Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J35: Focus Session: Iron Pnictides and Other Novel Superconductors VIII: Magnetism (Experiment) |
Hide Abstracts |
Sponsoring Units: DMP Chair: Jeff Lynn, National Institute of Standards and Technology Room: 405 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J35.00001: Nuclear and Magnetic structures of CeFeAs$_{1-x}$P$_{x}$O Clarina dela Cruz, Pengcheng Dai, Herbert Mook, Q. Huang, M. Green, J. Lynn, N.L. Wang, G.F. Chen, J.L. Lou A new class of superconductors (RO$_{1-x}$F$_{x}$FeAs) has been discovered very recently which has resulted to a flurry of activities in the scientific community. Our initial work on the La based parent compound successfully revealed, for the first time, the AFM order which is preceded by a structural distortion from tetragonal to orthorhombic nuclear structures. A model for the magnetic structure was also proposed. Both magnetic order and structural distortion are suppressed in lieu of the superconducting phase. Our systematic study of the doping dependence of both the nuclear and magnetic structure of CeO$_{1-x}$F$_{x}$FeAs as well as in the La system has established the nature of the competition between the static AFM order of the Fe spins and the superconductivity. This work looks at the effects of the tuning of the structural parameters in the compounds without doping carriers into the system. This is motivated by the belief that structurally driven electronic effects are very important in these highly correlated systems. We study the parent compound CeFeAsO and change the particulars about the structural distortion by substituting P on the As site until it's approach to CeFePO which is a nonmagnetic heavy Fermion. We present interesting details of the phase diagram of CeFeAs$_{1-x}$P$_{x}$O with respect to the As/P concentration. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J35.00002: Lattice and magnetic instabilities in CaFe$_{2}$As$_{2}$ Shibabrata Nandi, Alan Goldman, Dimitri Argyriou, Bachir Ouladdiaf, Tapan Chatterji, Andreas Kreyssig, Ni Ni, Sergey Bud'ko, Paul Canfield, Robert McQueeney Neutron diffraction measurements of a high quality single crystal of CaFe$_{2}$As$_{2}$ are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at T$_{S}$=172.5 K (on cooling) and 173.5 K (on warming). Concomitant with the structural transition we observe a rapid, but apparently continuous, ordering of the Fe moments, in a commensurate antiferromagnetic structure, with a saturated moment of 0.80(5) \textit{$\mu $}$_{B}$/Fe directed along the orthorhombic a-axis. The hysteresis of the structural transition is 1 K between cooling and warming and is consistent with previous thermodynamic, transport and single crystal x-ray studies. The temperature onset of magnetic ordering shifts rigidly with the structural transition providing clearest evidence to date of the coupling between the structural and magnetic transitions in this material and the broader class of arsenides. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J35.00003: High Energy Spin Waves in CaFe2As2 Single Crystals Souleymane Diallo, Vladimir Antropov, Collin Broholm, Toby Perring, Sergey Bud'ko, Ni Ni, Paul Canfield, Andreas Kreyssig, Alan Goldman, Robert McQueeney We present neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic ordered CaFe2As2 (TN = 172 K), the parent compound of the newly discovered iron-arsenide based superconductors. The data reveals steeply dispersive and well-defined spin waves up to an energy of approximately 120 meV. The data below 120 meV can be fit to a Heisenberg model consisting of nearest-neighbor interactions (J1a, J1b and J1c) and next-nearest neighbor interaction (J2), yielding constraining values on the magnetic exchange coupling constants. Above 120 meV, the excitations appear weaker or strongly damped. Ab-initio calculations of the dynamic magnetic susceptibility show that the high energy behavior arises from the damping of itinerant spin waves by particle-hole excitations. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J35.00004: Neutron Scattering Study of the Fe Oxypnictide Superconductors NdFeAsO$_{1-x}$F$_x$ and LaFeAsO$_{0.87}$F$_{0.13}$ Yiming Qiu, Wei Bao, Qingzhen Huang, Taner Yildirim, Jason Simmons, Mark Green, Ying Chen, Jeff Lynn, Maiko Kofu, Seunghun Lee, T. Wu, G. Wu, Xianhui Chen We report the neutron scattering studies of NdFeAsO$_{1-x}$F$_x$ (x=0, 0.2)[1] and LaFeAsO$_{0.87}$F$_{0.13}$[2]. In NdFeAsO, there is a tetragonal to orthorhombic structural transition at Ts$\approx$150 K, where an anomaly in resistivity also occurs. A long range magnetic order with the wave-vector (1/2, 1/2, 0)$_T$ forms below T$_N$=1.96 K. This long range order is dominated by the rare earth Nd ions, however, both the Nd and smaller Fe moments contribute to the antiferromagnetic structure. Neither the magnetic ordering nor the structural distortion occurs in the superconducting samples NdFeAsO$_{0.80}$F$_{0.20}$ and LaFeAsO$_{0.87}$F$_{0.13}$ at temperatures down to 1.6 K. In LaFeAsO$_{0.87}$F$_{0.13}$, no magnetic-resonance peak was observed in the superconducting state at 1.6 K. Two phonon peaks at 12 and 17 meV were observed, consistent with theoretical calculation. Reference: [1] Y. Qiu et al., arXiv:0806.2195 accpeted by PRL(2008) [2] Y. Qiu et al., Phys. Rev. B 78, 052508(2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J35.00005: The crystalline electric field as a probe for long range antiferromagnetic order and superconductivity in CeFeAsO$_{1-x}$F$_x$ Songxue Chi, Devashibhai Adroja, Titiana Guidi, Robert Bewley, Shiliang Li, Jun Zhao, Jeffrey Lynn, Craig Brown, Yiming Qiu, Gen Fu Chen, Jian Lin Luo, Nan Lin Wang, Pengcheng Dai We use inelastic neutron scattering to study the crystalline electric field (CEF) excitations of Ce$^{3+}$ in CeFeAsO$_{1-x}$F$_{x}$($x=0,0.16$). For nonsuperconducting CeFeAsO, the Ce CEF levels have three magnetic doublets in the paramagnetic state, but these doublets split into six singlets when Fe ions order antiferromagnetically. For superconducting CeFeAsO$_{0.84}$F$_{0.16}$ ($T_c=41$ K), where the static AF order is suppressed, the Ce CEF levels have three magnetic doublets at $\hbar\omega=0,18.7,58.4$ meV at all temperatures. Careful measurements of the intrinsic linewidth $\Gamma$ and the peak position of the 18.7 meV mode reveal clear anomaly at $T_c$, consistent with a strong enhancement of local magnetic susceptibility $\chi^{\prime\prime}(\hbar\omega)$ below $T_c$. These results suggest that CEF excitations in the rare-earth oxypnictides can be used as a probe of spin dynamics in the nearby FeAs planes. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J35.00006: Inelastic neutron scattering studies on spin excitations of Fe Pnictides Jun Zhao, Dao-Xin Yao, Shiliang Li, Tao Hong, Ying Chen, Sung Chang, William Ratcliff, Jeff Lynn, Herbert Mook, Genfu Chen, Jianlin Luo, Nanlin Wang, Erica Carlson, Jiangping Hu, Pengcheng Dai We used inelastic neutron scattering to study the evolution of spin excitations in the FeAs superconductors and their parent compounds. We show here not only the antiferromagnetic order is suppressed by the doping; the spin excitations also change dramatically with doping. We observed sharp spin-wave excitations in the antiferromagnetically ordered parent compound. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. We also studied the spin excitation spectrum in the superconducting sample and its relationship to the superconductivity. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J35.00007: Resonant spin excitation in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ R. Osborn, S. Rosenkranz, E.A. Goremychkin, D.Y. Chung, I.S. Todorov, H. Claus, A.D. Christianson, M.D. Lumsden, C.D. Malliakas, M.G. Kanatzidis, R.I. Bewley, T. Guidi In the iron arsenides, superconductivity occurs when the antiferromagnetism of a parent compound has been suppressed by chemical doping. We have investigated the evolution of the magnetic response with potassium doping in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. In the parent compound ($x$ = 0), there is evidence of a column of inelastic scattering at the AF wavevector, $Q$ = 1.2{\AA}$^{-1}$, consistent with a steep dispersion of gapped spin waves. A similar inelastic column is seen in the normal phase in the $x$ = 0.4 compound, but it persists to lower energy transfer. However, at the superconducting transition of 38K, there is a transfer of spectral weight into an excitation localized at $Q$ = 1.2{\AA}$^{-1}$ and $\omega $ = 14meV [A. D. Christianson \textit{et al}, Nature, in press]. Such resonant spin excitations, which are a universal feature of the copper oxide superconductors and seen in several heavy fermion superconductors, provide evidence that the energy gap has unconventional symmetry, with opposite sign on portions of the Fermi surface connected by the resonance wavevector. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J35.00008: Antiferromagnetic spin correlations in the tetragonal phase of CaFe$_{2}$As$_{2}$ R.J. McQueeney, S.O. Diallo, J.L. Zarestky, C. Broholm, T.G. Perring, S.L. Bud'ko, N. Ni, A. Kreyssig, P.C. Canfield, A.I. Goldman We present neutron scattering measurements of magnetic excitations in the tetragonal phase of CaFe$_{2}$As$_{2}$. Below $T_{S}$ = 173 K, CaFe$_{2}$As$_{2}$ undergoes a first-order transition to an orthorhombic structure with columnar antiferromagnetic ordering with a wavevector \textbf{Q}$_{AFM}$ = (101). This phase is characterized by strong magnetic interactions giving rise to steep spin waves. In the tetragonal phase above $T_{S}$, broad quasi-elastic excitations are observed near \textbf{Q}$_{AFM}$ that display a weak dependence on $L$, indicating two-dimensional antiferromagnetic correlations. These correlations are observed to persist up excitation energies of $\sim $50 meV and temperatures of at least 300 K. Our results indicate that strong magnetic interactions exist above $T_{S}$, and ordering is likely suppressed by magnetic frustration. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J35.00009: Magnetic order the iron spins in NdOFeAs Ying Chen, J.W. Lynn, J. Li, G. Li, G.F. Chen, J.L. Luo, N.L. Wang, Pengcheng Dai, C. dela Cruz, H.A. Mook Polarized and unpolarized powder neutron-diffraction measurements have been carried out to investigate the iron magnetic order in the parent compound of one of the highest Tc system, NdFeAsO. Antiferromagnetic order is observed below 141 K [1], which is in close proximity to the structural distortion observed in this material [2]. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c axis are antiparallel like LaFeAsO [3]. The ordered moment is 0.25 (7) $\mu$B, which is the smallest ordered moment found so far in these systems. \\[3pt] [1]Ying Chen, J. W. Lynn, J. Li, G. Li, G. F. Chen, J. L. Luo, N. L. Wang, Pengcheng Dai, C. dela Cruz and H. A. Mook, Phys. Rev. B ${\bf{78}}$, 064515 2008. \\[0pt] [2]Y. Qiu, W. Bao, Q. Huang, T. Yildirim, J. M. Simmons, M. A. Green, J.W. Lynn, Y.C. Gasparovic, J. Li, T. Wu, G. Wu, and X.H. Chen, arXiv:0806.2195 (Phys. Rev. Lett. accepted). \\[0pt] [3] C. dela Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II, J. L. Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, and P. Dai, Nature ${\bf{453}}$, 899 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J35.00010: Coexistence of the spin-density-wave and superconductivity in the $\rm Ba_{1-x}K_xFe_2As_2$ Yang Ren, H. Chen, Y. Qiu, Wei Bao, R.H. Liu, G. Wu, T. Wu, Y.L. Xie, X.F. Wang, Q. Huang, X.H. Chen The relation between the spin-density-wave (SDW) and superconducting order is a central topic in current research on the FeAs-based high $T_c$ superconductors. Conflicting results exist in the LaFeAs(O,F)-class of materials, for which whether the SDW and superconductivity are mutually exclusive or they can coexist has not been settled. Here we show that for the $\rm (Ba,K)Fe_2As_2$ system, the SDW and superconductivity can coexist in an extended range of compositions. The availability of single crystalline samples and high value of the energy gaps would make the materials a model system to investigate the high $T_c$ ferropnictide superconductivity. [arXiv:0807.3950 (2008)] [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J35.00011: High magnetic field vortex torque magnetometry in SmFeAsO$_{0.8}$F$_{0.2}$ single crystals Luis Balicas, Alex Gurevich, Younjung Jo, Jan Jaroszynski, David Larbalestier, R.H. Liu, H. Chen, Xianhui H. Chen, N.D. Zhigadlo, S. Katrych, Z. Bukowski, J. Karpinski To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $\tau(\theta)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $\gamma$ extracted from $\tau(\theta)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $\gamma$ depends on both $T$ and $H$, saturating at $\gamma \simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 2:03PM |
J35.00012: Inelastic Neutron Scattering from Fe Pnictide Superconductors Invited Speaker: We present inelastic neutron scattering data from both single crystal and polycrystalline specimens of several of the new Fe-based superconducting materials. The phonon density of states (PDOS) was determined for LaFeAsO$_{1-x}$F$_{x}$. The PDOS for the nonsuperconducting parent compound LaFeAsO was found to be nearly identical to that of superconducting LaFeAsO$_{0.89}$F$_{0.11}$. Good agreement was found between first principal calculations and the experimentally determined PDOS with the exception of a small difference in some of the Fe mode frequencies. The experimental PDOS is not consistent with conventional phonon mediated superconductivity. In the case of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$, a magnetic excitation appears below T$_{c}$ that is not present at any temperature in the parent compound BaFe$_{2}$As$_{2}$. The excitation occurs at an energy of 14 meV and at a wave vector consistent with antiferromagnetic correlations in the FeAs plane. The existence of this excitation is strong evidence for an unconventional superconducting gap symmetry and demonstrates that the superconducting order parameter is strongly coupled to magnetic degrees of freedom in the Fe-based superconductors. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700