Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y51: Invited Session: Origin and Manifestations of Electronic Nematicity in Iron-Based Superconductors |
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Sponsoring Units: DCMP Room: Grand Ballroom C1 |
Friday, March 6, 2015 8:00AM - 8:36AM |
Y51.00001: A New Magnetic Phase in Hole-Doped BaFe$_2$As$_2$: Implications for the Origin of Nematicity Invited Speaker: Raymond Osborn Establishing the origin of nematic order has emerged as one of the most important goals of research into iron pnictides and chalcogenides, because of its implications for the origin of their unconventional superconductivity [1]. It is well known that superconductivity emerges when antiferromagnetism is suppressed with doping or pressure. Across the phase diagram, the magnetic transition occurs just below, or is coincident with, a structural phase transition from tetragonal ($C_4$) to orthorhombic ($C_2$), or nematic, symmetry. A symmetry analysis indicates that the $C_2$ transition is electronically driven, but it could be due either to orbital interactions that then induce magnetic stripe order or to magnetic interactions that then induce orbital order. In the latter, magnetic fluctuations from interactions between the hole pockets at $Q=0$ and the electron pockets at $Q_X=(\pi,0)$ and $Q_Y=(0,\pi)$ break the Ising X/Y symmetry before time-reversal symmetry is broken. As part of a comprehensive neutron diffraction study of the phase diagram of hole-doped BaFe$_2$As$_2$ [2], we have recently observed an entirely new magnetic phase that occurs close to the suppression of the $C_2$ phase, in which the spins reorient along the $c$-axis and $C_4$ symmetry is restored [3]. This reentrant $C_4$ transition has now been observed in other hole-doped compounds as well. In spin-nematic theory, a restoration of $C_4$ symmetry is predicted to occur when doping weakens Fermi surface nesting, favoring an order parameter that involves $Q_X$ and $Q_Y$ simultaneously, so our observations provide evidence for magnetically-driven models of nematicity. \\[4pt] [1] R. M. Fernandes, \textit{et al.} Nat. Phys. \textbf{10}, 97 (2014);\\[0pt] [2] S. Avci \textit{et al.} Phys. Rev. B \textbf{88}, 094510 (2013);\\[0pt] [3] S. Avci \textit{et al} Nat. Comm. \textbf{5}, 3845 (2014) [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 9:12AM |
Y51.00002: Who is in charge of the nematic order in iron-based superconductors? Invited Speaker: Andrey Chubukov Although the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. Nematic order breaks the discrete lattice rotational symmetry by making the $x$- and $y$-directions in the iron plane non-equivalent. This can happen because of a regular structural transition or due to a electronically-driven instability -- in particular, orbital order and spin-driven Ising-nematic order. The latter is a magnetic state that breaks rotational symmetry but preserves time-reversal symmetry. Symmetry dictates that the development of one of these orders immediately induces the other two, making the origin of nematicity a physics realization of the ``chicken and egg problem.'' will argue that the evidence strongly points to an electronic mechanism of nematicity, placing nematic order in the class of correlation-driven electronic instabilities, like superconductivity and density-wave transitions. I will discuss different microscopic models for nematicity and link them to the properties of the magnetic and superconducting states, providing a unified perspective on the phase diagram of the iron pnictides. (Based on R.M. Fernandes, A.V. Chubukov, and J. Schmalian, Nature Physics 10, 97 (2014).) [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:48AM |
Y51.00003: Nematic charge fluctuations in iron-based superconductors by Raman scattering Invited Speaker: Yann Gallais Electronic analogues of nematic states, in which rotational symmetry is broken but translational invariance is preserved, have been proposed in a variety of correlated materials, such as quantum Hall systems, cuprates, ruthenates, heavy fermions, and, more recently, iron based superconductors. In the iron-based superconductors (Fe SC) several experiments have collected evidence that the tetragonal- to-orthorhombic structural transition is driven not by the lattice, but by electronic nematicity. However it remains a challenge to disentangle the roles of the lattice, spin and charge/orbital degrees of freedom in driving the nematic instability. In my talk I will discuss electronic Raman scattering measurements which demonstrate the presence of charge nematic fluctuations in the tetragonal phase of several Fe SC systems. I will discuss the implications of our results for the mechanism of the nematic/orthorhombic transition in these systems. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:24AM |
Y51.00004: Unusual disorder-limited transport in Fe-based superconducting materials Invited Speaker: Peter Hirschfeld The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, has been a longstanding puzzle. I discuss first the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave (SDW) metal. Within this framework[1], many of the qualitative features of the transport can be understood by accounting for the growth of spin correlations pinned by impurities above the Neel temperature, and SDW-induced Lifshitz transitions below. I then discuss implications of this picture for the anisotropy observed in untwinned crystals. [1] Y. Wang, M. N. Gastiasoro, B. M. Andersen, M. Tomic, H.O. Jeschke, Roser Valenti, I. Paul and P.J. Hirschfeld, arXiv:1408.1933 [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 11:00AM |
Y51.00005: Electronic nematicity in Iron Pnictide superconductors probed via STM Invited Speaker: Abhay Pasupathy The microscopic origin of electronic nematicity and its relationship to superconductivity in the iron pnictides remains poorly understood. I will present recent scanning tunneling microscopy (STM) and spectroscopy (STS) measurements that directly visualize the nematicity in the electronic states of the pnictide superconductor Na(Fe,Co)As. The spatial and energy dependence of features seen in the spectroscopic images sheds light on the nature of the important interactions responsible for nematicity in this material, and measurements taken above and below the superconducting transition temperature reveal the interaction between the nematic electronic structure and superconductivity. I will describe measurements across the entire temperature-doping phase diagram and present a simple, unified picture for understanding nematicity as visualized by STM in the pnictides. [Preview Abstract] |
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