Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N37: Focus Session: Fe-based Superconductors: Spin Fluctuations |
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Sponsoring Units: DMP DCOMP Chair: Guangyong Xu, Brookhaven Natl Lab Room: 345/346 |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N37.00001: Neutron Scattering as a Probe of Fermi Surface Nesting in Iron-Based Superconductors Invited Speaker: Raymond Osborn Superconductivity in the iron-based compounds is induced by suppressing a magnetically ordered phase by doping, pressure, or disorder, so it is no surprise that neutron scattering has had an important role in the field, elucidating both the origin of magnetic fluctuations and their role in the unconventional superconductivity. Our investigations of BaFe$_2$As$_2$ doped with potassium [1], sodium, and phosphor, can be interpreted within the framework of weakly correlated itinerant magnetism, in which Fermi surface nesting between hole pockets at the zone center and electron pockets at the zone boundary is responsible for both the magnetic (SDW) order and the superconductivity. Resonant spin excitations that occur when the superconducting energy gap changes sign on different parts of the Fermi surface were initially observed by inelastic neutron scattering in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ representing the first phase-sensitive evidence of s$_\pm$-symmetry [2]. We have since shown that the resonance splits into two with hole-doping because of the growing mismatch in the hole and electron Fermi surface volumes, accompanied by a decrease in the binding energy of the resonance and its spectral weight in accordance with RPA theory [3]. A detailed examination of the phase diagram close to the critical phase boundary for SDW order has identified a new phase that is further evidence of the role of Fermi surface nesting in generating magnetic order. \\[4pt] [1] S. Avci, O. Chmaissem, E. Goremychkin, \textit{et al}, Phys Rev B \textbf{83}, 172503 (2011).\\[0pt] [2] A. Christianson, E. Goremychkin, R. Osborn, \textit{et al}, Nature \textbf{456}, 930-932 (2008).\\[0pt] [3] J.-P. Castellan, S. Rosenkranz, E. Goremychkin, \textit{et al}, Phys Rev Lett \textbf{107}, 177003 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N37.00002: Spin fluctuations of BaFe2(As,P)2 studied by neutron scattering Chul-Ho Lee, P. Steffens, N. Qureshi, K. Kihou, M. Nakajima, A. Iyo, H. Eisaki, M. Braden Superconductivity can be induced in parent compounds of iron-based superconductors by several methods: carrier doping, external pressure and chemical pressure. To understand their superconducting mechanism, clarifying what is a common property for achieving high-Tc superconductivity is crucial. To date, studies on spin fluctuations have been mainly performed on carrier doped samples. On the other hand, there are only a few studies on chemical pressurized samples examined by powder samples. In this work, thus, we studied spin fluctuations of P doped BaFe$_2$(As,P)$_2$ ($T_c$ = 29.5K) using single crystal samples. Inelastic neutron scattering measurements were conducted using triple axis spectrometer IN8 of ILL. As results, well-defined commensurate peaks have been observed at (0.5,0.5,$L$), which is consistent with the nesting vector of the Fermi surface. Energy spectrums at $T$ = $T_c$ show $L$ dependence, suggesting a three dimensional character remains even in superconducting BaFe$_2$(As,P)$_2$. Clear spin gap has been observed below $T_c$, whose gap structure depends on $L$. Details will be discussed at the conference. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N37.00003: Role of the spin-orbit coupling in the spin-resonance formation in Fe-based superconductors M.M. Korshunov, Yu.N. Togushova, I. Eremin, P.J. Hirschfeld Determination of the gap symmetry is an important step towards uncovering mechanism of superconductivity in Fe-based materials. One of the key experiments in support of the $s_\pm$ spin-fluctuation-mediated gap was observation of the spin-resonance peak in many pnictides and chalcogenides, see P.J. Hirschfeld et al., Rep.Prog.Phys. 74, 124508 (2011). Recently, in inelastic polarized neutron scattering measurements by Lipscombe et al., Phys.Rev. B 82, 064515 (2010), it was found that the peaks in the transverse and longitudinal components of the spin susceptibility of BaFe$_{1.9}$Ni$_{0.1}$As$_2$ exhibit rather different behavior, and argued that the true spin resonance exists in the transverse channel only. Here, on the basis of the 5-orbital model, we argue that this disparity arises from spin-orbit coupling. It also leads to the relative shift of the two component's resonance frequency with lower frequency one exhibiting larger enhancement. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N37.00004: Compositional dependence of low energy spin fluctuations in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ Gregory Tucker, D.K. Pratt, A. Thaler, N. Ni, K. Marty, A.D. Christianson, M.D. Lumsden, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, A.I. Goldman, R.J. McQueeney The low energy magnetic fluctuation spectrum of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ samples in the range $x=(0.014,0.055)$ were studied in their antiferromagnetic ordered states using inelastic neutron scattering. The parent compound BaFe$_2$As$_2$ exhibits gapped spin-wave excitations at $\mathbf{Q}_{\mathrm{AFM}} = (1, 0, 1)$ with two gap energies [corresponding to in-Fe-plane and out-of-Fe-plane transverse excitations of the ordered moment]. Substitution of Co for Fe via doping acts to increase Landau damping without significantly modifying the parent compound spin-gap structure. For small amounts of Co the two-gap structure is resolvable in our measurements. For larger Co-doping the damping increases sufficiently to obscure the parent compound gap structure and the measured $\mathbf{Q}_{\mathrm{AFM}}$ excitation spectra is best described by a diffusive model. Interestingly, the change in excitation character corresponds in composition with the appearance of superconductivity in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As; these two effects may very well be related. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N37.00005: Spin Excitations in Overdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ A.D. Christianson, S. Calder, J.E. Mitchell, K. Marty, C.H. Wang, M.B. Stone, A.S. Sefat, B.C. Sales, M.D. Lumsden The relationship between spin excitations and unconventional superconductivity has been and continues to be the subject of considerable experimental and theoretical scrutiny. While the underdoped and optimally doped regions of the phase diagrams of unconventional superconductors have been extensively studied there have few studies of the spin excitations in the overdoped region. Here we report an inelastic neutron scattering study of an overdoped sample of the unconventional superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with x=0.15 and T$_c$ = 12 K. At energies below 40 meV the spin excitations are much broader and weaker when compared to samples close to optimal doping. Despite the weakness of the spin excitations a broad spin resonance is still observed at an energy of $\sim$8 meV at the wave vector (0.5 0.5 0). This corresponds to a value of 7.7 K$_B$T$_c$ which is nearly double the value of 4 K$_B$T$_c$ found for many Fe-based superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N37.00006: Magnetism in parent Fe-chalcogenides: quantum fluctuations select a plaquette order Natalia Perkins, Samuel Ducatman, Andrey Chubukov The analysis of magnetism in parent compounds of iron-based superconductors (FeSCs) is an integral part of the program to understand the origin of superconductivity in these materials. Here we analyze magnetic order in iron-chalcogenide Fe$_{1+y}$Te -- the parent compound of high-temperature superconductor Fe$_{1+y}$Te$_{1-x}$Se$_x$. Neutron scattering experiments show that magnetic order in this material contains components with momentum $Q_1=(\pi/2, \pi/2)$ and $Q_2 =(\pi/2, -\pi/2)$ in Fe-only Brillouin zone. The actual spin order depends on the interplay between these two components. Previous works argued that spin order is a single-$Q$ state (either $Q_1$ or $Q_2$). Such an order breaks rotational $C_4$ symmetry and order spins into a double diagonal stripe. We show that quantum fluctuations actually select another order -- a double $Q$ plaquette state with equal weight of $Q_1$ and $Q_2$ components, which preserves $C_4$ symmetry but breaks $Z_4$ translational symmetry. We argue that the plaquette state is consistent with recent neutron scattering experiments on Fe$_{1+y}$Te. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N37.00007: Unexpected ($\pi,\pi$) order in Fe$_{1.1}$Te David Fobes, Igor Zaliznyak, Zhijun Xu, Genda Gu, John M. Tranquada, Deepak Singh We have studied the evolution of the magnetic and crystal structure in single crystalline Fe$_{1.1}$Te, an iron-rich parent of the chalcogenide superconductor family. While a structural transition to a monoclinic symmetry occurs at $\sim 60$ K, magnetic peaks at $2\pi \cdot (0.48, 0)$ only arise below T$_{N}\approx 58$ K, and can be understood as bicollinear magnetism with discommensuration defects. \footnote{I. Zaliznyak \textit{et al.}, Phys. Rev. B \textbf{85}, 085105 (2012)} Unexpectedly, we have also observed resolution limited peaks at approximately $(\pi, \pi)$, arising at the same temperature T$_{N}$, and exhibiting temperature hysteresis similar to that seen in magnetic susceptibility, perhaps indicating that these peaks are of magnetic origin. Additionally, the position of these peaks is nearly the same as in the parent compounds of the iron pnictide family of superconductors, where magnetic order is simple collinear commensurate antiferromagnetism. The origin of these new peaks near ($\pi, \pi$) and their relationship to the dominant bicollinear magnetic order observed in Fe$_{1.1}$Te presents a puzzle. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N37.00008: Inelastic neutron scattering studies on the incommensurate-to-commensurate transformation of low energy magnetic excitations in $Fe_{1+\delta-y}(Ni/Cu)_{y}Te_{1-x}Se_{x}$ Zhijun Xu, Jinsheng Wen, John Schneeloch, Yang Zhao, Masaaki Matsuda, Wei Ku, Xuerong Liu, Genda Gu, D.-H. Lee, R.J. Birgeneau, J.M. Tranquada, Guangyong Xu We have performed a series of neutron scattering and magnetization measurements on $Fe_{1+\delta-y}(Ni/Cu)_{y}Te_{1-x}Se_{x}$ system to study the interplay between magnetism and superconductivity. Both non-superconducting and superconducting samples with T$_{c}$ 8$\sim$15K are studied. The low energy magnetic excitations of all samples at T$>$$>$T$_{c}$ consist of two incommensurate vertical columns. They change to a distinctly different U-shaped dispersion at T$>$T$_{c}$ for the superconducting samples and the transition temperature depend on the composition.[1] On the other hand, for all non-superconducting samples, there is no clear temperature dependence, and the low energy magnetic excitations remain two columns for temperatures down to 1.5 K. Work is supported by the Office of Basic Energy Sciences, DOE. [1]Zhijun Xu et al., arXiv:1201.4404(accepted by PRL). [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N37.00009: Cu-induced localization in the Fe-based superconductor FeTe$_{0.5}$Se$_{0.5}$ Jinsheng Wen, Zhijun Xu, Cheng Zhang, Masa Matsuda, Oleg Sobolev, Jitae Park, Edith Bourret, Dunghai Lee, Qiang Li, Genda Gu, Guangyong Xu, John Tranquada, Robert Birgeneau We report neutron scattering and resistivity results on the Cu-substitution effects in FeTe$_{0.5}$Se$_{0.5}$ with a $T_c$ of $\sim$15 K. With a 2$\%$ Cu substitution, the $T_c$ is reduced to 8 K, and for Fe$_{0.9}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$, it is not superconducting. In Fe$_{0.9}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$, the low-energy magnetic excitations around the in-plane wave vector (0.5, 0.5) is greatly enhanced. Upon heating, the magnetic scattering is weakened, which is different from the temperature dependences of the Cu-free and 2$\%$ Cu-doped sample. The spectral weight reduction upon warming decreases with increasing energy in the 10$\%$ Cu-doped sample. We take these as evidences that Cu drives the system towards localization, which is confirmed by our resistivity data. These observations probably explain why superconductivity is absent in the Cu-doped BaFe$_2$As$_2$ system and demonstrate the inadequacy of the rigid-band shift model on the substitution effects of the 3$d$ transition metals. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N37.00010: Neutron Scattering in Co Doped NaFeAs Yu Song, Chenglin Zhang, Scott Carr, Zachary Cole, Nicolas Luttrell, Pengcheng Dai In this presentation we will discuss some of our recent neutron scattering work done on Co doped NaFeAs, focusing on the evolution of magnetism and superconductivity with doping. The phase diagram of Co doped NaFeAs is similar to Co,Ni doped BaFe2As2, but also show significant differences in the magnetic response despite the similar Tc in both systems. Thus by comparing these systems, we attempt to find common features for unconventional superconductivity in both systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N37.00011: Magnetic excitation in Co-doped NaFeAs studied by neutron scattering Chenglin Zhang, Yu Song, Guotai Tan, Scott Car, Pengcheng Dai Even though NaFeAs ``111'' shares many similarities with BaFe2As2 ``122'' such as magnetic structure and phase diagram with doping , actually they are quite different from many aspects. For one example, the spin resonance is very sharp like delta function and well below 2detal, in sharp contrast with the broaden resonance observed in doped-122 systems. Our result provide a strong piece of evidence to support S$+$\_ and exclude S$++$ pairing symmetry in Fe-based superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N37.00012: NMR Search for the Spin Nematic State in LaFeAsO Single Crystal Mingxuan Fu, David A. Torchetti, Takashi Imai, Fanlong Ning, Jiaqian Yan, Athena S. Sefat The mechanism underlying high-$T_{c}$ superconductivity in iron-pnictides remains a major puzzle in condensed matter. Earlier NMR measurements provide evidence for a correlation between $T_{c}$ and the enhancement of low frequency spin fluctuations\footnote{F. L. Ning, T. Imai. et al., Phys. Rev. Lett. 104, 037001 (2010).}. However, slowing of spin fluctuations is accompanied by lattice softening, which is a major complication in this scenario. The intermediate temperature range between the tetragonal-orthorhombic structural phase transition at $T_{TO}$ and SDW transition at $T_{SDW}$ may be a realization of spin nematic state\footnote{C. Fang et al., Phys. Rev. B 77, 224509 (2008)}. We report $^{75}$As single crystal NMR study of LaFeAsO \footnote{M. Fu et al., arXiv:1208.5652, to appear in Phys. Rev. Lett.}. We have found that the low frequency spin dynamics exhibits a strong two-fold anisotropy within each orthorhombic domain below $T_{TO}$ This intermediate state then freezes progressively into a static SDW below $T_{SDW}$. Our results reveal the presence of an exotic intermediate spin state below $T_{TO}$ with the signature of spin nematicity. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N37.00013: Two-dimensional magnetic interactions in LaFeAsO Mehmet Ramazanoglu, Jagat Lamsal, Gregory S. Tucker, Stuart Calder, Jiaqiang Yan, Tatiana Guidi, Toby Perring, Thomas A. Lograsso, Andreas Kreyssig, Alan Goldman, Robert J. McQueeney The magnetic excitations in antiferromagnetic (AFM) ordered LaFeAsO (La1111) are mapped out by inelastic neutron scattering (INS) technique using both time-of-flight and triple-axis spectroscopies. The energy dependence of the observed intensity at the AFM ordering wavevector, Q$_{\mathrm{AFM}}$, yields a spin gap of $\sim$11 meV. The independence of the spin gap along the c-direction suggests nearly two-dimensional magnetic interactions. A steep magnetic excitation spectrum is observed for in-plane wavevectors for energy transfers up to 100 meV. The constant energy cuts of these steep excitations are elliptically shaped for the low energy transfers below 50 meV. As energy transfer increases, the elliptical anisotropy develops into a splitting above 50 meV. A phenomenological model based on highly damped diffuse spin dynamics is used to analyze the data. The calculated parameters of anisotropic spin wave velocities, spin gap and the damping values are similar to the previous results in BaFe$_{2}$As$_{2}$ and CaFe$_{2}$As$_{2}$ based materials but La111 has a more two-dimensional character. [Preview Abstract] |
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