer FeSe, as well as LiFeAs, indicating that orbital selective Cooper pairing plays a key role in the more strongly correlated iron-based superconductors. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L39.00005: Low energy model based on orbital selective spin fluctuations for iron superconductors Belen Valenzuela, Laura Fanfarillo, Lara Benfatto We propose a low energy model to study the magnetic and nematic phase in iron superconductors with the basic information to address the dificult problem of spin-orbital entanglement[1]. The model is based on the concept of orbital selective spin fluctuations[2]. The model turns out to have similar structure to the well-known spin-nematic model based upon band models (SNB) [3] without tensorial dependence in the orbitals. It has the advantage of addresing the orbital degree of freedom and the spin-orbital entanglement. The orbital information is encoded in the Landau parameters and in the definition of the different order parameters. This result explains in a transparent way why the well-known SNB model although simple has been very successful to address the physics of pnictides. As a result of the model we show how is able to explain the odd orbital ordering in FeSe.[4] [1] L. Fanfarillo, L. Benfatto, and B. Valenzuela, in preparation. [2] L. Fanfarillo, A. Cortijo, and B. Valenzuela, Phys. Rev. B 91, 214515 (2015) [3] R. M. Fernandes, A. V. Chubukov, J. Knolle, I. Eremin, and J. Schmalian, Phys. Rev. B 85, 024534 (2012). [4] L. Fanfarillo, J. Mansart, P. Toulemonde, H. Cercellier, P. Le Fevre, F. Bertran, B. Valenzuela, L. Benfatto, and V. Brouet, Phys. Rev. B 94, 155138 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L39.00006: Correlation-enhanced odd-parity inter-orbital singlet pairing in LiFeAs Reza Nourafkan, Gabi Kotliar, A.-M. S. Tremblay The rich variety of iron-based superconductors and their complex electronic structure lead to a wide range of possibilities for gap symmetry and pairing components. We solved, in the two-Fe Brillouin zone, the full frequency-dependent linearized Eliashberg equations to investigate spin-fluctuations mediated Cooper pairing for LiFeAs . The magnetic excitations were calculated with the random phase approximation on a correlated electronic structure obtained with density functional theory and dynamical mean field theory. The interaction between electrons through Hund's coupling promotes both the intra-orbital $d_{xz(yz)}$ and the inter-orbital magnetic susceptibility. As a consequence, the leading pairing channel, conventional $s^{+-}$, acquires sizeable inter-orbital $d_{xy}-d_{xz(yz)}$ singlet pairing with odd parity under glide-plane symmetry. The combination of intra- and inter-orbital components makes the results consistent with available experiments on the angular dependence of the gaps observed on the different Fermi surfaces [1]. We also explain the difference in pairing symmetry between LiFeAs and LiFeP [2]. \newline [1] R.~Nourafkan, G.~Kotliar, and A.-M.S.~Tremblay, Phys. Rev. Lett. \textbf{117}, 137001 (2016). [2] R.~Nourafkan, Phys. Rev. B \textbf{93}, 241116(R) (2016) [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L39.00007: Non-trivial role of interlayer cation states in iron-based superconductors Roser Valenti, Daniel Guterding, Harald O. Jeschke, J. K. Glasbrenner, E. Bascones, I. I. Mazin Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Under this assumption, one would expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors, but they aren't. In this talk we will argue that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond-angles or bond-distances with respect to iron pnictides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations) [1]. We will discuss the implications of these results in the general context of Fe-based superconductors. [1] D. Guterding, H.O. Jeschke, I.I. Mazin, J.K. Glasbrenner, E. Bascones, R. Valenti arXiv:1610.08626 [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L39.00008: Competing orders in Fe-based superconductors: a parquet renormalization group study of the full 4-pocket, 3-orbital low energy effective model. Rui-Qi Xing, Laura Classen, Maxim Khodas, Andrey Chubukov In Fe-based superconductors, superconductivity, magnetism and nematic orders are all observed. Understanding various competing orders in Fe-based superconductors may help to unveil the mechanism of high-temperature superconductivity. To understand these competing orders appeared in the phase diagram, we use parquet renormalization group, an unbiased approach, to study the full four-pocket, three-orbital low-energy model for Fe superconductors. We identified all symmetry-allowed interactions, derived and analyzed the RG flow of the couplings and susceptibilities, and obtained the hierarchy of competing instabilities. For parameters relevant to FeSe, we argue that the nematic order parameter has three components, and we found particular relations between these components. Our results are consistent with recent ARPES experiments [A. Fedorov et al, arxiv:1606.03022]. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L39.00009: Nematic quantum paramagnetic fluctuation mediated pairing in FeSe Jian-Huang She, Michael Lawler, Eun-Ah Kim Despite its seemingly simple composition and structure, the pairing mechanism of FeSe remains an open problem due to several striking phenomena. Among them are nematic order without magnetic order, nodeless gap and extremely anisotropic momentum dependence of inelastic neutron spectra. Here we propose a microscopic description of a nematic quantum paramagnet that reproduces key features of neutron spectra with the key insight of viewing it as a sum over contributions from nematic domains. We then study how the spin fluctuation of the local moments lead to pairing within a spin-fermion model. We find the resulting superconducting order parameter is nodeless $s\pm d$-wave within each domain. Furthermore we predict the gap magnitude to be very anisotropic at each Fermi pocket in a manner that reflects the distribution of orbital contents. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L39.00010: Nematicity, magnetism and superconductivity in FeSe under pressure: Unified explanation based on the self-consistent vertex correction theory Youichi Yamakawa, Hiroshi Kontani Rich electronic phase diagram in FeSe under pressure vividly demonstrates the strong interplay between the nematicity, magnetism and superconductivity in Fe-based superconductors. Here, we construct the multiorbital Hubbard model for FeSe under pressure by referring to the first-principles calculations, and analyze the electronic states by including the higher-order many-body effects called the vertex correction (VC). When the pressure-induced $\dxy$-orbital Fermi pocket appears, the spin fluctuations on the $\dxy$ orbital are enhanced, whereas those on $\dxz,\dyz$ orbitals are reduced. For this reason, nonmagnetic orbital order $O=n_{xz}-n_{yz}$, which is caused by the spin fluctuations on $\dxz,\dyz$ orbitals via the VC, is suppressed and replaced with the magnetism of $\dxy$-orbital $d$-electrons. The nodal $s$-wave state at ambient pressure ($O\ne0$) and the enhancement of $T_{\rm c}$ under pressure are driven by the cooperation between spin and orbital fluctuations. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L39.00011: Nematic fluctuation induced superconductivity in FeSe thin films with spin-orbital coupling Jian Kang, Rafael Fernandes Thin films of FeSe display the highest transition temperatures among iron-based superconductors. In contrast to most compounds, these systems have only electron-like Fermi-surface pockets, and their normal state shows little evidence for strong magnetic fluctuations. Indeed, bulk FeSe displays nematic order, but no long-range magnetic order. Motivated by recent experiments that revealed sizable nematic fluctuations in thin films of FeSe, we investigate whether nematic fluctuations can provide a suitable mechanism for the high-temperature superconducting state observed in these compounds. We show that, because nematic fluctuations are peaked at zero momentum, this mechanism leads to an intrinsic degeneracy between s-wave and d-wave states, which in turn results in a significant suppression of T$_{\mathrm{c}}$. We demonstrate, however, that this degeneracy is lifted in favor of the s-wave state by both the sizable spin-orbit coupling and the inversion symmetry-breaking that occurs at the interface. The resulting gap is anisotropic and qualitatively agrees with recent experiments. Finally, we discuss how this mechanism for superconductivity in FeSe thin films can be enhanced by forward-scattering phonon modes characteristic of titanium oxide substrates. [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L39.00012: Enhancement of superconductivity by interfacial phonons in perovskite-clad FeAs monolayer Jhinhwan Lee, Seokhwan Choi, Won-jun Jang, Yannis Semertzidis, Jong Mok Ok, Hyunjung Lee, Alireza Akbari, Jun-Sung Kim, Alex T. Lee, Ken Nakatsukasa, Steven Johnston, Yunkyu Bang The physics at interfaces between monolayer iron-based superconductors (FeSC) and perovskite substrates has received considerable attention due to the unusually high $T_{c}$ of \textasciitilde 100 K found recently in monolayer FeSe on SrTiO$_{\mathrm{3}}$. It has been suggested that forward-scattering interfacial phonons coupled with the Fe-layer electrons can enhance superconductivity from virtually any pre-existing electron-based pairing. Here we report a spectroscopic imaging scanning tunneling microscopy (SI-STM) study on a parent-compound superconductor Sr$_{\mathrm{2}}$VO$_{\mathrm{3}}$FeAs, a self-assembled bulk example of FeSC monolayers sandwiched by perovskite layers with substantially high $T_{c} \quad =$ 33 - 37 K. The quasiparticle interference (QPI) shows clear signatures of forward-scattering phonons with unprecedentedly strong coupling g$_{\mathrm{ph}}^{\mathrm{2}}$/$\Omega _{\mathrm{ph}}^{\mathrm{2}}$ \textasciitilde 0.7. Our masked QPI analysis based on the superconducting gap ($\Delta )$ and V-Fe hybridization strength ($\Gamma )$ maps show clear positive correlations between all pairs of $\Delta $, $\Gamma $ and g$_{\mathrm{ph}}$, which could be the hallmark of pairing enhancement due to interfacial phonons. A self-consistent Migdal-Eliashberg T-matrix QPI simulation reproduces most of the detailed features of the experimental QPI and shows that as much as half of the pairing in this material could be attributable to the electron-phonon coupling. [Preview Abstract] |
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