Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P26: Focus Session: Iron Based Superconductors -- Spin Dynamics |
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Sponsoring Units: DMP DCOMP Chair: Dmitri Argyriou, European Spallation Source Room: D162/164 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P26.00001: High Energy Spin Excitations in Optimal doped Superconducting BaFe$_{1.9}$ Ni$_{0.1}$As$_{2}$ Mengshu Liu, Huiqian Luo, Russell Ewings, Tatiana Guidi, Pengcheng Dai The recent discovered iron pnictide superconductor which shows a transition temperature up to 50K has drawn much attention in the community. There are indications that superconductivity in the iron arsenides family may be driven by a magnetic pairing mechanism, the nature of which remains poorly understood. In our recent inelastic neutron scattering experiment on optimal doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ sample, spin excitation data are collected throughout the Brillouin zone and up to energy transfer of 400meV which was not probed before. We found that the scattering persists as high as 300 meV in the superconducting sample, and a spectra transfer similar to the parent compound with a spin nematicity was also observed [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P26.00002: Spin-phonon coupling in iron arsenide superconductors Jennifer Niedziela, Daniel Parshall, Konstantin Lokshin, Athena Sefat, Ahmet Alatas, Takeshi Egami In this work we present the results of an inelastic x-ray scattering experiment measuring the softening of the TA[110] phonon in BaFe$_2$As$_2$ as a function of temperature. Cooling through the structural transition temperature yields a softening of the phonon energy of the transverse acoustic mode nearly 1 meV from the value at room temperature at $\mathbf{q}=0.1$. This phonon controls the structural phase transition, changing the symmetry from tetragonal to orthorhombic at the same temperature as the transition to long range antiferromanetic order. Even though the lattice distortion is minor, the anisotropy in the magnetic exchange constants is very large. We posit that this phonon mode couples to the orbital moment, and softening of this mode is required for the onset of long range antiferromagnetic ordering and the dramatic change in the exchange constants. This observation is suggestive that a mechanism of spin-phonon coupling is present in the pnictide systems, and is a possible contributor to the superconducting mechanism. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P26.00003: Collapse of the spin resonance spectral weight in overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ Ray Osborn, Stephan Rosenkranz, John-Paul Castellan, Eugene Goremychkin, Duck-Young Chung, Helmut Claus, Mercouri Kanatzidis, Tatiana Guidi We report inelastic neutron scattering measurements of magnetic excitations in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ over a broad range of electron band filling within the superconducting phase. In an itinerant model, these excitations are resonantly enhanced when the superconducting energy gap changes sign on different parts of the electron Fermi surface. They are therefore sensitive both to the superconducting gap symmetry and to the Fermi surface geometry. Our results show that, in addition to becoming incommensurate because of the growing mismatch in the hole and electron Fermi surface volumes, the resonant spectral weight decreases proportionally to the resonance binding energy, vanishing at $x \sim 0.72$. A tight-binding model including $s_{\pm}$-symmetry pairing is able to reproduce these observations confirming that the resonance arises from the pairing of band electrons. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P26.00004: Spin Correlations in Superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ Invited Speaker: Elastic and inelastic neutron scattering are used to study the spin correlations in superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. Measurements on the antiferromagnetically ordered parents of this system [1] show a strongly anisotropic spin-wave velocity. Here we report [2] measurements of the magnetic excitations in a the superconducting composition, $x$=0.065, up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wavevector- integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed. I will discuss the interpretation of our results in terms of the electronic structure and possible fluctuating nematic order.\\[4pt] [1] S. O. Diallo, \textit{et al.} Phys. Rev. Lett. \textbf{102}, 187206 (2009); J. Zhao , \textit{et al.} Nat. Phys. \textbf{5}, 555 (2009).\\[0pt] [2] Lester \textit{et al.} Phys. Rev. B \textbf{81}, 064505 (2010); unpublished. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P26.00005: Multiband effect on the magnetic resonance spectrum of pnictide superconductors Tanmoy Das, A.V. Balatsky The magnetic resonance behavior which is directly probed by the inelastic neutron scattering (INS) spectroscopy gives valuable information about the pairing mechanism of the unconventional superconductors. In high-$T_c$ cuprate superconductors, INS exhibits a clear signature of a magnetic resonance mode in addition to its characteristic dispersive feature (known as ``hour-glass'' behavior) which are enhanced dramatically below $T_c$ and the mode energy scales universally with the SC gap amplitude. In a multiband unconventional superconductors, the situation is more complex due to the presence of multi-orbital band structure, multiple-SC gaps as well as possibilities of having multiple pairing symmetries. We calculate magnetic susceptibility to show how does the magnetic resonance mode and its dispersion evolve both in energy as well as in momentum as a function of doping in both electron and holed doped pnictide superconductors. The inputs in our calculations are the Fermi surface information from ARPES or LDA and experimental values of superconducting gaps. We find that the magnetic resonance behavior is dramatically different in pnictide than in cuprates. The effects of multiple orbitals, gaps and different pairing symmetry play an important role. We argue that doping dependence of the resonance spectra can be understood from the topological change of the Fermi surface and the gap magnitudes, in good accord with experiments. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P26.00006: Spin-orbit coupling in Fe-based superconductors M.M. Korshunov, I. Eremin, P.J. Hirschfeld The recently discovered iron-based superconductors have attracted considerable attention mainly for their unconventional pairing state. In connection with the determination of pairing symmetry, the resonance peak observed in neutron scattering experiments [1] agrees well with predicted results for the extended s-wave ($s_\pm$) gap symmetry [2]. However, recent neutron measurements shows that there is anisotropy in the spin resonance [3]. In particular, $S^zS^z$ component is different from $S^+S^-$ component of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase without long-range order can occur due to spin-orbit (SO) coupling. We study the role of the SO coupling in the multiorbital model for Fe-pnictides, and discuss how it influences spin resonance feature and the relation to SC pairing. \\[4pt] [1]. A.D. Christianson \textit{et al.}, Nature \textbf{456}, 930 (2008). [2]. M.M. Korshunov and I. Eremin, Phys. Rev. B \textbf{78}, 140509(R) (2008); T.A. Maier and D.J. Scalapino, \textit{ibid}, 020514(R) (2008). [3]. O.J. Lipscombe \textit{et al.}, Phys. Rev. B \textbf{82}, 064515 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P26.00007: A LDA+DMFT+Vertex function study of dynamical magnetic susceptibility in iron based superconductors Hyowon Park, Kristjan Haule, Gabriel Kotliar We developed a method for computing dynamical magnetic susceptibility in complex correlated materials based on LDA+DMFT+Vertex function calculation. The dressed Greens function was obtained from the charge self-consistent LDA+DMFT calculation and the local Vertex function was computed from the quantum impurity model using a CTQMC impurity solver. We applied this method to compute the normal state magnetic susceptibility in iron pnictides and iron chalcogenides. Our result shows good agreement with inelastic neutron scattering data. At a low energy, the dynamical structure factor S(Q,w) is peaked at ($\pi$,0) momentum in BaFe$_{2}$As$_{2}$ and at ($\pi$/2,$\pi$/2) in FeTe, as expected for the distinct ordering of these compounds. At higher energy, the peak positions shifts to the ($\pi$,$\pi$) wave vector, in agreement with recent neutron experiments. We argue that this ($\pi$,$\pi$) magnetic response at high energy and the full spin dispersion above Neel temperature is captured by our realistic band structure method, LDA+DMFT. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P26.00008: Neutron scattering study of spin fluctuations on hole-overdoped KFe$_2$As$_2$ C.H. Lee, K. Kihou, A. Iyo, H. Eisaki, H. K.-Furukawa, H. Usui, K. Kuroki, T. Saito, H. Fukazawa, Y. Kohori, K. Yamada Spin fluctuations in Fe-based superconductors have attracted great attention since they can be a key factor of the formation of superconducting states. The inelastic neutron scattering technique is a powerful method to examine spin fluctuations, whereas measurements using a single crystal were restricted to Fe(Te,Se) or electron-doped AFe$_2$As$_2$ (A=Ba, Ca, or Sr) due to difficulty of growing a large single crystal. To overcome this problem, we have improved growth procedure and succeeded to grow single crystals of heavily hole-overdoped superconducting KFe$_2$As$_2$ ($T_c$ = 3.4 K). It was believed that no spin fluctuation can be observed in KFe$_2$As$_2$, since the nesting of the Fermi surface disappears. To confirm the hypothesis, we have studied spin fluctuations of KFe$_2$As$_2$ by neutron scattering using single crystals at JRR-3 reactor of JAERI in Tokai. As results, a well-defined low-energy incommensurate spin fluctuation has been observed at ($\pi(1\pm2\delta$),0) with $\delta$ = 0.16. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)$_2$As$_2$ at high energies. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped Fe-based superconductors, or a new type of spin fluctuation emerges by heavily hole doping. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P26.00009: Neutron Scattering Experiment on Magnetic Field Effect in Under-doped Superconducting BaFe$_{1.915}$Ni$_{0.085}$As$_{2}$ Miaoyin Wang, Pengcheng Dai, Meng Wang, Huiqian Luo, Jeffrey Lynn, Sung Chang, Songxue Chi, Deepak Singh, Jose Rodriguez In under-doped BaFe$_{2-X}$(Ni,Co)$_{X}$As$_{2}$, superconductivity coexist with the anti-ferromagnetic (AFM) order. By applying a $\sim $10 Tesla magnetic field parallel to a-b plan, we performed a series of elastic and inelastic neutron scattering measurement on BT-7 instrument in NCNR, NIST. We measured how magnetic bragg-peaks and spin excitation in BaFe$_{1.915}$Ni$_{0.085}$As$_{2}$ will change upon the change of the field. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P26.00010: Spin excitations as in hole-doped Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ superconductor Chenglin Zhang, Meng Wang, Miaoying Wang, Jun Zhao, Marty Karol, Mark Lumsden, Songxue Chi, Sung Chang, Jeffrey Lynn, Huiqian Luo, Tao Xiang, Jiangping Hu, Pengcheng Dai We used inelastic neutron scattering to study the optimally doped Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ (Tc=38K). In contrast to electron doped counterpart, we found that resonance is almost none-L dependence as shown in Fig.1 (b), but the spin gaps are. It is gaped along (0.5,0.5,0), however essentially gapless along (0.5,0.5,1). Meanwhile, the spin correlation is strongly temperature dependence which has not been observed in electron-doped 122 materials at all. The above findings clearly suggest that hole-doped region is indeed different from electron-doped region. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P26.00011: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P26.00012: Finite temperature spin dynamics of a square lattice $J_1-J_2$ antiferromagnet and its implications for iron arsenides Elihu Abrahams, Pallab Goswami, Rong Yu, Qimiao Si Motivated by recent inelastic neutron scattering measurements in the paramagnetic phase of iron arsenides, we have studied the finite temperature spin dynamics of a square lattice $J_1- J_2$ antiferromagnet in the parameter regime that gives rise to a collinear $(\pi, 0)$ ground state at zero temperature. We have calculated the dynamical structure factor $S(\mathbf{q}, \omega)$ in the paramagnetic state at finite temperatures using a modified spin wave theory. We have shown that short range antiferromagnetic correlations below the mean-field Ising transition temperature give rise to elliptic features for $S (\mathbf{q}, \omega)$ in momentum space. Employing an effective nonlinear sigma model analysis for the low energy and wave vector limit, we also account for fermion damping and circumvent the shortcoming of modified spin wave theory. Finally, considering a matrix $J_1-J_2$ model, we point out the connection between the Ising transition and the putative orbital ordering in iron arsenides. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P26.00013: Neutron and ARPES constraints on the couplings of the multiorbital Hubbard model for the iron pnictides Qinlong Luo, Adriana Moreo, Elbio Dagotto, George Martins, Dao-Xin Yao, Maria Daghofer, Rong Yu The results of neutron-scattering and angle-resolved photoemission experiments for the Fe-pnictide parent compounds are shown to impose severe constraints on the range of values that can be considered ``realistic'' for the intraorbital Hubbard repulsion U and Hund coupling J in multiorbital Hubbard models treated in the mean-field approximation. Phase diagrams for undoped three- and five-orbital models are discussed, and the physically realistic regime of couplings is highlighted [1]. \\[4pt] [1] Q. Luo {\it et al.}, Phys. Rev. B {\bf 82}, 104508 (2010), and references therein. [Preview Abstract] |
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