Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L11: Novel Physics of Fe-Pnictide SuperconductorsFocus
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Sponsoring Units: DMP Chair: Ming Yi, UC Berkeley Room: 307 |
Wednesday, March 16, 2016 11:15AM - 11:51AM |
L11.00001: Origin of superconductivity in KFe$_2$As$_2$ under positive and negative pressures and relation to other Fe-based families Invited Speaker: Roser Valenti KFe$_2$As$_2$ shows an intricate behavior as a function of pressure. At ambient pressure the system is superconductor with a low critical temperature T$_c$=3.4 K and follows a V-shaped pressure dependence of T$_c$ for moderate pressures with a local minimum at a pressure of 1.5 GPa. Under high pressures P$_c$=15 GPa, KFe$_2$As$_2$ exhibits a structural phase transition from a tetragonal to a collapsed tetragonal phase accompanied by a boost of the superconducting critical temperature up to 12 K. On the other hand, $negative$ pressures realized through substitution of K by Cs or Rb decrease T$_c$ down to 2.25K. In this talk we will discuss recent progress on the understanding of the microscopic origin of this pressure-dependent behavior by considering a combination of ab initio density functional theory with dynamical mean field theory and spin fluctuation theory calculations [1-3]. We will argue that a Lifshitz transition associated with the structural collapse changes the pairing symmetry from $d$-wave (tetragonal) to $s_\pm$ (collapsed tetragonal) at high pressures while at ambient and negative pressures correlation effects appear to be detrimental for superconductivity. Further, we shall establish cross-links to the chalcogenide family, in particular FeSe under pressure [4]. [1] S. Backes, D. Guterding, H. O. Jeschke, R. Valenti, New J. Phys. 16, 083025 (2014). [2] D. Guterding, S. Backes, H. O. Jeschke, R. Valenti, Phys. Rev. B 91, 140503(R) (2015). [3] S. Backes, H. O. Jeschke, R. Valenti, Phys. Rev. B (in press). [4] J. K. Glasbrenner, I. I. Mazin, H. O. Jeschke, P. J. Hirschfeld, R. Valenti Nature Physics 11, 953 (2015). [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L11.00002: ARPES investigation of Fe-based superconductor KFe$_2$As$_2$ and related compounds Pierre Richard, X. Shi, B.-Q. Lv, P. Zhang, T. Qian, H. Ding, T. K. Kim, M. Hoesch, D.-L. Fang, H.-H. Wen, X.-H. Chen, A. van Roekeghem, P. Seth, S. Biermann KFe$_2$As$_2$ is the end-member of the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ family of Fe-based superconductors. Despite a small $T_c$ of 3 K, this compound is of particular interest because unlike the other members of this family of superconductors, the Fermi surface of KFe$_2$As$_2$ is free of electron pocket. Interest for this material was intensified following various reports on possible nodal superconducting order parameters in this system. Due to its momentum-resolved capabilities, angle-resolved photoemission spectroscopy (ARPES) is particularly suitable for investigating the key aspects of the electronic structure of materials. In this work we present recent ARPES data of KFe$_2$As$_2$ and related materials. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L11.00003: Pauli Limiting and Multi-Band Superconductivity in KFe$_2$As$_2$ Studied by Small-Angle Neutron Scattering M.R. Eskildsen, S.J. Kuhn, H. Kawano-Furukawa, M. Ono, E.M. Forgan, E. Jellyman, R. Riyat, C.H. Lee, K. Kihou, F. Hardy, Th. Wolf, C. Meingast, J.L. Gavilano We have studied the intrinsic anisotropy of the superconducting state in KFe$_2$As$_2$, using used small-angle neutron scattering to image the vortex lattice (VL) as the applied magnetic field is rotated towards the FeAs planes. The anisotropy is found to be strongly field dependent, indicating multi-band superconductivity. Furthermore, the high field anisotropy significantly exceeds that of the upper critical field, providing further support for Pauli limiting in KFe$_2$As$_2$ for field applied along the basal plane. Finally, we are able determine the contribution to the field modulation in the mixed state due to Pauli Paramagnetic Effects by measuring both the non-spin flip and spin flip VL scattered intensity. This represents the first instance where all the effects listed above have been obtained simultaneously and in a comprehensive manner by a single experimental technique. [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L11.00004: Correlation-enhanced odd-parity inter-orbital singlet pairing in the iron-pnictide superconductor LiFeAs Reza Nourafkan, Gabriel 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. Here we solve in the 2-Fe Brillouin zone the full frequency-dependent linearized Eliashberg equations for LiFeAs with spin-fluctuation mediated pairing interactions. The magnetic excitations are calculated with the random phase approximation on a correlated electronic structure obtained with density functional theory and dynamical mean field theory. Correlations induce long-lived local moments with orbital-dependent dynamics. 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 acquires sizeable inter-orbital $d_{xy}$-$d_{xz(yz)}$ singlet pairing with odd parity under glide-plane symmetry. These components reduce the superconducting gap magnitude induced by the intra-orbital components of the gap function at the electron pockets intersection where the Fe-d $t_{2g}$ orbitals strongly mix. 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. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L11.00005: Superconducting mechanism due to the orbital and spin fluctuations in Fe-based superconductors Hiroshi Kontani, Youichi Yamakawa, Seiichiro Onari The rich variety of the phase diagrams in Fe-based superconductors, such as the nonmagnetic/magnetic nematic phase in FeSe/LaFeAsO, is not able to be explained by the mean-field level approximations. Recently, we explained the phase diagrams of FeSe and LaFeAsO in term of the orbital$+$spin fluctuation theory, by including the Aslamazov-Larkin vertex correction (AL-VC). The nematic orbital order without magnetization in FeSe is well explained [1]. In the present study, we analyze the superconducting states in FeSe and LaFeAsO, by applying the orbital$+$spin fluctuation theory. Rich variety of the superconducting gap structures are induced by the strong orbital and spin fluctuations driven by the AL-VC ($=$orbital-spin interplay). We find that the pairing interaction due to the orbital fluctuations is strongly enlarged by the AL-VC for the anomalous self-energy, so the Migdal theory is seriously violated in the orbital-fluctuation pairing mechanism. [1] Y. Yamakawa, S. Onari and H. Kontani, arXiv:1509.01161. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L11.00006: ABSTRACT WITHDRAWN |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L11.00007: The Magnetic Excitations in Optimal Doped BaFe$_{\mathrm{2}}$(As$_{\mathrm{0.7}}$P$_{\mathrm{0.3}})_{\mathrm{2}}$ Ding Hu, Shiliang Li, Pengcheng Dai High temperature superconductivity in iron based superconductors emerges near the boundary of static antiferromagnetic order which is suppressed by doping or pressure. Although spin fluctuations may be responsible for superconductivity, there is still no consensus on the mechanism. As a unique system in 122-type iron pnictides, the phosphorus doping in the arsenic position in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ does not induce external carrier and impurity scattering, but the maximum T$_{\mathrm{c}} \quad =$ 30K. We have carried out inelastic neutron scattering experiment on Time of Flight Spectrometers, and mapped out the whole spin fluctuation up to 300mev. Our results are consistent with the combined DFT and DMFT calculation results, which confirm that pnictogen height is correlated with the electron-electron correlation strength and consequently the effective bandwidth of magnetic excitations in iron pnictides. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L11.00008: Doping evolution of magnetization hysteresis in (Ba1-xKx)Fe2As2 single crystals: Crossover from the second magnetization peak to peak effect Yong Liu, Thomas Lograsso Magnetic hysteresis loops (MHLs) have been systematically measured in a series of (Ba1-xKx)Fe2As2 single crystals from underdoped x$=$0.177 to end member x$=$1 with applied magnetic fields parallel to c axis (H//c). The second magnetization peak (SMP) or fishtail effect was observed within the doping range 0.177$\le $x$\le $0.650. Remarkably, with further increasing doping the SMP becomes narrow and emerges very close to the irreversible field (Hirr) for the samples 0.692$\le $x$\le $0.910. The similar peak effect (PE) had been widely observed in various conventional or low Tc superconductors. Meanwhile, the magnetization curves change from symmetrical to asymmetric hysteresis loops, which suggests a dominant surface pinning instead of bulk pinning in the samples. Our findings demonstrate that (Ba1-xKx)Fe2As2 system is a very unique system that that links the SMP and PE by its doping dependence. Our results will lead to a better understanding of the underlying mechanisms for the origin of the SMP and PE. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L11.00009: Orbital and Pauli limiting effects in heavily doped Ba$_{\mathrm{0.05}}$K$_{\mathrm{0.95}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. S. Zhang, Y. P. Singh, X. Y. Huang, X. J. Chen, M. Dzero, C. C. Almasan We investigated the thermodynamic properties of the Fe-based lightly-disordered superconductor Ba$_{\mathrm{0.05}}$K$_{\mathrm{0.95}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ in external magnetic field H applied along the FeAs layers (H\textbar \textbar ab planes). The superconducting (SC) transition temperature for this doping level is T$_{\mathrm{c}} \quad =$ 6.6 K. Our analysis of the specific heat C(T,H) measured for T\textless T$_{\mathrm{c}}$ implies a sign change of the superconducting order parameter across different Fermi pockets. We provide experimental evidence for the three components superconducting order parameter. We find that all three components have values which are comparable with the previously reported ones for the stoichiometric compound KFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Our data for C(T,H) and resistivity $\rho $(T, H) can be interpreted in favor of the dominant orbital contribution to the pair-breaking mechanism at low fields, while Pauli limiting effect dominates at high fields, giving rise to a gapless superconducting state with only the leading non-zero gap. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L11.00010: Can oxygen deficient SmFeAsO$_{1-}_{x}$ be synthesized?: Unintentional incorporation of hydride ion at oxygen vacancy site Yoshinori Muraba, Soshi Iimura, Satoru Matsuishi, Hideo Hosono Hydrogen substitution and introduction of oxygen vacancy are effective electron doping methods for the \textit{Ln}FeAsO. However, their $T_{c}$ vs e$^{-}$/Fe diagrams do not entirely overlap each other, while $T_{c}$ vs lattice dimension relationships are very similar. These contradictions can be understood by assuming that unintentional hydrogen is incorporated into the oxygen vacancy. To examine the preferred electron-dopant species in \textit{Ln}FeAsO and the influence of the atmosphere during synthesis on the formation of \textit{Ln}FeAsO$_{1-}_{x}$, we try to synthesize the SmFeAsO$_{1-}_{x}$ under three well-controlled atmosphere (H$_{2}$O, H$_{2}$ and H$_{2}$O- and H$_{2}$-free). Under H$_{2}$O and H$_{2}$ atmosphere, hydrogen were incorporated at oxygen sites as the hydride ion and SmFeAsO$_{1-}_{x}$H$_{x}$ was formed. On the other hand, when H$_{2}$O and H$_{2}$ were removed from synthetic process, nearly stoichiometric SmFeAsO was formed. Furthermore, DFT calculations showed that H-substituted samples are more stable than oxygen deficient samples. These results strongly imply that the reported \textit{Ln}FeAsO$_{1-}_{x}$ was contaminated with unintentional hydrogen and \textit{Ln}FeAsO$_{1-}_{x}$H$_{x}$ was misidentified as \textit{Ln}FeAsO$_{1-}_{x}$. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L11.00011: Neutron diffraction study of 154SmFeAsO1-xDx SOSHI IIMURA, Hiroshi Okanishi, Satoru Matsuishi, Haruhiro Hiraka, Kazutaka Ikeda, Thomas Hansen, Toshiya Otomo, Hideo Hosono Hot issue in unconventional superconductors (SC) is why the 2nd highest-Tc of 56 K after cuprates is accomplished in the 1111-type iron-oxyarsenides LnFeAsO1-xFx (Ln $=$ lanthanide). Recently, utilizing a hydride-substitution-method (O2- $=$ H- $+$ e-) in the LnFeAsO1$-$xHx [1], we found a second SC phase in 0.18 $\le $ x $\le $ 0.45 at Ln $=$ La in addition to the first one adjacent to the antiferromagnetic (AFM) order [2], and another AFM order accompanying a unique structural transition in over-doped region x \textgreater 0.4 [3]. However, since the Tc of La-system is lower than the other systems, i.e., Ln $=$ Ce, Sm and so on, it is still unclear whether the second AFM phase is essential for their high-Tc or not. Thus, we synthesized the isotope-substituted 154SmFeAsO1-xDx and performed neutron powder diffraction (NPD) to examine the structural and magnetic properties of the high-Tc 1111 system. In this talk, we show the results of NPD data and discuss the relation between the superconducting, magnetic, and structural properties of the 154SmFeAsO1-xDx and electron-doping-effect on it. [1] T. Hanna, et al. PRB 85, 024521 (2011). [2] S. Iimura, et al. Nat.Commun. 3, 943 (2012). \newline [3] M. Hiraishi et al. Nat. Phys. 10, 300 (2014). [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L11.00012: Role of Hydrogen in the Electronic Properties of H-rich Pnictide Superconductors Yina Huang, Xianglong Yu, Dayong Liu, Liangjian Zou The electronic and magnetic properties of the parent material CaFeAsH and its La/Co-doped compounds are investigated using first-principles calculations based on the generalized gradient approximation (GGA). We predict that the ground state of CaFeAsH is a spin-density-wave (SDW)-type striped antiferromagnet driven by Fermi surface nesting. We find a sandglass-type hole pocket near the $\Gamma $point in CaFeAsH that is not present in CaFeAsF. In comparison with CaFeAsF, the sandglass-shaped pocket, mainly contributed from Fe d$_{\mathrm{xz}}+$d$_{\mathrm{yz}}$ orbitals, arises from the weak oxidization of CaH layers and the hybridization enhancement between FeAs layers. In contrast, the electronic properties of electron doped Ca$_{\mathrm{0.75}}$La$_{\mathrm{0.25}}$FeAsH and CaFe$_{\mathrm{0.75}}$Co$_{\mathrm{0.25}}$AsH indicate that La or Co doping almost does not affect the sandglass-type Fermi surface, while the suppression of Fermi surface nesting in Ca$_{\mathrm{0.75}}$La$_{\mathrm{0.25}}$FeAsH is weaker than that in CaFe$_{\mathrm{0.75}}$Co$_{\mathrm{0.25}}$AsH. This features may contribute to the higher T$_{\mathrm{c}}$ in La-substituted CaFeAsH. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L11.00013: Nanofabrication of Point Contact Junctions for Spectroscopic Studies of High-Temperature Superconductors Han Zhao, Omar Mehio, Wan Kyu Park, James Eckstein, Laura Greene Point contact spectroscopy (PCS) probes the superconducting order parameter from Andreev reflection conductance spectrum. A new method to achieve robust junctions with a precise control of the geometry of the point contact by focused ion beam (FIB) nanofabrication techniques is currently under development. Preliminary application on niobium thin films shows consistent data that is insensitive to thermal cycling. This opens the possibility to perform PCS on a series of materials as a function of external variables, including temperature, magnetic field as a function of angle, and stress. Our preliminary data as a function of junction size show the expected resistance dependence, which will help us to determine more precisely when junctions are in the ballistic, or spectroscopic regime. Our plan is to apply this newly-developed method to probe the electronic nematic state in iron-based superconductors under applied magnetic field and uniaxial stress, to further understand the origin of the nematicity. [Preview Abstract] |
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