Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W13: Focus Session: Fe-Based Superconductors-Moments/Fluctuations/NMR |
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Sponsoring Units: DMP Chair: Kenji Ishida, Kyoto University Room: 207 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W13.00001: Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors Yuting Tam, Dao-Xin Yao, Wei Ku The serious mismatch of large local magnetic moments and small ordered moments in iron-based superconductors is one of the unique and essential features of this new class of high-temperature superconductors. Here we demonstrate the active role of electron itinerancy in modulating strong anisotropic quantum spatial fluctuation and tuning the ordered anti-ferromagnetic moments. This is performed by first integrating out the itinerant degree of freedom of a degenerate spin-fermion model with Hund's coupling, followed by estimation of quantum fluctuation via spin-wave theory. Our results complement current emphasis on the temporal fluctuation in the literature, and highlight the essential interplay between itinerant and local degree of freedoms, paving the way to systematic studies of transport, superconductivity and other fluctuation dominant phenomena in iron-based superconductors. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W13.00002: Magnetic excitations in BaFe$_{2}$Se$_{3}$ Shan Wu, M. Mourigal, J.R. Nielson, M.B. Stone, T.M. McQueen, C. Broholm The ladder-like relative of the iron superconductors, BaFe2Se3, has recently been successfully synthesized and its thermo-magnetic and structural properties explored. Magnetic neutron diffraction and susceptibility data clearly reveal magnetic long-range order below $T_{N}=$256K. The proposed magnetic structure consists of antiferromagnetically aligned ferromagnetic blocks. To understand the origin of square plaquette formation, we carried out inelastic neutron scattering experiments on powder samples of BaFe$_{2}$Se$_{3}$. We identify spin-wave-like low energy excitations in the ordered state. The wave vector dependence of the low energy magnetic scattering is consistent with \textbf{\textit{k}}$=$(1/2,1/2,1/2) magnetic ordering. Comparison to a spin wave theory provides estimates for the dominant exchange interactions in BaFe$_{2}$Se$_{3}$. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W13.00003: Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides Meng Wang, Xingye Lu, Huiqian Luo, Xiaotian Zhang, Chenglin Zhang, Yu Song, Pengcheng Dai, Miaoyin Wang, Guotai Tan, E.A. Goremychkin, T.G. Perring, T.A. Maier, Zhiping Yin, Kristjan Haule, Gabriel Kotliar Since spin excitations may be responsible for electron pairing and superconductivity in iron pnictides, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe$_2$As$_2$ parent compound modifies the low-energy spin excitations and their correlation with superconductivity ($<50$ meV) without affecting the high-energy spin excitations ($>100$ meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy. These results suggest that high-Tc superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W13.00004: Local moments in iron-based superconductors probed with x-ray emission spectroscopy Invited Speaker: Young-June Kim We report Fe K$\beta$ x-ray emission spectroscopy studies of local magnetic moments in various iron-based superconductors. X-ray emission spectroscopy (XES) is a fast, local probe that is bulk-sensitive and couples directly to the d-electron moment. Recently developed integrated absolute difference method of analyzing the XES signal [1] allows us to study even small fluctuating moments found in metallic systems such as iron-based superconductors. In our survey of various materials in their paramagnetic phases [2], we found local magnetic moments in all samples studied: PrFeAsO, Ba(Fe,Co)$_2$As$_2$, LiFeAs, Fe$_{1+x}$(Te,Se), and A$_2$Fe$_4$Se$_5$ (where A = K, Rb, and Cs). The moment size shows very little dependence on temperature or carrier concentration, but varies significantly across different families. Specifically, all iron pnictide samples have local moments of about 1.5-2 $\mu_B$/Fe, while FeTe and K$_2$Fe$_4$Se$_5$ families have much larger local moments of ?3 $\mu_B$/Fe and ?5 $\mu_B$/Fe, respectively. The extracted moment sizes agree well with energy and momentum integrated inelastic neutron scattering results. In addition, XES was used to study the spin-state transition in rare-earths doped CaFe$_2$As$_2$ [3]. When about 10-20\% of Ca is replaced with Pr or Nd ions, this material goes through so-called collapsed tetragonal transition at 70 K, below which the c-lattice constant shrinks by almost 10\% [4]. The XES data show that the local magnetic moment is quenched in this collapsed tetragonal phase. We also found that the moment size exhibits unusually large temperature dependence even in the high temperature regime, indicating that the crystal field splitting and the Hund's rule coupling are of similar strength in this compound [5]. Our experimental results illustrate the importance of multiorbital physics in describing magnetism of iron-based superconductors.\\[4pt] [1] G. Vanko et al., J. Phys. Chem. B 110, 11 647 (2006);\\[0pt] [2] H. Gretarsson et al., Phys. Rev. B 84, 100509(R) (2011);\\[0pt] [3] H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013);\\[0pt] [4] S. R. Saha et al., Phys. Rev. B 85, 024525 (2012);\\[0pt] [5] J. Chaloupka and G. Khaliullin, arXiv:1208.1197v1. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W13.00005: Electronic and magnetic properties of Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ studied by $^{75}$As NMR Yuji Furukawa, Beas Roy, Shen Ran, Sergey L. Bud'ko, Paul C. Canfield Recently much attention has been paid to CaFe$_{2}$As$_{2}$ because the magnetic and electronic properties of the system can be controlled by changing the heat treatment conditions. CaFe$_{2}$As$_{2}$ annealed at 400 C for 24 hours undergoes a phase transition from a high-temperature tetragonal paramagnetic state to a low temperature orthorhombic antiferromagnetic state at T$_{\mathrm{N}}$ $\sim$ 160K. On the other hand, CaFe$_{2}$As$_{2}$ quenched from 960 C to room temperature shows a transition to a collapsed tetragonal non-magnetic phase below T$_{\mathrm{s}}$ $\sim$ 90 K. In order to investigate the difference in electronic and magnetic properties of the two different CaFe$_{2}$As$_{2}$ samples from a microscopic point of view, we have carried $^{75}$As-NMR spectra and spin-lattice relaxation measurements. We also performed $^{75}$As-NMR measurements on Co-doped CaFe$_{2}$As$_{2}$ superconductor. Based on our NMR data, we will discuss similarities and difference in magnetic fluctuations in the systems, and compare the NMR data with inelastic neutron scattering data. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W13.00006: NMR Evidence for Two Spin-Fluctuation Components and their Linear Scaling with Superconductivity in Fe Superconductors Weiqiang Yu, B. Normand, Long Ma, J. Dai, G.F. Ji, P. Fan, P.S. Wang, Y. Song, C.L. Zhang, Pengcheng Dai The relationship between spin fluctuations and superconductivity is one of the most fundamental questions in the study of iron-based superconductors. From a series of NMR measurements on different compounds and over wide ranges of temperature [1] and pressure [2], we have found strong evidence for two different components contributing to spin fluctuations in the normal state. One originates from Fermi-surface electrons and appears at low energies, while the other arises from the local magnetic moments and appears at higher energy scales [1]. In NaFe$_{1-x}$Co$_x$As, NMR studies demonstrate that both low-energy spin fluctuations and superconductivity are at first enhanced strongly under pressure, both scaling linearly, but decrease together above 2.2 GPa. A ``missing constant'' contribution to $T_c$ reveals the additional effect of the local spin fluctuations. This clear evidence for (i) separate itinerant and local spin fluctuations and (ii) the scaling between spin fluctuations and superconductivity provide essential input to support a magnetic origin of superconductivity based on a two-component model for spin fluctuations in Fe superconductors. \\[4pt] [1] L. Ma {\it et al.}, PRB {\bf 84}, 220505(R) (2011).\\[0pt] [2] G. F. Ji {\it et al.}, PRL {\bf 111}, 107004 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W13.00007: High-energy spin excitations in heavily hole-doped superconductor KFe$_{2}$As$_{2}$ Kazumasa Horigane, Chul-Ho Lee, Kunihiro Kihou, Kay Fujita, Ryoichi Kajimoto, Sungdae Ji, Yasuhiro Inamura, Masatoshi Arai, Jun Akimitsu The understanding of overall spin dynamics over the wide hole doping region is a key to progress in the study of iron based superconductors. In contrast to experimental works in parent compounds, spin dynamics in over hole doping region is not yet provided. In this research, we have performed inelastic neutron scattering of heavily hole-doped KFe$_{2}$As$_{2}$ at 4SEASONS. We revealed a well-defined low-energy incommensurate spin fluctuation at (0.32, 0.32) and (0.68, 0.68). This incommensurability was consistent with the previous triple-axis neutron scattering study. A practically vertical dispersion was discovered up to 80meV and spin fluctuations clearly exist even in heavily hole doped KFe$_{2}$As$_{2}$. The energy dependence of dynamical magnetic susceptibility $\chi $(q,w) can be explained by a phenomenological function applicable to correlated spin system in Fermi liquid without magnetic long-range ordering. Therefore, magnetism in KFe$_{2}$As$_{2}$ likely originates from itinerant nature. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W13.00008: Spin excitations in Ba(Fe$_{0.957}$Cu$_{0.043})_{2}$As$_{2}$ single crystals near a putative antiferromagnetic quantum critical point Min Gyu Kim, P. Valdivia, S. Chi, S. Ran, G.S. Tucker, A.D. Christianson, E. Bourret-Courchesne, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, R.J. McQueeney, A.I. Goldman, R.J. Birgeneau We report on inelastic neutron scattering measurements of the spin excitations in the non-superconducting Ba(Fe$_{0.957}$Cu$_{0.043})_{2}$As$_{2}$ compound that orders antiferromagnetically at $T_{\mathrm{N}}=$26(5) K in the near vicinity of a putative antiferromagnetic quantum critical point. These results are compared to those of previous studies on low Cu doped BaFe$_{2}$As$_{2}$ and high Co doped BaFe$_{2}$As$_{2}$ compounds. While Co doping induces superconductivity and Cu doping does not, we find that the Cu-doped and Co-doped spectra are qualitatively similar so additional factors must be considered to explain why the Cu-doped compound is not a superconductor. In addition, we show a possible E/T scaling in this compound, which has been observed in many heavy fermion and cuprate superconductors at/near putative quantum critical points. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W13.00009: NMR evidence for coexistence of cluster spin glass and superconductivity in Ba(Fe$_{1-\mathrm{x}}$Co$_{\mathrm{x}}$)$_2$As$_2$ Adam P. Dioguardi, John Crocker, Abigail C. Shockley, Ching H. Lin, Kent R. Shirer, David M. Nisson, Matthew M. Lawson, Nicholas apRoberts-Warren, Paul C. Canfield, Sergey L. Bud'ko, Sheng Ran, Nicholas J. Curro We present $^{75}$As nuclear magnetic resonance data from measurements of a series of Ba(Fe$_{1-\mathrm{x}}$Co$_{\mathrm{x}}$)$_2$As$_2$ crystals with $0 \leq \mathrm{x} \leq 0.075$. Spectral wipeout and the onset of stretched exponential spin-lattice relaxation as a function of decreasing temperature reveal the coexistence of frozen antiferromagnetic domains and superconductivity for $0.060 \leq \mathrm{x} \leq 0.071$. Although bulk probes reveal no long range antiferromagnetic order beyond $\mathrm{x} = 0.06$, we find that the local spin dynamics reveal no qualitative change across this transition. Domain sizes vary by more than an order of magnitude, reaching a maximum variation at $\mathrm{x} = 0.06$. This inhomogeneous glassy dynamics may be an intrinsic response to the competition between superconductivity and antiferromagnetism in this system. We also present field-dependent spin-lattice relaxation studies from 3.5 T to 30.4 T to further probe the glassy dynamics. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W13.00010: NMR study of the AF-SC-SC-AF phased transition in a pnictide superconductor LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ Naoki Fujiwara, Ryosuke Sakurai, Soushi Iimura, Satoru Matsuishi, Hideo Hosono, Youichi Yamakawa, Hiroshi Kontani We have performed $^{75}$As and $^{1}$H NMR measurements in LaFeAsO$_{\mathrm{1x}}$H$_{\mathrm{x}}$, an isomorphic compound of LaFeAsO$_{\mathrm{1x}}$F$_{\mathrm{x}}$. LaFeAsO$_{\mathrm{1x}}$H$_{\mathrm{x}}$ is an electron doped system, and O2- can be replaced with H$^{-}$ up to $x=$0.5. LaFeAsO$_{\mathrm{1x}}$H$_{\mathrm{x}}$ is known for having double superconducting (SC) domes on H doping. Recently, we discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric AF-SC-SC-AF phase alignment in the electronic phase diagram [1] Unlike the AF ordering in the lightly H-doped regime, the AF ordering in the highly H-doped regime is attributed to the nesting between electron pockets. In the conference, we will show the data of both NMR spectra and the relaxation rate 1/$T_{1}$ in the whole doping region. We will discuss the difference of electronic states between the lightly H-doped AF-SC phases and highly H-doped SC-AF phases.\\[4pt] [1] N. Fujiwara, et al., PRL \textbf{111} 097002 (2013)\\[0pt] [2] Y. Yamakawa, et al., PRB \textbf{88} 041106 (R) (2013) [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W13.00011: Neutron spin resonance study in Co-doped NaFeAs Chenglin Zhang, Egami Takeshi, Pengcheng Dai Since the discovery of iron superconductors, the (Ba,Sr,Ca)Fe2As2 (``122'') family especially electron doped side has been subjected to heavily study byneutron scattering. One of the pronounced features generally observed in bulk superconducting compositions is a broad resonance along antiferrromagnetic order wave vector. The resonance energy linearly scales with Tc. However, our neutron study shows that Co-doped NaFeAs system exhibits complexity, distinguishing itself from ``122'' system. We observed a sharp resonance in the electron-overdope regime, providing strong evidence for S$+$\textunderscore\ pairing symmetry in pnictide superconductors. In the underdoped regime, we find double resonances at commensurate wave vector, demonstrating the multi-orbital nature of pnictides. Our finding further suggests that the resonance energy and Tc may not be simply correlated in multiband superconductors such as iron pnictides. We will discuss in detail how resonances evolve with electron doping. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W13.00012: Compositional dependence of the superconducting resonance in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ G.S. Tucker, D.K. Pratt, A. Thaler, N. Ni, K. Marty, A.D. Christianson, M.D. Lumsden, B.C. Sales, A.S. Sefat, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, A.I. Goldman, R.J. McQueeney The low energy magnetic fluctuation spectra of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in the range $0.040 \le x \le 0.055$ were studied in their antiferromagnetically ordered state, above and below their superconducting transition temperature ($T_c$), using inelastic neutron scattering. The normal state excitation spectra are best described by a model of overdamped correlated spin fluctuations characteristic of the paramagnetic phase. Upon entering the superconducting state the excitation spectra is modified by the superconducting resonance. The superconducting resonance energy and its relation to $T_c$, the dispersion of the resonance and its dimensionality, and the absolute spectral weight of the resonance will be discussed in terms of their composition dependence.\\[0pt] Work at the Ames Laboratory was supported by US DOE, Basic Energy Sciences, DMSE under Contract No. DE-AC02-07CH11358. Part of the research conducted at ORNL's High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, BES, US DOE. Some work at Oak Ridge was supported by US DOE, Basic Energy Sciences, DMSE. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W13.00013: Orthorhombic distortion, superconductivity and magnetic fluctuations in FeSe A.E. B{\"o}hmer, F. Hardy, P. Schweiss, T. Wolf, C. Meingast, T. Arai, T. Iye, T. Hattori, K. Ishida The recently observed scaling between magnetic and lattice fluctuations observed for Ba(Fe,Co)$_2$As$_2$ provides evidence that its tetragonal-to-orthorhombic structural transition is magnetically driven\footnote{Fernandes et al. PRL 111, 137001 (2013)}. Here, we study the interplay between structure, magnetism and superconductivity in FeSe, an iron-based superconductor which is particularly interesting because it orders magnetically only under high pressure, while a tetragonal-to-orthorhombic structural transition takes place at 90 K at ambient pressure. In contrast to the 122-systems, our high-resolution thermal-expansion data clearly demonstrate that orthorhombic distortion and superconductivity do not compete in FeSe\footnote{B{\"o}hmer et al. PRB 87, 180505 (2013)}, while the shear modulus softening is similar to the 122 systems. By studying magnetic fluctuations using nuclear magnetic resonance, we investigate whether FeSe is simply a case of extreme splitting of magnetic and structural phase transitions - and thus comparable to 122 systems - or whether its structural transition has a qualitatively different origin. [Preview Abstract] |
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