Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X39: Focus Session: Iron Based Superconductors: Spin Excitation |
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Sponsoring Units: DMP DCMP Chair: Despina Louca, University of Virginia Room: F150 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X39.00001: Spin Excitations in Superconducting BaFe$_{2-x}$Co$_{x}$As$_{2}$ A.D. Christianson, M.D. Lumsden, D. Parshall, S.E. Nagler, G.J. MacDougall, H.A. Mook, K. Lokshin, T. Egami, M.B. Stone, D.L. Abernathy, E.A. Goremychkin, R. Osborn, M.A. McGuire, A.S. Sefat, R. Jin, B.C. Sales, D. Mandrus We present neutron scattering measurements on singe crystals of superconducting BaFe$_{2-x}$Co$_{x}$As$_{2}$. For optimally doped samples (x=0.16, T$_{C}$=21 K) the spin waves are gapless above T$_{C}$, while a gap opens below T$_{C}$ with the appearance of a spin resonance at an energy of 8.6 meV. The dispersion of the spin excitations is weak along the c-axis, but the in-plane dispersion is still very steep indicating a reduction in dimensionality of the spin excitations when compared to those found in the parent compound. On the other hand, in underdoped samples (x=0.08, T$_{C}$=11 K) the spin waves display a gap above T$_{C}$ with anisotropic three-dimensional interactions. Below T$_{C}$ no additional gap is observed for energies greater than 2 meV. However, below T$_{C}$ a spin resonance at 4.5 meV appears simultaneously with a reduction in the magnetic Bragg peak intensity suggesting that the spectral weight required for the spin resonance arises due to competition between superconductivity and magnetism. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X39.00002: Low-temperature spin excitations in Ba(Fe$_{0.972}$Cu$_{0.028})_{2}$As$_{2}$ single crystals M.G. Kim, A. Kreyssig, D.K. Pratt, K.J. Marty, M. D. Lumsden, W. Tian, J. Zarestky, S. Nandi, A. Thaler, N. Ni, S.L. Bud'ko, P.C. Canfield, R.J. McQueeney, A.I. Goldman We report on inelastic neutron scattering measurements of low temperature spin excitations in the non-superconducting Ba(Fe$_{0.972}$Cu$_{0.028})_{2}$As$_{2}$ compound that orders antiferromagnetically at $T_{N}$ = 62(1) K. These results are compared to Ba(Fe$_{0.953}$Co$_{0.047})_{2}$As$_{2}$, a compound that has a similar Neel temperature, but is superconducting below 17 K. Since the normal state spin fluctuations can possibly mediate superconducting pairing, it is interesting to compare the two compounds, considering that one is a superconductor and the other is not. While differing in some details, the spin excitation spectra for the Cu-doped and Co-doped samples are qualitatively similar, and can be understood as strongly damped spin waves. Given the similar spin excitations, additional factors must be considered to explain why the Cu-doped compound is not a superconductor. -- The work at the Ames Laboratory was supported by the US DOE, Office of Science, under contract No. DE-AC02-07CH11358. The work at ORNL was supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, US DOE. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X39.00003: Anisotropic spin fluctuations in superconducting pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_2$ Oliver Lipscombe, Chenglin Zhang, Leland Harriger, Paul Freeman, Pengcheng Dai Spin excitations in optimally doped superconducting BaFe$_{2-x} $Ni$_{x}$As$_2$ ($x=0.1$) were investigated with polarized neutron scattering. In previous (unpolarized) neutron scattering experiments a resonance and spin gap had been observed, but using polarization analysis we have been able to show that the response observed is of purely magnetic origin. Furthermore, we have found anisotropy in the magnetic response, showing different behavior between spin fluctuations with differing magnetic moment direction. This is an unexpected find, as this composition is not magnetic ordered. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X39.00004: Neutron Scattering Experiment on Doping Dependence of Spin Excitations in Superconducting BaFe$_{2-x}$Ni$_{x}$As$_{2}$ Shiliang Li, Jun Zhao, Pengcheng Dai, Songxue Chi, Jeffrey Lynn, Chenglin Zhang, Huiqian Luo, Meng Wang, Sung Chang, Deepak Singh Understanding the interplay between spin fluctuation and superconductivity in high transition temperature (Tc) superconductors is important because spin fluctuations may mediate electron paring for superconductivity. In copper-oxide based superconductor (cuprates), resonance modes are found at antiferromagnetic (AF) ordering vector Q = (1/2, 1/2), which are dispersionless along the c-direction and directly coupled to Tc. In the FeAs-based superconductors, although the energies of the spin resonance are also proportional to Tc, our recently experiments found that the resonance has c-axis dispersion and display distinct energies at Q$_{z}$=0 and 1. In this paper, we use inelastic neutron scattering to study electron-doping dependence of the resonance energy in underdoped and overdoped superconducting BaFe$_{2-x}$Ni$_{x}$As$_{2}$ (x=0.075, 0.15). We find that there exists similar energy dispersion in both underdoped and overdoped samples, suggesting that the c-axis coupling in BaFe$_{2-x}$Ni$_{x}$As$_{2}$ is weakly doping dependent. We discuss the microscopic origin for such behavior. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X39.00005: Inelastic Neutron Scattering Studies of High-Energy Spin Excitations in Superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ Mengshu Liu, Douglas Abernathy, Jun Zhao, Meng Wang, Chenglin Zhang, Miaoyin Wang, Pengcheng Dai Understanding how the spin fluctuations evolve with doping in iron pnictide superconductors is important because spin fluctuations may mediate electron pairing for superconductivity in these materials. Upon doping, the spin fluctuation persists long after the long-range antiferromagnetism is destroyed. More importantly, spin excitations are coupled to superconductivity in the appearance of a neutron magnetic resonance and a superconductivity-induced spin gap. However, all current neutron scattering results in iron based superconductors are confined to low energy excitations except for the ``11'' FeTe$_{1-x}$Se$_{x}$ system, which shows incommensurate excitations that are not found in other iron pnictide systems. Therefore, how the spin waves in parent compounds of the ``122'' (AFe$_{2}$As$_{2}$, A = Ca, Sr, Ba) system will evolve when the system becomes an optimal superconductor is still an open question. We use time-of-flight spectroscopy to determine S (Q,$\omega )$ at energy regions not accessed before. We compare spin fluctuations of iron arsenide superconductors with those of high-Tc copper oxides and discuss their role in the superconductivity of these materials. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X39.00006: Single Crystal Inelastic Neutron Scattering Measurements of the Unconventional Superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ J.P. Castellan, S. Rosenkranz, R. Osborn, F. Weber, E.A. Goremychkin, I.S. Todorov, H. Claus, D.Y. Chung, M.G. Kanatzidis, T. Guidi Iron arsenide superconductors have attracted a great deal of interest because of their similarities with high-Tc copper oxides. In both the copper oxides and iron arsenides, the parent compounds are antiferromagnetically ordered and superconductivity arises by suppressing this antiferromagnetic order by chemical doping. An important feature of the copper oxide superconductors is the existence of a resonant magnetic excitation. This resonance is localized both in energy and wavevector and is predicted to occur only when there is a change of sign of the superconducting energy gap on different parts of the Fermi surface. Following our observation of the resonance below Tc in a polycrystalline sample of Ba0.6K0.4Fe2As2 [Nature \textbf{456}, 930 (2008)], we have now performed inelastic neutron scattering measurements in a composite single crystal confirming that the resonance is centered at the $\Gamma$-M point with modulations along the $c$-axis. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 4:18PM |
X39.00007: Spin excitations in iron arsenide superconductors Invited Speaker: We used neutron scattering to study the spin excitations of the optimally electron doped BaFe1.9Ni0.1As2 (Tc =20 K) iron arsenide superconductor. We found a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. Our results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of superconductivity in iron arsenides. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X39.00008: Magnetic Excitation Spectrum of FeTe$_{1-x}$Se$_x$ M.D. Lumsden, A.D. Christianson, E.A. Goremychkin, S.E. Nagler, H.A. Mook, M.B. Stone, D.L. Abernathy, T. Guidi, G.J. MacDougall, C. de la Cruz, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus The magnetic excitation spectrum of single crystal samples of the bulk superconductor FeTe$_{0.51}$Se$_{0.49}$ and Fe$_{1.04} $Te$_{0.73}$Se$_{0.27}$ (not a bulk superconductor) have been studied using inelastic neutron scattering. Both samples exhibit incommensurate magnetic excitations despite the lack of long-range magnetic order. These incommensurate excitations are very two-dimensional in nature and persist to energies greater than 250 meV. The incommensurate excitations originate from a 2d wavevector near (1/2, 1/2), where the resonance was observed for the bulk superconductor FeTe$_{0.51}$Se$_{0.49}$, and broaden with increasing energy transfer. The spectrum exhibits four-fold symmetry about the (1,0) wavevector (the square lattice ($\pi$, $\pi$) point) consistent with the symmetry of the underlying Fe square lattice. The excitations can be characterized by the same model and the incommensuration is parameterized by the same wavevector as the high-T$_C$ cuprates demonstrating commonality in the magnetism of these two classes of unconventional superconductors. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X39.00009: Magnetic Excitations in FeSe$_{x}$Te$_{1-x}$ Leland Harriger, Oliver Lipscombe, Pengcheng Dai, Tao Hong, Karol Marty, Songxue Chi, Jeff Lynn The role of magnetism in high Tc superconductors has been widely debated. A ``resonance'' in the inelastic magnetic scattering in cuprates that tracks Tc is strong evidence that a relationship does exist. This resonance has now been seen in the iron arsenide superconductors as well, further strengthening theories that seek to correlate magnetism with superconductivity. Magnetism in both the pnictide and cuprates begins as a parent antiferromagnetic (AF) ground state. The resonance appears with superconductivity with a center that grows in energy upon doping but remains fixed at the Q = (0.5, 0.5) antiferromagnetic wavevector of the parent. At first glance, scattering in the superconducting FeSe$_{x}$Te$_{1-x}$ series appears to mimic that seen in the iron arsenides and cuprates; exhibiting both an AF ground state and resonance mode. However, the resonance in this system is peculiar in that it exists not at the Q = (0.5, 0, 0.5) wavevector of its AF parent but back at the (0.5, 0.5) position of the pnictides and cuprates. In this talk, I will discuss neutron scattering experiments that map out the evolution of spin excitations as spectral weight shifts from the Q = (0.5, 0, 0.5) to the Q = (0.5, 0.5) position with doping and contrast it with the scattering seen in other compounds. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X39.00010: Magnetic fluctuations and quasi-static magnetism in optimally doped FeSe$_{0.4}$Te$_{0.6}$ Vivek Thampy, Wei Bao, A.T. Savici, Y. Qiu, Jin Hu, Tijiang Liu, Z.Q. Mao, Collin Broholm Magnetic Fluctuations in the optimally doped 11-type iron superconductor FeSe$_{0.4}$Te$_{0.6}$ were examined using inelastic neutron scattering on the MACS instrument at NIST. In the normal state at T=25K we find strong low energy fluctuations through an extended area of the (hk0) zone that includes and connects the high symmetry (1/2,0,0) and (1/2,1/2,0) points. In the superconducting state intensity at the (1/2,1/2,0) location is depleted for $\hbar\omega$ = 1.5 meV as spectral weight is transferred to the 6.5 meV resonance. Low energy and quasi-elastic scattering however remains at (1/2,0,0). In the (HHL) zone we observed striped features indicating shorter range correlations along c. While glassy magnetism and superconductivity coexist in our samples, they are associated with distinct parts of momentum space. Work at JHU was supported by DoE through DE-FG02-08ER46544. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X39.00011: Magnetic correlations in the spin-glassy phase of Fe$_{1+\delta }$(Se,Te) Naoyuki Katayama, Sungdae Ji, Seunghun Lee, Masaki Fujita, Jinsheng Wen, Zhijun Xu, Guangyong Xu, Genda Gu, Taku Sato, Sung Chang, Kazutishi Yamada, John Tranquada Using elastic and inelastic neutron scattering techniques, we investigated magnetic correlations in Fe$_{1.02}$Se$_{0.3}$Te$_{0.7}$ and Fe$_{1.01}$Se$_{0.15}$Te$_{0.85}$, both of which exhibit spin-glassy (SG) behaviors at low temperatures. Below the phase transition temperature, T$_{SG}$, magnetic Bragg peaks appear at incommensurate wavevector (\textbf{Q}$_{m})$ positions with \textbf{Q}$_{m}$ = (0.46, 0, 0.50). The peaks are broader than the instrumental resolution, indicating short range magnetic ordering. Above T$_{SG}$, strong short range magnetic fluctuations exist around \textbf{Q}$_{m}$ = (0.46, 0, 0.50). The fluctuations have very weak L-dependence, indicating the good two-dimensionality of the magnetic correlations. The location of \textbf{Q}$_{m}$ contrasts with the characteristic wave vector, (0.5,0.5,L), of the magnetic fluctuations observed in the superconducting phase of Fe$_{1+d}$(Se,Te). [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X39.00012: Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe$_{0.6}$Se$_{0.4}$ superconductor Dimitri Argyriou, A. Hiess, A. Akbari, I. Eremin, M.M. Korshunov, J. Hu, B. Qian, Z. Mao, Y. Qiu, C. Broholm, W. Bao We report on inelastic neutron scattering measurements that find incommensurate itinerant like magnetic excitations in the normal state of superconducting FeTe$_{0.6}$Se$_{0.4}$ (Tc=14K) at an incommensurate wave-vector $\mathbf{Q}_{inc}=(1/2\pm\epsilon,1/2\mp\epsilon)$ with $\epsilon$=0.09(1). In the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal state excitations. We use a four band model to describe the Fermi surface topology of iron-based superconductors with the extended $s(\pm)$ symmetry and find that it qualitatively captures the salient features of these data. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X39.00013: Effect of magnetic field on the spin resonance in FeTe$_{0.5}$Se$_{0.5}$ as seen via inelastic neutron scattering Jinsheng Wen, Guangyong Xu, Zhijun Xu, Zhi Wei Lin, Qiang Li, Ying Chen, Songxue Chi, Genda Gu, John Tranquada Inelastic neutron scattering and susceptibility measurements have been performed on the optimally-doped Fe-based superconductor FeTe$_{0.5}$Se$_{0.5}$, which has a critical temperature, $T_c$ of 14~K. The magnetic scattering at the stripe antiferromagnetic wave-vector ${\bf Q}=(0.5,0.5)$ exhibits a ``resonance'' at $\sim6$~meV, where the scattering intensity increases abruptly when cooled below $T_c$. In a 7-T magnetic field, $T_c$ is slightly reduced to $\sim12$~K, based on susceptibility measurements. The resonance in the neutron scattering measurements is also affected by the field applied along the [1$\bar1$0] direction. The resonance intensity under field cooling starts to rise at a lower temperature $\sim12$~K, and the low temperature intensity is also reduced from the zero-field value. Our results provide clear evidence for the intimate relationship between superconductivity and the resonance measured in magnetic excitations of Fe-based superconductors. [Preview Abstract] |
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