Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X41: Fe-based Superconductivity: Magnetic ExcitationsFocus
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Sponsoring Units: DMP DCMP Chair: Ilya Eremin, University of Bochum Room: 388 |
Friday, March 17, 2017 8:00AM - 8:36AM |
X41.00001: Uniaxial pressure dependence of the magnetic ordered moment and transition temperatures in BaFe2-xNixAs2 Invited Speaker: Pengcheng Dai We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic phase in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and superconducting BaFe$_{\mathrm{1.915}}$Ni$_{\mathrm{0.085}}$As$_{\mathrm{2}}$. We find that the ordering temperature (TN) increases in both compounds, consistent with calculations of the static spin susceptibility within the random-phase approximation. At low temperature, where uniaxial pressure mechanically detwins the crystals, we find that the magnetic ordered moment in BaFe$_{\mathrm{1.915}}$Ni$_{\mathrm{0.085}}$As$_{\mathrm{2}}$ increases by as much as 20{\%} under 40 MPa of pressure, whereas no change was observed in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. The doping and uniaxial pressure dependence of the magnetic ordered moment is captured by DFT$+$DMFT calculations which suggests the pressure-induced increase near superconductivity is closely related to the diverging nematic susceptibility. We will also present new spin wave data on fully detwinned BaFe$_{\mathrm{2}}$As$_{\mathrm{2.}}$ [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X41.00002: Itinerant spin excitations and superconductivity in BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{\mathrm{2}}$ Huiqian Luo, Dongliang Gong, Tao Xie, Zhaoyu Liu, Shiliang Li, Kazuya Kamazawa, Kazuki Iida, Ryoichi Kajimoto, Alexandre Ivanov, Jiri Kulda, Mechthid Enderle, Devashibhai Adroja, Pengcheng Dai High-temperature superconductivity in iron pnictides emerges from electron or hole doped parent compounds with antiferromagnetic order, which is argued to be associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons. Recently, we have used time-of-flight neutron spectroscopy to carefully map out the spin excitations in the electron overdoped BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{\mathrm{2}}$ especially around the zone boundary of superconductivity. We have found a spin gap actually emerges after the vanishing of zero resistivity and directly responses to the disappearance of superconductivity. Further polarized neutron analysis indicate that the spin gap actually is anisotropic, and the longitudinal mode of spin fluctuations, as a hallmark of the itinerant magnetism from Fermi surface nesting, is totally eliminated together with the hole pockets near the electron-overdoped zone boundary of superconductivity. Our results suggest that the itinerant spin excitations originated from Fermi surface nesting are crucial to the superconductivity in iron pnictides. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X41.00003: Spin resonance of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ studied by neutron scattering Chul-Ho Lee, K. Kihou, J.T. Park, K. Horigane, F. Wasser, N. Qureshi, Y, Sidis, J. Akimitsu, M. Braden The remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase, so called spin resonance, is important to examin since it could include information of Cooper pairing. Here, we examined the spin fluctuation of hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by inelastic neutron scattering to clarify the doping dependence of spin resonance. Neutron scattering experiments were conducted using the triple-axis spectrometer PUMA at FRM ll, Germany and 2T1 at LLB, France. We have found that the behavior of the spin resonance dramatically changes around x = 0.66 [1]. Resonance peaks have been observed clearly below 2$\Delta$s in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2$\Delta$s to higher energies, peaking at values of 3$\Delta$s for x = 0.84. These results indicate a reduced impact of magnetism on Cooper pair formation in the overdoped region. [1] C.H.Lee et al., Sci. Rep. 6, 23424 (2016) [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X41.00004: Strong magnon-phonon coupling in NaFeAs studied by neutron scattering Yu Li, Zahra Yamani, Yu Song, Chenglin Zhang, Pengcheng Dai We carried on inelastic neutron scattering experiment on the triple axis spectrometer in CNBC in Chalk River. We measured both the phonon and magnon in NaFeAs single crystals and their temperature dependence. Since structural transition temperature (T$_{\mathrm{S}})$ and the magnetic transition temperature (T$_{\mathrm{\thinspace N}}$ ) are well separated in NaFeAs, it provides us an unique chance to exclude the consequence or magnetic order and focus on the so called nematic phase. As the previous paper on BaFe2As2, we observed the strong phonon softening nearby the structural transition temperature at very small q (q\textless 0.1). This makes the phonon in NaFeAs deviate from the classical linear dispersion relationship for acoustic phonons. Besides the phonon softening, we also observe phonon hardening at a larger q range when the temperature goes down. This is accompanied by the stiffening of the magnons which can be represented by the linewidth of the low energy magnetic peaks. Our results suggest that there is strong coupling between the phonons and magnons in NaFeAs. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X41.00005: Spin anisotropy due to spin-orbit coupling in optimally hole-doped Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ Yu Song, Haoran Man, Rui Zhang, Xingye Lu, Chenglin Zhang, Meng Wang, Guotai Tan, L.-P. Regnault, Yixi Su, Jian Kang, R. M. Fernandes, Pengcheng Dai We use polarized inelastic neutron scattering to study the temperature and energy dependence of spin space anisotropies in the optimally hole-doped iron pnictide Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ ($T_{{\rm c}}=38$ K). In the superconducting state, while the high-energy part of the magnetic spectrum is nearly isotropic, the low-energy part displays a pronouced anisotropy, manifested by a $c$-axis polarized resonance. We also observe that the spin anisotropy in superconducting Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ extends to higher energies compared to electron-doped BaFe$_{2-x}TM_{x}$As$_{2}$ ($TM=$Co, Ni) and isovalent-doped BaFe$_{2}$As$_{1.4}$P$_{0.6}$, suggesting a connection between $T_{\rm c}$ and the energy scale of the spin anisotropy. In the normal state, the low-energy spin anisotropy for optimally hole- and electron-doped iron pnictides onset at a temperature similar to the temperature in which the elastoresistance deviates from Curie-Weiss behavior, pointing to a possible connection between the two phenomena. Our results highlight the relevance of the spin-orbit coupling to the superconductivity of the iron pnictides. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X41.00006: Spin excitations in hole-overdoped iron-based superconductors Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ Kazumasa Horigane, Kunihiro Kihou, Kay Fujita, Ryoichi Kajimoto, Kazuhiko Ikeuchi, Sungdae Ji, Jun Akimitsu, Chul-Ho Lee Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ (x $=$ 0.5 and 1.0; $T_{\mathrm{c}} \quad =$ 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X41.00007: Spin resonance dispersion in hole-doped $Ba_{0.67}K_{0.33}Fe_{1.84}Co_{0.16}As_2$ Rui Zhang, Matthew Ston, Thomas Maier, Ming Yi, Meng Wang, Pengcheng Dai We present inelastic neutron scattering measurement in hole doped $Ba_{0.67}K_{0.33}Fe_{1.84}Co_{0.16}As_2$ where the spin resonance possesses an upward dispersion. Besides the universal enhanced signal of spin excitation at $\bm{Q_{AFM}}$ in superconducting state, a dispersive, ring-like feature along both transverse and longitudinal direction is found upon increasing energy. Our result shed some light on the origin of spin resonance as well as understanding its relationship with superconductivity in iron pnictides. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X41.00008: NMR study of over-doped NaFe$_{1-x}$Cu$_x$As Single Crystals Yizhou Xin, Peter Jeongseop Lee, Ingrid Stolt, William Halperin, Yu Song, Weiyi Wang, Pengcheng Dai, Chongde Cao, Arneil Reyes We present a preliminary $^{23}$Na and $^{75}$As NMR study of the over-doped NaFe$_{1-x}$Cu$_{x}$As pnictide, which has been demonstrated to be a possible Mott insulator near $x\approx0.5$.[1, 2] Our NMR spectra for three over-doped crystals (x = 0.15, 0.20, and 0.35) indicate that there are two inequivalent $^{23}$Na sites associated with a binomial distribution of Cu dopants. From NMR Knight shift and spin-lattice measurements of $^{23}$Na, we show that NaFe$_{1-x}$Cu$_{x}$As exhibits spin pseudo-gap behavior in the heavily Cu-doped regime. Comparison is made to earlier work on Ba(Fe$_{1-x}$Co$_{x}$)$_2$As. A possible presence of the coupling between charge density and spin density fluctuations is also discussed to account for the origin of the temperature dependence of the quadrupolar frequency. [1] Song, Yu, \textit{et al}., arXiv preprint arXiv:1504.05116 (2015). [2] C.E. Matt \textit{et al}.Phys. Rev. Lett. 117, 097001 (2016) \\ [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X41.00009: The influence of the structural transition on magnetic fluctuations in NaFeAs Wei Bao, Jinchen Wang, Juanjuan Liu, Wei Luo, Jieming Sheng, Aifeng Wang, Xianhui Chen, Sergey Danilkin NaFeAs belongs to a class of Fe-based superconductors which have parent compounds that show separated structural and magnetic transitions. Effects of the structural transition on spin dynamics therefore can be investigated separately from the magnetic transition. A plateau in dynamic spin response is observed in a critical region around the structural transition temperature $T_{\mathrm{S}}$. It is interpreted as being due to the stiffening of spin fluctuations along the in-plane magnetic hard axis due to the d$_{\mathrm{xz}}$ and d$_{\mathrm{yz}}$ orbital ordering. The appearance of anisotropic spin dynamics in the critical region above the $T_{\mathrm{S}}$ at $T$* offers a dynamic magnetic scattering mechanism for anisotropic electronic properties in the commonly referred `nematic phase'. Ref: J. Phys. Condens. Mater. 28 (2016) 27LT01 [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X41.00010: Spin excitations and pairing symmetry of sulfur-doped iron selenide superconductors Qisi Wang, Jitae Park, Yu Feng, Yao Shen, Yiqing Hao, Bingying Pan, Jeffrey Lynn, Alexandre Ivanov, Songxue Chi, Masaaki Matsuda, Huibo Cao, Robert Birgeneau, Dmitri Efremov, Jun Zhao In conventional BCS superconductors, electron-phonon coupling gives rise to a sign-preserved s-wave pairing. In high-temperature superconductors, weak coupling theories suggest that superconductivity is mediated by spin fluctuations which lead to a sign reversal between the superconducting order parameters on different parts of the Fermi surfaces. A strong evidence for this comes from the observation of a magnetic resonance mode in the spin excitation spectrum. Here, we report the observation of a transition from sign-reversed to sign-preserved Cooper-pairing symmetry in S-doped iron selenide superconductors K$_x$Fe$_{2-y}$(Se$_{1-z}$S${_z}$)$_2$ [1]. Our neutron scattering data show that a sharp magnetic resonance mode well below the superconducting gap (2Δ) in the undoped sample is replaced by a broad hump structure above 2Δ under 50\% S doping. These findings suggest that multiple channels are required to understand the superconductivity in this system. [1] Q. Wang et al., Phys. Rev. Letts. 116, 197004 (2016). [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X41.00011: Prominent role of spin-orbit coupling in FeSe revealed by inelastic neutron scattering Yuan Li, Mingwei Ma, Philippe Bourges, Yvan Sidis, Yang Xu, Shiyan Li, Biaoyan Hu, Jiarui Li, Fa Wang In most existing theories for iron-based superconductors, spin-orbit coupling (SOC) has been assumed insignificant. Here we use spin-polarized inelastic neutron scattering to show that collective low-energy spin fluctuations in the orthorhombic (or ``nematic") phase of FeSe possess nearly no in-plane component. Such spin-space anisotropy can only be caused by SOC. It is present over an energy range greater than the superconducting gap 2$\Delta _\mathrm{sc}$ and gets fully inherited in the superconducting state, resulting in a distinct $c$-axis polarized ``spin resonance". Our result demonstrates the importance of SOC in defining the low-energy spin excitations in FeSe, which helps to elucidate the nearby magnetic instabilities and the debated interplay between spin and orbital degrees of freedom. The prominent role of SOC also implies a possible unusual nature of the superconducting state. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X41.00012: L-dependence of low energy spin excitations in FeTe/Se superconductors Guangyong Xu, Zhijun Xu, John Schneeloch, Jinsheng Wen, Barry Winn, Yang Zhao, Robert Birgeneau, Genda Gu, John Tranquada We will present neutron scattering measurements on low energy magnetic excitations from FeTe$_{1-x}$Se$_x$ (``11'' system) samples. Our work shows that the low energy magnetic excitations are dominated by 2D correlations in the superconducting (SC) compound at low temperature, with the L-dependence well described by the Fe magnetic form factor. However, at temperatures much higher than T$_C$, the magnetic excitations become more three-dimensional with a clear change in the L-dependence. The low energy magnetic excitations from non-superconducting (NSC) samples, on the other hand, always exhibit three-dimensional features for the entire temperature range of our measurements. Our results suggest that in additional to in-plane correlations, the inter-plane spin correlations are also coupled to the superconducting properties in the ``11'' system. [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X41.00013: Spin waves and magnetic exchange interactions in the spin-ladder compounds BaFe$_{\mathrm{2}}$S$_{\mathrm{3}}$ and RbFe$_{\mathrm{2}}$Se$_{\mathrm{3}}$. Meng Wang, S. J. Jin, Dao-Xin Yao, Ming Yi, D. H. Lee, R. Birgeneau, H. C. Jiang, Yu Song, W. L. Zhang, H. Q. Luo, H. L. Sun, A. D. Christianson, C. de la Cruze, E. Bourret-Courchesne We report inelastic neutron scattering studies on the spin waves of BaFe$_{\mathrm{2}}$S$_{\mathrm{3}}$ and RbFe$_{\mathrm{2}}$Se$_{\mathrm{3}}$. Both of them exhibit similar stripe antiferromagnetic (AF) orders as that of BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. At ambient pressure BaFe$_{\mathrm{2}}$S$_{\mathrm{3}}$ can be viewed as the parent state of the superconductivity discovered under pressure. Fitting the spin wave spectra to a Heisenberg Hamiltonian reveals a strong ferromagnetic (FM) exchange interaction (SJ$_{\mathrm{R}}$\textasciitilde 71 meV) along the rung direction, an AF SJ$_{\mathrm{L}}$\textasciitilde 49 meV along the leg direction, and a FM SJ$_{\mathrm{2}}$\textasciitilde 15 meV along the diagonal direction. The results are in clear contrast to the exchange interactions of the parent compounds of the other iron-based superconductors, suggesting that specific relative values for the exchange interactions do not appear to be unique for the parent states of the superconducting materials. However, identical measurements for insulating RbFe$_{\mathrm{2}}$Se$_{\mathrm{3}}$ reveal comparable exchange interactions as that of BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$, indicating the irrelevance of the fermiology for the existence of the stripe AF order among various Fe-based materials. arXiv:1609.00465, PRB 94,041111(R)(2016). [Preview Abstract] |
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