Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z26: Focus Session: Iron Based Superconductors -- Electronic Anisotropy |
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Sponsoring Units: DMP DCOMP Chair: Rajiv Singh, University of California, Davis Room: D162/164 |
Friday, March 25, 2011 11:15AM - 11:51AM |
Z26.00001: In-plane electronic anisotropy of underdoped iron arsenide superconductors Invited Speaker: Common to the high Tc cuprates, superconductivity in the Fe arsenides and related compounds is associated with suppression of an antiferromagnetic ground state. On the underdoped side of the phase diagram, in addition to the antiferromagnetic transition, the materials also suffer a phase transition that breaks the 4-fold rotational symmetry of the high-temperature crystal structure, this occurring at either the same or higher temperature than the Neel transition. Emerging evidence based on measurements of detwinned single crystals reveals a dramatic in-plane electronic anisotropy associated with this nematic transition.\footnote{In collaboration with: J.-H. Chu, H.-H Kuo, J. Analytis, M. Yi, D. H. Lu, Z. X. Shen, A. P. Sorini, A. F. Kemper, S.-K. Mo, T. P. Devereaux, R. G. Moore, M. Hashimoto, W. S. Lee, Z. Hussain, K. De Greve, P. L. McMahon, Y. Yamamoto (Stanford); Z. Islam (APS,ANL); A. Dusza, A. Lucarelli, F. Pfuner \& L. Degiorgi (ETHZ).} [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z26.00002: Superconducting properties and the interplay between magnetism and superconductivity in 1111 Fe arsenides as revealed by torque magnetometry Gang Li, Gael Grisonnanche, Benjamin Conner, Nikolai Zhigadlo, Sergiy Katrych, Zbigniew Bukowski, Janusz Karpinski, Luis Balicas We performed a study of the angular dependence of the magnetic torque in LaFeAsO$_{0.9}$F$_{0.1}$ single crystals. We developed a method to separate the magnetic and the superconducting components inherent to the FeAs layers and which are superimposed onto the reversible torque signal $\tau_{\rm{rev}} (\theta, H, T)$. We show that by exploring the amplitude of the superconducting component in $\tau_{\rm{rev}} (\theta)$ as a function of $H$, it is possible to extract the thermodynamic value of the superconducting upper critical field $H_{c2}$. This so obtained value can be used to extract the field and the temperature dependencies of respectively, the superconducting anisotropy and the superfluid density through the Kogan formalism. We observe a strong temperature and field dependence of the superconducting anisotropy as expectable within a multi-band superconducting scenario. The resulting $T$-dependence of the superfluid-density resembles the behavior previously reported for LaFePO and which was ascribed to nodal superconductivity. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z26.00003: Properties of de-twinned iron-arsenide SrFe$_2$As$_2$ Erick Blomberg, M.A. Tanatar, A. Kreyssig, N. Ni, A. Thaler, R.W. Hu, P.C. Canfield, S.L. Bud'ko, A.I. Goldman, R. Prozorov The iron-pnictides, AFe$_2$As$_2$ (A = alkali earth metal), undergo a tetragonal to orthorhombic structural transition below $T_s$, which is in the range between 130~K to 210~K depending on the compound. To release elastic deformation the orthorhombic phase is spontaneously divided into four degenerate, equally populated, structural domains. This makes any measurements of in-plane anisotropy extremely difficult. Unlike high$-T_c$ Y-Ba-Cu-O which is orthorhombic already at room temperature, detwinning of pnictides is more difficult, because of lower $T_s$. We developed a technique of mechanical de-twinning of these materials that allows transport, x-ray and direct optical measurements [1]. Here we report polarized light microscopy, synchrotron X-ray analysis and AC transport measurements on SrFe$_2$As$_2$, which represents a clean case of first-order magnetic/structural transition. \\[4pt] [1] M. A. Tanatar, et al. Phys. Rev. B \textbf{81} 184508 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z26.00004: Magnetic Field Effect on the In-plane Electrical Resistivity of FeTe$_{1-x}$Se$_x$ Single Crystals Yimin Xiong, Amar Karki, Brian Sales, Rongying Jin The in-plane electrical resistivity ($\rho_{ab}$) of FeTe$_{1- x}$Se$_x$ single crystals is measured as a function of temperature (T), magnetic field (H), and the angle ($\theta$) between H and electric current (I). The results reveal that $\rho_{ab}$ strongly depends on both H and $\theta$, indicating the participation of spin scattering in the electrical transport. The underlying physics will be discussed. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z26.00005: Anisotropy of normal and superconducting states of FeSeTe M.A. Tanatar, E.C. Blomberg, J.-Ph. Reid, J. Hu, J. Quian, Z.Q. Mao, Louis Taillefer, R. Prozorov We report anisotropic electrical and thermal transport measurements in non-superconducting parent FeTe and superconduicting optimally doped FeTeSe. Intrinsic in- plane anisotropy of the electrical resistivity was measured in mechanically detwinned crystals of the parent compound [1]. In-plane and inter-plane heat transport was used to probe the symmetry of the superconducting gap in the material close to optimal doping (Tc=15~K). The results are compared to those of superconducting FeSe [2] and doping evolution of thermal conductivity in BaFe2As2 doped with cobalt [3,4]. [1] M.A. Tanatar, {\it et al.} Phys. Rev. B {\bf 81}, 184508 (2010). [2] J. K. Dong, {\it et al.} Phys. Rev. B {\bf 80}, 024518 (2009). [3] M. A. Tanatar, {\it et al.} Phys. Rev.Lett. {\bf 104}, 067002 (2010). [4] J.-Ph. Reid, {\it et al.} Phys. Rev. B {\bf 82}, 064501 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z26.00006: Pulsed-field contactless mapping of the anisotropic upper critical field in LiFeAs superconducting crystals K. Cho, H. Kim, M.A. Tanatar, R. Prozorov, Y.J. Song, Y.S. Kwon, W.A. Coniglio, C.C. Agosta, A. Gurevich Angle - resolved measurements of the upper critical field were performed using a tunnel diode resonator in the stoichiometric iron arsenide superconductor LiFeAs ($T_c$=18~K) in pulsed magnetic fields up to 50~T at temperatures down to 0.6~K. Complete $H^{\parallel c}_{c2}(T)$ and $H^{\bot c}_{c2}(T)$ curves with $T \to 0$ extrapolated values of $H^{\parallel c}_{c2}(0)=17 \pm 1 $ T and $H^{\bot c}_{c2}(T)=26 \pm 1$ T were obtained. The anisotropy, ${\gamma _{H_{c2}} } \equiv H_{c2}^{ \bot c}/H_{c2}^{\parallel c} \approx 2$, close to $T_c$ has revealed the essentially three-dimensional electronic structure of the material. Both temperature - dependent $H_{c2}(T)$ can be well fit within a single set of band structure, magnetism and scattering parameters. In a configuration with $H \parallel c$, $H^{\parallel c}_{c2} (T)$ is limited by orbital effects with modest scattering. In the perpendicular orientation, $H^{\bot c}_{c2}(T)$ shows a notable low-temperature saturation and a strong departure from the orbital Werthamer-Helfand-Hohenberg model. Instead, fitting results suggest paramagnetic Pauli limiting and the development of a spatially - modulated superconducting state. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z26.00007: Symmetry breaking orbital anisotropy observed in detwinned Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ above the spin density wave transition Ming Yi, D.H. Lu, J.-H. Chu, J.G. Analytis, A.P. Sorini, A.F. Kemper, S.-K. Mo, R.G. Moore, M. Hashimoto, W.-S. Lee, Z. Hussain, T.P. Devereaux, I.R. Fisher, Z.-X. Shen Nematicity has recently been observed in the competing phases in proximity to the superconducting phase in the cuprates. Similarly, the iron pnictides exhibit symmetry breaking competing phases in the form of a tetragonal to orthorhombic structural transition and a collinear spin density wave (SDW) transition in the underdoped regime. Evidence for strong in-plane anisotropy in the SDW state has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements, but the nature of this nematic behavior is still elusive. Here we present the results of an ARPES study of detwinned single crystals of underdoped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$, resolving single domain electronic structure in the orthorhombic SDW state which exhibits strong in-plane anisotropy consistent with other probes. The anisotropy is evident in a large splitting of the dxz and dyz bands, which is seen to develop almost fully above the onset of the long range magnetic order. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z26.00008: Nematic spin fluid in the tetragonal phase of BaFe$_{2}$As$_{2}$ Leland Harriger, Huiqian Luo, Mengshu Liu, Toby Perring, Chris Frost, Jiangping Hu, Mike Norman, Pengcheng Dai We use inelastic neutron scattering to demonstrate the presence of a large spin anisotropy above $T_{N}$ in the unstressed tetragonal phase of BaFe$_{2}$As$_{2}$. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the AF $a$-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest that the spin nematicity we find in the tetragonal phase of BaFe$_{2}$As$_{2}$ is the source of the electronic and orbital anisotropy observed above $T_{N}$ by other probes, and has profound consequences for the physics of these materials. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z26.00009: Reconstructed electronic structure from orbital ordering and antiferromagnetism in the iron pnictides Weicheng Lv, Philip Phillips Recent experimental developments have unambiguously demonstrated the in-plane electronic and magnetic anisotropy of the iron-based superconductors. It has been argued that this nematic state can arise from orbital ordering physics. Including an energy splitting term that breaks the degeneracy of the Fe $d_{xz}$ and $d_{yz}$ orbitals, we solve the multi-orbital Hubbard model within a mean-field approximation. Despite sensitivity of the resulting state to the input parameters, we find that a weak orbital order that places the $d_{yz}$ orbital slightly higher in energy than the $d_{xz}$ orbital, along with the interactions $U$ and $J$ being of intermediate strength, is compatible with current experimental results. In this regime, the stripe antiferromagnetism is further stabilized and the existence of the Dirac cones is preserved. Furthermore, this anisotropic electronic state leads to the observed resistivity anisotropy and STM interference patterns. Finally the relation between orbital order and superconductivity is discussed. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z26.00010: Magnetic Torque Evidence for Broken Rotational Symmetry in the Tetragonal Phase of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ Single Crystals Shigeru Kasahara, Hongjie Shi, Ryuji Okazaki, Kenichiro Hashimoto, Minoru Yamashita, Takasada Shibauchi, Takahito Terashima, Yuji Matsuda The emergence of broken four-fold symmetry is found in the tetragonal phase of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals [1] by in-plane anisotropy measurements of magnetic susceptibility. Magnetic torque detects a spontaneous growth of two-fold oscillations under in-plane field rotations, whose amplitude is linked to an order parameter of an electronic ``nematic'' phase. Our findings reveal that the spontaneous rotational symmetry breaking sets in far above the tetragonal to orthorhombic structural transition, which might be also linked to the unconventional superconductivity of this system [2,3]. \\[4pt] [1] S. Kasahara, et al., Phys. Rev. B 81, 184519 (2010). \\[0pt] [2] K. Hashimoto, et al., Phys. Rev. B 81, 220501(R) (2010). \\[0pt] [3] Y. Nakai, et al., Phys. Rev. B 81, 020503(R) (2010). [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z26.00011: Interplay of orbital ordering and magnetism in the parent compounds of the iron pnictides Andriy Nevidomskyy The neutron scattering experiments on the parent compounds of the 122 family of the iron pnictide superconductors show a pronounced orhthorhombic anisotropy in the spin wave spectra [1], also observed in resistivity measurements on detwinned crystals [2,3]. Orbital ordering of the d$_{xz}$ and d$_{yz}$ orbitals may be a possible explanation for this behaviour [4]. In this work, we establish the effect of orbital ordering on the magnetism and study their interplay through a combination of the first-principles band theory calculations and phenomenological analysis in the framework of the Landau theory. This enables us to establish the coupling between the orbital and magnetic degrees of freedom in these materials. Consequences for the symmetry of the superconducting order parameter are discussed.\\[4pt] [1] J. Zhao et al., Nature Phys. {\bf 5}, 555 (2009) \\[0pt] [2] M.A. Tanatar at al., Phys. Rev. B {\bf 81}, 184508 (2010).\\[0pt] [3] J.-H. Chu et al., Science {\bf 329}, 824 (2010). \\[0pt] [4] C.-C. Chen et al., Phys. Rev. B {\bf 82}, 100504(R) (2010). [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z26.00012: Orbital-ordering and In-plane Anisotropy in Low-moment Ground-state of Parent Compounds of Iron-based Superconductors Masahiko Machida, Hiroki Nakamura Since the discovery of the iron-based superconductor, a large discrepancy between experimental observations and first-principles calculations in the magnetic moment of the antiferromagnetic state of the parent compounds has been intensively debated. The observed moment values are about 3 to 5 times smaller than those of the calculation although there is a variety of the difference depending on the materials. Very recently, an interesting calculation data fully reproducing the observed low moment has been suggested by F. Cricchio et al., (Phys. Rev. B 81 (2010) 140403) who performed first-principles calculations using a LDA+U scheme. In this study, we suggest that the new state is a possible candidate to well explain the other data, e.g., strong anisotropy in spatial patterns measured by STM and magnetic excitations found by neutron scattering. Furthermore, we compare the result with other theoretical works reproducing similar low moment in terms of orbital ordering. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z26.00013: Conductivity Anisotropy in the Antiferromagnetic State of Iron Pnictides Belen Valenzuela, Elena Bascones, Maria J. Calderon Recent experiments on iron pnictides have uncovered a large in- plane resistivity anisotropy with a surprising result: the system conducts better in the antiferromagnetic $x$ direction than in the ferromagnetic $y$ direction [1]. We address this problem by calculating the ratio of the Drude weight along the $x$ and $y$ directions, $D_x/D_y$, for the mean-field $\bf{Q}= (\pi,0)$ magnetic phase diagram of a five-band model for the undoped pnictides [2,3]. We find [4] that $D_x/D_y$ ranges between $0.2 < D_x/D_y < 1.7$ for different interaction parameters. Large values of the orbital ordering favor an anisotropy opposite to the one found experimentally. On the other hand, $D_x/D_y$ is strongly dependent on the topology and morphology of the reconstructed Fermi surface. Our results point against orbital ordering as the origin of the observed conductivity anisotropy, which may be ascribed to the anisotropy of the Fermi velocity. [1] J.-H. Chu et al., Phys. Rev. B 81, 214502 (2010); J.-H. Chu et al., Science 329, 824 (2010); M. Tanatar et al., Phys. Rev. B 81, 184508 (2010). [2] M.J.Calderon, B. Valenzuela, E. Bascones, Phys. Rev. B 80, 094531 (2009). [3] E.Bascones, M.J. Calderon, B. Valenzuela, Phys. Rev. Lett. 104, 227201 (2010). [4] B.Valenzuela, E. Bascones, M.J. Calderon, Phys. Rev. Lett. 105, 207202 (2010). [Preview Abstract] |
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