Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W5: Focus Session: Magnetism and Transport in Fe-Based Superconductors |
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Sponsoring Units: DMP DCOMP Chair: Natalia Perkins, University of Minnesota Room: Juan Gorman Room 005 |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W5.00001: Intrinsic band pictures of (122) and (11) iron pnictides from magnetotransport measurements K. Huynh, Y. Tanabe, T. Urata, S. Heguri, K. Tanigaki, M. Hagiwara, T. Kida, H. Oguro, K. Watanabe In this report,the band picture of the typical (122) BaFe$_2$As$_2$ and (11) FeSe single crystals will be discussed from the view point of transport properties under high magnetic fields. By applying the technique of mobility spectrum analysis, we are able to describe the numbers of electrons and holes in terms of distribution functions of mobility; the partial contribution from each Fermi pocket to the overall transport properties is thus can be clarified. The analyses show that in both (122) and (11) materials the conduction of electron is much more complex than that of hole. The mobility spectra of holes always indicates isotropic pockets. On the other hand, in the electron side the spectra are broad and associated with a long tail extended to very high mobility region, highlighting the existence of Dirac cones [1, 2]. The unusual features of the mobility spectra will be discussed in comparison with various models and observations of band structures.\\ $[1]$ K. K. Huynh et al, New J. Phys. 16 0930262 (2014)\\ $[2]$ K. K. Huynh et al, PRB 90 144516 (2014) [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W5.00002: Sharp enhancement of spin fluctuations by nematic order in iron pnictides Qiang Zhang, Rafael. M. Fernandes, Jagat Lamsal, Jiaqiang Yan, Songxue Chi, Gregory. S. Tucker, Daniel. K. Pratt, Jeffrey. W. Lynn, R. W. McCallum, Paul. C. Canfield, Thomas A. Lograsso, Alan I. Goldman, David Vaknin, Robert J. McQueeney Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{{\rm S}}$, sets in well above the stripe antiferromagnetic ordering at $T_{{\rm N}}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{{\rm S}}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W5.00003: Electrodynamic response in the electronic nematic phase of BaFe$_2$As$_2$ C. Mirri We measure the in-plane optical reflectivity of BaFe$_2$As$_2$ beyond the MIR interval, studied so far, covering the spectral range from the far infrared (FIR) to the ultraviolet (UV), at several combinations of pressure, used to detwin the specimen, and temperature. Our goal is to probe the anisotropic response in the real part of the optical conductivity $\sigma_1(\omega)$, extracted from the reflectivity data via Kramers-Kronig transformations. We thus elucidate how the anisotropic optical metallic response evolves as a function of pressure, considered as an external symmetry breaking field, and across the ferro-elastic structural transition. At the center of our attention we then place the analysis of the spectral weight reshuffling over a large energy interval. We provide relevant information about the evolution of the effective metallic charge dynamics, upon tuning the degree of detwinning, in terms of scattering rate and plasma frequency of the itinerant charge carriers, which allows a direct link to the yet astonishing $dc$ transport properties. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:42PM |
W5.00004: Emergent Defect States as a Source of Resistivity Anisotropy in the Nematic Phase of Iron pnictides Invited Speaker: Brian M. Andersen The pronounced electronic anisotropy observed in several experiments probing the iron-based superconductors is currently a topic of great interest and controversy.[1] I will discuss novel disorder effects in the nematic phase above the transition temperature to the (pi, 0) stripe ordered magnetic state but below the orthorhombic structural transition. The anisotropic spin fluctuations in this region can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials that scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. From the calculated scattering potentials, relaxation rates, and conductivity in this region we conclude that such emergent defect states are essential for the transport anisotropy observed in experiments.[2] Thus, a full understanding of the transport anisotropy in iron pnictides requires both intrinsic nematic susceptibility and concomitant emergent impurity response. Below the spin density wave transition the nematogens freeze into dimer states that show many characteristics in agreement with STM measurements.[3] The talk will end with a discussion of theoretical results of other fascinating and highly unusual impurity aspects of iron-based superconductors. This includes, for example, unusual magnetic defect states in the different possibly magnetic structures at low temperatures, the induction of impurity-induced long-range ordered phases due to unconventional RKKY exchange couplings that would not be present without the disorder.[4] [1] R. M. Fernandes, A. V. Chubukov, and J. Schmalian, Nat. Phys. \textbf{10}, 97 (2014). [2] M. N. Gastiasoro,$^{\, }$I. Paul,$^{\, }$Y. Wang,$^{\, }$P. J. Hirschfeld,$^{\, }$and B. M. Andersen, Phys. Rev. Lett. \textbf{113}, 127001 (2014). [3] M. N. Gastiasoro, P. J. Hirschfeld,$^{\, }$and B. M. Andersen, Phys. Rev. B \textbf{89}, 100502(R) (2014). [4] M. N. Gastiasoro$^{\, }$and B. M. Andersen, Phys. Rev. Lett. \textbf{113}, 067002 (2014) [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W5.00005: Ising-nematic order and spin excitations in the bilinear-biquadratic model for the iron pnictides Patricia Bilbao Ergueta, Andriy H. Nevidomskyy Motivated by the recent inelastic neutron scattering (INS) measurements in the iron pnictides which show a strong anisotropy of spin excitations above the N\'eel temperature {[1]}, we study the frustrated Heisenberg model with a biquadratic spin-spin exchange interaction. Using the Dyson-Maleev (DM) spin representation, which proves appropriate for all temperature regimes, we find that the spin dynamical structure factors are in excellent agreement with experiment, exhibiting spontaneous $C_4$ lattice symmetry breaking even into the paramagnetic region $T_N |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W5.00006: Competing magnetic double-Q phases in iron pnictides Maria N. Gastiasoro, Brian M. Andersen Recent experimental studies have reported several unusual properties in the magnetic phase of iron pnictides compatible with double-Q magnetic phases in a tetragonal crystal. Here we perform a theoretical study of the stability and electronic properties of the relevant double-Q phases of these compounds. The model consists of an unrestricted Hartree-Fock approximation of a realistic five-orbital band and the standard on-site multi-orbital Coulomb interaction. We find that the competing non-collinear and non-uniform double-Q phases exist generally at the foot of the magnetic single-Q stripe dome in agreement with recent experiments, and separated from it by weak first order transitions. We contrast the electronic properties of the three different magnetic phases and study the consequences of the charge modulations formed around non-magnetic impurities. Finally we discuss the role of magnetic disorder relevant for the reported tetragonal magnetic phase in the Mn-122 compound. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W5.00007: Tetragonal magnetic order in iron pnictides: phase diagram, spin-wave dispersion, and competition with superconductivity Xiaoyu Wang, Jian Kang, Andrey Chubukov, Rafael Fernandes Recent experiments in hole-doped iron pnictides have found a magnetic state near optimal doping that displays magnetic Bragg peaks at $(\pi,0)$ and $(0,\pi)$ but no splitting of the lattice Bragg peaks, implying that the system remains tetragonal. Here we use a multi-band itinerant microscopic model to discuss the origin, the phase diagram, and the manifestations of this tetragonal magnetic state. Starting from perfect nesting, we find that increasing the doping concentration leads to a change in the magnetic state from a single-Q orthorhombic ($C_2$) phase to a double-Q tetragonal ($C_4$) phase and then back to a single-Q $C_2$ phase. As temperature is lowered, we find that the $C_4$ phase is in general unstable towards the $C_2$ phase. Furthermore, we show that the superconducting state tends to be more strongly suppressed by the $C_4$ phase due to the interplay between tetragonal symmetry-breaking and the near-degeneracy of the superconducting ground state. We compare our results with the phase diagram of Na- and K-doped Ba122 materials, and discuss the theoretical spin-wave spectrum of the tetragonal magnetic phase, highlighting the features that allow for an unambiguous experimental distinction between the $C_4$ and $C_2$ phases. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W5.00008: Theory of the evolution of magnetic order in Fe$_{1+y}$Te compounds with increasing interstitial iron Samuel Ducatman, Rafael Fernandes, Natalia Perkins We examine the influence of the excess of interstitial Fe on the magnetic properties of Fe$_{1+y}$Te compounds. We assume in our model that some of the Fe orbitals give rise to localized magnetic moments. These moments interact with each other via exchange interactions as well as biquadratic interactions that favor a collinear double-stripe state, corresponding to the ordering vectors $\left(\pm\pi/2,\pm\pi/2\right)$. The remaining Fe orbitals are assumed to be itinerant, giving rise to the Fermi surface displaying nesting features at momenta $\left(\pi,0\right)/\left(0,\pi\right)$. Increasing the amount of itinerant electrons due to excess Fe, $y$, leads to changes in the Fermi surface and to the suppression of its nesting properties. As a result, due to the Hund's coupling between the itinerant and localized moments, increasing $y$ leads to modifications in the local moments' exchange interactions via the multi-orbital generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. We computed the RKKY corrections and minimized the resulting effective exchange Hamiltonian. We find the excess electrons change the classical magnetic ground state from a double-stripe state to an incommensurate spiral, consistent with the experimental observations. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W5.00009: Critical Charge Fluctuations and Ingap Collective Modes in the Superconducting State of the NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As Iron Pnictide Superconductor Verner Thorsmolle, Maxim Khodas, Zhiping Yin, Chenglin Zhang, Scott Carr, Pengcheng Dai, Girsh Blumberg We use polarization-resolved Raman spectroscopy to study the Raman susceptibility ($\chi (\omega $,T,x)) of the x-T phase diagram of NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As [1]. Above the structural T$_{\mathrm{S}}$(x) and the superconducting T$_{\mathrm{c}}$(x) transition, $\chi (\omega $,T,x) is dominated by a low-frequency quasielastic peak in B$_{\mathrm{2g}}$ symmetry displaying critical behavior across the entire phase diagram. Below T$_{\mathrm{c}}$(x), sharp ingap modes emerge for x$\ge $0.0165 in A$_{\mathrm{1g}}$ ($\approx $65 cm$^{-1})$ and B$_{\mathrm{2g}}$ ($\approx $25 and $\approx $55 cm$^{-1})$ symmetry. The critical charge fluctuations are interpreted in terms of plasma waves of quadrupole excitations which below T$_{\mathrm{c}}$(x) undergo a metamorphosis into the ingap modes. The A$_{\mathrm{1g}}$ mode is a particle-hole (p-h) charge exciton consistent with a non-conventional s$+$- superconducting groundstate. The minor B$_{\mathrm{2g}}$ mode is a Bardasis-Schrieffer Cooper pair exciton of d-wave symmetry which exists only in a narrow doping window of density wave and superconductivity coexistence. The major B$_{\mathrm{2g}}$ mode is a bound state of d$+$- p-h plasma oscillations. \\[4pt] [1] V. K. Thorsm{\o}lle \textit{et al.} arXiv:1410.6456v1. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W5.00010: Magnetic fluctuations under the superconducting dome of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)$_{2}$As$_{2}$ from flux-flow resistivity Xinyi Huang, Derek Haney, Yogesh Singh, Shuai Zhang, Hai-Hu Wen, Tao Hu, Maxim Dzero, Carmen Almasan We investigate the magnetism inside the superconducting phase of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)$_{2}$As$_{2}$ crystals (on both sides of the optimal doping) by inducing superconducting vortices via applied field and performing current-voltage measurements. This allows us to measure the quasiparticle scattering within the normal cores, despite being inside the superconducting dome. Analysis of the free-flux-flow resistivity within the superconducting phase shows a sharp increase in the quasiparticle scattering with decreasing temperature and applied field, which we attribute to the presence of critical spin fluctuations inside the vortex core. The fluctuations are strongest in the doping with the highest critical temperature, and the behavior is suppressed as the material is more underdoped. For each doping measured, at different temperatures and applied fields, the vortex dissipation curves scale and show an exponential relationship. We will discuss the physics behind the exponential relationship for each doping range. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W5.00011: Correlation, doping, and interband effects on the optical conductivity of iron superconductors Luca de 'Medici, Maria J. Calderon, Belen Valenzuela, Elena Bascones Optical conductivity is one of the tools traditionally used to study strongly correlated systems. For single band systems, the interpretation of these data is rather straightforward and very well known. This is not the case for multiorbital systems, where electronic interactions lead to nontrivial features in the optical spectrum. We have studied the case of iron superconductors by means of a model that introduces the orbital dependent interactions. We find that interband transitions make a non-negligible contribution to the low-energy plateau found in the optical spectrum of undoped compounds and account for a large fraction of the spectral weight at the cutoff frequencies currently used to determine the Drude weight. This fraction is strongly enhanced in hole-doped samples as the larger effect of interactions towards half-filling strongly suppresses the Drude weight. We analyze the relationship between the Drude weight and the kinetic energy and their renormalizations. We show that with orbital differentiation, the renormalization of both the Drude weight and the kinetic energy are not equal, not even within a Fermi liquid picture. Phys. Rev B 90, 115128 (2014) [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W5.00012: Direct characterization of photo-induced femtosecond lattice dynamics in BaFe$_2$As$_2$ Simon Gerber, Yan Zhang, Diling Zhu, Nachum Plonka, Ming Yi, Georgi L. Dakowski, Patrick S. Kirchmann, Robert G. Moore, Matthieu Chollet, James M. Glownia, Yipeng Feng, Joshua J. Turner, Jun-Sik Lee, Apurva Mehta, Hsueh-Hui Kuo, Ian R. Fisher, Thomas Wolf, Yi-De Chuang, Zahid Hussain, Chi-Chang Kao, Kyungwan Kim, Zhi-Xun Shen, Thomas P. Devereaux, Wei-Sheng Lee BaFe$_2$As$_2$ exhibits a strong coupling among nematic fluctuations, the spins and the lattice, serving as a playground for an ultrafast control of the electronic properties via optical excitation. Here we use ultrafast x-ray scattering to measure a lattice Bragg peak in photo-excited BaFe$_2$As$_2$. Ultrafast structural phase transitions do not occur upon excitation with an optical laser pulse, indicating that the change of the crystal structure may not be solely attributed to nematic fluctuations. In addition, an ultrafast increase and oscillation of the Bragg peak intensity is observed. Its frequency is consistent with the coherent excitation of an $A_{1g}$ optical phonon mode, that modulates the Fe-As-Fe bond angle. Thereby, we obtain direct and quantitative access on the bond angle variation. [Preview Abstract] |
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