Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F13: Focus Session: Fe Based Superconductors-Orbital Physics |
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Sponsoring Units: DMP Chair: Adriana Moreo, University of Tennessee Room: 207 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F13.00001: Orbital nematic order and its interplay with magnetism in iron based superconductors Zhentao Wang, Andriy Nevidomskyy The nematic order in the iron pnictide family of superconductors has received a lot of attention, with recent ARPES [1] and STM [2] experiments providing strong indication in favor of the orbital nature of the nematic phase. We study the spontaneous development of the orbital nematic order and its interplay with magnetism, using random phase approximation (RPA), mean field methods, and variational cluster approximation (VCA). We show that the orbital nematic order develops when inter-orbital Hubbard repulsion $U^\prime$ is strong enough, while the intra-orbital Hubbard $U$ and Hund's coupling $J$ tend to suppress nematicity. In addition to the pure orbital nematic phase and columnar antiferromagnetic phase, we find a broad region in the parameter space where the two orders coexist. We have studied the doping dependence of these phases and find that doping away from half-filling generally suppresses both orders, consistent with the experimental phase diagram of the pnictides. We also find that the doping effect on both orders is not particle-hole symmetric, also consistent with experiments.\\[4pt] [1] M. Yi {\it et al.}, PNAS \textbf{108}, 6878 (2011).\\[0pt] [2] T.-M. Chuang {\it et al.}, Science \textbf{327}, 181 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F13.00002: Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors Hu Miao, Pierre Richard, Shangfei Wu, Jun Ma, Tian Qian, Lingyi Xing, Xiancheng Wang, Changqing Jin, Hong Ding, Chungpin Chou, Limin Wang, Wei Ku, Ziqiang Wang In iron-based superconductors, local orbital fluctuations have been proposed to be directly responsible for the structural phase transition and closely related to the observed giant magnetic anisotropy and electronic nematicity. However, whether superconductivity can emerge from, or even coexist with orbital fluctuations, remains unclear. Here we report the angle-resolved photoemission spectroscopy observation of the lifting of symmetry-protected band degeneracy, and consequently the breakdown of local tetragonal symmetry in the SC state of Li(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)As. Supported by theoretical simulations, we analyse the doping and temperature dependences of this band-splitting and demonstrate an intimate connection between ferro-orbital correlations and superconductivity. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F13.00003: Effects of spin-orbit coupling and space group symmetry in multiorbital models for iron pnictides Rong Yu, Emilian Nica, Jian-Xin Zhu, Qimiao Si Motivated by recent experiments, we study the effects of spin-orbit coupling in multiorbital models for iron-based superconductors. We show that the spin-orbit coupling leads to a nontrivial hybridization among the three t2g bands in the Brilluion zone corresponding to the two-iron unit cell, as required by the space group symmetry of the system. We also consider the superconducting pairing in the presence of spin-orbit coupling, and in agreement with the space group symmetry. By calculating the dynamical spin susceptibility in the superconducting state, we find anisotropic spin resonance excitations in consequence of the breaking of spin rotational symmetry. We further discuss the connections between our results and recent ARPES and polarized inelastic neutron scattering measurements. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F13.00004: Linear Response Theory for Shear Modulus $C_{66} $ and Raman Quadrupole Susceptibility: Evidence for Orbital Fluctuations in Fe-Based Superconductors Hiroshi Kontani, Youichi Yamakawa, Seiichiro Onari Existence of strong nematic fluctuations in various Fe-based superconductors has been discussed as a central issue. To clarify the origin, we discuss both the softening of shear modulus $C_{66} $ and the enhancement of the charge quadrupole susceptibility observed by Raman spectroscopy $\chi _{x^{2}-y^{2}}^{\mbox{Raman}} $. Due to the Aslamazov-Larkin vertex correction (AL-VC), strong orbital nematic fluctuations are induced by spin fluctuations. The strong development of $1/C_{66} $ is given by the summation of the Pauli and Van-Vleck orbital susceptibilities due to AL-VC, whereas moderate enhancement of $\chi_{x^{2}-y^{2}}^{\mbox{Raman}} $ is induced by the Van-Vleck term only. Therefore, a consistent explanation for the difference behavior between two measurements is achieved based on the orbital fluctuation theory. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F13.00005: Orbital Resonance Mode in Superconducting Iron Pnictides Wei-Cheng Lee, Philip Phillips We show that the fluctuations associated with ferro orbital order in the $d_{xz}$ and $d_{yz}$ orbitals can develop a sharp resonance mode in the superconducting state with a nodeless gap on the Fermi surface. This orbital resonance mode appears below the particle-hole continuum and is analogous to the magnetic resonance mode found in various unconventional superconductors. If the pairing symmetry is s$_{\mathrm{\pm }}$ , a dynamical coupling between the orbital ordering and the d-wave subdominant pairing channels is present by symmetry. Therefore the nature of the resonance mode depends on the relative strengths of the fluctuations in these two channels, which could vary significantly for different families of the iron based superconductors. The application of our theory to a recent observation of a new $\delta $ -function-like peak in the B$_{\mathrm{1g}}$ Raman spectrum of Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ is discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F13.00006: Study of the multi-orbital Hubbard model at finite temperature Anamitra Mukherjee, Shuai Dong, Gonzalo Alvarez, Elbio Dagotto Research in pnictide superconductors have clearly established the need for the study of multi-orbital Hubbard models. With this motivation, here we apply a combination of the real-space Exact Diagonalization and Classical Monte Carlo (ED+MC) method, widely used in manganites, with the standard Hartree-Fock mean field (MF) theory to investigate the properties of multiorbital models as a function of temperature. In this approach the MF parameters are treated via a classical MC and the fermions moving in the MF background are solved by exact diagonalization. The temperature dependence of the dynamical spin susceptibility $S(\vec{q},\omega)$, orbital resolved single particle spectral function $A(\vec{k},\omega)$, optical conductivity, and real space charge/spin/orbital density maps are calculated at different dopings. These results are relevant in understanding the role of the multiple degrees of freedom in governing the magnetic and transport properties of the Fe based superconductor materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F13.00007: Selective Mottness as a key to iron superconductors: weak \textit{and} strong correlations Invited Speaker: Luca de Medici I will discuss the strength of electronic correlations in the normal phase of Fe-superconductors and trace a comparison with cuprates. The phase diagram of the high-Tc~cuprates is dominated by the Mott insulating phase of the parent compounds. Approaching it from large doping, a standard Fermi-liquid is seen to gradually turn into a bad non-Fermi liquid metal in which quasiparticles have heavily differentiated coherence depending on momentum, a process which culminates in the pseudogap regime, in which the antinodal region in momentum space acquires a gap before the material reaches a fully gapped Mott state. I will show that experiments for electron- and hole-doped BaFe2As2~support an analogous scenario. The doping evolution is dominated by the influence of a Mott insulator that would be realized for half-filled conduction bands, while the stoichiometric compound does not play a special role. Weakly and strongly correlated conduction electrons coexist in much of the phase diagram, a differentiation that increases with hole-doping. We identify the reason for this ``selective Mottness'' in a simple emergent mechanism, an ``orbital decoupling,'' triggered by the strong Hund's coupling. When this mechanism is active charge excitations in the different orbitals are decoupled and each orbital behaves as a single band Hubbard model, where the correlation degree almost only depends on how doped is each orbital from half-filling. This scenario reconciles contrasting evidences on the electronic correlation strength, implies a strong asymmetry between hole- and electron-doping and establishes a deep connection with the cuprates. L. de' Medici, G. Giovannetti and M. Capone, ArXiv:1212.3966 [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F13.00008: Spin excitation spectra of iron-based superconductors from the degenerate double-exchange model Zhidong Leong, Wei-Cheng Lee, Weicheng Lv, Philip Phillips Using a degenerate double-exchange model, we investigate the spin excitation spectra of iron pnictides. The model consists of local spin moments on each Fe site as well as itinerant electrons from the degenerate $d_{xz}$ and $d_{yz}$ orbitals. The local moments interact with each other through antiferromagnetic $J_1$-$J_2$ Heisenberg interactions, and they couple to the itinerant electrons through a ferromagnetic Hund's coupling. We employ the fermionic spinon representation for the local moments and perform a generalized RPA calculation on both spinons and itinerant electrons. We find that in the ($\pi$,0) magnetically-ordered state, the spin-wave excitation at ($\pi$,$\pi$) is pushed to a higher energy due to the presence of itinerant electrons, which is consistent with the previous study using Holstein-Primakoff transformation. In the non-ordered state, the particle-hole continuum keeps the collective spin excitation near ($\pi$,$\pi$) at a higher energy even without any $C_4$ symmetry breaking. The implications for the recent neutron scattering measurement at high temperature will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F13.00009: Orbital-dependent electronic correlations in iron chalcogenide superconductors M. Yi, Z.K. Liu, Y. Zhang, R. Yu, J.J. Lee, R.G. Moore, F.T. Schmitt, W. Li, S.C. Riggs, J.-H. Chu, B. Lv, J. Hu, T.J. Liu, M. Hashimoto, S.K. Mo, Z. Hussain, Z.Q. Mao, C.W. Chu, I.R. Fisher, Q. Si, Z.X. Shen, D.H. Lu The strength of electronic correlations is a fundamental question that has not been fully settled for the iron-based superconductors. There appears to be a systematic trend among the various families of FeSC, from relatively weak correlation in the iron phosphides, to moderate in iron arsenides, to relatively strong in iron chalcogenides. In this study using angle-resolved photoemission spectroscopy, we find a generic behavior in the three families of iron chalcogenides: KxFe2-ySe2, Fe(Te,Se), and FeSe thin films, in which there is an orbital-dependent correlation effect with the dxy orbital strongly renormalized. Moreover, with raised temperature, we find that the dxy orbital-dominated bands in all three materials lose spectral weight while the other orbitals remain metallic. Hence we find this crossover from a metallic superconducting state at low temperatures to an orbital-selective Mott phase at high temperatures to be universal for iron-chalcogenide superconductors. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F13.00010: Nematic state of the pnictides stabilized by the interplay of the lattice with the spin and orbital degrees of freedom Shuhua Liang, Adriana Moreo, Elbio Dagotto The nematic state of the iron-based superconductors is studied in the undoped limit of the three-orbital (xz, yz, xy) spin-fermion model [1] via the introduction of lattice degrees of freedom. Monte Carlo simulations show that in order to stabilize the experimentally observed lattice distortion and nematic order, and to reproduce photoemission experiments, both the spin-lattice and orbital-lattice couplings are needed. The interplay between their respective coupling strengths regulates the separation between the structural and Ne\'el transition temperatures. Experimental results for the temperature dependence of the resistivity anisotropy and the angle-resolved photoemission orbital spectral weight are reproduced by the present numerical simulations [2]. \\[4pt] [1] S. Liang, G. Alvarez, C. Sen, A. Moreo, and E. Dagotto, Phys. Rev. Lett. 109 047001 (2012)\\[0pt] [2] S. Liang, A. Moreo, and E. Dagotto, Phys. Rev. Lett. 111 047004 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F13.00011: Origin of Electronic Nematicity in the Iron Pnictide NaFe$_{1-x}$Co$_x$As Superconductor Verner Thorsmolle, Wei-Lu Zhang, Chenglin Zhang, Scott Carr, Pengcheng Dai, Girsh Blumberg Doped iron pnictides present a complex phase diagram with superconductivity in close proximity to antiferromagnetic and structural transitions (ST). In addition to these phases, an electronic nematic phase has been suggested to be associated with the tetragonal-to-orthorhombic transition at $T_S$. Electronic nematicity breaks $C_4$ rotational symmetry and is believed to be the driving force behind the ST. However, at present, the main interaction behind electronic nematicity and nematic fluctuations remain unexplained. Using electronic Raman spectroscopy we show nematic charge fluctuations in the $XY$ symmetry channel to follow a Curie-Weiss-like temperature dependence extending over a $\sim$200 K range above $T_S$ and in the entire phase diagram including the superconducting phase in NaFe$_{1-x}$Co$_{x}$As ($0 < x < 0.08$) single crystals. The nematicity is found to originate from orbital fluctuations, interconnected with local phonons, and are described in the frame of a classical Curie-Weiss law two-level system corresponding to the $d_{xz}$ and $d_{yz}$ Fe-orbitals. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F13.00012: Superconductivity in Ab initio Low-Energy Effective Model for Iron-Based Superconductor Takahiro Misawa, Masatoshi Imada To clarify microscopic mechanism of superconductivity in iron-based superconductors, we study the {\it ab initio} low-energy effective models for iron-based superconductor~[1], particularly for LaFeAsO by using multi-variable variational Monte Carlo (mVMC) method, which properly takes into account both spatial and dynamical quantum fluctuations. The calculated magnetic order was shown to correctly reproduce the experimental material dependences~[2,3]. By extending these normal state studies, we find that superconductivity emerges in the electron doped LaFeAsO in essential agreement with the experimental results. The pairing satisfies $s\pm$ symmetry. We discuss the role of antiferromagnetic correlations, Mott proximity, and charge and orbital fluctuations in stabilizing the superconductivity. The specific orbital ($d_{X^2-Y^2}$) is shown to play a role of orbital-selected doped Mott insulator in stabilizing the superconducting phase as well as the antiferromagnetic phase. We discuss similarity and dissimilarity to the cuprate superconductors. ~[1]~T. Miyake $et$ $al$., J. Phys. Soc. Jpn. {\bf 79}, 044705 (2010). ~[2]~T. Misawa $et$ $al$., J. Phys. Soc. Jpn. {\bf 80}, 023704 (2011). ~[3]~T. Misawa $et$ $al$., Phys. Rev. Lett. {\bf 108}, 177007 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F13.00013: Effect of electron-phonon interactions on orbital fluctuations in iron-based superconductors: a cDFPT study Yusuke Nomura, Kazuma Nakamura, Ryotaro Arita The pairing mechanism and symmetry of iron-based superconductors is still an open issue. So far, it has been shown that while spin-fluctuations mediate $s$-wave pairing with sign changes in the gap function ($s_{\pm}$-wave), orbital-fluctuations favor $s$-wave pairing without sign changes ($s_{++}$-wave). It has been recently proposed that electron-phonon (el-ph) interactions can enhance the orbital-fluctuations and thus contribute to the superconductivity. To examine the scenario quantitatively, it is highly important to derive, from first principles, an effective model including the phonon degrees of freedom. In this study, we develop an {\it ab initio} downfolding scheme for the el-ph coupled system, which we call constrained density-functional perturbation theory (cDFPT), and estimate the el-ph couplings and the phonon frequencies in the low-energy effective model for LaFeAsO [1]. We analyze the resulting model by the random phase approximation and show that, due to the small phonon-mediated effective exchange interaction, the $s_{\pm}$-wave instability is dominant. Therefore, we conclude that the el-ph interactions cannot be a driving force for the orbital-fluctuation-mediated $s_{++}$-wave state.\\[4pt] [1] Y. Nomura, K. Nakamura, R. Arita, arXiv:1305.2995 [Preview Abstract] |
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