Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A13: Focus Session: Fe based superconductors-Magnetism and Nematicity |
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Sponsoring Units: DMP Chair: Wei-Cheng Lee, University of Illinois Room: 207 |
Monday, March 3, 2014 8:00AM - 8:12AM |
A13.00001: Magnetic order without tetragonal symmetry-breaking in the iron pnictides Xiaoyu Wang, Rafael Fernandes In most iron pnictides, the magnetic state is orthorhombic, displaying domains of magnetic stripes with ordering vectors $Q_1=(\pi,0)$ or $Q_2=(0,\pi)$. However, some recent experiments on Mn and Na doped Ba-122 found evidence for magnetic order at $Q_1$ and $Q_2$ without tetragonal symmetry breaking. Such a state corresponds to a spin configuration $S(r)=M_1 e^{iQ_1\cdot r}+M_2e^{iQ_2\cdot r}$ with $|M_1|=|M_2|$, in contrast to the stripe case where either $M_1$ or $M_2$ vanish. Here we discuss possible microscopic mechanisms responsible for this unusual order and its manifestations in the electronic and spin-wave spectra, focusing on the Mn doped compound. We show that the coupling between the itinerant Fe electrons and the Neel fluctuations arising from local Mn moments can give rise to a tetragonal-symmetric magnetic state with $M_1\parallel M_2$ -- a non-uniform state that induces checkerboard charge order. In contrast, the state with $M_1\perp M_2$ is non-collinear and gives rise to peculiar spin-wave modes. These characteristic features can be used to unambiguously identify the magnetic states, without relying on the absence of orthorhombic distortion. The existence of such states implies that tetragonal symmetry breaking is a consequence, not a cause, of magnetism. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A13.00002: Itinerant scenario of magnetism and superconductivity in iron-based superconductors Yu-Zhong Zhang, Ming-Cui Ding, Hai-Qing Lin We will show in this talk that magnetic and superconducting phases of iron-based superconductors can be systematically understood from itinerant weak coupling limit except for the K-doped iron selenides where Fe vacancy order plays dominant roles, FeTe where excess Fe in the interstitial is responsible for the unique bicollinear antiferromagnetic order, and LaFePO where superconducting state at low temperature comes out of competitions of two instabilities between ($\pi$,$\pi$) and ($0$,$0$) which show tendency towards collinear antiferromagnetic state and N\'{e}el ordered anferromagnetic or ferromagnetic state, respectively. We also exhibits that Fermi surface nesting is not a necessary condition for the itinerant magnetism in multi-orbital systems. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A13.00003: Longitudinal spin excitations and magnetic anisotropy in antiferromagnetically ordered BaFe$_2$As$_2$ Yuan Li, Chong Wang, Rui Zhang, Huiqian Luo, Fa Wang, Pengcheng Dai, Louis-Pierre Regnault In the iron-based superconductors, there is an outstanding debate on the microscopic origin of the magnetism, whether it arises from local moments or itinerant electrons with Fermi-surface nesting. To answer this question, we performed a spin-polarized inelastic neutron scattering study of spin waves in the antiferromagnetically ordered state of BaFe$_2$As$_2$. Three distinct excitation components are identified, with spins fluctuating along the $c$-axis, perpendicular to the ordering direction in the $ab$-plane, and parallel to the ordering direction. While the first two ``transverse'' components can be described by a linear spin-wave theory with magnetic anisotropy and inter-layer coupling, the third ``longitudinal'' component is generically incompatible with the local moment picture. It points towards a contribution of itinerant electrons to the magnetism already in the parent compound of this family of Fe-based superconductors. (arXiv:1309.7553) [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A13.00004: Interplay between accidental gap nodes and nematic order in iron-based superconductors Jian Kang, Alexander Kemper, Rafael Fernandes In some iron-based materials, long-range nematic order coexists with superconductivity -- either via a spontaneous tetragonal symmetry-breaking taking place at temperatures above $T_{c}$ or via application of a small external uniaxial strain to detwin the sample. Here we discuss the impact of nematic order on the anisotropic properties of the superconducting state, focusing on the particular case where accidental nodes are present at the electron pockets. Using both a 5-orbital tight-binding model and a phenomenological 3-band model, we investigate how the $d_{xz}$-$d_{yz}$ orbital order triggered by nematic order affects the magnetically-mediated pairing interaction and the gap structure. We find that proximity between $s$ and $d$ superconducting instabilities enhances the effects of nematic order, which may even lift the nodes in one of the electron pockets. We also compute thermodynamic properties in the nematic- superconducting state, such as the penetration depth and the thermal conductivity, and discuss its experimental manifestations. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A13.00005: Double magnetic resonance and spin anisotropy in Fe-based superconductors due to static and fluctuating antiferromagnetic orders Weicheng Lv, Adriana Moreo, Elbio Dagotto Motivated by recent neutron scattering experiments in Fe-based superconductors, we study how the magnetic resonance in the superconducting state is affected by the simultaneous presence of either static or fluctuating magnetic orders using the random phase approximation. We find that for the underdoped materials with coexisting superconducting and antiferromagnetic orders, spin rotational symmetry is explicitly broken at the ordering momentum $\mathbf{Q}_1 = (\pi,0)$. Only the longitudinal susceptibility exhibits the resonance mode, whereas a spin-wave Goldstone mode develops in the transverse component. Meanwhile, at the frustrated momentum $\mathbf{Q}_2 = (0,\pi)$, the susceptibility becomes isotropic in spin space and the magnetic resonance exists for both components. Furthermore, the resonance energies at $\mathbf{Q}_1$ and $\mathbf{Q}_2$ have distinct scales, which provides a natural explanation for the recently observed double resonance peaks. In addition, we show that near optimal doping the existence of strong magnetic fluctuations, which are modelled here via a Gaussian mode, can still induce the spin anisotropy in the magnetic susceptibility. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A13.00006: Nature of magnetic excitations in the electron-doped superconductor BaFe$_{2-x}$Ni$_x$As$_2$ Huiqian Luo, Xingye Lu, Meng Wang, Pengcheng Dai Inelastic neutron scattering experiments are extensively carried out on electron doped BaFe$_{2-x}$Ni$_x$As$_2$ single crystals. The effect of electron doping was found to modify spin waves in the parent compound below $\sim $100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. Our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q width below Tc and disperses with increasing energy. Upon further electron doping, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above $\sim $100 meV are still not much affected. Together with RPA calculation, we conclude that the low energy spin excitations are more likely dominated by itinerant magnetism originating from Fermi surface nesting. In the heavily electron doping x$=$0.3, the low energy spin excitations are totally gapped out below 50 meV. The whole spin spectrum reveal that the low-energy spin excitation coupling with itinerant electron is important for superconductivity, even though the high-energy spin excitations are weakly doping dependent. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A13.00007: Magnetically driven nematicity in the iron-pnictide superconductors Invited Speaker: Joerg Schmalian While the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. In this talk we discuss the physical motivation and experimental implications of Ising nematic order caused by magnetic fluctuations. We demonstrate how emergent nematic order and nematic fluctuations are consistent with the overall phase diagram as well as numerous properties of both the normal and superconducting states of the iron pnictides. Due to its magnetic origin, nematic order enhances the strength of magnetic fluctuations and induces a highly anisotropic fluctuation spectrum, leaving distinctive signatures that affect elastic and transport properties, neutron scattering experiments and the NMR spin-lattice relaxation rate. In particular we show that scaling between magnetic and lattice fluctuations provides strong evidence for a magnetically-driven nematicity in the iron-pnictide superconductors. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A13.00008: The effect of magnetic order on the superconducting gap in the co-existence phase of Fe-pnictides Alberto Hinojosa Alvarado, Andrey Chubukov We study the structure and symmetry of the superconducting gap in the presence of spin density wave (SDW) order in iron-based superconductors. We show that SDW order generally induces a spin-triplet component of the gap, in addition to the conventional spin singlet. We further show that, in some range of temperatures below $T_c$, the phases of superconducting order parameters on different reconstructed Fermi surfaces differ by an amount other than 0 or $\pi$, i.e., superconductivity directly reflects the breaking of time-reversal symmetry by SDW order. We specifically consider co-existing SDW and superconducting orders in a model with circular hole pockets and elliptic electronic pockets and present analytical results for the phase diagram and the structure of the superconducting gap at various temperatures. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A13.00009: Resistivity Anisotropy and Novel Impurity-Induced States in Fe-based Superconductors Brian Andersen, Maria Gastiasoro, Peter Hirschfeld We investigate emergent impurity-induced states arising from point-like scatterers in the spin-density wave (SDW) phase of iron-based superconductors within a microscopic five-band model [1]. Independent of the details of the band-structure and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic a-axis, and have typical lengths of 10 lattice constants in excellent agreement with recent scanning tunnelling experiments [2]. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found e.g. in the 122 iron pnictides [3]. We also study the extension of the (pi,0) SDW state above the putative Neel transition temperature T$_{\mathrm{N}}$ by addition of magnetic impurities. This study is relevant for recent neutron scattering studies showing induced magnetic high-temperature phases for sufficient amounts of Mn substitution in 122 materials [4]. Below T$_{\mathrm{N}}$ neutron studies have found enhanced (pi,pi) scattering which also can be reproduced within our scenario [5]. \\[4pt] [1] M. N. Gastiasoro \textit{et al.}, arXiv:1307.4913 (2013).\\[0pt] [2] M. P. Allan \textit{et al.,} Nat. Phys. \textbf{9}, 220 (2013).\\[0pt] [3] S. Ishida \textit{et al.,} Phys. Rev. Lett. \textbf{101}, 207001 (2013).\\[0pt] [4] D. S. Inosov \textit{et al., }Phys. Rev. B \textbf{87}, 224425 (2013).\\[0pt] [5] G. S. Tucker \textit{et al., }Phys. Rev. B \textbf{86}, 020503 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A13.00010: The interfacial effects on the spin density wave in FeSe/SrTiO3 thin film Hai-Yuan Cao, Shiyong Tan, Hongjun Xiang, D.L. Feng, Xin-Gao Gong Recently, the signs of both superconducting transition temperature beyond 60 K and spin density wave (SDW) have been observed in FeSe thin film on SrTiO3 substrate, which suggests a strong interplay between superconductivity and magnetism. With the first-principles calculations, we find that the substrate-induced tensile strain tends to stabilize the SDW state in FeSe thin film by enhancing of the next-nearest-neighbor superexchange antiferromagnetic interaction bridged through Se atoms. On the other hand, we find that when there are oxygen vacancies in the substrate, the significant charge transfer from the substrate to the first FeSe layer would suppress the magnetic order there, and thus the high-temperature superconductivity could occur. In addition, the stability of the SDW is lowered when FeSe is on a defect-free STO substrate due to the redistribution of charges among the Fe 3d-orbitals. Normally, heavy electron doping would kill superconductivity as it suppresses the spin fluctuations as well, but the expanded lattice constants in this system enhance the magnetism and thus preserve the superconductivity. Our results build a foundation for the further exploration of the superconductivity and magnetism in this novel superconducting interface. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A13.00011: Scaling between magnetic and lattice/nematic fluctuations in iron pnictides Rafael Fernandes, Anna B\"ohmer, Christoph Meingast, J\"org Schmalian The origin of the tetragonal-to-orthorhombic transition in the iron pnictides, and its relationship to the magnetically ordered state, remains a subject of intense debate, with potential implications to the mechanism behind the unconventional superconducting state. Here we investigate the coupling between these two normal-state instabilities -- magnetic and structural -- by comparing their corresponding fluctuations in the tetragonal paramagnetic phase of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. We find for several doping compositions a robust scaling relation between shear modulus data -- which probes the orthorhombic lattice fluctuations -- and NMR spin-lattice relaxation rate data -- which probes magnetic fluctuations. We explain this scaling using a theoretical model where the tetragonal symmetry breaking is triggered by an electronic nematic transition that emerges from degenerate magnetic fluctuations. Therefore, our results provide strong evidence that the structural transition in the iron pnictides is magnetically-driven. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A13.00012: Effect of SDW order fluctuations on the specific heat jump in iron pnictides Dushko Kuzmanovski, Alex Levchenko, Maxim Khodas, Maxim Vavilov A conjecture about existence of a quantum critical point beneath a superconducting dome has recently attracted attention to the properties of iron-based pnictide superconductors (FeSC) near optimal doping. Spin-density wave fluctuations in the vicinity of the critical point are expected to significantly affect thermodynamics properties of FeSC, including magnetic penetration depth, effective mass, and specific heat. We study the effect of thermal fluctuations of the SDW order on the specific heat jump at the onset of superconducting transition in the iron-based superconductors (FeSCs) based on the minimal two-band model. We find that, beyond mean-field level, the discontinuity of $\Delta C/T_c$ at the tetra-critical point (the end point of the coexistence phase) transforms into a sharp peak. We demonstrate that specific heat jump scales not simply logarithmically with $x - x_c$, as expected for the quantum critical behavior, but it acquires an even more singular power-law dependence. We fit to the experimental data from P. Walmsley $\mathit{et \ al}$., Phys. Rev. Lett. $\mathbf{110}$, 257002 (2013) including this additional term, and the increased goodness of fit suggests significant importance of the latter effect. [Preview Abstract] |
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