Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H26: Focus Session: Iron Based Superconductors -- Anisotropic Spin Dynamics |
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Sponsoring Units: DMP DCOMP Chair: Eduardo Granado Monteiro da Silva, Universidade Estadual de Campinas Room: D162/164 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H26.00001: Impact of the Spin Density Wave Order on the Superconducting Gap of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Ludivine Chauviere, Yann Gallais, Maximilien Cazayous, Marie-Aude Measson, Alain Sacuto, Dorothee Colson, Anne Forget We report a doping dependent electronic Raman scattering measurements on iron-pnictide superconductor Ba(Fe$_{1-x} $Co$_x$)$_2$As$_2$ single crystals. The B$_{2g}$ Raman spectrum at optimal doping is consistent with a strongly anisotropic gap on the electron pocket. Upon entering the coexistence region between superconducting (SC) and spin-density-wave (SDW) orders, the effective pairing energy scale is strongly reduced. Our results are interpreted in terms of a competition between SC and SDW orders for electronic states at the Fermi level. Our findings advocate for a strong connection between the SC and SDW gaps anisotropies which are both linked to interband interactions. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H26.00002: Frustrated square lattice Heisenberg model and magnetism in Iron Telluride Igor Zaliznyak, Zhijun Xu, Genda Gu, John Tranquada, Matthew Stone We have measured spin excitations in iron telluride Fe1.1Te, the parent material of (1,1) family of iron-based superconductors. It has been recognized that J1-J2-J3 frustrated Heisenberg model on a square lattice might be relevant for the unusual magnetism and, perhaps, the superconductivity in cuprates [1,2]. Recent neutron scattering measurements show that similar frustrated model might also provide reasonable account for magnetic excitations in iron pnictide materials. We find that it also describes general features of spin excitations in FeTe parent compound observed in our recent neutron measurements, as well as in those by other groups. Results imply proximity of magnetic system to the limit of extreme frustration. Selection of spin ground state under such conditions could be driven by weak extrinsic interactions, such as lattice distortion, or strain [3]. Consequently, different nonuniversal types of magnetic order could arise, both commensurate and incommensurate. These are not necessarily intrinsic to an ideal J1-J2-J3 model, but might result from lifting of its near degeneracy by weak extrinsic perturbations.\\[4pt][1] A. V. Chubukov, Phys. Rev. B 48, 5588 (1992). [2] P. A. Lindgard, Phys. Rev. Lett. 95, 217001 (2005). [3] I. A. Zaliznyak, Phys. Rev. B 68, 134451 (2003); ibid. 69, 092404 (2004). [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H26.00003: Superconductivity and Magnetism in the Checkerboard Models for Iron-based Superconductors Chen Fang, Xiaoli Lu, Yongjin Jiang, Wei-Feng Tsai, JiangPing Hu We study three different checkerboard models for iron-based superconductors and obtain their phase diagrams in the solvable limit of weakly coupled checkerboards. We demonstrate that the strongest superconducting pairing is in the $A_{1g}$-S wave channel and the development of the superconductivity (SC) is correlated with the emergence of the next nearest neighbor antiferromagnetism (AFM). Moreover, this study suggests that the superconductivity and magnetism are orbital-selective. In the three-band model, the AFM is more robust in the $d_{xy}$ orbital and the superconductivity is easier to be generated in the $d_{xz}$ and $d_{yz}$ orbitals. Comparisons between our theoretical results and current experimental measurements are discussed. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H26.00004: Anisotropy of the spin dynamics in hole and electron-doped 122 Invited Speaker: Pengcheng Dai This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H26.00005: Magnetic properties in the Mott-insulating iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$OSe$_{2}$ Jian-Xin Zhu, Rong Yu, Qimiao Si The role of electron correlation and magnetism in high-temperature superconductivity of the iron pnictides has been a topic of discussion. It has also motivated interest to compare related compounds with the iron pnictides and chalcogenides. Recently both electronic structure calculations and experimental measurements have indicated that the iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$OSe$_{2}$, which contains an Fe square lattice with an enlarged unit cell, has a larger U/t and is a Mott insulator [1]. We focus here on the understanding of the magnetism of this system. Within the density functional theory, we consider the magnetic phase diagram. Using an effective frustrating spin-exchange model in a doubled checker-board lattice, we study the magnetic excitation spectrum. Our theoretical results are compared with the emerging elastic and inelastic neutron scattering data in this compound. \\[4pt] [1] J.-X. Zhu, R. Yu \textit{et. al}, Phys. Rev. Lett. \textbf{104}, 216405 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H26.00006: Consistent model of magnetism in ferropnictides Aleksander Wysocki, Kirill Belashchenko, Vladimir Antropov The character of magnetic interactions and spin fluctuations in ferropnictides has until now resisted understanding within any conventional model of magnetism. We show that the most puzzling features can be naturally reconciled within a rather simple effective spin model with biquadratic interaction, which is consistent with electronic structure calculations. While preserving the symmetry of the lattice, this model spin Hamiltonian stabilizes the collinear stripe ground state and generates an anisotropic spin wave spectrum. A natural reinterpretation of the measured spin wave spectra in ferropnictides is presented based on this model. Classical Monte Carlo simulations with experimentally motivated parameters produce reasonable Neel temperatures for 122 compounds. The model predicts that the phase transition to the paramagnetic phase changes from second to first order as the magnitude of the biquadratic term is increased. This property agrees with the observed behavior of the 122 compounds under doping. A clear signature of the separation of the nematic and antiferromagnetic phase transitions is also found. Preprint: arXiv:1011.1715. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H26.00007: Spin-resolved electron-phonon coupling in FeSe Timur Bazhirov, Jesse Noffsinger, Marvin L. Cohen FeSe is one of the simplest iron-based superconductors. There are previous studies indicating that including the iron magnetic moment ordering has a significant effect on electron-phonon interactions and thus might be important for superconductivity. To explore the role of spin-dependent phonon induced pairing of the electrons, we apply first principle techniques based on the pseudopotential density functional approach and the local spin density approximation to calculate the electron-phonon coupling properties of FeSe. Our results indicate that introducing magnetic moments leads to a significant increase in coupling at least for certain phonon modes. At the gamma point in the Brillouin zone the increase is two-fold. Both phonon renormalization and electron-phonon matrix elements increases are present. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H26.00008: Spin-phonon coupling and superconductivity in iron pnictides Taner Yildirim, Xuhui Luo, Serdar Ogut Early electron-phonon (el-ph) coupling calculations for iron pnictide system based on standard non-spin-polarized perturbation theory indicate that conventional el-ph coupling cannot explain the observed high Tc in these systems. However, the experimental phonon spectrum indicates features which are not produced in the standard linear response non-magnetic phonon calculations. The magnetic phonon calculations clearly indicate that the observed phonon-DOS at room temperature is much closer to the magnetic phonon-DOS rather than non-magnetic DOS and Fe-magnetism must present in the iron-pnictide systems all the time [1-2]. Thus we need to calculate the magnetic el-phonon coupling with the Fe-spins included before we can rule out any type of phonon-mediated mechanism. In order to carry out such complex self-consistent magnetic el-ph coupling calculations we are developing a finite-displacement method in which both the phonon energies and the corresponding el-ph coupling constant are easily calculated. Implications of our results on the mechanism of superconductivity in iron pnictides will be discussed. Finally, we will compare our calculations with the available phonon energy and line-width measurements. \\[4pt] [1] T. Yildirim, Phys. Rev. Lett. 102, 037003 (2009). \\[0pt] [2] T. Yildirim, Physica C 469, 425-441 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H26.00009: Gap structure of the iron-pnictide superconductor LiFeAs via low-temperature thermal conductivity J.-Ph. Reid, S. Ren\'e de Cotret, Louis Taillefer, M.A. Tanatar, H. Kim, K. Cho, R. Prozorov, Y.J. Song, Y.S. Kwon The thermal conductivity of the stoichiometric iron-pnictide superconductor LiFeAs was measured at temperatures down to $T \sim 50$~mK in magnetic fields up to $H=17$~T on high-quality single crystals with $T_c \simeq 18$~K. The absence of any residual linear term at $T \to 0$ shows that there are no nodal quasiparticles. The slow increase of thermal conductivity with magnetic field shows that the gap is large everywhere on the Fermi surface. The same behaviour is observed for both in- plane and out-of-plane directions. We conclude that the superconducting gap in LiFeAs is basically isotropic. This is similar to what has been found in the iron-pnictide superconductors Ba$_{1- x}$K$_x$Fe$_2$As$_2$ [1] and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ [2] at optimal doping (maximal $T_c$). \\[4pt] [1] X.-G. Luo {\it et al.}, Phys. Rev. B {\bf 80}, 140503 (2009). \\[0pt] [2] J.-Ph. Reid {\it et al.}, Phys. Rev. B {\bf 82}, 064501 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H26.00010: NMR Study of the SDW ordering and the Spin Fluctuations on NaFeAs single crytals Weiqiang Yu, L. Ma, S. Zhang, J. Zhang, T.-L. Xia, G.F. Chen, Dao-Xin Yao In iron pnictides, the nature of the spin density wave (SDW) ordering is still not clear. Recently, increasing attention has been drawn to the correlation between the SDW transition and the high-temperature tetragonal to the low-temperature orthorhombic structure transition. In NaFeAs, the magnetic moment is small and both transitions are well separated, and therefore NaFeAs could be a good candidate to study the interplay of different degrees of freedom microscopically. In this talk, we report our $^{23}$Na and $^{75}$As NMR observations on NaFeAs single crystals. We found that 1) the spin fluctuations are largely enhanced below the structure transition; 2) the SDW transition temperature and the magnetic moment increase significantly with pressure; and 3) the NMR linewidth and the temperature/field dependence of the spin- lattice relaxation rate show signatures of an incommensurate SDW ordering in a limited temperature range just below the SDW transition. Based on these results, we discuss the coupling between the magnetism and the lattice/band structure in NaFeAs. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H26.00011: Spin excitation in LiFeAs Meng Wang, Huiqian Luo, Xiancheng Wang, Yang Zhao, Jeffrey Lynn, Changqing Jin, Pengcheng Dai We used inelastic neutron scattering to study the spin excitations in LiFeAs. Clear spin excitations were found but there was no spin resonance. Surprisingly, very big spin gap exists in this material. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H26.00012: Neutron Diffraction Studies of PrFe(1-x)Ru(x)AsO Yuen Yiu, Ovidiu Garlea, Ashfia Huq, Michael McGuire, David Mandrus, Stephen Nagler We report neutron powder diffraction (NPD) studies of Ru doped PrFe(1- x)Ru(x)AsO. The parent compound PrFeAsO undergoes a structural transition as well as magnetic transitions involving Fe and Pr moments upon cooling. Previous measurements (M. A. McGuire et al, Jrnl of Solid State Chem, 182- 8, 2326-2331) showed that Ru doping suppresses the above transitions. However, unlike most 1111's, this does not lead to superconductivity. To investigate the origin of this odd behavior we performed NPD measurements as a function of temperature for values of x up to 0.75. The results showed that although the structural and magnetic transitions are suppressed, the c axis displayed apparent negative thermal expansion (NTE) for all values of x. Such NTE has been seen in the parent compound (S. A. J. Kimber et al, PRB 78-140503), but to our knowledge there are no reports of NTE in superconducting samples. This suggests that the mechanism producing the NTE could also be responsible for the absence of superconductivity. We also report data on the magnetic transitions for lightly doped samples with x up to 0.1. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H26.00013: Interactions between rare earth and iron magnetism in \textit{RE}FeAsO single crystals A. Kreyssig$^{2,3}$, M.G. Kim$^{2,3}$, W. Tian$^{2}$, W. Ratcliff II$^{4}$, J.-W. Kim$^{5}$, S. Nandi$^{2,3}$, J.-Q. Yan$^{2}$, B. Jensen$^{2}$, K.W. Dennis$^{2}$, R.W. McCallum$^{2,6}$, T.A. Lograsso$^{2}$, J.W. Lynn$^{4}$, J.L. Zarestky$^{2}$, R.J. McQueeney$^{2,3}$, A.I. Goldman$^{2,3}$ In iron-based pnictides high-temperature superconductors, magnetic fluctuations and magneto-elastic effects are believed to be important for the superconducting electron pairing mechanism. To gain insight into the interplay between the different ordering phenomena and the underlying couplings we studied the tetragonal-to-orthorhombic distortion and the magnetic order by x-ray and neutron diffraction on \textit{RE}FeAsO single crystals. The onset of rare earth (\textit{RE} = Nd, Pr) magnetic order is coupled to changes in the iron magnetic structure without affecting the lattice distortion. High-resolution neutron and x-ray resonant magnetic scattering measurements down to 0.4 K revealed complex magnetic structures with multiple propagation vectors at low temperatures. [Preview Abstract] |
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