Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A11: Pairing Interaction and Gap Symmetry in Febased SuperconductorsFocus

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Sponsoring Units: DMP Chair: Rafael Fernandes, University of Minnesota Room: 307 
Monday, March 14, 2016 8:00AM  8:36AM 
A11.00001: GlidePlane Symmetry and Superconducting Gap Structure of IronBased Superconductors Invited Speaker: Thomas Maier This talk will provide a review [1] of the implications of the glide plane symmetry of a single Fepnictide/chalcogen plane on the structure of the superconducting gap. It will be shown that “$\eta$pairing” with nonzero total momentum occurs inevitably in this system, but that its contribution to the superconducting condensate has the usual even parity symmetry and time reversal symmetry is preserved. I will demonstrate that for a single plane the gap function, which appears in physical quantities, is identical to that found in 1 Fe per unit cell pseudocrystal momentum calculations and discuss the effects of the symmetry breaking outofplane hopping integrals in three dimensions. [1] Y. Wang, T. Berlijn, P. J. Hirschfeld, D. J. Scalapino, T. A. Maier, Phys. Rev. Lett. 114, 107002 (2015). [Preview Abstract] 
Monday, March 14, 2016 8:36AM  8:48AM 
A11.00002: Robust measurement of superconducting gap sign changes via quasiparticle interference: an application to 111 compounds Ilya Eremin, Dustin Altenfeld, Peter Hirschfeld, Igor Mazin While quasiparticle interference (QPI) measurements based on scanning tunneling spectroscopy are often proposed as definitive tests of gap structure, the analysis typically relies on details of the model employed. Here using the simplified twoband model system we propose, that the temperature dependence of momentumintegrated QPI data can be used to identify gap sign changes in a qualitative way, and present an illustration for $s_{\pm}$ and $s_{++}$ states in a system with typical Fepnictide Fermi surface. Using ARPES derived band structures within 10 orbital model Hamiltonian we analyze the QPI spectra in LiFeAs and Codoped NaFeAs compounds and show that the signchanging gap can be clearly identified using nonmagnetic impurity scattering. [Preview Abstract] 
Monday, March 14, 2016 8:48AM  9:00AM 
A11.00003: Features of Superconducting Gaps Revealed by STM/STS in Iron Based Superconductors With and Without Hole Pockets HaiHu Wen The pairing mechanism and gap structure in iron based superconductors (IBS) remains unresolved. We have conducted extensive STM/STS study on the Na(Fe$_{\mathrm{1x}}$T$_{\mathrm{x}})$As (T$=$Co, Cu, Mn)[1], Ba$_{\mathrm{1x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$[2], KFe$_{\mathrm{2}}$As$_{\mathrm{2}}$[3], and Li$_{\mathrm{1x}}$Fe$_{\mathrm{x}}$OHFeSe[4] single crystals. We found the clear evidence of the ingap quasiparticle states induced by the nonmagnetic Cu impurities in Na(Fe$_{\mathrm{0.97}}_{x}$Co$_{\mathrm{0.03}}$Cu$_{x})$As, giving strong evidence of the S$^{\mathrm{\pm }}$ pairing. Furthermore, we show the presence of the bosonic mode with the energy identical to that of the neutron resonance and a simple linear relation $\Omega $/k$_{\mathrm{B}}$T$_{\mathrm{c\thinspace }}\approx $ 4.3, being explained a consequence of the S\textpm pairing. The STS spectrum in Li$_{\mathrm{1}}_{x}$Fe$_{x}$OHFeSe clearly indicates the presence of double superconducting gaps with $\Delta_{\mathrm{1}} \quad \approx $ 14.3 meV and $\Delta_{\mathrm{2}} \quad \approx $ 8.6 meV. Further analysis based on QPI allows us to assign the larger (smaller) gap to the outer (inner) hybridized electron pockets[4]. The huge value 2$\Delta _{\mathrm{1}}$/$k_{\mathrm{B}}T_{\mathrm{c\thinspace }}=$ 8.7 discovered here undoubtedly proves the strong coupling mechanism. [1] H. Yang et al., Nature Communications \textbf{4}, 2947 (2013). [2] Z. Y. Wang, et al., Nature Physics \textbf{9}, 42(2013). [3] D. L. Fang, X. Shi et al., arXiv: Condmat.1412.0945. Phys. Rev. B 2015. [4] Z. Y. Du et al., arXiv: Condmat. 1506.04645. [Preview Abstract] 
Monday, March 14, 2016 9:00AM  9:12AM 
A11.00004: Specific Heat in High Magnetic Fields of BaFe$_{\mathrm{2}}$(As$_{\mathrm{1x}}$P$_{\mathrm{x}})_{\mathrm{2}}$ Camilla M. Moir, Jose A. Galvis, Phillip Walmsley, James G. Analytis, JiunHaw Chu, Ian R. Fisher, Arkady Shekhter, Greg S. Boebinger, Scott C. Riggs We measure the magnetic field dependence of the specific heat in BaFe$_{\mathrm{2}}$(As$_{\mathrm{1x}}$P$_{\mathrm{x}})_{\mathrm{2}}$ with x ranging from x$=$0.31 to x$=$0.6 in fields up to 34.5T. We report three important observations: $\surd $H behavior indicating a nodal superconducting gap with a linear energy dispersion, saturation of the heat capacity at the magnetic field that corresponds to the resistive onset [1], and a calculated quasiparticle mass using the increase in the electronic specific heat coefficient when entering the normal state, $\Delta \gamma $ $= \quad \gamma $(34.5T)  $\gamma $(0T), as a measure of the normal state specific heat. [1] James G. Analytis, HH. Kuo, Ross D. McDonald, Mark Wartenbe, P. M. C. Rourke, N. E. Hussey {\&} I. R. Fisher. \textit{Nature Phys}. 10, 194197 (2014) [Preview Abstract] 
Monday, March 14, 2016 9:12AM  9:24AM 
A11.00005: Direct evidence for a pressure induced nodal superconducting gap in the Ba$_{0.65}$Rb$_{0.35}$Fe$_{2}$As$_{2}$ superconductor Zurab Guguchia, Alex Amato, Jian Kang, Hubertus Luetkens, Pabitra K. Biswas, Giacomo Prando, Fabian v. Rohr, Zbigniew Bukowski, Alexander Shengelaya, Hugo Keller, Elvezio Morenzoni, Rafael M. Fernandes, Rustem Khasanov In contrast to other unconventional superconductors, in the Febased superconductors (FeHTSs) both $d$wave and extended $s$wave pairing symmetries are close in energy. Probing the proximity between these different superconducting (SC) states and identifying experimental parameters that can tune them is of central interest. We report highpressure muon spin rotation experiments on the temperaturedependent magnetic penetration depth in the optimally doped nodeless $s$wave FeHTS Ba$_{0.65}$Rb$_{0.35}$Fe$_{2}$As$_{2}$. Upon pressure, a strong decrease of the penetration depth is observed, while the SC transition temperature remains nearly constant. More importantly, the lowtemperature behavior of the inverse squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linearin$T$ behavior at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the SC gap. [Preview Abstract] 
Monday, March 14, 2016 9:24AM  9:36AM 
A11.00006: Thermoelectric signatures of timereversal symmetry breaking states in multiband superconductors Mikhail Silaev, Julien Garaud, Egor Babaev We demonstrate that superconductors which break timereversal symmetry can exhibit thermoelectric properties, which are entirely different from the Ginzburg mechanism. As an example, we show that in the s$+$is superconducting state there is a reversible contribution to thermally induced supercurrent, whose direction is not invariant under timereversal operation. Moreover in contrast to Ginzburg mechanism it has a singular behavior near the timereversal symmetry breaking phase transition. A local hot spot in such superconductors is surrounded by a multipolar magnetic field, sensitive to the presence of domain walls and crystalline anisotropy of superconducting states. A nonstationary heating process produces an electric field and a charge imbalance in different bands. These effect can be measured and used to distinguish s$+$is and s$+$id superconducting states in the candidate materials such as Ba$_{\mathrm{1x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. [Preview Abstract] 
Monday, March 14, 2016 9:36AM  9:48AM 
A11.00007: Raman resonance due to magnetic fluctuations in ironbased superconductors Jiashen Cai, Alberto Hinojosa, Andrey Chubukov We perform theoretical analysis of polarizationsensitive Raman spectroscopy on NaFe$_{1x}$Co$_x$As and Ba(Fe$_{1x}$Co$_x$)$_2$As$_2$, focusing on two features seen in the $B_{1g}$ symmetry channel (in one Fe unit cell notation): the strong temperature dependence of the static, uniform Raman response in the normal state and the existence of a collective mode in the superconducting state. We show that both features can be explained by the coupling of fermions to pairs of magnetic fluctuations via the AslamazovLarkin process. We argue that the singular temperature dependence in the normal state comes from the AslamazovLarkin vertex, while the resonance is due to the interaction between two propagating spin fluctuations in an $s^{+}$ superconductor. [Preview Abstract] 
Monday, March 14, 2016 9:48AM  10:00AM 
A11.00008: Electron pairing in the presence of incipient bands in ironbased superconductors Andy Linscheid, Xiao Chen, Saurabh Maiti, Peter Hirschfeld Recent experiments on certain Febased superconductors (SC) have hinted at a role for paired electrons in “incipient” bands that are close to, but do not cross the Fermi level. Within a simple multiband weakcoupling BCS approximation, we categorize the problem into two cases: case(I) where SC arises from the incipient band pairing alone, and case(II) where it is induced on an incipient band by pairing due to Fermisurface based interactions. Negative conclusions regarding the importance of incipient bands are largely based on case(I). However, we show explicitly that models under case(II) can explain the mild suppression of Tc, as well as robust large gaps on an incipient band. We also model the interplay between phonon and spin fluctuation (SF) driven SC and describe the bootstrap of electronphonon SC by SF coupling the incipient and the regular bands. We argue that pairing on incipient bands may be important in several Febased materials, including LiFeAs, FeSe intercalates and FeSe monolayers on SrTiO$_3$, and indeed may contribute to high Tc in some cases. In addition, we address the question whether this conclusion holds if the SF interaction is derived explicitly in the incipient band scenario and retardation effects are included on the level of the Eliashberg equations. [Preview Abstract] 
Monday, March 14, 2016 10:00AM  10:12AM 
A11.00009: Smearing of the Lifshitz transition by superconductivity Alexei Koshelev, Konstantin Matveev We consider a multiband metal with deep primary bands and a shallow secondary one [1]. In the normal state the system undergoes Lifshitz transition when the bottom of the shallow band crosses the Fermi level. In the superconducting state Cooper pairing in the shallow band is induced by the deep ones. As a result, the density of electrons in the shallow band remains finite even when the bottom of the band is above the Fermi level. We study the density of states in the system and find qualitatively different behaviors on the two sides of the Lifshitz transition. On one side of the transition the density of states diverges at the energy equal to the induced gap, whereas on the other side it vanishes. We argue that this physical picture describes the recently measured gap structure in shallow bands of iron pnictides and selenides.\newline [1] A. E. Koshelev and K. A. Matveev Phys. Rev. B \textbf{90}, 140505(R) (2014) [Preview Abstract] 
Monday, March 14, 2016 10:12AM  10:24AM 
A11.00010: Microscopic theory of superconductivity near a Lifshitz transition Vivek Mishra, Thomas Maier, Doug Scalapino Observation of robust superconductivity in some of the iron based superconductors in the vicinity of a Lifshitz point has attracted many theoretical and experimental studies. The majority of these studies have been phenomenological. Here we discuss a microscopic treatment of the pairing mechanism for a bilayer Hubbard model, which goes through a Lifshitz transition. We study pairing driven by spinfluctuations by solving the strong coupling Eliashberg equations and make a systematic comparison of the results with nonperturbative dynamical cluster quantum Monte Carlo calculations. Our findings are quite general and we will discuss their application to some of the iron based superconductors. [Preview Abstract] 
Monday, March 14, 2016 10:24AM  10:36AM 
A11.00011: Topological Phase Transitions in Linenodal Superconductors Gil Young Cho, SangEun Han, EunGook Moon Fathoming interplay between symmetry and topology of manyelectron wavefunctions deepens our understanding in quantum nature of many particle systems. Topology often protects zeroenergy excitation, and in a certain class, symmetry is intrinsically tied to the topological protection. Namely, unless symmetry is broken, topological nature is intact. We study one specific case of such class, symmetryprotected linenodal superconductors in three spatial dimensions (3d). Mismatch between phase spaces of order parameter fluctuation and linenodal fermion excitation induces an exotic universality class in a drastic contrast to one of the conventional $\phi^4$ theory in 3d. {\it Hyperscaling violation} and {\it relativistic dynamic scaling} with unusually large quantum critical region are main characteristics, and their implication in experiments is discussed. For example, continuous phase transition out of linenodal superconductors has a {\it linear} phase boundary in a temperaturetuning parameter phasediagram. [Preview Abstract] 
Monday, March 14, 2016 10:36AM  10:48AM 
A11.00012: Orbitalselective pairing: a $\tau $3 B1g pairing candidate state for the alkaline iron selenides. Rong Yu, Emilian M Nica, Qimiao Si The ironbased unconventional superconductors are inherently multiorbital systems and show remarkable variation in the Fermisurfaces and pairing symmetries. In the alkaline iron selenides cases, ARPES experiments indicate fully gapped superconducting states, which suggests swave pairing, while neutronscattering studies show resonances in the spinspectrum with wave vectors across the electron Fermi pockets, suggesting dwave pairing. We propose a novel superconducting state composed of a direct product of an swave form factor and a rotational symmetrybreaking orbital matrix in the $d_{xz/yz\thinspace }$sectors [1]. It belongs to the $B_{1g}$ representation of the D$_{\mathrm{4h}}$ point group, allowing for the overall change in sign between the pairing field at the electron pockets close to the 1Fe BZ edge. While it supports a spin resonance, it also produces a fully gapped quasiparticle spectrum, making it a candidate pairing state for the alkaline iron selenide compounds. Our results also show how such a state can become energetically competitive in the regime of quasidegeneracy between the s and dwave pairing states. In a broader context, this pairing provides an alternative to the $s+$\textit{id }to reconstruct the degenerate pairing states, while preserving the timereversal symmetry. We discuss possible analogs in other multiband strongcoupling superconductors such as the heavy fermions. [1] ''Emergent superconducting state from quasidegenerate s$$ and d$$wave pairing channels in ironbased superconductors,'' E. M. Nica, R. Yu, and Q. Si, arXiv:1505.04170v1 (2015). [Preview Abstract] 
Monday, March 14, 2016 10:48AM  11:00AM 
A11.00013: Searching for the Genes of Unconventional High Temperature Superconductors Jiangping Hu In the past, both curates and ironbased superconductors were discovered accidentally. Lacking of successful predictions on new high Tc materials is one of major obstacles to reach a consensus on the high Tc mechanism. In this talk, we discuss two emergent principles, which are called as the correspondence principle and the selective magnetic pairing rule, to unify the understanding of both cuprates and ironbased superconductors. These two principles provide an unified explanation why the dwave pairing symmetry and the swave pairing symmetry are robust respectively in cuprates and ironbased superconductors. In the meanwhile, the above two principles explain the rareness of unconventional high Tc superconductivity, identify necessary electronic environments required for high Tc superconductivity and finally serve as direct guiding rules to search new high Tc materials. We predict that the third family of unconventional high Tc superconductors exist in the compounds which carry two dimensional hexagonal lattices formed by cationanion trigonal bipyramidal complexes with a d_ filling configuration on the cation ions. Their superconducting states are expected to be dominated by the d+id pairing symmetry and their maximum Tc should be higher than those of ironbased superconductors. Verifying the prediction can convincingly establish the high Tc superconducting mechanism and pave a way to design new high Tc superconductors [Preview Abstract] 
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