Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X23: Electron Correlations and Nematic Order in Iron-based SuperconductorsInvited
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Sponsoring Units: DCMP Chair: Qimiao Si, Rice University Room: New Orleans Theater B |
Friday, March 17, 2017 8:00AM - 8:36AM |
X23.00001: Electron correlations and magnetism in iron-based superconductors Invited Speaker: Robert Birgeneau We have carried out a comprehensive study of the phase diagram, structures and phase transitions in the system RbxFeySe2-zSz. We find that the iron content is crucial in stabilizing the stripe antiferromagnetic (AF) phase (y\textasciitilde 1.5), the block AF phase (y\textasciitilde 1,6) and the iron vacancy-free metallic phase (y\textasciitilde 2). These phases are separated by first order transitions.(1). In going from superconducting Rb0.8Fe2Se2 to non-superconducting Rb0.8Fe2S2 we observe in our ARPES experiments little change in the Fermi surface topology but an increase in the overall bandwidth by a factor of 2, hence demonstrating that moderate correlation is essential in achieving high Tc.(2). We show also using neutron scattering that for z$=$0 there is a sharp magnetic resonance mode well below the superconducting gap which is replaced by a broad hump structure above the gap for z\textasciitilde 1. (3). This is accompanied by an insignificant change in Tc. This implies a concomitant change from sign-reversed to sign preserved Cooper-Pairing symmetry driven by the change in electron band width. In this talk we will discuss the overall significance of this rich behavior observed in this alkali Fe-chalcogenide system. \begin{enumerate} \item Meng Wang et al., Phys. Rev. B 93, 075155 (2016) \item M. Yi et al., PRL 115, 256403 (2015) \item Qisi Wang et al., PRL 116, 197004 (2016) \end{enumerate} [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 9:12AM |
X23.00002: Unexpected Complexity in Iron Based Superconductors Invited Speaker: Elbio Dagotto Fermi surface nesting ideas guided the initial theoretical studies of iron-based high critical temperature superconductors and they may have captured correctly important properties such as the superconducting state symmetry. However, evidence is accumulating that these materials are more complex than previously anticipated. Along these lines, two areas of research that are receiving considerable attention will be addressed: (a) The spin nematic regime from the perspective of spin-fermion model simulations [1]. Recent efforts include the influence of disorder to enhance the nematicity window [2] and the case of FeTe [3] where we have shown that coupling electrons to the monoclinic lattice distortion reproduces the bicollinear magnetic order and the ``reversed'' resistivity anisotropy found experimentally. Moreover, a novel form of nematicity is predicted. (b) The two-leg ladder compound BaFe2S3 [4]. This material is the only member of the iron-based family that becomes superconducting (at high pressure) without having iron layers in its crystal structure. Recent results using the density matrix renormalization group for a two-orbital Hubbard model [5] will be discussed. They correctly reproduce the dominant magnetic order, as in neutron scattering, and, moreover, have revealed indications of pairing tendencies at intermediate/strong couplings upon doping [6]. [1] S. Liang, A. Moreo and E. Dagotto, Phys. Rev. Lett. 111, 047004 (2013), and references therein. [2] S. Liang et al., Phys. Rev. B 92, 104512 (2015). [3] C. B. Bishop, A. Moreo, and E. Dagotto, Phys. Rev. Lett.117, 117201 (2016). [4] H. Takahashi et al., Nat. Mater. 14, 1008 (2015); T. Yamauchi et al., Phys. Rev. Lett. 115, 246402 (2015). [5] N. D. Patel, A. Nocera, G. Alvarez, R. Arita, A. Moreo, and E. Dagotto, Phys. Rev. B. 94, 075119 (2016). [6] Density functional theory studies suggest that pressure may induce self-doping of BaFe2S3, favoring superconductivity: Y. Zhang, L. Lin, J-J. Zhang, E. Dagotto, and S. Dong, submitted to PRB. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:48AM |
X23.00003: Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe Invited Speaker: Andreas Kreyssig In iron-based superconductors, the lattice, magnetism and electronic system show a fascinating interplay. Nematic order breaks the tetragonal symmetry and yields an orthorhombic lattice distortion. The same symmetry is broken by the stripe-like antiferromagnetic order suggesting a symmetry-related coupling between both phenomena. The phase transitions in to both ordered states can be simultaneous and of first-order character like in CaFe$_{2}$As$_{2}$, or separated in temperature like in Co-doped BaFe$_{2}$As$_{2}$ with second or first-order character depending on the doping level. Stripe-type magnetic fluctuations are discussed as correlation-driven electronic mechanism of the nematicity and important for the superconducting electron pairing establishing a coupling mechanism. However, a universal picture has been confounded by measurements of FeSe where the nematic and magnetic transitions appear to be decoupled by the observation of the lattice distortion without antiferromagnetic order at ambient pressure. \\ In this talk I will present our recent study on the relation between the nematic and magnetic order in FeSe single crystals investigated by synchrotron-based high-energy x-ray diffraction and time-domain Moessbauer spectroscopy as function of temperature and pressure. Distinct nematic and magnetic transitions are observed for low pressures and merge into a single first-order transition for higher pressures reminiscent of what has been found for the evolution of these transitions in Co-doped BaFe$_{2}$As$_{2}$. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the interplay between the different ordering phenomena in the iron-based superconductors. \\ This work was performed in collaboration with K. Kothapalli, A. E. B\"{o}hmer, W. T. Jayasekara, B. G. Ueland, P. Das, A. Sapkota, V. Taufour, Y. Xiao, E. Alp, S. L. Bud'ko, P.C. Canfield, and A.I. Goldman; and supported by the Department of Energy, Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract No. DE-AC02-07CH11358. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:24AM |
X23.00004: Magnetic correlations in FeSe-based superconductors Invited Speaker: Jun Zhao Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for determining the mechanism behind superconductivity. It is well established that the parent compounds of the cuprate and iron-pnictide superconductors exhibit Neel and stripe magnetic order, respectively. In contrast, FeSe exhibits nematic order but not magnetic order in the parent phase, and its magnetic ground state is undetermined. In this work, we used inelastic neutron scattering to study the spin fluctuations in FeSe ($T_c$ = 8.7 K) and heavily electron-doped FeSe-based superconductor Li$_{0.8}$Fe$_{0.2}$ODFeSe ($T_c$ = 41 K). The results revealed the coexistence of spin fluctuations near ($\pi$, 0) and ($\pi$, $\pi$) in FeSe, both of which are coupled to nematicity. The quantitative measurements of energy and momentum dependence of the spin fluctuations above and below the nematic phase transition show that FeSe is an S = 1 nematic quantum-disordered paramagnet. In addition, in Li$_{0.8}$Fe$_{0.2}$ODFeSe, ring-shaped magnetic resonant excitations were observed at 21 meV at ($\pi$, 0.62$\pi$) and equivalent wavevectors surrounding ($\pi$, $\pi$). As the energy increased, the spin fluctuations display a twisted dispersion, which is different from that of iron pnictide superconductors, but rather analogous to that of hole-doped cuprates. The effect of electron doping on the spin fluctuations, nematicity, and superconductivity in this system will be discussed. References: 1) Q. Wang et al., Nature Communications 7, 12182 (2016) 2) Q. Wang et al., Nature Materials 15, 159, (2016) 3) B. Pan et al., arXiv:1608.01204 (2016) [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 11:00AM |
X23.00005: Elastoresistance measurements as a probe of electronic nematicity in Fe-based superconductors Invited Speaker: Ian Fisher Elastoresistance measurements provide a new and powerful insight to electronic nematicity in strongly correlated materials. For Fe-based superconductors, such measurements have directly revealed the divergence of the nematic susceptibility towards a thermally driven nematic phase transition for underdoped compositions, and inferred the presence of a quantum phase transition with a nematic character near optimal doping. The elastoresistivity tensor $m_{ij,kl}$ relates changes in resistivity to strains experienced by a material. As a fourth-rank tensor, it contains considerably more information than the simpler (second-rank) resistivity tensor; in particular, for a tetragonal material, the $B_{1g}$ and $B_{2g}$ components of the elastoresistivity tensor ($m_{xx,xx} - m_{xx,yy}$ and $2m_{xy,xy}$, respectively) can be related to the material's nematic susceptibility for those symmetry channels. Spurred by our initial observations of a large elastoresistivity in Fe-based superconductors, which is directly related to the large nematic susceptibility in these materials, we have further developed the necessary formalism to describe elastoresistivity and have established improved methods to measure the most relevant elastoresistivity coefficients. In this talk I will outline some of these most recent developments in the context of elastoresistivity measurements of Fe-based superconductors. In addition, I will describe the effect of anisotropic strain on the coupled nematic/structural phase transition found for underdoped compositions, and make the case that this is an important, and thus far largely neglected, tuning parameter for materials that undergo nematic order. [Preview Abstract] |
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